TCP Port 593 on AD DC's

I did port scan of my Windows Server 2003 Standard Edition servers and found
that TPC port 593 was open on the AD DOmain Controllers ... member servers
don't have that port open. The telnet banner is ncacn_http/1.0.

I've read lots of information about this being part of the RPC service for
routing "RPC over HTTP" requests, and that DCOM can use RPC over HTTP, but
in all cases "RPC over HTTP" is not configured on any of my Windows Server
2003 servers. Does anyone know what this port does and if we don't have
"RPC over HTTP" enabled what it is needed for? I'd like to shut the port
down but am concerned I'll 'break' AD replication or something similar.
Thanks.

PJ.