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<!DOCTYPE html>
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<head>
<title>ZIP 203: Transaction Expiry</title>
<meta charset="utf-8" />
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<body>
<section>
<pre>ZIP: 203
Title: Transaction Expiry
Owners: Daira Hopwood <daira@electriccoin.co>
Original-Authors: Jay Graber
Status: Final
Category: Consensus
Created: 2018-01-09
License: MIT</pre>
<section id="abstract"><h2><span class="section-heading">Abstract</span><span class="section-anchor"> <a rel="bookmark" href="#abstract"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h2>
<p>This is a Standards ZIP describing a new consensus rule to set an expiration time after which a transaction cannot be mined. If it is not mined within that time, the transaction will be removed from nodes' mempools.</p>
</section>
<section id="motivation"><h2><span class="section-heading">Motivation</span><span class="section-anchor"> <a rel="bookmark" href="#motivation"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h2>
<p>Transactions that have insufficient fees are often not mined. This indeterminism is a source of confusion for users and wallets. Allowing a transaction to set a block height after which it cannot be mined would provide certainty around how long a transaction has to confirm before it is rejected by the network and must be re-sent.</p>
<p>Advantages include optimizing mempool performance by removing transactions that will not be mined, and potentially simplifying bidirectional payment channels by reducing the need to store and compress revocations for past states, since transactions not committed to the chain could expire and become invalid after a period of time.</p>
<p>If the expiry is at block height
<span class="math">\(N\)</span>
, then the transaction must be included in block
<span class="math">\(N\)</span>
or earlier. Block
<span class="math">\(N+1\)</span>
will be too late, and the transaction will be removed from the mempool.</p>
<p>The new consensus rule will enforce that the transaction will not be considered valid if included in block of height greater than
<span class="math">\(N\)</span>
, and blocks that include expired transactions will not be considered valid.</p>
</section>
<section id="specification"><h2><span class="section-heading">Specification</span><span class="section-anchor"> <a rel="bookmark" href="#specification"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h2>
<p>Transactions will have a new field, <code>nExpiryHeight</code>, which will set the block height after which transactions will be removed from the mempool if they have not been mined.</p>
<p>The data type for <code>nExpiryHeight</code> will be <code>uint32_t</code>. If used in combination with <code>nLockTime</code>, both <code>nLockTime</code> and <code>nExpiryHeight</code> must be block heights. <code>nExpiryHeight</code> will never be a UNIX timestamp, unlike <code>nLockTime</code> values, and thus the maximum expiry height will be 499999999 (but see the exception for coinbase transactions described in <a href="#changes-for-nu5">Changes for NU5</a>).</p>
<p>For the example below, the last block that the transaction below could possibly be included in is 3539. After that, it will be removed from the mempool.</p>
<pre>"txid": "17561b98cc77cd5a984bb959203e073b5f33cf14cbce90eb32b95ae2c796723f",
"version": 3,
"locktime": 2089,
"expiryheight": 3539,</pre>
<p>Default: 20 blocks from the current height, or about 50 minutes at 2.5-minute target block spacing. A configuration option can be used to set the user's default.</p>
<p>Minimum: No minimum</p>
<p>Maximum: 499999999, which is about 1185 years from now at 75 seconds/block.</p>
<p>No limit: To set no limit on transactions (so that they do not expire), <code>nExpiryHeight</code> should be set to 0.</p>
<p>Coinbase: <code>nExpiryHeight</code> on coinbase transactions is ignored, and is set to 0 by convention.</p>
<p>Every time a transaction expires and should be removed from the mempool, so should all of its dependent transactions.</p>
<section id="changes-for-blossom"><h3><span class="section-heading">Changes for Blossom</span><span class="section-anchor"> <a rel="bookmark" href="#changes-for-blossom"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h3>
<p>On Blossom activation <a id="id1" class="footnote_reference" href="#zip-0206">2</a>, the default changes to 40 blocks from the current height, which still represents about 50 minutes at the revised 75-second target block spacing.</p>
</section>
<section id="changes-for-nu5"><h3><span class="section-heading">Changes for NU5</span><span class="section-anchor"> <a rel="bookmark" href="#changes-for-nu5"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h3>
<p>As mentioned above, <code>nExpiryHeight</code> is ignored for coinbase transactions. However, from NU5 activation <a id="id2" class="footnote_reference" href="#zip-0252">3</a>, the <code>nExpiryHeight</code> field of a coinbase transaction MUST be set equal to the block height. Also, for coinbase transactions only, the bound of 499999999 on <code>nExpiryHeight</code> is removed. The motivation is to ensure that transaction IDs remain unique, as explained in more detail in a note in Section 7.1 of the protocol specification <a id="id3" class="footnote_reference" href="#protocol-txnencodingandconsensus">4</a>.</p>
</section>
<section id="wallet-behavior-and-ui"><h3><span class="section-heading">Wallet behavior and UI</span><span class="section-anchor"> <a rel="bookmark" href="#wallet-behavior-and-ui"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h3>
<p>With the addition of this feature, zero-confirmation transactions with an expiration block height set will have even less guarantee of inclusion. This means that UIs and services must never rely on zero-confirmation transactions in Zcash.</p>
<p>Wallet should notify the user of expired transactions that must be re-sent.</p>
</section>
<section id="rpc"><h3><span class="section-heading">RPC</span><span class="section-anchor"> <a rel="bookmark" href="#rpc"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h3>
<p>For Overwinter, transaction expiry will be set to a default that can be overridden by a flag <cite>txexpirydelta</cite> set in the config file.</p>
<p><code>-txexpirydelta=</code> set the number of blocks after which a transaction that has not been mined will become invalid</p>
<p>To view: <cite>listtransactions</cite> has a new filter attribute, showing expired transactions only:</p>
<pre>listtransactions "*" 10 0 "expired"</pre>
<p>WalletTxToJSON shows a boolean expired true/false.</p>
</section>
<section id="config"><h3><span class="section-heading">Config</span><span class="section-anchor"> <a rel="bookmark" href="#config"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h3>
<p>The default will be user-configurable with the option <cite>txexpirydelta</cite>.</p>
<p><cite>--txexpirydelta=100</cite></p>
</section>
<section id="deployment"><h3><span class="section-heading">Deployment</span><span class="section-anchor"> <a rel="bookmark" href="#deployment"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h3>
<p>This feature will be deployed with Overwinter. The activation blockheight proposal is in <a id="id4" class="footnote_reference" href="#zip-0201">1</a>.</p>
</section>
</section>
<section id="reference-implementation"><h2><span class="section-heading">Reference Implementation</span><span class="section-anchor"> <a rel="bookmark" href="#reference-implementation"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h2>
<p><a href="https://github.com/zcash/zcash/pull/2874">https://github.com/zcash/zcash/pull/2874</a></p>
</section>
<section id="references"><h2><span class="section-heading">References</span><span class="section-anchor"> <a rel="bookmark" href="#references"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h2>
<table id="zip-0201" class="footnote">
<tbody>
<tr>
<th>1</th>
<td><a href="zip-0201">ZIP 201: Network Peer Management for Overwinter</a></td>
</tr>
</tbody>
</table>
<table id="zip-0206" class="footnote">
<tbody>
<tr>
<th>2</th>
<td><a href="zip-0206">ZIP 206: Deployment of the Blossom Network Upgrade</a></td>
</tr>
</tbody>
</table>
<table id="zip-0252" class="footnote">
<tbody>
<tr>
<th>3</th>
<td><a href="zip-0252">ZIP 252: Deployment of the NU5 Network Upgrade</a></td>
</tr>
</tbody>
</table>
<table id="protocol-txnencodingandconsensus" class="footnote">
<tbody>
<tr>
<th>4</th>
<td><a href="protocol/protocol.pdf#txnencodingandconsensus">Zcash Protocol Specification, Version 2020.2.6. Section 7.1: Transaction Encoding and Consensus</a></td>
</tr>
</tbody>
</table>
</section>
</section>
</body>
</html>