[ovs-dev] [cfm_v2 7/9] cfm: Allow accurate transmission intervals in extended mode.

Ben Pfaff blp at nicira.com
Fri Sep 9 11:28:04 PDT 2011

On Thu, Sep 08, 2011 at 06:59:51PM -0700, Ethan Jackson wrote:
> The standard CFM protocol only allows a handful of transmission
> rates.  This is particularly problematic if you want to support a
> transmission rate slower than 100 ms and faster than 1000 ms.
> This patch allows arbitrary transmission rates (between 1 ms and
> 65535 ms).  It does this by commandeering parts of a reserved
> "zero" field in the ccm message.  This breaks wire compatibility
> with standard 802.1ag implementations, and thus is only supported
> in extended mode.
> Bug #7014.

Looks good, thank you.

This brings up a design choice that's been on my mind a bit for this
series.  One design alternative is the one you've chosen, which is to
explicitly enable an "extended mode".  Another alternative would be to
use a standards compliant mode unless options that force the extended
mode are configured, e.g. automatically enable extended mode if a
nonstandard interval is configured.  I usually lean toward the latter,
which is why I bring it up, but it seems that there are good reasons
to prefer explicit configuration in this case.  First, it seems that
some users might want extended mode simply to avoid conflicting with
standards compliant implementations, even if they are not using
extended features.  Second, the options that would force the extended
mode are kind of hard to describe (you'd have to say something like
"if the interval is not exactly 3, 10, 100, 1000, 10000, 60000, or
600000 milliseconds") so users could easily get confused about which
mode they've configured.  Anyway, I think you've made a good choice,
but I bring it up just in case you have further thoughts.

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