Diffserv Working Group                                   Dan Grossman
Internet Draft                                           Motorola, Inc.
Expires: June, 2002

                                                         December, 2001

            New Terminology and Clarifications for Diffserv

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
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   This memo captures Diffserv working group agreements concerning new
   and improved terminology, and also provides minor technical
   clarifications.  It is intended to update RFC 2474, RFC 2475 and RFC
   2597.   When RFCs 2474 and 2597 advance on the standards track, and
   RFC 2475 is updated, it is intended that the revisions in this memo
   will be incorporated, and that this memo will be obsoleted by the new

Copyright Notice

   Copyright (C) The Internet Society (1999, 2001).  All Rights

1.  Introduction

   As the Diffserv work has evolved, there have been several cases where
   terminology has needed to be created or the definitions in Diffserv
   standards track RFCs have needed to be refined.  Some minor technical
   clarifications were also found to be needed.   This memo was created
   to capture group agreements, rather than attempting to revise the
   base RFCs and recycle them at proposed standard.  It updates in part
   RFC 2474, RFC 2475 and RFC 2597.  RFC 2598 has been updated by RFC
   XXXX (draft-ietf-diffserv-rfc2598bis), and clarifications agreed by
   the group were incorporated in that update.

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

   As work progressed in Diffserv (as well as in the Policy WG [6]), 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 Specification (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.

   Also note that the definition of a "Service Provisioning Policy" is
   unchanged from RFC 2475.  RFC 2475 defines a "Service Provisioning
   Policy as "a policy which defines how traffic conditioners are
   configured on DS boundary nodes and how traffic streams are mapped to
   DS behavior aggregates to achieve a range of services."  According to
   one definition given in RFC 3198 [6], a policy is "...a set of rules
   to administer, manage, and control access to network resources".
   Therefore, the relationship between an SLS and a service provisioning
   policy is that the latter is, in part, the set of rules that express
   the parameters and range of values that may be in the former.

   Further note that this definition is more restrictive than that in
   RFC 3198.

3. Usage of PHB Group

   RFC 2475 defines a Per-hop behavior (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."

One standards track PHB Group is defined in RFC 2597 [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
Diffserv Codepoints (DSCPs).

RFC 2597 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 2597 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.

4. 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 "Currently Unused" (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

The present text is also inconsistent with BCP0037, IANA Allocation
Guidelines for Values in the Internet Protocol and Related Headers [5].
The IPV4 Type-of-Service (TOS) field and the IPV6 traffic class field
are superceded by the 6 bit DS field and a 2 bit CU field.  The IANA
allocates values in the DS 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 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 BCP0037.

5. Ordered Aggregates and PHB Scheduling Classes

   Work on Diffserv support by MPLS Label Switched Routers (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 Behavior
   Agrregeate (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 at least those packets belonging to the same
     microflow must be preserved.

     Ordered Aggregate (OA):  A set of Behavior Aggregates that share an
     ordering constraint.  The set of PHBs that are applied to this set
     of Behavior Aggregates constitutes a PHB scheduling class.

6. Unknown/Improperly Mapped DSCPs

   Several implementors have pointed out ambiguities or conflicts in the
   Diffserv RFCs concerning behavior when a DS-node recieves a packet
   with a DSCP which it does not understand.

    RFC 2475 states:
      "Ingress nodes must condition all other inbound traffic to ensure
     that the DS codepoints are acceptable; packets found to have
     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 Default PHB  codepoint [DSFIELD]."

   On the other hand, RFC 2474 states:
     "Packets received with an unrecognized codepoint SHOULD be
     forwarded as if they were marked for the Default behavior (see Sec.
     4), and their codepoints should not be changed."

   The intent in RFC 2474 principally concerned DS-interior nodes.
   However, this behavior could also be performed in DS-ingress nodes
   AFTER the traffic conditioning required by RFC 2475 (in which case,
   an unrecognized DSCP would occur only in the case of
   misconfiguration).   If a packet arrives with a DSCP that hadn't been
   explicitly mapped to a particular  PHB, it should be treated the same
   way as a packet marked for Default. The alternatives were to assign
   it another PHB, which could result in misallocation of provisioned
   resources, or to drop it.  Those are the only alternatives within the
   framework of 2474. Neither alternative was considered desirable.
   There has been discussion of a PHB which receives worse service than
   the default; this might be a better alternative.   Hence the
   imperative was "SHOULD" rather than "SHALL".

   The intent in RFC 2475 clearly concerns DS-ingress nodes, or to be
   more precise, the ingress traffic conditioning function.  This is
   another context where the "SHOULD" in RFC 2474 gives the flexibility
   to do what the group intended.  Such tortured readings are not

   Therefore, the statement in RFC 2474 will be clarified to indicate
   that it is not intended to apply at the ingress traffic conditioning
   function at a DS-ingress node, and cross reference RFC 2475 for that

   There was a similar issue, which manifested itself with the first
   incarnation of Expedited Forwarding (EF). RFC 2598 states:
     To protect itself against denial of service attacks, the edge of a
     DS domain MUST strictly police all EF marked packets to a rate
     negotiated with the adjacent upstream domain.  (This rate must be
     <= the EF PHB configured rate.)  Packets in excess of the
     negotiated rate MUST be dropped.  If two adjacent domains have not
     negotiated an EF rate, the downstream domain MUST use 0 as the rate
     (i.e., drop all EF marked packets).

   The problem arose in the case of  misconfiguration or routing
   problems.   An egress DS-node at the edge of one DS-domain forwards
   packets to an ingress DS-node at the edge of another DS domain.
   These packets are marked with a DSCP that the egress node understands
   to map to EF, but which the ingress node does not recognize.  The
   statement in RFC 2475 would appear to apply to this case.  RFC XXXX
   [7] (draft-ietf-diffserv-rfc2598bis) clarifies this point.

7. No Backward Compatibility With RFC 1349

   At least one implementor has expressed confusion about the
   relationship of the DSField, as defined in RFC 2474, to the use of
   the TOS bits, as described in RFC 1349.  The RFC 1349 useage was
   intended to interact with OSPF extensions in RFC 1247, which were
   never widely deployed.  The processing of the TOS bits is described
   as a requirement in RFC 1812 [8], RFC 1122 [9] and RFC 1123 [10].
   RFC 2474 states:
         "No attempt is made to maintain backwards compatibility with
      the "DTR"
         or TOS bits of the IPv4 TOS octet, as defined in [RFC791].",

   In addition, RFC 2474 obsoletes RFC 1349 by IESG action.  For
   completeness, when RFC 2474 is updated, the sentence should read:
      "No attempt is made to maintain backwards compatibility with the
      "DTR/MBZ" or TOS bits of the IPv4 TOS octet, as defined in
      [RFC791] and [RFC1349].  This implies that TOS bit processing as
      described in [RFC1812], [RFC1122] and [RFC1123] is also obsoleted
      by this memo.  Also see [RFC2780]."

8. IANA Considerations

   IANA has requested clarification of a point in RFC 2474, concerning
   registration of experimental/local use DSCPs.  When RFC 2474 is
   revised, the following should be added to Section 6:
      IANA is requested to maintain a registry of RECOMMENDED DSCP
      values assigned by standards action.  EXP/LU values are not to be

9. Summary of Pending Changes

   The following standards track and informational RFCs are expected to
   be updated to reflect the agreements captured in this memo.  It is
   intended that these updates occur when each standards track RFC
   progresses to Draft (or if some issue arises that forces recycling at
   Proposed).  RFC 2475 is expected to be updated at about the same time
   as RFC 2474.  Those updates will also obsolete this memo.

     RFC 2474: revise definition of DS field.  Clarify that the
     suggested default forwarding in the event of an unrecognized DSCP
     is not intended to apply to ingress conditioning in DS-ingress
     nodes.   Clarify effects on RFC1349 and RFC1812. Clarify that only
     RECOMMENDED DSCPs assigned by standards action are to be registered
     by IANA.

     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

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

In addition, RFCXXXX [7] (draft-ietf-diffserv-rfc2598bis) put a
reference to RFC 2475 in the security considerations section to cover
the case of a DS egress node receiving an unrecognized DSCP which maps
to EF in the DS ingress node.

10. Security Considerations

Security considerations are addressed in RFC 2475.


This memo captures agreements of the Diffserv working group.  Many
individuals contributed to the discussions on the Diffserv list and in
the meetings.  The Diffserv chairs were Brian Carpenter and Kathie
Nichols.   Among many who participated actively in these discussions
were Lloyd Wood, Juha Heinanen, Grenville Armitage, Scott Brim, Sharam
Davari, David Black, Gerard Gastaud, Joel Halpern, John Schnizlein,
Francois Le Faucheur, and Fred Baker.  Mike Ayers, Mike Heard and Andrea
Westerinen provided valuable editorial comments.

Normative References

   [1]  Nichols, Blake, Baker, Black, "Defintion of the Differentiated
        Services Field (DS Field) in the IPv4 and IPv6 Headers" RFC
        2474, December 1998.

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

   [3] Heinanen, Baker, Weiss, Wrocklawski, "Assured Forwarding PHB
        Group", RFC 2597

   [4] Ramakrishnan and Floyd, "A proposal to add Explicit Congestion
        Notification (ECN)" to IP, RFC 2481, January 1999

   [5] Bradner and Paxon, "IANA Allocation Guidelines for Values in the
        Internet Protocol and Related Headers", BCP0037/RFC2780, March

   [6] Westerinen et al, "Terminology for Policy Based Management", RFC

   [7] Davie et al, "An Expedited Forwarding PHB", RFC XXXX (ex-draft-

   [8] Baker, "Requirements for IP Version 4 Routers", RFC 1812

   [9] Braden, "Requirements for Internet Hosts -- Communications
        Layers", RFC1122/STD003

   [10] Braden, "Requirements for Internet Hosts -- Application and
        Support", RFC1123/STD003

Author's Address

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

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