CLN interoperability: "unable to start peer: invalid remote features: missing feature dependency: 45"

[C-Otto]

Background

I'm running 115b041 which is a very recent commit in master (somewhere between 0.15.0 and 0.15.1). Somewhere after 0.15.0 the issue surfaced, which seems to be limited to CLN peers. As I'm still able to connect to "zero fee routing | CLN" (038fe1bd966b5cb0545963490c631eaa1924e2c4c0ea4e7dcb5d4582a1e7f2f1a5, @zerofeerouting), I guess that this issue is limited to some CLN versions.

Your environment

• version of lnd: 115b041
• which operating system (uname -a on *Nix): Linux server 5.10.0-13-amd64 #1 SMP Debian 5.10.106-1 (2022-03-17) x86_64 GNU/Linux
• version of btcd, bitcoind, or other backend: bitcoind v23
• any other relevant environment details: db.prune-revocation=true

Steps to reproduce

Connect to any of the following peers:

• 024271a1be2d7a3e2a276b241257be734d843885d252f50575e4c7db2691aedd3a - "running v.0.12"
• 027b0575bc6a9a6a371f306e01916b34522a3bc2e9ce7490ebfea6815465734e98
• 02fe6a27ddcb2dd9fa8479fe1d52549b5932079a493b51f1409fa2c5878f1bc07c - running "the development tip of cln"
• 03a465772d45616bf6c8450a69191db8f3cf8cca19ff92138735fd5f1d436fe4dc

Expected behaviour

Connection

Actual behaviour

[INF] PEER: Peer(PUBKEY): disconnecting PUBKEY@IP:PORT, reason: unable to start peer: invalid remote features: missing feature dependency: 45

[C-Otto]

With git bisect I get this commit introducing the issue:
a6c62eb

@Crypt-iQ any idea?

[C-Otto]

Looks like a CLN bug to me, see linked issue.

[positiveblue]

@C-Otto I think you are right, ScidAliasOptional (47) depends on ExplicitChannelTypeOptional (45) and ZeroConfOptional (51) on ScidAliasOptional (47).

[C-Otto]

According to https://github.com/lightning/bolts/blob/master/09-features.md 46/47 does not have any dependency.

[positiveblue]

That's true. It looks like the nodes would be able to negotiate if the channel that they are opening is zero conf or not based on the mindepth and not the channel_type @Crypt-iQ

[C-Otto]

See comments for deps.go in #5955 (I can't link to them)

[Crypt-iQ]

The problem here is the feature bit 47 doesn't depend on the feature bit 45, but the channel type 47 that requires feature bit 47 does depend on channel type 45 which requires feature bit 45

[C-Otto]

So lnd shouldn't include the 47->45 dependency for connections, just for new channels?

[Crypt-iQ]

I think the dependency can be removed and everything should just work

[C-Otto]

Without the dependency I'm now able to connect to 024271a1be2d7a3e2a276b241257be734d843885d252f50575e4c7db2691aedd3a (and others).

[Roasbeef]

The above PR should resolve things for now. However, we always want to use the explicit channel type stuff for zero conf negotiation. As a result, if the other peer doesn't support it, we'll fail out on the RPC level. Looks like CLN will start to set the feature bit now (given they actually understand/implement it): ElementsProject/lightning#5455. With that in, everything "should just work" now.