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Does HTTPS use TCP or UDP?

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Why is this a -? I wish all questions were this clear and consise – Mark Henderson Jan 3 '10 at 9:53
up vote 21 down vote accepted

HTTPS can run over any reliable stream transport protocol. Normally that's TCP, but it could also be SCTP. It is NOT expected to run over UDP, which is an unreliable datagram protocol (in fact, while that's not its official name, that's a good way to remember what it is).

The IANA assignment for UDP is historical; at the time, nearly every protocol was assigned both the TCP and UDP port numbers, even if it was expected that it would only ever use one. There has been discussion of merging the port number registries, and only ever assigning one port to one protocol from here on. That is to make it easier to deploy future transport protocols that would otherwise need their own registries. I'm not aware of how that discussion concluded.

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Most SSL implementations expose it as SSL socket, so it implies TCP. There are very rare occurrences of using any other transport. – Nasko May 14 '10 at 18:10

It uses TCP. It would be difficult to run it on UDP without the guarantee of packets arriving. If the packets don't arrive, the encrypted data will not be decipherable.

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Or if the packets arrive out of order, as UDP contains no provisions for reordering packets like TCP. – janneb Jan 3 '10 at 9:42
Not going to -1 you because it'd be a little pedantic, but its worth noting that there's no reason it couldn't be run over UDP. It may be atypical, but it is possible. – ThatGraemeGuy Jan 3 '10 at 12:01

For the next time, if you wonder if a default port service run on tcp or udp, you can look at /etc/services on a linux machine.

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Or a Windows machine, in a command prompt: find "https" %windir%\system32\drivers\etc\services – ThatGraemeGuy Jan 3 '10 at 11:57
My /etc/services also lists 80/udp. Actually, many services have allocated both ports, "just in case" I guess. – Peter Eisentraut Jan 9 '10 at 23:03

The OSI model allows higher layer protocols to run over any underlying protocol providing the correct services. HTTPS is HTTP using SSL/TLS security. SSL/TLS typically runs on top of TCP, but there is nothing to stop you from running it on UDP, SCTP or any other transport layer protocol.

As a matter of fact HTTPS over TCP and UDP are both defined as "well known" by IANA and have reserved port numbers.

See for the "official" ports/protocol combinations.

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See for example (Datagram Transport Layer Security) for how SSL/TLS can run over UDP. – pehrs Jan 3 '10 at 11:35

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