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Network Working Group A.Shoshani, SDC Request for Comment # 197 E. Harslem, Rand NIC # 7142 14 July 1971 Categories: C.3, D.1 Updates: None Obsoletes: None



   At the Network meeting preceding the SJCC '71, an

"ICP Committee" was established. It's purpose was to get "something" working fast with minimum modifications to the current ICP so as to minimize complaints. (This seems like a good definition for almost everything!) Consequently, those who had objections to the current ICP were interviewed and a compromise was reached in the form of RFC #165. The ICP committee didn't have a chance to think about an alter- native because of the above mentioned constraints. In this note we attempted a simple version of an ICP assuming that we can add commands to Host-Host protocol. We hope that this will be useful in the design of the next version of the Host-Host protocol.


   To establish a regular connection one party can issue

an INIT (NCP sends RTS or STR commands), then the other party can accept the request for connection by responding with an INIT or refusing it with a CLOSE. We think that

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a similar, simple mechanism is desirable for the ICP. Furthermore, the ICP should allow for simplex as well as duplex connections from user to server.

   The following commands are necessary for simplex


   ISC - Initiate Simplex Connection
   ASC - Accept Simplex Connection
   RSC - Refuse Simplex Connection
   The notation for parameters is similar to that

of RFC #165:

   L - Server socket name, in one special case the
       server is "logger".
   U - User socket.
   S - Socket assigned by server for the connection
       with user.
   X - Is the byte size if U is odd and is the link
       number if U is even.
  1. -

X - Is the complement of X (X is the link number

       if U is odd and byte size if U is even.
   To initiate a simplex connection the user's NCP


         ISC, L, U, X
   To refuse this connection the server's NCP issues:
         RSC, L, U
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   To accept this connection the server's NCP issues:
         ASC, L, U, S, X
   Similarly, for duplex connections, we have:
         IDC, L, U1, X1, U2, X2
         RDC, L, U1, U2
                         -           -
         ADC, L, U1, S1, X1, U2, S2, X2

where (U1,U2), (S1,S2), (U1,S1) and (U2,S2) are pairs of opposite gender.

   After the server accepts the connection(s), it (they)

goes immediately to a "connected state", and the appropriate ALL command(s) must be sent.


   The main advantage to this approach is that it mini-

mizes the dialog between user and server. The server socket L is used only as an address, not a socket to connect to, therefore eliminating the need to establish a connection to L, choose a byte size, send an ALL command, send and receive data on it and CLOSE it. Race conditions as mentioned in RFC #143 do not arise. Socket L is the server and should be in a "Listening for ICP" state when an ISC or IDC is received. If socket L is not in that state, the serving NCP should refuse to ICP request. The serving NCP should _not_ queue ICP requests.

   In the current ICP, when the user choses socket U, he

has to reserve sockets U+2 and U+3. In the above described approach no restrictions exist for U1 and U2 (except that they are of opposite gender); the same is true for S1 and S2.

   We think that duplex commands are necessary since both

connections are to be connected to the same server process. Their separation by using two ISCs, will add complications of correlating the two ISCs with the same process. Also, if two ISCs are used, the first might be accepted and the second refused. This leads to uncertainty on the user's part. This condition cannot occur with the duplex commands.

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   The minimum change we can think of to make the current

ICP look similar to the above is to add one NCP level com- mand – accept:

         ACC, L, U, S
   The exchange between NCPs in the notation of RFC #165

is now: <where the original uses a script lowercase "L" we use "l"> <where the original uses subscripts we use {} so that

 A-subscript-B is printed A{B} >
   Server NCP                   User NCP
   ----------                   --------
   Listen for connection on L   RST,U,L,l{A}
   ACC,L,U,S                    S is passed by NCP to the
                                user and connection from
                                U to L is closed.
   STR,S+1,U+2,B{s}             STR,U+3,S,B{u}
   RTS,S,U+3,l{B}               RTS,U+2,S+1,l{c}
   Wait for connection          Wait for connection
   ALL,l{B},m{B},b{B}           ALL,l{c},m{c},b{c}
   An alternative way to the ACC command is a CLS command

with an additional parameter (32 bits long). If parameter is zero the request for connection by the user is refused; if the parameter is non-zero, the request is accepted and socket S is the value of the parameter.

   All suggested changes improve the ICP dialog both from

the aesthetic and efficiency points of view. We lean strongly, however, to the first, more major ICP modification.

                                                              [Page 4]


   It seems appropriate to mention here for the purpose

of the next version of the Host-Host protocol that the CLS command has more than one function. We think that the CLS command should be reserved to close connections in the "connected state" only (i.e., "open" connections). Two additional commands can be used for "refusing" and "reject- ing" requests for connections:

         REJ<mysocket><yoursocket> -- when a request
         for connection is rejected unconditionally.
         REF<mysocket><yoursocket><reason> -- when a
         request for connection is refused temporarily
         because the NCP could not handle it.  For
         example:  no process LISTENed to it and it
         was timed-out, or NCP tables are full in which
         case the user process may try again.  The
         reason for refusing is indicated in the
         parameter "reason".

[ This RFC was put into machine readable form for entry ] [ into the online RFC archives by BBN Corp. under the ] [ direction of Alex McKenzie. 12/96 ]

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