What is the difference between IP_ADD_MEMBERSHIP/IP_DROP_MEMBERSHIP and IP_ADD_SOURCE_MEMBERSHIP/IP_DROP_SOURCE_MEMBERSHIP?

Working on a project that sends multicast data packets, and the receivers can opt in/out of the group as necessary. I noticed there are two ways of joining/leaving the groups: IP_ADD_MEMBERSHIP/IP_DROP_MEMB开发者_如何学运维ERSHIP and IP_ADD_SOURCE_MEMBERSHIP/IP_DROP_SOURCE_MEMBERSHIP. Couldn't get anything out the man pages here, so throwing this one out to the SO gurus. What's the difference, if any, and can you give an example of when to use one instead of the other?

IP_ADD_SOURCE_MEMBERSHIP and IP_DROP_SOURCE_MEMBERSHIP are for SSM (source specific multicast) support.