Ch 1. Computer Networks and the Internet Myungchul Kim

2 What is the Internet? o One sentence definition? – Software and hardware – Services o Hosts or end systems – Internet home appliances – Pervasive computing – Ubiquitous computing o Communication links – Bandwidth: bits/second o Routers

3 o Packet o Route or path – End system -> links and routers -> end system – Packet switching: sharing a path o Internet Service Providers (ISP) o Protocols – Specifies the format of the packets that are sent and received among routers and end systems – TCP (Transmission Control Protocol) – IP (Internet Protocol) o Intranet – Private networks o Internet Standards – IETF (Internet Engineering Task Force): – RFCs (Request for Comments)

4 o A service description – distributed applications: remote login, electronic mail, Web surfing, instant messaging, audio and video streaming, Internet telephony, distributed games, peer-to-peer (P2P) file sharing,… – Connection-oriented reliable services and a connectionless unreliable service o Protocols – Figure 1.2.

5 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

6 The Network Edge o Host = end system: clients and servers o Peer-to-peer: acts as both a client and a server o Transport layer protocols – Connectionless service – Connection-oriented service o Connection-oriented service: TCP – Handshaking protocol -> connection – End-to-end – Reliable data transfer, flow control and congestion control o Reliable data transfer – Acknowledgement o Flow control – Force the sending end system to reduce its rate whenever there is a risk keeping the sending pace. With buffers in the end systems.

7 o Congestion control – Congestion – Buffer overflow of routers -> packet loss – Force end systems to decrease the rate at which they send packets into the network during periods of congestion. o Connectionless Service: UDP – No handshaking – No reliable data transfer – Internet phone and video conferencing

8 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)

9 o Fig 1.6.

10 o Packet switching – Message -> packets – Routers = packet switches – Store-and-forward transmission -> delay – Output queue -> delay – Packet loss – Fig 1.7

11 o Packet switching vs Circuit switching – Real-time services – Sharing of network resources – Implementation

12 o Message switching – Message switching: sequential transmission – Packet switching: parallel transmission (pipelining) – Error handling? – Header overhead? – Figure 1.8 and Figure 1.9

13 – Figure 1.10

14 – Figure 1.11

15 o Packet forwarding – Packet-switched networks: datagram networks and virtual circuit networks – Datagram network: forwards packets according to host destination addresses – Virtual circuit networks: forwards packets according to virtual circuit numbers o Virtual circuit networks – Virtual circuit: a path + virtual circuit numbers + entries in VC-number translation tables – Fig 1.12 – Maintain state information

16 o Datagram networks – Similar to the postal service – Do not maintain connection-state information in their switches. o Network taxonomy – Fig 1.13 – Networks with VCs are always connection-oriented.

17 Network Access o Residential access – Dial-up modem – Digital subscriber line (DSL): point-to-point – Hybrid fiber coaxial cable (HFC): shared – Bottom of Page 30 o Company access – LAN – Ethernet o Mobile access – Wireless LAN – IEEE b, Wi-Fi

18 ISPs and Internet Backbones o Tier-1 ISPs – Internet Backbone o Tier-2 ISPs o Fig 1.17

19 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. – Microseconds to milliseconds.

20 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

21 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

22 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.

23 o Traceroute – Repeats experiment three times – eniac.seas.upenn.edu -> diane.ibp.fr (pp. 48, 49) – Queuing delay – Transatlantic link

24 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 N-PDUs o Protocol stack o Service model – Layer n-1 is said to offer services to layer n o Fig 1.22

25 o Layer functions – Error control – Flow control – Segmentation and reassembly – Multiplexing – Connection setup – Duplicate lower-layer functionality

26 o The internet protocol stack – Fig 1.23

27 – Application layer: HTTP, SMTP, FTP – Transport layer: TCP, UDP – Network layer: IP, routing – Link layer: Ethernet, PPP – Physical layer – Fig 1.24