<pre>
  BIP: 159
  Layer: Peer Services
  Title: NODE_NETWORK_LIMITED service bit
  Author: Jonas Schnelli <dev@jonasschnelli.ch>
  Comments-Summary: No comments yet.
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0159
  Status: Draft
  Type: Standards Track
  Created: 2017-05-11
  License: BSD-2-Clause
</pre>

== Abstract ==

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

==Motivation==

Pruned peers can offer the same services as traditional peer except of serving all historical blocks.
Bitcoin right now only offers the NODE_NETWORK service bit which indicates that a peer can serve
all historical blocks.
# Pruned peers can relay blocks, headers, transactions, addresses and can serve a limited number of historical blocks, thus they should have a way how to announce their service(s)
# Peers no longer in initial block download should consider connecting some of its 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).
|}

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 the peers configuration, disk space, etc.) which can result in a fingerprinting weakness (finding the prune depth through getdata requests). NODE_NETWORK_LIMITED supporting peers <I>SHOULD</I> avoid leaking the prune depth and therefore not serve blocks deeper than the signaled <code>NODE_NETWORK_LIMITED</code> threshold (244 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 unwillingly connect to a pruned peer and ask for (filtered) blocks at a depth below their pruned depth. Light clients should therefore check the service bits (and eventually connect to peers signaling <code>NODE_NETWORK_LIMITED</code> if they require [filtered] blocks around the tip). Light clients obtaining peer IPs though DNS seed 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.
