rfc442.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

where 13P(2) is interpreted to mean message #13, priority number(2).

   Note that there are only 2 classes of messages, priority and non-
   priority, and that the priority numbers simply allow ordering at the
   destination of multiple outstanding priority transmissions from the
   same site.

   If HOSTs use link numbers to de-multiplex messages to processes, then
   it would be a mistake to arbitrarily assign short messages priority.
   If a file transmission were carried out such that the last short
   message had priority, the file might not enter the receiving HOST in
   the same order it was sent!

ACK Mechanism

   IMPs treat their physical channels (phone lines) as if they were
   pairs of simplex communications paths.  Each IMPSYS has a sender and
   receiver module for each full duplex channel.  Each module has an
   "ODD/EVEN" bit which is used to keep track of the state of the last
   packet on the line.  The object is for the sender module to "block" a
   channel until the corresponding receiver has received a packet
   indicating that the send packet was received on the other end (i.e.
   an acknowledgment).



Cerf                                                            [Page 4]

RFC 442        The Current Flow-Control Scheme for IMPSYS   January 1973


   In the present system, acknowledgments are separate IMP-IMP packets.
   In the new system, they are a single bit in a packet flowing in the
   opposite direction on the reverse path of a full duplex channel.

   Every packet sent between IMPs has an ACK bit and an OE bit, as shown
   below.


                         P                              A
                          O                              C
                           E                              K
               +-------+-----+------------------------+-----+----------+
typical packet |       |     |                        |     |          |
               |       |     |                        |     |          |
               +-------+-----+------------------------+-----+----------+

   We need some terminology: Let POE be the packet OE bit, and SOE, ROE
   be the send module OE bit and Receive module OE bit respectively.
   For two IMPs, A and B, we distinguish SOE/A and SOE/B as the two send
   module OE bits at IMPs A and B respectively.

   The rules of operation are as follow:

   Sender
   ------
   if ACK != SOE then do nothing
   --
   else SOE <- !SOE (i.e. flip SOE bit) and free channel.
   ----

   Receiver
   --------
   if POE = ROE then packet is a duplicate so throw it away.
   --
   else ROE <- !ROE
   ----

   Whenever a packet is sent by the sent module, its two bits, POE and
   ACK are set up by:

                        POE <- SOE
                        ACK <- ROE

   The mechanism is designed to use real traffic to accomplish the
   acknowledgment protocol by piggy-backing the ACK bits in the header
   of real packets.  If there is no real packet waiting for transmission
   in the opposite direction, a fake packet is assembled which carries
   the ACK, but which is not acknowledged by the receiving side.



Cerf                                                            [Page 5]

RFC 442        The Current Flow-Control Scheme for IMPSYS   January 1973


   We give an example of the operation of this mechanism between two
   IMPs.

                     IMP A                           IMP B
                     -----                           -----
                   ROE | SOE                       ROE | SOE
                       |           POE   ACK           |
                       |         +-----------+         |
IMP A blocks send    1 | 0    (1)|  0      1 |->     1 | 0 IMP B NOPS,
channel.               |         +-----------+         |   flips ROE
                       |                               |
                       |           POE   ACK           |
                       |         +-----------+         |
IMP A frees send     0 | 1     <-|  0      0 |(2)    0 | 0 IMP B blocks
channel,               |         +-----------+         |   channel for
Flips SOE              |                               |   new traffic
                       |           POE   ACK           |
IMP A blocks send      |         +-----------+  crashes|
channel                |      (3)|  1      0 |->or gets|
                       |         +-----------+  lost   |
                       |                               |
                       |           POE   ACK           |
IMP A detects packet   |         +-----------+         |
duplicate (POE=ROE)  0 | 1     <-|  0      0 |(2)    0 | 0 IMP B
so does not change     |         +-----------+         |  retransmits no
SOE bit.               |                               |  ACK received
                       |           POE   ACK           |
IMP A retransmits      |         +-----------+         |   IMP B flips
packet 3               |      (3)|  1      0 |->     1 | 1 SOE, unblocks
                       |         +-----------+         |   channel, and
                       |                               |   flips ROE.
                       |           POE   ACK           |
IMP A flips ROE,       |         +-----------+         |
      flips SOE      1 | 0     <-|  1      1 |(4)      |
                       |         +-----------+         |
                       |                               |

   In fact each send/receive module has 8 OE bits, so up to 8 packets
   can be outstanding in either direction.

How things really work

   Actually, a single send module is responsible for trying to transmit
   packets out on the 8 pseudo-channels.  Each channel has a two-bit
   state (in addition to an OE bit).  Each channel is either FREE or IN
   USE and if IN USE, it may be sending OLD or NEW packet.





Cerf                                                            [Page 6]

RFC 442        The Current Flow-Control Scheme for IMPSYS   January 1973


 start state                                      F = free
        |                                         I = in use
        V                                         X = don_t care
       +-----+                 +------+           N = new packet
       |  FX | --------------> | I, N |           O = old packet
       +-----+                 +------+
          ^                       |
          |                       |
          |                       |
          |                       |
   ACK    |                       |
 received |                       |
          |                       V
          |                   +------+
          +-------------------| I, O |---+
                              +------+   |
                                  ^      | re-transmissions
                                  +------+

   Between IMPs, packets are sent repeatedly, until they are
   acknowledged.  However, the choice of what to send is ordered by
   priority as follows:

      1. Priority Packets (as marked by HOST)

      2. Non-Priority Packet

      3. Unacknowledged packets (on I,O state channels)

      4. Others

   It was pointed out that a heavy load of type (1) and (2) traffic
   might prevent retransmissions from occurring at all, and W. Crowther
   responded that the bug would be fixed by a 125 ms time-out which
   forces retransmission of old packets in class (3).

   Note that each packet must carry a "pseudo-channel" number to
   identify the POE-to-channel association, and 8 ACK bits (which are
   positionally associated with the pseudo-channels).  Thus a single
   packet can ACK up to 8 packets at once.




          [This RFC was put into machine readable form for entry]
     [into the online RFC archives by Helene Morin, Via Genie, 12/99]





Cerf                                                            [Page 7]