Method and apparatus to properly route ICMP messages in a tag-switching network

摘要

A tag-switching router on a tag-switching network properly generates and forwards an ICMP message when an input packet containing a faulty tagged IP datagram is received. The faulty IP datagram includes source and destination IP addresses and is prepended with a tag stack containing a top tag entry having a top tag. Using the top tag to point to an entry in a forwarding table, the tag-switching router retrieves both a replacement tag and the identity of a forwarding link, and replaces the top tag in the tag stack with the replacement tag. The tag-switching router prepends the tag stack containing the replacement tag to an output IP datagram, which contains the ICMP message and also includes the faulty IP datagram's source IP address as its destination IP address, and discards the faulty IP datagram. The tag-switching router forwards the output IP datagram in an output packet over the forwarding link so that the ICMP message continues over the route that was originally specified for the faulty IP datagram. When the ICMP message finally leaves the tag domain, i.e., when it reaches a router that does not perform tag switching, it is turned around and sent back onto the tag-switching network and, in accordance with the destination IP address of the output IP datagram, back over the original route to the source of the faulty IP datagram.