A Delay-Tolerant Network Architecture for

Challenged InternetsAuthor: Kevin Fall

Paper Presentation: Vinay Goel

Internet Service Model

TCP/IP basedEnd to end IPC using

concatenation of potentially dissimilar link layer technologies

Packet-switched model of serviceA number of key assumptions…

Key Assumptions

AssumptionsEnd to end path exists b/w data source and

its peer(s)Maximum RTT b/w any node pairs is not

excessiveEnd to end packet drop probability is low

A class of challenged networks violate one or more of the assumptions

Examples of challenged networks

Terrestrial Mobile NetworksUnexpectedly partitioned due to node mobility,

changes in signal strength etc.Exotic Media Networks

High latencies with predictable interruption, suffer outage due to environmental conditions etc.

Military Ad-hoc NetworksHostile environmentsMobility, environmental factors, or intentional

jamming may cause disconnectionData traffic competing for bandwidth

Characteristics of these networks

Path and Link characteristicsHigh latency, low data rateDisconnectionLong queuing times

Network ArchitecturesInteroperability considerationsSecurity

End system characteristicsLimited longevityLow duty cycle operationLimited resources

Adapt Internet to these environments?

Link-repair approachesEngineer problem links to appear more similar to

the types of links for which TCP/IP was designed“fool” the internet protocols: strive to maintain

end-to-end reliability etc.Attach these networks to the edge of the

InternetUse of a special proxy agentProvides access to and from challenged networks

from the InternetNo support for using such networks for data

transit

Link repair approaches

In-network entities (“middle boxes”)Performance Enhancing Proxies (PEPs)

& protocol boostersContain state necessary for connection

violating the Internet fate sharing principles Confound end-to-end diagnostics and reliability,

increase system complexity if mobility is frequentPose a significant challenge for end-to-end

security mechanisms

Application Layer proxies

Provide specialized Internet-to-”special network” name mapping & protocol translation

Used at the edge of special networksDisadvantage: their specificity

Either respond to a specialized set of commands or act as raw data conduits

Limit the ability to re-use proxies for different applications

Fail to take advantage of special resources (storage, processing capabilities etc.)

Electronic Mail

Asynchronous message delivery systemProvides an abstraction that comes

close to addressing many problemsFlexible naming, asynchronous message-

based operation etc.Falls short

Lack of dynamic routing Weakly defined delivery semantics lack of consistent API

What’s the most desirable framework?

A network service and API providing non-interactive messaging

System should combine some overlay routing capability (such as in P2P systems) with delay-tolerant and disconnection-tolerant properties of e-mail

Delay Tolerant Message Based Overlay

ArchitectureBased on abstraction of message

switchingMessage aggregates known as

“bundles”Routers that handle them are called

“bundle forwarders” or DTN gatewaysArchitecture provides a store-and-

forward gateway function between various network architectures

Regions and DTN gateways

Two nodes are in the same region if they can communicate without using DTN gateways

DTN gatewayPoint through which data must pass in order

to gain entry to a regionCan serve as a basis for both translation

and well as a point to enforce policy and control

Name Tuples

Identifiers for objects or groups of objectsDTN name tuple {Region Name, Entity

Name}First portion is a globally unique,

hierarchically structured region nameInterpreted by DTN gateways to find the path(s)

to one or more DTN gateways at the edge of the specified region

Second portion identifies a name resolvable within the specified regionNeed not be unique outside the region

Name resolution

Only region identifier is used for routing a message that is in transit across a collection of regions

Entity name information is locally interpreted in the destination region

Form of late binding

A Postal Class of Service

Priority based resource allocationAdopt a subset of the types of

services provided by US Postal Service

Attractive characteristicsLow, ordinary and high priority

deliveryReturn receipt, delivery records

Path Selection and Scheduling

Architecture targeted at networks where an end-to-end path can’t be assumed to exist

Routes are comprised of a cascade of time-dependent contacts (communication opportunities)

Particular details of path selection and scheduling - heavily influenced by region-specific routing protocols and algorithms

Custody Transfer and Reliability

Custody transfer: acknowledged delivery of a message from one DTN hop to the next and corresponding passing of reliable delivery responsibility.

End hosts do not ordinarily need to keep a copy of data that has been custodially transferred to a DTN next hop

Custody transfer can be viewed as a performance optimization for end-to-end reliability that involves endpoint movement

Convergence Layers and Retransmission

Facilities provided by transport protocols in use within the regions may vary significantly

Bundle forwarding assumes underlying reliable delivery capability with message boundaries when performing custody transferTransport protocols lacking these features

must be augmentedInclude transport-protocol-specific

convergence layers

Time Synchronization

Coarse LevelIdentifying message fragmentsPurging messages that have

exceeded their source specified lifetimes

Stringent constraintsScheduling, path selectionCongestion management

Security

Verifiable access to the carriage of traffic at a particular class of service

Avoid carrying traffic potentially long distances that is later found to be prohibited

Each message includes an immutable “postage stamp” containingVerifiable identity of sender, an approval, class

of service etc.Credentials checked at each DTN hop by

routers; use of public key cryptography

Congestion and Flow Control

Flow control: limiting the sending rate of a DTN node to its next (DTN) hopAttempt to take advantage of underlying

protocols’ mechanismsCongestion control: handling of

contention for the persistent storage of a DTN gatewayShared priority queue for allocating

custody storage

Application Interface

Applications must be careful not to expect timely responses

Must be capable of operating in a region where a request/response RTT may exceed the longevity of the client and server processes

Structured to continue operating in the face of reboots or network partitioning as much as possible

Conclusion

Design embraces notion of message switching with in-network storage & retransmission, late-binding of names & routing tolerant of network partitions

Puts forth several design decisions worthy of consideration

Questions?