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.

[Page 1]

---

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 ]

[Page 2]

---