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This document specifies Metalink in HTTP Headers, an alternative representation, instead of the usual XML-based download description format. Metalink describes alternate download locations (mirrors), Peer-to-Peer information, checksums, digital signatures, and other information. Clients can transparently use this information to reliably transfer files.
1.
Introduction
1.1.
Examples
1.2.
Notational Conventions
2.
Requirements
3.
Mirrors / Alternate Download Locations
4.
Peer-to-Peer
5.
OpenPGP Signatures
6.
Checksums
7.
Client / Server Multi-source Download Interaction
8.
Security Considerations
8.1.
URIs and IRIs
8.2.
Spoofing
8.3.
Cryptographic Hashes
8.4.
Signing
9.
References
9.1.
Normative References
9.2.
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
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Metalink in HTTP Headers is an alternative to Metalink, usually represented in 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.). Metalink in HTTP Headers attempts to duplicate as much functionality of the Metalink XML format with existing standards such as Web Linking [draft‑nottingham‑http‑link‑header] (Nottingham, M., “Web Linking,” July 2009.) and Instance Digests in HTTP [RFC3230] (Mogul, J. and A. Van Hoff, “Instance Digests in HTTP,” January 2002.). Metalinks list information about a file to be downloaded. This includes lists of alternate 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 At times, individual servers can be slow, outdated, or unreachable, but this can not be determined until the download has been initiated.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 alternate 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 discuss@apps.ietf.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 . ]]
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A brief Metalink server response with checksum, mirrors, .torrent, and OpenPGP signature:
Link: <http://www2.example.com/example.ext>; rel="alternate"; Link: <ftp://ftp.example.com/example.ext>; rel="alternate"; 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=
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This specification describes conformance of Metalink in HTTP Headers.
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.
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In this context, "Metalink" refers to Metalink 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.).
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 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.
Mirrors are 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 are typically FTP or HTTP servers. Mirror servers MAY be Metalink servers.
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Mirrors are specified with the Link header [draft‑nottingham‑http‑link‑header] (Nottingham, M., “Web Linking,” July 2009.) and a relation type of "alternate".
A brief Metalink server response with two mirrors only:
Link: <http://www2.example.com/example.ext>; rel="alternate"; Link: <ftp://ftp.example.com/example.ext>; rel="alternate";
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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";
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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";
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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. How will we provide chunk checksums? Content-MD5?]]
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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
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="alternate"; Link: <ftp://ftp.example.com/example.ext>; rel="alternate"; 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 requests to a mirror:
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 response, from a mirror, to the request above:
HTTP/1.1 206 Partial Content Accept-Ranges: bytes Content-Length: 7433801 Content-Range: bytes 7433802-14867602/14867603
[[Content-MD5 for chunk checksums? That 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.]]
Once the download has completed, the Metalink client should verify the checksum of the file.
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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.
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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.
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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 the IANA registry named "Hash Function Textual Names" 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.
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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.
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[BITTORRENT] | Cohen, B., “The BitTorrent Protocol Specification,” BITTORRENT 11031, February 2008. |
[RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997. |
[RFC2616] | Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” RFC 2616, June 1999. |
[RFC3174] | Eastlake, D. and P. Jones, “US Secure Hash Algorithm 1 (SHA1),” RFC 3174, September 2001. |
[RFC3230] | Mogul, J. and A. Van Hoff, “Instance Digests in HTTP,” RFC 3230, January 2002. |
[RFC3986] | Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” STD 66, RFC 3986, January 2005. |
[RFC3987] | Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” RFC 3987, January 2005. |
[draft-nottingham-http-link-header] | Nottingham, M., “Web Linking,” draft-nottingham-http-link-header-06 (work in progress), July 2009. |
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[draft-bryan-metalink] | Bryan, A., Ed., Tsujikawa, T., McNab, N., and P. Poeml, “The Metalink Download Description Format,” draft-bryan-metalink-15 (work in progress), August 2009. |
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Thanks!
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...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.) :
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[[ to be removed by the RFC editor before publication as an RFC. ]]
Known issues concerning this draft:
-00
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Anthony Bryan (editor) | |
Metalinker Project | |
Email: | anthonybryan@gmail.com |
URI: | http://www.metalinker.org |