Help understand Command in SIP Profile

[Gary Tong]

Hi all,

Recently we're deploying the CUBE SIP service but there is one command for SIP profile cannot be understood by us. This is one command requested by the service provider but we would like to understand the meaning of this command.

voice class sip-profiles 1

request CANCEL sip-header Max-Forwards modify ".*" "User-Agent: CUBE"

We thought Max-Forwards should be modified to some number.

Please kindly help. Thank you.

Regards,

Gary

[Ayodeji Okanlawon]

Garry,

In the CANCEL request, your ITSP wants to  modify the max forwards header from a number to the string user0agent:CUBE. I am not sure why they want to do this since maximum forwards should be a numerical value. The sample trace below is an example of a a CANCEL method. You can see the max forwards header..this is what they are trying to modify

Sent:
CANCEL sip:0171626666@10.136.78.24:5081 SIP/2.0
Via: SIP/2.0/UDP 10.105.40.74:5060;branch=z9hG4bK38996EB1EC
From: <sip:02270999990@10.10.10.14>;tag=259B0918-3C6
To: <sip:441216264431@0.136.78.24>
Date: Mon, 22 Sep 2014 10:29:55 GMT
Call-ID: 3A9895D7-417A11E4-B2A18F4D-5D7E5E41@10.10.10.14
CSeq: 101 CANCEL
Max-Forwards: 70
Timestamp: 1411381796
Reason: Q.850;cause=16
Content-Length: 0

According to the RFC 3261, your ITSP is wrong in attempting to change this to a value other than a number.

8.1.1.6 Max-Forwards

The Max-Forwards header field serves to limit the number of hops a request can transit on the way to its destination. It consists of an integer that is decremented by one at each hop. If the Max-Forwards value reaches 0 before the request reaches its destination, it will be rejected with a 483(Too Many Hops) error response. A UAC MUST insert a Max-Forwards header field into each request it originates with a value that SHOULD be 70. This number was chosen to be sufficiently large to guarantee that a request would not be dropped in any SIP network when there were no loops, but not so large as to consume proxy resources when a loop does occur. Lower values should be used with caution and only in networks where topologies are known by the UA.