Good morning fiatjaf,

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‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Friday, August 9, 2019 10:35 AM, fiatjaf <> wrote:

> Ok, here's another question/probably-bad-idea: how feasible is it for these 
> trampoline nodes to return the route they've calculated somehow to the 
> original caller so it can cache the route and use it without trampolines the 
> next time? I don't know if caching routes is a good way improve routing 
> overall in larger networks, but it seems to be working well for BLW[1] 
> currently and overall it does make sense: in day-to-day payments we tend to 
> pay the same people and stores over and over, rarely paying someone else (but 
> of course these rare cases are still very common to be ignored).
> Anyway, I don't enough even to ask the question, but I guess it's 
> theoretically possible for some information to be returned along with the 
> preimage in the payment route, right?
> There remains a question of if and why the trampoline nodes would be or not 
> interested in cheating Alice, sending back a bad route that favors them, or 
> not returning any route at all as that would undermine their profits as 
> trampolines.
> [1]:

It is doable, and potentially a good idea.

As to economic incentive: the route returned would pass through the node that 
provides the route, so there is still economic incentive to do so.
In addition, the trampoline node would not have to cache it itself; instead the 
original payer does the caching, so this is a mild reduction in resources 
shouldered by the trampoline node (it avoids having to choose between 
recalculating the route vs caching it in its own storage).

It is even doable to support multipart payment, by simply allowing multiple 
routes to be returned.

Though we need to redesign the route serialization.
Current route serialization contains exact amount to be sent, and cannot be 
reused if amount is changed.


> On Friday, August 2, 2019, Bastien TEINTURIER <> 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]

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