Layer 3, also known as the Internet Layer in the TCP/IP model, or the Network Layer in the OSI model, is varied in implementation. In today's Internet, layer 3 is most often the Internet Protocol, RFC791. However, ARP RFC826 is also used on Local Area Networks in order to reach the Internet. Several other Layer 3 implementations exist, such as DECnet Routing Protocol, AppleTalk DDP (Datagram Delivery Protocol), and Vines Internetwork Protocol (VIP - part of the Banyan Vines suite). Although, many of these are falling into disuse as legacy protocols.
In the Internet, Layer 3 is responsible for the host-to-host routing of packets towards their destination. Which involves packet switching to the appropriate interface on the local router, and packet forwarding to the next node, or in IP verbage, hop.
Layer 3 interfaces with Layer 4 of the OSI/Internet models above, which it receives the Application layer PDU and the Transport Layer header from. Below, it passes data to the Data Link, or Link layer interface, which is concerned with end-to-end delivery and physical addressing, along with providing an interface to the physical layer. for transmission of bits to/from the medium.
The IP Header consists of 14 fields, comprising a total of 186 bits, or about 24 bytes.
During the design of the Internet Protocol, the Department of Defense wanted a way to differentiate high priority communications from lower priority ones, so the Type Of Service field was included. The Type of Service field, made up of 8 different levels of priority, is no longer used in its original intent to denote a Nuclear War initiation or cancellation that couldn't be overriden by a lower officer. Today it is used as an additional type of Quality of Service class supplementing the DSCP field. The DSCP field was added later, in 1998. RFC2474
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