[R-C] SCTP as an RTCWEB option (Re: Modular Congestion Control in rtcweb)

[Michael Welzl]

Hi,

I changed the subject as we're really talking about SCTP, not DCCP  
here now. More below:

[snip]


>> Yes, this means an optional different ACKing behavior, with some
>> congestion control logic on the receiver side, but only for the  
>> case of
>> (partially) unreliable transfers. We're blowing up SCTP more and more
>> here... still, this strikes me as a less blown up than having SCTP +
>> DCCP, and possibly (note, only possibly) more efficient than having
>> SCTP-for-data + some_new_cc-over-RTP/UDP

Note that the above paragraph by me captures a view that has meanwhile  
evolved: I now think that there should probably be only one single  
congestion control instance for everything (and then it wouldn't  
matter much whether it's on the sender or receiver side). With that in  
mind, I'll answer below:


> It is an interesting idea which had not really been considered  
> before, and would obviously solve the congestion issue between  
> rtcweb data channels and media channels.
>
> There is an efficiency argument about RTP/SCTP/DTLS/(ICE)UDP  
> compared to plain SRTP/UDP (keyed with DTLS-SRTP or SDES, a whole  
> other religious war over in rtcweb).  It's not a huge impact on  
> video, especially at higher bandwidths, but could be significant for  
> audio-only calls.
>
> In addition, some currently non-critical issues with SCTP would  
> become more critical, such as an issue flagged about large reliable  
> datagrams hogging the packet queues; and there might also be startup  
> speed issues.

I don't see why (a potentially adapted) SCTP couldn't be made to  
always fragment packets beyond a certain size to avoid that hogging  
problem. And I don't understand how that would become more critical?
As for startup speed, I think that this is not true (au contraire! no  
need for slow starting into a network that's already used by another  
stream) if we have only one congestion control instance.


On Apr 8, 2012, at 9:26 AM, Timothy B. Terriberry wrote:

> Keep in mind that one of the arguments against the shim approach to  
> multiple RTP sessions on a single port was that it would break all  
> existing traffic management software at gateways, firewalls, etc.,  
> that tries to inspect RTP/SRTP packets. The same would be true of  
> RTP/SCTP/DTLS/UDP, except moreso (as you would no longer even be  
> able to get at the fields that are normally unencrypted in an SRTP  
> session, regardless of your ability to upgrade software on the  
> gateways and firewalls).
>
> To me this is at least as big a disadvantage as the efficiency  
> argument, though adding that many more bytes of overhead to on the  
> order of 100 packets a second is already a pretty big disadvantage.

I don't know much about and have no strong opinion about that first  
argument, but regarding the overhead, note that there is significant  
overhead in the other variant too: you have SCTP data traffic causing  
receiver-side ACKs that convey pretty much the same information as  
RTCP feedback, but both streams mutually ignore each other...

Cheers,
Michael