A DelayTolerant Network Architecture for Challenged Internets

1
A Delay-Tolerant Network Architecture for
Challenged Internets

• Author Kevin Fall
• Paper Presentation Vinay Goel

2
Internet Service Model

• TCP/IP based
• End to end IPC using concatenation of potentially
  dissimilar link layer technologies
• Packet-switched model of service
• A number of key assumptions

3
Key Assumptions

• Assumptions
• End to end path exists b/w data source and its
  peer(s)
• Maximum RTT b/w any node pairs is not excessive
• End to end packet drop probability is low
• A class of challenged networks violate one or
  more of the assumptions

4
Examples of challenged networks

• Terrestrial Mobile Networks
• Unexpectedly 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 Networks
• Hostile environments
• Mobility, environmental factors, or intentional
  jamming may cause disconnection
• Data traffic competing for bandwidth

5
Characteristics of these networks

• Path and Link characteristics
• High latency, low data rate
• Disconnection
• Long queuing times
• Network Architectures
• Interoperability considerations
• Security
• End system characteristics
• Limited longevity
• Low duty cycle operation
• Limited resources

6
Adapt Internet to these environments?

• Link-repair approaches
• Engineer 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 Internet
• Use of a special proxy agent
• Provides access to and from challenged networks
  from the Internet
• No support for using such networks for data
  transit

7
Link repair approaches

• In-network entities (middle boxes)
• Performance Enhancing Proxies (PEPs) protocol
  boosters
• Contain state necessary for connection violating
  the Internet fate sharing principles
• Confound end-to-end diagnostics and reliability,
  increase system complexity if mobility is
  frequent
• Pose a significant challenge for end-to-end
  security mechanisms

8
Application Layer proxies

• Provide specialized Internet-to-special network
  name mapping protocol translation
• Used at the edge of special networks
• Disadvantage 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.)

9
Electronic Mail

• Asynchronous message delivery system
• Provides an abstraction that comes close to
  addressing many problems
• Flexible naming, asynchronous message-based
  operation etc.
• Falls short
• Lack of dynamic routing
• Weakly defined delivery semantics
• lack of consistent API

10
Whats 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

11
Delay Tolerant Message Based Overlay Architecture

• Based on abstraction of message switching
• Message aggregates known as bundles
• Routers that handle them are called bundle
  forwarders or DTN gateways
• Architecture provides a store-and-forward gateway
  function between various network architectures

12
(No Transcript)
13
Regions and DTN gateways

• Two nodes are in the same region if they can
  communicate without using DTN gateways
• DTN gateway
• Point through which data must pass in order to
  gain entry to a region
• Can serve as a basis for both translation and
  well as a point to enforce policy and control

14
Name Tuples

• Identifiers for objects or groups of objects
• DTN name tuple Region Name, Entity Name
• First portion is a globally unique,
  hierarchically structured region name
• Interpreted 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 region
• Need not be unique outside the region

15
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

16
A Postal Class of Service

• Priority based resource allocation
• Adopt a subset of the types of services provided
  by US Postal Service
• Attractive characteristics
• Low, ordinary and high priority delivery
• Return receipt, delivery records

17
Path Selection and Scheduling

• Architecture targeted at networks where an
  end-to-end path cant 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

18
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

19
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 transfer
• Transport protocols lacking these features must
  be augmented
• Include transport-protocol-specific convergence
  layers

20
Time Synchronization

• Coarse Level
• Identifying message fragments
• Purging messages that have exceeded their source
  specified lifetimes
• Stringent constraints
• Scheduling, path selection
• Congestion management

21
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 containing
• Verifiable identity of sender, an approval, class
  of service etc.
• Credentials checked at each DTN hop by routers
  use of public key cryptography

22
Congestion and Flow Control

• Flow control limiting the sending rate of a DTN
  node to its next (DTN) hop
• Attempt to take advantage of underlying
  protocols mechanisms
• Congestion control handling of contention for
  the persistent storage of a DTN gateway
• Shared priority queue for allocating custody
  storage

23
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

24
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

25
Questions?