OSI Model 7層的主要功能 OSI Layer Equipment Data Unit 7 Application Protocol Gateway Program HTTP, FTP, Telnet, SMTP, POP 3, 4 Transport Layer 4 Switch Message TCP, UDP, 3 Network Router Packet or Datagram IP, ARP, RARP 2 Data Link Switch, Bridge Frame Ethernet, 1 Physical Repeater, hub Bit Fiber 6 Presentation 5 Session