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Cisco Unified Border Element (SP Edition) provides support for 100rel (SIP Provisional Message Reliability) interworking. This feature provides support to resolve the interoperability problem of inconsistent support for SIP reliable provisional responses encountered when SBC works with different SIP networks.
SIP defines two types of responses: provisional and final. Final responses (2xx-6xx) convey the result of the request processing and are sent reliably. SIP provisional responses (1xx) do not have an acknowledgement system so they are not reliable. There are certain scenarios in which the provisional SIP responses (1xx) must be delivered reliably. For example in a SIP/PSTN interworking scenario it is crucial that the 180 and 183 messages are not dropped. The use of the Provisional Response ACKnowledgment (PRACK) method enables reliability to be offered to SIP provisional responses.
The 100rel option is used to indicate that the reliable provisional responses are supported or required, and the PRACK message is used to acknowledge receipt of a reliable provisional response.
Note This feature is supported in the unified model for Cisco IOS XE Release 2.5 and later.
Cisco Unified Border Element (SP Edition) was formerly known as Integrated Session Border Controller and may be commonly referred to in this document as the session border controller (SBC).
Feature History for 100rel Interworking Support
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The following restrictions apply when you configure the 100rel interworking support on the Cisco Unified Border Element (SP Edition):
– The Cisco Unified Border Element (SP Edition) configuration must be set up on the inbound adjacency of the applicable call to act as a PRACK User Agent Server (UAS) during 100rel interworking.
– The Cisco Unified Border Element (SP Edition) configuration must be set up on the outbound adjacency of the applicable call to act as a PRACK User Agent Client (UAC) during 100rel interworking.
The 100rel interoperability feature performs the following functions on individual SIP adjacencies:
Figure 30-1 shows SBC acting as UAS, and Figure 30-2 shows SBC acting as UAC.
Cisco Unified Border Element (SP Edition) sets up calls from a network that requires 100rel, but needs to be routed to networks that do not support 100rel. To facilitate this, SBC strips the 100rel option from Supported and Require headers in SIP requests. After stripping the 100rel option, SBC still sends reliable provisional responses with a “Require: 100rel” header if required. Cisco Unified Border Element can also be configured to send reliable provisional responses to requests that include a "Supported: 100rel" header when such requests do not include a "Require: 100rel" header and responses are received as unreliable provisional responses.
Send Reliable Responses if Required
If a SIP request includes a “Require: 100rel” header and SBC strips the 100rel option then it must send provisional responses as reliable provisional responses with a “Require: 100rel” header. In this case the called UAS sends unreliable provisional responses because SBC has stripped the 100rel option from the request.
Send Reliable Responses if Supported
If a SIP request includes a “Supported: 100rel” header then SBC must send reliable provisional responses to the caller UAC even when the SIP request does not include a “Require: 100rel” header and the called UAS sends unreliable provisional responses.
Cisco Unified Border Element (SP Edition) sends requests to networks that require 100rel from networks that do not. To facilitate this, the following functions are required:
SIP Requests passing through SBC should have “Supported: 100rel” header added to them.
If SBC advertises support for 100rel, then it also ensures that the non-PRACK network receives non-100rel messages.
Cisco Unified Border Element (SP Edition) requires following configurations to enable 100rel interworking support:
– whether to strip 100rel option from Supported and Require headers in the incoming SIP INVITE request.
– whether to enable 100rel interworking if incoming SIP INVITE request contains “Supported:100rel” header.
– add “Supported:100rel” in the outgoing SIP INVITE request.
– add “Require:100rel” in the outgoing SIP INVITE request.
This section contains the steps to configure the 100rel interworking support.
4. adjacency sip adjacency-name
5. 100rel inbound {strip | support}
6. 100rel outbound {require-add | support-add}
8. show sbc sbc-name sbe adjacencies { adjacency-name } [detail]