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IP Multicasting

Part of this presentation based on: Cisco, Forouzan TCP/IP textbook, Anthony Chung, Dale Buchholz.

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Topics IP Class D Group Addressing IGMP (Internet Group Management Protocol) Multicast Strategies:

Broadcast/Flooding Multiple Unicast Distribution trees Multicast forwarding (based on Reverse Path Forwarding)

Multicast Routing Protocols DVMRP (Distance Vector Multicast Routing Protocol) MOSPF (Multicast OSPF) PIM (Protocol Independent Multicast)

MBONE (Multicast Backbone) Implementation and Configuration (for Cisco)

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Practical Applications of IP Multicast

Multimedia: A number of users “tunes” to a video or audio transmission from a multimedia source station.

Teleconferencing: A group of workstations form a multicast group such that a transmission from any member is received by all other group members.

Database: All copies of a replicated file or database are updated at the same time.

Distributed computation: Intermediate results are sent to all participants.

Real-time workgroup: Files, graphics, and message are exchanged among active group members in real time.

MOST prevalent usage right now: trading – Used to multicast market data.

Can be VERY large; OPRA (Options Price Reporting Authority) feed is over 80 Mbps SUSTAINED

Very latency sensitive. Trading platforms are trying to optimize at the micro-second levels.

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Multicasting within single LAN

In 802.3, MAC address can specify a broadcast/multicast 0xFFFF.FFFF.FFFF is for broadcast Bit 0 of octet 0 indicates Broadcast/Multicast Bit 1 of octet 0 indicates Locally Administer Address (LAA) vs use of

Universally Administered Addresses (UAA) IANA was given MAC address block 01:00:5E IANA defined range

available: 0100.5E00.0000 to 0100.5E7F.FFFF – That is 23 bits

Multicast IP address is defined by 28 bits (1110 defines class D) The lowest 23 bits of the IP multicast address maps to a MAC address All stations that recognise the MAC address of the multicast address

accept the packet – Done at L2 admission instead of L3 like it would be in broadcast. (a broadcast is accepted by all stations, then sent to L3 for processing where it will be dropped if no process uses it)

Works because of broadcast nature of LAN Packet only sent once Much harder on Internet or routed infrastructure

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IP Class D Group Addressing (multicast address or group ID)

Mapping IP Multicast Address to Ethernet Address

Figure 10.10

This prefix is given for multicast MAC addresses

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MAC address Ambiguities Because upper 5 bits in IP multicast address are

dropped, result MAC address is not unique: 32 multicast group IDs map to a single MAC address.

Example all groups below maps to 0x0100.5E01.0101

224.1.1.1 224.129.1.1 225.1.1.1 225.129.1.1 … 238.1.1.1 238.129.1.1 239.1.1.1 239.129.1.1

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Multicast Strategies across routers

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Multicast Strategies across routers

Broadcast Multiple Unicasts True Multicast

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Broadcast Assume location of recipients not know Send packet to every network Packet addressed to N3 traverses N1,

link L3, N3 Router B translates IP multicast address

to MAC multicast address Repeat for each network Generates lots of packets Delivery of packets done via Routing

Protocols

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Multicast Strategies Broadcast: send one copy to each network (not knowing which network has members)

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Multiple Unicast

Location of each member of multicast group known to source

Table maps multicast address to list of networks

Only need to send to networks containing members of multicast group

Reduced traffic (a bit)

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Multicast Strategies Multiple Unicast: send one copy to each network where there is a member.

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True Multicast Least cost path from source to each

network containing member of group is determined Gives spanning tree configuration

For networks containing group members only

Source transmits packet along spanning tree

Packet replicated by routers at branch points of spanning tree

Reduced traffic

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Multicast Strategies Multicast along a spanning tree (A sub-graph that reaches all nodes without cycles) to networks with members only

S

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Multicast Transmission Example

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Requirements for Multicasting

Router must forward one or more copies of incoming packet

Addressing IPv4 uses class D

Start 1110 plus 28 bit group id IPv6 uses 8 bit prefix of all 1s, 4 bit flags field, 4 bit scope

field 112 bit group id Router must translate between IP multicast

address and list of networks containing members of group – router must keep track of membership subscriptions and tree.

Multicast addresses may be permanent or dynamic

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Requirements for Multicasting

Individual hosts may join or leave dynamically Need mechanism to inform routers

Routers exchange information on which subnets contain members of groups

Routers exchange information to calculate shortest path to each network

Need routing protocol and algorithm Routes determined based on source and

destination addresses Avoids unnecessary duplication of packets

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IGMP Operation

Host uses IGMP (Internet Group Management Protocol) to make itself know as member of group to other hosts and routers

To join, send IGMP membership report message Send to multicast destination of group being joined

Routers periodically issue IGMP query To all-hosts multicast address Hosts respond with report message for each group to which it

belongs Only one host in group needs to respond to keep group alive Host keeps timer and responds if no other reply heard in time

Host sends leave group message Group specific query from router determines if any members

remain Note IGMP v1 did not include “leave”: Multicast would be sent until

a timer expires

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IGMP – a protocol designed to help a multicast router identify the hosts in a LAN that are the members of a group.

Figure 10.2 IGMP (v2) message types

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IGMP – a protocol designed to help a multicast router identify the hosts in a LAN that are the members of a group.

From routers to hosts. Asking for group membership info

From hosts to routers. Reporting group membership info

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Reserved Link Local Addresses

IANA reserved 224.0.0.0 through 224.0.0.255 for routing protocols on local networks. These packets should NEVER be forwarded and should have TTL of 1.

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IP multicast address Description

224.0.0.0 Base address (reserved)

224.0.0.1The All Hosts multicast group that contains all systems on the same

network segment

224.0.0.2The All Routers multicast group that contains all routers on the same

network segment

224.0.0.5The Open Shortest Path First (OSPF) AllSPFRouters address. Used to

send Hello packets to all OSPF routers on a network segment

224.0.0.6The OSPF AllDRouters address. Used to send OSPF routing information

to OSPF designated routers on a network segment

224.0.0.9

The RIP version 2 group address. Used to send routing information using the RIP protocol to all RIP v2-aware routers on a network segment

224.0.0.10EIGRP group address. Used to send EIGRP routing information to all

EIGRP routers on a network segment

224.0.0.13 PIM Version 2 (Protocol Independent Multicast)

224.0.0.18 Virtual Router Redundancy Protocol

224.0.0.19 - 21 IS-IS over IP

224.0.0.22 IGMP Version 3 (Internet Group Management Protocol)

224.0.0.102 Hot Standby Router Protocol Version 2

224.0.0.251 Multicast DNS address

224.0.0.252 Link-local Multicast Name Resolution address

224.0.1.1 Network Time Protocol address

224.0.1.39 Cisco Auto-RP-Announce address

224.0.1.40 Cisco Auto-RP-Discovery address

224.0.1.41 H.323 Gatekeeper discovery address

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Glop Addressing (RFC 2770) RFC 2770 (revised 3180) proposes that 233.0.0.0/8 be

reserved for static Multicast groups for organizations that have a reserved, registered public AS number.

That AS # form the 2nd and 3rd Octet of the IP range. Gives an AS owner 255 unique static and global

Multicast groups. Example: AS #62010

Decimal 62010 = 0xF23A F2 is 2nd octet = 242 3A is 3rd octet = 58 233.242.58.0 is globally reserved for AS 62010 to use

Question: what does “GLOP” stands for?

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Using IGMP, R learns about the list of groups on this LAN. When R receives a packet with a destination multicast address, it checks against the list and determine if the packet should be propagated to the LAN.

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Four situations of IGMP operations VERSION 1!!

(periodic)

(in response to a query)

(initially)

(time out) Router erase after no responses to 3 queries

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(A host waits for a random period of time before sending out a report)

(they hear that the group has been reported)

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IGMP v2 improvements

IGMPv2-Defines a procedure for the election of multicast queriers for each LAN (the router with the lowest IP address).-Group-specific query message-Leave group message (to lower IGMP’s “leave latency”)

IGMPv3- Enables hosts to listen only to a specified subset of the hosts sending to the group

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IGMP v2 and V3 improvements

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Multicast Distribution Trees

Multicast Distribution Trees Control the path which IP Multicast traffic takes through the network in order to deliver traffic to all receivers.

Source Trees

o A source tree with its root at the source and branches forming a spanning tree through the network to the receivers.

o Also referred to as a shortest path tree (SPT).

o Remind you of anything???

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o Advantage of creating the optimal path between the source and the receivers.

o The routers must maintain path information for each source.

Memory consumption: O(S * G)

Multicast Source Tree

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Shared Trees o A single common root placed at some chosen point in the network. This shared root is called a Rendezvous Point (RP).

o Advantage of requiring the minimum amount of state in each router. Memory requirements: O(G)

o The disadvantage of shared trees is that under certain circumstances the paths between the source and receivers might not be the optimal paths.

o The placement of the RP must be carefully considered.

o THIS IS THE PREFERED MANNER to implement Multicasts

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Multicast ForwardingIn unicast routing, a router does not really care about the source address.In multicast routing, the multicast router must determine which direction is upstream (towards the source) and which direction (or directions) is downstream.

Multicast Forwarding

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Reverse Path Forwarding (RPF) oAn approximation of the source tree.

oMake use of the existing unicast routing table to determine the upstream and downstream neighbors. A router will only forward a multicast packet if it is received on the upstream interface. This RPF check helps to guarantee that the distribution tree will be loop free.

Step 1.   Router looks up the source address in the unicast routing table to determine if it has arrived on the interface that is on the reverse path back to the source.

Step 2.   If packet has arrived on the interface leading back to the source, the RPF check is successful and the packet will be forwarded.

Step 3.   If the RPF check in 2 fails, the packet is dropped.

Multicast Forwarding

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Enhancement

Only forward to your neighbor on a link if the neighbor uses this link to reach the source.

o How does the node know?

For link state routing: easy

For distance vector routing

Modify to include next hop information in messages; or

“Poison reverse”

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Figure 15.7 Taxonomy of common multicast routing protocols

MOSPF (RFC 1584)•Extension to OSPF unicast routing protocol

•Includes multicast information in OSPF link state advertisements to construct multicast distribution trees

•Group membership LSAs are flooded throughout the OSPF routing domain so MOSPF routers can compute outgoing interface lists•Uses Dijkstra algorithm to compute shortest-path tree

•Separate calculation is required for each (S Net , G) pair (Source, Group address pair)•Data-driven – a router construct a tree the first time it sees a (S Net , G) pair.

JP note: Core Based Trees – Never seen live

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Multicast Routing Protocols

Dense-Mode (DM) Uses “Push” Model Traffic Flooded throughout network Pruned back where it is unwanted Flood & Prune behavior (typically every 3

minutes)

Sparse-Mode (SM) Uses “Pull” Model Traffic sent only to where it is requested Explicit Join behavior

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DVRMPv3 (Internet Draft)

Dense Mode Protocol Distance vector-based

Similar to RIP Infinity = 32 hops Subnet masks in route advertisements

DVMRP Routes used: For RPF Check To build Truncated Broadcast Trees (TBTs)

Uses special “Poison-Reverse” mechanism Uses Flood and Prune operation

Traffic initially flooded down TBT’s TBT branches are pruned where traffic is unwanted.

Prunes periodically time-out causing reflooding.

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Multicast Extension to OSPF (MOSPF) Enables routing of IP multicast

datagrams within single AS Each router uses MOSPF to maintain

local group membership information Each router periodically floods this to all

routers in area Routers build shortest path spanning

tree from a source network to all networks containing members of group (Dijkstra) Takes time, so on demand only

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Forwarding Multicast Packets

If multicast address not recognised, discard

If router attaches to a network containing a member of group, transmit copy to that network

Consult spanning tree for this source-destination pair and forward to other routers if required

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Equal Cost Multipath Ambiguities

Dijkstra’ algorithm will include one of multiple equal cost paths Which depends on order of processing

nodes For multicast, all routers must

have same spanning tree for given source node

MOSPF has tiebreaker rule

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Inter-Area Multicasting

Multicast groups may contain members from more than one area

Routers only know about multicast groups with members in its area

Subset of area’s border routers forward group membership information and multicast datagrams between areas Interarea multicast forwarders

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Inter-AS Multicasting Certain boundary routers act as inter-AS

multicast forwarders Run an inter-AS multicast routing protocol as

well as MOSPF and OSPF MOSPF makes sure they receive all multicast

datagrams from within AS Each such router forwards if required Use reverse path routing to determine source

Assume datagram from X enters AS at point advertising shortest route back to X

Use this to determine path of datagram through MOSPF AS

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MOSPF (RFC 1584) Extension to OSPF unicast routing protocol

Includes multicast information in OSPF LSAs to construct multicast distribution trees

Group membership LSAs are flooded throughout the OSPF routing domain so MOSPF routers can compute outgoing interface lists.

Uses Dijkstra’s algorithm to compute shortest-path tree for each multicast group.

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Protocol Independent Multicast (PIM) Independent of unicast routing

protocols Extract required routing information

from any unicast routing protocol Work across multiple AS with

different unicast routing protocols JP Note: this is usually how we do

Multicasting in real life

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PIM Strategy Flooding is inefficient over large sparse

internet Little opportunity for shared spanning trees Focus on providing multiple shortest path

unicast routes Two operation modes

Dense mode For intra-AS Alternative to MOSPF

Sparse mode Inter-AS multicast routing

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PIM-DM (Internet Draft) Protocol Independent

Supports all underlying unicast routing protocols including: static, RIP, IGRP, EIGRP, IS-IS, BGP, and OSPF

Uses reverse path forwarding Floods network and prunes back based on

multicast group membership Assert mechanism used to prune off

redundant flows Appropriate for...

Smaller implementations and pilot networks

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PIM-SM (RFC 2362) Supports both source and shared trees

Assumes no host wants multicast traffic unless it specifically ask for it

Uses a Rendezvous Point (RP) Senders and Receivers “rendezvous” at this point to learn of

each others existence. Senders are “registered” with RP by their first-hop router. Receivers are “joined” to the Shared Tree (rooted at the RP) by

their local Designated Router (DR). Appropriate for…

Wide scale deployment for both densely and sparsely populated groups in the enterprise

Optimal choice for all production networks regardless of size and membership density.

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Sparse Mode

A sparse group: Number of networks/domains with

group members present significantly small than number of networks/domains in internet

Internet spanned by group not sufficiently resource rich to ignore overhead of current multicast schemes

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Group Routers Group Destination Router

Has local group members Router becomes destination router for

given group when at least one host joins group

Using IGMP or similar Group source router

Attaches to network with at least one host transmitting on multicast address via that router

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PIM Approach For a group, one router designated rendezvous point

(RP) Group destination router sends join message towards

RP requesting its members be added to group Use unicast shortest path route to send Reverse path becomes part of distribution tree for this RP to

listeners in this group Node sending to group sends towards RP using shortest

path unicast route Destination router may replace group-shared tree with

shortest path tree to any source By sending a join back to source router along unicast shortest

path Selection of RP dynamic

Not critical

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MBONE consists of multicast islands connected by tunnels

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L2 switching implementation Considerations

Issue: Default behavior of a l2 switch is to flood multicast to all port on a VLAN. This beats the purpose of a switch (to limit traffic)

Solution: Cisco Group Management Protocol (CGMP) and IGMP Snooping

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CGMP Proprietary Cisco solution Basically uses IGMP information to generate

CGMP messages to tell a switch to enable a port for a Mcast group and add CAM entry.

Disabled by default Must be configured on switch and routers Cisco recommend that for low end switches JP opinion: pretty poor and would not want to

use it. I’d rather just enable Mcast flooding on VLAN and if switch too low end to support such high Mcast, we have architecture issue.

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IGMP Snooping LAN switch looks at L3 information of

multicast packet to see the “join” requests

When it sees it: it enables forwarding on the port for that Mcast group

Pro: transparent to administrator, enabled by default.

Con: can get processor intensive unless you can do snooping in special ASIC (high end switches do that)

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Sparse or Dense Mode? What is best?

It depends! It is a global config for a given

VLAN/interface What if you want both?

Cisco: Sparse-Dense mode In this case it is by default Dense but if a group gets an RP, switch to Sparse

Recommendation: Sparse mode

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Enabling and configuring Mcast

Global command: Ip multicast-routing

Per interface/VLAN: Ip pim {sparse-mode; dense-mode;

sparse-dense-mode} If you don’t know: use sparse-dense

will work with all.

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Troubleshooting Mcast … Even more fun than troubleshooting VPN

problems! (if you want an unforgettable life experience: try putting Multicast traffic in a GRE tunnel over an IPSEC VPN! You only have one life: you got to try it!)

First make sure end-to-end unicast routing is fine.

Then consider the mode and signaling used to propagate Mcast groups.

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Troubleshooting Mcast Different aspects to check:

Source packet flow - At upstream router: “show ip igmp groups interface-name” “show ip igmp Interface interface-name”

Check Multicast Signaling: Show ip pim neighbor Show ip pim rp mapping Show ip rpf <source> (to make sure RPF is correct – alternatively look

at routing table) Look at active network mroutes:

Show ip mroute active Show ip mroute count

Check receiver: “show ip igmp groups interface-name” “show ip igmp Interface interface-name”

Advanced CLI tool: Mstat mtrace