Connected: An Internet Encyclopedia
RFC 1323 - APPENDIX E: EVENT PROCESSING
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APPENDIX E: EVENT PROCESSING
OPEN Call
...
An initial send sequence number (ISS) is selected. Send a SYN
segment of the form:
<SEQ=ISS><CTL=SYN><TSval=my.TSclock><WSopt=Rcv.Wind.Scale>
...
SEND Call
CLOSED STATE (i.e., TCB 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
containing the options: <TSval=my.TSclock> and
<WSopt=Rcv.Wind.Scale>. Set SND.UNA to ISS, SND.NXT to ISS+1.
Enter SYN-SENT state. ...
SYN-SENT STATE
SYN-RECEIVED STATE
...
ESTABLISHED STATE
CLOSE-WAIT STATE
Segmentize the buffer and send it with a piggybacked
acknowledgment (acknowledgment value = RCV.NXT). ...
If the urgent flag is set ...
If the Snd.TS.OK flag is set, then include the TCP Timestamps
option <TSval=my.TSclock,TSecr=TS.Recent> in each data segment.
Scale the receive window for transmission in the segment header:
SEG.WND = (SND.WND >> Rcv.Wind.Scale).
SEGMENT ARRIVES
...
If the state is LISTEN then
first check for an RST
...
second check for an ACK
...
third check for a SYN
if the SYN bit is set, check the security. If the ...
...
If the SEG.PRC is less than the TCB.PRC then continue.
Check for a Window Scale option (WSopt); if one is found, save
SEG.WSopt in Snd.Wind.Scale and set Snd.WS.OK flag on.
Otherwise, set both Snd.Wind.Scale and Rcv.Wind.Scale to zero
and clear Snd.WS.OK flag.
Check for a TSopt option; if one is found, save SEG.TSval in the
variable TS.Recent and turn on the Snd.TS.OK bit.
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>
If the Snd.WS.OK bit is on, include a WSopt option
<WSopt=Rcv.Wind.Scale> in this segment. If the Snd.TS.OK bit is
on, include a TSopt <TSval=my.TSclock,TSecr=TS.Recent> in this
segment. Last.ACK.sent is set to RCV.NXT.
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
...
If the state is SYN-SENT then
first check the ACK bit
...
fourth check the SYN bit
...
If the SYN bit is on and the security/compartment and precedence
are acceptable then, RCV.NXT is set to SEG.SEQ+1, IRS is set to
SEG.SEQ, and any acknowledgements on the retransmission queue
which are thereby acknowledged should be removed.
Check for a Window Scale option (WSopt); if is found, save
SEG.WSopt in Snd.Wind.Scale; otherwise, set both Snd.Wind.Scale
and Rcv.Wind.Scale to zero.
Check for a TSopt option; if one is found, save SEG.TSval in
variable TS.Recent and turn on the Snd.TS.OK bit in the
connection control block. If the ACK bit is set, use my.TSclock
- SEG.TSecr as the initial RTT estimate.
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. If the Snd.Echo.OK bit is on, include a TSopt
option <TSval=my.TSclock,TSecr=TS.Recent> in this ACK segment.
Last.ACK.sent is set to RCV.NXT.
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 the Snd.Echo.OK bit is on, include a TSopt
option <TSval=my.TSclock,TSecr=TS.Recent> in this segment. If
the Snd.WS.OK bit is on, include a WSopt option
<WSopt=Rcv.Wind.Scale> in this segment. Last.ACK.sent is set to
RCV.NXT.
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.
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.
Rescale the received window field:
TrueWindow = SEG.WND << Snd.Wind.Scale,
and use "TrueWindow" in place of SEG.WND in the following steps.
Check whether the segment contains a Timestamps option and bit
Snd.TS.OK is on. If so:
If SEG.TSval < TS.Recent, then test whether connection has
been idle less than 24 days; if both are true, then the
segment is not acceptable; follow steps below for an
unacceptable segment.
If SEG.SEQ is equal to Last.ACK.sent, then save SEG.ECopt in
variable TS.Recent.
There are four cases for the acceptability test for an incoming
segment:
...
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>
Last.ACK.sent is set to SEG.ACK of the acknowledgment. If the
Snd.Echo.OK bit is on, include the Timestamps option
<TSval=my.TSclock,TSecr=TS.Recent> in this ACK segment. Set
Last.ACK.sent to SEG.ACK and send the ACK segment. After
sending the acknowledgment, drop the unacceptable segment and
return.
...
fifth check the ACK field.
if the ACK bit is off drop the segment and return.
if the ACK bit is on
...
ESTABLISHED STATE
If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <- SEG.ACK.
Also compute a new estimate of round-trip time. If Snd.TS.OK
bit is on, use my.TSclock - SEG.TSecr; otherwise use the
elapsed time since the first segment in the retransmission
queue was sent. Any segments on the retransmission queue
which are thereby entirely acknowledged...
...
Seventh, process the segment text.
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
...
Send an acknowledgment of the form:
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
If the Snd.TS.OK bit is on, include Timestamps option
<TSval=my.TSclock,TSecr=TS.Recent> in this ACK segment. Set
Last.ACK.sent to SEG.ACK of the acknowledgment, and send it.
This acknowledgment should be piggy-backed on a segment being
transmitted if possible without incurring undue delay.
...
Next: Security Considerations
Connected: An Internet Encyclopedia
RFC 1323 - APPENDIX E: EVENT PROCESSING