Echelon LNS - Manual (Page 172)

LNS Programmer's Guide 158
propagates all outgoing packets using domain broadcast addressing, the device only
needs one address table entry per target domain since LNS only supports a single
domain, plus one for turn-around connections directed to itself.
Therefore, a good way to conserve address table space is to avoid using group
connections, and instead build connections that use subnet broadcast addressing and the
unacknowledged/repeat messaging service. This strategy saves group identifiers, and
also creates an address table entry that is likely to be re-usable by other connections
originating from the same device.
However, the address table does not only accommodate the destination address. The
second set of data being kept in each address table entry is the set of transport properties
like the repeat count, repeat timer and the transmit timer. In summary, address table
shortages can be avoided or overcome by the following means:
• Use broadcast addressing whenever possible to obtain re-usable address table
entries.
• Avoid group connections (for the group membership information itself is kept in
the address table on each device that is a member of said group).
• Keep variations on transport properties to a minimum.
When groups must be avoided by splitting a group-addressed multicast connection into
multiple unicast connections as explained in the previous section, it requires alias table
space, and may also quickly consume a large amount of address table space on the sender
device. This is because each unicast connection may require its own address table entry.
One way to conserve address table entries is to combine the lcaBroadcastAlways and
lcaAliasForUnicasts values, as described in Table 7.2. This combination will help
avoid the effect of address table consumption resulting from the use of multiple unicast
connections instead of a single (group-addressed) multicast connection. In this case, LNS
will split the multicast connection into multiple unicast connections, but it will also use
broadcast addressing for each of these unicast messages. Assuming at least two targets
reside on the same subnet, this will result in re-use of address table entries.
Shortage of Aliases
Since aliases are defined at device development time, their number is finite and fixed. At
integration time, the tools available are those that re-gain aliases, and those that avoid
alias consumption.
Consider the case of a new, connection C
2
, and assume C
2
requires an alias on a particular
device. Assume the device supports aliases, but all available ones are exhausted by
previously defined connections. These connections might contain a connection C
1
that can be
disconnected and reestablished, using different connection policy preferences, resulting in
C
1*
. This re-designed connection must connect the same set of senders and recipients, but
may not require an alias on the device in question.
Summary of Resource Shortage Recommendations
Resource limitations cannot easily be overcome at integration time, but most limitations
can be overcome with some trade-off. If a network is characterized by three critical
resources (e.g. group IDs, address table entries and alias table space), shortages in one of