From 08abdffa1c16b9603ee9bb33f3226ff186d98fa1 Mon Sep 17 00:00:00 2001 From: =?utf8?q?S=C3=A9bastien=20Barr=C3=A9?= Date: Mon, 12 Jan 2015 10:30:40 +0100 Subject: [PATCH] tcp: avoid reducing cwnd when ACK+DSACK is received MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit With TLP, the peer may reply to a probe with an ACK+D-SACK, with ack value set to tlp_high_seq. In the current code, such ACK+DSACK will be missed and only at next, higher ack will the TLP episode be considered done. Since the DSACK is not present anymore, this will cost a cwnd reduction. This patch ensures that this scenario does not cause a cwnd reduction, since receiving an ACK+DSACK indicates that both the initial segment and the probe have been received by the peer. The following packetdrill test, from Neal Cardwell, validates this patch: // Establish a 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 +0 > S. 0:0(0) ack 1 +.020 < . 1:1(0) ack 1 win 257 +0 accept(3, ..., ...) = 4 // Send 1 packet. +0 write(4, ..., 1000) = 1000 +0 > P. 1:1001(1000) ack 1 // Loss probe retransmission. // packets_out == 1 => schedule PTO in max(2*RTT, 1.5*RTT + 200ms) // In this case, this means: 1.5*RTT + 200ms = 230ms +.230 > P. 1:1001(1000) ack 1 +0 %{ assert tcpi_snd_cwnd == 10 }% // Receiver ACKs at tlp_high_seq with a DSACK, // indicating they received the original packet and probe. +.020 < . 1:1(0) ack 1001 win 257 +0 %{ assert tcpi_snd_cwnd == 10 }% // Send another packet. +0 write(4, ..., 1000) = 1000 +0 > P. 1001:2001(1000) ack 1 // Receiver ACKs above tlp_high_seq, which should end the TLP episode // if we haven't already. We should not reduce cwnd. +.020 < . 1:1(0) ack 2001 win 257 +0 %{ assert tcpi_snd_cwnd == 10, tcpi_snd_cwnd }% Credits: -Gregory helped in finding that tcp_process_tlp_ack was where the cwnd got reduced in our MPTCP tests. -Neal wrote the packetdrill test above -Yuchung reworked the patch to make it more readable. Cc: Gregory Detal Cc: Nandita Dukkipati Tested-by: Neal Cardwell Reviewed-by: Yuchung Cheng Reviewed-by: Eric Dumazet Signed-off-by: Sébastien Barré Acked-by: Eric Dumazet Acked-by: Neal Cardwell Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 38 +++++++++++++++++++------------------- 1 file changed, 19 insertions(+), 19 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 075ab4d5af5e..71fb37c70581 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -3358,34 +3358,34 @@ static void tcp_replace_ts_recent(struct tcp_sock *tp, u32 seq) } /* This routine deals with acks during a TLP episode. + * We mark the end of a TLP episode on receiving TLP dupack or when + * ack is after tlp_high_seq. * Ref: loss detection algorithm in draft-dukkipati-tcpm-tcp-loss-probe. */ static void tcp_process_tlp_ack(struct sock *sk, u32 ack, int flag) { struct tcp_sock *tp = tcp_sk(sk); - bool is_tlp_dupack = (ack == tp->tlp_high_seq) && - !(flag & (FLAG_SND_UNA_ADVANCED | - FLAG_NOT_DUP | FLAG_DATA_SACKED)); - /* Mark the end of TLP episode on receiving TLP dupack or when - * ack is after tlp_high_seq. - */ - if (is_tlp_dupack) { - tp->tlp_high_seq = 0; + if (before(ack, tp->tlp_high_seq)) return; - } - if (after(ack, tp->tlp_high_seq)) { + if (flag & FLAG_DSACKING_ACK) { + /* This DSACK means original and TLP probe arrived; no loss */ + tp->tlp_high_seq = 0; + } else if (after(ack, tp->tlp_high_seq)) { + /* ACK advances: there was a loss, so reduce cwnd. Reset + * tlp_high_seq in tcp_init_cwnd_reduction() + */ + tcp_init_cwnd_reduction(sk); + tcp_set_ca_state(sk, TCP_CA_CWR); + tcp_end_cwnd_reduction(sk); + tcp_try_keep_open(sk); + NET_INC_STATS_BH(sock_net(sk), + LINUX_MIB_TCPLOSSPROBERECOVERY); + } else if (!(flag & (FLAG_SND_UNA_ADVANCED | + FLAG_NOT_DUP | FLAG_DATA_SACKED))) { + /* Pure dupack: original and TLP probe arrived; no loss */ tp->tlp_high_seq = 0; - /* Don't reduce cwnd if DSACK arrives for TLP retrans. */ - if (!(flag & FLAG_DSACKING_ACK)) { - tcp_init_cwnd_reduction(sk); - tcp_set_ca_state(sk, TCP_CA_CWR); - tcp_end_cwnd_reduction(sk); - tcp_try_keep_open(sk); - NET_INC_STATS_BH(sock_net(sk), - LINUX_MIB_TCPLOSSPROBERECOVERY); - } } } -- 2.20.1