Network Working Group L. Wood
Internet-Draft University of Surrey
Intended status: Experimental P. Holliday
Expires: November 13, 2011 Cisco Systems
May 12, 2011
Using HTTP for delivery in Delay/Disruption-Tolerant Networks
draft-wood-dtnrg-http-dtn-delivery-07
Abstract
This document describes how to use the Hypertext Transfer Protocol,
HTTP, for communication across delay- and disruption-tolerant
networks, by making every transit node in the network HTTP-capable,
and doing peer HTTP transfers between nodes to move data hop-by-hop
or subnet-by-subnet towards its final destination. HTTP is well-
known and straightforward to implement in these networks.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on November 13, 2011.
Copyright Notice
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Background and Introduction . . . . . . . . . . . . . . . . . 3
2. Adapting the HTTP delivery mechanism for DTNs . . . . . . . . 5
3. Other useful proposed additional HTTP headers . . . . . . . . 7
4. Other suggestions on using MIME in DTN networks . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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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 lack of reliability
and its complexity have been made [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 in many cases be easily leveraged for DTN networks [Wood09a].
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 or subnet-by-subnet
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 support for
error-detection currently missing from the alternative 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 in the traditional
web caching sense.
Although HTTP-DTN use as as a hop-by-hop message carrier between
caches implementing some form of routing protocol between them, the
distinction between client, server and proxy is replaced by peer
intermediate caches using HTTP to communicate in separate sessions
that together combine over time to make the full path between
original source and final destination for the data.
HTTP is a session layer, running over a transport layer providing
reliable delivery of the HTTP stream between hops. This transport
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layer is commonly (and almost universally) TCP in the terrestrial
Internet, although alternative transport layers, such as SCTP, can
also be used under HTTP [I-D.natarajan-http-over-sctp]. 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 ordered delivery; how that
reliable streaming is provided is up to the local transport layer in
the local subnet, and multiple different transport layers can be used
across the multiple hops between nodes to transfer data from source
to final destination.
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 e.g. HDLC or a CCSDS reliable bitstream.
Given the prevalence of IP in many networks, it is likely that two
waists exist; IP and HTTP are likely choices, but the transport
protocol and physical enviroment will vary more. An expansion of
this argument is given in [Wood09b]
Separation of HTTP from the underlying transport layer to make HTTP a
layer in its own right is increasingly likely to happen; this is
analogous to the use of different "convergence layers" under the
Bundle Protocol. Being able to set what transport layer to use
depending on conditions is useful, and one simple configuration
approach to this, able to support HTTP-DTN, was outlined in
[I-D.wood-tae-specifying-uri-transports].
HTTP use here relies on the three P's - Persistence, Pipelining and
the PUT directive. These are all present in the HTTP/1.1
specification.
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.
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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. HTTP/
1.1's use of MIME is specified in [RFC2616] rather than in the
separate MIME documents.
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 across a local hop or a subnet, 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 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-DTN is a superset of HTTP/1.1. 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. (Note that [I-D.natarajan-http-over-sctp] also takes advantage
of HTTP pipelining and persistence.)
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 available links the file should be sent. Content-*
headers are special, in that they may not be ignored (section 9.6 of
[RFC2616]). 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
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DTN nodes MUST support the Content-Destination: header. Files that
are PUT are cached and then relayed onwards by intermediate peers to
the final destination that is indicated by the Content-Destination:
header. GET requests for files can be forwarded by intermediate
peers to that final destination that is indicated by the Content-
Destination: header.
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 a textual identification
of the original source of the data. HTTP-DTN nodes MUST support the
Content-Source: header.
For DTN use, DTN HTTP nodes MUST also implement and use Content-
Length: and Content-Range: headers. These permit partial delivery of
files and resends of missing pieces of files. The Content-MD5:
header must be supported. This 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 is not
recomputed at other nodes.
DTN HTTP nodes MUST implement the Host: header, in line with current
HTTP specifications. This header 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 matching the advertised
Content-Source: header. 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 between peer nodes talking
directly. 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 from and PUT to link-local IP
multicast addresses when used over IP subnets. This permits
efficient sharing of files on shared LANs, 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.
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(Note that in the alternative Bundle Protocol, the Bundle Endpoint
Identifier (EID) can identify a group of endpoints, rather than just
one; mapping the Bundle EID onto multicast IP adddresses on IP
subnets is possible. Placing textual EIDs directly in HTTP-DTN's
Content-Source: and Content-Destination: headers, or in a Host:
field, would be possible to interwork HTTP-DTN and bundling.)
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.
Many existing HTTP/1.1 headers are directly useful with HTTP-DTN.
For example, ETag: headers are useful for identifying unique copies
of files in the network, and can be used to provide globally unique
identifiers (GUIDs) for each version of a file. Age: headers are
useful for estimating the amount of time a MIME object has been in
the network - indicating both transmission and storage times. Last-
Modified: times refer to the times on the origin server - that is,
the Content-Source: - and should be preserved during onward
forwarding. Max-Forwards: provides a TTL hop count and propagation
limitation mechanism.
3. Other useful proposed additional HTTP headers
A number of other additional HTTP headers are proposed here, as
likely to be useful. These SHOULD be implemented. These would
benefit from being specified more completely, in line with the
suggestions in [RFC2774].
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 Protocol specification), we can then define
simple headers to be sent before each object:
Package-ID: - provides a unique textual identifier for the package
Package-Item: n of m (e.g. 1 of 7) - order of this HTTP file in the
package
Package-MD5: - MD5 hash across all Content-MD5: headers added
together in order of Package-Item: precedence.
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A way to request missing Package-Items (from the previous node or
from the source) is likely to be very useful.
Precedence: headers could set importance of objects - very-high,
high, normal low, very-low - to give simple quality of service and
prioritization.
Some sort of header protection may be a good idea; Content-MD5:
covers the message body (entity-body), but not the headers. Header-
MD5: could cover some important HTTP headers. Header-MD5 could be
preserved across hops if possible, avoiding unnecessary header
reordering. Changing timestamps would invalidate the Header-MD5:
end-to-end, however - this needs more thought, particularly on where
timestamps are placed in HTTP headers.
For larger files, stronger mechanisms than MD5 should be examined.
There may be a need to send HTTP-DTN transfers across paths that
include hops with unidirectional one-way links with no return path,
e.g. when a wireless sender knows that a receiver is available, but
cannot hear it. Using: Connection: cannot-hear-response could be
used across that hop to indicate that the sender cannot hear
receivers.
Timestamps and how they are handled needs to be examined here in
greater detail. HTTP has the same basic assumption as the Bundle
Protocol - that all nodes are expected to know the current UTC time.
4. Other suggestions on using MIME in DTN networks
x-application-dtn has previously been proposed as a MIMEtype
identifying Bundle Protocol bundles delivered by HTTP. This provides
a way to support 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 - HTTP can be used directly in DTN
networks without using the Bundle Protocol at all.
HTTP is a popular way to carry MIME, but support for MIME exists in
other protocols, including email, SIP and BEEP. BEEP can be thought
of as a more formalised and exactly-specified replacement for HTTP
for machine-machine interaction - and this detailed formal
specification makes BEEP complex [RFC3080]. BEEP provides an
alternative to HTTP to support XML-RPC and SOAP. BEEP's
specification is formally intended to support multiple different
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transports, but only TCP transport of BEEP has been agreed [RFC3081].
HTTP's simplicity of use and popularity appear to be compelling
advantages over BEEP.
5. Security Considerations
Better-Than-Nothing Security [RFC5386][RFC5387] is likely to be
useful here for ad-hoc communications without the availability of an
existing authentication infrastructure.
Security considerations and detailed examination of HTTP over TLS
(HTTPS) [RFC2817][RFC2818] and secure HTTP [RFC2660] are required
here.
Many existing security mechanism for HTTP could be used unchanged for
HTTP-DTN, if local conditions permit and the supporting
infrastructure, e.g. DNS, is available. However, reusing these
directly protects a single-hop transfer between peer nodes. To
protect an end-to-end transfer, the security mechanisms would need to
be applied using the information used in the Content-Source: and
Content-Destination: headers, before applying the local security
mechanism for the first peer-peer HTTP transfer.
6. IANA Considerations
Despite the Content-* rule for rejecting unfamiliar headers that
separates HTTP-DTN peers from traditional HTTP servers, it may be
desirable to use a non-standard port for DTN 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 a desirable
feature.
7. Acknowledgements
We thank Wes Eddy and Kevin Fall for their review comments.
Work on the Saratoga protocol inspired some of the concepts that are
reused here, and we thank everyone involved in Saratoga's development
and implementation.
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8. References
8.1. Normative References
[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.
[RFC2774] Nielsen, H., Leach, P., and S. Lawrence, "An HTTP
Extension Framework", RFC 2774, February 2000.
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, "Reliability-only
Ciphersuites for the Bundle Protocol",
draft-irtf-dtnrg-bundle-checksum-09 (work in progress),
May 2011.
[I-D.natarajan-http-over-sctp]
Natarajan, P., Amer, P., Leighton, J., and F. Baker,
"Using SCTP as a Transport Layer Protocol for HTTP",
draft-natarajan-http-over-sctp-02 (work in progress),
July 2009.
[I-D.wood-tae-specifying-uri-transports]
Wood, L., "Specifying transport mechanisms in Uniform
Resource Identifiers",
draft-wood-tae-specifying-uri-transports-08 (work in
progress), May 2010.
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[I-D.wood-tsvwg-saratoga]
Wood, L., Eddy, W., Smith, C., Ivancic, W., and C.
Jackson, "Saratoga: A Scalable File Transfer Protocol",
draft-wood-tsvwg-saratoga-08 (work in progress),
December 2010.
[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.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2660] Rescorla, E. and A. Schiffman, "The Secure HyperText
Transfer Protocol", RFC 2660, August 1999.
[RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within
HTTP/1.1", RFC 2817, May 2000.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3080] Rose, M., "The Blocks Extensible Exchange Protocol Core",
RFC 3080, March 2001.
[RFC3081] Rose, M., "Mapping the BEEP Core onto TCP", RFC 3081,
March 2001.
[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.
[RFC5386] Williams, N. and M. Richardson, "Better-Than-Nothing
Security: An Unauthenticated Mode of IPsec", RFC 5386,
November 2008.
[RFC5387] Touch, J., Black, D., and Y. Wang, "Problem and
Applicability Statement for Better-Than-Nothing Security
(BTNS)", RFC 5387, November 2008.
[Wood09a] Wood, L., Eddy, W., and P. Holliday, "A Bundle of
Problems", IEEE Aerospace Conference, Big Sky, Montana,
March 2009.
[Wood09b] Wood, L., Holliday, P., Floreani, D., and I. Psaras,
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"Moving data in DTNs with HTTP and MIME: Making use of
HTTP for delay- and disruption-tolerant networks with
convergence layers", Workshop on the Emergence of Delay-/
Disruption-Tolerant Networks (e-DTN 2009), St Petersburg,
Russia, October 2009.
Authors' Addresses
Lloyd Wood
Centre for Communication Systems Research, University of Surrey
Guildford, Surrey GU2 7XH
United Kingdom
Phone: +44-1483-689123
Email: L.Wood@surrey.ac.uk
Peter Holliday
Cisco Systems
Level 12
300 Adelaide Street
Brisbane, Queensland 4000
Australia
Phone: +61-2-6216-0604
Email: phollida@cisco.com
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