#Terminology

The key words "MUST", "SHALL", "SHOULD", and "SHOULD NOT" in this document are to be interpreted as described in BCP 14 ¹ when, and only when, they appear in all capitals.

The term "network upgrade" is to be interpreted as defined in ZIP 200. ²

The term "block target spacing" is to be interpreted as defined in ZIP 218. ³

The character § is used when referring to sections of the Zcash Protocol Specification. ⁴

The terms "Mainnet" and "Testnet" are to be interpreted as described in § 3.12 'Mainnet and Testnet'. ⁵

"NU8" is the code name of the network upgrade that deploys this proposal.

#Abstract

This proposal changes the block target spacing from 25 seconds to 10 seconds in NU8. The block subsidy, the block size limit, and the shielded action limits of each block are scaled by the same factor, so that issuance, block space, and shielded capacity per unit of time are unchanged.

#Motivation

ZIP 218 reduces the block target spacing from 75 seconds to 25 seconds in NU7 in order to reduce transaction latency. ³ This proposal reduces it to 10 seconds, which reduces the expected time to a first confirmation from 25 seconds to 10 seconds.

The proposal changes latency only. Each quantity that NU7 specifies per unit of time is preserved: issuance, funding stream address periods, reissuance by the Network Sustainability Mechanism, the duration of the difficulty averaging window, and the resource limits of the block chain.

#Specification

The changes below are made to the Zcash Protocol Specification ⁴ and to ZIPs 207, 214, and 237 ^{6 7 8}, as amended for NU7 by ZIP 218 ³ and ZIP 259 ⁹.

#Block target spacing

In § 5.3 'Constants' ¹⁰, define:

$𝖭𝖴\mathrm{𝟪}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝗂𝗈𝗇𝖧𝖾𝗂𝗀𝗁𝗍:\mathbb{N}$ $𝖯𝗈𝗌𝗍𝖭𝖴\mathrm{𝟪}𝖯𝗈𝖶𝖳𝖺𝗋𝗀𝖾𝗍𝖲𝗉𝖺𝖼𝗂𝗇𝗀:\mathbb{N}:=10\text{ 𝗌𝖾𝖼𝗈𝗇𝖽𝗌}$

In § 7.7.3 'Difficulty adjustment' ¹¹, define:

$𝖭𝖴\mathrm{𝟪}𝖯𝗈𝖶𝖳𝖺𝗋𝗀𝖾𝗍𝖲𝗉𝖺𝖼𝗂𝗇𝗀𝖱𝖺𝗍𝗂𝗈:\mathbb{Q}:=𝖯𝗈𝗌𝗍𝖭𝖴\mathrm{𝟩}𝖯𝗈𝖶𝖳𝖺𝗋𝗀𝖾𝗍𝖲𝗉𝖺𝖼𝗂𝗇𝗀/𝖯𝗈𝗌𝗍𝖭𝖴\mathrm{𝟪}𝖯𝗈𝖶𝖳𝖺𝗋𝗀𝖾𝗍𝖲𝗉𝖺𝖼𝗂𝗇𝗀=5/2$

$𝖨𝗌𝖭𝖴\mathrm{𝟪}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽:\mathbb{N}\to \{𝖿𝖺𝗅𝗌𝖾,𝗍𝗋𝗎𝖾\}$ $𝖨𝗌𝖭𝖴\mathrm{𝟪}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽(𝗁𝖾𝗂𝗀𝗁𝗍):=(𝗁𝖾𝗂𝗀𝗁𝗍\ge 𝖭𝖴\mathrm{𝟪}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝗂𝗈𝗇𝖧𝖾𝗂𝗀𝗁𝗍)$

On each network, $𝖭𝖴\mathrm{𝟪}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝗂𝗈𝗇𝖧𝖾𝗂𝗀𝗁𝗍$ SHALL be even.

In the definition of $𝖯𝗈𝖶𝖳𝖺𝗋𝗀𝖾𝗍𝖲𝗉𝖺𝖼𝗂𝗇𝗀$ in ZIP 218, replace the case "otherwise" with "if $𝖨𝗌𝖭𝖴\mathrm{𝟩}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽(𝗁𝖾𝗂𝗀𝗁𝗍)$ and not $𝖨𝗌𝖭𝖴\mathrm{𝟪}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽(𝗁𝖾𝗂𝗀𝗁𝗍)$", and add the case $𝖯𝗈𝗌𝗍𝖭𝖴\mathrm{𝟪}𝖯𝗈𝖶𝖳𝖺𝗋𝗀𝖾𝗍𝖲𝗉𝖺𝖼𝗂𝗇𝗀$ "otherwise".

#Difficulty averaging window

In § 5.3 'Constants', define:

$𝖯𝗈𝗌𝗍𝖭𝖴\mathrm{𝟪}𝖯𝗈𝖶𝖠𝗏𝖾𝗋𝖺𝗀𝗂𝗇𝗀𝖶𝗂𝗇𝖽𝗈𝗐:\mathbb{N}:=255$

In the definition of $𝖯𝗈𝖶𝖠𝗏𝖾𝗋𝖺𝗀𝗂𝗇𝗀𝖶𝗂𝗇𝖽𝗈𝗐$ in ZIP 218, replace the case "otherwise" with "if $𝖨𝗌𝖭𝖴\mathrm{𝟩}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽(𝗁𝖾𝗂𝗀𝗁𝗍)$ and not $𝖨𝗌𝖭𝖴\mathrm{𝟪}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽(𝗁𝖾𝗂𝗀𝗁𝗍)$", and add the case $𝖯𝗈𝗌𝗍𝖭𝖴\mathrm{𝟪}𝖯𝗈𝖶𝖠𝗏𝖾𝗋𝖺𝗀𝗂𝗇𝗀𝖶𝗂𝗇𝖽𝗈𝗐$ "otherwise".

The averaging window timespan therefore remains 2,550 seconds.

#Halving interval and block subsidy

Define:

$𝖯𝗈𝗌𝗍𝖭𝖴\mathrm{𝟪}𝖧𝖺𝗅𝗏𝗂𝗇𝗀𝖨𝗇𝗍𝖾𝗋𝗏𝖺𝗅:\mathbb{N}:=𝖯𝗈𝗌𝗍𝖭𝖴\mathrm{𝟩}𝖧𝖺𝗅𝗏𝗂𝗇𝗀𝖨𝗇𝗍𝖾𝗋𝗏𝖺𝗅\cdot 𝖭𝖴\mathrm{𝟪}𝖯𝗈𝖶𝖳𝖺𝗋𝗀𝖾𝗍𝖲𝗉𝖺𝖼𝗂𝗇𝗀𝖱𝖺𝗍𝗂𝗈=\mathrm{12,600,000}$

In the definition of $𝖧𝖺𝗅𝗏𝗂𝗇𝗀$ in ZIP 218, replace the case "otherwise" with "if $𝖨𝗌𝖭𝖴\mathrm{𝟩}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽(𝗁𝖾𝗂𝗀𝗁𝗍)$ and not $𝖨𝗌𝖭𝖴\mathrm{𝟪}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽(𝗁𝖾𝗂𝗀𝗁𝗍)$", and add the case:

In the definition of $𝖲𝖼𝗁𝖾𝖽𝗎𝗅𝖾𝖽𝖡𝗅𝗈𝖼𝗄𝖲𝗎𝖻𝗌𝗂𝖽𝗒$ in ZIP 237, replace the condition "if $𝖨𝗌𝖭𝖴\mathrm{𝟩}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽(𝗁𝖾𝗂𝗀𝗁𝗍)$" of the case added by ZIP 218 with "if $𝖨𝗌𝖭𝖴\mathrm{𝟩}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽(𝗁𝖾𝗂𝗀𝗁𝗍)$ and not $𝖨𝗌𝖭𝖴\mathrm{𝟪}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽(𝗁𝖾𝗂𝗀𝗁𝗍)$", and add the case:

The denominator is $15\cdot 2^{𝖧𝖺𝗅𝗏𝗂𝗇𝗀(𝗁𝖾𝗂𝗀𝗁𝗍)}$.

#Block size and shielded action limits

For a block at a height for which $𝖨𝗌𝖭𝖴\mathrm{𝟪}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽(𝗁𝖾𝗂𝗀𝗁𝗍)$, the size of the block MUST be less than or equal to 800,000 bytes, in place of the limit of 2,000,000 bytes in § 7.6 'Block Header Encoding and Consensus'. ¹²

For such a block, the shielded action limits of ZIP 218 apply with $𝖦𝗅𝗈𝖻𝖺𝗅𝖲𝗁𝗂𝖾𝗅𝖽𝖾𝖽𝖡𝗎𝖽𝗀𝖾𝗍=132$, $𝖮𝗋𝖼𝗁𝖺𝗋𝖽𝖯𝗋𝗈𝗍𝗈𝖼𝗈𝗅𝖡𝗅𝗈𝖼𝗄𝖠𝖼𝗍𝗂𝗈𝗇𝖫𝗂𝗆𝗂𝗍=132$, and $𝖲𝖺𝗉𝗅𝗂𝗇𝗀𝖡𝗅𝗈𝖼𝗄𝖨𝖮𝖫𝗂𝗆𝗂𝗍=120$. $𝖲𝗉𝗋𝗈𝗎𝗍𝖡𝗅𝗈𝖼𝗄𝖩𝗈𝗂𝗇𝖲𝗉𝗅𝗂𝗍𝖫𝗂𝗆𝗂𝗍$ remains 0.

#Funding streams

Let $A$, $R$, $C$, $I$, and $H_1$ be as in the definition of $𝖠𝖽𝖽𝗋𝖾𝗌𝗌𝖯𝖾𝗋𝗂𝗈𝖽$ in Revision 2 of ZIP 207, and let $B=𝖭𝖴\mathrm{𝟪}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝗂𝗈𝗇𝖧𝖾𝗂𝗀𝗁𝗍$. In that definition, replace the case "otherwise" with "if $𝖨𝗌𝖭𝖴\mathrm{𝟩}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽(𝗁𝖾𝗂𝗀𝗁𝗍)$ and not $𝖨𝗌𝖭𝖴\mathrm{𝟪}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽(𝗁𝖾𝗂𝗀𝗁𝗍)$", and add the case:

This preserves the address index at NU8 activation. Each later address period lasts $5RC/2=\mathrm{262,500}$ blocks.

For each funding stream that is active at $B$ and whose end height is $𝖧𝖾𝗂𝗀𝗁𝗍𝖥𝗈𝗋𝖧𝖺𝗅𝗏𝗂𝗇𝗀(3)$ by Revision 3 of ZIP 214, the end height SHALL be $𝖧𝖾𝗂𝗀𝗁𝗍𝖥𝗈𝗋𝖧𝖺𝗅𝗏𝗂𝗇𝗀(3)$ calculated using the $𝖧𝖺𝗅𝗏𝗂𝗇𝗀$ function of this proposal. If $H_3$ is the third-halving height under ZIP 218 (6,132,390 on Mainnet and 4,497,948 on Testnet), this end height is $B+5(H_3-B)/2$. The number of addresses that ZIP 207 requires for each such stream is unchanged.

#Network Sustainability Mechanism

In ZIP 237, $𝖧𝖺𝗅𝗏𝗂𝗇𝗀𝖨𝗇𝗍𝖾𝗋𝗏𝖺𝗅(𝗁𝖾𝗂𝗀𝗁𝗍)$ is $𝖯𝗈𝗌𝗍𝖭𝖴\mathrm{𝟪}𝖧𝖺𝗅𝗏𝗂𝗇𝗀𝖨𝗇𝗍𝖾𝗋𝗏𝖺𝗅$ if $𝖨𝗌𝖭𝖴\mathrm{𝟪}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝖾𝖽(𝗁𝖾𝗂𝗀𝗁𝗍)$. From NU8 activation, $𝖡𝖫𝖮𝖢𝖪\_𝖲𝖴𝖡𝖲𝖨𝖣𝖸\_𝖥𝖱𝖠𝖢𝖳𝖨𝖮𝖭$ is therefore $550/{10}^{10}$.

If $B$ is less than the $𝖣𝖤𝖯𝖫𝖮𝖸𝖬𝖤𝖭𝖳\_𝖡𝖫𝖮𝖢𝖪\_𝖧𝖤𝖨𝖦𝖧𝖳$ that ZIP 237 specifies for a network, then $𝖣𝖤𝖯𝖫𝖮𝖸𝖬𝖤𝖭𝖳\_𝖡𝖫𝖮𝖢𝖪\_𝖧𝖤𝖨𝖦𝖧𝖳$ for that network SHALL instead be calculated as in ZIP 237, with $B$ in place of $A$, with $𝖲𝖼𝗁𝖾𝖽𝗎𝗅𝖾𝖽𝖡𝗅𝗈𝖼𝗄𝖲𝗎𝖻𝗌𝗂𝖽𝗒$, $H_3$, and $H_4$ as specified by this proposal, and with 550 in place of 1375. NU8 activation heights SHALL be chosen so that this height exists before $H_4$. The NU8 deployment ZIP specifies the resulting height for each network.

#Minimum-difficulty blocks on Testnet

On Testnet, from NU8 activation, the minimum-difficulty threshold specified by ZIP 218 is $45\cdot 𝖯𝗈𝖶𝖳𝖺𝗋𝗀𝖾𝗍𝖲𝗉𝖺𝖼𝗂𝗇𝗀(𝗁𝖾𝗂𝗀𝗁𝗍)$ seconds, which remains 450 seconds.

#Unchanged consensus rules

Consensus rules that are expressed in blocks and not changed above, including the coinbase maturity of 100 blocks and $𝖯𝗈𝖶𝖬𝖾𝖽𝗂𝖺𝗇𝖡𝗅𝗈𝖼𝗄𝖲𝗉𝖺𝗇$, are unchanged.

#Non-consensus changes

From NU8 activation:

• Wallets SHOULD use a default expiry delta of 300 blocks, in place of the 120 blocks recommended by ZIP 218.
• Nodes SHOULD support rolling back at least 1,500 blocks, in place of the 600 blocks recommended by ZIP 218.
• Implementations SHOULD multiply the block-count constants that ZIP 218 multiplies by 3 by $𝖭𝖴\mathrm{𝟪}𝖯𝗈𝖶𝖳𝖺𝗋𝗀𝖾𝗍𝖲𝗉𝖺𝖼𝗂𝗇𝗀𝖱𝖺𝗍𝗂𝗈$, rounding up.
• Nodes SHOULD NOT admit to their mempools or relay, and wallets SHOULD NOT create, a transaction that alone exceeds the block size or shielded action limits above.

#Rationale

A target spacing of 10 seconds makes each scaled quantity an integer: the averaging window of 255 blocks, the halving interval of 12,600,000 blocks, the address period of 262,500 blocks, and the limits of 800,000 bytes, 132 actions, and 120 Sapling inputs and outputs.

Every post-NU7 halving height is even on both networks. An even activation height $B$ therefore maps each such height $H$ to the integer height $B+5(H-B)/2$, so that no halving boundary falls within a 25-second block.

Scaling the per-block limits by 2/5 keeps the worst-case validation work, compact sync bandwidth, and chain growth per unit of time at their NU7 values. ZIP 218 bounds worst-case compact sync bandwidth at 169.10 MB per day. ³ Without the scaling, each of these would increase by a factor of 5/2.

The post-NU8 block subsidy differs from 2/5 of the post-NU7 block subsidy by less than one zatoshi, due to rounding. If NU8 activates before the third halving, this reduces total scheduled issuance, relative to ZIP 218, by $(H_3-B)/2$ zatoshi before the third halving and by 1.134 ZEC after it.

#Security and Privacy Considerations

The stale block rate increases with the ratio of block propagation time to the block target spacing. ZIP 218 reports a stale block rate of 4.86% for 2 MB blocks at 25-second spacing on a 99-node devnet. ³ The rate at 10-second spacing has not been measured; it needs to be measured before deployment. Smaller blocks and compact block relay ¹³ reduce propagation time.

A given number of confirmations corresponds to 2.5 times less time. Where propagation time is significant relative to the target spacing, it also provides less security against an attacker with a given fraction of the hash power.

Wallets that select an anchor three blocks behind the chain tip, as recommended by ZIP 315 ¹⁴, select one about 30 seconds old. A rollback past the anchor invalidates the transaction, and links its revealed nullifiers to any later transaction that spends the same notes. More frequent blocks also give observers finer timing information.

#Deployment

This proposal is intended to be deployed in NU8. The NU8 deployment ZIP will specify $𝖭𝖴\mathrm{𝟪}𝖠𝖼𝗍𝗂𝗏𝖺𝗍𝗂𝗈𝗇𝖧𝖾𝗂𝗀𝗁𝗍$ for each network.

#References