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Category: networking fundamentals

Lesson Learned: Some Services Are Not Worth Delivering

Here’s one of the secrets to AWS’s unprecedented scale and financial success: they figured out very early on that some services are not worth delivering. Most everyone else believes in building snowflake single-customer solutions to solve imaginary problems, effectively losing money while doing so.

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Video: Early Data-Link-Layer Addressing

After a brief coverage of the theoretical aspects of network addressing, it’s time to pay a brief visit to the early data-link-layer addressing solutions, from one address per datagram/frame (SDLC, HDLC) and ignore this address (PPP) to no address on P2P links (SLIP).

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Big Picture: BFD, Non-Stop Forwarding, and Graceful Restart

We have school holidays this week, so I’m reposting wonderful comments that would otherwise be lost somewhere in the page margins. Today: Erik Auerswald’s excellent summary of BFD, NSF, and GR.

I’d suggest to step back a bit and consider the bigger picture: What is BFD good for? What is GR/NSF/NSR/SSO good for?

BFD and GR/NSF/NSR/SSO have different goals: one enables quick fail over, the other prevents fail over. Combining both promises to be interesting.

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Graceful Restart and BFD

The whole High Availability Switching series started with a question along the lines of “does it make sense to run BFD together with Graceful Restart”. After Non-Stop Forwarding 101, Graceful Restart 101, and Graceful Restart and Convergence Speed we finally have enough information to answer that question.

TL&DR: Most probably not.

A more nuanced answer depends (as always) on a gazillion implementation details.

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Lessons Learned: Complexity Will Kill Your System

You wouldn’t believe the intricate network designs I created decades ago until I learned that having an uninterrupted sleep is worth more than proving I can get the impossible to work (see also: using EBGP instead of IGP in a 4-node data center fabric).

Once I started valuing my free time, I tried to design things to be as simple as possible. However, as my friend Nicola Modena once said, “Consultants must propose new technologies because they must be seen as bringing innovation,” and we all know complexity sells. Go figure.

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Graceful Restart and Routing Protocol Convergence

I’m always amazed when I encounter networking engineers who want to have a fast-converging network using Non-Stop Forwarding (which implies Graceful Restart). It’s even worse than asking for smooth-running heptagonal wheels.

As we discussed in the Fast Failover series, any decent router uses a variety of mechanisms to detect adjacent device failure:

  • Physical link failure;
  • Routing protocol timeouts;
  • Next-hop liveliness checks (BFD, CFM…)
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Video: Theoretical View of Network Addressing

After explaining the basics of (network) names, addresses and routes, I wasted a few minutes of everyone’s time discussing the theoretical aspects of layered addressing, and then got back to practical issues like address scopes, namespaces, and address provisioning.

The video ends with a simple (and unappreciated) truth: if you have a point-to-point link between two nodes you don’t need data-link-layer addresses. The consequences of that fact are left as an exercise for the viewer (or you can wait till the next video ;)

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Graceful Restart and Other Control Plane Protocols

In the Graceful Restart 101 blog post, I promised to discuss the ugly parts of this concept in a follow-up post. It turns out we’ll need more than one; today, we’ll focus on other control plane protocols in an access network scenario.

Imagine an access router with multiple uplinks serving a bunch of non-redundantly-connected customers:

Non-redundant access network

Non-redundant access network

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Graceful Restart (GR) 101

In the Non-Stop Forwarding (NSF) article, I mentioned that the routers adjacent to the device using NSF have to play along to make the idea work. That capability is called Graceful Restart. Today we’ll explore its intricate details, be diplomatic, and leave the shortcomings and tradeoffs for the next blog post.

The Problem

Imagine an access (provider edge) router providing connectivity services to its clients and running a routing protocol with one or more upstream devices.

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Stateful Switchover (SSO) 101

Stateful Switchover (SSO) is another seemingly awesome technology that can help you implement high availability when facing a broken non-redundant network design. Here’s how it’s supposed to work:

  • A network device runs two copies of the control plane (primary and backup);
  • Primary control plane continuously synchronizes its state with the backup control plane;
  • When the primary control plane crashes, the backup control plane already has all the required state and is ready to take over in moments.

Delighted? You might be disappointed once you start digging into the details.

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Lessons Learned: Fundamentals Haven't Changed

Here’s another bitter pill to swallow if you desperately want to believe in the magic powers of unicorn dust: laws of physics and networking fundamentals haven’t changed (see also: RFC 1925 Rule 11).

Whenever someone is promising a miracle solution, it’s probably due to them working in marketing or having no clue what they’re talking about (or both)… or it might be another case of adding another layer of abstraction and pretending the problems disappeared because you can’t see them anymore.

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Non-Stop Forwarding (NSF) 101

Non-Stop Forwarding (NSF) is one of those ideas that look great in a slide deck and marketing collaterals, but might turn into a giant can of worms once you try to implement them properly (see also: stackable switches or VMware Fault Tolerance).

NSF has been around for at least 15 years, so I’m positive at least some vendors got most of the details right; I’m also pretty sure a few people have scars to prove they’ve been around the non-optimal implementations.
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Video: Introduction to Network Addressing

A friend of mine pointed out this quote by John Shoch when I started preparing the Network Stack Addressing slide deck for my How Networks Really Work webinar:

The name of a resource indicates what we seek, an address indicates where it is, and a route tells us how to get there.

You might wonder when that document was written… it’s from January 1978. They got it absolutely right 42 years ago, and we completely messed it up in the meantime with the crazy ideas of making IP addresses resource identifiers.

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