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Internet protocol suite
From Wikipedia, the free encyclopedia
When
computers connect and transmit
data between each other on the
Internet,
they follow a set of rules to do so. These rules are universal; all
computers throughout the Internet must follow them. Otherwise, the
Internet would not function as computers would not be able to transmit
data in a meaningful and useful way. These rules are called
protocols. There are many different protocols, each for different purposes, and they all together are called the
Internet protocol suite. The two most important protocols are the
Transmission Control Protocol (TCP) and the
Internet Protocol (IP), which ensure data is delivered at the right place, and without errors, and is what computers use when they access
servers (computers that have the data that is accessed on the Internet) on the
World Wide Web, as well as for
email, and the like. Other protocols include the
Network Time Protocol, which ensures clock synchronisation in computers, and there are many others.
The TCP/IP model and other related protocols are maintained by the
Internet Engineering Task Force, whose parent organisation is the
Internet Society, and which also cooperates closely with other standards bodies such as the
W3C (World Wide Web Consortium) and
ISO/IEC.
Other websites
Abstraction layers
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.
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Encapsulation of application data descending through the layers described in RFC 1122
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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 details 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. One frequently
referenced document,
RFC 1958,
does not contain a stack of layers. The lack of emphasis on layering is
a major 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:
- The Application layer is the scope within which applications create
user data and communicate this data to other applications on another or
the same host. The applications, or processes, make use of the services
provided by the underlying, lower layers, especially the Transport Layer
which provides reliable or unreliable pipes to other processes. The communications partners are characterized by the application architecture, such as the client-server model and peer-to-peer networking. This is the layer in which all higher level protocols, such as SMTP, FTP, SSH, HTTP, operate. Processes are addressed via ports which essentially represent services.
- The Transport Layer performs host-to-host communications on either
the same or different hosts and on either the local network or remote
networks separated by routers. It provides a channel for the
communication needs of applications. UDP is the basic transport layer
protocol, providing an unreliable datagram service. The Transmission
Control Protocol provides flow-control, connection establishment, and
reliable transmission of data.
- The Internet layer has the task of exchanging datagrams across
network boundaries. It provides a uniform networking interface that
hides the actual topology (layout) of the underlying network
connections. 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.
- The Link layer defines the networking methods within the scope of
the local network link on which hosts communicate without intervening
routers. This layer includes the protocols used to describe the local
network topology and the interfaces needed to effect transmission of
Internet layer datagrams to next-neighbor hosts.
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 and goals, including the relative importance of
strict layering.
This abstraction also allows upper layers to provide services that
the lower layers do not provide. While the original OSI model was
extended to include connectionless services (OSIRM CL),
[16] 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 how to provide reliability. 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 illustrated 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.
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).
[17]
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)
[18]
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.
The link is treated like a black box. The IETF explicitly does not
intend to discuss transmission systems, which is a less academic
[citation needed] 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 has the networking scope of the local network connection to which a host is attached. This regime is called the
link
in TCP/IP literature. It is the lowest component layer of the Internet
protocols, as TCP/IP is designed to be hardware independent. As a result
TCP/IP may 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 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). All other aspects below that level, however, 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.
The TCP/IP model's link layer corresponds to the Open Systems
Interconnection (OSI) model physical and data link layers, layers one
and two of the OSI model.
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.
[19]
The Internet Protocol performs two basic functions:
- Host addressing and identification: This is accomplished with a hierarchical IP addressing system.
- Packet routing: This is the basic task of sending packets of data
(datagrams) from source to destination by forwarding them to the next
network router closer to the final destination.
The internet layer is not only agnostic of data structures at the
transport layer, but it also does not distinguish between operation of
the various transport layer protocols. IP carries 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 which is used to
transmit diagnostic information, and IGMP which is 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 TCP/IP model's internet layer
corresponds to layer three of the Open Systems Interconnection (OSI)
model, where it is referred to as the network layer.
The internet layer provides only 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 a basic data channel that an
application uses in its task-specific data exchange. The layer
establishes process-to-process connectivity, meaning it provides
end-to-end services 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.
For the purpose of providing process-specific transmission channels for applications, the layer establishes the concept of the
port.
This is 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.
Because IP provides only a
best effort delivery, some transport layer protocols offer reliability. However, 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 in providing a
reliable byte stream:
- data arrives in-order
- data has minimal error (i.e., correctness)
- duplicate data is discarded
- lost or 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.
The 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.
The TCP/IP model's transport or host-to-host layer corresponds to the
fourth layer in the Open Systems Interconnection (OSI) model, also
called the transport layer.
Application layer
The
application layer
includes the protocols used by most applications for providing user
services or exchanging application data over the network connections
established by the lower level protocols, but this may include some
basic network support services, such as many routing protocols, and host
configuration protocols. Examples of application layer protocols
include the
Hypertext Transfer Protocol (HTTP), the
File Transfer Protocol (FTP), the
Simple Mail Transfer Protocol (SMTP), and the
Dynamic Host Configuration Protocol (DHCP).
[20] Data coded according to application layer protocols are
encapsulated into transport layer protocol units (such as TCP or UDP messages), which in turn use
lower layer protocols to effect actual data transfer.
The IP model does not consider the specifics of formatting and
presenting data, and does not define additional layers between the
application and transport layers as in the OSI model (presentation and
session layers). Such functions are the realm of
libraries and
application programming interfaces.
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. Application layer protocols are often associated with
particular
client–server applications, and common services have
well-known port numbers reserved by the
Internet Assigned Numbers Authority (IANA). For example, the
HyperText Transfer Protocol uses server port 80 and
Telnet uses server port 23.
Clients connecting to a service usually use
ephemeral ports,
i.e., port numbers assigned only for the duration of the transaction at
random or from a specific range configured in the application.
The transport layer and lower-level layers are unconcerned with the specifics of application layer protocols. Routers and
switches do not typically examine the encapsulated traffic, rather they just provide a conduit for it. However, some
firewall and
bandwidth throttling applications must interpret application data. An example is the
Resource Reservation Protocol (RSVP). It is also sometimes necessary for
network address translator (NAT) traversal to consider the application payload.
The application layer in the TCP/IP model is often compared as
equivalent to a combination of the fifth (Session), sixth
(Presentation), and the seventh (Application) layers of the Open Systems
Interconnection (OSI) model.
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.
| RFC 1122, Internet STD 3 (1989) |
Cisco Academy[21] |
Kurose,[22] Forouzan [23] |
Comer,[24] Kozierok[25] |
Stallings[26] |
Tanenbaum[27] |
Mike Padlipsky's 1982 "Arpanet Reference Model" (RFC 871) |
OSI model |
| Four layers |
Four layers |
Five layers |
Four+one layers |
Five layers |
Five layers |
Three layers |
Seven layers |
| "Internet model" |
"Internet model" |
"Five-layer Internet model" or "TCP/IP protocol suite" |
"TCP/IP 5-layer reference model" |
"TCP/IP model" |
"TCP/IP 5-layer reference model" |
"Arpanet reference model" |
OSI model |
| Application |
Application |
Application |
Application |
Application |
Application |
Application/Process |
Application |
| Presentation |
| Session |
| Transport |
Transport |
Transport |
Transport |
Host-to-host or transport |
Transport |
Host-to-host |
Transport |
| Internet |
Internetwork |
Network |
Internet |
Internet |
Internet |
Network |
| Link |
Network interface |
Data link |
Data link (Network interface) |
Network access |
Data link |
Network interface |
Data link |
| (n/a) |
|
Physical |
(Hardware) |
Physical |
Physical |
|
Physical |
Some of the networking models are from textbooks, which are secondary sources that may conflict with the intent of
RFC 1122 and other
IETF primary sources.
[28]
Comparison of TCP/IP and OSI layering
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 safely use 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 whether 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".
[28]
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
The Internet protocol suite does not presume any specific hardware or
software environment. It only requires that hardware and a software
layer exists that is capable of sending and receiving packets on a
computer network. As a result, the suite has been implemented on
essentially every computing platform. A minimal implementation of TCP/IP
includes the following:
Internet Protocol (IP),
Address Resolution Protocol (ARP),
Internet Control Message Protocol (ICMP),
Transmission Control Protocol (TCP),
User Datagram Protocol (UDP), and
IGMP.
In addition to IP, ICMP, TCP, UDP, Internet Protocol version 6 requires
NDP, ICMPv6, and IGMPv6 and is often accompanied by an integrated
IPSec security layer.
Application programmers are typically 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 operating 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
References
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