[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 RFC 3260

Diffserv Working Group                                   Dan Grossman
Internet Draft                                           Motorola, Inc.
Expires: April 2000
draft-ietf-diffserv-new-terms-01.txt
                                                         October, 1999

                      New Terminology for Diffserv

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   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.''

     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/ietf/1id-abstracts.txt

     The list of Internet-Draft Shadow Directories can be accessed at
     http://www.ietf.org/shadow.html.

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

Grossman                                                        [Page 1]


draft-ietf-diffserv-new-terms-01.txt                        October 1999

   whereever appropriate.

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 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

Grossman                                                        [Page 2]


draft-ietf-diffserv-new-terms-01.txt                        October 1999

     special case of a PHB group."

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

RFC 2497 defines twelve DSCPs, corresponding to four independent AF
classes.  The AF classes are referred to as AF1x, AF2x, AF3x, and AF4x
(where 'x' is 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 among AF classes 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 removed in future revisions of the AF specification.
It should  now be understood that AF is a _type_ of PHB group, and each
AF class is an _instance_ of the AF type.

Authors of new PHB specifications should be careful to adhere to the RFC
2475 definition of PHB Group. RFC 2475 does not prohibit new PHB
specifications from assigning enough DSCPs to represent multiple
independent instances of their PHB Group. However, such a set of DSCPs
must not be referred to as a single PHB 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".

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

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

Grossman                                                        [Page 3]


draft-ietf-diffserv-new-terms-01.txt                        October 1999

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 is that [5] correctly
restates the structure of 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 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

Grossman                                                        [Page 4]


draft-ietf-diffserv-new-terms-01.txt                        October 1999

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

Full Copyright Statement

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

Grossman                                                        [Page 5]


draft-ietf-diffserv-new-terms-01.txt                        October 1999

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain
   itor assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Grossman                                                        [Page 6]


Html markup produced by rfcmarkup 1.129b, available from https://tools.ietf.org/tools/rfcmarkup/