Minimum difficulty blocks on Testnet

On Testnet from block height 299188 onward, the difficulty adjustment algorithm 6 allows minimum-difficulty blocks, as described in 10, when the block time is greater than a given threshold. This specification changes this threshold to be proportional to the block target spacing.

That is, if the block time of a block at height \(\mathsf{height} \geq 299188\) is greater than \(6 \cdot \mathsf{PoWTargetSpacing}(\mathsf{height})\) seconds after that of the preceding block, then the block is a minimum-difficulty block. In that case its nBits field MUST be set to \(\mathsf{ToCompact}(\mathsf{PoWLimit})\!\) , where \(\mathsf{PoWLimit}\) is the value defined for Testnet in section 5.3 of the Zcash Protocol Specification 5, and \(\mathsf{ToCompact}\) is as defined in section 7.7.4 of that specification 7.

Note: a previous revision of this ZIP (and 10) incorrectly said that only the target threshold of minimum-difficulty blocks is affected. In fact the nBits field is modified as well, and this affects difficulty adjustment for subsequent blocks.

This change does not affect Mainnet.

End-of-Service halt

zcashd implements an "End-of-Service halt" behaviour that halts the node at a block height that corresponds approximately to a given time after release. This interval SHOULD be adjusted in releases where the End-of-Service halt time will follow Blossom activation.

Default expiry delta

When not overridden by the -txexpirydelta option, zcashd RPC calls that create transactions use a default value for the number of blocks after which a transaction will expire. The default in recent versions of zcashd is 20 blocks, which at the pre-Blossom block target spacing corresponds to roughly 50 minutes.

This default SHOULD change to BlossomPoWTargetSpacingRatio · 20 blocks after Blossom activation, to maintain the approximate expiry time of 50 minutes.

If the -txexpirydelta option is set, then the set value SHOULD be used both before and after Blossom activation.

Sprout to Sapling migration

ZIP 308 12 defines a procedure for migrating funds from Sprout to Sapling z-addresses. In that procedure, migration transactions are sent every 500 blocks, which corresponded to roughly 20.83 hours before Blossom.

The 500-block constant has not been changed. Therefore, migration transactions are now sent roughly every 10.42 hours after Blossom activation. This has been noted in the ZIP, and a table showing the expected time to complete migration has been updated accordingly.

Fingerprinting mitigation

A "fingerprinting attack" is a network analysis technique in which nodes are identified across network sessions, for example using information about which blocks they request or send.

zcashd inherits from Bitcoin Core the following behaviour, described in a comment in main.cpp, intended as a fingerprinting mitigation:

// To prevent fingerprinting attacks, only send blocks outside of the active
// chain if they are valid, and no more than a month older (both in time, and in
// best equivalent proof of work) than the best header chain we know about.

We make no assertion about the significance of fingerprinting for Zcash, and (despite the word "prevent" in the above comment) no claim about the effectiveness of this mitigation.

In any case, to estimate the "best equivalent proof of work" of a given block chain (measured in units of time), we take the total work of the chain as defined in section 7.7.5 of the Zcash Protocol Specification 8, divide by the work of the block at the active tip, and multiply by the target block spacing of that block.

It is not a requirement of the Zcash protocol that this fingerprinting mitigation is used; however, if it is used, then it SHOULD use the target block spacing at the same block height that is used for the current work estimate.

Monitoring for quicker- or slower-than-expected blocks

zcashd previously did this monitoring every 150 seconds; it is now done every 60 seconds.

Block timeout

The timeout for a requested block is calculated as the target block time, multiplied by 2 + (the number of queued validated headers)/2.

Latency optimization when requesting blocks

When zcashd sees an announced block that chains from headers that it does not already have, it will first ask for the headers, and then the block itself. A latency optimization is performed only if the chain is "nearly synced":

// First request the headers preceding the announced block. In the normal fully-synced
// case where a new block is announced that succeeds the current tip (no reorganization),
// there are no such headers.
// Secondly, and only when we are close to being synced, we request the announced block directly,
// to avoid an extra round-trip. Note that we must *first* ask for the headers, so by the
// time the block arrives, the header chain leading up to it is already validated. Not
// doing this will result in the received block being rejected as an orphan in case it is
// not a direct successor.

The heuristic for "nearly synced" is that the timestamp of the block at the active tip is no more than 20 block times before the current "adjusted time". In zcashd this calculation uses the block target spacing as of the best known header. Around Blossom activation when the block target spacing changes, this could cause the heuristic to be based on the pre-Blossom block target spacing until the node has synced headers past the activation block, but this is not anticipated to cause any problem.

Response to getblocks message when pruning

If pruning is enabled, when zcashd responds to an "getblocks" peer-to-peer message, it will only include blocks that it has on disk, and is likely to still have on disk an hour after responding to the message:

// If pruning, don't inv blocks unless we have on disk and are likely to still have
// for some reasonable time window (1 hour) that block relay might require.

For each block, when estimating whether it will still be on disk after an hour, we take MIN_BLOCKS_TO_KEEP = \(288\) blocks, minus approximately the number of blocks expected in one hour at the target block spacing as of that block. Around Blossom activation, this might underestimate the number of blocks in the next hour, but given the value of MIN_BLOCKS_TO_KEEP, this is not anticipated to cause any problem.

Estimation of fully synced chain height

zcashd uses the EstimateNetHeight function to estimate the approximate height of the fully synced chain, so that the progress of block download can be displayed to the node operator. This function has been rewritten, simplified, and changed to take account of cases where the time period that needs to be estimated crosses Blossom activation.

Other block-related constants

The following constants, measured in number of blocks, were reviewed and a decision was made not to change them:

/** The number of blocks within expiry height when a tx is considered to be expiring soon */
TX_EXPIRING_SOON_THRESHOLD = 3

/** Maximum reorg length we will accept before we shut down and alert the user. */
MAX_REORG_LENGTH = COINBASE_MATURITY - 1;

static const int COINBASE_MATURITY = 100;

/** Number of blocks that can be requested at any given time from a single peer. */
static const int MAX_BLOCKS_IN_TRANSIT_PER_PEER = 16;

static const unsigned int BLOCK_DOWNLOAD_WINDOW = 1024;

/** Block files containing a block-height within MIN_BLOCKS_TO_KEEP of chainActive.Tip() will not be pruned. */
static const unsigned int MIN_BLOCKS_TO_KEEP = 288;

/**
 * The period before a network upgrade activates, where connections to upgrading peers are preferred (in blocks).
 * This was three days for upgrades up to and including Blossom, and is 1.5 days from Heartwood onward.
 */
static const int NETWORK_UPGRADE_PEER_PREFERENCE_BLOCK_PERIOD = 1728;