See also: UDP

The User Datagram Protocol (UDP, in French protocol of datagram user ) is one of principal the protocol S of telecommunication used by Internet. It belongs to the layer transport of the pile of protocol TCP/IP: in the approximate adaptation of the latter to OSI model, it would belong to layer 4, like TCP. It is detailed in the RFC 768.

The role of this protocol is to allow the transmission of Paquet S in manner very simple between two entities, each one being defined by a Adresse IP and a number of port (to differentiate various users on the same machine). Contrary to the protocol TCP, he works in not-connected mode: it is not possible to check if all the packages sent arrived well at destination and nor in which order (the sequencing can however be assured by a protocol network sub-base). It is not envisaged any Contrôle of flow nor Contrôle of congestion. Therefore it is often described as being a not-reliable protocol. On the other hand, for a given package UDP, the exactitude of the contents of the data is ensured thanks to a sum of control ( checksum ).

Structure of a segment UDP

Package UDP is encapsulated in a datagram IP. It comprises a heading followed by the data themselves to transport.

The heading ( header in English) of a segment UDP is much simpler than that of TCP:

It contains the 4 following fields:

; Port Source: it indicates since which port the package was sent.

; Wearing of Destination: it indicates to which port the package must be sent

; Length: it indicates the overall length of segment UDP (heading and data). The minimal length is thus of 8 bytes (size of the heading)

; Summon of control: this one (CRC, Cyclic Redundancy Check) makes it possible to make sure of the integrity of the received package. It is calculated on the whole of heading UDP and the data, but also on a pseudo heading (extracted heading IP)
Note: the presence of this pseudo heading, interaction between two layers IP and UDP, is one of the reasons which make that model TCP/IP does not apply perfectly to OSI model. Required in ipV6 but optional in ipV4 (screen contains already a checksum)

Use

It is used when it is necessary is to transmit data very quickly, and where the loss of part of these data does not have great importance, that is to say to transmit minor amounts of data, where connection “3-WAY” TCP would be too heavy. For example, in the case of the transmission of the Voice on IP, it is not the serious if one or the other package is lost (there exist mechanisms of substitution of the missing data), on the other hand speed of transmission is a paramount criterion for the quality of listening.

Examples of use:

• the program Traceroute,
• the protocols DNS, TFTP,
• the network games (example: subjective plays of shooting)
• the Streaming

External bonds

• RFC 768 - User Datagram Protocol
• French Translation of the RFC 768
• IANA Assignments Port - list of the preset ports and their uses, by IANA