connection from ip phone at work to ip phone on client

connection from ip phone at work to ip phone on client
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connection from ip phone at work to ip phone on client beheer 05-01-2007
Posted by on May 1, 2007, 8:45 am
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Hi,

At work there's a Cisco callmanager server. I connect from home to
work using VPN.
I have a Cisco ip phone at home. When my VPN is up, I can call anybody
at work. They can hear me, but I cannot hear them.
If I'm correct, ip phones talk to each other directly after a call is
set up.
I suspect my client side is blocking incoming ip traffic which tries
to initiate a connection to my ip phone.

LAN at work: 192.168.1.x
LAN at home: 192.168.20.x (wireless)
Ip phone is directly connected to my computer on a separate subnet at.
192.168.21.4
VPN client ip: 192.168.1.32

ICS is turned on, on my VPN interface. IP routing in Windows XP is
enabled. Windows Firewall is off.
Gateway 192.168.1.1 at work has a static route (send 192.168.21.4 to
192.168.1.32).
I somehow need to allow connections coming from my VPN interface to
the 192.168.21.x subnet.
Can this be done?

thanks.
Arne


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Posted by Wolfgang S. Rupprecht on May 1, 2007, 11:07 am
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beheer@netbasics.nl writes:
> If I'm correct, ip phones talk to each other directly after a call is
> set up.

They do if they are set up that way. They can also keep any number of
intermediate proxies "in the loop". That obviously adds delay and
isn't as desirable.

> I suspect my client side is blocking incoming ip traffic which tries
> to initiate a connection to my ip phone.
>
> LAN at work: 192.168.1.x
> LAN at home: 192.168.20.x (wireless)
> Ip phone is directly connected to my computer on a separate subnet at.
> 192.168.21.4
> VPN client ip: 192.168.1.32

Well my first guess would be that work doesn't know how to route to
192.168.21.4. Can you ping that address from the phone (or a computer
that is put on the same network as the phone)?

In general, you really want real routeable IP addresses all the way
around. If not, you have to make sure everybody's routing table has
routes for all the involved private networks, and none of them
conflict with other private nets that other folks are using.

-wolfgang
--
Wolfgang S. Rupprecht http://www.wsrcc.com/wolfgang/

Posted by on May 2, 2007, 4:30 am
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> Well my first guess would be that work doesn't know how to route to
> 192.168.21.4. Can you ping that address from the phone (or a computer
> that is put on the same network as the phone)?

All computers and ip phones at work have 192.168.1.1 as default
gateway. On the default gateway I've made a static route
(192.168.21.4/255.255.255.255 to 192.168.1.32 on the LAN interface)
I can't ping my phone from work. I tried to ping directly from the
gateway to my phone. Also doesn't work.
I tried a packet sniffer on my computer. I don't see any packets when
pinging. I suspect Windows is blocking because I think it's like
trying to reach a local computer directly from the internet.

> In general, you really want real routeable IP addresses all the way
> around. If not, you have to make sure everybody's routing table has
> routes for all the involved private networks, and none of them
> conflict with other private nets that other folks are using.

Other options:
- when my phone is used, the call manager server should act as a proxy
(I can't find any settings to do that)
- my phone is known to the call manager server as 192.168.21.4. If I
could somehow translate the return packets to 192.168.1.32, then I
think it would work. I would forward the packets coming in on
192.168.1.32 to 192.168.21.4
- do some kind of NAT on my computer so that my phone identifies
itself as 192.168.1.32

All looks complicated, if feasible at all.


Posted by Wolfgang S. Rupprecht on May 2, 2007, 10:27 am
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beheer@netbasics.nl writes:
>> Well my first guess would be that work doesn't know how to route to
>> 192.168.21.4. Can you ping that address from the phone (or a computer
>> that is put on the same network as the phone)?
>
> All computers and ip phones at work have 192.168.1.1 as default
> gateway. On the default gateway I've made a static route
> (192.168.21.4/255.255.255.255 to 192.168.1.32 on the LAN interface)

Sounds ok, so far.

> I can't ping my phone from work. I tried to ping directly from the
> gateway to my phone. Also doesn't work.
> I tried a packet sniffer on my computer. I don't see any packets when
> pinging. I suspect Windows is blocking because I think it's like
> trying to reach a local computer directly from the internet.

Ah, I didn't realize you are NAT-ing the phone? I don't know anything
about MS software, but isn't there some way to turn off the NAT? You
really don't need it or want it. Straight routing would remove quite
a lot of issues.

> - when my phone is used, the call manager server should act as a proxy
> (I can't find any settings to do that)
> - my phone is known to the call manager server as 192.168.21.4. If I
> could somehow translate the return packets to 192.168.1.32, then I
> think it would work. I would forward the packets coming in on
> 192.168.1.32 to 192.168.21.4
> - do some kind of NAT on my computer so that my phone identifies
> itself as 192.168.1.32

NAT-ing voip's SIP (control) and RTP (voice data) packets requires
more work in the NAT than just slapping a new IP address in the IP
header. There are copies of the IP address in the data part of the
SIP packet that needs to be translated too. NAT can be made to work,
but only if you have the right kind of NAT and some external help from
a STUN server. If you can turn off the NAT you save yourself a world
of hurt.

> All looks complicated, if feasible at all.

When you hit a brick wall, walk around it. ;-) Change the rules of
the game.

-wolfgang
--
Wolfgang S. Rupprecht http://www.wsrcc.com/wolfgang/

Posted by Martin² on May 1, 2007, 10:19 pm
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>They can hear me, but I cannot hear them.

That's usually question of opening the right ports on your computer.
You may need to open 5060, 5004, 5082, 3478, 3479, 8000, and 10000 if Stun
is needed.
Regards,
Martin



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