It’s not a CLOS, it’s a Clos

17 June 2019 | 0 Comments

Way back in the day, when telephone lines were first being installed, running the physical infrastructure was quite expensive. The first attempt to maximize the infrastructure was the party line. In modern terms, the party line is just an Ethernet segment for the telephone. Anyone can pick up and talk to anyone else who happens to be listening. In order to schedule things, a user could contact an operator, who could then “ring” the appropriate phone to signal another user to “pick up.” CSMA/CA, in essence, with a human scheduler.

This proved to be somewhat unacceptable to everyone other than various intelligence agencies, so the operator’s position was “upgraded.” A line was run to each structure (house or business) and terminated at a switchboard. Each line terminated into a jack, and patch cables were supplied to the operator, who could then connect two telephone lines by inserting a jumper cable between the appropriate jacks.

An important concept: this kind of operator driven system is nonblocking. If Joe calls Susan, then Joe and Susan cannot also talk to someone other than one another for the duration of their call. If Joe’s line is tied up, when someone tries to call him, they will receive a busy signal. The network is not blocking in this case, the edge is—because the node the caller is trying to reach is already using 100% of its available bandwidth for an existing call. Blocking networks did exist in the form of trunk connections, or connections between these switch panels. Trunk connections not only consume ports on the switchboard, they are expensive to build, and they require a lot of power to run. Hence, making a “long distance call” costs money because it consumes a blocking resource. It is only when we get to packet switched digital networks that the cost of a “long distance call” drops to the rough equivalent of a “normal” call, and we see “long distance” charges fade into memory (many of my younger readers have never been charged for “long distance calls,” in fact, and may not even know what I’m talking about).

Gall’s Law and the Network

13 May 2019 | Comments Off on Gall’s Law and the Network

In Systemantics: How Systems Really Work and How They Fail, John Gall says: A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over with a working…

NTP Best Practices

7 May 2019 | Comments Off on NTP Best Practices

Throwing the baby out with the bathwater (No, you’re not Google, but why does this matter?)

6 May 2019 | Comments Off on Throwing the baby out with the bathwater (No, you’re not Google, but why does this matter?)

It was quite difficult to prepare a tub full of bath water at many points in recent history (and it probably still is in some many parts of the world). First, there was the water itself—if you do not have plumbing, then the water must be manually transported, one bucket at a time, from a…

About that Easy Button …

29 April 2019 | Comments Off on About that Easy Button …

We love layers and abstraction. After all, building in layers and it’s corollary, abstraction, are the foundation of large-scale system design. The only way to build large-scale systems is to divide and conquer, which means building many different component parts with clear and defined interaction surfaces (most often expressed as APIs) and combining these many…

Practical Simplification

22 April 2019 | Comments Off on Practical Simplification

Simplification is a constant theme not only here, and in my talks, but across the network engineering world right now. But what does this mean practically? Looking at a complex network, how do you begin simplifying? The first option is to abstract, abstract again, and abstract some more. But before diving into deep abstraction, remember…

Grey Failure Lessons Learned

25 March 2019 | Comments Off on Grey Failure Lessons Learned

Grey Failures in the Real World Most “smaller scale” operators probably believe they are not impacted by grey failures, but this is probably not true. Given the law of large numbers, there must be some number of grey failures in some percentage of smaller networks simply because there are so many of them. What is…

BGP Optimal Route Reflection

3 December 2018 | 1 Comment

Policy at the Edge

20 November 2018 | Comments Off on Policy at the Edge

Is BGP Good Enough?

3 September 2018 | Comments Off on Is BGP Good Enough?

In a recent podcast, Ivan and Dinesh ask why there is a lot of interest in running link state protocols on data center fabrics. They begin with this point: if you have less than a few hundred switches, it really doesn’t matter what routing protocol you run on your data center fabric. Beyond this, there…