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Twilight Zone: File Transfer Never Completes

Ages ago when we were building networks using super-expensive 64kbps WAN links, a customer sent us a weird bug report:

Everything works fine, but we cannot transfer one particular file between two locations – the file transfer stalls and eventually times out. At the same time, we’re seeing increased number of CRC errors on the WAN link.

My chat with the engineer handling the ticket went along these lines:

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Worth Exploring: Akvorado Flow Collector and Visualizer

The results you can get when you know how to apply proper glue to a bunch of open-source tools never cease to amaze me. The latest entrant in that category: Akvorado, a Netflow/IPFIX collector and analyzer by Vincent Bernat.

Some of the sample graphs (shown in the GitHub repo) are not far off from those that knocked our socks off during the first Kentik Networking Field Day presentation. Definitely a tool worth exploring ;)

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Twilight Zone: File Transfer Causes Link Drop

Long long time ago, we built a multi-protocol WAN network for a large organization. Everything worked great, until we got the weirdest bug report I’ve seen thus far:

When trying to transfer a particular file with DECnet to the central location, the WAN link drops. That does not happen with any other file, or when transferring the same file with TCP/IP. The only way to recover is to power cycle the modem.

Try to figure out what was going on before reading any further ;)

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Worth Reading: On the Dangers of Cryptocurrencies...

Bruce Schneier wrote an article on the dangers of cryptocurrencies and the uselessness of blockchain, including this gem:

From its inception, this technology has been a solution in search of a problem and has now latched onto concepts such as financial inclusion and data transparency to justify its existence, despite far better solutions to these issues already in use.

Please feel free to tell me how he’s just another individual full of misguided opinions… after all, what does he know about crypto?

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Repost: Buffers, Congestion, Jitter, and Shapers

Béla Várkonyi left a great comment on a blog post discussing (among other things) whether we need large buffers on spine switches. I don’t know how many people read the comments; this one is too valuable to be lost somewhere below the fold


You might want to add another consideration. If you have a lot of traffic aggregation even when the ingress and egress port are roughly at the same speed or when the egress port has more capacity, you could still have congestion. Then you have two strategies, buffer and suffer jitter and delay, or drop and hope that the upper layers will detect it and reduce the sending by shaping.

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Worth Reading: Smart Highways or Smart Cars?

I stumbled upon an interesting article in one of my RSS feeds: should we build smart highways or smart cars?

The article eloquently explains how ridiculous and expensive it would be to put the smarts in the infrastructure, and why most everyone is focused on building smart cars. The same concepts should be applied to networking, but of course the networking vendors furiously disagree – the network should be as complex, irreplaceable, and expensive as possible. I collected a few examples seven years ago, and nothing changed in the meantime.

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VLAN Module in netsim-tools Is Complete

One of the last things I did before starting the 2022 summer break was to push out the next release of netsim-tools.

It includes support for routed VLAN subinterfaces (needed to implement router-on-a-stick) and routed VLANs (needed to implement multi-hop VRF lite), completing the lengthy (and painful) development of the VLAN configuration module. Stefano Sasso added VLAN support for Mikrotik RouterOS and VyOS, and Jeroen van Bemmel completed VLAN implementation for Nokia SR Linux. Want to see VLANs on other platforms? Read the contributor guidelines and VLAN developer docs, and submit a PR.

I’ll be back in September with more blog posts, webinars, and cool netsim-tools features. In the meantime, automate everything, get away from work, turn off the Internet, and enjoy a few days in your favorite spot with your loved ones!

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MLAG Deep Dive: Layer-3 Forwarding

The layer-2 forwarding and flooding in an MLAG cluster are intricate but still reasonably easy to understand. Layer-3 gets more interesting; its quirks depend heavily on layer-2 implementation. While most MLAG implementations exhibit similar bridging behavior, expect interesting differences in routing behavior.

We’ll have to expand by-now familiar network topology to cover layer-3 edge cases. We’ll still work with two switches in an MLAG cluster, but we’ll have an external router attached to both of them. The hosts connected to the switches belong to two subnets (red and blue).

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VXLAN-to-VXLAN Bridging in DCI Environments

Almost exactly a decade ago I wrote that VXLAN isn’t a data center interconnect technology. That’s still true, but you can make it a bit better with EVPN – at the very minimum you’ll get an ARP proxy and anycast gateway. Even this combo does not address the other requirements I listed a decade ago, but maybe I’m too demanding and good enough works well enough.

However, there is one other bit that was missing from most VXLAN implementations: LAN-to-WAN VXLAN-to-VXLAN bridging. Sounds weird? Supposedly a picture is worth a thousand words, so here we go.

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Help Appreciated: netsim-tools Device Features

There are (at least) two steps to get new functionality (like VLANs) implemented in netsim-tools:

  • We have to develop a data transformation module that takes high-level lab-, node-, link- or interface attributes and transforms them into device data.
  • Someone has to create Jinja2 templates for each supported device that transform per-device netsim-tools data into device configurations.

I usually implement new features on Cisco IOSv and Arista EOS1, Stefano Sasso adds support for VyOS, Dell OS10, and Mikrotik RouterOS, and Jeroen van Bemmel adds Nokia SR Linux and/or SR OS support. That’s less than half of the platforms supported by netsim-tools, and anything you could do to help us increase the coverage would be highly appreciated.

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Worth Reading: Is IPv6 Faster Than IPv4?

In a recent blog post, Donal O Duibhir claims IPv6 is faster than IPv4… 39% of the time, which at a quick glance makes as much sense as “60% of the time it works every time”. The real reason for his claim is that there was no difference between IPv4 and IPv6 in ~30% of the measurements.

Unfortunately he measured only the Wi-Fi part of the connection (until the first-hop gateway); I hope he’ll keep going and measure response times from well-connected dual-stack sites like Google’s public DNS servers.

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Video: IPv6 RA Guard and Extension Headers

Last week’s IPv6 security video introduced the rogue IPv6 RA challenges and the usual countermeasure – RA guard. Unfortunately, IPv6 tends to be a wonderfully extensible protocol, creating all sorts of opportunities for nefarious actors and security researchers.

For years, the networking vendors were furiously trying to plug the holes created by the academically minded IPv6 designers in love with fragmented extension headers. In the meantime, security researches had absolutely no problem finding yet another weird combination of IPv6 headers that would bypass any IPv6 RA guard implementation until IETF gave up and admitted one cannot have “infinitely extensible” and “secure” in the same sentence.

For more details watch the video by Christopher Werny describing how one could use IPv6 extension headers to circumvent IPv6 RA guard

You need Free ipSpace.net Subscription to watch the video.
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MLAG Deep Dive: Layer-2 Flooding

In the previous blog post of the MLAG Technology Deep Dive series, we explored the intricacies of layer-2 unicast forwarding. Now let’s focus on layer-2 BUM1 flooding functionality of an MLAG system.

Our network topology will have two switches and five hosts, some connected to a single switch. That’s not a good idea in an MLAG environment, but even if you have a picture-perfect design with everything redundantly connected, you will have to deal with it after a single link failure.

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When You Find Yourself on Mount Stupid

The early October 2021 Facebook outage generated a predictable phenomenon – couch epidemiologists became experts in little-known Bridging the Gap Protocol (BGP), including its Introvert and Extrovert variants. Unfortunately, I also witnessed several unexpected trips to Mount Stupid by people who should have known better.

To set the record straight: everyone’s been there, and the more vocal you tend to be on social media (including mailing lists), the more probable it is that you’ll take a wrong turn and end there. What matters is how gracefully you descend and what you’ve learned on the way back.

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