[pjsip] Symmetric RTP

Alan J. Bond alan.bond at wintology.com
Wed May 7 14:03:00 EDT 2008

Hi Nanang,

Good!  It's already symmetric then (by my definition anyway).  I just noticed pjsua opened 4 ports at start up.  I can see now that's directly related to the --max-calls default.
A lot of UAs default to two lines,  I saw 4 ports and alarm bells went off in my head.  Here's why ...

An early attempt at NAT traversal (now non-compliant) used a technique of fiddling with the address  in Contact: headers on registration (replacing internal with external on reply) in conjunction with an RTP relay on a public IP to eliminate NAT-to-NAT scenarios.  That is I think the origin of the fix_nated_contact() option in (Open)SER. However, that's SIP.  For media streams the RTP proxy part only works with symmetric RTP on some NATs (most domestic ADSL routers these days).  Wherever the client outgoing stream appears to the proxy is where the incoming stream gets sent back.  No STUN or similar required in theory, but very heavy traffic hit on the provider's infrastructure.  Some carrier class RTP relays use megabucks hardware built specifically to handle the load, which won't get scrapped in a hurry (especially with core bandwidth getting ever cheaper).

While ICE will be wonderful anytime soon, some commercial platforms will carry on using this method regardless - well after ICE has been ratified and adopted.  It's therefore perhaps important for widespread adoption that we try hard to accommodate this sort of thing as long as it doesn't compromise anything compliant in any way.  No worries on this issue anyway :-)

Anyone with other non-compliant behaviours we ought to be thinking about?  Any  thoughts on UPnP support (no rants please)?


From: pjsip-bounces at lists.pjsip.org [mailto:pjsip-bounces at lists.pjsip.org] On Behalf Of Nanang Izzuddin
Sent: 06 May 2008 09:16
To: pjsip list
Subject: Re: [pjsip] Symmetric RTP

Hi Alan,

Not sure about symmetric RTP definition, but current pjmedia transport implementation does use single port to send & receive RTP.


2008/5/5 Alan J. Bond <alan.bond at wintology.com<mailto:alan.bond at wintology.com>>:

Has anyone any thoughts on adapting the stack to do symmetric RTP (i.e. media stream in and out of the same UDP port)?

Best Regards,


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