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

4.4 Explicit Rate Feedback Schemes
In July 1994, Charny, Clark and Jain [18] argued that the binary feedback was too
slow for rate-based control in high-speed networks and that an explicit rate indication
would not only be faster but would o er more exibility to switch designers.
In addition to providing a solution to the problems of the bit-based feedback
schemes described in the previous section, explicit rate schemes are attractive for
other reasons. First, policing is straight forward. The entry switches can monitor
the returning RM cells and use the rate directly in their policing algorithm. Second
with fast convergence time, the system come to the optimal operating point quickly.
Initial rate has less impact. Third, the schemes are robust against errors in or loss of
RM cells. The next RM cell carrying \correct" feedback will bring the system to the
correct operating point ina single step.
Further, one of the reasons for choosing the rate-based framework was that ABR
could be used for applications other that just data applications - to provide a cost-
e ective alternative to applications that traditionally use higher priority classes. Typ-
ical applications are compressed video, which could tolerate variable quality. The
explicit rate schemes could reduce the variation in the rates seen at the end-systems,
higher throughput and a controlled delay through the network. Further, the video
applications could directly use the rate values to tune their parameters as opposed to
the credit value, which cannot be directly used without knowledge of the round trip
delay.
In the following sections we survey several rate-based explicit feedback schemes.
In each section, we will have a brief discussion of the key techniques used by the
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