Network Working Group David D. Clark
Request for Comments: 969 Mark L. Lambert
Lixia Zhang
M. I. T. Laboratory for Computer Science
December 1985
NETBLT: A Bulk Data Transfer Protocol
1. STATUS OF THIS MEMO
This RFC suggests a proposed protocol for the ARPA-Internet
community, and requests discussion and suggestions for improvements.
This is a preliminary discussion of the NETBLT protocol. It is
published for discussion and comment, and does not constitute a
standard. As the proposal may change, implementation of this
document is not advised. Distribution of this memo is unlimited.
2. INTRODUCTION
NETBLT (Network Block Transfer) is a transport level protocol
intended for the rapid transfer of a large quantity of data between
computers. It provides a transfer that is reliable and flow
controlled, and is structured to provide maximum throughput over a
wide variety of networks.
The protocol works by opening a connection between two clients the
sender and the receiver), transferring the data in a series of large
data aggregates called buffers, and then closing the connection.
Because the amount of data to be transferred can be arbitrarily
large, the client is not required to provide at once all the data to
the protocol module. Instead, the data is provided by the client in
buffers. The NETBLT layer transfers each buffer as a sequence of
packets, but since each buffer is composed of a large number of
packets, the per-buffer interaction between NETBLT and its client is
far more efficient than a per-packet interaction would be.
In its simplest form, a NETBLT transfer works as follows. The
sending client loads a buffer of data and calls down to the NETBLT
layer to transfer it. The NETBLT layer breaks the buffer up into
packets and sends these packets across the network in Internet
datagrams. The receiving NETBLT layer loads these packets into a
matching buffer provided by the receiving client. When the last
packet in the buffer has been transmitted, the receiving NETBLT
checks to see that all packets in that buffer have arrived. If some
packets are missing, the receiving NETBLT requests that they be
resent. When the buffer has been completely transmitted, the
receiving client is notified by its NETBLT layer. The receiving
client disposes of the buffer and provides a new buffer to receive
more data. The receiving NETBLT notifies the sender that the buffer
arrived, and the sender prepares and sends the next buffer in the
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same manner. This continues until all buffers have been sent, at
which time the sender notifies the receiver that the transmission has
been completed. The connection is then closed.
As described above, the NETBLT protocol is "lock-step"; action is
halted after a buffer is transmitted, and begins again after
confirmation is received from the receiver of data. NETBLT provides
for multiple buffering, in which several buffers can be transmitted
concurrently. Multiple buffering makes packet flow essentially
continuous and can improve performance markedly.
The remainder of this document describes NETBLT in detail. The next
sections describe the philosophy behind a number of protocol
features: packetization, flow control, reliability, and connection
management. The final sections describe the protocol format.
3. BUFFERS AND PACKETS
NETBLT is designed to permit transfer of an essentially arbitrary
amount of data between two clients. During connection setup the
sending NETBLT can optionally inform the receiving NETBLT of the
transfer size; the maximum transfer length is imposed by the field
width, and is 2**32 bytes. This limit should permit any practical
application. The transfer size parameter is for the use of the
receiving client; the receiving NETBLT makes no use of it. A NETBLT
receiver accepts data until told by the sender that the transfer is
complete.
The data to be sent must be broken up into buffers by the client.
Each buffer must be the same size, save for the last buffer. During
connection setup, the sending and receiving NETBLTs negotiate the
buffer size, based on limits provided by the clients. Buffer sizes
are in bytes only; the client is responsible for breaking up data
into buffers on byte boundaries.
NETBLT has been designed and should be implemented to work with
buffers of arbitrary size. The only fundamental limitation on buffer
size should be the amount of memory available to the client. Buffers
should be as large as possible since this minimizes the number of
buffer transmissions and therefore improves performance.
NETBLT is designed to require a minimum of its own memory, allowing
the client to allocate as much memory as possible for buffer storage.
In particular, NETBLT does not keep buffer copies for retransmission
purposes. Instead, data to be retransmitted is recopied directly
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from the client buffer. This does mean that the client cannot
release buffer storage piece by piece as the buffer is sent, but this
has not proved a problem in preliminary NETBLT implementations.
Buffers are broken down by the NETBLT layer into sequences of DATA
packets. As with the buffer size, the packet size is negotiated
between the sending and receiving NETBLTs during connection setup.
Unlike buffer size, packet size is visible only to the NETBLT layer.
All DATA packets save the last packet in a buffer must be the same
size. Packets should be as large as possible, since in most cases
(including the preliminary protocol implementation) performance is
directly related to packet size. At the same time, the packets
should not be so large as to cause Internet fragmentation, since this
normally causes performance degrada- tion.
All buffers save the last buffer must be the same size; obviously the
last buffer can be any size required to complete the transfer. Since
the receiving NETBLT does not know the transfer size in advance, it
needs some way of identifying the last packet in each buffer. For
this reason, the last packet of every buffer is not a DATA packet but
rather an LDATA packet. DATA and LDATA packets are identical save
for the packet type.
4. FLOW CONTROL
NETBLT uses two strategies for flow control, one internal and one at
the client level.
The sending and receiving NETBLTs transmit data in buffers; client
flow control is therefore at a buffer level. Before a buffer can be
transmitted, NETBLT confirms that both clients have set up matching
buffers, that one is ready to send data, and that the other is ready
to receive data. Either client can therefore control the flow of
data by not providing a new buffer. Clients cannot stop a buffer
transfer while it is in progress.
Since buffers can be quite large, there has to be another method for
flow control that is used during a buffer transfer. The NETBLT layer
provides this form of flow control.
There are several flow control problems that could arise while a
buffer is being transmitted. If the sending NETBLT is transferring
data faster than the receiving NETBLT can process it, the receiver's
ability to buffer unprocessed packets could be overflowed, causing
packets to be lost. Similarly, a slow gateway or intermediate
network could cause packets to collect and overflow network packet
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buffer space. Packets will then be lost within the network,
degrading performance. This problem is particularly acute for NETBLT
because NETBLT buffers will generally be quite large, and therefore
composed of many packets.
A traditional solution to packet flow control is a window system, in
which the sending end is permitted to send only a certain number of
packets at a time. Unfortunately, flow control using windows tends
to result in low throughput. Windows must be kept small in order to
avoid overflowing hosts and gateways, and cannot easily be updated,
since an end-to-end exchange is required for each change.
To permit high throughput over a variety of networks and gateways of
differing speeds, NETBLT uses a novel flow control ethod: rate
control. The transmission rate is negotiated by the sending and
receiving NETBLTs during connection setup and after each buffer
transmission. The sender uses timers, rather than messages from the
receiver, to maintain the negotiated rate.
In its simplest form, rate control specifies a minimum time period
per packet transmission. This can cause performance problems for
several reasons: the transmission time for a single packet is very
small, frequently smaller than the granularity of the timing
mechanism. Also, the overhead required to maintain timing mechanisms
on a per packet basis is relatively high, which degrades performance.
The solution is to control the transmission rate of groups of
packets, rather than single packets. The sender transmits a burst of
packets over negotiated interval, then sends another burst. In this
way, the overhead decreases by a factor of the burst size, and the
per-burst transmission rate is large enough that timing mechanisms
will work properly. The NETBLT's rate control therefore has two
parts, a burst size and a burst rate, with (burst size)/(burst rate)
equal to the average transmission rate per packet.
The burst size and burst rate should be based not only on the packet
transmission and processing speed which each end can handle, but also
on the capacities of those gateways and networks intermediate to the
transfer. Following are some intuitive values for packet size,
buffer size, burst size, and burst rate.
Packet sizes can be as small as 128 bytes. Performance with packets
this small is almost always bad, because of the high per-packet
processing overhead. Even the default Internet Protocol packet size
of 576 bytes is barely big enough for adequate performance. Most
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networks do not support packet sizes much larger than one or two
thousand bytes, and packets of this size can also get fragmented when
traveling over intermediate networks, degrading performance.
The size of a NETBLT buffer is limited only by the amount of memory
available to a client. Theoretically, buffers of 100K bytes or more
are possible. This would mean the transmission of 50 to 100 packets
per buffer.
The burst size and burst rate are obviously very machine dependent.
There is a certain amount of transmission overhead in the sending and
receiving machines associated with maintaining timers and scheduling
processes. This overhead can be minimized by sending packets in
large bursts. There are also limitations imposed on the burst size
by the number of available packet buffers. On most modern operating
systems, a burst size of between five and ten packets should reduce
the overhead to an acceptable level. In fact, a preliminary NETBLT
implementation for the IBM PC/AT sends packets in bursts of five. It
could send more, but is limited by available memory.
The burst rate is in part determined by the granularity of the
sender's timing mechanism, and in part by the processing speed of the
receiver and any intermediate gateways. It is also directly related
to the burst size. Burst rates from 60 to 100 milliseconds have been
tried on the preliminary NETBLT implementation with good results
within a single local-area network. This value clearly depends on
the network bandwidth and packet buffering available.
All NETBLT flow control parameters (packet size, buffer size, burst
size, and burst rate) are negotiated during connection setup. The
negotiation process is the same for all parameters. The client
initiating the connection (the active end) proposes and sends a set
of values for each parameter with its open connection request. The
other client (the passive end) compares these values with the
highest-performance values it can support. The passive end can then
modify any of the parameters only by making them more restrictive.
The modified parameters are then sent back to the active end in the
response message. In addition, the burst size and burst rate can be
re-negotiated after each buffer transmission to adjust the transfer
rate according to the performance observed from transferring the
previous buffer. The receiving end sends a pair of burst size and
burst rate values in the OK message. The sender compares these
values with the values it can support. Again, it may then modify any
of the parameters only by making them more restrictive. The modified
parameters are then communicated to the receiver in a NULL-ACK
packet, described later.
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Obviously each of the parameters depend on many factors-- gateway and
host processing speeds, available memory, timer granularity--some of
which cannot be checked by either client. Each client must therefore
try to make as best a guess as it can, tuning for performance on
subsequent transfers.
5. RELIABILITY
Each NETBLT transfer has three stages, connection setup, data
transfer, and connection close. Each stage must be completed
reliably; methods for doing this are described below.
5.1. Connection Setup
A NETBLT connection is set up by an exchange of two packets
between the active client and the passive client. Note that
either client can send or receive data; the words "active" and
"passive" are only used to differentiate the client initiating the
connection process from the client responding to the connection
request. The first packet sent is an OPEN packet; the passive end
acknowledges the OPEN packet by sending a RESPONSE packet. After
these two packets have been exchanged, the transfer can begin.
As discussed in the previous section, the OPEN and RESPONSE
packets are used to negotiate flow control parameters. Other
parameters used in the transfer of data are also negotiated.
These parameters are (1) the maximum number of buffers that can be
sending at any one time (this permits multiple buffering and
higher throughput) and (2) whether or not DATA/LDATA packet data
will be checksummed. NETBLT automatically checksums all
non-DATA/LDATA packets. If the negotiated checksum flag is set to
TRUE (1), both the header and the data of a DATA/LDATA packet are
checksummed; if set to FALSE (0), only the header is checksummed.
NETBLT uses the same checksumming algorithm as TCP uses.
Finally, each end transmits its death-timeout value in either the
OPEN or the RESPONSE packet. The death-timeout value will be used
to determine the frequency with which to send KEEPALIVE packets
during idle periods of an opened connection (death timers and
KEEPALIVE packets are described in the following section).
The active end specifies a passive client through a
client-specific "well-known" 16 bit port number on which the
passive end listens. The active end identifies itself through a
32 bit Internet address and a 16 bit port number.
In order to allow the active and passive ends to communicate
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miscellaneous useful information, an unstructured, variable-
length field is provided in OPEN and RESPONSE messages for an
client-specific information that may be required.
Recovery for lost OPEN and RESPONSE packets is provided by the use
of timers. The active end sets a timer when it sends an OPEN
packet. When the timer expires, another OPEN packet is sent, until
some pre-determined maximum number of OPEN packets have been sent.
A similar scheme is used for the passive end when it sends a
RESPONSE packet. When a RESPONSE packet is received by the active
end, it clears its timer. The passive end's timer is cleared
either by receipt of a GO or a DATA packet, as described in the
section on data transfer.
To prevent duplication of OPEN and RESPONSE packets, the OPEN
packet contains a 32 bit connection unique ID that must be
returned in the RESPONSE packet. This prevents the initiator from
confusing the response to the current request with the response to
an earlier connection request (there can only be one connection
between any two ports). Any OPEN or RESPONSE packet with a
destination port matching that of an open connection has its
unique ID checked. A matching unique ID implies a duplicate
packet, and the packet is ignored. A non-matching unique ID must
be treated as an attempt to open a second connection between the
same port pair and must be rejected by sending an ABORT message.
5.2. Data Transfer
The simplest model of data transfer proceeds as follows. The
sending client sets up a buffer full of data. The receiving
NETBLT sends a GO message inside a CONTROL packet to the sender,
signifying that it too has set up a buffer and is ready to receive
data into it. Once the GO message has been received, the sender
transmits the buffer as a series of DATA packets followed by an
LDATA packet. When the last packet in the buffer has been
received, the receiver sends a RESEND message inside a CONTROL
packet containing a list of packets that were not received. The
sender resends these packets. This process continues until there
are no missing packets, at which time the receiver sends an OK
message inside a CONTROL packet to the sender, sets up another
buffer to receive data and sends another GO message. The sender,
having received the OK message, sets up another buffer, waits for
the GO message, and repeats the process.
There are several obvious flaws with this scheme. First, if the
LDATA packet is lost, how does the receiver know when the buffer
has been transmitted? Second, what if the GO, OK, or RESEND
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messages are lost? The sender cannot act on a packet it has not
received, so the protocol will hang. Solutions for each of these
problems are presented below, and are based on two kinds of
timers, a data timer and a control timer.
NETBLT solves the LDATA packet loss problem by using a data timer
at the receiving end. When the first DATA packet in a buffer
arrives, the receiving NETBLT sets its data timer; at the same
time, it clears its control timer, described below. If the data
timer expires, the receiving end assumes the buffer has been
transmitted and all missing packets lost. It then sends a RESEND
message containing a list of the missing packets.
NETBLT solves the second problem, that of missing OK, GO, and
RESEND messages, through use of a control timer. The receiver can
send one or more control messages (OK, GO, or RESEND) within a
single CONTROL packet. Whenever the receiver sends a control
packet, it sets a control timer (at the same time it clears its
data timer, if one has been set).
The control timer is cleared as follows: Each control message
includes a sequence number which starts at one and increases by
one for each control message sent. The sending NETBLT checks the
sequence number of every incoming control message against all
other sequence numbers it has received. It stores the highest
sequence number below which all other received sequence numbers
are consecutive, and returns this number in every packet flowing
back to the receiver. The receiver is permitted to clear the
control timer of every packet with a sequence number equal to or
lower than the sequence number returned by the sender.
Ideally, a NETBLT implementation should be able to cope with
out-of-sequence messages, perhaps collecting them for later
processing, or even processing them immediately. If an incoming
control message "fills" a "hole" in a group of message sequence
numbers, the implementation could even be clever enough to detect
this and adjust its outgoing sequence value accordingly.
When the control timer expires, the receiving NETBLT resends the
control message and resets the timer. After a predetermined
number of resends, the receiving NETBLT can assume that the
sending NETBLT has died, and can reset the connection.
The sending NETBLT, upon receiving a control message, should act
as quickly as possible on the packet; it either sets up a new
buffer (upon receipt of an OK packet for a previous buffer),
resends data (upon receipt of a RESEND packet), or sends data
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(upon receipt of a GO packet). If the sending NETBLT is not in a
position to send data, it sends a NULL-ACK packet, which contains
a
high-received-sequence-number as described above (this permits the
receiving NETBLT to clear the control timers of any packets which
are outstanding), and waits until it can send more data. In all
of these cases, the overhead for a response to the incoming
control message should be small; the total time for a response to
reach the receiving NETBLT should not be much more than the
network round-trip transit time, plus a variance factor.
The timer system can be summarized as follows: normally, the
receiving NETBLT is working under one of two types of timers, a
control timer or a data timer. There is one data timer per buffer
transmission and one control timer per control packet. The data
timer is active while its buffer is being transferred; a control
timer is active while it is between buffer transfers.
The above system still leaves a few problems. If the sending
NETBLT is not ready to send, it sends a single NULL-ACK packet to
clear any outstanding control timers at the receiving end. After
this the receiver will wait. The sending NETBLT could die and the
receiver, with all its control timers cleared, would hang. Also,
the above system puts timers only on the receiving NETBLT. The
sending NETBLT has no timers; if the receiving NETBLT dies, the
sending NETBLT will just hang waiting for control messages.
The solution to the above two problems is the use of a death timer
and a keepalive packet for both the sending and receiving NETBLTs.
As soon as the connection is opened, each end sets a death timer;
this timer is reset every time a packet is received. When a
NETBLT's death timer at one end expires, it can assume the other
end has died and can close the connection.
It is quite possible that the sending or receiving NETBLTs will
have to wait for long periods of time while their respective
clients get buffer space and load their buffers with data. Since
a NETBLT waiting for buffer space is in a perfectly valid state,
the protocol must have some method for preventing the other end's
death timer from expiring. The solution is to use a KEEPALIVE
packet, which is sent repeatedly at fixed intervals when a NETBLT
is waiting for buffer space. Since the death timer is reset
whenever a packet is received, it will never expire as long as the
other end sends packets.
The frequency with which KEEPALIVE packets are transmitted is
computed as follows: At connection startup, each NETBLT chooses a
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death-timeout value and sends it to the other end in either the
OPEN or the RESPONSE packet. The other end takes the
death-timeout value and uses it to compute a frequency with which
to send KEEPALIVE packets. The KEEPALIVE frequency should be high
enough that several KEEPALIVE packets can be lost before the other
end's death timer expires.
Both ends must have some way of estimating the values of the death
timers, the control timers, and the data timers. The timer values
obviously cannot be specified in a protocol document since they
are very machine- and network-load-dependent. Instead they must
be computed on a per-connection basis. The protocol has been
designed to make such determination easy.
The death timer value is relatively easy to estimate. Since it is
continually reset, it need not be based on the transfer size.
Instead, it should be based at least in part on the type of
application using NETBLT. User applications should have smaller
death timeout values to avoid forcing humans to wait long periods
of time for a death timeout to occur. Machine applications can
have longer timeout values.
The control timer must be more carefully estimated. It can have
as its initial value an arbitrary number; this number can be used
to send the first control packet. Subsequent control packets can
have their timer values based on the network round-trip transit
time (i.e. the time between sending the control packet and
receiving the acknowledgment of the corresponding sequence number)
plus a variance factor. The timer value should be continually
updated, based on a smoothed average of collected round-trip
transit times.
The data timer is dependent not on the network round-trip transit
time, but on the amount of time required to transfer a buffer of
data. The time value can be computed from the burst rate and the
number of bursts per buffer, plus a variance value <1>. During the
RESENDing phase, the data timer value should be set according to
the number of missing packets.
The timers have been designed to permit reasonable estimation. In
particular, in other protocols, determination of round-trip delay
has been a problem since the action performed by the other end on
receipt of a particular packet can vary greatly depending on the
packet type. In NETBLT, the action taken by the sender on receipt
of a control message is by and large the same in all cases, making
the round-trip delay relatively independent of the client.
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Timer value estimation is extremely important, especially in a
high-performance protocol like NETBLT. If the estimates are too
low, the protocol makes many unneeded retransmissions, degrading
performance. A short control timer value causes the sending
NETBLT to receive duplicate control messages (which it can reject,
but which takes time). A short data timer value causes the
receiving NETBLT to send unnecessary RESEND packets. This causes
considerably greater performance degradation since the sending
NETBLT does not merely throw away a duplicate packet, but instead
has to send a number of DATA packets. Because data timers are set
on each buffer transfer instead of on each DATA packet transfer,
we afford to use a small variance value without worrying about
performance degradation.
5.3. Closing the Connection
There are three ways to close a connection: a connection close, a
"quit", or an "abort".
The connection close occurs after a successful data transfer.
When the sending NETBLT has received an OK packet for the last
buffer in the transfer, it sends a DONE packet <2>. On receipt of
the DONE packet, the receiving NETBLT can close its half of the
connection. The sending NETBLT dallies for a predetermined amount
of time after sending the DONE packet. This allows for the
possibility of the DONE packet's having been lost. If the DONE
packet was lost, the receiving NETBLT will continue to send the
final OK packet, which will cause the sending end to resend the
DONE packet. After the dally period expires, the sending NETBLT
closes its half of the connection.
During the transfer, one client may send a QUIT packet to the
other if it thinks that the other client is malfunctioning. Since
the QUIT occurs at a client level, the QUIT transmission can only
occur between buffer transmissions. The NETBLT receiving the QUIT
packet can take no action other than to immediately notify its
client and transmit a QUITACK packet. The QUIT sender must time
out and retransmit until a QUITACK has been received or a
predetermined number of resends have taken place. The sender of
the QUITACK dallies in the manner described above.
An ABORT takes place when a NETBLT layer thinks that it or its
opposite is malfunctioning. Since the ABORT originates in the
NETBLT layer, it can be sent at any time. Since the ABORT implies
that the NETBLT layer is malfunctioning, no transmit reliability
is expected, and the sender can immediately close it connection.
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6. MULTIPLE BUFFERING
In order to increase performance, NETBLT has been designed in a
manner that encourages a multiple buffering implementation. Multiple
buffering is a technique in which the sender and receiver allocate
and transmit buffers in a manner that allows error recovery of
previous buffers to be concurrent with transmission of current
buffer.
During the connection setup phase, one of the negotiated parameters
is the number of concurrent buffers permitted during the transfer.
The simplest transfer allows for a maximum of one buffer to be
transmitted at a time; this is effectively a lock-step protocol and
causes time to be wasted while the sending NETBLT receives permission
to send a new buffer. If there are more than one buffer available,
transfer of the next buffer may start right after the current buffer
finishes. For example, assume buffer A and B are allowed to transfer
concurrently, with A preceding B. As soon as A finishes transferring
its data and is waiting for either an OK or a RESEND message, B can
start sending immediately, keeping data flowing at a stable rate. If
A receives an OK, it is done; if it receives a RESEND, the missing
packets specified in the RESEND message are retransmitted. All
packets flow out through a priority pipe, with the priority equal to
the buffer number, and with the transfer rate specified by the burst
size and burst rate. Since buffer numbers increase monotonically,
packets from an earlier buffer in the pipe will always precede those
of the later ones. One necessary change to the timing algorithm is
that when the receiving NETBLT set data timer for a new buffer, the
timer value should also take into consideration of the transfer time
for all missing packets from the previous buffers.
Having several buffers transmitting concurrently is actually not that
much more complicated than transmitting a single buffer at a time.
The key is to visualize each buffer as a finite state machine;
several buffers are merely a group of finite state machines, each in
one of several states. The transfer process consists of moving
buffers through various states until the entire transmission has
completed.
The state sequence of a send-receive buffer pair is as follows: the
sending and receiving buffers are created independently. The
receiving NETBLT sends a GO message, putting its buffer in a
"receiving" state, and sets its control timer; the sending NETBLT
receives the GO message, putting its buffer into a "sending" state.
The sending NETBLT sends data until the buffer has been transmitted.
If the receiving NETBLT's data timer goes off before it received the
last (LDATA) packet, or it receives the LDATA packet in the buffer
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and packets are missing, it sends a RESEND packet and moves the
buffer into a "resending" state. Once all DATA packets in the buffer
and the LDATA packet have been received, the receiving NETBLT enters
its buffer into a "received" state and sends an OK packet. The
sending NETBLT receives the OK packet and puts its buffer into a
"sent" state.
7. PROTOCOL LAYERING STRUCTURE
NETBLT is implemented directly on top of the Internet Protocol (IP).
It has been assigned a temporary protocol number of 255. This number
will change as soon as the final protocol specification has been
determined.
8. PACKET FORMATS
NETBLT packets are divided into three categories, each of which share
a common packet header. First, there are those packets that travel
only from sender to receiver; these contain the control message
sequence numbers which the receiver uses for reliability. These
packets are the NULL-ACK, DATA, and LDATA packets. Second, there is
a packet that travels only from receiver to sender. This is the
CONTROL packet; each CONTROL packet can contain an arbitrary number
of control messages (GO, OK, or RESEND), each with its own sequence
number. Finally, there are those packets which either have special
ways of insuring reliability, or are not reliably transmitted. These
are the QUIT, QUITACK, DONE, KEEPALIVE, and ABORT packets. Of these,
all save the DONE packet can be sent by both sending and receiving
NETBLTs.
connection unique ID: 4 bytes
standard buffer size: 4 bytes
transfer size: 4 bytes
DATA packet data segment size: 2 bytes
burst size: 2 bytes
burst rate: 2 bytes
death timeout value in seconds: 2 bytes
transfer mode (1 = SEND, 0 = RECEIVE): 1 byte
maximum number of concurrent buffers: 1 byte
checksum entire DATA packet / checksum
DATA packet data only (1/0): 1 byte
client-specific data: arbitrary
DONE, QUITACK, KEEPALIVE:
standard header only
ABORT, QUIT:
reason: arbitrary bytes
CONTROL packet format:
CONTROL packets consist of a standard NETBLT header of type
CONTROL, followed by an arbitrary number of control messages with
the following formats:
Control message numbers:
GO: 0
OK: 1
RESEND: 2
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OK message:
message type (OK): 1 byte
buffer number: 4 bytes
sequence number: 2 bytes
new burst size: 2 bytes
new burst interval: 2 bytes
message type (RESEND): 1 byte
buffer number: 4 bytes
sequence number: 2 bytes
number of missing packets: 2 bytes
packet numbers...: n * 2 bytes
DATA, LDATA packet formats:
buffer number: 4 bytes
highest consecutive sequence number received: 2 bytes
packet number within buffer: 2 bytes
data: arbitrary bytes
NULL-ACK packet format:
highest consecutive sequence number received: 2 bytes
acknowledged new burst size: 2 bytes
acknowledged new burst interval: 2 bytes
NOTES:
<1> When the buffer size is large, the variances in the round trip
delays of many packets may cancel each other out; this means the
variance value need not be very big. This expectation can be
verified in further testing.
<2> Since the receiving end may not know the transfer size in
advance, it is possible that it may have allocated buffer space
and sent GO messages for buffers beyond the actual last buffer
sent by the sending end. Care must be taken on the sending
end's part to ignore these extra GO messages.
