[sip-comm-dev] RTP ports in SC ?


#1

Hi Emil,

you said a long time ago that SC would grab UDP ports 5000 and
5001 for one direction and 5002 and 5003 for the other direction,
if these are available. If another instance of SC on the LAN
needed some RTP ports, it would take 5004 + 5005 and 5006
+ 5007 for the other direction, since 5000-5003 are already
occupied.

I do not see how a router can forward ports if the ports are
assigned dynamically. If I tell the router to forward 5000-
5003 to LAN IP 192.168.3.73 and ports 5004-5007 to LAN IP
192.168.3.155, but SC assigns ports dynamically and randomly,
this can not function correctly.

How does the router + firewall know how to send incoming
packets to the correct LAN IP?

Is it not necessary for the user to be able to specify which
ports to use for audio RTP and video RTP? Some SIP pgms
allow the user to specify audio and video ports.

Regards, Earl

···

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#2

Hey Earl,

Earl написа:

Hi Emil,

you said a long time ago that SC would grab UDP ports 5000 and
5001 for one direction and 5002 and 5003 for the other direction,

SIP Communicator would indeed start allocation ports at 5000 and then
move up in case it fails to bind for some reason. This means that in a
default case two instance of SIP Communicator are likely to both use
ports 5000 and 5001 for their audio stream.

if these are available. If another instance of SC on the LAN
needed some RTP ports, it would take 5004 + 5005 and 5006
+ 5007 for the other direction, since 5000-5003 are already
occupied.

I do not see how a router can forward ports if the ports are
assigned dynamically. If I tell the router to forward 5000-
5003 to LAN IP 192.168.3.73 and ports 5004-5007 to LAN IP
192.168.3.155, but SC assigns ports dynamically and randomly,
this can not function correctly.

It is generally not necessary to do this on your router. Media streams
are bidirectional which means that SIP Communicator would be sending
media from the same port that it expects to receive media on. A NAT
would there for let incoming packets for that port pass. Of course this
also means that you need a media relay in the middle that would be smart
enough to realize that you are sending from behind a NAT and that the
port number you advertise is likely to be different from the one you
actually use for sending and receiving. Luckily this would be the case
for most providers out there.

If, however, you need to absolutely map ports for some reason then you'd
need to do this on a range basis and forward all ports from 5000 to 6000
to the PC running your SC instance.

Hope this helps,
Emil

···

How does the router + firewall know how to send incoming
packets to the correct LAN IP?

Is it not necessary for the user to be able to specify which
ports to use for audio RTP and video RTP? Some SIP pgms
allow the user to specify audio and video ports.

Regards, Earl

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Emil Ivov, Ph.D. 67000 Strasbourg,
Project Lead France
SIP Communicator
emcho@sip-communicator.org PHONE: +33.1.77.62.43.30
http://sip-communicator.org FAX: +33.1.77.62.47.31

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