User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. Using UDP, programs on networked computers can send short messages sometimes known as datagrams (using Datagram Sockets) to one another. UDP is sometimes called the Universal Datagram Protocol. It has been suggested that Internet Protocols be merged into this article or section. ... In information technology, a packet is a formatted block of information carried by a computer network. ... Datagram socket is a type of Internet socket, which is the sending or receiving point for packet delivery services. ...

UDP does not guarantee reliability or ordering in the way that TCP does. Datagrams may arrive out of order, appear duplicated, or go missing without notice. Avoiding the overhead of checking whether every packet actually arrived makes UDP faster and more efficient, at least for applications that do not need guaranteed delivery. Time-sensitive applications often use UDP because dropped packets are preferable to delayed packets. UDP's stateless nature is also useful for servers that answer small queries from huge numbers of clients. Unlike TCP, UDP supports packet broadcast (sending to all on local network) and multicasting (send to all subscribers). The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data using Stream Sockets. ... A stateless server is one that treats each request as an independent transaction, unrelated to any previous request. ... The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data using Stream Sockets. ... Broadcasting in a computer network refers to transmiting a packet that will be received (conceptionally) by every device on the network. ... Routing Schemes anycast broadcast multicast unicast Multicast is sometimes also used to refer to a multiplexed broadcast, although that is a very different thing and should not be confused. ...

Common network applications that use UDP include the Domain Name System (DNS), streaming media applications such as IPTV, Voice over IP (VoIP), Trivial File Transfer Protocol (TFTP) and online games. It has been suggested that this article be split into multiple articles. ... Streaming media is multimedia that is continuously received by, and normally displayed to, the end-user while it is being delivered by the provider. ... IPTV (Internet Protocol Television) is a system where a digital television service is delivered by using Internet Protocol over a network infrastructure, which may include delivery by a broadband connection. ... An overview of how VoIP works A typical analog telephone adapter for connecting an ordinary phone to a VoIP network Ciscos implementation of VoIP - IP Phone Voice over Internet Protocol, also called VoIP, IP Telephony, Internet telephony, Broadband telephony, Broadband Phone and Voice over Broadband is the routing of... Trivial File Transfer Protocol (TFTP) is a very simple file transfer protocol, with the functionality of a very basic form of FTP; it was first defined in 1980. ... Online games refer to video games that are played over some form of computer network, most commonly the Internet. ...

Ports

Main article: List of TCP and UDP port numbers

UDP uses ports to allow application-to-application communication. The mostafa port field is 16-bits so the valid range is 0 to 65,535. Port 0 is reserved, but is a permissible source port value if the sending process does not expect messages in response. TCP and UDP are transport protocols used for communication between computers. ... It has been suggested that this article or section be merged into Computer port (software). ... This article is about the unit of information. ...

Ports 1 through 1023 are named "well-known" ports and on Unix-derived operating systems, binding to one of these ports requires root access. Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®) is a computer operating system originally developed in the 1960s and 1970s by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ... // An operating system (OS) is a set of computer programs that manage the hardware and software resources of a computer. ... On many computer operating systems, superuser, or root, is the term used for the special user account that is controlled by the system administrator. ...

Ports 1024 through 49,151 are registered ports. Registered port numbers are those network ports in the range 1024-49151. ...

Ports 49,152 through 65,535 are ephemeral ports and are used as temporary ports primarily by clients when communicating to servers. An ephemeral river is one in which lasts only a few days. ... In computing, a client is a system that accesses a (remote) service on another computer by some kind of network. ... This article or section does not cite any references or sources. ...

Packet structure

UDP is a minimal message-oriented transport layer protocol that is currently documented in IETF RFC 768. In computing and telecommunications, the transport layer is layer four of the seven layer OSI model. ... The Internet Engineering Task Force (IETF) is charged with developing and promoting Internet standards. ...

In the Internet protocol suite, UDP provides a very simple interface between a network layer below (e.g., IPv4) and a session layer or application layer above. It has been suggested that Internet Protocols be merged into this article or section. ... The network layer is level three of the seven level OSI model. ... Internet Protocol version 4 is the fourth iteration of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. ... The session layer is level five of the seven level OSI model. ... The application layer is the seventh level of the seven-layer OSI model. ...

UDP provides no guarantees to the upper layer protocol for message delivery and a UDP sender retains no state on UDP messages once sent (for this reason UDP is sometimes called the Unreliable Datagram Protocol). UDP adds only application multiplexing and checksumming of the header and payload. If any kind of reliability for the information transmitted is needed, it must be implemented in upper layers. In computer networking, the upper layer protocol (ULP) refers to the more abstract protocol when performing encapsulation. ... In computer networking, a reliable protocol is one that ensures data arrival via some internal method, as opposed to an unreliable protocol, which does not guarantee that all the data will arrive intact (or indeed, at all). ... In telecommunications, multiplexing (also muxing or MUXing) is the combining of two or more information channels onto a common transmission medium using hardware called a multiplexer or (MUX). ... A checksum is a form of redundancy check, a very simple measure for protecting the integrity of data by detecting errors in data that is sent through space (telecommunications) or time (storage). ...

The UDP header consists of only 4 fields. The use of two of those is optional (pink background in table).

Source port 

This field identifies the sending port when meaningful and should be assumed to be the port to reply to if needed. If not used, then it should be zero.

Destination port 

This field identifies the destination port and is required.

Length 

A 16-bit field that specifies the length in bytes of the entire datagram: header and data. The minimum length is 8 bytes since that's the length of the header. The field size sets a theoretical limit of 65,527 bytes for the data carried by a single UDP datagram.

Checksum 

The 16-bit checksum field is used for error-checking of the header and data.

With IPv4

When UDP runs over IPv4, the method used to compute the checksum is defined within RFC 768:

Checksum is the 16-bit one's complement of the one's 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.

In other words, all 16-bit words are summed together using one's complement (with the checksum field set to zero). The sum is then one's complemented. This final value is then inserted as the checksum field. Algorithmically speaking, this is the same as for IPv6.

The difference is in the data used to make the checksum. Included is a pseudo-header that mimics the IPv4 header:

The source and destination addresses are those in the IPv4 header. The protocol is that for UDP (see List of IPv4 protocol numbers): 17. The UDP length field is the length of the UDP header and data.

If the checksum is calculated to be zero (all 0's) it should be sent as negative zero (all 1's). If a checksum is not used it should be sent as zero (all 0's) as zero indicates an unused checksum.

With IPv6

When UDP runs over IPv6, the checksum is no longer considered optional, and the method used to compute the checksum is changed, as per RFC 2460:

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, a pseudo-header that mimics the IPv6 header is included:

The source address is the one in the IPv6 header. The destination address is the final destination; if the IPv6 packet doesn't contain a Routing header, that will be the destination address in the IPv6 header; otherwise, at the originating node, it will be the address in the last element of the Routing header, and, at the receiving node, it will be the destination address in the IPv6 header. The Next Header value is the protocol value for UDP: 17. The UDP length field is the length of the UDP header and data.

If the checksum is calculated to be zero (all 0's) it should be sent as negative zero (all 1's).

Lacking reliability, UDP applications must generally be willing to accept some loss, errors or duplication. Some applications such as TFTP may add rudimentary reliability mechanisms into the application layer as needed. Most often, UDP applications do not require 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. If an application requires a high degree of reliability, a protocol such as the Transmission Control Protocol or erasure codes may be used instead. A checksum is a form of redundancy check, a very simple measure for protecting the integrity of data by detecting errors in data that is sent through space (telecommunications) or time (storage). ... In mathematics, signed numbers in some arbitrary base is done in the usual way, by prefixing it with a - sign. ... Internet Protocol version 6 (IPv6) is a network layer protocol for packet-switched internetworks. ... This is a list of IP protocol numbers that defines the number used in the protocol field of IPv4 packets. ... Trivial File Transfer Protocol (TFTP) is a very simple file transfer protocol, with the functionality of a very basic form of FTP; it was first defined in 1980. ... Streaming media is multimedia that is continuously received by, and normally displayed to, the end-user while it is being delivered by the provider. ... An overview of how VoIP works A typical analog telephone adapter for connecting an ordinary phone to a VoIP network Ciscos implementation of VoIP - IP Phone Voice over Internet Protocol, also called VoIP, IP Telephony, Internet telephony, Broadband telephony, Broadband Phone and Voice over Broadband is the routing of... The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data using Stream Sockets. ... In general, an erasure code transforms a message of n blocks into a message with > n blocks such that the original message can be recovered from a subset of those blocks. ...

Lacking any congestion avoidance and control mechanisms, network-based mechanisms are required to minimize potential congestion collapse effects of uncontrolled, high rate UDP traffic loads. In other words, since UDP senders cannot detect congestion, network-based elements such as routers using packet queuing and dropping techniques will often be the only tool available to slow down excessive UDP traffic. The Datagram Congestion Control Protocol (DCCP) is being designed as a partial solution to this potential problem by adding end host TCP-friendly congestion control behavior to high-rate UDP streams such as streaming media. The Datagram Congestion Control Protocol (DCCP) is a message-oriented transport layer protocol that is currently under development in the IETF. Applications that might make use of DCCP include those with timingconstraints on the delivery of data such that reliable in-order delivery, when combined with congestion control, is likely...

While the total amount of UDP traffic found on a typical network is often in the order of only a few percent, numerous key applications use UDP, including the Domain Name System (DNS), the simple network management protocol (SNMP), the Dynamic Host Configuration Protocol (DHCP) and the Routing Information Protocol (RIP), to name just a few. It has been suggested that this article be split into multiple articles. ... The simple network management protocol (SNMP) forms part of the internet protocol suite as defined by the Internet Engineering Task Force (IETF). ... (DHCP) is a set of rules used by a communications device such as a computer, router or network adapter to allow the device to request and obtain an IP address from a server which has a list of addresses available for assignment. ... This article is chiefly about the Routing Information Protocol for IPv4 and IPv6. ...

Sample code (python language)

The following, minimalistic example shows how to use UDP for client/server communication under Unix: Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®) is a computer operating system originally developed in the 1960s and 1970s by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ...

The server:

 import socket PORT = 10000 BUFFLEN = 512 server = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP) server.bind(('', PORT)) while True: (message, address) = server.recvfrom(BUFFLEN) print 'Received packet from %s:%d' % (address[0], address[1]) print 'Data: %s' % message 

The client (replace "127.0.0.1" by the IP address of the server):

 import socket SERVER_ADDRESS = '127.0.0.1' SERVER_PORT = 10000 client = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP) for i in range(3): print 'Sending packet %d' % i message = 'This is packet %d' % i client.sendto(message, (SERVER_ADDRESS, SERVER_PORT)) client.close() 

Difference between TCP and UDP

TCP is a connection-oriented protocol; a connection can be made from client to server, and from then on any data can be sent along that connection.

• Reliable - TCP manages message acknowledgement, retransmission and timeout. Many attempts to realiably 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 along a connection, one after the other, the first message will reach the receiving application first. When data packets arrive in the wrong order, the TCP layer holds the later data until the earlier data can be rearranged and delivered to the application.
• Heavyweight - TCP requires three packets just to set up a socket, before any actual data can be sent. It handles connections, reliability and congestion control. It is a large transport protocol designed on top of IP.
• Streaming - Data is read as a "stream," with nothing distinguishing where one packet ends and another begins. Packets may be split or merged into bigger or smaller data streams arbitrarily.

UDP is a simpler message-based connectionless protocol. With UDP messages (packets) cross the network in independent units.

• Unreliable - When a message is sent, it can't be known if it will reach its destination; it could get lost along the way. There's no concept of acknowledgement, retransmission and 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. This means it's a lot quicker. It's a small transport layer designed on top of IP.
• Datagrams - Packets are sent individually and are guaranteed to be whole if they arrive. Packets have definite bounds and no split or merge into data streams may exist.

See also

• TCP and UDP port numbers for a partial (growing) listing of ports/services
• Connectionless protocol
• UDP flood attack
• UDP Data Transport
• UDP Lite, a variant that will deliver packets even if they are malformed
• Reliable User Datagram Protocol (RUDP)
• Transmission Control Protocol
• IP or Internet Protocol, on top of which rests UDP
• Transport protocol comparison table

IANA is responsible for assigning TCP and UDP port numbers to specific uses. ... In telecommunications, connectionless describes communication between two network end points in which a message can be sent from one end point to another without prior arrangement. ... UDP is a session-less protocol. ... UDT stands for UDP Data Transport. ... UDP Lite is a connectionless protocol, a variant form of UDP, that will deliver packets even if their checksum is invalid. ... In computer networking, the Reliable User Datagram Protocol (RUDP) is a transport layer protocol designed at Bell Labs for the Plan 9 operating system. ... The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data using Stream Sockets. ... The Internet Protocol (IP) is a data-oriented protocol used for communicating data across a packet-switched internetwork. ... In computing and telecommunications, the transport layer is layer four of the seven layer OSI model. ...

External links

• RFC 768
• IANA Port Assignments
• The Trouble with UDP Scanning (PDF)
• Breakdown of UDP frame
• UDP on MSDN Magazine Sockets and WCF