When UDP is used for NTP service and when TCP is used for NTP service?

I check from /etc/services and I find that there are 2 lines for ntp services:
ntp             123/tcp
ntp             123/udp

I got an answer from EE expert that I cannot telnet to NTP server using port 123 as it is UDP. I wonder when TCP is used for NTP service.
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tballahAsked:
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Pétur Ingi EgilssonConnect With a Mentor Software Engineer -- ConsultantCommented:
use `netstat -na|grep LISTEN` to see if anything is listening on the 123/TCP

As i stated.. 123/TCP is only listed in /etc/services as a reservation...
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Pétur Ingi EgilssonSoftware Engineer -- ConsultantCommented:
ntp/tcp is listed as a reserved port for ntp.
Does not mean anything is listening on it.

It would confuse people if you would have different reserved ports for both the TCP and UDP protocols.
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Pétur Ingi EgilssonSoftware Engineer -- ConsultantCommented:
example:
port 80 TCP is reserved for HTTPd
It would be confusing if you had port 80 TCP for HTTPd and port 80 UDP running something completely diffrent.
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tballahAuthor Commented:
I know a bit from DNS service that it uses port 53/UDP for query and 53/TCP for zone transfer between primary and secondary DNS server. I was thinking the same way for NTP service as defined in /etc/services. By the way who could I know the nature of the service whether it is pure UDP or TCP?
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