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Flow Control - Fixed Versus Demand Allocation (RFC0059) Disclosure Number: IPCOM000003666D
Original Publication Date: 1970-Jun-27
Included in the Prior Art Database: 2000-Sep-13
Document File: 6 page(s) / 17K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

E. Meyer: AUTHOR


The flow control argument is based on the following premises:

This text was extracted from a ASCII document.
This is the abbreviated version, containing approximately 16% of the total text.

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


(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

NWG/RFC 59 Flow Control - Fixed Versus Demand Allocation

manner. IMP messages for a particular "receive" link may be coming in

to the destination HOST faster than the attached process is reading them

out of the NCP's buffers. At some point the NCP will decide that the

input queue for that link is too large in relation to the total amount

of free NCP buffer space remaining. At this time the NCP initiates

quenching by sending a "cease on link" IMP message to its IMP. This does

nothing until the next message for that link comes in to the destination

IMP. The message still gets transmitted to the receiving HOST. However,

the RFNM returned to the transmitting HOST has a special bit set. This

indicates to the originating NCP that it should stop sending over that

link. As a way of confirming the suspension, the NCP sends an SPD

"suspended" NCP control message to...