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RFC 442 - Current flow-control scheme for IMPSYS


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Network Working Group                                            V. Cerf
Request for Comments: 442                                24 January 1973
NIC: 13774

               The Current Flow-Control Scheme for IMPSYS

   BB&N quarterly report #13 outlines part of the current flow control
   scheme in the IMP operating system.  A meeting held March 16, 1972,
   at BB&N was devoted to the description of this new scheme for the
   benefit of interested network participants.

   This note represents my understanding of the flow control mechanism.
   The essential goal is to eliminate unnecessary retransmissions when
   the load is heavy, eliminate the retransmission time-out period when
   the load is light, increase bandwidth, prevent re-assembly lock-up,
   control traffic from HOSTS into the net more strictly than the
   earlier link blocking method, and secure the rights of life, liberty,
   and the pursuit of happiness for ourselves and our posterity,...oops.

Source IMP-to-Destination IMP Protocol

   There are two different protocols depending on message length (i.e.
   single or multi-packet).  We illustrate first the single packet case.

          Source Imp                        Destination Imp
          ----------                        ---------------

case 1)   message (1) + implicit req (1)--->
                                        <--- RFNM (arrived ok)
          [discard copy of msg]

case 2)   message (1) + implicit req (1)---> no room, don't respond
                                        <--- All (1)  (room available)
          message (1)                   --->
          [discard copy of msg]         <--- RFNM (arrived ok)

   In the first case, a single packet message is sent to the destination
   IMP.  This message acts as an implicit request for single packet
   buffer space.  If there is room, as in case 1, the destination IMP
   responds with a RFNM.  The source IMP, which has retained a copy of
   the message, deletes its copy and goes on.

   The second case illustrates what happens when the source IMP sends a
   message to a destination IMP at which there is no room for the one-
   packet message.  The arrival of the single packet message constitutes
   a request for single packet buffer space, and is recorded as such by
   the destination IMP in a first-come-first-served buffer reservation

   request queue.  When space is available, the destination IMP will
   transmit an ALL (1) to the requesting source IMP which can then send
   the single packet message again, this time knowing that space has
   been reserved at the destination.

   For multi-packet messages, the procedure is somewhat different.  When
   a message enters an IMP from a HOST, and the "last bit" flag is not
   set when the number of bits in a maximum length single packet have
   arrived, the IMP halts the HOST->IMP transmission line while it
   determines whether space has been reserved at the dest. IMP.  If
   space (8 packets worth) has been reserved, the HOST->IMP line is re-
   opened, and the message is sent out normally.  If space has not been
   reserved, the HOST->IMP line is kept closed while the source IMP
   makes a request for multi-packet buffer storage at the destination
   IMP.  When 8 buffers are available, the destination IMP responds with
   an ALL (8).  The source IMP then transmits the message, and waits for
   a combination RFNM and ALL (8) from the destination IMP.  The
   destination IMP will delay its RFNM, if necessary, until it has
   another 8 buffers available for the next multipacket message.

   This sequence is illustrated below:

            Source IMP                   Destination IMP
            ----------                   ---------------

H-> I line
----------> First packet of multipacket
            arrives. Halt H->I line and
            send REQ (8)  -------------->
            start 30 sec. Time-out

            If time-out, resend
            REQ (8) and restart -------->
            time-out.
                                <--------ALL (8) when available. Start
                                         long term (2 min.) time-out.
                                         On time-out, reset all
                                         outstanding reservations.

            Send the message:
                        |   ----------->
            Start 30 sec. time-out
            for INComplete transmission.
            If time-out, send INC?----->

                                  <------On recept of message, send
                                         RFNM + implicit ALL (8). On
                                         receipt of INC? send RFNM +
                                         ALL(8) if MSG(8) received,
                                         or send INC! if MSG(8) not
                                         received. Start 2 min. time-out
                                         on ALL(8).

            Queue ALL(8); start 125 ms.
            time-out when it reaches
            head of queue. If time-out
            on ALL(8), send GVB(8)----->
                                  <----- Ack.
            else send next message ----->

   A key point in this protocol is that a source IMP, after receipt of a
   RFNM and implicit ALL(8) from the destination IMP, has 125 msec. in
   which to initiate the transfer of at least the first packet of a
   multi-packet message to the destination IMP.  The source IMP may have
   several allocate responses queued up in which case these time-outs
   occur one after the other (one has to time-out before the next 125
   msec time-out starts).

   Time-outs exist in the source IMP which cause it to send INC?
   messages to the destination IMP if it has received no response from
   some earlier message.

Buffer Allocation

   A total of 40 buffers are available for store/forward and re-assembly
   purposes.  At most 32 can be allocated for re-assembly, and at most
   24-25 can be allocated for store and forward use.  This prevents
   either kind of traffic from completely shutting out the other kind.

Message Ordering (Source IMP-to-Destination IMP).

   As an aid to congestion control, an IMP can have at most 4 messages
   outstanding (un-RFNMed) for each other IMP.  Link numbers in the
   message leader are ignored by the IMPs.  Instead, IMPs mark messages
   leaving for other destinations with an 8-bit message number.  In
   addition, a 2-bit priority number is also used in case a HOST has
   marked a message as a priority message.  The key notion here is that
   the IMPs treat all HOSTs on a given IMP as if they were a single
   HOST.  A single sequence of message and priority numbers is used in
   each direction between each pair of sites.

   The receiving IMP remembers the message number of the last message
   delivered, as well as the priority number of the last priority
   message delivered.  It uses this information to correctly sequence
   messages out the IMP-HOST line (s).  Since there is only one sequence
   of numbers for each pair of sites, messages for one HOST at a site
   may get in the way of messages for another HOST at the same site.  In
   fact, if some message, m, is the next in line to go to some HOST, and
   that HOST delays receipt for 30 seconds, any messages for another
   HOST may be delayed that long also.  However, only the first message
   is lost, since the second one could not even start into its
   destination HOST until the first one had been delivered.  There is a
   tighter coupling between HOSTs sharing an IMP than before, but not
   much tighter.

   An example of the use of message and priority numbers is given below.

Order sent by           Order received by       Order received by
Source IMP              Dest. IMP               HOST
----------              ---------               ----

11,12P(1),13P(2),14 --> 13P(2),12P(1),14,11 --> 12P(1),13P(2),11,14

11,12P(1),13P(2),14 --> 13P(2),11,14,12P(1) --> 11,12P(1),13P(2),14

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).

   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.

   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.

 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]

 

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