Appendix E. Event Processing

Appendix E: Event Processing

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.


     ...