One document matched: draft-ietf-sip-resource-priority-05.txt
Differences from draft-ietf-sip-resource-priority-04.txt
SIP Working Group H. Schulzrinne
Internet-Draft Columbia U.
Expires: April 25th, 2005 J. Polk
Cisco
October 25th, 2004
Communications Resource Priority for the Session Initiation Protocol
(SIP)
draft-ietf-sip-resource-priority-05
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document defines two new SIP header fields for communicating
resource priority, namely "Resource-Priority" and "Accept-Resource-
Priority". The "Resource-Priority" header field can influence the
behavior of SIP user agents, such as telephone gateways and IP
telephones, and Session Initiation Protocol (SIP) proxies. It does
not directly influence the forwarding behavior of IP routers.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. The Resource-Priority and Accept-Resource-Priority SIP
Header Fields . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 The 'Resource-Priority' Header Field . . . . . . . . . . . 5
3.2 The 'Accept-Resource-Priority' Header Field . . . . . . . 6
3.3 Usage of the 'Resource-Priority' and
'Accept-Resource-Priority' Header Fields . . . . . . . . . 7
3.4 The 'resource-priority' Option Tag . . . . . . . . . . . . 8
4. Behavior of SIP Elements that Receive Prioritized Requests . . 8
4.1 General Rules . . . . . . . . . . . . . . . . . . . . . . 8
4.1.1 Policy Guidelines Involving Preemption Policy . . . . . 9
4.2 Rejection Messages . . . . . . . . . . . . . . . . . . . . 9
4.3 Error Conditions . . . . . . . . . . . . . . . . . . . . . 10
4.3.1 Known Namespace and Priority Value . . . . . . . . . . 10
4.3.2 Handling Unknown Namespaces and Priority Values . . . 11
4.3.3 Strict Mode . . . . . . . . . . . . . . . . . . . . . 11
4.3.4 Semi-Strict Mode . . . . . . . . . . . . . . . . . . . 12
4.3.5 Loose Mode . . . . . . . . . . . . . . . . . . . . . . 14
4.4 User Agent Client Behavior . . . . . . . . . . . . . . . . 14
4.5 User Agent Server Behavior . . . . . . . . . . . . . . . . 14
4.5.1 User Agent Servers and Preemption Policy . . . . . . . . 15
4.6 Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 15
4.6.1 Proxies and Preemption Policy . . . . . . . . . . . . . 16
5. Third-Party Authentication . . . . . . . . . . . . . . . . . . 16
6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 16
7. Namespace Description . . . . . . . . . . . . . . . . . . . . 17
7.1 Multiple Namespaces in a Message . . . . . . . . . . . . . . 19
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.1 Simple Call . . . . . . . . . . . . . . . . . . . . . . . 21
8.2 Receiver Does Not Understand Namespace . . . . . . . . . . 22
9. Security Considerations . . . . . . . . . . . . . . . . . . . 24
9.1 Authentication and Authorization . . . . . . . . . . . . . 25
9.2 Confidentiality and Integrity . . . . . . . . . . . . . . 26
9.3 Anonymity . . . . . . . . . . . . . . . . . . . . . . . . 27
9.4 Denial-of-Service Attacks . . . . . . . . . . . . . . . . 27
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
10.1 IANA Registration of 'Resource-Priority' and
'Accept-Resource-Priority' Header Fields . . . . . . . . . 27
10.2 IANA Registration for Option Tag resource-priority . . . . 28
10.3 IANA Registration for Response Code 417 . . . . . . . . . 28
10.4 IANA Namespace Registrations . . . . . . . . . . . . . . . 28
10.5 IANA Priority-Value Registrations . . . . . . . . . . . . 29
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 29
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
12.1 Normative References . . . . . . . . . . . . . . . . . . . . 30
12.2 Informative References . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 32
Intellectual Property and Copyright Statements . . . . . . . . 32
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1. Introduction
During emergencies, communications resources including telephone
circuits, IP bandwidth and gateways between the circuit-switched and
IP networks may become congested. Congestion can occur due to heavy
usage, loss of resources caused by the natural or man-made disaster
and attacks on the network during man-made emergencies. This
congestion may make it difficult for persons charged with emergency
assistance, recovery or law enforcement to coordinate their efforts.
As IP networks become part of converged or hybrid networks along with
public and private circuit-switched (telephone) networks, it becomes
necessary to ensure that these networks can assist during such
emergencies.
Also, users may want to interrupt their lower-priority communications
activities and dedicate their end system resources to the
high-priority communications attempt if a high-priority
communications request arrives at their end system.
There are many IP-based services that can assist during emergencies.
This memo only covers real-time communications applications involving
the Session Initiation Protocol (SIP) [RFC3261], including
voice-over-IP, multimedia conferencing, instant messaging and
presence.
SIP applications may involve at least five different resources that
may become scarce and congested during emergencies. These resources
include gateway resources, circuit-switched network resources, IP
network resources, receiving end system resources and SIP proxy
resources. IP network resources are beyond the scope of SIP
signaling and are therefore not considered here.
In order to improve emergency response, it may become necessary to
prioritize access to SIP-signaled resources during periods of
emergency-induced resource scarcity. We call this "resource
prioritization".
The mechanism itself may well be in place at all times, but only
materially affect call handling during times of resource scarcity.
Currently, SIP does not include a mechanism that allows a request
originator to indicate to a SIP element that it wishes the request to
invoke such resource prioritization. To address this need, this
document adds a SIP protocol element that labels certain SIP
requests.
This document defines (Section 3) a new SIP [RFC3261] header field
for communications resource priority, called 'Resource-Priority' This
header field MAY be used by SIP user agents, including General
Switched Telephone Network (GSTN) gateways and terminals, and SIP
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proxy servers to influence their treatment of SIP requests, including
the priority afforded to GSTN calls. For GSTN gateways, the behavior
translates into analogous schemes in the GSTN, for example the ITU
Recommendation Q.735.3 [Q.735.3] prioritization mechanism, in both
the GSTN-to-IP and IP-to-GSTN directions. ITU Recommendation I.255.3
[I.255.3] is another example.
The 'Resource-Priority' header field may be used in several
situations. A SIP request with such an indication can be treated
differently in these situations:
1. The request can be given elevated priority for access to GSTN
gateway resources such as trunk circuits.
2. The request can interrupt lower-priority requests at a user
terminal, such as an IP phone.
3. The request can carry information from one multi-level priority
domain in the telephone network, e.g., using the facilities of
Q.735.3 [Q.735.3], to another, without the SIP proxies themselves
inspecting or modifying the header field.
4. In SIP proxies and back-to-back user agents, requests of higher
priorities may displace existing signaling requests or bypass
GSTN gateway capacity limits in effect for lower priorities.
This header field is related to, but differs in semantics from, the
'Priority' header field (RFC 3261 [RFC3261], Section 20.26). The
'Priority' header field describes the importance that the SIP request
should have to the receiving human or its agent. For example, that
header may be factored into decisions about call routing to mobile
devices and assistants and call acceptance when the call destination
is busy. The 'Priority' header field does not affect the usage of
GSTN gateway or proxy resources, for example. In addition, any User
Agent Client (UAC) can assert any 'Priority' value, while access to
resource priority values are subject to authorization.
While the 'Resource-Priority' header does not directly influence the
forwarding behavior of IP routers or the use of communications
resources such as packet forwarding priority, procedures for using
this header to cause such influence may be defined in other
documents.
Existing implementations of RFC 3261 that do not participate in the
resource priority mechanism follow the normal rules of RFC 3261,
Section 8.2.2: "If a UAS does not understand a header field in a
request (that is, the header field is not defined in this
specification or in any supported extension), the server MUST ignore
that header field and continue processing the message." Thus, the use
of this mechanism is wholly invisible to existing implementations
unless the request includes the Require header field with the
resource-priority option tag.
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The mechanism described here can be used for emergency preparedness
in emergency telecommunications systems, but is only a small part of
an emergency preparedness network and is not restricted to such use.
The mechanism aims to satisfy the requirements in [RFC3487]. It is
structured so that it works in all SIP and Real-Time Transport
Protocol (RTP) [RFC3550] transparent networks defined in [RFC3487].
In such networks, all network elements and SIP proxies let valid SIP
requests pass through unchanged. This is important since it is
likely that this mechanism will often be deployed in networks where
the edge networks are unaware of the resource priority mechanism and
provide no special privileges to such requests. The request then
reaches a GSTN gateway or set of SIP elements that are aware of the
mechanism.
For conciseness, we refer to SIP proxies and user agents (UAs) that
act on the 'Resource-Priority' header field as RP actors.
We define the header field syntax in Section 3 and then describe the
behavior of user agents and proxies in Section 4.3 through Section
4.5. Section 6 briefly describes how this feature affects existing
systems that do not support it. Third-party authentication is
discussed in Section 5, while general security issues are enumerated
in Section 8. This specification does not propose any new SIP
security mechanisms. Examples can be found in Section 7.
2. Terminology
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
[RFC2119] and indicate requirement levels for compliant
implementations.
3. The Resource-Priority and Accept-Resource-Priority SIP Header Fields
This document defines the 'Resource-Priority' and
'Accept-Resource-Priority' SIP header fields.
The SIP element behavior is described for user agent clients (UACs)
in Section 4.3, for UAS in Section 4.4 and for SIP proxy servers in
Section 4.5.
3.1 The 'Resource-Priority' Header Field
The 'Resource-Priority' header field marks a SIP request as desiring
prioritized resource access, as described in the introduction. In
responses, the 'Resource-Priority' header fields indicates the actual
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resource priority that was granted to the request. While it is
likely those responses contain the same 'Resource-Priority' header
field value as the requests, local policy MAY call for the UAS to
insert no header field or a different value.
There is no requirement that all requests within a SIP dialog or
session use the 'Resource-Priority' header field. Again, local
policy dictates the appropriate behavior; thus, implementations MUST
be configurable accordingly.
The syntax of the 'Resource-Priority' header field is described
below. The "token-nodot" production is copied from [RFC3265].
Resource-Priority = "Resource-Priority" HCOLON
r-value *(COMMA r-value)
r-value = namespace "." r-priority
namespace = token-nodot
r-priority = token-nodot
token-nodot = 1*( alphanum / "-" / "!" / "%" / "*"
/ "_" / "+" / "`" / "'" / "~" )
An example 'Resource-Priority' header field is shown below:
Resource-Priority: q735.1, dsn.flash
The 'Resource-value' parameter in the 'Resource-Priority' header
indicates the resource priority desired by the request originator.
Since a request may traverse multiple administrative domains with
multiple different namespaces, it is necessary to be able to
enumerate several different namespaces. However, each namespace MUST
NOT appear more than once in a SIP message.
Each resource value is formatted as 'namespace' '.' 'priority value'.
The value is drawn from the namespace identified by the 'namespace'
token. Namespaces and priorities are case-insensitive ASCII. Each
namespace has at least one priority value. Namespaces and priority
values within each namespace MUST be registered with IANA (Section
9). Initial namespace registrations are described in Section 9.5.
There may be multiple resource values or, equivalently, multiple
'Resource-Priority' header field instances.
3.2 The 'Accept-Resource-Priority' Header Field
The 'Accept-Resource-Priority' response header field enumerates the
resource values a SIP user agent server is willing to accept. The
syntax of the 'Accept-Resource-Priority' header field is as follows:
Accept-Resource-Priority = "Accept-Resource-Priority" HCOLON
[r-value *(COMMA r-value)]
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An example is given below:
Accept-Resource-Priority: dsn.flash-override,
dsn.flash, dsn.immediate, dsn.priority, dsn.routine
3.3 Usage of the 'Resource-Priority' and 'Accept-Resource-Priority'
Header Fields
The following table extends the values in Table 2 of RFC3261
[RFC3261]. (The PRACK method, labeled as PRA, is defined in
[RFC3262], the SUBSCRIBE (labeled SUB) and NOTIFY (labeled NOT)
methods in [RFC3265], the UPDATE (UPD) method in [RFC3311], the
MESSAGE (MSG) method in [RFC3428], the REFER (REF) method in
[RFC3515], the INFO (INF) method in [RFC2976], and the PUBLISH (PUB)
method in [I-D.ietf-sip-publish].)
Header field where proxy INV ACK CAN BYE REG OPT PRA
----------------------------------------------------------------
Resource-Priority R amdr o o o o o o o
Resource-Priority r amdr c c c c c c c
Resource-Priority 200 - o - o o o o o
Accept-Resource-Priority 200 - o o o o o o o
Accept-Resource-Priority 417 - m - m m m m m
Accept-Resource-Priority 420 - o - o o o o o
Header field where proxy SUB NOT UPD MSG REF INF PUB
----------------------------------------------------------------
Resource-Priority R amdr o o o o o o o
Resource-Priority r amdr c c c c c c c
Resource-Priority 200 - o o o o o o o
Accept-Resource-Priority 200 - o o o o o o o
Accept-Resource-Priority 417 - m m m m m m m
Accept-Resource-Priority 420 - o o o o o o o
Section 4 will change this table depending on the 'mode' a SIP
element is operating under.
Other request methods MAY define their own handling rules; unless
otherwise specified, recipients MAY ignore these header fields.
'Accept-Resource-Priority' MUST be returned in 420 (Not Supported)
responses marked as 'o' in table above if the element implements the
resource priority mechanism with some other namespaces or priority
values, but does not implement the particular namespace or priority
value contained in the request.
While all methods listed above allow the optional use of the
'Resource-Priority' header fields, only request methods that start a
dialog or deliver content, such as MESSAGE, are likely to benefit
from this mechanism and other methods SHOULD NOT use them. This
consideration also applies to methods not listed above.
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The 'Resource-Priority' header field included in a Request message
MUST be copied in the SIP response, subject to further rules in each
mode of operation (listed in section 4 of this document).
3.4 The 'resource-priority' Option Tag
This document also defines the "resource-priority" option tag. The
behavior is described in Section 4.2.2 and the IANA registration is
in Section 9.2.
4. Behavior of SIP Elements that Receive Prioritized Requests
All SIP user agents and proxy servers that support this
specification share certain common behavior, which we describe
below. Behavior specific to user agent clients is covered in
section 4.4, for user agent servers in Section 4.5, while Section
4.6 deals with proxy behavior.
4.1 General Rules
The Resource-Priority header field is potentially applicable to all
SIP request messages. As a minimum, implementations of this
specification supporting any of the following request types MUST
support the use of this specification for those request types:
1) INVITE [RFC3261]
2) ACK [RFC3261]
3) PRACK [RFC3262]
4) MESSAGE [RFC3428]
5) UPDATE [RFC3311]
6) SUBSCRIBE [RFC3265]
7) NOTIFY [RFC3265]
8) REFER [RFC3515]
Note that this does not require all implementations to support every
request type listed.
If a SIP element receives the Resource-Priority header in a Request
message other than what is listed above, the header MAY be ignored,
but not the message because the header was present (but could be for
other reasons not listed here).
An OPTIONS request can be used to determine if an element supports
the mechanism. A compliant implementation MUST return a
'Accept-Resource-Priority' header field in OPTIONS responses
enumerating all valid resource values. An implementation MAY reveal
this capability only to authorized UACs.
Note that an overloaded UAS may not be able to provide this
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information at all times.)
Note that according to RFC 3261, proxies reached with a Max-Forwards
value of zero answer the OPTIONS request, allowing a UAC to discover
the capabilities of both proxy and user agent servers.
4.1.1 Policy Guidelines Involving Preemption Policy
Not every implementation of the Resource Priority Header involves
preemption. In fact, it is expected that a minority of domains
implementing this specification will enable preemption, though
critical in those domains; thus preemption policies cannot be
minimized. With that said, a word or two is needed to address the
expected general behavior of a policy that includes preemption of
resources for SIP elements supporting this specification:
SIP messages themselves are not preempted due to this header.
For SIP elements compliant with this specification, SIP messages
containing a valid Resource Priority header are prioritized higher
or lower than other messages. Since [RFC3261] states SIP headers
not understood are ignored, the presence of this header SHOULD NOT
adversely affect SIP elements that are not aware of, nor supports
this specification. If the presence of this header is supported,
but either the namespace or the priority value are not recognized,
section 4.3 of this document addresses appropriate error responses.
SIP messages that create or have created a dialog can cause
preemption of another dialog with the usage of this specification.
The specific behaviors of SIP elements with regard to preemption
policy is included in 4.5 for UAS and gateway behavior, and section
4.6 for Proxy Servers.
4.2 Rejection Messages
The following is a list of rejections to SIP messages that contain a
Resource-Priority header specifically because of the contents of the
header.
If a UA is currently occupied with another session and receives a
dialog generating message containing a valid Resource-Priority
header of equal or lower relative priority value, the response is
the same as stated in section 13.3.1.3 of [RFC3261]:
- a 486 (Busy Here) is returned if the UAS knows it cannot or will
not accept the request,
- a 600 (Busy Everywhere) if the UAS knows there are no other SIP
elements that can accept the INVITE, and
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- a 488 (Not Acceptable Here) if the UAS is rejecting the INVITE.
[RFC3261] advises that a 488 response SHOULD include a warning
header with a reason for the rejection.
If the message from the UAC contains a known namespace, and the
priority-value is higher than is authorized, this error condition is
addressed in the next subsection (4.3).
In the case in which a UA is currently occupied with another session
and receives an INVITE containing a valid Resource-Priority header
of higher relative priority than that of the current dialog, the
current dialog is rejected with a BYE Request as per
[I-D. ietf-sipping-reason-header-for-preemption] and the new Request
is successful responded to.
If a Proxy Server is currently experiencing process difficulties
(perhaps due to overload), this is an error condition that will be
addressed in section 4.3.1.
4.3 Error Conditions
4.3.1 Known Namespace and Priority Value
Two error conditions can occur if a request reaches an element that
supports the namespace and resource priority. Elements receiving
requests with namespaces or priority values that they do not
understand act according to the rules in the next section.
Insufficient authorization: If the element receives a request with a
namespace and priority value it recognizes, but the originator is
not authorized for that level of service, the element MUST return
a 403 (Forbidden) response.
Insufficient resources: If there are insufficient resources at an
element for a given priority, a request might be delayed or
refused, depending on local policy or the definition of the
namespace. If it is refused, the element returns a 503 (Service
Unavailable) response. 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 than insufficient signaling capacity.
The 503 (Service Unavailable) response provides sufficient
indication to the originator to re-attempt with a higher
appropriate resource priority or to add a resource priority
indication, if authorized.
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4.3.2 Handling Unknown Namespaces and Priority Values
When handling requests with unknown namespaces or priority values,
elements can operate in one of three modes, "strict", "semi-strict"
and "loose". 'Strict' and 'semi-strict' are identified by the
presence or absence of a 'Require' header field with the 'Resource-
Priority' option tag. 'Loose' mode does not contain a Require
header. If the request includes a 'Require' header field with the
'Resource-Priority' option tag, a UAS MUST follow the strict or
semi-strict mode rules, otherwise UAS and proxies MUST operate in
loose mode. Stating the presence of the Require header (with the
'Resource-Priority' option tag) in a SIP message explicitly
determines adherence to one of two modes seems counterintuitive;
however, the differences are slight between the two modes and are
detailed below.
The use of the 'resource-priority' option tag in 'Proxy-Require'
header field is NOT RECOMMENDED in any mode.
4.3.3 Strict Mode
Strict Mode is for administrative domains (or realms) that require
the presence of the Resource-Priority Header with a known namespace
and priority-value in all SIP messages listed in section 4.1. UACs
will include a Requires header of 'Resource-Priority' in all such
SIP Requests to ensure all Responses include the 'Resource-Priority'
header. Domains that require inclusion of the 'Resource-Priority'
header in these message types have a proactive mechanism for
preferential treatment of SIP messages even in congestion instances.
When operating in strict mode, a SIP element MAY provide
preferential processing treatment of messages regardless of
processing load.
The following table extends the values in Table 2 of RFC3261
[RFC3261] operating in 'strict mode'. This table supercedes the
table in section 3.2 of this document.
Header field where proxy INV ACK CAN BYE REG OPT PRA
----------------------------------------------------------------
Resource-Priority R amdr m m m m m m m
Resource-Priority r amdr c c c c c c c
Resource-Priority 200 - m - m m m m m
Header field where proxy SUB NOT UPD MSG REF INF PUB
----------------------------------------------------------------
Resource-Priority R amdr m m m m m m m
Resource-Priority r amdr c c c c c c c
Resource-Priority 200 - m - m m m m m
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The 'Resource-Priority' header field included in a Request message
MUST be copied in the reply unchanged in 'strict' mode.
SIP elements in this mode will verify the namespace contained in the
SIP message is exactly as the domain expects, as well the supplied
priority-value MUST be one of the known priority-values for that
namespace (registered via IANA).
If a SIP element receives a Resource-Priority with unknown values
for either the namespace or priority-value, an error message of 417
(Unknown Resource-Priority) MUST be returned without exception.
Following standard SIP behavior (Section 8.2.2.3 of [RFC3261]), a UAS
operating in strict mode MUST reject a request with response code
420 (Bad Extension) if it does not support the 'Resource-Priority'
option tag.
For example, a gateway that is unaware of a resource priority
namespace might accept a request at non-elevated priority, but
then the request could later be preempted by other requests.
Also, use of the 'Require' restriction ensures that in parallel
forking, only branches that support the resource-priority
mechanism succeed.
In strict mode operations, any failure response MUST NOT include an
'Accept-Resource-Priority' header field on the final hop of
signaling. In other words, if an 'Accept-Resource-Priority' header
field is included in a SIP failure Response message, it MUST be
removed by the last Proxy (generally element in the second to last
Via entry). Revelation of this information to an unauthorized UAC
is not to occur as it would provide too much information to a UA
that might not be authorized to access that network.
UAS and Proxy implementations SHOULD support returning a 'Accept-
Resource-Priority' header field enumerating all the resource values
that the server is willing to process. This is particularly useful
for operational testing of systems supporting resource priority, as
otherwise it might be difficult to detect under non-overload
conditions whether an element supports the functionality or not.
4.3.4 Semi-Strict Mode
Semi-Strict Mode SHOULD be the operational mode when strict
adherence to the usage of the Resource-Priority header is granted
to authorized personnel in an otherwise public domain. In this mode,
the Resource-Priority header MUST only be used when a user is
authorized for preferential treatment of their SIP messages.
The following table extends the values in Table 2 of RFC3261
[RFC3261] operating in 'semi-strict mode' for the authorized UAC.
This table supercedes the table in section 3.2 of this document. A
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UAS will copy the 'Resource-Priority' header into the SIP Response
message from an authorized Request message. It is not expected that
a UAS can authenticate the Request, but the Response needs to be
given the same preferential treatment possibilities as the Request.
Header field where proxy INV ACK CAN BYE REG OPT PRA
----------------------------------------------------------------
Resource-Priority R amdr m m m m m m m
Resource-Priority r amdr c c c c c c c
Resource-Priority 200 - m - m m m m m
Header field where proxy SUB NOT UPD MSG REF INF PUB
----------------------------------------------------------------
Resource-Priority R amdr m m m m m m m
Resource-Priority r amdr c c c c c c c
Resource-Priority 200 - m - m m m m m
The 'Resource-Priority' header field included in a Request message
MUST be copied in the reply unchanged in 'semi-strict' mode.
One of the following MUST be included to differentiate the normal
users of a domain from those that will receive preferential
treatment:
1) Inclusion of the authorization header, or
2) What will come out of the Trait-Based Authorization
[I-D.ietf-sipping-trait-authz] effort currently under way
Semi-strict mode differs from Strict mode in that it is not required
that every SIP message have a Resource-Priority header. When the
Resource-Priority header is used, is MUST be exactly as is expected
by the SIP elements within a domain - otherwise a 417 (Unknown
Resource Priority) error is the response. If no Resource-Priority
header is present, the message does not receive preferential
treatment. Thus, the modified table 2 above would be filled with
'-' in each column for all Methods for the unauthorized UA.
An example usage of this mode is the US-based Government Emergency
Telephone Service (GETS), in which a very small number of
individuals have the ability to authorize with a centralized service
to provide to them preferential access for their next voice call.
This includes the ability to camp on a busy trunk group or circuit
in a Class 5 or Tandem switch until any one of the circuits becomes
free. The authorized user then is given access to the newly-
available circuit before anyone else has knowledge of it becoming
free. [RFC3487] is based on this GETS-type service.
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4.3.5 Loose Mode
In loose mode, unknown priority values or namespaces are ignored and
the request is treated as if these values were not included. If
there are no valid priority values or namespaces, the request is
treated as if it had no 'Resource-Priority' header field. Thus, no
417 (Unknown Resource-Priority) is generated. There is no
modification to the [RFC3261] table 2 extension in section 3.3 of
this document for loose mode.
Loose mode maintains the value extension table in section 3.3 of
this document (which is an extension of Table 2 of RFC3261).
4.4 User Agent Client Behavior
SIP UACs supporting this specification MUST be able to generate the
'Resource-Priority' header field for requests that require elevated
resource access priority. As stated previously, the UAC SHOULD be
able to generate more than one valid 'Resource-Priority' header
field in a single SIP Request, depending on the nature of the SIP
message.
If the request is returned with 417 (Unknown Resource-Priority), the
UAC MAY retry the request with a different set of namespace/priority
combinations, drawing from the values returned by the
'Accept-Resource-Priority' header field in the response, if included.
4.5 User Agent Server Behavior
If the UAS understands the resource value, but refuses to honor the
request with elevated priority for this particular user, it returns
the 403 (Forbidden) response code. It MAY include the list of
resource values that the user is allowed to use in the
'Accept-Resource-Priority' response header field. If the UAS
refuses to honor the request for a reason other than the Resource
Priority header, the proper response is a 488 (Not Acceptable Here).
A warning header MAY be included.
The lookup of the authorized values may take significant resources
since it may involve an AAA interaction. Thus, it seems imprudent
to require that the list is customized to the user. In general,
legitimate users know their highest resource value that they are
entitled to.
The precise effect of the 'Resource-Priority' indication depends on
the type of UAS, the namespace and local policy. For example, a
circuit-switched telephony gateway might move requests with elevated
priority to the front of the queue of requests waiting for outbound
lines, it may utilize additional resources or it may preempt existing
calls. For a terminal, such as a SIP phone, requests with elevated
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priority might trigger a special alert tone or preempt other,
lower-priority existing calls. The generic protocol mechanism
described here does not mandate the particular element behavior, but
namespace definitions, such as the ones in Section 9.5, MUST describe
the desired behavioral properties of user agents and proxy servers.
4.5.1 User Agent Servers and Preemption Policy
Within a UAS, the priority value (if higher in relative priority)
MUST cause a session to be terminated in favor of a newly signaled
session set-up (in strict mode). Consistent SIP termination
indications will be sent in these cases (using the BYE Method).
[I-D.ietf-sipping-reason-header-for-preemption] provides additional
information in the case of purposeful or administrative termination
of a session by including the Reason header in the BYE message that
states what happened (in this case, a preemption event). That
document offers several reasons for where the termination occurred
('at the UA', 'in the network', 'at a IP/PSTN gateway'), and
includes call flow examples of each reason.
4.6 Proxy Behavior
SIP proxies MAY ignore or inspect the 'Resource-Priority' header
field. SIP proxies MAY reject any unauthenticated request bearing
the header field.
If there are multiple namespace or priority choices available to the
user agent client, a proxy MAY return the request with an appropriate
'Accept-Resource-Priority' header field. Details are a matter of
local policy.
If a Proxy is expecting a message to have a 'Resource-Priority'
header and the header is protected via S/MIME encapsulation in a SIP
message fragment [RFC3420], the Proxy MUST error this message. This
MUST always be the case in 'strict-mode' (in which all messages will
have a valid 'Resource-Priority' header). Therefore, the use of
SIPFRAG in 'strict-mode' is not recommended. In 'semi-strict' mode,
SIP elements do not expect the 'Resource-Priority' header, so there
will not be any preferential treatment of that message by that SIP
element.
If no S/MIME is used, SIP proxies MAY downgrade or upgrade the
'Resource-Priority' of a request or insert a new 'Resource-Priority'
header if allowed by local policy.
This behavior is similar to that for any header field, as a UA can
decide to reject a request for the presence, absence or value of any
information in the request. The session policy mechanism does not
fit well, as user agents may not have a choice in the namespace or
priority available to them, there are no privacy concerns and the
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resource priority mechanism does not involve message bodies or
session descriptions.
If a stateful proxy has authorized a particular resource priority
level and if it offers differentiated treatment to responses
containing resource priority levels, the proxy SHOULD ignore any
higher value contained in responses, to avoid that colluding user
agents artificially raise the priority level.
It is unlikely that the resource priority value in responses will
have any influence on response handling.
A SIP proxy MAY use the 'Resource-Priority' indication in its
routing decisions, e.g., to retarget to a SIP node or SIP URI that
is reserved for a particular resource priority.
There do not appear to be any special considerations when forking
requests containing a resource priority indication.
Otherwise, the proxy behavior is the same as for user agent servers
Section 4.4).
4.6.1 Proxies and Preemption Policy
Within a SIP Server, a preemption policy means (authorized) messages
with higher priority values MUST be processed before messages
containing lower priority values. It does not mean the lower
priority SIP messages are deleted because they contain lower
priority values. That said, see section 4.3 for the error codes to
be returned to a UAC if the request had an insufficient priority to
be processed.
5. Third-Party Authentication
In some case, the RP actor may not be able to authenticate the
requestor or determine whether an authenticated user is authorized to
make such a request. In these circumstances, the SIP entity may
avail itself of general SIP mechanisms that are not specific to this
application. The authenticated identity management mechanism
[RFC3893] allows a third party to verify the identity of the
requestor and certify this towards an RP actor. In networks with
mutual trust, the SIP asserted identity mechanism [RFC3325] can help
the RP actor determine the identity of the requestor.
6. Backwards Compatibility
The resource priority mechanism described in this document is fully
backwards compatible with SIP systems following [RFC3261]. Systems
that do not understand the mechanism can only deliver standard, not
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elevated, service priority. User agent servers and proxies can ignore
any 'Resource-Priority' header field just like any other unknown
header field and then treat the request like any other request.
Naturally, the request may still succeed.
Introducing 'Require' or 'Proxy-Require' would not help backwards
compatibility, as systems that do not support these mechanism are no
better off rejecting the request due to feature failure. Since the
intent of resource priority indications is to increase the
probability of call completion, adding failure modes appears
counterproductive.
7. Namespace Description
A 'Resource-Priority' namespace MUST be unique with respect to other
'Resource-Priority' namespaces. There are 5 unique namespaces
defined in this specification. This section will define each, as
well as their individual parameters. The case insensitive
namespaces are as follows:
- dsn
- drsn
- q735
- ets
- wps
Each namespace has a finite list of relative priority-values. Each
is listed here in the order of lowest priority to highest priority:
(lowest) dsn.routine
dsn.priority
dsn.immediate
dsn.flash
(highest) dsn.flash-override
(lowest) drsn.routine
drsn.priority
drsn.immediate
drsn.flash
drsn.flash-override
(highest) drsn.flash-override-override
(lowest) q735.4
q735.3
q735.2
q735.1
(highest) q735.0
(lowest) ets.4
ets.3
ets.2
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ets.1
(highest) ets.0
(lowest) wps.4
wps.3
wps.2
wps.1
(highest) wps.0
More than one namespace listed here adheres to a 'strict mode' of
operation (dsn, drsn and q735). More than one adheres to a 'semi-
strict mode' of operation (ets and wps). No namespaces listed here
operate under 'loose mode'.
The dsn, drsn and q735 namespaces operate under a policy of
providing preferential message treatment to the point of preempting
lower relative priority requests in favor of processing higher
relative priority requests.
In SIP Servers, this translates to giving preferential processing
treatment to those messages containing higher priority values (a
"move to the front of the line" scenario). During light to medium
processing loads, this should not be evident at all to other
messages. During heavier loads on servers, these labels provide for
a domain ensuring (by their own configuration) a certain
classification(s) of messages are processed before other
classification(s) of messages.
In UAs, just as in SIP servers, this translates to giving
preferential processing treatment to those messages containing
higher priority values (a "move to the front of the line" scenario).
But for dialog requests, this behavior translates into preemption of
dialog events if a new INVITE is received that is indicating a
higher priority value than the one assigned to the current dialog.
The ets and wps namespaces operate with preferential treatment but
without preemption. These namespaces operate under a policy of
provide preferential treatment to higher relative priority requests
instead of processing lower relative priority requests. Messages
are not preempted, or deleted, except under extreme loads in which
all available processing is taken up with higher priority messages.
An example of this is at a IP/PSTN gateway with all of the PSTN side
circuits utilized. In strict mode one of the lower priority
circuits would be freed using preemption. In semi-strict mode, the
SIP request May be granted access to the next available circuit
based on this header's presence in the authorized message,
regardless of how many other "regular" requests are received at that
gateway.
There are no unique rejection messages to a SIP message containing
any one of these namespaces (outside of the rules within the mode
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SIP elements adhere to).
The 417 (Unknown Resource Priority) error message is the proper
response to any SIP request (authorized if that domain requires it,
and it SHOULD) if the namespace is unknown to that SIP element in
strict or semi-strict mode. A 417 is also the proper error response
in the case the namespace is known, but the priority-value is either
unknown, or does not correspond to the list of priority-values
associated with that particular namespace in strict or semi-strict
mode.
The following table will be repeated in the IANA considerations
section as the new registry for 'Resource-Priority' in the SIP
parameters section.
New New Error
Namespace Levels Mode Operation Rej. code code Reference
--------- ------ ---- --------- --------- ---- ---------
dsn 5 strict preemption no 417 [This RFC]
drsn 6 strict preemption no 417 [This RFC]
q735 5 strict preemption no 417 [This RFC]
ets 5 semi-strict preferential no 417 [This RFC]
wps 5 semi-strict preferential no 417 [This RFC]
Table 1. Namespace Parameters
New namespace/priority-value combinations proposed in the future
MUST be defined in a Standards Track RFC and MUST include an
augmentation to Table 1 of this document in that effort, as well as
list the finite set of priority values in relative priority order
(with no wildcards) for IANA Registration. New priority-values MUST
NOT be added to any previously (IANA) registered finite list
associated with a particular namespace. This will cause
interoperability problems.
7.1 Multiple Namespaces in a Message
The only rules stipulated here regarding more than one namespace in
a SIP message are:
o There MUST be one order of priorities a SIP element has to
process to.
o It MUST be configurable to have more than zero namespaces be
ignored, while the SIP element adheres to any number of Resource-
Priority headers (if placed in one overall relative order).
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o The header order in the message of which Resource-Priority header
is chosen MUST not matter. SIP elements MUST parse to any header
location it has visibility to.
o
For example, if a SIP element supports two namespaces (foo and bar).
Foo's priority-values are 1, 2 and 3 (lowest to highest), and bar's
priority-values are A , B and C (lowest to highest).
The following (not all inclusive list of) acceptable priority orders
the SIP element MAY process are:
Foo.3 Foo.3 Bar.C (highest)
Foo.2 Bar.C Foo.3
Foo.1 or Foo.2 or Foo.2
Bar.C Bar.B Foo.1
Bar.B Foo.1 Bar.B
Bar.A Bar.A Bar.A (lowest)
Bar.C (highest)
Foo.3 Bar.B <= these 2 are considered equivalent)
or Foo.2 Bar.A <= these 2 are considered equivalent)
Foo.1 (lowest)
Bar.C (highest)
Foo.3
or Foo.2
Foo.1 (lowest)
Where Bar.A and Bar.B are ignored
The following (not all inclusive list) are not acceptable priority
orders the SIP element MUST NOT process are:
Foo.3 Foo.3 Bar.C
Foo.2 Bar.A Foo.1
Foo.1 or Foo.2 or Foo.3
Bar.C Bar.B Foo.2
Bar.A Foo.1 Bar.A
Bar.B Bar.C Bar.B
Bar.C
Foo.1 Bar.B
or Foo.3 Bar.A
Foo.2
Local policy determines which namespace/priority-value combinations
are acceptable (if more than one is authorized for use within a
domain). The relative order of priority of the priority-values
within the same namespace MUST never change.
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8. Examples
The SDP message body and the BYE and ACK exchanges are the same as in
RFC 3665 [RFC3665] and omitted for brevity.
8.1 Simple Call
User A User B
| |
| INVITE F1 |
|----------------------->|
| 180 Ringing F2 |
|<-----------------------|
| |
| 200 OK F3 |
|<-----------------------|
| ACK F4 |
|----------------------->|
| Both Way RTP Media |
|<======================>|
| |
In this scenario, User A completes a call to User B directly. The
call from A to B is marked with a resource priority indication.
F1 INVITE User A -> User B
INVITE sip:UserB@biloxi.example.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: BigGuy <sip:UserA@atlanta.example.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Resource-Priority: dsn.flash
Contact: <sip:UserA@client.atlanta.example.com;transport=tcp>
Content-Type: application/sdp
Content-Length: ...
...
F2 180 Ringing User B -> User A
SIP/2.0 180 Ringing
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: BigGuy <sip:UserA@atlanta.example.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Resource-Priority: dsn.flash
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Contact: <sip:UserB@client.biloxi.example.com;transport=tcp>
Content-Length: 0
F3 200 OK User B -> User A
SIP/2.0 200 OK
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: BigGuy <sip:UserA@atlanta.example.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Resource-Priority: dsn.flash
Contact: <sip:UserB@client.biloxi.example.com;transport=tcp>
Content-Type: application/sdp
Content-Length: ...
...
8.2 Receiver Does Not Understand Namespace
In this example, the receiving UA does not understand the "dsn"
namespace and thus returns a 417 (Unknown Resource-Priority) status
code. We omit the message details for messages F5 through F7 since
they are essentially the same as in the first example.
User A User B
| |
| INVITE F1 |
|----------------------->|
| 417 R-P failed F2 |
|<-----------------------|
| ACK F3 |
|----------------------->|
| |
| INVITE F4 |
|----------------------->|
| 180 Ringing F5 |
|<-----------------------|
| 200 OK F6 |
|<-----------------------|
| ACK F7 |
|----------------------->|
| |
| Both Way RTP Media |
|<======================>|
F1 INVITE User A -> User B
INVITE sip:UserB@biloxi.example.com SIP/2.0
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Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: BigGuy <sip:UserA@atlanta.example.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Resource-Priority: dsn.flash
Contact: <sip:UserA@client.atlanta.example.com;transport=tcp>
Content-Type: application/sdp
Content-Length: ...
...
F2 417 Resource-Priority failed User B -> User A
SIP/2.0 417 Resource-Priority failed
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: BigGuy <sip:UserA@atlanta.example.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Accept-Resource-Priority: q735.0, q735.1, q735.2, q735.3, q735.4
Contact: <sip:UserB@client.biloxi.example.com;transport=tcp>
Content-Type: application/sdp
Content-Length: 0
F3 ACK User A -> User B
ACK sip:UserB@biloxi.example.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bd5
Max-Forwards: 70
From: BigGuy <sip:UserA@atlanta.example.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 ACK
Content-Length: 0
F4 INVITE User A -> User B
INVITE sip:UserB@biloxi.example.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: BigGuy <sip:UserA@atlanta.example.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Resource-Priority: q735.3
Contact: <sip:UserA@client.atlanta.example.com;transport=tcp>
Content-Type: application/sdp
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Content-Length: ...
...
9. Security Considerations
Any resource priority mechanism can be abused to obtain resources and
thus deny service to other users. An adversary may be able to take
over a particular gateway, cause additional congestion during PSTN
emergencies or deny service to legitimate users. In the Internet,
or any IP domain, this mechanism can cause certain messages or
sessions (calls) to be given a higher relative priority of
processing within a SIP element (move to the head of the line
scenario), to the point of prematurely terminating an existing
session in favor of a newer one. In some domains, this will be the
expected behavior for authenticated and authorized users (see
section 7). Unauthorized users MUST NOT be given this opportunity to
abuse network/element resources.
While the indication itself does not have to provide separate
authentication, any SIP request containing this header has higher
authentication requirements than regular requests.
A poor implementation of authentication and authorization will
likely cause illegitimate higher priority messages to process
without being successfully challenged for the privilege to do so.
While this will not likely have a great affect on the performance of
SIP Servers, this could have some (to a great) impact on the
premature termination of existing sessions. Those domains wishing
to utilize a namespace with an operation of preemption of lower
relative priority sessions should use caution to ensure only the
proper usage of this header is granted.
Care should be taken on 3 fronts:
1) To the domain that enables usage of the Resource-Priority header
such that adequate control exists to prevent unwanted
preferential message treatment from internal users.
Without an authentication and authorization mechanism to validate
each user request, unwanted usage (and potentially user hacked
settings) can have undesired affects on any internal network.
2) To the domain that enables usage of the Resource-Priority header
such that inadequate control exists to prevent unwanted
preferential message treatment from SIP messages from outside the
domain coming into the domain (and outside the area of direct AAA
control).
3) In the choosing of a namespace itself, to make sure the desired
behavior of SIP elements have equivalent behaviors defined in this
document to ensure interoperability and no surprises.
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An example of this is choosing a namespace throughout a domain and
configuring it for preferential treatment with no preemption, even
though a neighbor domain uses it as it is IANA defined (with
preemption as one expected behavior), resulting in poor performance
of fist domain's calls into the second domain's network.
Below, we describe authentication and authorization aspects,
confidentiality and privacy requirements, protection against denial
of service attacks and anonymity requirements. Naturally, the
general discussion in RFC 3261 [RFC3261] applies.
All user agents and proxy servers which support this extension MUST
implement SIP over TLS [RFC3546] and the sips: URI scheme as
described in Section 26.2 of RFC 3261, and Digest Authentication
[RFC2617] as described in Section 22 of RFC 3261. In addition, user
agents which support this extension SHOULD also implement S/MIME
[RFC2633] as described in Section 23 of RFC 3261 to allow for signing
and verification of signatures over requests which use this
extension.
9.1 Authentication and Authorization
Prioritized access to network and end system resources imposes
particularly stringent requirements on authentication and
authorization mechanisms since access to prioritized resources may
impact overall system stability and performance, not just result in
theft of, say, a single phone call.
Under certain emergency conditions, the network infrastructure,
including its authentication and authorization mechanism, may be
under attack.
Given the urgency during emergency events, normal statistical fraud
detection may be less effective, thus placing a premium on reliable
authentication.
Common requirements for authentication mechanisms apply, such as
resistance to replay, cut-and-paste and bid-down attacks.
Authentication MAY be SIP-based or use other mechanisms. Use of
Digest authentication and/or S/MIME is RECOMMENDED for UAS
authentication. Digest authentication requires that the parties
share a common secret, thus limiting its use across administrative
domains. SIP systems employing resource priority SHOULD implement S/
MIME at least for integrity, as described in Section 23 of [RFC3261].
However, in some environments, receipt of asserted identity [RFC3325]
from a trusted entity may be sufficient authorization. Section 5
describes third-party authentication.
Trait-based authorization [I-D.ietf-sipping-trait-authz] "entails an
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assertion by a authorization service of attributes associated with an
identity" and may be appropriate for this application. With
trait-based authorization, a network element can directly determine,
by inspecting the certificate, that a request is authorized to obtain
a particular type of service, without having to consult a mapping
mechanism that converts user identities to authorizations.
Authorization may be based on factors beyond the identity of the
caller, such as the requested destination. Namespaces MAY also
impose particular authentication or authorization consideration that
are stricter than the baseline described here.
9.2 Confidentiality and Integrity
Calls which use elevated resource priority levels provided by the
'Resource-Priority' header field are likely to be sensitive and often
need to be protected from intercept and alteration. In particular,
requirements for protecting the confidentiality of communications
relationships may be higher than for normal commercial service. For
SIP, the 'To', 'From', 'Organization' and 'Subject' header fields are
examples of particularly sensitive information. Systems MUST
implement encryption at the transport level using TLS and MAY
implement other transport-layer or network-layer security mechanisms.
UACs SHOULD use the "sips" URI to request a secure transport
association to the destination.
The 'Resource-Priority' header field can be carried in the SIP
message header or can be encapsulated in a message fragment carried
in the SIP message body [RFC3420]. To be considered valid
authentication for the purposes of this specification, S/MIME signed
SIP messages or fragments MUST contain, at a minimum, the Date, To,
From, Call-ID, and Resource-Priority header fields. Encapsulation in
S/MIME body parts allows the user to protect this header field
against inspection or modification by proxies. However, in many
cases, proxies will need to authenticate and authorize the request,
so that encapsulation is undesirable.
Removal of a Resource-Priority header field or downgrading its
priority value affords no additional opportunities to an adversary
since that man-in-the-middle could simply drop or otherwise
invalidate the SIP request and thus prevent call completion.
Only SIP elements within the same administrative trust domain
employing a secure channel between their SIP elements will trust a
Resource-Priority header field that is not appropriately signed.
Others will need to authenticate the request independently. Thus,
insertion of a Resource-Priority header field or upgrading the
priority value has no further security implications except causing a
request to fail (see discussion in the previous paragraph).
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9.3 Anonymity
Some users may wish to remain anonymous to the request destination.
Anonymity for requests with resource priority is no different than
for any other authenticated SIP request. For the reasons noted
earlier, users have to authenticate themselves towards the SIP
elements carrying the request where they desire resource priority
treatment. The authentication may be based on capabilities and noms,
not necessarily their civil name. Clearly, they may remain anonymous
towards the request destination, using the network-asserted identity
and general privacy mechanism described in [RFC3323].
9.4 Denial-of-Service Attacks
As noted, systems described here are likely to be subject to
deliberate denial-of-service (DoS) attacks during certain types of
emergencies. DoS attacks may be launched on the network itself as
well as its authentication and authorization mechanism. As noted,
systems should minimize the amount of state, computation and network
resources that an unauthorized user can command. The system must not
amplify attacks by causing the transmission of more than one packet
to a network address whose reachability has not been verified.
10. IANA Considerations
This section defines two new SIP headers (10.1), one SIP OPTION tag
(10.2), one new 4XX error code (10.3), and a new registry within the
sip-parameters section of IANA for Resource-Priority namespaces and
priority-values (10.4).
Additional namespaces and priority values MUST be registered with
IANA. Within each namespace, the registration MUST indicate the
relative priority levels, expressed as an ordered list. New
priority-values MUST NOT be added to existing namespace registries.
The registration MUST describe, in the registration itself, how SIP
elements should treat requests from that namespace in 'operation',
e.g., whether preemption or only preferential queuing are allowed.
The SIP Change Process [RFC 3427] establishes a policy for the
registration of new SIP extension headers. Resource priority
namespaces and priority values have similar interoperability
requirements to those of SIP extension headers. Consequently,
registration of new resource priority namespaces and priority values
requires documentation in an RFC using the extension header approval
process specified in RFC 3427.
10.1 IANA Registration of 'Resource-Priority' and
'Accept-Resource-Priority' Header Fields
[NOTE TO RFC EDITOR: Replace RFC XXXX with RFC number of this
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document.]
The following is the registration for the 'Resource-Priority' header
field:
RFC number: XXXX
Header name: 'Resource-Priority'
Compact form: none
The following is the registration for the 'Accept-Resource-Priority'
header field:
RFC number: XXXX
Header name: Accept-Resource-Priority
Compact form: none
10.2 IANA Registration for Option Tag resource-priority
RFC number: XXXX
Name of option tag: 'resource-priority'
Descriptive text: Indicates or requests support for the resource
priority mechanism.
10.3 IANA Registration for Response Code 417
RFC number: XXXX
Response code: 417
Default reason phrase: Unknown Resource-Priority
10.4 IANA Namespace and Priority-Value Registrations
A new registry ("Resource-Priority Namespaces") in the
sip-parameters section of IANA is to be created taking a form
similar to this table below:
New New Error
Namespace Levels Mode Operation Rej. code code Reference
--------- ------ ---- --------- --------- ---- ---------
dsn 5 strict preemption no 417 [This RFC]
drsn 6 strict preemption no 417 [This RFC]
q735 5 strict preemption no 417 [This RFC]
ets 5 semi-strict preferential no 417 [This RFC]
wps 5 semi-strict preferential no 417 [This RFC]
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Legend
------
Namespace = the name of the namespace
Levels = the number of priority-values within the namespace
Operation = Expected operational behavior of this namespace
New Rej. Code = New Rejection Codes introduced for this namespace
New Error Code = New Error Codes introduced for this namespace
Reference = Document reference for this namespace
10.5 IANA Priority-Value Registrations
A new registry ("Resource-Priority Priority-values") in the
sip-parameters section of IANA is to be created taking a form
similar to this table below (Reference [XXXX] is this RFC):
Namespace: drsn
Reference: [XXXX]
Priority-Values (least to greatest): "routine", "priority",
"immediate", "flash", "flash-override", "flash-override-override"
Namespace: dsn
Reference: [XXXX]
Priority-Values (least to greatest): "routine", "priority",
"immediate", "flash", "flash-override",
Namespace: q735
Reference: [XXXX]
Priority values (least to greatest): "4", "3", "2", "1", "0"
Namespace: ets
Reference: [XXXX]
Priority values (least to greatest): "4", "3", "2", "1", "0"
Namespace: wps
Reference: [XXXX]
Priority values (least to greatest): "4", "3", "2", "1", "0"
11. Acknowledgments
Ben Campbell, Janet Gunn, Paul Kyzivat, Rohan Mahy, Mike Pierce,
Samir Srivastava and Allison Mankin provided helpful comments.
Dean Willis provided much help with this effort.
Martin Dolly, An Nguyen and Niranjan Sandesara assisted with the
ets and wps namespaces.
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12. References
12.1 Normative References
[I.255.3] International Telecommunications Union, "Integrated
Services Digital Network (ISDN) - General Structure and
Service Capabilities - Multi-Level Precedence and
Preemption", Recommendation I.255.3, July 1990.
[Q.735.3] International Telecommunications Union, "Stage 3
description for community of interest supplementary
services using Signaling System No. 7: Multi-level
precedence and preemption", Recommendation Q.735.3, March
1993.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] 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.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, June 2002.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
UPDATE Method", RFC 3311, October 2002.
[RFC3420] Sparks, R., "Internet Media Type message/sipfrag", RFC
3420, November 2002.
[RFC3428] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C.
and D. Gurle, "Session Initiation Protocol (SIP) Extension
for Instant Messaging", RFC 3428, December 2002.
12.2 Informative References
[I-D.ietf-ieprep-framework]
Carlberg, K., Brown, I. and C. Beard, "Framework for
Supporting ETS in IP Telephony",
draft-ietf-ieprep-framework-09 (work in progress), April
2004.
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[RFC3893] Peterson, J., "SIP Authenticated Identity Body (AIB)
Format", RFC 3893, May 2004.
[I-D.ietf-sip-publish]
Niemi, A., "An Event State Publication Extension to the
Session Initiation Protocol (SIP)",
draft-ietf-sip-publish-04 (work in progress), May 2004.
[I-D.ietf-sipping-trait-authz]
Peterson, J., "Trait-based Authorization Requirements for
the Session Initiation Protocol (SIP)",
draft-ietf-sipping-trait-authz-00 (work in progress),
February 2004.
[I-D.ietf-sipping-reason-header-for-preemption]
Polk, J., "Reason Header for Preemption within the Session
Initiation Protocol (SIP)",
draft-ietf-sipping-reason-header-for-preemption-02 (work
in progress), August 2004
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A. and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC2633] Ramsdell, B., "S/MIME Version 3 Message Specification",
RFC 2633, June 1999.
[RFC2976] Donovan, S., "The SIP INFO Method", RFC 2976, October
2000.
[RFC3323] Peterson, J., "A Privacy Mechanism for the Session
Initiation Protocol (SIP)", RFC 3323, November 2002.
[RFC3324] Watson, M., "Short Term Requirements for Network Asserted
Identity", RFC 3324, November 2002.
[RFC3325] Jennings, C., Peterson, J. and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
November 2002.
[RFC3487] Schulzrinne, H., "Requirements for Resource Priority
Mechanisms for the Session Initiation Protocol (SIP)", RFC
3487, February 2003.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 3546, June 2003.
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[RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3665] Johnston, A., Donovan, S., Sparks, R., Cunningham, C. and
K. Summers, "Session Initiation Protocol (SIP) Basic Call
Flow Examples", BCP 75, RFC 3665, December 2003.
Authors' Addresses
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
US
Phone: +1 212 939 7042
EMail: hgs@cs.columbia.edu
URI: http://www.cs.columbia.edu
James Polk
Cisco Systems
2200 East President George Bush Turnpike
Richardson, TX 75082
US
EMail: jmpolk@cisco.com
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Schulzrinne & Polk [Page 33]
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