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Routing in IP networks Routage dans les réseaux IP
In a large IP network such as the global Internet, routers need to exchange routing information. The Internet is an interconnection of networks, often called domains, that are under different responsibilities. As of this writing, the Internet is composed on more than 40,000 different domains and this number is still growing [#fas]_. A domain can be a small enterprise that manages a few routers in a single building, a larger enterprise with a hundred routers at multiple locations, or a large Internet Service Provider managing thousands of routers. Two classes of routing protocols are used to allow these domains to efficiently exchange routing information. Dans un réseau IP large tel que l'Internet global, les routeurs ont besoin d'échanger des informations. L'internet est une interconnexion de réseaux, souvent appelés domaines, qui sont sous différentes responsabilités. Au moment d’écrire ces lignes, l'Internet est composé de plus de 40,000 domaines différents et ce nombre continue d'augmenter [#fas]_. Un domaine peut être une entreprise gérant quelques routeurs dans un seul bâtiment, une grande entreprise avec une centaine de routeurs à plusieurs endroits, ou un fournisseur de services Internet gérant des milliers de routeurs. Deux classes de protocoles de routage sont utilisés pour permettre à ces domaines d'échanger des informations de routage de manière efficace.
Organisation of a small Internet Organisation d'un petit Internet
The first class of routing protocols are the `intradomain routing protocols` (sometimes also called the interior gateway protocols or :term:`IGP`). An intradomain routing protocol is used by all routers inside a domain to exchange routing information about the destinations that are reachable inside the domain. There are several intradomain routing protocols. Some domains use :term:`RIP`, which is a distance vector protocol. Other domains use link-state routing protocols such as :term:`OSPF` or :term:`IS-IS`. Finally, some domains use static routing or proprietary protocols such as :term:`IGRP` or :term:`EIGRP`.
These intradomain routing protocols usually have two objectives. First, they distribute routing information that corresponds to the shortest path between two routers in the domain. Second, they should allow the routers to quickly recover from link and router failures.
The second class of routing protocols are the `interdomain routing protocols` (sometimes also called the exterior gateway protocols or :term:`EGP`). The objective of an interdomain routing protocol is to distribute routing information between domains. For scalability reasons, an interdomain routing protocol must distribute aggregated routing information and considers each domain as a black box.
A very important difference between intradomain and interdomain routing are the `routing policies` that are used by each domain. Inside a single domain, all routers are considered equal, and when several routes are available to reach a given destination prefix, the best route is selected based on technical criteria such as the route with the shortest delay, the route with the minimum number of hops or the route with the highest bandwidth.
When we consider the interconnection of domains that are managed by different organizations, this is no longer true. Each domain implements its own routing policy. A routing policy is composed of three elements : an `import filter` that specifies which routes can be accepted by a domain, an `export filter` that specifies which routes can be advertised by a domain and a ranking algorithm that selects the best route when a domain knows several routes towards the same destination prefix. As we will see later, another important difference is that the objective of the interdomain routing protocol is to find the `cheapest` route towards each destination. There is only one interdomain routing protocol : :term:`BGP`.
Intradomain routing
In this section, we briefly describe the key features of the two main intradomain unicast routing protocols : RIP and OSPF. The basic principles of distance vector and link-state routing have been presented earlier.
The Routing Information Protocol (RIP) is the simplest routing protocol that was standardized for the TCP/IP protocol suite. RIP is defined in :rfc:`2453`. Additional information about RIP may be found in [Malkin1999]_.
RIP routers periodically exchange RIP messages. The format of these messages is shown below. A RIP message is sent inside a UDP segment whose destination port is set to `521`. A RIP message contains several fields. The `command` field indicates whether the RIP message is a request or a response. When a router boots, its routing table is empty and it cannot forward any packet. To speedup the discovery of the network, it can send a request message to the RIP IPv6 multicast address, ``FF02::9``. All RIP routers listen to this multicast address and any router attached to the subnet will reply by sending its own routing table as a sequence of RIP messages. In steady state, routers multicast one of more RIP response messages every 30 seconds. These messages contain the distance vectors that summarize the router's routing table. The current version of RIP is version 2 defined in :rfc:`2453` for IPv4 and :rfc:`2080` for IPv6.
The RIP message format Le format de message de RIP
Each RIP message contains a set of route entries. Each route entry is encoded as a 20 bytes field whose format is shown below. RIP was initially designed to be suitable for different network layer protocols. Some implementations of RIP were used in XNS or IPX networks :rfc:`2453`. The format of the route entries used by :rfc:`2080` is shown below. `Prefix length` is the length of the subnet identifier in bits and the `metric` is encoded as one byte. The maximum metric supported by RIP is `15`.
Format of the RIP IPv6 route entries
A note on timers
The first RIP implementations sent their distance vector exactly every 30 seconds. This worked well in most networks, but some researchers noticed that routers were sometimes overloaded because they were processing too many distance vectors at the same time [FJ1994]_. They collected packet traces in these networks and found that after some time the routers' timers became synchronized, i.e. almost all routers were sending their distance vectors at almost the same time. This synchronization of the transmission times of the distance vectors caused an overload on the routers' CPU but also increased the convergence time of the protocol in some cases. This was mainly due to the fact that all routers set their timers to the same expiration time after having processed the received distance vectors. `Sally Floyd`_ and `Van Jacobson`_ proposed in [FJ1994]_ a simple solution to solve this synchronization problem. Instead of advertising their distance vector exactly after 30 seconds, a router should send its next distance vector after a delay chosen randomly in the [15,45] interval :rfc:`2080`. This randomization of the delays prevents the synchronization that occurs with a fixed delay and is now a recommended practice for protocol designers.


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