Network Working Group L. Wood
Internet-Draft Cisco Systems
Intended status: Experimental J. McKim
Expires: May 1, 2008 RSIS
W. Eddy
Verizon
W. Ivancic
NASA
C. Jackson
SSTL
October 29, 2007
Using Saratoga with a Bundle Agent
as a Convergence Layer for Delay-Tolerant Networking
draft-wood-dtnrg-saratoga-02
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Copyright (C) The IETF Trust (2007).
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Internet-Draft Using Saratoga with a Bundle Agent October 2007
Abstract
Saratoga is a simple, lightweight, UDP-based transfer protocol. This
is a companion document to the Saratoga specification, given in
draft-wood-tsvwg-saratoga. This document describes how to use
Saratoga as a Delay-Tolerant Networking (DTN) "convergence layer"
with the DTN Bundle Protocol, and with DTN Bundle Agents.
Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Applicability Statement . . . . . . . . . . . . . . . . . . . 3
3. Using Saratoga with a DTN Bundle Agent . . . . . . . . . . . . 4
4. Reactive Fragmentation . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. A Note on Naming . . . . . . . . . . . . . . . . . . . . . . . 7
8. Informative References . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Background
The Saratoga protocol is specified in [SaratogaProtocol]. Saratoga
was originally designed prior to the DTNRG's work on the Bundle
Protocol [I-D.irtf-dtnrg-bundle-spec]. It was later recognized that
Saratoga could be used to reliably exchange bundles between DTN
Bundle Agents by using a logical mapping between DTN bundles and
Saratoga files and back. Saratoga can be used in Delay/
Disruption-Tolerant Networking (DTN) [RFC4838], as a "convergence
layer" to exchange DTN bundles [I-D.irtf-dtnrg-bundle-spec] between
peer nodes. The DTN concept is applicable to networks where ad-hoc,
intermittent connectivity is expected, connections may be
infrequently established or short-lived, and end-to-end paths are not
present. This, coincidentally, is Saratoga's operating environment,
making the Saratoga transfer protocol a natural fit as a convergence
layer for DTN.
This document contains notes on use of Saratoga for the bundle
transfer procedure.
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
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. A simple UDP convergence
layer, able to be used over dedicated private links where congestion
control is not required, is also present. However, this simple UDP
convergence layer presumes that a bundle will always fit into a
single UDP packet, and does not support segmentation of bundles
across multiple UDP packets.
Two protocols capable of supporting segmentation of large bundles
across multiple UDP packets, with ARQ-based flexible delivery robust
to packet loss, are Saratoga [SaratogaProtocol] and the Licklider
Transmission Protocol (LTP) [I-D.irtf-dtnrg-ltp-motivation].
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
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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 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
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 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
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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.
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
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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
should include all leapseconds.
We expect that Saratoga instances will often 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 is 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 reliability checks or internal
robustness. See [I-D.irtf-dtnrg-bundle-checksum].
4. Reactive Fragmentation
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
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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 used with a bundle agent, the security
considerations outlined in [I-D.irtf-dtnrg-bundle-checksum] should be
borne in mind. 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., "Checksum Ciphersuites for the Bundle Protocol",
draft-irtf-dtnrg-bundle-checksum-00 (work in progress),
September 2007.
[I-D.irtf-dtnrg-bundle-spec]
Scott, K. and S. Burleigh, "Bundle Protocol
Specification", draft-irtf-dtnrg-bundle-spec-10 (work in
progress), July 2007.
[I-D.irtf-dtnrg-ltp-motivation]
Burleigh, S., Ramadas, M., and S. Farrell, "Licklider
Transmission Protocol - Motivation",
draft-irtf-dtnrg-ltp-motivation-05 (work in progress),
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October 2007.
[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.
[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.
[SaratogaProtocol]
Wood, L., McKim, J., Eddy, W., Ivancic, W., and C.
Jackson, "Saratoga: A Scalable File Transfer Protocol",
draft-wood-tsvwg-saratoga, work in progress as an
internet-draft, October 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
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
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Wesley M. Eddy
Verizon Federal Network Systems
NASA Glenn Research Center
21000 Brookpark Road, MS 54-5
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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