From: http://www.ietf.org/internet-drafts/draft-bryan-metalinkhttp-00.txt Title: Metalink in HTTP Headers Reference: IETF Network Working Group, Internet Draft 'draft-bryan-metalinkhttp-00' Date: August 28, 2009 I-D Tracker: http://ietfreport.isoc.org/idref/draft-bryan-metalinkhttp/ Tools: http://tools.ietf.org/html/draft-bryan-metalinkhttp-00 (HTML) Announced: http://www.ietf.org/mail-archive/web/i-d-announce/current/msg26978.html See also: The Metalink Download Description Format http://xml.coverpages.org/draft-bryan-metalink-15.txt Google Groups Metalink Discussion http://groups.google.com/group/metalink-discussion SourceForge SCM Repositories (includes XML source files http://metalinks.svn.sourceforge.net/viewvc/metalinks/internetdraft/ ============================================================================== Network Working Group A. Bryan, Ed. Internet-Draft Metalinker Project Intended status: Standards Track August 28, 2009 Expires: March 1, 2010 Metalink in HTTP Headers draft-bryan-metalinkhttp-00 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. 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 March 1, 2010. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Bryan Expires March 1, 2010 [Page 1] Internet-Draft Metalink in HTTP Headers August 2009 Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Notational Conventions . . . . . . . . . . . . . . . . . . 4 2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Mirrors / Alternate Download Locations . . . . . . . . . . . . 5 4. Peer-to-Peer . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. OpenPGP Signatures . . . . . . . . . . . . . . . . . . . . . . 5 6. Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 5 7. Client / Server Multi-source Download Interaction . . . . . . 6 8. Security Considerations . . . . . . . . . . . . . . . . . . . 7 8.1. URIs and IRIs . . . . . . . . . . . . . . . . . . . . . . 7 8.2. Spoofing . . . . . . . . . . . . . . . . . . . . . . . . . 7 8.3. Cryptographic Hashes . . . . . . . . . . . . . . . . . . . 7 8.4. Signing . . . . . . . . . . . . . . . . . . . . . . . . . 8 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 9.1. Normative References . . . . . . . . . . . . . . . . . . . 8 9.2. Informative References . . . . . . . . . . . . . . . . . . 8 Appendix A. Acknowledgements and Contributors . . . . . . . . . . 9 Appendix B. What's different...?! (to be removed by RFC Editor before publication) . . . . . . . . . . . . . 9 Appendix C. Document History (to be removed by RFC Editor before publication) . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10 Bryan Expires March 1, 2010 [Page 2] Internet-Draft Metalink in HTTP Headers August 2009 1. Introduction Metalink in HTTP Headers is an alternative to Metalink, usually represented in an XML-based document format [draft-bryan-metalink]. 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] and Instance Digests in HTTP [RFC3230]. 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 . ]] Bryan Expires March 1, 2010 [Page 3] Internet-Draft Metalink in HTTP Headers August 2009 1.1. Examples A brief Metalink server response with checksum, mirrors, .torrent, and OpenPGP signature: Link: ; rel="alternate"; Link: ; rel="alternate"; Link: ; rel="describedby"; type="torrent"; Link: ; rel="describedby"; type="application/pgp-signature"; Digest: SHA=thvDyvhfIqlvFe+A9MYgxAfm1q5= 1.2. Notational Conventions 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], as scoped to those conformance targets. 2. Requirements 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]. Metalink servers are HTTP servers that MUST have lists of mirrors and use the Link header [draft-nottingham-http-link-header] to indicate them. They also MUST provide checksums of files via 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 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] 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. Bryan Expires March 1, 2010 [Page 4] Internet-Draft Metalink in HTTP Headers August 2009 3. Mirrors / Alternate Download Locations Mirrors are specified with the Link header [draft-nottingham-http-link-header] and a relation type of "alternate". A brief Metalink server response with two mirrors only: Link: ; rel="alternate"; Link: ; rel="alternate"; 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] and a relation type of "describedby" and a type parameter of "torrent" for .torrent [BITTORRENT] files. A brief Metalink server response with .torrent only: Link: ; rel="describedby"; type="torrent"; 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: ; rel="describedby"; type="application/pgp-signature"; 6. Checksums Instance Digests in HTTP [RFC3230] 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 Bryan Expires March 1, 2010 [Page 5] Internet-Draft Metalink in HTTP Headers August 2009 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?]] 7. Client / Server Multi-source Download Interaction Metalink clients begin a download with a standard HTTP [RFC2616] 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: ; rel="alternate"; Link: ; rel="alternate"; Link: ; rel="describedby"; type="torrent"; Link: ; 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: Bryan Expires March 1, 2010 [Page 6] Internet-Draft Metalink in HTTP Headers August 2009 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. 8. Security Considerations 8.1. 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. 8.2. 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. 8.3. Cryptographic Hashes Currently, some of the hash types defined in Instance Digests in HTTP [RFC3230] 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, it SHOULD include "sha" which is SHA-1, as specified in [RFC3174]. It MAY also include other hashes. Bryan Expires March 1, 2010 [Page 7] Internet-Draft Metalink in HTTP Headers August 2009 8.4. Signing Metalinks should include digital signatures, as described in Section 5. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. 9. References 9.1. Normative References [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. 9.2. Informative References [draft-bryan-metalink] Bryan, A., Ed., Tsujikawa, T., McNab, N., and P. Poeml, "The Metalink Download Description Format", Bryan Expires March 1, 2010 [Page 8] Internet-Draft Metalink in HTTP Headers August 2009 draft-bryan-metalink-15 (work in progress), August 2009. Appendix A. Acknowledgements and Contributors Thanks! Appendix B. What's different...?! (to be removed by RFC Editor before publication) ...or missing, compared to the Metalink XML format [draft-bryan-metalink] : o (+) Reuses existing standards without defining much new stuff. It's more of a collection/coordinated feature set. o (+) No XML dependency. 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 changes to server software. o (-?) 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. o (-) Metalink XML could 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 to all download information with no changes needed to the server. o (-) Not portable/archivable/emailable. Not as easy for search engines to index? o (-) No way to show mirror/p2p priority or geographical location (yet). o (---) No chunk checksums/download repair (yet). o (-) No checksums besides MD5/SHA-1 (yet). o (-) Not as rich metadata. o (-) Not able to add multiple files to a download queue or create directory structure. Appendix C. Document History (to be removed by RFC Editor before publication) [[ to be removed by the RFC editor before publication as an RFC. ]] Known issues concerning this draft: Bryan Expires March 1, 2010 [Page 9] Internet-Draft Metalink in HTTP Headers August 2009 o None. -00 o Initial draft. Author's Address Anthony Bryan (editor) Metalinker Project Email: anthonybryan@gmail.com URI: http://www.metalinker.org Bryan Expires March 1, 2010 [Page 10]