Question

a) What other protocols are just fine with UDP, and why?

b) There are some applications protocols (e.g., HTTP, SMTP) that use TCP as their transport layer protocol and cannot use UDP instead. Why?

Answer 1

Qa) What other protocols are just fine with UDP, and why?
Answer:------
The User Datagram Protocol, or UDP for short, is a protocol that allows datagrams to be sent without connection in IP-based networks. To achieve the desired services on the target hosts, it uses ports that are listed as one of the core components in the UDP header. Like many other network protocols, UDP belongs to the internet protocol family, where it is classified as a mediator between the network layer and the application layer at the transport level.
UDP is a direct alternative to the more common TCP, although the two protocols differ by one point: While transmission via TCP takes place only after the obligatory three-way handshake (mutual authentication between sender and addressee including connection setup), UDP forgoes this procedure in order to keep the transmission time as short as possible.

Qb) There are some applications protocols (e.g., HTTP, SMTP) that use TCP as their transport layer protocol and cannot use UDP instead. Why?
Answer:------
TCP uses a three-way handshake before it starts to transfer data. UDP just blasts away without any formal preliminaries. Thus UDP does not introduce any delay to establish a connection. This is probably the principle reason why DNS runs over UDP rather than TCP -- DNS would be much slower if it ran over TCP. HTTP uses TCP rather than UDP, since reliability is critical for Web pages with text. The TCP connection establishment delay in HTTP is an important contributor to the "world wide wait".
TCP maintains connection state in the end systems. This connection state includes receive and send buffers, congestion control parameters, and sequence and acknowledgment number parameters. We will see in Section 3.5 that this state information is needed to implement TCP's reliable data transfer service and to provide congestion control. UDP, on the other hand, does not maintain connection state and does not track any of these parameters. For this reason, a server devoted to a particular application can typically support many more active clients when the application runs over UDP rather than TCP.