Description of the problem

After the new Ethernet private line service, often use FTP software to test the download rate, often because of the delay is too large resulting in the download rate does not meet the standard, why the delay will have an impact on the download rate?

Solution

1, first of all, first recognize the concept of delay, delay is a message or packet from one end of a network to the other end of the transmission time required, corresponding to the SDH network, the delay calculation formula is generally: t = (5us * k + 250us * n) * 2 + 1ms. where K indicates the distance between the two ends with units of KM, n indicates the number of intermediate through the network elements, 1ms indicates that the sending and receiving ends. Equipment for Ethernet encapsulation and de-encapsulation delay, testing the delay between the two points AB generally use the PING test method.

2, FTP software uses TCP protocol, TCP has the concept of sliding window, that is, the sender must send the corresponding bytes in accordance with the size of the sliding window, and need to be received by the receiver to confirm that you can continue to send more bytes. Example: Assuming that the PCs at both ends of the Ethernet leased line are docked, the MTU of the PC = 1500 Byte, the maximum number of windows w = 16, then the maximum number of bytes that can be transmitted by the sending end each time = 1500 * 16 = 24000 Byte = 192000 Bit If the delay is 30ms, that is, the data is sent from the sending end to the receiving end to receive the confirmation of the information for the time of 30ms, then this time the Download rate = the maximum number of bits can be transmitted / delay, that is, the download rate = 192000 / 30 = 6.4Mbit / S = 0.8MB / S. From the above, we can see that when the use of FTP single-threaded download, the sliding window is fixed, the greater the delay, the smaller the corresponding download rate.

3, synthesize the above, in the download rate test, it is recommended to try to use multi-threaded download method or increase the TCP sliding window for testing.

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