MSS, MSS Clamping, PMTUD, and MTU

Maximum Segment Size (MSS) and MSS clamping are concepts that can be confusing. How do they relate to the MTU (Maximum Transmission Unit)? Before we setup a lab to demonstrate these concepts, let’s give some background. Note that this entire post assumes a maximum frame size of 1518 bytes, the original Ethernet definition, and does not cover jumbo frames.

Ethernet frame

Almost all interfaces today are Ethernet. The original 802.3 standard from 1985 defined the minimum size- and maximum size frame as the following:

  • minFrameSize – 64 octets.
  • maxFrameSize – 1518 octets.

With a maximum frame size of 1518 octets (bytes), that leaves 1500 bytes of payload as the Ethernet frame adds 18 bytes, 14 bytes of header and 4 bytes of trailer. The Ethernet frame is shown below:

IP header

An IPv4 IP header adds at least 20 bytes to the frame. The IPv4 header is shown below:

Note that more than 20 bytes can be used if the header has IP options. With no options in the IP header, there’s 1480 bytes remaining for the L4 protocol such as UDP or TCP.

TCP header

TCP also adds a minimum of 20 bytes, meaning that the maximum payload Continue reading

TACC Fires Up “Vista” Bridge To Future “Horizon” Supercomputer

The Texas Advanced Computing Center at the University of Austin is the flagship datacenter for supercomputing for the US National Science Foundation, and so what TACC does – and doesn’t do – is a kind of bellwether for academic supercomputing. …

TACC Fires Up “Vista” Bridge To Future “Horizon” Supercomputer was written by Timothy Prickett Morgan at The Next Platform.

D2DO250: The Realities of Responsible Disclosure in the Cloud

Cloud security and responsible disclosure are the focus of today’s conversation with guest Kat Traxler. Kat shares her insights on identifying vulnerabilities in cloud services, particularly Google Cloud, and the importance of curiosity in her research. The episode explores the role of bug bounty programs and the shift towards issuing CVEs for cloud vulnerabilities. Lastly,... Read more »

Routing Table and BGP RIB on SR Linux

Ages ago, I described how “traditional” network operating systems used the BGP Routing Information Base (BGP RIB), the system routing table (RIB), and the forwarding table (FIB). Here’s the TL&DR:

  1. Routes received from BGP neighbors are stored in BGP RIB.
  2. Routes redistributed into BGP from other protocols are (re)created in the BGP RIB.
  3. BGP selects the best routes in BGP RIB using its convoluted set of rules.
  4. Best routes from the BGP RIB are advertised to BGP neighbors
  5. Best routes from the BGP RIB compete (based on their administrative distance) against routes from other routing protocols to enter the IP routing table (system RIB)
  6. Routes from the system RIB are copied into FIB after their next hops are fully evaluated (a process that might involve multiple recursive lookups).
Read more …

HW035: The Experience Paradox

In this episode of Heavy Wireless, host Keith Parsons and guest Tom Hollingsworth delve into the “experience paradox” in the tech industry: to get experience, you need to have experience. They highlight the disconnect between job requirements and candidates’ actual experience. Tom emphasizes the importance of hands-on learning, home labs, and making mistakes as essential... Read more »

Tech Bytes: Smarter Networking and Automation With SuzieQ’s Network Observability (Sponsored)

Today on the Tech Bytes podcast: network observability. SuzieQ is an open-source project that collects, normalizes, stores, and analyzes multi-vendor network data. This data becomes a foundation for network observability and automation. We talk with Dinesh Dutt, creator of SuzieQ and founder of Stardust Systems, which makes a commercial version of SuzieQ. Stardust Systems is... Read more »

PP029: Translating Security Objectives into Business Outcomes

On today’s Packet Protector we talk about how to talk about security objectives in ways that resonate with business and non-technical leaders in your organization. Tying security objectives to business outcomes can help you maintain (or increase) budgets, build trust and credibility with executives, and better align your risk management efforts with the organization’s broader... Read more »

Making progress on routing security: the new White House roadmap

The Internet can feel like magic. When you load a webpage in your browser, many simultaneous requests for data fly back and forth to remote servers. Then, often in less than one second, a website appears. Many people know that DNS is used to look up a hostname, and resolve it to an IP address, but fewer understand how data flows from your home network to the network that controls the IP address of the web server.

The Internet is an interconnected network of networks, operated by thousands of independent entities. To allow these networks to communicate with each other, in 1989, on the back of two napkins, three network engineers devised the Border Gateway Protocol (BGP). It allows these independent networks to signal directions for IP prefixes they own, or that are reachable through their network. At that time, Internet security wasn’t a big deal — SSL, initially developed to secure websites, wasn’t developed until 1995, six years later. So BGP wasn’t originally built with security in mind, but over time, security and availability concerns have emerged.

Today, the White House Office of the National Cyber Director issued the Roadmap to Enhancing Internet Routing Security, and Continue reading

Python OOP – Method vs Function and the Mystery of ‘self’

Python OOP - Method vs Function and the Mystery of ‘self’

I just realized how much I didn't know about Python Object-Oriented Programming. I thought I knew the basics, but a few days ago, while going through a Python course, I found out I was wrong. Before I forget what I’ve learned, I wanted to write this blog post and share it with you.

In this blog post, we’ll cover the difference between functions and methods, and what exactly ‘self’ means in Python. So, let’s get to it.

Functions vs Method

class MyClass:
    def say_hello():
        print('Hello')

In this snippet, we’ve defined a Class called MyClass with a function named say_hello. But here’s a question for you - what do you call say_hello? Is it a function or a method?

It’s a common misconception to think that simply defining a function inside a Class automatically makes it a method. However, the distinction lies in how the function is accessed.

Python OOP - Method vs Function and the Mystery of ‘self’
  1. Function - When you define a function inside a class, it’s just a regular function until it’s accessed through an instance of the class.
  2. Method - When you access that function via an instance of the class (e.g., `obj.say_hello'), it becomes a method. This is Continue reading

Poor man’s Traffic Engineering

Segment Routing allows the network operator to deploy Traffic Engineering even with the most basic routers that support the bare minimum of features.

What is traffic engineering

Traffic engineering is a set of techniques to influence the path a particular …

Napalm Configuration Management With Arista EOS

Napalm Configuration Management With Arista EOS

Hi all, welcome back to the Packetswitch blog. In today's post, we'll explore how to use NAPALM for managing device configurations. We'll focus on Arista EOS as our example. We'll cover the methods available in NAPALM and how to push, commit and revert configurations on Arista devices.

We'll start by explaining what NAPALM is and why you might want to use it. Then we'll move on to a few examples and take a look at what happens behind the scenes. This approach will give you a clear understanding of NAPALM's role in network configuration management and how it works with Arista EOS devices.

What is Napalm?

NAPALM stands for Network Automation and Programmability Abstraction Layer with Multivendor support. It's a Python library that helps network engineers manage and automate different network devices using a common set of functions. NAPALM solves the problem of dealing with multiple vendor-specific interfaces by providing a unified way to interact with network devices from various manufacturers. This means you can use the same code to manage devices from Cisco, Juniper, Arista, and others, saving time and reducing the complexity of network automation tasks.

A Beginner’s Guide to NAPALM Network Automation
In this blog post, we Continue reading

An AX.25 implementation in Rust

After having written a user space AX.25 stack in C++, I got bitten by the Rust bug. So this is the third time I’ve written an AX.25 stack, and I’ve become exceedingly efficient at it.

Here it is:

The reason for a user space stack remains from last time, but this time:

  1. It’s written in Rust. Yay! I know people say Rust has a honeymoon period, but I guess that’s where I am, still.
  2. It’s a normal library first. The previous C++ implementation started off as microservices, which in retrospect was needlessly complex and put the cart before the horse.

I’ve added almost an excessive amount of comments to the code, to cross reference with the specs. The specs that have a few bugs, by the way.

Rust

I’m not an expert in Rust, but it allows for so much more confidence in your code than any other language I’ve tried.

I think I know enough Rust to know what I don’t fully know. Sure, I’ve successfully added lifetime annotations, created macros, and built async code, but I’m not fluent in those yet.

Interestingly, Continue reading

Fixing layout shifts caused by web fonts

In 2020, Google introduced Core Web Vitals metrics to measure some aspects of real-world user experience on the web. This blog has consistently achieved good scores for two of these metrics: Largest Contentful Paint and Interaction to Next Paint. However, optimizing the third metric, Cumulative Layout Shift, which measures unexpected layout changes, has been more challenging. Let’s face it: optimizing for this metric is not really useful for a site like this one. But getting a better score is always a good distraction. 💯

To prevent the “flash of invisible text” when using web fonts, developers should set the font-display property to swap in @font-face rules. This method allows browsers to initially render text using a fallback font, then replace it with the web font after loading. While this improves the LCP score, it causes content reflow and layout shifts if the fallback and web fonts are not metrically compatible. These shifts negatively affect the CLS score. CSS provides properties to address this issue by overriding font metrics when using fallback fonts: size-adjust, ascent-override, descent-override, and line-gap-override.

Two comprehensive articles explain each property and their computation methods in detail: Creating Perfect Font Fallbacks in CSS and Improved Continue reading

Weekend Reads 083024


Microsoft on Thursday disclosed four medium-severity security flaws in the open-source OpenVPN software that could be chained to achieve remote code execution (RCE) and local privilege escalation (LPE).


Cybersecurity researchers have discovered multiple critical flaws in Amazon Web Services (AWS) offerings that, if successfully exploited, could result in serious consequences.


As many as 10 security flaws have been uncovered in Google’s Quick Share data transfer utility for Android and Windows that could be assembled to trigger remote code execution (RCE) chain on systems that have the software installed.


SiFive has announced the launch of its latest core for datacenters, the P870-D, and claims it has a leg up on Arm’s Neoverse N2 in density for AI.


Unstoppable Domains (UD), a provider of Web3 domain names and digital identities, has been officially accredited by the Internet Corporation for Assigned Names and Numbers (ICANN).


CENTR, the association overseeing European country code top-level domain (ccTLD) registries, has announced the public release of its Domain Crawler Project code.


Remote SIM provisioning (RSP) for consumer devices is the protocol specified by the GSM Association for downloading SIM profiles into a secure element in a mobile device. The process is commonly known as eSIM, and it Continue reading

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