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rfc:rfc1007

Network Working Group Wayne McCoy Request for Comments: 1007 June 1987

                       MILITARY SUPPLEMENT
                             TO THE
                     ISO TRANSPORT PROTOCOL

Status of this Memo

 This RFC is being distributed to members of the Internet community
 in order to solicit comments on the Draft Military Supplement.
 While this document may not be directly relevant to the research
 problems of the Internet, it may  be of some interest to a number
 of researchers and implementors. Distribution of this memor is
 unlimited.

1. SCOPE

1.1 Purpose.

 This document supplements the Transport Service and Protocol of the
 International Standards Organization (ISO), IS 8072 and IS 8073,
 respectively, and their formal descriptions by providing conventions,
 option selections and parameter values to be used when the protocol
 is operated within the scope of the applicability statement in
 Paragraph 1.3 below.  Paragraph 1.4, below, describes the ISO
 standards.  Full implementation detail is not provided in this
 document, but reference is made to a separate document, entitled
 "Implementation Guide for the ISO Transport Protocol", in which
 guidance for implementation is given.

1.2 Organization.

 Five sections comprise this supplement.  In Section 1, the role and
 purpose of the Transport Protocol are stated and the international
 standards upon which the protocol is based are described.  These
 documents, as well as others supporting  the international standards
 and this supplement are listed in  Section 2.  Other definitions not
 already included in the international standards and supporting
 documents are given in Section 3.  The international standards cover
 a very wide variety of network environments and situations.  There
 is, thus, a collection of options and parameters provided by the
 standards which must be determined for specific uses.  Section 4
 states the options and parameters relevant to those implementations
 to which this supplement applies, and defines usage conventions.

McCoy [Page 1] RFC 1007 June 1987

 Conventions for addressing and Transport connection reference
 number usage and recovery of the Transport connection from peer
 deactivation are covered in Section 5.

1.3 Application.

 The use of the Transport Protocol Class 4 and the Protocol for
 Providing the Connectionless-Mode Network Service (IS 8473) is
 mandatory foruse in all DOD packet-switched data networks where
 there is a potential for host-to-host connectivity across network
 or subnetwork boundaries.  The term "network" as used here shall
 include Local Area Networks but not integrated weapons systems.
 The use of the Transport Protocol Class 4 and IS 8473 is
 strongly encouraged, particularly  where a need for equipment
 interchangeability or survivability is perceived. Use of the
 Transport Protocol Class 4 and IS 8473 in weapons systems, where
 such usage does not diminish required performance, is also
 encouraged.

1.4 International Standards Organization Transport Protocol.

 The international standard upon which this supplement is based is
 described in four documents:
     a. IS 8072, the Transport Service Definition, which defines the
        service that Transport provides to a user, described in
        English text;
     b. WG4 N53, the Formal Description of the Transport Service, in
        which the Transport Service is described using a formal
        description language;
     c. IS 8073, the Transport Protocol, in which the protocol is
        specified in English text; and
     d. N123, the formal description of the Transport Protocol, in
        which the specification IS 8073 is written in a formal
        description language.
 The ISO protocol has five classes of service, named Class 0 through
 Class 4.  Only Classes 4 and 2 will apply to this supplement.  The
 formal description language, Estelle, DP 9074, provides for protocol
 descriptions in terms of communicating finite state automata. It
 contains a subset language which corresponds to the international
 standard Pascal.  The Class 4 protocol operation when supported by a
 connectionless network service is described in an addendum to IS
 8073, N3339(rev).

McCoy [Page 2] RFC 1007 June 1987

2. REFERENCED DOCUMENTS

2.1 Issues of Documents.

 The following documents of the issue in effect on date of invitation
 for bids or request for proposal form a part of this supplement to
 the extent specified herein.
               FED-STD-1037  - Federal Standard - 1037,
                 Glossary of Telecommunication Terms.
          Implementation Guide for the ISO Transport Protocol

2.2 Other Publications.

 The following documents form part of this standard to the extent
 specified herin.  Unless otherwise indicated, the issue in effect on
 the date of invitation for bids or request for proposal shall apply.
 IS 8072 - Information Processing Systems -
      Open Systems Interconnection - Transport Service Definition.
      Available from: ANSI ISO TC97/SC6 Secretariat 1430 Broadway
      New York, NY 10018 (212) 354-3343
 IS 8073 - Information Processing Systems -
      Open Systems Interconnection - Transport  Protocol
      Specification. Available from ANSI (SC6 Secretariat).
 N3339(rev) - Draft Proposed Addendum to IS 8073
      to Enable Class 4 Operation Over Connectionless Mode Network
      Service as Defined in ISO/ISO 8348/AD1.  Available from ANSI
      (SC6 Secretariat).
 DP 9074 - Estelle - A Formal Description
      Technique Based on an Extended State Transition Model.
      Available from ANSI (SC21 Secretariat), address as for SC6,
      above.
 WG4 N53 - Information Processing Systems -
      Open Systems Interconnection - Formal Description of IS 8072
      in Estelle. (Working draft, ISO TC 97/SC 6/WG 4)
 N123 - Information Processing Systems -
      Open Systems Interconnection - Formal Description of IS 8073
      in Estelle. (Working draft, ISO TC 97/SC 6)
 IS 8473 - Information Processing Systems -
      Data Communications - Protocol for Providing the
      Connectionless-mode Network Service. Available from ANSI
      (SC6 Secretariat).

McCoy [Page 3] RFC 1007 June 1987

3. DEFINITIONS

3.1 Definition of terms.

 The definition of terms used in this standard shall comply with
 FED-STD-1037, ISO IS 8072, IS 8073 and IS 8473.  Other terms and
 definitions unique to N3756, WG4 N53 and N3339(rev) appear in
 those documents.

3.2 Abbreviations and acronyms.

 The following abbreviations and acronyms are used in this
 supplement:
   a.  ISO.  The International Standards Organization;
   b.  OSI.  Open Systems Interconnection;
   c.  TS.  Transport service;
   d.  TSAP.  Transport service access point;
   e.  NSAP.  Network service access point;
   f.  TPDU.  Transport protocol data unit;
   g.  CR.  Connect request;
   h.  CC.  Connect confirm;
   i.  DR.  Disconnect request;
   j.  ER.  Error;
   k.  AK.  Acknowledgement;
   l.  IP.  Internetwork protocol;
   m.  LAN.  Local area network.
   n.  CONS. Connection oriented network service.
   o.  CLNS. Connectionless network service.
   (Other provisions of this Section are under consideration.)

McCoy [Page 4] RFC 1007 June 1987

4. GENERAL REQUIREMENTS

4.1 Conformance.

 Implementations to which this supplement applies shall satisfy the
 conformance requirements (Clause 14, of IS 8073 and N3339(rev), as
 adapted for this supplement) in the following statements.
   a.  A system claiming to implement the procedures specified
       in this standard shall comply with the requirements in
       b. - d., below.
   b.  The system shall implement:
       b.1  Class 2 or Class 0 or both, if operated over a connection
            oriented network service; or
       b.2  Class 4 if operated over a connectionless network service.
   c.  If the system implements Class 4, it shall also implement
       Class 2, if it is operated over a connection oriented network
       service.  Class 2 shall not be implemented if operation is
       exclusively over a connectionless network service.
   d.  For each class which the system claims to implement, the
       system shall be capable of:
       d.1  initiating CR TPDUs or responding to CR TPDUs with TPDUs
            or both;
       d.2  responding to any other TPDU and operating network
            service in accordance with procedures for the class;
       d.3  operating all the procedures for the class listed as
            mandatory in the Provisions of Options table below;
       d.4  operating those procedures for the class, listed as as
            optional in the Provisions of Options table, for which
            conformance is claimed; and
       d.5  handling all TPDUs of lengths up to the lesser value of:
            d.5.1  the maximum length for the class;
            d.5.2  the maximum for which conformance is claimed.
   e.  Claims of conformance shall state:
       e.1  whether or not operation over connectionless service is
            implemented;
       e.2  which class or classes of protocol are implemented, if

McCoy [Page 5] RFC 1007 June 1987

            operation over a connection oriented network is
            implemented;
       e.3  whether the system is capable of initiating or responding
            to CR TPDUs or both;
       e.4  which of the procedures listed in the Provisions of
            Options table are implemented;
       e.5  the maximum size of TPDU implemented; the value shall be
            chosen from the following list and all values in the list
            which are less than this maximum shall be implemented:
            128, 256, 512, 1024, 2048, 4096, or 8192 octets.
 Provision of options (adapted from IS 8073, Table 9)
  __________________________________________________________________
 |        PROCEDURE         |     CLASS 2        |     CLASS 4     |
 |__________________________|____________________|_________________|
 |                          |                    |                 |
 |TPDU with checksum        |not applicable      |mandatory        |
 |TPDU without checksum     |mandatory           |optional         |
 |__________________________|____________________|_________________|
 |                          |                    |                 |
 |Expedited data transfer   |mandatory           |mandatory        |
 |No expedited data transfer|mandatory           |mandatory        |
 |__________________________|____________________|_________________|
 |                          |                    |                 |
 |Flow control in Class 2   |mandatory           |not applicable   |
 |No flow control in Class 2|optional            |not applicable   |
 |__________________________|____________________|_________________|
 |                          |                    |                 |
 |Normal formats            |mandatory           |mandatory        |
 |Extended formats          |optional            |optional         |
 |__________________________|____________________|_________________|
 The explicit manner in which implementations, to which this
 supplement applies, shall satisfy these conformance statements is
 given in Paragraph 4.4.  The options are described in more detail in
 Paragraph 4.3.

4.2 Transport Service access parameters.

 Each of the services of transport has parameters that identify
 communicating peers, express options for operation of the transport
 connection, or transmit data from one peer user to the other.  The
 conventions for these parameters for usage in implementations to
 which this supplement applies are given below.

McCoy [Page 6] RFC 1007 June 1987

4.2.1 Connect Service.

 The Connect Service is summarized below (refer to IS 8072 for
 detailed discussion):
 __________________________________________________________________
 |             Primitives                        Parameters       |
 |________________________________________________________________|
 | T-CONNECT          request     |  Called Address,              |
 |                    indication  |  Calling Address,             |
 |                                |  Expedited Data Option,       |
 |                                |  Quality of Service,          |
 |                                |  TS User-Data                 |
 |________________________________|_______________________________|
 | T-CONNECT          response    |  Responding Address,          |
 |                    confirm     |  Quality of Service,          |
 |                                |  Expedited Data Option,       |
 |                                |  TS User-Data                 |
 |________________________________|_______________________________|
 Conventions for Called Address, Calling Address and Responding
 Address will appear in Paragraph 5.1.1.  Use of the Expedited
 Data Option is dependent on the nature of the transport user;
 this supplement does not define how transport users will decide
 on such usage.  The parameters that define Quality of Service are
 discussed in IS 8072.  However, the manner in which these
 parameters are to be applied in an implementation issue , and the
 mechanisms to be used to maintain the requested quality of sevice
 are not defined.  It is thus recommended that these parameters
 not be used in implementations until such time that usage
 definition exists. The amount of data passed in TS User-Data is
 constrained to 32 octets or less.  (This TS User-Data parameter
 shall not be used for any data that requires any security protection
 whatever.)  No implementation is required to be able to send such
 data received from its user, but each implementation shall be
 capable of passing data received from the remote peer user during
 connection establishment to its user.

4.2.2 Disconnect Service.

 __________________________________________________________________
 |             Primitives                        Parameters       |
 |________________________________________________________________|
 | T-DISCONNECT       request     |  TS User-Data                 |
 |________________________________|_______________________________|
 | T-DISCONNECT       indication  |  TS User-Data,                |
 |                                |  Disconnect reason            |
 |________________________________|_______________________________|
 The Disconnect Service is abrupt in the sense that data may be lost

McCoy [Page 7] RFC 1007 June 1987

 whenever the service is invoked.  Transport user processes should
 therefore ensure that all data intended to be received has in fact
 been received before issuing a T-DISCONNECT-request.  The data used
 in the TS User-Data parameter is constrained to be 64 octets or less
 in length.  (The TS User-Data parameter shall not be used for data
 that requires any security protection whatever.)  Disconnect reasons
 are discussed in IS 8073, and reasons other than those listed in IS
 8073 are permitted.

4.2.3 Data Transfer Service.

 __________________________________________________________________
 |             Primitives                      Parameters         |
 |________________________________________________________________|
 | T-DATA             request     |  TS User-Data                 |
 |                    indication  |                               |
 |________________________________|_______________________________|
 The length of the data that is carried by the TS User-Data parameter
 is not constrained by the ISO Standard, but interface considerations
 may impose practical limits.  This is discussed further in the
 Implementors guide, Part 3.1.  For the purposes of this supplement,
 the TS User-Data parameter in this service is considered to be
 protected and should be used for any data requiring security
 protection.

4.2.4 Expedited Data Service.

 __________________________________________________________________
 |               Primitives                   Parameters          |
 |________________________________________________________________|
 | T-EXPEDITED-DATA   request     |  TS User-Data                 |
 |                    indication  |                               |
 |________________________________|_______________________________|
 The TS User-Data parameter is constrained to be no longer than
 16 octets and shall not be used for data requiring any security
 protection whatever.  The T-EXPEDITED-DATA-request cannot be used
 whenever non-use of expedited data was called for in either the
 T-CONNECT-request or T-CONNECT-response primitive.

4.3 Options.

 The protocol described in IS 8073 and N3756 permits certain options
 which qualify or enhance the service to be provided.  Negotiated
 options are those which both communicating peer transport entities
 agree upon during connection establishment. Local options are those
 which apply to a particular implementation of transport that may
 be used to enhance performance, optimize resource utilization or
 improve resilience to network failures. The election of a local
 option is invisible to the remote peer entity.

McCoy [Page 8] RFC 1007 June 1987

4.3.1 Negotiated options.

 The options in IS 8073 that shall be negotiated between peer
 transport entities are given in the following list.  The elections
 of these options to be taken in  an implementation to which this
 supplement applies are defined in Paragraph 4.4.
   a.   a. Class of service--agreement as to one of five classes of
        transport service, depending on which classes are supported by
        the entities, the quality of the network service available and
        the degree of resilience to network errors and failure
        required by the peer transport users.
   b.   b. Use of extended formats--agreement to use or not use
        extended formats for sequence numbering and flow control
        credit; normal formats provide sequence numbers in the range 0
        - (2**7 - 1) and flow control credit in the range 0 - (2**15 -
        1); extended  formats provided sequence numbers in the range 0
        - (2**31 - 1)  and credit in the range 0 - (2**16 - 1).
   c.   Use of expedited data transfer--agreement to use or not to use
        expedited data transfer during normal data transfer
        procedures.
   d.   Maximum size of protocol data units to be exchanged--agreement
        to limit size of exchanged protocol data units, depending on
        buffer resources that the entities have and network quality of
        service; values negotiated are in the range 2**7 - 2**13
        octets (total length).
   e.   Use of checksum--agreement to use or not to use a 16-bit
        checksum on each protocol data unit exchange between the
        entities, depending on expected residual error rate in the
        network service used.
   f.   Protection parameters--agreement as to how protection will be
        defined and maintained on the transport connection; these
        parameters are defined by the communicants which elect to use
        them.
   g.   Use of flow control in Class 2--agreement to use or not to use
        flow control in Class 2 when Class 2 operation has been
        negotiated. Conformance to the ISO Standard requires that if
        Class 4 is supported over CONS, then Class 2 shall also be
        supported.
   h.   Service quality parameters--agreement as to the quality of
        service to be expected on the transport connection; the ISO
        Standard does not state how these parameters are to be used by
        the transport entities or their users.

McCoy [Page 9] RFC 1007 June 1987

4.3.2 Local options, Class 2.

 The options that an implementor may decide in a particular Class 2
 implementation are given in the following list.  Recommendations
 and requirements for these options for the purposes of this
 this supplement are given in Paragraph 4.5.1.
   a.   Multiplexing on network connection--for better usage of of
        network resources, an implementation may elect to share a
        network connection among two or more transport connections.
   b.   Acknowledgement strategy--an implementation is not required by
        IS 8073 to use any particular strategy for sending
        acknowledgements for received data:  each data transfer
        protocol data unit may be explicitly acknowledged (one-for-
        one) or may be implicitly acknowledged by a group
        acknowledgement (one-for-N).
   c.   Concatenation of protocol data units--when network service
        data units are large compared to the protocol data units to be
        sent, an implementation may elect to concatenate these
        protocol data units into a single network service data unit.
   d.   Lockup prevention timer--when the wait-before-closing state is
        entered, there is a possibility of deadlock if the peer
        transport entity never responds to the CR TPDU.  The standard
        provides for an optional timer to alleviate this situation.

4.3.3 Local options, Class 4.

 The options that an implementor may decide in a particular Class 4
 implementation are given in the list below.  Recommendations and
 requirements for use of these options in implementations to which
 this supplement applies are given in Paragraph 4.5.2.
   a.   Withdrawal of flow control credit--when supporting several
        connections of differing precedence or priority, resource
        management must be practiced so as to maintain the precedence
        or priority relationships.
   b.   Flow control confirmation--when flow control credit is
        reduced, extra delay may be encountered because
        acknowledgements carrying new flow control information are
        lost; this procedure aids in speeding up resynchronization of
        the flow control.
   c.   Subsequenced acknowledgements--when the flow control window
        has been closed this procedure alleviates ambiguity due to
        lost or out-of-order acknowledgements.

McCoy [Page 10] RFC 1007 June 1987

   d.   Splitting over network connection--when operating over a
        connection-oriented network service, a Class 4 implementation
        is permitted to use more than one network connection, for
        better performance and better resilience to network connection
        failure.
   e.   Acknowledgement strategy--an implementation is not required by
        the standard to use any particular strategy for sending
        acknowledgements for received data:  each data transfer
        protocol data unit may be explicitly acknowledged (one-for-
        one) or may be implicitly acknowledged by a group
        acknowledgement (one-for-N).
   f.   Wait-before-closing state--when a connect request has been
        sent to the peer and the user has requested a disconnection
        before the connect confirmation has been received, an
        implementation may elect to wait until the confirmation has
        arrived before sending the disconnection request to the peer,
        to ensure positive identification of the connection to be
        released.
   g.   Multiplexing on network connection--for better usage of
        network resources, an implementation may elect to share a
        network connection among two or more transport connections.
   h.   Concatenation of protocol data units--when network service
        data units are large compared to the protocol data units to be
        sent, an implementation may elect to concatenate these
        protocol data units into a single network service data unit.
   i.   Checksum algorithm--the Fletcher checksum algorithm provided
        in an annex to the standard is not part of the standard and is
        provided for information only.  The checksum algorithm used
        nature of network errors expected and need only satisfy the
        summation criterion given in the standard.
   j.   Send network RESET when bad checksum received--it may not be
        possible to know with certainty which of several transport
        connections multiplexed on a network connection is to receive
        a protocol data unit which carries a bad checksum.  A N-RESET
        or N-DISCONNECT may be sent on the network connection to all
        transport entities on the connection to indicate the error.
   k.   Protocol data unit retransmission policy--protocol data units
        for which no acknowledgement has been received may be
        retransmitted in case the originals were never received.
        Whether to retransmit only the oldest unacknowledged protocol
        data unit or all those that are outstanding has implications
        for buffer management in the sending entity and for
        utilization of the bandwidth in the network transmission

McCoy [Page 11] RFC 1007 June 1987

        medium.

4.4 Negotiations.

 Paragraph 4.2.1 lists those options that shall be negotiatied by
 communicating transport entities.  Below,  conventions are given for
 these options, in usage to which this  supplement applies.  These
 requirements reflect the conformance statement of IS 8073 and the
 needs of the DOD.

4.4.1 Options.

4.4.1.1 Class of service.

   a.   An implementation operating on CONS shall be capable of
        offering Class 2 and may optionally support Class 0.
   b.   An implementation shall not respond by a proposal of Class 0
        and shall not respond by a proposal of Class 2 if the connect
        request was received on a CLNS.
   c.   An implementation may offer Class 2 as an alternative class of
        operation in a connect request when operating over CONS. No
        alternative class may be offered if operation over a CLNS.
   d.   An implementation shall respond to a connect request that
        proposes Class 1 or 3 as primary choice with a disconnect
        request, reason code 128+2 (see p. 87 of IS 8073).
   e.   An implementation shall not propose Class 1 or Class 3 in
        response to a connect request carrying Class 1 or Class 3 as
        an alternative class of service.
   f.   An implementation which proposes Class 4 in a connect request
        need not accept a proposal for Class 2 from its peer if Class
        2 was not offered as an alternative in the connect request, or
        if operation is over a CLNS. Class 2 shall be accepted when
        proposed by the responding peer if it  was offered as an
        alternative in the connect request.

4.4.1.2 Extended formats.

   a.   An implementation shall always propose use of extended formats
        when either Class 4 or Class 2 is proposed in a connect
        request.
   b.   An implementation shall always accept the use of extended
        formats when so proposed in a received connect request.

McCoy [Page 12] RFC 1007 June 1987

4.4.1.3 Expedited data.

   a.   Use of expedited data is subject to negotiation by users of
        Transport Service.
   b.   Expedited data shall be supported in Class 2.

4.4.1.4 Maximum protocol data unit size.

 (The provisions of this paragraph are under consideration.)

4.4.1.5 Use of checksum.

 An implementation shall propose use of checksums consistent with the
 expected quality of service and security requirements.
   a.   Checksums should be used when operating with the IP on
        catenated networks.
   b.   Checksums should not be used if high performance is required,
        except when required by high error rates in the network
        service.
   c.   Checksums should always be used when any encryption is being
        used.

4.4.1.6 Protection parameters.

 Use of the security parameters is not defined in this supplement.

4.4.1.7 Use of flow control in Class 2.

   a.   An implementation shall always propose the use of flow control
        in Class 2 whenever Class 2 is proposed as either primary or
        alternative choice of service.
   b.   An implementation shall accept use of flow control in Class 2
        whenever negotiation to Class 2 occurs.

4.4.1.8 Service quality parameters.

   a.   Use of the service quality parameters in the CR and CC
        protocol data units is not defined except for the residual
        error rate parameter and the priority parameter.
   b.   Residual error rate (the use of this parameter is under
        consideration).

McCoy [Page 13] RFC 1007 June 1987

   c.   Priority (the use of this parameter is under consideration).

4.4.2 Parameters.

        This paragraph defines the values to be used in the CR and CC
        TPDUs.

4.4.2.1 Class 2 parameters.

4.4.2.1.1 Connect request (CR) protocol data unit.

4.4.2.1.1.1 Fixed part of header.

   a.   Connect request code: as in IS 8073.
   b.   Initial credit allocation: this field defines the number of
        TPDUs offered as initial credit by the connection initiator.
        Since the field is of length  4, the maximum credit that can
        be initially offered is limited to 15.  These TPDUs are
        constrained in length to the maximum size defined in the TPDU
        size field, listed below in Paragraph 4.4.2.1.1.2.
   c.   Destination reference: as in IS 8073.
   d.   Source reference: this reference shall be selected pursuant to
        the provisions of Paragraph 5.2.1.
   e.   Class and option: the class field shall take binary value
        0010; the option field shall take binary value 0010. (These
        values select Class 2, and the options of extended formats and
        flow control in Class 2.)

4.4.2.1.1.2 Variable part of header.

   a.   TSAP identifiers: the parameter values shall follow the
        conventions given in Paragraphs 5.1.1 and 5.1.2.
   b.   TPDU size: (The values to be used are under consideration.)
   c.   Version number: as in IS 8073.
   d.   Protection parameters: should not be used.
   e.   Checksum: shall not be used.
   f.   Additional options: this field shall take binary value 0001 if
        the initiating user has proposed the use of expedited data,
        and shall take value 0000 otherwise.

McCoy [Page 14] RFC 1007 June 1987

   g.   Alternative protocol classes: this field shall not be used
        unless Class 0 is to be proposed as an alternate class of
        operation.
   h.   Throughput: should not be used.
   i.   Residual error rate: should not be used.
   j.   Priority: (Use of this parameter is under consideration.)
   k.   Transit delay: should not be used.

4.4.2.1.1.3 User data.

 The CR TPDU shall not carry user data which has any requirement
 whatever for security protection.

4.4.2.1.2 Connect Confirm (CC) TPDU.

4.4.2.1.2.1 Fixed part of header.

   a.   Connect confirm code: as in IS 8073.
   b.   Initial credit allocation: same as Paragraph 4.4.2.1.1.1.
   c.   Destination reference: this reference shall be the "Source
        reference" number from the received CR TPDU.
   d.   Source reference: this reference shall be selected pursuant to
        the provisions of Paragraph 5.2.1.
   e.   Class and option: the class field shall take binary value 0010
        and the option field shall take binary value  0010 (selects
        Class 2 and options of extended formats and flow control in
        Class 2).

4.4.2.1.2.2 Variable part of header.

   a.   TSAP identifier(s): the parameter values shall follow the
        conventions given in Paragraphs 5.1.1 and 5.1.2.
   b.   b. TPDU size: (The values for this parameter are under
        consideration.)
   c.   Version number: as in  IS 8073.
   d.   Protection parameters: should not be used.

McCoy [Page 15] RFC 1007 June 1987

   e.   Checksum : shall not be used.
   f.   Additional options: This field shall take binary value 0001 if
        the responding transport entity has proposed the use of
        expedited data, and shall take binary value 0000 otherwise.
   g.   Alternative protocol classes: shall not be used.
   h.   Throughput: should not be used.
   i.   Residual error rate: should not be used.
   j.   Priority: (The use of this parameter is under consideration.)
   k.   Transit delay: should not be used.

4.4.2.1.2.3 User data.

   The CC TPDU shall not carry any data which has any requirement
   whatever for security protection.

4.4.2.2 Class 4 parameters.

4.4.2.2.1 Connect request (CR) TPDU.

4.4.2.2.1.1 Fixed part of header.

   a.   Connect request code: as in IS 8073.
   b.   Initial credit allocation: this field defines the number of
        TPDUs offered as initial credit by the connection initiator.
        Since the field is of length 4, the maximum credit that can be
        initially offered is limited to 15. These TPDUs are
        constrained in length to the maximum size defined in the TPDU
        size field, listed below in Paragraph 4.4.2.2.1.2.
   c.   Destination reference: as in  IS 8073.
   d.   Source reference: this reference shall be selected pursuant to
        the provisions of Paragaph 5.2.1.
   e.   Class and option: the class field shall take binary value
        0100; the option field shall take binary value 0010. (These
        values select Class 4, and the options  of extended formats
        and flow control in Class 2.  This latter option is ignored if
        the class negotiated is Class 2.)

McCoy [Page 16] RFC 1007 June 1987

4.4.2.2.1.2 Variable part of header.

   a.   TSAP identifiers: the parameter values shall follow the
        conventions given in Paragraphs 5.1.1 and 5.1.2.
   b.   PDU size: (The values for this parameter are under
        consideration.)
   c.   Version number: as in IS 8073.
   d.   Protection parameters: should not be used.
   e.   Checksum: if Class 4 has been selected, this parameter may be
        used.  If Class 2 (or Class) has been selected, this parameter
        shall not be used.
   f.   Additional options: this field shall take binary value 0001 if
        the initiating user has proposed the use of expedited data,
        and shall take binary value 0000 otherwise.
   g.   Alternative protocol classes: this field shall be used only if
        Class 2 (or Class 0) is to be proposed as an alternate class
        of operation, conformant to the conditions of Paragraph
        4.4.1.1.  If Class 2 is proposed, the field shall take binary
        value 00000010 (1 octet).
   h.   Acknowledge time:  should not be used.
   i.   Throughput: should not be used.
   j.   Residual error rate: (The use of this parameter is under
        consideration.)
   k.   Priority: (The use of this parameter is under consideration.)
   l.   Transit delay: should not be used.

4.4.2.2.1.3 User data.

   The CR TPDU shall not carry user data which has any requirement
   whatever for security protection.

4.4.2.2.2 Connect confirm (CC) TPDU.

4.4.2.2.2.1 Fixed part of header.

   a.   Connect confirm code: as in IS 8073.
   b.   Initial credit allocation: same as Paragraph 4.4.2.2.1.1.b.

McCoy [Page 17] RFC 1007 June 1987

   c.   Destination reference: this reference shall be the number in
        "Source reference" from the received CR TPDU.
   d.   Source reference: this reference shall be selected pursuant to
        the provisions of Paragraph 5.2.1.
   e.   Class and option: if Class 2 has been selected, then the class
        field shall take binary value 0010 and the option field shall
        take binary value 0010.  If Class 4 has been selected, then
        the class field shall take binary value 0100 and the option
        field shall take binary value 0010.

4.4.2.2.1.2 Variable part of header.

   a.   TSAP identifier(s): the parameters values shall follow the
        conventions given in Paragraphs 5.1.1 and 5.1.2.
   b.   TPDU size: (The values for this parameter are under
        consideration.)
   c.   Version number: as in IS 8073.
   d.   Protection parameters: should not be used.
   e.   Checksum: if Class 4 has been selected, this parameter may be
        used. If Class 2 (or Class 0) has been selected, this
        parameter shall not be used.
   f.   Additional options: if Class 4 or Class 2 has been selected,
        this field shall take binary value 0001 if the responding user
        has proposed use of expedited data and shall take binary value
        0000 otherwise.
   g.   Alternate protocol classes: shall not be used.
   h.   Acknowledgement time: should not be used.
   i.   Throughput: should not be used.
   j.   Residual error rate: (The use of this parameter is under
        consideration.)
   k.   Priority: (The use of this parameter is under consideration.)
   l.   Transit delay: should not be used.

4.4.2.2.1.3 User data.

 The CC TPDU shall not carry user data which has any requirement
 whatever for security protection.

McCoy [Page 18] RFC 1007 June 1987

4.5 Use of local options.

 The paragraphs that follow give policy and guidance in the election
 of local options.

4.5.1 Local options, Class 2.

4.5.1.1 Multiplexing.

 Any Class 2 connections may be multiplexed on the same network
 connection to the limits provided by the network service.
 Multiplexing Class 2 and Class 4 connections together on the same
 network connection is not recommended.

4.5.1.2 Acknowledgement strategy.

 (The provisions of this paragraph are under consideration.)

4.5.1.3 Concatenation.

 This permits placing certain TPDUs into a single network service
 data unit with a data-bearing TPDU.  It is useful for reducing
 the overhead of separate transmission of the individual TPDUs.

4.5.1.4 Lockup prevention timer.

 It is strongly recommended that this timer be used for all Class 2
 connections.  A description of the timer has been included in the
 transport formal description.  (This timer corresponds to the
 optional TS1 timer that IS 8073 recommends.)

4.5.1.5 Treatment of protocol errors.

 Protocol errors detected by a Class 2 transport connection shall
 result in that connection being terminated, without sending an ER
 TPDU.

4.5.1.6 Action on receipt of Error transport protocol data unit.

 The receipt of an ER TPDU for a Class 2 transport connection shall
 cause immediate termination of that transport connection.

4.5.2 Local options, Class 4.

4.5.2.1 Withdrawal of flow control credit.

 Because of the need to serve transport connections of various
 levels of operating priority, an implementation shall support
 the withdrawal of flow control credit from any Class 4 transport
 connection as a means of managing resource allocation among
 Class 4 connections.

McCoy [Page 19] RFC 1007 June 1987

4.5.2.2 Flow control confirmation.

 The requirement to support withdrawal of flow control credit
 strongly indicates the need to use flow control confirmation.
 An implementation should support and use the flow control
 confirmation procedures of IS 8073, consistent with quality of
 service and other requirements.

4.5.2.3 Subsequenced acknowledgements.

 The possibility of credit withdrawal strongly indicates the
 requirement for subsequence numbers on acknowledgements.  An
 implementation shall support and use subsequence numbers as
 defined in IS 8073.

4.5.2.4 Splitting over network connection.

 Implementations may use splitting as necessary or useful in the
 operating environment. (Splitting is defined only for operation
 over a CONS.

4.5.2.5 Acknowledgement strategy.

 (The provisions of this paragraph are under consideration.)

4.5.2.6 Wait-before-closing state.

 It is recommended that this state be used.  A lockup prevention
 timer, such as used in Class 2, is not necessary, since the CR
 TPDU retransmission timer serves this purpose.

4.5.2.7 Multiplexing on network connection.

 Multiplexing of Class 4 connections on a single network
 connection may be used as necessary or useful, within the limits
 permitted by the network service.  Class 4 connections should not
 be multiplexed onto network connections serving Class 2 transport
 connections.

4.5.2.8 Concatenation of protocol data units.

 Concatenation may be useful when operating over a CLNS that has
 large capacity service data units.  Concatenation on networks
 that areconnection-oriented may be useful if transport
 connections are being multiplexed.  A careful analysis of the
 treatment of the network service data unit in internetwork
 environments should be done to determine whether concatenation
 of TPDUs provides sufficient benefit to justify its usage in
 those circumstances.

McCoy [Page 20] RFC 1007 June 1987

4.5.2.9 Checksum algorithm.

 It is strongly recommended that the algorithm described in the
 Implementors Guide Part 7, be used rather than the algorithm
 given in the Annex to IS 8073.  The algorithm in  Part 7
 computes the same checksum as the one in IS 8073 but  has been
 optimized.  Guidance on the use and non-use of  checksum is
 given in the Implementors Guide, Part 7.

4.5.2.10 Send network RESET when bad checksum received.

 It is recommended that only an N-RESET be sent when encountering
 a TPDU with a bad checksum on a CONS.  An implementation shall
 not send an N-DISCONNECT-request in such situations, since the
 TPDU with the bad checksum may have come from some entity
 intending to interfere with communications.  When  operating
 Class 4 over a CLNS, no action shall be taken on the receipt of
 a TPDU with a bad checksum,  i.e., the TPDU shall be discarded.

4.5.2.11 Protocol data unit retransmission policy.

 (The provisions of this paragraph are under consideration.)

4.5.2.12 Treatment of protocol errors.

 In Class 4, a protocol error arising from a TPDU containing
 unrecognized parameters shall cause a DR TPDU to be sent to the
 sender, if the TPDU is otherwise valid.  All other erroneous TPDUs
 shall be discarded.

4.5.2.13 Action on receipt of Error transport protocol data unit.

 If an ER TPDU is received from a remote transport entity, an
 implementation to which this supplement applies shall release the
 transport connection with which the ER TPDU is associated, if such
 association can be made.  When association cannot be made, the ER
 TPDU shall be discarded.

5. SPECIAL REQUIREMENTS

5.1 Addressing conventions.

 (The provisions of Paragraph 5.1 and its subparagraphs are under
 consideration.)

5.1.1 Transport Service Access Point.

5.1.2 Connect-request/confirm protocol data units.

5.1.3 Network Service Access Point.

McCoy [Page 21] RFC 1007 June 1987

5.2 Convention for use of transport connection reference numbers.

 The ISO Transport Protocol provides for freezing reference numbers
 by means of a timer, so that re-use of a reference number does not
 cause ambiguity in communications. However, certain requirements
 are imposed on DOD implementations, so that this means of reference
 number control is inadequate alone. The ISO standard defines only
 those actions to be followed if a timer is used.  Other means of
 reference number control are not prohibited, providing that the
 minimum freeze time, as defined in IS 8073, is exceeded for each
 reference number used.

5.2.1 Specification of convention.

 An implementation adhering to the applications definitions in
 this supplement, Paragraph 1.3, shall not re-use a transport
 connection reference number until the set of available reference
 numbers has recycled to that point.  Expressed more formally,
 if all reference numbers are defined to be within the interval
 [1,N] and a reference number R in this interval is used, then
 R shall be prohibited from being selected again until all the
 numbers R+1,...,N,1,2,...,R-1 shall have been used.  The choice
 of N should be sufficiently large that the expected recycle period
 exceeds the minimum freeze time as specified in IS 8073.  This
 requirement is in addition to and does not supersede the freeze
 requirement of IS 8073.  A simple means of implementing this
 convention is given in Part 9.3 of the Implementors Guide.

5.3 Operation over connectionless network service.

 Implementations to which this supplement applies are required to
 operate over connectionless network services in addition to being
 able to operate over connection-oriented network services. The ISO
 standard specifies transport only for operation over a
 connection-oriented network.  However, the specification for Class
 4 has been written in such a way that use with connectionless
 network service is not precluded.  The formal description offers
 even more flexibility in this regard.  Consequently, operation over
 connectionless network services, whether a LAN or IP, is primarily
 an implementation issue for Class 4.  Operation of Class 2
 transportover a connectionless network service is not considered
 to be a reasonable  option because of the lack of sufficent error
 recovery in Class 2.   For the purposes of this supplement,
 operation of Class 2 on a  connectionless network service is
 not recommended. Operation of Class  4  over a connectionless
 network service is discussed further in parts 1.2.2.2,  3.4,
 and 6 of the accompanying Implementors Guide.

McCoy [Page 22] RFC 1007 June 1987

5.4 Recovery from peer deactivation.

 The ISO Standard does not provide for re-establishment of the
 transport connection  when one of the communicating peers is
 deactivated ("crashes").  However, the state tables for Class
 4 transport in Annex A to IS 8073  are flexible enough that
 simple adaptations in an implementation can yield some degree
 of crash recovery without change to the protocol.  These
 adaptations are discussed in Part 9.2 of the Implementors Guide.

McCoy [Page 23]

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