Internet protocols

For more details on this topic, see Internet Protocols.

In this context, there are three layers of protocols:

* At the lower level (OSI layer 3) is IP (Internet Protocol), which defines the datagrams or packets that carry blocks of data from one node to another. The vast majority of today's Internet uses version four of the IP protocol (i.e. IPv4), and although IPv6 is standardized, it exists only as "islands" of connectivity, and there are many ISPs without any IPv6 connectivity. [7]. ICMP (Internet Control Message Protocol) also exists at this level. ICMP is connectionless; it is used for control, signaling, and error reporting purposes.

* TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) exist at the next layer up (OSI layer 4); these are the protocols by which data is transmitted. TCP makes a virtual 'connection', which gives some level of guarantee of reliability. UDP is a best-effort, connectionless transport, in which data packets that are lost in transit will not be re-sent.

* The application protocols sit on top of TCP and UDP and occupy layers 5, 6, and 7 of the OSI model. These define the specific messages and data formats sent and understood by the applications running at each end of the communication. Examples of these protocols are HTTP, FTP, and SMTP.