Subject: Multicast:draft-ietf-pilc-link-design

From: Dr G Fairhurst <[email-masked]>
Date: Mon, 08 May 2000 15:05:14 +0100

I believe the current IP multicasting section is good, but it would
benefit from more clearer explanation of what is required.

I've tried to write something concisely, but may be others also have
views?

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IP multicasting is a network layer service which delivers a single
received packet to one or more destinations based on the packet’s
destination group address.   IP multicast provides dynamic many-to-many
connectivity between a set of senders (at least 1) and a group of
receivers.  The format of IP multicast packets is identical to that of
unicast packets and is distinguished only by the use of a special class
of destination address (class D) which denotes a specific multicast
group.

A point-to-point network may use unicast to transfer multicast packets.
Unicast may also be used to send IP Multicast packets over shared
networks by replicating each packet and sending a copy to a set of
receivers, however it is more efficient for subnetworks to support
multicast natively.  The minimal requirement for native multicast is the
support of a broadcast link. However, this simply delivers all packets
to all receivers.  Receivers who do not wish to receive the packets must
discard them (in the driver/ IP forwarding software). Like unicast, IP
multicast performance may be improved by enhancements to the link layer.
There are three enhancements:

A set of subnetwork addresses may be allocated to carry multicast IP
packets.  One address should be assigned to the all-nodes broadcast
service.  The assignment of IP multicast addresses to other subnetwork
addresses may be deterministic (e.g. as used in Ethernet) or dynamic (as
sometimes used in ATM).  The number of subnetwork addresses used may be
less than the number of active IP multicast sessions, since the same
subnetwork address may be used to support multiple IP multicast
addresses.  The optimal number of subnetwork addresses is a function of
the number of active multicast sessions, and the number of advertised
multicast sessions.

The IP receiver (driver or forwarding engine) removes IP multicast
packets for groups which are not known to be active at the receiver. To
minimise network processing, subnetwork receivers may request the
network to filter unwanted received multicast packets (i.e Class D
multicast addresses which the receiver has not requested to be
forwarded).  To ease implementation when a large number of subnetwork
addresses are used, some implementors choose to forward/discard using
hash functions.

For multiple-hop subnetworks (i.e. where the subnetwork is formed from a
set of active subnetwork devices, e.g. Ethernet switches or radio
repeaters) addresses may be used by a subnetwork to construct a delivery
tree. This optimises transmission resources by ensuring that packets are
only delivered to parts of the subnetwork where they will not be
subsequently filtered. A multicast routing/membership protocol will be
required to identify which edge nodes in the subnetwork require packets
for each subnetwork address.  This information may then be used to
configure appropriate egress filters on each out-going link within the
subnetwork devices.

N.B. 1) I believe that if the subnet can support native multicast in an
efficient way, it should.

N.B. 2) The current draft speaks of “links” at this section.

Gorry Fairhurst

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From: Rajkumar - Paulraj <[email-masked]>
Date: Mon, 8 May 2000 21:03:53 -0500 (CDT)

> the number of active multicast sessions, and the number of advertised

IMHO ..

Using a single subnetwork address for multiple IP multicast addresses
may introduce problems in building layer 2 forwarding trees due to
overlap of the IP addressses when mapped to the subnetwork addresses.
Because of this the IP addresses of the multicast sessions need
to be carefully selected ( eg: as in ethernet addresses
224.0.0.1, 224.128.0.1 , 225.0.0.1, 225.128.0.1 , etc all map
map to 01-00-5e-00-00-01 ).

Ideally, I would say we should have unique mappings. 

The optimal number of subnetwork addresses really depends on
the number of unique groups.

For eg:

we can have 100 sessions from one source to 100 groups

OR

we can have 100 sessions from 100 sources to 1 group.

The requirements on the subnetwork addresses are quite different
even though they have the same number of sessions.

The above is true for ISM model.

For the SSM model, the number of subnetwork addresses is probably
a function of the number of advertised sessions & number
of active sessions.

Rajkumar

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From: Dr G Fairhurst <[email-masked]>
Date: Tue, 09 May 2000 11:28:31 +0100

Thanks for your  feedback.

Please do see below:

> > the number of active multicast sessions, and the number of advertised

For a subnetwork involving multiple hops...

> Ideally, I would say we should have unique mappings.

I tired to cover this in my final paragraph, but perhaps it wasn't clear.

> even though they have the same number of sessions.

Yes, I wonder if this is a goog way to express this simply?

> a function of the number of advertised sessions & number

Yes, I think what you say is true, but I wonder if "SHOULD" is a bit strong,
I wanted
to make sure that people unambiguously realised it would work - but that this
was highly
desirable, particularly when we have a shared link with a common medium
between a set of nodes
or a switched subnetwork where some intermediate node links two or more
subnetwork areas.

What do you think?