TCP/IP

WordNet

1. the 9th letter of the Roman alphabet (同)i
2. the 20th letter of the Roman alphabet (同)t

PrepTutorEJDIC

1. 『私は』私が
2. iodineの化学記号
3. tritiumの化学記号
4. Teachers College 教員養成大学
5. (野球で)innings pitched

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2012/12/14 16:23:39」(JST)

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TCP/IP群
アプリケーション層
BGP / DHCP / DNS / FTP / HTTP / IMAP / IRC / LDAP / MGCP / NNTP / NTP / POP / RIP / RPC / RTP / SIP / SMTP / SNMP / SSH / Telnet / TFTP / TLS/SSL / XMPP カテゴリ
トランスポート層
TCP / UDP / DCCP / SCTP / RSVP / ECN カテゴリ
ネットワーク層
IP (IPv4, IPv6) / ICMP / ICMPv6 / IGMP / IPsec カテゴリ
リンク層
ARP/InARP / NDP / OSPF / トンネリング (L2TP) / PPP / MAC (イーサネット, IEEE 802.11, DSL, ISDN, FDDI) カテゴリ

インターネット・プロトコル・スイート（英: Internet protocol suite）とは、インターネットおよび大多数の商用ネットワークで稼動するプロトコルスタックを実装する通信プロトコルの一式である。 インターネット・プロトコル・スイートは、インターネットの黎明期に定義され、現在でも標準的に用いられている2つのプロトコル、Transmission Control Protocol (TCP) とInternet Protocol (IP) にちなんで、TCP/IPプロトコル・スイートとも呼ばれる。今日のIPネットワーキングは、1960年代と1970年代に発展し始めたLAN (Local Area Network) とインターネットの開発が統合されたものである。それは1989年のティム・バーナーズ＝リーによるWorld Wide Webの発明と共にコンピュータに革命をもたらした。

インターネット・プロトコル・スイート（類似した多くのプロトコル群）は、階層の一式として見ることができる。各層はデータ転送に伴い生じる一連の問題を解決し、下位層プロトコルのサービスを使用する上位層プロトコルに明確なサービスを提供する。上位層は利用者と論理的に近く、より理論的なデータを処理する。また最終的に物理的に転送できる形式へデータを変換するため、下位層プロトコルに依存する。 TCP/IP参照モデルは4つの階層で構成される。

概要

TCP/IPはあらゆるベンダーのコンピュータや、全く異なるOSで相互に通信することを可能にする。1990年代には一般的な通信として広く使われるようになり、プロトコルが無料で開放されていることからオープンシステムであると言える。

歴史

インターネット・プロトコル・スイートは、1970年代初期に米国国防高等研究計画局 (DARPA) による研究から登場した。1960年代後半に先駆的なARPANETの構築後、DARPAはその他様々なデータ転送技術における研究を開始した。1972年、ロバート・カーン (Robert E. Kahn) はDARPA情報処理技術室 (IPTO: Information Processing Technology Office) に雇われた。そこで彼は衛星パケット網と地上の無線パケット網の研究に取り組み、それらを横断して通信ができる事の価値を認識した。1973年春、ヴィントン・サーフ（Vinton Cerf。その当時既に完成していたARPANET Network Control Program (NCP) プロトコルの開発者）は、ARPANETの次世代プロトコルを設計する事を目標に、オープン・アーキテクチャ相互接続モデルに取り組むためにカーンと合流した。

1973年の夏までに、カーンとサーフはすぐに基本的な改良を解決した。ネットワーク・プロトコル間の違いは、共通の相互接続ネットワーク・プロトコルを用いる事で隠蔽された。そしてARPANETにおいては、信頼性についてネットワークが責任を持つ代わりに、ホストが責任を持つようになった（サーフはHubert Zimmermanとルイ・プザン（CYCLADESネットワーク設計者）が、この設計に対して重要な役割を果たした功績を認めている）。

ネットワークの役割を最低限まで減らす事で、それらの特性が何であろうとも、殆どどのネットワークも統合できるようになった。それによりカーンの当初の問題も解決した。よく言われる事は、TCP/IP（サーフとカーンの取り組みの最終成果）は「two tin cans and a string」（2つの空き缶と1本の紐、すなわち糸電話）ででも機能するだろうという事である。伝書鳩を用いて稼動するための実装案「鳥類キャリアによるIP」さえ存在する。(RFC 1149)

（その他の型のゲートウェイとの混同を避けるためにゲートウェイから改名された）ルータと呼ばれるコンピュータ はそれぞれのネットワークへインターフェースを提供し、ネットワーク間で行き来するパケットを転送する。 ルータに関する必要条件はRFC 1812 で定義された。

その着想は1973～74年度にスタンフォード大学のサーフ ネットワーク研究グループによってより詳細な構造が作り上げられ、最初のTCP/IP仕様RFC 675 を生み出した。 （PARC Universal Packet プロトコル群を生み出した、ゼロックス パロアルト研究所における初期のネットワーク研究も、大半が同時期に行われ技術的に重要な影響を与えた。人々はその2つに注目した。）

その後、異なったハードウェア上の実用プロトコルを開発するため、DARPAはBBNテクノロジーズ、スタンフォード大学およびユニヴァーシティ・カレッジ・ロンドンと契約した。 4バージョンが開発された。TCP v1、TCP v2、1978年春にはTCP v3とIP v3に分離、そして安定版のTCP/IP v4 - これは今日のインターネットでもまだ使われる標準プロトコルである。

1975年、スタンフォード大学とユニヴァーシティ・カレッジ・ロンドン間で、2拠点のTCP/IP通信試験が実施された。 1977年11月、アメリカ、イギリス、ノルウェー間で、3拠点のTCP/IP試験が実施された。1978年から1983年にかけて、複数の研究施設でその他いくつかのTCP/IPの試作が開発された。1983年1月1日、ARPANETはTCP/IPへ完全に切り替えられた。

1982年3月、アメリカ国防総省は全ての軍用コンピュータ網のためにTCP/IP標準を作成した。 1985年、インターネットアーキテクチャ委員会は、コンピュータ産業のために3日間のTCP/IPワークショップを挙行した。250の業者代表が参加し、TCP/IPの普及を助け、商用利用の増加に繋がった。

2005年11月9日、アメリカ文化への貢献を称え、カーンとサーフに大統領自由勲章が授与された。

日本における普及

TCP/IPをサポートしたUNIX系オペレーティングシステム (OS) である4.2BSDは1983年9月に登場している。この時期のUNIX系OSは大学機関を中心に発展してきた経緯があるが、1980年代後半には日本の大学でもUNIX系OSが用いられているところでは大学内ネットワークにTCP/IPが用いられていた。1988年8月2日、JUNETに大きく関わった村井純によって日本からのインターネットへのTCP/IP接続試験が行われ、その後、日本でもインターネットを取り巻く環境の整備が進むとともにTCP/IPが普及していくことになった。

1989年9月、最初の日本語による解説書である西田竹志著「TCP/IP」が発行された。

1997年3月、全国銀行協会連合会が傘下銀行の企業・銀行相互間のオンラインデータ交換において使用できる新しい標準通信プロトコルとして、全銀TCP/IP手順を制定した。それまで利用されてきた全銀手順に代わり、電子データ交換でもTCP/IPが使われるようになった。

（日本におけるインターネットの歴史については日本のインターネットを参照）

インターネット・プロトコル・スイートの階層

2つのホストが2つのルータを経由してネットワーク接続する場合に、それぞれのホップで使用する階層毎に対応する階層同士で論理的に接続される概念図

任意のデータがUDPデータグラム中に、UDPデータグラムがIPパケット中にカプセル化される概念図

IP群はプロトコルとサービスをカプセル化する事によって抽象化する。 通常、より上位層のプロトコルはその目的の達成に役立てるために、より下位層のプロトコルを用いる。 これまでIETFはインターネット・プロトコル・スタックをRFC 1122で定義された4層から変更した事はない。 IETFは7層からなるOSI参照モデルに従うような試みはせず、また標準化過程 (Standards Track) にあるプロトコル仕様やその他の構造上の文書をOSI参照モデルに対して参照する事もしない。

4. アプリケーション	DNS, TFTP, TLS/SSL, FTP, Gopher, HTTP, IMAP, IRC, NNTP, POP3, SIP, SMTP, SNMP, SSH, TELNET, ECHO, RTP, PNRP, rlogin, ENRP
	さまざまな理由でTCP上で稼動する BGPなどのルーティング・プロトコルも、アプリケーションまたはネットワーク層の一部と考えられる場合も有る。
3. トランスポート	TCP, UDP, DCCP, SCTP, IL, RUDP
2. インターネット	IP上で稼動するOSPFなどのルーティング・プロトコルも、経路選択を提供するため、ネットワーク層の一部であると考えられる事も有る。 ICMPとIGMPはIP上で稼動し、また制御情報を提供するため、ネットワーク層の一部であると考えられる。
	IP (IPv4, IPv6)
	ARPとRARPはIPの下、リンク層の上で動作するため、それらはどこか中間に属する。
1. リンク	イーサネット, Wi-Fi, トークンリング, PPP, SLIP, FDDI, ATM, フレームリレー, SMDS

いくつかの教科書ではインターネット・プロトコル・スイート・モデルを7層のOSI参照モデルへ対応付ける事を試みた事がある。 その対応付けは、インターネット・プロトコル・スイートのリンク層を物理層の上のデータリンク層へ、またインターネット層はOSI参照モデルのネットワーク層へ割り当てられる事が多い。 それらの教科書はRFC 1122やその他IETFの一次情報の意図と矛盾する二次情報である。 IETFは再三にわたりインターネット・プロトコルと構造の開発はOSI参照モデルに準拠する事は意図しないという事を述べている

実装

今日、ほとんどの商用および非商用のオペレーティングシステム (OS) に組み込まれ、そして標準でTCP/IPスタックが導入されている。 殆どの利用者は実装するために探す必要は無い。 TCP/IPはMicrosoft Windowsはもとより、全ての商用UNIXシステム、 Mac OS X、そしてLinuxディストリビューションやBSDシステムなどの全てのフリーのUNIXライクオペレーティングシステムシステムに組み込まれている。

独特な実装には、組み込みシステムのために設計されたオープンソースのプロトコル・スタックであるLightweight TCP/IP、アマチュアパケット無線システムおよびシリアルケーブルで接続されたパーソナルコンピュータのためのプロトコル・スタックおよび関連プロトコルであるKA9Q NOSが有る。

イントラネット

TCP/IPを使用して構築されたプライベートネットワークをイントラネットと呼び、これは今日のLANにおける事実上の標準と言える。イントラネットで用いられるプロトコルの代表例を下記に挙げる。

• E-mail (SMTP/POP3/IMAP4) (MAP)
• ドメイン・ネーム・システム (DNS)
• ハイパーテキスト転送プロトコル (HTTP)
• ファイル転送プロトコル (FTP)
• ネットニュース (NNTP)
• インターネット・リレー・チャット (IRC)
• Gopher (Gopher)
• ネットワーク・タイム・プロトコル (NTP)

関連項目

• Internet Engineering Task Force (IETF)
• TCPやUDPにおけるポート番号の一覧
• OSI参照モデル
• TCP/IPモデル
• エンドツーエンド原理
• プロトコルスタック

脚注

参考文献

• Internet History -- Pages on Robert Kahn, Vinton Cerf, and TCP/IP (reviewed by Cerf and Kahn).
• Forouzan, Behrouz A. (2003). TCP/IP Protocol Suite (2nd ed.). McGraw-Hill. ISBN 0-07-246060-1. 

追加知識

• アンドリュー・S・タネンバウム 『コンピュータネットワーク第4版』 日経BP社、2003年。ISBN 978-4822221065。
• Douglas E. Comer, David L.Stevens 『TCP/IPによるネットワーク構築 Vol.I 原理・プロトコル・アーキテクチャ 第4版』 村井純, 榎本博之訳、共立出版、2002年。ISBN 978-4320120549。
• Douglas E. Comer, David L.Stevens 『TCP/IPによるネットワーク構築 Vol.II 設計・実装・内部構造』 村井純, 榎本博之訳、共立出版、1995年。ISBN 978-4320027343。
• Douglas E. Comer, David L.Stevens 『TCP/IPによるネットワーク構築 Vol.III Windowsソケットバージョン クライアントサーバプログラミングとアプリケーション』 村井純, 榎本博之訳、共立出版、2001年。ISBN 978-4320029996。
• Douglas E. Comer, David L.Stevens 『/IPによるネットワーク構築 Vol.III Linux/POSIXソケットバージョン クライアントサーバプログラミングとアプリケーション』 村井純, 榎本博之訳、共立出版、2003年。ISBN 978-4320120846。
• Joseph G. Davies, Thomas F. Lee (2003). Microsoft Windows Server 2003 TCP/IP Protocols and Services Technical Reference. ISBN 0-7356-1291-9. 
• Craig Hunt (1998). TCP/IP Network Administration. O'Reilly. ISBN 1-56592-322-7. 
• W・リチャード・スティーヴンス 『詳解TCP/IP Vol.1 プロトコル』、ピアソンエデュケーション』 橘康雄, 井上尚司訳、2000年。ISBN 4894713208。
• W・リチャード・スティーヴンス, ゲーリー・R. ライト 『詳解TCP/IP Vol.2 実装』 徳田英幸, 戸辺義人訳、ピアソンエデュケーション、2002年。ISBN 4894714957。
• W・リチャード・スティーヴンス 『詳解TCP/IP Vol.3 トランザクションTCP、HTTP、NNTP、UNIXドメインプロトコル』 中本幸一, 田中伸佳, 石川裕次訳、ピアソンエデュケーション、2002年。ISBN 4894716674。
• Ian McLean (2000). Windows(R) 2000 TCP/IP Black Book. Coriolis Group. ISBN 1-57610-687-X. 

外部リンク

• Learn Networking - An Introduction to TCP/IP
• RFC 675 - Specification of Internet Transmission Control Program, December 1974 Version
• TCP/IP State Transition Diagram (PDF)
• RFC 1180 A TCP/IP Tutorial - from the Internet Engineering Task Force (January 1991)
• TCP/IP FAQ
• The TCP/IP Guide - A comprehensive look at the protocols and the procedures/processes involved
• A Study of the ARPANET TCP/IP Digest
• TCP/IP Sequence Diagrams
• The Internet in Practice
• TCP/IP - Directory & Informational Resource
• Daryl's TCP/IP Primer - Intro to TCP/IP LAN administration, conversational style
• Introduction to TCP/IP
• TCP/IP commands from command prompt
• TCP/IPに係る既知のぜい弱性に関する調査報告書　改訂第2版 情報処理推進機構 (IPA)

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Internet protocols
Application layer
DHCP DHCPv6 DNS FTP HTTP IMAP IRC LDAP MGCP NNTP NTP POP RPC RTP RTCP RTSP SIP SMTP SNMP SOCKS SSH Telnet TLS/SSL XMPP (more)
Transport layer
TCP UDP DCCP SCTP RSVP (more)
Routing protocols *
BGP OSPF RIP (more)
Internet layer
IP IPv4 IPv6 ICMP ICMPv6 ECN IGMP IPsec (more)
Link layer
ARP/InARP NDP Tunnels L2TP PPP Media access control Ethernet DSL ISDN FDDI (more)
* Not a layer. A routing protocol belongs either to application or network layer.

The Internet protocol suite is the set of communications protocols used for the Internet and similar networks, and generally the most popular protocol stack for wide area networks. It is commonly known as TCP/IP, because of its most important protocols: Transmission Control Protocol (TCP) and Internet Protocol (IP), which were the first networking protocols defined in this standard. It is occasionally known as the DoD model due to the foundational influence of the ARPANET in the 1970s (operated by DARPA, an agency of the United States Department of Defense).

TCP/IP provides end-to-end connectivity specifying how data should be formatted, addressed, transmitted, routed and received at the destination. It has four abstraction layers, each with its own protocols. From lowest to highest, the layers are:

1. The link layer (commonly Ethernet) contains communication technologies for a local network.
2. The internet layer (IP) connects local networks, thus establishing internetworking.
3. The transport layer (TCP) handles host-to-host communication.
4. The application layer (for example HTTP) contains all protocols for specific data communications services on a process-to-process level (for example how a web browser communicates with a web server).

The TCP/IP model and related protocols are maintained by the Internet Engineering Task Force (IETF).

History

Diagram of the first internetworked connection

A Stanford Research Institute packet radio van, site of the first three-way internetworked transmission.

Early research

The Internet protocol suite resulted from research and development conducted by the Defense Advanced Research Projects Agency (DARPA) in the early 1970s. After initiating the pioneering ARPANET in 1969, DARPA started work on a number of other data transmission technologies. In 1972, Robert E. Kahn joined the DARPA Information Processing Technology Office, where he worked on both satellite packet networks and ground-based radio packet networks, and recognized the value of being able to communicate across both. In the spring of 1973, Vinton Cerf, the developer of the existing ARPANET Network Control Program (NCP) protocol, joined Kahn to work on open-architecture interconnection models with the goal of designing the next protocol generation for the ARPANET.

By the summer of 1973, Kahn and Cerf had worked out a fundamental reformulation, where the differences between network protocols were hidden by using a common internetwork protocol, and, instead of the network being responsible for reliability, as in the ARPANET, the hosts became responsible. Cerf credits Hubert Zimmerman and Louis Pouzin, designer of the CYCLADES network, with important influences on this design.

The network's design included the recognition it should provide only the functions of efficiently transmitting and routing traffic between end nodes and that all other intelligence should be located at the edge of the network, in the end nodes. Using a simple design, it became possible to connect almost any network to the ARPANET, irrespective of their local characteristics, thereby solving Kahn's initial problem. One popular expression is that TCP/IP, the eventual product of Cerf and Kahn's work, will run over "two tin cans and a string." As a joke, the IP over Avian Carriers formal protocol specification was created and successfully tested.

A computer called a router is provided with an interface to each network. It forwards packets back and forth between them. Originally a router was called gateway, but the term was changed to avoid confusion with other types of gateways.

Specification

From 1973 to 1974, Cerf's networking research group at Stanford worked out details of the idea, resulting in the first TCP specification. A significant technical influence was the early networking work at Xerox PARC, which produced the PARC Universal Packet protocol suite, much of which existed around that time.

DARPA then contracted with BBN Technologies, Stanford University, and the University College London to develop operational versions of the protocol on different hardware platforms. Four versions were developed: TCP v1, TCP v2, TCP v3 and IP v3, and TCP/IP v4. The last protocol is still in use today.

In 1975, a two-network TCP/IP communications test was performed between Stanford and University College London (UCL). In November, 1977, a three-network TCP/IP test was conducted between sites in the US, the UK, and Norway. Several other TCP/IP prototypes were developed at multiple research centers between 1978 and 1983. The migration of the ARPANET to TCP/IP was officially completed on flag day January 1, 1983, when the new protocols were permanently activated.

Adoption

In March 1982, the US Department of Defense declared TCP/IP as the standard for all military computer networking. In 1985, the Internet Architecture Board held a three-day workshop on TCP/IP for the computer industry, attended by 250 vendor representatives, promoting the protocol and leading to its increasing commercial use.

In 1985, the first Interop conference was held, focusing on network interoperability via further adoption of TCP/IP. It was founded by Dan Lynch, an early Internet activist. From the beginning, it was attended by large corporations, such as IBM and DEC. Interoperability conferences have been held every year since then. Every year from 1985 through 1993, the number of attendees tripled.^{[citation needed]}

IBM, ATT and DEC were the first major corporations to adopt TCP/IP, despite having competing internal protocols (SNA, XNS, etc.). In IBM, from 1984, Barry Appelman's group did TCP/IP development. (Appelman later moved to AOL to be the head of all its development efforts.) They navigated the corporate politics to get a stream of TCP/IP products for various IBM systems, including MVS, VM, and OS/2. At the same time, several smaller companies began offering TCP/IP stacks for DOS and MS Windows, such as the company FTP Software, and the Wollongong Group. The first VM/CMS TCP/IP stack came from the University of Wisconsin.

Back then, most of these TCP/IP stacks were written single-handedly by a few talented programmers. For example, John Romkey of FTP Software was the author of the MIT PC/IP package. John Romkey's PC/IP implementation was the first IBM PC TCP/IP stack. Jay Elinsky and Oleg Vishnepolsky of IBM Research wrote TCP/IP stacks for VM/CMS and OS/2, respectively.

The spread of TCP/IP was fueled further in June 1989, when AT&T agreed to put into the public domain the TCP/IP code developed for UNIX. Various vendors, including IBM, included this code in their own TCP/IP stacks. Many companies sold TCP/IP stacks for Windows until Microsoft released its own TCP/IP stack in Windows 95. This event was a little late in the evolution of the Internet, but it cemented TCP/IP's dominance over other protocols, which eventually disappeared. These protocols included IBM's SNA, OSI, Microsoft's native NetBIOS, and Xerox' XNS.^{[citation needed]}

Key architectural principles

An early architectural document, RFC 1122, emphasizes architectural principles over layering.

• End-to-end principle: This principle has evolved over time. Its original expression put the maintenance of state and overall intelligence at the edges, and assumed the Internet that connected the edges retained no state and concentrated on speed and simplicity. Real-world needs for firewalls, network address translators, web content caches and the like have forced changes in this principle.
• Robustness Principle: "In general, an implementation must be conservative in its sending behavior, and liberal in its receiving behavior. That is, it must be careful to send well-formed datagrams, but must accept any datagram that it can interpret (e.g., not object to technical errors where the meaning is still clear)." "The second part of the principle is almost as important: software on other hosts may contain deficiencies that make it unwise to exploit legal but obscure protocol features."

Layers in the Internet protocol suite

Two Internet hosts connected via two routers and the corresponding layers used at each hop. The application on each host executes read and write operations as if the processes were directly connected to each other by some kind of data pipe. Every other detail of the communication is hidden from each process. The underlying mechanisms that transmit data between the host computers are located in the lower protocol layers.

Encapsulation of application data descending through the layers described in RFC 1122

The Internet protocol suite uses encapsulation to provide abstraction of protocols and services. Encapsulation is usually aligned with the division of the protocol suite into layers of general functionality. In general, an application (the highest level of the model) uses a set of protocols to send its data down the layers, being further encapsulated at each level.

The "layers" of the protocol suite near the top are logically closer to the user application, while those near the bottom are logically closer to the physical transmission of the data. Viewing layers as providing or consuming a service is a method of abstraction to isolate upper layer protocols from the nitty-gritty detail of transmitting bits over, for example, Ethernet and collision detection, while the lower layers avoid having to know the details of each and every application and its protocol.

Even when the layers are examined, the assorted architectural documents—there is no single architectural model such as ISO 7498, the Open Systems Interconnection (OSI) model—have fewer and less rigidly defined layers than the OSI model, and thus provide an easier fit for real-world protocols. In point of fact, one frequently referenced document, RFC 1958, does not contain a stack of layers. The lack of emphasis on layering is a strong difference between the IETF and OSI approaches. It only refers to the existence of the "internetworking layer" and generally to "upper layers"; this document was intended as a 1996 "snapshot" of the architecture: "The Internet and its architecture have grown in evolutionary fashion from modest beginnings, rather than from a Grand Plan. While this process of evolution is one of the main reasons for the technology's success, it nevertheless seems useful to record a snapshot of the current principles of the Internet architecture."

RFC 1122, entitled Host Requirements, is structured in paragraphs referring to layers, but the document refers to many other architectural principles not emphasizing layering. It loosely defines a four-layer model, with the layers having names, not numbers, as follows:

• Application layer (process-to-process): This is the scope within which applications create user data and communicate this data to other processes or applications on another or the same host. The communications partners are often called peers. This is where the "higher level" protocols such as SMTP, FTP, SSH, HTTP, etc. operate.
• Transport layer (host-to-host): The transport layer constitutes the networking regime between two network hosts, either on the local network or on remote networks separated by routers. The transport layer provides a uniform networking interface that hides the actual topology (layout) of the underlying network connections. This is where flow-control, error-correction, and connection protocols exist, such as TCP. This layer deals with opening and maintaining connections between Internet hosts.
• Internet layer (internetworking): The internet layer has the task of exchanging datagrams across network boundaries. It is therefore also referred to as the layer that establishes internetworking, indeed, it defines and establishes the Internet. This layer defines the addressing and routing structures used for the TCP/IP protocol suite. The primary protocol in this scope is the Internet Protocol, which defines IP addresses. Its function in routing is to transport datagrams to the next IP router that has the connectivity to a network closer to the final data destination.
• Link layer: This layer defines the networking methods within the scope of the local network link on which hosts communicate without intervening routers. This layer describes the protocols used to describe the local network topology and the interfaces needed to effect transmission of Internet layer datagrams to next-neighbor hosts. (cf. the OSI data link layer).

The Internet protocol suite and the layered protocol stack design were in use before the OSI model was established. Since then, the TCP/IP model has been compared with the OSI model in books and classrooms, which often results in confusion because the two models use different assumptions, including about the relative importance of strict layering.

This abstraction also allows upper layers to provide services that the lower layers cannot, or choose not, to provide. Again, the original OSI model was extended to include connectionless services (OSIRM CL). For example, IP is not designed to be reliable and is a best effort delivery protocol. This means that all transport layer implementations must choose whether or not to provide reliability and to what degree. UDP provides data integrity (via a checksum) but does not guarantee delivery; TCP provides both data integrity and delivery guarantee (by retransmitting until the receiver acknowledges the reception of the packet).

This model lacks the formalism of the OSI model and associated documents, but the IETF does not use a formal model and does not consider this a limitation, as in the comment by David D. Clark, "We reject: kings, presidents and voting. We believe in: rough consensus and running code." Criticisms of this model, which have been made with respect to the OSI model, often do not consider ISO's later extensions to that model.

1. For multiaccess links with their own addressing systems (e.g. Ethernet) an address mapping protocol is needed. Such protocols can be considered to be below IP but above the existing link system. While the IETF does not use the terminology, this is a subnetwork dependent convergence facility according to an extension to the OSI model, the internal organization of the network layer (IONL).
2. ICMP & IGMP operate on top of IP but do not transport data like UDP or TCP. Again, this functionality exists as layer management extensions to the OSI model, in its Management Framework (OSIRM MF)
3. The SSL/TLS library operates above the transport layer (uses TCP) but below application protocols. Again, there was no intention, on the part of the designers of these protocols, to comply with OSI architecture.
4. The link is treated like a black box here. This is fine for discussing IP (since the whole point of IP is it will run over virtually anything). The IETF explicitly does not intend to discuss transmission systems, which is a less academic but practical alternative to the OSI model.

The following is a description of each layer in the TCP/IP networking model starting from the lowest level.

Link layer

The link layer is the networking scope of the local network connection to which a host is attached. This regime is called the link in Internet literature. This is the lowest component layer of the Internet protocols, as TCP/IP is designed to be hardware independent. As a result TCP/IP is able to be implemented on top of virtually any hardware networking technology.

The link layer is used to move packets between the Internet layer interfaces of two different hosts on the same link. The processes of transmitting and receiving packets on a given link can be controlled both in the software device driver for the network card, as well as on firmware or specialized chipsets. These will perform data link functions such as adding a packet header to prepare it for transmission, then actually transmit the frame over a physical medium. The TCP/IP model includes specifications of translating the network addressing methods used in the Internet Protocol to data link addressing, such as Media Access Control (MAC), however all other aspects below that level are implicitly assumed to exist in the link layer, but are not explicitly defined.

This is also the layer where packets may be selected to be sent over a virtual private network or other networking tunnel. In this scenario, the link layer data may be considered application data which traverses another instantiation of the IP stack for transmission or reception over another IP connection. Such a connection, or virtual link, may be established with a transport protocol or even an application scope protocol that serves as a tunnel in the link layer of the protocol stack. Thus, the TCP/IP model does not dictate a strict hierarchical encapsulation sequence.

Internet layer

The internet layer has the responsibility of sending packets across potentially multiple networks. Internetworking requires sending data from the source network to the destination network. This process is called routing.

In the Internet protocol suite, the Internet Protocol performs two basic functions:

• Host addressing and identification: This is accomplished with a hierarchical addressing system (see IP address).
• Packet routing: This is the basic task of sending packets of data (datagrams) from source to destination by sending them to the next network node (router) closer to the final destination.

The internet layer is not only agnostic of application data structures at the transport layer, but it also does not distinguish between operation of the various transport layer protocols. So, IP can carry data for a variety of different upper layer protocols. These protocols are each identified by a unique protocol number: for example, Internet Control Message Protocol (ICMP) and Internet Group Management Protocol (IGMP) are protocols 1 and 2, respectively.

Some of the protocols carried by IP, such as ICMP (used to transmit diagnostic information about IP transmission) and IGMP (used to manage IP Multicast data) are layered on top of IP but perform internetworking functions. This illustrates the differences in the architecture of the TCP/IP stack of the Internet and the OSI model.

The internet layer only provides an unreliable datagram transmission facility between hosts located on potentially different IP networks by forwarding the transport layer datagrams to an appropriate next-hop router for further relaying to its destination. With this functionality, the internet layer makes possible internetworking, the interworking of different IP networks, and it essentially establishes the Internet. The Internet Protocol is the principal component of the internet layer, and it defines two addressing systems to identify network hosts computers, and to locate them on the network. The original address system of the ARPANET and its successor, the Internet, is Internet Protocol version 4 (IPv4). It uses a 32-bit IP address and is therefore capable of identifying approximately four billion hosts. This limitation was eliminated by the standardization of Internet Protocol version 6 (IPv6) in 1998, and beginning production implementations in approximately 2006.

Transport layer

The transport layer establishes host-to-host connectivity, meaning it handles the details of data transmission that are independent of the structure of user data and the logistics of exchanging information for any particular specific purpose. Its responsibility includes end-to-end message transfer independent of the underlying network, along with error control, segmentation, flow control, congestion control, and application addressing (port numbers). End to end message transmission or connecting applications at the transport layer can be categorized as either connection-oriented, implemented in TCP, or connectionless, implemented in UDP.

The transport layer can be thought of as a transport mechanism, e.g., a vehicle with the responsibility to make sure that its contents (passengers/goods) reach their destination safely and soundly, unless another protocol layer is responsible for safe delivery. The layer simply establishes a basic data channel that an application uses in its task-specific data exchange.

For this purpose the layer establishes the concept of the port, a numbered logical construct allocated specifically for each of the communication channels an application needs. For many types of services, these port numbers have been standardized so that client computers may address specific services of a server computer without the involvement of service announcements or directory services.

Since IP provides only a best effort delivery, the transport layer is the first layer of the TCP/IP stack to offer reliability. IP can run over a reliable data link protocol such as the High-Level Data Link Control (HDLC).

For example, the TCP is a connection-oriented protocol that addresses numerous reliability issues to provide a reliable byte stream:

• data arrives in-order
• data has minimal error (i.e. correctness)
• duplicate data is discarded
• lost/discarded packets are resent
• includes traffic congestion control

The newer Stream Control Transmission Protocol (SCTP) is also a reliable, connection-oriented transport mechanism. It is message-stream-oriented — not byte-stream-oriented like TCP — and provides multiple streams multiplexed over a single connection. It also provides multi-homing support, in which a connection end can be represented by multiple IP addresses (representing multiple physical interfaces), such that if one fails, the connection is not interrupted. It was developed initially for telephony applications (to transport SS7 over IP), but can also be used for other applications.

User Datagram Protocol is a connectionless datagram protocol. Like IP, it is a best effort, "unreliable" protocol. Reliability is addressed through error detection using a weak checksum algorithm. UDP is typically used for applications such as streaming media (audio, video, Voice over IP etc.) where on-time arrival is more important than reliability, or for simple query/response applications like DNS lookups, where the overhead of setting up a reliable connection is disproportionately large. Real-time Transport Protocol (RTP) is a datagram protocol that is designed for real-time data such as streaming audio and video.

The applications at any given network address are distinguished by their TCP or UDP port. By convention certain well known ports are associated with specific applications. (See List of TCP and UDP port numbers.)

Application layer

The application layer contains the higher-level protocols used by most applications for network communication. Examples of application layer protocols include the File Transfer Protocol (FTP) and the Simple Mail Transfer Protocol (SMTP). Data coded according to application layer protocols are then encapsulated into one or (occasionally) more transport layer protocols (such as TCP or UDP), which in turn use lower layer protocols to effect actual data transfer.

Since the IP stack defines no layers between the application and transport layers, the application layer must include any protocols that act like the OSI's presentation and session layer protocols. This is usually done through libraries.

Application layer protocols generally treat the transport layer (and lower) protocols as black boxes which provide a stable network connection across which to communicate, although the applications are usually aware of key qualities of the transport layer connection such as the end point IP addresses and port numbers. As noted above, layers are not necessarily clearly defined in the Internet protocol suite. Application layer protocols are most often associated with client–server applications, and the commoner servers have specific ports assigned to them by the IANA: HTTP has port 80; Telnet has port 23; etc. Clients, on the other hand, tend to use ephemeral ports, i.e. port numbers assigned at random from a range set aside for the purpose.

Transport and lower level layers are largely unconcerned with the specifics of application layer protocols. Routers and switches do not typically "look inside" the encapsulated traffic to see what kind of application protocol it represents, rather they just provide a conduit for it. However, some firewall and bandwidth throttling applications do try to determine what's inside, as with the Resource Reservation Protocol (RSVP). It's also sometimes necessary for Network Address Translation (NAT) facilities to take account of the needs of particular application layer protocols. (NAT allows hosts on private networks to communicate with the outside world via a single visible IP address using port forwarding, and is an almost ubiquitous feature of modern domestic broadband routers).

Layer names and number of layers in the literature

The following table shows various networking models. The number of layers varies between three and seven.

Kurose, Forouzan	Comer, Kozierok	Stallings	Tanenbaum	RFC 1122, Internet STD 3 (1989)	Cisco Academy	Mike Padlipsky's 1982 "Arpanet Reference Model" (RFC 871)	OSI model
Five layers	Four+one layers	Five layers	Five layers	Four layers	Four layers	Three layers	Seven layers
"Five-layer Internet model" or "TCP/IP protocol suite"	"TCP/IP 5-layer reference model"	"TCP/IP model"	"TCP/IP 5-layer reference model"	"Internet model"	"Internet model"	"Arpanet reference model"	OSI model
Application	Application	Application	Application	Application	Application	Application/Process	Application
							Presentation
							Session
Transport	Transport	Host-to-host or transport	Transport	Transport	Transport	Host-to-host	Transport
Network	Internet	Internet	Internet	Internet	Internetwork		Network
Data link	Data link (Network interface)	Network access	Data link	Link	Network interface	Network interface	Data link
Physical	(Hardware)	Physical	Physical				Physical

Some of the networking models are from textbooks, which are secondary sources that may contravene the intent of RFC 1122 and other IETF primary sources.

OSI and TCP/IP layering differences

The three top layers in the OSI model—the application layer, the presentation layer and the session layer—are not distinguished separately in the TCP/IP model where it is just the application layer. While some pure OSI protocol applications, such as X.400, also combined them, there is no requirement that a TCP/IP protocol stack must impose monolithic architecture above the transport layer. For example, the NFS application protocol runs over the eXternal Data Representation (XDR) presentation protocol, which, in turn, runs over a protocol called Remote Procedure Call (RPC). RPC provides reliable record transmission, so it can run safely over the best-effort UDP transport.

Different authors have interpreted the RFCs differently, about whether the link layer (and the TCP/IP model) covers OSI model layer 1 (physical layer) issues, or if a hardware layer is assumed below the link layer.

Several authors have attempted to incorporate the OSI model's layers 1 and 2 into the TCP/IP model, since these are commonly referred to in modern standards (for example, by IEEE and ITU). This often results in a model with five layers, where the link layer or network access layer is split into the OSI model's layers 1 and 2.

The session layer roughly corresponds to the Telnet virtual terminal functionality^{[citation needed]}, which is part of text based protocols such as the HTTP and SMTP TCP/IP model application layer protocols. It also corresponds to TCP and UDP port numbering, which is considered as part of the transport layer in the TCP/IP model. Some functions that would have been performed by an OSI presentation layer are realized at the Internet application layer using the MIME standard, which is used in application layer protocols such as HTTP and SMTP.

The IETF protocol development effort is not concerned with strict layering. Some of its protocols may not fit cleanly into the OSI model, although RFCs sometimes refer to it and often use the old OSI layer numbers. The IETF has repeatedly stated^{[citation needed]} that Internet protocol and architecture development is not intended to be OSI-compliant. RFC 3439, addressing Internet architecture, contains a section entitled: "Layering Considered Harmful".

Conflicts are apparent also in the original OSI model, ISO 7498, when not considering the annexes to this model (e.g., ISO 7498/4 Management Framework), or the ISO 8648 Internal Organization of the Network layer (IONL). When the IONL and Management Framework documents are considered, the ICMP and IGMP are neatly defined as layer management protocols for the network layer. In like manner, the IONL provides a structure for "subnetwork dependent convergence facilities" such as ARP and RARP.

IETF protocols can be encapsulated recursively, as demonstrated by tunneling protocols such as Generic Routing Encapsulation (GRE). GRE uses the same mechanism that OSI uses for tunneling at the network layer.

Implementations

No specific hardware or software implementation is required by the protocols or the layered model, so there are many. Most computer operating systems in use today, including all consumer-targeted systems, include a TCP/IP implementation.

A minimally acceptable implementation includes the following protocols, listed from most essential to least essential: IP, ARP, ICMP, UDP, TCP and sometimes IGMP. In principle, it is possible to support only one transport protocol, such as UDP, but this is rarely done, because it limits usage of the whole implementation. IPv6, beyond its own version of ARP (NDP), ICMP (ICMPv6) and IGMP (IGMPv6), has some additional required functions, and often is accompanied by an integrated IPSec security layer. Other protocols could be easily added later (possibly being implemented entirely in userspace), such as DNS for resolving domain names to IP addresses, or DHCP for automatically configuring network interfaces.

Normally, application programmers are concerned only with interfaces in the application layer and often also in the transport layer, while the layers below are services provided by the TCP/IP stack in the operating system. Most IP implementations are accessible to programmers through sockets and APIs.

Unique implementations include Lightweight TCP/IP, an open source stack designed for embedded systems, and KA9Q NOS, a stack and associated protocols for amateur packet radio systems and personal computers connected via serial lines.

Microcontroller firmware in the network adapter typically handles link issues, supported by driver software in the operational system. Non-programmable analog and digital electronics are normally in charge of the physical components below the link layer, typically using an application-specific integrated circuit (ASIC) chipset for each network interface or other physical standard. High-performance routers are to a large extent based on fast non-programmable digital electronics, carrying out link level switching.

See also

• List of network protocols
• List of automation protocols
• List of TCP and UDP port numbers
• FLIP (protocol) (fast local Internet protocol stack)
• List of information technology acronyms
• BBN Report 1822

References

Further reading

• Douglas E. Comer. Internetworking with TCP/IP - Principles, Protocols and Architecture. ISBN 86-7991-142-9
• Joseph G. Davies and Thomas F. Lee. Microsoft Windows Server 2003 TCP/IP Protocols and Services. ISBN 0-7356-1291-9
• Forouzan, Behrouz A. (2003). TCP/IP Protocol Suite (2nd ed.). McGraw-Hill. ISBN 0-07-246060-1.
• Craig Hunt TCP/IP Network Administration. O'Reilly (1998) ISBN 1-56592-322-7
• Maufer, Thomas A. (1999). IP Fundamentals. Prentice Hall. ISBN 0-13-975483-0.
• Ian McLean. Windows(R) 2000 TCP/IP Black Book. ISBN 1-57610-687-X
• Ajit Mungale Pro .NET 1.1 Network Programming. ISBN 1-59059-345-6
• W. Richard Stevens. TCP/IP Illustrated, Volume 1: The Protocols. ISBN 0-201-63346-9
• W. Richard Stevens and Gary R. Wright. TCP/IP Illustrated, Volume 2: The Implementation. ISBN 0-201-63354-X
• W. Richard Stevens. TCP/IP Illustrated, Volume 3: TCP for Transactions, HTTP, NNTP, and the UNIX Domain Protocols. ISBN 0-201-63495-3
• Andrew S. Tanenbaum. Computer Networks. ISBN 0-13-066102-3
• Clark, D. (1988). "The Design Philosophy of the DARPA Internet Protocols". SIGCOMM '88 Symposium proceedings on Communications architectures and protocols (ACM): 106–114. doi:10.1145/52324.52336. http://www.cs.princeton.edu/~jrex/teaching/spring2005/reading/clark88.pdf. Retrieved 2011-10-16.

External links

• Internet History—Pages on Robert Kahn, Vinton Cerf, and TCP/IP (reviewed by Cerf and Kahn).
• RFC 675 - Specification of Internet Transmission Control Program, December 1974 Version
• TCP/IP State Transition Diagram (PDF)
• RFC 1180 A TCP/IP Tutorial - from the Internet Engineering Task Force (January 1991)
• TCP/IP FAQ
• The TCP/IP Guide - A comprehensive look at the protocols and the procedures/processes involved
• A Study of the ARPANET TCP/IP Digest
• TCP/IP Sequence Diagrams
• The Internet in Practice
• TCP/IP - Directory & Informational Resource
• Daryl's TCP/IP Primer - Intro to TCP/IP LAN administration, conversational style
• Introduction to TCP/IP
• TCP/IP commands from command prompt
• cIPS — Robust TCP/IP stack for embedded devices without an Operating System

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• Ubiquitous monitoring of electrical household appliances.

• Lloret J, Macías E, Suárez A, Lacuesta R.SourceInstituto de Investigación para la Gestión Integrada de Zonas Costeras (IGIC), Universidad Politécnica de Valencia, Valencia, Spain. jlloret@dcom.upv.es
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• Sammarco C, Iera A.SourceARTS Laboratory, Department DIMET, University of Reggio Calabria, Loc Feo di Vito, 89100 Reggio Calabria, Italy. chiara.sammarco@unirc.it
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• 情報処理学会研究報告. 情報学基礎研究会報告 2013-IFAT-111(18), 1-6, 2013-07-15
• … クセスプロトコルを TCP/IP 上で転送する手法を用いれば,ネットワーク越しのストレージアクセスを行うことができ,インターネットなどの公衆網を用いて遠隔地のストレージにアクセスが可能なとなる.しかし,TCP/IP 通信の性能は RTT に依存し,RTT公平性が必ずしも高くないと考えられる.本稿では TCP/IP の上に SCSI …
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• … クセスプロトコルを TCP/IP 上で転送する手法を用いれば,ネットワーク越しのストレージアクセスを行うことができ,インターネットなどの公衆網を用いて遠隔地のストレージにアクセスが可能なとなる.しかし,TCP/IP 通信の性能は RTT に依存し,RTT公平性が必ずしも高くないと考えられる.本稿では TCP/IP の上に SCSI …
• NAID 110009585826

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