Tcp/ip explanation (version 2) notes / 3 link layer / 3.3 full duplex, energy saving, automatic negotiation mechanism, 802.1x flow control / 3.3.3 link layer flow control

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3.3.3 Link layer flow control

Run extended Ethernet in full duplex mode and span different speed segments , It may be necessary for the switch to buffer frames （ Storage ） A span
for example , When multiple sites are sent to the same destination （ Output port contention ）, That's what happens
If the total traffic rate to a site exceeds the link rate of the site , Then the frame will start to be stored in the middle switch
If this continues for a long time , Some frames may be discarded

One way to alleviate this situation is to apply flow control （flow control）（ namely ： Slow them down ）
Some Ethernet switches （ And interface ） Through the switch and Network interface card （NIC） Send special signal frames to realize flow control
Although the specification does not specify specific implementation details , But when the flow control sends a signal to the sender , The sender must reduce its transmission rate
Ethernet is used by 802.3x [802.3-2008] The specified is called PAUSE news （PAUSE messages）（ Also known as PAUSE frame （PAUSE frames）） The realization of flow control based on

PAUSE The message is contained in MAC Control frame , By value 0x8808 And use MAC Control opcode 0x0001 Ethernet Length/Type Field to identify
It is recommended that stations receiving such signals reduce the transmission rate
PAUSE Frames are always sent to MAC Address 01:80:C2:00:00:01, And only for full duplex links
PAUSE The frame includes a Delay （hold-off） Time value （ With quantum （quantas） Appoint , be equal to 512 Bit time ）, This value indicates how long the sender should pause before resuming transmission

MAC The frame format of the control frame is shown in figure 3-3 The conventional packaging method shown in , But in the Length/Type The field is followed by a 2 Bytes of opcode （opcode）
actually ,PAUSE Frames are the only ones used MAC Control the frame type of the frame
They include a coding delay time 2 Byte quantity
“ Whole ” MAC Control layer （ It's basically just 802.3x flow control ） The implementation of is optional

Because the Ethernet layer flow control may have a great negative impact , Therefore, it is generally not used
When multiple sites pass through an overloaded switch （ See the next section ） When sending data , The switch sends... To all hosts PAUSE frame
However, the memory usage ratio of the switch may be uneven for different sending hosts , Even though some hosts are not responsible for most of the traffic through the switch , But may still be punished （ flow control ）

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