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.

   (4)  providing an initial server window.

   A recently proposed set of enhancements [RFC-1323] defines a TCP
   Timestamps option that carries two 32-bit timestamp values.  The
   Timestamps option is used to accurately measure round-trip time
   (RTT).  The same option is also used in a procedure known as "PAWS"
   (Protect Againsts Wrapped Sequence) to prevent erroneous data
   delivery due to a combination of old duplicate segments and sequence
   number reuse at very high bandwidths.  The particular approach to
   transactions chosen in this memo does not require the RFC-1323
   enhancements; however, they are important and should be implemented
   in every TCP, with or without the transaction extensions described
   here.

3.  BYPASSING THE 3-WAY HANDSHAKE

   To avoid 3-way handshakes for transactions, we introduce a new
   mechanism for validating initial SYN segments, i.e., for enforcing
   at-most-once semantics without a 3-way handshake.  We refer to this
   as the TCP Accelerated Open, or TAO, mechanism.

   3.1 Concept of TAO

      The basis of TAO is this: a TCP uses cached per-host information
      to immediately validate new SYNs [Clark89].  If this validation
      fails, e.g., because there is no current cached state or the
      segment is an old duplicate, the procedure falls back to a normal
      3-way handshake to validate the SYN.  Thus, bypassing a 3-way
      handshake is considered to be an optional optimization.




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RFC 1379              Transaction TCP -- Concepts          November 1992


      The proposed TAO mechanism uses a finite sequence-like space of
      values that increase monotonically with successive transactions
      (connections) between a given (client, server) host pair.  Call
      this monotonic space M, and let each initial SYN segment carry an
      M value SEG.M.  If M is not the existing sequence (SEG.SEQ) field,
      SEG.M may be carried in a TCP option.

      When host B receives from host A an initial SYN segment containing
      a new value SEG.M, host B compares this against cache.M[A], the
      latest M value that B has cached for host A.  This comparison is
      the "TAO test".  Because the M values are monotonically
      increasing, SEG.M > cache.M[A] implies that the SYN must be new
      and can be accepted immediately.  If not, a normal 3-way handshake
      is performed to validate the initial SYN segment.  Figure 2
      illustrates the TAO mechanism; cached M values are shown enclosed
      in square brackets.  The M values generated by host A satisfy
      x0 < x1, and the M values generated by host B satisfy y0 < y1.

      An appropriate choice for the M value space is discussed in
      Section 5.  M values are drawn from a finite number space, so
      inequalities must be defined in the usual way for sequence numbers
      [STD-007].  The M space must not wrap so quickly that an old
      duplicate SYN will be erroneously accepted.  We assume that some
      maximum segment lifetime (MSL) is enforced by the IP layer.

        ____T_C_P__A_____                                ____T_C_P__B_____

            cache.M[B]                                  cache.M[A]
               V                                            V

            [ y0 ]                                       [ x0 ]

      1.             -->  <SYN,data1,M=x1> -->       ( (x1 > x0) =>
                                                      data1 -> user_B;
                                                      cache.M[A]= x1)

            [ y0 ]                                       [ x1 ]
      2.            <-- <SYN,ACK(data1),data2,M=y1> <--

         (data2 -> user_A,
          cache.M[B]= y1)

            [ y1 ]                                       [ x1 ]
                              ... (etc.) ...


                   Figure 2. TAO: Three-Way Handshake is Bypassed




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RFC 1379              Transaction TCP -- Concepts          November 1992


      Figure 2 shows the simplest case: each side has cached the latest
      M value of the other, and the SEG.M value in the client's SYN
      segment is greater than the value in the cache at the server host.
      As a result, B can accept the client A's request data1 immediately
      and pass it to the server application.  B's reply data2 is shown
      piggybacked on the <SYN,ACK> segment.  As a result of this 2-way
      exchange, the cached M values are updated at both sites; the
      client side becomes relevant only if the client/server roles
      reverse.  Validation of the <SYN,ACK> segment at host A is
      discussed later.

      Figure 3 shows the TAO test failing but the consequent 3-way
      handshake succeeding.  B updates its cache with the value x2 >= x1
      when the initial SYN is known to be valid.


           _T_C_P__A                                     _T_C_P__B

            cache.M[B]                                  cache.M[A]
               V                                           V

            [ y0 ]                                       [ x0 ]
      1.                 --> <SYN,data1,M=x1> -->   ( (x1 <= x0) =>
                                                    data1 queued;
                                                    3-way handshake)

            [ y0 ]                                       [ x0 ]
      2.                <-- <SYN,ACK(SYN),M=y1> <--
         (cache.M[B]= y1)

            [ y1 ]                                       [ x0 ]
      3.                  --> <ACK(SYN),M=x2> -->  (Handshake OK =>
                                                   data1->user_B,
                                                   cache.M[A]= x2)

            [ y1 ]                                       [ x2 ]
                            ...  (etc.)  ...

          Figure 3. TAO Test Fails but 3-Way Handshake Succeeds.

      There are several possible causes for a TAO test failure on a
      legitimate new SYN segment (not an old duplicate).

      (1)  There may be no cached M value for this particular client
           host.

      (2)  The SYN may be the one of a set of nearly-simultaneous SYNs
           for different connections but from the same host, which



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RFC 1379              Transaction TCP -- Concepts          November 1992


           arrived out of order.

      (3)  The finite M space may have wrapped around between successive
           transactions from the same client.

      (4)  The M values may advance too slowly for closely-spaced
           transactions.

      None of these TAO failures will cause a lockout, because the
      resulting 3-way handshake will succeed.  Note that the first
      transaction between a given host pair will always require a 3-way
      handshake; subsequent transactions can take advantage of TAO.

      The per-host cache required by TAO is highly desirable for other
      reasons, e.g., to retain the measured round trip time and MTU for
      a given remote host.  Furthermore, a host should already have a
      per-host routing cache [HR-COMM] that should be easily extensible
      for this purpose.

      Figure 4 illustrates a complete TCP transaction sequence using the
      TAO mechanism.  Bypassing the 3-way handshake leads to new
      connection states; Figure 4 shows three of them, "SYN-SENT*",
      "CLOSE-WAIT*", and "LAST-ACK*".  Explanation of these states is
      deferred to Section 6.


          TCP A  (Client)                                 TCP B (Server)
          _______________                                 ______________

          CLOSED                                                  LISTEN

      1.  SYN-SENT*    --> <SYN,data1,FIN,M=x1> -->          CLOSE-WAIT*
                                                         (TAO test OK=>
                                                          data1->user_B)

                   <-- <SYN,ACK(FIN),data2,FIN,M=y1> <--       LAST-ACK*
      2.  TIME-WAIT
       (data2->user_A)


      3.  TIME-WAIT          --> <ACK(FIN),M=x2> -->              CLOSED

          (timeout)
            CLOSED


               Figure 4: Minimal Transaction Sequence Using TAO




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RFC 1379              Transaction TCP -- Concepts          November 1992


   3.2 Cache Initialization

      The first connection between hosts A and B will find no cached
      state at one or both ends, so both M caches must be initialized.
      This requires that the first transaction carry a specially marked
      SEG.M value, which we call SEG.M.NEW.  Receiving a SEG.M.NEW value
      in an initial SYN segment, B will cache this value and send its
      own M back to initialize A's cache.  When a host crashes and
      restarts, all its cached M values cache.M[*] must be invalidated
      in order to force a re-synchronization of the caches at both ends.

      This cache synchronization procedure is illustrated in Figure 5,
      where client host A has crashed and restarted with its cache
      entries undefined, as indicated by "??".  Since cache.TS[B] is
      undefined, A sends a SEG.M.NEW value instead of SEG.M in the <SYN>
      segment of its first transaction request to B.  Receiving this
      SEG.M.NEW, the server host B invalidates cache.TS[A] and performs
      a 3-way handshake.  SEG.M in segment #2 updates A's cache, and
      when the handshake completes successfully, B updates its cached M
      value to x2 >= x1.


           _T_C_P__A                                     _T_C_P__B

            cache.M[B]                                  cache.M[A]
               V                                           V
            [ ?? ]                                       [ x0 ]

      1.           --> <SYN,data1,M.NEW=x1> -->   (invalidate cache;
                                                        queue data1;
            [ ?? ]                                  3-way handshake)

                                                         [ ?? ]
      2.              <-- <SYN,ACK(SYN),M=y1> <--
         (cache.M[B]= y1)

            [ y1 ]                                       [ ?? ]

      3.                  --> <ACK(SYN),M=x2> -->  data1->user_B,
                                                   cache.M[A]= x2)

            [ y1 ]                                       [ x2 ]
                            ...  (etc.)  ...

                  Figure 5.  Client Host Crashed


      Suppose that the 3-way handshake failed, presumably because



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RFC 1379              Transaction TCP -- Concepts          November 1992


      segment #1 was an old duplicate.  Then segment #3 from host A
      would be an RST segment, with the result that both side's caches
      would be left undefined.

      Figure 6 shows the procedure when the server crashes and restarts.
      Upon receiving a <SYN> segment from a host for which it has no
      cached M value, B initiates a 3-way handshake to validate the
      request and sends its own M value to A.  Again the result is to
      update cached M values on both sides.


              _T_C_P__A                                     _T_C_P__B

               cache.M[B]                                  cache.M[A]
                  V                                           V
               [ y0 ]                                       [ ?? ]

         1.               --> <SYN,data1,M=x1> -->      (data1 queued;
                                                       3-way handshake)

               [ y0 ]                                       [ ?? ]
         2.              <-- <SYN,ACK(SYN),M=y1> <--
            (cache.M[B]= y1)

               [ y1 ]                                       [ ?? ]
         3.                --> <ACK(SYN),M=x2> -->   (data1->user_B,
                                                      cache.M[A]= x2)

               [ y1 ]                                       [ x2 ]
                               ...  (etc.)  ...


                        Figure 6. Server Host Crashed


   3.3  Accepting <SYN,ACK> Segments

      Transactions introduce a new hazard of erroneously accepting an
      old duplicate <SYN,ACK> segment.  To be acceptable, a <SYN,ACK>
      segment must arrive in SYN-SENT state, and its ACK field must