I am failing to see how you actually will detect a hard fork with this 
system.

Maybe its because of this sentence not being very clear to me;
 «If a generalized block header chain with non-trivial total proof-of-work
   is emerging»

Also, can you explain what you are actually trying to accomplish?

I want to point out that the following part from your motivation is 
incorrect. A full node would reject a hard forked chain (by definition), 
there is no risk of them transacting on it.

«When such features are implemented with a hardfork, existing full node 
implementations would consider such blocks as invalid, and may even ban a 
peer for relaying such blocks. They are effectively blind to such hardfork 
rule changes, leaving users to unknowingly transact on a system with 
potentially different token value. »



On Friday, 2 December 2016 01:20:31 CET Johnson Lau via bitcoin-dev wrote:
> This BIP defines a change in consensus rules regarding to block nVersion,
> and define a concept of generalized block header to implement a hardfork
> warning system for full nodes and light nodes.
> 
> For better formatting, visit github
> https://github.com/jl2012/bips/blob/hfwarning/bip-hfwarning.mediawiki
> 
> 
> 
> BIP: ?
> Title: Hardfork warning system
> Author: Johnson Lau <jl2...@xbt.hk>
> Status: Draft
> Type: Standard
> Created: 2016-12-01
> 
> Abstract
> 
> This BIP defines a change in consensus rules regarding to block nVersion,
> and define a concept of generalized block header to implement a hardfork
> warning system for full nodes and light nodes.
> 
> Motivation
> 
> Softfork and hardfork are the 2 majors categories of consensus rules
> change. Generally, softforks make some previously valid blocks invalid,
> while hardforks make some previously invalid blocks valid. Bitcoin has
> successfully introduced a number of new functions through softforks. A
> built-in warning system is also available in many implementations to warn
> users for the activation of any unknown softforks.
> 
> Some features, however, may not be easily introduced with a softfork.
> Examples include expanding maximum block resources limits, and changing
> the average block time interval. When such features are implemented with
> a hardfork, existing full node implementations would consider such blocks
> as invalid, and may even ban a peer for relaying such blocks. They are
> effectively blind to such hardfork rule changes, leaving users to
> unknowingly transact on a system with potentially different token value.
> On the other hand, light nodes may blindly follow a hardfork with unknown
> rule changes and lose the right to choose the previous system.
> 
> This BIP defines a change in consensus rules regarding to block nVersion,
> and define a concept of generalized block header to implement a hardfork
> warning system for full nodes and light nodes.
> 
> Definitions
> 
> Valid block
> A block that satisfies all the consensus rules being enforced by a bitcoin
> protocol implementation. An implementation may intentionally (e.g. a
> light node) or unintentionally (e.g. unaware of a softfork) not enforcing
> any part of the current netwrok rules.
> 
> Valid blockchain
> A blockchain constituting of only valid blocks.
> 
> Best valid blockchain
> The valid blockchain with highest total proof-of-work.
> 
> Valid blockchain fork
> A valid blockchain sharing a common ancestral block with the best valid
> blockchain, but with less total proof-of-work
> 
> Generalized block header
> Any serialized hexadecimal data with exactly 80 bytes (byte 0 to byte 79).
> The bytes 4 to 35 are the double-SHA256 hash of another generalized block
> header. The bytes 72 to 75 are nBits, the target of this generalized
> block header encoded in the same way as normal bitcoin block header. The
> 2 most significant bits of the byte 3 are the hardfork notification bits.
> The semantics of other data in a generalized block header is not defined
> in any general way. It should be noted that a normal bitcoin block header
> is a special case of generalized block header.
> 
> Generalized block header chain
> A chain of generalized block header. A header chain of valid blocks is a
> special case of a generalized block header chain.
> 
> 
> Specifications
> 
> 
> Block nVersion softfork
> 
> A softfork is deployed to restrict the valid value of block nVersion. Upon
> activation, any block with the second highest nVersion bit set becomes
> invalid (nVersion & 0x40000000)
> 
> This softfork will be deployed by "version bits" BIP9 with the name
> "hfbit" and using bit 2.
> 
> For Bitcoin mainnet, the BIP9 starttime will be midnight TBC UTC (Epoch
> timestamp TBC) and BIP9 timeout will be midnight TBC UTC (Epoch timestamp
> TBC).
> 
> For Bitcoin testnet, the BIP9 starttime will be midnight TBC UTC (Epoch
> timestamp TBC) and BIP9 timeout will be midnight TBC UTC (Epoch timestamp
> TBC).
> 
> Any bitcoin implementation (full nodes and light nodes) supporting this
> softfork should also implement a hardfork warning system described below.
> 
> 
> Validation of generalized block header
> 
> A bitcoin protocol implementation should consider a generalized block
> header as valid if it satisfies all of the following criteria:
> 
>       • It is a descendant of the header of a valid block in a valid 
blockchain
> (the best valid blockchain or a valid blockchain fork). • It satisfies
> the proof-of-work requirement: its double-SHA256 value MUST be smaller
> than its target (encoded as nBits). • Its target MUST NOT be greater than
> the target of its last ancestral valid block by more than 1024 times. An
> implementation may decide to use a different threshold (or dynamic
> threshold), depending on its tolerance against potential DoS attacks by
> generating many low difficulty headers. However, if the value is set too
> low, a hardfork with lower difficulty may not be detected.[1] In general,
> a bitcoin protocol implementation should keep an index of all known
> generalized block header chains, along with the valid blockchain(s).
> However, if a generalized block header chain is grown on top of a very
> old valid block, with total proof-of-work much lower than the current
> best valid bloackchain, it may be safely discarded.
> 
> 
> Hardfork warning system in full nodes
> 
> Hardfork with unknown rules
> If a generalized block header chain with non-trivial total proof-of-work
> is emerging, and is not considered as a valid blockchain, a hardfork with
> unknown rules may be happening.
> 
> A wallet implementation should issue a warning to its users and stop
> processing incoming and outgoing transactions, until further instructions
> are given. It should not attempt to conduct transactions on or otherwise
> interpreting any block data of the hardfork with unknown rules.
> 
> A mining implementation should issue a warning to its operator. Until
> further instructions are given, it may either stop mining, or ignore the
> hardfork with unknown rules. It should not attempt to confirm a
> generalized block header with unknown rules.
> 
> Setting of one or both hardfork notification bits is, as defined by BIP34
> and this BIP, a hardfork, and should be considered as an indication of a
> planned hardfork. If a hardfork with unknown rules is happening without
> any hardfork notification bits set, it is probably an accidental
> consensus failure, such as the March 2013 fork due to a block database
> bug (BIP50), and the July 2015 fork following the BIP66 activation.[2]
> 
> 
> Hardfork with multiple valid blockchains
> If a valid blockchain fork is emerging with non-trivial total
> proof-of-work, a consensus disagreement may be happening among different
> miners.
> 
> A wallet implementation should issue a warning to its users and stop
> processing incoming and outgoing transactions, until further instructions
> are given.
> 
> A mining implementation should issue a warning to its operator. Until
> further instructions are given, it may either stop mining, or mine on top
> of the best valid chain by its own standard.
> 
> Hardfork warning system in light nodes
> 
> Light node (usually wallet implementations) is any bitcoin protocol
> implementations that intentionally not fully enforcing the network rules.
> As an important part of the hardfork warning system, a light node should
> observe the hardfork notification bits in block header, along with any
> other rules it opts to validate. If any of the hardfork notification bits
> is set, it should issue a warning to its users and stop processing
> incoming and outgoing transactions, until further instructions are given.
> It should not attempt to conduct transactions on or otherwise
> interpreting any block data of the hardfork blockchain, even if it might
> be able to decode the block data.
> 
> 
> Applications
> 
> Hardfork notification bits
> There are 2 hardfork notification bits defined in this BIP. The higher bit
> has been forbidden since BIP34, and the lower bit is disabled by this
> BIP. For nodes supporting this BIP, the semantics of the 2 bits are the
> same: a hardfork is happening. For legacy node, however, setting the
> higher bit would make them fail to follow the hardforking chain. In a
> soft-hardfork design (described below), the lower notification bit should
> be used. The hardfork warning system is able to detect the following
> types of hardforks:
> 
> Soft-hardfork (with the lower hardfork notification bit)
> A soft-hardfork is a technique to implement a hardfork by pretending to
> create blocks with only a zero output value coinbase transaction, and
> commit the real transaction Merkle root in the coinbase scriptSig field.
> With the lower hardfork notification bit set, a node following this BIP
> will consider this as a hardfork and enter the safe mode, while a legacy
> node not following this BIP will be effectively broken due to seeing the
> continuously empty blockchain.
> 
> Redefining the nTime field
> As the warning system does not interpret the nTime field, redefining it
> through a hardfork would be detectable. For example, overflow may be
> allowed to overcome the year 2106 problem.
> 
> Redefining the Merkle root hash field and changing block content
> validation rules The 32-byte Merkle root hash could be redefined, for
> example, with a different hashing algorithm. Any block resources
> limitation and transaction validation rules may also be changed. All such
> hardforks would be detected by the warning system.
> 
> Changing average block interval or difficulty reset
> Since the warning system is not bound to a particular proof-of-work target
> adjustment schedule, a hardfork changing the average block interval or
> resetting the difficulty will be detectable.
> 
> Introducing secondary proof-of-work
> Introducing secondary proof-of-work (with non-SHA256 algorithm or fixing
> the block withholding attack against mining pools) may be detectable, as
> long as the generalized block header format is preserved.
> 
> Accidental hardfork
> An accidental hardfork may be detectable, if the generalized block headers
> in both forks are valid but no hardfork notification bit is set.
> 
> 
> Limitations
> 
> The only function of this system is to inform the users that a hardfork
> might be happening and prompt for further instructions. It does not
> guarantee that the hardfork will be successful and not end up with two
> permanent incompatible forks. This requires broad consensus of the whole
> community and is not solvable with technical means alone.
> 
> The following types of hardfork are not detectable with this warning
> system:
> 
>       • Changing of proof-of-work algorithm
>       • Changing the encoding of previous block header hash or nBits
>       • A coercive soft-hardfork without setting any hardfork notification bit
> 
> 
> Backward compatibility
> 
> The softfork described in the BIP would only affect miners. As the
> disabled nVersion bit is never used in the main network, it is unlikely
> that any miner would unintentionally find an invalid block due to the new
> rules.
> 
> BIP9 is disabled when any of the hardfork notification bits is set, which
> may interrupt any ongoing softfork support signalling process. Developers
> should pay attention to this when desinging a hardfork. For example, they
> may redefine the counting of signal, or move the signalling bitfield to a
> different location.
> 
> Legacy nodes would not be benefited from this softfork and warning system.
> However, no additional risks are introduced to legacy node either.
> 
> 
> Reference implementation
> 
> To be done
> 
> 
> Footnotes
> 
> 
>       • ^ Please note that a hardfork may have lower difficulty but higher
> total proof-of-work, such as by decreasing the average block interval. •
> ^ In the March 2013 fork, pre-0.8 full nodes would see that as a hardfork
> with unknown rules, while light nodes and 0.8.0 full nodes would see that
> as multiple valid blockchains. In the July 2015 fork, BIP66-complying
> full nodes would see that as a hardfok with unknown rules, while legacy
> full nodes would see that as multiple valid blockchains.
> 
> 
> Copyright
> 
> This document is placed in the public domain.
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev


-- 
Tom Zander
Blog: https://zander.github.io
Vlog: https://vimeo.com/channels/tomscryptochannel
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