You have the trunk defined by the headers. Once a range from genesis to block n is fully downloaded,
you may validate upto block n. Furthermore after validation you can prune transactions spent until block n.

You would approach the highest block with validation and stop pruning say 100 blocks before it, to leave room for reorgs.

Tamas Blummer
http://bitsofproof.com

On 07.04.2014, at 21:13, Mark Friedenbach <mark@monetize.io> wrote:

> 
> 
> On 04/07/2014 12:20 PM, Tamas Blummer wrote:
>> Validation has to be sequantial, but that step can be deferred until the
>> blocks before a point are loaded and continous.
> 
> And how do you find those blocks?
> 
> I have a suggestion: have nodes advertise which range of full blocks
> they possess, then you can perform synchronization from the adversed ranges!
> 
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