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

[Gregory Maxwell <greg@xiph.org>]

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.