ELECCIÓN DEL MATERIAL Y PERFIL DE LAS BARRAS

internetwork with various maximum transfer units.

Some of the main ICMP messages are as follows:

◆ Destination Unreachable. These messages provide information when a

host, net, port, or protocol is (are) unreachable.

◆ Time Exceeded. These messages notify the source if a datagram is undeliver- able because its Time to Live expired.

◆ Parameter Problem. These messages report a parameter problem and the

octet in which the error was detected.

◆ Source Quench. These messages may be sent by destination routers or hosts forced to discard datagrams due to limitations in available buffer space, or if for any reason a datagram cannot be processed.

◆ Redirect. These messages are sent to a host when a router receives a datagram that could be routed more directly through another gateway. The message advises the host that was the source of the datagram of a more appropriate router to receive the datagram.

◆ Echo Request and Echo Reply Messages. These messages exchange data

between hosts.

◆ Timestamp Request and Timestamp Reply. These messages exchange

timestamp data between hosts.

◆ Information Request and Information Reply. These messages can be

used to enable a host to discover the network to which it is attached.

ICMP provides basic connectivity utilities used by upper-layer protocols—utilities such as PING use these to perform their functions.

Figure 5.5

Parts of the ICMP datagram.

Summary

This chapter has looked at the IP layer and introduced the concept of routing. You have also seen that no matter where the IP datagram is sent, it always goes to a MAC address, not an IP address. This layer has three main protocols: IP, ARP, and ICMP. As you have seen, these protocols all work together to provide the capability to actually send data. The next two chapters look at subnetting and routing in great detail, and explain the basis for using TCP/IP in wide-area networking.

Test Yourself

1. On what type of system is the IP layer present? 2. What are some of the main functions of the IP layer? 3. What is the function of a subnet mask?

4. What is the standard subnet mask for a Class C address? 5. On what types of systems do you find a routing table? 6. What command can you use to look at the routing table?

7. What command would you issue to add a route to network 142.53.0.0 (subnet mask 255.255.0.0) so that data goes to a local router at 125.32.45.7?

8. In the routing table, what does the interface refer to? 9. What is the entry 0.0.0.0 in the routing table for?

10. What happens after IP determines that the system is a local system? 11. How long is an entry in the ARP cache retained?

12. Why would it be unwise for ARP to retain an entry permanently?

13. Other than responding to the request, what else does ARP do on the target host? 14. The header of the IP datagram has many fields. What is the purpose of the

following ones? Fragment Offset Time to Live Protocol

15. When is fragmentation required?

16. Which IP layer protocol is used to handle the reporting of errors? 17. What are examples of some of the errors that might be handled?

Test Yourself Answers

1. All devices that have an IP address have the IP layer, including both computers and routers.

2. The IP layer is responsible for routing datagrams, resolving IP address to MAC addresses, fragmentating and re-assembing packets too large for the underlying topology, and error detection and reporting.

3. The subnet mask allows IP to strip the host ID from the IP address, leaving the network ID.

4. The Class C subnet mask is 255.255.255.0.

5. All systems that use IP have a routing table. The routing table for most comput- ers, however, points all data for networks other than the local network to the default gateway (router).

6. The ROUTE command enables the viewing and modifying of the routing table. 7. The command would be

ROUTE ADD 142.53.0.0 MASK 255.255.0.0 125.32.45.7

8. The interface refers to the network card in the local machine that sends out the information.

9. This is the entry for the default gateway.

10. The IP layer will now use ARP to find the MAC address.

11. Entries in the ARP cache have a maximum lifetime of ten minutes. If the entry is not used again within two minutes, however, it is removed.

12. ARP resolves the IP address to the MAC address; the MAC address is part of the network card. If the network card ever needed to be changed, the entry would have to be removed by hand and then re-entered.

13. ARP adds the IP address and MAC address of the sending host. It does this assuming that more communications will follow.

14. Fragment Offset is used to re-assemble datagrams that have been fragmented in transit.

Time to Live is the maximum number of “seconds” that a packet can remain active on the network. When the TTL reaches zero, the packet is discarded. Protocol Used is used to tell the IP layer on the target hosts which protocol to send the information up to.

15. Fragmentation occurs if the datagram travels over networks of different topolo- gies. The topologies all have different maximum transfer unit sizes. In the case of a packet that moves from a network with a large MTU to a small one, the

datagram must be fragmented.

16. Error reporting is a function of the Internet Control Messaging Protocol. 17. The ICMP protocol handles many errors and other messages including Destina-

tion Unreachable, Time Exceeded, Parameter Problem, Source Quench, Redirect, Echo Request, Echo Reply, Timestamp Request, Timestamp Reply, Information Request, and Information Reply.