Are there any protocol specifications that allow either TCP or UDP to be utilized?

Question

Are there any networking protocols that are not strictly TCP or UDP but can be used with either one?

For example, HTTP, FTP, STMP, RTMP are always TCP.

DNS, SNMP, DHCP, RIP are always UDP.

Is there anything that can be either TCP or UDP? Or am I wrong in the above assertions?

RTSP is one weird one I know of that uses both, TCP for the control port but UDP for audio/video/quality, but it has strict requirements of what gets sent of each.

I'm asking about standard, published, or at least commonly used protocols, not custom ones.

Answer 1

DNS can use either UDP or TCP; TCP is required when the response data exceeds 512 bytes.

Answer 2

If you examine a Windows' services file you will see a number of protocols registered for both TCP and UDP. Path: C:\Windows\System32\drivers\etc In fact, most of the listings in the services file use both TCP and UDP protocols.

As far as well known apps that use both, I would think that most chat applications use both. sms-chat definitely does but probably most others.

Edit:

From that file, here's a few of the protocols that can be sent over either TCP or UDP (there are exactly 100 listed protocols that use both in the file, many internal MS protocols):

• echo
• discard
• daytime
• qotd (Quote of the day)
• chargen (Character generator)
• time