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Network Working Group                                           A. Bryan
Internet-Draft                                                  N. McNab
Intended status: Standards Track                            H. Nordstrom
Expires: July 24, 2011                                      T. Tsujikawa

                                                                P. Poeml
                                                             MirrorBrain
                                                                 A. Ford
                                                     Roke Manor Research
                                                        January 20, 2011


 Metalink/HTTP: Mirrors and Cryptographic Hashes in HTTP Header Fields
                      draft-bryan-metalinkhttp-19

Abstract

   This document specifies Metalink/HTTP: Mirrors and Cryptographic
   Hashes in HTTP header fields, a different way to get information that
   is usually contained in the Metalink XML-based download description
   format.  Metalink/HTTP describes multiple download locations
   (mirrors), Peer-to-Peer, cryptographic hashes, digital signatures,
   and other information using existing standards for HTTP header
   fields.  Clients can use this information to make file transfers more
   robust and reliable.

Editorial Note (To be removed by RFC Editor)

   Discussion of this draft should take place on the HTTPBIS working
   group mailing list (ietf-http-wg@w3.org), althought this draft is not
   a WG item.

   The changes in this draft are summarized in Appendix C.

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."



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   This Internet-Draft will expire on July 24, 2011.

Copyright Notice

   Copyright (c) 2011 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.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Operation Overview . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Notational Conventions . . . . . . . . . . . . . . . . . .  5
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Mirrors / Multiple Download Locations  . . . . . . . . . . . .  7
     3.1.  Mirror Priority  . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Mirror Geographical Location . . . . . . . . . . . . . . .  7
     3.3.  Coordinated Mirror Policies  . . . . . . . . . . . . . . .  7
     3.4.  Mirror Depth . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Peer-to-Peer / Metainfo  . . . . . . . . . . . . . . . . . . .  8
     4.1.  Metalink/XML Files . . . . . . . . . . . . . . . . . . . .  9
   5.  OpenPGP Signatures . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Cryptographic Hashes of Whole Files  . . . . . . . . . . . . .  9
   7.  Client / Server Multi-source Download Interaction  . . . . . . 10
     7.1.  Error Prevention, Detection, and Correction  . . . . . . . 12
       7.1.1.  Error Prevention (Early File Mismatch Detection) . . . 12
       7.1.2.  Error Correction . . . . . . . . . . . . . . . . . . . 14
   8.  Multi-server Performance . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 15
     10.1. URIs and IRIs  . . . . . . . . . . . . . . . . . . . . . . 15
     10.2. Spoofing . . . . . . . . . . . . . . . . . . . . . . . . . 15
     10.3. Cryptographic Hashes . . . . . . . . . . . . . . . . . . . 16
     10.4. Signing  . . . . . . . . . . . . . . . . . . . . . . . . . 16
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     11.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Acknowledgements and Contributors . . . . . . . . . . 17
   Appendix B.  Comparisons to Similar Options  . . . . . . . . . . . 17
   Appendix C.  Document History  . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20

















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1.  Introduction

   Metalink/HTTP is an alternative representation of Metalink
   information, which is usually presented as an XML-based document
   format [RFC5854].  Metalink/HTTP attempts to provide as much
   functionality as the Metalink/XML format by using existing standards
   such as Web Linking [RFC5988], Instance Digests in HTTP [RFC3230],
   and Entity Tags (also known as ETags) [RFC2616].  Metalink/HTTP is
   used to list information about a file to be downloaded.  This can
   include lists of multiple URIs (mirrors), Peer-to-Peer information,
   cryptographic hashes, and digital signatures.

   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.

   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
   coordinated HTTP header fields hereafter referred to as 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



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   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.
   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.  Furthermore,
   in order to provide better load distribution across servers and
   potentially faster downloads to users, Metalink/HTTP facilitates
   multi-source downloads, where portions of a file are downloaded from
   multiple mirrors (and optionally, Peer-to-Peer) simultaneously.

1.1.  Operation Overview

   Detailed discussion of Metalink operation is covered in Section 2;
   this section will present a very brief, high-level overview of how
   Metalink achieves its goals.

   Upon connection to a Metalink/HTTP server, a client will receive
   information about other sources of the same resource and a
   cryptographic hash of the whole resource.  The client will then be
   able to request chunks of the file from the various sources,
   scheduling appropriately in order to maximise the download rate.

1.2.  Examples

   A brief Metalink server response with ETag, mirrors, .metalink,
   OpenPGP signature, and a cryptographic hash of the whole file:

   Etag: "thvDyvhfIqlvFe+A9MYgxAfm1q5="
   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="application/x-bittorrent"
   Link: <http://example.com/example.ext.metalink>; rel=describedby;
   type="application/metalink4+xml"
   Link: <http://example.com/example.ext.asc>; rel=describedby;
   type="application/pgp-signature"
   Digest: SHA-256=MWVkMWQxYTRiMzk5MDQ0MzI3NGU5NDEyZTk5OWY1ZGFmNzgyZTJlO
   DYzYjRjYzFhOTlmNTQwYzI2M2QwM2U2MQ==

1.3.  Notational Conventions

   This specification describes conformance of Metalink/HTTP.

   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.



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2.  Requirements

   In this context, "Metalink" refers to Metalink/HTTP which consists of
   mirrors and cryptographic hashes in HTTP header fields as described
   in this document.  "Metalink/XML" refers to the XML format described
   in [RFC5854].

   Metalink resources include Link header fields [RFC5988] to present a
   list of mirrors in the response to a client request for the resource.
   Metalink servers MUST include the cryptographic hash of a resource
   via Instance Digests in HTTP [RFC3230].  Valid algorithms are found
   in the IANA registry named "Hypertext Transfer Protocol (HTTP) Digest
   Algorithm Values" at
   <http://www.iana.org/assignments/http-dig-alg/http-dig-alg.xhtml>.
   SHA-256 and SHA-512 were added by [RFC5843].

   Metalink servers are HTTP servers with one or more Metalink
   resources.  Metalink servers MUST support the Link header fields for
   listing mirrors and MUST support Instance Digests in HTTP [RFC3230].
   Metalink servers MUST return the same Link header fields and Instance
   Digests on HEAD requests.  Metalink servers and their associated
   mirror servers SHOULD all share the same ETag policy.  To have the
   same ETag policy means that ETags are synchronized across servers for
   resources that are mirrored, i.e. byte-for-byte identical files will
   have the same ETag on mirrors that they have on the Metalink server.
   ETags could be based on the file contents (cryptographic hash) and
   not server-unique filesystem metadata.  The emitted ETag could be
   implemented the same as the Instance Digest for simplicity.  Metalink
   servers can offer Metalink/XML documents that contain cryptographic
   hashes of parts of the file and other information.

   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 can
   also be Metalink servers.  Mirror servers SHOULD support serving
   partial content.  HTTP mirror servers SHOULD share the same ETag
   policy as the originating Metalink server.  HTTP Mirror servers
   SHOULD support Instance Digests in HTTP [RFC3230].

   Metalink clients use the mirrors provided by a Metalink server with
   Link header fields [RFC5988].  Metalink clients MUST support HTTP 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 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 MUST support Instance Digests in HTTP



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   [RFC3230] by requesting and verifying cryptographic hashes.  Metalink
   clients MAY make use of digital signatures if they are offered.


3.  Mirrors / Multiple Download Locations

   Mirrors are specified with the Link header fields [RFC5988] and a
   relation type of "duplicate" as defined in Section 9.

   A brief Metalink server response with two mirrors only:

   Link: <http://www2.example.com/example.ext>; rel=duplicate;
   pri=1; pref
   Link: <ftp://ftp.example.com/example.ext>; rel=duplicate;
   pri=2; geo=gb; depth=1

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

   It is up to the server to choose how many Link header fieldss to
   send.  Such a decision could be a hard-coded limit, a random
   selection, based on file size, or based on server load.

3.1.  Mirror Priority

   Entries for mirror servers are listed in order of priority (from most
   preferred to least) or have a "pri" value, where mirrors with lower
   values are used first.

   This is purely an expression of the server's preferences; it is up to
   the client what it does with this information, particularly with
   reference to how many servers to use at any one time.

3.2.  Mirror Geographical Location

   Entries for a mirror servers can have a "geo" value, which is a
   [ISO3166-1] alpha-2 two letter country code for the geographical
   location of the physical server the URI is used to access.  A client
   can use this information to select a mirror, or set of mirrors, that
   are geographically near (if the client has access to such
   information), with the aim of reducing network load at inter-country
   bottlenecks.

3.3.  Coordinated Mirror Policies

   There are two types of mirror servers: preferred and normal.
   Preferred mirror servers are HTTP mirror servers that MUST share the
   same ETag policy as the originating Metalink server.  Preferred



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   mirrors make it possible to detect early on, before data is
   transferred, if the file requested matches the desired file.  Entries
   for preferred HTTP mirror servers have a "pref" value.  By default,
   if unspecified then mirrors are considered "normal" and do not
   necessarily share the same ETag policy.  FTP mirrors, as they do not
   emit ETags, are considered "normal". ([draft-ietf-ftpext2-hash]
   allows for FTP mirrors to be coordinated and provide file hashes).

   HTTP Mirror servers SHOULD support Instance Digests in HTTP
   [RFC3230].  Optimally, mirror servers will share the same ETag policy
   and support Instance Digests in HTTP.

3.4.  Mirror Depth

   Some mirrors can mirror single files, whole directories, or multiple
   directories.

   Entries for mirror servers can have a "depth" value, where "depth=0"
   is the default.  A value of 0 means ONLY that file is mirrored and
   that other URI path segments are not.  A value of 1 means that file
   and all other files and URI path segments contained in the rightmost
   URI path segment are mirrored.  For values of N, you go up N-1 URI
   path segments above.  A value of 2 means means going up one URI path
   segment above, and all files and URI path segments contained are
   mirrored.  For each higher value, another URI path segment closer to
   the Host is mirrored.

   A mirror with a depth value of 4:

   Link: <http://www2.example.com/dir1/dir2/dir3/dir4/dir5/example.ext>;
   rel=duplicate; pri=1; pref; depth=4

   In the above example, 4 URI path segments up are mirrored, from
   /dir2/ on down.


4.  Peer-to-Peer / Metainfo

   Entries for metainfo files, which describe ways to download a file
   over Peer-to-Peer networks or otherwise, are specified with the Link
   header fields [RFC5988] and a relation type of "describedby" and a
   type parameter that indicates the MIME type of the metadata available
   at the URI.  Since metainfo files can sometimes describe multiple
   files, or the filename may not be the same on the Metalink server and
   in the metainfo file but still have the same content, an optional
   name parameter can be used.





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   A brief Metalink server response with .torrent and .metalink:

   Link: <http://example.com/example.ext.torrent>; rel=describedby;
   type="application/x-bittorrent"; name="differentname.ext"
   Link: <http://example.com/example.ext.metalink>; rel=describedby;
   type="application/metalink4+xml"

   Metalink clients MAY support the use of metainfo files for
   downloading files.

4.1.  Metalink/XML Files

   Full Metalink/XML files for a given resource can be specified as
   shown in Section 4.  This is particularly useful for providing
   metadata such as cryptographic hashes of parts of a file, allowing a
   client to recover from partial errors (see Section 7.1.2).


5.  OpenPGP Signatures

   OpenPGP signatures [RFC3156] are specified with the Link header
   fields [RFC5988] and a relation type of "describedby" and a type
   parameter of "application/pgp-signature".

   A brief Metalink server response with OpenPGP signature only:

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

   Metalink clients MAY support the use of OpenPGP signatures.


6.  Cryptographic Hashes of Whole Files

   Metalink servers MUST provide Instance Digests in HTTP [RFC3230] for
   files they describe with mirrors via Link header fields.  Mirror
   servers SHOULD as well.  If Instance Digests are not provided by the
   Metalink servers, the Link header fields MUST be ignored.

   A brief Metalink server response with cryptographic hash:

   Digest: SHA-256=MWVkMWQxYTRiMzk5MDQ0MzI3NGU5NDEyZTk5OWY1ZGFmNzgyZTJlO
   DYzYjRjYzFhOTlmNTQwYzI2M2QwM2U2MQ==








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7.  Client / Server Multi-source Download Interaction

   Metalink clients begin a download with a standard HTTP [RFC2616] GET
   request to the Metalink server.  A Range limit is optional, not
   required.  Alternatively, Metalink clients can begin with a HEAD
   request to the Metalink server to discover mirrors via Link header
   fieldss.  After that, the client follows with a GET request to the
   desired mirrors.


   GET /distribution/example.ext HTTP/1.1
   Host: www.example.com

   The Metalink server responds with the data and these header fields:

   HTTP/1.1 200 OK
   Accept-Ranges: bytes
   Content-Length: 14867603
   Content-Type: application/x-cd-image
   Etag: "thvDyvhfIqlvFe+A9MYgxAfm1q5="
   Link: <http://www2.example.com/example.ext>; rel=duplicate; pref
   Link: <ftp://ftp.example.com/example.ext>; rel=duplicate
   Link: <http://example.com/example.ext.torrent>; rel=describedby;
   type="application/x-bittorrent"
   Link: <http://example.com/example.ext.metalink>; rel=describedby;
   type="application/metalink4+xml"
   Link: <http://example.com/example.ext.asc>; rel=describedby;
   type="application/pgp-signature"
   Digest: SHA-256=MWVkMWQxYTRiMzk5MDQ0MzI3NGU5NDEyZTk5OWY1ZGFmNzgyZTJlO
   DYzYjRjYzFhOTlmNTQwYzI2M2QwM2U2MQ==

   From the Metalink server response the client learns some or all of
   the following metadata about the requested object, in addition to
   also starting to receive the object:

   o  Object size.
   o  ETag.
   o  Mirror profile link, which can describe the mirror's priority,
      whether it shares the ETag policy of the originating Metalink
      server, geographical location, and mirror depth.
   o  Peer-to-peer information.
   o  Metalink/XML, which can include partial file cryptographic hashes
      to repair a file.
   o  Digital signature.
   o  Instance Digest, which is the whole file cryptographic hash.

   (Alternatively, the client could have requested a HEAD only, and then
   skipped to making the following decisions on every available mirror



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   server found via the Link header fieldss)

   If the object is large and gets delivered slower than expected then
   the Metalink client starts a number of parallel ranged downloads (one
   per selected mirror server other than the first) using mirrors
   provided by the Link header fields with "duplicate" relation type,
   using the location of the original GET request in the "Referer"
   header field.  The size and number of ranges requested from each
   server is for the client to decide, based upon the performance
   observed from each server.  Further discussion of performance
   considerations is presented in Section 8.

   If no range limit was given in the original request then work from
   the tail of the object (the first request is still running and will
   eventually catch up), otherwise continue after the range requested in
   the first request.  If no Range was provided, the original connection
   must be terminated once all parts of the resource have been
   retrieved.  It is recommended that a HEAD request is undertaken
   first, so that the client can find out if there are any Link header
   fieldss, and then Range-based requests are undertaken to the mirror
   servers as well as on the original connection.

   Preferred mirrors have coordinated ETags, as described in
   Section 3.3, and If-Match conditions based on the ETag SHOULD be used
   to quickly detect out-of-date mirrors by using the ETag from the
   Metalink server response.  If no indication of ETag syncronisation/
   knowledge is given then If-Match should not be used, and optimally
   there will be an Instance Digest in the mirror response which we can
   use to detect a mismatch early, and if not then a mismatch won't be
   detected until the completed object is verified.  Early file mismatch
   detection is described in detail in Section 7.1.1.

   One of the client requests to a mirror server:

   GET /example.ext HTTP/1.1
   Host: www2.example.com
   Range: bytes=7433802-
   If-Match: "thvDyvhfIqlvFe+A9MYgxAfm1q5="
   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 server response, with data, to the above request:








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   HTTP/1.1 206 Partial Content
   Accept-Ranges: bytes
   Content-Length: 7433801
   Content-Range: bytes 7433802-14867602/14867603
   Etag: "thvDyvhfIqlvFe+A9MYgxAfm1q5="
   Digest: SHA-256=MWVkMWQxYTRiMzk5MDQ0MzI3NGU5NDEyZTk5OWY1ZGFmNzgyZTJlO
   DYzYjRjYzFhOTlmNTQwYzI2M2QwM2U2MQ==

   If the first request was not Range limited then abort it by closing
   the connection when it catches up with the other parallel downloads
   of the same object.

   Downloads from mirrors that do not have the same file size as the
   Metalink server are considered unusable and the client can deal with
   it as it sees fit.

   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.

   Once the download has completed, the Metalink client MUST verify the
   cryptographic hash of the file.  If the cryptographic hash offered by
   the Metalink server with Instance Digests does not match the
   cryptographic hash of the downloaded file, see Section 7.1.2 for a
   possible way to repair errors.

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

7.1.  Error Prevention, Detection, and Correction

   Error prevention, or early file mismatch detection, is possible
   before file transfers with the use of file sizes, ETags, and
   cryptographic hashes.  Error detection requires Instance Digests, or
   cryptographic hashes, to determine after transfers if there has been
   an error.  Error correction, or download repair, is possible with
   partial file cryptographic hashes.

   Note that cyptographic hashes obtained from Instance Digests are in
   base64 encoding, while those from Metalink/XML and FTP HASH are in
   hexadecimal.

7.1.1.  Error Prevention (Early File Mismatch Detection)

   In HTTP terms, the requirement is that merging of ranges from
   multiple responses must be verified with a strong validator, which in
   this context is the same as either Instance Digest or a strong ETag.



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   In most cases it is sufficient that the Metalink server provides
   mirrors and Instance Digest information, but operation will be more
   robust and efficient if the mirror servers do implement a
   synchronized ETag as well.  In fact, the emitted ETag can be
   implemented the same as the Instance Digest for simplicity, but there
   is no need to specify how the ETag is generated, just that it needs
   to be shared among the mirror servers.  If the mirror server provides
   neither synchronized ETag or Instance Digest, then early detection of
   mismatches is not possible unless file length also differs.  Finally,
   the error is still detectable, after the download has completed, when
   the merged response is verified.

   ETags can not be used for verifying the integrity of the received
   content.  But it is a guarantee issued by the Metalink server that
   the content is correct for that ETag.  And if the ETag given by the
   mirror server matches the ETag given by the master server, then we
   have a chain of trust where the master server authorizes these
   responses as valid for that object.

   This guarantees that a mismatch will be detected by using only the
   synchronized ETag from a master server and mirror server, even
   alerted by the mirror servers themselves by responding with an error,
   preventing accidental merges of ranges from different versions of
   files with the same name.  This even includes many malicious attacks
   where the data on the mirror has been replaced by some other file,
   but not all.

   Synchronized ETag can not strictly protect against malicious attacks
   or server or network errors replacing content, but neither can
   Instance Digest on the mirror servers as the attacker most certainly
   can make the server seemingly respond with the expected Instance
   Digest even if the file contents have been modified, just as he can
   with ETag, and the same for various system failures also causing bad
   data to be returned.  The Metalink client has to rely on the Instance
   Digest returned by the Metalink master server in the first response
   for the verification of the downloaded object as a whole.

   If the mirror servers do return an Instance Digest, then that is a
   bonus, just as having them return the right set of Link header
   fieldss is.  The set of trusted mirrors doing that can be substituted
   as master servers accepting the initial request if one likes.

   The benefit of having slave mirror servers (those not trusted as
   masters) return Instance Digest is that the client then can detect
   mismatches early even if ETag is not used.  Both ETag and slave
   mirror Instance Digest do provide value, but just one is sufficient
   for early detection of mismatches.  If none is provided then early
   detection of mismatches is not possible unless the file length also



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   differs, but the error is still detected when the merged response is
   verified.

   If FTP servers support the FTP HASH command [draft-ietf-ftpext2-hash]
   and the same hash algorithm as the originating Metalink server, then
   that information can be used for early file mismatch detection.

7.1.2.  Error Correction

   Partial file cryptographic hashes can be used to detect errors during
   the download.  Metalink servers are not required to offer partial
   file cryptographic hashes in Metalink/XML as specified in
   Section 4.1, but they are encouraged to do so.

   If the object cryptographic hash does not match the Instance Digest
   then fetch the Metalink/XML if available, where partial file
   cryptographic hashes can be found, allowing detection of which server
   returned incorrect data.  If the Instance Digest computation does not
   match then the client needs to fetch the partial file cryptographic
   hashes, if available, and from there figure out what of the
   downloaded data can be recovered and what needs to be fetched again.
   If no partial cryptographic hashes are available, then the client
   MUST fetch the complete object from other mirrors.


8.  Multi-server Performance

   When opting to download simultaneously from multiple mirrors, there
   are a number of factors (both within and outside the influence of the
   client software) that are relevant to the performance achieved:

   o  The number of servers used simultaneously.
   o  The ability to pipeline sufficient or sufficiently large range
      requests to each server so as to avoid connections going idle.
   o  The ability to pipeline sufficiently few or sufficiently small
      range requests to servers so that all the servers finish their
      final chunks simultaneously.
   o  The ability to switch between mirrors dynamically so as to use the
      fastest mirrors at any moment in time

   Obviously we do not want to use too many simultaneous connections, or
   other traffic sharing a bottleneck link will be starved.  But at the
   same time, good performance requires that the client can
   simultaneously download from at least one fast mirror while exploring
   whether any other mirror is faster.  Based on laboratory experiments,
   we suggest a good default number of simultaneous connections is
   probably four, with three of these being used for the best three
   mirrors found so far, and one being used to evaluate whether any



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   other mirror might offer better performance.

   The size of chunks chosen by the client should be sufficiently large
   that the chunk request header fields and reponse header fields
   represent neglible overhead, and sufficiently large that they can be
   pipelined effectively without needing a very high rate of chunk
   requests.  At the same time, the amount of time wasted waiting for
   the last chunk to download from the last server after all the other
   servers have finished should be minimized.  Note that Range requests
   impose an overhead on servers and clients need to be aware of that
   and not abuse them.


9.  IANA Considerations

   Accordingly, IANA will make the following registration to the Link
   Relation Type registry.

   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.


10.  Security Considerations

10.1.  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.

10.2.  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 URIs into
   Metalinks.



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10.3.  Cryptographic Hashes

   Currently, some of the digest values defined in Instance Digests in
   HTTP [RFC3230] 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 whole file hashes as described in Section 6,
   it SHOULD include SHA-256, as specified in [FIPS-180-3], or stronger.
   It MAY also include other hashes.

10.4.  Signing

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

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


11.  References

11.1.  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.



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   [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.

   [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.

   [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.

   [draft-ietf-ftpext2-hash]
              Bryan, A., Kosse, T., and D. Stenberg, "FTP Extensions for
              Cryptographic Hashes", draft-ietf-ftpext2-hash-00 (work in
              progress), November 2010.

11.2.  Informative References

   [RFC5843]  Bryan, A., "Additional Hash Algorithms for HTTP Instance
              Digests", RFC 5843, April 2010.


Appendix A.  Acknowledgements and Contributors

   Thanks to the Metalink community, Alexey Melnikov, Julian Reschke,
   Mark Nottingham, Daniel Stenberg, Matt Domsch, Micah Cowan, and David
   Morris.

   Mark Handley and Javier Vela Diago did work on simultaneous download
   from multiple mirrors, which also provided validation of the benefits
   of this approach.


Appendix B.  Comparisons to Similar Options

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



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   This draft, compared to the Metalink/XML format [RFC5854] :

   o  (+) Reuses existing HTTP standards without much new besides a Link
      Relation Type.  It's more of a collection/coordinated feature set.
   o  (?)  The existing standards don't seem to be widely implemented.
   o  (+) No XML dependency, except for Metalink/XML for partial file
      cryptographic hashes.
   o  (+) Existing Metalink/XML clients can be easily converted to
      support this as well.
   o  (+) Coordination of mirror servers is preferred, but not required.
      Coordination could be difficult or impossible unless you are in
      control of all servers on the mirror network.
   o  (-) Requires software or configuration changes to originating
      server.
   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 server-side support.  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, this method
      still gives access to all download information (with no changes
      needed to servers) from all mirrors (FTP included).
   o  (-) Not portable/archivable/emailable.  Metalink/XML is used to
      import/export transfer queues.  Not as easy for search engines to
      index?
   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 the RFC editor before publication as an RFC. ]]

   Known issues concerning this draft:
   o  Some organizations have many mirrors.  Should all be sent, or only
      a certain number?  All should be included in the Metalink/XML, if
      used.
   o  Using Metalink/XML for partial file cryptographic hashes.  That
      adds XML dependency to apps for an important feature.  Is there a
      better method?

   -19 : January 20, 2011.
   o  Julian Reschke's review.

   -18 : January 1, 2010.





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   o  AD review by Alexey Melnikov.

   -17 : September 13, 2010.
   o  RFC 5854 Metalink/XML.

   -16 : April 16, 2010.
   o  Add draft-ietf-ftpext2-hash reference and FTP mirror coordination.

   -15 : February 20, 2010.
   o  Update references and terminology.

   -14 : December 31, 2009.
   o  Baseline file hash: SHA-256.

   -13 : November 22, 2009.
   o  Metalink/XML for partial file cryptographic hashes.

   -12 : November 11, 2009.
   o  Clarifications.

   -11 : October 23, 2009.
   o  Mirror changes.

   -10 : October 15, 2009.
   o  Mirror coordination changes.

   -09 : October 13, 2009.
   o  Mirror location, coordination, and depth.
   o  Split HTTP Digest Algorithm Values Registration into
      draft-bryan-http-digest-algorithm-values-update.

   -08 : October 4, 2009.
   o  Clarifications.

   -07 : September 29, 2009.
   o  Preferred mirror servers.

   -06 : September 24, 2009.
   o  Add Mismatch Detection, Error Recovery, and Digest Algorithm
      values.
   o  Remove Content-MD5 and Want-Digest.

   -05 : September 19, 2009.
   o  ETags, preferably matching the Instance Digests.

   -04 : September 17, 2009.





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   o  Temporarily remove .torrent.

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

   -02 : September 7, 2009.
   o  Content-MD5 for partial file cryptographic hashes.

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

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


Authors' Addresses

   Anthony Bryan
   Pompano Beach, FL
   USA

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


   Neil McNab

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


   Henrik Nordstrom

   Email: henrik@henriknordstrom.net
   URI:   http://www.henriknordstrom.net/


   Tatsuhiro Tsujikawa
   Shiga
   Japan

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








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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/


   Alan Ford
   Roke Manor Research
   Old Salisbury Lane
   Romsey, Hampshire  SO51 0ZN
   UK

   Phone: +44 1794 833 465
   Email: alan.ford@roke.co.uk
































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