Zeroconf Peer Advertising and Discovery

Abstract

Zeroconf is a collection of protocols including ipv4 link local

networking, mDNS, and DNS service-discovery. This document aims

to outline an extension for BitTorrent clients to allow them to find

peers on a local-link network using the zeroconf `DNS

service-discovery`_ system. It was written in the hope that clients

could standardise the way in which they found peers on the local-link

in order to maximize the functionality of the BitTorrent protocol.

Rationale

Many networks today consist of computers that have a fast network

connection (tens or hundreds of megabits/s) to a local area network,

in addition to a relatively slower (hundreds of kilobits/s or a few

megabits/s) connection to the wider internet. Such networks exist in

places such as university colleges, defence barracks and community

wifi networks. If two hosts on the same (fast) local-link network

wish to download the same torrent at the moment they only share

packets across their slow internet connection. The BitTorrent Zeroconf

Peer Advertising and Discovery Extension will allow automatic

discovery of these peers to each other so that packets can be quickly

transfered across the fast local network as opposed to across their

slow internet connections. Effectively the effort of downloading the

torrent can now be distributed between the hosts on a local-link

network.

Requirements

In order to facilitate the BitTorrent Zeroconf Peer Advertising and

Discovery Extension each host is going to be required to run a

zeroconf service discovery daemon. This is relatively trivial as there

are many available for free but two particularly good zeroconf

implementations are:

1. Apple's Bonjour for Mac OS X (included by default), Windows and POSIX systems

2. Avahi recommended for Linux and BSD clients and already available in many Linux and BSD distributions.

Note that you can only run one zeroconf instance per host due to it

binding to specific ports on your network interfaces, so it is not

recommended to build the whole zeroconf daemon/server into your

client. It is much better plug into a system wide service using the

API's: it cuts down your code and allows other services e.g. http, ftp

to advertise themselves using the same daemon.

Method

Each host will be required to publish (via zeroconf) the service type

_bittorrent._tcp. with the instance name as the torrent peer-id (in

hex).

<peer-id>._bittorrent._tcp.

Then clients need publish a subservice that is the torrent info hash (in hex). The result will be something in the form of:

<peer-id>._<info-hash>._sub._bittorrent._tcp.

e.g.

4d336d342d312d2d343834616435313564343437._32f17bbf96bdc77de85bb91ff8d56f124e817c0a._sub._bittorrent._tcp.

The above-mentioned zeroconf implementations provide good

documentation for developers on how to do this. If the client uses the

same peer-id for all torrents it has loaded, then for each additional

torrent file, only the subservice needs to be added. If the client

uses a different peer-id for each torrent then a

_bittorrent._tcp. service will have to be added for each torrent file

with the respective subservice. This is also true if each torrent uses

a different port.

In simple terms you can publish subtypes using the Apple Bonjour API

by passing them as a comma separated list after the service type (you

don't need to pass in the "_sub" label, since mDNSResponder will

automatically add it for you).

"_bittorrent._tcp,_info-hash1,_info-hash2,_info-hash3"

For the Avahi C API use the function

avahi_entry_group_add_service() to establish the parent

_bittorrent._tcp service and

avahi_entry_group_add_service_subtype() with

_<info-hash>._sub._bittorrent._tcp. as the 'type' parameter to

establish the subservices. For both functions the <peer-id> should be

used for the 'name' parameter.

Clients will need to browse for _<info-hash>._sub._bittorrent._tcp in

order to discover peers participating in the same torrent. Once again

good documentation is provided on how to browse for zeroconf services

in the above implementations. The replies will be in the form of

<peer-id>._bittorrent._tcp. Note: Some Macintosh official bittorrent

clients have a zeroconf implementation built in that publishes its

services in the form of:

_bittorrent-<info-hash>._tcp.

i.e.

_bittorrent-32f17bbf96bdc77de85bb91ff8d56f124e817c0a._tcp.

However this does not comply with RFC 2782 as the service name needs

to be 14 characters or less in addition to the fact that it is not a

service type but in fact a service instance it is advertising. Clients

may choose to also browse for these services for backwards

compatibility purposes (minimal effect - only one client is known to

do this) however they should not publish in this format.

Considerations

Clients should consider a couple of options when implementing the

BitTorrent Zeroconf Peer Advertising and Discovery Extension

specifically:

an option to not advertise torrents via zeroconf

an option not to upload limit local-link peers (or possibly have a separate rule).

Thoughts

I suspect the published peer-id need not be the hexed string as I

have it now. A simple unique string may be all that is required

(i.e. a hostname or a human readable computer name) with any

uniqueness conflicts handled by zeroconf automatically. We

ultimately only need to know what is on offer (as denoted by the

info-hash) and where to get it (the resolved IP address).

The mainline client and others may like to examine

pybonjour. pybonjour provides a pure-Python interface to Apple

Bonjour and compatible DNS-SD libraries (such as Avahi).

Newer zeroconf implementations such as Bonjour include support for

wide area service discovery. (Avahi can browse but not publish these

yet). This may be another consideration in support of this BEP.

May make a nice companion to BEP 9.

References

http://www.zeroconf.org

http://www.multicastdns.org

http://www.dns-sd.org

http://www.avahi.org

http://developer.apple.com/networking/bonjour/

http://o2s.csail.mit.edu/o2s-wiki/pybonjour

Copyright

This document has been placed in the public domain.

..

Local Variables:

mode: indented-text

indent-tabs-mode: nil

sentence-end-double-space: t

fill-column: 70

coding: utf-8

End: