CS 716 Advanced Computer Networks By Dr Amir
CS 716 Advanced Computer Networks By Dr. Amir Qayyum
Lecture No. 27 2
Multicast 3
Internetworking • Basics of internetworking (heterogeneity) – IP protocol, address resolution, control messages … • Routing • Global internets (scale) – Virtual geography and addresses – Hierarchical routing • Future internetworking: IPv 6 • Multicast traffic • MPLS 4
Internet Multicast Outline • • • Motivation and challenges Support strategy IP multicast service model Multicast in the Internet Routing – Review of ELAN techniques – Multicast routing • Limitations 5
Multicast • • Unicast: one destination Broadcast: all destinations Multicast: subset of destinations When is multicast useful ? – Send data to multiple receivers at once • Videoconferencing, video-on-demand, telecollaboration • Software update to group of customers – Limited broadcast/self-defined multicast • Send question to unknown receiver • Resource discovery; Distributed database 6
Multicast • Why not just use broadcast/unicast ? – Broadcast not supported outside of LAN – Unicast sends multiple copies across common links • Multicast support – Often supported by hardware in LAN’s (as broadcast, if not multicast) – But difficult to extend in scalable manner • Multicast challenges – Efficient distribution on an internetwork – Specification of recipient group (abstraction must support self-definition) 7
Multicast Support Strategy • IPv 4 used as basis for experimental solutions – Use class D addresses (1110 <28 bits>) – Demonstrated with MBone – Uses tunneling • Multicast integrated into IPv 6 • Internet Group Management Protocol (IGMP) • Several routing/forwarding schemes: – Distance-vector – Link-state – Protocol-independent 8
IP Multicast Service Model • Each group uses a single address – Class D addresses (1110 <28 bits>) – Some are well-known, some are dynamically assigned • Group membership – Members located anywhere in the Internet – Number of receivers is arbitrary – Members can join/leave dynamically – Hosts can belong to more than one group 9
IP Multicast Service Model • Senders simply use group address as destination – Sender need not be in group – LAN loopback needed for sender in group • Multicast scope – LAN (local scope) – Administrative scope (e. g. campus), may overlap, can assign group addresses dynamically – TTL scope (no more than N hops) • Scope is exposed to protocols and applications (by 10 exposing IP TTL)
IP Multicast Service Model • Multicast reception requires membership in group – Internet Group Management Protocol (IGMP), RFC 1112 – New operations to join and leave group – LAN routers track local membership – Forwarding depends on routing scheme – Last hop typically uses LAN broadcast • Packet reception same as IP unicast 11
Internet Multicast Backbone - MBone • • Existing infrastructure for multicast in the Internet Multicast route propagation using DVMRP Problem: most IP routers do not support multicast Solution: tunneling by multicast-capable routers – Encapsulate multicast traffic in IP packets – Send to other multicast-capable routers – Recipients unpack & forward original multicast packet • Passes through multicast-incapable areas of Internet 12
ELAN Multicast Techniques • Direct support (Ethernet) – Application subscribes to group – IP layer notifies Ethernet card to listen to packets with group address • Support through broadcast (LANE) • Flooding in ELANs – Each packet sent on all but incoming link – Switches must remember each packet! • Spanning tree: every host gets one copy 13
ELAN Multicast Techniques • Spanning tree selection – Elect a leader; spanning tree is shortest path to leader (Perlman) – Distribute topology everywhere, compute in parallel (link-state) • Problems with spanning trees – Bandwidth wasted for groups with few receivers; Solution: prune LAN’s with no receivers from tree – For very large ELAN’s, no single tree is efficient; Solution: define tree per group or tree per source • The same solutions are used in the Internet! 14
Spanning Tree Tradeoffs • Tree per group or tree per source ? • Per group advantage – One routing entry per group • Per source advantages – More efficient distribution – Spreads load better across links – Leverage unicast routing tables 15
Multicast Routing in the Internet • Multicast Open Shortest Path First (MOSPF) • Distance-Vector Multicast Routing Protocol (DVMRP, used in MBONE) • Protocol-Independent Multicast (PIM) – Deals with scalability issues of above protocols – Dense Mode (PIM-DM) 16 – Sparse Mode (PIM-SM)
Multicast Routing in the Internet • How do senders find receivers? – Receivers inform all senders of interest (MOSPF) – Send to all receivers; uninterested receivers prune (DVMRP, PIM-DM) – Agree on set of rendezvous points (PIM-SM) • Types of distribution trees – Separate tree from each sender (DVMRP, MOSPF, PIM-DM, PIM-SM) – Tree rooted at rendezvous point (PIM-SM) 17
Link State Multicast (MOSPF) • Each host on a LAN – Periodically announces its group memberships, via Internet Group Management Protocol (IGMP) • Extend LSP to include set of groups with members on a given LAN • MOSPF routing extends OSPF – Uses Dijkstra’s algorithm – Computes shortest-path spanning tree for sourcegroup pairs – Forward packet on local portion of tree 18
Link State Multicast (MOSPF) • Tree computation – Can’t precompute for all source-group pairs – Compute on demand when first packet from a source S to a group G arrives – Cache trees for active source-group pairs – Recompute when link-state changes • Scalability limitations – Reasonable intra-AS scalability – But meaningless for inter-AS – Source-group pairs scale with sources (needs to be 19 hierarchical)
Distance Vector Multicast (DVMRP) • Idea – Graph of directed next-hop edges to a destination S form a tree – Use reverse edges to broadcast from S • Implementation (Reverse Path Broadcast, or RPB) – Forward multicast packet on all links – If and only if packet came from next hop for packet source • Avoid repetition on LAN’s – Assign parent router for each LAN – Has shortest path to source, ties broken by ID – Track parenthood via vector exchanges 20
RPB and RPM M M G M RPM from S to G M Member of multicast group G RPB from S S Unicast route to S Pruned 21
RPB to RPM (reverse path multicast) • Identify leaf networks – Only one router on network – Thus no distance packets received on interface • Prune leaf networks – Without hosts in a group – Hosts must self-identify using IGMP • Forward pruning information – Extend distance vector with group information – Forward packets only to interested parties – Only when multicast source active 22
Distance Vector Multicast RPM Implementation • Assume that everyone is interested • Respond to unwanted packets with prune requests • Prune requests – Canceled by graft request – Time out periodically • Need ARQ for prune or graft ? 23
Distance Vector Multicast - Scalability • Packets are periodically broadcast (thus guaranteed to reach all interested members) • High overhead for sparse groups, consider: – Multicast group of 10 members – Scattered around the world – Packets periodically reach all routers in Internet • High overhead for routers – All off-tree routers maintain pruning state – And periodically retransmit 24
Protocol Independent Multicast (PIM) • Approach – Define rendezvous points (RP) for each group – Need multiple RP’s to handle failures • Two versions – Dense mode • Explicit prune messages • Shared tree – Sparse mode • Explicit join messages • Shared or source-specific tree 25
Protocol Independent Multicast (PIM) • Rendezvous points (RP) for each multicast group RP RP Specific multicast tree RP S 26
Protocol Independent Multicast • Joins – Receiver: send packet to one RP – Source: send to all RP’s • Tree selection – Rooted at rendezvous points – Shared for infrequent traffic – Source-specific if merited by traffic level 27
Limitations on Multicast • Scalability (addressed to some extent by PIM) – Explosive growth of the Internet population – Explosive growth of multicast, multimedia applications • Control of network resources – Applications have different performance needs – Different resource commitments by clients and/or organizations – Different ASs provide different Qo. S … 28
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