From: Ilpo Järvinen Date: Thu, 8 May 2008 08:09:11 +0000 (-0700) Subject: tcp FRTO: SACK variant is errorneously used with NewReno X-Git-Tag: v2.6.26-rc2~23^2~8 X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=62ab22278308a40bcb7f4079e9719ab8b7fe11b5;p=linux-2.6 tcp FRTO: SACK variant is errorneously used with NewReno Note: there's actually another bug in FRTO's SACK variant, which is the causing failure in NewReno case because of the error that's fixed here. I'll fix the SACK case separately (it's a separate bug really, though related, but in order to fix that I need to audit tp->snd_nxt usage a bit). There were two places where SACK variant of FRTO is getting incorrectly used even if SACK wasn't negotiated by the TCP flow. This leads to incorrect setting of frto_highmark with NewReno if a previous recovery was interrupted by another RTO. An eventual fallback to conventional recovery then incorrectly considers one or couple of segments as forward transmissions though they weren't, which then are not LOST marked during fallback making them "non-retransmittable" until the next RTO. In a bad case, those segments are really lost and are the only one left in the window. Thus TCP needs another RTO to continue. The next FRTO, however, could again repeat the same events making the progress of the TCP flow extremely slow. In order for these events to occur at all, FRTO must occur again in FRTOs step 3 while the key segments must be lost as well, which is not too likely in practice. It seems to most frequently with some small devices such as network printers that *seem* to accept TCP segments only in-order. In cases were key segments weren't lost, things get automatically resolved because those wrongly marked segments don't need to be retransmitted in order to continue. I found a reproducer after digging up relevant reports (few reports in total, none at netdev or lkml I know of), some cases seemed to indicate middlebox issues which seems now to be a false assumption some people had made. Bugzilla #10063 _might_ be related. Damon L. Chesser had a reproducable case and was kind enough to tcpdump it for me. With the tcpdump log it was quite trivial to figure out. Signed-off-by: Ilpo Järvinen Signed-off-by: David S. Miller --- diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 8ac15a604e..26c936930e 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -114,8 +114,6 @@ int sysctl_tcp_abc __read_mostly; #define FLAG_FORWARD_PROGRESS (FLAG_ACKED|FLAG_DATA_SACKED) #define FLAG_ANY_PROGRESS (FLAG_FORWARD_PROGRESS|FLAG_SND_UNA_ADVANCED) -#define IsSackFrto() (sysctl_tcp_frto == 0x2) - #define TCP_REMNANT (TCP_FLAG_FIN|TCP_FLAG_URG|TCP_FLAG_SYN|TCP_FLAG_PSH) #define TCP_HP_BITS (~(TCP_RESERVED_BITS|TCP_FLAG_PSH)) @@ -1686,6 +1684,11 @@ static inline void tcp_reset_reno_sack(struct tcp_sock *tp) tp->sacked_out = 0; } +static int tcp_is_sackfrto(const struct tcp_sock *tp) +{ + return (sysctl_tcp_frto == 0x2) && !tcp_is_reno(tp); +} + /* F-RTO can only be used if TCP has never retransmitted anything other than * head (SACK enhanced variant from Appendix B of RFC4138 is more robust here) */ @@ -1702,7 +1705,7 @@ int tcp_use_frto(struct sock *sk) if (icsk->icsk_mtup.probe_size) return 0; - if (IsSackFrto()) + if (tcp_is_sackfrto(tp)) return 1; /* Avoid expensive walking of rexmit queue if possible */ @@ -1792,7 +1795,7 @@ void tcp_enter_frto(struct sock *sk) /* Earlier loss recovery underway (see RFC4138; Appendix B). * The last condition is necessary at least in tp->frto_counter case. */ - if (IsSackFrto() && (tp->frto_counter || + if (tcp_is_sackfrto(tp) && (tp->frto_counter || ((1 << icsk->icsk_ca_state) & (TCPF_CA_Recovery|TCPF_CA_Loss))) && after(tp->high_seq, tp->snd_una)) { tp->frto_highmark = tp->high_seq; @@ -3124,7 +3127,7 @@ static int tcp_process_frto(struct sock *sk, int flag) return 1; } - if (!IsSackFrto() || tcp_is_reno(tp)) { + if (!tcp_is_sackfrto(tp)) { /* RFC4138 shortcoming in step 2; should also have case c): * ACK isn't duplicate nor advances window, e.g., opposite dir * data, winupdate