TCP/session improvements

- Added svm fifo flag for tracking fifo dequeue events (replaces event
  length). Updated all code to switch to the new scheme.
- More session debugging
- Fix peek index wrap
- Add a trivial socket test client
- Fast retransmit/cc fixes
- tx and rx SACK fixes and unit testing
- SRTT computation fix
- remove dupack/ack burst filters
- improve ack rx
- improved segment rx
- builtin client test code

Change-Id: Ic4eb2d5ca446eb2260ccd3ccbcdaa73c64e7f4e1
Signed-off-by: Florin Coras <fcoras@cisco.com>
Signed-off-by: Dave Barach <dbarach@cisco.com>

1 files changed, 327 insertions, 180 deletions

diff --git a/src/vnet/tcp/tcp_input.c b/src/vnet/tcp/tcp_input.c
index 67af4321..5d11985f 100644
--- a/src/vnet/tcp/tcp_input.c
+++ b/src/vnet/tcp/tcp_input.c
@@ -95,13 +95,21 @@ vlib_node_registration_t tcp6_established_node;
  * or the rcv_nxt at last ack sent instead of rcv_nxt since that's the
  * peer's reference when computing our receive window.
  *
- * This accepts only segments within the window.
+ * This:
+ *  seq_leq (end_seq, tc->rcv_las + tc->rcv_wnd) && seq_geq (seq, tc->rcv_las)
+ * however, is too strict when we have retransmits. Instead we just check that
+ * the seq is not beyond the right edge and that the end of the segment is not
+ * less than the left edge.
+ *
+ * N.B. rcv_nxt and rcv_wnd are both updated in this node if acks are sent, so
+ * use rcv_nxt in the right edge window test instead of rcv_las.
+ *
  */
 always_inline u8
 tcp_segment_in_rcv_wnd (tcp_connection_t * tc, u32 seq, u32 end_seq)
 {
-  return seq_leq (end_seq, tc->rcv_las + tc->rcv_wnd)
-    && seq_geq (seq, tc->rcv_nxt);
+  return (seq_geq (end_seq, tc->rcv_las)
+	  && seq_leq (seq, tc->rcv_nxt + tc->rcv_wnd));
 }
 
 void
@@ -253,6 +261,7 @@ tcp_segment_validate (vlib_main_t * vm, tcp_connection_t * tc0,
 	    {
 	      tcp_make_ack (tc0, b0);
 	      *next0 = tcp_next_output (tc0->c_is_ip4);
+	      TCP_EVT_DBG (TCP_EVT_DUPACK_SENT, tc0);
 	      return -1;
 	    }
 	}
@@ -262,13 +271,25 @@ tcp_segment_validate (vlib_main_t * vm, tcp_connection_t * tc0,
   if (!tcp_segment_in_rcv_wnd (tc0, vnet_buffer (b0)->tcp.seq_number,
 			       vnet_buffer (b0)->tcp.seq_end))
     {
-      if (!tcp_rst (th0))
+      /* If our window is 0 and the packet is in sequence, let it pass
+       * through for ack processing. It should be dropped later.*/
+      if (tc0->rcv_wnd == 0
+	  && tc0->rcv_nxt == vnet_buffer (b0)->tcp.seq_number)
 	{
-	  /* Send dup ack */
-	  tcp_make_ack (tc0, b0);
-	  *next0 = tcp_next_output (tc0->c_is_ip4);
+	  /* Make it look as if there's nothing to dequeue */
+	  vnet_buffer (b0)->tcp.seq_end = vnet_buffer (b0)->tcp.seq_number;
+	}
+      else
+	{
+	  /* If not RST, send dup ack */
+	  if (!tcp_rst (th0))
+	    {
+	      tcp_make_ack (tc0, b0);
+	      *next0 = tcp_next_output (tc0->c_is_ip4);
+	      TCP_EVT_DBG (TCP_EVT_DUPACK_SENT, tc0);
+	    }
+	  return -1;
 	}
-      return -1;
     }
 
   /* 2nd: check the RST bit */
@@ -326,13 +347,13 @@ tcp_estimate_rtt (tcp_connection_t * tc, u32 mrtt)
 
       /* XXX Drop in RTT results in RTTVAR increase and bigger RTO.
        * The increase should be bound */
-      tc->rttvar += (clib_abs (err) - tc->rttvar) >> 2;
+      tc->rttvar += ((int) clib_abs (err) - (int) tc->rttvar) >> 2;
     }
   else
     {
       /* First measurement. */
       tc->srtt = mrtt;
-      tc->rttvar = mrtt << 1;
+      tc->rttvar = mrtt >> 1;
     }
 }
 
@@ -394,7 +415,11 @@ tcp_dequeue_acked (tcp_connection_t * tc, u32 ack)
     }
 }
 
-/** Check if dupack as per RFC5681 Sec. 2 */
+/**
+ * Check if dupack as per RFC5681 Sec. 2
+ *
+ * This works only if called before updating snd_wnd.
+ * */
 always_inline u8
 tcp_ack_is_dupack (tcp_connection_t * tc, vlib_buffer_t * b, u32 new_snd_wnd)
 {
@@ -429,10 +454,10 @@ scoreboard_remove_hole (sack_scoreboard_t * sb, sack_scoreboard_hole_t * hole)
 }
 
 sack_scoreboard_hole_t *
-scoreboard_insert_hole (sack_scoreboard_t * sb, sack_scoreboard_hole_t * prev,
+scoreboard_insert_hole (sack_scoreboard_t * sb, u32 prev_index,
 			u32 start, u32 end)
 {
-  sack_scoreboard_hole_t *hole, *next;
+  sack_scoreboard_hole_t *hole, *next, *prev;
   u32 hole_index;
 
   pool_get (sb->holes, hole);
@@ -442,6 +467,7 @@ scoreboard_insert_hole (sack_scoreboard_t * sb, sack_scoreboard_hole_t * prev,
   hole->end = end;
   hole_index = hole - sb->holes;
 
+  prev = scoreboard_get_hole (sb, prev_index);
   if (prev)
     {
       hole->prev = prev - sb->holes;
@@ -462,28 +488,35 @@ scoreboard_insert_hole (sack_scoreboard_t * sb, sack_scoreboard_hole_t * prev,
   return hole;
 }
 
-static void
+void
 tcp_rcv_sacks (tcp_connection_t * tc, u32 ack)
 {
   sack_scoreboard_t *sb = &tc->sack_sb;
   sack_block_t *blk, tmp;
-  sack_scoreboard_hole_t *hole, *next_hole;
-  u32 blk_index = 0;
+  sack_scoreboard_hole_t *hole, *next_hole, *last_hole, *new_hole;
+  u32 blk_index = 0, old_sacked_bytes, hole_index;
   int i, j;
 
-  if (!tcp_opts_sack (tc) && sb->head == TCP_INVALID_SACK_HOLE_INDEX)
+  sb->last_sacked_bytes = 0;
+  sb->snd_una_adv = 0;
+  old_sacked_bytes = sb->sacked_bytes;
+
+  if (!tcp_opts_sack (&tc->opt) && sb->head == TCP_INVALID_SACK_HOLE_INDEX)
     return;
 
   /* Remove invalid blocks */
-  vec_foreach (blk, tc->opt.sacks)
-  {
-    if (seq_lt (blk->start, blk->end)
-	&& seq_gt (blk->start, tc->snd_una)
-	&& seq_gt (blk->start, ack) && seq_lt (blk->end, tc->snd_nxt))
-      continue;
-
-    vec_del1 (tc->opt.sacks, blk - tc->opt.sacks);
-  }
+  blk = tc->opt.sacks;
+  while (blk < vec_end (tc->opt.sacks))
+    {
+      if (seq_lt (blk->start, blk->end)
+	  && seq_gt (blk->start, tc->snd_una)
+	  && seq_gt (blk->start, ack) && seq_leq (blk->end, tc->snd_nxt))
+	{
+	  blk++;
+	  continue;
+	}
+      vec_del1 (tc->opt.sacks, blk - tc->opt.sacks);
+    }
 
   /* Add block for cumulative ack */
   if (seq_gt (ack, tc->snd_una))
@@ -498,7 +531,7 @@ tcp_rcv_sacks (tcp_connection_t * tc, u32 ack)
 
   /* Make sure blocks are ordered */
   for (i = 0; i < vec_len (tc->opt.sacks); i++)
-    for (j = i; j < vec_len (tc->opt.sacks); j++)
+    for (j = i + 1; j < vec_len (tc->opt.sacks); j++)
       if (seq_lt (tc->opt.sacks[j].start, tc->opt.sacks[i].start))
 	{
 	  tmp = tc->opt.sacks[i];
@@ -506,10 +539,22 @@ tcp_rcv_sacks (tcp_connection_t * tc, u32 ack)
 	  tc->opt.sacks[j] = tmp;
 	}
 
-  /* If no holes, insert the first that covers all outstanding bytes */
   if (sb->head == TCP_INVALID_SACK_HOLE_INDEX)
     {
-      scoreboard_insert_hole (sb, 0, tc->snd_una, tc->snd_una_max);
+      /* If no holes, insert the first that covers all outstanding bytes */
+      last_hole = scoreboard_insert_hole (sb, TCP_INVALID_SACK_HOLE_INDEX,
+					  tc->snd_una, tc->snd_una_max);
+      sb->tail = scoreboard_hole_index (sb, last_hole);
+    }
+  else
+    {
+      /* If we have holes but snd_una_max is beyond the last hole, update
+       * last hole end */
+      tmp = tc->opt.sacks[vec_len (tc->opt.sacks) - 1];
+      last_hole = scoreboard_last_hole (sb);
+      if (seq_gt (tc->snd_una_max, sb->max_byte_sacked)
+	  && seq_gt (tc->snd_una_max, last_hole->end))
+	last_hole->end = tc->snd_una_max;
     }
 
   /* Walk the holes with the SACK blocks */
@@ -526,10 +571,10 @@ tcp_rcv_sacks (tcp_connection_t * tc, u32 ack)
 	      next_hole = scoreboard_next_hole (sb, hole);
 
 	      /* Byte accounting */
-	      if (seq_lt (hole->end, ack))
+	      if (seq_leq (hole->end, ack))
 		{
-		  /* Bytes lost because snd wnd left edge advances */
-		  if (seq_lt (next_hole->start, ack))
+		  /* Bytes lost because snd_wnd left edge advances */
+		  if (next_hole && seq_leq (next_hole->start, ack))
 		    sb->sacked_bytes -= next_hole->start - hole->end;
 		  else
 		    sb->sacked_bytes -= ack - hole->end;
@@ -539,35 +584,78 @@ tcp_rcv_sacks (tcp_connection_t * tc, u32 ack)
 		  sb->sacked_bytes += scoreboard_hole_bytes (hole);
 		}
 
+	      /* snd_una needs to be advanced */
+	      if (seq_geq (ack, hole->end))
+		{
+		  if (next_hole && seq_lt (ack, next_hole->start))
+		    sb->snd_una_adv = next_hole->start - ack;
+		  else
+		    sb->snd_una_adv = sb->max_byte_sacked - ack;
+
+		  /* all these can be delivered */
+		  sb->sacked_bytes -= sb->snd_una_adv;
+		}
+
+	      /* About to remove last hole */
+	      if (hole == last_hole)
+		{
+		  sb->tail = hole->prev;
+		  last_hole = scoreboard_last_hole (sb);
+		  /* keep track of max byte sacked in case the last hole
+		   * is acked */
+		  if (seq_gt (hole->end, sb->max_byte_sacked))
+		    sb->max_byte_sacked = hole->end;
+		}
 	      scoreboard_remove_hole (sb, hole);
 	      hole = next_hole;
 	    }
-	  /* Partial overlap */
+	  /* Partial 'head' overlap */
 	  else
 	    {
-	      sb->sacked_bytes += blk->end - hole->start;
-	      hole->start = blk->end;
+	      if (seq_gt (blk->end, hole->start))
+		{
+		  sb->sacked_bytes += blk->end - hole->start;
+		  hole->start = blk->end;
+		}
 	      blk_index++;
 	    }
 	}
       else
 	{
 	  /* Hole must be split */
-	  if (seq_leq (blk->end, hole->end))
+	  if (seq_lt (blk->end, hole->end))
 	    {
 	      sb->sacked_bytes += blk->end - blk->start;
-	      scoreboard_insert_hole (sb, hole, blk->end, hole->end);
-	      hole->end = blk->start - 1;
+	      hole_index = scoreboard_hole_index (sb, hole);
+	      new_hole = scoreboard_insert_hole (sb, hole_index, blk->end,
+						 hole->end);
+
+	      /* Pool might've moved */
+	      hole = scoreboard_get_hole (sb, hole_index);
+	      hole->end = blk->start;
+
+	      /* New or split of tail */
+	      if ((last_hole->end == new_hole->end)
+		  || seq_lt (last_hole->end, new_hole->start))
+		{
+		  last_hole = new_hole;
+		  sb->tail = scoreboard_hole_index (sb, new_hole);
+		}
+
 	      blk_index++;
+	      hole = scoreboard_next_hole (sb, hole);
 	    }
 	  else
 	    {
-	      sb->sacked_bytes += hole->end - blk->start + 1;
-	      hole->end = blk->start - 1;
+	      sb->sacked_bytes += hole->end - blk->start;
+	      hole->end = blk->start;
 	      hole = scoreboard_next_hole (sb, hole);
 	    }
 	}
     }
+
+  sb->last_sacked_bytes = sb->sacked_bytes + sb->snd_una_adv
+    - old_sacked_bytes;
 }
 
 /** Update snd_wnd
@@ -577,72 +665,94 @@ tcp_rcv_sacks (tcp_connection_t * tc, u32 ack)
 static void
 tcp_update_snd_wnd (tcp_connection_t * tc, u32 seq, u32 ack, u32 snd_wnd)
 {
-  if (tc->snd_wl1 < seq || (tc->snd_wl1 == seq && tc->snd_wl2 <= ack))
+  if (seq_lt (tc->snd_wl1, seq)
+      || (tc->snd_wl1 == seq && seq_leq (tc->snd_wl2, ack)))
     {
       tc->snd_wnd = snd_wnd;
       tc->snd_wl1 = seq;
       tc->snd_wl2 = ack;
+      TCP_EVT_DBG (TCP_EVT_SND_WND, tc);
     }
 }
 
-static void
+void
 tcp_cc_congestion (tcp_connection_t * tc)
 {
+  tc->snd_congestion = tc->snd_nxt;
   tc->cc_algo->congestion (tc);
+  TCP_EVT_DBG (TCP_EVT_CC_EVT, tc, 4);
 }
 
-static void
+void
 tcp_cc_recover (tcp_connection_t * tc)
 {
-  if (tcp_in_fastrecovery (tc))
-    {
-      tc->cc_algo->recovered (tc);
-      tcp_recovery_off (tc);
-    }
-  else if (tcp_in_recovery (tc))
-    {
-      tcp_recovery_off (tc);
-      tc->cwnd = tcp_loss_wnd (tc);
-    }
+  tc->cc_algo->recovered (tc);
+
+  tc->rtx_bytes = 0;
+  tc->rcv_dupacks = 0;
+  tc->snd_nxt = tc->snd_una;
+
+  tc->cc_algo->rcv_ack (tc);
+  tc->tsecr_last_ack = tc->opt.tsecr;
+
+  tcp_fastrecovery_1_smss_off (tc);
+  tcp_fastrecovery_off (tc);
+
+  TCP_EVT_DBG (TCP_EVT_CC_EVT, tc, 3);
 }
 
 static void
-tcp_cc_rcv_ack (tcp_connection_t * tc)
+tcp_cc_rcv_ack (tcp_connection_t * tc, vlib_buffer_t * b)
 {
   u8 partial_ack;
 
   if (tcp_in_recovery (tc))
     {
-      partial_ack = seq_lt (tc->snd_una, tc->snd_una_max);
+      partial_ack = seq_lt (tc->snd_una, tc->snd_congestion);
       if (!partial_ack)
 	{
 	  /* Clear retransmitted bytes. */
-	  tc->rtx_bytes = 0;
 	  tcp_cc_recover (tc);
 	}
       else
 	{
+	  TCP_EVT_DBG (TCP_EVT_CC_PACK, tc);
+
 	  /* Clear retransmitted bytes. XXX should we clear all? */
 	  tc->rtx_bytes = 0;
 	  tc->cc_algo->rcv_cong_ack (tc, TCP_CC_PARTIALACK);
 
-	  /* Retransmit first unacked segment */
-	  tcp_retransmit_first_unacked (tc);
+	  /* In case snd_nxt is still in the past and output tries to
+	   * shove some new bytes */
+	  tc->snd_nxt = tc->snd_una;
+
+	  /* XXX need proper RFC6675 support */
+	  if (tc->sack_sb.last_sacked_bytes)
+	    {
+	      tcp_fast_retransmit (tc);
+	    }
+	  else
+	    {
+	      /* Retransmit first unacked segment */
+	      tcp_retransmit_first_unacked (tc);
+	      /* If window allows, send 1 SMSS of new data */
+	      if (seq_lt (tc->snd_nxt, tc->snd_congestion))
+		tc->snd_nxt = tc->snd_congestion;
+	    }
 	}
     }
   else
     {
       tc->cc_algo->rcv_ack (tc);
+      tc->tsecr_last_ack = tc->opt.tsecr;
+      tc->rcv_dupacks = 0;
     }
-
-  tc->rcv_dupacks = 0;
-  tc->tsecr_last_ack = tc->opt.tsecr;
 }
 
 static void
 tcp_cc_rcv_dupack (tcp_connection_t * tc, u32 ack)
 {
-  ASSERT (tc->snd_una == ack);
+//  ASSERT (seq_geq(tc->snd_una, ack));
 
   tc->rcv_dupacks++;
   if (tc->rcv_dupacks == TCP_DUPACK_THRESHOLD)
@@ -688,20 +798,39 @@ tcp_rcv_ack (tcp_connection_t * tc, vlib_buffer_t * b,
 {
   u32 new_snd_wnd;
 
-  /* If the ACK acks something not yet sent (SEG.ACK > SND.NXT) then send an
-   * ACK, drop the segment, and return  */
+  /* If the ACK acks something not yet sent (SEG.ACK > SND.NXT) */
   if (seq_gt (vnet_buffer (b)->tcp.ack_number, tc->snd_nxt))
     {
-      tcp_make_ack (tc, b);
-      *next = tcp_next_output (tc->c_is_ip4);
-      *error = TCP_ERROR_ACK_INVALID;
-      return -1;
+      /* If we have outstanding data and this is within the window, accept it,
+       * probably retransmit has timed out. Otherwise ACK segment and then
+       * drop it */
+      if (seq_gt (vnet_buffer (b)->tcp.ack_number, tc->snd_una_max))
+	{
+	  tcp_make_ack (tc, b);
+	  *next = tcp_next_output (tc->c_is_ip4);
+	  *error = TCP_ERROR_ACK_INVALID;
+	  TCP_EVT_DBG (TCP_EVT_ACK_RCV_ERR, tc, 0,
+		       vnet_buffer (b)->tcp.ack_number);
+	  return -1;
+	}
+
+      tc->snd_nxt = vnet_buffer (b)->tcp.ack_number;
+      *error = TCP_ERROR_ACK_FUTURE;
+      TCP_EVT_DBG (TCP_EVT_ACK_RCV_ERR, tc, 2,
+		   vnet_buffer (b)->tcp.ack_number);
     }
 
-  /* If old ACK, discard */
+  /* If old ACK, probably it's an old dupack */
   if (seq_lt (vnet_buffer (b)->tcp.ack_number, tc->snd_una))
     {
       *error = TCP_ERROR_ACK_OLD;
+      TCP_EVT_DBG (TCP_EVT_ACK_RCV_ERR, tc, 1,
+		   vnet_buffer (b)->tcp.ack_number);
+      if (tcp_in_fastrecovery (tc) && tc->rcv_dupacks == TCP_DUPACK_THRESHOLD)
+	{
+	  TCP_EVT_DBG (TCP_EVT_DUPACK_RCVD, tc);
+	  tcp_cc_rcv_dupack (tc, vnet_buffer (b)->tcp.ack_number);
+	}
       return -1;
     }
 
@@ -712,32 +841,40 @@ tcp_rcv_ack (tcp_connection_t * tc, vlib_buffer_t * b,
 
   if (tcp_ack_is_dupack (tc, b, new_snd_wnd))
     {
+      TCP_EVT_DBG (TCP_EVT_DUPACK_RCVD, tc, 1);
       tcp_cc_rcv_dupack (tc, vnet_buffer (b)->tcp.ack_number);
       *error = TCP_ERROR_ACK_DUP;
       return -1;
     }
 
-  /* Valid ACK */
+  /*
+   * Valid ACK
+   */
+
   tc->bytes_acked = vnet_buffer (b)->tcp.ack_number - tc->snd_una;
-  tc->snd_una = vnet_buffer (b)->tcp.ack_number;
+  tc->snd_una = vnet_buffer (b)->tcp.ack_number + tc->sack_sb.snd_una_adv;
 
-  /* Dequeue ACKed packet and update RTT */
+  /* Dequeue ACKed data and update RTT */
   tcp_dequeue_acked (tc, vnet_buffer (b)->tcp.ack_number);
-
   tcp_update_snd_wnd (tc, vnet_buffer (b)->tcp.seq_number,
 		      vnet_buffer (b)->tcp.ack_number, new_snd_wnd);
 
-  /* Updates congestion control (slow start/congestion avoidance) */
-  tcp_cc_rcv_ack (tc);
+  /* If some of our sent bytes have been acked, update cc and retransmit
+   * timer. */
+  if (tc->bytes_acked)
+    {
+      TCP_EVT_DBG (TCP_EVT_ACK_RCVD, tc, vnet_buffer (b)->tcp.ack_number);
 
-  TCP_EVT_DBG (TCP_EVT_ACK_RCVD, tc);
+      /* Updates congestion control (slow start/congestion avoidance) */
+      tcp_cc_rcv_ack (tc, b);
 
-  /* If everything has been acked, stop retransmit timer
-   * otherwise update */
-  if (tc->snd_una == tc->snd_una_max)
-    tcp_timer_reset (tc, TCP_TIMER_RETRANSMIT);
-  else
-    tcp_timer_update (tc, TCP_TIMER_RETRANSMIT, tc->rto);
+      /* If everything has been acked, stop retransmit timer
+       * otherwise update */
+      if (tc->snd_una == tc->snd_una_max)
+	tcp_retransmit_timer_reset (tc);
+      else
+	tcp_retransmit_timer_update (tc);
+    }
 
   return 0;
 }
@@ -757,9 +894,7 @@ static void
 tcp_update_sack_list (tcp_connection_t * tc, u32 start, u32 end)
 {
   sack_block_t *new_list = 0, block;
-  u32 n_elts;
   int i;
-  u8 new_head = 0;
 
   /* If the first segment is ooo add it to the list. Last write might've moved
    * rcv_nxt over the first segment. */
@@ -768,7 +903,6 @@ tcp_update_sack_list (tcp_connection_t * tc, u32 start, u32 end)
       block.start = start;
       block.end = end;
       vec_add1 (new_list, block);
-      new_head = 1;
     }
 
   /* Find the blocks still worth keeping. */
@@ -782,20 +916,19 @@ tcp_update_sack_list (tcp_connection_t * tc, u32 start, u32 end)
 	  || seq_leq (tc->snd_sacks[i].start, end))
 	continue;
 
-      /* Save subsequent segments to new SACK list. */
-      n_elts = clib_min (vec_len (tc->snd_sacks) - i,
-			 TCP_MAX_SACK_BLOCKS - new_head);
-      vec_insert_elts (new_list, &tc->snd_sacks[i], n_elts, new_head);
-      break;
+      /* Save to new SACK list. */
+      vec_add1 (new_list, tc->snd_sacks[i]);
     }
 
+  ASSERT (vec_len (new_list) < TCP_MAX_SACK_BLOCKS);
+
   /* Replace old vector with new one */
   vec_free (tc->snd_sacks);
   tc->snd_sacks = new_list;
 }
 
 /** Enqueue data for delivery to application */
-always_inline u32
+always_inline int
 tcp_session_enqueue_data (tcp_connection_t * tc, vlib_buffer_t * b,
 			  u16 data_len)
 {
@@ -812,6 +945,8 @@ tcp_session_enqueue_data (tcp_connection_t * tc, vlib_buffer_t * b,
 					 vlib_buffer_get_current (b),
 					 data_len, 1 /* queue event */ );
 
+  TCP_EVT_DBG (TCP_EVT_INPUT, tc, 0, data_len, written);
+
   /* Update rcv_nxt */
   if (PREDICT_TRUE (written == data_len))
     {
@@ -824,38 +959,61 @@ tcp_session_enqueue_data (tcp_connection_t * tc, vlib_buffer_t * b,
 
       /* Send ACK confirming the update */
       tc->flags |= TCP_CONN_SNDACK;
+    }
+  else if (written > 0)
+    {
+      /* We've written something but FIFO is probably full now */
+      tc->rcv_nxt += written;
 
-      /* Update SACK list if need be */
-      if (tcp_opts_sack_permitted (&tc->opt))
-	{
-	  /* Remove SACK blocks that have been delivered */
-	  tcp_update_sack_list (tc, tc->rcv_nxt, tc->rcv_nxt);
-	}
+      /* Depending on how fast the app is, all remaining buffers in burst will
+       * not be enqueued. Should we inform peer of the damage? XXX */
+      return TCP_ERROR_PARTIALLY_ENQUEUED;
     }
   else
     {
-      ASSERT (0);
       return TCP_ERROR_FIFO_FULL;
     }
 
+  /* Update SACK list if need be */
+  if (tcp_opts_sack_permitted (&tc->opt))
+    {
+      /* Remove SACK blocks that have been delivered */
+      tcp_update_sack_list (tc, tc->rcv_nxt, tc->rcv_nxt);
+    }
+
   return TCP_ERROR_ENQUEUED;
 }
 
 /** Enqueue out-of-order data */
-always_inline u32
+always_inline int
 tcp_session_enqueue_ooo (tcp_connection_t * tc, vlib_buffer_t * b,
 			 u16 data_len)
 {
   stream_session_t *s0;
   u32 offset, seq;
+  int rv;
+
+  /* Pure ACK. Do nothing */
+  if (PREDICT_FALSE (data_len == 0))
+    {
+      return TCP_ERROR_PURE_ACK;
+    }
 
   s0 = stream_session_get (tc->c_s_index, tc->c_thread_index);
   seq = vnet_buffer (b)->tcp.seq_number;
   offset = seq - tc->rcv_nxt;
 
-  if (svm_fifo_enqueue_with_offset (s0->server_rx_fifo, s0->pid, offset,
-				    data_len, vlib_buffer_get_current (b)))
-    return TCP_ERROR_FIFO_FULL;
+  rv = svm_fifo_enqueue_with_offset (s0->server_rx_fifo, s0->pid, offset,
+				     data_len, vlib_buffer_get_current (b));
+
+  /* Nothing written */
+  if (rv)
+    {
+      TCP_EVT_DBG (TCP_EVT_INPUT, tc, 1, data_len, 0);
+      return TCP_ERROR_FIFO_FULL;
+    }
+
+  TCP_EVT_DBG (TCP_EVT_INPUT, tc, 1, data_len, data_len);
 
   /* Update SACK list if in use */
   if (tcp_opts_sack_permitted (&tc->opt))
@@ -875,20 +1033,23 @@ tcp_session_enqueue_ooo (tcp_connection_t * tc, vlib_buffer_t * b,
 }
 
 /**
- * Check if ACK could be delayed. DELACK timer is set only after frame is
- * processed so this can return true for a full bursts of packets.
+ * Check if ACK could be delayed. If ack can be delayed, it should return
+ * true for a full frame. If we're always acking return 0.
  */
 always_inline int
 tcp_can_delack (tcp_connection_t * tc)
 {
-  /* If there's no DELACK timer set and the last window sent wasn't 0 we
-   * can safely delay. */
-  if (!tcp_timer_is_active (tc, TCP_TIMER_DELACK)
-      && (tc->flags & TCP_CONN_SENT_RCV_WND0) == 0
-      && (tc->flags & TCP_CONN_SNDACK) == 0)
-    return 1;
+  /* Send ack if ... */
+  if (TCP_ALWAYS_ACK
+      /* just sent a rcv wnd 0 */
+      || (tc->flags & TCP_CONN_SENT_RCV_WND0) != 0
+      /* constrained to send ack */
+      || (tc->flags & TCP_CONN_SNDACK) != 0
+      /* we're almost out of tx wnd */
+      || tcp_available_snd_space (tc) < 2 * tc->snd_mss)
+    return 0;
 
-  return 0;
+  return 1;
 }
 
 static int
@@ -900,23 +1061,33 @@ tcp_segment_rcv (tcp_main_t * tm, tcp_connection_t * tc, vlib_buffer_t * b,
   /* Handle out-of-order data */
   if (PREDICT_FALSE (vnet_buffer (b)->tcp.seq_number != tc->rcv_nxt))
     {
+      /* Old sequence numbers allowed through because they overlapped
+       * the rx window */
+      if (seq_lt (vnet_buffer (b)->tcp.seq_number, tc->rcv_nxt))
+	{
+	  error = TCP_ERROR_SEGMENT_OLD;
+	  *next0 = TCP_NEXT_DROP;
+	  goto done;
+	}
+
       error = tcp_session_enqueue_ooo (tc, b, n_data_bytes);
 
-      /* Don't send more than 3 dupacks per burst
-       * XXX decide if this is good */
-      if (tc->snt_dupacks < 3)
-	{
-	  /* RFC2581: Send DUPACK for fast retransmit */
-	  tcp_make_ack (tc, b);
-	  *next0 = tcp_next_output (tc->c_is_ip4);
+      /* N.B. Should not filter burst of dupacks. Two issues 1) dupacks open
+       * cwnd on remote peer when congested 2) acks leaving should have the
+       * latest rcv_wnd since the burst may eaten up all of it, so only the
+       * old ones could be filtered.
+       */
 
-	  /* Mark as DUPACK. We may filter these in output if
-	   * the burst fills the holes. */
-	  vnet_buffer (b)->tcp.flags = TCP_BUF_FLAG_DUPACK;
+      /* RFC2581: Send DUPACK for fast retransmit */
+      tcp_make_ack (tc, b);
+      *next0 = tcp_next_output (tc->c_is_ip4);
 
-	  tc->snt_dupacks++;
-	}
+      /* Mark as DUPACK. We may filter these in output if
+       * the burst fills the holes. */
+      if (n_data_bytes)
+	vnet_buffer (b)->tcp.flags = TCP_BUF_FLAG_DUPACK;
 
+      TCP_EVT_DBG (TCP_EVT_DUPACK_SENT, tc);
       goto done;
     }
 
@@ -924,63 +1095,45 @@ tcp_segment_rcv (tcp_main_t * tm, tcp_connection_t * tc, vlib_buffer_t * b,
    * segments can be enqueued after fifo tail offset changes. */
   error = tcp_session_enqueue_data (tc, b, n_data_bytes);
 
-  TCP_EVT_DBG (TCP_EVT_INPUT, tc, n_data_bytes);
+  if (n_data_bytes == 0)
+    {
+      *next0 = TCP_NEXT_DROP;
+      goto done;
+    }
+
+  if (PREDICT_FALSE (error == TCP_ERROR_FIFO_FULL))
+    *next0 = TCP_NEXT_DROP;
 
   /* Check if ACK can be delayed */
-  if (tcp_can_delack (tc))
+  if (!tcp_can_delack (tc))
     {
-      /* Nothing to do for pure ACKs */
+      /* Nothing to do for pure ACKs XXX */
       if (n_data_bytes == 0)
 	goto done;
 
-      /* If connection has not been previously marked for delay ack
-       * add it to the list and flag it */
-      if (!tc->flags & TCP_CONN_DELACK)
-	{
-	  vec_add1 (tm->delack_connections[tc->c_thread_index],
-		    tc->c_c_index);
-	  tc->flags |= TCP_CONN_DELACK;
-	}
+      *next0 = tcp_next_output (tc->c_is_ip4);
+      tcp_make_ack (tc, b);
     }
   else
     {
-      /* Check if a packet has already been enqueued to output for burst.
-       * If yes, then drop this one, otherwise, let it pass through to
-       * output */
-      if ((tc->flags & TCP_CONN_BURSTACK) == 0)
-	{
-	  *next0 = tcp_next_output (tc->c_is_ip4);
-	  tcp_make_ack (tc, b);
-	  error = TCP_ERROR_ENQUEUED;
-
-	  /* TODO: maybe add counter to ensure N acks will be sent/burst */
-	  tc->flags |= TCP_CONN_BURSTACK;
-	}
+      if (!tcp_timer_is_active (tc, TCP_TIMER_DELACK))
+	tcp_timer_set (tc, TCP_TIMER_DELACK, TCP_DELACK_TIME);
     }
 
 done:
   return error;
 }
 
-void
-delack_timers_init (tcp_main_t * tm, u32 thread_index)
+always_inline void
+tcp_established_inc_counter (vlib_main_t * vm, u8 is_ip4, u8 evt, u8 val)
 {
-  tcp_connection_t *tc;
-  u32 i, *conns;
-  tw_timer_wheel_16t_2w_512sl_t *tw;
-
-  tw = &tm->timer_wheels[thread_index];
-  conns = tm->delack_connections[thread_index];
-  for (i = 0; i < vec_len (conns); i++)
-    {
-      tc = pool_elt_at_index (tm->connections[thread_index], conns[i]);
-      ASSERT (0 != tc);
+  if (PREDICT_TRUE (!val))
+    return;
 
-      tc->timers[TCP_TIMER_DELACK]
-	= tw_timer_start_16t_2w_512sl (tw, conns[i],
-				       TCP_TIMER_DELACK, TCP_DELACK_TIME);
-    }
-  vec_reset_length (tm->delack_connections[thread_index]);
+  if (is_ip4)
+    vlib_node_increment_counter (vm, tcp4_established_node.index, evt, val);
+  else
+    vlib_node_increment_counter (vm, tcp6_established_node.index, evt, val);
 }
 
 always_inline uword
@@ -1027,7 +1180,7 @@ tcp46_established_inline (vlib_main_t * vm, vlib_node_runtime_t * node,
 	  if (PREDICT_FALSE (tc0 == 0))
 	    {
 	      error0 = TCP_ERROR_INVALID_CONNECTION;
-	      goto drop;
+	      goto done;
 	    }
 
 	  /* Checksum computed by ipx_local no need to compute again */
@@ -1061,18 +1214,22 @@ tcp46_established_inline (vlib_main_t * vm, vlib_node_runtime_t * node,
 	  if (PREDICT_FALSE (tcp_segment_validate (vm, tc0, b0, th0, &next0)))
 	    {
 	      error0 = TCP_ERROR_SEGMENT_INVALID;
-	      goto drop;
+	      TCP_EVT_DBG (TCP_EVT_SEG_INVALID, tc0,
+			   vnet_buffer (b0)->tcp.seq_number,
+			   vnet_buffer (b0)->tcp.seq_end);
+	      goto done;
 	    }
 
 	  /* 5: check the ACK field  */
 	  if (tcp_rcv_ack (tc0, b0, th0, &next0, &error0))
 	    {
-	      goto drop;
+	      goto done;
 	    }
 
 	  /* 6: check the URG bit TODO */
 
 	  /* 7: process the segment text */
+
 	  vlib_buffer_advance (b0, n_advance_bytes0);
 	  error0 = tcp_segment_rcv (tm, tc0, b0, n_data_bytes0, &next0);
 
@@ -1088,7 +1245,7 @@ tcp46_established_inline (vlib_main_t * vm, vlib_node_runtime_t * node,
 	      tcp_timer_set (tc0, TCP_TIMER_WAITCLOSE, TCP_CLOSEWAIT_TIME);
 	    }
 
-	drop:
+	done:
 	  b0->error = node->errors[error0];
 	  if (PREDICT_FALSE (b0->flags & VLIB_BUFFER_IS_TRACED))
 	    {
@@ -1103,17 +1260,7 @@ tcp46_established_inline (vlib_main_t * vm, vlib_node_runtime_t * node,
     }
 
   errors = session_manager_flush_enqueue_events (my_thread_index);
-  if (errors)
-    {
-      if (is_ip4)
-	vlib_node_increment_counter (vm, tcp4_established_node.index,
-				     TCP_ERROR_EVENT_FIFO_FULL, errors);
-      else
-	vlib_node_increment_counter (vm, tcp6_established_node.index,
-				     TCP_ERROR_EVENT_FIFO_FULL, errors);
-    }
-
-  delack_timers_init (tm, my_thread_index);
+  tcp_established_inc_counter (vm, is_ip4, TCP_ERROR_EVENT_FIFO_FULL, errors);
 
   return from_frame->n_vectors;
 }
@@ -1602,7 +1749,7 @@ tcp46_rcv_process_inline (vlib_main_t * vm, vlib_node_runtime_t * node,
 	      stream_session_accept_notify (&tc0->connection);
 
 	      /* Reset SYN-ACK retransmit timer */
-	      tcp_timer_reset (tc0, TCP_TIMER_RETRANSMIT);
+	      tcp_retransmit_timer_reset (tc0);
 	      break;
 	    case TCP_STATE_ESTABLISHED:
 	      /* We can get packets in established state here because they
@@ -1668,7 +1815,7 @@ tcp46_rcv_process_inline (vlib_main_t * vm, vlib_node_runtime_t * node,
 	      tcp_timer_update (tc0, TCP_TIMER_WAITCLOSE, TCP_CLEANUP_TIME);
 
 	      /* Stop retransmit */
-	      tcp_timer_reset (tc0, TCP_TIMER_RETRANSMIT);
+	      tcp_retransmit_timer_reset (tc0);
 
 	      goto drop;