From: Ilpo Järvinen Date: Thu, 22 Feb 2007 07:13:47 +0000 (-0800) Subject: [TCP] FRTO: Fake cwnd for ssthresh callback X-Git-Tag: v2.6.22-rc1~1128^2~433 X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=66e93e45c09affa407750cc06398492e8b897848;p=linux-2.6 [TCP] FRTO: Fake cwnd for ssthresh callback TCP without FRTO would be in Loss state with small cwnd. FRTO, however, leaves cwnd (typically) to a larger value which causes ssthresh to become too large in case RTO is triggered again compared to what conventional recovery would do. Because consecutive RTOs result in only a single ssthresh reduction, RTO+cumulative ACK+RTO pattern is required to trigger this event. A large comment is included for congestion control module writers trying to figure out what CA_EVENT_FRTO handler should do because there exists a remote possibility of incompatibility between FRTO and module defined ssthresh functions. 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 3ef7e9e079..055721d849 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -1289,7 +1289,31 @@ void tcp_enter_frto(struct sock *sk) ((icsk->icsk_ca_state == TCP_CA_Loss || tp->frto_counter) && !icsk->icsk_retransmits)) { tp->prior_ssthresh = tcp_current_ssthresh(sk); - tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk); + /* Our state is too optimistic in ssthresh() call because cwnd + * is not reduced until tcp_enter_frto_loss() when previous FRTO + * recovery has not yet completed. Pattern would be this: RTO, + * Cumulative ACK, RTO (2xRTO for the same segment does not end + * up here twice). + * RFC4138 should be more specific on what to do, even though + * RTO is quite unlikely to occur after the first Cumulative ACK + * due to back-off and complexity of triggering events ... + */ + if (tp->frto_counter) { + u32 stored_cwnd; + stored_cwnd = tp->snd_cwnd; + tp->snd_cwnd = 2; + tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk); + tp->snd_cwnd = stored_cwnd; + } else { + tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk); + } + /* ... in theory, cong.control module could do "any tricks" in + * ssthresh(), which means that ca_state, lost bits and lost_out + * counter would have to be faked before the call occurs. We + * consider that too expensive, unlikely and hacky, so modules + * using these in ssthresh() must deal these incompatibility + * issues if they receives CA_EVENT_FRTO and frto_counter != 0 + */ tcp_ca_event(sk, CA_EVENT_FRTO); }