Messages in this thread Patch in this message | | | From | Jon Maxwell <> | Subject | [PATCH net-next] tcp: Improve setsockopt() TCP_USER_TIMEOUT accuracy | Date | Tue, 3 Jul 2018 11:17:26 +1000 |
| |
Every time the TCP retransmission timer fires. It checks to see if there is a timeout before scheduling the next retransmit timer. The retransmit interval between each retransmission increases exponentially. The issue is that in order for the timeout to occur the retransmit timer needs to fire again. If the user timeout check happens after the 9th retransmit for example. It needs to wait for the 10th retransmit timer to fire in order to evaluate whether a timeout has occurred or not. If the interval is large enough then the timeout will be inaccurate.
For example with a TCP_USER_TIMEOUT of 10 seconds without patch:
1st retransmit:
22:25:18.973488 IP host1.49310 > host2.search-agent: Flags [.]
Last retransmit:
22:25:26.205499 IP host1.49310 > host2.search-agent: Flags [.]
Timeout:
send: Connection timed out Sun Jul 1 22:25:34 EDT 2018
We can see that last retransmit took ~7 seconds. Which pushed the total timeout to ~15 seconds instead of the expected 10 seconds. This gets more inaccurate the larger the TCP_USER_TIMEOUT value. As the interval increases.
Fix this by recalculating the last retransmit interval so that it fires when the timeout should occur. Only implement when icsk->icsk_user_timeout is set.
Test results with the patch is the expected 10 second timeout:
1st retransmit:
01:37:59.022555 IP host1.49310 > host2.search-agent: Flags [.]
Last retransmit:
01:38:06.486558 IP host1.49310 > host2.search-agent: Flags [.]
Timeout:
send: Connection timed out Mon Jul 2 01:38:09 EDT 2018
Signed-off-by: Jon Maxwell <jmaxwell37@gmail.com> --- net/ipv4/tcp_timer.c | 7 +++++++ 1 file changed, 7 insertions(+)
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c index 3b3611729928..94491a481722 100644 --- a/net/ipv4/tcp_timer.c +++ b/net/ipv4/tcp_timer.c @@ -407,6 +407,7 @@ void tcp_retransmit_timer(struct sock *sk) struct tcp_sock *tp = tcp_sk(sk); struct net *net = sock_net(sk); struct inet_connection_sock *icsk = inet_csk(sk); + __u32 time_remaining = 0; if (tp->fastopen_rsk) { WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV && @@ -535,6 +536,12 @@ void tcp_retransmit_timer(struct sock *sk) /* Use normal (exponential) backoff */ icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX); } + if (icsk->icsk_user_timeout) { + time_remaining = jiffies_to_msecs(icsk->icsk_user_timeout) - + (tcp_time_stamp(tcp_sk(sk)) - tcp_sk(sk)->retrans_stamp); + if (time_remaining < icsk->icsk_rto) + icsk->icsk_rto = time_remaining; + } inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, TCP_RTO_MAX); if (retransmits_timed_out(sk, net->ipv4.sysctl_tcp_retries1 + 1, 0)) __sk_dst_reset(sk); -- 2.13.6
| |