Transparently mantain "Via" header field across CUCM

[mmenozzi]

Hi team.

I'm not an expert here and sorry for the silly question.

My customer would like to understand if via SIP LUA or other ways it could be possible to "mantain" the "Via" header field when routing a call across CUCM.

this the scenario

Phone -- > Expressway --- > CUCM1 -- > SIP trunk -- > CUCM2-- > phone

When the call comes in from Expressway to CUCM1 there is a specific "via" header field like this

"Via: SIP/2.0/TLS 192.1.2.1:54200; branch=z9hG4bk00000012; received=172.1.1.2;rport=52696;..........snip"

when the call is routed through CUCM the same header is changed as:

"Via: SIP/2.0/TLS 10.1.1.1:5060; branch=z9hG4bk0234566712....snip"

Question is: is there a script could help to mantain the same Via header field  ?

thanks

regards

Marco

[Mark Stover]

This is possible. You have to use transparency and place the 'known' Via: header in an 'unknown' header to pass it through CUCM. I'm not sure why you would need to remove CUCM from the Via: headers; it typically doesn't impact the call setup on the two UAs you're trying to connect.

There is an example in the Developer Guide that shows how you do this with Reason: header, but it would work similarly for Via:

[mmenozzi]

If I understood well, when you mean "known" you mean the one that is sent by CUCM1 ?

Do you think this is possible even if the incoming call is NOT coming from a SIP trunk ?

Marco

[Mark Stover]

'known' means that CUCM recognizes and will process this header. It's why you see the header change when the call traverses CUCM. Where is the call coming from if it's not a SIP trunk? You can do Normalization & Transparency scripts on a SIP line or a SIP trunk.

[mmenozzi]

The SIP INVITE comes from an Expressway configured as MRA. In this scenario there is NOT any SIP trunk to CUCM. Expressway is in theory seen as "SIP line" or "endpoint".

[Mark Stover]

I'm not sure why you would want to change the Via: in this scenario. It should be working as designed.

[mmenozzi]

The objective is to maintain the original "Via" header filed to be able to keep track of original IP addressing information.

I'm wondering if it would be helpful to build a script that replace the outbound SIP invite "via" header field with the same one of incoming SIP invite and if someone could suggest me how to do.

thanks.

[Mark Stover]

Okay, but that kind of breaks the purpose of Via:, which may break the ability to get back to the source...

http://tools.ietf.org/html/rfc3261#section-8.1.1.7

If you really want to do this, you would want to use the method in the Developer Guide:

https://developer.cisco.com/fileMedia/download/7e805b95-8bd1-46ea-a759-89797326f58b

If you look beginning on Page 63 of that guide, it has examples on how to use the pass-through APIs to accomplish this.

Mark