RFC793 (TCP) [pt 3 of 3]
Brian Kantor
brian at sdcc3.UUCP
Fri Jan 11 10:21:24 AEST 1985
September 1981
Transmission Control Protocol
Functional Specification
TCP/Lower-Level Interface
The TCP calls on a lower level protocol module to actually send and
receive information over a network. One case is that of the ARPA
internetwork system where the lower level module is the Internet
Protocol (IP) [2].
If the lower level protocol is IP it provides arguments for a type
of service and for a time to live. TCP uses the following settings
for these parameters:
Type of Service = Precedence: routine, Delay: normal, Throughput:
normal, Reliability: normal; or 00000000.
Time to Live = one minute, or 00111100.
Note that the assumed maximum segment lifetime is two minutes.
Here we explicitly ask that a segment be destroyed if it cannot
be delivered by the internet system within one minute.
If the lower level is IP (or other protocol that provides this
feature) and source routing is used, the interface must allow the
route information to be communicated. This is especially important
so that the source and destination addresses used in the TCP
checksum be the originating source and ultimate destination. It is
also important to preserve the return route to answer connection
requests.
Any lower level protocol will have to provide the source address,
destination address, and protocol fields, and some way to determine
the "TCP length", both to provide the functional equivlent service
of IP and to be used in the TCP checksum.
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Transmission Control Protocol
Functional Specification
3.9. Event Processing
The processing depicted in this section is an example of one possible
implementation. Other implementations may have slightly different
processing sequences, but they should differ from those in this
section only in detail, not in substance.
The activity of the TCP can be characterized as responding to events.
The events that occur can be cast into three categories: user calls,
arriving segments, and timeouts. This section describes the
processing the TCP does in response to each of the events. In many
cases the processing required depends on the state of the connection.
Events that occur:
User Calls
OPEN
SEND
RECEIVE
CLOSE
ABORT
STATUS
Arriving Segments
SEGMENT ARRIVES
Timeouts
USER TIMEOUT
RETRANSMISSION TIMEOUT
TIME-WAIT TIMEOUT
The model of the TCP/user interface is that user commands receive an
immediate return and possibly a delayed response via an event or
pseudo interrupt. In the following descriptions, the term "signal"
means cause a delayed response.
Error responses are given as character strings. For example, user
commands referencing connections that do not exist receive "error:
connection not open".
Please note in the following that all arithmetic on sequence numbers,
acknowledgment numbers, windows, et cetera, is modulo 2**32 the size
of the sequence number space. Also note that "=<" means less than or
equal to (modulo 2**32).
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Transmission Control Protocol
Functional Specification
A natural way to think about processing incoming segments is to
imagine that they are first tested for proper sequence number (i.e.,
that their contents lie in the range of the expected "receive window"
in the sequence number space) and then that they are generally queued
and processed in sequence number order.
When a segment overlaps other already received segments we reconstruct
the segment to contain just the new data, and adjust the header fields
to be consistent.
Note that if no state change is mentioned the TCP stays in the same
state.
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Transmission Control Protocol
Functional Specification
OPEN Call
OPEN Call
CLOSED STATE (i.e., TCB does not exist)
Create a new transmission control block (TCB) to hold connection
state information. Fill in local socket identifier, foreign
socket, precedence, security/compartment, and user timeout
information. Note that some parts of the foreign socket may be
unspecified in a passive OPEN and are to be filled in by the
parameters of the incoming SYN segment. Verify the security and
precedence requested are allowed for this user, if not return
"error: precedence not allowed" or "error: security/compartment
not allowed." If passive enter the LISTEN state and return. If
active and the foreign socket is unspecified, return "error:
foreign socket unspecified"; if active and the foreign socket is
specified, issue a SYN segment. An initial send sequence number
(ISS) is selected. A SYN segment of the form <SEQ=ISS><CTL=SYN>
is sent. Set SND.UNA to ISS, SND.NXT to ISS+1, enter SYN-SENT
state, and return.
If the caller does not have access to the local socket specified,
return "error: connection illegal for this process". If there is
no room to create a new connection, return "error: insufficient
resources".
LISTEN STATE
If active and the foreign socket is specified, then change the
connection from passive to active, select an ISS. Send a SYN
segment, set SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT
state. Data associated with SEND may be sent with SYN segment or
queued for transmission after entering ESTABLISHED state. The
urgent bit if requested in the command must be sent with the data
segments sent as a result of this command. If there is no room to
queue the request, respond with "error: insufficient resources".
If Foreign socket was not specified, then return "error: foreign
socket unspecified".
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Transmission Control Protocol
Functional Specification
OPEN Call
SYN-SENT STATE
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Return "error: connection already exists".
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Transmission Control Protocol
Functional Specification
SEND Call
SEND Call
CLOSED STATE (i.e., TCB does not exist)
If the user does not have access to such a connection, then return
"error: connection illegal for this process".
Otherwise, return "error: connection does not exist".
LISTEN STATE
If the foreign socket is specified, then change the connection
from passive to active, select an ISS. Send a SYN segment, set
SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. Data
associated with SEND may be sent with SYN segment or queued for
transmission after entering ESTABLISHED state. The urgent bit if
requested in the command must be sent with the data segments sent
as a result of this command. If there is no room to queue the
request, respond with "error: insufficient resources". If
Foreign socket was not specified, then return "error: foreign
socket unspecified".
SYN-SENT STATE
SYN-RECEIVED STATE
Queue the data for transmission after entering ESTABLISHED state.
If no space to queue, respond with "error: insufficient
resources".
ESTABLISHED STATE
CLOSE-WAIT STATE
Segmentize the buffer and send it with a piggybacked
acknowledgment (acknowledgment value = RCV.NXT). If there is
insufficient space to remember this buffer, simply return "error:
insufficient resources".
If the urgent flag is set, then SND.UP <- SND.NXT-1 and set the
urgent pointer in the outgoing segments.
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Transmission Control Protocol
Functional Specification
SEND Call
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Return "error: connection closing" and do not service request.
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Transmission Control Protocol
Functional Specification
RECEIVE Call
RECEIVE Call
CLOSED STATE (i.e., TCB does not exist)
If the user does not have access to such a connection, return
"error: connection illegal for this process".
Otherwise return "error: connection does not exist".
LISTEN STATE
SYN-SENT STATE
SYN-RECEIVED STATE
Queue for processing after entering ESTABLISHED state. If there
is no room to queue this request, respond with "error:
insufficient resources".
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
If insufficient incoming segments are queued to satisfy the
request, queue the request. If there is no queue space to
remember the RECEIVE, respond with "error: insufficient
resources".
Reassemble queued incoming segments into receive buffer and return
to user. Mark "push seen" (PUSH) if this is the case.
If RCV.UP is in advance of the data currently being passed to the
user notify the user of the presence of urgent data.
When the TCP takes responsibility for delivering data to the user
that fact must be communicated to the sender via an
acknowledgment. The formation of such an acknowledgment is
described below in the discussion of processing an incoming
segment.
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Transmission Control Protocol
Functional Specification
RECEIVE Call
CLOSE-WAIT STATE
Since the remote side has already sent FIN, RECEIVEs must be
satisfied by text already on hand, but not yet delivered to the
user. If no text is awaiting delivery, the RECEIVE will get a
"error: connection closing" response. Otherwise, any remaining
text can be used to satisfy the RECEIVE.
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Return "error: connection closing".
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Transmission Control Protocol
Functional Specification
CLOSE Call
CLOSE Call
CLOSED STATE (i.e., TCB does not exist)
If the user does not have access to such a connection, return
"error: connection illegal for this process".
Otherwise, return "error: connection does not exist".
LISTEN STATE
Any outstanding RECEIVEs are returned with "error: closing"
responses. Delete TCB, enter CLOSED state, and return.
SYN-SENT STATE
Delete the TCB and return "error: closing" responses to any
queued SENDs, or RECEIVEs.
SYN-RECEIVED STATE
If no SENDs have been issued and there is no pending data to send,
then form a FIN segment and send it, and enter FIN-WAIT-1 state;
otherwise queue for processing after entering ESTABLISHED state.
ESTABLISHED STATE
Queue this until all preceding SENDs have been segmentized, then
form a FIN segment and send it. In any case, enter FIN-WAIT-1
state.
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
Strictly speaking, this is an error and should receive a "error:
connection closing" response. An "ok" response would be
acceptable, too, as long as a second FIN is not emitted (the first
FIN may be retransmitted though).
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Transmission Control Protocol
Functional Specification
CLOSE Call
CLOSE-WAIT STATE
Queue this request until all preceding SENDs have been
segmentized; then send a FIN segment, enter CLOSING state.
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Respond with "error: connection closing".
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Transmission Control Protocol
Functional Specification
ABORT Call
ABORT Call
CLOSED STATE (i.e., TCB does not exist)
If the user should not have access to such a connection, return
"error: connection illegal for this process".
Otherwise return "error: connection does not exist".
LISTEN STATE
Any outstanding RECEIVEs should be returned with "error:
connection reset" responses. Delete TCB, enter CLOSED state, and
return.
SYN-SENT STATE
All queued SENDs and RECEIVEs should be given "connection reset"
notification, delete the TCB, enter CLOSED state, and return.
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
Send a reset segment:
<SEQ=SND.NXT><CTL=RST>
All queued SENDs and RECEIVEs should be given "connection reset"
notification; all segments queued for transmission (except for the
RST formed above) or retransmission should be flushed, delete the
TCB, enter CLOSED state, and return.
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Respond with "ok" and delete the TCB, enter CLOSED state, and
return.
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Transmission Control Protocol
Functional Specification
STATUS Call
STATUS Call
CLOSED STATE (i.e., TCB does not exist)
If the user should not have access to such a connection, return
"error: connection illegal for this process".
Otherwise return "error: connection does not exist".
LISTEN STATE
Return "state = LISTEN", and the TCB pointer.
SYN-SENT STATE
Return "state = SYN-SENT", and the TCB pointer.
SYN-RECEIVED STATE
Return "state = SYN-RECEIVED", and the TCB pointer.
ESTABLISHED STATE
Return "state = ESTABLISHED", and the TCB pointer.
FIN-WAIT-1 STATE
Return "state = FIN-WAIT-1", and the TCB pointer.
FIN-WAIT-2 STATE
Return "state = FIN-WAIT-2", and the TCB pointer.
CLOSE-WAIT STATE
Return "state = CLOSE-WAIT", and the TCB pointer.
CLOSING STATE
Return "state = CLOSING", and the TCB pointer.
LAST-ACK STATE
Return "state = LAST-ACK", and the TCB pointer.
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Transmission Control Protocol
Functional Specification
STATUS Call
TIME-WAIT STATE
Return "state = TIME-WAIT", and the TCB pointer.
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Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES
SEGMENT ARRIVES
If the state is CLOSED (i.e., TCB does not exist) then
all data in the incoming segment is discarded. An incoming
segment containing a RST is discarded. An incoming segment not
containing a RST causes a RST to be sent in response. The
acknowledgment and sequence field values are selected to make the
reset sequence acceptable to the TCP that sent the offending
segment.
If the ACK bit is off, sequence number zero is used,
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If the ACK bit is on,
<SEQ=SEG.ACK><CTL=RST>
Return.
If the state is LISTEN then
first check for an RST
An incoming RST should be ignored. Return.
second check for an ACK
Any acknowledgment is bad if it arrives on a connection still in
the LISTEN state. An acceptable reset segment should be formed
for any arriving ACK-bearing segment. The RST should be
formatted as follows:
<SEQ=SEG.ACK><CTL=RST>
Return.
third check for a SYN
If the SYN bit is set, check the security. If the
security/compartment on the incoming segment does not exactly
match the security/compartment in the TCB then send a reset and
return.
<SEQ=SEG.ACK><CTL=RST>
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Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES
If the SEG.PRC is greater than the TCB.PRC then if allowed by
the user and the system set TCB.PRC<-SEG.PRC, if not allowed
send a reset and return.
<SEQ=SEG.ACK><CTL=RST>
If the SEG.PRC is less than the TCB.PRC then continue.
Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ and any other
control or text should be queued for processing later. ISS
should be selected and a SYN segment sent of the form:
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
SND.NXT is set to ISS+1 and SND.UNA to ISS. The connection
state should be changed to SYN-RECEIVED. Note that any other
incoming control or data (combined with SYN) will be processed
in the SYN-RECEIVED state, but processing of SYN and ACK should
not be repeated. If the listen was not fully specified (i.e.,
the foreign socket was not fully specified), then the
unspecified fields should be filled in now.
fourth other text or control
Any other control or text-bearing segment (not containing SYN)
must have an ACK and thus would be discarded by the ACK
processing. An incoming RST segment could not be valid, since
it could not have been sent in response to anything sent by this
incarnation of the connection. So you are unlikely to get here,
but if you do, drop the segment, and return.
If the state is SYN-SENT then
first check the ACK bit
If the ACK bit is set
If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset (unless
the RST bit is set, if so drop the segment and return)
<SEQ=SEG.ACK><CTL=RST>
and discard the segment. Return.
If SND.UNA =< SEG.ACK =< SND.NXT then the ACK is acceptable.
second check the RST bit
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Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES
If the RST bit is set
If the ACK was acceptable then signal the user "error:
connection reset", drop the segment, enter CLOSED state,
delete TCB, and return. Otherwise (no ACK) drop the segment
and return.
third check the security and precedence
If the security/compartment in the segment does not exactly
match the security/compartment in the TCB, send a reset
If there is an ACK
<SEQ=SEG.ACK><CTL=RST>
Otherwise
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If there is an ACK
The precedence in the segment must match the precedence in the
TCB, if not, send a reset
<SEQ=SEG.ACK><CTL=RST>
If there is no ACK
If the precedence in the segment is higher than the precedence
in the TCB then if allowed by the user and the system raise
the precedence in the TCB to that in the segment, if not
allowed to raise the prec then send a reset.
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If the precedence in the segment is lower than the precedence
in the TCB continue.
If a reset was sent, discard the segment and return.
fourth check the SYN bit
This step should be reached only if the ACK is ok, or there is
no ACK, and it the segment did not contain a RST.
If the SYN bit is on and the security/compartment and precedence
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Functional Specification
SEGMENT ARRIVES
are acceptable then, RCV.NXT is set to SEG.SEQ+1, IRS is set to
SEG.SEQ. SND.UNA should be advanced to equal SEG.ACK (if there
is an ACK), and any segments on the retransmission queue which
are thereby acknowledged should be removed.
If SND.UNA > ISS (our SYN has been ACKed), change the connection
state to ESTABLISHED, form an ACK segment
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
and send it. Data or controls which were queued for
transmission may be included. If there are other controls or
text in the segment then continue processing at the sixth step
below where the URG bit is checked, otherwise return.
Otherwise enter SYN-RECEIVED, form a SYN,ACK segment
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
and send it. If there are other controls or text in the
segment, queue them for processing after the ESTABLISHED state
has been reached, return.
fifth, if neither of the SYN or RST bits is set then drop the
segment and return.
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Functional Specification
SEGMENT ARRIVES
Otherwise,
first check sequence number
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Segments are processed in sequence. Initial tests on arrival
are used to discard old duplicates, but further processing is
done in SEG.SEQ order. If a segment's contents straddle the
boundary between old and new, only the new parts should be
processed.
There are four cases for the acceptability test for an incoming
segment:
Segment Receive Test
Length Window
------- ------- -------------------------------------------
0 0 SEG.SEQ = RCV.NXT
0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
>0 0 not acceptable
>0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
If the RCV.WND is zero, no segments will be acceptable, but
special allowance should be made to accept valid ACKs, URGs and
RSTs.
If an incoming segment is not acceptable, an acknowledgment
should be sent in reply (unless the RST bit is set, if so drop
the segment and return):
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
After sending the acknowledgment, drop the unacceptable segment
and return.
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Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES
In the following it is assumed that the segment is the idealized
segment that begins at RCV.NXT and does not exceed the window.
One could tailor actual segments to fit this assumption by
trimming off any portions that lie outside the window (including
SYN and FIN), and only processing further if the segment then
begins at RCV.NXT. Segments with higher begining sequence
numbers may be held for later processing.
second check the RST bit,
SYN-RECEIVED STATE
If the RST bit is set
If this connection was initiated with a passive OPEN (i.e.,
came from the LISTEN state), then return this connection to
LISTEN state and return. The user need not be informed. If
this connection was initiated with an active OPEN (i.e., came
from SYN-SENT state) then the connection was refused, signal
the user "connection refused". In either case, all segments
on the retransmission queue should be removed. And in the
active OPEN case, enter the CLOSED state and delete the TCB,
and return.
ESTABLISHED
FIN-WAIT-1
FIN-WAIT-2
CLOSE-WAIT
If the RST bit is set then, any outstanding RECEIVEs and SEND
should receive "reset" responses. All segment queues should be
flushed. Users should also receive an unsolicited general
"connection reset" signal. Enter the CLOSED state, delete the
TCB, and return.
CLOSING STATE
LAST-ACK STATE
TIME-WAIT
If the RST bit is set then, enter the CLOSED state, delete the
TCB, and return.
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Functional Specification
SEGMENT ARRIVES
third check security and precedence
SYN-RECEIVED
If the security/compartment and precedence in the segment do not
exactly match the security/compartment and precedence in the TCB
then send a reset, and return.
ESTABLISHED STATE
If the security/compartment and precedence in the segment do not
exactly match the security/compartment and precedence in the TCB
then send a reset, any outstanding RECEIVEs and SEND should
receive "reset" responses. All segment queues should be
flushed. Users should also receive an unsolicited general
"connection reset" signal. Enter the CLOSED state, delete the
TCB, and return.
Note this check is placed following the sequence check to prevent
a segment from an old connection between these ports with a
different security or precedence from causing an abort of the
current connection.
fourth, check the SYN bit,
SYN-RECEIVED
ESTABLISHED STATE
FIN-WAIT STATE-1
FIN-WAIT STATE-2
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
If the SYN is in the window it is an error, send a reset, any
outstanding RECEIVEs and SEND should receive "reset" responses,
all segment queues should be flushed, the user should also
receive an unsolicited general "connection reset" signal, enter
the CLOSED state, delete the TCB, and return.
If the SYN is not in the window this step would not be reached
and an ack would have been sent in the first step (sequence
number check).
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Functional Specification
SEGMENT ARRIVES
fifth check the ACK field,
if the ACK bit is off drop the segment and return
if the ACK bit is on
SYN-RECEIVED STATE
If SND.UNA =< SEG.ACK =< SND.NXT then enter ESTABLISHED state
and continue processing.
If the segment acknowledgment is not acceptable, form a
reset segment,
<SEQ=SEG.ACK><CTL=RST>
and send it.
ESTABLISHED STATE
If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <- SEG.ACK.
Any segments on the retransmission queue which are thereby
entirely acknowledged are removed. Users should receive
positive acknowledgments for buffers which have been SENT and
fully acknowledged (i.e., SEND buffer should be returned with
"ok" response). If the ACK is a duplicate
(SEG.ACK < SND.UNA), it can be ignored. If the ACK acks
something not yet sent (SEG.ACK > SND.NXT) then send an ACK,
drop the segment, and return.
If SND.UNA < SEG.ACK =< SND.NXT, the send window should be
updated. If (SND.WL1 < SEG.SEQ or (SND.WL1 = SEG.SEQ and
SND.WL2 =< SEG.ACK)), set SND.WND <- SEG.WND, set
SND.WL1 <- SEG.SEQ, and set SND.WL2 <- SEG.ACK.
Note that SND.WND is an offset from SND.UNA, that SND.WL1
records the sequence number of the last segment used to update
SND.WND, and that SND.WL2 records the acknowledgment number of
the last segment used to update SND.WND. The check here
prevents using old segments to update the window.
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Functional Specification
SEGMENT ARRIVES
FIN-WAIT-1 STATE
In addition to the processing for the ESTABLISHED state, if
our FIN is now acknowledged then enter FIN-WAIT-2 and continue
processing in that state.
FIN-WAIT-2 STATE
In addition to the processing for the ESTABLISHED state, if
the retransmission queue is empty, the user's CLOSE can be
acknowledged ("ok") but do not delete the TCB.
CLOSE-WAIT STATE
Do the same processing as for the ESTABLISHED state.
CLOSING STATE
In addition to the processing for the ESTABLISHED state, if
the ACK acknowledges our FIN then enter the TIME-WAIT state,
otherwise ignore the segment.
LAST-ACK STATE
The only thing that can arrive in this state is an
acknowledgment of our FIN. If our FIN is now acknowledged,
delete the TCB, enter the CLOSED state, and return.
TIME-WAIT STATE
The only thing that can arrive in this state is a
retransmission of the remote FIN. Acknowledge it, and restart
the 2 MSL timeout.
sixth, check the URG bit,
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
If the URG bit is set, RCV.UP <- max(RCV.UP,SEG.UP), and signal
the user that the remote side has urgent data if the urgent
pointer (RCV.UP) is in advance of the data consumed. If the
user has already been signaled (or is still in the "urgent
mode") for this continuous sequence of urgent data, do not
signal the user again.
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Functional Specification
SEGMENT ARRIVES
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT
This should not occur, since a FIN has been received from the
remote side. Ignore the URG.
seventh, process the segment text,
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
Once in the ESTABLISHED state, it is possible to deliver segment
text to user RECEIVE buffers. Text from segments can be moved
into buffers until either the buffer is full or the segment is
empty. If the segment empties and carries an PUSH flag, then
the user is informed, when the buffer is returned, that a PUSH
has been received.
When the TCP takes responsibility for delivering the data to the
user it must also acknowledge the receipt of the data.
Once the TCP takes responsibility for the data it advances
RCV.NXT over the data accepted, and adjusts RCV.WND as
apporopriate to the current buffer availability. The total of
RCV.NXT and RCV.WND should not be reduced.
Please note the window management suggestions in section 3.7.
Send an acknowledgment of the form:
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
This acknowledgment should be piggybacked on a segment being
transmitted if possible without incurring undue delay.
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Functional Specification
SEGMENT ARRIVES
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
This should not occur, since a FIN has been received from the
remote side. Ignore the segment text.
eighth, check the FIN bit,
Do not process the FIN if the state is CLOSED, LISTEN or SYN-SENT
since the SEG.SEQ cannot be validated; drop the segment and
return.
If the FIN bit is set, signal the user "connection closing" and
return any pending RECEIVEs with same message, advance RCV.NXT
over the FIN, and send an acknowledgment for the FIN. Note that
FIN implies PUSH for any segment text not yet delivered to the
user.
SYN-RECEIVED STATE
ESTABLISHED STATE
Enter the CLOSE-WAIT state.
FIN-WAIT-1 STATE
If our FIN has been ACKed (perhaps in this segment), then
enter TIME-WAIT, start the time-wait timer, turn off the other
timers; otherwise enter the CLOSING state.
FIN-WAIT-2 STATE
Enter the TIME-WAIT state. Start the time-wait timer, turn
off the other timers.
CLOSE-WAIT STATE
Remain in the CLOSE-WAIT state.
CLOSING STATE
Remain in the CLOSING state.
LAST-ACK STATE
Remain in the LAST-ACK state.
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Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES
TIME-WAIT STATE
Remain in the TIME-WAIT state. Restart the 2 MSL time-wait
timeout.
and return.
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September 1981
Transmission Control Protocol
Functional Specification
USER TIMEOUT
USER TIMEOUT
For any state if the user timeout expires, flush all queues, signal
the user "error: connection aborted due to user timeout" in general
and for any outstanding calls, delete the TCB, enter the CLOSED
state and return.
RETRANSMISSION TIMEOUT
For any state if the retransmission timeout expires on a segment in
the retransmission queue, send the segment at the front of the
retransmission queue again, reinitialize the retransmission timer,
and return.
TIME-WAIT TIMEOUT
If the time-wait timeout expires on a connection delete the TCB,
enter the CLOSED state and return.
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September 1981
Transmission Control Protocol
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September 1981
Transmission Control Protocol
GLOSSARY
1822
BBN Report 1822, "The Specification of the Interconnection of
a Host and an IMP". The specification of interface between a
host and the ARPANET.
ACK
A control bit (acknowledge) occupying no sequence space, which
indicates that the acknowledgment field of this segment
specifies the next sequence number the sender of this segment
is expecting to receive, hence acknowledging receipt of all
previous sequence numbers.
ARPANET message
The unit of transmission between a host and an IMP in the
ARPANET. The maximum size is about 1012 octets (8096 bits).
ARPANET packet
A unit of transmission used internally in the ARPANET between
IMPs. The maximum size is about 126 octets (1008 bits).
connection
A logical communication path identified by a pair of sockets.
datagram
A message sent in a packet switched computer communications
network.
Destination Address
The destination address, usually the network and host
identifiers.
FIN
A control bit (finis) occupying one sequence number, which
indicates that the sender will send no more data or control
occupying sequence space.
fragment
A portion of a logical unit of data, in particular an internet
fragment is a portion of an internet datagram.
FTP
A file transfer protocol.
[Page 79]
September 1981
Transmission Control Protocol
Glossary
header
Control information at the beginning of a message, segment,
fragment, packet or block of data.
host
A computer. In particular a source or destination of messages
from the point of view of the communication network.
Identification
An Internet Protocol field. This identifying value assigned
by the sender aids in assembling the fragments of a datagram.
IMP
The Interface Message Processor, the packet switch of the
ARPANET.
internet address
A source or destination address specific to the host level.
internet datagram
The unit of data exchanged between an internet module and the
higher level protocol together with the internet header.
internet fragment
A portion of the data of an internet datagram with an internet
header.
IP
Internet Protocol.
IRS
The Initial Receive Sequence number. The first sequence
number used by the sender on a connection.
ISN
The Initial Sequence Number. The first sequence number used
on a connection, (either ISS or IRS). Selected on a clock
based procedure.
ISS
The Initial Send Sequence number. The first sequence number
used by the sender on a connection.
leader
Control information at the beginning of a message or block of
data. In particular, in the ARPANET, the control information
on an ARPANET message at the host-IMP interface.
[Page 80]
September 1981
Transmission Control Protocol
Glossary
left sequence
This is the next sequence number to be acknowledged by the
data receiving TCP (or the lowest currently unacknowledged
sequence number) and is sometimes referred to as the left edge
of the send window.
local packet
The unit of transmission within a local network.
module
An implementation, usually in software, of a protocol or other
procedure.
MSL
Maximum Segment Lifetime, the time a TCP segment can exist in
the internetwork system. Arbitrarily defined to be 2 minutes.
octet
An eight bit byte.
Options
An Option field may contain several options, and each option
may be several octets in length. The options are used
primarily in testing situations; for example, to carry
timestamps. Both the Internet Protocol and TCP provide for
options fields.
packet
A package of data with a header which may or may not be
logically complete. More often a physical packaging than a
logical packaging of data.
port
The portion of a socket that specifies which logical input or
output channel of a process is associated with the data.
process
A program in execution. A source or destination of data from
the point of view of the TCP or other host-to-host protocol.
PUSH
A control bit occupying no sequence space, indicating that
this segment contains data that must be pushed through to the
receiving user.
RCV.NXT
receive next sequence number
[Page 81]
September 1981
Transmission Control Protocol
Glossary
RCV.UP
receive urgent pointer
RCV.WND
receive window
receive next sequence number
This is the next sequence number the local TCP is expecting to
receive.
receive window
This represents the sequence numbers the local (receiving) TCP
is willing to receive. Thus, the local TCP considers that
segments overlapping the range RCV.NXT to
RCV.NXT + RCV.WND - 1 carry acceptable data or control.
Segments containing sequence numbers entirely outside of this
range are considered duplicates and discarded.
RST
A control bit (reset), occupying no sequence space, indicating
that the receiver should delete the connection without further
interaction. The receiver can determine, based on the
sequence number and acknowledgment fields of the incoming
segment, whether it should honor the reset command or ignore
it. In no case does receipt of a segment containing RST give
rise to a RST in response.
RTP
Real Time Protocol: A host-to-host protocol for communication
of time critical information.
SEG.ACK
segment acknowledgment
SEG.LEN
segment length
SEG.PRC
segment precedence value
SEG.SEQ
segment sequence
SEG.UP
segment urgent pointer field
[Page 82]
September 1981
Transmission Control Protocol
Glossary
SEG.WND
segment window field
segment
A logical unit of data, in particular a TCP segment is the
unit of data transfered between a pair of TCP modules.
segment acknowledgment
The sequence number in the acknowledgment field of the
arriving segment.
segment length
The amount of sequence number space occupied by a segment,
including any controls which occupy sequence space.
segment sequence
The number in the sequence field of the arriving segment.
send sequence
This is the next sequence number the local (sending) TCP will
use on the connection. It is initially selected from an
initial sequence number curve (ISN) and is incremented for
each octet of data or sequenced control transmitted.
send window
This represents the sequence numbers which the remote
(receiving) TCP is willing to receive. It is the value of the
window field specified in segments from the remote (data
receiving) TCP. The range of new sequence numbers which may
be emitted by a TCP lies between SND.NXT and
SND.UNA + SND.WND - 1. (Retransmissions of sequence numbers
between SND.UNA and SND.NXT are expected, of course.)
SND.NXT
send sequence
SND.UNA
left sequence
SND.UP
send urgent pointer
SND.WL1
segment sequence number at last window update
SND.WL2
segment acknowledgment number at last window update
[Page 83]
September 1981
Transmission Control Protocol
Glossary
SND.WND
send window
socket
An address which specifically includes a port identifier, that
is, the concatenation of an Internet Address with a TCP port.
Source Address
The source address, usually the network and host identifiers.
SYN
A control bit in the incoming segment, occupying one sequence
number, used at the initiation of a connection, to indicate
where the sequence numbering will start.
TCB
Transmission control block, the data structure that records
the state of a connection.
TCB.PRC
The precedence of the connection.
TCP
Transmission Control Protocol: A host-to-host protocol for
reliable communication in internetwork environments.
TOS
Type of Service, an Internet Protocol field.
Type of Service
An Internet Protocol field which indicates the type of service
for this internet fragment.
URG
A control bit (urgent), occupying no sequence space, used to
indicate that the receiving user should be notified to do
urgent processing as long as there is data to be consumed with
sequence numbers less than the value indicated in the urgent
pointer.
urgent pointer
A control field meaningful only when the URG bit is on. This
field communicates the value of the urgent pointer which
indicates the data octet associated with the sending user's
urgent call.
[Page 84]
September 1981
Transmission Control Protocol
REFERENCES
[1] Cerf, V., and R. Kahn, "A Protocol for Packet Network
Intercommunication", IEEE Transactions on Communications,
Vol. COM-22, No. 5, pp 637-648, May 1974.
[2] Postel, J. (ed.), "Internet Protocol - DARPA Internet Program
Protocol Specification", RFC 791, USC/Information Sciences
Institute, September 1981.
[3] Dalal, Y. and C. Sunshine, "Connection Management in Transport
Protocols", Computer Networks, Vol. 2, No. 6, pp. 454-473,
December 1978.
[4] Postel, J., "Assigned Numbers", RFC 790, USC/Information Sciences
Institute, September 1981.
[Page 85]
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