zips/zip-0203.rst

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::
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ZIP: 203
Title: Transaction Expiry
Owners: Daira Emma Hopwood <daira@electriccoin.co>
Original-Authors: Jay Graber
Status: Final
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Category: Consensus
Created: 2018-01-09
License: MIT
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Abstract
========
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.
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Motivation
==========
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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.
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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.
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If the expiry is at block height :math:`N`, then the transaction must be included in block
:math:`N` or earlier. Block :math:`N+1` will be too late, and the transaction will be
removed from the mempool.
The new consensus rule will enforce that the transaction will not be considered valid if
included in block of height greater than :math:`N`, and blocks that include expired
transactions will not be considered valid.
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Specification
=============
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Transactions will have a new field, ``nExpiryHeight``, which will set the block height
after which transactions will be removed from the mempool if they have not been mined.
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The data type for ``nExpiryHeight`` will be ``uint32_t``. ``nExpiryHeight`` will never
be a UNIX timestamp, unlike ``nLockTime`` values, and thus the maximum expiry height
will be 499999999 (but see the exception for coinbase transactions described in
`Changes for NU5`_).
Note: a previous version of this ZIP incorrectly specified an interaction between
``nExpiryHeight`` and ``nLockTime`` that was never implemented.
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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.
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::
"txid": "17561b98cc77cd5a984bb959203e073b5f33cf14cbce90eb32b95ae2c796723f",
"version": 3,
"locktime": 2089,
"expiryheight": 3539,
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Default: (Before Blossom) 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.
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Minimum: No minimum
Maximum: 499999999, which is about 1185 years from now at 75 seconds/block.
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No limit: To set no limit on transactions (so that they do not expire), ``nExpiryHeight``
should be set to 0.
Coinbase: ``nExpiryHeight`` on coinbase transactions is ignored, and is set to 0 by
convention.
Every time a transaction expires and should be removed from the mempool, so should all
of its dependent transactions.
Changes for Blossom
-------------------
On Blossom activation [#zip-0206]_, the default changes to 40 blocks from the current
height, which still represents about 50 minutes at the revised 75-second target block
spacing.
Changes for NU5
---------------
As mentioned above, ``nExpiryHeight`` is ignored for coinbase transactions. However, from
NU5 activation [#zip-0252]_, the ``nExpiryHeight`` field of a coinbase transaction MUST
be set equal to the block height. Also, for coinbase transactions only, the bound of
499999999 on ``nExpiryHeight`` 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 [#protocol-txnencoding]_.
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Wallet behavior and UI
----------------------
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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.
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Wallet should notify the user of expired transactions that must be re-sent.
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RPC
---
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For Overwinter, transaction expiry will be set to a default that can be overridden by a
flag `txexpirydelta` set in the config file.
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``-txexpirydelta=`` set the number of blocks after which a transaction that has not been
mined will become invalid
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To view: `listtransactions` has a new filter attribute, showing expired transactions only::
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listtransactions "*" 10 0 "expired"
WalletTxToJSON shows a boolean expired true/false.
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Config
------
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The default will be user-configurable with the option `txexpirydelta`.
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`--txexpirydelta=100`
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Deployment
----------
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This feature will be deployed with Overwinter. The activation blockheight proposal is in
[#zip-0201]_.
Reference Implementation
========================
https://github.com/zcash/zcash/pull/2874
References
==========
.. [#zip-0201] `ZIP 201: Network Peer Management for Overwinter <zip-0201.rst>`_
.. [#zip-0206] `ZIP 206: Deployment of the Blossom Network Upgrade <zip-0206.rst>`_
.. [#zip-0252] `ZIP 252: Deployment of the NU5 Network Upgrade <zip-0252.rst>`_
.. [#protocol-txnencoding] `Zcash Protocol Specification, Version 2021.2.16. Section 7.1: Transaction Encoding and Consensus <protocol/protocol.pdf#txnencoding>`_