github.com/aakash4dev/cometbft@v0.38.2/spec/consensus/proposer-based-timestamp/pbts-algorithm_001_draft.md (about)

     1  # Proposer-Based Time - Part II
     2  
     3  ## Updated Consensus Algorithm
     4  
     5  ### Outline
     6  
     7  The algorithm in the [arXiv paper][arXiv] evaluates rules of the received messages without making explicit how these messages are received. In our solution, we will make some message filtering explicit. We will assume that there are message reception steps (where messages are received and possibly stored locally for later evaluation of rules) and processing steps (the latter roughly as described in a way similar to the pseudo code of the arXiv paper).
     8  
     9  In contrast to the original algorithm the field `proposal` in the `PROPOSE` message is a pair `(v, time)`, of the proposed consensus value `v` and the proposed time `time`.
    10  
    11  #### **[PBTS-RECEPTION-STEP.0]**
    12  
    13  In the reception step at process `p` at local time `now_p`, upon receiving a message `m`:
    14  
    15  - if the message `m` is of type `PROPOSE` and satisfies `now_p - PRECISION <  m.time < now_p + PRECISION + MSGDELAY`, then mark the message as `timely`
    16  
    17  > if `m` does not satisfy the constraint consider it `untimely`
    18  
    19  
    20  #### **[PBTS-PROCESSING-STEP.0]**
    21  
    22  In the processing step, based on the messages stored, the rules of the algorithms are
    23  executed. Note that the processing step only operates on messages
    24  for the current height. The consensus algorithm rules are defined by the following updates to arXiv paper.
    25  
    26  #### New `StartRound`
    27  
    28  There are two additions
    29  
    30  - in case the proposer's local time is smaller than the time of the previous block, the proposer waits until this is not the case anymore (to ensure the block time is monotonically increasing)
    31  - the proposer sends its time `now_p` as part of its proposal
    32  
    33  We update the timeout for the `PROPOSE` step according to the following reasoning:
    34  
    35  - If a correct proposer needs to wait to make sure its proposed time is larger than the `blockTime` of the previous block, then it sends by realtime `blockTime + ACCURACY` (By this time, its local clock must exceed `blockTime`)
    36  - the receiver will receive a `PROPOSE` message by `blockTime + ACCURACY + MSGDELAY`
    37  - the receiver's local clock will be `<= blockTime + 2 * ACCURACY + MSGDELAY`
    38  - thus when the receiver `p` enters this round it can set its timeout to a value `waitingTime => blockTime + 2 * ACCURACY + MSGDELAY - now_p`
    39  
    40  So we should set the timeout to `max(timeoutPropose(round_p), waitingTime)`.
    41  
    42  > If, in the future, a block delay parameter `BLOCKDELAY` is introduced, this means
    43  that the proposer should wait for `now_p > blockTime + BLOCKDELAY` before sending a `PROPOSE` message.
    44  Also, `BLOCKDELAY` needs to be added to `waitingTime`.
    45  
    46  #### **[PBTS-ALG-STARTROUND.0]**
    47  
    48  ```go
    49  function StartRound(round) {
    50    blockTime ← block time of block h_p - 1
    51    waitingTime ← blockTime + 2 * ACCURACY + MSGDELAY - now_p
    52    round_p ← round
    53    step_p ← propose
    54    if proposer(h_p, round_p) = p {
    55      wait until now_p > blockTime // new wait condition
    56      if validValue_p != nil {
    57        proposal ← (validValue_p, now_p) // added "now_p"
    58      }
    59      else {
    60        proposal ← (getValue(), now_p)   // added "now_p"
    61      }
    62      broadcast ⟨PROPOSAL, h_p, round_p, proposal, validRound_p⟩
    63    }
    64    else {
    65      schedule OnTimeoutPropose(h_p,round_p) to be executed after max(timeoutPropose(round_p), waitingTime)
    66    }
    67  }
    68  ```
    69  
    70  #### New Rule Replacing Lines 22 - 27
    71  
    72  - a validator prevotes for the consensus value `v` **and** the time `t`
    73  - the code changes as the `PROPOSAL` message carries time (while `lockedValue` does not)
    74  
    75  #### **[PBTS-ALG-UPON-PROP.0]**
    76  
    77  ```go
    78  upon timely(⟨PROPOSAL, h_p, round_p, (v,t), −1⟩) from proposer(h_p, round_p) while step_p = propose do {
    79    if valid(v) ∧ (lockedRound_p = −1 ∨ lockedValue_p = v) {
    80      broadcast ⟨PREVOTE, h_p, round_p, id(v,t)⟩ 
    81    }
    82    else {
    83      broadcast ⟨PREVOTE, h_p, round_p, nil⟩ 
    84    }
    85    step_p ← prevote
    86  }
    87  ```
    88  
    89  #### New Rule Replacing Lines 28 - 33
    90  
    91  In case consensus is not reached in round 1, in `StartRound` the proposer of future rounds may propose the same value but with a different time.
    92  Thus, the time `tprop` in the `PROPOSAL` message need not match the time `tvote` in the (old) `PREVOTE` messages.
    93  A validator may send `PREVOTE` for the current round as long as the value `v` matches.
    94  This gives the following rule:
    95  
    96  #### **[PBTS-ALG-OLD-PREVOTE.0]**
    97  
    98  ```go
    99  upon timely(⟨PROPOSAL, h_p, round_p, (v, tprop), vr⟩) from proposer(h_p, round_p) AND 2f + 1 ⟨PREVOTE, h_p, vr, id((v, tvote)⟩ 
   100  while step_p = propose ∧ (vr ≥ 0 ∧ vr < round_p) do {
   101    if valid(v) ∧ (lockedRound_p ≤ vr ∨ lockedValue_p = v) {
   102      broadcast ⟨PREVOTE, h_p, roundp, id(v, tprop)⟩
   103    }
   104    else {
   105      broadcast ⟨PREVOTE, hp, roundp, nil⟩
   106    }
   107    step_p ← prevote
   108  }
   109  ```
   110  
   111  #### New Rule Replacing Lines 36 - 43
   112  
   113  - As above, in the following `(v,t)` is part of the message rather than `v`
   114  - the stored values (i.e., `lockedValue`, `validValue`) do not contain the time
   115  
   116  #### **[PBTS-ALG-NEW-PREVOTE.0]**
   117  
   118  ```go
   119  upon timely(⟨PROPOSAL, h_p, round_p, (v,t), ∗⟩) from proposer(h_p, round_p) AND 2f + 1 ⟨PREVOTE, h_p, round_p, id(v,t)⟩ while valid(v) ∧ step_p ≥ prevote for the first time do {
   120    if step_p = prevote {
   121      lockedValue_p ← v
   122      lockedRound_p ← round_p
   123      broadcast ⟨PRECOMMIT, h_p, round_p, id(v,t))⟩ 
   124      step_p ← precommit
   125    }
   126    validValue_p ← v 
   127    validRound_p ← round_p
   128  }
   129  ```
   130  
   131  #### New Rule Replacing Lines 49 - 54
   132  
   133  - we decide on `v` as well as on the time from the proposal message
   134  - here we do not care whether the proposal was received timely.
   135  
   136  > In particular we need to take care of the case where the proposer is untimely to one correct validator only. We need to ensure that this validator decides if all decide.
   137  
   138  #### **[PBTS-ALG-DECIDE.0]**
   139  
   140  ```go
   141  upon ⟨PROPOSAL, h_p, r, (v,t), ∗⟩ from proposer(h_p, r) AND 2f + 1 ⟨PRECOMMIT, h_p, r, id(v,t)⟩ while decisionp[h_p] = nil do {
   142    if valid(v) {
   143      decision_p [h_p] = (v,t) // decide on time too
   144      h_p ← h_p + 1
   145      reset lockedRound_p , lockedValue_p, validRound_p and validValue_p to initial values and empty message log 
   146      StartRound(0)
   147    }
   148  }
   149  ```
   150  
   151  **All other rules remains unchanged.**
   152  
   153  Back to [main document][main].
   154  
   155  [main]: ./pbts_001_draft.md
   156  
   157  [arXiv]: https://arxiv.org/abs/1807.04938
   158  
   159  
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