From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp1.osuosl.org (smtp1.osuosl.org [140.211.166.138]) by lists.linuxfoundation.org (Postfix) with ESMTP id 9DFFFC0001 for ; Thu, 25 Feb 2021 22:33:33 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by smtp1.osuosl.org (Postfix) with ESMTP id 7D3E18423E for ; Thu, 25 Feb 2021 22:33:33 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org X-Spam-Flag: NO X-Spam-Score: -1.698 X-Spam-Level: X-Spam-Status: No, score=-1.698 tagged_above=-999 required=5 tests=[BAYES_50=0.8, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001] autolearn=ham autolearn_force=no Authentication-Results: smtp1.osuosl.org (amavisd-new); dkim=pass (2048-bit key) header.d=posteo.net Received: from smtp1.osuosl.org ([127.0.0.1]) by localhost (smtp1.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id 5dLFYibkbi29 for ; Thu, 25 Feb 2021 22:33:31 +0000 (UTC) X-Greylist: domain auto-whitelisted by SQLgrey-1.8.0 Received: from mout02.posteo.de (mout02.posteo.de [185.67.36.66]) by smtp1.osuosl.org (Postfix) with ESMTPS id B83E884243 for ; Thu, 25 Feb 2021 22:33:30 +0000 (UTC) Received: from submission (posteo.de [89.146.220.130]) by mout02.posteo.de (Postfix) with ESMTPS id 0126A2400FD for ; Thu, 25 Feb 2021 23:33:26 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=posteo.net; s=2017; t=1614292407; bh=ZtEmjvbZw0Y9wwE4YsV3xcl/w2A4A1JUOvg8UeOx6ik=; h=From:Subject:To:Date:From; b=T2281ealVAUtfsSmpcAvBclgOQ0jO8LEclkltMKJw4ito8Z0UKfAQMObELfADIxLi FXzIHKLfjoXCOWm9xttoRtuFI/NE9QjFMdw5EYIQLBDoI3ST7aBqOqIY0MYp9u2nq4 a7nBzCdPULKeRUNabtMn0M330Vd4p9ZcWpXZdO9uCL63nzPeZXTG6R8KrYuDwxXJ+s JHBSF/5YeqMRjVp85o1RWS7qaIK4pfK9k/ChPfwZa4UpPmenDll1opJbtTHEv8f3Qf LSOLgejYrDcnvSOX9Fa7IWhuj80NSc2GQ3jhXy70Ip+XLo1wkUwfMKDAawwkiNEd4+ S8HCBrGivwS6g== Received: from customer (localhost [127.0.0.1]) by submission (posteo.de) with ESMTPSA id 4Dmnck3ssqz9rxP for ; Thu, 25 Feb 2021 23:33:26 +0100 (CET) From: Gregorio Guidi To: bitcoin-dev@lists.linuxfoundation.org Message-ID: Date: Thu, 25 Feb 2021 23:33:25 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US X-Mailman-Approved-At: Thu, 25 Feb 2021 23:12:56 +0000 Subject: [bitcoin-dev] Exploring alternative activation mechanisms: decreasing threshold X-BeenThere: bitcoin-dev@lists.linuxfoundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Bitcoin Protocol Discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 25 Feb 2021 22:33:33 -0000 Hello, I followed the debate on LOT=false / LOT=true trying to get a grasp of the balance of risks and advantages. The summary by Aaron van Wirdum [1] explains well the difficulties to find a good equilibrium... it concludes that "perhaps, a new possibility will present itself". Thinking about such a "new possibility" that overcomes the LOT=true/false dichotomy, I would like to offer the following proposal. It could be called "decreasing threshold activation". Decreasing threshold activation works similarly to BIP8, with the difference that the threshold that triggers the STARTED -> LOCKED_IN transition starts at 100% for the first retargeting period, and then is gradually reduced on each period in steps of 24 blocks (~1,2%). More precisely: On the 1st period (starting on start_height): if 2016 out of 2016 blocks signal, the state is changed to LOCKED_IN on the next period (otherwise stays STARTED) On the 2nd period: if 1992 out of 2016 blocks signal (~98.8%), the state transitions to LOCKED_IN on the next period On the 3rd period: if 1968 out of 2016 blocks signal (~97.6%), the state transitions to LOCKED_IN on the next period ... On the 14th period (~6 months): if 1704 out of 2016 blocks signal (~84.5%), the state transitions to LOCKED_IN on the next period ... On the 27th period (~12 months): if 1392 out of 2016 blocks signal (~69.0%), the state transitions to LOCKED_IN on the next period ... On the 40th period (~18 months): if 1080 out of 2016 blocks signal (~53.6%), the state transitions to LOCKED_IN on the next period ... On the 53th period (~24 months): if 768 out of 2016 blocks signal (~38.1%), the state transitions to LOCKED_IN on the next period ... On the 66th period (~30 months): if 456 out of 2016 blocks signal (~22.6%), the state transitions to LOCKED_IN on the next period ... On the 79th period (~36 months): if 144 out of 2016 blocks signal (~7.1%), the state transitions to LOCKED_IN on the next period ... On the 84th and final period (~39 months): if 24 out of 2016 blocks signal (~1.2%), the state transitions to LOCKED_IN on the next period, otherwise goes to FAILED (For reference, I include below a snippet of pseudocode for the decreasing thresholds in the style of BIP8 and BIP9.) Here are the main features and advantages of this approach: 1. It is relatively conservative at the beginning: for activation to happen in the first year, it requires a clear majority of signaling hashrate, indicating that the activation is relatively safe. Only later the threshold starts to move towards "unsafe" territory, accepting the tradeoff of less support from existing hashrate in exchange for ensuring that the activation eventually happens. 2. Like LOT=true, the activation will always occur in the end (except in the negligible case where less than 1.2% of hashrate supports it). 3. This approach is quite easy to implement, in particular it avoids the extra code to deal with the MUST_SIGNAL period. 4. There are no parameters to set (except startheight). I am a KISS fan, so this is a plus for me, making the activation mechanism robust and predictable with less chance for users to shoot themselves in the foot. It is also a plus for me that - if adopted as the default mechanism - it would require very little discussion on how to activate future soft-forks. In fact I think it would be a winning move for Core to commit to such a scheme, to avoid getting lost in game-theoretic rabbit holes. 5. Since there is no MUST_SIGNAL period, no automatic chain split occurs around activation when not all miners have upgraded (so activation is generally as benign as a MASF). A chain split will occur only when/if an invalid block is created (and this requires dedicated effort! it can only happen by circumventing the normal policy rules [2]). This mitigates the risk of reorgs and involuntary forks around activation, even with low miner signaling. 6. It removes motivation to create UASF clients that force activation. While individual nodes could still try to force a quicker activation, the motivation to do so is reduced since the same result is obtained just by waiting a little more. 7. Compared to LOT=true, activation is cleaner and quicker when it is relatively safe to do so (when the signaling hashrate is - let's say - in the 70%-80% range). On the other hand, activation is pushed further and further in time when it is less safe (when signaling hashrate is <50%, meaning that there is a serious risk that users/miners that did not upgrade start following an alternative chain). This gives everyone time to prepare properly for such a potentially disruptive event. 8. If a significant number of users and miners consciously decide (for whatever reasons) that they don't want to upgrade and want to fork themselves off from the chain followed by Core (as is their prerogative), they will have time to do so safely. 9. Compared to the strategy of doing LOT=false and then LOT=true if it fails, using the decreasing threshold approach may not seem very different. But it completely removes the need to fiddle with different client releases and with the issues associated with deployed nodes with different consensus parameters. All in all, reading the various perspectives on this mailing list and outside I have the feeling that the strongest arguments against LOT=true have at their core a certain uneasiness with the MUST_SIGNAL mechanism and the related automatic chain split on activation, which is something that greatly complicates the analysis (but please tell me if I am wrong...). In this sense, this proposal achieves the big objective of always ending in activation (like LOT=true) without resorting to MUST_SIGNAL and chain splits. A final note: this proposal should be seen as somewhat independent from the discussion on taproot activation. Personally I would be happy with a LOT=false activation for taproot that succeeds quickly, while the decreasing threshold approach could be evaluated as potential default activation mechanism for the future. I would be happy to hear what you think about this. What are the possible issues/drawbacks of using this mechanism? Thanks, Gregorio [1] https://bitcoinmagazine.com/articles/lottrue-or-lotfalse-this-is-the-last-hurdle-before-taproot-activation [2] This was not the case in the past for upgrades such as BIP16 (P2SH), which generated frequent reorgs due to a combination of low activation threshold (55%) and no policy protection. But for upgrades such as taproot the normal policy rules prevent the creation of invalid blocks by non-upgraded miners. See https://blog.bitmex.com/the-arts-of-making-softforks-protection-by-policy-rule/ Pseudocode: case STARTED: int elapsed_periods = (block.height - startheight) / 2016; if (elapsed_periods > 2016 / 24) { return FAILED; } int threshold = 2016 - 24 * (elapsed_periods - 1); int count = 0; walk = block; for (i = 0; i < 2016; i++) { walk = walk.parent; if (walk.nVersion & 0xE0000000 == 0x20000000 && (walk.nVersion >> bit) & 1 == 1) { ++count; } } if (count >= threshold) { return LOCKED_IN; } return STARTED;