TCP is a replacement of UDP as bottom layer in cases where IP Multicast based on UDP is not desired. This may be the case when operating over a WAN, where routers will discard IP MCAST. However, as 2 connections are created for each member (one for incoming, one for outgoing messages), the number of group members should be relatively small.
Since TCP already offers some of the reliability guarantees that UDP doesn't, the FRAG and UNICAST layers are not needed on top of it. Actually, NAKACK could be discarded as well, however it was not cleanly written to only do message retransmission, but also view ID checking, therefore it is still needed.3.10