+.IP \fBtun_id=\fItunnel-id\fR[\fB/\fImask\fR]
+Matches tunnel identifier \fItunnel-id\fR. Only packets that arrive
+over a tunnel that carries a key (e.g. GRE with the RFC 2890 key
+extension) will have a nonzero tunnel ID. If \fImask\fR is omitted,
+\fItunnel-id\fR is the exact tunnel ID to match; \fImask\fR is
+specified, then a 1-bit in \fImask\fR indicates that the corresponding
+bit in \fItunnel-id\fR must match exactly, and a 0-bit wildcards that
+bit.
+.IP
+\fBtun_id\fR requires use of one of two Nicira extensions to OpenFlow:
+.RS
+.IP "NXM (Nicira Extended Match)"
+This extension fully supports \fBtun_id\fR.
+.IP "Tunnel ID from Cookie"
+This extension supports \fBtun_id\fR with three caveats: the top 32 bits
+of the \fBcookie\fR (see below) are used for \fItunnel-id\fR and thus
+unavailable for other use, specifying \fBtun_id\fR on
+\fBdump\-flows\fR or \fBdump\-aggregate\fR has no effect, and
+\fImask\fR is not supported.
+.RE
+.IP
+When \fBtun_id\fR is specified, \fBovs\-ofctl\fR will automatically
+attempt to negotiate use of one of these extensions. It will use the
+``tunnel ID from cookie'' extension if neither caveat applies and NXM
+otherwise. If the switch does not support the needed extension, then
+\fBovs\-ofctl\fR will report a fatal error.
+.IP "\fBreg\fIidx\fB=\fIvalue\fR[\fB/\fImask\fR]"
+Matches \fIvalue\fR either exactly or with optional \fImask\fR in
+register number \fIidx\fR. The valid range of \fIidx\fR depends on
+the switch. \fIvalue\fR and \fImask\fR are 32-bit integers, by
+default in decimal (use a \fB0x\fR prefix to specify hexadecimal).
+Arbitrary \fImask\fR values are allowed: a 1-bit in \fImask\fR
+indicates that the corresponding bit in \fIvalue\fR must match
+exactly, and a 0-bit wildcards that bit.
+.IP
+When a packet enters an OpenFlow switch, all of the registers are set
+to 0. Only explicit Nicira extension actions change register values.
+.IP
+Register matches require support for the NXM (Nicira Extended Match)
+extension to OpenFlow. When a register match is specified,
+\fBovs\-ofctl\fR will automatically attempt to negotiate use of this
+extension. If the switch does not support NXM, then \fBovs\-ofctl\fR
+will report a fatal error.