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Tsvwg Working Group                                               J. You
Internet-Draft                                                    Huawei
Intended status: Standards Track                                M. Welzl
Expires: September 14, 2016                           University of Oslo
                                                             B. Trammell
                                                           M. Kuehlewind
                                                              ETH Zurich
                                                                K. Smith
                                                          Vodafone Group
                                                          March 13, 2016

                    Latency Loss Tradeoff PHB Group


   This document defines a PHB (Per-Hop Behavior) group called Latency
   Loss Tradeoff (LLT).  The LLT group is intended to provide delivery
   of IP packets in two classes of services: a low-loss service (Lo
   service) and a low-latency service (La service).  The LLT group
   enables an application to request treatment for either low-loss or
   low-latency at a congested network link.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 14, 2016.

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Copyright Notice

   Copyright (c) 2016 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
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Abbreviations and acronyms  . . . . . . . . . . . . . . .   3
     2.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Existing DSCP PHBs  . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Default PHB . . . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  Class Selector PHB  . . . . . . . . . . . . . . . . .   5
       3.1.3.  Assured Forwarding PHB Group  . . . . . . . . . . . .   5
       3.1.4.  Expedited Forwarding PHB  . . . . . . . . . . . . . .   6
       3.1.5.  Voice Admit PHB . . . . . . . . . . . . . . . . . . .   6
       3.1.6.  Delay Bound PHB . . . . . . . . . . . . . . . . . . .   6
     3.2.  Incentives  . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Definition of LLT PHB . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Goal and Scope of LLT . . . . . . . . . . . . . . . . . .   7
     4.2.  Description of LLT behavior . . . . . . . . . . . . . . .   8
       4.2.1.  Implementation Considerations . . . . . . . . . . . .   8
     4.3.  Microflow misordering . . . . . . . . . . . . . . . . . .   9
     4.4.  Recommended Codepoints  . . . . . . . . . . . . . . . . .   9
     4.5.  Mutability  . . . . . . . . . . . . . . . . . . . . . . .   9
     4.6.  Tunneling . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.7.  Interaction with other PHBs . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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1.  Introduction

   Different applications have different communication requirements
   [QoS].  In interactive applications of real-time sound transmission,
   as well as in virtual reality, the overall one-way delay needs to be
   short in order to give the user an impression of a real-time
   response.  Yet, these applications may be able to tolerate high loss
   rates.  In conventional text and data networking, delay thresholds
   are the least stringent.  The response time in these types of
   applications can increase from 2 to 5 seconds before becoming
   unacceptable.  However, given that increased loss reduces the
   throughput of TCP, these applications desire minimal loss.

   The network resources consist primarily of buffers and link
   bandwidth.  Operators often favor high utilization of bottleneck
   links at the price of high queuing delay.  This is beneficial for
   non-real time applications.  However, this may be considered
   unacceptable for some real-time applications.  The proposed LLT group
   enables an application to choose between low-latency and low-loss at
   a congested network link [ABE] [RD].  Typically, an interactive
   application with real-time deadlines, such as audio, will mark most
   of its packets as a low-latency service.  In contrast, an application
   that transfers bulk data will mark most of its packets as a low-loss
   service.  The LLT group can be thought of as allowing an application
   to trade loss for delay by marking packets low-latency service (La)
   or to trade delay for loss by marking packets low-loss service (Lo).

2.  Terminology

   This section contains definitions for terms used frequently
   throughout this document.

2.1.  Abbreviations and acronyms

      DS: Differentiated Service

      PHB: Per-Hop Behavior

      LLT: Latency Loss Tradeoff

      TCA: Traffic Conditioning Agreement

      TCP: Transmission Control Protocol

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2.2.  Definitions

      DS-capable: capable of implementing differentiated services as
      described in this architecture; usually used in reference to a
      domain consisting of DS-compliant nodes.

      DS codepoint: a specific value of the DSCP portion of the DS
      field, used to select a PHB.

      DS-compliant: enabled to support differentiated services functions
      and behaviors as defined in [RFC2474], this document, and other
      differentiated services documents; usually used in reference to a
      node or device.

      DS field: the IPv4 header TOS octet or the IPv6 Traffic Class
      octet when interpreted in conformance with the definition given in
      [RFC2474].  The bits of the DSCP field encode the DS codepoint,
      while the remaining bits are currently unused.

      Low-latency service (La service): puts an emphasis on low queuing
      delay at a congested network link.  It allows an application to
      trade loss for delay.

      Low-loss service (Lo service): puts an emphasis on low packet loss
      rate at a congested network link.  It allows an application to
      trade delay for loss.

      Per-Hop-Behavior (PHB): the externally observable forwarding
      behavior applied at a DS-compliant node to a DS behavior

      PHB group: a set of one or more PHBs that can only be meaningfully
      specified and implemented simultaneously, due to a common
      constraint applying to all PHBs in the set such as a queue
      servicing or queue management policy.  A PHB group provides a
      service building block that allows a set of related forwarding
      behaviors to be specified together (e.g., four dropping
      priorities).  A single PHB is a special case of a PHB group.

      Traffic Conditioning Agreement (TCA): an agreement specifying
      classifier rules and any corresponding traffic profiles and
      metering, marking, discarding and/or shaping rules which are to
      apply to the traffic streams selected by the classifier.  A TCA
      encompasses all of the traffic conditioning rules explicitly
      specified within a SLA along with all of the rules implicit from
      the relevant service requirements and/or from a DS domain's
      service provisioning policy.

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3.  Problem Statement

3.1.  Existing DSCP PHBs

3.1.1.  Default PHB

   A default Per-Hop Behavior (PHB) [RFC2474] MUST be available in a
   DiffServ (DS)-compliant node.  This is the common, best-effort
   forwarding behavior available in existing routers as standardized in
   [RFC1812].  Codepoint '000000' from Pool 1 is used as the default PHB
   value.  In this document, packets received with the Default PHB is
   treated as Lo service on the LLT-compliant router.

3.1.2.  Class Selector PHB

   The Class Selector (CS) PHB [RFC2474] is introduced for backwards
   compatibility with use of the IPv4 Precedence field.  Any of the
   eight codepoints in the range 'xxx000' (where 'x' may equal '0' or
   '1') from Pool 1 is assigned as Class Selector codepoint.  The CS PHB
   does not match the services that LLT PHB is trying to deliver.

3.1.3.  Assured Forwarding PHB Group

   The Assured Forwarding (AF) PHB group [RFC2597] allows the operator
   to provide assured forwarding of IP packets as long as the aggregate
   traffic does not exceed the subscribed rate.  Traffic that exceeds
   the subscribed rate is not delivered with as high a probability as
   the traffic that is within the rate.

   The AF PHB group provides delivery of IP packets in four
   independently forwarded AF classes.  Within each AF class, an IP
   packet can be assigned one of three different levels of drop
   precedence.  The combination of classes and drop precedence yields
   twelve separate DSCP encodings from AF11 through AF43 as follows:

                          Class 1     Class 2     Class 3    Class 4
    Low Drop Prec      |  001010  |  010010   |  011010   |  100010  |
    Medium Drop Prec   |  001100  |  010100   |  011100   |  100100  |
    High Drop Prec     |  001110  |  010110   |  011110   |  100110  |

   The AF PHB does not match the services that LLT PHB is trying to

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3.1.4.  Expedited Forwarding PHB

   Expedited Forwarding (EF) PHB [RFC3246] is intended to provide a
   building block for low delay, low jitter and low loss services by
   ensuring that the EF aggregate is served at a certain configured
   rate.  EF traffic requires a strict admission control mechanism.
   Codepoint '101110' is recommended for the EF PHB.  The EF PHB does
   not match the services that LLT PHB is trying to deliver.

3.1.5.  Voice Admit PHB

   The Voice Admit (VA) PHB [RFC5865] has identical characteristics to
   the Expedited Forwarding PHB.  However Voice Admit traffic is also
   admitted by the network using a Call Admission Control (CAC)
   procedure.  The recommended DSCP for Voice Admit is '101100',
   parallel with the existing EF codepoint '101110'.  The VA PHB does
   not match the services that LLT PHB is trying to deliver.

3.1.6.  Delay Bound PHB

   The Delay Bound (DB) PHB [RFC3248] requires a bound on the delay of
   packets due to other traffic in the network.  Two parameters - capped
   arrival rate (R) and a 'score' (S), are defined and related to the
   target delay variation bound.  An experimental codepoint '101111' is
   suggested for DB behavior.  In this document, there's no specific
   bound on the delay, the LLT PHB only indicates the tradeoff.

3.2.  Incentives

   The primary goal of differentiated services is to allow different
   levels of service to be provided for traffic streams on a common
   network infrastructure.  Hence, an adversary may be able to obtain
   better service by modifying the DS field to codepoints indicating
   behaviors used for enhanced services or by injecting packets with the
   DS field set to such codepoints.  Such theft-of-service ([RFC2474],
   [RFC2475]) becomes a denial-of-service attack when the modified or
   injected traffic depletes the resources available to forward it and
   other traffic streams.

   DS ingress nodes must condition all traffic entering a DS domain to
   ensure that it has acceptable DS codepoints.  This means that the
   codepoints must conform to the applicable TCA(s) (Traffic
   Conditioning Agreement) [RFC2475] and the domain's service
   provisioning policy.  Packets received with an unacceptable
   codepoints must either be discarded or must have their DS codepoints
   modified to acceptable values before being forwarded.  For example,
   an ingress node receiving traffic from a domain with which no
   enhanced service agreement exists may reset the DS codepoint to the

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   Default PHB codepoint.  However, the Default PHB (i.e. best-effort
   forwarding) cannot meet the diverse needs of different Internet

   The objective of the LLT PHB group is to retain the best-effort
   service while providing low delay to real-time applications at the
   expense of increased loss or providing low loss to non real-time
   applications at the expense of increased delay.  This requires
   Internet applications to mark their traffic with appropriate
   codepoint values.  Since the low-loss service is neither better nor
   worse than the low-latency service but is merely different, there is
   no incentive for Internet applications to abuse such codepoints, and
   no need for admission control.

4.  Definition of LLT PHB

   The LLT group provides forwarding of IP packets in two classes of
   service: a low-loss service (Lo) and a low-latency service (La).  The
   LLT group enables an application to choose between low latency and
   low loss at a congested network link.  The packets marked as low-
   latency service receive little queuing delay.  The packets marked as
   low-loss service receive at least as much throughput as they would in
   a legacy best effort network.  La-marked packets are more likely to
   be dropped during periods of congestion than the Lo-marked packets.
   Note that among the two services, neither of the two has priority
   over the other.

4.1.  Goal and Scope of LLT

   The LLT group may be used by a network operator in two distinct ways:
   either as a separate service, or as a replacement of the flat
   (existing) best-effort IP service.

   A DS (Differentiated Services) node SHOULD implement the LLT group.
   It MAY allocate a configurable, minimum amount of forwarding
   resources (buffer space and bandwidth) to LLT group.

   The LLT group MAY also be configurable to receive more forwarding
   resources than the minimum when excess resources are available from
   other PHB groups.  This is beyond the scope of this document.

   The LLT PHB definition does NOT mandate or recommend any particular
   method for achieving LLT behavior.

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4.2.  Description of LLT behavior

   To support the LLT group on an output link, the router can maintain
   two FIFO (First-In First-Out) queues referred to as a Lo (Loss-
   sensitive) queue and La (Latency-sensitive) queue for packets
   destined to the link.  Depending on whether an incoming packet is
   marked for the low-loss or low-latency service, the router appends
   the packet to the Lo or La queue respectively.  The packets within
   each queue are served in the FIFO order.  The scheduling is work-

   A router can support the desired delay differentiation between the Lo
   and La services through buffer sizing for the Lo and La queues, and
   by ensuring that the La queue does not grow larger than the Lo queue.
   As common in current Internet routers, the size of the Lo buffer is
   chosen large enough so that the oscillating transmission of TCP
   (Transmission Control Protocol) and other legacy end-to-end
   congestion control protocols utilizes the available link rate fully.
   The La buffer is configured to a much smaller dynamic size to ensure
   that queuing delay for each forwarded packet of the La class is low.
   The assurance of low maximum queuing delay is attractive for delay-
   sensitive applications and easily verifiable by outside parties.

4.2.1.  Implementation Considerations

   This document does not specify any particular implementation method
   for achieving LLT behavior.  Some LLT-like implementations may refer
   to [I-D.hurley-alternative-best-effort], [RD] and

   [I-D.hurley-alternative-best-effort] marks every best effort packet
   as either green or blue.  Green packets receive a low, bounded delay
   at every hop, the value of the per-hop delay bound configured by the
   operator.  However, when transmitting more aggressively, the green
   users can enjoy both a higher rate and lower queuing delay than those
   of the blue users, which weakens the incentives for incremental
   deployment.  [RD] proposes Rate-Delay (RD) service enabling a user to
   choose either a higher transmission rate or low queuing delay.  The R
   (Rate) service is like Lo service while D (Delay) service is like La

   Note that both classes defined in this document do not provide any
   absolute guarantees on the loss rate or delay a packet will
   experience.  Using these classes only provides a relative treatment
   compared to the other class.  Depending on the amount of traffic
   arriving per class, it is possible for traffic in the La class to
   experience more delay than traffic in the Lo class.  However, this
   may be circumvented by using scheduling mechanisms, for example, by

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   adjusting the scheduling function that assigns traffic to the Lo and
   La queues, or by adjusting the scheduling weight based on the average
   load in each class.  Moreover, the delay experienced by La traffic is
   always bounded by the length of the La queue.  The particular
   implementation is beyond the scope of this document.

   When a DS-compliant node claims to implement the LLT PHB, the
   implementation MUST conform to the specification given in this

4.3.  Microflow misordering

   The packets within each queue are served in the FIFO order.  Packets
   belonging to a single microflow within the LLT aggregate SHOULD NOT
   experience re-ordering in normal operation of the device when passing

4.4.  Recommended Codepoints

   Recommended codepoints for the LLT PHB group are given below.

      Low-loss service: 000001
      Low-latency service:000101

4.5.  Mutability

   Packets marked for LLT PHB MAY be remarked at a DS domain boundary
   only to other codepoints that satisfy the LLT PHB.  Packets marked
   for LLT PHBs SHOULD NOT be demoted or promoted to another PHB by a DS

4.6.  Tunneling

   When LLT packets are tunneled, the tunneling packets must be marked
   as LLT.

4.7.  Interaction with other PHBs

   Other PHBs and PHB groups may be deployed in the same DS node or
   domain with the LLT PHB.

   Packets received with the Default PHB SHOULD be treated as Lo service
   as default by the LLT PHB aware device.  [RD] has proved that La
   service does not hurt Lo service.

   Packets received with the LLT PHB SHOULD be treated as Default PHB as
   default by the LLT PHB unaware device.

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5.  Security Considerations

   Internet applications cannot benefit from wrongly indicating low-loss
   or low-latency as they have to pay the expense of high delay or high
   loss as a tradeoff.  Hence there is no incentive for Internet
   applications to set the wrong codepoints.

6.  IANA Considerations

   This document suggests two experimental codepoints, 000001/000101, in
   Pool 3 of the code space defined by [RFC2474].

7.  References

7.1.  Normative References

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, 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,
              DOI 10.17487/RFC2474, December 1998,

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,

   [RFC2597]  Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
              "Assured Forwarding PHB Group", RFC 2597,
              DOI 10.17487/RFC2597, June 1999,

   [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, DOI 10.17487/RFC3246, March 2002,

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   [RFC3248]  Armitage, G., Carpenter, B., Casati, A., Crowcroft, J.,
              Halpern, J., Kumar, B., and J. Schnizlein, "A Delay Bound
              alternative revision of RFC 2598", RFC 3248,
              DOI 10.17487/RFC3248, March 2002,

   [RFC5865]  Baker, F., Polk, J., and M. Dolly, "A Differentiated
              Services Code Point (DSCP) for Capacity-Admitted Traffic",
              RFC 5865, DOI 10.17487/RFC5865, May 2010,

7.2.  Informative References

   [ABE]      Hurley, P., Le Boudec, J., Thiran, P., and M. Kara, "ABE:
              Providing a Low-Delay Service within Best Effort", IEEE
              Network Magazine 15(3): 60-69, May 2001.

              Schepper, K., Briscoe, B., Bondarenko, O., and I. Tsang,
              "DualQ Coupled AQM for Low Latency, Low Loss and Scalable
              Throughput", draft-briscoe-aqm-dualq-coupled-00 (work in
              progress), August 2015.

              Hurley, P., Iannaccone , G., Kara , M., Le Boudec , J.,
              Thiran , P., and C. Diot , "The ABE Service", November

   [QoS]      Chen, C., Farley, T., and N. Ye, "QoS Requirements of
              Network Applications on the Internet", Information
              Knowledge Systems Management 2004, 4(1): 55-76, 2004.

   [RD]       Podlesny, M. and S. Gorinsky, "RD network services:
              differentiation through performance incentives", ACM
              SIGCOMM Computer Communication Review,  38(4): 255-266,

Authors' Addresses

   Jianjie You
   101 Software Avenue, Yuhua District
   Nanjing  210012

   Email: youjianjie@huawei.com

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   Michael Welzl
   University of Oslo
   PO Box 1080 Blindern
   Oslo  N-0316

   Email: michawe@ifi.uio.no

   Brian Trammell
   ETH Zurich

   Email: ietf@trammell.ch

   Mirja Kuehlewind
   ETH Zurich

   Email: mirja.kuehlewind@tik.ee.ethz.ch

   Kevin Smith
   Vodafone Group
   One Kingdom Street,

   Email: kevin.smith@vodafone.com

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