Communication Networks Recitation 11

Multicast & QoS Routing

The Problem Traditional Unicast model does not scale –Millions of clients –Server and network meltdown

Solution: IP Multicast Source sends single stream Routers split stream towards all clients Guarantee only one copy in each link

Multicast Routing Tree On tree relay router Router with directly attached group members IGMP Multicast Routing Protocol

Internet Group Management Protocol (IGMP) Used by routers to learn about Multicast Group Memberships on their directly attached subnets Implemented over IP Designated Router –Each network has one Querier –All routers begin as Queriers –The router with the lowest IP address chosen

How IGMP Works one router is elected the “querier” querier periodically sends a Membership Query message to the all-systems group ( ), with TTL = 1 on receipt, hosts start random timers (between 0 and 10 seconds) for each multicast group to which they belong Qrouters: hosts:

How IGMP Works (cont.) when a host’s timer for group G expires, it sends a Membership Report to group G, with TTL = 1 other members of G hear the report and stop their timers routers hear all reports, and time out non-responding groups Q GGGG

Type of Service (TOS) Routing “ low delay ” “ high throughput ” Does not support real QoS

Multicast Tree with QoS QoS constraints –Link: minimum bandwidth; available buffer space. –Tree constraints: end-to-end delay; jitter. Optimization objectives –Link: maximize bandwidth. –Tree optimization: minimize the cost.

Core-Based Trees (CBT) Core-based multicast routing: –One router is selected as the core for each multicast group. –A tree rooted at the core spans all group members. –Data packets are forwarded on all on-tree interfaces except the one on which packets arrive.

CBT Multicast Routing Core On tree relay router On tree router Router with directly attached group member Sender

Member Join in CBT Core Requesting router with a new member join-request join-ack

QoS-Aware Member Join Core On tree relay router On tree group router join-request u v Eligibility Test Only after the join-request passes the eligibility tests will a join-acknowledgement be returned.

Shortest Path Tree (SPT) Source Based Tree: Rooted at the source, composed of the shortest paths between the source and each of the receivers in the multicast group. If the routing metric used is the latency between neighbors, the resulted tree will minimize delay over the multicast group. Example: DVMRP.

Distance-Vector Multicast Routing Protocol (DMVRP) DVMRP consists of two major components: (1) a conventional distance-vector routing protocol (like RIP) (2) a protocol for determining how to forward multicast packets, based on the routing table and routing messages of (1)

Example Topology gg s g

Phase 1: Flooding gg s g

Phase 2: Pruning gg s prune (s,g) g

Steady State gg s g g

graft (s,g) Joining on New Receivers gg s g g report (g)

Steady State after Joining gg s g g

Steiner Minimal Tree (SMT) Shared Tree: All sources use the same shared tree. SMT is defined to be the minimal cost subgraph (tree) spanning a given subset of nodes in a graph Approximate SMT: KMB

An example of a Steiner Tree A B DG H I C F KJ E Mcast group members Relay Nodes *

Step 1: Construct a complete directed distance graph G 1 =(V 1,E 1,c 1 ). Step 2: Find the min spanning tree T 1 of G 1. Step3: Construct a subgraph G S of G by replacing each edge in T 1 by its corresponding shortest path in G. Step 4: Find the min spanning tree T S of G S. Step 5: Construct a Steiner tree T H from T S by deleting edges in T S if necessary, so that all the leaves in T H are Steiner points. KMB Algorithm

Due to [Kou, Markowsky and Berman 81 ’ ] Worst case time complexity O(|S||V| 2 ). Cost no more than 2(1 - 1/l) *optimal cost where l = number of leaves in the steiner tree. KMB Algorithm Cont.

KMB Example A C D B A C D B A B CD EF G H I /2 2 1 B CD E F G H I A Destination Nodes Intermediate Nodes

KMB Example Cont. B CD E F G H I /2 2 A B CD E F I A Destination Nodes Intermediate Nodes

Protocol Independent Multicast (PIM)  PIM-DM (Dense Mode) uses RPM.  PIM-SM (Sparse Mode) designed to be more efficient than DVMRP. –Routers explicitly join multicast tree by sending unicast Join and Prune messages. –Routers join a multicast tree via a RP (Rendezvous Point) for each group. –Several RPs per domain (picked in a complex way). –Provides either: l Shared tree for all senders (default). l Source-specific tree.

PIM-SM Example R2 R1 RP IGMP Join Unicast to RP: (*, G) RP knows R1 has joined R2 learns to send (*,G) packets to R1 Sender/Source S Unicast to R3: (S,G) Source knows to send all G packets to RP. Source “tunnels” mcast-in-ucast packets to RP. RP unwraps mcast pkt and forwards to local tree.

PIM-SM Example R2 R1 RP Sender/Source S Optional Source- specific join to bypass RP router: Routers along the way learn new path for (S,G).