Internet DRAFT - draft-wood-dtnrg-saratoga
draft-wood-dtnrg-saratoga
Network Working Group L. Wood
Internet-Draft Surrey alumni
Intended status: Experimental J. McKim
Expires: October 21, 2014 RSIS
W. Eddy
MTI Systems
W. Ivancic
NASA
C. Jackson
SSTL
April 19, 2014
Using Saratoga with a Bundle Agent as a Convergence Layer for Delay-
Tolerant Networking
draft-wood-dtnrg-saratoga-14
Abstract
Saratoga is a simple, lightweight, UDP-based transfer protocol. This
describes how to use Saratoga as a Delay-Tolerant Networking (DTN)
"convergence layer" with the Bundle Protocol and its Bundle Agents,
building on the Saratoga specification in draft-wood-tsvwg-saratoga.
Status of This Memo
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Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Applicability Statement . . . . . . . . . . . . . . . . . . . 2
3. Using Saratoga with a DTN Bundle Agent . . . . . . . . . . . 4
4. Proactive and Reactive Fragmentation . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. A Note on Naming . . . . . . . . . . . . . . . . . . . . . . 7
8. Informative References . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Background
The Saratoga protocol is specified in [I-D.wood-tsvwg-saratoga].
Saratoga can optionally be used for Delay/Disruption-Tolerant
Networking (DTN) [RFC4838], as a "convergence layer" to exchange
bundles between peer nodes. Saratoga was originally designed prior
to work on the Bundle Protocol [RFC5050] by the DTN Research Group
(DTNRG). It was later recognized that Saratoga could also be used to
reliably exchange bundles between DTNRG Bundle Agents by using a
logical mapping from DTNRG bundles to Saratoga files and back. The
DTN concept encompasses networks where ad-hoc, intermittent
connectivity is expected, connections may be infrequently established
or short-lived, and end-to-end paths are not present. As Saratoga
was designed for intermittent, disrupted, space communications,
Saratoga's operating environment is a DTN network. This makes the
Saratoga transfer protocol a natural fit as a convergence layer for
the DTNRG's Bundle Protocol, although the Bundle Protocol is not
necessary for operation of Saratoga.
This document contains notes on use of Saratoga for the bundle
transfer procedure.
Bundle transfers over Saratoga from space and the UK-DMC satellite,
with use of proactive fragmentation, have now been demonstrated.
More details are provided in [IAC-2008] [SSTL-2008] [Ivancic10].
2. Applicability Statement
Why use Saratoga as a DTN convergence layer? The DTN architecture
already has a number of choices of convergence layer. Convergence
layers have been proposed for various link types, e.g. Ethernet or
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Bluetooth. As IP already runs over many link types, a convergence
layer that can run over many links using IP is likely to take
advantage of TCP or UDP.
For traversing the terrestrial Internet while supporting congestion
control, a simple TCP convergence layer has been implemented in the
DTN software reference implementation [I-D.irtf-dtnrg-tcp-clayer]. A
simple UDP convergence layer, able to be used over dedicated private
links where congestion control is not required, is also present.
However, that simple UDP convergence layer presumes that a bundle
will always fit into a single UDP packet, does not support
segmentation of bundles across multiple UDP packets, and does not
guarantee reliable delivery with retransmissions. Its use is
discouraged [I-D.irtf-dtnrg-udp-clayer].
Two protocols capable of supporting segmentation of large bundles
across multiple UDP packets, with ARQ-based flexible delivery robust
to packet loss, are Saratoga [I-D.wood-tsvwg-saratoga] and the
Licklider Transmission Protocol (LTP) [RFC5325].
Both Saratoga and LTP were designed based on experience gained with
using the CCSDS File Delivery Protocol (CFDP), which was developed
for the Consultative Committee for Space Data Systems (CCSDS). The
main design difference between LTP and Saratoga is that LTP transfers
arbitrary un-named data blobs (binary large objects), requiring a
higher layer (normally delay-tolerant-networking bundling) to handle
naming, while Saratoga transfers named files including file metadata,
and can be independent of higher layers. Both protocols can run over
the User Datagram Protocol, UDP [RFC0768], though LTP also is
intended to run at other layers in the stack (including directly over
the link), while Saratoga is only intended to run above the UDP or
UDP-Lite transport protocols. If errors in delivered content can be
tolerated (perhaps because the data being transferred has its own
integrity checks), Saratoga can also be used to transfer an entire
file or stream without error checking, using UDP-Lite [RFC3828],
which can protect only header content from errors.
Saratoga includes a file checksum mechanism to detect transfer errors
and to provide an overall degree of reliability. Licklider has no
similar reliability mechanism, although Licklider's optional security
mechanism [RFC5327] can be implemented to give some error detection.
Saratoga can also be used for delivery over unidirectional broadcast
links. Another UDP-based convergence layer proposed for
unidirectional links is Uni-DTN [I-D.kutscher-dtnrg-uni-clayer].
Uni-DTN is based on FLUTE forward layered coding for multicast
delivery. Saratoga presumes that the forward error coding needed to
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prevent errors in transmission is present at another layer in the
stack, usually near the physical layer.
3. Using Saratoga with a DTN Bundle Agent
While Saratoga was first developed for efficient file transfer, the
similarity between bundle payloads and files, in that both are
arbitrary blobs of some number of octets, allows Saratoga to be used
as a convergence layer for exchanging bundles between DTN bundle
agents. This section explains the basic concepts involved in mapping
bundle exchange onto the file transfer mechanism.
Routing of bundles is outside the scope of Saratoga and of this
document. Once a complete bundle file has been transferred between
peers using Saratoga, that bundle can be forwarded onwards along a
next available hop in any way. Saratoga provides a mechanism for
forwarding, but does not provide input to routing or forwarding
decisions.
A DTN bundle agent can work alongside a Saratoga peer to move
bundles. One simple method of communicating bundles between the
bundle agent and the Saratoga peer is to have a shared directory that
is accessible to both the bundle and Saratoga processes. To send a
bundle, the bundle agent can place the complete bundle (the
concatenated set of Bundle Protocol blocks) into a file in this
shared directory. The local Saratoga instance is then able to _put_
this bundle to peers or allow them to _get_ it. A flag bit in the
Saratoga METADATA and DATA packets indicates whether a particular
file is a bundle or not. This enables the receiving Saratoga peer to
know whether to handle the file itself, or to pass it to the local
bundle agent.
When using Saratoga as a convergence layer to transfer bundles, the
local bundle agent will either place bundles as files for Saratoga to
transfer from its directory, or otherwise use interprocess
communication to notify Saratoga of and provide a bundle to be
transferred.
Key to the use of Saratoga for bundle transfer are:
- indicating the capability to interoperate with a local bundle
agent. This involves advertising the capability to handle bundles
via setting Flag bit 10 in Saratoga BEACON packets, and indicating
when a bundle is being transferred by setting Flag bits 10 and 11 in
the METADATA and DATA packets.
- identifying the Bundle Agent in use, by providing an Endpoint
Identifier (EID) in the Saratoga BEACON packet.
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Note that the name of a file holding a bundle is actually
unimportant, as long as it can be determined that it does hold a
bundle. The filename becomes temporary, and local only to the
transfer. One implementation strategy is to name each bundle file
with a file name constructed from two fields of the Primary Bundle
Header: the DTN Endpoint Identifier (EID) of the destination node and
the bundle's creation time field. In the rare case of filename
collisions in using this scheme, additional octets can be appended to
the filename following some arbitrary local scheme. Bundle files
might be placed in different directories with different Saratoga-peer
access controls depending on the intended next-hop, if this
information is known ahead of time. In any case, Saratoga only
provides the transfer mechanism, and any forwarding decisions based
on routing intelligence would be made within the DTN bundle agents.
All of this detail is considered a matter of implementation for the
bundle agent, and is not specified here.
The identity field in the Saratoga BEACON packet allows a local DTN
bundle agent to advertise its administrative EID via Saratoga. Other
Saratoga peers that hear that BEACON can then notify their local DTN
bundle agents of the contact. These notifications might be used to
integrate contact information into a routing information base, as
they are similar to the "hello" packets used in several routing
protocols. However, this is outside the scope of this document.
The "epoch" format used in Saratoga timestamps in file object records
is the number of seconds since January 1, 2000 in UTC, which is the
same epoch used in the DTN Bundle Protocol for timestamps. This must
include all leapseconds.
Saratoga instances can work in conjunction with DTN bundle agents to
fill the role of a convergence-layer adapter between bundle agents
connected via point-to-point links. Saratoga implementations
designed to work this way should have a way of notifying bundle
agents when they receive BEACONs from other nodes, and when transfers
have completed. In order for custody transfer to function properly,
notifications between the Saratoga instances and bundle agents on
both sides of a fully-successful bundle file transfer are required.
When Saratoga is used as a convergence-layer adapter, it is desirable
to turn on Saratoga's end-to-end checksum facilty to provide an
indication of correct bundle transfer. This is necessary due to the
bundle protocol design not including its own reliability checks for
internal robustness. See discussion of this problem with the bundle
protocol in [I-D.irtf-dtnrg-bundle-checksum].
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4. Proactive and Reactive Fragmentation
Proactive fragmentation - splitting a bundle, before transfer, into
multiple separate Saratoga transfers that are reassembled after
delivery at the receiving peer - is relatively straightforward.
Reactive fragmentation - recovery of disrupted partial transfers - is
less so.
For bundle file transfers, the local bundle agent could interact with
Saratoga in order to perform a reactive fragmentation of the bundle
whose transfer was interrupted by expiration of the inactivity timer.
For DTN custody transfer, we expect complications to be encountered
in making this reactive fragmentation work properly, and the details
required to implement this functionality are left out of this
specification until more experience has been obtained with reactive
fragmentation in general.
This document does not specify the functionality required for
reactive fragmentation of bundles as described in [RFC4838], other
than what is needed to support disrupted delivery and hop-by-hop
custody transfer of complete bundles. Reactive fragmentation of
bundles lies outside the scope of custody transfer of complete
bundles, and of this document.
However, the status of a transfer that Saratoga provides to a bundle
agent could be used to trigger the reactive fragmentation of bundles
if a bundle file transfer is interrupted part-way through (assuming
at least the bundle protocol headers and some portion of the data was
successfully transferred first). This would allow for efficient
recovery when unplanned interruptions occur. This requires some
coordination between the Saratoga node and the local bundle agent at
each end. The local API or coupling between the Saratoga peer and
its bundle agent does not affect the interoperability between either
the Saratoga peers or the DTN bundle agents, assuming that both sides
agree that fragmentation will occur at the lowest un-acknowledged
octet of the bundle file after the disruption. Reactive
fragmentation and any forwarding of the fragments onwards for
reassembly at some downstream node is solely a bundle-agent problem.
5. IANA Considerations
This document has no actions for IANA.
6. Security Considerations
When Saratoga is also used with a bundle agent, the security and
reliability considerations that have been outlined in detail in
[I-D.irtf-dtnrg-bundle-checksum] should be borne in mind.
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Security in DTNs is in general considered an open issue. If a
framework of techniques for handling security in DTN scenarios
emerges, Saratoga might be adapted to conform to this.
7. A Note on Naming
Saratoga is named for the USS Saratoga (CV-3), the aircraft carrier
sunk at Bikini Atoll and now a popular diving site.
The philosophy behind the protocol and its use described here can be
summarized as Saratoga Carries Upper Bundles Adequately, or SCUBA.
8. Informative References
[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.irtf-dtnrg-tcp-clayer]
Demmer, M., Ott, J., and S. Perreault, "Delay Tolerant
Networking TCP Convergence Layer Protocol", draft-irtf-
dtnrg-tcp-clayer-09 (work in progress), March 2014.
[I-D.irtf-dtnrg-udp-clayer]
Kruse, H. and S. Ostermann, "UDP Convergence Layers for
the DTN Bundle and LTP Protocols", draft-irtf-dtnrg-udp-
clayer-00 (work in progress), November 2008.
[I-D.kutscher-dtnrg-uni-clayer]
Kutscher, D., "Uni-DTN: A DTN Convergence Layer Protocol
for Unidirectional Transport", draft-kutscher-dtnrg-uni-
clayer-00 (work in progress), April 2007.
[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-15 (work in progress) , April
2014.
[IAC-2008]
Wood, L., Ivancic, W., Eddy, W., Stewart, D., Northam, J.,
Jackson, C., and A. da Silva Curiel, "Use of the Delay-
Tolerant Networking Bundle Protocol from Space",
conference paper IAC-08-B.2.3.10, 59th International
Astronautical Congress, Glasgow , September 2008.
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[Ivancic10]
Ivancic, W., Eddy, W., Stewart, D., Wood, L., Northam, J.,
and C. Jackson, "Experience with delay-tolerant networking
from orbit", International Journal of Satellite
Communications and Networking, Special Issue on best
papers of the Fourth Advanced Satellite Mobile Systems
Conference (ASMS 2008), vol. 28, issues 5-6, pp. 335-351,
September-December 2010.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
G. Fairhurst, "The Lightweight User Datagram Protocol
(UDP-Lite)", RFC 3828, July 2004.
[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.
[RFC5325] Burleigh, S., Ramadas, M., and S. Farrell, "Licklider
Transmission Protocol - Motivation", RFC 5325, September
2008.
[RFC5327] Farrell, S., Ramadas, M., and S. Burleigh, "Licklider
Transmission Protocol - Security Extensions", RFC 5327,
September 2008.
[SSTL-2008]
"UK-DMC satellite first to transfer sensor data from space
using 'bundle' protocol", press release, Surrey Satellite
Technology Ltd , September 2008.
Authors' Addresses
Lloyd Wood
University of Surrey alumni
Sydney, New South Wales
Australia
Email: L.Wood@society.surrey.ac.uk
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Jim McKim
RS Information Systems
NASA Glenn Research Center
21000 Brookpark Road, MS 142-1
Cleveland, OH 44135
USA
Phone: +1-216-433-6536
Email: James.H.McKim@grc.nasa.gov
Wesley M. Eddy
MTI Systems
MS 500-ASRC
NASA Glenn Research Center
21000 Brookpark Road
Cleveland, OH 44135
USA
Phone: +1-216-433-6682
Email: weddy@grc.nasa.gov
Will Ivancic
NASA Glenn Research Center
21000 Brookpark Road, MS 54-5
Cleveland, OH 44135
USA
Phone: +1-216-433-3494
Email: William.D.Ivancic@grc.nasa.gov
Chris Jackson
Surrey Satellite Technology Ltd
Tycho House
Surrey Space Centre
20 Stephenson Road
Guildford, Surrey GU2 7YE
United Kingdom
Phone: +44-1483-803-803
Email: C.Jackson@sstl.co.uk
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