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Myths About Congestion Management in High-Speed Networks
Raj Jain, DEC
Provides substance on both sides of several religious arguments on
congestion control, including open-loop vs. closed-loop, one scheme
vs. many, router vs. source, backpressure vs. explicit feedback;
concludes that heterogeneity of traffic and network types is making
congestion worse, and "one size fits all" will never work.
- Previous myths exploded: Congestion will go away when buffers get
bigger, WANs get faster, CPU's get faster, or all of the above.
- Economic arguments for fast WANs:
- high-speed means more sharing, therefore
better cost amortization. Analogy: highways (hi speed) connect
cities (low speed), even
though most traffic is local.
- Today's data networks carry voice traffic as packets;
voice networks carry data as coded voice signals. This
separation cannot persist because it is not economical.
- Rate vs. window based congestion control: Rate-based should
become more popular because
memory is no longer the bottleneck, because window based
flow control is more susceptible to burstiness, and
because much high-speed traffic will require rate rather
than window guarantees.
(One possibility: rate-based with fallback to a large
window size, to be used in case of severe congestion.)
|Control||Window||n packets every T seconds
(most models incorrectly collapse these to single param L=n/T)|
|Required if||Memory is bottleneck||CPU,
link, or rate-based devices are bottleneck|
|Max Q length||determined by
|Burstiness||yes||not at source|
|Controlled||end-to-end, hop-by-hop, or both
|Network layer||Connectionless or
- Router- vs. source-based:
- Arguments against source based: significant delays; sources
may not cooperate; feedback may require additional packets
to be injected (explicit feedback notification); fairness
can't be achieved.
- Argument against router-based: adds instructions to
critical path of packet forwarding; congestion continues
until sources reduce their traffic anyway.
- Summary: use router-based to ensure fairness and for
overload; source-based (and at higher network layers) for
- Backpressure is a datalink level hop-by-hop on/off flow
control that works well in small-diameter networks for
short durations. Unfair in that traffic following
uncongested path can be adversely affected.
- Reservation vs. walk-in: Reservations good for steady,
predictable traffic; walk-in
good for shorter, bursty traffic.
- One scheme vs. many: need one, since congestion control deals
with shared resources. Rule of thumb: the longer the duration of
congestion, the higher the layer in which congestion control
should be implemented.
Collection of "revealed wisdom" about congestion control following
flurry of prior religious papers.
- Most arguments, though intuitively satisfying, are qualitative rather
than quantitative, though there are references.
- Not a prescriptive paper; given all this information, and the
existence of the heterogeneous wide-area Internet, what do we do?
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