Routing
Copyright (C) 1987, Charles L. Hedrick. Anyone may reproduce this
document, in whole or in part, provided that: (1) any copy or
republication of the entire document must show Rutgers University as
the source, and must include this notice; and (2) any other use of
this material must reference this manual and Rutgers University, and
the fact that the material is copyright by Charles Hedrick and is used
by permission.
The description above indicated that the IP implementation is
responsible for getting datagrams to the destination indicated by the
destination address, but little was said about how this would be done.
The task of finding how to get a datagram to its destination is
referred to as "routing". In fact many of the details depend upon the
particular implementation. However some general things can be said.
First, it is necessary to understand the model on which IP is based.
IP assumes that a system is attached to some local network. We assume
that the system can send datagrams to any other system on its own
network. (In the case of Ethernet, it simply finds the Ethernet
address of the destination system, and puts the datagram out on the
Ethernet.) The problem comes when a system is asked to send a
datagram to a system on a different network. This problem is handled
by gateways. A gateway is a system that connects a network with one
or more other networks. Gateways are often normal computers that
happen to have more than one network interface. For example, we have
a Unix machine that has two different Ethernet interfaces. Thus it is
connected to networks 128.6.4 and 128.6.3. This machine can act as a
gateway between those two networks. The software on that machine must
be set up so that it will forward datagrams from one network to the
other. That is, if a machine on network 128.6.4 sends a datagram to
the gateway, and the datagram is addressed to a machine on network
128.6.3, the gateway will forward the datagram to the destination.
Major communications centers often have gateways that connect a number
of different networks. (In many cases, special-purpose gateway
systems provide better performance or reliability than general-purpose
systems acting as gateways. A number of vendors sell such systems.)
Routing in IP is based entirely upon the network number of the
destination address. Each computer has a table of network numbers.
For each network number, a gateway is listed. This is the gateway to
be used to get to that network. Note that the gateway doesn't have to
connect directly to the network. It just has to be the best place to
go to get there. For example at Rutgers, our interface to NSFnet is
at the John von Neuman Supercomputer Center (JvNC). Our connection to
JvNC is via a high-speed serial line connected to a gateway whose
address is 128.6.3.12. Systems on net 128.6.3 will list 128.6.3.12 as
the gateway for many off-campus networks. However systems on net
128.6.4 will list 128.6.4.1 as the gateway to those same off-campus
networks. 128.6.4.1 is the gateway between networks 128.6.4 and
128.6.3, so it is the first step in getting to JvNC.
When a computer wants to send a datagram, it first checks to see if
the destination address is on the system's own local network. If so,
the datagram can be sent directly. Otherwise, the system expects to
find an entry for the network that the destination address is on. The
datagram is sent to the gateway listed in that entry. This table can
get quite big. For example, the Internet now includes several hundred
individual networks. Thus various strategies have been developed to
reduce the size of the routing table. One strategy is to depend upon
"default routes". Often, there is only one gateway out of a network.
This gateway might connect a local Ethernet to a campus-wide backbone
network. In that case, we don't need to have a separate entry for
every network in the world. We simply define that gateway as a
"default". When no specific route is found for a datagram, the
datagram is sent to the default gateway. A default gateway can even
be used when there are several gateways on a network. There are
provisions for gateways to send a message saying "I'm not the best
gateway -- use this one instead." (The message is sent via ICMP. See
RFC 792.) Most network software is designed to use these messages to
add entries to their routing tables. Suppose network 128.6.4 has two
gateways, 128.6.4.59 and 128.6.4.1. 128.6.4.59 leads to several other
internal Rutgers networks. 128.6.4.1 leads indirectly to the NSFnet.
Suppose we set 128.6.4.59 as a default gateway, and have no other
routing table entries. Now what happens when we need to send a
datagram to MIT? MIT is network 18. Since we have no entry for
network 18, the datagram will be sent to the default, 128.6.4.59. As
it happens, this gateway is the wrong one. So it will forward the
datagram to 128.6.4.1. But it will also send back an error saying in
effect: "to get to network 18, use 128.6.4.1". Our software will then
add an entry to the routing table. Any future datagrams to MIT will
then go directly to 128.6.4.1. (The error message is sent using the
ICMP protocol. The message type is called "ICMP redirect.")
Most IP experts recommend that individual computers should not try to
keep track of the entire network. Instead, they should start with
default gateways, and let the gateways tell them the routes, as just
described. However this doesn't say how the gateways should find out
about the routes. The gateways can't depend upon this strategy. They
have to have fairly complete routing tables. For this, some sort of
routing protocol is needed. A routing protocol is simply a technique
for the gateways to find each other, and keep up to date about the
best way to get to every network. RFC 1009 contains a review of
gateway design and routing. However rip.doc is probably a better
introduction to the subject. It contains some tutorial material, and
a detailed description of the most commonly-used routing protocol.
Back to the Index
Steven E. Newton /
<snewton@oac.hsc.uth.tmc.edu> / 1-20-94
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