Internet Multicasting Chapter 17

Hardware Broadcast Broadcast delivery –Network delivers a copy of a packet to each destination Bus technology - single packet transmission Switches and point-to-point - copies of the packet are forwarded across individual connections until all switches have received a copy –A computer specifies broadcast delivery by sending a packet to a broadcast address In Ethernet the address field is all ones Hardware on each machine recognizes its own or a broadcast address –Disadvantage - Demand on resources

Multicasting A different type of multi-point delivery –Allows each system to choose if it wants to participate –The network interface hardware is configured to recognize a multicast address for machines in a group –Forwarding with unicast and broadcast depends on the network topology

Ethernet Multicast An example of hardware multicasting –Half of Ethernet addresses are reserved for multicasting –The low order bit of the high-order octet distinguishes unicast (0) from multicast(1) addresses In dotted hexadecimal: –Device driver software can reconfigure a device to allow it to recognize one or more multicast addresses Example: 01.5E

IP Multicast The internet abstraction of hardware multicasting Characteristics –Group address: class D address, some permanent, some temporary –Number of groups: limited by practical constraints on routing table size, actual is 2 28 –Dynamic group membership: hosts can join or leave anytime –Use of hardware: if the underlying hardware permits multicast, IP uses hardware to send, else it uses broadcast or unicast to send!

IP Multicast –Inter-network forwarding: multicast routers are required to forward IP multicast (capability is usually added to conventional routers) –Delivery semantics: uses the same best-effort IP delivery - multicast datagrams may be lost, delayed, duplicated, delivered out of order –Membership and transmission: any may send to the multicast group; only the group may receive

Conceptual Pieces Requirements for internet multicasting –a multicast addressing scheme allowing local autonomy in assigning addresses, but having global meaning –a notification and delivery mechanism making effective use of hardware multicast, but allowing IP multicasting over networks without the hardware support –internetwork forwarding facility routing over shortest paths, along paths that lead only to group members, allowing hosts to join and leave when they choose

IP Multicast Addresses IP multicasting uses class D addresses - see Figures 4.1 and 17.1 –The first four bits are 1110 –The remaining 28 bits specify a multicast group –Multicast addresses: range is from to lowest address reserved up to for routing /group maintenance see permanently assigned addresses in Figure 17.2 –notice all systems and all routers

IP Multicast Addresses A multicast address –can only be used as a destination address –cannot appear in the source address field or in a source route or record route option –cannot have ICMP error messages generated for them (a ping to a multicast address will go unanswered) TTL field in multicast messages is decremented, when it reaches 0, no message is sent

Mapping IP Multicast to Ethernet Multicast The low-order 23 bits of the IP multicast address is placed in the low-order 23 bits of the Ethernet multicast address E Thus, IP multicast address becomes E The mapping is not unique and IP software must check for unwanted multicast datagrams

Multicast Scope Members have scope –Members on a single network, scope is the network –Members are in a single organization, scope is the organization Each datagram also has scope (also called range) –Control messages for communication between a host and router on the same network have a TTL = 1 –Interprocess communication (two applications on the same host) can have a TTL = 0

Extending Host Software to Handle Multicasting Hosts participate in IP multicast at one of 3 levels –Level 0 - host cannot send or receive multicast –Level 1 - host can send, but cannot receive multicast –Level 2 - host can send and receive multicast Modifications to allow sending –application program specifies multicast address and network interface software maps the IP into hardware Modifications to allow receiving –application program indicates that it wants to join or leave a group, and membership must be handled

Internet Group Management Protocol (IGMP) To participate in a multicast that spans multiple networks, the host must inform local multicast routers –Local routers pass membership information to other routers IGMP is used to communicate group membership information –IGMP is similar to ICMP in that it uses IP datagrams to carry messages is a standard for TCP/IP and is required on all machines that receive IP multicast –IGMP is considered an integral part of IP, not separate

Internet Group Management Protocol (IGMP) IGMP has two phases –Phase 1: A host joins a multicast group It sendsan IGMP message declaring its membership Local multicast routers receive the message and propagate group membership information –Phase 2: Local multicast routers poll hosts to see who the remaining members are As long as at least one host responds, the router keeps the group active If none respond, the router stops advertising

Internet Group Management Protocol (IGMP) IGMP is designed to avoid adding overhead by –datagrams that carry IGMP messages use hardware multicast, if it is available –multicast routers send a single query to request information about all groups, not separate messages –a single router is chosen to poll host membership –hosts respond to IGMP queries at random intervals –hosts listen for responses from other hosts and avoid sending repetitive information (only one host in a group needs to respond about membership)

Group Membership State Transitions Three states of an entry in a host’s multicast group tables are shown in Figure 17.4 –Nonmember joining –Delaying member waits for random time to respond to router’s query –Before moving to Member state, the host knows that another host responded, or its timer expires and this host sends a response Routers generate queries every 125 seconds Hosts remain in Member state most of the time

IGMP Message Format Type - 1 octet –General membership query, specific group query, membership report, leave group, version 1 membership report (backward compatibility) Response time -1 octet –Maximum interval for which random response time is determined (if 10, random value from 0 to 10 is chosen) –If there are many hosts, delay value is larger Checksum - 2 octets Group address - 4 octets - specific group or all groups

Multicast Forwarding and Routing Information Why is multicast routing difficult? Consider the architecture shown in Figure 17.7 –The router should not forward packets intended for the dotted group across network 2 –If a host on network 2 joins the dotted group, routing must change to accommodate it –A multicast router must examine more than just the destination address –A host that is not a member may send a datagram to a group

Basic Multicast Routing Reverse Path Forwarding (RPF) –a multicast router has a conventional routing table with shortest paths to all destinations the router extracts the source address and finds the interface that leads to the source the router forwards a copy of the datagram over all other interfaces, but not the one back to the source this means that RPF wastes bandwidth by transmitting over networks that don’t lead to group members Truncated RPF (TRPF) –checks to see that paths lead to group members

Consequences of TRPF Because TRPF relies on RPF to prevent loops, TRPF delivers and extra copy of datagrams to some networks –duplicates are sent when the network is as Figure 17.8 –R 3 and R 4 both send copies to host B on network 4 The source determines delivery path –Figure 17.9 (a) shows what happens when X is source 2 copies go to network 5 –Figure 17.9 (b) shows what happens when Z is source 2 copies go to networks 2 and 4

Multicast Trees Paths define a forwarding tree, or a delivery tree –the tree contains no cycles –each multicast router corresponds to a node in the tree –a network connecting the routers is an edge in the tree –the source of a datagram is the root –the last router on the path is a leaf –Figure 17.9(a) is a tree, technically part (b) is not A forwarding tree defines a set of paths through multicast routers from a source to all members of a multicast group (size of tables is a concern)

The Essence of Multicast Routing A Multicast Router must have knowledge of group membership Group membership information must be propagated across the internet –Because membership can change rapidly, information at a given router is imperfect and routing may lag changes –Design tradeoff: routing overhead and inefficient data transmission

Reverse Path Multicasting Underlying assumptions –It is more important for a datagram to reach each member of the group than it is to eliminate unnecessary transmission –Multicast routers contain a routing table with correct information –Needless transmission is eliminated when possible RPM uses a two-step process –Copies of datagrams are broadcast to the internet –Multicast routers inform each other of paths that don’t lead to group members

Reverse Path Multicasting Membership information is propagated bottom-up –It starts with hosts that join or leave the group –Hosts communicate with their local router using IGMP –When a router learns that no group member lie beyond a given network interface, it stops forwarding and notifies the router on the path back to the root When a router learns that there are no members along a path, that path is pruned The system is data-driven –A router does not send group information until datagrams arrive for this group (data arrives and we know where to send messages)

Distance Vector Multicast Routing Protocol DVMRP passes information about current membership and the cost to transfer datagrams between routers –Resembles RIP –Passes information about current membership and cost of transferring datagrams –For each possible (group, source) pair, the routers impose a forwarding tree –It defines an extended IGMP for multicasting

Mrouted Implements DVMRP for UNIX systems Mrouted provides –Route propagation Entries in the table include a (group, source) pair and a corresponding set of interfaces for forwarding Mrouted is used in addition to standard routing practice –Multicast tunneling Not all internet routers can forward multicast datagrams Multicast datagrams can be tunneled through intermediate routers that don’t participate in multicast routing

Mrouted Mrouted may be configured by a manager to specify –multicast groups that are permitted to advertise on each interface –how to forward datagrams associates a metric and threshold with each route –a manager can assign a cost to each path –threshold gives minimum TTL to complete the path A tunnel is shown in Figure (IP-in-IP) –a datagram is encapsulated in a unicast datagram and sent across the nonparticipating network-Figure 17.11

Mrouted Multicast tunnels form the basis of the Internet’s Muticast Backbone (MBONE) –Often used in teleconferencing Consequences of tunneling –Tunnel costs –DVMRP must compute its own version of unicast forwarding that includes the tunnels?

Limitations of DVMRP The volume of information kept is high Entries for previously active groups are stored –So that it knows where to send a graft message when a host joins a gorup that was pruned Propagation is slow Does not scale well Following are designs for overcoming these limitations

Core Based Trees (CBT) Avoids broadcasting –Instead of broadcasting until negative information is propagated, it only propagates where hosts have joined –This is demand driven When a host joins, the local router must inform other routers before datagrams will be forwarded (membership information sent first) Which routers should be informed? –The internet is divided into regions –Within each region there is a core router –Tree is built as hosts send join requests to core Each router sends an echo request to check connectivity

Protocol Independent Multicast (PIM) PIM consists of two protocols –PIM Dense Mode (PIM-DM) Most networks have hosts that listen to each multicast group Uses RPF to broadcast datagrams to every group Strops sending when it receives prune requests Assumes router also uses conventional routing protocols –PIM Sparse Mode (PIM-SM) Members of multicast groups occupy a small subset of possible networks Like CBT, requiresa point to which joins are sent Builds a forwarding tree, trees rooted at rendezvous point

Multicast Extensions to OSPF (MOSPF) Uses OSPF to form a forwarding tree for each source Demand driven Sends all group information to all routers

Reliable Multicast and ACK Implosions In theory –Guarantees that group members receive data in order, without loss, duplication or corruption –More efficient than broadcast In actuality –If a group has >1 sender, the notion of delivering in sequence is meaningless –Duplication happens even on small internets –Certain applications are susceptible to delay or jitter (audio and video) –Reliable protocols require ACK’s => ACK implosion

Reliable Multicast and ACK Implosions To overcome the ACK Implosion problem –Multicasting is restricted to a single source –Acknowledgement points are identified ACKs are processed further down the tree –Datagrams are assigned unique number so that loss can be detected

Summary IP multicasting uses class D address space Hardware multicast is used, if available Multicast groups are dynamic –Hosts may join or leave at any time Not limited to a single physical network Group membership is communicated using IGMP Two approaches to propagation –data-driven –demand-driven

Summary More information is required in a multicast forwarding table than in a unicast routing table –Entries are required for each (group, source) pair Not all routers forward multicast traffic –Tunnelling could be used A multicast datagram is encapsulated in a unicast datagram Reliable multicast uses a heirarchy of acknowledgement points or send redundant information

For Next Time Read Chapter 19