Comparison of TCP and UDP

Transmission Control Protocol is a connection-oriented protocol, which means that it requires handshaking to set up end-to-end communications. Once a connection is set up, user data may be sent bi-directionally over the connection.

• Reliable – TCP manages message acknowledgement, retransmission and timeout. Multiple attempts to deliver the message are made. If it gets lost along the way, the server will re-request the lost part. In TCP, there's either no missing data, or, in case of multiple timeouts, the connection is dropped.
• Ordered – If two messages are sent over a connection in sequence, the first message will reach the receiving application first. When data segments arrive in the wrong order, TCP buffers delay the out-of-order data until all data can be properly re-ordered and delivered to the application.
• Heavyweight – TCP requires three packets to set up a socket connection, before any user data can be sent. TCP handles reliability and congestion control.
• Streaming – Data is read as a byte stream, no distinguishing indications are transmitted to signal message (segment) boundaries.

User Datagram Protocol is a simpler message-based connectionless protocol. Connectionless protocols do not set up a dedicated end-to-end connection. Communication is achieved by transmitting information in one direction from source to destination without verifying the readiness or state of the receiver.

• Unreliable – When a UDP message is sent, it cannot be known if it will reach its destination; it could get lost along the way. There is no concept of acknowledgement, retransmission, or timeout.
• Not ordered – If two messages are sent to the same recipient, the order in which they arrive cannot be predicted.
• Lightweight – There is no ordering of messages, no tracking connections, etc. It is a small transport layer designed on top of IP.
• Datagrams – Packets are sent individually and are checked for integrity only if they arrive. Packets have definite boundaries which are honored upon receipt, meaning a read operation at the receiver socket will yield an entire message as it was originally sent.
• No congestion control – UDP itself does not avoid congestion, unless they implement congestion control measures at the application level.
• Broadcasts - being connectionless, UDP can broadcast - sent packets can be addressed to be receivable by all devices on the subnet.