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

From: Gregory Maxwell <[email-masked]>
Date: Mon, 6 Aug 2018 02:15:22 +0000

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.