Ch 1. Computer Networks and the Internet Myungchul Kim

2 What is the Internet? o One sentence definition? – A nuts-and-bolts description – A service description o A nuts-and-bolts description – Hosts or end systems – A network of communication links and packet switches – Transmission rate – Packets – Packet switches: routers and link-layer switches – Route or path – Internet Service Providers (ISPs) – Protocols: TCP and IP – Internet Standards: Request for comments (RFCs) by IETF – Intranet

3 o A service description – An infrastructure for providing services to distributed applications: remote login, electronic mail, Web surfing, instant messaging, VoIP, audio and video streaming, Internet telephony, distributed games, peer-to-peer (P2P) file sharing, IPTV… – Application Programming Interface (API) o Protocols – Figure 1.2.

4 o Definition of a Protocol – Defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event – Similar to a human analogy: there are specific messages we send, and specific actions we take in response to the received reply messages or other events

5 The Network Edge o Host = end system: clients and servers o Peer-to-peer: acts as both a client and a server o Access networks: connect an end system to its edge router – Residential access – Company access – Wireless access o Residential access – Digital subscriber line (DSL): point-to-point – Hybrid fiber-coaxial cable (HFC): cable modems, shared – Very-high speed DSL (VDSL)

6 o Company access – Ethernet : shared o Wireless access – Wireless LAN – IEEE WiFi – 3G Wireless: HSDPA (High-Speed Downlink Packet Access) – IEEE WiMax – WiBro o Physical media – Twisted-pair copper wire – Coaxial cable – Fiber optics – Terretrial radio channels: wireless LAN, the cellular access – Satellite Radio channels

7 The Network Core o Circuit switching – Reserved for the communication session – A circuit: at the guaranteed constant rate – Telephone network o Packet switching – The network resources on demand – Internet – Best effort o Multiplexing in Circuit-switched networks – The dedicated circuits are idle during silent periods – Frequency-division multiplexing (FDM) or Time-division multiplexing (TDM)

8 o Fig 1.6.

9 o Packet switching – Message -> packets – Routers = packet switches – Store-and-forward transmission: the switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link -> store-and-forward delay – Output queue -> queueing delay – Packet loss – Fig 1.7

10 o Packet switching vs Circuit switching – Packet switching is not suitable for real-time services? – Sharing of network resources -> statistical multiplexing of resources

11 – Figure 1.11

12 ISPs and Internet Backbones o Tier-1 ISPs: Internet Backbone o Tier-2 ISPs: regional or national coverage o Access ISPs o Points of Presence (POPs): the points at which the ISP connects to other ISPs

13 Delay and loss in Packet-switched networks o Fig 1.18 o Processing delay – Examine the packet’s header and determine where to direct the packet – Check for bit-level errors – Microseconds or less o Queuing delay – A packet waits to be transmitted onto the link – Depends on the number of earlier-arriving packets that are queued and waiting for transmission across the link. – congestion – Microseconds to milliseconds.

14 o Transmission delay – Store-and-forward delay – Transmit all of the packet’s bits into the link – L/R where L bits = length of the packet, R = 10 Mbps for a 10 Mbps Ethernet link – Microseconds to milliseconds o Propagation delay – Propagation speed of the link – d/s where d = distance and s = the propagation speed of the link – Milliseconds o Comparing transmission and propagation delay – d nodal = d proc + d queue + d trans + d prop – d prop : hundreds of milliseconds for two routers by a satellite link – d trans : hundreds of milliseconds for low-speed dial-up modem links – d proc : at the max rate of a router

15 o Queuing delay – Traffic intensity La/R where a = the average rate of packets arrival at the queue (packets/sec), L bits of a packet, R = the transmission rate (bits/sec), and the infinite queue. – If La/R > 1, the queue will tend to increase without bound and the queuing delay will approach infinity. – If La/R ≤ 1, the nature of the arriving traffic impacts the queuing delay. Periodically or in bursts or random – Fig 1.19

16 o Packet loss – A queue has finite capacity. – Performance of a node = delay + packet loss o End-to-end delay – d end-end = N (d proc + d trans + d prop ) for N-1 routers where the network is uncongested.

17 o Traceroute – Repeats experiment three times to get the round-trip delays between souce and destination – The queuing delay is varying with time. -> the round-trip delays are varying. – (Next slide) o Other delays – Media accessing delays in WiFi, Ethernet, … – Packetization delays in VoIP

18 “ Real ” Internet delays and routes 1 cs-gw ( ) 1 ms 1 ms 2 ms 2 border1-rt-fa5-1-0.gw.umass.edu ( ) 1 ms 1 ms 2 ms 3 cht-vbns.gw.umass.edu ( ) 6 ms 5 ms 5 ms 4 jn1-at wor.vbns.net ( ) 16 ms 11 ms 13 ms 5 jn1-so wae.vbns.net ( ) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu ( ) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu ( ) 22 ms 22 ms 22 ms ( ) 104 ms 109 ms 106 ms 9 de2-1.de1.de.geant.net ( ) 109 ms 102 ms 104 ms 10 de.fr1.fr.geant.net ( ) 113 ms 121 ms 114 ms 11 renater-gw.fr1.fr.geant.net ( ) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renater.fr ( ) 111 ms 114 ms 116 ms 13 nice.cssi.renater.fr ( ) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr ( ) 126 ms 126 ms 124 ms 15 eurecom-valbonne.r3t2.ft.net ( ) 135 ms 128 ms 133 ms ( ) 126 ms 128 ms 126 ms 17 * * * 18 * * * 19 fantasia.eurecom.fr ( ) 132 ms 128 ms 136 ms traceroute: gaia.cs.umass.edu to Three delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu * means no response (probe lost, router not replying) trans-oceanic link

19 Throughput o throughput: rate (bits/time unit) at which bits transferred between sender/receiver – instantaneous: rate at given point in time – average: rate over long(er) period of time server, with file of F bits to send to client link capacity R s bits/sec link capacity R c bits/sec pipe that can carry fluid at rate R s bits/sec) pipe that can carry fluid at rate R c bits/sec) server sends bits (fluid) into pipe

20 Throughput (more) o R s < R c What is average end-end throughput? R s bits/sec R c bits/sec o R s > R c What is average end-end throughput? R s bits/sec R c bits/sec link on end-end path that constrains end-end throughput bottleneck link

21 Protocol layers and their service models o A layered architecture allows us to discuss a well-defined, specific part of a large and complex system. o Protocol stack o Service model – Layer (n-1) is said to offer services to layer (n)

22 o Layer functions – Error control – Flow control – Segmentation and reassembly – Multiplexing – Connection setup – Drawbacks of layering  Duplicated lower-layer functionality  Accessing an information in another layer

23 – Application layer: HTTP, SMTP, FTP, DNS – Transport layer: TCP, UDP – Network layer: IP, routing – Link layer: Ethernet, PPP, WiFi – Physical layer

24 o The internet protocol stack

25 source application transport network link physical HtHt HnHn M segment HtHt datagram destination application transport network link physical HtHt HnHn HlHl M HtHt HnHn M HtHt M M network link physical link physical HtHt HnHn HlHl M HtHt HnHn M HtHt HnHn M HtHt HnHn HlHl M router switch Encapsulation message M HtHt M HnHn frame

26 Networks under attack o Network security o The bad guys can put malware into your host via the Internet – Botnet, Self-replicating, Viruses, Worms, Trojan hoars o The bad guys can attack servers and network infrastructure – Denial-of-service (DoS) attacks, Distributed DoS attacks o The bad guys can sniff packets – A packet sniffer: Ethereal o The bad guys can masquerade as someone you trust – IP spoofing: with a false source address – Authentication o The bad guys can modify or delete messages – Man-in-the-middle attacks – Integrity of the data