Flow control mechanism

Switch flow control mechanism

Network congestion is generally due to rate mismatch (such as 100M to 10M port to send data) and burst of centralized transmission, it may lead to these situations: delay increase, packet loss, retransmission increase, network resources can not be effectively utilized.

Ieee

802.3X Specifies the format of a 64-byte "PAUSE" Mac control frame. When a port is blocked, the switch sends a "pause" frame to the information source, telling the source to pause for a period of time before sending the message. In the actual network, especially the general LAN, the situation of network congestion is very small, so some manufacturers of switches do not support traffic control. High-performance switches should support reverse pressure and full-duplex ieee802.3x flow control in half-duplex mode. The traffic control of some switches will block the input of the whole LAN, reduce the performance of the whole LAN, the strategy of high performance switch is to block only the port that enters the frame to the switch congestion port, and ensure the normal operation of other port users.

Back pressure algorithm (backpressure)

Bridged or switched half-duplex Ethernet uses the CSMA/CD mechanism to handle transmission problems between stations with different speeds, using a so-called "back Pressure (backpressure)" concept. For example, if a high-speed 100Mbps server sends data to a 10Mbps client through a switch, the switch will buffer its frames as much as possible, and it notifies the server to suspend sending once the switch's buffer is about to fill.

There are two ways to do this: the switch can force a conflict with the server, causing the server to retreat, or the switch will keep the server's port busy by inserting a "carrier check" so that the server senses that the switch is sending the data. With both methods, the server pauses the send for a period of time, allowing the switch to process the data that accumulates in its buffer

IEEE802.3x-Send Pause frame

In a full-duplex environment, the connection between the server and the switch is a collision-free send and receive channel. Because there is no collision detection and the switch is not allowed to cause the server to stop sending through a single conflict, the server will always be sent to the switch frame buffer overflow. Therefore, the IEEE has developed a combined full-duplex flow control standard of 802.3x. The IEEE802.3X standard defines a new method to achieve flow control in a full-duplex environment. The switch generates a pause frame, and the pause frame uses a reserved multicast address: 01-80-c2-00-00-01, which is sent to the station being sent, and the sending station pauses or stops sending after it receives the frame. Pause frames Take advantage of a reserved multicast address, which is not forwarded by the bridge and the switch so that the pause frame does not generate additional information.

IEEE802.3X defines a 64-byte pause frame, and when the port is blocked, the switch sends a pause frame to tell the other party that it is busy. Pause for a period of time in the send.

In the actual network, because there are few ports blocking, the general manufacturer's switch does not match the function. High-performance switches should support back pressure and ieee802.3x flow control. The traffic control of the normal switch will block the input of the entire LAN, while the high-performance switch blocks only one port input. A half-duplex switch or bridge uses 1 ways to avoid blocking, and one is to reverse the pressure.

such as: a hundred MB server to a 10MB client computer to send data, the switch will be as much as possible to buffer its debugging,

If the switch's buffer is nearly full, the server will be notified to suspend sending and continue when the frame in the buffer is finished processing

Send.

There are 2 ways to implement this feature. One is to forge a blocking message to the server to stop the server from sending, and the other

The type is the frame that sends a carrier reconnaissance. Keeping the server ports busy makes the server feel like the switch is ready to send data.

All 2 of these methods allow the server to pause the sending of frames, so that the switch has data to process the buffers.

In full duplex due to the fact that the server and the switch are a collision-free information channel, the switch cannot be sent because there is no collision

Send a conflict to let the server temporarily stop sending, then the server will always send frame to buffer benefit. So the IEEE defines

802.3x.

Pause frame in order to prevent buffering benefit, pause frame can exceed the design level of the device, can short delay the flow of the long,

The device prevents its own buffering benefit by sending pause to the peer device.

The pause feature does not resolve the following issues: • Stable

The amount of data, easing the instantaneous overload

Rather than the flow control can solve the question

Network congestion, the setting of the Pause protocol

Case If the continuous flow exceeds the set

Problem. Pause frame does not resolve persistent stream

By reducing the arrival time of the buffer overflow

Design capability, then this is a configuration problem,

The amount of overload.

• Provide end

To the end of the flow control problem, nor

-to-end flow control. The pause operation only depends on

Can coordinate operations on multiple links.

On a direct-attached full-duplex link, it does not resolve the

• Provides more than simple "stop-and-start"

Rate of flow control and so on.

Complex mechanisms.