rfc1661.html

ppp协议英文文档.介绍ppp的和ldc等相关协议 好东西啊

   desired to control specific signals (such as DTR), additional actions
   are likely to be required.

   [p]   Passive option; see Stopped state discussion.

   [r]   Restart option; see Open event discussion.

   [x]   Crossed connection; see RCA event discussion.

4.2.  States

   Following is a more detailed description of each automaton state.

   Initial

      In the Initial state, the lower layer is unavailable (Down), and
      no Open has occurred.  The Restart timer is not running in the
      Initial state.

   Starting

      The Starting state is the Open counterpart to the Initial state.
      An administrative Open has been initiated, but the lower layer is
      still unavailable (Down).  The Restart timer is not running in the
      Starting state.

      When the lower layer becomes available (Up), a Configure-Request
      is sent.

   Closed

      In the Closed state, the link is available (Up), but no Open has
      occurred.  The Restart timer is not running in the Closed state.

      Upon reception of Configure-Request packets, a Terminate-Ack is
      sent.  Terminate-Acks are silently discarded to avoid creating a
      loop.

   Stopped

      The Stopped state is the Open counterpart to the Closed state.  It
      is entered when the automaton is waiting for a Down event after
      the This-Layer-Finished action, or after sending a Terminate-Ack.
      The Restart timer is not running in the Stopped state.

      Upon reception of Configure-Request packets, an appropriate
      response is sent.  Upon reception of other packets, a Terminate-
      Ack is sent.  Terminate-Acks are silently discarded to avoid
      creating a loop.

      Rationale:

         The Stopped state is a junction state for link termination,
         link configuration failure, and other automaton failure modes.
         These potentially separate states have been combined.

         There is a race condition between the Down event response (from

         the This-Layer-Finished action) and the Receive-Configure-
         Request event.  When a Configure-Request arrives before the
         Down event, the Down event will supercede by returning the
         automaton to the Starting state.  This prevents attack by
         repetition.

      Implementation Option:

         After the peer fails to respond to Configure-Requests, an
         implementation MAY wait passively for the peer to send
         Configure-Requests.  In this case, the This-Layer-Finished
         action is not used for the TO- event in states Req-Sent, Ack-
         Rcvd and Ack-Sent.

         This option is useful for dedicated circuits, or circuits which
         have no status signals available, but SHOULD NOT be used for
         switched circuits.

   Closing

      In the Closing state, an attempt is made to terminate the
      connection.  A Terminate-Request has been sent and the Restart
      timer is running, but a Terminate-Ack has not yet been received.

      Upon reception of a Terminate-Ack, the Closed state is entered.
      Upon the expiration of the Restart timer, a new Terminate-Request
      is transmitted, and the Restart timer is restarted.  After the
      Restart timer has expired Max-Terminate times, the Closed state is
      entered.

   Stopping

      The Stopping state is the Open counterpart to the Closing state.
      A Terminate-Request has been sent and the Restart timer is
      running, but a Terminate-Ack has not yet been received.

      Rationale:

         The Stopping state provides a well defined opportunity to
         terminate a link before allowing new traffic.  After the link
         has terminated, a new configuration may occur via the Stopped
         or Starting states.

   Request-Sent

      In the Request-Sent state an attempt is made to configure the
      connection.  A Configure-Request has been sent and the Restart
      timer is running, but a Configure-Ack has not yet been received

      nor has one been sent.

   Ack-Received

      In the Ack-Received state, a Configure-Request has been sent and a
      Configure-Ack has been received.  The Restart timer is still
      running, since a Configure-Ack has not yet been sent.

   Ack-Sent

      In the Ack-Sent state, a Configure-Request and a Configure-Ack
      have both been sent, but a Configure-Ack has not yet been
      received.  The Restart timer is running, since a Configure-Ack has
      not yet been received.

   Opened

      In the Opened state, a Configure-Ack has been both sent and
      received.  The Restart timer is not running.

      When entering the Opened state, the implementation SHOULD signal
      the upper layers that it is now Up.  Conversely, when leaving the
      Opened state, the implementation SHOULD signal the upper layers
      that it is now Down.

4.3.  Events

   Transitions and actions in the automaton are caused by events.

   Up

      This event occurs when a lower layer indicates that it is ready to
      carry packets.

      Typically, this event is used by a modem handling or calling
      process, or by some other coupling of the PPP link to the physical
      media, to signal LCP that the link is entering Link Establishment
      phase.

      It also can be used by LCP to signal each NCP that the link is
      entering Network-Layer Protocol phase.  That is, the This-Layer-Up
      action from LCP triggers the Up event in the NCP.

   Down

      This event occurs when a lower layer indicates that it is no

      longer ready to carry packets.

      Typically, this event is used by a modem handling or calling
      process, or by some other coupling of the PPP link to the physical
      media, to signal LCP that the link is entering Link Dead phase.

      It also can be used by LCP to signal each NCP that the link is
      leaving Network-Layer Protocol phase.  That is, the This-Layer-
      Down action from LCP triggers the Down event in the NCP.

   Open

      This event indicates that the link is administratively available
      for traffic; that is, the network administrator (human or program)
      has indicated that the link is allowed to be Opened.  When this
      event occurs, and the link is not in the Opened state, the
      automaton attempts to send configuration packets to the peer.

      If the automaton is not able to begin configuration (the lower
      layer is Down, or a previous Close event has not completed), the
      establishment of the link is automatically delayed.

      When a Terminate-Request is received, or other events occur which
      cause the link to become unavailable, the automaton will progress
      to a state where the link is ready to re-open.  No additional
      administrative intervention is necessary.

      Implementation Option:

         Experience has shown that users will execute an additional Open
         command when they want to renegotiate the link.  This might
         indicate that new values are to be negotiated.

         Since this is not the meaning of the Open event, it is
         suggested that when an Open user command is executed in the
         Opened, Closing, Stopping, or Stopped states, the
         implementation issue a Down event, immediately followed by an
         Up event.  Care must be taken that an intervening Down event
         cannot occur from another source.

         The Down followed by an Up will cause an orderly renegotiation
         of the link, by progressing through the Starting to the
         Request-Sent state.  This will cause the renegotiation of the
         link, without any harmful side effects.

   Close

      This event indicates that the link is not available for traffic;

      that is, the network administrator (human or program) has
      indicated that the link is not allowed to be Opened.  When this
      event occurs, and the link is not in the Closed state, the
      automaton attempts to terminate the connection.  Futher attempts
      to re-configure the link are denied until a new Open event occurs.

      Implementation Note:

         When authentication fails, the link SHOULD be terminated, to
         prevent attack by repetition and denial of service to other
         users.  Since the link is administratively available (by
         definition), this can be accomplished by simulating a Close
         event to the LCP, immediately followed by an Open event.  Care
         must be taken that an intervening Close event cannot occur from
         another source.

         The Close followed by an Open will cause an orderly termination
         of the link, by progressing through the Closing to the Stopping
         state, and the This-Layer-Finished action can disconnect the
         link.  The automaton waits in the Stopped or Starting states
         for the next connection attempt.

   Timeout (TO+,TO-)

      This event indicates the expiration of the Restart timer.  The
      Restart timer is used to time responses to Configure-Request and
      Terminate-Request packets.

      The TO+ event indicates that the Restart counter continues to be
      greater than zero, which triggers the corresponding Configure-
      Request or Terminate-Request packet to be retransmitted.

      The TO- event indicates that the Restart counter is not greater
      than zero, and no more packets need to be retransmitted.

   Receive-Configure-Request (RCR+,RCR-)

      This event occurs when a Configure-Request packet is received from
      the peer.  The Configure-Request packet indicates the desire to
      open a connection and may specify Configuration Options.  The
      Configure-Request packet is more fully described in a later
      section.

      The RCR+ event indicates that the Configure-Request was
      acceptable, and triggers the transmission of a corresponding
      Configure-Ack.

      The RCR- event indicates that the Configure-Request was

      unacceptable, and triggers the transmission of a corresponding
      Configure-Nak or Configure-Reject.

      Implementation Note:

         These events may occur on a connection which is already in the
         Opened state.  The implementation MUST be prepared to
         immediately renegotiate the Configuration Options.

   Receive-Configure-Ack (RCA)

      This event occurs when a valid Configure-Ack packet is received
      from the peer.  The Configure-Ack packet is a positive response to
      a Configure-Request packet.  An out of sequence or otherwise
      invalid packet is silently discarded.

      Implementation Note:

         Since the correct packet has already been received before
         reaching the Ack-Rcvd or Opened states, it is extremely
         unlikely that another such packet will arrive.  As specified,
         all invalid Ack/Nak/Rej packets are silently discarded, and do
         not affect the transitions of the automaton.

         However, it is not impossible that a correctly formed packet
         will arrive through a coincidentally-timed cross-connection.
         It is more likely to be the result of an implementation error.
         At the very least, this occurance SHOULD be logged.

   Receive-Configure-Nak/Rej (RCN)

      This event occurs when a valid Configure-Nak or Configure-Reject
      packet is received from the peer.  The Configure-Nak and
      Configure-Reject packets are negative responses to a Configure-
      Request packet.  An out of sequence or otherwise invalid packet is
      silently discarded.

      Implementation Note:

         Although the Configure-Nak and Configure-Reject cause the same
         state transition in the automaton, these packets have
         significantly different effects on the Configuration Options
         sent in the resulting Configure-Request packet.

   Receive-Terminate-Request (RTR)

      This event occurs when a Terminate-Request packet is received.
      The Terminate-Request packet indicates the desire of the peer to

      close the connection.

      Implementation Note:

         This event is not identical to the Close event (see above), and
         does not override the Open commands of the local network
         administrator.  The implementation MUST be prepared to receive
         a new Configure-Request without network administrator
         intervention.