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Cooperative Linux
Posted Date:
30 Mar 2008
Total Responses:
1
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Posted By: Girish Patil Member Level: Diamond Points: 1
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ABSTRACT
This seminar describes Cooperative Linux, a port of the Linux kernel that allows it to run as an unprivileged lightweight virtual machine in kernel mode, on top of another OS kernel. It allows Linux to run under any operating system that supports loading drivers, such as Windows or Linux, after minimal porting efforts. The paper includes the present and future implementation details, its applications, and its comparison with other Linux virtualization methods. Among the technical details, it also presents the CPU-complete context switch code, hardware interrupt forwarding, the interface between the host OS and Linux, and the management of the VM’s pseudo physical RAM.
TABLE OF CONTENTS
1. INTRODUCTION…………………………………………… (3)
2. COLINUX…………………………………………………… (4)
3. SYSTEM REQUIREMENTS……………………….………(5)
4. WORKING OF COLINUX……………………….…………(7)
5. DESIGN OVERVIEW………………………….….……….(9)
6. COMPARISON………………………………………………(14)
7. PLANNED FEATURES……………………….…………… (15)
8. USEFULL COMMANDS………………………..…..………(17)
9. COLINUX INSTALLATION……………..….……………..(19)
10. VIRTUALIZATION WITH COLINUX……….………….(23)
11. USES………………………………….………….…………(26)
12. ADVANTAGES &DISADVANTAGES………………….(29)
13. CONCLUSION……………………………………………..(31)
14. REFERENCES……………………………………………..(32)
1. INTRODUCTION
Cooperative Linux utilizes the rather underused concept of a Cooperative Virtual Machine (CVM), in contrast to traditional VM’s that are unprivileged and being under the complete control of the host machine. The term Cooperative is used to describe two entities working in parallel, e.g. co routines. In that sense the plainest description of Cooperative Linux is turning two operating system kernels into two big co routines. In that mode, each kernel has its own complete CPU context and address space, and each kernel decides when to give control back to its partner. However, only one of the two kernels has control on the physical hardware, where the other is provided only with virtual hardware abstraction. Seminar represents to these two kernels as the host operating system, and the guest Linux VM respectively. The host can be every OS kernel that exports basic primitives that provide the Cooperative Linux portable driver to run in CPL0 mode (ring 0) and allocate memory. The special CPL0 approach in Cooperative Linux makes it significantly different than traditional virtualization solutions such as VMware, plex86, Virtual PC, and other methods such as Xen. All of these approaches work by running the guest OS in a less privileged mode than of the host kernel. This approach allowed for the extensive simplification of Cooperative Linux’s design and its short early beta development cycle.
2. COLINUX:
Cooperative Linux is a relatively new means to provide Linux services on a Linux or Windows host. CoLinux is not an emulator or virtual machine. A real Linux kernel will run on your system alongside Windows. CoLinux is a system that consists of a Windows kernel driver, a Linux kernel and some userspace daemon programs. When the coLinux-daemon.exe is started, it will parse the given config file, load the linux.sys (Windows) kernel driver and tell it to reserve the specified amount of RAM for the Linux system. Then the daemon loads the Linux kernel in the reserved RAM area and starts it. The Linux kernel is a slightly modified version of the standard x86 kernel. It has been stripped of all parts that directly access the hardware. Instead the kernel uses (with the help of linux.sys) either defined APIs in the Windows kernel or user space programs to access needed resources like storage or network. This approach has the advantage, that there is no need for virtualization or emulation. The system behaves (from the user space point of view) like a normal Linux system, because there is one.
3. SYSTEM REQUIREMENTS
CoLinux requires Windows 2000, XP, or Windows Server 2003 in order to work, although I was unsuccessful at getting it to work under Windows Server 2003. I have yet to find out why. These NT-based operating systems include support for asynchronous operations and include security features not found in DOS-based versions of Windows. "Asynchronous operations" is just a fancy way of saying that the computer can work on new tasks while waiting for other tasks to complete rather than queuing tasks up.
coLinux also requires the use of the TAP-Win32 virtual Ethernet adapter driver. TAP-Win32 allows processes running under a Windows server to access that Windows server as if it were accessing it over the network and vice-versa. This probably brings some insight into how coLinux achieves its capability of appearing to run as a separate computer. The version of the coLinux installer I used for this article installed the TAP-Win32 driver as a part of the installation.
Winpcap is a utility for Windows that provides packet capture and analysis functions and is offers programmers a way to directly access a packet driver independent of Microsoft's APIs.
coLinux can be configured to be bridged to a specific physical network adapter in host computer. This is accomplished by making changes to the coLinux configuration file, as discussed later. For this article, the TAP-Win32 driver is used, which is installed automatically as a part of the coLinux installation.
As far as RAM and disk space go, this is where things get a little hairy with coLinux. Depending on how things work out, coLinux can use anywhere from 16MB to 64MB of RAM. In the docs, the authors use the phrase "It used to default to 29MB which was sufficient for most things, now it seems to default to 64MB", which probably doesn't inspire a whole lot of confidence.
This doesn't mean that a system with a total of 128MB of RAM will cut it. Operating system need to be run and other required processes, so be sure to have sufficient physical RAM to accommodate everything. Shoot for a minimum of 512MB, which, these days, shouldn't be much of a problem.
For disk space, a partition is required with a few gigs available, just like for a normal Linux server. Since coLinux works using disk images we can download, we can check the size of the image we want to use to determine exact requirements. Officially supported distributions include Gentoo, Debian, and Fedora Core in the 2.4 series kernel and just Gentoo and Debian if we prefer to use a kernel based on Linux 2.6. We can modify other kernels to work with coLinux, if you want, but I won't be going over that in this article.
4. WORKING OF COLINUX:
Unlike in other Linux virtualization solutions such as User Mode Linux (or the forementioned VMware), special driver software on the host operating system is used to execute the CoLinux kernel in a privileged mode (known as ring 0 or supervisor mode).
By constantly switching the machine's state between the host OS state and the CoLinux kernel state, CoLinux is given full control of the physical machine's MMU (i.e., paging and protection) in its own specially allocated address space, and is able to act just like a native kernel, achieving almost the same performance and functionality that can be expected from a regular Linux which could have ran on the same machine standalone. Since CoLinux uses the same binary format for user-space executables as native Linux, CoLinux can load and run an existing unmodified Linux distribution concurrently with the host OS.
HARDWARE VIRTUALIZATION
To cooperatively share hardware with the host operating system, CoLinux does not access I/O devices directly. Instead, it interfaces with emulated devices provided by the CoLinux drivers in the host OS. For example, a regular file in Windows can be used as a block device in CoLinux. All real hardware interrupts are transparently forwarded to the host OS, so this way the host OS's control of the real hardware is not being disturbed and thus it continues to run smoothly.
PORTABILITY
Unlike User Mode Linux, CoLinux always utilizes only one process of the host OS for all its Linux processes, privately managing their scheduling, resources, and faults in a manner which is contained and entirely independent of the way the host OS is implemented. In fact, CoLinux only requires a very small set of commonly exported primitives from the host OS kernel in order to work, thus, it can be rather easily ported to run under any operating system, such as Solaris, or even Linux itself. CoLinux is being ported to run under ReactOS, a standalone Open Source Windows implementation.
5. DESIGN OVERVIEW:
In this section the basic methods behind Cooperative Linux are described, which include Linux Symposium 2004 • Volume One • 25 complete context switches, handling of hardware interrupts by forwarding, physical address translation and the pseudo physical memory RAM.
DEVICE DRIVER:
The device driver port of Cooperative Linux is used for accessing kernel mode and using the kernel primitives that are exported by the host OS kernel. Most of the driver is OSindependent code that interfaces with the OS dependent primitives that include page allocations, debug printing, and interfacing with user space. When a Cooperative Linux VM is created, the driver loads a kernel image from a VMlinux file that was compiled from the patched kernel with CONFIG_COOPERATIVE. The VMlinux file doesn’t need any cross platform tools in order to be generated, and the same VMlinux file can be used to run a Cooperative Linux VM on several Operating Systems of the same architecture. The VM is associated with a per-process resource—a file descriptor in Linux, or a device handle in Windows. The purpose of this association makes sense: if the process running the VM ends abnormally in any way, all resources are cleaned up automatically from a callback when the system frees the per-process resource.
CONTEXT SWITCHING:
The Cooperative Linux VM uses only one host OS process in order to provide a context for itself and its processes. That one process, named CoLinux-daemon, can be called a Super Process since it frequently calls the kernel driver to perform a context switch from the host kernel to the guest Linux kernel and back. With the frequent (HZ times a second) host kernel entries, it is able to completely control the CPU and MMU without affecting anything else in the host OS kernel.
On the Intel 386 architecture, a complete context switch requires that the top page directory table pointer register—CR3—is changed. However, it is not possible to easily change both the instruction pointer (EIP) and CR3 in one instruction, so it implies that the code that changes CR3 must be mapped in both contexts for the change to be possible. It’s problematic to map that code at the same virtual address in both contexts due to design limitations—the two contexts can divide the kernel and user ad Linux Symposium 2004 • Volume One • 27 dress space differently, such that one virtual address can contain a kernel mapped page in one OS and a user mapped page in another. In Cooperative Linux the problem was solved by using an intermediate address space during the switch (referred to as the ‘passage page,’ see Figure 1). The intermediate address space is defined by a specially created page tables in both the guest and host contexts and maps the same code that is used for the switch (passage code) at both of the virtual addresses that are involved. When a switch occurs, first CR3 is changed to point to the intermediate address space. Then, EIP is relocated to the other mapping of the passage code using a jump. Finally, CR3 is changed to point to the top page table directory of the other OS. The single MMU page that contains the passage page code, also contains the saved state of one OS while the other is executing. Upon the beginning of a switch, interrupts are turned off, and a current state is saved to the passage page by the passage page code. The state includes all the general purpose registers, the segment registers, the interrupt descriptor table register (IDTR), the global descriptor table (GDTR), the local descriptor register (LTR), the task register (TR), and the state of the FPU / MMX / SSE registers. In the middle of the passage page code, it restores the state of the other OS and interrupts are turned back on. This process is akin to a “normal” process to process context switch. Since control is returned to the host OS on every hardware interrupt, it is the responsibility of the host OS scheduler to give time slices to the Cooperative Linux VM just as if it was a regular process. 0xFFFFFFFF Guest Linux Intermediate Host OS 0x80000000
INTERRUPT HANDLING AND FORWARDING
Since a complete MMU context switch also involves the IDTR, Cooperative Linux must set an interrupt vector table in order to handle the hardware interrupts that occur in the system during its running state. However, Cooperative Linux only forwards the invocations of interrupts to the host OS, because the latter needs to know about these interrupts in order to keep functioning and support the CoLinux-daemon process itself, regardless to the fact that external hardware interrupts are meaningless to the Cooperative Linux virtual machine. The interrupt vectors for the internal processor exceptions (0x0–0x1f) and the system call vector (0x80) are kept like they are so that Cooperative Linux handles its own page faults and other exceptions, but the other interrupt vectors point to special proxy ISRs (interrupt service routines). When such an ISR is invoked during the Cooperative Linux context by an external hardware interrupt, a context switch is made to the host OS using the passage code. On the 28 • Linux Symposium 2004 • Volume One other side, the address of the relevant ISR of the host OS is determined by looking at its IDT. An interrupt call stack is forged and a jump occurs to that address. Between the invocation of the ISR in the Linux side and the handling of the interrupt in the host side, the interrupt flag is disabled. The operation adds a tiny latency to interrupt handling in the host OS, but it is quite neglectable. Considering that this interrupt forwarding technique also involves the hardware timer interrupt, the host OS cannot detect that its CR3 was hijacked for a moment and therefore no exceptions in the host side would occur as a result of the context switch. To provide interrupts for the virtual device drivers of the guest Linux, the changes in the arch code include a virtual interrupt controller which receives messages from the host OS on the occasion of a switch and invokes do_IRQ() with a forged struct pt_args. The interrupt numbers are virtual and allocated on a per-device basis.
6. COMPARISONS:
Cooperative Linux is significantly different from traditional virtualization solutions such as VMware, plex86, Virtual PC, and other methods such as Xen, which generally work by running the guest OS in a less privileged mode than that of the host kernel. In contrast, the CPL0 approach simplified design with an early-beta development cycle of only one month -- starting from scratch by modifying the vanilla Linux 2.4.23-pre9 release until reaching to the point where KDE could run.
The only downsides to the CPL0 approach are stability and security. If it's unstable, it has the potential to crash the system (on earlier releases before ioperm was disabled attempting to start a normal X server under CoLinux would crash the host). However, measures can be taken, such as cleanly shutting it down on the first internal Oops or panic. Another disadvantage is security. Acquiring root user access on a Cooperative Linux machine can potentially lead to root on the host machine if the attacker loads specially crafted kernel module or (if the CoLinux kernel was compiled without module support) the attacker finds some other way to inject code into the running CoLinux kernel.
7. PLANNED FEATURES
Since Cooperative Linux is a new project (2004–), most of its features are still waiting to be implemented.
SUSPENSION
Software-suspending Linux is a challenge on standalone Linux systems, considering the entire state of the hardware need to be saved and restored, along with the space that needs to be found for storing the suspended image. On User Mode Linux suspending is easier— only the state of a few processes needs saving, and no hardware is involved. However, in Cooperative Linux, it will be even easier to implement suspension, because it will involve its internal state almost entirely. The procedure will involve serializing the pseudo physical RAM by enumerating all the page table entries that are used in Cooperative Linux, either by itself (for user space and VMalloc page tables) or for itself (the page tables of the pseudo physical RAM), and change them to contain the pseudo value instead of the real value. The purpose of this suspension procedure is to allow no notion of the real physical memory to be contained in any of the pages allocated for the Cooperative Linux VM, since Cooperative Linux will be given a different set of pages when it will resume at a later time. At the suspended state, the pages can be saved to a file and the VM could be resumed later. Resuming will involve loading that file, allocating the memory, and fix-enumerate all the page tables again so that the values in the page table entries point to the newly allocated memory. 30 • Linux Symposium 2004 • Volume One Another implementation strategy will be to just dump everything on suspension as it is, but on resume—enumerate all the page table entries and adjust between the values of the old RPPFNs2 and new RPPFNs.
A suspended image could be created under one host OS and be resumed in another host OS of the same architecture. One could carry a suspended Linux on a USB memory device and resume/suspend it on almost any computer.
USER MODE LINUX INSIDE COOPERATIVE LINUX
The possibility of running UML inside Cooperative Linux is not far from being immediately possible. It will allow bringing UML with all its glory to operating systems that cannot support it otherwise because of their user space APIs.
Combining UML and Cooperative Linux cancels the security downside that running Cooperative Linux could incur.
LIVE COOPERATIVE DISTRIBUTIONS
Live-CD distributions like KNOPPIX can be used to boot on top of another operating system and not only as standalone, reaching a larger sector of computer users considering the host operating system to be Windows NT/2000/XP.
8. USEFUL COMMANDS
Ifconfig:
Determine which a CoLinux adapter is running.
Df-h
Disk space usage
Gnome-terminals
Opens additional external windows from the CoLinux console. Requires an X server application on Windows. A few short cuts are listed here:
Ctrl-shift-c copy text
Ctrl-shift-v paste text
Ctrl-shift-t Create a new terminal tab in the same window.
Ctrl-shift-w close current tab
Ctrl-Page Up/Ctr-PageDown move between tabs
System-config-rootpassword
Change the root password. This screenshot demonstrates system-config-rootpassword being run from withen gnome-terminal.
System-config-users
Create, modify or delete users groups.
System-config-security level
Graphical interface for managing CoLinux firewall setting.
system-config-services
Graphical interface for starting and stopping services.
Yumex
Graphical interface for adding, removing and keeping CoLinux software up-to-date.
9. COLINUX INSTALLATION
CoLinux is a port of the Linux kernel to the Windows Os. This means you can run Linux within Windows without the use of VMware. This step by step guide will show you how to install CoLinux on your system. It does not show you how to setup your network connections nor does it show you how to use x-windows with CoLinux.
Install CoLinux
1. The 1st step you need to do is download the CoLinux installer.
2. When you run the installer and it ask what components you want to install make sure all of them are to be installed.
3. This is a very important step. When you run the setup the default installation path is c:\program files\CoLinux. Do not install CoLinux to this directory. Instead install CoLinux to C:\CoLinux.
4. This next step will download a Linux file system image. This step is optional. You do need a root file system image though in order to use CoLinux. There are two ways you can get a CoLinux compatible file system.
A. You can download one of the file systems that they provide in the installer.
B. You can go to there projects page on source forge and download one of the avaliable file systems.
STARTING COLINUX
Once you have downloaded CoLinux and have it installed to C:\CoLinux you will need a tool called Winrar. Winrar is like winzip but allows you to extract many more formats then programs like Winzip. Once you have winrar downloaded then follow these directions.
1. Open the directory c:\CoLinux. If you downloaded debian you will see a file called "Debian-3.0r0.ext3.1gb.bz2" This is the debian file system that we need in order to run CoLinux. Open this file with winrar and extract the contents of it to the same directory.
2. Once you have extracted it then your almost set. The next step you need to do is download a swap file. The swap file needs to be the size of your physical ram. So if I have 256 mb of ram I download the swap file that is 256mb in size. You can download the swap files here. Once you have downloaded the swap file you will need to extract the file to the same directory that you installed CoLinux to.
3. There is one more step into installing CoLinux. You must configure it. When CoLinux starts it reads from a config file. Open this file default.coLinux.xml in Windows notepad. default.coLinux.xml can be found in the same directory were you installed CoLinux to. There are comments in the xml file that tell you what each part is for. If you downloaded debian then your config should look something similar to this
10. VIRTUALIZATION WITH COLINUX
Virtualization with VMWare, Xen, and Kernel-based Virtual Machine (KVM) are all the rage these days. But did you know that you can run Linux cooperatively with Microsoft Windows? This article explores Cooperative Linux (coLinux), starting with a quick introduction to virtualization and then looking at the approach taken by coLinux. coLinux is a port of the Linux operating system that executes as a single process in the Microsoft operating system. The operating systems cooperate with each other by giving each other the central processing unit (CPU), as shown in Figure 1.
Figure 1. Microsoft Windows and Linux cooperate with each other in coLinux
Through the use of kernel drivers, physical resources such as the networking interface are also shared.
VIRTUALIZATION
Virtualization is an over-used term. In the context of this article, I'm referring to the platform variation. Virtualizing a platform (or hardware) means that the hardware is abstracted from a physical platform into a collection of logical platforms onto which operating systems can be run. In the simplest sense, this means that you can run multiple operating systems (of the same or different types) on the same hardware platform.
The element of the system that provides the virtualization is commonly known as a virtual machine monitor or hypervisor. Each operating system uses its own virtual machine that cooperates with the hypervisor to arbitrate access to the physical hardware (see Figure 2).
Figure 2. The hypervisor arbitrates access to the physical hardware
coLinux is a port of the standard Linux kernel. In other words, coLinux is the Linux kernel that's modified to run cooperatively with another operating system. The host operating system (Windows or Linux) maintains control of the physical resources of the operating system, while the guest operating system (coLinux) is provided with a virtual abstraction of the hardware. The host operating system must provide the means to execute a driver in the privileged ring (ring 0) and export the means to allocate memory.
Figure 3. COLINUX EXECUTES AS A PROCESS OF THE HOST OPERATING SYSTEM
The root file system for coLinux is a regular file within the host operating system. To Windows it's just a regular file, but to coLinux it's an ext3 file system that can be read and written to.
Other features needed by the Linux kernel, such as networking or video access, are proxied externally. Networking is made accessible to coLinux through a TUN/TAP driver (which is covered in "Networking," below). In short, this driver provides user-space access to the Ethernet device so that packets can be transmitted and received. Access to the display is also proxied. Recall that X Window System is a protocol by which video output can be sent from one host to another. Therefore, by using an X Window System server on the host operating system, video output can be redirected to the available X server.
11. USES:
Cooperative Linux in its current early state can already provide some of the uses that User Mode Linux provides, such as virtual hosting, kernel development environment, research, and testing of new distributions or buggy software. It also enabled new uses:
RELATIVELY EFFORTLESS MIGRATION PATH FROM WINDOWS.
In the process of switching to another OS, there is the choice between installing another computer, dual booting, or using virtualization software. The first option costs money, the second is tiresome in terms of operation, but the third can be the most quick and easy method—especially if it’s free. This is where Cooperative Linux comes in? It is already used in workplaces to convert Windows users to Linux.
ADDING WINDOWS MACHINES TO LINUX CLUSTERS.
The Cooperative Linux patch is minimal and can be easily combined with others such as the MOSIX or Open- MOSIX patches that add clustering capabilities to the kernel. This work in progress allows adding Windows machines to super-computer clusters, where one illustration could tell about a secretary workstation computer that runs Cooperative Linux as a screen saver—when the secretary goes home at the end of the day and leaves the computer unattended, the office’s cluster gets more CPU cycles for free.
RUNNING AN OTHERWISE-DUAL-BOOTED
Linux system from the other OS. The Windows port of Cooperative Linux allows it to mount real disk partitions as block devices. Numerous people are using this in order to access, rescue, or just run their Linux system from their ext3 file systems.
USING LINUX AS A WINDOWS FIREWALL ON THE SAME MACHINE.
As a likely competitor to other out-of-the-box Windows firewalls, iptables along with a stripped-down Cooperative Linux system can potentially serve as a network firewall.
LINUX KERNEL DEVELOPMENT / DEBUGGING/ RESEARCH AND STUDY ON ANOTHER OPERATING SYSTEMS.
Digging inside a running Cooperative Linux kernel, you can hardly tell the difference between it and a standalone Linux. All virtual addresses are the same-Oops reports look familiar and the architecture dependent code works in the same manner, excepts some transparent conversions.
12. ADVANTAGES AND DISADVANTAGES
ADVANTAGES
? coLinux is a great way to use and experiment with Linux. Like Cygwin, it allows you to develop and execute Linux applications on the Windows operating system (through CoLinux). You can also maintain the Linux operating system by installing, upgrading, or removing applications with apt-get.
? Unlike Cygwin, you can execute Linux applications on CoLinux without rebuilding. In this respect, CoLinux is a real Linux operating system that runs (or cooperates) with the Windows operating system.
? Another interesting advantage of cCoLinux is its portability. You can have a CoLinux distribution with a custom set of applications on a given Windows host (within the root file system). You can move the root file system to another host, and then restart it. This allows for a mobile development platform where the compressed root file system fits on a standard Universal Serial Bus (USB) memory stick.
? Finally, CoLinux is fast because it's essentially running on the native hardware.
DISADVANTAGES:
? The CoLinux runs aside the Windows kernel.
? It runs on the same hardware abstraction layer that is also called ring0.
? Inside this CPU abstraction layer, there is no hardware memory protection.
? This is the same situation as between the Linux or Windows kernel and their device drivers. So a problem in the Linux kernel can bring the Windows kernel down, if Linux violates the Windows address space.
? Also there are security implications. If a malice user gains root access inside CoLinux he can compromise the security of the Windows system as well. Also to load and use CoLinux the user has to have administrator rights. (There is the possibility to start CoLinux as a service, so it is possible to start CoLinux as a normal user, if the user has been granted the right to start the service)
? The primary disadvantage of CoLinux is that it has the ability to crash the entire machine (all cooperating operating systems) because the guest operating system runs in a privileged mode in the host kernel. It also has some dependencies on external software for normal operation (windows and networking support). Outside of this, it's quite easy to install and configure. In the many hours that I've used it, I've never seen a crash.
13. CONCLUSIONS
We have discussed how Cooperative Linux works and its benefits—apart from being a BSKH4, Cooperative Linux has the potential to become an alternative to User Mode Linux that enhances on portability and performance, rather than on security. Moreover, the implications that Cooperative Linux has on what is the media defines as ‘Linux on the Desktop’—are massive, as the world’s most dominant albeit proprietary desktop OS supports running Linux distributions
for free, as another software, with the aimed for possibility that the Linux newbie would switch to the standalone Linux. As user friendliness of the Windows port will improve, the exposure that Linux gets by the average computer user can increase tremendously.
14. REFERENCES:
www.google.co.in
www.linux.com
www.wikipedia.com
www.howstuffworks.com
www.colinux.org
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Responses
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| Author: Saranya 19 May 2008 | Member Level: Gold | Rating:  Points: 2 | Hi Girish Patil,
This is really very informative.
Thank you.
Regards,
Saranya...
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