YAPP(tm) protocol for Packet Radio binary file transfer
(c) 1986, Jeff Jacobsen - WA7MBL
Revision 1.1   06/23/86
Size of packet data ranges from 2-258 bytes.
len = length of following bytes (not including len)
RR Rcv_Rdy      ACK  01
RF Rcv_File     ACK  02
AF Ack_EOF      ACK  03   [1.1]
AT Ack_EOT      ACK  04   [1.1]
SI Send_Init    ENQ  01
HD Send_Hdr     SOH  len  (Filename)  NUL  (File Size in ASCII)  NUL  (Opt)
DT Send_Data    STX  len  (Data)    {if len=0 then data length = 256}
EF Send_EOF     ETX  01
ET Send_EOT     EOT  01
NR Not_Rdy      NAK  len  (Optional Reason in ASCII)
RE Resume       NAK  len  R  NULL  (File size in ASCII)  NULL
CN Cancel       CAN  len  (Optional Reason in ASCII)
CA Can_Ack      ACK  05
{Following are for use in Server mode}
RI Rcv_Init     ENQ   02   len   (Filespec requested - wildcard allowed)
TX Text         DLE   len  (ASCII text for display)  {to send text from server}
{Unimplement - Reserved for Server Commands}
UU Comands      ENQ   03
                ENQ   04
                ENQ   05
                ENQ   FF     
State         Send Pkt        Upon                  Next State
S (Send)      SI              RR                    SH
                              RF                    SD
                              NR                    Start/Done
                              First few timeouts    S
                              RI                    S
                              Other/Timeout         Abort
SH (Hdr)      HD              RF                    SD
                              NR                    Start/Done
IF (RESUME)                   RE                    Move file position then SD
                              Other/Timeout         Abort
SD (Data)     DT              (Not EOF)             SD
                              (EOF)                 SE
SE (EOF)      EF              AF (More files)       SH
                              AF (No More Files)    ST
                              Other/Timeout         Abort
ST (EOT)      ET              AT                    Start/Done
                              Other/Timeout         Start/Done

State      On    If           Do          Send Pkt       Next State
R (Rec)    SI (ready)                       RR              RH
           SI (opt)*       open file        RF              RD
           SI (not ready)                   NR              Start/Done
           NR                                               Start/Done
           Other/Timeout                                    Abort
(opt)*  If header info not desired it is ok to
        open file and indiate ready for file
RH (Hdr)   HD               open file       RF              RD
           HD (no room)                     NR              Start/Done
           HD Already       open file for   RE              RD
              have file!    append at -256
                            byte from EOF
ELSE       HD Already have file!!           NR              Start/Done
           SI                                               RH
           ET                               AT              Start/Done
           Other/Timeout                                    Abort
RD (Data)  DT              store data                       RD
           EF              close file       AF              RH
           Other/Timeout                                    Abort

Any State          On CN         Send CA                   Start/Done
except CW
CW (Can_Wait)      On CA                                   Start/Done
                   On CN         Send CA                   CW
                   Timeout       Display Err Msg           Start/Done
Abort                            Send CN                   CW

Starting States
For 2 users not using Remote Server
    Sender goes to State S, Receiver to State R after manually
    exchanging file names.  Since File Name is known, Recevier
    may optionally send RF from state R and bypass file header.
    Sender should give Receiver ample time to get into Receive

For Remote Server Mode
  USER (Start)                                       Next State
  User wishing to Send                               S
  User wishing to Receive          Send RI           R
  Other Server Request             Send COMMAND      Start
  SERVER (Start)
  Gets SI                                            R
  Gets RI             (file fnd)                     S
                      (no file)    Send NR           Start
  Gets COMMAND                    (Do Command)       Start
All states except Send Data should timeout if nothing is heard
for a period of time Tc (Crash timer).  Send Data isn't expecting
any input and if it should receive some it must be a Cancel, or
a TX (text) packet, otherwise Abort.
Revision 1.1 - Added AF & AT to acknowledge End of File and
End of Transfer.  This allows an automated sender to know that
a file was received successfully.  It also helps get past a
problem in the TNC-2 code by requiring the sender to wait for the
Ack-EOT before switching from transparent mode back to converse
The new state introduced from FC1EBN in TPK are explained and is inserted in
state table for recieve and send a file.
Note that it's valid only if RESUME is true. This will implie or not
the cancellation of file in case of transfer abort.
This metode is ok for resume a file transfert but it have some limit if
the two station hasn't the same RESUME state ON or OFF or not!
1st example:
A is a sender with RESUME OFF and B is receiver with RESUME ON.
A initiate transfer with B and at certain time this will abort.
In this case B (RESUME ON) don't erase the file and retry to download from A.
Now the transfer always abort because B that already have a part of file
request the resume that A interpret as Abort.
2nd example:
A is a sender and B is receiver both with resume ON.
Today B download PINCO.ZIP (1st version, 10KByte) from A.
Tomorrow in A there is the same filename but with lenght changed due a
new version of program (PINCO.ZIP 2nd version, 11Kbyte).
If B never delete the previous version of PINCO.ZIP from download directory
the last 1000 byte of new file are append to the 10000 of the old one make
simply caos!!
This because Yapp never check if it is the same file (eg comparing filedate
and time).
YappC extension introduced in F6FBB BBS 5.14
When a user request a resume of a previuos file transfer, every Yapp frame
long pklen byte is followed from a simple checksum byte.
So during a resume a Yapp Send_Data frame is:
STX len (Data) chksum
If len = 0 then data lenght = 256, note that len not include chksum byte and
this is the checksum of Data byte.
Extensions to the Yapp protocol used in TPK v 1.65a and FBB v 5.14
HEADER extension:
The header now carry the DATE and TIME of the file being transmitted.
[SOH] [Len] [Filename] [NUL] [File Size] [NUL] [Date] [Time] [NUL]
Date and Time are the values reported by DOS, coded in 4 hexadecimal
characters and sent in ASCII.
Receiver has the choice to use extended Yapp with checksum or normal Yapp.
- Normal Yapp reply is RF as before and the receiver can keep the
  date and time information .
- Extended Yapp reply is : RT  Receive_TPK
            and is coded : [ACK] [ACK]
If receiver reply is RT the protocol used will be what i have called YappC
for Yapp with checksum. When sender get this packet he MUST use YappC.
Data Packets extension:
If receiver reply is RT the protocol used will be YappC. The checksum allow
detection of packets corrupted along the link, particularly on the RS232
lines where there is no error control or correction (or very poor !)....
Data packets : [STX] [Len] [Datas] [Checksum]
Checksum is the sum of all datas bytes on 8 bits like Xmodem.
If checksum is bad receiver must send a Cancel and go in the CW state.
Transfer Resuming:
A new field has been added to the resume reply to tell sender if receiver
can use YappC or not.
- Resume reply for Yapp: (as used before by TPK and FBB)
    [NAK] [Len] [R] [NUL] [Received Length] [NUL]
            I    I                I
            I    I                +-- in ASCII as in the header
            I    +-- as Resume !
            +------- len of the following bytes
- Resume reply for YappC:
    [NAK] [Len] [R] [NUL] [Received Length] [NUL] [C] [NUL]
                Tell sender i can go in YappC -----+
    When sender get this packet he must use YappC.
Any comments or suggestions : FC1EBN @ F6ABJ.FRPA.FRA.EU
73's Gerard FC1EBN