1. Introductory Concepts Introductory Concepts Rudra Dutta ECE/CSC 570 - Fall 2010, Sections 001, 601

2. Copyright Fall 2010, Rudra Dutta, NCSU2 Digital Communication Digital representation of information – Reduces diverse information to same form – Allows infinite replication – Allows general purpose manipulation (computers) – Allows transmission from one computer to another Digital communication – Revolution in our times – Created digital economy – Large number of products and services seen as information

3. Copyright Fall 2009, Rudra Dutta, NCSU3 Components for Communication Nodes (computers, …) Links Communication primitive

4. Copyright Fall 2009, Rudra Dutta, NCSU4 Scalability

5. 5 Cooperation Nodes - endpoint nodes and intermediate nodes Links Communication primitive

6. Copyright Fall 2009, Rudra Dutta, NCSU6 Forwarding Performed by intermediate node – Process of copying data off one incoming link, on some outgoing link May be performed at physical layer – (Sub-bitpipe) – Utilize physical phenomena - “Switching” – Electrical circuit, etc. May be at higher layer – Bits or some organization of bits – “Forwarding” – “Routing” (ambiguity)

7. Copyright Fall 2009, Rudra Dutta, NCSU7 The Need for Sharing Taking turns “Multiplexing” – Time division, frequency division

8. Copyright Fall 2009, Rudra Dutta, NCSU8 Reserved Timeslots Two modes of TDM – Timeslots are reserved for end stations - “slotted” – First-come-first-serve - “statistical”

9. Copyright Fall 2009, Rudra Dutta, NCSU9 Protocols “Rule set”, “language” Standards for communication hardware/software Common guidelines for implementations – Different implementations of same protocol – Standards bodies make protocols ITU-T, IETF, IEEE – Software manufacturers produce implementations On = “1” or “0” ?

10. Copyright Fall 2009, Rudra Dutta, NCSU10 Delay Two meanings – How fast can successive bits be put into the pipe? – How long does a bit take to traverse the pipe? Time Sender Receiver “Distance”

11. Copyright Fall 2009, Rudra Dutta, NCSU11 Throughput Total number of bits transferred, over given time – Related obviously to the transmission delay – Propagation delay often referred to simply as “delay” or “latency” – Throughput sometimes referred to as “bandwidth” – “Bandwidth-delay product” - filling the pipe

12. Copyright Fall 2009, Rudra Dutta, NCSU12 Software Protocols Syntax of a message – what fields does it contain? – in what format? Semantics of a message – what does a message mean? – for example, not-OK message means receiver got a corrupted file Actions to take on receipt of a message – for example, on receiving not-OK message, retransmit the entire file

13. Copyright Fall 2009, Rudra Dutta, NCSU13 Service view of protocol A protocol is a software module providing a service ftp provides reliable file transfer service Peer entities use a protocol to provide a service to a higher-level peer entity – for example, postal workers use a protocol to present customers with the abstraction unreliable letter transfer service Service interaction between modules constrained to very specific pattern  layering

14. Copyright Fall 2009, Rudra Dutta, NCSU14 Some terminology Service access point (SAP) – Protocol data units (PDUs) – Service data units (SDUs) – Encapsulation – PDU = SDU + optional header or trailer

15. Copyright Fall 2009, Rudra Dutta, NCSU15 The importance of being layered Breaks up a complex problem into smaller manageable pieces – can compose simple service to provide complex ones – for example, WWW (HTTP) is Java layered over TCP over IP (and uses DNS, ARP, DHCP, RIP, OSPF, BGP, PPP, ICMP) Abstraction (hiding) of implementation details – separation of implementation and specification – can change implementation as long as service interface is maintained Can reuse functionality

16. Copyright Fall 2009, Rudra Dutta, NCSU16 ISO OSI reference model A set of protocols is open if – protocol details are publicly available – changes are managed by an organization whose membership and transactions are open to the public A system that implements open protocols is called an open system International Organization for Standards (ISO) prescribes a standard to connect open systems – open system interconnect (OSI) Has greatly influenced thinking on protocol stacks

17. Copyright Fall 2009, Rudra Dutta, NCSU17 The seven layers ApplicationTop level user of the system PresentationResolve platform issues (data representation, possibly encryption) SessionFull-duplex, expedited data delivery, session synchronization TransportError control, flow control, multiplex Reliability NetworkConcatenates links to form end-to-end abstraction Data Link ControlOrganizes bit transmissions into frame transmissions (LLC, MAC sublayers) PhysicalMoves bits between physically connected end-systems End-to-end Node-to-node

18. Copyright Fall 2009, Rudra Dutta, NCSU18 Peer Processes End Node Intermediate Node End Node DLC Phy DLC

19. Copyright Fall 2009, Rudra Dutta, NCSU19 Network Components Different network components function at different layers Embody service at different protocol layers Nomenclature is not very well standardized, and is changing Mixed, “layer-blurring” devices complicate matters

20. Copyright Fall 2009, Rudra Dutta, NCSU20 DLC PHY NET DLC PHY NET PHY DLC PHY NET DLC PHY NET ABCDE A B C D E Component C in this figure is a wire, or hub.

21. Copyright Fall 2009, Rudra Dutta, NCSU21 DLC PHY NET DLC PHY NET DLC PHY DLC PHY DLC PHY NET DLC PHY NET ABCDE A B C D E Computer C in this figure is a bridge. (Could be a switch, if dumb component.)

22. Copyright Fall 2009, Rudra Dutta, NCSU22 DLC PHY NET DLC PHY NET DLC PHY DLC PHY DLC PHY NET DLC PHY NET ABCDE Computer C in this figure is a router, or switch A B CD E DLC PHY NET

23. Copyright Fall 2009, Rudra Dutta, NCSU23 Layers in a Router/Switch DLC PHY NET DLC PHY DLC PHY NET ARD DLC PHY NET

24. Copyright Fall 2009, Rudra Dutta, NCSU24 Software Operation Some of L2 and all of L3 protocols are software processes Exchange of data requires IPC, and blocking Buffering may be employed between layers – Almost certainly at higher than the bitpipe layer

25. Copyright Fall 2009, Rudra Dutta, NCSU25 Buffering at L3 Operation of L3 itself may require buffering data – Store-and-forward Input-buffer-process-buffer-output cycle – May fall behind Discard data  loss DLC PHY NET DLC PHY DLC PHY NET ARD DLC PHY NET

26. Copyright Fall 2009, Rudra Dutta, NCSU26 Layering We have broken a complex problem into smaller, simpler pieces – Provides the application with sophisticated services – Each layer provides a clean abstraction to the layer above HOWEVER - sacrifices efficiency – Might even sacrifice functionality, or optimality – Recently, cross-layer approaches have started gaining ground Example - power-aware routing in ad-hoc networks

27. Copyright Fall 2009, Rudra Dutta, NCSU27 Network Performance Ultimately, measured in quantities the end-user cares about – Delay, throughput – Other metrics derived from these More sophisticated metrics – Predictability / Reliability / Survivability – Variability of delay or throughput – Guarantees - Quality of Service contracts – Security

28. Copyright Fall 2009, Rudra Dutta, NCSU28 Traffic Characterization Traffic - that which is carried by network – Generated and consumed by end-nodes – “Demand” for networking services: b/w and switching Magnitude (bandwidth) – Could vary with time, if “reasonably long” life Lifetime – How long it is resident in the network Arrival and departure patterns – Call (like telephony) arrival and departure – Increment and decrement – Periodic (scheduled) – Static (long-term) Requirement of performance – Hard or statistical

29. Copyright Fall 2009, Rudra Dutta, NCSU29 Network View Connectivity is always less than full (esp. in large networks) Because of scalability, hierarchy seems inevitable Nature of end-nodes and intermediate nodes vary All links are TDM (FDM modeled as separate links) 1 2 3 4

30. Copyright Fall 2009, Rudra Dutta, NCSU30 Traffic Aggregation - Static Traffic Consider lowest level networks Assume each station injects traffic steadily – Number of bits injected per time unit is constant for each source Due to aggregation, magnitude increases as traffic climbs hierarchy – But constant nature of traffic remains Aggregation/dis-aggregation process is straightforward for intermediate nodes – Effectively same as slotted TDM Therefore static traffic is stable - remains static at higher levels of hierarchy Magnitude and therefore capacity, of course, must increase at higher levels

31. Copyright Fall 2009, Rudra Dutta, NCSU31 Bursty Traffic Traffic is generated intermittently at each end node – Assume (peak) rates are known Question of capacity and aggregation become intertwined – One approach: pretend each end node is a steady source at its peak rate, then provision as before Aggregation will be easy – Another approach: provision for average Do bursts arrive deterministically? Sometimes link will be busy when traffic arrives to use it Must store-and-forward, or discard Question of slotting TDM comes in - work conservation

32. Copyright Fall 2009, Rudra Dutta, NCSU32 A View of Aggregation magnitude (“bandwidth”) burstiness 1 1 2 2 3 3 4 4 4

33. Copyright Fall 2009, Rudra Dutta, NCSU33 Static Traffic in Real Networks Aggregation can tend to cancel out bursts Finite capacity of pipe will appear as static-ness of traffic to next level of aggregation – Also: Concept of “elastic traffic” Source-to-destination traffic flows in the Internet are not static as generated, but congestion slows down bursts In response, flow duration will increase Empirical observation tends to confirm – For example, CAIDA data – Exhibits “busy hour” traffic patterns Changes from hour to hour, but each pattern stable over days and weeks

34. Copyright Fall 2009, Rudra Dutta, NCSU34 About Loss Loss may occur on the link – Usually very little in guided medium - ignore – Usually handled by L2 transmissions or ignored Loss may occur at intermediate nodes – Store-and-forward buffers are finite - may overflow – Other mechanism at intermediate node may discard Does retransmission occur? – May not be required / desired – If desired, May be at L2, on link May be at L4, E2E

35. Copyright Fall 2009, Rudra Dutta, NCSU35 Providing Guarantees - About Delay Controversial proposition: – “If delay is not important, capacity is not important” – “If delay is important, capacity must be large OR aggregation must be slotted OR both” Links must – Provide connectivity – Have capacity to carry traffic Routers must have memory and processor capacity to switch traffic Network design / resource provisioning problem

36. Copyright Fall 2009, Rudra Dutta, NCSU36 Two Modes of Networking Traffic Networks and Transport Networks Traffic networks: where stochastic demand picture is operative – Short term switching/routing – Design for connectivity Transport networks: where traffic demands of static magnitude are seen to be operative – (Semi-) Permanent – QoS considerations paramount – Demands seen to be injected at transport network nodes, lower level networks not visible – Design for connectivity and capacity

37. Network Design Static traffic / slotted TDM aggregation – Small, constant delay should result – Performance metric is global – demands satisfied, global delay Bursty traffic / statistical TDM – Larger range of delay, but statistically smaller – Design for statistical delay, or simply connectivity Circuits – canonical telephony – Very small and constant delay – Operative performance metric is blocking ratio Copyright Fall 2009, Rudra Dutta, NCSU37

38. Copyright Fall 2009, Rudra Dutta, NCSU38 Multi-layer Networks Generalized protocol layering can create complicated networks – Better thought of as multiple layers – Each layer satisfies flow constraints Generally, demand is specified in one layer and capacity in another – Must assume some mapping method - possibly constrained Digital Transmission Optics Media Cross- Connect IP Networks Ckt-switched voice Private Line

39. Copyright Fall 2009, Rudra Dutta, NCSU39 Review Networks have varied and extensive current and future uses Computers communicate digital information over physical media / links Network components perform cooperative effort Protocols enable communication, arranged in layers Holistic concepts such as connection-orientation, circuit-switching, emerge Traffic is “demand” for network Resource provisioning + protocols and policies produce measurable performance metrics experienced by traffic demand