Network2

Introduction

Last updated: Michael Stone 05:11, 18 July 2009 (UTC)

This page proposes a design for networking based on Scott's Network Principles. It then elaborates on the design with analysis, example configuration, and experimental results and, lastly, it credits those who have contributed to this design and explains some potential future work inspired by the design.

(If you've contributed and don't see your name, don't fret -- just add yourself with a word or two explaining your contribution!)

Please judge this design according to three criteria:

1. How well does it accord with its underlying network principles?
2. How well does it conform to the physical (bandwidth, latency, error) and social (use cases) realities which underly its niche?
3. What costs does it impose on its adopters in exchange for what benefits? In particular, whose time does it waste most profligately and how so?

When judging, please also note that the design is not yet complete in several important respects:

• it has only a stub of a bandwidth model,
• its self-test algorithm is not yet written, (though good diagnostic primitives are systematically identified)
• it lacks truly clear implementation guidance and comprehensive sample code, and
• there are unresolved questions about
  • how routing and timeouts should be configured so that clients search their target address space in a useful fashion
  • how communications security might best be provided.

Design

Protocols

We take IPv6 and DNS as the basis of our system and rely heavily the Linux Documentation Project's IPv6 documentation.

A couple of key facts and expectations inform the rest of this design, like:

• hosts will have multiple interfaces,
• interfaces will have multiple addresses,
• DNS queries (used via getaddrinfo()) will return multiple results
• these results will be sorted in a sane order, and
• hosts will choose routes for packets based on how specifically the routes match the destination and on any QoS information available to the routing node.

Client IPv6 Configuration

Your job is to be an IPv6 node. Consequently, when you bring up your interfaces,

1. You might discover an IPv6 router advertising on one of your links.
   • (See sysctl net.ipv6.conf.all.accept_ra and related variables.)
2. You might try out dhcp6c.
3. You might have some kind of IPv4 connectivity. If so, connect to the Internet or to other internetworks of your choice.
   • (miredo and openvpn seem particularly easy to configure and hence to experiment with...)
4. Use dnshash to add guessable link-local addresses to all your links.

Server IPv6 Configuration

Your job is to be an IPv6 router and a DNS server. One of several situations might obtain:

1. You might discover an IPv6 router advertising one or more IPv6 prefixes on your outbound link(s).
2. You might have some kind of IPv4 connectivity. If so, connect to the Internet or to other internetworks of your choice.
3. You might be under a tree. If so, generate a Unique Local Address prefix.
4. (Use dnshash to add guessable link-local addresses to all your links?)

When done, use radvd or dhcp6d to share addresses.

Server DNS Configuration

One of the server's most important jobs is to get itself on appropriate internetworks so that it can dynamically map stable (DNS) names to unstable names (IPv6 addresses) for itself and its clients.

Unfortunately, the most reliable and secure means of updating these mappings is likely to be bespoke -- RFC 2136 is not widely implemented and specifies no concrete security protocol while DNSSEC seems immature at present.

Consequently, I propose the following strawman update protocol -- exchange an RFC-2136 UPDATE packet and response over your favorite authenticated RPC protocol with the nameserver.

(My favorite protocol for this sort of thing is currently "json-over-SSH-to-python-and-make", but variations (ucspi-ssl, 9p, etc.) make me smile.)

(Other possibilities: maybe DNSSEC isn't so hard? Maybe DNSCurve will be usable? See ipcheck and ddclient for contemporary work...)

Client DNS Configuration

Clients which have been registered with one or more servers need to update those servers when their addresses change using the protocol described above.

Security

1. Spoofing, Integrity, Confidentiality. See communications security and petnames for some background. A very rough road along which something reasonable might lie:
   • Use physical introduction to CNAME cscott.michael.laptop.org to <key>.cscott.laptop.org.
   • Then, my dnscurve-compatible DNS resolver will refuse to give me addresses unless the nameserver I contact for cscott proves knowledge of cscott's private key.
   • Then I have a nice basis with which to configure IPsec security associations.
2. System Integrity
3. DoS

Analysis

Bandwidth Usage

Several important numbers that we need to predict and to measure:

tx == transmit, rx == receive, btx == broadcast

btx/tx/rx - ICMPv6+IPv6+phys           - router discovery (RD)
btx/rx    - ICMPv6+IPv6+phys           - duplicate address detection (DAD)
tx/rx     - ICMPv6+IPv6+phys           - NS neighbor discovery (ND)
tx/rx     - UDP+IPv6+phys              - DNS query
tx/rx     - JSON+SSH+TCP+IPv6+phys     - DNS update

where "phys" describes the equations' dependence on the "physical" layer's 
frame overhead and MTU

notable "phys" layers:

Ethernet           -- ad-hoc wifi, infra wifi, 802.11s mesh, switch, hub
TLS+UDP+IPv4       -- openvpn
L2TP+IPsec+IPv4    -- raccoon, isakmpd, openswan, etc.
UDP+IPv4           -- teredo

Debugging Techniques

Start recording a typescript so that we can see what you did.

TESTDIR=`pwd`/testing
mkdir -p $TESTDIR && cd TESTDIR
script
ulimit -c unlimited

Check that you've got the right DNS name for the person you want to talk to.

NAME=the.right.person
echo $NAME > peer

Dump your addresses, routes, and perhaps your open connections.

hostname --fqdn | tee host
ip addr show | tee addrs
ip route show | tee ipv4_routes
ip -6 route show | tee ipv6_routes
netstat -anp | tee conns

If you have wireless devices,

iwconfig | tee iwconfig
iwlist scan | tee iwlist_scan

Fire up tcpdump:

tcpdump -w packets -s0 &

Resolve that name to addresses. Check that the addresses seem sane.

dnshash lookup $NAME | tee peer_addrs_dnshash
dig $NAME | tee peer_addrs_dig

See who's answering broadcasts:

ping6 -I $IFACE ff02::1

Route to the addresses:

ping6 -I $IFACE $ADDR | tee ping
traceroute6 $ADDR | tee traceroute
tracepath6 $ADDR | tee tracepath

Connect to the address:

nc6 $ADDR $PORT
# echo "SSH-2.0-Hi" | nc6 $ADDR 22
# printf "GET / HTTP/1.0\r\n\r\n" | nc6 $ADDR 80
# ssh $ADDR
# curl -I http://$ADDR/
# ...

Conduct a bandwidth test:

iperf -c -V $ADDR

Collect logs from your application and send them to developers:

kill -SIGINT %1
cd ..
tar c $TESTDIR | lzma -c > logs.tar.lzma

Self-Test Algorithm

As we gain experience with the system, we'll write a decision-list here which inspects the output of the diagnostic procedures listed above and which identifies the proximate cause of networking failure based on those results.

Advice for Coders

There are two critical changes that you'll need to make to your design in order to really make it sing.

First, you'll want to add some mechanism for your users to type in hostnames that they want you to connect to. This lets them do all sorts of cool stuff like:

• copy-and-paste links from websites or cerebro
• type in names from a physical display like a blackboard or a handout,

Second, you'll want to be prepared to re-resolve names in order to get fresh addresses each time your connectivity changes. For the time being, you should do this by calling libc's getaddrinfo() function.

Third, go check out SCTP (wikipedia, man page). It's support for multi-homing, multi-streaming with and without ordering guarantees, and for updating the addresses you're using to talk to your peer on the fly seem particularly serendipitous.

Advice for Deployers

Ask your ISPs to provide IPv6 prefixes or tunnel endpoints. After all -- if none of their customers ask, then what incentive will they ever have to upgrade?

Failing that, see if you (or a local university?) can afford a public IPv4 address -- even if it's dynamic. If so, you can be many sorts of tunnel endpoint.

Regardless, if you manage to get a globally reachable IPv6 address by any means, then you can provide a DNS server for your kids and it can direct them to one another and to any other services that you feel like pointing them at.

Experiments

Link-local configuration

Try out dnshash on an isolated access point, ad-hoc network, switch, or hub.

Observations: very pleasant!

VPN server configuration

In this experiment, we're going to configure openvpn and radvd on a machine (teach.laptop.org) with a public IPv4 address. Truthfully, this combination is probably overkill, but the task of constructing it seemed like it might to offer valuable experience, e.g. for someone who wants to bridge multiple kinds of tunnel endpoint or who wants to load-balance lots of clients between a couple of endpoints.

# Install our VPN and route advertisement software.
apt-get install openvpn radvd
# yum -y install openvpn radvd
 
# add nobody:nobody
groupadd nobody
useradd nobody
usermod -a -G nobody nobody

# Configure radvd
cat > /etc/radvd.conf <<EOF
interface tap0
{
        AdvSendAdvert on;
        MinRtrAdvInterval 30;
        MaxRtrAdvInterval 100;
        prefix 1234:db8:1:0::/64
        {
                AdvOnLink on;
        };
};
EOF

# enable forwarding everywhere
sysctl -w net.ipv6.conf.all.forwarding=1

# flush the forwarding table
ip6tables -F FORWARD

# really, I /want/ a multi-user version of
# openvpn --dev tap --user nobody --group nobody --verb 6
# but I'm not sure how to get that. instead, I'll use some fake keys and no ciphers.
mkdir -P keys && cd keys
wget http://teach.laptop.org/~mstone/sample-keys.tar.bz2
tar xf sample-keys.tar.bz2 && cd sample-keys

# create a multi-user tunnel
openvpn --mode server --client-to-client --dev tap --user nobody --group nobody --verb 6 --opt-verify --tls-server --client-connect /bin/true --auth-user-pass-optional --duplicate-cn --auth-user-pass-verify /bin/true via-env --dh ./dh1024.pem --ca ./ca.crt --cert client.crt  --key client.key --script-security 3 --auth none --cipher none &

# at any rate, bring up the interface so that we get link-local addresses
ip link set tap0 up

# turn on the route advertisement daemon
radvd -d 5 -m stderr &

VPN client configuration

The purpose of this experiment was to test the VPN configuration described immediately above.

# install vpn client
apt-get install openvpn
# yum -y install openvpn

# add nobody:nobody
groupadd nobody
useradd nobody
usermod -a -G nobody nobody

# download fake keys.
mkdir -P keys && cd keys
wget http://teach.laptop.org/~mstone/sample-keys.tar.bz2
tar xf sample-keys.tar.bz2 && cd sample-keys

# connect to the vpn
openvpn --user nobody --group nobody --dev tap --remote teach.laptop.org --tls-client --ca ca.crt --cert ./client.crt --key client.key --auth none --cipher none &

# bring up the interface
ip link set tap0 up

# find other people
ping6 -I tap0 ff02::1

# if using dnshash, attach
dnshash attach <your>.<domain>.<name>

# ... test, as described above ...

Observations:

• TLS imposes a high latency cost, even with null algorithms.
• TAP devices work rather nicely.
• Be careful of firewall rules!
• radvd is perhaps unnecessary with a single virtual ethernet -- dnshash "suffices" -- though it might be useful for routing between several load-balanced ethernets.
• The default IP sorting rules and route priorities mean that it may take a long time for a client app like ssh or nc6 to connect to the /correct/ dnshash address.

Credits

• Michael Stone [none] (writing)
• C. Scott Ananian [OLPC] (architecture,teaching)
• John Watlington [OLPC] (architecture)
• Robert McQueen [Collabora] (prior work,critique)
• Dafydd Harries [Collabora] (prior work,critique)
• Polychronis Ypodimatopolous [MIT] (prior work,critique)
• Cortland Setlow [Tower Research Capital] (testing)
• Andres Ambrois [] (design,testing)
• Benjamin Schwartz [Harvard] (critique,publicity)
• Tabitha Roder [] (testing)

Future Work

• Per-host networks and per-app IPs and names.
• Sample code.