[PATCHv2] sctp: Do not create SACK chunk if the final packet size

[Wei Yongjun <yjwei@cn.fujitsu.com>]

The sender should create a SACK only if the size of the final SCTP
packet does not exceed the current MTU. Base on RFC 4960:

  6.1.  Transmission of DATA Chunks

    Before an endpoint transmits a DATA chunk, if any received DATA
    chunks have not been acknowledged (e.g., due to delayed ack), the
    sender should create a SACK and bundle it with the outbound DATA
    chunk, as long as the size of the final SCTP packet does not exceed
    the current MTU.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
---
 net/sctp/output.c |   28 ++++++++++++++++------------
 1 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/net/sctp/output.c b/net/sctp/output.c
index 94c110d..21b9efd 100644
--- a/net/sctp/output.c
+++ b/net/sctp/output.c
@@ -222,6 +222,16 @@ static sctp_xmit_t sctp_packet_bundle_auth(struct sctp_packet *pkt,
 	return retval;
 }
 
+static int can_append_chunk(struct sctp_packet *pkt, size_t size)
+{
+	size_t psize = pkt->size;
+	size_t pmtu = ((pkt->transport->asoc) ?
+		       (pkt->transport->asoc->pathmtu) :
+		       (pkt->transport->pathmtu));
+
+	return (psize + size <= pmtu);
+}
+
 /* Try to bundle a SACK with the packet. */
 static sctp_xmit_t sctp_packet_bundle_sack(struct sctp_packet *pkt,
 					   struct sctp_chunk *chunk)
@@ -235,11 +245,15 @@ static sctp_xmit_t sctp_packet_bundle_sack(struct sctp_packet *pkt,
 	    !pkt->has_cookie_echo) {
 		struct sctp_association *asoc;
 		struct timer_list *timer;
+		__u16 chunk_len;
+
 		asoc = pkt->transport->asoc;
 		timer = &asoc->timers[SCTP_EVENT_TIMEOUT_SACK];
+		chunk_len = WORD_ROUND(ntohs(chunk->chunk_hdr->length)) +
+			    sizeof(struct sctp_sack_chunk);
 
 		/* If the SACK timer is running, we have a pending SACK */
-		if (timer_pending(timer)) {
+		if (timer_pending(timer) && can_append_chunk(pkt, chunk_len)) {
 			struct sctp_chunk *sack;
 			asoc->a_rwnd = asoc->rwnd;
 			sack = sctp_make_sack(asoc);
@@ -262,9 +276,6 @@ sctp_xmit_t sctp_packet_append_chunk(struct sctp_packet *packet,
 {
 	sctp_xmit_t retval = SCTP_XMIT_OK;
 	__u16 chunk_len = WORD_ROUND(ntohs(chunk->chunk_hdr->length));
-	size_t psize;
-	size_t pmtu;
-	int too_big;
 
 	SCTP_DEBUG_PRINTK("%s: packet:%p chunk:%p\n", __func__, packet,
 			  chunk);
@@ -279,15 +290,8 @@ sctp_xmit_t sctp_packet_append_chunk(struct sctp_packet *packet,
 	if (retval != SCTP_XMIT_OK)
 		goto finish;
 
-	psize = packet->size;
-	pmtu  = ((packet->transport->asoc) ?
-		 (packet->transport->asoc->pathmtu) :
-		 (packet->transport->pathmtu));
-
-	too_big = (psize + chunk_len > pmtu);
-
 	/* Decide if we need to fragment or resubmit later. */
-	if (too_big) {
+	if (!can_append_chunk(packet, chunk_len)) {
 		/* It's OK to fragmet at IP level if any one of the following
 		 * is true:
 		 * 	1. The packet is empty (meaning this chunk is greater
-- 
1.6.2.2