handle communicator status, address a few FIXMEs

[oweals/gnunet.git] / src / transport / gnunet-service-tng.c

/*
 This file is part of GNUnet.
 Copyright (C) 2010-2016, 2018, 2019 GNUnet e.V.

 GNUnet is free software: you can redistribute it and/or modify it
 under the terms of the GNU Affero General Public License as published
 by the Free Software Foundation, either version 3 of the License,
 or (at your option) any later version.

 GNUnet is distributed in the hope that it will be useful, but
 WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 Affero General Public License for more details.

 You should have received a copy of the GNU Affero General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.

     SPDX-License-Identifier: AGPL3.0-or-later
 */
/**
 * @file transport/gnunet-service-tng.c
 * @brief main for gnunet-service-tng
 * @author Christian Grothoff
 *
 * TODO:
 * Implement next:
 * - retransmission logic
 * - track RTT, distance, loss, etc. => requires extra data structures!
 *
 * Later:
 * - change transport-core API to provide proper flow control in both
 *   directions, allow multiple messages per peer simultaneously (tag
 *   confirmations with unique message ID), and replace quota-out with
 *   proper flow control;
 * - if messages are below MTU, consider adding ACKs and other stuff
 *   (requires planning at receiver, and additional MST-style demultiplex
 *    at receiver!)
 * - could avoid copying body of message into each fragment and keep
 *   fragments as just pointers into the original message and only
 *   fully build fragments just before transmission (optimization, should
 *   reduce CPU and memory use)
 * - When we passively learned DV (with unconfirmed freshness), we
 *   right now add the path to our list but with a zero path_valid_until
 *   time and only use it for unconfirmed routes.  However, we could consider
 *   triggering an explicit validation mechansim ourselves, specifically routing
 *   a challenge-response message over the path (OPTIMIZATION-FIXME).
 *
 * FIXME (without marks in the code!):
 * - proper use/initialization of timestamps in messages exchanged
 *   during DV learning
 * - persistence of monotonic time obtained from other peers
 *   in PEERSTORE (by message type)
 *
 * Optimizations:
 * - use shorthashmap on msg_uuid's when matching reliability/fragment ACKs
 *   against our pending message queue (requires additional per neighbour
 *   hash map to be maintained, avoids possible linear scan on pending msgs)
 * - queue_send_msg and route_message both by API design have to make copies
 *   of the payload, and route_message on top of that requires a malloc/free.
 *   Change design to approximate "zero" copy better...
 *
 * Design realizations / discussion:
 * - communicators do flow control by calling MQ "notify sent"
 *   when 'ready'. They determine flow implicitly (i.e. TCP blocking)
 *   or explicitly via backchannel FC ACKs.  As long as the
 *   channel is not full, they may 'notify sent' even if the other
 *   peer has not yet confirmed receipt. The other peer confirming
 *   is _only_ for FC, not for more reliable transmission; reliable
 *   transmission (i.e. of fragments) is left to _transport_.
 * - ACKs sent back in uni-directional communicators are done via
 *   the background channel API; here transport _may_ initially
 *   broadcast (with bounded # hops) if no path is known;
 * - transport should _integrate_ DV-routing and build a view of
 *   the network; then background channel traffic can be
 *   routed via DV as well as explicit "DV" traffic.
 * - background channel is also used for ACKs and NAT traversal support
 * - transport service is responsible for AEAD'ing the background
 *   channel, timestamps and monotonic time are used against replay
 *   of old messages -> peerstore needs to be supplied with
 *   "latest timestamps seen" data
 * - if transport implements DV, we likely need a 3rd peermap
 *   in addition to ephemerals and (direct) neighbours
 *   ==> check if stuff needs to be moved out of "Neighbour"
 * - transport should encapsualte core-level messages and do its
 *   own ACKing for RTT/goodput/loss measurements _and_ fragment
 *   for retransmission
 */
#include "platform.h"
#include "gnunet_util_lib.h"
#include "gnunet_statistics_service.h"
#include "gnunet_transport_monitor_service.h"
#include "gnunet_peerstore_service.h"
#include "gnunet_hello_lib.h"
#include "gnunet_signatures.h"
#include "transport.h"


/**
 * What is the size we assume for a read operation in the
 * absence of an MTU for the purpose of flow control?
 */
#define IN_PACKET_SIZE_WITHOUT_MTU 128

/**
 * Minimum number of hops we should forward DV learn messages
 * even if they are NOT useful for us in hope of looping
 * back to the initiator?
 *
 * FIXME: allow initiator some control here instead?
 */
#define MIN_DV_PATH_LENGTH_FOR_INITIATOR 3

/**
 * Maximum DV distance allowed ever.
 */
#define MAX_DV_HOPS_ALLOWED 16

/**
 * Maximum number of DV learning activities we may
 * have pending at the same time.
 */
#define MAX_DV_LEARN_PENDING 64

/**
 * Maximum number of DV paths we keep simultaneously to the same target.
 */
#define MAX_DV_PATHS_TO_TARGET 3

/**
 * If a queue delays the next message by more than this number
 * of seconds we log a warning. Note: this is for testing,
 * the value chosen here might be too aggressively low!
 */
#define DELAY_WARN_THRESHOLD \
  GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_SECONDS, 5)

/**
 * We only consider queues as "quality" connections when
 * suppressing the generation of DV initiation messages if
 * the latency of the queue is below this threshold.
 */
#define DV_QUALITY_RTT_THRESHOLD \
  GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_SECONDS, 1)

/**
 * How long do we consider a DV path valid if we see no
 * further updates on it? Note: the value chosen here might be too low!
 */
#define DV_PATH_VALIDITY_TIMEOUT \
  GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_MINUTES, 5)

/**
 * How long before paths expire would we like to (re)discover DV paths? Should
 * be below #DV_PATH_VALIDITY_TIMEOUT.
 */
#define DV_PATH_DISCOVERY_FREQUENCY \
  GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_MINUTES, 4)

/**
 * How long are ephemeral keys valid?
 */
#define EPHEMERAL_VALIDITY \
  GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_HOURS, 4)

/**
 * How long do we keep partially reassembled messages around before giving up?
 */
#define REASSEMBLY_EXPIRATION \
  GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_MINUTES, 4)

/**
 * What is the fastest rate at which we send challenges *if* we keep learning
 * an address (gossip, DHT, etc.)?
 */
#define FAST_VALIDATION_CHALLENGE_FREQ \
  GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_MINUTES, 1)

/**
 * What is the slowest rate at which we send challenges?
 */
#define MAX_VALIDATION_CHALLENGE_FREQ \
  GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_DAYS, 1)

/**
 * What is the non-randomized base frequency at which we
 * would initiate DV learn messages?
 */
#define DV_LEARN_BASE_FREQUENCY GNUNET_TIME_UNIT_MINUTES

/**
 * How many good connections (confirmed, bi-directional, not DV)
 * do we need to have to suppress initiating DV learn messages?
 */
#define DV_LEARN_QUALITY_THRESHOLD 100

/**
 * When do we forget an invalid address for sure?
 */
#define MAX_ADDRESS_VALID_UNTIL \
  GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_MONTHS, 1)

/**
 * How long do we consider an address valid if we just checked?
 */
#define ADDRESS_VALIDATION_LIFETIME \
  GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_HOURS, 4)

/**
 * What is the maximum frequency at which we do address validation?
 * A random value between 0 and this value is added when scheduling
 * the #validation_task (both to ensure we do not validate too often,
 * and to randomize a bit).
 */
#define MIN_DELAY_ADDRESS_VALIDATION GNUNET_TIME_UNIT_MILLISECONDS

/**
 * How many network RTTs before an address validation expires should we begin
 * trying to revalidate? (Note that the RTT used here is the one that we
 * experienced during the last validation, not necessarily the latest RTT
 * observed).
 */
#define VALIDATION_RTT_BUFFER_FACTOR 3

/**
 * How many messages can we have pending for a given communicator
 * process before we start to throttle that communicator?
 *
 * Used if a communicator might be CPU-bound and cannot handle the traffic.
 */
#define COMMUNICATOR_TOTAL_QUEUE_LIMIT 512

/**
 * How many messages can we have pending for a given queue (queue to
 * a particular peer via a communicator) process before we start to
 * throttle that queue?
 */
#define QUEUE_LENGTH_LIMIT 32


GNUNET_NETWORK_STRUCT_BEGIN

/**
 * Outer layer of an encapsulated backchannel message.
 */
struct TransportBackchannelEncapsulationMessage
{
  /**
   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_BACKCHANNEL_ENCAPSULATION.
   */
  struct GNUNET_MessageHeader header;

  /**
   * Distance the backchannel message has traveled, to be updated at
   * each hop.  Used to bound the number of hops in case a backchannel
   * message is broadcast and thus travels without routing
   * information (during initial backchannel discovery).
   */
  uint32_t distance;

  /**
   * Target's peer identity (as backchannels may be transmitted
   * indirectly, or even be broadcast).
   */
  struct GNUNET_PeerIdentity target;

  /**
   * Ephemeral key setup by the sender for @e target, used
   * to encrypt the payload.
   */
  struct GNUNET_CRYPTO_EcdhePublicKey ephemeral_key;

  /**
   * We use an IV here as the @e ephemeral_key is re-used for
   * #EPHEMERAL_VALIDITY time to avoid re-signing it all the time.
   */
  struct GNUNET_ShortHashCode iv;

  /**
   * HMAC over the ciphertext of the encrypted, variable-size
   * body that follows.  Verified via DH of @e target and
   * @e ephemeral_key
   */
  struct GNUNET_HashCode hmac;

  /* Followed by encrypted, variable-size payload */
};


/**
 * Body by which a peer confirms that it is using an ephemeral key.
 */
struct EphemeralConfirmation
{

  /**
   * Purpose is #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_EPHEMERAL
   */
  struct GNUNET_CRYPTO_EccSignaturePurpose purpose;

  /**
   * How long is this signature over the ephemeral key valid?
   *
   * Note that the receiver MUST IGNORE the absolute time, and only interpret
   * the value as a mononic time and reject "older" values than the last one
   * observed.  This is necessary as we do not want to require synchronized
   * clocks and may not have a bidirectional communication channel.
   *
   * Even with this, there is no real guarantee against replay achieved here,
   * unless the latest timestamp is persisted.  While persistence should be
   * provided via PEERSTORE, we do not consider the mechanism reliable!  Thus,
   * communicators must protect against replay attacks when using backchannel
   * communication!
   */
  struct GNUNET_TIME_AbsoluteNBO ephemeral_validity;

  /**
   * Target's peer identity.
   */
  struct GNUNET_PeerIdentity target;

  /**
   * Ephemeral key setup by the sender for @e target, used
   * to encrypt the payload.
   */
  struct GNUNET_CRYPTO_EcdhePublicKey ephemeral_key;
};


/**
 * Plaintext of the variable-size payload that is encrypted
 * within a `struct TransportBackchannelEncapsulationMessage`
 */
struct TransportBackchannelRequestPayload
{

  /**
   * Sender's peer identity.
   */
  struct GNUNET_PeerIdentity sender;

  /**
   * Signature of the sender over an
   * #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_EPHEMERAL.
   */
  struct GNUNET_CRYPTO_EddsaSignature sender_sig;

  /**
   * How long is this signature over the ephemeral key valid?
   *
   * Note that the receiver MUST IGNORE the absolute time, and only interpret
   * the value as a mononic time and reject "older" values than the last one
   * observed.  This is necessary as we do not want to require synchronized
   * clocks and may not have a bidirectional communication channel.
   *
   * Even with this, there is no real guarantee against replay achieved here,
   * unless the latest timestamp is persisted.  While persistence should be
   * provided via PEERSTORE, we do not consider the mechanism reliable!  Thus,
   * communicators must protect against replay attacks when using backchannel
   * communication!
   */
  struct GNUNET_TIME_AbsoluteNBO ephemeral_validity;

  /**
   * Current monotonic time of the sending transport service.  Used to
   * detect replayed messages.  Note that the receiver should remember
   * a list of the recently seen timestamps and only reject messages
   * if the timestamp is in the list, or the list is "full" and the
   * timestamp is smaller than the lowest in the list.
   *
   * Like the @e ephemeral_validity, the list of timestamps per peer should be
   * persisted to guard against replays after restarts.
   */
  struct GNUNET_TIME_AbsoluteNBO monotonic_time;

  /* Followed by a `struct GNUNET_MessageHeader` with a message
     for a communicator */

  /* Followed by a 0-termianted string specifying the name of
     the communicator which is to receive the message */
};


/**
 * Outer layer of an encapsulated unfragmented application message sent
 * over an unreliable channel.
 */
struct TransportReliabilityBox
{
  /**
   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_BOX
   */
  struct GNUNET_MessageHeader header;

  /**
   * Number of messages still to be sent before a commulative
   * ACK is requested.  Zero if an ACK is requested immediately.
   * In NBO.  Note that the receiver may send the ACK faster
   * if it believes that is reasonable.
   */
  uint32_t ack_countdown GNUNET_PACKED;

  /**
   * Unique ID of the message used for signalling receipt of
   * messages sent over possibly unreliable channels.  Should
   * be a random.
   */
  struct GNUNET_ShortHashCode msg_uuid;
};


/**
 * Confirmation that the receiver got a
 * #GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_BOX. Note that the
 * confirmation may be transmitted over a completely different queue,
 * so ACKs are identified by a combination of PID of sender and
 * message UUID, without the queue playing any role!
 */
struct TransportReliabilityAckMessage
{
  /**
   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_ACK
   */
  struct GNUNET_MessageHeader header;

  /**
   * Reserved. Zero.
   */
  uint32_t reserved GNUNET_PACKED;

  /**
   * How long was the ACK delayed relative to the average time of
   * receipt of the messages being acknowledged?  Used to calculate
   * the average RTT by taking the receipt time of the ack minus the
   * average transmission time of the sender minus this value.
   */
  struct GNUNET_TIME_RelativeNBO avg_ack_delay;

  /* followed by any number of `struct GNUNET_ShortHashCode`
     messages providing ACKs */
};


/**
 * Outer layer of an encapsulated fragmented application message.
 */
struct TransportFragmentBox
{
  /**
   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_FRAGMENT
   */
  struct GNUNET_MessageHeader header;

  /**
   * Unique ID of this fragment (and fragment transmission!). Will
   * change even if a fragement is retransmitted to make each
   * transmission attempt unique! Should be incremented by one for
   * each fragment transmission. If a client receives a duplicate
   * fragment (same @e frag_off), it must send
   * #GNUNET_MESSAGE_TYPE_TRANSPORT_FRAGMENT_ACK immediately.
   */
  uint32_t frag_uuid GNUNET_PACKED;

  /**
   * Original message ID for of the message that all the1
   * fragments belong to.  Must be the same for all fragments.
   */
  struct GNUNET_ShortHashCode msg_uuid;

  /**
   * Offset of this fragment in the overall message.
   */
  uint16_t frag_off GNUNET_PACKED;

  /**
   * Total size of the message that is being fragmented.
   */
  uint16_t msg_size GNUNET_PACKED;
};


/**
 * Outer layer of an fragmented application message sent over a queue
 * with finite MTU.  When a #GNUNET_MESSAGE_TYPE_TRANSPORT_FRAGMENT is
 * received, the receiver has two RTTs or 64 further fragments with
 * the same basic message time to send an acknowledgement, possibly
 * acknowledging up to 65 fragments in one ACK.  ACKs must also be
 * sent immediately once all fragments were sent.
 */
struct TransportFragmentAckMessage
{
  /**
   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_FRAGMENT_ACK
   */
  struct GNUNET_MessageHeader header;

  /**
   * Unique ID of the lowest fragment UUID being acknowledged.
   */
  uint32_t frag_uuid GNUNET_PACKED;

  /**
   * Bitfield of up to 64 additional fragments following the
   * @e msg_uuid being acknowledged by this message.
   */
  uint64_t extra_acks GNUNET_PACKED;

  /**
   * Original message ID for of the message that all the
   * fragments belong to.
   */
  struct GNUNET_ShortHashCode msg_uuid;

  /**
   * How long was the ACK delayed relative to the average time of
   * receipt of the fragments being acknowledged?  Used to calculate
   * the average RTT by taking the receipt time of the ack minus the
   * average transmission time of the sender minus this value.
   */
  struct GNUNET_TIME_RelativeNBO avg_ack_delay;

  /**
   * How long until the receiver will stop trying reassembly
   * of this message?
   */
  struct GNUNET_TIME_RelativeNBO reassembly_timeout;
};


/**
 * Content signed by the initator during DV learning.
 *
 * The signature is required to prevent DDoS attacks. A peer sending out this
 * message is potentially generating a lot of traffic that will go back to the
 * initator, as peers receiving this message will try to let the initiator
 * know that they got the message.
 *
 * Without this signature, an attacker could abuse this mechanism for traffic
 * amplification, sending a lot of traffic to a peer by putting out this type
 * of message with the victim's peer identity.
 *
 * Even with just a signature, traffic amplification would be possible via
 * replay attacks. The @e monotonic_time limits such replay attacks, as every
 * potential amplificator will check the @e monotonic_time and only respond
 * (at most) once per message.
 */
struct DvInitPS
{
  /**
   * Purpose is #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_INITIATOR
   */
  struct GNUNET_CRYPTO_EccSignaturePurpose purpose;

  /**
   * Time at the initiator when generating the signature.
   *
   * Note that the receiver MUST IGNORE the absolute time, and only interpret
   * the value as a mononic time and reject "older" values than the last one
   * observed.  This is necessary as we do not want to require synchronized
   * clocks and may not have a bidirectional communication channel.
   *
   * Even with this, there is no real guarantee against replay achieved here,
   * unless the latest timestamp is persisted.  Persistence should be
   * provided via PEERSTORE if possible.
   */
  struct GNUNET_TIME_AbsoluteNBO monotonic_time;

  /**
   * Challenge value used by the initiator to re-identify the path.
   */
  struct GNUNET_ShortHashCode challenge;
};


/**
 * Content signed by each peer during DV learning.
 *
 * This assues the initiator of the DV learning operation that the hop from @e
 * pred via the signing peer to @e succ actually exists.  This makes it
 * impossible for an adversary to supply the network with bogus routes.
 *
 * The @e challenge is included to provide replay protection for the
 * initiator. This way, the initiator knows that the hop existed after the
 * original @e challenge was first transmitted, providing a freshness metric.
 *
 * Peers other than the initiator that passively learn paths by observing
 * these messages do NOT benefit from this. Here, an adversary may indeed
 * replay old messages.  Thus, passively learned paths should always be
 * immediately marked as "potentially stale".
 */
struct DvHopPS
{
  /**
   * Purpose is #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_HOP
   */
  struct GNUNET_CRYPTO_EccSignaturePurpose purpose;

  /**
   * Identity of the previous peer on the path.
   */
  struct GNUNET_PeerIdentity pred;

  /**
   * Identity of the next peer on the path.
   */
  struct GNUNET_PeerIdentity succ;

  /**
   * Challenge value used by the initiator to re-identify the path.
   */
  struct GNUNET_ShortHashCode challenge;
};


/**
 * An entry describing a peer on a path in a
 * `struct TransportDVLearn` message.
 */
struct DVPathEntryP
{
  /**
   * Identity of a peer on the path.
   */
  struct GNUNET_PeerIdentity hop;

  /**
   * Signature of this hop over the path, of purpose
   * #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_HOP
   */
  struct GNUNET_CRYPTO_EddsaSignature hop_sig;
};


/**
 * Internal message used by transport for distance vector learning.
 * If @e num_hops does not exceed the threshold, peers should append
 * themselves to the peer list and flood the message (possibly only
 * to a subset of their neighbours to limit discoverability of the
 * network topology).  To the extend that the @e bidirectional bits
 * are set, peers may learn the inverse paths even if they did not
 * initiate.
 *
 * Unless received on a bidirectional queue and @e num_hops just
 * zero, peers that can forward to the initator should always try to
 * forward to the initiator.
 */
struct TransportDVLearn
{
  /**
   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_DV_LEARN
   */
  struct GNUNET_MessageHeader header;

  /**
   * Number of hops this messages has travelled, in NBO. Zero if
   * sent by initiator.
   */
  uint16_t num_hops GNUNET_PACKED;

  /**
   * Bitmask of the last 16 hops indicating whether they are confirmed
   * available (without DV) in both directions or not, in NBO.  Used
   * to possibly instantly learn a path in both directions.  Each peer
   * should shift this value by one to the left, and then set the
   * lowest bit IF the current sender can be reached from it (without
   * DV routing).
   */
  uint16_t bidirectional GNUNET_PACKED;

  /**
   * Peers receiving this message and delaying forwarding to other
   * peers for any reason should increment this value by the non-network
   * delay created by the peer.
   */
  struct GNUNET_TIME_RelativeNBO non_network_delay;

  /**
   * Signature of this hop over the path, of purpose
   * #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_INITIATOR
   */
  struct GNUNET_CRYPTO_EddsaSignature init_sig;

  /**
   * Identity of the peer that started this learning activity.
   */
  struct GNUNET_PeerIdentity initiator;

  /**
   * Challenge value used by the initiator to re-identify the path.
   */
  struct GNUNET_ShortHashCode challenge;

  /* Followed by @e num_hops `struct DVPathEntryP` values,
     excluding the initiator of the DV trace; the last entry is the
     current sender; the current peer must not be included. */
};


/**
 * Outer layer of an encapsulated message send over multiple hops.
 * The path given only includes the identities of the subsequent
 * peers, i.e. it will be empty if we are the receiver. Each
 * forwarding peer should scan the list from the end, and if it can,
 * forward to the respective peer. The list should then be shortened
 * by all the entries up to and including that peer.  Each hop should
 * also increment @e total_hops to allow the receiver to get a precise
 * estimate on the number of hops the message travelled.  Senders must
 * provide a learned path that thus should work, but intermediaries
 * know of a shortcut, they are allowed to send the message via that
 * shortcut.
 *
 * If a peer finds itself still on the list, it must drop the message.
 */
struct TransportDVBox
{
  /**
   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_DV_BOX
   */
  struct GNUNET_MessageHeader header;

  /**
   * Number of total hops this messages travelled. In NBO.
   * @e origin sets this to zero, to be incremented at
   * each hop.
   */
  uint16_t total_hops GNUNET_PACKED;

  /**
   * Number of hops this messages includes. In NBO.
   */
  uint16_t num_hops GNUNET_PACKED;

  /**
   * Identity of the peer that originated the message.
   */
  struct GNUNET_PeerIdentity origin;

  /* Followed by @e num_hops `struct GNUNET_PeerIdentity` values;
     excluding the @e origin and the current peer, the last must be
     the ultimate target; if @e num_hops is zero, the receiver of this
     message is the ultimate target. */

  /* Followed by the actual message, which itself may be
     another box, but not a DV_LEARN or DV_BOX message! */
};


/**
 * Message send to another peer to validate that it can indeed
 * receive messages at a particular address.
 */
struct TransportValidationChallenge
{

  /**
   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_ADDRESS_VALIDATION_CHALLENGE
   */
  struct GNUNET_MessageHeader header;

  /**
   * Zero.
   */
  uint32_t reserved GNUNET_PACKED;

  /**
   * Challenge to be signed by the receiving peer.
   */
  struct GNUNET_ShortHashCode challenge;

  /**
   * Timestamp of the sender, to be copied into the reply
   * to allow sender to calculate RTT.
   */
  struct GNUNET_TIME_AbsoluteNBO sender_time;
};


/**
 * Message signed by a peer to confirm that it can indeed
 * receive messages at a particular address.
 */
struct TransportValidationPS
{

  /**
   * Purpose is #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_CHALLENGE
   */
  struct GNUNET_CRYPTO_EccSignaturePurpose purpose;

  /**
   * How long does the sender believe the address on
   * which the challenge was received to remain valid?
   */
  struct GNUNET_TIME_RelativeNBO validity_duration;

  /**
   * Challenge signed by the receiving peer.
   */
  struct GNUNET_ShortHashCode challenge;
};


/**
 * Message send to a peer to respond to a
 * #GNUNET_MESSAGE_TYPE_ADDRESS_VALIDATION_CHALLENGE
 */
struct TransportValidationResponse
{

  /**
   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_ADDRESS_VALIDATION_RESPONSE
   */
  struct GNUNET_MessageHeader header;

  /**
   * Zero.
   */
  uint32_t reserved GNUNET_PACKED;

  /**
   * The peer's signature matching the
   * #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_CHALLENGE purpose.
   */
  struct GNUNET_CRYPTO_EddsaSignature signature;

  /**
   * The challenge that was signed by the receiving peer.
   */
  struct GNUNET_ShortHashCode challenge;

  /**
   * Original timestamp of the sender (was @code{sender_time}),
   * copied into the reply to allow sender to calculate RTT.
   */
  struct GNUNET_TIME_AbsoluteNBO origin_time;

  /**
   * How long does the sender believe this address to remain
   * valid?
   */
  struct GNUNET_TIME_RelativeNBO validity_duration;
};


GNUNET_NETWORK_STRUCT_END


/**
 * What type of client is the `struct TransportClient` about?
 */
enum ClientType
{
  /**
   * We do not know yet (client is fresh).
   */
  CT_NONE = 0,

  /**
   * Is the CORE service, we need to forward traffic to it.
   */
  CT_CORE = 1,

  /**
   * It is a monitor, forward monitor data.
   */
  CT_MONITOR = 2,

  /**
   * It is a communicator, use for communication.
   */
  CT_COMMUNICATOR = 3,

  /**
   * "Application" telling us where to connect (i.e. TOPOLOGY, DHT or CADET).
   */
  CT_APPLICATION = 4
};


/**
 * When did we launch this DV learning activity?
 */
struct LearnLaunchEntry
{

  /**
   * Kept (also) in a DLL sorted by launch time.
   */
  struct LearnLaunchEntry *prev;

  /**
   * Kept (also) in a DLL sorted by launch time.
   */
  struct LearnLaunchEntry *next;

  /**
   * Challenge that uniquely identifies this activity.
   */
  struct GNUNET_ShortHashCode challenge;

  /**
   * When did we transmit the DV learn message (used to calculate RTT) and
   * determine freshness of paths learned via this operation.
   */
  struct GNUNET_TIME_Absolute launch_time;
};


/**
 * Entry in our cache of ephemeral keys we currently use.  This way, we only
 * sign an ephemeral once per @e target, and then can re-use it over multiple
 * #GNUNET_MESSAGE_TYPE_TRANSPORT_BACKCHANNEL_ENCAPSULATION messages (as
 * signing is expensive and in some cases we may use backchannel messages a
 * lot).
 */
struct EphemeralCacheEntry
{

  /**
   * Target's peer identity (we don't re-use ephemerals
   * to limit linkability of messages).
   */
  struct GNUNET_PeerIdentity target;

  /**
   * Signature affirming @e ephemeral_key of type
   * #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_EPHEMERAL
   */
  struct GNUNET_CRYPTO_EddsaSignature sender_sig;

  /**
   * How long is @e sender_sig valid
   */
  struct GNUNET_TIME_Absolute ephemeral_validity;

  /**
   * Our ephemeral key.
   */
  struct GNUNET_CRYPTO_EcdhePublicKey ephemeral_key;

  /**
   * Our private ephemeral key.
   */
  struct GNUNET_CRYPTO_EcdhePrivateKey private_key;

  /**
   * Node in the ephemeral cache for this entry.
   * Used for expiration.
   */
  struct GNUNET_CONTAINER_HeapNode *hn;
};


/**
 * Client connected to the transport service.
 */
struct TransportClient;


/**
 * A neighbour that at least one communicator is connected to.
 */
struct Neighbour;


/**
 * Entry in our #dv_routes table, representing a (set of) distance
 * vector routes to a particular peer.
 */
struct DistanceVector;

/**
 * One possible hop towards a DV target.
 */
struct DistanceVectorHop
{

  /**
   * Kept in a MDLL, sorted by @e timeout.
   */
  struct DistanceVectorHop *next_dv;

  /**
   * Kept in a MDLL, sorted by @e timeout.
   */
  struct DistanceVectorHop *prev_dv;

  /**
   * Kept in a MDLL.
   */
  struct DistanceVectorHop *next_neighbour;

  /**
   * Kept in a MDLL.
   */
  struct DistanceVectorHop *prev_neighbour;

  /**
   * What would be the next hop to @e target?
   */
  struct Neighbour *next_hop;

  /**
   * Distance vector entry this hop belongs with.
   */
  struct DistanceVector *dv;

  /**
   * Array of @e distance hops to the target, excluding @e next_hop.
   * NULL if the entire path is us to @e next_hop to `target`. Allocated
   * at the end of this struct. Excludes the target itself!
   */
  const struct GNUNET_PeerIdentity *path;

  /**
   * At what time do we forget about this path unless we see it again
   * while learning?
   */
  struct GNUNET_TIME_Absolute timeout;

  /**
   * For how long is the validation of this path considered
   * valid?
   * Set to ZERO if the path is learned by snooping on DV learn messages
   * initiated by other peers, and to the time at which we generated the
   * challenge for DV learn operations this peer initiated.
   */
  struct GNUNET_TIME_Absolute path_valid_until;

  /**
   * Number of hops in total to the `target` (excluding @e next_hop and `target`
   * itself). Thus 0 still means a distance of 2 hops (to @e next_hop and then
   * to `target`).
   */
  unsigned int distance;
};


/**
 * Entry in our #dv_routes table, representing a (set of) distance
 * vector routes to a particular peer.
 */
struct DistanceVector
{

  /**
   * To which peer is this a route?
   */
  struct GNUNET_PeerIdentity target;

  /**
   * Known paths to @e target.
   */
  struct DistanceVectorHop *dv_head;

  /**
   * Known paths to @e target.
   */
  struct DistanceVectorHop *dv_tail;

  /**
   * Task scheduled to purge expired paths from @e dv_head MDLL.
   */
  struct GNUNET_SCHEDULER_Task *timeout_task;

  /**
   * Task scheduled to possibly notfiy core that this queue is no longer
   * counting as confirmed.  Runs the #core_queue_visibility_check().
   */
  struct GNUNET_SCHEDULER_Task *visibility_task;

  /**
   * Quota at which CORE is allowed to transmit to this peer
   * (note that the value CORE should actually be told is this
   *  value plus the respective value in `struct Neighbour`).
   * Should match the sum of the quotas of all of the paths.
   *
   * FIXME: not yet set, tricky to get right given multiple paths,
   *        many of which may be inactive! (=> Idea: measure???)
   * FIXME: how do we set this value initially when we tell CORE?
   *    Options: start at a minimum value or at literally zero?
   *         (=> Current thought: clean would be zero!)
   */
  struct GNUNET_BANDWIDTH_Value32NBO quota_out;

  /**
   * Is one of the DV paths in this struct 'confirmed' and thus
   * the cause for CORE to see this peer as connected? (Note that
   * the same may apply to a `struct Neighbour` at the same time.)
   */
  int core_visible;
};


/**
 * A queue is a message queue provided by a communicator
 * via which we can reach a particular neighbour.
 */
struct Queue;

/**
 * Message awaiting transmission. See detailed comments below.
 */
struct PendingMessage;

/**
 * Entry identifying transmission in one of our `struct
 * Queue` which still awaits an ACK.  This is used to
 * ensure we do not overwhelm a communicator and limit the number of
 * messages outstanding per communicator (say in case communicator is
 * CPU bound) and per queue (in case bandwidth allocation exceeds
 * what the communicator can actually provide towards a particular
 * peer/target).
 */
struct QueueEntry
{

  /**
   * Kept as a DLL.
   */
  struct QueueEntry *next;

  /**
   * Kept as a DLL.
   */
  struct QueueEntry *prev;

  /**
   * Queue this entry is queued with.
   */
  struct Queue *queue;

  /**
   * Pending message this entry is for, or NULL for none.
   */
  struct PendingMessage *pm;

  /**
   * Message ID used for this message with the queue used for transmission.
   */
  uint64_t mid;
};


/**
 * A queue is a message queue provided by a communicator
 * via which we can reach a particular neighbour.
 */
struct Queue
{
  /**
   * Kept in a MDLL.
   */
  struct Queue *next_neighbour;

  /**
   * Kept in a MDLL.
   */
  struct Queue *prev_neighbour;

  /**
   * Kept in a MDLL.
   */
  struct Queue *prev_client;

  /**
   * Kept in a MDLL.
   */
  struct Queue *next_client;

  /**
   * Head of DLL of unacked transmission requests.
   */
  struct QueueEntry *queue_head;

  /**
   * End of DLL of unacked transmission requests.
   */
  struct QueueEntry *queue_tail;

  /**
   * Which neighbour is this queue for?
   */
  struct Neighbour *neighbour;

  /**
   * Which communicator offers this queue?
   */
  struct TransportClient *tc;

  /**
   * Address served by the queue.
   */
  const char *address;

  /**
   * Task scheduled for the time when this queue can (likely) transmit the
   * next message. Still needs to check with the @e tracker_out to be sure.
   */
  struct GNUNET_SCHEDULER_Task *transmit_task;

  /**
   * Task scheduled to possibly notfiy core that this queue is no longer
   * counting as confirmed.  Runs the #core_queue_visibility_check().
   */
  struct GNUNET_SCHEDULER_Task *visibility_task;

  /**
   * Our current RTT estimate for this queue.
   */
  struct GNUNET_TIME_Relative rtt;

  /**
   * How long do *we* consider this @e address to be valid?  In the past or
   * zero if we have not yet validated it.  Can be updated based on
   * challenge-response validations (via address validation logic), or when we
   * receive ACKs that we can definitively map to transmissions via this
   * queue.
   */
  struct GNUNET_TIME_Absolute validated_until;

  /**
   * Message ID generator for transmissions on this queue.
   */
  uint64_t mid_gen;

  /**
   * Unique identifier of this queue with the communicator.
   */
  uint32_t qid;

  /**
   * Maximum transmission unit supported by this queue.
   */
  uint32_t mtu;

  /**
   * Messages pending.
   */
  uint32_t num_msg_pending;

  /**
   * Bytes pending.
   */
  uint32_t num_bytes_pending;

  /**
   * Length of the DLL starting at @e queue_head.
   */
  unsigned int queue_length;

  /**
   * Network type offered by this queue.
   */
  enum GNUNET_NetworkType nt;

  /**
   * Connection status for this queue.
   */
  enum GNUNET_TRANSPORT_ConnectionStatus cs;

  /**
   * How much outbound bandwidth do we have available for this queue?
   */
  struct GNUNET_BANDWIDTH_Tracker tracker_out;

  /**
   * How much inbound bandwidth do we have available for this queue?
   */
  struct GNUNET_BANDWIDTH_Tracker tracker_in;
};


/**
 * Information we keep for a message that we are reassembling.
 */
struct ReassemblyContext
{

  /**
   * Original message ID for of the message that all the
   * fragments belong to.
   */
  struct GNUNET_ShortHashCode msg_uuid;

  /**
   * Which neighbour is this context for?
   */
  struct Neighbour *neighbour;

  /**
   * Entry in the reassembly heap (sorted by expiration).
   */
  struct GNUNET_CONTAINER_HeapNode *hn;

  /**
   * Bitfield with @e msg_size bits representing the positions
   * where we have received fragments.  When we receive a fragment,
   * we check the bits in @e bitfield before incrementing @e msg_missing.
   *
   * Allocated after the reassembled message.
   */
  uint8_t *bitfield;

  /**
   * Task for sending ACK. We may send ACKs either because of hitting
   * the @e extra_acks limit, or based on time and @e num_acks.  This
   * task is for the latter case.
   */
  struct GNUNET_SCHEDULER_Task *ack_task;

  /**
   * At what time will we give up reassembly of this message?
   */
  struct GNUNET_TIME_Absolute reassembly_timeout;

  /**
   * Average delay of all acks in @e extra_acks and @e frag_uuid.
   * Should be reset to zero when @e num_acks is set to 0.
   */
  struct GNUNET_TIME_Relative avg_ack_delay;

  /**
   * Time we received the last fragment.  @e avg_ack_delay must be
   * incremented by now - @e last_frag multiplied by @e num_acks.
   */
  struct GNUNET_TIME_Absolute last_frag;

  /**
   * Bitfield of up to 64 additional fragments following @e frag_uuid
   * to be acknowledged in the next cummulative ACK.
   */
  uint64_t extra_acks;

  /**
   * Unique ID of the lowest fragment UUID to be acknowledged in the
   * next cummulative ACK.  Only valid if @e num_acks > 0.
   */
  uint32_t frag_uuid;

  /**
   * Number of ACKs we have accumulated so far.  Reset to 0
   * whenever we send a #GNUNET_MESSAGE_TYPE_TRANSPORT_FRAGMENT_ACK.
   */
  unsigned int num_acks;

  /**
   * How big is the message we are reassembling in total?
   */
  uint16_t msg_size;

  /**
   * How many bytes of the message are still missing?  Defragmentation
   * is complete when @e msg_missing == 0.
   */
  uint16_t msg_missing;

  /* Followed by @e msg_size bytes of the (partially) defragmented original
   * message */

  /* Followed by @e bitfield data */
};


/**
 * A neighbour that at least one communicator is connected to.
 */
struct Neighbour
{

  /**
   * Which peer is this about?
   */
  struct GNUNET_PeerIdentity pid;

  /**
   * Map with `struct ReassemblyContext` structs for fragments under
   * reassembly. May be NULL if we currently have no fragments from
   * this @e pid (lazy initialization).
   */
  struct GNUNET_CONTAINER_MultiShortmap *reassembly_map;

  /**
   * Heap with `struct ReassemblyContext` structs for fragments under
   * reassembly. May be NULL if we currently have no fragments from
   * this @e pid (lazy initialization).
   */
  struct GNUNET_CONTAINER_Heap *reassembly_heap;

  /**
   * Task to free old entries from the @e reassembly_heap and @e reassembly_map.
   */
  struct GNUNET_SCHEDULER_Task *reassembly_timeout_task;

  /**
   * Head of list of messages pending for this neighbour.
   */
  struct PendingMessage *pending_msg_head;

  /**
   * Tail of list of messages pending for this neighbour.
   */
  struct PendingMessage *pending_msg_tail;

  /**
   * Head of MDLL of DV hops that have this neighbour as next hop. Must be
   * purged if this neighbour goes down.
   */
  struct DistanceVectorHop *dv_head;

  /**
   * Tail of MDLL of DV hops that have this neighbour as next hop. Must be
   * purged if this neighbour goes down.
   */
  struct DistanceVectorHop *dv_tail;

  /**
   * Head of DLL of queues to this peer.
   */
  struct Queue *queue_head;

  /**
   * Tail of DLL of queues to this peer.
   */
  struct Queue *queue_tail;

  /**
   * Task run to cleanup pending messages that have exceeded their timeout.
   */
  struct GNUNET_SCHEDULER_Task *timeout_task;

  /**
   * Quota at which CORE is allowed to transmit to this peer
   * (note that the value CORE should actually be told is this
   *  value plus the respective value in `struct DistanceVector`).
   * Should match the sum of the quotas of all of the queues.
   *
   * FIXME: not yet set, tricky to get right given multiple queues!
   *        (=> Idea: measure???)
   * FIXME: how do we set this value initially when we tell CORE?
   *    Options: start at a minimum value or at literally zero?
   *         (=> Current thought: clean would be zero!)
   */
  struct GNUNET_BANDWIDTH_Value32NBO quota_out;

  /**
   * What is the earliest timeout of any message in @e pending_msg_tail?
   */
  struct GNUNET_TIME_Absolute earliest_timeout;

  /**
   * Do we have a confirmed working queue and are thus visible to
   * CORE?
   */
  int core_visible;
};


/**
 * A peer that an application (client) would like us to talk to directly.
 */
struct PeerRequest
{

  /**
   * Which peer is this about?
   */
  struct GNUNET_PeerIdentity pid;

  /**
   * Client responsible for the request.
   */
  struct TransportClient *tc;

  /**
   * Handle for watching the peerstore for HELLOs for this peer.
   */
  struct GNUNET_PEERSTORE_WatchContext *wc;

  /**
   * What kind of performance preference does this @e tc have?
   */
  enum GNUNET_MQ_PreferenceKind pk;

  /**
   * How much bandwidth would this @e tc like to see?
   */
  struct GNUNET_BANDWIDTH_Value32NBO bw;
};


/**
 * Types of different pending messages.
 */
enum PendingMessageType
{

  /**
   * Ordinary message received from the CORE service.
   */
  PMT_CORE = 0,

  /**
   * Fragment box.
   */
  PMT_FRAGMENT_BOX = 1,

  /**
   * Reliability box.
   */
  PMT_RELIABILITY_BOX = 2,

  /**
   * Any type of acknowledgement.
   */
  PMT_ACKNOWLEDGEMENT = 3,

  /**
   * Control traffic generated by the TRANSPORT service itself.
   */
  PMT_CONTROL = 4

};


/**
 * Transmission request that is awaiting delivery.  The original
 * transmission requests from CORE may be too big for some queues.
 * In this case, a *tree* of fragments is created.  At each
 * level of the tree, fragments are kept in a DLL ordered by which
 * fragment should be sent next (at the head).  The tree is searched
 * top-down, with the original message at the root.
 *
 * To select a node for transmission, first it is checked if the
 * current node's message fits with the MTU.  If it does not, we
 * either calculate the next fragment (based on @e frag_off) from the
 * current node, or, if all fragments have already been created,
 * descend to the @e head_frag.  Even though the node was already
 * fragmented, the fragment may be too big if the fragment was
 * generated for a queue with a larger MTU. In this case, the node
 * may be fragmented again, thus creating a tree.
 *
 * When acknowledgements for fragments are received, the tree
 * must be pruned, removing those parts that were already
 * acknowledged.  When fragments are sent over a reliable
 * channel, they can be immediately removed.
 *
 * If a message is ever fragmented, then the original "full" message
 * is never again transmitted (even if it fits below the MTU), and
 * only (remaining) fragments are sent.
 */
struct PendingMessage
{
  /**
   * Kept in a MDLL of messages for this @a target.
   */
  struct PendingMessage *next_neighbour;

  /**
   * Kept in a MDLL of messages for this @a target.
   */
  struct PendingMessage *prev_neighbour;

  /**
   * Kept in a MDLL of messages from this @a client (if @e pmt is #PMT_CORE)
   */
  struct PendingMessage *next_client;

  /**
   * Kept in a MDLL of messages from this @a client  (if @e pmt is #PMT_CORE)
   */
  struct PendingMessage *prev_client;

  /**
   * Kept in a MDLL of messages from this @a cpm (if @e pmt is
   * #PMT_FRAGMENT_BOx)
   */
  struct PendingMessage *next_frag;

  /**
   * Kept in a MDLL of messages from this @a cpm  (if @e pmt is
   * #PMT_FRAGMENT_BOX)
   */
  struct PendingMessage *prev_frag;

  /**
   * This message, reliability boxed. Only possibly available if @e pmt is
   * #PMT_CORE.
   */
  struct PendingMessage *bpm;

  /**
   * Target of the request.
   */
  struct Neighbour *target;

  /**
   * Set to non-NULL value if this message is currently being given to a
   * communicator and we are awaiting that communicator's acknowledgement.
   * Note that we must not retransmit a pending message while we're still
   * in the process of giving it to a communicator. If a pending message
   * is free'd while this entry is non-NULL, the @e qe reference to us
   * should simply be set to NULL.
   */
  struct QueueEntry *qe;

  /**
   * Client that issued the transmission request, if @e pmt is #PMT_CORE.
   */
  struct TransportClient *client;

  /**
   * Head of a MDLL of fragments created for this core message.
   */
  struct PendingMessage *head_frag;

  /**
   * Tail of a MDLL of fragments created for this core message.
   */
  struct PendingMessage *tail_frag;

  /**
   * Our parent in the fragmentation tree.
   */
  struct PendingMessage *frag_parent;

  /**
   * At what time should we give up on the transmission (and no longer retry)?
   */
  struct GNUNET_TIME_Absolute timeout;

  /**
   * What is the earliest time for us to retry transmission of this message?
   */
  struct GNUNET_TIME_Absolute next_attempt;

  /**
   * UUID to use for this message (used for reassembly of fragments, only
   * initialized if @e msg_uuid_set is #GNUNET_YES).
   */
  struct GNUNET_ShortHashCode msg_uuid;

  /**
   * Counter incremented per generated fragment.
   */
  uint32_t frag_uuidgen;

  /**
   * Type of the pending message.
   */
  enum PendingMessageType pmt;

  /**
   * Size of the original message.
   */
  uint16_t bytes_msg;

  /**
   * Offset at which we should generate the next fragment.
   */
  uint16_t frag_off;

  /**
   * #GNUNET_YES once @e msg_uuid was initialized
   */
  int16_t msg_uuid_set;

  /* Followed by @e bytes_msg to transmit */
};


/**
 * One of the addresses of this peer.
 */
struct AddressListEntry
{

  /**
   * Kept in a DLL.
   */
  struct AddressListEntry *next;

  /**
   * Kept in a DLL.
   */
  struct AddressListEntry *prev;

  /**
   * Which communicator provides this address?
   */
  struct TransportClient *tc;

  /**
   * The actual address.
   */
  const char *address;

  /**
   * Current context for storing this address in the peerstore.
   */
  struct GNUNET_PEERSTORE_StoreContext *sc;

  /**
   * Task to periodically do @e st operation.
   */
  struct GNUNET_SCHEDULER_Task *st;

  /**
   * What is a typical lifetime the communicator expects this
   * address to have? (Always from now.)
   */
  struct GNUNET_TIME_Relative expiration;

  /**
   * Address identifier used by the communicator.
   */
  uint32_t aid;

  /**
   * Network type offered by this address.
   */
  enum GNUNET_NetworkType nt;
};


/**
 * Client connected to the transport service.
 */
struct TransportClient
{

  /**
   * Kept in a DLL.
   */
  struct TransportClient *next;

  /**
   * Kept in a DLL.
   */
  struct TransportClient *prev;

  /**
   * Handle to the client.
   */
  struct GNUNET_SERVICE_Client *client;

  /**
   * Message queue to the client.
   */
  struct GNUNET_MQ_Handle *mq;

  /**
   * What type of client is this?
   */
  enum ClientType type;

  union
  {

    /**
     * Information for @e type #CT_CORE.
     */
    struct
    {

      /**
       * Head of list of messages pending for this client, sorted by
       * transmission time ("next_attempt" + possibly internal prioritization).
       */
      struct PendingMessage *pending_msg_head;

      /**
       * Tail of list of messages pending for this client.
       */
      struct PendingMessage *pending_msg_tail;

    } core;

    /**
     * Information for @e type #CT_MONITOR.
     */
    struct
    {

      /**
       * Peer identity to monitor the addresses of.
       * Zero to monitor all neighbours.  Valid if
       * @e type is #CT_MONITOR.
       */
      struct GNUNET_PeerIdentity peer;

      /**
       * Is this a one-shot monitor?
       */
      int one_shot;

    } monitor;


    /**
     * Information for @e type #CT_COMMUNICATOR.
     */
    struct
    {
      /**
       * If @e type is #CT_COMMUNICATOR, this communicator
       * supports communicating using these addresses.
       */
      char *address_prefix;

      /**
       * Head of DLL of queues offered by this communicator.
       */
      struct Queue *queue_head;

      /**
       * Tail of DLL of queues offered by this communicator.
       */
      struct Queue *queue_tail;

      /**
       * Head of list of the addresses of this peer offered by this
       * communicator.
       */
      struct AddressListEntry *addr_head;

      /**
       * Tail of list of the addresses of this peer offered by this
       * communicator.
       */
      struct AddressListEntry *addr_tail;

      /**
       * Number of queue entries in all queues to this communicator. Used
       * throttle sending to a communicator if we see that the communicator
       * is globally unable to keep up.
       */
      unsigned int total_queue_length;

      /**
       * Characteristics of this communicator.
       */
      enum GNUNET_TRANSPORT_CommunicatorCharacteristics cc;

    } communicator;

    /**
     * Information for @e type #CT_APPLICATION
     */
    struct
    {

      /**
       * Map of requests for peers the given client application would like to
       * see connections for.  Maps from PIDs to `struct PeerRequest`.
       */
      struct GNUNET_CONTAINER_MultiPeerMap *requests;

    } application;

  } details;
};


/**
 * State we keep for validation activities.  Each of these
 * is both in the #validation_heap and the #validation_map.
 */
struct ValidationState
{

  /**
   * For which peer is @a address to be validated (or possibly valid)?
   * Serves as key in the #validation_map.
   */
  struct GNUNET_PeerIdentity pid;

  /**
   * How long did the peer claim this @e address to be valid? Capped at
   * minimum of #MAX_ADDRESS_VALID_UNTIL relative to the time where we last
   * were told about the address and the value claimed by the other peer at
   * that time.  May be updated similarly when validation succeeds.
   */
  struct GNUNET_TIME_Absolute valid_until;

  /**
   * How long do *we* consider this @e address to be valid?
   * In the past or zero if we have not yet validated it.
   */
  struct GNUNET_TIME_Absolute validated_until;

  /**
   * When did we FIRST use the current @e challenge in a message?
   * Used to sanity-check @code{origin_time} in the response when
   * calculating the RTT. If the @code{origin_time} is not in
   * the expected range, the response is discarded as malicious.
   */
  struct GNUNET_TIME_Absolute first_challenge_use;

  /**
   * When did we LAST use the current @e challenge in a message?
   * Used to sanity-check @code{origin_time} in the response when
   * calculating the RTT.  If the @code{origin_time} is not in
   * the expected range, the response is discarded as malicious.
   */
  struct GNUNET_TIME_Absolute last_challenge_use;

  /**
   * Next time we will send the @e challenge to the peer, if this time is past
   * @e valid_until, this validation state is released at this time.  If the
   * address is valid, @e next_challenge is set to @e validated_until MINUS @e
   * validation_delay * #VALIDATION_RTT_BUFFER_FACTOR, such that we will try
   * to re-validate before the validity actually expires.
   */
  struct GNUNET_TIME_Absolute next_challenge;

  /**
   * Current backoff factor we're applying for sending the @a challenge.
   * Reset to 0 if the @a challenge is confirmed upon validation.
   * Reduced to minimum of #FAST_VALIDATION_CHALLENGE_FREQ and half of the
   * existing value if we receive an unvalidated address again over
   * another channel (and thus should consider the information "fresh").
   * Maximum is #MAX_VALIDATION_CHALLENGE_FREQ.
   */
  struct GNUNET_TIME_Relative challenge_backoff;

  /**
   * Initially set to "forever". Once @e validated_until is set, this value is
   * set to the RTT that tells us how long it took to receive the validation.
   */
  struct GNUNET_TIME_Relative validation_rtt;

  /**
   * The challenge we sent to the peer to get it to validate the address. Note
   * that we rotate the challenge whenever we update @e validated_until to
   * avoid attacks where a peer simply replays an old challenge in the future.
   * (We must not rotate more often as otherwise we may discard valid answers
   * due to packet losses, latency and reorderings on the network).
   */
  struct GNUNET_ShortHashCode challenge;

  /**
   * Claimed address of the peer.
   */
  char *address;

  /**
   * Entry in the #validation_heap, which is sorted by @e next_challenge. The
   * heap is used to figure out when the next validation activity should be
   * run.
   */
  struct GNUNET_CONTAINER_HeapNode *hn;

  /**
   * Handle to a PEERSTORE store operation for this @e address.  NULL if
   * no PEERSTORE operation is pending.
   */
  struct GNUNET_PEERSTORE_StoreContext *sc;

  /**
   * We are technically ready to send the challenge, but we are waiting for
   * the respective queue to become available for transmission.
   */
  int awaiting_queue;
};


/**
 * Head of linked list of all clients to this service.
 */
static struct TransportClient *clients_head;

/**
 * Tail of linked list of all clients to this service.
 */
static struct TransportClient *clients_tail;

/**
 * Statistics handle.
 */
static struct GNUNET_STATISTICS_Handle *GST_stats;

/**
 * Configuration handle.
 */
static const struct GNUNET_CONFIGURATION_Handle *GST_cfg;

/**
 * Our public key.
 */
static struct GNUNET_PeerIdentity GST_my_identity;

/**
 * Our private key.
 */
static struct GNUNET_CRYPTO_EddsaPrivateKey *GST_my_private_key;

/**
 * Map from PIDs to `struct Neighbour` entries.  A peer is
 * a neighbour if we have an MQ to it from some communicator.
 */
static struct GNUNET_CONTAINER_MultiPeerMap *neighbours;

/**
 * Map from PIDs to `struct DistanceVector` entries describing
 * known paths to the peer.
 */
static struct GNUNET_CONTAINER_MultiPeerMap *dv_routes;

/**
 * Map from PIDs to `struct ValidationState` entries describing
 * addresses we are aware of and their validity state.
 */
static struct GNUNET_CONTAINER_MultiPeerMap *validation_map;

/**
 * Map from challenges to `struct LearnLaunchEntry` values.
 */
static struct GNUNET_CONTAINER_MultiShortmap *dvlearn_map;

/**
 * Head of a DLL sorted by launch time.
 */
static struct LearnLaunchEntry *lle_head;

/**
 * Tail of a DLL sorted by launch time.
 */
static struct LearnLaunchEntry *lle_tail;

/**
 * MIN Heap sorted by "next_challenge" to `struct ValidationState` entries
 * sorting addresses we are aware of by when we should next try to (re)validate
 * (or expire) them.
 */
static struct GNUNET_CONTAINER_Heap *validation_heap;

/**
 * Database for peer's HELLOs.
 */
static struct GNUNET_PEERSTORE_Handle *peerstore;

/**
 * Heap sorting `struct EphemeralCacheEntry` by their
 * key/signature validity.
 */
static struct GNUNET_CONTAINER_Heap *ephemeral_heap;

/**
 * Hash map for looking up `struct EphemeralCacheEntry`s
 * by peer identity. (We may have ephemerals in our
 * cache for which we do not have a neighbour entry,
 * and similar many neighbours may not need ephemerals,
 * so we use a second map.)
 */
static struct GNUNET_CONTAINER_MultiPeerMap *ephemeral_map;

/**
 * Task to free expired ephemerals.
 */
static struct GNUNET_SCHEDULER_Task *ephemeral_task;

/**
 * Task run to initiate DV learning.
 */
static struct GNUNET_SCHEDULER_Task *dvlearn_task;

/**
 * Task to run address validation.
 */
static struct GNUNET_SCHEDULER_Task *validation_task;


/**
 * Free cached ephemeral key.
 *
 * @param ece cached signature to free
 */
static void
free_ephemeral (struct EphemeralCacheEntry *ece)
{
  GNUNET_CONTAINER_multipeermap_remove (ephemeral_map, &ece->target, ece);
  GNUNET_CONTAINER_heap_remove_node (ece->hn);
  GNUNET_free (ece);
}


/**
 * Free validation state.
 *
 * @param vs validation state to free
 */
static void
free_validation_state (struct ValidationState *vs)
{
  GNUNET_CONTAINER_multipeermap_remove (validation_map, &vs->pid, vs);
  GNUNET_CONTAINER_heap_remove_node (vs->hn);
  vs->hn = NULL;
  if (NULL != vs->sc)
  {
    GNUNET_PEERSTORE_store_cancel (vs->sc);
    vs->sc = NULL;
  }
  GNUNET_free (vs->address);
  GNUNET_free (vs);
}


/**
 * Lookup neighbour record for peer @a pid.
 *
 * @param pid neighbour to look for
 * @return NULL if we do not have this peer as a neighbour
 */
static struct Neighbour *
lookup_neighbour (const struct GNUNET_PeerIdentity *pid)
{
  return GNUNET_CONTAINER_multipeermap_get (neighbours, pid);
}


/**
 * Details about what to notify monitors about.
 */
struct MonitorEvent
{
  /**
   * @deprecated To be discussed if we keep these...
   */
  struct GNUNET_TIME_Absolute last_validation;
  struct GNUNET_TIME_Absolute valid_until;
  struct GNUNET_TIME_Absolute next_validation;

  /**
   * Current round-trip time estimate.
   */
  struct GNUNET_TIME_Relative rtt;

  /**
   * Connection status.
   */
  enum GNUNET_TRANSPORT_ConnectionStatus cs;

  /**
   * Messages pending.
   */
  uint32_t num_msg_pending;

  /**
   * Bytes pending.
   */
  uint32_t num_bytes_pending;
};


/**
 * Free a @dvh. Callers MAY want to check if this was the last path to the
 * `target`, and if so call #free_dv_route to also free the associated DV
 * entry in #dv_routes (if not, the associated scheduler job should eventually
 * take care of it).
 *
 * @param dvh hop to free
 */
static void
free_distance_vector_hop (struct DistanceVectorHop *dvh)
{
  struct Neighbour *n = dvh->next_hop;
  struct DistanceVector *dv = dvh->dv;

  GNUNET_CONTAINER_MDLL_remove (neighbour, n->dv_head, n->dv_tail, dvh);
  GNUNET_CONTAINER_MDLL_remove (dv, dv->dv_head, dv->dv_tail, dvh);
  GNUNET_free (dvh);
}


/**
 * Free entry in #dv_routes.  First frees all hops to the target, and
 * if there are no entries left, frees @a dv as well.
 *
 * @param dv route to free
 */
static void
free_dv_route (struct DistanceVector *dv)
{
  struct DistanceVectorHop *dvh;

  while (NULL != (dvh = dv->dv_head))
    free_distance_vector_hop (dvh);
  if (NULL == dv->dv_head)
  {
    GNUNET_assert (
      GNUNET_YES ==
      GNUNET_CONTAINER_multipeermap_remove (dv_routes, &dv->target, dv));
    if (NULL != dv->visibility_task)
      GNUNET_SCHEDULER_cancel (dv->visibility_task);
    if (NULL != dv->timeout_task)
      GNUNET_SCHEDULER_cancel (dv->timeout_task);
    GNUNET_free (dv);
  }
}


/**
 * Notify monitor @a tc about an event.  That @a tc
 * cares about the event has already been checked.
 *
 * Send @a tc information in @a me about a @a peer's status with
 * respect to some @a address to all monitors that care.
 *
 * @param tc monitor to inform
 * @param peer peer the information is about
 * @param address address the information is about
 * @param nt network type associated with @a address
 * @param me detailed information to transmit
 */
static void
notify_monitor (struct TransportClient *tc,
                const struct GNUNET_PeerIdentity *peer,
                const char *address,
                enum GNUNET_NetworkType nt,
                const struct MonitorEvent *me)
{
  struct GNUNET_MQ_Envelope *env;
  struct GNUNET_TRANSPORT_MonitorData *md;
  size_t addr_len = strlen (address) + 1;

  env = GNUNET_MQ_msg_extra (md,
                             addr_len,
                             GNUNET_MESSAGE_TYPE_TRANSPORT_MONITOR_DATA);
  md->nt = htonl ((uint32_t) nt);
  md->peer = *peer;
  md->last_validation = GNUNET_TIME_absolute_hton (me->last_validation);
  md->valid_until = GNUNET_TIME_absolute_hton (me->valid_until);
  md->next_validation = GNUNET_TIME_absolute_hton (me->next_validation);
  md->rtt = GNUNET_TIME_relative_hton (me->rtt);
  md->cs = htonl ((uint32_t) me->cs);
  md->num_msg_pending = htonl (me->num_msg_pending);
  md->num_bytes_pending = htonl (me->num_bytes_pending);
  memcpy (&md[1], address, addr_len);
  GNUNET_MQ_send (tc->mq, env);
}


/**
 * Send information in @a me about a @a peer's status with respect
 * to some @a address to all monitors that care.
 *
 * @param peer peer the information is about
 * @param address address the information is about
 * @param nt network type associated with @a address
 * @param me detailed information to transmit
 */
static void
notify_monitors (const struct GNUNET_PeerIdentity *peer,
                 const char *address,
                 enum GNUNET_NetworkType nt,
                 const struct MonitorEvent *me)
{
  for (struct TransportClient *tc = clients_head; NULL != tc; tc = tc->next)
  {
    if (CT_MONITOR != tc->type)
      continue;
    if (tc->details.monitor.one_shot)
      continue;
    if ((0 != GNUNET_is_zero (&tc->details.monitor.peer)) &&
        (0 != GNUNET_memcmp (&tc->details.monitor.peer, peer)))
      continue;
    notify_monitor (tc, peer, address, nt, me);
  }
}


/**
 * Called whenever a client connects.  Allocates our
 * data structures associated with that client.
 *
 * @param cls closure, NULL
 * @param client identification of the client
 * @param mq message queue for the client
 * @return our `struct TransportClient`
 */
static void *
client_connect_cb (void *cls,
                   struct GNUNET_SERVICE_Client *client,
                   struct GNUNET_MQ_Handle *mq)
{
  struct TransportClient *tc;

  (void) cls;
  tc = GNUNET_new (struct TransportClient);
  tc->client = client;
  tc->mq = mq;
  GNUNET_CONTAINER_DLL_insert (clients_head, clients_tail, tc);
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "Client %p connected\n", tc);
  return tc;
}


/**
 * Free @a rc
 *
 * @param rc data structure to free
 */
static void
free_reassembly_context (struct ReassemblyContext *rc)
{
  struct Neighbour *n = rc->neighbour;

  GNUNET_assert (rc == GNUNET_CONTAINER_heap_remove_node (rc->hn));
  GNUNET_assert (GNUNET_OK ==
                 GNUNET_CONTAINER_multishortmap_remove (n->reassembly_map,
                                                        &rc->msg_uuid,
                                                        rc));
  GNUNET_free (rc);
}


/**
 * Task run to clean up reassembly context of a neighbour that have expired.
 *
 * @param cls a `struct Neighbour`
 */
static void
reassembly_cleanup_task (void *cls)
{
  struct Neighbour *n = cls;
  struct ReassemblyContext *rc;

  n->reassembly_timeout_task = NULL;
  while (NULL != (rc = GNUNET_CONTAINER_heap_peek (n->reassembly_heap)))
  {
    if (0 == GNUNET_TIME_absolute_get_remaining (rc->reassembly_timeout)
               .rel_value_us)
    {
      free_reassembly_context (rc);
      continue;
    }
    GNUNET_assert (NULL == n->reassembly_timeout_task);
    n->reassembly_timeout_task =
      GNUNET_SCHEDULER_add_at (rc->reassembly_timeout,
                               &reassembly_cleanup_task,
                               n);
    return;
  }
}


/**
 * function called to #free_reassembly_context().
 *
 * @param cls NULL
 * @param key unused
 * @param value a `struct ReassemblyContext` to free
 * @return #GNUNET_OK (continue iteration)
 */
static int
free_reassembly_cb (void *cls,
                    const struct GNUNET_ShortHashCode *key,
                    void *value)
{
  struct ReassemblyContext *rc = value;

  (void) cls;
  (void) key;
  free_reassembly_context (rc);
  return GNUNET_OK;
}


/**
 * Release memory used by @a neighbour.
 *
 * @param neighbour neighbour entry to free
 */
static void
free_neighbour (struct Neighbour *neighbour)
{
  struct DistanceVectorHop *dvh;

  GNUNET_assert (NULL == neighbour->queue_head);
  GNUNET_assert (GNUNET_YES ==
                 GNUNET_CONTAINER_multipeermap_remove (neighbours,
                                                       &neighbour->pid,
                                                       neighbour));
  if (NULL != neighbour->timeout_task)
    GNUNET_SCHEDULER_cancel (neighbour->timeout_task);
  if (NULL != neighbour->reassembly_map)
  {
    GNUNET_CONTAINER_multishortmap_iterate (neighbour->reassembly_map,
                                            &free_reassembly_cb,
                                            NULL);
    GNUNET_CONTAINER_multishortmap_destroy (neighbour->reassembly_map);
    neighbour->reassembly_map = NULL;
    GNUNET_CONTAINER_heap_destroy (neighbour->reassembly_heap);
    neighbour->reassembly_heap = NULL;
  }
  while (NULL != (dvh = neighbour->dv_head))
  {
    struct DistanceVector *dv = dvh->dv;

    free_distance_vector_hop (dvh);
    if (NULL == dv->dv_head)
      free_dv_route (dv);
  }
  if (NULL != neighbour->reassembly_timeout_task)
    GNUNET_SCHEDULER_cancel (neighbour->reassembly_timeout_task);
  GNUNET_free (neighbour);
}


/**
 * Send message to CORE clients that we lost a connection.
 *
 * @param tc client to inform (must be CORE client)
 * @param pid peer the connection is for
 * @param quota_out current quota for the peer
 */
static void
core_send_connect_info (struct TransportClient *tc,
                        const struct GNUNET_PeerIdentity *pid,
                        struct GNUNET_BANDWIDTH_Value32NBO quota_out)
{
  struct GNUNET_MQ_Envelope *env;
  struct ConnectInfoMessage *cim;

  GNUNET_assert (CT_CORE == tc->type);
  env = GNUNET_MQ_msg (cim, GNUNET_MESSAGE_TYPE_TRANSPORT_CONNECT);
  cim->quota_out = quota_out;
  cim->id = *pid;
  GNUNET_MQ_send (tc->mq, env);
}


/**
 * Send message to CORE clients that we gained a connection
 *
 * @param pid peer the queue was for
 * @param quota_out current quota for the peer
 */
static void
cores_send_connect_info (const struct GNUNET_PeerIdentity *pid,
                         struct GNUNET_BANDWIDTH_Value32NBO quota_out)
{
  for (struct TransportClient *tc = clients_head; NULL != tc; tc = tc->next)
  {
    if (CT_CORE != tc->type)
      continue;
    core_send_connect_info (tc, pid, quota_out);
  }
}


/**
 * Send message to CORE clients that we lost a connection.
 *
 * @param pid peer the connection was for
 */
static void
cores_send_disconnect_info (const struct GNUNET_PeerIdentity *pid)
{
  for (struct TransportClient *tc = clients_head; NULL != tc; tc = tc->next)
  {
    struct GNUNET_MQ_Envelope *env;
    struct DisconnectInfoMessage *dim;

    if (CT_CORE != tc->type)
      continue;
    env = GNUNET_MQ_msg (dim, GNUNET_MESSAGE_TYPE_TRANSPORT_DISCONNECT);
    dim->peer = *pid;
    GNUNET_MQ_send (tc->mq, env);
  }
}


/**
 * We believe we are ready to transmit a message on a queue. Double-checks
 * with the queue's "tracker_out" and then gives the message to the
 * communicator for transmission (updating the tracker, and re-scheduling
 * itself if applicable).
 *
 * @param cls the `struct Queue` to process transmissions for
 */
static void
transmit_on_queue (void *cls);


/**
 * Schedule next run of #transmit_on_queue().  Does NOTHING if
 * we should run immediately or if the message queue is empty.
 * Test for no task being added AND queue not being empty to
 * transmit immediately afterwards!  This function must only
 * be called if the message queue is non-empty!
 *
 * @param queue the queue to do scheduling for
 * @param inside_job set to #GNUNET_YES if called from
 *            #transmit_on_queue() itself and NOT setting
 *            the task means running immediately
 */
static void
schedule_transmit_on_queue (struct Queue *queue, int inside_job)
{
  struct Neighbour *n = queue->neighbour;
  struct PendingMessage *pm = n->pending_msg_head;
  struct GNUNET_TIME_Relative out_delay;
  unsigned int wsize;

  GNUNET_assert (NULL != pm);
  if (queue->tc->details.communicator.total_queue_length >=
      COMMUNICATOR_TOTAL_QUEUE_LIMIT)
  {
    GNUNET_STATISTICS_update (
      GST_stats,
      "# Transmission throttled due to communicator queue limit",
      1,
      GNUNET_NO);
    return;
  }
  if (queue->queue_length >= QUEUE_LENGTH_LIMIT)
  {
    GNUNET_STATISTICS_update (GST_stats,
                              "# Transmission throttled due to queue queue limit",
                              1,
                              GNUNET_NO);
    return;
  }

  wsize = (0 == queue->mtu) ? pm->bytes_msg /* FIXME: add overheads? */
                            : queue->mtu;
  out_delay = GNUNET_BANDWIDTH_tracker_get_delay (&queue->tracker_out, wsize);
  out_delay = GNUNET_TIME_relative_max (GNUNET_TIME_absolute_get_remaining (
                                          pm->next_attempt),
                                        out_delay);
  if ((GNUNET_YES == inside_job) && (0 == out_delay.rel_value_us))
    return; /* we should run immediately! */
  /* queue has changed since we were scheduled, reschedule again */
  queue->transmit_task =
    GNUNET_SCHEDULER_add_delayed (out_delay, &transmit_on_queue, queue);
  if (out_delay.rel_value_us > DELAY_WARN_THRESHOLD.rel_value_us)
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "Next transmission on queue `%s' in %s (high delay)\n",
                queue->address,
                GNUNET_STRINGS_relative_time_to_string (out_delay, GNUNET_YES));
  else
    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
                "Next transmission on queue `%s' in %s\n",
                queue->address,
                GNUNET_STRINGS_relative_time_to_string (out_delay, GNUNET_YES));
}


/**
 * Check whether the CORE visibility of @a n changed. If so,
 * check whether we need to notify CORE.
 *
 * @param n neighbour to perform the check for
 */
static void
update_neighbour_core_visibility (struct Neighbour *n);


/**
 * Free @a queue.
 *
 * @param queue the queue to free
 */
static void
free_queue (struct Queue *queue)
{
  struct Neighbour *neighbour = queue->neighbour;
  struct TransportClient *tc = queue->tc;
  struct MonitorEvent me = {.cs = GNUNET_TRANSPORT_CS_DOWN,
                            .rtt = GNUNET_TIME_UNIT_FOREVER_REL};
  struct QueueEntry *qe;
  int maxxed;

  if (NULL != queue->transmit_task)
  {
    GNUNET_SCHEDULER_cancel (queue->transmit_task);
    queue->transmit_task = NULL;
  }
  if (NULL != queue->visibility_task)
  {
    GNUNET_SCHEDULER_cancel (queue->visibility_task);
    queue->visibility_task = NULL;
  }
  GNUNET_CONTAINER_MDLL_remove (neighbour,
                                neighbour->queue_head,
                                neighbour->queue_tail,
                                queue);
  GNUNET_CONTAINER_MDLL_remove (client,
                                tc->details.communicator.queue_head,
                                tc->details.communicator.queue_tail,
                                queue);
  maxxed = (COMMUNICATOR_TOTAL_QUEUE_LIMIT >=
            tc->details.communicator.total_queue_length);
  while (NULL != (qe = queue->queue_head))
  {
    GNUNET_CONTAINER_DLL_remove (queue->queue_head, queue->queue_tail, qe);
    queue->queue_length--;
    tc->details.communicator.total_queue_length--;
    if (NULL != qe->pm)
    {
      GNUNET_assert (qe == qe->pm->qe);
      qe->pm->qe = NULL;
    }
    GNUNET_free (qe);
  }
  GNUNET_assert (0 == queue->queue_length);
  if ((maxxed) && (COMMUNICATOR_TOTAL_QUEUE_LIMIT <
                   tc->details.communicator.total_queue_length))
  {
    /* Communicator dropped below threshold, resume all queues */
    GNUNET_STATISTICS_update (
      GST_stats,
      "# Transmission throttled due to communicator queue limit",
      -1,
      GNUNET_NO);
    for (struct Queue *s = tc->details.communicator.queue_head; NULL != s;
         s = s->next_client)
      schedule_transmit_on_queue (s, GNUNET_NO);
  }
  notify_monitors (&neighbour->pid, queue->address, queue->nt, &me);
  GNUNET_BANDWIDTH_tracker_notification_stop (&queue->tracker_in);
  GNUNET_BANDWIDTH_tracker_notification_stop (&queue->tracker_out);
  GNUNET_free (queue);

  update_neighbour_core_visibility (neighbour);
  cores_send_disconnect_info (&neighbour->pid);

  if (NULL == neighbour->queue_head)
  {
    free_neighbour (neighbour);
  }
}


/**
 * Free @a ale
 *
 * @param ale address list entry to free
 */
static void
free_address_list_entry (struct AddressListEntry *ale)
{
  struct TransportClient *tc = ale->tc;

  GNUNET_CONTAINER_DLL_remove (tc->details.communicator.addr_head,
                               tc->details.communicator.addr_tail,
                               ale);
  if (NULL != ale->sc)
  {
    GNUNET_PEERSTORE_store_cancel (ale->sc);
    ale->sc = NULL;
  }
  if (NULL != ale->st)
  {
    GNUNET_SCHEDULER_cancel (ale->st);
    ale->st = NULL;
  }
  GNUNET_free (ale);
}


/**
 * Stop the peer request in @a value.
 *
 * @param cls a `struct TransportClient` that no longer makes the request
 * @param pid the peer's identity
 * @param value a `struct PeerRequest`
 * @return #GNUNET_YES (always)
 */
static int
stop_peer_request (void *cls,
                   const struct GNUNET_PeerIdentity *pid,
                   void *value)
{
  struct TransportClient *tc = cls;
  struct PeerRequest *pr = value;

  GNUNET_PEERSTORE_watch_cancel (pr->wc);
  GNUNET_assert (
    GNUNET_YES ==
    GNUNET_CONTAINER_multipeermap_remove (tc->details.application.requests,
                                          pid,
                                          pr));
  GNUNET_free (pr);

  return GNUNET_OK;
}


/**
 * Called whenever a client is disconnected.  Frees our
 * resources associated with that client.
 *
 * @param cls closure, NULL
 * @param client identification of the client
 * @param app_ctx our `struct TransportClient`
 */
static void
client_disconnect_cb (void *cls,
                      struct GNUNET_SERVICE_Client *client,
                      void *app_ctx)
{
  struct TransportClient *tc = app_ctx;

  (void) cls;
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "Client %p disconnected, cleaning up.\n",
              tc);
  GNUNET_CONTAINER_DLL_remove (clients_head, clients_tail, tc);
  switch (tc->type)
  {
  case CT_NONE:
    break;
  case CT_CORE: {
    struct PendingMessage *pm;

    while (NULL != (pm = tc->details.core.pending_msg_head))
    {
      GNUNET_CONTAINER_MDLL_remove (client,
                                    tc->details.core.pending_msg_head,
                                    tc->details.core.pending_msg_tail,
                                    pm);
      pm->client = NULL;
    }
  }
  break;
  case CT_MONITOR:
    break;
  case CT_COMMUNICATOR: {
    struct Queue *q;
    struct AddressListEntry *ale;

    while (NULL != (q = tc->details.communicator.queue_head))
      free_queue (q);
    while (NULL != (ale = tc->details.communicator.addr_head))
      free_address_list_entry (ale);
    GNUNET_free (tc->details.communicator.address_prefix);
  }
  break;
  case CT_APPLICATION:
    GNUNET_CONTAINER_multipeermap_iterate (tc->details.application.requests,
                                           &stop_peer_request,
                                           tc);
    GNUNET_CONTAINER_multipeermap_destroy (tc->details.application.requests);
    break;
  }
  GNUNET_free (tc);
}


/**
 * Iterator telling new CORE client about all existing
 * connections to peers.
 *
 * @param cls the new `struct TransportClient`
 * @param pid a connected peer
 * @param value the `struct Neighbour` with more information
 * @return #GNUNET_OK (continue to iterate)
 */
static int
notify_client_connect_info (void *cls,
                            const struct GNUNET_PeerIdentity *pid,
                            void *value)
{
  struct TransportClient *tc = cls;
  struct Neighbour *neighbour = value;

  core_send_connect_info (tc, pid, neighbour->quota_out);
  return GNUNET_OK;
}


/**
 * Initialize a "CORE" client.  We got a start message from this
 * client, so add it to the list of clients for broadcasting of
 * inbound messages.
 *
 * @param cls the client
 * @param start the start message that was sent
 */
static void
handle_client_start (void *cls, const struct StartMessage *start)
{
  struct TransportClient *tc = cls;
  uint32_t options;

  options = ntohl (start->options);
  if ((0 != (1 & options)) &&
      (0 != GNUNET_memcmp (&start->self, &GST_my_identity)))
  {
    /* client thinks this is a different peer, reject */
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (tc->client);
    return;
  }
  if (CT_NONE != tc->type)
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (tc->client);
    return;
  }
  tc->type = CT_CORE;
  GNUNET_CONTAINER_multipeermap_iterate (neighbours,
                                         &notify_client_connect_info,
                                         tc);
  GNUNET_SERVICE_client_continue (tc->client);
}


/**
 * Client asked for transmission to a peer.  Process the request.
 *
 * @param cls the client
 * @param obm the send message that was sent
 */
static int
check_client_send (void *cls, const struct OutboundMessage *obm)
{
  struct TransportClient *tc = cls;
  uint16_t size;
  const struct GNUNET_MessageHeader *obmm;

  if (CT_CORE != tc->type)
  {
    GNUNET_break (0);
    return GNUNET_SYSERR;
  }
  size = ntohs (obm->header.size) - sizeof (struct OutboundMessage);
  if (size < sizeof (struct GNUNET_MessageHeader))
  {
    GNUNET_break (0);
    return GNUNET_SYSERR;
  }
  obmm = (const struct GNUNET_MessageHeader *) &obm[1];
  if (size != ntohs (obmm->size))
  {
    GNUNET_break (0);
    return GNUNET_SYSERR;
  }
  return GNUNET_OK;
}


/**
 * Free fragment tree below @e root, excluding @e root itself.
 *
 * @param root root of the tree to free
 */
static void
free_fragment_tree (struct PendingMessage *root)
{
  struct PendingMessage *frag;

  while (NULL != (frag = root->head_frag))
  {
    free_fragment_tree (frag);
    GNUNET_CONTAINER_MDLL_remove (frag, root->head_frag, root->tail_frag, frag);
    GNUNET_free (frag);
  }
}


/**
 * Release memory associated with @a pm and remove @a pm from associated
 * data structures.  @a pm must be a top-level pending message and not
 * a fragment in the tree.  The entire tree is freed (if applicable).
 *
 * @param pm the pending message to free
 */
static void
free_pending_message (struct PendingMessage *pm)
{
  struct TransportClient *tc = pm->client;
  struct Neighbour *target = pm->target;

  if (NULL != tc)
  {
    GNUNET_CONTAINER_MDLL_remove (client,
                                  tc->details.core.pending_msg_head,
                                  tc->details.core.pending_msg_tail,
                                  pm);
  }
  GNUNET_CONTAINER_MDLL_remove (neighbour,
                                target->pending_msg_head,
                                target->pending_msg_tail,
                                pm);
  free_fragment_tree (pm);
  if (NULL != pm->qe)
  {
    GNUNET_assert (pm == pm->qe->pm);
    pm->qe->pm = NULL;
  }
  GNUNET_free_non_null (pm->bpm);
  GNUNET_free (pm);
}


/**
 * Send a response to the @a pm that we have processed a
 * "send" request with status @a success. We
 * transmitted @a bytes_physical on the actual wire.
 * Sends a confirmation to the "core" client responsible
 * for the original request and free's @a pm.
 *
 * @param pm handle to the original pending message
 * @param success status code, #GNUNET_OK on success, #GNUNET_SYSERR
 *          for transmission failure
 * @param bytes_physical amount of bandwidth consumed
 */
static void
client_send_response (struct PendingMessage *pm,
                      int success,
                      uint32_t bytes_physical)
{
  struct TransportClient *tc = pm->client;
  struct Neighbour *target = pm->target;
  struct GNUNET_MQ_Envelope *env;
  struct SendOkMessage *som;

  if (NULL != tc)
  {
    env = GNUNET_MQ_msg (som, GNUNET_MESSAGE_TYPE_TRANSPORT_SEND_OK);
    som->success = htonl ((uint32_t) success);
    som->bytes_msg = htons (pm->bytes_msg);
    som->bytes_physical = htonl (bytes_physical);
    som->peer = target->pid;
    GNUNET_MQ_send (tc->mq, env);
  }
  free_pending_message (pm);
}


/**
 * Checks the message queue for a neighbour for messages that have timed
 * out and purges them.
 *
 * @param cls a `struct Neighbour`
 */
static void
check_queue_timeouts (void *cls)
{
  struct Neighbour *n = cls;
  struct PendingMessage *pm;
  struct GNUNET_TIME_Absolute now;
  struct GNUNET_TIME_Absolute earliest_timeout;

  n->timeout_task = NULL;
  earliest_timeout = GNUNET_TIME_UNIT_FOREVER_ABS;
  now = GNUNET_TIME_absolute_get ();
  for (struct PendingMessage *pos = n->pending_msg_head; NULL != pos; pos = pm)
  {
    pm = pos->next_neighbour;
    if (pos->timeout.abs_value_us <= now.abs_value_us)
    {
      GNUNET_STATISTICS_update (GST_stats,
                                "# messages dropped (timeout before confirmation)",
                                1,
                                GNUNET_NO);
      client_send_response (pm, GNUNET_NO, 0);
      continue;
    }
    earliest_timeout =
      GNUNET_TIME_absolute_min (earliest_timeout, pos->timeout);
  }
  n->earliest_timeout = earliest_timeout;
  if (NULL != n->pending_msg_head)
    n->timeout_task =
      GNUNET_SCHEDULER_add_at (earliest_timeout, &check_queue_timeouts, n);
}


/**
 * Client asked for transmission to a peer.  Process the request.
 *
 * @param cls the client
 * @param obm the send message that was sent
 */
static void
handle_client_send (void *cls, const struct OutboundMessage *obm)
{
  struct TransportClient *tc = cls;
  struct PendingMessage *pm;
  const struct GNUNET_MessageHeader *obmm;
  struct Neighbour *target;
  uint32_t bytes_msg;
  int was_empty;

  GNUNET_assert (CT_CORE == tc->type);
  obmm = (const struct GNUNET_MessageHeader *) &obm[1];
  bytes_msg = ntohs (obmm->size);
  target = lookup_neighbour (&obm->peer);
  if (NULL == target)
  {
    /* Failure: don't have this peer as a neighbour (anymore).
       Might have gone down asynchronously, so this is NOT
       a protocol violation by CORE. Still count the event,
       as this should be rare. */
    struct GNUNET_MQ_Envelope *env;
    struct SendOkMessage *som;

    env = GNUNET_MQ_msg (som, GNUNET_MESSAGE_TYPE_TRANSPORT_SEND_OK);
    som->success = htonl (GNUNET_SYSERR);
    som->bytes_msg = htonl (bytes_msg);
    som->bytes_physical = htonl (0);
    som->peer = obm->peer;
    GNUNET_MQ_send (tc->mq, env);
    GNUNET_SERVICE_client_continue (tc->client);
    GNUNET_STATISTICS_update (GST_stats,
                              "# messages dropped (neighbour unknown)",
                              1,
                              GNUNET_NO);
    return;
  }
  was_empty = (NULL == target->pending_msg_head);
  pm = GNUNET_malloc (sizeof (struct PendingMessage) + bytes_msg);
  pm->client = tc;
  pm->target = target;
  pm->bytes_msg = bytes_msg;
  pm->timeout =
    GNUNET_TIME_relative_to_absolute (GNUNET_TIME_relative_ntoh (obm->timeout));
  memcpy (&pm[1], &obm[1], bytes_msg);
  GNUNET_CONTAINER_MDLL_insert (neighbour,
                                target->pending_msg_head,
                                target->pending_msg_tail,
                                pm);
  GNUNET_CONTAINER_MDLL_insert (client,
                                tc->details.core.pending_msg_head,
                                tc->details.core.pending_msg_tail,
                                pm);
  if (target->earliest_timeout.abs_value_us > pm->timeout.abs_value_us)
  {
    target->earliest_timeout.abs_value_us = pm->timeout.abs_value_us;
    if (NULL != target->timeout_task)
      GNUNET_SCHEDULER_cancel (target->timeout_task);
    target->timeout_task = GNUNET_SCHEDULER_add_at (target->earliest_timeout,
                                                    &check_queue_timeouts,
                                                    target);
  }
  if (! was_empty)
    return; /* all queues must already be busy */
  for (struct Queue *queue = target->queue_head; NULL != queue;
       queue = queue->next_neighbour)
  {
    /* try transmission on any queue that is idle */
    if (NULL == queue->transmit_task)
      queue->transmit_task =
        GNUNET_SCHEDULER_add_now (&transmit_on_queue, queue);
  }
}


/**
 * Communicator started.  Test message is well-formed.
 *
 * @param cls the client
 * @param cam the send message that was sent
 */
static int
check_communicator_available (
  void *cls,
  const struct GNUNET_TRANSPORT_CommunicatorAvailableMessage *cam)
{
  struct TransportClient *tc = cls;
  uint16_t size;

  if (CT_NONE != tc->type)
  {
    GNUNET_break (0);
    return GNUNET_SYSERR;
  }
  tc->type = CT_COMMUNICATOR;
  size = ntohs (cam->header.size) - sizeof (*cam);
  if (0 == size)
    return GNUNET_OK; /* receive-only communicator */
  GNUNET_MQ_check_zero_termination (cam);
  return GNUNET_OK;
}


/**
 * Communicator started.  Process the request.
 *
 * @param cls the client
 * @param cam the send message that was sent
 */
static void
handle_communicator_available (
  void *cls,
  const struct GNUNET_TRANSPORT_CommunicatorAvailableMessage *cam)
{
  struct TransportClient *tc = cls;
  uint16_t size;

  size = ntohs (cam->header.size) - sizeof (*cam);
  if (0 == size)
    return; /* receive-only communicator */
  tc->details.communicator.address_prefix =
    GNUNET_strdup ((const char *) &cam[1]);
  tc->details.communicator.cc =
    (enum GNUNET_TRANSPORT_CommunicatorCharacteristics) ntohl (cam->cc);
  GNUNET_SERVICE_client_continue (tc->client);
}


/**
 * Communicator requests backchannel transmission.  Check the request.
 *
 * @param cls the client
 * @param cb the send message that was sent
 * @return #GNUNET_OK if message is well-formed
 */
static int
check_communicator_backchannel (
  void *cls,
  const struct GNUNET_TRANSPORT_CommunicatorBackchannel *cb)
{
  const struct GNUNET_MessageHeader *inbox;
  const char *is;
  uint16_t msize;
  uint16_t isize;

  (void) cls;
  msize = ntohs (cb->header.size) - sizeof (*cb);
  if (UINT16_MAX - msize >
      sizeof (struct TransportBackchannelEncapsulationMessage) +
        sizeof (struct TransportBackchannelRequestPayload))
  {
    GNUNET_break (0);
    return GNUNET_SYSERR;
  }
  inbox = (const struct GNUNET_MessageHeader *) &cb[1];
  isize = ntohs (inbox->size);
  if (isize >= msize)
  {
    GNUNET_break (0);
    return GNUNET_SYSERR;
  }
  is = (const char *) inbox;
  is += isize;
  msize -= isize;
  GNUNET_assert (msize > 0);
  if ('\0' != is[msize - 1])
  {
    GNUNET_break (0);
    return GNUNET_SYSERR;
  }
  return GNUNET_OK;
}


/**
 * Remove memory used by expired ephemeral keys.
 *
 * @param cls NULL
 */
static void
expire_ephemerals (void *cls)
{
  struct EphemeralCacheEntry *ece;

  (void) cls;
  ephemeral_task = NULL;
  while (NULL != (ece = GNUNET_CONTAINER_heap_peek (ephemeral_heap)))
  {
    if (0 == GNUNET_TIME_absolute_get_remaining (ece->ephemeral_validity)
               .rel_value_us)
    {
      free_ephemeral (ece);
      continue;
    }
    ephemeral_task = GNUNET_SCHEDULER_add_at (ece->ephemeral_validity,
                                              &expire_ephemerals,
                                              NULL);
    return;
  }
}


/**
 * Lookup ephemeral key in our #ephemeral_map. If no valid one exists, generate
 * one, cache it and return it.
 *
 * @param pid peer to look up ephemeral for
 * @param private_key[out] set to the private key
 * @param ephemeral_key[out] set to the key
 * @param ephemeral_sender_sig[out] set to the signature
 * @param ephemeral_validity[out] set to the validity expiration time
 */
static void
lookup_ephemeral (const struct GNUNET_PeerIdentity *pid,
                  struct GNUNET_CRYPTO_EcdhePrivateKey *private_key,
                  struct GNUNET_CRYPTO_EcdhePublicKey *ephemeral_key,
                  struct GNUNET_CRYPTO_EddsaSignature *ephemeral_sender_sig,
                  struct GNUNET_TIME_Absolute *ephemeral_validity)
{
  struct EphemeralCacheEntry *ece;
  struct EphemeralConfirmation ec;

  ece = GNUNET_CONTAINER_multipeermap_get (ephemeral_map, pid);
  if ((NULL != ece) &&
      (0 == GNUNET_TIME_absolute_get_remaining (ece->ephemeral_validity)
              .rel_value_us))
  {
    free_ephemeral (ece);
    ece = NULL;
  }
  if (NULL == ece)
  {
    ece = GNUNET_new (struct EphemeralCacheEntry);
    ece->target = *pid;
    ece->ephemeral_validity =
      GNUNET_TIME_absolute_add (GNUNET_TIME_absolute_get_monotonic (GST_cfg),
                                EPHEMERAL_VALIDITY);
    GNUNET_assert (GNUNET_OK ==
                   GNUNET_CRYPTO_ecdhe_key_create2 (&ece->private_key));
    GNUNET_CRYPTO_ecdhe_key_get_public (&ece->private_key, &ece->ephemeral_key);
    ec.purpose.purpose = htonl (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_EPHEMERAL);
    ec.purpose.size = htonl (sizeof (ec));
    ec.target = *pid;
    ec.ephemeral_key = ece->ephemeral_key;
    GNUNET_assert (GNUNET_OK == GNUNET_CRYPTO_eddsa_sign (GST_my_private_key,
                                                          &ec.purpose,
                                                          &ece->sender_sig));
    ece->hn =
      GNUNET_CONTAINER_heap_insert (ephemeral_heap,
                                    ece,
                                    ece->ephemeral_validity.abs_value_us);
    GNUNET_assert (GNUNET_OK ==
                   GNUNET_CONTAINER_multipeermap_put (
                     ephemeral_map,
                     &ece->target,
                     ece,
                     GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
    if (NULL == ephemeral_task)
      ephemeral_task = GNUNET_SCHEDULER_add_at (ece->ephemeral_validity,
                                                &expire_ephemerals,
                                                NULL);
  }
  *private_key = ece->private_key;
  *ephemeral_key = ece->ephemeral_key;
  *ephemeral_sender_sig = ece->sender_sig;
  *ephemeral_validity = ece->ephemeral_validity;
}


/**
 * Send the control message @a payload on @a queue.
 *
 * @param queue the queue to use for transmission
 * @param pm pending message to update once transmission is done, may be NULL!
 * @param payload the payload to send (encapsulated in a
 *        #GNUNET_MESSAGE_TYPE_TRANSPORT_SEND_MSG).
 * @param payload_size number of bytes in @a payload
 */
static void
queue_send_msg (struct Queue *queue,
                struct PendingMessage *pm,
                const void *payload,
                size_t payload_size)
{
  struct Neighbour *n = queue->neighbour;
  struct GNUNET_TRANSPORT_SendMessageTo *smt;
  struct GNUNET_MQ_Envelope *env;

  env = GNUNET_MQ_msg_extra (smt,
                             payload_size,
                             GNUNET_MESSAGE_TYPE_TRANSPORT_SEND_MSG);
  smt->qid = queue->qid;
  smt->mid = queue->mid_gen;
  smt->receiver = n->pid;
  memcpy (&smt[1], payload, payload_size);
  {
    /* Pass the env to the communicator of queue for transmission. */
    struct QueueEntry *qe;

    qe = GNUNET_new (struct QueueEntry);
    qe->mid = queue->mid_gen++;
    qe->queue = queue;
    if (NULL != pm)
    {
      qe->pm = pm;
      GNUNET_assert (NULL == pm->qe);
      pm->qe = qe;
    }
    GNUNET_CONTAINER_DLL_insert (queue->queue_head, queue->queue_tail, qe);
    GNUNET_assert (CT_COMMUNICATOR == queue->tc->type);
    queue->queue_length++;
    queue->tc->details.communicator.total_queue_length++;
    GNUNET_MQ_send (queue->tc->mq, env);
  }
}


/**
 * Which transmission options are allowable for transmission?
 * Interpreted bit-wise!
 */
enum RouteMessageOptions
{
  /**
   * Only confirmed, non-DV direct neighbours.
   */
  RMO_NONE = 0,

  /**
   * We are allowed to use DV routing for this @a hdr
   */
  RMO_DV_ALLOWED = 1,

  /**
   * We are allowed to use unconfirmed queues or DV routes for this message
   */
  RMO_UNCONFIRMED_ALLOWED = 2,

  /**
   * Reliable and unreliable, DV and non-DV are all acceptable.
   */
  RMO_ANYTHING_GOES = (RMO_DV_ALLOWED | RMO_UNCONFIRMED_ALLOWED),

  /**
   * If we have multiple choices, it is OK to send this message
   * over multiple channels at the same time to improve loss tolerance.
   * (We do at most 2 transmissions.)
   */
  RMO_REDUNDANT = 4
};


/**
 * Pick a queue of @a n under constraints @a options and schedule
 * transmission of @a hdr.
 *
 * @param n neighbour to send to
 * @param hdr message to send as payload
 * @param options whether queues must be confirmed or not,
 *        and whether we may pick multiple (2) queues
 */
static void
route_via_neighbour (const struct Neighbour *n,
                     const struct GNUNET_MessageHeader *hdr,
                     enum RouteMessageOptions options)
{
  struct GNUNET_TIME_Absolute now;
  unsigned int candidates;
  unsigned int sel1;
  unsigned int sel2;

  /* Pick one or two 'random' queues from n (under constraints of options) */
  now = GNUNET_TIME_absolute_get ();
  /* FIXME-OPTIMIZE: give queues 'weights' and pick proportional to
     weight in the future; weight could be assigned by observed
     bandwidth (note: not sure if we should do this for this type
     of control traffic though). */
  candidates = 0;
  for (struct Queue *pos = n->queue_head; NULL != pos;
       pos = pos->next_neighbour)
  {
    /* Count the queue with the visibility task in all cases, as
       otherwise we may end up with no queues just because the
       time for the visibility task just expired but the scheduler
       just ran this task first */
    if ((0 == (options & RMO_UNCONFIRMED_ALLOWED)) ||
        (pos->validated_until.abs_value_us > now.abs_value_us) ||
        (NULL != pos->visibility_task))
      candidates++;
  }
  if (0 == candidates)
  {
    /* Given that we above check for pos->visibility task,
       this should be strictly impossible. */
    GNUNET_break (0);
    return;
  }
  sel1 = GNUNET_CRYPTO_random_u32 (GNUNET_CRYPTO_QUALITY_WEAK, candidates);
  if (0 == (options & RMO_REDUNDANT))
    sel2 = candidates; /* picks none! */
  else
    sel2 = GNUNET_CRYPTO_random_u32 (GNUNET_CRYPTO_QUALITY_WEAK, candidates);
  candidates = 0;
  for (struct Queue *pos = n->queue_head; NULL != pos;
       pos = pos->next_neighbour)
  {
    /* Count the queue with the visibility task in all cases, as
       otherwise we may end up with no queues just because the
       time for the visibility task just expired but the scheduler
       just ran this task first */
    if ((pos->validated_until.abs_value_us > now.abs_value_us) ||
        (NULL != pos->visibility_task))
    {
      if ((sel1 == candidates) || (sel2 == candidates))
        queue_send_msg (pos, NULL, hdr, ntohs (hdr->size));
      candidates++;
    }
  }
}


/**
 * Given a distance vector path @a dvh route @a payload to
 * the ultimate destination respecting @a options.
 * Sets up the boxed message and queues it at the next hop.
 *
 * @param dvh choice of the path for the message
 * @param payload body to transmit
 * @param options options to use for control
 */
static void
forward_via_dvh (const struct DistanceVectorHop *dvh,
                 const struct GNUNET_MessageHeader *payload,
                 enum RouteMessageOptions options)
{
  uint16_t mlen = ntohs (payload->size);
  char boxram[sizeof (struct TransportDVBox) +
              (dvh->distance + 1) * sizeof (struct GNUNET_PeerIdentity) +
              mlen] GNUNET_ALIGN;
  struct TransportDVBox *box = (struct TransportDVBox *) boxram;
  struct GNUNET_PeerIdentity *path = (struct GNUNET_PeerIdentity *) &box[1];

  box->header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_DV_BOX);
  box->header.size = htons (sizeof (boxram));
  box->total_hops = htons (0);
  box->num_hops = htons (dvh->distance + 1);
  box->origin = GST_my_identity;
  memcpy (path, dvh->path, dvh->distance * sizeof (struct GNUNET_PeerIdentity));
  path[dvh->distance] = dvh->dv->target;
  memcpy (&path[dvh->distance + 1], payload, mlen);
  route_via_neighbour (dvh->next_hop, &box->header, options);
}


/**
 * Pick a path of @a dv under constraints @a options and schedule
 * transmission of @a hdr.
 *
 * @param n neighbour to send to
 * @param hdr message to send as payload
 * @param options whether path must be confirmed or not
 *        and whether we may pick multiple (2) paths
 */
static void
route_via_dv (const struct DistanceVector *dv,
              const struct GNUNET_MessageHeader *hdr,
              enum RouteMessageOptions options)
{
  struct DistanceVectorHop *h1;
  struct DistanceVectorHop *h2;
  uint64_t num_dv;
  uint64_t choice1;
  uint64_t choice2;

  /* Pick random vectors, but weighted by distance, giving more weight
     to shorter vectors */
  num_dv = 0;
  for (struct DistanceVectorHop *pos = dv->dv_head; NULL != pos;
       pos = pos->next_dv)
  {
    if ((0 == (options & RMO_UNCONFIRMED_ALLOWED)) &&
        (GNUNET_TIME_absolute_get_remaining (pos->path_valid_until)
           .rel_value_us == 0))
      continue; /* pos unconfirmed and confirmed required */
    num_dv += MAX_DV_HOPS_ALLOWED - pos->distance;
  }
  if (0 == num_dv)
  {
    GNUNET_break (0);
    return;
  }
  choice1 = GNUNET_CRYPTO_random_u64 (GNUNET_CRYPTO_QUALITY_WEAK, num_dv);
  choice2 = GNUNET_CRYPTO_random_u64 (GNUNET_CRYPTO_QUALITY_WEAK, num_dv);
  num_dv = 0;
  h1 = NULL;
  h2 = NULL;
  for (struct DistanceVectorHop *pos = dv->dv_head; NULL != pos;
       pos = pos->next_dv)
  {
    uint32_t delta = MAX_DV_HOPS_ALLOWED - pos->distance;

    if ((0 == (options & RMO_UNCONFIRMED_ALLOWED)) &&
        (GNUNET_TIME_absolute_get_remaining (pos->path_valid_until)
           .rel_value_us == 0))
      continue; /* pos unconfirmed and confirmed required */
    if ((num_dv <= choice1) && (num_dv + delta > choice1))
      h1 = pos;
    if ((num_dv <= choice2) && (num_dv + delta > choice2))
      h2 = pos;
    num_dv += delta;
  }
  forward_via_dvh (h1, hdr, options & (~RMO_REDUNDANT));
  if (0 == (options & RMO_REDUNDANT))
    forward_via_dvh (h2, hdr, options & (~RMO_REDUNDANT));
}


/**
 * We need to transmit @a hdr to @a target.  If necessary, this may
 * involve DV routing.
 *
 * @param target peer to receive @a hdr
 * @param hdr header of the message to route and #GNUNET_free()
 * @param options which transmission channels are allowed
 */
static void
route_message (const struct GNUNET_PeerIdentity *target,
               struct GNUNET_MessageHeader *hdr,
               enum RouteMessageOptions options)
{
  struct Neighbour *n;
  struct DistanceVector *dv;

  n = GNUNET_CONTAINER_multipeermap_get (neighbours, target);
  dv = (0 != (options & RMO_DV_ALLOWED))
         ? GNUNET_CONTAINER_multipeermap_get (dv_routes, target)
         : NULL;
  if (0 == (options & RMO_UNCONFIRMED_ALLOWED))
  {
    /* if confirmed is required, and we do not have anything
       confirmed, drop respective options */
    if ((NULL != n) && (GNUNET_NO == n->core_visible))
      n = NULL;
    if ((NULL != dv) && (GNUNET_NO == dv->core_visible))
      dv = NULL;
  }
  if ((NULL == n) && (NULL == dv))
  {
    GNUNET_STATISTICS_update (GST_stats,
                              "# Messages dropped in routing: no acceptable method",
                              1,
                              GNUNET_NO);
    GNUNET_free (hdr);
    return;
  }
  /* If both dv and n are possible and we must choose:
     flip a coin for the choice between the two; for now 50/50 */
  if ((NULL != n) && (NULL != dv) && (0 == (options & RMO_REDUNDANT)))
  {
    if (0 == GNUNET_CRYPTO_random_u32 (GNUNET_CRYPTO_QUALITY_WEAK, 2))
      n = NULL;
    else
      dv = NULL;
  }
  if ((NULL != n) && (NULL != dv))
    options &= ~RMO_REDUNDANT; /* We will do one DV and one direct, that's
                                  enough for redunancy, so clear the flag. */
  if (NULL != n)
  {
    route_via_neighbour (n, hdr, options);
  }
  if (NULL != dv)
  {
    route_via_dv (dv, hdr, options);
  }
  GNUNET_free (hdr);
}


/**
 * Structure of the key material used to encrypt backchannel messages.
 */
struct BackchannelKeyState
{
  /**
   * State of our block cipher.
   */
  gcry_cipher_hd_t cipher;

  /**
   * Actual key material.
   */
  struct
  {

    /**
     * Key used for HMAC calculations (via #GNUNET_CRYPTO_hmac()).
     */
    struct GNUNET_CRYPTO_AuthKey hmac_key;

    /**
     * Symmetric key to use for encryption.
     */
    char aes_key[256 / 8];

    /**
     * Counter value to use during setup.
     */
    char aes_ctr[128 / 8];

  } material;
};


static void
bc_setup_key_state_from_km (const struct GNUNET_HashCode *km,
                            const struct GNUNET_ShortHashCode *iv,
                            struct BackchannelKeyState *key)
{
  /* must match #dh_key_derive_eph_pub */
  GNUNET_assert (GNUNET_YES ==
                 GNUNET_CRYPTO_kdf (&key->material,
                                    sizeof (key->material),
                                    "transport-backchannel-key",
                                    strlen ("transport-backchannel-key"),
                                    &km,
                                    sizeof (km),
                                    iv,
                                    sizeof (*iv)));
  gcry_cipher_open (&key->cipher,
                    GCRY_CIPHER_AES256 /* low level: go for speed */,
                    GCRY_CIPHER_MODE_CTR,
                    0 /* flags */);
  gcry_cipher_setkey (key->cipher,
                      &key->material.aes_key,
                      sizeof (key->material.aes_key));
  gcry_cipher_setctr (key->cipher,
                      &key->material.aes_ctr,
                      sizeof (key->material.aes_ctr));
}


/**
 * Derive backchannel encryption key material from @a priv_ephemeral
 * and @a target and @a iv.
 *
 * @param priv_ephemeral ephemeral private key to use
 * @param target the target peer to encrypt to
 * @param iv unique IV to use
 * @param key[out] set to the key material
 */
static void
dh_key_derive_eph_pid (
  const struct GNUNET_CRYPTO_EcdhePrivateKey *priv_ephemeral,
  const struct GNUNET_PeerIdentity *target,
  const struct GNUNET_ShortHashCode *iv,
  struct BackchannelKeyState *key)
{
  struct GNUNET_HashCode km;

  GNUNET_assert (GNUNET_YES == GNUNET_CRYPTO_ecdh_eddsa (priv_ephemeral,
                                                         &target->public_key,
                                                         &km));
  bc_setup_key_state_from_km (&km, iv, key);
}


/**
 * Derive backchannel encryption key material from #GST_my_private_key
 * and @a pub_ephemeral and @a iv.
 *
 * @param priv_ephemeral ephemeral private key to use
 * @param target the target peer to encrypt to
 * @param iv unique IV to use
 * @param key[out] set to the key material
 */
static void
dh_key_derive_eph_pub (const struct GNUNET_CRYPTO_EcdhePublicKey *pub_ephemeral,
                       const struct GNUNET_ShortHashCode *iv,
                       struct BackchannelKeyState *key)
{
  struct GNUNET_HashCode km;

  GNUNET_assert (GNUNET_YES == GNUNET_CRYPTO_eddsa_ecdh (GST_my_private_key,
                                                         pub_ephemeral,
                                                         &km));
  bc_setup_key_state_from_km (&km, iv, key);
}


/**
 * Do HMAC calculation for backchannel messages over @a data using key
 * material from @a key.
 *
 * @param key key material (from DH)
 * @param hmac[out] set to the HMAC
 * @param data data to perform HMAC calculation over
 * @param data_size number of bytes in @a data
 */
static void
bc_hmac (const struct BackchannelKeyState *key,
         struct GNUNET_HashCode *hmac,
         const void *data,
         size_t data_size)
{
  GNUNET_CRYPTO_hmac (&key->material.hmac_key, data, data_size, hmac);
}


/**
 * Perform backchannel encryption using symmetric secret in @a key
 * to encrypt data from @a in to @a dst.
 *
 * @param key[in,out] key material to use
 * @param dst where to write the result
 * @param in input data to encrypt (plaintext)
 * @param in_size number of bytes of input in @a in and available at @a dst
 */
static void
bc_encrypt (struct BackchannelKeyState *key,
            const void *in,
            void *dst,
            size_t in_size)
{
  GNUNET_assert (0 ==
                 gcry_cipher_encrypt (key->cipher, dst, in_size, in, in_size));
}


/**
 * Perform backchannel encryption using symmetric secret in @a key
 * to encrypt data from @a in to @a dst.
 *
 * @param key[in,out] key material to use
 * @param ciph cipher text to decrypt
 * @param out[out] output data to generate (plaintext)
 * @param out_size number of bytes of input in @a ciph and available in @a out
 */
static void
bc_decrypt (struct BackchannelKeyState *key,
            void *out,
            const void *ciph,
            size_t out_size)
{
  GNUNET_assert (
    0 == gcry_cipher_decrypt (key->cipher, out, out_size, ciph, out_size));
}


/**
 * Clean up key material in @a key.
 *
 * @param key key material to clean up (memory must not be free'd!)
 */
static void
bc_key_clean (struct BackchannelKeyState *key)
{
  gcry_cipher_close (key->cipher);
  GNUNET_CRYPTO_zero_keys (&key->material, sizeof (key->material));
}


/**
 * Communicator requests backchannel transmission.  Process the request.
 *
 * @param cls the client
 * @param cb the send message that was sent
 */
static void
handle_communicator_backchannel (
  void *cls,
  const struct GNUNET_TRANSPORT_CommunicatorBackchannel *cb)
{
  struct TransportClient *tc = cls;
  struct GNUNET_CRYPTO_EcdhePrivateKey private_key;
  struct GNUNET_TIME_Absolute ephemeral_validity;
  struct TransportBackchannelEncapsulationMessage *enc;
  struct TransportBackchannelRequestPayload ppay;
  struct BackchannelKeyState key;
  char *mpos;
  uint16_t msize;

  /* encapsulate and encrypt message */
  msize = ntohs (cb->header.size) - sizeof (*cb) +
          sizeof (struct TransportBackchannelRequestPayload);
  enc = GNUNET_malloc (sizeof (*enc) + msize);
  enc->header.type =
    htons (GNUNET_MESSAGE_TYPE_TRANSPORT_BACKCHANNEL_ENCAPSULATION);
  enc->header.size = htons (sizeof (*enc) + msize);
  enc->target = cb->pid;
  lookup_ephemeral (&cb->pid,
                    &private_key,
                    &enc->ephemeral_key,
                    &ppay.sender_sig,
                    &ephemeral_validity);
  GNUNET_CRYPTO_random_block (GNUNET_CRYPTO_QUALITY_NONCE,
                              &enc->iv,
                              sizeof (enc->iv));
  dh_key_derive_eph_pid (&private_key, &cb->pid, &enc->iv, &key);
  ppay.ephemeral_validity = GNUNET_TIME_absolute_hton (ephemeral_validity);
  ppay.monotonic_time =
    GNUNET_TIME_absolute_hton (GNUNET_TIME_absolute_get_monotonic (GST_cfg));
  mpos = (char *) &enc[1];
  bc_encrypt (&key, &ppay, mpos, sizeof (ppay));
  bc_encrypt (&key,
              &cb[1],
              &mpos[sizeof (ppay)],
              ntohs (cb->header.size) - sizeof (*cb));
  bc_hmac (&key,
           &enc->hmac,
           mpos,
           sizeof (ppay) + ntohs (cb->header.size) - sizeof (*cb));
  bc_key_clean (&key);
  route_message (&cb->pid, &enc->header, RMO_DV_ALLOWED);
  GNUNET_SERVICE_client_continue (tc->client);
}


/**
 * Address of our peer added.  Test message is well-formed.
 *
 * @param cls the client
 * @param aam the send message that was sent
 * @return #GNUNET_OK if message is well-formed
 */
static int
check_add_address (void *cls,
                   const struct GNUNET_TRANSPORT_AddAddressMessage *aam)
{
  struct TransportClient *tc = cls;

  if (CT_COMMUNICATOR != tc->type)
  {
    GNUNET_break (0);
    return GNUNET_SYSERR;
  }
  GNUNET_MQ_check_zero_termination (aam);
  return GNUNET_OK;
}


/**
 * Ask peerstore to store our address.
 *
 * @param cls an `struct AddressListEntry *`
 */
static void
store_pi (void *cls);


/**
 * Function called when peerstore is done storing our address.
 *
 * @param cls a `struct AddressListEntry`
 * @param success #GNUNET_YES if peerstore was successful
 */
static void
peerstore_store_own_cb (void *cls, int success)
{
  struct AddressListEntry *ale = cls;

  ale->sc = NULL;
  if (GNUNET_YES != success)
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "Failed to store our own address `%s' in peerstore!\n",
                ale->address);
  /* refresh period is 1/4 of expiration time, that should be plenty
     without being excessive. */
  ale->st =
    GNUNET_SCHEDULER_add_delayed (GNUNET_TIME_relative_divide (ale->expiration,
                                                               4ULL),
                                  &store_pi,
                                  ale);
}


/**
 * Ask peerstore to store our address.
 *
 * @param cls an `struct AddressListEntry *`
 */
static void
store_pi (void *cls)
{
  struct AddressListEntry *ale = cls;
  void *addr;
  size_t addr_len;
  struct GNUNET_TIME_Absolute expiration;

  ale->st = NULL;
  expiration = GNUNET_TIME_relative_to_absolute (ale->expiration);
  GNUNET_HELLO_sign_address (ale->address,
                             ale->nt,
                             expiration,
                             GST_my_private_key,
                             &addr,
                             &addr_len);
  ale->sc = GNUNET_PEERSTORE_store (peerstore,
                                    "transport",
                                    &GST_my_identity,
                                    GNUNET_PEERSTORE_TRANSPORT_HELLO_KEY,
                                    addr,
                                    addr_len,
                                    expiration,
                                    GNUNET_PEERSTORE_STOREOPTION_MULTIPLE,
                                    &peerstore_store_own_cb,
                                    ale);
  GNUNET_free (addr);
  if (NULL == ale->sc)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "Failed to store our address `%s' with peerstore\n",
                ale->address);
    ale->st =
      GNUNET_SCHEDULER_add_delayed (GNUNET_TIME_UNIT_SECONDS, &store_pi, ale);
  }
}


/**
 * Address of our peer added.  Process the request.
 *
 * @param cls the client
 * @param aam the send message that was sent
 */
static void
handle_add_address (void *cls,
                    const struct GNUNET_TRANSPORT_AddAddressMessage *aam)
{
  struct TransportClient *tc = cls;
  struct AddressListEntry *ale;
  size_t slen;

  slen = ntohs (aam->header.size) - sizeof (*aam);
  ale = GNUNET_malloc (sizeof (struct AddressListEntry) + slen);
  ale->tc = tc;
  ale->address = (const char *) &ale[1];
  ale->expiration = GNUNET_TIME_relative_ntoh (aam->expiration);
  ale->aid = aam->aid;
  ale->nt = (enum GNUNET_NetworkType) ntohl (aam->nt);
  memcpy (&ale[1], &aam[1], slen);
  GNUNET_CONTAINER_DLL_insert (tc->details.communicator.addr_head,
                               tc->details.communicator.addr_tail,
                               ale);
  ale->st = GNUNET_SCHEDULER_add_now (&store_pi, ale);
  GNUNET_SERVICE_client_continue (tc->client);
}


/**
 * Address of our peer deleted.  Process the request.
 *
 * @param cls the client
 * @param dam the send message that was sent
 */
static void
handle_del_address (void *cls,
                    const struct GNUNET_TRANSPORT_DelAddressMessage *dam)
{
  struct TransportClient *tc = cls;

  if (CT_COMMUNICATOR != tc->type)
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (tc->client);
    return;
  }
  for (struct AddressListEntry *ale = tc->details.communicator.addr_head;
       NULL != ale;
       ale = ale->next)
  {
    if (dam->aid != ale->aid)
      continue;
    GNUNET_assert (ale->tc == tc);
    free_address_list_entry (ale);
    GNUNET_SERVICE_client_continue (tc->client);
  }
  GNUNET_break (0);
  GNUNET_SERVICE_client_drop (tc->client);
}


/**
 * Context from #handle_incoming_msg().  Closure for many
 * message handlers below.
 */
struct CommunicatorMessageContext
{
  /**
   * Which communicator provided us with the message.
   */
  struct TransportClient *tc;

  /**
   * Additional information for flow control and about the sender.
   */
  struct GNUNET_TRANSPORT_IncomingMessage im;

  /**
   * Number of hops the message has travelled (if DV-routed).
   * FIXME: make use of this in ACK handling!
   */
  uint16_t total_hops;
};


/**
 * Given an inbound message @a msg from a communicator @a cmc,
 * demultiplex it based on the type calling the right handler.
 *
 * @param cmc context for demultiplexing
 * @param msg message to demultiplex
 */
static void
demultiplex_with_cmc (struct CommunicatorMessageContext *cmc,
                      const struct GNUNET_MessageHeader *msg);


/**
 * Send ACK to communicator (if requested) and free @a cmc.
 *
 * @param cmc context for which we are done handling the message
 */
static void
finish_cmc_handling (struct CommunicatorMessageContext *cmc)
{
  if (0 != ntohl (cmc->im.fc_on))
  {
    /* send ACK when done to communicator for flow control! */
    struct GNUNET_MQ_Envelope *env;
    struct GNUNET_TRANSPORT_IncomingMessageAck *ack;

    env = GNUNET_MQ_msg (ack, GNUNET_MESSAGE_TYPE_TRANSPORT_INCOMING_MSG_ACK);
    ack->reserved = htonl (0);
    ack->fc_id = cmc->im.fc_id;
    ack->sender = cmc->im.sender;
    GNUNET_MQ_send (cmc->tc->mq, env);
  }
  GNUNET_SERVICE_client_continue (cmc->tc->client);
  GNUNET_free (cmc);
}


/**
 * Communicator gave us an unencapsulated message to pass as-is to
 * CORE.  Process the request.
 *
 * @param cls a `struct CommunicatorMessageContext` (must call
 * #finish_cmc_handling() when done)
 * @param mh the message that was received
 */
static void
handle_raw_message (void *cls, const struct GNUNET_MessageHeader *mh)
{
  struct CommunicatorMessageContext *cmc = cls;
  uint16_t size = ntohs (mh->size);

  if ((size > UINT16_MAX - sizeof (struct InboundMessage)) ||
      (size < sizeof (struct GNUNET_MessageHeader)))
  {
    struct GNUNET_SERVICE_Client *client = cmc->tc->client;

    GNUNET_break (0);
    finish_cmc_handling (cmc);
    GNUNET_SERVICE_client_drop (client);
    return;
  }
  /* Forward to all CORE clients */
  for (struct TransportClient *tc = clients_head; NULL != tc; tc = tc->next)
  {
    struct GNUNET_MQ_Envelope *env;
    struct InboundMessage *im;

    if (CT_CORE != tc->type)
      continue;
    env = GNUNET_MQ_msg_extra (im, size, GNUNET_MESSAGE_TYPE_TRANSPORT_RECV);
    im->peer = cmc->im.sender;
    memcpy (&im[1], mh, size);
    GNUNET_MQ_send (tc->mq, env);
  }
  /* FIXME: consider doing this _only_ once the message
     was drained from the CORE MQs to extend flow control to CORE!
     (basically, increment counter in cmc, decrement on MQ send continuation! */
  finish_cmc_handling (cmc);
}


/**
 * Communicator gave us a fragment box.  Check the message.
 *
 * @param cls a `struct CommunicatorMessageContext`
 * @param fb the send message that was sent
 * @return #GNUNET_YES if message is well-formed
 */
static int
check_fragment_box (void *cls, const struct TransportFragmentBox *fb)
{
  uint16_t size = ntohs (fb->header.size);
  uint16_t bsize = size - sizeof (*fb);

  if (0 == bsize)
  {
    GNUNET_break_op (0);
    return GNUNET_SYSERR;
  }
  if (bsize + ntohs (fb->frag_off) > ntohs (fb->msg_size))
  {
    GNUNET_break_op (0);
    return GNUNET_SYSERR;
  }
  if (ntohs (fb->frag_off) >= ntohs (fb->msg_size))
  {
    GNUNET_break_op (0);
    return GNUNET_SYSERR;
  }
  return GNUNET_YES;
}


/**
 * Generate a fragment acknowledgement for an @a rc.
 *
 * @param rc context to generate ACK for, @a rc ACK state is reset
 */
static void
send_fragment_ack (struct ReassemblyContext *rc)
{
  struct TransportFragmentAckMessage *ack;

  ack = GNUNET_new (struct TransportFragmentAckMessage);
  ack->header.size = htons (sizeof (struct TransportFragmentAckMessage));
  ack->header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_FRAGMENT_ACK);
  ack->frag_uuid = htonl (rc->frag_uuid);
  ack->extra_acks = GNUNET_htonll (rc->extra_acks);
  ack->msg_uuid = rc->msg_uuid;
  ack->avg_ack_delay = GNUNET_TIME_relative_hton (rc->avg_ack_delay);
  if (0 == rc->msg_missing)
    ack->reassembly_timeout = GNUNET_TIME_relative_hton (
      GNUNET_TIME_UNIT_FOREVER_REL); /* signal completion */
  else
    ack->reassembly_timeout = GNUNET_TIME_relative_hton (
      GNUNET_TIME_absolute_get_remaining (rc->reassembly_timeout));
  route_message (&rc->neighbour->pid, &ack->header, RMO_DV_ALLOWED);
  rc->avg_ack_delay = GNUNET_TIME_UNIT_ZERO;
  rc->num_acks = 0;
  rc->extra_acks = 0LLU;
}


/**
 * Communicator gave us a fragment.  Process the request.
 *
 * @param cls a `struct CommunicatorMessageContext` (must call
 * #finish_cmc_handling() when done)
 * @param fb the message that was received
 */
static void
handle_fragment_box (void *cls, const struct TransportFragmentBox *fb)
{
  struct CommunicatorMessageContext *cmc = cls;
  struct Neighbour *n;
  struct ReassemblyContext *rc;
  const struct GNUNET_MessageHeader *msg;
  uint16_t msize;
  uint16_t fsize;
  uint16_t frag_off;
  uint32_t frag_uuid;
  char *target;
  struct GNUNET_TIME_Relative cdelay;
  int ack_now;

  n = GNUNET_CONTAINER_multipeermap_get (neighbours, &cmc->im.sender);
  if (NULL == n)
  {
    struct GNUNET_SERVICE_Client *client = cmc->tc->client;

    GNUNET_break (0);
    finish_cmc_handling (cmc);
    GNUNET_SERVICE_client_drop (client);
    return;
  }
  if (NULL == n->reassembly_map)
  {
    n->reassembly_map = GNUNET_CONTAINER_multishortmap_create (8, GNUNET_YES);
    n->reassembly_heap =
      GNUNET_CONTAINER_heap_create (GNUNET_CONTAINER_HEAP_ORDER_MIN);
    n->reassembly_timeout_task =
      GNUNET_SCHEDULER_add_delayed (REASSEMBLY_EXPIRATION,
                                    &reassembly_cleanup_task,
                                    n);
  }
  msize = ntohs (fb->msg_size);
  rc = GNUNET_CONTAINER_multishortmap_get (n->reassembly_map, &fb->msg_uuid);
  if (NULL == rc)
  {
    rc = GNUNET_malloc (sizeof (*rc) + msize + /* reassembly payload buffer */
                        (msize + 7) / 8 * sizeof (uint8_t) /* bitfield */);
    rc->msg_uuid = fb->msg_uuid;
    rc->neighbour = n;
    rc->msg_size = msize;
    rc->reassembly_timeout =
      GNUNET_TIME_relative_to_absolute (REASSEMBLY_EXPIRATION);
    rc->last_frag = GNUNET_TIME_absolute_get ();
    rc->hn = GNUNET_CONTAINER_heap_insert (n->reassembly_heap,
                                           rc,
                                           rc->reassembly_timeout.abs_value_us);
    GNUNET_assert (GNUNET_OK ==
                   GNUNET_CONTAINER_multishortmap_put (
                     n->reassembly_map,
                     &rc->msg_uuid,
                     rc,
                     GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
    target = (char *) &rc[1];
    rc->bitfield = (uint8_t *) (target + rc->msg_size);
    rc->msg_missing = rc->msg_size;
  }
  else
  {
    target = (char *) &rc[1];
  }
  if (msize != rc->msg_size)
  {
    GNUNET_break (0);
    finish_cmc_handling (cmc);
    return;
  }

  /* reassemble */
  fsize = ntohs (fb->header.size) - sizeof (*fb);
  frag_off = ntohs (fb->frag_off);
  memcpy (&target[frag_off], &fb[1], fsize);
  /* update bitfield and msg_missing */
  for (unsigned int i = frag_off; i < frag_off + fsize; i++)
  {
    if (0 == (rc->bitfield[i / 8] & (1 << (i % 8))))
    {
      rc->bitfield[i / 8] |= (1 << (i % 8));
      rc->msg_missing--;
    }
  }

  /* Compute cummulative ACK */
  frag_uuid = ntohl (fb->frag_uuid);
  cdelay = GNUNET_TIME_absolute_get_duration (rc->last_frag);
  cdelay = GNUNET_TIME_relative_multiply (cdelay, rc->num_acks);
  rc->last_frag = GNUNET_TIME_absolute_get ();
  rc->avg_ack_delay = GNUNET_TIME_relative_add (rc->avg_ack_delay, cdelay);
  ack_now = GNUNET_NO;
  if (0 == rc->num_acks)
  {
    /* case one: first ack */
    rc->frag_uuid = frag_uuid;
    rc->extra_acks = 0LLU;
    rc->num_acks = 1;
  }
  else if ((frag_uuid >= rc->frag_uuid) && (frag_uuid <= rc->frag_uuid + 64))
  {
    /* case two: ack fits after existing min UUID */
    if ((frag_uuid == rc->frag_uuid) ||
        (0 != (rc->extra_acks & (1LLU << (frag_uuid - rc->frag_uuid - 1)))))
    {
      /* duplicate fragment, ack now! */
      ack_now = GNUNET_YES;
    }
    else
    {
      rc->extra_acks |= (1LLU << (frag_uuid - rc->frag_uuid - 1));
      rc->num_acks++;
    }
  }
  else if ((rc->frag_uuid > frag_uuid) &&
           (((rc->frag_uuid == frag_uuid + 64) && (0 == rc->extra_acks)) ||
            ((rc->frag_uuid < frag_uuid + 64) &&
             (rc->extra_acks ==
              (rc->extra_acks &
               ~((1LLU << (64 - (rc->frag_uuid - frag_uuid))) - 1LLU))))))
  {
    /* can fit ack by shifting extra acks and starting at
       frag_uid, test above esured that the bits we will
       shift 'extra_acks' by are all zero. */
    rc->extra_acks <<= (rc->frag_uuid - frag_uuid);
    rc->extra_acks |= (1LLU << (rc->frag_uuid - frag_uuid - 1));
    rc->frag_uuid = frag_uuid;
    rc->num_acks++;
  }
  if (65 == rc->num_acks) /* FIXME: maybe use smaller threshold? This is very
                             aggressive. */
    ack_now = GNUNET_YES; /* maximum acks received */
  // FIXME: possibly also ACK based on RTT (but for that we'd need to
  // determine the queue used for the ACK first!)

  /* is reassembly complete? */
  if (0 != rc->msg_missing)
  {
    if (ack_now)
      send_fragment_ack (rc);
    finish_cmc_handling (cmc);
    return;
  }
  /* reassembly is complete, verify result */
  msg = (const struct GNUNET_MessageHeader *) &rc[1];
  if (ntohs (msg->size) != rc->msg_size)
  {
    GNUNET_break (0);
    free_reassembly_context (rc);
    finish_cmc_handling (cmc);
    return;
  }
  /* successful reassembly */
  send_fragment_ack (rc);
  demultiplex_with_cmc (cmc, msg);
  /* FIXME: really free here? Might be bad if fragments are still
     en-route and we forget that we finished this reassembly immediately!
     -> keep around until timeout?
     -> shorten timeout based on ACK? */
  free_reassembly_context (rc);
}


/**
 * Check the @a fa against the fragments associated with @a pm.
 * If it matches, remove the matching fragments from the transmission
 * list.
 *
 * @param pm pending message to check against the ack
 * @param fa the ack that was received
 * @return #GNUNET_YES if @a fa matched, #GNUNET_NO if not
 */
static int
check_ack_against_pm (struct PendingMessage *pm,
                      const struct TransportFragmentAckMessage *fa)
{
  int match;
  struct PendingMessage *nxt;
  uint32_t fs = ntohl (fa->frag_uuid);
  uint64_t xtra = GNUNET_ntohll (fa->extra_acks);

  match = GNUNET_NO;
  for (struct PendingMessage *frag = pm->head_frag; NULL != frag; frag = nxt)
  {
    const struct TransportFragmentBox *tfb =
      (const struct TransportFragmentBox *) &pm[1];
    uint32_t fu = ntohl (tfb->frag_uuid);

    GNUNET_assert (PMT_FRAGMENT_BOX == frag->pmt);
    nxt = frag->next_frag;
    /* Check for exact match or match in the 'xtra' bitmask */
    if ((fu == fs) ||
        ((fu > fs) && (fu <= fs + 64) && (0 != (1LLU << (fu - fs - 1) & xtra))))
    {
      match = GNUNET_YES;
      free_fragment_tree (frag);
    }
  }
  return match;
}


/**
 * Communicator gave us a fragment acknowledgement.  Process the request.
 *
 * @param cls a `struct CommunicatorMessageContext` (must call
 * #finish_cmc_handling() when done)
 * @param fa the message that was received
 */
static void
handle_fragment_ack (void *cls, const struct TransportFragmentAckMessage *fa)
{
  struct CommunicatorMessageContext *cmc = cls;
  struct Neighbour *n;
  int matched;

  n = GNUNET_CONTAINER_multipeermap_get (neighbours, &cmc->im.sender);
  if (NULL == n)
  {
    struct GNUNET_SERVICE_Client *client = cmc->tc->client;

    GNUNET_break (0);
    finish_cmc_handling (cmc);
    GNUNET_SERVICE_client_drop (client);
    return;
  }
  /* FIXME-OPTIMIZE: maybe use another hash map here? */
  matched = GNUNET_NO;
  for (struct PendingMessage *pm = n->pending_msg_head; NULL != pm;
       pm = pm->prev_neighbour)
  {
    if (0 != GNUNET_memcmp (&fa->msg_uuid, &pm->msg_uuid))
      continue;
    matched = GNUNET_YES;
    if (GNUNET_YES == check_ack_against_pm (pm, fa))
    {
      struct GNUNET_TIME_Relative avg_ack_delay =
        GNUNET_TIME_relative_ntoh (fa->avg_ack_delay);
      // FIXME: update RTT and other reliability data!
      // ISSUE: we don't know which of n's queues the message(s)
      // took (and in fact the different messages might have gone
      // over different queues and possibly over multiple).
      // => track queues with PendingMessages, and update RTT only if
      //    the queue used is unique?
      //    -> how can we get loss rates?
      //    -> or, add extra state to Box and ACK to identify queue?
      // IDEA: generate MULTIPLE frag-uuids per fragment and track
      //    the queue with the fragment! (-> this logic must
      //    be moved into check_ack_against_pm!)
      (void) avg_ack_delay;
    }
    else
    {
      GNUNET_STATISTICS_update (GST_stats,
                                "# FRAGMENT_ACKS dropped, no matching fragment",
                                1,
                                GNUNET_NO);
    }
    if (NULL == pm->head_frag)
    {
      // if entire message is ACKed, handle that as well.
      // => clean up PM, any post actions?
      free_pending_message (pm);
    }
    else
    {
      struct GNUNET_TIME_Relative reassembly_timeout =
        GNUNET_TIME_relative_ntoh (fa->reassembly_timeout);
      // OPTIMIZE-FIXME: adjust retransmission strategy based on
      // reassembly_timeout!
      (void) reassembly_timeout;
    }
    break;
  }
  if (GNUNET_NO == matched)
  {
    GNUNET_STATISTICS_update (GST_stats,
                              "# FRAGMENT_ACKS dropped, no matching pending message",
                              1,
                              GNUNET_NO);
  }
  finish_cmc_handling (cmc);
}


/**
 * Communicator gave us a reliability box.  Check the message.
 *
 * @param cls a `struct CommunicatorMessageContext`
 * @param rb the send message that was sent
 * @return #GNUNET_YES if message is well-formed
 */
static int
check_reliability_box (void *cls, const struct TransportReliabilityBox *rb)
{
  GNUNET_MQ_check_boxed_message (rb);
  return GNUNET_YES;
}


/**
 * Communicator gave us a reliability box.  Process the request.
 *
 * @param cls a `struct CommunicatorMessageContext` (must call
 * #finish_cmc_handling() when done)
 * @param rb the message that was received
 */
static void
handle_reliability_box (void *cls, const struct TransportReliabilityBox *rb)
{
  struct CommunicatorMessageContext *cmc = cls;
  const struct GNUNET_MessageHeader *inbox =
    (const struct GNUNET_MessageHeader *) &rb[1];

  if (0 == ntohl (rb->ack_countdown))
  {
    struct TransportReliabilityAckMessage *ack;

    /* FIXME: implement cummulative ACKs and ack_countdown,
       then setting the avg_ack_delay field below: */
    ack = GNUNET_malloc (sizeof (*ack) + sizeof (struct GNUNET_ShortHashCode));
    ack->header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_ACK);
    ack->header.size =
      htons (sizeof (*ack) + sizeof (struct GNUNET_ShortHashCode));
    memcpy (&ack[1], &rb->msg_uuid, sizeof (struct GNUNET_ShortHashCode));
    route_message (&cmc->im.sender, &ack->header, RMO_DV_ALLOWED);
  }
  /* continue with inner message */
  demultiplex_with_cmc (cmc, inbox);
}


/**
 * Communicator gave us a reliability ack.  Process the request.
 *
 * @param cls a `struct CommunicatorMessageContext` (must call
 * #finish_cmc_handling() when done)
 * @param ra the message that was received
 */
static void
handle_reliability_ack (void *cls,
                        const struct TransportReliabilityAckMessage *ra)
{
  struct CommunicatorMessageContext *cmc = cls;
  struct Neighbour *n;
  unsigned int n_acks;
  const struct GNUNET_ShortHashCode *msg_uuids;
  struct PendingMessage *nxt;
  int matched;

  n = GNUNET_CONTAINER_multipeermap_get (neighbours, &cmc->im.sender);
  if (NULL == n)
  {
    struct GNUNET_SERVICE_Client *client = cmc->tc->client;

    GNUNET_break (0);
    finish_cmc_handling (cmc);
    GNUNET_SERVICE_client_drop (client);
    return;
  }
  n_acks = (ntohs (ra->header.size) - sizeof (*ra)) /
           sizeof (struct GNUNET_ShortHashCode);
  msg_uuids = (const struct GNUNET_ShortHashCode *) &ra[1];

  /* FIXME-OPTIMIZE: maybe use another hash map here? */
  matched = GNUNET_NO;
  for (struct PendingMessage *pm = n->pending_msg_head; NULL != pm; pm = nxt)
  {
    int in_list;

    nxt = pm->next_neighbour;
    in_list = GNUNET_NO;
    for (unsigned int i = 0; i < n_acks; i++)
    {
      if (0 != GNUNET_memcmp (&msg_uuids[i], &pm->msg_uuid))
        continue;
      in_list = GNUNET_YES;
      break;
    }
    if (GNUNET_NO == in_list)
      continue;

    /* this pm was acked! */
    matched = GNUNET_YES;
    free_pending_message (pm);

    {
      struct GNUNET_TIME_Relative avg_ack_delay =
        GNUNET_TIME_relative_ntoh (ra->avg_ack_delay);
      // FIXME: update RTT and other reliability data!
      // ISSUE: we don't know which of n's queues the message(s)
      // took (and in fact the different messages might have gone
      // over different queues and possibly over multiple).
      // => track queues with PendingMessages, and update RTT only if
      //    the queue used is unique?
      //    -> how can we get loss rates?
      //    -> or, add extra state to MSG and ACKs to identify queue?
      //    -> if we do this, might just do the same for the avg_ack_delay!
      (void) avg_ack_delay;
    }
  }
  if (GNUNET_NO == matched)
  {
    GNUNET_STATISTICS_update (GST_stats,
                              "# FRAGMENT_ACKS dropped, no matching pending message",
                              1,
                              GNUNET_NO);
  }
  finish_cmc_handling (cmc);
}


/**
 * Communicator gave us a backchannel encapsulation.  Check the message.
 *
 * @param cls a `struct CommunicatorMessageContext`
 * @param be the send message that was sent
 * @return #GNUNET_YES if message is well-formed
 */
static int
check_backchannel_encapsulation (
  void *cls,
  const struct TransportBackchannelEncapsulationMessage *be)
{
  uint16_t size = ntohs (be->header.size);

  (void) cls;
  if ((size - sizeof (*be)) <
      (sizeof (struct TransportBackchannelRequestPayload) +
       sizeof (struct GNUNET_MessageHeader)))
  {
    GNUNET_break_op (0);
    return GNUNET_SYSERR;
  }
  return GNUNET_YES;
}


/**
 * Communicator gave us a backchannel encapsulation.  Process the request.
 * (We are not the origin of the backchannel here, the communicator simply
 * received a backchannel message and we are expected to forward it.)
 *
 * @param cls a `struct CommunicatorMessageContext` (must call
 * #finish_cmc_handling() when done)
 * @param be the message that was received
 */
static void
handle_backchannel_encapsulation (
  void *cls,
  const struct TransportBackchannelEncapsulationMessage *be)
{
  struct CommunicatorMessageContext *cmc = cls;
  struct BackchannelKeyState key;
  struct GNUNET_HashCode hmac;
  const char *hdr;
  size_t hdr_len;

  if (0 != GNUNET_memcmp (&be->target, &GST_my_identity))
  {
    /* not for me, try to route to target */
    /* FIXME: someone needs to update be->distance! */
    /* FIXME: BE routing can be special, should we put all of this
       on 'route_message'? Maybe at least pass some more arguments? */
    route_message (&be->target,
                   GNUNET_copy_message (&be->header),
                   RMO_DV_ALLOWED);
    finish_cmc_handling (cmc);
    return;
  }
  dh_key_derive_eph_pub (&be->ephemeral_key, &be->iv, &key);
  hdr = (const char *) &be[1];
  hdr_len = ntohs (be->header.size) - sizeof (*be);
  bc_hmac (&key, &hmac, hdr, hdr_len);
  if (0 != GNUNET_memcmp (&hmac, &be->hmac))
  {
    /* HMAC missmatch, disard! */
    GNUNET_break_op (0);
    finish_cmc_handling (cmc);
    return;
  }
  /* begin actual decryption */
  {
    struct TransportBackchannelRequestPayload ppay;
    char body[hdr_len - sizeof (ppay)];

    GNUNET_assert (hdr_len >=
                   sizeof (ppay) + sizeof (struct GNUNET_MessageHeader));
    bc_decrypt (&key, &ppay, hdr, sizeof (ppay));
    bc_decrypt (&key, &body, &hdr[sizeof (ppay)], hdr_len - sizeof (ppay));
    bc_key_clean (&key);
    // FIXME: verify signatures in ppay!
    // => check if ephemeral key is known & valid, if not
    // => verify sig, cache ephemeral key
    // => update monotonic_time of sender for replay detection

    // FIXME: forward to specified communicator!
    // (using GNUNET_MESSAGE_TYPE_TRANSPORT_COMMUNICATOR_BACKCHANNEL_INCOMING)
  }
  finish_cmc_handling (cmc);
}


/**
 * Task called when we should check if any of the DV paths
 * we have learned to a target are due for garbage collection.
 *
 * Collects stale paths, and possibly frees the entire DV
 * entry if no paths are left. Otherwise re-schedules itself.
 *
 * @param cls a `struct DistanceVector`
 */
static void
path_cleanup_cb (void *cls)
{
  struct DistanceVector *dv = cls;
  struct DistanceVectorHop *pos;

  dv->timeout_task = NULL;
  while (NULL != (pos = dv->dv_head))
  {
    GNUNET_assert (dv == pos->dv);
    if (GNUNET_TIME_absolute_get_remaining (pos->timeout).rel_value_us > 0)
      break;
    free_distance_vector_hop (pos);
  }
  if (NULL == pos)
  {
    free_dv_route (dv);
    return;
  }
  dv->timeout_task =
    GNUNET_SCHEDULER_add_at (pos->timeout, &path_cleanup_cb, dv);
}

/**
 * Task run to check whether the hops of the @a cls still
 * are validated, or if we need to core about disconnection.
 *
 * @param cls a `struct DistanceVector` (with core_visible set!)
 */
static void
check_dv_path_down (void *cls)
{
  struct DistanceVector *dv = cls;
  struct Neighbour *n;

  dv->visibility_task = NULL;
  GNUNET_assert (GNUNET_YES == dv->core_visible);
  for (struct DistanceVectorHop *pos = dv->dv_head; NULL != pos;
       pos = pos->next_dv)
  {
    if (0 <
        GNUNET_TIME_absolute_get_remaining (pos->path_valid_until).rel_value_us)
    {
      dv->visibility_task = GNUNET_SCHEDULER_add_at (pos->path_valid_until,
                                                     &check_dv_path_down,
                                                     dv);
      return;
    }
  }
  /* all paths invalid, make dv core-invisible */
  dv->core_visible = GNUNET_NO;
  n = GNUNET_CONTAINER_multipeermap_get (neighbours, &dv->target);
  if ((NULL != n) && (GNUNET_YES == n->core_visible))
    return; /* no need to tell core, connection still up! */
  cores_send_disconnect_info (&dv->target);
}


/**
 * The @a hop is a validated path to the respective target
 * peer and we should tell core about it -- and schedule
 * a job to revoke the state.
 *
 * @param hop a path to some peer that is the reason for activation
 */
static void
activate_core_visible_dv_path (struct DistanceVectorHop *hop)
{
  struct DistanceVector *dv = hop->dv;
  struct Neighbour *n;

  GNUNET_assert (GNUNET_NO == dv->core_visible);
  GNUNET_assert (NULL == dv->visibility_task);

  dv->core_visible = GNUNET_YES;
  dv->visibility_task =
    GNUNET_SCHEDULER_add_at (hop->path_valid_until, &check_dv_path_down, dv);
  n = GNUNET_CONTAINER_multipeermap_get (neighbours, &dv->target);
  if ((NULL != n) && (GNUNET_YES == n->core_visible))
    return; /* no need to tell core, connection already up! */
  cores_send_connect_info (&dv->target,
                           (NULL != n)
                             ? GNUNET_BANDWIDTH_value_sum (n->quota_out,
                                                           dv->quota_out)
                             : dv->quota_out);
}


/**
 * We have learned a @a path through the network to some other peer, add it to
 * our DV data structure (returning #GNUNET_YES on success).
 *
 * We do not add paths if we have a sufficient number of shorter
 * paths to this target already (returning #GNUNET_NO).
 *
 * We also do not add problematic paths, like those where we lack the first
 * hop in our neighbour list (i.e. due to a topology change) or where some
 * non-first hop is in our neighbour list (returning #GNUNET_SYSERR).
 *
 * @param path the path we learned, path[0] should be us,
 *             and then path contains a valid path from us to `path[path_len-1]`
 *             path[1] should be a direct neighbour (we should check!)
 * @param path_len number of entries on the @a path, at least three!
 * @param network_latency how long does the message take from us to
 * `path[path_len-1]`? set to "forever" if unknown
 * @param path_valid_until how long is this path considered validated? Maybe be
 * zero.
 * @return #GNUNET_YES on success,
 *         #GNUNET_NO if we have better path(s) to the target
 *         #GNUNET_SYSERR if the path is useless and/or invalid
 *                         (i.e. path[1] not a direct neighbour
 *                        or path[i+1] is a direct neighbour for i>0)
 */
static int
learn_dv_path (const struct GNUNET_PeerIdentity *path,
               unsigned int path_len,
               struct GNUNET_TIME_Relative network_latency,
               struct GNUNET_TIME_Absolute path_valid_until)
{
  struct DistanceVectorHop *hop;
  struct DistanceVector *dv;
  struct Neighbour *next_hop;
  unsigned int shorter_distance;

  if (path_len < 3)
  {
    /* what a boring path! not allowed! */
    GNUNET_break (0);
    return GNUNET_SYSERR;
  }
  GNUNET_assert (0 == GNUNET_memcmp (&GST_my_identity, &path[0]));
  next_hop = GNUNET_CONTAINER_multipeermap_get (neighbours, &path[1]);
  if (NULL == next_hop)
  {
    /* next hop must be a neighbour, otherwise this whole thing is useless! */
    GNUNET_break (0);
    return GNUNET_SYSERR;
  }
  for (unsigned int i = 2; i < path_len; i++)
    if (NULL != GNUNET_CONTAINER_multipeermap_get (neighbours, &path[i]))
    {
      /* Useless path, we have a direct connection to some hop
         in the middle of the path, so this one doesn't even
         seem terribly useful for redundancy */
      return GNUNET_SYSERR;
    }
  dv = GNUNET_CONTAINER_multipeermap_get (dv_routes, &path[path_len - 1]);
  if (NULL == dv)
  {
    dv = GNUNET_new (struct DistanceVector);
    dv->target = path[path_len - 1];
    dv->timeout_task = GNUNET_SCHEDULER_add_delayed (DV_PATH_VALIDITY_TIMEOUT,
                                                     &path_cleanup_cb,
                                                     dv);
    GNUNET_assert (GNUNET_OK ==
                   GNUNET_CONTAINER_multipeermap_put (
                     dv_routes,
                     &dv->target,
                     dv,
                     GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
  }
  /* Check if we have this path already! */
  shorter_distance = 0;
  for (struct DistanceVectorHop *pos = dv->dv_head; NULL != pos;
       pos = pos->next_dv)
  {
    if (pos->distance < path_len - 2)
      shorter_distance++;
    /* Note that the distances in 'pos' excludes us (path[0]) and
       the next_hop (path[1]), so we need to subtract two
       and check next_hop explicitly */
    if ((pos->distance == path_len - 2) && (pos->next_hop == next_hop))
    {
      int match = GNUNET_YES;

      for (unsigned int i = 0; i < pos->distance; i++)
      {
        if (0 != GNUNET_memcmp (&pos->path[i], &path[i + 2]))
        {
          match = GNUNET_NO;
          break;
        }
      }
      if (GNUNET_YES == match)
      {
        struct GNUNET_TIME_Relative last_timeout;

        /* Re-discovered known path, update timeout */
        GNUNET_STATISTICS_update (GST_stats,
                                  "# Known DV path refreshed",
                                  1,
                                  GNUNET_NO);
        last_timeout = GNUNET_TIME_absolute_get_remaining (pos->timeout);
        pos->timeout =
          GNUNET_TIME_relative_to_absolute (DV_PATH_VALIDITY_TIMEOUT);
        pos->path_valid_until =
          GNUNET_TIME_absolute_max (pos->path_valid_until, path_valid_until);
        GNUNET_CONTAINER_MDLL_remove (dv, dv->dv_head, dv->dv_tail, pos);
        GNUNET_CONTAINER_MDLL_insert (dv, dv->dv_head, dv->dv_tail, pos);
        if ((GNUNET_NO == dv->core_visible) &&
            (0 < GNUNET_TIME_absolute_get_remaining (path_valid_until)
                   .rel_value_us))
          activate_core_visible_dv_path (pos);
        if (last_timeout.rel_value_us <
            GNUNET_TIME_relative_subtract (DV_PATH_VALIDITY_TIMEOUT,
                                           DV_PATH_DISCOVERY_FREQUENCY)
              .rel_value_us)
        {
          /* Some peer send DV learn messages too often, we are learning
             the same path faster than it would be useful; do not forward! */
          return GNUNET_NO;
        }
        return GNUNET_YES;
      }
    }
  }
  /* Count how many shorter paths we have (incl. direct
     neighbours) before simply giving up on this one! */
  if (shorter_distance >= MAX_DV_PATHS_TO_TARGET)
  {
    /* We have a shorter path already! */
    return GNUNET_NO;
  }
  /* create new DV path entry */
  hop = GNUNET_malloc (sizeof (struct DistanceVectorHop) +
                       sizeof (struct GNUNET_PeerIdentity) * (path_len - 2));
  hop->next_hop = next_hop;
  hop->dv = dv;
  hop->path = (const struct GNUNET_PeerIdentity *) &hop[1];
  memcpy (&hop[1],
          &path[2],
          sizeof (struct GNUNET_PeerIdentity) * (path_len - 2));
  hop->timeout = GNUNET_TIME_relative_to_absolute (DV_PATH_VALIDITY_TIMEOUT);
  hop->path_valid_until = path_valid_until;
  hop->distance = path_len - 2;
  GNUNET_CONTAINER_MDLL_insert (dv, dv->dv_head, dv->dv_tail, hop);
  GNUNET_CONTAINER_MDLL_insert (neighbour,
                                next_hop->dv_head,
                                next_hop->dv_tail,
                                hop);
  if ((GNUNET_NO == dv->core_visible) &&
      (0 < GNUNET_TIME_absolute_get_remaining (path_valid_until).rel_value_us))
    activate_core_visible_dv_path (hop);
  return GNUNET_YES;
}


/**
 * Communicator gave us a DV learn message.  Check the message.
 *
 * @param cls a `struct CommunicatorMessageContext`
 * @param dvl the send message that was sent
 * @return #GNUNET_YES if message is well-formed
 */
static int
check_dv_learn (void *cls, const struct TransportDVLearn *dvl)
{
  uint16_t size = ntohs (dvl->header.size);
  uint16_t num_hops = ntohs (dvl->num_hops);
  const struct DVPathEntryP *hops = (const struct DVPathEntryP *) &dvl[1];

  (void) cls;
  if (size != sizeof (*dvl) + num_hops * sizeof (struct DVPathEntryP))
  {
    GNUNET_break_op (0);
    return GNUNET_SYSERR;
  }
  if (num_hops > MAX_DV_HOPS_ALLOWED)
  {
    GNUNET_break_op (0);
    return GNUNET_SYSERR;
  }
  for (unsigned int i = 0; i < num_hops; i++)
  {
    if (0 == GNUNET_memcmp (&dvl->initiator, &hops[i].hop))
    {
      GNUNET_break_op (0);
      return GNUNET_SYSERR;
    }
    if (0 == GNUNET_memcmp (&GST_my_identity, &hops[i].hop))
    {
      GNUNET_break_op (0);
      return GNUNET_SYSERR;
    }
  }
  return GNUNET_YES;
}


/**
 * Build and forward a DV learn message to @a next_hop.
 *
 * @param next_hop peer to send the message to
 * @param msg message received
 * @param bi_history bitmask specifying hops on path that were bidirectional
 * @param nhops length of the @a hops array
 * @param hops path the message traversed so far
 * @param in_time when did we receive the message, used to calculate network
 * delay
 */
static void
forward_dv_learn (const struct GNUNET_PeerIdentity *next_hop,
                  const struct TransportDVLearn *msg,
                  uint16_t bi_history,
                  uint16_t nhops,
                  const struct DVPathEntryP *hops,
                  struct GNUNET_TIME_Absolute in_time)
{
  struct DVPathEntryP *dhops;
  struct TransportDVLearn *fwd;
  struct GNUNET_TIME_Relative nnd;

  /* compute message for forwarding */
  GNUNET_assert (nhops < MAX_DV_HOPS_ALLOWED);
  fwd = GNUNET_malloc (sizeof (struct TransportDVLearn) +
                       (nhops + 1) * sizeof (struct DVPathEntryP));
  fwd->header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_DV_LEARN);
  fwd->header.size = htons (sizeof (struct TransportDVLearn) +
                            (nhops + 1) * sizeof (struct DVPathEntryP));
  fwd->num_hops = htons (nhops + 1);
  fwd->bidirectional = htons (bi_history);
  nnd = GNUNET_TIME_relative_add (GNUNET_TIME_absolute_get_duration (in_time),
                                  GNUNET_TIME_relative_ntoh (
                                    msg->non_network_delay));
  fwd->non_network_delay = GNUNET_TIME_relative_hton (nnd);
  fwd->init_sig = msg->init_sig;
  fwd->initiator = msg->initiator;
  fwd->challenge = msg->challenge;
  dhops = (struct DVPathEntryP *) &fwd[1];
  GNUNET_memcpy (dhops, hops, sizeof (struct DVPathEntryP) * nhops);
  dhops[nhops].hop = GST_my_identity;
  {
    struct DvHopPS dhp = {.purpose.purpose =
                            htonl (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_HOP),
                          .purpose.size = htonl (sizeof (dhp)),
                          .pred = dhops[nhops - 1].hop,
                          .succ = *next_hop,
                          .challenge = msg->challenge};

    GNUNET_assert (GNUNET_OK ==
                   GNUNET_CRYPTO_eddsa_sign (GST_my_private_key,
                                             &dhp.purpose,
                                             &dhops[nhops].hop_sig));
  }
  route_message (next_hop, &fwd->header, RMO_UNCONFIRMED_ALLOWED);
}


/**
 * Check signature of type #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_INITIATOR
 *
 * @param init the signer
 * @param challenge the challenge that was signed
 * @param init_sig signature presumably by @a init
 * @return #GNUNET_OK if the signature is valid
 */
static int
validate_dv_initiator_signature (
  const struct GNUNET_PeerIdentity *init,
  const struct GNUNET_ShortHashCode *challenge,
  const struct GNUNET_CRYPTO_EddsaSignature *init_sig)
{
  struct DvInitPS ip = {.purpose.purpose = htonl (
                          GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_INITIATOR),
                        .purpose.size = htonl (sizeof (ip)),
                        .challenge = *challenge};

  if (
    GNUNET_OK !=
    GNUNET_CRYPTO_eddsa_verify (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_INITIATOR,
                                &ip.purpose,
                                init_sig,
                                &init->public_key))
  {
    GNUNET_break_op (0);
    return GNUNET_SYSERR;
  }
  return GNUNET_OK;
}


/**
 * Communicator gave us a DV learn message.  Process the request.
 *
 * @param cls a `struct CommunicatorMessageContext` (must call
 * #finish_cmc_handling() when done)
 * @param dvl the message that was received
 */
static void
handle_dv_learn (void *cls, const struct TransportDVLearn *dvl)
{
  struct CommunicatorMessageContext *cmc = cls;
  enum GNUNET_TRANSPORT_CommunicatorCharacteristics cc;
  int bi_hop;
  uint16_t nhops;
  uint16_t bi_history;
  const struct DVPathEntryP *hops;
  int do_fwd;
  int did_initiator;
  struct GNUNET_TIME_Absolute in_time;

  nhops = ntohs (dvl->bidirectional); /* 0 = sender is initiator */
  bi_history = ntohs (dvl->bidirectional);
  hops = (const struct DVPathEntryP *) &dvl[1];
  if (0 == nhops)
  {
    /* sanity check */
    if (0 != GNUNET_memcmp (&dvl->initiator, &cmc->im.sender))
    {
      GNUNET_break (0);
      finish_cmc_handling (cmc);
      return;
    }
  }
  else
  {
    /* sanity check */
    if (0 != GNUNET_memcmp (&hops[nhops - 1].hop, &cmc->im.sender))
    {
      GNUNET_break (0);
      finish_cmc_handling (cmc);
      return;
    }
  }

  GNUNET_assert (CT_COMMUNICATOR == cmc->tc->type);
  cc = cmc->tc->details.communicator.cc;
  bi_hop = (GNUNET_TRANSPORT_CC_RELIABLE ==
            cc); // FIXME: add bi-directional flag to cc?
  in_time = GNUNET_TIME_absolute_get ();

  /* continue communicator here, everything else can happen asynchronous! */
  finish_cmc_handling (cmc);

  /* OPTIMIZE-FIXME: Technically, we only need to bother checking
     the initiator signature if we send the message back to the initiator... */
  if (GNUNET_OK != validate_dv_initiator_signature (&dvl->initiator,
                                                    &dvl->challenge,
                                                    &dvl->init_sig))
  {
    GNUNET_break_op (0);
    return;
  }
  // FIXME: asynchronously (!) verify hop-by-hop signatures!
  // => if signature verification load too high, implement random drop
  // strategy!?

  do_fwd = GNUNET_YES;
  if (0 == GNUNET_memcmp (&GST_my_identity, &dvl->initiator))
  {
    struct GNUNET_PeerIdentity path[nhops + 1];
    struct GNUNET_TIME_Relative host_latency_sum;
    struct GNUNET_TIME_Relative latency;
    struct GNUNET_TIME_Relative network_latency;

    /* We initiated this, learn the forward path! */
    path[0] = GST_my_identity;
    path[1] = hops[0].hop;
    host_latency_sum = GNUNET_TIME_relative_ntoh (dvl->non_network_delay);

    // Need also something to lookup initiation time
    // to compute RTT! -> add RTT argument here?
    latency = GNUNET_TIME_UNIT_FOREVER_REL; // FIXME: initialize properly
    // (based on dvl->challenge, we can identify time of origin!)

    network_latency = GNUNET_TIME_relative_subtract (latency, host_latency_sum);
    /* assumption: latency on all links is the same */
    network_latency = GNUNET_TIME_relative_divide (network_latency, nhops);

    for (unsigned int i = 2; i <= nhops; i++)
    {
      struct GNUNET_TIME_Relative ilat;

      /* assumption: linear latency increase per hop */
      ilat = GNUNET_TIME_relative_multiply (network_latency, i);
      path[i] = hops[i - 1].hop;
      learn_dv_path (path,
                     i,
                     ilat,
                     GNUNET_TIME_relative_to_absolute (
                       ADDRESS_VALIDATION_LIFETIME));
    }
    /* as we initiated, do not forward again (would be circular!) */
    do_fwd = GNUNET_NO;
    return;
  }
  else if (bi_hop)
  {
    /* last hop was bi-directional, we could learn something here! */
    struct GNUNET_PeerIdentity path[nhops + 2];

    path[0] = GST_my_identity;
    path[1] = hops[nhops - 1].hop; /* direct neighbour == predecessor! */
    for (unsigned int i = 0; i < nhops; i++)
    {
      int iret;

      if (0 == (bi_history & (1 << i)))
        break; /* i-th hop not bi-directional, stop learning! */
      if (i == nhops)
      {
        path[i + 2] = dvl->initiator;
      }
      else
      {
        path[i + 2] = hops[nhops - i - 2].hop;
      }

      iret = learn_dv_path (path,
                            i + 2,
                            GNUNET_TIME_UNIT_FOREVER_REL,
                            GNUNET_TIME_UNIT_ZERO_ABS);
      if (GNUNET_SYSERR == iret)
      {
        /* path invalid or too long to be interesting for US, thus should also
           not be interesting to our neighbours, cut path when forwarding to
           'i' hops, except of course for the one that goes back to the
           initiator */
        GNUNET_STATISTICS_update (GST_stats,
                                  "# DV learn not forwarded due invalidity of path",
                                  1,
                                  GNUNET_NO);
        do_fwd = GNUNET_NO;
        break;
      }
      if ((GNUNET_NO == iret) && (nhops == i + 1))
      {
        /* we have better paths, and this is the longest target,
           so there cannot be anything interesting later */
        GNUNET_STATISTICS_update (GST_stats,
                                  "# DV learn not forwarded, got better paths",
                                  1,
                                  GNUNET_NO);
        do_fwd = GNUNET_NO;
        break;
      }
    }
  }

  if (MAX_DV_HOPS_ALLOWED == nhops)
  {
    /* At limit, we're out of here! */
    finish_cmc_handling (cmc);
    return;
  }

  /* Forward to initiator, if path non-trivial and possible */
  bi_history = (bi_history << 1) | (bi_hop ? 1 : 0);
  did_initiator = GNUNET_NO;
  if ((1 < nhops) &&
      (GNUNET_YES ==
       GNUNET_CONTAINER_multipeermap_contains (neighbours, &dvl->initiator)))
  {
    /* send back to origin! */
    forward_dv_learn (&dvl->initiator, dvl, bi_history, nhops, hops, in_time);
    did_initiator = GNUNET_YES;
  }
  /* We forward under two conditions: either we still learned something
     ourselves (do_fwd), or the path was darn short and thus the initiator is
     likely to still be very interested in this (and we did NOT already
     send it back to the initiator) */
  if ((do_fwd) || ((nhops < MIN_DV_PATH_LENGTH_FOR_INITIATOR) &&
                   (GNUNET_NO == did_initiator)))
  {
    /* FIXME: loop over all neighbours, pick those with low
       queues AND that are not yet on the path; possibly
       adapt threshold to nhops! */
#if FIXME
    forward_dv_learn (NULL, // fill in peer from iterator here!
                      dvl,
                      bi_history,
                      nhops,
                      hops,
                      in_time);
#endif
  }
}


/**
 * Communicator gave us a DV box.  Check the message.
 *
 * @param cls a `struct CommunicatorMessageContext`
 * @param dvb the send message that was sent
 * @return #GNUNET_YES if message is well-formed
 */
static int
check_dv_box (void *cls, const struct TransportDVBox *dvb)
{
  uint16_t size = ntohs (dvb->header.size);
  uint16_t num_hops = ntohs (dvb->num_hops);
  const struct GNUNET_PeerIdentity *hops =
    (const struct GNUNET_PeerIdentity *) &dvb[1];
  const struct GNUNET_MessageHeader *inbox =
    (const struct GNUNET_MessageHeader *) &hops[num_hops];
  uint16_t isize;
  uint16_t itype;

  (void) cls;
  if (size < sizeof (*dvb) + num_hops * sizeof (struct GNUNET_PeerIdentity) +
               sizeof (struct GNUNET_MessageHeader))
  {
    GNUNET_break_op (0);
    return GNUNET_SYSERR;
  }
  isize = ntohs (inbox->size);
  if (size !=
      sizeof (*dvb) + num_hops * sizeof (struct GNUNET_PeerIdentity) + isize)
  {
    GNUNET_break_op (0);
    return GNUNET_SYSERR;
  }
  itype = ntohs (inbox->type);
  if ((GNUNET_MESSAGE_TYPE_TRANSPORT_DV_BOX == itype) ||
      (GNUNET_MESSAGE_TYPE_TRANSPORT_DV_LEARN == itype))
  {
    GNUNET_break_op (0);
    return GNUNET_SYSERR;
  }
  if (0 == GNUNET_memcmp (&dvb->origin, &GST_my_identity))
  {
    GNUNET_break_op (0);
    return GNUNET_SYSERR;
  }
  return GNUNET_YES;
}


/**
 * Create a DV Box message and queue it for transmission to
 * @ea next_hop.
 *
 * @param next_hop peer to receive the message next
 * @param total_hops how many hops did the message take so far
 * @param num_hops length of the @a hops array
 * @param origin origin of the message
 * @param hops next peer(s) to the destination, including destination
 * @param payload payload of the box
 * @param payload_size number of bytes in @a payload
 */
static void
forward_dv_box (struct Neighbour *next_hop,
                uint16_t total_hops,
                uint16_t num_hops,
                const struct GNUNET_PeerIdentity *origin,
                const struct GNUNET_PeerIdentity *hops,
                const void *payload,
                uint16_t payload_size)
{
  struct TransportDVBox *dvb;
  struct GNUNET_PeerIdentity *dhops;

  GNUNET_assert (UINT16_MAX < sizeof (struct TransportDVBox) +
                                sizeof (struct GNUNET_PeerIdentity) * num_hops +
                                payload_size);
  dvb = GNUNET_malloc (sizeof (struct TransportDVBox) +
                       sizeof (struct GNUNET_PeerIdentity) * num_hops +
                       payload_size);
  dvb->header.size =
    htons (sizeof (struct TransportDVBox) +
           sizeof (struct GNUNET_PeerIdentity) * num_hops + payload_size);
  dvb->header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_DV_BOX);
  dvb->total_hops = htons (total_hops);
  dvb->num_hops = htons (num_hops);
  dvb->origin = *origin;
  dhops = (struct GNUNET_PeerIdentity *) &dvb[1];
  memcpy (dhops, hops, num_hops * sizeof (struct GNUNET_PeerIdentity));
  memcpy (&dhops[num_hops], payload, payload_size);
  route_message (&next_hop->pid, &dvb->header, RMO_NONE);
}


/**
 * Communicator gave us a DV box.  Process the request.
 *
 * @param cls a `struct CommunicatorMessageContext` (must call
 * #finish_cmc_handling() when done)
 * @param dvb the message that was received
 */
static void
handle_dv_box (void *cls, const struct TransportDVBox *dvb)
{
  struct CommunicatorMessageContext *cmc = cls;
  uint16_t size = ntohs (dvb->header.size) - sizeof (*dvb);
  uint16_t num_hops = ntohs (dvb->num_hops);
  const struct GNUNET_PeerIdentity *hops =
    (const struct GNUNET_PeerIdentity *) &dvb[1];
  const struct GNUNET_MessageHeader *inbox =
    (const struct GNUNET_MessageHeader *) &hops[num_hops];

  if (num_hops > 0)
  {
    /* We're trying from the end of the hops array, as we may be
       able to find a shortcut unknown to the origin that way */
    for (int i = num_hops - 1; i >= 0; i--)
    {
      struct Neighbour *n;

      if (0 == GNUNET_memcmp (&hops[i], &GST_my_identity))
      {
        GNUNET_break_op (0);
        finish_cmc_handling (cmc);
        return;
      }
      n = GNUNET_CONTAINER_multipeermap_get (neighbours, &hops[i]);
      if (NULL == n)
        continue;
      forward_dv_box (n,
                      ntohs (dvb->total_hops) + 1,
                      num_hops - i - 1, /* number of hops left */
                      &dvb->origin,
                      &hops[i + 1], /* remaining hops */
                      (const void *) &dvb[1],
                      size);
      finish_cmc_handling (cmc);
      return;
    }
    /* Woopsie, next hop not in neighbours, drop! */
    GNUNET_STATISTICS_update (GST_stats,
                              "# DV Boxes dropped: next hop unknown",
                              1,
                              GNUNET_NO);
    finish_cmc_handling (cmc);
    return;
  }
  /* We are the target. Unbox and handle message. */
  cmc->im.sender = dvb->origin;
  cmc->total_hops = ntohs (dvb->total_hops);
  demultiplex_with_cmc (cmc, inbox);
}


/**
 * Client notified us about transmission from a peer.  Process the request.
 *
 * @param cls a `struct TransportClient` which sent us the message
 * @param obm the send message that was sent
 * @return #GNUNET_YES if message is well-formed
 */
static int
check_incoming_msg (void *cls,
                    const struct GNUNET_TRANSPORT_IncomingMessage *im)
{
  struct TransportClient *tc = cls;

  if (CT_COMMUNICATOR != tc->type)
  {
    GNUNET_break (0);
    return GNUNET_SYSERR;
  }
  GNUNET_MQ_check_boxed_message (im);
  return GNUNET_OK;
}


/**
 * Communicator gave us a transport address validation challenge.  Process the
 * request.
 *
 * @param cls a `struct CommunicatorMessageContext` (must call
 * #finish_cmc_handling() when done)
 * @param tvc the message that was received
 */
static void
handle_validation_challenge (void *cls,
                             const struct TransportValidationChallenge *tvc)
{
  struct CommunicatorMessageContext *cmc = cls;
  struct TransportValidationResponse *tvr;

  if (cmc->total_hops > 0)
  {
    /* DV routing is not allowed for validation challenges! */
    GNUNET_break_op (0);
    finish_cmc_handling (cmc);
    return;
  }
  tvr = GNUNET_new (struct TransportValidationResponse);
  tvr->header.type =
    htons (GNUNET_MESSAGE_TYPE_TRANSPORT_ADDRESS_VALIDATION_RESPONSE);
  tvr->header.size = htons (sizeof (*tvr));
  tvr->challenge = tvc->challenge;
  tvr->origin_time = tvc->sender_time;
  tvr->validity_duration = cmc->im.expected_address_validity;
  {
    /* create signature */
    struct TransportValidationPS tvp =
      {.purpose.purpose = htonl (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_CHALLENGE),
       .purpose.size = htonl (sizeof (tvp)),
       .validity_duration = tvr->validity_duration,
       .challenge = tvc->challenge};

    GNUNET_assert (GNUNET_OK == GNUNET_CRYPTO_eddsa_sign (GST_my_private_key,
                                                          &tvp.purpose,
                                                          &tvr->signature));
  }
  route_message (&cmc->im.sender,
                 &tvr->header,
                 RMO_ANYTHING_GOES | RMO_REDUNDANT);
  finish_cmc_handling (cmc);
}


/**
 * Closure for #check_known_challenge.
 */
struct CheckKnownChallengeContext
{
  /**
   * Set to the challenge we are looking for.
   */
  const struct GNUNET_ShortHashCode *challenge;

  /**
   * Set to a matching validation state, if one was found.
   */
  struct ValidationState *vs;
};


/**
 * Test if the validation state in @a value matches the
 * challenge from @a cls.
 *
 * @param cls a `struct CheckKnownChallengeContext`
 * @param pid unused (must match though)
 * @param value a `struct ValidationState`
 * @return #GNUNET_OK if not matching, #GNUNET_NO if match found
 */
static int
check_known_challenge (void *cls,
                       const struct GNUNET_PeerIdentity *pid,
                       void *value)
{
  struct CheckKnownChallengeContext *ckac = cls;
  struct ValidationState *vs = value;

  (void) pid;
  if (0 != GNUNET_memcmp (&vs->challenge, ckac->challenge))
    return GNUNET_OK;
  ckac->vs = vs;
  return GNUNET_NO;
}


/**
 * Function called when peerstore is done storing a
 * validated address.
 *
 * @param cls a `struct ValidationState`
 * @param success #GNUNET_YES on success
 */
static void
peerstore_store_validation_cb (void *cls, int success)
{
  struct ValidationState *vs = cls;

  vs->sc = NULL;
  if (GNUNET_YES == success)
    return;
  GNUNET_STATISTICS_update (GST_stats,
                            "# Peerstore failed to store foreign address",
                            1,
                            GNUNET_NO);
}


/**
 * Task run periodically to validate some address based on #validation_heap.
 *
 * @param cls NULL
 */
static void
validation_start_cb (void *cls);


/**
 * Set the time for next_challenge of @a vs to @a new_time.
 * Updates the heap and if necessary reschedules the job.
 *
 * @param vs validation state to update
 * @param new_time new time for revalidation
 */
static void
update_next_challenge_time (struct ValidationState *vs,
                            struct GNUNET_TIME_Absolute new_time)
{
  struct GNUNET_TIME_Relative delta;

  if (new_time.abs_value_us == vs->next_challenge.abs_value_us)
    return; /* be lazy */
  vs->next_challenge = new_time;
  if (NULL == vs->hn)
    vs->hn =
      GNUNET_CONTAINER_heap_insert (validation_heap, vs, new_time.abs_value_us);
  else
    GNUNET_CONTAINER_heap_update_cost (vs->hn, new_time.abs_value_us);
  if ((vs != GNUNET_CONTAINER_heap_peek (validation_heap)) &&
      (NULL != validation_task))
    return;
  if (NULL != validation_task)
    GNUNET_SCHEDULER_cancel (validation_task);
  /* randomize a bit */
  delta.rel_value_us =
    GNUNET_CRYPTO_random_u64 (GNUNET_CRYPTO_QUALITY_WEAK,
                              MIN_DELAY_ADDRESS_VALIDATION.rel_value_us);
  new_time = GNUNET_TIME_absolute_add (new_time, delta);
  validation_task =
    GNUNET_SCHEDULER_add_at (new_time, &validation_start_cb, NULL);
}


/**
 * Find the queue matching @a pid and @a address.
 *
 * @param pid peer the queue must go to
 * @param address address the queue must use
 * @return NULL if no such queue exists
 */
static struct Queue *
find_queue (const struct GNUNET_PeerIdentity *pid, const char *address)
{
  struct Neighbour *n;

  n = GNUNET_CONTAINER_multipeermap_get (neighbours, pid);
  if (NULL == n)
    return NULL;
  for (struct Queue *pos = n->queue_head; NULL != pos;
       pos = pos->next_neighbour)
  {
    if (0 == strcmp (pos->address, address))
      return pos;
  }
  return NULL;
}


/**
 * Task run periodically to check whether the validity of the given queue has
 * run its course. If so, finds either another queue to take over, or clears
 * the neighbour's `core_visible` flag. In the latter case, gives DV routes a
 * chance to take over, and if that fails, notifies CORE about the disconnect.
 *
 * @param cls a `struct Queue`
 */
static void
core_queue_visibility_check (void *cls)
{
  struct Queue *q = cls;

  q->visibility_task = NULL;
  if (0 != GNUNET_TIME_absolute_get_remaining (q->validated_until).rel_value_us)
  {
    q->visibility_task = GNUNET_SCHEDULER_add_at (q->validated_until,
                                                  &core_queue_visibility_check,
                                                  q);
    return;
  }
  update_neighbour_core_visibility (q->neighbour);
}


/**
 * Check whether the CORE visibility of @a n should change.  Finds either a
 * queue to preserve the visibility, or clears the neighbour's `core_visible`
 * flag. In the latter case, gives DV routes a chance to take over, and if
 * that fails, notifies CORE about the disconnect.  If so, check whether we
 * need to notify CORE.
 *
 * @param n neighbour to perform the check for
 */
static void
update_neighbour_core_visibility (struct Neighbour *n)
{
  struct DistanceVector *dv;

  GNUNET_assert (GNUNET_YES == n->core_visible);
  /* Check if _any_ queue of this neighbour is still valid, if so, schedule
     the #core_queue_visibility_check() task for that queue */
  for (struct Queue *q = n->queue_head; NULL != q; q = q->next_neighbour)
  {
    if (0 !=
        GNUNET_TIME_absolute_get_remaining (q->validated_until).rel_value_us)
    {
      /* found a valid queue, use this one */
      q->visibility_task =
        GNUNET_SCHEDULER_add_at (q->validated_until,
                                 &core_queue_visibility_check,
                                 q);
      return;
    }
  }
  n->core_visible = GNUNET_NO;

  /* Check if _any_ DV route to this neighbour is currently
     valid, if so, do NOT tell core about the loss of direct
     connectivity (DV still counts!) */
  dv = GNUNET_CONTAINER_multipeermap_get (dv_routes, &n->pid);
  if (GNUNET_YES == dv->core_visible)
    return;
  /* Nothing works anymore, need to tell CORE about the loss of
     connectivity! */
  cores_send_disconnect_info (&n->pid);
}


/**
 * Communicator gave us a transport address validation response.  Process the
 * request.
 *
 * @param cls a `struct CommunicatorMessageContext` (must call
 * #finish_cmc_handling() when done)
 * @param tvr the message that was received
 */
static void
handle_validation_response (void *cls,
                            const struct TransportValidationResponse *tvr)
{
  struct CommunicatorMessageContext *cmc = cls;
  struct ValidationState *vs;
  struct CheckKnownChallengeContext ckac = {.challenge = &tvr->challenge,
                                            .vs = NULL};
  struct GNUNET_TIME_Absolute origin_time;
  struct Queue *q;
  struct DistanceVector *dv;
  struct Neighbour *n;

  /* check this is one of our challenges */
  (void) GNUNET_CONTAINER_multipeermap_get_multiple (validation_map,
                                                     &cmc->im.sender,
                                                     &check_known_challenge,
                                                     &ckac);
  if (NULL == (vs = ckac.vs))
  {
    /* This can happen simply if we 'forgot' the challenge by now,
       i.e. because we received the validation response twice */
    GNUNET_STATISTICS_update (GST_stats,
                              "# Validations dropped, challenge unknown",
                              1,
                              GNUNET_NO);
    finish_cmc_handling (cmc);
    return;
  }

  /* sanity check on origin time */
  origin_time = GNUNET_TIME_absolute_ntoh (tvr->origin_time);
  if ((origin_time.abs_value_us < vs->first_challenge_use.abs_value_us) ||
      (origin_time.abs_value_us > vs->last_challenge_use.abs_value_us))
  {
    GNUNET_break_op (0);
    finish_cmc_handling (cmc);
    return;
  }

  {
    /* check signature */
    struct TransportValidationPS tvp =
      {.purpose.purpose = htonl (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_CHALLENGE),
       .purpose.size = htonl (sizeof (tvp)),
       .validity_duration = tvr->validity_duration,
       .challenge = tvr->challenge};

    if (
      GNUNET_OK !=
      GNUNET_CRYPTO_eddsa_verify (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_CHALLENGE,
                                  &tvp.purpose,
                                  &tvr->signature,
                                  &cmc->im.sender.public_key))
    {
      GNUNET_break_op (0);
      finish_cmc_handling (cmc);
      return;
    }
  }

  /* validity is capped by our willingness to keep track of the
     validation entry and the maximum the other peer allows */
  vs->valid_until = GNUNET_TIME_relative_to_absolute (
    GNUNET_TIME_relative_min (GNUNET_TIME_relative_ntoh (
                                tvr->validity_duration),
                              MAX_ADDRESS_VALID_UNTIL));
  vs->validated_until =
    GNUNET_TIME_absolute_min (vs->valid_until,
                              GNUNET_TIME_relative_to_absolute (
                                ADDRESS_VALIDATION_LIFETIME));
  vs->validation_rtt = GNUNET_TIME_absolute_get_duration (origin_time);
  vs->challenge_backoff = GNUNET_TIME_UNIT_ZERO;
  GNUNET_CRYPTO_random_block (GNUNET_CRYPTO_QUALITY_NONCE,
                              &vs->challenge,
                              sizeof (vs->challenge));
  vs->first_challenge_use = GNUNET_TIME_absolute_subtract (
    vs->validated_until,
    GNUNET_TIME_relative_multiply (vs->validation_rtt,
                                   VALIDATION_RTT_BUFFER_FACTOR));
  vs->last_challenge_use =
    GNUNET_TIME_UNIT_ZERO_ABS; /* challenge was not yet used */
  update_next_challenge_time (vs, vs->first_challenge_use);
  vs->sc = GNUNET_PEERSTORE_store (peerstore,
                                   "transport",
                                   &cmc->im.sender,
                                   GNUNET_PEERSTORE_TRANSPORT_URLADDRESS_KEY,
                                   vs->address,
                                   strlen (vs->address) + 1,
                                   vs->valid_until,
                                   GNUNET_PEERSTORE_STOREOPTION_MULTIPLE,
                                   &peerstore_store_validation_cb,
                                   vs);
  finish_cmc_handling (cmc);

  /* Finally, we now possibly have a confirmed (!) working queue,
     update queue status (if queue still is around) */
  q = find_queue (&vs->pid, vs->address);
  if (NULL == q)
  {
    GNUNET_STATISTICS_update (GST_stats,
                              "# Queues lost at time of successful validation",
                              1,
                              GNUNET_NO);
    return;
  }
  q->validated_until = vs->validated_until;
  q->rtt = vs->validation_rtt;
  n = q->neighbour;
  if (GNUNET_NO != n->core_visible)
    return; /* nothing changed, we are done here */
  n->core_visible = GNUNET_YES;
  q->visibility_task = GNUNET_SCHEDULER_add_at (q->validated_until,
                                                &core_queue_visibility_check,
                                                q);
  /* Check if _any_ DV route to this neighbour is
     currently valid, if so, do NOT tell core anything! */
  dv = GNUNET_CONTAINER_multipeermap_get (dv_routes, &n->pid);
  if ((NULL != dv) && (GNUNET_YES == dv->core_visible))
    return; /* nothing changed, done */
  /* We lacked a confirmed connection to the neighbour
     before, so tell CORE about it (finally!) */
  cores_send_connect_info (&n->pid,
                           (NULL != dv)
                             ? GNUNET_BANDWIDTH_value_sum (dv->quota_out,
                                                           n->quota_out)
                             : n->quota_out);
}


/**
 * Incoming meessage.  Process the request.
 *
 * @param im the send message that was received
 */
static void
handle_incoming_msg (void *cls,
                     const struct GNUNET_TRANSPORT_IncomingMessage *im)
{
  struct TransportClient *tc = cls;
  struct CommunicatorMessageContext *cmc =
    GNUNET_new (struct CommunicatorMessageContext);

  cmc->tc = tc;
  cmc->im = *im;
  demultiplex_with_cmc (cmc, (const struct GNUNET_MessageHeader *) &im[1]);
}


/**
 * Given an inbound message @a msg from a communicator @a cmc,
 * demultiplex it based on the type calling the right handler.
 *
 * @param cmc context for demultiplexing
 * @param msg message to demultiplex
 */
static void
demultiplex_with_cmc (struct CommunicatorMessageContext *cmc,
                      const struct GNUNET_MessageHeader *msg)
{
  struct GNUNET_MQ_MessageHandler handlers[] =
    {GNUNET_MQ_hd_var_size (fragment_box,
                            GNUNET_MESSAGE_TYPE_TRANSPORT_FRAGMENT,
                            struct TransportFragmentBox,
                            &cmc),
     GNUNET_MQ_hd_fixed_size (fragment_ack,
                              GNUNET_MESSAGE_TYPE_TRANSPORT_FRAGMENT_ACK,
                              struct TransportFragmentAckMessage,
                              &cmc),
     GNUNET_MQ_hd_var_size (reliability_box,
                            GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_BOX,
                            struct TransportReliabilityBox,
                            &cmc),
     GNUNET_MQ_hd_fixed_size (reliability_ack,
                              GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_ACK,
                              struct TransportReliabilityAckMessage,
                              &cmc),
     GNUNET_MQ_hd_var_size (backchannel_encapsulation,
                            GNUNET_MESSAGE_TYPE_TRANSPORT_BACKCHANNEL_ENCAPSULATION,
                            struct TransportBackchannelEncapsulationMessage,
                            &cmc),
     GNUNET_MQ_hd_var_size (dv_learn,
                            GNUNET_MESSAGE_TYPE_TRANSPORT_DV_LEARN,
                            struct TransportDVLearn,
                            &cmc),
     GNUNET_MQ_hd_var_size (dv_box,
                            GNUNET_MESSAGE_TYPE_TRANSPORT_DV_BOX,
                            struct TransportDVBox,
                            &cmc),
     GNUNET_MQ_hd_fixed_size (
       validation_challenge,
       GNUNET_MESSAGE_TYPE_TRANSPORT_ADDRESS_VALIDATION_CHALLENGE,
       struct TransportValidationChallenge,
       &cmc),
     GNUNET_MQ_hd_fixed_size (
       validation_response,
       GNUNET_MESSAGE_TYPE_TRANSPORT_ADDRESS_VALIDATION_RESPONSE,
       struct TransportValidationResponse,
       &cmc),
     GNUNET_MQ_handler_end ()};
  int ret;

  ret = GNUNET_MQ_handle_message (handlers, msg);
  if (GNUNET_SYSERR == ret)
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (cmc->tc->client);
    GNUNET_free (cmc);
    return;
  }
  if (GNUNET_NO == ret)
  {
    /* unencapsulated 'raw' message */
    handle_raw_message (&cmc, msg);
  }
}


/**
 * New queue became available.  Check message.
 *
 * @param cls the client
 * @param aqm the send message that was sent
 */
static int
check_add_queue_message (void *cls,
                         const struct GNUNET_TRANSPORT_AddQueueMessage *aqm)
{
  struct TransportClient *tc = cls;

  if (CT_COMMUNICATOR != tc->type)
  {
    GNUNET_break (0);
    return GNUNET_SYSERR;
  }
  GNUNET_MQ_check_zero_termination (aqm);
  return GNUNET_OK;
}


/**
 * Bandwidth tracker informs us that the delay until we should receive
 * more has changed.
 *
 * @param cls a `struct Queue` for which the delay changed
 */
static void
tracker_update_in_cb (void *cls)
{
  struct Queue *queue = cls;
  struct GNUNET_TIME_Relative in_delay;
  unsigned int rsize;

  rsize = (0 == queue->mtu) ? IN_PACKET_SIZE_WITHOUT_MTU : queue->mtu;
  in_delay = GNUNET_BANDWIDTH_tracker_get_delay (&queue->tracker_in, rsize);
  // FIXME: how exactly do we do inbound flow control?
}


/**
 * If necessary, generates the UUID for a @a pm
 *
 * @param pm pending message to generate UUID for.
 */
static void
set_pending_message_uuid (struct PendingMessage *pm)
{
  if (pm->msg_uuid_set)
    return;
  GNUNET_CRYPTO_random_block (GNUNET_CRYPTO_QUALITY_NONCE,
                              &pm->msg_uuid,
                              sizeof (pm->msg_uuid));
  pm->msg_uuid_set = GNUNET_YES;
}


/**
 * Fragment the given @a pm to the given @a mtu.  Adds
 * additional fragments to the neighbour as well. If the
 * @a mtu is too small, generates and error for the @a pm
 * and returns NULL.
 *
 * @param pm pending message to fragment for transmission
 * @param mtu MTU to apply
 * @return new message to transmit
 */
static struct PendingMessage *
fragment_message (struct PendingMessage *pm, uint16_t mtu)
{
  struct PendingMessage *ff;

  set_pending_message_uuid (pm);

  /* This invariant is established in #handle_add_queue_message() */
  GNUNET_assert (mtu > sizeof (struct TransportFragmentBox));

  /* select fragment for transmission, descending the tree if it has
     been expanded until we are at a leaf or at a fragment that is small enough
   */
  ff = pm;
  while (((ff->bytes_msg > mtu) || (pm == ff)) &&
         (ff->frag_off == ff->bytes_msg) && (NULL != ff->head_frag))
  {
    ff = ff->head_frag; /* descent into fragmented fragments */
  }

  if (((ff->bytes_msg > mtu) || (pm == ff)) && (pm->frag_off < pm->bytes_msg))
  {
    /* Did not yet calculate all fragments, calculate next fragment */
    struct PendingMessage *frag;
    struct TransportFragmentBox tfb;
    const char *orig;
    char *msg;
    uint16_t fragmax;
    uint16_t fragsize;
    uint16_t msize;
    uint16_t xoff = 0;

    orig = (const char *) &ff[1];
    msize = ff->bytes_msg;
    if (pm != ff)
    {
      const struct TransportFragmentBox *tfbo;

      tfbo = (const struct TransportFragmentBox *) orig;
      orig += sizeof (struct TransportFragmentBox);
      msize -= sizeof (struct TransportFragmentBox);
      xoff = ntohs (tfbo->frag_off);
    }
    fragmax = mtu - sizeof (struct TransportFragmentBox);
    fragsize = GNUNET_MIN (msize - ff->frag_off, fragmax);
    frag = GNUNET_malloc (sizeof (struct PendingMessage) +
                          sizeof (struct TransportFragmentBox) + fragsize);
    frag->target = pm->target;
    frag->frag_parent = ff;
    frag->timeout = pm->timeout;
    frag->bytes_msg = sizeof (struct TransportFragmentBox) + fragsize;
    frag->pmt = PMT_FRAGMENT_BOX;
    msg = (char *) &frag[1];
    tfb.header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_FRAGMENT);
    tfb.header.size = htons (sizeof (struct TransportFragmentBox) + fragsize);
    tfb.frag_uuid = htonl (pm->frag_uuidgen++);
    tfb.msg_uuid = pm->msg_uuid;
    tfb.frag_off = htons (ff->frag_off + xoff);
    tfb.msg_size = htons (pm->bytes_msg);
    memcpy (msg, &tfb, sizeof (tfb));
    memcpy (&msg[sizeof (tfb)], &orig[ff->frag_off], fragsize);
    GNUNET_CONTAINER_MDLL_insert (frag, ff->head_frag, ff->tail_frag, frag);
    ff->frag_off += fragsize;
    ff = frag;
  }

  /* Move head to the tail and return it */
  GNUNET_CONTAINER_MDLL_remove (frag,
                                ff->frag_parent->head_frag,
                                ff->frag_parent->tail_frag,
                                ff);
  GNUNET_CONTAINER_MDLL_insert_tail (frag,
                                     ff->frag_parent->head_frag,
                                     ff->frag_parent->tail_frag,
                                     ff);
  return ff;
}


/**
 * Reliability-box the given @a pm. On error (can there be any), NULL
 * may be returned, otherwise the "replacement" for @a pm (which
 * should then be added to the respective neighbour's queue instead of
 * @a pm).  If the @a pm is already fragmented or reliability boxed,
 * or itself an ACK, this function simply returns @a pm.
 *
 * @param pm pending message to box for transmission over unreliabile queue
 * @return new message to transmit
 */
static struct PendingMessage *
reliability_box_message (struct PendingMessage *pm)
{
  struct TransportReliabilityBox rbox;
  struct PendingMessage *bpm;
  char *msg;

  if (PMT_CORE != pm->pmt)
    return pm; /* already fragmented or reliability boxed, or control message:
                  do nothing */
  if (NULL != pm->bpm)
    return pm->bpm; /* already computed earlier: do nothing */
  GNUNET_assert (NULL == pm->head_frag);
  if (pm->bytes_msg + sizeof (rbox) > UINT16_MAX)
  {
    /* failed hard */
    GNUNET_break (0);
    client_send_response (pm, GNUNET_NO, 0);
    return NULL;
  }
  bpm = GNUNET_malloc (sizeof (struct PendingMessage) + sizeof (rbox) +
                       pm->bytes_msg);
  bpm->target = pm->target;
  bpm->frag_parent = pm;
  GNUNET_CONTAINER_MDLL_insert (frag, pm->head_frag, pm->tail_frag, bpm);
  bpm->timeout = pm->timeout;
  bpm->pmt = PMT_RELIABILITY_BOX;
  bpm->bytes_msg = pm->bytes_msg + sizeof (rbox);
  set_pending_message_uuid (bpm);
  rbox.header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_BOX);
  rbox.header.size = htons (sizeof (rbox) + pm->bytes_msg);
  rbox.ack_countdown = htonl (0); // FIXME: implement ACK countdown support
  rbox.msg_uuid = pm->msg_uuid;
  msg = (char *) &bpm[1];
  memcpy (msg, &rbox, sizeof (rbox));
  memcpy (&msg[sizeof (rbox)], &pm[1], pm->bytes_msg);
  pm->bpm = bpm;
  return bpm;
}


/**
 * Change the value of the `next_attempt` field of @a pm
 * to @a next_attempt and re-order @a pm in the transmission
 * list as required by the new timestmap.
 *
 * @param pm a pending message to update
 * @param next_attempt timestamp to use
 */
static void
update_pm_next_attempt (struct PendingMessage *pm,
                        struct GNUNET_TIME_Absolute next_attempt)
{
  struct Neighbour *neighbour = pm->target;

  pm->next_attempt = next_attempt;
  if (NULL == pm->frag_parent)
  {
    struct PendingMessage *pos;

    /* re-insert sort in neighbour list */
    GNUNET_CONTAINER_MDLL_remove (neighbour,
                                  neighbour->pending_msg_head,
                                  neighbour->pending_msg_tail,
                                  pm);
    pos = neighbour->pending_msg_tail;
    while ((NULL != pos) &&
           (next_attempt.abs_value_us > pos->next_attempt.abs_value_us))
      pos = pos->prev_neighbour;
    GNUNET_CONTAINER_MDLL_insert_after (neighbour,
                                        neighbour->pending_msg_head,
                                        neighbour->pending_msg_tail,
                                        pos,
                                        pm);
  }
  else
  {
    /* re-insert sort in fragment list */
    struct PendingMessage *fp = pm->frag_parent;
    struct PendingMessage *pos;

    GNUNET_CONTAINER_MDLL_remove (frag, fp->head_frag, fp->tail_frag, pm);
    pos = fp->tail_frag;
    while ((NULL != pos) &&
           (next_attempt.abs_value_us > pos->next_attempt.abs_value_us))
      pos = pos->prev_frag;
    GNUNET_CONTAINER_MDLL_insert_after (frag,
                                        fp->head_frag,
                                        fp->tail_frag,
                                        pos,
                                        pm);
  }
}


/**
 * We believe we are ready to transmit a message on a queue. Double-checks
 * with the queue's "tracker_out" and then gives the message to the
 * communicator for transmission (updating the tracker, and re-scheduling
 * itself if applicable).
 *
 * @param cls the `struct Queue` to process transmissions for
 */
static void
transmit_on_queue (void *cls)
{
  struct Queue *queue = cls;
  struct Neighbour *n = queue->neighbour;
  struct PendingMessage *pm;
  struct PendingMessage *s;
  uint32_t overhead;

  queue->transmit_task = NULL;
  if (NULL == (pm = n->pending_msg_head))
  {
    /* no message pending, nothing to do here! */
    return;
  }
  if (NULL != pm->qe)
  {
    /* message still pending with communciator!
       LOGGING-FIXME: Use stats? logging? Should this not be rare? */
    return;
  }
  schedule_transmit_on_queue (queue, GNUNET_YES);
  if (NULL != queue->transmit_task)
    return; /* do it later */
  overhead = 0;
  if (GNUNET_TRANSPORT_CC_RELIABLE != queue->tc->details.communicator.cc)
    overhead += sizeof (struct TransportReliabilityBox);
  s = pm;
  if ( ( (0 != queue->mtu) &&
         (pm->bytes_msg + overhead > queue->mtu) ) ||
       (pm->bytes_msg > UINT16_MAX - sizeof (struct GNUNET_TRANSPORT_SendMessageTo)) ||
       (NULL != pm->head_frag /* fragments already exist, should
                                 respect that even if MTU is 0 for
                                 this queue */) )
    s = fragment_message (s,
                          (0 == queue->mtu)
                            ? UINT16_MAX -
                                sizeof (struct GNUNET_TRANSPORT_SendMessageTo)
                            : queue->mtu);
  if (NULL == s)
  {
    /* Fragmentation failed, try next message... */
    schedule_transmit_on_queue (queue, GNUNET_NO);
    return;
  }
  if (GNUNET_TRANSPORT_CC_RELIABLE != queue->tc->details.communicator.cc)
    s = reliability_box_message (s);
  if (NULL == s)
  {
    /* Reliability boxing failed, try next message... */
    schedule_transmit_on_queue (queue, GNUNET_NO);
    return;
  }

  /* Pass 's' for transission to the communicator */
  queue_send_msg (queue, s, &s[1], s->bytes_msg);
  // FIXME: do something similar to the logic below
  // in defragmentation / reliability ACK handling!

  /* Check if this transmission somehow conclusively finished handing 'pm'
     even without any explicit ACKs */
  if ((PMT_CORE == s->pmt) &&
      (GNUNET_TRANSPORT_CC_RELIABLE == queue->tc->details.communicator.cc))
  {
    /* Full message sent, and over reliabile channel */
    client_send_response (pm, GNUNET_YES, pm->bytes_msg);
  }
  else if ((GNUNET_TRANSPORT_CC_RELIABLE ==
            queue->tc->details.communicator.cc) &&
           (PMT_FRAGMENT_BOX == s->pmt))
  {
    struct PendingMessage *pos;

    /* Fragment sent over reliabile channel */
    free_fragment_tree (s);
    pos = s->frag_parent;
    GNUNET_CONTAINER_MDLL_remove (frag, pos->head_frag, pos->tail_frag, s);
    GNUNET_free (s);
    /* check if subtree is done */
    while ((NULL == pos->head_frag) && (pos->frag_off == pos->bytes_msg) &&
           (pos != pm))
    {
      s = pos;
      pos = s->frag_parent;
      GNUNET_CONTAINER_MDLL_remove (frag, pos->head_frag, pos->tail_frag, s);
      GNUNET_free (s);
    }

    /* Was this the last applicable fragmment? */
    if ((NULL == pm->head_frag) && (pm->frag_off == pm->bytes_msg))
      client_send_response (
        pm,
        GNUNET_YES,
        pm->bytes_msg /* FIXME: calculate and add overheads! */);
  }
  else if (PMT_CORE != pm->pmt)
  {
    /* This was an acknowledgement of some type, always free */
    free_pending_message (pm);
  }
  else
  {
    /* Message not finished, waiting for acknowledgement.
       Update time by which we might retransmit 's' based on queue
       characteristics (i.e. RTT); it takes one RTT for the message to
       arrive and the ACK to come back in the best case; but the other
       side is allowed to delay ACKs by 2 RTTs, so we use 4 RTT before
       retransmitting.  Note that in the future this heuristic should
       likely be improved further (measure RTT stability, consider
       message urgency and size when delaying ACKs, etc.) */
    update_pm_next_attempt (s,
                            GNUNET_TIME_relative_to_absolute (
                              GNUNET_TIME_relative_multiply (queue->rtt, 4)));
  }

  /* finally, re-schedule queue transmission task itself */
  schedule_transmit_on_queue (queue, GNUNET_NO);
}


/**
 * Bandwidth tracker informs us that the delay until we
 * can transmit again changed.
 *
 * @param cls a `struct Queue` for which the delay changed
 */
static void
tracker_update_out_cb (void *cls)
{
  struct Queue *queue = cls;
  struct Neighbour *n = queue->neighbour;

  if (NULL == n->pending_msg_head)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
                "Bandwidth allocation updated for empty transmission queue `%s'\n",
                queue->address);
    return; /* no message pending, nothing to do here! */
  }
  GNUNET_SCHEDULER_cancel (queue->transmit_task);
  queue->transmit_task = NULL;
  schedule_transmit_on_queue (queue, GNUNET_NO);
}


/**
 * Bandwidth tracker informs us that excessive outbound bandwidth was
 * allocated which is not being used.
 *
 * @param cls a `struct Queue` for which the excess was noted
 */
static void
tracker_excess_out_cb (void *cls)
{
  (void) cls;

  /* FIXME: trigger excess bandwidth report to core? Right now,
     this is done internally within transport_api2_core already,
     but we probably want to change the logic and trigger it
     from here via a message instead! */
  /* TODO: maybe inform someone at this point? */
  GNUNET_STATISTICS_update (GST_stats,
                            "# Excess outbound bandwidth reported",
                            1,
                            GNUNET_NO);
}


/**
 * Bandwidth tracker informs us that excessive inbound bandwidth was allocated
 * which is not being used.
 *
 * @param cls a `struct Queue` for which the excess was noted
 */
static void
tracker_excess_in_cb (void *cls)
{
  (void) cls;

  /* TODO: maybe inform somone at this point? */
  GNUNET_STATISTICS_update (GST_stats,
                            "# Excess inbound bandwidth reported",
                            1,
                            GNUNET_NO);
}


/**
 * Queue to a peer went down.  Process the request.
 *
 * @param cls the client
 * @param dqm the send message that was sent
 */
static void
handle_del_queue_message (void *cls,
                          const struct GNUNET_TRANSPORT_DelQueueMessage *dqm)
{
  struct TransportClient *tc = cls;

  if (CT_COMMUNICATOR != tc->type)
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (tc->client);
    return;
  }
  for (struct Queue *queue = tc->details.communicator.queue_head; NULL != queue;
       queue = queue->next_client)
  {
    struct Neighbour *neighbour = queue->neighbour;

    if ((dqm->qid != queue->qid) ||
        (0 != GNUNET_memcmp (&dqm->receiver, &neighbour->pid)))
      continue;
    free_queue (queue);
    GNUNET_SERVICE_client_continue (tc->client);
    return;
  }
  GNUNET_break (0);
  GNUNET_SERVICE_client_drop (tc->client);
}


/**
 * Message was transmitted.  Process the request.
 *
 * @param cls the client
 * @param sma the send message that was sent
 */
static void
handle_send_message_ack (void *cls,
                         const struct GNUNET_TRANSPORT_SendMessageToAck *sma)
{
  struct TransportClient *tc = cls;
  struct QueueEntry *qe;
  struct PendingMessage *pm;

  if (CT_COMMUNICATOR != tc->type)
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (tc->client);
    return;
  }

  /* find our queue entry matching the ACK */
  qe = NULL;
  for (struct Queue *queue = tc->details.communicator.queue_head; NULL != queue;
       queue = queue->next_client)
  {
    if (0 != GNUNET_memcmp (&queue->neighbour->pid, &sma->receiver))
      continue;
    for (struct QueueEntry *qep = queue->queue_head; NULL != qep;
         qep = qep->next)
    {
      if (qep->mid != sma->mid)
        continue;
      qe = qep;
      break;
    }
    break;
  }
  if (NULL == qe)
  {
    /* this should never happen */
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (tc->client);
    return;
  }
  GNUNET_CONTAINER_DLL_remove (qe->queue->queue_head,
                               qe->queue->queue_tail,
                               qe);
  qe->queue->queue_length--;
  tc->details.communicator.total_queue_length--;
  GNUNET_SERVICE_client_continue (tc->client);

  /* if applicable, resume transmissions that waited on ACK */
  if (COMMUNICATOR_TOTAL_QUEUE_LIMIT - 1 ==
      tc->details.communicator.total_queue_length)
  {
    /* Communicator dropped below threshold, resume all queues
       incident with this client! */
    GNUNET_STATISTICS_update (
      GST_stats,
      "# Transmission throttled due to communicator queue limit",
      -1,
      GNUNET_NO);
    for (struct Queue *queue = tc->details.communicator.queue_head;
         NULL != queue;
         queue = queue->next_client)
      schedule_transmit_on_queue (queue, GNUNET_NO);
  }
  else if (QUEUE_LENGTH_LIMIT - 1 == qe->queue->queue_length)
  {
    /* queue dropped below threshold; only resume this one queue */
    GNUNET_STATISTICS_update (GST_stats,
                              "# Transmission throttled due to queue queue limit",
                              -1,
                              GNUNET_NO);
    schedule_transmit_on_queue (qe->queue, GNUNET_NO);
  }

  if (NULL != (pm = qe->pm))
  {
    struct Neighbour *n;

    GNUNET_assert (qe == pm->qe);
    pm->qe = NULL;
    /* If waiting for this communicator may have blocked transmission
       of pm on other queues for this neighbour, force schedule
       transmit on queue for queues of the neighbour */
    n = pm->target;
    if (n->pending_msg_head == pm)
    {
      for (struct Queue *queue = n->queue_head; NULL != queue;
           queue = queue->next_neighbour)
        schedule_transmit_on_queue (queue, GNUNET_NO);
    }
    if (GNUNET_OK != ntohl (sma->status))
    {
      GNUNET_log (
        GNUNET_ERROR_TYPE_INFO,
        "Queue failed in transmission, will try retransmission immediately\n");
      update_pm_next_attempt (pm, GNUNET_TIME_UNIT_ZERO_ABS);
    }
  }
  GNUNET_free (qe);
}


/**
 * Iterator telling new MONITOR client about all existing
 * queues to peers.
 *
 * @param cls the new `struct TransportClient`
 * @param pid a connected peer
 * @param value the `struct Neighbour` with more information
 * @return #GNUNET_OK (continue to iterate)
 */
static int
notify_client_queues (void *cls,
                      const struct GNUNET_PeerIdentity *pid,
                      void *value)
{
  struct TransportClient *tc = cls;
  struct Neighbour *neighbour = value;

  GNUNET_assert (CT_MONITOR == tc->type);
  for (struct Queue *q = neighbour->queue_head; NULL != q;
       q = q->next_neighbour)
  {
    struct MonitorEvent me = {.rtt = q->rtt,
                              .cs = q->cs,
                              .num_msg_pending = q->num_msg_pending,
                              .num_bytes_pending = q->num_bytes_pending};

    notify_monitor (tc, pid, q->address, q->nt, &me);
  }
  return GNUNET_OK;
}


/**
 * Initialize a monitor client.
 *
 * @param cls the client
 * @param start the start message that was sent
 */
static void
handle_monitor_start (void *cls,
                      const struct GNUNET_TRANSPORT_MonitorStart *start)
{
  struct TransportClient *tc = cls;

  if (CT_NONE != tc->type)
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (tc->client);
    return;
  }
  tc->type = CT_MONITOR;
  tc->details.monitor.peer = start->peer;
  tc->details.monitor.one_shot = ntohl (start->one_shot);
  GNUNET_CONTAINER_multipeermap_iterate (neighbours, &notify_client_queues, tc);
  GNUNET_SERVICE_client_mark_monitor (tc->client);
  GNUNET_SERVICE_client_continue (tc->client);
}


/**
 * Find transport client providing communication service
 * for the protocol @a prefix.
 *
 * @param prefix communicator name
 * @return NULL if no such transport client is available
 */
static struct TransportClient *
lookup_communicator (const char *prefix)
{
  for (struct TransportClient *tc = clients_head; NULL != tc; tc = tc->next)
  {
    if (CT_COMMUNICATOR != tc->type)
      continue;
    if (0 == strcmp (prefix, tc->details.communicator.address_prefix))
      return tc;
  }
  GNUNET_log (
    GNUNET_ERROR_TYPE_WARNING,
    "Somone suggested use of communicator for `%s', but we do not have such a communicator!\n",
    prefix);
  return NULL;
}


/**
 * Signature of a function called with a communicator @a address of a peer
 * @a pid that an application wants us to connect to.
 *
 * @param pid target peer
 * @param address the address to try
 */
static void
suggest_to_connect (const struct GNUNET_PeerIdentity *pid, const char *address)
{
  static uint32_t idgen;
  struct TransportClient *tc;
  char *prefix;
  struct GNUNET_TRANSPORT_CreateQueue *cqm;
  struct GNUNET_MQ_Envelope *env;
  size_t alen;

  prefix = GNUNET_HELLO_address_to_prefix (address);
  if (NULL == prefix)
  {
    GNUNET_break (0); /* We got an invalid address!? */
    return;
  }
  tc = lookup_communicator (prefix);
  if (NULL == tc)
  {
    GNUNET_STATISTICS_update (GST_stats,
                              "# Suggestions ignored due to missing communicator",
                              1,
                              GNUNET_NO);
    return;
  }
  /* forward suggestion for queue creation to communicator */
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "Request #%u for `%s' communicator to create queue to `%s'\n",
              (unsigned int) idgen,
              prefix,
              address);
  alen = strlen (address) + 1;
  env =
    GNUNET_MQ_msg_extra (cqm, alen, GNUNET_MESSAGE_TYPE_TRANSPORT_QUEUE_CREATE);
  cqm->request_id = htonl (idgen++);
  cqm->receiver = *pid;
  memcpy (&cqm[1], address, alen);
  GNUNET_MQ_send (tc->mq, env);
}


/**
 * The queue @a q (which matches the peer and address in @a vs) is
 * ready for queueing. We should now queue the validation request.
 *
 * @param q queue to send on
 * @param vs state to derive validation challenge from
 */
static void
validation_transmit_on_queue (struct Queue *q, struct ValidationState *vs)
{
  struct TransportValidationChallenge tvc;

  vs->last_challenge_use = GNUNET_TIME_absolute_get ();
  tvc.header.type =
    htons (GNUNET_MESSAGE_TYPE_TRANSPORT_ADDRESS_VALIDATION_CHALLENGE);
  tvc.header.size = htons (sizeof (tvc));
  tvc.reserved = htonl (0);
  tvc.challenge = vs->challenge;
  tvc.sender_time = GNUNET_TIME_absolute_hton (vs->last_challenge_use);
  queue_send_msg (q, NULL, &tvc, sizeof (tvc));
}


/**
 * Task run periodically to validate some address based on #validation_heap.
 *
 * @param cls NULL
 */
static void
validation_start_cb (void *cls)
{
  struct ValidationState *vs;
  struct Queue *q;

  (void) cls;
  validation_task = NULL;
  vs = GNUNET_CONTAINER_heap_peek (validation_heap);
  /* drop validations past their expiration */
  while (
    (NULL != vs) &&
    (0 == GNUNET_TIME_absolute_get_remaining (vs->valid_until).rel_value_us))
  {
    free_validation_state (vs);
    vs = GNUNET_CONTAINER_heap_peek (validation_heap);
  }
  if (NULL == vs)
    return; /* woopsie, no more addresses known, should only
               happen if we're really a lonely peer */
  q = find_queue (&vs->pid, vs->address);
  if (NULL == q)
  {
    vs->awaiting_queue = GNUNET_YES;
    suggest_to_connect (&vs->pid, vs->address);
  }
  else
    validation_transmit_on_queue (q, vs);
  /* Finally, reschedule next attempt */
  vs->challenge_backoff =
    GNUNET_TIME_randomized_backoff (vs->challenge_backoff,
                                    MAX_VALIDATION_CHALLENGE_FREQ);
  update_next_challenge_time (vs,
                              GNUNET_TIME_relative_to_absolute (
                                vs->challenge_backoff));
}


/**
 * Closure for #check_connection_quality.
 */
struct QueueQualityContext
{
  /**
   * Set to the @e k'th queue encountered.
   */
  struct Queue *q;

  /**
   * Set to the number of quality queues encountered.
   */
  unsigned int quality_count;

  /**
   * Set to the total number of queues encountered.
   */
  unsigned int num_queues;

  /**
   * Decremented for each queue, for selection of the
   * k-th queue in @e q.
   */
  unsigned int k;
};


/**
 * Check whether any queue to the given neighbour is
 * of a good "quality" and if so, increment the counter.
 * Also counts the total number of queues, and returns
 * the k-th queue found.
 *
 * @param cls a `struct QueueQualityContext *` with counters
 * @param pid peer this is about
 * @param value a `struct Neighbour`
 * @return #GNUNET_OK (continue to iterate)
 */
static int
check_connection_quality (void *cls,
                          const struct GNUNET_PeerIdentity *pid,
                          void *value)
{
  struct QueueQualityContext *ctx = cls;
  struct Neighbour *n = value;
  int do_inc;

  (void) pid;
  do_inc = GNUNET_NO;
  for (struct Queue *q = n->queue_head; NULL != q; q = q->next_neighbour)
  {
    ctx->num_queues++;
    if (0 == ctx->k--)
      ctx->q = q;
    /* OPTIMIZE-FIXME: in the future, add reliability / goodput
       statistics and consider those as well here? */
    if (q->rtt.rel_value_us < DV_QUALITY_RTT_THRESHOLD.rel_value_us)
      do_inc = GNUNET_YES;
  }
  if (GNUNET_YES == do_inc)
    ctx->quality_count++;
  return GNUNET_OK;
}


/**
 * Task run when we CONSIDER initiating a DV learn
 * process. We first check that sending out a message is
 * even possible (queues exist), then that it is desirable
 * (if not, reschedule the task for later), and finally
 * we may then begin the job.  If there are too many
 * entries in the #dvlearn_map, we purge the oldest entry
 * using #lle_tail.
 *
 * @param cls NULL
 */
static void
start_dv_learn (void *cls)
{
  struct LearnLaunchEntry *lle;
  struct QueueQualityContext qqc;
  struct TransportDVLearn dvl;

  (void) cls;
  dvlearn_task = NULL;
  if (0 == GNUNET_CONTAINER_multipeermap_size (neighbours))
    return; /* lost all connectivity, cannot do learning */
  qqc.quality_count = 0;
  qqc.num_queues = 0;
  GNUNET_CONTAINER_multipeermap_iterate (neighbours,
                                         &check_connection_quality,
                                         &qqc);
  if (qqc.quality_count > DV_LEARN_QUALITY_THRESHOLD)
  {
    struct GNUNET_TIME_Relative delay;
    unsigned int factor;

    /* scale our retries by how far we are above the threshold */
    factor = qqc.quality_count / DV_LEARN_QUALITY_THRESHOLD;
    delay = GNUNET_TIME_relative_multiply (DV_LEARN_BASE_FREQUENCY, factor);
    dvlearn_task = GNUNET_SCHEDULER_add_delayed (delay, &start_dv_learn, NULL);
    return;
  }
  /* remove old entries in #dvlearn_map if it has grown too big */
  while (MAX_DV_LEARN_PENDING >=
         GNUNET_CONTAINER_multishortmap_size (dvlearn_map))
  {
    lle = lle_tail;
    GNUNET_assert (GNUNET_YES ==
                   GNUNET_CONTAINER_multishortmap_remove (dvlearn_map,
                                                          &lle->challenge,
                                                          lle));
    GNUNET_CONTAINER_DLL_remove (lle_head, lle_tail, lle);
    GNUNET_free (lle);
  }
  /* setup data structure for learning */
  lle = GNUNET_new (struct LearnLaunchEntry);
  GNUNET_CRYPTO_random_block (GNUNET_CRYPTO_QUALITY_NONCE,
                              &lle->challenge,
                              sizeof (lle->challenge));
  GNUNET_CONTAINER_DLL_insert (lle_head, lle_tail, lle);
  GNUNET_break (GNUNET_YES ==
                GNUNET_CONTAINER_multishortmap_put (
                  dvlearn_map,
                  &lle->challenge,
                  lle,
                  GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
  dvl.header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_DV_LEARN);
  dvl.header.size = htons (sizeof (dvl));
  dvl.num_hops = htons (0);
  dvl.bidirectional = htons (0);
  dvl.non_network_delay = GNUNET_TIME_relative_hton (GNUNET_TIME_UNIT_ZERO);
  {
    struct DvInitPS dvip = {.purpose.purpose = htonl (
                              GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_INITIATOR),
                            .purpose.size = htonl (sizeof (dvip)),
                            .challenge = lle->challenge};

    GNUNET_assert (GNUNET_OK == GNUNET_CRYPTO_eddsa_sign (GST_my_private_key,
                                                          &dvip.purpose,
                                                          &dvl.init_sig));
  }
  dvl.initiator = GST_my_identity;
  dvl.challenge = lle->challenge;

  qqc.quality_count = 0;
  qqc.k = GNUNET_CRYPTO_random_u32 (GNUNET_CRYPTO_QUALITY_WEAK, qqc.num_queues);
  qqc.num_queues = 0;
  qqc.q = NULL;
  GNUNET_CONTAINER_multipeermap_iterate (neighbours,
                                         &check_connection_quality,
                                         &qqc);
  GNUNET_assert (NULL != qqc.q);

  /* Do this as close to transmission time as possible! */
  lle->launch_time = GNUNET_TIME_absolute_get ();

  queue_send_msg (qqc.q, NULL, &dvl, sizeof (dvl));
  /* reschedule this job, randomizing the time it runs (but no
     actual backoff!) */
  dvlearn_task = GNUNET_SCHEDULER_add_delayed (GNUNET_TIME_randomize (
                                                 DV_LEARN_BASE_FREQUENCY),
                                               &start_dv_learn,
                                               NULL);
}


/**
 * A new queue has been created, check if any address validation
 * requests have been waiting for it.
 *
 * @param cls a `struct Queue`
 * @param pid peer concerned (unused)
 * @param value a `struct ValidationState`
 * @return #GNUNET_NO if a match was found and we can stop looking
 */
static int
check_validation_request_pending (void *cls,
                                  const struct GNUNET_PeerIdentity *pid,
                                  void *value)
{
  struct Queue *q = cls;
  struct ValidationState *vs = value;

  (void) pid;
  if ((GNUNET_YES == vs->awaiting_queue) &&
      (0 == strcmp (vs->address, q->address)))
  {
    vs->awaiting_queue = GNUNET_NO;
    validation_transmit_on_queue (q, vs);
    return GNUNET_NO;
  }
  return GNUNET_OK;
}


/**
 * New queue became available.  Process the request.
 *
 * @param cls the client
 * @param aqm the send message that was sent
 */
static void
handle_add_queue_message (void *cls,
                          const struct GNUNET_TRANSPORT_AddQueueMessage *aqm)
{
  struct TransportClient *tc = cls;
  struct Queue *queue;
  struct Neighbour *neighbour;
  const char *addr;
  uint16_t addr_len;

  if (ntohl (aqm->mtu) <= sizeof (struct TransportFragmentBox))
  {
    /* MTU so small as to be useless for transmissions,
       required for #fragment_message()! */
    GNUNET_break_op (0);
    GNUNET_SERVICE_client_drop (tc->client);
    return;
  }
  neighbour = lookup_neighbour (&aqm->receiver);
  if (NULL == neighbour)
  {
    neighbour = GNUNET_new (struct Neighbour);
    neighbour->earliest_timeout = GNUNET_TIME_UNIT_FOREVER_ABS;
    neighbour->pid = aqm->receiver;
    GNUNET_assert (GNUNET_OK ==
                   GNUNET_CONTAINER_multipeermap_put (
                     neighbours,
                     &neighbour->pid,
                     neighbour,
                     GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
  }
  addr_len = ntohs (aqm->header.size) - sizeof (*aqm);
  addr = (const char *) &aqm[1];

  queue = GNUNET_malloc (sizeof (struct Queue) + addr_len);
  queue->tc = tc;
  queue->address = (const char *) &queue[1];
  queue->rtt = GNUNET_TIME_UNIT_FOREVER_REL;
  queue->qid = aqm->qid;
  queue->mtu = ntohl (aqm->mtu);
  queue->nt = (enum GNUNET_NetworkType) ntohl (aqm->nt);
  queue->cs = (enum GNUNET_TRANSPORT_ConnectionStatus) ntohl (aqm->cs);
  queue->neighbour = neighbour;
  GNUNET_BANDWIDTH_tracker_init2 (&queue->tracker_in,
                                  &tracker_update_in_cb,
                                  queue,
                                  GNUNET_BANDWIDTH_ZERO,
                                  GNUNET_CONSTANTS_MAX_BANDWIDTH_CARRY_S,
                                  &tracker_excess_in_cb,
                                  queue);
  GNUNET_BANDWIDTH_tracker_init2 (&queue->tracker_out,
                                  &tracker_update_out_cb,
                                  queue,
                                  GNUNET_BANDWIDTH_ZERO,
                                  GNUNET_CONSTANTS_MAX_BANDWIDTH_CARRY_S,
                                  &tracker_excess_out_cb,
                                  queue);
  memcpy (&queue[1], addr, addr_len);
  /* notify monitors about new queue */
  {
    struct MonitorEvent me = {.rtt = queue->rtt, .cs = queue->cs};

    notify_monitors (&neighbour->pid, queue->address, queue->nt, &me);
  }
  GNUNET_CONTAINER_MDLL_insert (neighbour,
                                neighbour->queue_head,
                                neighbour->queue_tail,
                                queue);
  GNUNET_CONTAINER_MDLL_insert (client,
                                tc->details.communicator.queue_head,
                                tc->details.communicator.queue_tail,
                                queue);
  /* check if valdiations are waiting for the queue */
  (void)
    GNUNET_CONTAINER_multipeermap_get_multiple (validation_map,
                                                &aqm->receiver,
                                                &check_validation_request_pending,
                                                queue);
  /* might be our first queue, try launching DV learning */
  if (NULL == dvlearn_task)
    dvlearn_task = GNUNET_SCHEDULER_add_now (&start_dv_learn, NULL);
  GNUNET_SERVICE_client_continue (tc->client);
}


/**
 * Communicator tells us that our request to create a queue "worked", that
 * is setting up the queue is now in process.
 *
 * @param cls the `struct TransportClient`
 * @param cqr confirmation message
 */
static void
handle_queue_create_ok (void *cls,
                        const struct GNUNET_TRANSPORT_CreateQueueResponse *cqr)
{
  struct TransportClient *tc = cls;

  if (CT_COMMUNICATOR != tc->type)
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (tc->client);
    return;
  }
  GNUNET_STATISTICS_update (GST_stats,
                            "# Suggestions succeeded at communicator",
                            1,
                            GNUNET_NO);
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "Request #%u for communicator to create queue succeeded\n",
              (unsigned int) ntohs (cqr->request_id));
  GNUNET_SERVICE_client_continue (tc->client);
}


/**
 * Communicator tells us that our request to create a queue failed. This usually
 * indicates that the provided address is simply invalid or that the
 * communicator's resources are exhausted.
 *
 * @param cls the `struct TransportClient`
 * @param cqr failure message
 */
static void
handle_queue_create_fail (
  void *cls,
  const struct GNUNET_TRANSPORT_CreateQueueResponse *cqr)
{
  struct TransportClient *tc = cls;

  if (CT_COMMUNICATOR != tc->type)
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (tc->client);
    return;
  }
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "Request #%u for communicator to create queue failed\n",
              (unsigned int) ntohs (cqr->request_id));
  GNUNET_STATISTICS_update (GST_stats,
                            "# Suggestions failed in queue creation at communicator",
                            1,
                            GNUNET_NO);
  GNUNET_SERVICE_client_continue (tc->client);
}


/**
 * We have received a `struct ExpressPreferenceMessage` from an application
 * client.
 *
 * @param cls handle to the client
 * @param msg the start message
 */
static void
handle_suggest_cancel (void *cls, const struct ExpressPreferenceMessage *msg)
{
  struct TransportClient *tc = cls;
  struct PeerRequest *pr;

  if (CT_APPLICATION != tc->type)
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (tc->client);
    return;
  }
  pr = GNUNET_CONTAINER_multipeermap_get (tc->details.application.requests,
                                          &msg->peer);
  if (NULL == pr)
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (tc->client);
    return;
  }
  (void) stop_peer_request (tc, &pr->pid, pr);
  GNUNET_SERVICE_client_continue (tc->client);
}


/**
 * Check #GNUNET_MESSAGE_TYPE_TRANSPORT_ADDRESS_CONSIDER_VERIFY
 * messages. We do nothing here, real verification is done later.
 *
 * @param cls a `struct TransportClient *`
 * @param msg message to verify
 * @return #GNUNET_OK
 */
static int
check_address_consider_verify (
  void *cls,
  const struct GNUNET_TRANSPORT_AddressToVerify *hdr)
{
  (void) cls;
  (void) hdr;
  return GNUNET_OK;
}


/**
 * Closure for #check_known_address.
 */
struct CheckKnownAddressContext
{
  /**
   * Set to the address we are looking for.
   */
  const char *address;

  /**
   * Set to a matching validation state, if one was found.
   */
  struct ValidationState *vs;
};


/**
 * Test if the validation state in @a value matches the
 * address from @a cls.
 *
 * @param cls a `struct CheckKnownAddressContext`
 * @param pid unused (must match though)
 * @param value a `struct ValidationState`
 * @return #GNUNET_OK if not matching, #GNUNET_NO if match found
 */
static int
check_known_address (void *cls,
                     const struct GNUNET_PeerIdentity *pid,
                     void *value)
{
  struct CheckKnownAddressContext *ckac = cls;
  struct ValidationState *vs = value;

  (void) pid;
  if (0 != strcmp (vs->address, ckac->address))
    return GNUNET_OK;
  ckac->vs = vs;
  return GNUNET_NO;
}


/**
 * Start address validation.
 *
 * @param pid peer the @a address is for
 * @param address an address to reach @a pid (presumably)
 * @param expiration when did @a pid claim @a address will become invalid
 */
static void
start_address_validation (const struct GNUNET_PeerIdentity *pid,
                          const char *address,
                          struct GNUNET_TIME_Absolute expiration)
{
  struct GNUNET_TIME_Absolute now;
  struct ValidationState *vs;
  struct CheckKnownAddressContext ckac = {.address = address, .vs = NULL};

  if (0 == GNUNET_TIME_absolute_get_remaining (expiration).rel_value_us)
    return; /* expired */
  (void) GNUNET_CONTAINER_multipeermap_get_multiple (validation_map,
                                                     pid,
                                                     &check_known_address,
                                                     &ckac);
  if (NULL != (vs = ckac.vs))
  {
    /* if 'vs' is not currently valid, we need to speed up retrying the
     * validation */
    if (vs->validated_until.abs_value_us < vs->next_challenge.abs_value_us)
    {
      /* reduce backoff as we got a fresh advertisement */
      vs->challenge_backoff =
        GNUNET_TIME_relative_min (FAST_VALIDATION_CHALLENGE_FREQ,
                                  GNUNET_TIME_relative_divide (vs->challenge_backoff,
                                                               2));
      update_next_challenge_time (vs,
                                  GNUNET_TIME_relative_to_absolute (
                                    vs->challenge_backoff));
    }
    return;
  }
  now = GNUNET_TIME_absolute_get ();
  vs = GNUNET_new (struct ValidationState);
  vs->pid = *pid;
  vs->valid_until = expiration;
  vs->first_challenge_use = now;
  vs->validation_rtt = GNUNET_TIME_UNIT_FOREVER_REL;
  GNUNET_CRYPTO_random_block (GNUNET_CRYPTO_QUALITY_NONCE,
                              &vs->challenge,
                              sizeof (vs->challenge));
  vs->address = GNUNET_strdup (address);
  GNUNET_assert (GNUNET_YES ==
                 GNUNET_CONTAINER_multipeermap_put (
                   validation_map,
                   &vs->pid,
                   vs,
                   GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
  update_next_challenge_time (vs, now);
}


/**
 * Function called by PEERSTORE for each matching record.
 *
 * @param cls closure
 * @param record peerstore record information
 * @param emsg error message, or NULL if no errors
 */
static void
handle_hello (void *cls,
              const struct GNUNET_PEERSTORE_Record *record,
              const char *emsg)
{
  struct PeerRequest *pr = cls;
  const char *val;

  if (NULL != emsg)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "Got failure from PEERSTORE: %s\n",
                emsg);
    return;
  }
  val = record->value;
  if ((0 == record->value_size) || ('\0' != val[record->value_size - 1]))
  {
    GNUNET_break (0);
    return;
  }
  start_address_validation (&pr->pid,
                            (const char *) record->value,
                            record->expiry);
}


/**
 * We have received a `struct ExpressPreferenceMessage` from an application
 * client.
 *
 * @param cls handle to the client
 * @param msg the start message
 */
static void
handle_suggest (void *cls, const struct ExpressPreferenceMessage *msg)
{
  struct TransportClient *tc = cls;
  struct PeerRequest *pr;

  if (CT_NONE == tc->type)
  {
    tc->type = CT_APPLICATION;
    tc->details.application.requests =
      GNUNET_CONTAINER_multipeermap_create (16, GNUNET_YES);
  }
  if (CT_APPLICATION != tc->type)
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (tc->client);
    return;
  }
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "Client suggested we talk to %s with preference %d at rate %u\n",
              GNUNET_i2s (&msg->peer),
              (int) ntohl (msg->pk),
              (int) ntohl (msg->bw.value__));
  pr = GNUNET_new (struct PeerRequest);
  pr->tc = tc;
  pr->pid = msg->peer;
  pr->bw = msg->bw;
  pr->pk = (enum GNUNET_MQ_PreferenceKind) ntohl (msg->pk);
  if (GNUNET_YES != GNUNET_CONTAINER_multipeermap_put (
                      tc->details.application.requests,
                      &pr->pid,
                      pr,
                      GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY))
  {
    GNUNET_break (0);
    GNUNET_free (pr);
    GNUNET_SERVICE_client_drop (tc->client);
    return;
  }
  pr->wc = GNUNET_PEERSTORE_watch (peerstore,
                                   "transport",
                                   &pr->pid,
                                   GNUNET_PEERSTORE_TRANSPORT_URLADDRESS_KEY,
                                   &handle_hello,
                                   pr);
  GNUNET_SERVICE_client_continue (tc->client);
}


/**
 * Given another peers address, consider checking it for validity
 * and then adding it to the Peerstore.
 *
 * @param cls a `struct TransportClient`
 * @param hdr message containing the raw address data and
 *        signature in the body, see #GNUNET_HELLO_extract_address()
 */
static void
handle_address_consider_verify (
  void *cls,
  const struct GNUNET_TRANSPORT_AddressToVerify *hdr)
{
  struct TransportClient *tc = cls;
  char *address;
  enum GNUNET_NetworkType nt;
  struct GNUNET_TIME_Absolute expiration;

  (void) cls;
  // OPTIMIZE-FIXME: checking that we know this address already should
  //        be done BEFORE checking the signature => HELLO API change!
  // OPTIMIZE-FIXME: pre-check: rate-limit signature verification / validation?!
  address =
    GNUNET_HELLO_extract_address (&hdr[1],
                                  ntohs (hdr->header.size) - sizeof (*hdr),
                                  &hdr->peer,
                                  &nt,
                                  &expiration);
  if (NULL == address)
  {
    GNUNET_break_op (0);
    return;
  }
  start_address_validation (&hdr->peer, address, expiration);
  GNUNET_free (address);
  GNUNET_SERVICE_client_continue (tc->client);
}


/**
 * Check #GNUNET_MESSAGE_TYPE_TRANSPORT_REQUEST_HELLO_VALIDATION
 * messages.
 *
 * @param cls a `struct TransportClient *`
 * @param m message to verify
 * @return #GNUNET_OK on success
 */
static int
check_request_hello_validation (void *cls,
                                const struct RequestHelloValidationMessage *m)
{
  (void) cls;
  GNUNET_MQ_check_zero_termination (m);
  return GNUNET_OK;
}


/**
 * A client encountered an address of another peer. Consider validating it,
 * and if validation succeeds, persist it to PEERSTORE.
 *
 * @param cls a `struct TransportClient *`
 * @param m message to verify
 */
static void
handle_request_hello_validation (void *cls,
                                 const struct RequestHelloValidationMessage *m)
{
  struct TransportClient *tc = cls;

  start_address_validation (&m->peer,
                            (const char *) &m[1],
                            GNUNET_TIME_absolute_ntoh (m->expiration));
  GNUNET_SERVICE_client_continue (tc->client);
}


/**
 * Free neighbour entry.
 *
 * @param cls NULL
 * @param pid unused
 * @param value a `struct Neighbour`
 * @return #GNUNET_OK (always)
 */
static int
free_neighbour_cb (void *cls,
                   const struct GNUNET_PeerIdentity *pid,
                   void *value)
{
  struct Neighbour *neighbour = value;

  (void) cls;
  (void) pid;
  GNUNET_break (0); // should this ever happen?
  free_neighbour (neighbour);

  return GNUNET_OK;
}


/**
 * Free DV route entry.
 *
 * @param cls NULL
 * @param pid unused
 * @param value a `struct DistanceVector`
 * @return #GNUNET_OK (always)
 */
static int
free_dv_routes_cb (void *cls,
                   const struct GNUNET_PeerIdentity *pid,
                   void *value)
{
  struct DistanceVector *dv = value;

  (void) cls;
  (void) pid;
  free_dv_route (dv);

  return GNUNET_OK;
}


/**
 * Free ephemeral entry.
 *
 * @param cls NULL
 * @param pid unused
 * @param value a `struct EphemeralCacheEntry`
 * @return #GNUNET_OK (always)
 */
static int
free_ephemeral_cb (void *cls,
                   const struct GNUNET_PeerIdentity *pid,
                   void *value)
{
  struct EphemeralCacheEntry *ece = value;

  (void) cls;
  (void) pid;
  free_ephemeral (ece);
  return GNUNET_OK;
}


/**
 * Free validation state.
 *
 * @param cls NULL
 * @param pid unused
 * @param value a `struct ValidationState`
 * @return #GNUNET_OK (always)
 */
static int
free_validation_state_cb (void *cls,
                          const struct GNUNET_PeerIdentity *pid,
                          void *value)
{
  struct ValidationState *vs = value;

  (void) cls;
  (void) pid;
  free_validation_state (vs);
  return GNUNET_OK;
}


/**
 * Function called when the service shuts down.  Unloads our plugins
 * and cancels pending validations.
 *
 * @param cls closure, unused
 */
static void
do_shutdown (void *cls)
{
  struct LearnLaunchEntry *lle;
  (void) cls;

  if (NULL != ephemeral_task)
  {
    GNUNET_SCHEDULER_cancel (ephemeral_task);
    ephemeral_task = NULL;
  }
  GNUNET_CONTAINER_multipeermap_iterate (neighbours, &free_neighbour_cb, NULL);
  if (NULL != peerstore)
  {
    GNUNET_PEERSTORE_disconnect (peerstore, GNUNET_NO);
    peerstore = NULL;
  }
  if (NULL != GST_stats)
  {
    GNUNET_STATISTICS_destroy (GST_stats, GNUNET_NO);
    GST_stats = NULL;
  }
  if (NULL != GST_my_private_key)
  {
    GNUNET_free (GST_my_private_key);
    GST_my_private_key = NULL;
  }
  GNUNET_CONTAINER_multipeermap_destroy (neighbours);
  neighbours = NULL;
  GNUNET_CONTAINER_multipeermap_iterate (validation_map,
                                         &free_validation_state_cb,
                                         NULL);
  GNUNET_CONTAINER_multipeermap_destroy (validation_map);
  validation_map = NULL;
  while (NULL != (lle = lle_head))
  {
    GNUNET_CONTAINER_DLL_remove (lle_head, lle_tail, lle);
    GNUNET_free (lle);
  }
  GNUNET_CONTAINER_multishortmap_destroy (dvlearn_map);
  dvlearn_map = NULL;
  GNUNET_CONTAINER_heap_destroy (validation_heap);
  validation_heap = NULL;
  GNUNET_CONTAINER_multipeermap_iterate (dv_routes, &free_dv_routes_cb, NULL);
  GNUNET_CONTAINER_multipeermap_destroy (dv_routes);
  dv_routes = NULL;
  GNUNET_CONTAINER_multipeermap_iterate (ephemeral_map,
                                         &free_ephemeral_cb,
                                         NULL);
  GNUNET_CONTAINER_multipeermap_destroy (ephemeral_map);
  ephemeral_map = NULL;
  GNUNET_CONTAINER_heap_destroy (ephemeral_heap);
  ephemeral_heap = NULL;
}


/**
 * Initiate transport service.
 *
 * @param cls closure
 * @param c configuration to use
 * @param service the initialized service
 */
static void
run (void *cls,
     const struct GNUNET_CONFIGURATION_Handle *c,
     struct GNUNET_SERVICE_Handle *service)
{
  (void) cls;
  (void) service;
  /* setup globals */
  GST_cfg = c;
  neighbours = GNUNET_CONTAINER_multipeermap_create (1024, GNUNET_YES);
  dv_routes = GNUNET_CONTAINER_multipeermap_create (1024, GNUNET_YES);
  ephemeral_map = GNUNET_CONTAINER_multipeermap_create (32, GNUNET_YES);
  ephemeral_heap =
    GNUNET_CONTAINER_heap_create (GNUNET_CONTAINER_HEAP_ORDER_MIN);
  dvlearn_map = GNUNET_CONTAINER_multishortmap_create (2 * MAX_DV_LEARN_PENDING,
                                                       GNUNET_YES);
  validation_map = GNUNET_CONTAINER_multipeermap_create (1024, GNUNET_YES);
  validation_heap =
    GNUNET_CONTAINER_heap_create (GNUNET_CONTAINER_HEAP_ORDER_MIN);
  GST_my_private_key =
    GNUNET_CRYPTO_eddsa_key_create_from_configuration (GST_cfg);
  if (NULL == GST_my_private_key)
  {
    GNUNET_log (
      GNUNET_ERROR_TYPE_ERROR,
      _ (
        "Transport service is lacking key configuration settings. Exiting.\n"));
    GNUNET_SCHEDULER_shutdown ();
    return;
  }
  GNUNET_CRYPTO_eddsa_key_get_public (GST_my_private_key,
                                      &GST_my_identity.public_key);
  GNUNET_log (GNUNET_ERROR_TYPE_INFO,
              "My identity is `%s'\n",
              GNUNET_i2s_full (&GST_my_identity));
  GST_stats = GNUNET_STATISTICS_create ("transport", GST_cfg);
  GNUNET_SCHEDULER_add_shutdown (&do_shutdown, NULL);
  peerstore = GNUNET_PEERSTORE_connect (GST_cfg);
  if (NULL == peerstore)
  {
    GNUNET_break (0);
    GNUNET_SCHEDULER_shutdown ();
    return;
  }
}


/**
 * Define "main" method using service macro.
 */
GNUNET_SERVICE_MAIN (
  "transport",
  GNUNET_SERVICE_OPTION_SOFT_SHUTDOWN,
  &run,
  &client_connect_cb,
  &client_disconnect_cb,
  NULL,
  /* communication with applications */
  GNUNET_MQ_hd_fixed_size (suggest,
                           GNUNET_MESSAGE_TYPE_TRANSPORT_SUGGEST,
                           struct ExpressPreferenceMessage,
                           NULL),
  GNUNET_MQ_hd_fixed_size (suggest_cancel,
                           GNUNET_MESSAGE_TYPE_TRANSPORT_SUGGEST_CANCEL,
                           struct ExpressPreferenceMessage,
                           NULL),
  GNUNET_MQ_hd_var_size (request_hello_validation,
                         GNUNET_MESSAGE_TYPE_TRANSPORT_REQUEST_HELLO_VALIDATION,
                         struct RequestHelloValidationMessage,
                         NULL),
  /* communication with core */
  GNUNET_MQ_hd_fixed_size (client_start,
                           GNUNET_MESSAGE_TYPE_TRANSPORT_START,
                           struct StartMessage,
                           NULL),
  GNUNET_MQ_hd_var_size (client_send,
                         GNUNET_MESSAGE_TYPE_TRANSPORT_SEND,
                         struct OutboundMessage,
                         NULL),
  /* communication with communicators */
  GNUNET_MQ_hd_var_size (communicator_available,
                         GNUNET_MESSAGE_TYPE_TRANSPORT_NEW_COMMUNICATOR,
                         struct GNUNET_TRANSPORT_CommunicatorAvailableMessage,
                         NULL),
  GNUNET_MQ_hd_var_size (communicator_backchannel,
                         GNUNET_MESSAGE_TYPE_TRANSPORT_COMMUNICATOR_BACKCHANNEL,
                         struct GNUNET_TRANSPORT_CommunicatorBackchannel,
                         NULL),
  GNUNET_MQ_hd_var_size (add_address,
                         GNUNET_MESSAGE_TYPE_TRANSPORT_ADD_ADDRESS,
                         struct GNUNET_TRANSPORT_AddAddressMessage,
                         NULL),
  GNUNET_MQ_hd_fixed_size (del_address,
                           GNUNET_MESSAGE_TYPE_TRANSPORT_DEL_ADDRESS,
                           struct GNUNET_TRANSPORT_DelAddressMessage,
                           NULL),
  GNUNET_MQ_hd_var_size (incoming_msg,
                         GNUNET_MESSAGE_TYPE_TRANSPORT_INCOMING_MSG,
                         struct GNUNET_TRANSPORT_IncomingMessage,
                         NULL),
  GNUNET_MQ_hd_fixed_size (queue_create_ok,
                           GNUNET_MESSAGE_TYPE_TRANSPORT_QUEUE_CREATE_OK,
                           struct GNUNET_TRANSPORT_CreateQueueResponse,
                           NULL),
  GNUNET_MQ_hd_fixed_size (queue_create_fail,
                           GNUNET_MESSAGE_TYPE_TRANSPORT_QUEUE_CREATE_FAIL,
                           struct GNUNET_TRANSPORT_CreateQueueResponse,
                           NULL),
  GNUNET_MQ_hd_var_size (add_queue_message,
                         GNUNET_MESSAGE_TYPE_TRANSPORT_QUEUE_SETUP,
                         struct GNUNET_TRANSPORT_AddQueueMessage,
                         NULL),
  GNUNET_MQ_hd_var_size (address_consider_verify,
                         GNUNET_MESSAGE_TYPE_TRANSPORT_ADDRESS_CONSIDER_VERIFY,
                         struct GNUNET_TRANSPORT_AddressToVerify,
                         NULL),
  GNUNET_MQ_hd_fixed_size (del_queue_message,
                           GNUNET_MESSAGE_TYPE_TRANSPORT_QUEUE_TEARDOWN,
                           struct GNUNET_TRANSPORT_DelQueueMessage,
                           NULL),
  GNUNET_MQ_hd_fixed_size (send_message_ack,
                           GNUNET_MESSAGE_TYPE_TRANSPORT_SEND_MSG_ACK,
                           struct GNUNET_TRANSPORT_SendMessageToAck,
                           NULL),
  /* communication with monitors */
  GNUNET_MQ_hd_fixed_size (monitor_start,
                           GNUNET_MESSAGE_TYPE_TRANSPORT_MONITOR_START,
                           struct GNUNET_TRANSPORT_MonitorStart,
                           NULL),
  GNUNET_MQ_handler_end ());


/* end of file gnunet-service-transport.c */