2009/235 dladm Possible Values List

[James Carlson]

Garrett D'Amore writes:
> Unless some specific real world tests (e.g. TCP throughput or UDP stress 
> tests) show otherwise, I'm disinclined to believe that there is any 
> reason an administrator would need to know about the underlying hardware 
> DMA limitations, or that the "optimum" value is anything other than the 
> largest supported value.

Even then, I suspect that this may be more of a man page or whitepaper
issue.

On a given L2 network, all of the links must be configured with
*exactly* the same MTU; neither larger nor smaller.  There's no good
way (at least today) to tell whether all of the systems are actually
configured that way.  You just have to hope so, because the penalty
for getting it wrong is a set of very obscure failure modes.  That's
part of the reason why the IEEE hasn't endorsed jumbograms -- they're
fragile.

The implication of that network-wide configuration issue is that the
administrator must choose an MTU value based not just on local
hardware considerations, but on the capabilities and configuration of
all of the other nodes on his L2 network.

That's hard, and it's certainly not as simple as a single "preferred"
value (unless that preferred value happens to be exactly 1500, the
standards-conforming and safe default).  In fact, I'd expect that
there are devices that have a set of multiple "sweet spot" MTU values
where buffers and other detailed components all happen to line up
perfectly.

Getting all of that information into a user interface seems hopeless
to me, so I'm in favor of having the dladm command line display what
the hardware can do, and leaving high-level network design and tuning
to other documentation and/or tools.

-- 
James Carlson, Solaris Networking              <james.d.carlson at sun.com>
Sun Microsystems / 35 Network Drive        71.232W   Vox +1 781 442 2084
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