There and back again: A packet's tale

I've got a new goal in life, and a new series of projects to achieve that goal. I'm planning to focus my free time to produce articles, screen casts and, hopefully, eventually, a book. They're all around answering my favourite interview question: "When I pull up my Internet Browser, type 'bbc.co.uk' into the address bar, and press return, what happens?"

In this article, I explore why I like it as an interview question, and some of the topics I'll be talking about in the coming months.

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I haven’t done any interviewing for a while1 but I went through a period of growth in one of the companies I worked for where we were feverishly expanding the development team, so we had to be a little more systematic in our approach to interviewing. Instead of just having an open conversation with candidates to see where it led (which is what I’d previously done in such situations), I wound up preparing a ‘standard’ set of questions. It took a few goes, but eventually I settled on a favourite question for the technical portion of the interview:

When I pull up my favourite Internet browser, type “bbc.co.uk” into the address bar, and press return, what happens?

I reckon it’s a doozy of a technical question. There’s so much breadth and depth in that answer. We could talk for hours on how the browser decides whether you’ve entered something which can be turned into a valid URL, or whether it’s intended to be a search term. From there, we can look at URL construction, then deconstruction, to figure out exactly what resource we’re looking for. Then it’s on to name resolution, to figure out who we should be talking to.

And then it gets really interesting. We start an HTTP conversation, which is encapsulated in a TCP session which is, in turn, encapsulated in a sequence of IP packets, which are, in turn, encapsulated in packets at the data link layer (some mixture of Ethernet and/or wireless protocols), which – finally! – causes some bits to fly through the air, travel along a copper wire, or become flashes of light through fibre optic.

Now our request emerges – fully formed again, if it survived the journey – at the data centre. The HTTP request is serviced (on which entire bookshelves have been written) and the response follows the same perilous journey back to the browser.

The story isn’t over, though. Once the browser has received the response, it still has to interpret it, create an internal representation of the document, apply visual style, and render it in a window. And then there’s the client side JavaScript code to be executed.

It’s an exciting story: one of daring journeys, lost packets, unanswerable questions, and the teamwork of many disjoint routers, distributed across the Internet.

T-shaped

There are so many different areas involved that it’s impossible to answer the question fully in an interview situation. But I hope an interviewee (for a technical role) would have a rough overview of the process involved, be able to make intelligent inferences as to how some bits work, and have depth of knowledge in some area(s). When you’re interviewing, as Valve’s Employee handbook (PDF) succinctly put it, you’re looking for a ’T’ shape:

people who are both generalists (highly skilled at a broad set of valuable things – the top of the T) and also experts (among the best in their field within a narrow discipline – the vertical leg of the T).

From their answer to the question, I’ll get an inkling about their generalist skills. As well as a general overview of the topics, I’ll get some insight into their ability to structure an answer, communication skills, and problem solving. And I get an idea of where their strengths lie:

  • If the answer is largely around the browser itself, how it interprets HTML and CSS to render the page, and how it interacts with JavaScript code, I’ll have a hint that the candidate is strong on front end development.

  • If they lean more towards the server side, talking in depth about how an HTTP request is serviced, then chances are I’m talking to a seasoned backend developer.

  • And if we get into the nitty gritty of IP protocols, packets and routing, I’ve found myself an Ops engineer.

This interview question has been sitting with me in my repertoire for the past five years now. More recently, I’ve been wondering: wouldn’t it be awesome if more of us2 had a deeper knowledge of the full stack? While it’s not necessary to know the wire layout of an ARP3 packet, for example, it’s occasionally useful to know what ARP is when you’re trying to figure out why two computers won’t talk to each other.

A Sneak Peek at The Internet

The classic text for understanding the network layer is TCP/IP Illustrated Volume 1: The Protocols, by W. Richard Stevens. It has been revised by Kevin Fall, with a second edition in 2011, and it definitely covers all the material, in meticulous depth. But it’s not an easy read with all that detail and, to many, is not an approachable book.

I think there’s an untapped market here. As it happens, I’m in the market for some new goals in life, so here’s my plan. For the next while, I’m going to focus on writing articles that explain various areas of the networking stack in some interesting level of detail. I’ll be covering topics around:

  • the high level protocols themselves (HTTP, SMTP, IMAP, SSH, etc);

  • securing communication at the transport layer (SSL/TLS);

  • name resolution at various layers (DNS, ARP, etc);

  • the predominant transport layers (TCP & UDP), plus a look into relatively new contenders like SCTP & QUIC;

  • the underlying Internet Protocol, IPv4 & IPv6, which will encompass topics like routing, tunneling, firewalls & configuration;

  • the physical network layers, like Ethernet, Wifi & cellular;

  • the standardisation process at the IETF; and

  • some of the tools that we can use to explore all these concepts.

I might even dive a little into the implementation of a network protocol. For some reason – and it couldn’t possibly have been because I was drunk at the time! – the note, “Implement a TCP/IP Stack in Go” appeared one day on my Someday/Maybe todo list. Perhaps creating a complete protocol stack is a bit much, but taking a look into the Linux kernel to see how things are implemented could be insightful.

I’d like to experiment a little with screen casting, too. I’m aiming to put together a series of short talks for conferences (lightning talks, and I’m submitting longer talks to every conference I see issuing a Call For Papers!) on various aspects of this overarching theme. I’ve got some notes together for a talk on DNS, for a lightning talk at the Bath Ruby Conference next month, for example. I’d like to turn these talks into short screen casts, too, if you can put up with my accent. ;-)

But the bigger goal is to write a book. I feel that this is an important topic, and it’s one that every developer could use some greater insight into. All of us use the Internet, and most of us develop applications which interact over the Internet. Wouldn’t it be awesome if we all had the confidence to understand how the network worked, instead of treating it like a (reliable) bit of cable directly connecting each endpoint?

The book is definitely in the very early stages. I’ve put together a rough outline, and I’ve written a few thousand words that will fit in … somewhere (I seem to have an unhealthy obsession with DNS). I’m expecting it to be a long project! My current working title is, “A Sneak Peek at The Internet”, though I was inspired by a new title while writing this morning: “There and back again: A Packet’s Tale.”

If you’re interested in keeping up to date with progress on my goal, I’ve put together a sign-up form (with LaunchRock which was delightfully easy to use!). You can sign up here: A Sneak Peek at The Internet.


  1. The mixed blessing of doing contract work instead of being a full time employee for the past few years. 

  2. Me included. I wasted my formative years printing out RFCs on a Star LC-10 dot matrix printer, then reading them – for fun! But I haven’t kept up with developments in the past decade, so I’m on a learning journey. And that’s kinda the point: that’s what I’m getting out of this deal. 

  3. Address Resolution Protocol. This is the mechanism used by Ethernet and Wifi networks to figure out who an IP address belongs to. If a host wants to communicate with 10.0.2.34 on the local network, it shouts “who has 10.0.2.34?” to everyone on that network. Hopefully, the owner of that IP address will answer, “I have 10.0.2.34, and my hardware address is 00:11:22:33:44:55.” 

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