Internet Core Protocols: The Definitive Guide

Name

Router Alert

Synopsis

Used to inform a router that the current IP packet has some peculiarities that should be studied before it is forwarded on.

Class and Code

Class 0, Code 20

Size

Thirty-two bits.

Copy to all fragments?

Yes.

Defined In

RFC 2113.

Status

Proposed Standard, Elective.

Notes

Typically, routers will blindly forward datagrams that are destined for a remote network host or network. They do not normally process datagrams unless those datagrams are explicitly addressed to the router (as indicated by the Destination Address field), or are broadcasts or multicasts that the router is participating in.

However, sometimes the data in a datagram is of such a nature that the router should examine it closely before simply forwarding it on. For example, an experimental form of Path MTU Discovery currently under development requires that routers return bandwidth information about the last network that the probe crossed before reaching the router. In order for this to work, the router has to process the datagram—which is actually destined for a remote host—see that it is a request for MTU information, and then return the requested data. Without this option, the router would simply pass the datagram on to the next-hop router or final destination system.

The two-byte Option-Data field used with Router Alert allows for 65,535 possible numeric codes. The only currently defined code is 0, which states that routers should examine the datagram before forwarding it on. The other 65,534 codes are currently ...