Network Working Group L. Wood Internet-Draft P. Holliday Intended status: Experimental Cisco Systems Expires: September 14, 2008 March 13, 2008 Using HTTP for delivery in Delay/Disruption-Tolerant Networks draft-wood-dtnrg-http-dtn-delivery-01 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of 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 September 14, 2008. Abstract This document describes how to use the Hypertext Transfer Protocol, HTTP, for communication across delay- and disruption-tolerant networks. HTTP is well-known and straightforward to implement in these networks. Wood & Holliday Expires September 14, 2008 [Page 1] Internet-Draft HTTP for DTN delivery March 2008 Table of Contents 1. Background and Introduction . . . . . . . . . . . . . . . . . 3 2. Adapting the HTTP delivery mechanism for DTNs . . . . . . . . 4 3. Other useful proposed HTTP headers . . . . . . . . . . . . . . 6 4. Other suggestions on HTTP for dtn use . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 8.1. Normative References . . . . . . . . . . . . . . . . . . . 8 8.2. Informative References . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9 Intellectual Property and Copyright Statements . . . . . . . . . . 10 Wood & Holliday Expires September 14, 2008 [Page 2] Internet-Draft HTTP for DTN delivery March 2008 1. Background and Introduction Delay- and Disruption-Tolerant Networks (DTNs) are networks where conditions are such that links between nodes are not always permanent, may be of very long delay or exist only during very short contact periods where the link is up, and may change over time [RFC4838]. Some DTNs can be thought of as sparse ad-hoc networks, with nodes communicating intermittently only when they come into contact. Store-and-forward delivery of data is a useful way of communicating across these networks. A specialised store-and-forward protocol for DTN delivery has been proposed in the IRTF DTN research group (DTNRG) - the Bundle Protocol [RFC5050]. Criticisms of the Bundle Protocol's reliability and complexity have been raised [I-D.irtf-dtnrg-bundle-checksum]. The Bundle Protocol is itself intended to be a routable data format, but the supporting architectures for node and application naming/ addressing, automated routing, security, QoS, and resource discovery have not yet been agreed upon or in some cases even significantly worked on. These things already exist for the Internet Protocol, and can be leveraged. This document outlines how the well-known Hypertext Transfer Protocol (HTTP) [RFC2616] can be used for store-and-forward communication across DTNs. HTTP is not used end-to-end as it is on the web. Instead, applications running on each node in the network communicate with their neighbours using dedicated hop-by-hop HTTP transfers to effect local data delivery. Additional HTTP header information adds context for onward forwarding and delivery to destination endpoints, and provides the reliability and error-detection missing from alternatives such as the Bundle Protocol. It must be stressed that this proposed use is distinct from proxy caching methods prevalent in the traditional web. Caching commands are not used; end-to-end HTTP requests are not intercepted by intermediate caches that attempt to fulfil them. The distinction between client, server and proxy is replaced by peers using HTTP to communicate hop-by-hop. HTTP is a session layer, running over a transport layer providing reliable delivery of the HTTP stream between hops. This transport layer is commonly (and almost universally) TCP in the terrestrial Internet, although alternative transport layers, such as SCTP, can also be used under HTTP. For long-delay networks, or for network conditions where TCP or an equivalent is not suitable, an alternative transport layer such as Saratoga [I-D.wood-tsvwg-saratoga] can be used under HTTP instead in hop-by-hop communications between nodes. HTTP requires only reliable streaming that can be used to provide Wood & Holliday Expires September 14, 2008 [Page 3] Internet-Draft HTTP for DTN delivery March 2008 ordered delivery. Steve Deering has often described IP as 'the waist in the hourglass' [Deering98] - what is above and touching on IP can be changed, what is below and touching on IP can be changed, but provided the new elements continue to interface to and work with IP, the hourglass remains complete and the network stack remains functional. Here, HTTP is the waist in this particular hourglass; applications can use HTTP to communicate, provided HTTP runs over a reliable transport stream. The applications can vary. The transport stream can be changed; HTTP does not have to run over TCP/IP, but could even be made to run directly over HDLC or a CCSDS reliable bitstream. This document contains an overview of how HTTP can be simply adapted to the DTN environment by the use of HTTP/1.1 with persistence and pipelining, the PUT and GET directives, and some trivial extra HTTP headers needed to indicate e.g. a destination in the DTN network. The remainder of this specification uses 'file' as a shorthand for 'binary object', which may be an HTTP 'object', file with an associated MIMEtype, or other type of contiguous binary data. A significant benefit to use of HTTP is that the well-known MIMEtype mechanism, integral to HTTP, provides hints on what received files are, and what applications should do with them [RFC2045]. The Bundle Protocol does not support MIMEtypes, or any similar mechanism. 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 RFC 2119. [RFC2119] 2. Adapting the HTTP delivery mechanism for DTNs Here, HTTP is used as a peer-to-peer protocol in the sense that multiple files may be transferred in both directions simultaneously between two communicating nodes using HTTP for DTN use. There is not intended to be a strict client/user-agent to server relationship as there is in the web. Instead, sending data across a path of six nodes, four nodes between source and destination, will require a minimum of five separate per-hop HTTP transactions between each pair of nodes to move the data onwards to the next node. This breaks the traditional end-to-end control loop and transfer into separate control loops and transfers suitable for the DTN environment. When two nodes come into contact, a request for files to be copied, stored, and carried onwards can be made by the receiving node issuing an HTTP GET request. Alternatively, the sending node can simply Wood & Holliday Expires September 14, 2008 [Page 4] Internet-Draft HTTP for DTN delivery March 2008 issue a series of HTTP PUT requests once a connection is established, if it believes that putting the data to the receiving node moves it closer to its eventual destination. The receiving node can always reject transfers with error codes. HTTP/1.1 pipelining and persistence permits multiple PUTs to be made in sequence. Support for these in implementations is crucial to the mechanisms outlined here. The key to enabling HTTP use for DTN networking is an added Content- Destination: header, which specifies the final destination of the file, and can be used by routing in the HTTP-using applications to decide over which links the file should be sent. Content-* headers are special, in that they may not be ignored (section 9.6 of [RFC2119]). Recipients not understanding Content-Destination: will generate a 501 (Not Implemented) error code. This separates HTTP use in DTNs described here from normal end-to-end HTTP web use. HTTP DTN nodes MUST support Content-Destination: The information provided in Content-Destination: identifying the destination may be an IP address, DNS name, Bundle Endpoint Identifier (EID) or other text-string identifier useful to the local DTN routing mechanisms being used. Similarly, a Content-Source: header provides the original source of the data. This MUST be implemented. For DTN use, DTN HTTP nodes MUST also implement Content-Length:, Content-Range: and Content-MD5 headers. This permits partial delivery of files and resends of missing pieces. The Content-MD5: header provides a simple end-to-end reliability check. The Content- MD5: header is intended to be generated by the source node first sending the data, and not recomputed at other nodes. DTN HTTP nodes MUST implement the Host: header. This field MAY be left blank to request available files from the peer node, rather than identifying a desired file from a distant source by hostname. A sender placing a new file into the DTN network for onward transmission MUST have the Content-Source: field of the data being sent match its Host: field. Hop-by-hop HTTP headers MAY be implemented. The headers described in section 13.5.1 of [RFC2616] are available. New hop-by-hop headers MUST use the Connection: header approach described in section 14.10 of [RFC2616]. DTN HTTP nodes may optionally GET and PUT to link-local multicast addresses. This permits efficient sharing of files on shared LANs, Wood & Holliday Expires September 14, 2008 [Page 5] Internet-Draft HTTP for DTN delivery March 2008 with recipients requesting resends via Content-Range: and checking assembly of file pieces using the Content-MD5: header. A GET to multicast can request a specific file from any available node that has it. The response to a multicast GET SHOULD be unicast, but a multicast HEAD MAY also be sent to inform other nodes that the sender has the file of interest. If other nodes also express interest in the file with GET requests to the sender, that file may later be PUT to a multicast address. The utility of HTTP with multicast has been recognised previously as a method of simple service discovery later adopted for the universal plug and play (UPnP) protocol [I-D.draft-goland-http-udp] [I-D.draft-cai-ssdp-v1]. Rather than call out multicast and unicast separately as different protocols to be used by HTTP, recognising that a given destination or address indicates multicast or broadcast use should suffice. 3. Other useful proposed HTTP headers A number of other HTTP headers are proposed here, as likely to be useful. These SHOULD be implemented. An HTTP object is just one binary file; the ability to group objects together is useful (and is done in bundles by the bundle protocol). If we call a group of related objects sent from the same source to the same destination a 'package' (a name chosen to avoid any confusion with the 'bundle' specification), we can then define simple headers to be sent before each object: Package-ID: - provides a unique text-string identifier for the package Package-Item: n of m (e.g. 1 of 7) - order of this HTTP file in the package Package-MD5: - checksum across all Content-MD5 headers added together in order A way to request missing Package-Items (from the previous node or from the source) is likely to be very useful. Some sort of header protection is likely also a good idea. So, Header-MD5: could cover some important HTTP headers. Header-MD5 could be preserved across hops if possible, avoiding unnecessary header reordering. Timestamps prevent this, however - this needs more thought, particularly on where timestamps are placed in HTTP headers. Wood & Holliday Expires September 14, 2008 [Page 6] Internet-Draft HTTP for DTN delivery March 2008 Timestamps and how they are handled needs to be examined here in greater detail. What if different machines have different notions of time? For larger files, stronger checksums than MD5 should be looked at. 4. Other suggestions on HTTP for dtn use x-application-dtn has previously been proposed as a MIMEtype identifying Bundle Protocol bundles delivered by HTTP. This provides a way to support legacy Bundle Protocol implementations in an HTTP infrastructure. Moving HTTP transfers over DTN networks using the Bundle Protocol has already been proposed [Ott06]. By changing how HTTP is used - hop- by-hop rather than end-to-end - this draft has outlined how HTTP can be used directly and independently in DTN networks without requiring the bundle protocol as a carrier. 5. Security Considerations Security considerations and examination of HTTP Security (HTTPS) is required here. Because there is a need for each node to validate that a file has been received correctly, privately-keyed hashes that can only be checked at the destination should be avoided, and HTTP security mechanisms should be used instead. 6. IANA Considerations It may make sense to use a non-standard port for HTTP use over IP, rather than the well-known port 80. If so, such a port should be requested from IANA. It may be necessary to request a dedicated IPv4 all-hosts multicast address and a dedicated IPv6 link-local multicast addresses for local HTTP DTN use, if local HTTP multicast is considered desirable. 7. Acknowledgements Work on the Saratoga protocol inspired some of the concepts used here. We thank Wes Eddy and Kevin Fall for their review comments. Wood & Holliday Expires September 14, 2008 [Page 7] Internet-Draft HTTP for DTN delivery March 2008 8. References 8.1. Normative References [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [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. 8.2. Informative References [Deering98] Deering, S., "Watching the Waist of the Protocol Hourglass", keynote, IEEE International Conference on Network Protocols (ICNP), Austin Texas, October 1998. [I-D.draft-cai-ssdp-v1] Goland, Y., Cai, T., Leach, P., Gu, Y., and S. Albright, "Simple Service Discovery Protocol/1.0 Operating without an Arbiter", draft-cai-ssdp-v1-03 (expired) , October 1999. [I-D.draft-goland-http-udp] Goland, Y., "Multicast and Unicast UDP HTTP Messages", draft-goland-http-udp-01 (expired) , November 1999. [I-D.irtf-dtnrg-bundle-checksum] Eddy, W., Wood, L., and W. Ivancic, "Checksum Ciphersuites for the Bundle Protocol", draft-irtf-dtnrg-bundle-checksum-02 (work in progress), March 2008. [I-D.wood-tsvwg-saratoga] Wood, L., McKim, J., Eddy, W., Ivancic, W., and C. Jackson, "Saratoga: A Scalable File Transfer Protocol", draft-wood-tsvwg-saratoga-01 (work in progress) , February 2008. [Ott06] Ott, J. and D. Kutscher, "Bundling the Web: HTTP over DTN", WNEPT 2006 Workshop on Networking in Public Transport, QShine Conference Ontario, August 2006. Wood & Holliday Expires September 14, 2008 [Page 8] Internet-Draft HTTP for DTN delivery March 2008 [RFC4838] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst, R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant Networking Architecture", RFC 4838, April 2007. [RFC5050] Scott, K. and S. Burleigh, "Bundle Protocol Specification", RFC 5050, November 2007. Authors' Addresses Lloyd Wood Cisco Systems 11 New Square Park, Bedfont Lakes Feltham, Middlesex TW14 8HA United Kingdom Phone: +44-20-8824-4236 Email: lwood@cisco.com Peter Holliday Cisco Systems Level 12 300 Adelaide Street Brisbane, Queensland 4000 Australia Phone: +61-2-6216-0604 Email: phollida@cisco.com Wood & Holliday Expires September 14, 2008 [Page 9] Internet-Draft HTTP for DTN delivery March 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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