Calculate the correct amount of bytes that are in-flight for a connection as

suggested by RFC 6675.

Currently differnt places in the stack tries to guess this in suboptimal ways.
The main problem is that current calculations don't take sacked bytes into
account. Sacked bytes are the bytes receiver acked via SACK option. This is
suboptimal because it assumes that network has more outstanding (unacked) bytes
than the actual value and thus sends less data by setting congestion window
lower than what's possible which in turn may cause slower recovery from losses.

As an example, one of the current calculations looks something like this:
snd_nxt - snd_fack + sackhint.sack_bytes_rexmit
New proposal from RFC 6675 is:
snd_max - snd_una - sackhint.sacked_bytes + sackhint.sack_bytes_rexmit
which takes sacked bytes into account which is a new addition to the sackhint
struct. Only thing we are missing from RFC 6675 is isLost() i.e. segment being
considered lost and thus adjusting pipe based on that which makes this
calculation a bit on conservative side.

The approach is very simple. We already process each ack with sack info in
tcp_sack_doack() and extract sack blocks/holes out of it. We'd now also track
this new variable sacked_bytes which keeps track of total sacked bytes reported.

One downside to this approach is that we may get incorrect count of sacked_bytes
if the other end decides to drop sack info in the ack because of memory pressure
or some other reasons. But in this (not very likely) case also the pipe
calculation would be conservative which is okay as opposed to being aggressive
in sending packets into the network.

Next step is to use this more accurate pipe estimation to drive congestion
window adjustments.

In collaboration with:	rrs
Reviewed by:		jason_eggnet dot com, rrs
MFC after:		2 weeks
Sponsored by:		Limelight Networks
Differential Revision:	https://reviews.freebsd.org/D3971

1 files changed, 5 insertions, 1 deletions

diff --git a/sys/netinet/tcp_sack.c b/sys/netinet/tcp_sack.c
index 223403cd4119..f9c71c84f0d4 100644
--- a/sys/netinet/tcp_sack.c
+++ b/sys/netinet/tcp_sack.c
@@ -369,6 +369,7 @@ tcp_sack_doack(struct tcpcb *tp, struct tcpopt *to, tcp_seq th_ack)
 	 * received new blocks from the other side.
 	 */
 	if (to->to_flags & TOF_SACK) {
+		tp->sackhint.sacked_bytes = 0;	/* reset */
 		for (i = 0; i < to->to_nsacks; i++) {
 			bcopy((to->to_sacks + i * TCPOLEN_SACK),
 			    &sack, sizeof(sack));
@@ -379,8 +380,11 @@ tcp_sack_doack(struct tcpcb *tp, struct tcpopt *to, tcp_seq th_ack)
 			    SEQ_GT(sack.start, th_ack) &&
 			    SEQ_LT(sack.start, tp->snd_max) &&
 			    SEQ_GT(sack.end, tp->snd_una) &&
-			    SEQ_LEQ(sack.end, tp->snd_max))
+			    SEQ_LEQ(sack.end, tp->snd_max)) {
 				sack_blocks[num_sack_blks++] = sack;
+				tp->sackhint.sacked_bytes +=
+				    (sack.end-sack.start);
+			}
 		}
 	}
 	/*