[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 
is critical:

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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