What is Traditional Cryptography?

- Cryptography is the practice that involves study and application of the techniques for making communication secure with the adversaries or the third parties. 

To be more general, it involves construction and the analyzation of the protocols for overcoming the impact of the adversaries and other aspects concerning the information security such as the following:

Ø  Data confidentiality

Ø  Data integrity

Ø  Authentication

Ø  Non – repudiation

- The modern cryptography in contrast to the traditional cryptography intersects the computer science, mathematical and the engineering disciplines. 

There are various applications of cryptography as in the following:

Ø  ATM cards

Ø  Computer passwords

Ø  Electronic commerce

- The traditional cryptography was synonymous with the process of encryption which involves converting the information which is in readable state to such a state in which it appears like utter nonsense. 

- The one who generated the encrypted message also shared the technique for decoding the message only with the desired recipients, thus the unwanted people are precluded from doing so.

- Cryptography is in use since the World War I and the methods that were used then now have become so complex and eventually its application increased. 

- Modern cryptography’s foundation is based up on the computer science and the mathematical theory. 

- The designing of the cryptographic algorithms is done around the computational hardness assumptions. 

- In practice, this makes these algorithms quite hard to break by any third party. 

- However, theoretically it is possible to break in to such a system but for doing so any known practical means are in-feasible.

- That is why, all these schemes are considered to be computationally safe and secure. 

For the following, the continuous adaptation of these methods is required:

Ø  Improvements in the algorithms for the integer factorization.

Ø  Faster computing technology.

- Also, there are schemes that are information – theoretically secure and even with unlimited computing power, these schemes cannot be broken.

- One such scheme is one time pad. 

- Also, the implementation of these schemes is also quite difficult when compared to the schemes that are computationally secure but are theoretically breakable. 

- Traditionally cryptography referred only to the encryption which involves conversion of the ordinary info in to cipher text or unintelligible text. 

- The reverse process of this is decryption. 

- The pair of algorithms that carry out these two processes is called the cipher. - Each instance of the operation of the cipher is controlled by a key which is kept secret between the communicants. 

- The purpose of this key lies in decryption of the cipher text. 

- Earlier the encryption and the decryption process were carried out directly by the ciphers without involvement of any integrity or authentication checks. 

- Before the advent of the modern cryptography, the traditional cryptography was known to be concerned only with the message confidentiality i.e., converting the message from comprehensible text in to incomprehensible text and vice versa. 

- The message was thus unreadable for the eavesdroppers and the interceptors without key. 

- For ensuring the secrecy in the communications, the encryption process was used. 

- But now the field expands far beyond the confidentiality issues.

- It now consists of techniques for authentication and message integrity checking, secure computation techniques, interactive proofs, digital signatures and so on. 

- Earlier two types of classical ciphers were used namely substitution ciphers and the transposition ciphers. 

- The former type involved replacing the letters by some other letters.

- The transposition ciphers involved rearrangement of the letters. 

- Some examples of early ciphers are caeser cipher, atbash cipher etc. 

- The early ciphers were assisted by some other physical aids and devices. 

- Eventually more complex ciphers could be developed with the development of the digital computers. 

- Any kind of data that could be represented in binary format could be encrypted.

What is fragmentation?

- The fragmentation technique is implemented in the IP (internet protocol) for breaking down the datagrams into smaller pieces. 

- This is done so that it becomes easy for the data packets to be passed through the link with a datagram size smaller than that of the original MTU or the maximum transmission unit. 

- The procedure for the IP fragmentation along with the procedures for reassembling and transmitting the datagrams is given in the RFC 791. 

- For determining the optimal MTU path, the IPv6 hosts are needed so that the packets can be sent. 

- If in case the PDU i.e., the protocol data unit received by the router is larger than the MTU of the next hop, then there are two options are available if IPv4 transport is being used:

Ø Dropping the PDU and sending an ICMP (internet control message protocol) message indicating that the condition packet is quite big.

Ø  Fragmenting the IP packet and then transmitting it over the link whose MTU is smaller. Any IPv6 packet with a size less than or equal to 1280 bytes can be delivered without having the need for using the IPv6 fragmentation.

- If a fragmented IP packet is received by the recipient host, its job is to reassemble the datagram and then send it over to the protocols at the higher layers. 

- The purpose of reassembling is expected to take place at the recipient’s host side but for some practical reasons it might be done by some intermediate router. 

- For example, the fragments might be reassembled by the NAT (network address translation) for translating the data streams. 

- Excessive re-transmission can result as a consequence of the IP fragmentation whenever packet loss might be encountered by the fragments. 

- It is required for all the reliable protocols (example, TCP) for re-transmitting the fragments in their correct order for recovering from the single fragment loss. 

Thus, typically two approaches are used by the senders for determining datagrams of what size should be transmitted over the network:

1. First approach: The sender must transmit an IP datagram of size as same as that of the first hop’s MTU.
2. Second approach: Running the path MTU discovery algorithm.

- Fragmentation does leave an impact on the network forwarding. 

- When there are multiple parallel paths for the internet router the traffic is split by the technologies such as the CEF and LAG throughout the links via some hash algorithms. 

- The major goal of this algorithm is to make sure that all the packets with the same flow are transmitted out on the same path for the minimization of the not so required packet reordering. 

- If the TCP or UDP port numbers are used by the hash algorithm, the fragmented packets might be forwarded through different paths. 

- This is so because the layer 4 information is contained only in the first fragment of the packet. 

- As a result of this, usually the initial fragment arrives after the non-initial fragments. 

- This condition is often treated as an error by most of the security devices in the hosts.  

- Therefore, they drop these packets.

- The fragmentation mechanism differs in IPv4 and IPv6. 

- In the former, the fragmentation is performed by the router. 

- On the other hand, in IPv6 fragments that are larger than MTU are dropped by the routers.

- Also, in both the cases there is a variation in the header format. 

- Since fragmentation is carried out using analogous fields, therefore the algorithm can be used again and again for the purpose of fragmentation and reassembling. 

- A best effort should be made by the IPv4 hosts for reassembling the datagram fragments. 

What is the concept of flow control?

- Flow control is an important concept in the field of data communications. 

- This process involves management of the data transmission rate between two communicating nodes. 

- Flow control is important to avoid a slow receiver from being outrun by a fast sender. 

- Using flow control, a mechanism is designed for the receiver using which it can control its speed of transmission.

- This prevents the receiving node from getting overwhelmed with traffic from the node that is transmitting.

- Do not confuse yourself with congestion control and flow control. Both are different concepts. 

- Congestion control comes in to play when in actual there is a problem of network congestion for controlling the data flow. 

On the other hand the mechanism of flow control can be classified in the following two ways:

1. The feedback is sent to the sending node by the receiving node.
2. The feedback is not sent to the sending node by the receiving node.

- The sending computer might tend to send the data at a faster rate than what can be received and processed by the other computer. 

- This is why we require flow control. 

- This situation arises when the traffic load is too much up on the receiving computer when compared to the computer that is sending the data. 

- It can also arise when the processing power of the receiving computer is slower than the processing power of the one that is sending the data.

Stop and Wait Flow Control Technique 

- This is the simplest type of the flow control technique. 

- Here, when the receiver is ready to start receiving data from the sender, the message is broken down in to a number of frames. 

- The sending system then waits for a specific time to get an acknowledgement or ACK from the receiver after sending each frame. 

- The purpose of the acknowledgement signal is to make sure that the frame has been received properly. 

- If during the transmission a packet or frame gets lost, then it has to be re-transmitted. 

- We call this process as the automatic repeat request or ARQ. 

- This technique has a problem which is that it is capable of transmitting only one frame in one go. 

- This makes the transmission channel very inefficient. 

- Therefore, until and unless the sender gets an acknowledgement it will not proceed further for transmitting another packet. 

- Both the transmission channel and the sender are left un-utilized during this period. 

- Simplicity of this method is its biggest advantage. 

- Disadvantage is the inefficiency resulting because of this simplicity. 

- Waiting state of the sender creates inefficiency. 

- This happens usually when the transmission delay is shorter than the propagation delay. 

- Sending longer transmissions is another cause for inefficiencies. 

- Also, it increases the chance for the errors to creep in this protocol. 

- In short messages, it is quite easy to detect the errors early. 

- By breaking down one big message in to various separate smaller frames, the inefficiency increases. 

- This is so because these pieces altogether take a long to be transmitted.

Sliding window Flow Control Technique 

- This is another method of flow control where permission is given to the sender by the receiver for continuously transmitting data until a window is filled up. 

- Once the window is full, sender stops transmission until a larger window is advertised. 

- This method can be utilized in a better way if the size of the buffer is kept limited. 

- During the transmission, space for say n frames is allocated to the buffer. 

- This means n frames can be accepted by the receiver without having to wait for ACK. 

- After n frames an ACK is sent consisting of the sequence number of the next frame that has to be sent. 

How can the problem of congestion be controlled?

Networks often get trapped in the situation of what we call network congestion. For avoiding such collapses, congestion avoidance and congestion control techniques are often used by the networks nowadays. 

In this article, we discuss about how we can control the problem of network congestion using these techniques. Few very common techniques are:

1. Exponential back off (used in CSMA/ CA protocols and Ethernet.)
2. Window reduction (used in TCP)
3. Fair queuing (used in devices such as routers)
4. The implementation of the priority schemes is another way of avoiding the negative effects of this very common problem. Priority schemes let the network transmit the packets having higher priority over the others. This way only the effects of the network congestion can be alleviated for some important transmissions. Priority schemes alone cannot solve this problem.
5. Another method is the explicit allocation of the resources of the network to certain flows. This is commonly used in CFTXOPs (contention – free transmission opportunities) providing very high speed for LAN (local area networks) over the coaxial cables and phone lines that already exist.

- The main cause of the problem of network congestion is the limited capacity of the network. 

- This is to say that the network has limited. 

- The resources also include the link throughput and the router processing time. 

- Congestion control is concerned with curbing the entry of the traffic in to the telecommunications network so that the problem of congestive collapse can be avoided. 

- The over-subscription of the link capabilities is avoided and steps are taken to reduce the resources. 

- One such step is reducing the packet transmission rate. 

- Even though if it sounds similar to flow control, it is not the same thing. 

- Frank Kelly is known as the pioneer of the theory of congestion control. 

- For describing the way in which the network wide rate allocation can be optimized by the individuals by controlling their rates, he used two theories namely the convex optimization theory and the micro economics theory. 

Some optimal rate allocation methods are:

Ø  Max – min fair allocation

Ø  Kelly’s proportional fair allocation

Ways to Classify Congestion Control Algorithm

There are 4 major ways for classifying the congestion control algorithms:

1. Amount as well as type of feedback: This classification involves judging the algorithm on the basis of multi-bit or single bit explicit signals, delay, loss and so on.
2. The performance aspect taken for improvement: Includes variable rate links, short flow advantage, fairness, links that can cause loss etc.
3. Incremental deployability: Modification is the need of sender only, modification is required by receiver and the sender, modification is needed only by the router, and modification is required by all three i.e., the sender, receiver and the router.
4. Fairness criterion being used: It includes minimum potential delay, max – min, proportional and so on.

Two major components are required for preventing network congestive collapse:

1. End to end flow control mechanism: This mechanism has been designed such that it can respond well to the congestive collapse and thus behave accordingly.
2. Mechanism in routers: This mechanism is used for dropping or reordering packets under the condition of overload.

- For repeating the dropped information correct behavior of the end point is required. 

- This indeed slows down the information transmission rate. 

- If all the end points exhibit this kind of behavior, the congestion would be lifted from the network. 

- Also, all the end points would be able to share the available bandwidth fairly. - Slow start is another strategy using which it can be ensured that the router is not overwhelmed by the new connections before congestion can be detected. 

What is optimality principle?

A network consists of nodes which require communicating with other on various grounds. This communication is established via communication channels that exist between them. The communication involves data transfers. In a network a node may or may not have a link with every other node in the network. 

Applications that require communicating over a network include:

1. Telecommunication network applications such as POTS/ PSTN, local area networks (LANs), internet, mobile phone networks and so on.

2. Distributed system applications

3. Parallel system applications

- As we mentioned above, each and every node might not be linked with every other nodes since for doing so a lot of wires and cables are required which will the whole network more complicated. 

- Therefore, we bring in the concept of the intermediate nodes. 

- The data transmitted by the source node is forwarded to the destination by these intermediate nodes. 

Now the problem that arises is which path or route will be the best to use i.e., the path with the least cost. 

- This is determined using the routing process. 

- The best path thus obtained is called the optimal route. 

- Today, we have a number of algorithms available for determining the optimal path. 

These algorithms have been classified in to two major types:

1. Non – adaptive or static algorithms
2. Adaptive or dynamic algorithms

Concept of Optimality Principle

- This is the principle followed while determining the optimal router between the two routes. 

The general statement of the principle of optimality is stated below:

“An optimal policy has the property that whatever the initial state and initial decision are, the remaining decision must constitute an optimal policy with regard to the state resulting from the first decision.”

- This means if P is an optimal state that results in another state say Q, and then the portion of the original from that state to this state i.e., from P to Q must be optimum. 

- This only means the optimality of the part of the optimal policy is preserved. - The initial state and the final state are the most important parts of the optimum. 

- Consider an example, suppose we have problem with 3 inputs and 26 states. - Here, the state is associated with the optimum and the total cost is associated with the optimum policy.

- If brute force method is used for 3 inputs and 100 stages we have the total number of computations as 3¹⁰⁰. 

- That means for solving this problem, a super computer is required.

- Therefore, the approach used for solving this problem is a parallel processing approach. 

- Here, for the each state the least step is computed and stored during the programming. 

- This reduces the number of possibilities and hence reducing the amount of computation.

- The problems become complex if the initial and the final states are undefined. - It is necessary for the problem to follow the principle of optimality in order to use the dynamic programming. 

- This implies that whatever the state may be, the decisions that follow must be optimal in regard with the state obtained from the previous decision. 

- This property is found in combinatorial problems but since they use a lot of time and memory, this method is inefficient for them. 

- These problems can be solved efficiently if some sort of best first search and pruning technique is applied.

- In regard to the routing in networks, it follows from the optimality principle if a router B lies between router A and C which lie on an optimal path, then the path between the router B and C is also an optimal path and lies on the same path. 

- Sink tree is formed as a result of all optimal routes which is the ultimate goal of all the routing algorithms.

Give advantages of frame relay over a leased phone line?

Frame relay and leased phone lines are two of the physical connection media for setting up the connections. 

Advantages of Frame Relay over Leased Phone Line

- Frame relay is a kind of the standardized WAN (wide area network) technology for specifying the logical link as well as physical link layers of the digital telecommunication channels. 

- It is done by the means of a packet switching methodology.

- The frame relay technology has been designed for transportation across the ISDN (integrated services digital network) infrastructure. 

- Today, it is used in the context of a number of network interfaces. 

- Frame relays are commonly implemented for VoFR (voice over frame relay).  - It is used as an encapsulation technique for the data. 

- The frame relays are used between the WANs and the LANS.

- A private line or a leased line is provided to the user that connects to the frame relay node. 

- The frequently changing path is transparent to the WAN protocols used extensively by the end users. 

- Data is transmitted via these networks and the frame relay network handles all this.

- One advantage of the frame relays over the leased lines is that they are less expensive and this is what that makes the frame relays so popular in the telecommunications industry.

- Another advantage of the frame relays over the leased lines that make them popular is that they have user equipment that can be configured with extreme simplicity in the frame relay network. 

- The usage of the Ethernet over the fiber optics communication is high. 

- This has led to using the frame relay protocol and encapsulation by the dedicated broadband services like DSL and cable modem, VPN, MPLS etc. 

- However, there are a number of rural regions in India where there is still an absence of the cable modem and DSL services.

- In such areas, the only option for the non-dial-up connection is the frame relay line of 64 Kbit/ s.

- Thus, it might be used by some retail chain to connect with the WAN of their corporate. 

- The aim of the designers of the frame relay is to offer a telecommunication service for transmitting the cost efficient data between the various end points in the WAN and the local area networks in an intermittent traffic. 

- The data is put in to units of variable sizes called the frames by the frame relay process. 

- The required error correction process is left up to the end points. 

- This error correction includes re-transmission of the data. 

- This increases the speed of the overall transmission of data. 

- A PVC or the permanent virtual circuit is provided by the network so that when a customer looks at a dedicated connection and not having to pay for leased line that is full time engaged. 

- The route by which each frame travels to the destined end point is figured out by the service provider and thus he decides the charges based up on the usage. 

- A level of the service quality can be selected by the enterprise. 

- The frames can be prioritized while the importance of the other frames is reduced. 

- The frame relay can run on systems such as the following:

Ø  Fractional T – 1

Ø  Full T – carrier

Ø  E – 1

Ø  Full E carrier

- A frame relay provides mid-range services between ATM (asynchronous transfer mode) and the ISDN operating at a speed of 128 Kbps. 

- Not only it provides the services, it also complements them. 

- The base of the frame relay technology is provided by the X.25 packet switching that has been designed for data transmission over the analog voice lines.

Explain about demand paging and page replacements

These are two very important concepts of memory management strategies in the computer operating systems namely demand paging and paging replacements. 

About Demand Paging

- Demand paging is just the opposite concept of the anticipatory paging. 

- Demand paging is actually a memory management strategy developed for managing the virtual memory.

- The operating system that makes use of demand paging technique, a copy of the disk page is made and kept in the physical memory whenever a request is made for it i.e., whenever a page fault occurs. 

- It is obvious that the execution of a process starts with none of its page loaded in to the main memory and follows by a number of page faults occurring one after the other until all of its required pages have been loaded in to the main memory. 

- Demand paging comes under the category of the lazy loading techniques. 

- This strategy follows that only if the process in execution demands a page, then only it should be brought in to the main memory. 

- That’s why the strategy has been named as demand paging. Sometimes it is even called as the lazy evaluation. 

- Page table implementation is required for using the demand paging technique.

- The purpose of this table is to map the physical memory to the logical memory. 

- This table uses a bit wise operator for marking a page as valid or invalid. 

The following steps are carried out whenever a process demands for a page:

1. An attempt is made for accessing the page.
2. If page is present in the memory the usual instructions are followed.
3. If page is not there i.e., is invalid then a page fault is generated.
4. Memory reference to a location in the virtual memory is checked if it is valid or not. If it’s an illegal memory access then the process is terminated. If not the requested page has to be paged in.
5. The disk operations are scheduled for reading the requested page in to the physical memory.
6. Restarting the instruction that raised the page fault trap.

- The nature of this strategy is itself of great advantage. 

- Upon availability of more space in the physical memory, it allows execution of many processes leading to a decrease in the context switching time.

- At the time of program start up, less latency occurs during loading. 

- This is because the inflow and outflow of the data between main memory and secondary memory is very less.

About Page Replacement

- When less number of real memory frames is available, it leads to invoking a page stealer. 

- This stealer searches through the PFT (page frame table) for pages to steal. 

- This table stores references to the pages which are required and modified. 

- If the requested page is found by the page stealer, it does not steal it but the reference flag is reset for that page. 

- So in the pass when the page stealer comes across this page, it steals this page. 

- Note that in this pass the page was flagged as un-referenced. 

- Any change made to the page is indicated by means of the modify flag.

- If the modify flag of the page to be stolen is set, then a page out call has to be made before the page stealer does its work. 

- Thus, the pages that form a part of the currently executing segments are written to so called paging space and the persisting segments are in turn written to the disk. 

- The page replacement is carried by the algorithms called the page replacement algorithms. 

- Besides this, these also keep a track of the faults. 

Explain about the Paged Memory and Segmentation techniques?

Paging and segmentation, both are memory management techniques. 

What is Paging?

- This technique has been designed so that the system can store or retrieve data from the virtual memory or secondary memory of the system to be loaded in the main memory and used. 

- In this scheme, the data from the secondary memory is retrieved by the operating system in blocks of same size commonly known as the paging. 

- This is why the technique has been called the paging memory – management scheme. 

- This memory management scheme has a major advantage over the segmentation scheme. 

- The advantage is that non-contiguous address spaces are allowed. 

- In segmentation, non-contiguous physical address spaces are not allowed. 

- Before the paging actually came in to use, the whole program had to be fitted in to the contiguous memory space by the systems. 

- This in turn led to a number of issues related to fragmentation and storage. 

- Paging is very important for the implementation of the virtual memory in many of the operating systems that are general purpose. 

- With the help of paging memory management technique, the data that cannot be fitted in to the physical memory i.e., RAM can be easily used. 

- Paging actually comes in to play whenever a program makes an attempt for accessing the pages that have not been presently mapped to the main memory (RAM). 

- Such situation is termed as the page fault. 

- At this point the control is handed over to the operating system for handling the page fault.

- This is done in a way that it is not visible to the interrupt raising program. 

The operating system has to carry out the following instructions:

1. Determining the location of the requested data from the auxiliary storage.
2. Obtaining a page frame in the main memory that is empty to be used for storing the requested data.
3. Loading the data requested in to the empty page obtained above.
4. Making updates to the page table so that new data is only available.
5. Returning the control interrupting program and retrying to execute the same instruction that caused the fault.

What is Segmentation?

- This memory management technique involves dividing the main memory in to various sections or segments.

- In the system that makes use of this management technique, a value identifying the segment and its offset is contained in the reference to that memory location. 

- Object files that are produced during the compilation of the programs make use of the segments when they have to be linked together to form an image of the program and this image has to be loaded in to the memory.  

- For different program modules, different segments might be created. 

- Some programs may even share some of the segments.

- In one way, memory protection is implemented by means of memory segmentation only.

- Paging and segmentation can be combined together for memory protection. 

- The size of memory segment is not always fixed and can be as small as a byte. 

- Natural divisions such as the data tables or the individual routines are represented by the segments.

- This is to make the segmentation visible to the programmer. 

- With every segment, a set of permissions and length is associated. 

- A segment can be referred to by the process only in a way that is permitted by this set of permissions. 

- If this is not done, a segmentation fault is raised by the operating system. 

- Segments also consist of a flag that indicates the presence of the segment in the main memory of the system.