| 1 | BEP: 24 |
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| 2 | Title: Tracker Returns External IP |
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| 3 | Version: $Revision$ |
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| 4 | Last-Modified: $Date$ |
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| 5 | Author: David Harrison <dave@bittorrent.com> |
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| 6 | Status: Draft |
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| 7 | Type: Standards track |
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| 8 | Content-Type: text/x-rst |
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| 9 | Created: 12-May-2008 |
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| 10 | Post-History: |
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| 11 | |
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| 12 | |
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| 13 | So that peers may easily determine their IP address. A BitTorrent |
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| 14 | tracker may return the requesting peer's public IP address. |
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| 15 | |
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| 16 | |
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| 17 | Overview |
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| 18 | ======== |
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| 19 | |
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| 20 | A BitTorrent client can easily learn the IP address used when sending, |
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| 21 | but because of intervnening Network Address Translators (NATs) the IP |
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| 22 | address of the client's host seen inside the client's private network |
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| 23 | may differ from the IP address used to route the client's packets |
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| 24 | through the public Internet. |
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| 25 | |
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| 26 | In BitTorrent as described in BEP 3 [#BEP-3]_, peers wishing to |
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| 27 | transfer a file contact a central tracker. This tracker returns a |
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| 28 | list of peers that are currently transferring the file. For all peers |
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| 29 | that reach the tracker across the public Internet, the source address |
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| 30 | in each request seen by the tracker carries the publicly routable IP |
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| 31 | address of the requesting peer. |
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| 32 | |
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| 33 | A tracker implementing this extension returns the key *external ip* |
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| 34 | with a bencoded string value containing the IPv4 address as a 32 bit |
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| 35 | binary or containing the IPv6 address as a 128 bit binary. |
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| 36 | |
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| 37 | Using a 32 or 128 packed binary is idiosyncratic since the value for |
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| 38 | the *ip* key returned in a dict within the *peers* list is returned as |
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| 39 | a string using ipv4 dotted notation or a domain name. However the |
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| 40 | packed representation poses no semantic disadvantage and is negligibly |
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| 41 | smaller. |
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| 42 | |
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| 43 | The IPv4 part of this extension has been supported by uTorrent since |
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| 44 | May of 2007. |
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| 45 | |
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| 46 | The tracker is not unique in its ability to see a peer's public IP |
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| 47 | address. Any peer reached over the public Internet sees the public IP |
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| 48 | address of the peer on the far side of the connection. The *Extension |
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| 49 | Protocol* proposed in [#BEP-10]_ provides a mechanism for a client to |
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| 50 | obtains its public IP from any of its peers. There is no conflict for |
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| 51 | a BitTorrent client to support both tracker and peer methods, and they |
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| 52 | have slightly different advantages. A peer can learn its external IP |
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| 53 | from a tracker even when there are no peers. For trackerless swarms, |
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| 54 | a peer can learn its IP from any peer whose reference was obtained from the DHT |
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| 55 | [#BEP-5]_. |
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| 56 | |
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| 57 | |
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| 58 | References |
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| 59 | ========== |
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| 60 | |
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| 61 | .. [#BEP-3] BEP_0003. The BitTorrent Protocol Specification, Cohen. |
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| 62 | (http://www.bittorrent.org/beps/bep_0003.html) |
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| 63 | |
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| 64 | .. [#BEP-5] BEP_0005. DHT Protocol. Loewenstern. |
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| 65 | (http://www.bittorrent.org/beps/bep_0005.html) |
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| 66 | |
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| 67 | .. [#BEP-10] BEP_0010. Extension Protocol. Norberg, Strigeus, Hazel |
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| 68 | (http://www.bittorrent.org/beps/bep_0010.html) |
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| 69 | |
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| 70 | |
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| 71 | Copyright |
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| 72 | ========= |
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| 73 | |
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| 74 | This document has been placed in the public domain. |
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| 75 | |
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| 76 | |
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| 77 | |
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| 78 | .. |
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| 79 | Local Variables: |
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| 80 | mode: indented-text |
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| 81 | indent-tabs-mode: nil |
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| 82 | sentence-end-double-space: t |
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| 83 | fill-column: 70 |
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| 84 | coding: utf-8 |
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| 85 | End: |
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| 86 | |
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