sctp: update order of adjustments of partial_bytes_acked and cwnd

RFC4960 Errata 3.12 says RFC4960 is unclear about the order of
adjustments applied to partial_bytes_acked and cwnd in the congestion
avoidance phase, and that the actual order should be:
partial_bytes_acked is reset to (partial_bytes_acked - cwnd). Next, cwnd
is increased by MTU.

We were first increasing cwnd, and then subtracting the new value pba,
which leads to a different result as pba is smaller than what it should
and could cause cwnd to not grow as much.

See-also: https://tools.ietf.org/html/draft-ietf-tsvwg-rfc4960-errata-01#section-3.12
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/sctp/transport.c b/net/sctp/transport.c
index 721eeebfcd8a50609877db61ede41575e012606a..04b6dd1a07ded25fe5874518b0944a6d9df4099b 100644 (file)
--- a/net/sctp/transport.c
+++ b/net/sctp/transport.c
@@ -452,17 +452,18 @@ void sctp_transport_raise_cwnd(struct sctp_transport *transport,
                 * chunks acknowledged by the new Cumulative TSN Ack and by
                 * Gap Ack Blocks.
                 *
-                * When partial_bytes_acked is equal to or greater than cwnd
-                * and before the arrival of the SACK the sender had cwnd or
-                * more bytes of data outstanding (i.e., before arrival of the
-                * SACK, flightsize was greater than or equal to cwnd),
-                * increase cwnd by MTU, and reset partial_bytes_acked to
-                * (partial_bytes_acked - cwnd).
+                * When partial_bytes_acked is equal to or greater than
+                * cwnd and before the arrival of the SACK the sender
+                * had cwnd or more bytes of data outstanding (i.e.,
+                * before arrival of the SACK, flightsize was greater
+                * than or equal to cwnd), partial_bytes_acked is reset
+                * to (partial_bytes_acked - cwnd). Next, cwnd is
+                * increased by MTU. (RFC 4960 Errata 3.12)
                 */
                pba += bytes_acked;
                if (pba >= cwnd) {
+                       pba = pba - cwnd;
                        cwnd += pmtu;
-                       pba = ((cwnd < pba) ? (pba - cwnd) : 0);
                }
 
                pr_debug("%s: congestion avoidance: transport:%p, "