rfc969.txt

RFC 相关的技术文档

RFC 969                                                    December 1985
NETBLT: A Bulk Data Transfer Protocol


      Timer value estimation is extremely important, especially in a
      high-performance protocol like NETBLT.  If the estimates are too
      low, the protocol makes many unneeded retransmissions, degrading
      performance.  A short control timer value causes the sending
      NETBLT to receive duplicate control messages (which it can reject,
      but which takes time).  A short data timer value causes the
      receiving NETBLT to send unnecessary RESEND packets.  This causes
      considerably greater performance degradation since the sending
      NETBLT does not merely throw away a duplicate packet, but instead
      has to send a number of DATA packets.  Because data timers are set
      on each buffer transfer instead of on each DATA packet transfer,
      we afford to use a small variance value without worrying about
      performance degradation.

   5.3. Closing the Connection

      There are three ways to close a connection: a connection close, a
      "quit", or an "abort".

      The connection close occurs after a successful data transfer.
      When the sending NETBLT has received an OK packet for the last
      buffer in the transfer, it sends a DONE packet <2>.  On receipt of
      the DONE packet, the receiving NETBLT can close its half of the
      connection.  The sending NETBLT dallies for a predetermined amount
      of time after sending the DONE packet.  This allows for the
      possibility of the DONE packet's having been lost.  If the DONE
      packet was lost, the receiving NETBLT will continue to send the
      final OK packet, which will cause the sending end to resend the
      DONE packet.  After the dally period expires, the sending NETBLT
      closes its half of the connection.

      During the transfer, one client may send a QUIT packet to the
      other if it thinks that the other client is malfunctioning.  Since
      the QUIT occurs at a client level, the QUIT transmission can only
      occur between buffer transmissions.  The NETBLT receiving the QUIT
      packet can take no action other than to immediately notify its
      client and transmit a QUITACK packet.  The QUIT sender must time
      out and retransmit until a QUITACK has been received or a
      predetermined number of resends have taken place.  The sender of
      the QUITACK dallies in the manner described above.

      An ABORT takes place when a NETBLT layer thinks that it or its
      opposite is malfunctioning.  Since the ABORT originates in the
      NETBLT layer, it can be sent at any time.  Since the ABORT implies
      that the NETBLT layer is malfunctioning, no transmit reliability
      is expected, and the sender can immediately close it connection.



Clark & Lambert & Zhang                                        [Page 11]



RFC 969                                                    December 1985
NETBLT: A Bulk Data Transfer Protocol


6. MULTIPLE BUFFERING

   In order to increase performance, NETBLT has been designed in a
   manner that encourages a multiple buffering implementation.  Multiple
   buffering is a technique in which the sender and receiver allocate
   and transmit buffers in a manner that allows error recovery of
   previous buffers to be concurrent with transmission of current
   buffer.

   During the connection setup phase, one of the negotiated parameters
   is the number of concurrent buffers permitted during the transfer.
   The simplest transfer allows for a maximum of one buffer to be
   transmitted at a time; this is effectively a lock-step protocol and
   causes time to be wasted while the sending NETBLT receives permission
   to send a new buffer.  If there are more than one buffer available,
   transfer of the next buffer may start right after the current buffer
   finishes.  For example, assume buffer A and B are allowed to transfer
   concurrently, with A preceding B. As soon as A finishes transferring
   its data and is waiting for either an OK or a RESEND message, B can
   start sending immediately, keeping data flowing at a stable rate.  If
   A receives an OK, it is done; if it receives a RESEND, the missing
   packets specified in the RESEND message are retransmitted.  All
   packets flow out through a priority pipe, with the priority equal to
   the buffer number, and with the transfer rate specified by the burst
   size and burst rate.  Since buffer numbers increase monotonically,
   packets from an earlier buffer in the pipe will always precede those
   of the later ones.  One necessary change to the timing algorithm is
   that when the receiving NETBLT set data timer for a new buffer, the
   timer value should also take into consideration of the transfer time
   for all missing packets from the previous buffers.

   Having several buffers transmitting concurrently is actually not that
   much more complicated than transmitting a single buffer at a time.
   The key is to visualize each buffer as a finite state machine;
   several buffers are merely a group of finite state machines, each in
   one of several states.  The transfer process consists of moving
   buffers through various states until the entire transmission has
   completed.

   The state sequence of a send-receive buffer pair is as follows: the
   sending and receiving buffers are created independently.  The
   receiving NETBLT sends a GO message, putting its buffer in a
   "receiving" state, and sets its control timer; the sending NETBLT
   receives the GO message, putting its buffer into a "sending" state.
   The sending NETBLT sends data until the buffer has been transmitted.
   If the receiving NETBLT's data timer goes off before it received the
   last (LDATA) packet, or it receives the LDATA packet in the buffer


Clark & Lambert & Zhang                                        [Page 12]



RFC 969                                                    December 1985
NETBLT: A Bulk Data Transfer Protocol


   and packets are missing, it sends a RESEND packet and moves the
   buffer into a "resending" state.  Once all DATA packets in the buffer
   and the LDATA packet have been received, the receiving NETBLT enters
   its buffer into a "received" state and sends an OK packet.  The
   sending NETBLT receives the OK packet and puts its buffer into a
   "sent" state.

7. PROTOCOL LAYERING STRUCTURE

   NETBLT is implemented directly on top of the Internet Protocol (IP).
   It has been assigned a temporary protocol number of 255.  This number
   will change as soon as the final protocol specification has been
   determined.

8. PACKET FORMATS

   NETBLT packets are divided into three categories, each of which share
   a common packet header.  First, there are those packets that travel
   only from sender to receiver; these contain the control message
   sequence numbers which the receiver uses for reliability.  These
   packets are the NULL-ACK, DATA, and LDATA packets.  Second, there is
   a packet that travels only from receiver to sender.  This is the
   CONTROL packet; each CONTROL packet can contain an arbitrary number
   of control messages (GO, OK, or RESEND), each with its own sequence
   number. Finally, there are those packets which either have special
   ways of insuring reliability, or are not reliably transmitted.  These
   are the QUIT, QUITACK, DONE, KEEPALIVE, and ABORT packets.  Of these,
   all save the DONE packet can be sent by both sending and receiving
   NETBLTs.

   Packet type numbers:

      OPEN:           0
      RESPONSE:       1
      KEEPALIVE:      2
      DONE:           3
      QUIT:           4
      QUITACK:        5
      ABORT:          6
      DATA:           7
      LDATA:          8
      NULL-ACK:       9
      CONTROL:        10






Clark & Lambert & Zhang                                        [Page 13]



RFC 969                                                    December 1985
NETBLT: A Bulk Data Transfer Protocol


   Standard header:

      local port:       2 bytes
      foreign port:     2 bytes
      checksum:         2 bytes
      version number:   1 byte
      packet type:      1 byte
      packet length:    2 bytes

   OPEN and RESPONSE packets:

      connection unique ID:                   4 bytes
      standard buffer size:                   4 bytes
      transfer size:                          4 bytes
      DATA packet data segment size:          2 bytes
      burst size:                             2 bytes
      burst rate:                             2 bytes
      death timeout value in seconds:         2 bytes
      transfer mode (1 = SEND, 0 = RECEIVE):  1 byte
      maximum number of concurrent buffers:   1 byte
      checksum entire DATA packet / checksum
      DATA packet data only (1/0):         1 byte
      client-specific data:                   arbitrary

   DONE, QUITACK, KEEPALIVE:

      standard header only

   ABORT, QUIT:

      reason:       arbitrary bytes

   CONTROL packet format:

      CONTROL packets consist of a standard NETBLT header of type
      CONTROL, followed by an arbitrary number of control messages with
      the following formats:

      Control message numbers:

         GO:             0
         OK:             1
         RESEND:         2






Clark & Lambert & Zhang                                        [Page 14]



RFC 969                                                    December 1985
NETBLT: A Bulk Data Transfer Protocol


         OK message:

            message type (OK):  1 byte
            buffer number:      4 bytes
            sequence number:    2 bytes
            new burst size:     2 bytes
            new burst interval: 2 bytes

         GO message:

            message type (GO):  1 byte
            buffer number:      4 bytes
            sequence number:    2 bytes

         RESEND message:

            message type (RESEND):     1 byte
            buffer number:             4 bytes
            sequence number:           2 bytes
            number of missing packets: 2 bytes
            packet numbers...:         n * 2 bytes

   DATA, LDATA packet formats:

      buffer number:                                4 bytes
      highest consecutive sequence number received: 2 bytes
      packet number within buffer:                  2 bytes
      data:                                         arbitrary bytes

   NULL-ACK packet format:

      highest consecutive sequence number received: 2 bytes
      acknowledged new burst size:                  2 bytes
      acknowledged new burst interval:              2 bytes

NOTES:

   <1>  When the buffer size is large, the variances in the round trip
        delays of many packets may cancel each other out; this means the
        variance value need not be very big.  This expectation can be
        verified in further testing.

   <2>  Since the receiving end may not know the transfer size in
        advance, it is possible that it may have allocated buffer space
        and sent GO messages for buffers beyond the actual last buffer
        sent by the sending end.  Care must be taken on the sending
        end's part to ignore these extra GO messages.


Clark & Lambert & Zhang                                        [Page 15]