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lab: printable exercise sheet for the Hands-on BGP lecture

Browse Source 
docs/lab/handout/handout.tex, typeset with Springer's LNCS class (title
block, abstract and keywords, figure, booktabs tables, RFC references), six
A4 pages so it prints double-sided on three sheets. The built PDF is
committed alongside for printing.

Three sections students work through with the slides:
1. Who holds which addresses: whois their own /48 and ethz.ch's prefix in
   the RIPE database, then compare with what is actually routed.
2. Announcing the prefix on the lab page, reading the UPDATE bytes, finding
   it from outside, and reaching it.
3. Steering with communities until the prefix is reachable only from the
   university network (AS559 via the SwissIX route servers), with
   reachability checkboxes for laptop and phone at each step.
Plus a one-page reference appendix: IPv6 notation and hex, the UPDATE
layout, path attributes, glossary.

The lab host, the lab address space and the prefix pattern are macros at
the top of the file. Expected answers were checked against the RIPE
database and RIPEstat (ETH: 2001:67c:10ec::/48, ASSIGNED PI, origin AS559;
lab: 2a0d:d741::/40, AGGREGATED-BY-LIR, route6 origin AS210036).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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% Exercise sheet for the "Hands-on BGP" lecture.
% Springer LNCS class (llncs.cls ships with TeX Live). Build: latexmk -pdf handout.tex
\documentclass[runningheads,a4paper]{llncs}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{microtype}
\usepackage{booktabs}
\usepackage{enumitem}
\usepackage{amssymb}
\usepackage{xcolor}
\usepackage{listings}
\usepackage{tikz}
\usetikzlibrary{positioning,arrows.meta,fit}
\PassOptionsToPackage{hyphens}{url}
\usepackage[hidelinks]{hyperref}
\renewcommand\UrlFont{\ttfamily\small}
% ---- things to change per class -------------------------------------------
\newcommand{\labhost}{bgplab.rayhaan.net}
\newcommand{\laburl}{https://\labhost/statusz}
\newcommand{\labspace}{2a0d:d741::/40}
\newcommand{\myprefix}{\texttt{2a0d:d741:}\underline{\texttt{\,n\,}}\texttt{::/48}}
% ---- exercise machinery ------------------------------------------------------
\lstset{basicstyle=\ttfamily\small,columns=fullflexible,keepspaces=true,
frame=single,framerule=0.3pt,xleftmargin=1.2em,xrightmargin=0.4em,
aboveskip=0.6em,belowskip=0.4em,breaklines=true}
\newcounter{task}[section]
\renewcommand{\thetask}{\thesection.\arabic{task}}
\newcommand{\task}[1]{\refstepcounter{task}\medskip\noindent
\textbf{Task \thetask}\enspace\textit{#1}\par\nobreak\smallskip\noindent\ignorespaces}
% Dotted lines to write on: \answer[3] gives three.
\newcommand{\answer}[1][1]{\par\nobreak
\foreach \i in {1,...,#1}{\vspace{1.05em}\noindent\makebox[\linewidth]{\dotfill}\par}\smallskip}
% Fill-in rows: the dots always run to the margin, never past it.
\newcommand{\fillin}[1]{\par\nopagebreak\noindent #1~\dotfill\par\vspace{0.5em}}
\newcommand{\fillintwo}[2]{\par\nopagebreak\noindent
\parbox[t]{0.485\linewidth}{#1~\dotfill}\hfill\parbox[t]{0.485\linewidth}{#2~\dotfill}\par\vspace{0.5em}}
\newcommand{\fillinthree}[3]{\par\nopagebreak\noindent
\parbox[t]{0.42\linewidth}{#1~\dotfill}\hfill\parbox[t]{0.27\linewidth}{#2~\dotfill}\hfill\parbox[t]{0.27\linewidth}{#3~\dotfill}\par\vspace{0.5em}}
\newcommand{\cbox}{$\square$\;}
% The reachability check that recurs through the sheet.
\newcommand{\reach}{\par\nopagebreak\smallskip\noindent
\begin{tabular}{@{}l@{\hspace{2em}}l@{}}
\cbox laptop, university network: reachable & \cbox phone, mobile data: reachable
\end{tabular}\par\smallskip}
\begin{document}
\title{Hands-on BGP:\\Announcing and Steering a Prefix on the Internet}
\subtitle{Exercise sheet}
\titlerunning{Hands-on BGP: exercise sheet}
\author{Rayhaan Jaufeerally}
\authorrunning{Network Security, 2026}
\institute{Network Security, ETH Z\"urich\\
\email{Lab page: \url{\laburl}}}
\maketitle
\begin{abstract}
In this exercise you take an IPv6 prefix that is registered for you, find out
what the Internet's registries say about it, announce it to the real Internet
with BGP, and then use BGP communities to decide which networks may learn it.
By the end your prefix should be reachable from the university network and
from nowhere else. Work through the tasks in order and write your answers on
this sheet; the questions marked \emph{Think} are for discussion.
\keywords{BGP \and IPv6 \and RIPE database \and route servers \and BGP communities}
\end{abstract}
\noindent
\begin{tabular}{@{}p{0.47\linewidth}@{\hspace{0.06\linewidth}}p{0.47\linewidth}@{}}
Name: \dotfill & Student number $n$ (decimal): \dotfill \\[0.9em]
$n$ in hexadecimal: \dotfill & Your prefix: \myprefix
\end{tabular}
\medskip\noindent
Every student has one \texttt{/48} out of the lab's \texttt{\labspace}. The
third group of your prefix is your student number \emph{written in
hexadecimal}: student 10 has \texttt{2a0d:d741:a::/48}, student 16 has
\texttt{2a0d:d741:10::/48}. Fill in the fields above before you continue.
% ==============================================================================
\section{Who Holds Which Addresses?}
Address space and AS numbers are handed down from IANA to five regional
registries and on to their members. For Europe the registry is the RIPE NCC,
and its database is public~\cite{ripedb}.
\task{Look up your own prefix.}
Query the RIPE database, with your prefix in place of the example:
\begin{lstlisting}
whois -h whois.ripe.net 2a0d:d741:1::/48
\end{lstlisting}
(No \texttt{whois} on your machine? The same query works in a
browser~\cite{ripedb}.) You get an \texttt{inet6num} object describing a block of addresses and a
\texttt{route6} object saying who may announce a prefix.
\enlargethispage{2\baselineskip}
\fillintwo{\texttt{inet6num} block:}{its \texttt{status}:}
\fillintwo{\texttt{route6} \texttt{origin} AS:}{maintained by (\texttt{mnt-by}):}
\task{Look up a prefix you use every day.}
Find an address of \texttt{ethz.ch} and look it up the same way:
\begin{lstlisting}
dig +short ethz.ch AAAA
whois -h whois.ripe.net <the address you got>
\end{lstlisting}
\fillinthree{\texttt{inet6num} block:}{\texttt{status}:}{\texttt{origin}:}
\task{Compare the database with the routing table.}
The database says who \emph{may} announce a prefix. Whether anybody \emph{does}
is a different question. Look up both prefixes on RIPEstat
(\texttt{stat.ripe.net}) or on \texttt{bgp.tools}.
\fillin{Is ETH's prefix announced, and by which AS?}
\fillintwo{Is yours?}{That AS belongs to:}
\medskip\noindent\textbf{Think.}
ETH's prefix is originated by an AS that is not called ETH. Whose is it, and
what does that tell you about how the university reaches the Internet? Your
\texttt{route6} object exists although nobody announces the prefix: what is
such an object good for?
\answer[3]
% ==============================================================================
\section{Announcing Your Prefix}
\begin{figure}[t]
\centering
\begin{tikzpicture}[font=\footnotesize, node distance=6mm and 8.5mm,
box/.style={draw, rounded corners=1.5pt, minimum height=8mm, inner xsep=3pt, align=center},
bgp/.style={-{Stealth[length=2mm]}, dashed}, web/.style={-{Stealth[length=2mm]}}]
\node[box] (you) {you\\[-1pt]\scriptsize browser};
\node[box, right=of you] (ferrix) {lab daemon\\[-1pt]\scriptsize AS4200000001};
\node[box, right=of ferrix] (pr) {\texttt{pr01.rue}\\[-1pt]\scriptsize AS210036};
\node[box, right=of pr] (rs) {SwissIX\\[-1pt]\scriptsize route servers\\[-1pt]\scriptsize AS42476};
\node[box, right=of rs] (sw) {SWITCH\\[-1pt]\scriptsize AS559\\[-1pt]\scriptsize (ETH Z\"urich)};
\node[box, below=5mm of rs] (other) {other exchanges,\\[-1pt]transit};
\draw[web] (you) -- node[above, inner sep=1.5pt]{\tiny HTTPS} (ferrix);
\draw[bgp] (ferrix) -- node[above, inner sep=1.5pt]{\tiny BGP} (pr);
\draw[bgp] (pr) -- (rs);
\draw[bgp] (rs) -- (sw);
\draw[bgp] (pr) |- (other);
\end{tikzpicture}
\caption{The lab. Your browser talks to a small BGP daemon, which announces
your prefix to the router \texttt{pr01.rue}. That router peers at several
Internet exchanges and buys transit. Dashed arrows are BGP sessions.}
\label{fig:lab}
\end{figure}
Open \url{\laburl} and log in with the credentials given in class. The page
belongs to the lab daemon of Fig.~\ref{fig:lab}. It speaks
BGP~\cite{rfc4271} to \texttt{pr01.rue}, and whatever you announce here
reaches the real Internet within a minute or two. Use the \emph{auto-refresh}
link at the top right to keep the tables current.
\task{Look before you touch.}
\noindent\begin{minipage}{\linewidth}
Read the \emph{Peers} table, then open the \emph{Configuration} line under
the header.
\fillintwo{Neighbour's AS number:}{session state:}
\fillintwo{The daemon's own AS number:}{it only accepts prefixes in:}
\end{minipage}
\task{Announce.}
Enter your name as \emph{owner} and your prefix, leave everything else
untouched, and press \emph{Announce}. Only the owner can withdraw a route, so
spell your name the same way each time.
\task{Read the message you just sent.}
Click \emph{bytes} next to your route and hover over the coloured segments.
An UPDATE is a header, two length fields, a list of path attributes, and a
list of IPv4 prefixes.
\fillintwo{Length of the message, in bytes:}{AS number(s) in \texttt{AS\_PATH}:}
\fillin{Your prefix is carried in the attribute named}
\noindent It is not in the prefix list at the end of the message. Why?
\answer[2]
\task{See it from the outside.}
Wait two minutes, then look your prefix up again on RIPEstat or
\texttt{bgp.tools}.
\fillintwo{Origin AS seen by the world:}{an AS path you found:}
\noindent The daemon's AS4200000001 is missing from that path. Where did it go, and why
is that necessary? (Hint: RFC~6996~\cite{rfc6996}.)
\answer[2]
\task{Reach it.}
The router answers for every address in the lab space, so your prefix is
reachable exactly when your announcement got out. From your laptop, and from
your phone \emph{on mobile data}:
\begin{lstlisting}
ping -6 2a0d:d741:<n>::1 traceroute -6 2a0d:d741:<n>::1
\end{lstlisting}
\reach
% ==============================================================================
\section{Steering Your Prefix with Communities}
% Let the opening paragraph finish on this page; the table then opens the next.
\enlargethispage{4\baselineskip}
{\interlinepenalty=10000
A community is a tag on a route. It means whatever the network that reads it
has decided it means~\cite{rfc1997,rfc8092}. Table~\ref{tab:communities}
lists the ones you need. Two networks act on them in turn: first our router
\texttt{pr01.rue}, then the SwissIX route servers~\cite{swissix}, which pass
routes between the members of the exchange.\par}
\begin{table}[!htb]
\centering
\caption{Large communities for this exercise, written
\texttt{who:function:target}. Within one network, \emph{never} beats
\emph{announce}, which beats \emph{nowhere}. The page's preset menu has all of
them.}
\label{tab:communities}
\begin{tabular}{@{}l@{\hspace{1.4em}}l@{\hspace{1.4em}}l@{}}
\toprule
Community & Read by & Effect \\
\midrule
\texttt{210036:0:0} & \texttt{pr01.rue} & announce nowhere \ldots \\
\texttt{210036:1:42476} & \texttt{pr01.rue} & \ldots except to AS42476 (SwissIX route servers) \\
\texttt{210036:0:}\textit{as} & \texttt{pr01.rue} & never announce to neighbour \textit{as} \\
\midrule
\texttt{42476:0:0} & SwissIX & send to no member \ldots \\
\texttt{42476:1:559} & SwissIX & \ldots except AS559, SWITCH \\
\texttt{42476:0:}\textit{as} & SwissIX & never send to member \textit{as} \\
\midrule
\texttt{NO\_EXPORT} & everyone & keep the route, pass it to no other AS \\
\bottomrule
\end{tabular}
\end{table}
\task{One exchange only.}
Announce your prefix again, now with the large communities
\texttt{210036:0:0} and \texttt{210036:1:42476}. Announcing a prefix you
already own replaces it. \emph{Before} you look: who do you expect to still
see it?
\answer[1]
\noindent Wait two minutes and check.
\fillin{Origin and path now:}
\reach
\task{The university only.}
Add \texttt{42476:0:0} and \texttt{42476:1:559} to the two you already have,
and announce again. Our router now tells only the route servers, and the route
servers tell only SWITCH, the network ETH is part of.
\reach
\noindent Is your prefix still visible on RIPEstat or bgp.tools? Explain what you see.
\answer[2]
\task{Find the communities in the bytes.}
Open \emph{bytes} again and find \texttt{LARGE\_COMMUNITIES}. Each community
is three 32-bit numbers. $42476$ is \texttt{a5ec} in hexadecimal; write down
the twelve bytes of \texttt{42476:1:559}:
\answer[1]
\noindent Now compare with what the rest of the world received: the
\texttt{210036:\ldots} communities are gone but the \texttt{42476:\ldots} ones
are still there. Who removed them, and why only those?
\answer[2]
\task{A well-known community.}
Remove all large communities and set the (standard) community
\texttt{NO\_EXPORT} instead. Predict who will learn the route, then check.
\answer[2]
\task{Clean up.}
Withdraw your prefix. How long does it take to disappear from the looking
glass, and what did the withdrawal look like compared to the announcement?
\answer[2]
\medskip\noindent\textbf{Think.}
You steered routing in two networks you do not operate, using nothing but
numbers attached to a route. What would happen if you attached a community
that one of \emph{their} upstreams acts on? Look at the \emph{Configuration}
on the lab page and at Table~\ref{tab:communities}: which communities were you
\emph{not} allowed to send, and who decided that?
\answer[3]
\begin{thebibliography}{8}
\bibitem{rfc4271} Rekhter, Y., Li, T., Hares, S.: A Border Gateway Protocol 4 (BGP-4). RFC 4271 (2006)
\bibitem{rfc1997} Chandra, R., Traina, P., Li, T.: BGP Communities Attribute. RFC 1997 (1996)
\bibitem{rfc8092} Heitz, J., Snijders, J., Patel, K., Bagdonas, I., Hilliard, N.: BGP Large Communities Attribute. RFC 8092 (2017)
\bibitem{rfc6996} Mitchell, J.: Autonomous System (AS) Reservation for Private Use. RFC 6996 (2013)
\bibitem{ripedb} RIPE NCC: RIPE Database Query, \url{https://apps.db.ripe.net/db-web-ui/query}
\bibitem{swissix} SwissIX: Action BGP Communities, \url{https://www.swissix.ch/resources/action-bgp-communities/}
\end{thebibliography}
\clearpage
\appendix
\section{Reference}
\small
\noindent\textbf{IPv6 notation.} An address is 128 bits, written as eight
groups of four hexadecimal digits. Leading zeros in a group may be dropped,
and one run of all-zero groups may be written \texttt{::}. A prefix
\texttt{2a0d:d741:1::/48} fixes the first 48 bits and leaves the rest free.
Hexadecimal digits: \texttt{a}\,=\,10, \texttt{b}\,=\,11, \texttt{c}\,=\,12,
\texttt{d}\,=\,13, \texttt{e}\,=\,14, \texttt{f}\,=\,15; for a two-digit value,
$n = 16 \cdot \mathit{first} + \mathit{second}$, so $26 = \texttt{1a}$.
\begin{table}[!htb]
\centering
\caption{Anatomy of a BGP UPDATE message~\cite{rfc4271}.}
\begin{tabular}{@{}l@{\hspace{1.4em}}l@{\hspace{1.4em}}l@{}}
\toprule
Bytes & Field & Contents \\
\midrule
16 & marker & all ones; marks a message boundary \\
2 & length & of the whole message, 19 to 4096 bytes \\
1 & type & 1 OPEN, 2 UPDATE, \\
& & 3 NOTIFICATION, 4 KEEPALIVE \\
\midrule
2 + $w$ & withdrawn routes & IPv4 prefixes no longer reachable \\
2 + $a$ & path attributes & everything about the route \\
rest & NLRI & IPv4 prefixes being announced \\
\bottomrule
\end{tabular}
\end{table}
\begin{table}[!htb]
\centering
\caption{Path attributes you will meet. Each is sent as flags, type, length,
value. Flag bits: \texttt{0x80} optional, \texttt{0x40} transitive (passed on
even by routers that do not understand it), \texttt{0x10} two-byte length.}
\begin{tabular}{@{}r@{\hspace{1.2em}}l@{\hspace{1.4em}}l@{}}
\toprule
Type & Name & Meaning \\
\midrule
1 & \texttt{ORIGIN} & how the route entered BGP \\
2 & \texttt{AS\_PATH} & networks crossed: nearest first, origin last \\
3 & \texttt{NEXT\_HOP} & where to send the packets (IPv4) \\
4 & \texttt{MULTI\_EXIT\_DISC} & hint to the neighbour; lower is preferred \\
8 & \texttt{COMMUNITIES} & 32-bit tags, written \texttt{a:b}~\cite{rfc1997} \\
14 & \texttt{MP\_REACH\_NLRI} & IPv6 (and other) prefixes, with their next hop \\
32 & \texttt{LARGE\_COMMUNITIES} & three 32-bit numbers, written \texttt{a:b:c}~\cite{rfc8092} \\
\bottomrule
\end{tabular}
\end{table}
\noindent\textbf{Glossary.}
\begin{description}[leftmargin=2.4cm,style=sameline,itemsep=0pt,topsep=2pt,parsep=0pt]
\item[AS] Autonomous system: a network under one administration, identified
by a number. Numbers from 4200000000 upwards are for private use~\cite{rfc6996}.
\item[Prefix] Addresses sharing their leading bits. The longest matching prefix wins.
\item[Transit] Paying a provider to carry your traffic to and from the whole Internet.
\item[Peering] Two networks exchanging their own and their customers' routes, usually for free.
\item[Exchange] An IX: a shared Ethernet network where many networks meet to peer.
\item[Route server] Passes routes between members of an exchange; carries no traffic.
\item[\texttt{route6}] A database object stating that an AS may originate an IPv6 prefix.
\end{description}
\end{document}