Network Working Group Wayne McCoy
Request for Comments: 1007 June 1987
MILITARY SUPPLEMENT
TO THE
ISO TRANSPORT PROTOCOL
Status of this Memo
This RFCis 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.
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).
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).
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.)
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
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 transfermandatory mandatory
_______________________________________________________________
Flow control in Class 2 mandatory not applicable
No flow control in Class 2optional 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.
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
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.
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.
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.
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
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.
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).
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.
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.
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.)
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.
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.
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.
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.
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.
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.
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.
