OSI参照モデルは通信を専門としている場合に、避けて通ることができないかもしれない。
ある年の情報工学の技術士補の試験で、OSI参照モデルが問題に出た。
近畿大学が試験会場で、会場に行くまで、アプセトネデフ（一拍休止）、アプレトネデフ（一拍休止）、と８拍子で歌いながら会場に行った。そこには、問題があった。問題があったのではない、問題に出ていたのだった。
ちょうど、その頃の流行りだったのだ。
電気・電子の技術士補試験の時は、ちょうどその年の中部電力の見学会のバスの中で、中部電力の方が「２００V配電」について説明された。問題に載っていた。
ちょうど、その頃の流行りだったのだ。
技術士補試験は、その分野の専門家として活動し、本、雑誌、研究会の３拍子で受かるということが分かった。その頃の流行りの技術を説明できることが最低限の素養のようだ。
今回、OSI参照モデルの資料を作りはじめたのは、AUTOSARで参照しているからである。
今回、本気で作業を始めた理由は２つあります。
一つは、セキュリティ。ネットワークを守るのに、壁はたくさんあった方がいいかもという仮定。これまで、プレゼンテーション層って意識して使ったことがなかった。セキュリティに役立つかどうか、使ってみようという試み。
一つは、自動車の診断通信。セッションの上のアプリに、プレゼンテーション層って使った方がいいのかどうか。採用するのなら今がいいだろうし、採用しないのなら、この際、OSIを参照するのを辞める提案をした方がいいかも。
セッション層については、MBL研究会で発表した際に、
神戸大学塚本昌彦教授から、ネットワーク層とトランスポート層の関係はわかったけど、セッション層との関係はどうか、SIPについてはどう検討しているかという質問をいただいた。
研究室は主にデータリンク層を扱っており、ネットワーク層、トランスポート層を扱うのは、自分一人例外的で、セッション層まで手が回っておらず、慌てて調べることになった。
当時、研究室ではネットワーク層、トランスポート層を規定しているRFC群を参照して、論文になるのかという質問を受けており、学会で座長の方から、RFCになっているSIPについて室温をいただいたのは、本当に幸運だった。
座長から質問されたので対応していると、RFCをばちばち引用して文章を書くのを、大手を降ってできるようになったからである。後に、SWESTという行事で、基調講演をしていただいた際に、学会での質問のお礼を述べることができたのは、これも幸運だった。
提示(presentation)層を深掘りするにあたっても、なんかの幸運がめぐってくることを期待している。
20220116 追記
集会(session)層まで翻訳ができそう。
ISO 7498
OSIの参照モデルは、ISOの国際規格とITU(当時はCCITT）の国際規格として制定してきた。ITUの方が、先進的で、国際規格のかなりの範囲を無償で公開している。WTO/TBT協定により、国際貿易において、技術仕様において非関税障壁を作らないように国際規格を尊重して記述することをうたっている。その仕様の文書を入手するのに、多額のお金がかかるのであれば、国際規格が非関税障壁になってしまう。
OSI参照モデルは、一つ前の版はJISになっており日本語で公開している。
最新の規格はITUで公開している。
ISO、IECで公開していないからといって諦めないでください。
ISO 7498:1984
Information processing systems — Open Systems Interconnection — Basic Reference Model
ISO 7498:1984/ADD 1:1987
Information processing systems — Open Systems Interconnection — Basic Reference Model — Addendum 1
JIS X5003として、同一内容を定義している。
ISO 7498:1984/COR 1:1988
Information processing systems — Open Systems Interconnection — Basic Reference Model — Technical Corrigendum 1
ITU Recommendation X.200 (11/88)
ISO/IEC 7498-1:1994
Information technology — Open Systems Interconnection — Basic Reference Model: The Basic Model
ITU Recommendation X.200 (07/94)
TCP/IP/Ethernet
自分の意識の中では、OSIとTCP/IP/Ethernetの関係はこんな感じ。
| layer | OSI | Ethernet |
|---|---|---|
| 7 | Application(応用) | HTTP, SMTP,SSH |
| 6 | Presentation（提示） | |
| 5 | Session(集会) | SIP |
| 4 | Transport(転送) | TCP,UDP |
| 3 | Network(網) | IP, ICMP |
| 2 | DataLink（与件連携） | Ethernet |
| 1 | Physical（物理） | 1000 base T, Wi-Fi |
Presentation Layerは一度も、意識して使ったことがない。
そこで、Presentation Layerを訳してみる。JISの改訂版がなく、独自の訳である。
presentation:提示
aspects:様子
concrete:具象（JIS)
syntax:構文（JIS)
transfer:転送（JIS）
abstract:抽象
formal:形式的（JIS)
system:系
open:開放
entity:体
mode 方法
connection 連結
less 無し
size 大きさ
data 与件
token 伝送印(もうちょっと気の利いた用語探し中)
segmentation 分割
reassemble 再構成
activity 活動
unit 単位
resource 資源
support 対応
address 住所
ほとんどがshall文ではない。候補を示しているだけである。
7 Detailed description of the resulting OSI architecture
7.1 Application Layer
7.1.1 Definitions
7.1.1.1 application-entity: An active element, within an application process, embodying a set of capabilities which is pertinent to OSI and which is defined for the Application Layer, that corresponds to a specific application-entity-type (without any extra capabilities being used).
7.1.1.2 abstract syntax: The specification of Application-protocol-data-units by using notation rules which are independent of the encoding technique used to represent them.
7.1.2 Purpose
7.1.2.1 As the highest layer in the Reference Model of Open Systems Interconnection, the Application Layer provides the sole means for the application process to access the OSIE.
Hence the Application Layer has no boundary with a higher layer.
7.1.2.2 The aspects of an application process which need to be taken into account for the purpose of OSI are represented by one or more application-entities.
7.1.2.3 An application-entity represents one and only one application process in the OSIE.
Different application processes may be represented by application-entities of the same application-entity-type.
An application process may be represented by a set of application-entitites: each application-entity in this set may be, but need not be, of different
application-entity-type.
7.1.3 Services provided by application-entities
7.1.3.1 General
7.1.3.1.1 Application-processes exchange information by means of application-entities which use application-protocols and presentation services.
7.1.3.1.2 As the only layer in the Reference Model that directly provides services to the application-processes, the Application Layer necessarily provides all OSI services directly usable by application-processes.
7.1.3.1.3 There exists no Application Layer service in the sense of (N)-layer service in that there is neither a general service provided to an upper layer nor a relation to a service-access-point.
NOTE – The related concept of OSI-service, defined in ISO/IEC 10731, is applicable in the Application Layer.
7.1.3.2 Connection-mode facilities
In addition to information transfer, such facilities may include, but are not limited to the following:
a) identification of the intended communication partners (for example by name, by address, by definite description, by generic description);
b) determination of the acceptable quality of service (for example response time, tolerable error rate, cost vis-a-vis the previous considerations);
c) synchronization of cooperating applications;
d) agreement on responsibility for error recovery;
e) agreement on security aspects (e.g. authentication, access control, data integrity);
f) selection of mode of dialogue; and
g) identification of abstract syntaxes.
7.1.3.3 Connectionless-mode facilities
7.1.3.3.1 Where appropriate for connectionless-mode operation, equivalent facilities to those provided for connectionmode operation are provided in the Application Layer to application-processes.
7.1.3.3.2 In addition to information transfer, such facilities constituting the service may include, but are not limited to, the following:
a) identification of intended communication partners;
b) establishment of authority to communicate;
c) authorization of intended communication partners;
d) determination of the acceptable quality of service; and
e) identification of abstract syntaxes.
7.1.4 Functions within the Application Layer
7.1.4.1 The Application Layer contains all those functions which imply communication in either mode between open systems that are not already performed by the lower layers. These include functions performed by programs as well as functions performed by human beings.
7.1.4.2 In particular, application-entities maintain, as part of the pre-knowledge necessary in order to communicate,
(or have access to, via use of a directory facility) information on the use of connection-mode and/or connectionless mode transmission by the peer entities with which they may need to communicate.
7.1.4.3 Groupings of functions in the Application Layer
An application-entity can be structured internally into Application Layer objects representing groups of functions.
Use of one grouping of functions may depend on use of some other functions, and the active functions may vary during the lifetime of the application-association.
7 OSIアーキテクチャ結果詳細説明
7.1 応用層
7.1.1定義
7.1.1.1応用体：応用作業内のアクティブな要素であり、OSIに関連し、特定の応用体型に対応する応用層用に定義された一連の機能を具体化します（追加の機能はありません）。使用されています）。
7.1.1.2抽象構文：それらを表すために使用されるエンコーディング技術に依存しない表記規則を使用することによるApplication-protocol-data-unitsの仕様。
7.1.2目的
7.1.2.1開放系相互接続の参照モデルの最上位層として、応用層は、応用作業がOSIEに接続するための唯一の手段を提供します。
したがって、応用層には上位層との境界がありません。
7.1.2.2 OSIの目的で考慮する必要のある応用作業の側面は、1つ以上の応用体によって表されます。
7.1.2.3応用体は、OSIE内の唯一の応用作業を表します。
異なる応用作業は、同じ応用体型の応用体によって表される場合があります。
応用作業は、応用体のセットで表すことができます。このセットの各応用体は、異なる場合がありますが、異なる必要はありません。
応用体型。
7.1.3応用体によって提供される奉仕
7.1.3.1一般
7.1.3.1.1応用作業は、応用規約と提示奉仕を使用する応用体を使用して情報を交換します。
7.1.3.1.2応用作業に奉仕を直接提供する参照モデルの唯一の層として、応用層は必然的に応用作業によって直接使用可能なすべてのOSI奉仕を提供します。
7.1.3.1.3上位層に提供される一般的な奉仕も奉仕接続点との関係もないという点で、（N）層奉仕の意味での応用層奉仕は存在しません。
注– ISO / IEC 10731で定義されているOSI奉仕の関連概念は、応用層に適用できます。
7.1.3.2接続モード機能
情報の転送に加えて、そのような施設には以下が含まれる場合がありますが、これらに限定されません。
a）意図された通信相手の識別（例えば、名前、住所、明確な説明、一般的な説明による）。
b）許容可能な奉仕品質の決定（たとえば、応答時間、許容可能なエラー率、以前の考慮事項に対するコスト）。
c）連携する応用の同期。
d）エラー回復の責任に関する合意。
e）セキュリティの側面（認証、接続制御、与件の整合性など）に関する合意。
f）対話モードの選択。と
g）抽象構文の識別。
7.1.3.3連結なし方法機能
7.1.3.3.1連結なし方法操作に適切な場合、連結なし方法操作に提供される機能と同等の機能が、応用層で応用作業に提供されます。
7.1.3.3.2情報転送に加えて、奉仕を構成するそのような施設には、以下が含まれる場合がありますが、これらに限定されません。
a）意図された通信相手の特定。
b）通信する権限の確立。
c）意図された通信相手の承認。
d）許容可能な奉仕品質の決定。と
e）抽象構文の識別。
7.1.4応用層内の機能
7.1.4.1応用層には、下位層によってまだ実行されていない開放系間のいずれかのモードでの通信を意味するすべての機能が含まれています。これには、プログラムによって実行される機能だけでなく、人間によって実行される機能も含まれます。
7.1.4.2特に、応用体は、通信するために必要な事前知識の一部として、
（またはディレクトリ機能を使用して接続できる）通信する必要のあるピア体によるコネクションモードおよび/または連結なし方法の送信の使用に関する情報。
7.1.4.3応用層の機能の班化
応用体は、関数の班を表す応用層オブジェクトに内部的に構造化できます。
機能の1つの班の使用は、他のいくつかの機能の使用に依存する場合があり、アクティブな機能は、応用の関連付けの存続期間中に変化する場合があります。
7.2 Presentation Layer
7.2 提示層 。
7.2.1 Definitions
7.2.1 用語定義
7.2.1.1 concrete syntax: Those aspects of the rules used in the formal specification of data which embody a specific representation of that data.
(1) 具象構文 [concrete syntax] 特定の表現を具体的に表す与件を形式的に規定するときに使用する規則の様子。
7.2.1.2 transfer syntax: The abstract and concrete syntax used in the transfer of data between open systems.
(2) 転送構文 [transfer syntax] 開放系間で与件を転送するときに用いる抽象構文と具象構文。
7.2.1.3 presentation context: An association of an abstract syntax with a transfer syntax.
(3) 提示構文 [presentation syntax] 転送構文で抽象構文を関連づける。
7.2.2 Purpose
7.2.2 目的
7.2.2.1 The Presentation Layer provides for the representation of information that application-entities either communicate or refer to in their communication.
7.2.2.1 提示層は，応用体同士が通信したり又は通信のときに参照する情報表現機能を提供する。
7.2.2.2 The Presentation Layer provides for common representation of the data transferred between application entities. This relieves application-entities of any concern with the problem of “common” representation of information, i.e. it provides them with syntax independence.
7.2.2.2 提示層は、応用体間転送与件の一般的な表現を提供する。 これにより、応用体を、情報の「共通の」表現の問題から解放します。つまり、構文に依存しなくなる。
7.2.2.3 The Presentation Layer ensures that the information content of the Application Layer data is preserved during transfer.
7.2.2.3 提示層は、転送中に応用層与件の情報内容を保持することを保証する。
Cooperating application-entities are responsible for determining the set of abstract syntaxes they employ in their communication.
協力する応用体は、通信で使用する一連の抽象構文を決定する責任がある。
The Presentation Layer is informed of the abstract syntaxes that are to be employed.
応用層には、使用する抽象構文が通知する。
Knowing the set of abstract syntaxes to be used by the application-entities, the Presentation Layer is responsible for selecting mutually acceptable transfer syntaxes.
応用体が使う抽象構文集合を知っている応用層は、相互に受け入れ可能な転送構文を選択する責任がある。
NOTE – Presentation-entities have no role in determining the set of abstract syntaxes to be used by the application-entities.
注–応用体は、応用体が使う抽象構文集合を決定しない。
7.2.3 Services provided to the Application Layer
7.2.3 応用層に提供する奉仕
7.2.3.1 The Presentation Layer provides the following facilities:
提示層は，集会奉仕（7.3参照）と次の機能を提供する。
a) identification of a set of transfer syntaxes;
a) 転送構文集合の識別。
b) selection of transfer syntax; and
b) 転送構文の選択
c) access to session services.
c) 集会奉仕接続
7.2.3.2 Identification of a set of transfer syntaxes provides one or more means of representing an abstract syntax.
7.2.3.2 転送構文集合識別は、抽象構文を表す1つ以上の手段を提供する。
Selection of transfer syntax provides the means of initially selecting a transfer syntax and subsequently modifying the selection.
転送構文の選択は、最初の転送構文選択、及びその後選択変更手段を提供する。
7.2.3.3 Session services are provided to application-entities in the form of presentation services.
7.2.3.3 集会奉仕は、提示奉仕の形で応用体に提供する。
7.2.3.4 In connectionless-mode, segmentation and reassembly are not provided in the Presentation Layer.
7.2.3.4 連結無し方法では、分割と再構成は定時層で提供しません。
Thus, the size of presentation-service-data-units is limited by the size of presentation-protocol-data-units and presentationprotocol-control-information.
したがって、提示奉仕与件単位の大きさは、提示規約与件単位及び提示規約制御情報の大きさの制限があります。
7.2.4 Functions within the Presentation Layer
7.2.4 提示層内の機能
The Presentation Layer performs the following functions to help accomplish the presentation-services:
提示層は、提示奉仕の実現を支援する次の機能を実行する。
a) negotiation and re-negotiation of transfer syntax;
a) 転送構文の交渉と再交渉。
b) representation of the abstract syntax chosen by the application-entities in the transfer syntax negotiated or renegotiated, including format and special purpose transformations (for example, data compression);
b) 形式及び特殊目的の変換（与件圧縮など）を含む、交渉または再交渉する転送構文の応用体が選択する抽象構文の表現。
c) restoration of previously negotiated syntaxes on the occurrence of certain events; and
c) 特定の出来事の発生に関して以前に交渉した構文の復元。
d) use of session-services.
d) 集合奉仕の利用
7.2.4.1 Representation of abstract syntax
7.2.4.1 抽象構文の表現
7.2.4.1.1 Application-entities agree on the abstract syntaxes that will be used for their communication.
7.2.4.1.1 応用体は、通信に使う抽象構文に同意する。
It is necessary that these abstract syntaxes are represented in appropriate transfer syntaxes for communication to take place.
これらの抽象構文は、通信を行うために適切な転送構文で表す必要がある。
NOTE – Within a real open system, data defined in terms of an abstract syntax will be represented within the local system environment by a local concrete syntax.
実際の公開系内では、抽象構文で定義した与件は、局所系環境内で局所具体構文で表す。
A transformation may be necessary between the local concrete syntax and the transfer syntax.
局所具体構文と転送構文の間で変換が必要になる場合がある。
Thus, in communication between real open systems there are three concrete syntax versions of the data:
したがって、実際の開放系間の通信には、与件の3つの具体構文版があります。
the concrete syntax used by the originating application-entity, the concrete syntax used by the receiving application-entity, and the concrete syntax used between the presentation-entities (the transfer syntax).
元の応用体で使う具体構文、受信する応用体で使う具体構文、および応用体間で使う具体構文（転送構文）。
It is clearly possible that any or all of these syntaxes be identical.
これらの構文のいずれかまたはすべてが同一である可能性は明らかです。
The local concrete syntaxes are not visible within the OSIE.
局所具体構文は、OSIE内で表示しません。
7.2.4.1.2 The fact that there is or is no actual transformation of concrete syntax has no impact on the presentation protocol.
7.2.4.1.2 具体構文の実際の変換がある又は無いという事実は提示規約に影響を与えない。
7.2.4.1.3 There is not a single predetermined transfer syntax for all OSI. In connection-mode, the transfer syntax to be used on a presentation-connection is negotiated between the correspondent presentation-entities.
7.2.4.1.4 In connectionless-mode, the transfer syntax is selected, but cannot be negotiated.
7.2.4.2 Negotiation of transfer syntax
7.2.4.2.1 Negotiation (or selection) of transfer syntax takes place between two presentation-entities when an applicationentity
provides the name of an abstract syntax for which a transfer syntax is required.
7.2.4.2.2 In general, there may be more than one combination of abstract syntax and transfer syntax. It may be possible
to represent a specific abstract syntax by one or more transfer syntaxes; also it may be possible to use one transfer syntax
to represent more than one abstract syntax. Each combination of abstract syntax and transfer syntax is called a
presentation context. From the viewpoint of the application-entity, a presentation context represents a specific distinct
use of an abstract syntax.
7.2.4.3 Addressing and Multiplexing
There is no multiplexing or splitting in the Presentation Layer.
7.2.4.1.3 すべてのOSIに対して、事前に決定した単一の転送構文はありません。
接続方法では、提示接続で使う転送構文は、対応する応用間で交渉する。
7.2.4.1.4 連結無し方法では、転送構文を選択していますが、交渉できません。
7.2.4.2 転送構文の交渉
7.2.4.2.1 転送構文の交渉（または選択）を、応用体が2つの提示体間で行う場合、転送構文が必要な抽象構文の名前を提供する。
7.2.4.2.2 一般に、抽象構文と転送構文の組み合わせは複数存在する可能性がある。
1つ以上の転送構文で特定の抽象構文を表すため。1つの転送構文を使用することもある。
複数の抽象構文を表すために。抽象構文と転送構文の各組み合わせは、提示文脈。応用体の立場から、提示体は特定の異なる抽象構文を使う。
7.2.4.3 住所指定と多重化
応用層には多重化や分割はありません。
7.3 Session Layer
7.3.1 Definitions
7.3.1.1 token management:
A facility of the session-service which allows correspondent presentation-entities to control explicitly whose right it is to exercise certain session-services.
7.3.1.2 duplex mode:
A mode of interaction where both presentation-entities may concurrently send and receive normal data.
7.3.1.3 half-duplex mode:
A mode of interaction where, at a given instance, only one of the two correspondent presentation-entities is allowed to send normal data.
7.3.1.4 session-connection synchronization:
A facility of the session-service which allows presentation-entities to define and to identify synchronization points and to reset a session-connection to a predefined state and to agree on a resynchronization point.
7.3.2 Purpose
7.3.2.1 The purpose of the Session Layer is to provide the means necessary for cooperating presentation-entities to organize and to synchronize their dialogue and to manage their data exchange.
To do this, the Session Layer provides services to establish a session-connection between two presentation-entities, to support orderly data exchange interactions, and to release the connection in an orderly manner.
7.3.2.2 The only function of the Session Layer for connectionless-mode communication is to provide a mapping of transport-addresses to session-addresses.
7.3.2.3 A session-connection is created when requested by a presentation-entity at a session-service-access-point.
The session-connection exists until it is released by either the presentation-entities or the session-entities.
7.3.2.4 The initiating presentation-entity designates the destination presentation-entity by a session address.
In general, there is a many-to-one correspondence between the session-addresses and transport-addresses.
This does not imply multiplexing of session-connections onto transport-connections, but does imply that at session-connection establishment
time, more than one presentation-entity is a potential target of a session-connection establishment request arriving on a given transport-connection. However, when needed, there may be a one-to-one correspondence between the sessionaddress and the transport-address.
7.3集会層
7.3.1定義
7.3.1.1 伝送印管理：
対応する提示体が、特定の集会奉仕を実行する権利を、明示的に制御する集会奉仕の機能。
7.3.1.2 多重化方法：
両方の提示体が通常の与件を同時に送受信できる相互作用の方法。
7.3.1.3 半二重方法：
特定の実例で、2つの対応する提示体のうちの1つだけが通常の与件を送信できる相互作用の方法。
7.3.1.4 集会接続の同期：
提示体が同期点を定義及び識別し、集会接続を事前定義した状態に再設定し、再同期点について合意できるようにする集会奉仕の機能。
7.3.2.2 無連結方法通信の集会層の唯一の機能は、転送住所から集会住所への対応付けを提供すること。
7.3.2.3 集会接続は、集会奉仕接続点の提示体が要求したときに作る。
集会接続は、提示体又は集会体のいずれかが解放するまで存在する。
7.3.2.4 提示体は、集会住所で宛先提示体を指定する。
一般に、集会住所と転送住所の間には多対1の対応がる。
これは、集会接続を転送接続に多重化するのではなく、集会接続の確立時に多重化する。
場合によっては、複数の提示体が、特定の転送接続に到着する集会接続確立要求の潜在的な対象になる。
ただし、必要な場合は、集会住所と転送住所の間に1対1の対応がある。
7.3.2目的
7.3.2.1 集会層の目的は、協力する提示体が対話を整理および同期し、与件交換を管理するために必要な手段を提供する。
これを行うために、集会層は、2つの提示体間の集会接続を確立し、整然とした与件交換相互作用を支援し、整然とした方法で接続を解放する奉仕を提供する。
7.3.3 Services provided to the Presentation Layer
7.3.3.1 General
7.3.3.1.1 In connection-mode, the services provided by the Session Layer are described below:
a) session-connection establishment;
b) session-connection release;
c) normal data transfer;
d) expedited data transfer;
e) token management;
f) session-connection synchronization;
g) exception reporting;
h) activity management;
j) typed data transfer; and
k) resynchronization.
7.3.3.1.2 In connectionless-mode, the Session Layer provides the following services:
a) connectionless-mode transmission using the transport-connectionless-mode service; and
b) exception reporting.
7.3.3.1.3 In connectionless-mode, segmentation and resassembly are not provided in the Session Layer. Thus, the size of
session-service-data-units is limited by the size of session-protocol-data-units and session-protocol-control-information.
7.3.3 提示層に提供する奉仕
7.3.3.1 概要
7.3.3.1.1 接続方法では、集会層が提供する奉仕を次のように説明する。
a）集会接続の確立。
b）集会接続の解放。
c）通常の与件転送。
d）迅速な与件転送。
e）伝送印管理。
f）集会接続の同期。
g）例外報告;
h）活動管理;
j）型付き与件転送。 と
k）再同期。
7.3.3.1.2連結無し方法では、集会層は次の奉仕を提供します。
a）転送連結無し方法奉仕を使用した連結無し方法伝送。 と
b）例外の報告。
7.3.3.1.3連結無し方法では、分割と再構成は集会層で提供しません。 したがって、集会奉仕与件単位の大きさは、集会規約与件単位及び集会規約制御情報の大きさが制限します。
7.3.3.2 Session-connection establishment
7.3.3.2.1 The session-connection establishment service enables two presentation-entities to establish a sessionconnection between them.
The presentation-entities are identified by session-addresses used to request the establishment of the session-connection.
7.3.3.2.2 The session-connection establishment service allows the presentation-entities to determine cooperatively the unique values of session-connection parameters at the time the session-connection is established.
7.3.3.2.3 The session-connection establishment service provides a session-connection identification parameter which allows the presentation-entities to identify the session-connection.
7.3.3.3 Session-connection release
7.3.3.3.1 The session-connection release service allows presentation-entities to release a session-connection in a orderly way without loss of data.
It also allows either presentation-entity to request at any time that a session-connection be aborted; in this case, data may be lost.
7.3.3.3.2 A session-connection may also be aborted by one of the session-entities supporting it.
7.3.3.4 Normal data transfer
The normal data transfer service allows a sending presentation-entity to transfer a session-service-data-unit to a receiving
presentation-entity.
7.3.3.5 Expedited data transfer
The expedited data transfer service provides expedited handling of the transfer of expedited session-service-data-units.
A specific size restriction is placed on expedited session-service-data-units.
7.3.3.6 Token management
The token management service allows the presentation-entities to control explicitly whose turn it is to exercise certain control functions.
7.3.3.7 Session-connection synchronization
7.3.3.7.1 The session-connection synchronization service allows presentation-entities:
a) to define and identify synchronization points; and
b) to reset the session-connection to a defined state and agree on a resynchronization point with the potential of the loss of data.
7.3.3.7.2 Any semantics which session-service users may give to their synchronization points are transparent to the session-service provider.
7.3.3.7.3 The Session Layer is not responsible for any associated checkpointing or commitment action associated with
synchronization.
7.3.3.7.4 Symmetric synchronization allows synchronization points to be set independently on the two directions of the flow.
7.3.3.8 Exception reporting
The exception reporting service permits the presentation-entities to be notified of exceptional situations.
7.3.3.9 Activity management
The activity concept allows session-service users to distinguish logical pieces of work called activities. Each activity
consists of one or more dialog units. Only one activity is allowed on a session-connection at a time, but there may be serveral consecutive activities during a session-connection.
An activity may also span more than one session-connection.
Activities can also be interrupted and then resumed on the same or a subsequent session-connection.
7.3.3.10 Typed Data Transfer
The typed data transfer service allows a sending presentation-entity to transfer a session-service-data-unit to a receiving presentation-entity independent of the provisions of token management.
7.3.3.11 Resynchronization
Resynchronization may be initiated by either session-service user. It sets the session-connection to a defined state and therefore includes reassignment of tokens and setting the synchronization point serial number to a new value.
Resynchronization may purge undelivered data.
7.3.4 Functions within the Session Layer
7.3.4.1 The functions within the Session Layer are those which shall be performed by session-entities in order to provide the session-services.
When providing the connectionless-mode service, the Session Layer provides a one-to-one mapping of Session-connectionless-mode-transmissions onto transport-connectionless-mode-transmissions.
7.3.4.2 Most of the functions required are readily implied by the services provided. Additional description is given below for the following functions:
a) session-connection to transport-connection mapping; and
b) session-connection flow control.
7.3.4.3 Session-connection to transport-connection mapping
There is a one-to-one mapping between a session-connection and a transport-connection at any given instant.
However, the lifetime of a transport-connection and that of a related session-connection can be distinguished so that a transportconnection supports several consecutive session-connections.
7.3.4.4 Session-connection flow control
There is no peer flow control in the Session Layer.
To prevent the receiving presentation-entity from being overloaded with data, the receiving session-entity applies back pressure across the transport-connection using the transport flow control.
7.3.3.2集会接続の確立
7.3.3.2.1集会接続確立奉仕により、2つの提示体がそれらの間の集会接続を確立できます。
提示体は、確立を要求するために使用される集会住所によって識別されます
集会接続の。
7.3.3.2.2集会接続確立奉仕により、提示体は、集会接続が確立されたときに、集会接続パラメータの一意の値を協調的に決定できます。
7.3.3.2.3集会接続確立奉仕は、提示体が集会接続を識別できるようにする集会接続識別パラメータを提供します。
7.3.3.3集会接続の資源
7.3.3.3.1集会接続解放奉仕により、提示体は与件を失うことなく規則正しい方法で集会接続を解放できます。
また、いずれかの提示体が集会接続の中止をいつでも要求できるようにします。この場合、与件が失われる可能性があります。
7.3.3.3.2集会接続は、それを対応する集会体の1つによって中止される場合もあります。
7.3.3.4通常の与件転送
通常の与件転送奉仕では、送信側の提示体が集会奉仕与件ユニットを受信側に転送できます
提示体。
7.3.3.5迅速な与件転送
優先与件転送奉仕は、優先集会奉仕与件ユニットの転送の迅速な処理を提供します。
優先集会奉仕与件ユニットには、特定の大きさ制限が設定されています。
7.3.3.6伝送印管理
伝送印管理奉仕を使用すると、提示体は、特定の制御機能を実行する順番を明示的に制御できます。
7.3.3.7集会接続の同期
7.3.3.7.1集会接続同期奉仕により、提示体は次のようになります。
a）同期点を定義および識別する。と
b）集会接続を定義された状態に再設定し、与件が失われる可能性のある再同期点について合意します。
7.3.3.7.2 集会奉仕利用者が同期点に与える可能性のある意味は、集会奉仕プロバイダーに対して透過的です。
7.3.3.7.3 集会層は、関連する検査点またはコミットメントアクションについて責任を負いません。
同期。
7.3.3.7.4 対称同期により、同期点を流れの2つの方向に独立して設定できます。
7.3.3.8 例外報告
例外報告奉仕により、提示体に例外的な状況を通知することができます。
7.3.3.9 活動管理
活動の概念により、集会奉仕利用者は活動と呼ばれる論理的な作業を区別できます。各活動
1つ以上のダイアログユニットで構成されます。集会接続では一度に1つの活動のみが許可されますが、集会接続中にサーバーの連続した活動が存在する場合があります。
活動は、複数の集会接続にまたがることもあります。
活動を中断して、同じまたは後続の集会接続で再開することもできます。
7.3.3.10型付き与件転送
型付き与件転送奉仕を使用すると、送信側の提示体は、伝送印管理の規定に関係なく、集会奉仕与件ユニットを受信側の提示体に転送できます。
7.3.3.11再同期
再同期は、いずれかの集会奉仕利用者が開始できます。集会接続を定義された状態に設定するため、伝送印の再割り当てと同期店のシリアル番号の新しい値への設定が含まれます。
再同期により、未配信の与件が削除される場合があります。
7.3.4集会層内の機能
7.3.4.1集会層内の機能は、集会奉仕を提供するために集会体によって実行される機能です。
非接続方法奉仕を提供する場合、集会層は、非接続方法伝送から転送非接続方法伝送への1対1の対応づけを提供します。
7.3.4.2必要な機能のほとんどは、提供される奉仕によって容易に暗示されます。以下の機能について、以下に追加の説明を示します。
a）集会接続から転送接続への対応づけ。と
b）集会接続流れ制御。
7.3.4.3集会接続から転送接続への対応づけ
任意の時点で、集会接続と転送接続の間に1対1の対応づけがあります。
ただし、転送接続の存続期間と関連する集会接続の存続期間を区別して、転送接続が複数の連続した集会接続に対応するようにすることができます。
7.3.4.4集会接続流れ制御
集会層にはピア流れ制御はありません。
受信側の提示体が与件で過負荷になるのを防ぐために、受信側の集会体は、転送流れ制御を使用して転送接続全体にバックプレッシャを適用します。
7.4 Transport Layer
7.4.1 Definitions
No Transport Layer specific terms are identified.
7.4.2 Purpose
7.4.2.1 The transport-service provides transparent transfer of data between session-entities and relieves them from any
concern with the detailed way in which reliable and cost effective transfer of data is achieved.
7.4.2.2 The Transport Layer optimizes the use of the available network-service to provide the performance required by
each session-entity at minimum cost. This optimization is achieved within the constraints imposed by the overall
demands of all concurrent session-entities and the overall quality and capacity of the network-service available to the
Transport Layer.
7.4.2.3 All protocols defined in the Transport Layer have end-to-end significance, where the ends are defined as
transport entities having transport associations. Therefore, the Transport Layer is OSI end open system oriented and
transport-protocols operate only between OSI end open systems.
7.4.2.4 The Transport Layer is relieved of any concern with routing and relaying since the network-service provides
data transfer from any transport-entity to any other, including the case of tandem subnetworks (see 7.5.1).
7.4.2.5 The transport functions invoked in the Transport Layer to provide a requested service quality depend on the
quality of the network-service. The quality of the network-service depends on the way the network-service is achieved
(see 7.5.3).
7.4.3 Services provided to the Session Layer
7.4.3.1 Introduction
7.4.3.1.1 The Transport Layer uniquely identifies each session-entity by its transport-address. When providing the
connectionless-mode service, the Transport Layer provides a connectionless-mode service which maps a request for
transmission of a transport-service-data-unit onto a request to the connectionless-mode network-service. In connectionmode,
the transport-service provides the means to establish, maintain, and release transport-connections. Transportconnections
provide duplex transmission between a pair of session-entites (through transport-SAPs).
7.4.3.1.2 More than one transport-connection can be established between the same pair of transport-addresses. A
session-entity uses transport-connection-endpoint-identifiers provided by the Transport Layer to distinguish between
transport-connection-endpoints.
7.4.3.1.3 The operation of one transport-connection is independent of the operation of all others except for the
limitations imposed by the finite resources available to the Transport Layer.
7.4.3.1.4 The quality of service provided on a transport-connection depends on the service class requested by the
session-entities when establishing the transport-connection. The selected quality of service is maintained throughout the
lifetime of the transport-connection. The session-entity is notified of any failure to maintain the selected quality of
service on a given transport-connection.
7.4.3.1.5 In connection-mode, the following facilities provided by the Transport Layer are described below:
a) transport-connection establishment;
b) transport-connection release;
c) data transfer;
d) expedited data transfer; and
e) suspend facility.
7.4.3.1.6 In connectionless-mode, segmentation and resassembly are not provided in the Transport Layer. Thus, the size
of transport-service-data-units is limited by the size of transport-protocol-data-units and transport-protocol-controlinformation.
7.4.3.2 Transport-connection establishment
7.4.3.2.1 Transport-connections are established between session-entities identified by transport-addresses. The quality
of service of the transport-connection is negotiated between the session-entities and the transport-service.
7.4.3.2.2 At the time of establishment of a transport-connection the class of transport-service to be provided can be
selected from a defined set of available classes of service.
7.4.3.2.3 These service classes are characterized by combinations of selected values of parameters such as throughput,
transit delay, and connection set-up delay and by guaranteed values of parameters such as residual error rate and service
availability.
7.4.3.2.4 These classes of service represent globally predefined combinations of parameters controling quality of
service. These classes of service are intended to cover the transport-service requirements of the various types of traffic
generated by the session-entities.
7.4.3.3 Transport-connection release
This facility provides the means by which either session-entity can release a transport-connection and have the
correspondent session-entity informed of the release.
7.4.3.4 Data transfer
This facility provides data transfer in accordance with the agreed quality of service. When the quality of service cannot
be maintained and all possible recovery attempts have failed, the transport-connection is terminated and the sessionentities
are notified.
a) The transport-service-data-unit transfer service provides the means by which transport-service-data-units
of arbitrary length are delimited and transparently transferred in sequence from one sending transportservice-
access-point to the receiving transport-service-access-point over a transport-connection. This
service is subject to flow control.
b) The expedited transport-service-data-unit transfer service provides an additional means of information
exchange on a transport-connection. The expedited transport-data-units are subject to their own set of
transport-service and flow control characteristics. The maximum size of expedited transport-service-dataunits
is limited.
7.4.3.5 Expedited Data
An expedited service is provided by the Transport Layer. However, it should be used in accordance with the constraints
described in 5.8.8.3.
7.4.4 Functions within the Transport Layer
7.4.4.1 General
7.4.4.1.1 In connection-mode, the Transport Layer functions may include:
a) mapping transport-address onto a network-address;
b) multiplexing (end-to-end) transport-connections onto network-connections;
c) establishment and release of transport-connections;
d) end-to-end sequence control on individual connections;
e) end-to-end error detection and any necessary monitoring of the quality of service;
f) end-to-end error recovery;
g) end-to-end segmenting, blocking, and concatenation;
h) end-to-end flow control on individual connections;
j) supervisory functions;
k) expedited transport-service-data-unit transfer; and
l) suspend/resume.
7.4.4.1.2 In connectionless-mode, the Transport Layer provides the following functions to support connectionless-mode
transmission:
a) mapping between transport-addresses and network-addresses;
b) mapping end-to-end transport-connectionless-mode transmissions on to network-connectionless-mode
transmissions;
NOTE – There may be specific situations where performing conversion from connection-mode to
connectionless-mode operation in the Transport Layer can be justified and may thus be permitted provided that this
requires only limited extensions to existing protocols. In such cases it is accepted that communication using such
conversions can only take place betweeen OSI end systems supporting them (see 6.4).
c) end-to-end error detection and monitoring of the quality of service;
d) transport-service-data-unit delimiting; and
e) supervisory functions.
7.4.4.2 Addressing
7.4.4.2.1 When a session-entity requests the Transport Layer to establish a transport-connection with another
session-entity identified by its transport-address, the Transport Layer determines the network-address identifying the
transport-entity which serves the correspondent session-entity.
7.4.4.2.2 Because transport-entities support services on an end-to-end basis no intermediate transport-entity is involved
as a relay between the end transport-entities. Therefore the Transport Layer maps transport-addresses to the networkaddresses
which identify the end transport-entities (see Figure 13).
TISO2940-94
Session-entity Session-entity
Transport-entity Transport-entity
End transport-address
End network-address
Network-Layer
Figure 13 – Association of transport-addresses and network address
7.4.4.2.3 One transport-entity may serve more than one session-entity. Several transport-addresses may be associated
with one network-address within the scope of the same transport-entity. Corresponding mapping functions are performed
within the transport-entities to provide these facilities (see Figure 14).
TISO2950-94/d13
Session-entity A Transport-addresses Session-entity B
Transport-entity
Network-entity
Networkaddress
Figure 14 – Association of one network address with several transport addresses
7.4.4.3 Connection multiplexing and splitting
In order to optimize the use of network-connections, the mapping of transport-connections onto network-connections
need not be on a one-to-one basis. Both splitting and multiplexing may be performed, namely for optimizing the cost of
usage of the network-service.
7.4.4.4 Phases of operation
In connection-mode operation, the phases of operation within the Transport Layer are:
a) establishment phase;
b) data transfer phase; and
c) release phase.
The transfer from one phase of operation to another is specified in detail within the protocol for the Transport Layer.
7.4.4.5 Establishment phase
During the establishment phase, the Transport Layer establishes a transport-connection between two session-entities.
The functions of the Transport Layer during this phase match the requested class of service with the services provided
by the Network Layer. The following functions can be performed during this phase:
a) obtain a network-connection which best matches requirements of the session-entity, taking into account
cost and quality of service;
b) decide whether multiplexing or splitting is needed to optimize the use of network-connections;
c) establish the optimum transport-protocol-data-unit size;
d) select the functions that will be operational upon entering the data transfer phase;
e) map transport-addresses onto network-addresses;
f) provide identification of different transport-connections between the same pair of transport-serviceaccess-
points (connection identification function); and
g) transfer of data.
7.4.4.6 Data transfer phase
The purpose of the data transfer phase is to transfer transport-service-data-units between the two session-entities
connected by the transport-connection. This is achieved by the transportation of transport-protocol-data-units and by the
following functions, each of which is used or not used according to the class of service selected in the establishment
phase:
a) sequencing;
b) blocking;
c) concatenation;
d) segmenting;
e) multiplexing or splitting;
f) flow control;
g) error detection;
h) error recovery;
j) expedited data transfer;
k) transport-service-data-unit delimiting; and
m) transport-connection identification.
7.4.4.7 Release phase
The purpose of the release phase is to release the transport-connection. It may include the following functions:
a) notification of reason for release;
b) identificaton of the transport-connection released; and
c) transfer of data.
7.4.4.8 Transport Layer management
The Transport Layer protocols deal with some management activities of the layer (such as activation and error control).
See clause 8 and ITU-T Rec. X.700 | ISO 7498-4 for the relationship with other management aspects.
7.5 Network Layer
7.5.1 Definitions
7.5.1.1 real subnetwork: A collection of equipment and physical media which forms an autonomous whole and
which can be used to interconnect real systems for the purpose of data transfer.
7.5.1.2 subnetwork: An abstraction of a real subnetwork.
NOTES
1 A subnetwork is a representation within the OSI Reference Model of a real network such as a carrier network, a
private network, or a local area network.
2 A subnetwork may itself be an open system, although this is not necessarily always the case. See ISO 8648 – Internal
Organization of the Network Layer.
7.5.1.3 subnetwork-connection: A communication path through a subnetwork which is used by entities in the
Network Layer in providing a network-connection.
7.5.2 Purpose
7.5.2.1 The Network Layer provides the functional and procedural means for connectionless-mode or connectionmode
transmission among transport-entities and, therefore, provides to the transport-entities independence of routing and
relay considerations.
7.5.2.2 The Network Layer provides the means to establish, maintain, and terminate network-connections between
open systems containing communicating application-entities and the functional and procedural means to exchange
network-service-data-units between transport-entities over network-connections.
7.5.2.3 It provides to the transport-entities independence from routing and relay consideration associated with the
establishment and operation of a given network-connection. This includes the case where several subnetworks are used
in tandem (see 7.5.4.2) or in parallel. It makes invisible to transport-entities how underlying resources such as data-linkconnections
are used to provide network-connections.
7.5.2.4 Any relay functions and hop-by-hop service enhancement protocols used to support the network-service
between the OSI end systems are operating below the Transport Layer, i.e. within the Network Layer or below.
7.5.3 Service provided to the Transport Layer
7.5.3.1 Introduction
7.5.3.1.1 The basic service of the Network Layer is to provide the transparent transfer of data between transport-entities.
This service allows the structure and detailed content of submitted data to be determined exclusively by layers above the Network Layer.
7.5.3.1.2 All facilities are provided to the Transport Layer at a known cost.
7.5.3.1.3 The Network Layer contains functions necessary to provide the Transport Layer with a firm
Network/Transport Layer boundary which is independent of the underlying communications media in all things other
than quality of service. Thus the Network Layer contains functions necessary to mask the differences in the
characteristics of different transmission and subnetwork technologies into a consistent network service.
7.5.3.1.4 The service provided at each end of a network-connection is the same even when a network-connection spans
several subnetworks, each offering dissimilar services (see 7.5.4.2).
NOTE – It is important to distinguish the specialized use of the term “service” within the OSI Reference Model from its
common use by suppliers of private networks and carriers.
7.5.3.1.5 The quality of service is negotiated between the transport-entities and the network-service at the time of
establishment of a network-connection. While this quality of service may vary from one network-connection to another
it will be agreed for a given network-connection and be the same at both network-connection-endpoints.
7.5.3.1.6 In connection-mode, the facilities provided by the Network Layer are described below:
a) network-addresses;
b) network-connections;
c) network-connection-endpoint-identifiers;
d) network-service-data-unit transfer;
e) quality of service parameters;
f) error notification;
g) expedited network-service-data-unit transfer ;
h) reset;
j) release; and
k) receipt of confirmation.
7.5.3.1.7 Some of these facilities are optional. This means that:
a) the user has to request the facilities; and
b) the network-service provider may honour the request or indicate that the service is not available.
7.5.3.1.8 In connectionless-mode, the facilities provided by the Network Layer, operating among network-serviceaccess-
points, are:
a) transmission of network-service-data-units of a defined maximum size;
b) quality of service parameters; and
c) local error notification.
7.5.3.2 Network-addresses
Transport-entities are known to the Network Layer by means of network-addresses. Network-addresses are provided by
the Network Layer and can be used by transport-entities to identify uniquely other transport-entities, i.e. networkaddresses
are necessary for transport-entities to communicate using the network-service. The Network Layer uniquely
identifies each of the end open systems (represented by transport-entities) by their network-addresses. This may be
independent of the addressing needed by the underlaying layers.
7.5.3.3 Network-connections
7.5.3.3.1 A network-connection provides the means of transferring data between transport-entities identified by
network-SAP-addresses. The Network Layer provides the means to establish, maintain and release network-connections.
7.5.3.3.2 A network-connection is point-to-point.
7.5.3.3.3 More than one network-connection may exist between the same pair of transport-entities (through the network-
SAP-addresses).
7.5.3.4 Network-connection-endpoint-identifiers
The Network Layer provides to the transport-entity a network-connection-endpoint-identifier which uniquely identifies
the network-connection-endpoint with the associated network-SAP-address.
7.5.3.5 Network-service-data-unit transfer
7.5.3.5.1 On a network-connection, the Network Layer provides for the transmission of network-service-data-units.
These units have a distinct beginning and end and the integrity of the unit’s content is maintained by the Network Layer.
7.5.3.5.2 In connection-mode, no limit is imposed on the maximum size of network-service-data-units.
7.5.3.5.3 The network-service-data-units are transferred transparently between transport-entities.
7.5.3.6 Quality of service parameters
7.5.3.6.1 The Network Layer establishes and maintains a selected qualilty of service for the duration of the networkconnection.
7.5.3.6.2 The quality of service parameters include residual error rate, service availability, reliability, throughput, transit
delay (including variations), and delay for network-connection establishment.
7.5.3.7 Error notification
7.5.3.7.1 Unrecoverable errors detected by the Network Layer are reported to the transport-entities.
7.5.3.7.2 Error notification may or may not lead to the release of the network-connection, according to the specification
of a particular network-service.
7.5.3.8 Expedited network-service-data-unit transfer
7.5.3.8.1 The expedited network-service-data-unit transfer is optional and provides an additional means of information
exchange on a network-connection. The transfer of a expedited network-service-data-units is subject to a different set of
network-service characteristics and to separate flow control.
7.5.3.8.2 The maximum size of an expedited network-service-data-unit is limited.
7.5.3.8.3 This service is an optional service that may not always be available.
7.5.3.9 Reset
The reset facility is optional and when invoked causes the Network Layer to discard all network-service-data-units in
transit on the network-connection and to notify the transport-entity at the other end of the network-connection that a
reset has occurred.
7.5.3.10 Release
7.5.3.10.1 A transport-entity may request release of a network-connection. The network-service does not guarantee
delivery of data preceding the release request and still in transit. The network-connection is released regardless of the
action taken by the correspondent transport-entity.
7.5.3.10.2 This facility is optional and may not always be available.
7.5.3.11 Receipt confirmation
7.5.3.11.1 A transport-entity may confirm receipt of data over a network-connection. The use of the receipt
confirmation service is agreed by the two users of the network-connection during connection establishment.
7.5.3.11.2 This service is optional and may not always be available.4)
7.5.4 Functions within the Network Layer
7.5.4.1 Introduction
7.5.4.1.1 Network Layer functions provide for the wide variety of configurations supporting network-connections
ranging from network-connections supported by point-to-point configurations to network-connections supported by
complex combinations of subnetworks with different characteristics.
NOTE – In order to cope with this wide variety of cases, network functions should be structured into sublayers. The
subdivision of the Network Layer into sublayers need only be done when this is useful. In particular, sublayering need not be used
when the access protocol to the subnetwork supports the complete functionality of the OSI network-service.
- This service is included in the network-service only to support existing features of ITU-T Rec. X.25.
7.5.4.1.2 The following are functions performed by the Network Layer:
a) routing and relaying;
b) network-connections;
c) network-connection multiplexing;
d) segmenting and blocking;
e) error detection;
f) error recovery;
g) sequencing;
h) flow control;
j) expedited data transfer;
k) reset;
m) service selection;
n) mapping between network-addresses and data-link addresses;
o) mapping network-connectionless-mode transmissions to data-link-connectionless-mode transmissions;
p) converting from data-link-connection-mode service to network-connectionless-mode service;
q) enhancing a data-link-connectionless mode service to provide a network-connection-mode service; and
r) network layer management.
7.5.4.2 Routing and relaying
7.5.4.2.1 Network-connections are provided by network-entities in OSI end systems and by intermediate open systems
which provide relaying. These intermediate open systems may interconnect subnetwork-connections, data-linkconnections,
and data-circuits (see 7.7). Routing functions determine an appropriate route between network-addresses. In
order to set up the resulting communication, it may be necessary for the Network Layer to use the services of the Data
Link Layer to control the interconnection of data-circuits (see 7.6.4.10 and 7.7.3.1).
7.5.4.2.2 The control of interconnection of data-circuits (which are in the Physical Layer) from the Network Layer
requires interaction between a network-entity and a physical-entity in the same open system. Since the Reference Model
permits direct interaction only between adjacent layers, the network-entity cannot interact directly with the physicalentity.
This interaction is thus described through the Data Link Layer which intervenes transparently to convey the
interaction between the Network Layer and the Physical Layer.
7.5.4.2.3 This representation of the control of data-circuit interconnection is an abstract representation. It is a local
matter in an open system. It does not model the functioning of real open systems and as such has no impact on the
standardization of OSI protocols.
NOTE – When Network Layer functions are performed by combinations of several individual subnetworks, the
specification of routing and relaying functions could be facilitated by using sublayers, isolating individual subnetworks routing and
relaying functions from internetwork routing and relaying functions. However, when subnetworks have access protocols supporting
the complete functionality of the OSI network service, there need be no sublayering in the Network Layer.
7.5.4.3 Network-connections
7.5.4.3.1 This function provides network-connections between transport-entities, making use of data-link-connections
provided by the Data Link Layer.
7.5.4.3.2 A network-connection may also be provided as subnetwork-connections in tandem, i.e. using several
individual subnetworks in series. The interconnected individual subnetworks may have the same or different service
capabilities. Each end of a subnetwork-connection may operate with a different subnetwork protocol.
7.5.4.3.3 The interconnection of a pair of subnetworks of differing qualities may be achieved in two ways. To illustrate
these, consider a pair of subnetworks, one of high quality and the other of low quality:
a) The two subnetworks are interconnected as they stand. The quality of the resulting network-connection is
not higher than that of the lower quality subnetwork (see Figure 15).
b) The lower quality subnetwork is enhanced to equal the higher quality subnetwork and the subnetworks
are then interconnected. The quality of the resulting network-connection is approximately that of the
higher quality subnetwork (see Figure 16).
The choice between these two alternatives depends on the degree of difference in quality, the cost of enhancement, and
other economic factors.
TISO2960-94/d14
Low quality
network connection
Low quality subnetwork High quality subnetwork
Figure 15 – Interconnection of a low quality and a high quality subnetwork
FIGURE 15/X.200...[D14] = 3.5 CM
TISO2970-94/d15
High quality
network connection
Low quality subnetwork
High quality subnetwork
Enhanced subnetwork
Figure 16 – Interconnection of an enhaned low quality subnetwork
and a high quality subnetwork
FIGURE 16/X.200...[D15] = 5 CM
7.5.4.4 Network-connection multiplexing
7.5.4.4.1 This function may be used to multiplex network-connections onto data-link-connections in order to optimize
their use.
7.5.4.4.2 In the case of subnetwork-connections in tandem, multiplexing onto individual subnetwork-connections may
also be performed in order to optimize their use.
7.5.4.5 Segmenting and blocking
The Network Layer may segment and/or block network-service-data-units for the purpose of facilitating the transfer.
However, the network-service-data-unit delimiters are preserved over the network-connection.
7.5.4.6 Error detection
Error detection functions are used to check that the quality of service provided over a network-connection is maintained.
Error detection in the Network Layer uses error notification from the Data Link Layer. Additional error detection
capabilities may be necessary to provide the required quality of service.
7.5.4.7 Error recovery
This function provides for the recovery from detected errors. This function may vary depending on the quality of the
network service provided.
7.5.4.8 Sequencing
This function provides for the sequenced delivery of network-service-data-units over a given network-connection when
requested by a transport-entity.
7.5.4.9 Flow control
If flow control is required, this function may need to be performed.
7.5.4.10 Expedited data transfer
This function provides for the expedited data transfer facility.
7.5.4.11 Reset
This function provides for the reset service.
7.5.4.12 Service selection
This function allows service selection to be carried out to ensure that the service provided at each end of a networkconnection
is the same when a network-connection spans several subnetworks of dissimilar quality.
7.5.4.13 Network Layer management
The Network Layer protocols deal with some management activities of the layer (such as activation and error control).
See clause 8 and ITU-T Rec. X.700 | ISO 7498-4 for the relationship with other management aspects.
7.6 Data Link Layer
7.6.1 Definitions
No Data Link Layer specific terms are identified.
7.6.2 Purpose
7.6.2.1 The Data Link Layer provides functional and procedural means for connectionless-mode among
network-entities, and for connection-mode for the establishment, maintenance, and release data-link-connections among
network-entities and for the transfer of data-link-service-data-units. A data-link-connection is built upon one or several
physical-connections.
7.6.2.2 The Data Link Layer detects and possibly corrects errors which may occur in the Physical Layer.
7.6.2.3 In addition, the Data Link Layer enables the Network Layer to control the interconnection of data-circuits
within the Physical Layer.
7.6.3 Service provided to the Network Layer
7.6.3.1 In connection-mode, the facilities provided by the Data Link Layer are:
a) data-link-addresses;
b) data-link-connection;
c) data-link-service-data-units;
d) data-link-connection-endpoint-identifiers;
e) error notification;
f) quality of service parameters; and
g) reset.
7.6.3.2 In connectionless-mode, the facilities provided by the Data Link Layer are:
a) data-link-addresses;
b) transmission of data-link-service-data-units of a defined maximum size; and
c) quality of service parameters.
7.6.3.3 Data-link-addresses
Network-entities are known to the Data Link Layer by means of data-link-addresses. Data-link-addresses are provided
by the Data Link Layer and can be used by network-entities to identify other network-entities which communicate using
the data link service. A data-link-address is unique within the scope of the set of Open Systems attached to a common
Data Link Layer. The notion of a data-link-address is distinct from that of a data-link-service-access-point-address
(DLSAP address).
7.6.3.4 Data-link-connection
A data-link-connection provides the means of transferring data between network-entities identified by data-linkaddresses.
A data-link-connection is established and released dynamically.
7.6.3.5 Data-link-service-data-units
7.6.3.5.1 The Data Link Layer allows exchange of data-link-service-data-units over a data-link-connection or exchange
of data-link-service-data-units (that bear no relation to any other data-link-service-data-units) using the connectionlessmode
data-link-service.
7.6.3.5.2 The size of the data-link-service-data-units may be limited by the relationship between the physical-connection
error rate and the Data Link Layer error detection capability.
7.6.3.6 Data-link-connection-endpoint-identifiers
If needed, the Data Link Layer provides data-link-connection-endpoint-identifiers that can be used by a network-entity
to identify a correspondent network-entity.
7.6.3.7 Error notification
Notification is provided to the network-entity when any unrecoverable error is detected by the Data Link Layer.
7.6.3.8 Quality of service parameters
Quality of service parameters may be optionally selectable. The Data Link Layer establishes and maintains a selected
quality of service for the duration of the data-link-connection. The quality of service parameters include mean time
between detected but unrecoverable errors, residual error rate (where errors may arise from alteration, loss, duplication,
misordering, misdelivery of data-link-service-data-units, and other causes), service availability, transit delay and
throughput.
7.6.3.9 Reset
The network-entity can force the data-link-entity-invocation into a known state by invoking the reset facility.
7.6.4 Functions within the Data Link Layer
In connection-mode and connectionless-mode, the functions performed by the Data Link Layer are:
a) data-link-service-data-unit mapping;
b) identification and parameter exchange;
c) control of data-circuit interconnection;
d) error detection;
e) routing and relaying; and
f) data Link Layer management.
In connection-mode, the following functions are also performed by the Data Link Layer:
a) data-link-connection establishment and release;
b) connection-mode data-link data transmission;
c) data-link-connection splitting;
d) sequence control;
e) delimiting and synchronization;
f) flow control;
g) error recovery; and
h) reset.
In connectionless-mode, the following function is also performed by the Data Link Layer:
a) connectionless-mode data-link data transmission.
7.6.4.1 Data-link-connection establishment and release
These functions establish and release data-link-connections on activated physical-connections. When a physicalconnection
has multiple endpoints (for example multipoint connection) a specific function is needed within the Data
Link Layer to identify the data-link-connections using such a physical-connection.
7.6.4.2 Connectionless-mode data-link-data transmission
The connectionless-mode data-link-data transmission provides the means for the transmission of data-link-service-dataunits
between data-link-service-access-points without establishing a data-link-connection.
7.6.4.3 Data-link-service-data-unit mapping
This function maps data-link-service-data-units into data-link-protocol-data-units on a one-to-one basis.
NOTE – More general mappings are for further study.
7.6.4.4 Data-link-connection splitting
This function performs splitting of one data-link-connection onto several physical-connections.
7.6.4.5 Delimiting and synchronization
These functions provide recognition of a sequence of physical-service-data-units (i.e. bits, see 7.7.3.2) transmitted over
the physical-connection, as a data-link-protocol-data-unit.
NOTE – These functions are sometimes referred to as framing.
7.6.4.6 Sequence control
This function maintains the sequential order of data-link-service-data-units across a data-link-connection.
7.6.4.7 Error detection
This function detects transmission, format and operational errors occurring either on the physical-connection, or as a
result of a malfunction of the correspondent data-link-entity.
7.6.4.8 Error recovery
This function attempts to recover from detected transmission, format and operational errors and notifies the networkentities
of errors which are unrecoverable.
7.6.4.9 Flow control
In connection-mode, each network-entity can dynamically control (up to the agreed maximum) the rate at which it
receives data-link-service-data-units from a data-link-connection. This control may be reflected in the rate at which
the Data Link Layer accepts data-link-service-data-units at the correspondent data-link-connection-endpoint. In
connectionless-mode, there is service boundary flow control, but no peer flow control.
7.6.4.10 Identification and parameter exchange
This function performs data-link-entity identification and parameter exchange.
7.6.4.11 Reset
This function performs a data-link reset forcing the data-link-entity-invocation to a known state.
7.6.4.12 Control of data-circuit interconnection
This function conveys to network-entities the capability of controlling the interconnection of data-circuits within the
Physical Layer.
NOTE – This function is used in particular when a physical-connection is established/released across a circuit-switched
subnetwork by relaying within an intermediate system between data-circuits. These data-circuits are elements of the end-to-end path.
A network-entity in the intermediate system makes the appropriate routing decisions as a function of the path requirements derived
from the network signalling protocols.
7.6.4.13 Routing and Relaying
Some subnetworks, and particularly some configurations of local area networks, require that routing and relaying
between individual local networks be performed in the data-link-layer.
7.6.4.14 Data Link Layer management
The Data Link Layer protocols deal with some management activities of the layer (such as activiation and error control).
See clause 8 and ITU-T Rec. X.700 | ISO 7498-4 for the relationship with other management aspects.
7.7 Physical Layer
7.7.1 Definition
7.7.1.1 data-circuit: A communication path in the physical media for OSI among two or more physical-entities,
together with the facilities necessary in the Physical Layer for the transmission of bits on it.
7.7.2 Purpose
The Physical Layer provides the mechanical, electrical, functional and procedural means to activate, maintain, and
de-activate physical-connections for bit transmission between data-link-entities. A physical-connection may involve
intermediate open systems, each relaying bit transmission within the Physical Layer. Physical Layer entities are
interconnected by means of a physical medium.
7.7.3 Services provided to the Data Link Layer
7.7.3.1 The services provided by the Physical Layer are determined by the characteristics of the underlying medium
and are too diverse to allow categorization into connection-mode and connectionless-mode.
7.7.3.2 The services or elements of services are provided by the Physical Layer:
a) physical-connections;
b) physical-service-data-units;
c) physical-connection-endpoints;
d) data-circuit identification;
e) sequencing;
f) fault condition notification; and
g) quality of service parameters.
7.7.3.3 Physical-connections
7.7.3.3.1 The Physical Layer provides for the transparent transmission of bit streams between data-link-entities across
physical-connections.
7.7.3.3.2 A data-circuit is a communication path in the physical media for OSI among two or more physical-entities,
together with the facilities necessary in the Physical Layer for the transmission of bits on it.
7.7.3.3.3 A physical-connection may be provided by the interconnection of data-circuits using relaying functions in the
Physical Layer. The provision of a physical-connection by such an assembly of data-circuits is illustrated in Figure 17.
7.7.3.3.4 The control of the interconnection of data-circuits is offered as a service to data-link-entities.
7.7.3.4 Physical-service-data-units
7.7.3.4.1 A physical-service-data-unit consists of one bit or a string of bits.
NOTE – Serial or parallel transmission can be accommodated by the design of the protocol within the Physical Layer.
TISO2980-94/d16
Physical-serviceaccess-
point
Physical connection-endpoint
Physical
Layer
Datacircuit
Physicalentity
Data
circuit
Physical
Media
Figure 17 – Interconnection of data circuits within the Physical Layer
7.7.3.4.2 A physical-connection may allow duplex or half-duplex transmission of bit streams.
7.7.3.5 Physical-connection-endpoints
7.7.3.5.1 The Physical Layer provides physical-connection-endpoint-identifiers which may be used by a data-link-entity
to identify physical-connection-endpoints.
7.7.3.5.2 A physical-connection will have two (point-to-point) or more (multi-endpoint) physical-connection-endpoints
(see Figure 18).
7.7.3.6 Data-circuit identification
The Physical Layer provides identifiers which uniquely specify the data-circuits between two adjacent open systems.
NOTE – This identifier is used by network-entities in adjacent open systems to refer to data-circuits in their dialogue.
7.7.3.7 Sequencing
The Physical Layer delivers bits in the same order in which they were submitted.
7.7.3.8 Fault condition notification
Data-link-entities are notified of fault conditions detected within the Physical Layer.
7.7.3.9 Quality of service parameters
The quality of service of a physical-connection is derived from the data-circuits forming it. The quality of service can be
characterized by:
a) error rate, where errors may arise from alteration, loss, creation, and other causes;
b) service availability;
c) transmission rate; and
d) transit delay.
7.7.4 Functions within the Physical Layer
7.7.4.1 The functions of the Physical Layer are determined by the characteristics of the underlying medium and are too
diverse to allow categorization into connection-mode and connectionless-mode.
A B
A B C
TISO2990-94/d17
Data Link
Layer
Data-Link-
Entity
Physical-connection-endpoints
Physical-connection
%XAMPLEOFATWOENDPOINTPHYSICALCONNECTION
CONNECTIONEXISTSBETWEEN!AND"
A
Data Link
Layer
Data-Link-
Entity
Physical-connection-endpoints
Physical-connection
%XAMPLEOFAMULTI
ENDPOINT
PHYSICAL
CONNECTION
CONNECTIONEXISTSBETWEEN!"AND#
B
Figure 18 – Examples of physical connections
7.7.4.2 The functions provided by the Physical Layer are:
a) physical-connection activation and deactivation;
b) physical-service-data-unit transmission;
c) multiplexing; and
d) Physical Layer management.
7.7.4.3 Physical-connection activation and deactivation
These functions provide for the activation and deactivation of physical-connections between two data-link-entities upon
request from the Data Link Layer. These include a relay function which provides for interconnection of data-circuits.
7.7.4.4 Physical-service-data-unit transmission
The transmission of physical-service-data-units (i.e. bits) may be synchronous or asynchronous. Optionally, the function
of physical-service-data-unit transmission provides recognition of the protocol-data-unit corresponding to a mutually
agreed sequence of physical-service-data-units that are being transmitted.
7.7.4.5 Multiplexing
This function provides for two or more physical-connections to be carried on a single data-circuit. This function provides
the recognition of the framing required to enable identification of the PhPDUs conveyed by the individual physicalconnections
over the single data-circuit. The multiplexing function is optional.
NOTE – A particular example of the use of multiplexing is offered when a transmission media is divided into data-circuits
in support of the different data link protocols used in the signalling phase and in the data transfer phase when using circuit switched
subnetworks. In such usage of multiplexing, flows of different nature are permanently assigned to different elements of the multiplex group.
7.7.4.6 Physical Layer management
7.7.4.6.1 The Physical Layer protocols deal with some management activities of the layer (such as activation and error
control). See clause 8 and ITU-T Rec. X.700 | ISO 7498-4 for the relationship with other management aspects.
NOTE – The above text deals with interconnection between open systems as illustrated in Figure 11. For open systems to
communicate in the real environment, real physical connections should be made as, for example, in Figure 19 a). Their logical
representation is as shown in Figure 19 b) and is called the physical media connection. The mechanical, electromagnetic and other
media dependent characteristics of physical media connections are defined at the boundary between the Physical Layer and the
physical media. Definitions of such characteristics are specified in other standards.
B C E F
TISO3000-94/d18
Real Open
System A
Metallic cable Optical fibre
Real Open
System B
Data modems Transducers
A 2EALENVIRONMENT
Physical Layer Protocols
Physical
Layer
Physical-Layer-
Entity
Physical-Layer-
Entity
Physical media
connections
Figure 19 – Examples of interconnection
NOTE – The area of physical media connections in OSI requires further study.
作業中の記録
下記、古いJIS表現が新しい英文のどの表現に変わったか未確認。
プレゼンテーション層は，この情報表現に関して次の二つの相補的な側面をもつ。 (1) 応用エンティティ間で転送されるデータの表現 (2) 応用エンティティが通信のときに参照するデータ構造の表現と，このデータ構造に対して施される可能性のある動作の集合の表現 これら二つの相補的な側面が，一つの一般的な概念“転送構文”を指す。
プレゼンテーション層は，データの構文すなわち表現だけに関係し，その意味すなわち応用層にとっての意味には関与しない。その意味は，応用エンティティだけが知っている。プ
レゼンテーション層は，応用エンティティ間で使われる一つの共通の表現を提供する。これによって，応用エンティティは，情報を両者に共通の表現で表すという問題から逃れることができる。
すなわち，この層は，応用エンティティに構文上の独立性を提供する。この独立性の実現には，次の二つの方法がある。 (1) プレゼンテーション層は，応用エンティティ間で使用される共通の構文要素を提供する。 (2) 応用エンティティは，任意の構文を用いることができ，プレゼンテーション層は，これらの構文と，応用エンティティ間の通信に必要な共通の構文との間の変換機能を提供する。この変換機能は，開放型システムの内部で実行される。この機能は，他の開放型システムからは見えないので，プレゼンテーションプロトコルの標準化に影響を与えない。 この規格では，(2) の方法を用いる。
参考資料
@kazuo_reve 自動車の故障診断に関連するプログラマーになりたての方が参照するとよさそうな情報
自己参照
@kazuo_reveさんの「自動車の故障診断に関連するプログラマーになりたての方が参照するとよさそうな情報」の読み方
ISO 14229-1:2020 Road vehicles — Unified diagnostic services (UDS) — Part 1: Application layer
ISO/IEC OSIに学ぶ
IT業界における国際規格等の利用。仮説・検証（22）
＜この記事は個人の過去の経験に基づく個人の感想です。現在所属する組織、業務とは関係がありません。＞
文書履歴
ver. 0.01 初稿 20220108
ver. 0.02 参考資料追記 20220109
ver. 0.03 数行翻訳 20220110
ver. 0.04 数行翻訳 ISO 14229-1 UDS 追記20220115
ver. 0.05 数十行翻訳 20210116
ver. 0.06 ありがとう追記 20230504
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