IT. Expert System.

 Network Working Group                                        A. Shoshani
Request for Comments: 161                                            SDC
NIC #6772                                                    19 May 1971
              A SOLUTION TO THE RACE CONDITION IN THE ICP
   In NWG/RFC #143 a race condition in the ICP was described and a
   solution was suggested.  The problem arises because the Host-Host
   protocol does not specify what the NCP should do when it gets more
   than one request of STR (or RTS) to the same socket.  As a result
   this decision depends on the particular implementation: some may
   queue these requests (SDC for example), some will refuse a request if
   the socket is already connected (UCLA for example), etc.
   The solution is not to change the Host-Host protocol, but find a
   third level ICP which does not depend on this issue.  Such a solution
   is the following: the INITs from server to user and user to server
   ((S5), (S6), (U5), (U6) on page 3 in RFC #143) should use another
   socket -- say U+2 and U+3.  The sequences in RFC #143 would be:
      Server                             User
      ------                             ----
      (S1) LISTEN(L,32)                  (U1) INIT(U,L,32)
      (S2) [wait for match]              (U2)
      (S3) SEND(L,S)                     (U3) RECEIVE(U,S)
      (S4) CLOSE(L)                      (U4) CLOSE(U)
      (S5) INIT(S,U+3,Bu)                (U5) INIT(U+3,S,Bu)
      (S6) INIT(S+1,U+2,Bs)              (U6) INIT(U+2,S+1,Bs)
This solution will solve the problems pointed out in RFC #143 without
any assumptions made about the NCP implementation.  The solution in RFC
#143 assumes that the NCP can notify a process when a command (e.g.,
close) comes in, which is implementation dependent.
       [ This RFC was put into machine readable form for entry ]
          [ into the online RFC archives by Alan Ford 08/99]
Shoshani                                                        [Page 1]