RFC on 100rel and precondition in the Supported header

RFC 3312, section 11:


Option Tag for Preconditions


   We define the option tag "precondition" for use in the Require and

   Supported header fields.  An offerer MUST include this tag in the

   Require header field if the offer contains one or more "mandatory"

   strength-tags.  If all the strength-tags in the description are

   "optional" or "none", the offerer MUST include this tag in either a

   Supported header field or in a Require header field.  It is, however,

   RECOMMENDED that the Supported header field be used in this case.

   The lack of preconditions in the answer would indicate that the

   answerer did not support this extension.


   The mapping of offers and answers to SIP requests and responses is

   performed following the rules given in [5]. Therefore, a user agent

   including preconditions in the SDP MUST support the PRACK and UPDATE

   methods. Consequently, it MUST include the "100rel" [7] tag in the

   Supported header field and SHOULD include an Allow header field with

   the "UPDATE" tag [5].

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