Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 334
Handling Errors

RSVP messages are sent unreliably because the program uses IP directly, similar to IGMP or ICMP. The error messages that RSVP uses are PathErr and ResvErr. This is a simplex process in that an error message is sent upstream to the sender that created the error.