Internet Multicasting
Chapter 16
Hardware Broadcast
Many HW technologies support sending
packets to multi destinations concurrently

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Broadcasting: most common form
Copy of packet delivered to each destination
Easy on bus technologies (Ethernet)
Done with single packet transmission

Others: SW must forward copies of the packet
Broadcast Address


Reserved destination address
Specifies broadcast delivery
Ex. – Ethernet: address of all 1’s

HW at each machine accepts packet for:
The machine’s address
The broadcast address
Chief disadvantage of broadcasting

Demand on resources
Network bandwidth
Computational resources on all machines
Hardware Origins of Multicast
Multicasting
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Less common form of multi-point delivery
Also supported by hardware technologies
Allows each system to choose if it wants to
participate in a given multicast
Large set of addresses reserved for multicast
One address used for each group of machines that
want to communicate


Multicast is a generalization of all addressing
Unicast: multicast with one computer in the group
Broadcast: multicast with all computers in the group
Multicast: arbitrary computers in the group
Multicast cannot replace conventional addressing
Difference in underlying mechanisms
Forwarding and delivery done differently


Unicast and broadcast
Forwarding depends on network topology
Multicast
Forwarding must send packet to all segments
Conclusion:
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
Multicast may be a generalization of addressing schemes
However, underlying forwarding and delivery
mechanisms make it less efficient
Ethernet Multicast
½ of Ethernet addresses reserved for multicast
 Low-order bit of high-order byte distinguishes
0 for unicast; 1 for multicast

Multicast in dotted hexadecimal notation:
01.00.00.00.00.0016

If interface configured for Ethernet multicast:
01.00.5E.00.00.0116
Will accept any packet sent to computer’s unicast
address, broadcast address, or above multicast
address
IP Multicast
IP Multicasting

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Internet abstraction of hardware multicasting
Allows transmission to subset of hosts
Generalizes subset
Can spread across arbitrary physical networks
Anywhere throughout the internet

Given subset is called a multicast group

IP multicasting has following characteristics:
Group address

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Each multicast group has unique class D address
Some groups permanent; some temporary
Number of groups
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Up to 228 simultaneous multicast groups
Number limited by constraints on routing table size
Dynamic group membership
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Host can join or leave anytime
Host can be member of arbitrary number of groups
Use of hardware

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If underlying network HW supports multicast; IP uses it
If not; IP uses broadcast or unicast to deliver IP multicast
Inter-network forwarding
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Members of IP multicast group can attach to multiple nets
Multicast routers are required to forward IP multicast
Capability usually added to conventional routers
Delivery semantics

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Uses same best-effort delivery as other IP datagram delivery
Multicast datagrams can be lost, delayed, duplicated, etc.
Membership and transmission
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Arbitrary host can send datagrams to any multicast group
Membership only used to determine who receives
Conceptual Pieces
Three conceptual pieces required
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Multicast addressing scheme
Effective notification & delivery mechanism
Efficient internetwork forwarding facility
Many goals, details, and constraints
present challenges for an overall design

Addressing scheme has conflicting goals
Allow local autonomy in assigning addresses
Define addresses that have global meaning

Notification and delivery has same problem
Make effective use of hardware when available
Allow IP multicast over networks w/o HW support

Forwarding facility presents biggest challenge
Want both efficient and dynamic scheme
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Route packets along shortest path
Not send copy on path not leading to member of the group
Allow hosts to join and leave a group at any time
IP multicast includes all three aspects
Rest of chapter considers each in more detail
IP Multicast Addresses
Permanently assigned addresses
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Called well-known
Used for major services on global Internet
Other groups created when needed

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Transient multicast groups
Discarded when group member count = 0
Class D addresses reserved for multicast
Figure 16.1

No identification information in the group bits
Not identify the origin or owner of a group
No info about whether all members on one physical network

Range: 224.0.0.0 through 239.255.255.255
Lowest address reserved
Others up through 224.0.0.255: routing & group maintenance
Figure 16.2 shows examples of permanently assigned addresses
Figure 16.2
Multicast Address Semantics
Multicast treated differently than unicast

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Multicast address can only be destination
No ICMP messages for multicast datagrams
Ping sent to multicast address never answered

Time-to-live field is honored
Reaches zero; datagram discarded
No ICMP message sent
Mapping IP Multicast to Ethernet Multicast
IP multicast standard does not cover all
types of network hardware


Does specify mapping to Ethernet multicast
Efficient and easy
Place low-order 23 bits of IP multicast address into the
low-order 23 bits of the special Ethernet multicast
address 01.00.5E.00.00.0016
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Example:
224.0.0.2 becomes 01.00.5E.00.00.0216
11101010 00000000 00000000 00000010
Mapping is not unique

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IP multicast uses 28 significant bits
More than one IP multicast group may map to
same Ethernet multicast address
11100000
0xxxxxxx
xxxxxxxx xxxxxxxx
.
.
.
.
.
.
11101111 1xxxxxxx xxxxxxxx xxxxxxxx
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Scheme chosen as a compromise
Uses 23 of the 28 bits
Chances are small that two groups will be the same
Using fixed part of Ethernet address helps
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Makes debugging easier
Eliminates interference between IP and other protocols
Consequences: host may get msg in error
IP software must check incoming datagrams
Hosts and Multicast Delivery
IP multicast on single physical network
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Host uses HW multicast address
Receiver always listening to it
Multicast throughout the internet
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Special multicast routers forward the datagrams
Host must send datagram to multicast router
Does via the hardware, like in local multicast
Diff between local & non-local in the routers
Multicast Scope
Scope of multicast group

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Refers to dispersion of group members
Perhaps restricted to a network or organization
Scope of multicast datagram

Set of networks over which datagram will be sent
Informally, datagram’s scope is called its range
IP uses two techniques to control scope
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Time-to-live field
Administrative scoping
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TTL field control
Set to small value, host can limit distance it’s routed
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Control messages have TTL of 1 (host-router comm)
Applications on single host can use IP multicast
Set TTL value to 0; never leave host
Can configure site routers such that a certain TTL is
needed or else the datagram will never leave the site
TTL field gives course-grain control over scope

Administrative scoping
Reserve parts of address space
Do for local groups or groups part of a single org
Routers forbidden from forwarding datagrams with
addresses from this space
Extending Host SW to Handle
Multicasting
Host participates in IP multicast in 1 of 3 levels
(1) Host can neither send nor receive IP multicast
(2) Host can send but not receive IP multicast
(3) Host can both send and receive IP multicast
 Modifications for host sending are not difficult
IP SW allows application to specify multicast as
destination
Network interface SW must be able to map IP
multicast address into HW multicast address

Extending host to receive is more complex
Host IP SW must have API allowing programs to join &
leave multicast groups
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If multiple applications join, must pass all a copy
If all leave, host must know it no longer participates
Host must inform multicast routers of membership
Hosts join specific IP groups on specific networks
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Host may have multiple network connections
May join group on one network and not on another
Keep group membership associated with networks
Allows multicast use with local machines on one net
SW must keep separate lists of multicast addresses for
each network
Application must specify a particular network when
joining or leaving a multicast group
Internet Group Management Protocol
To participate in IP multicast, host must:
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Local network multicast:
Have SW allowing send/receive multicast datagrams

Multicast that spans physical networks
Must inform local multicast routers
Multicast routers must know memberships
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Use IGMP to communicate group membership
Current version is 3; knows as IGMPv3
IGMP is like ICMP

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Uses IP datagrams to carry messages
IGMP provides a service used by IP
Think of as integral part of IP, not separate protocol

IGMP is a standard for TCP/IP
Required on all machines receiving IP multicast
Conceptually, IGMP has two phases
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Host joins group; sends message
Routers get and establish routing
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Routers poll hosts to see if still members
If none report, router stops advertising membership
IGMP Implementation
IGMP designed to avoid adding overhead
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May have multiple multicast routers on a net
May have hosts participating, too
Must avoid having all participants generate
control traffic
Several way IGMP minimizes effect on network
All host/router communication uses IP multicast
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IP destination address is a multicast address
Datagrams with IGMP messages transmitted via HW
multicast
Hosts not using IP multicast never receive IGMP messages
When polling, single query sent about all groups
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Not send a separate query for each group
Default polling rate is 125 seconds
Single polling router used
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Even if multiple multicast routers attach to same network
Hosts respond to queries at different times
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Query contains value N
Hosts pick random number between 0 and N to wait
Have multiple groups; pick different number for each
Reports for multiple group memberships can be
sent in a single packet
Group Membership State
Transitions
IGMP must remember status of each
multicast group to which host belongs
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Keeps a table to record group membership
When host joins, allocates entry
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Keeps group reference counter; initializes to 1
Another application joins; increment counter
Application terminates execution or drops out; decrement
Counter reaches zero; informs multicast router
Figure 16.4 The three possible states of an entry in a host’s multicast group
table and transitions among them, where each transition is
labeled with an event and an action. The state transitions do not
show messages sent when joining and leaving a group.
IGMP Message Format
Skip:
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16.16: IGMP Membership Query Message
Format
16.17: IGMP Membership Report Message
Format
Multicast Forwarding & Routing Information
Unanswered questions
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How do routers exchange membership info?
How do routers ensure datagrams get to all
group members?
No single standard for propagation
No real agreement on an overall plan
Protocols differ in goals and basic approach
Why is multicast routing so difficult?

Multicast routing differs from conventional routing
because multicast forwarding differs
Figure 16.9
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Need dynamic routing
No “dots” on net 2; not send packets there
But, host can join at any time
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Unicast routing: changes when topology changes
Multicast: change when application leaves/joins group
Insufficiency of destination routing
F & E can send to cross group
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Same destination; different actions
A send to cross group: still another action
Multicast forwarding requires a router to look at
more than destination address
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Arbitrary senders
Any host can send to the group; not just members
G can send to dotted group
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Not a member of the group
No members on G’s network
Datagram may pass through other networks with no
members of the group
So:
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Multicast datagram may originate on a computer that is
not part of the group
May be forwarded across networks that do not have any
group members attached
Basic Multicast Forwarding
Paradigms
Routers must use more than destination
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What exactly do they use?
Multicast destination represents a set of
computers
Want to reach all members of the set
Do not want to route over same network twice
Partial solution: do not send back on arriving interface
Will not prevent problem if set of routers form loop
Rely on datagram’s source address to avoid loops
One idea: Reverse Path Forwarding
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Uses source address to prevent loop
Multicast router must have conventional table
Have shortest path to all destinations
Datagram arrives
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Looks up in table; finds interface I which leads to source
If it arrived on I; forward copy to all other interfaces
Otherwise, discard the copy
Each multicast datagram goes to every network
Every host in multicast group will receive it
RPF alone not used – wastes bandwidth
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Transmits to nets with no members & lead to no members
Truncated Reverse Path Forwarding
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Modified version of RPF
Avoids sending datagrams where not needed
Multicast router needs to have:
Conventional routing table
List of multicast groups reachable thru each NI
When a multicast datagram arrives:
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Router first applies RPF rule
If should discard, does so
If not, checks to see if one or more members in the
destination are reachable over each interface
Truncates sending when no more members lie along path
Uses both source and destination addresses in decision
Consequences of TRPF
Guarantees each member gets datagram
Has two surprising consequences
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Delivers an extra copy to some networks
Delivery depends on the datagram’s source
If A is source, Net 4 & B will get duplicate copies
Figure 16.10
Behavior of delivery depends on the source
Figure 16.11
Multicast Trees
Use graph theory to describe a tree
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Set of paths from a given source to all members
Graph is tree if no cycles appear
That is, router is on no more than one path
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Called forwarding tree or delivery tree
Each multicast router is a node in the tree
Network that connects two routers is an edge
Last router along each path from source is a leaf
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Network hanging off leaf router is a leaf network
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Tree with root X
Leaves R3, R4, R5, and R6
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Technically not a tree
R3 lies along two paths
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Informally, referred to as a tree anyway
Important principle
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Forwarding tree is defined as set of paths
through multicast routers from source to all
members of a multicast group
For a given multicast group, each possible
source can determine a different forwarding tree
An immediate consequence concerns the
size of tables used for forwarding
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Each entry in multicast table is a pair:
(multicast group, source)
Conceptually, source identifies a single host
that can send datagram
In practice, do not keep separate entry for each host
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All forwarding trees defined by all hosts on a single network
are identical
To save space, use a network prefix as source
Router defines on forwarding entry for all hosts on same net
Aggregating entries by net prefix reduces table size
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Multicast tables can still be much larger than conventional
Conventional: size proportional to number of networks
Multicast: proportional to product of number of networks and
number of multicast groups
Essence of Multicast Routing
Inconsistency between IP multicast & TRPF

TRPF not forward datagram to a network
unless that network leads to a member
Multicast router must know about group membership

IP allows host to leave/join at any time
Leads to rapid changes
Group membership must be propagated

Since membership does not follow local scope
Membership issue central to routing
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All multicast routing schemes propagate info
In addition to using info for forwarding
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Rapid changes may make router info imperfect
Routing updates may lag changes
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Multicast design represents a tradeoff
Routing traffic vs. inefficient data transmission
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Must propagate group membership info or routers will not
forward datagrams efficiently
If scheme communicates with every member, resulting traffic
will overwhelm an internet
Every design is a compromise
Reverse Path Multicasting
Derived from TRPF
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Extensions make it more dynamic
Three underlying assumptions:
Receipt by every member more important than
eliminating unnecessary transmissions
Multicast routers have conventional tables with correct
information
Multicast routing should improve efficiency when
possible
RPM uses two step process
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At start, uses RPF broadcast scheme
Ensures all networks get copy of datagram

At same time, RPM has routers inform one
another about paths not leading to members
Routers stop forwarding along such paths
How do routers learn location of members?
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RPM propagates information bottom-up
Starts with hosts that join or leave groups
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Sends info via IGMP to local router
Thus, routers know about local members, but not distant
Leaf routers can decide if forward to leaf networks
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Does not forward if no members of the group
Leaf router informs next router along path back to source
Whenever router learns no members lie beyond it
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Stops forwarding; informs next router back toward root
Called pruning a path from the forwarding tree
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RPM called broadcast and prune
Router broadcasts using RPF
Until get information that allows a path to be pruned
RPM system also called data-driven
Router does not send membership info to any other
routers until datagrams arrive for the group
Data-driven model must also handle case of host
joining after path has been pruned
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RPM handles joins bottom-up
Host informs local router it has joined
Router consults its record of the group
Obtains address of router previously sent prune msg to
Send new message that undoes prune
Known as graft requests
DV Multicast Routing Protocol
Like RIP protocol; extended for multicast

Allows multicast routers to pass info
Group membership & datagram transfer cost

Routers make up forwarding tree for each
possible (group, source) pair
Sends datagrams out over networks that correspond to
branches in the forwarding tree
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Routers talk with extended form of IGMP
New msg types to enter group, leave group, query
other routers, & carry routing info (including cost)
Mrouted Program
Implements DVMRP for UNIX systems
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Like routed
Cooperates closely with OS to install routing info
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Unlike routed
Does not use standard routing table
Can only be used with a multicast kernel
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Has special multicast routing table
Code needed to forward multicast datagrams
Uses multicast tunneling
Mrouted handles two tasks:
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Route propagation
Uses DVMRP to propagate routing information
Constructs multicast routing table
Usually has mrouted in addition to standard routing

Multicast tunneling
Not all routers can forward multicast datagrams
Can tunnel a datagram from one router to another

Go through routers that do not participate in multicast
Both tasks may not be needed at a computer

Configuration file used to specify how operate
Tunneling
Used when routers on the path between
participating hosts do not run multicast routing
software

Tunnel consists of agreement between
mrouted programs running on two routers
Both listen on local net for datagram sent to specified
multicast destination
When one arrives, it is encapsulated by mrouted

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Put in conventional unicast datagram
Sent to other router
Other mrouted program receives datagram through its
tunnel
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Extracts the multicast datagram
Forwards according to its multicast routing table
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Outer, unicast datagram has own TTL counter
Inner, multicast datagram has separate TTL
counter
Tunnel treated like single physical network
Alternative Protocols
DVMRP has some limitations
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Uses small value for infinity (like RIP)
Keeps lots of information
Does broadcast and prune
Lots of traffic to propagate membership information
Since use DV, propagation is slow

Does not scale well
Other multicast protocols
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Core Based Trees (CBT)
Protocol Independent Multicast (PIM)
Multicast extensions to OSPF (MOSPF)
None are a required standard
Core Based Trees (CBT)
Avoids broadcasting

Uses demand-driven paradigm
Does not forward along path until host(s) join
Instead of forward until negative info received,
does not forward until positive info received

Which routers should be informed when a
host informs local router (via IGMP) that it has
joined a group?
Divides the internet into regions
Designates a core router for each region
Protocol Independent Multicast
(PIM)
Really two protocols

PIM-DM (dense mode)
LAN environment
Most all networks are listening to each group

PIM-SM (sparse mode)
WAN environment
Members are small subset of possible networks
PIM-DM

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
Assumes low-delay networks with plenty of
bandwidth
Optimized for guaranteed delivery
Uses broadcast and prune
Not worry about overhead
PIM-SM


Demand-driven like CBT
Designates router to be rendezvous point (RP)
Like CBT core
RP is equivalent of CBT core router
Multicast Extensions to OSPF
(MOSPF)
Let multicast routing benefit from info
gathered by conventional protocols



Uses OSPF’s topology database to build
forwarding tree for each source
Has advantage of being demand-driven
Disadv:
Amount of routing information to propagate
Information must be synchronized

Works well within area; cannot scale to internet
Reliable Multicast
Refers to system that:


Uses multicast delivery
Guarantees all group members receive data:
In order
No loss, duplication, or corruption
Theory


More efficient forwarding scheme than broadcasting
All data still arrives intact
In practice


Not as easy as sounds
“In sequence” delivery may be meaningless
If group has multiple senders



Multicast schemes easily produce duplication
Need to bound the delay for some applications
Reliability requires acknowledgements
Arbitrary number of group members
Thus, send must handle arbitrary number of ACKs
This is the ACK implosion problem
Use hierarchical approach


Multicasting limited to single source
ACK points identified
Router in tree that agrees to cache copies of data
Processes ACK from hosts or routers down the tree
ACK point accomplishes necessary retransmissions

NACK usually used
Host detects lost datagram; requests retransmission
Choice of branching topology and ACK
points is crucial to success of reliable
multicast
Other approaches to reliability

Send multiple datagram copies
Works well if RED is used by routers
Probability of more than one copy being discarded is
small

Forward error-correcting codes
Sender incorporates error-correction info into each
datagram
If one is lost, error correcting code contains sufficient
redundant information to allow sender to reconstruct
the missing datagram
Summary
IP multicast is an abstraction of hardware
multicasting


Allows delivery to multiple destinations
Uses class D addresses to specify delivery
Actual transmission uses hardware, if available
Multicast groups are dynamic

Hosts can join/leave at any time

For local multicast:
Host only need ability to send/receive multicast

Over multiple networks:
Multicast routers propagate group information
Arrange routing so all members get copy of datagram
Hosts communicate membership via IGMP


Efficient
Only periodic message from a multicast router and a single
reply for each multicast group per network
Variety of protocols for routing
information propagation


Either data-driven or demand-driven
Multicast forwarding table is much larger than
unicast routing table
Needs entries for each (group, source) pair

Not all routers propagate multicast routes or
forward multicast datagrams
IP tunnel used to transfer datagrams
Multicast datagram encapsulated in unicast datagram
Reliable multicast



Uses multicast forwarding
But offers reliable delivery semantics
Must avoid ACK implosion
Use hierarchy of acknowledgement points
Other approaches send redundant information