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The Network Layer The main design goals for Network Layer services: (see the book “Computer Networks by Tanenbaum” P345). Two kinds of subnets: 1- Datagram subnet (Internet community). The subnet job should be delivering packets only. No connection setup or reservation is required. (Internet community). 2- Virtual Circuit subnet (VC). There should be one reserved path from source to destination to provide quality of service. (telephone companies). Example of the first kind is the internet. Second kind example is ATM networks. Connection less services: 1- Each packet is inserted to the network individually and also the routers deal with each packet individually. 2- Packets are called Datagrams. 3- Each router holds a routing table which allow packets to move from source to destination. 4- Routing tables are updated and modified based on the network conditions by ROUTING algorithms. 5- No bandwidth is wasted. 6- Congestion can occur and quality of service is harder to guarantee. Connection oriented services: 1- In this kind the route from source to destination should be established before any packet is sent. 2- The connection is called virtual circuit (VC) and the subnet is called a virtual circuit subnet. 3- Avoid choosing a new path for every packet. The path from source to destination is fixed and established at the beginning of the connection. 4- Provide quality of service and can avoid congestions easily. 5- Network resources maybe wasted if nobody is using the reserved line. The Optimality Principle: Sink Tree: Gives the optimal paths from several sources to one destination (the sink). There may be more than one sink tree at the same time. Router algorithms aims to find and use sink trees for different routes. Using sink trees will allow packets to be delivered in finite number of hops. Shortest Path Routing: Different ways to measure the path length (number of hops, distance, time, cost.. etc). To put a standard way of measuring the path length we put weight for each link. This weight could be distance, cost… etc. or any number of them. See example Figure 5-7 page 353-354 (Tanenbaum) Flooding Routing: Packets are sent on every outgoing link. Problems: can send multiple unneeded packets. Packets may take loops. Solutions: to reduce outgoing lines be estimating the packet direction (selective flooding). Keep track of which packet has been flooded so routers avoid sending them again. Example of flooding useful applications are: o Military applications. o Distributed databases. o Wireless networks. o For performance comparison with other routing algorithms. Distance Vector Routing: Dynamic routing: takes new network conditions into account. Each router creates a table of two entries (Destination, Line, Distance) for all destinations. Then each router sends its table to neighbor routers. When the network conditions change, the affected router update its distance vector table and distribute it again. Link State Routing: Widely used in today’s networks. The procedure used in Link State Routing is as following: o Discovering neighbors and learning their network address. This is done by sending a HELLO packet to each directly connected (point-to-point) router. The receiver replies with its information (address). o Calculating the delay of each neighbor. To calculate the delay, each router sends an ECHO packet and the receiver should reply so that the sender can calculate how long it takes the packet to travel from the sender to the receiver (round trip time). The sender then calculate the delay by dividing this time by 2 delay = round trip time /2. o Creating a packet to tell all neighbors about the new findings. This packet has three entries: The sender address, a list of neighbors with delays for each one and age. o Sending the packet to neighbors. For that the FLOODING technique is used. However to solve the problem of having loops each routers make sure it does not send a flooding packet twice by checking the source router and packet sequence number. o Computing shortest path to every other router using Sorters Path Algorithm (explained before).