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The spam attacks prove that its possible to attach a transaction to whichever tip you want. Normal operation of the tangle is to attach a transaction to two random tips and then wait for another node to randomly choose your transaction as a tip for someone else's transaction. Even under best conditions there is a chance that it is just never picked as a tip and under current conditions it is 90% likely to be ignored.

Why couldn’t you forcibly choose your own transaction as a tip for a second transaction to force its confirmation?

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A transaction is confirmed once it is transitively approved from a milestone transaction. (At the moment, milestone transactions are chosen by the coordinator, but in the future the MCMC algorithm will be used to choose them on every node).

Therefore, if your transaction is merely approved by another transaction, you have won nothing. To get your transaction confirmed, that other transaction needs to be approved by another transaction until eventually the resulting transaction is picked as a milestone.

So, if you just send another transaction to approve your new transaction, the problem stays the same (there is still a tip that has to be chosen by others to ultimately get your transaction confirmed). Only that you have spent PoW one more time than you would have to.

On the other hand, if you have to perform another transaction right now (with different addresses so that they can be processed both at the same time), and your first transaction is not yet confirmed, it might be wise to pick your first transaction as one of the tips to confirm by your second one. Then you only need one tip to be picked and not two. [The current wallet does not allow you to do things like that, though.]

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  • "To get your transaction confirmed, that other transaction needs to be approved by another transaction until eventually the resulting transaction is picked as a milestone." Slightly confused. You say my 2nd transaction needs to be confirmed by a 3rd, but also you say that non of that matters until a milestone transaction comes along. So why the 3rd is necessary?
    – Spamalot
    Commented Dec 7, 2017 at 18:48
  • If your transaction is picked as a milestone, there is no 3rd transaction necessary. But there is only one milestone per minute, and the coordinator tends to pick the milestone that has most transactions below it, so (as every transaction confirms 2 others) it is probably not your transaction that is picked, but some other one that has a big tree (including your transaction) below it.
    – mihi ♦
    Commented Dec 7, 2017 at 22:40
  • Isn’t that what „promote“ is doing now?
    – Akkumulator
    Commented Dec 20, 2017 at 17:57
  • @Akkumulator More or less, yes. But as I said, it will not guarantee immediate confirmation either, only make the tip more recent to make it higher chance to be chosen by others.
    – mihi ♦
    Commented Dec 20, 2017 at 19:14
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It seems like my question has been made obsolete by changes to the current wallet. The new Promote function does actually send a second transaction that attaches to the first and also to the latest milestone. This does apparently improve confirmation chances.

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