rfc1379.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Braden                                                         [Page 22]

RFC 1379              Transaction TCP -- Concepts          November 1992


                TABLE I: Summary of Monotonic Sequences

      APPROACH              TRmax (Tps)    Required MSL      COMMENTS
   __________________________________________________________________

   1. Timestamp & PAWS        1              24 days         TRmax is
                                                            too small
   __________________________________________________________________

   2. Current TCP Sequence Numbers

     (a) clock-driven
       ISN: eq. [3]           268           240 secs      TRmax & MSL
                                                            too small

     (b) Timestamps& clock-
         driven ISN [3] &     10**9         24 days           Hard to
         R=10**9                                            implement

     (c) Timestamps & c-dr
         ISN: eq. [4]        2**30/(R*Ts)   24 days         TRmax too
                                                               small.
   __________________________________________________________________

   3. 64-bit TCP Sequence Numbers

                          2**63/(MSL*R*Ts)      MSL        Significant
                                                          TCP change
                           e.g., R=10**9 Bps,
                               MSL = 4.4 hrs,
                               Ts = 0.5 sec=>
                               TRmax = 10**6
   __________________________________________________________________

   4. Connection Counts

     (a) no timestamps       2**31/MSL        MSL        3rd sequence
                        e.g., MSL=2000 sec                      space
                             TRmax = 10**6

     (b) with timestamps     2**30           24 days     (ditto)
                 and PAWS
   __________________________________________________________________








Braden                                                         [Page 23]

RFC 1379              Transaction TCP -- Concepts          November 1992


6.  CONNECTION STATES

   TCP has always allowed a connection to be half-closed.  TAO makes a
   significant addition to TCP semantics by allowing a connection to be
   half-synchronized, i.e., to be open for data transfer in one
   direction before the other direction has been opened.  Thus, the
   passive end of a connection (which receives an initial SYN) can
   accept data and even a FIN bit before its own SYN has been
   acknowledged.  This SYN, data, and FIN may arrive on a single segment
   (as in Figure 4), or on multiple segments; packetization makes no
   difference to the logic of the finite-state machine (FSM) defining
   transitions among connection states.

   Half-synchronized connections have several consequences.

   (a)  The passive end must provide an implied initial data window in
        order to accept data.  The minimum size of this implied window
        is a parameter in the specification; we suggest 4K bytes.

   (b)  New connection states and transitions are introduced into the
        TCP FSM at both ends of the connection.  At the active end, new
        states are required to piggy-back the FIN on the initial SYN
        segment.  At the passive end, new states are required for a
        half-synchronized connection.

   This section develops the resulting FSM description of a TCP
   connection as a conventional state/transition diagram.  To develop a
   complete FSM, we take a constructive approach, as follows: (1) write
   down all possible events; (2) write down the precedence rules that
   govern the order in which events may occur; (3) construct the
   resulting FSM; and (4) augment it to support TAO.  In principle, we
   do this separately for the active and passive ends; however, the
   symmetry of TCP results in the two FSMs being almost entirely
   coincident.

   Figure 8 lists all possible state transitions for a TCP connection in
   the absence of TAO, as elementary events and corresponding actions.
   Each transition is labeled with a letter.  Transitions a-g are used
   by the active side, and c-i are used by the passive side.  Without
   TAO, transition "c" (event "rcv ACK(SYN)") synchronizes the
   connection, allowing data to be accepted for the user.

   By definition, the first transition for an active (or passive) side
   must be "a" (or "i", respectively).  During a single instance of a
   connection, the active side will progress through some permutation of
   the complete sequence of transitions {a b c d e f } or the sequence
   {a b c d e f g}.  The set of possible permutations is determined by
   precedence rules governing the order in which transitions can occur.



Braden                                                         [Page 24]

RFC 1379              Transaction TCP -- Concepts          November 1992


          Label              Event / Action
          _____              ________________________
            a                OPEN / snd SYN

            b                rcv SYN [No TAO]/ snd ACK(SYN)

            c                rcv ACK(SYN) /

            d                CLOSE / snd FIN

            e                rcv FIN / snd ACK(FIN)

            f                rcv ACK(FIN) /

            g                timeout=2MSL / delete TCB
        ___________________________________________________
            h                passive OPEN / create TCB

            i                rcv SYN [No TAO]/ snd SYN, ACK(SYN)
        ___________________________________________________

           Figure 8.  Basic TCP Connection Transitions


   Using the notation "<." to mean "must precede", the precedence rules
   are:

   (1)  Logical ordering: must open connection before closing it:

        b <. e

   (2)  Causality -- cannot receive ACK(x) before x has been sent:

        a <. c and i <. c and d <. f

   (3)  Acknowledgments are cumulative

        c <. f

   (4)  First packet in each direction must contain a SYN.

        b <. c and b <. f

   (5)  TIME-WAIT state

        Whenever d precedes e in the sequence, g must be the last
        transition.




Braden                                                         [Page 25]

RFC 1379              Transaction TCP -- Concepts          November 1992


   Applying these rules, we can enumerate all possible permutations of
   the events and summarize them in a state transition diagram.  Figure
   9 shows the result, with boxes representing the states and directed
   arcs representing the transitions.

          ________            ________
         |        |    h     |        |
         | CLOSED |--------->| LISTEN |
         |________|          |________|
              |                   |
              | a                 | i
          ____V____           ____V___                 ________
         |        |    b     |        |      e        |        |
         |        |--------->|        |-------------->|        |
         |________|          |________|               |________|
            /                    /   |                /       |
           /                    /    | c           d /        | c
          /                    /   __V_____          |    ____V___
         /                    /   |        | e       |   |        |
      d  |                d  /    |        |------------>|        |
         |                   |    |________|         |   |________|
         |                   |       |               |         |
         |                   |       |            ___V____     |
         |                   |       |           |        |    |
         |                   |       |           |        |    |
         |                   |       |           |________|    |
         |                   |       |                   |     |
     ____V___          ______V_      |     ________      |     |
    |        |    b   |        | e   |    |        |     |     |
    |        |------->|        |--------->|        |     |     |
    |________|        |________|     |    |________|     |     |
                              |      /          |        |     |
                            c |     / d       c |      c |   d |
                              |    /            |        |     |
                             _V___V__       ____V___     V_____V_
                            |        |  e  |        |   |        |
                            |        |---->|        |   |        |
                            |________|     |________|   |________|
                                 |              |           |
                                 | f            | f         | f
                             ____V___       ____V___     ___V____
                            |        |  e  | TIME-  | g |        |
                            |        |---->|   WAIT |-->| CLOSED |
                            |________|     |________|   |________|


               Figure 9: Basic State Diagram




Braden                                                         [Page 26]

RFC 1379              Transaction TCP -- Concepts          November 1992


   Although Figure 9 gives a correct representation of the possible
   event sequences, it is not quite correct for the actions, which do
   not compose as shown.   In particular, once a control bit X has been
   sent, it must continue to be sent until ACK(X) is received.  This
   requires new transitions with modified actions, shown in the
   following list.  We use the labeling convention that transitions with
   the same event part all have the same letter, with different numbers
   of primes to indicate different actions.

          Label              Event / Action
          _____              _______________________________________
            b' (=i)          rcv SYN [No TAO] / snd SYN,ACK(SYN)
            b''              rcv SYN [No TAO] / snd SYN,FIN,ACK(SYN)
            d'               CLOSE / snd SYN,FIN
            e'               rcv FIN / snd FIN,ACK(FIN)
            e''              rcv FIN / snd SYN,FIN,ACK(FIN)


   Figure 10 shows the state diagram of Figure 9, with the modified
   transitions and with the states used by standard TCP [STD-007]
   identified. Those states that do not occur in standard TCP are
   numbered 1-5.

   Standard TCP has another implied restriction: a FIN bit cannot be
   recognized before the connection has been synchronized, i.e., c <. e.
   This eliminates from standard TCP the states 1, 2, and 5 shown in
   Figure 10.  States 3 and 4 are needed if a FIN is to be piggy-backed
   on a SYN segment (note that the states shown in Figure 1 are actually
   wrong; the states shown as SYN-SENT and ESTABLISHED are really states
   3 and 4).  In the absence of piggybacking the FIN bit, Figure 10
   reduces to the standard TCP state diagram [STD-007].

   The FSM described in Figure 10 is intended to be applied
   cumulatively; that is, parsing a single packet header may lead to
   more than one transition.  For example, the standard TCP state
   diagram includes a direct transition from SYN-SENT to ESTABLISHED:

       rcv SYN,ACK(SYN) / snd ACK(SYN).

   This is transition b followed immediately by c.











Braden