i2p.www/www.i2p2/pages/how_networkdatabase.html

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<p>I2P's netDb is a specialized distributed database, containing 
just two types of data - router contact information and destination contact
information.  Each piece of data is signed by the appropriate party and verified
by anyone who uses or stores it.  In addition, the data has liveliness information
within it, allowing irrelevent entries to be dropped, newer entries to replace
older ones, and, for the paranoid, protection against certain classes of attack.
(Note that this is also why I2P bundles in the necessary code to determine the
correct time).</p>

<p>
The netDb is distributed with a simple technique called "floodfill".
Previously, the netDb also used kademlia as a fallback algorithm. However,
it did not work will in our application, and it was completely disabled
in release 0.6.1.20. More information is <a href="status">below</a>.
</p>

<p>
Note that this document has been updated to include floodfill details,
but there are still some incorrect statements about the use of kademlia that
need to be fixed up.
</p>

<h2><a name="routerInfo">RouterInfo</a></h2>

<p>When an I2P router wants to contact another router, they need to know some 
key pieces of data - all of which are bundled up and signed by the router into
a structure called the "RouterInfo", which is distributed under the key derived 
from the SHA256 of the router's identity.  The structure itself contains:</p><ul>
<li>The router's identity (a 2048bit ElGamal encryption key, a 1024bit DSA signing key, and a certificate)</li>
<li>The contact addresses at which it can be reached (e.g. TCP: dev.i2p.net port 4108)</li>
<li>When this was published</li>
<li>A set of arbitrary (uninterpreted) text options</li>
<li>The signature of the above, generated by the identity's DSA signingkey</li>
</ul>

<p>The arbitrary text options are currently used to help debug the network,
publishing various stats about the router's health.  These stats will be disabled
by default once I2P 1.0 is out, and can be disabled by adding 
"router.publishPeerRankings=false" to the router 
<a href="http://localhost:7657/configadvanced.jsp">configuration</a>.  The data 
published can be seen on the router's <a href="http://localhost:7657/netdb.jsp">netDb</a>
page, but should not be trusted.</p>

<h2><a name="leaseSet">LeaseSet</a></h2>

<p>The second piece of data distributed in the netDb is a "LeaseSet" - documenting
a group of tunnel entry points (leases) for a particular client destination.  
Each of these leases specify the tunnel's gateway router (with the hash of its 
identity), the tunnel ID on that router to send messages (a 4 byte number), and
when that tunnel will expire.  The LeaseSet itself is stored in the netDb under
the key derived from the SHA256 of the destination.</p>
  
<p>In addition to these leases, the LeaseSet also includes the destination 
itself (namely, the destination's 2048bit ElGamal encryption key, 1024bit DSA 
signing key, and certificate) as well as an additional pair of signing and 
encryption keys.  These additional keys can be used for garlic routing messages
to the router on which the destination is located (though these keys are <b>not</b>
the router's keys - they are generated by the client and given to the router to
use).  End to end client messages are still, of course, encrypted with the 
destination's public keys.</p>

<h2><a name="bootstrap">Bootstrapping</a></h2>

<p>The netDb, being a DHT, is completely decentralized, however you do need at
least one reference to a peer so that you can let the DHT's integration process
tie you in.  This is accomplished by "reseeding" your router with the RouterInfo
of an active peer - specifically, by retrieving their <code>routerInfo-$hash.dat</code>
file and storing it in your <code>netDb/</code> directory.  Anyone can provide
you with those files - you can even provide them to others by exposing your own
netDb directory.  To simplify the process of finding someone with a RouterInfo,
an alias has been made to the <a href="http://dev.i2p.net/i2pdb/">netDb</a> dir
of one of the routers on dev.i2p.net.</p>

<h2><a name="floodfill">Floodfill</a></h2>
<p>
(Adapted from a post by jrandom in the old Syndie, Nov. 26, 2005)
<br />
The floodfill netDb is really just a simple and perhaps temporary measure, 
using the simplest possible algorithm - send the data to a peer in the 
floodfill netDb, wait 10 seconds, pick a random peer in the netDb and ask them 
for the entry to be sent, verifying its proper insertion / distribution.  If the 
verification peer doesn't reply, or they don't have the entry, the sender 
repeats the process.  When the peer in the floodfill netDb receives a netDb 
store from a peer not in the floodfill netDb, they send it to all of the peers 
in the floodfill netDb.
</p><p>
Peers still do netDb exploration and bootstrapping as before.
</p><p>
At one point, the kademlia 
search/store functionality was still in place.  The peers 
considered the floodfill peers as always being 'closer' to every key than any 
peer not participating in the netDb.  We fell back on the kademlia 
netDb if the floodfill peers fail for some reason or another.
However, kademlia has since been disabled completely (see below).
</p><p>
Determining who is part of the floodfill netDb is trivial - its exposed in each 
router's published routerInfo.  If too many peers publish that flag, we may 
have to have peers publish the list of peers they consider as being in the 
netDb, but perhaps not, since these peers are not anonymous - if router X 
publishes the flag and they suck, we know router X's IP and can handle them 
accordingly.
</p><p>
As for efficiency, this algorithm is optimal when the netDb peers are known and 
their quantity is appropriate for the appropriate uptime demands.  Regarding 
scaling, we've got two peers who participate in the netDb right now, and 
they'll be able to handle the load by themselves until we've got 10k+ eepsites 
- at that point, we can toss on a few more or work out some more aggressive 
load balancing among the netDb peers, but worrying about it when we have dozens 
of eepsites may be a bit premature.  It's not as sexy as the old 
kademlia netDb, but there are subtle anonymity attacks against non-flooded 
netDbs.
</p>

<h2><a name="healing">Healing</a></h2>
<i>Needs update since kademlia is disabled.</i>

<p>While the kademlia algorithm is fairly efficient at maintaining the necessary
links, we keep additional statistics regarding the netDb's activity so that we 
can detect potential segmentation and actively avoid it.  This is done as part of
the peer profiling - with data points such as how many new and verifiable 
RouterInfo references a peer gives us, we can determine what peers know about
groups of peers that we have never seen references to.  When this occurs, we can
take advantage of kademlia's flexibility in exploration and send requests to that
peer so as to integrate ourselves further with the part of the network seen by
that well integrated router.</p>

<h2><a name="migration">Migration</a></h2>

<p>Unlike traditional DHTs, the very act of conducting a search distributes the
data as well, since rather than passing IP+port # pairs, references are given to
the routers on which to query (namely, the SHA256 of those router's identities).
As such, iteratively searching for a particular destination's LeaseSet or 
router's RouterInfo will also provide you with the RouterInfo of the peers along
the way.</p>

<p>In addition, due to the time sensitivity of the data, the information doesn't
often need to be migrated - since a LeaseSet is only valid for the 10 minutes 
that the referenced tunnels are around, those entries can simply be dropped at
expiration, since they will be replaced at the new location when the router
publishes a new LeaseSet.</p>

<p>To address the concerns of <a href="http://citeseer.ist.psu.edu/douceur02sybil.html">Sybil attacks</a>,
the location used to store entries varies over time.  Rather than storing the
RouterInfo on the peers closest to SHA256(router identity), they are stored on
the peers closest to SHA256(router identity + YYYYMMdd), requiring an adversary
to remount the attack again daily so as to maintain closeness to the "current"
keyspace.  In addition, entries are probabalistically distributed to an additional
peer outside of the target keyspace, so that a successful compromise of the K
routers closest to the key will only degrade the search time.</p>

<h2><a name="delivery">Delivery</a></h2>

<p>As with DNS lookups, the fact that someone is trying to retrieve the LeaseSet
for a particular destination is sensitive (the fact that someone is <i>publishing</i>
a LeaseSet even more so!).  To address this, netDb searches and netDb store 
messages are simply sent through the router's exploratory tunnels.</p>

<h2><a name="status">History and Status</a></h2>

<h3>The Introduction of the Floodfill Algorithm</h3>
<p>
Floodfill was introduced in release 0.6.0.4, keeping Kademlia as a backup algorithm.
</p>

<p>
(Adapted from a post by jrandom in the old Syndie, Nov. 26, 2005)
<br />
As I've often said, I'm not particularly bound to any specific technology - 
what matters to me is what will get results.  While I've been working through 
various netdb ideas over the last few years, the issues we've faced in the last 
few weeks have brought some of them to a head.  On the live net, 
with the netdb redundancy factor set to 4 peers (meaning we keep sending an 
entry to new peers until 4 of them confirm that they've got it) and the 
per-peer timeout set to 4 times that peer's average reply time, we're 
<b>still</b> getting an average of 40-60 peers sent to before 4 ack the store.  
That means sending 36-56 times as many messages as should go out, each using 
tunnels and thereby crossing 2-4 links.  Even further, that value is heavily 
skewed, as the average number of peers sent to in a 'failed' store (meaning 
less than 4 people acked the message after 60 seconds of sending messages out) 
was in the 130-160 peers range.
</p><p>
This is insane, especially for a network with only perhaps 250 peers on it.  
</p><p>
The simplest answer is to say "well, duh jrandom, it's broken.  fix it", but 
that doesn't quite get to the core of the issue.  In line with another current 
effort, it's likely that we have a substantial number of network issues due to 
restricted routes - peers who cannot talk with some other peers, often due to 
NAT or firewall issues.  If, say, the K peers closest to a particular netdb 
entry are behind a 'restricted route' such that the netdb store message could 
reach them but some other peer's netdb lookup message could not, that entry 
would be essentially unreachable.  Following down those lines a bit further and 
taking into consideration the fact that some restricted routes will be created 
with hostile intent, its clear that we're going to have to look closer into a 
long term netdb solution.
</p><p>
There are a few alternatives, but two worth mentioning in particular.  The 
first is to simply run the netdb as a kademlia DHT using a subset of the full 
network, where all of those peers are externally reachable.  Peers who are not 
participating in the netdb still query those peers but they don't receive 
unsolicited netdb store or lookup messages.  Participation in the netdb would 
be both self-selecting and user-eliminating - routers would choose whether to 
publish a flag in their routerInfo stating whether they want to participate 
while each router chooses which peers it wants to treat as part of the netdb 
(peers who publish that flag but who never give any useful data would be 
ignored, essentially eliminating them from the netdb).
</p><p>
Another alternative is a blast from the past, going back to the DTSTTCPW 
mentality - a floodfill netdb, but like the alternative above, using only a 
subset of the full network.  When a user wants to publish an entry into the 
floodfill netdb, they simply send it to one of the participating routers, wait 
for an ACK, and then 30 seconds later, query another random participant in the 
floodfill netdb to verify that it was properly distributed.  If it was, great, 
and if it wasn't, just repeat the process.  When a floodfill router receives a 
netdb store, they ACK immediately and queue off the netdb store to all of its 
known netdb peers.  When a floodfill router receives a netdb lookup, if they 
have the data, they reply with it, but if they don't, they reply with the 
hashes for, say, 20 other peers in the floodfill netdb.
</p><p>
Looking at it from a network economics perspective, the floodfill netdb is 
quite similar to the original broadcast netdb, except the cost for publishing 
an entry is borne mostly by peers in the netdb, rather than by the publisher.  
Fleshing this out a bit further and treating the netdb like a blackbox, we can 
see the total bandwidth required by the netdb to be:<pre>
  recvKBps = N * (L + 1) * (1 + F) * (1 + R) * S / T
</pre>where<pre>
  N = number of routers in the entire network
  L = average number of client destinations on each router 
      (+1 for the routerInfo)
  F = tunnel failure percentage 
  R = tunnel rebuild period, as a fraction of the tunnel lifetime
  S = average netdb entry size
  T = tunnel lifetime
</pre>Plugging in a few values:<pre>
  recvKBps = 1000 * (5 + 1) * (1 + 0.05) * (1 + 0.2) * 2KB / 10m
           = 25.2KBps
</pre>That, in turn, scales linearly with N (at 100,000 peers, the netdb must 
be able to handle netdb store messages totalling 2.5MBps, or, at 300 peers, 
7.6KBps).  
</p><p>
While the floodfill netdb would have each netdb participant receiving only a 
small fraction of the client generated netdb stores directly, they would all 
receive all entries eventually, so all of their links should be capable of 
handling the full recvKBps.  In turn, they'll all need to send 
<tt>(recvKBps/sizeof(netdb)) * (sizeof(netdb)-1)</tt> to keep the other 
peers in sync.
</p><p>
A floodfill netdb would not require either tunnel routing for netdb operation 
or any special selection as to which entries it can answer 'safely', as the 
basic assumption is that they are all storing everything.  Oh, and with regards 
to the netdb disk usage required, its still fairly trivial for any modern 
machine, requiring around 11MB for every 1000 peers <tt>(N * (L + 1) * 
S)</tt>.
</p><p>
The kademlia netdb would cut down on these numbers, ideally bringing them to K 
over M times their value, with K = the redundancy factor and M being the number 
of routers in the netdb (e.g. 5/100, giving a recvKBps of 126KBps and 536MB at 
100,000 routers).  The downside of the kademlia netdb though is the increased 
complexity of safe operation in a hostile environment.  
</p><p>
What I'm thinking about now is to simply implement and deploy a floodfill netdb 
in our existing live network, letting peers who want to use it pick out other 
peers who are flagged as members and query them instead of querying the 
traditional kademlia netdb peers.  The bandwidth and disk requirements at this 
stage are trivial enough  (7.6KBps and 3MB disk space) and it will remove the 
netdb entirely from the debugging plan - issues that remain to be addressed 
will be caused by something unrelated to the netdb.
</p><p>
How would peers be chosen to publish that flag saying they are a part of the 
floodfill netdb?  At the beginning, it could be done manually as an advanced 
config option (ignored if the router is not able to verify its external 
reachability).  If too many peers set that flag, how do the netdb participants 
pick which ones to eject?  Again, at the beginning it could be done manually as 
an advanced config option (after dropping peers which are unreachable).  How do 
we avoid netdb partitioning?  By having the routers verify that the netdb is 
doing the flood fill properly by querying K random netdb peers.  How do routers 
not participating in the netdb discover new routers to tunnel through?  Perhaps 
this could be done by sending a particular netdb lookup so that the netdb 
router would respond not with peers in the netdb, but with random peers outside 
the netdb.  
</p><p>
I2P's netdb is very different from traditional load bearing DHTs - it only 
carries network metadata, not any actual payload, which is why even a netdb 
using a floodfill algorithm will be able to sustain an arbitrary amount of 
eepsite/irc/bt/mail/syndie/etc data.  We can even do some optimizations as I2P 
grows to distribute that load a bit further (perhaps passing bloom filters 
between the netdb participants to see what they need to share), but it seems we 
can get by with a much simpler solution for now.
</p>

<h3>The Disabling of the Kademlia Algorithm</h3>
<p>
Kademlia was completely disabled in release 0.6.1.20.
</p><p>
(this is adapted from an IRC conversation with jrandom 11/07)
<br />
Kademlia requires a minimum level of service that the baseline could not offer (bw, cpu),
even after adding in tiers (pure kad is absurd on that point).
Kademlia just wouldn't work.  It was a nice idea, but not for a hostile and fluid environment.
</p>

<h3>Current Status</h3>
<p>The netDb plays a very specific role in the I2P network, and the algorithms
have been tuned towards our needs.  This also means that it hasn't been tuned 
to address the needs we have yet to run into.  I2P is currently
fairly small (a few hundred routers).
There were some calculations that 3-5 floodfill routers should be able to handle
10,000 nodes in the network.
The netDb implementation more than adequately meets our
needs at the moment, but there will likely be further tuning and bugfixing as 
the network grows.</p>

<h3>The Return of the Kademlia Algorithm?</h3>
<p>
(this is adapted from <a href="meeting195.html">the I2P meeting Jan. 2, 2007</a>)
<br />
The Kademlia netdb just wasn't working properly.
Is it dead forever or will it be coming back?
If it comes back, the peers in the kademlia netdb would be a very limited subset
of the routers in the network (basically an expanded number of floodfill peers, if/when the floodfill peers
cannot handle the load).
But until the floodfill peers cannot handle the load (and other peers cannot be added that can), it's unnecessary.
</p>

<h3>The Future of Floodfill</h3>
<p>
(this is adapted from an IRC conversation with jrandom 11/07)
<br />
Here's a proposal: Capacity class O is automatically floodfill.
Hmm.
Unless we're sure, we might end up with a fancy way of DDoS'ing all O class routers.
This is quite the case: we want to make sure the number of floodfill is as small as possible while providing sufficient reachability.
If/when netdb requests fail, then we need to increase the number of floodfill peers, but atm, I'm not aware of a netdb fetch problem.
There are 33 "O" class peers according to my records.
33 is a /lot/ to floodfill to.
</p><p>
So floodfill works best when the number of peers in that pool is firmly limited?
And the size of the floodfill pool shouldn't grow much, even if the network itself gradually would?
3-5 floodfill peers can handle 10K routers iirc (I posted a bunch of numbers on that explaining the details in the old syndie).
Sounds like a difficult requirement to fill with automatic opt-in,
especially if nodes opting in cannot trust data from others.
e.g. "let's see if I'm among the top 5",
and can only trust data about themselves (e.g. "I am definitely O class, and moving 150 KB/s, and up for 123 days").
And top 5 is hostile as well.  Basically, it's the same as the tor directory servers - chosen by trusted people (aka devs).
Yeah, right now it could be expoited by opt-in, but that'd be trivial to detect and deal with.
Seems like in the end, we might need something more useful than kademlia, and have only reasonably capable peers join that scheme.
N class and above should be a big enough quantity to suppress risk of an adversary causing denial of service, I'd hope.
But it would have to be different from floodfill then, in the sense that it wouldn't cause humongous traffic.
Large quantity?  For a DHT based netdb?
Not necessarily DHT-based.
</p>

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