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Proposed protocol extension
We extend the protocol to enable indexers to store and provide access to the BTC's mempool.
We add following messages.
Message types:
| Message ID | Name |
|---|---|
| 15 | FullFilterInv |
| 16 | GetFullFilter |
| 17 | FullFilter |
| 18 | GetMemFilters |
| 19 | MemFilters |
| 20 | GetMempool |
| 21 | MempoolChunk |
The three stage Notification-Request-Response scheme for full filters was chosen since a full mempool filter might get about 1Mb in size and it is not the best idea to include it in every notification.
When the indexer computes a new set of filters, it notifies all clients with FullFilterInv message.
This message carries no payload.
When the client wants to download the new full filter it sends GetFullFilter. It is not required that GetFullFilter is sent after FullFilterInv. The client might keep track of the mempool itself and ignore FullFilterInv. Or it might decide to download filters tree directly. The server must answer with FullFilter message.
This message carries no payload.
This message is sent by the server in response to GetFullFilter message. The whole payload is a single filter.
| Field | Type | Size | Description |
|---|---|---|---|
| Length | uint32 | VarInt | Length of the filter |
| Filter | ubyte[] | Length | GZIP encoded bytes of Filters |
If the client spots, via the full filter, that there is a client's transaction in the mempool, it sends a request to get all filters for mempool branches. The server must respond with MemFilters.
This message carries no payload.
Sent in response to GetMemFilters. The payload contains a list (a map) of filters with corresponding prefixes. Filters are compressed with GZIP.
| Field | Type | Size | Description |
|---|---|---|---|
| FiltersLen | uint32 | VarInt | Amount of filter-prefix pairs |
| FilterPrefPairs | FilterPrefPair[] | ? | Array of pairs |
| Field | Type | Size | Description |
|---|---|---|---|
| TxPrefix | ubyte[] | 2 | TxPrefix for the filter |
| Length | uint32 | VarInt | Length of the filter |
| Filter | ubyte[] | Length | GZIP encoded bytes of Filters |
If the client wants to get parts (or all) of the mempool, it has to send GetMempool with specific TxPrefixes. The indexer must respond with Mempool message.
Zero prefixes shall by convention mean "Send all chunks in response"
| Field | Type | Size | Description |
|---|---|---|---|
| Amount | uint32 | VarInt | Amount of prefixes |
| Prefixes | TxPrefix[] | Amount*sizeof(TxPrefix) | TxPrefixes |
sizeof(TxPrefix) = 2 bytes
This message contains transactions from mempool chunks requested via GetMempool. In response the indexer sends as many MempoolChunks messages as requested.
To save traffic, each chunk is compressed with GZIP
| Field | Type | Size | Description |
|---|---|---|---|
| Prefix | TxPrefix | 2 | The TxPrefix of the chunk |
| Amount | uint32 | VarInt | Amount of transactions in the chunk |
| Length | uint32 | VarInt | Length of the data |
| Data | ubyte[] | ? | GZIP encoded bytes of chunk data |
The data in MempoolChunk is constructed as follows:
- All transactions in a chunk are encoded as per Bitcoin protocol
- Resulting bytes are concatenated
- Resulting bytearray is compressed/encoded with GZIP