commit c8144356338995a277593bc08315d3b1ddcc8cb3
author Florin Coras <fcoras@cisco.com> Thu Jan 06 11:58:24 2022 -0800
committer Florin Coras <fcoras@cisco.com> Thu Jan 06 16:38:24 2022 -0800
tree 4eed6c04bf1e924e9d7312efe20af34db429c88b
parent ed5be47009aca45008ba9d44e76ac51ec1f7f156

tcp: update snd_congestion only during congestion

If running without sacks, if snd_una does not cover snd_congestion fast
recovery can be missed but the two heuristics from RFC6582 should avoid
that.

Also snd_congestion was used as a means of inferring if the connection
recently exited congestion while setting the persist timer but that does
not always work correctly if not congested.

Type: fix

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: I94d4ac738cdd4f7f23f62e97dd63059de1cd4af9

src/vnet/tcp/tcp.c

diff --git a/src/vnet/tcp/tcp.c b/src/vnet/tcp/tcp.c
index 3d3cc88..ffe5c89 100644
--- a/src/vnet/tcp/tcp.c
+++ b/src/vnet/tcp/tcp.c
@@ -71,6 +71,10 @@
 static void
 tcp_cc_init (tcp_connection_t * tc)
 {
+  /* As per RFC 6582 initialize "recover" to iss */
+  if (tcp_opts_sack_permitted (&tc->rcv_opts))
+    tc->snd_congestion = tc->iss;
+
   tc->cc_algo->init (tc);
 }
 

src/vnet/tcp/tcp_input.c

diff --git a/src/vnet/tcp/tcp_input.c b/src/vnet/tcp/tcp_input.c
index 11491a6..64b9334 100644
--- a/src/vnet/tcp/tcp_input.c
+++ b/src/vnet/tcp/tcp_input.c
@@ -802,15 +802,6 @@
 
   /* If a cumulative ack, make sure dupacks is 0 */
   tc->rcv_dupacks = 0;
-
-  /* When dupacks hits the threshold we only enter fast retransmit if
-   * cumulative ack covers more than snd_congestion. Should snd_una
-   * wrap this test may fail under otherwise valid circumstances.
-   * Therefore, proactively update snd_congestion when wrap detected. */
-  if (PREDICT_FALSE
-      (seq_leq (tc->snd_congestion, tc->snd_una - tc->bytes_acked)
-       && seq_gt (tc->snd_congestion, tc->snd_una)))
-    tc->snd_congestion = tc->snd_una - 1;
 }
 
 /**

src/vnet/tcp/tcp_timer.h

diff --git a/src/vnet/tcp/tcp_timer.h b/src/vnet/tcp/tcp_timer.h
index 5f14a71..7f7dbf1 100644
--- a/src/vnet/tcp/tcp_timer.h
+++ b/src/vnet/tcp/tcp_timer.h
@@ -51,6 +51,13 @@
 						    timer_id, interval);
 }
 
+always_inline u8
+tcp_timer_is_active (tcp_connection_t *tc, tcp_timers_e timer)
+{
+  return tc->timers[timer] != TCP_TIMER_HANDLE_INVALID ||
+	 (tc->pending_timers & (1 << timer));
+}
+
 always_inline void
 tcp_retransmit_timer_set (tcp_timer_wheel_t * tw, tcp_connection_t * tc)
 {
@@ -74,19 +81,6 @@
 }
 
 always_inline void
-tcp_persist_timer_update (tcp_timer_wheel_t * tw, tcp_connection_t * tc)
-{
-  u32 interval;
-
-  if (seq_leq (tc->snd_una, tc->snd_congestion + tc->burst_acked))
-    interval = 1;
-  else
-    interval = clib_max ((u32) tc->rto * TCP_TO_TIMER_TICK, 1);
-
-  tcp_timer_update (tw, tc, TCP_TIMER_PERSIST, interval);
-}
-
-always_inline void
 tcp_persist_timer_reset (tcp_timer_wheel_t * tw, tcp_connection_t * tc)
 {
   tcp_timer_reset (tw, tc, TCP_TIMER_PERSIST);
@@ -98,21 +92,15 @@
   if (tc->snd_una == tc->snd_nxt)
     {
       tcp_retransmit_timer_reset (tw, tc);
-      if (tc->snd_wnd < tc->snd_mss)
-	tcp_persist_timer_update (tw, tc);
+      if (tc->snd_wnd < tc->snd_mss &&
+	  !tcp_timer_is_active (tc, TCP_TIMER_PERSIST))
+	tcp_persist_timer_set (tw, tc);
     }
   else
     tcp_timer_update (tw, tc, TCP_TIMER_RETRANSMIT,
 		      clib_max ((u32) tc->rto * TCP_TO_TIMER_TICK, 1));
 }
 
-always_inline u8
-tcp_timer_is_active (tcp_connection_t * tc, tcp_timers_e timer)
-{
-  return tc->timers[timer] != TCP_TIMER_HANDLE_INVALID
-    || (tc->pending_timers & (1 << timer));
-}
-
 always_inline void
 tcp_timer_expire_timers (tcp_timer_wheel_t * tw, f64 now)
 {