1. Architectures for Delay/Disruption Tolerant Networking Implementations for Space
Authors
Unnikrishnan E, Ravichandran V, Sudhakar S, Subramanya Udupa, (Indian Space Research Organization-ISRO, India)

2. Overview Introduction to DTN DTN architecture Underlying protocols
Study on DTN2 reference implementation
Architectural components
Design models for flight implementation
Hardware implementation using VHDL
Embedded processor based implementation
Software implementation on SBC
Conclusion

3. DTN Introduction
DTN protocol objective is to integrate networks of different communication characteristics, called regional networks, for seamless operation
Eg. regional network: mobile network, sensor network, space network, terrestrial networks like Internet, etc.
Communication characteristics of a network include
connectivity (disruption or intermittency)
path delay (long)
error rate BER or packet loss (high)
data rate (asymmetric)
The underlying protocols are optimized for its characteristic for each network
Applications are made transparent to the above communication characteristics of individual networks for inter regional operation.

4. A typical DTN scenario Region Network B (Satellite Network) Network C
DTN GATEWAY
DTN GATEWAY B C A B
Network C
(Internet)
Region Network A
(Sensor Network)
Node C2
(user)
NodeA1
(user)
An illustrative diagram of DTN network

5. How DTN works
Transport layer is terminated when there is a network partition (disruption /delay)
A store and forward message switching mechanism is embedded in the network architecture
Messages are stored in a persistent storage in this new architecture when networks are partitioned
A new overlay protocol, called Bundle layer, is introduced above transport layer
Bundles are moved towards destination for delivery on opportunistic and scheduled contacts

6. DTN Architecture Bundle layer Region Specific Layers Region Specific
Application Layer
Application Layer
Bundle layer
Region Specific
Layers
Region Specific
Layers
Region Specific
Layers
Region Specific
Layers
Application
Application
Bundle layer
Persistent storage
Transport (TCP)
Transport
Network ( IP)
Network
Network specific layers
Data link
Data Link
Physical
Physical
Internet layers
DTN layers

7. Protocol Features Store and forward message switching
Reliability and custody transfer
Time stamp and time synchronization
Routing and Forwarding
Fragmentation and reassembly
Congestion control
Security

8. Bundle Block Format

9. Bundle Processing control flags
0 -- Bundle is a fragment.
1 -- Application data unit is an administrative record.
2 -- Bundle must not be fragmented.
3 -- Custody transfer is requested.
4 -- Destination endpoint is a singleton.
5 -- Acknowledgement by application is requested.
6 -- Reserved for future use.
Bits in positions 7 through 13 are used to indicate the bundle's class of service.
Bit positions 8 &7 as follows:
00 = bulk, 01 = normal, 10 = expedited, 11 is reserved for future use.
14 -- Request reporting of bundle reception.
15 -- Request reporting of custody acceptance.
16 -- Request reporting of bundle forwarding.
17 -- Request reporting of bundle delivery.
18 -- Request reporting of bundle deletion.
19 -- Reserved for future use.

10. Administrative records
Administrative record types :
Bundle status report.
Custody signal.
Custody signal:
Redundant reception
Depleted storage.
Destination endpoint ID unintelligible.
No known route to destination
No timely contact with next route
Block unintelligible.
Bundle status report:
Reporting node received bundle.
Reporting node accepted custody of bundle
Reporting node forwarded the bundle
Reporting node delivered the bundle
Reporting node deleted the bundle.

11. Evaluation of DTN2 reference implementation
Configured and evaluated reference implementation DTN2 on a Linux test bed
Simulated DTN operation for a typical configuration of a ground node, orbiter and a rover on the test bed
Simulated command file delivery to rover and telemetry reception at ground for intermittently connected links (LAN)
Proved the concept of DTN operation over disrupted link where other protocols fail to operate
Contact Graph based routing was configured

12. DTN simulation test bed
Simulated as
Ground node
Simulated as
Orbiter
Simulated as
Rover
received
command
file PC
DTN Node-1 PC
DTN Node-2 PC
DTN Node-3
link 1
link 2
telemetry
file
command
file
LAN
Links are disrupted using test scripts
Contact graphs are maintained at each node with scheduled connectivity
DTN simulation test bed 12

13. DTN prototype on FPGA (hardware implementation)
Specification (scaled down)
EID is singleton (only unicast is supported)
Storage – 4 memory blocks
DTN Timer resolution - 1ms
Priority levels – 3
Life time of bundle – 10 minutes
Bundle size – 1024 bytes (primary block 64 bytes + 4 bytes payload block bytes payload ADU)
SDNV encoding / decoding
Contact graph based routing
Security (not addressed)

14. Test setup for prototype validation
Test setup of DTN prototype implementations on FPFA
Hardware Xilinx vertex-4 FPGA XC4VFX60
Software ISE Design Suite 13.4, ModelSim SE 6.4
Coding Language – VHDL
System frequency – 4MHz
Simulation test done

15. Major VHDL modules Top bundle protocol module Top transmitter module
DTN Node
UART TX/RX
Top bundle protocol module
Top transmitter module
Top receiver module
Administrative record
Persistent storage
Header formation
Payload block formation
RAM modules
SDNV encoding
SDNV decoding
Clock generation
UART module (testing with I/O data)
UART RX
SDNV encode & Bundle formation
Persistent storage
Forward Module
SDNV and block decoding
UART TX
Extract ADU

16. DTN prototype hardware-software implementation
Core8051, an Intel compatible 8 bit microcontroller, executes all ASM51 instructions with RISC like design
System frequency 12MHz
Coding language –VHDL
Libero IDE 9.0, ModelSim SE 6.5d
Open Keil uVision 3
H/W Xilinx vertex4 FPGA XC4VFX60
FPGA (Xilinx)
Core8051soft core
RAM (component )
PROM (component )
Core8051 is chosen as an embedded processor platform to prove the concept
DTN basic modules ‘hex code’ is loaded on PROM to validate the platform
DTN modules are tested independently to port to the target

17. Bundle Protocol Server
Software Design approach
Bundle Protocol Server
Bundle Protocol Agent
Application Agent
Convergence
Contacts
Registration
Naming
Routing
Commands
Utilities
Convergence layer
Top level package level design

18. Bundle Protocol Sequence Diagram

19. Design approaches Embedded processor Standard defines a DTN Node as
Single process running on a general purpose computer
Thread in a process
An object in an object oriented system
A special purpose hardware device
....
Embedded processor OS
Monolithic hardware design
(collection of hardware modules)
Processor
H/W
OS (optional)
software modules
software modules
Hardware implementation
Hardware software implementation
Software implementation

20. Comparison of different DTN design architectures
Hardware based design
Hardware- software design
Software based design
No processor chip or software component in the design
A processor is embedded in FPGA
Design is based on a general purpose processor
Higher reliability
Reliability of software component to be addressed
Reliability of a fully software system
Fast realization of a flight worthy implementation with minimum features
Faster realization as protocol features are implemented as software
Software testing and qualification takes longer time with more features
Difficult to provide all features due to complexity
Easy to provide more features
Most easy to provide all the features
Not possible to accommodate changes
Flexible to accommodate changes as it is limited to software modifications
Most flexible to changes as a software implementation

21. Protocol stack diagram for envisaged space DTN network
DTN Application envisaged
DTN /BP
LTP
TM /TC
DTN/BP
Prox-1
APPLN
DTN/BP
Prox-1
APPLN
Orbiter
DTN/BP
AX.25
Lander
APPLN
rover
DTN/BP
AX.25
DTN/BP
DTN /BP
LTP
TM /TC
planetary surface
TCP/IP
Ground station
APPLN
DTN/BP
TCP/IP
MCC
Protocol stack diagram for envisaged space DTN network
Earth 21

22. Conclusion
Results of the study on DTN-2 reference implementation are promising for its usage on a disconnected network.
For DTN for space a highly reliable and light weight version of the protocol suit is required.
Different design approaches are compared and efforts for prototyping were summarized
Protocol implementations need to undergo sufficient testing on simulators for all possible end to end scenarios
More simulations at ground are required for introducing dynamic routing of bundles for a large network
CCSDS standards on the DTN protocols help the space agencies to deploy an interoperable network for future

23. References
Rationale, Scenarios, and Requirements for DTN in Space Informational Report CCSDS G-1August 2010
CCSDS Bundle Protocol Specification Recommended Standard CCSDS B-1 September 2015
RFC 5050 Bundle Protocol Specification ,Network Working Group Category: Experimental 2007
RFC 4838 Delay-Tolerant Networking Architecture, Network Working Group Category: Informational 2007
Solar System Internetwork Architecture Informational Report CCSDS G-1July 2014
Report of the Interagency Operations Advisory Group Space Internetworking Strategy Group Recommendations on a Strategy for Space Internetworking July 2008
Licklider Transport Protocol (LTP) for CCSDS B-1 May 2015
Proximity-1 Space Link Protocol— Data Link Layer CCSDS B-5 December 2013
“Design of DTN for Interplanetary Communication” ,M Tech Thesis, Samreen Fiza , Project work done at TMD/DSG/CDA/ISAC
“Design of a Platform for embedded applications using 8051 IP core”, M Tech Thesis, Pranam Amin, Project work done at TMD/CDEG/CDA/ISAC
“Design and Modeling of DTN Bundling Protocol”, Deepak N A, Unnikrishnan E, et al International Conference on Modeling and Simulation August 2007

24. Thank you Acknowledgement to
Dr. M Annadurai, Director ISRO, Satellite Centre