Network Working Group Rajendra K. Kanodia
Request for Comments #663 MIT, Project MAC
NIC #31387 November 29, 1974
A LOST MESSAGE DETECTION AND RECOVERY PROTOCOL
1.0 INTRODUCTION
The current Host-to-Host protocol does not provide for the
following three aspects of network communication:
1. detection of messages lost in the transmission path
2. detection of errors in the data
3. procedures for recovery in the event of lost messages or
data errors.
In this memo we propose an extension to the Host-to-Host protocol
that will allow detection of lost messages and an orderly
recovery from this situation. If Host-to-Host protocol were to
be amended to allow for detection of errors in the data, it is
expected that the recovery procedures proposed here will apply.
With the present protocol, it may some times be possible to
detect loss of messages in the transmission path. However, often
a lost message (especially one on a control link) simply results
in an inconsistent state of a network connection. One frequent
(and frustrating) symptom of a message loss on a control link has
been the "lost allocate" problem which results in a "paralyzed"
connection. The NCP (Network Control Program) at the receiving
site believes that sender has sufficient allocation for a
connection, whereas the NCP of the sending host believes that it
has no allocation (due to either loss of or error in a message
that contained the allocate command). The result is that the
sending site can not transmit any more messages over the
connection. This problem was reported in the NWG-RFC #467 by
Burchfiel and Tomlinson. They also proposed an extension to the
Host-to-Host protocol which allows for resynchronization of the
connection status. Their proposed solution was opposed by Edwin
Meyer (NWG-RFC #492) and Wayne Hathaway (NWG-RFC #512) on the
grounds that it tended to mask the basic problem of loss of
messages and they suggested that the fundamental problem of
message loss should be solved rather than its symptoms. As an
alternative to the solution proposed in NWG-RFC #467, Wayne
Hathaway suggested that Host-to-Host protocol header could be
extended to include a "Sequence Control Byte" to allow detection
of lost messages. At about the same time Jon Postel suggested a
similar scheme using message numbers (NWG-RFC #516). A little
later David Walden proposed that four unused bits of the message
sequence number (in the IMP leader) be utilized for sequencing
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messages (NWG-RFC #534). His scheme is similar to those proposed
by Postel and Hathaway; however it has the advantage that
Host-to-Host protocol mechanisms can be tied into the IMP-to-Host
protocol mechanisms.
The protocol extension proposed here uses the four bits of the
message sequence number in the message leader for detection of
lost messages. However, to facilitate recovery, it uses another
eight bit field (presently unused) in the 72 bit header of the
regular messages. In the next section of this paper we discuss
some of the basic ideas underlying our protocol. In section 3,
we provide a description of the protocol. It is our intention
that section 3 be a self-contained and complete description of
the protocol.
2.0 BASIC IDEAS
The purpose of this section is to provide a gentle introduction
to the central ideas on which this protocol is based. Roughly
speaking, our protocol can be divided into three major
components. First is the mechanism for detecting loss of
messages. Second is the exchange of information between the
sender and the receiver in the event of a message loss. For
reasons that will soon become obvious, we have termed this area
as "Exchange of Control Messages". The third component of our
protocol is the method of retransmission of lost messages. In
this section, we have reversed the order of discussion for the
second and third components, because the mechanisms for exchange
of control messages depend heavily upon the retransmission
methods.
A careful reader will find that several minor issues have been
left unresolved in this section. He (or she) should remember
that this section is not intended to be a complete description of
the protocol. Hopefully, we have resolved most of these issues
in the formal description of the protocol provided in the section
3.
2.1 DETECTION OF LOSS OF MESSAGES
The 32 bit Host-to-IMP and IMP-to-Host leaders contain a 12 bit
message-id in bit positions 17 to 28 (BBN Report #1822). The
Host-to-Host protocol (NIC 8246) uses 8 bits of the message-id
(bit positions 17 to 24) as a link number. The remaining 4 bits
of the message-id (bits 25 to 28) are presently unused. For the
purposes of the protocol to be presented here, we define these
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four bits to be the message sequence number (MSN in short)
associated with the link. Thus message-id consists of an eight
bit link number and a four bit message sequence number. The four
bit MSN provides a sixteen element sequence number for each link.
A network connection has a sending host (referred to as "sender"
henceforth), a receiving host (referred to as receiver
henceforth), and a link on which messages are transmitted. In
our protocol the sender starts communication with the value of
MSN set to one (i.e. the first message on any link has one in its
MSN field.) For the next message on the same link the value of
MSN is increased by one. When the value of MSN becomes 15 the
next value chosen is one. This results in the following sequence
1, 2, ...., 13, 14, 15, 1, 2, ...., etc. The receiver can detect
loss of messages by examining this sequence. Each hole
corresponds to a lost message. Notice that the detection
mechanism will fail if a sequence of exactly 15 messages were to
be lost. For the time being, we shall assume that the
probability of loosing a sequence of exactly 15 messages is
negligible. However, we shall later provide a status exchange
mechanism (Section 2.6) that can be used to prevent this failure.
Notice that in the sequence described above we have omitted the
value zero. Following a suggestion made by Hathaway (NWG-RFC
#512) and Walden (NWG-RFC #534) the new protocol uses the value
zero to indicate to the receiving host that the sending host is
not using message sequence numbers. We, in fact, extend the
meaning associated with the MSN value zero to imply that the
sending host has not implemented the detection and error recovery
protocol being proposed here.
2.2 COMPATIBILITY
The discussion above brings us to the issue of compatibility
between the present and the new protocols. Let us define the
hosts with the present protocol to be type A and the hosts with
the new protocol to be type B. We have three situations:
1. Type A communicating with type A: there is no
difference from the present situation.
2. Type A communicating with type B: from the zero value
MSNs in messages sent by the type A host, the type B
host can detect the fact that the other host is a type A
host. Therefore the type B host can simulate the
behaviour of a type A host in its communication with the
other host, and the type A host will not be confused.
As we will see later that this simulation is really
simple and can be easily applied selectively.
3. Type B host communicating with type B: Both hosts can
detect the fact that the other host is a type B host and
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use the message detection and error recovery protocol.
There is one difficulty here that we have not yet resolved. When
starting communication how does a type B host know whether the
other host is type A or type B? This difficulty can be resolved
by assuming that a type A host will not be confused by a non-zero
MSN in the messages that it receives. This assumption is not
unreasonable because a type A host can easily meet this
requirement by making a very simple change to its NCP (the
Network Control Program), if it does not already satisfy this
requirement. Another assumption that is crucial to our protocol,
is that the type A hosts always set the MSN field of messages
(they send out) to zero. As of this writing, the author believes
that no hosts are using the MSN field and therefore no
compatibility problem should arise.
2.3 RETRANSMISSION OF MESSAGES
Before getting down to the details of the actual protocol, we
will attempt here to explain the essential ideas underlying this
protocol by considering a somewhat simplified situation.
Consider a logical communication channel X, which has at its
disposal an inexhaustible supply of physical communication
channels C(1), C(2), C(3), ........, etc. (See footnote #1)
Channel X is to be used for transmission of messages. In
addition to carrying the data, these messages contain (1) the
channel name X, and (2) a Message Sequence Number (MSN). Let us
denote the sender on this channel by S and the receiver by R.
Let us also assume that at the start of the communication, R and
S are synchronized such that R is prepared to receive messages
for logical channel X on the physical channel C(1) and S is
prepared for sending these messages on C(1). S starts by pumping
a sequence of messages M(1), M(2), M(3), ........, M(n) into
channel C(1). Since these messages contain sequence numbers, R
is able to detect loss of messages on the channel C(1). Suppose
now that R discovers that message number K (where K <n) was lost
in the transmission path. Let us further assume that having
_________________________________________________________________
(1) One method of recovery may be to let the receiver save all
properly received messages and require the sender to retransmit
only those messages that were lost. This method requires the
receiver to have the ability to reassemble the messages to build
the data stream. A second method of recovery may be to abort and
restart the transmission at the error point. This method
requires that the receiving host be able to distinguish between
legitimate messages and messages to be ignored. For simplicity
we have chosen the second method and an inexhaustible supply of
physical channels serves to provide the distinction among
messages.
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discovered loss of a message, R can communicate this fact to S by
sending an appropriate control message on another logical channel
that is explicitly reserved for transmission of control messages
from R to S. This channel, named Y, is assumed to be completely
reliable.
We now provide a rather simplistic recovery protocol for the
scenario sketched above. Having detected the loss of message M(K)
on channel X, R takes the following series of actions:
1- R stops reading messages on C(1),
2- R discards those messages that were received on C1 and
are placed after M(K) in the logical message sequence,
3- R prepares itself to read messages M(K), M(K+1), .....,
etc. on the physical channel C(2),
and 4- R sends a control message to S on control channel Y,
which will inform S to the effect that there was an
error on logical channel X while using physical channel
C(1) in message number K.
When S receives this control message on Y, it takes the following
action:
1- S stops sending messages on C(1),
and 2- begins transmission of messages starting with the
sequence number K, on the physical channel C(2).
This resynchronization protocol is executed every time R detects
an error. If physical channel C(CN) was being used at the time
of the error, then the next channel to be used is C(CN+1). We
can define a "receiver synchronization state" for the channel X,
as the triplet R(C, CN, MSN), where C is the name of the group of
physical channels, CN is the number of the physical channel in
use, and MSN is the number of message expected. (See footnote #1)
We can specify a message received on a given C-channel as M(MSN).
When R receives the message M(R.MSN) on the channel C(R.CN), the
synch-state changes from R(C, CN, MSN) to R(C, CN, MSN+1).
However if M.MSN for the message received is greater than R.MSN
then a message has been lost, and R changes the synch-state to
R(C, CN+1, MSN). What really happens may be described as
follows: upon detection of error in a logical channel X, we
merely discard the physical channel that was in use at the time
of error, and restart communication on a new physical channel at
the point where break occurred.
_________________________________________________________________
(1) Notice that we have prefixed this triplet by the letter R
(for the receiver.) We will prefix other similarly defined
quantities by different letters. For example M can be used for
messages. This notation permits us to write expressions like
M.MSN = R.MSN, where M.MSN stands for the message sequence number
of the message.
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This scheme provides a reliable transmission path X, even though
the physical channels involved are unreliable. In this scheme we
have assumed that (1) a completely reliable channel Y is
available for exchange of control messages, and (2) that there is
a large supply of physical channels available for use of X. In
the paragraphs that follow we shall revise our protocol to use a
single physical channel and then apply this protocol to the
channel Y in such a way that Y would become "self-correcting."
Now suppose that channel X has only one physical channel (named
X') available for its use rather than the inexhaustible supply of
physical channels. Our protocol would still work, if we could
somehow simulate the effect of a large number of C-channels using
the single channel X'. One method of providing this simulation
is to include in each message the name of the C-channel on which
it is being sent, and send it on X'. Now the receiver must
examine each message received on X' to determine the C-channel on
which this message was sent. Our protocol still works except for
one minor difference, namely, the receiver must now discard
messages corresponding to C-channels that are no longer in use,
whereas in the previous system the C-channels no longer being
used were simply discarded. To be sure, X' can be multiplexed
among only a finite number of C-channels; however, we can provide
a sufficiently large number of C-channels so that during the life
time of the logical channel X, the probability of exhausting the
supply of C-channels would be very low. And even if we were to
exhaust the supply of C-channels, we could recycle them just as
we recycle the message sequence numbers.
A physical message received on X' can now be characterized by a
pair of C-channel number and a message sequence number, as M(CN,
MSN). The receiver synchronization state becomes a triplet R(X',
CN, MSN). This state tells us that R is ready to receive a
message for X on the physical channel X' and for this message
M.CN should be equal to R.CN and M.MSN should be equal to R.MSN.
All messages with M.CN less than R.CN will be ignored. If for
the next message received on X', M.CN = R.CN and M.MSN = R.MSN,
then R changes the synch state to R(X', CN, MSN+1). If M.CN =
R.CN but M.MSN > R.MSN then a message has been lost and the
synch-state R(X', CN, MSN) changes to R(X', CN+1, MSN). Notice
that we have not yet said anything about the situation M.CN >
R.CN. We will later describe a scheme for using this case to
provide for error correction on the control channel itself.
2.4 EXCHANGE OF CONTROL INFORMATION
So far we have discussed two schemes for the detection and
retransmission aspects of the lost-message problem. In this
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section, we discuss methods by which the receiver communicates to
the sender the fact of loss of messages.
We continue with the scenario developed in the above section with
a small change. For the purposes of the discussion that is about
to follow we shall assume that there are actually two perfect
channels available for exchange of control messages. One channel
from S to R named S->R, and the other from R to S named R->S.
The purpose of S->R will become clear in a moment. In order to
let R communicate the fact of loss of messages to S, We provide a
control message called Lost Message from Receiver (LMR) which is
__________________________
of the following form: LMR(X, CN, MSN), where X is the name of
the channel, CN is the new C-channel number, and MSN is the
message sequence number of the lost message. If more than one
message has been lost, then R uses the MSN of the first message
only. When S receives this message, it can restart communication
at the point where the break occurred using the C-channel
specified by the LMR message. This will restore the
communication path X. What happens if S can not restore
communication at the break point because it does not have the
relevant messages any more? This issue can be solved in one of
the two ways: either let the protocol specify a fixed rule that S
will be required to close the connection, or the protocol could
allow S and R (and may be the users on whose behalf S and R are
communicating on X) to negotiate the action to be taken. For the
protocol to be presented here, we have taken the approach that S
may, at its option, either close the connection or negotiate with
R. What action a specific host takes can either be built into
its NCP or determined dynamically. Those hosts that do not have
very powerful machines will probably chose the static option of
closing the connection; other hosts may make the decision
depending upon the circumstances. For example, a host may decide
that loss of messages is not acceptable during file transfers
whereas loss of a single message can be ignored for terminal
output to interactive users. A host might even let the user
processes decide the course of action to be taken. If S
determines that it can not retransmit lost messages, it may want
to let R decide what action is to be taken. If S so decides,
then it may communicate this fact to R by transmitting a
Lost Message from Sender (LMS) control message to R on the
________________________
channel S->R. An LMS message is of the following form: LMS(X,
CN, MSN, COUNT), where X is the name of the channel, CN is the
name of the C-channel obtained from the LMR message, MSN is the
message sequence number of the first message in the sequence of
lost messages, and COUNT is the number of messages in the
sequence. S resets its own synch-state for connection X to S(X,
CN, MSN+COUNT). When S has informed R of its inability to
retransmit lost messages, the burden of the decision falls on R,
and S simply awaits R's reply before taking any action for the
channel X. When R receives the LMS, it may either decide that
loss is unacceptable and close the connection, or it may decide
to let S continue. In the later case R informs S of its decision
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to continue by transmitting a Loss of Message Acceptable (LMA)
__________________________
control message to S. Upon receiving the LMA control message, S
resumes transmission on X. To avoid the possibility of errors in
exchange of control messages, the LMA control message is
specified to be an exact replica of the LMS control message,
except for the message code which determines whether a control
message is LMA or LMS or something else.
In general, a sending host has only a limited amount of memory
available for storing messages for possible retransmission later.
In section 2.6 we provide a status exchange mechanism that can be
used to determine when to discard these messages.
2.5 RECOVERY ON CONTROL LINKS
We continue our discussion with the scenario developed in the
previous section. The receiver R may detect loss of messages on
control channels by examining the message sequence numbers on the
messages. When R detects that a message has been lost on the
channel S->R, it (R) may transmit an LMR to S on R->S
communicating the fact of loss of messages. When S receives the
LMR for the control link, it must either retransmit the lost
messages, or "close" the connection. (In the context of
Host-to-Host protocol, closing the network connection for control
link implies exchange of reset commands, which has the effect of
reinitializing all communication between R and S.) For control
links, S does not have the option of sending an LMS to the
receiver. If S can not retransmit the lost messages then it
aborts the output queue (if any) for the channel S->R, and
inserts a Reset command at the break point. Essentially, we are
specifying that if S can not retransmit lost control messages
then the error would be considered irrecoverable and fatal. All
user communication between S and R is broken and must be
restarted from the beginning.
In the above paragraph, we considered the situation in which R
was able to detect the loss of messages. It is however possible
that it is the last message transmitted on S->R that is lost. In
this case, R will not be aware of the loss. In this situation,
recovery can be initiated only if S can either positively
determine or simply suspect that a message has been lost. In
general, after having transmitted a control message, S would be
expecting some sort of response from R. For example, if S
transmits a Request-for-Connection (RFC) control message to R, S
expects R to reply either with a Close (CLS) command or another
RFC. If, after an appropriate time interval has elapsed and S
has received no reply from R, our protocol specifies that S may
retransmit the control message. In retransmitting, S must use
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the same C-channel and MSN values that were used for the original
message. Since R can, now, receive duplicate copies, we
stipulate that if R receives a duplicate of the message that it
has already received, it may simply ignore the duplicate.
2.6 SOME OTHER PROBLEMS
There are two problems that have not yet been solved. First, a
sending host will usually have only a limited amount of buffer
space in which it can save messages for possible later
retransmission. So far, we have not provided any method by which
a host may positively determine whether the receiver has
correctly received a certain message or not. Thus it has no firm
basis on which it may decide to release the space used up by
messages that have been already transmitted. The second problem
is created by our recycling the message sequence numbers. For
the MSN mechanism to work correctly, it is essential that at any
given instant of time there be no more than 15 messages in the
transmission path. If there were more than 15 messages, some of
these messages would have same MSN and LRN, and if one of these
messages were to be lost, it would be impossible to identify the
lost message. However, the second problem should not arise in
the ARPA Network, since the IMP sub-network will not allow more
than eight outstanding messages between any host pair (NWG-RFC
#660).
We can solve both these problems by a simple yet highly flexible
scheme. In this scheme, there are two new control messages. One
of these, called Request Status from Sender (RSS), can be used by
__________________________
the sending host to query the receiver regarding the receiver's
synch-state. The receiver can supply its copies of C-channel
number and MSN for a transmission path by sending a Status from
___________
Receiver (SFR) control message over the control channel. An SFR
________
provides positive acknowledgement; differing with the usual
acknowledgement schemes in only that here acknowledgement is
provided only upon demand. Upon receiving SFR, the sender knows
exactly which messages have been properly delivered, and it may
free the buffer space used by these messages. The RSS and SFR
scheme can also be used to ensure that there are no more than
fifteen messages in the transmission path at any given time.
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3.0 LOST MESSAGE DETECTION AND RECOVERY PROTOCOL
This protocol is proposed as an amendment to the Host-to-Host
Protocol for the purpose of letting hosts detect the loss of
messages in the network. It also provides recovery procedures
from such losses. This protocol is divided into two parts. Part
1 states the compatibility requirements. Part 2 states the new
protocol and must be implemented by hosts that desire to have the
ability to recover from loss of messages in the network. The
reader will find many comments interspersed throughout the
description of this protocol. These comments are not part of the
protocol and are provided solely for the purpose of improving the
reader's understanding of this protocol.
The terminology used in this protocol is similar to that used in
the Host-to-Host protocol. We speak of a "network connection"
between a pair of hosts, called the "receiver" and the "sender."
A network connection is described by a pair of socket numbers,
and once established, a network connection is associated with a
link (which is described by a link number.) A network connection
is a logical communication path and the link assigned to it is a
physical communication path. In addition to links associated
with the network connections, there are "control-links" for the
transmission of "control commands." When we use the term
"connection" it may refer to either a network connection or the
link assigned to it; the context decides which one. The term
"links" encompasses the connection-associated-links as well as
control-links. Notice that a receiver of a connection may
transmit control commands regarding this connection.
3.1 DEFINTIONS
3.1.1 HOST TYPE "A"
Those hosts that have not adopted the part 2 of this protocol
amendment will be known as type A hosts.
(Comment: All existing hosts are type A hosts.)
3.1.2 HOST TYPE "B"
Those hosts, that adopt the part 2 of this protocol amendment
will be known as type B hosts.
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3.1.3 MESSAGE SEQUENCE NUMBER (MSN)
A four bit number associated with regular messages and contained
in the bits 25 through 28 of the Host-to-IMP and IMP-to-Host
leaders for regular messages [BBN Report No. 1822]. This number
is used by the type B hosts to detect loss of messages on a
given link. Type A hosts always set the MSN (for the messages
they send out) to zero. When in use by a type B host, the first
message on a link (after the connection has been established) has
the MSN value of one. For each successive message on this link,
the MSN value is increased by one until it reaches a value of 15.
The next message has the MSN value of one.
(Comments: Type B hosts will use the MSN mechanism only when
communicating with other type B hosts. If a type B host is
communicating with a type A host, the type B host will
essentially simulate the behaviour of a type A host and use zero
MSN values for this communication.)
3.1.4 LINK RESYNCH NUMBER (LRN)
The Link Resynch Number is an eight bit number associated with a
link and used for resynchronizing the communication. For links
associated with a network connection (i.e. user links), it is
intially set to zero. For control links, it is set to zero at
the time of the Network Control Program (NCP) initialization.
For a given link, its current LRN value is copied into the LRN
field of all messages sent out on the link. The LRN values may
be manipulated by type B hosts in accordance with the protocol
described later.
(Comments: Our protocol specifies that for all communication
involving a type A host, the LRN value will never change from
zero. Since the LRN field is presently unused, all hosts set it
to zero even though they do not explicitly recognize this field
as an LRN field. This guarantees compatibility.)
3.1.5 LRN FIELD
An eight bit field in the bits 33 through 40 of the Host-to-Host
message header.
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3.2 NEW CONTROL COMMANDS
3.2.1 LMR - LOST MESSAGE FROM RECEIVER
___8______8_______8_______8____
| I I I I
I LMR | link | LRN | MSN I
I______I_______I_______I______I_
The LMR command is sent by a receiving host to let the sending
host know that one or more messages have been lost. The MSN
field specifies the message sequence number of the first message
lost. The LRN field specifies the new LRN value that must be
used if and when communication is restored.
(Comments: As will be seen later, the LMR command also has the
effect of resetting the bit and message allocation in the sending
host to zero. In order to permit a sender to restore
communication, an LMR command will be usually accompanied with an
allocate command. However notice that these comments do not
apply to the control links because there is no allocation
mechanism for the control links.)
3.2.2 LMS - LOST MESSAGE FROM SENDER
____8_________8________8__________8_________8_____
I I I I I I
I LMS I Link I LRN I MSN I COUNT I
I__________I________I_________I__________I________I_
This command is sent by a sender host in reply to an LMR command
if it (the sender) can not retransmit the lost messages specified
by the LMR command. The purpose of this command is to query the
receiver whether or not the loss of messages is acceptable.
After sending this command, the sender waits for a reply before
transmitting any messages over the link involved. This command
may not be sent for control links. The LRN and MSN values are
same as those specified by the LMR command. COUNT specifies the
number of messages lost.
3.2.3 LMA - LOSS OF MESSAGES ACCEPTABLE
This command is identical to the LMS command accept for the
command code. Upon receipt of an LMS command, a receiver may
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send this command to the sender to let the sender know that loss
of messages is acceptable. All fields in this command are set to
corresponding values in the LMS command.
3.2.4 CLS2 - CLOSE2
____8___________3_2_______________3_2_____________8_______8______
I I I I I I
I CLS2 I my socket I your socket I LRN I MSN I
I________I_______________I__________________I________I_______I_
The CLS2 command is similar to the current CLS command except for
the LRN and MSN fields included in the new command. Both the
receiving and sending hosts copy their values of LRN and MSN into
the CLS2 command. Upon receiving a CLS2 command, a host compares
the LRN and MSN values contained in the CLS2 command with its own
values for the connection involved. If these values do not
match, then an error has occurred and a host may initiate
recovery procedures.
(Comments: The purpose of this command is to ensure that the
last message transmitted on a connection has been received
succesfully.)
3.2.5 ECLS - ERROR CLOSE
_____8___________3_2___________3_2_________
I I I I
I ECLS I my socket I your socketI
I_________I______________I______________I_
The ECLS command is similar to the current CLS command. It is
used for closing connections which have sufferred an
irrecoverable loss of messages.
(Comments: A connection may be closed in any one of the following
three ways:
1. A connection which has not yet been opened succesfully
may be closed by the CLS command. All connections
involving type A hosts must be closed using the CLS
command.
2. Connections between type B hosts may be closed using
CLS2 command. A connection is considered closed only
if matching CLS2 commands have been exchanged between
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the sender and the receiver.
3. Those connections between type B hosts, that have
sufferred an irrecoverable loss of messages, must be
closed by the ECLS command.)
3.2.6 RSS - REQUEST STATUS FROM SENDER
____8_______8______
I I I
I RSS I LINK I
I_______I_________I_
A sending host may issue an RSS command to find out about the
status of transmission on a particular connection or the control
link.
3.2.7 RSR - REQUEST STATUS FROM RECEIVER
____8_________8_____
I I I
I RSR I LINK |
I________I_________I_
A receiver can issue an RSR command to find out about the status
of transmission on a particular connection or the control link.
3.2.8 SFR - STATUS FROM RECEIVER
____8_________8_________8_________8____
I I I I I
I SFR I LINK I LRN I MSN I
I_________I_________I_________I________I_
A receiving host may issue this command to inform the sender
about the state of a particular connection or the control link.
3.2.9 SFS - STATUS FROM SENDER
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____8_________8_________8_________8____
I I I I I
I SFS I LINK I LRN I MSN I
I_________I_________I_________I________I_
A sending host may issue this command to inform the receiver
about the state of a particular connection or the control link.
3.3 THE PROTOCOL
3.3.1 PART ONE
All type A hosts must use zero MSN and LRN values on the messages
sent out by them. When communicating with a host of type A, a
type B host must simulate the behaviour of type A host.
(Comments: Notice that this simulation is not complicated at
all. All that is required is that hosts that adopt this
protocol must not use it when communicating with the hosts that
have not adopted it.)
3.3.2 PART TWO
This part of the protocol is stated as a set of rules which must
be observed by all type B hosts when communicating with other
type B hosts.
3.3.2.1 RESPONSIBILITIES OF HOSTS AS SENDERS
(1). A type B sending host must use message sequence numbers
on all regular messages that it sends to other type B
hosts as specified in the definition of the message
sequence numbers (Section 3.1.3).
(2). A type B sending host must use link resynch numbers on
all regular messages that it sends to other type B
hosts as specified in the definition of link resynch
number (Section 3.1.4).
(3). A sending host may retransmit a message if it suspects
that the message may have been lost in the network
during previous transmission.
(4). A sending host may issue an RSS command to the receiver
to determine the state of transmission on any link.
(5). A sending host must use the ECLS command to close a
connection, if the connection is being closed due to an
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irrecoverable transmission error. Otherwise, it must
the CLS2 command.
3.3.2.2 RESPONSIBILITIES OF HOSTS AS RECEIVERS
(1). A receiver host will maintain LRN and MSN values for
each link on which it receives messages. Initial value
of LRN will be zero, and initial value of MSN will be
one. For each receive link, the host should be
prepared to receive a message with LRN and MSN values
specified by its tables. When the host has received
the expected message on a given link, it will change
its table MSN value as specified in the definition of
MSN.
(2). On a given link, if a host receives a message with an
LRN value smaller than the one in use, it will ignore
the message.
(3). If a host receives a duplicate message (same LRN and
MSN values), it will ignore the duplicate.
(4). A host will examine the MSN values on all regular
messages that it receives to detect loss of messages.
If on any link, one or more messages are found missing,
it will concern itself with only the first message lost
and take the following series of action:
1. Increase its own LRN value for this link by
one.
2. Send an LMR command to the sending host with
LRN field set to the new value and MSN field
set to the sequence number of the first
message lost.
3. Realizing that LMR command will cause the
allocation to be reset to zero, it will send
more allocation. This is not applicable to
the control links.
However, if a host does not want to initiate the
recovery procedures, it may simply close the connection
by an ECLS command.
(5). A receiver host may issue the RSR command to determine
the state of transmission on any link.
(6). If a connection is being closed due to an irrecoverable
error, a receiving host must use the ECLS command.
Otherwise it must use the CLS2 command.
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3.3.2.3 SENDING HOST'S RESPONSE TO CONTROL MESSAGES
(1). RSR command: the sender must transmit a SFS command to
the receiver for the link involved.
(2). ECLS command: The sender must cease transmission, if it
has not already done so, and issue an ECLS command if
it has not already issues either a ECLS or CLS2
command.
(3) CLS2 command: The sender must compare the LRN and MSN
values of the CLS2 command with its own values of the
LRN and MSN for the connection involved. If an error
is indicated, it may either close the connection with
an ECLS, or initiate recovery action as specified in
the section 3.3.2.1.
(4). LMR command for a connection (i.e., not a control
link): The sender may follow any one of the following
three courses of action:
1. Close the connection with an ECLS command.
2. Set the allocations for the link involved to
zero, set LRN to that specified in the LMR
command, and restart communication at the
point of break.
3. Set the allocations for the link involved to
zero, set the LRN to that specified in the
LMR command, and send an LMS command to the
receiver host informing him that one or more
of the lost messages can not be
retransmitted. After sending an LMS command,
a sending host must not transmit any more
messages on the link involved until and
unless it receives an LMA command from the
receiver host.
(Comments: As we have mentioned before (Section 2.3), the
decision regarding which course of action to follow depends upon
a number of factors. For example, if a host has implemented only
the detection of lost messages aspect of our protocol (and no
recovery), then it will chose the first option of closing the
connection.)
(5). LMR for a control link: The sender may take one of the
following two actions:
1. Set the LRN to that specified in the LMR
command and begin retransmission of lost
messages
2. Set the LRN to that specified by the LMR
command, and insert a Reset command at the
break point.
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(Comment: If a sending host can not retransmit messages lost on
a control link, then this protocol requires that all
communication between the two host be broken, and reinitialized.
We do not explicitly specify the actions that are associated with
the exchange of Reset commands. These actions are specified by
the Host-to-Host protocol.)
(6). LMA command: When a sending host receives an LMA
command matching an LMS command previously issued by
it, it may resume transmission.
(Comments: The protocol does not require the sending host to take
any specific action with regard to a SFR. However, a sending host
may use the information contained in the SFR command regarding
the state of transmission. From a SFR command a sender host may
determine what messages have been received properly. The sender
may use this information to cleanup its buffer space or
retransmit messages that it might suspect are lost.)
3.3.2.4 RECEIVING HOST'S RESPONSE TO CONTROL MESSAGES
(1). RSS command: A receiver is obligated to transmit a SFR
to the sender for the link involved.
(2). ECLS command: The receiver must close the connection
by issuing an ECLS command if it has not already done
so.
(3). CLS2 command: A receiver must compare the LRN and MSN
values of the command with its own values for the
connection involved. If an error is indicated, it may
either close the connection by an ECLS command or
initiate recovery procedures as specified in section
3.3.2.2.
(4). LMS command: The receiver may take one of the following
two courses of action:
(1). Close the connection specified by the LMS
command, by issuing an ECLS command.
(2). Set the link involved to be prepared to
receive messages starting with the sequence
number MSN + COUNT, where MSN and COUNT are
those specified by the LMS command.
(Comment: This action implies that receiver
is willing to accept the loss of messages
specified by the LMS command.)
(Comments: The protocol does not require the receiver to take any
specific action with regard to a SFS command. However a receiver
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host may use the information contained in it.)
4.0 CONCLUDING REMARKS
The design of this protocol has been governed by three major
principles. First, we believe that to be useful within the ARPA
Network, any new protocol must be compatible with the existing
protocols, so that each host can make the transition to the new
protocol at its own pace and without large investment. Secondly,
the protocol should tie into the recovery mechanism of the
IMP-to-Host Protocol. The price we pay for this is the small MSN
field and a message oriented protocol rather than a byte stream
oriented protocol. The third consideration has been flexibility.
While this protocol guarantees detection of lost messages, the
philosophy behind the recovery procedures is that a host should
have several options, each option providing a different degree of
sophistication. A host can implement a recovery procedure that
is most suitable for its needs and the capabilities of its
machine. Even though two hosts may have implemented different
recovery procedures, they can communicate with each other in a
compatible way. In a network of independent machines of widely
varying capabilities and requirements, this seems to be the only
way of implementing such a protocol. Even though this protocol
provides a variety of options in a given error situation, the
choice of a specific action must be consistent with the basic
requirements of the communication path. For example, partial
recovery is not acceptable during file transfers. We fully
expect the File Transfer Protocol to specify that if an
irrecoverable error occurs, the file transfer must be aborted.
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