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dip-0025.md 
  DIP: 0025
  Title: Compressed Block Headers
  Author(s): gabriel-bjg, Thephez, UdjinM6
  Special-Thanks: Will Clark
  Comments-Summary: No comments yet.
  Status: Proposed
  Type: Standard
  Created: 2022-06-06
  License: MIT License

Motivation

Block headers as exchanged by nodes over the p2p network are currently 81 bytes each.

For low bandwidth nodes who are doing a headers-only sync, reducing the size of the headers can provide a significant bandwidth saving. Also, nodes can support more header-only peers for IBD and protection against eclipse attacks if header bandwidth is reduced.

Background

Currently headers are sent over the p2p network as a vector of block_headers, which are composed of the following sized fields:

Field Size
Version 4 bytes
Previous block hash 32 bytes
Merkle root hash 32 bytes
Time 4 bytes
nBits 4 bytes
nonce 4 bytes
txn_count 1 byte
Total 81 bytes

Some fields can be removed completely, others can be compressed under certain conditions.

Prior work

This work is a derivation of the following:

Proposed specification

block_header2 data type

The following table illustrates the proposed block_header2 data type specification.

Field Size Compressed
Bitfield 1 byte 1 byte
Version 4 bytes 0 | 4 bytes
Previous block hash 32 bytes 0 | 32 bytes
Merkle root hash 32 bytes 32 bytes
Time 4 bytes 2 | 4 bytes
nBits 4 bytes 0 | 4 bytes
nonce 4 bytes 4 bytes
Total 81 bytes range: 39 - 81 bytes

This compression results in a maximum reduction from an 81 byte header to best-case 39 byte header. In bitcoin a continuous header sync from genesis (requiring a single full 81 byte header followed by only compressed block_header2) to height 629,474 was tested using this method and it resulted in a bandwidth reduction from 50.98MB down to 25.86MB, a saving of 49%.

Bitfield

To make parsing of header messages easier and further increase header compression, a single byte bitfield was suggested by gmaxwell. We propose the following amended bitfield meanings (bits re-ordered to match headers2 field order):

Bit Meaning + field size to read
0
1
2		 Version: same as the last distinct value 1st ... 7th (0 byte field) or a new 32-bit distinct value (4 byte field).
3 Previous block hash: Omitted (0 byte field) when bit is 0; Included (32 byte field) when bit is 1
4 Timestamp: Small offset (2 byte field) when bit is 0; Full timestamp (4 byte field) when bit is 1
5 nBits: Same as last header (0 byte field) when bit is 0; New value (4 byte field) when bit is 1
6+ Undefined

This bitfield adds 1 byte for every block in the chain.

Version

In most cases the Version field will be identical to one referenced in one of the previous 7 unique versions, as indicated by bits 0, 1, and 2 of the Bitfield.

In bitcoin testing to block 629,474, there were 616,137 blocks whose version was in the previous 7 distinct versions and only 13,338 blocks that were not.

Genesis to block Current (B) Compressed (B) Saving (%)
629,474 2,517,896 53,352 98

Previous block hash

The previous block hash will always be the X11 hash of previous_header so it is redundant given that you have the previous header in the chain.

Genesis to block Current (B) Compressed (B) Saving (%)
629,474 20,143,168 0 100

Time

The timestamp (in seconds) is consensus bound, based both on the time in the previous header: MAX_FUTURE_BLOCK_TIME = 2 * 60 * 60 = 7200, and being greater than the MedianTimePast of the previous 11 blocks. Therefore this can be safely represented as an offset from the previous headers' timestamp using a 2 byte signed short int.

Genesis to block Current (B) Compressed (B) Saving (%)
629,474 2,517,896 1,258,952 50

nBits

In Dash use of the Dark Gravity Wave (DGW) difficulty adjustment algorithm results in nBits generally changing in every block. Consequently, this field will usually contain the full value.

There is a period of time at the beginning of the chain where the bitcoin difficulty adjustment algorithm was used. During this time compression of the nBits field will occur.

txn_count

txn_count is included to make parsing of these messages compatible with parsing of block messages as explained on the Bitcoin StackExchange. Therefore this field and its associated byte can be removed for transmission of compact headers.

Genesis to block Current (B) Compressed (B) Saving (%)
629,474 629,474 0 100

Service Bit

A new service bit is required so that the nodes can advertise their ability to supply compact headers. Dash would use bit 11 and designate it NODE_HEADERS_COMPRESSED.

P2P Messages

Three new messages would be used by nodes that enable compact block header support, two query messages: getheaders2 and sendheaders2 and one response: headers2.

getheaders2 -- Requesting compact headers

The new p2p message required to request compact block headers would require the same fields as the current getheaders message:

Field Size Description Data type Comments
4 version uint32_t The protocol version
1+ hash count var_int Number of block locator hash entries
32+ block locator hashes char[32] Block locator object; newest back to genesis block (dense to start, but then sparse)
32 hash_stop char[32] Hash of the last desired block header; set to zero to get as many blocks as possible (2000)

sendheaders2 -- Request compact header announcements

Since BIP-130, nodes have been able to request to receive new headers directly in headers messages, rather than via an inv of the new block hash and subsequent getheader request and headers response (followed by a final getdata to get the tip block itself, if desired). This is requested by transmitting an empty sendheaders message after the version handshake is complete.]

Upon receipt of this message, the node is permitted, but not required, to preemptively announce new headers with the headers2 message (instead of inv). Preemptive header announcement has been supported by the protocol version ≥ 70206 | Dash Core version ≥ 0.12.1.

For the motivational use-case it makes sense to also update this mechanism to support sending header updates using compact headers using a new message.

headers2 -- Receiving compact headers

A headers2 message is returned in response to getheaders2 or at new header announcement following a sendheaders2 request. It contains both length and headers fields. The headers field contains a variable length vector of block_header2:

Field Size Description Data type Comments
1+ length var_int Length of headers
39-81x? headers block_header2[] Compressed block headers in block_header2 data type format

Implementation

  • The first header in each block_header2[] vector MUST contain the full nBits, timestamp, version and prev_block_hash fields, along with a correctly populated bitfield byte.
  • Subsequent headers in a contiguous vector SHOULD follow the compressed block_header2 data type format.
  • If a peer sends both a sendheaders and a sendheaders2 message, they SHOULD be sent block announcements using compressed headers regardless of which message was sent first.

Copyright

Copyright (c) 2022 Dash Core Group, Inc. Licensed under the MIT License

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