Acme Packet 6300
 

PLA-Support>Sun Systems>CommsGBU>Session Delivery Network>SN-SND: Acme Service Provider
 

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Goal

Solution

Applies to:

Goal

Does SBC allows multiple to-tags within same SIP call flow to caller when call forking?

In case of  sip recursion call flow, kind of a serial forking on core side of SBC. After SBC receives provisional response (180/183) from outbound session-agent, followed by 503 which triggers SBC to go to next hop agent in a Session-agent group.
SBC changes the `TO' tag in dialog between itself and the caller whenever it forwards provisioning responses (180/183) to the caller from each SA.

Solution

This behaviour is compliant with  RFC3261 

Section 13.1 Overview

"Before sending a final response, the UAS can also send provisional responses (1xx) to advise the UAC of progress in contacting the called user.
After possibly receiving one or more provisional responses, the UAC will get one or more 2xx responses or one non-2xx final response."

1xx Responses Zero, one or multiple provisional responses may arrive before one or more final responses
are received. Provisional responses for an INVITE request can create “early dialogs”.

As defined in RFC 3261 [RFC3261], a Session Initiation Protocol (SIP) early dialog is created when a non-100 provisional response is sent to the initial dialog initiation request (e.g., INVITE, outside an existing dialog).
The dialog is considered to be in early state until a final response is sent. When a proxy receives an initial dialog initiation request, it can forward the request towards multiple remote destinations.
When the proxy does that, it performs forking [RFC3261].

When an initial INVITE causes multiple dialogs due to forking, an offer/answer exchange is carried out independently in each distinct dialog.

The different to-tags should be sufficient for the client to recognize that the 2nd 183 is in a different early-dialog.

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