Term
| What scenarios are delay tolerant networks normally implemented in? |
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Definition
- 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
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Term
| What is the Internet Research Task Force (IRTF)? |
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Definition
- A parallel research organisation to the IETF
- Studies architectures and protocols that are a bit "further out"
- IRTF has DTN RG
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Term
| What are some assumptions of the TCP/IP model? |
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Definition
- 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
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Term
| What are some characteristics of Delay tolerant network architecture (DTN)? |
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Definition
- 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
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Term
| What are DTN design principles? |
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Definition
- Support for varriable length messages
- Flexible naming syntax
- Presence of persistent storage in the network for "store and forward"
- 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
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Term
| What are some characteristics of DTN messages? |
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Definition
- 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
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Term
| What are some underlying transports which could be used between any two DTN nodes? |
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Definition
- 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
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Term
| What are the key capabilities of the Bundle Protocol? |
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Definition
- 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
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Term
| What does the bunlle header contain? |
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Definition
- 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
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Term
| What is the purpose of the Applicaiton interface? |
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Definition
- 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"
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Term
| What QoS support is available in the DTN architecture? |
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Definition
- 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
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Term
| What are the priority classes of DTN QoS? |
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Definition
- Bulk
- Only sent when all other bundles of other classes are sent
- Normal
- Expedited
- Shipped prior to bundles marked with other classes
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Term
| What are the requirements for DTN naming? |
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Definition
- Endpoint identifiers (EID):
- Need to be flexible
- Support variable length names
- Multiple type of identifiers
- Ability to identify the namespace being used
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Term
| What are characteristics of Naming and Addressing for DTNs? |
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Definition
- 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://rover3.mars.net/cmdApp
- These can be wildcarded (certificated)
- A DTN node's routing table contains
- EID destination (next hop)
- Next hop can be EID
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Term
| What are some characteristics of Name Resolution for DTNs? |
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Definition
- 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
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Term
| How does Routing and Forwarding work with DTNs? |
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Definition
- 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
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Term
| What are the principles of routing in DTNs? |
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Definition
- 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
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Term
| What are some routing approaches to DTNs? |
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Definition
- 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
- PRoPHET
- Pass bundle on based on neighbour's estimated probability to reach destination
- If probability > threshold, replicate bundle
- Maxprop and RAPID
- Constrained replication based on limited storage/bandwidth, using historical data
- Adapted link-state routing
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Term
| Give an Example of application of link-state principles to DTN |
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Definition
- 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
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Term
| What do Link State Advertisements (LSA) consist of in DTLSR? |
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Definition
- 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
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Term
| How to compute best paths for DTLSR? |
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Definition
- 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
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Term
| What are some DTN security considerations? |
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Definition
- 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
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Term
| What are some characteristics of administrative messages in DTNs? |
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Definition
- DTN framework supports (optional) administrative and diagnostic messages
- For example:
- Bundle status report (bsr)
- Received, forwarded, deleted, delivered, etc
- Custody Signal
- 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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