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From: Andy Parkins <andyparkins@gmail.com>
To: Wladimir <laanwj@gmail.com>
Date: Sat, 16 Jun 2012 10:54:11 +0100
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Cc: bitcoin-development@lists.sourceforge.net
Subject: Re: [Bitcoin-development] Proposed new P2P command and response:
	getcmds, cmdlist
Precedence: list

On Saturday 16 Jun 2012 09:42:21 Wladimir wrote:

> Which is a perfectly reasonable requirement. However, one could simply
> standardize what a 'thin client' and what a 'thick client' does and
> offers (at a certain version level), without having to explicitly
> enumerate everything over the protocol.

My problem is that that I suspect the spectrum of clients will be far more 
than simply "thin" or "thick".  What about thick-pruned, thick-full?  What 
about thin-blocks-on-demand and thin-headers-on-demand?  These are just what 
I can think of now; it seems unwise to limit the functionality of clients 
not yet designed with a binary designation.  So... we make a field that can 
hold more than just a bit; with each possible value representing a specific 
(possibly overlapping) set of features?  Why not just enumerate the features 
then?

I did write responses to each of your following points; but they just 
sounded like me being contrary.  The short version is that I think too much 
emphasis is being placed on defining a specific set of feature->version 
mapping.  That's going to make it hard for future clients that want to 
implement some of the features but not all, and yet still want to be good 
bitcoin citizens and be able to tell their peers what they don't support.  
For example, there is no easy way for a node to tell another that it doesn't 
have the whole block chain available, so requesting it from it will fail. 

> I'm just afraid that the currently simple P2P protocol will turn into a
> zoo of complicated (and potentially buggy/insecure) interactions.

Fair enough.

> So maybe a capability system is a good idea but then the granularity
> should be large, not command-level. The interaction between protocol
> versions and capabilities needs to be defined as well. Does offering
> "getdata" at protocol version 10 mean the same as offering it at
> protocol version 11"? Probably not guaranteed. The arguments might have
> changed. So it's not entirely self-documenting either.

That problem doesn't go away just because you don't have a capabilities 
system.  Either version 11 can speak version 10 or it can't.  I don't see 
how having a system for finding out that fact changes anything other than 
removing a load of protocol noise.

"I support getdata10" makes it far easier to discover that the peer supports 
getdata10 than sending getdata11 and watching it fail does.


Andy
-- 
Dr Andy Parkins
andyparkins@gmail.com