1.5 Encapsulation
nUser data -> application
header/trailer -> TCP (or UDP) header/trailer ->
IP header/trailer -> data link layer header (e.g. Ethernet header)/trailer.
nWhen the TCP/UDP header is attached the packet is called the TCP/UDP segment.
nWhen the IP header is attached it is referred to as the IP datagram.
nWhen the data link layer header is attached, it is called the link frame
(e.g., Ethernet frame)