Computer Network Design - 6

Transcription

1 opyright Review of fundamentals working concepts ndrea ianco Paolo Giaccone Telecommunication Networks Group Quest opera è protetta dalla licenza reative ommons Noerivs-Nonommercial. Per vedere una copia di questa licenza, consultare oppure inviare una lettera a: reative ommons, 559 Nathan bbott Way, Stanford, alifornia 94305, US. This work is licensed under the reative ommons Noerivs-Nonommercial License. To view a copy of this license, visit: or send a letter to reative ommons, 559 Nathan bbott Way, Stanford, alifornia 94305, US. omputer Network esign - 1 omputer Network esign - 2 Review What is a (modern) telecommunication and computer work? Topologies Multipleing/multiple Switching techniques User (traffic) characterization Properties of telephone and computer works Protocol architectures for computer works Physical layer (multipleing) ata link layer (multiple ) Network layer (routing, congestion) Transport Layer (congestion) pplication Layer (client server, P2P, N, ata enter) omputer Network esign - 3 Networks Focus on modern telecommunication and computer works Telephone and Inter One possible work definition (service oriented view) Infrastructure that provides services to applications: Web, VoIP, , games, e-commerce, social s, phone calls, fa, Inter provides programming interface to apps program to connect to Inter provides service options omputer Network esign - 4 Networks nother work definition: 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) ifferent levels of detail Key issue in works is resource sharing Node is the point where Multipleing /multiple (link sharing) and switching (node sharing) occurs 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 multipleing omputer Network esign - 5 omputer Network esign - 6 Pag. 1

2 Type of channels roadcast channel Many T and many R Sometimes one (many) T and many (one) R 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 hannel properties Many quality indices ttenuation, robusteness to mechanical stress, ease of installation, robusteness to interference, cost, etc, Mainly interested in bit rate [bit/s] lso named bandwidth, throughput, with slightly different meaning delay [s] propagation delay depends on the channel length andwidth delay [bit] channel size how much we can push on the channel omputer Network esign - 7 omputer Network esign - 8 Topologies in TL works The work topology is defined by the relative position of nodes and channels work topology is a graph G=(V,) V = set of vertices (represented as circles - nodes) = set of edges (represented as segments - channels) Edges may be: direct (directed segments (arrow) unidirectional channels) undirect (non directed segments bidirectional channels) bstraction of real works Several levels of abstraction are possible Many topologies Full mesh, mesh, tree, bus, star, E E E E E E E omputer Network esign - 9 omputer Network esign - 10 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 Physical topology E.g., satellite work omputer Network esign - 11 omputer Network esign - 12 Pag. 2

3 Logical topology Private work over satellite work Logical topology Router interconnection for an ISP Router Node Router Node Frame Relay or TM work Router Node Virtual ircuits Node Router omputer Network esign - 13 omputer Network esign - 14 Logical topology Logical topology Overlay among peers in a P2P work ISP IXP IXP ISP ISP regional omputer Network esign - 15 omputer Network esign - 16 Logical topology Tier-1 ISP: e.g., Sprint Tier 1 ISP Tier 1 ISP Google IXP Regional ISP IXP Regional ISP IXP ISP ISP ISP ISP ISP ISP ISP ISP omputer Network esign - 17 omputer Network esign - 18 Pag. 3

4 Topologies and performance The amount of traffic that can be succesfully transferred (throughput) in a work is for a given channel available capacity inversely proportional to the average distance among node pairs weighted by the amount of traffic echanged between the two node For uniform traffic and regular topologies the average distance on the topology establish the throughput omputer Network esign - 19 Topologies and performance omparison among topologies, with the same number of nodes (4) and (almost) the same number of channels Uniform traffic Every node pair echange bit/s. Total generated traffic is 12. Every unidirectional channel has capacity bit/s. ompute: average distance, work capacity (maimum throughput), maimum channel load,maimum node load omputer Network esign - 20 Topologies and performance apacity: 32=6 verage distance: 20/12=1.66 onsider only traffic from left to right (simmetry) maimum channel load is 4. Thus, <= /4 Node 3 (or 2) must handle 7/4 of traffic unit Uniform traffic, non regular topology, unbalanced channel load, unbalanced node load omputer Network esign - 21 Topologies and performance apacity: 32=6 verage distance: 1.5 onsidering the traffic from node 4. Maimum load on (all) channel is 3. Thus <=/3 The same holds for the other direction Node 4 must handle 3 of traffic unit Uniform traffic, non regular topology, balanced channel load, unbalanced node load omputer Network esign - 22 Topologies and performance apacity: 42=8 verage distance: 1.33 For clockwise traffic the maimum channel load is 2. Thus <= /2. The same holds for counter clock wise traffic Each node must handle 2 unit of traffic Uniform traffic, regular topology, balanced channel load, balanced node load 3/2 /2 hannel, sharing Multipleing and multiple 3/2 /2 3/2 /2 3/2 /2 omputer Network esign - 23 omputer Network esign - 24 Pag. 4

5 Sharing channel resources hannel sharing techniques Sharing of channel resources among data flows comes in two different flavours Multipleing ll flows the channel from a single point Single transmitter scenario entralized problem radio from an antenna (base station in a cellular work, point in a WI-FI work, satellite transmission), an output link in a switch or a router Multiple- Flows the channel from different points Many transmitters are active istributed problems Local area works (if not switched), mobile phones in a cellulare work, P ing via a Wi-FI hot-spot Frequency (FM - FM) Time (TM - TM) ode (M - M) f channel t omputer Network esign - 25 omputer Network esign - 26 Frequency division Each flow is transmitted using different frequency bands Need for band guard f Time division (TM TM) Each flow eploits different time intervals (slots) efine frame over which slot allocations are repeated Need for time guard f t t omputer Network esign - 27 omputer Network esign - 28 ode division (M M) Each flow eploits a different code (waveform with higher frequency than the bit t rate) Need for orthogonal codes c f f ode division (M M) t t omputer Network esign - 29 omputer Network esign - 30 Pag. 5

6 ode division Eample ode word used by user i: oded sequence = information bit code word Information bit: oded sequence: ode multipleing Eample: ode word for user 1: ode word for user 2: ode word for user 3: ode word for user 4: We are interested in receiving data from user 1 Over the channel, transmitted signals sum up (need to equalize power) T of 1+2+3: T of 2+3 (noise): omputer Network esign - 31 omputer Network esign - 32 ode multipleing 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: orrelation with = 4 The noise Transmissions of 2+3: orrelation with = 0 Multipleing 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 ode 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) omputer Network esign - 33 omputer Network esign - 34 FM TM frequency FM and TM Eample: 4 users time Statistical multipleing Multipleing can be deterministic, fied in time, on the basis of requirements determined at connection setup statistical, variable in time, to adapt to instantaneous traffic requirements 125 ms frequency time omputer Network esign - 35 omputer Network esign - 36 Pag. 6

7 Statistical Multipleing 100 Mb/s Ether statistical multipleing queue of packets waiting for output link 1.5 Mb/s Switching techniques ircuit and packet switching Sequence of & packets does not have fied pattern, bandwidth shared on demand ynamic TM scheme E omputer Network esign - 37 omputer Network esign - 38 Switching techniques ircuit switching Telephone works Packet switching Two flavours (datagram and virtual circuit service) Inter, computer works Three phases Opening ata transfer losing Network transparently forwards user generated data No operation on user data Fied bit rate 64kbit/s ircuit switching U1 N1 N2 U2 t t t t omputer Network esign - 39 omputer Network esign - 40 Space vs time switching U1 N1 N2 U2 125 ms U1 N1 N2 U2 125 ms Switching techniques ircuit switching Resources allocated uniquely to a circuit Physical channel, time-slot in TM frame Resources are shared among connections but are statically allocated to data onnection oriented Need to open (and close) the circuit prior (after) data transmission Store state information on each circuit (stateful approach) ddress (unique for each user in the work) used only when opening the circuit, not carried in data ata unit identified by position Routing (choice of the best route) performed only when opening the circuit one through routing table lookup ata forwarding Through forwarding table look-up (one entry for each active circuit) Static (always the same scheduling, unless circuits are closed or opened) omputer Network esign - 41 omputer Network esign - 42 Pag. 7

8 Packet switching ata transfer over virtual circuits Transmission time Propagation time U1 N1 N2 U2 Processing time TE TE t t t t Forwarding table In Label Out Label omputer Network esign - 43 omputer Network esign - 44 Grouping virtual circuits virtual circuit is logically identified by a label The label may change value over each link Relabeling Label = often a pair of identifiers (VI-VPI in TM) Virtual channel (V): identifies a single connection Virtual path (VP): identifies a group of virtual channels Grouping virtual circuits The grouping allows flow aggregation Eases work management Increases scalability Possible use LN inter-connection to crete a VPN (Virtual Private Network) Multimedia flows (video, audio, data) omputer Network esign - 45 omputer Network esign - 46 Virtual circuits and paths (TM) TM: VP switching VPI 1 VI 1 VI 21 VI 22 VPI 1 VPI 4 VI 23 VI 24 VPI 6 VI 2 VI 3 VI 4 VI 5 VI 23 VI 24 VI 25 VI 24 VPI 2 VPI 5 VPI 3 VPI 8 VP SWITHING VI 25 VI 24 VI 21 VI 22 omputer Network esign - 47 omputer Network esign - 48 Pag. 8

9 VI 23 VI 24 TM: V switching VI 25 VI 25 VPI 4 VI 21 VI 21 VPI 5 VI 23 VPI 2 VI 24 V SWITHING Virtual circuits Switched virtual circuit (SV) Established on-demand, through signaling, in real-time Three phases Virtual circuit opening ata transfer Virtual circuit closing Users (and work) echange signaling packets (over dedicated VI/VPI) to establish a virtual circuit; then, data transfer can occur Permanent virtual circuit (PV) Established through agreement among user and work provider Off-line, through management procedures efine a semi-static work Logical topology Users can immediately echange data, with no delay omputer Network esign - 49 omputer Network esign - 50 Switching techniques Packet switching, with datagram service Shared resources Ideally the full work is available to a single user Resources are continuously shared with all other users at the data level onnectionless Free to send data when available, no need to check work or user availability Stateless approach Each packet must carry the destination (and source) address ata unit identified through source and destination addresses (unique for each pair of users in the work) Routing and forwarding performed independently over each packet Through routing table look-up Switching techniques Packet switching, with virtual circuit service Shared resources Resources are shared with all virtual circuits sharing the same link onnection 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) ddress (unique for each user in the work) used only when opening the virtual circuit, not carried in data ata unit identified through a label (unique for each eisting virtual circuit on each link in the work) 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 one through routing table lookup ata forwarding Through forwarding table look-up (one entry for each active virtual circuit) Re-labelling needed omputer Network esign - 51 omputer Network esign - 52 Fundamentals of packet switching ata sent as packets Nodes operate in store&forward (almost always) uffers elay Many operations on data in the work (not in circuit switching) Error detection, error recovery, flow control, routing, forwarding, congestion control, packet inspection.. Need to define a work architecture to organize functionalities (see later) Packet size Packet size P Measured in bit Packet size in time T TX Transmission time measured in s ifferent 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 T TX omputer Network esign - 53 omputer Network esign - 54 Pag. 9

10 urstiness omputer Network esign Review elays elays 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 compleity of the eecuted procedures on the packet header Normally negligible with respect to transmission time Queuing delays epend on the traffic generated by all users Highly variable omputer Network esign - 55 Traffic characterization and QoS Provisioning omputer Network esign - 56 User traffic characterization Need to know user behaviour to design a work Traffic sources R (onstant it Rate) VR (Variable it Rate) haracterized by their rate [bit/s] User traffic characterization R (onstant it Rate) sources: Rate (bit/s) Perfectly known all duration (s) all generation process Only statistically known omputer Network esign - 57 omputer Network esign - 58 User traffic characterization VR sources: verage rate (bit/s) Known? Over which period? Peak rate (bit/s) or urstiness (Peak rate/ average rate) Known (worst case) urst duration Known? all duration (s) all generation process Only statistically known User traffic characterization low speed data urstiness= Peak rate/ verage rate alphanumeric terminals connectionless LN alta velocità immagini data graphical terminal transmission compressed VIEO non compressed supercomputer interconnection HTV LN voice audio Peak rate [bit/s] omputer Network esign - 59 omputer Network esign - 60 Pag. 10

11 Integrated vs dedicated works Telecommunications works were traditionally defined to provide a specific service one service one work paradigm Telephone work for the interactive human voice transportation service Inter for data echange among computers TV or radio distribution for the TV or radio system Integrated works one work for any service narrowband ISN o N-ISN broadband ISN o -ISN Integrated vs dedicated works edicated works Easier to optimize for the specific service Optimal engineering solutions for the specific requirements of the service Integrated works advantages No need to create an independent infrastructure for each service Supporting different requirements implies sub/optimal choices Integrated works trade fleibility and infrastructure cost reduction with perfomance and increased control compleity omputer Network esign - 61 omputer Network esign - 62 Quality of service provided by works Networks used as eamples Fied telephone work: POTS Inter -ISN escribe in an informal way the quality of service provided by these works omputer Network esign - 63 POTS haracteristics R source completely known (generated by the work) ircuit switching onstant, 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 omputer Network esign - 64 Inter haracteristics Source behavior unknown Packet switching with datagram service omplete sharing of work resources it rate and delay unknown Possible congestion Loss probability may be significant Requirements Error probability negligible in wired works Zero blocking probability QoS largely dependent of other users Network utilization can be very high, user satisfaction can be very low omputer Network esign ISN Intermediate situation Source known (either deterministically or statistically) Packet switching with virtual circuit service May introduce algorithms to control work resources sharing it rate and delay negotiable Loss probability negotiable Requirements locking probability reasonably small Error probability negligible QoS dependent of other user behavior and of algorithms used to manage work resources Trade work utilization and user satisfaction omputer Network esign - 66 Pag. 11

12 Network design esign problem Given: Network topology (nodes, link speed) Traffic characterization Jointly obtain: Guarantee some sort of QoS for user connection High work utilization Without the objective of high work utilization, the problem becomes trivial overprovisioning (power line or water distribution works) omputer Network esign - 67 Network design Several flavour Running at different time scale (with different level of compleity) Mainly focus on work design and planning (resource deployment) On the basis of traffic estimates and cost constraints Eploits routing criteria and traffic engineering Other eamples Network management (running a work) Measurements Fault management (protection and restoration) Includes re-design and re-planning onnection management ata unit transport omputer Network esign - 68 esign to obtain QoS ifferent time scale (with different level of compleity) Network design and planning (resource deployment) On the basis of traffic estimates and cost constraints Eploits routing criteria and traffic engineering Network management (running a work) Measurements Fault management (protection and restoration) Includes re-design and re-planning onnection management ata unit transport omputer Network esign - 69 Our definition of QoS ssume that a work has been designed and is properly managed vailable resources are given Mainly study algorithms operating at the following timescale: onnection management ata unit transport lso named traffic control problem Must define what is meant by connection. lso named data classification problem. Two different traffic control principles: Preventive control : mainly eecuted at work ingress, with fairly tight traffic control to avoid congestion insurgence in the work Reactive control: react when congestion situation occur, to reduce or eliminate congestion negative effects omputer Network esign - 70 Traffic control: essential elements onnection oriented work User-work service interface Traffic characterization QoS negotiation Resource allocation (bit rate and buffer) lgorithms for traffic control (onnection dmission ontrol) and routing Scheduling and buffer management (allocation, discard) in switching nodes onformance verification (policing or UP: Usage Parameter ontrol) Traffic shaping to adapt it to a given model ongestion control Traffic control: connection oriented work The connection oriented paradigm permits to know which are the work elements over which traffic control algorithms must be eecuted (path known) ircuit switching Packet switching with virtual circuit service If high utilization is a major objective: Packet switching s such, the most suited switching technique to obtain QOS is packet switching with virtual circuit service omputer Network esign - 71 omputer Network esign - 72 Pag. 12

13 Traffic control: user-work service interface The capability to control the work increases with the knowledge of user traffic. Limiting factor is the compleity. Over the service interface Traffic characterization QoS parameters negotiation an be defined on a call basis or on a contract basis POTS: implicit, on a contract basis Inter: not eisting Frame relay: negotiable, normally on a contract basis -ISN: negotiable with traffic contract on both contract and call basis Inter etended to support QoS: negotiable through a SL (Service Level greement) mainly on a contract basis omputer Network esign - 73 Traffic control: resource allocation Main resources: it rate over transmission links uffer Resources can be allocated On a contract basis (booking) On a call basis Packet by packet llocation Eclusive (dedicated resource) Shared omputer Network esign - 74 lgorithms: and routing Routing QoS based path selection to router a connection etermine whether to accept a connection or not, depending on The path chosen by the routing algorithm Traffic characterization QoS requests Network status onstraints It is not acceptable to destroy or even reduce the quality of service guaranteed to already accepted connections an be relinquished onnection must be refused to avoid work overload or congestion Preventive control (but can become reactive) lgorithms: scheduling and buffer management Scheduling hoice of the data unit to be transmitted among data unit stored in the switch uffer management llocation (partial/total, eclusive/shared) of memories in the switch ropping policies Mandatory in an heterogeneous environment to support different QOS requests FIFO (First In First Out) or FFS (First ame First Served) policy with drop-tail discard is optimal in a homogeneous environment ounter for less than 10 pieces at supermarket Preventive and reactive omputer Network esign - 75 omputer Network esign - 76 lgorithms: policing e shaping Policing (traffic verification) Network control of user behavior to guarantee conformance to traffic characterization Shaping (traffic conditioning) User/work 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 eecuted? Only at work edge, i.e., when user work? Multipleing point modify traffic shape oth at work and internally to the work Mainly preventive, but they can become reactive if QoS level may change over time omputer Network esign - 77 lgorithms: congestion control ongestion Traffic ecess over a given channel (link) an occur due to Short term traffic variability llocation policies that share resources to increase work utilization ongestion effects: uffer occupancy increase elay increase ata loss Needed to obtain high link utilization Must eecute at work edge, within the work or.? Reactive omputer Network esign - 78 Pag. 13

14 Protocol architectures omputer Network esign - 79 rchitectures and protocols ommunication requires cooperation One abstract description of the communication paradigm between two or more users requires the definition of a reference model specifying a work architecture work architecture defines the communication process the relation among objects used in communication the functionalities to support the communication Layered architectures! omputer Network esign - 80 Layered architectures Layered architectures are used because of simple design simple management simple standardization separation among functions pplication Presentation Session Transport Network ata Link Physical OSI User Netw. ppl. Session End to End Routing ata Link Physical ENET pplication Service Interwork Network RP Transaction Service Presentation Service ata Flow Trans. ontrol SN Manag. Service Virtual Route Eplicit Route Transm. Group ata Link Physical omputer Network esign - 81 half session path control host 3 Separation among functions: Inter host 4 sub 4 sub 1 router 1 router 2 sub 3 host 2 router 3 host 1 sub 2 applications error control routing packet transfer omputer Network esign ISN Management plane ontrol plane User plane High layers High layers L TM Physical omputer Network esign - 83 Protocols Formal definition of the procedures adopted to guarantee the communication between two or more objects on the same hierarchical level to eecute a specific function Protocol definition: semantics set of commands and answers synta structure of commands and answers timing temporal sequence of commands and answers omputer Network esign - 84 Pag. 14

15 Layer and entities ctive elements in a subsystem Run the functions of the layer (eploiting service provided by lower layers) Interact (cooperate) within the same layer (N) - layer System (N) - entity System Services service can be: connection-oriented (O): a preliminary agreement (connection) is established between the work and the communication end-points, then the data is transferred and finally the connection is released connectionless (L): data is sent to the work without any preliminary agreement and is treated independently from each other transmission media omputer Network esign - 85 omputer Network esign - 86 Services Services (N) - service (N-1) - service N+1 N+1 N N N (N) service provider N N-1 (N-1) service provider N-1 lack-bo for the (N+1) - entity lack-bo for the (N) - entity omputer Network esign - 87 omputer Network esign - 88 PU creation Information transfer (N) - PU (N) - layer (N-1) - SU Transmitter Receiver interface (N-1) - layer (N-1) - PI SP (N-1) - SU pplication Presentation Session Transport Network ata link Physical data PI SU PPI PSU SPI SSU TPI TSU NPI NSU LPI LSU bit or symbols pplication Presentation Session Transport Network ata link Physical (N-1) - PU omputer Network esign - 89 omputer Network esign - 90 Pag. 15

16 The seven OSI layers application presentation session transport work data link physical pplication protocol Presentation protocol Session protocol Transport protocol Network protocol ata link protocol Physical layer protocol transmission media application presentation session transport work data link physical Functions (OSI-like) Physical layer: allows to transfer binary digits echanged among the data link entities deals with bits or symbols defines transmission codes, connectors, voltage levels, etc ata link layer error detection and error correction flow control data unit (packet, cell, datagram) delimitation addressing omputer Network esign - 91 omputer Network esign - 92 Functions Network layer routing congestion control (moved to layer 4 in the Inter) pricing addressing Transport layer error control sequence control flow control (end to end) congestion control (Inter) Functions (OSI) Session layer provides synchronization points to recover for interruption of the transport layer Presentation layer eals with data representation pplication layer provides the application processes with the means to the OSI environment Often merged in a single layer in the Inter omputer Network esign - 93 omputer Network esign - 94 Pag. 16