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Computer Networking 2014/2015 Departamento de Tecnología Electrónica Contents Chapter 1: Introduction Chapter 2: The Application Layer Chapter 3: The Transport Layer Chapter 4: The Network Layer Chapter 5: The Link Layer and Local Area Networks Introduction 1-2 Departamento de Tecnología Electrónica Computer Networking Chapter 1 Computer Networks and the Internet Some of these slides have copyright from: Computer Networking: A Top Down Approach , 5th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009. Introduction 1-3 Chapter 1: Introduction Our goal: get used to terminology more depth, detail later in course approach: use Internet as example Overview: what’s the Internet? what’s a protocol? network edge; hosts, network access, physical media network core: packet/circuit switching, Internet structure performance: loss, delay, throughput protocol layers, service models history Introduction 1-4 Chapter 1. Computer networks and the internet 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 History Introduction 1-5 What’s the Internet? Overview PC millions of connected server wireless laptop cellular handheld computing devices: hosts = end systems running network apps communication access points wired links router links fiber, copper, radio, satellite transmission rate = bandwidth routers: forward packets (chunks of data) Mobile network Global ISP Home network Regional ISP Institutional network Introduction 1-6 “Fun” Internet appliances Web-enabled toaster + weather forecaster IP picture frame Slingbox: watch, control cable TV remotely Internet refrigerator Internet phones Introduction 1-7 What’s the Internet? HW and SW components protocols control sending, receiving of msgs Mobile network Global ISP e.g., TCP, IP, HTTP, Ethernet Internet: “network of networks” hierarchical public Internet versus private intranet Internet standards Home network Regional ISP Institutional network RFC: Request for comments IETF: Internet Engineering Task Force Introduction 1-8 What’s the Internet? A service view communication infrastructure enables distributed applications: Web, VoIP, email, online games, e-commerce, file sharing communication services provided to apps: reliable data delivery from source to destination “best effort” (unreliable) data delivery Introduction 1-9 What’s a protocol? human protocols: “what’s the time?” “I have a question” introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols: machines rather than humans all communication activity in Internet governed by protocols protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt Introduction 1-10 What’s a protocol? a human protocol and a computer network protocol: Hi TCP connection request Hi TCP connection response Time? Get http://www.awl.com/kurose-ross 2:00 <file> time Q: Other human protocols? Introduction 1-11 Chapter 1. Computer networks and the internet 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 History Introduction 1-12 A closer look at network structure network edge: applications and hosts access networks, physical media: wired or wireless communication links network core: interconnected routers network of networks Introduction 1-13 The network edge end systems (hosts): run application programs e.g. Web, email at “edge of network” peer-peer client/server model client host requests, receives service from always-on server client/server e.g. Web browser/server; email client/server peer-peer model: minimal (or no) use of dedicated servers e.g. BitTorrent Introduction 1-14 Access networks and physical media Q: How to connect end systems to an edge router? residential access nets institutional access networks (schools, companies,…) mobile access networks Keep in mind: bandwidth (bits per second) of access network? shared or dedicated? Introduction 1-15 Residential access networks Dial-up Modem central office home PC home dial-up modem telephone network Internet ISP modem (e.g., AOL) uses existing telephony infrastructure home directly-connected to central office up to 56Kbps direct access to router (often less) can’t browse, phone at same time: not “always on” Introduction 1-16 Residential access networks Digital Subscriber Line (DSL) Existing phone line: 0-4KHz phone; 4-50KHz upstream data; 50KHz-1MHz downstream data home phone Internet DSLAM telephone network splitter DSL modem home PC central office uses existing telephone infrastructure up to 1 Mbps upstream (today typically < 256 kbps) up to 8 Mbps downstream (today typically < 1 Mbps) dedicated physical line to telephone central office Introduction 1-17 Residential access networks Cable modems uses cable TV infrastructure, rather than telephone infrastructure HFC: hybrid fiber coax asymmetric: up to 30Mbps downstream, 2 Mbps upstream network of cable, fiber attaches homes to the ISP router homes share access to the router unlike DSL, which has dedicated access Introduction 1-18 Residential access networks Cable modems Typically 500 to 5,000 homes Diagram: http://www.cabledatacomnews.com/cmic/diagram.html Introduction 1-19 Residential access networks Fiber to the Home optical fibers Internet OLT ONT optical fiber central office optical splitter ONT: Optical Network Terminator OLT: Optical Line Terminator ONT ONT optical links from central office to the home two competing optical technologies: Passive Optical network (PON) Active Optical Network (AON) much higher Internet rates; fiber also carries television and phone services (Gpbs: down 10-20 Mpbs, up 2-10 Mbps) Introduction 1-20 Institutional access networks Ethernet Internet access 100 Mbps Ethernet switch institutional router to institution’s ISP 100 Mbps 1 Gbps 100 Mbps typically used in companies, universities, etc server Also used in residential networks 10 Mbps, 100Mbps, 1Gbps, 10Gbps Ethernet today, end systems typically connect to Ethernet switch Introduction 1-21 Mobile access networks Wireless access networks shared wireless access network connects end system to router via base station a.k.a “access point” wireless LANs: 802.11b/g (WiFi): 11 or 54 Mbps wider-area wireless access provided by telecom operator ~1Mbps over cellular system (EVDO, HSDPA) next up (?): WiMAX (10’s Mbps) over wide area router base station mobile hosts EVDO: Evolution-Data Optimized HSDPA: High Speed Downlink Packet Access Introduction 1-22 Home networks Typical home network components: DSL or cable modem router/firewall/NAT Ethernet wireless access point to/from cable headend cable modem router/ Firewall / NAT Ethernet Home Station Fibra Óptica Teldat i-1104W (MoviStar) http://www.movistar.es/particulares/ayuda/internet/adsl/equipami ento-adsl/routers/Teldat-i-1104w wireless laptops wireless access point Introduction 1-23 Physical Media bit: propagates between transmitter/rcvr pairs physical link: what lies between transmitter & receiver guided media: signals propagate in solid media: copper, fiber, coax unguided media: signals propagate freely, e.g., radio Introduction 1-24 Guided Physical Media Twisted Pair (TP) two insulated copper wires Category 3: traditional phone wires, 10 Mbps Ethernet Category 5: 100Mbps Ethernet Coaxial cable: two concentric copper conductors bidirectional baseband: single channel on cable legacy Ethernet broadband: multiple channels on cable HFC Fiber-optic cable: glass fiber carrying light pulses, each pulse a bit high-speed operation: high-speed point-to-point transmission (e.g., 10’s-100’s Gpbs) low error rate: repeaters spaced far apart; immune to electromagnetic noise Introduction 1-25 Unguided Physical Media: radio signal carried on electromagnetic spectrum no physical “wire” bidirectional propagation environment effects: reflection obstruction by objects interference Radio link types: terrestrial microwave e.g. up to 45 Mbps channels LAN (e.g., WiFi) 11Mbps, 54 Mbps wide-area (e.g., cellular) 3G cellular: ~ 1 Mbps satellite Kbps to 45Mbps channel (or multiple smaller channels) 270 msec end-end delay geosynchronous versus low altitude Introduction 1-26 Chapter 1. Computer networks and the internet 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 History Introduction 1-27 The Network Core mesh of interconnected routers the fundamental question: how is data transferred through network? circuit switching: dedicated circuit per call: telephone network packet-switching: data sent through network in discrete “chunks” Introduction 1-28 Network Core: Circuit Switching end-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required Introduction 1-29 Network Core: Circuit Switching network resources (e.g., bandwidth) divided into “pieces” pieces allocated to calls resource piece idle if not used by owning call (no sharing) dividing link bandwidth into “pieces” frequency division time division Introduction 1-30 Baseband and broadband 1 path 3 channels DEMULTIPLEXOR Broadband MULTIPLEXOR Baseband - No modulation - Original signal Introduction 1-31 Circuit Switching: FDM and TDM Example: FDM 4 users frequency time TDM frequency time Introduction 1-32 Network Core: Packet Switching each end-end data stream divided into packets user A, B packets share network resources each packet uses full link bandwidth resources used as needed Bandwidth division into “pieces” Dedicated allocation Resource reservation resource contention: aggregate resource demand can exceed amount available congestion: packets queue, wait for link use store and forward: packets move one hop at a time node receives complete packet before forwarding Introduction 1-33 Packet Switching: Statistical Multiplexing 100 Mb/s Ethernet A C statistical multiplexing 1.5 Mb/s B queue of packets waiting for output link D E sequence of A & B packets has no fixed timing pattern bandwidth shared on demand: statistical multiplexing. Introduction 1-34 Packet-switching: store-and-forward L R R takes L/R seconds to transmit (push out) packet of L bits on to link at R bps store and forward: entire packet must arrive at router before it can be transmitted on next link delay = 3L/R (assuming zero propagation delay) R Example: L = 7.5 Mbits R = 1.5 Mbps transmission delay = 15 sec more on delay shortly … Introduction 1-35 Packet switching versus circuit switching Packet switching allows more users to use network! Example: 1 Mb/s link each user: • 100 kb/s when “active” • active 10% of time N users 1 Mbps link circuit-switching: 10 users packet switching: with 35 users, probability > 10 active at same time is less than .0004 Q: what happens if > 35 users ? Introduction 1-36 Packet switching versus circuit switching Is packet switching the best option by far? great for bursts of data resource sharing more simple, no call setup excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)? Introduction 1-37 Internet structure: network of networks NAP or IXP Internet Service Provider (ISP) (Suministrador de Servicio de Internet) Regional Service Provider (RSP) (Suministrador de Servicio Regional) Network Service Provider (NSP) (Suministrador de Servicio de Red) Network Access Point (NAP) o Internet eXchange Point (IXP) (Punto de acceso a la red) NSP RSP ISP Customers Tier 1 RSP ISP Customers NSP Tier 2 RSP ISP Tier 3 ISP Customers Customers ISP Customers Introduction 1-38 Internet structure: network of networks Introduction 1-39 Chapter 1. Computer networks and the internet 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 History Introduction 1-40 How do loss and delay occur? packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turn packet being transmitted (delay) A B packets queuing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers Introduction 1-41 Four sources of packet delay transmission A propagation B nodal processing queuing dnodal = dproc + dqueue + dtrans + dprop dproc: nodal processing dqueue: queueing delay check bit errors determine output link typically < msec time waiting at output link for transmission depends on congestion level of router Introduction 1-42 Four sources of packet delay transmission A propagation B nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop dtrans: transmission delay: dprop: propagation delay: L: packet length (bits) R: link bandwidth (bps) dtrans = L/R d: length of physical link s: propagation speed in medium (~2x108 m/sec to 2x108 m/sec) dprop = d/s dtrans and dprop very different Introduction 1-43 R: link bandwidth (bps) L: packet length (bits) a: average packet arrival rate average queuing delay Queuing delay (revisited) traffic intensity = La/R L·a/R ~ 0: avg. queuing delay small L·a/R -> 1: avg. queuing delay large L·a/R > 1: more “work” arriving than can be serviced, average delay infinite! La/R ~ 0 La/R -> 1 Introduction 1-44 Packet loss queue (a.k.a buffer) preceding link in buffer has finite capacity packet arriving to full queue dropped (a.k.a lost) lost packet may be retransmitted by previous node, by source end system, or not at all buffer (waiting area) A B packet being transmitted packet arriving to full buffer is lost Introduction 1-45 Throughput throughput: rate (bits/time unit) at which bits transferred between sender/receiver instantaneous: rate at given point in time average: rate over longer period of time server, with server sends bits file ofinto F bits (fluid) pipe to send to client pipe can carry link that capacity at rate Rfluid s bits/sec Rs (bits/sec) link capacity pipe that can carry Rcfluid bits/sec at rate Rc (bits/sec) Introduction 1-46 Throughput (more) Rs < Rc What is average end-end throughput? Rs bits/sec Rc bits/sec Rs > Rc What is average end-end throughput? Rs bits/sec Rc bits/sec bottleneck link link on end-end path that constrains end-end throughput Introduction 1-47 Throughput: Internet scenario per-connection endend throughput: min(Rc,Rs,R/10) in practice: Rc or Rs is often bottleneck Rs Rs Rs R Rc Rc Rc 10 connections (fairly) share backbone bottleneck link R bits/sec Introduction 1-48 Chapter 1. Computer networks and the internet 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 History Introduction 1-49 Protocol Layers Networks are complex, with many “pieces”: hosts routers links of various media applications protocols hardware, software Question: Is there any hope of organizing the structure of the network? Or at least our discussion of networks? Introduction 1-50 Organization of air travel ticket (purchase) ticket (complain) baggage (check) baggage (claim) gates (load) gates (unload) runway takeoff runway landing airplane routing airplane routing airplane routing a set of steps Introduction 1-51 Layering of airline functionality ticket (purchase) ticket (complain) ticket baggage (check) baggage (claim baggage gates (load) gates (unload) gate runway (takeoff) runway (land) takeoff/landing airplane routing airplane routing airplane routing departure airport airplane routing airplane routing intermediate air-traffic control centers arrival airport Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below Introduction 1-52 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 Introduction 1-53 Layer architecture scheme (I) Entity It’s an active element in the system It uses protocols to supply services Peer entity (one for every level) Functions Set of tasks for every level Not all the functions are held in both ends E.g., the baggage layer implements check-in and check out functions. Services Set of provisions provided by a layer (supplier) to its inmediate upper layer (user). They are formally specified by a set of primitives or operations. E.g.: service to check in baggage Introduction 1-54 Layer architecture scheme (II) Protocol: Set of rules that determines format and meaning of data exchange between two peer entities that are “talking”. This way, the protocol defines: The message format to exchange. The rules for exchanging the messages. Entities use a protocol to provide services (to the upper layer). Transmitter Receiver Layer N + 1 Layer N + 1 ... N_SAP N layer entity Layer N Layer N - 1 ... ... N layer protocol N_PDU Layer N Layer N - 1 ... Introduction 1-55 Services of a layer Primitives: Set of structures of information that implements the services of a layer Types: Request issued by user of service in source Indication issued by supplier of service (by own initiative or not) Response issued by user of service in destination Confirm issued by supplier of service Destination end Source end Request Layer N+1 Layer N 1 Service Layer N+1 Layer N 4 Confirm To the bottom in transmission 2 Response Service 3 Indication To the bottom in transmission To the top in reception To the top in reception Introduction 1-56 Functions, Services and Primitives Layer N+1 Confirmed: They require a response They implement the four primitives Not confirmed: They don’t require response They implement request and indication Partially confirmed: Provider responds They implement request, indication and confirmation Initiated by provider: When triggering a condition They implement indication in both directions N-Service User Layer N N-Service Provider Service.Request Layer N+1 N-Service User Service.Indication Service.Confirmation Service.Request Service.Request Confirmed Service.Response Not Confirmed Service.Indication Service.Indication Partially confirmed Service.Confirmation Service.Indication Initiated by provider time Transmission Service.Indication time Transmission Introduction 1-57 Layers, entities and SAPs Vertical communication: Physical Inside the own system Horizontal communication: Logical Peer protocol; layer N protocol Among different systems End A End B N-service primitive N-service primitive N layer protocol N-PDU N-1-service primitive Layer peer entity N-1-service primitive N-1 layer protocol N-1 -PDU N-2-service primitive Layer N-1 peer entity N-2 layer protocol N-2-service primitive N-2 -PDU Layer N-2 peer entity Introduction 1-58 Encapsulation Transmitter (N+1)-PDU Layer N+1 (N)-ICI IDU: Interface Data Unit ICI: Interface Control Information PCI: Protocol Control Information UD: User Data SAP: Service Access Point SAP (N)-IDU Encapsulation Layer N (N)-ICI (N)-SDU (N)-PCI (N)-UD (N)-PDU Introduction 1-59 De-encapsulation Receiver (N+1)-PDU Layer N+1 (N)-ICI IDU: Interface Data Unit ICI: Interface Control Information PCI: Protocol Control Information UD: User Data SAP (N)-IDU De-encapsulation Layer N (N)-ICI (N)-SDU (N)-PCI (N)-UD (N)-PDU Introduction 1-60 Segmentation Transmitter (N+1)-PDU Layer N+1 (N)-ICI IDU: Interface Data Unit ICI: Interface Control Information PCI: Protocol Control Information UD: User Data SAP (N)-IDU (N)-ICI (N)-SDU (N)-PCI (N)-PCI (N)-PDU Encapsulation Layer N (N)-PDU Introduction 1-61 Desegmentation Receiver (N+1)-PDU Nivel N+1 (N)-ICI IDU: Interface Data Unit ICI: Interface Control Information PCI: Protocol Control Information UD: User Data SAP Nivel N (N)-ICI (N)-SDU (N)-PCI (N)-PCI (N)-PDU De-encapsulation (N)-IDU (N)-PDU Introduction 1-62 How many layers? Depending on the desired set of functions for the network architecture Two main network architectures: TCP/IP: used on the Internet. Five layers. Describe functions, services and protocols Modelo de referencia OSI (Open System Interconnection). Seven layers. ISO standard (International Organization for Standardization). Describe functions and services. Internet protocol stack application: supporting network applications FTP, SMTP, HTTP transport: process-process data transfer application transport TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements network link physical Ethernet, 802.111 (WiFi), PPP physical: bits “on the wire” Introduction 1-64 ISO/OSI reference model presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machinespecific conventions session: synchronization, checkpointing, recovery of data exchange Internet stack “missing” these layers! these services, if needed, must be implemented in application needed? application presentation session transport network link physical Introduction 1-65 How are layers implemented? Note aplication transport network data link physical Programs Operating System Not all layers are present in the devices used on the Internet. Software Note hardware Hardware that implements data link and physical layers is know as network interface, or NIC (Network Interface Card). source message segment M Ht M datagram M frame Hn Ht Hl Hn Ht M Encapsulation application transport network link physical link physical switch destination M Ht M Hn Ht Hl Hn Ht M M application transport network link physical Hn Ht Hl Hn Ht M M network link physical Hn Ht M router Introduction 1-67 Chapter 1. Computer networks and the internet 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6. History Introduction 1-68 Internet History 1961-1972: Early packet-switching principles 1961: Kleinrock - queueing theory shows effectiveness of packet-switching 1964: Baran - packetswitching in military nets 1967: ARPAnet conceived by Advanced Research Projects Agency 1969: first ARPAnet node operational 1972: ARPAnet public demonstration NCP (Network Control Protocol) first host-host protocol first e-mail program ARPAnet has 15 nodes Introduction 1-69 Internet History 1972-1980: Internetworking, new and proprietary nets 1970: ALOHAnet satellite network in Hawaii 1974: Cerf and Kahn - architecture for interconnecting networks 1976: Ethernet at Xerox PARC late70’s: proprietary architectures: DECnet, SNA, XNA late 70’s: switching fixed length packets (ATM precursor) 1979: ARPAnet has 200 nodes Cerf and Kahn’s internetworking principles: minimalism, autonomy - no internal changes required to interconnect networks best effort service model stateless routers decentralized control define today’s Internet architecture Introduction 1-70 Internet History 1980-1990: new protocols, a proliferation of networks 1983: deployment of TCP/IP 1982: smtp e-mail protocol defined 1983: DNS defined for name-to-IP-address translation 1985: ftp protocol defined 1988: TCP congestion control new national networks: Csnet, BITnet, NSFnet, Minitel 100,000 hosts connected to confederation of networks Introduction 1-71 Internet History 1990, 2000’s: commercialization, the Web, new apps early 1990’s: ARPAnet decommissioned 1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995) early 1990s: Web hypertext [Bush 1945, Nelson 1960’s] HTML, HTTP: Berners-Lee 1994: Mosaic, later Netscape late 1990’s: commercialization of late 1990’s – 2000’s: more killer apps: instant messaging, P2P file sharing network security to forefront est. 50 million host, 100 million+ users backbone links running at Gbps the Web Introduction 1-72 Internet History 2010: ~750 million hosts voice, video over IP P2P applications: BitTorrent (file sharing) Skype (VoIP), PPLive (video) more applications: YouTube, gaming, Twitter wireless, mobility Introduction 1-73 Introduction: Summary Covered a “ton” of material! Internet overview what’s a protocol? network edge, core, access network packet-switching versus circuit-switching Internet structure performance: loss, delay, throughput layering, service models history You now have: context, overview, “feel” of networking more depth, detail to follow! Introduction 1-74 Departamento de Tecnología Electrónica Computer Networking – Unit 1: Computer Networks and Internet PROBLEMS AND EXERCISES Introduction 1-75 Pr1: TDM How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? all link speeds: 1.536 Mbps each link uses TDM with 24 slots/sec 500 msec to establish end-to-end circuit Introduction 1-76 Pr2: End-to-end delay Consider that packet of length L transmitted by end system A is going through three links to a destination end system. These three links are connected through two packet switches. Let di, si and Ri denote the length, propagation speed, and the transmission rate of link i, for i=1,2,3. The packet switch delays each packet by dproc. Assuming no queuing delays, what is the total end-to-end delay for the packet in terms of di, si, Ri (i=1,2,3), and L? Suppose now the packet is 1500 bytes, the propagation speed on both links is 2.5x108 m/s, the transmission rate of the three links is 2 Mbps, the packet switch processing delay is 3 msec, the length of the first link is 5000 km, the length of the second link is 4000 km and the length of the last link is 1000 km. For these values, what is the end-to-end delay? Introduction 1-77 Pr3: Queuing delay A router is receiving a packet and has to determine the output interface to which the packet should be forwarded. When the packet arrives, another packet is being transmitted (half packet has already been transmitted) and three other packets are waiting to be transmitted. Packets are transmitted in order of arrival. Suppose all packets are 1500 bytes and the link rate is 2Mbps. What is the queuing delay for the packet? More generally, what is the queuing delay when all packets have length L, the transmission rate is R, x bits of the currentlybeing-transmitted packet have been transmitted, and n packets are already in the queue? Introduction 1-78 Pr4: Encapsulation and segmentation Suppose that, in the OSI model, a data link protocol has the maximum size of D_SDU limited to 1000 bytes, and the network protocol doesn’t have any maximum limitation for N_SDU to the higher level. If T_PDUs are always 2000 bytes and N_PCI are 100 bytes, how many N_PDUs will network layer send? What is the content of each N_PDU? Introduction 1-79 Pr5: Segmentation In modern packet-switched networks, the source host segments long, application-layer messages (for example, an image or a music file) into smaller packets and sends the packets into the network. The receiver then reassembles the packet back into the original message. We refer to this process a message segmentation. Figure illustrates the end-to-end transport of a message with or without message segmentation: Packet without segmentation Source host Packet switch Packet switch Destination host Packet with segmentation Source host Packet switch Packet switch Destination host Introduction 1-80 Pr6: Segmentation Consider a message that is 8·106 bits long that is to be sent from source to destination. Use the figure scheme. Suppose each link in the figure is 2 Mbps. Ignore propagation, queuing and processing delays. a) Consider sending the message from source to destination without message segmentation. How long does it take to move the message from the source host to the first packet switch? Keeping in mind that each switch uses store and forward packet switching, what is the total time to move the message from source host to destination host? b) Now suppose that the message is segmented into 4000 packets, with each packet being 2000 bits long. How long does it take the first packet to get from the source host to the first switch? When the packet is sent from the source host to the first switch, the second packet is sent from the source host to the first switch. When is the second packet fully received at the first switch? c) How long does it take the file to get from source host to destination host when message segmentation is used? Compare this result with your answer in part a) and discuss about it. d) Discuss the drawbacks of message segmentation. Introduction 1-81