Specification of the Controlled-Load Network Element Service
RFC 2211

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Network Working Group                                      J. Wroclawski
Request For Comments: 2211                                       MIT LCS
Category: Standards Track                                 September 1997

      Specification of the Controlled-Load Network Element Service

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.

Abstract

   This memo specifies the network element behavior required to deliver
   Controlled-Load service in the Internet.  Controlled-load service
   provides the client data flow with a quality of service closely
   approximating the QoS that same flow would receive from an unloaded
   network element, but uses capacity (admission) control to assure that
   this service is received even when the network element is overloaded.

1. Introduction

   This document defines the requirements for network elements that
   support the Controlled-Load service.  This memo is one of a series of
   documents that specify the network element behavior required to
   support various qualities of service in IP internetworks.  Services
   described in these documents are useful both in the global Internet
   and private IP networks.

   This document is based on the service specification template given in
   [1]. Please refer to that document for definitions and additional
   information about the specification of qualities of service within
   the IP protocol family.

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RFC 2211                Controlled-Load Network           September 1997

2. End-to-End Behavior

   The end-to-end behavior provided to an application by a series of
   network elements providing controlled-load service tightly
   approximates the behavior visible to applications receiving best-
   effort service *under unloaded conditions* from the same series of
   network elements.  Assuming the network is functioning correctly,
   these applications may assume that:

     - A very high percentage of transmitted packets will be
     successfully delivered by the network to the receiving end-nodes.
     (The percentage of packets not successfully delivered must closely
     approximate the basic packet error rate of the transmission
     medium).

     - The transit delay experienced by a very high percentage of the
     delivered packets will not greatly exceed the minimum transmit
     delay experienced by any successfully delivered packet. (This
     minimum transit delay includes speed-of-light delay plus the fixed
     processing time in routers and other communications devices along
     the path.)

   To ensure that these conditions are met, clients requesting
   controlled-load service provide the intermediate network elements
   with a estimation of the data traffic they will generate; the TSpec.
   In return, the service ensures that network element resources
   adequate to process traffic falling within this descriptive envelope
   will be available to the client. Should the client's traffic
   generation properties fall outside of the region described by the
   TSpec parameters, the QoS provided to the client may exhibit
   characteristics indicative of overload, including large numbers of
   delayed or dropped packets. The service definition does not require
   that the precise characteristics of this overload behavior match
   those which would be received by a best-effort data flow traversing
   the same path under overloaded conditions.

      NOTE: In this memo, the term "unloaded" is used in the sense of
      "not heavily loaded or congested" rather than in the sense of "no
      other network traffic whatsoever".

3. Motivation

   The controlled load service is intended to support a broad class of
   applications which have been developed for use in today's Internet,
   but are highly sensitive to overloaded conditions.  Important members
   of this class are the "adaptive real-time applications" currently

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RFC 2211                Controlled-Load Network           September 1997

   offered by a number of vendors and researchers. These applications
   have been shown to work well on unloaded nets, but to degrade quickly
   under overloaded conditions. A service which mimics unloaded nets
   serves these applications well.

   The controlled-load service is intentionally minimal, in that there
   are no optional functions or capabilities in the specification. The
   service offers only a single function, and system and application
   designers can assume that all implementations will be identical in
   this respect.