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```
Filename: 278-directory-compression-scheme-negotiation.txt
Title: Directory Compression Scheme Negotiation
Author: Alexander Færøy
Created: 2017-03-06
Status: Closed
Implemented-In: 0.3.1.1-alpha
0. Overview
This document describes a method to provide and use different
compression schemes in Tor's directory specification[0] and let it be
up the client and server to negotiate a mutually supported scheme
using the semantics of the HTTP protocol.
Furthermore this proposal also extends Tor's directory protocol with
support for the LZMA and Zstandard compression schemes.
1. Motivation
Currently Tor serves each directory client with its different document
flavours in either an uncompressed format or, if the client adds a
".z"-suffix to the URL file path, a zlib-compressed document.
This have historically been non-problematic, but it disallows us from
easily extending the set of supported compression schemes.
Some of the problems this proposal is trying to aid:
- We currently only support zlib-based compression schemes and there
is no way for directory servers or clients to announce which
compression schemes they support. Zlib might not be the ideal
compression scheme for all purposes.
- It is not easily possible to add support for additional
compression schemes without adding additional file extensions or
flavours of the directory documents.
- In low-bandwidth and/or low-memory client scenarios it is useful
to be able to limit the amount of supported compression schemes to
have a client only support the most efficient compression scheme
for the given use-case and have the directory servers support the
most commonly available compression schemes used throughout the
network.
- We add support for the LZMA compression scheme, which yields
better compressed size and decompression time at the expensive of
higher compression time and higher memory usage.
- We add support for the Zstandard compression scheme, which yields
better compression ratio than GZip, but slightly worse than LZMA,
but with a smaller CPU and memory footprint than LZMA.
2. Analysis
We investigated the compression ratio, memory usage, memory allocation
strategies, and execution time for compression and decompression of
the GZip, BZip2, LZMA, and Zstandard compression schemes at
compression levels 1 through 9.
The data used in this analysis can be found in [1] and the `bench`
tool for generating the data can be found in [2].
During the preparation for this proposal Nick have analysed
compressing consensus diffs using both GZip, LZMA, and Zstandard. The
result of Nick's analysis can be found in [3].
We must continue to support both "gzip", "deflate", and "identity"
which are the currently available compression schemes in the Tor
network.
Further to enhance the compression ratio Nick have also worked on
proposal #274 (Rotate onion keys less frequently), #275 (Stop
including meaningful "published" time in microdescriptor consensus),
#276 (Report bandwidth with lower granularity in consensus documents),
and #277 (Detect multiple relay instances running with same ID) which
all aid in making our consensus documents less dynamic.
3. Proposal
We extend the directory client requests to include the
"Accept-Encoding" header as part of its request. The "Accept-Encoding"
header should contain a comma-separated list of names of the
compression schemes of which the client supports.
For example:
GET / HTTP/1.0
Accept-Encoding: x-zstd, x-tor-lzma, gzip, deflate
When a directory server receives a request with the "Accept-Encoding"
header included, to either the ".z" compressed or the uncompressed
version of any given document, it must decide on a mutually supported
compression scheme and add the "Content-Encoding" header to its
response and thus notifying the client of its decision. The
"Content-Encoding" header can at most contain one supported
compression scheme. If no mutual compression scheme can be negotiated
the server must respond with an HTTP error status code of 406
"Not Acceptable".
For example:
HTTP/1.0 200 OK
Content-Length: 1337
Connection: close
Content-Encoding: x-zstd
Currently supported compression scheme names includes "identity",
"gzip", and "deflate". This proposal adds two additional compression
scheme named "x-tor-lzma" (LZMA) and "x-zstd" (Zstandard).
All compression scheme names are case-insensitive.
The "deflate", "gzip", and "identity" compression schemes must be
supported by directory servers for backwards compatibility.
We use the name "x-tor-lzma" instead of just "x-lzma" because we
require a defined upper bound of memory usage that is available for
decompression of LZMA compressed data. The upper bound for memory
available for LZMA decompression is defined as 16 MB. This currently
means that will not use the LZMA compression scheme with a "preset"
value higher than 6.
Additionally, when a client, that supports this proposals, makes a
request to a directory document with the ".z"-suffix it must send an
ordered set of supported compression schemes where the last elements
in the set contains compression schemes that are supported by all of
the currently available Tor nodes ("gzip", "deflate", "identity"). In
this way older relays will simply respond with the document compressed
using zlib deflate without any prior knowledge of the newly added
compression schemes.
If a directory server receives a request to a document with the ".z"
suffix, where the client does not include an "Accept-Encoding" header,
the server should respond with the zlib compressed version of the
document for backwards compatibility with client that does not support
this proposal.
The "Content-Length" header contains the number of compressed bytes
sent to the client.
The new compression schemes will be available for directory clients
over both clearnet and BEGIN_DIR-style connections.
4. Security Implications
4.1 Compression and Decompression Bombs
We currently detect compression and decompression "bombs" and must
continue to do so with any additional compression schemes that we add.
The detection of compression and decompression bombs are handled in
`is_compression_bomb()` in torgzip.c and the same functionality is
used both for compression and decompression. These functions must be
extended to support LZMA and Zstandard.
4.2 Detection of Compression Algorithms
To ensure that we do not pass compressed data through the incorrect
decompression handler, when we have received data from another peer,
Tor tries to detect the compression scheme in
`detect_compression_method()`` in torgzip.c. This function should be
extended to also detect the LZMA and Zstandard formats. Possible
methods of applying this detection is looking at xz-tools, zstd's CLI,
and the libmagic 'compress' module.
4.3 Fingerprinting
All clients should aim at supporting the same set of supported
compression schemes to avoid fingerprinting.
5. Compatibility
This proposal does not break any backwards compatibility.
Tor will continue to support serving uncompressed and zlib-compressed
objects using the method defined in the directory specification[0],
but will allow newer clients to negotiate a mutually supported
compression scheme.
6. Performance and Scalability
Each newly added compression scheme adds to the compression cache of a
relay, which increases the memory requirements of a relay.
The LZMA compression scheme yields better compression ratio at the
expense of higher memory and CPU requirements for compression and
slightly higher memory and CPU requirements for decompression.
The Zstandard compression scheme yields better compression ratio than
GZip does, but does not suffer from the same high CPU and memory
requirements for compression as LZMA does.
Because of the high requirements for CPU and memory usage for LZMA it
is possible that we do not support this scheme for all available
documents or that we only support it in situations where it is
possible to pre-compute and cache the compressed document.
7. References
[0]: https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt
[1]: https://docs.google.com/spreadsheets/d/1devQlUOzMPStqUl9mPawFWP99xSsRM8xWv7DNcqjFdo
[2]: https://gitlab.com/ahf/tor-sponsor4-compression
[3]: https://github.com/nmathewson/consensus-diff-analysis
8. Acknowledgements
This research was supported in part by NSF grants CNS-1111539,
CNS-1314637, CNS-1526306, CNS-1619454, and CNS-1640548.
```
|
