Commit 66205121 authored Jun 15, 2020 by Eric Dumazet Committed by David S. Miller Jun 16, 2020

tcp: grow window for OOO packets only for SACK flows



Back in 2013, we made a change that broke fast retransmit
for non SACK flows.

Indeed, for these flows, a sender needs to receive three duplicate
ACK before starting fast retransmit. Sending ACK with different
receive window do not count.

Even if enabling SACK is strongly recommended these days,
there still are some cases where it has to be disabled.

Not increasing the window seems better than having to
rely on RTO.

After the fix, following packetdrill test gives :

// Initialize connection
    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
   +0 bind(3, ..., ...) = 0
   +0 listen(3, 1) = 0

   +0 < S 0:0(0) win 32792 <mss 1000,nop,wscale 7>
   +0 > S. 0:0(0) ack 1 <mss 1460,nop,wscale 8>
   +0 < . 1:1(0) ack 1 win 514

   +0 accept(3, ..., ...) = 4

   +0 < . 1:1001(1000) ack 1 win 514
// Quick ack
   +0 > . 1:1(0) ack 1001 win 264

   +0 < . 2001:3001(1000) ack 1 win 514
// DUPACK : Normally we should not change the window
   +0 > . 1:1(0) ack 1001 win 264

   +0 < . 3001:4001(1000) ack 1 win 514
// DUPACK : Normally we should not change the window
   +0 > . 1:1(0) ack 1001 win 264

   +0 < . 4001:5001(1000) ack 1 win 514
// DUPACK : Normally we should not change the window
    +0 > . 1:1(0) ack 1001 win 264

   +0 < . 1001:2001(1000) ack 1 win 514
// Hole is repaired.
   +0 > . 1:1(0) ack 5001 win 272

Fixes: 4e4f1fc2 ("tcp: properly increase rcv_ssthresh for ofo packets")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Venkat Venkatsubra <venkat.x.venkatsubra@oracle.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent b8ad540d

net/ipv4/tcp_input.c

+10 −2

Original line number	Diff line number	Diff line
		@@ -4605,6 +4605,10 @@ static void tcp_data_queue_ofo(struct sock sk, struct sk_buff skb)
		if (tcp_ooo_try_coalesce(sk, tp->ooo_last_skb,
		skb, &fragstolen)) {
		coalesce_done:
		/* For non sack flows, do not grow window to force DUPACK
		* and trigger fast retransmit.
		*/
		if (tcp_is_sack(tp))
		tcp_grow_window(sk, skb);
		kfree_skb_partial(skb, fragstolen);
		skb = NULL;
		@@ -4689,6 +4693,10 @@ add_sack:
		tcp_sack_new_ofo_skb(sk, seq, end_seq);
		end:
		if (skb) {
		/* For non sack flows, do not grow window to force DUPACK
		* and trigger fast retransmit.
		*/
		if (tcp_is_sack(tp))
		tcp_grow_window(sk, skb);
		skb_condense(skb);
		skb_set_owner_r(skb, sk);

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