All Categories :
Intranets
Chapter 4
How Intranet Routers Work
CONTENTS
Routers are the traffic cops of intranets. They make sure that
all data gets sent to where it's supposed to go and that it gets
sent via the most efficient route. Routers are also useful tools
to make the most efficient use of the intranet. Routers are used
to segment traffic and provide redundancy of routes. Routers use
encapsulation to permit different protocols to be sent across
otherwise incompatible networks.
When you sit down at your computer on an intranet and send or
receive data, that information generally must first go through
at least one router, and often more than one router before it
reaches its final destination. Routers can be simple or quite
sophisticated. Factors that determine the required complexity
of a router include the size of the intranet, the type and quantity
of traffic on segments, and security concerns of the intranet.
The more complex the intranet, and, in particular, the greater
number of possible destinations for data, the greater the need
for sophisticated router hardware and software.
Routers open the IP packet to read the destination address, calculate
the best route, and then send the packet toward the final destination.
If the destination is on the same part of an intranet, the packet
would be sent directly to the destination computer by the router.
If the packet is destined for another intranet or subnetwork (or
if the destination is on the Internet), the router considers factors
like traffic congestion and the number of hops-a term that
refers to the number of routers or gateways on any given path.
The IP packet carries with it a segment that holds the hop count
and a router will not use a path that would exceed a predefined
number of hops. Multiple routes within an acceptable hop count
range are desirable in intranets to provide redundancy and assure
that data can get through. For example, if a direct route between
San Francisco and New York were unavailable, sophisticated routers
would send data to New York via another router probably in another
city on the intranet-and this would all be transparent to the
users.
Routers have two or more physical ports: receiving (input) ports
and sending (output) ports. In actuality, every port is bi-directional
and can receive or send data. When a packet is received at an
input port, a software routine called a routing process is run.
This process looks inside the header information in the IP packet
and finds the address where the data is being sent. It then compares
this address against an internal database called a routing
table that has information detailing to which port packets
with various IP addresses should be sent. Based on what it finds
in the routing table, it sends the packet to a specific output
port. This output port then sends the data to the next router
or to the destination itself.
At times, packets are sent to a router's input port faster than
it can process them. When this happens, the packets are sent to
a special holding area called an input queue, an area of
RAM on the router. That specific input queue is associated with
a specific input port. A router can have more than one input queue,
if several input ports are being sent packets faster than the
router can process them. Each input port will process packets
from the queue in the order in which they were received.
If the traffic through the router is very heavy, the number of
packets in the queue can be greater than the capacity of the queue.
(The capacity of the queue is called the queue's length.)
When this happens, there is a possibility that packets may be
dropped and so will not be processed by the router, and won't
be sent to their destination. This doesn't mean, though, that
the information has be to lost. The TCP protocol was designed
to take into account that packets can be lost en route to their
final destination. If not all the packets are sent to the receiving
end, TCP at the receiving computer recognizes that and asks that
the missing packets be re-sent. It will keep requesting that the
packets be re-sent until they are all received. Sophisticated
routers can be managed and problems diagnosed and resolved using
special software, such as SNMP (Simple Network Management Protocol).
TCP can tell what actions to take because there are various flags
in the packet, like the hop count in IP, that tell TCP what it
needs to know to act. For example, the ack flag, set to
"on," indicates that it is responding to (acknowledging)
a previous communication.
A wide variety of routing hardware and software is available.
In some cases, a variety of different kinds of routing software
can be run on a given piece of hardware. For example, Novell's
Multi Protocol Router is routing software that runs on router
hardware. In other instances-and particularly when routers are
high-performance routers-the routing software is built directly
into a router's hardware or firmware.
There are several kinds of tables used in routing. In the simplest
kind of intranet, an exceedingly simple routing table can be used,
called a minimal routing table. When an intranet is composed
of a single TCP/IP network, and when that network is not connected
to any other TCP/IP network or to the Internet, minimal routing
can be used. In minimal routing, a program called ifconfig
automatically creates the table, which contains only a few basic
entries. Since there are very few places that data can be sent,
only a minimal number of routes need to be configured.
If an intranet has only a limited number of other TCP/IP networks,
then a static routing table can be used. In this case,
packets with specific addresses are sent to specific routers-the
routers do not redirect packets to adjust to changing network
traffic. Static routing should be used when there is only one
route to each given destination. A static routing table allows
an intranet administrator to add or take away entries in the routing
table.
Dynamic routing tables are the most sophisticated
routing tables. They should be used when there is more than one
way in which data can be sent from a router to the final destination,
and in more complex intranets. These tables constantly change
as network traffic and conditions change, so that they always
route data the most efficient way possible, taking into account
the current state of traffic on the intranet.
Dynamic routing tables are built using routing protocols. These
protocols are ways in which routers communicate with one another,
giving each other information about the most efficient way of
routing data given the current state of the intranet. A router
with a dynamic routing table can automatically switch data to
a backup route if the primary route is down. It can also always
determine the most efficient way of routing data toward its final
destination. Routers advertise their IP addresses and know the
IP addresses of their neighbors. Routers can use this information
in an algorithm to calculate the best route to send packets.
The most common routing protocol that performs these best-case
calculations is known as RIP (Routing Information Protocol). When
RIP determines the most efficient route for data, it calculates
the hop count for the route. RIP always chooses the path with
the lowest hop count as the route to send data over. It assumes
that the fewer the hops, the more efficient the path. RIP will
not allow any path with a hop count of over 16. If there is a
hop count of over 16, it will discard the route. On most intranets
this shouldn't be a problem.
The Exterior Gateway Protocol (EGP) is used for the Internet where
many more routers might have to be traversed before a packet reaches
its final destination. It will not be needed on an intranet since
it's rare that an intranet would be so large that data would have
to pass through more than 16 routers or gateways in order to be
sent to the final destination.
The thing to keep in mind about intranets and routing technology
is that it is not an either/or situation. Many different kinds
of routing technologies can be used on a single intranet, depending
on the needs of that particular part of the network. Some parts
may be able to use routers with static routing tables, while other
parts may require dynamic routing tables. As in anything having
to do with intranets, the whole point is flexibility and using
the right tool for the job.
Just as routers direct traffic on the Internet, sending information
to its proper destination, routers on an intranet perform the
same function. Routers-equipment that is a combination of hardware
and software-can send the data to a computer on the same subnetwork
inside the intranet, to another network on the intranet, or outside
to the Internet. They do this by examining header information
in IP packets, and then sending the data on its way. Typically,
a router will send the packet to the next router closest to the
final destination, which in turn sends it to an even closer router,
and so on, until the data reaches its intended recipient.
- A router has input ports for receiving IP packets, and output
ports for sending those packets toward their destination. When
a packet comes to the input port, the router examines the packet
header, and checks the destination in it against a routing table-a
database that tells the router how to send packets to various
destinations.
- Based on the information in the routing table, the packet
is sent to a particular output port, which sends the packet to
the next closest router to the packet's destination.
- If packets come to the input port more quickly than the router
can process them, they are sent to a holding area called an input
queue. The router then processes packets from the queue in the
order they were received. If the number of packets received exceeds
the capacity of the queue (called the length of the queue), packets
may be lost. When this happens, the TCP protocol on the sending
and receiving computers will have the packets re-sent.
- In a simple intranet that is a single, completely self-contained
network, and in which there are no connections to any other network
or the intranet, only minimal routing need be done, and so the
routing table in the router is exceedingly simple with very few
entries, and is constructed automatically by a program called
ifconfig.
- In a slightly more complicated intranet which is composed
of a number of TCP/IP-based networks, and connects to a limited
number of TCP/IP-based networks, static routing will be required.
In static routing, the routing table has specific ways of routing
data to other networks. Only those pathways can be used. Intranet
administrators can add routes to the routing table. Static routing
is more flexible than minimal routing, but it can't change routes
as network traffic changes, and so isn't suitable for many intranets.
- In more complex intranets, dynamic routing will be required.
Dynamic routing is used to permit multiple routes for a packet
to reach its final destination. Dynamic routing also allows routers
to change the way they route information based on the amount of
network traffic on some paths and routers. In dynamic routing,
the routing table is called a dynamic routing table and changes
as network conditions change. The tables are built dynamically
by routing protocols, and so constantly change according to network
traffic and conditions.
- There are two broad types of routing protocols: interior and
exterior. Interior routing protocols are typically used on internal
routers inside an intranet that routes traffic bound only for
inside the intranet. A common interior routing protocol is the
Routing Information Protocol (RIP). Exterior protocols are typically
used for external routers on the Internet., AÊcommon exterior
protocol is the Exterior Gateway Protocol (EGP).
Contact
reference@developer.com with questions or comments.
Copyright 1998
EarthWeb Inc., All rights reserved.
PLEASE READ THE ACCEPTABLE USAGE STATEMENT.
Copyright 1998 Macmillan Computer Publishing. All rights reserved.
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