An Alternate BitTorrent Cache Discovery Protocol

Motivation

Some Internet Service Providers (ISPs) may be interested in deploying

BitTorrent caches to lower transit costs, reduce internal traffic, and

improve user experience by speeding up downloads.

A cache is simply 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.

With this extension, BitTorrent clients are able to discover caches

nearby on the network. 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.

This is meant as a simpler alternative than presented in BEP-22

[#BEP-22]_ though further from the intended usage of the Domain Name

System (DNS) and existing standards.

The Discovery Mechanism

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

DNS lookup on its external IP address, prepends "bittorrent-tracker"

and resolves the resulting domain name to find the tracker.

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 resolves the domain name

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

If no IP address(es) are found, one or more subsequent queries take place as

described in Iterative Queries.

The returned tracker(s) are called cache trackers, but the protocol

to talk to these trackers 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

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

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

A cache is a BitTorrent peer. A client MAY treat it preferentially.

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

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 bittorrent-tracker A or AAAA resource record for every querying host.

The most obvious solution is to use a wildcard of the form

  bittorrent-tracker.*.pacbell.net

However, 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.*.pacbell.net

matches.

We propose an alternative that avoids wildcards and allows

suborganizations to override mappings 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 A/AAAA records are returned

when querying for

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

then the client queries for

  bittorrent-tracker.dsl.pltn13.pacbell.net

and then

  bittorrent-tracker.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*.

When finding a cache, the BitTorrent client must use its host's

external IP address. A BitTorrent client can obtain its host's

external IP either from the external ip key returned from a tracker

implementing BEP 24 [#BEP-24]_ or from peers using 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

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

pacbell.net,

  bittorrent-tracker.pacbell.net.      IN  A   206.13.28.15

Now when a client performs cache 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 cache tracker's address can be obtained using the following

  import socket
  
  tlds = ["com", "net", "org"]  # add more here.
  
  name, aliases, ipaddrs = socket.gethostbyaddr("69.107.0.14")
  names = name.split('.')
  while names and names[0] not in tlds:
     name = "bittorrent-tracker." + ".".join(names)
     try:
       ip = socket.gethostbyname(name)
       break
     except:
       del names[0]
  
  print "response=", ip

which might generate output like

  response='151.164.129.4'

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

implement SRV records for BitTorrent trackers.

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-22] BEP_0022. BitTorrent Cache Discovery Protocol. Harrison,

Shalunov, 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-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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