loops. However, if a loop occurs, the Time To Live (TTL) field in the IP header is used to time
out looping packets so that the packets do not loop forever.
of the two codes for Time Exceeded will be described here--namely, the Time To Live (TTL)
code option.
Run was created. When they turned 30, citizens on this planet participated in a religious
ceremony in which they were cremated; the reason was for population control. Logan turned 30
and decided that he did not like the rules--so he ran.
counter expires, so does the packet. Each router decrements the TTL field in each packet header.
(The router does not actually calculate a time that should be decremented; it just decrements by
1.) However, if TTL decrements to 0, the packet is discarded. (For those who remember Logan's
Run, you can think of TTL as the Logan's Run field.)
when TTL expires. The router sends the "ICMP Time Exceeded, code Time To Live Exceeded"
message to the originator of the discarded packet. TTL is used to ensure that packets that are
looping do not do so forever. TTL exceeded lets the originating host know that a routing loop
may be occurring.
sending IP packets (with a UDP transport layer) with TTL set to 1, an "ICMP Time Exceeded"
message is returned by the first router in the route. That's because that router decrements TTL
to 0, causing it to discard the packet, and also sends the "TTL exceeded" message. The trace
command learns the IP address of the first router by receiving the "TTL exceeded" message
from that router. (The trace command actually sends three successive packets with TTL=1.)
Another set of three IP packets, this time with TTL=2, is sent by the trace command. The first
fragment to forward the packet.
Unreachable
transport layer protocol is not available on that host.
destination port has not been opened by an application.
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