1)Describe the difference between Stateless and Stateful protocols, and
which type of protocol is susceptible to which types of PDU faults.
Ans 
Stateless Protocols:When a server does not maintain any state of a
transaction, we term it stateless.
Subject to : syntactically faulty PDUs
Stateful Protocols: In these protocols, the server maintains state about the
client until the entire transaction is over,is aborted, or times out.
Subject to : syntactically and semantically faulty PDUs

2) Paragraph #3, page 2, define robustness as "... continue normal protocol
operation", is it a valid interpretation of IEEE's definition of robustness
"function correctly in the presence of invalid input..." ? I agree that
crashing the server process is not robust. But terminating a connection and
exiting gracefully if the client misbehave is arguably safer because that is
very likely an attempt to attack the server as any ordinary client must
conform to the protocol.
Ans It is important that the server can notify the client of the
inappropriate action in cases when the client is not an intruder and can
also recover from such setbacks rather than terminating the connection which
can test the robustness of the server under such circumstances as reflected
in the definition.

3) Why are stateless protocols susceptible to syntax faults,  and stateful
protocols susceptible to semantic faults?
Ans If a protocol is stateless, it is susceptible to syntactically faulty
PDUs. An incorrect sequence of syntactically correct PDUs cannot cause
problems for a stateless protocol since it maintains no history of PDUs.
However,if a protocol is stateful(which maintains state), it is susceptible
to syntactically and semantically faulty PDUs.