Subject: [Sip] Clarification with respect to "To" and "From" headers

From: Abhishek Gupta <[email-masked]>
Date: Mon, 30 May 2011 21:16:38 +0530

Hi Forum,

I have a simple scenario as mentioned below -

EP1-------------------->SBC-------------------->EP2

As we all know that an SBC is a B2BUA device and its main function is
Topology Hiding that is achieved through header manipulation.

Now, for EP1, SBC acts as a UAS and for EP2 it acts as a UAC.

I have configured two endpoints EP1 with IP1 and EP2 with IP2 that access
the SBC through AP1 and AP2 respectively (Pls read AP as Access Point).  An
Access Point is nothing but an IP through which EP1 and EP2 communicate.

Say, EP1 sends INVITE to SBC containing as follows -

INVITE sip:5555@AP1 SIP/2.0
Via: SIP/2.0/UDP IP1;branch=z9hG4bK776asdhds
Max-Forwards: 70
To: <sip:5555@AP1>
From: <sip:1111@IP1>;tag=1928301774
Call-ID: a84b4c76e66710@IP1
CSeq: 1 INVITE
Contact: <sip:1111@IP1>
Content-Type: application/sdp

(SDP not shown)

Upon receiving INVITE, SBC forms a request and forwards to the EP2 as
follows -

INVITE sip:5555@IP2 SIP/2.0
Via: SIP/2.0/UDP AP2;branch=z9hG4bKhoiasdb
Max-Forwards: 69
To: <sip:5555@AP2>
From: <sip:1111@AP2>;tag=98765479
Call-ID: ufs8doi8afjnfd67asdfhu
CSeq: 1 INVITE
Contact: <sip:1111@AP2>
Content-Type: application/sdp

(SDP not shown)

Although the call was successful after responses from EP2, I would kindly
like to know from your side how correct is "To" and "From" header values
with respect to leg2 as highlighted ??  Though there is no impact due of
this behavior.

If it is correct, no issues !
If it is incorrect what should be the correct behavior ?

Thanks & Regards,

Abhishek Kumar Gupta

---

From: Iñaki Baz Castillo <[email-masked]>
Date: Mon, 30 May 2011 18:06:21 +0200

> If it is incorrect what should be the correct behavior ?

Hi Abhishek. First of all, I strongly think this kind of questions
should take place in sip-implementors maillist rather than here (this
maillist is to discuss about SIP related specifications).

Replying your question (but please, next one ask in sip-implementors)
a B2BUA can behave as it wants, so if it desires it can change From/To
or whatever. From the point of view of pure SIP, EP1 just cares about
signalling with B2BUA (UAS) and EP2 just cares about signalling with
B2BUA (UAC).

-- 
Iñaki Baz Castillo
<[email-masked]>