rfc1661.txt

this is a linux pptp software

Simpson                                                        [Page 18]
RFC 1661                Point-to-Point Protocol                July 1994


      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



Simpson                                                        [Page 19]
RFC 1661                Point-to-Point Protocol                July 1994


      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.

   Receive-Terminate-Ack (RTA)

      This event occurs when a Terminate-Ack packet is received from the
      peer.  The Terminate-Ack packet is usually a response to a
      Terminate-Request packet.  The Terminate-Ack packet may also
      indicate that the peer is in Closed or Stopped states, and serves
      to re-synchronize the link configuration.

   Receive-Unknown-Code (RUC)

      This event occurs when an un-interpretable packet is received from
      the peer.  A Code-Reject packet is sent in response.

   Receive-Code-Reject, Receive-Protocol-Reject (RXJ+,RXJ-)

      This event occurs when a Code-Reject or a Protocol-Reject packet
      is received from the peer.

      The RXJ+ event arises when the rejected value is acceptable, such
      as a Code-Reject of an extended code, or a Protocol-Reject of a
      NCP.  These are within the scope of normal operation.  The
      implementation MUST stop sending the offending packet type.

      The RXJ- event arises when the rejected value is catastrophic,
      such as a Code-Reject of Configure-Request, or a Protocol-Reject
      of LCP!  This event communicates an unrecoverable error that
      terminates the connection.

   Receive-Echo-Request, Receive-Echo-Reply, Receive-Discard-Request
   (RXR)

      This event occurs when an Echo-Request, Echo-Reply or Discard-
      Request packet is received from the peer.  The Echo-Reply packet
      is a response to an Echo-Request packet.  There is no reply to an
      Echo-Reply or Discard-Request packet.






Simpson                                                        [Page 20]
RFC 1661                Point-to-Point Protocol                July 1994


4.4.  Actions

   Actions in the automaton are caused by events and typically indicate
   the transmission of packets and/or the starting or stopping of the
   Restart timer.

   Illegal-Event (-)

      This indicates an event that cannot occur in a properly
      implemented automaton.  The implementation has an internal error,
      which should be reported and logged.  No transition is taken, and
      the implementation SHOULD NOT reset or freeze.

   This-Layer-Up (tlu)

      This action indicates to the upper layers that the automaton is
      entering the Opened state.

      Typically, this action is used by the LCP to signal the Up event
      to a NCP, Authentication Protocol, or Link Quality Protocol, or
      MAY be used by a NCP to indicate that the link is available for
      its network layer traffic.

   This-Layer-Down (tld)

      This action indicates to the upper layers that the automaton is
      leaving the Opened state.

      Typically, this action is used by the LCP to signal the Down event
      to a NCP, Authentication Protocol, or Link Quality Protocol, or
      MAY be used by a NCP to indicate that the link is no longer
      available for its network layer traffic.

   This-Layer-Started (tls)

      This action indicates to the lower layers that the automaton is
      entering the Starting state, and the lower layer is needed for the
      link.  The lower layer SHOULD respond with an Up event when the
      lower layer is available.

      This results of this action are highly implementation dependent.

   This-Layer-Finished (tlf)

      This action indicates to the lower layers that the automaton is
      entering the Initial, Closed or Stopped states, and the lower
      layer is no longer needed for the link.  The lower layer SHOULD
      respond with a Down event when the lower layer has terminated.



Simpson                                                        [Page 21]
RFC 1661                Point-to-Point Protocol                July 1994


      Typically, this action MAY be used by the LCP to advance to the
      Link Dead phase, or MAY be used by a NCP to indicate to the LCP
      that the link may terminate when there are no other NCPs open.

      This results of this action are highly implementation dependent.

   Initialize-Restart-Count (irc)

      This action sets the Restart counter to the appropriate value
      (Max-Terminate or Max-Configure).  The counter is decremented for
      each transmission, including the first.

      Implementation Note:

         In addition to setting the Restart counter, the implementation
         MUST set the timeout period to the initial value when Restart
         timer backoff is used.

   Zero-Restart-Count (zrc)

      This action sets the Restart counter to zero.

      Implementation Note:

         This action enables the FSA to pause before proceeding to the
         desired final state, allowing traffic to be processed by the
         peer.  In addition to zeroing the Restart counter, the
         implementation MUST set the timeout period to an appropriate
         value.

   Send-Configure-Request (scr)

      A Configure-Request packet is transmitted.  This indicates the
      desire to open a connection with a specified set of Configuration
      Options.  The Restart timer is started when the Configure-Request
      packet is transmitted, to guard against packet loss.  The Restart
      counter is decremented each time a Configure-Request is sent.

   Send-Configure-Ack (sca)

      A Configure-Ack packet is transmitted.  This acknowledges the
      reception of a Configure-Request packet with an acceptable set of
      Configuration Options.

   Send-Configure-Nak (scn)

      A Configure-Nak or Configure-Reject packet is transmitted, as
      appropriate.  This negative response reports the reception of a



Simpson                                                        [Page 22]
RFC 1661                Point-to-Point Protocol                July 1994


      Configure-Request packet with an unacceptable set of Configuration
      Options.

      Configure-Nak packets are used to refuse a Configuration Option
      value, and to suggest a new, acceptable value.  Configure-Reject
      packets are used to refuse all negotiation about a Configuration
      Option, typically because it is not recognized or implemented.
      The use of Configure-Nak versus Configure-Reject is more fully
      described in the chapter on LCP Packet Formats.

   Send-Terminate-Request (str)

      A Terminate-Request packet is transmitted.  This indicates the
      desire to close a connection.  The Restart timer is started when
      the Terminate-Request packet is transmitted, to guard against
      packet loss.  The Restart counter is decremented each time a
      Terminate-Request is sent.

   Send-Terminate-Ack (sta)

      A Terminate-Ack packet is transmitted.  This acknowledges the
      reception of a Terminate-Request packet or otherwise serves to
      synchronize the automatons.

   Send-Code-Reject (scj)

      A Code-Reject packet is transmitted.  This indicates the reception
      of an unknown type of packet.

   Send-Echo-Reply (ser)

      An Echo-Reply packet is transmitted.  This acknowledges the
      reception of an Echo-Request packet.



4.5.  Loop Avoidance

   The protocol makes a reasonable attempt at avoiding Configuration
   Option negotiation loops.  However, the protocol does NOT guarantee
   that loops will not happen.  As with any negotiation, it is possible
   to configure two PPP implementations with conflicting policies that
   will never converge.  It is also possible to configure policies which
   do converge, but which take significant time to do so.  Implementors
   should keep this in mind and SHOULD implement loop detection
   mechanisms or higher level timeouts.





Simpson                                                        [Page 23]
RFC 1661                Point-to-Point Protocol                July 1994


4.6.  Counters and Timers

   Restart Timer

      There is one special timer used by the automaton.  The Restart
      timer is used to time transmissions of Configure-Request and
      Terminate-Request packets.  Expiration of the Restart timer causes
      a Timeout event, and retransmission of the corresponding
      Configure-Request or Terminate-Request packet.  The Restart timer
      MUST be configurable, but SHOULD default to three (3) seconds.

      Implementation Note:

         The Restart timer SHOULD be based on the speed of the link.
         The default value is designed for low speed (2,400 to 9,600
         bps), high switching latency links (typical telephone lines).
         Higher speed links, or links with low switching latency, SHOULD
         have correspondingly faster retransmission times.

         Instead of a constant value, the Restart timer MAY begin at an
         initial small value and increase to the configured final value.
         Each successive value less than the final value SHOULD be at
         least twice the previous value.  The initial value SHOULD be
         large enough to account for the size of the packets, twice the
         round trip time for transmission at the link speed, and at
         least an additional 100 milliseconds to allow the peer to
         process the packets before responding.  Some circuits add
         another 200 milliseconds of satellite delay.  Round trip times
         for modems operating at 14,400 bps have been measured in the
         range of 160 to more than 600 milliseconds.

   Max-Terminate

      There is one required restart counter for Terminate-Requests.
      Max-Terminate indicates the number of Terminate-Request packets
      sent without receiving a Terminate-Ack before assuming that the
      peer is unable to respond.  Max-Terminate MUST be configurable,
      but SHOULD default to two (2) transmissions.

   Max-Configure

      A similar counter is recommended for Configure-Requests.  Max-
      Configure indicates the number of Configure-Request packets sent
      without receiving a valid Configure-Ack, Configure-Nak or
      Configure-Reject before assuming that the peer is unable to
      respond.  Max-Configure MUST be configurable, but SHOULD default
      to ten (10) transmissions.




Simpson                                                        [Page 24]
RFC 1661                Point-to-Point Protocol                July 1994


   Max-Failure

      A related counter is recommended for Configure-Nak.  Max-Failure
      indicates the number of Configure-Nak packets sent without sending
      a Configure-Ack before assuming that configuration is not
      converging.  Any further Configure-Nak packets for peer requested
      options are converted to Configure-Reject packets, and locally
      desired options are no longer appended.  Max-Failure MUST be
      configurable, but SHOULD default to five (5) transmissions.