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Network Working Group                                            C. ShueRequest for Comments: 1240                      Open Software Foundation                                                             W. Haggerty                                                 Wang Laboratories, Inc.                                                              K. Dobbins                                                 Cabletron Systems, Inc.                                                               June 1991OSI Connectionless Transport Services on top of UDPVersion: 1Status of this Memo   This document describes a protocol for running OSI Connectionless   service on UDP.  This RFC specifies an IAB standards track protocol   for the Internet community, and requests discussion and suggestions   for improvements.  Please refer to the current edition of the "IAB   Official Protocol Standards" for the standardization state and status   of this protocol.  Distribution of this memo is unlimited.1.  Introduction and Philosophy   The Internet community has a well-developed, mature set of layered   transport and network protocols, which are quite successful in   offering both connection-oriented (TCP) and connectionless (UDP)   transport services over connectionless network services (IP) to end-   users.  Many popular network applications have been built directly on   top of the TCP and UDP over the past decade.  These have helped the   Internet services and protocols to become widely-spread de facto   standards.  In the past few years, the ISO and CCITT have defined a   well-architected set of upper layer standards which include   connection-oriented and connectionless session, presentation, and   application layer services and protocols.  These OSI upper layer   standards offer valuable services to application developers (e.g.,   dialogue control, transfer syntax, peer authentication, directory   services, etc.) which are not currently offered by the TCP/IP   standards.   As indicated inRFC 1006, it is desirable to offer the OSI upper   layer services directly in the Internet without disrupting existing   facilities.  This permits a more graceful convergence and transition   strategy from IP-based networks to OSI-based networks in the future.   Using the approach ofRFC 1006, this memo specifies how to offer OSI   connectionless transport service using the User Datagram Protocol   (UDP) [RFC768] of the TCP/IP suite.   We will define a Transport Service Access Point (TSAP) which appearsShue, Haggerty & Dobbins                                        [Page 1]

RFC 1240                   OSI on top of UDP                   June 1991   to be identical to the services and interfaces defined in ISO 8072   and its Addendum 1, but we will in fact implement the ISO T-UNIT-DATA   protocol on top of UDP.  By this means, OSI TPDU's can be delivered   across the Internet network, and OSI connectionless upper layers can   operate fully without knowledge of the fact that they are running on   top of UDP/IP.  In essence, the OSI T-UNIT-DATA service will use UDP   as its connectionless network service provider.2.  Motivation   The primary motivation for the standard described in this memo is to   facilitate the process of gaining experience with OSI connectionless   upper layers protocols, i.e., S-UNIT-DATA [ISO9548], P-UNIT-DATA   [ISO9576] and A-UNIT-DATA [ISO10035], and connectionless transport   protocol T-UNIT-DATA [ISO8602].   Though many OSI standard applications such as X.400 and FTAM are   connection-oriented, it is recognized in the OSI reference model   [ISO7498/AD1] as well as in practice that the connectionless-mode   operations are appropriate for certain distributed application   classes and environments.  The following connectionless application   classes were identified by ISO SC21/WG6 [ISOSC21/WG6 N184]:         -    Request-Response Applications         -    Broadcast/Multicast         -    Inward Data Collection         -    Migratory/Unreliable Processes   Among them, the "request/response" client-server application class is   the most prominent one, which is gaining popularity and importance.   It is observed that the connection setup and tear-down protocol   exchanges and complex connection-oriented protocol machines become   unnecessary overheads for a simple request/response exchange between   a client application and a server application, especially in reliable   communications environments such as LAN and ISDN.  The OSI   connectionless upper layers are thought to be highly effective and   efficient, both in time and space, for the distributed application   classes mentioned above.   The stability, maturity and wide availability of UDP/IP are ideal for   providing solid connectionless transport services independent of   actual implementations.3.  The Model   The [ISO 8072/AD1] standard describes the OSI connectionless   transport services definition. The [ISO 8602] standard describes the   OSI connectionless transport protocols.  A defining characteristic ofShue, Haggerty & Dobbins                                        [Page 2]

RFC 1240                   OSI on top of UDP                   June 1991   transport connectionless-mode transmission is the independent nature   of each invocation of the connectionless transport service.   The OSI transport service definition describes the services offered   by the TS-provider and the interfaces used to access those services.   It also describes the services required.  This memo focuses on how   UDP [RFC 768] can be used to offer the required services and provide   the interfaces.   The following is the model:        +-----------+                               +-----------+        |  TS-user  |                               |  TS-user  |        +-----------+                               +-----------+              |                                           |              |CLTS interface                             |              |[ISO 8072/AD1]                             |              |                                           |     _________________________________________________________________    |         |                                           |           |    |         |                                           |           |    |   +-----------+            UD TPDU            +-----------+     |    |   |  TS-peer  |   <----------------------->   |  TS-peer  |     |    |   +-----------+                               +-----------+     |    |         |                                           |           |    |         |                                           |           |    |         |                                           |           |    |         |UDP interface [RFC 768]                    |           |    |         |                                           |           |    |   +-----------+          UDP datagram         +-----------+     |    |   |    UDP    |   <----------------------->   |    UDP    |     |    |   +-----------+     (UD TPDU encapsulated)    +-----------+     |    |         |                                           |           |    |         |                                           |           |    |         |                                           |           |    |         |                                           |           |    |                                                                 |    |                                                                 |    |                           TS-provider                           |    |_________________________________________________________________|The following abbreviations are used:    CLTS          Connectionless TransportShue, Haggerty & Dobbins                                        [Page 3]

RFC 1240                   OSI on top of UDP                   June 1991    TS            Transport Services (implies connectionless transport                  service in this memo)    TSAP          Transport Service Access Point    TS-peer       a process which implements the mapping of CLTS                  protocols onto the UDP interface as described by                  this memo    TS-user       a process using the services of a TS-provider    TS-provider   the abstraction of the totality of those entities                  which provide the overall service between the two                  TS-users    UD TPDU       Unit Data TPDU (Transport Protocol Data Unit)   Each TS-user gains access to the TS-provider at a TSAP.  The two TS-   users can communicate with each other using a connectionless   transport provided that there is pre-arranged knowledge about each   other (e.g., protocol version, formats, options, ... etc.), since   there is no negotiation before data transfer.  In the above diagram   one TS-user passes a message to the TS-provider, and the peer TS-user   receives the message from the TS-provider.  The interactions between   TS-user and TS-provider are described by connectionless TS   primitives.   All aspects of [ISO 8072/AD1] are supported in this memo with one   exception: QOS (Quality of Service) parameter, which is left for   future study.   The OSI standards do not specify the format of a TSAP selector.   Neither does this memo.  However, implementors should consult the   GOSIP 1.0 specification [GOSIP88/FIPS146] for an interpretation of   this parameter, wherein the TSAP selector consists of two octets and   a value of (binary) 1 identifies the service interface between OSI   transport layer and session layer.4.  The Primitives   This RFC assumes that UDP [RFC768] offers the following service   primitives:    send datagram  -  datagram is sent to the IP address/destination                      port    read datagram  -  datagram is read from UDPShue, Haggerty & Dobbins                                        [Page 4]

RFC 1240                   OSI on top of UDP                   June 1991   Data can only be read from a receive port after the port has been   created.  This is a local matter.   This memo reserves the use of UDP port 102 for the use of   applications which realize the CLTS over UDP.  However as withRFC1006, other port values may be used by prior agreement (e.g., through   use of the OSI Directory).   This RFC describes how to use these services to emulate the following   connectionless-mode network service primitives, which are required by   [ISO8602]:    N-UNIT-DATA.REQUEST     -  A NS-user requests unit data to be sent    N-UNIT-DATA.INDICATION  -  A NS-user is notified that unit data                               can be read from the NSAP   The mapping between the UDP service primitives and the service   primitives expected by the connectionless transport peer entity are   quite straightforward:    connectionless network service  UDP    ------------------------------  ---    N-UNIT-DATA.REQUEST             send datagram    N-UNIT-DATA.INDICATION          read datagramThe parameter mapping is:    connectionless network service  UDP    ------------------------------  ---    Source address                  source IP address from                                    calling TS-address    Destination address             destination IP address from                                    called TS-address    Quality of service              (ignored)    NS-user data                    UD TPDU constructed from T-UNIT-DATA   When the T-UNIT-DATA.REQUEST primitive is issued, the TS-peer   constructs a UD TPDU and sends it as a single datagram to the desired   IP address using UDP.Shue, Haggerty & Dobbins                                        [Page 5]

RFC 1240                   OSI on top of UDP                   June 1991   When UDP indicates that a datagram has been received, a UD TPDU is   read from UDP, and a T-UNIT-DATA.INDICATION primitive is generated.5.  Packet Format   The following is the UD TPDU structure which is encapsulated in UDP   data field:          1       2         3                m m+1          n          +--------------------------------------------------+          |  LI  |  UD      |  Variable Part  |  User Data   |          |      | 01000000 |                 |              |          +--------------------------------------------------+    LI (octet 1) -  the length of the header including parameters, but                    excluding the LI and user data, with a maximum                    value of 254    UD (octet 2) -  the type of TPDU    Variable Part (octets 3 to m) - the source and destination TSAP id's          Parameter code:   source TSAP       11000001                            destination TSAP  11000010          Parameter length: the length of the parameter, not including                            the parameter code or length fields, with a                            maximum value of 254          Parameter value:  source or destination T-selector          The optional checksum parameter is not required in the          variable part since the UDP checksum field in the UDP header          already performs the checking.    User Data (octets m+1 to n) -  all the data of the TSDU.   The maximum NS-user data allowed in the OSI connectionless network   service is 64,512 octets.  This limit is further constrained by the   lesser maximum datagram size supported by the two communicating UDP   peers, which should be known by a priori agreement.6.  Conclusion   There is a general trend towards support of the OSI protocol suite in   the Internet.  This direction is being fostered by the Internet   Activities Board (IAB) and its Internet Engineering Task Force, and   by the Federal Networking Council.  By offering an OSI connectionless   transport service on top of the Internet, this RFC will allow future   applications to use the OSI connectionless upper-layer services,   which are required to be conformant to the OSI upper layerShue, Haggerty & Dobbins                                        [Page 6]

RFC 1240                   OSI on top of UDP                   June 1991   architecture.  Currently, T-UNIT-DATA, S-UNIT-DATA, P-UNIT-DATA, and   A-UNIT-DATA have reached International Standard (IS).  As   applications based on OSI connectionless services become widely   available and OSI lower-layer service is widely implemented in the   Internet, the underlying UDP/IP services can be simply replaced with   the OSI lower layers.7.  Acknowledgements   Marshall T. Rose of PSI, Inc., provided many valuable comments and   corrections.8.  References   [GOSIP88]  U.S. Department of Commerce/National Bureau of Standards,   [FIPS146]  "Government Open Systems Interconnection Profile (GOSIP)",              August 1988.   [ISO7498/AD1]  ISO, "International Standard 7498 - Information              Processing Systems - OSI:  Basic Reference Model              Addendum 1: Connectionless-mode Transmission",              May 1986.   [ISO8072]  ISO, "International Standard 8072 - Information Processing              Systems - OSI:  Transport Service Definition", June 1984.   [ISO8072/AD1]  ISO, "International Standard 8072 - Information              Processing Systems - OSI:  Transport Service Definition              Addendum 1: Connectionless-mode Transmission",              December 1986.   [ISO8602]  ISO, "International Standard 8602 - Information Processing              Systems - OSI:  Connectionless Transport Protocol              Specification", December 1986.   [ISO9548]  ISO, "International Standard 9548 - Information Processing              Systems - OSI:  Connectionless Session Protocol              Specification", April 1989.   [ISO9576]  ISO, "Draft International Standard 9576 - Information              Processing Systems - OSI:  Connectionless Presentation              Protocol Specification", April 1989.   [ISO10035]  ISO, "Draft International Standard 10035 - Information              Processing Systems - OSI:  Connectionless ACSE Protocol              Specification", April 1989.   [ISOSC21/WG6 N184]  ISO SC21 WG6, "Justification for ConnectionlessShue, Haggerty & Dobbins                                        [Page 7]

RFC 1240                   OSI on top of UDP                   June 1991              Services in the Upper Layers", June 1986.   [RFC768]   Postel, J., "User Datagram Protocol",RFC 768,              USC/Information Sciences Institute, September 1981.   [RFC791]   Postel, J., "Internet Protocol",RFC 791,              USC/Information Sciences Institute, September 1981.   [RFC1006]  Rose, M., and D. Cass, "ISO Transport Service on top of              the TCP - Version 3",RFC 1006, Northrop Research and              Technology Center, May 1987.Security Considerations   Security issues are not discussed in this memo.Authors' Addresses   Chikong Shue   Open Software Foundation, Inc.   11 Cambridge Center   Cambridge, MA 02142   Phone: (617) 621-8972   EMail: chi@osf.org   William Haggerty   Wang Laboratories, Inc.   1 Industrial Ave   Lowell, MA 01851   Phone: (508) 967-3403   EMail: bill@comm.wang.com   Kurt Dobbins   Cabletron, Inc.   35 Industrial Way   Rochester, NH 03867   Phone: (603) 332-9400Shue, Haggerty & Dobbins                                        [Page 8]