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The initial design goal of TCP/IP was to provide an internetworking architecture. With many networks TCP/IP gateways become involved. A TCP/IP gateway is equivalent to an OSI router and so in this handout TCP/IP gateways are referred to as routers.
 

A router will have at least two network cards and be connected to at least two networks. A router will merely forward IP datagrams between networks. This forwarding is known as routing.

Repeaters and Bridges

Before looking at routers it is worth reviewing other technology for interconnecting physical networks. With thick Ethernet a length of cable ( a segment) can be a maximum of 500m in length. To overcome this restriction repeaters may be used. A repeater just amplifies a signal. A repeater sits at layer 1.

Note: that repeaters are "invisible" to TCP/IP.

The TCP/IP protocols have no knowledge of the presence or otherwise of repeaters. The problem with repeaters is that as well as increasing the length of an Ethernet LAN they can also increase the traffic on the LAN resulting in performance problems.

Bridges overcome the problems of repeaters. Bridges sit a layer 2 in the OSI/RM. Bridges still connect physical LAN segments to form a single logical LAN. Bridges therefore deal with frames rather than individual bits.

Bridges can obtain source and destination addresses from frame headers. Using these addresses a bridge can determine where workstations are in terms of which side of the bridge a workstation is. Once a bridge knows where a workstation and it sees a frame is going to that workstation the bridge can decide whether the frame needs to be passed on to the other side of the bridge. The net result is that a bridge will filter traffic allowing multiple traffic on a single Ethernet LAN.

Note: that bridges are not just for Ethernet.

The basic rule is that only similar networks can be bridged. For example Ethernet can not be bridged to X.25

Note: however that a feature of IEEE 802 LANs is that they share a common Logical Link Control Layer - IEEE 802.2

This means that bridging is possible between, say Ethernet (802.3) and Token Ring(802.5). The presence of bridges can have a marked effect on performance. As with repeaters bridges are invisible to TCP/IP. The bridge creates the illusion of a single network to the higher layers.

Remote bridges

Bridges can be used to connect geographically remote LANs together. In this case two bridges would sit connected to their respective LANs. The bridges would be connected via a leased line or fibre optic link.

Routers and Gateways

A router is used to connect two physically separate networks. The router will forward packets on between these networks. Routers sit at layer three in the OSI/RM. This means that they are protocol dependent. This is because with TCP/IP it is IP which decides the routing to be done.

There are number of IP routers on the market. One of the decisions to make is whether to use normal hosts as your routers or to use dedicated routers. Note that the TCP/IP documentation often refers to routers as gateways. Gateways in OSI terminology convert protocols at layers above layer 3. A good example would be a mail gateway, for example converting smtp mail formats into X.400 formats.

 

How Indirect Routing Works

Previous to the chapter we have seen how when two hosts communicate ARP is used to determine the physical address of the destination host. This is sometimes referred to as direct routing.

Before a host will send an IP datagram the IP address is studied. The net id of the destination IP address is compared to our local net id. If they are the same then IP knows that no routing is required and that the datagram can be sent using the direct routing method where ARP will find the physical address of the destination host.

Routing will be used if when the destination and source addresses are compared they are found to be different. If this is the case the routing table of the host will be used to find the intermediate destination of this datagram.
 

The initial design goal of TCP/IP was to provide an internetworking architecture. With many networks TCP/IP gateways become involved. A TCP/IP gateway is equivalent to an OSI router and so in this handout TCP/IP gateways are referred to as routers.
 

This might seem daunting at first in terms of each host must have a routing table but in fact in most cases the routing table will have one simple entry. The simple entry will be that of the default gateway. The default gateway is often specified at installation of DOS based TCP/IP implementations.

The result is that the IP datagram will hop onto a router. Note that to get to the router direct routing (i.e. ARP) will be used. The router will then compare the IP address and see if it is for a network to which we are directly connected. If the IP datagram is not local then the routing tables of the router will be used to determine the next intermediate destination of the datagram. The datagram then hops on between routers until it reaches a router which is directly connected to the destination host in which case the direct routing method using ARP is used.

The TTL field is continually decremented by one and if this field reaches zero then the datagram is thrown away and an error is returned.

Routing Tables

The routing tables are used to find out which router the datagram should be passed on to. All hosts have routing tables but the normal hosts will have one entry defining the default route to take. The routing tables do not contain a list of all hosts. Instead routing tables only contain the routes to get to a particular network. This makes the routing tables smaller and more manageable.

Routers will have a complete routing table containing all routes in your internet. If required host specific routing can be employed. Host specific routing is where a hosts IP address is in the routing table. This technique might be handy when debugging.

The question is who updates the routing tables? The routing tables can be updated by hand but this is an unrequired extra burden placed on the administrator. Routers routing tables are normally dynamically updated by the use of routing protocols.

 

Routing Protocols

Routing protocols are often called gateway protocols as TCP/IP calls routers gateways. Routing protocols dynamically update routing tables. This means that extra software will run on the routers.

If your routers are UNIX machines then this routing software often comes in the shape of a program called routed. Another common UNIX routing protocol program is gated.

There are two main type of routing protocol

  Interior gateway protocols
  Exterior gateway protocols

In an internet there will be groups of networks managed by a particular organisation. This group of networks will be called an autonomous system.

Interior gateway protocols exchange routing information in an autonomous system. The routers in an autonomous system know all about all routes within the autonomous system.

There are many interior gateway protocols.

  RIP - Routing Information Protocol. The most common?
  OSPF - The best?
  IGRP - Proprietary to Cisco routers.
  HELLO - Not used much.
  GGP - Used to be used within the core Internet.

Exterior gateway protocols are used for connections to outside of an autonomous system.

RIP

RIP is an example Interior Gateway Protocol.
RIP stands for Routing Information Protocol.
RIP is only suitable for small networks.
RIP is popular only because it comes with the UNIX implementation of TCP/IP.

Routers running RIP broadcast their routing tables to neighbours once every 30 seconds. Each entry in the routing table consists of a destination network address and the number of hops that it will take to get there.

There are a number of problems with RIP. One is that it takes routing data a long time to work its way through the network.

OSPF

OSPF is the Open Shortest Path First protocol.
OSPF was designed to overcome the limitations of previous routing protocols.
OSPF overcomes the problems of RIP and is much more suitable for larger networks.

 

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