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Merge PR #3103: Tombstone specification
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# Staking Tombstone | ||
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## Abstract | ||
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In the current implementation of the `slashing` module, when the consensus engine | ||
informs the state machine of a validator's consensus fault, the validator is | ||
partially slashed, and put into a "jail period", a period of time in which they | ||
are not allowed to rejoin the validator set. However, because of the nature of | ||
consensus faults and ABCI, there can be a delay between an infraction occurring, | ||
and evidence of the infraction reaching the state machine (this is one of the | ||
primary reasons for the existence of the unbonding period). | ||
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> Note: The tombstone concept, only applies to faults that have a delay between | ||
the infraction occurring and evidence reaching the state machine. For example, | ||
evidence of a validator double signing may take a while to reach the state machine | ||
due to unpredictable evidence gossip layer delays and the ability of validators to | ||
selectively reveal double-signatures (e.g. to infrequently-online light clients). | ||
Liveness slashing, on the other hand, is detected immediately as soon as the | ||
infraction occurs, and therefore no slashing period is needed. A validator is | ||
immediately put into jail period, and they cannot commit another liveness fault | ||
until they unjail. In the future, there may be other types of byzantine faults | ||
that have delays (for example, submitting evidence of an invalid proposal as a transaction). | ||
When implemented, it will have to be decided whether these future types of | ||
byzantine faults will result in a tombstoning (and if not, the slash amounts | ||
will not be capped by a slashing period). | ||
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In the current system design, once a validator is put in the jail for a consensus | ||
fault, after the `JailPeriod` they are allowed to send a transaction to `unjail` | ||
themselves, and thus rejoin the validator set. | ||
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One of the "design desires" of the `slashing` module is that if multiple | ||
infractions occur before evidence is executed (and a validator is put in jail), | ||
they should only be punished for single worst infraction, but not cumulatively. | ||
For example, if the sequence of events is: | ||
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1. Validator A commits Infraction 1 (worth 30% slash) | ||
2. Validator A commits Infraction 2 (worth 40% slash) | ||
3. Validator A commits Infraction 3 (worth 35% slash) | ||
4. Evidence for Infraction 1 reaches state machine (and validator is put in jail) | ||
5. Evidence for Infraction 2 reaches state machine | ||
6. Evidence for Infraction 3 reaches state machine | ||
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Only Infraction 2 should have its slash take effect, as it is the highest. This | ||
is done, so that in the case of the compromise of a validator's consensus key, | ||
they will only be punished once, even if the hacker double-signs many blocks. | ||
Because, the unjailing has to be done with the validator's operator key, they | ||
have a chance to re-secure their consensus key, and then signal that they are | ||
ready using their operator key. We call this period during which we track only | ||
the max infraction, the "slashing period". | ||
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Once, a validator rejoins by unjailing themselves, we begin a new slashing period; | ||
if they commit a new infraction after unjailing, it gets slashed cumulatively on | ||
top of the worst infraction from the previous slashing period. | ||
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However, while infractions are grouped based off of the slashing periods, because | ||
evidence can be submitted up to an `unbondingPeriod` after the infraction, we | ||
still have to allow for evidence to be submitted for previous slashing periods. | ||
For example, if the sequence of events is: | ||
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1. Validator A commits Infraction 1 (worth 30% slash) | ||
2. Validator A commits Infraction 2 (worth 40% slash) | ||
3. Evidence for Infraction 1 reaches state machine (and Validator A is put in jail) | ||
4. Validator A unjails | ||
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We are now in a new slashing period, however we still have to keep the door open | ||
for the previous infraction, as the evidence for Infraction 2 may still come in. | ||
As the number of slashing periods increase, it creates more complexity as we have | ||
to keep track of the highest infraction amount for every single slashing period. | ||
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> Note: Currently, according to the `slashing` module spec, a new slashing period | ||
is created every time a validator is unbonded then rebonded. This should probably | ||
be changed to jailed/unjailed. See issue [#3205](https://github.com/cosmos/cosmos-sdk/issues/3205) | ||
for further details. For the remainder of this, I will assume that we only start | ||
a new slashing period when a validator gets unjailed. | ||
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The maximum number of slashing periods is the `len(UnbondingPeriod) / len(JailPeriod)`. | ||
The current defaults in Gaia for the `UnbondingPeriod` and `JailPeriod` are 3 weeks | ||
and 2 days, respectively. This means there could potentially be up to 11 slashing | ||
periods concurrently being tracked per validator. If we set the `JailPeriod >= UnbondingPeriod`, | ||
we only have to track 1 slashing period (i.e not have to track slashing periods). | ||
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Currently, in the jail period implementation, once a validator unjails, all of | ||
their delegators who are delegated to them (haven't unbonded / redelegated away), | ||
stay with them. Given that consensus safety faults are so egregious | ||
(way more so than liveness faults), it is probably prudent to have delegators not | ||
"auto-rebond" to the validator. Thus, we propose setting the "jail time" for a | ||
validator who commits a consensus safety fault, to `infinite` (i.e. a tombstone state). | ||
This essentially kicks the validator out of the validator set and does not allow | ||
them to re-enter the validator set. All of their delegators (including the operator themselves) | ||
have to either unbond or redelegate away. The validator operator can create a new | ||
validator if they would like, with a new operator key and consensus key, but they | ||
have to "re-earn" their delegations back. To put the validator in the tombstone | ||
state, we set `DoubleSignJailEndTime` to `time.Unix(253402300800)`, the maximum | ||
time supported by Amino. | ||
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Implementing the tombstone system and getting rid of the slashing period tracking | ||
will make the `slashing` module way simpler, especially because we can remove all | ||
of the hooks defined in the `slashing` module consumed by the `staking` module | ||
(the `slashing` module still consumes hooks defined in `staking`). | ||
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### Single slashing amount | ||
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Another optimization that can be made is that if we assume that all ABCI faults | ||
for Tendermint consensus are slashed at the same level, we don't have to keep | ||
track of "max slash". Once an ABCI fault happens, we don't have to worry about | ||
comparing potential future ones to find the max. | ||
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Currently the only Tendermint ABCI fault is: | ||
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- Unjustified precommits (double signs) | ||
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It is currently planned to include the following fault in the near future: | ||
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- Signing a precommit when you're in unbonding phase (needed to make light client bisection safe) | ||
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Given that these faults are both attributable byzantine faults, we will likely | ||
want to slash them equally, and thus we can enact the above change. | ||
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> Note: This change may make sense for current Tendermint consensus, but maybe | ||
not for a different consensus algorithm or future versions of Tendermint that | ||
may want to punish at different levels (for example, partial slashing). |