For a paper describing the general principles of the Scalable
Reliable Multicast implemented in wb, see the following:

Floyd, S., Jacobson, V., Liu, C., McCanne, S., and Zhang, L., A
Reliable Multicast Framework for Light-weight Sessions and Application
Level Framing.  ACM SIGCOMM 95, pp. 342-356, August 1995.  
URL ftp://ftp.ee.lbl.gov/papers/link.ps.Z.