Differentiate between transparent and nontransparent fragmentation?

A number of problems are encountered because of the size of the data packets. There is no ability in the data link layer by means of which it could handle these problems and so the bridges also don’t work here. 

The Ethernet also experiences a number of problems because of the following:

Ø  Different way in which the maximum packet size is defined.

Ø  Maximum packet size that can be handled by a router.

Ø  The maximum length slot that are used for transmission

Ø  Errors due to the packet length

Ø  Standards

The data packets can be fragmented in two ways namely:

1. Transparent and
2. Non – transparent

Both these ways can be followed based on a network by network basis. We can also say that no such end – to – end agreement exists based up on which it can be decided which process is to be used.

Transparent Fragmentation: 

- This type of fragmentation is followed when a packet is split in to smaller fragments by a router.

- These fragments are sent to the next router which does just the opposite i.e., it reassembles the fragments and combine them to form original packet. 

- Here, the next network does not come to know whether any fragmentation has taken place. 

- Transparency is maintained between the small packet networks when compared to the other subsequent networks.

- For example, transparent fragmentation is used by the ATM networks by means of some special hardware. 

- There are some issues with this type of fragmentation. 

- It puts some burden on the performance of the network since all the fragments have to be transmitted through the same gateway. 

- Also, sometimes the repeated fragmentation and reassembling has to be done for small packet network in series. 

- Whenever an over-sized packet reaches a router, it is broken up in to small fragments. 

- These fragments are transported to the next exit router. 

- The fragments are assembled by this exit router which then forwards them to the next router.

- Awareness regarding this fragmentation is not maintained for the subsequent networks. 

- For a single packet fragmentation is done many times before the destination is finally reached. 

- This of course consumes a lot of time because the repeated fragmentation and assembling has to be carried out. 

- Sometimes, it also presents the reason of corrupting the packet’s integrity.

Non-Transparent Fragmentation: 

- In this type, the packet is split in to fragments by one router. 

- But the difference is that these fragments are not reassembled until the fragments reach their destination. 

- They remain split till then. 

- Since in this type of fragmentation the fragments are assembled only at the destination host, the fragments can be routed independent of each other. 

- This type of fragmentation also experiences some problems such as header has to be carried by each of the fragments till they reach their destination. 

- Numbering has to be done for all the fragments so that no problem is experienced in reconstructing the data stream.

Whichever type of fragmentation we use, one thing has to be made sure which is that later we should be able to form the original packets using the fragments. This insists on having some type of labeling for the fragments. 

Segmentation is another name for the fragmentation. A packet is injected in to the data link layer by the IP layer but it is not responsible for reliable transmission of the packets. Some maximum value on the size of the packets is imposed by each layer for their reasons. For a large packet that travels through the network for which the MTU is small, fragmentation is very much needed. 

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. 

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.

What is fragmentation? What are different types of fragmentation?

In the field of computer science, the fragmentation is an important factor concerning the performance of the system. It has a great role to play in bringing the performance of the computers. 

What is Fragmentation?

- It can be defined as a phenomenon involving the inefficient use of the storage space that in turn reduces the capacity of the system and also brings down its performance.  

- This phenomenon leads to the wastage of the memory and the term itself means the ‘wasted space’.

- Fragmentation is of three different forms as mentioned below:

1. The external fragmentation
2. Internal fragmentation and
3. Data fragmentation

- All these forms of fragmentation might be present in conjunction with each other or in isolation. 

- In some cases, the fragmentation might be accepted in exchange of simplicity and speed of the system. 

Basic principle behind the fragmentation concept. 

- The CPU allocates the memory in form of blocks or chunks whenever requested by some computer program. 

- When this program has finished executing, the allocated chunk can be returned back to the system memory. 

- The size of memory chunk required by every program varies.

- In its lifetime, a program may request any number of memory chunks and free them after use. 

- When a program begins with its execution, the memory areas that are free to allocated, are contiguous and long. 

- After prolonged usage, these contiguous memory locations get fragmented in to smaller parts. 

- Later, a stage comes when it becomes almost impossible to serve the large memory demands of the program. 

Types of Fragmentation

1.External Fragmentation: 

- This type of fragmentation occurs when the available memory is divided in to smaller blocks and then interspersed. 

- Certain memory allocation algorithms have a minus point that they are at times unable to order the memory used by the programs in such a way that its wastage is minimized. 

- This leads to an undesired situation where even though we have free memory, it cannot be used effectively since being divided in to very small parts that alone cannot satisfy the memory demands of the programs.  

- Since here, the unusable storage lies outside the allocated memory regions, this type of fragmentation is called external fragmentation. 

- This type of fragmentation is also very common in file systems since here many files with different sizes are created as well as deleted. 

- This has a worse effect if the file deleted was in many small pieces. 

- This is so because this leaves similar small free memory chunks which might be of no use.

2. Internal Fragmentation: 

- There are certain rules that govern the process of memory allocation. 

- This leads to the allocation of more computer memory what is required. 

- For example, as the rule memory that is allocated to programs should be divisible by 4, 8 or 16. So if some program actually requires 19 bytes, it gets 20 bytes. 

- This leads to the wastage of extra 1 byte of memory. 

- In this case, this memory becomes unusable and is contained in the allocated region itself and therefore this type of fragmentation is called as the internal fragmentation.

- In computer forensic investigation, the slack space is the most useful source for evidence. 

- However, it is often difficult to reclaim the internal fragmentation. 

- Making a change in the design is the most effective way for preventing it. 

- Memory pools in dynamic memory allocation are the most effective methods for cutting down the internal fragmentation. 

- In this the space overhead is spread by a large number of objects.

3. Data Fragmentation: 

This occurs because of breaking up of the data in many pieces that lie far enough from each other.

                                                                                                               

Introduction to Packet Fragmentation

Each network imposes some maximum size on its packets. The network designers are not free to choose any maximum packet size they wish as there are various factors like hardware, operating system, protocols, compliance with some (inter)national standard, desire to reduce error induced transmissions to some level and desire to prevent one packet from occupying the channel too long.
Packets larger than the allowable MTU (Maximum Transmission Unit) must be divided into multiple smaller packets, or fragments, to enable them to traverse the network.
If a packet that is about to be sent (for eg : over an Ethernet link) is bigger than that, the router which is about to send the packet over that link will fragment the packet i.e. the router will split the packet up into smaller messages (known as fragments) that are each small enough to be transmitted over the link. When the fragments arrive at their destination (the computer to which they are being sent), that computer can reassemble the fragments to recover the original message - assuming none of the messages are lost in transit.

How can be fragmentation avoided ?
If the option of "don't fragment" is set ON in IP version 4, and the router wants to send the packet over a link for which the packet is too large, the router will not send the packet at all. Instead, the router will send a message back to the sender of the packet that was too large. The sending computer can then respond to this by sending out smaller packets. This is known as "path MTU discovery".

Strategies for recombining fragments :
- Transparent Fragmentation : When an oversized packet arrives at gateway, the gateway breaks it into smaller fragments, each fragment is addressed to same exit gateway, where pieces are recombined. In this way passage through the small packet network has been made transparent.
Benefits : It maximizes bandwidth on higher links and deterministic fragmentation unlikely.
Drawbacks : Packets may be reassembled/fragmented, gateways more complex, performance gains bounded because the max TU will be the MTU of the first hop. Plus, the IP layer at the destination may still have to perform reassembly if the last link had a smaller MTU than the first link. Only use on links with unusually small MTUs.



- Non transparent Fragmentation : This strategy includes refraining the recombining of fragmented packets at intermediate gateway. Once a packet is fragmented, each fragment is treated as an original packet. All fragments are passed through the exit gateway. Recombination exists only at destination host.
Benefits : Multiple exit gateways can now be used and higher performance can be achieved.
Drawbacks : Overhead increases. Also, it requires every host to be able to do reassembly.