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

                                                         January, 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). 2002).  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". 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 subsequent to the
   publication of RFC 2474, they were assigned an
experimental use for an explicit congestion notification scheme
   notification, as defined in RFC 3168 [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 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 superseded 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 2474, as clarified in the CU field are
allocated following a Standards Action process. section 8 of this

   The consensus of the DiffServ working group is that [5] BCP0037 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.

   When RFC 2474 is updated, consideration should be given to changing
   the designation "currently unused (CU)" to "explicit congestion
   notification (ECN)" and referencing RFC 3168 (or its successor).

   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 1247.  These were
   never widely deployed. deployed and thus removed by standards action when STD
   0054 was published.  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

   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 aggregate.

      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 Ramakrishnan, Floyd, "A proposal to add Black "The Addition of Explicit Congestion
        Notification (ECN)" (ECN) to IP, IP", RFC 2481, January 1999 3168, September 2001

   [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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