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