Protokol Routing Unicast (RIP, OSPF dan BGP)

Chapter 11

11.1

                Cost or Metric

                A router receives a packet from a network and passes it to another network. A router is usually attached to several networks.

                One approach is to assign a Cost for passing through a network. We call this cost a metric. High cost can be thought of as something bad; low cost can be thought of something good. The high throughput means low cost and the low throughput means high cost and  low delay is low cost and high delay is high cost.

Static versus Dynamic Routing Tables

A routing table can be either static or dynamic. A static table is one with manual entries. A dynamic table, on the other hand, is one that is updated automatically when there is a change somewhere in the internet.

                Routing Protocol

                Routing protocols have been created in response to the demand for dynamic routing tables. A routing protocol is a combination of rules and procedures that lets routers in the internet inform each other of changes. It allows routers to share whatever they know about the internet or their neighborhood.

Routing protocols can be either an interior protocol or an exterior protocol. An interior protocol handles intradomain routing; an exterior protocol handles interdomain routing.

11.2

Autonomous system (AS) is a group of networks and routers under the authority of a single administration. Routing inside an autonomous system is referred to as intra-domain routing. Routing between autonomous systems is referred to as inter-domain routing.

Routing Information Protocol (RIP) is the implementation of the distance vector protocol. Open Shortest Path First (OSPF) is the implementation of the link state protocol. Border Gateway Protocol (BGP) is the implementation of the path vector protocol. RIP and OSPF are interior routing protocols; BGP is an exterior routing protocol.

11.3

A router can normally be represented by a node and a network by a link connecting two nodes, although other representations are also possible. The graph theory used an algorithm called Bellman-Ford (also called Ford-Fulkerson) for a while to find the shortest path between nodes in a graph given the distance between nodes.

Count to Infinity

A problem with distance vector routing is that any decrease in cost (good news) propagates quickly, but any increase in cost (bad news) propagates slowly. The problem is referred to as count to infinity. It takes several updates before the cost for a broken link is recorded as infinity by all routers.

11.4

                The Routing Information Protocol (RIP) is an intradomain (interior) routing protocol used inside an autonomous system.

                RIP implements distance vector routing directly with some considerations:

1.       In an autonomous system, we are dealing with routers and networks (links), what was described as a node.

2.       The destination in a routing table is a network, which means the first column defines a network address.

3.       3. The metric used by RIP is very simple; the distance is defined as the number of links (networks) that have to be used to reach the destination. For this reason, the metric in RIP is called a hop count.

4.       Infinity is defined as 16, which means that any route in an autonomous system using RIP cannot have more than 15 hops.

5.       The next node column defines the address of the router to which the packet is to be sent to reach its destination.

RIP Message Format

1.       Command. This 8-bit field specifies the type of message: request (1) or response (2).

2.       Version. This 8-bit field defines the version. In this book we use version 1, but at the end of this section, we give some new features of version 2.

3.       Family. This 16-bit field defines the family of the protocol used. For TCP/IP the value is 2.

4.       Network address. The address field defines the address of the destination network. RIP has allocated 14 bytes for this field to be applicable to any protocol. However, IP currently uses only 4 bytes. The rest of the address is filled with 0s.

5.       Distance. This 32-bit field defines the hop count (cost) from the advertising router to the destination network.

Requests and Responses

                Request

A request message is sent by a router that has just come up or by a router that has some time-out entries.

Response

A response can be either solicited or unsolicited. A solicited response is sent only in answer to a request. It contains information about the destination specified in the corresponding request. An unsolicited response, on the other hand, is sent periodically, every 30 seconds or when there is a change in the routing table.

                Timers in RIP

                                Periodic Timer

                                                The periodic timer controls the advertising of regular update messages.

                                Expiration Timer

                The expiration timer governs the validity of a route. When a router receives update information for a route, the expiration timer is set to 180 s for that particular route.

Garbage Collection Timer

When the information about a route becomes invalid, the router does not immediately purge that route from its table. Instead, it continues to advertise the route with a metric value of 16. At the same time, a timer called the garbage collection timer is set to 120 s for that route. When the count reaches zero, the route is purged from the table. This timer allows neighbors to become aware of the invalidity of a route prior to purging.

RIP Version 2

RIP version 2 was designed to overcome some of the shortcomings of version 1. The designers of version 2 have not augmented the length of the message for each entry.

Message Format

1.       Route tag. This field carries information such as the autonomous system number. It can be used to enable RIP to receive information from an interdomain routing protocol.

2.       Subnet mask. This is a 4-byte field that carries the subnet mask (or prefix). This means that RIP2 supports classless addressing and CIDR.

3.       Next-hop address. This field shows the address of the next hop. This is particularly useful if two autonomous systems share a network (a backbone, for example). Then the message can define the router, in the same autonomous system or another autonomous system, to which the packet next goes.

Classless Addressing

Probably the most important difference between the two versions of RIP is classful versus classless addressing. RIPv1 uses classful addressing. The only entry in the message format is the network address (with a default mask). RIPv2 adds one field for the subnet mask, which can be used to define a network prefix length.

                                Authentication

                Authentication is added to protect the message against unauthorized advertisement. No new fields are added to the packet; instead, the first entry of the message is set aside for authentication information.