Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers
RFC 2474

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Network Working Group                                         K. Nichols
Request for Comments: 2474                                 Cisco Systems
Obsoletes: 1455, 1349                                           S. Blake
Category: Standards Track                Torrent Networking Technologies
                                                                F. Baker
                                                           Cisco Systems
                                                                D. Black
                                                         EMC Corporation
                                                           December 1998

    Definition of the Differentiated Services Field (DS Field)
                      in the IPv4 and IPv6 Headers

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 (1998).  All Rights Reserved.

Abstract

   Differentiated services enhancements to the Internet protocol are
   intended to enable scalable service discrimination in the Internet
   without the need for per-flow state and signaling at every hop.  A
   variety of services may be built from a small, well-defined set of
   building blocks which are deployed in network nodes.  The services
   may be either end-to-end or intra-domain; they include both those
   that can satisfy quantitative performance requirements (e.g., peak
   bandwidth) and those based on relative performance (e.g., "class"
   differentiation).  Services can be constructed by a combination of:

   - setting bits in an IP header field at network boundaries
     (autonomous system boundaries, internal administrative boundaries,
     or hosts),
   - using those bits to determine how packets are forwarded by the
     nodes inside the network, and
   - conditioning the marked packets at network boundaries in accordance
     with the requirements or rules of each service.

Nichols, et. al.            Standards Track                     [Page 1]
RFC 2474             Differentiated Services Field         December 1998

   The requirements or rules of each service must be set through
   administrative policy mechanisms which are outside the scope of this
   document.  A differentiated services-compliant network node includes
   a classifier that selects packets based on the value of the DS field,
   along with buffer management and packet scheduling mechanisms capable
   of delivering the specific packet forwarding treatment indicated by
   the DS field value.  Setting of the DS field and conditioning of the
   temporal behavior of marked packets need only be performed at network
   boundaries and may vary in complexity.

   This document defines the IP header field, called the DS (for
   differentiated services) field.  In IPv4, it defines the layout of
   the TOS octet; in IPv6, the Traffic Class octet.  In addition, a base
   set of packet forwarding treatments, or per-hop behaviors, is
   defined.

   For a more complete understanding of differentiated services, see
   also the differentiated services architecture [ARCH].

Table of Contents

   1.  Introduction .................................................  3
   2.  Terminology Used in This Document ............................  5
   3.  Differentiated Services Field Definition .....................  7
   4.  Historical Codepoint Definitions and PHB Requirements ........  9
     4.1  A Default PHB .............................................  9
     4.2  Once and Future IP Precedence Field Use ................... 10
       4.2.1  IP Precedence History and Evolution in Brief .......... 10
       4.2.2  Subsuming IP Precedence into Class Selector  .......... 11
              Codepoints
         4.2.2.1  The Class Selector Codepoints ..................... 11
         4.2.2.2  The Class Selector PHB Requirements ............... 11
         4.2.2.3  Using the Class Selector PHB Requirements ......... 12
                  for IP Precedence Compatibility
         4.2.2.4  Example Mechanisms for Implementing Class ......... 12
                  Selector Compliant PHB Groups
     4.3  Summary ................................................... 13
   5.  Per-Hop Behavior Standardization Guidelines .................. 13
   6.  IANA Considerations .......................................... 14
   7.  Security Considerations ...................................... 15
     7.1  Theft and Denial of Service ............................... 15
     7.2  IPsec and Tunneling Interactions .......................... 16
   8.  Acknowledgments .............................................. 17
   9.  References ................................................... 17