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.

         ...