Computer Networks and Internets 5 e By Douglas

Chapter 27 Internet Routing and Routing Protocols © 2009 Pearson Education Inc. , Upper

• • • • 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. 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 Intermediate System - Intermediate System (IS-IS) 27. 16 Multicast Routing © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 3

27. 1 Introduction • This chapter – explores an important aspect of internetworking technology: • the propagation of routing information that is used to create and update forwarding tables – – – discusses how forwarding tables are built explains how routing software updates the tables, as needed focuses on the propagation of routing information in the Internet describes several routing update protocols that are used explains the distinction between interior and exterior routing protocols © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 4

27. 2 Static Vs. Dynamic Routing • IP routing can be partitioned into two broad categories: – Static routing – Dynamic routing • Static routing forwarding table is created before the system starts to forward packets – and does not change entries, unless manually altering them • In dynamic routing route propagation software runs on the system and continuously updates the forwarding table – to insure that each datagram follows an optimum route – the software communicates with other systems to learn optimum routes to each destination – it continually checks for network failures that cause routes to change – dynamic routing begins exactly like static routing • by loading an initial set of routes into a forwarding table when the system boots © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 5

27. 3 Static Routing in Hosts and a Default Route • Static routing is straightforward and easy to specify – It does not require extra routing software – It does not consume bandwidth • and no CPU cycles are required to propagate routing information • However, static routing is relatively inflexible – it cannot accommodate network failures or changes in topology • Where is static routing used? • Most hosts use static routing – especially in cases where the host has one network connection and a single router connects the network to the rest of the Internet • Consider the architecture in Figure 27. 1 © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 6

27. 3 Static Routing in Hosts and a Default Route © 2009 Pearson Education

27. 3 Static Routing in Hosts and a Default Route • As Figure 27. 1 shows • A static table with two entries suffices for a typical host – One entry specifies the address of the directly connected network – The other entry specifies that router R 1 • provides a default route for all other destinations • When an application generates a datagram for a computer on the local net (e. g. , a local printer) – the first entry in the forwarding table directs IP to deliver the datagram directly to its destination • When a datagram is destined for any other destination in the Internet – the second entry in the table directs IP to send the datagram to the router, R 1 © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 8

27. 4 Dynamic Routing and Routers • Can a router in the Internet use static routing the same way a host does? • Most routers use dynamic routing – but in some exceptional cases static routing can be used • As an exception (case where static routing does suffice for a router) – Look at Figure 27. 1 again • We can imagine that the figure corresponds to a small organization that is a customer of an ISP • All traffic leaving the customer's site through router R 1 must travel to the ISP – Because routes never change • the forwarding table in router R 1 can be static – Furthermore, the forwarding table in R 1 can use a default route • just as the forwarding table in a host does © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 9

27. 4 Dynamic Routing and Routers • Despite a few exceptions – static routing and default routes do not suffice for most routers – use is limited to special configurations such as the one above • When two ISPs interconnect – both need to exchange routing information dynamically • Consider three networks interconnected by two routers as in Figure 27. 2 © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 10

27. 4 Dynamic Routing and Routers • In Fig 27. 2, each router knows about directly connected networks – – Router R 1 knows about networks 1 and 3 Router R 1 does not know about network 2 (no direct connection) Router R 2 knows about networks 2 and 3 Router R 2 does not know about network 1 (no direct connection) • For this example, it may seem that static routing will suffice • The static approach does not scale to handle thousands of networks – In particular, each time an ISP adds a new customer's network • the information must be passed throughout the Internet – More important, a manual process is far too slow to accommodate network failures and congestion in the Internet © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 11

27. 4 Dynamic Routing and Routers • Each router exchanges information with other routers • When it learns about changes in routes – the routing software updates the local forwarding table • Routers exchange information periodically – the local forwarding table is updated continuously • In Figure 27. 2 routers R 1 and R 2 will exchange routing information – As a result, routing software in R 2 will install a route to network 1 and software running in R 1 will install a route to network 2 – If router R 2 crashes, the route propagation software in R 1 will detect that network 2 is no longer reachable • and will remove the route from its forwarding table – Later, when R 2 comes back on line, the routing software in R 1 will determine that network 2 is reachable again • and will reinstall the route © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 12

27. 5 Routing in the Global Internet • How is routing in the global Internet performed? – The section considers general principles – Later sections explain specific route propagation protocols • A route propagation protocol allows one router to exchange routing information with another • However, such a scheme cannot scale to the entire Internet – if a router in the Internet attempted to exchange routing information with all other routers, the resulting traffic would overwhelm the Internet • To limit routing traffic – – The Internet uses a routing hierarchy Routers and networks in the Internet are divided into groups All routers within a group exchange routing information Then, at least one router (possibly more) in each group summarizes the information before passing it to other groups © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 13

27. 5 Routing in the Global Internet • • • 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? The designers of the Internet routing system did not dictate an exact size nor did they specify an exact data representation or protocol – Instead, the designers purposefully kept the architecture flexible enough to handle a wide variety of organizations • For example, to accommodate organizations of various size – the designers avoided specifying a minimum or maximum size for a group • To accommodate arbitrary routing protocols – the designers decided to permit each organization to choose a routing protocol independently © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 14

27. 6 Autonomous System Concept • Term Autonomous System (AS) to specify groups of routers • One can think of an AS as a contiguous set of networks and routers all under control of one administrative authority • There is no exact meaning for administrative authority • The term is sufficiently flexible to accommodate many possibilities – For example, an AS can correspond to an ISP, an entire corporation, or a university – Alternatively, a large organization with multiple sites may choose to define one AS for each site – In particular, each ISP is usually a single AS, but it is possible for a large ISP to divide itself into multiple ASs • The choice of AS size can be made for – economic, technical, or administrative reasons © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 15

27. 6 Autonomous System Concept • For example – consider a multi-national corporation • It may be less expensive for the corporation to divide into multiple ASs, each of which has a connection to an ISP in a given country, than to act as a single AS with one connection to the rest of the Internet • Another reason for a specific size arises from the routing protocol to be used – a protocol may generate excessive routing traffic when used on many routers (i. e. , routing traffic may grow as the square of the number of routers) © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 16

27. 7 The Two Types of Internet Routing Protocols • All Internet routing protocols are divided into two major categories: – Interior Gateway Protocols (IGPs) – Exterior Gateway Protocols (EGPs) • After defining the two categories – we will examine a set of example routing protocols that illustrate each category • • 27. 7. 1 27. 7. 2 27. 7. 3 27. 7. 4 Interior Gateway Protocols (IGPs) Exterior Gateway Protocols (EGPs) Illustration of How IGPs and EGPs Are Used Optimal Routes, Routing Metrics, and IGPs © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 17

27. 7 The Two Types of Internet Routing Protocols 27. 7. 1 Interior Gateway Protocols (IGPs) • Routers within an AS use an IGP exchange routing information • Several IGPs are available – each AS is free to choose its own IGP • Usually, an IGP is easy to install and operate • IGP may limit the size or routing complexity of an AS © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 18

27. 7 The Two Types of Internet Routing Protocols 27. 7. 2 Exterior Gateway Protocols (EGPs) • A router in one AS uses an EGP to exchange routing information with a router in another AS • EGPs are more complex to install and operate than IGPs – but EGPs offer more flexibility and lower overhead (i. e. , less traffic) • To save traffic – an EGP summarizes routing information from an 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 © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 19

27. 7 The Two Types of Internet Routing Protocols 27. 7. 3 Illustration of How IGPs and EGPs Are Used • Figure 27. 3 illustrates the two-level routing hierarchy used in the Internet by showing two routers in two AS © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 20

27. 7 The Two Types of Internet Routing Protocols 27. 7. 3 Illustration of How IGPs and EGPs Are Used In Figure 27. 3 • AS 1 has chosen IGP 1 to use internally, and AS 2 has chosen IGP 2 • All routers in AS 1 communicate using IGP 1 • All routers in AS 2 communicate using IGP 2 • Routers R 1 and R 4 use an EGP to communicate between the two ASs – That is, R 1 must summarize information from its AS 1 • and send the summary to R 4 – In addition, R 4 accepts a summary from R 1 • and uses IGP 2 to propagate the information to routers in AS 2 – R 1 performs the same service for AS 1 © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 21

27. 7 The Two Types of Internet Routing Protocols 27. 7. 4 Optimal Routes, Routing Metrics, and IGPs • Routing software should find all possible paths and then choose one that is optimal • Although the Internet usually has multiple paths between any source and destination – there is no universal agreement about which path is optimal • Consider the requirements of various applications – For a remote desktop 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 © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 22

27. 7 The Two Types of Internet Routing Protocols 27. 7. 4 Optimal Routes, Routing Metrics, and IGPs • Routing metric to refer to a measure of the path – that routing software uses when choosing a route • It is possible to use throughput, delay, or jitter as a routing metric – but most Internet routing software does not use them as it is • Typical Internet routing uses a combination of two metrics: – administrative cost and hop count • A hop corresponds to an intermediate network (or router) – the hop count for a destination gives the number of intermediate networks on the path to the destination • Administrative costs are assigned manually – Often to control which paths traffic can use – Routing software will choose the path with the lower cost • Thus, traffic will follow the corporate policy © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 23

27. 7 The Two Types of Internet Routing Protocols 27. 7. 4 Optimal Routes, Routing Metrics, and IGPs • IGPs and EGPs differ in an important way with respect to routing metrics: – IGPs use routing metrics, but EGPs do not • each AS chooses a routing metric and arranges internal routing software to send the metric with each route so receiver 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 • Each AS is free to choose a routing metric • An EGP cannot make meaningful comparisons – 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 least 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 © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 24

27. 8 Routes and Data Traffic • Data traffic for a given destination flows in exactly the opposite direction of routing traffic. For example – suppose an AS owned by ISP 1 contains network N • Before traffic can arrive destined for N, ISP 1 must advertise a route to N • That is, when the routing advertisement flows out, data will begin to flow in • Figure 27. 4 (below) illustrates the flow of data in response to routing advertisements © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 25

27. 9 The Border Gateway Protocol (BGP) • Most widely used EGP in the Internet is BGP – Current standard is version 4, abbreviated BGP-4 • BGP has the following characteristics: • Routing Among AS – Because it is intended for use as an EGP – BGP provides routing information at the AS level – No way for BGP to provide details about the routers within each AS on the path • Provision for Policies – BGP allows the sender and receiver to enforce policies • a manager can configure BGP to restrict which routes BGP advertises to outsiders • Reliable Transport – BGP uses TCP for all communication • a BGP program on a router in one AS forms a TCP connection to a BGP program on a router in another AS and then sends data across the connection © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 26

27. 9 The Border Gateway Protocol (BGP) • Facilities for Transit Routing – BGP classifies each AS • as a transit system if it agrees to pass traffic through to another AS or • as a stub system if it does not – Traffic passing through on its way to another AS is classified as transit traffic – Classification allows BGP to distinguish between ISPs and other AS – BGP allows a corporation to classify itself as a stub • even if it is multi-homed (i. e. , a corporation with multiple external connections can refuse to accept transit traffic) • BGP provides the glue that holds Internet routing together at the center of the Internet – Tier-1 ISPs use BGP to exchange routing information and learn about each other's customers © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 27

27. 10 The Routing Information Protocol (RIP) • RIP was among the first IGP used in the Internet • RIP has the following characteristics: • Routing within an AS – RIP is designed as an IGP to be used among routers within an AS • Hop Count Metric – RIP measures distance in network hops • where each network between the source and destination counts as a single hop – RIP counts a directly connected network as one hop away • Unreliable Transport – RIP uses UDP to transfer messages among routers • Broadcast or Multicast Delivery – RIP is intended for use over LAN technologies • that support broadcast or multicast (e. g. , Ethernet) – Version 1 of RIP broadcasts messages – Version 2 allows delivery via multicast © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 28

27. 10 The Routing Information Protocol (RIP) • Support for CIDR and Subnetting – RIP version 2 includes an address mask with each destination • Support for Default Route Propagation – RIP allows a router to advertise a default route • Distance Vector Algorithm – RIP uses the distance-vector approach to routing defined in Algorithm 18. 3 • Passive Version for Hosts – RIP allows a host to listen passively and update its forwarding table – Passive RIP is useful on networks where a host selects among multiple routers © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 29

27. 10 The Routing Information Protocol (RIP) • How RIP propagates routes? (Distance Vector Routing) – Each outgoing message contains an advertisement that lists the networks the sender can reach along with a distance to each – When it receives an advertisement • RIP software uses the list of destinations to update the local forwarding table • Each entry in a RIP advertisement consists of a pair: (destination network, distance) where distance is the number of hops to the destination • 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 © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 30

27. 10 The Routing Information Protocol (RIP) • Advantage of RIP is simplicity, requires little configuration – A manager merely • 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 in the organization will have routes to all destinations • RIP also handles the propagation of a default route – The organization needs to configure one of its routers to have a default (an organization chooses a router that connects to an ISP) – RIP propagates the default route to all other routers in the organization • which means that any datagram sent to a destination outside the organization will be forwarded to the ISP © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 31

27. 11 RIP Packet Format • The RIP message format helps explain how a distance vector routing protocol operates • Figure 27. 5 illustrates a RIP update message • As the figure shows – each entry contains the IP address of a destination and a distance to that destination • To permit RIP to be used with CIDR or subnet addressing – an entry contains a 32 -bit address mask • Each entry also has a next hop address • Two 16 -bit fields that identify the entry as an IP address and provide a tag used to group entries together • Each entry contains 20 octets (bytes) © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 32

27. 11 RIP Packet Format © 2009 Pearson Education Inc. , Upper Saddle River,

27. 12 The Open Shortest Path First Protocol (OSPF) • The RIP message format illustrates a disadvantage of DVR protocols: – The size of a message is proportional to the number of networks that can be reached – Sending RIP messages introduces delay – The delay means that route changes propagate slowly – RIP works well among a few routers, but it does not scale well – Processing RIP messages consumes many CPU cycles • To satisfy demand for a routing protocol that can scale to large organizations – IETF devised an IGP known as the OSPF – The name is derived from the use of Dijkstra's SPF algorithm © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 34

27. 12 The Open Shortest Path First Protocol (OSPF) • OSPF has the following characteristics: – Routing within an AS (OSPF is an IGP) – CIDR Support • OSPF includes a 32 -bit address mask with each address – Authenticated Message Exchange • A pair of routers using OSPF can authenticate each message – Imported Routes • OSPF allows a router to introduce routes learned from another means (e. g. , from BGP) – Link-State Algorithm – Support for Metrics • OSPF allows an administrator to assign a cost to each route – Support for Multi-access Networks • LSR is inefficient across a multi-access network, such as an Ethernet, because all routers attached to the network broadcast link status • OSPF optimizes by designating a single router to broadcast on the network © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 35

27. 13 An Example OSPF Graph • Consider the network and associated graph in

27. 13 An Example OSPF Graph • Figure 27. 6 shows a typical OSPF graph – each node corresponds to a router – an edge in the graph is a connection between a pair of routers • To follow a link-state algorithm – each pair of routers connected by a network periodically probe one another and then broadcast a link-state message to other routers • 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 © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 37

27. 14 OSPF Areas • Hierarchical routing makes OSPF more complex than other routing protocols but also makes it more powerful • To achieve a hierarchy – – OSPF allows an AS to be partitioned for routing purposes We can divide routers in an AS into subsets, called areas Each router is configured to know the area boundary Routers within a given area exchange link-state messages periodically • OSPF allows communication between areas – One router in each area communicates with a router in one or more other area(s) – The two routers summarize routing information learned from other routers within their respective area • and then exchange the summary – Instead of broadcasting to all routers in the AS • OSPF limits link-state broadcasts to routers within an area • As a result of the hierarchy – OSPF can scale to handle much larger AS than other routing protocols © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 38

27. 15 Intermediate System (IS-IS) • Originally designed by DEC to be part of DECNET V, • The IS-IS is an IGP – naming follows Digital's terminology in which a router was called an IS and a host was called an End System • IS-IS was created around the same time as OSPF • The two protocols are similar in many ways – Both use the link-state approach • employ Dijkstra's algorithm to compute shortest paths – Both protocols require two adjacent routers to periodically test the link between them and broadcast a status message © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 39

27. 15 Intermediate System (IS-IS) • The chief differences between OSPF and the original IS-IS can be summarized as: • IS-IS was proprietary (owned by DEC) – and OSPF was created as an open standard available to all vendors • OSPF was designed to run over IP – IS-IS was designed to run over CLNS (part of the ill-fated OSI protocol) • OSPF was designed to propagate IPv 4 routes – IS-IS was designed to propagate routes for OSI protocols • Over time, OSPF gained many features – As a result, IS-IS now has less overhead • When the protocols were initially invented – OSPF's openness made it much more popular than IS-IS – In fact, IS-IS was almost completely forgotten © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 40

27. 15 Intermediate System (IS-IS) • As the years progressed – OSPF's popularity encouraged the IETF to additional features • Ironically, in the early 2000's, ten years after the protocols were designed – several things changed to give IS-IS a second chance • DEC had dissolved, and IS-IS was no longer considered valuable proprietary property • A newer version of IS-IS was defined to integrate it with IP and the Internet • Because OSPF was built for IPv 4 – a completely new version had to be developed to handle larger IPv 6 addresses • The largest ISPs have grown to a size where the extra overhead in OSPF makes IS-IS more attractive • As a result, IS-IS has started to make a comeback © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 41

27. 16 Multicast Routing 27. 16. 1 IP Multicast Semantics • So far, we have discussed unicast routing – That is, we have considered routing protocols that propagate information about destinations that each have a static address and a location that does not change • One of the design 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 the route until a failure makes the path unusable • Propagating multicast routing information differs dramatically from unicast route propagation – 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 © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 42

27. 16 Multicast Routing 27. 16. 1 IP Multicast Semantics • An arbitrary computer can Join to a multicast group at any time and begin receiving packets sent to the group • To join a group, a host informs a nearby router – If multiple applications on the same host decide to join a group • the host receives one copy of each datagram sent to the group and makes a local copy for each application • A host periodically sends group membership messages to the local router • An arbitrary computer can Leave a multicast group at any time – Once the last application on the host leaves the group • the host informs the local router that it is no longer participating in the group © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 43

27. 16 Multicast Routing 27. 16. 1 IP Multicast Semantics • An IP multicast group is anonymous in a number of ways: • First – neither a sender nor a receiver knows (nor can they 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 • 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 © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 44

27. 16 Multicast Routing 27. 16. 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 – protocol is known as the Internet Group Multicast Protocol (IGMP) • The protocol is used only on the network between a host and a router • The protocol defines the host – not the application, to be a group member • The protocol does not specify anything about applications • If multiple applications on a given host join a multicast group – the host must make copies of each datagram it receives for local applications • When the last application on a host leaves a group – the host uses IGMP to inform the local router that it is no longer a member of the group © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 45

27. 16 Multicast Routing 27. 16. 3 Forwarding and Discovery Techniques • When a router learns that a host on one of its networks has joined a multicast group – the router must establish a path to the group and propagate datagrams it receives for the group to the host – thus, routers, not hosts, have responsibility for the propagation of multicast routing information • Dynamic group membership and support for anonymous senders makes general-purpose multicast routing extremely difficult • Moreover, the size and topology of groups vary considerably among applications – For example, teleconferencing often creates small groups • who may be geographically dispersed or in the same organization – A webcast application can potentially create a group with millions of members that span the globe © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 46

27. 16 Multicast Routing 27. 16. 3 Forwarding and Discovery Techniques • To accommodate dynamic membership – multicast routing protocols must be able to change routes quickly and continually • For example, if a user in France joins a multicast group that has members in the U. S. and Japan – multicast routing software must first find other members of the group, and then create an optimal forwarding structure • An arbitrary user can send a datagram to the group – information about routes 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 © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 47

27. 16 Multicast Routing 27. 16. 3 Forwarding and Discovery Techniques Flood-and-Prune • Flood-and-prune is ideal in a situation where the group is small and all members are attached to contiguous LANs • Initially, routers forward each datagram to all networks – That is, when a multicast datagram arrives • a router transmits the datagram on all directly attached LANs via hardware 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 hosts on a given network are members of the group – the router stops forwarding multicast to the network (i. e. , “prunes” the network from the set) © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 48

27. 16 Multicast Routing 27. 16. 3 Forwarding and Discovery Techniques Configuration-and-Tunneling • Configuration-and-tunneling is ideal in a situation where the group is geographically dispersed (i. e. , has a few members at each site, with sites far apart) • 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 hardware multicast • The router then consults its configuration table to – determine which remote sites should receive a copy – and uses IP-in-IP tunneling to transfer a copy of the multicast datagram to each of the remote sites © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 49

27. 16 Multicast Routing 27. 16. 3 Forwarding and Discovery Techniques Core-Based Discovery • Flood-and-prune and configuration-and-tunneling handle extreme cases well • But a technique is needed that allows multicast to scale gracefully from a small group in one area to a large group with members at arbitrary locations • To provide smooth growth – some multicast routing protocols designate a core unicast address for each multicast group. • Whenever a router R 1 receives a multicast datagram that must be transmitted to a group – R 1 encapsulates the multicast datagram in a unicast datagram • and forwards the unicast datagram to the group's core unicast address © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 50

27. 16 Multicast Routing 27. 16. 3 Forwarding and Discovery Techniques • As the unicast datagram travels through the Internet – each router examines the contents • When the datagram reaches a router R 2 that participates in the group – R 2 removes and processes the multicast message – R 2 uses multicast routing to forward the datagram to members of the group • Requests to join the group follow the same pattern – if it receives a request to join a group • R 2 adds a new route to its multicast forwarding table and begins to forward a copy of each multicast datagram to R 1 • The set of routers receiving a particular multicast group grows from the core outward – In graph theoretic terms, the routers form a tree © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 51

27. 16 Multicast Routing 27. 16. 4 Multicast Protocols • Many multicast routing protocols have been proposed – But no Internet-wide multicast routing currently exists • A few of the proposed protocols are: • Distance Vector Multicast Routing Protocol (DVMRP) – A protocol used by the UNIX program mrouted and the Internet Multicast back. BONE (MBONE) – DVMRP performs local multicast, and uses IP-in-IP encapsulation to send multicast datagrams from one site on the Internet to another • Core-Based Trees (CBT) – A particular protocol in which routers build a delivery tree from a central point for each group – CBT relies on unicast routing to reach a central point © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 52

27. 16 Multicast Routing 27. 16. 4 Multicast Protocols • Protocol Independent Multicast-Sparse Mode (PIM-SM) – A protocol that uses the same approach as CBT to form a multicast routing tree – The designers chose the term protocol independent to emphasize that although unicast datagrams are used to contact remote destinations when establishing multicast forwarding, PIM-SM does not depend on any particular unicast routing protocol • Protocol Independent Multicast-Dense Mode (PIM-DM) – A protocol designed for use within an organization – Routers that use PIM-DM broadcast (i. e. , flood) multicast packets to all locations within the organization – Each router that has no member of a particular group sends back a message to prune the multicast routing tree – The scheme works well for short-lived multicast sessions • because it does not require setup before transmission begins © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 53

27. 16 Multicast Routing 27. 16. 4 Multicast Protocols • Multicast Extensions to the OSPF (MOSPF) Protocol – Rather than a general-purpose multicast routing protocol • MOSPF is designed to pass multicast routes among routers within an organization – Thus, instead of a general-purpose multicast approach • MOSPF builds on OSPF and uses the LSR facilities. • Figure 27. 7 summarizes the multicast routing protocols described above • Despite many years of research and many experiments – general-purpose Internet multicast has not been successful – even collaborative applications have not provided sufficient incentive © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 54