Thanks for this Alex.
Here's a transcript of your recent presentation at Bitcoin++ on Minisketch and
Lightning gossip:
https://btctranscripts.com/bitcoinplusplus/2022/2022-06-07-alex-myers-minisketch-lightning-gossip/
Having followed Gleb's work on using Minisketch for Erlay in Bitcoin Core [0]
for a while now I was especially interested in how the challenges of using
Minisketch for Lightning gossip (node_announcement, channel_announcement,
channel_update messages) would differ to the challenges of using Minisketch for
transaction relay on the base layer.
I guess one of the major differences is full nodes are trying to verify a block
every 10 minutes (on average) and so there is a sense of urgency to get the
transactions of the next block to be mined. With Lightning gossip unless you
are planning to send a payment (or route a payment) across a certain route you
are less concerned about learning about the current state of the network
urgently. If a new channel pops up you might choose not to route through it
regardless given its "newness" and its lack of track record of successfully
routing payments. There are parts of the network you care less about (if they
can't help you get to your regular destinations say) whereas with transaction
relay you have to care about all transactions (paying a sufficient fee rate).
"The problem that Bitcoin faced with transaction relay was pretty similar but
there are a few differences.For one, any time you introduce that short hash
function that produces a 64 bit fingerprint you have to be concerned with
collisions between hash functions. Someone could potentially take advantage of
that and grind out a hash that would resolve to the same fingerprint."
Could you elaborate on this? Why are hash collisions a concern for Lightning
gossip and not for Erlay? Is it not a DoS vector for both?
It seems you are leaning towards per-peer sketches with inventory sets (like
Erlay) rather than global sketches. This makes sense to me and seems to be
moving in a direction where your peer connections are more stable as you are
storing data on what your peer's understanding of the network is. There could
even be centralized APIs which allow you to compare your current understanding
of the network to the centralized service's understanding. (Of course we don't
want to have to rely on centralized services or bake them into the protocol if
you don't want to use them.) Erlay falls back to flooding if the set
reconciliation algorithm doesn't work which I'm assuming you'll do with
Lightning gossip.
I was also surprised to hear that channel_update made up 97 percent of gossip
messages. Isn't it recommended that you don't make too changes to your channel
as it is likely to result in failed routed payments and being dropped as a
routing node for future payments? It seems that this advice isn't being
followed if there are so many channel_update messages being sent around. I
almost wonder if Lightning implementations should include user prompts like
"Are you sure you want to update your channel given this may affect your
routing success?" :)
Thanks
Michael
P.S. Are we referring to "routing nodes" as "forwarding nodes" now? I've
noticed "forwarding nodes" being used more recently on this list.
[0]: https://github.com/bitcoin/bitcoin/pull/21515
--
Michael Folkson
Email: michaelfolkson at [protonmail.com](http://protonmail.com/)
Keybase: michaelfolkson
PGP: 43ED C999 9F85 1D40 EAF4 9835 92D6 0159 214C FEE3
------- Original Message -------
On Thursday, April 14th, 2022 at 22:00, Alex Myers <[email protected]> wrote:
> Hello lightning developers,
>
> I’ve been investigating set reconciliation as a means to reduce bandwidth and
> redundancy of gossip message propagation. This builds on some earlier work
> from Rusty using the minisketch library [1]. The idea is that each node will
> build a sketch representing it’s own gossip set. Alice’s node will encode and
> transmit this sketch to Bob’s node, where it will be merged with his own
> sketch, and the differences produced. These differences should ideally be
> exactly the latest missing gossip of both nodes. Due to size constraints, the
> set differences will necessarily be encoded, but Bob’s node will be able to
> identify which gossip Alice is missing, and may then transmit exactly those
> messages.
>
> This process is relatively straightforward, with the caveat that the sets
> must otherwise match very closely (each sketch has a maximum capacity for
> differences.) The difficulty here is that each node and lightning
> implementation may have its own rules for gossip acceptance and propagation.
> Depending on their gossip partners, not all gossip may propagate to the
> entire network.
>
> Core-lightning implements rate limiting for incoming channel updates and node
> announcements. The default rate limit is 1 per day, with a burst of 4. I
> analyzed my node’s gossip over a 14 day period, and found that, of all
> publicly broadcasting half-channels, 18% of them fell afoul of our
> spam-limiting rules at least once. [2]
>
> Picking several offending channel ids, and digging further, the majority of
> these appear to be flapping due to Tor or otherwise intermittent connections.
> Well connected nodes may be more susceptible to this due to more frequent
> routing attempts, and failures resulting in a returned channel update (which
> otherwise might not have been broadcast.)A slight relaxation of the rate
> limit resolves the majority of these cases.
>
> A smaller subset of channels broadcast frequent channel updates with minor
> adjustments to htlc_maximum_msat and fee_proportional_millionths parameters.
> These nodes appear to be power users, with many channels and large balances.
> I assume this is automated channel management at work.
>
> Core-Lightning has updated rate-limiting in the upcoming release to achieve a
> higher acceptance of incoming gossip, however, it seems that a broader
> discussion of rate limits may now be worthwhile. A few immediate ideas:
>
> - A common listing of current default rate limits across lightning network
> implementations.
>
> - Internal checks of RPC input to limit or warn of network propagation issues
> if certain rates are exceeded.
>
> - A commonly adopted rate-limit standard.
>
> My aim is a set reconciliation gossip type, which will use a common, simple
> heuristic to accept or reject a gossip message. (Think one channel update per
> block, or perhaps one per block_height << 5.) See my github for my current
> draft. [3] This solution allows tighter consensus, yet suffers from the same
> problem as original anti-spam measures – it remains somewhat arbitrary. I
> would like to start a conversation regarding gossip propagation,
> channel_update and node_announcement usage, and perhaps even bandwidth goals
> for syncing gossip in the future (how about a million channels?) This would
> aid in the development of gossip set reconciliation, but could also benefit
> current node connection and routing reliability more generally.
>
> Thanks,
>
> Alex
>
> [1] https://github.com/sipa/minisketch
>
> [2]
> https://github.com/endothermicdev/lnspammityspam/blob/main/sampleoutput.txt
>
> [3]
> https://github.com/endothermicdev/lightning-rfc/blob/gossip-minisketch/07-routing-gossip.md#set-reconciliation
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