Traffic Management for the Available Bit Rate (ABR) Service in ...

4. Large legacy switches have a problem in that they are very expensive to replace,
but provide few hooks for adding new functionality. Counters, registers and
pin-outs are always at a premium on a chip, and are rarely left unused. For
example, one might decide to use a switch hook (a pin in the chip) to implement
an improved switch scheme. However, if the scheme requires measurement of
di erent quantities, it cannot be done due to the lack of hooks. For example, the
ERICA switch scheme requires the measurement ofquantities such as the input
rate and the number of active ABR sources. But, the only available metric in the
switch might be just the queue length, which doesnotallow the implementation
of the algorithm to be retro tted on the switch. Simple algorithms which use
just the queue metric need to be used for such cases.
5. Some ABR features such as the Virtual Source/Virtual Destination feature re-
quire the implementation of per-VC queueing. Recall that the requirement of
per-VC queueing was one of the key reasons why the credit-based framework
was rejected. It might seemcontradictory to see per-VC queueing implemented
by all major vendors. However, note that the current switches typically support
upto 128K VCs per port. When the number of VCs grows further (millions of
VCs), the accounting information required and the scheduling overhead is ex-
pected to become prohibitively expensive. In such cases, VCs will be aggregated
into classes and supported by afewthousand class queues.
VS/VD, on the other hand, requires the implementation of the source end sys-
tem rules, scheduling of VC cells at a variable ACR, and the maintainence of
a large amount of state per-VC. This has resulted in the VS/VD option to
be implemented only in very large switches (like satellite switches) where the
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