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User Datagram Protocol

User Datagram Protocol (UDP) provides a connectionless, unreliable transport service. It is often used for one-to-many communications that use broadcast or multicast IP datagrams. As delivery of UDP datagrams is not guaranteed, applications using UDP must compensate for dropped UDP datagrams through simple retransmission or other reliable mechanisms. Microsoft networking uses UDP for logging on, browsing, and NetBIOS name resolution. UDP can also used to carry IP multicast streams for applications such as Microsoft® NetShow™.

UDP and Name Resolution

UDP is used for NetBIOS name resolution via unicast to a NetBIOS name server or via subnet broadcasts, and for Domain Name System (DNS) host name to IP address resolution. NetBIOS name resolution is accomplished over UDP port 137. DNS queries use UDP port 53. Because UDP itself does not guarantee delivery of datagrams, both of these services use their own retransmission schemes if they receive no answer to queries. Broadcast UDP datagrams are not usually forwarded over IP routers, so NetBIOS name resolution in a routed environment requires a name server such as a Windows Internet Name Service (WINS) server, or the use of a static database file such as the Lmhosts file.

Mailslots Over UDP

Many NetBIOS applications use mailslot messaging. A second-class mailslot is a simple mechanism for sending a message from one NetBIOS name to another over UDP. Mailslot messages can be broadcast on a subnet or directed to a specific host.

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