Peter Lothberg <roll@Stupi.SE> wrote:
On the radio1:: "101 north is congested, take 280 instead"
Guess what hapens 5 minutes laster; On the radio2:: "280 north is congested, take 101 instead"
goto on the radio1
Goto the theory of control systems :) This is a textbook case of a system with too strong negative feedback loop. There are two ways to deal with that -- reduce the feedback (say, make the announcement in a barely intelligible speech, so only 1/3rd of drivers will get it :), or increase inertia of the system (by posting 10MPH speed limit :), so announcements can come fast enough to compensate each other. Seriously, the load-sensitive traffic management _can_ be done, but to do it you a) need full knowledge of the load patterns, plus some historical data, and b) have a way to control system in steps less drastic than rerouting an entire flow. Can't be done with ciscos, or any existing routers... --vadim
Is this really a discussion for this list-server? I don't know about all of you, but I don't have the time to continually read a re-hash of age-old discussion. Peter Lothberg <roll@Stupi.SE> wrote:
On the radio1:: "101 north is congested, take 280 instead"
Guess what hapens 5 minutes laster; On the radio2:: "280 north is congested, take 101 instead"
goto on the radio1
Goto the theory of control systems :) This is a textbook case of a system with too strong negative feedback loop. There are two ways to deal with that -- reduce the feedback (say, make the announcement in a barely intelligible speech, so only 1/3rd of drivers will get it :), or increase inertia of the system (by posting 10MPH speed limit :), so announcements can come fast enough to compensate each other. Seriously, the load-sensitive traffic management _can_ be done, but to do it you a) need full knowledge of the load patterns, plus some historical data, and b) have a way to control system in steps less drastic than rerouting an entire flow. Can't be done with ciscos, or any existing routers... --vadim
Seriously, the load-sensitive traffic management _can_ be done,
Can't be done with ciscos, or any existing routers...
Just the other day an engineer was telling me that he was adjusting metrics to better balance load and account for congestion. I was doing the same last week. As always, it seemed to work quite well. So it can and is being done. It wasn't enough of a mental effort that I believe that computers couldn't do it better. I agree that "make small changes slowly" is good advice.
Seriously, the load-sensitive traffic management _can_ be done,
Can't be done with ciscos, or any existing routers...
Just the other day an engineer was telling me that he was adjusting metrics to better balance load and account for congestion. I was doing the same last week. As always, it seemed to work quite well. So it can and is being done. It wasn't enough of a mental effort that I believe that computers couldn't do it better.
I agree that "make small changes slowly" is good advice.
This will be my last followup on this topic. And this is not meant as a flame. There are some rather simple problems that humans can do OK (not provably optimally but OK) that are exponential wrt the number of data points to code mathematically/algorithmically/to a computer. In fact, many of those problems (the class "NP-Complete") are really in some sense the same problem. I don't know, but I suspect that someone has proven already that many optimum route-selection algorithms (or worse, many parts of route-selection algorithms) are really the traveling salesman problem in disguise. Or the arbitrary object recognition problem. Or any NP-complete problem. Of course, noone KNOWS for sure that there isn't a sub-exponential time solution to the class NP-complete. If you can solve the hard routing problems, you may just have won the Nobel Prize by showing that NP-Complete problems can be solved in less than exponential time. At some point, the IETF (or specific working groups) is/are a much better place to discuss this than NANOG. But in general, you'll (the general you here, not one person in specific) get more sympathy if you say "I have an idea to base route selection on X, Y, and Z - and to not have it melt all the other routers by not having it send the new idea of best next-hop except to routers with the flooby flag set in the bgp neighbor table" than "I can do it, I want my Ciscos to do it". Avi
Vadim, are you shure about this? Withouth the friction - what would you got this case? Very big pendulum, isn't it?
There are two ways to deal with that -- reduce the feedback (say, make the announcement in a barely intelligible speech, so only 1/3rd of drivers will get it :), or increase inertia ^^^^^^^^^^^^^^^^
of the system (by posting 10MPH speed limit :), so announcements can come fast enough to compensate each other.
Seriously, the load-sensitive traffic management _can_ be done, Exactly it's done in the lot of different systems.
but to do it you a) need full knowledge of the load patterns, plus some historical data, and b) have a way to control system in steps less drastic than rerouting an entire flow.
Can't be done with ciscos, or any existing routers... I am not shure about EGRP, through.
--vadim
--- Aleksei Roudnev, Network Operations Center, Relcom, Moscow (+7 095) 194-19-95 (Network Operations Center Hot Line),(+7 095) 239-10-10, N 13729 (pager) (+7 095) 196-72-12 (Support), (+7 095) 194-33-28 (Fax)
participants (5)
-
alex＠relcom.eu.net -
Avi Freedman -
Daryn D. Fisher -
jon＠branch.net -
Vadim Antonov