Memory buffers, aka mbufs mbufs are a linked list of structures extensively used in the networking code. The linked list is usually referred to as an mbuf chain. When the data in the chain exceeds a set threshold the structures then don't store data internally but each point to a external chunk of memory of MCLBYTES long known as a cluster. Several chains form a queue, the header of each mbuf chain contains a pointer to the next mbuf chain in the queue. bus_dma gives us bus_dmamap_load_mbuf() which lets us map an mbuf chains for DMA relatively easily. See the mbuf(9) man page for more discussion.