RFC 1254 (rfc1254) - Page 2 of 25

Gateway Congestion Control Survey

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
   finite.  The nature of the internet protocol is to drop packets when
   these buffers overflow.




Performance and Congestion Control Working Group