The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)                 9  ICMP Concepts and General Operation

ICMP Overview, History, Versions and Standards 1 2 ICMP Message Classes, Types and Codes

ICMP Error-Reporting Limitations

One interesting general characteristic of ICMP's operation is that when errors are detected, they can be reported using ICMP, but only back to the original source of a datagram. This is actually a big drawback in how ICMP works. Refer back to Figure 137 and consider again client host A sending a message to server host B, with a problem detected in the datagram by router R3. Even if R3 suspects that the problem was caused by one of the preceding routers that handled the message, such as R2, it cannot send a problem report to R2. It can only send an ICMP message back to host A.

This limitation is an artifact of how the Internet Protocol works. You may recall from looking at the IP datagram format that the only address fields are for the original source and ultimate destination of the datagram. (The only exception is if the IP Record Route option is used, but devices cannot count on this.) When R3 receives a datagram from R2 that R2 in turn received from R1 (and prior to that, from A), it is only A's address in the datagram. Thus, R3 must send a problem report back to A, and A must decide what to do with it. Device A may decide to change the route it uses, or to generate an error report that an administrator can use to troubleshoot the R2 router.

In addition to this basic limitation, several special rules and conventions have been put in place to govern the circumstances under which ICMP messages are generated, sent and processed.

ICMP Overview, History, Versions and Standards 1 2 ICMP Message Classes, Types and Codes