Channel Jamming: add hold-time-dependent bidirectional upfront payment (#14)

spam-prevention.md

@@ -24,6 +24,7 @@ of the network's capacity.
   * [Naive upfront payment](#naive-upfront-payment)
   * [Reverse upfront payment](#reverse-upfront-payment)
   * [Bidirectional upfront payment](#bidirectional-upfront-payment)
+  * [Hold-time-dependent bidirectional upfront payment](#hold-time-dependent-bidirectional-upfront-payment)
   * [Web of trust HTLC hold fees](#web-of-trust-htlc-hold-fees)
 
 ## Description of the attack
@@ -327,6 +328,24 @@ Drawbacks:
     and let them get away with it, but record it. If they're doing it too often, close channels and
     ban them; stealing upfront fees should never be worth losing channels.
 
+### Hold-time-dependent bidirectional upfront payment
+
+One characteristic of bidirectional upfront payments as described above is that
+the `hold_fees` are time-independent. If an htlc doesn't resolve within the
+`grace_period`, the receiver of the htlc will be forced to pay the full hold
+fee. The hold fee should cover the expenses for locking up an htlc for the
+maximum duration (could be 2000 blocks), so this can be a significant penalty.
+Applications such as atomic onchain/offchain swaps (Lightning Loop and others)
+rely on locking funds for some time and could get expensive with a fixed hold
+fee.
+
+A different variant of bidirectional upfront payments uses a time-proportional hold
+fee rate to address the limitation above. It aims to relate the fees paid more
+directly to the actual costs incurred and thereby reduce the number of
+parameters.
+
+The complete proposal can be found [here](https://lists.linuxfoundation.org/pipermail/lightning-dev/2021-February/002958.html).
+
 ### Web of trust HTLC hold fees
 
 This [proposal](https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-October/002826.html) introduces fees depending on the amount of time HTLCs are kept pending.