Introduction to Computer Networks

• The notes cover introductory material for a computer networking course.
• Goal is to gain a feel for networking terminology to be covered in more detail later.
• The course will use the Internet as its primary example.

Course Roadmap

• The course will cover the main topics, including:
• What is the Internet?
• Network edge details covering hosts, access networks, and links.
• Network core concepts, packet/circuit switching, and overall network structure.
• Network performance factors like delay, loss and throughput.
• Protocol layers and service models.
• Network security considerations and common attacks.
• A brief history of the Internet.

What is the Internet?

• The Internet consists of billions of computing devices like PCs, servers, laptops, and smartphones
• These devices are connected via communication links:
• Fiber optic
• Copper
• Radio waves
• Satellite.
• Data transmission rate is bandwidth.
• End systems are also called hosts which run network apps.
• routers and switches are specialized packet switches forward data chunks
• Internet: a network of interconnected ISPs.
• Protocols control the sending and receiving of messages (e.g., TCP, IP, HTTP, Skype, 802.11)
• The Internet uses standards like RFCs and those from the IETF.
• Provides infrastructure and programming interface for applications like web, VoIP, email, games and social networks
• Connection provided that allow program to use internet like postal service

Protocols

• Protocols are used in Internet governed communication activity and define the format and order of messages.
• Includes messages sent and received among network entities and actions taken on message transmission and receipt.

Network Structure

• The network edge includes client and server hosts, with servers often in data centers.
• Access networks and physical media use wired and wireless communication links.
• The network core consists of interconnected routers which form a network of networks.

Access Networks and Physical Media

• Connections into the edge router are established by residential access nets, institutional access networks (school, company), and mobile access networks.
• Bandwidth (bits per second) of the access network is an important factor.
• Networks can be shared or dedicated.

Digital Subscriber Line (DSL)

• DSL uses existing telephone lines.
• The DSL connects subscriber to a central office Digital Subscriber Line Access Multiplexer (DSLAM).
• Data travels over the DSL phone line to the Internet, while voice goes to the telephone net.
• Upstream transmission rate < 2.5 Mbps, downstream < 24 Mbps.

Cable Network

• Cable networks use cable and fiber to attach homes to ISP router.
• They share access network to cable headend.
• Unlike DSL, which has dedicated access to a central office, cable is shared.
• Hybrid fiber coax (HFC) is asymmetric with up to 30Mbps downstream and 2 Mbps upstream.
• Frequency division multiplexing transmits different channels in different frequency bands.

Home Network

• A typical home network includes: Wireless devices, a combined modem/router, a firewall that uses Network Address Translation, wireless access point, and wired Ethernet.

Enterprise Access Networks (Ethernet)

• These are typically used in companies and universities
• Common transmission rates are 10 Mbps, 100 Mbps, 1 Gbps and 10 Gbps.
• End systems often connect into an Ethernet switch.

Wireless Access Networks

• Shared wireless access networks connect an end system to a router.
• It uses a base station, also known as access point.
• Wireless LANs operate within a building (100 ft.) using 802.11b/g/n (WiFi) at 11, 54, or 450 Mbps.
• Wide-area wireless access is provided by cellular operators over 10's of kilometers. Data rate is between 1 and 10 Mbps using 3G, 4G or LTE.

Data Transmission

• A host takes an application message and breaks it into smaller packets of length L bits.
• Then it transmits the packets into the access network at a transmission rate R.
• Link transmission rate is also known as link capacity or link bandwidth.
• Packet transmission delay = the time needed to transmit an L-bit packet into the link.
• Packet transmission delay = L (bits) / R (bits/sec)

Physical Media Overview

• A bit propagates between transmitter/receiver pairs.
• The physical link is what lies between the transmitter and receiver.
• Guided media uses signals that propagate in solid media like copper, fiber, or coax.
• Unguided media signals propagate freely, like radio.

Twisted Pair (TP) Cable

• Uses two insulated copper wires.
• Category 5: 100 Mbps, 1 Gbps Ethernet.
• Category 6: 10Gbps.

Coaxial Cable

• Coaxial cable composed of two concentric copper conductors.
• Bidirectional and broadband supporting multiple channels (HFC).

Fiber Optic Cable

• Fiber optic cable uses glass fiber carrying light pulses.
• High-speed and high-bandwidth, supporting high-speed point-to-point transmission like 10's-100's Gbps transmission rate.
• Low error rate and immune to electromagnetic noise, with widely spaced repeaters.

Radio Media

• Radio transmits electromagnetic spectrum without a physical wire.
• Affected by reflection, obstructions, and interference.
• Data rate varies from Kbps to 45Mbps in satellite channels
• 270 msec end-end delay
• Geosynchronous vs low altitude affect delay

Network Core Switching

• Packet-switching is where hosts break application-layer messages into packets.
• It uses routers to forward packets from one router to the next
• Each packet is transmitted at full link capacity.

Packet Switching: Store-and-Forward

• In this model, the switch takes L/R seconds to transmit an L-bit packet into a link at a rate R bps.
• The entire packet must arrive at the router before being transmitted on the next link.
• End-to-end delay = 2L/R, assuming zero propagation delay.
• In a one-hop example:
• L = 7.5 Mbits
• R = 1.5 Mbps
• One-hop transmission delay = 5 seconds
• The queue of packets waiting to exit link will increase delay waiting

Two Key Network-Core Functions

• Routing determines the source-destination route taken by packets.
• Forwarding moves packets from router's input to appropriate router output using a forwarding table.

Alternative Core: Circuit Switching

• With circuit switching, end-to-end resources are allocated and reserved for the "call" between source and destination.
• Each link has multiple circuits.
• Resources are dedicated, not shared, leading to guaranteed performance.
• Circuit segment remains idle if not used by allocating call.
• Commonly used in traditional telephone networks.
• FDM, Frequency Division Multiplexing and TDM, Time Division Multiplexing are two flavors

Packet Switching vs. Circuit Switching

• Packet switching allows more users to use the network.
• In a 1 Mb/s link with each user needing 100 kb/s when active and active only 10% of the time:
• Circuit-switching can support 10 users.
• Packet switching can support more by sharing

Packet-Switching vs. Circuit Switching Analysis

• Packet switching is great for bursty data, since it shares resource.

Packet Switching: Issues

• Packets can be delayed given possibility of packet delay and loss issues
• Protocols are important for reliable data transfer and congestion control
• Excessive congestion is excessive packet delay and loss
• Packet require protocols for reliable data transfer, congestion control
• Bandwidth guarantees for audio/video apps is unsolved problem

Internet Structure: Network of Networks

• End systems connect to the Internet via access ISPs (Internet Service Providers).
• Access ISPs must be interconnected.
• Resulting network of networks has evolved by economics and national policies.
• Internet structure is described stepwise.

Possible connection solutions

• Connecting each access ISP to every other access ISP is not scalable O(N^2) connections.
• Connecting each access ISP to one global transit ISP requires an economic agreement between customer and provider ISP.

Competing Networks

• Multiple global ISP competitors leads to the formation of:
• Internet exchange points (IXP).
• Regional networks used to connect access networks to ISPs.
• Content provider networks (e.g., Google, Microsoft, Akamai) may run own network.

Tiered Structure

• Internet has a tier structure, with a small number of interconnected tier-1 commercial ISPs
• Tier-1 ISP examples: Level 3, Sprint, AT&T and NTT
• Tier-1 ISPs have national and international coverage.
• Content provider networks (e.g., Google) also use private networks to connect data centers, bypassing tier-1 ISPs.

Network Performance Factors

• Delays are due to packet queue up in router buffers when packet arrival rate to link temporarily exceeds link capacity.
• Factors include: packet arrival rate to link exceeding output link capacity and packets queue up, wait for turn

Four Sources of Packet Delay

• Nodal processing (d_proc): Includes checking bit errors and determining output link; typically < msec.
• Queueing delay (d_queue): Time waiting at output link for transmission.
• Transmission delay (d_trans): L: packet length (bits) and R: link bandwidth (bps) where d_trans = L/R
• Propagation delay (d_prop): d: length of physical link and s: propagation speed (~2x108 m/sec) where d_prop = d/s
• The variables, d_trans and d_prop are different
• dnodal = dproc + dqueue + dtrans + dprop

Simple Caravan Analogy

• Models a cars "propagate" at 100 km/hr, where a toll booth take 12 sec to service car
• Cars ~ bit; caravan ~ packet
• Used to understand propagation delay and transmission delay
• Time spent is affected by capacity to "propagate" and service

Queueing Delay

• R is Link bandwidth (bps)
• L is Packet length (bits)
• a is average packet arrival rate
• traffic intensity = La/R

Queueing Delay Revisited

• La/R ~ 0: average queueing delay small
• La/R -> 1: average queueing delay grows large.
• La/R > 1: more "work" arriving than can be serviced, so average delay is infinite.

Internet Path and Loss

• Traceroute provides measurement from source to router along end-end Internet path.
• Sends three packets that will reach router i on path towards destination.
• Router i returns packets to sender.
• Sender times interval between transmission and reply.

Packet Loss

• Queue (aka buffer) preceding link in buffer has finite capacity.
• Packet arriving to full queue is dropped (aka lost).
• Lost packet may be retransmitted by previous node, by source end system, or not at all.

Throughput

• The rate (bits/time unit) at which bits transfer between sender/receiver.
• Rate at given point in time is instantaneous throughtput.
• Rate over longer period of time is average throughput.

Throughput (2)

• If sender data rate is less than the receive rate: R < R then sender is limiting s

• If sender data rate is more than the receive rate: R > R then receiver is limiting s

• Bottleneck link: Link on end-end path that constrains end-end throughput.

• Per-connection end-end throughput: min(Rc,Rs,R/10)

• Practice: bandwidth to computer Rc or bandwidth from serve Rs is often bottleneck

Protocol "Layers"

• Networks are complex, including many "pieces" such as hosts, routers, links, applications, protocols, hardware, and software.
• They are modular via identification to establish communication and maintain order
• Explicit structure through layering makes it easier to identify and establish relationships of pieces within compmlex system
• Layering allows easy maintenance and updating of system

Layered system of air travel

• Each layer implements a service, through: its own internal-layer actions and relying on services provided by layer below

Internet Protocol Stack

• Application: supporting network applications (FTP, SMTP, HTTP).
• Transport: process-process data transfer (TCP, UDP).
• Network: routing of datagrams from source to destination (IP, routing protocols).
• Link: data transfer between neighboring network elements (Ethernet, 802.111 (WiFi), PPP).
• Physical: bits "on the wire".

ISO/OSI Reference Model

• Presentation: allows applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions.
• Session: synchronization, checkpointing, recovery of data exchange.
• The Internet stack doesn't include these layers, so any needed services must be implemented in the application.

Encapsulation

• Process of adding new properties to datagram

Network Security Considerations

• The field involves studying attacks to computer networks, how to defend against these attacks, and how to design architectures immune to attacks.
• The internet was not originally designed with much security in mind.

Network Security Attacks

• Malware enters hosts from:
  • Viruses: replicate by receiving/executing an object (e.g., email attachment).
  • Worms: replicate by passively receiving an object that gets itself executed.
• Spyware malware records keystrokes, web sites visited, and uploads info to collection site.
• Infected hosts can then be enrolled in a botnet and used for spam or DDoS attacks.
• Denial of Service (DoS) attacks make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming the resource with bogus traffic.
  • Targets are selected, then hosts around the network are broken into like a botnet which sends packets from compromised hosts
• Packet "sniffing" on broadcast media (shared Ethernet, wireless) used for reading/recording packets
• Promiscuous network interface reads/records all packets.
• Wireshark software is available for packet sniffing. IP spoofing: sending a packet with a false source address.

Internet History

• Early packet-switching principles
  • 1961: Kleinrock's queueing theory shows effectiveness of packet-switching.
  • 1964: Baran - packet-switching in military nets.
  • 1967: ARPAnet conceived by Advanced Research Projects Agency.
  • 1969: first ARPAnet node operational.
  • 1972: • ARPAnet public demo. • NCP (Network Control Protocol) first host-host protocol. • first e-mail program. • ARPAnet has 15 nodes.
• Internetworking, new and proprietary nets
  • 1970: ALOHAnet satellite network in Hawaii.
  • 1974: Cerf and Kahn architecture for interconnecting networks.
  • 1976: Ethernet at Xerox PARC.
  • Late 70's proprietary architectures: DECnet, SNA, XNA.
  • Late 70's switching fixed length packets (ATM precursor).
• Cerf and Kahn's internetworking principles: • minimalism, autonomy - no internal changes required to interconnect networks • best effort service model and stateless routers • decentralized controls define today's Internet architechture •New protocols, a proliferation of networks
  • New national networks: CSnet, BITnet, NSFnet, Minitel.
  • 100,000 hosts connected to confederation of networks. 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 the Web • Late 1990's-2000's • More killer app-instant messaging. • P2P file sharing. • Network Security to forefront. • est. 50 million host, 100 million + users • backbone links running at GbPs.
• The period since 2005 up to the present covers: •~5B devices attached to internet(2016) -smartphones and tablets • aggressive deployment of broadband access •Service providers create own networks, bypassing internet providing "instantaneous" access to search, video content, email, etc •e-commerce,universtities,enterprises,services in "cloud (example/amazon)"