From: Ellen K. 

That's pretty sneaky!    :)

On Fri, 18 Feb 2005 19:48:02 -0500, "Geo" 
wrote in message :

>"Ellen K."  wrote in message
>news:dr8c1117v3gogk0i5qdgti6m35fake7gc6{at}4ax.com...
>> This is interesting, so upfront it already actually knows the route
>> (based on IP address)?   I imagined it learned each next hop from the
>> current one.
>>
>> But waitaminnit, the internet has dynamic routing, right?   (See, I
>> remember stuff I learned here before!!!)   So how can the complete exact
>> route be known upfront?
>
>The short version.
>
>Each packet sent across the internet has a Time To Live (TTL) value. If the
>packet passes thru a router the TTL is decreased by 1, when it reaches zero
>the router that just decreased it to zero trashes the packet and sends an
>icmp packet back to the source (time exceeded ICMP packet).
>
>So tracert sends out a packet with a TTL of 1 (or maybe it starts at 2 I
>forget), which makes the first router return the timed out message, it then
>sends a packet with a TTL of 2 which the second router responds with the
>icmp timeout message, it then sends a TTL of 3 and the third router
>responds, etc. until it gets to the end and then the receiving end sends
>back an ACK saying packet was received ok.
>
>So after this initial burst, tracert knows the IP of every router along the
>initial path and it can then start asking DNS to lookup those IP addresses
>or whatever else it wants to know about them. It also knows the round trip
>time the packet took to go to the router and get back so it can give you
>information about where bottlenecks are and such.
>
>You are right though, the routes are dynamic and may change so if you
>tracert an hour later it may take a different path. But the path it takes is
>usually stable and won't change much unless there is a traffic jam or
>something that makes another path faster.
>
>Ok that's the short version, in reality I believe it actually sends out a
>bunch of packets with the same TTL instead of a single packet to get the
>most likely route and the other stuff has a few more options as well but you
>get the basics.
>
>Geo. (cool stuff, huh?)
>

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