Add anchor outputs announcement blog post (#214)

Author: wpaulino

docs/_blog/anchor-outputs-channels-are-here.md

@@ -0,0 +1,57 @@
+---
+title: "Anchor Outputs Channels Are Here"
+description: "LDK v0.0.116 added support for opening channels with anchor outputs."
+date: "2023-08-01"
+authors:
+  - Wilmer Paulino
+tags:
+  - Channels
+  - Anchor Outputs
+  - Fees
+---
+
+`v0.0.116` of LDK with added support for opening channels with [anchor
+outputs](https://bitcoinops.org/en/topics/anchor-outputs) is live. This much-requested feature
+brings multiple security and usability improvements to Lightning channels.
+
+Lightning channels rely on pre-signed transactions that participants broadcast to the network if
+they wish to close a channel unilaterally, e.g., when their counterparty is offline. Before
+introducing anchor outputs to the Lightning protocol, participants continually negotiated their
+commitment transaction’s fees based on the current block-space demand. This fee negotiation
+unfortunately came with its own set of problems. If participants disagreed on the proposed fee
+range, a unilateral close made the channel unusable. Underestimating the required fee could result
+in loss of funds if an HTLC needed to be resolved on-chain, and overestimating would misallocate the
+channel balance towards fees, potentially rendering the channel useless.
+
+With anchor outputs, participants can now contribute the majority of fees required at the time of
+broadcast by bumping the commitment transaction's fee via a child transaction using the
+[Child-Pays-For-Parent (CPFP)](https://bitcoinops.org/en/topics/cpfp) fee-bumping mechanism. A small
+portion of fees must still be allocated to commitment transactions to ensure they can enter nodes'
+mempools independently. This will be required until [package
+relay](https://bitcoinops.org/en/topics/package-relay) is deployed network-wide. At that point, we
+can have a fixed 1 sat/vB commitment transaction that is likely to do away with the fee negotiation
+once and for all, eliminating the most common cause of unilateral closes that we see today.
+
+Given LDK's modularized design, support for anchor outputs posed a few challenges. As a
+Lightning-only library, we do not include an on-chain wallet as a traditional node implementation
+would, requiring manual integration work by the application developer. However, we were able to
+leverage our existing [event-based
+API](https://docs.rs/lightning/latest/lightning/events/index.html) to communicate that a commitment
+or HTLC transaction's fee needs to be bumped. The handling of these events is often non-trivial, so
+we added a utility
+([`BumpTransactionEventHandler`](https://docs.rs/lightning/latest/lightning/events/bump_transaction/struct.BumpTransactionEventHandler.html))
+to simplify required integration work. With this utility, an application developer only needs to
+provide a view into their on-chain wallet for LDK to access confirmed UTXOs and sign them.
+
+Note that node operators need to maintain a reserve of confirmed funds if they wish to exit a
+channel unilaterally. LDK will not ensure this reserve is enforced, but we’re exploring [possible
+ways to do so](https://github.com/lightningdevkit/rust-lightning/issues/2320). Without such
+enforcement, a node may open or accept more channels than its provisioned reserve allows it to
+handle, potentially resulting in a loss of funds if any HTLCs need to be resolved on-chain. In the
+meantime, application developers must determine whether their use case warrants such enforcement,
+then implement it themselves. For example, a mobile user connected to a LSP could always defer to
+the LSP to broadcast the latest state so that the user wouldn’t need to maintain a reserve.
+
+While the feature is now available, we still consider it experimental. We're always seeking feedback
+on how to improve our API and provide a better developer experience.
+