[SR-Users] kamailio with mediaproxy-ng, 488 Not Acceptable Here

[Richard Fuchs]

On 02/18/14 11:12, Mihai Marin wrote:
> Hello Sirs,
> Thank you, one step forward but still buggy - half buggy :)
> 
> Now, it's working just one way. If bob calls alice, alice will receive
> video but bob won't. If I stop mediaproxy-ng process (without any other
> modification) and redo the call, everything is just fine.
> 
> The invite sdp that alice receives is here http://pastebin.com/BgZZbbEA
> The sdp from OK that bob receive is here http://pastebin.com/ADGt3D3E
> 
> I noticed something strange in those 2 SDPs (just audio candidates,
> video are the same):
> 1. offer
> 1.a. stun candidates
> a=candidate:1855365925 1 udp 1845501695 188.215.94.132 63015 typ srflx
> raddr 192.168.0.5 rport 63015 generation 0
> a=candidate:1855365925 2 udp 1845501695 188.215.94.132 63015 typ srflx
> raddr 192.168.0.5 rport 63015 generation 0
> 1.b. turn candidates
> a=candidate:Om0baCmE7kgKNPvv 1 UDP 2130706432 93.187.138.214 30008 typ host
> a=candidate:Om0baCmE7kgKNPvv 2 UDP 2130706431 93.187.138.214 30009 typ host
> 
> 2. answer
> 2.a. stun candidates
> a=candidate:1855365925 1 udp 1845501695 188.215.94.132 63028 typ srflx
> raddr 192.168.0.5 rport 63028 generation 0
> a=candidate:1855365925 2 udp 1845501694 188.215.94.132 63031 typ srflx
> raddr 192.168.0.5 rport 63031 generation 0
> 2.b. turn candidates
> a=candidate:Om0baCmE7kgKNPvv 1 UDP 2130706432 93.187.138.214 30010 typ host
> 
> No bundle in sdp but: 
>  - offer has 1 single port for stun candidates and 2 for turn candidates
>  - answer has 2 ports for stun candidates and 1 for turn candidates
> 
> The kamailio log can be accessed at http://pastebin.com/tVgE3BGt
> 
> Do you have any idea why this behavior?

Yes, this is caused by MP-NG demultiplexing RTCP. I haven't thought of
this before, as in our case we only use MP-NG with the '+' flag, which
would eliminate this issue.

What happens is that MP-NG supports rtcp-mux in that it answers to an
rtcp-mux offer and handles it correctly, but it never offers rtcp-mux to
anyone, unless that endpoint is already talking rtcp-mux.

So from Alice's offer, MP-NG knows that Alice supports rtcp-mux and
gladly accepts the offer in the answer sent to Alice. In this case, the
same port is used for RTP and RTCP, which implies that only one ICE
candidate is given for that port (only in the answer though -- in the
offer, two must be given as a fallback).

However, MP-NG doesn't offer rtcp-mux to Bob, which means that Bob's
answer includes separate ICE candidates for RTP and RTCP (due to
separate ports). As those ICE candidates are left in place by MP-NG when
translating the answer (which will include rtcp-mux), there's now a
disagreement between the ICE candidates from Bob (two per candidate due
to lack of rtcp-mux) and the ICE candidate inserted by MP-NG (only one
due to rtcp-mux).

There's two possible solutions without resorting to the '+' flag...
Either make MP-NG offer rtcp-mux to Bob initially (who may or may not
accept it -- if he doesn't, nothing is fixed), or make it reject the
rtcp-mux offer coming from Alice (which violates the WebRTC spec).

Another thing that MP-NG could do is to remove the RTCP ICE candidates
in the answer going to Alice, hoping that Bob knows to demultiplex the
RTCP packets coming in on the RTP port, despite rtcp-mux not having been
negotiated.

I can't say I like any of these options :)

cheers

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