Interworking Sip Prack Messages; Interworking Sip Session Timer; Interworking Sip Early Media - AudioCodes Mediant 4000 SBC User Manual

CHAPTER 22 SBC Overview
The handling of REFER can be configured for all calls, using the global parameter
[SBCReferBehavior]. To configure different REFER handling options for different UAs (i.e., IP
Groups), use the IP Profiles table parameter, 'Remote REFER Mode'.
■ Local handling of REFER: This option is used for UAs that do not support REFER. Upon
receipt of a REFER request, instead of forwarding it to the IP Group, the device handles it
locally. It generates a new INVITE to the alternative destination according to the rules in the IP-
to-IP Routing table (where the 'Call Trigger' field is set to REFER). It is also possible to specify
the IP Group that sent the REFER request, as matching criteria for the re-routing rule in this
table ('ReRoute IP Group ID' field).
■ Transparent handling: The device forwards the REFER with the Refer-To header unchanged.
■ Re-routing through SBC: The device changes the Refer-To header so that the re-routed
INVITE is sent through the SBC application.
■ IP Group Name: The device sets the host part in the REFER message to the name configured
for the IP Group in the IP Groups table.

Interworking SIP PRACK Messages

The device supports interworking of SIP Provisional Response ACKnowledgement (PRACK)
messages (18x). While some UAs may not support PRACK (RFC 3262) others may require it. The
device can be configured to resolve this interoperable issue and enable sessions between such
endpoints. SIP PRACK handling is configured using the IP Profile parameter, 'SBC Prack Mode':
■ Optional: PRACK is optional for these UAs. If required, the device performs the PRACK
process on behalf of the destination UA.
■ Mandatory: PRACK is required for these UAs. Calls from UAs that do not support PRACK are
rejected. Calls destined to these UAs are also required to support PRACK.
■ Transparent (default): The device does not intervene with the PRACK process and forwards
the request as is.

Interworking SIP Session Timer

The device supports interworking of the SIP signaling keep-alive mechanism. The SIP standard
provides a signaling keep-alive mechanism using re-INVITE and UPDATE messages. In certain
setups, keep-alive may be required by some SIP UAs while for others it may not be supported. The
device can resolve this mismatch by performing the keep-alive process on behalf of SIP UAs that
do not support it.
To configure the handling of session expires, use the IP Profile parameter, 'SBC Session Expires
Mode'.

Interworking SIP Early Media

The device supports early media. Early media is when the media flow starts before the SIP call is
established (i.e., before the 200 OK response). This occurs when the first SDP offer-answer
transaction completes. The offer-answer options can be included in the following SIP messages:
■ Offer in first INVITE, answer on 180, and no or same answer in the 200 OK
■ Offer in first INVITE, answer on 180, and a different answer in the 200 OK (not standard)
■ INVITE without SDP, offer in 180, and answer in PRACK
■ PRACK and UPDATE transactions can also be used for initiating subsequent offer-answer
transactions before the INVITE 200 OK response.
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Mediant 4000 SBC | User's Manual