CS4254
Computer Network Architecture and
Programming
Dr. Ayman A. Abdel-Hamid
Computer Science Department
Virginia Tech
Multicasting
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Outline
•Multicasting (Chapter 21)
Multipoint Communications
IP Multicast
IPv4 Multicast addresses
Sending and Receiving Messages
Multicasting on a LAN
Multicasting on a WAN
Multicast Issues
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Multipoint Communications
• Multipoint communications support communications between
more than two hosts
One-to-many
Many-to-many
•Unlike broadcast, allows a proper subset of hosts to participate
•Example standards
IP Multicast (RFC 1112, standard)
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Logical Multipoint Communications
•Two basic logical organizations
Rooted: hierarchy (perhaps just two
levels) that structures communications
Non-rooted: peer-to-peer (no
distinguished nodes)
•Different structure could apply to control
and data “planes”
Control plane determines how
multipoint session is created
Data plane determines how data is
transferred between hosts in the
multipoint session
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Logical Multipoint Communications
Control Plane
•The control plane manages creation of a multipoint session
Rooted control plane
One member of the session is the root, c_root
Other members are the leafs, c_leafs
Normally c_root establishes a session
Root connects to one or more c_leafs
c_leafs join c_root after session established
Non-rooted control plane
All members are the same (c_leafs)
Each leaf adds itself to the session
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Logical Multipoint Communications
Data Plane
The data plane is concerned with data transfer
•Rooted data plane
Special root member, d_root
Other members are leafs, d_leafs
Data transferred between d_leafs and d_roots
d_leaf to d_root
d_root to d_leaf
There is no direct communication between d_leafs
•Non-rooted data plane
No special members, all are d_leafs
Every d_leafs communicate with all d_leafs
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Forms of Multipoint Communications
•Server-based -- rooted multipoint communications with server as
d_root
Passive or inactive
Relay
Reflector
Active
Bridge or multipoint control unit (MCU)
•Strictly peer-to-peer multipoint – Non-rooted
Multicast
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Multipoint Servers
Passive Multipoint Server
•a relay or reflector service
•Provides no processing of the
data
•Minimum requirement is for
transport-level semantics, so
can operate at the transport or
application level
Multicasting
Active Multipoint Server
•Does application-level
processing
transcoding
•uses application-level
semantics
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Multicast Communication
•Multicast abstraction is peer-to-peer
Application-level multicast
Network-level multicast
Requires router support (multicast-enabled routers)
Multicast provided at network protocol level  IP multicast
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Multicast Communication
•Transport mechanism and network layer must support multicast
•Internet multicast limited to UDP (not TCP)
Unreliable: No acknowledgements or other error recovery
schemes (perhaps at application level)
Connectionless: No connection setup (although there is
routing information provided to multicast-enabled routers)
Datagram: Message-based multicast
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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IP Multicast
•IP supports multicasting
Uses only UDP, not TCP
Special IP addresses (Class D) identify multicast groups
Internet Group Management Protocol (IGMP) to provide
group routing information
Multicast-enabled routers selectively forward multicast
datagrams
IP TTL field limits extent of multicast
•Requires underlying network and adapter to support broadcast or,
preferably, multicast
Ethernet (IEEE 802.3) supports multicast
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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IP Multicast: Group Address
•How to identify the receivers of a multicast datagram?
•How to address a datagram sent to these receivers?
Each multicast datagram to carry the IP addresses of all
recipients?  Not scalable for large number of recipients
 Use address indirection
A single identifier used for a group of receivers
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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IP Multicast: IGMP Protocol
•RFC 3376 (IGMP v3): operates between a
host and its directly attached router
•host informs its attached router that an
application running on the host wants to
join or leave a specific multicast group
•another protocol is required to coordinate
multicast routers throughout the Internet 
network layer multicast routing algorithms
•Network layer multicast  IGMP
and multicast routing protocols
•IGMP enables routers to populate
multicast routing tables
•Carried within an IP datagram
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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IP Multicast: IGMP Protocol
IGMP v2 Message types
•membership query: general
•Sent by routers  router query multicast groups joined by
attached hosts
•membership query: specific
•Sent by routers  query if specific multicast group joined by
attached hosts
•membership report
•Sent by host  report host wants to join or is joined to given
multicast group
•leave group (optional)
•Sent by host  report leaving given multicast group
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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IP Multicast: IGMP Protocol
•Joining a group
Host sends group report when the first process joins a given
group
Application requests join, service provider (end-host) sends
report
•Maintaining table at the router
Multicast router periodically queries for group information
Host (service provider) replies with an IGMP report for each
group
Host does not notify router when the last process leaves a
group  this is discovered through the lack of a report for a
query
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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IP Multicast: Multicast Routing
•Multicast routers do not maintain a list of individual members of
each host group
•Multicast routers do associate zero or more host group addresses
with each interface
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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IP Multicast: Multicast Routing
•Multicast router maintains table of multicast groups that are active
on its networks
•Datagrams forwarded only to those networks with group members
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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IP Multicast: Multicast Routing
•How multicast routers route traffic amongst themselves to ensure
delivery of group traffic?
Find a tree of links that connects all of the routers that have
attached hosts belonging to the multicast group
Group-shared trees
Source-based trees
Shared Tree
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
Source Trees
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MBONE: Internet Multicast Backbone
•The MBone is a virtual network on top of the Internet (section B.2)
Routers that support IP multicast
IP tunnels between such routers and/or subnets
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Unicast versus Broadcast versus Multicast
•A unicast address identifies a single IP interface
•A broadcast address identifies all IP interfaces on the subnet
•A multicast address identifies a set of IP interfaces
•A multicast datagram is received only by those interfaces
interested in the datagram (applications wishing to participate in
the multicast group)
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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IPv4 Multicast Addresses
1/3
•Class D addresses in the range 224.0.0.0 through
239.255.255.255
•Low order 28 bits of class D Naddress (see appendix A) form
the multicast group ID (32-bit address is the group address)
•Mapping of IPv4 multicast address to Ethernet address
High-order 24 bits of Ethernet address are always 01:00:5E
Next bit always 0
Low-order 23 bits are copied from low-order 23 bits of multicast group
address
High-order 5 bits of group address are ignored in the mapping
Mapping not one-to-one
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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IPv4 Multicast Addresses
2/3
224.0.1.88 mapped into an Ethernet address?
•Remember an Ethernet address is 48 bits
•The address 224 is E0 in hex, 0 is 00 in hex, 1 is 01 in hex, and
88 is 58 in hex. However, only the low-order 23 bits are used
•Therefore, the IP address of 224.0.1.88 converted to a MAC
address is 01-00-5E-00-01-58.
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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IPv4 Multicast Addresses
3/3
Some special IPv4 multicast addresses
•224.0.0.0 reserved
•224.0.0.1 all-host group
•224.0.0.2 all-routers group
•224.0.0.1 through 224.0.0.255 reserved for routing-protocols
•Datagrams destined to any of theses addresses are never
forwarded by a multicast router
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Sending & Receiving Multicast Messages
Receiving Multicast Messages
•Create a UDP socket
•Bind it to a UDP port, e.g., 1234
All processes must bind to the same port in order to
receive the multicast messages
•Join a multicast group address
•Use recv or recvfrom to read the messages
Sending Multicast Messages
•You may use the same socket (you used for receiving) for
sending multicast messages or you can use any other UDP
socket (it does not have to join any multicast group)
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Multicast on a LAN
1/3
•Receiving application creates a UDP socket, binds to port 123
and joins multicast group 224.0.1.1
•IPv4 layers saves the information internally and tells appropriate
datalink to receive Ethernet frames destined to 01:00:5E:00:01:01
•Sending applications creates a UDP socket and sends a
datagram to 224.0.1.1, port 123
•Ethernet frame contains destination Ethernet address,
destination IP address, and destination port
•A host on the LAN that did not express interest in receiving
multicast from that group will ignore such datagram
•Destination Ethernet address does not match the interface address
•Destination Ethernet address is not the ethernet broadcast address
•The interface has not been told to receive any group addresses
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Multicast on a LAN
2/3
•Ethernet frame received by datalink of receiver based on imperfect
filtering (When interface told to receive frames destined to one specific
Ethernet multicast address, it can receive frames destined to other
Ethernet multicast addresses)
Ethernet interface cards apply a hash function to group address,
calculating a value between 0 and 511. This information turns on a bit in a
512-bit array
Small size bit-array implies receiving unwanted frames
Some network cards provide perfect filtering
Some network cards have no multicast filtering at all (multicast
promiscuous mode)
•Packet passed to IP layer (IP layer compares group address against all
multicast addresses that applications on this host have joined  perfect
filtering)
•Packet passed to UDP layer, which passes it to socket bound to port 123
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Multicast on a LAN
3/3
Some Other scenarios
•A host running an application that has joined 225.0.1.1 
Ethernet address 01:00:5E:00:01:01. Packet will be discarded by
perfect filtering in IP layer
•A host running an application that has joined some multicast
group which the Ethernet address produces the same hash value as
01:00:5E:00:01:01. Packet will be discarded by datalink layer or
by IP layer
•A packet destined to the same group, but a different port.
Accepted by IP layer, but discarded by UDP layer (no socket has
bound the different port)
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Multicast on a WAN
•A program started on five hosts belonging to different LANs
•Multicast routers communicate with neighbor routers using a multicast routing
protocol (MRP)
•When a process on a host joins a multicast group, that host sends an IGMP
message to any attached multicast routers, which in turn exchange this
information using MRP with neighbor routers
•When a sender sends a multicast message, mutlicast routing information is used
to direct the message
H1
S
MR1
MR2
MR5
MR3
MR4
H4
H2
Multicasting
H5
H3
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Some Multicast Issues
Time To Live
Set TTL for outgoing multicast datagrams (default is 1  local
subnet)
Loopback mode
•Enable or disable local loopback of multicast datagrams
•By default loopback is enabled
•A copy of each multicast datagram sent by a process on the
host will also be looped back and processed as a received
datagram by that host
Port Reuse
•Allow the same multicast application to have several instances
running on the same host
•In Java, Port reuse is enabled by default
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Socket Options
•Various attributes that are used to determine the behavior of
sockets (see chapter 7)
#include <sys/socket.h>
int getsockopt (int sockfd, int level, int optname, void * optval,
socklen_t *optlen);
int setsockopt (int sockfd, int level, int optname, const void * optval,
socklen_t optlen);
Both return 0 if OK, -1 on error
•sockfd: an open socket descriptor
•level: code in the system that interprets the option (general socket
code, or protocol-specific code) (SOL_SOCKET, IPPROTO_IP,
IPPROTO_IPv6, IPPROTO_TCP are examples)
•optname: see page 193
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Socket Options
Some socket options examples (see table on page 193 and 194)
For multicast socket options see section 21.6 on page 559
•Socket Level
SO_SNDBUF, SO_RCVBUF, SO_KEEPALIVE,
SO_BROADCAST, SO_REUSEADDR,
SO_RESUEPORT
•IP Level
IP_TTL, IPMULTICAST_IF, IPMUTLICAST_TTL,
IP_MULTICAST_LOOP, IP_ADD_MEMBERSHIP,
IP_DROP_MEMBERSHIP
•TCP Level
TCP_KEEPALIVE, TCP_MAXSEG, TCP_NODELAY
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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Sending and Receiving
•Section 21.10 page 575
•A program to send and receive multicast datagrams
Send datagram to a specific group every five seconds
(datagram contains sender’s hostname and process ID)
An infinite loop that joins the multicast group to which the
sending part is sending and prints every received datagram
•Create a UDP socket then set multicast socket options for address
reuse, joining the group, and setting loopback
•See mcast/main.c, mcast/send.c, and mcast/recv.c
Multicasting
© Dr. Ayman Abdel-Hamid, CS4254 Spring 2006
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