What are transport and application gateways?

- Hosts and routers are separated in TCP/IP architecture. 

- For private networks, more protection is required to maintain an access control over it. 

- Firewall is one of the components of this TCP/IP architecture. 

- Internet is separated from Intranet by this firewall.

- This means all the incoming traffic must pass through this firewall. 

- The traffic that is authorized is allowed to pass through. 

- It is not possible penetrate the firewall simply. 

Firewall has two components namely:

Ø  Filtering router and

Ø  Two types of gateways namely application and transport gateways.

- All the packets are checked by the router and filtered based up on any of the attributes such as protocol type, port numbers, and TCP header and so on. 

- Designing the rules for filtering of the packets is quite a complex task. 

- A little protection is offered by this packet filtering since with the filtering rules on one side, it is difficult to cater to the services of the users on other side.

About Application Gateways

- Application layer gateways consist of 7 layer intermediate system designed mainly for the access control. 

- However, these gateways are not commonly used in the TCP/ IP architecture. 

- These gateways might be used sometimes for solving some inter-networking issues. 

- The application gateways follow a proxy principle for supporting the authentication, restrictions on access controls, encryption and so on. 

- Consider two users A and B. 

- A generates an HTTP request which is first sent to the application layer gateway rather than being send to its destination. 

- The gateway checks about the authorization of this request and performs encryption. 

- After the request has been authorized, it is sent to user B from the gateway just at it would have been sent by A.

- B responds back with a MIME header and data which might be de-crypted or rejected by the gateway.

- If the gateway accepts, it is sent to A as if from B. 

- These gateways are designed for all the protocols of application level.

About Transport Gateways

- The working of the transport gateway is similar to application gateway but it works at the TCP connection level. 

- These gateways are not dependent up on the application code but they do need client software so as to maintain awareness about the gateway. 

- Transport gateways are intermediate systems at layer 4. 

- An example is the SOCKS gateways. 

- IETF has defined it as a standard transport gateway.

- Again, consider two clients A and B. 

- A TCP connection is opened by A to the gateway. 

- The SOCKS server port is nothing but the destination port. 

- A sends a request to this port for opening the connection to B indicating the port number of the destination. 

- After checking the request, the request for connection from A is either accepted or rejected. 

- If accepted, a new connection is opened to B. 

- The server also informs A that the connection has been established successfully. 

- The data relay between the clients is kept transparent. 

- But in actual there are two TCP connections having their own sequence numbers as well as acknowledgements. 

- The transport gateways are simpler when compared with the application layer gateways. 

- This is so because the transport gateways are not concerned with the data units at the application layer. 

- It has to act on the packets simply once the connection has been established. 

- Also, this is the reason why it also gives higher performance in comparison with the application layer gateways. 

- But it is important that the client must be aware of its presence since there is no transparency here. 

- If between the two networks the only border existing is the application gateway, it alone can act as the firewall. 

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. 

What are the metrics that can be used during performance testing?

One of the most important parts that together make up the complete and effective software testing is the performance testing. Perhaps no software system or application can do without performance testing and testing of any software system or application is incomplete without this one. 

"The determination of the performance of a software system or application in terms of how and when and responsiveness and stability under different particular work loads is nothing but performance testing".

Apart from this there are several other attributes that can be taken care of by the performance testing like for example:

1. Scalability
2. Reliability
3. Resource usage and so on.

The concept of performance testing falls under the concept of performance engineering. The practice of performance testing is aimed at building the performance in to the architecture as well as design of the software system or applications before the actual coding of the software system or application begins. 

We have so many other types of testing that together make up the complete performance testing and have been mentioned below:

1. Load testing
2. Stress testing
3. Soak testing or Endurance testing
4. Spike testing
5. Isolation testing and
6. Configuration testing

Metrics used during Performance Testing

The performance testing cannot be carried out all alone by itself! Rather it is supported by some specific metrics called performance metrics. In this article we shall discuss about the metrics that are to be used during the performance testing.  We shall discuss them one by one:

 1. Average response time: 

This is the time that takes in to consideration all the response cycle ups or round trip requests until a particular point is reached and is the mean of all the response times. Response times can be measured in any of the following way:

      a) Time to last byte or 

      b) Time to first byte

   

      2. Peak response time: 

    This is similar to the previously mentioned metric and represents the longest cycle at a particular point in a particular test. Peak response time is an indication of some potential problem in our software system or application.

   

     3. Error rate: 

   The occurrence of errors under load during the processing of the requests is perhaps the most expected thing during the testing. it has been noted that the most of the errors are reported when the load on the application reaches a peak point and further from there the software system or application is not able to process any requests. Thus error rate provides a percentage of the HTTP status code responses as errors on the servers.

    

     4. Throughput: 

   This performance metric gives you a percentage of the flow of data back and forth from the servers and is measured in units of KB per second.

   

    5. Requests per second (RPS): 

    It gives a measure of the requests that are sent to the target server including requests for CSS style sheets, HTML pages, JavaScript libraries, flash files, XML documents and so on.

    6. Concurrent users: 

    This performance metric is perhaps that best way for expressing the load that is being levied on the software system or application during the performance testing. This metric is not to be equated to the RPS since there are possibilities of one user only generating a high number of requests and on the other hand a user not constantly generating requests.

     7. Cross result graphs: 

     It shows difference between post tuning and pre-tuning tests. 

What are different HTML errors?

HTML or hyper text mark up language is perhaps one of the most used mark up languages for the web pages, sites and applications with which most of us are familiar. This language is written using the HTML elements which mainly constitute of the tags enclosed in the angle brackets like:

These elements are housed in the web page or site. Most of the HTML tags are implemented in pairs of two.

HTML ELEMENTS AND CONTENT
- Some tags are even empty and are commonly known as empty elements.
- These are usually unpaired unlike the filled elements which are paired.
- In the paired tags, the first tag is called the start tag and the second one is then called the end tag.
- Between these two tags, any text, comments, tags etc can be added by the designer of the web sites.
- The content that is to be added should only be of textual type.
- Due to some wrong designing principles often some errors are introduced in to the html of the web site or the page.
- One of the most common errors is the insertion of the graphical content in to the html.
- Such content is neither displayed nor is it interpreted.
- It simply causes the malfunctioning of the web site.
- Other elements of the html allow the addition of the graphical content and only these should be used whenever some graphics are to be inserted in to the page.
- Html elements provide a means for the creation of the structured documents that denote the structural semantics for textual content like:

1. Lists
2. Links
3. Paragraphs
4. Headings
5. Quotes and so on.

DIFFERENT HTML ERRORS AND THEIR IMPACT

- Html can also be embedded in to the scripts like javascript which also some times leads to errors when inserted incorrectly.
- Such errors affect the behaviour of the web sites and cause them to behave abnormally.
- Whenever the site is affected by an error or a bug is encountered, a set of error messages is generated by the business logic component which is then stored as a string in any of the available scopes.
- For using such functions, you need to define an application scope with the name of default attribute, because if such a scope is not found, then nothing is to be rendered by the business logic.
- Some designers forget to define such scopes and therefore end up with unnecessary errors in their web sites.
- Many more errors occur whenever a requested is generated by the client to the web server.
- In such cases the server responds with some status messages, few of which have been mentioned below along with the possible errors:

1. 400:
Bad syntax of the request is preventing the server to process it.

2. 401:
The request though being cannot be processed because of unavailability of the authentication processes.

3. 404 not found:
The page requested by the client is found to be unavailable at that moment.

4. 405 method not allowed:
The client made request in such a way that is not supported by that site.

5. 407 proxy authentication required:
The client has not authenticated its proxy.

6. 408 request time out:
The server’s request time expired while waiting for the client to generate one.

7. 409 conflict:
The server is not able to process the request because of the presence of some conflict in it.

8. 410 gone:
The page that was requested is no longer available on the web.

Introduction to Class-Responsibility-Collaborator (CRC) Modeling

Class-responsibility-collaborator (CRC) modeling is a means to identify and organize classes relevant to system requirements. CRC model is a collection of index cards and it consists of three parts:

Classes : It is a collection of similar objects.
- Entity classes or business classes are obtained directly from statement of the problem. The information contained in these classes are important to users but they do not display themselves.
- Boundary classes are used to create interface which user sees and interacts with as software is used.
- Controller classes are designed to manage creation or update of entity objects, instantiation of boundary objects, communication between objects and validation of data.

Responsibility : something that a class knows or does. Some guidelines that can be applied for allocating responsibilities to classes are:
- System intelligence should be distributed across classes to best address the needs of the problem.
- Each responsibility should be stated as generally as possible.
- Information and the behavior related to it should reside within the same class.
- Information about one thing should be localized with a single class not distributed across multiple classes.
- Responsibilities should be shared among related classes when appropriate.

Collaborator : another class that the class interacts with to fulfill the responsibilities.
- It takes one of two forms : a request for information or a request to do something.
- If a class cannot fulfill all of its obligations itself, then a collaboration is required.
- Collaboration identifies relationship between classes.

What comprises Test Ware Development : Test Plan - Integration Test Plan

The integration test plan is the overall plan for carrying out the activities in the integration test level, which contains the following sections:

- What is to be tested?
This section clearly specifies the kinds of interfaces fall under the scope of testing internal, external interfaces, with request and response is to be explained. This need not go deep in terms of technical; details but the general approach how the interfaces are triggered.

- Sequence of Integration
When there are multiple modules present in an application, the sequence in which they are to be integrated will be specified in this section. In this, the dependencies between the modules play a vital role. If a unit B has to be executed, it may need the data that is fed by unit A and unit X. In this case, the units A and X have to be integrated and then using the data, the unit B has to be tested. This has to be stated to the whole set of units in the program. Given this correctly, the testing activities will lead to the product, slowly building the product, unit by unit and then integrating them.

- List of modules and interface functions
There may be N number of units in the application but the units that are going to communicate with each other, alone are tested in this phase. If the units are designed in such a way that they are manually independent, then the interfaces do not come into picture.This is almost impossible in any system, as the units have to communicate to other units, in order to get different types of functionalities executed. In this section, we need to list the units and for what purpose it talks to the others needs to be mentioned. This will not go into technical aspects, but at a higher level, this has to be explained in plain English.

Apart from above sections, it also includes:
- Integration Testing Tools
- Priority of Program Interfaces
- Naming Convention for test cases
- Status reporting mechanism
- Regression test approach
- ETVX criteria
- Build/Refresh criteria.

RARP : Reverse Address Resolution Protocol

- RARP (Reverse Address Resolution Protocol) is a protocol by which a physical machine in a local area network can request to learn its IP address from a gateway server's Address Resolution Protocol (ARP) table or cache.
- A reverse address resolution protocol (RARP) is used for disk less computers to determine their IP address using the network. The RARP message format is very similar to the ARP format.
- When a new machine is set up, its RARP client program requests from the RARP server on the router to be sent its IP address.
- The RARP server will return the IP address to the machine which can store it for future use assuming that the entry has been put in the router table.
- RARP is available for Ethernet, Fiber Distributed-Data Interface, and Token Ring LANs.
- The 'operation' field in the RARP packet is used to differentiate between a RARP request and a RARP reply packet.
- Since a RARP request packet is a broadcast packet, it is received by all the hosts in the network. But only a RARP server processes a RARP request packet, all the other hosts discard the packet.
- The RARP reply packet is not broadcast, it is sent directly to the host, which sent the RARP request.

When a RARP server receives a RARP request packet, it performs the following steps:
- The MAC address in the request packet is looked up in the configuration file and
mapped to the corresponding IP address.
- If the mapping is not found, the packet is discarded.
- If the mapping is found, a RARP reply packet is generated with the MAC and IP
address. This packet is sent to the host, which originated the RARP request.

When a host receives a RARP reply packet, it gets its IP address from the packet and completes the booting process.