1 - Computer Networks and the Internet

1 - Computer Networks and the Internet#

1.1 - What is the Internet?#

1.1.1 - A “Nuts and Bolts” Description#

The Internet is a computer network that connects billions of devices across the world.

The devices, including computer, phones, TVs, gaming consoles, thermostats, and others, that are connected to the Internet are called hosts or end systems/nodes.

The end systems in the network are connected together through a network of communication links and packet switches. Different links transmit data at different rates, with the transmission rate of each link measured in bits/second. The packages of information exchanged are known as packets.

  • A packet switch takes a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links.

    • The most prominent type of packet switches are routers and link-layer switches.

  • The sequence of communication links and packet switches traversed by a packet from the sending end system to the receiving end system is known as its route or path through a network.

End systems access the Internet through Internet Service Providers (ISPs). Each ISP is itself a network of packet switches and communication links.

End systems, packet switches, and other pieces of the Internet run protocols that control the sending and receiving of information within the Internet.

  • The Transmission Control Protocol (TCP) and the Internet Protocol (IP) are two of the most important protocols in the Internet. The IP protocol specifies the format of the packets that are sent and received amoung routers and end systems. These two protocols are collectively known as TCP/IP.

  • The Internet standards that define these protocols are developed by the Internet Engineering Task Force (IETF) in documents called requests for comments (RFCs).

1.1.2 - A “Services” Description#

  • Infrastructure that provides services to applications

    • Web, streaming video, multimedia teleconferencing, email, games, e-commerce, social media, interconnected appliances, etc.

  • Provides programming interface to distributed applications:

    • “Hooks” allowing sending/receiving apps to “connect” to, use Internet transport service

    • Provides service options, analogous to postal service

1.1.3 - What is a Protocol?#

Protocols define the format, order of messages sent and received among network entities, and actions taken on message transmission and receipt.

1.2 - The Network Edge#

End systems, which are the network edges in the Internet, are commonly divided into two categories:

  • Hosts: desktops, smartphones, laptops, etc.

  • Servers: often in large data centers

Access networks, physical media:

  • Wired, wireless communications links

Network core:

  • Interconnected routers

  • Network of networks

1.2.1 - Access Networks#

How to connect end systems to edge router?

  • Residential access nets

  • Institutional access networks (school, company)

  • Mobile access networks (WiFi, 4G,/5G)

Digital Subscriber Line (DSL)#

Digital subscriber line (DSL) uses an existing phone line, where data goes to the Internet and voice goes to the telephone network. For DSL, the ISP is typically the local telephone company (telco).

A residence that is connected to DSL will have the telephone and Internet run through the same wire, and the telephone and Internet data is separated with a DSL access multiplexer (DSLAM) located in the telco’s local central office (CO).

The DSL standards define multiple transmission rates:

  • 24 or 52 Mbps dedicated downstream transmission rate

  • 3.5 or 16 Mbps dedicated upstream transmission rate

DSL is designed for residences to be within 5-10 miles of their CO. Residences outside of this range typically must resort to other means of Internet access.

Cable-Based Access#

Cable internet access makes use of existing cable infrastructure from television companies. Both fiber obtic and coaxial cables are used in the development of these networks, so the system is called hybrid fiber coax (HFC).

Frequency division multiplexing (FDM): Different channels transmitted in different frequency bands.

  • 40 Mbps - 1.2 Gbps downstream transmission rate

  • 30-100 Mbps upstream transmission rate

Wireless Access Networks#

Shared wireless access network connects end system to router via base station

  • Wireless local area networks (WLAN)

    • Typically within or around a building (~100 ft)

    • 802.11b/g/n (WiFi): 11, 54, 450 Mbps transmission rate

  • Wide-area cellular access networks

    • Provided by mobile, cellular network operator (10s of kilometers)

    • 10s of Mbps

    • 4G/5G cellular networks

Wide-Area Access Networks#

Telecommunications companies manage fourth-generation (4G) wireless networks, which provide real-world download speeds of up to 60 Mbps.

Enterprise Networks#

Mix of wired, wireless link technologies, connecting a mix of switches and routers.

  • Ethernet: wired access at 100Mbps, 1Gbps, 10Gbps

  • WiFi: wireless access points at 11, 54, 450 Mbps

Data Center Networks#

High-bandwidth links (10s to 100s Gbps) connect hundreds to thousands of servers together and to the Internet.

1.2.2 - Physical Media#

Hosts#

Hosts send packets of data:

  • Takes application message

  • Breaks into smaller chunks, known as packets, of length \(L\) bits

  • Transmits packet into access network at transmission rate \(R\)

    • Link transmission rate, aka capacity, aka link bandwidth

\[ \text{Packet transmission delay} = \text{Time needed to transmit } L \text{-bit packet into link} = \frac{L}{R} \]

1.3 - The Network Core#

The network core is a mesh of interconnected routers.

Packet-switching is when hosts break application-layer messages into packets.

  • Network forwards packets from one router to the next, across links on path from source to destination

Two key network-core functions:

  • Forwarding (switching): A local action that moves arriving packets from the router’s input link to appropriate router output link.

  • Routing: A global action that determines source-destination paths taken by packets.

1.3.1 - Packet Switching#

Store-and-Forward#

  • Packet transmission delay: 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 the next link. This makes the transmission rate \(2L/R\) when there is one packet switch that employs store-and-forward transmission.

If we consider there to be \(N\) routers between source and destination:

\[ d_{\text{end-to-end}} = (N + 1) \frac{L}{R} \]

Queueing#

Queueing occurs when work arrives faster than it can be serviced. Each packet switch has an output buffer or output queue, which stores packets that the

If the arrival rate (in bps) to link exceeds transmission rate (bps) of link for some period of time:

  • There will be a queueing delay, since packets will wait to be serviced

  • Packet loss will occur if memory (buffer) in router fills up

1.3.2 - Circuit Switching#

End-to-end resources are allocated to and reserved for “call” between source and destination.

  • Commonly used in traditional telephone networks

  • Dedicated resources provide guaranteed performance

  • Circuit segment sits idle if unused by call

Frequency Division Multiplexing (FDM):

  • Optical, electromagnetic frequencies divided into narrow frequency bands.

  • Each call allocated at its own band, can transmit at max rate of that narrow band.

Time Division Multiplexing (TDM):

  • Time divided into slots.

  • Each call allocated periodic slots, can transmit at maximum rate of wider frequency band during its time slot.

Packet Switching vs. Circuit Switching#

Consider a system with 10 users with a total capacity of 1 Gb/s for its link. Each user uses 100 Mb/s when active, and each user is active 10% of the time. How many users can use this network under circuit-switching and packet-switching?

  • Circuit-switching: 10 users

  • Packet-switching: With 35 users, the probability that more than 10 users are active at the same time is less than 0.0004.

Packet-switching is great for “bursty” data, because it is better at resource sharing and simpler, with no call setup.

  • Excessive congestion possible: packet delay and loss due to buffer overflow

  • Protocols are needed for reliable data transfer

  • There are ways of providing circuit-like behavior with packet-switching that will be covered later in the course

1.3.3 - A Network of Networks#

End systems connect to the Internet via access ISPs, which can be a telco, cable company, university, or company among others. To connect every end system to each other, these access ISPs must also be connected, which is called the network of networks.

A naive approach to connect ISPs would be to have a mesh, i.e., to connect every ISP to each other through a physical connection.

In Network Structure 1, a single global transit ISP is constructed that connects all ISPs to each other. Each access ISP would then pay the global transit ISP based on the amount of traffic it exchanges with the global ISP.

In Network Structure 2, there are multiple global transit ISPs that compete with each other on price. Note that the global transit ISPs must also connect with each other.

Network Structure 4: In any given city, there is typically one regional ISP to which the access ISPs connect. These regional ISPs then connect to tier-1 ISPs, of which there are about a dozen in the world including AT&T, Sprint, and others. To more accurately approximate today’s Internet, we must consider the following:

  • A point of presence (PoP) is a group of one or more routers in the provider’s network where customer ISPs can connect to the provider ISP.

  • Any ISP can choose to multi-home, i.e., to connect to two or more provider ISPs. This allows the ISP to send and receive packets even if one of the provider ISPs fails.

  • Two ISPs of the same tier can peer with each other. This means that they directly connect their networks and can access each other’s resources without paying a fee. Note that tier-1 ISPs peer with each other.

    • An Internet Exchange Point (IXP) is a meeting point where multiple ISPs peer with each other.

Network Structure 5 describes today’s Internet. It works similarly to Network Structure 4, but includes content-provider networks. For example, Google has 19 major data centers (and many more smaller data centers) around the world that connect to each other through a private TCP/IP network that is separate from the Internet. Google can then peer with lower-tier ISPs, typically at IXPs, allowing them to pay fewer fees to higher-tier ISPs and gain more control over the way they send data to end systems.

1.4 - Delay, Loss, and Throughput in Packet-Switched Networks#

1.4.1 - Overview of Delay in Packet-Switched Networks#

As a packet travels from its source to its destination, it suffers several types of delays at each node along the path.

\[ d_{\text{nodal}} = d_{\text{proc}} + d_{\text{queue}} + d_{\text{trans}} + d_{\text{prop}} \]

  • The time required to examine the packet’s header and determine where to direct it is part of the processing delay. It can also include the time needed to check for bit-level errors in the packet. These delays are typically microseconds or less.

  • A packet experiences queuing delay as it waits to be transmitted onto the link. The queuing delay depends on the number of items in the queue when the packet arrives, and is zero if it is empty.

  • Denote the length of the packet (in bits) as \(L\) and the transmission rate (in bits/second) \(R\). The transmission delay is the amount time it takes to push the packet’s contents to the link, and is \(L/R\).

  • The time required to travel across the physical links is called the propagation delay, which is equal to or just a little more than the distance between the nodes divided by the speed of light. This can be expressed as \(d/s\), where \(d\) is the distance between the links and \(s\) is the speed of light.

1.4.2 - Queueing Delay and Packet Loss#

The most interesting component of nodal delay is the queuing delay.

If a packet arrives to a router with a full queue, the router will drop the packet, resulting in packet loss.

1.4.3 - End-to-End Delay#

End-to-end delay considers the full delay of a packet as it travels from its source to its destination, rather than just at a node.

Suppose there are \(N-1\) routers on the packet’s path. If we suppose that there are no queuing delays, we can say

\[ d_{\text{end-to-end} = N (d_{\text{proc}} + d_{\text{trans}} + d_{\text{prop}})} \]

Note that Traceroute is a program that sends a message back to the source with the name and address of each router it encounters.

It can be run in Windows with:

tracert mtmoen.com

Or in Linux with

traceroute mtmoen.com

1.4.4 - Throughput in Computer Networks#

Throughput is the rate (bits/time) at which bits are being sent from sender to receiver.

  • Instantaneous: Rate at given point in time

  • Average: Rate over long period of time

Note that the throughput is determined by the link with the smallest transmission rate along the entire path. This link is called the bottleneck link. Note that this bottleneck link may be due to a shared link.

1.5 - Protocol Layers and Their Service Models#

1.5.1 - Layered Architecture#

Network protocols are designed to fit in certain layers. This is part of the service model, wherein each layer provides a specific service. These protocol layers are defined by a combination of hardware and software. The collection of protocol layers is called the protocol stack.

Protocol layering simplifies the process of understanding complex interactions over networks, but has the drawback of potentially causing redundancies.

Layered Internet protocol stack:

  1. Application Layer: supporting network applications

    • HTTP, IMAP, SMTP, DNS

    • A packet of information at this layer is called a message.

  2. Transport Layer: process-process data transfer

    • TCP, UDP

    • A packet of information at this layer is called a segment.

  3. Network Layer: routing of datagrams from source to destination

    • IP, routing protocols

  4. Link Layer: data transfer between neighboring network elements

    • Ethernet, 802.11 (WiFi), PPP

  5. Physical Layer: bits “on the wire”

    • Copper wire, Coaxial cable, fiber optic cable

1.5.2 - Encapsulation#

Encapsulation allows developers to abstract away lower levels of the protocol stack, so long as the messages from the higher levels contain all the information needed at the lower levels.

1.6 - Networks Under Attack#

The Internet was not originally designed with much security in mind.

  • Original vision: “A group of mutually trusting users attached to a transparent network”

A compromised host can be enrolled in a network of thousands of similarly compromised devices in what is collectively known as a botnet.

One broad class of security threats are known as denial-of-service (DoS) attacks. These attacks render a network or host unusable by legitimate users. In a distributed denial-of-service (DDoS) attack, the attacker controls multiple sources that it uses.

Packet sniffers can detect any packets sent over a network and passively record any information sent.

The ability to inject packets into the Internet with a false source address is called IP spoofing.

1.7 - History of Computer Networking and the Internet#

1.7.1 - The Development of Packet Switching: 1961-1972#

In the early 1960s, as computers became more prevalent, researchers sought efficient ways to connect them, moving beyond traditional circuit-switched telephone networks. Three independent research groups—Leonard Kleinrock at MIT, Paul Baran at Rand, and Donald Davies at the National Physical Laboratory—developed the foundational concepts of packet switching, which proved more efficient for bursty computer traffic.

This work led to the creation of ARPAnet, the first packet-switched network, funded by the Advanced Research Projects Agency (ARPA). The first node was installed at UCLA in 1969, with additional nodes soon following. By 1972, ARPAnet had grown to 15 nodes, with Robert Kahn demonstrating its capabilities publicly. The network-control protocol (NCP), ARPAnet’s first host-to-host protocol, was also introduced, enabling applications such as Ray Tomlinson’s pioneering email program.

1.7.2 - Proprietary Networks and Internetworking: 1972-1980#

In the early to mid-1970s, multiple independent packet-switched networks emerged, including ALOHANet in Hawaii, DARPA’s packet-satellite and packet-radio networks, and commercial networks like Telenet and Cyclades. This proliferation of networks highlighted the need for an overarching architecture to enable inter-network communication.

Vinton Cerf and Robert Kahn, funded by DARPA, pioneered the concept of internetting, leading to the development of TCP. Initially, TCP combined both transport and forwarding functions, but later, IP was separated from TCP, and the lightweight UDP protocol was introduced. By the late 1970s, TCP, IP, and UDP were established as the foundational Internet protocols.

During this period, key innovations also emerged in local networking. Norman Abramson’s ALOHAnet introduced a multiple-access protocol for wireless communication, which influenced the development of Ethernet by Robert Metcalfe and David Boggs. Ethernet became the dominant protocol for local area networks (LANs), providing the foundation for modern PC networking.

1.7.3 - A Proliferation of Networks: 1980-1990#

During the 1980s, the number of hosts connected to the Internet grew exponentially, from a few hundred to over 100,000. This expansion was driven by efforts to link universities and research institutions, including BITNET for email and file transfers, CSNET for computer science researchers, and NSFNET, which provided access to supercomputing centers with an upgraded 1.5 Mbps backbone by the decade’s end.

Key advancements in Internet architecture occurred in this period. On January 1, 1983, ARPAnet officially transitioned from the NCP protocol to TCP/IP, standardizing modern Internet communication. TCP was also enhanced with host-based congestion control, and the Domain Name System (DNS) was introduced to map human-readable names to IP addresses.

Outside the U.S., France developed the Minitel network, a public packet-switched system based on the X.25 protocol. Launched in 1984, Minitel provided online services, including home banking and research databases, and was widely adopted in French households—predating widespread Internet use in the U.S. by a decade.

1.7.4 - The Internet Explosion: The 1990s#

The 1990s marked the transition of the Internet from a research network to a commercialized, global infrastructure. ARPAnet was decommissioned, and in 1991, NSFNET lifted restrictions on commercial use, allowing private Internet Service Providers (ISPs) to handle backbone traffic. By 1995, NSFNET was fully decommissioned, solidifying the Internet’s commercial expansion.

The defining event of the decade was the emergence of the World Wide Web, invented by Tim Berners-Lee at CERN between 1989 and 1991. The Web, built on HTML, HTTP, browsers, and servers, quickly gained traction. By 1993, only 200 Web servers existed, but the development of GUI-based browsers like Netscape Navigator in 1994 accelerated adoption. By the mid-1990s, companies began using the Web for commerce, leading to a rapid expansion of online businesses. The so-called “browser wars” between Netscape and Microsoft began, with Microsoft ultimately dominating by the late 1990s.

The second half of the decade saw unprecedented innovation and investment in the Internet. By 2000, the Internet supported key applications, including email with attachments, Web browsing and e-commerce, instant messaging, and peer-to-peer file sharing, popularized by Napster. The late 1990s also saw the rise and subsequent collapse of the dot-com bubble, where numerous startups, many without profits, gained and lost billions in market valuation. Despite the crash in 2000-2001, major tech giants such as Microsoft, Cisco, Yahoo, eBay, Google, and Amazon emerged as long-term winners.

1.7.5 - The New Millenium#

The 21st century has seen the Internet and Internet-connected smartphones revolutionize society. Advances in networking continue at a rapid pace, with faster routers, higher transmission speeds, and expanded access networks. Several key developments stand out:

  • Broadband Expansion & Video Applications: The deployment of broadband Internet (cable modems, DSL, fiber, and now 5G fixed wireless) has enabled widespread video applications, including user-generated content (YouTube), on-demand streaming (Netflix), and video conferencing (Skype, FaceTime, Google Meet).

  • Wireless Internet & Mobile Devices: High-speed wireless connectivity has made continuous, on-the-go Internet access a reality, fostering location-based applications such as Yelp, Tinder, and Waze. By 2011, the number of wireless Internet-connected devices surpassed wired devices, leading to the explosion of smartphones, tablets, and other mobile devices.

  • Rise of Social Networks: Platforms like Facebook, Instagram, Twitter, and WeChat have transformed communication and social interaction, often becoming the primary online environment for users. Their APIs have enabled new applications, including mobile payments and social gaming.

  • Private Network Infrastructure: Tech giants like Google and Microsoft have built extensive private networks, directly peering with ISPs to optimize performance. This infrastructure enables near-instant search results, seamless email access, and efficient global data center operations.

  • Cloud Computing: The rise of cloud services (Amazon EC2, Microsoft Azure, Alibaba Cloud) has transformed how businesses and institutions deploy applications. Cloud platforms offer scalable computing and storage while leveraging high-performance private networks for speed and reliability.

These advancements have shaped the modern digital landscape, driving innovations in communication, commerce, and entertainment.

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