CSC 401 Data and Computer Communications Networks Computer Networks and The Inter Sec 1.3 Prof. Lina Battestilli Fall 2017
Outline Computer Networks and the Inter (Ch 1) 1.1 What is the Inter? 1.2 work edge end systems, works, links 1.3 work core circuit switching, packet switching, work structure 1.4 delay, loss, throughput in works 1.5 protocol layers, service models 1.6 works under attack: security 1.7 history Previous Lecture NCSU CSC401 Lina Battestilli 2
A closer look at work structure: mobile work home work National or global ISP Local or regional ISP work edge: applications and hosts servers in data centers works, physical media: wired, wireless communication links work core: interconnected routers work of works institutional work NCSU CSC401 Lina Battestilli 3
Outline Computer Networks and the Inter (Ch 1) 1.1 What is the Inter? 1.2 work edge end systems, works, links 1.3 work core circuit switching, packet switching, work structure 1.4 delay, loss, throughput in works 1.5 protocol layers, service models 1.6 works under attack: security 1.7 history NCSU CSC401 Lina Battestilli 6
The work core mesh of interconnected routers Two possibilities Circuit switched Packet switched NCSU CSC401 Lina Battestilli 7
Circuit Switching: Telephone Network Dedicated Wire Dedicated Wire telephones are connected by a dedicated wire to a local exchange Early days, switchboard operators used a big patch-panel to manually connect the wire is dedicated to the phone conversation from the start to the end of the phone call. NCSU CSC401 Lina Battestilli 8
Circuit Switching: Telephone Network Dedicated Wire Dedicated Wire Circuit Switch 1. Dial a number, which creates a dedicated circuit between the two phones. Each switch maintains state to map the incoming circuit to the correct outgoing circuit. 2. Talk: digital phone system, our voice is sampled and digitized, and sent over the dedicated circuit, typically 64kb/s for voice. Our phone conversation has a dedicated circuit 3. Hang up: the circuit is removed, and any state is removed at the switches along the path. NCSU CSC401 Lina Battestilli 9
Circuit Switching: Telephone Network Each phone call allocated 64kb/s. A 10Gb/s trunk line can carry over 150,000 calls. big fat pipes 2.4Gb/s-100Gb/s Source Caller Destination Callee Central Office (C.O.) Central Office (C.O.) Trunk Exchange 10
Circuit switching for the Inter s core? end-end resources reserved for, call between source, destination: dedicated resources: no sharing circuit-like (guaranteed) performance resource piece idle if not used by owning call (no sharing) dividing link bandwidth into pieces frequency division time division Restaurant Analogy
Circuit switching: FDM versus TDM FDM Example: 4 users frequency FM radio stations use FDM TDM time frequency frame time time is divided into frames of certain duration e.g 8000 frames/sec, each slot 8bits -> 64Kbps circuit
Circuit Switching Example A dedicated resources: no sharing, circuit-like (guaranteed) performance, idle if not used Host A to Host B Each link is 1.536Mbps and uses TDM, 24 slots End-to-end circuit establishment 500msec How long will it take to transmit a 640 Kbits file? B NCSU CSC401 Lina Battestilli 13
15 Circuit Switching Summary Each call has its own private, guaranteed, isolated data rate from end-to-end. A call has three phases: 1. Establish circuit from end-to-end ( dialing ) 2. Communicate 3. Close circuit ( tear down ) Originally, a circuit was an end-to-end physical wire. Nowadays, a circuit is like a virtual private wire.
16 Circuit Switching Issues for the Inter 1. Inefficient. Computer communication tends to be very bursty. e.g. typing over an ssh connection, or viewing a sequence of web pages. If each communication has a dedicated circuit, it will be used very inefficiently. 2. Diverse Rates. Computers communicate at many different rates. e.g. a web server streaming video at 6Mb/s, or typing at 1 character per second over ssh. A fixed rate circuit will not be much use! 3. State Management. Circuit switches maintain percommunication state, which must be managed.
Outline Computer Networks and the Inter (Ch 1) 1.1 What is the Inter? 1.2 work edge end systems, works, links 1.3 work core circuit switching, packet switching, work structure 1.4 delay, loss, throughput in works 1.5 protocol layers, service models 1.6 works under attack: security 1.7 history NCSU CSC401 Lina Battestilli 18
19 Packet Switching Data Data B Header A Packet Switch B Address Next-hop S 2 B S 2 C S 3 S 1 S 4 D S 3 C S 3 D hosts break application-layer messages into packets mesh of interconnected routers forward packets from one router to the next, across links on path from source to destination
20 Packet Switching Packet Switch
Packet Switching Advantages A Source R1 R2 R3 B Destination R4 - Packets are routed individually, by looking up address in router s local table. - All packets share the full capacity of a link. - The routers maintain no per-communication state. 21
Inter uses Packet Switching Efficient use of expensive links Links were assumed to be expensive and scarce. Packet switching allows many, bursty flows to share the same link efficiently. Circuit switching is rarely used for data works,... because of very inefficient use of the links, Bertsekas/Gallager Resilience to failure of links & routers For high reliability,... [the Inter] was to be a datagram sub, so if some lines and [routers] were destroyed, messages could be... rerouted, Tanenbaum 22
Host: sends packets of data host sending function: takes application message breaks into smaller chunks, known as packets, of length L bits transmits packet into work at transmission rate R link transmission rate, aka link capacity, aka link bandwidth 2 host 1 two packets, L bits each R: link transmission rate packet transmission delay time needed to transmit L-bit packet into link = = L (bits) R (bits/sec) 1-23
Packet-switching: store-and-forward L bits per packet source 3 2 1 R bps R bps destination takes L/R seconds to transmit (push out) L-bit packet into link at R bps store and forward: entire packet must arrive at router before it can be transmitted on next link (assuming zero propagation delay) delay = 2L R NCSU CSC401 Lina Battestilli 24
Packet-switching: store-and-forward (assuming zero propagation delay) L bits per packet source 3 2 1 R bps R bps destination Q: Time between when the source begins to send the first packet until the destination has received all three packets? NCSU CSC401 Lina Battestilli 27
Packet Switching: queueing delay, loss A R = 100 Mb/s C B queue of packets waiting for output link, output buffer R = 1.5 Mb/s D E resource contention: aggregate resource demand (use of transmission link) can exceed amount available congestion: packets will queue, wait for link use packets can be dropped (lost) if no memory to store them NCSU CSC401 Lina Battestilli 28
Forwarding Tables and Routing Protocols routing: determines sourcedestination route taken by packets routing algorithms forwarding: move packets from router s input to appropriate router output routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 3 2 1 Analogues to a driver that wants to ask for directions instead of using a map. dest address in arriving packet s header
Packet Switching versus Circuit Switching packet switching allows more users to use work! 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
Packet switching versus circuit switching is packet switching a slam dunk winner? great for bursty data resource sharing simpler, no call setup excessive congestion possible: 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 (ch 7, Multimedia Networking) Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?
Outline Computer Networks and the Inter (Ch 1) 1.1 What is the Inter? 1.2 work edge end systems, works, links 1.3 work core circuit switching, packet switching, work structure 1.4 delay, loss, throughput in works 1.5 protocol layers, service models 1.6 works under attack: security 1.7 history NCSU CSC401 Lina Battestilli 34
Inter structure: work of works End systems connect to Inter via ISPs (Inter Service Providers) Residential, company and university ISPs Access ISPs in turn must be interconnected. So that any two hosts can send packets to each other Resulting work of works is very complex Evolution was driven by economics and national policies Let s take a stepwise approach to describe current Inter structure
Inter structure: work of works Question: given millions of ISPs, how to connect them together?
Inter structure: work of works Option: connect each ISP to every other ISP? connecting each ISP to each other directly doesn t scale: O(N 2 ) connections.
Inter structure: work of works Option: connect each ISP to a global transit ISP? Customer and provider ISPs have economic agreement. global ISP
Inter structure: work of works But if one global ISP is viable business, there will be competitors. ISP A ISP B ISP C
Inter structure: work of works But if one global ISP is viable business, there will be competitors. which must be interconnected Inter exchange point ISP A IXP IXP ISP B ISP C peering link
Inter structure: work of works and regional works may arise to connect s to ISPS ISP A IXP IXP ISP B ISP C regional
Inter structure: work of works and content provider works (e.g., Google, Microsoft, Akamai ) may run their own work, to bring services, content close to end users ISP A ISP B ISP B IXP Content provider work IXP regional
Inter structure: work of works Tier 1 ISP Tier 1 ISP Google IXP IXP IXP Regional ISP Regional ISP ISP ISP ISP ISP ISP ISP ISP ISP at center: small # of well-connected large works tier-1 commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage content provider work (e.g, Google): private work that connects it data centers to Inter, often bypassing tier-1, regional ISPs
Tier-1 ISP: e.g., Sprint POP: point-of-presence to/from backbone peering to/from customers
References Some of the slides are identical or derived from 1. Slides for the 7 th edition of the book Kurose & Ross, Computer Networking: A Top-Down Approach, 2. Slides by Jim Kurose for his CSC453 course at Umass 3. Slides from Nick McKeown, CS144 at Stanford University NCSU CSC401 Lina Battestilli