path: root/doc/rfc/rfc468.txt

Network Working Group                                         R.  Braden
Request for Comment: 468                                        UCLA/CCN
NIC: 14742                                                 March 8, 1973


                          FTP DATA COMPRESSION

I.  INTRODUCTION

APOLOGIA

   Major design objectives of the proposed File Transfer Protocol (FTP)
   are reliability and efficiency for transmission of large files.
   Efficiency has two faces: efficiency of the host CPU's, and efficient
   use of the Network bandwidth.  Block mode is intended to minimize CPU
   overhead for bandwidth efficiency, there is a mode called "HASP" in
   RFC 454.  The "HASP" mode of FTP is really transmission with data
   compression, i.e., an encoding scheme to reduce the information
   redundancy in the messages.

   RFC 454 contains no explicit definition of the "HASP" or compressed
   mode, but instead notes that a future RFC by yours truly will define
   the mode.  Students of FTP may find this scarcely credible, but you
   are now reading the promised RFC.  It turned out to be much farther
   in the future than any of us expected.  Mea Culpa.

GENERAL CONSIDERATIONS

   In the early years of the Network, its major uses have been remote
   terminal interactions and the small-to-medium-sized file transmission
   typical of remote job entry.  As facilities such as the Illiac IV and
   the Data Machine become operational on the Network, and the Network
   community begins to include users with heavy data transmission
   requirements, large file transmission will become a major mode of
   Network use.  For example, one user of CCN expects to send 2 x 10**8
   bits of data _each_ _day_ over the Network.

   Local byte compression of the type proposed here is particular
   effective for reducing the size of "printer" files such as those
   transmitted under the Network RJE protocol.  Experience with CCN's
   RJS service has shown a typical compression of print files by a
   factor of between two and three.  Since FTP was intended to contain
   the data transfer part of Network RJE protocol as a subset, it is
   appropriate to include a print file compression mechanism in FTP.
   These considerations led the FTP committee to include a compressed
   mode within FTP.





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RFC 468                   FTP Data Compression                March 1973


   The two main arguments for data compression are economics and
   convenience (usability).  Consider first economics, which is
   essentially a trade-off between CPU time and transmission costs.  Of
   course, as long as Network use is a free commodity, the economics of
   data compression are all bad.  That happy state won't last forever.
   What does data compression cost?

   Let us consider only simple linear compression schemes, such as the
   one proposed here.  By linear, I mean that the CPU time to examine a
   source record is proportional to number of bytes in the record.  A
   simple linear scheme could detect repeated single characters, for
   example.  One could imagine quadratic schemes, which detected
   repeated substrings; but except for possible special circumstance
   where the source stings have some structure known to the compression
   algorithm, the CPU economics don't favor quadratic compression.

   Assuming a reasonable figure for large-scale CPU costs in the
   generation of CCN's 360/91, we concluded that an upper bound on CPU
   costs for total compression and decompression would be 5 cents per
   megabit; this is based on very loose coding of a simple linear
   algorithm.  This may be compared with the projected Network
   transmission costs of over 30 cents per megabit (possibly a lot
   over).

   Thus, the CPU time to conserve bandwidth costs significantly less
   than the bandwidth saved.  Both CPU costs and bandwidth costs are
   trending downward, but it seems exceedingly unlikely that the ratio
   of CPU cost to bandwidth cost for linear compression will reverse in
   the next few years.  On the other hand, this calculation clearly
   discourages one from using quadratic compression.

WHY HASP

   CCN's batch remote job entry protocol NETRJS (see RFC #189, July 15,
   1971) was designed to include two data transfer modes, truncated and
   compressed.  The NETRJS truncated mode is essentially identical to
   current FTP block mode record structure (except for minor bit format
   differences).  The compressed mode of NETRJS uses an adaptation of
   the particular compression scheme which is incorporated in the
   "Multileaving protocol" of the binary synchronous rje support in
   IBM's HASP system.

   Although it isn't really necessary for the purpose of defining a
   compression scheme in FTP, I have included an appendix summarizing
   very briefly the nature of HASP and its rje package.  That appendix
   may be considered cultural enrichment for those in the Network
   Community who have been denied the privilege of being an IBM
   customer.  After all, I know a lot of HASP experts who never heard of



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RFC 468                   FTP Data Compression                March 1973


   TENEX! More seriously, because HASP is widely used on IBM machines,
   the HASP compression scheme is also widely used.  In designing
   NETRJS, we chose the HASP scheme of compression because of its
   ubiquity and plausibility.

   However, certain details of the HASP bit formats are inappropriate or
   sub-optimal for FTP.  Therefore, our proposal for compressed mode of
   FTP is only an adaptation of the HASP compression scheme.

   It should be clear from Appendix A that the compression scheme of
   HASP, even if used literally, is a very minor and incidental part of
   that system.  Although we ought to properly credit the intellectual
   origin of FTP's compressed mode, it seems a little strange to call
   the compressed mode in FTP the "HASP mode".  I trust this will be
   rectified by the forthcoming FTP meeting.

II.  PROPOSED FTP COMPRESSED DATA MODE

   Byte size is B bits.  Figures above boxes are field lengths in bits.

                                  n bytes of data
                               /--------/\--------\
              1    B-1        /   B              B \
             +---+------+    +--------+     +--------+
Byte String: | 0 |  n   |    |   d    |. . .|   d    |
             |   |      |    |    1   |     |    n   |
             +---+------+    +--------+     +--------+
                  String of n data bytes d(1),...,d(n)
                  Count n must be positive

                     2     B-2            B
                   +----+------+    +---------+
Replicated Byte:   | 1 0|   n  |    |    d    |
                   +----+------+    +---------+
                 String consisting of n replications of the data byte d

                     2    B-2
                  +----+------+
Filler String:    | 1 1|   n  |
                  +----+------+
                 String of n filler bytes.  The filler byte is a "space"
                 character for ASCII or EBCDIC type, or a binary zero
                 byte for Image or Local Byte Type.

                                B            B
                          +----------+ +----------+
Control Escape Sequence:  | 0......0 | |      C   |    (see below)
                          +----------+ +----------+



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RFC 468                   FTP Data Compression                March 1973


   The control byte "C" which is the second byte of a control escape
   sequence is to have the same coding as the descriptor byte in Block
   Mode.  This includes end-of-file and end-of-record indications.  I
   will not specify this further because there is some question at
   present about the exact coding of the Block Mode descriptor byte.

   Following the example of APL*, we have let the meaning of the filler
   (blank or 0) be determined by the type: character (ASCII|EBCDIC) vs.
   binary (Image|Local Byte).  If byte size is equal to the word size of
   the transmitting host, the compressed mode allows one to send sparse
   notices with reasonable efficiency.

   * Compare 1 (take) 0 1\`A' with 1 (take) 0 1\2






































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RFC 468                   FTP Data Compression                March 1973


APPENDIX A: HASP MULTILEAVING

   HASP (Houston Automatic Spooling Program) is a subsystem which
   essentially runs within OS/360 as a job but takes over the batch
   processing management functions from the operating system.  That is,
   HASP handles spooling of card input and printer and punch output,
   queueing and scheduling of job execution, and the operator control
   interface.  It is a tightly-written and efficient system for running
   a large and varied job load through a large-scale machine.  The name
   results from the historical fact that HASP was originally by a local
   IBM group for one particular customer, NASA Houston.

   HASP has always been an anomaly in the IBM scheme of things.  The
   system was written around 1965 by two programmers; the HASP group has
   probably averaged three programmers during most of its life.  The
   leader of the group has been "Mr. HASP", Tom Simpson.  The HASP
   system spread rapidly through (more or less) underground channels to
   many of the medium and large scale 360's.  At least once, only
   intense customer pressure prevented IBM from killing the HASP effort.
   HASP generated an astonishing emotional mystique among its users.
   The HASP sessions at SHARE Meetings were reminiscent of revival
   meetings.  For years every SHARE Meeting has included HASP song
   sessions around the piano during the nightly open bar.  HASP forms a
   fascinating chapter in the history of IBM's large machine business.

   The core concepts in HASP are pseudo-devices, and the general
   technique of intercepting supervisor calls to augment operating
   system functions without changing the operating system itself.  A
   generation of OS/360 system programmers learned these techniques from
   HASP.  (These important techniques are hardly ever described in the
   literature, and "practical" programmers don't read the literature
   anyway).

   When HASP starts up (in supervisor state), it overlays an instruction
   in the I/O Supervisor with a branch to its own code.  A user program
   is written as if it were doing real I/O to card readers and printers.
   HASP intercepts and interprets these I/O operations to handle job I/O
   in a manner transparent to OS/360.  It similarly intercepts and
   interprets operator console I/O.

   HASP includes batch remote job entry using binary synchronous
   communication.  The HASP communication protocol and message formats
   use a scheme developed by Simpson's group called "Multileaving
   Protocol".  The HASP rje system, by far the best rje package IBM has
   produced, finally replaced two competitive IBM packages and has
   effectively become the IBM standard for rje.  CCN's RJS system not
   only adopted the Multileaving Protocol but essentially copied its
   binary synchronous communication line handler directly form HASP.



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RFC 468                   FTP Data Compression                March 1973


   The Multileaving Protocol is described in the HASP manual(1) as the
   "fully synchronized, pseudo-simultaneous, bidirectional transmission
   of a variable number of data streams between two or more computers
   using binary synchronous communications facilities".  It allows a
   remote batch terminal to operate a variable number of card readers
   and printers simultaneously at different speeds over one
   communication line.  It is not surprising that HASP Multileaving
   contains in miniature many of the features of IMP-IMP Protocol and a
   little host-host protocol.  Specifically, Multileaving includes the
   following general features:

      (1) "Conversational" transmission line protocol using transparency
          (DLE STX, etc.).

      (2) "Strong" error control and retransmission using a 16-bit CRC
          and a modulo-16 block sequence number.

      (3) Flow control for multiple streams in both directions.  This
          includes the interchanging of matching control records
          ("RFC's") to open a stream, and a set of flow control bits in
          each block.  Each flow control bit is logically equivalent to
          an ALLOcate command for one "message" (buffer) for a
          particular stream.

      (4) Optional Special Control Information for remote devices.  This
          includes printer carriage control, switching card reader
          hoppers, etc.

      (5) Multiplexing ("multileaving") multiple streams into a single
          block for transmission.

      (6) Marking end of file and ends of records within each stream.

      (7) Compressing transmitted text by encoding repeated blanks and
          repeated single characters.
















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RFC 468                   FTP Data Compression                March 1973


   Finally, we have reached the (only) aspect of HASP relevant to FTP:
   its compression scheme.  HASP uses the following encoding:

                       8
                  +---------+
   End of Record: | 0 ... 0 |
                  +---------+
                    2     6             8              8
                  +---+---------+   +-------+     +--------+
   Data String:   |1 1|     N   |   |    d  | ... |   d    |
                  |   |         |   |     1 |     |    N   |
                  +---+---------+   +-------+     +--------+
                         3      5
                       +---+--------+
   N Duplicate Blanks  |100|     N  |
                       +---+--------+
                               3       5           8
                             +---+---------+  +---------+
   N Replicated Characters D |101|    N    |  |    D    |
                             +---+---------+  +---------+

   HASP is concerned only with 8-bit bytes.  However, there is a
   provision (which was never implemented) in the Multileaving Protocol
   to set the unit of the counts N as 1 byte, 2 bytes, or 4 bytes.

   Reference:

   (1) HASP II System Manual, IBM Corporation (February 26, 1971)






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