1. Seminar Mobilkommunikation Reliable Multicast in Wireless Networks
1 . General points about reliable multicast
2 . A protocol designed for reliable multicast in wireless networks
Christophe Le Roquais

2. What is Multicast? One sender and a group of receivers Applications:
Video-conferences
Distribution of news
Software updates
Multi-players games
et cetera...
Typical problems:
How to address group members?
Reliability

3. Reliability in the Transport Layer
The Transport Layer enables two end-system applications to communicate. For example, TCP is a protocol designed to offer a reliable packet transmission between two machines.
Error recovery
Transport layer treats lost, erroneous, duplicated packets. Waiting of an acknowledgement from the receiver (ACK)
Flow control
The receiver sends a „window“ (reception capacity) with each ACK
Packet order
Receiver can reorder packets due to the sequence number

4. Reliability classes
Multicast cannot be the sum of unicast connections. Otherwise there will be a traffic congestion.
Unreliable
No check of the packet delivery
Partial-reliable
k-reliable
K group members are expected to receive packets correctly
statistically reliable
A percentage of all group members is expected to receive packets correctly
sufficiently reliable
Unknown number of receivers who received packets correctly. At least 1 group member received the packet.
Reliable
Every group member receives packets without errors, no duplicated message and in the correct order

5. Delivery Orders
Packet delivery order is a major aspect in reliable multicast. There are four delivery order classes :
FIFO order or source order
If a group member sends a message m2 after sending a message m1, then any group member that delivers both messages delivers first m1 and then m2
Causal order (includes FIFO order)
If the transmission of a message m2 causally follows the transmission of a message m1, then any group member that delivers both messages delivers first m1 and then m2
Total order (includes FIFO and Causal orders)
Any two group members that deliver a pair of messages m1 and m2, deliver them in the same order, e.g either they both deliver m1 before m2, or they both deliver m2 before m1
Global order (includes FIFO, Causal and Total orders)
Every group member delivers messages in the same order as they were sent

6. Example: FIFO order

7. Example: Causal order

8. Example: Total order

9. Global order

10. Wireless Network Architecture
A wired backbone
Wired stations:
Coordinators (C)
Manage a few of MHs
Coordinator Boss (CB)
Manage the whole network
Mobile Support Stations (MSS)
Gateways between the wired backbone and mobile hosts
Mobile Hosts (MH)
User terminals (laptops, PDAs...)

11. A Protocol for Reliable Multicast
G. Anastasi, A. Bartoli and F. Spadoni have designed a reliable protocol for reliable mobile multicast. Features:
Multicast communication is reliable: All multicast messages are delivered and there are no duplicates
The sender of multicast datagram may select among FIFO order, Causal order and Total order
The set of senders and receivers may be dynamic: A mobile host may join and leave such group at will
Each group member is associated to a Coordinator C
Cell switchings of MHs do not trigger any message exchange in the network

12. Messages structure
The protocol uses eight important messages. The Tag is the first field of each message. It indicates the nature of the message.
Tag
From To
Use
JOIN MH CB
For joining the group
NEW C
For multicasting in the group
NACK
MSS
Notify about missing multicast datagrams
ACK
Acknowledge that a message has arrived at MSS
NORMAL
C or CB
Propagate a multicast or a membership change
STABINFO
Propagate stability information
JOINSYNC
Allocate data structures at coordinators
JOINOK
Communicate initial sequence number

13. Dynamic membership Actions at MH Actions at Coordinators
MH sends a JOIN message to CB. This message includes a unique member identifier Mid composed of the MAC-address and of a unique number
Actions at Coordinators
CB receives a JOIN message from a new MH
CB sends a JOINSYNC message to all coordinators. CB choose a coordinator C for the new MH and sends a NORMAL message to every MSSs
C adds MH in its member list and responds to CB with a JOINOK message
Actions at MSSs
MSSs add the new MH in their member lists

14. Sending of a FIFO/Causal ordered message
Mid sends a NEW message to the local MSS
The MSS replies with an ACK
The MSS forwards the message to the coordinator (C1) associated with MH1
C1 changes the TAG of the NEW message to NORMAL, appends a sequence number and multicasts it to all MSSs
MSSs broadcast the NORMAL message to MHs
A MH which receives a NORMAL message determines if it has to be buffered or delivered
6’. F-Cast : a MH delivers the NORMAL message if MH has already delivered the last message sent by C1
6’’. C-Cast : same condition as F-Cast and if MH has already delivered all messages sent by every Ci. NEW message includes delivered array.

15. Sending of a Total ordered message
Mid sends a NEW message to the local MSS
The MSS replies with an ACK
The MSS forwards the message to the coordinator (C1) associated with MH1
C1 changes the TAG of the NEW message to NORMAL, appends a sequence number and sends it to CB
CB appends a sequence number and multicasts the resulting message to MSSs and C1
A MH which receives a NORMAL message determines if it has to be buffered or delivered
T-Cast : a MH delivers the NORMAL message if :
MH has already delivered the last message sent by C1
MH has already delivered the last-1 message sent by CB
MH has already delivered all messages sent by every Ci

16. A MH receives a NORMAL message (details)

17. Summary
Reliable multicast is a powerful communication primitive for structuring distributing programs
The protocol is a reliable multicast protocol for distributed mobile systems that supports dynamic membership
The protocol accomodates three delivery ordering guarantees: FIFO, Causal and Total