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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 RFC 6249

Network Working Group                                      A. Bryan, Ed.
Internet-Draft                                        Metalinker Project
Intended status: Standards Track                      September 16, 2009
Expires: March 20, 2010


         MetaLinkHeader: Mirrors and Checksums in HTTP Headers
                      draft-bryan-metalinkhttp-03

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   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   This document specifies MetaLinkHeader: Mirrors and Checksums in HTTP
   Headers, an alternative to the Metalink XML-based download
   description format.  MetaLinkHeader describes multiple download
   locations (mirrors), Peer-to-Peer, checksums, digital signatures, and
   other information using existing standards.  Clients can
   transparently use this information to make file transfers more robust
   and reliable.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Notational Conventions . . . . . . . . . . . . . . . . . .  4
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Mirrors / Multiple Download Locations  . . . . . . . . . . . .  5
   4.  Peer-to-Peer . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  OpenPGP Signatures . . . . . . . . . . . . . . . . . . . . . .  5
   6.  Checksums  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     6.1.  Checksums of Whole Files . . . . . . . . . . . . . . . . .  6
     6.2.  Checksums of Chunks of Files . . . . . . . . . . . . . . .  6
   7.  Client / Server Multi-source Download Interaction  . . . . . .  7
   8.  Link Relation Type Registration: "duplicate" . . . . . . . . .  8
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
     9.1.  URIs and IRIs  . . . . . . . . . . . . . . . . . . . . . .  8
     9.2.  Spoofing . . . . . . . . . . . . . . . . . . . . . . . . .  9
     9.3.  Cryptographic Hashes . . . . . . . . . . . . . . . . . . .  9
     9.4.  Signing  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     10.1. Normative References . . . . . . . . . . . . . . . . . . .  9
     10.2. Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Acknowledgements and Contributors . . . . . . . . . . 10
   Appendix B.  What's different...?! (to be removed by RFC
                Editor before publication)  . . . . . . . . . . . . . 10
   Appendix C.  Document History (to be removed by RFC Editor
                before publication) . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11







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1.  Introduction

   MetaLinkHeader is an alternative to Metalink, usually an XML-based
   document format [draft-bryan-metalink].  MetaLinkHeader attempts to
   provide as much functionality as the Metalink XML format by using
   existing standards such as Web Linking
   [draft-nottingham-http-link-header], Instance Digests in HTTP
   [RFC3230], and Content-MD5 [RFC1864].  MetaLinkHeader is used to list
   information about a file to be downloaded.  This includes lists of
   multiple URIs (mirrors), Peer-to-Peer information, checksums, and
   digital signatures.

   Identical copies of a file are frequently accessible in multiple
   locations on the Internet over a variety of protocols (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.  At
   times, individual servers can be slow, outdated, or unreachable, but
   this can not be determined until the download has been initiated.
   This can lead to the user canceling the download and needing to
   restart it.  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.

   All the information about a download, including mirrors, checksums,
   digital signatures, and more can be transferred in coordinated HTTP
   Headers.  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 is done transparently to the
   user and the download is much more reliable and efficient.  In
   contrast, a traditional HTTP redirect to a mirror conveys only
   extremely minimal information - one link to one 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.

   [[ Discussion of this draft should take place on IETF HTTP WG mailing
   list at ietf-http-wg@w3.org or the Metalink discussion mailing list
   located at metalink-discussion@googlegroups.com.  To join the list,
   visit http://groups.google.com/group/metalink-discussion . ]]




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1.1.  Examples

   A brief Metalink server response with checksum, mirrors, .torrent,
   and OpenPGP signature:

   Link: <http://www2.example.com/example.ext>; rel="duplicate";
   Link: <ftp://ftp.example.com/example.ext>; rel="duplicate";
   Link: <http://example.com/example.ext.torrent>; rel="describedby";
   type="torrent";
   Link: <http://example.com/example.ext.asc>; rel="describedby";
   type="application/pgp-signature";
   Digest: SHA=thvDyvhfIqlvFe+A9MYgxAfm1q5=

1.2.  Notational Conventions

   This specification describes conformance of MetaLinkHeader.

   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 BCP 14, [RFC2119], as
   scoped to those conformance targets.


2.  Requirements

   In this context, "MetaLink" refers to a MetaLinkHeader which consists
   of mirrors and checksums in HTTP Headers as described in this
   document.  "Metalink XML" refers to the XML format described in
   [draft-bryan-metalink].

   Metalink servers are HTTP servers that MUST have lists of mirrors and
   use the Link header [draft-nottingham-http-link-header] to indicate
   them.  They also MUST provide checksums of files via Instance Digests
   in HTTP [RFC3230].  Mirror and checksum information provided by the
   originating Metalink server is considered authoritative.

   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 MAY
   be Metalink servers.  Mirror servers MUST support serving partial
   content.  Mirror servers SHOULD support Instance Digests in HTTP
   [RFC3230].

   Metalink clients use the mirrors provided by a Metalink server with
   Link header [draft-nottingham-http-link-header].  Metalink clients
   MUST support HTTP and MAY support FTP, BitTorrent, or other download
   methods.  Metalink clients MUST switch downloads from one mirror to
   another if the one mirror becomes unreachable.  Metalink clients are



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   RECOMMENDED to support multi-source, or parallel, downloads, where
   chunks of a file are downloaded from multiple mirrors simultaneously
   (and optionally, Peer-to-Peer).  Metalink clients MUST support
   Instance Digests in HTTP [RFC3230] by requesting and verifying
   checksums.  Metalink clients MAY make use of digital signatures if
   they are offered.


3.  Mirrors / Multiple Download Locations

   Mirrors are specified with the Link header
   [draft-nottingham-http-link-header] and a relation type of
   "duplicate" as defined in Section 8.

   A brief Metalink server response with two mirrors only:

   Link: <http://www2.example.com/example.ext>; rel="duplicate";
   Link: <ftp://ftp.example.com/example.ext>; rel="duplicate";

   Mirror servers are listed in order of priority.

   [[Some organizations have many mirrors.  Only send a few mirrors, or
   only use the Link header if Want-Digest is used?]]


4.  Peer-to-Peer

   Ways to download a file over Peer-to-Peer networks are specified with
   the Link header [draft-nottingham-http-link-header] and a relation
   type of "describedby" and a type parameter of "torrent" for .torrent
   [BITTORRENT] files.

   A brief Metalink server response with .torrent only:

   Link: <http://example.com/example.ext.torrent>; rel="describedby";
   type="torrent";


5.  OpenPGP Signatures

   OpenPGP signatures are specified with the Link header
   [draft-nottingham-http-link-header] and a relation type of
   "describedby" and a type parameter of "application/pgp-signature".








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   A brief Metalink server response with OpenPGP signature only:

   Link: <http://example.com/example.ext.asc>; rel="describedby";
   type="application/pgp-signature";


6.  Checksums

6.1.  Checksums of Whole Files

   Instance Digests in HTTP [RFC3230] are used to request and retrieve
   whole file checksums.

   A brief Metalink client request that prefers SHA-1 checksums over
   MD5:

   Want-Digest: MD5;q=0.3, SHA;q=0.8

   A brief Metalink server response with checksum:

   Digest: SHA=thvDyvhfIqlvFe+A9MYgxAfm1q5=

   [[Some publishers will probably desire stronger hashes.]]

6.2.  Checksums of Chunks of Files

   The Content-MD5 header [RFC1864] provides checksums for a chunk, or
   portion, of a file, when requested with a Range header field.

   Negotiation of Content-MD5 is described in [RFC3230].  A checksum for
   a chunk of a file can determine if there has been an error in
   transmission, which means the file is corrupt.  If an error is
   detected in a chunk, then just that chunk can be requested again from
   the current mirror, or a different mirror.

   A brief Metalink client request for Content-MD5 of a portion of a
   file:

   Range: bytes=7433802-
   Want-Digest: contentMD5;q=0.8











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   A brief Metalink server response with checksum:

   HTTP/1.1 206 Partial Content
   Accept-Ranges: bytes
   Content-Length: 7433801
   Content-Range: bytes 7433802-14867602/14867603
   Content-MD5:  Q2hlY2sgSW50ZWdyaXR5IQ==

   [[Content-MD5 for chunk checksums could lead to many random size
   chunk checksum requests.  Use consistent chunk sizes?  Could we get
   all chunk checksums from the referring Metalink server with Content-
   MD5?  Otherwise, this could also be a lot to ask on a mirror network
   if you don't control it and most servers might not have this feature
   enabled.]]

   [[Alternatively, Metalink XML could be used for chunk checksums but
   that complicates things.]]


7.  Client / Server Multi-source Download Interaction

   Metalink clients begin a download with a standard HTTP [RFC2616] GET
   request to the Metalink server.  Here the client prefers SHA-1
   checksums over MD5:


   GET /distribution/example.ext HTTP/1.1
   Host: www.example.com
   Want-Digest: MD5;q=0.3, SHA;q=0.8

   Alternatively, Metalink clients can use a HEAD request to discover
   mirrors via Link headers.  After that, it follows with a GET request
   as usual.

   The Metalink server responds with this:

   HTTP/1.1 200 OK
   Accept-Ranges: bytes
   Content-Length: 14867603
   Content-Type: application/x-cd-image
   Link: <http://www2.example.com/example.ext>; rel="duplicate";
   Link: <ftp://ftp.example.com/example.ext>; rel="duplicate";
   Link: <http://example.com/example.ext.torrent>; rel="describedby";
   type="torrent";
   Link: <http://example.com/example.ext.asc>; rel="describedby";
   type="application/pgp-signature";
   Digest: SHA=thvDyvhfIqlvFe+A9MYgxAfm1q5=




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   The Metalink client then contacts the other mirrors requesting a
   portion of the file with the "Range" header field, and using the
   location of the original GET request in the "Referer" header field.
   One of the client requests to a mirror server:

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

   The mirror servers respond with a 206 Partial Content HTTP status
   code and appropriate "Content-Length" and "Content Range" header
   fields.  The mirror response to the above request:

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

   Once the download has completed, the Metalink client MUST verify the
   checksum of the file.


8.  Link Relation Type Registration: "duplicate"

   o Relation Name: duplicate

   o Description: Refers to a resource whose available representations
   are byte-for-byte identical with the corresponding representations of
   the context IRI.

   o Reference: This specification.

   o Notes: This relation is for static resources.  That is, an HTTP GET
   request on any duplicate will return the same representation.  It
   does not make sense for dynamic or POSTable resources and should not
   be used for them.


9.  Security Considerations

9.1.  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.





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9.2.  Spoofing

   There is potential for spoofing attacks where the attacker publishes
   Metalinks with false information.  In that case, this could deceive
   unaware downloaders that they are downloading a malicious or
   worthless file.  Also, malicious publishers could attempt a
   distributed denial of service attack by inserting unrelated IRIs into
   Metalinks.

9.3.  Cryptographic Hashes

   Currently, some of the hash types defined in Instance Digests in HTTP
   [RFC3230] and Content-MD5 header [RFC1864] are considered insecure.
   These include the whole Message Digest family of algorithms which are
   not suitable for cryptographically strong verification.  Malicious
   people 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.

   If a Metalink contains hashes as described in Section 6, it SHOULD
   include "sha" which is SHA-1, as specified in [RFC3174].  It MAY also
   include other hashes.

9.4.  Signing

   Metalinks should include digital signatures, as described in
   Section 5.

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


10.  References

10.1.  Normative References

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

   [RFC1864]  Myers, J. and M. Rose, "The Content-MD5 Header Field",
              RFC 1864, October 1995.

   [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.,



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              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3174]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, September 2001.

   [RFC3230]  Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
              RFC 3230, January 2002.

   [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.

   [draft-nottingham-http-link-header]
              Nottingham, M., "Web Linking",
              draft-nottingham-http-link-header-06 (work in progress),
              July 2009.

10.2.  Informative References

   [draft-bryan-metalink]
              Bryan, A., Ed., Tsujikawa, T., McNab, N., and P. Poeml,
              "The Metalink Download Description Format",
              draft-bryan-metalink-16 (work in progress), August 2009.


Appendix A.  Acknowledgements and Contributors

   Thanks to Daniel Stenberg, Mark Nottingham, and Neil McNab.


Appendix B.  What's different...?! (to be removed by RFC Editor before
             publication)

   ...or missing, compared to the Metalink XML format
   [draft-bryan-metalink] :

   o  (+) Reuses existing standards without defining much new stuff.
      It's more of a collection/coordinated feature set.
   o  (+) No XML dependency.
   o  (-?)  Tied to HTTP, not as generic.  FTP/P2P clients won't be
      using it unless they also support HTTP, unlike Metalink XML.
   o  (---) Requires changes to server software.





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   o  (-?)  Could require some coordination of all mirror servers for
      all features, which may be difficult or impossible unless you are
      in control of all servers on the mirror network.
   o  (-) Metalink XML can be created by user (or server, but server
      component/changes not required).
   o  (-) Also, Metalink XML files are easily mirrored on all servers.
      Even if usage in that case is not as transparent, it still gives
      access to users at all mirrors (FTP included) to all download
      information with no changes needed to the server.
   o  (-) Not portable/archivable/emailable.  Not as easy for search
      engines to index?
   o  (-) No way to show mirror/p2p geographical location (yet).
   o  (-) No checksums besides MD5/SHA-1 (yet).
   o  (-) Not as rich metadata.
   o  (-) Not able to add multiple files to a download queue or create
      directory structure.


Appendix C.  Document History (to be removed by RFC Editor before
             publication)

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

   Known issues concerning this draft:
   o  None.

   -03 : September 16, 2009.
   o  Mention HEAD request, negotiate mirrors if Want-Digest is used.

   -02 : September 6, 2009.
   o  Content-MD5 for chunk checksums.

   -01 : September 1, 2009.
   o  Link Relation Type Registration: "duplicate"

   -00 : August 24, 2009.
   o  Initial draft.


Author's Address

   Anthony Bryan (editor)
   Metalinker Project

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





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