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

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Abstract

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

Table of Contents

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

1.  Introduction

MetaLinkHeader is an alternative to Metalink, usually an XML-based document format [draft‑bryan‑metalink] (Bryan, A., Ed., Tsujikawa, T., McNab, N., and P. Poeml, “The Metalink Download Description Format,” August 2009.). MetaLinkHeader attempts to provide as much functionality as the Metalink XML format by using existing standards such as Web Linking [draft‑nottingham‑http‑link‑header] (Nottingham, M., “Web Linking,” July 2009.), Instance Digests in HTTP [RFC3230] (Mogul, J. and A. Van Hoff, “Instance Digests in HTTP,” January 2002.), and Content-MD5 [RFC1864] (Myers, J. and M. Rose, “The Content-MD5 Header Field,” October 1995.). MetaLinkHeader is used to list information about a file to be downloaded. This includes lists of multiple URIs (mirrors), Peer-to-Peer information, checksums, and digital signatures.

Identical copies of a file are frequently accessible in multiple locations on the Internet over a variety of protocols (FTP, HTTP, and Peer-to-Peer). In some cases, Users are shown a list of these multiple download locations (mirrors) and must manually select a single one on the basis of geographical location, priority, or bandwidth. This distributes the load across multiple servers. At times, individual servers can be slow, outdated, or unreachable, but this can not be determined until the download has been initiated. This can lead to the user canceling the download and needing to restart it. During downloads, errors in transmission can corrupt the file. There are no easy ways to repair these files. For large downloads this can be extremely troublesome. Any of the number of problems that can occur during a download lead to frustration on the part of users.

All the information about a download, including mirrors, checksums, digital signatures, and more can be transferred in coordinated HTTP Headers. This Metalink transfers the knowledge of the download server (and mirror database) to the client. Clients can fallback to other mirrors if the current one has an issue. With this knowledge, the client is enabled to work its way to a successful download even under adverse circumstances. All this is done transparently to the user and the download is much more reliable and efficient. In contrast, a traditional HTTP redirect to a mirror conveys only extremely minimal information - one link to one server, and there is no provision in the HTTP protocol to handle failures. Other features that some clients provide include multi-source downloads, where chunks of a file are downloaded from multiple mirrors (and optionally, Peer-to-Peer) simultaneously, which frequently results in a faster download.

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

1.1.  Examples

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

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

1.2.  Notational Conventions

This specification describes conformance of MetaLinkHeader.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.), as scoped to those conformance targets.

2.  Requirements

In this context, "MetaLink" refers to a MetaLinkHeader which consists of mirrors and checksums in HTTP Headers as described in this document. "Metalink XML" refers to the XML format described in [draft‑bryan‑metalink] (Bryan, A., Ed., Tsujikawa, T., McNab, N., and P. Poeml, “The Metalink Download Description Format,” August 2009.).

Metalink servers are HTTP servers that MUST have lists of mirrors and use the Link header [draft‑nottingham‑http‑link‑header] (Nottingham, M., “Web Linking,” July 2009.) to indicate them. They also MUST provide checksums of files via Instance Digests in HTTP [RFC3230] (Mogul, J. and A. Van Hoff, “Instance Digests in HTTP,” January 2002.). Mirror and checksum information provided by the originating Metalink server is considered authoritative.

Mirror servers are typically FTP or HTTP servers that "mirror" another server. That is, they provide identical copies of (at least some) files that are also on the mirrored server. Mirror servers MAY be Metalink servers. Mirror servers MUST support serving partial content. Mirror servers SHOULD support Instance Digests in HTTP [RFC3230] (Mogul, J. and A. Van Hoff, “Instance Digests in HTTP,” January 2002.).

Metalink clients use the mirrors provided by a Metalink server with Link header [draft‑nottingham‑http‑link‑header] (Nottingham, M., “Web Linking,” July 2009.). Metalink clients MUST support HTTP and MAY support FTP, BitTorrent, or other download methods. Metalink clients MUST switch downloads from one mirror to another if the one mirror becomes unreachable. Metalink clients are RECOMMENDED to support multi-source, or parallel, downloads, where chunks of a file are downloaded from multiple mirrors simultaneously (and optionally, Peer-to-Peer). Metalink clients MUST support Instance Digests in HTTP [RFC3230] (Mogul, J. and A. Van Hoff, “Instance Digests in HTTP,” January 2002.) 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] (Nottingham, M., “Web Linking,” July 2009.) and a relation type of "duplicate" as defined in Section 8 (Link Relation Type Registration: "duplicate").

A brief Metalink server response with two mirrors only:

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

Mirror servers are listed in order of priority.

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

4.  Peer-to-Peer

Ways to download a file over Peer-to-Peer networks are specified with the Link header [draft‑nottingham‑http‑link‑header] (Nottingham, M., “Web Linking,” July 2009.) and a relation type of "describedby" and a type parameter of "torrent" for .torrent [BITTORRENT] (Cohen, B., “The BitTorrent Protocol Specification,” February 2008.) files.

A brief Metalink server response with .torrent only:

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

5.  OpenPGP Signatures

OpenPGP signatures are specified with the Link header [draft‑nottingham‑http‑link‑header] (Nottingham, M., “Web Linking,” July 2009.) 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";

6.  Checksums

6.1.  Checksums of Whole Files

Instance Digests in HTTP [RFC3230] (Mogul, J. and A. Van Hoff, “Instance Digests in HTTP,” January 2002.) are used to request and retrieve whole file checksums.

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

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

A brief Metalink server response with checksum:

Digest: SHA=thvDyvhfIqlvFe+A9MYgxAfm1q5=

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

6.2.  Checksums of Chunks of Files

The Content-MD5 header [RFC1864] (Myers, J. and M. Rose, “The Content-MD5 Header Field,” October 1995.) provides checksums for a chunk, or portion, of a file, when requested with a Range header field.

Negotiation of Content-MD5 is described in [RFC3230] (Mogul, J. and A. Van Hoff, “Instance Digests in HTTP,” January 2002.). A checksum for a chunk of a file can determine if there has been an error in transmission, which means the file is corrupt. If an error is detected in a chunk, then just that chunk can be requested again from the current mirror, or a different mirror.

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

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

A brief Metalink server response with checksum:

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

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

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

7.  Client / Server Multi-source Download Interaction

Metalink clients begin a download with a standard HTTP [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) GET request to the Metalink server. Here the client prefers SHA-1 checksums over MD5:

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

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

The Metalink server responds with this:

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

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

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

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

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

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

8.  Link Relation Type Registration: "duplicate"

o Relation Name: duplicate

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

o Reference: This specification.

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

9.  Security Considerations

9.1.  URIs and IRIs

Metalink clients handle URIs and IRIs. See Section 7 of [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) and Section 8 of [RFC3987] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) 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 that they are downloading a malicious or worthless file. Also, malicious publishers could attempt a distributed denial of service attack by inserting unrelated IRIs into Metalinks.

9.3.  Cryptographic Hashes

Currently, some of the hash types defined in Instance Digests in HTTP [RFC3230] (Mogul, J. and A. Van Hoff, “Instance Digests in HTTP,” January 2002.) and Content-MD5 header [RFC1864] (Myers, J. and M. Rose, “The Content-MD5 Header Field,” October 1995.) are considered insecure. These include the whole Message Digest family of algorithms which are not suitable for cryptographically strong verification. Malicious people could provide files that appear to be identical to another file because of a collision, i.e. the weak cryptographic hashes of the intended file and a substituted malicious file could match.

If a Metalink contains hashes as described in Section 6 (Checksums), it SHOULD include "sha" which is SHA-1, as specified in [RFC3174] (Eastlake, D. and P. Jones, “US Secure Hash Algorithm 1 (SHA1),” September 2001.). It MAY also include other hashes.

9.4.  Signing

Metalinks should include digital signatures, as described in Section 5 (OpenPGP Signatures).

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

10.  References

10.1. Normative References

10.2. Informative References

Appendix A.  Acknowledgements and Contributors

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

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

...or missing, compared to the Metalink XML format [draft‑bryan‑metalink] (Bryan, A., Ed., Tsujikawa, T., McNab, N., and P. Poeml, “The Metalink Download Description Format,” August 2009.) :

• (+) Reuses existing standards without defining much new stuff. It's more of a collection/coordinated feature set.
• (+) No XML dependency.
• (-?) Tied to HTTP, not as generic. FTP/P2P clients won't be using it unless they also support HTTP, unlike Metalink XML.
• (---) Requires changes to server software.
• (-?) Could require some coordination of all mirror servers for all features, which may be difficult or impossible unless you are in control of all servers on the mirror network.
• (-) Metalink XML can be created by user (or server, but server component/changes not required).
• (-) 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.
• (-) Not portable/archivable/emailable. Not as easy for search engines to index?
• (-) No way to show mirror/p2p geographical location (yet).
• (-) No checksums besides MD5/SHA-1 (yet).
• (-) Not as rich metadata.
• (-) Not able to add multiple files to a download queue or create directory structure.

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

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

Known issues concerning this draft:

• None.

-03 : September 16, 2009.

• Mention HEAD request, negotiate mirrors if Want-Digest is used.

-02 : September 6, 2009.

• Content-MD5 for chunk checksums.

-01 : September 1, 2009.

• Link Relation Type Registration: "duplicate"

-00 : August 24, 2009.

• Initial draft.

Author's Address