[e2e] Is a control theoretic approach sound?

[Shivkumar Kalyanaraman]

The congestion adaptation takes place on the time scale of RTTs. If you
switch over to a longer RTT path or more congested path, or your window
size is mismatched to that path, you will do damage for at most 1 or 2
RTTs (orders of 10s-100s of milliseconds), whereas the expected time-scale
for routing change "under your feet" is in the order of 10s of seconds
minimum. Change in RTT (except in vegas-like schemes) can be detected
within an RTT; change in congestion or window mismatched is detected the
moment you lose a packet or bunch of packets.

i do not see a significant problem, unless the time-scale of routing
changes becomes smaller (100s of ms) and frequency of routing changes
increases dramatically. But this points to a poorly designed
routing protocol not a poorly designed congestion control scheme. So, as
congestion control people, we view this as out-of-scope for our robustness
considerations.

But to be fair on the paranoia: There is some danger of this kind of
routing behavior with the trend towards high frequency hellos, shorter
timers in routing protocols etc aiming for quicker routing convergence,
with unknown effects on routing stability/robustness. I know KK
Ramakrishnan at AT&T labs was doing a study of congestion control and
routing interactions (OSPF and BGP) on this particular topic.

-Shiv

> There are two different points: whether routing should adapt to the
> congestion state of the network, and whether congestion control scheme
> should be evaluated as to how they react to a routing change, or more
> generally to a change in topology.
>
> The result of the Arpanet experience was indeed that adapting routes to
> congestion state is tricky. Pushing packets from a short congested path
> to a longer less congested path means that each of the individual
> packets will consume more network resource, and thus will contribute
> more to network congestion. Also, moving traffic around in reaction to
> congestion notifications creates a feedback loop between routing and
> congestion, which has a significant potential for creating oscillations.
> There is however a limited amount of load balancing going on today, e.g.
> OSPF or ISIS load-balancing between equal length paths.
>
> But, on the other hand, the topology can change "under your feet", and
> it actually does from time to time. You mention that, on a routing
> change, the TCP window will quickly go down to 1. That is a bit
> cavalier. Will it actually go down? How long does it take to adapt? What
> is the effect of the transients? What if the transition actually results
> in higher capacity? The halving of the window on a congestion
> notification in VJ's algorithm was precisely designed to quickly react
> to such transitions. Some proposed schemes don't have this feature. How
> do they compare?
>
> > -----Original Message-----
> > From: Shivkumar Kalyanaraman [mailto:shivkuma at ecse.rpi.edu]
> > Sent: Friday, August 01, 2003 8:53 AM
> > To: Christian Huitema
> > Cc: John T. Wen; Saverio Mascolo; end2end-interest at postel.org; John
> Wen;
> > Murat Arcak
> > Subject: RE: [e2e] Is a control theoretic approach sound?
> >
> >
> > I have never seen any congestion control simulation in the last 10
> years
> > which also considers effects of routing as well. After the original
> > ARPAnet experience of congestion-sensitive route changes, we have
> always
> > de-coupled the design/analysis of routing from congestion control.
> Routing
> > operates at time-scales much larger than CC.
> >
> > In any case, if there is a route change, TCP sessions will most likely
> > reset their windows to 1, or rapidly detect new congestion and cut
> their
> > windows down.
> >
> > -Shiv
> > ===
> > Shivkumar Kalyanaraman
> > Associate Professor, Dept of ECSE, Rensselaer Polytechnic Institute
> (RPI)
> > 110, 8th Street, Room JEC 6003, Troy NY 12180-3590
> > Ph: 518 276 8979   Fax: 518 276 4403
> > WWW: http://www.ecse.rpi.edu/Homepages/shivkuma
> >
> > A goal is a dream with a deadline -C. Knight
> >
> >
> > On Fri, 1 Aug 2003, Christian Huitema wrote:
> >
> > > There is a big problem with a lot of the control approaches: they
> ignore
> > > the dynamic nature of the network. (The average NS simulation
> certainly
> > > does.) At any given time, some link somewhere is going to be lost, a
> BGP
> > > update will reroute a fraction of the traffic on another path, a new
> > > network will join the Internet, etc.
> > >