Network Working Group                             H. Brodie
Request for Comments #250                         UCLA-NMC
NIC #7691                                         Computer Science
Categories:  D5, D7                               7 October 71
Updates:  None
Obsoletes:  None

                     Some Thoughts on File Transfer
	
   There are several aspects of the proposed Data Transfer Protocol (RFC
   #171) and File Transfer Protocol (RFC #172) which we believe could
   use further clarification and perhaps revision.  Interest in
   transferring larger amounts of data than is typically sent via the
   usual TELNET connection is increasing, and at least at UCLA-NMC
   implementation attempts have pointed out several difficulties with
   the proposed protocols.

   First, and probably most easily decided, is the ambiguity in RFC #171
   with regards to the sequence number field of the descriptor and count
   transaction.  The description provided for the transaction header
   provides for 16 bit sequence number.  However, the sequence number
   field in the error codes transaction only provides for 8 bits.  We
   are of the opinion that 8 bits is sufficient for a sequence number
   field.  If the sequence number is reduced to 8 bits, and the two NUL
   bytes are deleted from the descriptor and count header, then its size
   is reduced to 48 bits, which would seem to be as convenient to handle
   as the proposed 72 bit transaction header.

   Another source of difficulty lies in the implementation of the (the
   SEX time-sharing system) the 'end' of a file (which presumably would
   be the begin point of an Append transaction) is almost com- pletely
   context-defined--i.e., the program reading the file determines when
   it has reached the end of the file.  Therefore, the meaning of
   'Append' is somewhat hazy, and since the proposed Mail Box Protocol
   uses the Append feature, not implementing this command in a File
   Transfer service is costly in terms of lost useability.

   We believe that resolution of these ambiguities will lead to a
   greatly accelerated implementation schedule, at least here at UCLA-
   NMC.

       [ This RFC was put into machine readable form for entry ]
       [ into the online RFC archives by BBN Corp. under the   ]
       [ direction of Alex McKenzie.                   12/96   ]