Doug Graham wrote:
>> This patch corrects the conditions under which a SACK will be piggybacked
>> on a DATA packet. The previous condition was incorrect due to a
>> misinterpretation of RFC 4960 and/or RFC 2960. Specifically, the
>> following paragraph from section 6.2 had not been implemented correctly:
>>
>> 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. See Section 6.2.
>>
>>
>
> The above text said that SACK is create when the size of the final SCTP
> packet
> does not exceed the current MTU. With the patch, what will happend?
> If the packet is too large for bundle with SACK, the packet sequence will
> like this:
>
> Endpoint A Endpoint B
>
> DATA ------------->
> <------------- SACK
> <------------- DATA (size=1452)
> SACK ------------->
> DATA (size=1452)------------->
> <------------- SACK
> <------------- DATA (size=1452)
>
> The behavior is the same as no delayed ack support. So I think
> you also need to check the packet size before append the SACK.
The patch should be like this:
[PATCH] sctp: Do not create SACK chunk if the final packet size exceed current MTU
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@xxxxxxxxxxxxxx>
---
net/sctp/output.c | 13 ++++++++-----
1 files changed, 8 insertions(+), 5 deletions(-)
diff --git a/net/sctp/output.c b/net/sctp/output.c
index 94c110d..1aa3c6e 100644
--- a/net/sctp/output.c
+++ b/net/sctp/output.c
@@ -274,16 +274,19 @@ sctp_xmit_t sctp_packet_append_chunk(struct sctp_packet *packet,
if (retval != SCTP_XMIT_OK)
goto finish;
- /* Try to bundle SACK chunk */
- retval = sctp_packet_bundle_sack(packet, chunk);
- if (retval != SCTP_XMIT_OK)
- goto finish;
-
psize = packet->size;
pmtu = ((packet->transport->asoc) ?
(packet->transport->asoc->pathmtu) :
(packet->transport->pathmtu));
+ /* Try to bundle SACK chunk if not exceed the current MTU */
+ if (psize + chunk_len + sizeof(struct sctp_sack_chunk) <= pmtu) {
+ retval = sctp_packet_bundle_sack(packet, chunk);
+ if (retval != SCTP_XMIT_OK)
+ goto finish;
+ psize = packet->size;
+ }
+
too_big = (psize + chunk_len > pmtu);
/* Decide if we need to fragment or resubmit later. */
--
1.6.2.2
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