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::
ZIP: 301
Title: Zcash Stratum Protocol
Owner: Jack Grigg <str4d@electriccoin.co>
Credits: 5a1t
Daira Hopwood
Marek Palatinus (slush) and colleagues
Jelle Bourdeaud'hui (razakal)
ocminer
Status: Final
Category: Standards / Ecosystem
Created: 2016-09-23
License: MIT
Terminology
===========
The key words "MUST", "MUST NOT", "SHOULD", "MAY", and "RECOMMENDED" in this
document are to be interpreted as described in RFC 2119. [#RFC2119]_
Abstract
========
This ZIP describes the Zcash variant of the Stratum protocol, used by miners to
communicate with mining pool servers.
Motivation
==========
Many existing cryptocurrency miners and pools use the original Stratum protocol
[#Slushpool-Stratum]_ [#Bitcointalk-Stratum]_ for communication, in situations
where the miner does not require any control over what they mine (for example, a
miner connected to a local [#P2Pool]_ node). However, the protocol is very
specific to Bitcoin, in that it makes assumptions about the block header format,
and the available nonce space [#Bitcoin-Block]_. Zcash has made changes that
invalidate these assumptions.
Having a formal specification for a Zcash-compatible Stratum-style mining
protocol means that existing pool operators and miner authors can quickly and
easily migrate their frameworks to the Zcash network, with no ambiguity about
interoperability.
Specification
=============
The Stratum protocol is an instance of [#JSON-RPC-1.0]_. The miner is a JSON-RPC
client, and the Stratum server is a JSON-RPC server. The miner starts a session
by opening a standard TCP connection to the server, which is then used for
two-way line-based communication:
- The miner can send requests to the server.
- The server can respond to requests.
- The server can send notifications to the client.
All communication for a particular session happens through a single connection,
which is kept open for the duration of the session. If the connection is broken
or either party disconnects, the active session is ended. Servers MAY support
session resuming; this is negotiated between the client and server during
initial setup (see `Session Resuming`_).
Each request or response is a JSON string, terminated by an ASCII LF character
(denoted in the rest of this specification by ``\n``). The LF character MUST NOT
appear elsewhere in a request or response. Client and server implementations MAY
assume that once they read a LF character, the current message has been
completely received.
Per [#JSON-RPC-1.0]_, there is no requirement for the ``id`` property in requests
and responses to be unique; only that servers MUST set ``id`` in their responses
equal to that in the request they are responding to (or ``null`` for
notifications). However, it is RECOMMENDED that clients use unique ids for their
requests, to simplify their response parsing.
In the protocol messages below, ``(content)`` indicates that ``content`` is
optional. Variable names are indicated in *EMPHASIS*. All other characters are
part of the protocol message.
Error Objects
~~~~~~~~~~~~~
The [#JSON-RPC-1.0]_ specification allows for error objects in responses, but
does not specify their format. The original Stratum protocol uses the following
format for error responses [#Slushpool-Stratum]_:
{"id": ##, "result": null, "error": [*ERROR_CODE*, "*ERROR_MESSAGE*", *TRACEBACK*]} ``\n``
For compatibility, this format is retained. We therefore define an error object
as an array:
[*ERROR_CODE*, "*ERROR_MESSAGE*", *TRACEBACK*]
``ERROR_CODE`` (int)
Indicates the type of error that occurred.
The error codes are to be interpreted as described in [#JSON-RPC-2.0]_.
The following application error codes are defined:
- 20 - Other/Unknown
- 21 - Job not found (=stale)
- 22 - Duplicate share
- 23 - Low difficulty share
- 24 - Unauthorized worker
- 25 - Not subscribed
``ERROR_MESSAGE`` (str)
A human-readable error message. The message SHOULD be limited to a concise
single sentence.
``TRACEBACK``
Additional information for debugging errors. The format is server-specific.
Miners MAY attempt to parse the field for displaying to the user, and SHOULD
fall back to rendering it as a JSON string.
Servers MUST set this to ``null`` if they have no additional information.
Miners SHOULD display a human-readable message to the user. This message can be
derived from either ``ERROR_CODE`` or ``ERROR_MESSAGE``, or both. An example of
using ``ERROR_CODE`` over ``ERROR_MESSAGE`` might be that the miner UI offers
localization.
Protocol Flow
~~~~~~~~~~~~~
- Client sends ``mining.subscribe`` to set up the session.
- Server replies with the session information.
- Client sends ``mining.authorize`` for their worker(s).
- Server replies with the result of authorization.
- Server sends ``mining.set_target``.
- Server sends ``mining.notify`` with a new job.
- Client mines on that job.
- Client sends ``mining.submit`` for each solution found.
- Server replies with whether the solution was accepted.
- Server sends ``mining.notify`` again when there is a new job.
Nonce Parts
~~~~~~~~~~~
In Bitcoin, blocks contain two nonces: the 4-byte block header nonce, and an
extra nonce in the coinbase transaction [#Bitcoin-Block]_. The original Stratum
protocol splits this extra nonce into two parts: one set by the server (used
for splitting the search space amongst connected miners), and the other iterated
by the miner [#Slushpool-Stratum]_. The nonce in Zcash's block header is 32 bytes
long [#protocol-blockheader]_, and thus can serve both purposes simultaneously.
We define two nonce parts:
``NONCE_1``
The server MUST pick such that ``len(NONCE_1) < 32`` in bytes.
``NONCE_2``
The miner MUST pick such that ``len(NONCE_2) = 32 - len(NONCE_1)`` in bytes.
In hex, ``lenHex(NONCE_2) = 64 - lenHex(NONCE_1)``, and both lengths are even.
The nonce in the block header is the concatenation of ``NONCE_1`` and
``NONCE_2`` in hex. This means that a miner using bignum representations of nonce
MUST increment by ``1 << len(NONCE_1)`` to avoid altering ``NONCE_1`` (because
the encoding of the nonce in the block header is little endian, in line with the
other 32-byte fields [#Bitcoin-Block]_ [#protocol-blockheader]_).
Session Resuming
~~~~~~~~~~~~~~~~
Servers that support session resuming identify this by setting a ``SESSION_ID``
in their initial response. Servers MAY set ``SESSION_ID`` to ``null`` to
indicate that they do not support session resuming. Servers that do not set
``SESSION_ID`` to ``null`` MUST cache the following information:
- The session ID.
- ``NONCE_1``
- Any active job IDs.
Servers MAY drop entries from the cache on their own schedule.
When a miner connects using a previous ``SESSION_ID``:
- If the cache contains the ``SESSION_ID``, the server's initial response MUST
be constructed from the cached information.
- If the server does not recognise the session, the ``SESSION_ID`` in the
server's initial response MUST NOT equal the ``SESSION_ID`` provided by the
miner.
Miners MUST re-authorize all workers upon resuming a session.
Methods
~~~~~~~
``mining.subscribe()``
----------------------
Request:
{"id": 1, "method": "mining.subscribe", "params": ["*MINER_USER_AGENT*", "*SESSION_ID*", "*CONNECT_HOST*", *CONNECT_PORT*]} ``\n``
``MINER_USER_AGENT`` (str)
A free-form string specifying the type and version of the mining software.
Recommended syntax is the User Agent format used by Zcash nodes.
Example: ``MagicBean/1.0.0``
``SESSION_ID`` (str)
The id for a previous session that the miner wants to resume (e.g. after a
temporary network disconnection) (see `Session Resuming`_).
This MAY be ``null`` indicating that the miner wants to start a new session.
``CONNECT_HOST`` (str)
The host that the miner is connecting to (from the server URL).
Example: ``pool.example.com``
``CONNECT_PORT`` (int)
The port that the miner is connecting to (from the server URL).
Example: ``3337``
Response:
{"id": 1, "result": ["*SESSION_ID*", "*NONCE_1*"], "error": null} ``\n``
``SESSION_ID`` (str)
The session id, for use when resuming (see `Session Resuming`_).
``NONCE_1`` (hex)
The first part of the block header nonce (see `Nonce Parts`_).
``mining.authorize()``
----------------------
A miner MUST authorize a worker in order to submit solutions. A miner MAY
authorize multiple workers in the same session; this could be for statistical
purposes on the particular server being used. Details of such purposes are
outside the scope of this specification.
Request:
{"id": 2, "method": "mining.authorize", "params": ["*WORKER_NAME*", "*WORKER_PASSWORD*"]} ``\n``
``WORKER_NAME`` (str)
The worker name.
``WORKER_PASSWORD`` (str)
The worker password.
Response:
{"id": 2, "result": *AUTHORIZED*, "error": *ERROR*} ``\n``
``AUTHORIZED`` (bool)
This MUST be ``true`` if authorization succeeded. Per [#JSON-RPC-1.0]_, it MUST be
``null`` if there was an error.
``ERROR`` (obj)
An error object. This MUST be ``null`` if authorization succeeded.
If authorization failed, the server MUST provide an error object describing
the reason. See `Error Objects`_ for the object format.
``mining.set_target()``
-----------------------
Server message:
{"id": null, "method": "mining.set_target", "params": ["*TARGET*"]} ``\n``
``TARGET`` (hex)
The server target for the next received job and all subsequent jobs (until the
next time this message is sent). The miner compares proposed block hashes with
this target as a 256-bit big-endian integer, and valid blocks MUST NOT have
hashes larger than (above) the current target (in accordance with the Zcash
network consensus rules [#protocol-difficulty]_).
Miners SHOULD NOT submit work above this target. Miners SHOULD validate their
solutions before submission (to avoid both unnecessary network traffic and
wasted miner time).
Servers MUST NOT accept submissions above this target for jobs sent after this
message. Servers MAY accept submissions above this target for jobs sent before
this message, but MUST check them against the previous target.
When displaying the current target in the UI to users, miners MAY convert the
target to an integer difficulty as used in Bitcoin miners. When doing so, miners
SHOULD use ``powLimit`` (as defined in ``src/chainparams.cpp``) as the basis for
conversion.
``mining.notify()``
-------------------
Server message:
{"id": null, "method": "mining.notify", "params": ["*JOB_ID*", "*VERSION*", "*PREVHASH*", "*MERKLEROOT*", "*RESERVED*", "*TIME*", "*BITS*", *CLEAN_JOBS*]} ``\n``
``JOB_ID`` (str)
The id of this job.
``VERSION`` (hex)
The block header version, encoded as in a block header (little-endian
``int32_t``).
Used as a switch for subsequent parameters. At time of writing, the only
defined block header version is 4. Miners SHOULD alert the user upon receiving
jobs containing block header versions they do not know about or support, and
MUST ignore such jobs.
Example: ``04000000``
The following parameters are only valid for ``VERSION == "04000000"``:
``PREVHASH`` (hex)
The 32-byte hash of the previous block, encoded as in a block header.
``MERKLEROOT`` (hex)
The 32-byte Merkle root of the transactions in this block, encoded as in a
block header.
``RESERVED`` (hex)
A 32-byte reserved field, encoded as in a block header. Zero by convention (in
hex, ``0000000000000000000000000000000000000000000000000000000000000000``).
``TIME`` (hex)
The block time suggested by the server, encoded as in a block header.
``BITS`` (hex)
The current network difficulty target, represented in compact format, encoded
as in a block header.
``CLEAN_JOBS`` (bool)
If true, a new block has arrived. The miner SHOULD abandon all previous jobs.
``mining.submit()``
-------------------
Request:
{"id": 4, "method": "mining.submit", "params": ["*WORKER_NAME*", "*JOB_ID*", "*TIME*", "*NONCE_2*", "*EQUIHASH_SOLUTION*"]} ``\n``
``WORKER_NAME`` (str)
A previously-authenticated worker name.
Servers MUST NOT accept submissions from unauthenticated workers.
``JOB_ID`` (str)
The id of the job this submission is for.
Miners MAY make multiple submissions for a single job id.
``TIME`` (hex)
The block time used in the submission, encoded as in a block header.
MAY be enforced by the server to be unchanged.
``NONCE_2`` (hex)
The second part of the block header nonce (see `Nonce Parts`_).
``EQUIHASH_SOLUTION`` (hex)
The Equihash solution, encoded as in a block header (including the compactSize
at the beginning in canonical form [#Bitcoin-CompactSize]_).
Result:
{"id": 4, "result": *ACCEPTED*, "error": *ERROR*} ``\n``
``ACCEPTED`` (bool)
This MUST be ``true`` if the submission was accepted. Per [#JSON-RPC-1.0]_, it
MUST be ``null`` if there was an error.
``ERROR`` (obj)
An error object. Per [#JSON-RPC-1.0]_, this MUST be ``null`` if the submission
was accepted without error.
If the submission was not accepted, the server MUST provide an error object
describing the reason for not accepting the submission. See `Error Objects`_
for the object format.
``client.reconnect()``
----------------------
Server message:
{"id": null, "method": "client.reconnect", "params": [("*HOST*", *PORT*, *WAIT_TIME*)]} ``\n``
``HOST`` (str)
The host to reconnect to.
Example: ``pool.example.com``
``PORT`` (int)
The port to reconnect to.
Example: ``3337``
``WAIT_TIME`` (int)
Time in seconds that the miner should wait before reconnecting.
If ``client.reconnect`` is sent with an empty parameter array, the miner SHOULD
reconnect to the same host and port it is currently connected to.
``mining.suggest_target()``
---------------------------
Request (optional):
{"id": 3, "method": "mining.suggest_target", "params": ["*TARGET*"]} ``\n``
``TARGET`` (hex)
The target suggested by the miner for the next received job and all subsequent
jobs (until the next time this message is sent).
The server SHOULD reply with ``mining.set_target``. The server MAY set the
result id equal to the request id.
Rationale
=========
Why does ``mining.subscribe`` include the host and port?
- It has the same use cases as the ``Host:`` header in HTTP. Specifically, it
enables virtual hosting, where virtual pools or private URLs might be used
for DDoS protection, but that are aggregated on Stratum server backends.
As with HTTP, the server CANNOT trust the host string.
- The port is included separately to parallel the ``client.reconnect`` method;
both are extracted from the server URL that the miner is connecting to (e.g.
``stratum+tcp://pool.example.com:3337``).
Why use the 256-bit target instead of a numerical difficulty?
- There is no protocol ambiguity when using a target. A server can pick a
specific target (by whatever algorithm), and enforce it cleanly on submitted
jobs.
- A numerical difficulty must be converted into a target by miners, which adds
unnecessary complexity, results in a loss of precision, and leaves ambiguity
over the conversion and the validity of resulting submissions.
- The minimum numerical difficulty in Bitcoin's Stratum protocol is 1, which
corresponds to ``powLimit``. This makes it harder to test miners and servers.
A target can represent difficulties lower than the minimum.
Does a 256-bit target waste bandwidth?
- The target is generally not set as often as solutions are submitted, so any
effect is minimal.
- Zcash's proof-of-work, Equihash, is much slower than Bitcoin's, so any latency
caused by the size of the target is minimal compared to the overall solver
time.
- For the current Equihash parameters (200/9), the Equihash solution dominates
bandwidth usage.
Why does ``mining.submit`` include ``WORKER_NAME``?
- ``WORKER_NAME`` is only included here for statistical purposes (like
monitoring performance and/or downtime). ``JOB_ID`` is used for pairing
server-stored jobs with submissions.
Reference Implementation
========================
- `str4d's standalone miner <https://github.com/str4d/zcash/tree/standalone-miner>`_
Acknowledgements
================
Thanks to:
- 5a1t for the initial brainstorming session.
- Daira Hopwood for hir input on API selection and design.
- Marek Palatinus (slush) and his colleagues for their refinements, suggestions, and
robust discussion.
- Jelle Bourdeaud'hui (razakal) and ocminer for their help with testing and
finding implementation bugs in the specification.
References
==========
.. [#RFC2119] `RFC 2119: Key words for use in RFCs to Indicate Requirement Levels <https://www.rfc-editor.org/rfc/rfc2119.html>`_
.. [#protocol-blockheader] `Zcash Protocol Specification, Version 2020.1.15. Section 7.3: Block Headers <protocol/protocol.pdf#blockheader>`_
.. [#protocol-difficulty] `Zcash Protocol Specification, Version 2020.1.15. Section 7.6.2: Difficulty filter <protocol/protocol.pdf#difficulty>`_
.. [#Slushpool-Stratum] `Stratum Mining Protocol. Slush Pool <https://slushpool.com/help/stratum-protocol/>`_
.. [#Bitcointalk-Stratum] `Stratum protocol documentation. Bitcoin Forum <https://bitcointalk.org/index.php?topic=557866.0>`_
.. [#P2Pool] `P2Pool. Bitcoin Wiki <https://en.bitcoin.it/wiki/P2Pool>`_
.. [#Bitcoin-Block] `Block Headers - Bitcoin Developer Reference. <https://developer.bitcoin.org/reference/block_chain.html#block-headers>`_
.. [#Bitcoin-CompactSize] `Variable length integer. Bitcoin Wiki <https://en.bitcoin.it/wiki/Protocol_documentation#Variable_length_integer>`_
.. [#JSON-RPC-1.0] `JSON-RPC 1.0 Specification (2005). <https://www.jsonrpc.org/specification_v1>`_
.. [#JSON-RPC-2.0] `JSON-RPC 2.0 Specification. The JSON-RPC Working Group. <https://www.jsonrpc.org/specification>`_
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