Good morning fiatjaf,

This is a good question, I'm glad you asked.

As ZmnSCPxj points out, Alice doesn't know. By not syncing the full network
graph, Alice has to accept
"being in the dark" for some decisions. She is merely hoping that RT2 *can
find a route* to Bob. Note that
it's quite easy to help Alice make informed decision by proving routing
hints in the invoice and in gossip
messages (which we already do for "normal" routing).

The graph today is strongly connected, so it's quite a reasonable
assumption (and Alice can easily retry
with another choice of trampoline node if the first one fails - just like
we do today with normal payments).

I fully agree with ZmnSCPxj though that in the future this might not be
true anymore. When/if the network
becomes too large we will likely lose its strongly connected nature. When
that happens, the Lightning
Network will need some kind of hierarchical / packet switched routing
architecture and we won't require
trampoline nodes to know the whole network graph and be able to route to
mostly anyone.
I argue that trampoline routing is a first step towards enabling that. It's
a good engineering trade-off between
ease of implementation and deployment, fixing a problem we have today and
enabling future scaling for
problems we'll have tomorrow. It's somewhat easy once we have trampoline
payments to evolve that to a
system closer to the internet's packet switching infrastructure, so we'll
deal with that once the need for it
becomes obvious.

Does that answer your question?

Cheers,
Bastien

Le sam. 3 août 2019 à 05:48, ZmnSCPxj <zmnsc...@protonmail.com> a écrit :

> Good morning fiatjaf,
>
> I proposed before that we could institute a rule where nodes are mapped to
> some virtual space, and nodes should preferably retain the part of the
> network graph that connects itself to those nodes near to it in this
> virtual space (and possibly prefer to channel to those nodes).
>
>
> https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-April/001959.html
>
> Thus Alice might **not** know that some route exists between T1 and T2.
>
> T1 itself might not know of a route from itself to T2.
> But if T1 knows a route to T1.5, and it knows that T1.5 is nearer to T2
> than to itself in the virtual space, it can **try** to route through T1.5
> in the hope T1.5 knows a route from itself to T2.
> This can be done if T1 can remove itself from the trampoline route and
> replace itself with T1.5, offerring in exchange some of the fee to T1.5.
>
> Other ways of knowing some distillation of the public network without
> remembering the channel level details are also possible.
> My recent pointlessly long spam email for example has a section on
> Hierarchical Maps.
>
>
> https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-August/002095.html
>
> Regards,
> ZmnSCPxj
>
>
> Sent with ProtonMail Secure Email.
>
> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> On Saturday, August 3, 2019 12:29 AM, fiatjaf <fiat...@alhur.es> wrote:
>
> > Ok, since you seem to imply each question is valuable, here's mine: how
> does Alice know RT2 has a route to Bob? If she knows that, can she also
> know T1 has a route to Bob? In any case, why can't she just build her small
> onion with Alice -> T1 -> Bob? I would expect that to be the most common
> case, am I right?
> >
> > On Friday, August 2, 2019, Bastien TEINTURIER <bast...@acinq.fr> wrote:
> >
> > > Good morning list,
> > >
> > > I realized that trampoline routing has only been briefly described to
> this list (credits to cdecker and pm47 for laying
> > > out the foundations). I just published an updated PR [1] and want to
> take this opportunity to present the high level
> > > view here and the parts that need a concept ACK and more feedback.
> > >
> > > Trampoline routing is conceptually quite simple. Alice wants to send a
> payment to Bob, but she doesn't know a
> > > route to get there because Alice only keeps a small area of the
> routing table locally (Alice has a crappy phone,
> > > damn it Alice sell some satoshis and buy a real phone). However, Alice
> has a few trampoline nodes in her
> > > friends-of-friends and knows some trampoline nodes outside of her
> local area (but she doesn't know how to reach
> > > them). Alice would like to send a payment to a trampoline node she can
> reach and defer calculation of the rest of
> > > the route to that node.
> > >
> > > The onion routing part is very simple now that we have variable-length
> onion payloads (thanks again cdecker!).
> > > Just like russian dolls, we simply put a small onion inside a big
> onion. And the HTLC management forwards very
> > > naturally.
> > >
> > > It's always simpler with an example. Let's imagine that Alice can
> reach three trampoline nodes: T1, T2 and T3.
> > > She also knows the details of many remote trampoline nodes that she
> cannot reach: RT1, RT2, RT3 and RT4.
> > > Alice selects T1 and RT2 to use as trampoline hops. She builds a small
> onion that describes the following route:
> > >
> > > Alice -> T1 -> RT2 -> Bob
> > >
> > > She finds a route to T1 and builds a normal onion to send a payment to
> T1:
> > >
> > > Alice -> N1 -> N2 -> T1
> > >
> > > In the payload for T1, Alice puts the small trampoline onion.
> > > When T1 receives the payment, he is able to peel one layer of the
> trampoline onion and discover that he must
> > > forward the payment to RT2. T1 finds a route to RT2 and builds a
> normal onion to send a payment to RT2:
> > >
> > > T1 -> N3 -> RT2
> > >
> > > In the payload for RT2, T1 puts the peeled small trampoline onion.
> > > When RT2 receives the payment, he is able to peel one layer of the
> trampoline onion and discover that he must
> > > forward the payment to Bob. RT2 finds a route to Bob and builds a
> normal onion to send a payment:
> > >
> > > RT2 -> N4 -> N5 -> Bob
> > >
> > > In the payload for Bob, RT2 puts the peeled small trampoline onion.
> > > When Bob receives the payment, he is able to peel the last layer of
> the trampoline onion and discover that he is
> > > the final recipient, and fulfills the payment.
> > >
> > > Alice has successfully sent a payment to Bob deferring route
> calculation to some chosen trampoline nodes.
> > > That part was simple and (hopefully) not controversial, but it left
> out some important details:
> > >
> > > 1.  How do trampoline nodes specify their fees and cltv requirements?
> > > 2.  How does Alice sync the fees and cltv requirements for her remote
> trampoline nodes?
> > >
> > > To answer 1., trampoline nodes needs to estimate a fee and cltv that
> allows them to route to (almost) any other
> > > trampoline node. This is likely going to increase the fees paid by
> end-users, but they can't eat their cake and
> > > have it too: by not syncing the whole network, users are trading fees
> for ease of use and payment reliability.
> > >
> > > To answer 2., we can re-use the existing gossip infrastructure to
> exchange a new node_update message that
> > > contains the trampoline fees and cltv. However Alice doesn't want to
> receive every network update because she
> > > doesn't have the bandwidth to support it (damn it again Alice, upgrade
> your mobile plan). My suggestion is to
> > > create a filter system (similiar to BIP37) where Alice sends gossip
> filters to her peers, and peers only forward to
> > > Alice updates that match these filters. This doesn't have the issues
> BIP37 has for Bitcoin because it has a cost
> > > for Alice: she has to open a channel (and thus lock funds) to get a
> connection to a peer. Peers can refuse to serve
> > > filters if they are too expensive to compute, but the filters I
> propose in the PR are very cheap (a simple xor or a
> > > node distance comparison).
> > >
> > > If you're interested in the technical details, head over to [1].
> > > I would really like to get feedback from this list on the concept
> itself, and especially on the gossip and fee estimation
> > > parts. If you made it that far, I'm sure you have many questions and
> suggestions ;).
> > >
> > > Cheers,
> > > Bastien
> > >
> > > [1] https://github.com/lightningnetwork/lightning-rfc/pull/654
>
>
>
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