Core Concepts in Network Layer Protocols and Routing

1)A virtual-circuit network (VCN) is a hybrid network model that combines features of both circuit-switched and datagram networks. It provides a balance between connection-oriented and connectionless transmission methods :-Three Phases in a Virtual-Circuit Network In a virtual-circuit network, the communication between a source and destination involves three phases: setup, data transfer, and teardown. These phases ensure that a reliable path is established and maintained for the communication session. 1. Setup Phase: o The source and destination use their global addresses to establish a connection. During this phase, switches along the path create table entries to store information about the virtual circuit. This phase ensures that each switch is prepared to route the data properly. 2. Data Transfer Phase: o After the setup phase, data is transferred between the source and destination. The switches use the table entries created during the setup phase to route the frames. The switches maintain information like the incoming and outgoing ports and Virtual Circuit Identifiers (VCI). Each frame is processed the same way, with the VCIs changing at each switch to ensure the data follows the correct path. 3. Teardown Phase: o Once the data transfer is complete, the source and destination send signals to the switches to remove the corresponding table entries, effectively ending the virtual circuit.

2)PACKET SWITCHING When the Internet was first designed, the network layer provided a connectionless service. Each packet is treated independently, with no relationship to other packets. The network layer’s role is to deliver packets from the source to the destination, and packets may take different paths to get there. . Datagram Approach: Connectionless Service ➔ When the network layer provides a connectionless service, each packet traveling in the Internet is an independent entity; there is no relationship between packets belonging to the same message. ➔ The switches in this type of network are called routers. ➔ A packet belonging to a message may be followed by a packet belonging to the same message or to a different message. ➔ A packet may be followed by a packet coming from the same or from a different source. ➔ Each packet is routed based on the information contained in its header: source and destination addresses. The destination address defines where it should go; the source address defines where it comes from. T ➔ he router in this case routes the packet based only on the destination address. The source address may be used to send an error message to the source if the packet is discarded. ➔ Figure shows the forwarding process in a router in this case. We have used symbolic addresses such as A and B. 

3)ROUTING ALGORITHMS Several routing algorithms have been designed in the past. The differences between these methods are in the way they interpret the least cost and the way they create the least-cost tree for each node. The Distance-Vector (DV) Routing Algorithm ➔ The distance vector (DV) algorithm is iterative, asynchronous, and distributed. ➔ It is iterative that the process continues on until no more information is exchanged between neighbours. ➔ The algorithm is asynchronous in that it does not require all of the nodes to operate in lockstep with each other. ➔ Let dx(y) be the cost of the least-cost path from node x to node y. Then the least costs are related by the celebrated Bellman-Ford equation, namely, dx(y) = minv{c(x,v) + dv(y)} ➔ With the DV algorithm, each node x maintains the following routing information: o For each neighbour v, the cost c(x,v) from x to directly attached neighbour, v o Node x’s distance vector, that is, Dx = [Dx(y): y in N], containing x’s estimate of its cost to all destinations, y, in N o The distance vectors of each of its neighbours, that is, Dv = [Dv(y): y in N] for each neighbour v of x 4)THE IPv6 PROTOCOL:- The change of the IPv6 address size requires the change in the IPv4 packet format. The following shows other changes implemented in the protocol in addition to changing address size and format. Better header format. IPv6 uses a new header format in which options are separated from the base header and inserted, when needed, between the base header and the data. New options. IPv6 has new options to allow for additional functionalities. Allowance for extension. IPv6 is designed to allow the extension of the protocol if required by new technologies or applications. Support for resource allocation. In IPv6, the type-of-service field has been removed, but two new fields, traffic class and flow label, have been added to enable the source to request special handling of the packet. This mechanism can be used to support traffic such as real-time audio and video. Support for more security. The encryption and authentication options in IPv6 provide confidentiality and integrity of the packet. Packet Format The IPv6 packet is shown in Figure. Each packet is composed of a base header followed by the payload. The base header occupies 40 bytes, whereas payload can be up to 65,535 bytes of information. The description of fields follows. Version. 

5)Open Shortest Path First (OSPF) Open Shortest Path First (OSPF) is an intradomain routing protocol like RIP but is based on the link-state routing protocol. It is an open protocol, meaning the specification is publicly available. Unlike RIP, OSPF allows each link to be assigned a weight based on factors such as throughput, round-trip time, or reliability, though administrators can use hop count as a cost. Different Types of Service (TOS) can have different weights for cost calculation. OSPF Implementation: OSPF operates at the network layer and uses IP for message propagation. OSPF messages are encapsulated in IP datagrams with a protocol field value of 89. OSPF has two versions, with version 2 being the most widely implemented. Link-State Update (Type 4): The primary message used to build the LSDB, with versions for each type of link. Link-State Acknowledgment (Type 5): Provides reliability by confirming receipt of link-state updates. Authentication: OSPF supports message authentication to prevent unauthorized routers from joining the routing system. 

OSPF Messages: OSPF uses five message types, each with a distinct format : 

Border Gateway Protocol Version 4 (BGP4) BGP4 is the predominant interdomain routing protocol used across the internet. It is a path vector protocol, which means it provides the entire path to the destination instead of just the next hop. This path information is useful for avoiding routing loops and ensuring stable paths between autonomous systems (AS). Figure illustrates a network with four ASs: AS1 is a transient AS, and AS2, AS3, and AS4 are stub ASs. Data exchanges between the stub ASs occur through AS1. External BGP (eBGP) Operation BGP operates over Transmission Control Protocol (TCP) connections, ensuring reliable communication between routers. The protocol defines two types of operations: external (eBGP) and internal (iBGP). ll Internal BGP (iBGP) Operation Within the same AS, routers establish iBGP sessions to disseminate the routes learned from eBGP sessions. A fully connected mesh of iBGP sessions is required to prevent routing loops. Each iBGP router must have a direct session with every other iBGP router,