TTL expiry and ICMP time-exceeded

Objective

Ping with TTL 2 across three routers: the packet dies in R2 and the sender gets 'time exceeded' instead of a reply.

Procedure

1. Run: no echo reply, but 'TTL exceeded' answers.
2. Raise ttl to 4 in H1's ping line: the pings are answered.

Theory

The Time-To-Live field stops packets circling forever in routing loops. R1 makes it 1, R2 makes it 0 and drops the packet, sending ICMP type 11 back to the sender.

Network blocks used

2 × Host / PC, 3 × Router

More Computer Networks Lab experiments

• RTT and packet loss on a lossy link
• Packet size versus latency
• Queueing delay and loss on an overloaded link
• TCP congestion control (slow start and loss recovery)
• File transfer over TCP: LAN versus WAN