rfc2597.txt

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Network Working Group                                        J. Heinanen
Request for Comments: 2597                                 Telia Finland
Category: Standards Track                                       F. Baker
                                                           Cisco Systems
                                                                W. Weiss
                                                     Lucent Technologies
                                                           J. Wroclawski
                                                                 MIT LCS
                                                               June 1999


                      Assured Forwarding PHB Group

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

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

Abstract

   This document defines a general use Differentiated Services (DS)
   [Blake] Per-Hop-Behavior (PHB) Group called Assured Forwarding (AF).
   The AF PHB group provides delivery of IP packets in four
   independently forwarded AF classes.  Within each AF class, an IP
   packet can be assigned one of three different levels of drop
   precedence.  A DS node does not reorder IP packets of the same
   microflow if they belong to the same AF class.

1. Purpose and Overview

   There is a demand to provide assured forwarding of IP packets over
   the Internet.  In a typical application, a company uses the Internet
   to interconnect its geographically distributed sites and wants an
   assurance that IP packets within this intranet are forwarded with
   high probability as long as the aggregate traffic from each site does
   not exceed the subscribed information rate (profile).  It is
   desirable that a site may exceed the subscribed profile with the
   understanding that the excess traffic is not delivered with as high
   probability as the traffic that is within the profile.  It is also





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RFC 2597              Assured Forwarding PHB Group             June 1999


   important that the network does not reorder packets that belong to
   the same microflow, as defined in [Nichols], no matter if they are in
   or out of the profile.

   Assured Forwarding (AF) PHB group is a means for a provider DS domain
   to offer different levels of forwarding assurances for IP packets
   received from a customer DS domain.  Four AF classes are defined,
   where each AF class is in each DS node allocated a certain amount of
   forwarding resources (buffer space and bandwidth). IP packets that
   wish to use the services provided by the AF PHB group are assigned by
   the customer or the provider DS domain into one or more of these AF
   classes according to the services that the customer has subscribed
   to. Further background about this capability and some ways to use it
   may be found in [Clark].

   Within each AF class IP packets are marked (again by the customer or
   the provider DS domain) with one of three possible drop precedence
   values.  In case of congestion, the drop precedence of a packet
   determines the relative importance of the packet within the AF class.
   A congested DS node tries to protect packets with a lower drop
   precedence value from being lost by preferably discarding packets
   with a higher drop precedence value.

   In a DS node, the level of forwarding assurance of an IP packet thus
   depends on (1) how much forwarding resources has been allocated to
   the AF class that the packet belongs to, (2) what is the current load
   of the AF class, and, in case of congestion within the class, (3)
   what is the drop precedence of the packet.

   For example, if traffic conditioning actions at the ingress of the
   provider DS domain make sure that an AF class in the DS nodes is only
   moderately loaded by packets with the lowest drop precedence value
   and is not overloaded by packets with the two lowest drop precedence
   values, then the AF class can offer a high level of forwarding
   assurance for packets that are within the subscribed profile (i.e.,
   marked with the lowest drop precedence value) and offer up to two
   lower levels of forwarding assurance for the excess traffic.

   This document describes the AF PHB group. An otherwise DS-compliant
   node is not required to implement this PHB group in order to be
   considered DS-compliant, but when a DS-compliant node is said to
   implement an AF PHB group, it must conform to the specification in
   this document.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [Bradner].




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RFC 2597              Assured Forwarding PHB Group             June 1999


2. The AF PHB Group

   Assured Forwarding (AF) PHB group provides forwarding of IP packets
   in N independent AF classes.  Within each AF class, an IP packet is
   assigned one of M different levels of drop precedence.  An IP packet
   that belongs to an AF class i and has drop precedence j is marked
   with the AF codepoint AFij, where 1 <= i <= N and 1 <= j <= M.
   Currently, four classes (N=4) with three levels of drop precedence in
   each class (M=3) are defined for general use.  More AF classes or
   levels of drop precedence MAY be defined for local use.

   A DS node SHOULD implement all four general use AF classes.  Packets
   in one AF class MUST be forwarded independently from packets in
   another AF class, i.e., a DS node MUST NOT aggregate two or more AF
   classes together.

   A DS node MUST allocate a configurable, minimum amount of forwarding
   resources (buffer space and bandwidth) to each implemented AF class.
   Each class SHOULD be serviced in a manner to achieve the configured
   service rate (bandwidth) over both small and large time scales.

   An AF class MAY also be configurable to receive more forwarding
   resources than the minimum when excess resources are available either
   from other AF classes or from other PHB groups.  This memo does not
   specify how the excess resources should be allocated, but
   implementations MUST specify what algorithms are actually supported
   and how they can be parameterized.

   Within an AF class, a DS node MUST NOT forward an IP packet with
   smaller probability if it contains a drop precedence value p than if
   it contains a drop precedence value q when p < q.  Note that this
   requirement can be fulfilled without needing to dequeue and discard
   already-queued packets.

   Within each AF class, a DS node MUST accept all three drop precedence
   codepoints and they MUST yield at least two different levels of loss
   probability.  In some networks, particularly in enterprise networks,
   where transient congestion is a rare and brief occurrence, it may be
   reasonable for a DS node to implement only two different levels of
   loss probability per AF class.  While this may suffice for some
   networks, three different levels of loss probability SHOULD be
   supported in DS domains where congestion is a common occurrence.

   If a DS node only implements two different levels of loss probability
   for an AF class x, the codepoint AFx1 MUST yield the lower loss
   probability and the codepoints AFx2 and AFx3 MUST yield the higher
   loss probability.




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RFC 2597              Assured Forwarding PHB Group             June 1999


   A DS node MUST NOT reorder AF packets of the same microflow when they
   belong to the same AF class regardless of their drop precedence.
   There are no quantifiable timing requirements (delay or delay
   variation) associated with the forwarding of AF packets.

   The relationship between AF classes and other PHBs is described in
   Section 7 of this memo.

   The AF PHB group MAY be used to implement both end-to-end and domain
   edge-to-domain edge services.

3. Traffic Conditioning Actions

   A DS domain MAY at the edge of a domain control the amount of AF
   traffic that enters or exits the domain at various levels of drop
   precedence.  Such traffic conditioning actions MAY include traffic
   shaping, discarding of packets, increasing or decreasing the drop
   precedence of packets, and reassigning of packets to other AF
   classes.  However, the traffic conditioning actions MUST NOT cause
   reordering of packets of the same microflow.

4. Queueing and Discard Behavior

   This section defines the queueing and discard behavior of the AF PHB
   group.  Other aspects of the PHB group's behavior are defined in
   Section 2.

   An AF implementation MUST attempt to minimize long-term congestion
   within each class, while allowing short-term congestion resulting
   from bursts. This requires an active queue management algorithm.  An
   example of such an algorithm is Random Early Drop (RED) [Floyd].
   This memo does not specify the use of a particular algorithm, but
   does require that several properties hold.

   An AF implementation MUST detect and respond to long-term congestion
   within each class by dropping packets, while handling short-term
   congestion (packet bursts) by queueing packets.  This implies the
   presence of a smoothing or filtering function that monitors the
   instantaneous congestion level and computes a smoothed congestion
   level.  The dropping algorithm uses this smoothed congestion level to
   determine when packets should be discarded.

   The dropping algorithm MUST be insensitive to the short-term traffic
   characteristics of the microflows using an AF class.  That is, flows
   with different short-term burst shapes but identical longer-term
   packet rates should have packets discarded with essentially equal
   probability.  One way to achieve this is to use randomness within the
   dropping function.



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RFC 2597              Assured Forwarding PHB Group             June 1999


   The dropping algorithm MUST treat all packets within a single class
   and precedence level identically.  This implies that for any given
   smoothed congestion level, the discard rate of a particular
   microflow's packets within a single precedence level will be
   proportional to that flow's percentage of the total amount of traffic
   passing through that precedence level.

   The congestion indication feedback to the end nodes, and thus the
   level of packet discard at each drop precedence in relation to
   congestion, MUST be gradual rather than abrupt, to allow the overall
   system to reach a stable operating point.  One way to do this (RED)
   uses two (configurable) smoothed congestion level thresholds.  When
   the smoothed congestion level is below the first threshold, no
   packets of the relevant precedence are discarded.  When the smoothed
   congestion level is between the first and the second threshold,
   packets are discarded with linearly increasing probability, ranging
   from zero to a configurable value reached just prior to the second
   threshold.  When the smoothed congestion level is above the second
   threshold, packets of the relevant precedence are discarded with 100%
   probability.

   To allow the AF PHB to be used in many different operating
   environments, the dropping algorithm control parameters MUST be
   independently configurable for each packet drop precedence and for
   each AF class.

   Within the limits above, this specification allows for a range of
   packet discard behaviors.  Inconsistent discard behaviors lead to
   inconsistent end-to-end service semantics and limit the range of
   possible uses of the AF PHB in a multi-vendor environment.  As
   experience is gained, future versions of this document may more
   tightly define specific aspects of the desirable behavior.

5. Tunneling

   When AF packets are tunneled, the PHB of the tunneling packet MUST
   NOT reduce the forwarding assurance of the tunneled AF packet nor
   cause reordering of AF packets belonging to the same microflow.













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RFC 2597              Assured Forwarding PHB Group             June 1999


6. Recommended Codepoints

   Recommended codepoints for the four general use AF classes are given
   below. These codepoints do not overlap with any other general use PHB
   groups.

   The RECOMMENDED values of the AF codepoints are as follows: AF11 = '
   001010', AF12 = '001100', AF13 = '001110', AF21 = '010010', AF22 = '
   010100', AF23 = '010110', AF31 = '011010', AF32 = '011100', AF33 = '
   011110', AF41 = '100010', AF42 = '100100', and AF43 = '100110'.  The
   table below summarizes the recommended AF codepoint values.

                        Class 1    Class 2    Class 3    Class 4
                      +----------+----------+----------+----------+
     Low Drop Prec    |  001010  |  010010  |  011010  |  100010  |
     Medium Drop Prec |  001100  |  010100  |  011100  |  100100  |
     High Drop Prec   |  001110  |  010110  |  011110  |  100110  |
                      +----------+----------+----------+----------+

7. Interactions with Other PHB Groups

   The AF codepoint mappings recommended above do not interfere with the
   local use spaces nor the Class Selector codepoints recommended in
   [Nichols].  The PHBs selected by those Class Selector codepoints may