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Using Virtual Circuit Window Mechanisms for Metering

IP.com Disclosure Number: IPCOM000053036D
Original Publication Date: 1981-Aug-01
Included in the Prior Art Database: 2005-Feb-12
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Bharath-Kumar, K: AUTHOR [+3]

Abstract

Window mechanisms cause severe buffer congestion when the window size is large. This new mechanism prevents this by sending out only a few messages at a time.

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Using Virtual Circuit Window Mechanisms for Metering

Window mechanisms cause severe buffer congestion when the window size is large. This new mechanism prevents this by sending out only a few messages at a time.

In computer networks, the number of messages permitted on a Virtual Circuit is often regulated by "Window mechanisms". There is a buffering problem if the window size, w, is too large (we will refer to a specific example with w=5O). When a "window permit" arrives at the source node, 50 messages may be sent into the network. This might be too much of a short-term demand on the buffers of nearby intermediate nodes.

This problem occurs when large windows are needed. Examples are virtual circuits with satellite links and circuits that traverse many hops.

The source of the problem is the fact that many messages must be on the virtual circuit at once (requiring a large window size), but only few may be sent off by the source node at once (requiring a small window size). The basic solution is to satisfy the latter constraint by maintaining a small window size and to satisfy the former constraint by permitting many windows to be outstanding at once. Thus a window of windows is to be implemented at the source node.

The details of the technique are best explained with an example. Assume that a window size of 50 is desired from the point of view of a satellite link, but a window size of 5 is desired from the point of view of the intermediate buffers. Then a window size of 5 would be used, but 10 windows could always be outstanding. Each time a window permit arrives, five new messages may be sent out. If there are no messages available for transmission and two window permits arrive, then there are message permits for 10 messages to be sent out as soon as they become ready for transmission.

Generally speaking, the 10 window permits for the 10 outstanding windows should arrive spread apart fairly evenly. It is unlikely that all would arrive at once, since there is a natural spacing apart in the network due to the inherent transmission and propagation delays of the links. Thus in essence, a "metering" scheme is being implemented on the message rate, by taking advantage of the simplicity of window mechanisms and the natural functioning of the network.

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