RFC 3261  allows new SIP URI and SIPS URI parameters, and new
parameter values to be defined. However, RFC 3261 omitted an IANA
registry for them. This document creates such a registry.
RFC 3427  documents the process to extend SIP. This document
updates RFC 3427 by specifying how to define and register new SIP and
SIP URI parameters and their values.
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
 and indicate requirement levels for compliant SIP
3. Use of the Registry
SIP and SIPS URI parameters and values for these parameters MUST be
documented in a standards-track RFC in order to be registered by
IANA. This documentation MUST fully explain the syntax, intended
usage, and semantics of the parameter. The intent of this
requirement is to assure interoperability between independent
implementations, and to prevent accidental namespace collisions
between implementations of dissimilar features.
Note that this registry, unlike other protocol registries, only
deals with parameters and parameter values defined in RFCs (i.e.,
it lacks a vendor-extension tree). RFC 3427  documents
concerns with regards to new SIP extensions which may damage
security, greatly increase the complexity of the protocol, or
both. New parameters and parameter values need to be documented
in RFCs as a result of these concerns.
RFCs defining SIP URI, SIPS URI parameters, or parameter values MUST
register them with IANA as described below.
Registered SIP and SIPS URI parameters and their values are to be
considered "reserved words". In order to preserve interoperability,
registered parameters MUST be used in a manner consistent with that
described in their defining RFC. Implementations MUST NOT utilize
"private" or "locally defined" URI parameters that conflict with
Note that although unregistered SIP and SIPS URI parameters may be
used in implementations, developers are cautioned that usage of
such parameters is risky. New SIP and SIPS URI parameters and new
values for them may be registered at any time, and there is no
assurance that these new registered URI parameters will not
conflict with unregistered parameters currently in use.
Some SIP and SIPS URI parameters only accept a set of predefined
parameter values. For example, a parameter indicating the transport
protocol in use may only accept the predefined tokens TCP, UDP, and
SCTP as valid values. Registering all parameter values for all SIP
and SIPS URI parameters of this type would require a large number of
subregistries. Instead, we have chosen to register URI parameter
values by reference. That is, the entry in the URI parameter
registry for a given URI parameter contains references to the RFCs
defining new values of that parameter. References to RFCs defining
parameter values appear in double brackets in the registry.
So, the SIP and SIPS URI parameter registry contains a column that
indicates whether or not each parameter only accepts a set of
predefined values. Implementers of parameters with a "yes" in that
column need to find all the valid parameter values in the RFCs
provided as references.
4. IANA Considerations
Section 27 of RFC 3261  creates an IANA registry for method names,
header field names, warning codes, status codes, and option tags.
This specification creates a new sub-registry under the SIP
o SIP/SIPS URI Parameters
4.1. SIP and SIPS URI Parameters Sub-Registry
New SIP and SIPS URI parameters and new parameter values are
registered by the IANA. When registering a new SIP or SIPS parameter
or a new value for a parameter, the following information MUST be
o Name of the parameter.
o Whether the parameter only accepts a set of predefined values.
o Reference to the RFC defining the parameter and to any RFC that
defines new values for the parameter. References to RFCs
defining parameter values appear in double brackets in the
Table 1 contains the initial values for this sub-registry.
Parameter Name Predefined Values Reference
comp Yes [RFC 3486]
lr No [RFC 3261]
maddr No [RFC 3261]
method Yes [RFC 3261]
transport Yes [RFC 3261]
ttl No [RFC 3261]
user Yes [RFC 3261]
Table 1: IANA SIP and SIPS URI parameter sub-registry
Note that any given parameter name is registered both as a SIP and as
a SIPS URI parameter. Still, some parameters may not apply to one of
the schemes. We have chosen to register any parameter as both a SIP
and SIPS URI parameter anyway to avoid having two parameters with the
same name, one applicable to SIP URIs and one to SIPS URIs, but with
different semantics. Implementors are urged to read the parameter
specifications for a detailed description of the semantics of any
4.2. Registration Policy for SIP and SIPS URI Parameters
As per the terminology in RFC 2434 , the registration policy for
SIP and SIPS URI parameters shall be "Specification Required".
For the purposes of this registry, the parameter for which IANA
registration is requested MUST be defined by a standards-track RFC.
5. Security Considerations
The registry in this document does not in itself have security
considerations. However, as mentioned in RFC 3427, an important
reason for the IETF to manage the extensions of SIP is to ensure that
all extensions and parameters are able to provide secure usage. The
supporting RFC publications for parameter registrations described
this specification MUST provide detailed security considerations for
Jonathan Rosenberg, Henning Schulzrinne, Rohan Mahy, Dean Willis, and
Allison Mankin provided useful comments on this document.
7. Normative References
 Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
 Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J., and B.
Rosen, "Change Process for the Session Initiation Protocol
(SIP)", BCP 67, RFC 3427, December 2002.
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
Advanced Signalling Research Lab.
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