MPLS Fundamentals

TTL Behavior of Labeled Packets

Time To Live (TTL) is a well-known mechanism thanks to IP. In the IP header is a field of 8 bits that signifies the time that a packet still has before its life ends and is dropped. When an IP packet is sent, its TTL is usually 255 and is then decremented by 1 at each hop. If the TTL reaches 0, the packet is dropped. In such a case, the router that dropped the IP packet for which the TTL reached 0 sends an Internet Control Message Protocol (ICMP) message type 11 and code 0 (time exceeded) to the originator of the IP packet.

With the introduction of MPLS, labels are added to IP packets. This calls for a mechanism in which the TTL is propagated from the IP header into the label stack and vice versa. This ensures ...