JP2832592B2 - Back pressure control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a back pressure control system, and more particularly to a back pressure control system for temporarily stopping cell transfer by notifying a congestion occurring on a link to a preceding switching node. Things.

[0002]

2. Description of the Related Art Generally, in a switching node such as a packet switching device or an ATM switching device, as a congestion control for a link to be accommodated, first, a waiting cell queue of cells stored in a link buffer provided for each link of the switching node. If the length of the waiting cell queue exceeds a predetermined threshold (threshold), it is determined that congestion has occurred on the link, and a predetermined rate control is performed. The transmission rate is reduced to an adaptive rate, and congestion recovery of a congested link is performed.

However, since the transmission rate of the transmitting terminal is reduced by such rate control, unexpected traffic fluctuations, propagation delays, control delays, and the like are interposed. Cell dropping may occur. In particular, in high-speed data communication, when a cell drop occurs, it is necessary to perform retransmission in packet units, and congestion may further spread.

Conventionally, in order to prevent occurrence of such a cell drop, a back pressure control corresponding to a link is activated for a preceding switching node to temporarily stop cell transfer (back pressure). By doing so, a back-pressure type congestion control method for avoiding cell discard has been proposed.

[0005]

However, in such a conventional back pressure control system, when the back pressure control is independently performed in each switching node, the switching of the preceding switch instructed to stop the cell transfer is performed. The node temporarily suspends all cells that are forwarded to the downstream switching node that accommodates the congested link, regardless of whether the cell passes through the congested link at the downstream switching node. A similar waiting state (Head Of Line Blocking) occurs in a cell to another link that has not been connected, and there is a problem that the throughput is reduced.

In order to avoid such a phenomenon,
It is conceivable that the back pressure control is performed only during transient congestion by using the back pressure control together with the rate control. However,
If wide-area, ultra-high-speed data communication is assumed, unpredictable traffic fluctuations and propagation delays will occur until the rate is reduced. There is a concern that this will be prolonged.

[0007] When a huge buffer is held, it is necessary to provide a buffer for each link of the switching node, so that the economic burden increases, and when the back pressure is prolonged, new congestion occurs. To promote
There is a problem that the influence is spread to cells and links not related to the congested link, and congestion of the entire network becomes more serious. An object of the present invention is to solve such a problem, and an object of the present invention is to provide a back pressure control method capable of avoiding a decrease in throughput in a preceding switching node even when cell transfer is stopped. And

[0008]

In order to achieve the above object, a back pressure control system according to the present invention comprises a congestion detecting unit for detecting occurrence of congestion in each link to be accommodated, and a congestion detecting unit for detecting congestion. When the occurrence is detected, a predetermined back pressure notification instructing stop of cell transfer to each connection accommodated in the congestion occurrence link is sent to each preceding switching node in which each connection is accommodated. A congestion notification transmitting unit for notifying, a congestion notification receiving unit for receiving a back pressure notification from another switching node, and a cell transfer to a designated connection based on the back pressure notification received by the congestion notification receiving unit. And an inter-device back pressure execution unit for stopping the operation.

Therefore, in response to the detection of the occurrence of congestion by the congestion detecting unit, the congestion notification transmitting unit issues a predetermined back pressure notification for instructing stop of cell transfer to each connection accommodated in the congestion-occurring link. Is notified to each of the preceding switching nodes in which is accommodated. Further, based on the back pressure notification from the other switching nodes received by the congestion notification receiving unit, the inter-device back pressure executing unit stops cell transfer to the instructed connection.

A congestion detecting unit for detecting the occurrence of congestion on each link accommodated in the switching node, and, when the congestion detected by the congestion detecting unit is that of the input side link, the congestion occurrence A congestion notification transmitting unit for notifying a predetermined back pressure notification for instructing a cell transfer stop to each connection accommodated in the link to each preceding switching node accommodating each connection, and a congestion detection unit If the occurrence of congestion detected by is the output side link, the back pressure execution unit in the device to stop cell transfer to each connection accommodated in the congestion occurrence link, and from the other switching node A congestion notification receiving unit that receives a back pressure notification, and an instruction is given based on the back pressure notification received by the congestion notification receiving unit. It is intended and a device between the back pressure execution unit perform cell transfer stop to connection.

Therefore, when the occurrence of congestion detected by the congestion detecting unit is that of the input side link, the congestion notification transmitting unit instructs to stop cell transfer to each connection accommodated in the congestion occurring link. A predetermined back pressure notification is sent to each preceding switching node in which each connection is accommodated. If the occurrence of congestion detected by the congestion detection unit is that of the output side link, the intra-device backpressure execution unit stops cell transfer to each connection accommodated in the congestion occurrence link. Further, based on the back pressure notification from another switching node received by the congestion notification receiving unit, the inter-device back pressure execution unit stops cell transfer to the instructed connection.

The congestion detecting unit detects the occurrence of congestion on each link, and when the cell transfer is not stopped on the link based on the back pressure control, the used capacity of the link buffer of the link is reduced to the second. The congestion is detected only when the threshold of 1 is exceeded, and when the cell transfer is stopped, the second threshold in which the used capacity of the link buffer of the link is smaller than the first threshold. Even when the value is exceeded, the occurrence of congestion is detected. Therefore, when the congestion detection unit does not stop the cell transfer based on the back pressure control for the link, the congestion occurs only when the used capacity of the link buffer of each link exceeds the first threshold. If the occurrence is detected and the cell transfer is stopped, the occurrence of congestion is detected even when the used capacity of the link buffer of the link exceeds a second threshold smaller than the first threshold. You.

Further, when the back pressure notification is received by the congestion notification receiving unit, the congestion notification transmitting unit sends each of the preceding switching nodes accommodating each of the connections indicated by the back pressure notification to each of the preceding switching nodes. A new back pressure notification instructing stop of cell transfer to a corresponding connection among the connections is sent. Therefore, when the back pressure notification is received by the congestion notification receiving unit, the inter-device back pressure execution unit sends each connection to the preceding switching node in which each connection indicated by the back pressure notification is accommodated. Among them, a new back pressure notification instructing stop of cell transfer to the corresponding connection is notified.

[0014]

Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a back pressure control method in a switching node according to an embodiment of the present invention. Here, an input / output buffer type switching node having link buffers on both an input port side and an output port side is shown. An example is described.

In FIG. 1, reference numeral 1 denotes a congestion detecting unit for monitoring a queue length of a waiting cell stored in a link buffer provided for each link, and detecting the occurrence of congestion when the length exceeds a predetermined threshold. 2 extracts the connection subject to regulation from the connection table 5 of the memory 4 in response to the occurrence of congestion, updates and registers this connection in the regulation registration table 6 of the memory 4, and based on the contents of the regulation registration table 6 It is a congestion notification transmitting unit that notifies a back pressure to the node.

Reference numeral 3 denotes an internal back pressure execution unit for executing back pressure (stop cell transfer) in a link buffer of a link to be accommodated based on the contents of the regulation registration table 6 in response to the notification from the congestion notification transmission unit 2. 7 is a congestion notification receiving unit that receives the back pressure notification notified from the subsequent switching node, and updates and registers the connection to be restricted in the restriction registration table 6 based on the content of the back pressure notification;
Reference numeral 8 denotes an inter-apparatus back pressure execution unit that executes back pressure (stops cell transfer) in the link buffer of the accommodated link based on the contents of the regulation registration table 6 in response to the notification from the congestion notification reception unit 7.

FIG. 2 is an explanatory diagram showing a connection table, and FIG. 3 is an explanatory diagram showing a regulation registration table. The connection table 5 includes a channel identifier (VPI / VCI) of each connection relayed in the switching node.
And the link information (input port and output port) of the link accommodating the connection, and are registered as connection information for each link.

In this case, for example, channel a # 1 is accommodated in link x # 1 on the output port side and connected to channel b # 1 accommodated in link y # 1 on the input port side. It is shown that. In the regulation registration table 6, a channel identifier indicating a connection that requires back pressure control, that is, a connection subject to regulation, is registered for each link. In this case,
Channels b # 1 and b # 2 are registered as connections requiring back pressure control on link y # 1.

Next, the operation of the present invention will be described with reference to FIG. FIG. 4 is an explanatory diagram showing the back pressure control method, in which ST1 and ST2 are transmitting terminals, SW1 is an exchange node accommodating transmitting terminals ST1 and ST2 via a link w # 1, ST3 and ST4 are receiving terminals, and SW3 is Link x
An exchange node accommodating the receiving terminals ST3 and ST4 via # 1 and x # 2, and SW2 is a relay connection exchange node connecting the exchange nodes SW1 and SW3 via links y # 1 and z # 1.

Also, 11 to 15 are the respective links x #
1, link buffers provided in x # 2, y # 1, z # 1, 21 to 25 are thresholds (first thresholds) for congestion detection provided in each link buffer 11 to 15, 31-
Reference numeral 35 denotes a threshold (second threshold) for detecting congestion provided in each of the link buffers 11 to 15, reference numerals 41 to 45 denote congestion release thresholds provided in the respective link buffers 11 to 15, Reference numeral 55 denotes a back pressure notification from the switching nodes SW3 and SW2 to the preceding switching nodes SW2 and SW1, and reference numeral 59 denotes a congestion notification by the conventional rate control.

Now, the receiving terminals ST3 from the transmitting terminals ST1 and ST2 via the link w # 1, the switching node SW1, the link z # 1, the switching node SW2, the link y # 1, the switching node SW3 and the link x # 1. It is assumed that the cell has been transferred to The congestion detection unit 1 of the switching node SW3 constantly monitors the waiting cell queue length in the link buffer of each link.

Here, when the queued cell queue length of the cells stored in the buffer 11 of the link x # 1 exceeds the threshold value 21 and becomes congested, the congestion detecting unit 1 detects this and determines the occurrence of congestion. The notification is sent to the notification transmitting unit 2. The congestion notification transmitting unit 2 reads from the connection table 5 connection information indicating each connection accommodated in the link x # 1 in which congestion has occurred. For example, when the connection table 5 of the switching node SW3 is set as shown in FIG. 2, the connection information of the link x # 1 in the congested state is (a # 1, a # 2, a # 3). See

Subsequently, the connection information (a # 1, a #
2, connection information (b # 1, b # 2, b # 3) on the input port side corresponding to the link y # 1.
As a subject of back pressure control corresponding to
The connection information (b # 1, b #
2) is registered, and connection information (b # 3) is registered as a control target of the link y # 2 (not shown).

Further, since the detected congestion is the link x # 1 on the output port side, the congestion notification transmitting unit 1 notifies the back pressure execution unit 3 in the apparatus of the occurrence of the congestion. In response to this, the in-device backpressure execution unit 3 reads the restricted connection information from the restricted registration table 6. For example, when the restriction target table of the switching node SW3 is set as shown in FIG. 3, (b # 1,
b # 2) is read out, and (b # 3) is read out as the restricted connection information of the link y # 2.

Subsequently, the in-apparatus back pressure executing unit 3 links the links y # 1 and y # connected to the corresponding input port based on the restricted connection information.
Cell transfer is stopped in each link buffer of No. 2. For example, in the link buffer 13 of the link y # 1, based on the above-described restricted connection information (b # 1 and b # 2), the channels b # 1 and
The cell transfer of b # 2 is stopped.

Therefore, in the switching node SW3,
Each cell transferred to the link x # 1 is stopped (back-pressured) in the link buffer on each input port side. However, in this case, the transfer is stopped only for the transfer destination of the link x # 1, and for the cells having the other transfer destinations, for example, the cells of the channel b # 4, the transfer destinations are the respective transfer destinations. , Here link x # 2.

In addition, according to the transfer destination of such a cell,
As a method of transferring an arbitrary cell from the link buffer, a known technique such as a random-in / random-out (RandomIn Random Out) method may be used, and as a simple method, the cells are stored in the link buffer in the order of arrival. A method of sequentially reading cells, skipping cells corresponding to the connection to be restricted, and transferring cells not to be restricted is conceivable.

Next, a case will be described in which a congestion state occurs in a link in which cell transfer suspension for a predetermined connection is executed by back pressure control. As described above, the link buffer 1 of the link y # 1
In 3, when the cell transfer stop is executed for the cells of the channels b # 1 and b # 2 based on the back pressure control, the link buffer 13
May exceed the threshold value 33, and new congestion may occur.

When the congestion detection unit 1 of the switching node SW3 detects the congestion of the link buffer 13 based on the threshold value 33, the detected congestion is converted to the link y # on the input port side.
1, the occurrence of this congestion is notified to the congestion notification transmitting unit 2
Notify to In response, the congestion notification transmitting unit 2
Is the connection information (b # 1, b #) registered in the regulation registration table 6 corresponding to the congestion link y # 1.
2) as back pressure notification 53 between the devices,
The switching node SW2 located before the link y # 1 is notified.

On the other hand, the congestion notification receiving section 7 of the switching node SW2 receives the back pressure notification 53 from the switching node SW3, and receives the restriction object notified by this notification, that is, the connection information (b # 1, b # 2). Link y
The information is registered in the regulation registration table 6 corresponding to # 1. Further, the congestion notification receiving unit 7 notifies the inter-device back pressure execution unit 8. The inter-device back pressure execution unit 8 refers to the regulation registration table 6 and stops cell transfer between the devices b # 1 and b # 2 toward the congestion link x # 1.

The link buffer 14 of the link y # 1
In the case where the cell transfer suspension is executed for the cells of the channels b # 1 and b # 2 based on the back pressure control, the waiting cell queue length of the link buffer 14 exceeds the threshold value 34 due to the subsequent cell arrival. , New congestion may occur. For example, if the connection table 5 of the switching node SW2 is registered as shown in FIG. 5, b # 1, b # 2 heading toward the congestion link x # 1
C # 1 and c # 2 are subject to in-device back pressure, while c # 3 is not targeted and becomes the switching node S
Transferred to W3.

In the switching node SW2, when the waiting cell queue length of the link buffer 14 of the link y # 1 further increases and exceeds the threshold value 24 (threshold value 24> threshold value 34). The new congestion is detected by the congestion detection unit 1 of the switching node SW2.

In this case, the link buffer 1
Similarly to 1, in order to stop cell transfer for all connections provided on the link y # 1, connection information (c # 1, c # 2, c # 3) corresponding to the link y # 1 is obtained from the connection table 5. Is read by the congestion notification transmitting unit 2 and registered in the regulation registration table 6 corresponding to the link z # 1.

Further, in response to the notification of the occurrence of congestion from the congestion detecting unit 1, the congestion notification transmitting unit 2 refers to the connection information to be restricted (c # 1, c # 2, c # 3) and executes the back pressure in the apparatus. In section 3, channels c # 1, c
In addition to # 2, the cell transfer is stopped for channel c # 3.

As described above, each switching node is provided with the congestion detecting unit 1 for detecting the occurrence of congestion in each of the accommodated links.
When the congestion detection unit 1 detects the occurrence of congestion, the congestion notification transmission unit 2 sends a predetermined back pressure notification instructing stop of cell transfer to each of the connections accommodated in the congestion occurrence link. Is notified to each of the preceding switching nodes that are accommodated.

Further, each switching node is provided with a congestion notification receiving unit 7 for receiving a back pressure notification from another switching node, and based on the back pressure notification received by the congestion notification receiving unit 7, an inter-device back-up is performed. The pressure execution unit 8 stops the cell transfer to the specified connection.

Further, when the congestion occurrence detected by the congestion detection unit 1 is for the output side link, the back pressure execution unit 3 in the device transmits cells to each connection accommodated in the congestion occurrence link. When the transfer is stopped and the occurrence of congestion detected by the congestion detecting unit 1 is that of the input side link, the congestion notification transmitting unit 2
A predetermined back pressure notification instructing a stop of cell transfer to each connection accommodated in the congestion link is sent to each preceding switching node accommodating each connection. .

Therefore, by the back pressure notification indicating the cell transfer stop regulation target, only the cell transfer to the congestion-occurring link is performed in the switching node located before the connection accommodated in the congestion-occurring link. While being stopped, cells to links other than the congestion-occurring link will be transferred, and, as in the past, in response to back pressure notification, compared to the case where cell transfer to all links is stopped, It is possible to avoid a decrease in throughput in the preceding switching node.

Further, thresholds 31 to 35 (second threshold: smaller than thresholds 21 to 25 (first threshold: for
Threshold value: for deriving congestion), and when the transfer of cells to the congestion-occurring link is stopped in accordance with the occurrence of congestion in the subsequent link buffer, the threshold values 31 to 31 are set.
35, the occurrence of congestion is monitored by a stricter standard than in the case where cell transfer suspension is not performed, and the resources of the entire network are controlled by the back pressure control. Can be used effectively, and even if the buffer capacity is small,
It is possible to quickly respond to burst traffic and avoid cell drop.

The switching node S that first detects congestion
Rate control notification 5 notified from receiving terminal ST3 to transmitting terminals ST1 and ST2 in response to the congestion occurrence notification from W3.
9, the transmission rates at the transmitting terminals ST1 and ST2 are reduced. Therefore, the back pressure notification source,
That is, in the link buffer of the switching node SW3,
If the waiting cell queue length is smaller than the thresholds 41 to 45 (thresholds 31 to 35> thresholds 41 to 45), this is detected by the congestion detector 1 and the congestion notification transmitter 2 Is notified of the congestion release to the back pressure execution unit 3 in the apparatus.

In response, the intra-device back pressure execution unit 3 releases the suspension of cell transfer (back pressure) for the congestion link x # 1 executed in the switching node SW3. Also, the exchange node SW3
Of the back pressure is sent to the preceding switching node SW2 by the congestion notification transmitting unit 2, and the congestion notification receiving unit 7 of the switching node SW2 receives the notification, and the inter-device back pressure executing unit 8 transfers the cell. The suspension is released.

In the above description, an input / output buffer type switching node having link buffers on both the input port side and the output port side has been described as an example. However, the present invention is not limited to this. An input buffer type switching node having a link buffer only on the port side,
Alternatively, the present invention can be applied to an output buffer type switching node having a link buffer only on the input port side, and the same operation and effect as described above can be obtained. However, in the case of the input buffer type and output buffer type switching node, the switching node that detects the occurrence of congestion does not perform the back pressure control in the device and immediately notifies the preceding switching node of the back pressure. Becomes

In the case of any of the input buffer type, output buffer type, and input / output buffer type switching nodes, thresholds 31 to 35, 41 to 45 are set in accordance with back pressure notifications from other switching nodes. Instead of waiting for the congestion detection based on the notification, the back pressure notification may be immediately performed to the preceding switching node based on the restriction target information of the notification. As a result, it is possible to perform a quicker back pressure notification in the forward direction, and it is possible to suppress the amount of cells stored in the network related to the connection accommodated in the congestion-occurring link and to use the resources of the entire network effectively. It can be used.

[0044]

As described above, the present invention provides a congestion detection unit for detecting the occurrence of congestion in each link accommodated in a switching node, and a congestion notification receiving unit for receiving a back pressure notification from another switching node. When the congestion detection unit detects the occurrence of congestion, each connection accommodates a predetermined back pressure notification that instructs to stop cell transfer to each connection accommodated on the congestion occurrence link. In addition to notifying the preceding switching nodes that are performed, the cell transfer to the designated connection is stopped based on the back pressure notification received by the congestion notification receiving unit.

If the occurrence of congestion detected by the congestion detection unit is that of the output side link, cell transfer to each connection accommodated in the congestion occurrence link is stopped, and the congestion detection unit detects the congestion. If the generated congestion occurrence is that of the input side link, a predetermined back pressure notification instructing stop of cell transfer to each connection accommodated in the congestion occurrence link is sent before the connection is accommodated. This is to notify each switching node of the order.

Therefore, by the back pressure notification instructing the target of the cell transfer stop regulation, only the cell transfer to the congestion link is performed at the switching node located before the connection accommodated in the congestion link. While being stopped, cells to links other than the congestion-occurring link will be transferred, and, as in the past, in response to back pressure notification, compared to the case where cell transfer to all links is stopped, It is possible to avoid a decrease in throughput in the preceding switching node.

Also, the congestion detection unit normally detects the occurrence of congestion only when the used capacity of the link buffer of the link exceeds the first threshold, and when the cell transfer is stopped by the back pressure control, Since the occurrence of congestion is detected even when the used capacity of the link buffer of the link exceeds the second threshold value smaller than the first threshold value, when the cell transfer is stopped, compared with the normal time, The occurrence of congestion is monitored according to strict standards, and these back pressure controls enable effective use of resources as a whole network. Even if the buffer capacity is small, respond quickly to burst traffic and avoid cell loss. Becomes possible.

Further, when the back pressure notification is received by the congestion notification receiving unit, the corresponding connection among the respective connections is sent to the preceding switching node in which each connection indicated by the back pressure notification is accommodated. A new back pressure notification that instructs the cell transfer to stop, it is possible to carry out a quick back pressure notification in the forward direction, and the cell related to the connection accommodated in the congestion occurrence link Can be suppressed in the network, and the resources of the entire network can be effectively used.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a back pressure control method in a switching node according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a connection table.

FIG. 3 is an explanatory diagram showing a regulation registration table.

FIG. 4 is an explanatory diagram showing a back pressure control method.

FIG. 5 is an explanatory diagram showing a connection table of another switching node.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Congestion detection part, 2 ... Congestion notification transmission part, 3 ... In-device back pressure execution part, 4 ... Memory, 5 ... Connection table, 6 ... Regulation registration table, 7 ... Congestion notification reception part, 8 ... Inter-device back pressure Execution unit, SW1 to S
W3: switching node, ST1, ST2: transmitting terminal, ST
3, ST4: receiving terminal, x # 1, x # 2, y # 1, z #
1, w # 1 link, 11-15 link buffer, 2
1 to 25... Threshold value (first threshold value), 31 to 35.
Thresholds (second thresholds), 41 to 45: thresholds (congestion release thresholds), 53, 55: back pressure notification, 59: congestion notification by rate control.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-285532 (JP, A) JP-A-3-175584 (JP, A) JP-A-7-183886 (JP, A) JP-A-1- 272245 (JP, A) Proceedings of the IEICE General Conference, B-741 (March 11, 1996) "Proposal of a backpressure control method using congestion connection notification" Masami Kawami (58) Int.Cl. ⁶ , DB name) H04L 12 / 28,12 / 56

Claims (4)

(57) [Claims] In a back pressure control method for instructing each preceding switching node to stop cell transfer when congestion occurs in a link accommodated in the switching node, the switching node A congestion detection unit that detects the occurrence of congestion on the link, and if the occurrence of congestion is detected by the congestion detection unit,
A congestion notification transmitting unit that notifies a predetermined back pressure notification instructing stop of cell transfer to each connection accommodated in the congestion occurrence link to each of the preceding switching nodes in which each connection is accommodated, A congestion notification receiving unit that receives a back pressure notification from another switching node; and, based on the back pressure notification received by the congestion notification receiving unit, executing inter-device back pressure that stops cell transfer to the specified connection. Back pressure control system characterized by comprising 2. A back pressure control method for instructing each preceding switching node to stop cell transfer when congestion occurs in a link accommodated in the switching node. A congestion detection unit for detecting the occurrence of congestion on the link; and, when the congestion occurrence detected by the congestion detection unit is that of the input side link, cell transfer to each connection accommodated in the congestion occurrence link. A congestion notification transmitting unit for notifying a predetermined back pressure notification for instructing a stop to each of the preceding switching nodes in which each of the connections is accommodated, and the occurrence of congestion detected by the congestion detection unit indicating that the congestion has occurred. If it is, a back pressure execution unit in the device that stops cell transfer to each connection accommodated in the congestion link, A congestion notification receiving unit that receives a back pressure notification from the switching node, based on the back pressure notification received by the congestion notification receiving unit, an inter-device back pressure execution unit that stops cell transfer to the instructed connection; Back pressure control system characterized by comprising: 3. The back pressure control method according to claim 1, wherein the congestion detection unit, when detecting occurrence of congestion on each link, suspends cell transfer for the link based on the back pressure control. If no, the congestion is detected only when the used capacity of the link buffer of the link exceeds the first threshold, and when the cell transfer is stopped, the link buffer of the link is A back pressure control method, wherein the occurrence of congestion is detected even when the used capacity exceeds a second threshold value smaller than the first threshold value. 4. The back pressure control method according to claim 1, wherein the congestion notification transmitting unit is configured to, when a back pressure notification is received by the congestion notification receiving unit, connect each connection indicated by the back pressure notification. A back pressure control method, wherein a new back pressure notification for instructing each of the preceding switching nodes to be accommodated to stop cell transfer to a corresponding one of the connections is notified.