Networking The Complete Reference, Third Edition by Bobbi Sandberg

Author:Bobbi Sandberg [Sandberg, Bobbi]
Language: eng
Format: epub, pdf
Publisher: McGraw-Hill Education
Published: 2015-03-22T22:00:00+00:00

ICMP Query Messages

ICMP query messages are not generated in response to other activities, as are the error messages. Systems use them for self-contained request/reply transactions in which one computer requests information from another, which responds with a reply containing that information.

Because they are not associated with other IP transmissions, ICMP queries do not contain datagram information in their Data fields. The data they do carry is specific to the function of the message. The following sections examine some of the more common ICMP query messages and their functions.

Echo Requests and Replies Echo Request and Echo Reply messages are the basis for the TCP/IP ping utility, which sends test messages to another host on the network to determine whether it is capable of receiving and responding to messages. Each ping consists of an ICMP Echo Request message (Type 8, Code 0) that, in addition to the standard ICMP Type, Code, and Checksum fields, adds Identifier and Sequence Number fields that the systems use to associate requests and replies.

If the system receiving the message is functioning normally, it reverses the Source and Destination IP Address fields in the IP header, changes the value of the ICMP Type field to 0 (Echo Reply), and recomputes the checksum before transmitting it back to the sender.

Router Solicitations and Advertisements These messages make it possible for a host system to discover the addresses of the routers connected to the local network. Systems can use this information to configure the default gateway entry in their routing tables. When a host broadcasts or multicasts a Router Solicitation message (Type 10, Code 0), the routers on the network respond with Router Advertisement messages (Type 9, Code 0). Routers continue to advertise their availability at regular intervals (typically seven to ten minutes). A host may stop using a router as its default gateway if it fails to receive continued advertisements.

The Router Solicitation message consists only of the standard Type, Code, and Checksum fields, plus a 4-byte pad in the Data field. Figure 14-2 shows the Router Advertisement message format.

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