[ovs-discuss] Issue: ovs send many packets of a table miss flow to controller

[Big Strong]

In some cases, the controller may not set up the flow as fast as the packet
arrives. Then multiple pktIn messages for the same flow will be send to
controller. Won't the flow be installed repeatedly? I see  another post
discussed about the linked buffering of ovs
<https://mail.openvswitch.org/pipermail/ovs-discuss/2016-April/040744.html>,
I'm wondering if it's being implemented now.

Besides, during my test of generating pktIn messages about 100/s, the
buffer seems to be overwritten by new packets when it is filled full, which
makes the "buffer id unknown" error as the controller decide to tell the
buffered packets to forward but cannot find it anymore in the buffer.

2017-03-17 23:16 GMT+08:00 Ben Pfaff <blp at ovn.org>:

> Table-miss packet-in messages most commonly pass the first packet in a
> microflow to the OpenFlow controller, which then sets up an OpenFlow
> flow that handles remaining traffic in the microflow without further
> controller intervention.  In such a case, the packet that initiates the
> microflow is usually small, which means that the switch sends the entire
> packet to the controller and the buffer only saves a small number of
> bytes in the reverse direction.
>
> On Fri, Mar 17, 2017 at 02:14:52PM +0800, Big Strong wrote:
> > Why are buffers usefulness? I thought it is useful  to cache table-missed
> > packets and transfer the packets after controller's decision. Without it,
> > the controller has to receive all bytes of the packets and re-inject them
> > afterwards. Or else the connections may be broken for loss of packets.
> >
> > 2017-03-17 11:54 GMT+08:00 Ben Pfaff <blp at ovn.org>:
> >
> > > You can change anything in OVS by modifying the source code, so this
> > > isn't really a surprise.
> > >
> > > Buffers aren't very useful, so we've never made them adjustable, and
> OVS
> > > 2.7 removes them entirely.
> > >
> > > On Fri, Mar 17, 2017 at 10:21:49AM +0800, Big Strong wrote:
> > > > From the source code I found the way to modify n_buffers by
> adjusting the
> > > > PKTBUF_BITS. For example, I can set it to 1024 by setting
> PKTBUF_BITS to
> > > > 10.
> > > > Does the modification have any some side-effects? Why doesn't OVS
> make it
> > > > adjustable?
> > > >
> > > > /* Buffers are identified by a 32-bit opaque ID.  We divide the ID
> > > > >  * into a buffer number (low bits) and a cookie (high bits).  The
> > > buffer
> > > > > number
> > > > >  * is an index into an array of buffers.  The cookie distinguishes
> > > between
> > > > >  * different packets that have occupied a single buffer.  Thus, the
> > > more
> > > > >  * buffers we have, the lower-quality the cookie... */
> > > > > #define PKTBUF_BITS     8
> > > > > #define PKTBUF_MASK     (PKTBUF_CNT - 1)
> > > > > #define PKTBUF_CNT      (1u << PKTBUF_BITS)
> > > > > #define COOKIE_BITS     (32 - PKTBUF_BITS)
> > > > > #define COOKIE_MAX      ((1u << COOKIE_BITS) - 1)
> > > >
> > > >
> > > > 2017-03-16 23:07 GMT+08:00 Ben Pfaff <blp at ovn.org>:
> > > >
> > > > > OVS sends a packet to the controller if the flow table tells it to
> do
> > > > > so.  There's nothing in OpenFlow that restricts this on the basis
> of a
> > > > > flow or a response from the controller.
> > > > >
> > > > > n_buffers is not adjustable.
> > > > >
> > > > > On Thu, Mar 16, 2017 at 10:22:38PM +0800, Big Strong wrote:
> > > > > > @Ben, does it mean ovs would repeatedly send packet in messages
> as
> > > long
> > > > > as
> > > > > > no response is got from controller? What will happen when a same
> > > packet
> > > > > > comes to ovs while the old packet in message is still not
> handled?
> > > > > >
> > > > > > Another question, is the n_buffers size adjustable using
> > > > > > ovs-ofctl/ovs-vsctl command? I want a larger n_buffers rather
> than
> > > the
> > > > > > default 256.
> > > > > >
> > > > > > Regards.
> > > > > >
> > > > > > 2017-03-16 4:54 GMT+08:00 Ben Pfaff <blp at ovn.org>:
> > > > > >
> > > > > > > On Wed, Mar 15, 2017 at 01:34:56PM +0800, yiyefuping198425
> wrote:
> > > > > > > > I'm asking for assistance to fix this issue: ovs send many
> > > packets
> > > > > of a
> > > > > > > table miss flow instead of the first packet.
> > > > > > >
> > > > > > > That's how OpenFlow works.
> > > > > > > _______________________________________________
> > > > > > > discuss mailing list
> > > > > > > discuss at openvswitch.org
> > > > > > > https://mail.openvswitch.org/mailman/listinfo/ovs-discuss
> > > > > > >
> > > > >
> > >
>
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