Clarifications for when to use the name-addr production in SIP messages

defines several header fields that contain URIs to allow both a form that contains the bare URI (addr-spec) and one that provides a name and the URI (name-addr). This subset, taken from the ABNF specified in shows the relevant part of the definition of the syntax of the "From" header field:

The prose in section 20.20 of , which discusses the "From" header field, constrains how the production may be used by saying:

Section 20.39, which discusses the "To" header field contains no such constraining text. This constraint is specified slightly differently, but with the same intent, in the introduction to section 20:

). ]]> Unfortunately, this can be read to only apply to the Contact, From, and To header fields, making it necessary to provide the constraint explicitly in the prose discussing any other header field using the name-addr or addr-spec alternative. As extension header fields were standardized, the specifications sometimes failed to include the constraint. Many errata have been entered to correct this omission. When the constraint was called out, the form has not been consistent. This memo updates the specifications of SIP and its extensions to clarify that the constraint to use the name-addr form applies anywhere there is a choice between the name-addr and addr-spec production rules in the grammar for SIP header fields. It is important to note that a message formed without honoring the constraint will still be syntactically valid, but would very likely be interpreted differently. The characters after the comma, question mark, or semicolon will, in most cases, be interpreted as header field parameters or additional header field values as discussed in section 7.3.1 of . (An exception is the degenerate case of a URL like sip:10.0.0.1,@10.0.0.0 where it is possible to parse the comma via the 'user' production).

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

This text from the introduction to section 20 of :

). Any URI parameters are contained within these brackets. If the URI is not enclosed in angle brackets, any semicolon-delimited parameters are header-parameters, not URI parameters. ]]> is replaced with:

). If the URI is not enclosed in angle brackets, any semicolon-delimited parameters are header-parameters, not URI parameters. The header fields defined in this specification that allow this choice are "To", "From", "Contact", and "Reply-To". ]]>

The following standards track RFCs: , , , and and the following informational RFCS: , , , and are updated to include:

The status of the Informational RFCs remains Informational.

This memo has no considerations for IANA.

The updates specified in this memo clarify a constraint on the grammar for producing SIP messages. It introduces no new security considerations. One pre-existing consideration is worth reiterating: messages produced without honoring the constraint will very likely be mis-interpreted by the receiving element.

Brett Tate identified this issue in several extension documents, submitted several corresponding errata, and drove the discussion that led to this memo. Substantive comments leading to this text were provided by Paul Kyzivat, Gonzalo Camarillo, Dale Worley, and Yehoshua Gev.

Please remove this section in its entirety before publication as an RFC. Please replace any instances of RFCxxxx with the RFC number assigned to this memo. This memo, if it is approved, obviates Errata 3744, 3894, and 4648-4652 inclusive.