Hi Gloria,

>  1. Transaction relay rate limiting

I have a similar concern as yours, that this could prevent higher fee-rate transactions from being broadcast. 

> 2. Staggered broadcast of replacement transactions: within some time interval, maybe accept multiple replacements for the same prevout, but only relay the original transaction.

By this do you mean basically having a batching window where, on receiving a replacement transaction, a node will wait for a period of time, potentially receiving many replacements for the same transaction (or many separate conflicting transactions), and only broadcasting the "best" one(s) at the end of that time window? 

Its an interesting idea, but it would produce a problem. Every hop that replacement transaction takes would be delayed by this staggered window. If the window were 3 minutes and transactions generally take 20 hops to get to the majority of miners, a "best-case average" delay might be 3.75 minutes (noting that among your 8 nodes, its quite likely one of them would have a window ending much sooner than 3 minutes). Some (maybe 3% of) nodes would experience delays of more than 20 minutes. That kind of delay isn't great. 

However it made me think of another idea: a transaction replacement broadcast cooldown. What if nodes kept track of the time they broadcasted the last replacement for a package and had a relay cooldown after the last replacement was broadcasted? A node receiving a replacement would relay the replacement immediately if the package its replacing was broadcasted more than X seconds ago, and otherwise it would wait until the time when that package was broadcasted at least X seconds ago to broadcast it. Any replacements it receives during that waiting period would replace as normal, meaning the unrebroadcasted replacement would never be broadcasted, and only the highest value replacement would be broadcasted at the end of the cooldown.

This wouldn't prevent a higher-fee-rate transaction from being broadcasted (like rate limiting could), but would still be effective at limiting unnecessary data transmission. Another benefit is that in the non-adversarial case, replacement transactions wouldn't be subject to any delay at all (while in the staggered broadcast idea, most replacements would experience some delay). And in the adversarial case, where a malicious actor broadcasts a low-as-possible-value replacement just before yours, the worst case delay is just whatever the cooldown period is. I would imagine that maybe 1 minute would be a reasonable worst-case delay. This would limit spam for a transaction that makes it into a block to ~10x (9 to 1). I don't see much of a downside to doing this beyond just the slight additional complexity of relay rules (and considering it could save substantial additional code complexity, even that is a benefit). 

All a node would need to do is keep a timestamp on each transaction they receive for when it was broadcasted and check it when a replacement comes in. If now-broadcastDate < cooldown, set a timer for cooldown - (now-broadcastDate) to broadcast it. If another replacement comes in, clear that timer and repeat using the original broadcast date (since the unbroadcast transaction doesn't have a broadcast date yet). 

I think it might also be useful to note that eliminating "extra data" caused by careless or malicious actors (spam or whatever you want to call it) should not be the goal. It is impossible to prevent all spam. What we should be aiming for is more specific: we should attempt to design a system where spam is manageable. Eg if our goal is to ensure that a bitcoin node uses no more than 10% of the bandwidth of a "normal" user, if current non-spam traffic only requires 1% of a "normal" users's bandwidth, then the network can bear a 9 to 1 ratio of spam. When a node spins up, there is a lot more data to download and process. So we know that all full nodes can handle at least as much traffic as they handle during IBD. What's the difference between those amounts? I'm not sure, but I would guess that IBD is at least a couple times more demanding than a fully synced node. So I might suggest that as long as spam can be kept below a ratio of maybe 2 to 1, we should consider the design acceptable (and therefore more complexity unnecessary). 

The 1 minute broadcast cooldown I mentioned before wouldn't be quite sufficient to achieve that ratio. But a 3.33 minute cooldown would be. Whether this is "too much" is something that would have to be discussed, I suspect a worst-case adversarial 3.33 minute delay would not be "too much". Doing this could basically eliminate any risk of actual service denial via replacement transactions. 

However, I do think that these DOS concerns are quite overblown. I wrote up a comment on your rbf-improvements.md detailing my thought process on that. The summary is that as long as the fee-rate relay rule is maintained, any "spam" is actually paid for, either by the "first" transaction in the spam chain, or by the "spam" itself. Even without something like a minimum RBF relay delay limiting how much spam could be created, the economics of the fee-rate rule already sufficiently mitigate the issue of spam. 
On Wed, Mar 9, 2022 at 9:37 AM Gloria Zhao via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
Hi RBF friends,

Posting a summary of RBF discussions at coredev (mostly on transaction relay rate-limiting), user-elected descendant limit as a short term solution to unblock package RBF, and mining score, all open for feedback:

One big concept discussed was baking DoS protection into the p2p level rather than policy level. TLDR: The fees are not paid to the node operator, but to the miner. While we can use fees to reason about the cost of an attack, if we're ultimately interested in preventing resource exhaustion, maybe we want to "stop the bleeding" when it happens and bound the amount of resources used in general. There were two main ideas:

1. Transaction relay rate limiting (i.e. the one you proposed above or some variation) with a feerate-based priority queue
2. Staggered broadcast of replacement transactions: within some time interval, maybe accept multiple replacements for the same prevout, but only relay the original transaction.

Looking to solicit feedback on these ideas and the concept in general. Is it a good idea (separate from RBF) to add rate-limiting in transaction relay? And is it the right direction to think about RBF DoS protection this way?

A lingering concern that I have about this idea is it would then be possible to impact the propagation of another person’s transaction, i.e., an attacker can censor somebody’s transaction from ever being announced by a node if they send enough transactions to fill up the rate limit. Obviously this would be expensive since they're spending a lot on fees, but I imagine it could be profitable in some situations to spend a few thousand dollars to prevent anyone from hearing about a transaction for a few hours. This might be a non-issue in practice if the rate limit is generous and traffic isn’t horrendous, but is this a problem?

And if we don't require an increase in (i.e. addition of "new") absolute fees, users are essentially allowed to “recycle” fees. In the scenario where we prioritize relay based on feerate, users could potentially be placed higher in the queue, ahead of other users’ transactions, multiple times, without ever adding more fees to the transaction. Again, maybe this isn’t a huge deal in practice if we set the parameters right, but it seems… not great, in principle.

---------

It's probably also a good idea to point out that there's been some discussion happening on the gist containing my original post on this thread (https://gist.github.com/glozow/25d9662c52453bd08b4b4b1d3783b9ff).

Suhas and Matt [proposed][0] adding a policy rule allowing users to specify descendant limits on their transactions. For example, some nth bit of nSequence with nVersion 3 means "this transaction won't have more than X vbytes of descendants" where X = max(1000, vsizeof(tx)) or something. It solves the pinning problem with package RBF where the attacker's package contains a very large and high-fee descendant.

We could add this policy and deploy it with package RBF/package relay so that LN can use it by setting the user-elected descendant limit flag on commitment transactions. (Otherwise package RBF is blocked until we find a more comprehensive solution to the pinning attack).

It's simple to [implement][1] as a mempool policy, but adds some complexity for wallets that use it, since it limits their use of UTXOs from transactions with this bit set.

---------

Also, coming back to the idea of "we can't just use {individual, ancestor} feerate," I'm interested in soliciting feedback on adding a “mining score” calculator. I've implemented one [here][2] which takes the transaction in question, grabs all of the connected mempool transactions (including siblings, coparents, etc., as they wouldn’t be in the ancestor nor descendant sets), and builds a “block template” using our current mining algorithm. The mining score of a transaction is the ancestor feerate at which it is included.

This would be helpful for something like ancestor-aware funding and fee-bumping in the wallet: [3], [4]. I think if we did the rate-limited priority queue for transaction relay, we'd want to use something like this as the priority value. And for RBF, we probably want to require that a replacement have a higher mining score than the original transactions. This could be computationally expensive to do all the time; it could be good to cache it but that could make mempool bookkeeping more complicated. Also, if we end up trying to switch to a candidate set-based algorithm for mining, we'd of course need a new calculator.

[0]: https://gist.github.com/glozow/25d9662c52453bd08b4b4b1d3783b9ff?permalink_comment_id=4058140#gistcomment-4058140
[1]: https://github.com/glozow/bitcoin/tree/2022-02-user-desclimit
[2] https://github.com/glozow/bitcoin/tree/2022-02-mining-score
[3]: https://github.com/bitcoin/bitcoin/issues/9645
[4]: https://github.com/bitcoin/bitcoin/issues/15553

Best,
Gloria

On Tue, Feb 8, 2022 at 4:58 AM Anthony Towns <aj@erisian.com.au> wrote:
On Mon, Feb 07, 2022 at 11:16:26AM +0000, Gloria Zhao wrote:
> @aj:
> > I wonder sometimes if it could be sufficient to just have a relay rate
> > limit and prioritise by ancestor feerate though. Maybe something like:
> > - instead of adding txs to each peers setInventoryTxToSend immediately,
> >   set a mempool flag "relayed=false"
> > - on a time delay, add the top N (by fee rate) "relayed=false" txs to
> >   each peer's setInventoryTxToSend and mark them as "relayed=true";
> >   calculate how much kB those txs were, and do this again after
> >   SIZE/RATELIMIT seconds

> > - don't include "relayed=false" txs when building blocks?

The "?" was me not being sure that point is a good suggestion...

Miners might reasonably decide to have no rate limit, and always relay,
and never exclude txs -- but the question then becomes is whether they
hear about the tx at all, so rate limiting behaviour could still be a
potential problem for whoever made the tx.

> Wow cool! I think outbound tx relay size-based rate-limiting and
> prioritizing tx relay by feerate are great ideas for preventing spammers
> from wasting bandwidth network-wide. I agree, this would slow the low
> feerate spam down, preventing a huge network-wide bandwidth spike. And it
> would allow high feerate transactions to propagate as they should,
> regardless of how busy traffic is. Combined with inbound tx request
> rate-limiting, might this be sufficient to prevent DoS regardless of the
> fee-based replacement policies?

I think you only want to do outbound rate limits, ie, how often you send
INV, GETDATA and TX messages? Once you receive any of those, I think
you have to immediately process / ignore it, you can't really sensibly
defer it (beyond the existing queues we have that just build up while
we're busy processing other things first)?

> One point that I'm not 100% clear on: is it ok to prioritize the
> transactions by ancestor feerate in this scheme? As I described in the
> original post, this can be quite different from the actual feerate we would
> consider a transaction in a block for. The transaction could have a high
> feerate sibling bumping its ancestor.
> For example, A (1sat/vB) has 2 children: B (49sat/vB) and C (5sat/vB). If
> we just received C, it would be incorrect to give it a priority equal to
> its ancestor feerate (3sat/vB) because if we constructed a block template
> now, B would bump A, and C's new ancestor feerate is 5sat/vB.
> Then, if we imagine that top N is >5sat/vB, we're not relaying C. If we
> also exclude C when building blocks, we're missing out on good fees.

I think you're right that this would be ugly. It's something of a
special case:

 a) you really care about C getting into the next block; but
 b) you're trusting B not being replaced by a higher fee tx that
    doesn't have A as a parent; and
 c) there's a lot of txs bidding the floor of the next block up to a
    level in-between the ancestor fee rate of 3sat/vB and the tx fee
    rate of 5sat/vB

Without (a), maybe you don't care about it getting to a miner quickly.
If your trust in (b) was misplaced, then your tx's effective fee rate
will drop and (because of (c)), you'll lose anyway. And if the spam ends
up outside of (c)'s range, either the rate limiting won't take effect
(spam's too cheap) and you'll be fine, or you'll miss out on the block
anyway (spam's paying more than your tx rate) and you never had any hope
of making it in.

Note that we already rate limit via INVENTORY_BROADCAST_MAX /
*_INVENTORY_BROADCAST_INTERVAL; which gets to something like 10,500 txs
per 10 minutes for outbound connections. This would be a weight based
rate limit instead-of/in-addition-to that, I guess.

As far as a non-ugly approach goes, I think you'd have to be smarter about
tracking the "effective fee rate" than the ancestor fee rate manages;
maybe that's something that could fall out of Murch and Clara's candidate
set blockbuilding ideas [0] ?

Perhaps that same work would also make it possible to come up with
a better answer to "do I care that this replacement would invalidate
these descendents?"

[0] https://github.com/Xekyo/blockbuilding

> > - keep high-feerate evicted txs around for a while in case they get
> >   mined by someone else to improve compact block relay, a la the
> >   orphan pool?
> Replaced transactions are already added to vExtraTxnForCompact :D

I guess I was thinking that it's just a 100 tx LRU cache, which might
not be good enough?

Maybe it would be more on point to have a rate limit apply only to
replacement transactions?

> For wallets, AJ's "All you need is for there to be *a* path that follows
> the new relay rules and gets from your node/wallet to perhaps 10% of
> hashpower" makes sense to me (which would be the former).

Perhaps a corollarly of that is that it's *better* to have the mempool
acceptance rule only consider economic incentives, and have the spam
prevention only be about "shall I tell my peers about this?"

If you don't have that split; then the anti-spam rules can prevent you
from getting the tx in the mempool at all; whereas if you do have the
split, then even if the bitcoind anti-spam rules are blocking you at
every turn, you can still send your tx to miners by some other route,
and then they can add it to their mempool directly without any hassle.

Cheers,
aj

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