Description of the problem

The customer tests the flow control frames of the EFS4 and EFT4 veneers of Huawei OSN3500 devices through STT tester with the following networking scenarios:

STT------EFS4-----EFS4----STT

STT------EFT4-----EFT4-----STT

When both ends are EFS4 single boards, flow control is running fine, 100M ports, bound 1 VC3 speed is around 48M.

When both ends are EFT4 single boards, the flow control is running normally, 100M port, bound to 1 VC3 speed changes back and forth around 30-60M, packet loss on the receiving end.

The version of the network element is V1R8C02SPC500. Since the rate of the EFT4 single board has been changing, the customer suspects that there is a problem with the processing of flow control on the EFT4 single board.

Alarm Information

None

Processing

Confirmed that it is not a product problem, STT meter responds slowly to ON/OFF pause frames, which leads to a big dynamic change of bandwidth when testing EFT4 single board. It is recommended that subsequent customers use smartbit meters to test Ethernet-related services.

Root Cause

1, through the STT tester on the packet capture, you can see that the EFT4 board and EFS4 sent Pause frame content is different, see attached. It can be seen that EFT4 sends pause frames with pause time=0 quanta and pause time=65535 quanta in a loop, while EFS4 keeps sending pause frames with pause time=2048 quanta. (The Quanta value represents the time it takes to transmit 512bit of data, 512bit/Ethernet Speed). The customer suspects that because the pause time sent by EFT4 keeps jumping, the rate keeps going back and forth and is unstable.

2, query the IEEE802.3 standard, found that the pause time can support the change between 0 and 65535, see annex. As EFS4 and EFT4 single boards use different chips to send flow control frames in different ways, but they are in line with the standard recommendations. EFT4 single board and EFS4 single board two flow control specifications: EFT4 is ON/OFF form: when the EFT4 single board to enable the flow control, EFT4 will be sent to the other end of the Quanta value of 0 or 65535 PAUSE frames; send the value of 0, that is, to let the other end of the PAUSE frame; send the value of 0, that is, to let the other end of the PAUSE frame. value of 0 means to let the peer send the message. A value of 0 allows the peer to send the message, and a value of 65535 stops the peer from sending. When the Quanta value is a value in the middle of 0~65535, it means that the message will be sent only after a certain period. For example, if EFS4 sends a value of 2048, it means that when the peer receives a PAUSE frame, it will take 2048 Quanta cycles to send the message. The minimum Quanta period is 512 bits/pair port rate.

3, suspected that the STT instrument on the ON/OFF form of the pause frame slow response. After letting the customer coordinate with the SMBit instrument flow control frame test, found that no matter which single board Ethernet rate is relatively stable, the results are normal.

Suggestions and Summary

When traffic congestion occurs on a data veneer port, the port mitigates the congestion pressure on the port by sending a PAUSE frame to the peer to stop data transmission for a certain period of time.A PAUSE frame is a specific kind of MAC control frame with a specific multicast destination address of 0180-C200-0001, a type field of 0x8808, and an operation code of 0x0001, and a 2-byte pause-time.

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