1	Pervasive Computing Vincenzo Liberatore Case Western Reserve University
2	Pervasive Computing Computing in the physical world Components Sensors, actuators Controllers Networks Enables Operations in hazardous environments Timely remote support Continuous operations Remote monitoring Troubleshooting Reduce time, effort, cost to develop and upgrade applications Merge cyber- and physical- worlds
3	Example Physical environment Pipes, levers Switches Sample task Close lever Robot Actuators Arm, gripper Sensing Force feedback Visual feedback Control Local compliant control Remote supervision
4	Expertise Embedded Systems Devices embedded in physical world Control Theory Interaction physics and computation Computer Networks Interconnection of embedded systems, controllers
5	Roadmap Protocol Layers Network and data link layers Transport layer: TCP, RTP/UDP Middleware Quality-of-Service
6	Protocol “Layers” Networks are complex! Many “pieces”: hosts routers links of various media applications protocols hardware, software Question: Is there any hope of organizing structure of network? Or at least our discussion of networks?
7	Organization of air travel A series of steps
8	Organization of air travel: a different view Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below
9	Layered air travel: services
10	Distributed implementation of layer functionality
11	Why layering? Dealing with complex systems: Explicit structure allows identification, relationship of complex system’s pieces Layered reference model for discussion Modularization eases maintenance, updating of system Change of implementation of layer’s service transparent to rest of system E.g., change in gate procedure doesn’t affect rest of system Layering considered harmful?
12	Internet protocol stack application: supporting network applications ftp, smtp, http transport: host-host data transfer tcp, udp network: routing of datagrams from source to destination ip, routing protocols link: data transfer between neighboring network elements ppp, ethernet physical: bits “on the wire”
13	Layering: logical communication Each layer: distributed “entities” implement layer functions at each node entities perform actions, exchange messages with peers
14	Layering: logical communication E.g.: transport take data from app add addressing, reliability check info to form “datagram” send datagram to peer wait for peer to ack receipt analogy: post office
15	Layering: physical communication
16	Pervasive Computing Stack Network stack Layers of network software Middleware and above User space Transport and below Operating Systems Design choice Choose protocol at each layer Modular design
17	Roadmap Protocol Layers Network and data link layers Transport layer: TCP, RTP/UDP Middleware Quality-of-Service
18	Network Layer Network layer End-to-end connectivity Multi-hop networks E.g., wireless laptops in U.S., Sweden linked as 802.11 ↔ Ethernet ↔ satellite ↔ Ethernet ↔ 802.11 Convergence layer (IP) IP (Internet Protocol) IP supports multiple data links Multiple transports support IP
19	Network Layer Design space Choose transport Choose data link Convergence Transport independent of data link Mix and match E.g., RTP/UDP over 802.11
20	Roadmap Protocol Layers Network and data link layers Transport layer: TCP, RTP/UDP Middleware Quality-of-Service
21	Transport Layer Design choice TCP or RTP/UDP Three rules: Real-time data: RTP/UDP Sensor data Video, sound, I/O Control Bulk data: TCP Log download Software upgrades Coarse supervision Middleware rules E.g., RealAudio over UDP E.g., HTTP over TCP Usually, right choice made by middleware designer However, bad choices can propagate
22	TCP TCP’s objectives Reliable, bi-directional, byte stream Simplicity Do not worry about reliability A problematic choice Reliability and timeliness Data delivered successfully but late Head-of-line blocking Flow control Low bandwidth applications Overhead Memory, time
23	Real-Time Transport RTP-RTCP Problems above solved Support for Feedback on connection quality Timing, synchronization End-point description, aggregation Frame boundaries Flexibility Application-level framing Custom-tailored reaction to congestion, packet losses, but No error detection, recovery Small header Multicast support
24	RTP Objectives Support real-time communication Active in Internet video and audio RTP header Includes sequence number, timestamp Supports an extension header, padding RTCP Control protocol Library support RTPCreate(), RTPOpenConnection() RTPSend(), RTPReceive() RTPCloseConnection(), RTPDestroy() URL: www.cs.columbia.edu/~hgs/rtp/
25	Roadmap Protocol Layers Network and data link layers Transport layer: TCP, RTP/UDP Middleware Quality-of-Service
26	Resource Discovery Plug-and-play Add new resources on the fly Example: USB Plug in a USB camera on a USB port But now we want: on a network, with arbitrary units Example Locate the Paradex on the network
27	Jini Operations Discover, Join, Look-up, Use Programming Include a library Use functions Fault-tolerance Leases Join only last for a certain time period Renew the lease Multiple look-up servers JavaSpaces Distributed shared memory URL: www.jini.org
28	Middleware Between application and transport Libraries to provide advanced functionality Hide communication Applications Resource Discovery Remote Procedure Calls Security Interoperability (e.g., since Real-Time Corba) Scheduling, resource management, performance analysis Multicast Software development Simpler, faster State-of-the-art functionality Middleware over IP Wealth of libraries for IP Critical advantage of the Internet Protocol
29	Roadmap Protocol Layers Network and data link layers Transport layer: TCP, RTP/UDP Middleware Quality-of-Service
30	Quality-of-Service (QoS) Best-effort model Prevalent in the Internet No end-to-end circuit Packets can be delayed Delay is often unpredictable No bandwidth reservation QoS Ameliorate best-effort model E.g., packet priorities Impact on control Delays E.g., forces small feedback gains Packet drop-outs Infinite delays Jitter Delay variability Often more critical than average delays Bandwidth Relevance?
31	QoS Application and middleware Remediate network vagaries Network layer Map to data link QoS QoS in multihop networks Data link E.g., priorities
32	QoS at IP QoS at data link E.g., priorities, token passing, VPN E.g., CAN, Ethernet QoS at IP Protocol IPv4: 4 basic Types-of-Service (ToS) IPv6: many flow ids, Classes-of-Service (CoS) QoS: from data link to IP Code-point mapping IP addresses
33	QoS in Multi-hop Networks Queuing Packets are forwarded from one network to another Packets are queued at routers (store-and-forward) Protect from cross-traffic cause congestion Per-Hop Behaviors (PHB) Behavior of individual router E.g., Forward before all other queued packets Maps into data link QoS Depends on Class-of-Service Class of service Assigned by source or shapers Imply PHB
34	Middleware and QoS End-point middleware Assume best-effort, unreliable network with no explicit QoS provision End-points compensate for vagaries Previous work (example) Voice-over-IP Replay buffer Sampling and control methods Joint work with M. Branicky and S. Phillips On-line papers through vincenzo.liberatore.org
35	QoS: LAN and WAN Data link Network layers Data link QoS independent of IP, transport, middleware Middleware and transport motivate IP Multiple possible solutions E.g., Ethernet switches with 802.1p, 802.1q Administrative Can do anything (e.g., RVSP) within any single administrative domain Need much cooperation with multiple administrative domains
36	QoS Assurances Objectives QoS protects from cross-traffic QoS does not protect from failures, interferences Worst-case Is it cross-traffic? Is it failures (e.g., network partition)? Reasoning Reason about system timing Priorities make it easy QoS vs. fault-tolerance Failures as first class citizens Average-case Non-sense Exacerbated by TCP Tail E.g., 99-percentile Does not account for failure It is not meant to account for failures Must be paired with middleware, fault-tolerance
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