Does HTTP use UDP?

Question

From RFC 2616:

HTTP communication usually takes place over TCP/IP connections. The default port is TCP 80, but other ports can be used. This does not preclude HTTP from being implemented on top of any other protocol on the Internet, or on other networks. HTTP only presumes a reliable transport; any protocol that provides such guarantees can be used; the mapping of the HTTP/1.1 request and response structures onto the transport data units of the protocol in question is outside the scope of this specification.

So although it doesn't explicitly say so, UDP is not used because it is not a "reliable transport".

EDIT - more recently, the QUIC protocol (which is more strictly a pseudo-transport or a session layer protocol) does use UDP for carrying HTTP/2.0 traffic and much of Google's traffic already uses this protocol. It's currently progressing towards standardisation as HTTP/3.

Typically, no.

Streaming is seldom used over HTTP itself, and HTTP is seldom run over UDP. See, however, RTP.

For something as your example (in the comment), you're not showing a protocol for the resource. If that protocol were to be HTTP, then I wouldn't call the access "streaming"; even if it in some sense of the word is since it's sending a (possibly large) resource serially over a network. Typically, the resource will be saved to local disk before being played back, so the network transfer is not what's usually meant by "streaming".

As commenters have pointed out, though, it's certainly possible to really stream over HTTP, and that's done by some.

Maybe just a bit of trivia, but UPnP will use HTTP formatted messages over UDP for device discovery.

Yes, HTTP, as an application protocol, can be transferred over UDP transport protocol. Here are some of the services that use UDP and an underlying protocol for transferring HTTP data and streaming it to the end-user:

• XMPP's Jingle Raw UDP Transport Method
• A number for services that use UDT --- UDP-based Data Transfer Protocol, which is the a superset of UDP protocol.
• The Transport Layer Security (TLS) protocol encapsulating HTTP as well as the above mentioned XMPP and other application protocols does have an implementation that uses UDP in its transport layer; this implementation is called Datagram Transport Layer Security (DTLS).
• Push notifications in GNUTella are HTTP requests sent over UDP transport.

This article contains further details on streaming over UDP and its reliable superset, the RUDP: Reliable UDP (RUDP): The Next Big Streaming Protocol?