Trickle and transaction propogation

Trickle and transaction propogation
2014-07-20 22:02 ` Mike Hearn
  
From: Kaz Wesley @ 2014-07-20 21:01 UTC (permalink / raw)
  To: Bitcoin Dev

The inv trickling mechanism currently serves two purposes:
- protect casual users' privacy by slightly obscuring a tx's originating node
- reduce invs unnecessarily sent both directions for a connection
It has some drawbacks:
- it slows transaction propagation
- it delays knowledge between two nodes of what txes are mutually known
These drawbacks will be especially costly once optimizations based on
mutually-known transactions are available (in progress, see "sparse
blocks" thread).

Both of the benefits of trickling can be achieved more efficiently and
without the costs to transaction propagation and mutual transaction
knowledge.

Privacy: trickling helps hide the origin of 3/4 of the transactions a
node is pushing by preventing most of the node's neighbors from seeing
the transactions from that node right away; by the time a peer becomes
the trickle node, it may have received the same inv from another of
its peers.
This staggering of introduction of new invs to the network could be
made more effective by scheduling staggered pushes of wallet
transactions to each peer in a structure similar to mapAskFor.
This does have the drawback that someone who has established multiple
connections to a node can observe that some invs are pushed at
different times, suggesting they are in the stagger set. I don't see
any straightforward way to remedy this, but trickling is also
vulnerable to sybil attacks, and floods 1/4 of its transactions
immediately anyway -- so I think staggered push would be an overall
privacy improvement.
Likelihood of a partial sybil obtaining inv origin information could
be reduced by a policy of ending staggering and pushing to all peers
once another peer has received the tx from elsewhere and inved the
transaction back to the original node; if the staggering is
sufficiently slow, only one or two nodes would receive the initial
push to the network and after that the inv would be treated
indistinguishably from if it originated externally.

Redundant invs: without trickling, when two nodes receive transactions
at around the same time they may each send each other an inv before
receiving the other's. Trickling reduces this by giving all
non-trickleSend nodes a chance to send first. Thus just eliminating
trickling would at most double inv traffic. Although invs are small
they are numerous, being the only common message potentially sent from
every node to all its neighbors.
A more efficient solution to the who-sends-first problem would be for
connections to have directional parity:
- a node initiating a connection would announce its parity (even or odd)
- an inv is sent right away to peers with matching parity, but only
sent against parity if a certain timeout has elapsed without the inv
being received
In order to allow for nodes with few peers (i.e. -connect) or nodes on
local connections that might as well flood everything to each other,
parity could be specified as a mask (fEven << 1 & fOdd). Peers from
pre-directional-parity versions can be treated as having the mask
fully set.

Both push staggering and directional parity admit simple
implementations. The specific staggering delay distribution would need
some thought; it could be set slower than the typical trickle cycle
period for better than current privacy, since general transaction
propagation would not impeded by heavy staggering. What do you think
of this approach? Any gotchas/improvements/alternatives?



 permalink raw reply	[|nested]
2014-07-20 21:01 [Bitcoin-development] Trickle and transaction propogation Kaz Wesley

  
From: Mike Hearn @ 2014-07-20 22:02 UTC (permalink / raw)
  To: Kaz Wesley;  Bitcoin Dev

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No comment on the proposal itself, which sounds reasonable but I didn't
think about it much yet. Instead, just an observation that by now most
users are not using the Core wallet anymore, but rather use either SPV
wallets or more centralised blockchain/coinbase style gateways to the
network.

SPV wallets don't relay thus you know any tx sent from them must be
originated by that wallet. Centralised services accept tx submissions via
SSL and can easily improve their users privacy by sending transactions out
via a node that isn't listening.

So IMHO we should be optimising the network for the common use case rather
than stuff that only helps Core wallet users, and actively slows down
everyone else. If your proposed techniques let us have our cake and eat it,
fantastic, otherwise I still think we should remove tx trickling.



On Sun, Jul 20, 2014 at 11:01 PM, Kaz Wesley <keziahw@gmail•com> wrote:

> The inv trickling mechanism currently serves two purposes:
> - protect casual users' privacy by slightly obscuring a tx's originating
> node
> - reduce invs unnecessarily sent both directions for a connection
> It has some drawbacks:
> - it slows transaction propagation
> - it delays knowledge between two nodes of what txes are mutually known
> These drawbacks will be especially costly once optimizations based on
> mutually-known transactions are available (in progress, see "sparse
> blocks" thread).
>
> Both of the benefits of trickling can be achieved more efficiently and
> without the costs to transaction propagation and mutual transaction
> knowledge.
>
> Privacy: trickling helps hide the origin of 3/4 of the transactions a
> node is pushing by preventing most of the node's neighbors from seeing
> the transactions from that node right away; by the time a peer becomes
> the trickle node, it may have received the same inv from another of
> its peers.
> This staggering of introduction of new invs to the network could be
> made more effective by scheduling staggered pushes of wallet
> transactions to each peer in a structure similar to mapAskFor.
> This does have the drawback that someone who has established multiple
> connections to a node can observe that some invs are pushed at
> different times, suggesting they are in the stagger set. I don't see
> any straightforward way to remedy this, but trickling is also
> vulnerable to sybil attacks, and floods 1/4 of its transactions
> immediately anyway -- so I think staggered push would be an overall
> privacy improvement.
> Likelihood of a partial sybil obtaining inv origin information could
> be reduced by a policy of ending staggering and pushing to all peers
> once another peer has received the tx from elsewhere and inved the
> transaction back to the original node; if the staggering is
> sufficiently slow, only one or two nodes would receive the initial
> push to the network and after that the inv would be treated
> indistinguishably from if it originated externally.
>
> Redundant invs: without trickling, when two nodes receive transactions
> at around the same time they may each send each other an inv before
> receiving the other's. Trickling reduces this by giving all
> non-trickleSend nodes a chance to send first. Thus just eliminating
> trickling would at most double inv traffic. Although invs are small
> they are numerous, being the only common message potentially sent from
> every node to all its neighbors.
> A more efficient solution to the who-sends-first problem would be for
> connections to have directional parity:
> - a node initiating a connection would announce its parity (even or odd)
> - an inv is sent right away to peers with matching parity, but only
> sent against parity if a certain timeout has elapsed without the inv
> being received
> In order to allow for nodes with few peers (i.e. -connect) or nodes on
> local connections that might as well flood everything to each other,
> parity could be specified as a mask (fEven << 1 & fOdd). Peers from
> pre-directional-parity versions can be treated as having the mask
> fully set.
>
> Both push staggering and directional parity admit simple
> implementations. The specific staggering delay distribution would need
> some thought; it could be set slower than the typical trickle cycle
> period for better than current privacy, since general transaction
> propagation would not impeded by heavy staggering. What do you think
> of this approach? Any gotchas/improvements/alternatives?
>
>
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