Download What is Multicast?

Multicast
for
IP Networks
6th April 2000
John A. Clark - Technical Account Manager
Technical Presentation Series: Multicast for IP Networks
Agenda
• Overview of IP Multicast
— IP Multicast Addressing
— IGMP
• IP Multicast Routing Protocols
— DVMRP
— MOSPF
— PIM-DM
— PIM-SM
— PGM
• Protocol Comparison
• Summary
Technical Presentation Series: Multicast for IP Networks - Page 2
IP Multicast Overview
… What is it & Why do we need it?
Technical Presentation Series: Multicast for IP Networks
What is Multicast?
Multicast is a method for
sending identical data
streams to groups of
multiple end-stations
•
•
•
•
Delivers data only to interested users
Enables mass data distribution without specialised processors
Eases administration for data sources
Efficient use of bandwidth for transport of common data
Technical Presentation Series: Multicast for IP Networks - Page 4
Why use Multicasting?
• Multicasting fits many of today’s business activities
• Multicast support on Operating Systems
— Windows 98, NT, Solaris, Linux, etc.
• Nortel customers are using multicast
applications in production environments
— Real-time data push
– SIAC, Stock Ticker
– NASA for Mission Critical satellite telemetry,
command and control and shuttle mission data.
— Multimedia broadcast distribution
– Marshall Space Center, IP TV,
– UC Davis, distance learning
Technical Presentation Series: Multicast for IP Networks - Page 5
An Endstations View
• To Send Multicast Data
– Host Endstations simply transmit to a Class D Multicast Address
• To Receive Multicast Data
– Receiving Endstations register interest in data stream associated
with a Class D Multicast address (group membership)
Technical Presentation Series: Multicast for IP Networks - Page 6
Multicast Addressing
IANA defined
DVMRP
OSPF
PIM Sparse
Applications
RFC 1700,
“Assigned
Numbers”
224.0.0.1
224.0.0.2
224.0.0.4
224.0.0.5
224.0.0.6
224.0.0.13
224.0.1.1
224.0.1.11
224.0.1.12
224.0.1.7
224.0.1.16
224.0.13.000
to
224.0.13.255
All Local Hosts
All Routers
Routing Protocol
All OSPF Routers
Designated Routers
All PIM Routers
Network Time Protocol
IETF-1-Audio
IETF-2-Video
Audionews
Music-Service
Net News
• Class D Addresses (starting 1110)
• 224.0.0.0 - 239.255.255.255
• Multicast Addresses are also mapped to Ethernet addresses ……...
Technical Presentation Series: Multicast for IP Networks - Page 7
Multicast Ethernet Addresses
Class D Multicast IP Address
Multicast MAC Address
(starts with IANA Prefix)
224
10
8
5
11100000 00001010 00001000 00000101
00000001 00000000 01011110
1
0
5E
0A
08
05
• IANA (InterNet Assigned Numbers Authority) owns the reserved
block 01-00-5E
• The low order 23 bits of the IP Address are mapped to the low
order 23 bits of the Ethernet address
Technical Presentation Series: Multicast for IP Networks - Page 8
End-to-end Protocol Involvement
• Routers at both the source host LAN and
receiving end-station LANs use IGMP to learn
the existence of host group members on their
directly attached subnets
IGMP
• Routers use a Multicast routing protocol (e.g.
DVMRP, MOSPF etc.) to establish
DVMRP
MOSPF
• Routers know there is a receiver - not how many
• Receivers don’t know who the host sender is
• Senders don’t know who the receivers are
Technical Presentation Series: Multicast for IP Networks - Page 9
IGMP
IGMP - Internet Group Management Protocol
Memberships
224.1.1.1
224.100.1.1
Memberships
228.1.1.1
224.1.2.1
224.1.90.5
Memberships
228.1.1.1
224.1.2.1
224.1.90.5
Memberships
224.1.200.1
224.100.1.1
224.1.90.5
224.100.1.1
224.1.200.1
228.1.1.1
224.1.2.1
224.1.1.1
Host Membership Reports
Host Membership
Query
Memberships
224.1.90.5
224.1.1.1
224.100.1.1
• Multicast router periodically sends a data link layer IGMP Host
Membership Query to all nodes on its LAN
— query is sent to the all-hosts group (network address 224.0.0.1)
—
TTL of 1 - queries are not propagated outside of the LAN
• Each host sends back one IGMP Host Membership Report
message per host group
— sent to the group address i.e. only one member reports membership
Technical Presentation Series: Multicast for IP Networks - Page 10
IGMP Protocol Format
• Sends IGMP queries and IP hosts report their host group
memberships.
• IGMP is loosely analogous to ICMP (rfc 1112)
• IGMP messages are encapsulated in IP datagrams.
• IGMP has only two kinds of packets: Host Membership Query
and Host Membership Report,
• Simple fixed format - payload 1st word: control information 2nd
word: class D address
Technical Presentation Series: Multicast for IP Networks - Page 11
Routers and Multicast
Multicast routers:
• Translate multicast addresses into host
addresses (Class D addresses identify a
multicast stream not a specific destination)
• Make copies of multicast frames to forward
throughout the forwarding tree
• Use a choice of routing protocols to provide
forwarding throughout the network (e.g. DVMRP,
MOSPF etc.)
• Exchange information about neighboring routers
• Keep ‘state’ for all group memberships
• Elect a Designated Router for each LAN (via
IGMP) to avoid duplication of multicast tables
Technical Presentation Series: Multicast for IP Networks - Page 12
The Forwarding Tree
Host sends single packet to Multicast Group
Router table has members of this Group
Router forwards one copy of packet
Downstream Router table has members of this Group
Router forwards one copy of packet on each
downstream interface
Router forwards a single copy of the packet onto the attached LAN
where a group member resides
Technical Presentation Series: Multicast for IP Networks - Page 13
Reverse Path Forwarding
Source of Multicast Data
Data flows in the direction
of the destination endstation
Routers consult the routing
table to the source to make
forwarding decision
Destination
Technical Presentation Series: Multicast for IP Networks - Page 14
Destination
Two Types of Multicast Routing Protocols
using Two Types of Multicast Trees
• Dense Mode
— Multicast group members are densely distributed
— Assumes many of the subnets contain at least one group member
— Uses Source Tree
– Shortest path tree from source to all receivers
• Sparse Mode
— Multicast group members are sparsely distributed
— May be many members - but assumes they are widely dispersed
— Uses Shared Tree
– Core delivers data to receivers on shared tree
Technical Presentation Series: Multicast for IP Networks - Page 15
Dense Mode Protocols
• Protocols that use a “Flood & Prune”
mechanism for packet delivery
• Useful in campus and environments where
bandwidth is plentiful
• Source Tree - Shortest Path
Technical Presentation Series: Multicast for IP Networks - Page 16
campus
Sparse Mode Protocols
• Protocols that use Explicit routing
and forwarding
WAN
• Useful in WANs and environments
where bandwidth is not plentiful
• Shared Tree from Core
(Rendezvous Point)
WAN
WAN
Technical Presentation Series: Multicast for IP Networks - Page 17
WAN
Source Tree
Red group
source host
• Shortest path tree from source
to all receivers
• Data flooded to all end-stations
where not pruned
• Dense Mode Protocols
Blue group
source host
— DVMRP, MOSPF, PIM-DM
Technical Presentation Series: Multicast for IP Networks - Page 18
Shared Tree
• Data sent and received via Core
(Rendezvous Point)
• Data explicitly forwarded to
endstations
Red group
source host
• Sparse Mode Protocols
— CBT, PIM-SM
Rendezvous
Point (Core)
Shared tree
– Receivers build shared
trees to Core
– Source sends to Core
– Core delivers data to
receiver on shared tree
Core to receivers
Blue group
source host
Technical Presentation Series: Multicast for IP Networks - Page 19
Source to core
Flooding, Pruning and Grafting
no receivers
pruning
leaf
• Flooding
leaf
— Sending multicast packets to all
router interfaces - except that on
which the packet arrived
leaf
leaf
• Pruning
— Explicitly removing router
interfaces for which there are no
multicast group members
Source tree
source host
leaf
• Grafting
— Reconnecting router interfaces to a
pruned multicast group
Grafting
pruning
new
receiver
no receivers
Technical Presentation Series: Multicast for IP Networks - Page 20
DVMRP
Distance Vector Multicast
Routing Protocol
Technical Presentation Series: Multicast for IP Networks
Distance Vector Multicast Routing Protocol
• “RIP for IP Multicast” - defined in RFC 1075
• Separate Multicast routing protocol, in addition to
unicast RIP
• Collects information about multicast group membership
via IGMP
• State maintained on all routers
• Can tunnel multicast data through non-multicast
networks using IP-in-IP
• Uses Reverse Path Multicasting
Technical Presentation Series: Multicast for IP Networks - Page 22
DVMRP Forwarding
• Assumes initially every host is part of the
multicast group
• Designated router on source subnet transmits
a multicast message to all adjacent routers
• Each router selectively forwards the message
downstream, until it is passed to all multicast
group members
• Receiving routers check unicast routing
tables to determine shortest path back to the
source
• Forwards multicast message to all adjacent
routers, other than the one that sent the
message
• Ensures loop-free tree with shortest paths
from the source to all recipients
Technical Presentation Series: Multicast for IP Networks - Page 23
DVMRP Protocol Operation ...
• Routers send prune messages resulting in a source specific tree
• Graft messages are used to reconnect to the pruned tree
• DVMRP routing table shows reverse path tree from the router
• Leaf detection relies upon “poison reverse”
— Multicast source network advertised at infinity
— Advertisements not at infinity are for leaf networks
• Leaf timeout 200 seconds (default) - flooding and pruning follows
• Neighbor timeout 240 seconds (default) - DVMRP route flushing
follows
Technical Presentation Series: Multicast for IP Networks - Page 24
MOSPF
Multicast Extensions
to OSPF
Technical Presentation Series: Multicast for IP Networks
Multicast Extensions to OSPF
• Link state Multicast routing protocol defined in rfc1584
• Utilizes the unicast OSPF link state database
• Can mix MOSPF and OSPF routers in the same network
• Routers indicate MOSPF capabilities by setting MC bit in
Router-LSA
• Group membership LSAs are distributed throughout the
OSPF area
• When data for a group arrives the forwarding tree is
calculated by running the Dijkstra algorithm
Technical Presentation Series: Multicast for IP Networks - Page 26
MOSPF Protocol Operation
• MOSPF routers collects information about
multicast group membership via IGMP
• Routers update their internal link-state
information based on information flooded by
adjacent routers
• Each router can independently calculate a
least-cost tree with the multicast source as
the root and the group members as leaves
• All routers will calculate exactly the same
tree, since they share link-state information
• A Dijkstra calculation is required to compute
a shortest-path tree for each (source,
destination group) pair.
Technical Presentation Series: Multicast for IP Networks - Page 27
MOSPF Protocol Operation ...
• Wildcard multicast forwarders (ABRs) ensure that all
multicast data is flooded into the backbone area
• All ABRs advertise group membership into the backbone
area ensuring multicast data flows across areas
• Designated router selection is as per OSPF
• Designated routers solicit and listen to IGMP group
membership messages. Backup Designated routers listen
only
• In a mixed OSPF/MOSPF the designated must be an
MOSPF router
Technical Presentation Series: Multicast for IP Networks - Page 28
PIM
Protocol Independent Multicast
Technical Presentation Series: Multicast for IP Networks
Protocol Independent Multicast/Dense Mode
• Similar to DVMRP - also employs Reverse Path
Multicasting (RPM) to construct source trees.
• Uses pruning to create the Shortest Path tree
• Designate router is elected using PIM Router-Query
messages transmitted every 30 seconds
• Introduces new message type call “assert” to resolve
looping …..
Technical Presentation Series: Multicast for IP Networks - Page 30
PIM Dense Mode Assert Messages
• An assert message is sent indicating the metric to
the source
• Upon receiving an assert metric value is compared
- if lower the interface is pruned. Designate router
wins equal cost
• Downstream routers must listen to asserts to
ensure knowledge of the correct upstream router
Technical Presentation Series: Multicast for IP Networks - Page 31
Protocol Independent Multicast/Sparse Mode
• Uses unicast routing table
• Utilizes Rendezvous Points (RP) to build the
forwarding tree
• Provides both native and encapsulated data
transport
• Allows routers to bypass the RP when high data
volumes are present
Technical Presentation Series: Multicast for IP Networks - Page 32
PIM Sparse Protocol Operation
• Transmitted data is forwarded to the RP for
distribution to the RP-Tree
• If the data rate is warrants, the RP or endpoint router can switch to the SPF tree
• Timers are used for tree maintenance
• Designate router election is identical to
PIM/Dense (PIM Router-Query messages
transmitted every 30 seconds)
• Assert messages are used for loop
detection
Technical Presentation Series: Multicast for IP Networks - Page 33
PGM
Pragmatic General Multicast
Technical Presentation Series: Multicast for IP Networks
Introduction to PGM
• Reliable Multicast transport protocol
• Original title, Pretty Good Multicast
• Re-labeled
Pragmatic
General
Multicast
• Provides ordered, duplicate free, multicast data delivery
from multiple sources to multiple receivers.
• Guarantees that a receiver in a multicast group either
receives all data from transmissions and retransmissions,
or is capable of detecting unrecoverable data packet loss.
• Standard is in Internet-Draft status - future??
Technical Presentation Series: Multicast for IP Networks - Page 35
PGM Basic Operation
Receiver
Receiver
Receiver
Source
SPM Frame
• Group members & sources periodically
interleave Source Path Messages (SPM’s) with
multicast data
• Numbered data frames are used to detect
missing data and issue No Acknowledgement
(NAK) messages if data is lost
• Hosts will continue to send NAK messages
Multicast Data Frame 1
Multicast Data Frame 2
Multicast Data Frame 3
NAK Frame 2
until they receive a NAK Confirmation (NCF)
• The source will then retransmit the requested
frame
NCF Frame
Multicast Data Frame 2
Technical Presentation Series: Multicast for IP Networks - Page 36
PGM Protocol Messages
• Source Path Messages (SPMs)
— Destination Address; Multicast Group Address.
— Transmitted by Sources to establish source-path state
— Must send SPM, before sending Multicast Data Stream
• Negative Acknowledgement (NAK)
— Destination Address; Unicast, PGM-hop by PGM-hop, back to the source
• Negative Acknowledgement Confirmation (NCF)
— Destination Address; Multicast Group Address
— Transmitted by Network Elements and sources in response to NAKs
— Designated Local Transmitters (DLRs) may respond to a NCF with their own
NCF making themselves available as a DLR (Redirection NCF)
Technical Presentation Series: Multicast for IP Networks - Page 37
PGM Network Element
NAK Frame 2
NCF Frame
NCF Frame
NAK Frame 2
NAK Frame 2
Receiver
NCF Frame
Receiver
Receiver
Mcast Data
Frame 2
Receiver
Receiver
Mcast Data
Frame 2
Receiver
Mcast Data
Frame 2
Source
SPM Frame
Mcast
Data
Mcast
Data
Mcast
Data
Technical Presentation Series: Multicast for IP Networks - Page 38
Modified
SPM
FRAME
Modified
SPM
FRAME
Designated Local Retransmitter
Source
Receiver
SPM Frame
Receiver
DLR
Null NAK
Null NAK
Receiver
Receiver
Mcast
Data
Mcast
Data
Modified
SPM
FRAME
Rdata
Receiver
NCF Frame
NAK Frame 2
Mcast
Data
Redirecting NCF Frame
Technical Presentation Series: Multicast for IP Networks - Page 39
Modified
SPM
FRAME
Multicast Routing Protocol
Comparison
Technical Presentation Series: Multicast for IP Networks
Making the Protocol Decision
• DVMRP is the most widely implement Multicast routing protocol
available today
• DVMRP tunnels can provide the connection between MOSPF
networks
• MOSPF was the first multicast routing protocol to achieve full
standard
• Multicast extensions to OSPF have the convergence and reliability
characteristics of OSPF- proven OSPF technology for multicast
forwarding
• MOSPF provides state for both Unicast and Multicast forwarding in
a single database
• PIM-SM supports shared tree & source path trees
• PIM-SM can switch from shared tree to source tree
Technical Presentation Series: Multicast for IP Networks - Page 41
Multicasting and the Internet
• Multicasting is carried through the internet overlaid on
the unicast network using DVMRP tunnels
• MBONE consist of around 2750 routes
• Because of the MBONE’s size, it suffers from reliability
problems
• Public domain software is used in the internet for video,
voice, shared text and whiteboard session
• A recommended set of TTL’s exist for use in the internet
Technical Presentation Series: Multicast for IP Networks - Page 42
Summary - IP Multicast in One Page
• IP Multicast is receiver oriented
— Receivers & hosts “join” multicast groups
— Group is defined by a multicast address
— Uses Internet Group Mgmt Protocol (IGMP) to communicate group interest to
serving router
— Routers know there is a receiver, not how many
— Receivers don’t know who the host sender is
— Senders don’t know who the receivers are
• IP Multicast routing is the “glue”
— Transparently forwards data from sender to receivers
Technical Presentation Series: Multicast for IP Networks - Page 43
Thank you
Questions ?
Technical Presentation Series: Multicast for IP Networks