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.
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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
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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
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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.
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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.
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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
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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
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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
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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-
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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.
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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
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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.
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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.
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.
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.
