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 claim.
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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SUMMARY
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. link.
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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