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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