Data Transfer

After a payment session is established via verification, the Seeder begins serving pieces to the Leecher while tracking payment checks. This phase reuses the normal BitTorrent request/piece/cancel messages and adds only local accounting.

Mapping Pieces to Cost

SeedPay doesn't modify BitTorrent wire messages for data transfer. Instead, the Seeder observes each request message and converts the byte length into monetary cost:

cost = bytes / (1024 × 1024) × price_per_mb

The Seeder maintains per payment session:

Field	Description
`channel_id`	Payment channel identifier
`channel_deposit`	Total amount locked in escrow
`last_check_nonce`	Highest nonce from valid payment checks
`last_check_amount`	Highest amount authorized by valid checks
`bytes_downloaded`	Cumulative bytes served
`price_per_mb`	Agreed rate from handshake

Tracking on successful piece send is RECOMMENDED for accuracy.

Serving Requests

For each request(index, begin, length) from the Leecher, the Seeder:

Looks up the payment session for this connection
Computes the cost of serving this block
Computes cumulative cost: (bytes_downloaded + length) / (1024 × 1024) × price_per_mb
Checks whether last_check_amount covers the cumulative_cost

If Payment is Sufficient

The Seeder increments bytes_downloaded and sends the piece message as normal.

If Payment is Insufficient

The Seeder MAY send a payment_check_required message:

{
  "type": "payment_check_required",
  "required_amount": 0.005,
  "current_check_amount": 0.003,
  "estimated_remaining_mb": 20.0
}

Before choking, the Seeder SHOULD wait a short grace period (e.g. 5 seconds) to allow in-flight payment checks to arrive. This prevents unnecessary choke/unchoke cycles.

After the grace period, if no valid check has been received, the Seeder chokes the Leecher until a new payment check arrives.

Payment Check Processing

When the Seeder receives a payment_check during data transfer:

Verify the signature using the Leecher's public key from channel state
Verify nonce is greater than last_check_nonce (prevents replay)
Verify amount is greater than or equal to last_check_amount (monotonically increasing)
Verify amount does not exceed channel_deposit

If all validations pass:

Update last_check_nonce and last_check_amount
Unchoke the Leecher if previously choked
Continue serving pieces

If validation fails:

Reject the check and keep the Leecher choked
MAY send a payment_check_rejected error (see Message Types)

Session Lifetime

A Seeder SHOULD treat a session as expired if:

The channel timeout has been reached
The connection is closed or idle for too long
The ephemeral keys have been deleted

When a session ends, the Seeder SHOULD:

Submit the highest valid payment check to close the channel cooperatively
Discard session state (channel tracking, usage counters)
Delete the ephemeral secret key (ensures forward secrecy)

The Seeder MAY continue to serve as a free seeder (if local policy allows) or close the connection entirely.