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SEGMENT ARRIVES Connected: An Internet Encyclopedia
SEGMENT ARRIVES

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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>

        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

        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
        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.

    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.

        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.

    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).

    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.


        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.


      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.


      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.

        TIME-WAIT STATE

          Remain in the TIME-WAIT state.  Restart the 2 MSL time-wait
          timeout.

    and return.


Next: USER TIMEOUT

Connected: An Internet Encyclopedia
SEGMENT ARRIVES

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