[ovs-discuss] Hello version 0

[Anup Khadka]

Ben,
It was more about sanity tests on OVS than anything else (and hardware
switches using OVS as its stack). Naturally, if a controller is trying to
negotiate version 0x00, there is something wrong with the controller, no
question about it.

The issue is not that. The issue was more about how OVS should respond to a
request as such (or few other bad requests that I am testing), and if there
was anything in the standard that I could have missed that explains the
right behavior of the switch in such scenarios.

0x00 doesn't represent an OpenFlow version, no question, but there are
error codes for bad versions under bad request. When and why those codes
should be used isn't completely clear, at least to me, and I was seeking
insight.

Just reading through the standard,   the non-experimental versions are:
0x01, 0x02, 0x03, 0x04, 0x05, and 0x06. The experimental versions are 0x81
- 0x99.

The OVS as currently constructed, I believe, sends a Hello failed with
incompatible error for every version 0x01 - 0xFF.

Why does the switch silently ignore a Hello message with version 0, but
responds with a hello failed for version 0x01 - 0xFF even though its not
explicitly stated as a valid openflow version. In other words, why is a
version, say 0xFE, a valid version?

If you think, the switch's current behavior is expected, then its alright.
I am trying to get a better insight, and make sure its consistent.

Thanks,
Anup






On Thu, Jul 24, 2014 at 11:07 AM, Ben Pfaff <blp at nicira.com> wrote:

> On Wed, Jul 23, 2014 at 07:59:10PM -0400, Anup Khadka wrote:
> > If OVS receives a Hello packet with version set to 0, it logs an error
> > "received message while expecting hello" and change's vconn's state
> without
> > sending any message back to the controller.
> >
> > Is this a desired behavior? The standard isn't really clear on how to
> > process a Hello packet with version == 0 (or if its even a valid OpenFlow
> > packet at that point), but I think its reasonable for a controller to
> > expect either one of the following two messages:
> >
> > 1. An OFPT_ERROR message with OFPET_HELLO_FAILED error type, and
> > OFPHFC_INCOMPATIBLE error code (this was the behavior of OVS 1.4.0 stack,
> > but the code base, and checks were significantly different then. The old
> > code didn't really check for version number range, simply transitioned to
> > VCS_SEND_ERROR state, and sent an error back to controller)
> >
> > 2. An OFPT_ERROR message with OFPET_BAD_REQUEST error type, and
> > OFPBRC_BAD_VERSION error code.
>
> It's not great behavior.
>
> I don't know how OVS would send OFPT_ERROR in this case, though, because
> 0x00 doesn't represent an OpenFlow version and so there aren't any error
> codes to use.
>
> If your controller is trying to negotiate version 0x00, it's buggy.  Fix
> it.
>
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