Fixing incorrect IPv6 routing tables

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:

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Exploring BGP with IPMininet

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:

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Playing with Ethernet Organisation Unique Identifiers

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.

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Exploring static routing with IPMininet

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.

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Observing IPv6 link local addresses

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.

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Alternatives to man pages

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.

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Experimenting with Mininet and IPv6 routes

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.

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TCP's initial congestion window

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.

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We will eventually deprecate IPv4

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.

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Deploying new TCP options takes time

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.

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Even more bandwidth accross the oceans

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.

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Using public-key crypto remains difficult

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.

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Networking notes for readers of Computer Networking - Principles, Protocols and Practice

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.

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Beyond today's add-supported web

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.

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A 100-hops IPv6 wireless mesh

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.

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