Traceroute works by increasing the "time-to-live" value of
each successive batch of packets sent. The first three
packets sent have a time-to-live (TTL) value of one
(implying that they are not forwarded by the next router and
make only a single hop). The next three packets have a TTL
value of 2, and so on. When a packet passes through a host,
normally the host decrements the TTL value by one, and
forwards the packet to the next host. When a packet with a
TTL of one reaches a host, the host discards the packet and
sends an ICMP time exceeded (type 11) packet to the sender.
The traceroute utility uses these returning packets to
produce a list of hosts that the packets have traversed en
route to the destination. The three timestamp values
returned for each host along the path are the delay (aka
latency) values typically in milliseconds (ms) for each
packet in the batch. If a packet does not return within the
expected timeout window, a star (asterisk) is traditionally
printed. Traceroute may not list the real hosts. It
indicates that the first host is at one hop, the second host
at two hops, etc. IP does not guarantee that all the packets
take the same route. Also note that if the host at hop
number N does not reply, the hop will be skipped in the output.
It works by sending ICMP “echo request” packets to the
target host and listening for ICMP “echo response” replies.
Ping estimates the round-trip time, generally in
milliseconds, and records any packet loss, and prints a
statistical summary when finished.
Added 3/26/2013 1:38:18 AM