Network Working Group R. Sparks
Internet-Draft Oracle
Updates: 3261, 3325, 3515, 3892, 4508, June 01, 2017
5002, 5318, 5360, 5502 (if
Intended status: Standards Track
Expires: December 3, 2017

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


RFC3261 constrained several SIP header fields whose grammar contains the "name-addr / addr-spec" alternative to use name-addr when certain characters appear. Unfortunately it expressed the constraints with prose copied into each header field definition, and at least one header field was missed. Further, the constraint has not been copied into documents defining extension headers whose grammar contains the alternative.

This document updates RFC3261 to state the constraint generically, and clarifies that the constraint applies to all SIP header fields where there is a choice between using name-addr or addr-spec. It also updates the RFCs that define extension SIP header fields using the alternative to clarify that the constraint applies (RFCs 3325, 3515, 3892, 4508, 5002, 5318, 5360, and 5502).

Status of This Memo

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This Internet-Draft will expire on December 3, 2017.

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Table of Contents

1. Introduction

[RFC3261] 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 [RFC5234] specified in [RFC3261] shows the relevant part of the definition of the syntax of the "From" header field:

  From        =  ( "From" / "f" ) HCOLON from-spec
  from-spec   =  ( name-addr / addr-spec )
                 *( SEMI from-param )
  name-addr      =  [ display-name ] LAQUOT addr-spec RAQUOT
  addr-spec      =  SIP-URI / SIPS-URI / absoluteURI

The prose in section 20.20 of [RFC3261], which discusses the "From" header field, constrains how the production may be used by saying:

   Even if the "display-name" is empty, the "name-addr" form 
   MUST be used if the "addr-spec" contains a comma, question 
   mark, or semicolon.

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:

  The Contact, From, and To header fields contain a URI.  If the URI
  contains a comma, question mark or semicolon, the URI MUST be
  enclosed in angle brackets (< and >). 

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 [RFC3261]. (An exception is the degenerate case of a URL like sip:,@ where it is possible to parse the comma via the 'user' production).

2. Terminology

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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. Updates to RFC3261

This text from the introduction to section 20 of [RFC3261]:

  The Contact, From, and To header fields contain a URI.  If the URI
  contains a comma, question mark or semicolon, the URI MUST be
  enclosed in angle brackets (< and >).  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:

 When constructing the value of any SIP header field whose grammar
 allows choosing between name-addr and addr-spec, such as those
 that use the form '(name-addr / addr-spec)', the "addr-spec" form
 MUST NOT be used if its value would contain a comma, semicolon,
 or question mark.

 When a URI appears in such a header field, any URI parameters MUST
 be contained within angle brackets (< and >).  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". 

4. Updates to RFCs defining SIP Extension header fields

The following standards track RFCs: [RFC3515], [RFC3892], [RFC4508], and [RFC5360]

and the following informational RFCS: [RFC3325], [RFC5002], [RFC5318], and [RFC5502]

are updated to include:

  This RFC contains the definition of one or more SIP header fields
  that allow choosing between addr-spec and name-addr when
  constructing header field values. As specified in RFCxxxx,
  the "addr-spec" form MUST NOT be used if its value would contain
  a comma, semicolon, or question mark.

The status of the Informational RFCs remains Informational.

5. IANA Considerations

This memo has no considerations for IANA.

6. Security Considerations

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.

7. Acknowledgments

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.

8. Instructions to the RFC Editor

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.

9. Normative References

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer Method", RFC 3515, DOI 10.17487/RFC3515, April 2003.
[RFC3892] Sparks, R., "The Session Initiation Protocol (SIP) Referred-By Mechanism", RFC 3892, DOI 10.17487/RFC3892, September 2004.
[RFC4508] Levin, O. and A. Johnston, "Conveying Feature Tags with the Session Initiation Protocol (SIP) REFER Method", RFC 4508, DOI 10.17487/RFC4508, May 2006.
[RFC5360] Rosenberg, J., Camarillo, G. and D. Willis, "A Framework for Consent-Based Communications in the Session Initiation Protocol (SIP)", RFC 5360, DOI 10.17487/RFC5360, October 2008.
[RFC3325] Jennings, C., Peterson, J. and M. Watson, "Private Extensions to the Session Initiation Protocol (SIP) for Asserted Identity within Trusted Networks", RFC 3325, DOI 10.17487/RFC3325, November 2002.
[RFC5002] Camarillo, G. and G. Blanco, "The Session Initiation Protocol (SIP) P-Profile-Key Private Header (P-Header)", RFC 5002, DOI 10.17487/RFC5002, August 2007.
[RFC5318] Hautakorpi, J. and G. Camarillo, "The Session Initiation Protocol (SIP) P-Refused-URI-List Private-Header (P-Header)", RFC 5318, DOI 10.17487/RFC5318, December 2008.
[RFC5502] van Elburg, J., "The SIP P-Served-User Private-Header (P-Header) for the 3GPP IP Multimedia (IM) Core Network (CN) Subsystem", RFC 5502, DOI 10.17487/RFC5502, April 2009.

Author's Address

Robert Sparks Oracle EMail: