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Posted by Gilles Ganault on April 15, 2007, 7:22 am
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Hi
I was wondering: How do SIP devices open the ports they need for RTP?
I'm about to send an IP phone to someone who doesn't know much about
things computers, so I need to find out if his router can handle
SIP/RTP automagically, or if he'll need to map incoming ports to this
device.
Is there a way to check this easily? Some special words to look for in
the router's documentation ("UPnP?")? Some utility I can run from the
outside or that he can download and run on his computer to check if
his router will open ports dynamically?
Thank you.
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Posted by Wolfgang S. Rupprecht on April 15, 2007, 4:03 pm
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> I was wondering: How do SIP devices open the ports they need for RTP?
Ideally the phones would be put in front of any firewalls or NAT
translations. That's how SIP and RTP are meant to be run and doing it
any other way is a bit of an uphill battle.
If you do need to run SIP and RTP behind a firewall or NAT box you'll
need to read up on "stun" and configure the phones to use one. If
things are working correctly stun should take care of opening up
any stateful firewall and working around any NAT translations.
http://www.voip-info.org/wiki-STUN
-wolfgang
--
Wolfgang S. Rupprecht http://www.wsrcc.com/wolfgang/
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Posted by Gilles Ganault on April 16, 2007, 7:15 pm
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>If you do need to run SIP and RTP behind a firewall or NAT box you'll
>need to read up on "stun" and configure the phones to use one. If
>things are working correctly stun should take care of opening up
>any stateful firewall and working around any NAT translations.
Thanks guys for the tip. I though STUN was simply a way for an SIP
device in a private LAN to learn its public IP address by connecting
out to a STUN server on the Net, but I guess that, when running in
STUN mode, an SIP device keeps sending packets out to keep some ports
open on the router.
In that case, how can the remote SIP device use those open ports? Is
an open UDP port available for use by any remote host, even if it's
not the STUN server that was used to open them in the first place?
I'll read up on STUN. Thank you.
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Posted by Wolfgang S. Rupprecht on April 16, 2007, 9:06 pm
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> In that case, how can the remote SIP device use those open ports? Is
> an open UDP port available for use by any remote host, even if it's
> not the STUN server that was used to open them in the first place?
I believe the STUN hack works as such: Initially both sides talk to
the stun server and it relays the packets. After a while the two
endpoints try to talk to each other directly using the same ports that
they used when talking indirectly via the stun server. The act of the
local phone sending a UDP packet out to the remote phone through the
local stateful firewall should cause the firewall to create a second
opening, this time for the remote phone. Since both sides are doing
the same thing, both firewalls should end up having pass rules
inserted for packets from both the stun server and for the direct UDP
link to the other side. (This is just from my cursory reading, I may
have some of the details wrong. Corrections welcome.)
-wolfgang
--
Wolfgang S. Rupprecht http://www.wsrcc.com/wolfgang/
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Posted by Gilles Ganault on April 18, 2007, 6:58 am
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>I believe the STUN hack works as such: Initially both sides talk to
>the stun server and it relays the packets.
Thanks for the explanation, but most calls are made from two phones
that don't use the same STUN server, if at all. In that case, I wonder
how the STUN trick works.
I'll see what documentation I can gather about STUN, UPnP and possibly
other tricks SIP phones can use to open RTP ports dynamically.
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