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Network Working Group                                           A. Bryan
Internet-Draft                                                  N. McNab
Intended status: Standards Track                            H. Nordstrom
Expires: April 26, 2010
                                                                 A. Ford
                                                     Roke Manor Research
                                                        October 23, 2009

          Metalink/HTTP: Mirrors and Checksums in HTTP Headers

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 26, 2010.

Copyright Notice

   Copyright (c) 2009 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 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
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   This document specifies Metalink/HTTP: Mirrors and Checksums in HTTP
   Headers, an alternative to the Metalink XML-based download
   description format.  Metalink/HTTP describes multiple download
   locations (mirrors), Peer-to-Peer, checksums, digital signatures, and
   other information using existing standards for HTTP headers.  Clients
   can transparently use this information to make file transfers more
   robust and reliable.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Operation Overview . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Notational Conventions . . . . . . . . . . . . . . . . . .  4
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Mirrors / Multiple Download Locations  . . . . . . . . . . . .  5
     3.1.  Mirror Priority  . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Mirror Geographical Location . . . . . . . . . . . . . . .  6
     3.3.  Coordinated Mirror Policies  . . . . . . . . . . . . . . .  6
     3.4.  Mirror Depth . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Peer-to-Peer / Metainfo  . . . . . . . . . . . . . . . . . . .  7
     4.1.  Metalink/XML Files . . . . . . . . . . . . . . . . . . . .  8
   5.  OpenPGP Signatures . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Checksums of Whole Files . . . . . . . . . . . . . . . . . . .  8
   7.  Client / Server Multi-source Download Interaction  . . . . . .  8
     7.1.  Error Prevention, Detection, and Correction  . . . . . . . 11
       7.1.1.  Error Prevention (Early File Mismatch Detection) . . . 11
       7.1.2.  Error Correction . . . . . . . . . . . . . . . . . . . 12
   8.  Multi-server Performance . . . . . . . . . . . . . . . . . . . 12
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 13
     10.1. URIs and IRIs  . . . . . . . . . . . . . . . . . . . . . . 14
     10.2. Spoofing . . . . . . . . . . . . . . . . . . . . . . . . . 14
     10.3. Cryptographic Hashes . . . . . . . . . . . . . . . . . . . 14
     10.4. Signing  . . . . . . . . . . . . . . . . . . . . . . . . . 14
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     11.2. Informative References . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Acknowledgements and Contributors . . . . . . . . . . 15
   Appendix B.  Comparisons to Similar Options  . . . . . . . . . . . 15
   Appendix C.  Document History  . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17

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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 [draft-bryan-metalink].  Metalink/HTTP 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 ETags.  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,
   checksums, 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
   may 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, 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

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   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.  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.

   [[ 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 . ]]

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 checksum
   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 whole file checksum:

   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";
   Link: <http://example.com/example.ext.metalink>; rel="describedby";
   Link: <http://example.com/example.ext.asc>; rel="describedby";
   Digest: SHA=thvDyvhfIqlvFe+A9MYgxAfm1q5=

1.3.  Notational Conventions

   This specification describes conformance of Metalink/HTTP.

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

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   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 Metalink/HTTP which consists of
   mirrors and checksums in HTTP Headers as described in this document.
   "Metalink/XML" refers to the XML format described in

   Metalink servers are HTTP servers that use the Link header
   [draft-nottingham-http-link-header] to present a list of mirrors of a
   resource to a client.  They MUST provide checksums of files via
   Instance Digests in HTTP [RFC3230], whether requested or not.  Mirror
   and checksum information provided by the originating Metalink server
   MUST be considered authoritative.  Metalink servers and their
   associated mirror servers SHOULD all share the same ETag policy (ETag
   Synchronization), i.e. based on the file contents (checksum) and not
   server-unique filesystem metadata.  The emitted ETag MAY be
   implemented the same as the Instance Digest for simplicity.

   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.  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 [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 mirror becomes unreachable.  Metalink clients SHOULD
   support multi-source, or parallel, downloads, where portions of a
   file are downloaded from multiple mirrors simultaneously (and
   optionally, from Peer-to-Peer sources).  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

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   "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=1
   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 if Want-Digest is used?]]

   It is up to the server to choose how many Link headers 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

   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.  A client MUST
   respect the server's priority ordering, however.

   [[Would it make more sense to use qvalue-style policies here, i.e.
   q=1.0 through q=0.0 ?]]

3.2.  Mirror Geographical Location

   Mirror servers MAY have a "geo" value, which is a [ISO3166-1] alpha-2
   two letter country code for the geographical location of the physical
   server the IRI is used to access.  A client may 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.  Optimally, they
   will do both.  Preferred mirrors make it possible to detect early on,
   before data is transferred, if the file requested matches the desired
   file.  Preferred mirror servers Preferred HTTP mirror servers have a
   "pref" value of 1.  By default, if unspecified then mirrors are

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   considered "normal" and do not share the same ETag policy.  FTP
   mirrors, as they do not emit ETags, MUST always be considered

   HTTP Mirror servers SHOULD support Instance Digests in HTTP

   [[Suggestion: In order for clients to identify servers that have
   coordinated ETag policies, the ETag MUST begin with "Metalink:", e.g.

   ETag: "Metalink:SHA=thvDyvhfIqlvFe+A9MYgxAfm1q5="


3.4.  Mirror Depth

   Some mirrors may mirror single files, whole directories, or multiple

   Mirror servers MAY have a "depth" value, where "depth=0" is the
   default.  A value of 0 means ONLY that file is mirrored.  A value of
   1 means that file and all other files and subdirectories in the
   directory are mirrored.  A value of 2 means the directory above, and
   all files and subdirectories, are 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=1; depth=4

   Is the above example, 4 directories up are mirrored, from /dir2/ on

4.  Peer-to-Peer / Metainfo

   Metainfo files, which describe ways to download a file over Peer-to-
   Peer networks or otherwise, are specified with the Link header
   [draft-nottingham-http-link-header] and a relation type of
   "describedby" and a type parameter that indicates the MIME type of
   the metadata available at the IRI.

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

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

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 checksums of chunks, allowing a client to recover
   from partial errors (see Section 7.1.2).

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".

   A brief Metalink server response with OpenPGP signature only:

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

6.  Checksums of Whole Files

   Metalink servers MUST provide Instance Digests in HTTP [RFC3230] for
   files they describe with mirrors.  Mirror servers SHOULD as well.

   A brief Metalink server response with checksum:

   Digest: SHA=thvDyvhfIqlvFe+A9MYgxAfm1q5=

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

   GET /distribution/example.ext HTTP/1.1

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   Host: www.example.com

   The Metalink server responds with the data and these headers:

   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=1
   Link: <ftp://ftp.example.com/example.ext>; rel="duplicate"
   Link: <http://example.com/example.ext.torrent>; rel="describedby";
   Link: <http://example.com/example.ext.metalink>; rel="describedby";
   Link: <http://example.com/example.ext.asc>; rel="describedby";
   Digest: SHA=thvDyvhfIqlvFe+A9MYgxAfm1q5=

   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 may 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 checksums to repair a
   o  Digital signature.
   o  Instance Digest, which is the whole file checksum.

   (Alternatively, the client could have requested a HEAD only, and then
   skipped to making the following decisions on every available mirror
   server found via the Link headers)

   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 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.

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   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 headers,
   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:

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

   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 MUST be aborted.

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

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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 Instance
   Digests.  Error dectection requires Instance Digests, or checksums,
   to determine after transfers if there has been an error.  Error
   correction, or download repair, is possible with partial file

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.
   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 may 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.

   ETag 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

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   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 headers 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
   differs, but the error is still detected when the merged response is

7.1.2.  Error Correction

   If the object checksum does not match the Instance Digest then fetch
   the Metalink/XML or other recovery profile link, where partial file
   checksums can be found, allowing detection of which server returned
   bad information.  If the Instance Digest computation does not match
   then the client needs to fetch the partial file checksums and from
   there figure out what of the downloaded data can be recovered and
   what needs to be fetched again.  If no partial checksums are
   available, then the client MUST fetch the complete object from a
   trusted Metalink server.

   Partial file checksums can be used to detect errors during the

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

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   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
   other mirror might offer better performance.

   The size of chunks chosen by the client should be sufficiently large
   that the chunk request headers and reponse headers 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.  Thus we currently recommend that a
   chunk size of at least 10KBytes should be used.  If the file being
   transfered is very large, or the download speed very high, this can
   be increased to perhaps 1MByte.  As network bandwidths increase, we
   expect these numbers to increase appropriately, so that the time to
   transfer a chunk remains significantly larger than the latency of
   requesting a chunk from a server.

9.  IANA Considerations

   Accordingly, IANA has made 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

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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 IRIs into

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" which is SHA-1, as specified in [RFC3174], 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

              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.

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   [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.

   [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.

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

11.2.  Informative References

              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 the Metalink community, Mark Handley, Mark Nottingham,
   Daniel Stenberg, Tatsuhiro Tsujikawa, Peter Poeml, and Matt Domsch.

   Support for simultaneous download from multiple mirrors is based upon
   work by Mark Handley and Javier Vela Diago, who 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. ]]

   This draft, compared to the Metalink/XML format

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   [draft-bryan-metalink] :

   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, unless we use Metalink/XML for partial file
   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 may be difficult or impossible unless you are in
      control of all servers on the mirror network.
   o  (-) Requires software or configuration changes to originating
   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, 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.  Metalink/XML is used to
      import/export transfer queues.  Not as easy for search engines to
   o  (-) No way to show mirror geographical location (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 the RFC editor before publication as an RFC. ]]

   Known issues concerning this draft:
   o  Use of Link header to describe Mirrors.  Only send a few mirrors
      with Link header, or only send them if Want-Digest is used?  Some
      organizations have many mirrors.
   o  Would it make more sense to use qvalue-style policies to describe
      mirror priority, i.e. q=1.0 through q=0.0 ?
   o  Will we use Metalink/XML for partial file checksums?  That would
      add XML dependency to apps for an important feature.
   o  Do we need an official MIME type for .torrent files or allow

   -11 : October 23, 2009.

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   o  Mirror changes.

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

   -09 : October 12, 2009.
   o  Mirror location, coordination, and depth.
   o  Split HTTP Digest Algorithm Values Registration into

   -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
   o  Remove Content-MD5 and Want-Digest.

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

   -04 : September 17, 2009.
   o  Temporarily remove .torrent.

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

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

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

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

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Authors' Addresses

   Anthony Bryan
   Pompano Beach, FL

   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/

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

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

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