Subject: [SIP] meaning of IP address in SDP

From: Jonathan Rosenberg <[email-masked]>
Date: Fri, 27 Apr 2001 16:46:04 -0400

Folks,

There appears to be some continuing confusion about the interpretation of
the IP address in the SDP of an INVITE or 200 OK.

When user A sends an INVITE with SDP, and the stream is sendrecv (which is
the default), the IP address in the c line for that stream is the address
where A expects to receive media for that stream. It DOES NOT imply that the
source address of media that A sends will be equal to that IP address. The
source IP address is chosen by the underlying OS based on its routing
tables. As such, a multihomed host has no way to know what the source
address of its RTP packets will be, until it actually tries to send them.
So, clearly, the IP address in the c line in the INVITE cannot possibly be
the source address, since A doesn't even know yet where it will send
packets.

Unfortunately, since rfc2543 does not explicitly say that the source address
won't be the address in the c line, several implementations appear to have
assumed that it will be. I want to make sure we are clear that this
assumption is false.

Please, in your implementations, do not make any assumptions about the
source address of media.

Thanks,
Jonathan R.

---
Jonathan D. Rosenberg                       72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
[email-masked]                     FAX:   (973) 952-5050
http://www.jdrosen.net                      PHONE: (973) 952-5000
http://www.dynamicsoft.com

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