Thursday, October 14, 2010

User Datagram Protocol (UDP)

The User Datagram Protocol (UDP) is one of the core members of the Internet Protocol Suite and used at the Transport layer.

UDP uses a simple transmission model without implicit hand-shaking 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. When UDP sends segments over a network, it just sends them and forgets about them. It doesn't follow through, check on them, or even allow for an acknowledgment of safe arrival, This does not mean that UDP is ineffective.

UDP's stateless nature is also useful for servers answering small queries from huge numbers of clients. Unlike TCP, UDP is compatible with packet broadcast (sending to all on local network) and multicasting (send to all subscribers).

The only services it provides over IP are checksumming of data and multiplexing by port number. Therefore, an application program running over UDP must deal directly with end-to-end communication problems that a connection-oriented protocol would have handled -- e.g., retransmission for reliable delivery, packetization, reassembly, flow control, congestion avoidance, etc., when these are required.

UDP is a minimal message-oriented Transport Layer protocol. 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.

bits0 – 1516 – 31
0Source Port NumberDestination Port Number

The UDP header consists of 4 fields, all of which are 2 bytes (16 bits). The use of two of those is optional in IPv4 (pink background in table)

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.

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).

The checksum field is used for error-checking of the header and data. If the checksum is omitted in IPv4.

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