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<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 (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 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.
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