A PDU can include different information as it goes up or down the OSI model. It is given a different name according to the information it is carrying (the layer it is at). When the transport layer receives upper layer data, it adds a TCP header to the data; this is called a segment. The segment is then passed to the network layer, and an IP header is added; thus, the data becomes a packet. The packet is passed to the data link layer, thus becoming a frame. This frame is then converted into bits and is passed across the network medium. This is data encapsulationApplication layer -- Data
- Transport layer -- Segment
- Network layer -- Packet
- Data link layer -- Frame
- There is also the Physical Layer -- Bits
Routing protocols determine how to route traffic to the best location of a routed protocol. Examples of routing protocols are RIP, EIGRP, OSFP, and BGP. Examples of routed protocols are IP and IPX.