3.3 Connectionless Transport: UDP

UDP Overview

• Protocol: User Datagram Protocol (UDP) defined in RFC $768$.

• Characteristics:

  • Connectionless: No handshaking between sender and receiver before sending data. Each segment is handled independently of those that preceded or followed it.

  • Best-Effort Service: There is no guarantee that the segment will reach its destination, nor is there a guarantee on the order of delivery.

  • Minimalist Design: No congestion control or flow control. This allows the application to send data at any rate, restricted only by the link bandwidth and the underlying IP layer.

  • No Connection State: Servers do not need to track sequence numbers, acknowledgement numbers, or congestion window sizes, allowing a single server to handle significantly more active clients compared to TCP.

Sender and Receiver Actions

• Sender Process:

  • The application layer passes a message to UDP.

  • UDP wraps the message with a header containing multiplexing and error-detection fields.

  • The resulting segment (header + payload) is then passed to the IP layer for encapsulation.

• Receiver Process:

  • The transport layer receives the UDP segment from the IP datagram.

  • Checksum Verification: The receiver calculates its own checksum for the received segment. If it does not match the value in the header, the segment is usually discarded.

  • Demultiplexing: The segment is directed to the appropriate application-layer socket based on the destination port number.

UDP Segment Structure

• Total Header Size: Fixed at $8$ bytes ($64$ bits).

• Header Fields (each $16$ bits):

  • Source Port: Identifies the port on the sender's side to which replies should be sent.

  • Destination Port: Identifies the specific receiving process on the destination host.

  • Length: Specifies the total number of bytes in the segment (Header + Data). The minimum length is $8$ bytes.

  • Checksum: Used to provide error detection for the segment.

Internet Checksum Details

• Objective: Detect errors (e.g., bit flips) incurred during transmission.

• Computation at Sender:

  1. The segment is treated as a series of $16$-bit integers.

  2. These integers are summed. If a carry bit is produced from the most significant bit, it is added to the least significant bit (end-around carry).

  3. The final sum is inverted (one’s complement).

• Verification at Receiver: The receiver sums all $16$-bit words including the checksum itself. If no errors occurred, the resulting sum should be all ones ($1111111111111111$).

UDP Applications and Advantages

• Why choose UDP?:

  • No Connection Establishment: No three-way handshake means no RTT (Round Trip Time) delay before data transfer begins.

  • Low Overhead: The small $8$-byte header minimizes bandwidth waste compared to TCP's $20$-byte header.

  • Application-Level Control: Fine-grained control over when data is sent; ideal for real-time applications where timeliness is preferred over reliability.

• Common Use Cases:

  • Streaming Multimedia: Can tolerate some data loss while requiring steady delivery rates (VoIP, Video Conferencing).

  • Network Services: DNS (for speed) and SNMP (to operate even when the network is congested).

  • Reliable UDP: Reliability can be implemented at the application layer. HTTP/3 uses the QUIC protocol, which runs on top of UDP to provide congestion control and error recovery without the setup delays of traditional TCP.