RFC 998 (rfc998) - Page 2 of 21

NETBLT: A bulk data transfer protocol

RFC 998                                                       March 1987


   The protocol works by opening a connection between two "clients" (the
   "sender" and the "receiver"), transferring the data in a series of
   large data aggregates called "buffers", and then closing the
   connection.  Because the amount of data to be transferred can be very
   large, the client is not required to provide at once all the data to
   the protocol module.  Instead, the data is provided by the client in
   buffers.  The NETBLT layer transfers each buffer as a sequence of
   packets; since each buffer is composed of a large number of packets,
   the per-buffer interaction between NETBLT and its client is far more
   efficient than a per-packet interaction would be.

   In its simplest form, a NETBLT transfer works as follows:  the
   sending client loads a buffer of data and calls down to the NETBLT
   layer to transfer it.  The NETBLT layer breaks the buffer up into
   packets and sends these packets across the network in Internet
   datagrams.  The receiving NETBLT layer loads these packets into a
   matching buffer provided by the receiving client.  When the last
   packet in the buffer has arrived, the receiving NETBLT checks to see
   that all packets in that buffer have been correctly received.  If
   some packets are missing, the receiving NETBLT requests that they be
   resent.  When the buffer has been completely transmitted, the
   receiving client is notified by its NETBLT layer.  The receiving
   client disposes of the buffer and provides a new buffer to receive
   more data.  The receiving NETBLT notifies the sender that the new
   buffer is ready, and the sender prepares and sends the next buffer in
   the same manner.  This continues until all the data has been sent; at
   that time the sender notifies the receiver that the transmission has
   been completed.  The connection is then closed.

   As described above, the NETBLT protocol is "lock-step".  Action halts
   after a buffer is transmitted, and begins again after confirmation is
   received from the receiver of data.  NETBLT provides for multiple
   buffering, a transfer model in which the sending NETBLT can transmit
   new buffers while earlier buffers are waiting for confirmation from
   the receiving NETBLT.  Multiple buffering makes packet flow
   essentially continuous and markedly improves performance.

   The remainder of this document describes NETBLT in detail.  The next
   sections describe the philosophy behind a number of protocol
   features:  packetization, flow control, transfer reliability, and
   connection management. The final sections describe NETBLT's packet
   formats.

3. Buffers and Packets

   NETBLT is designed to permit transfer of a very large amounts of data
   between two clients.  During connection setup the sending NETBLT can
   inform the receiving NETBLT of the transfer size; the maximum
   transfer length is 2**32 bytes.  This limit should permit any
   practical application.  The transfer size parameter is for the use of
   the receiving client; the receiving NETBLT makes no use of it.  A



Clark, Lambert, & Zhang