Re: Question on SCTP ABORT chunk is generated when the association_max_retrans is reached

[Michael Tuexen]

> On 23 Jan 2015, at 19:30, Vlad Yasevich <vyasevich@xxxxxxxxx> wrote:
> 
> On 01/23/2015 12:10 PM, Daniel Borkmann wrote:
>> On 01/23/2015 05:05 PM, Vlad Yasevich wrote:
>>> On 01/23/2015 06:50 AM, Daniel Borkmann wrote:
>>>> On 01/23/2015 11:25 AM, Sun Paul wrote:
>>>> ...
>>>>> I would like to check the behave in LKSCTP.
>>>>> 
>>>>> we are running DIAMETER message over SCTP, and we have set the
>>>>> parameter "net.sctp.association_max_retrans = 4" in the LinuxOS.
>>>>> 
>>>>> We noticed that when remote peer have retry to send the same request
>>>>> for 4 times, the LKSCTP will initiate an ABORT chunk with reason
>>>>> "association exceeded its max_retrans count".
>>>>> 
>>>>> We would like to know whether this is the correct behavior? is there
>>>>> any other option that we can alter in order to avoid the ABORT chunk
>>>>> being sent?
>>>> 
>>>> I don't recall the RFC saying to send an ABORT, but let me double
>>>> check in the mean time.
>>> 
>>> The RFC is silent on the matter.  The abort got added in 3.8 so
>>> it's been there for a while.
>> 
>> I see, commit de4594a51c90 ("sctp: send abort chunk when max_retrans
>> exceeded") added the behaviour.
>> 
>>>> Hmm, untested, but could you try something like that?
>>>> 
>>>> diff --git a/net/sctp/sm_sideeffect.c b/net/sctp/sm_sideeffect.c
>>>> index fef2acd..5ce198d 100644
>>>> --- a/net/sctp/sm_sideeffect.c
>>>> +++ b/net/sctp/sm_sideeffect.c
>>>> @@ -584,7 +584,8 @@ static void sctp_cmd_assoc_failed(sctp_cmd_seq_t *commands,
>>>>          sctp_add_cmd_sf(commands, SCTP_CMD_EVENT_ULP,
>>>>                  SCTP_ULPEVENT(event));
>>>> 
>>>> -    if (asoc->overall_error_count >= asoc->max_retrans) {
>>>> +    if (asoc->overall_error_count >= asoc->max_retrans &&
>>>> +        error != SCTP_ERROR_NO_ERROR) {
>>>>          abort = sctp_make_violation_max_retrans(asoc, chunk);
>>>>          if (abort)
>>>>              sctp_add_cmd_sf(commands, SCTP_CMD_REPLY,
>>> 
>>> This would pretty much stop all ABORTs due to excessive rtx.  Might
>>> as well take the code out :).
>>> 
>>> I was a bit concerned about this ABORT when it went in.
>> 
>> So effectively, if I understand the argument from the commit, the
>> assumption is that the ABORT would never reach the peer anyway, but
>> is a way for tcpdump users to see on the wire that rtx limit has
>> been exceeded and since there's not mentioned anything in the RFC
>> about this, it doesn't break it. Hm.
>> 
> 
> Additionally I seem to recall BSD sending this type of ABORT for pretty
> much the same reason.
Yepp.

Best regards
Michael
> 
> -vlad
> 
>> Sun Paul, what exactly broke in your scenario? Can you be more explicit?
>> 
>> Thanks,
>> Daniel
> 
> 

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