JOHNSON@NUHUB.ACS.NORTHEASTERN.EDU.UUCP (02/19/87)
Hi people.
From time to time there has been discussion on the list about what
happens when you try to solve a congestion problem at a particular
layer. Vary often you end up solving the problem at the layer you
wanted only to find that a new layer has become congested. This can
be called Vertically Moving Congestion or VMC.
We've probably all seen VMC happen in many different networks. I
went to a DECUS not long ago and talked to some DECNET wizards. They of
course knew of the problem but hadn't addressed it. I don't think the
x.25 gang have and I'm not sure about ISO. I haven't heard a lot about
this from the TCP/IP bunch either.
Some time ago I tried to arouse a little interest in VMC on the
list and that's exactly what I got, a little interest.
Time has passed and to the best of my knowledge the problem still
exists (unless somebody snuck the solution by when I was asleep). I was
wondering whether or not anyone had given any more thought to the
following question: What kind of traffic monitoring/resource use
information needs to be passed vertically to help alleviate the problem?
Another interesting question: Is the problem solvable?
I've been thinking about this for some years now, ever since I
first ran into it. I'm not even sure I've gotten to the rule of thumb
stage for an answer yet.
Am I showing my ignorance of the literature here? Has someone
solved this behind my back? Does anybody really care?
Any takers?
Chris Johnson
Northeastern University
csnet: johnson@nuhub.acs.northeastern.edu
arpa: johnson%nuhub.acs.northeastern.edu@relay.cs.net
at&t: (617) 437-2335Lixia@XX.LCS.MIT.EDU.UUCP (02/21/87)
> Any takers? Sure. I'm working on the problem. In fact I received your msg just when I came back from picking up a new print of my draft of a group memo, titled "The Interactions of Date Flow Control at Different Protocol Layers" I can give you a copy when it's done (be a bit patient; I have other things to do too). Some brief comments on your message: - I do not agree with the view of "vertically moving congestion". Generally speaking, each data transmission layer (link, network, internet, and transport) should have a flow control mechanism; each control has its own goals and responsibilities; none should be missing. > Vary often you end up solving the problem at the layer you >wanted only to find that a new layer has become congested. This can >be called Vertically Moving Congestion or VMC. You got this feeling because in today's networks the control is often missing at one layer or another (lack of coordiation between layers is also bad). Missing control at one layer can cause problems showing up at other layers. - I agree with you that one hardly can find studies that address this problem. I wouldn't bother to ask standardization people about it, because they don't (SHOULD NOT!) make solutions themselves to unsolved research problems; they (should) only take over solutions after the research is done and the running experience with the solutions has been collected. > I was >wondering whether or not anyone had given any more thought to the >following question: What kind of traffic monitoring/resource use >information needs to be passed vertically to help alleviate the problem? I consider we need two things here: one is a vertical communication channel that can pass info across protocol layers, another is a good flow control mechanism that has control parameters that are meaningful to other layers. I may not have made the second point clear, let me try an example: with a file transfer, can you figure out the actual rate at which data are dumped to the net, by looking at the TCP window size? You probably can't, because the throughput depends on the round trip delay too, and because window doesn't tell you how many retransmissions are made. But if you can't, the network cannot either, which means window size is not very meaningful when passed across layers. > Another interesting question: Is the problem solvable? I feel the question is a bit too broad or unclear -- what is your definition of "solvable"? Anyway, I think the answer is yes by my definition, that is, congestion can be controlled. Lixia -------