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Commit 66e93e45 authored by Ilpo Järvinen's avatar Ilpo Järvinen Committed by David S. Miller
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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: default avatarIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent d1a54c6a

net/ipv4/tcp_input.c

+25 −1
Original line number Diff line number Diff line
@@ -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);

		/* 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);

	}