S305 – Network Infrastructure Chapter 5 Network and Transport Layers

Network Layer Computer 1Computer 2

5.4 Routing Process of identifying what path to have a packet take through a network from sender to receiver Routing Tables  Used to make routing decisions  Shows which path to send packets on to reach a given destination  Kept by computers making routing decisions Routers  Special purpose devices used to handle routing decisions on the Internet  Maintain their own routing tables Dest. B C D E F G Next B D B

5.4 Routing DestinationInterface X X X X X All Others X Routing Table

5.4 Routing 5 - 5

Types of Routing Centralized routing  Decisions made by one central computer  Used on small, mainframe-based networks Decentralized routing  Decisions made by each node independently of one another  Information needs to be exchanged to prepare routing tables  Used by the Internet

5.4.2 Routing Protocols – how tables are set up Static routing:  Uses fixed routing tables developed by network managers Each node has its own routing table Changes when computers added or removed Dynamic routing or Adaptive routing:  Uses routing tables at each node that are updated dynamically  Based on routing condition information exchanged between routing devices  Types 5 - 7

Routing Information Protocol (RIP) A dynamic distance vector interior routing protocol Uses the least number of hops to decide how to route a packet Operations:  Manager builds a routing table by using RIP  Routing tables broadcast periodically (every minute or so) by all nodes  When a new node added, RIP counts number of hops between computers and updates routing tables 5 - 8

Open Shortest Path First (OSPF) A dynamic link state interior routing protocol  Uses a variety of information types to decide how to route a packet (more sophisticated) e.g., number of hops, congestion, speed of circuit  Links state info exchanged periodically by each node to keep every node in the network up to date  Provides more reliable, up to date paths to destinations Became more popular on Internet  More reliable paths  Less burdensome to the network 5 - 9

Other Interior Routing Protocols Enhanced Interior Gateway Routing Protocol (EIGRP)  A dynamic link state protocol (developed by Cisco)  Records transmission capacity, delay time, reliability and load for all paths  Keeps the routing tables for its neighbors and uses this information in its routing decisions as well Internet Control Message Protocol (ICMP)  Simplest and most basic  An error reporting protocol (report routing errors to message senders)  Limited ability to update routing tables

5.4.2 Exterior Routing Each network may be running a different protocol locally so how are they able to talk?

Sending Messages using TCP/IP Required Network layer addressing information  Computer’s own IP address  Its subnet mask To determine what addresses are part of its subnet  Local DNS server’s IP address To translate URLs into IP addresses  IP address of the router (gateway) on its subnet To route messages going outside of its subnet Address information is obtained from a configuration file or provided by a DHCP server  Servers also need to know their own application layer addresses (domain names)

TCP/IP Configuration Information

5.5.1 Subnet Masks Tells the computer what part of an Internet Protocol address to be used to determine whether the destination is on the same subnet or on a different subnet Example  Subnet: x  Subnet mask: Example  Subnets: ,  Subnet mask

5.5.1 TCP/IP Network Example

5.5.1 TCP/IP Network Example

Moving Messages - TCP/IP and Layers How layers are handled in a LAN: Host Computers Packets move through all layers  Gateways, Routers Packet moves from Physical layer to Data Link Layer through the network Layer At each stop along the way (e.g. from router to router):  Ethernet packets is removed and a new one is created for the next node  IP and above packets never change in transit (created by the original sender and destroyed by the final receiver)

5.5.1 Message Moving Through Layers

5.5.2 Known IP Address Transmit from A => E (A knows E’s IP Address) How many Hops will it take? PATHIP Source IP Destination Ethernet Source Ethernet Destination

5.5.3 Unknown IP Address Transmit from A => E (A doesn’t know E’s IP address) How do we do this? PATHIP Source IP Destination Ethernet Source Ethernet Destination DNS Request PATHIP Source IP Destination Ethernet Source Ethernet Destination DNS Response

5.5.4 Unknown Data Link Address Transmit from A => E (doesn’t know E’s Ethernet address) An ARP request from D will occur once the message sent by A is received by D PATHIP Source IP Destination Ethernet Source Ethernet Destination

Known IP Address and Ethernet Address Transmit from B => F (B knows F’s IP Address) PATHIP Source IP Destination Ethernet Source Ethernet Destination

Unknown IP Address Transmit from B => F (B doesn’t know F’s IP address) PATHIP Source IP Destination Ethernet Source Ethernet Destination

Unknown Ethernet Address Transmit from B => F (doesn’t know F’s Ethernet address) PATHIP Source IP Destination Ethernet Source Ethernet Destination

Implications for Management Most organizations moving toward a single standard based on TCP/IP  Decreased cost of buying and maintaining network equipment  Decreased cost of training networking staff Telephone companies with non-TCP/IP networks are also moving toward TCP/IP  Significant financial implications for telcos  Significant financial implications for networking equipment manufacturers