[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>

1 files changed, 25 insertions, 1 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 3ef7e9e07964..055721d8495e 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);
         }