These are my notes for IPv6 multicast for the CCDE exam. Overview

  • Prefix FF::/8 reserved for multicast
  • Multicast Listener Discovery (MLD) replaces IGMP
    • MLD is part of ICMPv6
    • MLDv1 equivalent to IGMPv2
    • MLDv2 equivalent to IGMPv3
  • ASM, SSM and Bidir supported
  • PIM identified by IPv6 next header 103
  • BSR and static RP supported
  • No support for MSDP
    • Anycast supported through PIM, defined in RFC4610
  • Any Source Multicast (ASM)
    • PIM-SM, PIM-BiDir
    • Default for generic multicast and unicast prefix-based multicast
    • Starts with FF3x::/12
  • Source Specific Multicast (SSM)
    • PIM-SSM
    • FF3X::/32 is allocated for SSM by IANA
    • Currently prefix and plen is zero so FF3X::/96 is useable for SSM
  • Embedded RP groups
    • PIM-SM, PIM-BIDir
    • Starts with FF70::/12

IPv6 Multicast Addressing

IPv6 multicast address format includes variable bits to define what type of address it is and what the scope is of the multicast group. The scope can be:

1 – Node

2 – Link

3 – Subnet

4 – Admin

5 – Site

8 – Organization

E – Global

The flags define if embedded RP is used, if the address is based on unicast and if the address is IANA assigned or not (temporary). The unicast based IPv6 multicast address allows an organization to create globally unique IPv6 multicast groups based on their unicast prefixes. This is similar to GLOP addressing in IPv4 but does not require an Autonomous System Number (ASN). IPv6 also allows for embedding the RP address into the multicast address itself. This provides a static RP to multicast group mapping mechanism and can be used to provide interdomain IPv6 multicast as there is no MSDP in IPv6. When using Ethernet, the destination MAC address of the frame will start with 33:33 and the remaining 32 bits will consist of the low order 32 bits of the IPv6 multicast address.

Well Known Multicast Addresses

FF02::1 – All Nodes

FF02::2 – All Routers

FF02::5 – OSPF All Routers

FF02::6 – OSPF DR Routers

FF02::A – EIGRP Routers

FF02::D – PIM Routers

Neighbor Solicitation and DAD

IPv6 also uses multicast to replace ARP through the neighbor solicitation process. To do this the solicited node multicast address is used and the prefix is FF02::1:FF/104 and the last 24 bits are taken from the lower 24 bits of the IPv6 unicast address. If Host A needs to get the MAC of Host B, Host A will send the NS to the solicited node multicast address of B. IPv6 also does Duplicate Address Detection (DAD) to check that noone else is using the same IPv6 address and this also uses the solicited node multicast address. If Host A is checking uniqueness of its IPv6 address, the message will be sent to the solicited node multicast address of Host A.

Multicast Listener Discovery (MLD)

  • MLDv1 messages
    • Listener Query
    • Listener Report
    • Listener Done
  • MLDv2 messages
    • Listener Query
    • Listener Report

MLDv2 does not use a specific Done message which is equivalent to the Leave message in IGMP. It will stop sending Reports or send a Report which excludes the source it was previously interested in.

Protocol Independent Multicast (PIM) for IPv6

  • PIM-SM (RP is required)
    • Many to many applications (multiple sources, single group)
    • Uses shared tree initially but may switch to source tree
  • PIM-BiDir (RP is required)
    • Bidirectional many to many applications (hosts can be sources and receivers)
    • Only uses shared tree, less state
  • PIM-SSM
    • One to many applications (single source, single group)
    • Always uses source tree
    • Source must be learnt through out of band mechanism

Anycast RP

IPv6 does not have support for MSDP. It can support anycast RP through the use of PIM which can implement this feature. All the RPs doing anycast will use the same IPv6 address but they also require a unique IPv6 address that will be used to relay the PIM Register messages coming from the multicast sources. A RP-set is defined with the RPs that should be included in the Anycast RP and the PIM Register messages will be relayed to all the RPs defined in the RP-set. If the PIM Register message comes from an IPv6 address that is defined in the RP-set, the Register will not be sent along which is a form of split horizon to prevent looping of control plane messages. When a RP relays a PIM Register, this is done from a unique IPv6 address which is similar to how MSDP works.

Sources will find the RP based on the unicast metric as is normally done when implementing anycast RP. If a RP goes offline, messages will be routed to the next RP which now has the best metric.

Interdomain Multicast

These are my thoughts on interdomain multicast since there is no MSDP for IPv6. Embedded RP can be used which means that other organization needs to use your RP. Define a RP prefix that is used for interdomain multicast only or use a prefix that is used for internal usage but implement a data plane filter to filter out requests for groups that should not cross organizational boundaries. This could also be done by filtering on the the scope of the multicast address.

Another option would be to anycast RP with the other organization but this could get a lot messier unless a RP is defined for only a set of groups that are used for interdomain multicast. Each side would then have a RP defined for the groups and PIM Register messages would be relayed. The drawback would be that both sides could have sources but the policy may be that only one side should have sources and the other side only has listeners. This would be difficult to implement in a data plane filter. It might be possible to solve in the control plane by defining which sources the RP will allow to Register.

If using SSM, there is no need for a RP which makes it easier to implement interdomain multicast. There is always the consideration of joining two PIM domains but this could be solved by using static joins at the edge and implementing data plane filtering. Interdomain multicast is not something that is implemented a lot and it requires some thought to not merge into one failure domain and one administrative domain.

Final Thoughts

Multicast is used a lot in IPv6, multicast is more tightly integrated into the protocol than in IPv4, and it’s there even if you see it or not. The addressing, flags and scope can be a bit confusing at first but it allows for using multicast in a better way in IPv6 than in IPv4.

IPv6 Multicast
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4 thoughts on “IPv6 Multicast

  • July 14, 2015 at 1:32 pm
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    Personally, I wouldn’t say that for IPv4, multicast was an afterthought. The anecdote about Mr. Deering’s request to his supervisor to purchase 16 OUIs to map the entire IPv4 Class D into MAC address space uniquely (and the supervisor buying just a single OUI and keeping a half of it to himself) is well known but it shows that multicast started off in IPv4 world. Whether it has gained wide acceptance is a different issue.

    Reply
    • July 14, 2015 at 3:27 pm
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      Maybe I took it a bit to far. There’s the anecdote which you mentioned but the point was that multicast is more tightly integrated into IPv6 and it’s there whether you deploy it or not.

      Thanks, Peter!

      Reply
  • January 10, 2016 at 5:55 am
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    I wanna know how does a Multicast ping work?

    Reply
    • January 11, 2016 at 12:49 pm
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      When you ping with a destination IP that is a multicast address, you are simulating a multicast source sending packets to the group. This will then trigger PIM to build the source tree towards the RP (if using ASM).

      Please note though that the reply to the ping is unicasted back towards the source.

      Reply

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