Circuit Breaker

circuitbreaker is to Lightning what firewalls are to the internet.

It allows nodes to protect themselves from being flooded with htlcs. With circuitbreaker a maximum to the number of in-flight htlcs can be set on a per-peer basis. Known and trusted peers for example can be assigned a higher maximum, while a new channel from a previously unseen node may be limited to only a few pending htlcs.

Furthermore it is possible to apply rate limits to the number of forwarded htlcs. This offers protection against DoS/spam attacks that rely on large numbers of fast-resolving htlcs. Rate limiting is implemented with a Token bucket. In configuration the minimum interval between htlcs and a burst size can be specified.

Large numbers of htlcs are also required for probing channel balances. Reducing the information leakage through probing could be another reason to put in place a rate limit for untrusted peers.

Why are limits needed?

In today's Lighting Network payments are routed via a series of hops. Each of those hops will incur a cost for forwarding that payment. While the htlc of an hop is in-flight, the associated amount is locked in the hop's outgoing channel. Those funds cannot be used for another purpose. This can be considered to be an opportunity cost.

Furthermore each channel has a limited number of htlc 'slots'. The current maximum is 483 slots. This means that regardless of channel capacity, there can never be more than 483 htlcs pending. With large channels in particular, it can happen that all slots are occupied while only a fraction of the channel capacity is used. In that case the whole channel is considered to be locked. The duration of the lock can vary from a few seconds to as long as 2 weeks or even more.

When the payment is completed successfully, each hop will collect a routing fee. But depending on the length of the lock and the htlc amounts, this may be far from sufficient to cover the costs.

This is where circuitbreaker comes in. It puts up a defense around that valuable channel liquidity and helps to keep the locked coins at work to maximize routing revenue.

Hold fees

An alternative to lowering limits is to charge peers for the actual costs that they generate in both the success and failure cases. For more information about this idea, see thread Hold fees: 402 Payment Required for Lightning itself on the lightning-dev mailing list.

Circuit Breaker does not support 'breaking the circuits' when hold fees aren't paid, but this is a potential direction for the future. It would roughly entail requiring peers to deposit money for hold fees and blocking forwards once the peer's balance is zero.

What is currently implemented is only the reporting of these (virtual) hold fees. A fee schedule can be defined (see configuration below) and the hold fee that could have been charged is logged for every forward. Additionally a periodic report is printed that contains the hold fees charged to peers during the reporting period. Peers that did not offer any htlcs in that period will be omitted.

When reportingInterval is not set, no hold fee reporting will take place.

2020-10-17T20:45:15.708+0200	INFO	Forwarding htlc	{"channel": 39778131669745664, "htlc": 52, "peer_alias": "tester", "peer": "03afe7da13950201562df3fdd6c8b209aab248daee82d773b9dadebba3eeecbb4c", "pending_htlcs": 1, "max_pending_htlcs": 5}
2020-10-17T20:45:15.852+0200	INFO	Resolving htlc	{"channel": 39778131669745664, "htlc": 52, "peer_alias": "tester", "peer": "03afe7da13950201562df3fdd6c8b209aab248daee82d773b9dadebba3eeecbb4c", "pending_htlcs": 0, "hold_time": "143.396033ms", "hold_fee_msat": 4}
2020-10-17T20:45:20.000+0200	INFO	Hold fees report	{"next_report_time": "2020-10-17T20:45:25.000+0200"}
2020-10-17T20:45:20.000+0200	INFO	Report	{"peer_alias": "tester", "peer": "03afe7da13950201562df3fdd6c8b209aab248daee82d773b9dadebba3eeecbb4c", "total_fees_msat": 74, "interval_fees_msat": 4}

How to use

Requirements

• go 1.13
• lnd version 0.11.0-beta or above.

Configuration

circuitbreaker by default reads its configuration from ~/.circuitbreaker/circuitbreaker.yaml. An example configuration can be found here

Run

• Clone this repository
• go install
• Execute circuitbreaker with the correct command line flags to connect to lnd. See circuitbreaker --help for details.

Limitations

• This software is alpha quality. Use at your own risk and be careful in particular on mainnet.
• The interfaces on lnd aren't optimized for this purpose. Therefore the use of a combination of different endpoints is required. This may lead to certain corner cases.
• circuitbreaker is currently unaware of htlcs that are already in flight when it is started.