The NAK layer provides reliable FIFO multicast and point-to-point communication over unreliable communication mechanisms. It does not use a window-based protocol (in fact, no acknowledgements are sent). Instead, it only sends negative acknowledgements when message omission errors are detected, and occasional protocol status update reports (much like XTP [14]). The NAK layer does not provide fragmentation of messages (this is done by the FRAG layer, which is not discussed in this paper). Other layers can implement flow control (see, for example, the FC layer below). The NAK layer reports potential communication problems when it has not received a protocol status update for some time from a member.
If the underlying communication system already provides reliable communication (such as in the case of TCP), the NAK and FRAG layers can be omitted.