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 are the advantages and disadvantages of datagram approach?

- Today’s packet switching networks make use of a basic transfer unit commonly known as the datagram. 

- In such packet switched networks, the order of the data packets arrival, time of arrival and delivery comes with no guarantee. 

- The first packet switching network to use the datagrams was CYCLADES. 

- Datagrams are known by different names at different levels of the OSI model. 

- For example, at layer 1 we call it Chip, at layer 2 it is called Frame or cell, data packet at layer 3 and data segment at layer 4. 

- The major characteristic of a datagram is that it is independent i.e., it does not rely on any other thing for the information required for exchange.

- The duration of a connection between any two points is not fixed such as in telephone conversations. 

- Virtual circuits are just the opposite of the datagrams. 

- Thus, a datagram can be called as a self containing entity. 

- It consists of information sufficient for routing it from the source to the destination without depending up on the exchanges made earlier. 

- Often, a comparison is drawn between the mail delivery service and the datagram service. 

- The user’s work is to just provide the address of the destination. 

- But he/she is not guaranteed the delivery of the datagram and if the datagram is successfully delivered, no confirmation is sent to the user. 

- The data gram are routed to some destination without help of a predetermined path. 

- The order in which the data has to be sent or received is given no consideration. 

- It is because of this that the datagrams belonging to a single group might travel over different routes before they reach their common destination. 

Advantages of Datagram Approach

1. Datagrams can contain the full destination address rather than using some number.
2. There is no set up phase required for the datagram circuits. This means that no resources are consumed.
3. If it happens during a transmission that one router goes down, the datagrams that will suffer will include only those routers which would have been queued up in that specific router. The other datagrams will not suffer.
4. If any fault or loss occurs on a communication line, the datagrams circuits are capable of compensating for it.
5. Datagrams play an important role in the balancing of the traffic in the subnet. This is so because halfway the router can be changed.

Disadvantages of Datagram Approach

1. Since the datagrams consist of the full destination address, they generate more overhead and thus lead to wastage of the bandwidth. This in turn makes using datagram approach quite costly.
2. A complicated procedure has to be followed for datagram circuits for determining the destination of the packet.
3. In a subnet using the datagram approach, it is very difficult to keep congestion problems at bay.
4. The any-to-any communication is one of the key disadvantages of the datagram subnets. This means that if a system can communicate with any device, any of the devices can communicate with this system. This can lead to various security issues.
5. Datagram subnets are prone to losing or re - sequencing the data packets during the transition. This puts a great burden on the end systems for monitoring, recovering, and reordering the packets as they were originally.
6. Datagram subnets have less capability of dealing with congestion control as well as flow control. This happens because the direction of the incoming traffic is not specified. In the virtual circuit subnets, the flow of the packets is directed only along the virtual circuits thus making it comparatively easy for controlling it.
7. The unpredictable nature of the flow of the traffic makes it difficult to design the datagram networks. 

What are the differences between inter-network routing and intra-network routing?

- The individual networks when combined together form the inter-network. 

- Intermediate inter networking devices are used for making connections between them. 

- All these networking elements combine to work as single large unit. 

- The creation of the internetworking has been made possible because of the packet switching technology. 

- The router is the most common and important device used for performing inter-network routing and intranetwork routing.

- Routing across various networks in the inter network is termed as internetwork routing and routing within the same network is termed intranetwork routing. 

In this article we discuss about the differences between internetwork routing and intranetwork routing. 

- Just like inter-network, intranetwork also uses IP (internet protocol) technology for computing services and sharing information. 

- But what makes it different from internetworking is that it is limited to some organization whereas internetwork extends beyond all i.e., it is not limited.

- Or we can put it in other words: Internetwork is spread across organizations and Intranetwork lies within an organization. 

- In some cases, the term intranetwork might mean only the internal website of the organization, but in other cases it might be a larger part of the IT infrastructure of the organization. 

- Sometimes, it may span over a number of LANs (local area networks). 

- The intranetwork is driven by the goal of minimizing the time, effort and cost of the individual’s desktop in order to make it more competitive, cost efficient, timely as well as productive.

- An intranetwork is capable of hosting multiple websites that are private to organizations and may even constitute an important part of the collaboration and communication between the members of the organization. 

- Intranetwork also makes use of various well known protocols such as the FTP, SMTP and HTTP. 

- The intranets are often incorporated with the technologies for lending a modern interface to the systems that host the corporate data. 

- These systems are known as the legacy systems. 

- We can see intranetwork to be a private analog of the internetwork. 

- It means the internetwork has been simply extended to an organization for its private use. 

- Extranetworks are a modified version of the intranetworks.

- Here, the website might be accessed by the non-members i.e., the suppliers, customers or some other approved third parties and so on. 

- Intranetworks are well equipped with a special protocol called the AAA protocol. 

- The 3 As stand for authentication, authorization and accounting. 

- There are a number of organizations who are concerned about the security of their intranetworks. 

- They have deployed a firewall and a network gateway for controlling the access to their services. 

- The intermediate systems when connect together form the internetwork whereas they may bound together a part of the internetwork which might be an intranetwork. 

- The intranetwork routing involves routing between two routers which lie in the same network whereas in internetwork routing, routing is done between routers which reside across different networks. 

- Intranetwork routing is quite easy when compared to the internetwork routing. 

- Protocols used in both the types of routing are different.

- Interior gateway protocol is responsible for routing in the intranetworks whereas the exterior gateway protocol takes the responsibility of routing across the internetwork. 

- Most common example of interior gateway protocol is the OSPF or the open shortest path first protocol. 

- And most common example of exterior gateway protocol is the border gateway protocol or BGP. 

- Also, the routing graphs for both the types are different. 

- In the intranetwork’s graph, all the routers are simply linked to one another in the same network. 

- There is less mess.

- On the other hand, the inter network’s graph is quite tedious. 

- This is so because routers of different networks have to be inter-linked with one another. 

What is inter-network routing?

In this article we shall discuss about inter-network routing. Before moving to that there are certain terms with which you should be familiar:

Ø  End systems: The ISO (the international standards of organization) defines the end systems as the network elements that do not have the ability of forwarding the packets across the networks. Sometimes the term host is used to refer to the end systems.

Ø  Intermediate systems: These are the network elements that have the ability of forwarding the packets across the network. Most common examples are routers, switches, bridges and so on.

Ø  Network: It can be defined as a part of the inter-network infrastructure encompassing various elements including hubs, repeaters, bridges and so on. The networks are bounded by the intermediate systems.

Ø Router: This is one of the intermediate systems that is used for connecting various networks with each other. It might support one protocol (router) or many protocols (multi-protocol router). Its hardware part is optimized especially for performing routing. The software part is responsible for carrying out the routing and takes care of the routing tables.

Apart from these devices, there are 3 types of addresses involved in inter-network routing:

Ø  The inter-network address: The host address and the network address are combined together to form this address. This is used for unique identification of a host over the inter-network.

Ø The host address or host ID: This ID might be assigned by the administrator or might be simply the physical address of the host. It is used for the unique identification of the host on its network.

Ø  The network address or network ID: This is address of a network for identifying it in an inter-network.

All the data packets consist of a network layer header. This network layer header consists of the following when the packet is transmitted from one host to another:

ØThe address of the source inter-network: This address combines the address of the source host and the source network.

ØThe address of the destination inter-network: This address combines the address of the destination host and the destination network.

ØThe hop count: This usually begins at zero and is numerically incremented when the packet crosses a router. Or in the opposite case it might be assigned some maximum value which might be decremented on reaching a router. The purpose of using the hop count is to make sure that the packet does not keeps on circulating endlessly in the network.

- For inter-network routing, two things have to be known.

- Firstly, how do you reach other routers which lie in the same network and secondly, how do you reach other routers which lie in other networks? 

- The answer to the first question is easy as it is the common routing problem among two hosts residing over the same network. 

- This routing is handled by the interior gateway protocol and it is different for different networks since only local routing info is required. 

- In this case, the commonly used protocol is the open shortest path first or OSPF protocol. 

- The routing between two different networks is performed using the exterior gateway protocol. 

- This is actually the problem of inter-network routing. 

- Here, the commonly used protocol is the BGP or the border gateway protocol. 

- The graph for inter-network routing is quite different from the one that is used in the network routing. 

- This is so because the routers which lie in the same network can be thought of as being directly connected to one another for routing across inter-network. - All the networks in an inter-network function as though they are one large unit. 

Explain Border Gateway Protocol (BGP)?

- BGP or Border gateway protocol is the set of rules that is implemented for making the routing decisions at the core of the internet. 

- It involves the use of the IP networks table or we can say prefixes which are used for designating the reach-ability of the network to the autonomous systems. 

- This protocol falls under the category of the path vector protocol or sometimes classified as a variant of the distance vector routing protocols. 

- The metrics of the IGP or the interior gateway protocol are not used by the border gateway protocol rather paths, rule sets or polices are used for making decisions for routing. 

- This is why the border gateway protocol is often called a reach-ability protocol rather than being termed as a routing protocol. 

- The BGP has ultimately replaced the EGP or the exterior gateway protocol. 

- This is so because it allows the full decentralization of the routing process for making transition between the ARPANET model’s core and the decentralized system that consists of a NSFNET backbone and the regional networks associated with it. 

- The present version of the BGP that is being used is the version 4. 

- The earlier versions were discarded for being obsolete. 

- The major advantage is of the classless inter-domain routing and availability of a technique called the route aggregation for making reductions in the routing size. 

- The use of the BGP has made the whole routing system a decentralized system.

- BGP is used by most of the internet service providers for establishing a route between them. 

- This is done especially when the ISPs are multi-homed. 

- That’s why even though it is not used directly by the users; it is still one of the most important protocols in networking. 

- The BGP is used internally by a number of large private IP networks. 

- For example, it is used to combine many large open shortest path first or OSPF networks where these networks do not have the capability to scale to the size by themselves. 

- BGP is also used for multi-homing a network so as to provide a better redundancy. 

- This can be either to many ISPs or to a single ISP’s multi access points. 

- Neighbors of the border gateway protocol are known as the peers. 

- They are created by manually configuring the two routers so as to establish a TCP session on the port. 

- Messages called the 19 byte keep alive messages are sent to the port periodically by the BGP speaker for maintaining the connection. 

- Among the various routing protocols, the most unique is BGP since it relies up on TCP for transporting. 

- When the protocol is implemented in the autonomous system among two peers, it is called IBGP or the internal border gateway protocol. 

- The protocol is termed as the EBGP or the external border gateway protocol when it runs between many autonomous systems.

- Border edge routers are the routers that are implemented on the boundary for exchanging information between various autonomous systems.

- BGP speakers have the capability for negotiating with the session’s option capabilities such as the multi-protocol extensions and a number of recovery modes. 

- The NLRI (network layer reach-ability information) can be prefixed by the BGP speaker if at the time of the creation itself, the multi-protocol extensions are negotiated. 

- The NLRI is advertised along with some address family prefix. 

The family consists of the following:

Ø  IPv4

Ø  IPv6

Ø  Multicast BGP

Ø  IPv4/ IPv6 virtual private networks

- These days the border gateway protocol is being commonly employed as the generalized signaling protocol whose purpose is to carry information via the routes that might not form the global internet’s part. 

What is Portability Testing?

- Portability Testing is the testing of a software/component/application to determine the ease with which it can be moved from one machine platform to another. 

- In other words, it’s a process to verify the extent to which that software implementation will be processed the same way by different processors as the one it was developed on.  

- It can also be understood as amount of work done or efforts made in order to move software from one environment to another without making any changes or modifications to the source code but in real world this is seldom possible.

- For example, moving a computer application from Windows XP environment to Windows 7 environment, thereby measuring the efforts and time required to make the move and hence determining whether it is re usable with ease or not.

- Portability testing is also considered to be one of the sub parts of System testing as this covers the complete testing of software and also it’s re-usability over different computer environments that include different Operating systems, web browsers.

What needs to be done before Portability testing is performed (pre requisites/pre conditions)? 

1.   Keep in mind portability requirements before designing and coding of software.

2.   Unit and Integration Testing must have been performed.

3.   Test environment has been set up.

Objectives of Portability Testing

1. To validate the system partially i.e. to determine if the system under consideration fulfills the portability requirements and can be ported to environments with different :-

a). RAM and disk space

b). Processor and Processor speed

c). Screen resolution

d). Operating system and its version in use.

e). Browser and its version in use.

To ensure that the look and feel of the web pages is similar and functional in the various browser types and their versions.

2.   To identify the causes of failures regarding the portability requirements, this in turn helps in identifying the flaws that were not found during unit and integration testing.

3.   The failures must be reported to the development teams so that the associated flaws can be fixed.

4.   To determine the potential or extent to which the software is ready for launch.

5.   Help in providing project status metrics (e.g., percentage of use case paths that were successfully tested).

6.   To provide input to the defect trend analysis effort.

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.