-- binary
01 02 # version / type
00 0b # length
12 34 56 78 # xid
61 62 01 # data
-- python
ofp.message.echo_request(xid=0x12345678, data="ab\x01")
-- python pretty-printer
echo_request { xid = 0x12345678, data = 'ab\x01' }
-- c
obj = of_echo_request_new(OF_VERSION_1_0);
of_echo_request_xid_set(obj, 0x12345678);
{
    of_octets_t data = { .data=(uint8_t *)"ab\x01", .bytes=3 };
    of_echo_request_data_set(obj, &data);
}