The issue you're having is described on page 24 of the OpenFlow spec as well as on the ovs-ofctl man page, but without some context, it may not entirely make sense. Specifically, the spec says: The nw_tos fi eld is the 6 upper bits of the ToS field to set, in the original bit positions (shifted to the left by 2).
To understand this better: nw_tos is actually not really to do with the TOS field (which is long retired), but with the DiffServ field which superseded it. The bottom two bits of this field were originally unused, and later became the ECN bits. The top six bits are the DiffServ Code Point (DSCP), which is what nw_tos actually matches (right; it should actually probably be called nw_dscp). So the value you give to OpenFlow for nw_tos is actually a six bit field -- the top six bits of eight. One might imagine that the values of this field would therefore be 0 through 63, and would be shifted up two bits when written to the packet. However, as the spec says, these are "in the original bit positions". Thus, the smallest value legal for this field is 4 (followed by 8 and then 12, etc.). TLDR: You can't set the bottom two bits. You want one of the following, though it's hard to say which for sure: 1) to shift what you think the value should be to the left by two, or 2) to set the ECN bits and not the "TOS" (DSCP) bits. -- Murphy On Dec 8, 2013, at 11:12 PM, Sameer Katti <ska...@usc.edu> wrote: > Hi All, > > I am trying to set a flow that matches based on the nw_tos field of the match > object. But when I dump the flows on the switch it has nw_tos field set as 0. > I am using OpenvSwitch 1.10 on Mininet 2.0 > The code snippet I am using is:- > > msg = of.ofp_flow_mod() > #msg.match.dl_dst = packet.dst > msg.match.dl_type = ethernet.IP_TYPE > msg.match.nw_tos = 1; > > Even after installing a flow with such a match the nw_tos is still set to > zero. > > *** s1 > ------------------------------------------------------------------------ > NXST_FLOW reply (xid=0x4): > cookie=0x0, duration=13.677s, table=0, n_packets=1, n_bytes=98, idle_age=12, > ip,dl_dst=d2:35:53:03:83:bc,nw_tos=0 actions=output:1 > cookie=0x0, duration=6.652s, table=0, n_packets=3, n_bytes=294, idle_age=3, > ip,dl_dst=da:79:78:e1:7c:4f,nw_tos=0 actions=output:2 > > > How should I set the nw_tos field in ofp_match object? > -- > -Sameer
