This class contains messages that are used in IppOperation and related classes.
Most of messages can be found in RFC 2911/2910/3380
http://ietf.org/rfc/rfc2911.txt?number=2911
http://ietf.org/rfc/rfc2910.txt?number=2910
http://ietf.org/rfc/rfc3380.txt?number=3380