Subject: Re: [bitcoin-dev] Optimized Header Sync

From: Jim Posen <[email-masked]>
Date: Thu, 29 Mar 2018 17:50:30 -0700

Thanks for giving it a read and for sparking the discussion with your
observation about the 40% savings from dropping prev_hash!

> Thought this wasn't effective in case overt asic boost get widely adopted,
> but then I understood that at the moment only two bits of version get
> scrambled by that technique so this looks fine, maybe add a comment about
> this so the reader doesn't get the same initial doubt I got.
>

I still need to compute for historical blocks how many could have an
omitted version. Will post back with that when I get results. If overt ASIC
Boost made this less effective, that would be unfortunate, but so be it.

> My feeling is that encoding of the headers and checkpoints/parallel
> download are separate subjects for two BIPS.
> About the checkpoints I don't grasp why they are useful since an attacker
> could lie about them but maybe I am missing something...
>

Yeah, I guess the background wasn't explained in the BIP itself. After your
original post on the mailing list, there were suggestions that instead of
modifying the format of existing messages, it would be better do create a
new headers message. And as long as we're designing a new headers message,
we should change the semantics to allow parallel download. But if you want
to download from peers in parallel, you need to get a summary of the blocks
that they have. Hence the checkpoints message. So that is why both of these
messages are in the same BIP -- only together can they perform an efficient
sync.

Regarding the reliability of the checkpoints, I think it's strictly better
than what we have now. Let's say a node is connected to 6 honest peers and
2 malicious peers. Even if the node does not know which ones are good or
bad until it validates the headers, it sees that 6 of the peers are on the
same chain, and can download those headers in parallel from 6 different
sources. So that's already a win.

Taken a step further though, I'm really interested in treating the
checkpoints as commitments to chain work and using random sampling to
detect lying peers before downloading all of their headers. So imagine you
are connected to two peers, one good one bad, where the good one claims a
chain with X total work and the bad one claims a chain with Y total work.
To determine quickly which is correct, you can randomly sample ranges of
headers and check the proofs of work to see whether it matches what the
peer claimed. So basically you pick a checkpoint at random (weighted by the
work delta) which commits to a total amount of work from the last
checkpoint, then request all headers in between. If the peer responds with
headers with the correct start hash, end hash, and start height (from the
coinbase tx of the first header), then you can be somewhat more confident
their total PoW matches the claimed amount.

How many times do you need to sample? I don't know yet, but I've heard
Benedikt Bunz is exploring this question with his research on FlyClients
[1], which was an inspiration for this.

> Bitfield allows great savings, however the encoding depends on the headers
> height a client ask for, this cause a little computational burden on the
> node and the undesirable side effect of difficult caching. Variable length
> encoding cause caching difficulties too...
> A simpler approach could be to encode the headers in groups of 2016
> headers (the difficulty period) where the first header is complete and the
> others 2015 are missing the previous hash and the difficulty, this achieve
> comparable savings ~45%, allows better caching and has fixed length
> encoding. This could be useful for the node by caching headers on a single
> file on disk and simply stream out the relative range when requested or to
> serve the same encoded headers format in other context like http,
> leveraging http caching infrastructure.
>

I don't see too much of a problem with caching. Most node implementations I
know of keep all headers in memory anyway, often in contiguous segments of
RAM for historical headers, so it should be fairly inexpensive to serve
queries. Beyond that, the response for a particular query (start_height,
end_hash, encoding version) can be cached, so if some service wants to
precompute max size responses for all start_height multiples of 1,000, they
could cache those.

-Jim

[1] https://www.youtube.com/watch?time_continue=8400&v=BPNs9EVxWrA

> 2018-03-28 1:31 GMT+02:00 Jim Posen via bitcoin-dev <bitcoin-dev@lists.
> linuxfoundation.org>:
>
>> Based on some ideas that were thrown around in this thread (
>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/
>> 2017-December/015385.html), I have been working on a P2P extension that
>> will allow faster header sync mechanisms. The one-sentence summary is that
>> by encoding headers more efficiently (eg. omitting prev_hash) and
>> downloading evenly spaced checkpoints throughout history (say every
>> 1,000th) from all peers first, we could speed up header sync, which would
>> be a huge improvement for light clients. Here is a draft of the BIP:
>> https://github.com/jimpo/bips/blob/headers-sync/headersv2.mediawiki. The
>> full text is below as well.
>>
>> I'd love to hear any feedback people have.
>>
>> ----------------------------------------------------------
>>
>> == Abstract ==
>>
>> This BIP describes a P2P network extension enabling faster, more reliable methods for syncing the block header chain. New P2P messages are proposed as more efficient replacements for <code>getheaders</code> and <code>headers</code> during initial block download. The proposed header download protocol reduces bandwidth usage by ~40%-50% and supports downloading headers ranges from multiple peers in parallel, which is not possible with the current mechanism. This also enables sync strategies with better resistance to denial-of-service attacks.
>>
>> == Motivation ==
>>
>> Since 2015, optimized Bitcoin clients fetch all block headers before blocks themselves in order to avoid downloading ones that are not part of the most work chain. The protocol currently in use for fetching headers leaves room for further optimization, specifically by compressing header data and downloading more headers simulaneously<ref>https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-December/015385.html</ref>. Any savings here should have a large impact given that both full nodes and light clients must sync the header chain as a first step, and that the time to validate and index the headers is negligible compared to the time spent downloading them from the network. Furthermore, some current implementations of headers syncing rely on preconfigured checkpoints to discourage attackers attempting to fill up a victim's disk space with low-work headers. The proposed messages enable sync strategies that are resilient against these types of attacks. The P2P messages are designed to be flexible, supporting multiple header sync strategies and leaving room for future innovations, while also compact.
>>
>> == Definitions ==
>>
>> ''double-SHA256'' is a hash algorithm defined by two invocations of SHA-256: <code>double-SHA256(x) = SHA256(SHA256(x))</code>.
>>
>> == Specification ==
>>
>> The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
>>
>> === New Structures ===
>>
>> ==== Compressed Headers ====
>>
>> Bitcoin headers are serialized by default in 80 bytes as follows:
>>
>> {| class="wikitable"
>> ! Field Name
>> ! Data Type
>> ! Byte Size
>> ! Description
>> |-
>> | version
>> | int32_t
>> | 4
>> | Block version information
>> |-
>> | prev_block
>> | uint256
>> | 32
>> | The hash of the previous block
>> |-
>> | merkle_root
>> | uint256
>> | 32
>> | The root hash of the transaction Merkle tree
>> |-
>> | timestamp
>> | uint32_t
>> | 4
>> | A Unix timestamp of the block creation time, as reported by the miner
>> |-
>> | bits
>> | uint32_t
>> | 4
>> | The calculated difficulty target for this block
>> |-
>> | nonce
>> | uint32_t
>> | 4
>> | A nonce that is set such that the header's hash matches the difficulty target
>> |}
>>
>> When deserializing a correctly-formed sequence of block headers encoded in this way, it can be noted that:
>>
>> * The prev_block field should always match the double-SHA256 hash of the previous header, making it redundant
>> * According to Bitcoin consensus rules, the bits field only changes every 2016 blocks
>> * The version often matches that of a recent ancestor block
>> * The timestamp is often a small delta from the preceding header's timestamp
>>
>> To take advantage of these possible savings, this document defines a variable-sized ''compressed encoding'' of block headers that occur in a range. Note that no savings are possible when serializing a single header; it should only be used for vectors of sequential headers. The full headers are reconstructed using data from previous headers in the range. The serialization begins with an ''encoding indicator'', which is a bitfield specifying how each field is serialized. The bits of the indicator have the following semantics:
>>
>> {| class="wikitable"
>> ! Bit Index
>> ! Reconstruction
>> ! Description
>> |-
>> | 0
>> | <code>prev_block[i] = DSHA256(header[i-1])</code>
>> | The prev_block field is ommitted and assigned to the double-SHA256 hash of the previous uncompressed header.
>> |-
>> | 1
>> | <code>nbits[i] = nbits[i-1]</code>
>> | The nbits field is omitted and matches that of the previous header.
>> |-
>> | 2
>> | <code>timestamp[i] = timestamp[i-1] + value</code>
>> | The timestamp is replaced by a 2-byte signed short int, representing an offset from the previous block's timestamp
>> |-
>> | 3
>> |
>> | Interpreted along with bits 4 & 5.
>> |-
>> | 4
>> |
>> | Interpreted along with bits 3 & 5.
>> |-
>> | 5
>> | <code>version[i] = version[i - ((bit[3] << 2) + (bit[4] << 1) + bit[5])]</code>
>> | Bits 3, 4, and 5 are first interpreted as a 3-bit offset, with bit index 3 as the most significant and bit index 5 as the least significant. If the offset is non-zero, the version field is omitted and assigned to the version of the block at the offset number of blocks prior.
>> |-
>> | 6
>> |
>> | Reserved.
>> |-
>> | 7
>> |
>> | Reserved. May be used in a future encoding version to signal another indicator byte.
>> |}
>>
>> The compressed header format is versioned by a 256-bit unsigned integer. This document defines version 0.
>>
>> ==== VarInt ====
>>
>> ''VarInt'' is a variable-length unsigned integer encoding that supports a greater range of numbers than the standard ''CompactSize''. This encoding was introduced at the database layer in Bitcoin Core<ref>https://github.com/bitcoin/bitcoin/commit/4d6144f97faf9d2a6c89f41d7d2360f21f0b71e2</ref> in 2012, but is new to the Bitcoin P2P layer.
>>
>> This definition is per the code comments in Bitcoin Core written by Pieter Wuille:
>>
>> <pre>
>> Variable-length integers: bytes are a MSB base-128 encoding of the number.
>> The high bit in each byte signifies whether another digit follows. To make
>> the encoding is one-to-one, one is subtracted from all but the last digit.
>> Thus, the byte sequence a[] with length len, where all but the last byte
>> has bit 128 set, encodes the number:
>>
>>   (a[len-1] & 0x7F) + sum(i=1..len-1, 128^i*((a[len-i-1] & 0x7F)+1))
>>
>> Properties:
>> * Very small (0-127: 1 byte, 128-16511: 2 bytes, 16512-2113663: 3 bytes)
>> * Every integer has exactly one encoding
>> * Encoding does not depend on size of original integer type
>> * No redundancy: every (infinite) byte sequence corresponds to a list
>>   of encoded integers.
>>
>> 0:         [0x00]  256:        [0x81 0x00]
>> 1:         [0x01]  16383:      [0xFE 0x7F]
>> 127:       [0x7F]  16384:      [0xFF 0x00]
>> 128:  [0x80 0x00]  16511: [0x80 0xFF 0x7F]
>> 255:  [0x80 0x7F]  65535: [0x82 0xFD 0x7F]
>> 2^32:           [0x8E 0xFE 0xFE 0xFF 0x00]
>> </pre>
>>
>> ==== Checkpoints ====
>>
>> A ''checkpoint'' is defined for a block as a tuple of its hash and the chain work:
>>
>> {| class="wikitable"
>> ! Field Name
>> ! Data Type
>> ! Byte Size
>> ! Description
>> |-
>> | block_hash
>> | uint256
>> | 32
>> | The hash of the block
>> |-
>> | chain_work
>> | VarInt
>> | Variable(1-20)
>> | A delta between the total work in the chain at the checkpoint block and a previous checkpoint, determined by context
>> |}
>>
>> === Service Bit ===
>>
>> This BIP allocates a new service bit:
>>
>> {| class="wikitable"
>> |-
>> | NODE_HEADERS_V2
>> | <code>1 << ?</code>
>> | If enabled, the node MUST respond to <code>getcheckpts</code> and <code>getheaders2</code> queries
>> |}
>>
>> === New Messages ===
>>
>> ==== getcheckpts ====
>> <code>getcheckpts</code> is used to request block headers at a specified distance from each other which serve as checkpoints during parallel header download. The message contains the following fields:
>>
>> {| class="wikitable"
>> ! Field Name
>> ! Data Type
>> ! Byte Size
>> ! Description
>> |-
>> | block_locator
>> | uint256[]
>> | Variable
>> | A vector of block hashes in descending order by height used to identify the header chain of the requesting node
>> |-
>> | interval
>> | uint32_t
>> | 4
>> | The distance in block height between requested block hashes
>> |}
>>
>> # Nodes SHOULD NOT send <code>getcheckpts</code> unless the peer has set the <code>NODE_HEADERS_V2</code> service bit
>> # The hashes in <code>block_locator</code> MUST be in descending order by block height
>> # The block locator SHOULD be generated as it is in <code>getheaders</code> requests
>> # The receiving node MUST respond to valid requests with a <code>checkpts</code> response where the interval is the same as in the request and the first checkpoint hash matches the first common block hash in the block locator
>>
>> ==== checkpts ====
>> <code>checkpts</code> is sent in response to <code>getcheckpts</code>, listing block hashes at the specified interval. The message contains the following fields:
>>
>> {| class="wikitable"
>> ! Field Name
>> ! Data Type
>> ! Byte Size
>> ! Description
>> |-
>> | start_height
>> | uint32_t
>> | 4
>> | The height of the first block in the active chain matching the request's block locator
>> |-
>> | end_height
>> | uint32_t
>> | 4
>> | The height of the last block in the active chain
>> |-
>> | start_checkpoint
>> | Checkpoint
>> | 48
>> | The checkpoint structure for the block in the active chain at height start_height
>> |-
>> | end_checkpoint
>> | Checkpoint
>> | 48
>> | The checkpoint structure for the block in the active chain at height end_height
>> |-
>> | interval
>> | uint32_t
>> | 4
>> | The distance in block height between checkpoints
>> |-
>> | checkpoints_length
>> | CompactSize
>> | Variable(1-5)
>> | The number of checkpoints to follow
>> |-
>> | checkpoints
>> | Checkpoint[]
>> | checkoints_length * Variable(33-52)
>> | The checkpoints as specified below
>> |}
>>
>> # The interval SHOULD match the field in the <code>getcheckpts</code> request
>> # The start_checkpoint SHOULD correspond to the first block hash in the locator from the <code>getcheckpts</code> request that is part of the active chain
>> # The end_checkpoint SHOULD correspond to the tip of the node's active chain
>> # The start_height MOST be set to the block height of the start_checkpoint
>> # The end_height MOST be set to the block height of the end_checkpoint
>> # If the interval is zero, the checkpoints vector MUST be empty
>> # If the interval is non-zero, checkpoints MUST correspond to blocks on the active chain between the start_checkpoint and the end_checkpoint (exclusive), where the difference in block height between each entry and the previous one is equal to the interval
>> # The checkpoints_length MUST be less than or equal to 2,000
>> # The node SHOULD include as many checkpoints on its active chain as are available, up to the limit of 2,000
>> # The chain_work field in the first checkpoint MUST be the total work in the chain ending at that block
>> # The chain_work field in each subsequent checkpoint MUST be the difference in chain work between that block and the previous checkpoint
>> # The chain_work field in each checkpoint MUST be a properly-encoded VarInt, not exceeding 20 bytes
>>
>> ==== getheaders2 ====
>> <code>getheaders2</code> is used to request compressed headers for a range of blocks. The message contains the following fields:
>>
>> {| class="wikitable"
>> ! Field Name
>> ! Data Type
>> ! Byte Size
>> ! Description
>> |-
>> | max_version
>> | uint8_t
>> | 1
>> | The maximum supported encoding version of the headers
>> |-
>> | flags
>> | uint8_t
>> | 1
>> | A bitfield of message encoding flags
>> |-
>> | start_height
>> | uint32_t
>> | 4
>> | The height of the first block header in the requested range
>> |-
>> | end_hash
>> | uint256
>> | 32
>> | The hash of the last block header in the requested range
>> |}
>>
>> # Nodes SHOULD NOT send <code>getheaders2</code> unless the peer has set the <code>NODE_HEADERS_V2</code> service bit
>> # The height of the block with hash end_hash MUST be greater than or equal to start_height, and the difference MUST be strictly less than 3,000
>> # The end_hash SHOULD match one in a previously received <code>checkpts</code> message, otherwise the receiving node MAY disconnect
>> # The 0th bit (least significant order) of the flags field MAY be set to request the coinbase transaction and merkle branch for the block at height start_height
>>
>> ==== headers2 ====
>> <code>headers2</code> is sent in response to <code>getheaders2</code>, listing the compressed headers in the requested range. The message contains the following fields:
>>
>> {| class="wikitable"
>> ! Field Name
>> ! Data Type
>> ! Byte Size
>> ! Description
>> |-
>> | version
>> | uint8_t
>> | 1
>> | The encoding version of the headers
>> |-
>> | flags
>> | uint8_t
>> | 1
>> | A bitfield of message encoding flags
>> |-
>> | start_height
>> | uint32_t
>> | 4
>> | The height of the first block header returned
>> |-
>> | headers_length
>> | CompactSize
>> | 1-3
>> | The number of block headers to follow
>> |-
>> | headers
>> | CompressedHeader[]
>> | Variable
>> | The compressed block headers
>> |-
>> | start_block_coinbase_tx
>> | CTransaction
>> | Variable
>> | The coinbase transaction in the block at start_height
>> |-
>> | start_block_coinbase_branch
>> | uint256[]
>> | Variable
>> | A merkle branch linking the coinbase transaction in the block at start_height to its header
>> |}
>>
>> # The version MUST be less than or equal to the max_version field of the <code>getheaders2</code> request
>> # Any bits set in the flags field of the <code>getheaders2</code> request MAY be set in the response field
>> # Any bits not set in the flags field of the <code>getheaders2</code> request MUST NOT be set in the response field
>> # The first header MUST be encoded with a 0-byte indicator (ie. the header is uncompressed)
>> # start_height MUST be set to the block height of the first header
>> # The hash of the last block SHOULD equal the end_hash of the <code>getheaders2</code> request, ''even if the block is no longer part of the active chain''
>> # The length of the headers vector MUST be less than or equal to 3,000
>> # The headers MUST be sequential in order of height, with each header a successor of the previous one
>> # Each header SHOULD be optimally compressed
>> # The start_block_coinbase_tx should be the serialized coinbase transaction in the block corresponding to the first header
>> # The start_block_coinbase_branch should be a vector of right-hand-side hashes in the merkle branch linking the coinbase transaction to the first header, in order from bottom of the tree to top
>> # If the 0th bit (least significant order) of the flags field is unset, the start_block_coinbase_tx and start_block_coinbase_branch fields MUST be omitted
>>
>> === Sync Strategies ===
>>
>> The general header sync protocol for clients now is to first request checkpoints from all peers with <code>getcheckpts</code>, then decide which peers to fetch ranges of headers from and download them with <code>getheaders2</code>.
>>
>> ==== Forward Sequential Syncing ====
>>
>> Similar to the current sync protocol, a client may choose one peer to download headers from, then fetch them in forward sequential order. Once this peer is out of headers, the client performs the same routine with any peers offering more headers.
>>
>> With this strategy, the client is able to fully validate the block headers in order and abort if the peer serves an invalid one. On the other hand, the peer may be able to serve a longer, lower-work chain than the global active chain, wasting the client's time, memory, and storage space.
>>
>> ==== Parallel Header Download ====
>>
>> In order to increase the throughput of header downloads, a node may download multiple header ranges in parallel from all peers serving the same checkpoints, then validate them in sequential order.
>>
>> ==== Random Sampling Proof-of-Work  ====
>>
>> Similar the FlyClient<ref>https://www.youtube.com/watch?time_continue=8400&v=BPNs9EVxWrA</ref> header download protocol, clients can select the peer claiming the greatest total work chain and use random sampling to efficiently determine if the peer is likely to be reporting its chain work honestly.
>>
>> The client treats the checkpoint message as a commitment to chain work of intermediate ranges of headers, the client then randomly samples ranges of headers weighted by total work to determine whether the total chain work is valid before downloading all headers. To defend against malicious peers attempting to reuse earlier headers later in the chain to fake greater total work, the client should check the block height in the coinbase transaction for all headers after the BIP 34 activation height. If the peer is found to be dishonest, they can be banned before the client downloads too many headers, otherwise the client chooses this as the primary sync peer for forward sequential sync or parallel download.
>>
>> == Rationale ==
>>
>> * '''Why include the coinbase transaction in the headers messages?''' The primary reason is that after BIP 34<ref>https://github.com/bitcoin/bips/blob/master/bip-0034.mediawiki</ref> activation at block height 227,835, coinbase transactions constitute cryptographic commitments to a block's height in the chain, which mitigates certain attacks during header sync. Furthermore, the <code>getheaders2</code> message can be used as a simple way of requesting a coinbase transaction for a single header, which may be independently useful.
>>
>> * '''Why not omit nBits entirely?''' The compression is designed to permit full decompression of all headers in a <code>headers2</code> message ''without'' requiring any other chain context. This is desirable so that proofs of work may be validated for arbitrary header ranges. While nBits can be computed knowing previous headers, this requires block headers that may not be sent in the same message.
>>
>> == Compatibility ==
>>
>> This is backwards compatible, as it defines new P2P messages which are available if a service bit is signaled. There are no changes to consensus rules.
>>
>> == Acknowledgements ==
>>
>> Thanks to Gregory Maxwell for suggestions on the compressed header encoding and the DOS-resistant sync strategies. Thanks to Suhas Daftuar for helpful discussions.
>>
>> Credit for the VarInt encoding goes to Pieter Wuille.
>>
>>
>> _______________________________________________
>> bitcoin-dev mailing list
>> [email-masked]
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
>>
>
>
> --
> Riccardo Casatta - @RCasatta <https://twitter.com/RCasatta>
>