[Docs] [txt|pdf] [draft-ietf-tsvwg-...] [Diff1] [Diff2]

PROPOSED STANDARD

Internet Engineering Task Force (IETF)                    F. Le Faucheur
Request for Comments: 6401                                       J. Polk
Category: Standards Track                                          Cisco
ISSN: 2070-1721                                              K. Carlberg
                                                                     G11
                                                            October 2011


                 RSVP Extensions for Admission Priority

Abstract

   Some applications require the ability to provide an elevated
   probability of session establishment to specific sessions in times of
   network congestion.  When supported over the Internet Protocol suite,
   this may be facilitated through a network-layer admission control
   solution that supports prioritized access to resources (e.g.,
   bandwidth).  These resources may be explicitly set aside for
   prioritized sessions, or may be shared with other sessions.  This
   document specifies extensions to the Resource reSerVation Protocol
   (RSVP) that can be used to support such an admission priority
   capability at the network layer.

   Based on current security concerns, these extensions are intended for
   use in a single administrative domain.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6401.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  4
   3.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  4
   4.  Overview of RSVP Extensions and Operations . . . . . . . . . .  4
     4.1.  Operations of Admission Priority . . . . . . . . . . . . .  6
   5.  New Policy Elements  . . . . . . . . . . . . . . . . . . . . .  7
     5.1.  Admission Priority Policy Element  . . . . . . . . . . . .  8
       5.1.1.  Admission Priority Merging Rules . . . . . . . . . . .  9
     5.2.  Application-Level Resource Priority Policy Element . . . . 10
       5.2.1.  Application-Level Resource Priority Modifying and
               Merging Rules  . . . . . . . . . . . . . . . . . . . . 11
     5.3.  Default Handling . . . . . . . . . . . . . . . . . . . . . 12
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
     6.1.  Use of RSVP Authentication between RSVP Neighbors  . . . . 13
     6.2.  Use of INTEGRITY object within the POLICY_DATA Object  . . 13
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 18
   Appendix A.  Examples of Bandwidth Allocation Model for
                Admission Priority  . . . . . . . . . . . . . . . . . 19
     A.1.  Admission Priority with Maximum Allocation Model (MAM) . . 19
     A.2.  Admission Priority with Russian Dolls Model (RDM)  . . . . 23
     A.3.  Admission Priority with Priority Bypass Model (PrBM) . . . 26
   Appendix B.  Example Usages of RSVP Extensions . . . . . . . . . . 29



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1.  Introduction

   Some applications require the ability to provide an elevated
   probability of session establishment to specific sessions in times of
   network congestion.

   Solutions to meet this requirement for elevated session establishment
   probability may involve session-layer capabilities prioritizing
   access to resources controlled by the session control function.  As
   an example, entities involved in session control (such as SIP user
   agents, when the Session Initiation Protocol (SIP) [RFC3261], is the
   session control protocol in use) can influence their treatment of
   session establishment requests (such as SIP requests).  This may
   include the ability to "queue" session establishment requests when
   those can not be immediately honored (in some cases with the notion
   of "bumping", or "displacement", of less important session
   establishment requests from that queue).  It may include additional
   mechanisms such as alternate routing and exemption from certain
   network management controls.

   Solutions to meet the requirement for elevated session establishment
   probability may also take advantage of network-layer admission
   control mechanisms supporting admission priority.  Networks usually
   have engineered capacity limits that characterize the maximum load
   that can be handled (say, on any given link) for a class of traffic
   while satisfying the quality-of-service (QoS) requirements of that
   traffic class.  Admission priority may involve setting aside some
   network resources (e.g., bandwidth) out of the engineered capacity
   limits for the prioritized sessions only.  Or alternatively, it may
   involve allowing the prioritized sessions to seize additional
   resources beyond the engineered capacity limits applied to normal
   sessions.  This document specifies the necessary extensions to
   support such admission priority when network-layer admission control
   is performed using the Resource reSerVation Protocol (RSVP)
   [RFC2205].

   [RFC3181] specifies the Signaled Preemption Priority Policy Element
   that can be signaled in RSVP so that network node may take into
   account this policy element in order to preempt some previously
   admitted low-priority sessions in order to make room for a newer,
   higher-priority session.  In contrast, this document specifies new
   RSVP extensions to increase the probability of session establishment
   without preemption of existing sessions.  This is achieved by
   engineered capacity techniques in the form of bandwidth allocation
   models.  In particular, this document specifies two new RSVP policy
   elements allowing the admission priority to be conveyed inside RSVP
   signaling messages so that RSVP nodes can enforce a selective
   bandwidth admission control decision based on the session admission



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   priority.  Appendix A of this document also provides examples of
   bandwidth allocation models that can be used by RSVP-routers to
   enforce such admission priority on every link.  A given reservation
   may be signaled with the admission priority extensions specified in
   the present document, with the preemption priority specified in
   [RFC3181], or with both.

1.1.  Terminology

   This document assumes the terminology defined in [RFC2753].  For
   convenience, the definitions of a few key terms are repeated here:

   o  Policy Decision Point (PDP): The point where policy decisions are
      made.

   o  Local Policy Decision Point (LPDP): The PDP local to the network
      element.

   o  Policy Enforcement Point (PEP): The point where the policy
      decisions are actually enforced.

   o  Policy Ignorant Node (PIN): A network element that does not
      explicitly support policy control using the mechanisms defined in
      [RFC2753].

2.  Applicability Statement

   A subset of RSVP messages are signaled with the Router Alert Option
   (RAO) ([RFC2113], [RFC2711]).  The security aspects and common
   practices around the use of the current IP Router Alert Option and
   consequences on the use of IP Router Alert by applications such as
   RSVP are discussed in [RFC6398].  Based on those, the extensions
   defined in this document are intended for use within a single
   administrative domain.  Thus, in particular, the extensions defined
   in this document are not intended for end-to-end use on the Internet.

3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

4.  Overview of RSVP Extensions and Operations

   Let us consider the case where a session requires elevated
   probability of establishment, and more specifically that the
   preference to be granted to this session is in terms of network-layer
   "admission priority" (as opposed to preference granted through



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   preemption of existing sessions).  By "admission priority" we mean
   allowing the priority session to seize network-layer resources from
   the engineered capacity that has been set aside for priority sessions
   (and not made available to normal sessions) or, alternatively,
   allowing the priority session to seize additional resources beyond
   the engineered capacity limits applied to normal sessions.

   Session establishment can be made conditional on resource-based and
   policy-based network-layer admission control achieved via RSVP
   signaling.  In the case where the session control protocol is SIP,
   the use of RSVP-based admission control in conjunction with SIP is
   specified in [RFC3312].

   Devices involved in the session establishment are expected to be
   aware of the application-level priority requirements of prioritized
   sessions.  For example, considering the case where the session
   control protocol is SIP, the SIP user agents may be made aware of the
   resource priority requirements of a given session using the
   "Resource-Priority" header mechanism specified in [RFC4412].  The
   end-devices involved in the upper-layer session establishment simply
   need to copy the application-level resource priority requirements
   (e.g., as communicated in the SIP "Resource-Priority" header) inside
   the new RSVP Application-Level Resource Priority Policy Element
   defined in this document.

   Conveying the application-level resource priority requirements inside
   the RSVP message allows this application-level requirement to be
   mapped/remapped into a different RSVP "admission priority" at a
   policy boundary based on the policy applicable in that policy area.
   In a typical model (see [RFC2753]) where PDPs control PEPs at the
   periphery of the policy area (e.g., on the first hop router), PDPs
   would interpret the RSVP Application-Level Resource Priority Policy
   Element and map the requirement of the prioritized session into an
   RSVP "admission priority" level.  Then, PDPs would convey this
   information inside the new Admission Priority Policy Element defined
   in this document.  This way, the RSVP admission priority can be
   communicated to downstream PEPs (i.e., RSVP routers) of the same
   policy domain that have LPDPs but no controlling PDP.  In turn, this
   means the necessary RSVP Admission priority can be enforced at every
   RSVP hop, including all the (possibly many) hops that do not have any
   understanding of application-level resource priority semantics.  It
   is not expected that the RSVP Application-Level Resource Priority
   Header Policy Element would be taken into account at RSVP hops within
   a given policy area.  It is expected to be used at policy area
   boundaries only in order to set/reset the RSVP Admission Priority
   Policy Element.





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   Remapping by PDPs of the Admission Priority Policy Element from the
   Application-Level Resource Priority Policy Element may also be used
   at boundaries with other signaling protocols, such as the NSIS
   Signaling Layer Protocol (NSLP) for QoS Signaling [RFC5974].

   As can be observed, the framework described above for mapping/
   remapping application-level resource priority requirements into an
   RSVP admission priority can also be used together with [RFC3181] for
   mapping/remapping application-level resource priority requirements
   into an RSVP preemption priority (when preemption is indeed deemed
   necessary by the prioritized session handling policy).  In that case,
   when processing the RSVP Application-Level Resource Priority Policy
   Element, the PDPs at policy boundaries (or between various QoS
   signaling protocols) can map it into an RSVP "preemption priority"
   information.  This preemption priority information comprises a setup
   preemption level and a defending preemption priority level that can
   then be encoded inside the Preemption Priority Policy Element of
   [RFC3181].

   Appendix B provides examples of various hypothetical policies for
   prioritized session handling, some of them involving admission
   priority, some of them involving both admission priority and
   preemption priority.  Appendix B also identifies how the application-
   level resource priority needs to be mapped into RSVP policy elements
   by the PDPs to realize these policies.

4.1.  Operations of Admission Priority

   The RSVP Admission Priority Policy Element defined in this document
   allows admission bandwidth to be allocated preferentially to
   prioritized sessions.  Multiple models of bandwidth allocation MAY be
   used to that end.

   A number of bandwidth allocation models have been defined in the IETF
   for allocation of bandwidth across different classes of traffic
   trunks in the context of Diffserv-aware MPLS Traffic Engineering.
   Those include the Maximum Allocation Model (MAM) defined in
   [RFC4125], the Russian Dolls Model (RDM) specified in [RFC4127], and
   the Maximum Allocation model with Reservation (MAR) defined in
   [RFC4126].  However, these same models MAY be applied for allocation
   of bandwidth across different levels of admission priority as defined
   in this document.  Appendix A provides an illustration of how these
   bandwidth allocation models can be applied for such purposes and also
   introduces an additional bandwidth allocation model that we term the
   Priority Bypass Model (PrBM).  It is important to note that the
   models described and illustrated in Appendix A are only informative
   and do not represent a recommended course of action.




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   We can see in these examples how the RSVP Admission Priority can be
   used by RSVP routers to influence their admission control decision
   (for example, by determining which bandwidth pool is to be used by
   RSVP for performing its bandwidth allocation) and therefore to
   increase the probability of reservation establishment.  In turn, this
   increases the probability of application-level session establishment
   for the corresponding session.

5.  New Policy Elements

   The Framework document for policy-based admission control [RFC2753]
   describes the various components that participate in policy decision
   making (i.e., PDP, PEP, and LPDP).

   As described in Section 4 of the present document, the Application-
   Level Resource Priority Policy Element and the Admission Priority
   Policy Element serve different roles in this framework:

   o  The Application-Level Resource Priority Policy Element conveys
      application-level information and is processed by PDPs.

   o  The emphasis of Admission Priority Policy Element is to be simple,
      stateless, and lightweight such that it can be processed
      internally within a node's LPDP.  It can then be enforced
      internally within a node's PEP.  It is set by PDPs based on
      processing of the Application-Level Resource Priority Policy
      Element.

   [RFC2750] defines extensions for supporting generic policy-based
   admission control in RSVP.  These extensions include the standard
   format of POLICY_DATA objects and a description of RSVP handling of
   policy events.

   The POLICY_DATA object contains one or more policy elements, each
   representing a different (and perhaps orthogonal) policy.  As an
   example, [RFC3181] specifies the Preemption Priority Policy Element.
   This document defines two new policy elements called:

   o  the Admission Priority Policy Element

   o  the Application-Level Resource Priority Policy Element










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5.1.  Admission Priority Policy Element

   The format of the Admission Priority Policy Element is as shown in
   Figure 1:

          0           0 0           1 1           2 2           3
          0   . . .   7 8   . . .   5 6   . . .   3 4   . . .   1
         +-------------+-------------+-------------+-------------+
         |     Length                | P-Type = ADMISSION_PRI    |
         +-------------+-------------+-------------+-------------+
         | Flags       | M. Strategy | Error Code  | Reserved    |
         +-------------+-------------+-------------+-------------+
         |              Reserved                   |Adm. Priority|
         +---------------------------+---------------------------+

                Figure 1: Admission Priority Policy Element

   where:

   o  Length: 16 bits

      *  Always 12.  The overall length of the policy element, in bytes.

   o  P-Type: 16 bits

      *  ADMISSION_PRI = 0x05 (see the "IANA Considerations" section).

   o  Flags: Reserved

      *  SHALL be set to zero on transmit and SHALL be ignored on
         reception.

   o  Merge Strategy: 8 bits (applicable to multicast flows)

      *  values are defined in the corresponding registry maintained by
         IANA (see the "IANA Considerations" section).

   o  Error code: 8 bits (applicable to multicast flows)

      *  values are defined in the corresponding registry maintained by
         IANA (see the "IANA Considerations" section).

   o  Reserved: 8 bits

      *  SHALL be set to zero on transmit and SHALL be ignored on
         reception.





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   o  Reserved: 24 bits

      *  SHALL be set to zero on transmit and SHALL be ignored on
         reception

   o  Adm. Priority (Admission Priority): 8 bits (unsigned)

      *  The admission control priority of the flow, in terms of access
         to network bandwidth in order to provide higher probability of
         session completion to selected flows.  Higher values represent
         higher priority.  Bandwidth allocation models such as those
         described in Appendix A are to be used by the RSVP router to
         achieve increased probability of session establishment.  The
         admission priority value effectively indicates which bandwidth
         constraint(s) of the bandwidth constraint model in use is/are
         applicable to admission of this RSVP reservation.

   Note that the Admission Priority Policy Element does NOT indicate
   that this RSVP reservation is to preempt any other RSVP reservation.
   If a priority session justifies both admission priority and
   preemption priority, the corresponding RSVP reservation needs to
   carry both an Admission Priority Policy Element and a Preemption
   Priority Policy Element.  The Admission Priority and Preemption
   Priority are handled by LPDPs and PEPs as separate mechanisms.  They
   can be used one without the other, or they can be used both in
   combination.

5.1.1.  Admission Priority Merging Rules

   This section discusses alternatives for dealing with RSVP admission
   priority in case of merging of reservations.  As merging applies to
   multicast, this section also applies to multicast sessions.

   The rules for merging Admission Priority Policy Elements are defined
   by the value encoded inside the Merge Strategy field in accordance
   with the corresponding IANA registry.  This registry applies both to
   the Merge Strategy field of the Admission Priority Policy Element
   defined in the present document and to the Merge Strategy field of
   the Preemption Priority Policy Element defined in [RFC3181].  The
   registry initially contains the values already defined in [RFC3181]
   (see the "IANA Considerations" section).

   The only difference from [RFC3181] is that this document does not
   recommend a given merge strategy over the others for Admission
   Priority, while [RFC3181] recommends the first of these merge
   strategies for Preemption Priority.  Note that with the Admission
   Priority (as is the case with the Preemption Priority), "take highest
   priority" translates into "take the highest numerical value".



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5.2.  Application-Level Resource Priority Policy Element

   The format of the Application-Level Resource Priority Policy Element
   is as shown in Figure 2:

          0           0 0           1 1           2 2           3
          0   . . .   7 8   . . .   5 6   . . .   3 4   . . .   1
         +-------------+-------------+-------------+-------------+
         | Length                    | P-Type = APP_RESOURCE_PRI |
         +-------------+-------------+-------------+-------------+
         //     ALRP List                                        //
         +---------------------------+---------------------------+

       Figure 2: Application-Level Resource Priority Policy Element

   where:

   o  Length:

      *  The length of the policy element (including the Length and
         P-Type) is in number of octets (MUST be a multiple of 4) and
         indicates the end of the ALRP list.

   o  P-Type: 16 bits

      *  APP_RESOURCE_PRI = 0x06 (see the "IANA Considerations"
         section).

   o  ALRP List:

      *  List of ALRPs where each ALRP is encoded as shown in Figure 3.

   ALRP:
          0           0 0           1 1           2 2           3
          0   . . .   7 8   . . .   5 6   . . .   3 4   . . .   1
         +---------------------------+-------------+-------------+
         |     ALRP Namespace        | Reserved    |ALRP Value   |
         +---------------------------+---------------------------+

               Figure 3: Application-Level Resource Priority











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   where:

   o  ALRP Namespace (Application-Level Resource Priority Namespace): 16
      bits (unsigned)

      *  Contains a numerical value identifying the namespace of the
         application-level resource priority.  This value is encoded as
         per the "Resource Priority Namespaces" IANA registry.  (See the
         "IANA Considerations" section; IANA has extended the registry
         to include this numerical value).

   o  Reserved: 8 bits

      *  SHALL be set to zero on transmit and SHALL be ignored on
         reception.

   o  ALRP Value (Application-Level Resource Priority Value): 8 bits
      (unsigned)

      *  Contains the priority value within the namespace of the
         application-level resource priority.  This value is encoded as
         per the "Resource Priority Priority-Value" IANA registry.  (See
         the "IANA Considerations" section; IANA has extended the
         registry to include this numerical value).

5.2.1.  Application-Level Resource Priority Modifying and Merging Rules

   When POLICY_DATA objects are protected by integrity, LPDPs should not
   attempt to modify them.  They MUST be forwarded without modification
   to ensure their security envelope is not invalidated.

   In case of multicast, when POLICY_DATA objects are not protected by
   integrity, LPDPs MAY merge incoming Application-Level Resource
   Priority Elements to reduce their size and number.  When they do
   merge those elements, LPDPs MUST do so according to the following
   rule:

   o  The ALRP List in the outgoing APP_RESOURCE_PRI element MUST
      contain all the ALRPs appearing in the ALRP List of an incoming
      APP_RESOURCE_PRI element.  A given ALRP MUST NOT appear more than
      once.  In other words, the outgoing ALRP List is the union of the
      incoming ALRP Lists that are merged.

   As merging applies to multicast, this rule also applies to multicast
   sessions.






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5.3.  Default Handling

   As specified in Section 4.2 of [RFC2750], Policy Ignorant Nodes
   (PINs) implement a default handling of POLICY_DATA objects ensuring
   that those objects can traverse PINs in transit from one PEP to
   another.  This applies to the situations where POLICY_DATA objects
   contain the Admission Priority Policy Element and the ALRP Policy
   Element specified in this document, so that those objects can
   traverse PINs.

   Section 4.2 of [RFC2750] also defines a similar default behavior for
   policy-capable nodes that do not recognize a particular policy
   element.  This applies to the Admission Priority Policy Element and
   the ALRP Policy Element specified in this document, so that those
   elements can traverse policy-capable nodes that do not support these
   extensions defined in the present document.

6.  Security Considerations

   As this document defines extensions to RSVP, the security
   considerations of RSVP apply.  Those are discussed in [RFC2205],
   [RFC4230], and [RFC6411].  Approaches for addressing those concerns
   are discussed further below.

   A subset of RSVP messages are signaled with the Router Alert Option
   (RAO) ([RFC2113], [RFC2711]).  The security aspects and common
   practices around the use of the current IP Router Alert Option and
   consequences on the use of IP Router Alert by applications such as
   RSVP are discussed in [RFC6398].  As discussed in Section 2, the
   extensions defined in this document are intended for use within a
   single administrative domain.

   [RFC6398] discusses router alert protection approaches for service
   providers.  These approaches can be used to protect a given network
   against the potential risks associated with the leaking of router
   alert packets resulting from the use of the present extensions in
   another domain.  Also, where RSVP is not used, by simply not enabling
   RSVP on the routers of a given network, generally that network can
   isolate itself from any RSVP signaling that may leak from another
   network that uses the present extensions (since the routers will then
   typically ignore RSVP messages).  Where RSVP is to be used internally
   within a given network, the network operator can activate, on the
   edge of his network, mechanisms that either tunnel or, as a last
   resort, drop incoming RSVP messages in order to protect the given
   network from RSVP signaling that may leak from another network that
   uses the present extensions.





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   The ADMISSION_PRI and APP_RESOURCE_PRI Policy Elements defined in
   this document are signaled by RSVP through encapsulation in a
   POLICY_DATA object as defined in [RFC2750].  Therefore, like any
   other policy elements, their integrity can be protected as discussed
   in Section 6 of [RFC2750] by two optional security mechanisms.  The
   first mechanism relies on RSVP authentication as specified in
   [RFC2747] and [RFC3097] to provide a chain of trust when all RSVP
   nodes are policy capable.  With this mechanism, the INTEGRITY object
   is carried inside RSVP messages.  The second mechanism relies on the
   INTEGRITY object within the POLICY_DATA object to guarantee integrity
   between RSVP PEPs that are not RSVP neighbors.

6.1.  Use of RSVP Authentication between RSVP Neighbors

   RSVP authentication can be used between RSVP neighbors that are
   policy capable.  RSVP authentication (defined in [RFC2747] and
   [RFC3097]) SHOULD be supported by an implementation of the present
   document.

   With RSVP authentication, the RSVP neighbors use shared keys to
   compute the cryptographic signature of the RSVP message.  [RFC6411]
   discusses key types and key provisioning methods as well as their
   respective applicabilities.

6.2.  Use of INTEGRITY object within the POLICY_DATA Object

   The INTEGRITY object within the POLICY_DATA object can be used to
   guarantee integrity between non-neighboring RSVP PEPs.  This is
   useful only when some RSVP nodes are Policy Ignorant Nodes (PINs).
   The INTEGRITY object within the POLICY_DATA object MAY be supported
   by an implementation of the present document.

   Details for computation of the content of the INTEGRITY object can be
   found in Appendix B of [RFC2750].  This states that the Policy
   Decision Point (PDP), at its discretion, and based on the destination
   PEP/PDP or other criteria, selects an Authentication Key and the hash
   algorithm to be used.  Keys to be used between PDPs can be
   distributed manually or via a standard key management protocol for
   secure key distribution.

   Note that where non-RSVP hops may exist in between RSVP hops, as well
   as where RSVP-capable PINs may exist in between PEPs, it may be
   difficult for the PDP to determine what is the destination PDP for a
   POLICY_DATA object contained in some RSVP messages (such as a Path
   message).  This is because in those cases the next PEP is not known
   at the time of forwarding the message.  In this situation, key shared
   across multiple PDPs may be used.  This is conceptually similar to
   the use of a key shared across multiple RSVP neighbors as discussed



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   in [RFC6411].  We observe also that this issue may not exist in some
   deployment scenarios where a single (or low number of) PDP is used to
   control all the PEPs of a region (such as an administrative domain).
   In such scenarios, it may be easy for a PDP to determine what is the
   next-hop PDP, even when the next-hop PEP is not known, simply by
   determining what is the next region that will be traversed (say,
   based on the destination address).

7.  IANA Considerations

   As specified in [RFC2750], standard RSVP policy elements (P-type
   values) are to be assigned by IANA as per "IETF Consensus" policy as
   outlined in [RFC2434] (this policy is now called "IETF Review" as per
   [RFC5226]) .

   IANA has allocated two P-Types from the standard RSVP policy element
   range:

   o  0x05 ADMISSION_PRI for the Admission Priority Policy Element

   o  0x06 APP_RESOURCE_PRI for the Application-Level Resource Priority
      Policy Element

   In Section 5.1, the present document defines a Merge Strategy field
   inside the Admission Priority Policy Element.  This registry is to be
   specified as also applicable to the Merge Strategy field of the
   Preemption Priority Policy Elements defined in [RFC3181].  Since it
   is conceivable that, in the future, values will be added to the
   registry that only apply to the Admission Priority Policy Element or
   to the Preemption Priority Policy Element (but not to both), IANA has
   listed the applicable documents for each value.  IANA has allocated
   the following values:

   o  0: Reserved

   o  1: Take priority of highest QoS [RFC3181] [RFC6401]

   o  2: Take highest priority [RFC3181] [RFC6401]

   o  3: Force Error on heterogeneous merge [RFC3181] [RFC6401]

   Following the policies outlined in [RFC5226], numbers in the range
   0-127 are allocated according to the "IETF Review" policy, numbers in
   the range 128-240 as "First Come First Served", and numbers in the
   range 241-255 are "Reserved for Private Use".






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   In Section 5.1, the present document defines an Error Code field
   inside the Admission Priority Policy Element.  IANA has created a
   registry for this field and allocate the following values:

   o  0: NO_ERROR - Value used for regular ADMISSION_PRI elements

   o  2: HETEROGENEOUS - This element encountered heterogeneous merge

   Following the policies outlined in [RFC5226], numbers in the range
   0-127 are allocated according to the "IETF Review" policy, numbers in
   the range 128-240 as "First Come First Served", and numbers in the
   range 241-255 are "Reserved for Private Use".  Value 1 is Reserved
   (for consistency with [RFC3181] Error Code values).

   The present document defines an ALRP Namespace field in Section 5.2
   that contains a numerical value identifying the namespace of the
   application-level resource priority.  The IANA already maintains the
   Resource-Priority Namespaces registry (under the SIP Parameters)
   listing all such namespaces.  That registry has been updated to
   allocate a numerical value to each namespace.  To be exact, the IANA
   has extended the Resource-Priority Namespaces registry in the
   following ways:

   o  A new column has been added to the registry.

   o  The title of the new column is "Namespace Numerical Value *".

   o  In the Legend, a line has been added stating "Namespace Numerical
      Value = the unique numerical value identifying the namespace".

   o  In the Legend, a line has been added stating "* : [RFC6401]".

   o  An actual numerical value has been allocated to each namespace in
      the registry and is listed in the new "Namespace Numerical Value
      *" column.

   A numerical value has been allocated by IANA for all existing
   namespaces.  In the future, IANA should automatically allocate a
   numerical value to any new namespace added to the registry.

   The present document defines an ALRP Priority field in Section 5.2
   that contains a numerical value identifying the actual application-
   level resource priority within the application-level resource
   priority namespace.  The IANA already maintains the Resource-Priority
   Priority-Values registry (under the SIP Parameters) listing all such
   priorities.  That registry has been updated to allocate a numerical
   value to each priority-value.  To be exact, the IANA has extended the
   Resource-Priority Priority-Values registry in the following ways:



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   o  For each namespace, the registry is structured with two columns.

   o  The title of the first column is "Priority Values (least to
      greatest)".

   o  The first column lists all the values currently defined in the
      registry (e.g., for the drsn namespace: "routine", "priority",
      "immediate", "flash", "flash-override", and "flash-override-
      override")

   o  The title of the second column is "Priority Numerical Value *".

   o  At the bottom of the registry, a "Legend" has been added with a
      line stating "Priority Numerical Value = the unique numerical
      value identifying the priority within a namespace".

   o  In the Legend, a line has been added stating "* : [RFC6401]".

   o  An actual numerical value has been allocated to each priority
      value and is listed in the new "Priority Numerical Value *"
      column.

   A numerical value has been allocated by IANA to all existing
   priorities.  In the future, IANA should automatically allocate a
   numerical value to any new namespace added to the registry.  The
   numerical value must be unique within each namespace.  Within each
   namespace, values should be allocated in decreasing order ending with
   0 (so that the greatest priority is always allocated value 0).  For
   example, in the drsn namespace, "routine" is allocated numerical
   value 5, and "flash-override-override" is allocated numerical value
   0.

8.  Acknowledgments

   We would like to thank An Nguyen for his encouragement to address
   this topic and ongoing comments.  Also, this document borrows heavily
   from some of the work of S. Herzog on the Preemption Priority Policy
   Element [RFC3181].  Dave Oran and Janet Gunn provided useful input
   for this document.  Ron Bonica, Magnus Westerlund, Cullen Jennings,
   Ross Callon and Tim Polk provided specific guidance for the
   applicability statement of the mechanisms defined in this document.










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9.  References

9.1.  Normative References

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

   [RFC2205]  Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
              Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
              Functional Specification", RFC 2205, September 1997.

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

   [RFC2747]  Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
              Authentication", RFC 2747, January 2000.

   [RFC2750]  Herzog, S., "RSVP Extensions for Policy Control", RFC
              2750, January 2000.

   [RFC3097]  Braden, R. and L. Zhang, "RSVP Cryptographic
              Authentication -- Updated Message Type Value", RFC 3097,
              April 2001.

   [RFC3181]  Herzog, S., "Signaled Preemption Priority Policy Element",
              RFC 3181, October 2001.

   [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.

   [RFC3312]  Camarillo, G., Marshall, W., and J. Rosenberg,
              "Integration of Resource Management and Session Initiation
              Protocol (SIP)", RFC 3312, October 2002.

   [RFC4412]  Schulzrinne, H. and J. Polk, "Communications Resource
              Priority for the Session Initiation Protocol (SIP)", RFC
              4412, February 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC6398]  Le Faucheur, F., Ed., "IP Router Alert Considerations and
              Usage", BCP 168, RFC 6398, October 2011.




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9.2.  Informative References

   [RFC2113]  Katz, D., "IP Router Alert Option", RFC 2113, February
              1997.

   [RFC2711]  Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
              RFC 2711, October 1999.

   [RFC2753]  Yavatkar, R., Pendarakis, D., and R. Guerin, "A Framework
              for Policy-based Admission Control", RFC 2753, January
              2000.

   [RFC4125]  Le Faucheur, F. and W. Lai, "Maximum Allocation Bandwidth
              Constraints Model for Diffserv-aware MPLS Traffic
              Engineering", RFC 4125, June 2005.

   [RFC4126]  Ash, J., "Max Allocation with Reservation Bandwidth
              Constraints Model for Diffserv-aware MPLS Traffic
              Engineering & Performance Comparisons", RFC 4126, June
              2005.

   [RFC4127]  Le Faucheur, F., "Russian Dolls Bandwidth Constraints
              Model for Diffserv-aware MPLS Traffic Engineering", RFC
              4127, June 2005.

   [RFC4230]  Tschofenig, H. and R. Graveman, "RSVP Security
              Properties", RFC 4230, December 2005.

   [RFC4495]  Polk, J. and S. Dhesikan, "A Resource Reservation Protocol
              (RSVP) Extension for the Reduction of Bandwidth of a
              Reservation Flow", RFC 4495, May 2006.

   [RFC5974]  Manner, J., Karagiannis, G., and A. McDonald, "NSIS
              Signaling Layer Protocol (NSLP) for Quality-of-Service
              Signaling", RFC 5974, October 2010.

   [RFC6411]  Behringer, M., Le Faucheur, F., and B. Weis,
              "Applicability of Keying Methods for RSVP Security", RFC
              6411, October 2011.












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Appendix A.  Examples of Bandwidth Allocation Model for Admission
             Priority

   Appendices A.1 and A.2 respectively illustrate how the Maximum
   Allocation Model (MAM) [RFC4125] and the Russian Dolls Model (RDM)
   [RFC4127] can be used for support of admission priority.  The Maximum
   Allocation model with Reservation (MAR) [RFC4126] can also be used in
   a similar manner for support of admission priority.  Appendix A.3
   illustrates how a simple "Priority Bypass Model" can also be used for
   support of admission priority.

   For simplicity, operations with only a single "priority" level
   (beyond non-priority) are illustrated here; however, the reader will
   appreciate that operations with multiple priority levels can easily
   be supported with these models.

   In all the figures below:

      "x" represents a non-priority session
      "o" represents a priority session

A.1.  Admission Priority with Maximum Allocation Model (MAM)

   This section illustrates operations of admission priority when a
   Maximum Allocation Model (MAM) is used for bandwidth allocation
   across non-priority traffic and priority traffic.  A property of the
   Maximum Allocation Model is that priority traffic cannot use more
   than the bandwidth made available to priority traffic (even if the
   non-priority traffic is not using all of the bandwidth available for
   it).

                -----------------------
           ^  ^  ^  |              |  ^
           .  .  .  |              |  .
    Total  .  .  .  |              |  .   Bandwidth
          (1)(2)(3) |              |  .   available
    Engi-  .  .  .  |              |  .   for non-priority use
   neered  .or.or.  |              |  .
           .  .  .  |              |  .
   Capacity.  .  .  |              |  .
           v  .  .  |              |  v
              .  .  |--------------| ---
              v  .  |              |  ^
                 .  |              |  .   Bandwidth available for
                 v  |              |  v   priority use
                -------------------------

                    Figure 4: MAM Bandwidth Allocation



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   Figure 4 shows a link that is within a routed network and conforms to
   this document.  On this link are two amounts of bandwidth available
   to two types of traffic: non-priority and priority.

   If the non-priority traffic load reaches the maximum bandwidth
   available for non-priority, no additional non-priority sessions can
   be accepted even if the bandwidth reserved for priority traffic is
   not fully utilized currently.

   With the Maximum Allocation Model, in the case where the priority
   load reaches the maximum bandwidth reserved for priority sessions, no
   additional priority sessions can be accepted.

   As illustrated in Figure 4, an operator may map the MAM to the
   engineered capacity limits according to different policies.  At one
   extreme, where the proportion of priority traffic is reliably known
   to be fairly small at all times and where there may be some safety
   margin factored in the engineered capacity limits, the operator may
   decide to configure the bandwidth available for non-priority use to
   the full engineered capacity limits, effectively allowing the
   priority traffic to ride within the safety margin of this engineered
   capacity.  This policy can be seen as an economically attractive
   approach as all of the engineered capacity is made available to non-
   priority sessions.  This policy is illustrated as (1) in Figure 4.
   As an example, if the engineered capacity limit on a given link is X,
   the operator may configure the bandwidth available to non-priority
   traffic to X, and the bandwidth available to priority traffic to 5%
   of X.  At the other extreme, where the proportion of priority traffic
   may be significant at times and the engineered capacity limits are
   very tight, the operator may decide to configure the bandwidth
   available to non-priority traffic and the bandwidth available to
   priority traffic such that their sum is equal to the engineered
   capacity limits.  This guarantees that the total load across non-
   priority and priority traffic is always below the engineered capacity
   and, in turn, guarantees there will never be any QoS degradation.
   However, this policy is less attractive economically as it prevents
   non-priority sessions from using the full engineered capacity, even
   when there is no or little priority load, which is the majority of
   time.  This policy is illustrated as (3) in Figure 4.  As an example,
   if the engineered capacity limit on a given link is X, the operator
   may configure the bandwidth available to non-priority traffic to 95%
   of X, and the bandwidth available to priority traffic to 5% of X.  Of
   course, an operator may also strike a balance anywhere in between
   these two approaches.  This policy is illustrated as (2) in Figure 4.







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   Figure 5 shows some of the non-priority capacity of this link being
   used.

                -----------------------
           ^  ^  ^  |              |  ^
           .  .  .  |              |  .
    Total  .  .  .  |              |  .   Bandwidth
           .  .  .  |              |  .   available
    Engi-  .  .  .  |              |  .   for non-priority use
   neered  .or.or.  |xxxxxxxxxxxxxx|  .
           .  .  .  |xxxxxxxxxxxxxx|  .
   Capacity.  .  .  |xxxxxxxxxxxxxx|  .
           v  .  .  |xxxxxxxxxxxxxx|  v
              .  .  |--------------| ---
              v  .  |              |  ^
                 .  |              |  .   Bandwidth available for
                 v  |              |  v   priority use
                -------------------------

               Figure 5: Partial Load of Non-Priority Calls

   Figure 6 shows the same amount of non-priority load being used at
   this link and a small amount of priority bandwidth being used.

                -----------------------
           ^  ^  ^  |              |  ^
           .  .  .  |              |  .
    Total  .  .  .  |              |  .   Bandwidth
           .  .  .  |              |  .   available
    Engi-  .  .  .  |              |  .   for non-priority use
   neered  .or.or.  |xxxxxxxxxxxxxx|  .
           .  .  .  |xxxxxxxxxxxxxx|  .
   Capacity.  .  .  |xxxxxxxxxxxxxx|  .
           v  .  .  |xxxxxxxxxxxxxx|  v
              .  .  |--------------| ---
              v  .  |              |  ^
                 .  |              |  .   Bandwidth available for
                 v  |oooooooooooooo|  v   priority use
                -------------------------

     Figure 6: Partial Load of Non-Priority Calls and Partial Load of
                              Priority Calls









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   Figure 7 shows the case where non-priority load equates or exceeds
   the maximum bandwidth available to non-priority traffic.  Note that
   additional non-priority sessions would be rejected even if the
   bandwidth reserved for priority sessions is not fully utilized.

                -----------------------
           ^  ^  ^  |xxxxxxxxxxxxxx|  ^
           .  .  .  |xxxxxxxxxxxxxx|  .
    Total  .  .  .  |xxxxxxxxxxxxxx|  .   Bandwidth
           .  .  .  |xxxxxxxxxxxxxx|  .   available
    Engi-  .  .  .  |xxxxxxxxxxxxxx|  .   for non-priority use
   neered  .or.or.  |xxxxxxxxxxxxxx|  .
           .  .  .  |xxxxxxxxxxxxxx|  .
   Capacity.  .  .  |xxxxxxxxxxxxxx|  .
           v  .  .  |xxxxxxxxxxxxxx|  v
              .  .  |--------------| ---
              v  .  |              |  ^
                 .  |              |  .   Bandwidth available for
                 v  |oooooooooooooo|  v   priority use
                -------------------------

    Figure 7: Full Non-Priority Load and Partial Load of Priority Calls

   Figure 8 shows the case where the priority traffic equates or exceeds
   the bandwidth reserved for such priority traffic.

   In that case, additional priority sessions could not be accepted.
   Note that this does not mean that such sessions are dropped
   altogether: they may be handled by mechanisms, which are beyond the
   scope of this particular document (such as establishment through
   preemption of existing non-priority sessions or such as queueing of
   new priority session requests until capacity becomes available again
   for priority traffic).


















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                -----------------------
           ^  ^  ^  |xxxxxxxxxxxxxx|  ^
           .  .  .  |xxxxxxxxxxxxxx|  .
    Total  .  .  .  |xxxxxxxxxxxxxx|  .   Bandwidth
           .  .  .  |xxxxxxxxxxxxxx|  .   available
    Engi-  .  .  .  |xxxxxxxxxxxxxx|  .   for non-priority use
   neered  .or.or.  |xxxxxxxxxxxxxx|  .
           .  .  .  |xxxxxxxxxxxxxx|  .
   Capacity.  .  .  |              |  .
           v  .  .  |              |  v
              .  .  |--------------| ---
              v  .  |oooooooooooooo|  ^
                 .  |oooooooooooooo|  .   Bandwidth available for
                 v  |oooooooooooooo|  v   priority use
                -------------------------

        Figure 8: Partial Non-Priority Load and Full Priority Load

A.2.  Admission Priority with Russian Dolls Model (RDM)

   This section illustrates operations of admission priority when a
   Russian Dolls Model (RDM) is used for bandwidth allocation across
   non-priority traffic and priority traffic.  A property of the RDM is
   that priority traffic can use the bandwidth that is not currently
   used by non-priority traffic.

   As with the MAM, an operator may map the RDM onto the engineered
   capacity limits according to different policies.  The operator may
   decide to configure the bandwidth available for non-priority use to
   the full engineered capacity limits.  As an example, if the
   engineered capacity limit on a given link is X, the operator may
   configure the bandwidth available to non-priority traffic to X, and
   the bandwidth available to non-priority and priority traffic to 105%
   of X.

   Alternatively, the operator may decide to configure the bandwidth
   available to non-priority and priority traffic to the engineered
   capacity limits.  As an example, if the engineered capacity limit on
   a given link is X, the operator may configure the bandwidth available
   to non-priority traffic to 95% of X, and the bandwidth available to
   non-priority and priority traffic to X.

   Finally, the operator may decide to strike a balance in between.  The
   considerations presented for these policies in the previous section
   in the MAM context are equally applicable to RDM.






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   Figure 9 shows the case where only some of the bandwidth available to
   non-priority traffic is being used, and a small amount of priority
   traffic is in place.  In that situation, both new non-priority
   sessions and new priority sessions would be accepted.

               --------------------------------------
               |xxxxxxxxxxxxxx|  .                 ^
               |xxxxxxxxxxxxxx|  . Bandwidth       .
               |xxxxxxxxxxxxxx|  . available for   .
               |xxxxxxxxxxxxxx|  . non-priority    .
               |xxxxxxxxxxxxxx|  . use             .
               |xxxxxxxxxxxxxx|  .                 . Bandwidth
               |              |  .                 . available for
               |              |  v                 . non-priority
               |--------------| ---                . and priority
               |              |                    . use
               |              |                    .
               |oooooooooooooo|                    v
               ---------------------------------------

      Figure 9: Partial Non-Priority Load and Partial Aggregate Load

   Figure 10 shows the case where all of the bandwidth available to non-
   priority traffic is being used and a small amount of priority traffic
   is in place.  In that situation, new priority sessions would be
   accepted, but new non-priority sessions would be rejected.

               --------------------------------------
               |xxxxxxxxxxxxxx|  .                 ^
               |xxxxxxxxxxxxxx|  . Bandwidth       .
               |xxxxxxxxxxxxxx|  . available for   .
               |xxxxxxxxxxxxxx|  . non-priority    .
               |xxxxxxxxxxxxxx|  . use             .
               |xxxxxxxxxxxxxx|  .                 . Bandwidth
               |xxxxxxxxxxxxxx|  .                 . available for
               |xxxxxxxxxxxxxx|  v                 . non-priority
               |--------------| ---                . and priority
               |              |                    . use
               |              |                    .
               |oooooooooooooo|                    v
               ---------------------------------------

       Figure 10: Full Non-Priority Load and Partial Aggregate Load








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   Figure 11 shows the case where only some of the bandwidth available
   to non-priority traffic is being used, and a heavy load of priority
   traffic is in place.  In that situation, both new non-priority
   sessions and new priority sessions would be accepted.  Note that, as
   illustrated in Figure 10, priority sessions use some of the bandwidth
   currently not used by non-priority traffic.

               --------------------------------------
               |xxxxxxxxxxxxxx|  .                 ^
               |xxxxxxxxxxxxxx|  . Bandwidth       .
               |xxxxxxxxxxxxxx|  . available for   .
               |xxxxxxxxxxxxxx|  . non-priority    .
               |xxxxxxxxxxxxxx|  . use             .
               |              |  .                 . Bandwidth
               |              |  .                 . available for
               |oooooooooooooo|  v                 . non-priority
               |--------------| ---                . and priority
               |oooooooooooooo|                    . use
               |oooooooooooooo|                    .
               |oooooooooooooo|                    v
               ---------------------------------------

       Figure 11: Partial Non-Priority Load and Heavy Aggregate Load

   Figure 12 shows the case where all of the bandwidth available to non-
   priority traffic is being used, and all of the remaining available
   bandwidth is used by priority traffic.  In that situation, new non-
   priority sessions would be rejected, and new priority sessions could
   not be accepted right away.  Those priority sessions may be handled
   by mechanisms, which are beyond the scope of this particular document
   (such as established through preemption of existing non-priority
   sessions or such as queueing of new priority session requests until
   capacity becomes available again for priority traffic).


















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               --------------------------------------
               |xxxxxxxxxxxxxx|  .                 ^
               |xxxxxxxxxxxxxx|  . Bandwidth       .
               |xxxxxxxxxxxxxx|  . available for   .
               |xxxxxxxxxxxxxx|  . non-priority    .
               |xxxxxxxxxxxxxx|  . use             .
               |xxxxxxxxxxxxxx|  .                 . Bandwidth
               |xxxxxxxxxxxxxx|  .                 . available for
               |xxxxxxxxxxxxxx|  v                 . non-priority
               |--------------| ---                . and priority
               |oooooooooooooo|                    . use
               |oooooooooooooo|                    .
               |oooooooooooooo|                    v
               ---------------------------------------

         Figure 12: Full Non-Priority Load and Full Aggregate Load

A.3.  Admission Priority with Priority Bypass Model (PrBM)

   This section illustrates operations of admission priority when a
   simple Priority Bypass Model (PrBM) is used for bandwidth allocation
   across non-priority traffic and priority traffic.  With the PrBM,
   non-priority traffic is subject to resource-based admission control,
   while priority traffic simply bypasses the resource-based admission
   control.  In other words:

   o  when a non-priority session arrives, this session is subject to
      bandwidth admission control and is accepted if the current total
      load (aggregate over non-priority and priority traffic) is below
      the engineered/allocated bandwidth.

   o  when a priority session arrives, this session is admitted
      regardless of the current load.

   A property of this model is that a priority session is never
   rejected.

   The rationale for this simple scheme is that, in practice, in some
   networks:

   o  The volume of priority sessions is very low for the vast majority
      of time, so it may not be economical to completely set aside
      bandwidth for priority sessions and preclude the utilization of
      this bandwidth by normal sessions in normal situations.

   o  Even in congestion periods where priority sessions may be more
      heavily used, those sessions always still represent a fairly small
      proportion of the overall load that can be absorbed within the



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      safety margin of the engineered capacity limits.  Thus, even if
      they are admitted beyond the engineered bandwidth threshold, they
      are unlikely to result in noticeable QoS degradation.

   As with the MAM and RDM, an operator may map the PrBM onto the
   engineered capacity limits according to different policies.  The
   operator may decide to configure the bandwidth limit for admission of
   non-priority traffic to the full engineered capacity limit.  As an
   example, if the engineered capacity limit on a given link is X, the
   operator may configure the bandwidth limit for non-priority traffic
   to X.  Alternatively, the operator may decide to configure the
   bandwidth limit for non-priority traffic to below the engineered
   capacity limits (so that the sum of the non-priority and priority
   traffic stays below the engineered capacity).  As an example, if the
   engineered capacity limit on a given link is X, the operator may
   configure the bandwidth limit for non-priority traffic to 95% of X.
    Finally, the operator may decide to strike a balance in between.
   The considerations presented for these policies in the previous
   sections in the MAM and RDM contexts are equally applicable to the
   PrBM.

   Figure 13 illustrates the bandwidth allocation with the PrBM.

                -----------------------
           ^     ^  |              |  ^
           .     .  |              |  .
    Total  .     .  |              |  .   Bandwidth limit
          (1)   (2) |              |  .   (on non-priority + priority)
    Engi-  .     .  |              |  .   for admission
   neered  . or  .  |              |  .   of non-priority traffic
           .     .  |              |  .
   Capacity.     .  |              |  .
           v     .  |              |  v
                 .  |--------------| ---
                 .  |              |
                 v  |              |
                    |              |

           Figure 13: Priority Bypass Model Bandwidth Allocation












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   Figure 14 shows some of the non-priority capacity of this link being
   used.  In this situation, both new non-priority and new priority
   sessions would be accepted.

                -----------------------
           ^     ^  |xxxxxxxxxxxxxx|  ^
           .     .  |xxxxxxxxxxxxxx|  .
    Total  .     .  |xxxxxxxxxxxxxx|  .   Bandwidth limit
          (1)   (2) |xxxxxxxxxxxxxx|  .   (on non-priority + priority)
    Engi-  .     .  |              |  .   for admission
   neered  . or  .  |              |  .   of non-priority traffic
           .     .  |              |  .
   Capacity.     .  |              |  .
           v     .  |              |  v
                 .  |--------------| ---
                 .  |              |
                 v  |              |
                    |              |

               Figure 14: Partial Load of Non-Priority Calls

   Figure 15 shows the same amount of non-priority load being used at
   this link and a small amount of priority bandwidth being used.  In
   this situation, both new non-priority and new priority sessions would
   be accepted.

                 -----------------------
           ^     ^  |xxxxxxxxxxxxxx|  ^
           .     .  |xxxxxxxxxxxxxx|  .
    Total  .     .  |xxxxxxxxxxxxxx|  .   Bandwidth limit
          (1)   (2) |xxxxxxxxxxxxxx|  .   (on non-priority + priority)
    Engi-  .     .  |oooooooooooooo|  .   for admission
   neered  . or  .  |              |  .   of non-priority traffic
           .     .  |              |  .
   Capacity.     .  |              |  .
           v     .  |              |  v
                 .  |--------------| ---
                 .  |              |
                 v  |              |
                    |              |

     Figure 15: Partial Load of Non-Priority Calls and Partial Load of
                              Priority Calls








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   Figure 16 shows the case where aggregate non-priority and priority
   load exceeds the bandwidth limit for admission of non-priority
   traffic.  In this situation, any new non-priority session is
   rejected, while any new priority session is admitted.

                -----------------------
           ^     ^  |xxxxxxxxxxxxxx|  ^
           .     .  |xxxxxxxxxxxxxx|  .
    Total  .     .  |xxxxxxxxxxxxxx|  .   Bandwidth limit
          (1)   (2) |xxxxxxxxxxxxxx|  .   (on non-priority + priority)
    Engi-  .     .  |oooooooooooooo|  .   for admission
   neered  . or  .  |xxxooxxxooxxxo|  .   of non-priority traffic
           .     .  |xxoxxxxxxoxxxx|  .
   Capacity.     .  |oxxxooooxxxxoo|  .
           v     .  |xxoxxxooxxxxxx|  v
                 .  |--------------| ---
                 .  |oooooooooooooo|
                 v  |              |
                    |              |

                     Figure 16: Full Non-Priority Load

Appendix B.  Example Usages of RSVP Extensions

   This section provides examples of how RSVP extensions defined in this
   document can be used (in conjunction with other RSVP functionality
   and SIP functionality) to enforce different hypothetical policies for
   handling prioritized sessions in a given administrative domain.  This
   appendix does not provide additional specification.  It is only
   included in this document for illustration purposes.

   We assume an environment where SIP is used for session control and
   RSVP is used for resource reservation.

   We refer here to "Session Queueing" as the set of "session-layer"
   capabilities that may be implemented by SIP user agents to influence
   their treatment of SIP requests.  This may include the ability to
   "queue" session requests when those cannot be immediately honored (in
   some cases with the notion of "bumping", or "displacement", of less
   important session requests from that queue).  It may include
   additional mechanisms such as alternate routing and exemption from
   certain network management controls.

   We only mention below the RSVP policy elements that are to be
   enforced by PEPs.  It is assumed that these policy elements are set
   at a policy area boundary by PDPs.  The Admission Priority and





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   Preemption Priority RSVP policy elements are set by PDPs as a result
   of processing the Application-Level Resource Priority Policy Element
   (which is carried in RSVP messages).

   If one wants to implement a prioritized service purely based on
   Session Queueing, one can achieve this by signaling prioritized
   sessions:

   o  using the "Resource-Priority" header in SIP

   o  not using the Admission-Priority Policy Element in RSVP

   o  not using the Preemption Policy Element in RSVP

   If one wants to implement a prioritized service based on Session
   Queueing and "prioritized access to network-layer resources", one can
   achieve this by signaling prioritized sessions:

   o  using the "Resource-Priority" header in SIP

   o  using the Admission-Priority Policy Element in RSVP

   o  not using the Preemption Policy Element in RSVP

   Establishment of prioritized sessions will not result in preemption
   of any session.  Different bandwidth allocation models can be used to
   offer different "prioritized access to network-layer resources".
   Just as examples, this includes setting aside capacity exclusively
   for prioritized sessions as well as simple bypass of admission limits
   for prioritized sessions.

   If one wants to implement a prioritized service based on Session
   Queueing and "prioritized access to network-layer resources", and
   wants to ensure that (say) "Prioritized-1" sessions can preempt
   "Prioritized-2" sessions, but non-prioritized sessions are not
   affected by preemption, one can do that by signaling prioritized
   sessions:

   o  using the "Resource-Priority" header in SIP

   o  using the Admission-Priority Policy Element in RSVP

   o  using the Preemption Policy Element in RSVP with:

      *  setup (Prioritized-1) > defending (Prioritized-2)

      *  setup (Prioritized-2) <= defending (Prioritized-1)




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      *  setup (Prioritized-1) <= defending (Non-Prioritized)

      *  setup (Prioritized-2) <= defending (Non-Prioritized)

   If one wants to implement a prioritized service based on Session
   Queueing and "prioritized access to network-layer resources", and
   wants to ensure that prioritized sessions can preempt regular
   sessions, one could do that by signaling Prioritized sessions:

   o  using the "Resource-Priority" header in SIP

   o  using the Admission-Priority Policy Element in RSVP

   o  using the Preemption Policy Element in RSVP with:

      *  setup (Prioritized) > defending (Non-Prioritized)

      *  setup (Non-Prioritized) <= defending (Prioritized)

   If one wants to implement a prioritized service based on Session
   Queueing and "prioritized access to network-layer resources", and
   wants to ensure that prioritized sessions can partially preempt
   regular sessions (i.e., reduce their reservation size), one could do
   that by signaling prioritized sessions:

   o  using the "Resource-Priority" header in SIP

   o  using the Admission-Priority Policy Element in RSVP

   o  using the Preemption Policy Element in RSVP with:

      *  setup (Prioritized) > defending (Non-Prioritized)

      *  setup (Non-Prioritized) <= defending (Prioritized)

   o  activate [RFC4495] RSVP bandwidth reduction mechanisms















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Authors' Addresses

   Francois Le Faucheur
   Cisco Systems
   Greenside, 400 Avenue de Roumanille
   Sophia Antipolis  06410
   France

   Phone: +33 4 97 23 26 19
   EMail: flefauch@cisco.com


   James Polk
   Cisco Systems
   2200 East President George Bush Highway
   Richardson, TX  75082-3550
   United States

   Phone: +1 972 813 5208
   EMail: jmpolk@cisco.com


   Ken Carlberg
   G11
   123a Versailles Circle
   Towson, MD  21204
   United States

   EMail: carlberg@g11.org.uk






















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