CSE 679 Multicast and Multimedia r Basics r

CSE 679: Multicast and Multimedia r Basics r Addressing r Routing r Hierarchical multicast r Qo. S multicast

Multicast -motivation r Point-to-point delivery not efficient for events/transmissions of general interest r Examples m m News multicast IETF sessions/rock concerts r Many receivers may share the same path r Point-to-point delivery too expensive

Multicast motivation

Multicast issues r In point-to-point delivery, receiver address is known => routing is straightforward r In Multicast, many receivers r How to transmit data to all the receivers? r How to identify the group of receivers? m m At the sender? In the network?

Multicast issues r Identify multicast by a group/multicast address r The membership changes as the receivers join/leave the group r Routers/Network need to recognize the multicast address r Receivers need to receive on their own IP address and on the multicast address

Multicast addressing r A multicast sender uses the group address as the receiver’s address when sending packets r Network/routers keep track of actual receiving subnets/paths for this group address (not the actual receivers) r Receivers can reply to sender’s address or to group address

Multicast addressing r Part of IP address space reserved for multicast r Multicast routers recognize multicast addresses r Need to get a multicast address for a transmission before multicast can start r Protocols exist for obtaining multicast addresses and finding out a multicast address

Class D addresses r High order 4 bits = 1110, followed by a 28 -bit r r multicast group ID. 224. 0. 0. 0 - 239. 255 224. 0. 0. 1 - all systems on this subnet 224. 0. 0. 2 - all routers on this subnet 224. 0. 0. 4 - all DVMRP routers 224. 0. 0. 5 - all OSPF routers

IGMP r Internet Group Membership Protocol r Used to join a multicast group and to check on the r r r current status of receivers on a subnet IGMP -join message propagated up the routers until the multicast tree reached. Routers periodically poll hosts on subnets to see if any active receivers remain If no active receivers remain, routers propagate leave messages upstream to reduce unnecessary traffic Frequent polling can increase overhead Separate protocols for finding group membership address - sd

Multicast routing r For point-to-point delivery, a router looks up routing table and knows how to forward r For multicast, many receivers m may have to forward packets on multiple outgoing links too hard to keep track of individual receivers at each router for each multicast group remember just the links on which to be forwarded change as receivers join/leave

Multicast routing r Need to recognize multicast addresses r The routing tables change as the receivers join/leave a multicast group r Every router keeps track of “downstream” links on which to forward a packet r Routing table and multicast address “expire” at the end of session

Multicast Routing Protocols r DVMRP - Distance Vector Multicast r MOSPF - Multicast Extensions to OSPF r PIM - Protocol Independent Multicast

Multicast routing approaches r Flooding m send on all links to reach the receivers m not efficient r Spanning tree m efficient m could concentrate traffic on a few links

Spanning tree based routing r Spanning trees rooted at the sender r When a receiver wants to join a group, may have to broadcast on all upstream links to find a path to the sender m m could cause a lot of overhead in sparse groups need better solutions

Sparse Mode routing r Use a Core-based tree (CBT) r Use a rendezvous point or a core router r Direct all joins to RP which knows how to reach the sender m m m can avoid broadcasting multicast group information to all routers in the network can result in non-optimal paths would need to modify multicast tree over time

Reliable Multicast r In unicast, receiver ACKs give feedback ---Sender takes responsibility in transmitting data r In Multicast, many receivers -- too difficult for sender to keep track of every receiver’s status r ACK Implosion

Receiver-driven Multicast r Sender based schemes don’t scale well as number of receivers increase r Receiver based schemes scale better r Receivers can decide the level of reliability needed as well as the level of quality desired etc.

Send NAKs r Sender keeps no information of receivers’ status r Receivers send NAKs to reduce ACK implosion problem r How to send NAKs? m m Unicast NAKs to sender Multicast NAKs

Unicast NAKs to sender r Reduces overhead when packet losses are isolated and rare r Packet loss early in the tree will result in too many NAKs

Multicast NAKs r Others missing packets need not send NAKs r if every receiver, sends a NAK immediately after r r getting an out-of-sequence packet, too many NAKS at once! Wait for a random time, send a NAK If some one else sends a NAK, suppress your NAK Getting random timers tricky business Could cause burden on receivers if only one receiver doesn’t get the packet

Hierarchical Multicast r Organize multicast into a number of groups r One Designated Receiver (DR) takes responsibility for reliability r On packet loss, NAK propagated to DR r If DR has data, retransmits or re-multicasts with limited scope to the group r If DR doesn’t have data, sends NAK to sender

Hierarchical Multicast r More scalable than other multicast protocols r Specially useful when multicast over wide r r geographic boundaries, keep one DR in each country for example DR nodes may need more power than other receivers Need mechanisms to find out DR Need mechanisms to delegate DR function to another node as primary DR node leaves multicast RMTP: Reliable Multicast Transport Protocol - Bell Labs

Congestion control r Layered multicast r Arrange layers in an exponentially increasing data rates r TCP-friendly Multicast m m In steady state, packet drop => congestion, drop a layer If layers are doubling in data rates, dropped layer = reducing multicast rate by half => TCP friendly

Qo. S-Sensitive Multicast r The key issue is to construct a multicast tree with Qo. S constraints r Goal is to build a tree of paths to destinations such that sum of link costs (e. g. consumed bandwidth) is minimum and Qo. S constraints (e. g. delay) are satisfied r Exact solutions to such multi-constrained optimization problems are prohibitively expensive r Need heuristics that provide fast solutions of high quality

An Example for Constructing A Tree r Application Qo. S requirements: end-to-end delay 13, jitter 7 r example 1 10 10 10 r example 2 2 6 6 10 10

Mbone r Multicast Backbone r Consists of all the multicast-enabled routers r If two multicast routers are not directly connected, uses tunneling over non-multicast routers r Allows gradual deployment

Conclusion r Multicast basics m Motivation and Issues m Addressing m Routing r Hierarchical multicast r Qo. S multicast