Connection and connectionless svc. (Reliability)
1: Message boundaries are maintained.
2: Mess. bound. not maint. 1 2KB mess. could be 2 1KB mess or other
3: Possible to have noise or garbling yet have reliable communication
4: Elect. junk mail need not be reliable just have hi prob. of arrival
5: Like sending registered post by USPS
6: Client server model. Client issues request, server replies.
Network software/connection+connectionless.