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Request for Comments # 210                                     W. ConradCategories  C.4                                                  HarvardNIC 7189                                                    16 August 71                      Improvement of Flow ControlThe current "give back" - "return" scheme seems to put the cart beforethe horse in that the "return" command indicates the amount of spacethe sending host is returning rather than the amount of space it hasleft after decrementing by the amount specified in the "give back"command.  Considering the fact that allocation counters at sending andreceiving hosts may get out of synchronization and the fact that thereceiving host has a preemptive priority in the allocation of itsresources, it is only logical that the receiving host be able to findout exactly how much of its buffer space a sending host thinks it canclaim.If the "return" command is to accurately reflect a sending host'scurrent allocation, and if successive "give backs" are to produce"return" commands which can be properly interpreted, certain raceconditions must be avoided. A "give back" must be answered by a"return" and no more "give backs" can be issued until that "return" isreceived.  In some sense, a "return" command as here proposed isreally a give back reply, and, perhaps, should implemented under thatname. On the sending side, the "return" command must not be issued aslong 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" maybe sent and data transmission may continue when the RFNM for thismessage containing the "return" command is received.The current "give back" command uses fractions and has a formatdifferent from the "allocate" and "return" commands making processingunnecessarily complicated. By adopting the convention that allocationscan not be decremented below zero, one can safely specify absolutedecrements in a format like that of the "allocate" command. If thereceiving host's estimate of a suitable decrement is inaccurate, noharm is done and the "return" command in response to the "give back"provides immediate corrective information.                                                                [Page 1]

SUMMARY          Proposal                       Advantage1    "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 ]                                                                [Page 2]