Aims. The aims of this unit are to: Outline the usage and importance of routing protocols. Understand the problems caused with routing protocols.

Transcription

1 Aims The aims of this unit are to: Outline the usage and importance of routing protocols. Understand the problems caused with routing protocols. Unit 9: Routing Protocols 167

2 Introduction Routers filter network traffic so that the only internetwork traffic flows into and out of a network. In many cases, there are several possible routes that can be taken between two nodes on different networks. Consider the network in Figure 1. In this case, the upper network shows the connection between two nodes A and B through routers 1 to 6. It can be seen from the lower diagram that there are four routes that the data can take. To stop traffic taking a long route or even one that does not exist, each router must maintain a routing table so that it knows where the data must be sent when it receives data destined for a remote node. For routers to find the best route they must communicate with their neighbors to find the best way through the network. This measure can be defined in a number of ways, such as the number of router hops to the remote node, the bandwidth on each link, latency, average error rates, current network traffic, and so on. Many routers use the number of router hops to determine the route, which is not always the best measure, as it may include a congested route. As with road traffic, it is often better to take the freeway (which is equivalent to a high-bandwidth route) than it is to take a route which has lower speed limits, or has a great deal of traffic congestion and/or traffic lights. Each router communicates with its neighbors to build-up a routing table. For example in Figure 1 the routing table for router 1 could be: 1 AA 1 Net1 Net2 2 Net5 Net4 4 Net6 6 Net8 BB 3 Net3 5 Net BB AA BB BB B Figure 1 Example routing Destination Distance Next router Output port (hops) Net5 1 2 (Net1) Net7 2 3 (Net2) Net8 3 2 (Net1) Net3 1 3 (Net2) And so on. 168 Computer Networks CO33006

3 It can be seen that the best route (measured by hops) from node A to Net8 is to go via Net1. This is the only information that the router needs to store. When the data gets to router 2 it has the choice of whether to send it to Net4 or Net5, as both routes get to Net8 in two hops. A better method of determining the best route is to have some measure of the delay. For this routers pass delay information about their neighbors. For example, if the relative delay in Net5 was 1.5 and in Net6 it was 1.25, and the relative delay in Net4 was 1.1 and in Net7 was 1.3, then the relative delay between router 2 and router 6 can be calculated as: Route(2,4,6) = = 2.75 Route(2,5,6) = = 2.4 Thus the best route is via router 5. Another technique used to determine the best route is error probability. In this case, the probability of an error is multiplied to give the total probability. The route with the lowest error probability will be the most reliable route. For example: P e (2 5)=0.01 P e (5 6)=0.15 P e (2 4)=0.05 P e (4 6)=0.1 Thus, P noerror (2,5,6) =(1 0.01) (1 0.15) = P noerror (2,4,6) =(1 0.05) (1 0.1) = Thus, the route via router 4 is the most reliable. Dijstra s Algorithm As we have seen, network topology and network architecture have a great impact on the performance of networks. The interconnection of these networks is often complex, where routers must pass information to each other about the best way to get to a destination. They could thus base their judgement on many things, such as the: Hop count. Number of routers that they cross over to get to the destination (a hop count). Latency. Time taken for a data packet to cross a route (the latency). Bandwidth. This defines the capacity of routes. Cost. This is the financial cost of routes. Error rate. This defines the error rate on routes. And so on. For example in Figure 2, there are several ways that can be used to travel between the two networks (Network A and Network B), such as: Unit 9: Routing Protocols 169

4 Router Hops ABH 3 ABCDFGH 7 ABEH 4 If we were basing our routing on the number of hops, then the route ABH would be chosen. For this Router H would tell Router B and Router G that it can reach Network B in one hop. Router B would then inform Router A that it could reach Network B in two hops, and so on. Thus Router A would choose Router B to go to Network B, and Network B would choose Router H rather than Router E, and so on. BB HH Network B F F Network A A A DD E E CC GG Figure 2 Example network infrastructure Most routers can implement the hop count principle to determine the best route, but this is not an efficient method, as it is too simplistic, and does not take into account other metrics. Thus routing algorithms are used to determine the best way to get to a node. One of the best known was invented in the 1950s by EW Dijstra, and is known as Dijkstra s Algorithm. For this he defined: Analyse a part of a route that goes from one routing node to another, and reject the worst routes, so that it reduces the complexity of the problem This continually reduces the complexity of the route until it can be simplified into a number of alternative routes which can be easily measured against each other. For example, let s say that we need to find the best route from Glasgow to Edinburgh. Figure 3 illustrates the routes which are defined as: Glasgow to Falkirk Glasgow to Oban Oban to Inverness Oban to Stirling Inverness to Stirling 50 miles 100 miles 90 miles 120 miles 100 miles 170 Computer Networks CO33006

5 Stirling to Falkirk Stirling to Edinburgh Stirling to Dunfermline Falkirk to Edinburgh Dunfermline to Edinburgh 5 miles 60 miles 20 miles 70 miles 15 miles Our task is to determine the shortest route. This of course may not be the best metric, as a route may be shorter, but the speed limit on the route may reduce it so much that a longer route may be quicker. Inverness Oban 120 Stirling 20 Dunfermline Falkirk Glasgow Edinburgh Routing fundamentals Figure 3 Routing around a transport network Layer 3 of the OSI model covers the network layer. There are two types of protocol at this level. These are: 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 Unit 9: Routing Protocols 171

6 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. A route can either use static or dynamic routing. These are: 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. In order to achieve dynamic routing, each router uses a metric for a route to a destination. The route with the lowest metric wins and the router sends the data packet onto the next router in the best path. There are many ways to define the best route, these include: 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. Routers can use a single metric (such as hop count with RIP) or multiple metrics, where the updates in routing information can be sent by: 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. 172 Computer Networks CO33006

7 Routing protocols suffer from several problems. The main problem with dynamic routing protocols is the amount of time that a network will take to change its routing to take into account changes in topology, whether it is due to failure, growth or reconfiguration. The knowledge that is passed between routers must be accurate and represent the true nature of any changes. This is known as convergence, and occurs when all of the routers on an internetwork have the same knowledge. An efficient network should have fast convergence, as it reduces the time that routers use outdated information, which would be used to send data packets on an incorrect route, or is sent over an inefficient route. Classless interdomain routing, route summarization and supernetting Routers can use classless interdomain routing (CIDR), where a router determines the class of an IP address and uses this to identify the network and host parts of the address. With CIDR, the router uses a bitmask to identify the network and the host parts of the address, Also the usage of this bitmask is not restricted to multiples of octets. This bitmask allows the router to summarize routing information, where an address and a mask can represent multiple networks / / / / / It can be seen from the above addresses that they have the first 12 bits in common. For a CIDR-based router the router can summarize these routes using a 12-bit prefix, with the value of: The subnet mask can then be set to , and the routes would be summarized with: /12 This method make routing more efficient, in terms of processing of the routing table, and also reduces the memory requirements. Supernetting, or route aggregation, uses a bitmask to group multiple classful networks as a single network address, and can be thought of as the inverse of subnetting, as subnetting splits networks into subnetworks, whereas supernetting builds them back up into a complete network. For example, if an organisation required 1000 hosts on a network. They could be granted a Class B address, but this would be wasteful, as a Class B address allows for Unit 9: Routing Protocols 173

8 hosts (65534). An alternative approach would be for them to be granted four contiguous Class C address, which had a common part, which could be aggregated. For example, if they were granted the address: There now 10 bits which can be used for hosts which gives (1022) hosts, which is enough for all the hosts in the organisation. This address can be aggregated as they have a common address part. The subnet mask will thus be: and the address can be aggregated with: /22 This type of approach allows ISPs to simplify their routing by allocated grouping addresses. Variable-length subnet masks Variable-length subnet masks (VLSM) allows for different subnet masks to be used within an organization. For example, using the previous example / It is possible to subnet the network within this address space to create six subnets (001, 011, ), by / / / / :::: / Computer Networks CO33006

9 Figure 2 illustrates this, where one side of the router is using on size of subnet mask, whereas the other side uses another size / / /22 Router Router / /27 Routing protocol techniques Figure 4 Example of VSLM There are three main types of routing protocols (as illustrated in Figure 3): Distance-vector. Distance-vector routing uses a distance-vector algorithm (such as the Bellman-Ford routing algorithm), which uses a direction (vector) and distance to any link in the internetwork to determine the best route. Each router periodically sends information to each of its neighbors on the cost that it takes to get to a distance node. Typically, this cost relates to the hop count (as with RIP). The main problem with distance-vector is that updates to the network are step-by-step, and it has high bandwidth requirements as each router sends its complete routing table to all of its neighbors at regular intervals. Link-state. Link-state involves each router building up the complete topology of the entire internetwork (or at least of the partition on which the router is situated), thus each router contains the same information. With this method, routers only send information to all of the other routers when there is a change in the topology of the network. Link-state is also known as shortest path first. Typical link-state protocols are OSPF, BGP and EGP. With OSPF, each router builds a hierarchical topology of the internetwork, with itself at the top of the tree. The main problem with link-state is that routers require much more processing power to update the database, and more memory as routers require to build a database with details of all the routers on the network. Hybrid. A mixture of distance-vector and link-state. Typical hybrid routing protocols are IS-IS and Enhanced IGRP. An outline of the main techniques used in routing protocols is outlined in Figure 5. Unit 9: Routing Protocols 175

10 H ybrid (IS-IS) Layer Layer 3 protocols Types Types Link-state Distance-vector Routed (IP, IPX, N etbeui) Routing (RIP, OSPF) Session Session Transport T ransport Network Network Data Data link link Physical Physical Routing HTTP HTTP TCP TCP Distance IP IP RIP RIP metrics Ethernet/ Ethernet/ FDDI FDDI Hop count Delay Updates Each router Each router transmits routing periodically sends information to information to all other routers each of its neighbors only when ther e (RIP). are changes (OSPF/BGP/EGP) Pr oblems: Bandwidth Pr oblems: Step-by-step updates Initial flooding Processing/memory Tick Bandwidth Event driven v. broadcast Cost Reliability Static.v. dynamic Figure 5 Example routing Distance-vector In the distance-vector routing protocol, each router initially identifies each of its neighbors. Next, it defines each of its ports which connect to a network as having a distance of zero. The discovery process continues by communicating with each of its neighbors. A typical distance-vector algorithm is RIP, which uses a hop count as a metric, but other algorithms can use different cost metrics (such as bandwidth or delay). An example network is shown in Figure 6. In this case, Router Z communicates with Router W and Router Y and receives their routing tables. Each router will maintain a routing table which defines the number of hops that it takes to get to a destination and the port of the router that the router should use. An example sequence may be: All routers communicate to determine their neighbors. For example, Router X knows that its neighbors are Router Y and Router W. All routers set the count to network that they connect to, at zero. Router X then tells Router Y and Router W that it takes zero hops to get to Network A. Router Y will add one hop for the destination to Network A (hop count to A is one), and Router W will add one hop for the destination to Network A (hop count to A is one). Router Y then tells Router X and Router Z that it takes zero hops to get to Network B. Router X will add one hop for the destination to Network B (hop count to B is one), and Router Z will add one hop for the destination to Network B (hop count to B is one). Router Z then tells Router W and Router Y that it takes zero hops to get to Network C. Router W will add one hop for the destination to Network C (hop count to C is one), and Router Y will add one hop for the destination to Network C (hop count to C is one). Router X sends its new updated routing table to W and Y, and informs them that it takes one hop to get to Network B. Router Y will not change its entry for Network B 176 Computer Networks CO33006

11 as it currently has a hop count of zero, but Router W will update its routing table for Network B to two hops. and so on. After the convergence time, the routers will then build up the tables shown in Figure 6. If a router gets information that it can reach the route from more than one path, it will take the lowest hop count for its table entry. In the example in Figure 4, there are two equal paths to get to Network B from Router W, each with a hop count of two. The router can thus use any one of these. It should be noted that distance-vector is inefficient, as each router will send out its entire routing table at regular intervals. This is inefficient when there are no updates in the topology of the network. Updates also take some time, as they must be passed from router-to-router. Dest Hops Next Dest Hops Next A 1 B 2 C 1 x z z A 0 B 1 C 2 Network A y y W 2 X Network A 1 3 Dest Hops A 2 B 1 C 0 Next w y Network C Z 4 Y Dest Hops A 1 B 0 C 1 Next x Network B z Network C Problems with distance-vector routing protocols Network B Figure 6 Example routing The major problems with distance-vector routing protocols (such as RIP) are: Routing Loops. These occur when slow convergence causes inconsistent routing entities when a new configuration occurs (Figure 7). In this case, Network A becomes unavailable. Router V will report this to Router Y, which will then report to Router Z and Router X. Both Routers X and Z will stop routing to Network A. Unfortunately Router W still thinks it can reach Network A with 3 hops, thus Router Z will receive information that says that Router W can get to Network A in 3 hops, and that it is unreachable from Router Y. Thus Router Z updates its routing table so that Network A is reachable in 4 hops, and that the next router to the destination is Router W. Router Z will then send its updated information to Router Y which informs it that there is a path to Network A from Router Z to Router W, and so on. Router Y will then inform Router X, and so on. Thus, any data packet which is destined for Network A will now loop around the loop from Router Z to Router W to Router X to Router Y to Router Z, and so on. Counting to Infinity. As has been seen in Figure 7, data packets can loop around forever, because of incorrect routing information. In this loop, the distance-vector of the hop count will increment each time the packet goes through a router. Unit 9: Routing Protocols 177

12 Timing of events A B E. E. Net Net wor wor k k A A reachable reachable C D E B. B. I I can can reach reach Network Network A A in in 3 3 hops hops W Z X Y A. A. Network Network A A unreachable unreachable D. D. Network Network A A reachable reachable Router Z thinks it can A. A. Network Network A A unreachable reach Network A in 4 hops, unreachable as Router W says it can reach it in 3 hops, this overrules the information from Router Y which says it cannot C. C. Network Network A A Reachable reach Network A Reachable via via Router Router W V Network unreachable Network A Figure 7 Routing loops There are several solutions to the count-to-infinity and routing loop problems. These are: 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 7, 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. 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: If, 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. If 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. If, 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. 178 Computer Networks CO33006

13 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. Link-state concepts The link-state algorithm (known as shortest path first) maintains a complex database on the topology of a network. Each router thus has the complete picture of the whole of the network and has knowledge of distant routers, and how they interconnect. The distancevector, on the other hand, has nonspecific information on distant networks, and no knowledge of distant routers. The link-state algorithm uses link-state advertisements (LSAs) to advertise routing information from routers. From this each router build-up a topological database with themselves at the top of the tree, and uses a shortest path first (SPF) algorithm to determine the best route to get to a destination. A typical implementation is Open Shortest Path First (OSPF), which is defined in RFC1583, and uses Dijkstra s algorithm to determine the best path. An outline of it is given in Figure 8. Initially, on start-up, each router must discover the routes which it connects to. Next, each router advertises its connection to all the other routers on the network. From this, it builds up a topology database with the information. There may be multiple routes to get to a destination, thus the SPF algorithm is used to determine the best route. It is this route that is then used in the topological database. All the routers on the network will thus have the same information. The distance-vector approach uses constant updates of routing tables between neighbors, whereas the link-state approach only sends routing information when there is a change in the topology of the network. This information floods to all the routers on the network (the discovery process), thus all routers are updated with the new information. In order for the link-state algorithm to work, a router must keep communicating with its neighbors to determine if they are still responding to communications, and if they have any changes in their link metrics. Each router only uses the most up-to-date information from LSA packets to build up the topological database. The main problem with link-state, as apposed to distance-vector, is that the algorithm requires much more processing power, and also increased amount of memory to store up-to-date LSAs and the complete topological database. With Dijkstra s algorithm the processing task is proportional to the number of links in the internetwork multiplied by the number of routers in the internetwork. Link-state routing is much more efficient in bandwidth, though, as the routing information is only passed when there is a change in the topology of the internetwork. Initially, though, there is a high requirement for bandwidth, as each of the routers must flood routing information through the internetwork. After the internetwork has converged, there is a reduced requirement for bandwidth, as apposed to the distancevector method which has a constant demand on bandwidth. Unit 9: Routing Protocols 179

14 LSP (Link state packets) T opological database (for SPF) Methods Problem Link-state OSPF OSPF (RFC1583) (RFC1583) Ver. Ver. Type Type Message Message Len. Len. Router Router ID ID Area Area ID ID Checksum Checksum Auth. Auth. Type Type Concerns Authentication Authentication Processing Memory Increased processing power required to build trees Incr eased amount of storage memory for tree Network 1 becomes unreachable for a short time LSP:Network LSP:Network LSP:Network LSP:Network Reachable Reachable Unreachable Unreachable W X 2 Network unreachable 1 3 arrives after 4 network Z Y reachable LSP:Network LSP:Network Unreachable Unreachable Operation LSP A change in topology causes updates to all other routers Each rout er builds up a tree topology of the subnetworks and find shortest path The main problems with link-state are: Figure 8 Link-state overview Link-state updates. This problem is illustrated in Figure 8. In this case, Network link 1 becomes unavailable for a short time. Thus Router W and Router Z transmit the information that this link is unavailable to the rest of the network. In this case the information will be received by Router X and Router Y. If the network then becomes available from Router W, Router W will send out Network Reachable advertisement. If Router X receives this before it receives the Network Unreachable then Router X thinks that the network is still unavailable, even though it is available via Router W. This problem can cause whole sections of a network to become unavailable. Scaling. A problem with link-state occurs when scaling-up large internetworks when one network comes up before other parts of the network. This causes a timing problem where differing reachability information is sent between routers, thus routers might learn about different versions of the topology before they construct their SPF trees and routing tables. On a large internetwork, parts that update more quickly can cause problems for parts that update more slowly. Hybrid routing An important third classification of routing algorithm is hybrid routing, which is a combination of both distance-vector and link-state routing, and is named balancedhybrid routing. These routing protocols use distance-vectors for more accurate metrics to determine the best paths to destination networks, and report routing information only when there is a change in the topology of the network. The event-driven nature of hybrid routing allows for rapid convergence (as with link-state protocols), but it requires much less processing power and memory than a link-state protocol would require. Examples of hybrid protocols are Intermediate System-to-Intermediate System (IS-IS) routing and Enhanced Interior Gateway Routing Protocol (EIGRP). The IS-IS routing protocol has 180 Computer Networks CO33006

15 been defined OSI link-state hierarchical routing protocol which is based on DECnet Phase V routing whereby ISs (routers) exchange routing information based on a single metric to determine network topology. RIP Most routers support RIP and EGP. In the past, RIP was the most popular router protocol standard. Its widespread use is due, in no small part, to the fact that it was distributed along with the Berkeley Software Distribution (BSD) of UNIX (from which most commercial versions of UNIX are derived), and was originally defined in RFC Unfortunately, it suffers from several disadvantages and has been largely replaced by OSFP and EGB, which have the advantage over RIP in that they can handle large internetworks, as well as reducing routing table update traffic. By default, in RIP, each router transmits its complete routing table to their neighbor once every 30 seconds (although this time is configurable in most routers). RIP uses a distance-vector algorithm which measures the number of network jumps (known as hops), up to a maximum of 16, to the destination router. This has the disadvantage that the smallest number of hops may not be the best route from source to destination. The OSPF and EGB protocols use a link state algorithm that can decide between multiple paths to the destination router, which are based, not only on hops, but also on other parameters such as delay, capacity, reliability and throughput. With distance-vector routing each router maintains tables by communicating with neighboring routers. The number of hops in its own table is then computed, as it knows the number of hops to local routers. Unfortunately, the routing table can take some time to be updated when changes occur, because it takes time for all the routers to communicate with each other (known as slow convergence). RIP packets, which use TCP port 520, generally add to the general network traffic as each router broadcasts its entire routing table every seconds. Figure 9 outlines the RIP packet format. The fields are: Operation (2 bytes) this field gives an indication that the RIP packet is either a request or a response. The first 8 bits of the field give the command/request name and the next 8 bits give the version number. Network number (4 bytes of IP addresses) this field defines the assigned network address number to which the routing information applies (note that, although 4 bytes are shown, there are in fact 14 bytes reserved for the address. In RIP version 1 (RIPv1), with IP traffic, 10 of the bytes were unused; RIPv2 uses the 14-byte address field for other purposes, such as subnet masks. Number of router hops (2 bytes) this field indicates the number of routers that a packet must go through in order to reach the required destination. Each router adds a single hop, the minimum number is 1 and the maximum is 16. The maximum number of hops to a destination is thus limited to 15. Number of ticks (2 bytes) this field indicates the amount of time (in 1/18 second) it will take for a packet to reach a given destination. Note that a route which has the fewest hops may not necessarily be the fastest route. Unit 9: Routing Protocols 181

16 2 Operation 4 Network number 2 Number of router hops 2 Number of ticks 4 Network number 2 Number of router hops 2 Number of ticks Routing table entry 1 Routing table entry 2 Figure 9 RIP packet format OSPF The OSPF (Open Shortest Path First) is an open, non-proprietary standard which was created by the IETF (Internet Engineering Task Force), a task force of the IAB. It is a link state routing protocol and is thus able to maintain a complete and more current view of the total internetwork, than distance-vector routing protocols. Link state routing protocols have these features: They use link state packets (LSPs) which are special datagrams that determine the names of and the cost or distance to any neighboring routers and associated networks. Any information learned about the network is then passed to all known routers, and not just neighboring routers, using LSPs. Thus all routers have a fuller knowledge of the entire internetwork than the view of only the immediate neighbors (as with distance-vector routing). OSPF adds to these features with: Additional hierarchy. OSPF allows the global network to be split into areas. Thus, a router in a domain does not necessarily have to know how to reach all the networks with a domain, it simply has to send to the right area. Authentication of routing messages using an 8-byte password. This length is not long enough to stop unauthorized users from causing damage. Its main purpose is to reduce the traffic from misconfigured routers. Typically, a misconfigured router will inform the network that it can reach all nodes with no overhead. Load balancing. OSPF allows multiple routes to the same place to be assigned the same cost and will cause traffic to be distributed evenly over those routes. Figure 8 shows the OSPF header. The fields in the header are: A version number (1 byte) which, in current implementations, has the version number of 2. The type field (1 byte) which can range from 1 to 5. Type 1 is the Hello message and the others are to request, send and acknowledge the receipt of link state messages. Nodes, to convince their neighbors that they are alive and reachable, use hello messages. If a router fails to receive these messages from one of its neighbors for a period of time, it assumes that the node is no longer directly reachable and updates its link state information accordingly. Router ID (4 bytes) identifies the sender of the message. Area ID (4 bytes) is an identifier to the area in which the node is located. 182 Computer Networks CO33006

17 Authentication field can either be set to 0 (none) or 1. If it is set to 1 then the authentication contains an 8-byte password. The Hello packet is used to establish and maintain a connection. It is used to determine the routers that are connected to the current router. The connected routers then agree on HelloInterval and RouterDeadInterval values. The HelloIntervalue defines the number of seconds between Hello packets. The smaller the value, the faster the detection of topological changes. For example, X.25 typically uses 30sec and LANs use 10sec. The RouterDeadInterval defines the number of seconds before a router assumes that a route is down. It should be a multiple of HelloInterval (such as four times). OSPF header OSPF OSPF (RFC1583) (RFC1583) Ver. Ver. Type Type Message Message Len. Len. Router Router ID ID (unique (unique in in AS) AS) Area Area ID ID (similar (similar to to subnetting) subnetting) Checksum Checksum Auth. Auth. Type Type Authentication Authentication Additional Information (depends on packet type) 32 bits Gateways OSPFis an IGP (Interior Gateway Protocol) which distributes routing information between r outer s in a single autonomous system. All r outer s have the same database. Hello [1]. Used to establish and maintain a connection. Routers agree HelloInterval and RouterDeadInterval. HelloInterval. N umber of seconds between Hello packets. The smaller the value, the fastest the detection of topological changes. X.25 uses 30 sec, LANs uses 10 sec. RouterDeadInterval. Number of seconds before a router assumes that a route is down. It should be a multiple of HelloInterval (such as four times). Database Description [2]. Used to send database between routers. Link-state Request [3]. Request parts of a neighbor 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. Separ at e domains Aut onomous Aut onomous System System Aut Aut onomous Aut onomous onomous Aut onomous System System System System Internet EGP used between AS s Figure 10 OSPF overview When a router thinks that it does not have the correct information on a part of a route it sends Link-state Request, which request parts of a neighbor s database. The requested neighbor then sends back a Database Description which describes the requested part of its database. When a router detects a change in its connections it sends a Link-state Update message, which is then flooded to all the routers on the internetwork. Routers return back a Link-state Acknowledgement to acknowledge the flooded advertisements. It would of course be impossible for every router in the world to know about every other router and every link, thus each internetwork is segmented into Automomous Systems (ASs), which are bounded by a gateway. In these each router knows the complete topology of the AS. An interior routing protocol (such as OSPF) is used to transmit routing information within the AS, and an exterior routing protocol (such as EGP) is used to route between ASs, as illustrated in Figure 8. The Network Information Center (NIC) assigns a unique 16-bit number to enterprises for ASs. The usage of ASs provides for a similar architecture for the Internet, where data packets are routed from one domain into another. For example, all the universities in France could define one domain. Anyone communicating with them will be routed in a Unit 9: Routing Protocols 183

18 defined domain through a designated gateway. The routers within the domain would then have a complete picture of all the internetworks within the domain. ASs also help to hide the architecture of the interior network from other routers outside the AS. IGRP IGRP (Interior Gateway Routing Protocol) is a distance-vector routing protocol (as RIP), which is used as an interior routing within an AS. Like RIP, it transmits routing information at regular intervals, but unlike RIP, it uses a much better measure of the metric, these include: Bandwidth. Delay. Load. Reliability. Maximum transmission unit (MTU), which defines the maximum data packet size that an interface can handle. It advertises routing information every 90 seconds. The key points of IGRP are: Handles complex networks, as the metric can define problems (rather than the basic hop count used in RIP). It allows for more efficient routing, as the network can cope with different delays and bandwidths. Scaleable for very large networks. EGP/BGP The two main interdomain routing protocols in recent history are EGP and BGP. EGP suffers from several limitations, and its principal one is that it treats the Internet as a treelike structure, as illustrated in Figure 11. This assumes that the structure of the Internet is made up of parents and children, with a single backbone. A more typical topology for the Internet is illustrated in Figure 12. BGP is now one of the most widely accepted exterior routing protocol, and has largely replaced EGP. BGP is an improvement on EGP (the fourth version of BGP is known as BGP-4), and is defined in RFC1772. Unfortunately it is more complex than EGP, but not as complex as OSPF. BGP assumes that the Internet is made up of an arbitrarily interconnected set of nodes. It then assumes the Internet connects to a number of AANs (autonomously attached networks), as illustrated in Figure 13, which create boundaries around organizations, Internet service providers, and so on. It then assumes that, once they are in the AAN, the packets will be properly routed. 184 Computer Networks CO33006

19 Single backbone Org1 Org2 Site1 Site2 Site3 Site1 Site2 Site3 LAN1 LAN2 LAN3 LAN1 LAN2 LAN3 Figure 11 Tree-like topology Org 3 Org1 Org2 Site1 Site2 Site3 Site1 Site2 Site3 LAN1 LAN2 LAN3 LAN1 LAN2 LAN3 Figure 12 Network with multiple backbones Autonomously Autonomously attached attached network network (AAN (AAN ) ) G/W Gateway (G/W) G/W AAN AAN G/W G/W G/W AAN AAN G/W AAN AAN Figure 13 Autonomously attached networks Most routing algorithms try to find the quickest way through the network, whereas BGP tries to find any path through the network. Thus, the main goal is reachability instead of the number of hops to the destination. So finding a path which is nearly Unit 9: Routing Protocols 185

20 optimal is a good achievement. The AAN administrator selects at least one node to be a BGP speaker and also one or more border gateways. These gateways simply route the packet into and out of the AAN. The border gateways are the routers through which packets reach the AAN. The speaker on the AAN broadcasts its reachability information to all the networks within its AAN. This information states only whether a destination AAN can be reached; it does not describe any other metrics. An important point is that BGP is not a distancevector or link state protocol because it transmits complete routing information instead of partial information. The BGP update packet also contains information on routes which cannot be reached (withdrawn routes), and the content of the BGP-4 update packet is: Unfeasible routes length (2 bytes). Withdrawn routes (variable length). Total path attribute length (2 bytes). Path attributes (variable length). Network layer reachability information (variable length). This can contain extra information, such as use AAN 1 in preference to AAN 2. Routers within AS s share similar routing policies, and thus operate as a single administrative unit. All the routers outside the AS treat the AS as a single unit. The AS identification number is assigned by the Internet Assigned Numbers Authority (IANA) in the range of 1 to 65,535, where 64,512 to 65,535 are reserved for private use. The private numbers are only used within private domain, and must be translated to registered numbers when leaving the domain. 186 Computer Networks CO33006

21 Activity 9.1: Test The end of unit test contains questions on the material in this unit. Unit 9: Routing Protocols 187