On 6/5/2019 1:18 PM, Ben Pfaff wrote:
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.
Ah, I stand corrected then. Thanks for clearing that up for me.
- Greg
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.
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