mux_first_header
total_length = sizeof(struct mux_first_header) + data_len;
struct mux_first_header *hdr = (struct mux_first_header *)frame->data;
memset(hdr, 0, sizeof(struct mux_first_header));
frame->n_bytes += sizeof(struct mux_first_header);
struct mux_first_header *hdr = (void*)&frame->data[0];
struct mux_first_header *first;
frame_size = sizeof(struct mux_first_header) + xn->queued_bytes + sizeof(struct mux_next_header) + 4 + sizeof(struct mux_bounds)*xn->queued_packets;