Subject: Re: [bitcoin-dev] Do we really need a mempool? (for relay nodes)

From: odinn <[email-masked]>
Date: Sun, 19 Jul 2015 01:59:49 -0700

Some brief thoughts, adding here a suggestion for a dynamic approach
to the issue. (e.g. each additional tx relayed has some thing
associated with it, that is, a "doubling" for each additional tx
relayed that spends a given UTXO, doesn't sound like it would be the
most dynamic approach to the issue; considering that full nodes use
the (UTXOs) to establish if transactions are valid – all inputs to a
transaction must be in the UTXO database for it to be valid, but
rather, would end up ratcheting upward the fee/kB for each additional
tx relayed, as proposed).

A more dynamic fee approach would be a better one, imho, but how is
this to occur?

Quoting from Gavin Andresen's http://gavinandresen.ninja/utxo-uhoh,
"The entire UTXO set doesn’t have to be in RAM; it can be stored on an
SSD or spinning hard disk. The access pattern to the UTXO is not
random; outputs that were spent recently are more likely to be
re-spent than outputs that have not been spent in a long time. Bitcoin
Core already has a multi-level UTXO cache, thanks to the hard work of
Pieter Wuille."

The relay nodes would, in this scenario that is proposed here in this
message, be confirming and discarding; the wallet nodes, if I
understand properly, in this scenario, as proposed, should be
retaining (keeping a record of the transactions they've relayed and
using a mempool).

There are some questions here:

- - How is the mempool to be limited?
- - What is the mechanism by which the UTXO set is stored (or proposed
to be stored)?
- - How would dynamic fee determinations be calculated?
- - Please describe more the general purpose messaging network?

Thank you

On 07/18/2015 12:46 PM, Patrick Strateman via bitcoin-dev wrote:
> Relay nodes do not need a mempool, but do need some mechanism to
> avoid DoS issues.
> 
> Wallet nodes can use the mempool for fee estimation (in addition
> to looking at past blocks).
> 
> On 07/18/2015 11:52 AM, Peter Todd via bitcoin-dev wrote:
>> As in, do relay nodes need to keep a record of the transactions
>> they've relayed? Strictly speaking, the answer is no: one a tx is
>> relayed modulo DoS concerns the entire thing can be discarded by
>> the node. (unconfirmed txs spending other unconfirmed txs can be
>> handled by creating packages of transactions, evaluated as a
>> whole)
>> 
>> To mitigate DoS concerns, we of course have to have some per-UTXO
>> limit on bandwidth relayed, but that task can be accomplished by
>> simply maintaining some kind of per-UTXO record of bandwidth
>> used. For instance if the weighted fee and fee/KB were recorded,
>> and forced to - say - double for each additional tx relayed that
>> spent a given UTXO you would have a clear and simple upper limit
>> of lifetime bandwidth. Equally it's easy to limit bandwidth
>> moment to moment by asking peers for highest fee/KB transactions
>> they advertise first, stopping when our bandwidth limit is
>> reached.
>> 
>> You probably could even remove IsStandard() pretty much entirely
>> with the right increasingly expensive "replacement" policy,
>> relying on it alone to provide anti-DoS. Obviously this would
>> simplify some of the debates around mining policy! This could
>> even be re-used for scalable a general-purpose messaging network
>> paid by coin ownership if the UTXO set is split up, and some kind
>> of expiration over time policy is implemented.
>> 
>> Miners of course would still want to have a mempool, but that
>> codebase may prove simpler if it doesn't have to work double-duty
>> for relaying as well.
>> 
>> 
>> 
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> 
> 
> 
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