Internet Engineering Task Force
Internet Draft Polk/Schulzrinne
Cisco/Columbia U.
draft-polk-sipping-resource-01.txt
March 2, 2002
Expires: August 2002
SIP Communications Resource Priority Header
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document defines a new SIP header field for communications
resource priority, called "Resource-Priority". This header field
influences the behavior of gateways and SIP proxies. It does not
influence the forwarding behavior of IP routers.
Table of Contents
1 Conventions used in this document ................... 2
2 Introduction ........................................ 2
3 The Resource-Priority Header Field .................. 3
4 IANA Considerations ................................. 4
5 Security Considerations ............................. 4
A Namespace dsn ....................................... 4
B Namespace q735 ...................................... 4
C Bibliography ........................................ 4
D Acknowledgements .................................... 5
E Authors' Addresses .................................. 5
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Internet Draft Resource Priority March 2, 2002
1 Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
2 Introduction
This document defines a new SIP [2] header field for communications
resource priority, called "Resource-Priority". This header MAY be
used by GSTN gateways and SIP proxy servers to influence their
treatment of SIP requests, including the priority afforded to GSTN
calls. For GSTN gateways, the behavior translates into the ITU
Recommendation Q.735.3 [3] prioritization mechanism, in both GSTN-
to-IP and IP-to-GSTN directions. For IP networks, proxies may offer
mechanisms beyond the scope of this document to influence, for
example, admission control or IP packet marking.
The Resource-Priority header field may be inserted by proxies and SIP
user agents.
The Resource-Priority header field may be used in several situations:
1. Requesting elevated priority for access to GSTN gateway
resources such as trunk circuits.
2. Carrying information from one multi-level priority domain
in the telephone network, e.g., using the facilities of
Q.735.3 [3], to another, without the SIP proxies themselves
inspecting the header field.
3. Indicating signaling priority in SIP proxies and back-to-
back user agents, with higher priorities displacing
existing signaling requests or bypassing GSTN gateway
capacity limits in effect for lower priorities.
This header is related to, but differs in semantics from, the
Priority header field (RFC 2543, Section 6.25). The Priority header
field describes the priority that the SIP request should have to the
receiving human or its agent. For example, it may be factored into
decisions about call routing and acceptance. It does not influence
the use of communications resources such as packet forwarding
priority in routers.
Header field where proxy ACK BYE CAN INV OPT REG
_____________________________________________________________
Resource-Priority Rr a - - - o - -
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The mechanism described here can be used for emergency preparedness,
but is only a small part of an emergency preparedness network.
SIP entities supporting this specification MUST be able to generate
and process this header.
3 The Resource-Priority Header Field
This document defines the Resource-Priority general header field.
Resource-Priority _ "Resource-Priority" HCOLON Resource-value
Resource-value _ namespace "." priority
namespace _ alphanum / "-"
priority _ alphanum / "-"
As a response header, the value indicates the actual priority
selected by the recipient. Implementations MAY change the value
offered in the request; in some environments, the response value is
known to be the same as in the request.
The resource value is formatted as "namespace" "." "priority value".
Name space and priority values are registered with IANA (see IANA
Considerations).
Header field where proxy ACK BYE CAN INV OPT REG
_____________________________________________________________
Resource-Priority c ar - - - o - -
SIP elements MAY downgrade the Resource-Priority of or reject
unauthenticated requests. Details are a matter of local policy.
Proxies MAY downgrade the Resource-Priority value of unauthenticated
requests. Details are specific to each administrative domain and
beyond the scope of this document. Proxies SHOULD NOT reject requests
with such headers but instead downgrade the resource priority value.
SIP elements MUST ignore a Resource-Priority header value with an
unknown name space or priority value.
A proxy or user agent MAY return status code 503 (Service
Unavailable) if there are insufficient resources at the resource
priority level specified. The response MAY also include a Warning
header with warning code 370 (Insufficient Bandwidth) if the request
failed due to insufficient capacity for the media streams, rather
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Internet Draft Resource Priority March 2, 2002
than insufficient signaling capacity.
4 IANA Considerations
Additional name spaces and priority values are registered with IANA.
Within each namespace, The registration MUST indicate the relative
precedence levels, expressed as an ordered list. New labels SHOULD
NOT be added to existing namespaces; as noted above, implementations
predating the addition will ignore such values.
5 Security Considerations
The Resource-Priority header field can be abused to consume scarce
communications resources. Thus, authentication of the requester is of
particular importance. Authentication MAY be SIP-based.
A Namespace dsn
This document defines the namespace "dsn". The namespace "dsn"
(Defense Switched Network), contains the priority values "critic-
ecp", "flash-override", "flash", "immediate", "priority", "routine".
The values are adopted from RFC 791 [4], omitting the levels "network
control" and "internetwork control", as these are inappropriate here.
The values are prioritized in the order "critic-ecp" (highest),
"flash-override", "flash", "immediate", "priority" and "routine"
(lowest).
The value "critic-ecp" stands for "Critical and Emergency Call
Processing" [4]. This value SHOULD only be used for authorized
emergency communications, for example in the United States Government
Emergency Telecommunications Service (GETS) [5], the United Kingdom
Government Telephone Preference Scheme (GTPS) and similar government
emergency preparedness or reactionary implementations elsewhere.
B Namespace q735
This document also defines the namespace "q735". The namespace "q735"
supports interworking with Q.735.3 (or equivalent) GSTN (ISDN)
entities; this allows, for example, carrying information between
Q.735.3 entities without loss of information. One or both of the SIP
endpoints might be PSTN gateways. The namespace contains the priority
values "0", "1", "2", "3" and "4", with "4" representing the lowest
priority and "0" the highest.
C Bibliography
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[1] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," Request for Comments 2119, Internet Engineering Task Force,
Mar. 1997.
[2] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
session initiation protocol," Request for Comments 2543, Internet
Engineering Task Force, Mar. 1999.
[3] International Telecommunication Union, "Stage 3 description for
community of interest supplementary services using signalling system
no. 7: Multi-level precedence and preemption," Recommendation
Q.735.3, Telecommunication Standardization Sector of ITU, Geneva,
Switzerland, Mar. 1993.
[4] J. Postel, "Internet protocol," Request for Comments 791,
Internet Engineering Task Force, Sept. 1981.
[5] K. Carlberg and I. Brown, "Framework for supporting IEPS in IP
telephony," Internet Draft, Internet Engineering Task Force, Oct.
2001. Work in progress.
D Acknowledgements
Mike Pierce provided helpful comments.
E Authors' Addresses
James Polk
Cisco Systems
2200 East President George Bush Turnpike
Richardson, TX 75082 USA
electronic mail: jmpolk@cisco.com
Henning Schulzrinne
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue
New York, NY 10027
USA
electronic mail: schulzrinne@cs.columbia.edu
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