| 1 | BEP: 21 |
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| 2 | Title: Extension for partial seeds |
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| 3 | Version: $Revision$ |
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| 4 | Last-Modified: $Date$ |
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| 5 | Author: Arvid Norberg <arvid@bittorrent.com>, Greg Hazel <greg@bittorrent.com>, Aaron Grunthal |
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| 6 | Status: Draft |
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| 7 | Type: Standards Track |
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| 8 | Created: 8-May-2008 |
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| 9 | Post-History: |
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| 10 | |
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| 11 | The purpose of this extension is to allow further optimizations of |
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| 12 | bittorrent swarms when peers are *partial seeds*. A partial seed |
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| 13 | is a peer that is incomplete without downloading anything more. This |
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| 14 | happens for multi file torrents where users only download some of |
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| 15 | the files. |
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| 16 | |
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| 17 | Extension Header |
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| 18 | ================ |
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| 19 | |
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| 20 | A peer that is a partial seed SHOULD include an extra header in the extension |
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| 21 | handshake (specified in `BEP 0010`_), 'upload_only'. Setting the value of |
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| 22 | this key to 1 indicates that this peer is not interested in downloading anything. |
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| 23 | |
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| 24 | Example extension handshake message:: |
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| 25 | |
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| 26 | {'m': {'ut_metadata', 3}, 'upload_only': 1} |
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| 27 | |
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| 28 | Tracker Scrapes |
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| 29 | =============== |
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| 30 | |
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| 31 | The tracker scrape conventions defines three values per torrent, 'complete', |
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| 32 | 'incomplete' and 'downloaded'. The purpose of this extensions is to let clients |
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| 33 | distinguish between partial seeds and downloaders, both of which currently would |
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| 34 | be classified as *incomplete*. |
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| 35 | |
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| 36 | If the tracker supports this extension, it MUST add a fourth field, 'downloaders'. |
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| 37 | This field is the number of active downloaders in the swarm, it does not include |
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| 38 | partial seeds. The number of partial seeds can be calculated by: |
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| 39 | ``incomplete - downloaders``. |
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| 40 | |
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| 41 | Tracker Announce |
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| 42 | ================ |
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| 43 | |
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| 44 | In order to tell the tracker that a peer is a partial seed, it MUST send an |
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| 45 | ``event=paused`` parameter in every announce while it is a partial seed. |
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| 46 | |
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| 47 | Rationale |
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| 48 | ========= |
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| 49 | |
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| 50 | Allowing peers to scrape a tracker and distinguish between active downloaders |
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| 51 | and partial seeds makes it more efficient to determine what to seed based |
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| 52 | on the downloader/seed ratio. |
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| 53 | |
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| 54 | The reason why every announce should contain ``event=paused`` is to avoid |
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| 55 | relying on the state being stored in the tracker. In case there's a failure |
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| 56 | and a backup tracker is used, it can recover all of the swarm state because |
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| 57 | the clients are announcing that they are partial seeds. |
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| 58 | |
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| 59 | References |
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| 60 | ========== |
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| 61 | |
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| 62 | .. _`BEP 0010`: http://www.bittorrent.org/beps/bep_0010.html |
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| 63 | |
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| 64 | |
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| 65 | Copyright |
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| 66 | ========= |
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| 67 | |
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| 68 | This document has been placed in the public domain. |
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| 69 | |
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| 70 | |
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| 71 | .. |
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| 72 | Local Variables: |
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| 73 | mode: indented-text |
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| 74 | indent-tabs-mode: nil |
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| 75 | sentence-end-double-space: t |
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| 76 | fill-column: 70 |
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| 77 | coding: utf-8 |
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| 78 | End: |
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