# Mempool/TX-pool synchronization

On startup the æternity node needs to update (or get from scratch if it is a\
completely new node) its list of unconfirmed transactions. New transactions it\
will get through gossip, but by construction older transactions that (for some\
reason) have not made it onto the chain is not gossiped.

Potentially there is a big overlap between the list of transactions held by the\
starting node (transactions are persisted) and the peer it is synchronizing\
with; thus to save network bandwidth it is preferable to only send the missing\
transactions. The protocol is straightforward:

1. the node picks one peer to synchronize with
2. they each create a MP tree holding their local transactions - In the MP\
   tree, each element has as key the hash of the signed transaction and as\
   (placeholder) value the empty list.
3. the initiator asks for a partial unfolding of the peer's tree
4. the initiator computes what parts of the tree it doesn't know about and\
   either goes back to 3. and asks for further information, or
5. the initiator asks for the missing transactions.

The P2P messages used are listed in [p2p\_messages](/developer-documentation/protocol/sync-1/p2p_messages.md).

## Unfold serialization

The instructions for unfolding an MP tree are sent over the P2P protocol, thus\
they need to be serialized. There are four different messages sent, they are\
listed below. Except for the types mentioned in[p2p\_messages](/developer-documentation/protocol/sync-1/p2p_messages.md) we also use the type `path` which is path to\
a node in an MP-tree, thus it could be an even or odd number of *nibbles*. RLP\
require an even number of nibbles, i.e. whole bytes, so we pad with a parity:

* even number of nibbles add byte with 0
* odd number of nibbles add a nibble with value 1

The result is an even number of nibbles and the first nibble indicate how it\
should be deserialized.

### UNFOLD\_NODE

Message is RLP encoded, fields:

* `type :: int` = 0
* `path :: path`
* `node :: byte_array`

### LEAF\_NODE

Message is RLP encoded, fields:

* `type :: int` = 1
* `leaf :: path`

### SUBTREE\_NODE

Message is RLP encoded, fields:

* `type :: int` = 2
* `subtree :: path`

### KEY\_NODE

Message is RLP encoded, fields:

* `type :: int` = 3
* `key :: byte_array`


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.aeternity.com/developer-documentation/protocol/sync-1/tx_pool_sync.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.