UDP datagram communication

UDP datagrams are sent without acknowledgement or retries

Issues relating to datagram communication

• Message size: not bigger than 64k in size, otherwise truncated on arrival
• blocking: non-blocking sends (message could be discarded at destination if there is not a socket bound to the port ) and blocking receives (could be timeout)
• Timeout: receiver set on socket
• Receive from any: not specify an origin for messages

Failure model

• omission failure: message be dropped due to checksum error or no buffer space at sender side or receiver side
• ordering: message be delivered out of sender order
• application maintains the reliability of UDP communication channel by itself

TCP stream communication

The API to the TCP

• Provide the abstraction of a stream of bytes to which data may be written and from which data may be read

Hidden network characteristics

• message sizes
• lost messages
• flow control
• message duplication and ordering
• message destinations 

Issues related to stream communication

• Matching of data items: agree to the contents of the transmitted data
• Blocking: send blocked until the data is written in the receiver’s buffer, receive blocked until the data in the local buffer becomes available
• Threads: server create a new thread when it accept a connection

Failure model

• Integrity and validity have been achieved by checksum, sequence number, timeout and retransmission in TCP protocol
• Connection could be broken due to unknown failures
  • Can’t distinguish between network failure and the destination process failure 
  • Can’t tell whether its recent messages have been received or not