Multicasting

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Multicasting
Anthony Choi David Strole Werner Kalyan Landeck
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Broadcasting Delivering one packet to each
destination
Multicasting Allows each system to choose
whether it wants to participate in a given
multicast
A multicast address identifies an arbitrary set
of listeners, so forwarding mechanism must
propagate the packet to all segments
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Ethernet Multicast the low order bit of the
high-order octet distinguishes conventional
unicast address (0) from multicast address (1)
01-00-00-00-00-0016
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IP Multicasting The Internet abstraction of
hardware multicasting. The subset of systems
that are receiving a multicast is called
multicast group
General Characteristics

• Group Address Each multicast group is a unique
  class D address
• Number of Groups IP provides for up to 228
  simultaneous multicast groups
• Dynamic Group Membership A host can join or
  leave an IP multicast at any time
• Use of hardware IP uses hardware multicast to
  send IP multicast, if hardware supports
  multicast, otherwise, it uses broadcast or
  unicast to deliver IP multicast

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General Characteristics (continued)

• Inter-network Forwarding Special multicast
  routers are used to forward multicasts to systems
  in different physical networks
• Delivery Semantics IP multicast uses the same
  best-effort delivery semantics as other IP
  datagrams
• Membership And Transmission An arbitrary host
  may send datagrams to any multicast group group
  membership is only used to determine whether the
  host receives datagrams sent to the group

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Three Conceptual Pieces

1. Addressing Scheme
2. Effective notification and delivery system
3. Efficient internetworking forwarding facility

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Multicasting Addresses

• Two kinds Permanent (well known) or Create when
  needed (transient)

1
1
1
0
Group Identification
Multicast Address Range 224.0.0.0 to
239.255.255.255
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Example of permanent IP multicast address
assignments
Address Meaning
224.0.0.0 Base Address (Reserved)
224.0.0.1 All Systems on this Subnet
224.0.0.2 All Routers on this Subnet
224.0.0.3 Unassigned
224.0.0.4 DVMRP Routers
224.0.0.5 OSPFIGP All Routers
224.0.0.6 OSPFIGP Designated Routers
224.0.0.7 ST Routers
224.0.0.8 ST Hosts
224.0.0.9 RIP2 Routers
224.0.0.10 IGRP Routers
224.0.0.11 Mobile-Agents
224.0.0.12 DHCP Server / Relay Agent
224.0.0.13 All PIM Routers
224.0.0.14 RSVP-Encapsulation
224.0.0.15 All-CBT-Routers
224.0.0.16 Designated-Sbm
224.0.0.17 All-Sbm
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Example of permanent IP multicast address
assignments (continued)
Address Meaning
224.0.0.18 VRRP
through 244.0.0.255 Other Link Local Address
224.0.1.0 Through 238.255.255.255 Globally Scoped Addresses
239.0.0.0 Through 239.55.255.255 Scope restricted to one organization
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To map an IP multicast address to the
corresponding Ethernet multicast address, place
the low-order 23 bits of the IP multicast address
into the low-order 23 bits of the special
Ethernet multicast address 01-00-5E-00-00-0016
Example 224.0.0.2 becomes 01-00-5E-00-00-0216
The mapping of IP to Ethernet multicast address
is NOT unique It is up to the host to disregard
unwanted multicast datagrams
Primary difference between local and nonlocal
multicast lies in multicast routers, not in hosts
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Multicast Scope
Scope refers to the range of group members
Two techniques to control multicast scope

• The Time To Live (TTL) field
• Limiting the number of hops allowed
• e.g. 1 to never leave a network, 0 to never leave
  host
• Administrative scoping
• Reserving parts of the address space for groups
  that are local to a given site or local to a
  given organization
• Assign the group an address that has local scope

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• A host can join a multicast in three different
  levels
• Level 1 Can neither receive nor send IP
  multicast
• Level 2 Can send but not receive IP multicast
• Level 3 Can send and receive IP multicast
• Hosts join specific IP multicast groups on
  specific networks
• A host must have multicast capability software
  and a multicast router would be needed if a
  multicast spans multiple networks

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Internet Group Management Protocol (IGMP)

• Multicast routers and hosts must use IGMP to
  communicate
• Although IGMP uses IP datagrams to carry
  messages, it is thought of as an integral part of
  IP, not a separate protocol
• IGMP is standard for TCP/IP
• Two phases
• Phase 1
• A host sends an IGMP message to the group to
  declare its membership when joining
• Local multicast routers propagate the membership
  information to the rest of the internet
• Phase 2
• Local multicast routers must poll hosts to
  determine if a host is still a member, due to
  dynamic membership
• Source address filter

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IGMP Implementation
Five ways IGMP use to minimize its traffic on
the network

• All communications between hosts and multicast
  routers use IP multicast
• When polling, a multicast router sends a single
  query about all groups instead of sending a
  separate message to each
• If multiple multicast routers attach to the same
  network, they quickly and efficiently choose a
  single router to poll host membership
• Hosts do not respond to a routers IGMP query at
  the same time
• Spaced randomly over 10 seconds.
• Reports for multiple group memberships sent in a
  single packet

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Group Membership State Transitions
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IGMP Membership Query Message
TYPE (0x11)
RESP CODE
CHECKSUM
GROUP ADDRESS
RES
S
QQIC
NUM SOURCES
QRV
SOURCE ADDRESS 1 SOUCRE ADDRESS 2 SOUCRE
ADDRESS N
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TYPE (0X22)
RESERVED
CHECKSUM
NUM GROUP RECORDS (K)
RESERVED
GROUP RECORD 1
GROUP RECORD 2
GROUP RECORD K
REC TYPE
NUM OF SOURCES
ZEROES
MULTICAST ADDRESS
SOURCE ADDRESS 1
SOURCE ADDRESS 2
SOURCE ADDRESS N
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Multicast Forwarding and Routing

• Unlike unicast forwarding in which routes change
  only when the topology changes or equipment
  fails, multicast routes can change simply because
  an application program joins or leaves a
  multicast
• Multicast forwarding requires a router to examine
  more than the destination address
• A multicast datagram may originate on a computer
  that is not part of the multicast group, and may
  be forwarded across networks that do not have any
  group members attached

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Multicast Forwarding Paradigms
A multicast destination is a set of hosts, an
optimal forwarding system will reach all of it
destinations without sending a datagram across a
given network twice.
Reverse Path Forwarding (RPF) uses the source
address to avoid loops.

• Not used alone because it still does not solve
  the problem of multicast datagram being sent to
  non member groups

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Multicast Forwarding Paradigms (contd)
Truncated Reverse Path Forwarding (TRPF)
follows the basic RPF algorithm, but avoids
(truncates) paths that do not lead to group
members.

• To use TRPF, router needs 2 pieces of
  information
• Conventional routing table
• List of reachable multicast groups of each
  interface
• For the TRPF algorithm, first RPF is used
• If no group members are reachable over an
  interface, the router skips over the interface,
  checking the next one

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Multicast Forwarding Paradigms (contd)
Consequences of TRPF

• Because it relies on RPF to prevent loops, it
  delivers an extra copy of datagrams to some
  networks just like conventional RPF
• Delivery depends on a datagrams source

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Multicast Forwarding Paradigms (contd)
Multicast Trees the other paradigm

• Multicast router node to the tree
• Connection between routers edge to the tree
• Last router along each of the paths from the
  source leaf of the tree

A multicast forwarding tree is defined as a set
of paths through multicast routers from a source
to all members of a multicast group. For a given
multicast group, each possible source of
datagrams can determine a different forward tree
Forwarding protocols use a network prefix as a
source
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Reverse Path Multicasting

• More important to ensure that a multicast
  datagram reaches each member of the group than to
  eliminate unnecessary transmission
• Multicast routers each contain a conventional
  routing table that has correct information
• Multicast routing should improve inefficiency
  when possible
• Two step process
• Uses RPF scheme to send a copy of each datagram
  across all networks in the net.
• Simultaneously have multicast routers to inform
  one another about paths that do not lead to group
  members

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• RPM propagates membership information bottom-up
• A multicast router only knows about local group
  members
• If a leaf network contains no members for a
  given group the router connecting that network to
  the rest of the internet does not forward on the
  network
• RPM is called a broadcast and prune strategy
  because a router broadcasts until it receives
  information that allows it to prune a path
• Also known as data driven because router does not
  send group information until a datagram arrives
  for that group

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Multicast Routing Protocols

• Multiple Multicast Routing Protocols exists, but
  none of them is a required standard.
• Distance Vector Multicast Routing Protocol
  (DVMRP)
• Core Based Trees (CBT)
• Protocol Independant Multicast (PIM)
• Multicast extensions to OSPF (MOSPF)

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Distance Vector Multicast Routing Protocol (DVMRP)

• Uses Data-Driven Paradigm
• Allows for Multicast Router to pass along group
  membership and routing imformation to other
  Multicast Routers
• Resembles the RIP protocol, but it has been
  extended for Multicasting.
• Basically the protocol passes information about
  current multicast group membership as well as the
  cost to transfer datagrams between routers
• A forwarding tree is imposed on top of the
  physical interconnection for every possible
  (group, sorce) pair. So when a datagram destined
  for an IP multicast group is received by a
  router, it will send a copy of the datagram to
  very network link corresponding to a branch in
  the forwarding tree.

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Distance Vector Multicast Routing Protocol
(DVMRP) (contd)

• DVMRP is implemented by a Unix program named
  mrouted which uses a special multicast kernel
• Uses multicast tunneling to forward multicasts
  acroos the World Wide Web
• At each site an mrouted tunnel to other sites has
  to be configured manually
• The mrouted tunnel uses IP in IP encapsulation to
  send multicasts.
• When a multicast datagram is generated it is
  encapsulated in a unicast datagram and then
  forwared to other sites
• When a unicast datagram is received through one
  of the tunnels, the multicast datagram is
  extracted and forwarded according to the
  multicast routing table

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Core Based Trees (CBT)

• Uses Demand-Driven Paradigm
• Avoids broadcasting and allows all sources to
  share the same forwarding tree if possible
• Does not forward multicasts until at least one
  host along the path joins the multicast group
• When a host uses IGMP to join a multicast group,
  the host does not receive data until the local
  router has propagated the new hosts information
  to the other routers
• CBT uses a combination of static and dynamic
  algorithms to build a multicast forwarding tree.
  In order to make this scheme scalable CBT devides
  the underlying internet into regions of a size
  specified by the network administrator
• Each region has a designated core router and all
  other routers in the region have to know about
  the core router

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Core Based Trees (CBT) (contd)

• The use of a core router in each region allows
  for the region to have a shared tree
• So router in a region dynamically build the
  forwarding trees by sending join requests to the
  core router

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Protocol Independant Multicast (PIM)

• Actually is 2 independant protocols, which have
  little in common exept for the name and message
  header
• PIM Defense Mode (PIM DM)
• Designed for LAN environments in which all or
  almost all networks have hosts which are lisening
  to each of the multicast group
• PIM Sparse Mode (PIM SP)
• Designed for a wide area environment in which the
  member of a given multicast group occupy a small
  subset of all possible networks
• 2 Independant Protocols exists, because neither
  works well for all situations

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Protocol Independant Multicast (PIM) (contd)

• PIM functions independantly from the unicast
  routing protocol or protocols that a rounter
  uses, because PIM does not specify a routing
  table form. Thus PIM can use an arbitary routing
  protocol
• To accomandate many listenners
• PIM-DM uses a broadcast and prune approach.
  Datagrams are forwarded to all routers using RPF
  until a router sends an explicit request
• PIM-SM designates a router, called a Renderview
  Point (RP), which functions similar to the CBT
  core Router

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Multicast extensions to OSPF (MOSPF)

• The idea behind MOSPF is when the router know the
  topology of its OSPF area, MOSPF can use this to
  compute a forwarding tree.
• MOSPF is Demand-Driven , so traffic will not be
  propagted until requested
• Problems with this scheme are that all router
  must maintain the membership information of all
  groups and the information has to be synchronized
  to ensure every router has the exatly same
  database

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Reliable Multicast and ACK Implosions

• Reliable Multicast referes to a system which
  provides multicast delivery and guarantees that
  all members of the group receive the data in
  order without any loss, duplication or
  corruption.
• In theory sound greath but hard to implement,
  since many problems arise
• If a multicast group has multiple sender datagram
  arrival in sequence becomes meaning less
• Widely used multicast forwarding schemes, such as
  RPF, produce duplication even on small internets
• Applications like audio or video expect relioable
  systems to bound the delay and jitter

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Reliable Multicast and ACK Implosions (contd)

• Reliabiliy requires acknowledgements, since
  multicastr groups can have an arbitary number of
  members, traditional reliable protocols require
  the sender to handle an arbitary number of
  acknowledgements. This problem is refered to as
  the ACK Implosion, since no computer has enough
  processing power to handle this problem.
• How to overcome ACk implosion
• Reliable multicost protocols restrict multicating
  to a single source. Before data is sent a
  forwarding tree is established from the source to
  all group members and acknowledgement points need
  to be established.

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Reliable Multicast and ACK Implosions (contd)

• An acknowledgement point, also know as
  acknowledgement aggregator or designated route
  (DR), is a router that agrees to cache copies of
  the data and process acknowledgements from
  routers and hosts further down the forwarding
  tree. If a retransmission is need it is taken
  from the routers cache. This occurs when the host
  or router send a NACK. The NACK will propogate up
  the tree until a acknowledgement point or the
  source is reached.
• The selection of sufficient acknowledgement is
  important, since an insufficient number points
  will cause an ACK Implosion. Thus reliable
  multicast protocols require manual configuratio,
  as a result if you have a fast growing network
  this can be a problem

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Reliable Multicast and ACK Implosions (contd)

• Other methos of reliable multicast are still
  theoretical or experimental. Here are 2 protocols
  being considered.
• One was is to send multiple copies to reduce the
  need for retransmission
• Works good with Random Early Discard (RED), since
  the probability of more than one copy being
  discarded is extremly small
• Another way is to incorporate forward
  error-correcting codes