+(Output actions are obviously side effects too, but
+the trace commands never execute them, even when one specifies a
+packet.)
+.
+.IP "Incomplete information."
+Most of the time, Open vSwitch can figure out everything about the
+path of a packet using just the flow, but in some special
+circumstances it needs to look at parts of the packet that are not
+included in the flow. When this is the case, and you do not supply a
+packet, then a trace command will tell you it needs a packet.
+.RE
+.
+.IP
+If you wish to include a packet as part of a trace operation, there
+are two ways to do it:
+.
+.RS
+.IP \fB\-generate\fR
+This option, added to one of the ways to specify a flow already
+described, causes Open vSwitch to internally generate a packet with
+the flow described and then to use that packet. If your goal is to
+execute side effects, then \fB\-generate\fR is the easiest way to do
+it, but \fB\-generate\fR is not a good way to fill in incomplete
+information, because it generates packets based on only the flow
+information, which means that the packets really do not have any more
+information than the flow.
+.
+.IP \fIpacket\fR
+This form supplies an explicit \fIpacket\fR as a sequence of hex
+digits. An Ethernet frame is at least 14 bytes long, so there must be
+at least 28 hex digits. Obviously, it is inconvenient to type in the
+hex digits by hand, so the \fBovs\-pcap\fR(1) and
+\fBovs\-tcpundump\fR(1) utilities provide easier ways.
+.IP
+With this form, packet headers are extracted directly from
+\fIpacket\fR, so the \fIodp_flow\fR or \fIbr_flow\fR should specify
+only metadata. The metadata can be:
+.RS
+.IP \fIskb_priority\fR
+Packet QoS priority.
+.IP \fIpkt_mark\fR
+Mark of the packet.
+.IP \fItun_id\fR
+The tunnel ID on which the packet arrived.
+.IP \fIin_port\fR
+The port on which the packet arrived.
+.RE
+.RE
+.
+.IP
+The in_port value is kernel datapath port number for the first format
+and OpenFlow port number for the second format. The numbering of these
+two types of port usually differs and there is no relationship.
+.