cnp3-ebook/protocols/routing — English @ Weblate

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6 ../../protocols/routing.rst:21

	English 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. 397/10000 · 397
7 ../../protocols/routing.rst:23

	English 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. 442/10000 · 442
8 ../../protocols/routing.rst:25

	English 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`. 718/10000 · 718
9 ../../protocols/routing.rst:29

	English Intradomain routing 19/10000 · 19
10 ../../protocols/routing.rst:31

	English 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. 215/10000 · 215
11 ../../protocols/routing.rst:36

	English RIP 3/10000 · 3
12 ../../protocols/routing.rst:38

	English 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]_. 219/10000 · 219
13 ../../protocols/routing.rst:40

	English 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. 931/10000 · 931
14 ../../protocols/routing.rst:46

	English The RIP message format 22/10000 · 22
15 ../../protocols/routing.rst:50

	English 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`. 498/10000 · 498
16 ../../protocols/routing.rst:56

	English Format of the RIP IPv6 route entries 36/10000 · 36
17 ../../protocols/routing.rst:58

	English A note on timers 16/10000 · 16
18 ../../protocols/routing.rst:60

	English 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. 1250/10000 · 1250
19 ../../protocols/routing.rst:65

	English OSPF 4/10000 · 4
20 ../../protocols/routing.rst:67

	English Link-state routing protocols are used in IP networks. Open Shortest Path First (OSPF), defined in :rfc:`2328`, is the link state routing protocol that has been standardized by the IETF. The last version of OSPF, which supports IPv6, is defined in :rfc:`5340`. OSPF is frequently used in enterprise networks and in some ISP networks. However, ISP networks often use the IS-IS link-state routing protocol [ISO10589]_ , which was developed for the ISO CLNP protocol but was adapted to be used in IP :rfc:`1195` networks before the finalization of the standardization of OSPF. A detailed analysis of ISIS and OSPF may be found in [BMO2006]_ and [Perlman2000]_. Additional information about OSPF may be found in [Moy1998]_. 719/10000 · 719
21 ../../protocols/routing.rst:71

	English Compared to the basics of link-state routing protocols that we discussed in section :ref:`linkstate`, there are some particularities of OSPF that are worth discussing. First, in a large network, flooding the information about all routers and links to thousands of routers or more may be costly as each router needs to store all the information about the entire network. A better approach would be to introduce hierarchical routing. Hierarchical routing divides the network into regions. All the routers inside a region have detailed information about the topology of the region but only learn aggregated information about the topology of the other regions and their interconnections. OSPF supports a restricted variant of hierarchical routing. In OSPF's terminology, a region is called an `area`. 796/10000 · 796
22 ../../protocols/routing.rst:73

	English OSPF imposes restrictions on how a network can be divided into areas. An area is a set of routers and links that are grouped together. Usually, the topology of an area is chosen so that a packet sent by one router inside the area can reach any other router in the area without leaving the area [#fvirtual]_ . An OSPF area contains two types of routers :rfc:`2328`: 364/10000 · 364
23 ../../protocols/routing.rst:75

	English Internal router : A router whose directly connected networks belong to the area 79/10000 · 79
24 ../../protocols/routing.rst:76

	English Area border routers : A router that is attached to several areas. 65/10000 · 65
25 ../../protocols/routing.rst:78

	English For example, the network shown in the figure below has been divided into three areas : `area 0`, containing routers `RA`, `RB`, `RC` and `RD`, `area 1`, containing routers `R1`, `R3`, `R4`, `R5` and `RA`, and `area 2` containing `R7`, `R8`, `R9`, `R10`, `RB` and `RC`. OSPF areas are identified by a 32 bit integer, which is sometimes represented as an IP address. Among the OSPF areas, `area 0`, also called the `backbone area`, has a special role. The backbone area groups all the area border routers (routers `RA`, `RB` and `RC` in the figure below) and the routers that are directly connected to the backbone routers but do not belong to another area (router `RD` in the figure below). An important restriction imposed by OSPF is that the path between two routers that belong to two different areas (e.g. `R1` and `R8` in the figure below) must pass through the backbone area. 880/10000 · 880

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