IP Multicast. Overview. Casts. Tarik Čičić University of Oslo December 2001

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1 IP Multicast Tarik Čičić University of Oslo December 00 Overview One-to-many communication, why and how Algorithmic approach (IP) multicast protocols: host-router intra-domain (router-router) inter-domain Scalability and manageability issues Casts Unicast: one-to-one Broadcast: one-to-all Multicast: one-to-group the group is a subset of all most general notion (Anycast is a special unicast)

2 One-to-many communication needs to send the same information to all three receivers Receiver Router Receiver transmissions over the same link! Receiver One-to-many communication needs to send the same information to all three receivers Receiver Router Receiver Only one transmission Receiver 5 Link utilization Three receivers: unicast case: (+)/ =.5 msg./link multicast case (+)/ = msg./link Thousand receivers: unicast case: ( )/00 = msg./link multicast case: (+000)/00 = msg./link 6

3 Graph-algorithmic background Network routers and switches map to a set of nodes (V) Links map to edges E Problem: given graph (V, E), node-set G, where G subset V, create a spanning tree that interconnects G 7 Steiner trees All links in the tree have an associated cost The minimal spanning tree we call Steiner tree Constructing this tree is a well-known NP-complete problem 8 Approximate algorithms

4 Approximate algorithms () Cost: 7 Optimal cost: 0 Approximate algorithms () Spanning tree in practice Creating a minimal spanning tree is an NPcomplete problem More than one metric adds to the complexity (cost/delay optimization) Research shows that best effort trees are often quite good (0% costlier on average)

5 Shortest Path Trees (SPT) Use communication delay as the metric Find the (unicast) shortest path between all node pairs in the group G Remove possible cycles Result: delay-optimized spanning tree! Core Based Trees (CBT) Removing loops in the SPT algorithm leads to sub-optimal tree for some nodes Alternative : keep all n trees (n= G ) Alternative : select a central node in the graph (tree core) find the shortest paths from this node to all other nodes in G merge the paths, creating a CBT More problems In the Internet, nodes do not know the network state " we need a distributed calculation network dynamics group dynamics (members joining and leaving) 5 5

6 IP multicast concepts In the Internet, one-to-many communication is called IP multicast multicast group is a set of receivers interested in the same data group membership is dynamic, everybody can join and leave as desired senders can but need not be members of the group 6 IP multicast history Introduced in 988 (!) Flood-and-Prune mechanisms (DVM) Mbone established in 99 Audio broadcast from IETF meeting in 99 Exciting but limited manageability (flat structure, bandwidth consumption) scalability (many routes in the routing tables, BW) Mbone collapse in ~997 7 Modern IP multicast New (PIM) protocols being introduced from ~996 Due to its explicit control messages, PIM- SM is the most popular intra-domain multicast routing protocol today Inter-domain multicast: Multicast Source Discovery Protocol (MSDP) Border Gateway Multicast Protocol (BGMP) 8 6

7 Host/network and network protocols How does a sender learn where the receivers are? all receivers make up a multicast group receivers use IGMP (Group Membership Protocol) to mark their interest to the closest router routers construct a multicast distribution tree rooted in the source, spanning all receivers the tree is constructed using a multicast routing protocol (+ inter-domain issues) 9 IGMP OK, I do not need to send G 5 G 7 G 7 G Have G 5 Have G 7 Have G Group report? On a local, well-known multicast address: Query interval: 5s Response: (0, 0s) CISCO xxxx 0 IGMP in switched LANs IGMP was designed for multiple access LANs There are three approaches for switched LAN optimization: proprietary (ICMP, Cisco) IGMP snooping (the switch reads IP headers searching for IGMP control messages) Generic Attribute Registration Protocol (GA), and GM, having drawback that all hosts need to have modified kernels, but otherwise able to handle complex switched LANs 7

8 Multicast addresses In IPv: Class D addresses (prefix 0) per today, random address assignment scope through TTL field in the IP header IPv6: prefix (+ -bit flags + -bit scope) # -group address Scope: how far to transmit the packet (e.g. link local, organization local scope) Flood-and-Prune (s) start transmitting packets on all adjacent links Flood-and-Prune (s) start transmitting packets on all adjacent links Candidate branch 8

9 Flood-and-Prune Routers forward data on all links except where the data arrived Candidate branch 5 Flood-and-Prune Routing loops must be avoided! Candidate branch 6 Flood-and-Prune Prune control messages are sent downstream on inactive links Candidate branch 7 9

10 Flood-and-Prune Prunes are also sent downstream if prunes are received on all upstream interfaces Candidate branch 8 Flood-and-Prune Candidate branch Tree branch 9 Flood-and-Prune Candidate branch Tree branch 0 0

11 Flood-and-Prune What if a new receiver wants to join? - flooding is periodically repeated Candidate branch Tree branch Flood-and-Prune problem Can we flood the Internet in order to reach 5 receivers? NO! this is one of the reasons why the original Mbone collapsed PIM-SM Protocol Independent Multicast Sparse Mode does not flood the network it is based on explicit Join/Prune messages receivers wanting to join the group send a Join message where? to the well-known core router ( Rendezvous Point )

12 PIM-SM operation send a Join message to the Rendezvous Point Core () PIM-SM operation The sender uses unicast to reach the core Core () Tree branch pp stream 5 PIM-SM operation If the traffic generated by the source is large enough, the group switches to Shortest Path Tree (instead of CBT) Core () Tree branch 6

13 Comparison Control-driven tree construction (PIM-SM) saves a significant amount of resources compared to data-driven (DVM) this is particularly true in large networks in the case of small multicast groups 7 Sample multicast group 8 9

14 Hierarchical multicast We still cannot cover the Internet by PIM- SM: state and control traffic (periodic state refreshing) overhead in transient routers where to place the s? policing solution: divide the Internet in domains, quite as in the unicast case 0 MSDP PIM-SM s act as the MSDP speakers (note: simplified view in this presentation) The speakers announce local active sources to the other domains, these forward the SAs If an announcement is received, and if local receivers exist, send a PIM-Join towards the announced source (in the other domain) MSDP operation D R D S MSDP /TCP D D R

15 MSDP operation D R D S MSDP_SA (, S, G) D D R MSDP operation D R D S D PIM_join (S, G) D R MSDP operation D R D S D D R 5 5

16 BGMP Main idea: build a global shared tree of domains The root domain can be determined from the group address Pure inter-domain protocol Needs support of an address allocation scheme/system/protocol (MASC, ++) 6 BGMP operation D (root) R D S D D R 7 BGMP operation D (root) R D S BGMP_join D PIM_join BGMP_join D R 8 6

17 BGMP operation D (root) R D S D D R 9 Property Intra-domain Generality Control Information Forwarding the control information Data Forwarding Join latency State Comparison MSDP PIM-SM Only Flooding of SA over the whole MSDP tree Bidirectional, between the MSDP peers, periodic Unidirectional, PIM-SM Low when caching, high when not Medium to high 50 BGMP Full Joins/Prunes where needed Bidirectional, between the border gateways, triggered Bidirectional between the domains, any within Low Low to medium Reliable multicast Needed for many new applications, but difficult: ACK implosion End-to-end argument seems to does not work with multicast more intelligence in the network? 5 7

18 Future of multicast Multicast will be an integral part of the Internet a ubiquitous service if there is use for it multicast will become widespread if a functional, scalable reliable multicast transport protocol is ever to be constructed use today: Real-time media streaming (unreliable streams) use tomorrow: reliable services (e.g. auctions, distribution lists) 5 Summary Algorithms: optimal multicast NP complete problem good behavior in practice using simple means SPT vs. CBT Protocols: host/network, network and inter-domain protocols flooding vs. explicit messaging IP multicast addressing issues Scalability, manageability and hierarchical solutions References: IEEE Network, Jan/Feb 000, thematic issue on multicasting! 5 8