Advanced Packet Data Testing with Linux

At Nortel Networks, we have developed a Linux-based system for testing a second-generation packet radio service. During system development we explored the details of packet radio, the IP internals of the Linux operating system and device-driver development.
Testing the Gb Interface

Data transportation in the world of mobility is often treacherous. The Gb protocol stack (see Figure 3) is designed to support reliable data transportation for multiple network addressing schemes (e.g., IP or X.25), serving innumerable applications in the packet data network.

Another large piece of the puzzle was the development of a test server and test services to test the Gb protocol stack. This development began about halfway through the development of the GGSN test server. Naturally, Linux was our primary development environment, and we planned on reusing the USN device and the Linux UDP/TCP/IP stacks all over again. However, this time they would be used to test the Gb stack by simulating calls via virtual mobiles, cells and base stations.

The server portion of the Gb test interface is a distributed application consisting of two primary components. One component resides on a PowerPC single board computer (SBC) housed in a VME chassis running a commercial RTOS. This component drives a frame-relay interface running on PCI (peripheral component interconnect) Mezzanine cards that support multiple T1 or E1 carriers. The second component performs resource management, proxies the frame-relay services and implements the BSSGP (base station system GPRS protocol) and NS (network service) layers of the Gb protocol stack. Thanks to the diversity of TTF, this component runs on either a PC running Linux 2.0 or a PowerPC SBC target running an RTOS.

Figure 7. Gb Test System

Within the test case as shown in Figure 7, the developer creates instances of mobiles, cells and base stations using Gb test services, all of which act as clients to the Gb test server. These test services complete the Gb stack by implementing the LLC, SNDCP and GMM (GPRS mobility management) Gb protocol layers per virtual mobile.

Reserving mobiles, cells and base stations, the developer can bring up simulated calls within their test case. Once the calls are up and the PDP contexts are established, the developer uses basic UDP or TCP sockets to send and receive data. The data is encapsulated by the Linux UDP/TCP/IP stack and is routed to the USN device. From there, test services retrieves the data from the USN device, passes it through the Gb SNDCP, the LLC layers of the virtual mobile, and then the Gb test server. At the server, the data is passed through the BSSGP and NS protocol layers and then transported over frame relay to the SGSN. This process is reversed in the receive direction.

All of this was very nifty, but a larger problem loomed on the horizon. What if we wanted to do something “normal”? Perhaps something that could stress the SUT. What about bringing up several simulated calls and using Netscape over them to browse the Internet? That would mean binding an application's execution environment to a specific PDP context.

Kernel Modifications

The test-case machine sits on both the user-space internal Gb LAN and the Gi LAN. In an example where the test case has two mobiles, each with an active context, several routing problems become immediately apparent. In this case, the user might want to launch two instances of Netscape, “binding” each instance to a different mobile's context (i.e., IP address). If one of these Netscape sessions opens a TCP socket to a Gi LAN web server, the kernel has three choices for routing the TCP packets: the kernel could send the packets to either one of the two mobile's USN devices, or to the Gi LAN's real Ethernet device.

For UDP or TCP sockets opened by the test-case developer, the developer can use the SO_BINDTODEVICE option by calling setsockopt. Binding to a specific device forces all packets sent on that socket to use the bound device; however, applications such as FTP, TELNET and Netscape do not use this option.

After careful observation of a Linux Virtual Private Network software package, we noticed programs such as TELNET and Netscape routed all their IP traffic out the VPN tunnel. Using nm on the provided shared library, we found that the shared library contained a complete replacement for the socket API calls.

As web browsing is the major motivator for mobile data services, we had to provide a way to surf the Web through a mobile's context. After many hours looking through the Linux networking code to forego rewriting the entire socket API, we decided to hook into the IPv4 socket creation. Our hook searches the current task's environment table for an environment variable (ND), and if it exists, binds it to the device specified by the ND variable.

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