docs: ADR for throttle with retries

docs/docs/adrs/adr-008-throttle-retries.md

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+---
+sidebar_position: 7
+title: Throttle with retries
+---
+
+# ADR {008}: {Throttle with retries}
+
+## Changelog
+
+* {6/9/23}: Initial draft
+
+## Status
+
+> A decision may be "proposed" if it hasn't been agreed upon yet, or "accepted" once it is agreed upon. If a later ADR changes or reverses a decision, it may be marked as "deprecated" or "superseded" with a reference to its replacement.
+
+Proposed
+
+## Context
+
+For context on why the throttling mechanism exists, see [ADR 002](./adr-002-throttle.md).
+
+Note the terms slash throttling and jail throttling are synonymous, since in replicated security a `SlashPacket` simply jails a validator for downtime infractions.  
+
+Currently the throttling mechanism is designed so that provider logic (slash meter, etc.) dictates how many slash packets can be handled over time. Throttled slash packets are persisted on the provider, leading to multiple possible issues. Namely:
+
+* If slash or vsc matured packets are actually throttled/queued on the provider, state can grow and potentially lead to a DoS attack. We have short term solutions around this, but overall they come with their own weaknesses. See [#594](https://github.com/cosmos/interchain-security/issues/594).
+* If a jailing attack described in [ADR 002](adr-002-throttle.md) were actually to be carried out with the current throttling design, we'd likely have to halt the provider, and perform an emergency upgrade and/or migration to clear the queues of slash packets that were deemed to be malicious. Alternatively, validators would just have to _tough it out_ and wait for the queues to clear, during which all/most validators would be jailed. Right after being jailed, vals would have to unjail themselves promptly to ensure safety. The synchronous coordination required to maintain safety in such a scenario is not ideal.
+
+So what's the solution? We can improve the throttling mechanism to instead queue/persist relevant data on each consumer, and have consumers retry slash requests as needed.
+
+## Decision
+
+> This section explains all of the details of the proposed solution, including implementation details.
+It should also describe affects / corollary items that may need to be changed as a part of this.
+If the proposed change will be large, please also indicate a way to do the change to maximize ease of review.
+(e.g. the optimal split of things to do between separate PR's)
+
+### Provider changes
+
+The main change needed for the provider is the removal of queuing logic for slash and vsc matured packets upon being received. Instead, the provider will consult the slash meter to determine if a slash packet can be handled immediately. If not, the provider will return an ack message to the consumer communicating that the slash packet could not be handled, and needs to be sent again in the future (retried). VSCMatured packets will always be handled immediately upon being received by the provider.
+
+Note [spec](https://github.com/cosmos/ibc/blob/main/spec/app/ics-028-cross-chain-validation/system_model_and_properties.md#consumer-initiated-slashing). Specifically the section on _VSC Maturity and Slashing Order_. Previously the onus was on the provider to maintain this property via its queuing. Now the onus will be on the consumer to send packets in the correct order and block packet sending as needed. Then the ordered IBC channel will ensure that the packets are received in the correct order on the provider.
+
+The provider's main responsibility regarding throttling will now be to determine if a recv slash packet can be handled via slash meter etc., and appropriately ack to the sending consumer.
+
+### Consumer changes
+
+Note the consumer already queues up both slash and vsc matured packets via `AppendPendingPacket`. Those packets are dequeued every endblock in `SendPackets` and sent to the provider.
+
+Instead, we will now introduce the following logic on endblock:
+
+* Slash packets will always be sent to the provider once they're at the head of the queue. However, once sent, the consumer will not send any trailing vsc matured packets from the queue until the provider responds with an ack that the slash packet has been handled (ie. val was jailed). That is, slash packets block the sending of trailing vsc matured packets in the consumer queue.
+* If two slash packets are at the head of the queue, the consumer will send the first slash packet, and then wait for a success ack from the provider before sending the second slash packet. This seems like it'd simplify implementation.
+* VSC matured packets at the head of the queue (ie. NOT trailing a slash packet) can be sent immediately, and do not block any other packets in the queue, since the provider always handles them immediately.
+
+To prevent the provider from having to keep track of what slash packets have been rejected, the consumer will have to retry the sending of slash packets over some period of time. This can be achieved with an on-chain consumer param. The suggested param value would probably be 1/2 of the provider's `SlashMeterReplenishmentPeriod`, although it doesn't matter too much as long as the param value is sane.
+
+Note to prevent weird edge case behavior, a retry would not be attempted until either a success ack or failure ack has been recv from the provider.
+
+With the behavior described, we maintain very similar behavior to the current throttling mechanism regarding the timing that slash and vsc matured packets are handled on the provider. Obviously the queueing and blocking logic is moved, and the two chains do have to send more messages between one another.
+
+In the nominal case (no or minimal slash packets being sent), the vsc matured packets will not be delayed, and hence unbonding will not be delayed.
+
+### Splitting of PRs
+
+We could split this feature into two PRs, one affecting the consumer and one affecting the provider, along with a third PR which could setup a clever way to upgrade the provider in multiple steps, ensuring that queued slash packets at upgrade time are handled properly. 
+
+## Consequences
+
+> This section describes the consequences, after applying the decision. All consequences should be summarized here, not just the "positive" ones.
+
+* Consumers will now have to manage their own queues, and retry logic.
+* Consumers still aren't trustless, but the provider is now less susceptible to mismanaged or malicious consumers.
+* Recovering from the "jailing attack" is more elegant.
+* Some issues like [#1001](https://github.com/cosmos/interchain-security/issues/1001) will now be handled implicitly by the improved throttling mechanism.
+* Slash and vsc matured packets can be handled immediately once recv by the provider if the slash meter allows.
+* In general, we reduce the amount of computation that happens in the provider end-blocker.
+
+### Positive
+
+* We no longer have to reason about a "global queue" and a "chain specific queue", and keeping those all in-sync. Now slash and vsc matured packet queuing is handled on each consumer individually.
+* Due to the above, the throttling protocol becomes less complex overall.
+* We no longer have to worry about throttle related DoS attack on the provider, since no queuing exists on the provider.
+
+### Negative
+
+* Increased number of IBC packets being relayed anytime throttling logic is triggered.
+* Consumer complexity increases, since consumers now have manage queuing themselves, and implement packet retry logic.
+
+### Neutral
+
+* Core throttling logic on the provider remains unchanged, ie. slash meter, replenishment cycles, etc.
+
+## References
+
+* [EPIC](https://github.com/cosmos/interchain-security/issues/713)
+* [ADR 002](./adr-002-throttle.md)
+* [#594](https://github.com/cosmos/interchain-security/issues/594)