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Date: Fri, 31 Oct 2014 11:51:56 +0800 From: Yuchung Cheng <ycheng@...gle.com> To: Marcelo Ricardo Leitner <mleitner@...hat.com> Cc: Neal Cardwell <ncardwell@...gle.com>, netdev <netdev@...r.kernel.org>, Eric Dumazet <edumazet@...gle.com> Subject: Re: TCP NewReno and single retransmit On Thu, Oct 30, 2014 at 7:24 PM, Marcelo Ricardo Leitner <mleitner@...hat.com> wrote: > On 30-10-2014 00:03, Neal Cardwell wrote: >> >> On Mon, Oct 27, 2014 at 2:49 PM, Marcelo Ricardo Leitner >> <mleitner@...hat.com> wrote: >>> >>> Hi, >>> >>> We have a report from a customer saying that on a very calm connection, >>> like >>> having only a single data packet within some minutes, if this packet gets >>> to >>> be re-transmitted, retrans_stamp is only cleared when the next acked >>> packet >>> is received. But this may make we abort the connection too soon if this >>> next >>> packet also gets lost, because the reference for the initial loss is >>> still >>> for a big while ago.. >> >> ... >>> >>> @@ -2382,31 +2382,32 @@ static inline bool tcp_may_undo(const struct >>> tcp_sock *tp) >>> static bool tcp_try_undo_recovery(struct sock *sk) >> >> ... >>> >>> if (tp->snd_una == tp->high_seq && tcp_is_reno(tp)) { >>> /* Hold old state until something *above* high_seq >>> * is ACKed. For Reno it is MUST to prevent false >>> * fast retransmits (RFC2582). SACK TCP is safe. */ Or we can just remove this strange state-holding logic? I couldn't find such a "MUST" statement in RFC2582. RFC2582 section 3 step 5 suggests exiting the recovery procedure when an ACK acknowledges the "recover" variable (== tp->high_seq - 1). Since we've called tcp_reset_reno_sack() before tcp_try_undo_recovery(), I couldn't see how false fast retransmits can be triggered without this state-holding. Any insights? >>> tcp_moderate_cwnd(tp); >>> + tp->retrans_stamp = 0; >>> return true; >>> } >>> tcp_set_ca_state(sk, TCP_CA_Open); >>> return false; >>> } >>> >>> We would still hold state, at least part of it.. WDYT? >> >> >> This approach sounds OK to me as long as we include a check of >> tcp_any_retrans_done(), as we do in the similar code paths (for >> motivation, see the comment above tcp_any_retrans_done()). > > > Yes, okay. I thought that this would be taken care of already by then but > reading the code again now after your comment, I can see what you're saying. > Thanks. > >> So it sounds fine to me if you change that one new line to the following >> 2: >> >> + if (!tcp_any_retrans_done(sk)) >> + tp->retrans_stamp = 0; > > > Will do. > >> Nice catch! > > > A good part of it (including the diagram) was done by customer. :) > I'll post the patch as soon as we sync with them (credits). > > Marcelo > -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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