The treats User-Agent and Server headers as known headers beginning in release 4.1.0. The 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 relaxes the ABNF grammar formatting to allow invalid syntax server-vals plus an unlimited number of server-vals for these two headers.