RIP Message format

R I P uses special packets to collect and share the distance information to the destination. Figure 1 2-1 shows the r I P packet with only one destination in the routing information domain.

Figure 12-1 RIP packet structure

Up to two or five a f I, interconnected network addresses, and measurement fields can appear in the r I P message. This allows you to use an r I P report to update Multiple Route table entries in a vro. The r I p packet containing multiple route table items simply repeats the structure from a f I to the measurement domain, including all the zero fields. The duplicate structure is appended to the structure in Figure 1 2-1. The r I P packet 1 2-2 has two table items.

Figure 12-2 RIP packets with two table items

The address field can include both the sender's address and a series of I P addresses in the sender's route table. The request message contains a table entry and the requester's address. A response message can contain up to two or five r I P route table items.

The entire r I p packet size limit is 5 1 2 B. Therefore, in a larger r I p network, the update request for the entire route table needs to transmit multiple r I P packets. When packets arrive at their destination, they are not ordered. One route table entry is not separated in two r I P packets. Therefore, the content of any r I P packet is complete, even if they are only a subset of the entire route table. When a message is received, the receiving node can process updates at will without the need to sort the updates.

For example, the routing table of an r I P router can contain 1 0 items. To share this information with other r I p routers, four r I P packets are required. Each packet includes two or five items. If a receiving node (node) first receives the packet No. 4 (including table items ranging from 7 6 to 1 0 0), it will first update the corresponding part of the route table, there is no sequential correlation between these packets. In this way, the forwarding of r I P packets can save the overhead of transmission protocols such as T C P.

1. Command domain
The command field specifies whether the r I P packet is a request message or a response packet to the request. In both cases, the same frame structure is used:
The request message requests the router to send the entire or part of the route table.
The response packet includes route table items shared with other r I P nodes in the network. The response message can be a response to the request or an active update.

2. Version Number Field
The version field includes the version used to generate the r I P packet. R I P is an open standard routing protocol that is updated over time. These updates are reflected in the version number. Although there are many routing protocols like r I p, r I P only has two versions: Version 1 and version 2. This chapter describes the commonly used version 1.

3. 0 domain
Multiple 0 fields embedded in the r I p message prove that many protocols, such as r I P, existed before RFC 1058 appeared. Most 0 domains are used to be backward compatible with old protocols such as r I P. 0 domains do not support all their private features.

For example, the two old mechanisms t r a c e o n and t r a c e o ff. These mechanisms are abandoned by RFC 1058, but open standard r I P needs to be backward compatible with the protocols that support these mechanisms. Therefore, RFC 1058 reserves space for the packets, but requires that these spaces be invariably set to 0. When these fields are not 0 in the received message, they are simply discarded.

Not all 0 fields are for backward compatibility. At least one zero domain is reserved for future use.

4. AFI domain
The Address Family Identifier (a f I) domain indicates the Address Family in the Address domain of the interconnected network. Although RFC 1058 was created by I e t f and therefore applies to Internet Protocol (I P), it is designed to provide compatibility with previous versions. This means that it must provide transmission of route information consisting of a large number of interconnected network addresses or a family. Therefore, open standard r I P requires a mechanism to determine the types of addresses carried in the packets.

5. Internet address domain
A 4-byte interconnected network address domain contains an interconnected network address. The address can be the host, network, or even the address code of a default gateway. Two examples of how the content of this domain changes are as follows:
In a single table request message, this domain includes the address of the packet sender.
In a multi-Table response message, these domains will include the I P address stored in the packet sender routing table.

6. Measurement Standard domain
The last field in the r I P packet is the measurement standard field, which contains the measurement count of the packet. This value is incremented when it passes through the router. The valid range of the quantity standard is 1 ~ Between 1 and 5. The measurement standard can actually increase to 1 6, but this value corresponds to the invalid route. Therefore, 1 6 is the error value in the measurement standard field and is not within the valid range.

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