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Request for Comments # 210                                     W. Conrad
Categories  C.4                                                  Harvard
NIC 7189                                                    16 August 71

                      Improvement of Flow Control

The current "give back" - "return" scheme seems to put the cart before
the horse in that the "return" command indicates the amount of space
the sending host is returning rather than the amount of space it has
left after decrementing by the amount specified in the "give back"
command.  Considering the fact that allocation counters at sending and
receiving hosts may get out of synchronization and the fact that the
receiving host has a preemptive priority in the allocation of its
resources, it is only logical that the receiving host be able to find
out exactly how much of its buffer space a sending host thinks it can

If the "return" command is to accurately reflect a sending host's
current allocation, and if successive "give backs" are to produce
"return" commands which can be properly interpreted, certain race
conditions must be avoided. A "give back" must be answered by a
"return" and no more "give backs" can be issued until that "return" is
received.  In some sense, a "return" command as here proposed is
really a give back reply, and, perhaps, should implemented under that
name. On the sending side, the "return" command must not be issued as
long as a data RFNM is awaited on the link to which the "return"
refers. As soon as the net is clear of data messages, the "return" may
be sent and data transmission may continue when the RFNM for this
message containing the "return" command is received.

The current "give back" command uses fractions and has a format
different from the "allocate" and "return" commands making processing
unnecessarily complicated. By adopting the convention that allocations
can not be decremented below zero, one can safely specify absolute
decrements in a format like that of the "allocate" command. If the
receiving host's estimate of a suitable decrement is inaccurate, no
harm is done and the "return" command in response to the "give back"
provides immediate corrective information.

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          Proposal                       Advantage

1    "Return" specifies amount       Provides more pertinent
     of space left after             information and a means
     decrementing.                   of resynchronization other
                                     than closing connection.

2    "Give Back" must get            Provide more accurate
     "return" in reply and           allocation information
     "return" must not be            by eliminating race
     sent with data on the           conditions.

3    Eliminate fractions             Eliminate messy math
     from "give back".               and provide symmetry
                                     to allocation commands
                                     making processing easier.

       [ This RFC was put into machine readable form for entry ]
        [ into the online RFC archives by Gunnar Reichert 6/97 ]

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