Synchronous and asynchronous

• a queue associated with message destination, Sending process add message to remote queue, Receiving process remove message from local queue
• Synchronous: send and receive are blocking operations
• asynchronous: send is unblocking, receive could be blocking or unblocking (receive notification by polling or interrupt)

Message destination

• Internet address + local port
• service name: help by name service at run time
• location independent identifiers, e.g. in Mach

Reliability

• validity: messages are guaranteed to be delivered despite a reasonable number of packets being dropped or lost
• Integrity: messages arrive uncorrupted and without duplication

Ordering 

• the messages be delivered in sender order