Re: [B.A.T.M.A.N.] A few questions on TVLV

["Linus Lüssing" <linus.luessing@web.de>]

On Tue, Oct 22, 2013 at 11:26:04PM +0200, Linus Lüssing wrote:
> On Sun, Oct 20, 2013 at 06:26:41PM +0800, Marek Lindner wrote:
> > On Sunday 20 October 2013 10:25:07 Linus Lüssing wrote:
> > > I think I had posted some thoughts on the TVLV feature on the IRC
> > > channel before with some suggestions for changes (e.g. moving the
> > > TVLV feature from the individual packet types down to the common
> > > batman header). And had tried to come up with examples for
> > > features where TVLVs for batman broadcast packets could be useful
> > > - which might have been potential, but not the most convincing,
> > > not the most awesome features.
> > > 
> > > So now I have a feature which I would like to have / would like to
> > > work on in the future [0], and I would like to know:
> > 
> > Before we discuss how we get TVLVs 'down to the common batman header' we 
> > should talk about why that would be useful. What are the advantages and 
> > disadvantages ? Adding tvlvs everywhere for the mere sake of having them 
> > around isn't good enough as they increase code complexity, network traffic 
> > overhead and encapsulation speed penalties.
> > 
> > Cheers,
> > Marek
> 
> Agreed, bloated, never used code is never a good thing. And yes,
> before we do get into discussing the specific proposal, we should
> discuss the current limitations, (perceived) issues.
> 
> To be honest, I'm not 100% sure about the advantages and
> disadvantages, that's why I had decided to ask questions,
> initially :).
> 
> 
> This is my current (probably wrong) understanding about the
> disadvantages of the current implementation:
> 
> With the current TVLV approach it is still going to be difficult
> to introduce new features / changes to batman-adv broadcast /
> coded / fragment / ... packets as the current approach only
> provides TVLVs for OGMs and unicast packets.
> 
> How difficult, I'm not quite sure. Initially I thought compat
> bumps would be needed for new broadcast features for instance,
> but now I realized, that this is not the case, because bcast features
> could be signalized via OGM TVLVs and like with the mcast patches,
> could be enabled only if all nodes have that bcast-feature OGM
> TVLV.
> 
> So I guess the only disadvantage is, that you would either only be
> able to use a new bcast feature, if all nodes had it (which might
> be painful in a heterogeneous mesh network). Or if it is an
> optimization which can be applied "locally", then the according
> nodes would need to convert the broadcast packets back and forth.
> 
> 
> So vice versa, the advantage of having TVLVs for broadcast packets
> would be to have ignorable features, no need for conversions
> because the TVLV could be skipped, no compat bumps, not always a
> need to wait for all nodes to be upgraded to make the feature effective
> - just like for the current OGM and unicast TVLVs. We'd have the
> same flexibility provided by a uniform framework for all packet
> types from an "infrastructure point of view".
> 
> --
> 
> For the disadvantages of moving 'TVLVs down to the common header',
> I'm actually having trouble getting them. Maybe others could list
> them again and I try to explain which of these I'm having trouble
> understanding and why?

Or actually I just go ahead from the things which got mentioned in
your email and on IRC by Antonio already.

# Code complexity

Wouldn't this actually reduce code complexity? Now the code needs
to distinguish which packet type it got and based on that needs to
perform TVLV parsing. "Moving TVLVs down to the common header"
does not duplicating the code to for every packet type in my
understanding, but the contrary, unifying things, "just moving" the
current concept.

# Network Traffic overhead

It's just 2 Bytes extra for the tvlv_len which doesn't seem much
to me. Furthermore I'm wondering, whether saving these two bytes
but not being able to run some new, more efficiant feature
could be a net loss in terms of overhead in the future.

# TVLV parsing speed overhead

If no TVLV were actually added, then this would be nearly zero
parsing overhead, checking whether tvlv_len == 0 is trivial. Of
course, if no other packet types than the current TVLV capable
ones were actually having TVLVs one day, then wouldn't need this
discussion, true. Broadcast packets, for instance, aren't really
on the fastest fastpath, thre it wouldn't harm TVLVs, I think. But
also in general, I don't quite see, why packets on the fastpath
shouldn't have some TVLVs in the future: For one thing the parsing
speed overhead heavily depends on the kind of TVLV, how complex it
is to parse it. And why not having some easy to parse TVLVs for
those too? Embedded systems already need to be able to parse
Ethernet and IP headers as well as IP options on a hop-by-hop
basis in layer 3 routing. Also, even embedded systems become
faster and faster every day.

# Encapsulation speed overhead

Again, heavily depends on the kind of TVLV, doesn't it?


Cheers, Linus