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Re: [Diffserv] Model - queues


A quick comment on your model (which btw is quite comprehensive) and in
particular the FIFO and buffer management component.

It seems that the description you have is limited to buffer management
schemes that make acceptance decisions only at the time of packet
arrival.  This clearly represent the bulk of existing schemes, but there
are other options that i don't think we want to preclude in the future. 
In general, you can have a buffer management scheme that continuously
makes state-dependent decisions on packets already in the buffer, i.e.,
you have the option of changing your mind even after accepting the
packet.  One of the simplest example of such a scheme that has been used
(with limited success though) in ATM switches is the pushout scheme, but
there are other variations that are possible and I think we should try
to have a structure capable of accommodating this.

So I would say that a discarding element is an element which for every
state change in the buffer, i.e., a packet leaving or arriving, is
making decisions on which packets to keep/accept in the buffer based on
a discarding discipline.  This is not much different from what you have,
but I thought it might be important to clarify things.

One maybe trickier implication is wrt the FIFO structure as more complex
discard decisions may require additional information.  For example, a
pushout scheme may require multiple linked lists, one for each pushout
priority.  I don't think that we want to change the FIFO assumption but
it seems that there may be a need for additional "control" structures
together with it.  I don't think this drastically affects your outline,
but this is again something I felt might be useful to keep in mind.

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