1. Chapter 9: Multicast Sockets
Network Programming
Chapter 9: Multicast Sockets

2. Agenda Unicasts, Broadcasts, Multicasts
Application Models with Multicasting
Architecture of Multicast Sockets
Using Multicast Sockets with .NET
Creating a Chat Application
Creating a Picture Show Application

3. Introduction
Multicasts can be used for group communications over the Internet, where every node participating in the multicast must join the group set up for the purpose
In multicasts, a sender sends a message to a GROUP of receivers (group distribution)
Broadcasts may be achieved via UDP
Multicasts may be achieved via UDP

4. Unicasts, Broadcasts and Multicasts
Introduction
IP Protocol supports
Unicast
Broadcast
Multicast
Unicast addresses
Broadcast addresses
Multicast addresses

5. Unicasts, Broadcasts and Multicasts
Introduction
Unicast addresses
Network packets sent to a single destination
Server maintains a separate connection for each client
TCP can only send unicast messages
UDP can send unicast messages (no reliability checks)
Broadcast addresses
Multicast addresses

6. Unicasts, Broadcasts and Multicasts
Introduction
Unicast addresses
Broadcast addresses
Broadcast datagrams are sent to all nodes in a subnetwork
Broadcasts always use UDP
Low overheads
No guarantee that message has been received
Broadcasts don’t cross subnets
Multicast addresses

7. Unicasts, Broadcasts and Multicasts
Introduction
Unicast addresses
Broadcast addresses
Multicast addresses
Multicast datagrams are sent to every node that belongs to a group (regardless of subnet)
Every host (node) that wants to receive multicast messages must register with the Internet Group Management Protocol (IGMP)
Class D addresses ( to ) are used for multicast gateways

8. Agenda Unicasts, Broadcasts, Multicasts
Application Models with Multicasting
Architecture of Multicast Sockets
Using Multicast Sockets with .NET
Creating a Chat Application
Creating a Picture Show Application

9. Application Models with Multicasting
Chat server/client
Many-to-Many Applications
Allows every system in a group to send data to every other system in the group
A multicast can be used
E.g. a peer-to-peer chat system (everyone in the group can see what everyone else is saying)
One-to-Many Applications
Chat server/client
Chat server/client
Chat server/client
Chat server/client

10. Application Models with Multicasting
Many-to-Many Applications
One-to-Many Applications
One system may send a message to a group of systems
This can be useful for video, audio or other large files/data types
Sender
Receiver
Receiver
Receiver
Receiver
Receiver

11. Agenda Unicasts, Broadcasts, Multicasts
Application Models with Multicasting
Architecture of Multicast Sockets
Using Multicast Sockets with .NET
Creating a Chat Application
Creating a Picture Show Application

12. Architecture of Multicast Sockets
Introduction
Multicast messages send using UDP and class D addresses
Can also use IGMP to send multicast messages
IGMP
Multicast Address Allocation
Routing
Scoping
Routing Protocols
Scalability
Reliability
Security

13. IGMP
Part of IP module and allows clients to leave a group as well as join a group
IGMP = Internet Group Management Protocol
IGMP messages are encapsulated in IP datagrams with the IP protocol number 2.
Versions of IGMP
IGMP version 2 (IGMPv2): client can explicitly leave the group (and almost immediately)
IGMP version 3 (IGMPv3): RFC 3376 – ) Allows specific joins and leaves with the source address(es). Possible now to run Source-Specific Multicast (SSM) protocol.

14. IGMPv3 Message Format 8-Bit Type 8-Bit Max Response Time
16-Bit Checksum
32-Bit Group Address
Reserved S
QRV
QQIC
Number of Sources
Source Address 1
Source Address 2
Source Address 3
The IGMPv3 message adds a varying number of 32-bit values to the message. The 32 bits that follow the group address are separated as follows:
A 4-bit reserved field
A 1-bit S flag that defines that the router should suppress updating the time field
A 3-bit QRV (Querier’s Robustness Variable) that defines the robustness value of the sender of the query
An 8-bit QQIC (Querier’s Query Interval Code) value that specifies the number of seconds of the query interval
A 16-bit number that defines the number of source addresses that continue with the following bytes of the message

15. IGMP Message Header Hex Value Message Description 0x11
Membership Query
These are used by the router to see whether any group members exist. Two types of membership queries can be differentiated by the group address in the 32-bit group address field. A general query has a group address in the ICMP header of all zeros, and asks which groups have members on an attached network. A group-specific query returns information about whether a particular group has members
0x22
Version 3 Membership Report
When a host joins a multicast group, a membership report is sent to the router to inform the router that a system on the network is listening to multicast messages. The values for IGMPv1 and v2 membership reports are 0x12 and 0x16. IGMPv3 implementations must also understand membership reports for IGMPv1 and IGMPv2
0x17
Leave Group
The last host of a multicast group inside a subnet must send a leave group message to all routers ( ) when it leaves a group. A host may remember the hosts of the multicast group (received in membership reports in response t o membership queries), so that it knows when it is the last one in the grup, but this is not a requirement. If the group members are not remembered, every host leaving a group sends a leave group message. In any case, the router checks if it was the last host in the group, and stops forwarding multicast messages if so.

16. Multicast Address Allocation
Class D address range to
Special purpose range: to (cannot be used and routers will not pass these messages on)
RFC 3171 defines the use of specific ranges of IP multicast addresses and their purposes
Ways in which addresses can be allocated:
Static
Dynamic
Scope-relative

17. Multicast Address Allocation
Static Multicast Addresses
Usually are globally assigned by the IANA
Usually for messages that are of global interest
Used for protocols that need well-known addresses
IANA-assigned addresses may be hard-coded into applications and devices
Two blocks of static multicast addresses are used:
Local Network Control Block
Internetwork Control Block
Dynamic Multicast Addresses
Scope-Relative Multicast Addresses
IANA = Internet Assigned Numbers Authority Local Network control Block: This includes all systems on the subnet. Addresses starting with belong to this block, and they are never forwarded by a router. Examples of these are to send a message to all systems on the subnet and to send a message to all routers on a subnet. They Dynamic Host Configuration Protocol (DHCP) server answers messages on the IP address , but only on a subnet
Internetwork Control Block: The addresses in the CIDR range /24 belong to this block. Messages sent to these addresses can be forwarded by a router. Examples are Network Time Protocol (NTP) and Windows Internet Name Service (WINS) requests.
CIDR = Classless Inter-Domain Routing

18. Multicast Address Allocation
Static Multicast Addresses
Dynamic Multicast Addresses
Often better suited to circumstances than static address
Request-on-demand addresses have a “life-time”
Similar concept to DHCP
Multicast Address Dynamic Client Allocation Protocol (MADCAP)
MADCAP: the client sends a unicast or multicast message out to a MADCAP server to request a multicast address. The server answers with a lease-based address
Scope-Relative Multicast Addresses

19. Multicast Address Allocation
Static Multicast Addresses
Dynamic Multicast Addresses
Scope-Relative Multicast Addresses
Multicast addresses that are only used within a local group or organisation
Address range to is reserved for administrative scope-relative addresses. (These addresses can be reused with other local groups because the routers filter this address range out and prevent the message from leaving the local network)

20. Architecture of Multicast Sockets
Introduction
Multicast messages send using UDP and class D addresses
Can also use IGMP to send multicast messages
IGMP
Multicast Address Allocation
Routing
Scoping
Routing Protocols
Scalability
Reliability
Security

21. Routing Multicast backbone (MBone)
Routers don’t necessarily implement multicasting (manufacturing issue)
Multicasting over the Internet
Multicast messages forwarded using unicast connections
MBone tools are available
MBone used for audio and video multicasts, technical talks, seminars, NASA space shuttle missions, etc

22. Scoping Done via Class D addresses (issued by IANA) TTL = 1
Message does not leave the network
Each router that receives the group report decrements the TTL by 1

23. Routing Protocols
Distance Vector Multicast Routing Protocol (DVMRP): uses a reverse path-flooding algorithm, where the router sends a copy of the network packet out to all paths, except the one from which the packet originated. If no node in a router’s network is a member of the multicast group, the router sends a prune message back to the sending router so that it knows it does not need to receive packets for that multicast group. DVMRP periodically refloods attached networks to reach new nodes that may be added to the multicast group. DVMRP does not scale very well.
Multicast Open Shortest Path First (MOSPF)
Protocol-Independent Multicast (PIM)

24. Routing Protocols Distance Vector Multicast Routing Protocol (DVMRP)
Multicast Open Shortest Path First (MOSPF): This protocol is an extension of the Open Shortest Path First (OSPF) Protocol. With MOSPF, all routers must be aware of all available links to networks hosting members of multicast groups. MOSPF calculates routes when multicast traffic is received. This protocol can be used only in networks where OSPF is used as a unicast routing protocol, because MOSPF routes are exchanged between routers using OSPF. MOSPF does not scale well if many multicast groups are used or if groups change often.
Protocol-Independent Multicast (PIM)

25. Routing Protocols Distance Vector Multicast Routing Protocol (DVMRP)
Multicast Open Shortest Path First (MOSPF)
Protocol-Independent Multicast (PIM): This protocol uses two different algorithms for sending messages to group members. When the members are widely distributed across different networks, PIM-SM (Sparse Mode) is employed; when a group uses only a few networks, PIM-DM (Dense Mode) is used. PIM-DM uses a reverse path-flooding algorithm similar to DVMRP, except that any unicast routing protocol can be used. PIM-SM defines a registration point for proper routing of packets.

26. Architecture of Multicast Sockets
Introduction
Multicast messages send using UDP and class D addresses
Can also use IGMP to send multicast messages
IGMP
Multicast Address Allocation
Routing
Scoping
Routing Protocols
Scalability
Reliability
Security

27. Scalability It is an advantage of multicasting
Multicasting is the most efficient and scalable way to send messages to multiple clients
Broadcasting has the disadvantage that messages cannot cross networks and that clients who are not interested pick up the broadcast as well.

28. Reliability
IP multicasting does not offer any compatible transport-level protocol that is both reliable and implements a flow mechanism
UDP offers no guarantees
Sometimes need a highly reliable mechanism for transporting packets (e.g. multicasting an application to several workstations simultaneously)
If guaranteed delivery is needed, then must use custom handshaking using a reliable protocol
Add packet numbers
Add a checksum
If the receiver detects a corrupted message, it sends a NACK message to the sender
NACK = Negative Acknowledgement

29. Security Multicasting security
Internet multicasting vs Intranet multicasting
Firewall prevents “leaks” into Internet
Clients need to register with group
Internet Engineering Task Force (IETF) working group working on Multicast Security (MSEC) proposal
Multicast Group Key management

30. Agenda Unicasts, Broadcasts, Multicasts
Application Models with Multicasting
Architecture of Multicast Sockets
Using Multicast Sockets with .NET
Creating a Chat Application
Creating a Picture Show Application

31. Using Multicast Sockets with .NET
Multicast Sender Applications
No special tasks
Use the UdpClient class
Use a multicast address
IPEndPoint object, remoteEP, will point to the group address and the port number that will be used by the group
Multicast address must be known to clients joining the group
Can use a MADCAP server to get addresses dynamically
Use a stream socket that the client connects to, to tell the client about the multicast address
Multicast Receiver Applications

32. Using Multicast Sockets with .NET
Multicast Sender Applications
Multicast Receiver Applications
Using the UdpClient Class
Need to program UdpClient Class
Clients must join the multicast group
Need to be able to drop a group member
Using the Socket Class
May use the Socket class instead of UdpClient class
A UDP socket is created (AddMembership and MulticastTimeToLive are programmable options)
IGMP protocol is implemented

33. Agenda Unicasts, Broadcasts, Multicasts
Application Models with Multicasting
Architecture of Multicast Sockets
Using Multicast Sockets with .NET
Creating a Chat Application
Creating a Picture Show Application

34. Creating a Chat Application
Setting Up the User Interface
Creating the Configuration File
Joining the Multicast Group
Receiving Multicast Messages
Sending Multicast Messages
Starting the Chat Application

35. Agenda Unicasts, Broadcasts, Multicasts
Application Models with Multicasting
Architecture of Multicast Sockets
Using Multicast Sockets with .NET
Creating a Chat Application
Creating a Picture Show Application

36. Creating a Picture Show Application
Creating a Picture Protocol
Building the Picture Show Server
Creating the Picture Show Client
Starting the Picture Show