Why do http ftp smtp pop3 imap run on top of Tcp rather than on udp?

tcp is more reliable than udp,udp may have failures or data loss, so we can't afford to have losses in http,smtp,pop3 and so on.Accurate data is very important is all this protocols

ImprovementThe above answer being true, it is worth mentioning that none of the Protocols in your question actually require TCP as its Transport protocol. They simply require the use of a reliable protocol at the Transport Layer (refer ISO::OSI network model) and TCP is the best there is right now.

And also, since these protocols were developed after the reign of TCP as the primary transport protocol started, integrating them with TCP is relatively easier. If used with other protocols, it has to be explicitly configured into the App protocols.

Regards

Binaek Sarkar

Foundation

http://www.foundation.net.in/