A Two-bit Differentiated Services Architecture for the Internet
RFC 2638

Document Type RFC - Informational (July 1999; Errata)
Authors Lixia Zhang  , Van Jacobson  , Kathleen Nichols 
Last updated 2020-01-21
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Network Working Group                                          K. Nichols
Request for Comments: 2638                                    V. Jacobson
Category: Informational                                             Cisco
                                                                 L. Zhang
                                                                July 1999

    A Two-bit Differentiated Services Architecture for the Internet

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 (1999).  All Rights Reserved.


   This document was originally submitted as an internet draft in
   November of 1997. As one of the documents predating the formation of
   the IETF's Differentiated Services Working Group, many of the ideas
   presented here, in concert with Dave Clark's subsequent presentation
   to the December 1997 meeting of the IETF Integrated Services Working
   Group, were key to the work which led to RFCs 2474 and 2475 and the
   section on allocation remains a timely proposal. For this reason, and
   to provide a reference, it is being submitted in its original form.
   The forwarding path portion of this document is intended as a record
   of where we were at in late 1997 and not as an indication of future

   The postscript version of this document includes Clark's slides as an
   appendix. The postscript version of this document also includes many
   figures that aid greatly in its readability.

1. Introduction

   This document presents a differentiated services architecture for the
   internet. Dave Clark and Van Jacobson each presented work on
   differentiated services at the Munich IETF meeting [2,3]. Each
   explained how to use one bit of the IP header to deliver a new kind
   of service to packets in the internet. These were two very different
   kinds of service with quite different policy assumptions. Ensuing
   discussion has convinced us that both service types have merit and
   that both service types can be implemented with a set of very similar

Nichols, et al.              Informational                      [Page 1]
RFC 2638      Two-bit Differentiated Services Architecture     July 1999

   mechanisms. We propose an architectural framework that permits the
   use of both of these service types and exploits their similarities in
   forwarding path mechanisms. The major goals of this architecture are
   each shared with one or both of those two proposals: keep the
   forwarding path simple, push complexity to the edges of the network
   to the extent possible, provide a service that avoids assumptions
   about the type of traffic using it, employ an allocation policy that
   will be compatible with both long-term and short-term provisioning,
   make it possible for the dominant Internet traffic model to remain

   The major contributions of this document are to present two distinct
   service types, a set of general mechanisms for the forwarding path
   that can be used to implement a range of differentiated services and
   to propose a flexible framework for provisioning a differentiated
   services network. It is precisely this kind of architecture that is
   needed for expedient deployment of differentiated services: we need a
   framework and set of primitives that can be implemented in the
   short-term and provide interoperable services, yet can provide a
   "sandbox" for experimentation and elaboration that can lead in time
   to more levels of differentiation within each service as needed.

   At the risk of belaboring an analogy, we are motivated to provide
   services tiers in somewhat the same fashion as the airlines do with
   first class, business class and coach class. The latter also has
   tiering built in due to the various restrictions put on the purchase.
   A part of the analogy we want to stress is that best effort traffic,
   like coach class seats on an airplane, is still expected to make up
   the bulk of internet traffic. Business and first class carry a small
   number of passengers, but are quite important to the economics of the
   airline industry. The various economic forces and realities combine
   to dictate the relative allocation of the seats and to try to fill
   the airplane. We don't expect that differentiated services will
   comprise all the traffic on the internet, but we do expect that new
   services will lead to a healthy economic and service environment.

   This document is organized into sections describing service
   architecture, mechanisms, the bandwidth allocation architecture, how
   this architecture might interoperate with RSVP/int-serv work, and
   gives recommendations for deployment.

Nichols, et al.              Informational                      [Page 2]
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