The MPLS WG Archive

[Curtis Villamizar <curtis@avici.com>]

In message <39AD27F1.CEE36138@marconi.com>, David Charlap writes:
> Curtis Villamizar wrote:
> > 
> > TCP backs off very nicely when congestion occurs so long duration
> > transfer get a little slow for a while (minutes, hours) and then go
> > faster as load lessens.  A fair amount of traffic is still
> > asynchronous bulk transfer (last I looked) and the majority (HTTP) is
> > interactive bulk transfer and is not perceptibly impacted by mild
> > congestion and only gets annoying when moderate congestion occurs.
> 
> And what does this have to do with how a transit router makes
> reservations?

The reservation amounts are based on historic measurements (ie: over
the course of days or longer) of LSP utilization.  This is the way
many ISPs set the bandwidth parameter as reported in the TE WG.  These
bandwidth amounts can be significantly different from the actual
offerred load.  My first point (not quoted above) was that the
bandwidth on reservations is often wrong.  My second point (above) is
that things work even though the reservation amounts are wrong.
That's the only thing it has to do with making reservations.

> Once data enters a tunnel, it is supposed to be opaque to the routers
> until it exits that tunnel.  The fact that it may be using a layer-4
> protocol with congestino control does not mean a switch can ignore its
> reservations.

If a resource (a link or a queue implementing a shore on that link) is
overbooked, some data is lost.  That's where active queue management
(RED) comes into play.  This is an expected behaviour for BE and for
services such as AF.  My point was that the behavior observed by the
end user for mild congestion is imperceptibly different from
uncongested operation.  This is why it makes sense to an ISP as a
service offering and is in fact a very attractive service offering.

The Internet works and is a viable and growing business in case nobody
on this thread has noticed.  My comments above were just intended as
insights into why it works despite not offering guarentees.

Curtis