Supported SIP Headers

The SBC supports up to 20 parameters per SIP header to accommodate enhanced services such as IMS and VoLTE using call flows such as ICS and SRVCC.

The SBC treats User-Agent and Server headers as known headers beginning in release 4.1.0. The SBC accepts these headers based on ABNF grammar format and restricts them to a maximum of 10 server-vals. You must explicitly turn ON these headers for transparency. Additionally, if a header requires more than 10 server-vals in compliance with ABNF grammar, the header must convert to an unknown header using SMM.

SIP provides a mechanism to represent common header field names in an abbreviated form. This may be useful when messages are too large to be carried on the transport available to it. A compact form may be substituted for the longer form of a header field name at any time without changing the semantics of the message. A header field name may appear in both long and short forms within the same message. A compact form of some common header field names is also defined for use when overall message size is an issue. Any headers marked with an alternate like: ' | ' indicates the long form or the short form of the header.

In order to maintain backward capability, as of 4.1.6R0 and 4.2.4R0 releases, the SBC relaxes the ABNF grammar formatting to allow invalid syntax server-vals plus an unlimited number of server-vals for these two headers.

The SBC does not process or send the Date header natively. If you need to pass the Date Header transparently, use the SBC’s Header Transparency feature.

The SBC uses the Cause Indicators parameter(CAI) of REL message received in P-N-ISUP-R header of any SIP error message. The CAI is used in the following scenarios:

• The CAI is recorded onto CAM.
• The CAI makes the error response toward ingress side (SIP-T) based on the mapping of “cpcToSipCauseMapProfile”.
• The CAI is triggered by a crankback (“crankbackProfile” refers to the CAI).