Simple file transfer protocol TFTP

One . Introduction to simple file transfer protocol

File transfer protocol (FTP) regulates the way to access files locally from remote servers .

File transfer protocols fall into two categories ：1.online-access and 2.whole file copying. The representative agreement of the former is NFS, Similar to sharing documents , Changes to the file locally will affect the file on the server . The latter is essentially a reproduction ： Upload files to the server （ Put the local file copy To the server ） And downloading files from the server （ The file copy To local ）. Modification of a copy , Will not affect the original file .

TFTP The full name is Trivial File Transfer Protocol, And FTP In comparison, it is simple ：

1. be based on UDP Designed application layer protocol . Port is 69. Of course, other transport layer protocols are also supported , such as TCP.

2. No, access control, That is, there is no login verification .

3. There are no folders , All the files are in the root directory .

4. The concept of design is simple, simple and simple , So the cost is very small , It is commonly used in diskless workstations and has strict memory requirements （ Small available memory ） Environment .

Two .TFTP packet format

TFTP packet from TFTP header and TFTP data form . There is a two byte opcode Marked with the TFTP packet The type of , All in all 5 Kind of . We will introduce them one by one .

2.1 RRQ(read request)

 opcode = 01( In the decimal sense ), Indicates that the packet by RRQ type , It is used to locally apply to the server for downloading files . The file name is from filename Field assignment , by ASCII Code string , Variable length , With NULL ending .mode The field is also a variable length ASCII Code string , With NULL ending , It defines the file transfer mode （ Transfer mode of files in different formats , Or the way must be different , Such as binary files and txt file ）, We'll talk about later .

2.2 WRQ(write request)

It is used to apply for uploading files to the server locally ,opcode by 02. The rest and RRQ Agreement . 

2.3 DATA(data)

opcode = 03. Used for data transmission when uploading or downloading files . File is too large. , Will be split into multiple data segments for transmission .block# The field identifies the sequence number of the data segment , Facilitate the reorganization of subsequent data .data Field placement 0-512 Bytes of data . For sizes other than 512 Byte multiples of the file , the last one data packet Of data The size of the field is less than 512 byte ; For exactly 512 Byte multiple size file , In the end, they often send a message data field by 0 Bytes of data packet, To avoid a data field by 512 Byte size data packet As the last of the file data packet. 

2.4 ACK(acknowledgement)

 opcode = 04. Used by the server for local transport data packet Response . For example, there is a local block# by xxx Of data packet, The server will confirm after receiving it , Return to one block# The field is also xxx Of data packet.

Attention is only to data packet The confirmation of , Yes WRQ and RRQ And other types will not return ACK.

2.5 ERROR(error)

 opcode = 05.errcode Field indicates the error type ,errorstring Field gives an error message . among errorstring In order to ASCII Error prompt in the form of code string , Its variable length , And in NULL ending .

2.6 transfer mode explain

Introduction to transmission txt Of documents Netascii And transfer binary files Octet Pattern .

Netascii Mode is used to transmit txt File format , It is transmitted by line . The sender will add... At the end of each line \r\n, And the receiver will remove it .

Octet For binary file transfer , Is transmitted in bytes , The sender and the receiver do not need to do similar addition and removal processing .

3.TFTP Agreement that

TFTP Agreed packet format Stated , The following instructions TFTP Of operations.

We know TFTP Is a simple file transfer for point-to-point , These two points are actually symmetrical , Can be used as a server or a client .TFTP Is based on UDP The design of the , This is very strange , In particular, it is necessary to ensure the reliability of file transmission , and UDP Reliability is not guaranteed . So who guarantees the reliability of file transfer ？ Nature is the application layer TFTP agreement , Its use ACK type packet Guarantee data packet Reliability of transmission .

3.1 TFTP The basic process of transferring documents and the guarantee of reliability

We only consider the process of data transmission （ Just consider ACK and DATA packet）, Not for now WRQ and RRQ etc. packet.

Remember two facts ：

1. First ,server and client Each end maintains a timer , It is used to judge whether the data it sends has timed out .

2. in addition ,server and client The end transmission data adopts stop and wait The way , No confirmation of the last data was received , Don't send the next one .

Server End send data[1], And start timing , can do with client End ack(1) Response . If timer timeout Still not received after ack(1), be server It is believed that data[1] The loss of , And retransmit ; Once received within the time limit ack(1),server Will be sent immediately ack(2).

Client Termination received data(1), Send it now ack(1), And start timing , Hope to receive server End data(2) Response . If timer timeout Still not received after data(2), be client It is believed that ack(1) The loss of , And retransmit ; Once received within the time limit data(2),client The client will immediately send ack(2).

3.2 SAS(sorcerer's apprentice syndrome)

This represents the original TFTP Defects in the agreement . The original TFTP Protocol design ：client Once the end receives ack(n), Whether received or not , All sent immediately data(n+1), Even if data(n+1) Already sent ;server Once the end is data(n), Whether received or not , All sent immediately ack(n). and dealyed but not lost ack May trigger SAS, Make the following data Are transmitted twice .

  The solution is in sender Add a judgment to the end algorithm , if ack(n) repeat , No data will be sent data(n+1).

3.3 State transition diagram

Five kinds packet There is no end of transmission in , Need to rely on DATA packet Implicitly represent （data Insufficient domains 512B）.

 

for fear of SAS, You can put server Terminal ACK(n) Change it to ：ACK(n) and not duplicate.

3.4 Message sequence diagram