📄 rfc1180.txt
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5.6 IP Route Table How does IP know which lower network interface to use when sending out a IP packet? IP looks it up in the route table using a search key of the IP network number extracted from the IP destinationSocolofsky & Kale [Page 17] RFC 1180 A TCP/IP Tutorial January 1991 address. The route table contains one row for each route. The primary columns in the route table are: IP network number, direct/indirect flag, router IP address, and interface number. This table is referred to by IP for each outgoing IP packet. On most computers the route table can be modified with the "route" command. The content of the route table is defined by the network manager, because the network manager assigns the IP addresses to the computers.5.7 Direct Routing Details To explain how it is used, let us visit in detail the routing situations we have reviewed previously. --------- --------- | alpha | | beta | | 1 | | 1 | --------- --------- | | --------o---------------o- Ethernet 1 IP network "development" Figure 8. Close-up View of One IP Network The route table inside alpha looks like this: -------------------------------------------------------------- |network direct/indirect flag router interface number| -------------------------------------------------------------- |development direct <blank> 1 | -------------------------------------------------------------- TABLE 8. Example Simple Route Table This view can be seen on some UNIX systems with the "netstat -r" command. With this simple network, all computers have identical routing tables. For discussion, the table is printed again without the network number translated to its network name.Socolofsky & Kale [Page 18] RFC 1180 A TCP/IP Tutorial January 1991 -------------------------------------------------------------- |network direct/indirect flag router interface number| -------------------------------------------------------------- |223.1.2 direct <blank> 1 | -------------------------------------------------------------- TABLE 9. Example Simple Route Table with Numbers5.8 Direct Scenario Alpha is sending an IP packet to beta. The IP packet is in alpha's IP module and the destination IP address is beta or 223.1.2.2. IP extracts the network portion of this IP address and scans the first column of the table looking for a match. With this network a match is found on the first entry. The other information in this entry indicates that computers on this network can be reached directly through interface number 1. An ARP table translation is done on beta's IP address then the Ethernet frame is sent directly to beta via interface number 1. If an application tries to send data to an IP address that is not on the development network, IP will be unable to find a match in the route table. IP then discards the IP packet. Some computers provide a "Network not reachable" error message.5.9 Indirect Routing Details Now, let's take a closer look at the more complicated routing scenario that we examined previously.Socolofsky & Kale [Page 19] RFC 1180 A TCP/IP Tutorial January 1991 --------- --------- --------- | alpha | | delta | |epsilon| | 1 | |1 2 3| | 1 | --------- --------- --------- | | | | | --------o---------------o- | -o----------------o-------- Ethernet 1 | Ethernet 2 IP network "Development" | IP network "accounting" | | -------- | | iota | | | 1 | | -------- | | --o--------o-------- Ethernet 3 IP network "factory" Figure 9. Close-up View of Three IP Networks The route table inside alpha looks like this: --------------------------------------------------------------------- |network direct/indirect flag router interface number| --------------------------------------------------------------------- |development direct <blank> 1 | |accounting indirect devnetrouter 1 | |factory indirect devnetrouter 1 | --------------------------------------------------------------------- TABLE 10. Alpha Route Table For discussion the table is printed again using numbers instead of names. -------------------------------------------------------------------- |network direct/indirect flag router interface number| -------------------------------------------------------------------- |223.1.2 direct <blank> 1 | |223.1.3 indirect 223.1.2.4 1 | |223.1.4 indirect 223.1.2.4 1 | -------------------------------------------------------------------- TABLE 11. Alpha Route Table with Numbers The router in Alpha's route table is the IP address of delta's connection to the development network.Socolofsky & Kale [Page 20] RFC 1180 A TCP/IP Tutorial January 19915.10 Indirect Scenario Alpha is sending an IP packet to epsilon. The IP packet is in alpha's IP module and the destination IP address is epsilon (223.1.3.2). IP extracts the network portion of this IP address (223.1.3) and scans the first column of the table looking for a match. A match is found on the second entry. This entry indicates that computers on the 223.1.3 network can be reached through the IP-router devnetrouter. Alpha's IP module then does an ARP table translation for devnetrouter's IP address and sends the IP packet directly to devnetrouter through Alpha's interface number 1. The IP packet still contains the destination address of epsilon. The IP packet arrives at delta's development network interface and is passed up to delta's IP module. The destination IP address is examined and because it does not match any of delta's own IP addresses, delta decides to forward the IP packet. Delta's IP module extracts the network portion of the destination IP address (223.1.3) and scans its route table for a matching network field. Delta's route table looks like this: ---------------------------------------------------------------------- |network direct/indirect flag router interface number| ---------------------------------------------------------------------- |development direct <blank> 1 | |factory direct <blank> 3 | |accounting direct <blank> 2 | ---------------------------------------------------------------------- TABLE 12. Delta's Route Table Below is delta's table printed again, without the translation to names. ---------------------------------------------------------------------- |network direct/indirect flag router interface number| ---------------------------------------------------------------------- |223.1.2 direct <blank> 1 | |223.1.3 direct <blank> 3 | |223.1.4 direct <blank> 2 | ---------------------------------------------------------------------- TABLE 13. Delta's Route Table with Numbers The match is found on the second entry. IP then sends the IP packet directly to epsilon through interface number 3. The IP packet contains the IP destination address of epsilon and the EthernetSocolofsky & Kale [Page 21] RFC 1180 A TCP/IP Tutorial January 1991 destination address of epsilon. The IP packet arrives at epsilon and is passed up to epsilon's IP module. The destination IP address is examined and found to match with epsilon's IP address, so the IP packet is passed to the upper protocol layer.5.11 Routing Summary When a IP packet travels through a large internet it may go through many IP-routers before it reaches its destination. The path it takes is not determined by a central source but is a result of consulting each of the routing tables used in the journey. Each computer defines only the next hop in the journey and relies on that computer to send the IP packet on its way.5.12 Managing the Routes Maintaining correct routing tables on all computers in a large internet is a difficult task; network configuration is being modified constantly by the network managers to meet changing needs. Mistakes in routing tables can block communication in ways that are excruciatingly tedious to diagnose. Keeping a simple network configuration goes a long way towards making a reliable internet. For instance, the most straightforward method of assigning IP networks to Ethernet is to assign a single IP network number to each Ethernet. Help is also available from certain protocols and network applications. ICMP (Internet Control Message Protocol) can report some routing problems. For small networks the route table is filled manually on each computer by the network administrator. For larger networks the network administrator automates this manual operation with a routing protocol to distribute routes throughout a network. When a computer is moved from one IP network to another, its IP address must change. When a computer is removed from an IP network its old address becomes invalid. These changes require frequent updates to the "hosts" file. This flat file can become difficult to maintain for even medium-size networks. The Domain Name System helps solve these problems.6. User Datagram Protocol UDP is one of the two main protocols to reside on top of IP. It offers service to the user's network applications. Example network applications that use UDP are: Network File System (NFS) and SimpleSocolofsky & Kale [Page 22] RFC 1180 A TCP/IP Tutorial January 1991 Network Management Protocol (SNMP). The service is little more than an interface to IP. UDP is a connectionless datagram delivery service that does not guarantee delivery. UDP does not maintain an end-to-end connection with the remote UDP module; it merely pushes the datagram out on the net and accepts incoming datagrams off the net. UDP adds two values to what is provided by IP. One is the multiplexing of information between applications based on port number. The other is a checksum to check the integrity of the data.6.1 Ports How does a client on one computer reach the server on another? The path of communication between an application and UDP is through UDP ports. These ports are numbered, beginning with zero. An application that is offering service (the server) waits for messages to come in on a specific port dedicated to that service. The server waits patiently for any client to request service.⌨️ 快捷键说明
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