It's complex, although Aaron should chime in here. The history is Eric Schwartz tried to open source the CARP plugin but the code was such a mess that we never pushed through on that. Aaron has done a lot of cleanup in the plugin so we may try open sourcing it again. However, everyone (including myself and Aaron) view that as a short term fix. There's a group working on the Layer 7 Routing project [1] which has as its goal to replace CARP as a plugin with support directly for that kind of thing in the core. Talking with Aaron, he thinks that despite the work he's already done, it will need another round of clean up before it's suitable for use.
[1] https://github.com/apache/trafficserver/projects/7
