Value,Description,Reference
0,Reserved,[RFC3331]
1,Registration Request (REG REQ),[RFC3331]
2,Registration Response (REG RSP),[RFC3331]
3,Deregistration Request (DEREG REQ),[RFC3331]
4,Deregistration Response (DEREG RSP),[RFC3331]
5-127,Unassigned,
128-255,Reserved for IETF-Defined IIM extensions,