Multicast Routing Protocols

Many applications transmit the same data at one time to multiple receivers Broadcasts of Radio or Video Videoconferencing Shared Applications A network must have mechanisms to support such applications in an efficient manner Applications with multiple receivers

Multicasting Multicast communications refers to one-to-many communications. IP Multicasting refers to the implementation of multicast communication in the Internet Multicast is driven by receivers: Receivers indicate interest in receiving data Unicast Broadcast Multicast

Multicast Groups The set of receivers for a multicast transmission is called a multicast group A multicast group is identified by a multicast address A user that wants to receive multicast transmissions joins the corresponding multicast group, and becomes a member of that group After a user joins, the network builds the necessary routing paths so that the user receives the data sent to the multicast group

Multicasting over a Packet Network Without support for multicast at the network layer: Multiple copies of the same message is transmitted on the same link

Multicasting over a Packet Network With support for multicast at the network layer: Requires a set of mechanisms: (1) Packet forwarding can send multiple copies of same packet (2) Multicast routing algorithm which builds a spanning tree (dynamically)

Semantics of IP Multicast Multicast groups are identified by IP addresses in the range 224.0.0.0 - 239.255.255.255 (class D address) Every host ( more precisely: interface) can join and leave a multicast group dynamically no access control Every IP datagram send to a multicast group is transmitted to all members of the group no security, no “floor control” Sender does not need to be a member of the group The IP Multicast service is unreliable

The IP Protocol Stack IP Multicasting only supports UDP as higher layer There is no multicast TCP ! Network Interface User Layer IP IP Multicast UDP TCP Socket Layer Stream Sockets Datagram Sockets Multicast Sockets

IP Multicasting There are three essential components of the IP Multicast service: IP Multicast Addressing IP Group Management Multicast Routing

Multicast Addressing All Class D addresses are multicast addresses: Multicast addresses are dynamically assigned. An IP datagram sent to a multicast address is forwarded to everyone who has joined the multicast group If an application is terminated, the multicast address is (implicitly) released.

Types of Multicast addresses The range of addresses between 224.0.0.0 and 224.0.0.255, inclusive, is reserved for the use of routing protocols and other low-level topology discovery or maintenance protocols Multicast routers should not forward any multicast datagram with destination addresses in this range. Examples of special and reserved Class D addresses, e.g,

Multicast Address Translation In Ethernet MAC addresses, a multicast address is identified by setting the lowest bit of the “most left byte” Not all Ethernet cards can filter multicast addresses in hardware - Then: Filtering is done in software by device driver.

IGMP The Internet Group Management Protocol (IGMP) is a simple protocol for the support of IP multicast. IGMP is defined in RFC 1112. IGMP operates on a physical network (e.g., single Ethernet Segment. IGMP is used by multicast routers to keep track of membership in a multicast group. Support for: Joining a multicast group Query membership Send membership reports

A host sends an IGMP report when it joins a multicast group (Note: multiple processes on a host can join. A report is sent only for the first process). No report is sent when a process leaves a group A multicast router regularly multicasts an IGMP query to all hosts (group address is set to zero). A host responds to an IGMP query with an IGMP report . Multicast router keeps a table on the multicast groups that have joined hosts. The router only forwards a packet, if there is a host still joined. Note: Router does not keep track which host is joined. IGMP Protocol

IGMP Packet Format IGMP messages are only 8 bytes long Type: 1 = sent by router, 2 = sent by host

Networks with multiple multicast routers Only one router responds to IGMP queries ( Querier ) Router with smallest IP address becomes the querier on a network. One router forwards multicast packets to the network ( Forwarder ) .

Multicast Routing Protocols Goal: Build a spanning tree between all members of a multicast group

Multicast Routing protocols Source based tree DVMRP Reverse path forwarding (RPF) Reverse path Broadcasting (RPB) Reverse path multicasting (RPM) MOSP Core based Tree (CBT) PIM ( Protocol Independent Multicast) PIM-DM PIM-SM

Objectives Every member should receive only one copy Non members should not receive a copy There should be no loops Source to destination must be shortest path

Multicast routing as a graph problem Problem : Embed a tree such that all multicast group members are connected by the tree

Multicast routing as a graph problem Problem : Embed a tree such that all multicast group members are connected by the tree Solution 1: Shortest Path Tree or source-based tree Build a tree that minimizes the path cost from the source to each receiver Good tree if there is a single sender If there are multiple senders, need one tree per sender Easy to compute

Multicast routing as a graph problem Problem : Embed a tree such that all multicast group members are connected by the tree Solution 2: Minimum-Cost Tree Build a tree that minimizes the total cost of the edges Good solution if there are multiple senders Very expensive to compute (not practical for more than 30 nodes)

Multicast routing in practice Routing Protocols implement one of two approaches: Source Based Tree: Essentially implements Solution 1. Builds one shortest path tree for each sender Tree is built from receiver to the sender  reverse shortest path / reverse path forwarding Core-based Tree: Build a single distribution tree that is shared by all senders Does not use Solution 2 (because it is too expensive) Selects one router as a “core” (also called “rendezvous point”) All receivers build a shortest path to the core  reverse shortest path / reverse path forwarding

Multicast Routing table Routing table entries for source-based trees and for core-based trees are different Source-based tree : (Source, Group) or (S, G) entry. Core-based tree: (*, G) entry. I1, I3 I2 G2 * I2, I3 I1 G1 S1 Outgoing interface list Incoming interface (RPF interface) Multicast group Source IP address

Multicast routing in practice Routing algorithms in practice implement one of two approaches: Source Based Tree Tree: Establish a reverse path to the source Core-based Tree: Establish a reverse path to the core router

Reverse Path Forwarding (RPF) RPF builds a shortest path tree in a distributed fashion by taking advantage of the unicast routing tables. Main concept: Given the address of the root of the tree (e.g., the sending host), a router selects as its upstream neighbor in the tree the router which is the next-hop neighbor for forwarding unicast packets to the root. This concept leads to a reverse shortest path from any router to the sending host. The union of reverse shortest paths builds a reverse shortest path tree . RPF Forwarding: Forward a packet only if it is receives from an RPF neighbor

Building a source-based tree Set routing tables according to RPF forwarding Flood-and-Prune

Building a source-based tree Set routing tables according to RPF forwarding Flood-and-Prune Flood= Forward packets that arrive on RPF interface on all non-RPF interfaces

Building a source-based tree Set routing tables according to RPF forwarding Flood-and-Prune Flood= Forward packets on all non-RPF interfaces Receiver drops packets not received on RPF interface

Building a source-based tree Set routing tables according to RPF forwarding Flood-and-Prune Prune= Send a prune message when a packet is received on a non-RPF interface or when there are no receivers downstream Prune message disables routing table entry

Pruning Prune message temporarily disables a routing table entry Effect : Removes a link from the multicast tree No multicast messages are sent on a pruned link Prune message is sent in response to a multicast packet Question: Why is routing table only temporarily disabled? Who sends prune messages? A router with no group members in its local network and no connection to other routers (sent on RPF interface) A router with no group members in its local network which has received a prune message on all non-RPF interfaces (sent on RPF interface) A router with group members which has received a packet from a non-RPF neighbor (to non-RPF neighbor)

Building a source-based tree When a receiver joins, one needs to re-activate a pruned routing table entry Grafting Sending a Graft message disables prune, and re-activates routing table entry.

Alternative method for building a source-based tree This only works if the receiver knows the source Explicit-Join Receiver sends a Join message to RPF neighbor Join message creates (S,G) routing table entry Join message is passed on

Building a core-based tree One route is the core Receiver sends a Join message to RPF neighbor with respect to core Join message creates (*, G) routing table entry

Building a core-based tree Source sends data to the core Core forwards data according to routing table entry

Multicast routing protocols in the Internet Distance Vector Multicast Routing Protocol (DVMRP): First multicast routing protocol Implements flood-and-prune Multicast Open Shortest Path First (MOSPF): Multicast extensions to OSPF. Each router calculates a shortest-path tree based on link state database Not widely used Core Based Tree (CBT): First core-based tree routing protocol Protocol Independent Multicast (PIM): Runs in two modes: PIM Dense Mode (PIM-DM) and PIM Sparse Mode (PIM-SM). PIM-DM builds source-based trees using flood-and-prune PIM-SM builds core-based trees as well as source-based trees with explicit joins.

PIM Messages (PIM version 2) Encapsulated in IP datagrams with protocol number 103. PIM messages can be sent as unicast or multicast packet 224.0.0.13 is reserved as the ALL-PIM-Routers group  8 Candidate-RP-Advertisement  7 Graft-Ack  6 Graft   5 Assert  4 Bootstrap   3 Join/Prune  2 Register-Stop  1 Register   0 Hello PIM-SM PIM-DM Type PIM-DM messages

PIM-DM: PIM Dense Mode PIM-DM implements flood-and-prune Orange packet: Multicast packet (=Data) Blue packet: PIM message

PIM-SM: PIM Sparse Mode Core is called rendezvous-point (RP) Receivers know RP (statically configured or dynamically elected) When receiver joins, a Join message is sent to RP on RPF.

PIM-SM: PIM Sparse Mode Host H3 joins: Join message is only forwarded until the first router that is part of the core-based tree.

PIM-SM: Data transmission Source sends multicast packet to RP Packet is attached to an RP Register message When packet reaches RP, it is forwarded in the tree Also: RP sends a Join message on reverse path to S1

PIM-SM: Data transmission When Join messages reaches R1, it sends a native multicast packet to the RP (in addition to the packet attached to the register message)

PIM-SM: Data transmission When RP receives native multicast packet it sends a register stop message to R1. This message stops the transmission of register messages from R1.

PIM-SM: Switching to source-based tree When data to receivers exceeds a threshold, routers switch to a source-based tree This is done by sending an explicit join message to the source There may be duplicate packets being sent for some time

PIM-SM: Switching to source-based tree When data arrives from source (as opposed to RP), a Prune message is sent to the RPT Now: data is forwarded only along the shortest-path tree

Multicast Routing Protocols

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Multicast Routing Protocols