Network Working Group A. Bryan, Ed. Internet-Draft Metalinker Project Intended status: Standards Track September 7, 2009 Expires: March 11, 2010 MetaLinkHeader: Mirrors and Checksums in HTTP Headers draft-bryan-metalinkhttp-02 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. 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The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on March 11, 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 11, 2010 [Page 1] Internet-Draft MetaLinkHeader: Mirrors and Checksums September 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 MetaLinkHeader: Mirrors and Checksums in HTTP Headers, an alternative representation of Metalink, instead of the usual 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Notational Conventions . . . . . . . . . . . . . . . . . . 4 2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Mirrors / Multiple Download Locations . . . . . . . . . . . . 5 4. Peer-to-Peer . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. OpenPGP Signatures . . . . . . . . . . . . . . . . . . . . . . 5 6. Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6.1. Checksums of Whole Files . . . . . . . . . . . . . . . . . 6 6.2. Checksums of Chunks of Files . . . . . . . . . . . . . . . 6 7. Client / Server Multi-source Download Interaction . . . . . . 7 8. Link Relation Type Registration: "duplicate" . . . . . . . . . 8 9. Security Considerations . . . . . . . . . . . . . . . . . . . 8 9.1. URIs and IRIs . . . . . . . . . . . . . . . . . . . . . . 8 9.2. Spoofing . . . . . . . . . . . . . . . . . . . . . . . . . 8 9.3. Cryptographic Hashes . . . . . . . . . . . . . . . . . . . 8 9.4. Signing . . . . . . . . . . . . . . . . . . . . . . . . . 9 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 10.1. Normative References . . . . . . . . . . . . . . . . . . . 9 10.2. Informative References . . . . . . . . . . . . . . . . . . 10 Appendix A. Acknowledgements and Contributors . . . . . . . . . . 10 Appendix B. What's different...?! (to be removed by RFC Editor before publication) . . . . . . . . . . . . . 10 Appendix C. Document History (to be removed by RFC Editor before publication) . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11 Bryan Expires March 11, 2010 [Page 2] Internet-Draft MetaLinkHeader: Mirrors and Checksums September 2009 1. Introduction MetaLinkHeader is an alternative to Metalink, usually represented in an XML-based document format [draft-bryan-metalink]. 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], Instance Digests in HTTP [RFC3230], and Content-MD5 [RFC1864]. 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 . ]] Bryan Expires March 11, 2010 [Page 3] Internet-Draft MetaLinkHeader: Mirrors and Checksums September 2009 1.1. Examples A brief Metalink server response with checksum, mirrors, .torrent, and OpenPGP signature: Link: ; rel="duplicate"; Link: ; rel="duplicate"; 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 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], 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]. 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]. 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]. 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 Bryan Expires March 11, 2010 [Page 4] Internet-Draft MetaLinkHeader: Mirrors and Checksums September 2009 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] 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] and a relation type of "duplicate" as defined in Section 8. A brief Metalink server response with two mirrors only: Link: ; rel="duplicate"; Link: ; rel="duplicate"; Mirror servers are listed in order of priority. 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"; Bryan Expires March 11, 2010 [Page 5] Internet-Draft MetaLinkHeader: Mirrors and Checksums September 2009 6. Checksums 6.1. Checksums of Whole Files 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 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] 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]. 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 Bryan Expires March 11, 2010 [Page 6] Internet-Draft MetaLinkHeader: Mirrors and Checksums September 2009 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] 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="duplicate"; Link: ; rel="duplicate"; 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 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 Bryan Expires March 11, 2010 [Page 7] Internet-Draft MetaLinkHeader: Mirrors and Checksums September 2009 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 an identical resource that is a byte-for- byte equivalence of representations. 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] 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 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 the IANA registry named "Hash Function Textual Names", Instance Digests in HTTP [RFC3230], and Content-MD5 header [RFC1864] 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. Bryan Expires March 11, 2010 [Page 8] Internet-Draft MetaLinkHeader: Mirrors and Checksums September 2009 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. 9.4. Signing Metalinks should include digital signatures, as described in Section 5. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. 10. References 10.1. Normative References [BITTORRENT] Cohen, B., "The BitTorrent Protocol Specification", BITTORRENT 11031, February 2008, . [RFC1864] Myers, J. and M. Rose, "The Content-MD5 Header Field", RFC 1864, October 1995. [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), Bryan Expires March 11, 2010 [Page 9] Internet-Draft MetaLinkHeader: Mirrors and Checksums September 2009 July 2009. 10.2. Informative References [draft-bryan-metalink] Bryan, A., Ed., Tsujikawa, T., McNab, N., and P. Poeml, "The Metalink Download Description Format", draft-bryan-metalink-16 (work in progress), August 2009. Appendix A. Acknowledgements and Contributors Thanks to Daniel Stenberg and Mark Nottingham. 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 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. Not as easy for search engines to index? o (-) No way to show mirror/p2p geographical location (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) Bryan Expires March 11, 2010 [Page 10] Internet-Draft MetaLinkHeader: Mirrors and Checksums September 2009 [[ to be removed by the RFC editor before publication as an RFC. ]] Known issues concerning this draft: o None. -02 : September 6, 2009. o Content-MD5 for chunk checksums. -01 : September 1, 2009. o Link Relation Type Registration: "duplicate" -00 : August 24, 2009. o Initial draft. Author's Address Anthony Bryan (editor) Metalinker Project Email: anthonybryan@gmail.com URI: http://www.metalinker.org Bryan Expires March 11, 2010 [Page 11]