bLIP-0029: zero-reserve channels

Nodes can opt-in to remove the channel reserve requirements, which makes better use of their channel liquidity.
lightning · Oct 24, 2023 · 4c59a11 · 4c59a11
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diff --git a/README.md b/README.md
@@ -25,3 +25,4 @@ For more detail on the process, please read [bLIP-0001](./blip-0001.md) and
 | [10](./blip-0010.md)     | Podcasting 2.0            | Satoshis Stream             | Active |
 | [11](./blip-0011.md)     | NameDesc                  | Hampus Sjöberg              | Active |
 | [17](./blip-0017.md)     | Hosted Channels           | Anton Kumaigorodskiy        | Active |
+| [xx](./blip-00xx.md)     | Zero-reserve channels     | Bastien Teinturier          | Active |
diff --git a/blip-0002.md b/blip-0002.md
@@ -32,8 +32,9 @@ network split.
 * [Messages](#messages)
 * [TLV fields in BOLT messages](#tlv-fields-in-bolt-messages)
   * [`init`](#init)
+  * [`open_channel2`](#open_channel2)
+  * [`accept_channel2`](#accept_channel2)
   * [`ping`](#ping)
-  * [`update_add_htlc`](#update_add_htlc)
 
 ### Feature bits
 
@@ -48,6 +49,7 @@ bLIPs may reserve feature bits by adding them to the following table:
 | ---------   | ---------------------- | ------------------------------------------------- | ---------------- | -------------------------------- | -------------------------------- |
 | 54/55       | `keysend`              | A form of spontaneous payment                     | N                | `var_onion_optin`                | [bLIP 3](./blip-0003.md)         |
 | 256/257     | `hosted_channels`      | This node accepts requests for hosted channels    | IN               |                                  | [bLIP 17](./blip-0017.md)        |
+| 258/259     | `zero_reserve`         | This node may accept zero-reserve channels        | IN               |                                  | [bLIP xx](./blip-00xx.md)        |
 
 ### Messages
 
@@ -92,6 +94,22 @@ The following table contains extension tlv fields for the `init` message:
 |-------|-----------------------------|--------------------------------|
 | 65536 | `tlv_field_name`            | Link to the corresponding bLIP |
 
+#### `open_channel2`
+
+The following table contains extension tlv fields for the `open_channel2` message:
+
+| Type  | Name                        | Link                      |
+|-------|-----------------------------|---------------------------|
+| 32768 | `zero_reserve`              | [bLIP xx](./blip-00xx.md) |
+
+#### `accept_channel2`
+
+The following table contains extension tlv fields for the `accept_channel2` message:
+
+| Type  | Name                        | Link                      |
+|-------|-----------------------------|---------------------------|
+| 32768 | `zero_reserve`              | [bLIP xx](./blip-00xx.md) |
+
 #### `payment_onion_payload`
 
 The following table contains extension tlv fields for the `payment_onion_payload` message:
@@ -101,7 +119,6 @@ The following table contains extension tlv fields for the `payment_onion_payload
 | 7629169     | `podcasting_2_0`            | [bLIP 10](./blip-0010.md)      |
 | 5482373484  | `keysend_preimage`          | [bLIP 3](./blip-0003.md)       |
 
-
 #### `ping`
 
 The following table contains extension tlv fields for the `ping` message:

diff --git a/blip-00xx.md b/blip-00xx.md
@@ -0,0 +1,132 @@
+```
+bLIP: xx
+Title: Zero-reserve channels
+Status: Active
+Author: Bastien Teinturier <bastien@acinq.fr>
+Created: 2023-10-24
+License: CC0
+```
+
+## Abstract
+
+Standard lightning channels require nodes to lock some of their channel funds
+into a channel reserve, which cannot be used for payments made on that channel.
+This guarantees that both nodes always have an output in the commitment
+transaction, which they will lose if they publish a revoked commitment.
+
+While this requirement is generally useful, it creates some inefficiencies
+since that liquidity can't be used to relay payments, and provides a bad user
+experience. In some settings, we may want to remove that channel reserve and
+allow nodes to use all of their channel funds.
+
+## Copyright
+
+This bLIP is licensed under the CC0 license.
+
+## Specification
+
+### TLV extensions
+
+Additional TLV fields for the `open_channel2` message:
+
+1. `tlv_stream`: `open_channel2_tlvs`
+2. types:
+    1. type: 32768 (`zero_reserve`)
+    2. data:
+        * [`byte`:`use_zero_reserve`]
+
+Additional TLV fields for the `accept_channel2` message:
+
+1. `tlv_stream`: `accept_channel2_tlvs`
+2. types:
+    1. type: 32768 (`zero_reserve`)
+    2. data:
+        * [`byte`:`use_zero_reserve`]
+
+### Requirements
+
+A node that wants to support zero-reserve channels:
+
+* MUST set the `zero_reserve` feature bit
+
+When sending `open_channel`:
+
+* If `zero_reserve` was negotiated:
+  * MAY set `channel_reserve_satoshis` to `0`
+
+When receiving `open_channel`:
+
+* If `channel_reserve_satoshis` is set to `0`:
+  * If it wants to use `zero_reserve`:
+    * MUST set `channel_reserve_satoshis` to `0` in `accept_channel`
+  * Otherwise:
+    * MUST send an `error` and forget the channel
+
+When sending `open_channel2`:
+
+* If `zero_reserve` was negotiated:
+  * MAY set the `zero_reserve` TLV field to `1`
+
+When receiving `open_channel2`:
+
+* If `zero_reserve` is set to `1`:
+  * If it wants to use `zero_reserve`:
+    * MUST set the `zero_reserve` TLV field to `1` in `accept_channel2`
+  * Otherwise:
+    * MUST send an `error` and forget the channel
+
+When sending or receiving `update_add_htlc`:
+
+* If `zero_reserve` has been negotiated:
+  * MUST ignore any channel reserve standard requirement
+
+If the channel is not public, both nodes:
+
+* When the funding transaction confirms:
+  * MUST send a `channel_update` using the final `short_channel_id`
+
+### Rationale
+
+The use of zero-reserve is symmetrical: it is either offered to both nodes or
+unused.
+
+### Fraud proofs
+
+If one of the nodes publishes a revoked commitment, the other node can create
+a fraud proof that shows which node tried to cheat. This proof may be shared
+publicly to harm the cheating node's reputation.
+
+That proof contains:
+
+1. the revoked commitment transaction
+2. a proof of knowledge of the revocation secret
+3. a proof of knowledge of the private key associated to the main output of the
+   honest participant
+4. if the channel is public, its `channel_announcement`
+5. if the channel is not public, a `channel_update` from the malicious peer
+   that uses the final `short_channel_id`
+
+The second and third items prove the identity of the honest user in that
+channel, while the last two items tie the identity of the malicious user to
+its public `node_id`.
+
+## Motivation
+
+In some cases, there may be some trust between nodes that the other node won't
+try to publish a revoked commitment: when that is the case, it is wasteful to
+enforce a channel reserve.
+
+In other cases, different incentives may be sufficient to remove the need for
+channel reserves.
+
+A mobile wallet using a service provider is a good candidate for removing the
+reserve requirements. The wallet user is regularly paying fees to the service
+provider: this incentivizes the service provider to offer zero-reserve, which
+provides a better user experience. The service provider isn't taking any risk
+here, as they should always be online and able to punish revoked transactions.
+It also makes sense for the wallet user to offer zero-reserve to the service
+provider: even on a mobile wallet, users should be able to react to revoked
+transactions. If the service provider publishes a revoked transaction, the
+wallet user can additionnally create a public proof that the service provider
+tried to cheat: this harms the service provider's reputation, which is another
+incentive for them to avoid cheating.