Gateway Congestion Control Survey
RFC 1254

Document Type RFC - Informational (August 1991; No errata)
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Network Working Group                                          A. Mankin
Request for Comments: 1254                                         MITRE
                                                         K. Ramakrishnan
                                           Digital Equipment Corporation
                                                                 Editors
                                                             August 1991

                   Gateway Congestion Control Survey

Status of this Memo

   This memo provides information for the Internet community.  It is a
   survey of some of the major directions and issues.  It does not
   specify an Internet standard.  Distribution of this memo is
   unlimited.

Abstract

   The growth of network intensive Internet applications has made
   gateway congestion control a high priority.  The IETF Performance and
   Congestion Control Working Group surveyed and reviewed gateway
   congestion control and avoidance approaches.  The purpose of this
   paper is to present our review of the congestion control approaches,
   as a way of encouraging new discussion and experimentation.  Included
   in the survey are Source Quench, Random Drop, Congestion Indication
   (DEC Bit), and Fair Queueing.  The task remains for Internet
   implementors to determine and agree on the most effective mechanisms
   for controlling gateway congestion.

1.  Introduction

   Internet users regularly encounter congestion, often in mild forms.
   However, severe congestion episodes have been reported also; and
   gateway congestion remains an obstacle for Internet applications such
   as scientific supercomputing data transfer.  The need for Internet
   congestion control originally became apparent during several periods
   of 1986 and 1987, when the Internet experienced the "congestion
   collapse" condition predicted by Nagle [Nag84].  A large number of
   widely dispersed Internet sites experienced simultaneous slowdown or
   cessation of networking services for prolonged periods.  BBN, the
   firm responsible for maintaining the then backbone of the Internet,
   the ARPANET, responded to the collapse by adding link capacity
   [Gar87].

   Much of the Internet now uses as a transmission backbone the National
   Science Foundation Network (NSFNET). Extensive monitoring and
   capacity planning are being done for the NSFNET backbone; still, as

Performance and Congestion Control Working Group                [Page 1]
RFC 1254           Gateway Congestion Control Survey         August 1991

   the demand for this capacity grows, and as resource-intensive
   applications such as wide-area file system management [Sp89]
   increasingly use the backbone, effective congestion control policies
   will be a critical requirement.

   Only a few mechanisms currently exist in Internet hosts and gateways
   to avoid or control congestion.  The mechanisms for handling
   congestion set forth in the specifications for the DoD Internet
   protocols are limited to:

      Window flow control in TCP [Pos81b], intended primarily for
      controlling the demand on the receiver's capacity, both in terms
      of processing and buffers.

      Source quench in ICMP, the message sent by IP to request that a
      sender throttle back [Pos81a].

   One approach to enhancing Internet congestion control has been to
   overlay the simple existing mechanisms in TCP and ICMP with more
   powerful ones.  Since 1987, the TCP congestion control policy, Slow-
   start, a collection of several algorithms developed by Van Jacobson
   and Mike Karels [Jac88], has been widely adopted. Successful Internet
   experiences with Slow-start led to the Host Requirements RFC [HREQ89]
   classifying the algorithms as mandatory for TCP.  Slow-start modifies
   the user's demand when congestion reaches such a point that packets
   are dropped at the gateway.  By the time such overflows occur, the
   gateway is congested.  Jacobson writes that the Slow-start policy is
   intended to function best with a complementary gateway policy
   [Jac88].

1.1  Definitions

   The characteristics of the Internet that we are interested in include
   that it is, in general, an arbitrary mesh-connected network.  The
   internetwork protocol is connectionless.  The number of users that
   place demands on the network is not limited by any explicit
   mechanism; no reservation of resources occurs and transport layer
   set-ups are not disallowed due to lack of resources.  A path from a
   source to destination host may have multiple hops, through several
   gateways and links.  Paths through the Internet may be heterogeneous
   (though homogeneous paths also exist and experience congestion).
   That is, links may be of different speeds.  Also, the gateways and
   hosts may be of different speeds or may be providing only a part of
   their processing power to communication-related activity.  The
   buffers for storing information flowing through Internet gateways are
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