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rfc:rfc983

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,

Cass & Rose [Page 1]

RFC 983 April 1986 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
  1. a TS-user (server) is notified that connection establishment

is in progress

    T-DISCONNECT.INDICATION
  1. a TS-user is notified that the connection is closed
    T-CONNECT.CONFIRMATION
  1. a TS-user (client) is notified that the connection has been

established

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

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

connection

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

the connection

 Actions
    T-CONNECT.RESPONSE
  1. a TS-user (server) indicates that it will honor the request
    T-DISCONNECT.REQUEST
  1. a TS-user indicates that the connection is to be closed
    T-CONNECT.REQUEST
  1. a TS-user (client) indicates that it wants to establish a

connection

    T-DATA.REQUEST
  1. a TS-user sends data
    T-EXPEDITED DATA.REQUEST
  1. 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:
  1. the TSAP ID of the TS-user on the client's host (the "caller")
  2. the TSAP ID of the TS-user that the client wants to talk to

(the "called")

  1. if the expedited data option was requested, the TSAP ID of the

expedited port for the client's host

  1. 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:
  1. the TSAP ID of the TS-user responding to the connection

(usually the "called")

  1. 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

Cass & Rose [Page 10]

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

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

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

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

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

 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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RFC 983 April 1986 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

Cass & Rose [Page 22]

RFC 983 April 1986 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.

Cass & Rose [Page 23]

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

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.

Cass & Rose [Page 24]

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

 [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

Cass & Rose [Page 26]

RFC 983 April 1986 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.

Cass & Rose [Page 27]

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