10 feb 2009 kl. 13.44 skrev Iñaki Baz Castillo:
2009/2/10 Johansson Olle E <o...@edvina.net>:alice --- (NAT A) --- ProxyA & RtpProxyA --- ProxyB & RtpProxyB --- (NAT B) --- bobIn this case, when alice calls bob, ProxyA will apply RtpProxyA so theSDP will contain a public IP. Since ProxyB knows that bob is registered behind NAT, it will try toapply RtpProxyB but this will "fail" because the incoming SDP containsa line: a=nortpproxy:yes This line was added by ProxyA when applying RtpProxyA.When ProxyB executes "force_rtpproxy()" this function will not modifythe SDP since that line is present. If not, there will be no audio because RtpProxyA would be waiting for audio from RtpProxyB and vice versa (lock condition).On the INVITE there's no need for ProxyB to allocate an rtpproxy, sincethe SDP already has public IP - fixed by Proxy A.No, that's not an excuse. RtpProxy must be applied in both the request and response. If a PSTN gateway calls a SIP user behind NAT, you must apply RtpProxy even if the incoming SDP has public address.
Not in the INVITE sdp - the device behind the NAT can always send to a public IP address, right?
ProxyB must be well configured, this means that since "force_rtpproxy()" didn't success on the incoming INVITE, it must noexecute "force_rtpproxy()" on the 18X/2XX response from bob. For this, I use a bflag(RTPPROXY_SET) which only set to 1 if "force_rtpproxy()"succeded in the incoming INVITE, and only run "force_rtpproxy()" for responses from bob if that bflag is on.It should force RTPproxy on teh response from Bob, since Bob is a local user and the SDP includes a private IP.Not again, please re-read my explanation above :) In the example of a PSTN gateway calling a natted SIP user, if the proxy only applies RtpProxy in the user response then you will get asymmetric audio: user (NAT) RtpProxy Gateway --------------------------(RTP)-------------------> <---------(RTP)---------- <---------(RTP)---------- So the RTP from the gateway will not arrive to the user since the NAT router will not allow it (there is not an existing UDP mapping to allow UDP traffic from the RtpProxy, but just from the Gateway address).
The gateway will (in teh case of Asterisk) listen to the audio coming from the user and change to the port and IP we're receiving audio from. That way, we'll have symmetric audio and will bypass the NAT.
So RtpProxy must be present in the request and response, then the NAT router mapping will work and allow incoming data from RtpProxy after the user sends RTP data to the RtpProxy. Am I wrong?
We are mixing cases here. One case is a gateway scenario, where the RTP proxy isn't really needed, since the gateway may take care of it by itself, provided that the gateway is on a public IP. If you have two users both behind NAT, each user applies an RTP proxy to the incoming audio stream, not the outbound stream. /O
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