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author | Ilpo Järvinen <ilpo.jarvinen@helsinki.fi> | 2007-02-21 23:13:47 -0800 |
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committer | David S. Miller <davem@sunset.davemloft.net> | 2007-04-25 22:23:14 -0700 |
commit | 66e93e45c09affa407750cc06398492e8b897848 (patch) | |
tree | bcf65f7c29a3bd27eb386198560463f3eece7d41 /net | |
parent | d1a54c6a0a3f9c2c4ef71982d89b8571bd9eaa51 (diff) | |
download | kernel_samsung_smdk4412-66e93e45c09affa407750cc06398492e8b897848.zip kernel_samsung_smdk4412-66e93e45c09affa407750cc06398492e8b897848.tar.gz kernel_samsung_smdk4412-66e93e45c09affa407750cc06398492e8b897848.tar.bz2 |
[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 <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net')
-rw-r--r-- | net/ipv4/tcp_input.c | 26 |
1 files changed, 25 insertions, 1 deletions
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 3ef7e9e..055721d 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); } |