- /* Attempts to receive a packet from 'rx' into the 'size' bytes in
- * 'buffer'. If successful, returns the number of bytes in the received
- * packet, otherwise a negative errno value. Returns -EAGAIN immediately
- * if no packet is ready to be received.
+ /* Attempts to receive a packet from 'rx' into the tailroom of 'buffer',
+ * which should initially be empty. If successful, returns 0 and
+ * increments 'buffer->size' by the number of bytes in the received packet,
+ * otherwise a positive errno value. Returns EAGAIN immediately if no
+ * packet is ready to be received.
+ *
+ * Must return EMSGSIZE, and discard the packet, if the received packet
+ * is longer than 'ofpbuf_tailroom(buffer)'.
+ *
+ * Implementations may make use of VLAN_HEADER_LEN bytes of tailroom to
+ * add a VLAN header which is obtained out-of-band to the packet. If
+ * this occurs then VLAN_HEADER_LEN bytes of tailroom will no longer be
+ * available for the packet, otherwise it may be used for the packet
+ * itself.