Routing around the Internet is a fairly straightforward business, but not easy to understand. What follows is my understanding, and it may be far from perfect. :-)

When a packet is sent/received from a distant machine (IP Address) the route it takes is determined by the routers in between. From your PC, the only available route is through your modem to the PoP or through your Cable Modem to the UBR.

From there, the process is roughly as follows:

1. Packet reaches the next router.

2. Router examines the packet to see if it's arrived at its destination.

3. If yes, stop.

4. If not, see if an existing "best" route exists. All routes have a "TTL" (Time to Live), and if a next-best-hop is known, and its TTL hasn't expired, the router sends the packet to the next hop on that route. Routes are based on "subnets", so a request to route to 193.38.113.6 will be based on finding the best route to 193.*, or 193.38.* or 193.38.113.* first.

   GOTO 1

5. If no "best" route exists, ask other ("primary") routers if they know of a best route. "Best" routes are "advertised" by key nodes such as LINX (London), "VIX" (Vienna), "MAE EAST" and "MAE WEST" (Noo Yawk, I think) etc.

6. If a "best" route is known by a "primary" router, cache that information ("learning the route") and send the packet to the next-best-hop on that route.

   GOTO 1

So, poor routing can be caused by three things:

• A cached "best" route degrades in quality, but is still within its TTL, so the routers will not seek another route. As long as the data gets through to the other end, the route remains valid. Only if the route dies completely will a new route be sought.

• A poor route is advertised as "best" by poorly-configured primary routers. This can be accidental or malicious. The latter can cause data to be "black-hole" (i.e. never reach its destination) and is a subtle but effective Denial of Service attack.

• No "best" route is advertised at all, and the router system has to find its own route. Some may find better routes than others.

http://www.freesoft.org/CIE/Topics/87.htm and http://www.faqs.org/rfcs/rfc1786.html for (much) more detail.

In summary, poor routing can arise without any poor practice. If Support are alerted to poor routes, they can ask the Networking people to expire that route in an attempt to learn a new, faster route. If the degradation in the original route is transient (i.e. it goes slow for only a few minutes), this may actually do more harm than good. Once the original route is back up and running, packets are now going by a second-best route, and will continue to do so until that route expires.

And now...it's Analogy Time. (Yay!)

You send an airmail letter to your Auntie Adrienne in Nepean, a suburb of Ottawa in Canada. You address the letter correctly, and post it.

Your local sorting office doesn't know of Nepean, nor the exact geographic location of Ottawa ("It's in America, innit?"), but they do know that Canada is abroad. They therefore redirect the letter to the Overseas sorting office.

The overseas sorting office knows where Canada is, so directs the letter into the "Canada" pile. The letter is put onto a plane, and flown to Toronto.

In Toronto, the sorting office looks at the letter, and knows it's for Ottawa, so they send it off to Ottawa.

Once in Ottawa, the sorting office knows the letter is for Nepean, so puts the letter in the Nepean pile.

The Nepean pile is then sorted by road names, and given to the postman. He knows all the roads and houses, so makes the final delivery.

Note that, until the postman gets involved, nobody has a clear idea of where the letter is supposed to go; just a good idea of the best direction to send it to get it there fastest.

Now, suppose the airmail plane is forced down in storms, and has to make an emergency landing in Iceland. Repairs take four days before the plane can take off and continue to Toronto to make its (late) delivery. Had the letter gone surface mail, it might have arrived sooner. Air Mail was, in this case, not the best route. It was the best route before, and will be again; but it would be a foolish person who decided, based on that once incident, always to send letters to their Auntie by surface mail because it's faster.

Here endeth the analogy. <grin>

Last Amended : 2002-08-31 by elfin
Original Author : Unknown