Differentiate between upward and downward multiplexing?

The process of multiplexing is carried out at the transport layer. Several conversations are multiplexed in to one connection or physical links or virtual circuit. For example, suppose the host has only one network address available for use. Then it has to be used by all the transport connections originating at that host. For multiplexing the following two main strategies are followed:

Ø  Upward multiplexing and

Ø  Downward multiplexing

Upward Multiplexing 

- In upward multiplexing, the different transport connections are multiplexed in to one network connection. 

- These transport connections are grouped by the transport layer as per their destinations. 

- It then maps the groups with the minimum number of network connections possible.

- The upward multiplexing is quite useful where the network connections come very expensive.

Downward Multiplexing 

- It is only used when the connections with high bandwidth are required. 

- In case of the downward multiplexing, the multiple network connections are opened by the transport layer and the traffic is distributed among them. 

- But for using downward multiplexing, it is necessary that this capacity must be handled well by the subnet’s data links.

Another Technique 

- In either of the cases it is not guaranteed that the segments will be delivered in order. 

- Therefore, another technique is adopted. 

- The segments are numbered sequentially. 

- Each octet is numbered by the TCP sequentially. 

- Segments are then numbered based up on the number of the first octet present in that segment. 

- The segments might get damaged in the transition or some may even fail to arrive at the destination. 

- This failure is not acknowledged by the transmitter. 

- However, the successful receipt of the segment is does acknowledged by the receiver. 

- Sometimes, the cumulative acknowledgements might be used. 

- If the ACK triggers a time out interrupt, the re-transmission of the segment is done. 

- Also the re-transmission is done when an ACK is lost. 

- The receiver must have the ability to recognize the duplicate ACKs. 

- If such thing occurs, the receiver assumes by itself the ACK might have been lost.

- This happens when the ACK duplicate is received before the connection is closed. 

- If the duplicate is received after the closure of the connection, the situation is dealt differently. 

- In this case, the sender and receiver are allowed to know about each other’s existence. 

- They negotiate about the parameters and the transport entity resources are allocated based up on some mutual agreement. 

The connection release is of two types:

Ø Asymmetric release: 

This is the one used in the telephone systems. However it does not works well for the network that use packet switching.

Ø  Symmetric release: 

- This is certainly better than the previous one.

- Here, all the directions are released independently with respect to each other. 

- The host continues receiving data after the disconnection TPDU has been sent. 

- But the symmetric release has another problem which is related with indirection levels and fake messages. 

- There are no proper solutions for this problem in case of the unreliable communication media. 

- Note that this has nothing to do with the protocol. 

- Putting a reliable protocol over an unreliable medium can actually guarantee the delivery of the message. 

- Another thing to be noted is that it the time limit within which the message will be delivered cannot be guaranteed by any protocol. 

- Error conditions might prolong the delivery period. 

- Restarting the connections can lead to the loss of all the state info and the connection might remain as half-open. 

- Since no protocol has been designed to deal with this problem therefore one has to go forward with the risks associated with releasing the connections. 

Explain the technique of admission control to control congestion in virtual circuit subnets?

- Virtual circuits can be thought of as a virtual channel in the telecommunication networks as well as computer networks. 

- Virtual circuit sub-nets represent a communication service that is connection oriented. 

- This service is used through the packet mode communication.

- A stream of data bytes can be exchanged between the two nodes only if a virtual circuit has been established between them. 

- Without the presence of the higher level protocols, data division has to be dealt with unnecessarily. 

- Therefore, the virtual circuits always allow the high level protocols. 

- There is a resemblance between the circuit switching mode and the virtual circuits because of the fact that both of them are based up on connection. 

- The packets that are transmitted through a virtual circuit sub-net consist of a circuit number and not the destination address. 

- This is why the memory requirement of the packets is less when using virtual circuits and more in the others. 

- This also makes the virtual circuit sub nets less expensive when compared to other sub nets. 

In this article we discuss about the technique for congestion control in virtual circuit sub nets. 

- One most popular technique is of the admission control. 

- Most of the congestion control methods are based up on an open loop i.e., the congestion is prevented rather than managing it after it has occurred. 

- Admission control is a dynamic method for controlling the congestion problems in the virtual circuit sub nets. 

- Admission control technique has been widely accepted for preventing the congestion problem from getting worse over the time. 

- The technique is based up on a very simple idea which is that no virtual circuit is set up until and unless the congestion problem that was detected has been resolved. 

- Therefore, any attempt that is made for establishing a new virtual connection with the transport layer is failed. 

- The things get even worse if the access is granted to more and more people. 

- Simplicity of the technique is one of its characteristic that makes it easy to be implemented. 

- The technique can be implemented in a straightforward manner. 

- The admission control technique is also used by the telephone systems for combating with the congestion problems. 

- The admission control technique is implemented whenever a switch in the network gets overloaded. 

- At this time no dial tone is heard. 

- Establishing new virtual connections represents another way of coping with this problem. 

- Here, the new connections have to be routed carefully such that there are no problems. 

Another method for eliminating the problem of congestion is to strike an agreement between the virtual circuit subnet and the host. 

- By this we mean establishing a new virtual circuit. 

- But this arrangement requires specifying how the traffic has to be shaped and what would be its volume, QoS (quality of service), other parameters and so on. 

- The virtual circuit sub net has to reserve the resources on its part of the agreement established. 

- These resources lie on the route where the virtual circuit has been established. 

- The resources might include the following:

Ø  Space in the router’s buffer.

Ø  Tables

Ø  Bandwidth of the lines and so on.

- The newly virtual circuits are less likely to experience the congestion problems. 
- This is because to them the availability of the resources has been guaranteed.
- Resources can be reserved in this way only if the existing sub nets are experiencing congestion problem or when the standard operating procedure is being followed. 
- One disadvantage of the admission control technique is that it leads to the wastage of the resources. 
- Also, sometime the bandwidth is left unused. 

What is meant by load shedding?

The network is monitored by the network monitoring systems. These systems need to be robust and must be capable of inevitably coping with the situations in which the overload occurs. The network gets overloaded because of the nodes generating large volumes of data at high rates. Overload might also occur because of the burstiness of the traffic in its normal course of operation. For reducing the load of the network, load shedding techniques are applied. 

- Load shedding techniques have to be followed if the network is under a lot of stress. 

- This has to be done while monitoring the network for avoiding the packet loss that otherwise might be uncontrollable. 

- Load shedding involves sampling the incoming traffic. 

- CoMo or continuous monitoring has been developed to serve this purpose. 

- It uses such a load shedding scheme which can infer the query’s cost using the relation between the set of features of the traffic and the actual resource usage without having any knowledge of the plug-ins. 

- Here, traffic feature can be defined as a counter describing the incoming traffic’s particular property. 

The property might be any of the following:

Ø  Number of packets

Ø  Number of bytes

Ø  Flows

Ø  Unique IP destination address and so on.

- The CoMo consists of a prediction and the load shedding sub-system for intercepting the packets prior to sending them to the plug-in from the filter.

- A traffic query is implemented by this plug-in. 

- The system completes the process in 4 phases. 

- In the first phase, it forms a batch of packets for each 100ms of the traffic. - It then processes each of these batches for extracting a predefined traffic features’ set that is quite large. 

- From these, the most relevant sets are selected by the feature selection sub-system based up on the present stats of the CPU usage of the query. 

- The selected subset is then supplied as input for the “multiple linear regression subsystem”. 

- This is done for the prediction of the CPU cycles that the query requires for processing the whole batch. 

- If the prediction is greater than the capacity of the system, the batch is pre-processed by the load shedding subsystem for discarding the packet’s portion. 

- The batch is discarded through packet or flow sampling. 

Load shedding is now being seen as an effective method for curbing the overload situations even in the real time systems. 

- It involves shedding excess of the load in such a way that the stability of the system is not disturbed and also the system buffers do not experience any overflows. 

- The idea for applying the technique of load shedding in the field of networking has been adopted from the concept of the electric power management.

- Here, the electric current is intentionally disconnected on particular lines when the demands for the power supply are higher than what is being supplied.

- CoMo is an open source system and can be quickly implemented and can be used for further deploying other network monitoring applications. 

- The system has been written using C language and uses a feature rich API. 

- The system works by predicting the CPU usage of the system and thus anticipates about the resource requirements bursts that might occur in future. 

- The load shedding scheme used by the CoMo has the capability of automatically identifying the features using which the resource usage can be best modeled for each monitoring application.

- This identification is made according to the previous resource usage measurements. 

- These measurements are then used for determining the system’s overall load and by what percentage the load must be shed. 

Explain the concept of piggybacking?

- Piggybacking is a well known technique used in the transmission of data in the third layer of the OSI model i.e., the network layer. 

- It is employed in making a majority of the frames that are transmitted from receiver to the emitter. 

- It adds to the data frame, the confirmation that the sender sent on successful delivery of data frame. 

- This confirmation is called the ACK or acknowledge signal. 

- Practically, this ACK signal is piggybacked on the data frame rather than sending it individually by some other means. 

Principle behind Piggybacking

- The piggybacking technique should not be confused with the sliding window protocols that are also employed in the OSI model. 

- In piggybacking, an additional field for the ACK or the acknowledgement signal is incorporated in to the data frame itself. 

- There is only a difference of bit between the sliding window protocol and piggybacking.

- Whenever some data has to be sent from party to another, the data will be sent along with the field for ACK. 

The piggybacking data transfer is governed by the following three rules:

Ø  If both the data as well as the acknowledgement have to be sent by the party A, it has to include both the fields in the same frame.

Ø  If only the acknowledgement has to be sent by the party A, then it will have use a separate frame i.e., an ACK for that.

Ø  If only the data has to be by the party A, then the ACK field will be included within the data frame and thus transmitted along with it. This duplicate ACK frame is simply ignored by the receiving party B.

- The only advantage of using this technique is that it helps in improving efficiency. 

- The disadvantage is that is the service can be blocked or jammed by the receiving party if there is no data to be transmitted. 

- Enabling a receiver timeout by means of a counter the moment when the party receives the data frame can solve this problem to a great extent. 

- An ACK control frame will be sent by the receiver if the timeout occurs and still there is no data for transfer. 

- A counter called the emitter timeout is also set up by the sender which if ends without getting any confirmation from the receiver will make the sender assume that the data packet got lost in the way and therefore will have to re-transmitted.

- Piggybacking is also used in accessing the internet.

- It is used in establishment of a wireless internet connection by means of wireless internet access service of the subscriber without taking explicit permission from the subscriber. 

- However, according to the various jurisdiction laws around the world, this practice is under ethical and legal controversy. 

- In some places it is completely regulated or outlawed while at other places it is allowed.  

- A business customer who provides services related to hotspots, as of cafe and hotels, cannot be thought of using piggybacking technique via non – customers. - A number of such locations provide services for a fee. 

Define the Virtual Memory technique?

Modern operating systems come with multitasking kernels. These multitasking kernels often run in to the problems related to memory management. Physical memory does not suffice for them to execute the tasks assigned to them because of being fragmented. So they have to take some additional from the secondary memory. But they cannot use this memory directly. Virtual memory offers a solution to this problem. 

What is Virtual Memory technique?

- Using this technique makes the fragmented main memory available to the kernels as a contiguous main memory. 

- Since it is really not the main memory but just appears to be, it has been named as the virtual memory and this technique is called the virtual memory technique. 

- Since, it helps in managing the memory, it is essential a memory management technique. 

- The main storage gets fragmented because of many programming and processing problems. 

- The main memory available to the processes and the tasks is virtualized by the virtual memory technique and then it appears to the process as a contiguous memory location. 

- This memory is a global address space. 

- Virtual address spaces such as these are managed by the operating system. 

- The real memory is assigned to the virtual memory by the operating system itself. 

- The virtual addresses of the allocated virtual address spaces are translated in to the physical addresses automatically by the CPU. 

- It achieves this with the help of some memory management hardware specially designed for this purpose. 

- The processes continue to execute uninterrupted as long as this hardware properly translates the virtual addresses in to real memory addresses properly. 

- If it fails in doing so at any point of time, the execution comes to a halt and the control is transferred to the operating system. 

- The duty of the operating system now is to move the requested memory page to the main memory from the backing store. 

- Once done with this, it then returns the control again to the process that was interrupted. 

- It greatly simplifies the whole execution process. 

- Even if the application would require more data or code that would fit in real memory, it does not have to be moved to and fro between the backing store and the real memory. 

- Furthermore, this technique also offers protection to the processes that are provided distinct address spaces by the isolation of the memory allocate to them from other tasks.

- Application programming has been made a lot easier with the help of the virtual memory technique since it hides the fragmentation defects of the real memory. 

- The burden of memory hierarchy management is delegated to the kernel which eliminates the need for the explicit handling of the overlays by the program. 

- Thus each process can execute in an address space that is dedicated to it. 

- The need for relocating the code of the program is obviated along with using relative addressing for accessing the memory. 

- The concept of virtual memory was generalized and eventually named as memory virtualization. 

- Gradually, the virtual memory has become an inseparable part of the architecture of the modern computers. 

- For implementing it, dedicated hardware support is absolutely necessary. 

- This hardware is built in to the CPU in some sort of memory management hardware. - If required for boosting the performance of the virtual memory, some virtual machines and emulators may employ some additional hardware support. 

- The older mainframe computers did not have any support for the virtual memory concept. 

- In virtual memory technique, each program can solely access the virtual memory.

Describe some Caching Issues?

We all are familiar with what a cache is? 

"A cache can be defined as a memory component that is held for storing the data transparently in order to speed up the future serving of the data requests."

The data stored in a cache might be the data that has been required earlier for some operations. 

There are two events related to cache as mentioned below:

1. Cache hit: When the requested data is available in cache and

2. Cache miss: When the requested data is not available in cache and has to be looked up in to the RAM.

- The speed of processing is directly proportional to the number of requests that can be served via the cache. 

- Cache memories are quite costly and hence to make it cost efficient and keep the data usage as efficient as possible, smaller caches are used. 

- But since the time of its advent, cache has proven itself in the field of computing. 

"Caching can be thought of as a technique that is aimed at increasing the computing performance by keeping in itself the frequently accessed data."

There are basically three kinds of caching as we have stated below:

1. Caching output caching: In this type of caching the dynamic output that had been generated up on a request.

2. Fragment caching: In this type of caching the portion of the page that is generated by the request is cached since in many situations it is not practical to cache the whole page at once.

3. Data caching: In this type of caching the objects are cached pro grammatically.

What are different caching issues?

In this article we have taken in to discussion some very prominent caching issues. Most of the people experience problem in server caching of certain files. 

There are four major caching issues have been recognized which have been mentioned below:

1. Designing of a custom cache.

2. Securing of a custom cache.

3. Monitoring of a custom cache and lastly.

4. Synchronization of the caches in a server farm.

Besides these four major caching issues, there are many other minor caching issues.

- Some times it happens that the package delivery fails or an object or element appears like it has been corrupted and seems like such a failure has not got anything to do with the connection! 

- In such cases you can go on with a cache clear up. 

- If the situation is worse, you may also require clearing up the proxy cache!

- Web sites and browsers are looked up as a means of optimizing the resources which is done by them so well that they end up breaking down your dynamic web site content. 

- Your web site is not updated as you thought it will be done. 

Let us see an example, suppose you own a music web site which you frequently update with new music. Your clients come to your site every day so what happens is that the cache forces the web site to list the cached version of the play list and so the clients would never be able to listen to new and the latest music.

Such situation though enhances your internet experience, can also cause many other problems! Some times the cache will stash up an old page of the web site instead of showing up the latest one. You should make it a point to empty your browser’s cache from time to time. There are many internet service providers that cache pages to speed up the internet access like AOL. All the web pages that you visit are stashed up in the cache. 

Explain the concepts of URL manipulation?

Today in this internet savvy world, I guess almost everybody is familiar with what is an URL or uniform resource locator. 

If you see an URL, you can make out that it is nothing but a string of characters. These characters add up to mark up a reference string which points to a source from internet. A uniform resource locator was previously known as uniform resource identifier.

The URLs came in to existence in the year of 1994 along with the introduction of the World Wide Web by Sir Tim Berners – Lee along with the contributions from the internet engineering task force. 

The format of a typical URL consists of the domain names along with the file paths and the forward slashes are used to distinguish between the different file names and folders. Name of the servers are preceded by a double slash. 

Components of URL

Let us now list the components of a typical URL in the order in which they are lined up in the URL:

1. The scheme name which is usually a protocol.
2. The scheme is followed by a colon
3. Two slashes
4. Name of the domain (if any depending on the scheme).
5. A port number
6. CGI (common gateway scripts) scripts
7. Query string
8. Fragment identifier (optional)

Categories of URL

- The URLs are categorized under two categories namely relative URLs and absolute URLs. 

- The relative URLs are used whenever the references contained in the resources refer to another resource. 

- These relative URLs are often conceived from the absolute URLs. 

- The URLs locate a resource based on their primary access mechanism. 

- There are various issues related to URLs like URL normalization, URL manipulation etc. 

What is meant by URL Manipulation?

- URL manipulation is just another name for URL rewriting.

- As the term itself suggests it is all about altering the parameters of the URL.

- The URL manipulation is used for good purposes also and for bad ones also. 

- It is a technique that is usually employed by the web server administrator for convenience and is often used by the hackers for nefarious purposes. 

- The original URLs of the resources are quite complicated and complex. 

- Therefore, a purpose of this technique is also to make it easy for the user to access a web resource by providing a simple URL. 

- URL manipulation technique is used so that the user does not require cutting, copying or pasting long and arcane string of characters. 

- This technique is also employed since remembering complex URLs is a difficult task and they are quite lengthy which makes it quite a tedious task for the users to remember or store it and use. 

- Therefore, using the technique of URL manipulation they are modified in to simple and short URLs which are comparatively easy for the users to remember. 

Wrong Use of URL Manipulation

- A nefarious use of URL manipulation is to use the URL of a legitimate site or web resource without the prior permission or knowledge of the site owner or administrator to redirect the users to an illegitimate web site or web resource. 

- Such illegitimate sites then may install malicious code on the hard drive of the user’s system.

- This may also have an intended purpose of increase the traffic on the attacker’s illegitimate web site or application.

- There is a term similar to the term URL manipulation called URL poisoning. These two terms may sound similar in meaning, though they are not. 

What is URL Poisoning?

- URL poisoning is a technique that is employed to track the activities of the user on the web. 

- This technique involves the addition of an identification number to the current URL of the web browser when that particular web site is visited by the user. 

- This URL with the ID number is then used for tracking the visits of that user on the sites.