|      |        |         |         |
             |      |        |         |         v
             |      |        |         |    +---------+
             |      |        |         |    |         |
             |      |        |         |    +---------+
             |      |        |         |
             |      |        |         \    +---------+
             |      |        |          `-> |         |
             |      |        |              +---------+
             |      |        |
             |      |        \              +---------+
             |      |         `-----------> |         |
             |      |                       +---------+
             |      |
             |      \                       +---------+
             |       `--------------------->|         |
             |                              +---------+
             |
             \                                    +---+
              `-----CARRY------------------------>|   |
                                                  +---+
                                  ADD
                                  +---------+---------+
                                  |         |         |
                                  +--CARRY--+---------+
                                       |
                                       \        +-----+
                                    ADD `-----> |     |
                                                +-----+

                                            +---------+
                                            |         |
                                            +-RESULT--+





 
RFC 2


                  3b1a2a Using this scheme, it is assumed that, if there
                  are n fields, the carries from the first n-1 fields
                  are automatically added into the low order position of
                  the next higher field, so that in folding, one need
                  only add the [n] result fields to the carry from the
                  nth field, and then add in an appropiately sized carry
                  from that addition (and repeat the desired number of
                  times to achieve the result.

            3b1a3 A checksum computed in this manner has the advantage
            that the word lengths of different machines may each be used
            optimally:

               3b1a3a If a string of suitable length is chosen for
               computing the checksum, and a suitable checksum field
               length is selected, the checksum technique for each of
               the machines will be relatively optimal.

                  3b1a3a1 Field length: 288 bits (lowest common
                  denomenator of (24,32,36)

                  3b1a3a2 Checksum length: 8 bits (convenient field size
                  for all machines)

         3b1b If a message is divided into groups of fields, and each
         group is checksummed in this manner, an order dependent
         checksum may be got by shifting the checksum for each group,
         and adding it in (successively) to the checksum of the next
         group

   3c A facility will be provided where two HOSTs may enter a mode which
   requires positive verification of all messages. This verification is
   sent over the control link.

4 MONITOR FUNCTIONS

   4a Network I/O drivers

      4a1 Input

         4a1a Input message from IMP.

         4a1b Do error checking on message.

            4a1b1 Verify checksum,

            4a1b2 Send "message recieved" aknowledgement over control
            link if aknowledge mode is in effect.




 
RFC 2


         4a1c (trans)character translation