I was investigating a sig mismatch between eclair and lnd and I think we need to better specify how the outputs are to be ordered in the commitment tx.

BOLT 3 only refers to BIP 69, but I think it is not sufficient, because offered htlcs with the same amount, same payment_hash and a different cltv_expiry will have the same pubkey script but a different HTLC-Timeout (given that only the latter depends on cltv_expiry), resulting in different incompatible sigs.

I'm not entirely sure of this, but if I'm right fix is easy, e.g. we could just order following BIP69, and then by cltv_expiry.