Symmetric messages are sent in any direction without any solicitation. There are four messages that exist here: the hello, echo, error, and experimenter messages.

The hello message (OFPT_HELLO), echo request/reply, and vendor message are symmetric OpenFlow messages. In the OpenFlow reference implementation that includes a user space process and a kernel module, echo request/reply is implemented in the kernel module. This implementation consideration provides more accurate end-to-end latency timing. The vendor field in the OFPT_VENDOR message is a 32-bit value that uniquely identifies the vendor. If the most significant byte is zero, the next three bytes (24 bits) are the vendor's IEEE OUI. If a switch does not understand a vendor extension, it must send an OFPT_ERROR message with an OFPET_BAD_REQUEST error type and an OFPBRC_BAD_VENDOR error code.