Re: [MMUSIC] SCTP question: Where does it multiplex?

Paul Kyzivat <pkyzivat@alum.mit.edu> Tue, 11 December 2012 19:26 UTC

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Subject: Re: [MMUSIC] SCTP question: Where does it multiplex?
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On 12/11/12 1:53 PM, Dan Wing wrote:
>> -----Original Message-----
>> From: mmusic-bounces@ietf.org [mailto:mmusic-bounces@ietf.org] On Behalf
>> Of Markus.Isomaki@nokia.com
>> Sent: Tuesday, December 11, 2012 2:34 AM
>> To: salvatore.loreto@ericsson.com; mmusic@ietf.org
>> Cc: randell@jesup.org
>> Subject: Re: [MMUSIC] SCTP question: Where does it multiplex?
>>
>> Hi Sal,
>>
>> Salvatore Loreto wrote:
>>>
>>> based on the current stack the SCTP traffic is the only traffic that
>>> runs directly over the DTLS stack.
>>> What I am trying to do is to include the Randell Jesup (I am including
>>> him in CC as I am not sure he is subscribed to this mailing list)
>>>   suggestion to give the possibility to have multiple SCTP
>>> *associations* running  on top of the same DTLS session and of course
>>> providing a way to signal it in SDP.
>>>
>>> to be clear: at moment WebRTC allows only one SCTP association per PC,
>>> so this is something that would be nice to define just to be ready for
>> the future.
>>>
>>
>> It's a bit unclear to me what the benefit of this would be. SCTP itself
>> allows multiple streams to be transported in parallel within a single
>> SCTP association. So, in what type of situations would we need multiple
>> parallel SCTP associations? I thought the main point of SCTP is that it
>> provides the parallelism and multiplexing by itself.
>>
>> In other words: I do understand the desire to multiplex RTP and SCTP
>> within the same UDP flow, so we can for instance reduce the number of
>> needed NAT/FW bindings.
>
> Sending bulk data (SCTP) and interactive audio/video over the same port
> will break prioritization of the audio/video flow above the bulk data.

Yeah, probably. In theory you could use DSCP to mark the packets 
differently.

This indeed might be a reason to *not* multiplex the SCTP association 
for data channels with the media. Assuming we find a way to do that 
multiplexing, this would be an argument for making that multiplexing 
*optional*.

>> I also understand why multiple independent non-
>> HOL blocking streams within SCTP are useful for applications. But I
>> don't yet understand the additional benefit of multiple parallel SCTP
>> associations. No doubt it can be technically done, but to what purposes?
>
> One example:  transferring a big file while, at the same time, doing
> screen sharing or an instant message chat.

When using DTLS/SCTP this is an argument for using SCTP associations 
over separate DTLS connections. Two SCTP associations (differing by SCTP 
port) over the same DTLS connection wouldn't solve this problem.

	Thanks,
	Paul