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Comp2005 - Lecture 12 - Delay Tolerant Networking
Alejandro Saucedo - Comp2005 Lecture 12 FlashCard Set
Computer Science
Undergraduate 2

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What scenarios are delay tolerant networks normally implemented in?
  • Low power and lossy networks (LLN)
  • Wireless sensor networks (WSN) - with intermittent but scheduled connectivity
  • Highly stressed networking environments
    • Intermittent connectivity
      • Devices may have low power, or mobile
    • Large and/or variable delays
      • Potentially inter-planetary
    • High bit error rates
What is the Internet Research Task Force (IRTF)?
  • A parallel research organisation to the IETF
  • Studies architectures and protocols that are a bit "further out"
  • IRTF has DTN RG
What are some assumptions of the TCP/IP model?
  • End-to-end path exists for duration of communcation
    • Affects design of protocols such as TCP
    • e.g. retransmission can be based on timely feedback
  • End-to-end loss is (relatively) small
  • All routers and hosts support TCP/IP
  • Packet switching is the most appropriate abstraction for interoperability and performance
  • Selecting a single route is sufficient
    • Based on a path determined by IP routing protocols
What are some characteristics of Delay tolerant network architecture (DTN)?
  • Defines a message overlay network that sits on top of existing transport protocols
    • DTN nodes create and tarnsmit "bundles"
    • Assumption of some bundle storage capability on DTN nodes
    • Using a flexible naming scheme based on URIs
    • The RFCs don't talk about specific link or routing technologies
What are DTN design principles?
  • Support for varriable length messages
  • Flexible naming syntax
  • Presence of persistent storage in the network for "store and forward"
    • Disk, flash memory, etc
  • Security mechanisms that discard unauthorised traffic as quickly as possible
  • Need to minimise number of round trip messages
  • Cope with restarts while network transactions remain pending
    • Stored bundles must thus survive restarts
  • Coarse-grained classes of service, and a means to express useful lifetime of data
    • Allows network to make better forwarding decisions
What are some characteristics of DTN messages?
  • Known as applciation data units (ADUs)
    • Can be arbitrary length
    • May not be ordered; make no assumptions
  • Bundle layer transforms ADUs into protocol data unit(s) known "bundles"
    • Bundles use an endpoint identifier (EID) name space
  • Bundles contain two or more "blocks" of data
    • Blocks serve classic header or payload functions
    • A bit like IPv6 chained header/payload design
    • Important to consider impact of header block overhead
  • Bundles may be fragmented
    • Fragments can become new bundles
What are some underlying transports which could be used between any two DTN nodes?
  • Examples of transport mediums can be:
    • IP
    • Some specific proprietary space protocol
    • Serial lines
    • Hand-carried service
  • Convergence layer adapter (CLA) is used to send bundles over the underlying transport
    • Bundle Protocol Agents (BPAs) that provide that interface
    • Bundle layer is abstracted to URI namespace
What are the key capabilities of the Bundle Protocol?
  • Runs over dynamic, intermittent, constituent networks
  • Cornerstone of the store and foward overlay
  • Custody-based retransmission
    • If reliable communications are needed
  • Ability to cope with intermittent connectivity
  • Ability to take advantage of opportunistic connectivity
  • Late binding of overlay network endpoint identifiers to underlying transport addresses
What does the bunlle header contain?
  • Creation timestamp
  • Lifespan
  • Class of service flags
  • Source EID
  • Destination EID
  • Report-to EID - for administrative/diagnostic use
  • Custodian EID
  • Payload length is in the payload block
What is the purpose of the Applicaiton interface?
  • Applications on DTN nodes "register" with Bundle Protocol  Agents (BPAs)
    • Registration maintained by the DTN node
    • Should survive restarts
  • Applications then pass data to their BPA
  • BPA creates bundles and performs transmissions
    • A form of "packetisation"
  • In DTN, the basic service is unacknowledged, possibly prioritised, but not guaranteed, unicast
    • Reliable transport requires BPA acknowledgements for "custody transfer" between "custodians"
What QoS support is available in the DTN architecture?
  • Similar to differentiated services on the IP approach
  • Relative priority included in block header:
    • Low(bulk), medium(normal), high(expedited)
    • By default, applied to bundles form same source
What are the priority classes of DTN QoS?
  • Bulk
    • Only sent when all other bundles of other classes are sent
  • Normal
  • Expedited
    • Shipped prior to bundles marked with other classes
What are the requirements for DTN naming?
  • Endpoint identifiers (EID):
    • Need to be flexible
    • Support variable length names
    • Multiple type of identifiers
    • Ability to identify the namespace being used
What are characteristics of Naming and Addressing for DTNs?
  • Endpoint Identifiers (EID) are tokens by which bundles are routed to their destination
  • May use DNS-like names, but these are just strings
    • e.g. dtn://
  •  These can be wildcarded (certificated)
  • A DTN node's routing table contains
    • EID destination (next hop)
    • Next hop can be EID
What are some characteristics of Name Resolution for DTNs?
  • DNS not appropriate for DTN environments
    • Unlike normal IP networking, you don't resolve the destination namespace to a transport address at the source
    • Nor would recursive resolution be practical
  • Use of "Late binding"
    • Resolve the namespace at each node - Map EID to a next hop EID or to a lower layer address for transmission
    • Allows cases where transmit time may exceed validity time of a binding
    • Likely to have stale data
    • Currently work in progress
How does Routing and Forwarding work with DTNs?
  • DTN framework supports unicast, anycast and multicast
  • DTN network can still form a graph
    • Graph may change over time
    • Links may vary in delay, capacity and be directional
    • These factors should be included in forwarding decisions
  • Different type of connectivity
    • Persistent
    • On-demand
    • Intermittent - scheduled
    • Intermittent - Opportunistic
    • Intermittent - probabilistic, based on past behaviour
What are the principles of routing in DTNs?
  • Challenge is the large set of DTN scenarios
    • Many environments, many characteristics
  • Classic distance-vector or link-state approaches not appropriate
    • Variability of connectivity, transmission delays
  • Cannot assume end-to-end live path
    • But we can note that in some cases that such a path exists "on average" over time
  • May replicate bundle
    • Not forward and discard  (As per IP networks)
    • Can then pass to ther nodes as they're encountered
    • Bundles then "live" in the network
What are some routing approaches to DTNs?
  • Flooding, epidemic flooding
    • Pass bundles to nodes that do not have them
    • Statistically, bundles will get to all randomly moving nodes
    • But not scalable with low storage
    • Pass bundle on based on neighbour's estimated probability to reach destination
      • "Have you seen Node X?"
    • If probability > threshold, replicate bundle
  • Maxprop and RAPID
    • Constrained replication based on limited storage/bandwidth, using historical data
  • Adapted link-state routing
Give an Example of application of link-state principles to DTN
  • Delay Tolerant Link State Routing (DTLSR)
    • Nodes can be assigned to link-state areas
    • Constrains network graph and scope of LSAs
    • Can then use gateways between areas
What do Link State Advertisements (LSA) consist of in DTLSR?
  • Contains EID, sequence number, area id, vector of link-state information
  • Per-link information can include measured bandwidth, latency and average/current queue occupancy
  • LSAs can be forwarded as DTN bundles with long lifetime
  • Assumption is that a path can be valid route, even if it is only available on an intermitent basis
How to compute best paths for DTLSR?
  • Can use Dijkstra
    • DTLSR version focuses on minimising the expected delay for all messages
    • With the goal of maximising the overall delivery rate
    • Can utilise last known queue status, to estimate to drain current queue, and then to send a new bundle
    • Can discount links known to be down for a long time
  • Classic link state rouding, but can include links to currently unreachable nodes
  • Approach well0suited if connectivity "predictable"
    • e.g. routing over a bus network
    • Not so good for a more "random" scenario, which is where flooding approaches may be more appropriate
What are some DTN security considerations?
  • Prevent unauthorized applications having data carried through or asserting control over the DTN
    • Considering the nature/cost of the links here
  • Prevent applications sending bundles at a rate or class of service for which they lack permission
  • Discard damaged or improperly modified bundles
  • Detect and de-authorise compromised entities
  • Challenges due to the DTN environment, e.g.
    • Updating ACLs (Access control lists)
    • Revoking permissions
What are some characteristics of administrative messages in DTNs?
  • DTN framework supports (optional) administrative and diagnostic messages
  • For example:
    • Bundle status report (bsr)
      • Received, forwarded, deleted, delivered, etc
    • Custody Signal
      • Successfully transferred
  • Can give information about delivery of bundles
  • Similar to ICMP (Internet Control message protocol) for notmal IP
    • But sent to Report-to EID, not to the source EID
    • Gives flexible reporting capability
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