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<div class="document" id="bittorrent-dht-security-extension">
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<div id="container">
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<table id="header">
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<tr><td id="orange"></td>
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<td id="logo">libtorrent</td></tr>
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</table>
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<div id="main">
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<h1 class="title">BitTorrent DHT security extension</h1>
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<table class="docinfo" frame="void" rules="none">
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<col class="docinfo-name" />
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<col class="docinfo-content" />
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<tbody valign="top">
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<tr><th class="docinfo-name">Author:</th>
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<td>Arvid Norberg, <a class="last reference external" href="mailto:arvid@libtorrent.org">arvid@libtorrent.org</a></td></tr>
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<tr><th class="docinfo-name">Version:</th>
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<td>1.1.0</td></tr>
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</tbody>
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</table>
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<div class="contents topic" id="table-of-contents">
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<p class="topic-title first">Table of contents</p>
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<ul class="simple">
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<li><a class="reference internal" href="#id1" id="id2">BitTorrent DHT security extension</a></li>
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<li><a class="reference internal" href="#considerations" id="id3">considerations</a></li>
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<li><a class="reference internal" href="#node-id-restriction" id="id4">Node ID restriction</a></li>
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<li><a class="reference internal" href="#bootstrapping" id="id5">bootstrapping</a></li>
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<li><a class="reference internal" href="#rationale" id="id6">rationale</a></li>
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<li><a class="reference internal" href="#enforcement" id="id7">enforcement</a></li>
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<li><a class="reference internal" href="#backwards-compatibility-and-transition" id="id8">backwards compatibility and transition</a></li>
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<li><a class="reference internal" href="#forward-compatibility" id="id9">forward compatibility</a></li>
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</ul>
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</div>
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<div class="section" id="id1">
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<h1>BitTorrent DHT security extension</h1>
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<p>The purpose of this extension is to make it harder to launch a few
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specific attacks against the BitTorrent DHT and also to make it harder
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to snoop the network.</p>
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<p>Specifically the attack this extension intends to make harder is launching
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8 or more DHT nodes which node-IDs selected close to a specific target
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info-hash, in order to become the main nodes hosting peers for it. Currently
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this is very easy to do and lets the attacker not only see all the traffic
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related to this specific info-hash but also block access to it by other
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peers.</p>
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<p>The proposed guard against this is to enforce restrictions on which node-ID
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a node can choose, based on its external IP address.</p>
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</div>
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<div class="section" id="considerations">
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<h1>considerations</h1>
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<p>One straight forward scheme to tie the node ID to an IP would be to hash
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the IP and force the node ID to share the prefix of that hash. One main
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draw back of this approach is that an entities control over the DHT key
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space grows linearly with its control over the IP address space.</p>
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<p>In order to successfully launch an attack, you just need to find 8 IPs
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whose hash will be <em>closest</em> to the target info-hash. Given the current
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size of the DHT, that is quite likely to be possible by anyone in control
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of a /8 IP block.</p>
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<p>The size of the DHT is approximately 8.4 million nodes. This is estmiated
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by observing that a typical routing table typically has about 20 of its
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top routing table buckets full. That means the key space is dense enough
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to contain 8 nodes for every combination of the 20 top bits of node IDs.</p>
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<blockquote>
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<tt class="docutils literal">2^20 * 8 = 8388608</tt></blockquote>
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<p>By controlling that many IP addresses, an attacker could snoop any info-hash.
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By controlling 8 times that many IP addresses, an attacker could actually
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take over any info-hash.</p>
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<p>With IPv4, snooping would require a /8 IP block, giving access to 16.7 million
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Ips.</p>
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<p>Another problem with hashing the IP is that multiple users behind a NAT are
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forced to run their DHT nodes on the same node ID.</p>
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</div>
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<div class="section" id="node-id-restriction">
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<h1>Node ID restriction</h1>
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<p>In order to avoid the number node IDs controlled to grow linearly by the number
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of IPs, as well as allowing more than one node ID per external IP, the node
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ID can be restricted at each class level of the IP.</p>
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<p>Another important property of the restriction put on node IDs is that the
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distribution of the IDs remoain uniform. This is why CRC32C (Castagnoli) was
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chosen as the hash function.</p>
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<p>The expression to calculate a valid ID prefix (from an IPv4 address) is:</p>
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<pre class="literal-block">
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crc32c((ip & 0x030f3fff) | (r << 29))
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</pre>
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<p>And for an IPv6 address (<tt class="docutils literal">ip</tt> is the high 64 bits of the address):</p>
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<pre class="literal-block">
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crc32c((ip & 0x0103070f1f3f7fff) | (r << 61))
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</pre>
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<p><tt class="docutils literal">r</tt> is a random number in the range [0, 7]. The resulting integer,
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representing the masked IP address is supposed to be big-endian before
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hashed. The "|" operator means bit-wise OR.</p>
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<p>The details of implementing this is to evaluate the expression, store the
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result in a big endian 64 bit integer and hash those 8 bytes with CRC32C.</p>
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<p>The first (most significant) 21 bits of the node ID used in the DHT MUST
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match the first 21 bits of the resulting hash. The last byte of the hash MUST
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match the random number (<tt class="docutils literal">r</tt>) used to generate the hash.</p>
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<img alt="ip_id_v4.png" src="ip_id_v4.png" />
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<img alt="ip_id_v6.png" src="ip_id_v6.png" />
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<p>Example code code for calculating a valid node ID:</p>
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<pre class="literal-block">
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uint8_t* ip; // our external IPv4 or IPv6 address (network byte order)
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int num_octets; // the number of octets to consider in ip (4 or 8)
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uint8_t node_id[20]; // resulting node ID
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uint8_t v4_mask[] = { 0x03, 0x0f, 0x3f, 0xff };
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uint8_t v6_mask[] = { 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff };
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uint8_t* mask = num_octets == 4 ? v4_mask : v6_mask;
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for (int i = 0; i < num_octets; ++i)
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ip[i] &= mask[i];
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uint32_t rand = std::rand() & 0xff;
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uint8_t r = rand & 0x7;
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ip[0] |= r << 5;
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uint32_t crc = 0;
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crc = crc32c(crc, ip, num_octets);
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// only take the top 21 bits from crc
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node_id[0] = (crc >> 24) & 0xff;
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node_id[1] = (crc >> 16) & 0xff;
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node_id[2] = ((crc >> 8) & 0xf8) | (std::rand() & 0x7);
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for (int i = 3; i < 19; ++i) node_id[i] = std::rand();
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node_id[19] = rand;
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</pre>
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<p>test vectors:</p>
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<pre class="literal-block">
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IP rand example node ID
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============ ===== ==========================================
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124.31.75.21 1 <strong>5fbfbf</strong> f10c5d6a4ec8a88e4c6ab4c28b95eee4 <strong>01</strong>
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21.75.31.124 86 <strong>5a3ce9</strong> c14e7a08645677bbd1cfe7d8f956d532 <strong>56</strong>
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65.23.51.170 22 <strong>a5d432</strong> 20bc8f112a3d426c84764f8c2a1150e6 <strong>16</strong>
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84.124.73.14 65 <strong>1b0321</strong> dd1bb1fe518101ceef99462b947a01ff <strong>41</strong>
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43.213.53.83 90 <strong>e56f6c</strong> bf5b7c4be0237986d5243b87aa6d5130 <strong>5a</strong>
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</pre>
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<p>The bold parts of the node ID are the important parts. The rest are
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random numbers. The last bold number of each row has only its most significant
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bit pulled from the CRC32C function. The lower 3 bits are random.</p>
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</div>
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<div class="section" id="bootstrapping">
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<h1>bootstrapping</h1>
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<p>In order to set ones initial node ID, the external IP needs to be known. This
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is not a trivial problem. With this extension, <em>all</em> DHT responses SHOULD include
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a <em>top-level</em> field called <tt class="docutils literal">ip</tt>, containing a compact binary representation of
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the requestor's IP and port. That is big endian IP followed by 2 bytes of big endian
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port.</p>
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<p>The IP portion is the same byte sequence used to verify the node ID.</p>
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<p>It is important that the <tt class="docutils literal">ip</tt> field is in the top level dictionary. Nodes that
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enforce the node-ID will respond with an error message ("y": "e", "e": { ... }),
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whereas a node that supports this extension but without enforcing it will respond
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with a normal reply ("y": "r", "r": { ... }).</p>
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<p>A DHT node which receives an <tt class="docutils literal">ip</tt> result in a request SHOULD consider restarting
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its DHT node with a new node ID, taking this IP into account. Since a single node
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can not be trusted, there should be some mechanism to determine whether or
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not the node has a correct understanding of its external IP or not. This could
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be done by voting, or only restart the DHT once at least a certain number of
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nodes, from separate searches, tells you your node ID is incorrect.</p>
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</div>
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<div class="section" id="rationale">
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<h1>rationale</h1>
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<p>The choice of using CRC32C instead of a more traditional cryptographic hash
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function is justified primarily of these reasons:</p>
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<ol class="arabic simple">
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<li>it is a fast function</li>
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<li>produces well distributed results</li>
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<li>there is no need for the hash function to be one-way (the input set is
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so small that any hash function could be reversed).</li>
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<li>CRC32C (Castagnoli) is supported in hardware by SSE 4.2, which can
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significantly speed up computation</li>
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</ol>
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<p>There are primarily two tests run on SHA-1 and CRC32C to establish the
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distribution of results. The first one is the number of bits in the output
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set that contain every possible combination of bits. The CRC32C function
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has a longer such prefix in its output than SHA-1. This means nodes will still
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have well uniformly distributed IDs, even when IP addresses in use are not
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uniformly distributed.</p>
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<p>The following graph illustrate a few different hash functions with regard
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to this property.</p>
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<img alt="complete_bit_prefixes.png" src="complete_bit_prefixes.png" />
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<p>This test takes into account IP addresses that are not globally routable, i.e.
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reserved for local networks, multicast and other things. It also takes into
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account that some /8 blocks are not in use by end-users and exremely unlikely
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to ever run a DHT node. This makes the results likely to be very similar to
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what we would see in the wild.</p>
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<p>These results indicate that CRC32C provides the best uniformity in the results
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in terms of bit prefixes where all possibilities are represented, and that
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no more than 21 bits should be used from the result. If more than 21 bits
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were to be used, there would be certain node IDs that would be impossible to
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have, which would make routing sub-optimal.</p>
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<p>The second test is more of a sanity test for the uniform distribution property.
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The target space (32 bit interger) is divided up into 1000 buckets. Every valid
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IP and <tt class="docutils literal">r</tt> input is run through the algorithm and the result is put in the
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bucket it falls in. The expectation is that each bucket has roughly an equal
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number of results falling into it. The following graph shows the resulting
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histogram, comparing SHA-1 and CRC32C.</p>
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<img alt="hash_distribution.png" src="hash_distribution.png" />
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<p>The source code for these tests can be found <a class="reference external" href="https://github.com/arvidn/hash_complete_prefix">here</a>.</p>
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<p>The reason to use CRC32C instead of the CRC32 implemented by zlib is that
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Intel CPUs have hardware support for the CRC32C calculations. The input
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being exactly 4 bytes is also deliberate, to make it fit in a single
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instruction.</p>
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</div>
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<div class="section" id="enforcement">
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<h1>enforcement</h1>
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<p>Once enforced, write tokens from peers whose node ID does not match its external
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IP should be considered dropped. In other words, a peer that uses a non-matching
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ID MUST never be used to store information on, regardless of which request. In the
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original DHT specification only <tt class="docutils literal">announce_peer</tt> stores data in the network,
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but any future extension which stores data in the network SHOULD use the same
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restriction.</p>
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<p>Any peer on a local network address is exempt from this node ID verification.
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This includes the following IP blocks:</p>
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<dl class="docutils">
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<dt>10.0.0.0/8</dt>
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<dd>reserved for local networks</dd>
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<dt>172.16.0.0/12</dt>
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<dd>reserved for local networks</dd>
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<dt>192.168.0.0/16</dt>
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<dd>reserved for local networks</dd>
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<dt>169.254.0.0/16</dt>
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<dd>reserved for self-assigned IPs</dd>
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<dt>127.0.0.0/8</dt>
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<dd>reserved for loopback</dd>
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</dl>
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</div>
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<div class="section" id="backwards-compatibility-and-transition">
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<h1>backwards compatibility and transition</h1>
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<p>During some transition period, this restriction should not be enforced, and
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peers whose node ID does not match this formula relative to their external IP
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should not be blocked.</p>
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<p>Requests from peers whose node ID does not match their external IP should
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always be serviced, even after the transition period. The attack this protects
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from is storing data on an attacker's node, not servicing an attackers request.</p>
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</div>
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<div class="section" id="forward-compatibility">
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<h1>forward compatibility</h1>
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<p>If the total size of the DHT grows to the point where the inherent size limit
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in this proposal is too small, the modulus constants can be updated in a new
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proposal, and another transition period where both sets of modulus constants
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are accepted.</p>
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