Internet Core Protocols: The Definitive Guide

Redirect error messages

The Redirect error message is used whenever a router needs to inform a sender of a shorter path to the specified destination. This message is typically seen when users only have a single (default) route defined on a network with multiple routers, and they should be sending datagrams for a specific network to a router other than the default. If the users don’t send the datagrams to the “better” router, then the default router may use Redirect error messages to inform the sender of the correct router to use.

There are some rules that have to be taken into consideration with this design. First and foremost among them is the fact that Redirect error messages can only be sent when the “better” router is on the same subnet as the host that is sending the packets. This is pretty obvious; if the specified router were on a remote subnet, then the host would not be able to forward packets through it to the destination system. Furthermore, Redirect error messages should be sent only from a router on the same subnet as the original sender, since other routers on other subnets are not likely to know the routing paths available to the sender.

RFC 1122 even goes so far as to state that systems should discard any redirect messages that do not originate from first-hop routers, or messages with a suggested route that does not point to another first-hop router. In addition, RFC 1122 also states that whenever a system receives a Redirect error message, it must update its routing ...