From 1f37bf87aa7523d28e7e4c4f7bb5dba98faa3e00 Mon Sep 17 00:00:00 2001 From: Marcelo Leitner Date: Tue, 4 Nov 2014 17:15:08 -0200 Subject: [PATCH] tcp: zero retrans_stamp if all retrans were acked Ueki Kohei reported that when we are using NewReno with connections that have a very low traffic, we may timeout the connection too early if a second loss occurs after the first one was successfully acked but no data was transfered later. Below is his description of it: When SACK is disabled, and a socket suffers multiple separate TCP retransmissions, that socket's ETIMEDOUT value is calculated from the time of the *first* retransmission instead of the *latest* retransmission. This happens because the tcp_sock's retrans_stamp is set once then never cleared. Take the following connection: Linux remote-machine | | send#1---->(*1)|--------> data#1 --------->| | | | RTO : : | | | ---(*2)|----> data#1(retrans) ---->| | (*3)|<---------- ACK <----------| | | | | : : | : : | : : 16 minutes (or more) : | : : | : : | : : | | | send#2---->(*4)|--------> data#2 --------->| | | | RTO : : | | | ---(*5)|----> data#2(retrans) ---->| | | | | | | RTO*2 : : | | | | | | ETIMEDOUT<----(*6)| | (*1) One data packet sent. (*2) Because no ACK packet is received, the packet is retransmitted. (*3) The ACK packet is received. The transmitted packet is acknowledged. At this point the first "retransmission event" has passed and been recovered from. Any future retransmission is a completely new "event". (*4) After 16 minutes (to correspond with retries2=15), a new data packet is sent. Note: No data is transmitted between (*3) and (*4). The socket's timeout SHOULD be calculated from this point in time, but instead it's calculated from the prior "event" 16 minutes ago. (*5) Because no ACK packet is received, the packet is retransmitted. (*6) At the time of the 2nd retransmission, the socket returns ETIMEDOUT. Therefore, now we clear retrans_stamp as soon as all data during the loss window is fully acked. Reported-by: Ueki Kohei Cc: Neal Cardwell Cc: Yuchung Cheng Signed-off-by: Marcelo Ricardo Leitner Acked-by: Neal Cardwell Tested-by: Neal Cardwell Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 60 +++++++++++++++++++++++--------------------- 1 file changed, 31 insertions(+), 29 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index a12b455928e5..88fa2d160685 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2315,6 +2315,35 @@ static inline bool tcp_packet_delayed(const struct tcp_sock *tp) /* Undo procedures. */ +/* We can clear retrans_stamp when there are no retransmissions in the + * window. It would seem that it is trivially available for us in + * tp->retrans_out, however, that kind of assumptions doesn't consider + * what will happen if errors occur when sending retransmission for the + * second time. ...It could the that such segment has only + * TCPCB_EVER_RETRANS set at the present time. It seems that checking + * the head skb is enough except for some reneging corner cases that + * are not worth the effort. + * + * Main reason for all this complexity is the fact that connection dying + * time now depends on the validity of the retrans_stamp, in particular, + * that successive retransmissions of a segment must not advance + * retrans_stamp under any conditions. + */ +static bool tcp_any_retrans_done(const struct sock *sk) +{ + const struct tcp_sock *tp = tcp_sk(sk); + struct sk_buff *skb; + + if (tp->retrans_out) + return true; + + skb = tcp_write_queue_head(sk); + if (unlikely(skb && TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS)) + return true; + + return false; +} + #if FASTRETRANS_DEBUG > 1 static void DBGUNDO(struct sock *sk, const char *msg) { @@ -2410,6 +2439,8 @@ static bool tcp_try_undo_recovery(struct sock *sk) * is ACKed. For Reno it is MUST to prevent false * fast retransmits (RFC2582). SACK TCP is safe. */ tcp_moderate_cwnd(tp); + if (!tcp_any_retrans_done(sk)) + tp->retrans_stamp = 0; return true; } tcp_set_ca_state(sk, TCP_CA_Open); @@ -2430,35 +2461,6 @@ static bool tcp_try_undo_dsack(struct sock *sk) return false; } -/* We can clear retrans_stamp when there are no retransmissions in the - * window. It would seem that it is trivially available for us in - * tp->retrans_out, however, that kind of assumptions doesn't consider - * what will happen if errors occur when sending retransmission for the - * second time. ...It could the that such segment has only - * TCPCB_EVER_RETRANS set at the present time. It seems that checking - * the head skb is enough except for some reneging corner cases that - * are not worth the effort. - * - * Main reason for all this complexity is the fact that connection dying - * time now depends on the validity of the retrans_stamp, in particular, - * that successive retransmissions of a segment must not advance - * retrans_stamp under any conditions. - */ -static bool tcp_any_retrans_done(const struct sock *sk) -{ - const struct tcp_sock *tp = tcp_sk(sk); - struct sk_buff *skb; - - if (tp->retrans_out) - return true; - - skb = tcp_write_queue_head(sk); - if (unlikely(skb && TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS)) - return true; - - return false; -} - /* Undo during loss recovery after partial ACK or using F-RTO. */ static bool tcp_try_undo_loss(struct sock *sk, bool frto_undo) {