Re: [AVTCORE] Mandatory nature of RTCP

[Paul Kyzivat <[e-mail]>]

Magnus,

IMO the first question in this area that comes up for siprec is:

1) must we be able to record the RTP of a call when that RTP
    doesn't have RTCP?

    I think the answer is clearly yes.
    There seems to be enough wiggle room in 3550 that we can't
    argue that the call is invalid and we don't need to record it.
    (And in general we should try to record calls even if they
    *are* invalid.)

The next question that comes up is:

2) when recording a call with a media stream that doesn't have
    RTCP, must the siprec SRC synthesize RTCP in order that it will be
    available to the receiver? Or can we leave it to the siprec SRS
    to deal with it?

I think we need AVTxxx input into siprec on such issues.
Decisions on these things can potentially have significant architectural 
impact on the way recording is done.

	Thanks,
	Paul

On 5/9/2011 8:29 AM, Magnus Westerlund wrote:
> On 2011-05-09 13:20, Schwarz, Albrecht (Albrecht) wrote:
>> RTCP is NOT MANDATORY according RFC 3550.
>> I fail to see any normative statement, too.
>> I could also imagine RTP use cases which do not need at all RTCP.
>
> I think I agree that RTCP is not mandated directly. However, in reality
> it is required. Only in a few very limited usages can one skip to use RTCP.
>
> If one uses UDP as underlying transport then RTCP is needed to fulfill
> the congestion control requirements in RFC 3550 and the profiles
> (RFC3551). Both has a SHOULD requirement and that SHOULD exist to allow
> cases where you have dedicated resource.
>
> Then we have all the binding information that RTCP provides, like time
> synchronization (RTCP SR), which to be synchronized (SDES CNAME). Some
> single media applications can live without this, but any multi-media do
> need some of this.
>
> There is a reason why RTP doesn't contain so much hard requirements. RTP
> is a toolbox and applications are suppsed to pick the pieces it needs.
> And general RTCP is needed in most cases. So I would recommend any one
> deciding on RTP/RTCP should start with describing what application
> requirements they have. Then pick the RTP parts.
>
> But I will guess that SIPREC is likely to need RTCP. But please do your
> analysis of what you need.
>
> Cheers
>
> Magnus Westerlund
>
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