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Transcript
A Delay-Tolerant Network Architecture for Challenged Internets Kevin Fall May 25, 2017 Anshul Kantawala 1 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 May 25, 2017 Anshul Kantawala 2 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 May 25, 2017 Anshul Kantawala 3 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 May 25, 2017 Anshul Kantawala 4 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 May 25, 2017 Anshul Kantawala 5 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 May 25, 2017 Anshul Kantawala 6 Characteristics of Challenged Networks Path and Link characteristics Network architectures End System characteristics May 25, 2017 Anshul Kantawala 7 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 May 25, 2017 Anshul Kantawala 8 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 May 25, 2017 Anshul Kantawala 9 Path and Link characteristics Long queueing times Conventional networks rarely greater than a second Challenged network could be hours or days due to disconnection May 25, 2017 Anshul Kantawala 10 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 May 25, 2017 Anshul Kantawala 11 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 May 25, 2017 Anshul Kantawala 12 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 May 25, 2017 Anshul Kantawala 13 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 May 25, 2017 Anshul Kantawala 14 Delay Tolerant Message-Oriented Overlay Architecture May 25, 2017 Anshul Kantawala 15 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 May 25, 2017 Anshul Kantawala 16 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 May 25, 2017 Anshul Kantawala 17 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/ } May 25, 2017 Anshul Kantawala 18 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 May 25, 2017 Anshul Kantawala 19 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 May 25, 2017 Anshul Kantawala 20 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 May 25, 2017 Anshul Kantawala 21 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 May 25, 2017 Anshul Kantawala 22 Protocol Translation and Convergence Layers Bundle forwarding function assumes underlying reliable delivery capability with message boundaries Convergence layer augments underlying network protocols appropriately May 25, 2017 Anshul Kantawala 23 Time Synchronization Need for time synchronization Provide a mechanism to deliver preprogrammed 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 May 25, 2017 Anshul Kantawala 24 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 May 25, 2017 Anshul Kantawala 25 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 May 25, 2017 Anshul Kantawala 26 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 May 25, 2017 Anshul Kantawala 27 Implementation May 25, 2017 Anshul Kantawala 28 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 May 25, 2017 Anshul Kantawala 29 Conclusion DTN architecture attempts to provide interoperable communications between and among challenged networks Design uses message switching with in-network retransmission, latebinding of names and routing tolerant of network partitioning May 25, 2017 Anshul Kantawala 30