Commit 56f8c5d7 authored May 16, 2018 by Yuchung Cheng Committed by David S. Miller May 17, 2018

tcp: don't mark recently sent packets lost on RTO



An RTO event indicates the head has not been acked for a long time
after its last (re)transmission. But the other packets are not
necessarily lost if they have been only sent recently (for example
due to application limit). This patch would prohibit marking packets
sent within an RTT to be lost on RTO event, using similar logic in
TCP RACK detection.

Normally the head (SND.UNA) would be marked lost since RTO should
fire strictly after the head was sent. An exception is when the
most recent RACK RTT measurement is larger than the (previous)
RTO. To address this exception the head is always marked lost.

Congestion control interaction: since we may not mark every packet
lost, the congestion window may be more than 1 (inflight plus 1).
But only one packet will be retransmitted after RTO, since
tcp_retransmit_timer() calls tcp_retransmit_skb(...,segs=1). The
connection still performs slow start from one packet (with Cubic
congestion control).

This commit was tested in an A/B test with Google web servers,
and showed a reduction of 2% in (spurious) retransmits post
timeout (SlowStartRetrans), and correspondingly reduced DSACKs
(DSACKIgnoredOld) by 7%.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Soheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: Priyaranjan Jha <priyarjha@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent b8fef65a

net/ipv4/tcp_input.c

+8 −4

Original line number	Diff line number	Diff line
		@@ -1929,11 +1929,11 @@ static bool tcp_is_rack(const struct sock *sk)
		static void tcp_timeout_mark_lost(struct sock *sk)
		{
		struct tcp_sock *tp = tcp_sk(sk);
		struct sk_buff *skb;
		struct sk_buff skb, head;
		bool is_reneg; /* is receiver reneging on SACKs? */

		skb = tcp_rtx_queue_head(sk);
		is_reneg = skb && (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED);
		head = tcp_rtx_queue_head(sk);
		is_reneg = head && (TCP_SKB_CB(head)->sacked & TCPCB_SACKED_ACKED);
		if (is_reneg) {
		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSACKRENEGING);
		tp->sacked_out = 0;
		@@ -1943,9 +1943,13 @@ static void tcp_timeout_mark_lost(struct sock *sk)
		tcp_reset_reno_sack(tp);
		}

		skb = head;
		skb_rbtree_walk_from(skb) {
		if (is_reneg)
		TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED;
		else if (tcp_is_rack(sk) && skb != head &&
		tcp_rack_skb_timeout(tp, skb, 0) > 0)
		continue; /* Don't mark recently sent ones lost yet */
		tcp_mark_skb_lost(sk, skb);
		}
		tcp_verify_left_out(tp);
		@@ -1972,7 +1976,7 @@ void tcp_enter_loss(struct sock *sk)
		tcp_ca_event(sk, CA_EVENT_LOSS);
		tcp_init_undo(tp);
		}
		tp->snd_cwnd = 1;
		tp->snd_cwnd = tcp_packets_in_flight(tp) + 1;
		tp->snd_cwnd_cnt = 0;
		tp->snd_cwnd_stamp = tcp_jiffies32;

Admin message