Multiplex communications – Data flow congestion prevention or control – Control of data admission to the network
Reexamination Certificate
1998-12-11
2003-09-02
Kizou, Hassan (Department: 2662)
Multiplex communications
Data flow congestion prevention or control
Control of data admission to the network
C370S235000, C370S391000
Reexamination Certificate
active
06614755
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for controlling congestion from a terminal in a frame relay network, and in particular, to a method for controlling congestion from a frame relay terminal (hereinafter referred as a terminal) connected to a digital network providing a frame relay bearer service, which is called a frame relay network (hereinafter referred as a “network”). The terminal may be a terminal device such as a terminal adapter, a communication control device, and a communication protocol conversion device.
This application is based on Japanese Patent Application No. Hei 9-363107, the contents of which are incorporated herein by reference.
BACKGROUND ART
In congestion control on a frame relay network, when the network becomes congested, a congestion notification is sent to a terminal, which then restricts data traffic (ITU-T recommendation: Q.922 ISDN data link layer specification for frame mode bearer service).
In one conventional method for providing congestion control in the frame relay network, discarding of data caused by the congestion is avoided to enhance network throughput (Japanese Patent Application, First Publication No. Hei 8-149156), and, in another conventional method, a frame relay communication processing device executes an interrupt to alleviate the congestion (Japanese Patent Application, First Publication No. Hei 8-85337).
In general, there are three types of method for explicitly sending a congestion notification through a network to a terminal:
1) setting to 1 a forward explicit congestion notification (FECN) bit, which is included in a frame transmitted to the terminal,
2) setting to 1 a backward explicit congestion notification (BECN) bit, which is included in a frame transmitted to the terminal, and
3) voluntarily transmitting a consolidated link layer management (CLLM) message to the terminal.
There are two types of method for controlling congestion from the terminal which receives the notification explicitly indicating the congestion in the network:
a) a throughput control method for controlling a quantity of transmitted data during a certain period of time (throughput), and
b) a window control method for altering a transmission window size WS which represents the number of frames transmittable without requesting acknowledgement of receipt from the terminal.
The two congestion control methods are intended to restrict the quantity of transmitted data when the network becomes congested and to restore the quantity of transmitted data after the congestion subsides.
With reference to
FIG. 2
, the basic steps of the conventional window control method will be explained from the occurrence of the congestion during data transmission from the terminal to the restoring the normal quantity of transmitted data to the normal level after the subsidence of the network congestion.
Before the occurrence of the congestion, the terminal transmits data while maintaining a transmission window size WS at the maximum window size MW (for example, 32) (in step P
201
). When the network becomes congested (in step P
202
), the terminal recognizes the congestion by the congestion notification transmitted through the network (in step P
203
). The terminal, after the recognition of the congestion, reduces the quantity of transmitted data to decrease a load to the network. The reduction is executed by, for example, the following window control process for reducing the transmission window size WS (in step P
204
).
In case of the congestion notification using a BECN bit, on receipt of a frame with the BECN bit set to 1 (congestion notification frame), the terminal alters the transmission window size WS according to WS=present WS×0.675 (where WS≧1). Further, when the stepcount S number of consecutive frames are received with the BECN bits set to 1, the alteration is repeated. The stepcount S indicates the number of frames to be received within twice the length of time of a frame transmission delay from one terminal to another terminal under the maximum throughput.
When the subsidence of the congestion is detected (in step P
205
), the terminal detects the subsidence by receiving no congestion notification frame (in step P
206
). On receipt of the congestion subsidence notification through the network, the terminal restores the quantity of the transmitted data to normal. The transmission window size WS is enlarged (in step
207
) to the initial maximum transmission window size MW (in step P
208
) by, for example, the following process.
On receipt of S/2 (half of the stepcount S) consecutive frames with the BECN bits=0, the terminal alters the transmission window size WS according to WS=present WS+1.
In the above-mentioned conventional window control method based on the congestion notification using the BECN bit, when there are few frames traveling in the direction opposite to the congestion, there is a problem in that the terminal cannot sufficiently implement the flow control. For example, when the stepcount S is 100 and there are two frames per second traveling in the direction opposite to the congestion, it takes 50 seconds from the reduction of the transmission window size WS on receipt of the first BECN bit set to 1 until the next reduction of the transmission window size WS. This applies to the avoidance of the congestion, under which it takes 25 seconds to enlarge the transmission window size WS to the next step size. Thus, even though congestion may occur, the transmission window size WS is not reduced immediately, and neither the transmission window size WS is increased immediately after the congestion subsides, resulting in inefficient throughput.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method for controlling congestion from a terminal in a frame relay network, which can provide efficient window control according to the status of a network irrespective of a quantity of frames traveling in the direction opposite to the congestion.
In order to accomplish the above object, a method for controlling congestion from a terminal in a frame relay network, according to the present invention comprises the step of reducing and increasing a quantity of transmitted data, eliminating the influence of the quantity of data transmitted through the frame relay network. In one aspect of the present invention, a transmission window size is reduced based on a ratio of frames with BECN (backward explicit congestion notification) bits set to frames received in a certain period of time.
In another aspect of the present invention, a method for controlling congestion from a terminal in a frame relay network, comprises the steps of: detecting congestion in the network by receiving a congestion notification frame with a BECN bit=1; reducing a transmission window size based on the ratio of the congestion notification frames with the BECN bits=1 to received frames; enlarging the transmission window size, when the congestion subsides and frames with the BECN=0 in the received frames increase; and stopping the enlargement of the transmission window size when the transmission window size attains the maximum transmission window size.
In another aspect of the present invention, a method for controlling congestion from a terminal in a frame relay network, comprises the steps of: detecting congestion in the network by receiving a congestion notification frame with a BECN bit=1; reducing a transmission window size on receipt of the first congestion notification frame; starting a timer; resetting a frame number counter and a congestion notification frame number counter; determining whether a measurement interval has passed since starting the timer; determining whether a frame is received when the measurement interval has not passed; increasing the frame number counter incrementally when the frame is received; determining whether the BECN bit of the received frame is set to 1; increasing the congestion notification frame number counter incrementally when the BECN bit
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