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.