RFC 59 (rfc59) - Page 1 of 7

Flow Control - Fixed Versus Demand Allocation

Edwin W. Meyer, Jr.
                            MIT Project MAC
                              27 June 1970


The method of flow control described in RFC 54, prior allocation of
buffer space by the use of ALL network commands, has one particular
advantage. If no more than 100% of an NCP's buffer space is allocated,
the situation in which more messages are presented to a HOST then it can
handle will never arise.

However, this scheme has very serious disadvantages:


(i)  chronic underutilization of resources,

(ii) highly restricted bandwidth,

(iii)considerable overhead under normal operation,

(iv) insufficient flexibility under conditions of increasing load,

(v)  it optimizes for the wrong set of conditions, and

(vi) the scheme breaks down because of message length indeterminacy.

Several people from Project MAC and Lincoln Laboratories have discussed
this topic, and we feel that the "cease on link" flow control scheme
proposed in RFC 35 by UCLA is greatly preferable to this new plan for
flow control.

The method of flow control proposed in RFC 46, using BLK and RSM control
messages, has been abandoned because it can not guarantee to quench flow
within a limited number of messages.


The advantages of "cease on link" to the fixed allocation proposal are
that:

(i)  it permits greater utilization of resources,

(ii) does not arbitrarily limit transmission bandwidth,

(iii)is highly flexible under conditions of changing load,

(iv) imposes no overhead on normal operation, and

(v)  optimizes for the situations that most often occur.

Its single disadvantage is that under rare circumstances an NCP's input
buffers can become temporarily overloaded. This should not be a serious
drawback for network operation.

The "cease on link" method of flow control operates in the following