Determining gateway buffer capacity


Stuart Levy (slevy@umn-rei-uc.arpa)
Fri, 11 Dec 87 02:23:21 CST


Some recent messages on this list described efforts to define a scheme for
hosts to find a network path's maximum preferred message size, e.g. the
largest datagram which could be sent without some gateway fragmenting it.

Has anyone considered having gateways give advice to hosts on how -much- data
they should send, either in terms of total amount of outstanding data
(which TCPs could use to limit windows) or data flow rate (which, say,
NETBLTs could use to set rate parameters)?

This seems like a natural function to piggyback onto a max-message-size
probe. It suffers from some of the same limitations, that there may be
multiple paths with different behavior.

A gateway giving buffering advice would have to make some assumption about
what fraction of its capacity (or its links' capacity) should be available
to a given session. It might make a static decision ("I expect to have <= 10
active connections passing through me at once, so I advise everybody to use
no more than 1/10th of my capacity") or a dynamic one ("Things are getting
too crowded, so I'll tell everyone to send no more than 1000 bytes per second").
If implemented as an IP option it could be hung on Source Quenches to give
some crude quantitative information. Capacity assumptions could be wrong,
but at least they would prevent hosts with 32K TCP windows from swamping
gateways with 20K of buffer space.

Is anything along these lines being discussed? (Or has it already been
discussed and abandoned?)

                        Stuart Levy, Minnesota Supercomputer Center
                        slevy@uc.msc.umn.edu, (612) 626-0211



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