Network2

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Introduction

Last updated: Michael Stone 19:10, 25 July 2009 (UTC)

What is this document?

This document proposes a design for networking based on previously realized Network Principles. It then explores and elaborates the design with analysis, example configuration, and experimental results after which it concludes by crediting those who have contributed to the design and by explaining future work inspired by current results.

Who is this document for?

This document is written for several kinds of people.

  • deployment teachers who want networking to "just work" in their classrooms, so they can focus on teaching rather than troubleshooting and working around the situation when their students' computers can't collaborate for technical reasons. This document will try to explain several reasons why things may not be working for you, and give you several ways to start fixing them (usually knowing who you should be asking to do what). It will also give you a way to give your feedback on networking to developers.
  • deployment engineers who have the responsibility of making networking "just work" for deployment teachers, and want to make those responsibilities easier so they can spend more time on other technical aspects of the deployment. This document will try to explain a way of implementing networking at a deployment that will save you work and time. It also explains things we don't know about this method of implementation, and things we're trying to figure out that you can help with.
  • Activity authors who want it to be easier to program collaboration features into their software. This document will explain a different way of doing the networking stack, and then describe why and how this would make it easier for you to get collaboration into your Activity, and what you can do to make that happen.
  • core developers interested in improving the networking stack. This document will explain an alternative proposal to the current stack, as well as where this proposal is along the development timeline and what you can contribute to move it forward.
  • others who would like to see XO/Sugar-based networking improve. This document will give you an overview of the current networking situation for teachers and technologists alike, show you what different groups are working on, and hopefully give you ideas on where and how you can contribute. Can you organize a code sprint for developers, gather feedback from teachers, make a prototype for Activity authors, clarify any of this documentation, ask questions about things that you don't understand in this document so that we can improve it?

What is this document for?

Its abstract purpose is to advance the Network Principles project by explaining how you might build a system based on those principles with currently available tools and by doing a first round of modeling and prototyping in order to gain some analytic and empirical evidence about whether those principles are sound.

Its concrete purpose is to provide internetworking and naming technology to XO-users (and other interested parties) that seamlessly and predictably supports the XO's most important low-latency network scenarios as well as is possible with existing software.

Its social and conceptual purpose is to provide a design that is satisfactory in several ways in which previous networking and collaboration substrates were not, as described in the following principles of design quality:

do no harm -- our users are not volunteers, so don't waste their time
play well with others, since we want a large ecosystem and lots of testing
be realistic, so that we don't promise the impossible
be predictable, so that we can tell people what will work and what will fail in advance
prevent failure, by means of proof, simulation, and wise habits
tolerate failure, by removing inappropriate single points of failure
route around failure, by means of self-test procedures and preplanned maneuvers (manual overrides)

Notes on quality principles:

  1. do no harm and play well with others mean that we believe that previous designs unnecessarily harmed their users by wasting scarce resources (e.g. time, trust, and the capacity to learn) and opportunities (e.g. to grow the ecosystem). Other "harms" inflicted by previous designs include having badly confused their users, having over-promised their scalability, and having been unable to articulate or to meet basic "go/no-go" availability requirements.
  2. realism and predictability are intended to evoke the following "litmus test" questions:
    • how well does the design conform to the physical realities (bandwidth, latency, power, failure, and error) and to the social realities (ignorance, interdiction, authority, and autonomy) that define its niche?
    • is there a public, written, and peer-reviewed design document describing the design?
  3. prevent, tolerate, and route around are all direct usability goals that no networking design intended for real humans (particularly by teachers!) should ignore

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, hence we yet know how much it costs to scale it up
  • 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 peers search their target address space in a useful fashion
    • how communications security might best be provided.
  • it lacks an "integration and deployment" plan outlining how to get it adopted.

Design

Network Architecture

We want to offer maximally efficient and robust support for our ideal network scenarios (nos. 1 and 9, denoted with bold text, below) while offering seamless support for optional network enhancements like fancy links, routers, tunnel endpoints, and transit agreements that may be provided by the surrounding ecosystem of deployment organizations, universities, individuals, and commercial entities.

Network Scenarios:

  1. access to at least one shared-media link.
  2. a more efficient link, like an 802.3 switch or an 802.11 access point.
  3. a bridge, like an XS or a good access point, between two or more otherwise separate single-link networks.
  4. a local router, like an XS, routing between two or more otherwise separate (but potentially complicated) local networks
  5. a restrictive local router which provides some IPv4 connectivity but which drops IPv6 traffic
  6. credentials for some sort of dedicated local tunnel endpoint (like a SOCKS proxy or an HTTP proxy)
  7. a remote router offering us some sort of access to a larger internetwork, typically via (perhaps restricted) IPv4
  8. credentials for some sort of dedicated remote tunnel endpoint (like an SSL or IPsec VPN or a 6to4 tunnel, etc.)
  9. a remote router offering great access to a larger internetwork

Based on these scenarios, we imagine our network as being organized into three kinds of composable layers:

  1. a link layer, usually implemented via 802.3 wired Ethernet, 802.11b/g wifi in either ad-hoc or infrastructure mode, or various sorts of tunneling over IPv4, perhaps across NATs and firewalls,
  2. an internetworking layer, based on IPv6 (tutorial documentation), and
  3. a naming layer, based on DNS, for binding logical addresses from networks with different failure modes to stable human-memorable names

We find this layered conceptual model helpful for estimating dependency ("what has to work before this layer can work?") and cost ("what does it cost to traverse this layer?").

IPv6 Configuration

Peers:

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 interfaces.

Servers:

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.

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 peers.

Discovery:

Peers need help locating one or more DNS servers. See RFC 4339 for available mechanisms; pay particular attention to RDNSS discovery.

Update

Here are two approaches for solving the update problem, based on how peers might want to communicate with DNS servers:

  1. Use a DNS UPDATE client like ipcheck or ddclient with shared keys with a DNS server like BIND.
  2. Run a bespoke control protocol over an existing secure tunnel, e.g. something based on with XML-RPC over HTTPS + client certs or on access to a restricted shell over SSH.

(NB: In order to perform this update, it will usually have been necessary for the peer to have been cryptographically introduced to the server.)

Unfinished Ideas

Security

This optional section is included merely to offer some hints about where we think communications security ought to be headed.

  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

Performance

Wad points out that people writing software are probably going to want some help figuring out what routes are best to use. (e.g. in terms of bandwidth, latency, jitter, integrity, confidentiality, availability, ...)

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

In order for things to "just work", there are many subgoals that need to be satisfied. The purpose of the self-test algorithm is to speed up debugging by quickly and reliably identifying subgoals whose named requirements are satisfied but whose characteristic test fails.

The form of the self-test algorithm will be a decision-list which may, in the future, be incorporated into software.

A rough outline of that decision list is: 

Do we have all the network interfaces that we should?
Is each interface attached to a link?
Does each interface have a link-local address?

Is every interface able to ping itself?
Does link-layer broadcast return responses?
Does network-layer broadcast return responses?

# assuming that we have a partner on the same link
Can we ping our partner?
Can we hear our partner pinging us?
Does there seem to be reasonable bandwidth on our link?

# assuming we have a link-local partner with a name
Do we and our partner have byte-identical names written down?
Can we both resolve the name to a link-local address?
Do we get the same address?
Can we both ping the address?
Can I connect to a service running at the address (e.g. ssh)

# assuming that we have a router
Can we ping our router?
Can we traceroute someone upstream of the router?

...

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 peers 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, at least for tiny networks.
  • 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 connecting app like ssh or nc6 to connect to the /correct/ dnshash address.

Credits

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

Future Work

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