pass only CadetTunnelAxolotl if it suffices, preparation for having ambiguous KX...
[oweals/gnunet.git] / src / cadet / gnunet-service-cadet-new_channel.c
index b97b2b577307c67387c7f4971859485a8f2566d9..f5e310cfc51b1855183b01881a26463a3fe83d88 100644 (file)
  * @author Christian Grothoff
  *
  * TODO:
- * - FIXME: send ACKs back to loopback clients!
- *
- * - introduce shutdown so we can have half-closed channels, modify
- *   destroy to include MID to have FIN-ACK equivalents, etc.
- * - estimate max bandwidth using bursts and use to for CONGESTION CONTROL!
- * - check that '0xFFULL' really is sufficient for flow control!
- * - revisit handling of 'unreliable' traffic!
- * - revisit handling of 'out-of-order' option, especially in combination with/without 'reliable'.
- * - figure out flow control without ACKs (unreliable traffic!)
+ * - Congestion/flow control:
+ *   + calculate current RTT if possible, use that for initial retransmissions
+ *     (NOTE: needs us to learn which connection the tunnel uses for the message!)
+ *   + estimate max bandwidth using bursts and use to for CONGESTION CONTROL!
+ *     (and figure out how/where to use this!)
+ *   + figure out flow control without ACKs (unreliable traffic!)
+ * - revisit handling of 'unbuffered' traffic!
+ *   (need to push down through tunnel into connection selection)
+ * - revisit handling of 'buffered' traffic: 4 is a rather small buffer; maybe
+ *   reserve more bits in 'options' to allow for buffer size control?
  */
 #include "platform.h"
 #include "gnunet_util_lib.h"
  */
 #define TIMEOUT_CLOSED_PORT GNUNET_TIME_relative_multiply(GNUNET_TIME_UNIT_SECONDS, 30)
 
+/**
+ * How long do we wait at least before retransmitting ever?
+ */
+#define MIN_RTT_DELAY GNUNET_TIME_relative_multiply(GNUNET_TIME_UNIT_MILLISECONDS, 75)
+
+/**
+ * Maximum message ID into the future we accept for out-of-order messages.
+ * If the message is more than this into the future, we drop it.  This is
+ * important both to detect values that are actually in the past, as well
+ * as to limit adversarially triggerable memory consumption.
+ *
+ * Note that right now we have "max_pending_messages = 4" hard-coded in
+ * the logic below, so a value of 4 would suffice here. But we plan to
+ * allow larger windows in the future...
+ */
+#define MAX_OUT_OF_ORDER_DISTANCE 1024
+
 
 /**
  * All the states a connection can be in.
@@ -192,6 +210,11 @@ struct CadetChannelClient
    */
   struct GNUNET_CADET_ClientChannelNumber ccn;
 
+  /**
+   * Number of entries currently in @a head_recv DLL.
+   */
+  unsigned int num_recv;
+
   /**
    * Can we send data to the client?
    */
@@ -306,6 +329,12 @@ struct CadetChannel
    */
   enum CadetChannelState state;
 
+  /**
+   * Count how many ACKs we skipped, used to prevent long
+   * sequences of ACK skipping.
+   */
+  unsigned int skip_ack_series;
+
   /**
    * Is the tunnel bufferless (minimum latency)?
    */
@@ -391,6 +420,7 @@ free_channel_client (struct CadetChannelClient *ccc)
     GNUNET_CONTAINER_DLL_remove (ccc->head_recv,
                                  ccc->tail_recv,
                                  com);
+    ccc->num_recv--;
     GNUNET_MQ_discard (com->env);
     GNUNET_free (com);
   }
@@ -526,6 +556,7 @@ send_channel_open (void *cls)
                                   &msgcc.header,
                                   &channel_open_sent_cb,
                                   ch);
+  GNUNET_assert (NULL == ch->retry_control_task);
 }
 
 
@@ -578,6 +609,7 @@ GCCH_channel_local_new (struct CadetClient *owner,
   ccco->client_ready = GNUNET_YES;
 
   ch = GNUNET_new (struct CadetChannel);
+  ch->mid_recv.mid = htonl (1); /* The OPEN_ACK counts as message 0! */
   ch->nobuffer = (0 != (options & GNUNET_CADET_OPTION_NOBUFFER));
   ch->reliable = (0 != (options & GNUNET_CADET_OPTION_RELIABLE));
   ch->out_of_order = (0 != (options & GNUNET_CADET_OPTION_OUT_OF_ORDER));
@@ -745,13 +777,20 @@ send_channel_data_ack (struct CadetChannel *ch)
 {
   struct GNUNET_CADET_ChannelDataAckMessage msg;
 
+  if (GNUNET_NO == ch->reliable)
+    return; /* no ACKs */
   msg.header.type = htons (GNUNET_MESSAGE_TYPE_CADET_CHANNEL_APP_DATA_ACK);
   msg.header.size = htons (sizeof (msg));
   msg.ctn = ch->ctn;
-  msg.mid.mid = htonl (ntohl (ch->mid_recv.mid) - 1);
+  msg.mid.mid = htonl (ntohl (ch->mid_recv.mid));
   msg.futures = GNUNET_htonll (ch->mid_futures);
   if (NULL != ch->last_control_qe)
     GCT_send_cancel (ch->last_control_qe);
+  LOG (GNUNET_ERROR_TYPE_DEBUG,
+       "Sending DATA_ACK %u:%llX via %s\n",
+       (unsigned int) ntohl (msg.mid.mid),
+       (unsigned long long) ch->mid_futures,
+       GCCH_2s (ch));
   ch->last_control_qe = GCT_send (ch->t,
                                   &msg.header,
                                   &send_ack_cb,
@@ -836,15 +875,24 @@ send_ack_to_client (struct CadetChannel *ch,
   struct GNUNET_CADET_LocalAck *ack;
   struct CadetChannelClient *ccc;
 
+  ccc = (GNUNET_YES == to_owner) ? ch->owner : ch->dest;
+  if (NULL == ccc)
+  {
+    /* This can happen if we are just getting ACKs after
+       our local client already disconnected. */
+    GNUNET_assert (GNUNET_YES == ch->destroy);
+    return;
+  }
   env = GNUNET_MQ_msg (ack,
                        GNUNET_MESSAGE_TYPE_CADET_LOCAL_ACK);
-  ccc = (GNUNET_YES == to_owner) ? ch->owner : ch->dest;
   ack->ccn = ccc->ccn;
   LOG (GNUNET_ERROR_TYPE_DEBUG,
-       "Sending CADET_LOCAL_ACK to %s (%s) at ccn %X\n",
+       "Sending CADET_LOCAL_ACK to %s (%s) at ccn %X (%u/%u pending)\n",
        GSC_2s (ccc->c),
        (GNUNET_YES == to_owner) ? "owner" : "dest",
-       ntohl (ack->ccn.channel_of_client));
+       ntohl (ack->ccn.channel_of_client),
+       ch->pending_messages,
+       ch->max_pending_messages);
   GSC_send_to_client (ccc->c,
                       env);
 }
@@ -898,7 +946,7 @@ GCCH_bind (struct CadetChannel *ch,
                         options);
   GNUNET_assert (ntohl (cccd->ccn.channel_of_client) <
                  GNUNET_CADET_LOCAL_CHANNEL_ID_CLI);
-  ch->mid_recv.mid = htonl (1); /* The CONNECT counts as message 0! */
+  ch->mid_recv.mid = htonl (1); /* The OPEN counts as message 0! */
   if (GNUNET_YES == ch->is_loopback)
   {
     ch->state = CADET_CHANNEL_OPEN_SENT;
@@ -1036,26 +1084,24 @@ GCCH_handle_channel_open_ack (struct CadetChannel *ch)
 /**
  * Test if element @a e1 comes before element @a e2.
  *
- * TODO: use opportunity to create generic list insertion sort
- * logic in container!
- *
- * @param cls closure, our `struct CadetChannel`
+ * @param cls closure, to a flag where we indicate duplicate packets
  * @param e1 an element of to sort
  * @param e2 another element to sort
  * @return #GNUNET_YES if @e1 < @e2, otherwise #GNUNET_NO
  */
 static int
 is_before (void *cls,
-           void *e1,
-           void *e2)
+           struct CadetOutOfOrderMessage *m1,
+           struct CadetOutOfOrderMessage *m2)
 {
-  struct CadetOutOfOrderMessage *m1 = e1;
-  struct CadetOutOfOrderMessage *m2 = e2;
+  int *duplicate = cls;
   uint32_t v1 = ntohl (m1->mid.mid);
   uint32_t v2 = ntohl (m2->mid.mid);
   uint32_t delta;
 
-  delta = v1 - v2;
+  delta = v2 - v1;
+  if (0 == delta)
+    *duplicate = GNUNET_YES;
   if (delta > (uint32_t) INT_MAX)
   {
     /* in overflow range, we can safely assume we wrapped around */
@@ -1063,6 +1109,7 @@ is_before (void *cls,
   }
   else
   {
+    /* result is small, thus v2 > v1, thus e1 < e2 */
     return GNUNET_YES;
   }
 }
@@ -1082,10 +1129,30 @@ GCCH_handle_channel_plaintext_data (struct CadetChannel *ch,
   struct GNUNET_MQ_Envelope *env;
   struct GNUNET_CADET_LocalData *ld;
   struct CadetChannelClient *ccc;
-  struct CadetOutOfOrderMessage *com;
   size_t payload_size;
+  struct CadetOutOfOrderMessage *com;
+  int duplicate;
+  uint32_t mid_min;
+  uint32_t mid_max;
+  uint32_t mid_msg;
+  uint32_t delta;
 
   GNUNET_assert (GNUNET_NO == ch->is_loopback);
+  if ( (GNUNET_YES == ch->destroy) &&
+       (NULL == ch->owner) &&
+       (NULL == ch->dest) )
+  {
+    /* This client is gone, but we still have messages to send to
+       the other end (which is why @a ch is not yet dead).  However,
+       we cannot pass messages to our client anymore. */
+    LOG (GNUNET_ERROR_TYPE_DEBUG,
+         "Dropping incoming payload on %s as this end is already closed\n",
+         GCCH_2s (ch));
+    /* send back DESTROY notification to stop further retransmissions! */
+    GCT_send_channel_destroy (ch->t,
+                              ch->ctn);
+    return;
+  }
   payload_size = ntohs (msg->header.size) - sizeof (*msg);
   env = GNUNET_MQ_msg_extra (ld,
                              payload_size,
@@ -1100,8 +1167,9 @@ GCCH_handle_channel_plaintext_data (struct CadetChannel *ch,
          (msg->mid.mid == ch->mid_recv.mid) ) )
   {
     LOG (GNUNET_ERROR_TYPE_DEBUG,
-         "Giving %u bytes of payload from %s to client %s\n",
+         "Giving %u bytes of payload with MID %u from %s to client %s\n",
          (unsigned int) payload_size,
+         ntohl (msg->mid.mid),
          GCCH_2s (ch),
          GSC_2s (ccc->c));
     ccc->client_ready = GNUNET_NO;
@@ -1109,59 +1177,208 @@ GCCH_handle_channel_plaintext_data (struct CadetChannel *ch,
                         env);
     ch->mid_recv.mid = htonl (1 + ntohl (ch->mid_recv.mid));
     ch->mid_futures >>= 1;
+    send_channel_data_ack (ch);
+    return;
   }
-  else
-  {
-    /* FIXME-SECURITY: if the element is WAY too far ahead,
-       drop it (can't buffer too much!) */
-    LOG (GNUNET_ERROR_TYPE_DEBUG,
-         "Queuing %s payload of %u bytes on %s (mid %u, need %u first)\n",
-         (GNUNET_YES == ccc->client_ready)
-         ? "out-of-order"
-         : "client-not-ready",
-         (unsigned int) payload_size,
-         GCCH_2s (ch),
-         ntohl (msg->mid.mid),
-         ntohl (ch->mid_recv.mid));
 
-    com = GNUNET_new (struct CadetOutOfOrderMessage);
-    com->mid = msg->mid;
-    com->env = env;
-    /* sort into list ordered by "is_before" */
-    if ( (NULL == ccc->head_recv) ||
-         (GNUNET_YES == is_before (ch,
-                                   com,
-                                   ccc->head_recv)) )
+  if (GNUNET_YES == ch->reliable)
+  {
+    /* check if message ought to be dropped because it is ancient/too distant/duplicate */
+    mid_min = ntohl (ch->mid_recv.mid);
+    mid_max = mid_min + ch->max_pending_messages;
+    mid_msg = ntohl (msg->mid.mid);
+    if ( ( (uint32_t) (mid_msg - mid_min) > ch->max_pending_messages) ||
+         ( (uint32_t) (mid_max - mid_msg) > ch->max_pending_messages) )
     {
-      GNUNET_CONTAINER_DLL_insert (ccc->head_recv,
-                                   ccc->tail_recv,
-                                   com);
+      LOG (GNUNET_ERROR_TYPE_DEBUG,
+           "%s at %u drops ancient or far-future message %u\n",
+           GCCH_2s (ch),
+           (unsigned int) mid_min,
+           ntohl (msg->mid.mid));
+
+      GNUNET_STATISTICS_update (stats,
+                                "# duplicate DATA (ancient or future)",
+                                1,
+                                GNUNET_NO);
+      GNUNET_MQ_discard (env);
+      send_channel_data_ack (ch);
+      return;
     }
-    else
+    /* mark bit for future ACKs */
+    delta = mid_msg - mid_min - 1; /* overflow/underflow are OK here */
+    if (delta < 64)
     {
-      struct CadetOutOfOrderMessage *pos;
-
-      for (pos = ccc->head_recv;
-           NULL != pos;
-           pos = pos->next)
+      if (0 != (ch->mid_futures & (1LLU << delta)))
       {
-        if (GNUNET_YES !=
-            is_before (NULL,
-                       pos,
-                       com))
-          break;
+        /* Duplicate within the queue, drop also */
+        LOG (GNUNET_ERROR_TYPE_DEBUG,
+             "Duplicate payload of %u bytes on %s (mid %u) dropped\n",
+             (unsigned int) payload_size,
+             GCCH_2s (ch),
+             ntohl (msg->mid.mid));
+        GNUNET_STATISTICS_update (stats,
+                                  "# duplicate DATA",
+                                  1,
+                                  GNUNET_NO);
+        GNUNET_MQ_discard (env);
+        send_channel_data_ack (ch);
+        return;
       }
-      if (NULL == pos)
-        GNUNET_CONTAINER_DLL_insert_tail (ccc->head_recv,
-                                          ccc->tail_recv,
-                                          com);
-      else
-        GNUNET_CONTAINER_DLL_insert_after (ccc->head_recv,
-                                           ccc->tail_recv,
-                                           com,
-                                           pos->prev);
+      ch->mid_futures |= (1LLU << delta);
+      LOG (GNUNET_ERROR_TYPE_DEBUG,
+           "Marked bit %llX for mid %u (base: %u); now: %llX\n",
+           (1LLU << delta),
+           mid_msg,
+           mid_min,
+           ch->mid_futures);
+    }
+  }
+  else /* ! ch->reliable */
+  {
+    /* Channel is unreliable, so we do not ACK. But we also cannot
+       allow buffering everything, so check if we have space... */
+    if (ccc->num_recv >= ch->max_pending_messages)
+    {
+      struct CadetOutOfOrderMessage *drop;
+
+      /* Yep, need to drop. Drop the oldest message in
+         the buffer. */
+      drop = ccc->head_recv;
+      GNUNET_CONTAINER_DLL_remove (ccc->head_recv,
+                                   ccc->tail_recv,
+                                   drop);
+      ccc->num_recv--;
+      GNUNET_MQ_discard (drop->env);
+      GNUNET_free (drop);
     }
   }
+
+  /* Insert message into sorted out-of-order queue */
+  com = GNUNET_new (struct CadetOutOfOrderMessage);
+  com->mid = msg->mid;
+  com->env = env;
+  duplicate = GNUNET_NO;
+  GNUNET_CONTAINER_DLL_insert_sorted (struct CadetOutOfOrderMessage,
+                                      is_before,
+                                      &duplicate,
+                                      ccc->head_recv,
+                                      ccc->tail_recv,
+                                      com);
+  ccc->num_recv++;
+  if (GNUNET_YES == duplicate)
+  {
+    /* Duplicate within the queue, drop also (this is not covered by
+       the case above if "delta" >= 64, which could be the case if
+       max_pending_messages is also >= 64 or if our client is unready
+       and we are seeing retransmissions of the message our client is
+       blocked on. */
+    LOG (GNUNET_ERROR_TYPE_DEBUG,
+         "Duplicate payload of %u bytes on %s (mid %u) dropped\n",
+         (unsigned int) payload_size,
+         GCCH_2s (ch),
+         ntohl (msg->mid.mid));
+    GNUNET_STATISTICS_update (stats,
+                              "# duplicate DATA",
+                              1,
+                              GNUNET_NO);
+    GNUNET_CONTAINER_DLL_remove (ccc->head_recv,
+                                 ccc->tail_recv,
+                                 com);
+    ccc->num_recv--;
+    GNUNET_MQ_discard (com->env);
+    GNUNET_free (com);
+    send_channel_data_ack (ch);
+    return;
+  }
+  LOG (GNUNET_ERROR_TYPE_DEBUG,
+       "Queued %s payload of %u bytes on %s-%X(%p) (mid %u, need %u first)\n",
+       (GNUNET_YES == ccc->client_ready)
+       ? "out-of-order"
+       : "client-not-ready",
+       (unsigned int) payload_size,
+       GCCH_2s (ch),
+       ntohl (ccc->ccn.channel_of_client),
+       ccc,
+       ntohl (msg->mid.mid),
+       ntohl (ch->mid_recv.mid));
+  /* NOTE: this ACK we _could_ skip, as the packet is out-of-order and
+     the sender may already be transmitting the previous one.  Needs
+     experimental evaluation to see if/when this ACK helps or
+     hurts. (We might even want another option.) */
+  send_channel_data_ack (ch);
+}
+
+
+/**
+ * Function called once the tunnel has sent one of our messages.
+ * If the message is unreliable, simply frees the `crm`. If the
+ * message was reliable, calculate retransmission time and
+ * wait for ACK (or retransmit).
+ *
+ * @param cls the `struct CadetReliableMessage` that was sent
+ */
+static void
+data_sent_cb (void *cls);
+
+
+/**
+ * We need to retry a transmission, the last one took too long to
+ * be acknowledged.
+ *
+ * @param cls the `struct CadetChannel` where we need to retransmit
+ */
+static void
+retry_transmission (void *cls)
+{
+  struct CadetChannel *ch = cls;
+  struct CadetReliableMessage *crm = ch->head_sent;
+
+  ch->retry_data_task = NULL;
+  GNUNET_assert (NULL == crm->qe);
+  LOG (GNUNET_ERROR_TYPE_DEBUG,
+       "Retrying transmission on %s of message %u\n",
+       GCCH_2s (ch),
+       (unsigned int) ntohl (crm->data_message->mid.mid));
+  crm->qe = GCT_send (ch->t,
+                      &crm->data_message->header,
+                      &data_sent_cb,
+                      crm);
+  GNUNET_assert (NULL == ch->retry_data_task);
+}
+
+
+/**
+ * We got an PLAINTEXT_DATA_ACK for a message in our queue, remove it from
+ * the queue and tell our client that it can send more.
+ *
+ * @param ch the channel that got the PLAINTEXT_DATA_ACK
+ * @param crm the message that got acknowledged
+ */
+static void
+handle_matching_ack (struct CadetChannel *ch,
+                     struct CadetReliableMessage *crm)
+{
+  GNUNET_CONTAINER_DLL_remove (ch->head_sent,
+                               ch->tail_sent,
+                               crm);
+  ch->pending_messages--;
+  GNUNET_assert (ch->pending_messages < ch->max_pending_messages);
+  LOG (GNUNET_ERROR_TYPE_DEBUG,
+       "Received DATA_ACK on %s for message %u (%u ACKs pending)\n",
+       GCCH_2s (ch),
+       (unsigned int) ntohl (crm->data_message->mid.mid),
+       ch->pending_messages);
+  if (NULL != crm->qe)
+  {
+    GCT_send_cancel (crm->qe);
+    crm->qe = NULL;
+  }
+  GNUNET_free (crm->data_message);
+  GNUNET_free (crm);
+  send_ack_to_client (ch,
+                      (NULL == ch->owner)
+                      ? GNUNET_NO
+                      : GNUNET_YES);
 }
 
 
@@ -1177,6 +1394,11 @@ GCCH_handle_channel_plaintext_data_ack (struct CadetChannel *ch,
                                         const struct GNUNET_CADET_ChannelDataAckMessage *ack)
 {
   struct CadetReliableMessage *crm;
+  struct CadetReliableMessage *crmn;
+  int found;
+  uint32_t mid_base;
+  uint64_t mid_mask;
+  unsigned int delta;
 
   GNUNET_break (GNUNET_NO == ch->is_loopback);
   if (GNUNET_NO == ch->reliable)
@@ -1185,12 +1407,51 @@ GCCH_handle_channel_plaintext_data_ack (struct CadetChannel *ch,
     GNUNET_break_op (0);
     return;
   }
+  /* mid_base is the MID of the next message that the
+     other peer expects (i.e. that is missing!), everything
+     LOWER (but excluding mid_base itself) was received. */
+  mid_base = ntohl (ack->mid.mid);
+  mid_mask = GNUNET_htonll (ack->futures);
+  found = GNUNET_NO;
   for (crm = ch->head_sent;
         NULL != crm;
-       crm = crm->next)
-    if (ack->mid.mid == crm->data_message->mid.mid)
-      break;
-  if (NULL == crm)
+       crm = crmn)
+  {
+    crmn = crm->next;
+    delta = (unsigned int) (ntohl (crm->data_message->mid.mid) - mid_base);
+    if (delta >= UINT_MAX - ch->max_pending_messages)
+    {
+      /* overflow, means crm was a bit in the past, so this ACK counts for it. */
+      LOG (GNUNET_ERROR_TYPE_DEBUG,
+           "Got DATA_ACK with base %u satisfying past message %u on %s\n",
+           (unsigned int) mid_base,
+           ntohl (crm->data_message->mid.mid),
+           GCCH_2s (ch));
+      handle_matching_ack (ch,
+                           crm);
+      found = GNUNET_YES;
+      continue;
+    }
+    delta--;
+    if (delta >= 64)
+      continue;
+    LOG (GNUNET_ERROR_TYPE_DEBUG,
+         "Testing bit %llX for mid %u (base: %u)\n",
+         (1LLU << delta),
+         ntohl (crm->data_message->mid.mid),
+         mid_base);
+    if (0 != (mid_mask & (1LLU << delta)))
+    {
+      LOG (GNUNET_ERROR_TYPE_DEBUG,
+           "Got DATA_ACK with mask for %u on %s\n",
+           ntohl (crm->data_message->mid.mid),
+           GCCH_2s (ch));
+      handle_matching_ack (ch,
+                           crm);
+      found = GNUNET_YES;
+    }
+  }
+  if (GNUNET_NO == found)
   {
     /* ACK for message we already dropped, might have been a
        duplicate ACK? Ignore. */
@@ -1203,26 +1464,17 @@ GCCH_handle_channel_plaintext_data_ack (struct CadetChannel *ch,
                               GNUNET_NO);
     return;
   }
-  GNUNET_CONTAINER_DLL_remove (ch->head_sent,
-                               ch->tail_sent,
-                               crm);
-  GNUNET_free (crm->data_message);
-  GNUNET_free (crm);
-  ch->pending_messages--;
-  send_ack_to_client (ch,
-                      (NULL == ch->owner)
-                      ? GNUNET_NO
-                      : GNUNET_YES);
-  GNUNET_assert (ch->pending_messages < ch->max_pending_messages);
-  LOG (GNUNET_ERROR_TYPE_DEBUG,
-       "Received DATA_ACK on %s for message %u (%u ACKs pending)\n",
-       GCCH_2s (ch),
-       (unsigned int) ntohl (ack->mid.mid),
-       ch->pending_messages);
-  send_ack_to_client (ch,
-                      (NULL == ch->owner)
-                      ? GNUNET_NO
-                      : GNUNET_YES);
+  if (NULL != ch->retry_data_task)
+  {
+    GNUNET_SCHEDULER_cancel (ch->retry_data_task);
+    ch->retry_data_task = NULL;
+  }
+  if ( (NULL != ch->head_sent) &&
+       (NULL == ch->head_sent->qe) )
+    ch->retry_data_task
+      = GNUNET_SCHEDULER_add_at (ch->head_sent->next_retry,
+                                 &retry_transmission,
+                                 ch);
 }
 
 
@@ -1265,35 +1517,22 @@ GCCH_handle_remote_destroy (struct CadetChannel *ch)
 
 
 /**
- * Function called once the tunnel has sent one of our messages.
- * If the message is unreliable, simply frees the `crm`. If the
- * message was reliable, calculate retransmission time and
- * wait for ACK (or retransmit).
- *
- * @param cls the `struct CadetReliableMessage` that was sent
- */
-static void
-data_sent_cb (void *cls);
-
-
-/**
- * We need to retry a transmission, the last one took too long to
- * be acknowledged.
+ * Test if element @a e1 comes before element @a e2.
  *
- * @param cls the `struct CadetChannel` where we need to retransmit
+ * @param cls closure, to a flag where we indicate duplicate packets
+ * @param crm1 an element of to sort
+ * @param crm2 another element to sort
+ * @return #GNUNET_YES if @e1 < @e2, otherwise #GNUNET_NO
  */
-static void
-retry_transmission (void *cls)
+static int
+cmp_crm_by_next_retry (void *cls,
+                       struct CadetReliableMessage *crm1,
+                       struct CadetReliableMessage *crm2)
 {
-  struct CadetChannel *ch = cls;
-  struct CadetReliableMessage *crm = ch->head_sent;
-
-  ch->retry_data_task = NULL;
-  GNUNET_assert (NULL == crm->qe);
-  crm->qe = GCT_send (ch->t,
-                      &crm->data_message->header,
-                      &data_sent_cb,
-                      crm);
+  if (crm1->next_retry.abs_value_us <
+      crm2->next_retry.abs_value_us)
+    return GNUNET_YES;
+  return GNUNET_NO;
 }
 
 
@@ -1310,15 +1549,16 @@ data_sent_cb (void *cls)
 {
   struct CadetReliableMessage *crm = cls;
   struct CadetChannel *ch = crm->ch;
-  struct CadetReliableMessage *off;
 
   GNUNET_assert (GNUNET_NO == ch->is_loopback);
+  GNUNET_assert (NULL != crm->qe);
   crm->qe = NULL;
   GNUNET_CONTAINER_DLL_remove (ch->head_sent,
                                ch->tail_sent,
                                crm);
   if (GNUNET_NO == ch->reliable)
   {
+    GNUNET_free (crm->data_message);
     GNUNET_free (crm);
     ch->pending_messages--;
     send_ack_to_client (ch,
@@ -1329,41 +1569,32 @@ data_sent_cb (void *cls)
   }
   if (0 == crm->retry_delay.rel_value_us)
     crm->retry_delay = ch->expected_delay;
+  else
+    crm->retry_delay = GNUNET_TIME_STD_BACKOFF (crm->retry_delay);
+  crm->retry_delay = GNUNET_TIME_relative_max (crm->retry_delay,
+                                               MIN_RTT_DELAY);
   crm->next_retry = GNUNET_TIME_relative_to_absolute (crm->retry_delay);
 
-  /* find position for re-insertion into the DLL */
-  if ( (NULL == ch->head_sent) ||
-       (crm->next_retry.abs_value_us < ch->head_sent->next_retry.abs_value_us) )
+  GNUNET_CONTAINER_DLL_insert_sorted (struct CadetReliableMessage,
+                                      cmp_crm_by_next_retry,
+                                      NULL,
+                                      ch->head_sent,
+                                      ch->tail_sent,
+                                      crm);
+  LOG (GNUNET_ERROR_TYPE_DEBUG,
+       "Message %u sent, next transmission on %s in %s\n",
+       (unsigned int) ntohl (crm->data_message->mid.mid),
+       GCCH_2s (ch),
+       GNUNET_STRINGS_relative_time_to_string (GNUNET_TIME_absolute_get_remaining (ch->head_sent->next_retry),
+                                               GNUNET_YES));
+  if (NULL == ch->head_sent->qe)
   {
-    /* insert at HEAD, also (re)schedule retry task! */
-    GNUNET_CONTAINER_DLL_insert (ch->head_sent,
-                                 ch->tail_sent,
-                                 crm);
     if (NULL != ch->retry_data_task)
       GNUNET_SCHEDULER_cancel (ch->retry_data_task);
     ch->retry_data_task
-      = GNUNET_SCHEDULER_add_delayed (crm->retry_delay,
-                                      &retry_transmission,
-                                      ch);
-    return;
-  }
-  for (off = ch->head_sent; NULL != off; off = off->next)
-    if (crm->next_retry.abs_value_us < off->next_retry.abs_value_us)
-      break;
-  if (NULL == off)
-  {
-    /* insert at tail */
-    GNUNET_CONTAINER_DLL_insert_tail (ch->head_sent,
-                                      ch->tail_sent,
-                                      crm);
-  }
-  else
-  {
-    /* insert before off */
-    GNUNET_CONTAINER_DLL_insert_after (ch->head_sent,
-                                       ch->tail_sent,
-                                       off->prev,
-                                       crm);
+      = GNUNET_SCHEDULER_add_at (ch->head_sent->next_retry,
+                                 &retry_transmission,
+                                 ch);
   }
 }
 
@@ -1424,11 +1655,24 @@ GCCH_handle_local_data (struct CadetChannel *ch,
     GNUNET_memcpy (&ld[1],
                    buf,
                    buf_len);
-    /* FIXME: this does not provide for flow control! */
-    GSC_send_to_client (receiver->c,
-                        env);
-    send_ack_to_client (ch,
-                        to_owner);
+    if (GNUNET_YES == receiver->client_ready)
+    {
+      GSC_send_to_client (receiver->c,
+                          env);
+      send_ack_to_client (ch,
+                          to_owner);
+    }
+    else
+    {
+      struct CadetOutOfOrderMessage *oom;
+
+      oom = GNUNET_new (struct CadetOutOfOrderMessage);
+      oom->env = env;
+      GNUNET_CONTAINER_DLL_insert_tail (receiver->head_recv,
+                                        receiver->tail_recv,
+                                        oom);
+      receiver->num_recv++;
+    }
     return GNUNET_OK;
   }
 
@@ -1445,17 +1689,24 @@ GCCH_handle_local_data (struct CadetChannel *ch,
   GNUNET_memcpy (&crm->data_message[1],
                  buf,
                  buf_len);
-  GNUNET_CONTAINER_DLL_insert (ch->head_sent,
-                               ch->tail_sent,
-                               crm);
+  GNUNET_CONTAINER_DLL_insert_tail (ch->head_sent,
+                                    ch->tail_sent,
+                                    crm);
   LOG (GNUNET_ERROR_TYPE_DEBUG,
-       "Sending %u bytes from local client to %s\n",
-       buf_len,
-       GCCH_2s (ch));
+       "Sending message %u from local client to %s with %u bytes\n",
+       ntohl (crm->data_message->mid.mid),
+       GCCH_2s (ch),
+       buf_len);
+  if (NULL != ch->retry_data_task)
+  {
+    GNUNET_SCHEDULER_cancel (ch->retry_data_task);
+    ch->retry_data_task = NULL;
+  }
   crm->qe = GCT_send (ch->t,
                       &crm->data_message->header,
                       &data_sent_cb,
                       crm);
+  GNUNET_assert (NULL == ch->retry_data_task);
   return GNUNET_OK;
 }
 
@@ -1483,46 +1734,81 @@ GCCH_handle_local_ack (struct CadetChannel *ch,
   else
     GNUNET_assert (0);
   ccc->client_ready = GNUNET_YES;
-  LOG (GNUNET_ERROR_TYPE_DEBUG,
-       "Got LOCAL_ACK, client ready to receive more data!\n");
   com = ccc->head_recv;
   if (NULL == com)
+  {
+    LOG (GNUNET_ERROR_TYPE_DEBUG,
+         "Got LOCAL_ACK, %s-%X ready to receive more data, but none pending on %s-%X(%p)!\n",
+         GSC_2s (ccc->c),
+         ntohl (client_ccn.channel_of_client),
+         GCCH_2s (ch),
+         ntohl (ccc->ccn.channel_of_client),
+         ccc);
     return; /* none pending */
+  }
+  if (GNUNET_YES == ch->is_loopback)
+  {
+    int to_owner;
+
+    /* Messages are always in-order, just send */
+    GNUNET_CONTAINER_DLL_remove (ccc->head_recv,
+                                 ccc->tail_recv,
+                                 com);
+    ccc->num_recv--;
+    GSC_send_to_client (ccc->c,
+                        com->env);
+    /* Notify sender that we can receive more */
+    if (ccc->ccn.channel_of_client ==
+        ch->owner->ccn.channel_of_client)
+    {
+      to_owner = GNUNET_NO;
+    }
+    else
+    {
+      GNUNET_assert (ccc->ccn.channel_of_client ==
+                     ch->dest->ccn.channel_of_client);
+      to_owner = GNUNET_YES;
+    }
+    send_ack_to_client (ch,
+                        to_owner);
+    GNUNET_free (com);
+    return;
+  }
+
   if ( (com->mid.mid != ch->mid_recv.mid) &&
        (GNUNET_NO == ch->out_of_order) )
+  {
+    LOG (GNUNET_ERROR_TYPE_DEBUG,
+         "Got LOCAL_ACK, %s-%X ready to receive more data (but next one is out-of-order %u vs. %u)!\n",
+         GSC_2s (ccc->c),
+         ntohl (ccc->ccn.channel_of_client),
+         ntohl (com->mid.mid),
+         ntohl (ch->mid_recv.mid));
     return; /* missing next one in-order */
+  }
 
-  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
-              "Passing payload message to client on %s\n",
-              GCCH_2s (ch));
+  LOG (GNUNET_ERROR_TYPE_DEBUG,
+       "Got LOCAL_ACK, giving payload message %u to %s-%X on %s\n",
+       ntohl (com->mid.mid),
+       GSC_2s (ccc->c),
+       ntohl (ccc->ccn.channel_of_client),
+       GCCH_2s (ch));
 
   /* all good, pass next message to client */
   GNUNET_CONTAINER_DLL_remove (ccc->head_recv,
                                ccc->tail_recv,
                                com);
+  ccc->num_recv--;
   /* FIXME: if unreliable, this is not aggressive
      enough, as it would be OK to have lost some! */
+
   ch->mid_recv.mid = htonl (1 + ntohl (com->mid.mid));
   ch->mid_futures >>= 1; /* equivalent to division by 2 */
   ccc->client_ready = GNUNET_NO;
   GSC_send_to_client (ccc->c,
                       com->env);
   GNUNET_free (com);
-  if ( (0xFFULL == (ch->mid_futures & 0xFFULL)) &&
-       (GNUNET_YES == ch->reliable) )
-  {
-    /* The next 15 messages were also already received (0xFF), this
-       suggests that the sender may be blocked on flow control
-       urgently waiting for an ACK from us. (As we have an inherent
-       maximum of 64 bits, and 15 is getting too close for comfort.)
-       So we should send one now. */
-    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
-                "Sender on %s likely blocked on flow-control, sending ACK now.\n",
-                GCCH_2s (ch));
-    if (GNUNET_YES == ch->reliable)
-      send_channel_data_ack (ch);
-  }
-
+  send_channel_data_ack (ch);
   if (NULL != ccc->head_recv)
     return;
   if (GNUNET_NO == ch->destroy)