SIP name-addr: Clarifications for When to Use the name-addr Production in SIP Messages
Clarifying when SIP headers must use the formal address format.
Sparks, R., "Clarifications for When to Use the name-addr Production in SIP Messages", RFC 8217, DOI 10.17487/RFC8217, August 2017, <https://www.rfc-editor.org/info/rfc8217>.
@misc{rfc8217,
series = {Request for Comments},
number = 8217,
howpublished = {RFC 8217},
publisher = {RFC Editor},
doi = {10.17487/RFC8217},
url = {https://www.rfc-editor.org/info/rfc8217},
author = {R. Sparks},
title = {{Clarifications for When to Use the name-addr Production in SIP Messages}},
pagetotal = 6,
year = 2017,
month = aug,
} RFC text
Contents
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.
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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
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.
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.
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.
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>.
[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
AI ELI5
On this page
§1The Problem with Split Rules
RFC 3261 defines SIP (Session Initiation Protocol) header fields that contain URIs. These headers allow two ways to write the URI: a bare URI (called addr-spec) or a named URI with angle brackets (called name-addr). For example, the From header can be either sip:alice@example.com or Alice <sip:alice@example.com>.
RFC 3261 included a constraint: if the URI contains a comma, question mark, or semicolon, the named form must be used. This constraint was written separately for each header that has the choice. Because it was repeated instead of stated once, the rule was missed in at least one header definition, and later extension specifications did not consistently include it.
A rule was written in many places in the handbook, but people still forgot to follow it in one place. Now it is written once at the start.
§2Why Special Characters Need the Named Form
When a SIP message is parsed, a parser reads the header field and stops at certain punctuation marks (commas, semicolons, question marks). If a URI contains these characters and is not enclosed in angle brackets, the parser will split the URI at the wrong place, breaking it into separate values or parameters.
For instance, a bare URI sip:alice@example.com,bob@example.com looks like two addresses separated by a comma. With the named form <sip:alice@example.com,bob@example.com>, the angle brackets tell the parser to treat the entire thing as one URI.
Special marks mean 'stop here' to a reader. Parentheses tell the reader to keep going and read everything inside as one thing.
§3The Generic Rule
This RFC updates RFC 3261 with a single, clear rule: When a SIP header field grammar allows choosing between addr-spec and name-addr (such as in (name-addr / addr-spec)), the addr-spec form MUST NOT be used if its value contains a comma, semicolon, or question mark.
This rule applies to all such header fields. The headers defined in RFC 3261 that allow this choice are: To, From, Contact, and Reply-To.
If your address label has special marks in it, put parentheses around it. That is the rule for all address fields.
§4Effect on Extension Headers
The RFC also updates nine other standards documents that define SIP extension header fields. Each extension (such as the Refer method, Referred-By mechanism, and others) adds one or more header fields that use the same addr-spec / name-addr choice. All of these extension header fields are now subject to the same rule.
The updates preserve the status of those RFCs (Standards Track or Informational) but add a note that the constraint applies to them.
The same rule now applies to extra headers added by other documents, so they all follow the same pattern.
§5Why Clarity Matters
Messages formed without honoring the constraint are still syntactically valid according to the grammar, but they will likely be misinterpreted. Characters after the comma, question mark, or semicolon will be read as header field parameters or additional header values instead of part of the URI. This misinterpretation can cause the message to be sent to the wrong destination or to have the wrong meaning, leading to routing errors or security issues.
The RFC emphasizes that implementations must follow the constraint to prevent these problems.
Breaking the rule does not make the message invalid, but it will be read the wrong way, which is worse.