[RTW] Symmetric RTP/RTCP (Re: Fwd: I-D Action:draft-perkins-rtcweb-rtp-usage-00.txt)

[Magnus Westerlund]

Harald Alvestrand skrev 2011-03-07 14:32:
> Having finished reading the rtp-usage draft, I am impressed. This is 
> good work, and will help a lot going forward.
> 
> One thing I want to check, because it seems that the only place that 
> says "Using ... is REQUIRED" rather than "Support ... is REQUIRED":
> 

It is primarily a question of inconsistent writing. It needs to be
supported, but likely also forced to be used.

This is one thing that needs to be tighten in the doc if it was to
become a specification. Which features needs to be supported, and which
needs to be always used, as there is a difference.


> 5.3.  Symmetric RTP/RTCP
> 
>     RTP entities choose the RTP and RTCP transport addresses, i.e., IP
>     addresses and port numbers, to receive packets on and bind their
>     respective sockets to those.  When sending RTP packets, however, they
>     may use a different IP address or port number for RTP, RTCP, or both;
>     e.g., when using a different socket instance for sending and for
>     receiving.  Symmetric RTP/RTCP requires that the IP address and port
>     number for sending and receiving RTP/RTCP packets are identical.
> 
>     Using Symmetric RTP and RTCP [RFC4961] is REQURIED.
> 
> In the STUN-based firewall traversal scenario, STUN will discover a 
> <sender address/port, recipient address/port> at the sender that will 
> cause delivery of packets with a corresponding (not necessarily 
> identical) <sender address/port, recipient address/port> at the 
> recipient, and that the recipient can swap those addresses around and 
> have the packets delivered to the sender (the STUN connectivity check is 
> bidirectional). No guarantees are made for any other <sender 
> address/port, recipient address/port> pair, so non-symmetric RTP/RTCP 
> seems likely to fail.
> 
> Is this the (only) reasoning that led to this requirement?

NATs and Firewalls are the core of the need for symmetric RTP. If they
didn't exist, then knowing the source address and port for a packet
would have less value. But today, then not using symmetric RTP leads to
failure to communicate. There is simple no realistic option to not use it.

> 
> If so, should it be inserted into the document so that people can 
> reproduce the thinking? (and if there are other reasons, which I haven't 
> thought of, it might be good to document those too).

Yes, we should add a bit of motivation and not only description why
symmetric RTP is necessary.

Cheers

Magnus Westerlund

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