ISO TP on TCP: ISO Transport Service on top of the TCP Version: 3

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.

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.

      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.

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
      providing solid transport services independent of actual
      implementation.

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
         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.

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

         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

                                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

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

                                                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.

          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.

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).

      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.

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.

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)

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