Network Working Group Deepinder P. Sidhu
Request for Comments: 964 Thomas P. Blumer
SDC - A Burroughs Company
November 1985
SOME PROBLEMS WITH THE SPECIFICATION OF THE
MILITARY STANDARD TRANSMISSION CONTROL PROTOCOL
STATUS OF THIS MEMO
The purpose of this RFC is to provide helpful information on the
Military Standard Transmission Control Protocol (MIL-STD-1778) so
that one can obtain a reliable implementation of this protocol
standard. Distribution of this note is unlimited.
Reprinted from: Proc. Protocol Specification, Testing and
Verification IV, (ed.) Y. Yemini, et al, North-Holland (1984).
ABSTRACT
This note points out three errors with the specification of the
Military Standard Transmission Control Protocol (MIL-STD-1778, dated
August 1983 [MILS83]). These results are based on an initial
investigation of this protocol standard. The first problem is that
data accompanying a SYN can not be accepted because of errors in the
acceptance policy. The second problem is that no retransmission
timer is set for a SYN packet, and therefore the SYN will not be
retransmitted if it is lost. The third problem is that when the
connection has been established, neither entity takes the proper
steps to accept incoming data. This note also proposes solutions to
these problems.
1. Introduction
In recent years, much progress has been made in creating an
integrated set of tools for developing reliable communication
protocols. These tools provide assistance in the specification,
verification, implementation and testing of protocols. Several
protocols have been analyzed and developed using such tools.
In a recent paper, the authors discussed the verification of the
connection management of NBS class 4 transport protocol (TP4). The
verification was carried out with the help of a software tool we
developed [BLUT82] [BLUT83] [SIDD83]. In spite of the very precise
specification of this protocol, our analysis discovered several
errors in the current specification of NBS TP4. These errors are
incompleteness errors in the specification, that is, states where
there is no transition for the reception of some input event. Our
analysis did not find deadlocks, livelocks or any other problem in
the connection management of TP4. In that paper, we proposed
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Some Problems with MIL-STD TCP
solutions for all errors except for errors associated with 2 states
whose satisfactory resolution may require redesigning parts of TP4.
Modifications to TP4 specification are currently underway to solve
the remaining incompleteness problems with 2 states. It is important
to emphasize that we did not find any obvious error in the NBS
specification of TP4.
The authors are currently working on the verification of connection
management of the Military Standard Transmission Control Protocol
(TCP). This analysis will be based on the published specification
[MILS83] of TCP dated 12 August 1983.
While studying the MIL standard TCP specification in preparation for
our analysis of the connection management features, we have noticed
several errors in the specification. As a consequence of these
errors, the Transmission Control Protocol (as specified in [MILS83])
will not permit data to be received by TCP entities in SYN_RECVD and
ESTAB states.
The proof of this statement follows from the specification of the
three-way handshake mechanism of TCP [MILS83] and from a decision
table associated with ESTAB state.
2. Transmission Control Protocol
The Transmission Control Protocol (TCP) is a transport level
connection-oriented protocol in the DoD protocol hierarchy for use in
packet-switched and other networks. Its most important services are
reliable transfer and ordered delivery of data over full-duplex and
flow-controlled virtual connections. TCP is designed to operate
successfully over channels that are inherently unreliable, i.e., they
can lose, damage, duplicate, and reorder packets.
TCP is based, in part, on a protocol discussed by Cerf and Kahn
[CERV74]. Over the years, DARPA has supported specifications of
several versions of this protocol, the last one appeared in [POSJ81].
Some issues in the connection management of this protocol are
discussed in [SUNC78].
A few years ago, DCA decided to standardize TCP for use in DoD
networks and supported formal specification of this protocol
following the design of this protocol discussed in [POSJ81]. A
detailed specification of this protocol given in [MILS83] has been
adopted as the DoD standard for the Transmission Control Protocol, a
reliable connection-oriented transport protocol for DoD networks.
A TCP connection progresses through three phases: opening (or
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synchronization), maintenance, and closing. In this note we consider
data transfer in the opening and maintenance phases of the
connection.
3. Problems with MIL Standard TCP
One basic feature of TCP is the three-way handshake which is used to
set up a properly synchronized connection between two remote TCP
entities. This mechanism is incorrectly specified in the current
specification of TCP. One problem is that data associated with the
SYN packet can not be delivered. This results from an incorrect
specification of the interaction between the accept_policy action
procedure and the record_syn action procedure. Neither of the 2
possible strategies suggested in accept_policy will give the correct
result when called from the record_syn procedure, because the
recv_next variable is updated in record_syn before the accept_policy
procedure is called.
Another problem with the specification of the three-way handshake is
apparent in the actions listed for the Active Open event (with or
without data) when in the CLOSED state. No retransmission timer is
set in these actions, and therefore if the initial SYN is lost, there
will be no timer expiration to trigger retransmission. This will
prevent connection establishment if the initial SYN packet is lost by
the network.
The third problem with the specification is that the actions for
receiving data in the ESTAB state are incorrect. The accept action
procedure must be called when data is received, so that arriving data
may be queued and possibly passed to the user.
A general problem with this specification is that the program
language and action table portions of the specification were clearly
not checked by any automatic syntax checking process. Several
variable and procedure names are misspelled, and the syntax of the
action statements is often incorrect. This can be confusing,
especially when a procedure name cannot be found in the alphabetized
list of procedures because of misspelling.
These are some of the very serious errors that we have discovered
with the MIL standard TCP.
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4. Detailed Discussion of the Problem
Problem 1: Problem with Receiving Data Accompanying SYN
The following scenario traces the actions of 2 communicating
entities during the establishment of a connection. Only the
simplest case is considered, i.e., the case where the connection
is established by the exchange of 3 segments.
TCP entity A TCP entity B
------------ ------------
state segment segment state
transition recvd or sent recvd or sent transition
by A by B
CLOSED -> LISTEN
CLOSED -> SYN_SENT SYN -->
SYN --> LISTEN -> SYN_RECVD
<-- SYN ACK
SYN_SENT -> ESTAB <-- SYN ACK
ACK -->
ACK --> SYN_RECVD -> ESTAB
As shown in the above diagram, 5 state transitions occur and 3 TCP
segments are exchanged during the simplest case of the three-way
handshake. We now examine in detail the actions of each entity
during this exchange. Special attention is given to the sequence
numbers carried in each packet and recorded in the state variables
of each entity.
In the diagram below, the actions occurring within a procedure are
shown indented from the procedure call. The resulting values of
sequence number variables are shown in square brackets to the
right of each statement. The sequence number variables are shown
with the entity name (A or B) as prefix so that the two sets of
state variables may be easily distinguished.
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Transition 1 (entity B goes from state CLOSED to state LISTEN).
The user associated with entity B issues a Passive Open.
Actions: (see p. 104)
open; (see p. 144)
new state := LISTEN;
Transition 2 (entity A goes from state CLOSED to SYN_SENT). The
user associated with entity A issues an Active Open with Data.
Actions: (see p. 104)
open; (see p. 144)
gen_syn(WITH_DATA); (see p. 141)
send_isn := gen_isn(); [A.send_isn = 100]
send_next := send_isn + 1; [A.send_next = 101]
send_una := send_isn; [A.send_una = 100]
seg.seq_num := send_isn; [seg.seq_num = 100]
seg.ack_flag := FALSE; [seg.ack_flag = FALSE]
seg.wndw := 0; [seg.wndw = 0]
amount := send_policy() [assume amount > 0]
new state := SYN_SENT;
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Transition 3 (Entity B goes from state LISTEN to state SYN_RECVD).
Entity B receives the SYN segment accompanying data sent by entity
A.
Actions: (see p. 106)
(since this segment has no RESET, no ACK, does have SYN, and
we assume reasonable security and precedence parameters, row
3 of the table applies)
record_syn; (see p. 147)
recv_isn := seg.seq_num; [B.recv_isn = seg_seq_num = 100]
recv_next := recv_isn + 1; [B.recv_next = 101]
if seg.ack_flag then
send_una := seg.ack_num; [no change]
accept_policy; (see p. 131)
Accept in-order data only:
Acceptance Test is
seg.seq_num = recv_next;
Accept any data within the receive window:
Acceptance Test has two parts
recv_next =< seg.seq_num =< recv_next +
recv_wndw
or
recv_next =< seg.seq_num + length =<
recv_next + recv_wndw
********************************************
An error occurs here, with either possible
strategy given in accept_policy, because
recv_next > seg.seq_num. Therefore
accept_policy will incorrectly indicate that
the data cannot be accepted.
********************************************
gen_syn(WITH_ACK); (see p. 141)
send_isn := gen_isn(); [B.send_isn = 300]
send_next := send_isn + 1; [B.send_next = 301]
send_una := send_isn; [B.send_una = 300]
seg.seq_num := send_next; [seg.seq_num = 301]
seg.ack_flag := TRUE; [seg.ack_flag = TRUE]
seg.ack_num := recv_isn + 1; [seg.ack_num = 102]
new state := SYN_RECVD;
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Transition 4 (entity A goes from state SYN_SENT to ESTAB) Entity A
receives the SYN ACK sent by entity B.
Actions: (see p. 107)
In order to select the applicable row of the table on p.
107, we first evaluate the decision function
ACK_status_test1.
ACK_status_test1();
if(seg.ack_flag = FALSE) then
return(NONE);
if(seg.ack_num <= send_una) or
(seg.ack_num > send_next) then
return(INVALID)
else
return(VALID);
... and so on.
The important thing to notice in the above scenario is the error
that occurs in transition 3, where the wrong value for recv_next
leads to the routine record_syn refusing to accept the data.
Problem 2: Problem with Retransmission of SYN Packet
The actions listed for Active Open (with or without data; see p.
103) are calls to the routines open and gen_syn. Neither of these
routines (or routines that they call) explicitly sets a
retransmission timer. Therefore if the initial SYN is lost there
is no timer expiration to trigger retransmission of the SYN. If
this happens, the TCP will fail in its attempt to establish the
desired connection with a remote TCP.
Note that this differs with the actions specified for transmission
of data from the ESTAB state. In that transition the routine
dispatch (p. 137) is called first which in turn calls the routine
send_new_data (p. 156). One of actions of the last routine is to
start a retransmission timer for the newly sent data.
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Problem 3: Problem with Receiving Data in TCP ESTAB State
When both entities are in the state ESTAB, and one sends data to
the other, an error in the actions of the receiver prohibits the
data from being accepted. The following simple scenario
illustrates the problem. Here the user associated with entity A
issues a Send request, and A sends data to entity B. When B
receives the data it replies with an acknowledgment.
TCP entity A TCP entity B
------------ ------------
state segment segment state
transition recvd or sent recvd or sent transition
by A by B
ESTAB -> ESTAB DATA -->
DATA --> ESTAB -> ESTAB
<-- ACK
Transition 1 (entity A goes from state ESTAB to ESTAB) Entity A
sends data packet to entity B.
Actions: (see p. 110)
dispatch; (see p. 137)
Transition 2 (entity B goes from state ESTAB to ESTAB) Entity B
receives data packet from entity B.
Actions: (see p. 111)
Assuming the data is in order and valid, we use row 6 of the
table.
update; (see p. 159)
************************************************************
An error occurs here, because the routine update does
nothing to accept the incoming data, or to arrange to
pass it on to the user.
************************************************************
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5. Solutions to Problems
The problem with record_syn and accept_policy can be solved by having
record_syn call accept_policy before the variable recv_next is
updated.
The problem with gen_syn can be corrected by having gen_syn or open
explicitly request the retransmission timer.
The problem with the reception of data in the ESTAB state is
apparently caused by the transposition of the action tables on pages
111 and 112. These tables should be interchanged. This solution
will also correct a related problem, namely that an entity can never
reach the CLOSE_WAIT state from the ESTAB state.
Syntax errors in the action statements and tables could be easily
caught by an automatic syntax checker if the document used a more
formal description technique. This would be difficult to do for
[MILS83] since this document is not based on a formalized description
technique [BREM83].
The errors pointed out in this note have been submitted to DCA and
will be corrected in the next update of the MIL STD TCP
specification.
6. Implementation of MIL Standard TCP
In the discussion above, we pointed out several serious errors in the
specification of the Military Standard Transmission Control Protocol
[MILS83]. These errors imply that a TCP implementation that
faithfully conforms to the Military TCP standard will not be able to
Receive data sent with a SYN packet.
Establish a connection if the initial SYN packet is lost.
Receive data when in the ESTAB state.
It also follows from our discussion that an implementation of MIL
Standard TCP [MILS83] must include corrections mentioned above to get
a running TCP.
The problems pointed out in this paper with the current specification
of the MIL Standard TCP [MILS83] are based on an initial
investigation of this protocol standard by the authors.
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REFERENCES
[BLUT83] Blumer, T. P., and Sidhu, D. P., "Mechanical Verification
and Automatic Implementation of Authentication Protocols
for Computer Networks", SDC Burroughs Report (1983),
submitted for publication.
[BLUT82] Blumer, T. P., and Tenney, R. L., "A Formal Specification
Technique and Implementation Method for Protocols",
Computer Networks, Vol. 6, No. 3, July 1982, pp. 201-217.
[BREM83] Breslin, M., Pollack, R. and Sidhu D. P., "Formalization of
DoD Protocol Specification Technique", SDC - Burroughs
Report 1983.
[CERV74] Cerf, V., and Kahn, R., "A Protocol for Packet Network
Interconnection", IEEE Trans. Comm., May 1974.
[MILS83] "Military Standard Transmission Control Protocol",
MIL-STD-1778, 12 August 1983.
[POSJ81] Postel, J. (ed.), "DoD Standard Transmission Control
Protocol", Defense Advanced Research Projects Agency,
Information Processing Techniques Office, RFC-793,
September 1981.
[SIDD83] Sidhu, D. P., and Blumer, T. P., "Verification of NBS Class
4 Transport Protocol", SDC Burroughs Report (1983),
submitted for publication.
[SUNC78] Sunshine, C., and Dalal, Y., "Connection Management in
Transport Protocols", Computer Networks, Vol. 2, pp.454-473
(1978).
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