Major Internet Applications

It is traditionally the most significant and remains the most commonly employed. This illustrates the simple structure in which client processes cooperate with individual server process in separate host computers in order to access the shared resources those they mange.

Servers may in twist be clients of other servers, as the figure indicates. For example, a web server is frequently a client of a local file server that manages the files in which the web pages are store.

For the applications discussed in Exercise 2.1 state how the servers cooperate in providing a service.

Web servers and most other Internet services are clients of the DNS service, which translate Internet Domain Names to network addresses. Also, search engines, which enable users to look up summary of information obtainable on web pages at site all over the Internet.

A search engine is a web server that responds to client requests to search in its stored indexes and (concurrently) runs several web crawler tasks to build and update the indexes. What are the requirements for synchronization between these concurrent activities?

What happen are the server tasks (respond to user queries) and the crawler tasks (making request to other web servers) are completely independent because there is small need to synchronize them and they may run concurrently. In reality, atypical search engine would normally include many concurrent threads of execution some serving its clients and others running web crawlers.

The host computers used in peer-to-peer systems is often simply desktop computers in users’ offices of homes. What are the implications of this for the availability and security of any shared data objects tat they hold and to what extent can any weaknesses be overcome through the use of replication?

List the types of local resource that are vulnerable to an attack by an untrusted program that is downloaded from a remote site and run in a local computer.

Network communication – the program might attempt to create sockets, connect to them, and send messages .Access to printers. It may also impersonate the user in various ways for example: sending receiving email

Objects in the file system for example files, directories can be read/written/created/deleted using the rights of the local user who runs the program.

Mobile agent is a running program (including both code and data) that movements from one computer to another in a network transport out a task on someone’s behalf, such as collect information, finally returning with the results. A mobile agent can make many invocations to local resources at each site it visits for example, access individual database entries.

Give some examples of applications where the use of mobile code is beneficial.

Applets one example of mobile code which means: the user running a browser selects a link to an applet whose code is stored on a web server so the code is downloaded to the browser and runs there

Accessing services which mean: running code that can invoke their operations.

What factors affect the responsiveness of an application that accesses shared data managed by a server? Describe remedies that are available and discuss their usefulness.

When the client accesses a server, it makes an invocation of a process in a server running in a remote computer. These things that affect the responsiveness:

Server overloaded, Latency in exchanging request and replies, Load on network.

The use of reserve helps with all of the above problems. In fact client reserve reduces all of them. Proxy server reserve helps with duplication of the service also helps with the use of lightweight communication protocols helps with.

Distinguish between buffering and caching.

Buffering: a method for store data transmit from a sending process to a receiving process in local memory or disk storage until the receiving process is prepared to use it. For example, when reading data from a file or transmitting messages during a network, it is useful to handle it in huge blocks. The blocks are held in buffer storage in the receiving process’ memory space. The buffer is free when the data has been used by the process.

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Caching: a technique for optimizing access to isolated data objects by hold a copy of them in local memory or disk storage. Accesses to parts of the remote object are translated into accesses to the related parts of the local copy. Unlike buffering, the local copy may be retained as long as there is local memory obtainable to hold it. A cache management algorithm and a release policy are wanted to run the use of the memory allocated to the cache.

Give some examples of faults in hardware and software that can/cannot be tolerated by the use of redundancy in a distributed system. To what extent does the use of redundancy in the appropriate cases make a system fault-tolerant?

Hardware faults – processors, disks, and network connections can use redundancy for example: run process on multiple computers, write to two disks, have two separate routes in the network available.

Software bugs, crashes. Redundancy is not high-quality with bugs because they will be replicated. Replicated processes help with crashes which may be due to bugs in unrelated parts of the system. Retransmitted messages help with lost messages. Redundancy makes faults less likely to occur

Consider a pair of processes X and Y that use the communication service B from Exercise 2.14 to communicate with one another. Suppose that X is a client and Y a server and that an invocation consists of a request message from X to Y (that carries out the request) followed by a reply message from Y to X. Describe the classes of failure that may be exhibited by an invocation.

An invocation may suffer from the following failures:

• Crash failures: X or Y may crash. Therefore an invocation may suffer from crash failures.
• Omission failures: as SB suffers from omission failures the request or reply message may be lost.

Describe possible occurrences of each of the main types of security threat (threats to processes, threats to communication channels, denial of service) that might occur in the Internet.

Threats to processes: not including authentication of main and servers, a lot of threats exist. An enemy could access other user’s files or mailboxes. Threats to communication channels: IP spoofing – sending requests to servers with a false source address.