TSVWG                                                       R. Geib, Ed.
Internet-Draft                                          Deutsche Telekom
Intended status: Informational                         February 22, 2013
Expires: August 26, 2013

             DiffServ interconnection classes and practice


   This document proposes a limited set of interconnection QoS PHBs and
   PHB groups.  It further introduces some DiffServ deployment aspects.
   The proposals made here should be integrated into a revised version
   of RFC5127.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  An Interconnection class and codepoint scheme  . . . . . . . .  5
   4.  Consolidation of QoS standards by the interconnection
       codepoint scheme . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  MPLS, Ethernet and IP Precedence for aggregated classes  . . .  8
   6.  QoS class name selection . . . . . . . . . . . . . . . . . . .  9
   7.  Allow for DiffServ extendability on MPLS and Ethernet level  .  9
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
     10.2. Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Change log  . . . . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   This draft proposes a DiffServ interconnection class and codepoint
   scheme.  At least one party of an interconnection often is a network
   provider.  Aggregated DiffServ classes are often deployed within
   provider networks.  To respect this, this draft also contains
   concepts and current practice relevant for a revised version of
   RFC5127 [RFC5127].  Its main purpose is to be considered as an input
   for the latter task.

   DiffServ sees deployment in many networks for the time being.  As
   described in the introduction of the draft diffserv problem statement
   [I-D.polk-tsvwg-diffserv-stds-problem-statement], remarking of
   packets at domain boundaries is a DiffServ feature.  This draft
   proposes a set of standard QoS classes and codepoints at
   interconnection points to which and from which locally used classes
   and codepoints should be mapped.  Such a scheme simplifies
   interconnection negotiations and ensures that end to end class
   properties remain roughly the same, even if codepoints change.

   The proposed Interconnection class and codepoint scheme tries to
   reflect and consolidate related DiffServ and QoS standardisation
   efforts outside of the IETF, namely MEF, GSMA and ITU.

   IP Precedence has been depricated when DiffServ was standardised.  It
   is common practice today however to copy the DSCPs "IP Precedence
   Bits" into MPLS TC or Ethernet P-Bits, whenever possible.  This is
   reflected by the DiffServ codepoint definitions of AF and EF.  This
   practice and it's limits deserve to be documented and disussed

   The draft further adds proposes a philosophy how to add or pick
   aggregated DiffServ classes.  The set of available router and traffic
   management tools to configure and operate DiffServ classes is
   limited.  This should be reflected by class definitions.  These may
   in the end be more related to transport properties than to
   application requirements.  Please interpret transport properties as
   "congestion aware" and "not congestion aware" rather then TCP or UDP.

   Finally, this draft proposes to leave some MPLS TC codepoint space to
   allow for future DiffServ extensions like ECN/PCN and domain internal
   classes (network management traffic is a good example for the

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Terminology

   A later version of this draft needs to be clearer on that.  The
   author prefers to talk of QoS classes.  PHB or PHB groups are not
   commonly used, although they are better defined.  An issue is, that
   PHB groups, which e.g. allow to offer two or more different drop
   levels (PHB's) within one PHB group , hardly saw commercial
   deployment.  This may change with more Ethernet services being

   | Term  | Definition                                                |
   | Class | A class is a set of one or more PHBs.  If a class         |
   |       | consists of a set of PHBs and these obey to an ordering   |
   |       | constraint.  In that sense, a class is a single AF class  |
   |       | (e.g.  AF4 consisting of AF41, AF41 and AF43) [RFC2597].  |
   |       | A class is a PHB group [RFC2575] and a PHB scheduling     |
   |       | class [RFC3260].  On IP level all DSCPs sharing the same  |
   |       | IP precedence value belong to a single class.  A class    |
   |       | may consist of one or more PHBs.  A single class uses     |
   |       | forwarding resources, which are independent of the        |
   |       | forwarding rseources of any other class.  Different       |
   |       | classes must not be aggregated.                           |
   | PHB   | A single Per Hop Behaviour [RFC2575] is identified by a   |
   |       | single DSCP on IP layer.                                  |

                                  Table 1

   Many DiffServ related RFCs introduce new terminolgy duplicating the
   existing one.  The above references are incomplete and refer to the
   early DiffServ RFCs only.  Stopping terminology duplication may
   simplify discussion.

   The following current practice issues relate to the concept of the
   DiffServ interconnection class propsal rather than to terminology.
   They serve as additional motivation of this activity:

   o  Abstract class names like "EF" are preferrential over those being
      close to an application, like "Voice".  Unfortunately, non QoS
      experts can't handle abstract class names.  Hence and usually
      sooner than later, classes are named for applications or groups of
      them.  One consequence however is, that people tend to combine
      application group class names and SLA parameters.  Based on an

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      application specific name and some worst case performance numbers
      on a paper, they often decide that their application needs a
      separate new QoS class.

   o  Worse than that, but very present in practice, is the class
      abstraction level which is preferred by those dealing with QoS (as
      experts or non experts): the DSCPs or the IP precedence values.
      These are the commodity abstractions applied for QoS classes.
      Most of these persons have fixed class to codepoint mappings in
      their minds, which they can't easily adapt on a per customer or
      interconnection partner basis.

   While these issues aren't to be solved by IETF (QoS experts could and
   should of course teach staff to use proper Diffserv terminology and
   concepts), a simple and comprehensible QoS interconnection class
   scheme also is helpful in this area.

3.  An Interconnection class and codepoint scheme

   DiffServ deployments mostly follow loose class specification schemes
   (often one or two AF classes, EF and Best Effort).  Especially DSCP
   assignment for the AF classes varies between deployments.  Basic AF
   class definitions are often similar however.  This is in line with
   the DiffServ architecture.  This document doesn't propose to change

   Interconnecting parties face the problem of matching classes to be
   interconnected and then to agree on codepoint mapping.  As stated by
   draft diffserv prolebm statement
   [I-D.polk-tsvwg-diffserv-stds-problem-statement], remarking is a
   standard behaviour at interconnection interfaces.  This draft propses
   a set of 4 QoS classes with a set of well defined DSCPs and IP-
   Precedence values as interconnection class and codepoint scheme.  A
   sending party remarks DSCPs from internal schemes to the
   Interconnection codepoints.  The receiving party remarks IP-
   Precedence and or DSCPs to their internal scheme.  Thus the
   interconnection codepoint scheme fully complies with the DiffServ
   architecture.  Such an interconnection class and codepoint scheme was
   introduced by ITU-T [Y.1566] (there also includuing Ethernet).  It is
   specified to a higher level of detail in this document.

   At first glance, this looks like an additional effort.  But there are
   obvious benefits: each party sending or receiving traffic has to
   specify the mapping from or to the interconnection class and
   codepoint scheme only once.  Without it, this is to be negotiated per
   interconnection party individually.  Further, end-to-end QoS in terms
   of traffic being classified for the same class in all passed domains

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   is likely to result if an interconnection codepoint scheme is used.
   It is not necessarily resulting from individual per network mapping

   The standards and deployments known to the author of this draft are
   limited to 4 DiffServ classes at interconnection points (or
   less).Draft RFC 4597 update [I-D.polk-tsvwg-rfc4594-update]doesn't
   seem to generally contradict to this, as it proposes to standardise
   "many services classes, not all will be used in each network at any
   period of time."  Some more good reasons fovour working with 4
   DiffServ interconnection classes for now:

   o  There should be a coding reserve for interconnection classes,
      leaving space for future standards, for bilateral agreements and
      for carrier internal classes.

   o  MPLS and Ethernet support only 8 PHBs, classes or ECN indications.
      Assignment of codepoints for whatever purpose must be well thought
      through.  Limiting interconnection QoS to four classes is MPLS and
      Ethernet friendly in that sense.

   o  Migrations from one codepoint scheme to another may require spare
      QoS codepoints.

4.  Consolidation of QoS standards by the interconnection codepoint

   The interconnection class and codepoint scheme proposed by Y.1566
   also tries to consolidate related DiffServ and QoS standardisation
   efforts outside of the IETF [Y.1566].  The interconnection class and
   codepoint scheme may be a suitable approach to consolidate these
   standards.  MEF 23.1 specifies 3 aggregated classes, consuming up to
   5 codepoints on Ethernet layer (EF, AF3 and AF1 and Best Effort) and
   6 PHBs [MEF23.1].  MEF aggregates AF1 and Default PHB in a single
   class.  This is not recommended for interconnection, as it is not in
   line with RFC 2597 (which requires separate forwarding resources for
   each AF class and doesn't forsee aggregation of Default PHB and an AF

   GSMA IR.34 proposes four classes, EF, AF4, another AF class and Best
   Effort with 7 PHBs in sum [IR.34].  IR.34 specifies an "Interactive"
   class consisting of 3 PHBs with fifferent drop priorities.  IR.34
   specfies the PHBS AF31, AF21 and AF11 for this Interactive class.
   This definitely breaks RFC 2597.  The interconnection class and
   codepoint scheme supports the Interactive class but assigns AF3 with
   PHBs AF31, AF32 and AF33.

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   The classes to be supported at interconnection interfaces are
   specified by Y.1566 as:

   | Class     | Properties                                            |
   | Priority  | EF, expecting the figures of merit describing the PHB |
   |           | to be in the range of low single digit milliseconds.  |
   |           | See [RFC3246].                                        |
   | Bulk      | Traffic load in this class must be controlled, e.g.   |
   | inelastic | by application servers.  One example could be flow    |
   |           | admission control.  There may be infrequent           |
   |           | retransmissions requested by the application layer to |
   |           | mitigate low levels of packet losses.  Discard of     |
   |           | packets through active queue management should be     |
   |           | avoided in this class.  Congestion in this class may  |
   |           | result in bursty packet loss.  If used to carry       |
   |           | multimedia traffic, it is recommended to carry audio  |
   |           | and video traffic in a single PHB.  All of these      |
   |           | properties influence the buffer design.               |
   | Assured   | This class may be optimised to transport traffic      |
   |           | without bandwidth requirements.  Retransmissions      |
   |           | after losses characterise the class and influence the |
   |           | buffer design.  Active queue management with          |
   |           | probabilistic dropping may be deployed.               |
   | Default   | Default.  This class may be optimised to transport    |
   |           | traffic without bandwidth requirements.               |
   |           | Retransmissions after losses characterise the class   |
   |           | and influence the buffer design.  Active queue        |
   |           | management with probabilistic dropping may be         |
   |           | deployed.                                             |

                                  Table 2

   Note that other DiffServ related standards trim down class
   requirements to SLA parameters.  To quote e.g RFC 4594-update, "A
   "service class" represents a similar set of traffic characteristics
   for delay, loss, and jitter as packets traverse routers in a
   network."  This draft adds traffic conditioning properties
   corresponding to expected transport layer characteristics as a key
   factor to a class definition: the desired class performance like
   delay, jitter and worst case loss are met only if conditioning and
   and transport properties meet the ones described by the class
   definition.  This is not to say, the other standards ignore
   conditioner properties.  They are e.g. a core part of RFC 4594-
   update.  They do not directly refer to tranport protocol properties,
   as most existing QoS standards prefer the approach of assigning QoS

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   classes to applications or application sets.  This may result in
   undesirable class mappings, if an e.g.  IP TV application demanding
   low loss is matched to a class whose low loss guarantees depend on
   AQM mechanisms.

   Y.1566 does not recommend all PHBs to be supported at an
   interconnection interface.  This information is added by this draft.
   At interconnection points, the following PHBs should be accepted
   between interconnected parties:

   | Class          | PHBs                                            |
   | Priority       | EF                                              |
   | Bulk inelastic | AF41 (AF42 and AF43 are reserved for extension) |
   | Assured        | AF31, AF32 and AF33                             |
   | Default        | Default                                         |

                                  Table 3

   Class names (and property specification) are picked from Y.1566.
   PHBs to the level of detail introduced here are not part of Y.1566.

5.  MPLS, Ethernet and IP Precedence for aggregated classes

   IP Precedence has been depricated when DiffServ was standardised.
   Ethernet and MPLS support 3 bit codepoint fields to differntiate
   service quality.  Mapping of the IP precedence to these 3 Bit fields
   has been a configuration restriction in the early days of DiffServ.
   The concept of paying attention to the three most significant bits of
   a DSCP has however been part of Diffserv from start on (EF's IP
   Precedence is 5, that of AF4 is 4 and so on).  The interconnection
   class and codepoint scheme respects this in different ways:

   o  it allows to classify four interconnection classes based on IP

   o  It supports a single PHB group (AF3), which may be mapped to up to
      three different MPLS TC's or Ethernet P-Bits.  Note that this
      draft doesn's favour or recommend doing that, but it is possible.
      The author isn't aware of deployed service offers with 3 different
      drop levels in a single class.

   This is of course no requirement to depricate any DSCP to MPLS TC or
   Ethernet P-Bit mapping functionality.  This functionality is very
   important as well.

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6.  QoS class name selection

   This is more of an informational discussion, proposed best practice,
   and mainly relates to human behviour (including QoS experts) rather
   than technical issues.  Above the human preference for conceivable
   class names has been mentioned.  Network engineers (including the
   former Diffserv WG authors) recommend to avoid application related
   QoS class names.  Focus should be put on class properties.  But these
   can be irritating again, as just looking at SLA parameters like
   Delay, Jitter and packet loss don't tell the reader, which
   conditioning and transport properties guideed the class engineering
   assumptions resulted in the conditioning of a class.  A router
   produces QoS with a scheduling mechanism, a settable queue depth and
   optional active queue management (including ECN), and may be a
   policer.  Some kind of resource management may be present (also in
   Diffserv domains).  It's beyond the imagination of the author how one
   would engineer more than half a dozen classes with distinguishable
   properties with this set of tools.

   There's no perfect solution to the problem, as conditioning
   configurations are not comprehensible to most readers, even if they
   were communicated (they are operational secrets of course).  There
   are (or should be) engineering assumptions, when designing QoS
   conditioners.  But they closer relate to layer 3 or layer 4 level
   properties than to specific applications.  In general, an application
   responds to congestion by reducing traffic, or it ignores congestion.
   Active queue management doesn't help to avoid congestion in the
   latter case, only resource management does.  EF may be a special
   case.  If the EF traffic is not responsive to congestion, and packets
   are assumed to be short, rather small jitter values can be reached if
   enginieering ensures that the packet arrival rate never exceeds the
   transmision rate of that queue (see RFC 3246 [RFC3246]).  There's
   other non congestion-responsive traffic, for which the EF engineering
   assumptions may not fit.  So conditioning) like bulk inelastic is

   Active queue management may be deployed for QoS classes, which are
   designed to transport traffic responding to congestion by traffic

   The class names of this document follow Y.1566.  TCP_optimised and
   especially UDP_optimised are inappropriate as clas names, as some UDP
   based application are or may be expected to become TCP friendly.

7.  Allow for DiffServ extendability on MPLS and Ethernet level

   Any aggregated Diffserv deployment faces codepoint depletion issues

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   rather soon, if deployed on MPLS or Ethernet.  Coding space should be
   left for new features, like ECN, PCN or Conex.  In addition to
   carrying customer traffic, internal routing and network management
   traffic may be protected by using a separate class.  Offering
   interconnection with up to four classes and 4 - 6 MPLS TC's (or
   Ethernet P-bits) to that respect is probably at least a fair

8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   This document does not introduce new features, it describes how to
   use existing ones.  The security section of RFC 4597 [RFC4597]

10.  References

10.1.  Normative References

   [RFC2575]  Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based
              Access Control Model (VACM) for the Simple Network
              Management Protocol (SNMP)", RFC 2575, April 1999.

   [RFC2597]  Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
              "Assured Forwarding PHB Group", RFC 2597, 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, March 2002.

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

   [min_ref]  authSurName, authInitials., "Minimal Reference", 2006.

10.2.  Informative References

              Polk, J., "The Problem Statement for the Standard
              Configuration of DiffServ Service Classes",
              draft-polk-tsvwg-diffserv-stds-problem-statement-00 (work

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              in progress), July 2012.

              Polk, J., "Standard Configuration of DiffServ Service
              Classes", draft-polk-tsvwg-rfc4594-update-02 (work in
              progress), October 2012.

   [IR.34]    GSMA Association, "IR.34 Inter-Service Provider IP
              Backbone Guidelines Version 7.0", GSMA,  GSMA IR.34 http:/
              ir.34.pdf, 2012.

   [MEF23.1]  MEF, "Implementation Agreement MEF 23.1 Carrier Ethernet
              Class of Service Phase 2", MEF,  MEF23.1 http://
              technical-specifications/MEF_23.1.pdf, 2012.

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

   [RFC4597]  Even, R. and N. Ismail, "Conferencing Scenarios",
              RFC 4597, August 2006.

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

   [Y.1566]   ITU-T, "Quality of service mapping and interconnection
              between Ethernet, IP and multiprotocol label switching
              networks", ITU,
               http://www.itu.int/rec/T-REC-Y.1566-201207-I/en, 2012.

Appendix A.  Change log

   | Versions | Changes                                                |
   | 00 to 01 | Added terminology and references.  Added details and   |
   |          | information to interconnection class and codepoint     |
   |          | scheme.  Editorial changes                             |

                                  Table 4

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Author's Address

   Ruediger Geib (editor)
   Deutsche Telekom
   Heinrich Hertz Str. 3-7
   Darmstdadt,   64297

   Phone: +49 6151 5812747
   Email: Ruediger.Geib@telekom.de

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