Re: [bitcoin-dev] Capping the size of locators [trivial protocol change BIP]

[Eric Voskuil <eric@voskuil.org>]

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Libbitcoin has implemented a 11 + log2(height) limit since version3 for
this reason. This message can be very costly if not constrained.

The presumed protocol inherently limits valid locator size for a given
recipient. IMO it's worth considering instead describing the expected
semantics of the message and thereby its *inherent* limits. Doing so
gives the recipient an upper bound on valid locator size, eliminating
the need to introduce an arbitrary limit.

I have commonly seen locators with 100 elements, I believe from
BitcoinJ. I recall posting a query on the issue to their IRC but got no
response. So it would seem that a quick survey and a limit of 64 would
not have prevented the issue of concern.

But in any case, I agree that implementations should enforce a limit.

e

On 08/05/2018 07:15 PM, Gregory Maxwell via bitcoin-dev wrote:
> Coinr8d posted on bct that the node software would process large
> locators limited only by the maximum message size yet sensible usage
> of locators only results in messages of log2(n_blocks) size. He was
> concerned that it might be a DOS vulnerability but quick measurements
> indicated to me that it likely wasn't worse than many other protocol
> messages.  It still seems silly to allow absurd locators. So I propose
> that the size of locators be limited.
> 
> However, capping them is a P2P change that could potentially result in
> network splits if older nodes would potentially produce larger
> locators (esp if triggered to produce unexpectedly large ones by
> forks).  A quick survey of node software indicated that no software I
> could find would ever produce a locator with more than 42 hashes
> before encountering other limits, so I think a limit of 64 will be
> automatically compatible with all or virtually all nodes on the
> network.
> 
> I'm bothering writing a BIP because there might be some naive
> implementation lurking out there that sends a crazy number due to
> sub-exponential backoff that would be broken by nodes enforcing a
> limit... particularly since the correct use of locators was never
> previously mandated and might not be obvious to all developers.
> 
> I take the opportunity to also specify that the locators be correctly
> ordered in terms of total work, but  don't specify that they all come
> from the same chain.
> 
> Cheers,
> 
> ==Introduction==
> 
> ===Abstract===
> 
> This document proposes limiting the locator messages used in the getblocks
> and getheaders to 64 entries and requiring that be ordered by total
> work.
> 
> ===Copyright===
> 
> This document is licensed under the 2-clause BSD license.
> 
> ==Motivation==
> 
> The Bitcoin P2P protocol uses a simple and efficient data structure
> to reconcile blockchains between nodes called a locator.  A locator
> communicates a list of known hashes which allows a peer to find a
> recent common ancestor between the best chains on two nodes.  By
> exponentially increasing the space between each entry, the locator
> allows a log() sized message to find the difference between two nodes
> with only a constant factor overhead.
> 
> Because short forks are much more common than long forks the typical
> usage of the locator includes a small number of topmost hashes before
> switching to exponential spacing.
> 
> The original Bitcoin implementation provided no explicit limit to the
> number of hashes in a locator message, allowing for absurd and
> wasteful uses like including
> all hashes in a chain.
> 
> Although locators are very inexpensive for existing node software to
> process there is no known utility for sending very large locators.
> To reduce the worst case cost of processing a locator message it would
> be useful if the size of locator messages were strictly
> bounded to sensible levels.
> 
> Common implementations have implicit limitations of 2^32 blocks and an
> exponent of 2 after the first 10 locators and so could never request
> more than 42 hashes in any case.
> 
> == Specification ==
> 
> A locator included in a getblock or getheaders message may include no more
> than 64 hashes, including the final hash_stop hash. Additionally, the blocks
> referenced by the locator must be in order of equal or decreasing total
> work.
> 
> Sending a locator that violates these requirements may result in normal
> processing, the message being ignored, a disconnection, or a ban.
> 
> Implementations that seek to handle larger numbers of blocks than afforded
> by this limit with an exponent of 2 can adaptively switch to a larger
> exponent as required to stay within the limit.
> 
> == Acknowledgements ==
> 
> Thanks to Coinr8d on bitcointalk for pointing out that node software would
> process and respond to locators with about 125,000 hashes in them.
> _______________________________________________
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> bitcoin-dev@lists.linuxfoundation.org
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> 


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