Zcash activates NU7, cutting target block time to 25 seconds

Zcash’s mainnet now targets a block every 25 seconds instead of every 75: the network activated its NU7 upgrade at block 3,543,405 early Friday, Nov. 6, according to CoinDesk’s report on the activation. The shorter interval is intended to reduce the wait for confirmations, while NU7 also changes shielded transaction capacity and how fees are allocated.

The Zcash Foundation said the target spacing change is specified in ZIP 218, and that block rewards are divided by three to keep daily issuance unchanged. CoinDesk reported that the first 225 blocks after activation averaged about 39 seconds apart by 04:59 UTC, as mining difficulty continued adjusting toward the new target.

How does NU7 change shielded transaction capacity?

NU7 sets per-block limits for Orchard, Ironwood and Sapling activity, according to the Zcash Foundation’s Zebra 7.0.0 release notes, and permits no new Sprout JoinSplits. The limits are designed to constrain the work wallets must do when scanning for payments, including under transaction spam conditions.

The Foundation says the limits more than double Orchard throughput, from 2.9 to 6.6 transactions per second, and cut worst-case sync bandwidth that a spammer could impose on light wallets by about 37%. CoinDesk gave a corresponding estimate for the worst-case daily download burden, which falls from 271 megabytes to 169 megabytes. It also reported that the specification estimates simple private-payment capacity could rise from about three to more than six transactions per second.

These are protocol capacity estimates, rather than a guarantee that every wallet or payment service will process funds at the same pace. CoinDesk noted that each service sets its own confirmation requirements; under the new target, one confirmation averages about 25 seconds, while three confirmations would average about 75 seconds.

What happens to transaction fees under NU7?

Under ZIP 235, 60% of each block’s transaction fees are removed from circulation and placed in a reserve; miners keep the other 40%, the Foundation said. ZIP 237 provides for part of that reserve to be reissued in future block rewards. The Foundation lists the mainnet start for that mechanism at block 8,939,664, and says the four-year halving schedule and 21 million ZEC supply cap remain unchanged.

Which older Zcash transactions are affected?

NU7 makes version 4 transactions invalid, while versions 5 and 6 remain valid, according to the Foundation. Because Sprout transfers can use only version 4, ZEC left in the original Sprout pool becomes unspendable. CoinDesk reported that about 12,224 ZEC remains there and that no recovery plan had been approved as of Friday.

Sources

• CoinDesk’s report on the activation — coindesk.com
• Zcash Foundation’s Zebra 7.0.0 release notes — zfnd.org