A Delay-Tolerant Network Architecture for Challenged Internets
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April 22, 2023 Anshul Kantawala 1
A Delay-Tolerant Network Architecture for Challenged Internets
Kevin Fall
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April 22, 2023 Anshul Kantawala 2
Challenged Networks Terrestrial mobile networks
Unexpected partitions due to node mobility or RF interference
Periodic, predictable partitions e.g. Commuter bus acting as store and
forward switch
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Challenged Networks (cont.) Exotic Media Networks
Near-Earth satellites, very long-distance radio (deep space) etc. High latencies with predictable interruption Outage due to environmental conditions Predictably available store and forward
network service – e.g. low-earth orbiting satellites
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Challenged Networks (cont.) Military Ad-Hoc Networks
Operate in hostile environments mobile nodes, environmental factors or
intentional jamming cause disconnections Data traffic may be pre-empted by
higher priority voice traffic Strong infrastructure protection
requirements
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April 22, 2023 Anshul Kantawala 5
Challenged Networks (cont.) Sensor networks
Limited end-node power, memory and CPU capability
Thousands or millions of nodes per network
Communication scheduled to conserve power
Interfaced to other networks using proxy nodes
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April 22, 2023 Anshul Kantawala 6
Current Solutions Link-repair approach
Engineer problem links to appear similar to regular links
Use proxy agents Attach challenged networks at edges
using proxy agents Does not provide a general way to use
these networks for data transit
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Characteristics of Challenged Networks Path and Link characteristics Network architectures End System characteristics
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Path and Link characteristics High latency, low data rate
e.g. 10 kbps, 1-2 second latencies Asymmetric data rates
e.g. remote instruments – large return channel, small uplink for device control
Protocols should be terse and dynamic control functions performed open-loop or hop-by-hop
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Path and Link characteristics Disconnection
Non-faulty disconnections Motion
Predictable: satellite passes, bus acts as router Random: motion of nodes/routers, interference
Low-duty-cycle operation Routing subsystem should not treat
predictable disconnections as faults and can use this information to pre-schedule messages
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Path and Link characteristics Long queueing times
Conventional networks rarely greater than a second
Challenged network could be hours or days due to disconnection
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April 22, 2023 Anshul Kantawala 11
Network Architectures Interoperability considerations
Networks may use application-specific framing formats, data packet size restrictions, limited node addressing and naming etc.
Security End-to-end approach not attractive
Require end-to-end exchanges of keys Undesirable to carry traffic to destination
before authentication/access control check
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End System Characteristics Limited longevity
Round-trip time may exceed node’s lifetime making ACK-based policies useless
Low duty cycle operation Disconnection affects routing protocols
Limited resources Affects ability to store and retransmit
data due to limited memory
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April 22, 2023 Anshul Kantawala 13
Can we use TCP/IP? Transport layer (TCP)
High latency and moderate to high loss rates severely limit TCP’s performance
Network layer (IP) Performance affected by loss of
fragments Routing
High latency will cause current routing protocols to incorrectly label links as non-operational
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Proxies and Protocol Boosters Proxies and protocol boosters are
inherently fragile Increase system complexity if mobility is
frequent May require both directions to flow
through the proxy – fail for asymmetric routing
Application proxies have limited re-use abilities and may fail to take advantage of special resources of the proxy node
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April 22, 2023 Anshul Kantawala 15
Delay Tolerant Message-Oriented Overlay Architecture
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Abstraction Message switching
Use message aggregates or “bundles” Allows network’s path selection and scheduling
functions a-priori knowledge of the size and performance requirements of data transfers
Overlay architecture DTN will operate over existing protocol stacks
and provide a gateway when a node touches two or more dissimilar networks
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Regions and DTN Gateways DTN gateways are interconnection points
between dissimilar network protocol and addressing families called regions e.g. Internet-like, Ad-hoc, Mobile etc.
DTN gateways Perform reliable message routing Perform security checks Store messages for reliable delivery Resolve globally-significant name tuples to
locally-resolvable names for internal destined traffic
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Name Tuples Two variable length portions
Region name Globally-unique hierarchically structured region
name Used by DTN gateways for forwarding messages
Entity name Resolvable within the specified region, need not
be unique outside it E.g. { internet.icann.int, http://www.ietf.org/ }
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Class of Service Similar to the Postal service
Delivery priority: low, ordinary, high Notifications of mailing, delivery to
receiver and route taken Reliable delivery using custody transfer
at each routing hop
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Path Selection and Scheduling End-to-end path routing path cannot
be assumed to exist Can solve a multicommodity flow
optimization problem using approximate algorithms, since the protocol is message based
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Custody Transfer Two types of message nodes
Persistent (P) and non-persistant (NP) P nodes assumed to contain persistent
memory storage and participate in custody transfer
Custody Transfer Acknowledged delivery of message from
one DTN hop to the next and passing of reliability delivery responsibility
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Custody Transfer (cont.) Advantages
Relieves potentially resource-poor end nodes from maintaining end-to-end connection states
Useful for overcoming high loss rates along the delivery path
As reliable as typical end-to-end reliability
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Protocol Translation and Convergence Layers
Bundle forwarding function assumes underlying reliable delivery capability with message boundaries Convergence layer augments underlying
network protocols appropriately
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Time Synchronization Need for time synchronization
Provide a mechanism to deliver pre-programmed control instructions to be executed at future points in time
Use for scheduling, path selection and to remove expired pending messages
Propose time synchronization on the order of 1 ms
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Security Each message contains
Identity of sender Requested class of service (CoS)
Use public key cryptography First DTN router verifies user and
validates CoS request Re-signs message using its key Core routers need only cache keys of
their neighbours
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Congestion and Flow Control Flow control is hop-by-hop
Uses underlying protocols mechanisms if they exist
Congestion control Refers to contention of persistent
storage at a DTN forwarder Current approach uses a priority queue Priority inversion and head-of-line
blocking can occur
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Application Interface Applications must be able to operate
in a regime where request/response time may exceed the longevity of the client and server processes
Application interface is non-blocking Also has registration and callback
functions between bundle-based applications and the local forwarding agent
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Implementation
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Implementation (cont.) Prototype DTN system under Linux
Application interface Rudimentary bundle forwarding across
scheduled and “always on” connections Detection of new and lost contacts Two convergence layers
TCP/IP Bundle-based proxy to the Berkeley mote
network
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Conclusion DTN architecture attempts to provide
interoperable communications between and among challenged networks
Design uses message switching with in-network retransmission, late-binding of names and routing tolerant of network partitioning