6. STATE DIAGRAMS
Connected: An Internet Encyclopedia
6. STATE DIAGRAMS
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6. STATE DIAGRAMS
6. STATE DIAGRAMS
Here we present state diagrams for a very simple minded FTP
implementation. Only the first digit of the reply codes is used.
There is one state diagram for each group of FTP commands or command
sequences.
The command groupings were determined by constructing a model for
each command then collecting together the commands with structurally
identical models.
For each command or command sequence there are three possible
outcomes: success (S), failure (F), and error (E). In the state
diagrams below we use the symbol B for "begin", and the symbol W for
"wait for reply".
We first present the diagram that represents the largest group of FTP
commands:
1,3 ++
> E 
 ++

++ cmd ++ 2 ++
 B > W > S 
++ ++ ++

 4,5 ++
> F 
++
This diagram models the commands:
ABOR, ALLO, DELE, CWD, CDUP, SMNT, HELP, MODE, NOOP, PASV,
QUIT, SITE, PORT, SYST, STAT, RMD, MKD, PWD, STRU, and TYPE.
The other large group of commands is represented by a very similar
diagram:
3 ++
> E 
 ++

++ cmd ++ 2 ++
 B > W > S 
++ >++ ++
  
   4,5 ++
 1  > F 
 ++
This diagram models the commands:
APPE, LIST, NLST, REIN, RETR, STOR, and STOU.
Note that this second model could also be used to represent the first
group of commands, the only difference being that in the first group
the 100 series replies are unexpected and therefore treated as error,
while the second group expects (some may require) 100 series replies.
Remember that at most, one 100 series reply is allowed per command.
The remaining diagrams model command sequences, perhaps the simplest
of these is the rename sequence:
++ RNFR ++ 1,2 ++
 B > W > E 
++ ++ >++
  
3   4,5 
  
   ++
 > S 
  1,3   ++
 2 
   
V   
++ RNTO ++ 4,5 >++
 > W > F 
++ ++ ++
The next diagram is a simple model of the Restart command:
++ REST ++ 1,2 ++
 B > W > E 
++ ++ >++
  
3   4,5 
  
   ++
 > S 
  3   ++
 2 
   
V   
++ cmd ++ 4,5 >++
 > W > F 
++ >++ ++
 
 1 

Where "cmd" is APPE, STOR, or RETR.
We note that the above three models are similar. The Restart differs
from the Rename two only in the treatment of 100 series replies at
the second stage, while the second group expects (some may require)
100 series replies. Remember that at most, one 100 series reply is
allowed per command.
The most complicated diagram is for the Login sequence:
1
++ USER ++>++
 B > W  2 > E 
++ ++  >++
    
3   4,5   
    
    
    
  
 1   
V    
++ PASS ++ 2  >++
 > W > S 
++ ++ >++
    
3  4,5  
  
    
    
 
 1,3   
V  2  
++ ACCT ++  >++
 > W  4,5 > F 
++ ++>++
Finally, we present a generalized diagram that could be used to model
the command and reply interchange:

 
Begin  
 V 
 ++ cmd ++ 2 ++ 
> > >  
   W   S 
>  >    
 ++  ++ 4,5  ++ 
      
   1 3  ++ 
        
    > F 
    
   ++



V
End
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Connected: An Internet Encyclopedia
6. STATE DIAGRAMS
