User Datagram Protocol

Applications can use

The Internet Assigned Numbers Authority (IANA) has divided port numbers into three ranges.^[6] Port numbers 0 through 1023 are used for common, well-known services. On Unix-like operating systems, using one of these ports requires superuser operating permission. Port numbers 1024 through 49151 are the registered ports used for IANA-registered services. Ports 49152 through 65535 are dynamic ports that are not officially designated for any specific service and may be used for any purpose. These may also be used as ephemeral ports, which software running on the host may use to dynamically create communications endpoints as needed.^[6]

A UDP datagram consists of a datagram header followed by a data section (the payload data for the application). The UDP datagram header consists of 4 fields, each of which is 2 bytes (16 bits):^[3]

The use of the Checksum and Source Port fields is optional in IPv4 (light purple background in table). In IPv6 only the Source Port field is optional. If not used, these fields should be set to zero.^[7]

Source Port: 16 bits

This field identifies the sender's port, when used, and should be assumed to be the port to reply to if needed. If the source host is the client, the port number is likely to be an ephemeral port. If the source host is the server, the port number is likely to be a

well-known port number from 0 to 1023.^[6]

Destination Port: 16 bits

This field identifies the receiver's port and is required. Similar to source port number, if the client is the destination host then the port number will likely be an ephemeral port number and if the destination host is the server then the port number will likely be a well-known port number.[6]

Length: 16 bits

This field specifies the length in bytes of the UDP datagram (the header fields and Data field) in

IP header).^[8]

Using IPv6 jumbograms it is possible to have UDP datagrams of size greater than 65,535 bytes. The length field is set to zero if the length of the UDP header plus UDP data is greater than 65,535.^[9]

Checksum: 16 bits

The checksum field may be used for error-checking of the header and data. This field is optional in IPv4, and mandatory in most cases in IPv6.^[10]

Data: Variable

The payload of the UDP packet.

The method used to compute the checksum is defined in RFC 768, and efficient calculation is discussed in RFC 1071:

Checksum is the 16-bit ones' complement of the ones' complement sum of a pseudo header of information from the IP header, the UDP header, and the data, padded with zero octets at the end (if necessary) to make a multiple of two octets.^[7]

In other words, all 16-bit words are summed using ones' complement arithmetic. Add the 16-bit values up. On each addition, if a carry-out (17th bit) is produced, swing that 17th carry bit around and add it to the least significant bit of the running total.^[11] Finally, the sum is then ones' complemented to yield the value of the UDP checksum field.

If the checksum calculation results in the value zero (all 16 bits 0) it should be sent as the ones' complement (all 1s) as a zero-value checksum indicates no checksum has been calculated.^[7] In this case, any specific processing is not required at the receiver, because all 0s and all 1s are equal to zero in 1's complement arithmetic.

The differences between IPv4 and IPv6 are in the pseudo header used to compute the checksum, and that the checksum is not optional in IPv6.^[12] Under specific conditions, a UDP application using IPv6 is allowed to use a zero UDP zero-checksum mode with a tunnel protocol.^[13]

When UDP runs over IPv4, the checksum is computed using a pseudo header that contains some of the same information from the real

Source Address: 32 bits

The source address from the IPv4 header.

Destination Address: 32 bits

The destination address from the IPv4 header.

Zeroes: 8 bits; Zeroes == 0

All zeroes.

Protocol: 8 bits

The protocol value for UDP: 17 (or 0x11).

UDP length: 16 bits

The length of the UDP header and data (measured in octets).

As IPv6 has larger addresses and a different header layout, the method used to compute the checksum is changed accordingly:^{[10]: §8.1}

Any transport or other upper-layer protocol that includes the addresses from the IP header in its checksum computation must be modified for use over IPv6, to include the 128-bit IPv6 addresses instead of 32-bit IPv4 addresses.

When computing the checksum, again a pseudo header is used that mimics the real

The checksum is computed over the following fields:

Source address: 128 bits

The address in the IPv6 header.

Destination address: 128 bits

The final destination; if the IPv6 packet does not contain a Routing header, TCP uses the destination address in the IPv6 header, otherwise, at the originating node, it uses the address in the last element of the Routing header, and, at the receiving node, it uses the destination address in the IPv6 header.

UDP length: 32 bits

The length of the UDP header and data (measured in octets).

Zeroes: 24 bits; Zeroes == 0

All zeroes.

Next Header: 8 bits

The transport layer protocol value for UDP: 17.

Lacking reliability, UDP applications may encounter some packet loss, reordering, errors or duplication. If using UDP, the end-user applications must provide any necessary handshaking such as real-time confirmation that the message has been received. Applications, such as

Most often, UDP applications do not employ reliability mechanisms and may even be hindered by them. Streaming media, real-time multiplayer games and voice over IP (VoIP) are examples of applications that often use UDP. In these particular applications, loss of packets is not usually a fatal problem. In VoIP, for example, latency and jitter are the primary concerns. The use of TCP would cause jitter if any packets were lost as TCP does not provide subsequent data to the application while it is requesting a re-send of the missing data.

Numerous key Internet applications use UDP, including: the Domain Name System (DNS), the Simple Network Management Protocol (SNMP), the Routing Information Protocol (RIP)^[3] and the Dynamic Host Configuration Protocol (DHCP).

Voice and video traffic is generally transmitted using UDP. Real-time video and

QUIC is a transport protocol built on top of UDP. QUIC provides a reliable and secure connection. HTTP/3 uses QUIC as opposed to earlier versions of HTTPS which use a combination of TCP and TLS to ensure reliability and security respectively. This means that HTTP/3 uses a single handshake to set up a connection, rather than having two separate handshakes for TCP and TLS, meaning the overall time to establish a connection is reduced.^[15]

Transmission Control Protocol is a connection-oriented protocol and 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 acknowledgment, retransmission and timeouts. Multiple attempts to deliver the message are made. If data gets lost along the way, data will be re-sent. 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 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

• 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 acknowledgment, retransmission, or timeout.
• Not ordered – If two messages are sent to the same recipient, the order in which they arrive cannot be guaranteed.
• Lightweight – There is no ordering of messages, no tracking connections, etc. It is a very simple transport layer designed on top of IP.
• Datagrams – Packets are sent individually and are checked for integrity on arrival. Packets have definite boundaries which are honored upon receipt; 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. Congestion control measures must be implemented at the application level or in the network.
• Broadcasts – being connectionless, UDP can broadcast - sent packets can be addressed to be receivable by all devices on the subnet.
• Multicast – a multicast mode of operation is supported whereby a single datagram packet can be automatically routed without duplication to a group of subscribers.

• RFC 768 – User Datagram Protocol
• RFC 2460 – Internet Protocol, Version 6 (IPv6) Specification
• RFC 2675 – IPv6 Jumbograms
• RFC 4113 – Management Information Base for the UDP
• RFC 8085 – UDP Usage Guidelines

External links

Wikiversity has learning resources about User Datagram Protocol

• IANA Port Assignments
• The Trouble with UDP Scanning (PDF)
• Breakdown of UDP frame
• UDP on MSDN Magazine Sockets and WCF