Unlimited Mobility; Support For Host Mask; Source Address Selection For Outbound Connections - Novell BUSINESS CONTINUITY CLUSTERING 1.1 SP2 - ADMINISTRATION Administration Manual

8.2.2 Unlimited Mobility

Unlike physical IP addresses, which are limited in their mobility, virtual IP addresses are highly
mobile. The degree of mobility is determined by the number of servers that an IP address on a
specific server could be moved to. In other words, if you choose a physical IP address as an IP
address of a network resource, you are limiting the set of potential servers to which this resource
could transparently fail over to.
If you choose a virtual IP address, the set of servers that the resource could be transparently moved
to is potentially unlimited. This is because of the nature of virtual IP addresses; they are not bound to
a physical wire and, as a result, they carry their virtual network to wherever they are moved. Again,
there is an implicit assumption that the location of a virtual IP address is advertised to the owning
server through some routing protocol. The ability to move an IP address across different machines
becomes particularly important when you need to transparently move or fail over a network resource
that is identified by an IP address (which could be a shared volume or a mission-critical service) to
another server on another network.
This unlimited mobility of virtual IP addresses is an advantage to network administrators, offering
more ease of manageability and greatly minimizing network reorganization overhead. For network
administrators, shuffling services between different IP networks is the rule rather than the exception.
The need often arises to move a machine hosting a particular service to some other IP network, or to
move a service hosted on a particular machine to be rehosted on some other machine connected to a
different IP network. If the service is hosted on a physical IP address, accommodating these changes
involves rehosting the service on a different IP address pulled out from the new network, and
appropriately changing the DNS entry for the service to point to the new IP address. However, if the
service is hosted on a virtual IP address, the necessity of changing the DNS entries for the service is
eliminated.

8.2.3 Support for Host Mask

Virtual boards support configuring virtual IP addresses with a host mask. This results in a single
address being used rather than an entire subnet. See Section 8.3, "Reducing the Consumption of
Additional IP Addresses," on page 98.

8.2.4 Source Address Selection for Outbound Connections

Full resilience of connections to interface failures can be ensured only when the connections are
established between machines through using virtual IP addresses as end point addresses. This means
an application that initiates outbound connections to a virtual IP address should also use a virtual IP
address as its local end point address.
This isn't difficult if the application binds its local socket end point address with a virtual IP address.
However, there are some legacy applications that bind their sockets to a wildcard address (such as
0.0.0.0). When these applications initiate an outbound connection to other machines, TCP/IP
chooses the outbound interface's IP address as the local socket end point address. In order for these
legacy applications to take advantage of the fault resilience provided by the virtual IP address
feature, the default source address selection behavior of TCP/IP has been enhanced to accommodate
the use of a virtual IP address as the source IP address. As a result, whenever a TCP or UDP
application initiates an outbound connection with a wildcard source IP address, TCP/IP chooses the
first bound virtual IP address as the source IP address for the connection.
Virtual IP Addresses 97