IP Multicast

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IP Multicast

• Fang Yan
• CSEE, UMBC
• 10/14/04

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topics

• Basics of multicasting
• MBone
• IPv4 routing protocols
• 6bone

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What is multicasting

• Application level one to many communication.
• A single packet is addressed to all intended
  recipientshost group
• The network replicates packets only as
  neededwhen paths diverge at a router

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Why Multicasting

• It is efficient
• better bandwidth utilization
• less host/router processing
• quicker participation
• Many applications send data to multiple
  receivers.
• Streaming multimedia
• Content delivery (meeting, lecture, speech)
• Stock quotes, news
• Database replication
• Software distribution

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B
Y
A
X
C
D
Z
E
Flow of data in multiple unicasting
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B
Y
A
X
C
D
Z
E
Flow of data in multicasting
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Basic components of IP multicasting

• Definition of a multicast host group
• Protocols for establishing and controlling
  multicast groups
• Router infrastructure for distribution of
  multicast traffic
• Application protocols and APIs that enable
  desktop computers and workstations to put
  multicasting to good use

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multicast address

• IPV4
• 4 28 bits
• class D
• From 224.0.0.0 to 239.255.255.255
• From 224.0.0.0 to 224.0.0.255 is reserved
• Totally 228-256268 million addresses

1110 Multicast Group ID
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Reserved address examples

• 224.0.0.1 all hosts
• 224.0.0.1 all multicast routers
• 224.0.0.4 DVMRP routers
• 224.0.0.5 OSPF routers
• 224.0.0.6 OSPF designated routers
• 224.0.0.22 IGMP

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multicast address (cont.)

• IPV6
• 8 4 4 112
  bits
• Support 4millionbillionbillionbillion addresses
• Flag
• Transit/permanent
• Scope
• 1 node-local
• 2 link-local
• 5 site-local (set by administrators of routers
  at the site)
• 8 organization local (set by administrators of
  routers of the organization)
• 14 global

11111111 flags scope Multicast Group ID
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• Ready to go?
• Unfortunately, there is a problem
• At the early 90s, the majority of the routers on
  the Internet don't know how to handle
  multicasting. Most routers are set up to move
  traditional IP unicast packets.
• Router manufacturers have been reluctant to
  create equipment that can do multicasting until
  there is a proven need for such equipment. But,
  as you might expect, it's difficult for users to
  try out a technology until they have a way to use
  it. Without the right routers, there's no
  multicasting. Without multicasting, there won't
  be the right routers.
• Solution?
• MBone !

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MBonea little bit history

• Stands for Multicast Backbone
• In 1992, some bright fellows on the Internet
  Engineering Task Force (IETF) decided that no one
  would do in hardware, they could do in software.
• Many scientific conferences, scientific events,
  concerts were broadcast over MBone since then.
• In 1997, the first ISPs started rolling out
  commercial services utilizing multicasting on the
  Internet.

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MBonearchitecture

• A virtual overlay network on top of the Internet.
• Consists of multicast-capable islands connected
  by tunnels
• Each island (typically a LAN or group of
  interconnected LANs) supports hardware multicast
  to its hosts.
• Each island contains one or more special routers
  called mrouters (multicast routers)
• Mrouters are often workstations running multicast
  routing daemon (mrouted)

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mrouter
C
B
LAN
A
D
Multicast island
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tunneling

• A tunnel is a connection between two mrouters
  using "IP over IP".
• Tunnel parameters
• Threshold The minimum TTL required for a packet
  to be forwarded across this tunnel
• Cost A metric used to compute routing
  distances
• encapsulate MBone packets within IP packet and
  send as regular unicast packets to the
  destination mrouters IP address

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Original packet
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Encapsulated packet
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Tunnel configure

• Tunnel is configured manually
• The administrator of the new island sends a
  message announcing its existence to the MBone
  mailing list.
• The administrators of nearby sites then contact
  him to arrange to set up tunnels.
• Existing tunnels may be reshuffled to take
  advantage of the new island to optimize the
  topology.

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MBone (cont.)

• Initially used DVMRP routing algorithm
• Migrated toward PIM
• Uses UDP protocol
• Problems
• Scalability
• Flow control
• Congestion control
• Future
• Universe support of multicast?

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IGMPv1

• Stands for Internet Group Management Protocol
• Described in RFC 1112.
• Manages multicast group membership
• Runs between hosts and their immediate
  neighboring mrouter
• Only two kinds of packets query and report
• Packet format
• 4 4 4 16 bits

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IGMPv1 How to join a host group

• Mrouter initiated
• Periodically, mrouter sends out a Query packet to
  its island asking who is interested in which
  channel.
• Hosts wishing to (continue to) receive one or
  more channels send back Report packet.
• Each mrouter keeps a table of which channels it
  must put out into its LAN
• Receiver initiated
• When a host first joins a group, it immediately
  transmits Report message rather than waiting from
  the next Query from the mrouter.
• The request may need to be forwarded up to a
  router thats already part of the host group
• Reduce join latency

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More IGMP

• IGMPv2
• Defined in RFC 2236
• Adds an explicit Leave message
• Routers can more easily determine when a group
  has no interested listeners on a LAN
• IGMPv3
• Defined in RFC 3376
• Optimizes support for a single source of content
  for a multicast group (SSM)
• Backward compatible

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IPv4 multicast routing protocols

• DVMRP (Distance Vector Multicast Routing
  Protocol)
• MOSPF (Multicast Open Shortest Path First)
• PIM-DM (Protocol Independent Multicast, Dense
  Mode)
• PIM-SM (Protocol Independent Multicast, Sparse
  Mode)
• CBT (Core-Based Tree)

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distance vector routing

• Dynamic routing algorithm
• Each router maintain a table giving the best
  known distance to each destination and which line
  to use to get there.
• These tables are updated by exchanging
  information with the neighbors periodically.
• Problem
• Count-to-infinity

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link state shortest-path-first routing

• Periodically, each router floods a link-state
  message, containing the routers identity and its
  associated connectivity information to each of
  its immediate neighbors.
• Each router runs the shortest-path-first
  algorithm to determine the shortest path from
  itself to all the other routers.
• Problem
• Computation intensive

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RPF

• Stands for reverse path forwarding
• Simple algorithm developed to avoid duplicate
  packets on multi-access links
• RPF algorithm takes advantage of the IP routing
  table to compute a multicast tree for each
  source.
• RPF check
• When a multicast packet is received, note its
  source (S) and interface (I)
• If I belongs to the shortest path from S,
  forward to all interfaces except I
• If test in step 2 is false, drop the packet
• Packet is never forwarded back out the RPF
  interface!

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DVMRPexchange distance vectors

• First multicast routing protocol ever deployed in
  the Internet
• Each router maintains a multicast routing table
  by exchanging distance vector information among
  routers
• Constructs a source tree for each group using
  reverse path forwarding
• There is a designated forwarder in each subnet
• Multiple routers on the same LAN select
  designated forwarder by lower metric or lower IP
  address (discover when exchanging metric info.)
• Once tree is created, it is used to forward
  messages from source to receivers

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DVMRPbroadcast prune

• Flood multicast packets based on RPF (Reverse
  path forwarding) rule to all interested routers.
• Leaf routers check and sends prune message to
  upstream router when no group member is on their
  network
• Upstream router prune the interface with no
  dependent downstream router.
• Graft message to create a new branch for late
  participants
• Restart forwarding after prune lifetime (standard
  720 minutes)

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DVMRP tables

• Routing table
• Source subnet the source of multicast datagrams
• From-Gateway previous hop router leading back to
  source
• Metric cost
• TTL
• InPort routers interface for incoming traffic
• OutPort routers interface for outgoing traffic
• Forwarding table
• Source subnet the sub network sending multicast
  datagrams
• Multicast group class D IP address
• TTL
• InPort parent interface, prune message
• OutPort child interface to forward, prune
  message

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DVMRP

• Advantage
• Relatively simple
• Modest processing demands
• Disadvantage
• Convergence performance
• Periodically flood multicast traffic to rebuild
  its trees scalability

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MOSPF

• Modified from OSPF
• Suitable for multicasting within a single
  autonomous system (AS)
• One router is selected as the designated router
  (DR)
• DR floods Group-Membership Link State
  Advertisements (LSAs)
• Each router maintains the up-to-date image of the
  topology of the entire network
• Each router floods its local state through the AS
• Source-rooted Shortest path tree for each source
  network, destination group pair
• Source-rooted SPT calculated at each router
• Based on Link-State database
• Provides the best route to any destination in AS
• Pruned SPT - Group membership LSAs

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MOSPF

• Each router makes its forwarding decision based
  on the contents of its forwarding cache.
• Forwarding cache is build from the source-based
  shortest-path tree for each (source, group) pair
  and the routers local group database
• Forwarding cache entry
• Destination
• Source
• Upstream receive from
• Downstream forward to
• TTL
• Advantage
• Adapt rapidly to changes in group membership
• Interoperate with OSPFused to forward normal
  unicast IP traffic
• Disadvantage
• Computationally intensive
• Not well-suited for handling sparse mode

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PIM-DM

• Similar to DVMRP
• Floods multicasts out of all interfaces except
  the source interface
• Uses RPM
• Prune message to eliminate unneeded branches
• Protocol-independent
• Needs to establish its own router-to-router
  dialogs

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PIM-DM Assert Mechanism

• Multiple routers are active on the same link
• Routers sends PIM assert messages
• Compare distance and metric values
• Router with best route to source wins
• If metric distance equal, highest IP address
  wins
• Losing router stops sending (prunes interface)

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PIM-SM

• Designed to provide efficient communication
  between members of sparsely distributed groups
• Rendezvous point (RP) are used by senders to
  announce their existence and by receivers to
  learn about new senders of a group
• Requires host group members explicitly join a
  delivery tree by transmitting Join message
• One set of RPs per sparse-mode domain, not per
  group.
• Each group has precisely one RP at any given
  time.
• DR sends Join/Prune messages toward the RP and
  maintain the active RP

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PIM-SM (cont.)

• Allows routers to create a source-based SPT on
  behalf of their attacked receivers
• Reduce delay
• Reduce concentration of traffic on RP-tree
• Advantages
• Less traffic
• disadvantages
• Bottle-neck at an RP router
• Single point of failure

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PIM-SMjoining

• DR receives IGMP Report message from receiver
• DR creates a multicast forwarding entry for the
  (,G) pair and transmits a unicast Join message
  toward the primary RP for this group
• Intermediate router forwards the Join message,
  creating a forwarding entry for the (,F) pair if
  not exists.

Source (S)
Designated Router
Rendezvous Point (RP) for group G
Host (receiver)
Join
Join
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PIM-SMsending

• DR encapsulates the initial multicast packets in
  a Register packet and sends them toward the
  primary RP
• The active RP transmits Join messages back toward
  the sources DR.
• Intermediate routers create a new (S, G) pair.
• When source gets a Join, it sends further packets
  without encapsulation

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CBT

• Construct a single tree shared by a Group
• Protocol independent
• Core router equivalent to RP
• Receiver cannot switch from RP-tree to SPT
• CBT state bi-directional
• data flows in either direction along the branch
• Advantage
• Less traffic
• Better scalability
• Disadvantage
• Bottleneck at CR
• Single point failure

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CBT

• Host sends IGMP report to Join a group
• JOIN-Request sent towards the core router
• JOIN-Request is explicitly acked using JOIN-ACK
  by core or on-tree router
• Intermediate routers set up Transient state
• ltGroup, Incoming interface, Outgoing interfacegt
• Transient state converted to Active state by
  JOIN-ACK
• Bi-directional state
• Keep-alive mechanismCBT Echo protocol
• Echo-Request
• Each on-tree router is responsible for
  maintaining its Upstream link
• Sent to the Upstream router
• Carries a list of groups for which the upstream
  router is the parent
• Echo-Reply
• From the parent with the list of groups attached
  to the child interface

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Comparison
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Inter-Domain Routing

• Hierarchical routing
• HDVMPR
• HPIM
• M-BGP (Multicast Border Gateway Protocol)
• MSDP (Multicast Source Discovery Protocol)
• Connect multiple PIM-SM domains together

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6bone

• Stands fro IPv6 backbone
• an IPv6 test bed to assist in the evolution and
  deployment of IPv6
• The network became a reality in March 1996
• started as a virtual network (using IPv6 over
  IPv4 tunneling/encapsulation) operating over the
  IPv4-based Internet to support IPv6 transport
• slowly migrating to native links for IPv6
  transport
• Now 1478 sites from 60 countries

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6bone hierarchy

• The 6Bone network is organized in three
  hierarchical layers
• the core backbone layer
• made up of a mesh of IPv6 over IPv4 tunnels which
  connect only the backbone nodes
• routing is based on BGP4 (Border Gateway
  Protocol version 4 extended to support IPv6) (RFC
  2858)
• the transit node layer
• connected to one or more backbone nodes
• provide transit service to the leaf nodes
• Routing outside of the backbone is mainly static
• the peripheral or leaf node layer.

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6bone addressing

• based on the format established for IPv6 unicast
  addresses (RFC 2073)
• backbone nodes act as experimental TLAs (Top
  Level Aggregators) called pseudo-TLA
• responsible for assigning IPv6 addresses to nodes
  belonging to lower hierarchical levels which are
  configured as their "clients".
• The entire 6Bone network is identified by a
  16-bit prefix assigned directly by IANA
• each backbone node is assigned a 24- or 28-bit
  long prefix

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6bone future

• Will stay in place until on longer needed
• excellent place for an ISP to get early
  experience before going into production

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• Thank you !
• question?

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• References
• IP Multicasting by Dave Kosiur
• Multimedia communications by Fred Halsall
• Computer Networks by Andrew S. Tanenbaum
• http//www.6bone.net
• http//www.savetz.com/mbone/
• http//www.juniper.net/techpubs/software/junos/jun
  os60/swconfig60-multicast/html/swconfig60-multicas
  tTOC.html