Expiring HTLCs without free relay

[josh]

@Ajian

First, where is the R coming from? We use the normal bit space (from bit 1 to bit 15 of nSequence, just like BIP68 relative timelock) to pass it ?

R is the lowest 16 bits of nSequence, per BIP68.

@Ajian

Second, if I understand correctly, the approach wants to constrain the nLockTime value, forces the value is bigger than a supposed value, but don’t limit its actual value, right ? In this way, we can use a Tx2 with bit 21 and a nLockTime value, to force the Tx1 must be included before the nLockTime height minus 100 (or a passed value bigger than 100, or the difference but capped by 100), otherwise(if later than that), the Tx2 will be invalid, right ?

You got the idea. This approach imposes a maximum coin height given a fixed nLockTime or, equivalently, a minimum nLockTime given a known coin height.

@Ajian

Final, how can we say, in this approcah, the “expiry” is binded to the parent transaction(Tx1) ? If I understand correctly, the expiry time is set by the nLockTime of Tx1, which is presigned, and not affected by the inclusion time of Tx1 , and we use this nLockTime to force Tx1 to be included before a blockheight ?

To clarify, Tx1 never expires. It merely publishes the preimage and creates the staging output for Tx2. Tx2 becomes invalid unless Tx1 has at least R confirmations by Tx2’s nLockTime.