Internet protocol suite

The Internet protocol suite is the set of communications protocols that implement the protocol stack on which the Internet runs. It is sometimes called the TCP/IP protocol suite, after the two most important protocols in it: the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which were also the first two defined.

The internet protocol suite can be described by analogy with the OSI model, which describes the layers of a protocol stack, not all of which correspond well with internet practice. In a protocol stack, each layer solves a set of problems involving the transmission of data, and provides a well-defined service to the higher layers. Higher layers are logically closer to the user and deal with more abstract data, relying on lower layers to translate data into forms that can eventually be physically manipulated.

The internet model was produced as the solution to a practical engineering problem. The OSI model, on the other hand, was a more theoretical approach, and was also produced at an earlier stage in the evolution of networks. Therefore, the OSI model is easier to understand, but the TCP/IP model is the one in actual use. It is helpful to have an understanding of the OSI model before learning TCP/IP, as the same principles apply, but are easier to understand in the OSI model.

Contents

1 Layers in the TCP/IP stack 1.1 The physical layer 1.2 The data link layer 1.3 The network layer 1.4 The transport layer 1.5 The application layer 2 Implementations 3 Related topics

Layers in the TCP/IP stack

There is some discussion about how to map the TCP/IP model onto the OSI model. Since the TCP/IP and OSI protocol suites do not match precisely, there is no one correct answer.

In addition, the OSI model is not really rich enough at the lower layers to capture the true layering; there needs to be an extra layer (the Internetworking layer) between the Transport and Network layers. Protocols specific to a particular network type, but which are run on top of the basic hardware framing, ought to be at the Network layer. Examples of such protocols are ARP, and the Spanning Tree Protocol (used to keep redundant bridges idle until they are needed). However, they are local protocols, and operate beneath the internetwork functionality. Admittedly, placing both groups (not to mention protocols which are logically part of the internetwork layer, but run on top of the internetwork protocol, such as ICMP) all at the same layer can be confusing, but the OSI model is not complex enough to do a better job.

The following diagram attempts to show where various TCP/IP and other protocols would reside in the original OSI model:

7	Application	e.g. HTTP, SMTP, SNMP, FTP, Telnet, SSH and Scp, NFS, RTSP
6	Presentation	e.g. XDR, ASN.1, SMB, AFP
5	Session	e.g. TLS, SSH, ISO 8327 / CCITT X.225, RPC, NetBIOS, ASP
4	Transport	e.g. TCP, UDP, RTP, SCTP, SPX, ATP
3	Network	e.g. IP, ICMP, IGMP, X.25, CLNP, ARP, RARP, BGP, OSPF, RIP, IPX, DDP
2	Data Link	e.g. Ethernet, Token ring, PPP, HDLC, Frame relay, ISDN, ATM, 802.11 WiFi, FDDI
1	Physical	e.g. wire, radio, fiber optic

Commonly, the top three layers of the OSI model (Application, Presentation and Session) are considered as a single Application Layer in the TCP/IP suite. Because the TCP/IP suite has no unified session layer on which higher layers are built, these functions are typically carried out (or ignored) by individual applications. The most notable difference between TCP/IP and OSI models is the Application layer, as TCP/IP integrates a few steps of the OSI model into its Application layer. A simplified TCP/IP interpretation of the stack is shown below:

	Application "layer 7"	e.g. HTTP, FTP, DNS (routing protocols like BGP and RIP, which for a variety of reasons run over TCP and UDP respectively, may also be considered part of the Network layer)
4	Transport	e.g. TCP, UDP, RTP, SCTP (routing protocols like OSPF, which run over IP, may also be considered part of the Network layer)
3	Network	For TCP/IP this is the Internet Protocol (IP) (required protocols like ICMP and IGMP run over IP, but may still be considered part of the network layer; ARP does not run over IP)
2	Data Link	e.g. Ethernet, Token ring, etc.
1	Physical	e.g. physical media, and encoding techniques, T1, E1

The physical layer

The Physical layer describes the physical characteristics of the communication, such as conventions about the nature of the medium used for communication (such as wires, fiber optic links or radio links), and all related details such as connectors, channel codes and modulation, signal strengths, wavelength, low-level sychronization and timing and maximum distances. The Internet protocol suite does not cover the physical layer of any network; see the articles on specific network technologies for detail on the physical layer of each particular technology.

The data link layer

The data link layer specifies how packets are transported over the physical layer, including the framing (i.e. the special bit patterns which mark the start and end of packets). Ethernet, for example, includes fields in the packet header which specify which machine or machines on the network a packet is destined for. Examples of Data link layer protocols are Ethernet, Wireless Ethernet, SLIP, Token Ring and ATM.

PPP is a little more complex, as it was originally specified as a separate protocol which ran on top of another data link layer, HDLC/SDLC.

This layer is sometimes further subdivided into Logical Link Control and Media Access Control.

The network layer

As originally defined, the Network layer solved the problem of getting packets across a single network. Examples of such protocols are X.25, and the ARPANET's Host/IMP Protocol.

With the advent of the concept of internetworking, additional functionality was added to this layer, namely getting data from the source network to the destination network. This generally involves routing the packet across a network of networks, known as an internet.

In the internet protocol suite, IP performs the basic task of getting packets of data from source to destination. IP can carry data for a number of different higher level protocols; these protocols are each identified by a unique IP Protocol Number. ICMP and 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 multicast data) are layered on top of IP but perform network layer functions, illustrating an incompatibility between the internet and OSI models. All routing protocols, such as BGP, OSPF, and RIP are also really part of the network layer, although they might seem to belong higher in the stack.

The transport layer

The protocols at the Transport layer can solve problems like reliability ("did the data reach the destination?") and ensure that data arrives in the correct order. In the TCP/IP protocol suite, transport protocols also determine which application any given data is intended for.

The dynamic routing protocols which technically fit at this layer in the TCP/IP Protocol Suite (since they run over IP) are generally considered to be part of the Network layer; an example is OSPF (IP protocol number 89).

TCP (IP protocol number 6) is a "reliable", connection-oriented, transport mechanism providing a reliable byte stream, which makes sure data arrives complete, undamaged, and in order. TCP tries to continuously measure how loaded the network is and throttles its sending rate in order to avoid overloading the network. Furthermore, TCP will attempt to deliver all data correctly in the specified sequence. These are its main differences from UDP, and can become disadvantages in real-time streaming or routing applications with high internetwork layer loss rates.

The newer SCTP is also a "reliable", connection-oriented, transport mechanism. It is record rather than byte oriented, and provides multiple sub-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.

UDP (IP protocol number 17) is a connectionless datagram protocol. It is a "best effort" or "unreliable" protocol - not because it is particularly unreliable, but because it does not verify that packets have reached their destination, and gives no guarantee that they will arrive in order. If an Application requires these characteristics, it must provide them itself, or use TCP.

UDP is typically used for applications such as streaming media (audio and video, 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.

DCCP is currently under development by IETF. It provides TCP's flow control semantics, while keeping UDP's datagram service model visible to the user.

Both TCP and UDP are used to carry a number of higher-level applications. The applications at any given network address are distinguished by their TCP or UDP Port Number. By convention certain well known ports are associated with specific applications.

RTP is a datagram protocol that is designed for real-time data such as streaming audio and video. Although RTP uses the UDP packet format as a basis, it provides a function that is at the same protocol layer.

The application layer

The Application layer is the layer that most common network-aware programs interface use in order to communicate across a network with other programs. Processes that occur in this layer are application specific; data is passed from the network-aware program, in the format used internally by this application, and is encoded into a standard protocol.

Some specific programs are considered to run in this layer. They provide services that directly support user applications. These programs and their corresponding protocols include HTTP (The World Wide Web), FTP (File transport), SMTP (Email), SSH (Secure remote login), DNS (Name <-> IP Address lookups) and many others.

Once the data from an application has been encoded into a standard application layer protocol it will be passed down to the next layer of the IP stack.

At the Transport Layer, applications will most commonly make use of TCP or UDP, and are often associated with a well-known port number. The most common ports are listed below:

• QOTD on port 17
• File Transfer Protocol (FTP) on port 21
• Secure Shell (SSH) on port 22
• Telnet on port 23
• Simple Mail Transport Protocol (SMTP) for outgoing e-mail on port 25
• Domain Name System (DNS) lookups on UDP (or sometimes TCP) port 53
• Dynamic Host Configuration Protocol (DHCP) on ports 67 and 68
• Gopher on port 70
• Finger on port 79
• HTTP on TCP port 80
• POP3 read e-mail on port 110
• Network News Transfer Protocol (NNTP) on port 119
• Network Time Protocol (NTP) on port 123
• NetBIOS on port 139
• IMAP read e-mail on port 143
• Simple network management protocol (SNMP) on port 161
• XDMCP on port 177
• HTTPS secure HTTP on port 443
• Routing Information Protocol (RIP) routing updates on UDP port 520
• RTSP on port 554
• IMAPS on port 993
• Universal Plug and Play (UPnP) on port 5000
• IRC on port 6667
• IDENT on port 113

These ports were originally allocated by the Internet Assigned Numbers Authority (IANA).

Feel free to add to this list: POPS.

Implementations

• KA9Q

Related topics

• Protocol (computing)
• List of well-known ports (computing)
• OSI Modelca:TCP/IP

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