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[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>
This commit is contained in:
Ilpo Järvinen 2007-02-21 23:13:47 -08:00 committed by David S. Miller
parent d1a54c6a0a
commit 66e93e45c0
1 changed files with 25 additions and 1 deletions
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26
net/ipv4/tcp_input.c
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@ -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);
}