wip
[oweals/gnunet.git] / src / transport / plugin_transport_tcp.c
index 05a7c2eb06e200009c8863a811245c626dec398b..9dc67dcdf4eb098305c64658889e0fa9a2493da6 100644 (file)
@@ -4,7 +4,7 @@
 
      GNUnet is free software; you can redistribute it and/or modify
      it under the terms of the GNU General Public License as published
-     by the Free Software Foundation; either version 2, or (at your
+     by the Free Software Foundation; either version 3, or (at your
      option) any later version.
 
      GNUnet is distributed in the hope that it will be useful, but
      Free Software Foundation, Inc., 59 Temple Place - Suite 330,
      Boston, MA 02111-1307, USA.
 */
-
 /**
  * @file transport/plugin_transport_tcp.c
  * @brief Implementation of the TCP transport service
  * @author Christian Grothoff
  */
-
 #include "platform.h"
 #include "gnunet_hello_lib.h"
 #include "gnunet_connection_lib.h"
 #include "gnunet_container_lib.h"
+#include "gnunet_nat_lib.h"
 #include "gnunet_os_lib.h"
 #include "gnunet_protocols.h"
 #include "gnunet_resolver_service.h"
 #include "gnunet_signatures.h"
 #include "gnunet_statistics_service.h"
 #include "gnunet_transport_service.h"
-#include "plugin_transport.h"
+#include "gnunet_transport_plugin.h"
 #include "transport.h"
 
 #define DEBUG_TCP GNUNET_NO
 
+#define DEBUG_TCP_NAT GNUNET_NO
+
 /**
  * How long until we give up on transmitting the welcome message?
  */
@@ -65,12 +66,151 @@ struct WelcomeMessage
 };
 
 
+/**
+ * Basically a WELCOME message, but with the purpose
+ * of giving the waiting peer a client handle to use
+ */
+struct TCP_NAT_ProbeMessage
+{
+  /**
+   * Type is GNUNET_MESSAGE_TYPE_TRANSPORT_TCP_NAT_PROBE.
+   */
+  struct GNUNET_MessageHeader header;
+
+  /**
+   * Identity of the sender of the message.
+   */
+  struct GNUNET_PeerIdentity clientIdentity;
+
+};
+
+
+/**
+ * Context for sending a NAT probe via TCP.
+ */
+struct TCPProbeContext
+{
+
+  /**
+   * Active probes are kept in a DLL.
+   */
+  struct TCPProbeContext *next;
+
+  /**
+   * Active probes are kept in a DLL.
+   */
+  struct TCPProbeContext *prev;
+
+  /**
+   * Probe connection.
+   */
+  struct GNUNET_CONNECTION_Handle *sock;
+
+  /**
+   * Message to be sent.
+   */
+  struct TCP_NAT_ProbeMessage message;
+
+  /**
+   * Handle to the transmission.
+   */
+  struct GNUNET_CONNECTION_TransmitHandle *transmit_handle;
+
+  /**
+   * Transport plugin handle.
+   */
+  struct Plugin *plugin;
+};
+
+
+/**
+ * Network format for IPv4 addresses.
+ */
+struct IPv4TcpAddress
+{
+  /**
+   * IPv4 address, in network byte order.
+   */
+  uint32_t ipv4_addr GNUNET_PACKED;
+
+  /**
+   * Port number, in network byte order.
+   */
+  uint16_t t_port GNUNET_PACKED;
+
+};
+
+
+/**
+ * Network format for IPv6 addresses.
+ */
+struct IPv6TcpAddress
+{
+  /**
+   * IPv6 address.
+   */
+  struct in6_addr ipv6_addr GNUNET_PACKED;
+
+  /**
+   * Port number, in network byte order.
+   */
+  uint16_t t6_port GNUNET_PACKED;
+
+};
+
+
 /**
  * Encapsulation of all of the state of the plugin.
  */
 struct Plugin;
 
 
+/**
+ * Local network addresses (actual IP address follows this struct).
+ * PORT is NOT included!
+ */
+struct LocalAddrList
+{
+
+  /**
+   * This is a doubly linked list.
+   */
+  struct LocalAddrList *next;
+
+  /**
+   * This is a doubly linked list.
+   */
+  struct LocalAddrList *prev;
+
+  /**
+   * Link to plugin.
+   */
+  struct Plugin *plugin;
+
+  /**
+   * Handle to NAT holes we've tried to punch for this address.
+   */
+  struct GNUNET_NAT_Handle *nat;
+
+  /**
+   * Pointer to a 'struct IPv4/V6TcpAddress' describing our external IP and port
+   * as obtained from the NAT by automatic port mapping.
+   */
+  void *external_nat_address;
+
+  /**
+   * Number of bytes in 'external_nat_address'
+   */
+  size_t ena_size;
+
+  /**
+   * Number of bytes of the address that follow
+   */
+  size_t size;
+
+};
+
+
 /**
  * Information kept for each message that is yet to
  * be transmitted.
@@ -126,6 +266,11 @@ struct PendingMessage
 struct Session
 {
 
+  /**
+   * API requirement.
+   */
+  struct SessionHeader header;
+
   /**
    * Stored in a linked list.
    */
@@ -165,9 +310,9 @@ struct Session
   struct GNUNET_PeerIdentity target;
 
   /**
-   * At what time did we reset last_received last?
+   * ID of task used to delay receiving more to throttle sender.
    */
-  struct GNUNET_TIME_Absolute last_quota_update;
+  GNUNET_SCHEDULER_TaskIdentifier receive_delay_task;
 
   /**
    * Address of the other peer (either based on our 'connect'
@@ -176,16 +321,10 @@ struct Session
   void *connect_addr;
 
   /**
-   * How many bytes have we received since the "last_quota_update"
-   * timestamp?
-   */
-  uint64_t last_received;
-
-  /**
-   * Number of bytes per ms that this peer is allowed
-   * to send to us.
+   * Last activity on this connection.  Used to select preferred
+   * connection.
    */
-  uint32_t quota_in;
+  struct GNUNET_TIME_Absolute last_activity;
 
   /**
    * Length of connect_addr.
@@ -197,6 +336,16 @@ struct Session
    */
   int expecting_welcome;
 
+  /**
+   * Was this a connection that was inbound (we accepted)? (GNUNET_YES/GNUNET_NO)
+   */
+  int inbound;
+
+  /**
+   * Was this session created using NAT traversal?
+   */
+  int is_nat;
+
 };
 
 
@@ -216,14 +365,29 @@ struct Plugin
   struct GNUNET_CONNECTION_Handle *lsock;
 
   /**
-   * List of open TCP sessions.
+   * stdout pipe handle for the gnunet-nat-server process
    */
-  struct Session *sessions;
+  struct GNUNET_DISK_PipeHandle *server_stdout;
+
+  /**
+   * stdout file handle (for reading) for the gnunet-nat-server process
+   */
+  const struct GNUNET_DISK_FileHandle *server_stdout_handle;
+
+  /**
+   * ID of select gnunet-nat-server stdout read task
+   */
+  GNUNET_SCHEDULER_TaskIdentifier server_read_task;
 
   /**
-   * Handle for the statistics service.
+   * The process id of the server process (if behind NAT)
+   */
+  struct GNUNET_OS_Process *server_proc;
+
+  /**
+   * List of open TCP sessions.
    */
-  struct GNUNET_STATISTICS_Handle *statistics;
+  struct Session *sessions;
 
   /**
    * Handle to the network service.
@@ -246,6 +410,54 @@ struct Plugin
    */
   struct GNUNET_RESOLVER_RequestHandle *hostname_dns;
 
+  /**
+   * Map of peers we have tried to contact behind a NAT
+   */
+  struct GNUNET_CONTAINER_MultiHashMap *nat_wait_conns;
+
+  /**
+   * The external address given to us by the user.  Used for HELLOs
+   * and address validation.
+   */
+  char *external_address;
+
+  /**
+   * The internal address given to us by the user (or discovered).
+   * Used for NAT traversal (ICMP method), but not as a 'validateable'
+   * address in HELLOs.
+   */
+  char *internal_address;
+
+  /**
+   * Address given for us to bind to (ONLY).
+   */
+  char *bind_address;
+
+  /**
+   * List of our IP addresses.
+   */
+  struct LocalAddrList *lal_head;
+
+  /**
+   * Tail of our IP address list.
+   */
+  struct LocalAddrList *lal_tail;
+
+  /**
+   * List of active TCP probes.
+   */
+  struct TCPProbeContext *probe_head;
+
+  /**
+   * List of active TCP probes.
+   */
+  struct TCPProbeContext *probe_tail;
+
+  /**
+   * Handle for (DYN)DNS lookup of our external IP.
+   */
+  struct GNUNET_RESOLVER_RequestHandle *ext_dns;
+
   /**
    * ID of task used to update our addresses when one expires.
    */
@@ -262,34 +474,298 @@ struct Plugin
    */
   uint16_t adv_port;
 
+  /**
+   * Is this transport configured to be behind a NAT?
+   */
+  int behind_nat;
+
+  /**
+   * Has the NAT been punched?
+   */
+  int nat_punched;
+
+  /**
+   * Is this transport configured to allow connections to NAT'd peers?
+   */
+  int enable_nat_client;
+
+  /**
+   * Should we run the gnunet-nat-server?
+   */
+  int enable_nat_server;
+
+  /**
+   * Are we allowed to try UPnP/PMP for NAT traversal?
+   */
+  int enable_upnp;
+
 };
 
 
 /**
- * Find a session handle for the given peer. 
- * FIXME: using a hash map we could do this in O(1).
+ * Our external IP address/port mapping has changed.
  *
- * @return NULL if no matching session exists
+ * @param cls closure, the 'struct LocalAddrList'
+ * @param add_remove GNUNET_YES to mean the new public IP address, GNUNET_NO to mean
+ *     the previous (now invalid) one
+ * @param addr either the previous or the new public IP address
+ * @param addrlen actual lenght of the address
  */
-static struct Session *
-find_session_by_target (struct Plugin *plugin,
-                        const struct GNUNET_PeerIdentity *target)
+static void
+nat_port_map_callback (void *cls,
+                      int add_remove,
+                      const struct sockaddr *addr,
+                      socklen_t addrlen)
 {
-  struct Session *ret;
+  struct LocalAddrList *lal = cls;
+  struct Plugin *plugin = lal->plugin;
+  int af;
+  struct IPv4TcpAddress t4;
+  struct IPv6TcpAddress t6;
+  void *arg;
+  uint16_t args;
 
-  ret = plugin->sessions;
-  while ( (ret != NULL) &&
-         ((GNUNET_SYSERR == ret->expecting_welcome) ||
-          (0 != memcmp (target,
-                        &ret->target, sizeof (struct GNUNET_PeerIdentity)))))
-    ret = ret->next;
-  return ret;
+  /* convert 'addr' to our internal format */
+  af = addr->sa_family;
+  switch (af)
+    {
+    case AF_INET:
+      t4.ipv4_addr = ((struct sockaddr_in *) addr)->sin_addr.s_addr;
+      t6.t6_port = ((struct sockaddr_in *) addr)->sin_port;
+      arg = &t4;
+      args = sizeof (t4);
+      break;
+    case AF_INET6:
+      memcpy (&t6.ipv6_addr,
+             &((struct sockaddr_in6 *) addr)->sin6_addr,
+             sizeof (struct in6_addr));
+      t6.t6_port = ((struct sockaddr_in6 *) addr)->sin6_port;
+      arg = &t6;
+      args = sizeof (t6);    
+      break;
+    default:
+      GNUNET_break (0);
+      return;
+    } 
+
+  /* modify our published address list */
+  if (GNUNET_YES == add_remove)
+    {
+      plugin->env->notify_address (plugin->env->cls,
+                                   "tcp",
+                                   arg, args, GNUNET_TIME_UNIT_FOREVER_REL);
+      GNUNET_free_non_null (lal->external_nat_address);
+      lal->external_nat_address = GNUNET_memdup (arg, args);
+      lal->ena_size = args;
+    }
+  else
+    {
+      plugin->env->notify_address (plugin->env->cls,
+                                   "tcp",
+                                   arg, args, GNUNET_TIME_UNIT_ZERO);
+      GNUNET_free_non_null (lal->external_nat_address);
+      lal->ena_size = 0;
+    }
+}
+
+
+/**
+ * Add the given address to the list of 'local' addresses, thereby
+ * making it a 'legal' address for this peer to have.  
+ * 
+ * @param plugin the plugin
+ * @param arg the address, either an IPv4 or an IPv6 IP address
+ * @param arg_size number of bytes in arg
+ */
+static void
+add_to_address_list (struct Plugin *plugin,
+                    const void *arg,
+                    size_t arg_size)
+{
+  struct LocalAddrList *lal;
+  struct sockaddr_in v4;
+  struct sockaddr_in6 v6;
+  const struct sockaddr *sa;
+  socklen_t salen;
+
+  lal = plugin->lal_head;
+  while (NULL != lal)
+    {
+      if ( (lal->size == arg_size) &&
+          (0 == memcmp (&lal[1], arg, arg_size)) )
+       return;
+      lal = lal->next;
+    }
+  lal = GNUNET_malloc (sizeof (struct LocalAddrList) + arg_size);
+  lal->plugin = plugin;
+  lal->size = arg_size;
+  memcpy (&lal[1], arg, arg_size);
+  GNUNET_CONTAINER_DLL_insert (plugin->lal_head,
+                              plugin->lal_tail,
+                              lal);
+  if (plugin->open_port == 0)
+    return; /* we're not listening at all... */
+  if (arg_size == sizeof (struct in_addr))
+    {
+      memset (&v4, 0, sizeof (v4));
+      v4.sin_family = AF_INET;
+      v4.sin_port = htons (plugin->open_port);
+      memcpy (&v4.sin_addr, arg, arg_size);
+#if HAVE_SOCKADDR_IN_SIN_LEN
+      v4.sin_len = sizeof (struct sockaddr_in);
+#endif
+      sa = (const struct sockaddr*) &v4;
+      salen = sizeof (v4);
+    }
+  else if (arg_size == sizeof (struct in6_addr))
+    {     
+      memset (&v6, 0, sizeof (v6));
+      v6.sin6_family = AF_INET6;
+      v6.sin6_port = htons (plugin->open_port);
+      memcpy (&v6.sin6_addr, arg, arg_size);
+#if HAVE_SOCKADDR_IN_SIN_LEN
+      v6.sin6_len = sizeof (struct sockaddr_in6);
+#endif
+      sa = (const struct sockaddr*) &v6;
+      salen = sizeof (v6);
+    }
+  else
+    {
+      GNUNET_break (0);
+      return;
+    }
+  if ( (plugin->behind_nat == GNUNET_YES) &&
+       (plugin->enable_upnp == GNUNET_YES) )
+    lal->nat = GNUNET_NAT_register (sa, salen,
+                                   &nat_port_map_callback,
+                                   lal);
+}
+
+
+/**
+ * Check if the given address is in the list of 'local' addresses.
+ * 
+ * @param plugin the plugin
+ * @param arg the address, either an IPv4 or an IPv6 IP address
+ * @param arg_size number of bytes in arg
+ * @return GNUNET_OK if this is one of our IPs, GNUNET_SYSERR if not
+ */
+static int
+check_local_addr (struct Plugin *plugin,
+                 const void *arg,
+                 size_t arg_size)
+{
+  struct LocalAddrList *lal;
+
+  lal = plugin->lal_head;
+  while (NULL != lal)
+    {
+      if ( (lal->size == arg_size) &&
+          (0 == memcmp (&lal[1], arg, arg_size)) )
+       return GNUNET_OK;
+      lal = lal->next;
+    }
+  return GNUNET_SYSERR;
+}
+
+
+/**
+ * Check if the given address is in the list of 'mapped' addresses.
+ * 
+ * @param plugin the plugin
+ * @param arg the address, either a 'struct IPv4TcpAddress' or a 'struct IPv6TcpAddress'
+ * @param arg_size number of bytes in arg
+ * @return GNUNET_OK if this is one of our IPs, GNUNET_SYSERR if not
+ */
+static int
+check_mapped_addr (struct Plugin *plugin,
+                  const void *arg,
+                  size_t arg_size)
+{
+  struct LocalAddrList *lal;
+
+  lal = plugin->lal_head;
+  while (NULL != lal)
+    {
+      if ( (lal->ena_size == arg_size) &&
+          (0 == memcmp (lal->external_nat_address, arg, arg_size)) )
+       return GNUNET_OK;
+      lal = lal->next;
+    }
+  return GNUNET_SYSERR;
+}
+
+
+/**
+ * Function called for a quick conversion of the binary address to
+ * a numeric address.  Note that the caller must not free the
+ * address and that the next call to this function is allowed
+ * to override the address again.
+ *
+ * @param cls closure ('struct Plugin*')
+ * @param addr binary address
+ * @param addrlen length of the address
+ * @return string representing the same address
+ */
+static const char*
+tcp_address_to_string (void *cls,
+                      const void *addr,
+                      size_t addrlen)
+{
+  static char rbuf[INET6_ADDRSTRLEN + 12];
+  char buf[INET6_ADDRSTRLEN];
+  const void *sb;
+  struct in_addr a4;
+  struct in6_addr a6;
+  const struct IPv4TcpAddress *t4;
+  const struct IPv6TcpAddress *t6;
+  int af;
+  uint16_t port;
+
+  if (addrlen == sizeof (struct IPv6TcpAddress))
+    {
+      t6 = addr;
+      af = AF_INET6;
+      port = ntohs (t6->t6_port);
+      memcpy (&a6, &t6->ipv6_addr, sizeof (a6));
+      sb = &a6;
+    }
+  else if (addrlen == sizeof (struct IPv4TcpAddress))
+    {
+      t4 = addr;
+      af = AF_INET;
+      port = ntohs (t4->t_port);
+      memcpy (&a4, &t4->ipv4_addr, sizeof (a4));
+      sb = &a4;
+    }
+  else
+    {
+      GNUNET_log_from (GNUNET_ERROR_TYPE_ERROR,
+                      "tcp",
+                      _("Unexpected address length: %u\n"),
+                      addrlen);
+      GNUNET_break (0);
+      return NULL;
+    }
+  if (NULL == inet_ntop (af, sb, buf, INET6_ADDRSTRLEN))
+    {
+      GNUNET_log_strerror (GNUNET_ERROR_TYPE_WARNING, "inet_ntop");
+      return NULL;
+    }
+  GNUNET_snprintf (rbuf,
+                  sizeof (rbuf),
+                  (af == AF_INET6) ? "[%s]:%u" : "%s:%u",
+                  buf,
+                  port);
+  return rbuf;
 }
 
 
 /**
  * Find the session handle for the given client.
  *
+ * @param plugin the plugin
+ * @param client which client to find the session handle for
  * @return NULL if no matching session exists
  */
 static struct Session *
@@ -308,28 +784,45 @@ find_session_by_client (struct Plugin *plugin,
 /**
  * Create a new session.  Also queues a welcome message.
  *
- * @param plugin us
+ * @param plugin the plugin
  * @param target peer to connect to
  * @param client client to use
+ * @param is_nat this a NAT session, we should wait for a client to
+ *               connect to us from an address, then assign that to
+ *               the session
  * @return new session object
  */
 static struct Session *
 create_session (struct Plugin *plugin,
                 const struct GNUNET_PeerIdentity *target,
-                struct GNUNET_SERVER_Client *client)
+                struct GNUNET_SERVER_Client *client, 
+               int is_nat)
 {
   struct Session *ret;
   struct PendingMessage *pm;
   struct WelcomeMessage welcome;
 
+  if (is_nat != GNUNET_YES)
+    GNUNET_assert (client != NULL);
+  else
+    GNUNET_assert (client == NULL);
+#if DEBUG_TCP
+  GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
+                  "tcp",
+                  "Creating new session for peer `%4s'\n",
+                  GNUNET_i2s (target));
+#endif
   ret = GNUNET_malloc (sizeof (struct Session));
+  ret->last_activity = GNUNET_TIME_absolute_get ();
   ret->plugin = plugin;
-  ret->next = plugin->sessions;
-  plugin->sessions = ret;
+  ret->is_nat = is_nat;
+  if (is_nat != GNUNET_YES) /* If not a NAT WAIT conn, add it to global list */
+    {
+      ret->next = plugin->sessions;
+      plugin->sessions = ret;
+    }
   ret->client = client;
   ret->target = *target;
-  ret->last_quota_update = GNUNET_TIME_absolute_get ();
-  ret->quota_in = plugin->env->default_quota_in;
   ret->expecting_welcome = GNUNET_YES;
   pm = GNUNET_malloc (sizeof (struct PendingMessage) + sizeof (struct WelcomeMessage));
   pm->msg = (const char*) &pm[1];
@@ -339,9 +832,18 @@ create_session (struct Plugin *plugin,
   welcome.clientIdentity = *plugin->env->my_identity;
   memcpy (&pm[1], &welcome, sizeof (welcome));
   pm->timeout = GNUNET_TIME_UNIT_FOREVER_ABS;
+  GNUNET_STATISTICS_update (plugin->env->stats,
+                           gettext_noop ("# bytes currently in TCP buffers"),
+                           pm->message_size,
+                           GNUNET_NO);
   GNUNET_CONTAINER_DLL_insert (ret->pending_messages_head,
                               ret->pending_messages_tail,
                               pm);
+  if (is_nat != GNUNET_YES)
+    GNUNET_STATISTICS_update (plugin->env->stats,
+                              gettext_noop ("# TCP sessions active"),
+                              1,
+                              GNUNET_NO);
   return ret;
 }
 
@@ -370,64 +872,123 @@ static size_t
 do_transmit (void *cls, size_t size, void *buf)
 {
   struct Session *session = cls;
-  struct PendingMessage *pm;
+  struct GNUNET_PeerIdentity pid;
+  struct Plugin *plugin;
+  struct PendingMessage *pos;
+  struct PendingMessage *hd;
+  struct PendingMessage *tl;
+  struct GNUNET_TIME_Absolute now;
   char *cbuf;
-
   size_t ret;
 
   session->transmit_handle = NULL;
+  plugin = session->plugin;
   if (buf == NULL)
     {
 #if DEBUG_TCP
       GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
-                       "tcp",
+                      "tcp",
                        "Timeout trying to transmit to peer `%4s', discarding message queue.\n",
                        GNUNET_i2s (&session->target));
 #endif
-      /* timeout */
-      while (NULL != (pm = session->pending_messages_head))
-        {
+      /* timeout; cancel all messages that have already expired */
+      hd = NULL;
+      tl = NULL;
+      ret = 0;
+      now = GNUNET_TIME_absolute_get ();
+      while ( (NULL != (pos = session->pending_messages_head)) &&
+             (pos->timeout.abs_value <= now.abs_value) )
+       {
          GNUNET_CONTAINER_DLL_remove (session->pending_messages_head,
                                       session->pending_messages_tail,
-                                      pm);
+                                      pos);
 #if DEBUG_TCP
           GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
-                           "tcp",
+                          "tcp",
                            "Failed to transmit %u byte message to `%4s'.\n",
-                          pm->message_size,
+                          pos->message_size,
                            GNUNET_i2s (&session->target));
 #endif
-          if (pm->transmit_cont != NULL)
-            pm->transmit_cont (pm->transmit_cont_cls,
-                               &session->target, GNUNET_SYSERR);
-          GNUNET_free (pm);
+         ret += pos->message_size;
+         GNUNET_CONTAINER_DLL_insert_after (hd, tl, tl, pos);
         }
+      /* do this call before callbacks (so that if callbacks destroy
+        session, they have a chance to cancel actions done by this
+        call) */
+      process_pending_messages (session);
+      pid = session->target;
+      /* no do callbacks and do not use session again since
+        the callbacks may abort the session */
+      while (NULL != (pos = hd))
+       {
+         GNUNET_CONTAINER_DLL_remove (hd, tl, pos);
+         if (pos->transmit_cont != NULL)
+           pos->transmit_cont (pos->transmit_cont_cls,
+                               &pid, GNUNET_SYSERR);
+         GNUNET_free (pos);
+       }
+      GNUNET_STATISTICS_update (plugin->env->stats,
+                               gettext_noop ("# bytes currently in TCP buffers"),
+                               - (int64_t) ret,
+                               GNUNET_NO);
+      GNUNET_STATISTICS_update (plugin->env->stats,
+                               gettext_noop ("# bytes discarded by TCP (timeout)"),
+                               ret,
+                               GNUNET_NO);
       return 0;
     }
+  /* copy all pending messages that would fit */
   ret = 0;
   cbuf = buf;
-  while (NULL != (pm = session->pending_messages_head))
+  hd = NULL;
+  tl = NULL;
+  while (NULL != (pos = session->pending_messages_head))
     {
-      if (size < pm->message_size)
+      if (ret + pos->message_size > size)
        break;
-      memcpy (cbuf, pm->msg, pm->message_size);
-      cbuf += pm->message_size;
-      ret += pm->message_size;
-      size -= pm->message_size;
       GNUNET_CONTAINER_DLL_remove (session->pending_messages_head,
                                   session->pending_messages_tail,
-                                  pm);
-      if (pm->transmit_cont != NULL)
-        pm->transmit_cont (pm->transmit_cont_cls,
-                           &session->target, GNUNET_OK);
-      GNUNET_free (pm);
+                                  pos);
+      GNUNET_assert (size >= pos->message_size);
+      /* FIXME: this memcpy can be up to 7% of our total runtime */
+      memcpy (cbuf, pos->msg, pos->message_size);
+      cbuf += pos->message_size;
+      ret += pos->message_size;
+      size -= pos->message_size;
+      GNUNET_CONTAINER_DLL_insert_after (hd, tl, tl, pos);
     }
-  if (session->client != NULL)
-    process_pending_messages (session);
+  /* schedule 'continuation' before callbacks so that callbacks that
+     cancel everything don't cause us to use a session that no longer
+     exists... */
+  process_pending_messages (session);
+  session->last_activity = GNUNET_TIME_absolute_get ();
+  pid = session->target;
+  /* we'll now call callbacks that may cancel the session; hence
+     we should not use 'session' after this point */
+  while (NULL != (pos = hd))
+    {
+      GNUNET_CONTAINER_DLL_remove (hd, tl, pos);
+      if (pos->transmit_cont != NULL)
+        pos->transmit_cont (pos->transmit_cont_cls,
+                           &pid, GNUNET_OK);
+      GNUNET_free (pos);
+    }
+  GNUNET_assert (hd == NULL);
+  GNUNET_assert (tl == NULL);
 #if DEBUG_TCP > 1
   GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
-                   "tcp", "Transmitting %u bytes\n", ret);
+                  "tcp",
+                  "Transmitting %u bytes\n", 
+                  ret);
 #endif
+  GNUNET_STATISTICS_update (plugin->env->stats,
+                           gettext_noop ("# bytes currently in TCP buffers"),
+                           - (int64_t) ret,
+                           GNUNET_NO);
+  GNUNET_STATISTICS_update (plugin->env->stats,
+                           gettext_noop ("# bytes transmitted via TCP"),
+                           ret,
+                           GNUNET_NO);
   return ret;
 }
 
@@ -442,11 +1003,13 @@ static void
 process_pending_messages (struct Session *session)
 {
   struct PendingMessage *pm;
+
   GNUNET_assert (session->client != NULL);
   if (session->transmit_handle != NULL)
     return;
   if (NULL == (pm = session->pending_messages_head))
     return;
+
   session->transmit_handle
     = GNUNET_SERVER_notify_transmit_ready (session->client,
                                            pm->message_size,
@@ -472,12 +1035,13 @@ disconnect_session (struct Session *session)
 
 #if DEBUG_TCP
   GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
-                   "tcp",
-                   "Disconnecting from `%4s' at %s (session %p).\n",
+                  "tcp",
+                   "Disconnecting from `%4s' at %s.\n",
                    GNUNET_i2s (&session->target),
                    (session->connect_addr != NULL) ?
-                   GNUNET_a2s (session->connect_addr,
-                               session->connect_alen) : "*", session);
+                   tcp_address_to_string (session->plugin,
+                                         session->connect_addr,
+                                         session->connect_alen) : "*");
 #endif
   /* remove from session list */
   prev = NULL;
@@ -491,6 +1055,9 @@ disconnect_session (struct Session *session)
     session->plugin->sessions = session->next;
   else
     prev->next = session->next;
+  session->plugin->env->session_end (session->plugin->env->cls,
+                                    &session->target,
+                                    session);
   /* clean up state */
   if (session->transmit_handle != NULL)
     {
@@ -502,13 +1069,20 @@ disconnect_session (struct Session *session)
     {
 #if DEBUG_TCP
       GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
-                       "tcp",
+                      "tcp",
                        pm->transmit_cont != NULL
                        ? "Could not deliver message to `%4s'.\n"
-                       :
-                       "Could not deliver message to `%4s', notifying.\n",
+                       : "Could not deliver message to `%4s', notifying.\n",
                        GNUNET_i2s (&session->target));
 #endif
+      GNUNET_STATISTICS_update (session->plugin->env->stats,
+                               gettext_noop ("# bytes currently in TCP buffers"),
+                               - (int64_t) pm->message_size,
+                               GNUNET_NO);
+      GNUNET_STATISTICS_update (session->plugin->env->stats,
+                               gettext_noop ("# bytes discarded by TCP (disconnect)"),
+                               pm->message_size,
+                               GNUNET_NO);
       GNUNET_CONTAINER_DLL_remove (session->pending_messages_head,
                                   session->pending_messages_tail,
                                   pm);
@@ -517,35 +1091,119 @@ disconnect_session (struct Session *session)
                            &session->target, GNUNET_SYSERR);
       GNUNET_free (pm);
     }
-  if (GNUNET_NO == session->expecting_welcome)
+  GNUNET_break (session->client != NULL);
+  if (session->receive_delay_task != GNUNET_SCHEDULER_NO_TASK)
     {
-#if DEBUG_TCP
-      GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
-                       "tcp",
-                       "Notifying transport service about loss of connection with `%4s'.\n",
-                       GNUNET_i2s (&session->target));
-#endif
-      /* Data session that actually went past the initial handshake;
-         transport service may know about this one, so we need to
-         notify transport service about disconnect */
-      // FIXME: we should have a very clear connect-disconnect
-      // protocol with gnunet-service-transport!
-      session->plugin->env->receive (session->plugin->env->cls,
-                                     &session->target, NULL,
-                                     1,
-                                    session->connect_addr,
-                                    session->connect_alen);
-    }
-  if (session->client != NULL)
-    {
-      GNUNET_SERVER_client_drop (session->client);
-      session->client = NULL;
+      GNUNET_SCHEDULER_cancel (session->receive_delay_task);
+      if (session->client != NULL)
+       GNUNET_SERVER_receive_done (session->client,
+                                   GNUNET_SYSERR);     
     }
+  else if (session->client != NULL)
+    GNUNET_SERVER_client_drop (session->client);
+  GNUNET_STATISTICS_update (session->plugin->env->stats,
+                           gettext_noop ("# TCP sessions active"),
+                           -1,
+                           GNUNET_NO);
   GNUNET_free_non_null (session->connect_addr);
   GNUNET_free (session);
 }
 
 
+/**
+ * Given two otherwise equivalent sessions, pick the better one.
+ *
+ * @param s1 one session (also default)
+ * @param s2 other session
+ * @return "better" session (more active)
+ */
+static struct Session *
+select_better_session (struct Session *s1,
+                      struct Session *s2)
+{
+  if (s1 == NULL)
+    return s2;
+  if (s2 == NULL)
+    return s1;
+  if ( (s1->expecting_welcome == GNUNET_NO) &&
+       (s2->expecting_welcome == GNUNET_YES) )
+    return s1;
+  if ( (s1->expecting_welcome == GNUNET_YES) &&
+       (s2->expecting_welcome == GNUNET_NO) )
+    return s2;
+  if (s1->last_activity.abs_value < s2->last_activity.abs_value)
+    return s2;
+  if (s1->last_activity.abs_value > s2->last_activity.abs_value)
+    return s1;
+  if ( (GNUNET_YES == s1->inbound) &&
+       (GNUNET_NO  == s2->inbound) )
+    return s1;
+  if ( (GNUNET_NO  == s1->inbound) &&
+       (GNUNET_YES == s2->inbound) )
+    return s2;
+  return s1;
+}
+
+
+/**
+ * We learned about a peer (possibly behind NAT) so run the
+ * gnunet-nat-client to send dummy ICMP responses.
+ *
+ * @param plugin the plugin for this transport
+ * @param sa the address of the peer (IPv4-only)
+ */
+static void
+run_gnunet_nat_client (struct Plugin *plugin, 
+                      const struct sockaddr_in *sa)
+{
+  char inet4[INET_ADDRSTRLEN];
+  char port_as_string[6];
+  struct GNUNET_OS_Process *proc;
+
+  if (plugin->internal_address == NULL)
+    {
+      GNUNET_log_from (GNUNET_ERROR_TYPE_WARNING,
+                      "tcp",
+                      _("Internal IP address not known, cannot use ICMP NAT traversal method\n"));
+      return;
+    }
+  GNUNET_assert (sa->sin_family == AF_INET);
+  if (NULL == inet_ntop (AF_INET,
+                        &sa->sin_addr,
+                        inet4, INET_ADDRSTRLEN))
+    {
+      GNUNET_log_strerror (GNUNET_ERROR_TYPE_WARNING, "inet_ntop");
+      return;
+    }
+  GNUNET_snprintf(port_as_string, 
+                 sizeof (port_as_string),
+                 "%d", 
+                 plugin->adv_port);
+#if DEBUG_TCP_NAT
+  GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
+                  "tcp",
+                  _("Running gnunet-nat-client %s %s %u\n"), 
+                  plugin->internal_address,
+                  inet4,
+                  (unsigned int) plugin->adv_port);
+#endif
+  proc = GNUNET_OS_start_process (NULL, 
+                                 NULL, 
+                                 "gnunet-nat-client",
+                                 "gnunet-nat-client",
+                                 plugin->internal_address, 
+                                 inet4,
+                                 port_as_string, 
+                                 NULL);
+  if (NULL == proc)
+    return;
+  /* we know that the gnunet-nat-client will terminate virtually
+     instantly */
+  GNUNET_OS_process_wait (proc);
+  GNUNET_OS_process_close (proc);
+}
+
+
 /**
  * Function that can be used by the transport service to transmit
  * a message using the plugin.   Note that in the case of a
@@ -557,18 +1215,22 @@ disconnect_session (struct Session *session)
  * @param cls closure
  * @param target who should receive this message
  * @param msg the message to transmit
+ * @param msgbuf_size number of bytes in 'msg'
  * @param priority how important is the message (most plugins will
  *                 ignore message priority and just FIFO)
  * @param timeout how long to wait at most for the transmission (does not
  *                require plugins to discard the message after the timeout,
  *                just advisory for the desired delay; most plugins will ignore
  *                this as well)
+ * @param session which session must be used (or NULL for "any")
  * @param addr the address to use (can be NULL if the plugin
  *                is "on its own" (i.e. re-use existing TCP connection))
  * @param addrlen length of the address in bytes
  * @param force_address GNUNET_YES if the plugin MUST use the given address,
- *                otherwise the plugin may use other addresses or
- *                existing connections (if available)
+ *                GNUNET_NO means the plugin may use any other address and
+ *                GNUNET_SYSERR means that only reliable existing
+ *                bi-directional connections should be used (regardless
+ *                of address)
  * @param cont continuation to call once the message has
  *        been transmitted (or if the transport is ready
  *        for the next transmission call; or if the
@@ -576,7 +1238,7 @@ disconnect_session (struct Session *session)
  * @param cont_cls closure for cont
  * @return number of bytes used (on the physical network, with overheads);
  *         -1 on hard errors (i.e. address invalid); 0 is a legal value
- *         and does NOT mean that the message was not transmitted (DV)
+ *         and does NOT mean that the message was not transmitted (DV and NAT)
  */
 static ssize_t
 tcp_plugin_send (void *cls,
@@ -585,57 +1247,195 @@ tcp_plugin_send (void *cls,
                  size_t msgbuf_size,
                  uint32_t priority,
                  struct GNUNET_TIME_Relative timeout,
+                struct Session *session,
                 const void *addr,
                 size_t addrlen,
                 int force_address,
                  GNUNET_TRANSPORT_TransmitContinuation cont, void *cont_cls)
 {
   struct Plugin *plugin = cls;
-  struct Session *session;
+  struct Session *cand_session;
+  struct Session *next;
   struct PendingMessage *pm;
   struct GNUNET_CONNECTION_Handle *sa;
   int af;
-
-  session = plugin->sessions;
-  /* FIXME: we could do this a cheaper with a hash table
+  const void *sb;
+  size_t sbs;
+  struct sockaddr_in a4;
+  struct sockaddr_in6 a6;
+  const struct IPv4TcpAddress *t4;
+  const struct IPv6TcpAddress *t6;
+  unsigned int is_natd;
+
+  GNUNET_STATISTICS_update (plugin->env->stats,
+                           gettext_noop ("# bytes TCP was asked to transmit"),
+                           msgbuf_size,
+                           GNUNET_NO);
+  /* FIXME: we could do this cheaper with a hash table
      where we could restrict the iteration to entries that match
      the target peer... */
-  while ( (session != NULL) &&
-         ( (0 != memcmp (target,
-                         &session->target, 
-                         sizeof (struct GNUNET_PeerIdentity))) ||
-           ( (GNUNET_YES == force_address) &&
-             (addr != NULL) &&
-             ( (addrlen != session->connect_alen) ||
-               (0 != memcmp (session->connect_addr,
-                             addr,
-                             addrlen)) ) ) ) )
-    session = session->next;
+  is_natd = GNUNET_NO;
+  if (session == NULL)
+    {
+      cand_session = NULL;
+      next = plugin->sessions;
+      while (NULL != (session = next))
+       {
+         next = session->next;
+         GNUNET_assert (session->client != NULL);
+         if (0 != memcmp (target,
+                          &session->target,
+                          sizeof (struct GNUNET_PeerIdentity)))
+           continue;
+         if ( ( (GNUNET_SYSERR == force_address) &&
+                (session->expecting_welcome == GNUNET_NO) ) ||
+              (GNUNET_NO == force_address) )
+           {
+             cand_session = select_better_session (cand_session,
+                                                   session);
+             continue;
+           }
+         if (GNUNET_SYSERR == force_address)
+           continue;
+         GNUNET_break (GNUNET_YES == force_address);
+         if (addr == NULL)
+           {
+             GNUNET_break (0);
+             break;
+           }
+         if ( (addrlen != session->connect_alen) && 
+              (session->is_nat == GNUNET_NO) )
+           continue;
+         if ((0 != memcmp (session->connect_addr,
+                          addr,
+                          addrlen)) && (session->is_nat == GNUNET_NO))
+           continue;
+         cand_session = select_better_session (cand_session,
+                                               session);       
+       }
+      session = cand_session;
+    }
   if ( (session == NULL) &&
        (addr == NULL) )
     {
 #if DEBUG_TCP
       GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
-                       "tcp",
-                       "Asked to transmit to `%4s' without address and I have no existing connection (failing).\n",
+                      "tcp",
+                      "Asked to transmit to `%4s' without address and I have no existing connection (failing).\n",
                        GNUNET_i2s (target));
 #endif
+      GNUNET_STATISTICS_update (plugin->env->stats,
+                               gettext_noop ("# bytes discarded by TCP (no address and no connection)"),
+                               msgbuf_size,
+                               GNUNET_NO);
       return -1;
     }
   if (session == NULL)
     {
-      if (sizeof (struct sockaddr_in) == addrlen)
-       af = AF_INET;
-      else if (sizeof (struct sockaddr_in6) == addrlen)
-       af = AF_INET6;
+      if (addrlen == sizeof (struct IPv6TcpAddress))
+       {
+         t6 = addr;
+         af = AF_INET6;
+         memset (&a6, 0, sizeof (a6));
+#if HAVE_SOCKADDR_IN_SIN_LEN
+          a6.sin6_len = sizeof (a6);
+#endif
+         a6.sin6_family = AF_INET6;
+         a6.sin6_port = t6->t6_port;
+         if (t6->t6_port == 0)
+           is_natd = GNUNET_YES;
+         memcpy (&a6.sin6_addr,
+                 &t6->ipv6_addr,
+                 sizeof (struct in6_addr));
+         sb = &a6;
+         sbs = sizeof (a6);
+       }
+      else if (addrlen == sizeof (struct IPv4TcpAddress))
+       {
+         t4 = addr;
+         af = AF_INET;
+         memset (&a4, 0, sizeof (a4));
+#if HAVE_SOCKADDR_IN_SIN_LEN
+          a4.sin_len = sizeof (a4);
+#endif
+         a4.sin_family = AF_INET;
+         a4.sin_port = t4->t_port;
+         if (t4->t_port == 0)
+           is_natd = GNUNET_YES;
+         a4.sin_addr.s_addr = t4->ipv4_addr;
+         sb = &a4;
+         sbs = sizeof (a4);
+       }
       else
        {
-         GNUNET_break_op (0);
+         GNUNET_log_from (GNUNET_ERROR_TYPE_ERROR,
+                          "tcp",
+                          _("Address of unexpected length: %u\n"),
+                          addrlen);
+         GNUNET_break (0);
          return -1;
        }
-      sa = GNUNET_CONNECTION_create_from_sockaddr (plugin->env->sched,
-                                                  af, addr, addrlen,
-                                                  GNUNET_SERVER_MAX_MESSAGE_SIZE);
+
+      if ((is_natd == GNUNET_YES) && (addrlen == sizeof (struct IPv6TcpAddress)))
+        return -1; /* NAT client only works with IPv4 addresses */
+
+
+      if ( (plugin->enable_nat_client == GNUNET_YES) && 
+          (is_natd == GNUNET_YES) &&
+           (GNUNET_NO == GNUNET_CONTAINER_multihashmap_contains(plugin->nat_wait_conns,
+                                                               &target->hashPubKey)) )
+        {
+#if DEBUG_TCP_NAT
+          GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
+                          "tcp",
+                           _("Found valid IPv4 NAT address (creating session)!\n"));
+#endif
+          session = create_session (plugin,
+                                    target,
+                                    NULL, 
+                                   GNUNET_YES);
+
+          /* create new message entry */
+          pm = GNUNET_malloc (sizeof (struct PendingMessage) + msgbuf_size);
+         /* FIXME: the memset of this malloc can be up to 2% of our total runtime */
+          pm->msg = (const char*) &pm[1];
+          memcpy (&pm[1], msg, msgbuf_size);
+         /* FIXME: this memcpy can be up to 7% of our total run-time
+            (for transport service) */
+          pm->message_size = msgbuf_size;
+          pm->timeout = GNUNET_TIME_relative_to_absolute (timeout);
+          pm->transmit_cont = cont;
+          pm->transmit_cont_cls = cont_cls;
+
+          /* append pm to pending_messages list */
+          GNUNET_CONTAINER_DLL_insert_after (session->pending_messages_head,
+                                             session->pending_messages_tail,
+                                             session->pending_messages_tail,
+                                             pm);
+
+          GNUNET_assert(GNUNET_CONTAINER_multihashmap_put(plugin->nat_wait_conns,
+                                                         &target->hashPubKey,
+                                                         session, 
+                                                         GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY) == GNUNET_OK);
+#if DEBUG_TCP_NAT
+          GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
+                          "tcp",
+                           "Created NAT WAIT connection to `%4s' at `%s'\n",
+                           GNUNET_i2s (target),
+                           GNUNET_a2s (sb, sbs));
+#endif
+          run_gnunet_nat_client (plugin, &a4);
+          return 0;
+        }
+      if ( (plugin->enable_nat_client == GNUNET_YES) && 
+          (is_natd == GNUNET_YES) && 
+          (GNUNET_YES == GNUNET_CONTAINER_multihashmap_contains(plugin->nat_wait_conns, 
+                                                                &target->hashPubKey)) )
+        {
+          /* Only do one NAT punch attempt per peer identity */
+          return -1;
+        }
+      sa = GNUNET_CONNECTION_create_from_sockaddr (af, sb, sbs);
       if (sa == NULL)
        {
 #if DEBUG_TCP
@@ -643,22 +1443,26 @@ tcp_plugin_send (void *cls,
                           "tcp",
                           "Failed to create connection to `%4s' at `%s'\n",
                           GNUNET_i2s (target),
-                          GNUNET_a2s (addr, addrlen));
+                          GNUNET_a2s (sb, sbs));
 #endif
+         GNUNET_STATISTICS_update (plugin->env->stats,
+                                   gettext_noop ("# bytes discarded by TCP (failed to connect)"),
+                                   msgbuf_size,
+                                   GNUNET_NO);
          return -1;
        }
-
-#if DEBUG_TCP
+#if DEBUG_TCP_NAT
       GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
-                       "tcp",
+                      "tcp",
                        "Asked to transmit to `%4s', creating fresh session using address `%s'.\n",
                       GNUNET_i2s (target),
-                      GNUNET_a2s (addr, addrlen));
+                      GNUNET_a2s (sb, sbs));
 #endif
       session = create_session (plugin,
                                target,
                                GNUNET_SERVER_connect_socket (plugin->server,
-                                                             sa));
+                                                             sa), 
+                               GNUNET_NO);
       session->connect_addr = GNUNET_malloc (addrlen);
       memcpy (session->connect_addr,
              addr,
@@ -667,7 +1471,10 @@ tcp_plugin_send (void *cls,
     }
   GNUNET_assert (session != NULL);
   GNUNET_assert (session->client != NULL);
-
+  GNUNET_STATISTICS_update (plugin->env->stats,
+                           gettext_noop ("# bytes currently in TCP buffers"),
+                           msgbuf_size,
+                           GNUNET_NO);
   /* create new message entry */
   pm = GNUNET_malloc (sizeof (struct PendingMessage) + msgbuf_size);
   pm->msg = (const char*) &pm[1];
@@ -684,7 +1491,7 @@ tcp_plugin_send (void *cls,
                                     pm);
 #if DEBUG_TCP
   GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
-                   "tcp",
+                  "tcp",
                   "Asked to transmit %u bytes to `%s', added message to list.\n",
                   msgbuf_size,
                   GNUNET_i2s (target));
@@ -711,10 +1518,12 @@ tcp_plugin_send (void *cls,
  *        to be cancelled
  */
 static void
-tcp_plugin_disconnect (void *cls, const struct GNUNET_PeerIdentity *target)
+tcp_plugin_disconnect (void *cls,
+                      const struct GNUNET_PeerIdentity *target)
 {
   struct Plugin *plugin = cls;
   struct Session *session;
+  struct Session *next;
   struct PendingMessage *pm;
 
 #if DEBUG_TCP
@@ -723,44 +1532,53 @@ tcp_plugin_disconnect (void *cls, const struct GNUNET_PeerIdentity *target)
                    "Asked to cancel session with `%4s'\n",
                    GNUNET_i2s (target));
 #endif
-  session = plugin->sessions;
-  while (NULL != session)
+  next = plugin->sessions;
+  while (NULL != (session = next))
     {
-      if (0 == memcmp (target,
+      next = session->next;
+      if (0 != memcmp (target,
                       &session->target,
                       sizeof (struct GNUNET_PeerIdentity)))
+       continue;
+      pm = session->pending_messages_head;
+      while (pm != NULL)
        {
-         pm = session->pending_messages_head;
-         while (pm != NULL)
-           {
-             pm->transmit_cont = NULL;
-             pm->transmit_cont_cls = NULL;
-             pm = pm->next;
-           }
-         if (session->client != NULL)
-           {
-             GNUNET_SERVER_client_drop (session->client);
-             session->client = NULL;
-           }
-         /* rest of the clean-up of the session will be done as part of
-            disconnect_notify which should be triggered any time now
-            (or which may be triggering this call in the first place) */
+         pm->transmit_cont = NULL;
+         pm->transmit_cont_cls = NULL;
+         pm = pm->next;
        }
-      session = session->next;
+      disconnect_session (session);
     }
 }
 
 
+/**
+ * Context for address to string conversion.
+ */
 struct PrettyPrinterContext
 {
+  /**
+   * Function to call with the result.
+   */
   GNUNET_TRANSPORT_AddressStringCallback asc;
+
+  /**
+   * Clsoure for 'asc'.
+   */
   void *asc_cls;
+
+  /**
+   * Port to add after the IP address.
+   */
   uint16_t port;
 };
 
 
 /**
  * Append our port and forward the result.
+ *
+ * @param cls the 'struct PrettyPrinterContext*'
+ * @param hostname hostname part of the address
  */
 static void
 append_port (void *cls, const char *hostname)
@@ -805,12 +1623,40 @@ tcp_plugin_address_pretty_printer (void *cls,
                                    void *asc_cls)
 {
   struct Plugin *plugin = cls;
-  const struct sockaddr_in *v4;
-  const struct sockaddr_in6 *v6;
   struct PrettyPrinterContext *ppc;
+  const void *sb;
+  size_t sbs;
+  struct sockaddr_in a4;
+  struct sockaddr_in6 a6;
+  const struct IPv4TcpAddress *t4;
+  const struct IPv6TcpAddress *t6;
+  uint16_t port;
 
-  if ((addrlen != sizeof (struct sockaddr_in)) &&
-      (addrlen != sizeof (struct sockaddr_in6)))
+  if (addrlen == sizeof (struct IPv6TcpAddress))
+    {
+      t6 = addr;
+      memset (&a6, 0, sizeof (a6));
+      a6.sin6_family = AF_INET6;
+      a6.sin6_port = t6->t6_port;
+      memcpy (&a6.sin6_addr,
+             &t6->ipv6_addr,
+             sizeof (struct in6_addr));
+      port = ntohs (t6->t6_port);
+      sb = &a6;
+      sbs = sizeof (a6);
+    }
+  else if (addrlen == sizeof (struct IPv4TcpAddress))
+    {
+      t4 = addr;
+      memset (&a4, 0, sizeof (a4));
+      a4.sin_family = AF_INET;
+      a4.sin_port = t4->t_port;
+      a4.sin_addr.s_addr = t4->ipv4_addr;
+      port = ntohs (t4->t_port);
+      sb = &a4;
+      sbs = sizeof (a4);
+    }
+  else
     {
       /* invalid address */
       GNUNET_break_op (0);
@@ -820,156 +1666,223 @@ tcp_plugin_address_pretty_printer (void *cls,
   ppc = GNUNET_malloc (sizeof (struct PrettyPrinterContext));
   ppc->asc = asc;
   ppc->asc_cls = asc_cls;
-  if (addrlen == sizeof (struct sockaddr_in))
-    {
-      v4 = (const struct sockaddr_in *) addr;
-      ppc->port = ntohs (v4->sin_port);
-    }
-  else
-    {
-      v6 = (const struct sockaddr_in6 *) addr;
-      ppc->port = ntohs (v6->sin6_port);
-
-    }
-  GNUNET_RESOLVER_hostname_get (plugin->env->sched,
-                                plugin->env->cfg,
-                                addr,
-                                addrlen,
+  ppc->port = port;
+  GNUNET_RESOLVER_hostname_get (plugin->env->cfg,
+                                sb,
+                                sbs,
                                 !numeric, timeout, &append_port, ppc);
 }
 
 
 /**
- * Update the last-received and bandwidth quota values
- * for this session.
+ * Check if the given port is plausible (must be either our listen
+ * port or our advertised port), or any port if we are behind NAT
+ * and do not have a port open.  If it is neither, we return
+ * GNUNET_SYSERR.
  *
- * @param session session to update
- * @param force set to GNUNET_YES if we should update even
- *        though the minimum refresh time has not yet expired
+ * @param plugin global variables
+ * @param in_port port number to check
+ * @return GNUNET_OK if port is either open_port or adv_port
  */
-static void
-update_quota (struct Session *session, int force)
+static int
+check_port (struct Plugin *plugin, 
+           uint16_t in_port)
 {
-  struct GNUNET_TIME_Absolute now;
-  unsigned long long delta;
-  unsigned long long total_allowed;
-  unsigned long long total_remaining;
-
-  now = GNUNET_TIME_absolute_get ();
-  delta = now.value - session->last_quota_update.value;
-  if ((delta < MIN_QUOTA_REFRESH_TIME) && (!force))
-    return;                     /* too early, not enough data */
-
-  total_allowed = session->quota_in * delta;
-  if (total_allowed > session->last_received)
-    {
-      /* got less than acceptable */
-      total_remaining = total_allowed - session->last_received;
-      session->last_received = 0;
-      delta = total_remaining / session->quota_in;      /* bonus seconds */
-      if (delta > MAX_BANDWIDTH_CARRY)
-        delta = MAX_BANDWIDTH_CARRY;    /* limit amount of carry-over */
-    }
-  else
-    {
-      /* got more than acceptable */
-      session->last_received -= total_allowed;
-      delta = 0;
-    }
-  session->last_quota_update.value = now.value - delta;
+  if ((in_port == plugin->adv_port) || (in_port == plugin->open_port))
+    return GNUNET_OK;
+  return GNUNET_SYSERR;
 }
 
 
 /**
- * Set a quota for receiving data from the given peer; this is a
- * per-transport limit.  The transport should limit its read/select
- * calls to stay below the quota (in terms of incoming data).
+ * Function that will be called to check if a binary address for this
+ * plugin is well-formed and corresponds to an address for THIS peer
+ * (as per our configuration).  Naturally, if absolutely necessary,
+ * plugins can be a bit conservative in their answer, but in general
+ * plugins should make sure that the address does not redirect
+ * traffic to a 3rd party that might try to man-in-the-middle our
+ * traffic.
  *
- * @param cls closure
- * @param target the peer for whom the quota is given
- * @param quota_in quota for receiving/sending data in bytes per ms
+ * @param cls closure, our 'struct Plugin*'
+ * @param addr pointer to the address
+ * @param addrlen length of addr
+ * @return GNUNET_OK if this is a plausible address for this peer
+ *         and transport, GNUNET_SYSERR if not
  */
-static void
-tcp_plugin_set_receive_quota (void *cls,
-                              const struct GNUNET_PeerIdentity *target,
-                              uint32_t quota_in)
+static int
+tcp_plugin_check_address (void *cls,
+                         const void *addr,
+                         size_t addrlen)
 {
   struct Plugin *plugin = cls;
-  struct Session *session;
+  struct IPv4TcpAddress *v4;
+  struct IPv6TcpAddress *v6;
 
-  session = find_session_by_target (plugin, target);
-  if (session == NULL)
-    return;                     /* peer must have disconnected, ignore */
-  if (session->quota_in != quota_in)
+  if ((addrlen != sizeof (struct IPv4TcpAddress)) &&
+      (addrlen != sizeof (struct IPv6TcpAddress)))
     {
-      update_quota (session, GNUNET_YES);
-      if (session->quota_in > quota_in)
-        session->last_quota_update = GNUNET_TIME_absolute_get ();
-      session->quota_in = quota_in;
+      GNUNET_break_op (0);
+      return GNUNET_SYSERR;
     }
+  if (addrlen == sizeof (struct IPv4TcpAddress))
+    {
+      v4 = (struct IPv4TcpAddress *) addr;
+      if (GNUNET_OK ==
+         check_mapped_addr (plugin, v4, sizeof (struct IPv4TcpAddress)))
+       return GNUNET_OK;
+      if (GNUNET_OK !=
+         check_port (plugin, ntohs (v4->t_port)))
+       return GNUNET_SYSERR;
+      if (GNUNET_OK !=
+         check_local_addr (plugin, &v4->ipv4_addr, sizeof (struct in_addr)))
+       return GNUNET_SYSERR;   
+    }
+  else
+    {
+      v6 = (struct IPv6TcpAddress *) addr;
+      if (IN6_IS_ADDR_LINKLOCAL (&v6->ipv6_addr))
+       {
+         GNUNET_break_op (0);
+         return GNUNET_SYSERR;
+       }
+      if (GNUNET_OK ==
+         check_mapped_addr (plugin, v6, sizeof (struct IPv6TcpAddress)))
+       return GNUNET_OK;
+      if (GNUNET_OK !=
+         check_port (plugin, ntohs (v6->t6_port)))
+       return GNUNET_SYSERR;
+      if (GNUNET_OK !=
+         check_local_addr (plugin, &v6->ipv6_addr, sizeof (struct in6_addr)))
+       return GNUNET_SYSERR;
+    }
+  return GNUNET_OK;
 }
 
 
 /**
- * Check if the given port is plausible (must be either
- * our listen port or our advertised port).  If it is
- * neither, we return one of these two ports at random.
- *
- * @return either in_port or a more plausible port
- */
-static uint16_t
-check_port (struct Plugin *plugin, uint16_t in_port)
-{
-  if ((in_port == plugin->adv_port) || (in_port == plugin->open_port))
-    return in_port;
-  return (GNUNET_CRYPTO_random_u32 (GNUNET_CRYPTO_QUALITY_WEAK,
-                                    2) == 0)
-    ? plugin->open_port : plugin->adv_port;
-}
-
-
-/**
- * Another peer has suggested an address for this peer and transport
- * plugin.  Check that this could be a valid address.
+ * We've received a nat probe from this peer via TCP.  Finish
+ * creating the client session and resume sending of queued
+ * messages.
  *
  * @param cls closure
- * @param addr pointer to the address
- * @param addrlen length of addr
- * @return GNUNET_OK if this is a plausible address for this peer
- *         and transport
+ * @param client identification of the client
+ * @param message the actual message
  */
-static int
-tcp_plugin_check_address (void *cls, void *addr, size_t addrlen)
+static void
+handle_tcp_nat_probe (void *cls,
+                     struct GNUNET_SERVER_Client *client,
+                     const struct GNUNET_MessageHeader *message)
 {
   struct Plugin *plugin = cls;
-  char buf[sizeof (struct sockaddr_in6)];
-  struct sockaddr_in *v4;
-  struct sockaddr_in6 *v6;
+  struct Session *session;
+  const struct TCP_NAT_ProbeMessage *tcp_nat_probe;
+  size_t alen;
+  void *vaddr;
+  struct IPv4TcpAddress *t4;
+  struct IPv6TcpAddress *t6;
+  const struct sockaddr_in *s4;
+  const struct sockaddr_in6 *s6;
 
-  if ((addrlen != sizeof (struct sockaddr_in)) &&
-      (addrlen != sizeof (struct sockaddr_in6)))
+#if DEBUG_TCP_NAT
+  GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG, 
+                  "tcp",
+                  "received tcp NAT probe\n");
+#endif
+  /* We have received a TCP NAT probe, meaning we (hopefully) initiated
+   * a connection to this peer by running gnunet-nat-client.  This peer
+   * received the punch message and now wants us to use the new connection
+   * as the default for that peer.  Do so and then send a WELCOME message
+   * so we can really be connected!
+   */
+  if (ntohs(message->size) != sizeof(struct TCP_NAT_ProbeMessage))
     {
-      GNUNET_break_op (0);
-      return GNUNET_SYSERR;
+      GNUNET_break_op(0);
+      return;
     }
-  memcpy (buf, addr, sizeof (struct sockaddr_in6));
-  if (addrlen == sizeof (struct sockaddr_in))
+  tcp_nat_probe = (const struct TCP_NAT_ProbeMessage *)message;
+  session = GNUNET_CONTAINER_multihashmap_get(plugin->nat_wait_conns, 
+                                             &tcp_nat_probe->clientIdentity.hashPubKey);
+  if (session == NULL)
     {
-      v4 = (struct sockaddr_in *) buf;
-      v4->sin_port = htons (check_port (plugin, ntohs (v4->sin_port)));
+#if DEBUG_TCP_NAT
+      GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
+                      "tcp",
+                      "Did NOT find session for NAT probe!\n");
+#endif
+      GNUNET_SERVER_receive_done (client, GNUNET_OK);
+      return;
     }
-  else
+#if DEBUG_TCP_NAT
+  GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG, 
+                  "tcp",
+                  "Found session for NAT probe!\n");
+#endif
+  GNUNET_assert(GNUNET_CONTAINER_multihashmap_remove(plugin->nat_wait_conns, 
+                                                    &tcp_nat_probe->clientIdentity.hashPubKey,
+                                                    session) == GNUNET_YES);
+  if (GNUNET_OK !=
+      GNUNET_SERVER_client_get_address (client, &vaddr, &alen))
     {
-      v6 = (struct sockaddr_in6 *) buf;
-      v6->sin6_port = htons (check_port (plugin, ntohs (v6->sin6_port)));
+      GNUNET_break (0);
+      GNUNET_free (session);
+      GNUNET_SERVER_receive_done (client, GNUNET_OK);
+      return;
     }
-#if DEBUG_TCP
+
+  GNUNET_SERVER_client_keep (client);
+  session->client = client;
+  session->last_activity = GNUNET_TIME_absolute_get ();
+  session->inbound = GNUNET_NO;
+
+#if DEBUG_TCP_NAT
   GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
-                   "tcp",
-                   "Informing transport service about my address `%s'.\n",
-                   GNUNET_a2s (addr, addrlen));
+                  "tcp",
+                  "Found address `%s' for incoming connection\n",
+                  GNUNET_a2s (vaddr, alen));
 #endif
-  return GNUNET_OK;
+  switch (((const struct sockaddr *)vaddr)->sa_family)
+    {
+    case AF_INET:
+      s4 = vaddr;
+      t4 = GNUNET_malloc (sizeof (struct IPv4TcpAddress));
+      t4->t_port = s4->sin_port;
+      t4->ipv4_addr = s4->sin_addr.s_addr;
+      session->connect_addr = t4;
+      session->connect_alen = sizeof (struct IPv4TcpAddress);
+      break;
+    case AF_INET6:    
+      s6 = vaddr;
+      t6 = GNUNET_malloc (sizeof (struct IPv6TcpAddress));
+      t6->t6_port = s6->sin6_port;
+      memcpy (&t6->ipv6_addr,
+             &s6->sin6_addr,
+             sizeof (struct in6_addr));
+      session->connect_addr = t6;
+      session->connect_alen = sizeof (struct IPv6TcpAddress);
+      break;
+    default:
+      GNUNET_break_op (0);
+#if DEBUG_TCP_NAT
+      GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
+                      "tcp",
+                      "Bad address for incoming connection!\n");
+#endif
+      GNUNET_free (vaddr);
+      GNUNET_SERVER_client_drop (client);
+      GNUNET_free (session);
+      GNUNET_SERVER_receive_done (client, GNUNET_SYSERR);
+      return;
+    }
+  GNUNET_free (vaddr);
+  
+  session->next = plugin->sessions;
+  plugin->sessions = session;
+  GNUNET_STATISTICS_update (plugin->env->stats,
+                           gettext_noop ("# TCP sessions active"),
+                           1,
+                           GNUNET_NO);
+  process_pending_messages (session);
+  GNUNET_SERVER_receive_done (client, GNUNET_OK);
 }
 
 
@@ -991,32 +1904,70 @@ handle_tcp_welcome (void *cls,
   struct Session *session;
   size_t alen;
   void *vaddr;
+  struct IPv4TcpAddress *t4;
+  struct IPv6TcpAddress *t6;
+  const struct sockaddr_in *s4;
+  const struct sockaddr_in6 *s6;
 
 #if DEBUG_TCP
   GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
-                   "tcp",
-                   "Received %s message from a `%4s/%p'.\n", 
+                  "tcp",
+                   "Received %s message from `%4s'.\n",
                   "WELCOME",
-                   GNUNET_i2s (&wm->clientIdentity), client);
+                   GNUNET_i2s (&wm->clientIdentity));
 #endif
+  GNUNET_STATISTICS_update (plugin->env->stats,
+                           gettext_noop ("# TCP WELCOME messages received"),
+                           1,
+                           GNUNET_NO);
   session = find_session_by_client (plugin, client);
+
   if (session == NULL)
     {
+#if DEBUG_TCP_NAT
+      GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
+                      "tcp",
+                       "Received %s message from a `%4s', creating new session\n",
+                       "WELCOME",
+                       GNUNET_i2s (&wm->clientIdentity));
+#endif
       GNUNET_SERVER_client_keep (client);
       session = create_session (plugin,
-                               &wm->clientIdentity, client);
+                               &wm->clientIdentity,
+                               client,
+                               GNUNET_NO);
+      session->inbound = GNUNET_YES;
       if (GNUNET_OK ==
          GNUNET_SERVER_client_get_address (client, &vaddr, &alen))
        {
-#if DEBUG_TCP
+#if DEBUG_TCP_NAT
          GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
                           "tcp",
-                          "Found address `%s' for incoming connection %p\n",
-                          GNUNET_a2s (vaddr, alen),
-                          client);
+                          "Found address `%s' for incoming connection\n",
+                          GNUNET_a2s (vaddr, alen));
 #endif
-         session->connect_addr = vaddr;
-         session->connect_alen = alen;
+         if (alen == sizeof (struct sockaddr_in))
+           {
+             s4 = vaddr;
+             t4 = GNUNET_malloc (sizeof (struct IPv4TcpAddress));
+             t4->t_port = s4->sin_port;
+             t4->ipv4_addr = s4->sin_addr.s_addr;
+             session->connect_addr = t4;
+             session->connect_alen = sizeof (struct IPv4TcpAddress);
+           }
+         else if (alen == sizeof (struct sockaddr_in6))
+           {
+             s6 = vaddr;
+             t6 = GNUNET_malloc (sizeof (struct IPv6TcpAddress));
+             t6->t6_port = s6->sin6_port;
+             memcpy (&t6->ipv6_addr,
+                     &s6->sin6_addr,
+                     sizeof (struct in6_addr));
+             session->connect_addr = t6;
+             session->connect_alen = sizeof (struct IPv6TcpAddress);
+           }
+
+         GNUNET_free (vaddr);
        }
       else
         {
@@ -1026,68 +1977,60 @@ handle_tcp_welcome (void *cls,
                           "Did not obtain TCP socket address for incoming connection\n");
 #endif
         }
-#if DEBUG_TCP
-      GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
-                       "tcp",
-                       "Creating new session %p for connection %p\n",
-                       session, client);
-#endif
       process_pending_messages (session);
     }
+  else
+    {
+#if DEBUG_TCP_NAT
+    if (GNUNET_OK ==
+        GNUNET_SERVER_client_get_address (client, &vaddr, &alen))
+      {
+        GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
+                        "tcp",
+                        "Found address `%s' (already have session)\n",
+                        GNUNET_a2s (vaddr, alen));
+      }
+#endif
+    }
+
   if (session->expecting_welcome != GNUNET_YES)
     {
       GNUNET_break_op (0);
       GNUNET_SERVER_receive_done (client, GNUNET_SYSERR);
       return;
     }
+  session->last_activity = GNUNET_TIME_absolute_get ();
   session->expecting_welcome = GNUNET_NO;
   GNUNET_SERVER_receive_done (client, GNUNET_OK);
 }
 
 
-/**
- * Calculate how long we should delay reading from the TCP socket to
- * ensure that we stay within our bandwidth limits (push back).
- *
- * @param session for which client should this be calculated
- */
-static struct GNUNET_TIME_Relative
-calculate_throttle_delay (struct Session *session)
-{
-  struct GNUNET_TIME_Relative ret;
-  struct GNUNET_TIME_Absolute now;
-  uint64_t del;
-  uint64_t avail;
-  uint64_t excess;
-
-  now = GNUNET_TIME_absolute_get ();
-  del = now.value - session->last_quota_update.value;
-  if (del > MAX_BANDWIDTH_CARRY)
-    {
-      update_quota (session, GNUNET_YES);
-      del = now.value - session->last_quota_update.value;
-      GNUNET_assert (del <= MAX_BANDWIDTH_CARRY);
-    }
-  if (session->quota_in == 0)
-    session->quota_in = 1;      /* avoid divison by zero */
-  avail = del * session->quota_in;
-  if (avail > session->last_received)
-    return GNUNET_TIME_UNIT_ZERO;       /* can receive right now */
-  excess = session->last_received - avail;
-  ret.value = excess / session->quota_in;
-  return ret;
-}
-
-
 /**
  * Task to signal the server that we can continue
  * receiving from the TCP client now.
+ *
+ * @param cls the 'struct Session*'
+ * @param tc task context (unused)
  */
 static void
-delayed_done (void *cls, const struct GNUNET_SCHEDULER_TaskContext *tc)
+delayed_done (void *cls, 
+             const struct GNUNET_SCHEDULER_TaskContext *tc)
 {
   struct Session *session = cls;
-  GNUNET_SERVER_receive_done (session->client, GNUNET_OK);
+  struct GNUNET_TIME_Relative delay;
+
+  session->receive_delay_task = GNUNET_SCHEDULER_NO_TASK;
+  delay = session->plugin->env->receive (session->plugin->env->cls,
+                                        &session->target,
+                                        NULL,
+                                        NULL, 0,
+                                        session,
+                                        NULL, 0);
+  if (delay.rel_value == 0)
+    GNUNET_SERVER_receive_done (session->client, GNUNET_OK);
+  else
+    session->receive_delay_task =
+      GNUNET_SCHEDULER_add_delayed (delay, &delayed_done, session);
 }
 
 
@@ -1106,58 +2049,55 @@ handle_tcp_data (void *cls,
 {
   struct Plugin *plugin = cls;
   struct Session *session;
-  uint16_t msize;
   struct GNUNET_TIME_Relative delay;
+  uint16_t type;
 
-  msize = ntohs (message->size);
-  session = find_session_by_client (plugin, client);
-
-  if (GNUNET_MESSAGE_TYPE_TRANSPORT_TCP_WELCOME == ntohs(message->type))
+  type = ntohs (message->type);
+  if ( (GNUNET_MESSAGE_TYPE_TRANSPORT_TCP_WELCOME == type) || 
+       (GNUNET_MESSAGE_TYPE_TRANSPORT_TCP_NAT_PROBE == type) )
     {
-      /* We don't want to propagate WELCOME messages up! */
+      /* We don't want to propagate WELCOME and NAT Probe messages up! */
       GNUNET_SERVER_receive_done (client, GNUNET_OK);
-      return; 
-    }    
-  if ( (NULL == session) || (GNUNET_NO != session->expecting_welcome))
+      return;
+    }
+  session = find_session_by_client (plugin, client);
+  if ( (NULL == session) || (GNUNET_YES == session->expecting_welcome) )
     {
-      GNUNET_break_op (0);
       GNUNET_SERVER_receive_done (client, GNUNET_SYSERR);
       return;
     }
-#if DEBUG_TCP
+  session->last_activity = GNUNET_TIME_absolute_get ();
+#if DEBUG_TCP > 1
   GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
-                   "tcp", 
+                  "tcp",
                   "Passing %u bytes of type %u from `%4s' to transport service.\n",
-                   (unsigned int) msize, 
+                   (unsigned int) ntohs (message->size),
                   (unsigned int) ntohs (message->type),
                   GNUNET_i2s (&session->target));
 #endif
-  plugin->env->receive (plugin->env->cls, &session->target, message, 1,
-                       session->connect_addr,
-                       session->connect_alen);
-  /* update bandwidth used */
-  session->last_received += msize;
-  update_quota (session, GNUNET_NO);
-  delay = calculate_throttle_delay (session);
-  if (delay.value == 0)
+  GNUNET_STATISTICS_update (plugin->env->stats,
+                           gettext_noop ("# bytes received via TCP"),
+                           ntohs (message->size),
+                           GNUNET_NO);
+  struct GNUNET_TRANSPORT_ATS_Information distance[2];
+  distance[0].type = htonl (GNUNET_TRANSPORT_ATS_QUALITY_NET_DISTANCE);
+  distance[0].value = htonl (1);
+  distance[1].type = htonl (GNUNET_TRANSPORT_ATS_ARRAY_TERMINATOR);
+  distance[1].value = htonl (0);
+  delay = plugin->env->receive (plugin->env->cls, &session->target, message,
+                               (const struct GNUNET_TRANSPORT_ATS_Information *) &distance,
+                               2,
+                               session,
+                               (GNUNET_YES == session->inbound) ? NULL : session->connect_addr,
+                               (GNUNET_YES == session->inbound) ? 0 : session->connect_alen);
+  if (delay.rel_value == 0)
     GNUNET_SERVER_receive_done (client, GNUNET_OK);
   else
-    GNUNET_SCHEDULER_add_delayed (session->plugin->env->sched,
-                                  delay, &delayed_done, session);
+    session->receive_delay_task =
+      GNUNET_SCHEDULER_add_delayed (delay, &delayed_done, session);
 }
 
 
-/**
- * Handlers for the various TCP messages.
- */
-static struct GNUNET_SERVER_MessageHandler my_handlers[] = {
-  {&handle_tcp_welcome, NULL, GNUNET_MESSAGE_TYPE_TRANSPORT_TCP_WELCOME,
-   sizeof (struct WelcomeMessage)},
-  {&handle_tcp_data, NULL, GNUNET_MESSAGE_TYPE_ALL, 0},
-  {NULL, NULL, 0, 0}
-};
-
-
 /**
  * Functions with this signature are called whenever a peer
  * is disconnected on the network level.
@@ -1166,7 +2106,8 @@ static struct GNUNET_SERVER_MessageHandler my_handlers[] = {
  * @param client identification of the client
  */
 static void
-disconnect_notify (void *cls, struct GNUNET_SERVER_Client *client)
+disconnect_notify (void *cls,
+                  struct GNUNET_SERVER_Client *client)
 {
   struct Plugin *plugin = cls;
   struct Session *session;
@@ -1178,12 +2119,13 @@ disconnect_notify (void *cls, struct GNUNET_SERVER_Client *client)
     return;                     /* unknown, nothing to do */
 #if DEBUG_TCP
   GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
-                   "tcp",
-                   "Destroying session of `%4s' with %s (%p) due to network-level disconnect.\n",
+                  "tcp",
+                   "Destroying session of `%4s' with %s due to network-level disconnect.\n",
                    GNUNET_i2s (&session->target),
                    (session->connect_addr != NULL) ?
-                   GNUNET_a2s (session->connect_addr,
-                               session->connect_alen) : "*", client);
+                   tcp_address_to_string (session->plugin,
+                                         session->connect_addr,
+                                         session->connect_alen) : "*");
 #endif
   disconnect_session (session);
 }
@@ -1191,7 +2133,14 @@ disconnect_notify (void *cls, struct GNUNET_SERVER_Client *client)
 
 /**
  * Add the IP of our network interface to the list of
- * our external IP addresses.
+ * our internal IP addresses.
+ *
+ * @param cls the 'struct Plugin*'
+ * @param name name of the interface
+ * @param isDefault do we think this may be our default interface
+ * @param addr address of the interface
+ * @param addrlen number of bytes in addr
+ * @return GNUNET_OK to continue iterating
  */
 static int
 process_interfaces (void *cls,
@@ -1201,28 +2150,104 @@ process_interfaces (void *cls,
 {
   struct Plugin *plugin = cls;
   int af;
-  struct sockaddr_in *v4;
-  struct sockaddr_in6 *v6;
+  struct IPv4TcpAddress t4;
+  struct IPv6TcpAddress t6;
+  struct IPv4TcpAddress t4_nat;
+  struct IPv6TcpAddress t6_nat;
+  void *arg;
+  uint16_t args;
+  void *arg_nat;
+  char buf[INET_ADDRSTRLEN];
 
   af = addr->sa_family;
-  if (af == AF_INET)
+  arg_nat = NULL;
+  switch (af)
     {
-      v4 = (struct sockaddr_in *) addr;
-      v4->sin_port = htons (plugin->adv_port);
-    }
-  else
-    {
-      GNUNET_assert (af == AF_INET6);
-      v6 = (struct sockaddr_in6 *) addr;
-      v6->sin6_port = htons (plugin->adv_port);
+    case AF_INET:
+      t4.ipv4_addr = ((struct sockaddr_in *) addr)->sin_addr.s_addr;
+      GNUNET_assert (NULL != inet_ntop(AF_INET, 
+                                      &t4.ipv4_addr, 
+                                      buf, 
+                                      sizeof (buf)));
+      if ( (plugin->bind_address != NULL) && 
+          (0 != strcmp(buf, plugin->bind_address)) )
+        {
+#if DEBUG_TCP
+          GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG, 
+                          "tcp",
+                          "Not notifying transport of address `%s' (redundant)\n",
+                          GNUNET_a2s (addr, addrlen));
+#endif
+          return GNUNET_OK;
+        }
+      if ( (plugin->internal_address == NULL) &&
+          (isDefault) )        
+       plugin->internal_address = GNUNET_strdup (buf); 
+      add_to_address_list (plugin, &t4.ipv4_addr, sizeof (struct in_addr));
+      if (plugin->behind_nat == GNUNET_YES) 
+        {
+         /* Also advertise as NAT (with port 0) */
+          t4_nat.ipv4_addr = ((struct sockaddr_in *) addr)->sin_addr.s_addr;
+          t4_nat.t_port = htons(0);
+          arg_nat = &t4_nat;
+        }      
+      t4.t_port = htons (plugin->adv_port);    
+      arg = &t4;
+      args = sizeof (t4);
+      break;
+    case AF_INET6:      
+      if ( (IN6_IS_ADDR_LINKLOCAL (&((struct sockaddr_in6 *) addr)->sin6_addr)) || 
+          (GNUNET_YES == GNUNET_CONFIGURATION_get_value_yesno(plugin->env->cfg, 
+                                                              "transport-tcp", 
+                                                              "DISABLEV6")) )
+       {
+         /* skip link local addresses */
+         return GNUNET_OK;
+       }
+      memcpy (&t6.ipv6_addr,
+             &((struct sockaddr_in6 *) addr)->sin6_addr,
+             sizeof (struct in6_addr));
+      add_to_address_list (plugin, 
+                          &t6.ipv6_addr, 
+                          sizeof (struct in6_addr));
+      if (plugin->behind_nat == GNUNET_YES)
+        {
+         /* Also advertise as NAT (with port 0) */
+          memcpy (&t6_nat.ipv6_addr,
+                  &((struct sockaddr_in6 *) addr)->sin6_addr,
+                  sizeof (struct in6_addr));
+          t6_nat.t6_port = htons(0);
+          arg_nat = &t6;
+        }
+      t6.t6_port = htons (plugin->adv_port);
+      arg = &t6;
+      args = sizeof (t6);
+      break;
+    default:
+      GNUNET_break (0);
+      return GNUNET_OK;
     }
-  GNUNET_log_from (GNUNET_ERROR_TYPE_INFO |
-                   GNUNET_ERROR_TYPE_BULK,
-                   "tcp", _("Found address `%s' (%s)\n"),
-                   GNUNET_a2s (addr, addrlen), name);
+#if DEBUG_TCP
+  GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
+                  "tcp",
+                  "Found address `%s' (%s) len %d\n",
+                   GNUNET_a2s (addr, addrlen), name, args);
+#endif
   plugin->env->notify_address (plugin->env->cls,
                                "tcp",
-                               addr, addrlen, GNUNET_TIME_UNIT_FOREVER_REL);
+                               arg, args, GNUNET_TIME_UNIT_FOREVER_REL);
+
+  if (arg_nat != NULL)
+    {
+      GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
+                      "tcp",
+                      _("Found address `%s' (%s) len %d\n"),
+                      GNUNET_a2s (addr, addrlen), name, args);
+      plugin->env->notify_address (plugin->env->cls,
+                                   "tcp",
+                                   arg_nat, args, GNUNET_TIME_UNIT_FOREVER_REL);
+    }
+
   return GNUNET_OK;
 }
 
@@ -1247,16 +2272,423 @@ process_hostname_ips (void *cls,
       plugin->hostname_dns = NULL;
       return;
     }
+  /* FIXME: Can we figure out our external address here so it doesn't need to be user specified? */
   process_interfaces (plugin, "<hostname>", GNUNET_YES, addr, addrlen);
 }
 
 
+/**
+ * We can now send a probe message, copy into buffer to really send.
+ *
+ * @param cls closure, a struct TCPProbeContext
+ * @param size max size to copy
+ * @param buf buffer to copy message to
+ * @return number of bytes copied into buf
+ */
+static size_t
+notify_send_probe (void *cls,
+                  size_t size,
+                  void *buf)
+{
+  struct TCPProbeContext *tcp_probe_ctx = cls;
+  struct Plugin *plugin = tcp_probe_ctx->plugin;
+  size_t ret;
+
+  tcp_probe_ctx->transmit_handle = NULL;
+  GNUNET_CONTAINER_DLL_remove (plugin->probe_head,
+                              plugin->probe_tail,
+                              tcp_probe_ctx);
+  if (buf == NULL)
+    {
+      GNUNET_CONNECTION_destroy (tcp_probe_ctx->sock, GNUNET_NO);
+      GNUNET_free(tcp_probe_ctx);
+      return 0;    
+    }
+  GNUNET_assert(size >= sizeof(tcp_probe_ctx->message));
+  memcpy(buf, &tcp_probe_ctx->message, sizeof(tcp_probe_ctx->message));
+  GNUNET_SERVER_connect_socket (tcp_probe_ctx->plugin->server,
+                                tcp_probe_ctx->sock);
+  ret = sizeof(tcp_probe_ctx->message);
+  GNUNET_free(tcp_probe_ctx);
+  return ret;
+}
+
+
+/**
+ * We have been notified that gnunet-nat-server has written something to stdout.
+ * Handle the output, then reschedule this function to be called again once
+ * more is available.
+ *
+ * @param cls the plugin handle
+ * @param tc the scheduling context
+ */
+static void
+tcp_plugin_server_read (void *cls, 
+                       const struct GNUNET_SCHEDULER_TaskContext *tc)
+{
+  struct Plugin *plugin = cls;
+  char mybuf[40];
+  ssize_t bytes;
+  size_t i;
+  int port;
+  const char *port_start;
+  struct sockaddr_in sin_addr;
+  struct TCPProbeContext *tcp_probe_ctx;
+  struct GNUNET_CONNECTION_Handle *sock;
+
+  if ( (tc->reason & GNUNET_SCHEDULER_REASON_SHUTDOWN) != 0)
+    return;
+  memset (mybuf, 0, sizeof(mybuf));
+  bytes = GNUNET_DISK_file_read(plugin->server_stdout_handle, 
+                               mybuf,
+                               sizeof(mybuf));
+  if (bytes < 1)
+    {
+#if DEBUG_TCP_NAT
+      GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
+                      "tcp",
+                      "Finished reading from server stdout with code: %d\n", 
+                      bytes);
+#endif
+      /* FIXME: consider process_wait here? */
+      return;
+    }
+
+  port_start = NULL;
+  for (i = 0; i < sizeof(mybuf); i++)
+    {
+      if (mybuf[i] == '\n')
+       {
+         mybuf[i] = '\0';
+         break;
+       }
+      if ( (mybuf[i] == ':') && (i + 1 < sizeof(mybuf)) )
+        {
+          mybuf[i] = '\0';
+          port_start = &mybuf[i + 1];
+        }
+    }
+
+  /* construct socket address of sender */
+  memset (&sin_addr, 0, sizeof (sin_addr));
+  sin_addr.sin_family = AF_INET;
+#if HAVE_SOCKADDR_IN_SIN_LEN
+  sin_addr.sin_len = sizeof (sin_addr);
+#endif
+  if ( (NULL == port_start) ||
+       (1 != sscanf (port_start, "%d", &port)) ||
+       (-1 == inet_pton(AF_INET, mybuf, &sin_addr.sin_addr)) )
+    {
+      /* should we restart gnunet-nat-server? */
+      GNUNET_log_from (GNUNET_ERROR_TYPE_WARNING,
+                      "tcp",
+                      _("gnunet-nat-server generated malformed address `%s'\n"),
+                      mybuf);
+      plugin->server_read_task 
+       = GNUNET_SCHEDULER_add_read_file (GNUNET_TIME_UNIT_FOREVER_REL,
+                                         plugin->server_stdout_handle,
+                                         &tcp_plugin_server_read, 
+                                         plugin);
+      return;
+    }
+  sin_addr.sin_port = htons((uint16_t) port);
+#if DEBUG_TCP_NAT
+  GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
+                  "tcp",
+                  "gnunet-nat-server read: %s:%d\n", 
+                  mybuf, port);
+#endif
+
+  /**
+   * We have received an ICMP response, ostensibly from a peer
+   * that wants to connect to us! Send a message to establish a connection.
+   */
+  sock = GNUNET_CONNECTION_create_from_sockaddr (AF_INET, 
+                                                (const struct sockaddr *)&sin_addr,
+                                                 sizeof (sin_addr));
+  if (sock == NULL)
+    {
+      /* failed for some odd reason (out of sockets?); ignore attempt */
+      plugin->server_read_task =
+          GNUNET_SCHEDULER_add_read_file (GNUNET_TIME_UNIT_FOREVER_REL,
+                                          plugin->server_stdout_handle, 
+                                         &tcp_plugin_server_read, 
+                                         plugin);
+      return;
+    }
+
+  GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
+                  "Sending TCP probe message to `%s:%u'!\n", 
+                  mybuf,
+                  (unsigned int) port);  
+  /* FIXME: do we need to track these probe context objects so that
+     we can clean them up on plugin unload? */
+  tcp_probe_ctx
+    = GNUNET_malloc(sizeof(struct TCPProbeContext));
+  tcp_probe_ctx->message.header.size
+    = htons(sizeof(struct TCP_NAT_ProbeMessage));
+  tcp_probe_ctx->message.header.type
+    = htons(GNUNET_MESSAGE_TYPE_TRANSPORT_TCP_NAT_PROBE);
+  memcpy (&tcp_probe_ctx->message.clientIdentity,
+         plugin->env->my_identity,
+         sizeof(struct GNUNET_PeerIdentity));
+  tcp_probe_ctx->plugin = plugin;
+  tcp_probe_ctx->sock = sock;
+  GNUNET_CONTAINER_DLL_insert (plugin->probe_head,
+                              plugin->probe_tail,
+                              tcp_probe_ctx);
+  tcp_probe_ctx->transmit_handle 
+    = GNUNET_CONNECTION_notify_transmit_ready (sock,
+                                              ntohs (tcp_probe_ctx->message.header.size),
+                                              GNUNET_TIME_UNIT_FOREVER_REL,
+                                              &notify_send_probe, tcp_probe_ctx);
+  
+  plugin->server_read_task =
+      GNUNET_SCHEDULER_add_read_file (GNUNET_TIME_UNIT_FOREVER_REL,
+                                      plugin->server_stdout_handle,
+                                     &tcp_plugin_server_read,
+                                     plugin);
+}
+
+
+/**
+ * Start the gnunet-nat-server process for users behind NAT.
+ *
+ * @param plugin the transport plugin
+ * @return GNUNET_YES if process was started, GNUNET_SYSERR on error
+ */
+static int
+tcp_transport_start_nat_server (struct Plugin *plugin)
+{
+  if (plugin->internal_address == NULL)
+    return GNUNET_SYSERR;
+  plugin->server_stdout = GNUNET_DISK_pipe (GNUNET_YES,
+                                           GNUNET_NO,
+                                           GNUNET_YES);
+  if (plugin->server_stdout == NULL)
+    return GNUNET_SYSERR;
+#if DEBUG_TCP_NAT
+  GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG,
+                  "tcp"
+                   "Starting %s %s\n", 
+                  "gnunet-nat-server", 
+                  plugin->internal_address);
+#endif
+  /* Start the server process */
+  plugin->server_proc = GNUNET_OS_start_process (NULL,
+                                                plugin->server_stdout,
+                                                "gnunet-nat-server", 
+                                                "gnunet-nat-server", 
+                                                plugin->internal_address, 
+                                                NULL);
+  if (plugin->server_proc == NULL)
+    {
+      GNUNET_log_from (GNUNET_ERROR_TYPE_WARNING,
+                      "tcp",
+                      _("Failed to start %s\n"),
+                      "gnunet-nat-server");
+      GNUNET_DISK_pipe_close (plugin->server_stdout);
+      plugin->server_stdout = NULL;    
+      return GNUNET_SYSERR;
+    }
+  /* Close the write end of the read pipe */
+  GNUNET_DISK_pipe_close_end(plugin->server_stdout, 
+                            GNUNET_DISK_PIPE_END_WRITE);
+  plugin->server_stdout_handle 
+    = GNUNET_DISK_pipe_handle (plugin->server_stdout, 
+                              GNUNET_DISK_PIPE_END_READ);
+  plugin->server_read_task 
+    = GNUNET_SCHEDULER_add_read_file (GNUNET_TIME_UNIT_FOREVER_REL,
+                                      plugin->server_stdout_handle,
+                                     &tcp_plugin_server_read, 
+                                     plugin);
+  return GNUNET_YES;
+}
+
+
+/**
+ * Return the actual path to a file found in the current
+ * PATH environment variable.
+ *
+ * @param binary the name of the file to find
+ * @return path to binary, NULL if not found
+ */
+static char *
+get_path_from_PATH (const char *binary)
+{
+  char *path;
+  char *pos;
+  char *end;
+  char *buf;
+  const char *p;
+
+  p = getenv ("PATH");
+  if (p == NULL)
+    {
+      GNUNET_log_from (GNUNET_ERROR_TYPE_ERROR,
+                      "tcp",
+                      _("PATH environment variable is unset.\n"));
+      return NULL;
+    }
+  path = GNUNET_strdup (p);     /* because we write on it */
+  buf = GNUNET_malloc (strlen (path) + 20);
+  pos = path;
+
+  while (NULL != (end = strchr (pos, PATH_SEPARATOR)))
+    {
+      *end = '\0';
+      sprintf (buf, "%s/%s", pos, binary);
+      if (GNUNET_DISK_file_test (buf) == GNUNET_YES)
+        {
+          GNUNET_free (path);
+          return buf;
+        }
+      pos = end + 1;
+    }
+  sprintf (buf, "%s/%s", pos, binary);
+  if (GNUNET_DISK_file_test (buf) == GNUNET_YES)
+    {
+      GNUNET_free (path);
+      return buf;
+    }
+  GNUNET_free (buf);
+  GNUNET_free (path);
+  return NULL;
+}
+
+
+/**
+ * Check whether the suid bit is set on a file.
+ * Attempts to find the file using the current
+ * PATH environment variable as a search path.
+ *
+ * @param binary the name of the file to check
+ * @return GNUNET_YES if the file is SUID, 
+ *         GNUNET_NO if not, 
+ *         GNUNET_SYSERR on error
+ */
+static int
+check_gnunet_nat_binary (const char *binary)
+{
+  struct stat statbuf;
+  char *p;
+#ifdef MINGW
+  SOCKET rawsock;
+  char *binaryexe;
+
+  GNUNET_asprintf (&binaryexe, "%s.exe", binary);
+  p = get_path_from_PATH (binaryexe);
+  free (binaryexe);
+#else
+  p = get_path_from_PATH (binary);
+#endif
+  if (p == NULL)
+    {
+      GNUNET_log_from (GNUNET_ERROR_TYPE_ERROR,
+                      "tcp",
+                      _("Could not find binary `%s' in PATH!\n"),
+                      binary);
+      return GNUNET_NO;
+    }
+  if (0 != STAT (p, &statbuf))
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_WARNING, 
+                 _("stat (%s) failed: %s\n"), 
+                 p,
+                 STRERROR (errno));
+      GNUNET_free (p);
+      return GNUNET_SYSERR;
+    }
+  GNUNET_free (p);
+#ifndef MINGW
+  if ( (0 != (statbuf.st_mode & S_ISUID)) &&
+       (statbuf.st_uid == 0) )
+    return GNUNET_YES;
+  return GNUNET_NO;
+#else
+  rawsock = socket (AF_INET, SOCK_RAW, IPPROTO_ICMP);
+  if (INVALID_SOCKET == rawsock)
+    {
+      DWORD err = GetLastError ();
+      GNUNET_log_from (GNUNET_ERROR_TYPE_WARNING, 
+                      "tcp",
+                      "socket (AF_INET, SOCK_RAW, IPPROTO_ICMP) failed! GLE = %d\n", err);
+      return GNUNET_NO; /* not running as administrator */
+    }
+  closesocket (rawsock);
+  return GNUNET_YES;
+#endif
+}
+
+
+/**
+ * Our (external) hostname was resolved.
+ *
+ * @param cls the 'struct Plugin'
+ * @param addr NULL on error, otherwise result of DNS lookup
+ * @param addrlen number of bytes in addr
+ */
+static void
+process_external_ip (void *cls,
+                    const struct sockaddr *addr,
+                    socklen_t addrlen)
+{
+  struct Plugin *plugin = cls;
+  const struct sockaddr_in *s;
+  struct IPv4TcpAddress t4;
+
+
+  plugin->ext_dns = NULL;
+  if (addr == NULL)
+    return;
+  GNUNET_assert (addrlen == sizeof (struct sockaddr_in));
+  s = (const struct sockaddr_in *) addr;
+  t4.ipv4_addr = s->sin_addr.s_addr;
+  if ( (plugin->behind_nat == GNUNET_YES) &&
+       (plugin->enable_nat_server == GNUNET_YES) )
+    {
+      t4.t_port = htons(0);
+      GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG, 
+                      "tcp",
+                      "Notifying transport of address %s:%d\n", 
+                      plugin->external_address,
+                      0);
+    }
+  else
+    {
+      t4.t_port = htons(plugin->adv_port);
+      GNUNET_log_from (GNUNET_ERROR_TYPE_DEBUG, 
+                      "tcp",
+                      "Notifying transport of address %s:%d\n",
+                      plugin->external_address, 
+                      (int) plugin->adv_port);
+    }
+  add_to_address_list (plugin, 
+                      &t4.ipv4_addr, 
+                      sizeof (struct in_addr));
+  plugin->env->notify_address (plugin->env->cls,
+                              "tcp",
+                              &t4, sizeof(t4),
+                              GNUNET_TIME_UNIT_FOREVER_REL);
+}
+
+
 /**
  * Entry point for the plugin.
+ *
+ * @param cls closure, the 'struct GNUNET_TRANSPORT_PluginEnvironment*'
+ * @return the 'struct GNUNET_TRANSPORT_PluginFunctions*' or NULL on error
  */
 void *
 libgnunet_plugin_transport_tcp_init (void *cls)
 {
+  static const struct GNUNET_SERVER_MessageHandler my_handlers[] = {
+    {&handle_tcp_welcome, NULL, GNUNET_MESSAGE_TYPE_TRANSPORT_TCP_WELCOME,
+     sizeof (struct WelcomeMessage)},
+    {&handle_tcp_nat_probe, NULL, GNUNET_MESSAGE_TYPE_TRANSPORT_TCP_NAT_PROBE, sizeof (struct TCP_NAT_ProbeMessage)},
+    {&handle_tcp_data, NULL, GNUNET_MESSAGE_TYPE_ALL, 0},
+    {NULL, NULL, 0, 0}
+  };
   struct GNUNET_TRANSPORT_PluginEnvironment *env = cls;
   struct GNUNET_TRANSPORT_PluginFunctions *api;
   struct Plugin *plugin;
@@ -1264,83 +2696,278 @@ libgnunet_plugin_transport_tcp_init (void *cls)
   unsigned long long aport;
   unsigned long long bport;
   unsigned int i;
+  int behind_nat;
+  int nat_punched;
+  int enable_nat_client;
+  int enable_nat_server;
+  int enable_upnp;
+  char *internal_address;
+  char *external_address;
+  struct sockaddr_in in_addr;
+  struct GNUNET_TIME_Relative idle_timeout;
+
+  behind_nat = GNUNET_CONFIGURATION_get_value_yesno (env->cfg,
+                                                    "transport-tcp",
+                                                    "BEHIND_NAT");
+  nat_punched = GNUNET_CONFIGURATION_get_value_yesno (env->cfg,
+                                                     "transport-tcp",
+                                                     "NAT_PUNCHED");
+  enable_nat_client = GNUNET_CONFIGURATION_get_value_yesno (env->cfg,
+                                                           "transport-tcp",
+                                                           "ENABLE_NAT_CLIENT");
+  enable_nat_server = GNUNET_CONFIGURATION_get_value_yesno (env->cfg,
+                                                           "transport-tcp",
+                                                           "ENABLE_NAT_SERVER");
+  enable_upnp = GNUNET_CONFIGURATION_get_value_yesno (env->cfg,
+                                                     "transport-tcp",
+                                                     "ENABLE_UPNP");
+  
+  if ( (GNUNET_YES == enable_nat_server) &&
+       (GNUNET_YES != check_gnunet_nat_binary("gnunet-nat-server")) )
+    {
+      enable_nat_server = GNUNET_NO;
+      GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+                 _("Configuration requires `%s', but binary is not installed properly (SUID bit not set).  Option disabled.\n"),
+                 "gnunet-nat-server");        
+    }
+
+  if ( (GNUNET_YES == enable_nat_client) &&
+       (GNUNET_YES != check_gnunet_nat_binary("gnunet-nat-client")) )
+    {
+      enable_nat_client = GNUNET_NO;
+      GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+                 _("Configuration requires `%s', but binary is not installed properly (SUID bit not set).  Option disabled.\n"),
+                 "gnunet-nat-client"); 
+    }
+  
+  external_address = NULL;
+  if (GNUNET_OK ==
+      GNUNET_CONFIGURATION_have_value (env->cfg,
+                                      "transport-tcp",
+                                      "EXTERNAL_ADDRESS"))
+    {
+      (void) GNUNET_CONFIGURATION_get_value_string (env->cfg,
+                                                   "transport-tcp",
+                                                   "EXTERNAL_ADDRESS",
+                                                   &external_address);
+    }
 
-  service = GNUNET_SERVICE_start ("transport-tcp", env->sched, env->cfg);
-  if (service == NULL)
+  if ( (external_address != NULL) && 
+       (inet_pton(AF_INET, external_address, &in_addr.sin_addr) != 1) ) 
     {
+      
       GNUNET_log_from (GNUNET_ERROR_TYPE_WARNING,
-                       "tcp",
-                       _
-                       ("Failed to start service for `%s' transport plugin.\n"),
-                       "tcp");
+                      "tcp",
+                      _("Malformed %s `%s' given in configuration!\n"), 
+                      "EXTERNAL_ADDRESS",
+                      external_address);
+      return NULL;   
+    }
+  if ( (external_address == NULL) &&
+       (nat_punched == GNUNET_YES) )
+    {
+      nat_punched = GNUNET_NO;
+      GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+                 _("Configuration says NAT was punched, but `%s' is not given.  Option ignored.\n"),
+                 "EXTERNAL_ADDRESS");  
+    }
+
+  if (GNUNET_YES == nat_punched)
+    {
+      enable_nat_server = GNUNET_NO;
+      enable_upnp = GNUNET_NO;
+    }
+
+  internal_address = NULL;
+  if (GNUNET_OK ==
+      GNUNET_CONFIGURATION_have_value (env->cfg,
+                                      "transport-tcp",
+                                      "INTERNAL_ADDRESS"))
+    {
+      (void) GNUNET_CONFIGURATION_get_value_string (env->cfg,
+                                                   "transport-tcp",
+                                                   "INTERNAL_ADDRESS",
+                                                   &internal_address);
+    }
+
+  if ( (internal_address != NULL) && 
+       (inet_pton(AF_INET, internal_address, &in_addr.sin_addr) != 1) )
+    {
+      GNUNET_log_from (GNUNET_ERROR_TYPE_WARNING,
+                      "tcp",
+                      _("Malformed %s `%s' given in configuration!\n"), 
+                      "INTERNAL_ADDRESS",
+                      internal_address);      
+      GNUNET_free_non_null(internal_address);
+      GNUNET_free_non_null(external_address);
       return NULL;
     }
+
   aport = 0;
-  if ((GNUNET_OK !=
-       GNUNET_CONFIGURATION_get_value_number (env->cfg,
-                                              "transport-tcp",
-                                              "PORT",
-                                              &bport)) ||
-      (bport > 65535) ||
-      ((GNUNET_OK ==
-        GNUNET_CONFIGURATION_get_value_number (env->cfg,
-                                               "transport-tcp",
-                                               "ADVERTISED-PORT",
-                                               &aport)) && (aport > 65535)))
+  if ( (GNUNET_OK !=
+       GNUNET_CONFIGURATION_get_value_number (env->cfg,
+                                              "transport-tcp",
+                                              "PORT",
+                                              &bport)) ||
+       (bport > 65535) ||
+       ((GNUNET_OK ==
+        GNUNET_CONFIGURATION_get_value_number (env->cfg,
+                                               "transport-tcp",
+                                               "ADVERTISED-PORT",
+                                               &aport)) && 
+       (aport > 65535)) )
     {
       GNUNET_log_from (GNUNET_ERROR_TYPE_ERROR,
-                       "tcp",
-                       _
-                       ("Require valid port number for service `%s' in configuration!\n"),
+                      "tcp",
+                       _("Require valid port number for service `%s' in configuration!\n"),
                        "transport-tcp");
-      GNUNET_SERVICE_stop (service);
+      GNUNET_free_non_null(external_address);
+      GNUNET_free_non_null(internal_address);
       return NULL;
     }
+
   if (aport == 0)
     aport = bport;
+  if (bport == 0)
+    aport = 0;
+
+  if (bport != 0)
+    {
+      service = GNUNET_SERVICE_start ("transport-tcp", env->cfg);
+      if (service == NULL)
+       {
+         GNUNET_log_from (GNUNET_ERROR_TYPE_WARNING,
+                          "tcp",
+                          _("Failed to start service.\n"));
+         return NULL;
+       }
+    }
+  else
+    service = NULL;
+
   plugin = GNUNET_malloc (sizeof (struct Plugin));
   plugin->open_port = bport;
   plugin->adv_port = aport;
+  plugin->external_address = external_address;
+  plugin->internal_address = internal_address;
+  plugin->behind_nat = behind_nat;
+  plugin->nat_punched = nat_punched;
+  plugin->enable_nat_client = enable_nat_client;
+  plugin->enable_nat_server = enable_nat_server;
+  plugin->enable_upnp = enable_upnp;
   plugin->env = env;
   plugin->lsock = NULL;
-  plugin->statistics = NULL;
   api = GNUNET_malloc (sizeof (struct GNUNET_TRANSPORT_PluginFunctions));
   api->cls = plugin;
   api->send = &tcp_plugin_send;
   api->disconnect = &tcp_plugin_disconnect;
   api->address_pretty_printer = &tcp_plugin_address_pretty_printer;
-  api->set_receive_quota = &tcp_plugin_set_receive_quota;
   api->check_address = &tcp_plugin_check_address;
+  api->address_to_string = &tcp_address_to_string;
   plugin->service = service;
-  plugin->server = GNUNET_SERVICE_get_server (service);
+  if (service != NULL)   
+    {
+      plugin->server = GNUNET_SERVICE_get_server (service);
+    }
+  else
+    {
+      if (GNUNET_OK !=
+         GNUNET_CONFIGURATION_get_value_time (env->cfg,
+                                              "transport-tcp",
+                                              "TIMEOUT",
+                                              &idle_timeout))
+       {
+         GNUNET_log_from (GNUNET_ERROR_TYPE_ERROR,
+                          "tcp",
+                          _("Failed to find option %s in section %s!\n"),
+                          "TIMEOUT",
+                          "transport-tcp");
+         GNUNET_free_non_null(external_address);
+         GNUNET_free_non_null(internal_address);
+         GNUNET_free (api);
+         return NULL;
+       }
+      plugin->server = GNUNET_SERVER_create_with_sockets (NULL, NULL, NULL,
+                                                         idle_timeout, GNUNET_YES);
+    }
   plugin->handlers = GNUNET_malloc (sizeof (my_handlers));
   memcpy (plugin->handlers, my_handlers, sizeof (my_handlers));
   for (i = 0;
-       i <
-       sizeof (my_handlers) / sizeof (struct GNUNET_SERVER_MessageHandler);
+       i < sizeof (my_handlers) / sizeof (struct GNUNET_SERVER_MessageHandler);
        i++)
     plugin->handlers[i].callback_cls = plugin;
   GNUNET_SERVER_add_handlers (plugin->server, plugin->handlers);
+  GNUNET_SERVER_disconnect_notify (plugin->server,
+                                  &disconnect_notify,
+                                  plugin);    
+  GNUNET_OS_network_interfaces_list (&process_interfaces, plugin);
 
-  GNUNET_log_from (GNUNET_ERROR_TYPE_INFO,
-                   "tcp", _("TCP transport listening on port %llu\n"), bport);
+  if ( (plugin->behind_nat == GNUNET_YES) &&
+       (plugin->enable_nat_server == GNUNET_YES) &&
+       (GNUNET_YES != tcp_transport_start_nat_server(plugin)) )
+    {
+      GNUNET_log_from (GNUNET_ERROR_TYPE_ERROR,
+                      "tcp",
+                      _("Failed to start %s required for NAT in %s!\n"),
+                      "gnunet-nat-server"
+                      "transport-tcp");
+      GNUNET_free_non_null(external_address);
+      GNUNET_free_non_null(internal_address);
+      if (service != NULL)
+       GNUNET_SERVICE_stop (service);
+      else
+       GNUNET_SERVER_destroy (plugin->server);
+      GNUNET_free (api);
+      return NULL;
+    }
+
+  if (enable_nat_client == GNUNET_YES)
+    {
+      plugin->nat_wait_conns = GNUNET_CONTAINER_multihashmap_create(16);
+      GNUNET_assert (plugin->nat_wait_conns != NULL);
+    }
+
+  if (bport != 0)
+    GNUNET_log_from (GNUNET_ERROR_TYPE_INFO, 
+                    "tcp",
+                    _("TCP transport listening on port %llu\n"), 
+                    bport);
+  else
+    GNUNET_log_from (GNUNET_ERROR_TYPE_INFO, 
+                    "tcp",
+                    _("TCP transport not listening on any port (client only)\n"));
   if (aport != bport)
     GNUNET_log_from (GNUNET_ERROR_TYPE_INFO,
-                     "tcp",
-                     _
-                     ("TCP transport advertises itself as being on port %llu\n"),
+                    "tcp",
+                     _("TCP transport advertises itself as being on port %llu\n"),
                      aport);
-  GNUNET_SERVER_disconnect_notify (plugin->server, &disconnect_notify,
-                                   plugin);
-  /* FIXME: do the two calls below periodically again and
-     not just once (since the info we get might change...) */
-  GNUNET_OS_network_interfaces_list (&process_interfaces, plugin);
-  plugin->hostname_dns = GNUNET_RESOLVER_hostname_resolve (env->sched,
-                                                           env->cfg,
+
+  if (GNUNET_YES == GNUNET_CONFIGURATION_get_value_string (env->cfg,
+                                                          "transport-tcp", 
+                                                          "BINDTO", 
+                                                          &plugin->bind_address))
+    {
+      GNUNET_log_from (GNUNET_ERROR_TYPE_INFO, 
+                      "tcp",
+                      _("Binding TCP plugin to specific address: `%s'\n"), 
+                      plugin->bind_address);
+    }
+
+  plugin->hostname_dns = GNUNET_RESOLVER_hostname_resolve (env->cfg,
                                                            AF_UNSPEC,
                                                            HOSTNAME_RESOLVE_TIMEOUT,
                                                            &process_hostname_ips,
                                                            plugin);
+
+  if (plugin->external_address != NULL) 
+    {
+      plugin->ext_dns = GNUNET_RESOLVER_ip_get (env->cfg,
+                                               plugin->external_address,
+                                               AF_INET,
+                                               GNUNET_TIME_UNIT_MINUTES,
+                                               &process_external_ip,
+                                               plugin);
+    }
   return api;
 }
 
@@ -1354,7 +2981,14 @@ libgnunet_plugin_transport_tcp_done (void *cls)
   struct GNUNET_TRANSPORT_PluginFunctions *api = cls;
   struct Plugin *plugin = api->cls;
   struct Session *session;
+  struct LocalAddrList *lal;
+  struct TCPProbeContext *tcp_probe;
 
+  if (plugin->ext_dns != NULL)
+    {
+      GNUNET_RESOLVER_request_cancel (plugin->ext_dns);
+      plugin->ext_dns = NULL;
+    }
   while (NULL != (session = plugin->sessions))
     disconnect_session (session);
   if (NULL != plugin->hostname_dns)
@@ -1362,8 +2996,40 @@ libgnunet_plugin_transport_tcp_done (void *cls)
       GNUNET_RESOLVER_request_cancel (plugin->hostname_dns);
       plugin->hostname_dns = NULL;
     }
-  GNUNET_SERVICE_stop (plugin->service);
+  if (plugin->service != NULL)
+    GNUNET_SERVICE_stop (plugin->service);
+  else
+    GNUNET_SERVER_destroy (plugin->server);
   GNUNET_free (plugin->handlers);
+  while (NULL != (lal = plugin->lal_head))
+    {
+      GNUNET_CONTAINER_DLL_remove (plugin->lal_head,
+                                  plugin->lal_tail,
+                                  lal);
+      if (lal->nat != NULL)
+       GNUNET_NAT_unregister (lal->nat);
+      GNUNET_free_non_null (lal->external_nat_address);
+      GNUNET_free (lal);
+    }
+  while (NULL != (tcp_probe = plugin->probe_head))
+    {
+      GNUNET_CONTAINER_DLL_remove (plugin->probe_head,
+                                  plugin->probe_tail,
+                                  tcp_probe);
+      GNUNET_CONNECTION_destroy (tcp_probe->sock, GNUNET_NO);
+      GNUNET_free (tcp_probe);
+    }
+
+  if ((plugin->behind_nat == GNUNET_YES) &&
+      (plugin->enable_nat_server == GNUNET_YES))
+    {
+      if (0 != GNUNET_OS_process_kill (plugin->server_proc, SIGTERM))
+        GNUNET_log_strerror (GNUNET_ERROR_TYPE_WARNING, "kill");
+      GNUNET_OS_process_wait (plugin->server_proc);
+      GNUNET_OS_process_close (plugin->server_proc);
+      plugin->server_proc = NULL;
+    }
+  GNUNET_free_non_null(plugin->bind_address);
   GNUNET_free (plugin);
   GNUNET_free (api);
   return NULL;