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Network Working Group                                     David D. ClarkRequest for Comments: 969                                Mark L. Lambert                                                             Lixia Zhang                                M. I. T. Laboratory for Computer Science                                                           December 1985NETBLT: A Bulk Data Transfer Protocol1. 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 theClark & Lambert & Zhang                                         [Page 1]

RFC 969                                                    December 1985NETBLT: 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 directlyClark & Lambert & Zhang                                         [Page 2]

RFC 969                                                    December 1985NETBLT: 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 packetClark & Lambert & Zhang                                         [Page 3]

RFC 969                                                    December 1985NETBLT: 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.  MostClark & Lambert & Zhang                                         [Page 4]

RFC 969                                                    December 1985NETBLT: 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.Clark & Lambert & Zhang                                         [Page 5]

RFC 969                                                    December 1985NETBLT: A Bulk Data Transfer Protocol   Obviously each of the parameters depend on many factors-- gateway and   host processing speeds, available memory, timer granularity--some of   which cannot be checked by either client.  Each client must therefore   try to make as best a guess as it can, tuning for performance on   subsequent transfers.5. RELIABILITY   Each NETBLT transfer has three stages, connection setup, data   transfer, and connection close.  Each stage must be completed   reliably; methods for doing this are described below.   5.1. Connection Setup      A NETBLT connection is set up by an exchange of two packets      between the active client and the passive client.  Note that      either client can send or receive data; the words "active" and      "passive" are only used to differentiate the client initiating the      connection process from the client responding to the connection      request.  The first packet sent is an OPEN packet; the passive end      acknowledges the OPEN packet by sending a RESPONSE packet.  After      these two packets have been exchanged, the transfer can begin.      As discussed in the previous section, the OPEN and RESPONSE      packets are used to negotiate flow control parameters.  Other      parameters used in the transfer of data are also negotiated.      These parameters are (1) the maximum number of buffers that can be      sending at any one time (this permits multiple buffering and      higher throughput) and (2) whether or not DATA/LDATA packet data      will be checksummed.  NETBLT automatically checksums all      non-DATA/LDATA packets.  If the negotiated checksum flag is set to      TRUE (1), both the header and the data of a DATA/LDATA packet are      checksummed; if set to FALSE (0), only the header is checksummed.      NETBLT uses the same checksumming algorithm as TCP uses.      Finally, each end transmits its death-timeout value in either the      OPEN or the RESPONSE packet.  The death-timeout value will be used      to determine the frequency with which to send KEEPALIVE packets      during idle periods of an opened connection (death timers and      KEEPALIVE packets are described in the following section).      The active end specifies a passive client through a      client-specific "well-known" 16 bit port number on which the      passive end listens.  The active end identifies itself through a      32 bit Internet address and a 16 bit port number.      In order to allow the active and passive ends to communicateClark & Lambert & Zhang                                         [Page 6]

RFC 969                                                    December 1985NETBLT: A Bulk Data Transfer Protocol      miscellaneous useful information, an unstructured, variable-      length field is provided in OPEN and RESPONSE messages for an      client-specific information that may be required.      Recovery for lost OPEN and RESPONSE packets is provided by the use      of timers.  The active end sets a timer when it sends an OPEN      packet. When the timer expires, another OPEN packet is sent, until      some pre-determined maximum number of OPEN packets have been sent.      A similar scheme is used for the passive end when it sends a      RESPONSE packet.  When a RESPONSE packet is received by the active      end, it clears its timer.  The passive end's timer is cleared      either by receipt of a GO or a DATA packet, as described in the      section on data transfer.      To prevent duplication of OPEN and RESPONSE packets, the OPEN      packet contains a 32 bit connection unique ID that must be      returned in the RESPONSE packet.  This prevents the initiator from      confusing the response to the current request with the response to      an earlier connection request (there can only be one connection      between any two ports).  Any OPEN or RESPONSE packet with a      destination port matching that of an open connection has its      unique ID checked.  A matching unique ID implies a duplicate      packet, and the packet is ignored.  A non-matching unique ID must      be treated as an attempt to open a second connection between the      same port pair and must be rejected by sending an ABORT message.   5.2. Data Transfer      The simplest model of data transfer proceeds as follows.  The      sending client sets up a buffer full of data.  The receiving      NETBLT sends a GO message inside a CONTROL packet to the sender,      signifying that it too has set up a buffer and is ready to receive      data into it. Once the GO message has been received, the sender      transmits the buffer as a series of DATA packets followed by an      LDATA packet.  When the last packet in the buffer has been      received, the receiver sends a RESEND message inside a CONTROL      packet containing a list of packets that were not received.  The      sender resends these packets.  This process continues until there      are no missing packets, at which time the receiver sends an OK      message inside a CONTROL packet to the sender, sets up another      buffer to receive data and sends another GO message.  The sender,      having received the OK message, sets up another buffer, waits for      the GO message, and repeats the process.      There are several obvious flaws with this scheme.  First, if the      LDATA packet is lost, how does the receiver know when the buffer      has been transmitted?  Second, what if the GO, OK, or RESENDClark & Lambert & Zhang                                         [Page 7]

RFC 969                                                    December 1985NETBLT: A Bulk Data Transfer Protocol      messages are lost?  The sender cannot act on a packet it has not      received, so the protocol will hang.  Solutions for each of these      problems are presented below, and are based on two kinds of      timers, a data timer and a control timer.      NETBLT solves the LDATA packet loss problem by using a data timer      at the receiving end.  When the first DATA packet in a buffer      arrives, the receiving NETBLT sets its data timer; at the same      time, it clears its control timer, described below.  If the data      timer expires, the receiving end assumes the buffer has been      transmitted and all missing packets lost.  It then sends a RESEND      message containing a list of the missing packets.      NETBLT solves the second problem, that of missing OK, GO, and      RESEND messages, through use of a control timer.  The receiver can      send one or more control messages (OK, GO, or RESEND) within a      single CONTROL packet.  Whenever the receiver sends a control      packet, it sets a control timer (at the same time it clears its      data timer, if one has been set).      The control timer is cleared as follows: Each control message      includes a sequence number which starts at one and increases by      one for each control message sent.  The sending NETBLT checks the      sequence number of every incoming control message against all      other sequence numbers it has received.  It stores the highest      sequence number below which all other received sequence numbers      are consecutive, and returns this number in every packet flowing      back to the receiver.  The receiver is permitted to clear the      control timer of every packet with a sequence number equal to or      lower than the sequence number returned by the sender.      Ideally, a NETBLT implementation should be able to cope with      out-of-sequence messages, perhaps collecting them for later      processing, or even processing them immediately.  If an incoming      control message "fills" a "hole" in a group of message sequence      numbers, the implementation could even be clever enough to detect      this and adjust its outgoing sequence value accordingly.      When the control timer expires, the receiving NETBLT resends the      control message and resets the timer.  After a predetermined      number of resends, the receiving NETBLT can assume that the      sending NETBLT has died, and can reset the connection.      The sending NETBLT, upon receiving a control message, should act      as quickly as possible on the packet; it either sets up a new      buffer (upon receipt of an OK packet for a previous buffer),      resends data (upon receipt of a RESEND packet), or sends dataClark & Lambert & Zhang                                         [Page 8]

RFC 969                                                    December 1985NETBLT: A Bulk Data Transfer Protocol      (upon receipt of a GO packet).  If the sending NETBLT is not in a      position to send data, it sends a NULL-ACK packet, which contains      a      high-received-sequence-number as described above (this permits the      receiving NETBLT to clear the control timers of any packets which      are outstanding), and waits until it can send more data.  In all      of these cases, the overhead for a response to the incoming      control message should be small; the total time for a response to      reach the receiving NETBLT should not be much more than the      network round-trip transit time, plus a variance factor.      The timer system can be summarized as follows: normally, the      receiving NETBLT is working under one of two types of timers, a      control timer or a data timer.  There is one data timer per buffer      transmission and one control timer per control packet.  The data      timer is active while its buffer is being transferred; a control      timer is active while it is between buffer transfers.      The above system still leaves a few problems.  If the sending      NETBLT is not ready to send, it sends a single NULL-ACK packet to      clear any outstanding control timers at the receiving end.  After      this the receiver will wait.  The sending NETBLT could die and the      receiver, with all its control timers cleared, would hang.  Also,      the above system puts timers only on the receiving NETBLT.  The      sending NETBLT has no timers; if the receiving NETBLT dies, the      sending NETBLT will just hang waiting for control messages.      The solution to the above two problems is the use of a death timer      and a keepalive packet for both the sending and receiving NETBLTs.      As soon as the connection is opened, each end sets a death timer;      this timer is reset every time a packet is received.  When a      NETBLT's death timer at one end expires, it can assume the other      end has died and can close the connection.      It is quite possible that the sending or receiving NETBLTs will      have to wait for long periods of time while their respective      clients get buffer space and load their buffers with data.  Since      a NETBLT waiting for buffer space is in a perfectly valid state,      the protocol must have some method for preventing the other end's      death timer from expiring. The solution is to use a KEEPALIVE      packet, which is sent repeatedly at fixed intervals when a NETBLT      is waiting for buffer space.  Since the death timer is reset      whenever a packet is received, it will never expire as long as the      other end sends packets.      The frequency with which KEEPALIVE packets are transmitted is      computed as follows: At connection startup, each NETBLT chooses aClark & Lambert & Zhang                                         [Page 9]

RFC 969                                                    December 1985NETBLT: A Bulk Data Transfer Protocol      death-timeout value and sends it to the other end in either the      OPEN or the RESPONSE packet.  The other end takes the      death-timeout value and uses it to compute a frequency with which      to send KEEPALIVE packets.  The KEEPALIVE frequency should be high      enough that several KEEPALIVE packets can be lost before the other      end's death timer expires.      Both ends must have some way of estimating the values of the death      timers, the control timers, and the data timers.  The timer values      obviously cannot be specified in a protocol document since they      are very machine- and network-load-dependent.  Instead they must      be computed on a per-connection basis.  The protocol has been      designed to make such determination easy.      The death timer value is relatively easy to estimate.  Since it is      continually reset, it need not be based on the transfer size.      Instead, it should be based at least in part on the type of      application using NETBLT.  User applications should have smaller      death timeout values to avoid forcing humans to wait long periods      of time for a death timeout to occur.  Machine applications can      have longer timeout values.      The control timer must be more carefully estimated.  It can have      as its initial value an arbitrary number; this number can be used      to send the first control packet.  Subsequent control packets can      have their timer values based on the network round-trip transit      time (i.e.  the time between sending the control packet and      receiving the acknowledgment of the corresponding sequence number)      plus a variance factor.  The timer value should be continually      updated, based on a smoothed average of collected round-trip      transit times.      The data timer is dependent not on the network round-trip transit      time, but on the amount of time required to transfer a buffer of      data. The time value can be computed from the burst rate and the      number of bursts per buffer, plus a variance value <1>. During the      RESENDing phase, the data timer value should be set according to      the number of missing packets.      The timers have been designed to permit reasonable estimation.  In      particular, in other protocols, determination of round-trip delay      has been a problem since the action performed by the other end on      receipt of a particular packet can vary greatly depending on the      packet type. In NETBLT, the action taken by the sender on receipt      of a control message is by and large the same in all cases, making      the round-trip delay relatively independent of the client.Clark & Lambert & Zhang                                        [Page 10]

RFC 969                                                    December 1985NETBLT: 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 1985NETBLT: A Bulk Data Transfer Protocol6. 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 bufferClark & Lambert & Zhang                                        [Page 12]

RFC 969                                                    December 1985NETBLT: 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:        10Clark & Lambert & Zhang                                        [Page 13]

RFC 969                                                    December 1985NETBLT: 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:         2Clark & Lambert & Zhang                                        [Page 14]

RFC 969                                                    December 1985NETBLT: 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 bytesNOTES:   <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]