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56
lessons/introduction-to-route-0.md
View File

@ -6,13 +6,12 @@ Linux, and Wireshark.
## Topology ## Topology
First, let's look at the topology that we will be using for this lesson, the 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 [`one_rtr` topology](../topology/one_rtr). The network is very
[README](../topology/one_rtr/README.md). The network is very simple. It simple. It consists of three nodes, but only one of them, `R1`, is a router,
consists of three nodes, but only one of them, `R1`, is a router, hence the hence the name of the topology. The other two are end-hosts. A host is not
name of the topology. The other two are end-hosts. A host is not necessarily necessarily a different device to a router, but it has a very different role in
a different device to a router, but it has a very different role in the the network. A host will only have one outgoing link and it will not forward
network. A host will only have one outgoing link and it will not forward IP IP packets which means that it can only be the source or destination of 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 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`. 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 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 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 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 addresses and default routes.
lesson.
Once the CLI prompt appears let us inspect Mininet's representation of the Once the CLI prompt appears let us inspect Mininet's representation of the
network by running network by running
@ -35,7 +33,7 @@ net
in the command prompt. The output tells us about all the nodes in the network 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 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 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 the output into this [web
tool](https://achille.github.io/mininet-dump-visualizer/) though its usefulness tool](https://achille.github.io/mininet-dump-visualizer/) though its usefulness
is limited for small networks such as this. 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 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 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 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 should belong to an interface on a directly connected neighbour. Note that you
routes in this network setup, because the network is too simple. 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 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 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 ### ping
The command `ping` sends a special IP packet to the specified destination to 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 verify connectivity with that end-host. Connectivity is verified if a response
IP address on `h1_2` by running 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 h1_1 ping 10.2.0.1
``` ```
@ -101,17 +100,17 @@ because it is directly connected to both of them.
## Wireshark ## 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 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 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. 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 Furthermore, it is able to present them in a human readable form rather than
simply dumping the binary representation directly from the wire. simply dumping the binary representation directly from the wire.
Start by running the command to trigger `h1_1` to start sending pings to Start by running the command to trigger `h1_1` to send pings to `h1_2`. Now
`h1_2`. Now open a new terminal window and navigate to the `route0` directory. open a new terminal window and navigate to the `route0` directory. We will use
We will use the `attach.py` helper script to run Wireshark on `R1` and `h1_2`. the `attach.py` helper script to run Wireshark on `R1` and `h1_2`. Let's start
Let's start with `R1` by running with `R1` by running
``` ```
sudo python attach.py --node R1 --cmd wireshark 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 the interface name or select the appropriate button on the menu bar in the
top-left corner. top-left corner.
Once the packet capture notice how the ping packets appear every second as a Once the packet capture window opens notice how the ping packets appear every
request/reply pair. Look at the source and destination IP addresses as well. second as a request/reply pair. Look at the source and destination IP
Note how the originating node has filled out the source address with the addresses as well. Note how the originating node has filled out the source
address of its interface `h1_2-eth1` and how the reply has the addresses address with the address of its interface `h1_1-eth1` and how the reply has the
flipped around. Have a look around and inspect the contents if you wish, but addresses flipped around. Have a look around and inspect the contents if you
we won't go into any detail on the form of the ping packets. 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 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 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 ## Conclusion
In this lesson you learned how to start up Route 0 experiments and learned how In this lesson you learned how to start Route 0 experiments and learned how to
to inspect your network using basic Linux commands and Wireshark. You will inspect your network using basic Linux commands and Wireshark. You will find
find these tools will come in handy at all times whenever dealing with these tools will come in handy at all times whenever dealing with networks.
networks.

74
lessons/ip-addresses-and-subnets.md
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@ -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 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 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 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. the `basic` network to illustrate all the different concepts involved.
### Assigning IP addresses ### Assigning IP addresses
A good place to start would be to simply assign all the IP addresses as per the 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 [`one_rtr` topology](../topology/one_rtr). The command to assign an IP address
assign an IP address to an interface in Linux has the form 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 This command assigns the address `<ip>` associated with the subnet defined by
`mask-digits` to the interface `if-name`. This should be pretty 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 self-explanatory except for the subnet which may be a new concept for some of
you. you.
An IPv4 address is basically a 32-bit number. The common representation An IPv4 address is a 32-bit number. The common representation `x.x.x.x` simply
`x.x.x.x` simply splits this number into four 8-bit numbers making it more splits this number into four 8-bit numbers making it more readable for a human.
readable for a human. This is why none of the four numbers ever exceed 255 as This is why none of the four numbers ever exceed 255 as that is the largest
that is the largest number you can represent with 8 bits. number you can represent with 8 bits.
A subnet is a subdivision of an IP network and determines all the possible IP 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. 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. subnet by using this interface.
The subnet of an IP address is determined by its prefix. The length in bits of 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 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 `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 `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. 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. `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 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 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 `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. 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. 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. The first thing you will notice is how `h1_1` keeps sending ARP protocol
ARP stands for the Address Resolution Protocol and is the mechanism by which a messages. ARP stands for the Address Resolution Protocol and is the mechanism
node finds the MAC address of the interface associated with the particular IP by which a node finds the MAC address of the interface associated with the
address. In order to send a packet over a link it must be addressed to the particular IP address. In order to send a packet over a link it must be
right MAC address as otherwise no interface on the local network will pick the addressed to the right MAC address as otherwise no interface on the local
packet up. In this case we see packets constantly asking "Who has 10.1.0.5? network will pick up the packet. In this case we see packets constantly asking
Tell 10.1.0.1", but nobody owns that IP address so nobody responds. "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 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 `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 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. 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 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 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 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. 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 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 `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 via `R1` which would work for `h1_2`, but would fail as soon as any new host is
new host is added to `R1`. added to `R1`.
Instead we will add a default route to our host. A default route is the route 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 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. 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 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 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 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 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 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 the other so that's not it. If you also inspect `h1_2` you will find that the
actually manage to make their way to the destination, but no response is sent. 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 Can you figure out what's going on? What happens if you try pinging `h1_1`'s
interface from `h1_2`? interface from `h1_2`?
The problem is that `h1_2` doesn't have a default route itself. It receives 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 the ping packets and it tries to send a response back to the source IP address,
then it finds out it doesn't know how which way to send a packet to that IP but then it finds out it doesn't know what to do with a packet addressed to
address. The solution is to install a default route just like we did for that IP address. The solution is to install a default route just like we did
`h1_1`. Once installed you will notice that pings from `h1_1` now succeed. for `h1_1`. Once installed you will notice that pings from `h1_1` now succeed.
## Conclusion ## Conclusion
In this lesson you learned how to assign IP addresses to interfaces, what At this point you should have the same network as was for the `basic` scenario.
subnet is and how it is used in routing, and you also learned how to install By building this network manually you learned how to assign IP addresses to
default routs on hosts. With these foundations we can move on to more complex interfaces, what a subnet is and how it is used in routing, and you also
routing where not all hosts are directly connected to the same router. 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.