DTTP: A Delay-Tolerant Transport Protocol for Space Internetworks

1. DTTP: A Delay-Tolerant Transport Protocol for Space Internetworks Christos Samaras ComNet Group, Democritus University of Thrace February 2008

2. Contents • Space Networking Environments • Standard Internet Protocols in Space • Space Agencies vs. Each Other • DTTP: Delay-Tolerant Transport Protocol • Simulation Results and Future Work

3. Space Networking Environments Challenged networks: • intermittent connectivity • long and/or variable propagation delays • asymmetric data rates • high error rates So, can standard Internet protocols operate in Space?

4. Standard Internet Protocols in Space? short answer no (at least in their current form) long answer maybe (adaptations needed: Mobile IP? TCP not suitable; etc.) Internet usability is based on the following assumptions: • continues, bidirectional end-to-end path • short round trip times • symmetric data rates • low error rates IP-inside

5. Space Agencies vs. Each Other Space missions interoperability: – common goal for many space agencies (CCSDS efforts) – increase in data return rates – offering flexible/alternative communication opportunities – might prove catalytic in critical situations ...no consensus (yet) among space agencies potential space communications convergence through: • deploying common protocol stacks (possibly IP-enabled) • hiding heterogeneous networks (e.g. Delay-Tolerant Networking (DTN) architecture as a message-oriented overlay) • other (to be conceived)... in any case, we need a specialized, efficient, reliable transport protocol

6. Why a Transport Layer Approach? • ease-of-use: programmers are familiar with developing applications which sit upon a transport layer • the DTN approach only disguises congestion; need for mechanisms that handle congestion or storage capacity depletion • there are cases where homogeneous networks (in terms of underlying protocol stacks) don’t require different DTN protocols for each hop: a multi-hop transport solution is therefore needed

7. DTTP: Delay-Tolerant Transport Protocol DTTP features: • reliability:asynchronous acknowldgement procedures (when compared to TCP’s Ack-clocking functionality) • custody transfer: based on in-network storage; robust against link disconnections; more efficient than end-to-end approaches • parallel data transfer: multiple data paths can be exploited in parallel • (time periods with) constant sending rate: rated-based protocol; fills the communication pipe (note: stateful sessions) • sending rate adaptivity: relies on explicit signals from (intermediate/final) receivers, e.g. storage exhaustion • application-oriented transmission behavior: provision of transmission tactics to reflect application needs

8. 1st Transmission Tactic immediate use of acknowledgment info; graduated reliability enhancements (help: redundant data); suitable for certain video or image applications etc. until (all application data is acknowledged) start transmitting new application data if (acknowledgment info arrives) send or multiply-send missing data end; end;

9. 2nd Transmission Tactic more efficient use of bandwidth resources (i.e., less retransmissions); potentially produces more gaps in the receive window; suitable for bulk data transfers. send all application data until (all application data is acknowledged) exploit current acknowledgment info send or multiply-send missing data end;

10. DTTP Deployed in an IP-Enabled Internetwork common network layer (IP in the figure) with potentially heterogeneous underlying protocols

11. DTTP Deployed in a DTN-Enabled Internetwork DTTP’s custody transfer functionality is deactivated (since offered by DTN); DTTP is essentially used as a delay-tolerant, transport protocol

12. Simulation Parameters and Topologies 10MByte file transfer; last link intermittent connectivity (70% on & 30% off) 2-hop and 5-hop Topologies

13. Simulation Results File delivery completion time (using different communication times)

14. Simulation Results RTT impact on file delivery completion time

15. Future Work • investigate various acknowledgment schemes (e.g. SACK, SNACK, other mechanisms...) to mitigate bandwidth asymmetries • improve retransmission behavior (in relation to delay-bandwidth product; incorporate relevant timers) • implement data forwarding via parallel paths, and explicit signaling for storage resources exhaustion • explore network dynamics in space environments: e.g. buffer resources and rate-based transmission trade-offs

16. ...any questions?