1. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html Networking Operating Systems (CO32010) 1. Operating Systems 2. Processes and scheduling 3. Distributed processing 4. Distributed file systems 5. Routing protocols 6. Routers 7. Encryption 8. NT, UNIX and NetWare 5.1Introduction 5.2Routing fundamentals 5.3Routing protocol techniques 5.4RIP 5.5OSPF 5.6IGRP 5.7EGP/BGP Objectives: To outline the fundamental techniques using in routing protocols. To define the main problem in routing protocol techniques, such as routing loops, and count-to-infinity, and how the may be overcome. To outline practical protocols, especially RIP and IGRP, and reflect on their strengths and weaknesses. Objectives: To outline the fundamental techniques using in routing protocols. To define the main problem in routing protocol techniques, such as routing loops, and count-to-infinity, and how the may be overcome. To outline practical protocols, especially RIP and IGRP, and reflect on their strengths and weaknesses.

2. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.1 Alternative Routes B B 1 Net1 Net2 Net3 Net4 Net5 Net6 Net7 Net8 4 3 6 B B A A 1 2 5 A A 1 1 2 2 3 3 4 4 6 6 5 5 6 6 B B B B 5 5 6 6 2 2 4 4 6 6 B B

3. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.2 Best route? Routing based on hops: Route (1,3,5,6) = 4 hops [BEST] Route (1,3,5,2,4,6) = 6 hops Routing based on delay (latency): Route(2,4,6) = 1.5+1.25 = 2.75 Route(2,5,6) = 1.1+1.3 = 2.4 [BEST] Routing based on error probability: P e (2 – 5)=0.01P e (5 – 6)=0.15 P e (2 – 4)=0.05P e (4 – 6)=0.1 P noerror (2,5,6)=(1 – 0.01)  (1 – 0.15) = 0.8415 P noerror (2,4,6)=(1 – 0.05)  (1 – 0.1) = 0.855 [BEST]

4. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.3 Layer 3 protocols Routing protocols. A routing protocol provides a mechanism for routers to share routing information. These protocols allow routers to pass information between themselves, and update their routing tables. Examples of routing protocols are Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Enhanced Interior Gateway Routing Protocol (EIGRP), and Open Shortest Path First (OSPF). Routed protocols. These protocols are any network layer protocol that allows for the addressing of a host and a destination on a network, such as IP and IPX. Routers are responsible for passing a data packet onto the next router in, if possible, an optimal way, based on the destination network address. The definition of an optimal way depends on many things, especially its reachability. With IP, routers on the path between a source and a destination, examine the network part of the IP address to achieve their routing. Only the last router, which is connected to the destination node network, examines the host part of the IP address.

5. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.4 Types of Routing Dynamic routing. In dynamic routing, the routers monitor the network, and can change their routing tables based on the current network conditions. The network thus adapts to changing conditions. Unfortunately, this method tends to reveal everything known about an internetwork to the rest of the network. This may be inappropriate for security reasons. Static routing. In static routing, a system administrator sets up a manual route when there is only one route to get to a network (a stub network). This type of configuring reduces the overhead of dynamic routing. Static routing also allows the internetwork administrator to specify the information that is advertised about restricted parts of a network. Default routing. These are manually defined by the system administrator and define the path that is taken if there is not a known route for the destination.

6. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.5 Best Route Parameters? Bandwidth. The data capacity of a link, which is typically defined in bps. Delay. The amount of time that is required to send a packet from the source to a destination. Load. A measure of the amount of activity on a route. Reliability. Relates to the error rate of the link. Hop count. Defined by the number of routers that it takes between the current router and the destination. Ticks. Defines the delay of a link by a number of ticks of a clock. Cost. An arbitrary value which defines the cost of a link, such as financial expense, bandwidth, and so on.

7. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.6 Type of Update? Broadcast. In broadcast, routers transmit their information to other routers at regular intervals. A typical broadcast routing protocol is RIP, in which routers send their complete routing table once every few minutes, to all of their neighbors. This technique tends to be wasteful in bandwidth, as changes in the route do not vary much over short amounts of time. Event-driven. In event-driven routing protocols, routing information is only sent when there is a change in the topology or state of the network. This technique tends to be more efficient than broadcast, as it does not use up as much bandwidth.

8. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.7 Routing protocol types Bandwidth Hop count + + + Event driven v. broadcast Static.v. dynamic + Link-state Distance-vector Each router transmits routing information to all other routers onlywhen there are changes (OSPF/BGP/EGP) Problems: Initial flooding Processing/memory Each router periodically sends information to each of itsneighbors (RIP). Problems: Bandwidth Step-by-step updates Hybrid (IS-IS) + + Routed (IP, IPX, NetBEUI) + Routing (RIP, OSPF) + + + Delay Reliability Tick Cost Session Transport Network Data link Physical HTTP TCP IP RIP Ethernet/ FDDI Ethernet/ FDDI Routing Layer 3 protocols Layer 3 protocols Types Updates Distance metrics Distance metrics Bandwidth Hop count + + + Event driven v. broadcast Static.v. dynamic + Link-stateLink-state Distance-vectorDistance-vector Each router transmits routing information to all other routers onlywhen there are changes (OSPF/BGP/EGP) Problems: Initial flooding Processing/memory Each router periodically sends information to each of itsneighbors (RIP). Problems: Bandwidth Step-by-step updates Hybrid (IS-IS) + + Routed (IP, IPX, NetBEUI) + Routing (RIP, OSPF) + + + Delay Reliability Tick Cost Session Transport Network Data link Physical HTTP TCP IP RIP Ethernet/ FDDI Ethernet/ FDDI Session Transport Network Data link Physical HTTP TCP IP RIP Ethernet/ FDDI Ethernet/ FDDI Routing Layer 3 protocols Layer 3 protocols Types Updates Distance metrics Distance metrics

9. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.8 Example routing

10. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.9 Routing loops WX ZY 13 2 4 A. Network A unreachable A. Network A unreachable Network unreachable Network A V A. Network A unreachable A. Network A unreachable B. I can reach Network A in 3 hops B. I can reach Network A in 3 hops Router Z thinks it can reach Network A in 4 hops, as Router W says it can reach it in 3 hops, this overrules the information from Router Y which says it cannot reach Network A C. Network A Reachable via Router W C. Network A Reachable via Router W D. Network A reachable D. Network A reachable E. Network A reachable E. Network A reachable A A B B C C D D E E Timing of events WX ZY 13 2 4 A. Network A unreachable A. Network A unreachable Network unreachable Network A V A. Network A unreachable A. Network A unreachable B. I can reach Network A in 3 hops B. I can reach Network A in 3 hops Router Z thinks it can reach Network A in 4 hops, as Router W says it can reach it in 3 hops, this overrules the information from Router Y which says it cannot reach Network A C. Network A Reachable via Router W C. Network A Reachable via Router W D. Network A reachable D. Network A reachable E. Network A reachable E. Network A reachable A A B B C C D D E E Timing of events

11. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.10 Overcoming Distance Vector Problems Setting infinity values. The count-to-infinity will eventually resolve itself when the routers have counted to infinity (as infinity will be constrained with the maximum definable value), but while the network is counting to this value, the routing information will be incorrect. To reduce the time that it takes to get to this maximum, a maximum value is normally defined. In RIP this value is set at 16 hops for hop-count distance-vectors, thus the maximum number of hops that can occur is 15. This leads to a problem in that a destination which has a distance of more than 15 hops is unreachable, as a value of 16 or more defines that the network is unreachable. Split horizon. This method tries to overcome routing loops. With this routers do not update their routing table with information on a destination if they know that the network is already connected to the router (that is, the router knows more about the state of the network than any other router, as it connects to it). Thus in Figure X, Router Z and Router X will not send routing information on Network B to Router Y, as they know that Network B is connected to Router Y.

12. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.11 Overcoming Distance Vector Problems Hold-Down Timers. This method overcomes the count-to-infinity problem. With a hold-time time, a router starts a hold-time timer when it receives an update from a neighbor indicating that a previously accessible network is now inaccessible. It also marks the route as inaccessible. There are then three possible situations: oIf, at any time before the hold-down timer expires, an update is sent from the same neighbor which alerted the initial problem saying that it is now accessible, the router marks the network as accessible and removes the hold-down timer. oIf an update arrives from a different neighboring router with a better metric than the original metric, the router marks the network as accessible and removes the hold-down timer. oIf, at any time before the hold-down timer expires, an update is sent from a different neighbor which alerted the initial problem saying that it is accessible, but has a poorer metric than the previously recorded metric, the update is ignored. Obviously after the timer has expired the network will still be prone to looping routes, but the timer allows for a longer time for the network to settle down and recover the correct information.

13. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.12 Link-state overview

14. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.13 OSPF overview OSPFis anIGP(Interior Gateway Protocol) which distributes routing information between routers in a single autonomous system. All routers have the same database. Gateways Separate domains OSPF (RFC1583) Ver.. Type Message Len. Router ID (unique in AS) Area ID (similar tosubnetting) Area ID (similar tosubnetting) Checksum Auth. Type Authentication Hello[1]. Used to establish and maintain aconnection. Routers agreeHelloIntervalandRouterDeadInterval. HelloInterval. Number of seconds between Hello packets. The smaller the value, the fastest the detection of topological changes. X.25 uses30 sec, LANsuses 10 sec. RouterDeadInterval. Number of seconds before arouter assumes that arouteis down. It should be a multiple ofHelloInterval(such as four times). Database Description[2]. Used to send database between routers. Link-state Request[3]. Request parts of aneighbor’s database, which may be more up-to-date. Link-state Update[4]. Used to flood link state advertisements. Link-state Acknowledgement[5]. Used to acknowledge flooded advertisements. + + + + + Additional Information (depends on packet type) 32 bits OSPF header Autonomous System Autonomous System Autonomous System Autonomous System Autonomous System Autonomous System EGP used between AS’s Internet OSPFis anIGP(Interior Gateway Protocol) which distributes routing information between routers in a single autonomous system. All routers have the same database. Gateways Separate domains OSPF (RFC1583) Ver.. Type Message Len. Router ID (unique in AS) Area ID (similar tosubnetting) Area ID (similar tosubnetting) Checksum Auth. Type Authentication OSPF (RFC1583) Ver.. Type Message Len. Router ID (unique in AS) Area ID (similar tosubnetting) Area ID (similar tosubnetting) Checksum Auth. Type Authentication Hello[1]. Used to establish and maintain aconnection. Routers agreeHelloIntervalandRouterDeadInterval. HelloInterval. Number of seconds between Hello packets. The smaller the value, the fastest the detection of topological changes. X.25 uses30 sec, LANsuses 10 sec. RouterDeadInterval. Number of seconds before arouter assumes that arouteis down. It should be a multiple ofHelloInterval(such as four times). Database Description[2]. Used to send database between routers. Link-state Request[3]. Request parts of aneighbor’s database, which may be more up-to-date. Link-state Update[4]. Used to flood link state advertisements. Link-state Acknowledgement[5]. Used to acknowledge flooded advertisements. + + + + + Additional Information (depends on packet type) 32 bits OSPF header Autonomous System Autonomous System Autonomous System Autonomous System Autonomous System Autonomous System EGP used between AS’s Internet

15. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.14 Tree-like topology v. Internet-like topology

16. bill@napier, 2002 http://www.soc.napier.ac.uk/~bill/nos.html 5.15 Autonomously attached networks