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
TCP “point-to-point”
retransmission
LTP point-to-point
retransmission
TM
TC
AOS
IP
Prox-1
R/F, optical
Ethernet
wire
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17 November 2004
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.
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17 November 2004
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.
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17 November 2004
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 DelayTolerant Networking (DTN) bundling protocol.
• Standard class-1 CFDP over reliable Bundling UT layer.
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17 November 2004
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.
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17 November 2004
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.
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17 November 2004
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.
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17 November 2004
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.
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17 November 2004
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.
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17 November 2004
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)
“UT layer”
TCP end-to-end
retransmission
4
IP network routing
3
LTP
point-to-point
retransmission
COP/P
retransmission
TM/TC, AOS
Prox-1
2
Ethernet
R/F, optical
wire
10
1
17 November 2004
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.
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17 November 2004