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RFC 0793

 
 
 

Transmission Control Protocol

Part 4 of 4, p. 54 to 85
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                                                               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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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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                                                               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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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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                                                            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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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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                                                              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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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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                                                              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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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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                                                             STATUS Call



    TIME-WAIT STATE

      Return "state = TIME-WAIT", and the TCB pointer.

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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>

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                                                         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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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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                                                         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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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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                                                         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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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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                                                         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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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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                                                         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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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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                                                         SEGMENT ARRIVES



        TIME-WAIT STATE

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

    and return.

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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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                                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.

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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.

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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

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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

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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

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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.

          

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                               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.