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RFC 998 - NETBLT: A bulk data transfer protocol

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Network Working Group                                     David D. Clark
Request for Comments:  998                               Mark L. Lambert
Obsoletes:  RFC 969                                          Lixia Zhang
                                                              March 1987

                 NETBLT: A Bulk Data Transfer Protocol

1. Status

   This document is a description of, and a specification for, the
   NETBLT protocol.  It is a revision of the specification published in
   NIC RFC-969.  The protocol has been revised after extensive research
   into NETBLT's performance over long-delay, high-bandwidth satellite
   channels.  Most of the changes in the protocol specification have to
   do with the computation and use of data timers in a multiple
   buffering data transfer model.

   This document is published for discussion and comment, and does not
   constitute a standard.  The proposal may change and certain parts of
   the protocol have not yet been specified; implementation of this
   document is therefore not advised.

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 designed to provide maximum throughput over a wide
   variety of networks.  Although NETBLT currently runs on top of the
   Internet Protocol (IP), it should be able to operate on top of any
   datagram protocol similar in function to IP.

   NETBLT's motivation is to achieve higher throughput than other
   protocols might offer.  The protocol achieves this goal by trying to
   minimize the effect of several network-related problems: network
   congestion, delays over satellite links, and packet loss.

   Its transmission rate-control algorithms deal well with network
   congestion; its multiple-buffering capability allows high throughput
   over long-delay satellite channels, and its various
   timeout/retransmit algorithms minimize the effect of packet loss
   during a transfer.  Most importantly, NETBLT's features give it good
   performance over long-delay channels without impairing performance
   over high-speed LANs.

   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

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

   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 placing data in
   buffers on byte boundaries.

   NETBLT has been designed and should be implemented to work with
   buffers of any 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 amount of 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
   from the client buffer.  This means that the client cannot release
   buffer storage piece by piece as the buffer is sent, but this has not
   been 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 DATA packet size is negotiated
   between the sending and receiving NETBLTs during connection setup.
   Unlike buffer size, DATA packet size is visible only to the NETBLT

   All DATA packets save the last packet in a buffer must be the same
   size.  Packets should be as large as possible, since NETBLT's
   performance is directly related to packet size.  At the same time,
   the packets should not be so large as to cause internetwork
   fragmentation, since this normally causes performance degradation.

   All buffers save the last buffer must be the same size; 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 once 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
   packet loss.  Similarly, a slow gateway or intermediate network could
   cause packets to collect and overflow network packet buffer space.
   Packets will then be lost within the network.  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 window change.

   To permit high throughput over a variety of networks and gateways,
   NETBLT uses a novel flow control method: 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.  First, 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 and lowers 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 a negotiated time interval, then sends another burst.
   In this way, the overhead decreases by a factor of the burst size,
   and the per-burst transmission time is long enough that timing
   mechanisms will work properly.  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 time 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 any intermediate gateways or networks.
   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
   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, lowering performance.

   The size of a NETBLT buffer is limited only by the amount of memory
   available to a client.  Theoretically, buffers of 100 Kbytes 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 in the operating system
   kernel. On most modern operating systems, a burst size of between
   five and ten packets should reduce the overhead to an acceptable
   level.  A preliminary NETBLT implementation for the IBM PC/AT sends
   packets in bursts of five.  It could send more, but is limited by the
   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 20 to 45 milliseconds per 5-
   packet burst have been tried on the IBM PC/AT and Symbolics 3600
   NETBLT implementations 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 in its 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, but only by making them more restrictive.  The
   modified parameters are then sent back to the active end in its
   response message.

   The burst size and burst rate can also 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 burst size and burst rate values in its OK
   messages (described later).  The sender compares these values with
   the values it can support.  Again, it may then modify any of the
   parameters, but only by making them more restrictive.  The modified
   parameters are then communicated to the receiver in a NULL-ACK
   packet, described later.

   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. The NETBLT Transfer Model

   Each NETBLT transfer has three stages, connection setup, data
   transfer, and connection close.  The stages are described in detail
   below, along with methods for insuring that each stage completes

5.1. Connection Setup

   A NETBLT connection is set up by an exchange of two packets between
   the active NETBLT and the passive NETBLT.  Note that either NETBLT
   can send or receive data; the words "active" and "passive" are only
   used to differentiate the end making the connection request from the
   end responding to the connection request.  The active end sends an
   OPEN packet; the passive end acknowledges the OPEN packet in one of
   two ways.  It can either send a REFUSED packet, indicating that the
   connection cannot be completed for some reason, or it can complete
   the connection setup by sending a RESPONSE packet.  At this point 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 data transfer are also negotiated.  These parameters are (1)
   the maximum number of buffers that can be sending at any one time,
   and (2) whether or not DATA 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.  The checksum value is the bitwise
   negation of the ones-complement sum of the 16-bit words being

   Finally, each end transmits its death-timeout value in seconds 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 unique 16 bit port number.

   In order to allow the active and passive ends to communicate
   miscellaneous useful information, an unstructured, variable-length
   field is provided in OPEN and RESPONSE packets for any client-
   specific information that may be required.  In addition, a "reason
   for refusal" field is provided in REFUSED packets.

   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
   predetermined maximum number of OPEN packets have been sent.  The
   timer is cleared upon receipt of a RESPONSE packet.

   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.  If the unique
   ID of the packet matches the unique ID of the connection, then the
   packet type is checked.  If it is a RESPONSE packet, it is treated as
   a duplicate and ignored.  If it is an OPEN packet, the passive NETBLT
   sends another RESPONSE (assuming that a previous RESPONSE packet was
   sent and lost, causing the initiating NETBLT to retransmit its OPEN
   packet).  A non-matching unique ID must be treated as an attempt to
   open a second connection between the same port pair and is 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.  Once the GO
   message is 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 that time the receiver sends
   an OK message inside a CONTROL packet, 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.

   The above data transfer model is effectively a lock-step protocol,
   and causes time to be wasted while the sending NETBLT waits for
   permission to send a new buffer.  A more efficient transfer model
   uses multiple buffering to increase performance.  Multiple buffering
   is a technique in which the sender and receiver allocate and transmit
   buffers in a manner that allows error recovery or successful
   transmission confirmation of previous buffers to be concurrent with
   transmission of the current buffer.

   During the connection setup phase, one of the negotiated parameters
   is the number of concurrent buffers permitted during the transfer.
   If there is more than one buffer available, transfer of the next
   buffer may start right after the current buffer finishes.  This is
   illustrated in the following example:

   Assume two buffers A and B in a multiple-buffer transfer, with A
   preceding B. When A has been transferred and the sending NETBLT is
   waiting for either an OK or a RESEND message for it, the sending
   NETBLT can start sending B immediately, keeping data flowing at a
   stable rate.  If the receiver of data sends an OK for A, all is well;
   if it receives a RESEND, the missing packets specified in the RESEND
   message are retransmitted.

   In the multiple-buffer transfer model, all packets to be sent are
   re-ordered by buffer number (lowest number first), with the transfer
   rate specified by the burst size and burst rate.  Since buffer
   numbers increase monotonically, packets from an earlier buffer will
   always precede packets from a later buffer.

   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

   There are several obvious flaws in the data transfer model as
   described above.  First, what if the GO, OK, or RESEND messages are
   lost?  The sender cannot act on a packet it has not received, so the
   protocol will hang.  Second, if an LDATA packet is lost, how does the
   receiver know when the buffer has been transmitted?  Solutions for
   each of these problems are presented below.

5.2.1. Recovering from Lost Control Messages

   NETBLT solves the problem of lost OK, GO, and RESEND messages in two
   ways.  First, it makes 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.  This timer is either "reset" (set again) or
   "cleared" (deactivated), under the following conditions:

   When the control timer expires, the receiving NETBLT resends the
   control packet and resets the timer.  The receiving NETBLT continues
   to resend control packets in response to control timer's expiration
   until either the control timer is cleared or the receiving NETBLT's
   death timer (described later) expires (at which time it shuts down
   the connection).

   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 (in following paragraphs this is called the
   high-acknowledged-sequence-number) and returns this number in every
   packet flowing back to the receiver.  The receiver is permitted to
   clear its control timer when it receives a packet from the sender
   with a high-acknowledged-sequence-number greater than or equal to the
   highest sequence number in the control packet just sent.

   Ideally, a NETBLT implementation should be able to cope with out-of-
   sequence control 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.

   The sending NETBLT, upon receiving a CONTROL packet, should act on
   the packet as quickly as possible.  It either sets up a new buffer
   (upon receipt of an OK message for a previous buffer), marks data for
   resending (upon receipt of a RESEND message), or prepares a buffer
   for sending (upon receipt of a GO message).  If the sending NETBLT is
   not in a position to send data, it should send a NULL-ACK packet,
   which contains its high-acknowledged-sequence-number (this permits
   the receiving NETBLT to acknowledge any outstanding control
   messages), and wait until it can send more data.  In all of these
   cases, the system overhead for a response to the incoming control
   message should be small and relatively constant.

   The small amount of message-processing overhead allows accurate
   control timers to be set for all types of control messages with a
   single, simple algorithm -- the network round-trip transit time, plus
   a variance factor.  This is more efficient than schemes used by other
   protocols, where timer value calculation has been a problem because
   the processing time for a particular packet can vary greatly
   depending on the packet type.

   Control timer value estimation is extremely important in a high-

   performance protocol like NETBLT.  A long control timer causes the
   receiving NETBLT to wait for long periods of time before
   retransmitting unacknowledged messages.  A short control timer value
   causes the sending NETBLT to receive many duplicate control messages
   (which it can reject, but which takes time).

   In addition to the use of control timers, NETBLT reduces lost control
   messages by using a single long-lived control packet; the packet is
   treated like a FIFO queue, with new control messages added on at the
   end and acknowledged control messages removed from the front.  The
   implementation places control messages in the control packet and
   transmits the entire control packet, consisting of any unacknowledged
   control messages plus new messages just added.  The entire control
   packet is also transmitted whenever the control timer expires.  Since
   control packet transmissions are fairly frequent, unacknowledged
   messages may be transmitted several times before they are finally
   acknowledged.  This redundant transmission of control messages
   provides automatic recovery for most control message losses over a
   noisy channel.

   This scheme places some burdens on the receiver of the control
   messages.  It must be able to quickly reject duplicate control
   messages, since a given message may be retransmitted several times
   before its acknowledgement is received and it is removed from the
   control packet.  Typically this is fairly easy to do; the sender of
   data merely throws away any control messages with sequence numbers
   lower than its high-acknowledged-sequence-number.

   Another problem with this scheme is that the control packet may
   become larger than the maximum allowable packet size if too many
   control messages are placed into it.  This has not been a problem in
   the current NETBLT implementations: a typical control packet size is
   1000 bytes; RESEND control messages average about 20 bytes in length,
   GO messages are 8 bytes long, and OK messages are 16 bytes long.
   This allows 50-80 control messages to be placed in the control
   packet, more than enough for reasonable transfers.  Other
   implementations can provide for multiple control packets if a single
   control packet may not be sufficient.

   The control timer value must be carefully estimated.  It can have as
   its initial value an arbitrary number.  Subsequent control packets
   should 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 all messages in the control packet)
   plus a variance factor.  The timer value should be continually
   updated, based on a smoothed average of collected round-trip transit

5.2.2. Recovering from Lost LDATA Packets

   NETBLT solves the problem of LDATA packet loss by using a data timer
   for each buffer at the receiving end.  The simplest data timer model
   has a data timer set when a buffer is ready to be received; if the
   data timer expires, the receiving NETBLT assumes a lost LDATA packet
   and sends a RESEND message requesting all missing DATA packets in the
   buffer.  When all packets have been received, the timer is cleared.

   Data timer values are not based on network round-trip transit time;
   instead they are based on the amount of time taken to transfer a
   buffer (as determined by the number of DATA packet bursts in the
   buffer times the burst rate) plus a variance factor <1>.

   Obviously an accurate estimation of the data timer value is very
   important.  A short data timer value causes the receiving NETBLT to
   send unnecessary RESEND packets.  This causes serious performance
   degradation since the sending NETBLT has to stop what it is doing and
   resend a number of DATA packets.

   Data timer setting and clearing turns out to be fairly complicated,
   particularly in a multiple-buffering transfer model.  In
   understanding how and when data timers are set and cleared, it is
   helpful to visualize each buffer as a finite-state machine and take a
   look at the various states.

   The state sequence for a sending buffer is simple.  When a GO message
   for the buffer is received, the buffer is created, filled with data,
   and placed in a SENDING state.  When an OK for that buffer has been
   received, it goes into a SENT state and is disposed of.

   The state sequence for a receiving buffer is a little more
   complicated.  Assume existence of a buffer A. When a control message
   for A is sent, the buffer moves into state ACK-WAIT (it is waiting
   for acknowledgement of the control message).

   As soon as the control message has been acknowledged, buffer A moves
   from the ACK-WAIT state into the ACKED state (it is now waiting for
   DATA packets to arrive).  At this point, A's data timer is set and
   the control message removed from the control packet.  Estimation of
   the data timer value at this point is quite difficult.  In a
   multiple-buffer transfer model, the receiving NETBLT can send several
   GO messages at once.  A single DATA packet from the sending NETBLT
   could acknowledge all the GO messages, causing several buffers to
   start up data timers.  Clearly each of the data timers must be set in
   a manner that takes into account each buffer's place in the order of
   transmission.  Packets for a buffer A - 1 will always be transmitted
   before packets in A, so A's data timer must take into account the
   arrival of all of A - 1's DATA packets as well as arrival of its own
   DATA packets.  This means that the timer values become increasingly
   less accurate for higher-numbered buffers.  Because this data timer

   value can be quite inaccurate, it is called a "loose" data timer.
   The loose data timer value is recalculated later (using the same
   algorithm, but with updated information), giving a "tight" timer, as
   described below.

   When the first DATA packet for A arrives, A moves from the ACKED
   state to the RECEIVING state and its data timer is set to a new
   "tight" value.  The tight timer value is calculated in the same
   manner as the loose timer, but it is more accurate since we have
   moved forward in time and those buffers numbered lower than A have
   presumably been dealt with (or their packets would have arrived
   before A's), leaving fewer packets to arrive between the setting of
   the data timer and the arrival of the last DATA packet in A.

   The receiving NETBLT also sets the tight data timers of any buffers
   numbered lower than A that are also in the ACKED state.  This is done
   as an optimization: we know that buffers are processed in order,
   lowest number first.  If a buffer B numbered lower than A is in the
   ACKED state, its DATA packets should arrive before A's.  Since A's
   have arrived first, B's must have gotten lost.  Since B's loose data
   timer has not expired (it would then have sent a RESEND message and
   be in the ACK-WAIT state), we set the tight timer, allowing the
   missing packets to be detected earlier.  An immediate RESEND is not
   sent because it is possible that A's packet was re-ordered before B's
   by the network, and that B's packets may arrive shortly.

   When all DATA packets for A have been received, it moves from the
   RECEIVING state to the RECEIVED state and is disposed of.  Had any
   packets been missing, A's data timer would have expired and A would
   have moved into the ACK-WAIT state after sending a RESEND message.
   The state progression would then move as in the above example.

   The control and data 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 in either the ACKED (loose
   data timer value is used) or the RECEIVING (tight data timer value is
   used) states; a control timer is active whenever the receiving NETBLT
   has any unacknowledged control messages in its control packet.

5.2.3. Death Timers and Keepalive Packets

   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 its control timer 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 hang
   while waiting for control messages to arrive.

   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 expires, it can assume the other end has died and can
   close the connection.

   It is possible that the sending or receiving NETBLTs will have to
   wait for long periods 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 cannot send other packets.  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 a
   death-timer 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 (e.g.
   death timer value divided by four).

   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.

5.3. Closing the Connection

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

5.3.1. Successful Transfer

   After a successful data transfer, NETBLT closes the connection.  When
   the sender is transmitting the last buffer of data, it sets a "last-
   buffer" flag on every DATA packet in the buffer.  This means that no
   NEW data will be transmitted.  The receiver knows the transfer has
   completed successfully when all of the following are true: (1) it has
   received DATA packets with a "last-buffer" flag set, (2) all its
   control messages have been acknowledged, and (3) it has no
   outstanding buffers with missing packets.  At that point, the
   receiver is permitted to close its half of the connection.  The
   sender knows the transfer has completed when the following are true:

   (1) it has transmitted DATA packets with a "last-buffer" flag set and
   (2) it has received OK messages for all its buffers.  At that point,
   it "dallies" for a predetermined period of time before closing its
   half of the connection.  If the NULL-ACK packet acknowledging the
   receiver's last OK message was lost, the receiver has time to
   retransmit the OK message, receive a new NULL-ACK, and recognize a
   successful transfer.  The dally timer value MUST be based on the
   receiver's control timer value; it must be long enough to allow the
   receiver's control timer to expire so that the OK message can be re-
   sent.  For this reason, all OK messages contain (in addition to new
   burst size and burst rate values), the receiver's current control
   timer value in milliseconds.  The sender uses this value to compute
   its dally timer value.

   Since the dally timer value may be quite large, the receiving NETBLT
   is permitted to "short-circuit" the sending NETBLT's dally timer by
   transmitting a DONE packet.  The DONE packet is transmitted when the
   receiver knows the transfer has been successfully completed.  When
   the sender receives a DONE packet, it is allowed to clear its dally
   timer and close its half of the connection immediately.  The DONE
   packet is not reliably transmitted, since failure to receive it only
   means that the sending NETBLT will take longer time to close its half
   of the connection (as it waits for its dally timer to clear)

5.3.2. Client QUIT

   During a NETBLT 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 immediately notifying its client
   and transmitting a QUITACK packet.  The QUIT sender must time out and
   retransmit until a QUITACK has been received or its death timer
   expires.  The sender of the QUITACK dallies before quitting, so that
   it can respond to a retransmitted QUIT.


   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.  The ABORT implies that the NETBLT
   layer is malfunctioning, so no transmit reliability is expected, and
   the sender can immediately close it connection.

6. Protocol Layering Structure

   NETBLT is implemented directly on top of the Internet Protocol (IP).
   It has been assigned an official protocol number of 30 (decimal).

7. Planned Enhancements

   As currently specified, NETBLT has no algorithm for determining its
   rate-control parameters (burst rate, burst size, etc.).  In initial
   performance testing, these parameters have been set by the person
   performing the test.  We are now exploring ways to have NETBLT set
   and adjust its rate-control parameters automatically.

8. Packet Formats

   NETBLT packets are divided into three categories, all of which share
   a common packet header.  First, there are those packets that travel
   only from data sender to receiver; these contain the high-
   acknowledged-sequence-numbers which the receiver uses for control
   message transmission 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 OPEN, RESPONSE,
   these, all save the DONE packet can be sent by both sending and
   receiving NETBLTs.

   All packets are "longword-aligned", i.e. all packets are a multiple
   of 4 bytes in length and all 4-byte fields start on a longword
   boundary.  All arbitrary-length string fields are terminated with at
   least one null byte, with extra null bytes added at the end to create
   a field that is a multiple of 4 bytes long.

   Packet Formats for NETBLT

   OPEN (type 0) and RESPONSE (type 1):

                      1                   2                   3
    1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
   |           Checksum            |    Version    |     Type      |
   |           Length              |           Local Port          |
   |        Foreign Port           | Longword Alignment Padding    |
   |                       Connection Unique ID                    |
   |                         Buffer Size                           |
   |                       Transfer Size                           |
   |        DATA packet size       |          Burst Size           |
   |           Burst Rate          |       Death Timer Value       |
   |       Reserved (MBZ)      |C|M| Maximum # Outstanding Buffers |
   | Client String ...
                                     Longword Alignment Padding    |

   Checksum: packet checksum (algorithm is described in the section
   "Connection Setup")

   Version: the NETBLT protocol version number

   Type: the NETBLT packet type number (OPEN = 0, RESPONSE = 1,

   Length: the total length (NETBLT header plus data, if present)
   of the NETBLT packet in bytes

   Local Port: the local NETBLT's 16-bit port number

   Foreign Port: the foreign NETBLT's 16-bit port number

   Connection UID: the 32 bit connection UID specified in the
   section "Connection Setup".

   Buffer size: the size in bytes of each NETBLT buffer (save the

   Transfer size: (optional) the size in bytes of the transfer.

   This is for client information only; the receiving NETBLT should
   NOT make use of it.

   Data packet size: length of each DATA packet in bytes

   Burst Size: Number of DATA packets in a burst

   Burst Rate: Transmit time in milliseconds of a single burst

   Death timer: Packet sender's death timer value in seconds

   "M": the transfer mode (0 = READ, 1 = WRITE)

   "C": the DATA packet data checksum flag (0 = do not checksum
   DATA packet data, 1 = do)

   Maximum Outstanding Buffers: maximum number of buffers that can
   be transferred before waiting for an OK message from the
   receiving NETBLT.

   Client string: an arbitrary, null-terminated, longword-aligned
   string for use by NETBLT clients.

   KEEPALIVE (type 2), QUITACK (type 4), and DONE (type 11)

                      1                   2                   3
    1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
   |           Checksum            |    Version    |     Type      |
   |           Length              |           Local Port          |
   |        Foreign Port           | Longword Alignment Padding    |

   QUIT (type 3), ABORT (type 5), and REFUSED (type 10)

                      1                   2                   3
    1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
   |           Checksum            |    Version    |     Type      |
   |           Length              |           Local Port          |
   |        Foreign Port           | Longword Alignment Padding    |
   | Reason for QUIT/ABORT/REFUSE...
                                     Longword Alignment Padding    |

   DATA (type 6) and LDATA (type 7):

                      1                   2                   3
    1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
   |           Checksum            |    Version    |     Type      |
   |           Length              |           Local Port          |
   |        Foreign Port           | Longword Alignment Padding    |
   |                       Buffer Number                           |
   | High Consecutive Seq Num Rcvd |         Packet Number         |
   |    Data Area Checksum Value   |      Reserved (MBZ)         |L|

   Buffer number: a 32 bit unique number assigned to every buffer.
   Numbers are monotonically increasing.

   High Consecutive Sequence Number Received: Highest control
   message sequence number below which all sequence numbers received
   are consecutive.

   Packet number: monotonically increasing DATA packet identifier

   Data Area Checksum Value: Checksum of the DATA packet's data.
   Algorithm used is the same as that used to compute checksums of
   other NETBLT packets.

   "L" is a flag set when the buffer that this DATA packet belongs
   to is the last buffer in the transfer.

   NULL-ACK (type 8)

                      1                   2                   3
    1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
   |           Checksum            |    Version    |     Type      |
   |           Length              |           Local Port          |
   |        Foreign Port           | Longword Alignment Padding    |
   | High Consecutive Seq Num Rcvd |        New Burst Size         |
   |       New Burst Rate          |  Longword Alignment Padding   |

   High Consecutive Sequence Number Received: same as in DATA/LDATA

   New Burst Size:  Burst size as negotiated from value given by
   receiving NETBLT in OK message

   New burst rate: Burst rate as negotiated from value given
   by receiving NETBLT in OK message.  Value is in milliseconds.

   CONTROL (type 9):

                      1                   2                   3
    1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
   |           Checksum            |    Version    |     Type      |
   |           Length              |           Local Port          |
   |        Foreign Port           | Longword Alignment Padding    |

   Followed by any number of messages, each of which is longword
   aligned, with the following formats:

   GO message (type 0):

                      1                   2                   3
    1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
   |    Type       | Word Padding  |       Sequence Number         |
   |                        Buffer Number                          |

   Type: message type (GO = 0, OK = 1, RESEND = 2)

   Sequence number: A 16 bit unique message number.  Sequence
   numbers must be monotonically increasing, starting from 1.

   Buffer number: as in DATA/LDATA packet

   OK message (type 1):

                      1                   2                   3
    1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
   |    Type       | Word Padding  |       Sequence Number         |
   |                        Buffer Number                          |
   |    New Offered Burst Size     |   New Offered Burst Rate      |
   | Current control timer value   | Longword Alignment Padding    |

   New offered burst size: burst size for subsequent buffer
   transfers, possibly based on performance information for previous
   buffer transfers.

   New offered burst rate: burst rate for subsequent buffer
   transfers, possibly based on performance information for previous
   buffer transfers.  Rate is in milliseconds.

   Current control timer value: Receiving NETBLT's control timer
   value in milliseconds.

   RESEND Message (type 2):

                      1                   2                   3
    1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
   |    Type       | Word Padding  |       Sequence Number         |
   |                        Buffer Number                          |
   |  Number of Missing Packets    | Longword Alignment Padding    |
   |       Packet Number (2 bytes) ...
                                   |    Padding (if necessary)     |

   Packet number:  the 16 bit data packet identifier found in each
   DATA packet.


   <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 will be
   explored in further testing.


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