Dear Ben,
I figured out that drops occur inside OVS. I see all packets entering one
interface of OVS, Packet_In generated for every packet, then Flow_Mods (or
Packet_Out in other tests) generated and sent for every Packet_In by
external controller and all this rules are installed to OVS. Namely 500
Packet_In --> 500 flows in OVS, but only part of ingress packets is
processed through their corresponding flow rules and leaves OVS.
(dump-ports and dump-flows both in kermel and user-space modules show this).
Drops occur only after some threshold of Packet_In per msec, that's why it
seems like OVS drops some packets due to buffer overloads (or probably due
to expired timeouts for arrived packets).
I read logs up to dbg level but the only thing that I figured out (in
ovs-vswitchd.log) that governor periodically expands hash table in response
to flow_mods increasing frequency.
Is there possibility to track drops in internal buffers of OVS or somehow
to debug it?
Or, probably, does OVS drop packets after expired timeout for Packet_In
residing in buffer? And what is the default value for such timeout if any?
--
Best regards,
Anton Matsiuk
On 21 November 2013 17:56, Ben Pfaff <b...@nicira.com> wrote:
> Please don't drop the mailing list.
>
> You have begun to narrow down where the drops occur, but it's still not
> clear exactly where. I suggest following the troubleshooting procedure
> in the FAQ.
>
> Q: I have a sophisticated network setup involving Open vSwitch, VMs or
> multiple hosts, and other components. The behavior isn't what I
> expect. Help!
>
> A: To debug network behavior problems, trace the path of a packet,
> hop-by-hop, from its origin in one host to a remote host. If
> that's correct, then trace the path of the response packet back to
> the origin.
>
> Usually a simple ICMP echo request and reply ("ping") packet is
> good enough. Start by initiating an ongoing "ping" from the origin
> host to a remote host. If you are tracking down a connectivity
> problem, the "ping" will not display any successful output, but
> packets are still being sent. (In this case the packets being sent
> are likely ARP rather than ICMP.)
>
> Tools available for tracing include the following:
>
> - "tcpdump" and "wireshark" for observing hops across network
> devices, such as Open vSwitch internal devices and physical
> wires.
>
> - "ovs-appctl dpif/dump-flows <br>" in Open vSwitch 1.10 and
> later or "ovs-dpctl dump-flows <br>" in earlier versions.
> These tools allow one to observe the actions being taken on
> packets in ongoing flows.
>
> See ovs-vswitchd(8) for "ovs-appctl dpif/dump-flows"
> documentation, ovs-dpctl(8) for "ovs-dpctl dump-flows"
> documentation, and "Why are there so many different ways to
> dump flows?" above for some background.
>
> - "ovs-appctl ofproto/trace" to observe the logic behind how
> ovs-vswitchd treats packets. See ovs-vswitchd(8) for
> documentation. You can out more details about a given flow
> that "ovs-dpctl dump-flows" displays, by cutting and pasting
> a flow from the output into an "ovs-appctl ofproto/trace"
> command.
>
> - SPAN, RSPAN, and ERSPAN features of physical switches, to
> observe what goes on at these physical hops.
>
> Starting at the origin of a given packet, observe the packet at
> each hop in turn. For example, in one plausible scenario, you
> might:
>
> 1. "tcpdump" the "eth" interface through which an ARP egresses
> a VM, from inside the VM.
>
> 2. "tcpdump" the "vif" or "tap" interface through which the ARP
> ingresses the host machine.
>
> 3. Use "ovs-dpctl dump-flows" to spot the ARP flow and observe
> the host interface through which the ARP egresses the
> physical machine. You may need to use "ovs-dpctl show" to
> interpret the port numbers. If the output seems surprising,
> you can use "ovs-appctl ofproto/trace" to observe details of
> how ovs-vswitchd determined the actions in the "ovs-dpctl
> dump-flows" output.
>
> 4. "tcpdump" the "eth" interface through which the ARP egresses
> the physical machine.
>
> 5. "tcpdump" the "eth" interface through which the ARP
> ingresses the physical machine, at the remote host that
> receives the ARP.
>
> 6. Use "ovs-dpctl dump-flows" to spot the ARP flow on the
> remote host that receives the ARP and observe the VM "vif"
> or "tap" interface to which the flow is directed. Again,
> "ovs-dpctl show" and "ovs-appctl ofproto/trace" might help.
>
> 7. "tcpdump" the "vif" or "tap" interface to which the ARP is
> directed.
>
> 8. "tcpdump" the "eth" interface through which the ARP
> ingresses a VM, from inside the VM.
>
> It is likely that during one of these steps you will figure out the
> problem. If not, then follow the ARP reply back to the origin, in
> reverse.
>
>
> On Thu, Nov 21, 2013 at 04:55:13PM +0100, Anton Matsiuk wrote:
> > I request log files up to debug level, namely:
> > ovs-vswitchd.log
> > ovs-dpctl.log
> > ovs-ofctl.log
> > but none of them shows any messages related to packet drops. All the
> > statistics shows that correct number of flows was installed and only part
> > of packets was processed.
> > That's why I am asking, is there any else possibilities (beyond log
> files)
> > to track packet drops in input buffers and probably to fix them? Or at
> > least in which direction I should search for a solution?
> >
> >
> > On 20 November 2013 18:13, Ben Pfaff <b...@nicira.com> wrote:
> >
> > > On Wed, Nov 20, 2013 at 12:35:25PM +0100, Anton Matsiuk wrote:
> > > > I test Open vSwitch in the following scheme: I use 2 hosts directly
> > > > connected to OVS and external OpenFlow Controller. Host1 generates
> UDP
> > > > datagrams with sequential ports towards Host2, Host 2 listens for
> these
> > > UDP
> > > > datagrams. In responce to every UDP datagram OVS generates Packet_In
> and
> > > > Controller sends Flow_Mod back with L4 granularity (so for every
> pair of
> > > > UDP port numbers it installs separate flow). I send bunch of UDP
> > > datagrams
> > > > from Host1 and calculate how many of them arrived to Host2. I tried
> both
> > > > with detached controller and running in the same machine as OVS. I
> tested
> > > > it on different machines (in Mininet and with separated real hosts).
> I
> > > use
> > > > out-of-band option for controller and disable-in-band=true.
> > > >
> > > >
> > > > Starting some number of packets ( around >300) packet drops are
> > > observed.
> > > > For instance, if I generate 500 UDP packets in 120 ms only around
> 350 of
> > > > them arrive to Host2 (Subsequent packets of the same flow can arrive
> to
> > > > Host2, but first packets of flows always experience drops)
> > > >
> > > >
> > > > ovs-ofctl dump-aggregate show that all the flows are installed but
> only
> > > > part of packets are processed through them:
> > > >
> > > > NXST_AGGREGATE reply (xid=0x4): packet_count=356 byte_count=42364
> > > > flow_count=500
> > > >
> > > >
> > > > ovs-ofctl dump-ports also shows that 500 packets arrive on ingress
> > > > interface and only 356 leave egress.
> > > >
> > > >
> > > > ovs-dpctl show ?s shows the same ? 500 flows installed and 356
> packets
> > > > processed.
> > > >
> > > >
> > > > Also I tried to replace Flow_Mods with Packet_Out messages for every
> > > > packet, but I experienced the same drops. It seems like OVS starts
> > > dropping
> > > > packets after some threshold (or buffer overload).
> > > >
> > > >
> > > > Is there any possibility to debug these drops and maybe to manipulate
> > > > ingress buffer sizes (or queue priorities) in order to avoid such
> drops?
> > >
> > > Yes, I think you will have to do the initial debugging yourself, to
> find
> > > out where the drop is occurring. When you report that back to us, we
> > > can help you figure out how to fix it.
> > >
> >
> >
> >
> > --
> > Best regards,
> > Anton Matsiuk
>
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