From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org [172.17.192.35]) by mail.linuxfoundation.org (Postfix) with ESMTPS id E745987A for ; Thu, 13 Apr 2017 12:03:05 +0000 (UTC) X-Greylist: delayed 00:20:05 by SQLgrey-1.7.6 Received: from slow1-d.mail.gandi.net (slow1-d.mail.gandi.net [217.70.178.86]) by smtp1.linuxfoundation.org (Postfix) with ESMTP id 0B6C91D0 for ; Thu, 13 Apr 2017 12:03:04 +0000 (UTC) Received: from relay2-d.mail.gandi.net (relay2-d.mail.gandi.net [217.70.183.194]) by slow1-d.mail.gandi.net (Postfix) with ESMTP id B9C374F6736 for ; Thu, 13 Apr 2017 13:36:40 +0200 (CEST) Received: from mfilter43-d.gandi.net (mfilter43-d.gandi.net [217.70.178.174]) by relay2-d.mail.gandi.net (Postfix) with ESMTP id 7AB33C5A63 for ; Thu, 13 Apr 2017 13:36:37 +0200 (CEST) X-Virus-Scanned: Debian amavisd-new at mfilter43-d.gandi.net Received: from relay2-d.mail.gandi.net ([IPv6:::ffff:217.70.183.194]) by mfilter43-d.gandi.net (mfilter43-d.gandi.net [::ffff:10.0.15.180]) (amavisd-new, port 10024) with ESMTP id n0Ce0Gs_52TJ for ; Thu, 13 Apr 2017 13:36:35 +0200 (CEST) X-Originating-IP: 134.60.149.33 Received: from [134.60.149.33] (wlan149-033.wlan.uni-ulm.de [134.60.149.33]) (Authenticated sender: thomasv@electrum.org) by relay2-d.mail.gandi.net (Postfix) with ESMTPSA id AE0BAC5A7F for ; Thu, 13 Apr 2017 13:36:35 +0200 (CEST) To: bitcoin-dev@lists.linuxfoundation.org From: Thomas Voegtlin Message-ID: <0a38c993-9269-efb5-7790-e49cd4f7987f@electrum.org> Date: Thu, 13 Apr 2017 13:36:36 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,RCVD_IN_DNSWL_LOW, RCVD_IN_SORBS_SPAM autolearn=ham version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on smtp1.linux-foundation.org Subject: [bitcoin-dev] Proposal: Soft Fork Threshold Signaling X-BeenThere: bitcoin-dev@lists.linuxfoundation.org X-Mailman-Version: 2.1.12 Precedence: list List-Id: Bitcoin Protocol Discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 13 Apr 2017 12:03:06 -0000 Disclaimer: I am fully supportive of Segregated Witness and its activation by users through BIP148. However, I also believe that a soft fork would be less risky if it was initially activated by miners, before the date set in BIP148. This proposal is not intended to replace UASF, but to mitigate the risks. The following idea might already have been proposed and discussed elsewhere. If that is the case, I am sorry for the noise. Background ========== BIP9 requires 95% of miner hashrate support in order to activate a soft-fork. So far, the lack of miner consensus about Segwit has been frustrating both users and developers. This had led some users to propose a soft fork activation regardless of the expressed level of miner support (UASF, BIP148). There are many risks associated with UASF. If the fork is activated with less than 50% of the hashing power, the blockchain will have two competing branches. In addition, if the hashrate on the forking branch is very low, that branch will be exposed to attacks, where non-empty blocks are systematically orphaned by adverse miners. This threat may be a strong deterrent for miners willing to support the fork. The main argument in favor of UASF is that users, not miners, give its value to Bitcoin. Therefore, users and markets should have the power to decide which branch of the fork has the most value, and profit-driven miners should follow. If the soft-forking branch is valued more than the non-forking branch, it will end up attracting a majority of the hashing power, and the non-forking branch will eventually be orphaned. Feedback through markets, however, will only work if the forking branch can effectively be used. If the forking branch is rendered unusable by adverse miners, there is little chance the new coins will ever reach markets. To make things worse, profit-driven miners might adopt a passive attitude and decide to mine on the forking branch only once a proper price has been set by markets, or only once they see that it has enough hashing power to be usable. Thus, the lack of hashrate information prior to the soft fork increases the risk. On the other hand, if a soft fork was initiated with more than 33% of the hashing power, then it would probably be viable, because the remaining two thirds of the hashing power cannot successfully be allocated to mine blocks on the non-forking branch and to orphan blocks on the forking branch. Therefore, users will be able to move coins on the forking branch, and markets will be able to set a price on these coins, thus creating the feedback needed by profit-driven miners. Today about 30% of the hashing power are signaling their intention to activate Segwit using BIP9. This hashrate is very close to the 33% threshold, and it would probably be enough to initiate a viable soft fork; indeed we can expect additional hashing power to be gained from miners mining on both branches of the fork. However, nothing suggests that a soft fork triggered with 30% of the hashrate would be followed by the miners who are currently signaling Segwit using BIP9. BIP9 signaling means that these miners are willing to soft fork if support reaches 95%; it does not say anything about their intentions if support is as low as 30%. In other words, BIP9 signaling does allow miners to properly signal their intentions. BIP9 signaling ============== The activation threshold is part of the semantics of BIP9. Miners who use BIP9 do not only signal their support for a soft fork; they also signal to other miners that they will activate the soft fork if and only if support reaches 95%. Some of these miners might actually be willing to activate a soft fork with a lower support, even at the cost of creating two chains. Other miners might not be supportive of that idea, because they consider that the danger of their blocks being orphaned is too high. The problem is that this information, at which level of support miners are willing to initiate a soft fork, is not available. Thus, miners who are willing to initiate a soft fork at a lower hashrate cannot coordinate their action. Proposal: Soft Fork Threshold Signaling ======================================= Miners signal the threshold at which they are willing to activate a soft fork. The value of the threshold is published in the coinbase transaction of the block, with the corresponding version bit. Miners activate a soft fork if their threshold has been reached over the last retargeting period. For example, if 504 of 2016 blocks signal a soft fork with a threshold equal or lower to 25%, then the soft fork is activated by these miners. If no activation threshold is reached, the current BIP9 signaling rate indicator is replaced by a distribution of signaling rates per threshold. The public availability of threshold information allows miners to adjust their own threshold, and to optimize their chances of activating the soft fork. UASF ==== Even if the soft fork is not activated by miners, this proposal will reduce the risks associated to a user activated soft fork (UASF). The public availability of hashrate threshold information prior to the soft fork will help miners decide whether they should join the fork right after it has been activated, before price information is available. Vulnerabilities =============== This proposal has similar vulnerabilities as BIP9: it is susceptible to fake signaling by miners, and to miners withholding hashing power before the fork.