Version 7.2 629 Mediant 1000B Gateway & E-SBC
User's Manual 28. SBC Overview
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.
Figure 28-7: SIP 3xx Response Handling
The process of this feature is described using an example:
1. The device receives the Redirect server's SIP 3xx response (e.g., Contact:
<sip:User@IPPBX:5060;transport=tcp;param=a>;q=0.5).
2. The device replaces the Contact header value with the special prefix and database
key value as user part, and with the device's URL as host part (e.g., Contact:
<sip:Prefix_Key_User@SBC:5070;transport=udp>;q=0.5).
3. The device sends this manipulated SIP 3xx response to the Far-End User (FEU).
4. The FEU sends a new request with the Request-URI set to the value of the received
3xx response's Contact header (e.g., RequestURI:
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