Delay-Tolerant Networking (DTN)

1. Delay-Tolerant Networking (DTN) • General-purpose capability for scalable, reliable communications across deep space. • Extending and streamlining the capabilities of CFDP: • Built-in security (authentication and confidentiality). • Flexible, dynamic multipath route selection. • Deferred transmission, store-and-forward routing for tolerance of intermittent connectivity. • Point-to-point retransmission for efficient reliability. • Custody transfer for early release of retransmission resources. • Will enable CFDP to scale up to large deployment configurations. Bundling store-and-forward LTP point-to-point retransmission TCP “point-to-point” retransmission IP TM TC AOS Prox-1 Ethernet R/F, optical wire

2. CFDP Basic Deployment • Premise: entities can communicate directly (R/F or optical). • Mutual line-of-sight visibility. • Compatible operating schedules: entity A can point at entity B and transmit at a time when entity B can point at entity A and receive. • Adequate links: the levels of transmitter power and receiver power combine to produce a data rate greater than zero. • Implementation: core CFDP over CCSDS TM/TC (or AOS) UT layer.

3. CFDP Advanced Deployment • Premise: entities cannot communicate directly. • No mutual visibility: intervening planetary mass, intervening Sun. • Incompatible operating schedules. • Insufficient signal power between sender and receiver. • So CFDP must support indirect communication, via “relay” or “waypoint” entities, using store-and-forward techniques. • Constraint: a single, serial end-to-end route from the sender to the receiver for the duration of each transaction. • Implementation options: • Extended procedures • Additional functionality built into CFDP itself. • Store-and-forward Overlay • CFDP is left unchanged. • Additional functionality built into standard user application layer.

4. CFDP Network Deployment • Premise: • As in Advanced Deployment, entities cannot communicate directly. • But the constraint on Advanced Deployment is removed: multiple forwarders may operate in parallel for a single CFDP transaction. • So data may routinely arrive out of transmission order. • Bad for end-to-end acknowledged CFDP: whenever EOF arrives before file data segments, unnecessary retransmission is triggered. • Implementation: core unacknowledged CFDP over Delay-Tolerant Networking (DTN) bundling protocol. • Standard class-1 CFDP over reliable Bundling UT layer.

5. Bundling • As in the Internet, there may be multiple possible routes (both in space and time) to the destination. • Multi-layer routing: • End-to-end routes are computed by “bundling” protocol. • Route to next hop within the same region – if not point-to-point – is performed by region-specific protocol, such as IP within the Internet. • Internal routing technology can be different in different regions. • Tuned for cost effectiveness. • Evolving independently. • This enables end-to-end routing complexity to scale up indefinitely without imposing excessive overhead within any single region.

6. Bundling (cont’d) • Bundle forwarding algorithms may consider: • requested delivery deadline • estimated time to destination on alternative paths • class of service, e.g., explicit transfer of custody • For example, bundling might withhold bundles from an impending low-rate contact in favor of a future high-rate contact. • Routing decisions are re-evaluated at each forwarding hop. Nature of connectivity may affect routing decisions: • continuous • opportunistic • scheduled • Schedules loaded via management interface or routing protocol.

7. Bundling (cont’d) • Additional features: • “Reply-to” address may differ from original source. • Optional interim progress reports (similar to SFO). • Optional end-to-end reception report, retransmission. • Support for multiple user applications: • CFDP • sensor webs • messaging • Explicit transfer of custody. • Not all forwarding nodes need be custodians.

8. LTP • LTP is Licklider (or “Long-haul”) Transmission Protocol. • Directly descended from CFDP Core reliability procedures, with a few simplifications: • It’s not file-oriented. LTP divides a block into segments for reliable transmission. No filestore commands, no metadata. (File-oriented mechanisms are left to CFDP, above bundling.) • Indications analogous to EOF, Finished, Prompt, etc. are combinations of bit flags in the standard header. • The last segment of a block carries an “end of block” flag. There’s no separate “EOF” segment, so a small block may be entirely contained in a single segment. • Negative acknowledgment segments are sent reliably, so there’s nothing like the NAK timer cycle. All timeout intervals can be computed from operational data: no guesswork. • No transaction-specific Suspend and Resume, no flow labels.

9. LTP (continued) • What’s retained from CFDP core reliability procedures: • Deferred transmission. • Parallel transactions, with a transaction cancellation mechanism. • Negative acknowledgment of missing data, positive acknowledgment of critical (e.g., end of block) segments. • Abstract interface to underlying transmission layer. • Simple analogs to the Prompt and Keepalive mechanisms. • All four “lost segment detection” options: deferred, prompted, immediate, asynchronous. • Link-specific Freeze and Thaw.

10. CFDP/DTN Architecture User application CFDP file system functions CFDP unacknowledged transmission 7 (no retransmission, no store-and-forward) UT adapter Bundling store-and-forward (bandwidth management) TCP end-to-end retransmission 4 IP network routing 3 “UT layer” LTP point-to-point retransmission COP/P retransmission 2 TM/TC, AOS Prox-1 Ethernet R/F, optical wire 1

11. DTN Status • Spring of 2002: Internet Research Task Force research group DTNRG formed to articulate DTN concepts. • Summer of 2002: first demonstration of initial Bundling implementation. • March 2003: peer review of DTN architecture Internet Draft. • May 2004: DARPA issues BAA (Broad Agency Announcement) for its DTN research program. • July 2004: version 01 of LTP Internet Draft published. • Version 02 editing is in progress. • Stephen Farrell is working on the first implementation. • September 2004: version 03 of Bundling protocol spec Internet Draft published. • November 2004: initial meeting of CCSDS DTN BOF.