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Network Working Group V. Cerf Request for Comments: 696 Stanford University NIC: 32962 July 1975

         Comments on IMP/HOST and HOST/IMP Protocol Changes

 With reference to RFC's 687, 690, and 692 (NIC's 32564, 32699, and
 32734, respectively) by D.C. Walden, J. Postel, and S. Wolfe
 (respectively), I would like to offer some observations relative to
 current international standards recommendations from working group
 6.1 of the International Federation of Information Processing.  In a
 meeting held last May at the NCC, this working group voted to present
 a recommendation to CCITT (International Consultative Committee on
 Telephony and Telegraphy of the International Telegraphics Union) for
 a standard packet (or DATAGRAM) header.

 The proposed packet header format is meant to interface hosts to
 packet networks.  It is not a header for Host-to-Host protocol, nor
 is it an IMP-to-IMP header.  The bulk of the header is taken up with
 addressing space(96 bits!)  since this will be compatible with the
 current maximum address space of the telephone system (14 digits).

 LOCAL NETWORK FIELD - 4 bits

    This field allows local networks to operate easily on multiple
    formats, since the 4 bits can be used in any fashion desired by
    the local network.

 DATAGRAM FORMAT - 4 bits

    This field could be used by ARPANET to contain "1001" binary, so
    as to maintain backward compatibility with the existing message
    leader format.

 PACKET TYPE CODE - 8 bits

    This could be used for the HOST/IMP and IMP/HOST code.

 FACILITIES - 16 bits

    These bits have not yet been specifically allocated.  Some will no
    doubt be for international services (e.g., tracing at gateways
    between networks, accounting, class of service).  It was the
    feeling of WG 6.1 members that some of these bits (e.g., 8) might
    be allocated to the originating network (or destination network)
    for its own use.

Cerf [Page 1]  RFC 696 Comments on IMP/HOST and HOST/IMP Protocol Changes July 1975

 TEXT LENGTH - 16 bits

    These bits count the number of octets in the text of the packet,
    not including octets in the header (which is fixed in length for
    any particular format).

 DESTINATION ADDRESS - 48 bits [1]

    These bits could be allocated in the following way: Destination
    Network Identifier - 8 bits; Destination Host Identifier - 8 bits;
    Destination IMP identifier - 16 bits; Reserved- 16 bits.

 SOURCE ADDRESS - 48 bits

    These bits would be used in a fashion similar to the destination
    address bits.

 The resulting packet is 144 bits long and adding the present 40-bit
 Host-to-Host header results in a total of 184 bits, which is not very
 pleasant.  A temporary fix (until we can introduce a new NCP design)
 might be to squeeze out the reserved 16-bit fields in the source and
 destination address fields, giving 32 bits to carry the byte size and
 byte count information for the present Host/Host protocol.
 Alternatively, the length field of the packet header and one of the
 facilities flags (or a whole field) could be used to indicate byte
 size and byte count.  Either idea would require some fairly
 substantial modification of existing NCP programs, so is probably not
 very palatable.

 Another alternative would be to add a dummy byte after the 144th bit
 of header, followed by 40 bits of NCP header, giving a total length
 of message leader and NCP header of 192 bits, a number divisible by
 12, 16, 24, 32, 48.

 With respect to the proposed text length field, although bit lengths
 are the most flexible, it seems reasonable to admit that nearly all
 data transmission is done in 8-bit quantities, and therefore that bit
 lengths are, in fact, an unnecessary luxury.  This is a weak argument
 when 36-bit and 32-bit machines must interface.

       [ This RFC was put into machine readable form for entry ]
       [ into the online RFC archives by Alex McKenzie with    ]
       [ support from GTE, formerly BBN Corp.            11/99 ]

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