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Routing

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Routing

• Routing is the act of deciding how each individual datagram finds its way through the multiple different paths to its destination.

• Routing is concerned with finding the right network for a datagram. Once the right network has been found the datagram can be delivered to the host.

• Most hosts (and gateways) on the Internet maintain a routing table. The entries in the routing table contain the information describing where to send datagrams for a particular network.

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One Example Network

Host A’s routing table

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Packet Forwarding

• netstat -rn shows the routing table for a given host.• At a minimum, it describes routing to the machine’s

own network and the machine itself. These entries exist so that forwarding to directly connected networks need not be handled as a special case.

• It also has a default route, which is used when the target network is not found in the routing table.

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Internet Control Message Protocol

The principal ICMP message types.

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Illustration Of ICMP Message Encapsulation

• Two levels of encapsulation• IP type field specifies ICMP

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ICMP Redirects

• Although IP generally does not concern itself with the management of routing information, it does define a small damage control feature called ICMP redirect. When a router forwards a packet to a machine on the same network from which the packet was originally received, something is clearly wrong. Since the sender, the router and the nexthop router are all on the same network, the packet could have been forwarded in one hop rather than two. In this situation, the router notifies the sender a ICMP redirect packet, which in turn will enter a host route into its routing table.

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Routing Daemons and Protocols

• When networks become too complicated, it becomes hard to manage routing tables manually. The routing daemons and routing protocols can be used to manage the networks.

• Routing protocols have a major advantage over static routing systems in that they can react and adapt to changing network conditions. If a link goes down, the routing daemons can quickly discover and propagate alternate routes to the networks that link served, if any exist.

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Routing Daemons and Protocols

• Routing daemons collect information from three sources: configuration files, the existing routing tables and routing daemons on other systems. This information is merged to compute an optimal set of routes, and the new routes are then fed back into the system routing table.

• Because network conditions change over time, routing daemons must periodically check in with each other to make sure that the routing information is current.

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Routing Protocols

• There are two types of protocols that are used commonly.– Distance vector protocol

• A machine announces how far it thinks it is from the network. If no one knows any better, they figure that you are the best gateway.

– Link state protocol• information is distributed in a relatively unprocessed

form. Each router computes its own routing table.

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RIP

• This is the protocol used by routed. RIP is a simple distance-vector protocol that uses hop counts as a cost metric. Therefore, large local networks that have more than 15 routers along any single path cannot use the RIP protocol.

• Although RIP is a resource hog because of its broadcasting, it is good for a network that changes often or when the topology of remote networks is not know. However, it can be slow to stabilize after a link goes down.

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RIP-2 and OSPF

• This is a revision of RIP, where the most important change is that netmasks along with next-hop addresses are provided.

• Open shortest path first is a link-state protocol.

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OSPF – The Interior Gateway Routing Protocol

(a) An autonomous system. (b) A graph representation of (a).

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OSPF (2)

The relation between ASes, backbones, and areas in OSPF.

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OSPF (3)

The five types of OSPF messages.

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routed

• This is the standard UNIX routing daemon. routed can be run in server mode or in quite mode. Both modes listen for broadcasts, but only servers distribute their own information.

• routed adds its discovered routes to the kernel’s routing table. Routes must be heard at lest every four minutes or they will be removed. It knows the routes that it adds and does not remove any routed added with the route command.

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Routing Strategy

• Routing for a network can be managed in many ways:– no routing

• a stand-alone network

– static routes only• If there is only one way out of a network

– mostly static routes, but clients listen for RIP updates• If you have several gateways, some to specific networks and some

to the outside

– dynamic routing everywhere• when networks cross political or administrative boundaries