[LINK] Congestion (was Re: NBN to cost 24 times South Korea's faster network, says research body)

Paul Brooks pbrooks-link at layer10.com.au
Mon Feb 21 12:39:09 AEDT 2011

On 20/02/2011 3:50 PM, George Bray wrote:
> On Mon, Feb 14, 2011 at 10:39 AM, Paul Brooks
> <pbrooks-link at layer10.com.au> wrote:
>> Note also that as far as the NBN is concerned, 'multicast' is ethernet multicast, not
>> IP multicast (although the two layers might need to be connected), and the multicast
>> capability might well be unidirectional - from the PoI towards the user, not from
>> user-to-user.
>> Paul
> Paul, what's the difference between ethernet multicast and IP multicast?
> George

Ethernet multicast can be driven from multicast IP addresses, but need not be - for
example, the NBN could set up a ethernet-multicast tunnel-tree to distribute some
streaming content to all listeners, while the content itself at the IP layer could use
non-multicast IP addresses as the destination, if it didn't require any response
(think one-way UDP streams being blasted out blind, with no return stream required).

Ethernet multicast can be set up independently of any particular layer 3 protocol - so
the NBN multicast service could carry IPv4, and/or IPv6, and/or AppleTalk or whatever
the proprietary Mac communications protocol on LANs is these days.

Whenever IP multicast addresses are used, the underlying network equipment has to
support recognising those addresses and translating them to the link-level equivalent
of multicast or broadcast for whatever the link-level technology is, or the
multicast/broadcast semantics don't work. If the router sending out a packet with a
IPv4 multicast IP destination address doesn't use the appropriate ethernet multicast
MAC address when the packet is put on the wire, it won't achieve the desired result -
so the two layers can be linked, but they are not the same.

Working out which ethernet multicast MAC address matches to a particular IP multicast
destination address is the function of a whole pile of helper protocols such as  IGMP
in IPv4 and MLD on IPv6, and it is not currently clear how these will be supported on
the NBN, and most real broadband implementations also need to rely on the service
provider's equipment doing IGMP-snooping to successfully match the layers together.
The NBN is fundamentally a layer-2 ethernet service and knows nothing about IP
addresses, leaving this to negotiation between the end-user's device and the service
provider, however the NBN documentation, and Comms Alliance documentation which is a
precurser to the NBN product description does talk of a possible requirement for the
NBN network terminal to do some limited Layer-3 functions like IGMP-snooping in order
to get the multicast service working properly.

A simplistic implementation might be for the NBN to ignore IGMP or MLD and establish
multicast replication from a particular service provider's inbound stream at the the
headend to a dedicated port for each user that subscribes to the particular service
provider - hard coded by the provisioning system so that each packet from the service
provider is delivered to each subscribed user, completely irrespective of the
destination IP address of the packet, or even whether it was IP at all.

This could also mean that if an end-user was to send out packets destined to a
ethernet-multicast or IP-multicast address, they may only be delivered to the service
provider, not to each of the other end-users in the same distribution tree.


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