Sticky pack issues, and how to invoke operating system commands in Python

A. New module ' subprocess '

review: Invoking system commands in the OS moduleImportOSF= Os.popen ('dir')Print(F.read ()) The result above is the same as the following notationImportSUBPROCESSR= subprocess. Popen ('ls', Shell=true,stdout=subprocess. Pipe,stderr=subprocess. PIPE)#subprocess. Popen (Cmd,shell=true,subprocess.stdout,subprocess.stderr)#cmd: Represents a System command#Shell = True Indicates that this command is a system command that tells the operating system to execute CMD as a system command#stdout is a pipe that is used to save results after a system command has been executed#stderr is a pipe used to hold error results after a system command has been executedstdout = R.stdout.read (). Decode ('GBK') stderr= R.stderr.read (). Decode ('GBK')Print('the correct return result:', stdout)Print('incorrect return result:', stderr)Print('incorrect return result:', stderr)

Two. Sticky packet problem (only TCP protocol will send sticky packets, UDP will not occur)

EX: Sending data , the receiver does not know how to receive , resulting in a data chaos phenomenon

1. The package mechanism (Nagle algorithm): The data that is sent multiple times and with smaller intervals is packaged into a single piece of data transfer .

2. Unpacking mechanism: Another mechanism is the unpacking mechanism , on the sending side , because the MTU limit of the network card , will be larger than the MTU Limit  of Data , split, split into multiple small data , for transmission . When transferred to the operating system layer of the target host, multiple small data are re-merged into the original data

 for sending data using the UDP protocol, how much is the one-time transceiver size appropriate? UDP does not occur sticky packets, the UDP protocol this layer to the size of the first time    to send and receive data is:65535-ip Baotou (8)-UDP packet packet = 65507 station at the data link layer, because the network card MTU is generally limited to 1500, So for the data link layer,
The size of a single transmit and receive  data is limited to 1500-ip Baotou (8) = 1472 :    if SendTo (num)     > 65507   Error      1472 < num < 65507   will be unpacking at the data link layer, and UDP itself is not a reliable protocol, so once the packet is split, the resulting multiple small packets in the network transmission, if you lose any one, then this data transfer failed      < 1472 is a more ideal state

Sticky pack issues, and how to invoke operating system commands in Python