Internet Engineering Task Force (IETF) A. Bryan

Internet Engineering Task Force (IETF) A. Bryan
Request for Comments: 6249 N. McNab
Category: Standards Track T. Tsujikawa
ISSN: 2070-1721
P. Poeml
MirrorBrain
H. Nordstrom
June 2011

Metalink/HTTP: Mirrors and Hashes

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. Metalink clients can use this information to make file
transfers more robust and reliable. Normative requirements for
Metalink/HTTP clients and servers are described here.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6249.

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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.

Table of Contents

1. Introduction ....................................................3
1.1. Example Metalink Server Response ...........................4
1.2. Notational Conventions .....................................4
1.3. Terminology ................................................5
2. Requirements ....................................................5
3. Mirrors / Multiple Download Locations ...........................7
3.1. Mirror Priority ............................................8
3.2. Mirror Geographical Location ...............................8
3.3. Coordinated Mirror Policies ................................8
3.4. Mirror Depth ...............................................9
4. Peer-to-Peer / Metainfo .........................................9
4.1. Metalink/XML Files ........................................10
5. Signatures .....................................................10
5.1. OpenPGP Signatures ........................................10
5.2. S/MIME Signatures .........................................10
6. Cryptographic Hashes of Whole Documents ........................11
7. Client / Server Multi-Source Download Interaction ..............11
7.1. Error Prevention, Detection, and Correction ...............15
7.1.1. Error Prevention (Early File Mismatch Detection) ...15
7.1.2. Error Correction ...................................16
8. IANA Considerations ............................................16
9. Security Considerations ........................................17
9.1. URIs and IRIs .............................................17
9.2. Spoofing ..................................................17
9.3. Cryptographic Hashes ......................................17
9.4. Signing ...................................................17
10. References ....................................................18
10.1. Normative References .....................................18
10.2. Informative References ...................................19
Appendix A. Acknowledgements and Contributors .....................20

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1. Introduction

Metalink/HTTP is an alternative and complementary 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
cannot 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 fall back to
other mirrors if the current one has an issue. With this knowledge,

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the client is enabled to work its way to a successful download even
under adverse circumstances. All this can be done without
complicated user interaction, and the download can be much more
reliable and efficient. In contrast, a traditional HTTP redirect to
a mirror conveys only 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.

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 maximize the download rate.

1.1. Example Metalink Server Response

This example shows a brief Metalink server response with ETag,
mirrors, Peer-to-Peer information, Metalink/XML, 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.meta4>; rel=describedby;
type="application/metalink4+xml"
Link: <http://example.com/example.ext.asc>; rel=describedby;
type="application/pgp-signature"
Digest: SHA-256=MWVkMWQxYTRiMzk5MDQ0MzI3NGU5NDEyZTk5OWY1ZGFmNzgyZTJlO
DYzYjRjYzFhOTlmNTQwYzI2M2QwM2U2MQ==

1.2. 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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1.3. Terminology

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

o Metalink server: HTTP server that provides a Metalink in HTTP
response header fields.

o Metalink : A collection of HTTP response header fields from a
Metalink server, which is the reply to a GET or HEAD request from
a client and which includes Link header fields listing mirrors and
Instance Digests listing a cryptographic hash.

o Link header field: HTTP response header field, defined in
[RFC5988], that can list mirrors and, potentially, other download
methods for obtaining a file, along with digital signatures.

o Instance Digest: HTTP response header field, defined in [RFC3230],
that contains the cryptographic hash of a file, which is used by
the Metalink client to verify the integrity of the file once the
download has completed.

o Entity Tag or ETag: HTTP response header field, defined in
[RFC2616], that, if synchronized between the Metalink server and
mirror servers, allows Metalink clients to provide advanced
features.

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

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

o Metalink/XML: An XML file that can contain similar information to
an HTTP response header Metalink, such as mirrors and
cryptographic hashes.

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].

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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]. Algorithms used in the
Instance Digest field are registered in the IANA registry named
"Hypertext Transfer Protocol (HTTP) Digest Algorithm Values" at
<http://www.iana.org/>. This document restricts the use of these
algorithms. SHA-256 and SHA-512 were added to the registry by
[RFC5843]. Metalinks contain whole file hashes as described in
Section 6, and MUST include SHA-256, as specified in [FIPS-180-3].
It MAY also include other hashes.

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
preferred mirror servers MUST all share the same ETag policy.
Metalink servers and their associated normal mirror servers SHOULD
all share the same ETag policy. (See Section 3.3 for the definition
of "preferred" and "normal" mirror servers.) It is up to the
administrator of the Metalink server to communicate the details of
the shared ETag policy to the administrators of the mirror servers so
that the mirror servers can be configured with 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. For example, it would be better to derive an
ETag from a cryptographic hash of the file contents than on server-
unique filesystem metadata. Metalink servers SHOULD offer Metalink/
XML documents that contain cryptographic hashes of parts of the file
(and other information) if error recovery is desirable.

Mirror servers are typically FTP or HTTP servers that "mirror"
another server. That is, they provide identical copies of (at least
some) files that are also on the mirrored server. Mirror servers
SHOULD support serving partial content. HTTP mirror servers SHOULD
share the same ETag policy as the originating Metalink server. HTTP
mirror servers SHOULD support Instance Digests in HTTP [RFC3230]
using the same algorithm as the Metalink server. Optimally, HTTP
mirror servers will share the same ETag policy and support Instance
Digests in HTTP. Mirror servers that share the same ETag policy
and/or support Instance Digests in HTTP using the same algorithm as a
Metalink server are known as preferred mirror servers.

Metalink clients use the mirrors provided by a Metalink server in
Link header fields [RFC5988] but these clients are restricted to
using the mirrors provided by the initial Metalink server they

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contacted. If Metalink clients find Link header fields [RFC5988]
from mirrors that in turn list mirrors, or from a Metalink server
listing itself as a mirror, they MUST discard such Link header fields
[RFC5988] to prevent a possible infinite loop. 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 [RFC3230] by requesting and verifying cryptographic
hashes. Metalink clients SHOULD support error recovery by using the
cryptographic hashes of parts of the file listed in Metalink/XML
files. Metalink clients SHOULD support checking digital signatures.

3. Mirrors / Multiple Download Locations

Mirrors are specified with the Link header fields [RFC5988] and a
relation type of "duplicate" as defined in Section 8.

The following list contains OPTIONAL attributes, which are defined
elsewhere in this document:

o "depth" : mirror depth (see Section 3.4).

o "geo" : mirror geographical location (see Section 3.2).

o "pref" : a preferred mirror server (see Section 3.3).

o "pri" : mirror priority (see Section 3.1).

This example shows 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

As some organizations can have many mirrors, it is up to the
organization to configure the amount of Link header fields the
Metalink server will provide. Such a decision could be a random
selection or a hard-coded limit based on network proximity, file
size, server load, or other factors.

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3.1. Mirror Priority

Entries for mirror servers MAY have a "pri" value to designate the
priority of a mirror. Valid ranges for the "pri" attribute are from
1 to 999999. Mirror servers with a lower value of the "pri"
attribute have a higher priority, while mirrors with an undefined
"pri" attribute are considered to have a value of 999999, which is
the lowest priority. For example, a mirror with "pri=10" has higher
priority than a mirror with "pri=20". Metalink clients SHOULD use
mirrors with lower "pri" values first, but depending on other
conditions, they MAY decide to use other mirrors.

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 server MAY have a "geo" value, which is an
[ISO3166-1] alpha-2 two-letter country code for the geographical
location of the physical server the URI is used to access. A client
MAY use this information to select a mirror, or set of mirrors, that
is 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. Entries
for preferred HTTP mirror servers have a "pref" value and entries for
normal mirrors don't. Preferred mirror servers are HTTP mirror
servers that MUST share the same ETag policy as the originating
Metalink server, or if the ETag is not used MUST provide an Instance
Digest using the same algorithm as the Metalink server. Preferred
mirrors make it possible for Metalink clients to detect early on,
before data is transferred, if the file requested matches the desired
file. This early file mismatch detection is described in
Section 7.1.1. Normal mirrors do not necessarily share the same ETag
policy or support Instance Digests using the same algorithm as the
Metalink server. FTP mirrors are considered "normal", as they do not
emit ETags or support Instance Digests.

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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 that only that file is mirrored
and that other URI path segments are not. A value of 1 means that
the file and all other files and URI path segments contained in the
rightmost URI path segment are mirrored. For values of N, N-1 URI
path segments closer to the Host are mirrored. A value of 2 means
one URI path segment closer to the Host is mirrored, and all files
and URI path segments contained are mirrored. For each higher value,
another URI path segment closer to the Host is mirrored.

This example shows 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, four URI path segments closer to the Host are
mirrored, from /dir2/ and all files and directories included.

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 MAY still have the same content, an OPTIONAL
"name" attribute can be used.

The following list contains an OPTIONAL attribute, which is defined
in this document:

o "name" : a file described within the metainfo file.

This example shows a brief Metalink server response with .torrent and
.meta4:

Link: <http://example.com/example.ext.torrent>; rel=describedby;
type="application/x-bittorrent"; name="differentname.ext"
Link: <http://example.com/example.ext.meta4>; rel=describedby;
type="application/metalink4+xml"

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

4.1. Metalink/XML Files

Metalink/XML files for a given resource MAY be provided in a Link
header field as shown in the example in Section 4. Metalink/XML
files are specified in [RFC5854], and they are particularly useful
for providing metadata such as cryptographic hashes of parts of a
file, allowing a client to recover from errors (see Section 7.1.2).
Metalink servers SHOULD provide Metalink/XML files with partial file
hashes in Link header fields, and Metalink clients SHOULD use them
for error recovery.

5. Signatures

5.1. OpenPGP Signatures

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

This example shows a brief Metalink server response with OpenPGP
signature only:

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

Metalink clients SHOULD support the use of OpenPGP signatures.

5.2. S/MIME Signatures

Secure/Multipurpose Internet Mail Extensions (S/MIME) signatures
[RFC5751] of requested files are specified with the Link header
fields [RFC5988] and a relation type of "describedby" and a type
parameter of "application/pkcs7-mime".

This example shows a brief Metalink server response with S/MIME
signature only:

Link: <http://example.com/example.ext.p7m>; rel=describedby;
type="application/pkcs7-mime"

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

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6. Cryptographic Hashes of Whole Documents

If Instance Digests are not provided by the Metalink servers, the
Link header fields pertaining to this specification MUST be ignored.

This example shows a brief Metalink server response with ETag,
mirror, and cryptographic hash:

Etag: "thvDyvhfIqlvFe+A9MYgxAfm1q5="
Link: <http://www2.example.com/example.ext>; rel=duplicate
Digest: SHA-256=MWVkMWQxYTRiMzk5MDQ0MzI3NGU5NDEyZTk5OWY1ZGFmNzgyZTJlO
DYzYjRjYzFhOTlmNTQwYzI2M2QwM2U2MQ==

7. Client / Server Multi-Source Download Interaction

Metalink clients begin a download with a standard HTTP [RFC2616] GET
request to the Metalink server. Metalink clients MAY use a range
limit if desired.

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.meta4>; rel=describedby;
type="application/metalink4+xml"
Link: <http://example.com/example.ext.asc>; rel=describedby;
type="application/pgp-signature"
Digest: SHA-256=MWVkMWQxYTRiMzk5MDQ0MzI3NGU5NDEyZTk5OWY1ZGFmNzgyZTJlO
DYzYjRjYzFhOTlmNTQwYzI2M2QwM2U2MQ==

Alternatively, Metalink clients can begin with a HEAD request to the
Metalink server to discover mirrors via Link header fields and then
skip to making the following decisions on every available mirror
server found via the Link header fields.

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

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

o Mirror locations, with optional attributes describing the mirror's
priority, whether it shares the ETag policy of the originating
Metalink server, geographical location, and mirror depth.

o Instance Digest, which is the whole file cryptographic hash.

o ETag.

o Object size from the Content-Length header field.

o Metalink/XML, which can include partial file cryptographic hashes
to repair a file.

o Peer-to-Peer information.

o Digital signature.

Next, the Metalink client requests a range of the object from a
preferred mirror server, so it can use If-Match conditions:

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

Metalink clients SHOULD use preferred mirrors, if possible, as they
allow early file mismatch detection as described in Section 7.1.1.
Preferred mirrors have coordinated ETags, as described in
Section 3.3, and Metalink clients SHOULD use If-Match conditions
based on the ETag to quickly detect out-of-date mirrors by using the
ETag from the Metalink server response. Metalink clients SHOULD use
partial file cryptographic hashes as described in Section 7.1.2, if
available, to detect if the mirror server returned the correct data.

Optimally, the mirror server also will include an Instance Digest in
the mirror response to the client GET request, which the client can
also use to detect a mismatch early. Metalink clients MUST reject
individual downloads from mirrors that support Instance Digests if
the Instance Digest from the mirror does not match the Instance
Digest as reported by the Metalink server and the same algorithm is

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used. If normal mirrors are used, then a mismatch cannot be detected
until the completed object is verified. Errors in transmission and
substitutions of incorrect data on mirrors, whether deliberate or
accidental, can be detected with error correction as described in
Section 7.1.2.

Here, the preferred mirror server has the correct file (the If-Match
conditions match) and responds with a 206 Partial Content HTTP status
code and appropriate "Content-Length", "Content-Range", ETag, and
Instance Digest header fields. In this example, the mirror server
responds, with data, to the above request:

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

Metalink clients MAY start a number of parallel range requests (one
per selected mirror server other than the first) using mirrors
provided by the Link header fields with "duplicate" relation type.
Metalink clients MUST limit the number of parallel connections to
mirror servers, ideally based on observing how the aggregate
throughput changes as connections are opened. It would be pointless
to blindly open connections once the path bottleneck is filled.
After establishing a new connection, a Metalink client SHOULD monitor
whether the aggregate throughput increases over all connections that
are part of the download. The client SHOULD NOT open additional
connections during this period. If the aggregate throughput has
increased, the client MAY open an additional connection and repeat
these steps. Otherwise, the client SHOULD NOT open a new connection
until an established one closes. Metalink clients SHOULD use the
location of the original GET request in the "Referer" header field
for these range requests.

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

Metalink clients SHOULD close all but the fastest connection if any
range requests generated after the first request end up with a
complete response, instead of a partial response (as some mirrors

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might not support HTTP ranges), if the goal is the fastest transfer.
Metalink clients MAY monitor mirror conditions and dynamically switch
between mirrors to achieve the fastest download possible. Similarly,
Metalink clients SHOULD abort extremely slow or stalled range
requests and finish the request on other mirrors. If all ranges have
finished except for the final one, the Metalink client can split the
final range into multiple range requests to other mirrors so the
transfer finishes faster.

If the first request was a GET, no Range header field was sent, and
the client determines later that it will issue a range request, then
the client SHOULD close the first connection when it catches up with
the other parallel range requests of the same object. This means the
first connection was sacrificed. Metalink clients can use a HEAD
request first, if possible, so that the client can find out if there
are any Link header fields, and then range-based requests are
undertaken to the mirror servers without sacrificing a first
connection.

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

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

Metalink clients MUST verify the cryptographic hash of the file once
the download has completed. If the cryptographic hash offered 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 cannot be repaired, it is considered corrupt. The
client can attempt to re-download the file.

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

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RFC 6249 Metalink/HTTP: Mirrors and Hashes June 2011

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 provided by Metalink servers. Error detection requires
Instance Digests to detect errors in transfer after the transfers
have completed. Error correction, or download repair, is possible
with partial file cryptographic hashes.

Note that cryptographic hashes obtained from Instance Digests are in
base64 encoding, while those from Metalink/XML are in hexadecimal.

7.1.1. Error Prevention (Early File Mismatch Detection)

In HTTP terms, the merging of ranges from multiple responses SHOULD
be verified with a strong validator, which in this context is either
an Instance Digest or a shared ETag from that Metalink server that
matches with the Instance Digest or ETag provided by a preferred
mirror server. 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 shared ETag policy or Instance Digests as well. There is
no need to specify how the ETag is generated, just that it needs to
be shared between the Metalink server and the mirror servers. The
benefit of having mirror servers return an Instance Digest is that
the client then can detect mismatches early even if ETags are not
used. Mirrors that support both a shared ETag and Instance Digests
do provide value, but just one is sufficient for early detection of
mismatches. If the mirror server provides neither shared ETag nor
Instance Digest, then early detection of mismatches is not possible
unless file length also differs. Finally, errors are still
detectable after the download has completed, when the cryptographic
hash of the merged response is verified.

ETags cannot be used for verifying the integrity of the received
content. If the ETag given by the mirror server matches the ETag
given by the Metalink server, then the Metalink client assumes the
responses are valid for that object.

This guarantees that a mismatch will be detected by using only the
shared ETag from a Metalink server and mirror server. Metalink
clients will detect an error if ETags do not match, which will
prevent accidental merges of ranges from different versions of files
with the same name.

A shared ETag or Instance Digest cannot strictly protect against
malicious attacks or server or network errors replacing content. An
attacker can make a mirror server seemingly respond with the expected

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Instance Digest or ETags even if the file contents have been
modified. The same goes for various system failures, which would
also cause bad data (i.e., corrupted files) to be returned. The
Metalink client has to rely on the Instance Digest returned by the
Metalink server in the first response for the verification of the
downloaded object as a whole. To verify the individual ranges, which
might have been requested from different sources, see Section 7.1.2.

7.1.2. Error Correction

Partial file cryptographic hashes can be used to detect errors during
the download. Metalink servers SHOULD provide Metalink/XML files
with partial file hashes in Link header fields as specified in
Section 4.1, and Metalink clients SHOULD use them for error
correction.

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

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

8. IANA Considerations

Accordingly, IANA has made the following registration to the "Link
Relation Types" registry at <http://www.iana.org/>.

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.

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9. Security Considerations

9.1. URIs and IRIs

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

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 into downloading a malicious or worthless file.
Metalink clients are advised to prevent loops, possibly from a mirror
server to a Metalink server and back again, in Section 2. As with
all downloads, users should only download from trusted sources.
Also, malicious publishers could attempt a distributed denial-of-
service attack by inserting unrelated URIs into Metalinks. [RFC4732]
contains information on amplification attacks and denial-of-service
attacks.

9.3. 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.

9.4. Signing

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

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

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RFC 6249 Metalink/HTTP: Mirrors and Hashes June 2011

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>.

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

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

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

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

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

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

[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.

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

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

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RFC 6249 Metalink/HTTP: Mirrors and Hashes June 2011

[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.

10.2. Informative References

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

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

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RFC 6249 Metalink/HTTP: Mirrors and Hashes June 2011

Appendix A. Acknowledgements and Contributors

Thanks to the Metalink community, Alexey Melnikov, Julian Reschke,
Mark Nottingham, Daniel Stenberg, Matt Domsch, Micah Cowan, David
Morris, Yves Lafon, Juergen Schoenwaelder, Ben Campbell, Lars Eggert,
Sean Turner, Robert Sparks, and the HTTPBIS Working Group.

Thanks to Alan Ford and Mark Handley for spurring us on to publish
this document.

This document is dedicated to Zimmy Bryan, Juanita Anthony, and Janie
Burnett.

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RFC 6249 Metalink/HTTP: Mirrors and Hashes June 2011

Authors' Addresses

Anthony Bryan
Pompano Beach, FL
USA

EMail: [email protected]
URI: http://www.metalinker.org

Neil McNab

EMail: [email protected]
URI: http://www.nabber.org

Tatsuhiro Tsujikawa
Shiga
Japan

EMail: [email protected]
URI: http://aria2.sourceforge.net

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

Phone: +49 221 6778 333 8
EMail: [email protected]
URI: http://mirrorbrain.org/~poeml/

Henrik Nordstrom

EMail: [email protected]
URI: http://www.henriknordstrom.net/

Bryan, et al. Standards Track [Page 21]