RFC 1254 (rfc1254) - Page 2 of 25
Gateway Congestion Control Survey
Alternative Format: Original Text Document
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