# 6. Job Declaration Protocol

The Job Declaration Protocol is used to coordinate the creation of custom work, avoiding scenarios where Pools are unilaterally imposing work on miners.

Pools that opt into this protocol are only responsible for accounting shares and distributing rewards.

This is a key feature of Stratum V2 that improves Bitcoin decentralization.

# 6.1 Job Declarator Server

The Job Declarator Server (JDS) is deployed on Pool side, although it could be theoretically outsourced to a third party that is trusted by the Pool.

In order to fully implement the Server side of the Job Declaration Protocol, the JDS also needs to exchange RPCs (or similar) with a Bitcoin Node.

It is responsible for:

• Allocating tokens that JDC will use to declare Custom Jobs.
• Acknowledging declaration of Custom Jobs associated with specific allocated tokens.
• Maintaining an internal mempool (via RPCs (or similar) to a Bitcoin Node).
• Requesting identification for transactions on some declared Custom Job.
• Requesting missing transactions on some declared Custom Job.
• Publishing valid block submissions received from JDC.

# 6.2 Job Declarator Client

The Job Declarator Client (JDC) is deployed on the miner side.

In order to fully implement the Client side of the Job Declaration Protocol, the JDC also needs to operate under the Template Distribution and Mining Protocols.

It is responsible for:

• Receiving Templates from the Template Provider (via Template Distribution Protocol).
• Declaring Custom Jobs to JDS (via Job Declaration Protocol under Full-Template mode).
• Notifying declared Custom Jobs to Pool (via Mining Protocol).
• Receiving Shares from downstream Mining Devices working on Custom Jobs (via Mining Protocol).
• Submitting Shares for Custom Jobs to Pool.
• Publishing valid blocks found by downstream Mining Devices (both to Template Provider (TP) and JDS).

Additionally, if:

• JDS fails to respond with an AllocateMiningJobToken.Success in a reasonable time.
• JDS rejects some Custom Job declaration via DeclareMiningJob.Error.
• Pool rejects valid shares under a Custom Job that was previously acknowledged via SetCustomMiningJob.Success and/or DeclareMiningJob.Success.

JDC is also responsible for switching to a new Pool+JDS (or solo mining as a last resort).

This fallback strategy incentivizes honesty on Pool side, otherwise it will lose hashrate by rejecting Shares for a Custom Job that was already acknowledged to be valid.

# 6.3 Job Declaration Modes

# 6.3.1 Coinbase-only Mode

Under Coinbase-only mode:

• JDS allocates mining_job_token to JDC (AllocateMiningJobToken / AllocateMiningJobToken.Success).
• Pool evaluates fee revenue of some proposed work (uniquely identified by mining_job_token) by looking at the coinbase.
• JDC never reveals the tx data contained in the template (in other words, the transaction set).
• proposed work is acknowledged as valid by Pool via SetCustomMiningJob.Success.
• the DeclareMiningJob message is never used.
• if a valid block is found, JDC propagates it unilaterally.

In other words, Pool + JDS operating under Coinbase-only mode do not require to ever know which transactions are included in the miner template, preserving the privacy around the miner's mempool.

This leaves Pool arguably vulnerable to an attack where miner declares a coinbase with some fee revenue while one of the following situations is true:

• the template actually has a different fee revenue.
• the template has invalid transactions.

This potential attack vector is in many ways identical to block withholding and defenses against block withholding generally apply.

Some pools may wish to utilize additional mitigations. For example, Zero-Knowledge-Proof based protocol extensions, where JDC proves that the fee revenue on the coinbase belongs to a valid template, without ever revealing the template itself.

As soon as JDC has:

• a token (allocated by JDS), so it can use it to identify some unique work that it wants to advertise
• a new template

It SHOULD start sending the jobs downstream for hashing right away. The proof-of-work (shares) SHOULD be cached until JDC receives an acknowledgement via SetCustomMiningJob.Success.

We call this optimistic mining.

# 6.3.2 Full-Template Mode

Under Full-Template mode:

• JDS allocates mining_job_token to JDC (AllocateMiningJobToken / AllocateMiningJobToken.Success).
• JDC sends DeclareMiningJob to JDS (containing the txid of every transaction in the template).
• JDS could potentially request for JDC to reveal txdata (via ProvideMissingTransactions).
• JDS acknowledges proposed work via DeclareMiningJob.Success.
• JDC notifies work to Pool via SetCustomMiningJob, acknowledged via SetCustomMiningJob.Success.
• if a valid block is found, both JDC and JDS propagate it.

Similarly to Coinbase-only, as soon as JDC has:

• a token (allocated by JDS), so it can use it to identify some unique work that it wants to advertise
• a new template

It SHOULD do optimistic mining by sending the jobs downstream for hashing right away. The proof-of-work (shares) SHOULD be cached until JDC receives an acknowledgement via SetCustomMiningJob.Success.

# 6.3.3 Coinbase-only vs Full-Template

The table below shows a comparison between the two Sv2 Job Declation Modes:

	Coinbase-only	Full-Template
knowledge of fee revenue	• JDC • Pool (while blind against tx data)	• JDC • Pool (while blind against tx data) • JDS (verifying against revealed tx data on behalf of Pool)
knowledge of txdata on the template	• JDC	• JDC • JDS (on behalf of Pool)
ability to broadcast mined block	• JDC	• JDC • JDS (on behalf of Pool)

# 6.4 Job Declaration Protocol Messages

# 6.4.1 SetupConnection Flags for Job Declaration Protocol

Flags usable in SetupConnection.flags and SetupConnection.Error::flags:

Field Name	Bit	Description
DECLARE_TX_DATA	0	JDC agrees to reveal the template's txdata via DeclareMiningJob and ProvideMissingTransactions.

No flags are yet defined for use in SetupConnection.Success.

# 6.4.2 AllocateMiningJobToken (JDC -> JDS)

A request to get an identifier for a future-submitted mining job. Rate limited to a rather slow rate and only available on connections where this has been negotiated. Otherwise, only mining_job_token(s) from AllocateMiningJobToken.Success are valid.

Field Name	Data Type	Description
user_identifier	STR0_255	Unconstrained sequence of bytes. Whatever is needed by the pool to identify/authenticate the client, e.g. "braiinstest". Additional restrictions can be imposed by the pool. It is highly recommended that UTF-8 encoding is used.
request_id	U32	Unique identifier for pairing the response

# 6.4.3 AllocateMiningJobToken.Success (Server -> Client)

The Server MUST NOT change the value of coinbase_output_max_additional_size in AllocateMiningJobToken.Success messages unless required for changes to the pool’s configuration. Notably, if the pool intends to change the space it requires for coinbase transaction outputs regularly, it should simply prefer to use the maximum of all such output sizes as the coinbase_output_max_additional_size value.

Field Name	Data Type	Description
request_id	U32	Unique identifier for pairing the response
mining_job_token	B0_255	Token that makes the client eligible for committing a mining job for approval/transaction declaration or for identifying custom mining job on mining connection.
coinbase_tx_outputs	B0_64K	Serialized Bitcoin transaction outputs added by the pool.

The following rules apply to AllocateMiningJobToken.Success.coinbase_tx_outputs:

• JDS MUST reserve the first output with a locking script where the pool payout will go. While this output is initally set with a 0 amount of sats, this convention designates this locking script as the pool payout output.
• JDS MAY add more 0 value outputs in addition to the pool payout output.

Once JDC receives AllocateMiningJobToken.Success, the following rules apply to the subsequent DeclareMiningJob.coinbase_tx_suffix (if under Full-Template mode) and SetCustomMiningJob.coinbase_tx_outputs (under both Full-Template and Coinbase-only modes) message fields:

• JDC MUST allocate sats into the pool payout output in order to qualify for pooled mining rewards. JDS and Pool SHOULD reject custom jobs that fail to do so.
• JDC MAY add more 0 value outputs in addition to the ones established by JDS.
• JDC MAY add more non-0 value outputs in addition to the ones established by JDS. In doing so, the template revenue is not fully allocated to the designated pool payout output, so Pool MAY pay proportionally smaller rewards for this job.
• JDC MAY arbitrarily reorder the outputs to something different from the original ordering of AllocateMiningJobToken.Success.coinbase_tx_outputs.
• Under Full-Template mode, the order of the outputs under DeclareMiningJob.coinbase_tx_suffix and SetCustomMiningJob.coinbase_tx_outputs MUST remain the same, even if they differ from the original AllocateMiningJobToken.Success.coinbase_tx_outputs.

In summary, the pool payout goes to one single output (standardized as the first output of AllocateMiningJobToken.Success.coinbase_tx_outputs), and JDC is free to add more outputs and shuffle them under any arbitrary ordering scheme.

Here's a few examples for clarification:

# Example A

Example A illustrates JDS and JDC adding arbitrary 0 valued outputs in addition to the pool payout output.

Outputs in AllocateMiningJobToken.Success.coinbase_tx_outputs:

Script	Amount (sats)	Description
P2WPKH	0	pool payout output
OP_RETURN X	0	arbitrary unspendable output imposed by JDS
OP_RETURN Y	0	arbitrary unspendable output imposed by JDS

Outputs in DeclareMiningJob.coinbase_tx_suffix and/or SetCustomMiningJob.coinbase_tx_outputs:

Script	Amount (sats)	Description
OP_RETURN Z	0	arbitrary unspendable output added by JDC
P2WPKH	template_revenue	pool payout output
OP_RETURN X	0	arbitrary unspendable output imposed by JDS
OP_RETURN Y	0	arbitrary unspendable output imposed by JDS
OP_RETURN W	0	SegWit commitment added by Template Provider

# Example B

Example B illustrates JDC allocating part of the template revenue to some outputs that are not the pool payout. Here, the Pool MAY pay proportionally smaller rewards for this custom job, since the template revenue is not fully allocated to the pool payout output.

Ordered outputs in AllocateMiningJobToken.Success.coinbase_tx_outputs:

Script	Amount (sats)	Description
P2TR	0	pool payout output

Ordered outputs in DeclareMiningJob.coinbase_tx_suffix and/or SetCustomMiningJob.coinbase_tx_outputs:

Script	Amount (sats)	Description
P2PKH	X	arbitrary output added by JDC
P2WSH	Y	arbitrary output added by JDC
P2TR	template_revenue - X - Y	pool payout output
OP_RETURN	0	SegWit commitment added by Template Provider

# 6.4.4 DeclareMiningJob (Client -> Server)

Only used in Full-Template mode.

A request sent by JDC that proposes a selected set of transactions to JDS.

Field Name	Data Type	Description
request_id	U32	Unique identifier for pairing the response
mining_job_token	B0_255	Previously reserved mining job token received by AllocateMiningJobToken.Success
version	U32	Version header field. To be later modified by BIP320-consistent changes.
coinbase_tx_prefix	B0_64K	Serialized bytes representing the initial part of the coinbase transaction (not including extranonce)
coinbase_tx_suffix	B0_64K	Serialized bytes representing the final part of the coinbase transaction (after extranonce)
tx_ids_list	SEQ0_64K[U256]	List of hashes of the transaction set contained in the template. JDS checks the list against its mempool and requests missing txs via ProvideMissingTransactions. Does not include the coinbase transaction (as there is no corresponding full data for it yet).
excess_data	B0_64K	Extra data which the Pool may require to validate the work (as defined in the Template Distribution Protocol)

# 6.4.5 DeclareMiningJob.Success (Server -> Client)

A response sent by JDS acknowledging some Custom Job declaration.

JDS MAY request txdata via ProvideMissingTransactions before making this commitment.

Since JDS could be third party (not necessarily integrated to Pool), DeclareMiningJob.Success.new_mining_job_token MAY carry a cryptographic commitment from JDS where:

• JDS acknowledges that the template declared under DeclareMiningJob.mining_job_token is valid
• it signals to JDC that it should use SetCustomMininingJob.mining_job_token = DeclareMiningJob.Success.new_mining_job_token when declaring Custom Jobs to Pool as a way to prove it went through the Full-Template declaration process

Field Name	Data Type	Description
request_id	U32	Identifier of the original request.
new_mining_job_token	B0_255	Unique custom work identifier.

# 6.4.6 DeclareMiningJob.Error (Server->Client)

A response sent by JDS rejecting some Custom Job declaration.

This should be a trigger for fallback into some other Pool+JDS or solo mining.

Field Name	Data Type	Description
request_id	U32	Identifier of the original request
error_code	STR0_255
error_details	B0_64K	Optional data providing further details to given error

Possible error codes:

• invalid-mining-job-token
• invalid-job-param-value-{} - {} is replaced by a particular field name from DeclareMiningJob message

# 6.4.7 ProvideMissingTransactions (Server->Client)

Only used in Full-Template mode.

If DeclareMiningJob includes some transactions that JDS's mempool has not yet seen, then JDS needs to request that JDC provides those missing ones.

Field Name	Data Type	Description
request_id	U32	Identifier of the original AllocateMiningJobToken request
unknown_tx_position_list	SEQ0_64K[U16]	A list of unrecognized transactions that need to be supplied by the Job Declarator in full. They are specified by their position in the original DeclareMiningJob message, 0-indexed not including the coinbase transaction transaction.

# 6.4.8 ProvideMissingTransactions.Success (Client->Server)

This is a message to push transactions that the server did not recognize and requested them to be supplied in ProvideMissingTransactions.

Field Name	Data Type	Description
request_id	U32	Identifier of the original AllocateMiningJobToken request ""
transaction_list	SEQ0_64K[B0_16M]	List of full transactions as requested by ProvideMissingTransactions, in the order they were requested in ProvideMissingTransactions

# 6.4.9 PushSolution (Client -> Server)

Only used in Full-Template mode.

Sent by JDC as soon as a valid block is found, so that it can be propagated also by JDS.

In the meantime, the block is also transmitted to the network by JDC through the SubmitSolution message under in Template Distribution Protocol.

In this way, a valid solution is immediately propagated on both client and server sides, decreasing the chance of the block being orphaned by the network.

Field Name	Data Type	Description
extranonce	B0_32	Extranonce bytes which need to be added to coinbase to form a fully valid submission. (This is the full extranonce)
prev hash	U256	Hash of the last block
nonce	U32	Nonce leading to the hash being submitted
ntime	U32	The nTime field in the block header.
nbits	U32	The nBits field in the block header.
version	U32	Version header field.