1. NETWORK SERVICE
   1. facilitate connection between
      1. software application
      2. lower-layer network services
   2. PROTOCOLS
      1. HTTP
      2. FTP
      3. SMTP
      4. others, required by services
         1. DNS
         2. DHCP
         3. NTP
         4. BOOTP
         5. MOUNT
   3. result
      1. application can intepret data sent from network
      2. network can intepret data requested from application
2. SERVICE ADVERTISMENT
   1. Active
   2. Passive
3. SERVICE USE
   1. OS call interception
   2. Reote Operation
   3. Collaborative