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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

Cerf [Page 1] RFC 442 The Current Flow-Control Scheme for IMPSYS January 1973

 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 -------->
                              <--------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?----->

Cerf [Page 2] RFC 442 The Current Flow-Control Scheme for IMPSYS January 1973

                                <------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.

Cerf [Page 3] RFC 442 The Current Flow-Control Scheme for IMPSYS January 1973

 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).

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

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