17 November 20041

Scott Burleigh, JPL

SIS Deep SpaceProtocols Status

17 November 20042

Overview

• CCSDS File Delivery Protocol (CFDP)– Unacknowledged CFDP Extensions (UCE) pink sheets were issued

for review and agency approval on 19 October 2004.

– Max Ciccone will report on progress in interoperability testing.

• Delay-tolerant networking (DTN)– BOF meets for the first time in November of 2004.

• Licklider Transmission Protocol (LTP)– A link-neutral point-to-point retransmission system designed to

support DTN operations in deep space.

– BOF will be proposed later this week.

• Asynchronous Message Service (AMS)– A companion protocol to CFDP, for message exchange over the

deep space delay-tolerant network.

– BOF will be proposed later this week.

17 November 20043

CFDP UCE (1 of 2)

• Motivated by processing requirements for Mars Reconnaissance Orbiter (MRO): MRO wants to run CFPD in unacknowledged mode over a reliable UT layer, in this case a non-standard AOS frame retransmission system.

• Problem:– Version B2 of CFDP closes an unacknowledged-mode transaction

as soon as the EOF PDU is received.

– But a reliable UT layer might cause missing file data PDUs to be retransmitted after EOF receipt. Because the transaction is closed, these PDUs would never be collected into the delivered file.

• Solution: add a “check timer” that works in unacknowledged mode the same way the NAK timer works in acknowledged mode. Transaction is closed only when examination finds that reception is complete – either on EOF arrival or on check timer expiration – or when check timer expiration limit is reached.

17 November 20044

CFDP UCE (2 of 2)

• Status:– BOF formed in May of 2003 and agreed on design.

– Working group approved in October of 2003.

– Implementation demonstrated in February of 2004.

– Initial pink sheets completed in March of 2004.

– Revised pink sheets completed in May of 2004.

– Pink sheets issued for agency review and approval on 19 October 2004.

17 November 20045

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 20046

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 20047

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 20048

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 20049

Network Deployment

Rover1Rover2 Motes

Lander1

Orbiter2Orbiter1

Ground Station

Investigator

Earth’sInternet

DeepSpaceBackbone

MarsIn-situInternet

Bundle

CFDP

Bundle

CFDP

Bundle

CFDP

Bundle

CFDP

MarsRelayNetwork

MarsSensorWeb

Bundle

Bundle

17 November 200410

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 200411

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 200412

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 200413

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 200414

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 200415

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 200416

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.

17 November 200417

Asynchronous Message Service (1 of 3)

• In addition to file transfer, event-driven asynchronous message exchange may also be useful for deep space communications with and among spacecraft :– streaming engineering (housekeeping) data– real-time commanding– continuous collaborative operation among robotic craft

• NASA’s proposed new Command, Control, Communications, and Information (C3I) architecture is based on this model.

• Challenges in large-scale asynchronous message exchange:– Heterogeneity: platforms, security regimes, communication

environments, QOS requirements, performance requirements, cost tolerance.

– Changing topology: requires autonomous discovery of communication endpoints, automatic reconfiguration.

– Publish/subscribe message exchange model scales better than client/server.

17 November 200418

Asynchronous Message Service (2 of 3)

• But most asynchronous message exchange systems are:– proprietary, licensed products (e.g., TIBCO Rendezvous, NDDS)

rather than open international standards;

– not designed for operation on deep space robots.

• Proposed CCSDS Asynchronous Message Service (AMS) standard is based on proven NASA technology: no commercial licensing, designed for spacecraft flight operations.

• Tramel (Task Remote Asynchronous Message Exchange Layer) was developed in JPL’s Flight Systems Testbed (FST) in 1995-1996; mature and stable since 1998.– Real-time spacecraft simulation in FST (1994-1999).

– Software fault tolerance experiments at JPL (1998).

– X-34 Integrated Vehicle Health Management testbed (2003).

– Baselined for inclusion in C3I.

17 November 200419

Asynchronous Message Service (3 of 3)

• AMS features:– Platform-neutral, UT-layer neutral.

– Designed to scale from very small to very large configurations.

– Self-configuring and fault-tolerant, via silent “meta-AMS” protocol.

– “Remote AMS” adaptations enable efficient, delay-tolerant publish/subscribe capability over interplanetary distances.

• Status:– Concept paper (tentative protocol specification) ready for review.

– Fully-functional, well-documented prototype (Tramel) has been mature for six years.

17 November 200420

Deep Space Communications 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)

AMS

UT adapter