[GitHub] [incubator-nuttx] pkarashchenko commented on a change in pull request #5311: net/tcp/sendfile: fast retransmit on duplicate acknowledgments (RFC 5681)

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pkarashchenko commented on a change in pull request #5311:
URL: https://github.com/apache/incubator-nuttx/pull/5311#discussion_r790054971



##########
File path: net/tcp/tcp_sendfile.c
##########
@@ -242,13 +252,14 @@ static uint16_t sendfile_eventhandler(FAR struct net_driver_s *dev,
         }
 #endif /* CONFIG_NET_IPv4 */
 
-      /* The current acknowledgement number number is the (relative) offset
+      /* The current acknowledgement number is the (relative) offset
        * of the of the next byte needed by the receiver.  The snd_isn is the
        * offset of the first byte to send to the receiver.  The difference
        * is the number of bytes to be acknowledged.
        */
 
-      pstate->snd_acked = TCP_SEQ_SUB(tcp_getsequence(tcp->ackno),
+      ackno = tcp_getsequence(tcp->ackno);
+      pstate->snd_acked = TCP_SEQ_SUB(ackno,

Review comment:
       can we combine to a singe line now?

##########
File path: net/tcp/tcp_sendfile.c
##########
@@ -264,15 +275,42 @@ static uint16_t sendfile_eventhandler(FAR struct net_driver_s *dev,
           goto end_wait;
         }
 
-      /* No. Fall through to send more data if necessary */
+      /* Fast Retransmit (RFC 5681): an acknowledgment is considered a
+       * "duplicate" when (a) the receiver of the ACK has outstanding data,
+       * (b) the incoming acknowledgment carries no data, (c) the SYN and
+       * FIN bits are both off, (d) the acknowledgment number is equal to
+       * the greatest acknowledgment received on the given connection
+       * and (e) the advertised window in the incoming acknowledgment equals
+       * the advertised window in the last incoming acknowledgment.
+       */
+
+      if (pstate->snd_acked < pstate->snd_sent &&
+          (flags & TCP_NEWDATA) == 0 &&
+          (tcp->flags & (TCP_SYN | TCP_FIN)) == 0 &&
+          ackno == pstate->snd_prev_ack &&
+          conn->snd_wnd == pstate->snd_prev_wnd)
+        {
+          if (++pstate->snd_dup_acks >=

Review comment:
       Should we carry for rollover here?




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