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Versions: 00 01 02

Network Working Group                                           A. Bryan
Internet-Draft                                              T. Tsujikawa
Intended status: Standards Track                                N. McNab
Expires: July 21, 2013
                                                                P. Poeml
                                                             MirrorBrain
                                                        January 17, 2013


              Metalink/XML Clients, Publishers, and Caches
                     draft-bryan-metalink-client-02

Abstract

   This document specifies behavior for Metalink/XML clients,
   publishers, and proxy caches.  Metalink XML files contain multiple
   download locations (mirrors and Peer-to-Peer), cryptographic hashes,
   digital signatures, and other information.  Metalink clients can use
   this information to make file transfers more robust and reliable.
   Normative requirements for Metalink/XML clients, publishers, and
   proxy caches are described here.

Editorial Note (To be removed by RFC Editor)

   Discussion of this draft should take place on the apps-discuss
   mailing list (apps-discuss@ietf.org), although this draft is not a WG
   item.

   The changes in this draft are summarized in Appendix B.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 21, 2013.

Copyright Notice



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   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Notational Conventions . . . . . . . . . . . . . . . . . .  4
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Metalink/XML Clients . . . . . . . . . . . . . . . . . . . . .  5
   3.  Metalink/XML Publishers and Generators . . . . . . . . . . . .  7
     3.1.  Transparent Metalink/XML Usage . . . . . . . . . . . . . .  8
     3.2.  Mirror Servers . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Metalink/XML Proxy Cache . . . . . . . . . . . . . . . . . . .  8
   5.  Client / Server Multi-source Download Interaction  . . . . . .  9
     5.1.  Error Prevention, Detection, and Correction  . . . . . . . 11
       5.1.1.  Error Prevention (Early File Mismatch Detection) . . . 11
       5.1.2.  Error Correction . . . . . . . . . . . . . . . . . . . 11
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
     7.1.  Directory Traversal and Safe file names  . . . . . . . . . 12
     7.2.  URIs and IRIs  . . . . . . . . . . . . . . . . . . . . . . 12
     7.3.  Spoofing . . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.4.  Cache Poisoning  . . . . . . . . . . . . . . . . . . . . . 12
     7.5.  Cryptographic Hashes . . . . . . . . . . . . . . . . . . . 12
     7.6.  Signing  . . . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  Acknowledgements and Contributors . . . . . . . . . . 14
   Appendix B.  Document History  . . . . . . . . . . . . . . . . . . 14











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1.  Introduction

   Metalink [RFC5854] is a document format based on Extensible Markup
   Language (XML) that describes a file or list of files to be
   downloaded from a server.  Metalinks can list a number of files, each
   with an extensible set of attached metadata.  Each listed file can
   have a description, multiple cryptographic hashes, and a list of
   Uniform Resource Identifiers (URIs) from which it is available.

   Identical copies of a file are frequently accessible in multiple
   locations on the Internet over a variety of protocols (such as FTP,
   HTTP, and Peer-to-Peer).  In some cases, users are shown a list of
   these multiple download locations (mirrors) and must manually select
   a single one on the basis of geographical location, priority, or
   bandwidth.  This distributes the load across multiple servers, and
   should also increase throughput and resilience.  At times, however,
   individual servers can be slow, outdated, or unreachable, but this
   can not be determined until the download has been initiated.  Users
   will rarely have sufficient information to choose the most
   appropriate server, and will often choose the first in a list which
   might not be optimal for their needs, and will lead to a particular
   server getting a disproportionate share of load.  The use of
   suboptimal mirrors can lead to the user canceling and restarting the
   download to try to manually find a better source.  During downloads,
   errors in transmission can corrupt the file.  There are no easy ways
   to repair these files.  For large downloads this can be extremely
   troublesome.  Any of the number of problems that can occur during a
   download lead to frustration on the part of users.

   Knowledge about availability of a download on mirror servers can be
   acquired and maintained by the operators of the origin server or by a
   third party.  This knowledge, together with cryptographic hashes,
   digital signatures, and more, can be stored in a machine-readable
   Metalink file.  The Metalink file can transfer this knowledge to the
   user agent, which can peruse it in automatic ways or present the
   information to a human user.  User agents can fall back to alternate
   mirrors if the current one has an issue.  Thereby, clients are
   enabled to work their way to a successful download under adverse
   circumstances.  All this can be done transparently to the human user
   and the download is much more reliable and efficient.  In contrast, a
   traditional HTTP redirect to one mirror conveys only comparatively
   minimal information -- a referral to a single server, and there is no
   provision in the HTTP protocol to handle failures.

   Other features that some clients provide include multi-source
   downloads, where chunks of a file are downloaded from multiple
   mirrors (and optionally, Peer-to-Peer) simultaneously, which
   frequently results in a faster download.  Metalinks can leverage



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   HTTP, FTP, and Peer-to-Peer protocols together, because regardless of
   the protocol over which the Metalink was obtained, it can make a
   resource accessible through other protocols.  If the Metalink was
   obtained from a trusted source, included verification metadata can
   solve trust issues when downloading files from replica servers
   operated by third parties.  Metalinks also provide structured
   information about downloads that can be indexed by search engines.

   Metalink/HTTP [RFC6249] is an alternative and complementary
   representation of Metalink information, using HTTP header fields
   instead of the XML-based document format [RFC5854].  Metalink/HTTP is
   used to list information about a file to be downloaded.  This can
   include lists of multiple URIs (mirrors and Peer-to-Peer
   information), cryptographic hashes, and digital signatures.

   Some popular sites automate the process of selecting mirrors using
   DNS load balancing, both to approximately balance load between
   servers, and to direct clients to nearby servers with the hope that
   this improves throughput.  Indeed, DNS load balancing can balance
   long-term server load fairly effectively, but it is less effective at
   delivering the best throughput to users when the bottleneck is not
   the server but the network.

   This document describes a mechanism by which the benefit of mirrors
   can be automatically and more effectively realized.  All the
   information about a download, including mirrors, cryptographic
   hashes, digital signatures, and more can be transferred in a
   Metalink.  This Metalink transfers the knowledge of the download
   server (and mirror database) to the client.  Clients can fallback to
   other mirrors if the current one has an issue.  With this knowledge,
   the client is enabled to work its way to a successful download even
   under adverse circumstances.  All this can be done without
   complicated user interaction and the download can be much more
   reliable and efficient.  Furthermore, in order to provide better load
   distribution across servers and potentially faster downloads to
   users, Metalinks facilitates multi-source downloads, where portions
   of a file are downloaded from multiple mirrors (and optionally, Peer-
   to-Peer) simultaneously.

   The client will then be able to request chunks of the file from the
   various sources, scheduling appropriately in order to maximize the
   download rate.

1.1.  Notational Conventions

   This specification describes conformance of Metalink/HTTP.

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



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   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, [RFC2119], as
   scoped to those conformance targets.

1.2.  Terminology

   In this context, "Metalink" refers to "Metalink/XML" refers to the
   XML format described in [RFC5854].

   The following terms as used in this document are defined here:

   o  Metalink Generator : Application that creates Metalink/XML files,
      and includes information about the files described in the Metalink
      such as locations (on Mirror servers or other methods like P2P),
      file sizes, and cryptographic hashes.

   o  Metalink Publisher : One who uses a Metalink Generator to create
      Metalink/XML files that are then offered to people to improve
      their download experience.

   o  Mirror server : Typically FTP or HTTP servers that "mirror" the
      Metalink server, as in they provide identical copies of (at least
      some) files that are also on the mirrored server.

   o  Metalink/XML : An XML file which describes a file that can contain
      information such as mirrors, cryptographic hashes, digital
      signatures, and more.

   o  Metalink Processors or Clients : Applications that process
      Metalink/XML and use them provide an improved download experience.
      They support HTTP and could also support other download protocols
      like FTP or various Peer-to-Peer methods.

   o  Metalink/HTTP : An HTTP header that contains information such as
      mirrors, cryptographic hashes, and digital signatures, which could
      also be in a Metalink/XML file.

2.  Metalink/XML Clients

   Metalink/XML clients use the mirrors provided by a Metalink/XML file.
   Metalink clients SHOULD support HTTP [RFC2616] and SHOULD support FTP
   [RFC0959].  Metalink clients MAY support BitTorrent [BITTORRENT], or
   other download methods.  Metalink clients SHOULD switch downloads
   from one mirror to another if a file is not available or if a mirror
   becomes unreachable.  Metalink clients MAY support multi-source, or
   parallel, downloads, where portions of a file can be downloaded from
   multiple mirrors simultaneously (and optionally, from Peer-to-Peer
   sources).  Metalink clients SHOULD support error recovery by using



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   the cryptographic hashes of parts of the file listed in Metalink/XML
   files, as described in [RFC5854], Section 4.1.3.

   Metalink clients SHOULD support checking digital signatures.  Digital
   signatures should be given preference over file hashes, if both are
   included in a Metalink/XML file.

   Metalink/XML clients MUST process metalinks available by URI.  They
   MAY process local Metalinks.

   Metalink/XML clients SHOULD recognize Metalink/XML files by MIME
   type.  [[What about misconfigured/unupdated servers that do not have
   correct MIME type?  SHOULD(?) clients recognize Metalinks by file
   extension as well?]]

   If Metalink/XML clients support HTTP, they SHOULD support
   "transparent metalink" usage to upgrade from a regular download to an
   enhanced download, using the information in the Metalink/XML
   file.(see Section 3.1).

   If a file with same name already exists locally, Metalink/XML clients
   SHOULD verify full file hash and if hash is correct, do not re-
   download the file.  If a file exists and full file hash is incorrect,
   Metalink/XML clients MAY repair file if partial file hashes exist.
   Otherwise, Metalink/XML clients MAY write to a different output file
   name (file_2 or file(2) like some apps already do).

   Metalink/XML clients SHOULD (or MUST?) verify full file hash after
   download completes.  If there is an error, MUST describe as corrupted
   and MAY re-download or keep download?  SHOULD verify chunk hash if
   available and re-get error parts.  SHOULD (or MAY?) be done during
   initial download process, MAY be done after download completed or to
   repair file downloaded another way?

   Metalink/XML clients SHOULD(?) use BitTorrent chunk hashes with HTTP/
   FTP downloads to repair file errors if the client supports torrents
   downloads.  (What if chunk hashes are present in torrent and
   metalink, should one be preferred?)

   If client supports Metalink/XML AND Metalink/HTTP, which should be
   preferred (in case mirrors/hashes differ)?

   Metalink/XML clients SHOULD make use of Metalink/XML origin element
   if dynamic="true" to check for updated Metalink.

   Metalink/XML clients MAY make use of the [ISO3166-1] alpha-2 two-
   letter country code for the geographical location of the physical
   server the URI is used to access, in an attempt to improve the



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   download experience.

   Metalink/XML clients could support multiple versions of Metalink/XML,
   and if they do they SHOULD prefer [RFC5854] Metalink/XML files.

   If a client supports Metalink/HTTP and Metalink/XML, it MAY prefer
   Metalink/HTTP but still use partial file hashes in the Metalink/XML
   files.

   Metalink/XML clients MAY create the directory structure described in
   the Metalink/XML at a relative download location, but see
   Section 7.1.

   Metalink clients could use the location of the original Metalink in
   the "Referer" header field for these ranged requests.

   Metalink clients MAY support the use of metainfo files (such as
   BitTorrent) for downloading files.

   Metalink clients SHOULD support the use of OpenPGP signatures.

   Metalink clients SHOULD support the use of S/MIME [RFC5751]
   signatures.

   Previously, Metalink/XML clients that support HTTP would do Accept
   header transparent content negotiation, but this has been deprecated.

   [[ NOTE: A number of requirements of Metalink clients are also in
   [RFC5854].  Should these be repeated or referenced?]]

3.  Metalink/XML Publishers and Generators

   Metalink/XML publishers MUST use correct MIME type for metalink files

   Metalink/XML publishers SHOULD advertise Metalink/XML file with Link
   HTTP header field from regular download for "transparent metalink"
   usage (see Section 3.1).

   Metalink/XML publishers SHOULD publish with chunk hashes if error
   recovery ability is desired (and files meet certain criteria like
   "large enough" - no point for 10k size file).

   Metalink Generators SHOULD offer Metalink/XML documents that contain
   cryptographic hashes of parts of the file (and other information) if
   error recovery is desirable.

   Metalink/XML publishers SHOULD publish with size element if it refers
   to a specific file.



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   Metalink/XML publishers MAY do Accept header transparent content
   negotiation (deprecated?)

   Metalink/XML publishers SHOULD include Metalink/XML origin element
   and dynamic="true" if updated metalinks will be offered.

   Metalink publishers SHOULD include digital signatures, as described
   in [RFC5854] Section 4.2.13.

3.1.  Transparent Metalink/XML Usage

   Metalink/XML files for a given resource MAY be provided in a Link
   header field [RFC5988] as shown in this example:

   This example shows a brief HTTP response header with .meta4:

   Link: <http://example.com/example.ext.meta4>; rel=describedby;
   type="application/metalink4+xml"

   Metalink/XML files are specified in [RFC5854], and they are
   particularly useful for providing metadata such as cryptographic
   hashes of parts of a file (see [RFC5854] Section 4.1.3), allowing a
   client to recover from errors (see Section 5.1.2).  Metalink servers
   SHOULD provide Metalink/XML files with partial file hashes in Link
   header fields, and Metalink clients SHOULD use them for error
   recovery.

3.2.  Mirror Servers

   Mirror servers are typically FTP or HTTP servers that "mirror"
   another server.  That is, they provide identical copies of (at least
   some) files that are also on the mirrored server.  Mirror servers
   SHOULD support serving partial content.

4.  Metalink/XML Proxy Cache

   Metalink/XML proxy cache could detect and log Metalink usage.

   Metalink/XML proxy cache MUST? use a whitelist for trusted sources by
   domain name (ie kde.org, ubuntu.com, fedoraproject.org) to prevent
   cache poisoning.

   Metalink/XML proxy cache SHOULD use preferred mirrors (those that are
   most cost efficient/better/local)

   Metalink/XML proxy cache MAY? repair errors or use hashes?  I guess
   so, but the client will also be verifying hashes.




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5.  Client / Server Multi-source Download Interaction

   Metalink clients begin with a Metalink/XML document.  They parse the
   XML and obtain a list of ways to retrieve a file or files from FTP or
   HTTP mirrors or P2P.

   After that, the client follows with a GET request to the desired
   mirrors.

   From the Metalink/XML file, the client learns some or all of the
   following metadata about the requested object:

   o  Mirror list, which can describe the mirror's priority and
      geographical location.

   o  Whole and partial file cryptographic hash.

   o  Object size.

   o  Peer-to-peer information.

   o  Digital signature.

   Next, the Metalink client requests a Range of the object from a
   mirror server:

   GET /example.ext HTTP/1.1
   Host: www2.example.com
   Range: bytes=7433802-
   Referer: http://www.example.com/distribution/example.ext

   Metalink clients SHOULD use partial file cryptographic hashes as
   described in Section 5.1.2, if available, to detect if the mirror
   server returned the correct data.  Errors in transmission and
   substitutions of incorrect data on mirrors, whether deliberate or
   accidental, can be detected with error correction as described in
   Section 5.1.2.

   Here, the mirror server has the correct file and responds with a 206
   Partial Content HTTP status code and appropriate "Content-Length" and
   "Content Range" header fields.  In this example, the mirror server
   responds, with data, to the above request:

   HTTP/1.1 206 Partial Content
   Accept-Ranges: bytes
   Content-Length: 7433801
   Content-Range: bytes 7433802-14867602/14867603




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   Metalink clients MAY start a number of parallel ranged downloads (one
   per selected mirror server other than the first) using mirrors
   provided by the Metalink/XML.  Metalink clients MUST limit the number
   of parallel connections to mirror servers, ideally based on observing
   how the aggregate throughput changes as connections are opened.  It
   would be pointless to blindly open connections once the path
   bottleneck is filled.  After establishing a new connection, a
   Metalink client SHOULD monitor whether the aggregate throughput
   increases over all connections that are part of the download.  The
   client SHOULD NOT open additional connections during this period.  If
   the aggregate throughput has increased, the client MAY open an
   additional connection and repeat these steps.  Otherwise, the client
   SHOULD NOT open a new connection until an established one closes.

   The Metalink client can determine the size and number of ranges
   requested from each server, based upon the type and number of mirrors
   and performance observed from each mirror.  Note that Range requests
   impose an overhead on servers and clients need to be aware of that
   and not abuse them.  When dowloading a particular file, metalink
   clients MUST NOT make more than one concurrent request to each mirror
   server that it downloads from.

   Metalink clients SHOULD close all but the fastest connection if any
   Ranged requests generated after the first request end up with a
   complete response, instead of a partial response (as some mirrors
   might not support HTTP ranges), if the goal is the fastest transfer.
   Metalink clients MAY monitor mirror conditions and dynamically switch
   between mirrors to achieve the fastest download possible.  Similarly,
   Metalink clients SHOULD abort extremely slow or stalled range
   requests and finish the request on other mirrors.  If all ranges have
   finished except for the final one, the Metalink client can split the
   final range into multiple range requests to other mirrors so the
   transfer finishes faster.

   Metalink clients MUST reject individual downloads from mirrors where
   the file size does not match the file size as reported by the
   Metalink server.

   If a Metalink client does not support certain download methods (such
   as FTP or BitTorrent) that a file is available from, and there are no
   available download methods that the client supports, then the
   download will have no way to complete.

   Metalink clients MUST verify the cryptographic hash of the file once
   the download has completed.  If the cryptographic hash offered in the
   Metalink/XML does not match the cryptographic hash of the downloaded
   file, see Section 5.1.2 for a possible way to repair errors.




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   If the download can not be repaired, it is considered corrupt.  The
   client can attempt to re-download the file.

   Metalink clients that support verifying digital signatures MUST
   verify digital signatures of requested files if they are included.
   Digital signatures MUST validate back to a trust anchor as described
   in the validation rules in [RFC3156] and [RFC5280].

5.1.  Error Prevention, Detection, and Correction

   Error prevention, or early file mismatch detection, is possible
   before file transfers with the use of file sizes provided in
   Metalink/XML.  Error detection requires full file cryptographic
   hashes in the Metalink/XML to detect errors in transfer after the
   transfers have completed.  Error correction, or download repair, is
   possible with partial file cryptographic hashes.

5.1.1.  Error Prevention (Early File Mismatch Detection)

   To verify the individual ranges of files, which might have been
   requested from different sources, see Section 5.1.2.

5.1.2.  Error Correction

   Partial file cryptographic hashes can be used to detect errors during
   the download.  Metalink servers SHOULD provide Metalink/XML files
   with partial file hashes in Link header fields as specified in
   Section 3.1, and Metalink clients SHOULD use them for error
   correction.

   An error in transfer or a substitution attack will be detected by a
   cryptographic hash of the object not matching the full file checksum
   from the Metalink/XML.  If the cryptographic hash of the object does
   not match the full file checksum from the Metalink/XML, then the
   client SHOULD use the partial file cryptographic hashes (if
   available).  This may contain partial file cryptographic hashes which
   will allow detection of which mirror server returned incorrect data.
   Metalink clients SHOULD use the Metalink/XML data to figure out what
   ranges of the downloaded data can be recovered and what needs to be
   fetched again.

   Other methods can be used for error correction.  For example, some
   other metainfo files also include partial file hashes that can be
   used to check for errors.







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6.  IANA Considerations

   None.

7.  Security Considerations

7.1.  Directory Traversal and Safe file names

   Metalink/XML clients MUST sanitize directory traversal information as
   specified in [RFC5854] Section 4.1.2.1.  Also see [RFC2183] Section 5
   and [RFC6266] Section 4.3 for similar considerations when creating
   file names supplied by Metalink/XML files which could be created
   without user input or awareness.

7.2.  URIs and IRIs

   Metalink clients handle URIs and IRIs.  See Section 7 of [RFC3986]
   and Section 8 of [RFC3987] for security considerations related to
   their handling and use.

7.3.  Spoofing

   There is potential for spoofing attacks where the attacker publishes
   Metalinks with false information.  In that case, this could deceive
   unaware downloaders into downloading a malicious or worthless file.
   As with all downloads, users should only download from trusted
   sources.  Also, malicious publishers could attempt a distributed
   denial of service attack by inserting unrelated URIs into Metalinks.
   [RFC4732] contains information on amplification attacks and denial of
   service attacks.

7.4.  Cache Poisoning

   Proxy caches MUST prevent cache poisoning.

7.5.  Cryptographic Hashes

   Currently, some of the hash types defined in the IANA registry named
   "Hash Function Textual Names" are considered insecure.  These include
   the whole Message Digest family of algorithms that are not suitable
   for cryptographically strong verification.  Malicious parties could
   provide files that appear to be identical to another file because of
   a collision, i.e., the weak cryptographic hashes of the intended file
   and a substituted malicious file could match.

   Metalink Generators and Processors MUST support "sha-256", which is
   SHA-256, as specified in [FIPS-180-3], and MAY support stronger
   hashes.



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   If a Metalink Document contains hashes, it SHOULD include "sha-256",
   which is SHA-256, or stronger.  It MAY also include other hashes from
   the IANA registry named "Hash Function Textual Names".

7.6.  Signing

   Metalinks SHOULD include digital signatures, as described in
   [RFC5854] Section 4.2.13.

   Digital signatures provide authentication, message integrity, and
   enable non-repudiation with proof of origin.

8.  References

8.1.  Normative References

   [BITTORRENT]  Cohen, B., "The BitTorrent Protocol Specification",
                 BITTORRENT 11031, February 2008,
                 <http://www.bittorrent.org/beps/bep_0003.html>.

   [FIPS-180-3]  National Institute of Standards and Technology (NIST),
                 "Secure Hash Standard (SHS)", FIPS PUB 180-3,
                 October 2008.

   [ISO3166-1]   International Organization for Standardization, "ISO
                 3166-1:2006.  Codes for the representation of names of
                 countries and their subdivisions -- Part 1: Country
                 codes", November 2006.

   [RFC0959]     Postel, J. and J. Reynolds, "File Transfer Protocol",
                 STD 9, RFC 0959, October 1985.

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]     Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
                 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3156]     Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
                 "MIME Security with OpenPGP", RFC 3156, August 2001.

   [RFC3986]     Berners-Lee, T., Fielding, R., and L. Masinter,
                 "Uniform Resource Identifier (URI): Generic Syntax",
                 STD 66, RFC 3986, January 2005.

   [RFC3987]     Duerst, M. and M. Suignard, "Internationalized Resource
                 Identifiers (IRIs)", RFC 3987, January 2005.



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   [RFC5280]     Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
                 Housley, R., and W. Polk, "Internet X.509 Public Key
                 Infrastructure Certificate and Certificate Revocation
                 List (CRL) Profile", RFC 5280, May 2008.

   [RFC5751]     Ramsdell, B. and S. Turner, "Secure/Multipurpose
                 Internet Mail Extensions (S/MIME) Version 3.2 Message
                 Specification", RFC 5751, January 2010.

   [RFC5854]     Bryan, A., Tsujikawa, T., McNab, N., and P. Poeml, "The
                 Metalink Download Description Format", RFC 5854,
                 June 2010.

   [RFC5988]     Nottingham, M., "Web Linking", RFC 5988, October 2010.

8.2.  Informative References

   [RFC2183]     Troost, R., Dorner, S., and K. Moore, "Communicating
                 Presentation Information in Internet Messages: The
                 Content-Disposition Header Field", RFC 2183,
                 August 1997.

   [RFC4732]     Handley, M., Rescorla, E., and IAB, "Internet Denial-
                 of-Service Considerations", RFC 4732, December 2006.

   [RFC6249]     Bryan, A., McNab, N., Tsujikawa, T., Poeml, P., and H.
                 Nordstrom, "Metalink/HTTP: Mirrors and Hashes",
                 RFC 6249, June 2011.

   [RFC6266]     Reschke, J., "Use of the Content-Disposition Header
                 Field in the Hypertext Transfer Protocol (HTTP)",
                 RFC 6266, June 2011.

Appendix A.  Acknowledgements and Contributors

   Some text borrowed from our previous RFCs: [RFC5854] and [RFC6249].

   Thanks to the Metalink community, Daniel Stenberg, Micah Cowan,
   Guiseppe Scrivano, Ilim Ugur, Jack Bates, and Mark Nottingham.

   This document is dedicated to Zimmy Bryan and Juanita Anthony.

Appendix B.  Document History

   [[ to be removed by the RFC editor before publication as an RFC. ]]

   Known issues concerning this draft:




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   o  Intro, term, downloads.

   o  Repeat requirements from RFC5854 or add pointer to them?

   -02 : January 17, 2013.

   o  Minor tweaks.

   -01 : July 16, 2012.

   o  Minor tweaks.

   -00 : July 4, 2012.

   o  Initial draft.

Authors' Addresses

   Anthony Bryan
   Pompano Beach, FL
   USA

   EMail: anthonybryan@gmail.com
   URI:   http://www.metalinker.org


   Tatsuhiro Tsujikawa
   Shiga
   Japan

   EMail: tatsuhiro.t@gmail.com
   URI:   http://aria2.sourceforge.net


   Neil McNab

   EMail: neil@nabber.org
   URI:   http://www.nabber.org













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   Dr. med. Peter Poeml
   MirrorBrain
   Venloer Str. 317
   Koeln  50823
   DE

   Phone: +49 221 6778 333 8
   EMail: peter@poeml.de
   URI:   http://mirrorbrain.org/~poeml/










































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