

Network Working Group                                  D. E. Cass (NRTC)
Request for Comments: 983                              M. T. Rose (NRTC)
                                                              April 1986

                ISO Transport Services on Top of the TCP


Status of This Memo

   This memo describes a proposed protocol standard for the ARPA
   Internet community.  The intention is that hosts in the ARPA-Internet
   that choose to implement ISO TSAP services on top of the TCP be
   expected to adopt and implement this standard.  Suggestions for
   improvement are encouraged.  Distribution of this memo is unlimited.

1.  Introduction and Philosophy

   The ARPA 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 services
   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 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,


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ISO Transport Services on Top of the TCP


      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 [ISO-8072]), but we will base the internals of
   this TSAP on TCP/IP (as defined in [RFC-793,RFC791]), not on the
   ISO/CCITT transport and network protocols.  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.

   The authors hope that the preceding paragraph will not come as a
   shock to most readers.  However, an ALARMING number of people seem to
   think that layering is just a way of cutting up a large problem into
   smaller ones, *simply* for the sake of cutting it up.  Although
   layering tends to introduce modularity into an architecture, and
   modularity tends to introduce sanity into implementations (both
   conceptual and physical implementations), modularity, per se, is not
   the end goal.  Flexibility IS.






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RFC 983                                                       April 1986
ISO Transport Services on Top of the TCP


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:

      a.  Experience teaches us that it takes just as long to get good
      implementations of the lower level protocols as it takes to get
      good 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.

      b.  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


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RFC 983                                                       April 1986
ISO Transport Services on Top of the TCP


   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 [ISO-8072] 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      [ISO-8072]      [X.214]
         Transport protocol     [ISO-8073]      [X.224]
         Session protocol       [ISO-8327]      [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) [RFC-793] can be used to offer
   the services and provide the interfaces.

   +-------------+                                      +-------------+
   |   TS-user   |                                      |   TS-user   |
   +-------------+                                      +-------------+
           |                                                   |
           | TSAP interface                     TSAP interface |
           |  [ISO-8072]                                       |
           |                                                   |
   +------------+   ISO Transport Services on the TCP    +------------+
   |   client   |----------------------------------------|   server   |
   +------------+              (this memo)               +------------+
           |                                                   |
           | TCP interface                       TCP interface |
           |  [RFC-793]                                        |
           |                                                   |

   For expository purposes, the following abbreviations are used:

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




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RFC 983                                                       April 1986
ISO Transport Services on Top of the TCP


      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 1 of the TSAP
   protocol, all aspects of [ISO-8072] are supported with one exception:

      Quality of Service parameters

   In the spirit of CCITT, this is left "for further study".  Version 2
   of the TSAP protocol will most likely support the QOS parameters for
   TP by mapping these onto various TCP parameters.

   Since TP supports the notion of a session port (termed a TSAP ID),
   but the list of reserved ISO TSAP IDs is not clearly defined at this
   time, this memo takes the philosophy of isolating the TCP port space
   from the TSAP ID space and uses a single TCP port.  This memo
   reserves TCP port 102 for this purpose.  This protocol manages its
   own TSAP ID space independent of the TCP.  Appendix A of this memo
   lists reserved TSAP IDs for version 1 of this TSAP protocol.  It is
   expected that future editions of the "Assigned Numbers" document
   [RFC-960] will contain updates to this list.  (Interested readers are
   encouraged to read [ISO-8073] and try to figure out exactly what a
   TSAP ID is.)

   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.














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RFC 983                                                       April 1986
ISO Transport Services on Top of the TCP


4.  The Primitives

   The protocol assumes that the TCP [RFC-793] 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)

   The protocol offers the following service primitives, as defined in
   [ISO-8072], to the TS-user:

   Events

      T-CONNECT.INDICATION

         - a TS-user (server) is notified that connection establishment
           is in progress

      T-DISCONNECT.INDICATION

         - a TS-user is notified that the connection is closed

      T-CONNECT.CONFIRMATION

         - a TS-user (client) is notified that the connection has been
           established


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ISO Transport Services on Top of the TCP


      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

   Actions

      T-CONNECT.RESPONSE

         - a TS-user (server) indicates that it will honor the request

      T-DISCONNECT.REQUEST

         - a TS-user indicates that the connection is to be closed

      T-CONNECT.REQUEST

         - a TS-user (client) indicates that it wants to establish a
           connection

      T-DATA.REQUEST

         - a TS-user sends data

      T-EXPEDITED DATA.REQUEST

         - a TS-user sends "expedited" data

















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RFC 983                                                       April 1986
ISO Transport Services on Top of the TCP


5.  The Protocol

   It is the goal of this memo to offer a TP interface on top of the
   TCP.  Fortunately, the TCP does just about everything that
   TS-provider offers to the TS-user, so the hard parts of the transport
   layer (e.g., three-way handshakes, choice of ISS, windowing,
   multiplexing, ad infinitum) are all taken care of by the TCP.

   Despite the symmetry of TP, it is useful to consider the protocol
   with the perspective of a client/server model.

   The information exchanged between TSAP-peers is in the form of
   packets termed "TPKT"s.  The format of these packets is described in
   the next section.  For the purposes of the description below, a TPKT
   has a code which is one of:

      CR - request connection
      CC - confirm connection
      DR - request disconnection
      DT - data
      ED - expedited data

   A TSAP server begins by LISTENing on TCP port 102.  When a TSAP
   client successfully connects to this port, the protocol begins.

   A client decides to connect to the port when a TS-user issues a
   T-CONNECT.REQUEST action.  This action specifies the TSAP ID of the
   remote TS-user, whether expedited data is to be supported, and
   (optionally) some initial TS-user data.  The client consults the TSAP
   ID given to ascertain the IP address of the server.  If the expedited
   data option was requested, the client opens a passive TCP port, in
   non-blocking mode, noting the port number.  This TCP port is termed
   the "expedited port".  The client then tries to open a TCP connection
   to the server on port 102.  If not successful, the client fires
   T-DISCONNECT.INDICATION for the TS-user specifying the reason for
   failure (and, closes the expedited port, if any).  If successful, the
   client sends a TPKT with code CR containing:

      - the TSAP ID of the TS-user on the client's host (the "caller")
      - the TSAP ID of the TS-user that the client wants to talk to
        (the "called")
      - if the expedited data option was requested, the TSAP ID of the
        expedited port for the client's host
      - any TS-user data from the T-CONNECT.REQUEST

   The client now awaits a response.



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RFC 983                                                       April 1986
ISO Transport Services on Top of the TCP


   The server, upon receipt of the TPKT, validates the contents of the
   TPKT (checking the version number, verifying that the code is CR, and
   so forth).  If the packet is invalid, the server sends a TPKT with
   code DR specifying "PROTOCOL ERROR", closes the TCP connection, and
   goes back to the LISTEN state.

   If the packet is valid, the server examines the TSAP ID that the
   remote TS-user wants to communicate with.  If the TS-user specified
   can be located and started (e.g., the appropriate program which
   implements the indicated protocol is present), then the server starts
   this TS-user by firing T-CONNECT.INDICATION.  Otherwise, the server
   sends a TPKT with code DR specifying "SESSION ENTITY NOT ATTACHED TO
   TSAP" or "REMOTE TRANSPORT ENTITY CONGESTED AT CONNECT REQUEST TIME"
   as appropriate, closes the TCP connection, and goes back to the
   LISTEN state.

   The server now waits for a T-CONNECT.RESPONSE or T-DISCONNECT.REQUEST
   from the TS-user it started.  if the latter is given, the server
   sends a TPKT with code DR containing the reason for the disconnect as
   supplied by the TS-user.

   The server then closes the TCP connection and goes back to the LISTEN
   state.

   Instead, if T-CONNECT.RESPONSE is given, the server sees if an
   expedited port was specified in the connection request.  If so, the
   server opens a second TCP connection and connects to the specified
   port.  If the connection fails, the server sends a TPKT with code DR
   specifying "CONNECTION NEGOTIATION FAILED", closes the TCP
   connection, and goes back to the LISTEN state.  If the connection
   succeeded, the server notes the local port number used to connect to
   the expedited port.

   If an expedited port was not specified in the TPKT with code CR, and
   the server's TS-user indicates that it wants to use expedited data,
   then the server sends a TPKT with code DR specifying "CONNECTION
   NEGOTIATION FAILED", fires T-DISCONNECT.INDICATION with this error to
   the TS-user, closes the TCP connection, and goes back to the LISTEN
   state.

   The server now sends a TPKT with code CC containing:

      - the TSAP ID of the TS-user responding to the connection
        (usually the "called")
      - if an expedited port was specified in the TPKT with code CR,




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RFC 983                                                       April 1986
ISO Transport Services on Top of the TCP


        the TSAP ID of the port number on the server's host that was
        used to connect to the expedited port
      - any TS-user data from the T-CONNECT.RESPONSE

   After sending the TPKT, the server enters the SYMMETRIC PEER state.

   The client, upon receipt of the TPKT, validates the contents of the
   TPKT (checking the version number, verifying that the code is CC or
   DR, and so forth).  If the packet is invalid, the client sends a TPKT
   with code DR specifying "PROTOCOL ERROR", fires
   T-DISCONNECT.INDICATION with this error to the TS-user, and closes
   the TCP connection (and the expedited port, if any).

   If the packet's code is DR, the client fires T-DISCONNECT.INDICATION
   with the reason given in the TPKT to the TS-user, and closes the TCP
   connection (and the expedited port, if any).

   If the packet's code is CC, the client checks if an expedited port
   was specified and that a connection is waiting on the expedited port.
   If not, a protocol error has occurred, a TPKT with code DR is
   returned, T-DISCONNECT.INDICATION is fired, and so on.  Otherwise,
   the client checks the remote address that connected to the expedited
   port.  If it differs from the port listed in the TPKT with code CC, a
   protocol error has occurred.  Otherwise, all is well, two TCP
   connections have been established, one for all TPKTs except expedited
   data, and the second for the exclusive use of expedited data.

   The client now fires T-CONNECT.CONFIRMATION, and enters the SYMMETRIC
   PEER state.

   Once both sides have reached the SYMMETRIC PEER state, the protocol
   is completely symmetric, the notion of client/server is lost.  Both
   TS-peers act in the following fashion:

   If the TCP indicates that data can be read, the TS-peer, upon receipt
   of the TPKT, validates the contents.  If the packet is invalid, the
   TS-peer sends a TPKT with code DR specifying "PROTOCOL ERROR", fires
   T-DISCONNECT.INDICATION with this error to the TS-user, and closes
   the TCP connection (and expedited data connection, if any).  If the
   TS-peer was the server, it goes back to the LISTEN state.

      NOTE:  If the expedited data option was requested, then there are
      two TCP connections that can supply data for reading.  The
      dialogue below assumes that only ED TPKTs are read from the
      expedited data connection. For simplicity's sake, when reading
      from TCP the relation between connections and TPKTs is unimportant
      and this memo URGES all implementations to be very lenient in this


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RFC 983                                                       April 1986
ISO Transport Services on Top of the TCP


      regard.  When writing to TCP, implementations should use the
      expedited data connection only to send TPKTs with code ED.
      Section 7 of this memo discusses the handling of expedited data in
      greater detail.

   If the packet's code is DR, the TS-peer fires T-DISCONNECT.INDICATION
   with the reason given in the TPKT to the TS-user, and closes the TCP
   connection (and expedited data connection, if any).  If the TS-peer
   was the server, it goes back to the LISTEN state.

   If the packet's code is ED or DT, the TS-peer fires T-DATA.INDICATION
   or T-EXPEDITED DATA.INDICATION as appropriate with the enclosed user
   data for the TS-user.  It then goes back to the SYMMETRIC PEER state.

   If the packet is invalid, the TS-peer sends a TPKT with code DR
   specifying "PROTOCOL ERROR", fires T-DISCONNECT.INDICATION with this
   error to the TS-user, and closes the TCP connection (and expedited
   data connection, if any).  If the TS-peer was the server, it goes
   back to the LISTEN state.

   If the TCP indicates that an error has occurred and the connection
   has closed, then the TS-peer fires T-DISCONNECT.INDICATION to the
   TS-user specifying the reason for the failure.  If the expedited data
   connection, if any, is still open, it is closed.  If the TS-peer was
   the server, it goes back to the LISTEN state.

   If the TS-user issues a T-DATA.REQUEST or T-EXPEDITED DATA.REQUEST
   action, the TS-peer sends a TPKT with code DT or ED containing the
   TS-user data.  It then goes back to the SYMMETRIC PEER state.

   If the TS-user issues a T-DISCONNECT.REQUEST action, the TS-peer
   sends a TPKT with code DR containing the reason for the disconnect as
   supplied by the TS-user.  The TS-peer then closes the TCP connection,
   (and expedited data connection, if any).  If the TS-peer was the
   server, it goes back to the LISTEN state.

   In terms of (augmented) state tables, the protocol can be explained
   as follows.  The server starts in state S0, the client starts in
   state C0.  "TCP:"  refers to an event or action from the TCP service,
   "SS:"  refers to an event or action from the TS-user (e.g., the ISO
   session service [ISO-8327]).








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RFC 983                                                       April 1986
ISO Transport Services on Top of the TCP


                        S E R V E R   S T A T E S

   state        event                   action                      goto
   -----        -----                   ------                      ----
   S0                                   TCP: listen on port 102     S1

   S1           TCP: connected          TCP: read TPKT
                                        parse, on error
                                          TCP: send DR, close       S0
                                        code is CR
                                          start session server
                                          SS: T-CONNECT             S2
                                                .INDICATION
                                        otherwise,
                                          TCP: send DR, close       S0

   S2           SS: T-CONNECT.RESPONSE  if expedited option,
                                          TCP: open port EXPD
                                        TCP: send CC                P0

   S2           SS: T-DISCONNECT        TCP: send DR, close         S0
                        .REQUEST

   Any event occuring for a state not listed above is considered an
   error, and handled thusly:

   state        event                   action                      goto
   -----        -----                   ------                      ----
   S*           TCP: other              if TCP is open, TCP: close  S0
                                        otherwise ignore            S0
   S*           SS: other               SS: T-DISCONNECT
                                              .INDICATION
                                        if TCP is open, close       S0
















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ISO Transport Services on Top of the TCP


                        C L I E N T   S T A T E S

   state        event                   action                      goto
   -----        -----                   ------                      ----
   C0           SS: T-CONNECT.REQUEST   if expedited option,
                                          TCP: non-blocking
                                               listen on port EXPD
                                        TCP: open port 102          C1

   C1           TCP: connected          TCP: send CR                C2

   C1           TCP: connect fails      TCP: close
                                        SS: T-DISCONNECT            C0
                                                 .INDICATION
   

   C2           TCP: data ready         TCP: read TPKT
                                        parse, on error
                                          TCP: send DR, close
                                          SS: T-DISCONNECT          C0
                                                .INDICATION
                                        code is CC
                                          if expedited option,
                                             verify port EXPD
                                             connected correctly,
                                             if not, treat as error
                                          SS: T-CONNECT             P0
                                                .CONFIRMATION
                                        code is DR
                                          TCP: close
                                          SS: T-DISCONNECT          C0
                                                .INDICATION
                                        otherwise
                                          TCP: send DR, close
                                          SS: T-DISCONNECT          C0
                                                .INDICATION













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RFC 983                                                       April 1986
ISO Transport Services on Top of the TCP


   Any event occuring for a state not listed above is considered an
   error, and handled thusly:

   state        event                   action                      goto
   -----        -----                   ------                      ----
   C*           TCP: other              if TCP is open, close       C0
                                        otherwise ignore            C0

   C*           SS: other               SS: T-DISCONNECT
                                              .INDICATION
                                        if TCP is open, close       C0






































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RFC 983                                                       April 1986
ISO Transport Services on Top of the TCP


                          P E E R   S T A T E S

   state        event                   action                      goto
   -----        -----                   ------                      ----
   P0           TCP: data ready         TCP: read TPKT
                                        parse, on error
                                          TCP: send DR, close
                                          SS: T-DISCONNECT          end
                                                .INDICATION
                                        code is DT
                                          SS: T-DATA.INDICATION      P0
                                        code is ED
                                          SS: T-EXPEDITED DATA       P0
                                                .INDICATION
                                        code is DR
                                          TCP: close
                                          SS: T-DISCONNECT          end
                                                .INDICATION
                                        otherwise
                                          TCP: send DR, close
                                          SS: T-DISCONNECT          end
                                                .INDICATION

   P0           TCP: other              TCP: close
                                        SS: T-DISCONNECT            end
                                                  .INDICATION

   P0           SS: T-DATA.REQUEST      TCP: send DT                 P0

   P0           SS: T-EXPEDITED DATA    TCP: send ED                 P0
                        .REQUEST

   P0           SS: T-DISCONNECT        TCP: send DR, close         end
                        .REQUEST

   P0           SS: other               TCP: send DR, close
                                        SS: T-DISCONNECT            end
                                                .INDICATION











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RFC 983                                                       April 1986
ISO Transport Services on Top of the TCP


6.  Packet Format

   Two TS-peers exchange information over a TCP connection by
   encapsulating information in well-defined packets.  A packet, denoted
   as "TPKT" in the previous sections, 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 TPDU be a multiple of 4 octets in length.

   A packet consists of two parts: a packet-header and a pseudo-TPDU.
   The format of the header is constant regardless of the type of
   packet.  The format of the pseudo-TPDU follows the [ISO-8073]
   recommendation very closely with the exceptions listed below.  As per
   [ISO-8073], each TPDU consists of two parts: header and data.

   It is EXTREMELY important to observe that TPKTs represent
   "indivisible" units of data to the TS-user.  That is, a
   T-DATA.REQUEST initiated by the TS-user at the sending end of a
   connection should result in exactly one T-DATA.INDICATION being
   generated (with exactly that data) for the TS-user at the receiving
   end.  To ensure this behavior, it is critical that any INDICATION
   event resulting from a TPKT be initiated ONLY after the entire TPKT
   is fully received.  Furthermore, exactly one such INDICATION event
   should be generated by the TS-peer.  The packet length field, as
   described below, can accommodate on the order of 65K octets of user
   data.  This should be well above the requirements of the size of any
   SPDU (Session Protocol Data Unit) for any real implementation.  As a
   result, version 1 of this protocol has no need to either fragment or
   re-assemble TS-user data.  If an application arises which requires an
   SPDU of size greater than 65K octets, this memo will be revised.

   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:

      1.  vrsn                  8 bits

         This field is always 1 for this memo.


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      2.  packet length         16 bits (min=8, max=65535)

         The length of entire packet in octets, including packet-header.

   The format of the TPDU (to re-phrase from [ISO-8073]) depends on the
   type of a TPDU.  All TPDUs start with a fixed-part header length and
   the code.  The information following after the code varies, depending
   on the value of the code.  In the context of this memo, the following
   codes are valid:

      CR: connect request
      CC: connect confirm
      DR: disconnect request
      DT: data
      ED: expedited data

   The format of a CR or CC TPDU is:

       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|     destination reference     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       source reference        | class |options| variable data |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    ...        |      ...      |      ...      |      ...      |
      |    ...        |      ...      |      ...      |      ...      |
      |    ...        |   user data   |      ...      |      ...      |
      |    ...        |      ...      |      ...      |      ...      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      3.  header length          8 bits (min=6, max=min(254,packet
      length-6))

         The TPDU-header length in octets including parameters but
         excluding the header length field and user data (if any).











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      4.  code                   4 bits

         The type of TPDU.  Values, in the context of this memo, are:

            value       meaning
            -----       -------
             14         CR: connection request  (binary 1110)
             13         CC: connection confirm  (binary 1101)
              8         DR: disconnect request  (binary 1000)
             15         DT: data                (binary 1111)
              1         ED: expedited data      (binary 0001)
            all other   reserved                             

      5.  credit                 4 bits

         This field is always ZERO on output and ignored on input.

      6.  destination reference 16 bits

         This field is always ZERO on output and ignored on input.

      7.  source reference      16 bits

         This field is always ZERO on output and ignored on input.

      8.  class                  4 bits

         This field is always 4 (binary 0100) on output and ignored on
         input. It is anticipated that future versions of this protocol
         will make use of this field.

      9.  options                4 bits

         This field is always ZERO on output and ignored on input.

      10.  variable data        (header length - 6) octets

         This portion of the TPDU is of variable length.  For most
         TPDUs, this portion is empty (the header length field of the
         TPDU is exactly 6).  The contents of the variable data consist
         of zero or more "parameters".  Each parameter has the following
         format:

            parameter code        1 octet in size
            parameter length      1 octet in size, value is the number
                                    of octets in parameter value
            parameter value       parameter data


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         Normally, the parameter length is 1 octet.  Any implementation
         conforming to this version of the protocol must recognize all
         parameter types listed in [ISO-8073].  With the exception of
         the parameters listed below, these parameters are simply
         ignored.

         o   Parameter name:       Transport service access point
                                   identifier (TSAP-ID) of the client
         TSAP

            Parameter code:        193 (binary 1100 0001)
            Parameter length:      variable
            Parameter value:       TSAP-ID attributes

            Each TSAP-ID consists of 1 or more attributes.  Each
            attribute has this format:

               Attribute code      1 octet in size
               Attribute length    1 octet in size, value is the number
                                     of octets in attribute value
               Attribute value     attribute data

            In version 1 of this protocol, only two attributes are
            defined. All others are reserved.

               Attribute name:     Internet Address

               Attribute code:     1
               Attribute length:   6
               Attribute value:    IP address (4 octets)
                                   session port (2 octets, unsigned
                                   integer)

                  This attribute is ALWAYS required.  Values for session
                  port can be found in Appendix A of this memo.

               Attribute name:     Internet Address for Expedited Data

               Attribute code:     2
               Attribute length:   6
               Attribute value:    IP address (4 octets)
                                   TCP port (2 octets, unsigned integer)

                  This attribute is required ONLY if expedited data is
                  to be exchanged.  The attribute value is an <IP
                  address, TCP port> pair designated by the TS-peer for
                  use with expedited data.


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         o   Parameter name:       TSAP-ID of the server TSAP

            Parameter code:        194 (binary 1100 0010)
            Parameter length:      variable
            Parameter value:       TSAP-ID attributes

         o   Parameter name:       Additional option selection

            Parameter code:        198 (binary 1100 0110)
            Parameter length:      1
            Parameter value:       additional flags

            The additional flags octet consists of 8-bits of optional
            flags.  Only one bit is of interest to this memo, the
            remaining bits should be ZERO on output and ignored on
            input.  This bit indicates if the transport expedited data
            service is to be used.  If this bit is set (bit mask 0000
            0001) or this parameter does not appear in the TPDU, then
            the expedited data service is requested.  If this parameter
            appears in the TPDU and the bit is not set then the service
            is disabled.  If the service is requested, then the TSAP-ID
            of the sender of the TPDU must include an attribute
            indicating the internet address to use for expedited data.

         o   Parameter name:       Alternative protocol classes

            Parameter code:        199 (binary 1100 0111)

            Parameter length:      variable
            Parameter value:       64 (binary 0100 0000) in each octet

               This is used as a NOOP in the variable data.  Its use is
               HIGHLY discouraged, but for those implementors who wish
               to align the user data portion of the TPDU on word (or
               page) boundaries, use of this parameter for filling is
               recommended.

      11.  user data            (packet length - header length - 5)
                                   octets

         This portion of the TPDU is actual user data, most probably one
         or more SPDUs (session protocol data units).







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   The format of a DR TPDU is:

       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|     destination reference     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       source reference        |     reason    | variable data |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    ...        |      ...      |      ...      |      ...      |
      |    ...        |      ...      |      ...      |      ...      |
      |      ...      |   user data   |      ...      |      ...      |
      |    ...        |      ...      |      ...      |      ...      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format of the fields is identical to those of a CR or CC TPDU,
   with the following exceptions:

   where:

      8.  reason                        8 bits

         This replaces the class/option fields of the CR or CC TPDU. Any
         value, as specified in [ISO-8073], may be used in this field.
         This memo makes use of several:

            value       meaning
            -----       -------
              1         Congestion at TSAP
              2         Session entity not attached to TSAP
              3         Address unknown (at TCP connect time)
            128+0       Normal disconnect initiated by the session
                        entity
            128+1       Remote transport entity congestion at connect
                        request time
            128+3       Connection negotiation failed
            128+5       Protocol Error
            128+8       Connection request refused on this network
                        connection










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ISO Transport Services on Top of the TCP


   The format of a DT or ED TPDU is:

       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   |      ...      |      ...      |      ...      |
      |    ...        |      ...      |      ...      |      ...      |
      |    ...        |      ...      |      ...      |      ...      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

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

      6.  TPDU-NR and EOT               16 bits

         This field is always ZERO on output and ignored on input.

7.  Expedited Data

   This memo utilizes a second TCP connection to handle expedited data
   and does not make use of the TCP URGENT mechanism.  The primary
   disadvantage of this decision is that single-threaded implementations
   of TCP may have some difficulty in supporting two simultaneous
   connections.  There are however several advantages to this approach:

      a.  Use of a single connection to implement the semantics of
      expedited data implies that the TSAP peer manage a set of buffers
      independent from TCP.  The peer would, upon indication of TCP
      urgent information, have to buffer all preceeding TPKTs until the
      TCP buffer was empty.  Expedited data would then be given to the
      TS-user.  When the expedited data was flushed, then the buffered
      (non-expedited) data could be passed up to the receiving user.

      b.  It assumes that implementations support TCP urgency correctly.
      This is perhaps an untrue assumption, particular in the case of
      TCP urgency occuring when the send window is zero-sized.  Further,
      it assumes that the implementations can signal this event to the
      TCP-user in a meaningful fashion.  In a single-threaded
      implementation, this is not likely.

   Given a reasonable TCP implementation, the TS-peer need listen on two




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ISO Transport Services on Top of the TCP


   TCP sockets in either polling or interrupt mode.  When the TS-peer is
   given data, the TCP must indicate which connection should be read
   from.

   The only tricky part of the protocol is that the client must be able
   to start a passive OPEN for the expedited port, and then wait to read
   from another connection.  In between the passive OPEN and the other
   connection supplying data, the server will connect to the expedited
   port, prior to sending data on the other connection.  To summarize
   from Section 5, the sequence of events, with respect to TCP, is:

      time      client                          Server
      ----      ------                          ------
      0.                                        passive OPEN of port 102

      1.        T-CONNECT.REQUEST from user
                passive OPEN of expedited
                port (non-blocking)

      2.        active OPEN of port 102

      3.        send CC TPKT

      4.                                        port 102 connected

      5.                                        receive CC TPKT
                                                T-CONNECT.INDICATION to
                                                user
                                                T-CONNECT.RESPONSE from
                                                user

      6.                                        active OPEN to expedited
                                                port

      7.        expedited port connected

      8.                                        send CR TPKT

      9.        receive CR TPKT
                verify expedited port
                connected correctly

   Multi-threaded implementations of TCP should be able to support this
   sequence of events without any great difficulty.





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8.  Conclusions

   There are two design decisions which should be considered.  The first
   deals with particular packet format used.  It should be obvious to
   the reader that the TP packet format was adopted for use in this
   memo.  Although this results in a few fields being ignored (e.g.,
   source reference), it does not introduce an unacceptable amount of
   overhead.  For example, on a connection request packet (the worst
   case) there are 6 bytes of "zero on output, ignore on input" fields.
   Considering that the packet overhead processing is fixed, requiring
   that implementations allocate an additional 1.5 words is not
   unreasonable!  Of course, it should be noted that some of these
   fields (i.e., class) may be used in future versions of the protocol
   as experience is gained.

   The second decision deals with how the TCP and TSAP port space is
   administered.  It is probably a very bad idea to take any
   responsibility, whatsoever, for managing this addressing space, even
   after ISO has stabilized.  There are two issues involved.  First, at
   what level do the TCP and TSAP port spaces interact; second, who
   defines what this interaction looks like.  With respect to the first,
   it wholly undesirable to require that each TSAP port map to a unique
   TCP port.  The administrative problems for the TCP "numbers czar (and
   czarina)" would be non-trivial.  Therefore, it is desirable to
   allocate a single TCP port, namely port 102, as the port where the
   "ISO Transport Services" live in the TCP domain. Second, the
   interaction defined in Appendix A of this memo is embryonic at best.
   It will no doubt be replaced as soon as the ISO world reaches
   convergence on how services are addressed in ISO TP. Therefore
   readers (and implementors) are asked to keep in mind that this aspect
   of the memo is guaranteed to change.  Unfortunately, the authors are
   not permitted the luxury of waiting for a consensus in ISO.  As a
   result, the minimal effort approach outlined in the appendix below
   was adopted.

   A prototype implementation of the protocol described by this memo is
   available for 4.2BSD UNIX.  Interested parties should contact the
   authors for a copy.  To briefly mention its implementation, a given
   ISO service is implemented as a separate program.  A daemon listens
   on TCP port 102, consults a database when a connection request packet
   is received, and fires the appropriate program for the ISO service
   requested.  Of course, given the nature of the BSD implementation of
   TCP, as the child fires, responsibility of that particular connection
   is delegated to the child; the daemon returns to listening for new
   connection requests.  The prototype implementation consists of both
   the daemon program and subroutine libraries which are loaded with
   programs providing ISO services.


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ISO Transport Services on Top of the TCP


9.  References

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

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

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

   [RFC-791]    Internet Protocol.
                Request for Comments 791
                (September, 1981)
                (See also: MIL-STD-1777)

   [RFC-793]    Transmission Control Protocol.
                Request for Comments 793
                (September, 1981)
                (See also: MIL-STD-1778)

   [RFC-960]    Assigned Numbers.
                Request for Comments 960
                (December, 1985)

   [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)




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

   [X.410]      CCITT.
                "Recommendation X.410.  Message Handling Systems: Remote
                Operations and Reliable Transfer Server."
                (October, 1984)

Appendix A:  Reserved TSAP IDs

   Version 1 of this protocol uses a relatively simple encoding scheme
   for TSAP IDs.  A TSAP ID is an attribute list containing two
   parameters, a 32-bit IP address, and a 16-bit port number.  This is
   used to identify both the client TSAP and the server TSAP.  When it
   appears in a TPKT with code CR or CC, the TSAP ID is encoded in the
   variable data part for the client TSAP as:

       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 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      193      |       8       |       1       |       6       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       a       |       b       |       c       |       d       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              port             |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         and for the server TSAP as:

       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 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      194      |       8       |       1       |       6       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       a       |       b       |       c       |       d       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              port             |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (Neither of these examples include an attribute for a TCP connection
   for expedited data.  If one were present, the length of the TSAP ID
   attribute would be 16 instead of 8, and the 8 bytes following the
   Internet address would be "2" for the attribute code, "6" for the



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ISO Transport Services on Top of the TCP


   attribute length, and then 6 octets for the Internet address to use
   for expedited data, 4 octets for IP address, and 2 octets for TCP
   port.)

   Where [a.b.c.d] is the IP address of the host where the respective
   TSAP peer resides, and port is a 16-bit unsigned integer describing
   where in the TSAP port space the TS-user lives.

      Port value        Designation
      ----------        -----------
          0             illegal
         1-4096         privileged
      4097-65535        user

   The following table contains the list of the "official" TSAP ID port
   numbers as of the first release of this memo.  It is expected that
   future editions of the "Assigned Numbers" document[RFC-960] will
   contain updates to this list.

      Port    name        ISO service
      ----    ----        -----------
      1       echo        unofficial echo
      2       sink        unofficial data sink
      3       FTAM        File Transfer, Access, and Management
      4       VTS         ISO-8571 Virtual Terminal Service
      5       MHS         Message Handling System [X.411]
                          CCITT X.400
      6       JTM         Job Transfer and Manipulation
                          ISO 8831/8832
      7       CASE        Common Application Service Elements
                          Kernel ISO-8650/2

   If anyone knows of a list of "official" ISO services, the authors
   would be very interested.















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