rfc2.txt

RFC 的详细文档！

Network Working Group                                        Bill Duvall
Request for Comments: 2                                              SRI


                            [unknown title]


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1 LINKS

   1a Control Links

      1a1 Logical link 0 will be a control link between any two HOSTs on
      the network

         1a1a Only one control link may exist between any two HOSTs on
         the network. Thus, if there are n HOSTs on the network, there
         are n-1 control links from each HOST.

      1a2 It will be primarily used for communication between HOSTs for
      the purposes of:

         1a2a Establishing user links

         1a2b Breaking user links

         1a2c Passing interrupts regarding the status of links and/or
         programs using the links

         1a2d Monitor communication

      1a3 Imps in the network may automatically trace all messages sent
      on link 0.

   1b Primary Links

      1b1 A user at a given HOST may have exactly 1 primary link to each
      of the other HOSTs on the network.

         1b1a The primary link must be the first link established
         between a HOST user and another HOST.

         1b1b Primary links are global to a user, i.e. a user program
         may open a primary link, and that link remains open until it is
         specifically closed.





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         1b1c The primary link is treated like a teletype connected over
         a normal data-phone or direct line by the remote HOST, i.e. the
         remote HOST considers a primary link to be a normal teletype
         user.

         1b1d The primary link is used for passing (user) control
         information to the remote HOST, e.g. it will be used for
         logging in to the remote host (using the remote hosts standard
         login procedure).

   1c Auxilliary Links

      1c1 A user program may establish any number of auxilliary links
      between itself and a user program in a connected HOST.

         1c1a These links may be used for either binary or character
         transmission.

         1c1b Auxilliary links are local to the sub-system which
         establishes them, and therefore are closed when that subsystem
         is left.

2 MANIPULATION OF LINKS

   2a Control links

      2a1 The control link is established at system load time.

      2a2 The status of a control link may be active or inactive

         2a2a The status of the control link should reflect the
         relationship between the HOSTs.

   2b Primary Links

      2b1 Primary links are established by a user or executive call to
      the monitor

         2b1a The network identification number of the HOST to be linked
         to must be included in the call

         2b1b An attempt to establish more than one primary link to a
         particular HOST will be regarded as an error, and the request
         will be defaulted

         2b1c Standard Transmission Character Set





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            2b1c1 There will be a standard character set for
            transmission of data over the primary links and control
            links.

               2b1c1a This will be full (8 bit) ASCII.

         2b1d (get link) The protocol for establishing a link to HOST B
         from HOST A is as follows

            2b1d1 A selects a currently unused link to HOST B from its
            allocation tables

            2b1d2 A transmits a link-connect message to B over link 0.

            2b1d3 A then waits for:

               2b1d3a A communication regarding that link from B

               2b1d3b A certain amount of time to elaspse

            2b1d4 If a communication regarding the link is received from
            B, it is examined to see if it is:

               2b1d4a A verification of the link from B.

                  2b1d4a1 This results in a successful return from the
                  monitor to the requestor. The link number is returned
                  to the requestor, and the link is established.

               2b1d4b A request from B to establish the link. This
               means: that B is trying to establish the same link as A
               independently of A.

                  2b1d4b1 If the network ID number of A(Na) is greater
                  than that of B(Nb), then A ignores the request, and
                  continues to await confirmation of the link from B.

                  2b1d4b2 If, on the other hand, Na<Nb, A:

                     2b1d4b2a Honors the request from B to establish the
                     link,

                     2b1d4b2b Sends verification as required,

                     2b1d4b2c Aborts its own request, and repeats the
                     allocation process.





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               2b1d4c Some other communication from B regarding the
               link.

                  2b1d4c1 This is an error condition, meaning that
                  either:

                     2b1d4c1a A has faulted by selecting a previously
                     allocated link for allocation,

                     2b1d4c1b B is transmitting information over an un-
                     allocated link,

                     2b1d4c1c Or a message regarding allocation from B
                     to A has been garbled in transmission.

                  2b1d4c2 In this case, A's action is to:

                     2b1d4c2a Send a link disconnect message to B
                     concerning the attempted connection

                     2b1d4c2b Consider the state of HOST B to be in
                     error and initiate entry to a panic routine(error).

               2b1d5 If no communication regarding the link is received
               from B in the prescribed amount of time, HOST B is
               considered to be in an error state.

                  2b1d5a A link disconnect message is sent to B from A.

                  2b1d5b A panic routine is called(error).

   2c Auxilliary Links

      2c1 Auxilliary links are established by a call to the monitor from
      a user program.

         2c1a The request must specify pertinent data about the desired
         link to the monitor

            2c1a1 The number of the primary link to B.

         2c1b The request for an auxilliary link must be made by a user
         program in each of the HOSTs (A and B).

         2c1c If Na > Nb, then HOST A proceeds to establish a link to
         HOST B in the manner outlined above (getlink).





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         2c1d If Na<Nb, then A waits:

            2c1d1 For HOST B to establish the link (after looking to see
            if B has already established the corresponding link).

            2c1d2 For a specified amount of time to elapse.

               2c1d2a This means that HOST B did not respond to the
               request of HOST A.

               2c1d2b The program in HOST A and B should be able to
               specifiy the amount of time to wait for the timeout.

3 ERROR CHECKING

   3a All messages sent over the network will be error checked initally
   so as to help isolate software and hardware bugs.

   3b A checksum will be associated with each message, which is order
   dependent.

      3b1 The following algorithm is one which might be used:

         3b1a A checksum of length 1 may be formed by adding successive
         fields in the string to be checked serially, and adding the
         carry bit into the lowest bit position of the sum.

              +--------------+
              |              |
              +---FIELD 1----+

          ADD +--------------+
              |              |
              +----FIELD 2---+

            +-+--------------+
            | |              |
            +-+--------------+
              \
               \          +--+
          ADD   CARRY---> |  |
                          +--+

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

            3b1a1 This process is known as folding.



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