<pre>
  BIP: 159
  Layer: Peer Services
  Title: NODE_NETWORK_LIMITED service bit
  Authors: Jonas Schnelli <dev@jonasschnelli.ch>
  Status: Deployed
  Type: Specification
  Assigned: 2017-05-11
  License: BSD-2-Clause
</pre>

== Abstract ==

Define a service bit that allows pruned peers to signal their limited services.

==Motivation==

Pruned peers can offer the same services as traditional peers, except that of serving all historical blocks.
Bitcoin right now only offers the <code>NODE_NETWORK</code> service bit to indicate that a peer can serve
all historical blocks.
# Pruned peers can relay blocks, headers, transactions, and addresses, but they only guarantee serving a minimum number of historical blocks; thus, they should have a way to announce their service(s)
# Peers no longer in initial block download should consider connecting some of their outbound connections to pruned peers, to allow other peers to bootstrap from non-pruned peers

== Specification ==

=== New service bit ===

This BIP proposes a new service bit

{|class="wikitable"
|-
| NODE_NETWORK_LIMITED || bit 10 (0x400) || If signaled, the peer <I>MUST</I> be capable of serving at least the last 288 blocks (~2 days).
|}

Pruned/limited peers <I>MUST NOT</I> set a service bit that signals serving the complete block chain (e.g., <code>NODE_NETWORK</code>). Rationale: nodes that signal serving the complete block chain may also signal <code>NODE_NETWORK_LIMITED</code>.

A safety buffer of 144 blocks to handle chain reorganizations <I>SHOULD</I> be taken into account when connecting to a peer signaling the <code>NODE_NETWORK_LIMITED</code> service bit.

=== Address relay ===

Full nodes following this BIP <I>SHOULD</I> relay address/services (<code>addr</code> message) from peers they would connect to (including peers signaling <code>NODE_NETWORK_LIMITED</code>).

=== Counter-measures for peer fingerprinting ===

Peers may have different prune depths (depending on their configuration, disk space, etc.), which can result in a fingerprinting weakness (finding the prune depth through getdata requests).

Pruned nodes should therefore avoid leaking the prune depth and <I>SHOULD NOT</I> serve blocks deeper than the signaled <code>NODE_NETWORK_LIMITED</code> threshold of 288 blocks.

=== Risks ===

Pruned peers following this BIP may consume more outbound bandwidth.

Light clients (and such) who are not checking the <code>nServiceFlags</code> (service bits) from a relayed <code>addr</code>-message may unwittingly connect to a pruned peer and ask for (filtered) blocks at a depth below the peer’s pruned depth. Light clients should therefore check the service bits and either (1) connect to peers signaling <code>NODE_NETWORK_LIMITED</code> that preferably do not also signal serving the full block chain, if they only require (filtered) blocks around the tip, or (2) connect to peers signaling serving the full block chain if they need data older than the latest 288 blocks. Light clients obtaining peer IPs through DNS seeds should use the DNS filtering option.

== Compatibility ==

This proposal is backward compatible.

== Reference implementation ==

* https://github.com/bitcoin/bitcoin/pull/11740 (signaling)
* https://github.com/bitcoin/bitcoin/pull/10387 (connection and relay)

== Copyright ==

This BIP is licensed under the 2-clause BSD license.
