Russ

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Administravia 030818: Added Navigation

I was asked by a reader to add categories and links for videos; I actually added three new categories, one for short videos, another for long videos, and a third for written posts. You can find these under the bottom menu item on the left. I am having a problem with the menu not showing up correctly, so I move the resources under the third menu item, as well.

Finally, I added a new archive page, which shows you all the posts in the “left” category across the three years this blog has been “in production.” I couldn’t figure out how to narrow things down so pictures and other stuff are not included, so there is more on the page than needed right now, but it’s a start.

DFS and Low Points

On a recent history of networking episode, Alia talked a little about Maximally Redundant Trees (MRTs), and the concept of Depth First Search (DFS) numbering, along with the idea of a low point. While low points are quickly explained in my new book in the context of MRTs, I thought it worthwhile to revisit the concept in a blog post. Take a look at the following network:

On the left side is a small network with the nodes (think of these as routers) being labeled from A through G. On the right side is the same network, only each node has been numbered by traversing the graph, starting at A. This process, in a network, would either require some device which knows about every node and edge (link) in the network, or it would require a distributed algorithm that “walks” the network from one node to another, numbering each node as it is touched, and skipping any node that has already been visited (again, for more details on this, please see the book).

Once this numbering has been done, the numbers now produce this interesting property: if you remove the parent of any node, and the node can still reach Continue reading

Responding to Readers: Questions on Microloops

Two different readers, in two different forums, asked me some excellent questions about some older posts on mircoloops. Unfortunately I didn’t take down the names or forums when I noted the questions, but you know who you are! For this discussion, use the network show below.

In this network, assume all link costs are one, and the destination is the 100::/64 Ipv6 address connected to A at the top. To review, a microloop will form in this network when the A->B link fails:

  1. B will learn about the link failure
  2. B will send an updated router LSP or LSA towards D, with the A->B link removed
  3. At about the same time, B will recalculate its best path to 100::/64, so its routing and forwarding tables now point towards D as the best path
  4. D, in the meantime, receives the updated information, runs SPF, and installs the new routing information into its forwarding table, with the new path pointing towards E

Between the third and fourth steps, B will be using D as its best path, while D is using B as its best path. Hence the microloop. The first question about microloops was—

Would BFD help prevent the microloop (or Continue reading

Weekend Reads 030218:

It’s been said that software is “eating the world.” More and more, critical systems that were once controlled mechanically, or by people, are coming to depend on code. This was perhaps never clearer than in the summer of 2015, when on a single day, United Airlines grounded its fleet because of a problem with its departure-management system; trading was suspended on the New York Stock Exchange after an upgrade; the front page of The Wall Street Journal’s website crashed; and Seattle’s 911 system went down again, this time because a different router failed. The simultaneous failure of so many software systems smelled at first of a coordinated cyberattack. Almost more frightening was the realization, late in the day, that it was just a coincidence. —James Somers @The Atlantic

The charges, and the confirmation that the Russians had used social media in an attempt to influence the 2016 election, is likely to fuel the call for government regulation of Twitter, Facebook and other social media outlets. When tweets and posts can hurt democracy, America should do something, right? Wrong. —Paul Levinson @Connecting

As we start to connect durable goods such as cars, medical devices, and electricity meters to the Internet, Continue reading

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