[e2e] link between Kelly's control and TCP's AIMD
David P. Reed
dpreed at reed.com
Fri Feb 25 07:06:12 PST 2005
Just two quick high-level observations, since framing the issue properly
1) TCP is a protocol that is designed for networks, not links. That
means that many diverse users with diverse and unpredictable (not
gaussian, not time-invariant, not stationary, not independent - i.e.
theoretical simplification is nice, but optimizing a theoretical model
is only relevant to the limited extent that the real world is like the
2) TCP is designed to be robust over a wide variety of network
implementation choices, and should not depend on assumptions like
"routes are stable and unchanging".
3) TCP, and the Internet itself, are not designed to maximize the
utilization of the wires. The mathematicians looking for objective
functions to optimize, and the elders who remember when wires were
expensive, but haven't paid attention for 50 years, tend to focus on
this objective function. They remind me of people who would decide
that a highway system was optimally operating when cars are going 1 mph
with 1 inch between them in all directions. Yes, the wires would be
full, and the throughput (in cars per minute) would be pretty high, but
the latency would suck.
A reasonable objective function of the network is the probability that a
user walking up to the network to send a message or get an MP3 file or a
web page is getting a service that costs less than what the value of the
service is to him/her. This has precious little to do with utilization
of the wires, it turns out, in any practical network. I'd fire any
graduate student or post-doc who thought that utilization was
"obviously" the best metric for network performance.
Peer reviewers in network protocol performance work who rate purely
mathematical exercises highly because they solve irrelevant problems
have accepted far too many papers that focus on "utilization" as a
metric. Read Andrew Odlyzko's many papers to get a better idea of what
metrics actually matter in practice - and understand why 10% peak
utilization is a typical, desirable corporate network operating point,
and 50% is a sign of bad management.
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