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

   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

   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.

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.

      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;

      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;

      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.

   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.
            ************************************************************

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.

   [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).