Network Working Group A. Bryan
Internet-Draft N. McNab
Intended status: Standards Track H. Nordstrom
Expires: August 24, 2010
A. Ford
Roke Manor Research
February 20, 2010
Metalink/HTTP: Mirrors and Cryptographic Hashes in HTTP Headers
draft-bryan-metalinkhttp-15
Abstract
This document specifies Metalink/HTTP: Mirrors and Cryptographic
Hashes in HTTP Headers, 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 headers.
Clients can transparently use this information to make file transfers
more robust and reliable.
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
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on August 24, 2010.
Copyright Notice
Bryan, et al. Expires August 24, 2010 [Page 1]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
Copyright (c) 2010 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 BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Operation Overview . . . . . . . . . . . . . . . . . . . . 4
1.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Notational Conventions . . . . . . . . . . . . . . . . . . 5
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Mirrors / Multiple Download Locations . . . . . . . . . . . . 6
3.1. Mirror Priority . . . . . . . . . . . . . . . . . . . . . 6
3.2. Mirror Geographical Location . . . . . . . . . . . . . . . 6
3.3. Coordinated Mirror Policies . . . . . . . . . . . . . . . 7
3.4. Mirror Depth . . . . . . . . . . . . . . . . . . . . . . . 7
4. Peer-to-Peer / Metainfo . . . . . . . . . . . . . . . . . . . 7
4.1. Metalink/XML Files . . . . . . . . . . . . . . . . . . . . 8
5. OpenPGP Signatures . . . . . . . . . . . . . . . . . . . . . . 8
6. Cryptographic Hashes of Whole Files . . . . . . . . . . . . . 8
7. Client / Server Multi-source Download Interaction . . . . . . 9
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 . . . . . . . . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10. Security Considerations . . . . . . . . . . . . . . . . . . . 14
10.1. URIs and IRIs . . . . . . . . . . . . . . . . . . . . . . 14
10.2. Spoofing . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.3. Cryptographic Hashes . . . . . . . . . . . . . . . . . . . 14
10.4. Signing . . . . . . . . . . . . . . . . . . . . . . . . . 15
11. Normative References . . . . . . . . . . . . . . . . . . . . . 15
Appendix A. Acknowledgements and Contributors . . . . . . . . . . 16
Appendix B. Comparisons to Similar Options . . . . . . . . . . . 16
Appendix C. Document History . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
Bryan, et al. Expires August 24, 2010 [Page 2]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
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,
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
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, cryptographic
hashes, 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
Bryan, et al. Expires August 24, 2010 [Page 3]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
download even under adverse circumstances. All this is done
transparently to the user and the download is much more reliable and
efficient. In contrast, a traditional HTTP redirect to a mirror
conveys only extremely minimal information - one link to one server,
and there is no provision in the HTTP protocol to handle failures.
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
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==
Bryan, et al. Expires August 24, 2010 [Page 4]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
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.
2. Requirements
In this context, "Metalink" refers to Metalink/HTTP which consists of
mirrors and cryptographic hashes in HTTP Headers as described in this
document. "Metalink/XML" refers to the XML format described in
[draft-bryan-metalink].
Metalink resources include a Link header
[draft-nottingham-http-link-header] to present a list of mirrors in
the response to a client request for the resource. The cryptographic
hash of a resource must be included via Instance Digests in HTTP
[RFC3230].
Metalink servers are HTTP servers with one or more Metalink
resources. Mirror and cryptographic hash 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 (cryptographic hash) and not server-unique filesystem
metadata. The emitted ETag MAY be implemented the same as the
Instance Digest for simplicity. Metalink servers MAY 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 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
Bryan, et al. Expires August 24, 2010 [Page 5]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
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 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
[draft-nottingham-http-link-header] 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=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 URI 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
Bryan, et al. Expires August 24, 2010 [Page 6]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
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 HTTP mirror servers have a "pref" value of 1. By
default, if unspecified then mirrors are considered "normal" and do
not share the same ETag policy. FTP mirrors, as they do not emit
ETags, MUST always be considered "normal".
HTTP Mirror servers SHOULD support Instance Digests in HTTP
[RFC3230].
[[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
directories.
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
down.
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
Bryan, et al. Expires August 24, 2010 [Page 7]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
[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 URI.
A brief Metalink server response with .torrent and .metalink:
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"
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 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";
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. Mirror servers SHOULD as well.
A brief Metalink server response with cryptographic hash:
Digest: SHA-256=MWVkMWQxYTRiMzk5MDQ0MzI3NGU5NDEyZTk5OWY1ZGFmNzgyZTJlO
DYzYjRjYzFhOTlmNTQwYzI2M2QwM2U2MQ==
Bryan, et al. Expires August 24, 2010 [Page 8]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
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
mirrors.
GET /distribution/example.ext HTTP/1.1
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";
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 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 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
Bryan, et al. Expires August 24, 2010 [Page 9]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
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.
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:
Bryan, et al. Expires August 24, 2010 [Page 10]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
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 MUST be aborted.
Once the download has completed, the Metalink client MUST verify the
cryptographic hash of 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 Instance
Digests. Error dectection 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.
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
Bryan, et al. Expires August 24, 2010 [Page 11]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
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 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
verified.
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, but they are encouraged to do so.
If the object cryptographic hash does not match the Instance Digest
then fetch the Metalink/XML as specified in Section 4.1, where
partial file cryptographic hashes may 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
Bryan, et al. Expires August 24, 2010 [Page 12]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
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
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.
Bryan, et al. Expires August 24, 2010 [Page 13]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
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.
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" which is SHA-256, as specified in
[FIPS-180-3], or stronger. It MAY also include other hashes.
Bryan, et al. Expires August 24, 2010 [Page 14]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
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. Normative References
[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.
[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.
[RFC3230] Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
RFC 3230, January 2002.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[draft-bryan-metalink]
Bryan, A., Ed., Tsujikawa, T., McNab, N., and P. Poeml,
"The Metalink Download Description Format",
draft-bryan-metalink-28 (work in progress), February 2010.
[draft-nottingham-http-link-header]
Nottingham, M., "Web Linking",
draft-nottingham-http-link-header-07 (work in progress),
January 2010.
Bryan, et al. Expires August 24, 2010 [Page 15]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
Appendix A. Acknowledgements and Contributors
Thanks to the Metalink community, Mark Handley, Mark Nottingham,
Daniel Stenberg, Tatsuhiro Tsujikawa, Peter Poeml, Matt Domsch, Micah
Cowan, and David Morris.
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
[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, 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 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
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, 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
index?
o (-) Not as rich metadata.
o (-) Not able to add multiple files to a download queue or create
directory structure.
Bryan, et al. Expires August 24, 2010 [Page 16]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
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 Would it make more sense to use qvalue-style policies to describe
mirror priority, i.e. q=1.0 through q=0.0 ?
o Using Metalink/XML for partial file cryptographic hashes. That
adds XML dependency to apps for an important feature. Is there a
better method?
o Do we need an "official" MIME type for .torrent files or allow
"application/x-bittorrent"?
-15 : December 31, 2009.
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 12, 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.
Bryan, et al. Expires August 24, 2010 [Page 17]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
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.
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 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/
Bryan, et al. Expires August 24, 2010 [Page 18]
Internet-Draft Metalink/HTTP: Mirrors and Hashes February 2010
Alan Ford
Roke Manor Research
Old Salisbury Lane
Romsey, Hampshire SO51 0ZN
UK
Phone: +44 1794 833 465
Email: alan.ford@roke.co.uk
Bryan, et al. Expires August 24, 2010 [Page 19]