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From: Antoine Riard <antoine.riard@gmail.com>
Date: Thu, 10 Jun 2021 17:45:04 -0400
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To: Lloyd Fournier <lloyd.fourn@gmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] A Stroll through Fee-Bumping Techniques :
 Input-Based vs Child-Pay-For-Parent
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Hi Lloyd,

Thanks for this tx mutation proposal extending the scope of fee-bumping
techniques. IIUC, the <output_index> serves as a pointer to increase the
output amount by value to recover the recompute the transaction hash
against which the original signature is valid ?

Let's do a quick analysis of this scheme.
* onchain footprint : one tapleaf per contract participant, with O(log n)
increase of witness size, also one output per contract participant
* tx-relay bandwidth rebroadcast : assuming aforementioned in-place mempool
substitution policy, the mutated transaction
* batching : fee-bumping value is extract from contract transaction itself,
so O(n) per contract
* mempool flexibility : the mutated transaction
* watchtower key management : to enable outsourcing, the mutating key must
be shared, in theory enabling contract value siphoning to miner fees ?

Further, I think tx mutation scheme can be achieved in another way, with
SIGHASH_ANYAMOUNT. A contract participant tapscript will be the following :

<contract_key> <finalizing_alice_key>

Where <contract_signature> is committed with SIGHASH_ANYAMOUNT, blanking
nValue of one or more outputs. That way, the fee-to-contract-value
distribution can be unilaterally finalized at a later time through the
finalizing key [0].

Note, I think that the tx mutation proposal relies on interactivity in the
worst-case scenario where a counterparty wants to increase its fee-bumping
output from the contract balance. This interactivity may lure a
counterparty to alway lock the worst-case fee-bumping reserve in the
output. I believe anchor output enables more "real-time" fee-bumping
reserve adjustment ?

Cheers,
Antoine

[0] Incautious sighash alleability is unsafe. Be careful, otherwise kitties
will perish by the thousands :
https://github.com/revault/practical-revault/pull/83

Le dim. 6 juin 2021 =C3=A0 22:28, Lloyd Fournier <lloyd.fourn@gmail.com> a
=C3=A9crit :

> Hi Antione,
>
> Thanks for bringing up this important topic. I think there might be
> another class of solutions over input based, CPFP and sponsorship. I'll
> call them tx mutation schemes. The idea is that you can set a key that ca=
n
> increase the fee by lowering a particular output after the tx is signed
> without invalidating the signature. The premise is that anytime you need =
to
> bump the fee of a transaction you must necessarily have funds in an outpu=
t
> that are going to you and therefore you can sacrifice some of them to
> increase the fee. This is obviously destructive to txids so child presign=
ed
> transactions will have to use ANYPREVOUT as in your proposal. The advanta=
ge
> is that it does not require keeping extra inputs around to bump the fee.
>
> So imagine a new opcode OP_CHECKSIG_MUTATED <output index> <publickey>
> <value> <signature>.
> This would check that <signature> is valid against <publickey> if the
> current transaction had the output at <output index> reduced by <value>. =
To
> make this more efficient, if the public key is one byte: 0x02 it referenc=
es
> the taproot *external key* (similar to how ANYPREVOUT uses 0x01 to refer =
to
> internal key[1]).
> Now for our protocol we want both parties (p1 and p2) to be able to fee
> bump a commitment transaction. They use MuSig to sign the commitment tx
> under the external key with a decent fee for the current conditions. But =
in
> case it proves insufficient they have added the following two leaves to
> their key in the funding output as a backup so that p1 and p2 can
> unilaterally bump the fee of anything they sign spending from the funding
> output:
>
> 1. OP_CHECKSIG_MUTATED(0, 0x02, <fee-bump-value>, <original-signature>)
> OP_CHECKSIGADD(p1-fee-bump-key, <p1-fee-bump-signature>)  OP_2
> OP_NUMEQUALVERIFY
> 2. OP_CHECKSIG_MUTATED(1, 0x02, <fee-bump-value>, <original-signature>)
> OP_CHECKSIGADD(p2-fee-bump-key, <p2-fee-bump-signature>) OP_2
> OP_NUMEQUALVERIFY
>
> where <...> indicates the thing comes from the witness stack.
> So to bump the fee of the commit tx after it has been signed either party
> takes the <original-signature> and adds a signature under their
> fee-bump-key for the new tx and reveals their fee bump leaf.
> <original-signature> is checked against the old transaction while the fee
> bumped transaction is checked against the fee bump key.
>
> I know I have left out how to change mempool eviction rules to accommodat=
e
> this kind of fee bumping without DoS or pinning attacks but hopefully I
> have demonstrated that this class of solutions also exists.
>
> [1] https://github.com/ajtowns/bips/blob/bip-anyprevout/bip-0118.mediawik=
i
>
> Cheers,
>
> LL
>
>
>
> On Fri, 28 May 2021 at 07:13, Antoine Riard via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> Hi,
>>
>> This post is pursuing a wider discussion around better fee-bumping
>> strategies for second-layer protocols. It draws out a comparison between
>> input-based and CPFP fee-bumping techniques, and their apparent trade-of=
fs
>> in terms of onchain footprint, tx-relay bandwidth rebroadcast, batching
>> opportunity and mempool flexibility.
>>
>> Thanks to Darosior for reviews, ideas and discussions.
>>
>> ## Child-Pay-For-Parent
>>
>> CPFP is a mature fee-bumping technique, known and used for a while in th=
e
>> Bitcoin ecosystem. However, its usage in contract protocols with
>> distrusting counterparties raised some security issues. As mempool's cha=
in
>> of unconfirmed transactions are limited in size, if any output is spenda=
ble
>> by any contract participant, it can be leveraged as a pinning vector to
>> downgrade odds of transaction confirmation [0].
>>
>> That said, contract transactions interested to be protected under the
>> carve-out logic require to add a new output for any contract participant=
,
>> even if ultimately only one of them serves as an anchor to attach a CPFP=
.
>>
>> ## Input-Based
>>
>> I think input-based fee-bumping has been less studied as fee-bumping
>> primitive for L2s [1]. One variant of input-based fee-bumping usable tod=
ay
>> is the leverage of the SIGHASH_ANYONECANPAY/SIGHASH_SINGLE malleability
>> flags. If the transaction is the latest stage of the contract, a bumping
>> input can be attached just-in-time, thus increasing the feerate of the
>> whole package.
>>
>> However, as of today, input-based fee-bumping doesn't work to bump first
>> stages of contract transactions as it's destructive of the txid, and as
>> such breaks chain of pre-signed transactions. A first improvement would =
be
>> the deployment of the SIGHASH_ANYPREVOUT softfork proposal. This new
>> malleability flag allows a transaction to be signed without reference to
>> any specific previous output. That way,  spent transactions can be
>> fee-bumped without altering validity of the chain of transactions.
>>
>> Even assuming SIGHASH_ANYPREVOUT, if the first stage contract transactio=
n
>> includes multiple outputs (e.g the LN's commitment tx has multiple HTLC
>> outputs), SIGHASH_SINGLE can't be used and the fee-bumping input value
>> might be wasted. This edge can be smoothed by broadcasting a preliminary
>> fan-out transaction with a set of outputs providing a range of feerate
>> points for the bumped transaction.
>>
>> This overhead could be smoothed even further in the future with more
>> advanced sighash malleability flags like SIGHASH_IOMAP, allowing
>> transaction signers to commit to a map of inputs/outputs [2]. In the
>> context of input-based, the overflowed fee value could be redirected to =
an
>> outgoing output.
>>
>> ## Onchain Footprint
>>
>> CPFP: One anchor output per participant must be included in the
>> commitment transaction. To this anchor must be attached a child transact=
ion
>> with 2 inputs (one for the commitment, one for the bumping utxo) and 1
>> output. Onchain footprint: 2 inputs + 3 outputs.
>>
>> Input-based (today): If the bumping utxo is offering an adequate feerate
>> point in function of network mempools congestion at time of broadcast, o=
nly
>> 1 input. If a preliminary fan-out transaction to adjust feerate point mu=
st
>> be broadcasted first, 1 input and 2 outputs more must be accounted for.
>> Onchain footprint: 2 inputs + 3 outputs.
>>
>> Input-based (SIGHASH_ANYPREVOUT+SIGHASH_IOMAP): As long as the bumping
>> utxo's value is wide enough to cover the worst-case of mempools congesti=
on,
>> the bumped transaction can be attached 1 input and 1 output. Onchain
>> footprint: 1 input + 1 output.
>>
>> ## Tx-Relay Bandwidth Rebroadcast
>>
>> CPFP: In the context of multi-party protocols, we should assume bounded
>> rationality of the participants w.r.t to an unconfirmed spend of the
>> contract utxo across network mempools. Under this assumption, the bumped
>> transaction might have been replaced by a concurrent state. To guarantee
>> efficiency of the CPFP the whole chain of transactions should be
>> rebroadcast, perhaps wasting bandwidth consumption for a still-identical
>> bumped transaction [3]. Rebroadcast footprint: the whole chain of
>> transactions.
>>
>> Input-based (today): In case of rebroadcast, the fee-bumping input is
>> attached to the root of the chain of transactions and as such breaks the
>> chain validity in itself. Beyond the rebroadcast of the updated root und=
er
>> replacement policy, the remaining transactions must be updated and
>> rebroadcast. Rebroadcast footprint: the whole chain of transactions.
>>
>> Input-based(SIGHASH_ANYPREVOUT+SIGHASH_IOMAP): In case of rebroadcast,
>> the fee-bumping is attached to the root of the chain of transactions but=
 it
>> doesn't break the chain validity in itself. Assuming a future mempool
>> acceptance logic to authorize in-place substitution, the rest of the cha=
in
>> could be preserved. Rebroadcast footprint: the root of the chain of
>> transactions.
>>
>> ## Fee-Bumping Batching
>>
>> CPFP: In the context of multi-party protocols, in optimistic scenarios,
>> we can assume aggregation of multiple chains of transactions. For e.g, a=
 LN
>> operator is desirous to non-cooperatively close multiple channels at the
>> same time and would like to combine their fee-bumping. With CPFP, one
>> anchor output and one bumping input must be consumed per aggregated chai=
n,
>> even if the child transaction fields can be shared. Batching perf: 1
>> input/1 output per aggregated chain.
>>
>> Input-based (today): Unless the contract allows interactivity, multiple
>> chains of transactions cannot be aggregated. One bumping input must be
>> attached per chain, though if a preliminary fan-out transaction is relie=
d
>> on to offer multiple feerate points, transaction fields can be shared.
>> Batching perf: 1 input/1 output per aggregated chain.
>>
>> Input-based (SIGHASH_ANYPREVOUT+SIGHASH_IOMAP): Multiple chains of
>> transactions might be aggregated together *non-interactively*. One bumpi=
ng
>> input and outgoing output can be attached to the aggregated root. Batchi=
ng
>> perf: 1 input/1 output per aggregation.
>>
>> ## Fee-Bumping Mempool Flexibility
>>
>> CPFP: In the context of multi-party protocols, one of your counterpartie=
s
>> might build a branch of transactions from one of the root outputs thus
>> saturating the in-mempool package limits. To avoid these shenanigans, LN
>> channels are relying on the carve-out mechanism. Though, the carve-out
>> mechanism includes its own limitation and doesn't scale beyond 2 contrac=
t
>> participants.
>>
>> Input-based: The root of the chain of transaction is the package's oldes=
t
>> ancestor, so package limits don't restrain its acceptance and it works
>> whatever the number of contract participants.
>>
>> To conclude, this post scores 2 fee-bumping primitives for multi-party
>> protocols on a range of factors. It hopes to unravel the ground for a re=
al
>> feerate performance framework of second-layers protocols .
>>
>> Beyond that, few points can be highlighted a) future soft forks allow
>> significant onchain footprint savings, especially in case of batching, b=
)
>> future package relay bandwidth efficiency should account for rebroadcast
>> frequency of CPFPing multi-party protocols. On this latter point one
>> follow-up might be to evaluate differing package relay *announcement*
>> schemes in function of odds of non-cooperative protocol broadcast/odds o=
f
>> concurrent broadcast/rebroadcast frequencies.
>>
>> Thoughts ?
>>
>> Cheers,
>> Antoine
>>
>> [0]
>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-November/01=
6518.html
>> [1] Beyond the revault architecture :
>> https://github.com/revault/practical-revault/blob/master/revault.pdf
>> [2] Already proposed a while back :
>> https://bitcointalk.org/index.php?topic=3D252960.0
>> [3] In theory, an already-relayed transaction shouldn't pass Core's
>> `filterInventoryKnown`. In practice, if the transaction is announced as
>> part of a package_id, the child might have changed, not the parent, lead=
ing
>> to a redundant relay of the latter.
>> _______________________________________________
>> bitcoin-dev mailing list
>> bitcoin-dev@lists.linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
>

--00000000000081d15b05c4704ead
Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

<div dir=3D"ltr">Hi Lloyd,<br><br>Thanks for this tx mutation proposal exte=
nding the scope of fee-bumping techniques. IIUC, the &lt;output_index&gt; s=
erves as a pointer to increase the output amount by value to recover the re=
compute the transaction hash against which the original signature is valid =
?<br><br>Let&#39;s do a quick analysis of this scheme.<br>* onchain footpri=
nt : one tapleaf per contract participant, with O(log n) increase of witnes=
s size, also one output per contract participant<br>* tx-relay bandwidth re=
broadcast : assuming aforementioned in-place mempool substitution policy, t=
he mutated transaction<br>* batching : fee-bumping value is extract from co=
ntract transaction itself, so O(n) per contract<br>* mempool flexibility : =
the mutated transaction<br>* watchtower key management : to enable outsourc=
ing, the mutating key must be shared, in theory enabling contract value sip=
honing to miner fees ?<br><br>Further, I think tx mutation scheme can be ac=
hieved in another way, with SIGHASH_ANYAMOUNT. A contract participant tapsc=
ript will be the following :<br><br>&lt;contract_key&gt; &lt;finalizing_ali=
ce_key&gt;<br><br>Where &lt;contract_signature&gt; is committed with SIGHAS=
H_ANYAMOUNT, blanking nValue of one or more outputs. That way, the fee-to-c=
ontract-value distribution can be unilaterally finalized at a later time th=
rough the finalizing key [0].<br><br>Note, I think that the tx mutation pro=
posal relies on interactivity in the worst-case scenario where a counterpar=
ty wants to increase its fee-bumping output from the contract balance. This=
 interactivity may lure a counterparty to alway lock the worst-case fee-bum=
ping reserve in the output. I believe anchor output enables more &quot;real=
-time&quot; fee-bumping reserve adjustment ?<br><br>Cheers,<br>Antoine<br><=
br>[0] Incautious sighash alleability is unsafe. Be careful, otherwise kitt=
ies will perish by the thousands :<br><a href=3D"https://github.com/revault=
/practical-revault/pull/83">https://github.com/revault/practical-revault/pu=
ll/83</a><br></div><br><div class=3D"gmail_quote"><div dir=3D"ltr" class=3D=
"gmail_attr">Le=C2=A0dim. 6 juin 2021 =C3=A0=C2=A022:28, Lloyd Fournier &lt=
;<a href=3D"mailto:lloyd.fourn@gmail.com">lloyd.fourn@gmail.com</a>&gt; a =
=C3=A9crit=C2=A0:<br></div><blockquote class=3D"gmail_quote" style=3D"margi=
n:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex=
"><div dir=3D"ltr"><div>Hi Antione,</div><div><br></div><div>Thanks for bri=
nging up this important topic. I think there might be another class of solu=
tions over input based, CPFP and sponsorship. I&#39;ll call them tx mutatio=
n schemes. The idea is that you can set a key that can increase the fee by =
lowering a particular output after the tx is signed without invalidating th=
e signature.=20
 The premise is that anytime you need to bump the fee of a transaction you =
must necessarily have funds in an output that are going to you and therefor=
e you can sacrifice some of them to increase the fee. This is obviously des=
tructive to txids so child presigned transactions will have to use ANYPREVO=
UT as in your proposal. The advantage is that it does not require keeping e=
xtra inputs around to bump the fee.<br></div><div><br></div><div>So imagine=
 a new opcode OP_CHECKSIG_MUTATED &lt;output index&gt;=20
&lt;publickey&gt; &lt;value&gt;  &lt;signature&gt;.</div><div>This would ch=
eck that &lt;signature&gt; is valid against &lt;publickey&gt; if the curren=
t transaction had the output at &lt;output index&gt; reduced by &lt;value&g=
t;. To make this more efficient, if the public key is one byte: 0x02 it ref=
erences the taproot *external key* (similar to how ANYPREVOUT uses 0x01 to =
refer to internal key[1]).<br></div><div>Now for our protocol we want both =
parties (p1 and p2) to be able to fee bump a commitment transaction. They u=
se MuSig to sign the commitment tx under the external key with a decent fee=
 for the current conditions. But in case it proves insufficient they have a=
dded the following two leaves to their key in the funding output as a backu=
p so that p1 and p2 can unilaterally bump the fee of anything they sign spe=
nding from the funding output:<br></div><div><br></div><div>1. OP_CHECKSIG_=
MUTATED(0, 0x02, &lt;fee-bump-value&gt;,=20

&lt;original-signature&gt;) OP_CHECKSIGADD(p1-fee-bump-key, &lt;p1-fee-bump=
-signature&gt;)=C2=A0
OP_2 OP_NUMEQUALVERIFY

</div><div>2.=20
OP_CHECKSIG_MUTATED(1, 0x02, &lt;fee-bump-value&gt;,=20

&lt;original-signature&gt;) OP_CHECKSIGADD(p2-fee-bump-key, &lt;p2-fee-bump=
-signature&gt;) OP_2 OP_NUMEQUALVERIFY</div><div><br></div><div>where &lt;.=
..&gt; indicates the thing comes from the witness stack.</div><div>So to bu=
mp the fee of the commit tx after it has been signed either party takes the=
 &lt;original-signature&gt; and adds a signature under their fee-bump-key f=
or the new tx and reveals their fee bump leaf. &lt;original-signature&gt; i=
s checked against the old transaction while the fee bumped transaction is c=
hecked against the fee bump key.<br></div><div><br></div><div>
I know I have left out how to change mempool eviction rules to accommodate =
this kind of fee bumping without DoS or pinning attacks but hopefully I hav=
e demonstrated that this class of solutions also exists.<br></div><div><br>=
</div><div>[1] <a href=3D"https://github.com/ajtowns/bips/blob/bip-anyprevo=
ut/bip-0118.mediawiki" target=3D"_blank">https://github.com/ajtowns/bips/bl=
ob/bip-anyprevout/bip-0118.mediawiki</a></div><div><br></div><div>Cheers,</=
div><div><br></div><div>LL<br></div><div><br></div><div><br></div></div><br=
><div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr">On Fri, 2=
8 May 2021 at 07:13, Antoine Riard via bitcoin-dev &lt;<a href=3D"mailto:bi=
tcoin-dev@lists.linuxfoundation.org" target=3D"_blank">bitcoin-dev@lists.li=
nuxfoundation.org</a>&gt; wrote:<br></div><blockquote class=3D"gmail_quote"=
 style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);p=
adding-left:1ex"><div dir=3D"ltr">Hi,<br><br>This post is pursuing a wider =
discussion around better fee-bumping strategies for second-layer protocols.=
 It draws out a comparison between input-based and CPFP fee-bumping techniq=
ues, and their apparent trade-offs in terms of onchain footprint, tx-relay =
bandwidth rebroadcast, batching opportunity and mempool flexibility.<br><br=
>Thanks to Darosior for reviews, ideas and discussions.<br><br>## Child-Pay=
-For-Parent<br><br>CPFP is a mature fee-bumping technique, known and used f=
or a while in the Bitcoin ecosystem. However, its usage in contract protoco=
ls with distrusting counterparties raised some security issues. As mempool&=
#39;s chain of unconfirmed transactions are limited in size, if any output =
is spendable by any contract participant, it can be leveraged as a pinning =
vector to downgrade odds of transaction confirmation [0].<br><br>That said,=
 contract transactions interested to be protected under the carve-out logic=
 require to add a new output for any contract participant, even if ultimate=
ly only one of them serves as an anchor to attach a CPFP.<br><br>## Input-B=
ased<br><br>I think input-based fee-bumping has been less studied as fee-bu=
mping primitive for L2s [1]. One variant of input-based fee-bumping usable =
today is the leverage of the SIGHASH_ANYONECANPAY/SIGHASH_SINGLE malleabili=
ty flags. If the transaction is the latest stage of the contract, a bumping=
 input can be attached just-in-time, thus increasing the feerate of the who=
le package.<br><br>However, as of today, input-based fee-bumping doesn&#39;=
t work to bump first stages of contract transactions as it&#39;s destructiv=
e of the txid, and as such breaks chain of pre-signed transactions. A first=
 improvement would be the deployment of the SIGHASH_ANYPREVOUT softfork pro=
posal. This new malleability flag allows a transaction to be signed without=
 reference to any specific previous output. That way,=C2=A0 spent transacti=
ons can be fee-bumped without altering validity of the chain of transaction=
s.<br><br>Even assuming SIGHASH_ANYPREVOUT, if the first stage contract tra=
nsaction includes multiple outputs (e.g the LN&#39;s commitment tx has mult=
iple HTLC outputs), SIGHASH_SINGLE can&#39;t be used and the fee-bumping in=
put value might be wasted. This edge can be smoothed by broadcasting a prel=
iminary fan-out transaction with a set of outputs providing a range of feer=
ate points for the bumped transaction.<br><br>This overhead could be smooth=
ed even further in the future with more advanced sighash malleability flags=
 like SIGHASH_IOMAP, allowing transaction signers to commit to a map of inp=
uts/outputs [2]. In the context of input-based, the overflowed fee value co=
uld be redirected to an outgoing output.<br><br>## Onchain Footprint<br><br=
>CPFP: One anchor output per participant must be included in the commitment=
 transaction. To this anchor must be attached a child transaction with 2 in=
puts (one for the commitment, one for the bumping utxo) and 1 output. Oncha=
in footprint: 2 inputs + 3 outputs.<br><br>Input-based (today): If the bump=
ing utxo is offering an adequate feerate point in function of network mempo=
ols congestion at time of broadcast, only 1 input. If a preliminary fan-out=
 transaction to adjust feerate point must be broadcasted first, 1 input and=
 2 outputs more must be accounted for. Onchain footprint: 2 inputs + 3 outp=
uts.<br><br>Input-based (SIGHASH_ANYPREVOUT+SIGHASH_IOMAP): As long as the =
bumping utxo&#39;s value is wide enough to cover the worst-case of mempools=
 congestion, the bumped transaction can be attached 1 input and 1 output. O=
nchain footprint: 1 input + 1 output.<br><br>## Tx-Relay Bandwidth Rebroadc=
ast<br><br>CPFP: In the context of multi-party protocols, we should assume =
bounded rationality of the participants w.r.t to an unconfirmed spend of th=
e contract utxo across network mempools. Under this assumption, the bumped =
transaction might have been replaced by a concurrent state. To guarantee ef=
ficiency of the CPFP the whole chain of transactions should be rebroadcast,=
 perhaps wasting bandwidth consumption for a still-identical bumped transac=
tion [3]. Rebroadcast footprint: the whole chain of transactions.<br><br>In=
put-based (today): In case of rebroadcast, the fee-bumping input is attache=
d to the root of the chain of transactions and as such breaks the chain val=
idity in itself. Beyond the rebroadcast of the updated root under replaceme=
nt policy, the remaining transactions must be updated and rebroadcast. Rebr=
oadcast footprint: the whole chain of transactions.<br><br>Input-based(SIGH=
ASH_ANYPREVOUT+SIGHASH_IOMAP): In case of rebroadcast, the fee-bumping is a=
ttached to the root of the chain of transactions but it doesn&#39;t break t=
he chain validity in itself. Assuming a future mempool acceptance logic to =
authorize in-place substitution, the rest of the chain could be preserved. =
Rebroadcast footprint: the root of the chain of transactions.<br><br>## Fee=
-Bumping Batching<br><br>CPFP: In the context of multi-party protocols, in =
optimistic scenarios, we can assume aggregation of multiple chains of trans=
actions. For e.g, a LN operator is desirous to non-cooperatively close mult=
iple channels at the same time and would like to combine their fee-bumping.=
 With CPFP, one anchor output and one bumping input must be consumed per ag=
gregated chain, even if the child transaction fields can be shared. Batchin=
g perf: 1 input/1 output per aggregated chain.<br><br>Input-based (today): =
Unless the contract allows interactivity, multiple chains of transactions c=
annot be aggregated. One bumping input must be attached per chain, though i=
f a preliminary fan-out transaction is relied on to offer multiple feerate =
points, transaction fields can be shared. Batching perf: 1 input/1 output p=
er aggregated chain.<br><br>Input-based (SIGHASH_ANYPREVOUT+SIGHASH_IOMAP):=
 Multiple chains of transactions might be aggregated together *non-interact=
ively*. One bumping input and outgoing output can be attached to the aggreg=
ated root. Batching perf: 1 input/1 output per aggregation.<br><br>## Fee-B=
umping Mempool Flexibility<br><br>CPFP: In the context of multi-party proto=
cols, one of your counterparties might build a branch of transactions from =
one of the root outputs thus saturating the in-mempool package limits. To a=
void these shenanigans, LN channels are relying on the carve-out mechanism.=
 Though, the carve-out mechanism includes its own limitation and doesn&#39;=
t scale beyond 2 contract participants.<br><br>Input-based: The root of the=
 chain of transaction is the package&#39;s oldest ancestor, so package limi=
ts don&#39;t restrain its acceptance and it works whatever the number of co=
ntract participants.<br><br>To conclude, this post scores 2 fee-bumping pri=
mitives for multi-party protocols on a range of factors. It hopes to unrave=
l the ground for a real feerate performance framework of second-layers prot=
ocols .<br><br>Beyond that, few points can be highlighted a) future soft fo=
rks allow significant onchain footprint savings, especially in case of batc=
hing, b) future package relay bandwidth efficiency should account for rebro=
adcast frequency of CPFPing multi-party protocols. On this latter point one=
 follow-up might be to evaluate differing package relay *announcement* sche=
mes in function of odds of non-cooperative protocol broadcast/odds of concu=
rrent broadcast/rebroadcast frequencies.<br><br>Thoughts ?<br><br>Cheers,<b=
r>Antoine<br><br>[0] <a href=3D"https://lists.linuxfoundation.org/pipermail=
/bitcoin-dev/2018-November/016518.html" target=3D"_blank">https://lists.lin=
uxfoundation.org/pipermail/bitcoin-dev/2018-November/016518.html</a><br>[1]=
 Beyond the revault architecture : <a href=3D"https://github.com/revault/pr=
actical-revault/blob/master/revault.pdf" target=3D"_blank">https://github.c=
om/revault/practical-revault/blob/master/revault.pdf</a><br>[2] Already pro=
posed a while back : <a href=3D"https://bitcointalk.org/index.php?topic=3D2=
52960.0" target=3D"_blank">https://bitcointalk.org/index.php?topic=3D252960=
.0</a><br>[3] In theory, an already-relayed transaction shouldn&#39;t pass =
Core&#39;s `filterInventoryKnown`. In practice, if the transaction is annou=
nced as part of a package_id, the child might have changed, not the parent,=
 leading to a redundant relay of the latter.<br></div>
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</blockquote></div>
</blockquote></div>

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