Re: [AVT] RFC3550 RTP Session defintion Q

[Stephen Casner <casner at acm.org>]

On Wed, 11 Aug 2004, Oren Peleg wrote:

> In the RFC 3550 RTP session definition it is written:
>
>      "A participant distinguishes
>       multiple RTP sessions by reception of different sessions using
>       different pairs of destination transport addresses"

This says that distinct sessions must use different port pairs.  It
does not say that a single session can use only one port pair.

> While at the end of the paragraph it is written:
>
>      "All participants in an RTP session may
>       share a common destination transport address pair, as in the case
>       of IP multicast, or the pairs may be different for each
>       participant, as in the case of individual unicast network
>       addresses and port pairs.  In the unicast case, a participant may
>       receive from all other participants in the session using the same
>       pair of ports, or may use a distinct pair of ports for each"

This says that in one session, the port pairs may be the same for all
participants or may be different.  When they are different, then
usually some signaling will be required to know about all the
participants that should be joined into the session.  With IP
multicast using a common port pair for all, as in the early days of
the MBone, a participant needs only to know the destination address
(pair) to join and then learns of all the participants.

> How can one have distinct pair of ports for each participant in a RTP
> session while still maintaining the definition that different sessions
> have different destination transport addresses? Can anyone help me
> understand this contradiction?

Do the additional words above explain it for you?  The next paragraph
in RFC 3550 after the one you quote explains the distinguishing
feature of an RTP session.  Also note the paragraph after that which
says that a particular application design is likely to impose further
constraints.

                                                        -- Steve

_______________________________________________
Audio/Video Transport Working Group
avt at ietf.org
https://www1.ietf.org/mailman/listinfo/avt