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56
lessons/introduction-to-route-0.md
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@ -6,13 +6,12 @@ Linux, and Wireshark.
## Topology
First, let's look at the topology that we will be using for this lesson, the
`one_rtr` topology. You can view it in this its
[README](../topology/one_rtr/README.md). The network is very simple. It
consists of three nodes, but only one of them, `R1`, is a router, hence the
name of the topology. The other two are end-hosts. A host is not necessarily
a different device to a router, but it has a very different role in the
network. A host will only have one outgoing link and it will not forward IP
packets which means that it can only be the source or destination of IP
[`one_rtr` topology](../topology/one_rtr). The network is very
simple. It consists of three nodes, but only one of them, `R1`, is a router,
hence the name of the topology. The other two are end-hosts. A host is not
necessarily a different device to a router, but it has a very different role in
the network. A host will only have one outgoing link and it will not forward
IP packets which means that it can only be the source or destination of IP
communication. The convention in Route 0 is to name routers with a name that
starts with the letter `R` and hosts with a name starting with `h`.
@ -24,8 +23,7 @@ sudo python route0.py --topology one_rtr --scenario basic
This command instructs the driver script `route0.py` to start a network with
the `one_rtr` topology running the `basic` scenario. The `basic` scenario is
special and simply means to run the network and set up all the interface
addresses and default routes. We will go over what this means later in this
lesson.
addresses and default routes.
Once the CLI prompt appears let us inspect Mininet's representation of the
network by running
@ -35,7 +33,7 @@ net
in the command prompt. The output tells us about all the nodes in the network
and the connections between them. We can see that `R1`'s `R1-eth1` interface
is connected to `h1_1`'s `h1_1-eth1` interface and `R1-eth2` is connected to
`h1_2`'s `h1_2-eth1` interface. You can visualise the network by copy pasting
`h1_2`'s `h1_2-eth1` interface. You can visualise the network by copy-pasting
the output into this [web
tool](https://achille.github.io/mininet-dump-visualizer/) though its usefulness
is limited for small networks such as this.
@ -73,8 +71,8 @@ will notice that the `lo` interface on `R1` actually has two IP addresses.
The `ip route` command is used to list all the routes installed on a particular
node. The basic format of a route is `x.x.x.x/y via z.z.z.z` which says that
to reach the IP network `x.x.x.x/y` you must go via the address `z.z.z.z` which
should resolve to a directly connected neighbour. Note that you won't see such
routes in this network setup, because the network is too simple.
should belong to an interface on a directly connected neighbour. Note that you
won't see such routes in this network setup, because the network is too simple.
The host nodes have a default route installed which looks like `default via
z.z.z.z` which means that the node should route all traffic it doesn't have a
@ -87,8 +85,9 @@ network connected to the interface `if-name`.
### ping
The command `ping` sends a special IP packet to the specified destination to
verify connectivity with that end-host. Try sending a ping from `h1_1` to an
IP address on `h1_2` by running
verify connectivity with that end-host. Connectivity is verified if a response
is received. Try sending a ping from `h1_1` to an IP address on `h1_2` by
running
```
h1_1 ping 10.2.0.1
```
@ -101,17 +100,17 @@ because it is directly connected to both of them.
## Wireshark
Before moving on to the next section it would be good to introduce a
Before moving on to the next lesson it would be good to introduce a
particularly useful tool in studying networks, Wireshark, by using it to look
at pings from `h1_1` to `h1_2`. Wireshark is a tool that lets you capture and
inspect packets sent and received over all interfaces on a device.
Furthermore, it is able to present them in a human readable form rather than
simply dumping the binary representation directly from the wire.
Start by running the command to trigger `h1_1` to start sending pings to
`h1_2`. Now open a new terminal window and navigate to the `route0` directory.
We will use the `attach.py` helper script to run Wireshark on `R1` and `h1_2`.
Let's start with `R1` by running
Start by running the command to trigger `h1_1` to send pings to `h1_2`. Now
open a new terminal window and navigate to the `route0` directory. We will use
the `attach.py` helper script to run Wireshark on `R1` and `h1_2`. Let's start
with `R1` by running
```
sudo python attach.py --node R1 --cmd wireshark
```
@ -123,12 +122,12 @@ connected to `h1_2`, the source of the packets. You can either double-click on
the interface name or select the appropriate button on the menu bar in the
top-left corner.
Once the packet capture notice how the ping packets appear every second as a
request/reply pair. Look at the source and destination IP addresses as well.
Note how the originating node has filled out the source address with the
address of its interface `h1_2-eth1` and how the reply has the addresses
flipped around. Have a look around and inspect the contents if you wish, but
we won't go into any detail on the form of the ping packets.
Once the packet capture window opens notice how the ping packets appear every
second as a request/reply pair. Look at the source and destination IP
addresses as well. Note how the originating node has filled out the source
address with the address of its interface `h1_1-eth1` and how the reply has the
addresses flipped around. Have a look around and inspect the contents if you
wish, but we won't go into any detail on the format of the ping packets.
Now let's look at the packet capture on the other interface on `R1`. You can
do this by stopping the current capture, finding the capture options button and
@ -147,7 +146,6 @@ This will shut down all the nodes and protocols that are running.
## Conclusion
In this lesson you learned how to start up Route 0 experiments and learned how
to inspect your network using basic Linux commands and Wireshark. You will
find these tools will come in handy at all times whenever dealing with
networks.
In this lesson you learned how to start Route 0 experiments and learned how to
inspect your network using basic Linux commands and Wireshark. You will find
these tools will come in handy at all times whenever dealing with networks.

74
lessons/ip-addresses-and-subnets.md
View File

@ -24,27 +24,27 @@ Start by having a look around using the commands you learned in the previous
lesson, `ip address` and `ip route`, and notice how none of the addresses or
routes are present on any of the nodes. Furthermore, if you try running the
pings between any of the nodes, you will find they do not work and fail with a
`Network is unreachable error`. In this lesson we will manually reconstruct
`Network is unreachable` error. In this lesson we will manually reconstruct
the `basic` network to illustrate all the different concepts involved.
### Assigning IP addresses
A good place to start would be to simply assign all the IP addresses as per the
`one_rtr` topology [README](../topology/one_rtr/README.md). The command to
assign an IP address to an interface in Linux has the form
[`one_rtr` topology](../topology/one_rtr). The command to assign an IP address
to an interface in Linux has the form
```
ip address add [ip]/[mask-digits] dev [if-name]
ip address add <ip>/<mask-digits> dev <if-name>
```
This command assigns the address `ip` associated with the subnet defined by the
`mask-digits` to the interface `if-name`. This should be pretty
This command assigns the address `<ip>` associated with the subnet defined by
the `<mask-digits>` to the interface `<if-name>`. This should be pretty
self-explanatory except for the subnet which may be a new concept for some of
you.
An IPv4 address is basically a 32-bit number. The common representation
`x.x.x.x` simply splits this number into four 8-bit numbers making it more
readable for a human. This is why none of the four numbers ever exceed 255 as
that is the largest number you can represent with 8 bits.
An IPv4 address is a 32-bit number. The common representation `x.x.x.x` simply
splits this number into four 8-bit numbers making it more readable for a human.
This is why none of the four numbers ever exceed 255 as that is the largest
number you can represent with 8 bits.
A subnet is a subdivision of an IP network and determines all the possible IP
addresses that can be connected directly to each other over a local network.
@ -54,8 +54,8 @@ subnet means that we can communicate with all the other addresses in that
subnet by using this interface.
The subnet of an IP address is determined by its prefix. The length in bits of
the prefix is determine by the `mask-digits.`. Thus, the IP address
`10.11.12.13/24` belongs to a subnet defined by its first 24 bits, that is
the prefix is determine by the `mask-digits`. Thus, the IP address
`10.11.12.13/24` belongs to a subnet defined by its first 24 bits,
`10.11.12.0/24`. The router will now forward all traffic to any IP address on
this subnet, such as `10.11.12.1` or `10.11.12.165`, over this interface.
@ -89,7 +89,7 @@ example we only have `10.1.0.1` and `10.1.0.254` on the network on the subnet
`10.1.0.0/24` which is effectively a local network of one point-to-point link.
Try pinging `10.100.0.5` and `10.1.0.5` from `h1_1`. Notice how both fail, but
only the first one returns the `Network is unreachable error`. Why does the
only the first one returns the `Network is unreachable` error. Why does the
second one appear to be stuck? Since `10.1.0.5` belongs to the same subnet as
`h1_1-eth1` the host tries to send the ping over this interface, but as the
other end does not exist, the response never arrives.
@ -102,21 +102,22 @@ sudo python attach.py --node h1_1 --cmd wireshark
```
and start a packet capture on the `h1_1-eth1` interface.
The first thing you will notice is how `h1_1` keeps send ARP protocol messages.
ARP stands for the Address Resolution Protocol and is the mechanism by which a
node finds the MAC address of the interface associated with the particular IP
address. In order to send a packet over a link it must be addressed to the
right MAC address as otherwise no interface on the local network will pick the
packet up. In this case we see packets constantly asking "Who has 10.1.0.5?
Tell 10.1.0.1", but nobody owns that IP address so nobody responds.
The first thing you will notice is how `h1_1` keeps sending ARP protocol
messages. ARP stands for the Address Resolution Protocol and is the mechanism
by which a node finds the MAC address of the interface associated with the
particular IP address. In order to send a packet over a link it must be
addressed to the right MAC address as otherwise no interface on the local
network will pick up the packet. In this case we see packets constantly asking
"Who has 10.1.0.5? Tell 10.1.0.1", but nobody owns that IP address so nobody
responds.
Let's now look at what happens when the IP address exists on the network. Set
`h1_1` to ping the other end of its link `10.1.0.254` (you don't have to close
wireshark). Most of the packets sent and received will be the already known
Wireshark). Most of the packets sent and received will be the already known
ping packets, but every now and then an ARP request is sent. However, this
time `h1_1` receives a response telling it the MAC address of the interface.
If you inspect the ping packets that originate at `h1_1` you will notice that
they do use that MAC address in the Ethernet header.
they do use that MAC address as the destination in the Ethernet header.
You may wonder why do the nodes need to do this? After all the IP address
already uniquely identifies the interface. This is because the IP protocol
@ -134,8 +135,8 @@ ping `10.2.0.1` from `h1_1` you will be told that the network is unreachable.
If you look at the output of `ip route` on the host this error makes sense.
The routing table doesn't know how to reach any subnet other than
`10.1.0.0/24`. We could just add a route for the `10.2.0.0/24` subnet to go
via `R1` to `h1_1` which would work for `h1_2`, but would fail as soon as any
new host is added to `R1`.
via `R1` which would work for `h1_2`, but would fail as soon as any new host is
added to `R1`.
Instead we will add a default route to our host. A default route is the route
used for IP addresses that do not match any other more specific route. To add
@ -153,26 +154,29 @@ the local network, but in principle we could have more. In that case,
specifying an interface would not uniquely identify the next hop.
Try pinging `10.2.0.1` from `h1_1` now. You will notice that it no longer
fails with a "Network unreachable error", but it still doesn't work. Let's
fails with a `Network is unreachable` error, but it still doesn't work. Let's
investigate using Wireshark. If you inspect the traffic at `h1_1` you will
notice that the requests are being sent, but no responses are received. Let's
check if `R1` is forwarding the packets. If you launch Wireshark on `R1` you
will notice that the packets are received on one interface and are forwarded to
the other. If you also inspect `h1_2` you will find that the request packets
actually manage to make their way to the destination, but no response is sent.
the other so that's not it. If you also inspect `h1_2` you will find that the
request packets do manage to make their way to the destination, but still no
response is sent.
Can you figure out what's going on? What happens if you try pinging `h1_1`'s
interface from `h1_2`?
The problem is that `h1_2` doesn't have a default route itself. It receives
the ping packets and it tries to send a response back to source IP address, but
then it finds out it doesn't know how which way to send a packet to that IP
address. The solution is to install a default route just like we did for
`h1_1`. Once installed you will notice that pings from `h1_1` now succeed.
the ping packets and it tries to send a response back to the source IP address,
but then it finds out it doesn't know what to do with a packet addressed to
that IP address. The solution is to install a default route just like we did
for `h1_1`. Once installed you will notice that pings from `h1_1` now succeed.
## Conclusion
In this lesson you learned how to assign IP addresses to interfaces, what
subnet is and how it is used in routing, and you also learned how to install
default routs on hosts. With these foundations we can move on to more complex
routing where not all hosts are directly connected to the same router.
At this point you should have the same network as was for the `basic` scenario.
By building this network manually you learned how to assign IP addresses to
interfaces, what a subnet is and how it is used in routing, and you also
learned how to install default routes on hosts. With these foundations we can
move on to more complex routing where not all hosts are directly connected to
the same router.