[SR-Users] Handling a non-standard SIP port

[Jean-Marie Baran]

Hi,

This is a question for the sake of comprehension, as I actually solved 
the problem with a workaround.

I am trying to get Kamailio working with a third party SIP provider. The 
SIP server of this provider pings my Kamailio with OPTIONS messages. 
Being located in a sub-network, the SIP messages are relayed to a router 
(not a SIP router, just a gateway to the Internet) which is not 
listening/sending on port 5060 but on 13377. When Kamailio answer to 
OPTIONS, it send back the message to the router on the port 5060 which 
is wrong. Here is the topology:

*Kamailio* *Router* *SIP Server*
/my.sip.domain               174.37.217.1 197.17.144.4/
    |                              | ? <-------------------- 5060 |
    | 5060 <---------------- 13377 |                              |
    | 5060 ----------------−> 5060 |                              |
    |                              | X                            |

You can see that, Kamailio answers to the wrong port, but I think this 
is somewhat coherent since the SIP message says that the port 5060 is in 
use. But still, I would have liked that the correct port was used:

*Kamailio* *Router* *SIP Server*
/my.sip.domain               174.37.217.1 197.17.144.4
/   |                              | ? <-------------------- 5060 |
    | 5060 <---------------- 13377 |                              |
    | 5060 ---------------−> 13377 |                              |
    |                              | ? --------------------> 5060 |

But I do not understand where Kamailio is supposed to find the correct 
port. Below are the transport frames, and SIP frames.

Kamailio receives:

Internet Protocol Version 4, Src: 174.37.217.1 (174.37.217.1), Dst: 
172.16.8.61 (172.16.8.61)
     Source: 174.37.217.1 (174.37.217.1)
     Destination: 172.16.8.61 (172.16.8.61)
User Datagram Protocol, Src Port: 13377 (13377), Dst Port: sip (5060)
     Source port: 13377 (13377)
     Destination port: sip (5060)

OPTIONS sip:my.sip.domain:5060 SIP/2.0
Via: SIP/2.0/UDP 197.17.144.4:5060;branch=z9hG4bKb9340cb29ba
From: <sip:197.17.144.4>;tag=1797904074
To: <sip:my.sip.domain>
Date: Mon, 10 Aug 2015 10:01:51 GMT
Call-ID: c7f38580-5c817680-b94-38512c6 at 197.17.144.4
User-Agent: Cisco-CUCM10.5
CSeq: 101 OPTIONS
Contact: <sip:197.17.144.4:5060>
Max-Forwards: 0
Content-Length: 0

...and then sends:

Internet Protocol Version 4, Src: 172.16.8.61 (172.16.8.61), Dst: 
174.37.217.1 (174.37.217.1)
     Source: 172.16.8.61 (172.16.8.61)
     Destination: 174.37.217.1 (174.37.217.1)
User Datagram Protocol, Src Port: sip (5060), Dst Port: sip (5060)
     Source port: sip (5060)
     Destination port: sip (5060)

SIP/2.0 200 OK
Via: SIP/2.0/UDP 
197.17.144.4:5060;branch=z9hG4bKb9340cb29ba;received=174.37.217.1
From: <sip:197.17.144.4>;tag=1797904074
To: <sip:my.sip.domain>;tag=b27e1a1d33761e85846fc98f5f3a7e58.de7a
Call-ID: c7f38580-5c817680-b94-38512c6 at 197.17.144.4
CSeq: 101 OPTIONS
Allow: INVITE,ACK,BYE,CANCEL,INFO,OPTIONS,REFER,SUBSCRIBE,NOTIFY
Accept: */*
Accept-Encoding:
Accept-Language: en
Supported:
Server: kamailio (4.3.1 (x86_64/linux))
Content-Length: 0

We see clearly that the port is wrong here. But in the received SIP 
frame, everything suggests that the correct port is 5060, and I do not 
think Kamailio will look at the transport frame to send the response 
back to the correct port. Am I correct stating this ? Maybe the Via 
header should contain a port along with the IP address ?

I solved the problem by using TCP instead of UDP, as I know that since 
the TCP connection is maintained, the port would be correct (because 
imposed by the gateway).

I am sorry if the question does not directly relates to Kamailio, but 
any explanation here would be appreciated, as I am a student trying to 
learn how all this is working :)

Cheers,
Jean-Marie.
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