RFC 3312, "Integration of Resource Management and Session Initiation Protocol (SIP)", October 2002

Errata ID: 4883
Status: Held for Document Update
Type: Technical
Publication Format(s) : TEXT
Reported By: Shraddha Soni
Date Reported: 2016-12-12
Held for Document Update by: Ben Campbell
Date Held: 2016-12-13

Section 11 says:

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].

It should say:

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 MUST include an Allow header field with
   the "UPDATE" tag [5].

Notes:

As stated in first line in the mentioned paragraph, the user agent MUST support the UPDATE method, hence even in Allow header field also, UPDATE method MUST be included.

As per RFC 2119
"SHOULD -This word, or the adjective "RECOMMENDED", mean that there
may exist valid reasons in particular circumstances to ignore a
particular item, but the full implications must be understood and
carefully weighed before choosing a different course. "

Here in precondition case, whether any chance of ignoring the UPDATE method happens ?

As per RFC 3261
Section 8.2.1 states -
"The Allow header field MUST list the set of methods supported by the UAS
generating the message. ... If the method is one supported by the server, processing continues."

and also in RFC 3261 Sections 20.5 states-
"All methods, including ACK and CANCEL, understood by the UA MUST be
included in the list of methods in the Allow header field, when
present."

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