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

[Harald Alvestrand]

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":

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?

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).

                  Harald