Colin Perkins

 

00:00:00.233 Welcome to network systems

 

00:00:02.700 for those who don't know me,

00:00:04.033 my name is Colin Perkins,

00:00:05.366 and i'm the lecturer and coordinator for this course.

 

00:00:08.833 In this first lecture

00:00:10.100 I'll review the aims, objectives,

00:00:11.966 and administration of the course.

 

00:00:14.533 Then, I'll recap some of the material

00:00:16.333 covered in the Networks and Operating Systems

 

00:00:18.200 Essentials course in lecture 2.

 

00:00:21.133 Finally, I'll briefly introduce some

00:00:23.066 of the changes occurring in the network

00:00:25.233 to set the scene for the remainder of the course.

 

00:00:30.500 In this part of the lecture

00:00:31.866 I'll start with some administrative details.

 

00:00:34.700 I'll talk about the aims, objectives,

00:00:36.633 and intended learning outcomes for the course

 

00:00:39.300 I'll outline the content of the lectures and labs,

00:00:41.966 the timetable for the lectures and laboratory sessions,

00:00:44.833 and the structure of the assessed exercises and exam.

 

00:00:48.600 Finally, I'll highlight

00:00:50.666 the recommended reading for the course.

 

00:00:55.200 As I mentioned at the start, I'm Colin Perkins

00:00:57.800 and I'm the lecturer and coordinator for the course.

 

00:01:01.200 If you have questions about the material

00:01:02.866 covered in the course,

00:01:03.933 my email addresses on the slide.

00:01:05.900 I'm happy to answer questions via email.

 

00:01:08.966 We also have office hours

00:01:10.300 and discussion sessions throughout the course.

00:01:12.600 and a chat room in Microsoft teams.

 

00:01:16.000 The course materials,

00:01:17.166 including lecture recordings,

00:01:18.766 copies of the slides,

00:01:20.200 lab handouts,

00:01:21.133 and assessed exercises,

00:01:22.866 will be uploaded to Moodle.

 

00:01:25.500 The material is also on my website

00:01:27.733 at csperkins.org/teaching

 

00:01:30.700 the version on my website has lecture transcripts

00:01:33.000 that are difficult to upload to Moodle,

00:01:35.200 as well as the other material.

 

00:01:39.966 The aims and objectives for the course of four-fold

 

00:01:44.500 Firstly, the course aims to introduce

00:01:46.966 the fundamental concepts and theory of communication.

 

00:01:51.000 We'll build on the material from the Networks

00:01:53.066 and Operating Systems Essentials course in level, 2

00:01:56.166 covering the concepts in more depth

00:01:58.133 and going into more details about the operation

00:02:00.300 of the Internet.

 

00:02:02.900 Second, the cost aims to

00:02:05.366 give you a good understanding

00:02:06.700 of the technologies that comprise the modern Internet,

 

00:02:10.000 and an understanding of how,

00:02:11.500 and why, the network is changing.

 

00:02:14.866 We're in the middle of a rapid

00:02:16.266 period of change in the Internet infrastructure.

 

00:02:19.200 This course will try to make it

00:02:20.566 clear what's changing,

00:02:21.800 and what are the drivers for those changes.

 

00:02:25.666 This should give you the ability to evaluate network systems

00:02:28.900 and understand what technologies and approaches

00:02:31.200 are suitable for particular scenarios,

00:02:33.533 so you can advice on what's appropriate to use.

 

00:02:38.000 Finally, the course will build on the

00:02:40.066 material in the Systems Programming course

00:02:42.233 to introduce low-level network programming.

00:02:44.700 and to give you practice with systems programming in C.

 

00:02:51.666 By the end of the course

00:02:53.200 you should be able to describe and compare

00:02:55.266 the capabilities of different communications technologies,

00:02:58.633 understand the implications of scale on network systems,

00:03:02.700 and the different quality of service needs

00:03:05.400 of different applications.

 

00:03:08.200 In concrete terms,

00:03:09.766 this means you should understand what are

00:03:11.633 the different protocols in use in the Internet,

00:03:14.433 and when it's appropriate to use one or the other.

 

00:03:18.266 For example,

00:03:19.666 you should understand the differences

00:03:21.200 between UDP, TCP, and QUIC,

00:03:24.366 and when it's appropriate to use each of these protocols.

 

00:03:28.600 You should understand the importance of

00:03:30.366 heterogeneity in the design of the Internet,

00:03:33.266 the importance of layered protocol stacks,

00:03:36.433 and the way that different components of the

00:03:38.300 network are combined to form a whole.

 

00:03:42.100 Finally, you should be able to write

00:03:44.266 simple low-level communication software in C,

00:03:47.700 showing awareness of good practices

00:03:50.000 for correct and secure programming.

 

00:03:55.333 The course is organized as 10 lectures

00:03:58.000 and 5 laboratory sessions.

 

00:04:00.700 There's one lecture a week for 10 weeks,

00:04:03.233 with a pre-recorded lecture

00:04:04.866 and time for discussion each week

 

00:04:07.866 Labs are more variable.

00:04:09.766 The first 2 lab exercises

00:04:11.366 are expected to take one week each,

00:04:13.700 then the next 3 exercises should take two weeks each.

 

00:04:17.600 The labs run for 8 weeks,

00:04:19.233 starting in week 2 of the semester.

 

00:04:23.733 Lecture recordings will be made available ahead of time,

00:04:27.100 and comprise an hour or so of material,

00:04:29.433 split into several shorter parts each week.

 

00:04:33.433 Each lecture will be accompanied

00:04:34.966 by a set of discussion questions

00:04:37.200 that will be available on Moodle

00:04:38.633 and on the course website.

 

00:04:41.166 These discussion questions are not assessed,

00:04:43.500 and are intended to help you understand the material.

 

00:04:47.500 We have a live lecture session timetabled

00:04:49.866 from 4:00 to 6:00 pm on Thursday afternoons.

 

00:04:53.366 This is intended for a discussion

00:04:55.100 of the lecture material and questions.

 

00:04:57.933 You should watch the lecture videos,

00:04:59.666 and think about the discussion questions,

00:05:01.666 before the timetabled sessions.

 

00:05:05.266 This discussion slot

00:05:06.700 is your main opportunity

00:05:07.833 to ask questions about the material.

 

00:05:10.566 I strongly encourage you to come prepared

00:05:12.766 to use the slot to talk.

 

00:05:15.033 The more questions and discussion we have,

00:05:17.300 the more useful it'll be for everyone.

 

00:05:22.666 There are also weekly labs.

 

00:05:25.566 The goal of the labs is twofold.

 

00:05:29.033 First, they are intended to help you practice

00:05:31.333 C programming.

00:05:32.700 building on the material in the systems programming course,

00:05:35.866 and to introduce you to network programming in C.

 

00:05:40.400 While languages like Python are popular for web development,

00:05:44.100 C is still by far the most

00:05:46.100 widely used language for writing low-level networking code.

 

00:05:49.766 So it's important that you're familiar with network

00:05:51.933 programming in C.

 

00:05:54.666 The second goal of the labs

00:05:56.466 is to complement the material covered in the lectures,

00:05:59.566 to allow you to explore certain topics

00:06:02.533 around congestion control

00:06:04.066 and the structure of the network,

00:06:05.600 in more detail.

 

00:06:07.800 As with the lectures,

00:06:09.366 the lab materials will be made available in advance

00:06:11.800 of the timetabled sessions.

 

00:06:13.966 The labs are to be completed in your own time.

 

00:06:17.933 The timetabled sessions,

00:06:19.533 from 1:00-3:00pm on Thursdays,

00:06:21.433 are for live support with the lab exercises.

 

00:06:24.766 You should try to solve the exercises

00:06:27.033 and think of questions you might need to ask

00:06:29.466 before the timetable support slot,

00:06:31.966 so the lab demonstrators and I

00:06:33.566 can effectively help you.

 

00:06:39.266 This course follows the usual model

00:06:41.900 of an exam worth of 80% of the marks

00:06:44.500 and an assessed exercise was the remaining 20%.

 

00:06:48.800 The assessed exercise will be made available in Lecture 5,

00:06:52.233 and will be due on the same day as Lecture 7 is held.

 

00:06:56.733 The usual penalties for late submission will be applied,

00:06:59.700 following the University's code of assessment.

 

00:07:03.233 If you're ill,

00:07:04.166 or have other circumstances that might affect

00:07:06.200 your on-time submission,

00:07:08.033 then you can contact me before the deadline

00:07:09.933 to request an extension

 

00:07:12.700 Also, following the School's policy,

00:07:16.066 note that submissions of assessed exercises

00:07:18.433 that do not follow the submission instructions

00:07:20.700 given in the handout

00:07:21.933 will be given a two-band penalty.

 

00:07:25.000 And I want to emphasize this last point,

00:07:27.166 because some people are surprised each year.

 

00:07:30.100 The penalties for late submission,

00:07:31.800 and for not following submission instructions,

00:07:34.333 will be strictly enforced.

 

00:07:40.866 As I said earlier,

00:07:42.000 the final exam is worth 80% of the marks for the course.

 

00:07:46.433 The format of the exam

00:07:47.966 is that there are 3 questions,

00:07:49.433 and you have to answer all questions.

 

00:07:52.200 There are past exam papers on Moodle,

00:07:54.466 including one with sample answers.

 

00:07:57.900 The curriculum has changed over the years,

00:08:00.300 and the pandemic has caused

00:08:02.833 a shift to to online open book exams,

00:08:06.733 and together these mean the style of questions has changed.

 

00:08:10.933 The more recent exam papers,

00:08:12.800 from 2020 onwards,

00:08:14.433 are most representative of

00:08:16.033 the style of questions you can expect

 

00:08:19.700 When studying the material,

00:08:21.166 and preparing for the exam,

00:08:23.000 remember that the goal of the course

00:08:24.900 is to help you understand how the Internet works.

 

00:08:28.133 The aim of the exam is to test that understanding,

00:08:31.600 not simply to test your memory of the details.

 

00:08:35.133 Explain why,

00:08:37.200 don't just recite what

 

00:08:39.933 To achieve an A grade in this course,

00:08:42.600 representing excellent performance,

00:08:45.066 you'll need to demonstrate

00:08:46.233 that you can make a reasoned argument,

00:08:48.266 develop logical conclusions,

00:08:50.600 and apply your learning to new situations,

00:08:53.200 to solve new problems.

 

00:08:56.100 The material in the lectures and the labs is examinable,

00:08:59.533 but you're also expected to follow the required readings

00:09:02.433 and to develop your broader understanding of the material.

 

00:09:09.233 So what are those required readings?

 

00:09:12.833 Well, the slide lists for good books on computer networking.

 

00:09:17.500 You should read one of them

 

00:09:19.866 The books by Peterson and Davie,

00:09:21.900 and by Olivier Bonventure,

00:09:23.866 are available for free online.

 

00:09:27.633 The lectures, labs, and discussion

00:09:29.900 can introduce you to the important concepts of networking.

00:09:33.700 but it's also important that you read about,

00:09:36.000 and around, the material covered,

00:09:38.100 so you understand the details.

 

00:09:40.900 The books explain the material in different ways,

 

00:09:44.366 and may help illustrate different aspects of the subjects.

00:09:47.533 and the different perspectives might make things clear

00:09:50.000 in cases where my explanations do not.

 

00:09:54.733 In addition to the books,

00:09:56.533 many of the lecture slides

00:09:57.900 also include links to standards documents,

00:10:00.366 research papers, blog posts,

00:10:02.533 or talks that go into more detail about the material.

 

00:10:07.466 You're not expected to read all of this material,

00:10:10.233 and much of it goes into far more detail than you need.

00:10:13.300 but you should at least look at it

00:10:15.066 to begin to get a feel for the depth of the subject.

 

00:10:19.866 There's only so much depth

00:10:21.566 that can be covered in a pre-recorded lecture.

 

00:10:24.666 I've chosen the reading material carefully

00:10:26.966 to complement the lectures.

 

00:10:29.333 Following along with the reading,

00:10:31.100 and participating in the discussion sessions,

00:10:33.500 will further your understanding.

 

00:10:37.200 The exam questions

00:10:38.400 will focus on application of the knowledge

00:10:40.466 you gained during the course,

00:10:41.866 not on rote memorization.

 

00:10:44.766 Reading further,

00:10:46.000 thinking about the ideas,

00:10:47.333 and discussing the material

00:10:48.633 covered in the lectures and the labs is essential.

 

00:10:55.033 That concludes the review

00:10:56.233 of the course structure and administration.

 

00:10:59.366 In the remaining parts of this lecture,

00:11:01.433 I'll review the traditional Internet architecture,

00:11:04.200 starting with a discussion of protocols and layering.

 

00:11:07.466 Then talk about some of the ways in which the network is changing.

 

00:00:01.333 I’d like to begin the course by reviewing

00:00:03.366 some of the fundamental principles

00:00:05.100 of networked systems.

 

00:00:06.900 In particular, I'd like to talk briefly

00:00:09.466 about what is a networked system,

00:00:11.366 and how networked systems are structured in

00:00:13.600 the form of a layered protocol stack.

 

00:00:17.000 So, what is a networked system?

 

00:00:20.433 A networked system is a set of cooperating,

00:00:23.233 autonomous, computing devices

00:00:25.433 that exchange data to perform some application goal.

 

00:00:29.600 I talk about computing devices,

00:00:31.800 because networked systems are not limited to traditional

00:00:34.500 PCs, laptops, or servers.

 

00:00:37.366 There are far more smart phones and tablets

00:00:39.566 connected to the network

00:00:40.733 than there are laptops and servers, for example

 

00:00:44.000 But there are also numerous sensors and controllers

00:00:46.466 that form the Internet of Things:

00:00:48.500 cameras, smart light bulbs, heating controllers,

00:00:51.633 weather stations, medical devices,

00:00:54.566 industrial automation, and so on.

 

00:00:57.633 The network is comprised of an increasingly

00:00:59.966 diverse set of devices, and has to

00:01:02.166 meet their diverse needs.

 

00:01:05.000 These devices are autonomous.

00:01:07.166 Each device acts independently

00:01:09.233 with no central control,

00:01:11.100 and can choose what data to send,

00:01:13.133 and when to send it.

 

00:01:15.333 And these devices are all running different applications

00:01:18.233 with different requirements.

00:01:20.066 They require different things from the network

00:01:22.433 and need different network protocols

00:01:24.466 to support their needs.

 

00:01:28.000 There are four aspects to the network.

 

00:01:30.900 The first is communication.

00:01:33.233 How can two devices,

00:01:34.800 that are connected to a single link,

00:01:37.100 reliably exchange data.

 

00:01:39.966 Second, is networking.

00:01:42.133 How can we combine multiple links

00:01:45.033 to form a wide area network,

00:01:46.866 around a building, a campus, or a region.

 

00:01:51.200 Third is internetworking.

00:01:53.433 How can we connect multiple networks

00:01:56.133 together to form an internet?

00:01:58.166 How can multiple independently operated

00:02:00.866 networks work together

00:02:02.466 to act as-if they were a single global network?

 

00:02:05.233 How can data be routed across this collection

00:02:07.900 of networks to reach its destination?

 

00:02:10.666 And, finally, there is the problem of transport.

00:02:14.000 How do the end systems ensure

00:02:16.366 that data is delivered across this network,

00:02:18.333 or networks, with appropriate reliability

00:02:21.366 to meet the needs of the application?

 

00:02:26.366 Networked systems are fundamentally

00:02:28.266 about communications protocols.

 

00:02:31.100 A sender is trying to talk to one of more receivers,

00:02:34.233 via some communications channel.

 

00:02:36.833 It does this by sending messages,

00:02:39.233 that have to fit within the constraints

00:02:40.833 of the communications channel.

 

00:02:43.166 These constraints provide limits on the speed

00:02:45.700 and reliability of transmission.

00:02:47.866 The channel is not infinitely fast

00:02:50.100 or perfectly reliable.

 

00:02:53.566 The messages being sent across the channel

00:02:55.966 have a well defined format

00:02:58.300 Much like programming languages,

00:03:00.333 network protocols have well defined syntax

00:03:02.900 that describes the structure and format

00:03:04.833 of the messages that can be sent.

 

00:03:07.766 They also have well defined semantics

00:03:09.766 that define the meaning of the messages,

00:03:12.066 the order in which they are sent,

00:03:13.733 and the patterns of communication.

 

00:03:16.600 Together the syntax and semantics of the messages

00:03:19.833 define a network protocol, such as

00:03:23.566 HTTP, TCP/IP, etc.

 

00:03:27.000 Each protocol has a particular purpose

00:03:29.466 and solves a particular problem.

 

00:03:32.100 Protocols can be combined,

00:03:34.100 and layered on one another,

00:03:35.700 to gradually raise the level of abstraction,

00:03:38.400 and provide more sophisticated services

00:03:40.533 to the applications.

 

00:03:44.900 The Open Systems Interconnection Reference Model,

00:03:48.133 the OSI Model,

00:03:49.933 is a common way of thinking about protocol layering.

 

00:03:54.366 The OSI model structures the network

00:03:56.500 as a set of seven layers.

 

00:03:59.300 At the bottom of the protocol stack

00:04:01.266 is the physical layer.

 

00:04:03.100 For two devices, two end systems,

00:04:06.200 that are directly connected, as shown on the slide,

00:04:09.566 the physical layer represents

00:04:11.333 the means of interconnection.

00:04:12.933 The type of cable,

00:04:14.333 if it’s a wired link,

00:04:15.800 or the details of the radio channel.

 

00:04:18.866 The purpose of the physical layer

00:04:20.566 is to exchange data between the devices.

 

00:04:24.766 Above this sits the data link layer.

 

00:04:28.366 The link layer structures that data into messages,

00:04:31.233 identifies the devices,

00:04:33.866 and coordinates when each device can send,

00:04:36.466 so they share access to the channel.

 

00:04:41.266 If a single network comprises more than one

00:04:43.166 physical link, devices known as switches

00:04:45.966 can be inserted into the network.

00:04:48.233 Switches operate at the data link layer,

 

00:04:51.466 adapting the message framing and arbitrating access

00:04:54.466 to the different channels, in order to

00:04:56.633 bridge the different links together.

 

00:04:59.966 Examples of this include Ethernet switches,

00:05:02.900 that connect multiple Ethernet links together

00:05:05.400 to form a larger Ethernet network,

00:05:08.200 and Wi-Fi base stations that bridge between Ethernet

00:05:11.033 and Wi-Fi networks.

 

00:05:15.966 The third layer in the OSI model

00:05:18.100 is the network layer.

 

00:05:20.500 The network layer supports the interconnection

00:05:22.800 of multiple independently operated networks.

 

00:05:26.466 It abstracts away the details

00:05:27.900 of the different types of ink layer,

00:05:29.833 and combines different networks into one,

00:05:32.933 to give the illusion

00:05:33.966 that there is a single global internetwork.

 

00:05:37.166 The Internet protocols,

00:05:38.566 IPv4 and IPv6,

00:05:40.833 are examples of network layer protocols,

00:05:43.566 and the devices that connect different networks

00:05:45.633 together are known as routers.

 

00:05:49.533 Above the network layer,

00:05:51.300 the transport layer ensures that data is delivered

00:05:53.766 with appropriate reliability between end systems.

 

00:05:57.100 The Transmission Control Protocol,

00:05:59.333 TCP,

00:06:00.333 is a widely used example of a transport protocol.

 

00:06:04.533 Finally, the Session, Presentation, and Application layers

00:06:09.066 support the coordination of multiple transport connections,

00:06:12.566 describe the data formats used,

00:06:14.733 and support application semantics.

 

00:06:20.400 The OSI reference model is a standard

00:06:22.466 model for layered protocol design.

00:06:25.066 It’s important to realise, though,

00:06:27.566 that real networks don’t follow the OSI model.

 

00:06:31.133 No deployed networked system has seven layers

 

00:06:34.366 structured in this way.

 

00:06:36.700 Real systems are more complex.

 

00:06:39.066 In some cases, especially around the upper layers of

00:06:42.266 the stack, the layers merge together,

00:06:44.700 and the boundaries between them are unclear.

 

00:06:48.266 In other cases, systems are designed with

00:06:50.766 more layers, or with clearly defined sub-layers,

00:06:54.266 to support features that don't fit neatly

00:06:56.433 into the layer boundaries defined by the OSI model.

 

00:07:00.433 Tunnelling solutions, that encapsulate data from a

00:07:02.900 lower layer, and transmit it within a higher layer

00:07:05.566 are also common.

 

00:07:06.766 Virtual Private Networks,

00:07:08.733 VPNs, are a good example of this,

00:07:11.000 and take network layer data,

00:07:13.033 in the form IP packets,

00:07:14.700 and tunnel them inside a transport layer

00:07:16.700 connection to some other part of the network,

00:07:19.166 giving the illusion that a device

00:07:21.166 is physically connected elsewhere.

 

00:07:22.900 There’s not a strict progression of layers.

 

00:07:26.700 We talk about the OSI model

00:07:28.600 because it's an extremely useful way to structure

00:07:30.933 discussion about networks,

00:07:32.566 not because it represents the structure

00:07:34.566 of any particular network.

 

00:07:38.266 To conclude, the network is a collection

00:07:41.433 of autonomous computing devices,

00:07:43.566 cooperating to exchange

00:07:45.400 messages to support application needs.

 

00:07:47.866 Those messages are structured in the form

00:07:50.666 of a layered protocol stack,

00:07:52.066 gradually building up features

00:07:54.233 from the exchange of data between directly

00:07:56.233 connected devices

00:07:57.466 until we have the global Internet.

 

00:08:00.200 The seven layer OSI model doesn’t reflect reality,

00:08:03.300 but is a useful way of thinking

00:08:05.500 about the protocol stack.

 

00:08:07.333 For this reason, I’ll use it to structure the

00:08:10.000 discussion in the other parts of this lecture

00:08:11.933 starting, in the next part,

00:08:14.000 with a discussion of the physical

00:08:15.600 and data link layers.

 

00:00:00.900 The physical and data link layers

00:00:03.100 occupy the lowest levels of the protocol stack,

00:00:05.600 and support data transmission

00:00:07.366 between directly connected devices.

 

00:00:10.066 In the following, I’ll briefly discuss the physical

00:00:12.566 characteristics of network links

00:00:14.366 and the modulation process by which data

00:00:16.266 is transmitted across those links.

 

00:00:18.500 Then, I’ll talk about features of the data link layer,

00:00:21.266 such as framing, addressing, and media access control.

 

00:00:27.300 The physical layer describes the properties of

00:00:29.900 the communications channel.

00:00:31.200 It’s the realm of the electrical engineer;

00:00:33.566 of cables, optical fibres,

00:00:35.533 and radio transmission.

 

00:00:37.666 When considering the physical layer,

 

00:00:39.666 we discuss the physical properties of the channel,

00:00:42.500 the way bits are encoded onto the channel,

00:00:44.733 and the capacity, and error rate, of the channel.

 

00:00:48.066 We begin by asking whether the link

00:00:50.066 is wired or wireless?

 

00:00:52.866 If it is a wired link,

00:00:54.433 we then consider the type of cable

00:00:56.366 or optical fibre used.

00:00:58.000 We ask what voltage is applied to the cable,

00:01:00.700 or what frequency of laser light

00:01:02.566 is sent down the fibre.

 

00:01:04.366 And we ask how the bits are encoded as

00:01:06.100 variations in that voltage

00:01:07.600 or in the intensity of the laser.

 

00:01:10.233 If, instead, the channel is wireless,

00:01:12.800 we consider what type of antenna is used,

00:01:15.300 the transmission power, the carrier frequency,

00:01:17.300 and the modulation scheme used to encode

00:01:19.466 data onto the carrier wave.

 

00:01:22.133 Given these details, we can then estimate

00:01:24.200 the capacity of the channel, and model

00:01:26.533 the physical limitations of the performance

00:01:28.566 of the transmission link.

 

00:01:32.733 When using a wired link,

00:01:34.533 whether an electrical cable or an optical fibre,

00:01:37.266 the signal to be transmitted is usually

00:01:39.466 directly encoded onto the channel.

 

00:01:41.966 That is, the voltage applied to an electrical cable,

00:01:44.900 or the brightness of a laser shining down an optical fibre,

00:01:48.266 is changed in a way that directly corresponds to the

 

00:01:50.733 signal to be transmitted.

 

00:01:53.433 The signal will occupy a certain frequency

00:01:55.600 range, known as its bandwidth.

 

00:01:58.200 This is measured in units of Hertz, Hz,

00:02:00.733 and directly corresponds to the complexity and

00:02:03.033 information content of the signal.

 

00:02:06.100 The more information being transmitted in a given

00:02:08.066 time interval, the greater the bandwidth.

 

00:02:11.200 A signal directly applied to a channel

00:02:13.366 occupies what’s known as the baseband frequency range:

00:02:15.933 the range starting at 0 Hz,

00:02:18.066 and reaching up to the maximum bandwidth of the signal.

 

00:02:21.833 Every channel also has a maximum bandwidth it can transmit.

 

00:02:26.100 This depends on the physical characteristics of the channel.

 

00:02:29.000 For example, the maximum bandwidth that can be sent

00:02:31.400 over a twisted pair electrical cable

00:02:33.466 depends on the length of the cable,

00:02:35.300 the tightness of the twists,

00:02:36.900 and the thickness of the wires.

 

00:02:39.333 A channel is only able to transmit a particular signal

00:02:42.266 if the bandwidth of the signal

00:02:43.933 is less than the bandwidth of the channel

 

00:02:46.300 There’s a maximum rate at which data can be transmitted,

00:02:49.000 depending on the physical characteristics of the channel.

 

00:02:52.766 That maximum rate is determined by Nyquist’s theorem.

 

00:02:56.533 This states that the maximum data rate

00:02:58.433 a channel can support,

00:02:59.866 Rmax,

00:03:01.033 cannot exceed 2 B Log2 V bits per second,

00:03:07.133 where B is the maximum bandwidth of the channel,

00:03:10.166 and V is the number of different values each symbol can take.

 

00:03:14.333 For binary data, each symbol can have

00:03:16.700 one of two values, it can be a zero

00:03:18.800 or a one. Accordingly, V is equal to two.

 

00:03:23.100 The value of “Log2 V” term, in this case,

00:03:25.966 evaluates to one,

00:03:27.400 and the maximum data rate directly

00:03:29.333 corresponds to the channel bandwidth.

 

00:03:34.000 So, how is data encoded onto the channel?

 

00:03:38.433 The simplest case is what’s known as

00:03:40.833 non-return to zero, NRZ, encoding,

00:03:43.666 as shown in the top figure on the slide.

 

00:03:46.833 When sending binary data, NRZ encoding

00:03:50.433 directly encodes the signal onto the channel.

 

00:03:53.433 If the binary value 1 is to

00:03:54.933 be sent over an electrical cable,

00:03:56.600 for example, a high voltage is applied

00:03:58.900 to the cable, whereas a low voltage

00:04:01.166 is applied to send the binary value zero.

 

00:04:03.633 The receiver simply measures the voltage,

00:04:05.700 and directly translates it into binary values.

 

00:04:10.600 Non-return to zero encoding is simple,

00:04:12.833 but has the problem that long runs

00:04:14.833 of ones or zeros result in signals

00:04:17.100 that maintain the same value for long periods of time.

 

00:04:20.666 The example has a run of four consecutive one bits,

00:04:23.733 followed a little later by a run of four consecutive zeros,

 

00:04:27.133 and each results in a signal that’s unchanging

00:04:29.400 for a significant period of time.

 

00:04:32.933 At high data rates, and with long

00:04:35.133 consecutive sequences of the same value,

00:04:37.166 it can be difficult to measure the exact

00:04:39.033 time for which the signal is unchaning

 

00:04:41.666 This leads to miscounting, where the receiver

00:04:44.066 believes one more, or one less, bit was sent than intended,

00:04:47.600 giving a corrupt signal.

 

00:04:50.300 To avoid this, more complex encodings are used.

 

00:04:54.500 One such scheme is Manchester Encoding.

 

00:04:57.433 This encodes every bit to be sent as a pair of values

 

00:05:01.633 A binary 1 is sent as a high-to-low transition

00:05:04.600 in the signal strength, whereas a binary 0

00:05:07.466 is sent as a low-to-high transition.

 

00:05:10.366 This avoids the miscounting problem of NRZ encoding,

00:05:13.600 since the signal always changes

00:05:15.433 irrespective of the data being sent,

00:05:17.700 but at the cost of requiring twice as many transitions,

00:05:20.700 and hence using twice the bandwidth.

 

00:05:23.800 There are many different methods of baseband data encoding,

00:05:27.666 of which the NRZ and Manchester encodings are the simplest.

 

00:05:31.800 The different encodings trade-off increased complexity

00:05:34.600 for better performance.

 

00:05:39.633 When encoding data onto a wireless channel,

00:05:42.300 carrier modulation is used rather than baseband encoding.

 

00:05:46.433 This allows multiple signals to be

00:05:48.166 carried on a single channel,

00:05:49.766 each modulated onto a carrier wave

00:05:52.066 transmitted at a different frequency.

 

00:05:55.233 Carrier modulation shifts the frequency range occupied

00:05:58.166 by a signal up, so it’s centred on a carrier frequency

 

00:06:01.633 Instead of occupying the baseband range, from 0 - B Hz,

00:06:05.500 the signal is shifted to occupy

00:06:07.233 a frequency range centred on the carrier frequncy.

 

00:06:10.566 This is done by varying some property of the carrier wave

00:06:13.133 to match the signal being sent.

 

00:06:15.400 The receiver tunes into the carrier frequency,

00:06:18.133 and measures the variation in the carrier wave

00:06:20.366 to extract the signal.

 

00:06:23.133 In much the same way as wired links,

00:06:25.266 there are limitations in the bandwidth

00:06:26.833 of the signal that can be sent,

00:06:28.400 depending on the carrier frequency,

00:06:30.266 the type of antenna used,

00:06:31.933 the transmission power,

00:06:33.500 the modulation scheme, etc.

 

00:06:37.000 Broadcast radio stations use carrier modulation to

00:06:39.800 transmit on different frequencies.

 

00:06:41.833 The principle is the same

00:06:43.533 for digital transmission, except that

00:06:45.600 it’s data being transmitted, rather than music.

 

00:06:51.466 There are three types of carrier modulation that can be used.

 

00:06:55.866 Amplitude modulation varies the loudness of the carrier

00:06:58.900 wave to directly match the signal being transmitted.

 

00:07:02.566 AM radio works the same way.

 

00:07:05.666 Amplitude modulation is simple,

00:07:07.733 but can perform poorly,

00:07:09.266 because radio noise takes the form

00:07:11.100 of changes in the loudness of

00:07:12.966 the signal that corrupt the received data.

 

00:07:16.766 Frequency modulation varies the frequency of

00:07:19.533 the carrier to match the signal.

 

00:07:21.900 In the example on the slide,

00:07:23.633 it switches between a high frequency,

 

00:07:25.533 that corresponds to binary zeros,

00:07:27.333 and a lower frequency that corresponds

00:07:29.233 to a binary ones.

 

00:07:30.700 FM radio works in a similar way,

00:07:33.166 varying the frequency to match

00:07:34.766 the speech or music being transmitted.

 

00:07:37.700 This is slightly more complex than amplitude modulation,

00:07:40.766 but more resistant to noise and interference.

 

00:07:44.400 Finally, phase modulation shifts forwards or backwards

00:07:47.633 in the cycle of the waveform to indicate different symbols.

 

00:07:51.566 Real systems tend to use a combination

00:07:53.600 of modulation techniques, perhaps varying both the

00:07:56.566 amplitude and phase, to increase the data rate.

 

00:08:02.333 Radio signals modulated onto a carrier

00:08:04.333 wave are prone to interference.

 

00:08:06.933 This is because signals sent at particular frequencies

00:08:09.600 tend to be blocked by vehicles, trees,

00:08:11.900 and people moving around, or by the weather

00:08:14.566 and other radio transmissions,

00:08:16.166 while signals sent on a carrier at a different

00:08:18.633 frequency may be unaffected.

 

00:08:21.000 This strength of this interference can change rapidly.

 

00:08:25.166 To avoid this problem, many wireless links

00:08:28.000 use a technique known as spread spectrum communication,

00:08:31.166 where the carrier frequency is changed

00:08:33.000 several times per second, following a pseudo-random sequence

00:08:36.200 known to both the sender and the receiver.

 

00:08:39.766 Spread spectrum communication limits

00:08:41.900 the impact of interference,

00:08:43.533 because the transmission will quickly switch away

00:08:46.433 from a poorly performing channel.

 

00:08:48.500 It adds a lot of complexity,

00:08:50.466 since both sender and receiver need to

00:08:52.700 continually change the carrier frequency,

00:08:55.066 and synchronise what frequencies they use,

00:08:57.666 but greatly improves performance.

 

00:09:00.666 The concept of spread spectrum communication

00:09:03.600 was invented by Hedy Lamarr,

00:09:05.500 a Hollywood actress turned inventor,

00:09:07.333 during World War 2.

 

00:09:09.166 It’s now widely used as part of the Wi-Fi-standards.

 

00:09:13.833 The impact of noise on the data rate

00:09:16.400 achievable over a particular channel can be predicted.

 

00:09:19.800 This is the case whether that noise is due to electrical

00:09:22.166 or radio interference, imperfections in an optical fibre,

00:09:25.366 or other means.

 

00:09:27.433 In the simplest case, where it’s assumed that

00:09:30.433 the noise affects all frequencies used by

00:09:32.233 the transmission to the same extent,

00:09:34.300 the maximum data rate of the channel

00:09:36.400 can be determined using the Shannon-Hartley theorem.

 

00:09:39.433 This states that the maximum data rate,

00:09:41.866 Rmax, is equal to the bandwidth of

00:09:44.133 the channel, B, multiplied by the log

00:09:47.333 of 1 plus the strength of the signal, S,

00:09:49.900 divided by the amount of noise, N.

 

00:09:53.266 The bandwidth and the amount of noise

00:09:55.300 depend on the channel and the environment.

 

00:09:58.633 For example, for a wireless link,

00:10:00.733 they depend on the carrier frequency,

00:10:02.733 the type of antenna, the weather,

00:10:04.833 the presence of obstacles between the sender and receiver,

00:10:07.533 and whether there are other

00:10:08.633 simultaneous radio transmissions.

 

00:10:11.400 The signal strength depends on the amount of power

00:10:14.066 applied at the transmitter.

 

00:10:16.233 This allows the sender to trade-off

00:10:18.066 battery life for performance,

00:10:20.033 saving power by transmitting more slowly.

 

00:10:26.500 We have seen that the physical layer enables communication.

00:10:29.566 It allows the sender and receiver to exchange

00:10:32.133 a sequence of bits of data across a channel,

00:10:34.033 but assigns no meaning to those bits.

 

00:10:37.600 The data link layer starts to provide

00:10:39.733 structure to the bitstream provided by the physical layer.

 

00:10:43.200 It provides framing,

00:10:44.600 splitting the bitstream into individual messages,

00:10:47.366 and gives the ability to detect,

00:10:49.366 and possibly correct,

00:10:50.600 errors in the transmission of those messages.

 

00:10:53.966 It provides addressing,

00:10:55.566 giving each device an identifier

00:10:57.466 that can be used to indicate

00:10:58.600 what device sent the message

00:11:00.533 and what device, or devices,

00:11:02.566 should act on the message.

 

00:11:04.766 And, finally, the data link layer provides

00:11:07.233 media access control.

 

00:11:09.033 It arbitrates access to the channel,

00:11:11.300 to make sure that more than one device

00:11:13.133 doesn’t try to send at the same time,

00:11:15.033 and to ensure that each gets its fair share to transmit.

 

00:11:21.533 A key role of the data link layer

00:11:23.900 is to separate the bitstream into

00:11:25.233 meaningful frames of data,

 

00:11:27.500 and to identify the devices that are

00:11:29.066 sending and receiving those frames.

 

00:11:32.066 For example, if we consider an Ethernet link,

00:11:35.666 the bitstream is split up into frames

00:11:38.166 that contain a number of different elements.

 

00:11:41.233 First is the start code.

 

00:11:43.533 This is a preamble, containing a particular pattern

00:11:46.033 that occurs only occurs at the start of a message,

00:11:48.500 and is used to alert the receiver

00:11:50.100 that a new message is starting.

 

00:11:53.166 This is followed by some header information.

 

00:11:55.966 The header comprises a source address,

00:11:58.033 specifying the identity of the device sending the frame,

00:12:01.033 and a destination address that identifies the receiver.

 

00:12:04.666 These are followed by a length field,

00:12:06.366 indicating the amount of data to follow.

 

00:12:09.766 The data comes next, up to 1500 bytes in length.

 

00:12:14.633 And, finally, a cyclic redundancy code

00:12:17.166 concludes the packet,

00:12:18.300 allowing the receiver to check

00:12:19.600 if the frame was received correctly

 

00:12:23.733 The start code provides for synchronisation

00:12:26.233 and timing recovery.

 

00:12:28.000 It’s a regular pattern that’s only

00:12:29.533 sent at the start of a frame,

00:12:31.033 and allows the receiver to precisely

00:12:32.900 measure the speed at which the frame is being sent.

 

00:12:38.300 The source and destination addresses

00:12:41.433 identify the devices sending and receiving the message.

 

00:12:46.200 Each is 48 bits, six bytes, in size,

00:12:49.300 and is globally unique.

 

00:12:51.633 The addresses are split into two 24-bit parts,

00:12:54.533 one indicating the vendor, and one indicating the device.

 

00:12:58.933 In this example,

00:13:00.266 the vendor ID of 00:14:51 indicates Apple,

00:13:05.100 and the device ID of 04:27:ea

00:13:08.666 indicates the laptop on which I’m recording this lecture.

 

00:13:12.933 Modern operating systems are starting to randomly

00:13:15.433 change the Ethernet addresses each time they

00:13:17.500 connect to the network, to limit tracking

00:13:19.766 and improve privacy.

 

00:13:22.833 Finally, the data part of an Ethernet frame

00:13:25.666 contains data for the next layer

00:13:27.366 up in the protocol stack, the network layer.

 

00:13:32.933 The last feature of the data link layer

00:13:35.166 that I want to discuss is media access control.

 

00:13:38.966 If you have a channel that’s shared

00:13:40.400 between multiple devices, such as a

00:13:42.600 common wireless link or a shared cable,

00:13:44.800 then there’s the risk that two devices

00:13:46.966 can try to send at once.

 

00:13:49.233 In this slide, for example, devices

00:13:51.566 A and B both try to send a message

00:13:53.566 to device C at the same time.

 

00:13:56.766 The centre image shows the signals sent

00:13:59.400 by the devices A and B,

00:14:01.300 each of which is sending using NRZ encoding.

 

00:14:04.966 These are entirely normal signals.

 

00:14:07.900 The right-hand image shows what’s received at C.

 

00:14:11.200 This is the superposition of the two signals,

00:14:13.966 the result of adding the two signals together,

00:14:16.433 and is corrupt and meaningless to the receiver.

 

00:14:20.166 Media access control

00:14:21.600 is the problem of avoiding such collisions.

 

00:14:26.400 A common way to perform media access

00:14:28.733 control is using a technique known as

00:14:30.433 carrier sense multiple access with collision detection,

 

00:14:34.066 CSMA/CD.

 

00:14:36.666 The idea is that when a device wants to send,

00:14:39.466 it first listens to see if another

00:14:41.466 device is sending already.

 

00:14:43.766 If another transmission is active,

00:14:45.600 then it waits before trying again.

 

00:14:48.666 If it doesn’t hear anything, it starts to send data.

 

00:14:52.666 While sending, it listens to see if another

00:14:54.866 device also starts to send.

 

00:14:57.300 If such a collision occurs,

00:14:58.900 the device stops sending,

00:15:00.600 waits, and tries again.

 

00:15:03.400 Collisions don’t usually happen,

00:15:05.300 because devices listen before sending,

00:15:07.533 but there’s always some chance that messages

00:15:10.033 might overlap because of the

00:15:11.366 time it takes a message to traverse the network.

 

00:15:15.766 As we see from the diagram on the right,

00:15:18.333 if device A starts to send,

00:15:20.366 and simultaneously B listens,

00:15:22.900 hears nothing because the message from A

00:15:25.066 hasn’t reached it yet, and starts sending,

00:15:27.700 then there’s the risk that both messages

00:15:29.366 collide and are corrupted.

 

00:15:32.033 Collisions are more likely to occur

00:15:33.833 in long distance networks,

00:15:35.433 with large propagation delays,

00:15:37.266 but can happen in any network with a shared channel.

 

00:15:42.300 If a collision occurs,

00:15:44.033 how long should a device wait

00:15:45.500 before trying to re-send a message?

 

00:15:48.333 Well, devices shouldn’t always wait for the

00:15:50.466 same amount of time.

 

00:15:52.533 Doing so would run the risk that two devices get stuck,

00:15:55.833 each repeatedly trying to send,

00:15:57.766 waiting the same time,

00:15:59.033 and then colliding again in a loop.

 

00:16:01.966 The amount of time to wait should be randomised,

00:16:04.166 to avoid deterministic collisions.

 

00:16:07.900 If such randomisation is used,

00:16:10.133 but another collision occurs after waiting,

00:16:12.500 this suggests that the network is busy.

 

00:16:15.533 It’s unlikely that two devices will

00:16:17.300 randomly wait for the same time,

00:16:19.500 so a subsequent collisions suggest that

00:16:21.333 there are many devices trying to send.

 

00:16:24.233 A sender should therefore increase the time it waits

00:16:26.600 after each collision,

00:16:27.666 to reduce the overall load on the network.

 

00:16:31.000 Many data link layer protocols

00:16:32.600 double the wait time after each repeated collision,

00:16:35.066 resetting when a successful transmission occurs.

 

00:16:39.266 This approach, known as CSMA/CD, is widely

00:16:44.066 used, including in Ethernet and WiFi networks.

 

00:16:47.333 A consequence of it

00:16:49.033 is that devices share access to a channel,

00:16:51.733 and how quickly they can send a message

00:16:53.866 depends on how busy is the network.

 

00:16:56.466 This introduces some element of unpredictability

00:16:58.833 into the timing of many messages sent over the network.

 

00:17:04.966 To conclude,

00:17:06.100 the physical layer provides

00:17:07.933 for encoding a sequence of bits onto a channel,

00:17:10.733 but says nothing about the meaning of those bits.

 

00:17:14.166 The data link layer starts to add structure,

00:17:16.800 separating the bit stream into frames,

00:17:19.166 checking those frames

00:17:20.466 to ensure they were correctly received,

00:17:22.400 identifying devices,

00:17:24.000 and arbitrating access to the channel.

 

00:17:26.766 Together, the physical and data link layers

00:17:29.400 enable local area networks,

00:17:31.133 where devices connected to a single link can communicate.

 

00:17:35.466 This forms the basis of the Internet.

 

00:17:38.733 In the next part, I’ll talk about the network layer,

00:17:42.000 that allows multiple networks to be combined into one.

 

00:00:01.466 The network layer allows several independently operated

00:00:04.733 networks to be combined to give the

00:00:06.866 appearance of a single network.

 

00:00:08.933 It provides an internetworking function that allows us to

00:00:11.600 build an internet.

 

00:00:13.666 In this part, I’ll talk about the Internet Protocols,

00:00:16.366 IPv4 and IPv6,

00:00:18.933 that provide the network layer in the Internet,

00:00:21.366 and briefly review the network layer concepts

00:00:24.000 of addressing, routing, and forwarding.

 

00:00:28.466 The network layer is the internetworking point

00:00:31.033 in the protocol stack.

 

00:00:32.866 The use of a common network layer

00:00:34.466 protocol allows us to decouple the operation

00:00:37.000 of the networks that comprise an internet,

00:00:39.133 from the operation of the applications that

00:00:41.166 run on the internet.

 

00:00:43.733 It allows each network to make its own choice

00:00:45.800 about what sort of data link

00:00:47.200 and physical layer technologies to use,

00:00:49.733 because that choice is hidden from the

00:00:51.466 applications and transport protocols

00:00:53.033 by the common network layer.

 

00:00:55.200 It doesn’t matter whether the underlying network

00:00:57.233 is Ethernet, WiFi, optical fibre,

00:00:59.966 or something else,

00:01:01.533 because the differences are hidden

00:01:02.933 from the upper-layer protocols.

 

00:01:05.533 Similarly, the use of a common network

00:01:07.800 layer makes it easy to deploy different

00:01:10.033 applications and transport protocols.

 

00:01:12.566 The lower layers must deliver network layer packets,

00:01:15.566 but are unaware of the type of application data

00:01:17.800 contained in those packets.

 

00:01:20.000 They cannot tell whether the packets being delivered

00:01:22.266 comprise an email message, a web page,

00:01:24.800 a phone call, streaming video, or whatever.

00:01:28.900 This approach is very flexible,

00:01:31.500 and makes it easy to support new physical

00:01:33.300 and data link layer technologies,

00:01:35.500 and new transport protocols and applications,

00:01:38.266 provided they can deliver packets for,

00:01:40.500 and operate over, the common network layer.

 

00:01:44.266 The disadvantage is that the network

00:01:46.533 layer is not optimised for any one application.

 

00:01:49.433 It emphasises generality and flexibility

00:01:52.333 to support many different uses,

00:01:54.600 rather than providing optimal performance

00:01:56.366 for any particular use case.

 

00:01:59.466 In the Internet,

00:02:00.533 the network layer is known as the Internet Protocol, IP.

 

00:02:04.233 This is the IP part of the

00:02:06.133 well known TCP/IP protocol suite.

 

00:02:10.100 The Internet Protocol provides a common way

00:02:12.433 to identify devices on the network,

00:02:14.500 using what’s known as an IP address.

 

00:02:17.400 It provides routing algorithms to direct packets

00:02:19.800 across the network from source to destination.

 

00:02:22.700 And it forwards those packets in a best effort manner,

00:02:25.333 accepting that the network may be unreliable.

 

00:02:29.066 At its core, the Internet Protocol

00:02:31.233 provides uniform connectivity.

00:02:33.600 Any host can send data to any other host,

00:02:36.066 subject to firewall policy,

00:02:38.166 but makes no quality guarantees.

 

00:02:43.366 There are two versions of the Internet Protocol in use.

00:02:46.600 The most commonly used version is IP version 4.

00:02:49.966 This was introduced into what became the Internet in 1983.

 

00:02:54.733 The figure shows the format of an IPv4 packet.

 

00:02:58.266 This is sent in the payload data section

00:03:00.300 of a data link layer packet,

00:03:01.933 such as an Ethernet or WiFi frame,

00:03:04.400 with the different parts of the IPv4 packet

00:03:06.866 being sent in the order shown,

00:03:08.766 left-to-right, top-to-bottom,

00:03:10.733 starting with the version number,

00:03:12.400 header length, DSCP field, and so on,

00:03:15.700 and concluding with the transport layer data.

 

00:03:19.533 Key parts of an IPv4 packet are

00:03:21.833 the source and destination addresses,

00:03:24.000 each 32 bits in size,

00:03:26.166 that denote the network interfaces

00:03:28.133 from which the packet was sent,

00:03:29.533 and to which it should be delivered.

 

00:03:32.200 The use of 32 bits for the address fields

00:03:34.500 allows 2-to-the-power-32 possible addresses,

00:03:37.333 around 4 billion,

00:03:38.900 which is not enough for the current Internet.

 

00:03:41.800 This is the motivation to switch to IPv6.

 

00:03:45.633 In addition to addressing,

00:03:47.333 IPv4 provides the fragment identifier,

00:03:50.233 fragment offset,

00:03:51.733 “don’t fragment” (DF),

00:03:53.800 and “more fragments” (MF) fields to allow

 

00:03:56.233 large IPv4 packets to be split into

00:03:58.966 pieces for delivery over networks

00:04:00.766 that can only deliver small packets.

 

00:04:03.366 It also includes a Differentiated Services Code Point

00:04:06.833 (DSCP) field to allow packets to request

00:04:10.066 special treatment by the network.

 

00:04:12.133 For example,

00:04:13.200 a packet that carries video conferencing or gaming data

00:04:16.100 might ask for low latency delivery,

00:04:18.633 while one carrying data that’s part of a background

00:04:20.966 software update might indicate that it’s low priority.

 

00:04:25.200 The time-to-live (TTL) field prevents packets

00:04:27.533 from circulating forever in the network in case a routing

00:04:30.266 problem causes them to go around in a loop,

00:04:32.566 and the header checksum detects transmission errors.

 

00:04:36.533 Finally, the upper layer protocol identifier

00:04:38.900 identifies the format of the transport layer data

00:04:41.000 that follows the IPv4 header.

00:04:43.600 This usually indicates that the transport

00:04:45.566 layer data is a TCP segment or a UDP datagram.

 

00:04:52.433 IPv6 was designed to solve the

00:04:54.500 problem that the IPv4 addresses are too small.

 

00:04:58.033 It replaces the 32 bit addresses used in IPv4

00:05:01.666 with 128 bit addresses.

 

00:05:04.100 This vastly increases the number of devices

00:05:06.033 that can be added to the network,

00:05:08.433 since each additional bit

00:05:10.000 doubles the number of addresses that are available.

 

00:05:13.300 In addition, IPv6 simplifies the header.

 

00:05:16.900 It removes the support for in-network fragmentation,

00:05:19.733 that was present in IPv4,

00:05:21.866 since it was difficult to implement efficiently,

00:05:24.733 and instead requires the hosts to adjust the size

00:05:26.966 of the packets they send to match the network path.

 

00:05:30.866 It also removes the header checksum,

00:05:32.500 since it’s usually redundant

00:05:33.900 with the checksum provided by the data link layer.

 

00:05:37.933 As of late 2020,

00:05:39.900 Google reports that about a third of their users access

00:05:42.433 Google over IPv6.

 

00:05:45.033 Statistics from Akamai,

00:05:46.633 a large content distribution network,

00:05:49.000 report around 60% of connections

00:05:51.266 to their network from India are over IPv6,

00:05:54.333 around 50% from the US, Germany,

00:05:57.333 Belgium, Greece, Taiwan, and Vietnam,

00:06:00.266 and around 35% from the UK.

 

00:06:03.766 IPv6 took a long time to start seeing deployment,

00:06:06.833 but its use has greatly accelerated over the last few years.

 

00:06:14.000 If we have IPv4 and IPv6,

00:06:17.400 you might ask what about IPv5?

 

00:06:20.900 Well, experiments with packet voice over the ARPAnet,

00:06:24.100 the precursor to the Internet,

00:06:25.966 started in the early 1970s, with the Network Voice Protocol

00:06:30.000 developed by Danny Cohen

00:06:31.433 at the University of Southern California’s

00:06:33.233 Information Sciences Institute.

 

00:06:36.233 This work eventually led to the Internet Stream Protocol,

00:06:39.100 ST-II, that was an experimental

00:06:41.466 multimedia streaming protocol

00:06:43.166 developed mostly in the 1980s and early 1990s.

 

00:06:47.333 ST-II ran in parallel to IPv4,

00:06:50.400 and used IP version 5 in its header.

 

00:06:54.433 ST-II was not widely deployed,

00:06:57.300 but it helped prototype a number of important ideas

00:06:59.933 around multimedia transport over packet networks.

 

00:07:04.100 Both what worked well, and what didn’t.

 

00:07:07.300 Steve Casner and Eve Schooler,

00:07:09.533 who both worked with Danny Cohen at ISI,

00:07:12.233 helped lead the development of the next wave of

00:07:14.366 multimedia transport protocols,

00:07:16.400 RTP and SIP,

00:07:18.166 based on experiences with ST-II and earlier protocols.

 

00:07:22.400 RTP and SIP are extremely widely used

00:07:25.733 in modern video conferencing services,

00:07:28.000 such as Zoom, Webex, and Microsoft Teams,

00:07:31.000 and as the basis for today’s mobile phone networks.

 

00:07:37.000 IPv4 and IPv6 have differently sized addresses,

00:07:41.533 but otherwise work similarly.

 

00:07:44.400 In both protocols, an IP address represents

00:07:47.100 ]the location of a particular network interface.

 

00:07:50.666 If a device has more than one network interface,

00:07:54.000 for example if it’s a smart phone with both 5G and WiFi,

00:07:57.566 it will have one IP address for each interface.

 

00:08:01.066 It may also have both IPv4 and IPv6 addresses

00:08:04.900 assigned to some, or all, of its network interfaces.

00:08:08.600 And, in some cases, can have more than one

00:08:10.966 address of each type assigned to an interface.

 

00:08:13.866 Importantly, the IP address identifies the location

00:08:17.433 at which a network interface is attached to the network.

00:08:20.066 It does not identify the device,

00:08:22.533 and if a device moves to a different place

00:08:24.766 it will acquire a different IP address.

 

00:08:27.700 IPv4 and IPv6 addresses

00:08:30.600 both comprise a network part and a host part.

 

00:08:33.933 The network part, often known as the network prefix,

00:08:37.200 identifies the network to which the device is attached,

00:08:40.300 while the host part of the address

00:08:42.066 identifies a particular attachment point on that network.

 

00:08:45.700 The fraction of the address tthe address that identifies

00:08:48.366 the network part, and the fraction left for the host varies,

00:08:52.466 with different networks being assigned

00:08:54.033 differing amounts of address space.

 

00:08:57.133 As an example,

00:08:58.366 the School of Computing Science operates an IPv4 network

00:09:01.700 in Lilybank Gardens and the Boyd Orr Building

00:09:04.166 where the first 20 bits of the IPv4 address

00:09:06.966 comprise the network part,

00:09:08.600 and the last 12 bits are the host part.

 

00:09:11.233 IPv4 addresses in the range 130.209.240.0

00:09:17.033 to 130.209.255.255,

00:09:21.433 that all share the same initial 20 bit prefix,

00:09:24.800 are all within that network.

 

00:09:27.133 The School also has an IPv6 network,

00:09:29.733 that works similarly, although the addresses are longer.

 

00:09:34.366 In the wide area,

00:09:35.833 Internet traffic is routed towards its destination

00:09:38.733 looking only at the network part of the IP address.

00:09:41.733 Only when it reaches the destination network

00:09:44.566 do the routers in the network inspect

00:09:46.400 the host part of the address,

00:09:48.066 to find the device that should receive the packet.

 

00:09:52.166 Finally, it’s important to remember that the network layer

00:09:55.133 does not use names such as example.com.

00:09:58.166 Traffic delivered over the Internet

00:10:00.300 contains source and destination IP addresses,

00:10:03.066 and is routed based on those IP addresses.

 

00:10:06.200 The host that sends an IP packet

00:10:08.366 resolves the name of the destination to an IP address,

00:10:12.166 and puts that address into the destination address field

00:10:14.733 of the IP packet.

 

00:10:16.433 The network delivers that packet using only that IP address.

 

00:10:21.400 The DNS, that resolves names,

00:10:23.500 is just another application that runs on the Internet,

00:10:26.166 and is not fundamental to its operation.

 

00:10:31.933 The Internet is a network of networks.

 

00:10:35.533 Each network is administered and operated separately.

 

00:10:39.133 It acts as what is known as an Autonomous System, an AS.

 

00:10:43.666 Each AS can choose to use different technologies internally,

00:10:47.400 and can have different rules and policies.

 

00:10:50.333 The commonality is that all run the Internet Protocol.

 

00:10:54.300 The University of Glasgow acts as an Autonomous System

00:10:57.000 in the Internet,

00:10:58.500 as do companies such as Google or Facebook,

00:11:01.166 and Internet Service Providers such as BT,

00:11:03.900 Virgin Media, O2, etc.

 

00:11:09.000 Within a network, the network operator

00:11:11.900 will seek to deliver data to its destination

00:11:14.300 as efficiently as possible.

 

00:11:16.633 The Internet places no requirements on how they do this,

00:11:20.166 or on what data link layer

00:11:21.500 or physical layer technologies they use.

 

00:11:24.500 Each network operator is free to use whatever technologies,

00:11:27.666 and whatever routing or forwarding algorithms,

00:11:29.966 that it chooses.

 

00:11:32.033 Typically, the network operator

00:11:34.133 will seek to ensure traffic follows the shortest path

00:11:36.600 from source to destination across their network,

00:11:39.733 using either a distance vector routing algorithm,

00:11:42.566 or, more likely, a link state routing algorithm

00:11:45.700 such as OSPF.

 

00:11:47.400 There are a wide variety of different approaches used,

00:11:51.133 though, depending on the size of the network,

00:11:53.766 and the needs the network operator and its customers.

 

00:11:59.533 Most Internet traffic is not confined to a single AS.

00:12:03.733 Rather, it’s common for traffic to

00:12:06.066 pass through several Autonomous Systems

00:12:07.933 on its path from source to destination.

 

00:12:11.333 For example, packets sent from the University of

00:12:13.733 Glasgow to Google will start in the University’s network,

00:12:17.666 then traverse a network known as JANET

00:12:19.800 (the Joint Academic NETwork; the University’s ISP),

00:12:22.666 and then finally reach Google.

 

00:12:25.666 Each of these three networks is an Autonomous System.

 

00:12:29.200 Each must cooperate to forward the packets,

00:12:31.633 and find an appropriate route

00:12:33.366 for the data across the network.

 

00:12:35.666 Indeed, all of the ASes that comprise the Internet

00:12:38.833 cooperate to ensure the network

00:12:40.366 can successfully route data to its destination,

00:12:42.966 wherever that destination is.

 

00:12:46.200 This cooperation is enabled by

00:12:48.400 the Border Gateway Protocol, BGP.

 

00:12:51.766 BGP is a routing protocol.

00:12:54.100 It allows each AS to advertise the network

00:12:56.900 prefixes that it owns,

00:12:58.466 to tell the rest of the network where to send packets

00:13:01.400 destined for IP addresses contained within those prefixes.

 

00:13:05.466 In addition, BGP allows ASes

00:13:07.933 to advertise the routes they can use to reach

00:13:10.000 the network prefixes owned by other ASes.

 

00:13:13.300 This allows, for example,

00:13:15.266 an ISP to advertise how to reach the IP

00:13:17.766 addresses used by its customers.

 

00:13:20.433 Similarly, BGP allows ASes to filter out

00:13:23.833 advertisements for network prefixes to which they will not

00:13:26.366 forward traffic.

 

00:13:29.200 The information exchanged in BGP

00:13:31.733 allows ASes to decide how to route

00:13:33.700 packets across the network.

 

00:13:36.166 Networks located near the edge of the Internet

00:13:38.866 need to maintain relatively little information

00:13:41.166 to participate in BGP.

00:13:43.333 They simply need to know their own network prefixes,

00:13:46.466 and those of their customers.

 

00:13:48.400 They then advertise those prefixes

00:13:50.466 to the rest of the network.

 

00:13:52.566 Those edge ASes know how to route traffic to themselves

00:13:56.066 and their customers,

00:13:57.500 and just pass everything else to a default route

00:13:59.700 that directs it out to the wider network.

 

00:14:02.933 For networks nearer the core of the Internet,

00:14:05.400 this approach is no longer sufficient.

 

00:14:08.133 These ASes form the so-called default free zone,

00:14:11.200 the DFZ,

00:14:12.433 where they use BGP to put together something

00:14:15.400 close to a complete map of the Internet,

00:14:17.800 so they know how to forward packets to reach any

00:14:20.000 possible destination.

 

00:14:22.700 When it comes to BGP routing,

00:14:24.666 and finding the correct route for data to

00:14:26.433 cross the wide area Internet,

00:14:28.733 policy, politics, and economics are often

00:14:31.633 more important than finding the shortest route.

 

00:14:37.000 A final feature of the network layer is forwarding.

 

00:14:40.766 Given a route through the network,

00:14:42.833 calculated by BGP for the inter-domain path,

00:14:45.733 and by an intra-domain routing protocol

00:14:47.900 such as OSPF within each network,

00:14:50.600 how are the packets actually forwarded along that path?

 

00:14:54.700 Forwarding in the Internet follows a best effort approach.

 

00:14:58.533 A router in the network receives a packet,

00:15:00.966 and makes its best effort to forward it

00:15:02.700 towards its destination.

 

00:15:05.600 The network is connectionless.

00:15:07.933 A sender doesn’t need to establish a connection

00:15:10.566 or ask permission before sending a packet,

00:15:12.900 and makes no attempt to reserve capacity.

 

00:15:16.100 As a results, the network makes no guarantees.

 

00:15:19.533 If there are insufficient resources available to

00:15:22.033 forward a packet, then it may be delayed

00:15:24.400 or will simply be discarded.

 

00:15:27.466 The figure shows an example,

00:15:29.133 showing the time packets take to traverse the network,

00:15:32.366 with the x-axis showing time since the start

00:15:35.166 of transmission, and the y-axis

00:15:37.533 showing the time taken to receive a response

00:15:39.500 from the destination.

 

00:15:42.033 As can be seen, the time taken to get a response

00:15:44.566 may vary significantly,

00:15:46.033 depending on the amount of other traffic in the network.

 

00:15:49.566 Packets may be delayed, reordered, lost,

00:15:52.766 duplicated, or corrupted in transit.

 

00:15:56.800 In a well engineered network,

00:15:58.800 there is little timing variation,

00:16:00.866 packets are rarely lost or corrupted,

00:16:03.033 and almost never arrive out of order.

00:16:05.733 If the packets traverse a poor quality network,

00:16:08.600 however, behaviour may be less predictable.

 

00:16:12.666 The Internet can provide extremely high quality,

00:16:16.200 but there’s no requirement that it does so.

 

00:16:19.000 This gives flexibility.

 

00:16:20.566 The Internet can encompass all sorts of different networks,

00:16:23.833 not just those in rich countries

00:16:25.733 with well-developed infrastructure.

00:16:28.000 But, it requires applications

00:16:30.300 to be able to cope with unpredictable quality.

 

00:16:34.500 To summarise,

00:16:36.200 the network layer provides the interworking function

00:16:39.300 that allows networks to cooperate,

00:16:41.366 and come together to build a global internet.

 

00:16:44.400 It provides a common addressing scheme

00:16:46.566 to identify endpoints of communication,

00:16:49.066 a routing scheme to allow systems to determine

00:16:51.833 the appropriate path for data to take

00:16:53.500 across the internetwork, and a forwarding scheme

00:16:56.333 to move packets towards their destination.

 

00:16:59.833 In the Internet,

00:17:00.833 the network layer is the Internet Protocols,

00:17:03.466 IPv4 and IPv6.

 

00:17:06.433 Data is routed between Autonomous Systems using BGP,

00:17:10.266 and within autonomous systems using distance vector

00:17:13.400 or link-state routing protocols, such as OSPF.

 

00:17:17.433 Finally, packet forwarding happens on a best effort basis,

00:17:21.466 giving the flexibility operate over any type of link layer

00:17:24.800 at the cost of potentially unpredictable behaviour.

 

00:17:28.800 The network layer provides connectivity.

 

00:17:31.666 Above it sits the transport layer,

00:17:33.600 that provides the abstractions that

00:17:35.433 applications use to deliver data.

 

00:00:00.566 The transport layer isolates the applications

00:00:03.566 from the vagaries of the network.

00:00:05.566 It ensures data is delivered with appropriate reliability,

00:00:08.833 and adapts the speed of transmission to match

00:00:11.033 the network capacity.

 

00:00:13.066 There are two widely used transport layer protocols

00:00:15.466 in the Internet: UDP and TCP.

 

00:00:19.300 UDP provides an unreliable packet delivery service,

00:00:22.666 essentially exposing the raw IP

00:00:25.033 network model to the applications.

00:00:27.500 It’s useful for applications that prefer

00:00:29.533 timeliness over reliability,

00:00:31.766 and as a building block for developing

00:00:33.433 new transport protocols.

 

00:00:35.833 TCP provides a reliable service,

00:00:38.300 retransmitting any lost packets,

00:00:40.466 putting reordered data back into the correct order,

00:00:43.200 and adapting its transmission rate

00:00:45.000 to match the available capacity.

00:00:47.166 It’s useful for applications that need data

00:00:49.633 to be delivered reliably

00:00:51.233 and as fast as possible.

 

00:00:53.566 In this part, I’ll talk about transport layer concepts,

00:00:56.866 and the UDP and TCP transport protocols

00:00:59.466 used in the Internet.

 

00:01:01.566 I’ll also briefly introduce the idea of congestion control,

00:01:04.700 adapting the sending rate of the transport

00:01:06.600 to match the available network capacity.

 

00:01:10.766 As discussed in the previous part,

00:01:13.600 an IP network provides only a

00:01:15.466 best effort packet delivery service.

 

00:01:18.000 IP packets can be lost,

00:01:19.800 duplicated, delayed, or re-ordered in transit.

 

00:01:24.033 The role of the transport protocol

00:01:26.100 is to isolate applications,

00:01:27.933 as much as is necessary, from the network.

 

00:01:31.333 The transport protocol demultiplexes traffic

00:01:33.866 destined for different applications.

00:01:35.933 It enhances the network quality of service

00:01:39.633 to offer appropriate reliability for those applications.

00:01:43.233 And it performs congestion control,

00:01:45.033 to adapt the transmission rate

00:01:46.400 to the available network capacity.

 

00:01:49.433 There are two transport protocols that have

00:01:51.366 been successfully deployed in the Internet.

 

00:01:53.700 UDP, the user datagram protocol,

00:01:57.200 provides an unreliable service.

 

00:02:00.200 TCP, the transmission control protocol,

00:02:03.133 provides a reliable, ordered,

00:02:05.333 and congestion controlled service.

 

00:02:07.966 Applications that run on the Internet

00:02:10.066 use one of these two transport protocols.

 

00:02:14.300 UDP is the simplest

00:02:16.366 possible transport protocol that can run on the Internet.

 

00:02:19.666 It exposes the raw IP service to applications,

00:02:23.733 adding only the concept of a port number,

00:02:26.133 to identify different applications running on a single host.

 

00:02:30.166 Like IPv4 and IPv6, UDP is connectionless.

00:02:35.300 An application doesn’t need to establish a UDP connection.

00:02:39.200 Rather, it can simply send a packet towards

00:02:41.633 a destination without asking permission

00:02:43.733 or establishing a connection.

 

00:02:46.933 UDP datagrams are delivered in a best effort manner.

 

00:02:50.600 Datagrams might not arrive at all,

00:02:52.800 and if they do arrive, they might not arrive in the order

00:02:55.733 they were sent.

 

00:02:57.266 The UDP transport protocol

00:02:58.866 doesn’t attempt to correct for this,

00:03:01.033 or to ensure reliable delivery.

 

00:03:03.733 It’s the responsibility of the application using UDP

00:03:07.133 to reconstruct the ordering, or to detect lost packets.

 

00:03:11.566 Similarly, UDP doesn’t perform any form of

00:03:14.700 congestion control.

00:03:16.366 If an application using UDP

00:03:18.566 tries to send faster than the network can deliver packets,

00:03:22.033 then those packets will simply be discarded.

00:03:25.033 UDP doesn’t help the application adapt

00:03:27.533 its sending rate to match the network capacity.

 

00:03:31.066 Accordingly, applications using UDP

00:03:34.433 must be able to tolerate some loss of data,

00:03:36.933 to receive data out of order,

00:03:39.400 and to be able to estimate and adapt

00:03:41.700 to the available network capacity.

 

00:03:44.533 Doing this well is extremely difficult,

00:03:47.033 and UDP is not suitable for many applications.

 

00:03:51.433 Where UDP is useful

00:03:53.233 is when the application prefers timeliness over reliability.

 

00:03:57.466 Because it doesn’t attempt to retransmit lost data,

00:04:00.533 and doesn’t buffer data

00:04:02.233 to allow it to be put into the correct order,

00:04:04.433 UDP offers the lowest possible latency for

00:04:07.433 data sent across the network.

 

00:04:10.300 That’s useful for applications like voice-over-IP,

00:04:13.366 video conferencing, and gaming,

00:04:15.633 that can tolerate some data loss but need low latency.

 

00:04:20.033 For most applications, though,

00:04:22.066 the unreliable nature of UDP

00:04:23.933 makes it poorly suited to their needs.

 

00:04:28.333 By way of contrast,

00:04:29.966 the TCP protocol provides a fully reliable,

00:04:33.266 ordered, byte stream delivery service

00:04:36.100 than runs over an IP network.

 

00:04:39.400 TCP is a connection oriented transport protocol.

00:04:42.800 Applications that use TCP as their transport

00:04:46.300 must first setup a connection from sender to receiver,

00:04:49.333 before they can send data.

00:04:51.900 Once a connection is established,

00:04:54.000 the TCP protocol ensures that any lost

00:04:56.533 data is retransmitted,

00:04:58.233 and that any reordered data

00:04:59.866 is put back into the correct order,

00:05:01.600 before delivering it to the receiving application.

 

00:05:05.600 The TCP protocol also adapts the rate at which

00:05:08.866 data is sent across the network

00:05:10.466 to match the available network capacity,

00:05:13.066 a process known as congestion control.

 

00:05:16.833 A TCP sender writes a sequence

00:05:19.133 of bytes into a TCP connection,

00:05:21.533 and that exact same sequence of bytes

00:05:23.400 is delivered to the receiver.

00:05:25.066 Reliably. In the order sent.

00:05:27.833 And at the maximum speed the network can support.

 

00:05:31.933 The overwhelming majority of applications

00:05:34.366 running on the Internet use TCP as their transport protocol.

 

00:05:39.366 TCP has two limitations.

 

00:05:42.600 The first is that TCP delivers a sequence of bytes,

00:05:46.100 not a sequence of messages.

00:05:48.400 If a sender writes 2000 bytes of data

00:05:51.200 onto a TCP connection,

00:05:53.500 comprising two messages of 1000 bytes each,

00:05:57.033 then TCP guarantees

00:05:58.966 that those 2000 bytes will be received reliably,

00:06:01.933 and in the order sent.

 

00:06:03.733 It does not guarantee that they will be received as

00:06:06.233 two messages of 1000 bytes each.

00:06:09.500 Indeed, it’s entirely possible that TCP

00:06:11.966 will deliver the data as a block of 1500 bytes

00:06:15.033 followed by a separate block containing

00:06:17.300 the remaining 500 bytes.

 

00:06:20.000 Applications that care about message boundaries must

00:06:23.233 structure the data they send over a TCP connection

00:06:26.300 to allow those message boundaries to be reconstructed.

 

00:06:30.066 Since reading and writing data to a file

00:06:32.600 also doesn’t preserve message boundaries,

00:06:35.266 doing this tends not to be difficult.

 

00:06:39.433 The other limitation of TCP is that,

00:06:42.133 because it delivers data reliably and in order,

00:06:45.100 it must retransmit any lost packets.

00:06:48.333 This delays any data following those lost packets,

00:06:51.700 since it can’t be delivered to the application

00:06:53.866 until the retransmission arrives.

00:06:56.966 This is an unavoidable trade-off,

00:06:58.800 if data is to be delivered reliably,

00:07:00.700 and in the order sent, across an unreliable network,

00:07:04.000 but does mean that TCP trades latency for reliability.

 

00:07:11.466 TCP delivers data in the form of segments,

00:07:14.600 each delivered within an IP packet.

 

00:07:17.966 Each TCP segment

00:07:20.033 includes a source and destination port numbers,

00:07:22.733 that identify the applications

00:07:24.966 that send and receive the data.

00:07:27.300 Each TCP segment also includes a sequence number,

00:07:30.433 that counts the number of bytes of data sent

00:07:32.900 and allows the receiver to detect loss

00:07:35.166 and reconstruct the original sending order,

00:07:37.600 and an acknowledgement number

00:07:39.400 that indicates the next segment it wishes to receive.

 

00:07:43.033 TCP segments also include the receiver window size,

00:07:46.900 that indicates the amount of buffer space

00:07:48.833 the receiver has to store incoming TCP segments,

00:07:52.100 a checksum to detect packet corruption,

00:07:55.366 a set of flags to manage connection setup,

00:07:58.166 and an urgent pointer to support

00:07:59.900 advance delivery of important data.

 

00:08:02.766 The actual TCP payload data follows this header.

 

00:08:07.400 As can be seen,

00:08:08.800 TCP packets carry a lot of information

00:08:11.366 in addition to that carried in an IP packet header.

 

00:08:15.066 TCP is a complex and sophisticated transport protocol,

00:08:18.500 that adds a lot of features to IP

00:08:20.800 in order to ensure that data is delivered effectively.

 

00:08:25.933 A TCP connection proceeds in three stages.

 

00:08:30.933 At the start of the connection

00:08:32.866 is an initial three-way handshake

00:08:34.766 that establishes the connection.

00:08:36.866 An initial TCP packet is sent

00:08:39.566 from the TCP client to the server,

00:08:41.666 with the SYN (“synchronise”) bit

00:08:44.500 set in the TCP header,

00:08:46.233 to indicate the start of a connection.

 

00:08:49.266 The server sends a response,

00:08:51.566 with its SYN bit set,

00:08:54.233 to indicate that it’s willing to establish a connection.

00:08:57.433 This response also has the ACK bit set,

00:09:00.300 indicating that the acknowledgement number is valid,

00:09:02.900 because it acknowledges receipt of that initial packet.

 

00:09:07.000 The client completes the three-way

00:09:09.133 SYN - SYN+ACK - ACK handshake

00:09:11.266 by sending an acknowledgement to the server.

00:09:13.800 This establishes the connection.

 

00:09:17.200 The client and server can then exchange data.

 

00:09:20.933 In this example, the client sends data to the server,

00:09:24.366 and the server acknowledges receipt of that data.

00:09:27.633 The initial packet had sequence number 0,

00:09:30.933 and each packet includes 1500 bytes of data.

 

00:09:35.133 We see the server sending acknowledgement packets,

00:09:37.900 and delivering data to the application in response

00:09:40.300 to recv() calls, as the data packets arrive.

 

00:09:44.366 We also see that the client

00:09:46.066 can send some number of data packets,

00:09:48.100 known as its congestion window,

00:09:50.100 before it receives an acknowledgement.

 

00:09:52.966 In this example, the congestion window is 3000 bytes:

00:09:56.533 the client is allowed to send up to 3000 bytes of data

00:09:59.633 without receiving an acknowledgement.

 

00:10:04.000 The packet with sequence number 4500,

00:10:07.533 sent from client to server, is lost.

00:10:10.800 As a result, when the next packet,

00:10:13.300 with sequence number 6000, arrives

00:10:16.033 the server generates another acknowledgment

00:10:18.433 indicating that it’s still expecting packet 4500.

 

00:10:23.233 This continues until three duplicate

00:10:25.166 acknowledgements have been received,

00:10:26.800 when the client retransmits the lost packet.

 

00:10:30.200 Eventually, the missing data arrives at the server.

00:10:33.933 This fills the gap, and the missing data,

00:10:36.533 and the three following packets that were already received,

00:10:39.533 are delivered to the application.

00:10:42.166 The server sees a delay,

00:10:43.866 while the missing data is retransmitted

00:10:46.000 then receives a burst of data.

00:10:48.533 Message boundaries are not preserved.

 

00:10:52.433 Finally, the connection is closed

00:10:54.333 using a three-way handshake

00:10:55.566 similar to that used to open the connection.

 

00:10:58.866 We’ll talk more about how TCP establishes connections,

00:11:02.166 and reliably transmits data, in later lectures.

 

00:11:08.533 The TCP protocol includes a sophisticated algorithm,

00:11:12.000 known as congestion control,

00:11:14.000 that adapts its transmission rate

00:11:15.766 to match the available network capacity.

 

00:11:18.766 This operates in two phases.

 

00:11:21.533 The first phase is known as slow-start.

 

00:11:24.800 At the start of a connection,

00:11:26.666 TCP starts by sending slowly,

00:11:29.066 but increases its sending rate exponentially

00:11:31.633 until it reaches the network capacity.

 

00:11:33.933 Once it’s sending at a speed

00:11:36.200 that matches the available network capacity,

00:11:38.533 TCP switches to a congestion avoidance phase,

00:11:41.666 adapting its sending rate following a sawtooth pattern

00:11:45.033 that allows it to gradually adapt to changes in capacity.

 

00:11:49.166 There are a lot of subtle details

00:11:51.300 in TCP congestion control behaviour,

00:11:53.233 that we’ll talk about later in the course.

 

00:11:56.033 What’s important for now is that TCP can effectively

00:11:59.366 adjust the rate at which data is sent

00:12:01.200 to match changes in the available network capacity.

 

00:12:04.533 The algorithm it uses to do this looks simple at first glance,

00:12:08.600 but actually contains a lot of complex features,

00:12:11.433 and not obvious behaviours, and is highly effective.

 

00:12:17.266 To conclude,

00:12:18.933 the transport layer protocols adapt the service

00:12:22.533 provided by the network layer

00:12:23.866 to meet the needs of applications.

 

00:12:26.266 The Internet provides two transport protocols,

00:12:28.833 TCP and UDP.

 

00:12:31.033 TCP is a complex and sophisticated protocol,

00:12:34.200 that is highly optimised to deliver reliable data quickly.

00:12:38.066 It’s well suited to the overwhelming majority

00:12:40.800 of Internet applications.

 

00:12:42.766 UDP is much less sophisticated,

00:12:45.400 and essentially exposes the best effort

00:12:47.433 packet delivery service

00:12:48.666 offered by the underlying IP layer to the application.

00:12:51.700 It’s useful when developing applications that

00:12:54.100 prefer timeliness to reliability,

00:12:56.566 or as a basis for building new transport protocols,

00:12:59.700 but is very difficult to use effectively.

 

00:13:02.866 The transport layer is the last

00:13:05.100 general purpose layer in the protocol stack.

 

00:13:07.533 Above it sit the application protocols,

00:13:10.033 that we’ll discuss briefly in the next part.

00:00:00.000 The higher layers of the OSI reference

00:00:03.696 model – the session, presentation, and application

00:00:06.393 layers – provide services to help applications

00:00:09.089 manage transport connections, data formats, and the

00:00:11.785 application logic.

00:00:12.556 In this part, I’ll talk briefly about

00:00:15.352 some issues to consider around higher layer

00:00:18.048 protocols, and of the importance of protocol

00:00:20.744 standards for interoperability.

 

00:00:23.000 The OSI reference model defines three layers

00:00:25.878 above the transport layer. These are the

00:00:28.655 session, presentation, and application layers.

00:00:30.639 The goal of protocols at these layers

00:00:33.517 is to support the needs of applications.

00:00:36.294 They manage transport connection, name and locate

00:00:39.072 resources used by the application, describe and

00:00:41.850 negotiate data formats, and present the data

00:00:44.627 in an appropriate manner. Essentially, they translate

00:00:47.405 the application’s needs into protocol mechanisms.

00:00:49.786 The protocols used at these layers tend

00:00:52.663 to be quite tightly bound to particular

00:00:55.441 classes of application, and are less general

00:00:58.218 purpose than protocols at the transport layer

00:01:00.996 and below. As a result, the boundaries

00:01:03.774 between these layers tend to be less

00:01:06.551 clear, and many systems implement these higher

00:01:09.329 layer protocols without a clear distinction between

00:01:12.106 the layers.

 

00:01:14.000 The session layer is about managing transport

00:01:16.991 layer connections.

00:01:17.816 Some applications are straight forward, with a

00:01:20.807 single client connecting to a single server,

00:01:23.698 or to a small set of servers.

00:01:26.588 HTTP is an example of such a

00:01:29.479 protocol. This class of protocols tends to

00:01:32.369 need relatively little session layer support,

00:01:34.847 often limited to being able to re-use

00:01:37.738 a transport layer connection to send and

00:01:40.628 receive multiple messages.

00:01:41.867 Others systems are more complex, involving multiple

00:01:44.858 clients and servers collaborating to meet the

00:01:47.748 application needs. Examples include video conferencing and

00:01:50.639 chat applications, and multiplayer games, that use

00:01:53.529 a cluster of servers to support end-user

00:01:56.420 applications. The session layer in these applications

00:01:59.311 tends to be concerned with finding the

00:02:02.201 participants in a call or players in

00:02:05.092 the game, and forwarding messages between those

00:02:07.982 participants and between the servers supporting each

00:02:10.873 user.

00:02:12.036 Peer-to-peer applications tend to have still more

00:02:15.026 sophisticated session layer features. These applications have

00:02:17.917 the problem of how to set-up peer-to-peer

00:02:20.808 connections in the presence of firewalls and

00:02:23.698 network address translators, and often need to

00:02:26.589 adapt to rapid changes in group membership.

00:02:29.479 BitTorrent is a well known example of

00:02:32.370 this class of application.

00:02:34.022 Finally, there are applications that use multicast

00:02:37.012 or broadcast transmission, where the network can

00:02:39.903 send messages to a group of receivers.

00:02:42.794 Like peer-to-peer applications, these tend to require

00:02:45.684 the session layer to manage group membership

00:02:48.575 changes.

00:02:49.738 Each session layer protocol is different,

00:02:52.315 and they have relatively little in common

00:02:55.206 with each other, since they tend to

00:02:58.096 be closely tied to particular classes of

00:03:00.987 application.

 

00:03:01.000 The presentation layer sits above the session

00:03:03.900 layer, and manages the presentation, representation,

00:03:06.300 and conversion of data.

00:03:07.900 Presentation layer features include the media type

00:03:10.800 descriptions that web servers and video conferencing

00:03:13.600 tools use to describe data formats.

00:03:16.000 They specify that a page is in

00:03:18.800 HTML, whether an image is a JPEG

00:03:21.600 or a PNG, or that the video

00:03:24.400 is compressed with H.264 rather than VP8.

00:03:27.200 And they allow applications to describe the

00:03:30.000 data formats they support, and to negotiate

00:03:32.800 an agreed format with the other participants

00:03:35.600 in the session.

00:03:36.800 Other presentation layer features include channel encodings,

00:03:39.700 where data is adapted to fit the

00:03:42.500 limitations of the communications channel. An example

00:03:45.300 is email, where the original design only

00:03:48.100 supported textual data in ASCII format.

00:03:50.500 When support for email attachments was added,

00:03:53.300 those attachments had to look like text,

00:03:56.100 so they could pass through mail servers

00:03:58.900 that hadn’t yet been upgraded. A channel

00:04:01.700 encoding scheme, known as BASE64 encoding,

00:04:04.100 was developed to do this, allowing arbitrary

00:04:06.900 data to be converted to a text-based

00:04:09.700 format for transmission.

00:04:10.900 Finally, the presentation layer is often where

00:04:13.800 support for internationalisation is implemented. There are

00:04:16.600 two sorts of concerns here. One is

00:04:19.400 around labelling the character set used to

00:04:22.200 represent textual data, whether it is Unicode

00:04:25.000 text in UTF-8 format, or some national

00:04:27.800 character set such as ASCII, Latin1,

00:04:30.200 or the Big5 system used in Taiwan

00:04:33.000 and Hong Kong. The other is around

00:04:35.800 labelling the language, and possibly the regional

00:04:38.600 variant or dialect, that is used.

00:04:41.000 The problems addressed by the presentation layer

00:04:43.900 are wide-ranging, and tend to relate to

00:04:46.700 the broader system, and to the environment

00:04:49.500 in which the application operates, not just

00:04:52.300 to the network communication.

 

00:04:55.000 The final layer in the 7-layer OSI

00:04:58.119 reference model is the application layer.

00:05:00.706 It is here that protocol functions that

00:05:03.725 are specific to the application logic are

00:05:06.744 implemented. The application layer protocols deliver email

00:05:09.763 messages, retrieve web pages, stream video,

00:05:12.350 support multiplayer games, and so on.

00:05:14.938 By definition, such messages are entirely application

00:05:18.056 specific, and there is little that is

00:05:21.075 general to say here.

 

00:05:24.000 The OSI model is a reasonable way

00:05:27.116 of thinking about network protocols. Having a

00:05:30.132 common model helps to frame and organise

00:05:33.148 discussion of network protocols. Real networks are

00:05:36.164 more complex, and the layer boundaries are

00:05:39.180 less clear cut than the model suggests,

00:05:42.196 especially around the higher layers, but the

00:05:45.212 OSI model is close enough to reality

00:05:48.228 to be useful.

00:05:49.521 But, it misses two key layers:

00:05:52.206 the financial and the political.

00:05:54.360 Network protocols are only successful to the

00:05:57.476 extent that they enable different devices to

00:06:00.492 communicate. This requires interoperability between different implementations

00:06:03.508 of a protocol, implemented by different groups

00:06:06.524 of people.

00:06:07.386 Getting the incentives right, so that different

00:06:10.502 vendors can work together to ensure their

00:06:13.518 products interoperate, is a problem that rapidly

00:06:16.534 goes beyond the technical, and into the

00:06:19.550 realms of organisational politics, market forces,

00:06:22.135 regulation, and economics.

00:06:23.427 It’s an area where standards setting organisations,

00:06:26.543 such as the Internet Engineering Task Force,

00:06:29.559 the International Telecommunications Union, the World-wide Web

00:06:32.575 Consortium, the 3rd Generation Partnership Project,

00:06:35.161 the Moving Picture Experts Group, and others,

00:06:38.177 play an important role.

 

00:06:41.000 Network protocol standards are a very human

00:06:44.141 process. They’re the result of much discussion

00:06:47.181 and negotiation, much argument and compromise.

00:06:49.787 The IETF describes the outcome as “rough

00:06:52.828 consensus and running code”. The Internet works

00:06:55.869 because thousands of engineers spend the effort

00:06:58.909 to make sure it works, to make

00:07:01.950 sure their products work together, and that

00:07:04.991 the protocols are described well enough to

00:07:08.031 support interoperability.

 

00:07:10.000 This concludes our brief review of the

00:07:12.260 Internet protocols. In the next part,

00:07:14.111 we’ll start to think about how the

00:07:16.271 network is changing and evolving, to set

00:07:18.431 the scene for the remainder of the

00:07:20.591 course.

00:00:00.000 We are in the middle of a

00:00:03.689 period of rapid change in the Internet

00:00:06.378 infrastructure. In this part, I want to

00:00:09.067 highlight some ways in which the network

00:00:11.756 is changing, to set the scene for

00:00:14.444 the remainder of the course.

00:00:16.365 In particular, I want to talk about

00:00:19.154 the exhaustion of the IPv4 address space,

00:00:21.843 and the accelerating transition to IPv6.

00:00:24.148 The increasing proportion of wireless, mobile,

00:00:26.552 Internet endpoints, such as smartphones, and the

00:00:29.241 implications of this shift to a network

00:00:31.930 of mobile devices.

00:00:33.083 The increasing centralisation of the network around

00:00:35.871 a small number of hypergiant content providers.

00:00:38.560 The increasing use of real-time applications,

00:00:40.965 and the corresponding need for low-latency transport.

00:00:43.654 And, finally, new approaches to protocol design

00:00:46.443 to support innovation in the face of

00:00:49.132 network ossification.

 

00:00:51.000 The designers of the Internet made certain

00:00:53.791 assumptions.

00:00:54.925 They assumed that devices would generally be

00:00:57.716 located at a fixed location in the

00:01:00.407 network, and would have a small number

00:01:03.099 of network interfaces that could be given

00:01:05.790 persistent and globally unique addresses.

00:01:07.712 They assumed the network, and the services

00:01:10.503 that run on it, would be operated

00:01:13.194 by many different organisations, working as peers,

00:01:15.885 and that there would be no central

00:01:18.576 points of control.

00:01:19.729 They assumed that best effort packet delivery

00:01:22.520 service would provide sufficient quality, and that

00:01:25.211 applications would be adaptive, and able to

00:01:27.902 cope with changes in the network capacity

00:01:30.593 and available bandwidth.

00:01:31.746 They assumed that the network was trusted

00:01:34.537 and secure.

00:01:35.306 And they assumed that innovation would happen

00:01:38.097 at the edges, and that the network

00:01:40.788 itself would provide only a simple packet

00:01:43.479 delivery service.

00:01:44.248 These assumptions generally made sense for a

00:01:47.039 research network in the mid-1980s, when the

00:01:49.730 original design of the Internet protocols was

00:01:52.421 being finalised. Do they still make sense

00:01:55.112 for a network today?

 

00:01:57.000 The first assumption was that devices were

00:02:00.231 located at fixed places in the network,

00:02:03.362 and that each device had a small

00:02:06.493 number of network interfaces, that could be

00:02:09.624 given persistent and globally unique IP addresses.

00:02:12.755 This gives the desirable property that every

00:02:15.986 device is addressable by every other device.

00:02:19.117 It assumes a network of peers,

00:02:21.800 with no conceptual difference between clients and

00:02:24.931 servers, where any device connected to the

00:02:28.062 network can take either role, depending on

00:02:31.193 the software it runs. And where,

00:02:33.877 in principle, any device can connect to

00:02:37.008 any other device.

00:02:38.350 Of course, addressable doesn’t necessarily imply reachable,

00:02:41.580 and the Internet has long supported firewalls

00:02:44.711 to provide access control, by blocking access

00:02:47.842 to certain devices, but the ability for

00:02:50.973 any device to be a server empowers

00:02:54.104 end-users.

00:02:55.301 If any device can act as a

00:02:58.532 server, it ought to be possible to

00:03:01.663 run a website, or other service,

00:03:04.347 anywhere in the network, including on a

00:03:07.478 home machine. There should be no requirement

00:03:10.609 to pay for a dedicated server,

00:03:13.293 located in a managed data centre,

00:03:15.976 to host a website or other service.

00:03:19.107 Unfortunately, this assumption failed.

00:03:20.996 It failed because IPv4 had insufficient addresses

00:03:24.227 to support it, and because devices became

00:03:27.358 mobile.

00:03:28.555 The problem with the lack of addresses

00:03:31.786 in IPv4 results in many hosts sharing

00:03:34.917 IP address with others, using a technique

00:03:38.048 known as network address translation (NAT).

00:03:40.732 In theory, IPv6 will solve this problem

00:03:43.863 by providing enough addresses for each host,

00:03:46.994 but IPv6 has been slow to deploy.

00:03:50.125 The result of this lack of addresses

00:03:53.356 is that connectivity becomes difficult.

00:03:55.592 It’s increasingly necessary to try both IPv4

00:03:58.823 and IPv6 addresses when connecting to a

00:04:01.954 device, perhaps racing connection attempts in parallel

00:04:05.085 to get good performance – a technique

00:04:08.216 known as Happy Eyeballs because it improves

00:04:11.347 end user web browsing performance.

00:04:13.583 It’s also necessary to think about NAT

00:04:16.814 traversal. A client behind a NAT can

00:04:19.945 easily connect to a server on the

00:04:23.076 public Internet but, as we’ll discuss in

00:04:26.207 Lecture 2, it’s difficult to connect to

00:04:29.338 a device located behind a NAT,

00:04:32.022 and to build peer-to-peer applications.

00:04:34.258 This combination greatly increases the complexity of

00:04:37.489 establishing a connection.

00:04:38.831 This makes networked applications slower, and less

00:04:42.062 reliable. And, perhaps more importantly, it forces

00:04:45.193 server applications to run in data centres,

00:04:48.324 and discourages peer-to-peer applications. This forces reliance

00:04:51.454 on cloud services, and encourages centralisation of

00:04:54.585 Internet services onto large cloud providers,

00:04:57.269 such as Amazon Web Services and Google.

 

00:05:00.000 How severe is the shortage of IPv4

00:05:03.391 addresses?

00:05:04.612 Well, IP address assignment follows a hierarchical

00:05:08.003 model. The Internet Assigned Numbers Authority (IANA)

00:05:11.295 assigns blocks of IP addresses to Regional

00:05:14.586 Internet Registries. Those regional registries then assign

00:05:17.878 addresses to ISPs, and other organisations within

00:05:21.169 their region, and those in turn allocate

00:05:24.461 addresses to end users.

00:05:26.341 The IANA assigned the last available blocks

00:05:29.733 of IP addresses to the regional registries

00:05:33.024 in 2011 – around ten years ago.

00:05:36.316 Since then, the regional registries have gradually

00:05:39.707 been running down their pool of available

00:05:42.999 addresses. As of late 2020, the regional

00:05:46.290 registries for Europe, North America, Latin America,

00:05:49.582 and the Asia-Pacific Region have entirely run

00:05:52.873 out of available IPv4 addresses. Africa is

00:05:56.165 projected to run out of IPv4 addresses

00:05:59.456 during 2021.

00:06:00.397 There is now a thriving market in

00:06:03.788 the transfer of Pv4 addresses, with networks

00:06:07.080 that were previously assigned IPv4 addresses,

00:06:09.901 and have more than they need,

00:06:12.722 selling the addresses on to others.

00:06:15.544 As of late 2020, a single IPv4

00:06:18.835 address can be sold for around $25.

00:06:22.127 The IPv4 address space owned by the

00:06:25.418 University of Glasgow, for example, is worth

00:06:28.710 around $1,600,000.

 

00:06:30.000 Despite this shortage of IPv4 addresses,

00:06:32.821 adoption of IPv6 has been relatively slow,

00:06:35.995 although it’s now reaching critical mass.

00:06:38.716 For example, as shown on the slide,

00:06:41.991 Google currently reports that around a third

00:06:45.165 of its users are on IPv6.

00:06:47.886 Availability of IPv6 is highly variable,

00:06:50.707 since network operators tend to switch their

00:06:53.881 customers all at once. Generally, either all

00:06:57.056 the users in a particular network have

00:07:00.230 IPv6, or none of them. In the

00:07:03.405 UK, for example, most mobile networks assign

00:07:06.579 IPv6 addresses to smartphones on their networks

00:07:09.753 by default, but many residential ISPs and

00:07:12.928 businesses still use IPv4. What type of

00:07:16.102 address you get depends on how you

00:07:19.277 connect to the network.

00:07:21.091 Other countries follow similar patterns, with some

00:07:24.365 networks switching wholesale to IPv6, while others

00:07:27.540 remain on IPv4.

 

00:07:30.000 This mixed use of IPv4 and IPv6,

00:07:33.276 with many IPv4 hosts being located behind

00:07:36.453 network address translators, greatly complicates connection establishment.

00:07:39.629 In the IPv4 Internet, peer-to-peer applications must

00:07:42.905 perform a complex process to discover NAT

00:07:46.082 bindings, exchange candidate addresses with their peer,

00:07:49.258 and probe to establish what addresses are

00:07:52.434 usable for a connection.

00:07:54.249 This uses a set of protocols,

00:07:57.072 known as STUN and TURN, and the

00:08:00.248 assistance of a central server with a

00:08:03.424 globally unique public IPv4 address, to detect

00:08:06.601 the presence of network address translation,

00:08:09.323 to determine what type of address translation

00:08:12.499 is being performed, and to derive a

00:08:15.676 set of candidate addresses that can be

00:08:18.852 used for peer to peer communication.

00:08:21.575 The peers use the central server to

00:08:24.751 exchange these candidate addresses with each other,

00:08:27.927 and then follow an algorithm known as

00:08:31.103 Interactive Connectivity Establishment (ICE) to setup direct,

00:08:34.280 low-latency, peer-to-peer flows.

00:08:35.641 This works, most of the time,

00:08:38.464 but is complicated, slow, power hungry,

00:08:41.186 and generates a lot of otherwise unnecessary

00:08:44.362 traffic – and all because there aren’t

00:08:47.539 enough IPv4 addresses.

 

00:08:50.000 The fix, of course, is to move

00:08:53.290 to IPv6.

00:08:54.201 Unfortunately, the move to IPv6 will take

00:08:57.490 a long time. And, while it’s happening,

00:09:00.680 there will be some devices, and some

00:09:03.869 networks, that support only IPv4, some that

00:09:07.059 support only IPv6, and some that support

00:09:10.248 both.

00:09:11.454 To reach users on both IPv4 and

00:09:14.744 IPv6, popular services tend to be hosted

00:09:17.933 on servers that have both IPv4 and

00:09:21.123 IPv6 addresses. This is known as dual

00:09:24.312 stack hosting, and it further encourages centralisation

00:09:27.502 onto large hosting providers with the resources

00:09:30.691 to provide both types of address.

00:09:33.425 To get good performance, clients must try

00:09:36.715 to connect using both IPv4 and IPv6

00:09:39.904 addresses simultaneously, or near simultaneously. This further

00:09:43.094 complicates connection setup, making it harder to

00:09:46.283 write networks applications.

 

00:09:48.000 The Internet Protocols are designed such that

00:09:50.907 IP addresses encode the location of a

00:09:53.714 network interface within the network. An IP

00:09:56.521 address does not represent a device,

00:09:58.927 it represents a location where a device

00:10:01.733 can attach to the network.

00:10:03.738 If a device is attached to the

00:10:06.645 network via a wireless connection, but moves

00:10:09.452 so that it changes the WiFi or

00:10:12.259 4G base station to which it connects,

00:10:15.066 then that device will be assigned a

00:10:17.873 new IP address.

00:10:19.076 This has some privacy benefits, but makes

00:10:21.983 it difficult to maintain long-lived connections with

00:10:24.790 that device. For example, TCP connections fail,

00:10:27.597 and must be re-established by the application,

00:10:30.403 when a device moves. And UDP applications

00:10:33.210 need to coordinate with their peers to

00:10:36.017 change the IP addresses they use for

00:10:38.824 communication.

00:10:39.975 Applications that want to maintain long-lived connections,

00:10:42.882 or that want to accept incoming connections,

00:10:45.689 must deal with the complexity of changing

00:10:48.496 IP addresses, and the need to signal

00:10:51.303 such changes to their peers and reestablish

00:10:54.110 connections as the device moves.

00:10:56.115 Something has to keep track of where

00:10:59.021 each device is, in order to route

00:11:01.828 traffic. The assumptions in the design of

00:11:04.635 the Internet mean that complexity is visible

00:11:07.442 to applications, rather than hidden inside the

00:11:10.249 network.

 

00:11:11.000 As we’ve seen, several aspects of the

00:11:14.324 Internet’s design push toward centralisation.

00:11:16.627 The network topology is gradually flattening,

00:11:19.491 and moving away from a complex mesh

00:11:22.715 of peer connections, towards a hub-and-spoke model

00:11:25.939 centred around a small number of large,

00:11:29.164 centralised, services, directly connecting to so-called “eyeball”

00:11:32.388 networks, at the edge of the network,

00:11:35.612 where consumers of those services live.

00:11:38.376 This enables the set of hypergiant content

00:11:41.700 providers, including Google, Facebook, Amazon, Akamai,

00:11:44.464 Apple, Netflix, and so on, to dominate,

00:11:47.688 and makes it difficult for new competitors

00:11:50.912 to gain a foothold. This has implications

00:11:54.136 for network neutrality, competition, and innovation.

 

00:11:58.000 We’re also seeing steady growth of real-time

00:12:01.077 traffic, with streaming video being, by far,

00:12:04.055 the dominant type of traffic in the

00:12:07.032 network – while still growing at ~40%

00:12:10.009 year on year.

00:12:11.285 Streaming video has reasonably strict timing and

00:12:14.362 quality constraints, and these push network operators

00:12:17.340 to improve the quality of their networks,

00:12:20.317 and push streaming video content providers to

00:12:23.294 peer directly with the residential edge networks.

00:12:26.271 This is a further incentive towards centralisation,

00:12:29.249 the flattening of the network, since such

00:12:32.226 direct peerings make it easier to ensure

00:12:35.203 high-quality video is delivered to viewers.

00:12:37.755 Increases in streaming video are also driving

00:12:40.832 changes in TCP congestion control, such as

00:12:43.810 Google’s BBR algorithm, and the development of

00:12:46.787 TCP replacements, such as QUIC, both aimed

00:12:49.764 at reducing latency and increasing quality.

00:12:52.316 Lectures 4 and 5 talk will about

00:12:55.294 some of these developments.

00:12:56.995 WebRTC-based video conferencing services, such as Zoom,

00:13:00.072 Webex, and Microsoft Teams, have even stricter

00:13:03.049 latency requirements.

 

00:13:05.000 The COVID-19 pandemic has accelerated these effects.

00:13:08.345 This graph shows measurement of Internet traffic

00:13:11.691 as the initial lockdowns started in March

00:13:14.936 2020. It shows that many residential networks

00:13:18.182 saw a 20-25% increase in the total

00:13:21.427 amount of Internet traffic they were carrying,

00:13:24.673 as people shifted to working from home.

00:13:27.918 It also shows a corresponding drop in

00:13:31.164 mobile traffic.

00:13:32.091 That shift in traffic wasn’t evenly distributed,

00:13:35.436 though.

 

00:13:37.000 This second graph shows the amount of

00:13:40.090 video data being sent over Webex,

00:13:42.653 one of the popular video conferencing platforms,

00:13:45.643 over a similar time period. Usage grew

00:13:48.633 by around a factor of 20 in

00:13:51.623 less than a week, and has continued

00:13:54.614 growing since.

00:13:55.468 Impressively, the Internet was flexible and robust

00:13:58.558 enough to support this rapid change in

00:14:01.548 how it is used.

00:14:03.257 The question is, can we maintain such

00:14:06.347 flexibility while also improving quality? Lectures 6

00:14:09.337 and 7 discuss this topic further.

 

00:14:13.000 The final shift in assumptions has been

00:14:16.191 around security.

00:14:17.074 The Internet Protocols were originally designed to

00:14:20.265 support a research network, with a relatively

00:14:23.355 small set of users, who had reasonably

00:14:26.446 closely aligned goals, and provided little in

00:14:29.537 the way of security.

00:14:31.303 Over the years, the protocols have changed

00:14:34.494 to provide increasingly sophisticated security and protection

00:14:37.585 from attacks. The Edward Snowden revelations accelerated

00:14:40.675 that trend, by increasing awareness of large-scale

00:14:43.766 government surveillance, but the increase in security

00:14:46.857 started before that, in response to hacking

00:14:49.948 and criminal activity.

00:14:51.272 A significant challenge going forward will be

00:14:54.463 in balancing the needs of law enforcement

00:14:57.554 to access and monitor some traffic,

00:15:00.203 in a targeted manner, while preserving privacy

00:15:03.294 and protecting against attackers.

00:15:05.060 We’ll talk about these topics more in

00:15:08.251 Lectures 3, 4, 8, and 9.

 

00:15:12.000 The Internet is now a globally deployed

00:15:15.103 network. Like any large system, it becomes

00:15:18.105 increasingly ossified, increasingly difficult to change,

00:15:20.679 over time.

00:15:21.537 The slow transition from IPv4 to IPv6

00:15:24.640 is one example of this ossification.

00:15:27.214 Another example would be the difficulty in

00:15:30.316 updating TCP, to better support low-latency services

00:15:33.319 and improve performance. The widespread use of

00:15:36.322 NATs, firewalls, and other middleboxes makes such

00:15:39.325 changes surprisingly difficult. We’re now starting to

00:15:42.327 see serious attempts to replace TCP,

00:15:44.901 with protocols such as QUIC that employ

00:15:47.904 pervasive encryption and tunnel over UDP to

00:15:50.907 avoid such interference by the legacy network,

00:15:53.909 but it’s not clear that these will

00:15:56.912 succeed.

00:15:58.091 Finally, we must consider the effects of

00:16:01.194 the push towards centralised services and applications,

00:16:04.196 driven by both technical and business considerations,

00:16:07.199 and whether these are beneficial to consumers

00:16:10.202 and users of the network, or not.

00:16:13.205 Is this shift towards a small number

00:16:16.207 of hypergiant service providers an inevitable consequence

00:16:19.210 of the design of the network,

00:16:21.784 or of the business and regulatory environment,

00:16:24.787 and to what extent should we attempt

00:16:27.789 to influence it through technological change in

00:16:30.792 the network?

 

00:16:32.000 The way the network is used is

00:16:34.890 changing, and the technologies that support that

00:16:37.580 use are necessarily shifting too. The network

00:16:40.269 has become more fragmented, there are more

00:16:42.959 serious security threats, more demanding applications,

00:16:45.265 and some significant shifts in the devices

00:16:47.955 and technologies we use to access the

00:16:50.644 network.

00:16:51.779 In the rest of this course,

00:16:54.184 I want to start to discuss how

00:16:56.874 the protocols that form the Internet are

00:16:59.564 evolving to meet these needs, and to

00:17:02.254 highlight some of the open issues and

00:17:04.944 challenges still to be addressed.

00:17:06.865 We’ll start, in the next lecture,

00:17:09.270 by discussing the increasing fragmentation of the

00:17:11.960 network and its implications for connection establishment.