Peter Tschipper [ARCHIVE] on Nostr: 📅 Original date posted:2015-11-30 📝 Original message:@gmaxwell Bip Editor, and ...

📅 Original date posted:2015-11-30
📝 Original message:@gmaxwell Bip Editor, and the Bitcoin Dev Community,

After several weeks of experimenting and testing with various
compression libraries I think there is enough evidence to show that
compressing blocks and transactions is not only beneficial in reducing
network bandwidth but is also provides a small performance boost when
there is latency on the network.

The following is a BIP Draft document for your review.
(The alignment of the columns in the tables doesn't come out looking
right in this email but if you cut and paste into a text document they
are just fine)


<pre>
BIP: ?
Title: Datastream compression of Blocks and Tx's
Author: Peter Tschipper <peter.tschipper at gmail.com>
Status: Draft
Type: Standards Track
Created: 2015-11-30
</pre>

==Abstract==

To compress blocks and transactions, and to concatenate them together
when possible, before sending.

==Motivation==

Bandwidth is an issue for users that run nodes in regions where
bandwidth is expensive and subject to caps, in addition network latency
in some regions can also be quite high. By compressing data we can
reduce daily bandwidth used in a significant way while at the same time
speed up the transmission of data throughout the network. This should
encourage users to keep their nodes running longer and allow for more
peer connections with less need for bandwidth throttling and in
addition, may also encourage users in areas of marginal internet
connectivity to run nodes where in the past they would not have been
able to.

==Specification==

Advertise compression using a service bit. Both peers must have
compression turned on in order for data to be compressed, sent, and
decompressed.

Blocks will be sent compressed.

Transactions will be sent compressed with the exception of those less
than 500 bytes.

Blocks will be concatenated when possible.

Transactions will be concatenated when possible or when a
MSG_FILTERED_BLOCK is requested.

Compression levels to be specified in "bitcoin.conf".

Compression and decompression can be completely turned off.

Although unlikely, if compression should fail then data will be sent
uncompressed.

The code for compressing and decompressing will be located in class
CDataStream.

Compression library LZO1x will be used.

==Rationale==

By using a service bit, compression and decompression can be turned
on/off completely at both ends with a simple configuration setting. It
is important to be able to easily turn off compression/decompression as
a fall back mechanism. Using a service bit also makes the code fully
compatible with any node that does not currently support compression. A
node that do not present the correct service bit will simply receive
data in standard uncompressed format.

All blocks will be compressed. Even small blocks have been found to
benefit from compression.

Multiple block requests that are in queue will be concatenated together
when possible to increase compressibility of smaller blocks.
Concatenation will happen only if there are multiple block requests from
the same remote peer. For example, if peer1 is requesting two blocks
and they are both in queue then those two blocks will be concatenated.
However, if peer1 is requesting 1 block and peer2 also one block, and
they are both in queue, then each peer is sent only its block and no
concatenation will occur. Up to 16 blocks (the max blocks in flight) can
be concatenated but not exceeding the MAX_PROTOCOL_MESSAGE_LENGTH.
Concatenated blocks compress better and further reduce bandwidth.

Transactions below 500 bytes do not compress well and will be sent
uncompressed unless they can be concatenated (see Table 3).

Multiple transaction requests that are in queue will be concatenated
when possible. This further reduces bandwidth needs and speeds the
transfer of large requests for many transactions, such as with
MSG_FILTERED_BLOCK requests, or when the system gets busy and is flooded
with transactions. Concatenation happens in the same way as for blocks,
described above.

By allowing for differing compression levels which can be specified in
the bitcoin.conf file, a node operator can tailor their compression to a
level suitable for their system.

Although unlikely, if compression fails for any reason then blocks and
transactions will be sent uncompressed. Therefore, even with
compression turned on, a node will be able to handle both compressed and
uncompressed data from another peer.

By Abstracting the compression/decompression code into class
"CDataStream", compression can be easily applied to any datastream.

The compression library LZO1x-1 does not compress to the extent that
Zlib does but it is clearly the better performer (particularly as file
sizes get larger), while at the same time providing very good
compression (see Tables 1 and 2). Furthermore, LZO1x-999 can provide
and almost Zlib like compression for those who wish to have more
compression, although at a cost.

==Test Results==

With the LZO library, current test results show up to a 20% compression
using LZO1x-1 and up to 27% when using LZO1x-999. In addition there is
a marked performance improvement when there is latency on the network.
>From the test results, with a latency of 60ms there is an almost 30%
improvement in performance when comparing LZO1x-1 compressed blocks with
uncompressed blocks (see Table 5).

The following table shows the percentage that blocks were compressed,
using two different Zlib and LZO1x compression level settings.

TABLE 1:
range = data size range
range Zlib-1 Zlib-6 LZO1x-1 LZO1x-999
----------- ------ ------ ------- --------
0-250 12.44 12.86 10.79 14.34
250-500 19.33 12.97 10.34 11.11
600-700 16.72 n/a 12.91 17.25
700-800 6.37 7.65 4.83 8.07
900-1KB 6.54 6.95 5.64 7.9
1KB-10KB 25.08 25.65 21.21 22.65
10KB-100KB 19.77 21.57 4.37 19.02
100KB-200KB 21.49 23.56 15.37 21.55
200KB-300KB 23.66 24.18 16.91 22.76
300KB-400KB 23.4 23.7 16.5 21.38
400KB-500KB 24.6 24.85 17.56 22.43
500KB-600KB 25.51 26.55 18.51 23.4
600KB-700KB 27.25 28.41 19.91 25.46
700KB-800KB 27.58 29.18 20.26 27.17
800KB-900KB 27 29.11 20 27.4
900KB-1MB 28.19 29.38 21.15 26.43
1MB -2MB 27.41 29.46 21.33 27.73

The following table shows the time in seconds that a block of data takes
to compress using different compression levels. One can clearly see
that LZO1x-1 is the fastest and is not as affected when data sizes get
larger.

TABLE 2:
range = data size range
range Zlib-1 Zlib-6 LZO1x-1 LZO1x-999
----------- ------ ------ ------- ---------
0-250 0.001 0 0 0
250-500 0 0 0 0.001
500-1KB 0 0 0 0.001
1KB-10KB 0.001 0.001 0 0.002
10KB-100KB 0.004 0.006 0.001 0.017
100KB-200KB 0.012 0.017 0.002 0.054
200KB-300KB 0.018 0.024 0.003 0.087
300KB-400KB 0.022 0.03 0.003 0.121
400KB-500KB 0.027 0.037 0.004 0.151
500KB-600KB 0.031 0.044 0.004 0.184
600KB-700KB 0.035 0.051 0.006 0.211
700KB-800KB 0.039 0.057 0.006 0.243
800KB-900KB 0.045 0.064 0.006 0.27
900KB-1MB 0.049 0.072 0.006 0.307

TABLE 3:
Compression of Transactions (without concatenation)
range = block size range
ubytes = average size of uncompressed transactions
cbytes = average size of compressed transactions
cmp% = the percentage amount that the transaction was compressed
datapoints = number of datapoints taken

range ubytes cbytes cmp% datapoints
---------- ------ ------ ------ ----------
0-250 220 227 -3.16 23780
250-500 356 354 0.68 20882
500-600 534 505 5.29 2772
600-700 653 608 6.95 1853
700-800 757 649 14.22 578
800-900 822 758 7.77 661
900-1KB 954 862 9.69 906
1KB-10KB 2698 2222 17.64 3370
10KB-100KB 15463 12092 21.80 15429

The above table shows that transactions don't compress well below 500
bytes but do very well beyond 1KB where there are a great deal of those
large spam type transactions. However, most transactions happen to be
in the < 500 byte range. So the next step was to appy concatenation for
those smaller transactions. Doing that yielded some very good
compression results. Some examples as follows:

The best one that was seen was when 175 transactions were concatenated
before being compressed. That yielded a 20% compression ratio, but that
doesn't take into account the savings from the unneeded 174 message
headers (24 bytes each) as well as 174 TCP ACKs of 52 bytes each which
yields and additional 76*174 = 13224 byte savings, making for an overall
bandwidth savings of 32%:

2015-11-18 01:09:09.002061 compressed data from 79890 to 67426
txcount:175

However, that was an extreme example. Most transaction aggregates were
in the 2 to 10 transaction range. Such as the following:

2015-11-17 21:08:28.469313 compressed data from 3199 to 2876 txcount:10

But even here the savings of 10% was far better than the "nothing" we
would get without concatenation, but add to that the 76 byte * 9
transaction savings and we have a total 20% savings in bandwidth for
transactions that otherwise would not be compressible. Therefore the
concatenation of small transactions can also save bandwidth and speed up
the transmission of those transactions through the network while keeping
network and message queue chatter to a minimum.

==Choice of Compression library==

LZO was chosen over Zlib. LZO is the fastest most scalable option when
used at the lowest compression setting which will be a performance boost
for users that prefer performance over bandwidth savings. And at the
higher end, LZO provides good compression (although at a higher cost)
which approaches that of Zlib.

Other compression libraries investigated were Snappy, LZOf, fastZlib and
LZ4 however none of these were found to be suitable, either because they
were not portable, lacked the flexibility to set compression levels or
did not provide a useful compression ratio.

The following two tables show results in seconds for syncing the first
200,000 blocks. Tests were run on a high-speed wireless LAN with very
little latency, and also run with a 60ms latency which was induced with
"Netbalancer".

TABLE 4:
Results shown in seconds on highspeed wireless LAN (no induced latency)
Num blks sync'd Uncmp Zlib-1 Zlib-6 LZO1x-1 LZO1x-999
--------------- ----- ------ ------ ------- ---------
10000 255 232 233 231 257
20000 464 414 420 407 453
30000 677 594 611 585 650
40000 887 787 795 760 849
50000 1099 961 977 933 1048
60000 1310 1145 1167 1110 1259
70000 1512 1330 1362 1291 1470
80000 1714 1519 1552 1469 1679
90000 1917 1707 1747 1650 1882
100000 2122 1905 1950 1843 2111
110000 2333 2107 2151 2038 2329
120000 2560 2333 2376 2256 2580
130000 2835 2656 2679 2558 2921
140000 3274 3259 3161 3051 3466
150000 3662 3793 3547 3440 3919
160000 4040 4172 3937 3767 4416
170000 4425 4625 4379 4215 4958
180000 4860 5149 4895 4781 5560
190000 5855 6160 5898 5805 6557
200000 7004 7234 7051 6983 7770

TABLE 5:
Results shown in seconds with 60ms of induced latency
Num blks sync'd Uncmp Zlib-1 Zlib-6 LZO1x-1 LZO1x-999
--------------- ----- ------ ------ ------- ---------
10000 219 299 296 294 291
20000 432 568 565 558 548
30000 652 835 836 819 811
40000 866 1106 1107 1081 1071
50000 1082 1372 1381 1341 1333
60000 1309 1644 1654 1605 1600
70000 1535 1917 1936 1873 1875
80000 1762 2191 2210 2141 2141
90000 1992 2463 2486 2411 2411
100000 2257 2748 2780 2694 2697
110000 2627 3034 3076 2970 2983
120000 3226 3416 3397 3266 3302
130000 4010 3983 3773 3625 3703
140000 4914 4503 4292 4127 4287
150000 5806 4928 4719 4529 4821
160000 6674 5249 5164 4840 5314
170000 7563 5603 5669 5289 6002
180000 8477 6054 6268 5858 6638
190000 9843 7085 7278 6868 7679
200000 11338 8215 8433 8044 8795

==Backward compatibility==

Being unable to present the correct service bit, older clients will
continue to receive standard uncompressed data and will be fully
compatible with this change.

==Fallback==

It is important to be able to entirely and easily turn off compression
and decompression as a fall back mechanism. This can be done with a
simple bitcoin.conf setting of "compressionlevel=0". Only one of the two
connected peers need to set compressionlevel=0 in order to turn off
compression and decompression completely.

==Deployment==

This enhancement does not require a hard or soft fork.

==Service Bit==

During the testing of this implementation, service bit 28 was used,
however this enhancement will require a permanently assigned service bit.

==Implementation==

This implementation depends on the LZO compression library: lzo-2.09

https://github.com/ptschip/bitcoin/tree/compress

==Copyright==

This document is placed in the public domain.