Diffserv Working Group                                   Dan Grossman
Internet Draft                                           Motorola, Inc.
Expires: April 2000 draft-ietf-diffserv-new-terms-00.txt

                                                         October, 1999

                      New Terminology for Diffserv

Status of this Memo

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Abstract

   This memo captures Diffserv working group agreements concerning new
   and improved terminology. It is intended as a living document for use
   by the Diffserv working group, and especially for use of authors of
   Diffserv drafts.  It is expected that the terminology in this memo
   will be incorporated into the existing Diffserv RFCs when they are
   updated.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

1.  Introduction
   As the Diffserv work has evolved, there have been
   several cases where terminology has needed to be created or the
   definitions in [1] and [2] have needed to be refined.   This memo was
   created to capture and test group agreements on terminology, rather
   than attempting to revise the base RFCs and recycle them at proposed
   standard.  Diffserv authors are encouraged to use the new terminology
   whereever appropriate.

   [Author's note:  the following represents in part the Author's
   understanding of  the agreements.  However, in some cases, the Author
   found it necessary to elaborate or expand.  The Author has also
   polled the Diffserv chairs and incorporated their recollection into
   this memo.  Every attempt will be made to refine this memo based on
   comments from the group. No claim is made that the 00 version of
   this memo represents a group consensus.)

2. Terminology related to Service Level Agreements (SLAs)

   The Diffserv Architecture [2] uses the term "Service Level Agreement"
   (SLA) to describe the "service contract... that specifies the
   forwarding service a customer should receive".  The SLA may include
   traffic conditioning rules which (at least  in part) constitute a
   Traffic Conditioning Agreement (TCA).  A TCA is "an agreement
   specifying classifier rules and any corresponding traffic profiles
   and metering, marking, discarding and/or shaping rules which are to
   apply...."

   As work progressed in Diffserv, it came to be believed that the
   notion of an "agreement" implied considerations that were of a
   pricing, contractual or other  business nature, as well as those that
   were strictly technical.  There also  could be other technical
   considerations in such an agreement (e.g., service availability)
   which are not addressed by Diffserv.  It was therefore agreed that
   the notions of SLAs and TCAs would be taken to represent the broader
   context, and that new terminology would be used to describe those
   elements of service and traffic conditioning that are addressed by
   Diffserv.

     - A Service Level Specfication (SLS) is a set of parameters and
     their values which together define the service offered to a traffic
     stream by a DS domain.

     - A Traffic Conditioning Specification (TCS) is a set of parameters
     and their values which together specify a set of classfier rules
     and a traffic profile.  A TCS is an integral element of an SLS.

Note that the definition of "Traffic stream" is unchanged from RFC 2475.
A traffic stream can be an individual microflow or a group of microflows
(i.e., in a source or destination  DS domain) or  it can be a BA.  Thus,
an SLS may apply in the source or destination DS domain to a single
microflow or group of microflows, as well as to a BA in any DS domain.

2. Usage of PHB Group

RFC 2475 deines defines a PHB group to be:

     "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."

The first standards track PHB Group is defined in RFC 2497 [3] is entitled [3], "Assured
Forwarding PHB Group", Group".   Assured Forwarding (AF) is a type of forwarding
behavior with some assigned level of queuing resources and uses the
term three drop
precedences.  An AF PHB group consistently in discussing the set Group consists of three PHBs, and uses three
DSCPs.

RFC 2497 defines twelve DSCPs, corresponding to four independent AF PHBs.
However, this usage
classes.  The AF classes are referred to as AF1x, AF2x, AF3x, and AF4x
(where 'x' is not consistent with RFC 2475. 1, 2, or 3 to represent drop precedence).  Each AF class
is one instance of an AF PHB Group.

There has been confusion expressed that RFC 2497 refers to all four AF
classes with their three drop precedences as being part of a single  PHB
Group. However, since each AF  class operates entirely independently of
the others, (and thus there is no common constraint which applies to BAs having different AF classes.  Indeed,
packets having different among AF classes must as
there is among drop precedences within an AF class) this usage is
inconsistent with RFC 2475.   The inconsistency exists  for historical
reasons and will be forwarded independently.
Therefore,  each removed in future revisions of the four AF classes constitutes specification.
It should  now be understood that AF is a separate _type_ of PHB group, and each having three PHBs corresponding to three drop precedences.

A new definition
AF class is thus needed an _instance_ of the AF type.

Authors of new PHB specifications should be careful to describe a set adhere to the RFC
2475 definition of related PHB groups. Group. RFC 2475 does not prohibit new PHB Group Family: a set
specifications from assigning enough DSCPs to represent multiple
independent instances of two or more PHB groups which are
     specified together and have similar relationships among their
     constituent PHBs, but which lack any common constraint.  A PHB
     group family provides a service building block that allows Group. However, such a set of
     related PHB groups to DSCPs
must not be specified together (e.g., three classes of referred to as a single PHB groups). Group.

3. Definition of the DS Field Diffserv uses six bits of the IPV4 or IPV6
header to convey the Diffserv Codepoint (DSCP), which selects a PHB.
RFC 2474 attempts to rename the TOS octet of the IPV4 header, and
Traffic Class octet of the IPV6 header, respectively, to the DS field.
The DS Field has a six bit Diffserv Codepoint and two "currently unused
bits".

Several participants in the Diffserv working group have

It has been pointed out that this leads to inconsistencies. inconsistencies and
ambiguities.  In particular, the CU bits of the DS Field have not been
assigned to Diffserv (and in fact are being used by
RFC 2481 [] Diffserv, and have been assigned an experimental use for an
explicit congestion notification).   A notification scheme [4].   In the current text, a
DSCP is, depending on context, either an encoding which selects a PHB or
a sub-
field sub-field in the DS field which contains that encoding.

[Author's note:  there was no

The present text is also inconsistent with the IANA allocation
guidelines draft [5].  In that draft, the IPV4 TOS field and the IPV6
traffic class field are superceded by the 6 bit DS field and a 2 bit CU
field.  The IANA alloctes values in the DE field following the IANA
considerations section in RFC 2474.  Experimental uses of the CU field

are assigned after IESG approval processes.  Permanent values in the CU
field are allocated following a Standards Action process.

The consensus of the DiffServ working group consensus on this subject.
This is my attempt at an intellectually satisfying solution, albeit one that will require readers to switch between two sets [5] correctly
restates the structure of terminology
until RFC 2474 can be updated]

For the former TOS and traffic class fields.

Therefore, for use in future drafts, including the next update to RFC
2474, the following definitions should apply:
     - the Differentiated Services Field (DSField) is the six most
     significant bits of either the (former) IPV4 TOS octet or the (former)
     IPV6 Traffic Class octet.

     - the Differentiated Services Codepoint (DSCP) 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.

The two least significant bits of the IPV4 TOS octet and the IPV6
Traffic Class octet are not presently used by Diffserv.

The update should also reference the IANA Allocation Guidelines,
assuming that they are published as an RFC.

4. Ordered aggregates and PHB scheduling classes

Work on Diffserv support by MPLS LSRs led to the realization that a
concept was needed in Diffserv to capture the notion of a set of BAs
with a common ordering constraint.  This presently applies to AF
behavior aggregates, since a DS node may not reorder packets of the same
microflow if they belong to the same AF class.  This would, for example,
prevent an MPLS LSR which was also a DS node from discriminating between
packets of an AF BA based on drop precedence and forwarding packets of
the same AF class but different drop precedence over different LSPs.
The following new terms are defined.

     PHB Scheduling Class: A PHB group for which a common constraint is
     that ordering of packets must be preserved

     Ordered Aggregate (OA):  A set of Behavior Aggregates that share an
     ordering constraint. All of the packets of an OA are members of the
     same PHB scheduling class.

5. Summary of pending changes The following standards track RFCs are
expected to be updated to reflect the agreements captured in this memo.
It is intended that these updates occur when each specification
progresses to Draft (or if some issue arises that forces recycling at

Proposed).

     RFC 2474: revise definition of DS field

     RFC 2475: revise definition of DS field.  Add SLS and TCS
     definitions.  Update body of document to use SLS and TCS
     appropriately.  Add definitions of PHB scheduling class and ordered
     aggregate.

     RFC 2497: revise to reflect understanding that AF classes are
     instances of the AF PHB group, and are not collectively a PHB
     group.

6. Security Considerations Security considerations are addressed in RFC
2475.

Acknowledgements

References

   [1]  RFC 2474.

   [2]  Blake, Black, Carlson, Davies, Wang and Weiss "An Architecture
        for Differentiated Services", RFC 2475, December 1998.

   [3] Heinanen and Guerin, "Assured Forwarding PHB Group", RFC 2497

   [4] RFC 2481

   [5] Bradner and Paxon, IANA Allocation Guidelines for Values in the
        Internet Protocol and Related Headers, draft-bradner-iana-
        allocation-02.txt, October 1999, work in progress

Author's Address

        Dan Grossman
        Motorola, Inc.
        20 Cabot Blvd.
        Mansfield, MA 02048
        Email: dan@dma.isg.mot.com

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