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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 Message Types and Formats                      9  ICMP Version 4 (ICMPv4) Error Message Types and Formats

ICMPv4 Time Exceeded Messages 1 2 3 ICMPv4 Parameter Problem Messages

Limitations of Redirect Messages

I must emphasize that ICMP Redirect messages are not a mechanism by which the general routing process in IP is implemented; they are a support function only. They are a convenient way for hosts to be given information about routes by local routers, but are not used to communicate route information between routers.

This means that a Redirect can tell a host to use a more efficient first-hop router, but cannot tell a router to use a more efficient second-hop router. In our example above, suppose that in addition to the connections mentioned, R2 is connected to R3 and R4 as shown in Figure 143. R2 sends the datagram in question to R3, which realizes it needs to send to R4, a router already directly connected to R2. R3 cannot send a Redirect to R2 telling it to use R4 next time. The messages are simply not designed for this purpose—remember that ICMP messages always go back to the source of the original datagram, which would not be R2 in this case. Such inefficiencies must be resolved using routing protocols.

ICMPv4 Time Exceeded Messages 1 2 3 ICMPv4 Parameter Problem Messages