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. 

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 the concept of inter-networking?

- The practice in which one computer network is connected with the other networks is called inter-networking. 

- The networks are connected with the help of gateways. 

- These gateways are used since they offer a common method for routing the data packets across the networks.

- The resulting system in which a number of networks are connected is called the inter-network or more commonly as the internet. 

- The terms “inter” and “networking” combine together to form the term “internet working”.  

- Internet is the best and the most popular example of the inter networking. 

- Internet has formed as a result of many networks connected with the help of numerous technologies. 

- Many types of hardware technologies underlie the internet. 

- The internet protocol suite (IP suite) is the inter networking protocol standard responsible for unifying the diverse networks. 

- This protocol is more commonly known as the TCP/ IP. 

- Two computer local area networks (LANs) connected to one another by means of a router form the smallest internet but not the inter network. 

- Inter networking is not formed by simply connecting two LANs together via a hub or a switch. 

- This is called expansion of the original local area network. 

- Inter networking was started as a means for connecting the disparate networking technologies. 

- Eventually, it gained widespread popularity because of the development needs of connecting many local area networks together through some kind of WAN (wide area network). 

- “Catenet” was the original term that was used for the inter network. 

- Inter network includes many types of other networks such as the PAN or personal area network. 

- Gateways were the network elements that were originally used for connecting various networks in predecessor of the internet called the ARPANET. 

- Today, these connecting devices are more commonly known as the internet routers. 

- There is a type of interconnection between the various networks at the link layer of the networking model. 

- This layer is particularly known as the hardware centric layer and it lies below the TCP/ IP logical interfaces level. 

Two devices are mainly used in establishing this interconnection:

Ø  Network switches and

Ø  Network bridges

- Even now this cannot be called as inter networking rather, the system is just a single and large sub-network. 

- Further, for traversing these devices no inter networking protocol is required. 

- However, it is possible to convert a single network in to an inter network. 

- This can be done by making various segments out of the network and also making logical divisions of the segment traffic using the routers. 

- The internet protocol suite has been particularly designed for providing a packet service. 

- This packet service offered by the IPS is quite unreliable. 

- The elements that maintain a network state and are intermediate in the network are avoided by the architecture. 

- The focus of the architecture is more on the end points of the active communication session.

- For a reliable transfer of the data, a proper transport layer protocol must be used by the applications. 

- One such protocol is the TCP (transmission control protocol) and it is capable of providing a reliable stream for communication. 

- Sometimes a simpler protocol such as the UDP (user datagram protocol) might be used by the applications. 

- The applications using this protocol carry out only those tasks for which reliable data delivery is not required or for which realtime is required. 

Examples of such tasks include voice chat or watching a video online etc. Inter networking uses two architectural models namely:

1. OSI or the open system interconnection model: This model comes with 7 layer architecture that covers the hardware and the software interface.
2. TCP/ IP model: The architecture of this model is somewhat loosely defined when compared with the OSI model. 

Quick Tech Tip: Concatenated Virtual Circuits

Two styles of internetworking are common:

* a connection-oriented concatenation of virtual circuit subnets,
* a datagram internet style.

In the concatenated virtual circuit model, a connection to a host in a distant network is set up in a way similar to the way connections are normally established. The virtual circuit consists of concatenated virtual circuits between the routers or gateways along the way from the source node to the destination node. Each gateway maintains tables telling which virtual circuits pass through it, where they are to be routed, and what the new virtual circuit number is. This process continues until the destination host has been reached.



Once data packets begin flowing along the path, each gateway relays incoming packets, converting between packet formats and virtual circuit numbers as needed. Clearly, all data packets must traverse the same sequence of gateways , and thus arrive in order.
This scheme works best when all the networks have roughly the same properties.
Concatenated virtual circuits are also common in the transport layer. In particular, it is possible to build a bit pipe using OSI, which terminates in a gateway, and have a TCP connection go from the gateway to the next gateway. In this manner, an end-to-end virtual circuit can be built spanning different networks and protocols.

Quick Tech Tip: InterNetworking

What are we talking about ? The interconnection of two or more networks, usually local area networks so that data can pass between hosts on the different networks as though they were one network. This requires some kind of router or gateway to facilitate the process of interconnecting two or more individual networks to facilitate communications among their respective nodes. Note: The interconnected networks may be different types. Each network is distinct, with its own addresses, internal protocols, access methods, and administration.
For different networks to interact, devices such as repeaters, bridges, routers, gateways are required. Repeaters are low level devices that just amplify or regenerate weak signals. They are needed to provide current to drive long cables. They copy individual bits between cable segments.
Unlike repeaters, bridges are store and forward devices. A bridge accepts the whole frame and passes it to the data link layer where the checksum is verified. Then the frame is sent down to the physical layer for forwarding on a different network.
Multiprotocol routers are conceptually similar to bridges, except that they are found in the network layer. They take incoming packets from one line to another, just as all routers do, but the lines may belong to different networks and use different protocols.
Transport gateways connect byte streams in the transport layer. Application gateways connects two parts of an application in the application layer.
When a gateway is between two WANs run by different organizations in different countries, the gateway is ripped apart in the middle and the two parts are connected with a wire.