A Delay-Tolerant Network Architecture for Challenged Internets

April 22, 2023 Anshul Kantawala 1

A Delay-Tolerant Network Architecture for Challenged Internets

Kevin Fall


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


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


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


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


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


Characteristics of Challenged Networks Path and Link characteristics Network architectures End System characteristics


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


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


Path and Link characteristics Long queueing times

Conventional networks rarely greater than a second

Challenged network could be hours or days due to disconnection


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


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


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


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


Delay Tolerant Message-Oriented Overlay Architecture


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


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


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/ }

http://www.ietf.org/


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


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


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


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


Protocol Translation and Convergence Layers

Bundle forwarding function assumes underlying reliable delivery capability with message boundaries Convergence layer augments underlying

network protocols appropriately


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


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


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


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


Implementation


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


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

A Delay-Tolerant Network Architecture for Challenged Internets

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