Multicast

Multicast

• The source IPs in multicast packets remain Unicast
• The destination IPs represent the Multicast Group
• Multicast Address range
• 224.0.0.0 - 239.255.255.255
• Class D is flat IP space; there is no concept of Subnetting
• IGMPv2 is the most common
• Every Single IP represents a multicast Group.
• The Querier is elected based on the IP, the router with the lower IP becomes the querier.
• There are some reserved sub-ranges
• 224.0.0.0/24 - Local Network control block (Routing protocols)
• Link local Scope
• TTL of 1 or 2 - TTL for NMBA
• Eg. 224.0.0.5 and 224.0.0.6 for OSPF and 224.0.0.9 for EIGRP
• 232.0.0.0/8 - SSM Block
• For multicast block whose source are already known
• 239.0.0.0/8 - Organization - Local Scope (Private IPs)
• Compared to RFC 1918
• Should be limited to use withing an AS
  
  
• A receiver already knows which L3 multicast it will accept.
• This is at the application layer
• Works its way down the TCP/IP stack
• Ultimately results in the NIC "tuned" to the L3 multicast IP address.
  
  
• Multicast L2 address must be dynamic and well known.
• Dynamic because the L3 multicast groups are highly arbitrary
• Well known because a unique L3 multicast address must have a consistent L2 address.
• To provide this, a simple mapping procedure is used to generate a well-known multicast MAC address.
  
  
• For IP multicast, there is a well-known MAC OUI that is used exclusively.
• 01-00-5E - These 24 bits identify IP multicast
• Looking into the first 8 bit 00000001, the last bit is the Individual group bit. The remaining bits 00-5E always indicate multicast.
• If the bit is 0, it identifies individual address or unicast
• If the bit is 1, it identifies group or multicast
  
  
• There are 48 bits in MAC address and 24 have been fixed for OUI and can only be 01-00-5E
• Of the remaining 24 bits, the most significant bit is fixed to 0 (01-00-5E-0 the 0 in this case).
• This leaves with 23 bits (48-24-1=23) to map the L3 IP.
• The Lower Order 23 bits of the IP are simply copied to LO 23 bits of the MAC.
• Since the 28 bits of the IP are being mapped onto 23 bits of the MAC, there is ambiguity. 28 bits = (11100000.0.0.0, the first half of the first octet is already fixed to multicast 224, so 32 - 4 = 28. )
• Eg. 230.1.2.3, 230.129.2.3 and 239.1.2.3 all map to 0100-5E01-0203
  
  
• Two major interface types per multicast source
• Upstream 
• Closest to the source
• Downstream 
• Where interested receivers are
• There is always a single incoming interface
• IIF or incoming interface
• There is an interface list which  can contain one or more interface as the receivers join or leave the group.
• OIL 
• Multicast routing protocol has three main responsibilities.
• Identify Upstream interface or IIF
• FIB is utilized
• The interface on the shortest path to the source is the upstream interface
• Can be overridden with MC specific interfaces
• Identify downstream Interface OIL
• Routing protocol will provide procedures for the receivers to signal interest
• As the request come into the router, OIL is created
• Maintain dynamic multicast trees
• Add and removes both IIF and OIL dynamically as the source and receivers come online and go offline.
• Multicast packet always arrives at router IIF and is forwarded to OIL
• Multicast tree
• (S,G) Trees are rooted at the source.
• Shortest path tree
• (*,G) Trees
• Shared tree
• RPF Check avoids loop.
• MC packets go through a reverse path forwarding check (RPF)
• As MC packet arrives, the router inspects the source IP of the packet.
• The IIF is identified
• If the packet was not received on a valid IIF, it is dropped.
• By Default, as with MC routing table, unicast FIB information is utilized.
• Information can be overridden with specific information.
• RPF check override
• There can be cases where the MC topology does not match the Unicast topology.
• RPF checks utilizing UC routing table will cause problem.
• Most vendors implementation allow for RPF overrides.
• Separate routing table for RPF
• cans be populated statically or via multi-topology IGP implementations
• MBGP can also be used
• Static RPF override
• This is the so called static multicast route
• ip mroute <Network> <Mask> <Next-hop> These are unicast IP.
• 
  

Layer 2 multicast

• L2 multicast Flows
• Mostly automatic
• Basic switches treat MC as BC
• Send it out all ports except the ingress
• All host in the vlan receive the stream
• Interested receivers accept the packet, other drop them.
• Not very efficient but works.
• MC aware switches
• Inspect the MC signaling between receivers and routers
• Create proper MC state for each switchport
• Only interested receivers are forwarded the MC packets.
• IGMP
• Once a receiver knows which MC group to receive, it must inform a local, multicast enabled router.
• Receiver Acquires the information from the application layer.
• T e router is called last hop router (LHR)
• Reachable at layer2
• this is the step that separates multicast from broadcast
• Makes joining the group explicit vs implicit
• Receiver sends an IGMP message to join the group.
• A MC router on the segment listens for all IGMP messages.
• Once it receives the relevant IGMP message on an interface, the interface is added to the OIL for this group.

• IGMPv1
• Silent leaves.
• MC routing protocol was responsible for querier selection.
• Messages
• Membership report
• How receivers signal their interest in a MC group to MC routers.
• Informally called the IGMP join.
• Membership query
• How MC router confirms continued interest of receivers in a MC group.
• V1 only support one form, which is the General.
• Generated periodically to update status of all MC groups
• IGMPv2
• Added few new features to V1.
• Mechanisms for receivers to leave a certain group.
• Election of a querier.
• This is the default version on a cisco device.
• Messages
• Membership report
• How receivers signal their interest in a MC group to MC routers.
• Informally called the IGMP join.
• Leave Group
• How receivers inform the MC router when they are no longer interested in a group.
• Membership query
• How MC router confirms continued interest of receivers in a MC group.
• Queries can be of two forms:
• General
• Generated periodically to update status of all MC groups
• Group specific
• Event-driven to update status of a particular MC group
• The event is a host leaving the group.

• IGMP Membership Report
• Membership report is how receivers join MC groups.
• The MC router on the local segment is being signaled
• The MC router listens to all MC traffic
• IP Packets
• Source IP - Hot's configured IP
• Destination IP - The group's MC IP.
• IP Options - Router Alert (TTL)
• The MC router accepts because it is listening to all MC traffic
• The MC router processes this because of the Router Alert option
• IGMP Leave Group
• Membership Report is how the receivers leave MC groups
• The MC router on the local segment is being signaled
• IGMPv2 specific enhancement - not used in IGMPv1
• IP Packet
• Source IP - Host's configured IP
• Destination IP - 224.0.0.2 (special IP known as ALL-ROUTERS)
• IP Option - Router Alert
• The MC router accepts as any MC router must accepts ALL-ROUTERS multicast
• The MC router processes this because of the Router Alert option
• IGMP Membership Query
• Membership Query is how MC router update IGMP states
• Sent to all MC hosts on a segment
• One host per group must respond for the group to remain active
• IP Packet
• Source IP - Designated querier's (MC router) IP.
• Destination IP - Depends on General vs Group-specific Query
• General Query - 224.0.0.1 (Special IP knows as ALL-SYSTEMS)
• Group-Specific Query - MC IP of the group being queried
• IP Options - Router Alert
  
  
• IGMPv1/v2 Mechanism
• Receiver Joins a new group
• A receiver wants to join a group and send a membership report (Join)
• Multicast on the Group IP address
• Unsolicited to reduce join latency
• Recipient
• MC routers - Listens to all MC messages
• Other group members - Listens to the MC group
• Actions
• Group members
• No action/Cancel any pending Reports for the group
• MC routers:
• Create new state for the group if none existed
• Refresh timers on current stated if group exists.
  
  
• Periodic General Queries and reports
• Every IGMP state (for each group) on a router has a timer
• Must be refreshed periodically to keep the state active
• General queries ensure the groups(s) still has interested receivers
• Sent on 224.0.0.1 (ALL-SYSTEMS) MC IP
• Recipients:
• All IP devices (routers and hosts)
• Actions
• Hosts:
• Start random timer (<Max Resp Time) per active group
• If timer expires, send and IGMP report
• Cancel timer if another IGMP report group is heard
• MC Routers
• Querier election
• Lower IP becomes the querier
• State refresh of the group in reception of a Report.
  
  
• Receiver Leaves a new group
• A receiver on the LAN decides to leave a group and multicasts a leave group message
• Unsolicited to reduce leave latency
• Sent on 244.0.0.2 (ALL-ROUTERS)
• Recipients:
• MC Routers - ALL-ROUTERS address
• Actions:
• MC routers:
• Send Group-Specific query if other members exist for the group
• Delete group state if no other member exists.
  
  
• Group Specific query
• When a member leaves a group, the router polls the remaining group members.
• Only sent if the host to leave was not the only host on that group.
• Unlike general queries, these are event driven and for specific groups.
• Multicasted on the group IP address
• Recipients
• All members subscribed to the group
• Actions:
• Hosts:
• Starte a random timer (< Max Resp Time) per active group
• If timer expires, send an IGMP Report for the group
• Cancel timer if another IGMP Report for group is heard
• MC Routers:
• No action
• IGMPv3
• It adds one major feature
• Allows for the signaling source information along with the group
• This was not possible with V1/V2
• Enhancement allows for the use of SSM
• It has a mechanism for excluding certain sources.
• New MC address for Membership Reports
• 224.0.0.22
• Backwards compatibility with IGMPv1 and v2
• IGMPv1/v2 messages are processed by the IGMPv3 LHR
• Can be encapsulated either in IGMPv1/2 or IGMPv3 packets
• IGMPv3 also provided mechanisms to replicate IGMPv1/2 functions natively.
• Messages
• IGMPv3 Membership reports (V3 Joins)
• Membership report is how receivers join MC groups
• The MC router local segment is being signaled
• The MC router listens to all MC traffic (224.0.0.22)
• IP Packet
• Source IP - Host's configured IP
• Destination IP - 224.0.0.22
• It eliminates the "one report per group" requirement of v1/v2.
• Multiple groups can now be included in a single report.
• IP Options - Router Alert
• The MC router accepts because it is listening to 224.0.0.22
• Packet structure
• More complex than IGMPv1/v2
• Record Type
• New fields in IGMPv3
• Provides great flexibility to IGMPv3 including the ability to signal SSM and ASM groups
• Allows the report also to carry Leave message.
• Concept of filter mode is used
• Include
• MC stream is required only from source included
• Exclude
• MC stream is requested from sources excluding the record.
• Record type
• Current-state Record (steady state signaling)
• MODE_IS_INCLUDE
• MODE_IS_EXCLUDE
• Filter-Mode-Change (when the mode of a group changes)
• CHANGE_MODE_IS_INCLUDE
• CHANGE_MODE_IS_EXCLUDE
• Source-List-Change (when sources for a group change)
• ALLOW_NEW_SOURCES
• BLOC_OLD_SOURCES
• IGMPv3 Membership Query (Hello! Is anyone there? v3 style?)
• This is how MC routers update IGMP states
• Sent to all MC hosts on a segment.
• One host per group must respond for the group to remain active
• IP packet
• Source IP - Designated querier's (MC router) IP
• Destination IP - Depends on General vs Group-Specific Query
• General Query - 224.0.0.1 (same as v1/v2)
• Group-specific Query - MC IP of the group being queried (same as v1/v2)
• Group-and-source-specific Query - MC IP of the g4oup being Queried (same as v1/v2)
• IP Options - Router alert
• IGMPv3 Mechanisms
• Receiver joins a new ASM Group "*,G" (Any-Source-Multicast) 
• A receiver wants to join a group and sends a membership report (join)
• Sent to 224.0.0.22
• Unsolicited to reduce join latency
• Record type (Majority cases):
• CHANGE_TO_EXCLUDE_MODE
• Default mode is include (majority cases)
• Number of sources 
• 0 to indicated ASM
• Recipients
• MC router - Listens to 224.0.0.22
• Actions 
• Create new state for the group if none existed
• Refresh timers for current state if group exists
• Receiver joins a new SSM Group "S,G" (Any-Source-Multicast) 
• A receiver wants to join a group and sends a membership report (join)
• Sent to 224.0.0.22
• Unsolicited to reduce join latency
• Record type (Majority cases):
• ALLOW_NEW_SOURCES
• Default mode was already include 
• Number of sources 
• >=1 to indicated SSM
• Recipients
• MC router - Listens to 224.0.0.22
• Actions 
• Create new SSM state for the group if none existed
• Refresh timers on current SSM state if group exists
  
  
• Periodic general Queries
• Every IGMP state (for each group) on the router has a timer.
• Must be refreshed periodically to keep the state active
• General queries ensure the groups(s) still has interested receivers
• Sent on 224.0.0.1 (ALL-SYSTEMS) MC IP
• Recipients
• All IP devices (routers and hosts)
• Actions
• Hosts
• Start a random timer (< Max Resp Time) per active group
• When timer expires, send a IGMP Report
• Major difference - The timer will expire (vs may expire in v1/v2)
• MC Router
• Querier election
• State refresh of the group on reception of a Report
• Tunning of Query interval to match that of the Designated Querier
  
  
• Receiver wants to leave an ASM group and sends a membership report
• Sent to 224.0.0.22
• No separate Leave Group message in v3
• Unsolicited to reduce leave latency.
• Record type
• CHANGE_TO_INCLUDE_MODE
• Mode for active ASM groups is exclude
• Number of sources 
• 0 to indicate a leave
• Recipient
• MC routers - Listen to 224.0.0.22
• Actions
• Group-specific-query for the Group
  
  
• Receiver wants to leave an SSM group and sends a membership report
• Sent to 224.0.0.22
• No separate Leave Group message in v3
• Unsolicited to reduce leave latency.
• Record type
• BLOCK_OLD_SOURCES
• Mode for SSM groups is exclude
• Number of sources 
• >1 to indicate an SSM leave
• Recipient
• MC routers - Listen to 224.0.0.22
• Actions
• Group-specific-query for the SSM Group
  
  
• Group Specific query
• When a member leaves an ASM group, the router polls the remaining group members.

• Unlike general queries, these are event driven and for specific groups.
• Multicasted on the group IP address
• Recipients
• All members subscribed to the group
• Actions:
• Hosts:
• Start a random timer (< Max Resp Time) for the ASM group
• When timer expires, send an IGMP Report
• Major difference - The timer will expire (vs. may expire in v1/2)
• Cancel timer if another IGMP Report for group is heard
• MC Routers:
• State refresh of the ASM group on reception of a report 
• Tuning of Query interval for the ASM group to match that of the Designated Querier
• Group-and-source-specific Query
• When a member leaves an SSM group, the Router polls the remaining members
• Unlike General Queries, these are event driven and for specific groups
• Multicasted on the group IP address
• Recipients
• All members subscribed to the group
• Actions
• Hosts:
• Start a random timer (<Max Resp Time) for the SSM group
• When timer expires, send an IGMP Report
• Major difference - The timer will expire (vs. may expire in v1/2)
• MC Routers:
• State refresh of the SSM group on reception of a report 
• Tuning of Query interval for the SSM group to match that of the Designated Querier
• Source Signaling
• MC Packet on an MC Enabled Interface
• MC source signaling is a much easier concept than MC receiver signaling.
  
• Happens entirely in the data plane.
• The router connecte4d tot he MC source is called First Hop Router on an FHR
• When this router receives a multicast packet on a multicast enabled interface, it creates and (S,G) entry.
• This router becomes the Root of the Source tree or the Shortest Path Tree (SPT)
• After this state is created, multicast routing protocol takes over.
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