Commit 0cfa5c07 authored May 30, 2014 by Yuchung Cheng Committed by David S. Miller Jun 02, 2014

tcp: fix cwnd undo on DSACK in F-RTO

This bug is discovered by an recent F-RTO issue on tcpm list
https://www.ietf.org/mail-archive/web/tcpm/current/msg08794.html



The bug is that currently F-RTO does not use DSACK to undo cwnd in
certain cases: upon receiving an ACK after the RTO retransmission in
F-RTO, and the ACK has DSACK indicating the retransmission is spurious,
the sender only calls tcp_try_undo_loss() if some never retransmisted
data is sacked (FLAG_ORIG_DATA_SACKED).

The correct behavior is to unconditionally call tcp_try_undo_loss so
the DSACK information is used properly to undo the cwnd reduction.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 2d7a85f4

net/ipv4/tcp_input.c

+5 −6

Original line number	Diff line number	Diff line
		@@ -2684,13 +2684,12 @@ static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack)
		bool recovered = !before(tp->snd_una, tp->high_seq);

		if (tp->frto) { /* F-RTO RFC5682 sec 3.1 (sack enhanced version). */
		if (flag & FLAG_ORIG_SACK_ACKED) {
		/* Step 3.b. A timeout is spurious if not all data are
		* lost, i.e., never-retransmitted data are (s)acked.
		*/
		tcp_try_undo_loss(sk, true);
		if (tcp_try_undo_loss(sk, flag & FLAG_ORIG_SACK_ACKED))
		return;
		}

		if (after(tp->snd_nxt, tp->high_seq) &&
		(flag & FLAG_DATA_SACKED \|\| is_dupack)) {
		tp->frto = 0; /* Loss was real: 2nd part of step 3.a */

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