RFC 2474 (rfc2474) - Page 2 of 20

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

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
   Authors' Addresses ............................................... 19
   Full Copyright Statement ......................................... 20





Nichols, et. al.            Standards Track