tcp: remove cwnd moderation after recovery

For non-SACK connections, cwnd is lowered to inflight plus 3 packets
when the recovery ends. This is an optional feature in the NewReno
RFC 2582 to reduce the potential burst when cwnd is "re-opened"
after recovery and inflight is low.

This feature is questionably effective because of PRR: when
the recovery ends (i.e., snd_una == high_seq) NewReno holds the
CA_Recovery state for another round trip to prevent false fast
retransmits. But if the inflight is low, PRR will overwrite the
moderated cwnd in tcp_cwnd_reduction() later regardlessly. So if a
receiver responds bogus ACKs (i.e., acking future data) to speed up
transfer after recovery, it can only induce a burst up to a window
worth of data packets by acking up to SND.NXT. A restart from (short)
idle or receiving streched ACKs can both cause such bursts as well.

On the other hand, if the recovery ends because the sender
detects the losses were spurious (e.g., reordering). This feature
unconditionally lowers a reverted cwnd even though nothing
was lost.

By principle loss recovery module should not update cwnd. Further
pacing is much more effective to reduce burst. Hence this patch
removes the cwnd moderation feature.

v2 changes: revised commit message on bogus ACKs and burst, and
            missing signature
Signed-off-by: Matt Mathis <mattmathis@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1039,17 +1039,6 @@ static inline __u32 tcp_max_tso_deferred_mss(const struct tcp_sock *tp)
 	return 3;
 }
 
-/* Slow start with delack produces 3 packets of burst, so that
- * it is safe "de facto".  This will be the default - same as
- * the default reordering threshold - but if reordering increases,
- * we must be able to allow cwnd to burst at least this much in order
- * to not pull it back when holes are filled.
- */
-static __inline__ __u32 tcp_max_burst(const struct tcp_sock *tp)
-{
-	return tp->reordering;
-}
-
 /* Returns end sequence number of the receiver's advertised window */
 static inline u32 tcp_wnd_end(const struct tcp_sock *tp)
 {

net/ipv4/tcp_input.c

@@ -2252,16 +2252,6 @@ static void tcp_update_scoreboard(struct sock *sk, int fast_rexmit)
 	}
 }
 
-/* CWND moderation, preventing bursts due to too big ACKs
- * in dubious situations.
- */
-static inline void tcp_moderate_cwnd(struct tcp_sock *tp)
-{
-	tp->snd_cwnd = min(tp->snd_cwnd,
-			   tcp_packets_in_flight(tp) + tcp_max_burst(tp));
-	tp->snd_cwnd_stamp = tcp_time_stamp;
-}
-
 static bool tcp_tsopt_ecr_before(const struct tcp_sock *tp, u32 when)
 {
 	return tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
@@ -2410,7 +2400,6 @@ static bool tcp_try_undo_recovery(struct sock *sk)
 		/* Hold old state until something *above* high_seq
 		 * is ACKed. For Reno it is MUST to prevent false
 		 * fast retransmits (RFC2582). SACK TCP is safe. */
-		tcp_moderate_cwnd(tp);
 		if (!tcp_any_retrans_done(sk))
 			tp->retrans_stamp = 0;
 		return true;