BitTorrent Local Tracker Discovery Protocol

Motivation

Some Internet Service Providers (ISPs) may wish to localize traffic

to reduce transit costs, reduce internal traffic, and improve user

experience by speeding up downloads.

With this extension, BitTorrent clients are able to discover a tracker

nearby on the network, and via this tracker discover nearby caches or

peers. A cache may simply be a fast peer in the middle of the network. It

might also have substantial disk space. The client communicates with a

cache using the normal BitTorrent protocol.

When a cache is present, the user benefits from having a high capacity

peer from which the user's client downloads and to which it can

delegate seeding. When a cache inside the user's ISP network seeds on

behalf of the client, it frees upstream capacity in the user's access

network benefiting the user and those that share the access network.

When subsequent peers transfer from their ISP's cache, the ISP

experiences less transit traffic.

The scope of this BEP is limited to the local tracker discovery

process. Extensions to the BitTorrent protocol suite to delegate

seeding or improve cache performance are beyond the scope of this BEP.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL

NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and

"OPTIONAL" in this document are to be interpreted as described in

IETF RFC 2119 [#RFC-2119]_.

Client implementation of local tracker discovery is OPTIONAL. Clients

MUST NOT announce private torrents to a local tracker. It is

RECOMMENDED that clients provide a user option for turning off local

tracker discovery. Local tracker discovery MAY be off by default.

Clients MAY automatically turn off caching if the performance benefit

is not obvious. Determining obviousness is beyond the scope of this

BEP.

The Discovery Mechanism

To find the tracker for its ISP, a BitTorrent client performs a reverse

DNS lookup on its external IP address and then obtains the BitTorrent

SRV resource record associated with the host's domain name. For

example, a host with address 69.107.0.14 obtains the PTR record at

  14.0.107.69.in-addr.arpa

The client's host IP address may not match the host's IP address as

seen outside the client's private network. We address this in Section

Network Address Translators.

The PTR resource record returned for this example contains domain name

  adsl-69-107-0-14.dsl.pltn13.pacbell.net

The client then looks up the SRV records at

  _bittorrent-tracker._tcp.adsl-69-107-0-14.dsl.pltn13.pacbell.net

If no SRV record is found, one or more subsequent queries take place as

described in Iterative Queries.

The target field in each returned SRV resource record contains the

domain name of a tracker and the port on which the tracker runs. This

tracker is called a local tracker, but the protocol to talk to this

tracker is no different from the standard BitTorrent tracker protocol

described in [#BEP-3]_.

When the BitTorrent client joins a swarm it announces to one or more

of the trackers referenced in the .torrent file and announces to the

local tracker. The local tracker returns peers which may be caches or

other peers that announced the same file to the local tracker.

A client MAY treat nearby peers or caches preferentially.

Reverse DNS lookups are described in RFC 1034 [#RFC-1034]_.

The SRV resource record type is described in RFC 2782 [#RFC-2782]_.

Iterative Queries

The domain name returned from the reverse DNS lookup is specific to

the querying host. In the naive implementation in DNS, there would be

one SRV resource record for every querying host. This would work but

is burdensome. A natural, seemingly less burdensome, but incorrect

solution is to use a wildcard of the form

  *.pacbell.net

If wildcards are implemented according to the algorithm in section

4.3.2 in [#RFC-1034]_ then all subdomains of pacbell.net that do not

have an exact label match will match the wildcard. Thus unless there

is an exact match then queries for

  _bittorrent-tracker._tcp.adsl-69-107-0-14.dsl.pltn13.pacbell.net

and

  _jabber._tcp.pacbell.net 

both match \*.pacbell.net and all SRV resource records with owner

\*.pacbell.net would be returned with the name set to the name in the

query. Thus it would be impossible to disambiguate Jabber from

BitTorrent SRV records without further information. This behavior is

implemented with BIND 9.4.1.

Another natural but incorrect solution is to specify domain names of

the type

  _bittorrent-tracker._tcp.*.pacbell.net

Section 4.3.3 in [#RFC-1034]_ specifies that wildcards only appear as

the first label in a domain name. This restriction was lifted in

[#RFC-4592]_, but not with semantics applicable to our use case. An

asterisk not at the beginning of a domain name is not treated like a

wildcard. Only a lookup for the exact domain name

  _bittorrent-tracker._tcp.*.pacbell.net

matches.

We propose an alternative that avoids wildcards and allows

suborganizations to override SRV records provided by parent

organizations: the peer starts by querying using its fully-qualified

domain name returned from the reverse DNS lookup, and if this fails

then it queries again after removing the most specific (leftmost)

label in the domain name. For example, if no SRV records are returned

when querying for

  _bittorrent-tracker._tcp.adsl-69-107-0-14.dsl.pltn13.pacbell.net

then the client queries for

  _bittorrent-tracker._tcp.dsl.pltn13.pacbell.net

and then

  _bittorrent-tracker._tcp.pltn13.pacbell.net

The search removes one label at a time terminating when one or more

resource records are found or before querying the root domain or

top-level domains that are not ccTLDs, e.g., .com, .org, .net. We

avoid querying the root or top-level domains given the low likelihood

that caches would be defined globally, and thus clients would

unnecessarily burden the root domain name servers with queries

generating negative results. We considered stopping before querying

country-level domains, but a country providing public infrastructure

might choose to provide caches.

Network Address Translators

Many hosts on the Internet sit in private networks that connect to the

Internet via a Network Address Translator (NAT). Such hosts may have

an IP address allocated from one of the private IP address ranges

defined by IANA, e.g., ranges with prefixes 10/8, 172.16/12, and

192.168/16. When communicating with hosts outside the private

network, the NAT translates the private IP to a globally-routable IP

address. This globally-routable address is the host's *external IP

address*.

The BitTorrent client must use its host's external IP address. A

BitTorrent client MAY obtain its host's external IP either from the

external ip key returned from a tracker implementing BEP 24

[#BEP-24]_ or from peers implementing the yourip extension defined

for the Extension Protocol proposed in [#BEP-10]_.

Example

In our example, we use AT&T's PacBell network. AT&T could implement

tracker discovery by adding the following lines to the zone file for

pacbell.net,

  ; name                                ttl  cls rr  pri weight port target
  _bittorrent-tracker._tcp.pacbell.net. 600  IN  SRV 5   0      6969 tracker

Now when a client performs tracker discovery, it performs three DNS

queries removing labels before reaching the domain name pacbell.net,

at which point the SRV record is returned and the client queries

tracker.pacbell.net to obtain the domain names of caches.

In Python, the local tracker's port and domain can be obtained using

PyDNS using the following code

  import DNS

  tlds = ["com", "net", "org"]  # add more TLDs here.
  
  name = DNS.revlookup( "69.107.0.14" )
  names = name.split('.')
  while names and names[0] not in tlds:
     name = "_bittorrent-tracker._tcp." + ".".join(names)
     req = DNS.Request( name=name, qtype="SRV", protocol="udp")
     response = req.req()
     if response.answers:
        break
     del names[0]

  print "response=", response.show()

which might generate output like

  response= ; <<>> PDG.py 1.0 <<>> _bittorrent._tcp.pacbell.net SRV
  ;; options: recurs 
  ;; got answer:
  ;; ->>HEADER<<- opcode 0, status NOERROR, id 0
  ;; flags: qr aa rd ra; Ques: 1, Ans: 1, Auth: 2, Addit: 3
  ;; QUESTIONS:
  ;;      _bittorrent-tracker._tcp.pacbell.net, type = SRV, class = IN
  
  ;; ANSWERS:
  _bittorrent-tracker._tcp.pacbell.net    600    SRV     (5, 0, 6969, 'cache.pacbell.net')
  
  ;; AUTHORITY RECORDS:
  pacbell.net             86400   NS      ns1.pbi.net
  pacbell.net             86400   NS      ns2.pbi.net
  
  ;; ADDITIONAL RECORDS:
  cache.pacbell.net       86400   A       69.107.0.1
  ns1.pacbell.net         86400   A       206.13.28.11
  ns2.pacbell.net         86400   A       206.13.29.11
  
  ;; Total query time: 0 msec
  ;; To SERVER: localhost
  ;; WHEN: Mon May 19 16:00:12 2008

The answer above is fictional since AT&T does not at this time

implement SRV records for BitTorrent trackers.

In Microsoft Windows, the port and domain name of the server can be

obtained using WinDNS (Dnsapi.lib) using DnsQuery(). In Unix, the

relevant call is res_query() from libresolv.

References

.. [#BEP-3] BEP_0003. The BitTorrent Protocol Specification, Cohen.

http://www.bittorrent.org/beps/bep_0003.html

.. [#BEP-10] BEP_0010. Extension Protocol. Norberg, Strigeus, Hazel.

http://www.bittorrent.org/beps/bep_0010.html

.. [#BEP-24] BEP_0024. Tracker Returns External IP. Harrison.

http://www.bittorrent.org/beps/bep_0024.html

.. [#RFC-1034] RFC-1034. DOMAIN NAMES - CONCEPTS AND FACILITIES. Mockapetris,

November 1987. http://tools.ietf.org/html/rfc1034

.. [#RFC-2119] RFC-2119. http://www.ietf.org/rfc/rfc2119.txt

.. [#RFC-2782] RFC-2782. A DNS RR for specifying the location of services (DNS

SRV). Gulbrandsen, Vixie, Esibov. February 2000.

http://tools.ietf.org/html/rfc2782

.. [#RFC-4592] RFC-4592. The Role of Wildcards in the Domain Name

System. Lewis. http://www.faqs.org/rfcs/rfc4592.html

Copyright

This document has been placed in the public domain.

..

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