[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>

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 3ef7e9e..055721d 100644 (file)
--- 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);
        }