    M. Rose & D. Cass                                               [Page 1]

    Network Working Group                    Marshall T. Rose, Dwight E. Cass
    Request for Comments: RFC 1006    Northrop Research and Technology Center
    Obsoletes: RFC 983                                               May 1987

                    ISO Transport Service on top of the TCP
                                   Version: 3

    Status of this Memo

       This memo specifies a standard for the Internet community. Hosts
       on the Internet that choose to implement ISO transport services
       on top of the TCP are expected to adopt and implement this
       standard.  TCP port 102 is reserved for hosts which implement this
       standard.  Distribution of this memo is unlimited.

       This memo specifies version 3 of the protocol and supersedes
       [RFC983].  Changes between the protocol as described in Request for
       Comments 983 and this memo are minor, but are unfortunately
       incompatible.

    M. Rose & D. Cass                                               [Page 1]

    RFC 1006                                                        May 1987

    1.  Introduction and Philosophy

          The Internet community has a well-developed, mature set of
          transport and internetwork protocols (TCP/IP), which are quite
          successful in offering network and transport services to
          end-users. The CCITT and the ISO have defined various session,
          presentation, and application recommendations which have been
          adopted by the international community and numerous vendors.
          To the largest extent possible, it is desirable to offer these
          higher level directly in the ARPA Internet, without disrupting
          existing facilities.  This permits users to develop expertise
          with ISO and CCITT applications which previously were not
          available in the ARPA Internet.  It also permits a more
          graceful convergence and transition strategy from
          TCP/IP-based networks to ISO-based networks in the
          medium-and long-term.

          There are two basic approaches which can be taken when "porting"
          an ISO or CCITT application to a TCP/IP environment.  One
          approach is to port each individual application separately,
          developing local protocols on top of the TCP.  Although this is
          useful in the short-term (since special-purpose interfaces to the
          TCP can be developed quickly), it lacks generality.

          A second approach is based on the observation that both the ARPA
          Internet protocol suite and the ISO protocol suite are both
          layered systems (though the former uses layering from a more
          pragmatic perspective).  A key aspect of the layering principle
          is that of layer-independence.  Although this section is
          redundant for most readers, a slight bit of background material
          is necessary to introduce this concept.

          Externally, a layer is defined by two definitions:

             a service-offered definition, which describes the services
             provided by the layer and the interfaces it provides to
             access those services; and,

             a service-required definitions, which describes the services
             used by the layer and the interfaces it uses to access those
             services.

          Collectively, all of the entities in the network which co-operate
          to provide the service are known as the service-provider.
          Individually, each of these entities is known as a service-peer.

          Internally, a layer is defined by one definition:

              a protocol definition, which describes the rules which each
              service-peer uses when communicating with other service-peers.

    M. Rose & D. Cass                                               [Page 2]

    RFC 1006                                                        May 1987

          Putting all this together, the service-provider uses the protocol
          and services from the layer below to offer the its service to the
          layer above.  Protocol verification, for instance, deals with
          proving that this in fact happens (and is also a fertile field
          for many Ph.D. dissertations in computer science).

          The concept of layer-independence quite simply is:

              IF one preserves the services offered by the service-provider

              THEN the service-user is completely naive with respect to the
              protocol which the service-peers use

          For the purposes of this memo, we will use the layer-independence
          to define a Transport Service Access Point (TSAP) which appears
          to be identical to the services and interfaces offered by the
          ISO/CCITT TSAP (as defined in [ISO8072]), but we will in fact
          implement the ISO TP0 protocol on top of TCP/IP (as defined in
          [RFC793,RFC791]), not on top of the the ISO/CCITT network
          protocol.  Since the transport class 0 protocol is used over the
          TCP/IP connection, it achieves identical functionality as
          transport class 4.  Hence, ISO/CCITT higher level layers (all
          session, presentation, and application entities) can operate
          fully without knowledge of the fact that they are running on a
          TCP/IP internetwork.

    M. Rose & D. Cass                                               [Page 3]

    RFC 1006                                                        May 1987

    2.  Motivation

          In migrating from the use of TCP/IP to the ISO protocols, there
          are several strategies that one might undertake.  This memo was
          written with one particular strategy in mind.

          The particular migration strategy which this memo uses is based
          on the notion of gatewaying between the TCP/IP and ISO protocol
          suites at the transport layer.  There are two strong arguments
          for this approach:

          1.  Experience teaches us that it takes just as long to get good
          implementations of the lower level protocols as it takes to get
          implementations of the higher level ones.  In particular, it has
          been observed that there is still a lot of work being done at the
          ISO network and transport layers.  As a result, implementations
          of protocols above these layers are not being aggressively
          pursued. Thus, something must be done "now" to provide a medium
          in which the higher level protocols can be developed.  Since
          TCP/IP is mature, and essentially provides identical
          functionality, it is an ideal medium to support this development.

          2.  Implementation of gateways at the IP and ISO IP layers are
          probably not of general use in the long term.  In effect, this
          would require each Internet host to support both TP4 and TCP.
          As such, a better strategy is to implement a graceful migration
          path from TCP/IP to ISO protocols for the ARPA Internet when the
          ISO protocols have matured sufficiently.

          Both of these arguments indicate that gatewaying should occur at
          or above the transport layer service access point.  Further, the
          first argument suggests that the best approach is to perform the
          gatewaying exactly AT the transport service access point to
          maximize the number of ISO layers which can be developed.

            NOTE:     This memo does not intend to act as a migration or
                      intercept document.  It is intended ONLY to meet the
                      needs discussed above.  However, it would not be
                      unexpected that the protocol described in this memo
                      might form part of an overall transition plan.  The
                      description of such a plan however is COMPLETELY
                      beyond the scope of this memo.

          Finally, in general, building gateways between other layers in the
          TCP/IP and ISO protocol suites is problematic, at best.

          To summarize: the primary motivation for the standard described in
          this memo is to facilitate the process of gaining experience with
          higher-level ISO protocols (session, presentation, and
          application). The stability and maturity of TCP/IP are ideal for

    M. Rose & D. Cass                                               [Page 4]

    RFC 1006                                                        May 1987

          providing solid transport services independent of actual
          implementation.

    M. Rose & D. Cass                                               [Page 5]

    RFC 1006                                                        May 1987

    3.  The Model

          The [ISO8072] standard describes the ISO transport service
          definition, henceforth called TP.

              ASIDE:    This memo references the ISO specifications rather
                        than the CCITT recommendations.  The differences
                        between these parallel standards are quite small,
                        and can be ignored, with respect to this memo,
                        without loss of generality.  To provide the reader
                        with the relationships:

                             Transport service    [ISO8072]       [X.214]
                             Transport protocol   [ISO8073]       [X.224]
                             Session protocol     [ISO8327]       [X.225]

          The ISO transport service definition describes the services
          offered by the TS-provider (transport service) and the interfaces
          used to access those services.  This memo focuses on how the ARPA
          Transmission Control Protocol (TCP) [RFC793] can be used to offer
          the services and provide the interfaces.

          +-----------+                                       +-----------+
          |  TS-user  |                                       |  TS-user  |
          +-----------+                                       +-----------+
               |                                                     |
               | TSAP interface                       TSAP interface |
               |  [ISO8072]                                          |
               |                                                     |
          +----------+   ISO Transport Services on the TCP     +----------+
          |  client  |-----------------------------------------|  server  |
          +----------+              (this memo)                +----------+
               |                                                     |
               | TCP interface                         TCP interface |
               |  [RFC793]                                           |
               |                                                     |

          For expository purposes, the following abbreviations are used:

             TS-peer      a process which implements the protocol described
                          by this memo

             TS-user      a process talking using the services of a TS-peer

    M. Rose & D. Cass                                               [Page 6]

    RFC 1006                                                        May 1987

             TS-provider  the black-box entity implementing the protocol
                          described by this memo

          For the purposes of this memo, which describes version 2 of the
          TSAP protocol, all aspects of [ISO8072] are supported with one
          exception:

              Quality of Service parameters

          In the spirit of CCITT, this is left "for further study".  A
          future version of the protocol will most likely support the QOS
          parameters for TP by mapping these onto various TCP parameters.

          The ISO standards do not specify the format of a session port
          (termed a TSAP ID).  This memo mandates the use of the GOSIP
          specification [GOSIP86] for the interpretation of this field.
          (Please refer to Section 5.2, entitled "UPPER LAYERS ADDRESSING".)

          Finally, the ISO TSAP is fundamentally symmetric in behavior.
          There is no underlying client/server model.  Instead of a server
          listening on a well-known port, when a connection is established,
          the TS-provider generates an INDICATION event which, presumably
          the TS-user catches and acts upon.  Although this might be
          implemented by having a server "listen" by hanging on the
          INDICATION event, from the perspective of the ISO TSAP, all TS-
          users just sit around in the IDLE state until they either generate
          a REQUEST or accept an INDICATION.

    M. Rose & D. Cass                                               [Page 7]

    RFC 1006                                                        May 1987

    4.  The Primitives

          The protocol assumes that the TCP[RFC793] offers the following
          service primitives:

                                        Events

             connected       - open succeeded (either ACTIVE or PASSIVE)

             connect fails   - ACTIVE open failed

             data ready      - data can be read from the connection

             errored         - the connection has errored and is now closed

             closed          - an orderly disconnection has started

                                         Actions

             listen on port  - PASSIVE open on the given port

             open port       - ACTIVE open to the given port

             read data       - data is read from the connection

             send data       - data is sent on the connection

             close           - the connection is closed (pending data is
                               sent)

    This memo describes how to use these services to emulate the following
    service primitives, which are required by [ISO8073]:

                                     Events

             N-CONNECT.INDICATION
                              - An NS-user (responder) is notified that
                                connection establishment is in progress

             N-CONNECT.CONFIRMATION
                              - An NS-user (responder) is notified that
                                the connection has been established

             N-DATA.INDICATION
                              - An NS-user is notified that data can be
                                read from the connection

    M. Rose & D. Cass                                               [Page 8]

    RFC 1006                                                        May 1987

             N-DISCONNECT.INDICATION
                              - An NS-user is notified that the connection
                                is closed

                                    Actions

             N-CONNECT.REQUEST
                              - An NS-user (initiator) indicates that it
                                wants to establish a connection

             N-CONNECT.RESPONSE
                              - An NS-user (responder) indicates that it
                                will honor the request

             N-DATA.REQUEST   - An NS-user sends data

             N-DISCONNECT.REQUEST
                              - An NS-user indicates that the connection
                                is to be closed

          The protocol offers the following service primitives, as defined
          in [ISO8072], to the TS-user:

                                        Events

             T-CONNECT.INDICATION
                              - a TS-user (responder) is notified that
                                connection establishment is in progress

             T-CONNECT.CONFIRMATION
                              - a TS-user (initiator) is notified that the
                                connection has been established

             T-DATA.INDICATION
                              - a TS-user is notified that data can be read
                                from the connection

             T-EXPEDITED DATA.INDICATION
                              - a TS-user is notified that "expedited" data
                                can be read from the connection

             T-DISCONNECT.INDICATION
                              - a TS-user is notified that the connection
                                is closed

    M. Rose & D. Cass                                               [Page 9]

    RFC 1006                                                        May 1987

                                    Actions

             T-CONNECT.REQUEST
                              - a TS-user (initiator) indicates that it
                                wants to establish a connection

             T-CONNECT.RESPONSE
                              - a TS-user (responder) indicates that it
                                will honor the request

             T-DATA.REQUEST   - a TS-user sends data

             T-EXPEDITED DATA.REQUEST
                              - a TS-user sends "expedited" data

             T-DISCONNECT.REQUEST
                              - a TS-user indicates that the connection
                                is to be closed

    M. Rose & D. Cass                                              [Page 10]

    RFC 1006                                                        May 1987

    5.  The Protocol

          The protocol specified by this memo is identical to the protocol
          for ISO transport class 0, with the following exceptions:

                - for testing purposes, initial data may be exchanged
                  during connection establishment

                - for testing purposes, an expedited data service is
                  supported

                - for performance reasons, a much larger TSDU size is
                  supported

                - the network service used by the protocol is provided
                  by the TCP

          The ISO transport protocol exchanges information between peers in
          discrete units of information called transport protocol data units
          (TPDUs).  The protocol defined in this memo encapsulates these
          TPDUs in discrete units called TPKTs.  The structure of these
          TPKTs and their relationship to TPDUs are discussed in the next
          section.

          PRIMITIVES

             The mapping between the TCP service primitives and the service
             primitives expected by transport class 0 are quite straight-
             forward:

                       network service              TCP
                       ---------------              ---
                       CONNECTION ESTABLISHMENT

                           N-CONNECT.REQUEST        open completes

                           N-CONNECT.INDICATION     listen (PASSIVE open)
                                                    finishes

                           N-CONNECT.RESPONSE       listen completes

                           N-CONNECT.CONFIRMATION   open (ACTIVE open)
                                                    finishes

                       DATA TRANSFER

                           N-DATA.REQUEST           send data

                           N-DATA.INDICATION        data ready followed by

    M. Rose & D. Cass                                              [Page 11]

    RFC 1006                                                        May 1987

                                                    read data

                       CONNECTION RELEASE

                           N-DISCONNECT.REQUEST     close

                           N-DISCONNECT.INDICATION  connection closes or
                                                    errors

              Mapping parameters is also straight-forward:

                         network service             TCP
                         ---------------             ---
                         CONNECTION RELEASE

                             Called address          server's IP address
                                                     (4 octets)

                             Calling address         client's IP address
                                                     (4 octets)

                             all others              ignored

                          DATA TRANSFER

                             NS-user data (NSDU)     data

                          CONNECTION RELEASE

                             all parameters          ignored

          CONNECTION ESTABLISHMENT

              The elements of procedure used during connection establishment
              are identical to those presented in [ISO8073], with three
              exceptions.

              In order to facilitate testing, the connection request and
              connection confirmation TPDUs may exchange initial user data,
              using the user data fields of these TPDUs.

              In order to experiment with expedited data services, the
              connection request and connection confirmation TPDUs may
              negotiate the use of expedited data transfer using the
              negotiation mechanism specified in [ISO8073] is used (e.g.,
              setting the "use of transport expedited data transfer service"
              bit in the "Additional Option Selection" variable part). The
              default is not to use the transport expedited data transfer
              service.

    M. Rose & D. Cass                                              [Page 12]

    RFC 1006                                                        May 1987

              In order to achieve good performance, the default TPDU size is
              65531 octets, instead of 128 octets.  In order to negotiate a
              smaller (standard) TPDU size, the negotiation mechanism
              specified in [ISO8073] is used (e.g., setting the desired bit
              in the "TPDU Size" variable part).

              To perform an N-CONNECT.REQUEST action, the TS-peer performs
              an active open to the desired IP address using TCP port 102.
              When the TCP signals either success or failure, this results
              in an N-CONNECT.INDICATION action.

              To await an N-CONNECT.INDICATION event, a server listens on
              TCP port 102.  When a client successfully connects to this
              port, the event occurs, and an implicit N-CONNECT.RESPONSE
              action is performed.

                  NOTE:      In most implementations, a single server will
                             perpetually LISTEN on port 102, handing off
                             connections as they are made

    DATA TRANSFER

          The elements of procedure used during data transfer are identical
          to those presented in [ISO8073], with one exception: expedited
          data may be supported (if so negotiated during connection
          establishment) by sending a modified ED TPDU (described below).
          The TPDU is sent on the same TCP connection as all of the other
          TPDUs. This method, while not faithful to the spirit of [ISO8072],
          is true to the letter of the specification.

          To perform an N-DATA.REQUEST action, the TS-peer constructs the
          desired TPKT and uses the TCP send data primitive.

          To trigger an N-DATA.INDICATION action, the TCP indicates that
          data is ready and a TPKT is read using the TCP read data
          primitive.

    CONNECTION RELEASE

       To perform an N-DISCONNECT.REQUEST action, the TS-peer simply closes
       the TCP connection.

       If the TCP informs the TS-peer that the connection has been closed or
       has errored, this indicates an N-DISCONNECT.INDICATION event.

    M. Rose & D. Cass                                              [Page 13]

    RFC 1006                                                        May 1987

    6.  Packet Format

          A fundamental difference between the TCP and the network service
          expected by TP0 is that the TCP manages a continuous stream of
          octets, with no explicit boundaries.  The TP0 expects information
          to be sent and delivered in discrete objects termed network
          service data units (NSDUs).  Although other classes of transport
          may combine more than one TPDU inside a single NSDU, transport
          class 0 does not use this facility.  Hence, an NSDU is identical
          to a TPDU for the purposes of our discussion.

          The protocol described by this memo uses a simple packetization
          scheme in order to delimit TPDUs.  Each packet, termed a TPKT, is
          viewed as an object composed of an integral number of octets, of
          variable length.

              NOTE:       For the purposes of presentation, these objects are
                          shown as being 4 octets (32 bits wide).  This
                          representation is an artifact of the style of this
                          memo and should not be interpreted as requiring
                          that a TPKT be a multiple of 4 octets in length.

          A TPKT consists of two parts:  a packet-header and a TPDU.  The
          format of the header is constant regardless of the type of packet.
          The format of the packet-header is as follows:

            0                   1                   2                   3
            0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |      vrsn     |    reserved   |          packet length        |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          where:

          vrsn                         8 bits

          This field is always 3 for the version of the protocol described in
          this memo.

          packet length                16 bits (min=7, max=65535)

          This field contains the length of entire packet in octets,
          including packet-header.  This permits a maximum TPDU size of
          65531 octets.  Based on the size of the data transfer (DT) TPDU,
          this permits a maximum TSDU size of 65524 octets.

          The format of the TPDU is defined in [ISO8073].  Note that only
          TPDUs formatted for transport class 0 are exchanged (different
          transport classes may use slightly different formats).

    M. Rose & D. Cass                                              [Page 14]

    RFC 1006                                                        May 1987

          To support expedited data, a non-standard TPDU, for expedited data
          is permitted.  The format used for the ED TPDU is nearly identical
          to the format for the normal data, DT, TPDU.  The only difference
          is that the value used for the TPDU's code is ED, not DT:

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          | header length | code  |credit |TPDU-NR and EOT|   user data   |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |      ...      |      ...      |      ...      |      ...      |
          |      ...      |      ...      |      ...      |      ...      |
          |      ...      |      ...      |      ...      |      ...      |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          After the credit field (which is always ZERO on output and ignored
          on input), there is one additional field prior to the user data.

          TPDU-NR and EOT         8 bits

          Bit 7 (the high-order bit, bit mask 1000 0000) indicates the end
          of a TSDU.  All other bits should be ZERO on output and ignored on
          input.

          Note that the TP specification limits the size of an expedited
          transport service data unit (XSDU) to 16 octets.

    M. Rose & D. Cass                                              [Page 15]

    RFC 1006                                                        May 1987

    7.  Comments

          Since the release of RFC983 in April of 1986, we have gained much
          experience in using ISO transport services on top of the TCP.  In
          September of 1986, we introduced the use of version 2 of the
          protocol, based mostly on comments from the community.

          In January of 1987, we observed that the differences between
          version 2 of the protocol and the actual transport class 0
          definition were actually quite small.  In retrospect, this
          realization took much longer than it should have:  TP0 is is meant
          to run over a reliable network service, e.g., X.25. The TCP can be
          used to provide a service of this type, and, if no one complains
          too loudly, one could state that this memo really just describes a
          method for encapsulating TPO inside of TCP!

          The changes in going from version 1 of the protocol to version 2
          and then to version 3 are all relatively small. Initially, in
          describing version 1, we decided to use the TPDU formats from the
          ISO transport protocol.  This naturally led to the evolution
          described above.

    M. Rose & D. Cass                                              [Page 16]

    RFC 1006                                                        May 1987

    8. References

       [GOSIP86]    The U.S. Government OSI User's Committee.
                    "Government Open Systems Interconnection Procurement
                    (GOSIP) Specification for Fiscal years 1987 and
                    1988." (December, 1986) [draft status]

       [ISO8072]    ISO.
                    "International Standard 8072.  Information Processing
                    Systems -- Open Systems Interconnection: Transport
                    Service Definition."
                    (June, 1984)

       [ISO8073]    ISO.
                    "International Standard 8073.  Information Processing
                    Systems -- Open Systems Interconnection: Transport
                    Protocol Specification."
                    (June, 1984)

       [ISO8327]    ISO.
                    "International Standard 8327.  Information Processing
                    Systems -- Open Systems Interconnection: Session
                    Protocol Specification."
                    (June, 1984)

       [RFC791]     Internet Protocol.
                    Request for Comments 791 (MILSTD 1777)
                    (September, 1981)

       [RFC793]     Transmission Control Protocol.
                    Request for Comments 793 (MILSTD 1778)
                    (September, 1981)

       [RFC983]     ISO Transport Services on Top of the TCP.
                    Request for Comments 983
                    (April, 1986)

       [X.214]      CCITT.
                    "Recommendation X.214.  Transport Service Definitions
                    for Open Systems Interconnection (OSI) for CCITT
                    Applications."
                    (October, 1984)

       [X.224]      CCITT.
                    "Recommendation X.224.  Transport Protocol
                    Specification for Open Systems Interconnection (OSI)
                    for CCITT Applications." (October, 1984)

    M. Rose & D. Cass                                              [Page 17]

    RFC 1006                                                        May 1987

       [X.225]      CCITT.
                    "Recommendation X.225.  Session Protocol Specification
                    for Open Systems Interconnection (OSI) for CCITT
                    Applications."
                    (October, 1984)

    M. Rose & D. Cass                                              [Page 18]