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Versions: (draft-baker-tsvwg-admitted-voice-dscp) 00 01 02 03 04 05 06 07 RFC 5865

Transport Working Group                                         F. Baker
Internet-Draft                                                   J. Polk
Updates: 4542,4594                                         Cisco Systems
(if approved)                                                   M. Dolly
Intended status: Standards Track                               AT&T Labs
Expires: May 5, 2009                                    November 1, 2008


                   DSCP for Capacity-Admitted Traffic
               draft-ietf-tsvwg-admitted-realtime-dscp-05

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on May 5, 2009.

Abstract

   This document requests one Differentiated Services Code Point (DSCP)
   from the Internet Assigned Numbers Authority (IANA) for real-time
   traffic classes similar to voice conforming to the Expedited
   Forwarding Per Hop Behavior, and admitted using a call admission
   procedure involving authentication, authorization, and capacity
   admission.

   This document also recommends that certain classes of video traffic
   described in RFC 4594 and which have similar requirements be changed
   to require admission using a Call Admission Control (CAC) procedure



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   involving authentication, authorization, and capacity admission.

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].


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Problem  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.  Candidate Implementations of the Admitted Telephony
       Service Class  . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.1.  Potential implementations of EF in this model  . . . . . .  7
     2.2.  Capacity admission control . . . . . . . . . . . . . . . .  9
     2.3.  Recommendations on implementation of an Admitted
           Telephony Service Class  . . . . . . . . . . . . . . . . . 10
   3.  Summary: changes from RFC 4594 . . . . . . . . . . . . . . . . 11
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 15






















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1.  Introduction

   This document requests one Differentiated Services Code Point (DSCP)
   from the Internet Assigned Numbers Authority (IANA) for a class of
   real-time traffic.  This class conforms to the Expedited Forwarding
   [RFC3246] [RFC3247] Per Hop Behavior.  It is also admitted using a
   CAC procedure involving authentication, authorization, and capacity
   admission.  This differs from a real-time traffic class conforming to
   the Expedited Forwarding Per Hop Behavior but not subject to capacity
   admission or subject to very coarse capacity admission.

   It also recommends that certain classes of video described in
   [RFC4594] be treated as requiring capacity admission as well.

   These applications have one or more potential congestion points
   between the endpoints.  Reserving capacity for them is important to
   application performance.  All of these applications have low
   tolerance to jitter (aka delay variation) and loss, as summarized in
   Section 2, and most (except for multimedia conferencing) have
   inelastic flow behavior from Figure 1 of [RFC4594].  Inelastic flow
   behavior and low jitter/loss tolerance are the service
   characteristics that define the need for admission control behavior.

   One of the reasons behind this is the need for classes of traffic
   that are handled under special policies, such as the non-preemptive
   Emergency Telecommunication Service, the US Department of Defense's
   Assured Service (which is similar to Multi-Level Precedence and
   Preemption [ITU.MLPP.1990] procedure), or e-911, in addition to
   normal routine calls that use call admission.  It is possible to use
   control plane protocols to generally restrict session admission such
   that admitted traffic should receive the desired service, and the
   policy (e.g.  Routine, National Security or Emergency Preparedness
   [NS/EP] communications, e-911, etc) need not be signaled in a DSCP.
   However, service providers need to distinguish between special-policy
   traffic and other classes, particularly the existing VoIP services
   that perform no capacity admission or only very coarse capacity
   admission and can exceed their allocated resources.

   The requested DSCP applies to the Telephony Service Class described
   in [RFC4594].

   Since video classes have not had the history of mixing admitted and
   non-admitted traffic in the same Per-Hop Behavior (PHB) as has
   occurred for EF, an additional DSCP code point is not recommended.
   Instead, the recommended "best practice" is to perform admission
   control for all traffic in three of [RFC4594]'s video classes: the





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   o  Interactive Real-Time Traffic (CS4, used for Video conferencing
      and Interactive gaming),

   o  Broadcast TV (CS3) for use in a video on demand context, and

   o  AF4 Multimedia Conferencing (video conferencing).

   Other video classes are believed to not have the current problem of
   confusion with unadmitted traffic and therefore would not benefit
   from the notion of a separate DSCP for admitted traffic.  Within an
   ISP and on inter-ISP links (i.e. within networks whose internal paths
   are uniform at hundreds of megabits or faster), one would expect all
   of this traffic to be carried in the Real Time Traffic Class
   described in [RFC5127].

1.1.  Definitions

   The following terms and acronyms are used in this document.

   PHB:  A Per-Hop-Behavior (PHB) is the externally observable
      forwarding behavior applied at a Differentiated Services compliant
      node to a DS behavior aggregate [RFC2475].  It may be thought of
      as a program configured on the interface of an Internet host or
      router, specified drop probabilities, queuing priorities or rates,
      and other handling characteristics for the traffic class.

   DSCP:  The Differentiated Services Code Point (DSCP), as defined in
      [RFC2474], is a value which is encoded in the DS field, and which
      each DS Node MUST use to select the PHB which is to be experienced
      by each packet it forwards [RFC3260].  It is a 6-bit number
      embedded into the 8-bit TOS field of an IPv4 datagram or the
      Traffic Class field of an IPv6 datagram.

   CAC:  Call Admission Control includes concepts of authorization and
      capacity admission.  "Authorization" refers to any procedure that
      identifies a user, verifies the authenticity of the
      identification, and determines whether the user is authorized to
      use the service under the relevant policy.  "Capacity Admission"
      refers to any procedure that determines whether capacity exists
      supporting a session's requirements under some policy.

      In the Internet, these are separate functions, while in the PSTN
      they and call routing are carried out together.

   UNI:  A User/Network Interface (UNI) is the interface (often a
      physical link or its virtual equivalent) that connects two
      entities that do not trust each other, and in which one (the user)
      purchases connectivity services from the other (the network).



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      Figure 1 shows two user networks connected by what appears to each
      of them to be a single network ("The Internet", access to which is
      provided by their service provider) that provides connectivity
      services to other users.

      UNIs tend to be the bottlenecks in the Internet, where users
      purchase relatively low amounts of bandwidth for cost or service
      reasons, and as a result are most subject to congestion issues and
      therefore issues requiring traffic conditioning and service
      prioritization.

   NNI:  A Network/Network Interface (NNI) is the interface (often a
      physical link or its virtual equivalent) that connects two
      entities that trust each other within limits, and in which the two
      are seen as trading services for value.  Figure 1 shows three
      service networks that together provide the connectivity services
      that we call "the Internet".  They are different administrations
      and are very probably in competition, but exchange contracts for
      connectivity and capacity that enable them to offer specific
      services to their customers.

      NNIs may not be bottlenecks in the Internet if service providers
      contractually agree to provision excess capacity at them, as they
      commonly do.  However, NNI performance may differ by ISP, and the
      performance guarantee interval may range from a month to a much
      shorter period.  Furthermore, a peering point NNI may not have
      contractual performance guarantees or may become overloaded under
      certain conditions.  They are also policy-controlled interfaces,
      especially in BGP.  As a result, they may require traffic
      prioritization policy.

   Queue:  There are multiple ways to build a multi-queue scheduler.
      Weighted Round Robin (WRR) literally builds multiple lists and
      visits them in a specified order, while a calendar queue (often
      used to implement Weighted Fair Queuing, or WFQ) builds a list for
      each time interval and enqueues at most a stated amount of data in
      each such list for transmission during that time interval.  While
      these differ dramatically in implementation, the external
      difference in behavior is generally negligible when they are
      properly configured.  Consistent with the definitions used in the
      Differentiated Services Architecture [RFC2475], these are treated
      as equivalent in this document, and the lists of WRR and the
      classes of a calendar queue will be referred to uniformly as
      "queues".







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                                        _.--------.
                                    ,-''           `--.
                                 ,-'                   `-.
           ,-------.           ,',-------.                `.
         ,'         `.       ,','         `.                `.
        /  User       \ UNI / /   Service   \                 \
       (    Network    +-----+    Network    )                 `.
        \             /  ;    \             /                    :
         `.         ,'   ;     `.         .+                     :
           '-------'    /        '-------'  \ NNI                 \
                       ;                     \                     :
                       ;     "The Internet"   \  ,-------.         :
                      ;                        +'         `.        :
        UNI: User/Network Interface           /   Service   \       |
                     |                       (    Network    )      |
        NNI: Network/Network Interface        \             /       |
                      :                        +.         ,'        ;
                       :                      /  '-------'         ;
                       :                     /                     ;
           ,-------.    \        ,-------.  / NNI                 /
         ,'         `.   :     ,'         `+                     ;
        /  User       \ UNI   /   Service   \                    ;
       (    Network    +-----+    Network    )                 ,'
        \             /     \ \             /                 /
         `.         ,'       `.`.         ,'                ,'
           '-------'           `.'-------'                ,'
                                 `-.                   ,-'
                                    `--.           _.-'
                                        `--------''

                     Figure 1: UNI and NNI interfaces

1.2.  Problem

   In short, the Telephony Service Class described in [RFC4594] permits
   the use of capacity admission in implementing the service, but
   present implementations either provide no capacity admission services
   or do so in a manner that depends on specific traffic engineering.
   In the context of the Internet backbone, the two are essentially
   equivalent; the edge network depends on specific engineering by the
   service provider that may not be present, especially in a mobile
   environment.

   However, services are being requested of the network that would
   specifically make use of capacity admission, and would distinguish
   among users or the uses of available Voice-over-IP or Video-over-IP
   capacity in various ways.  Various agencies would like to provide
   services as described in section 2.6 of [RFC4504] or in [RFC4190].



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   This requires the use of capacity admission to differentiate among
   users (which might be 911 call centers, other offices with
   preferential service contracts, or individual users gaining access
   with special credentials) to provide services to them that are not
   afforded to non-capacity admitted customer-to-customer IP telephony
   sessions.


2.  Candidate Implementations of the Admitted Telephony Service Class

2.1.  Potential implementations of EF in this model

   There are at least two possible ways to implement isolation between
   the Capacity Admitted PHB and the Expedited Forwarding PHB in this
   model.  They are to implement separate classes as a set of

   o  Multiple data plane traffic classes, each consisting of a policer
      and a queue, and the queues enjoying different priorities, or

   o  Multiple data plane traffic classes, each consisting of a policer
      but feeding into a common queue or multiple queues at the same
      priority.

   We will explain the difference, and describe in what way they differ
   in operation.  The reason this is necessary is that there is current
   confusion in the industry.

   The multi-priority model is shown in Figure 2.  In this model,
   traffic from each service class is placed into a separate priority
   queue.  If data is present in both queues, traffic from one of them
   will always be selected for transmission.  This has the effect of
   transferring jitter from the higher priority queue to the lower
   priority queue, and reordering traffic in a way that gives the higher
   priority traffic a smaller average queuing delay.  Each queue must
   have its own policer, however, to protect the network from errors and
   attacks; if a traffic class thinks it is carrying a certain data rate
   but an abuse sends significantly more, the effect of simple
   prioritization would not preserve the lower priorities of traffic,
   which could cause routing to fail or otherwise impact an SLA.












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                                             .
                     policers    priorities  |`.
           Unadmitted EF <=> ----------||----+  `.
                                         high|    `.
             Admitted EF <=> ----------||----+     .'-----------
                           .             medium  .'
              rate queues  |`.         +-----+ .' Priority
           AF1------>||----+  `.      /  low |'   Scheduler
                           |    `.   /
           AF2------>||----+     .'-+
                           |   .'
           CS0------>||----+ .' Rate Scheduler
                           |'   (WFQ, WRR, etc)

             Figure 2: Implementation as a data plane priority

   The multi-policer model is shown in Figure 3.  In this model, traffic
   from each service class is policed according to its SLA requirements,
   and then placed into a common priority queue.  Unlike the multi-
   priority model, the jitter experienced by the traffic classes in this
   case is the same, as there is only one queue, but the sum of the
   traffic in this higher priority queue experiences less average jitter
   than the elastic traffic in the lower priority.

                       policers    priorities  .
             Unadmitted EF <=> -------\        |`.
                                       --||----+  `.
               Admitted EF <=> -------/    high|    `.
                             .                 |     .'--------
                rate queues  |`.         +-----+   .'
             AF1------>||----+  `.      /  low | .' Priority
                             |    `.   /       |'   Scheduler
             AF2------>||----+     .'-+
                             |   .'
             CS0------>||----+ .' Rate Scheduler
                             |'   (WFQ, WRR, etc)

             Figure 3: Implementation as a data plane policer

   The difference between the two operationally is, as stated, the
   issues of loss due to policing and distribution of jitter.

   If the two traffic classes are, for example, voice and video,
   datagrams conaining video data can be relatively large (often of
   variable sizes up to the path MTU) while datagrams containing voice
   are relatively small, on the order of only 40 to 200 bytes, depending
   on the codec.  On lower speed links (less than 10 MBPS), the jitter
   introduced by video to voice can be disruptive, while at higher



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   speeds the jitter is nominal compared to the jitter requirements of
   voice.  At access network speeds, therefore, [RFC4594] recommends
   separation of video and voice into separate queues, while at optical
   speeds [RFC5127] recommends that they use a common queue.

   If, on the other hand, the two traffic classes are carrying the same
   type of application with the same jitter requirements, then giving
   one preference in this sense does not benefit the higher priority
   traffic and may harm the lower priority traffic.  In such a case,
   using separate policers and a common queue is a superior approach.

2.2.  Capacity admission control

   There are at least six major ways that capacity admission is done or
   has been proposed to be done for real-time applications.  Each will
   be described below, then Section 3 will judge which ones are likely
   to meet the requirements of the Admitted Telephony service class.
   These include:

   o  Drop Precedence used to force sessions to voluntarily exit,

   o  Capacity admission control by assumption or engineering,

   o  Capacity admission control by call counting,

   o  End-point capacity admission performed by probing the network,

   o  Centralized capacity admission control via bandwidth broker, and

   o  Distributed capacity admission control using protocols such as
      RSVP or NSIS.

   The problem with capacity admission by assumption, which is widely
   reployed in today's VoIP environment, is that it depends on the
   assumptions made.  One can do careful traffic engineering to ensure
   needed bandwidth, but this can also be painful, and has to be
   revisited when the network is changed or network usage changes.

   The problem with dropping traffic to force users to hang up is that
   it affects a broad class of users - if there is capacity for N calls
   and the N+1 calls are active, data is dropped randomly from all
   sessions to ensure that offered load doesn't exceed capacity.  On
   very fast links, that is acceptable, but on lower speed links it can
   seriously affect call quality.

   There are two fundamental problems with depending on the endpoint to
   perform capacity admission; it may not be able to accurately measure
   the impact of the traffic it generates on the network, and it tends



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   to be greedy (eg., it doesn't care).  If the network operator is
   providing a service, he must be able to guarantee the service, which
   means that he can't trust systems that are not controlled by his
   network.

   The problem with mechanisms that don't enable the association of a
   policy with the request is that they don't allow for multi-policy
   services, which are becoming important.

   The operator's choice of admission procedure must, for this DSCP,
   ensure the following:

   o  The actual links that a session uses have enough bandwidth to
      support it.

   o  New sessions are refused admission if there is inadequate
      bandwidth under the relevant policy.

   o  If multiple policies are in use in a network, that the user is
      identified and the correct policy applied.

   o  Under periods of network stress, the process of admission of new
      sessions does not disrupt existing sessions, unless the service
      explicitly allows for disruption of calls.

2.3.  Recommendations on implementation of an Admitted Telephony Service
      Class

   It is the belief of the authors that either PHB implementation
   described in Section 2.1, if coupled with adequate AAA and capacity
   admission procedures as described in Section 2.2, are sufficient to
   provide the services required for an Admitted Telephony service
   class.  If preemption is required, as described in section 2.3.5.2 of
   [RFC4542], this provides the tools for carrying out the preemption.
   If preemption is not in view, or if used in addition to preemptive
   services, the application of different thresholds depending on call
   precedence has the effect of improving the probability of call
   completion by admitting preferred calls at a time that other calls
   are being refused.  Routine and priority traffic can be admitted
   using the same DSCP value, as the choice of which calls are admitted
   is handled in the admission procedure executed in the control plane,
   not the policing of the data plane.

   On the point of what protocols and procedures are required for
   authentication, authorization, and capacity admission, we note that
   clear standards do not at this time exist for bandwidth brokers, NSIS
   has not at this time been finalized and in any event is limited to
   unicast sessions, and that RSVP has been standardized and has the



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   relevant services.  We therefore recommend the use of RSVP at the
   UNI.  Procedures at the NNI are business matters to be discussed
   between the relevant networks, and are recommended but not required.


3.  Summary: changes from RFC 4594

   To summarize, there are two changes to [RFC4594] discussed in this
   document:

   Telephony class:  The Telephony Service Class in RFC 4594 does not
      involve capacity admission, but depends on application layer
      admission that only estimates capacity, and that through static
      engineering.  In addition to that class, a separate Admitted
      Telephony Class is added which performs capacity admission
      dynamically.

   Video classes  Capacity admission is added to three video classes.
      These are the Interactive Real-Time Traffic class, Broadcast TV
      class when used for video on demand, and the Multimedia
      Conferencing class.


4.  IANA Considerations

   This note requests that IANA assign a DSCP value to a second EF
   traffic class consistent with [RFC3246] and [RFC3247] in the
   "Differentiated Services Field Codepoints" registry.  It implements
   the Telephony Service Class described in [RFC4594] at lower speeds
   and is included in the Real Time Treatment Aggregate [RFC5127] at
   higher speeds.  The recommended value for the code point is 101100,
   paralleling the EF code point, which is 101110.  The code point
   should be referred to as VOICE-ADMIT.

   This traffic class requires the use of capacity admission such as
   RSVP services together with AAA services at the User/Network
   Interface (UNI); the use of such services at the NNI is at the option
   of the interconnected networks.


5.  Security Considerations

   A major requirement of this service is effective use of a signaling
   protocol such as RSVP, with the capabilities to identify its user
   either as an individual or as a member of some corporate entity, and
   assert a policy such as "routine" or "priority".

   This capability, one has to believe, will be abused by script kiddies



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   and others if the proof of identity is not adequately strong or if
   policies are written or implemented improperly by the carriers.  This
   goes without saying, but this section is here for it to be said...


6.  Acknowledgements

   Kwok Ho Chan, Georgios Karagiannis, Dan Voce, and Bob Briscoe
   commented and offered text.  The impetus for including Video in the
   discussion, which initially only targeted voice, is from Dave
   McDysan,


7.  References

7.1.  Normative References

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

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              December 1998.

   [RFC3246]  Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
              J., Courtney, W., Davari, S., Firoiu, V., and D.
              Stiliadis, "An Expedited Forwarding PHB (Per-Hop
              Behavior)", RFC 3246, March 2002.

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              August 2006.

7.2.  Informative References

   [ITU.MLPP.1990]
              International Telecommunications Union, "Multilevel
              Precedence and Preemption Service", ITU-T Recommendation
              I.255.3, 1990.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, December 1998.

   [RFC3247]  Charny, A., Bennet, J., Benson, K., Boudec, J., Chiu, A.,
              Courtney, W., Davari, S., Firoiu, V., Kalmanek, C., and K.
              Ramakrishnan, "Supplemental Information for the New



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              Definition of the EF PHB (Expedited Forwarding Per-Hop
              Behavior)", RFC 3247, March 2002.

   [RFC3260]  Grossman, D., "New Terminology and Clarifications for
              Diffserv", RFC 3260, April 2002.

   [RFC4190]  Carlberg, K., Brown, I., and C. Beard, "Framework for
              Supporting Emergency Telecommunications Service (ETS) in
              IP Telephony", RFC 4190, November 2005.

   [RFC4504]  Sinnreich, H., Lass, S., and C. Stredicke, "SIP Telephony
              Device Requirements and Configuration", RFC 4504,
              May 2006.

   [RFC4542]  Baker, F. and J. Polk, "Implementing an Emergency
              Telecommunications Service (ETS) for Real-Time Services in
              the Internet Protocol Suite", RFC 4542, May 2006.

   [RFC5127]  Chan, K., Babiarz, J., and F. Baker, "Aggregation of
              DiffServ Service Classes", RFC 5127, February 2008.


Authors' Addresses

   Fred Baker
   Cisco Systems
   Santa Barbara, California  93117
   USA

   Phone: +1-408-526-4257
   Email: fred@cisco.com


   James Polk
   Cisco Systems
   Richardson, Texas  75082
   USA

   Phone: +1-817-271-3552
   Email: jmpolk@cisco.com











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   Martin Dolly
   AT&T Labs
   Middletown Township, New Jersey  07748
   USA

   Phone: +1-732-420-4574
   Email: mdolly@att.com












































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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
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