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 16.07.2002

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

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

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

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

Example: FIFO order

Example: Causal order

Example: Total order

Global order

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...)

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

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

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

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.

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

A MH receives a NORMAL message (details)

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