17 November 20041

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.

LTP point-to-pointretransmission

Bundling store-and-forward

TM TC Prox-1

R/F, optical

TCP “point-to-point”retransmission

Ethernet

IP

wire

AOS

17 November 20042

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.

17 November 20043

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.

17 November 20044

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.

17 November 20045

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.

17 November 20046

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.

17 November 20047

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.

17 November 20048

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.

17 November 20049

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.

17 November 200410

CFDP/DTN Architecture

(no retransmission, no store-and-forward)

User application

UT adapter

CFDP file system functions

“UT layer”

CFDP unacknowledged transmission

LTPpoint-to-point

retransmission

Bundling store-and-forward

TM/TC, AOS Prox-1

R/F, optical

TCP end-to-endretransmission

Ethernet

wire

COP/Pretransmission

IP network routing

7

4

3

2

1

(bandwidth management)

17 November 200411

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.