What is the lszrz protocol used at ordinary times? Talk about xmodem/ymodem/zmodem

XMODEM, YMODEM, and ZMODEM

Because of the usual use of rz/sz More ,r/s A good understanding , One send One receive. But the reason is not clear z What does that mean? , Therefore, there is this article. .
sx/rx, sb/rb (b=batch) and sz/rz Implemented separately xmodem,ymodem and zmodem File transfer protocol .

Xmodem(1977 Ward Christensen )->Ymodem(1985 Chuck Forsberg )->Zmodem(1986 Chuck Forsberg)

To have a first Xmodem, Later, it gradually evolved into Ymodem, however Ymodem It's not fast enough and has many shortcomings , Then it developed into Zmodem, This is one of the many file transfer protocols we use in our daily work .

Xmodem

One of the widely used file transfer protocols . The original Xmodem Agreement to use 128 Byte packets and simple The checksum Method as error detection . Subsequently improved to Xmodem-CRC, Use a more secure cyclic redundancy check （Cyclic Redundancy Check ） Method as error detection .Xmodem Protocols always try to take precedence CRC, If the sender does not receive the use CRC Mode request message , The receiver will switch to The checksum Mode to continue requesting transmission .

Checksum method

Package structure

byte1	byte2	byte3	bytes 4~131=128bytes	byte132
SOH(Start of Header)	block no	255-(block no)	data packet	checksum

For example , It takes points to send a document 5 block （block）, that block no It's from 1 To 5, And suppose this 5 The packets of the block are d1~d5, that checksum=sum(d1+d2+d3+d4+d5)/256 , here / It's a mold .

How does the receiver know whether a block is the last one ？

Take the above example , In the 5 After the block is sent , Then a message will be sent EOT（End of Transmission） Mark characters , This character is sent as a single byte .

Transmission process

CRC The way

Package structure

byte1	byte2	byte3	bytes 4~131=128bytes	bytes132~133
SOH	block no	255-(block no)	data packet	CRC-16

CRC In the original way checksum On the basis of , Updated from one byte to two bytes CRC, be relative to checksum More secure .CRC Cyclic redundancy check also belongs to a kind of checksum , But in the strict sense, it does not belong to and But to except

Xmodem-1K

Package structure

byte1	byte2	byte3	bytes 4~1027=1024bytes	bytes1028~1029
STX	block no	255-(block no)	data packet	CRC-16

Xmodem-1K Essentially, Xmodem-CRC, It's just data packet Increase the length to 1024bytes（1K）, Then the first 1 Bytes by SOH Updated to STX. In other systems and BBS It is also called Ymodem,XMODEM-1K Initially Chuck Forsberg In its YMODEM In the agreement XMODEM One of the many improvements made . Forsberg Suggest , Various improvements are optional , It is hoped that software authors can achieve as many improvements as possible . contrary , They usually only meet the minimum requirements , This leads to a large number of semi compatible implementations , And finally from the name “ YMODEM” Split into “ XMODEM-1K” And all kinds of YMODEM.

Ymodem

Ymodem yes Chuck Forsberg Yes Xmodem Improvement , but Ymodem Is named by Xmodem The original author of Ward Christensen give .Ymodem Essentially, Xmodem-1K, It allows you to work in the same conversation (session) Multiple files are transferred simultaneously in . On some systems , It's listed as Ymodem Batch.

Package structure

Ymodem stay Xmodem Develop on the basis of , Continue to use TeLink The method of adding short blocks to the protocol , That is to say, increase block 0. The block Identifies the of the file to be sent file name , file size and File creation timestamp , as follows ：

byte1	byte2	byte3	bytes 4~131=128bytes	bytes132~133
SOH	0x00	0xFF	file name file size Time stamp	CRC-16

Ymodem Of block n structure （ Not the second 0 block ）, data packet It can be either 128 It can also be 1024 position , Can be controlled by command

byte1	byte2	byte3	bytes 4~131=128bytes	bytes132~133
SOH	block no	255-(block no)	data packet	CRC-16

byte1	byte2	byte3	bytes 4~1027=1024bytes	bytes1028~1029
STX	block no	255-(block no)	data packet	CRC-16

Transmission process

Ymodem Allow in the same session (session) Simultaneous transfer of multiple files within , The following shows the transfer process by taking multiple files as an example , A single file is similar to .

With 128 Bit transfer

With 1K transmission

Ymodem-g

YMODEM-g yes Ymodem A variation of the , It's a streaming protocol , Send and receive as a continuous stream 1K Data packets , Until it indicates the need to stop . Before sending the next packet , It doesn't have to wait to receive ACK. The agreement ratio YMODEM faster , Because no delay is introduced between packets , But there is no ability to correct mistakes . If any block transfer fails , Cancel the transfer of the entire file .

Zmodem

As the network speeds up , Existing x/ymodem The speed of the agreement has not kept pace with the times .Zmodem emerge as the times require , It uses a window sliding mechanism ,Zmodem There are two most important characteristics ： Efficient , Provides crash recovery .

Efficient

As long as the receiver does not find any errors, the sender is allowed to send packets continuously , No need The sender receive The receiving party Of ACK Mark ( Because there may be delays or errors ). The number of packets allowed to be sent continuously is called window , In most Zmodem In the implementation protocol of 2~16 Number of packages .

Crash recovery

If Zmodem The transmission is cancelled or interfered with for some reason , The transmission can be resumed later , And there is no need to resend the previous content .

Reference resources

http://web.cecs.pdx.edu/~rootd/catdoc/guide/TheGuide_226.html#SEC237
https://en.wikipedia.org/wiki/XMODEM
https://en.wikipedia.org/wiki/YMODEM
https://en.wikipedia.org/wiki/ZMODEM