Review of fundamentals networking concepts

Transcription

1 Review of fundamentals networking concepts Andrea Bianco Paolo Giaccone Telecommunication Networks Group Computer Network Design - 1 Copyright Quest opera è protetta dalla licenza Creative Commons NoDerivs-NonCommercial. Per vedere una copia di questa licenza, consultare oppure inviare una lettera a: Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. This work is licensed under the Creative Commons NoDerivs-NonCommercial License. To view a copy of this license, visit: or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. Computer Network Design - 2 Pag. 1

2 Review What is a (modern) telecommunication and computer network? Topologies Multiplexing/multiple Switching techniques User (traffic) characterization Properties of telephone and computer networks Protocol architectures for computer networks Physical layer (multiplexing) Data link layer (multiple ) Network layer (routing, congestion) Transport Layer (congestion) Application Layer (client server, P2P, CDN, Data Center) Computer Network Design - 3 Networks Focus on modern telecommunication and computer networks Telephone and Internet One possible network definition (service oriented view) Infrastructure that provides services to applications: Web, VoIP, , games, e-commerce, social nets, phone calls, fax, Internet provides programming interface to apps program to connect to Internet provides service options Computer Network Design - 4 Pag. 2

3 Networks Another network definition: A set of nodes and channels that offer a connection among two or more points to make telecommunication possible among users (i.e. move infos (data/flows) among nodes) Represented as a topology (graph) Different levels of detail Key issue in networks is resource sharing Node is the point where Multiplexing /multiple (link sharing) and switching (node sharing) occurs Computer Network Design - 5 Type of channels Point-to-point channel Only two nodes connected to the channel The channel is used by both nodes (often in the same fashion) Sharing the channel among flows is called multiplexing A B Computer Network Design - 6 Pag. 3

4 Broadcast channel Type of channels Many Tx and many Rx Sometimes one (many) Tx and many (one) Rx Single communication channel shared by all nodes This room! The information sent by one node is received by all other nodes (with some delay) Sharing the channel among flows is called multiple Computer Network Design - 7 Many quality indices Channel properties Attenuation, robusteness to mechanical stress, ease of installation, robusteness to interference, cost, etc, Mainly interested in bit rate [bit/s] Also named bandwidth, throughput, with slightly different meaning delay [s] propagation delay depends on the channel length Bandwidth x delay [bit] channel size how much we can push on the channel Computer Network Design - 8 Pag. 4

5 Topologies in TLC networks The network topology is defined by the relative position of nodes and channels A network topology is a graph G=(V,A) V = set of vertices (represented as circles - nodes) A = set of edges (represented as segments - channels) Edges may be: direct (directed segments (arrow) unidirectional channels) undirect (non directed segments bidirectional channels) Abstraction of real networks Several levels of abstraction are possible Computer Network Design - 9 Many topologies Full mesh, mesh, tree, bus, star, E A B A A E B D C E B D C A D C E B E A B E A B D C D C D C A E B C D Computer Network Design - 10 Pag. 5

6 Physical and logical topology It is important to distinguish between the physical and the logical topology Logical topology: logical interconnection among nodes via logical channels Physical topology: takes into account transmission media constraints Computer Network Design - 11 Physical topology E.g., satellite network A B C D Computer Network Design - 12 Pag. 6

7 Logical topology Private network over satellite network A B C D Computer Network Design - 13 Logical topology Router interconnection for an ISP Router Router Node Node Frame Relay or ATM network Router Node Virtual Circuits Node Router Computer Network Design - 14 Pag. 7

8 Logical topology Overlay among peers in a P2P network Computer Network Design - 15 Logical topology net net net ISP A net net IXP net net net IXP ISP B net net net ISP C net regional net net net net net Computer Network Design - 16 Pag. 8

9 Logical topology Tier 1 ISP Tier 1 ISP Google IXP Regional ISP IXP Regional ISP IXP ISP ISP ISP ISP ISP ISP ISP ISP Computer Network Design - 17 Tier-1 ISP: e.g., Sprint Computer Network Design - 18 Pag. 9

10 Topologies and performance The amount of traffic that can be succesfully transferred (throughput) in a network is for a given channel available capacity inversely proportional to the average distance among node pairs weighted by the amount of traffic exchanged between the two node For uniform traffic and regular topologies the average distance on the topology establish the throughput Computer Network Design - 19 Topologies and performance Comparison among topologies, with the same number of nodes (4) and (almost) the same number of channels Uniform traffic Every node pair exchange x bit/s. Total generated traffic is 12x. Every unidirectional channel has capacity B bit/s. Compute: average distance, network capacity (maximum throughput), maximum channel load,maximum node load Computer Network Design - 20 Pag. 10

11 Topologies and performance Capacity: 3x2B=6B Average distance: 20/12=1.66 Consider only traffic from left to right (simmetry) maximum channel load is 4x. Thus, x <= B/4 Node 3 (or 2) must handle 7B/4 of traffic unit Uniform traffic, non regular topology, unbalanced channel load, unbalanced node load x 2x 2x x x x Computer Network Design - 21 Topologies and performance Capacity: 3x2B=6B Average distance: 1.5 Considering the traffic from node 4. Maximum load on (all) channel is 3x. Thus x <=B/3 The same holds for the other direction Node 4 must handle 3B of traffic unit Uniform traffic, non regular topology, balanced channel load, unbalanced node load 1 x x x 4 2 x x x x x x 3 Computer Network Design - 22 Pag. 11

12 Topologies and performance Capacity: 4x2B=8B Average distance: 1.33 For clockwise traffic the maximum channel load is 2x. Thus x <= B/2. The same holds for counter clock wise traffic Each node must handle 2B unit of traffic Uniform traffic, regular topology, balanced channel load, balanced node load 3x/2 x/2 3x/2 x/2 3x/2 x/2 3x/2 x/2 Computer Network Design - 23 Channel, sharing Multiplexing and multiple Computer Network Design - 24 Pag. 12

13 Sharing channel resources Sharing of channel resources among data flows comes in two different flavours Multiplexing All flows the channel from a single point Single transmitter scenario Centralized problem A radio from an antenna (base station in a cellular network, point in a WI-FI network, satellite transmission), an output link in a switch or a router Multiple- Flows the channel from different points Many transmitters are active Distributed problems Local area networks (if not switched), mobile phones in a cellulare network, PC ing via a Wi-FI hot-spot Computer Network Design - 25 Channel sharing techniques Frequency (FDM - FDMA) Time (TDM - TDMA) Code (CDM - CDMA) f channel t Computer Network Design - 26 Pag. 13

14 Frequency division Each flow is transmitted using different frequency bands Need for band guard f t Computer Network Design - 27 Time division (TDM TDMA) Each flow exploits different time intervals (slots) Define frame over which slot allocations are repeated Need for time guard f t Computer Network Design - 28 Pag. 14

15 Code division (CDM CDMA) Each flow exploits a different code (waveform with higher frequency than the bit tx rate) Need for orthogonal codes c f t Computer Network Design - 29 Code division (CDM CDMA) f t Computer Network Design - 30 Pag. 15

16 Code division Example Code word used by user i: Coded sequence = information bit x code word Information bit: Coded sequence: Computer Network Design - 31 Code multiplexing Example: Code word for user 1: Code word for user 2: Code word for user 3: Code word for user 4: We are interested in receiving data from user 1 Over the channel, transmitted signals sum up (need to equalize power) Tx of 1+2+3: Tx of 2+3 (noise): Computer Network Design - 32 Pag. 16

17 Code multiplexing Reception = correlation with code words Reception of user 1 = scalar product of the received sequence with the code word Everything we receive Transmissions of 1+2+3: Correlation with = 4 The noise Transmissions of 2+3: Correlation with = 0 Computer Network Design - 33 Multiplexing or multiple Time, frequency, code (and space: multiple parallel wires) are all equivalente alternatives Given a channel capacity we can choose one among the above techniques depending on technological constraints Code division permits to increase channel capacity (by allowing more users) if using pseudo-orthogonal codes but degrading the signal to noise ratio at the receiver (increase the bit error rate) Computer Network Design - 34 Pag. 17

18 FDM and TDM FDM Example: 4 users frequency TDM time 125 ms frequency time Computer Network Design - 35 Statistical multiplexing Multiplexing can be deterministic, fixed in time, on the basis of requirements determined at connection setup statistical, variable in time, to adapt to instantaneous traffic requirements Computer Network Design - 36 Pag. 18

19 Statistical Multiplexing A 100 Mb/s Ethernet statistical multiplexing C B queue of packets waiting for output link 1.5 Mb/s D Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand Dynamic TDM scheme E Computer Network Design - 37 Switching techniques Circuit and packet switching Computer Network Design - 38 Pag. 19

20 Switching techniques Circuit switching Telephone networks Packet switching Two flavours (datagram and virtual circuit service) Internet, computer networks Computer Network Design - 39 Three phases Opening Data transfer Closing Network transparently forwards user generated data No operation on user data Fixed bit rate 64kbit/s Circuit switching U1 N1 N2 U2 t t t t Computer Network Design - 40 Pag. 20

21 Space vs time switching U1 N1 N2 U2 125 ms U1 N1 N2 U2 125 ms Computer Network Design - 41 Switching techniques Circuit switching Resources allocated uniquely to a circuit Physical channel, time-slot in TDM frame Resources are shared among connections but are statically allocated to data Connection oriented Need to open (and close) the circuit prior (after) data transmission Store state information on each circuit (stateful approach) Address (unique for each user in the network) used only when opening the circuit, not carried in data Data unit identified by position Routing (choice of the best route) performed only when opening the circuit Done through routing table lookup Data forwarding Through forwarding table look-up (one entry for each active circuit) Static (always the same scheduling, unless circuits are closed or opened) Computer Network Design - 42 Pag. 21

22 Packet switching U1 N1 N2 U2 Transmission time Processing time Propagation time t t t t Computer Network Design - 43 Data transfer over virtual circuits DTE DTE Forwarding table In Label Out Label Computer Network Design - 44 Pag. 22

23 Grouping virtual circuits A virtual circuit is logically identified by a label The label may change value over each link Relabeling Label = often a pair of identifiers (VCI-VPI in ATM) Virtual channel (VC): identifies a single connection Virtual path (VP): identifies a group of virtual channels Computer Network Design - 45 Grouping virtual circuits The grouping allows flow aggregation Eases network management Increases scalability Possible use LAN inter-connection to crete a VPN (Virtual Private Network) Multimedia flows (video, audio, data) Computer Network Design - 46 Pag. 23

24 Virtual circuits and paths (ATM) VPI 1 VCI 1 VCI 2 VPI 6 VCI 3 VCI 4 VCI 5 Computer Network Design - 47 ATM: VP switching VCI 21 VCI 22 VPI 1 VPI 4 VCI 23 VCI 24 VCI 23 VCI 24 VPI 2 VPI 5 VCI 25 VCI 24 VCI 25 VCI 24 VPI 3 VPI 8 VP SWITCHING VCI 21 VCI 22 Computer Network Design - 48 Pag. 24

25 ATM: VC switching VCI 25 VCI 25 VPI 4 VCI 21 VCI 21 VPI 5 VCI 23 VCI 24 VPI 2 VCI 23 VCI 24 VC SWITCHING Computer Network Design - 49 Virtual circuits Switched virtual circuit (SVC) Established on-demand, through signaling, in real-time Three phases Virtual circuit opening Data transfer Virtual circuit closing Users (and network) exchange signaling packets (over dedicated VCI/VPI) to establish a virtual circuit; then, data transfer can occur Permanent virtual circuit (PVC) Established through agreement among user and network provider Off-line, through management procedures Define a semi-static network Logical topology Users can immediately exchange data, with no delay Computer Network Design - 50 Pag. 25

26 Switching techniques Packet switching, with datagram service Shared resources Ideally the full network is available to a single user Resources are continuously shared with all other users at the data level Connectionless Free to send data when available, no need to check network or user availability Stateless approach Each packet must carry the destination (and source) address Data unit identified through source and destination addresses (unique for each pair of users in the network) Routing and forwarding performed independently over each packet Through routing table look-up Computer Network Design - 51 Switching techniques Packet switching, with virtual circuit service Shared resources Resources are shared with all virtual circuits sharing the same link Connection oriented Need to open (and close) the virtual circuit prior (after) data transmission Permanent virtual circuits available Store state information on each virtual circuit (stateful approach) Address (unique for each user in the network) used only when opening the virtual circuit, not carried in data Data unit identified through a label (unique for each existing virtual circuit on each link in the network) Label is unique on each link, but has a local scope, i.e. the value assumed is different on each link for simplicity Routing (choice of the best route) performed only when opening the virtual circuit Done through routing table lookup Data forwarding Through forwarding table look-up (one entry for each active virtual circuit) Re-labelling needed Computer Network Design - 52 Pag. 26

27 Fundamentals of packet switching Data sent as packets Nodes operate in store&forward (almost always) Buffers Delay Many operations on data in the network (not in circuit switching) Error detection, error recovery, flow control, routing, forwarding, congestion control, packet inspection.. Need to define a network architecture to organize functionalities (see later) Computer Network Design - 53 Packet size P Measured in bit Packet size Packet size in time T TX Transmission time measured in s Different on every link T TX = P/V TX where V TX is the link bit rate Packet size in meter M on a given link M = Speed of light x T TX Computer Network Design - 54 Pag. 27

28 Delays Delays suffered by each packet from source to destination node In each link Transmission (and reception) delay It is a funciton of the packet size in bit and of the link bit rate Propagation delay It is a function of the link length in meters In each switching node Processing time Function of the processing speed and of the complexity of the executed procedures on the packet header Normally negligible with respect to transmission time Queuing delays Depend on the traffic generated by all users Highly variable Computer Network Design - 55 Traffic characterization and QoS Provisioning Computer Network Design - 56 Pag. 28

29 User traffic characterization Need to know user behaviour to design a network Traffic sources CBR (Constant Bit Rate) VBR (Variable Bit Rate) Characterized by their rate [bit/s] Computer Network Design - 57 User traffic characterization CBR (Constant Bit Rate) sources: Rate (bit/s) Perfectly known Call duration (s) Call generation process Only statistically known Computer Network Design - 58 Pag. 29

30 Burstiness Computer Network Design Review User traffic characterization VBR sources: Average rate (bit/s) Known? Over which period? Peak rate (bit/s) or Burstiness (Peak rate/ average rate) Known (worst case) Burst duration Known? Call duration (s) Call generation process Only statistically known Computer Network Design - 59 User traffic characterization 1000 Burstiness= Peak rate/ Average rate low speed data alphanumeric terminals connectionless LAN alta velocità immagini data graphical terminal transmission compressed VIDEO non compressed Peak rate [bit/s] supercomputer interconnection HDTV LAN voice audio Computer Network Design - 60 Pag. 30

31 Integrated vs dedicated networks Telecommunications networks were traditionally defined to provide a specific service one service one network paradigm Telephone network for the interactive human voice transportation service Internet for data exchange among computers TV or radio distribution for the TV or radio system Integrated networks one network for any service narrowband ISDN o N-ISDN broadband ISDN o B-ISDN Computer Network Design - 61 Integrated vs dedicated networks Dedicated networks Easier to optimize for the specific service Optimal engineering solutions for the specific requirements of the service Integrated networks advantages No need to create an independent infrastructure for each service Supporting different requirements implies sub/optimal choices Integrated networks trade flexibility and infrastructure cost reduction with perfomance and increased control complexity Computer Network Design - 62 Pag. 31

32 Quality of service provided by networks Networks used as examples Fixed telephone network: POTS Internet B-ISDN Describe in an informal way the quality of service provided by these networks Computer Network Design - 63 POTS Characteristics CBR source completely known (generated by the network) Circuit switching Constant, dedicated bit rate no congestion Minimum possible delay (only propagation): order of tens of ms (real time) Zero loss probability Requirements Error probability smaller than few % Small or negligible blocking probability QoS largely independent of other users (apart from blocking probability) Network utilization can be really low, user satisfaction very high Computer Network Design - 64 Pag. 32

33 Internet Characteristics Source behavior unknown Packet switching with datagram service Complete sharing of network resources Bit rate and delay unknown Possible congestion Loss probability may be significant Requirements Error probability negligible in wired networks Zero blocking probability QoS largely dependent of other users Network utilization can be very high, user satisfaction can be very low Computer Network Design - 65 B-ISDN Intermediate situation Source known (either deterministically or statistically) Packet switching with virtual circuit service May introduce algorithms to control network resources sharing Bit rate and delay negotiable Loss probability negotiable Requirements Blocking probability reasonably small Error probability negligible QoS dependent of other user behavior and of algorithms used to manage network resources Trade network utilization and user satisfaction Computer Network Design - 66 Pag. 33

34 Design problem Given: Network design Network topology (nodes, link speed) Traffic characterization Jointly obtain: Guarantee some sort of QoS for user connection High network utilization Without the objective of high network utilization, the problem becomes trivial overprovisioning (power line or water distribution networks) Computer Network Design - 67 Network design Several flavour Running at different time scale (with different level of complexity) Mainly focus on network design and planning (resource deployment) On the basis of traffic estimates and cost constraints Exploits routing criteria and traffic engineering Other examples Network management (running a network) Measurements Fault management (protection and restoration) Includes re-design and re-planning Connection management Data unit transport Computer Network Design - 68 Pag. 34

35 Design to obtain QoS Different time scale (with different level of complexity) Network design and planning (resource deployment) On the basis of traffic estimates and cost constraints Exploits routing criteria and traffic engineering Network management (running a network) Measurements Fault management (protection and restoration) Includes re-design and re-planning Connection management Data unit transport Computer Network Design - 69 Our definition of QoS Assume that a network has been designed and is properly managed Available resources are given Mainly study algorithms operating at the following timescale: Connection management Data unit transport Also named traffic control problem Must define what is meant by connection. Also named data classification problem. Two different traffic control principles: Preventive control : mainly executed at network ingress, with fairly tight traffic control to avoid congestion insurgence in the network Reactive control: react when congestion situation occur, to reduce or eliminate congestion negative effects Computer Network Design - 70 Pag. 35

36 Traffic control: essential elements Connection oriented network User-network service interface Traffic characterization QoS negotiation Resource allocation (bit rate and buffer) Algorithms for traffic control CAC (Connection Admission Control) and routing Scheduling and buffer management (allocation, discard) in switching nodes Conformance verification (policing or UPC: Usage Parameter Control) Traffic shaping to adapt it to a given model Congestion control Computer Network Design - 71 Traffic control: connection oriented network The connection oriented paradigm permits to know which are the network elements over which traffic control algorithms must be executed (path known) Circuit switching Packet switching with virtual circuit service If high utilization is a major objective: Packet switching As such, the most suited switching technique to obtain QOS is packet switching with virtual circuit service Computer Network Design - 72 Pag. 36

37 Traffic control: user-network service interface The capability to control the network increases with the knowledge of user traffic. Limiting factor is the complexity. Over the service interface Traffic characterization QoS parameters negotiation Can be defined on a call basis or on a contract basis POTS: implicit, on a contract basis Internet: not existing Frame relay: negotiable, normally on a contract basis B-ISDN: negotiable with traffic contract on both contract and call basis Internet extended to support QoS: negotiable through a SLA (Service Level Agreement) mainly on a contract basis Computer Network Design - 73 Traffic control: resource allocation Main resources: Bit rate over transmission links Buffer Resources can be allocated On a contract basis (booking) On a call basis Packet by packet Allocation Exclusive (dedicated resource) Shared Computer Network Design - 74 Pag. 37

38 Algorithms: CAC and routing Routing QoS based path selection to router a connection CAC Determine whether to accept a connection or not, depending on The path chosen by the routing algorithm Traffic characterization QoS requests Network status Constraints It is not acceptable to destroy or even reduce the quality of service guaranteed to already accepted connections Can be relinquished Connection must be refused to avoid network overload or congestion Preventive control (but can become reactive) Computer Network Design - 75 Algorithms: scheduling and buffer management Scheduling Choice of the data unit to be transmitted among data unit stored in the switch Buffer management Allocation (partial/total, exclusive/shared) of memories in the switch Dropping policies Mandatory in an heterogeneous environment to support different QOS requests FIFO (First In First Out) or FCFS (First Came First Served) policy with drop-tail discard is optimal in a homogeneous environment Counter for less than 10 pieces at supermarket Preventive and reactive Computer Network Design - 76 Pag. 38

39 Algorithms: policing e shaping Policing (traffic verification) Network control of user behavior to guarantee conformance to traffic characterization Shaping (traffic conditioning) User/network adaptation of data traffic to make it conformant to a given characterization Mandatory to control user honesty and to adapt traffic which is difficult to generate as conformant a priori Where algorithms must be executed? Only at network edge, i.e., when user network? Multiplexing point modify traffic shape Both at network and internally to the network Mainly preventive, but they can become reactive if QoS level may change over time Computer Network Design - 77 Algorithms: congestion control Congestion Traffic excess over a given channel (link) Can occur due to Short term traffic variability Allocation policies that share resources to increase network utilization Congestion effects: Buffer occupancy increase Delay increase Data loss Needed to obtain high link utilization Must execute at network edge, within the network or.? Reactive Computer Network Design - 78 Pag. 39

40 Protocol architectures Computer Network Design - 79 Architectures and protocols Communication requires cooperation One abstract description of the communication paradigm between two or more users requires the definition of a reference model specifying a network architecture A network architecture defines the communication process the relation among objects used in communication the functionalities to support the communication Layered architectures! Computer Network Design - 80 Pag. 40

41 Layered architectures Layered architectures are used because of simple design simple management simple standardization separation among functions Application Presentation Session Transport Network Data Link Physical OSI User Netw. Appl. Session End to End Routing Data Link Physical DECNET Application Service Internetwork Network ARPA Transaction Service Presentation Service Data Flow Trans. Control SNA Manag. Service Virtual Route Explicit Route Transm. Group Data Link Physical Computer Network Design - 81 half session path control Separation among functions: Internet subnet 1 subnet 2 host 4 router 2 router 3 applications error control host 3 host 1 routing subnet 4 router 1 subnet 3 host 2 packet transfer Computer Network Design - 82 Pag. 41

42 B - ISDN Management plane Control plane High layers User plane High layers AAL ATM Physical Computer Network Design - 83 Protocols Formal definition of the procedures adopted to guarantee the communication between two or more objects on the same hierarchical level to execute a specific function Protocol definition: semantics set of commands and answers syntax structure of commands and answers timing temporal sequence of commands and answers Computer Network Design - 84 Pag. 42

43 Layer and entities Active elements in a subsystem Run the functions of the layer (exploiting service provided by lower layers) Interact (cooperate) within the same layer System A System B (N) - layer (N) - entity transmission media Computer Network Design - 85 Services A service can be: connection-oriented (CO): a preliminary agreement (connection) is established between the network and the communication end-points, then the data is transferred and finally the connection is released connectionless (CL): data is sent to the network without any preliminary agreement and is treated independently from each other Computer Network Design - 86 Pag. 43

44 Services (N) - service N+1 N (N) service provider N+1 N Black-box for the (N+1) - entity Computer Network Design - 87 Services N N-1 (N-1) - service (N-1) service provider N N-1 Black-box for the (N) - entity Computer Network Design - 88 Pag. 44

45 PDU creation (N) - PDU (N) - layer interface (N-1) - SDU SAP (N-1) - layer (N-1) - PCI (N-1) - SDU (N-1) - PDU Computer Network Design - 89 Information transfer Transmitter Receiver Application Presentation Session Transport Network Data link Physical data APCI ASDU PPCI PSDU SPCI SSDU TPCI TSDU NPCI NSDU DLPCI DLSDU bit or symbols Application Presentation Session Transport Network Data link Physical Computer Network Design - 90 Pag. 45

46 The seven OSI layers application presentation session transport network data link physical Application protocol Presentation protocol Session protocol Transport protocol Network protocol Data link protocol Physical layer protocol transmission media application presentation session transport network data link physical Computer Network Design - 91 Physical layer: Functions (OSI-like) allows to transfer binary digits exchanged among the data link entities deals with bits or symbols defines transmission codes, connectors, voltage levels, etc Data link layer error detection and error correction flow control data unit (packet, cell, datagram) delimitation addressing Computer Network Design - 92 Pag. 46

47 Network layer Functions routing congestion control (moved to layer 4 in the Internet) pricing addressing Transport layer error control sequence control flow control (end to end) congestion control (Internet) Computer Network Design - 93 Functions (OSI) Session layer provides synchronization points to recover for interruption of the transport layer Presentation layer Deals with data representation Application layer provides the application processes with the means to the OSI environment Often merged in a single layer in the Internet Computer Network Design - 94 Pag. 47