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In-Space Cross Support UsingDelay / Disruption Tolerant Networking
Keith Scott
15 October, 2008Berlin, Germany
October 15, 2008
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[My Notion of] Context
CCSDS has defined, implemented, and is deploying
cross-support on the ground• Cross-support between one agency’s control center and
another agency’s ground station
• SLE / CSTS
No current standards for space-to-space cross
support above the data link layer
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Space-to-Space Cross Support
Mars Exploration Rovers / Mars Odyssey approach was expedient, but
inefficient
Packet-based service, as opposed to a bitstream service, desirable
Current Prox-1 implementations at Mars would make CFDP difficult to
cross-support, but in principle CFDP should be a cross-supported file
transfer protocol in space and on the ground
CFDP primarily implements file transfer together with metadata
AMS defines a messaging protocol for connected, low-latency
environments; Remote AMS can connect AMS continua
Routed service would support lander-orbiter-lander comms as well as
lander-orbiter-Earth comms
Given current CCSDS protocol suite, an internetworking layer (in the OSI
sense) is needed• Internetworking spans multiple data links, such as Proximity-1 and TC/TM
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Internetworking Layer Options
Internet Protocol (IP)• Pros: Very mature protocol suite• Cons: Implementations not well-suited for long-delay
and/or intermittently-connected environments
CCSDS Space Packets• Pros: Mature protocol for space communications• Cons: Lacks some features like source and destination
addresses in packets
Delay / Disruption Tolerant Networking (DTN)• Pros: Designed to handle intermittency and space
environment• Cons: Immature for space (but working on it…)
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Relevant Properties of DTN for Cross-Support in Space
“UDP-Like” messaging paradigmusing application-layer PDUscalled ‘bundles’• Unicast / multicast• DTN handles getting the bundles to
the destinations, regardless oflocation- DTN layer implements routing
• Optional (set by application)reliability
• 3-level priority• No guarantees of in-order delivery, duplicate suppression
CCSDS Space Packet can be used as an application-layer protocol• Data identification, application sequencing, …
Other protocols like CFDP / AMS can sit directly on top of DTN
source
destination
disrupted areas
Time t
Time t+n
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Capabilities vs. Policy
We need to specify the capabilities we want to provide now because:• It’s difficult to add new capabilities later
• It’s even more difficult to retrofit new capabilities into existing systems later
• Drive out advanced ops concepts now
We do not have to invoke all of those capabilities from the beginning• May use dynamic routing, can use static routing
• May provide cross-support to other agencies, may not (special case of next)
• Definitely policy, science constraints, contingency operations, … will all affect what cross-support
can be provided by a particular asset
Cross-support agreements between agencies (policy, not technical) need NOT be ‘hard’
commitments
Geometry
Mission Operations
• Policy• Science Constraints• Contingency Operations• Other
Actual Relay Opportunities
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Persistent StorageCT Custody Transfer Capability
Bundle PathCustody Acknowledgements
DTN for Multi-Hop Space Communications
Application
DTN
TCP
IPv6
Ethernet
UTP
DTN (potential delay)
TCP
IPv6
ATM
DS-1
IPv6
Ethernet
UTP
Orbit-to-SurfaceTerrestrial Network
LTP
Encap
AOS
Application
DTN
Prox-1
GroundStation
Deep Space
DTN (Potential delay)
LTP
Encap
AOS Prox-1
Mars RelaySatellite
IP Router
ATM
DS-1
CT CT CT CT
MissionControl
MarsRover
LTP
Encap
LTP
Encap
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Operations Concept
Users / applications emit data when it suits them, without regard to end-to-end connectivity• Applications don’t have to worry about the destination of
the location or whether there’s a network path or not• When the source and destination are connected, bundles
flow in “real-time”• When source and destination are not connected, bundles
move in store-and-forward fashion
For commands, applications may want to use time-triggered command sequences• Send command sequence ahead of time, allowing for
store-and-forward delivery• Sequence is invoked at a particular time carried as part of
the command
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Applications
CCSDS Space Packet can be used as an application-layer protocol
CFDP can be re-factored to use DTN• Solves advanced CFDP scenarios
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Multi-Agency Cross-Support
Control Center
Control Center
Control Center
Elements:AgenciesRoversSurface relaysOrbital relays
GEO / Direct Comm Mission
LEO/MEOEarth OrbitInter-Network
Mars Orbit And SurfaceInter-Network
Lunar OrbitAnd SurfaceInter-Network
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Status of DTN
Internet Research Task Force Working Group• Stable protocol specification• Active and ongoing research work for terrestrial applications
CCSDS DTN WG• Draft Green Book out – criteria for evaluating candidate protocols• Target is to adopt / adapt Internet RFC5050
NASA Constellation• Carrying DTN as a requirement in the C3I Interoperability Specification
NASA DTN-for-2010 project• Advance DTN to TRL-8 by 2010• DINET (Scott)
IOAG’s Space Internetworking Strategy Group (SISG)• Report / presentation to IOAG in September• Draft report / presentation to IOP in November• Conclusions: The agencies need to move towards a network-centric model
of communications using some combination of IP, Space Packets, and DTN
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Backup
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IP Packet Format
Version HeaderLength Total length
Identification
TTL Protocol Header Checksum
Source IP Address
Destination IP Address
Flags Fragment offset
DSCPECN
DATA
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CCSDS Space Packets
PacketVersionNumber
PacketIdentification
Packet Sequence Control
PacketData
LengthPkt
TypeSec.HdrFlag
Application Process Identifier
Sequence Flags
Packet Sequence Count or Packet
Name
3 bits 1 bit 1 bit 11 bits 2 bits 14 bits 16 bits
2 octets 2 octets 2 octets
Packet Primary Header
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Creation Stamp1
Version Flags
Blocklength
DestinationScheme
DestinationSSP
SourceScheme
SourceSSP
Report-toScheme
Report-toSSP
Custodianscheme
CustodianSSP
CreationStamp2 Lifetime
DictionaryLength
Dictionary
FragmentOffset
TotalADU length
BlockType
Primary BundleBlock
ControlFlags
Block Length
Payload
Bundle PayloadBlock
32 bits
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source
destination
disrupted areas
Time t
Time t+n
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Required Services (from the standpoint of Applications)
Applications need:• To send/receive delimited application-layer PDUs• To send those PDUs end-to-end through a possibly
multi-hop infrastructure• To be able to communicate when the infrastructure is
only intermittently-connected
The infrastructure needs to support:• Multiple applications at each end node• Multiple end nodes multiplexed onto the infrastructure
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What We Have Now
Space Packets• Addressing requires elements from the frame
(spacecraft ID)• 11-bit APID is available and could be re-purposed (but
11 bits isn’t a lot to identify end systems, intermediate systems, and applications)
CFDP (as a network layer)
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Stuff To Do
October 15, 2008
Moving the bits• Packet formats• Protocol definition
[Easy]
Exposing ‘resources’ to other projects / agencies
• SM&C
[Hard, independent of internetwork protocol]
Registering information
• End system IDs
[Easy]
Service Level Agreements
• What does AgencyA commit to providing
[Hard, independent of internetwork protocol]
Possible Mission Planning Models:• It’s a giant network free-for-all [no]• I plan my mission to use my agency’s resources only, and
throw any spare resources into the ‘common’ pot• And I sometimes take from the ‘common’ pot