> Perhaps I am not following what you"re saying here.
> If the receiver is paying a higher feerate than your replacement,
> he"ll get it confirmed as fast or faster than your replacement in any
> case.

It actually doesn't really matter much.
Let's say I want to pay Alice and Bob (unrelated entities), so I send it to 
them (together) with a low-fee transaction of paying 50 sat/byte. After an hour 
it's obvious that it's not confirmed (maybe there was a big backlog, or no 
blocks mined), so I want to replace my small transaction with one that pays 70 
sat/byte.
But in the mean time, Alice has swept her wallet (or used a service that has 
done so) and generated 50KB of descendant transactions paying 40 sat/byte (i.e. 
total fees of 0.02 BTC or $50).
According to the BIP125 rules, I would need to pay for the cost of invalidating 
all the transactions (an absolute higher amount of fee) along with the replay 
cost of the descendant transactions.
So in this case, for me to fee bump the transaction I'm looking at throwing 
away $50 because of descendant transactions that were totally out of my 
control. If I don't fee bump, I violate my promise to Bob to pay in a timely 
manner (and also probably Alice, as it wasn't in her control she was using an 
exchange that did this).
If I guarantee to fee bump, the amount I risk is effectively unbounded. And 
even if the descendant transactions have a higher fee rate, and are assisting 
by CPFP boosting my transaction -- it very well might not be enough.
--
The idea of this proposal comes from the problems *in practice* of trying to 
low-ball fee estimation with scheduled continuous fee bumps until it confirms. 
At the moment it's not viable, but if this proposal was adopted then it would 
be.
Personally I think it's of critical piece of having a stable fee market. Fee 
estimation is a fools game, even the new and improved fee estimation in master 
today was suggesting x30 fees to what was required (and I successfully made 
transactions with). People (and especially services) being able to be able to 
dynamically increase their fees sanely when dealing with withdrawals (and 
especially batched ones) will go a long way to helping the overall ecosystem.

-Ryan

> -------- Original Message --------
> Subject: Re: [bitcoin-dev] BIP proposal: No chaining off replaceable 
> transactions
> Local Time: July 2, 2017 9:28 PM
> UTC Time: July 3, 2017 2:28 AM
> From: g...@xiph.org
> To: Rhavar <rha...@protonmail.com>
> bitcoin-dev@lists.linuxfoundation.org <bitcoin-dev@lists.linuxfoundation.org>
> On Sun, Jul 2, 2017 at 9:01 PM, Rhavar <rha...@protonmail.com> wrote:
>> That"s not really realistic. In practice some receivers do big sweeps and
>> include unconfirmed inputs. Replacing the transaction means you need to pay
>> the cost of the sweep, which you probably don"t want to do (can be in the
>> order of $100s of dollars).
> Perhaps I am not following what you"re saying here.
> If the receiver is paying a higher feerate than your replacement,
> he"ll get it confirmed as fast or faster than your replacement in any
> case.
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