AudioCodes Mediant 500 E-SBC, User Manual

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User's Manual 27. SBC Overview
Version 6.8 457 Mediant 500 E-SBC
27.7 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.
27.7.1 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. For configuring different SIP 3xx handling options for different UAs (i.e.,
per IP Group), use the IP Profile table parameter, 'SBC Remote 3xx Behavior'.
27.7.1.1 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.