[Lightning-dev] Laundry list of inter-peer wire protocol changes

[CJP]

CJP schreef op di 02-02-2016 om 18:56 [+0100]:
> 
> > > * Reserving before locking: this is an optimization, to reduce the risk
> > > of locking funds in payment channels on a part of the route, and then
> > > having to undo the locking when it turns out that the remaining part of
> > > the route doesn't exist (anymore). Reserving is an informal(*),
> > > temporary locking of funds for use in the transaction, and can be done
> > > and undone very fast, without any channel operations. It is done
> > > together with route searching + establishment.
> > 
> > I think that trades one DoS for another, though.  It saves cryptographic
> > constructs, but latency is the real cost, and this increases it.
> > 
> > Of course, we'll have to revisit that if the network in practice proves
> > subject to these problems...
> 
> For one category of channel designs, reserving is absolutely essential:
> channels where bi-directional payments are made possible with a
> decrementing lock time. There, you want to make sure that failed routing
> attempts don't cause lock time decrements, since that would reduce the
> channel lifetime more than necessary. I'd have to check whether there is
> still any use case for this channel design, and whether the reserving
> step is important for some other reason.

After actually implementing some of these things, I'd like to revisit
this discussion.

The "reserving" stage used to be important for channel types where the
bi-directional nature of the channel is realized with decreasing lock
times (see above). Now that a large part of the community has put its
hope on Lightning, it's more likely that the extensions required for
better channel types will be included in Bitcoin, so this issue has
become less important.

Another reason why the "reserving" stage is important, is that it it
necessary for bi-directional routing. In order to make both payer and
payee anonymous, we need to provide this feature, where both route to a
meeting point. For the part between meeting point and payee, routing
goes from payee to meeting point(*), but locking goes from meeting point
to payee, so they can't be done in the same stage.

Finally, the "reserving" stage allows you to set a tighter value on the
HTLC time-outs. If you do routing together with locking, then routing
has to go in the payer->payee direction; the time-out has to be reduced
on every hop. Assuming(**) the payer doesn't know the number of hops in
advance, the time-out has to be set to a very high value by the payer,
to maximize chances of a successful transaction. If you do routing in
the reverse direction (payee->payer), you can start with a very low
time-out, and increment it on every hop. This will generally result in
lower time-out values, which is desirable.

That's about it, for the advantages offered by the separate reserving
stage in Amiko Pay. Let me finish with an attempt to make an ASCII-art
sequence diagram of reserving, bi-directional routing and locking in
Amiko Pay:

H(R) = the transaction hash (also included in other messages)
MR() = MakeRoute message without time info
MR(t) = MakeRoute message with time info
HR() = HaveRoute message without time info
HR(t) = HaveRoute message
L(t) = Lock message (adds HTLC to channel)
A = Alice (payer)
B = Bob (meeting point)
C = Carol (intermediate node). There can be many intermediate nodes
between A and B, and between B and D.
D = Dave (payee)

A               B               C             D
|<------------------ H(R) --------------------|
|------MR()---->|               |<-MR(T0+dt)--|
                |<-MR(T0+2*dt)--|
|<-HR(T0+3*dt)--|--HR(T0+2*dt)->|
                                |--HR(T0+dt)->|
|<------------------ HR() --------------------|
|--L(T0+3*dt)-->|--L(T0+2*dt)-->|--L(T0+dt)-->|

(...and then the sequence continues with removing the HTLCs and
releasing funds towards payee-side of each channel.)


CJP

(*) It can't go in the other direction, since the meeting point should
not know the identity (or network address) of the payee.

(**) This is the case for non-source routing. One advantage of
non-source routing is that nodes don't need a map of the entire network;
this improves privacy, and could help to keep the network open to new
participants, even against the will of many powerful existing
participants. In the end, I imagine a system where a mix of
source-routing and non-source routing can be used, to get the best of
both worlds.