Network Working Group H. Schulzrinne

Network Working Group H. Schulzrinne
Request for Comments: 4412 Columbia U.
Category: Standards Track J. Polk
Cisco Systems
February 2006

Communications Resource Priority for
the Session Initiation Protocol (SIP)

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2006).

Abstract

This document defines two new Session Initiation Protocol (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 SIP
proxies. It does not directly influence the forwarding behavior of
IP routers.

Table of Contents

1. Introduction ....................................................3
2. Terminology .....................................................6
3. The Resource-Priority and Accept-Resource-Priority SIP
Header Fields ...................................................6
3.1. The 'Resource-Priority' Header Field .......................6
3.2. The 'Accept-Resource-Priority' Header Field ................8
3.3. Usage of the 'Resource-Priority' and
'Accept-Resource-Priority' .................................8
3.4. The 'resource-priority' Option Tag .........................9
4. Behavior of SIP Elements That Receive Prioritized Requests ......9
4.1. Introduction ...............................................9
4.2. General Rules ..............................................9
4.3. Usage of Require Header with Resource-Priority ............10
4.4. OPTIONS Request with Resource-Priority ....................10

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RFC 4412 SIP Resource Priority February 2006

4.5. Approaches for Preferential Treatment of Requests .........11
4.5.1. Preemption .........................................11
4.5.2. Priority Queueing ..................................12
4.6. Error Conditions ..........................................12
4.6.1. Introduction .......................................12
4.6.2. No Known Namespace or Priority Value ...............13
4.6.3. Authentication Failure .............................13
4.6.4. Authorization Failure ..............................14
4.6.5. Insufficient Resources .............................14
4.6.6. Busy ...............................................14
4.7. Element-Specific Behaviors ................................15
4.7.1. User Agent Client Behavior .........................15
4.7.2. User Agent Server Behavior .........................15
4.7.3. Proxy Behavior .....................................16
5. Third-Party Authentication .....................................17
6. Backwards Compatibility ........................................17
7. Examples .......................................................17
7.1. Simple Call ...............................................18
7.2. Receiver Does Not Understand Namespace ....................19
8. Handling Multiple Concurrent Namespaces ........................21
8.1. General Rules .............................................21
8.2. Examples of Valid Orderings ...............................21
8.3. Examples of Invalid Orderings .............................22
9. Registering Namespaces .........................................23
10. Namespace Definitions .........................................24
10.1. Introduction .............................................24
10.2. The "DSN" Namespace ......................................24
10.3. The "DRSN" Namespace .....................................25
10.4. The "Q735" Namespace .....................................25
10.5. The "ETS" Namespace ......................................26
10.6. The "WPS" Namespace ......................................26
11. Security Considerations .......................................27
11.1. General Remarks ..........................................27
11.2. Authentication and Authorization .........................27
11.3. Confidentiality and Integrity ............................28
11.4. Anonymity ................................................29
11.5. Denial-of-Service Attacks ................................29
12. IANA Considerations ...........................................30
12.1. Introduction .............................................30
12.2. IANA Registration of 'Resource-Priority' and
'Accept-Resource-Priority' Header Fields .................30
12.3. IANA Registration for Option Tag resource-priority .......31
12.4. IANA Registration for Response Code 417 ..................31
12.5. IANA Resource-Priority Namespace Registration ............31
12.6. IANA Priority-Value Registrations ........................32
13. Acknowledgements ..............................................32
14. References ....................................................33

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RFC 4412 SIP Resource Priority February 2006

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.

Even if the resources at the SIP element itself are not scarce, a SIP
gateway may mark outgoing calls with an indication of priority, e.g.,
on an ISUP (ISDN User Part) IAM (Initial Address Message) originated
by a SIP gateway with the Public Switched Telephone Network (PSTN).

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 may 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.

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RFC 4412 SIP Resource Priority February 2006

This document defines (Section 3) two new SIP header fields for
communications resource priority, called 'Resource-Priority' and
'Accept-Resource-Priority'. The 'Resource-Priority' header field MAY
be used by SIP user agents, including Public Switched Telephone
Network (PSTN) gateways and terminals, and SIP proxy servers to
influence their treatment of SIP requests, including the priority
afforded to PSTN calls. For PSTN gateways, the behavior translates
into analogous schemes in the PSTN, for example, the ITU
Recommendation Q.735.3 [Q.735.3] prioritization mechanism, in both
the PSTN-to-IP and IP-to-PSTN directions. ITU Recommendation I.255.3
[I.255.3] is another example.

A SIP request with a 'Resource-Priority' indication can be treated
differently in these situations:

1. The request can be given elevated priority for access to PSTN
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
PSTN gateway capacity limits in effect for lower priorities.

This header field is related to, but differs in semantics from, the
'Priority' header field ([RFC3261], Section 20.26). The 'Priority'
header field describes the importance that the SIP request should
have for the receiving human or its agent. For example, that header
may be factored into decisions about call routing to mobile devices
and assistants and about call acceptance when the call destination is
busy. The 'Priority' header field does not affect the usage of PSTN
gateway or proxy resources, for example. In addition, any User Agent
Client (UAC) can assert any 'Priority' value, and usage of 'Resource-
Priority' header field values is subject to authorization.

While the 'Resource-Priority' header field 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 field to cause such influence may be
defined in other documents.

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RFC 4412 SIP Resource Priority February 2006

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.

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 PSTN 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.

It is likely to be common that the same SIP element will handle
requests that bear the 'Resource-Priority' header fields and those
that do not.

Government entities and standardization bodies have developed several
different priority schemes for their networks. Users would like to
be able to obtain authorized priority handling in several of these
networks, without changing SIP clients. Also, a single call may
traverse SIP elements that are run by different administrations and
subject to different priority mechanisms. Since there is no global
ordering among those priorities, we allow each request to contain
more than one priority value drawn from these different priority
lists, called a namespace in this document. Typically, each SIP
element only supports one such namespace, but we discuss what happens
if an element needs to support multiple namespaces in Section 8.

Since gaining prioritized access to resources offers opportunities to
deny service to others, it is expected that all such prioritized
calls are subject to authentication and authorization, using standard
SIP security (Section 11) or other appropriate mechanisms.

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RFC 4412 SIP Resource Priority February 2006

The remainder of this document is structured as follows. After
defining terminology in Section 2, we define the syntax for the two
new SIP header fields in Section 3 and then describe protocol
behavior in Section 4. The two principal mechanisms for
differentiated treatment of SIP requests (namely, preemption and
queueing) are described in Section 4.5. Error conditions are covered
in Section 4.6. Sections 4.7.1 through 4.7.3 detail the behavior of
specific SIP elements. Third-party authentication is briefly
summarized in Section 5. Section 6 describes how this feature
affects existing systems that do not support it.

Since calls may traverse multiple administrative domains with
different namespaces or multiple elements with the same namespace, it
is strongly suggested that all such domains and elements apply the
same algorithms for the same namespace, as otherwise the end-to-end
experience of privileged users may be compromised.

Protocol examples are given in Section 7. Section 8 discusses what
happens if a request contains multiple namespaces or an element can
handle more than one namespace. Section 9 enumerates the information
that namespace registrations need to provide. Section 10 defines the
properties of five namespaces that are registered through this
document. Security issues are considered in Section 11, but this
document does not define new security mechanisms. Section 12
discusses IANA considerations and registers parameters related to
this document.

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 section defines the 'Resource-Priority' and
'Accept-Resource-Priority' SIP header field syntax. Behavior is
described in Section 4.

3.1. The 'Resource-Priority' Header Field

The 'Resource-Priority' request header field marks a SIP request as
desiring prioritized access to resources, as described in the
introduction.

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There is no protocol requirement that all requests within a SIP
dialog or session use the 'Resource-Priority' header field. Local
administrative policy MAY mandate the inclusion of the
'Resource-Priority' header field in all requests. Implementations of
this specification MUST allow inclusion to be either by explicit user
request or automatic for all requests.

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: dsn.flash

The 'r-value' parameter in the 'Resource-Priority' header field
indicates the resource priority desired by the request originator.
Each resource value (r-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 tokens that do not contain periods. Thus,
"dsn.flash" and "DSN.Flash", for example, are equivalent. Each
namespace has at least one priority value. Namespaces and priority
values within each namespace MUST be registered with IANA
(Section 12). Initial namespace registrations are described in
Section 12.5.

Since a request may traverse multiple administrative domains with
multiple different namespaces, it is necessary to be able to
enumerate several different namespaces within the same message.
However, a particular namespace MUST NOT appear more than once in the
same SIP message. These may be expressed equivalently as either
comma-separated lists within a single header field, as multiple
header fields, or as some combination. The ordering of 'r-values'
within the header field has no significance. Thus, for example, the
following three header snippets are equivalent:

Resource-Priority: dsn.flash, wps.3

Resource-Priority: wps.3, dsn.flash

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RFC 4412 SIP Resource Priority February 2006

Resource-Priority: wps.3
Resource-Priority: dsn.flash

3.2. The 'Accept-Resource-Priority' Header Field

The 'Accept-Resource-Priority' response header field enumerates the
resource values (r-values) a SIP user agent server is willing to
process. (This does not imply that a call with such values will find
sufficient resources and succeed.) The syntax of the 'Accept-
Resource-Priority' header field is as follows:

Accept-Resource-Priority = "Accept-Resource-Priority" HCOLON
[r-value *(COMMA r-value)]

An example is given below:

Accept-Resource-Priority: dsn.flash-override,
dsn.flash, dsn.immediate, dsn.priority, dsn.routine

Some administrative domains MAY choose to disable the use of the
'Accept-Resource-Priority' header for revealing too much information
about that domain in responses. However, this behavior is NOT
RECOMMENDED, as this header field aids in troubleshooting.

3.3. Usage of the 'Resource-Priority' and 'Accept-Resource-Priority'
Header Fields

The following table extends the values in Table 2 of RFC 3261
[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 [RFC3903].)

Header field where proxy INV ACK CAN BYE REG OPT PRA
----------------------------------------------------------------
Resource-Priority R amdr o o o o o o o
Accept-Resource-Priority 200 amdr o - o o o o o
Accept-Resource-Priority 417 amdr 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
Accept-Resource-Priority 200 amdr o o o o o o o
Accept-Resource-Priority 417 amdr o o o o o o o

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RFC 4412 SIP Resource Priority February 2006

Other request methods MAY define their own handling rules; unless
otherwise specified, recipients MAY ignore these header fields.

3.4. The 'resource-priority' Option Tag

This document also defines the "resource-priority" option tag. The
behavior is described in Section 4.3, and the IANA registration is in
Section 12.3.

4. Behavior of SIP Elements That Receive Prioritized Requests

4.1. Introduction

All SIP user agents and proxy servers that support this specification
share certain common behavior, which we describe below in
Section 4.2. The behavior when a 'resource-priority' option tag is
encountered in a 'Require' header field is described in Section 4.3.
Section 4.4 describes the treatment of OPTIONS requests. The two
fundamental resource contention resolution mechanisms, preemption and
queueing, are described in Section 4.5. Section 4.6 explains what
happens when requests fail. Behavior specific to user agent clients,
servers, and proxy servers is covered in Section 4.7.

4.2. General Rules

The 'Resource-Priority' header field is potentially applicable to all
SIP request messages. At a minimum, implementations of the following
request types MUST support the Resource-Priority header to be in
compliance with this specification:

o INVITE [RFC3261]

o ACK [RFC3261]

o PRACK [RFC3262]

o UPDATE [RFC3311]

o REFER [RFC3515]

Implementations SHOULD support the 'Resource-Priority' header field
in the following request types:

o MESSAGE [RFC3428]

o SUBSCRIBE [RFC3265]

o NOTIFY [RFC3265]

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RFC 4412 SIP Resource Priority February 2006

Note that this does not imply that all implementations have to
support all request methods listed.

If a SIP element receives the 'Resource-Priority' header field in a
request other than those listed above, the header MAY be ignored,
according to the rules of [RFC3261].

In short, an RP actor performs the following steps when receiving a
prioritized request. Error behavior is described in Section 4.6.

1. If the RP actor recognizes none of the name spaces, it treats the
request as if it had no 'Resource-Priority' header field.

2. It ascertains that the request is authorized according to local
policy to use the priority levels indicated. If the request is
not authorized, it rejects it. Examples of authorization
policies are discussed in Security Considerations (Section 11).

3. If the request is authorized and resources are available (no
congestion), it serves the request as usual. If the request is
authorized but resources are not available (congestion), it
either preempts other current sessions or inserts the request
into a priority queue, as described in Section 4.5.

4.3. Usage of Require Header with Resource-Priority

Following standard SIP behavior, if a SIP request contains the
'Require' header field with the 'resource-priority' option tag, a SIP
user agent MUST respond with a 420 (Bad Extension) if it does not
support the SIP extensions described in this document. It then lists
"resource-priority" in the 'Unsupported' header field included in the
response.

The use of the 'resource-priority' option tag in 'Proxy-Require'
header field is NOT RECOMMENDED.

4.4. OPTIONS Request with Resource-Priority

An OPTIONS request can be used to determine if an element supports
the mechanism. A compliant implementation SHOULD return an 'Accept-
Resource-Priority' header field in OPTIONS responses enumerating all
valid resource values, but an RP actor MAY be configured not to
return such values or only to return them to authorized requestors.

Following standard SIP behavior, OPTIONS responses MUST include the
'Supported' header field that includes the 'resource-priority' option
tag.

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RFC 4412 SIP Resource Priority February 2006

According to RFC 3261, Section 11, proxies that receive a request
with a 'Max-Forwards' header field value of zero MAY answer the
OPTIONS request, allowing a UAC to discover the capabilities of both
proxy and user agent servers.

4.5. Approaches for Preferential Treatment of Requests

SIP elements may use the resource priority mechanism to modify a
variety of behaviors, such as routing requests, authentication
requirements, override of network capacity controls, or logging. The
resource priority mechanism may influence the treatment of the
request itself, the marking of outbound PSTN calls at a gateway, or
of the session created by the request. (Here, we use the terms
session and call interchangeably, both implying a continuous data
stream between two or more parties. Sessions are established by SIP
dialogs.)

Below, we define two common algorithms, namely, preemption and
priority queueing. Preemption applies only to sessions created by
SIP requests, while both sessions and request handling can be subject
to priority queueing. Both algorithms can sometimes be combined in
the same element, although none of the namespaces described in this
document do this. Algorithms can be defined for each namespace or,
in some cases, can be specific to an administrative domain. Other
behavior, such as request routing or network management controls, is
not defined by this specification.

Naturally, only SIP elements that understand this mechanism and the
namespace and resource value perform these algorithms. Section 4.6.2
discusses what happens if an RP actor does not understand priority
values contained in a request.

4.5.1. Preemption

An RP actor following a preemption policy may disrupt an existing
session to make room for a higher-priority incoming session. Since
sessions may require different amounts of bandwidth or a different
number of circuits, a single higher-priority session may displace
more than one lower-priority session. Unless otherwise noted,
requests do not preempt other requests of equal priority. As noted
above, the processing of SIP requests itself is not preempted. Thus,
since proxies do not manage sessions, they do not perform preemption.

[RFC4411] contains more details and examples of this behavior.

UAS behavior for preemption is discussed in Section 4.7.2.1.

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RFC 4412 SIP Resource Priority February 2006

4.5.2. Priority Queueing

In a priority queueing policy, requests that find no available
resources are queued to the queue assigned to the priority value.
Unless otherwise specified, requests are queued in first-come, first-
served order. Each priority value may have its own queue, or several
priority values may share a single queue. If a resource becomes
available, the RP actor selects the request from the highest-priority
non-empty queue according to the queue service policy. For first-
come, first-served policies, the request from that queue that has
been waiting the longest is served. Each queue can hold a finite
number of pending requests. If the per-priority-value queue for a
newly arriving request is full, the request is rejected immediately,
with the status codes specified in Section 4.6.5 and Section 4.6.6.
In addition, a priority queueing policy MAY impose a waiting time
limit for each priority class, whereby requests that exceed a
specified waiting time are ejected from the queue and a 408 (Request
Timeout) failure response is returned to the requestor.

Finally, an RP actor MAY impose a global queue size limit summed
across all queues and drop waiting lower-priority requests with a 408
(Request Timeout) failure response. This does not imply preemption,
since the session has not been established yet.

UAS behavior for queueing is discussed in Section 4.7.2.2.

4.6. Error Conditions

4.6.1. Introduction

In this section, we describe the error behavior that is shared among
multiple types of RP actors (including various instances of UAS such
as trunk gateways, line gateways, and IP phones) and proxies.

A request containing a resource priority indication can fail for four
reasons:

o the RP actor does not understand the priority value
(Section 4.6.2),

o the requestor is not authenticated (Section 4.6.3),

o an authenticated requestor is not authorized to make such a
request (Section 4.6.4), or

o there are insufficient resources for an authorized request
(Section 4.6.5).

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RFC 4412 SIP Resource Priority February 2006

We treat these error cases in the order that they typically arise in
the processing of requests with Resource-Priority headers. However,
this order is not mandated. For example, an RP actor that knows that
a particular resource value cannot be served or queued MAY, as a
matter of local policy, forgo authorization, since it would only add
processing load without changing the outcome.

4.6.2. No Known Namespace or Priority Value

If an RP actor does not understand any of the resource values in the
request, the treatment depends on the presence of the 'Require'
'resource-priority' option tag:

1. Without the option tag, the RP actor treats the request as if it
contained no 'Resource-Priority' header field and processes it
with default priority. Resource values that are not understood
MUST NOT be modified or deleted.

2. With the option tag, it MUST reject the request with a 417
(Unknown Resource-Priority) response code.

Making case (1) the default is necessary since otherwise there would
be no way to successfully complete any calls in the case where a
proxy on the way to the UAS shares no common namespaces with the UAC,
but the UAC and UAS do have such a namespace in common.

In general, as noted, a SIP request can contain more than one
'Resource-Priority' header field. This is necessary if a request
needs to traverse different administrative domains, each with its own
set of valid resource values. For example, the ETS namespace might
be enabled for United States government networks that also support
the DSN and/or DRSN namespaces for most individuals in those domains.

A 417 (Unknown Resource-Priority) response MAY, according to local
policy, include an 'Accept-Resource-Priority' header field
enumerating the acceptable resource values.

4.6.3. Authentication Failure

If the request is not authenticated, a 401 (Unauthorized) or 407
(Proxy Authentication Required) response is returned in order to
allow the requestor to insert appropriate credentials.

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RFC 4412 SIP Resource Priority February 2006

4.6.4. Authorization Failure

If the RP actor receives an authenticated 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.

4.6.5. Insufficient Resources

Insufficient resource conditions can occur on proxy servers and user
agent servers, typically trunk gateways, if an RP actor receives an
authorized request, has insufficient resources, and the request
neither preempts another session nor is queued. A request can fail
because the RP actor has either insufficient processing capacity to
handle the SIP request or insufficient bandwidth or trunk capacity to
establish the requested session for session-creating SIP requests.

If the request fails because the RP actor cannot handle the signaling
load, the RP actor responds with 503 (Service Unavailable).

If there is not enough bandwidth, or if there is an insufficient
number of trunks, a 488 (Not Acceptable Here) response indicates that
the RP actor is rejecting the request due to media path availability,
such as insufficient gateway resources. In that case, [RFC3261]
advises that a 488 response SHOULD include a 'Warning' header field
with a reason for the rejection; warning code 370 (Insufficient
Bandwidth) is typical.

For systems implementing queueing, if the request is queued, the UAS
will return 408 (Request Timeout) if the request exceeds the maximum
configured waiting time in the queue.

4.6.6. Busy

Resource contention also occurs when a call request arrives at a UAS
that is unable to accept another call, because the UAS either has
just one line appearance or has active calls on all line appearances.
If the call request indicates an equal or lower priority value when
compared to all active calls present on the UAS, the UAS returns a
486 (Busy here) response.

If the request is queued instead, the UAS will return a 408 (Request
Timeout) if the request exceeds the maximum configured waiting time
in the device queue.

If a proxy gets 486 (Busy Here) responses on all branches, it can
then return a 600 (Busy Everywhere) response to the caller.

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RFC 4412 SIP Resource Priority February 2006

4.7. Element-Specific Behaviors

4.7.1. 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 resource value in a single SIP
request.

Upon receiving a 417 (Unknown Resource-Priority) response, the UAC
MAY attempt a subsequent request with the same or different resource
value. If available, it SHOULD choose authorized resource values
from the set of values returned in the 'Accept-Resource-Priority'
header field.

4.7.1.1. User Agent Client Behavior with a Preemption Algorithm

A UAC that requests a priority value that may cause preemption MUST
understand a Reason header field in the BYE request explaining why
the session was terminated, as discussed in [RFC4411].

4.7.1.2. User Agent Client Behavior with a Queueing Policy

By standard SIP protocol rules, a UAC MUST be prepared to receive a
182 (Queued) response from an RP actor that is currently at capacity,
but that has put the original request into a queue. A UAC MAY
indicate this queued status to the user by some audio or visual
indication to prevent the user from interpreting the call as having
failed.

4.7.2. User Agent Server Behavior

The precise effect of the 'Resource-Priority' indication depends on
the type of UAS, the namespace, and local policy.

4.7.2.1. User Agent Servers and Preemption Algorithm

A UAS compliant with this specification MUST terminate a session
established with a valid namespace and lower-priority value in favor
of a new session set up with a valid namespace and higher relative
priority value, unless local policy has some form of call-waiting
capability enabled. If a session is terminated, the BYE method is
used with a 'Reason' header field indicating why and where the
preemption took place.

Implementors have a number of choices in how to implement preemption
at IP phones with multiple line presences, i.e., with devices that

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RFC 4412 SIP Resource Priority February 2006

can handle multiple simultaneous sessions. Naturally, if that device
has exhausted the number of simultaneous sessions, one of the
sessions needs to be replaced. If the device has spare sessions, an
implementation MAY choose to alert the callee to the arrival of a
higher-priority call. Details may also be set by local or namespace
policy.

[RFC4411] 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 why the BYE was sent (in this
case, a preemption event). The mechanisms in that document allow
indication of where the termination occurred ('at the UA', 'within a
reservation', 'at a IP/PSTN gateway') and include call flow examples
of each reason.

4.7.2.2. User Agent Servers and Queue-Based Policy

A UAS compliant with this specification SHOULD generate a 182
(Queued) response if that element's resources are busy, until it is
able to handle the request and provide a final response. The
frequency of such provisional messages is governed by [RFC3261].

4.7.3. Proxy Behavior

SIP proxies MAY ignore the 'Resource-Priority' header field. SIP
proxies MAY reject any unauthenticated request bearing that header
field.

When the 'Require' header field is included in a message, it ensures
that in parallel forking, only branches that support the resource-
priority mechanism succeed.

If S/MIME encapsulation is used according to Section 23 of RFC 3261,
special considerations apply. As tabulated in Section 3.3, the
'Resource-Priority' header field can be modified by proxies and thus
is exempted from the integrity checking described in Section 23.4.1.1
of RFC 3261. Since it may need to be inspected or modified by
proxies, the header field MUST also be placed in the "outer" message
if the UAC would like proxy servers to be able to act on the header
information. Similar considerations apply if parts of the message
are integrity protected or encrypted as described in [RFC3420].

If S/MIME is not used, or if the 'Resource-Priority' header field is
in the "outer" header, SIP proxies MAY downgrade or upgrade the
'Resource-Priority' of a request or insert a new 'Resource-Priority'
header if allowed by local policy.

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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 prevent colluding user agents
from artificially raising the priority level.

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 are no special considerations for proxies when forking requests
containing a resource priority indication.

Otherwise, the proxy behavior is the same as for user agent servers
described in Section 4.7.2.

5. Third-Party Authentication

In some cases, 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 to 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
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.

7. Examples

The SDP message body and the BYE and ACK exchanges are the same as in
RFC 3665 [RFC3665] and are omitted for brevity.

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RFC 4412 SIP Resource Priority February 2006

7.1. 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:[email protected] SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: BigGuy <sip:[email protected]>;tag=9fxced76sl
To: LittleGuy <sip:[email protected]>
Call-ID: [email protected]
CSeq: 1 INVITE
Resource-Priority: dsn.flash
Contact: <sip:[email protected];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:[email protected]>;tag=9fxced76sl
To: LittleGuy <sip:[email protected]>;tag=8321234356
Call-ID: [email protected]
CSeq: 1 INVITE
Contact: <sip:[email protected];transport=tcp>
Content-Length: 0

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RFC 4412 SIP Resource Priority February 2006

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:[email protected]>;tag=9fxced76sl
To: LittleGuy <sip:[email protected]>;tag=8321234356
Call-ID: [email protected]
CSeq: 1 INVITE
Contact: <sip:[email protected];transport=tcp>
Content-Type: application/sdp
Content-Length: ...

...

7.2. 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 |
|<======================>|

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RFC 4412 SIP Resource Priority February 2006

F1 INVITE User A -> User B

INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: BigGuy <sip:[email protected]>;tag=9fxced76sl
To: LittleGuy <sip:[email protected]>
Call-ID: [email protected]
CSeq: 1 INVITE
Require: resource-priority
Resource-Priority: dsn.flash
Contact: <sip:[email protected];transport=tcp>

Content-Type: application/sdp
Content-Length: ...

...

F2 417 Resource-Priority failed User B -> User A

SIP/2.0 417 Unknown Resource-Priority
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: BigGuy <sip:[email protected]>;tag=9fxced76sl
To: LittleGuy <sip:[email protected]>;tag=8321234356
Call-ID: [email protected]
CSeq: 1 INVITE
Accept-Resource-Priority: q735.0, q735.1, q735.2, q735.3, q735.4
Contact: <sip:[email protected];transport=tcp>
Content-Type: application/sdp
Content-Length: 0

F3 ACK User A -> User B

ACK sip:[email protected] SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bd5
Max-Forwards: 70
From: BigGuy <sip:[email protected]>;tag=9fxced76sl
To: LittleGuy <sip:[email protected]>;tag=8321234356
Call-ID: [email protected]
CSeq: 1 ACK
Content-Length: 0

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RFC 4412 SIP Resource Priority February 2006

F4 INVITE User A -> User B

INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: BigGuy <sip:[email protected]>;tag=9fxced76sl
To: LittleGuy <sip:[email protected]>
Call-ID: [email protected]
CSeq: 2 INVITE
Require: resource-priority
Resource-Priority: q735.3
Contact: <sip:[email protected];transport=tcp>

Content-Type: application/sdp
Content-Length: ...
...

8. Handling Multiple Concurrent Namespaces

8.1. General Rules

A single SIP request MAY contain resource values from multiple
namespaces. As noted earlier, an RP actor disregards all namespaces
it does not recognize. This specification only addresses the case
where an RP actor then selects one of the remaining resource values
for processing, usually choosing the one with the highest relative
priority.

If an RP actor understands multiple namespaces, it MUST create a
local total ordering across all resource values from these
namespaces, maintaining the relative ordering within each namespace.
It is RECOMMENDED that the same ordering be used across an
administrative domain. However, there is no requirement that such
ordering be the same across all administrative domains.

8.2. Examples of Valid Orderings

Below are a set of examples of an RP actor that supports two
namespaces, foo and bar. Foo's priority-values are 3 (highest), then
2, and then 1 (lowest), and bar's priority-values are C (highest),
then B, and then A (lowest).

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Below are five lists of acceptable priority orders the SIP element
may use:

Foo.3 Foo.3 Bar.C (highest priority)
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 priority)

Bar.C (highest priority)
Foo.3 Bar.B (both treated with equal priority (FIFO))
or Foo.2 Bar.A (both treated with equal priority (FIFO))
Foo.1 (lowest priority)

Bar.C (highest priority)
Foo.3
or Foo.2
Foo.1 (lowest priority)

In the last example above, Bar.A and Bar.B are ignored.

8.3. Examples of Invalid Orderings

Based on the priority order of the namespaces above, the following
combinations are examples of orderings that are NOT acceptable and
MUST NOT be configurable:

Example 1 Example 2 Example 3
--------- --------- ---------
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

Example 4
---------
Bar.C
Foo.1 Bar.B
or Foo.3 Bar.A
Foo.2

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RFC 4412 SIP Resource Priority February 2006

These examples are invalid since the following global orderings are
not consistent with the namespace-internal order:

o In Example 1, Bar.A is ordered higher than Bar.B.

o In Example 2, Bar.A is ordered higher than Bar.B and Bar.C.

o In Example 3, Foo.1 is ordered higher than Foo.2 and Foo.3.

o In Example 4, Foo.1 is ordered higher than Foo.3 and Foo.2.

9. Registering Namespaces

Organizations considering the use of the Resource-Priority header
field should investigate whether an existing combination of namespace
and priority-values meets their needs. For example, emergency first
responders around the world are discussing utilizing this mechanism
for preferential treatment in future networks. Jurisdictions SHOULD
attempt to reuse existing IANA registered namespaces where possible,
as a goal of this document is not to have unique namespaces per
jurisdiction serving the same purpose, with the same usage of
priority levels. This will greatly increase interoperability and
reduce development time, and probably reduce future confusion if
there is ever a need to map one namespace to another in an
interworking function.

Below, we describe the steps necessary to register a new namespace.

A new namespace MUST be defined in a Standards Track RFC, following
the 'Standards Action' policy in [RFC2434], and MUST include the
following facets:

o It must define the namespace label, a unique namespace label
within the IANA registry for the SIP Resource-Priority header
field.

o It must enumerate the priority levels (i.e., 'r-priority' values)
the namespace is using. Note that only finite lists are
permissible, not unconstrained integers or tokens, for example.

o The priority algorithm (Section 4.5), identifying whether the
namespace is to be used with priority queueing ("queue") or
preemption ("preemption"). If queueing is used, the namespace MAY
indicate whether normal-priority requests are queued. If there is
a new "intended algorithm" other than preemption or priority
queueing, the algorithm must be described, taking into account all
RP actors (UAC, UAS, proxies).

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RFC 4412 SIP Resource Priority February 2006

o A namespace may either reference an existing list of priority
values or define a new finite list of priority values in relative
priority order for IANA registration within the sip-parameters
Resource-Priority priority-values registry. New priority-values
SHOULD NOT be added to a previously IANA-registered list
associated with a particular namespace, as this may cause
interoperability problems. Unless otherwise specified, it is
assumed that all priority values confer higher priority than
requests without a priority value.

o Any new SIP response codes unique to this new namespace need to be
explained and registered.

o The reference document must specify and describe any new Warning
header field warn-codes (RFC 3261, Section 27.2).

o The document needs to specify a new row for the following table
that summarizes the features of the namespace and is included into
IANA Resource-Priority Namespace registration:

Intended New New resp.
Namespace Levels algorithm warn-code code Reference
--------- ------ ---------------- --------- -------- ---------
<label> <# of <preemption <new warn <new resp. <RFC>
levels> or queue> code> code>

If information on new response codes, rejection codes, or error
behaviors is omitted, it is to be assumed that the namespace defines
no new parameters or behaviors.

10. Namespace Definitions

10.1. Introduction

This specification defines five unique namespaces below: DSN, DRSN,
Q735, ETS, and WPS, constituting their registration with IANA. Each
IANA registration contains the facets defined in Section 9. For
recognizability, we label the namespaces in capital letters, but note
that namespace names are case insensitive and are customarily
rendered as lowercase in protocol requests.

10.2. The "DSN" Namespace

The DSN namespace comes from the name of a US government network
called "The Defense Switched Network".

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RFC 4412 SIP Resource Priority February 2006

The DSN namespace has a finite list of relative priority-values,
listed below from lowest priority to highest priority:

(lowest) dsn.routine
dsn.priority
dsn.immediate
dsn.flash
(highest) dsn.flash-override

The DSN namespace uses the preemption algorithm (Section 4.5.1).

10.3. The "DRSN" Namespace

The DRSN namespace comes from the name of a US government network,
called "The Defense RED Switched Network".

The DRSN namespace defines the following resource values, listed from
lowest priority to highest priority:

(lowest) drsn.routine
drsn.priority
drsn.immediate
drsn.flash
drsn.flash-override
(highest) drsn.flash-override-override

The DRSN namespace uses the preemption algorithm (Section 4.5.1).

The DRSN namespace differs in one algorithmic aspect from the DSN and
Q735 namespaces. The behavior for the 'flash-override-override'
priority value differs from the other values. Normally, requests do
not preempt those of equal priority, but a newly arriving 'flash-
override-override' request will displace another one of equal
priority if there are insufficient resources. This can also be
expressed as saying that 'flash-override-override' requests defend
themselves as 'flash-override' only.

10.4. The "Q735" Namespace

Q.735.3 [Q.735.3] was created to be a commercial version of the
operationally equivalent DSN specification for Multi-Level Precedence
and Preemption (MLPP). The Q735 namespace is defined here in the
same manner.

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RFC 4412 SIP Resource Priority February 2006

The Q735 namespace defines the following resource values, listed from
lowest priority to highest priority:

(lowest) q735.4
q735.3
q735.2
q735.1
(highest) q735.0

The Q735 namespace operates according to the preemption
(Section 4.5.1) algorithm.

10.5. The "ETS" Namespace

The ETS namespace derives its name indirectly from the name of the US
government telecommunications service, called "Government Emergency
Telecommunications Service" (or GETS), though the organization
responsible for the GETS service chose the acronym "ETS" for its GETS
over IP service, which stands for "Emergency Telecommunications
Service".

The ETS namespace defines the following resource values, listed from
lowest priority to highest priority:

(lowest) ets.4
ets.3
ets.2
ets.1
(highest) ets.0

The ETS namespace operates according to the priority queueing
algorithm (Section 4.5.2).

10.6. The "WPS" Namespace

The WPS namespace derives its name from the "Wireless Priority
Service", defined in GSM and other wireless technologies.

The WPS namespace defines the following resource values, listed from
lowest priority to highest priority:

(lowest) wps.4
wps.3
wps.2
wps.1
(highest) wps.0

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RFC 4412 SIP Resource Priority February 2006

The WPS namespace operates according to the priority queueing
algorithm (Section 4.5.2).

11. Security Considerations

11.1. General Remarks

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 PSTN gateway, cause additional congestion during
emergencies affecting the PSTN, or deny service to legitimate users.
In SIP end systems, such as IP phones, this mechanism could
inappropriately terminate existing sessions and calls.

Thus, while the indication itself does not have to provide separate
authentication, SIP requests containing this header are very likely
to have higher authentication requirements than those without.

These authentication and authorization requirements extend to users
within the administrative domain, as later interconnection with other
administrative domains may invalidate earlier assumptions on the
trustworthiness of users.

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 that support this extension MUST
implement SIP over TLS [RFC3546], 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 that
support this extension SHOULD also implement S/MIME [RFC3851] as
described in Section 23 of RFC 3261 to allow for signing and
verification of signatures over requests that use this extension.

11.2. 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 and 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.

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RFC 4412 SIP Resource Priority February 2006

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 [TRAIT] "entails an 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 besides the identity of the
caller, such as the requested destination. Namespaces MAY also
impose particular authentication or authorization considerations that
are stricter than the baseline described here.

11.3. Confidentiality and Integrity

Calls that 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 those 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

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RFC 4412 SIP Resource Priority February 2006

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 encapsulation would be 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).

11.4. Anonymity

Some users may wish to remain anonymous to the request destination.
Anonymity for requests with resource priority is no different from
that 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].

11.5. 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 on 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.

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RFC 4412 SIP Resource Priority February 2006

12. IANA Considerations

12.1. Introduction

This section defines two new SIP headers (Section 12.2), one SIP
option tag (Section 12.3), one new 4xx error code (Section 12.4), a
new registry within the sip-parameters section of IANA for Resource-
Priority namespaces (Section 12.5), and a new registry within the
sip-parameters section of IANA for Resource-Priority and priority-
values (Section 12.6).

Additional namespaces and priority values MUST be registered with
IANA, as described in Section 9.

The SIP Change Process [RFC3427] 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.

Registration policies for new namespaces are defined in Section 9.

12.2. IANA Registration of 'Resource-Priority' and 'Accept-Resource-
Priority' Header Fields

The following is the registration for the 'Resource-Priority' header
field:

RFC number: 4412
Header name: 'Resource-Priority'
Compact form: none

The following is the registration for the 'Accept-Resource-Priority'
header field:

RFC number: 4412
Header name: Accept-Resource-Priority
Compact form: none

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RFC 4412 SIP Resource Priority February 2006

12.3. IANA Registration for Option Tag resource-priority

RFC number: 4412
Name of option tag: 'resource-priority'
Descriptive text: Indicates or requests support for the resource
priority mechanism.

12.4. IANA Registration for Response Code 417

RFC number: 4412
Response code: 417
Default reason phrase: Unknown Resource-Priority

12.5. IANA Resource-Priority Namespace Registration

A new registry ("Resource-Priority Namespaces") in the sip-parameters
section of IANA has been created, taking a form similar to this table
below:

Intended New warn- New resp.
Namespace Levels Algorithm code code Reference
--------- ------ ---------------- --------- --------- ---------
dsn 5 preemption no no [RFC4412]

drsn 6 preemption no no [RFC4412]

q735 5 preemption no no [RFC4412]

ets 5 queue no no [RFC4412]

wps 5 queue no no [RFC4412]

Legend
------
Namespace The unique string identifying the namespace.
Levels The number of priority-values within the namespace.
Algorithm Intended operational behavior of SIP elements
implementing this namespace.
New Warn code New Warning Codes (warn-codes) introduced by
this namespace.
New Resp. code New SIP response codes introduced by this namespace.
Reference IETF document reference for this namespace.

Schulzrinne & Polk Standards Track [Page 31]

RFC 4412 SIP Resource Priority February 2006

12.6. IANA Priority-Value Registrations

A new registry ("Resource-Priority Priority-values") in the sip-
parameters section of IANA has been created, taking a form similar to
this table below:

Namespace: drsn
Reference: RFC 4412
Priority-Values (least to greatest): "routine", "priority",
"immediate", "flash", "flash-override", "flash-override-override"

Namespace: dsn
Reference: RFC 4412
Priority-Values (least to greatest): "routine", "priority",
"immediate", "flash", "flash-override"

Namespace: q735
Reference: RFC 4412
Priority values (least to greatest): "4", "3", "2", "1", "0"

Namespace: ets
Reference: RFC 4412
Priority values (least to greatest): "4", "3", "2", "1", "0"

Namespace: wps
Reference: RFC 4412
Priority values (least to greatest): "4", "3", "2", "1", "0"

13. Acknowledgements

Ben Campbell, Ken Carlberg, Paul Kyzivat, Rohan Mahy, Allison Mankin,
Xavier Marjou, Piers O'Hanlon, Mike Pierce, Samir Srivastava, and
Dale Worley 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.

Janet Gunn helped improve the text on queueing-based priority.

Schulzrinne & Polk Standards Track [Page 32]

RFC 4412 SIP Resource Priority February 2006

14. References

14.1. 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 Signalling System No. 7: Multi-level
precedence and preemption", Recommendation Q.735.3,
March 1993.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.

[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.B., "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.

[RFC4411] Polk, J., "Extending the Session Initiation Protocol (SIP)
Reason Header for Preemption Events", RFC 4411, February
2006.

Schulzrinne & Polk Standards Track [Page 33]

RFC 4412 SIP Resource Priority February 2006

14.2. Informative References

[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.

[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.

[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.

[RFC3427] 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.

[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.

[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.

[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.

[RFC3893] Peterson, J., "Session Initiation Protocol (SIP)
Authenticated Identity Body (AIB) Format", RFC 3893,
September 2004.

Schulzrinne & Polk Standards Track [Page 34]

RFC 4412 SIP Resource Priority February 2006

[RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension
for Event State Publication", RFC 3903, October 2004. for
Event State Publication", RFC 3903, October 2004.

[TRAIT] Peterson, J., Polk, J., Sicker, D., and H. Tschofenig,
"Trait-based Authorization Requirements for the Session
Initiation Protocol (SIP)", Work in Progress,
February 2005.

Authors' Addresses

Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
US

Phone: +1 212 939 7004
EMail: [email protected]
URI: http://www.cs.columbia.edu

James Polk
Cisco Systems
2200 East President George Bush Turnpike
Richardson, TX 75082
US

EMail: [email protected]

Schulzrinne & Polk Standards Track [Page 35]

RFC 4412 SIP Resource Priority February 2006

Full Copyright Statement

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Schulzrinne & Polk Standards Track [Page 36]