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<!DOCTYPE html>
<html>
<head>
<title>ZIP 205: Deployment of the Sapling Network Upgrade</title>
<meta charset="utf-8" />
<link rel="stylesheet" href="css/zip-style.css"><link rel="stylesheet" href="assets/css/style.css"></head>
<body>
<section>
<pre>ZIP: 205
Title: Deployment of the Sapling Network Upgrade
Owners: Daira Hopwood <[email protected]>
Credits: Simon Liu <[email protected]>
Status: Final
Category: Consensus
Created: 2018-10-08
License: MIT</pre>
<section id="terminology">
<h2>Terminology</h2>
<p>The key words "MUST", "MUST NOT", "SHOULD", and "MAY" in this document are to be interpreted as described in RFC 2119. <a href="#rfc2119" id="id1" class="footnote_reference">2</a></p>
<p>The terms "branch" and "network upgrade" in this document are to be interpreted as described in ZIP 200. <a href="#zip-0200" id="id2" class="footnote_reference">3</a></p>
<p>The terms below are to be interpreted as follows:</p>
<dl>
<dt>Sapling</dt>
<dd>Code-name for the second Zcash network upgrade, also known as Network Upgrade 1.</dd>
</dl>
</section>
<section id="abstract">
<h2>Abstract</h2>
<p>This proposal defines the deployment of the Sapling network upgrade. In addition, it describes a hard fork that occurred on testnet to allow "minimum-difficulty" blocks.</p>
</section>
<section id="specification">
<h2>Specification</h2>
<section id="sapling-deployment">
<h3>Sapling deployment</h3>
<p>The primary sources of information about Sapling consensus protocol changes are:</p>
<ul>
<li>The Zcash Protocol Specification <a href="#protocol" id="id3" class="footnote_reference">1</a>.</li>
<li>Transaction Signature Verification for Sapling <a href="#zip-0243" id="id4" class="footnote_reference">5</a>.</li>
<li>Network Upgrade Activation Mechanism <a href="#zip-0200" id="id5" class="footnote_reference">3</a>.</li>
</ul>
<p>The network handshake and peer management mechanisms defined in <a href="#zip-0201" id="id6" class="footnote_reference">4</a> also apply to this upgrade.</p>
<p>The following network upgrade constants <a href="#zip-0200" id="id7" class="footnote_reference">3</a> are defined for the Sapling upgrade:</p>
<dl>
<dt>BRANCH_ID</dt>
<dd><code>0x76b809bb</code></dd>
<dt>ACTIVATION_HEIGHT (Sapling)</dt>
<dd>
<p>Testnet: 280000</p>
<p>Mainnet: 419200</p>
</dd>
</dl>
<p>On testnet, Sapling had activated prior to this height, but that branch was rolled back. A subsequent hard fork occurred on testnet, changing the difficulty algorithm to accept "minimum-difficulty" blocks under certain conditions starting at block height 299188.</p>
<p>On both mainnet and testnet, Sapling-compatible nodes MUST advertise protocol version 170007 or later. The minimum peer protocol version that Sapling-compatible nodes will connect to, remains 170002.</p>
<p>Pre-Sapling nodes are defined as nodes advertising a protocol version less than 170007.</p>
<p>Approximately three days (defined in terms of block height) before the Sapling activation height, Sapling-compatible nodes will change the behaviour of their peer connection logic in order to prefer pre-Sapling peers for eviction from the set of peer connections.</p>
<pre>/** The period before a network upgrade activates, where connections to upgrading peers are preferred (in blocks). */
static const int NETWORK_UPGRADE_PEER_PREFERENCE_BLOCK_PERIOD = 24 * 24 * 3;</pre>
<p>The implementation is similar to that for Overwinter which was described in <a href="#zip-0201" id="id8" class="footnote_reference">4</a>.</p>
<p>Once Sapling activates on testnet or mainnet, Sapling nodes should take steps to:</p>
<ul>
<li>reject new connections from pre-Sapling nodes;</li>
<li>disconnect any existing connections to pre-Sapling nodes.</li>
</ul>
</section>
<section id="change-to-difficulty-adjustment-on-testnet">
<h3>Change to difficulty adjustment on testnet</h3>
<p>Section 7.6.3 of <a href="#protocol" id="id9" class="footnote_reference">1</a> describes the algorithm used to adjust the difficulty of a block (defined in terms of a "target threshold") based on the <code>nTime</code> and <code>nBits</code> fields of preceding blocks.</p>
<p>This algorithm changed on testnet, starting from block 299188, to allow "minimum-difficulty" blocks. If the block time of a block from this height onward is at least 15 minutes after that of the preceding block, then the block is a minimum-difficulty block, and its target threshold is set to the value of PoWLimit for testnet (see <a href="#protocol" id="id10" class="footnote_reference">1</a> section 5.3). However, its <code>nBits</code> field is still computed according to the original difficulty adjustment algorithm.</p>
<p>This does not affect how the minimum-difficulty block is treated for subsequent difficulty adjustments. In particular, only the <code>nBits</code> field computed by the original algorithm is used for the purpose of computing the MeanTarget values from which subsequent difficulty changes are calculated.</p>
<p>This change does not affect mainnet.</p>
</section>
</section>
<section id="backward-compatibility">
<h2>Backward compatibility</h2>
<p>Prior to the network upgrade activating, Sapling and pre-Sapling nodes are compatible and can connect to each other. However, Sapling nodes will have a preference for connecting to other Sapling nodes, so pre-Sapling nodes will gradually be disconnected in the run up to activation.</p>
<p>Once the network upgrades, even though pre-Sapling nodes can still accept the numerically larger protocol version used by Sapling as being valid, Sapling nodes will always disconnect peers using lower protocol versions.</p>
</section>
<section id="support-in-zcashd">
<h2>Support in zcashd</h2>
<p>Support for Sapling consensus rules was implemented in zcashd version 2.0.0. The majority of support for RPC calls and persistence of Sapling z-addresses was implemented in version 2.0.1. Both of these versions advertise protocol version 170007.</p>
</section>
<section id="references">
<h2>References</h2>
<table id="protocol" class="footnote">
<tbody>
<tr>
<th>1</th>
<td><a href="protocol/protocol.pdf">Zcash Protocol Specification, Version 2018.0-beta-37 [Overwinter+Sapling]</a></td>
</tr>
</tbody>
</table>
<table id="rfc2119" class="footnote">
<tbody>
<tr>
<th>2</th>
<td><a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a></td>
</tr>
</tbody>
</table>
<table id="zip-0200" class="footnote">
<tbody>
<tr>
<th>3</th>
<td><a href="zip-0200">ZIP 200: Network Upgrade Activation Mechanism</a></td>
</tr>
</tbody>
</table>
<table id="zip-0201" class="footnote">
<tbody>
<tr>
<th>4</th>
<td><a href="zip-0201">ZIP 201: Network Peer Management for Overwinter</a></td>
</tr>
</tbody>
</table>
<table id="zip-0243" class="footnote">
<tbody>
<tr>
<th>5</th>
<td><a href="zip-0243">ZIP 243: Transaction Signature Verification for Sapling</a></td>
</tr>
</tbody>
</table>
</section>
</section>
</body>
</html>