Interworking Sip Signaling; Interworking Sip 3Xx Redirect Responses; Resultant Invite Traversing Device - AudioCodes Mediant 4000 SBC User Manual

CHAPTER 22 SBC Overview

Interworking SIP Signaling

The device supports interworking of SIP signaling messages to ensure interoperability between
communicating SIP UAs or entities. This is critical in network environments where the UAs on
opposing SBC legs have different SIP signaling support. For example, some UAs may support
different versions of a SIP method while others may not event support a specific SIP method. The
configuration method for assigning specific SIP message handling modes to UAs, includes
configuring an IP Profile with the required interworking mode, and then assigning the IP Profile to
the relevant IP Group.
This section describes some of the device's support for handling SIP methods to ensure
interoperability.

Interworking SIP 3xx Redirect Responses

The device supports interworking of SIP 3xx redirect responses. By default, the device's handling
of SIP 3xx responses is to send the Contact header unchanged. However, some SIP UAs may
support different versions of the SIP 3xx standard while others may not even support SIP 3xx.
The handling of SIP 3xx can be configured for all calls, using the global parameter
SBC3xxBehavior. To configure different SIP 3xx handling options for different UAs (i.e., per IP
Group), use the IP Profiles table parameter, 'SBC Remote 3xx Mode'.

Resultant INVITE Traversing Device

The device can handle SIP 3xx responses so that the new INVITE message sent as a result of the
3xx traverses the device. The reasons for enforcing resultant INVITEs to traverse the device may
vary:
■ The user that receives the 3xx is unable to route to the 3xx contact (i.e., the user is on the LAN
and the new contact is on the WAN). In such a scenario, the device enables the user to reach
the WAN contact and overcome NAT problems.
■ Enforce certain SBC policies (e.g., call admission control, header manipulation, and
transcoding) on the resultant INVITE.
The device enforces this by modifying each Contact in the 3xx response as follows:
■ Changes the host part to the device's IP address – this change causes the remote user agent
to send the INVITE to the device.
■ Adds a special prefix ("T~&R_") to the Contact user part – to identify the new INVITE as a 3xx
resultant INVITE.
The SBC handling for the 3xx resultant INVITE is as follows:
1. The incoming INVITE is identified as a 3xx resultant INVITE according to the special prefix.
2. The device automatically replaces the SBC host part (in the Request-URI) with the host from
the 3xx Contact.
3. The prefix ("T~&R_") remains in the user part for the classification, manipulation, and routing
mechanisms.
4. The classification, manipulation, and routing processes are done exactly like any other INVITE
handling. The special prefix can be used for specific routing rules for 3xx resultant INVITEs.
5. The prefix is removed before the resultant INVITE is sent to the destination.
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Mediant 4000 SBC | User's Manual