IGMP and PIM

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IGMP and PIM

• Suman Pandey
• EECS702 Topics in Computer Systems - Future
  Internet (Spring 2008)
• DPNM Lab

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Modern Applications
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Outline

• Concepts behind protocols IGMP and PIM
• Multicasting
• Addressing scheme
• Group management
• IGMP
• Multicast Routing
• PIM

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RFC

• RFC
• IGMP v1 RC 1112
• IGMP v2 RFC 2236
• IGMP v3 draft
• PIM v2 SM RFC 2362
• PIM v2 DM RFC 2365
• Supported on Macs, PCs, UNIX

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Multicasting

• Multicast communications refers to one-to-many or
  many-to-many communications.
• Application level multicast
• Network multicast
• IP Multicasting refers to the implementation of
  multicast communication in the Internet

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Tools

• Multicasting Tools
• SDR, VIC and RAT for Sun, Linux and Windows
  multicasting.
• Quicktime will be the Macintosh application for
  viewing multicast sessions.
• Products
• Apple's QuickTime Conferencing software.
• ICAST Express Media, video, audio and text
  clients and servers, beta version available on
  request.
• Merit Network's mrouted, multicast router daemon
  (server).
• Microsoft's NetShow-- Windows video/audio client
  and server. Multicast-capable.
• Precept's IP/TV -- Windows client for receiving
  video/audio/slide broadcasts.
• Van Jacobson's popular multimedia multicasting
  tools for a Unix X Window server video (VIC),
  and audio (VAT).

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Multicast
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Network Multicast
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Multicast Semantics

• IP multicast works as follows
• 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
• Every IP datagram send to a multicast group is
  transmitted to all members of the group by
  routers
• no security, no floor control

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IP Multicast Protocol stack

• IP Multicasting only supports UDP as higher layer
• There is no multicast TCP !

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Multicasting

• There are three essential components of the IP
  Multicast service
• IP Multicast Addressing
• IP Group Management (IGMP v1 v2)
• Multicast Routing (PIM v1 v2)

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Addressing

• How do you talk to a group of hosts (our
  multicast group), where each host has a different
  MAC address. Ensure that the other hosts, which
  are not part of the multicast group, don't
  process the information ?
• Break things down
• Hardware/Ethernet Multicasting
• IP Multicasting
• Mapping IP Multicast to Ethernet Multicast

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Hardware/Ethernet multicasting

• The network card (NIC) must be multicast aware,
  it is configured, via its drivers, to watch out
  for particular multicast MAC addresses apart from
  its own
• Ethernet uses the low-order bit set to ZERO (0)
  for unicast and ONE (1) for multicast
• Lower order bit of the higher order octet is set
  to 1.
• The multicast MAC address is that can be
  recognized by computers that are part of the
  multicast group
• The IEEE group used a special Rule to determine
  the various MAC addresses that will be considered
  for multicasting for ex MAC address
  01005E000005 will be used for the OSPF
  protocol

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IP Multicast

• The IP Multicast combined with the hardware
  multicasting, gives us a multicasting model that
  works for our Ethernet network
• Once Layer 2 (Datalink) picks the multicast
  packet from the network (because it recognises
  it, as the destination MAC address is a
  multicast) it will strip the MAC addresses off
  and send the rest to the above layer, which is
  the Network Layer.
• Class D IP address is used for multicast
  addresses (decided by IANA)
• Class D is flat- that is, subnetting is not
  used, so no network and host partition.
• In class D address first 4 bits will always be
  1110, the rest of 28 bits are group id, and there
  can be 228 multicast groups
• The group can be permanent such as (assigned by
  IANA)
• 224.0.0.0 Base Address (Reserved) RFC1112,JBP ,
  224.0.0.1 All Systems on this Subnet
  RFC1112,JBP ,224.0.0.2 All Routers on this
  Subnet JBP ,224.0.0.3 Unassigned JBP
  ,224.0.0.4 DVMRP Routers RFC1075,JBP,224.0.0.5
  OSPFIGP OSPFIGP All Routers RFC2328,JXM1
• Group can be transient.
• IPv6 has similar address allocations

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Mapping IP multicast to Ethernet Multicast

• Rule for the mapping
• To map an IP Multicast address to the
  corresponding Hardward/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. The rest
  of the high-order bits are defined by the IEEE
  (yellow color in the example)
• 23 bits are mapped to the base MAC address of the
  computer
• When combined makes one multicast Mac address

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Group Management and IGMP

• We will examine issues of joining and leaving
  group
• Then see how these issues are handled by IGMP

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Advertise Group membership
One Way of Locating Multicast Groups Is Through
Web-Based Announcements, Such as This Schedule of
MBone Sessions at www.cilea.it/MBone/browse.htm
Applications Such as Multikit Listen for SDP and
SAP and Display the Multicast Sessions Advertised
by Those Protocols
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Local and wide area multicast
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IGMP Join and Leave
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Joining and Leaving a Group cont
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Increasing efficiency of Joining and Leaving a
Group cont

• Allowing host to sends message to Router to join
  a group without waiting for query
• If routers have no group member then it doesnt
  forward any message
• Allowing a host to explicitly notify its local
  router when it leaves a group can increase
  efficiency
• After receiving leave request, router sends query
  to subnet asking if there is any other group
  members, if it doest get response, it ceases the
  packet forwarding
• Then router can prune itself from the multicast
  tree

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IGMP v2 Host functions

• Membership Report messages
• when host want to join a group
• In response to the router query

• Leave Group message
• when host want to leave group

• Version 1 Membership Report messages
• IGMPv2 hosts support IGMPv1 Membership Reports
  for backward compatibility.

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IGMP v2 Host function cont..

• Join
• Multicast sessions are identified in the routers
  by a (source, group) pair of addresses
• source is the address of the session's originator
  
• group is the Class D group address
• The destination address of the Membership Report
  message's IP header is the group address
• other group members that might be on the subnet
  hear the report in addition to the router.
• message itself also contains the group address
• host sends one or two duplicate reports separated
  by a short interval 10 sec. for reinsurance.
• The local router periodically polls the subnet
  with queries
• Each query contains a value called the Max
  Response Time 10 sec
• If timer expires, the host responds to the query
  with one Membership Report for each group to
  which it belongs.
• Other group member also hear it, but the timer
  plays a role, If the host receives a Membership
  Report for a group before its delay timer
  expires, it does not send a Membership Report for
  that group. In this way, the router is informed
  of the presence of at least one group member on
  the subnet, without all members flooding the
  subnet with reports.
• Leave
• The message contains the address of the group
  being left, but unlike Membership Report
  messages, the Leave Group message is addressed to
  the "all routers on this subnet address of
  224.0.0.2
• because only the multicast routers on the subnet
  need to know that the host is leaving other
  group members do not.

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IGMP v2 Router Function
General Query

• Group-Specific Query
• when host want to leave group

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IGMP Querier Election

• Querier Election
• Only one IGMP Querier per LAN
• Querier with lowest numbered IP source address
  (v2)

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IGMP v2 Router function cont..

• General Query
• For checking the presence of the group members
• queries are sent every 60 seconds
• query also contains a value called the Max
  Response Time. By default, the Max Response Time
  is 10 seconds
• sent to the "all systems on this subnet" address
  of 224.0.0.1 and does not contain a reference to
  any specific group
• As a result, the single message polls for reports
  from members of any and all groups that might be
  active on the subnet.
• The router tracks known groups and the interfaces
  attached to subnets with active members
• If router does not hear anything from the group
  in the twice the query interval plus one Max
  Response Time interval, then it considers there
  is no members to the group
• Group-Specific Query
• When a router receives a Leave Group message, it
  must determine whether any remaining members of
  that group are on the subnet by sending this
  query
• It contains the group address. Uses that as a
  destination address

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IGMPv1 Vs IGMPv2

• IGMPv1 has no Leave Group message, meaning that
  there is a longer period between the time the
  last host leaves a group and the time the router
  stops forwarding the group traffic.
• IGMPv1 has no Group-Specific Query. This follows
  from the fact that there is no Leave Group
  message.
• IGMPv1 does not specify a Max Response Time in
  its query messages. Instead, hosts have a fixed
  Max Response Time of 10 seconds.
• IGMPv1 has no querier election process. Instead,
  it relies on the IP multicast routing protocol to
  elect a designated router on the subnet. Because
  different protocols use different election
  mechanisms, it is possible under IGMPv1 to have
  more than one querier on a subnet.

RFC 2236 describes several mechanisms that allow
IGMPv2 to adapt in earlier versions
IGMPv3 inclusion of group to be identified not
only by group address, but also by source
address. If certain member wants to receive
traffic from specific source or exclude to
receive traffic from some specific source. The
member can express these wants in a Membership
Report with Include or Exclude filter requests.
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IGMP message format
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PIM
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Multicast Routing concepts

• source-based tree one tree per source
• shortest path trees, reverse path forwarding
• group-shared tree group uses one tree
• minimal spanning (Steiner) , center-based trees

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Shortest Path Tree
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Reverse Path Forwarding

• Rule
• forward packet from Source (R1) to all interfaces
  if and only if packet arrives on the interface
  that corresponds to the shortest path to Source
• no need to remember past packets
• R5 need not forward packet received from R6

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RPF / Pruning
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Core-based Trees An Example

• One router identified as center of tree.
• To join
• edge router sends unicast join-msg addressed to
  center router
• join-msg processed by intermediate routers and
  forwarded towards center
• join-msg either hits existing tree branch for
  this center, or arrives at center
• path taken by join-msg becomes new branch of tree
  for this router

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Core-based Trees An Example
Suppose R6 chosen as center
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Core based tree pros and cons

• Pros
• routers not part of a group are not involved in
  pruning
• explicit join/leave makes membership changes
  faster
• router needs to store only one record per group
• Cons
• all multicast traffic traverses core, which is a
  bottleneck
• traffic travels on non-optimal paths

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PIM v2

• Protocol independent because other protocols are
  dependent on Link state routing (MOSPF) and
  distance vector routing (DVMRP)
• This is the only protocol fully supported by
  Cisco.
• PIMv1 Vs PIMv2
• Version 1 of the protocol encapsulates its
  messages in IP packets with protocol number 2
  (IGMP) and uses the multicast address 224.0.0.2.
• PIM v2 uses its own protocol number of 103 and
  the reserved multicast address 224.0.0.13.
• PIM Choose different strategies depending on
  whether multicast tree is dense or sparse
• In dense mode the receivers are densely situated
  and most f the routes need to participate in the
  multicast forwarding
• flood and prune good for dense groups
• only need a few prunes
• CBT needs explicit join per source/group
• In sparse mode receivers are sparsely situated
• Flood and prune is a wastage. Too many prune
  message.
• Join and prune is better
• Center based tree is good for sparse groups

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PIM v2 Continue

• PIM uses a notion of central node (rendezvous
  point) RP for each group, which maintains
  multicast shortest path tree for each group
• We assume in a domain of routers each router
  knows the unicast IP address for RP of a
  particular group
• In PIM sparse there are two type of trees
  shared tree for a group and source specific
  tree
• Typically shared tree is built first and then
  source specific tree if required

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PIM-DM

• Uses five PIMv2 message
• Hello
• Join/Prune
• Graft
• Graft-Ack
• Assert
• Uses flood-and-prune to build the multicast tree.
• Flood hello to entire PIM domain, because it does
  not have a build in routing protocol
• Join and Prune happens the same way as explained
  in the IGMP protocol, same mechanism of waiting
  and prune overriding happens.
• The Graft message used mainly when there is
  change in topology. Recalculation of the RPF
  interface when the unicast routing table changes
• Needs to elect a Designated router. IGMP needs a
  DR.
• If there are two router to reach the destination,
  then PIM uses Assert to decide which route to
  select. (This is required because there is no
  routing protocol)

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PIM-SM

• Supports both shared and source-based trees. This
  is why popular.
• It has following packets
• Hello
• Bootstrap
• Candidate-RP-Advertisement
• Join/Prune
• Assert
• Register
• Register-Stop
• In Shared tree model we have core router. This is
  called rendezvous point (RP) in PIM-SM.
• RP can be configured in routers statically
• Open standard bootstrap protocol can be used
• Cisco-proprietary Auto-RP protocol can be used to
  designate and advertise the RP.
• In shared Tree the multicast tree is rooted
  towards the core rather than source.
• CBT uses bidirectional tree, because source might
  need to deliver the packets to the RP over the
  branches of the tree. This can lead to a loop in
  topology, because RPF checks can not be
  performed if there is upstream and downstream
  as it is protocol independent.
• To achieve this PIM-SM uses register and register
  stop message, and encapsulate the multicast
  packet in these messages.
• If there are too many encapsulated message then
  it sends register-stop message to stop this
  process and chooses source based STP rather
  than Shared Once STP
• Depending on the situation different scheme can
  be chosen.

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Some experiments

• I wanted to see whether our network supports
  multicast of not http//detective.internet2.edu
• Try a simple multicast application
• Vic, rat, sdr - http//www.openmash.org/
• Listen to some multicast stream -
  http//people.internet2.edu/bdr/dvguide.html
• There is no multicast streams in our network, no
  IGMP no PIM

Tool for testing multicast enabled network
List of multicast server
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Some experiments cont

• Enabling IP multicast
• Configuring routers on your network
• Enable multicast for the network (globally).
• Determine the interfaces on which to use
  multicast, and enable multicast on those
  interfaces.
• Enable multicast routing protocols on specific
  devices. For example
• PIM Sparse mode for links that have limited
  bandwidth.
• PIM Dense mode for links that have broad
  bandwidth.
• IGMP or DVMRP4. Create access lists specifying
  the range of multicast group addresses allowed to
  cross the router.
• Associate access lists with specific interfaces
  on different routers.
• http//technet2.microsoft.com/windowsserver/en/lib
  rary/1eacd6dc-f51e-474f-9a49-ba524a15d6691033.mspx
  ?mfrtrue

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Finally we can see something like