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Network Working Group                                     David D. ClarkRequest for Comments:  998                               Mark L. LambertObsoletes:  RFC969                                          Lixia Zhang                                                                     MIT                                                              March 1987NETBLT: A Bulk Data Transfer Protocol1. Status   This document is a description of, and a specification for, the   NETBLT protocol.  It is a revision of the specification published in   NICRFC-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.Clark, Lambert, & Zhang                                         [Page 1]

RFC 998                                                       March 1987   The protocol works by opening a connection between two "clients" (the   "sender" and the "receiver"), transferring the data in a series of   large data aggregates called "buffers", and then closing the   connection.  Because the amount of data to be transferred can be very   large, the client is not required to provide at once all the data to   the protocol module.  Instead, the data is provided by the client in   buffers.  The NETBLT layer transfers each buffer as a sequence of   packets; since each buffer is composed of a large number of packets,   the per-buffer interaction between NETBLT and its client is far more   efficient than a per-packet interaction would be.   In its simplest form, a NETBLT transfer works as follows:  the   sending client loads a buffer of data and calls down to the NETBLT   layer to transfer it.  The NETBLT layer breaks the buffer up into   packets and sends these packets across the network in Internet   datagrams.  The receiving NETBLT layer loads these packets into a   matching buffer provided by the receiving client.  When the last   packet in the buffer has arrived, the receiving NETBLT checks to see   that all packets in that buffer have been correctly received.  If   some packets are missing, the receiving NETBLT requests that they be   resent.  When the buffer has been completely transmitted, the   receiving client is notified by its NETBLT layer.  The receiving   client disposes of the buffer and provides a new buffer to receive   more data.  The receiving NETBLT notifies the sender that the new   buffer is ready, and the sender prepares and sends the next buffer in   the same manner.  This continues until all the data has been sent; at   that time the sender notifies the receiver that the transmission has   been completed.  The connection is then closed.   As described above, the NETBLT protocol is "lock-step".  Action halts   after a buffer is transmitted, and begins again after confirmation is   received from the receiver of data.  NETBLT provides for multiple   buffering, a transfer model in which the sending NETBLT can transmit   new buffers while earlier buffers are waiting for confirmation from   the receiving NETBLT.  Multiple buffering makes packet flow   essentially continuous and markedly improves performance.   The remainder of this document describes NETBLT in detail.  The next   sections describe the philosophy behind a number of protocol   features:  packetization, flow control, transfer reliability, and   connection management. The final sections describe NETBLT's packet   formats.3. Buffers and Packets   NETBLT is designed to permit transfer of a very large amounts of data   between two clients.  During connection setup the sending NETBLT can   inform the receiving NETBLT of the transfer size; the maximum   transfer length is 2**32 bytes.  This limit should permit any   practical application.  The transfer size parameter is for the use of   the receiving client; the receiving NETBLT makes no use of it.  AClark, Lambert, & Zhang                                         [Page 2]

RFC 998                                                       March 1987   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   layer.   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 beClark, Lambert, & Zhang                                         [Page 3]

RFC 998                                                       March 1987   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.Clark, Lambert, & Zhang                                         [Page 4]

RFC 998                                                       March 1987   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.Clark, Lambert, & Zhang                                         [Page 5]

RFC 998                                                       March 1987   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   reliably.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   checksummed.   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 KEEPALIVEClark, Lambert, & Zhang                                         [Page 6]

RFC 998                                                       March 1987   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 GOClark, Lambert, & Zhang                                         [Page 7]

RFC 998                                                       March 1987   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   completed.   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 singleClark, Lambert, & Zhang                                         [Page 8]

RFC 998                                                       March 1987   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-Clark, Lambert, & Zhang                                         [Page 9]

RFC 998                                                       March 1987   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   times.Clark, Lambert, & Zhang                                        [Page 10]

RFC 998                                                       March 19875.2.2. 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 timerClark, Lambert, & Zhang                                        [Page 11]

RFC 998                                                       March 1987   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.Clark, Lambert, & Zhang                                        [Page 12]

RFC 998                                                       March 1987   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:Clark, Lambert, & Zhang                                        [Page 13]

RFC 998                                                       March 1987   (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.5.3.3. NETBLT ABORT   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).Clark, Lambert, & Zhang                                        [Page 14]

RFC 998                                                       March 19877. 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,   REFUSED, QUIT, QUITACK, DONE, KEEPALIVE, and ABORT packets.  Of   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.Clark, Lambert, & Zhang                                        [Page 15]

RFC 998                                                       March 1987   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,   etc.)   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   last)Clark, Lambert, & Zhang                                        [Page 16]

RFC 998                                                       March 1987   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    |   +---------------+---------------+---------------+---------------+Clark, Lambert, & Zhang                                        [Page 17]

RFC 998                                                       March 1987   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.Clark, Lambert, & Zhang                                        [Page 18]

RFC 998                                                       March 1987   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   packet   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)Clark, Lambert, & Zhang                                        [Page 19]

RFC 998                                                       March 1987   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.Clark, Lambert, & Zhang                                        [Page 20]

RFC 998                                                       March 1987NOTES:   <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.Clark, Lambert, & Zhang                                        [Page 21]