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Topic Name is While it is true that the DDS API specification does not mention a limit for Topic names. The complementary DDS Wire Protocol specification: Real-Time Publish Subscribe (RTPS) protocol (http://www.omg.org/spec/DDSI-RTPS/2.2), does state that Topic names shall not exceed 256 characters (see Table 9.12).
So the 256 character limit on Topic name lengths imposed by the RTI DDS implementation is not arbitrary. It is precisely what is required to be interoperable with other DDS implementations.
Source: Topic Name