rfc969.txt

RFC 相关的技术文档

Network Working Group                                     David D. Clark
Request for Comments: 969                                Mark L. Lambert
                                                             Lixia Zhang
                                M. I. T. Laboratory for Computer Science
                                                           December 1985

                 NETBLT: A Bulk Data Transfer Protocol


1. STATUS OF THIS MEMO

   This RFC suggests a proposed protocol for the ARPA-Internet
   community, and requests discussion and suggestions for improvements.
   This is a preliminary discussion of the NETBLT protocol.  It is
   published for discussion and comment, and does not constitute a
   standard.  As the proposal may change, implementation of this
   document is not advised.  Distribution of this memo is unlimited.

2. INTRODUCTION

   NETBLT (Network Block Transfer) is a transport level protocol
   intended for the rapid transfer of a large quantity of data between
   computers. It provides a transfer that is reliable and flow
   controlled, and is structured to provide maximum throughput over a
   wide variety of networks.

   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 arbitrarily
   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, but 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 been transmitted, the receiving NETBLT
   checks to see that all packets in that buffer have arrived.  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 buffer
   arrived, and the sender prepares and sends the next buffer in the


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RFC 969                                                    December 1985
NETBLT: A Bulk Data Transfer Protocol


   same manner.  This continues until all buffers have been sent, at
   which 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 is
   halted after a buffer is transmitted, and begins again after
   confirmation is received from the receiver of data.  NETBLT provides
   for multiple buffering, in which several buffers can be transmitted
   concurrently.  Multiple buffering makes packet flow essentially
   continuous and can improve performance markedly.

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

3. BUFFERS AND PACKETS

   NETBLT is designed to permit transfer of an essentially arbitrary
   amount of data between two clients.  During connection setup the
   sending NETBLT can optionally inform the receiving NETBLT of the
   transfer size; the maximum transfer length is imposed by the field
   width, and 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 NETBLT
   receiver accepts data until told by the sender that the transfer is
   complete.

   The data to be sent must be broken up into buffers by the client.
   Each buffer must be the same size, save for the last buffer.  During
   connection setup, the sending and receiving NETBLTs negotiate the
   buffer size, based on limits provided by the clients.  Buffer sizes
   are in bytes only; the client is responsible for breaking up data
   into buffers on byte boundaries.

   NETBLT has been designed and should be implemented to work with
   buffers of arbitrary size.  The only fundamental limitation on buffer
   size should be the amount of memory available to the client.  Buffers
   should be as large as possible since this minimizes the number of
   buffer transmissions and therefore improves performance.

   NETBLT is designed to require a minimum of its own memory, allowing
   the client to allocate as much memory as possible for buffer storage.
   In particular, NETBLT does not keep buffer copies for retransmission
   purposes.  Instead, data to be retransmitted is recopied directly




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RFC 969                                                    December 1985
NETBLT: A Bulk Data Transfer Protocol


   from the client buffer.  This does mean that the client cannot
   release buffer storage piece by piece as the buffer is sent, but this
   has not proved a problem in preliminary NETBLT implementations.

   Buffers are broken down by the NETBLT layer into sequences of DATA
   packets.  As with the buffer size, the packet size is negotiated
   between the sending and receiving NETBLTs during connection setup.
   Unlike buffer size, packet size is visible only to the NETBLT layer.

   All DATA packets save the last packet in a buffer must be the same
   size.  Packets should be as large as possible, since in most cases
   (including the preliminary protocol implementation) performance is
   directly related to packet size.  At the same time, the packets
   should not be so large as to cause Internet fragmentation, since this
   normally causes performance degrada- tion.

   All buffers save the last buffer must be the same size; obviously the
   last buffer can be any size required to complete the transfer. Since
   the receiving NETBLT does not know the transfer size in advance, it
   needs some way of identifying the last packet in each buffer.  For
   this reason, the last packet of every buffer is not a DATA packet but
   rather an LDATA packet.  DATA and LDATA packets are identical save
   for the packet type.

4. FLOW CONTROL

   NETBLT uses two strategies for flow control, one internal and one at
   the client level.

   The sending and receiving NETBLTs transmit data in buffers; client
   flow control is therefore at a buffer level.  Before a buffer can be
   transmitted, NETBLT confirms that both clients have set up matching
   buffers, that one is ready to send data, and that the other is ready
   to receive data.  Either client can therefore control the flow of
   data by not providing a new buffer.  Clients cannot stop a buffer
   transfer while it is in progress.

   Since buffers can be quite large, there has to be another method for
   flow control that is used during a buffer transfer.  The NETBLT layer
   provides this form of flow control.

   There are several flow control problems that could arise while a
   buffer is being transmitted.  If the sending NETBLT is transferring
   data faster than the receiving NETBLT can process it, the receiver's
   ability to buffer unprocessed packets could be overflowed, causing
   packets to be lost.  Similarly, a slow gateway or intermediate
   network could cause packets to collect and overflow network packet


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RFC 969                                                    December 1985
NETBLT: A Bulk Data Transfer Protocol


   buffer space.  Packets will then be lost within the network,
   degrading performance.  This problem is particularly acute for NETBLT
   because NETBLT buffers will generally be quite large, and therefore
   composed of many packets.

   A traditional solution to packet flow control is a window system, in
   which the sending end is permitted to send only a certain number of
   packets at a time.  Unfortunately, flow control using windows tends
   to result in low throughput.  Windows must be kept small in order to
   avoid overflowing hosts and gateways, and cannot easily be updated,
   since an end-to-end exchange is required for each change.

   To permit high throughput over a variety of networks and gateways of
   differing speeds, NETBLT uses a novel flow control ethod: rate
   control.  The transmission rate is negotiated by the sending and
   receiving NETBLTs during connection setup and after each buffer
   transmission.  The sender uses timers, rather than messages from the
   receiver, to maintain the negotiated rate.

   In its simplest form, rate control specifies a minimum time period
   per packet transmission.  This can cause performance problems for
   several reasons: the transmission time for a single packet is very
   small, frequently smaller than the granularity of the timing
   mechanism.  Also, the overhead required to maintain timing mechanisms
   on a per packet basis is relatively high, which degrades performance.

   The solution is to control the transmission rate of groups of
   packets, rather than single packets.  The sender transmits a burst of
   packets over negotiated interval, then sends another burst.  In this
   way, the overhead decreases by a factor of the burst size, and the
   per-burst transmission rate is large enough that timing mechanisms
   will work properly.  The NETBLT's rate control therefore has two
   parts, a burst size and a burst rate, with (burst size)/(burst rate)
   equal to the average transmission rate per packet.

   The burst size and burst rate should be based not only on the packet
   transmission and processing speed which each end can handle, but also
   on the capacities of those gateways and networks intermediate to the
   transfer.  Following are some intuitive values for packet size,
   buffer size, burst size, and burst rate.

   Packet sizes can be as small as 128 bytes.  Performance with packets
   this small is almost always bad, because of the high per-packet
   processing overhead.  Even the default Internet Protocol packet size
   of 576 bytes is barely big enough for adequate performance.  Most




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RFC 969                                                    December 1985
NETBLT: A Bulk Data Transfer Protocol


   networks do not support packet sizes much larger than one or two
   thousand bytes, and packets of this size can also get fragmented when
   traveling over intermediate networks, degrading performance.

   The size of a NETBLT buffer is limited only by the amount of memory
   available to a client.  Theoretically, buffers of 100K bytes or more
   are possible.  This would mean the transmission of 50 to 100 packets
   per buffer.

   The burst size and burst rate are obviously very machine dependent.
   There is a certain amount of transmission overhead in the sending and
   receiving machines associated with maintaining timers and scheduling
   processes.  This overhead can be minimized by sending packets in
   large bursts.  There are also limitations imposed on the burst size
   by the number of available packet buffers.  On most modern operating
   systems, a burst size of between five and ten packets should reduce
   the overhead to an acceptable level.  In fact, a preliminary NETBLT
   implementation for the IBM PC/AT sends packets in bursts of five.  It
   could send more, but is limited by available memory.

   The burst rate is in part determined by the granularity of the
   sender's timing mechanism, and in part by the processing speed of the
   receiver and any intermediate gateways.  It is also directly related
   to the burst size.  Burst rates from 60 to 100 milliseconds have been
   tried on the preliminary NETBLT implementation with good results
   within a single local-area network.  This value clearly depends on
   the network bandwidth and packet buffering available.

   All NETBLT flow control parameters (packet size, buffer size, burst
   size, and burst rate) are negotiated during connection setup.  The
   negotiation process is the same for all parameters.  The client
   initiating the connection (the active end) proposes and sends a set
   of values for each parameter with its open connection request.  The
   other client (the passive end) compares these values with the
   highest-performance values it can support.  The passive end can then
   modify any of the parameters only by making them more restrictive.
   The modified parameters are then sent back to the active end in the
   response message.  In addition, the burst size and burst rate can be
   re-negotiated after each buffer transmission to adjust the transfer
   rate according to the performance observed from transferring the
   previous buffer.  The receiving end sends a pair of burst size and
   burst rate values in the OK message.  The sender compares these
   values with the values it can support.  Again, it may then modify any
   of the parameters only by making them more restrictive.  The modified
   parameters are then communicated to the receiver in a NULL-ACK
   packet, described later.



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