Download Chapter 1 - Introduction

Computer Networks and Internets, 5e
By Douglas E. Comer
Lecture PowerPoints
By Lami Kaya, [email protected]
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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Chapter 27
Internet Routing
and
Routing Protocols
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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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
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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
–
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–
–
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
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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
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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
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27.3 Static Routing in Hosts
and a Default Route
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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 R1
• 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, R1
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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 R1 must
travel to the ISP
– Because routes never change
• the forwarding table in router R1 can be static
– Furthermore, the forwarding table in R1 can use a default route
• just as the forwarding table in a host does
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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
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27.4 Dynamic Routing and Routers
• In Fig 27.2, each router knows about directly connected
networks
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–
–
–
Router R1 knows about networks 1 and 3
Router R1 does not know about network 2 (no direct connection)
Router R2 knows about networks 2 and 3
Router R2 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
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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 R1 and R2 will exchange routing
information
– As a result, routing software in R2 will install a route to network 1 and
software running in R1 will install a route to network 2
– If router R2 crashes, the route propagation software in R1 will detect
that network 2 is no longer reachable
• and will remove the route from its forwarding table
– Later, when R2 comes back on line, the routing software in R1 will
determine that network 2 is reachable again
• and will reinstall the route
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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
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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
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27.5 Routing in the Global Internet
•
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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
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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
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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)
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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
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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
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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
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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
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27.7 The Two Types of Internet Routing Protocols
27.7.3 Illustration of How IGPs and EGPs Are Used
In Figure 27.3
• AS1 has chosen IGP1 to use internally, and AS2 has
chosen IGP2
• All routers in AS1 communicate using IGP1
• All routers in AS2 communicate using IGP2
• Routers R1 and R4 use an EGP to communicate between
the two ASs
– That is, R1 must summarize information from its AS1
• and send the summary to R4
– In addition, R4 accepts a summary from R1
• and uses IGP2 to propagate the information to routers in AS2
– R1 performs the same service for AS1
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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
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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
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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
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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 ISP1 contains network N
• Before traffic can arrive destined for N, ISP1 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
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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
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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
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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
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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
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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
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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
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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)
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27.11 RIP Packet Format
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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
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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
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27.13 An Example OSPF Graph
• Consider the network and associated graph in Figure 27.6
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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
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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
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27.15 Intermediate System Intermediate System (IS-IS)
• Originally designed by DEC to be part of DECNET
• 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
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27.15 Intermediate System 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 IPv4 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
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27.15 Intermediate System Intermediate System (IS-IS)
• As the years progressed
– OSPF's popularity encouraged the IETF to add 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
• OSPF was built for IPv4
– a completely new version had to be developed to handle IPv6 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
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27.16 Multicast Routing
27.16.1 IP Multicast Semantics
• So far, we have discussed unicast routing
– We 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
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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
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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
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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
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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
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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
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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)
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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
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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
• 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 R1 receives a multicast datagram that
must be transmitted to a group
– R1 encapsulates the multicast datagram in a unicast datagram
• and forwards the unicast datagram to the group's core unicast address
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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 R2 that participates in
the group
– R2 removes and processes the multicast message
– R2 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
• R2 adds a new route to its multicast forwarding table and begins to forward a
copy of each multicast datagram to R1
• The set of routers receiving a particular multicast group
grows from the core outward
– In graph theoretic terms, the routers form a tree
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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 backBONE (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
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27.16 Multicast Routing
27.16.4 Multicast Protocols
• Protocol Independent Multicast-Sparse Mode (PIM-SM)
– It 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
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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
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27.16 Multicast Routing
27.16.4 Multicast Protocols
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