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rfc:rfc998

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

                                                                   MIT
                                                            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.

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.  A

Clark, 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 be

Clark, 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 KEEPALIVE

Clark, 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 GO

Clark, 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 single

Clark, 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 1987

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

Clark, 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 1987

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,
 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 1987

NOTES:

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

Clark, Lambert, & Zhang [Page 21]

/data/webs/external/dokuwiki/data/pages/rfc/rfc998.txt · Last modified: 1987/03/05 18:59 by 127.0.0.1

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