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+Network Working Group                                        Bill Duvall
+Request for Comments: 2                                              SRI
+
+
+                            [unknown title]
+
+
+[page 1 missing]
+
+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.
+
+
+
+
+
+Duvall                                                          [Page 1]
+
+RFC 2
+
+
+         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
+
+
+
+
+
+Duvall                                                          [Page 2]
+
+RFC 2
+
+
+            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.
+
+
+
+
+
+Duvall                                                          [Page 3]
+
+RFC 2
+
+
+               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).
+
+
+
+
+
+Duvall                                                          [Page 4]
+
+RFC 2
+
+
+         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.
+
+
+
+Duvall                                                          [Page 5]
+
+RFC 2
+
+
+            3b1a2 Several fields may be added and folded in parallel, if
+            they are folded appropiately after the addition.
+
+              +---------+---------+---------+---------+
+              | FIELD 4 | FIELD 3 | FIELD 2 | FIELD 1 |
+              +---------+---------+---------+---------+
+
+              +---------+---------+---------+---------+
+              | FIELD 8 | FIELD 7 | FIELD 6 | FIELD 5 |
+              +---------+---------+---------+---------+
+            ADD
+            +-+---------+---------+---------+---------+
+            | |         |         |         |         |
+            +-+---------+---------+---------+---------+
+             |      |        |         |         |
+             |      |        |         |         v
+             |      |        |         |    +---------+
+             |      |        |         |    |         |
+             |      |        |         |    +---------+
+             |      |        |         |
+             |      |        |         \    +---------+
+             |      |        |          `-> |         |
+             |      |        |              +---------+
+             |      |        |
+             |      |        \              +---------+
+             |      |         `-----------> |         |
+             |      |                       +---------+
+             |      |
+             |      \                       +---------+
+             |       `--------------------->|         |
+             |                              +---------+
+             |
+             \                                    +---+
+              `-----CARRY------------------------>|   |
+                                                  +---+
+                                  ADD
+                                  +---------+---------+
+                                  |         |         |
+                                  +--CARRY--+---------+
+                                       |
+                                       \        +-----+
+                                    ADD `-----> |     |
+                                                +-----+
+
+                                            +---------+
+                                            |         |
+                                            +-RESULT--+
+
+
+
+
+Duvall                                                          [Page 6]
+
+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.
+
+
+
+Duvall                                                          [Page 7]
+
+RFC 2
+
+
+         4a1c (trans)character translation
+
+            4a1c1 There is a strong possibility that the character
+            translation may be done in the IMP.
+
+            4a1c2 This needs to be explored further with BBN.
+
+            4a1c3 There are two main considerations
+
+               4a1c3a Should the translation be done by table or
+               algorithm?
+
+                  4a1c3a1 Initially it seems as though the best way to
+                  go is table.
+
+               4a1c3b How should we decide which messages should be
+               translated, i.e. is it desirable to not translate
+               everything (YES!!) and by what means can we use to
+               differentiate?
+
+         4a1d Decode header, and pass message to correct recipient as
+         identified by source, and link.
+
+      4a2 Output
+
+         4a2a Build header
+
+         4a2b Character translation
+
+            4a2b1 See remarks under the section on output translation
+            (trans).
+
+         4a2c Create checksum
+
+         4a2d Check status of link
+
+            4a2d1 If there has not been a RFNM since the last message
+            transmitted out the link, wait for it.
+
+         4a2e Transmit message to IMP
+
+         4a2f If aknowledge mode is in effect,wait for
+
+            4a2f1 RFNM from destination IMP.
+
+            4a2f2 Response from destination HOST over control line 0.
+
+
+
+
+
+Duvall                                                          [Page 8]
+
+RFC 2
+
+
+   4b Network status
+
+      4b1 Maintain status of other HOSTs on network
+
+         4b1a If an IMP is down, then his HOST is considered to be down.
+
+      4b2 Maintain status of control lines.
+
+      4b3 Answer status queries from other HOSTs.
+
+      4b4 Inform other HOSTs as to status of primary and auxilliary
+      links on an interrupt basis.
+
+      4b5 Inform other HOSTs as to status of programs using primary and
+      secondary links
+
+5 EXECUTIVE PRIMITIVES
+
+   5a Primary Links
+
+      5a1 These require the HOST number as a parameter.
+
+         5a1a Establish primary link
+
+         5a1b Connect controlling teletype to primary link
+
+         5a1c INPUT/OUTPUT over primary link
+
+         5a1d Interrogate status of primary link
+
+            5a1d1 Don't know what, exactly, this should do, but it seems
+            as though it might be useful.
+
+         5a1e Disconnect controlling teletype from primary link
+
+         5a1f Kill primary link
+
+   5b Auxilliary Links.
+
+      5b1 Establish auxilliary link.
+
+         5b1a requires the HOST number as a parameter
+
+         5b1b It returns a logical link number which is similar to a
+         file index. It is this number which is passed to all of the
+         other Auxilliary routines as a parameter.
+
+      5b2 INPUT/OUTPUT over auxilliary link
+
+
+
+Duvall                                                          [Page 9]
+
+RFC 2
+
+
+      5b3 Interrogate status auxilliary link.
+
+         5b3a Don't know what, exactly, this should do, but it seems as
+         though it might be useful.
+
+      5b4 Kill auxilliary link.
+
+   5c Special executive functions
+
+      5c1 Transparent. INPUT/OUTPUT over link
+
+         5c1a This may be used to do block I/O transfers over a link
+
+         5c1b The function of the monitor in this instance is to
+         transfer a buffer directly to its IMP
+
+         5c1c At does not modify it in any way
+
+            5c1c1 This means that the header and other control
+            information must be in the buffer.
+
+         5c1d The indended use of this is for network debugging.
+
+6 INITIAL CHECKOUT
+
+   6a The network will be initially checked out using the links in a
+   simulated data-phone mode.
+
+      6a1 All messages will be one character in length.
+
+      6a2 Links will be transparent to the monitor, and controlled by
+      user program via a special executive primitive.
+
+         6a2a The initial test will be run from two user programs in
+         different HOSTs, e.g. DDT to DDT.
+
+         6a2b It will be paralleled by a telephone link or similar.
+
+
+           [  This RFC was put into machine readable form for entry  ]
+           [  into the online RFC archives by Robbie Bennet 10/1998  ]
+           [  This RFC was nroffed by Kelly Tardif, Viagenie 10/1999 ]
+
+
+
+
+
+
+
+
+
+Duvall                                                         [Page 10]
+