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full_node.md

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1 - What is a full node ?

A full client, or “full node” is a client that stores the entire history of bitcoin transactions (every transaction by every user, ever). It’s a ledger.
It can manage the user’s wallets. It’s a bank.
It can initiate transactions directly on the bitcoin network. It’s a bank + a p2p node.
After being validated by miners, blocks are broadcasted to the network, via the full nodes. It’s a broadcaster.

In John Newbery's presentation, he said that he would call a full node:

something that is on the block propagation network and is validating the blockchain.

He made a difference a difference between the block propagation network and the transaction relay network which is more linked to mining nodes. After being validated by miners, blocks are broadcasted to the network, via the full nodes.

2 - What are the incentives to run a full node?

In The Onion Model of Blockchain Security written by Hasu, the authors talked about 4 guarantees given by the bitcoin security model: cryptographic, consensus, economic and social.
Each one of them is a layer that protect bitcoin viability.
We can use the same point of vue, but for the incentive.

  • Social incentive

    • Being part of a community
    • Being part of a network that you contribute to strengthen
    • Have your word to say (vote) in case of disagreement with devs or miners

  • Economic

    • Protect your financial sovereignty
    • Being your own bank
    • Estimate your fees
      • By having a mempool, on which you can find unconfirmed transactions

  • Consensus

    • Being an honest node for the peers that want to download the blockchain and are open to incoming connections.
      • To fight sybil attacks (Thus, if a Sybil attacker completely surrounded a new node that was syncing from scratch, it could create some short blockchains at low heights at almost no cost, but only up to the various checkpointed blocks.). Bitcoin Security model by Jameson Lopp
    • To maintain and validate the blockchain, with the balance, the old transactions and by adding the new ones.
      • To fight Byzantine Fault tolerance
      • We talked about it in the Bitcoin’s Academic Pedigree paper where they said that in every distributed ledger system, there is a risk related to the state replication and the order of the state transition
      • The solution was to enable a set of nodes to apply the same state transitions in the same order

  • Cryptographic

    • Hold your private keys
    • Generate multiple addresses, multiple identities

3 - Why accept incoming connections?

You need to connect to others for the IBD (Initial Blockchain Download), to retrieve the blockchain.
Also, it is needed to relay blocks to other peers.
Finally, that’s how you retrieve unconfirmed transactions.
But, you can still protect yourself from sybil attacks by accepting only the DNS seeds that have been hard coded into the software.

  • Among the peers that run those seeds, you have Pieter Wuille and Luke Dashjr.

4 - Costs of accepting incoming connections

  • Memory & Storage Usage: The blockchain require a lot of available space and some important memory for the consensus checks
  • Bandwidth, with the incoming transactions and blocks
  • We can receive spamming alerts from network nodes, as a DoS attack
    • There is no protection against it.
  • Sybil attacks if fully surrounded by dishonest nodes
  • Privacy