This work is a derivation of these mailing list posts:
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bitcoin-dev: "Compressed" headers stream - 2017 (with resurrection here)
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.
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.
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. With 629,474 blocks in the current blockchain, a continuous header sync from genesis (requiring a single full 81 byte header followed by only compressed block_header2) has been tested to have its required bandwidth reduced from 50.98MB down to 25.86MB, a saving of 49%.
To make parsing of header messages easier and further increase header compression, a single byte bitfield was suggested by gmaxwell [1]. We propose the following amended bitfield meanings (bits re-ordered to match headers2 field order):
| Bit | Meaning + field size to read |
|---|---|
0 |
version: same as the last distinct value 1st … 7th (0 byte field) or a new 32bit distinct value (4 byte field). |
3 |
prev_block_hash: is omitted (0 byte field) or included (32 byte field) |
4 |
timestamp: as small offset (2 byte field) or full (4 byte field). |
5 |
nbits: same as last header (0 byte field) or new (4 byte field). |
6 |
possibly to signal "more headers follow" to make the encoding self-delimiting. |
7 |
currently undefined |
This bitfield adds 1 byte for every block in the chain, for a current total increase of 629,474B.
In most cases the Version field will be identical to one referenced in one of the previous 7 unique versions, as indicated by bits 0,1,2 of the Bitfield.
To block 629,474 there were 616,137 blocks whose version was in the previous 7 distinct versions, and only 13,338 blocks whose version was not, this includes any version bit manipulation done via overt ASIC boost.
| Genesis to block | Current (B) | Compressed (B) | Saving (%) |
|---|---|---|---|
629,474 |
2,517,896 |
53,352 |
98 |
The previous block hash will always be the
SHA256(SHA256(<previous_header>)) so is redundant, presuming you have the previous header in the chain.
| Genesis to block | Current (B) | Compressed (B) | Saving (%) |
|---|---|---|---|
629,474 |
20,143,168 |
0 |
100 |
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 currently changes once every 2016 blocks. It could be entirely calculated by the client from the timestamps of the previous 2015 blocks [2].
To simplify 'light' client implementations which would otherwise require consensus-valid calculation of the adjustments, we propose to transmit this according to the Bitfield specification above.
To block 629,474 there have been 298 nBits adjustments (vs an expected 311 — there was none before block 32,256).
| Genesis to block | Current (B) | Compressed (B) | Saving (%) |
|---|---|---|---|
629,474 |
2,517,896 |
1,196 |
99.6 |
txn_count is included to make parsing of these messages compatible with parsing of block messages [3]. 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 |
A new service bit would be required so that the nodes can advertise their ability to supply compact headers.
Three new messages would be used by nodes that enable compact block header support, two query messages: getheaders2 and sendheaders2 and one response: headers2.
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) |
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 is supported by the protocol version ≥ 70012 | Bitcoin Core version ≥ 0.12.0.
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.
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 |
39-81x? |
headers |
block_header2[] |
Compressed block headers in block_header2 data type format |
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The first header in the first
block_header2[]vector to a newly-connected client MUST contain the full nBits`,timestamp,versionandprev_block_hashfields, along with a correctly populatedbitfieldbyte. -
Subsequent headers in a contiguous vector SHOULD follow the compressed block_header2 data type format.
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Subsequent compressed headers supplied to an already-connected client (requesting compressed headers), SHOULD follow the compressed block_header2 data type format.