Congestion control is one of the key functions of transport protocols such as TCP or QUIC. Students need to get both theoretical and practical understanding on how congestion control algorithms operate.

ssh is the standard technique to connect securely to distant computers. The first version of ssh was designed in the mid-nineties to replace the rsh, rlogin and telnet solutions. Its main advantage for many users is that it provides security protections for the session with a distant server. However, many ssh deployments rely on the Trust on First Use (TOFU) principle. During the first connection with a server, the client assumes that the public key announced by the server is valid and can be trusted. This key is then stored in the client cache and used to validate future connections with the same server. However, this is not the only way to authenticate servers and ssh supports certificates like TLS. In a recent blog post , Mike Malone discusses the benefits of these ssh certificates and why they are deployed by large organizations.

Equal Cost Multipath (ECMP) is a a key feature of IP networks compared to other types of networks. With ECMP, routers can send the packets belonging to different flows over different paths provided that these paths have the same cost. This technique is widely used to spread the load in large networks. Dip Sing describes with a lot of details in a recent blog post how routers spread packets from different flows over different paths.

cURL and tcpdump are among the command-line tools that networking students need to master. Two recent blog posts provide excellent summaries of many use cases of using these tools.

Most IPv4 Internet accesses are behind NAT (Network Address Translation boxes. While students easily learn how NAT translate packets from TCP connections or UDP (https://beta.computer-networking.info/syllabus/default/protocols/udp.html?highlight=udp) flows, the interactions between NAT and the Internet Control Message Protocol is more complex.

With Wi-Fi, cellular and satellite access networks, there is a growing fraction of the Internet traffic which is carried over radio waves. Although a detailed understanding of all the technology behind radio transmission is outside the scope of most networking courses for computer scientists, it is useful to have some basics understanding of the principles behind the transmission of radio signals.

A TCP connection always starts with the three-way handshake. The client sends a SYN packet contains several TCP options including the Maximum Segment Size (MSS) that announces the largest segment that the client agrees to receive. The server provides its own MSS in the SYN+ACK.

The Border Gateway Protocol is probably the most important routing protocols. It allows ISPs and enterprise networks to exchange interdomain routes, but can also used inside enterprise or ISP networks. Much of BGP’s power resides in its flexibility and ability to support various types of filters. Operates use these filters to prevent some routes over others and many other objectives.

Artificial Intelligence tools like ChatGPT sometimes have expected results. Users have tested ChatGPT at a wide range of tasks, from generating code in various programming languages to solving mathematical problems. ChatGPT can also provide more funny results.

The IPv6 deployment continues globally. The measurements carried by Google and others indicate that IPv6 is now widely used in access networks, both fixed and wireless ones. Many home users use both IPv4 and IPv6 without knowing that there are two different versions of IP. In enterprise networks, IPv4 is still the dominant protocol and enterprises have been relunctant to deploy IPv6 with some exceptions.

To cross the oceans, the Internet mainly depends on transoceanic cables that connect different countries across the oceans. A recent study indicates that more than 500 of these cables have been deployed.

Internet Service Providers that serve end users like to use metrics to compare their service with their competitors. For many years, maximum throughput has been the metric of choice among ISPs. When dial-up links were popular a 56 kbps connections made a difference with a 34 kbps one. The same applied with the early deployments of xDSL and cable networks.

Netnews is a distributed bulletin board system that was deployed in the early 1980s. Netnews had an important influence on the Internet and served as a precursor of today’s social networks. Netnews was the platform of choice for major technical announcements. Linus Torvalds announced Linux on netnews, Marc Andreessen announced the first release of the Mosaic browser, …

Energy consumption is a major concern among network operators. While studies show that the total energy consumption of telecommunications networks tend to remain constant despite a growth in subscribers and a higher growth in traffic, operators look at different solutions to minimize their energy consumption.

Cellular and mobile networks play a growing role on today’s Internet with a growing fraction of the traffic produced or consumed in these networks. Every year, Ericsson publishes a mobility report that summarizes the recent statistics about these networks and makes some predictions.

The Request for Comments (RFC) contains all the specifications of the Internet protocols. Since the publication of RFC1 on 7 April 1969, these documents have been published in ASCII format. Compared to word processing tools, ASCII has the main advantage that old documents remain easily readable on any platform fifty years later. Unfortunately, it also means that RFC authors need to sometimes struggle to use ASCII art to prepare figures such as state machines, protocol messages, …

In an early version of TCP, published in RFC761, Jon Postel wrote a paragraph that became known as his robustness principle:

Dynamic web sites often evolve from a simple and not necessarily efficient proof-of-concept to a larger system that needs to serve many customers. Getting dynamic web sites to scale is not always easy and HTTP is rarely the culprit. In a very interesting talk, Willy Tarreau, the author of HA Proxy explores several of these factors with a simple use case.

The Dynamic Host Configuration Protocol (DHCP) is an important protocol from an operation viewpoint since most IP networks contain DHCP servers that allocate IP addresses. Network administrators know that there are sometimes problems with DHCP, but networking textbooks such as Computer Networking: Principles, Protocols and Practice rarely discuss DHCP in lots of details.

The Internet Engineering Task Force continues to develop new extensions and improvements for the Border Gateway Protocol (BGP). BGP is used by more than 80k Internet Service Providers and enterprise networks of various sizes to exchange routing information.

The Border Gateway Protocol (BGP) is probably the most important Internet routing protocol. It is used by more than 80k Internet Service Providers and enterprise networks of various sizes to exchange routing information. BGP enables all Internet routers to obtain routes to reach Internet destinations.

The web revolution started with the HTTP protocol. The first version of HTTP used a single TCP connection to transfer each HTML page. This was not acceptable for the first web pages that contained only text, but became inefficient when JavaScript, images and videos were added to all web pages. HTTP evolved and the main versions are HTTP/1.1 and HTTP/2.0 which uses a binary format and supports multiple streams over a single TCP connection.

Many Ethernet and Wi-Fi Local Area Networks (LAN) in enterprises use 802.1x to verify the user credentials before authorizing them to access the network. 802.1x defines a flexible set of protocols that support various forms of user authorization and authentication and can be used at scale.

Internet Service Providers (ISP) and content providers exchange traffic using private and public Internet peering links. On these peering links, they rely on the Border Gateway Protocol (BGP) to exchange routing information. Various websites provide information about the peering links used by these ISPs, notably PeeringDB. Anurag Bhatia published on twitter an interesting list of the top 25 ISPs with the largest public peering capacity :

Most of the security protocols that are used on the Internet, including TLS, ssh, IPSec and many others use secure hash functions and cryptographic techniques. The designers of these security protocols know that they must support different hash and cryptographic functions because some of them could be declared as insecure or worse broken after some or several years of analysis by cryptographers. The first important hash function was MD5, but it is now deprecated because it is not considered secure anymore. Most security protocols rely on SHA-1 and its descendants.

We are clearly moving to the Internet of Things with more and more devices equipped with network interfaces. While Bluetooth was initially designed to connect small devices like earphones or microphones to smartphones, its low energy consumption has enabled new use cases. One of these use cases are the tracking tags such Apple’s AirTag, Tiles or the trackers useable with Google’s find my device network.

The Computer Networking: Principles, Protocols, and Practice ebook continues to evolve in parallel with Internet technology. During the 2022-2023 academic year, UCLouvain experimented with a new way of organizing the teaching material. Instead of using a top-down or a bottom approach as most networking books do, we divided the course in two parts. The first part covers the protocols that are used by the endhosts and the network is considered as a black box. This part starts with the applications that the students already use. The second part focus on the network infrastructure with the routing protocols and the local area networks.

Ethernet is now the ubiquitous fixed Local Area Network technology. This technology was invented by Bob Metcalfe and David Boggs at the Xerox’s Palo Alto Research Center (PARC) to connect the ALTO workstations and the first laser printers. In 2021, Bob Metcalfe shared a dropbox with several historical documents:

Computer Science textbooks rarely take power consumption into account and the Computer Networking: Principles, Protocols, and Practice ebook is no exception. This is slowly changing as energy consumption is being more and more important in many deployments, even in fixed networks where this was not the case in the past.

Students and readers of the Computer Networking: Principles, Protocols, and Practice ebook who want to stay up-to-date on the evolution of the networking technology should subscribe to the Internet Protocol Journal. The first issue of this journal was published 25 years ago. Initially distributed on paper, it is now mainly available as pdf files from https://ipj.dreamhosters.com/. Each issue contains very interesting tutorial articles that describe new protocols or discuss the evolution of networking technologies.

Students and readers of the Computer Networking: Principles, Protocols, and Practice ebook know that the Domain Name System is one of the key protocols on today’s Internet. A good understanding of the DNS is key to correctly understand how the Internet operates and debug some networking problems.

Students and readers of the Computer Networking: Principles, Protocols, and Practice ebook often ask for exercises to enable them to evaluate their understanding of the different topics discussed in the ebook. Over the years, we have added various exercises on the INGInious. In the framework of their ongoing Master thesis, Clément Linsmeau and Matthieu Leclercq have developed an INGInious extension that uses statistical techniques to evaluate the students using simple questions. Their INGInious suggests a series of about a dozen of exercises to each student to evaluate hir/her knowledge. The exercises that are selected depend on the answers given by each student to each exercise. A brigth student will receive a set of challenging exercises to see whether she/he really masters the course while an average student will receive simpler exercises.

Welcome to the August 2021 edition of the Networking Notes newsletter.

Welcome to the June 2021 edition of the Networking Notes newsletter.

Welcome to the May 2021 edition of the Networking Notes newsletter.

Welcome to the April 2021 edition of the Networking Notes newsletter.

Welcome to the March 2021 edition of the Networking Notes newsletter.

The covid crisis has forced most educators to reconsider how they interact with students using online tools instead of in-class discussions. Many university courses have been reorganised as video podcasts during which the professor explains his slides to passive students. Our spring semester starts in February and when it was clear that I would need to teach the networking course online, I thought about possible solutions to provide a better experience to the students. During the previous months, I had attended some remote lectures where basically the professor was mainly explaining his slides and the students were taking notes and sometimes had to answer questions. I thought that there could be a better approach for the theoretical lessons and the exercise sessions.

Today, most of the telephone traffic is carried over IP networks using Voice over IP technologies. A few decades ago, it was the opposite. The first data connections were made using modems that convert a binary signal into an acoustic signal which can be carried over regular telephone lines.

Latency is one of the important quality of service metrics in IP networks. The latency of a network path depends on various factors :

On November 12th, 1990, Tim Berners Lee and Robert Caillau wrote the first project proposal of what became the web. You can read this historical document from https://www.w3.org/Proposal.html. It’s interesting to see how the web has evolved since this initial proposal and spend some time to try to predict how it would like thirty years from now…

A recent blog post on APNIC describes GRoot, a tool that checks various DNS configuration and zone files for errors. Would be very useful if you need to maintain DNS servers.

This blog aims at encouraging students to continue to explore the networking field after having followed their first networking course. Until November 2020, the blog mainly contained medium size articles that were published as time permitted.

The Transmission Control Protocol (TCP) is one of the most important protocols used on the Internet. The first TCP specification was published in RFC793 in September 1981. During the last 39 years, TCP has been regularly improved without a revision of RFC793. In 2013, Wesley Eddy started to work on rfc793bis. After almost two years, the TCPM IETF working group decided to adopt this effort. Five years later, we now have a consolidated version of the updated version of RFC793.

Routers are an important part of the Internet infrastructure as they carry all the packets that we exchange. Several vendors sell these routers. Some vendors supply different types of routers while others focus on specialised ones such as access routers or backbone routers. Industry analysts often publish market studies that provide the market share of each vendor.

Network engineers sometimes have fun in configuring networks. In 2013, Ryan Weber configured a set of Cisco VRFs to return a Star Wars story when running traceroute towards a specific IP address. You can find some technical information in an article published by The Register and a youtube video.

Computer networks and the TCP/IP protocol suite continue to evolve. This blog posts highlights some recent news and scientific articles which could be of interest for readers of Computer Networking: Principles, Protocols and Practice.

The TCP/IP protocol suite has been implemented on a wide range of devices, ranging from embedded systems to supercomputers. Besides the classical TCP/IP stacks, there are specialized stacks that are used on specific devices. mTCP is an example of such stacks. This software runs on older PCs that are still running MS-DOS or FreeDOS. The stack runs on a 35 years old IBM PC Junior that is reachable from http://50.125.82.27:8088/mtcp/

Most computer networking classes in universities include lab sessions and projects that enable students to learn how protocols are used in the real worl. For almost a decade, I’ve asked the students who follow the networking course at UCLouvain to analyze one web site. This project idea was suggested by Gisueppe Di Battista who also teaches computer networking at Roma III university.

Transport Layer Security (TLS) is used by a growing number of applications in today’s Internet. While updating the TLS chapter of the third editio of Computer Networking: Principles, Protocols and Practice , I had to look at interesting articles on TLS and version 1.3 in particular. Some of them could be very useful for readers of the e-book:

It is difficult to exactly track the first steps of the networking technology that laid the foundation for today’s Internet, but most experts consider the ARPART as the major ancestor to our current Internet. This network was funded by DARPA, the Advanced Research Project Agency of the US Department of Defense. The first experiments started in the late 1960s with the installation of a few ARPANET nodes in US universities and labs.

Passwords are used for a wide range of services. Every time I explain passwords to students, I strongly encourage them to never, ever design a software that stores passwords as clear text inside a file. Storing clear text passwords is a recipe for disaster. We’ve seen dozen myriads of websites having their “secure” password file being hacked. Since the publication o Password security: a case history every computer scientist should know that passwords should be hashed before being stored in a file, even if the file is protected by strict permissions. This is the default solution on Unix since the early days and has been adopted by all derivatives.

Many network experts consider the beginning of the ARPANet network in the US as the first days of the global network that we call the Internet today. The first ARPANet nodes were installed during the latest months of 1969. The Internet turns 50 this year. At the same time, the first networking researchers agreed to create a working group that later between ACM’s Special Interest Group in Datacommunications (SIGCOMM). Many of the articles that have influenced the development of computer networks and the Internet in particular have been published by ACM SIGCOMM. Computer Communication Review recently published a special issue that includes technical articles that summarise the evolution of computer networks. Several of these articles are particularly interesting for networking students:

The first version of the Computer Networking: Principles, Protocols and Practice was written almost ten years ago. At that time, my main objective was to provide a textbook that students could use to learn the networking concepts that I considered important for a first undergraduate course in computer networking. I tried to cover these concepts in details. I also provided references that students could read to get even more details.

Each new version of the Linux kernel brings new features. Version 5.3 includes a small change to the handling of IPv4 packets. IPv4 addresses are running out and some network engineers are trying to recover as much IPv4 addresses are possible.

When network engineers analyze log files, collect packets or observe traceroute data, they sometimes want to know the AS that announces a given IP address. This information can be extracted from BGP routing tables to by using services such as RIPE RIS or RouteViews. There is now an interesting alternative with the https://iptoasn.com website that provides both files containing the mappings between IP addresses and AS numbers and an API to retrieve this mapping.

In computer networks, bandwidth is usually measured in bits/sec. This metric reflects that amount of information which can be exchanged over a given network. The Morse code used by early telegraph systems and amateur radio operators among others operates at roughly 40 words per minute, which corresponds to 56 bits per second.

While collecting some DNS packet traces to prepare new DNS exercises for the students, I was surprised to notice that my Linux host was regularly doing DNS requests for [connectivity-check.ubuntu.com]

Many entreprise networks restrict the applications that users can use by blocking some TCP and UDP ports at the entreprise firewalls. This happens as well in campus networks. To cope with these restrictions, some applications, notably those running on smartphones, have moved to the well known and usually open HTTP or HTTPS ports. Over the years, firewalls have evolved. instad of simply looking at port numbers, most of today’s firewalls inspect the packets exchanged over a connection to ensure that HTTP is used on port 80 and TLS on port 443. If this is not the case, the connection is considered to be suspicious and blocked.

Web browsers are probably one of the most widely used applications on desktops and smartphones. While Google Chrome is currently the dominant web browser, users can decide to install different web browsers to improve their experience or for other reasons like privacy concerns, performance issues, …

In less than 20 years, Transport Layer Security (TLS) moved from a niche protocol that was only used by banks and e-commerce websites to almost a default solution. Today, a growing fraction of the Internet traffic is encrypted by using TLS or similar protocols. This encryption and the associated authentication improve the security of Internet users since attackers cannot observe or modify the packets that they exchange.

Network Interface Cards (NIC) play an important role in the protocol stack since contain all the hardware functions that are required to transmit and receive packets. In the early days, the NICs mainly implemented the physical layer and a fraction of the datalink layer (e.g. CSMA/CD for Ethernet or CSMA/CA for Wi-Fi). Over the years, a variety of functions have been added to the NICs, starting from the computation of the datalink layer checksums and CRCs. Then they have also be capable of fragmentation packets and even splitting large TCP segments in a series of IP packets. Some NICs can offload cryptographic computations for TLS or IPSec and the latest generation of NICs are fully programmables.

Transport Layer Security is a key part of the protocol stack. During the last years, a lot of effort have been invested in creating version 1.3 of this important protocol. TLS 1.3 RFC8446 was published in August 2018. In contrast with some protocols that are specified and then implemented, the TLS 1.3 implementations were written in parallel with the specification work and several operational issues influenced the protocol design. When RFC8446 was published, several TLS 1.3 implementations were available and large companies have quickly deployed it on clients and servers. On server side, RedHat has enabled TLS 1.3 in their latest Linux distribution. On the client side, Apple has enabled TLS 1.3 in March 2019 on both iOS and macOS.

One of the main advantages of IPv6 is that it uses 128 bits long addresses. The 64 high order bits of the address are used to identify the subnet while the low order 64 bits are reserved for the host identifier. This host identifier can be configured manually, allocated by DHCPv6 or auto-configured using the Stateless Address Autoconfiguration (SLAAC). SLAAC evolved over the years. The first versions, RFC1971, RFC2462 RFC4862 mainly computed the low order 64 bits of the address from the MAC address of the endhost. However, this utilisation of a stable identifier raised privacy concerns as a host would use the same low order bits in any IPv6 address that it generates RFC3041. Today’s stack implement the privacy extensions defined in RFC4941 and generate random identifiers that are used as the low order 64 bits of the IPv6 addresses that they generate.

One of the benefits of the Internet is that it lowers the cost of communicating between distant users. Today, anyone takes for granted that it is possible to send an email or an instant message to any other Internet user. All these exchanges can be done at a very low cost, which enabled a wide range of activities that would have not been possible without the Internet. Unfortunately, the ridiculously low cost of sending information to any Internet user has attracted a range of fraudulent users that send unsolicited commercial or phishing messages.

Securing network protocols remains a difficult task as illustrated by the KNOB Attack that was recently announced on a wide range of Bluetooth devices. Bluetooth is a widely used short-range wireless technology that is used to connect devices such as keyboard, mouse, headphones to computers. It also used to directly exchange data between mobile devices such as smartphones. It is also possible to use a Bluetooth link to exchange IP packets. The development of Bluetooth started almost thirty years ago and the first devices appeared 20 years ago.

TCP is today the most widely used transport protocol. It was defined in RFC793. It provides a connection-oriented bi-directional bytestream service on top of the unreliable IP layer. A TCP connection always starts with a three-way handshake. During this handshake, the client and the server can use TCP options to negotiate the utilisation of TCP extensions. These TCP options are also used to exchange a key parameter of a TCP connection: the Maximum Segment Size (MSS).

The Domain Name System is one of the key protocols in today’s Internet as it allows to map names onto IP addresses. Most networking students probably see DNS as the typical request-response application running over UDP. To retrieve the DNS record corresponding to a given name, a client sends a DNS request to its resolver inside a single UDP packet. DNS was defined in RFC1035. Over the years, the protocol has evolved have several extensions have been added. Several IETF working groups have been targeted around the Domain Name System:

Email is one of the oldest applications on the Internet. Initially, email was used to deliver emails between two Internet hosts by using the SMTP protocol defined in RFC821 and updated several times after. SMTP is used to exchange emails between servers. With the proliferation of client devices such as PCs, and later laptops or smartphones, several protocols have defined to enable clients to retrieve emails from servers. The most popular are POP and IMAP. These two protocols are classical ASCII-based protocols where clients and servers exchanged commands encoded as one ASCII line over a TCP connection.

TCP Fast Open is a TCP extension that enables client and servers to place data inside the SYN and SYN+ACK packets during the three-way handshake. This extension has been pushed by google to speedup short transfers. It is defined in RFC7413 and its Linux implementation is described in a nice LWN.net article.

The large size of the IPv6 addresses enables unexpected use cases and nice demos. Using a Raspberry PI, Markus Klock has designed an open IPv6 board where anyone can write short messages by sending IPv6 ICMP packets. The raspberry listens to IPv6 prefix 2001:6b0:1001:105::/64 and captures any packet sent to this prefix. To write a message on the board, simply encode the ASCII characters as the low order 64 bits of the address that you ping.

For many readers, ebooks are a simple variant of the traditionnal books that we are used to since Gutenberg and before. As a book owner, I am free to use the book as I want, share it with others, … Unfortunately, many ebooks are different. Companies like Amazon with the kindle, Apple with iBooks, Adobe, Microsoft and others consider that an ebook is a software product that is subject to a license. The ebook license allows you to read the ebook, take notes, … However, these licenses are not always perpetual. In April 2019, Microsoft announced that they stopped their ebook business, probably because it was not profitable enough and planned to stop the licensing servers for their ebooks in July 2019. Microsoft announced some refunds, but all ebook readers are now warned. Their preferred ebook might stop to be available due to a decision of the ebook publisher.

The Transport Layer Security (TLS) protocol plays a more and more important role in today’s Internet. It secures wbesites, but also mail servers and a wide range of other services. Like many security protocols, TLS can be configured in very different ways and a minor change in a configuration file might have an important impact on the security of a TLS deployment. Another factor that contributes to the complexity of configuring TLS is that there are different implementations of this protocol that can be integrated in very different web servers. Each web server uses its own configuration file and different web servers use different parameters.

In the early days of the web, the google web crawler that crawls public web pages to build the web index used by the search engine received complains from webmasters who did not agree to let google index their website or were concerned by the load that these crawlers put on their infrastructure. To cope with this problem, google engineers developed a simple text file called robots.txt which can be used by webmasters to specify the parts of the web site that can be crawled and by which crawler. Over the years, the robots.txt file evolved, but its format had never been formally specified despite its widespread usage. 25 years later, google fills this gap by releasing an Internet draft that eventually describes the format of this text file. It is likely that the IETF will discuss minor details on this document in the coming years or maybe months before accepting it.

The performance of the web protocols has been heavily optimised during the last decades. Given the importance of smartphones and mobile data networks, many web sites try to offer the best performance for mobile users. Many factors can influence the performance of web sites. One of the classical benchmark is the Page Load Time (PLT), i.e. the time required to completely dowload a web page. Many studies has analysed the PLT of web sites and proposed techniques to reduce it in specific scenarios. In a recently published paper, a group of researchers led by Mohammad Rajiullah has analysed a large dataset of measurements performed in mobile networks throughout Europe using the MONROE testbed. They summarise their findings in a paper entitled Web Experience in Mobile Networks: Lessons from Two Million Page Visits that they have recently presented at the Web Conference.

The HTTP protocol defines several error messages which can be returned by a server in response to a client request. Some webmasters have tuned their servers to generate special error messages under specific conditions. The French car manufacturer Peugeot has gone one step further by returning an image of their historial Peugeot 404 car when you request an invalid URL such as https://www.peugeot.fr/cnp3…

The Domain Name System (DNS) has historically been mainly used over UDP. When the DNS was designed, UDP provided a much lower latency than TCP. Over the years, notably with the utilisation of DNS extensions such as DNSSec, some DNS queries moved to TCP, but this remained the exception.

The security of interdomain routing has been an important operational problem for many years and several BGP errors have affected the global Internet, including the famous AS7007 incident or the Youtube hijack. Many smaller BGP errors or misconfigurations occur regularly as shown in a blog post by Andrei Robachevsky.

Measurements always reveal new insights or unexpected results about the system under study. This applies to a wide range of systems including the Internet. Two recent scientific articles provide unexpected results about two very different protocols: ICMP and BGP.

curl is one of the most versatile implementation of a wide range of application-layer networking protocols. It started as an open-source project twenty years ago and is now used on billions of devices. In a recent post on stackoverflow, Daniel Stenberg explains why he developed this project.

Most of the Internet protocols have been document in Requests For Comments (RFCs). Initially, these documents were simply a set of notes that were exchanged among networking researchers. The first of these RFCs was published on April 7th, 1969, 50 years ago.

During the last thirty years, computer networks and the Internet in particular evolved from a niche technology that was only known by few scientists into a mainstream technology that affects a large fraction of our society. Thirty years ago, Ethernet was already deployed in universities and many of them started to be connected to the Internet with bandwidth that would be considered as ridiculous today. In parallel, the number of users who have been able to access the Internet has grown tremedously and the Internet is slowly being considered as important as electricity in many countries.

The IPv4 addressing space was initially designed as being divided in four classes of addresses. RFC791 clearly defined those classes of addresses as follows:

In the early days of the Internet, governments considered it as a strange research experiment and did not really bother to try to understand how it worked. During the last years, governments all over the world have made more and more efforts to try to control its utilization. The numbers of laws that affect the Internet consider to grow and various governments restrict the utilization of the Internet. It is impossible to list all government interferences that affect the Internet, but here are a few notable ones.

The Border Gateway Protocol (BGP) is probably the most important routing protocol in today’s Internet. Its main role is the exchange of interdomain routes, but it also plays a key role inside ISP networks to support various services. This post provides pointers to recent articles and blog posts that are directly related to the evolution of BGP and could be of interest to the readers of Computer Networking : Principles, Protocols and Practice.

The Transport Layer Security protocol (TLS) plays a growing role in today’s Internet by securing many key application layer protocols. Since the publication of version 1.3 of TLS in RFC8446, it is interesting to track the deployment of this new version. This post provides pointers to recent articles and blog posts that are directly related to the evolution of TLS and could be of interest to the readers of Computer Networking : Principles, Protocols and Practice.

Wi-Fi and Ethernet adapters contain a unique MAC address that they use to when exchanging frames in the LAN. These addresses are assigned by IEEE to each manufacturer that is supposed to configure each adapter with a unique address. Everytime you use a laptop, smartphone, tablet or Wi-Fi equipped device, it sends frames with its unique MAC address. These MAC addresses do not leave the LAN where they are used, but they are used by services such as DHCP to allocate addresses. Some of these services log the MAC addresses that they have seen for security reasons.

The Internet is a dynamic system that continuously evolves. This evolution can be observed from several vantage points. A recent article, entitled Five Years at the Edge: Watching Internet from the ISP Network and co-authored by Martino Trevisan and his colleagues from Politechnico di Torino provides an unusual and very interesting perpective on the evolution of Internet traffic. This paper was presented last week at the Conext’2018 and can be downloaded from the conference program page

Networking students can learn a lot about Internet protocols by analyzing how they are actually deployed. For several years, Computer Science students at UCLouvain have analyzed different websites within their introductory networking course. This project considers several key Internet protocols, DNS, HTTP, TLS and TCP. In this post, we briefly analyze how TCP is used on some web sites as a starting point for these students.

Networking students can learn a lot about Internet protocols by analyzing how they are actually deployed. For several years, Computer Science students at UCLouvain have analyzed different websites within their introductory networking course. This project considers several key Internet protocols, DNS, HTTP, TLS and TCP. In this post, we briefly analyze how TLS is used on some websites as a starting point for these students.

Networking students can learn a lot about Internet protocols by analyzing how they are actually deployed. For several years, Computer Science students at UCLouvain have analyzed different websites within their introductory networking course. This project considers several key Internet protocols, DNS, HTTP, TLS and TCP. In this post, we briefly analyze how HTTP is used on some websites as a starting point for these students.

The Border Gateway Protocol (BGP) is an important protocol in today’s Internet. As such, it is part of the standard networking textbooks. At UCLouvain, timing constraints force me to explain BGP in two different courses. The students learn the basics of external BGP within the introductory networking course that is mandatory for all CS students. We mainly cover routing policies (customer provider and shared-cost peerings) and the basics of eBGP with the utilisation of the AS-Path and the local-pref attribute. Some students register for the advanced networking course that covers BGP in more details, MPLS, VPNs, multicast and other advanced topics.

During the last summer, we brainstormed about new types of exercises that could help students to better understand different networking topics. For these new exercices, we wanted to leverage the INGINIOUS code grading platform that is being developed at UCLouvain. The UDP socket and TCP socket exercises are good examples of the benefits of a flexible code grading platform like INGINIOUS.

During their first networking course, each CS student at UCLouvain writes a four-pages report that analyses the organisation of a popular web site and the optimisations or sometimes the errors that the maintainers of this website have made when configuring their DNS, HTTP, TLS or protocols TCP. This project lasts one month and the students receive every week guidelines and suggestions on how to carry their analysis. Here are a few examples which can be used to bootstrap the DNS analysis of such a website.

A growing fraction of our webservers are now reachable via https instead of http. With the http scheme, all the information is transported in plain, including the HTTP headers, cookies, web pages and other sensitive information. For many years, https, which combines http with Transport Layer Security, has been restricted to sensitive web sites such as those that require a password or e-commerce. During the last five years, the deployment of https changed significantly. Today, Mozilla’s telemetry reports that roughly 80% of the webpages downloaded by firefox users are served over https.

The TCP Timestamp option was proposed in RFC1323 in 1992 at the same time as the Window Scale option. There were two motivations for the initial TCP Timestamp option : improving round-trip-time estimation and protecting agains wrapped sequence numbers (PAWS). By adding timestamps to each packets, it becomes easier to estimate round-trip-times, especially when packets are lost because retransmissions of a packet carry different timestamps. The PAWS mechanism is less well understood. It is a direct consequence of the utilisation of 32 bits sequence numbers in TCP. TCP RFC793 was designed under the assumption that the IP layer guarantees that a packet will not live in the network for more than 2 minutes (the Maximum Segment Lifetime). TCP’s reliable transmission can be guaranteed provided that it does not use the same sequence number for different packets within MSL seconds. In 1981, with 32 bits sequence numbers, nobody thought that reusing the same sequence number over 2 minutes would become a problem. Today, this is a reality, even in wide area networks. PAWS RFC7323 solves this problem by using timestamps to detects spurious packets and prevent from problems where old packets are delayed within MSL seconds. It took more than a decade to reach a significant deployment of RFC1323.

In a previous post, we have described a first INGINIOUS exercise that enables students to check their understanding of the utilisation of the socket API with UDP. This API is more frequently used to interact with TCP. Interacting correctly with TCP is more challenging than interacting correctly with UDP. As TCP provides a reliable, connection-oriented, bystream service, there are several subtleties that the students need to consider to write code that interacts correctly with the TCP socket API.

Created in the early days of the TCP/IP protocol suite, the socket API remains the standard low-level API to interact with the underlying networking stack. Despite its age, it remains widely used and most networking students are exposed to it during their studies. Although more recent languages and higher-level APIs can simplify the interactions between applications and the networking stack, it remains important for students to understand its operation. At UCLouvain, we aks the students to write a simple transport protocol in C over UDP. This enables them to understand how to parse packets, but also how to manage timers and how to interact with the socket API.

Dropbox is a very popular file sharing service. Many users rely on its infrastructure to store large files, perform backups or share files. Like other commercial services such as Apple’s iCloud or Microsoft’s OneDrive, Dropbox uses a proprietary protocol to exchange information between client applications and its servers. The most detailed description of Dropbox’s protocol was published in Inside Dropbox: Understanding Personal Cloud Storage Services. This paper appeared in 2012 and it is unfortunately very likely that Dropbox’s protocols and architecture have evolved since then.

Various types of carreers are possible in the networking business. Some develop new applications, others deploy network services or manage enterprise networks. Most of the people who are active in the field work in established organisations that already have a running network. Some decide to create their own business or their own company. The same happens when considering Internet Service Providers. Most of the existing ISPs were created almost twenty years ago. While it is more difficult to launch an ISP business today that when the Internet was booming, there are still new ISPs that are created from scratch. In a series of two blog posts, Chris Hacken discusses many of the technical bareers that exist in this type of businesses. There are very few documents that describe those business, practical and operational issues.

Last week’s information about the breach that affected 50 millions of facebook users has been largely discussed in the press (see techcrunch, wired or eff.org).

A recent post on twitter shared a Swedish website that briefly describes how to disable IPv4 on Windows (see below), Linux and MacOS.

When IP routers forward packets, they inspect their destination address to determine the outgoing interface or the next step router towards the packet’s destination. Given this, a simple router does not need to look at the source address of the packets. The source address is mainly used by the destination to send the return packets or by intermediate routers to generate ICMP messages when problems are detected. This assumption was true in the the early days of the Internet and most routers only looked at destination addresses.

Internet protocols continue to be used in a variety of scenarios that go beyond the initial objectives of the TCP/IP protocol suite. Two recent scientific articles provide an insight at the performance of TCP/IP in challenging environments.

The Domain Name System is one on the venerable Internet protocols like IP or TCP. For performance reasons, the DNS protocol is usually used on top of UDP. This enables clients to send their DNS request in a single message to which the servers reply in a single message as well. Both the request and the response are sent in plain text, which raises obvious security and privacy concerns. Many of these are documented in RFC7626. In a recent Usenix Security article, B. Liu et al. revealed that 259 of the 3,047 ASes where they could perform measurements used some form of DNS interception. The IETF has explored several solutions to secure the information exchanged between DNS clients and servers. RFC7858 and [RFC8310] have specified solutions to transport DNS over TLS and DTLS. Some public resolvers already support these extensions. Apparently, Android P also supports it. Geoff Huston published an interesting blog post that compares different DNS securisation techniques.

There are many ways to wire Ethernet networks. When students create simple labs with a few cables and a few switches and a few hosts, they simply plug any suitable cable and run their experiment. In real networks, a good wiring strategy can help to avoid lots of problems and lost time debugging those problems.

Networks are composed of cables and equipment whose normal utilisation is sometimes disrupted by animals that view them from a different angle than humans. Fiber optic cables that are laid under the sea to connect continents attract a variety of animals. Sharks can be attracted by the shape of the cable or the magnetic fields that they emit. One of these cable biting sharks has even been stopped by under the sea surveillance cameras…

Internet protocols have traditionally been clear-text protocols and many protocols like SMTP or HTTP could be tested by using a simple telnet session. This feature was very handy when testing or debugging protocol implementations. However, it is difficult to implement a correct parser for plaintext protocols and many of these parsers have suffered from bugs. Binary protocols have a more precise syntax and are thus easier to parse at least if they do not contain lots of extensibility. All Internet security protocols including IPSec, TLS or ssh are binary protocols. With the Snowden revelations, the IETF has strongly encouraged the utilisation of security protocols to counter pervasive monitoring as explained in RFC7258.

Despite its age, TCP continues to evolve and the existing TCP implementations continue to be improved. Some recent blog posts provide useful information about the evolution of TCP in the wild.

There is a wide variety of file systems that store files on remote servers. NFS is very popular in the Unix world while Samba allows Windows clients to store files on Unix servers. Besides those regular file systems, some networkers have developed special file systems that use or abuse popular Internet protocols. A first example is pingfs, a filesystem that relies on ICMP request/response packets sent by the popular ping software to “store” information inside the network itself. To store a file, pingfs needs to split it in packets that are sent on a regular basis to remote hosts that return ICMP messages. This file is then “stored” as packets that are flying through the network but the entire file does not reside on a disk somewhere.

Students have sometimes difficulties to understand how IPv6 static routes work. A typical exam question to check their ability at understanding IPv6 static routes is to prepare a simple network containing static routes that have been incorrectly specified. Here is a simple IPMininet example network with four routers and two hosts:

IPMininet supports various routing protocols. In this post, we use it to study how the Border Gateway Protocol operates in a simple network containing only BGP routers. Our virtual lab contains four routers and four hosts:

Ethernet remains the mostly widely used LAN technology. Since the invention of Ethernet in the early 1970s, the only part of the specification that remains unchanged is the format of the addresses. Ethernet was the first Local Area Network technology to introduce 48 bits long addresses. These addresses, sometimes called MAC addresses, are divided in two parts. The high order bits contain an Organisation Unique Identifier which identifies a company or organisation. Any organisation can register a OUI from which it can allocate Ethernet addresses. Most OUIs identify companies selling networking equipment, but there are a few exceptions.

In this post, we continue our exploration of using IPMininet to prepare exercises that enable students to discover IPv6 routing. Our focus is now on OSPFv3 defined in RFC5340. We consider a simple network that contains four routers and two hosts. The network is created by the ospfv3_example.py script from the Routing Examples project.

In a previous post we have shown that IPMininet can be used to develop exercises that enable students to explore how IPv6 routers forward packets. We used a simple example with only three routers and very simple static routes. In this post, we build a larger network and introduce different static routes on the main routers. Our IPMininet network contains two hosts and five routers.

Link local addresses play an important role in IPv6 since they enable hosts that are attached to the same subnet to directly exchange packets without requiring any configuration. When an IPv6 host or router boots, the first thing that it tries to do is to create a link-local address for each of its interfaces. It is interesting to observe how those link-local addresses are used in a very simple network.

When I discovered Unix as a student, one of its most impressive features was the availability of the entire documentation through the man command. Compared with the other computers that I had use before, this online and searchable documentation was a major change. These Unix computers were also connected to the Internet, but the entire university had a few tens of kilobits per second of bandwidth and the Internet was not as interactive as it is today.

When students discover IPv6, they usually start playing with static routes to understand how routing tables are built. At UCL, we’ve used a variety of techniques to let the students understand routing tables. A first approach is to simply use the blackboard and let the students analyse routing tables and explain how packets will be forwarded in a given network. This works well, but students often ask for additional exercises to practice before the exam. Another approach is to use netkit. netkit was designed by researchers at Roma3 University as an experimental learning tool. It relies on User Mode Linux to run a Linux kernels as processes on a virtual machine. Several student labs were provided by the netkit authors. We have used it in the past, but the project does not seem to make progress anymore. A third approach is to use Mininet. Mininet is an emulation framework developed at Stanford University that leverages the namespaces features of recent Linux kernel. With those features, a single Linux kernel can support a variety of routers and hosts interconnected by virtual links. Mininet has been used by various universities as an educational tool, but unfortunately it was designed with IPv4 in mind while Computer Networking : Principles, Protocols and Practice has been focussed on IPv6.

TCP’s initial congestion window is a key performance factor for short TCP connections. For many years, the initial value of the congestion window was set to less than two segments RFC2581. In 2002, RFC3390 proposed to increase this value up to 4 segments. This conservative value was a compromise between a fast startup of the TCP connection and preventing congestion collapse. In 2010, Nandita Dukkipati and her colleagues argued in An Argument for Increasing TCP’s Initial Congestion Window for increasing this initial value and demonstrated its benefits on google servers. After the publication of this article, and a patch to include this modification in the Linux kernel, it took only three years for the IETF to adopt the change in RFC6928.

IPv4 has been a huge success that goes beyond the dreams of its inventors. However, the IPv4 addressing space is far too small to cope with all the needs for Internet connected hosts. IPv6 is slowly replacing IPv4 and deployment continues. The plot below shows the growth in the number of IPv6 browsers worldwide.

TCP is an extensible protocol. Since the publication of RFC793, various TCP extensions have been proposed, specified and eventually deployed. When looking at the deployment of TCP extensions, one needs to distinguish between the extensions that provide benefits once implemented on senders and receivers and the implementations that need to be supported by both client and servers to be actually used.

Fiber optics play a key role in Wide Area Networks. With very small exceptions, most of the links that compose WANs are composed of optical fibers. As the demand for bandwidth continues to grow, network operators and large cloud companies continue to deploy new optical fiber links, both on the ground and accross the oceans. The latest announcement came from Microsoft and Facebook. Together, they have commissioned a new fiber optical link between Virginia Beach, Virginia (USA) and Bilbao, Spain. The landing points chosen for this fiber are a bit unusual since many of the fiber optic cables that cross the Atlantic Ocean land in the UK for obvious geographical reasons. This new cable brings 160 Terabits/sec of capacity and adds diversity to the fiber routes between America and Europe. This diveristy is beneficial against unexpected failures but also against organisations that captures Internet traffic by tapping optical fibers as revealed by Edward Snowden.

Pretty Good Privacy, released in 1991, was probably one of the first software packages to make public-key cryptography available for regular users. Until then, crytography was mainly used by banks, soldiers and researchers. Public-key cryptography is a very powerful technique that plays a key role in securing the Internet. Despite of its importance, we still face issues to deploy it to all Internet users. The recent release of Adobe security team’s private key on a public web page is one example of this difficulty, but by far not the only one.

Networking education has changed a lot during the last twenty years. When a was still a student, before the invention of the web, students learned most from the explanations of their professors and teachning assistants. Additional information was available in scientific librairies, but few students could access it. Today’s students live in a completely different world. Computer networks and the Internet in particular have completely changed our society. Students have access to much more information that I could imagine when I was a student. Wikipedia provides lots of useful information, Internet drafts, RFCs and many scientific articles and open-source software are within the reach of all students provided that they understand the basics that enable them to navigate through this deluge of information.

The web was designed in the 20th century as a decentralised technique to freely share information. The initial audience for the web protocols were scientific researchers who needed to share scientific documents. HTTP was designed as a stateless protocol and Netscape added HTTP cookies to ease e-commerce. These cookies play a crucial role in today’s ad-supported Internet. They have also enabled companies like Google or Facebook to collect huge amount of data about the browsing habits of almost all Internet users in order to deliver targeted advertisements.

A recent study released by Adobe provides many data points about the utilisation of mobile devices (smartphones, tablets) to access websites.

IPv6 is used for a variety of services. Wireless mesh networks are networks were routers use wireless links between themselves. This blog post describes such a large mesh network and provides several experiments conducted over it.