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i2p.www/i2p2www/spec/proposals/140-invisible-multihoming.rst
str4d 334b6df0e4 Prop140: 
- Add definitions of user, client (with sub-definitions for balancer, frontend
  and backend), and router
- Replace "edge router" with "router"
- Add a second diagram showing a multi-client setup with split-out balancer,
  frontends and backend
2017-07-04 18:21:24 +00:00

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=====================
Invisible Multihoming
=====================
.. meta::
:author: str4d
:created: 2017-05-22
:thread: http://zzz.i2p/topics/2335
:lastupdated: 2017-07-04
:status: Open
.. contents::
Overview
========
This proposal outlines a design for a protocol enabling an I2P client, service
or external balancer process to manage multiple routers transparently hosting a
single [Destination]_.
The proposal currently does not specify a concrete implementation. It could be
implemented as an extension to [I2CP]_, or as a new protocol.
Motivation
==========
Multihoming is where multiple routers are used to host the same Destination.
The current way to multihome with I2P is to run the same Destination on each
router independently; the router that gets used by clients at any particular
time is the last one to publish a [LeaseSet]_.
This is a hack and presumably won't work for large websites at scale. Say we had
100 multihoming routers each with 16 tunnels. That's 1600 LeaseSet publishes
every 10 minutes, or almost 3 per second. The floodfills would get overwhelmed
and throttles would kick in. And that's before we even mention the lookup
traffic.
[Prop123]_ solves this problem with a meta-LeaseSet, which lists the 100 real
LeaseSet hashes. A lookup becomes a two-stage process: first looking up the
meta-LeaseSet, and then one of the named LeaseSets. This is a good solution to
the lookup traffic issue, but on its own it creates a significant privacy leak:
It is possible to determine which multihoming routers are online by monitoring
the published meta-LeaseSet, because each real LeaseSet has corresponds to a
single router.
We need a way for an I2P client or service to spread a single Destination across
multiple routers, in a way that is indistinguishable to using a single router
(from the perspective of the LeaseSet itself).
Design
======
Definitions
-----------
User
The person or organisation wanting to multihome their Destination(s). A
single Destination is considered here without loss of generality (WLOG).
Client
The application or service running behind the Destination. It may be a
client-side, server-side, or peer-to-peer application; we refer to it as
a client in the sense that it connects to the I2P routers.
The client consists of three parts, which may all be in the same process
or may be split across processes or machines (in a multi-client setup):
Balancer
The part of the client that manages peer selection and tunnel
building. There is a single balancer at any one time, and it
communicates with all I2P routers. There may be failover balancers.
Frontend
The part of the client that can be operated in parallel. Each
frontend communicates with a single I2P router.
Backend
The part of the client that is shared between all frontends. It has
no direct communication with any I2P router.
Router
An I2P router run by the user that sits at the boundary between the I2P
network and the user's network (akin to an edge device in corporate
networks). It builds tunnels under the command of a balancer, and routes
packets for a client or frontend.
High-level overview
-------------------
Imagine the following desired configuration:
- A client application with one Destination.
- Four routers, each managing three inbound tunnels.
- All twelve tunnels should be published in a single LeaseSet.
Single-client
`````````````
.. raw:: html
{% highlight lang='text' %}
-{ [Tunnel 1]===\
|-{ [Tunnel 2]====[Router 1]-----
|-{ [Tunnel 3]===/ \
| \
|-{ [Tunnel 4]===\ \
[Destination] |-{ [Tunnel 5]====[Router 2]----- \
\ |-{ [Tunnel 6]===/ \ \
[LeaseSet]--| [Client]
|-{ [Tunnel 7]===\ / /
|-{ [Tunnel 8]====[Router 3]----- /
|-{ [Tunnel 9]===/ /
| /
|-{ [Tunnel 10]==\ /
|-{ [Tunnel 11]===[Router 4]-----
-{ [Tunnel 12]==/
{% endhighlight %}
Multi-client
````````````
.. raw:: html
{% highlight lang='text' %}
-{ [Tunnel 1]===\
|-{ [Tunnel 2]====[Router 1]---------[Frontend 1]
|-{ [Tunnel 3]===/ \ \
| \ \
|-{ [Tunnel 4]===\ \ \
[Destination] |-{ [Tunnel 5]====[Router 2]---\-----[Frontend 2] \
\ |-{ [Tunnel 6]===/ \ \ \ \
[LeaseSet]--| [Balancer] [Backend]
|-{ [Tunnel 7]===\ / / / /
|-{ [Tunnel 8]====[Router 3]---/-----[Frontend 3] /
|-{ [Tunnel 9]===/ / /
| / /
|-{ [Tunnel 10]==\ / /
|-{ [Tunnel 11]===[Router 4]---------[Frontend 4]
-{ [Tunnel 12]==/
{% endhighlight %}
General client process
``````````````````````
- Load or generate a Destination.
- Open up a session with each router, tied to the Destination.
- Periodically (around every ten minutes, but more or less based on tunnel
liveness):
- Obtain the fast tier from each router.
- Use the superset of peers to build tunnels to/from each router.
- By default, tunnels to/from a particular router will use peers from
that router's fast tier, but this is not enforced by the protocol.
- Collect the set of active inbound tunnels from all active routers, and create a
LeaseSet.
- Publish the LeaseSet through one or more of the routers.
Differences to I2CP
```````````````````
To create and manage this configuration, the client needs the following new
functionality beyond what is currently provided by [I2CP]_:
- Tell a router to build tunnels, without creating a LeaseSet for them.
- Get a list of the current tunnels in the inbound pool.
Additionally, the following functionality would enable significant flexibility
in how the client manages its tunnels:
- Get the contents of a router's fast tier.
- Tell a router to build an inbound or outbound tunnel using a given list of
peers.
Protocol outline
----------------
.. raw:: html
{% highlight %}
Client Router
---------------------> Create Session
Session Status <---------------------
---------------------> Get Fast Tier
Peer List <---------------------
---------------------> Create Tunnel
Tunnel Status <---------------------
---------------------> Get Tunnel Pool
Tunnel List <---------------------
---------------------> Publish LeaseSet
---------------------> Send Packet
Send Status <---------------------
Packet Received <---------------------
{% endhighlight %}
Messages
````````
Create Session
Create a session for the given Destination.
Session Status
Confirmation that the session has been set up, and the client can now
start building tunnels.
Get Fast Tier
Request a list of the peers that the router currently would consider
building tunnels through.
Peer List
A list of peers known to the router.
Create Tunnel
Request that the router build a new tunnel through the specified peers.
Tunnel Status
The result of a particular tunnel build, once it is available.
Get Tunnel Pool
Request a list of the current tunnels in the inbound or outbound pool
for the Destination.
Tunnel List
A list of tunnels for the requested pool.
Publish LeaseSet
Request that the router publish the provided LeaseSet through one of the
outbound tunnels for the Destination. No reply status is needed; the
router should continue re-trying until it is satisfied that the LeaseSet
has been published.
Send Packet
An outgoing packet from the client. Optionally specifies an outbound
tunnel through which the packet must (should?) be sent.
Send Status
Informs the client of the success or failure of sending a packet.
Packet Received
An incoming packet for the client. Optionally specifies the inbound
tunnel through which the packet was received(?)
Security implications
=====================
From the perspective of the routers, this design is functionally equivalent to
the status quo. The router still builds all tunnels, maintains its own peer
profiles, and enforces separation between router and client operations. In the
default configuration is completely identical, because tunnels for that router
are built from its own fast tier.
From the perspective of the netDB, a single LeaseSet created via this protocol
is identical to the status quo, because it leverages pre-existing functionality.
However, for larger LeaseSets approaching 16 Leases, it may be possible for an
observer to determine that the LeaseSet is multihomed:
- The current maximum size of the fast tier is 75 peers. The Inbound Gateway
(IBGW, the node published in a Lease) is selected from a fraction of the tier
(partitioned randomly per-tunnel pool by hash, not count):
1 hop
The whole fast tier
2 hops
Half of the fast tier
(the default until mid-2014)
3+ hops
A quarter of the fast tier
(3 being the current default)
That means on average the IBGWs will be from a set of 20-30 peers.
- In a single-homed setup, a full 16-tunnel LeaseSet would have 16 IBGWs
randomly selected from a set of up to (say) 20 peers.
- In a 4-router multihomed setup using the default configuration, a full
16-tunnel LeaseSet would have 16 IBGWs randomly-selected from a set of at most
80 peers, though there are likely to be a fraction of common peers between
routers.
Thus with the default configuration, it may be possible through statistical
analysis to figure out that a LeaseSet is being generated by this protocol. It
might also be possible to figure out how many routers there are, although the
effect of churn on the fast tiers would reduce the effectiveness of this
analysis.
As the client has full control over which peers it selects, this information
leakage could be reduced or eliminated by selecting IBGWs from a reduced set of
peers.
Compatibility
=============
This design is completely backwards-compatible with the network, because there
are no changes to the [LeaseSet]_ format. All routers would need to be aware of
the new protocol, but this is not a concern as they would all be controlled by
the same entity.
Performance and scalability notes
=================================
The upper limit of 16 [Leases]_ per LeaseSet is unaltered by this proposal. For
Destinations that require more tunnels than this, there are two possible network
modifications:
- Increase the upper limit on the size of LeaseSets. This would be the simplest
to implement (though it would still require pervasive network support before
it could be widely used), but could result in slower lookups due to the larger
packet sizes. The maximum feasible LeaseSet size is defined by the MTU of the
underlying transports, and is therefore around 16kB.
- Implement [Prop123]_ for tiered LeaseSets. In combination with this proposal,
the Destinations for the sub-LeaseSets could be spread across multiple
routers, effectively acting like multiple IP addresses for a clearnet service.
Acknowledgements
================
Thanks to psi for the discussion that led to this proposal.
References
==========
.. [Destination]
{{ ctags_url('Destination') }}
.. [I2CP]
{{ site_url('docs/protocol/i2cp', True) }}
.. [Leases]
{{ ctags_url('Lease') }}
.. [LeaseSet]
{{ ctags_url('LeaseSet') }}
.. [Prop123]
{{ proposal_url('123') }}
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