Network Working Group                                        Steve Wolfe
Request for Comments:  #202                                     UCLA-CCN
NIC #7155                                                     Jon Postel
Categories:  D                                                  UCLA-NMC
References:  Document #2                                    26 July 1971
Obsoletes:  None

We have noticed a possible deadlock situation which may arise using the
Initial Connection Protocol (ICP) specified in Document #2 (NIC #7101 in
the Current Network Protocols Notebook NIC #7104).

If on both sides one RFC is issued and a "wait for match" is required
before the second RFC is issued, it is possible that the first RFC's
will not match.  In particular a deadlock will occur if both sides open
their send or both sides open their receive sockets first.

Briefly the ICP is:
<where the original uses a script lowercase letter with a single digit
subscript we use the lower case letter followed by {digit} so that
script-m-subscript-2 is printed m{2}>

Server                                User
------                                ----

S1:  Listen on socket L.              U1:  RTS(U, L, l{1})

S2:  Wait for a match.                U2:  Wait for match.

S3:  STR(L, U, s{1})

S4:  Wait for allocation.             U3:  All(l{1}, m{1}, b{1})

S5:  Send data S in s{1} bit          U4:  Receive data S in s{1}
     bytes as allowed by                   bit bytes.
     allocation m{1}, b{1}.

S6:  CLS(L, U)                        U5:  CLS(U, L)

S7:  RTS(S, U+3, l{2})                U6:  STR(U+3, S, s{2})

S8:  STR(S+1, U+2, s{3})              U7:  RTS(U+2, S+1, l{3})

"The labels here imply no ordering except that ordering required by the
Host-Host Protocol.  Note that steps S7 and S8 can be reversed as can U6
and U7.  Also, notice that at any time after S2 the server could
initiate steps S7 and S8 in parallel with steps S3 through S6, and that
at any time after U4 the user could initiate steps U6 and U7 in parallel
with step U5."

We recommend that the server perform steps 7 and 8 before waiting for
the user to perform step 6 or 7.  It is also suggested that the user
issue the RFC's in steps 6 and 7 without waiting for the server.  (If
the user is only Listening then both Listens should be issued without
waiting for the server.)  If for some reason a host must delay between
issuing RFC's it must issue the RFC's involving sockets S and U+3 first.

       [ This RFC was put into machine readable form for entry ]
         [ into the online RFC archives by Robert Barnes 6/97 ]