On Fri, 20 Dec 2002, at 9:44am, [EMAIL PROTECTED] wrote:
> Are the three different times returned between each hop, representative of
> three different probes that are sent from router to router?
Yes.
The traceroute(8) utility works as follows:
It starts with a TTL (Time To Live) set to zero (or one, I forget which),
and sends a packet to the destination address. The next hop will send back
an ICMP (Internet Control Message Protocol) "Time Exceeded" error, since TTL
has expired. Traceroute notes the RTT (Round Trip Time) and prints it on
the screen, along with the IP address of the host that sent the ICMP
message. It then does this two more times.
Next, traceroute increments the TTL by one, and repeats.
Eventually, the TTL will be high enough that the packet actually makes it
to its destination. At that point, the destination host sends back some
kind of reply, so traceroute knows to stop. (The traditional traceroute
implementation uses a UDP packet sent to port 33434, and expects an ICMP
"Port Unreachable" message. The Microsoft implementation uses ICMP "Echo
Request" packets ("ping"), and expects a "Echo Reply" message.)
If traceroute does not receive a reply of any kind within a timeout
period, it prints a star (*) and continues. If a link is down at some
point, replies never make it back, so you see an endless series of stars, up
to the hop count limit in traceroute (30 by default).
--
Ben Scott <[EMAIL PROTECTED]>
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