Possible Deadlock in ICP (RFC0202)

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

S.M. Wolfe: AUTHOR [+1][-]

J. Postel: AUTHOR

Abstract

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.

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:

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 ]