IP Multicast Angelos Vassiliou HMY 654

Overview Definitions Multicast routing Concepts IP Multicast Protocols

Definitions A multicast group is a set of receivers with a common interest. A source is an end user that originates a data stream. A receiver is an end user wishing to receive a data stream.

ASM vs. SSM Any source multicast (ASM) model - no limit on the number of multicast groups, or on the number of sources and receivers for a group. Source specific multicast (SSM) allows, for a given group g, a receiver to select the specific sources for this g from which it will receive a stream.

A sample network

Multicast group g, ASM

Multicast group g, SSM

Multicast routing Concepts Reverse Path Forwarding Shared Trees Source Trees Network coding

Broadcast

Not a good solution Result: Broadcast Storm Solutions: Switched LANs: Spanning Trees

Spanning Tree

Routed Networks Solution: Forbid / ignore Broadcasts. In case of dense Multicasting, all routers require multicast traffic. Links cannot be shut down in routed WANs. Routing protocols need some time to converge. Simple and viable solution: RPF

RPF Consider a source S for group g that is sending packets to a set of receivers that have joined g. Suppose some node N receives a packet from a neighbour C and must decide what to do with the packet. The reverse path forwarding (RPF) check is: if the shortest path from N to S has neighbour C as its next hop, the check passes, and N should forward the packet on each of the arcs specified in its multicast forwarding table for group g; otherwise, the check fails and N should discard the packet.

RPF: Reverse Path forwarding C’s Routing Table DestinationNext Hop SA B’s Routing Table DestinationNext Hop SG N’s Routing Table DestinationNext Hop SA

Source Trees Group g 1, source s 1, behind router S G,N,K,D belong in g 1

Source Trees Each router in the source tree must create a routing table, containing entries regarding (s 1,g 1 ) routing A’s Routing Table (s,g) pairNext Hop s1,g1s1,g1 N s1,g1s1,g1 C C’s Routing Table (s,g) pairNext Hop s1,g1s1,g1 K

Source Trees Suppose a host off of router F wants to also send to group g 1 A new source tree is created A’s Routing Table (s,g) pairNext Hop s1,g1s1,g1 N s1,g1s1,g1 C s2,g1s2,g1 S C’s Routing Table (s,g) pairNext Hop s1,g1s1,g1 K s2,g1s2,g1 K s2,g1s2,g1 A

Source Trees vs. Shared Trees When the number of sources and groups is large, considerable memory can be required at each node to store, for each (s, g) tree touching the node, the (s, g) pair, the interface to the RPF neighbour, and the outgoing interface list. The memory requirement can be greatly reduced by using shared trees.

Shared Tree A shared tree for a group g is a single tree used by all sources for g. A core node (Rendezvous Point) has to be selected.

Shared Trees When a new node joins the group or a new source needs to send, it joins the tree. Sub-optimal routing: Suppose a source exists off node D. Multicast traffic to node N must travel the D-K-C-A-N path. Shared trees solve the Scalability issues of Source Trees

Network Coding Example S 1 generates message x and S 2 generates message y t 1 and t 2 want to receive both messages. a message needs one time unit to traverse an arc (link)

Network Coding 3 time units are necessary for message x to reach node t 2

Network Coding 3 time units are necessary for message y to reach node t 1

Network Coding XOR is the simplest example of a network coding scheme 3 time units are necessary for both messages y and x to reach both nodes t 1 and t 2

IP Multicast Protocols Internet Group Management Protocol (IGMP) Multicast Listener Discovery (MLD) Multiple Registration Protocol (MRP) Protocol Independent Multicast (PIM) (and flavors) Distance Vector Multicast Routing Protocol (DVMRP) Multicast Open Shortest Path First (MOSPF) Multicast BGP (MBGP) Multicast Source Discovery Protocol(MSDP) Multicast DNS (mDNS)

3 main functions Multicast Addressing – Multicast address defines a multicast group of hosts Multicast Group Management – Mostly IGMP – MLD for IPv6 LANs Multicast Datagram processing and routing – Many routing protocols have been developed, deployed and “standardized” – More issues than unicast routing protocols

Multicast Datagram routing protocols IGP – DVMPR – MOSPF EGP – MBGP – MSDP PIM

IGMP used by hosts and adjacent routers to establish multicast group memberships. Hosts let their gateway (subnet router) to know they are interested in joining a Multicast group.

Multicast Listener Discovery (MLD) RFC 4604 defines MLDv2 used by IPv6 routers for discovering multicast listeners Multicast Listener Query - Sent by a multicast router to poll a network segment for group members. Queries can be general, requesting group membership for all groups, or can request group membership for a specific group. Multicast Listener Report - Sent by a host when it joins a multicast group, or in response to an MLD Multicast Listener Query sent by a router. Multicast Listener Done - Sent by a host when it leaves a host group and is the last member of that group on the network segment.

Multiple Registration Protocol (MRP) Registration framework defined by the IEEE 802.1ak amendment to the IEEE 802.1Q standard. MRP allows switches or similar devices to register and de-register attribute values, such as VLAN identifiers and multicast group membership across a large LAN. MRP operates at the Data Link Layer. The information distributed using MRP is used by “smart” switches. Better bandwidth utilization

MRP example Switches use IGMP snooping to recognize to which port they should forward multicast traffic They use MRP to exchange multicast Layer 2 switching information

PIM PIM Sparse Mode (PIM-SM) PIM Dense Mode (PIM-DM) Bidirectional PIM PIM source-specific multicast (PIM-SSM)

PIM Sparse Mode (PIM-SM) explicitly builds unidirectional shared trees rooted at a rendezvous point (RP) per group, and optionally creates shortest-path trees per source. Sparse mode means that the protocol is designed for situations where multicast groups are thinly populated across a large region. Sparse-mode protocols are most efficient over WANs.

PIM-SM source Join RP is chosen RP tree is built

PIM-SM group join Host A Joins group G Traffic goes trough RP

PIM-SM source tree setup After a threshold is reached, RP prunes itself from the shared tree Source tree is set up

PIM Dense Mode (PIM-DM) Dense protocols follow a flood-and-prune model. To inform the routers of multicast sources, this traffic is initially broadcast throughout the domain. Upon first receiving traffic to a dense group on its interface closest to the source, a router forwards this traffic out all of its interfaces except the interface on which it received the data. If traffic is received on the interface that is not the RPF interface toward the source, the traffic is discarded, and a Prune message is sent upstream. If a router has no interested receivers for the, it sends a Prune upstream. Periodic reflooding is used to refresh state. Wasteful but simple

Bidirectional PIM Builds shared bi-directional trees. It never builds a shortest path tree, so may have longer end-to-end delays than PIM-SM Scales better than PIM-SM because it needs no source-specific state

PIM source-specific multicast (PIM-SSM) Builds trees that are rooted in just one source, offering a more secure and scalable model for a limited amount of applications (mostly broadcasting of content). In SSM, an IP datagram is transmitted by a source S to an SSM destination address G, and receivers can receive this datagram by subscribing to channel (S,G)

Distance Vector Multicast Routing Protocol (DVMRP) Dense protocol Forms the basis of the Internet's multicast backbone (MBONE) based on RIP Does not scale well All routers in the network need global information about all multicast groups and their sources. Uses distance metrics (router hops by default). Maintains a table with all the router interfaces connected to a host interested in the group. most DVMRP deployments have been replaced by PIM-SM standard flood-and-prune behaviour

MOSPF RFC 1584 defines Multicast Extensions to OSPF For a given multicast datagram, all routers calculate an identical shortest-path tree. There is a single path between the datagram's source and any particular destination group member. Link State Not widely deployed

Multiprotocol Extensions for BGP (MBGP) defined in IETF RFC 4760 as an extension to Border Gateway Protocol enables the exchange of inter-domain multicast routing information other protocols (such as PIM) are needed to build trees and forward multicast traffic

Multicast Source Discovery Protocol (MSDP) a mechanism to connect multiple PIM sparse-mode (PIM-SM) domains

MSDP operation overview 1.When a source's first data packet is registered by the first-hop router, that same data packet is decapsulated by the RP and forwarded down the shared tree. 2.That packet is also re-encapsulated in a Source-Active (SA) message that is immediately forwarded to all MSDP peers. 3.The SA message identifies the source, the group the source is sending to, and the RP's own address 4.If the peer is an RP and has a member of that multicast group, the data packet is decapsulated and forwarded down the shared-tree in the remote domain. 5.Each MSDP peer receives and forwards the SA message away from the originating RP to achieve "peer-RPF flooding." 6.The concept of peer-RPF flooding is with respect to forwarding SA messages. 7.The router examines the BGP or MBGP routing table to determine which peer is the next hop toward the originating RP of the SA message.

MSDP

Multicast DNS (mDNS) A multicast application Suitable for environments where there in no central DNS server Each host looking for the IP address of a local host, multicasts a mDNS query The host that owns the queried hostname, multicasts a mDNS response with its IP addess Uses well-known multicast address Why not simply broadcast?

Broadcast

Multicast

References A Primer of Multicast Routing by Eric Rosenberg. Springer Press 2012 edition Network Information Flow. by Ahlswede R, Cai N, Li SYR, Yeung RW(2000)published in IEEE Transactions on Information Theory 46: Interdomain Multicast Routing: Practical Juniper Networks and Cisco Systems Solutions by Brian M. Edwards, Leonard A. Giuliano, Brian R. Wright. Addison-Wesley Professional 1st edition May, 2002 Wikipedia

12_0t7/feature/guide/msdp.html