Index: sys/dev/cxgbe/tom/t4_ddp.c
===================================================================
--- sys/dev/cxgbe/tom/t4_ddp.c
+++ sys/dev/cxgbe/tom/t4_ddp.c
@@ -405,6 +405,19 @@
 	}
 
 	tp = intotcpcb(inp);
+
+	/*
+	 * For RX_DDP_COMPLETE, len will be zero and rcv_nxt is the
+	 * sequence number of the next byte to receive.  The length of
+	 * the data received for this message must be computed by
+	 * comparing the new and old values of rcv_nxt.
+	 * 
+	 * For RX_DATA_DDP, len might be non-zero, but it is only the
+	 * length of the most recent DMA.  It does not include the
+	 * total length of the data received since the previous update
+	 * for this DDP buffer.  rcv_nxt is the sequence number of the
+	 * first received byte from the most recent DMA.
+	 */
 	len += be32toh(rcv_nxt) - tp->rcv_nxt;
 	tp->rcv_nxt += len;
 	tp->t_rcvtime = ticks;