User Datagram Protocol
Encyclopedia
The User Datagram Protocol (UDP) is one of the core members of the Internet Protocol Suite
, the set of network protocols used for the Internet
. With UDP, computer applications can send messages, in this case referred to as datagram
s, to other hosts on an Internet Protocol
(IP) network without requiring prior communications to set up special transmission channels or data paths. The protocol was designed by David P. Reed
in 1980 and formally defined in RFC 768.
UDP uses a simple transmission model without implicit handshaking
dialogues for providing reliability, ordering, or data integrity. Thus, UDP provides an unreliable service and datagrams may arrive out of order, appear duplicated, or go missing without notice. UDP assumes that error checking and correction is either not necessary or performed in the application, avoiding the overhead of such processing at the network interface level. Time-sensitive applications often use UDP because dropping packets is preferable to waiting for delayed packets, which may not be an option in a real-time system. If error correction facilities are needed at the network interface level, an application may use the Transmission Control Protocol
(TCP) or Stream Control Transmission Protocol
(SCTP) which are designed for this purpose.
UDP's stateless
nature is also useful for servers answering small queries from huge numbers of clients. Unlike TCP
, UDP supports packet broadcast
(sending to all on local network) and multicast
ing (send to all subscribers).
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), IP tunneling protocols and many online game
s.
s to establish host-to-host communications. An application binds a socket to its endpoint of data transmission, which is a combination of an IP address
and a service port. A port is a software structure that is identified by the port number, a 16 bit
integer value, allowing for port numbers between 0 and . Port 0 is reserved, but is a permissible source port value if the sending process does not expect messages in response.
The Internet Assigned Numbers Authority
has divided port numbers into three ranges. Port numbers 0 through 1023 are used for common, well-known services. On Unix
-like operating system
s, using one of these ports requires superuser
operating permission. Port numbers 1024 through are the registered port
s used for IANA-registered services. Ports through are dynamic ports that are not officially designated for any specific service, and can be used for any purpose. They are also used as ephemeral port
s, from which software running on the host may randomly choose a port in order to define itself. In effect, they are used as temporary ports primarily by clients
when communicating with servers
.
protocol that is documented in IETF RFC 768.
UDP provides no guarantees to the upper layer protocol
for message delivery and the UDP protocol layer retains no state of UDP messages once sent. For this reason, UDP is sometimes referred to as Unreliable
Datagram Protocol.
UDP provides application multiplexing
(via port numbers) and integrity verification (via checksum
) of the header and payload. If transmission reliability is desired, it must be implemented in the user's application.
The UDP header consists of 4 fields, each of which is 2 bytes (16 bits). The use of two of those is optional in IPv4 (pink background in table). In IPv6 only the source port is optional (see below).
Source port number : This field identifies the sender's port when meaningful and should be assumed to be the port to reply to if needed. If not used, then it should be zero. If the source host is the client, the port number is likely to be an ephemeral port number. If the source host is the server, the port number is likely to be a well-known port number.
Destination port number : 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.
Length : A 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 65,535 bytes (8 byte header + 65,527 bytes of data) for a UDP datagram. The practical limit for the data length which is imposed by the underlying IPv4
protocol is 65,507 bytes (65,535 − 8 byte UDP header − 20 byte IP header).
Checksum : The checksum
field is used for error-checking of the header and data. If no checksum is generated by the transmitter, the field uses the value all-zeros. This field is not optional for IPv6.
In other words, all 16-bit words are summed using one's complement arithmetic. The sum is then one's 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 one's complement (all 1s).
The difference between IPv4
and IPv6
is in the data used to compute the checksum.
The source and destination addresses are those in the IPv4 header. The protocol is that for UDP (see List of IP protocol numbers): 17 (0x11). The UDP length field is the length of the UDP header and data.
UDP checksum computation is optional for IPv4. If a checksum is not used it should be set to the value zero.
When computing the checksum, again a PSEUDO-HEADER is used that mimics the real IPv6 header:
The source address is the one in the IPv6 header. The destination address is the final destination; if the IPv6 packet does not 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 value of the Next Header field is the protocol value for UDP: 17. The UDP length field is the length of the UDP header and data.
may add rudimentary reliability mechanisms into the application layer as needed.
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. If an application requires a high degree of reliability, a protocol such as the Transmission Control Protocol
or erasure code
s may be used instead.
Potentially more seriously, unlike TCP, UDP-based applications don't necessarily have good congestion
avoidance and control mechanisms. Congestion insensitive UDP applications that consume a large fraction of available bandwidth could endanger the stability of the internet, as they frequently give a bandwidth load that is inelastic. Network-based mechanisms have been proposed to minimize potential congestion collapse effects of uncontrolled, high rate UDP traffic loads. Network-based elements such as routers using packet queuing and dropping techniques are often 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.
(DNS), where queries must be fast and only consist of a single request followed by a single reply packet, the Simple Network Management Protocol
(SNMP), the Routing Information Protocol
(RIP) and the Dynamic Host Configuration Protocol
(DHCP).
Voice and video traffic is generally transmitted using UDP. Real-time video and audio streaming protocols are designed to handle occasional lost packets, so only slight degradation in quality occurs, rather than large delays if lost packets were retransmitted. Because both TCP and UDP run over the same network, many businesses are finding that a recent increase in UDP traffic from these real-time applications is hindering the performance of applications using TCP, such as point of sale
, accounting
, and database
systems. When TCP detects packet loss, it will throttle back its data rate usage. Since both real-time and business applications are important to businesses, developing quality of service
solutions is seen as crucial by some.
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.
UDP 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. However, one primary benefit of UDP over TCP is the application to voice over internet protocol (VoIP) where any handshaking would hinder clear voice communication. It is assumed in VoIP UDP that the end users provide any necessary real time confirmation that the message has been received.
Internet protocol suite
The Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...
, the set of network protocols used for the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
. With UDP, computer applications can send messages, in this case referred to as datagram
Datagram
A datagram is a basic transfer unit associated with a packet-switched network in which the delivery, arrival time, and order are not guaranteed....
s, to other hosts on an Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP) network without requiring prior communications to set up special transmission channels or data paths. The protocol was designed by David P. Reed
David P. Reed
David P. Reed is an American computer scientist, educated at the Massachusetts Institute of Technology, known for a number of significant contributions to computer networking....
in 1980 and formally defined in RFC 768.
UDP uses a simple transmission model without implicit handshaking
Handshaking
In information technology, telecommunications, and related fields, handshaking is an automated process of negotiation that dynamically sets parameters of a communications channel established between two entities before normal communication over the channel begins...
dialogues for providing reliability, ordering, or data integrity. Thus, UDP provides an unreliable service and datagrams may arrive out of order, appear duplicated, or go missing without notice. UDP assumes that error checking and correction is either not necessary or performed in the application, avoiding the overhead of such processing at the network interface level. Time-sensitive applications often use UDP because dropping packets is preferable to waiting for delayed packets, which may not be an option in a real-time system. If error correction facilities are needed at the network interface level, an application may use the Transmission Control Protocol
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
(TCP) or Stream Control Transmission Protocol
Stream Control Transmission Protocol
In computer networking, the Stream Control Transmission Protocol is a Transport Layer protocol, serving in a similar role to the popular protocols Transmission Control Protocol and User Datagram Protocol...
(SCTP) which are designed for this purpose.
UDP's stateless
Stateless server
In computing, a stateless protocol is a communications protocol that treats each request as an independent transaction that is unrelated to any previous request so that the communication consists of independent pairs of requests and responses...
nature is also useful for servers answering small queries from huge numbers of clients. Unlike TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
, UDP supports packet broadcast
Broadcasting (networks)
right|250pxIn telecommunication and information theory, broadcasting refers to a method of transferring a message to all recipients simultaneously...
(sending to all on local network) and multicast
Multicast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
ing (send to all subscribers).
Common network applications that use UDP include: the Domain Name System
Domain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
(DNS), streaming media
Streaming media
Streaming media is multimedia that is constantly received by and presented to an end-user while being delivered by a streaming provider.The term "presented" is used in this article in a general sense that includes audio or video playback. The name refers to the delivery method of the medium rather...
applications such as IPTV
IPTV
Internet Protocol television is a system through which television services are delivered using the Internet protocol suite over a packet-switched network such as the Internet, instead of being delivered through traditional terrestrial, satellite signal, and cable television formats.IPTV services...
, Voice over IP
Voice over IP
Voice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...
(VoIP), Trivial File Transfer Protocol
Trivial File Transfer Protocol
Trivial File Transfer Protocol is a file transfer protocol known for its simplicity. It is generally used forautomated transfer of configuration or boot files between machines in a local environment....
(TFTP), IP tunneling protocols and many online game
Online game
An online game is a game played over some form of computer network. This almost always means the Internet or equivalent technology, but games have always used whatever technology was current: modems before the Internet, and hard wired terminals before modems...
s.
Service ports
UDP applications use datagram socketDatagram socket
A datagram socket is a type of connectionless Internet socket, which is the sending or receiving point for packet delivery services. Each packet sent or received on a datagram socket is individually addressed and routed...
s to establish host-to-host communications. An application binds a socket to its endpoint of data transmission, which is a combination of an IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
and a service port. A port is a software structure that is identified by the port number, a 16 bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
integer value, allowing for port numbers between 0 and . Port 0 is reserved, but is a permissible source port value if the sending process does not expect messages in response.
The Internet Assigned Numbers Authority
Internet Assigned Numbers Authority
The Internet Assigned Numbers Authority is the entity that oversees global IP address allocation, autonomous system number allocation, root zone management in the Domain Name System , media types, and other Internet Protocol-related symbols and numbers...
has divided port numbers into three ranges. Port numbers 0 through 1023 are used for common, well-known services. On Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
-like operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s, using one of these ports requires superuser
Superuser
On many computer operating systems, the superuser is a special user account used for system administration. Depending on the operating system, the actual name of this account might be: root, administrator or supervisor....
operating permission. Port numbers 1024 through are the registered port
Registered port
A registered port is a network port assigned by the Internet Assigned Numbers Authority for use with a certain protocol or application.Ports with numbers lower than those of the registered ports are called well known...
s used for IANA-registered services. Ports through are dynamic ports that are not officially designated for any specific service, and can be used for any purpose. They are also used as ephemeral port
Ephemeral port
An ephemeral port is a short-lived transport protocol port for Internet Protocol communications allocated automatically from a predefined range by the TCP/IP software...
s, from which software running on the host may randomly choose a port in order to define itself. In effect, they are used as temporary ports primarily by clients
Client (computing)
A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....
when communicating with servers
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...
.
Packet structure
UDP is a minimal message-oriented Transport LayerTransport layer
In computer networking, the transport layer or layer 4 provides end-to-end communication services for applications within a layered architecture of network components and protocols...
protocol that is documented in IETF RFC 768.
UDP provides no guarantees to the upper layer protocol
Upper layer protocol
In computer networking, the term upper layer protocol refers to a more abstract protocol when performing encapsulation, in particular it is often used to describe the protocols above the network layer....
for message delivery and the UDP protocol layer retains no state of UDP messages once sent. For this reason, UDP is sometimes referred to as Unreliable
Reliability (computer networking)
In computer networking, a reliable protocol is one that provides reliability properties with respect to the delivery of data to the intended recipient, as opposed to an unreliable protocol, which does not provide notifications to the sender as to the delivery of transmitted data.A reliable...
Datagram Protocol.
UDP provides application multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
(via port numbers) and integrity verification (via checksum
Checksum
A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and...
) of the header and payload. If transmission reliability is desired, it must be implemented in the user's application.
| offset (bits) | 0 – 15 | 16 – 31 | ||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | Source Port Number | Destination Port Number | ||||||||||||||||||||||||||||||
| 32 | Length | Checksum | ||||||||||||||||||||||||||||||
| 64+ | Data |
|||||||||||||||||||||||||||||||
The UDP header consists of 4 fields, each of which is 2 bytes (16 bits). The use of two of those is optional in IPv4 (pink background in table). In IPv6 only the source port is optional (see below).
Source port number : This field identifies the sender's port when meaningful and should be assumed to be the port to reply to if needed. If not used, then it should be zero. If the source host is the client, the port number is likely to be an ephemeral port number. If the source host is the server, the port number is likely to be a well-known port number.
Destination port number : 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.
Length : A 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 65,535 bytes (8 byte header + 65,527 bytes of data) for a UDP datagram. The practical limit for the data length which is imposed by the underlying IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
protocol is 65,507 bytes (65,535 − 8 byte UDP header − 20 byte IP header).
Checksum : The checksum
Checksum
A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and...
field is used for error-checking of the header and data. If no checksum is generated by the transmitter, the field uses the value all-zeros. This field is not optional for IPv6.
Checksum computation
The method used to compute the checksum is defined in RFC 768:- Checksum is the 16-bit one's complementOne's complementThe ones' complement of a binary number is defined as the value obtained by inverting all the bits in the binary representation of the number . The ones' complement of the number then behaves like the negative of the original number in most arithmetic operations...
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 using one's complement arithmetic. The sum is then one's 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 one's complement (all 1s).
The difference between IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
and IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
is in the data used to compute the checksum.
IPv4 PSEUDO-HEADER
When UDP runs over IPv4, the checksum is computed using a PSEUDO-HEADER that contains some of the same information from the real IPv4 header. The PSEUDO-HEADER is not the real IPv4 header used to send an IP packet. The following table defines the PSEUDO-HEADER used only for the checksum calculation.| bits | 0 – 7 | 8 – 15 | 16 – 23 | 24 – 31 | ||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | Source address | |||||||||||||||||||||||||||||||
| 32 | Destination address | |||||||||||||||||||||||||||||||
| 64 | Zeros | Protocol | UDP length | |||||||||||||||||||||||||||||
| 96 | Source Port | Destination Port | ||||||||||||||||||||||||||||||
| 128 | Length | Checksum | ||||||||||||||||||||||||||||||
| 160+ | Data |
|||||||||||||||||||||||||||||||
The source and destination addresses are those in the IPv4 header. The protocol is that for UDP (see List of IP protocol numbers): 17 (0x11). The UDP length field is the length of the UDP header and data.
UDP checksum computation is optional for IPv4. If a checksum is not used it should be set to the value zero.
IPv6 PSEUDO-HEADER
When UDP runs over IPv6, the checksum is mandatory. The method used to compute it is changed as documented in 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.
When computing the checksum, again a PSEUDO-HEADER is used that mimics the real IPv6 header:
| bits | 0 – 7 | 8 – 15 | 16 – 23 | 24 – 31 | ||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | Source address | |||||||||||||||||||||||||||||||
| 32 | ||||||||||||||||||||||||||||||||
| 64 | ||||||||||||||||||||||||||||||||
| 96 | ||||||||||||||||||||||||||||||||
| 128 | Destination address | |||||||||||||||||||||||||||||||
| 160 | ||||||||||||||||||||||||||||||||
| 192 | ||||||||||||||||||||||||||||||||
| 224 | ||||||||||||||||||||||||||||||||
| 256 | UDP length | |||||||||||||||||||||||||||||||
| 288 | Zeros | Next Header | ||||||||||||||||||||||||||||||
| 320 | Source Port | Destination Port | ||||||||||||||||||||||||||||||
| 352 | Length | Checksum | ||||||||||||||||||||||||||||||
| 384+ | Data |
|||||||||||||||||||||||||||||||
The source address is the one in the IPv6 header. The destination address is the final destination; if the IPv6 packet does not 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 value of the Next Header field is the protocol value for UDP: 17. The UDP length field is the length of the UDP header and data.
Reliability and congestion control solutions
Lacking reliability, UDP applications must generally be willing to accept some loss, errors or duplication. Some applications such as TFTPTrivial File Transfer Protocol
Trivial File Transfer Protocol is a file transfer protocol known for its simplicity. It is generally used forautomated transfer of configuration or boot files between machines in a local environment....
may add rudimentary reliability mechanisms into the application layer as needed.
Most often, UDP applications do not employ reliability mechanisms and may even be hindered by them. Streaming media
Streaming media
Streaming media is multimedia that is constantly received by and presented to an end-user while being delivered by a streaming provider.The term "presented" is used in this article in a general sense that includes audio or video playback. The name refers to the delivery method of the medium rather...
, real-time multiplayer games and voice over IP
Voice over IP
Voice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...
(VoIP) are examples of applications that often use UDP. In these particular applications, loss of packets is not usually a fatal problem. If an application requires a high degree of reliability, a protocol such as the Transmission Control Protocol
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
or erasure code
Erasure code
In information theory, an erasure code is a forward error correction code for the binary erasure channel, which transforms a message of k symbols into a longer message with n symbols such that the original message can be recovered from a subset of the n symbols...
s may be used instead.
Potentially more seriously, unlike TCP, UDP-based applications don't necessarily have good congestion
Network congestion
In data networking and queueing theory, network congestion occurs when a link or node is carrying so much data that its quality of service deteriorates. Typical effects include queueing delay, packet loss or the blocking of new connections...
avoidance and control mechanisms. Congestion insensitive UDP applications that consume a large fraction of available bandwidth could endanger the stability of the internet, as they frequently give a bandwidth load that is inelastic. Network-based mechanisms have been proposed to minimize potential congestion collapse effects of uncontrolled, high rate UDP traffic loads. Network-based elements such as routers using packet queuing and dropping techniques are often the only tool available to slow down excessive UDP traffic. The Datagram Congestion Control Protocol
Datagram Congestion Control Protocol
The Datagram Congestion Control Protocol is a message-oriented Transport Layer protocol. DCCP implements reliable connection setup, teardown, Explicit Congestion Notification , congestion control, and feature negotiation. DCCP was published as RFC 4340, a proposed standard, by the IETF in March,...
(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.
Applications
Numerous key Internet applications use UDP, including: the Domain Name SystemDomain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
(DNS), where queries must be fast and only consist of a single request followed by a single reply packet, the Simple Network Management Protocol
Simple Network Management Protocol
Simple Network Management Protocol is an "Internet-standard protocol for managing devices on IP networks. Devices that typically support SNMP include routers, switches, servers, workstations, printers, modem racks, and more." It is used mostly in network management systems to monitor...
(SNMP), the Routing Information Protocol
Routing Information Protocol
The Routing Information Protocol is a distance-vector routing protocol, which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops allowed for RIP is 15....
(RIP) and the Dynamic Host Configuration Protocol
Dynamic Host Configuration Protocol
The Dynamic Host Configuration Protocol is a network configuration protocol for hosts on Internet Protocol networks. Computers that are connected to IP networks must be configured before they can communicate with other hosts. The most essential information needed is an IP address, and a default...
(DHCP).
Voice and video traffic is generally transmitted using UDP. Real-time video and audio streaming protocols are designed to handle occasional lost packets, so only slight degradation in quality occurs, rather than large delays if lost packets were retransmitted. Because both TCP and UDP run over the same network, many businesses are finding that a recent increase in UDP traffic from these real-time applications is hindering the performance of applications using TCP, such as point of sale
Point of sale
Point of sale or checkout is the location where a transaction occurs...
, accounting
Accounting software
Accounting software is application software that records and processes accounting transactions within functional modules such as accounts payable, accounts receivable, payroll, and trial balance. It functions as an accounting information system...
, and database
Database management system
A database management system is a software package with computer programs that control the creation, maintenance, and use of a database. It allows organizations to conveniently develop databases for various applications by database administrators and other specialists. A database is an integrated...
systems. When TCP detects packet loss, it will throttle back its data rate usage. Since both real-time and business applications are important to businesses, developing quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
solutions is seen as crucial by some.
Comparison of UDP and TCP
Transmission Control ProtocolTransmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
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 acknowledgment, 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 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 byteByteThe byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
stream, no distinguishing indications are transmitted to signal message (segment) boundaries.
UDP is a simpler message-based connectionless protocol
Connectionless protocol
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. The device at one end of the communication transmits data addressed to the other, without first ensuring that the...
. 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. However, one primary benefit of UDP over TCP is the application to voice over internet protocol (VoIP) where any handshaking would hinder clear voice communication. It is assumed in VoIP UDP that the end users provide any necessary real time confirmation that the message has been received.
- Unreliable – When a 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 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, and it's possible for high bandwidth applications to trigger congestion collapse, unless they implement congestion control measures at the application level.
See also
- List of TCP and UDP port numbers
- Reliable User Datagram ProtocolReliable User Datagram ProtocolIn computer networking, the Reliable User Datagram Protocol is a transport layer protocol designed at Bell Labs for the Plan 9 operating system...
(RUDP) - Transport protocol comparison table
- UDP flood attackUDP flood attackA UDP flood attack is a denial-of-service attack using the User Datagram Protocol , a sessionless/connectionless computer networking protocol....
- UDP Data TransportUDP Data TransportUDT is a protocol developed by the National Center for Data Mining at University of Illinois at Chicago. The protocol was designed to sit on top of User Datagram Protocol to facilitate large data transfers over modern high-speed fiber optic computer networks...
- UDP LiteUDP LiteUDP-Lite is a connectionless protocol, very similar to UDP. Unlike UDP, where either all or none of a packet is protected by a checksum, UDP-Lite allows for partial checksums that only cover part of a datagram , and will therefore deliver packets that have been partially corrupted...
, a variant that will deliver packets even if they are malformed - UDP Helper AddressUDP Helper AddressA UDP Helper Address is a special router configuration used to forward broadcast network traffic from a client machine on one subnet to a server in another subnet. This is used primarily in business environments as way to save money by not having to buy a server for each subnet.-Usage Example:In...
RFC references
- RFC 768 – User Datagram Protocol
- RFC 2460 – Internet Protocol, Version 6 (IPv6) Specification
- RFC 4113 – Management Information Base for the UDP
- RFC 5405 – Unicast UDP Usage Guidelines for Application Designers