[ovs-dev] Fix for hashing fragmented UDP packets

[Ben Pfaff]

On Fri, May 31, 2019 at 01:18:53PM -0700, Gregory Rose wrote:
> 
> On 5/30/2019 10:37 PM, Van Bemmel, Jeroen (Nokia - US) wrote:
> > Hello all,
> > 
> > Back in 2015 I submitted some code to improve ECMP hashing algorithms used in OVS, see https://mail.openvswitch.org/pipermail/ovs-dev/2015-July/300748.html
> > 
> > We have now found an issue with the hashing of fragmented UDP packets: The first packet contains the UDP ports, but the rest of the fragments do not - so subsequent packets may hash to a different destination. This causes IMS SIP calls using UDP to fail ( for example )
> > 
> > To fix this, we need a 1-line patch at https://github.com/openvswitch/ovs/blob/master/lib/flow.c#L2374:
> > Old:   (inc_udp_ports && flow->nw_proto == IPPROTO_UDP)) {
> > New: (inc_udp_ports && flow->nw_proto == IPPROTO_UDP && !(flow->nw_frag & FLOW_NW_FRAG_MASK)) {
> > 
> > And something similar at https://github.com/openvswitch/ovs/blob/master/lib/flow.c#L2489
> > Old: if (is_ip_any(flow) && flow->nw_proto == IPPROTO_UDP) {
> > New: if (is_ip_any(flow) && flow->nw_proto == IPPROTO_UDP && !(flow->nw_frag & FLOW_NW_FRAG_MASK)) {
> > 
> > In other words: Don't include the ports for the first fragment
> > 
> > Comments or objections?
> 
> I guess if there were a lot of UDP flows with fragmented traffic then it
> would slow down the lookups.  

I don't understand that statement.  Why would changing the ECMP hash
value slow down lookups?  (What lookups?)

Oh, are you thinking of this as a hash function for use in hash tables?
I do not think that this particular hash function is used that way.

> I don't believe that there is a lot of fragmented UDP traffic in most
> scenarios, although I'm more familiar with datacenter than telco.  It
> seems like a reasonable approach but if we could get some sort of
> regression testing done to show how much of an impact it might have in
> scenarios with high numbers of UDP fragmented connections have then I
> think that might make us more comfortable with the change.

It would have the effect of putting all the fragmented UDP traffic into
a single ECMP hash.  If all the traffic were fragmented UDP, that would
waste a lot of capacity, but the traffic would not be reordered.