rfc3260.txt

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Network Working Group                                        D. Grossman
Request for Comments: 3260                                Motorola, Inc.
Updates: 2474, 2475, 2597                                     April 2002
Category: Informational


            New Terminology and Clarifications for Diffserv

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This memo captures Diffserv working group agreements concerning new
   and improved terminology, and 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
   RFCs.

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 obsoleted by RFC
   3246, and clarifications agreed by the group were incorporated in
   that revision.

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




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   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 (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
         classifier 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.







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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.




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   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 subsequent to the
   publication of RFC 2474, they were assigned for explicit congestion
   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 BCP 37, 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 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, as clarified in section 8 of this
   memo.

   The consensus of the DiffServ working group is that BCP 37 correctly
   restates the structure of the former TOS and traffic class fields.

   Therefore, for use in future documents, 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 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 BCP 37.

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



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   a DS node, from discriminating between packets of an AF Behavior
   Aggregate (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 receives 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."

   RFC 2474 is principally concerned with 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 RFC
   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".





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