Clarifications for When to Use the name-addr Production in SIP Messages
RFC 8217

Internet Engineering Task Force (IETF)                         R. Sparks
Request for Comments: 8217                                        Oracle
Updates: 3261, 3325, 3515, 3892, 4508,                       August 2017
         5002, 5318, 5360, 5502
Category: Standards Track
ISSN: 2070-1721

Clarifications for When to Use the name-addr Production in SIP Messages

Abstract

   RFC 3261 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 RFC 3261 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

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8217.

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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Updates to RFC 3261 . . . . . . . . . . . . . . . . . . . . .   4
   4.  Updates to RFCs Defining SIP Extension Header Fields  . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

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.

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   Section 20.39 of [RFC3261], 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 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 >).

   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 has been
   included, the requirement to use the name-addr form has not been
   consistently stated.

   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 it 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:10.0.0.1,@10.0.0.0 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.

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3.  Updates to RFC 3261

   This text from 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 RFC 8217,
      the "addr-spec" form MUST NOT be used if its value would contain
      a comma, semicolon, or question mark.

   The status of these RFCs remains unchanged.  In particular the status
   of the Informational RFCs remains Informational.

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5.  IANA Considerations

   This document does not require any IANA actions.

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
   misinterpreted by the receiving element.

7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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,
              <http://www.rfc-editor.org/info/rfc3261>.

   [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,
              <http://www.rfc-editor.org/info/rfc3325>.

   [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", RFC 3515, DOI 10.17487/RFC3515, April 2003,
              <http://www.rfc-editor.org/info/rfc3515>.

   [RFC3892]  Sparks, R., "The Session Initiation Protocol (SIP)
              Referred-By Mechanism", RFC 3892, DOI 10.17487/RFC3892,
              September 2004, <http://www.rfc-editor.org/info/rfc3892>.

   [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,
              <http://www.rfc-editor.org/info/rfc4508>.

   [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,
              <http://www.rfc-editor.org/info/rfc5002>.

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   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [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, <http://www.rfc-editor.org/info/rfc5318>.

   [RFC5360]  Rosenberg, J., Camarillo, G., Ed., and D. Willis, "A
              Framework for Consent-Based Communications in the Session
              Initiation Protocol (SIP)", RFC 5360,
              DOI 10.17487/RFC5360, October 2008,
              <http://www.rfc-editor.org/info/rfc5360>.

   [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, <http://www.rfc-editor.org/info/rfc5502>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <http://www.rfc-editor.org/info/rfc8174>.

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.

Author's Address

   Robert Sparks
   Oracle

   Email: rjsparks@nostrum.com

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