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InfoQ Homepage News Computer Networks: Myths, Missteps, and Mysteries - Radia Perlman at QCon London

Computer Networks: Myths, Missteps, and Mysteries - Radia Perlman at QCon London

Radia Perlman, EMC Fellow and one of the pioneers of early network design, presented a keynote at QCon London that explored how networking protocols and technologies have evolved to become today’s Internet. In her talk, she answered some common questions (e.g. Why do we need both Ethernet and IP?) and explored how things might have looked if they were designed today.

Perlman began by stating that she doesn’t always agree with the way professionals speak about network protocols, memorising the details of the way everything is deployed, such as:

TCP/IP arrived on tablets from the sky in its awesome perfection, nothing else ever existed

She believes that standards act more like moving targets than stable reference points. Also, the expert groups working on the standards are, at times, acting more like sports fans rather than professional technologists.

Next, Perlman delved into reviewing network layers. Even though real-world network implementations subdivide into additional layers, combine layers, or perform layer violation when looking at the headers of other layers, she suggested the following classification for the talk:

  • Layer 1. Physical layer - where bits are signalled.
  • Layer 2. Structures the bits signalled at layer 1 into packets: marks the beginning and end of a packet, maybe a checksum. The packet is sent to a neighbour.
  • Layer 3: Switches forward packets from link to link to deliver packets from a source to a destination across a network.

So, layer 2 only talks to neighbours and layer 3 forwards from link to link.

Even if Ethernet was initially designed for multiple machines to communicate over the same wire – layer 2 connectivity – it ended up working at layer 3 as well. At layer 3, Ethernet packets are forwarded having just an extra field – hop count – for keeping track of how many times a packet is forwarded. The initial protocol used was Carrier Sense Multiple Access/Collision Detection (CSMA/CD). In plain words: common sense implemented for networks - don’t speak if somebody else is speaking and if it happens, restart the conversation based on a randomly chosen time. This would work for a network situated within a building but not for the whole internet. Just looking at 60% of the traffic would mean too many collisions.

According to Perlman, the routing algorithms she wrote should work with any underlying protocol, not only IP or DECNet. When Ethernet came into play, she needed to integrate the nodes within a network. if she had considered each node combination in the algorithm, she would have ended up with an n*n combination of nodes, which would've increased the size of the lookup database too much. Instead, she considered the "pseudonode" approach, which meant just n+1 nodes, as shown by the diagram below.

After disagreeing with industry representatives that considered that there would be no need to communicate between different Ethernets, she was given the assignment to build a "magic box" that would enable Ethernets to talk to each other. An important constraint was also provided: Perlman could not modify the endnote or Ethernet packet. The basic functioning concept she developed was a bridge:

Bridges just listen promiscuously, and forward to other port(s) when the "ether" is free [...] This requires a topology without loops

The initial purpose of the spanning tree algorithm was to have a scalable, efficient algorithm (that requires the same amount of memory regardless of the number of nodes.) that identifies loop-free topologies. As the time required to design the algorithm was significantly less than what she had at hand, and her supervisor was on vacation and unavailable, she wrote an "algorhyme" that acted as the abstract of her research paper.


Perlman considers that Connectionless-mode Network Protocol would have been an improved alternative to IP and Ethernet, which would require zero configuration.

The world adopted IP as the standard for layer 3, even if it was configuration intensive. For instance, moving from one side of the router to the other would mean that the layer 3 address has to change. In her way of putting things:

People are saying: thank Goodness for IP, without it, the Internet would not be possible. It is the same thing as saying: thank Goodness for English, without it modern civilization would not be possible. And English is not a great language, but it kind of does the job.

Even if sticking to IP, rather than CLNP was "the worst possible decision" some good things came out of it:

  • Dynamic Host Configuration Protocol - was invented to compensate for the fact that IP needs to be configured. Besides allowing dynamic address assignment, the DHCP enhances privacy as well as the MAC address will not roam around in layer 3 conversations.
  • Network Address Translation (NAT) - it compensates for the fact that there are not enough IP addresses by reusing the same address in private networks and translating them into a global one on the public internet.

Perlman wrapped up her highly personal and funny presentation by stating:

There is no security at all on the internet

In order to defend her strong statement, she provided a refresher on the theory of secure communication on the Internet, including how DNS and cryptography come into play to ensure end-to-end security. Afterwards, she returned to the present day with a related personal anecdote. She shared how she was scammed while trying to renew her driver’s license on the web. This happened even though she understood the underlying network theory and even wrote part of this.

Using a popular web search engine, Perlman picked the first search result from her license renewal query, which was an advert pointing to a malicious site. Not realising this, she entered her bank details for payment, and the criminals attempted to make multiple charges to her account. Ultimately, her bank prevented the fraudulent transactions. Still, it was a reminder that even though the foundations of the modern Internet are built with security in mind, it is still open to criminal exploitation.

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