rfc1548.txt

<VC++网络游戏建摸与实现>源代码

               |        |             |        |
         +-------+      |   RTA +-------+      | Close +-------+
         |       |<-----+<------|       |<-str-+<------|       |
         |Closed |              |Closing|              |Opened |
         |       | Open         |       |              |       |
         |       |------+       |       |              |       |
         +-------+      |       +-------+              +-------+
                        |                                ^
                        |                                |
                        |         +-sca----------------->+
                        |         |                      ^
                RCN,TO+ V    RCR+ |     RCR-         RCA |    RCN,TO+
               +------->+         |   +------->+         |   +--scr-->+
               |        |         |   |        |         |   |        |
         +-------+      |   TO+ +-------+      |       +-------+      |
         |       |<-scr-+<------|       |<-scn-+       |       |<-----+
         | Req-  |              | Ack-  |              | Ack-  |
         | Sent  | RCA          | Rcvd  |              | Sent  |
  +-scn->|       |------------->|       |       +-sca->|       |
  |      +-------+              +-------+       |      +-------+
  |   RCR- |   | RCR+                           |   RCR+ |   | RCR-
  |        |   +------------------------------->+<-------+   |
  |        |                                                 |
  +<-------+<------------------------------------------------+

























Simpson                                                        [Page 13]

RFC 1548              The Point-to-Point Protocol          December 1993


4.2 State Transition Table

  The complete state transition table follows.  States are indicated
  horizontally, and events are read vertically.  State transitions and
  actions are represented in the form action/new-state.  Multiple
  actions are separated by commas, and may continue on succeeding lines
  as space requires; multiple actions may be implemented in any
  convenient order.  The state may be followed by a letter, which
  indicates an explanatory footnote.  The dash ('-') indicates an
  illegal transition.


         | State
         |    0         1         2         3         4         5
   Events| Initial   Starting  Closed    Stopped   Closing   Stopping
   ------+-----------------------------------------------------------
    Up   |    2     irc,scr/6     -         -         -         -
    Down |    -         -         0       tls/1       0         1
    Open |  tls/1       1     irc,scr/6     3r        5r        5r
    Close|    0         0         2         2         4         4
         |
     TO+ |    -         -         -         -       str/4     str/5
     TO- |    -         -         -         -       tlf/2     tlf/3
         |
    RCR+ |    -         -       sta/2 irc,scr,sca/8   4         5
    RCR- |    -         -       sta/2 irc,scr,scn/6   4         5
    RCA  |    -         -       sta/2     sta/3       4         5
    RCN  |    -         -       sta/2     sta/3       4         5
         |
    RTR  |    -         -       sta/2     sta/3     sta/4     sta/5
    RTA  |    -         -         2         3       tlf/2     tlf/3
         |
    RUC  |    -         -       scj/2     scj/3     scj/4     scj/5
    RXJ+ |    -         -         2         3         4         5
    RXJ- |    -         -       tlf/2     tlf/3     tlf/2     tlf/3
         |
    RXR  |    -         -         2         3         4         5














Simpson                                                        [Page 14]

RFC 1548              The Point-to-Point Protocol          December 1993


            | State
            |    6         7         8           9
      Events| Req-Sent  Ack-Rcvd  Ack-Sent    Opened
      ------+-----------------------------------------
       Up   |    -         -         -           -
       Down |    1         1         1         tld/1
       Open |    6         7         8           9r
       Close|irc,str/4 irc,str/4 irc,str/4 tld,irc,str/4
            |
        TO+ |  scr/6     scr/6     scr/8         -
        TO- |  tlf/3p    tlf/3p    tlf/3p        -
            |
       RCR+ |  sca/8   sca,tlu/9   sca/8   tld,scr,sca/8
       RCR- |  scn/6     scn/7     scn/6   tld,scr,scn/6
       RCA  |  irc/7     scr/6x  irc,tlu/9   tld,scr/6x
       RCN  |irc,scr/6   scr/6x  irc,scr/8   tld,scr/6x
            |
       RTR  |  sta/6     sta/6     sta/6   tld,zrc,sta/5
       RTA  |    6         6         8       tld,scr/6
            |
       RUC  |  scj/6     scj/7     scj/8       scj/9
       RXJ+ |    6         6         8           9
       RXJ- |  tlf/3     tlf/3     tlf/3   tld,irc,str/5
            |
       RXR  |    6         7         8         ser/9

   The states in which the Restart timer is running are identifiable by
   the presence of TO events.  Only the Send-Configure-Request, Send-
   Terminate-Request and Zero-Restart-Counter actions start or re-start
   the Restart timer.  The Restart timer is stopped when transitioning
   from any state where the timer is running to a state where the timer
   is not running.

      [p]   Passive option; see Stopped state discussion.

      [r]   Restart option; see Open event discussion.

      [x]   Crossed connection; see RCA event discussion.

4.3 A Day in the Life

   Here is an example of how a typical implementation might use the
   automaton to implement LCP in a dial-up environment:

   -  The Network Access Server is powered on (Initial state, Link Dead
      phase).

   -  A configuration file indicates that a particular link is to be



Simpson                                                        [Page 15]

RFC 1548              The Point-to-Point Protocol          December 1993


      used for PPP access (Open: tls/Starting).  The This-Layer-Started
      event turns on DTR to a modem, readying it for accepting calls.

   -  An incoming call is answered.  The modem CD triggers configuration
      negotiation (Up: irc,scr/Req-Sent, Link Establishment phase).

   -  A Configure-Request is received, which is acknowleged (RCR+:
      sca/Ack-Sent).

   -  The Request is acknowleged (RCA: irc,tlu/Opened).  The This-
      Layer-Up event starts authentication and quality monitoring
      protocols (Authentication phase).

   -  When authentication and quality monitoring are satisfied, they
      send an Up event to start the available NCPs (Network-Layer
      Protocol phase).

   -  Later, the peer is finished, and closes the link.  A Terminate-
      Request arrives (RTR: tld,zrc,sta/Stopping, Termination phase).
      The This-Layer-Down action sends the Down event to any NCPs, while
      the Terminate-Ack is sent.  The Zero-Restart-Counter action causes
      the link to wait for the peer to process the Terminate-Ack, with
      no retries.

   -  When the Restart Timer times out (TO-: tlf/Stopped), the This-
      Layer-Finished action signals the modem to hang up by dropping
      DTR.

   -  When the CD from the modem drops (Down: tls/Starting), the This-
      Layer-Started action raises DTR again, readying it for the next
      call (returning to the Link Dead phase).

4.4 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.



Simpson                                                        [Page 16]

RFC 1548              The Point-to-Point Protocol          December 1993


      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.





Simpson                                                        [Page 17]

RFC 1548              The Point-to-Point Protocol          December 1993


    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, this action may be
      skipped, and the Closed state may be 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 always running in the Ack-Sent state.

    Opened

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



Simpson                                                        [Page 18]

RFC 1548              The Point-to-Point Protocol          December 1993


      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.

4.5 Events