Hello Tor developers,

I am interested in becoming an open source contributor for Tor, but I don't know where to start some guidance would be appreciated.

Thank you,

On Sat, Aug 12, 2017 at 8:00 AM, tor-dev-request@lists.torproject.org wrote:

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Today's Topics:

1. Proposal 281: downloading microdescriptors in bulk (Nick Mathewson)

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Message: 1 Date: Fri, 11 Aug 2017 13:36:00 -0400 From: Nick Mathewson nickm@torproject.org To: tor-dev@lists.torproject.org Subject: [tor-dev] Proposal 281: downloading microdescriptors in bulk Message-ID: <CAKDKvux=eJBh_JsEeiqnhbEbcgRVeMRrbm2G7itZ8kX y4+M26A@mail.gmail.com> Content-Type: text/plain; charset="UTF-8"

Filename: 281-bulk-md-download.txt Title: Downloading microdescriptors in bulk Author: Nick Mathewson Created: 11-Aug-2017 Status: Draft

1. Introduction

This proposal describes a ways to download more microdescriptors at a time, using fewer bytes.

Right now, to download N microdescriptors, the client must send about 44*N bytes in its HTTP request. Because clients can request microdescriptors in any combination, the directory caches cannot pre-compress responses to these requests, and need to use less space-efficient on-the-fly compression algorithms.

Under this proposal, clients simply say "Send me the microdescriptors I need", given what I know.

2. Combined microdescriptor downloads

2.1. By diff

If a client has a consensus with base64 sha3-256 digest X, and it previously had a consensus with base64 sha3-256 digests Y then it may request all the microdescriptors listed in X but not Y, by asking for the resource: /tor/micro/diff/X/Y

Clients SHOULD only ask for this resource compressed.

Caches MUST NOT answer this request unless they recognize the consensus with digest X, and digest Y. digest Y. If answering, caches MUST reply with all of the microdescriptors that the cache holds that were listed by consensus X, and MUST omit all the microdescriptors that were omitted listed in consensus Y.

2.2. By consensus:

If a client has fewer than NMNM% of the microdescriptors listed in a consensus X, it should fetch the resource /tor/micro/full/X

Clients SHOULD only ask for this resource compressed.

Caches MUST NOT answer this request unless they recognize the consensus with digest X. They should send all the microdescriptors they have that are listed in that consensus.

2.3. When to make these requests

Clients should decide to use this format in preference to the old download-by-digest format if the consensus X lists their preferred directory cache as using a new DirCache subprotocol version. (See 5 below.)

3. Performance analysis

This is a back-of-the-envelope analysis using a month's worth of consensus documents, and a randomly chosen sample of microdescriptors.

On average, about 0.5% of the microdescriptors change between any two consensuses. Call it 50. That means 50*43 bytes == 2150 bytes to request the microdescriptors. It means ~24530 bytes of microdescriptors downloaded, compressed to ~13687 bytes by zstd.

With this proposal, we're down to 86 bytes for the request, and we can precompute the compressed output, making it save to use lzma2, getting a compressed result more like 13362.

It appears that this change would save about 15% for incremental microdescriptor downloads, most of that coming from the reduction in request size.

For complete downloads, a complete set of microdescriptors is about 7700 microdesciptors long. That makes the total number of bytes for the requests 7700*43 == 331100 bytes. The response, if compressed with lzma instead of zstd, would fall from 1659682 to 1587804 bytes, for a total savings of 20%.

5. Compatibility

   Caches supporting this download protocol need to advertise support of a new DirCache subprotocol version.

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