How to communicate with the NFVI
New chaining functionality
Communications with the NFVI are not trivial: there are different problematics that range from the initial connection to the way packets have to be transmitted. Here, during our thesis development, we come up with two approaches at these problems: using a tunnel (TUN/TAP section) to automatically encapsulating the data (at the cost of losing the NFVI transparency) or using a proxy to make the NFVI like a man-in-the-middle, handling the extremes of the communications with the sender and the receiver.
TUN/TAP
TUN and TAP are virtual kernel interfaces. These differs from the normal network interfaces because they are completely software and no physical components are required.
TUN (network TUNnel) operates with layer-3 packets (as IP packets), TAP instead (network tap) works on layer 2, with Ethernet packets. When used with user-space programs TUN/TAP devices injects packets to the operating-system network stack: this emulates incoming data from an external source.
In order to manage the communications on a tun/tap interface the program gets a file descriptor where its able to perform I/O operations. From the kernel point of view these packets are behave the same as incoming packets from an external source.
These interfaces can be both transient and persistent. Transient means that the interface is created, used and destroyed by the same program, persistent means that are used some utility to create them and then programs can attach to it.
Once created the interfaces can be used as normal "hardware" network interfaces.
TUN e TAP interfaces are used for VPNs, virtual machine networking, to connect physical machines to network emulators and NATs.
Network tunnel
Network tunneling protocol is a communication protocol that makes possible exchange of data in secure ways across public networks and to run a protocol that is not supported by a certain network thanks to the encapsulation. Basically, the tunneling protocol works wrapping the original packet inside the payload of a new one.
Network TAP
Network Terminal Access Point is a external monitoring device that mirrors the traffic that pass through two nodes, instead TAP is the device in the network that actually monitor the traffic.
TunConnector
The first change we performed was to create a TUN tunnel between VNFs. This allowed us to manipulate packages in better way encapsulating the original message sent from the sender as the payload of a UDP packet, and exchanging it among the VNFs. In that way we were able to maintain most of the original information, and to send the packet to the right destination.
In order to accomplish that, we create TunConnector: this is a naive implementation of a software that create a virtual tun interface on the host machine and than allows the connection to another machine or waits for connection. This software was developed following this tutorial and starting from simpletun code.
TunConnector requires to specify the port on the host used to create the TUN tunnel and the IP that will be used in it to reach the machine.
How it works
TunConnector can work as a client, server or both modality (for a certain link the host will be a client, for another the server). The first operation that it performs in each configuration is to create a TUN device using an OpenVPN utility:
In order to perform that operation TunConnector must run as root. First command create an interface, in this case called tun3. The second one turns up the newly created interface. Last one set the IP address (in the example above 192.168.0.2) that will be used in the tunnel. This is a naive way to create a persistent interface: in fact it is possible to do it using C++, but for the sake of simplicity and because of the controlled environment in which we work, that is the best choice.
After both in client and in server mode it allocates 2 socket, retrieving the correspondent file descriptors: one is used for the communication inside the tunnel, the other to communicate with the "usual" network. In case of server mode after it binds a socket to a specific port in order to listen for requests of connections.
Once a server receive a request of connection from a certain client, the tunnel will be created. This tunnel consist of a infinite loop in which data for the tunnel are redirected to the right file descriptor from the network and vice versa.
Example
Usage:
--serverinterface -p XTo set the name if the interface used by the server component--clientinterface -n XTo set the name if the interface used by the client component--serverip -s XTo set the IP on the server interface--clientip -c XTo set the IP on the client interface--inport -i XTo set the port used by the server component. Default55556--remoteip -r XTo set the ip of the remote server to connect to--mode -m [client|server|both]To set run mode. Defaultboth--help -hShow the help
with the following options
$ sudo ./TunConnector -p tun0 -s 10.0.0.2 -i 55555 -m server
In the example above we suppose to create a server on a host with IP 192.168.6.9 (the IP is required from the client), that accept requests on port 55555. The interface will be named tun0 and inside the tunnel the machine will have the address 10.0.0.2.
In an another machine the client run in a mirrored way the program, specifying the server port and IP address.
Once the tunnel is up, you can check that the connection is working for example pinging the two ends of the tunnel with the IP address used inside the tunnel.
Problems of this approach
At the end of the day we've choose to not use this approach, because the use of point-to-point connections made our entire structure stiff: our final goal is to have VNF chains that can dynamically adapt and change. Making point-to-point connections means this it's not possible, because every time a packet has to go to a new VNF first a new connection (tunnel) has to be established and next the packet has to be sent. This, multiplied for a great number of packets makes the whole system unsustainable. Finally, scalability has to be taken in consideration too: when creating a point-to-point connection between two pods these jeopardize a fair load distribution between replicas, since the tunnel is created only with one instance, and not all of them. Thus, to make the system really scalable, every connection should be service-to-service, and not point-to-point, making the total number of the connections between two VNF services equal to N * M, where N is the number of pods for one VNF service and M is for the other one.
This is why we opted for a proxy-oriented solution.
Proxy-oriented connection
As already said at the beginning of this article, a proxy connection makes the NFVI acting like a man-in-the-middle, where packages are first elaborated and secondly a proxy sends them to the receiver, managing all details regarding the connection. The packets follows this flow:
The client machine, with a local proxy installed, sends the packet to the receiver. The packet goes through the proxy and gets sent to the NFVI, where another proxy (called Ingress) listen.
The Ingress proxy receives the packet, stores the packet header in a specific back-end (Roulette) and performs packet classification, eventually choosing the VNF chain where the packet will be elaborated. Finally, the packet gets send to the right chain
For every VNF, the packet comes and gets elaborated. To determine the next hop, a request is made to Roulette, with replies with the name of the next VNF where the packet has to be forwarded. The last VNF forwards the packet to the proxy that communicates with the receiver, called Egress
The Egress receives the packet wrapped inside a UDP one, communicates with Roulette additional information about who received it (with the aim to maintain the session between the sender and the receiver when other packages) and sends the packet to the receiver.
If the receiver replies, the reply follows a symmetrical pattern, where now the Egress becomes the Ingress of the connection.
This flow, while it seems complicated at first, it allows to maintain the state of the connection in TCP communications, where the session is fundamental.
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