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rfc:rfc1006

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:
  1. for testing purposes, initial data may be exchanged

during connection establishment

  1. for testing purposes, an expedited data service is

supported

  1. for performance reasons, a much larger TSDU size is

supported

  1. 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]

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