ICMP extensions for MPLS

In MPLS networks, when a packet's TTL expires, MPLS strips the MPLS header, encapsulates the remaining datagram into an ICMP time exceeded message, and sends the message to the egress router of the MPLS tunnel. Then the egress router sends the message back to the ingress router of the tunnel. The ICMP message, however, does not contain the label information that is very important to the ingress router. With support for ICMP extensions enabled, the device appends the MPLS label to the ICMP time exceeded message before sending it back to the ingress router of the tunnel.

ICMP extensions are usually used for an enhanced traceroute implementation in MPLS networks, in which MPLS label information of each hop the original datagram arrives at is printed.