RFC1006

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.

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.

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.

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.

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

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.

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.

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.

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)