1. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.1 Computer Networks and Internets with Internet Applications, 4e By Douglas E. Comer Lecture PowerPoints By Lami Kaya, LKaya@ieee.org

2. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.2 Chapter 27 Internet Routing

3. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.3 Topics Covered 27.1 Introduction 27.2 Static Vs. Dynamic Routing 27.3 Static Routing In Hosts And A Default Route 27.4 Dynamic Routing And Routers 27.5 Routing In The Global Internet 27.6 Autonomous System Concept 27.7 The Two Types Of Internet Routing Protocols –27.7.1 Interior Gateway Protocols (IGPs) –27.7.2 Exterior Gateway Protocols (EGPs) –27.7.3 When EGPs And IGPs Are Used –27.7.4 Optimal Routes, Routing Metrics, and IGPs

4. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.4 Topics Covered (cont) 27.8 Routes And Data Traffic 27.9 The Border Gateway Protocol (BGP) 27.10 The Routing Information Protocol (RIP) 27.11 RIP Packet Format 27.12 The Open Shortest Path First Protocol (OSPF) 27.13 An Example OSPF Graph 27.14 OSPF Areas 27.15 Multicast Routing –27.15.1 IP Multicast Semantics –27.15.2 IGMP –27.15.3 Forwarding And Discovery Techniques –27.15.4 Multicast Protocols

5. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.5 27.1 Introduction This chapter explores important aspects of today’s internet: Discusses how routing tables are built initially Explains how routing SW updates the tables Focuses on the propagation of RI Explains the general concept of RI exchange Describes several routing update protocols used in the Internet Introduces multicasting and multicasting techniques

6. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.6 27.2 Static Vs. Dynamic Routing IP routing can be partitioned into two broad categories: Static routing (SR) –A SR table is loaded with values when the system starts –and the routes do not change unless an error is detected Dynamic routing (DR) –Can not change routing table (RT) information over time –DR begins exactly like SR by loading an initial set of routes into a RT when the system boots and starts operation Routing SW on computers interacts with each other –to learn about optimal routes to each location The SW then updates the local RT –to ensure that datagrams follow optimal routes

7. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.7 27.3 Static Routing In Hosts And A Default Route (1) SR is straightforward –easy to specify, and does not require extra routing SW It does not consume BW –No CPU cycles are required to propagate RI However, static routing is relatively inflexible –it cannot accommodate NW failures or changes in topology Where is static routing used? Most hosts use SR –especially in cases where the host has one NW connection and a single router connects the NW to the rest of the Internet –Ex: consider the architecture in Figure 27.1

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9. 9 27.3 Static Routing In Hosts And A Default Route (2) As the figure shows, a SR table can be extremely small –In the example, two entries suffice –one for the directly connected NW –the other for a default route that is followed for all other destinations When a datagram is for a host on the local net –first entry in the RT directs IP to deliver it directly to its destination When a datagram is destined for any other NW –second entry in the table directs IP to send the datagram to the router

10. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.10 27.3 Static Routing In Hosts And A Default Route (3) Is static routing used in the Internet? Yes, most PCs use SR –When configuring IP SW on a PC, a user enters a NW prefix, a subnet mask, and the address of a default IP router –When a PC boots, the OS reads the three values from a configuration file and uses them to construct a RT

11. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.11 27.4 Dynamic Routing And Routers (1) Can a router use SR the same way a host does? –If so, how large is the RT in a router? The answers to these questions are complex: –Although cases exist where a router uses SR, most use DR To understand an exceptional case, look at Figure 27.1 –Imagine that the figure corresponds to a small organization that is a customer of an ISP –All traffic leaving the customer's site through R 1 must travel to the ISP (e.g., across a DSL connection) –Because routes never change, the RT in R 1 can be static –RT in R 1 can use a default route just as the RT in a host does

12. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.12 27.4 Dynamic Routing And Routers (2) SR and default routes do not suffice for most routers: –Consider the case given in Figure 27.2 Each of the two routers belong to a separate ISP How can a router in one ISP have routes to NWs owned by customers of another ISP? In Figure 27.2 only three NW, SR seems to make sense Because SR requires manual installation of routes –scheme does not scale to ISPs with large number of customers –Each time an ISP adds a customer the information must be passed to the other ISP, which then updates its RT –Manual process is far too slow to accommodate NW failures or congestion

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14. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.14 27.4 Dynamic Routing And Routers (3) To ensure that all routers maintain information about how to reach each possible destination each router uses a route propagation protocol –to exchange information with other routers when it learns about changes in routes –updates the local RT Because routers exchange information periodically –the local RT is updated continuously

15. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.15 27.5 Routing In The Global Internet (1) To limit routing traffic, the Internet uses a two-level routing hierarchy –Routers and NW in the Internet are divided into groups –All routers within a group exchange RI –Then, at least one router (possibly more) in each group summarizes the RI before passing it to other groups How large is a group? What protocol do routers use within a group? How is routing information represented? What protocol do routers use between groups?

16. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.16 27.5 Routing In The Global Internet (2) Internet routing system –did not dictate an exact size –nor did they specify an exact data representation or protocol The architecture flexible to handle a wide variety of organizations –To accommodate organizations of various size, Avoided specifying a minimum or maximum size for a group. –To accommodate arbitrary routing protocols, Decided to permit each organization to choose their routing protocol

17. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.17 27.6 Autonomous System Concept (1) A contiguous set of NWs and routers all under the control of one “administrative authority” –called “autonomous system” (AS) There is no exact meaning for administrative authority The term is flexible to accommodate many possibilities –An AS can correspond to an entire corporation or to a university –A large organization with multiple sites may choose to define one AS for each site –Each ISP is usually a single AS but it is possible for a large ISP to divide itself into multiple AS

18. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.18 27.6 Autonomous System Concept (2) The choice of AS size can be made for different reasons: –Economic –Technical –Administrative Consider a large corporation with multiple physical sites: It may be less expensive to divide into multiple AS –each of which has a connection to an ISP than to act as a single AS with one connection to the rest of the Internet Selection of a routing protocol may determine whether an organization chooses to use multiple AS –the protocol may bound the maximum size of the NW –or may generate excessive routing traffic when used on a large number of routers

19. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.19 27.7 The Two Types Of Internet Routing Protocols After AS, Internet routing can be defined more precisely All Internet routing protocols fall into one of two categories –Interior Gateway Protocol (IGP) –Exterior Gateway Protocol (EGP) After defining the two categories –we examine specific protocols in each category Categories of Routing Algorithms, based on computation –Distance Vector Routing (DVR) –Link State Routing (LSR)

20. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.20 27.7.1 Interior Gateway Protocols (IGPs) The routers within an AS use an Interior Gateway Protocol (IGP) to exchange RI There are several IGP protocols available; –each AS is free to choose its own IGP Usually, an IGP is easy to install and operate –but an IGP may limit the size or routing complexity of an AS

21. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.21 27.7.2 Exterior Gateway Protocols (EGPs) A router in one AS uses an Exterior Gateway Protocol (EGP) to exchange RI with a router in another AS EGPs are usually more complex to manage than IGPs –but EGPs offer more flexibility and lower overhead (less traffic) To save traffic –EGP summarizes RI from the AS before passing it to another AS An EGP implements policy constraints –that allow a system manager to determine exactly what information is released outside the organization

22. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.22 27.7.3 When EGPs And IGPs Are Used Figure 27.3 illustrates a two-level routing hierarchy AS 1 has chosen IGP 1 to use internally and AS 2 has chosen IGP 2 R 1 and R 4 use an EGP to between the two AS –R 1 must summarize information from its AS and send the summary to R 4 In addition, R 1 accepts a summary from R 4, and uses IGP 1 to propagate the information to routers in AS 1 R 4 performs the same service for AS 2

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24. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.24 27.7.4 Optimal Routes, Routing Metrics, and IGPs (1) Although the Internet usually does have multiple paths between any source and destination There is no universal agreement about which path is optimal Why? consider the requirements of various applications –For an interactive login application a path with least delay is optimal –For a browser downloading a large graphics file a path with maximum throughput is optimal –For an audio webcast application that receives real-time audio, a path with least jitter is optimal

25. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.25 27.7.4 Optimal Routes, Routing Metrics, and IGPs (2) We use the term routing metric (RM) to refer to a measure of the path that routing SW uses when choosing a route Although it is possible to use –throughput, delay, or jitter as a RM, most Internet routing SW does not Instead, typical Internet routing uses a combination of two metrics –Administrative cost –Hop count A hop corresponds to an intermediate NW (or router) –the hop count for a destination gives the number of intermediate NW on the path to the destination Administrative costs are assigned manually, often to control which paths traffic can use

26. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.26 27.7.4 Optimal Routes, Routing Metrics, and IGPs (3) IGPs and EGPs differ in an important way with respect to RM –IGPs use RMs, but EGPs do not –Each AS chooses a RM –and arranges internal routing SW to send the metric with each route –so receiving SW can use the metric to choose optimal paths Outside an AS, an EGP does not attempt to choose an optimal path –Instead, the EGP merely finds a path Because each AS is free to choose a RM, an EGP can’t make comparisons Ex: suppose one AS reports the number of hops along a path to destination D and another AS reports the throughput along a different path to D An EGP that receives the two reports cannot choose which of the two paths has less cost –because there is no way to convert from hops to throughput –Thus, an EGP can only report the existence of a path and not its cost

27. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.27 27.8 Routes And Data Traffic An aphorism in NW suggests that the response to a routing advertisement is data –data traffic for a given destination flows in exactly the opposite direction of routing traffic Ex: suppose an AS owned by ISP contains NW N Before traffic can arrive destined for N –ISP must advertise a route to N –When the routing advertisement flows out, data will begin to flow –Figure 27.4 illustrates the flow

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29. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.29 27.9 Border Gateway Protocol (BGP) (1) Most popular EGP is Border Gateway Protocol (BGP) –the protocol has survived three major revisions –Version 4 is the current standard, and the abbreviation (BGP-4) BGP has the following characteristics: Routing Among AS Provision For Policies –BGP allows the sender/receiver to enforce policies: –We can configure BGP to restrict which routes BGP advertises to outsiders

30. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.30 27.9 Border Gateway Protocol (BGP) (2) BGP characteristics (cont): Facilities For Transit Routing –classifies each AS as a transit system if it agrees to pass traffic through, or as a stub system if it does not –BGP allows a corporation to classify itself as a stub even if it is multi-homed (refuse to accept transit traffic) Reliable Transport (uses TCP)

31. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.31 27.9 Border Gateway Protocol (BGP) (3) BGP is especially important in the global Internet –because major ISPs use BGP to exchange RI –Especially, ISPs that interconnect at NW Access Points use BGP To ensure that a datagram will be forwarded correctly –global RI must be consistent To ensure that routing remains consistent –several organizations attempt to maintain a database of all routes –Ex: an organization named Reseaux IP Europeens (RIPE) maintains a “routing registry” that contains a list of destinations and ISPs that owns them

32. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.32 27.10 The Routing Information Protocol (RIP) (1) RIP was used as an IGP. It’s characteristics Routing Within An AS Hop Count Metric –Uses origin-one counting; a connected NW is 1 hop away, not 0 Unreliable Transport (uses UDP) Broadcast Or Multicast Delivery –Intended for use over LAN that support broadcast or multicast Support For Default Route Propagation –A default routes all forward traffic to the organization's ISP Distance Vector Algorithm Passive Version For Hosts –allows a host to listen passively and update its routing table –Passive RIP is useful on NWs that have multiple routers

33. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.33 27.10 The Routing Information Protocol (RIP) (2) How RIP Works? Each outgoing RIP message contains an advertisement An advertisement consists of ( destination NW, distance ) Adjacent routers receive the RIP messages –and update their RTs: When a message arrives –if the receiver does not have a route to an advertised destination –or if an advertised distance is shorter than the distance of the current route the receiver replaces its route with a route to the sender

34. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.34 27.10 The Routing Information Protocol (RIP) (3) The chief advantage of RIP is simplicity –RIP requires little configuration –A manager starts RIP running on each router in the organization and allows the routers to broadcast messages to one another –After a short time, all routers will have routes to all destinations RIP also handles the propagation of a default route –An organization needs to configure one of its routers as a default –An organization usually chooses a router that connects to an ISP RIP propagates the default route to all routers –Any datagram sent to a destination outside the organization will be forwarded to the ISP

35. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.35 27.11 RIP Packet Format The RIP message format will help illustrate how a distance vector protocol operates. Figure 27.5 illustrates a RIP update message Although an update message consists of a list of entries –a few details complicate the format In addition, to permit RIP to be used with CIDR or subnet addressing –an entry contains a 32-bit subnet mask Each entry has a next hop address, and two 16-bit fields –that identify the entry as an IP address –and provide a tag used to group entries together

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37. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.37 27.12 The Open Shortest Path First Protocol (OSPF) (1) RIP illustrates some of the disadvantages of a DVR Because each message contains a complete list of destinations and distances, messages will be large Receiving router compares each entry in the incoming message to the current route for the destination –processing a message introduces delay Therefore, route changes propagate slowly, one router at a time –thus, although RIP works well among a few routers –it does not scale to a large internet, for large organizations –the Internet Engineering Task Force (IETF) devised OSPF

38. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.38 27.12 The Open Shortest Path First Protocol (OSPF) (2) Each router must periodically probe (check) adjacent routers –and then broadcast a link-status message All routers receive the broadcast message –each uses the message to update its local copy of the graph –and recomputes shortest paths when the status changes –Use Dijkstra's SPF algorithm to compute shortest paths

39. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.39 27.12 The Open Shortest Path First Protocol (OSPF) (3) Routing Within An Autonomous System Full CIDR And Subnet Support –OSPF includes a 32-bit address mask with each address, which allows the address to be classful, classless, or subnetted Authenticated Message Exchange –A pair of routers using OSPF can authenticate each message to ensure that messages are only accepted from a trusted source Imported Routes –Allows a router to introduce routes learned from another means (e.g., from BGP)

40. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.40 27.12 The Open Shortest Path First Protocol (OSPF) (4) Link-State Algorithm Support For Metrics –Allows an administrator to assign a cost to each route Support For Multi-access NWs –Traditional LSR is inefficient across a multi-access NW such as an Ethernet because all routers attached to the NW broadcast link status –OSPF optimizes by designating a single router to broadcast on the NW

41. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.41 27.13 An Example OSPF Graph Although OSPF allows a complex relationship between NWs and a graph –a simple example will help explain the basic concept In practice, OSPF graphs are much more complex than shown, you may check RFCs Ex: consider the NW and associated graph illustrated in Figure 27.6

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43. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.43 27.14 OSPF Areas (1) One particular feature that makes OSPF more complex than other routing protocols also makes it more powerful: –hierarchical routing OSPF allows an AS to be partitioned for routing purposes –We can divide routers and NWs in an AS into subsets –called “areas” Each router is configured to know the area boundary –i.e., exactly which other routers are in its area Routers within a given area exchange link-status messages periodically

44. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.44 27.14 OSPF Areas (2) In addition to exchanging information within an area OSPF allows communication between areas –One router in each area is configured to communicate with a router in one or more other area(s) –The two routers summarize routing information they have learned from other routers within their respective area and then exchange the summary –Thus, instead of broadcasting to all routers in the AS, OSPF limits link-status broadcasts to routers within an area As a result of the hierarchy –OSPF can scale to handle much larger internets than other routing protocols

45. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.45 27.15 Multicast Routing Multicasting in the global Internet is discussed What is multicasting Why do we need multicasting? Benefits What are multicasting strategies?

46. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.46 27.15.1 IP Multicast Semantics (1) One of the goals for unicast route propagation is stability –continual changes in routes are undesirable because they lead to higher jitter and datagrams arriving out of order –once a unicast routing protocol finds a shortest path, it usually retains it until a failure makes the path unusable Propagating multicast routing information differs –The difference arises because Internet multicast allows “dynamic group membership” and anonymous senders –Dynamic group membership means that an application can choose to participate in a group at any time and remain a participant for an arbitrary duration

47. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.47 27.15.1 IP Multicast Semantics (2) IP multicast allows an application running on a computer to join / leave a group Join a multicast group at any time by informing a nearby router Leave a multicast group at any time –Periodically sends membership messages to the local router –Once the last application on the computer leaves the group the computer informs the local router that it is no longer participating

48. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.48 27.15.1 IP Multicast Semantics (3) An IP multicast group is anonymous in two ways: First, neither a sender nor a receiver knows (or can find out) the identity or the number of group members Second, routers and hosts do not know which applications will send a datagram to a group –because an arbitrary application can send a datagram to any multicast group at any time That is, membership in a multicast group only defines a set of receivers –A sender does not need to join a multicast group before sending a message to the group

49. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.49 27.15.2 IGMP How does a host join or leave a multicast group? A protocol exists that allows a host to inform a nearby router –whenever the host needs to join or leave a particular multicast group –known as the Internet Group Multicast Protocol (IGMP) The protocol is used only on the NW between a host and a router Furthermore, the protocol defines the computer –not the application, to be a group member If multiple applications on a given computer join a multicast group, the computer must make copies of each datagram Only when the last application on the computer leaves a group does the computer use IGMP –to inform the local router that it is no longer a member of the group

50. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.50 27.15.3 Forwarding And Discovery Techniques (1) When a router learns that a host on one of its NWs has joined a multicast group –the router must establish a path to that group and propagate datagrams –routers, not hosts, propagates multicast routing information Dynamic group membership and support for anonymous senders makes general-purpose multicast routing extremely difficult Size and topology of groups vary considerably among applications: –Teleconferencing often creates small groups –An application, such as webcasting, can create a large group To accommodate dynamic membership –multicast routing protocols must be able to change routing quickly and continually

51. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.51 27.15.3 Forwarding And Discovery Techniques (2) Multicast routing SW first find other members of the group –and then create an optimal forwarding structure. More important, because an arbitrary user can send a datagram to the group –routing must extend beyond group members. In practice, multicast protocols have followed three different approaches for datagram forwarding: – Flood-And-Prune – Configuration-And-Tunneling – Core-Based Discovery

52. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.52 Multicasting Forwarding: Flood-And-Prune Flood-and-prune is ideal in a situation where the group is small and all members are attached to contiguous LAN Initially, routers forward each datagram to all NWs –That is, when a multicast datagram arrives, a router transmits the datagram on all directly attached LANs via HW multicast To avoid routing loops, flood-and-prune protocols use a technique –known as “Reverse Path Broadcasting” (RPB) that breaks cycles While the flooding stage proceeds –routers exchange information about group membership If a router learns that no computers on a given NW are members of the group –the router stops forwarding multicast to that NW –(i.e. ``prunes'' the NW from the set)

53. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.53 Multicasting Forwarding: Configuration-And-Tunneling Configuration-and-tunneling is ideal in a situation where the group is geographically dispersed –(has a few members at each site, separated by long distances) A router at each site is configured to know about other sites When a multicast datagram arrives, –the router at a site transmits the datagram on all directly attached LANs via HW multicast –the router then consults its configuration table to determine which other sites should receive a copy –the router uses “IP-in-IP tunneling” to transfer a copy of the multicast datagram to other sites

54. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.54 Multicasting Forwarding: Core-Based Discovery A technique is needed for multicast to scale gracefully from a small group in one area to a large group with members at arbitrary locations –some multicast routing protocols designate a core unicast address for each multicast group Whenever a router R 1 needs to reach a group, –R 1 sends a datagram to the group's core address As the datagram travels through the Internet –each router examines the contents When the datagram reaches a router R 2 that participates –R 2 removes and processes the message If the message contains a multicast datagram with a destination address equal to the group's address –R 2 forwards the datagram to members of the group If the message contains a request to join the group –R 2 adds the information to its routes, –and uses “IP-in-IP” to forward a copy of each multicast datagram to R 1

55. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.55 27.15.4 Multicast Protocols Many multicast routing protocols have been proposed –Distance Vector Multicast Routing Protocol (DVMRP) –Core Based Trees (CBT) –Protocol Independent Multicast - Sparse Mode (PIM-SM) –Protocol Independent Multicast - Dense Mode (PIM-DM) –Multicast extensions to the Open Shortest Path First protocol (MOSPF)