SEALINK                             File Transfer Protocol                                 9 February 1987          (C) COPYRIGHT 1986,87 by System Enhancement Associates, Inc.     This document describes briefly the  SEAlink  file  transfer  protocol     developers' package.  SEAlink is a sliding  window  protocol  that  is     fully  backwards compatible with XMODEM in all tested implementations.     The intent of SEAlink is to provide a file transfer protocol that does     not  suffer  from  propagation  delays,  such  as  are  introduced  by     satellite relays or packet switched  networks.  Actual  tests  of  the     enclosed  routines  has  shown  that  SEAlink  is capable of virtually     eliminating propagation delays and turnaround delays.  File  transfers     between New Jersey and Hawaii,  which normally suffer a degradation of     50% or more  due  to  satellite  relays,  proceed  as  fast  as  local     transfers.  Even transfers within the local exchange are speeded up by     up to 20% at 2400 baud by the elimination of turnaround delays.  Large     volume  tests  show  that SEAlink is capable of coming to within 2% of     the theoretical minimum time for data transfer.     The developers' package contains the following files:         SEALINK.DOC    This document.         SEALINK.C      A set of C routines for implementing SEAlink.         CLINK.EXE      A sample TTY program that implements SEAlink.     You are granted a license to use this code in your  programs,  and  to     adapt  it to your particular situation and needs,  subject only to the     following conditions:     1) You must refer to it as the SEAlink protocol,  and  you  must  give        credit to System Enhancement Associates.     2) If  you  modify  it in such a way that your version cannot converse        with the original code as supplied by us,  then you should refer to        it as "SEAlink derived",  or as a "variation of SEAlink",  or words        to that effect.     In short,  we're not asking for any money,  but we'd like to get  some     credit for our work.     This  document  is  not  meant  to  be  a  rigorous  definition of the     protocol.  The code provided should serve to document the details  and     fine  points  of  implementing SEAlink.  We will,  however,  present a     brief synopsis of how SEAlink adds sliding windows to XMODEM,  and why     XMODEM doesn't mind.     First of all,  SEAlink adds a block number to the ACK and NAK used  in     XMODEM.(1)  We  thus  create  "ACK/NAK  packets",  with  the following     structure:         Byte 0:   ACK, NAK, or C         Byte 1:   Block number         Byte 2:   One's compliment of block number     This is identical in form to the first three bytes of a  data  packet,     except that the SOH has been replaced with an ACK or NAK.(2)     From the receiver's point of view,  it does not matter if  the  trans-     mitter  is using sliding window or not.  The receiver simply sends ACK     and NAK packets as appropriate.  Any XMODEM driver tested to date will     simply ignore this excess data behind the ACK or NAK.     From the transmitter's point of view,  it just barely matters  if  the     receiver can handle sliding window.  The transmitter always acts as if     it  is  sending sliding window,  but varies the window size.  If it is     seeing valid block numbers and check values behind the  received  ACKs     and NAKs,  it sets the window size to six blocks.  Otherwise,  it sets     the  window  size  to  one  block.  The  result is that it only "sends     ahead" if the receiver can handle it.     It  should  be  a fairly simple matter to apply the underlying SEAlink     logic to almost any variant of XMODEM.     The  SEAlink  routines  provided  in  this package are also capable of     passing system dependent information,  such as true file size and time     of  last modification.  This data is passed in a special header block.     The header block looks exactly like any other block, except that it is     block number zero.     This is still backwards compatible with XMODEM,  as a SEAlink receiver     does  not  mind if block zero is missing,  and any XMODEM receiver yet     tested will regard block zero as a duplicate block and ACK it.     The data portion of block zero contains the following fields:         Offset    Size      Contents         ======    ====      ========            0        4       Original file length.            4        4       Date  and  time  file  was  last mofified,  in                             seconds since 1979.            8       17       Original  file  name,  as  a  null  terminated                             string.           25       15       Name  of  transmitting  program,   as  a  null                             terminated string.           40       88       Null filler and expansion area.     (1) XMODEM/CRC uses a "C" in place of  a  NAK  to  indicate  CRC  error         detection.  SEAlink  follows  this convention,  and supports either         checksum or CRC.  For brevity,  this document will use the term NAK         to mean either a true NAK (hex 15) or a C (hex 43).     (2) See previous footnote.     Any field which the transmitter cannot support should be  set  to  all     zeros.  Conversly,  the  receiver  should ignore any null fields.  The     receiver may ignore any field which he cannot support.     The  routines  enclosed  in  this package should be reasonably easy to     implement in your application.  We have attempted to exclude  compiler     dependent and system dependent logic from these routines.     You will need to alter our references to our communications driver  to     conform  to  your  own driver.  The communications related routines we     use are:         com_putc(c)         Output character c to comm port.         int com_getc(t)     Get character from comm port within  t  tenths                             of   a  second.   Return  EOF  if  time  limit                             expires.         com_dump()          Discard any pending output without sending it.     In  addition,  we  use  the  following  routines for controlling timed     loops:         long timerset(t)    Set a timer.  Returns a timer value which will                             expire in t tenths of a second.         int timeup(z)       Check a timer.  Returns true if  timer  z  has                             expired yet, or false otherwise.     These routines also make reference to the following functions for     system dependent information, which is optional:         filestat(name,&fs)  Read directory entry for  a  file  and  return                             system dependent information.         setstamp(f,dtg)     Set a file's date/time of last modification.     The SEAlink  implementation  provided  in  this  package  is  used  by     invoking the two primary routines:         int xmtfile(name)             /* transmit a file */         char *name;                   /* name of file to transmit */     This  routine is used to send a file.  One file is sent at a time.  If     the  name  is blank (name is null or *name points to a null),  then an     end of transmission marker is sent.     This routine returns a one if the file is successfully transmitted, or     a zero if a fatal error occurs.         char *rcvfile(name)           /* receive a file */         char *name;                   /* name of file (optional) */     This routine is used to receive a file.  One file is  received.  If  a     name is specified for the file,  then that name WILL be used,  and any     name  sent  by  the transmitter will be ignored.  If the name is blank     (name is null or *name points to a null),  then the  transmitter  must     provide a name for the file.     This routine returns a pointer to  the  name  of  the  file  that  was     received.  If the file transfer is not successful, then a null pointer     is returned.     The  pointer  returned  by  rcvfile()  points to a static data buffer.     This does not have to be freed (and should not be),  but  it  will  be     overwritten the next time rcvfile() is called.     The  rcvfile()  function  works  on a temporary file whose name is the     same as the final file,  but with a dash ("-") added at the beginning.     If  a  file  transfer  is  aborted,  then  this temporary file will be     retained.  An aborted file transfer will not harm a pre-existing  file     of the same name.     These  routines  can  be  used  for  either  single  or  multiple file     transfers.     To  send  multiple  files,  send  each  file one by one until either a     transmit fails or all files are sent.  If all  files  are  sent,  then     signal the end by calling xmtfile() with a null pointer.     To receive multiple files,  call rcvfile() repeatedly until it returns     a null pointer.     This  package includes a demonstration program named CLINK (pronounced     "clink"),  which is a  simple  TTY  program  for  doing  SEAlink  file     transfers.  CLINK  does  not  perform  any  sort of terminal emulation     whatsoever.  However,  she will make use of the ANSI.SYS screen driver     if you have it installed.     CLINK may be used in either of two ways: interactive mode or command     mode.     To use CLINK in the interactive mode, give the command "CLINK" with no     arguments.  Press  the  "ESCape"  key to give a command to CLINK.  The     command "?" (question mark) instructs CLINK to tell you what  commands     she understands.     To  use  CLINK  in the command mode,  give the command "CLINK" with an     argument.  There are three arguments you can give CLINK in the command     mode.  These are:      1) Receive files;  Do this with a command of the form:              CLINK R         CLINK  will  attempt  to receive one or more files from COM1,  and         will terminate as soon as all files  are  received,  or  when  the         transfer aborts.      2) Transmit files; Do this with a command of the form:              CLINK T <filename> ...         CLINK  will  attempt  to transmit the listed files over COM1,  and         will terminate as soon as all files are sent,  or the transfer  is         aborted.  <filename> may be one or more file names with or without         drive and path specifiers.  Wildcards may be used.      3) Give help;  If you type:              CLINK ?         or any invalid command,  CLINK will display a  brief  reminder  of         what arguments she understands in command mode.     In all cases, CLINK in the command mode will not alter the serial port     other than to set eight data bits,  one stop bit,  and no parity.  Any     previously installed serial drivers will be  replaced,  and  the  baud     rate will not be changed.     CLINK comes with her own serial driver built in for the IBM PC  family     and true compatibles,  but she is capable of using any standard FOSSIL     driver.