Retransmission based error control for large scale multicast applications is difficult because of implosion and exposure. Existing schemes (SRM, RMTP, TMTP LBRRM) have good solutions to implosion, but only approximate solutions to exposure. We present a scheme that achieves finer grain fault recovery by exploiting new forwarding services that allow us to create a dynamic hierarchy of receivers. We extend the IP multicast service model so that routers provide a more refined form of multicasting (which may be useful to other applications), that enables local recovery. The new services are simple to implement and do not require routers to examine or store application packets; hence, they do not violate layering. Besides providing better implosion control and less exposure than other schemes, our scheme integrates well with the current IP model, has small recovery latencies (it requires no back-off delays), and completely isolates group members from topology. Our scheme can be used with a variety of multicast routing protocols, including DVMRP and PIM. We have implemented our scheme in NetBSD Unix, using about 250 lines of new C-code. The implementation requires two new IP options, 4 additional bytes in each routing entry and a slight modification to IGMP reports. The forwarding overhead incurred by the new services is actually lower than forwarding normal multicast traffic.
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