RFC 981 (rfc981) - Page 2 of 22
Experimental multiple-path routing algorithm
Alternative Format: Original Text Document
RFC 981 March 1986
An Experimental Multiple-Path Routing Algorithm
virtual-circuit and datagram modes. The prototype implementation
provides primary and alternate routes, can route around congested
areas and can change routes during a connection. This document
describes the design, implementation and initial testing of the
algorithm.
1. Introduction
This document describes the design, implementation and initial
testing of the Wiretap Algorithm, a dynamic routing algorithm for the
AX.25 packet-radio channel [4]. The AX.25 channel operates in CSMA
contention mode at VHF frequencies using AFSK/FM modulation at 1200
bps. The AX.25 protocol itself is similar to X.25 link-layer protocol
LAPB, but with an extended frame header consisting of a string of
radio callsigns representing a path, usually selected by the
operator, between two end stations, possibly via one or more
intermediate packet repeaters or digipeaters. Most stations can
operate simultaneously as intermediate systems digipeaters) and as
end systems with respect to the ISO model.
Wiretap uses passive monitoring of frames transmitted on the channel
in order to build a dynamic data base which can be used to determine
optimum routes. The algorithm operates in real time and generates a
set of paths ordered by increasing total distance, as determined by a
shortest-path-first procedure similar to that used now in the ARPANET
and planned for use in the new Internet gateway system [2]. The
implementation provides optimum routes (with respect to the factors
and weights selected) at initial-connection time for virtual
circuits, as well as for each datagram transmission. This document
is an initial status report and overview of the prototype
implementation for the LSI-11 processor running the "fuzzball"
operating system.
The principal advantage in the use of routing algorithms like Wiretap
is that digipeater paths can be avoided when direct paths are
available, with digipeaters used only when necessary and also to
discover hidden stations. In the present exploratory stage of
evolution, the scope of Wiretap has been intentionally restricted to
passive monitoring. In a later stage the scope may be extended to
include the use of active probes to discover hidden stations and the
use of clustering techniques to manage the distribution of large
quantities of routing information.
The AX.25 channel interface is the 6809-based TAPR-1 processor
running the WA8DED firmware (version 1.0) and connected to the LSI-11
by a 4800-bps serial line. The WA8DED firmware produces as an option
a monitor report for each received frame of a selected type,
Mills