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RFC 4412 - Communications Resource Priority for the Session Init


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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

      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

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.

   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.

   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.

   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.

   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

     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

   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]

   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.

   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.

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).

   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.

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.

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

   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.

   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.

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: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
   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
   Contact: <sip:UserB@client.biloxi.example.com;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   |
     |<======================>|

   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
   Require: resource-priority
   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 Unknown Resource-Priority
   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
   Require: resource-priority
   Resource-Priority: q735.3
   Contact: <sip:UserA@client.atlanta.example.com;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).

   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

   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).

   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".

   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.

   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

   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.

   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

   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.

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

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.

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.

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.

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.

   [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: 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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