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Date: Mon, 12 Jan 2015 10:30:40 +0100
From: Sébastien Barré
<sebastien.barre@...ouvain.be>
To: David Miller <davem@...emloft.net>
CC: Sébastien Barré
<sebastien.barre@...ouvain.be>,
Neal Cardwell <ncardwell@...gle.com>,
Yuchung Cheng <ycheng@...gle.com>,
Eric Dumazet <eric.dumazet@...il.com>,
<netdev@...r.kernel.org>,
Gregory Detal <gregory.detal@...ouvain.be>,
Nandita Dukkipati <nanditad@...gle.com>
Subject: [PATCH net-next v3] tcp: avoid reducing cwnd when ACK+DSACK is received
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 <mss 1000,sackOK,nop,nop,nop,wscale 7>
+0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 6>
+.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 <sack 1:1001,nop,nop>
+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 <gregory.detal@...ouvain.be>
Cc: Nandita Dukkipati <nanditad@...gle.com>
Tested-by: Neal Cardwell <ncardwell@...gle.com>
Reviewed-by: Yuchung Cheng <ycheng@...gle.com>
Reviewed-by: Eric Dumazet <eric.dumazet@...il.com>
Signed-off-by: Sébastien Barré <sebastien.barre@...ouvain.be>
---
Changes:
-Added Neal's test in commit text
-applied Yuchung's changes to if conditions (for readability)
-removed delayed ack as main reason for triggering an ACK+DSACK, as
Eric mentioned that lost ack has higher chances to be the trigger.
Neal, Yuchung, Eric: I added Tested-by/Reviewed-by as I thought it was
appropriate, please correct if it is not. Thanks again for you help !
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 075ab4d..71fb37c 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);
- }
}
}
--
tg: (44d84d7..) net-next/tlp-dsack-handling (depends on: net-next/master)
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