What is an application-layer message? A transport-layer segment? A networklayer datagram? A link-layer frame?
Application layer message: HTTP, SMTP, and FTP protocols are used in application layer. it is used to send data over multiple end systems.
Transport layer segment: Transfer the content between two endpoints mainly. TCP and UDP protocols are used in transport layer.
Network layer datagram: Move the packets between any two hosts in the network. IP protocol is used in network layer.
link-layer frame: Move the packets from one node to the next another node. Point-to-point protocol (ppp) used in data link layer.