Internet protocol suite : TCP/IP

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	Internet protocol suite : TCP/IP The Internet protocol suite is the set of protocols that implement the protocol stack on which the Internet runs. It is sometimes called the TCP/IP protocol suite after two of the many protocols that make up the suite: the transport protocol TCP and the internet protocol IP. The authoritative reference on this subject is RFC 1122[?], which can be found at http://www.ietf.org/rfc/rfc1122.txt. 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. 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 designed as the solution to a practical engineering problem. The OSI model, on the other hand, was a more theoretical approach, and was built by committee. Therefore, the OSI model is easier to understand, but the TCP/IP model is more practical. 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.

The TCP/IP Stack

There is some argument as to where the distinctions between layers are drawn; there is no one correct answer. But roughly:

7 Application e.g. HTTP, SMTP, SNMP, FTP, Telnet, NFS

6 Presentation e.g. XDR[?], ASN.1, SMB[?], NCP[?], AFP[?]

5 Session e.g. ISO 8327 / CCITT X.225, RPC, NetBIOS, ASP[?]

4 Transport e.g. TCP, UDP, RTP, SPX, ATP

3 Network e.g. IP, ICMP, IGMP, X.25, CLNP[?], ARP, OSPF, RIP, IPX, DDP[?]

2 Data Link e.g. Ethernet, Token Ring, PPP, HDLC, Frame relay, ISDN, ATM

1 Physical e.g. electricity, radio, laser

This 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 channel codes[?] and modulation, signal strengths, wavelengths, low-level sychronization and timing.

The Data-Link Layer

This specifies how packets are transported over the physical layer. Ethernet, for example, includes details about specifying which machine on the network a packet is destined for. Some examples are Ethernet, PPP, Wireless Ethernet, SLIP, Token Ring and ATM.

The Network Layer

The Network Layer solves the problem of getting from the source network to the destination network. This likely 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 packetized data from source to destination, and also supports ICMP (used to transmit diagnostic information about IP transmission) and IGMP (used to manage multicast data). ICMP and IGMP are layered on top of IP but perform network layer functions, illustrating an incompatibility between the Internet and OSI models.

The Transport Layer

The protocols at this Layer can solve problems like reliability ("did the packet reach the destination?") and ensure that packets arrive in the correct order. It is also at this protocol where it is decided which application to connect to.

TCP is a very "sturdy" transport mechanism, which makes sure packets arrive in order, are re-transmitted if lost, and eliminates duplicates, as well as handling "emergency" content which must be handled out of order (out-of-band[?]). TCP will attempt to deliver all data correctly in the specified sequence - this is its purpose and main advantage over UDP, but it can be a disadvantage in real-time streaming or routing applications with high layer 3 loss rates.

UDP is a lower-overhead protocol that is less sturdy. There is no attempt to verify that packets have reached their destination, and no guarantee that they will arrive in order. If the Application requires these guarantees, it must provide them itself, or use TCP. UDP is typically used for applications such as streaming media (audio and video, etc) where the time TCP requires for retransmission and re-ordering might not be available.

RTP is an attempt to provide a compromise between TCP and raw UDP. Although it 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 where most common network programs reside. These programs and their corresponding protocols include HTTP (The World Wide Web), FTP (File Transport), SMTP (Email), SSH (Remote Login), DNS (Name<-->IP Lookups) and many others.

Feel free to add to this list: Echo, QOTD[?], Telnet, NTP, DHCP (Kind-of), Finger, IMAP, SNMP, IRC, POP, IMAPS[?], HTTPS, POPS[?], RTSP, NNTP, Gopher.

Implementations

This is an OK explanation, but that's the OSI (Open Standards Interconnect) model not the TCP/IP stack

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7	Application	e.g. HTTP, SMTP, SNMP, FTP, Telnet, NFS
6	Presentation	e.g. XDR[?], ASN.1, SMB[?], NCP[?], AFP[?]
5	Session	e.g. ISO 8327 / CCITT X.225, RPC, NetBIOS, ASP[?]
4	Transport	e.g. TCP, UDP, RTP, SPX, ATP
3	Network	e.g. IP, ICMP, IGMP, X.25, CLNP[?], ARP, OSPF, RIP, IPX, DDP[?]
2	Data Link	e.g. Ethernet, Token Ring, PPP, HDLC, Frame relay, ISDN, ATM
1	Physical	e.g. electricity, radio, laser