Subject: Re: [PATCH] sctp: fix the fast retransmit limit

From: Wei Yongjun <[email-masked]>
Date: Fri, 04 Mar 2011 13:46:30 +0800

> On 03/03/2011 04:29 AM, Wei Yongjun wrote:
>> If chunk is still lost after fast retransmit, SCTP stack will
>> never allow the second fast retransmit of this chunk, even if
>> the peer need we do this. This chunk will be retransmit until
>> the rtx timeout. This limit is introduce by the following patch:
>>   sctp: reduce memory footprint of sctp_chunk structure
>>   (c226ef9b83694311327f3ab0036c6de9c22e9daf)
>>
>> This patch revert this limit and removed useless SCTP_DONT_FRTX.
> NACK.  Please read the spec and how fast recovery is specified.

RFC 7.2.4. Fast Retransmit on Gap Reports said:
5) Mark the DATA chunk(s) as being fast retransmitted and thus
ineligible for a subsequent Fast Retransmit. Those TSNs marked
for retransmission due to the Fast-Retransmit algorithm that did
not fit in the sent datagram carrying K other TSNs are also
marked as ineligible for a subsequent Fast Retransmit. However,
as they are marked for retransmission they will be retransmitted
later on as soon as cwnd allows.

So we can treat this chunk can be Fast Retransmit again if not in
fast recovery?

> -vlad
>
>> Signed-off-by: Wei Yongjun <[email-masked]>
>> ---
>>  include/net/sctp/structs.h |    1 -
>>  net/sctp/outqueue.c        |    4 ++--
>>  2 files changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/net/sctp/structs.h b/include/net/sctp/structs.h
>> index cc9185c..82a0f84 100644
>> --- a/include/net/sctp/structs.h
>> +++ b/include/net/sctp/structs.h
>> @@ -751,7 +751,6 @@ struct sctp_chunk {
>>  
>>  #define SCTP_CAN_FRTX 0x0
>>  #define SCTP_NEED_FRTX 0x1
>> -#define SCTP_DONT_FRTX 0x2
>>  	__u16	rtt_in_progress:1,	/* This chunk used for RTT calc? */
>>  		has_tsn:1,		/* Does this chunk have a TSN yet? */
>>  		has_ssn:1,		/* Does this chunk have a SSN yet? */
>> diff --git a/net/sctp/outqueue.c b/net/sctp/outqueue.c
>> index 8c6d379..7ed5862 100644
>> --- a/net/sctp/outqueue.c
>> +++ b/net/sctp/outqueue.c
>> @@ -657,7 +657,7 @@ redo:
>>  			 * after it is retransmitted.
>>  			 */
>>  			if (chunk->fast_retransmit == SCTP_NEED_FRTX)
>> -				chunk->fast_retransmit = SCTP_DONT_FRTX;
>> +				chunk->fast_retransmit = SCTP_CAN_FRTX;
>>  
>>  			q->empty = 0;
>>  			break;
>> @@ -679,7 +679,7 @@ redo:
>>  	if (rtx_timeout || fast_rtx) {
>>  		list_for_each_entry(chunk1, lqueue, transmitted_list) {
>>  			if (chunk1->fast_retransmit == SCTP_NEED_FRTX)
>> -				chunk1->fast_retransmit = SCTP_DONT_FRTX;
>> +				chunk1->fast_retransmit = SCTP_CAN_FRTX;
>>  		}
>>  	}
>>  
> --
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