[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>

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);
+		/* 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);
 	}