72 Software Networks
3.9. Active networks
Active networks are similar to other packet- and frame-transfer
networks. The basic unit transferred in these networks is the packet.
The role of the nodes is to examine the different fields in the packet,
which are placed in accordance with a very rigid predefined system. In
particular, the address field determines the output port. The different
fields in the packet are interpreted by a virtual machine. We can
consider that this virtual machine is a packet interface, which is
typically called a “network API”, or “NAPI” (Network Application
Programming Interface). For an IP network, the IP API is the
language defined by the syntax and semantics of the IP header. In
typical networks, virtual machines are fixed and the language used is
basic.
With regard to active networks, we can say that the nodes provide
a programmable NAPI. If we consider that, in an IP network, the
header of the packet provides the input to the virtual machine, we can
define an active network as one in which the nodes possess a virtual
machine that executes the code contained in the packet headers.
Numerous categories of active networks can be defined on the
basis of the following attributes:
– expressive power of the language, which determines the degree
with which the network will be able to be programmed. The language
can range from simple orders to very highly-evolved languages.
The simpler the language, the shorter the computation time.
Conversely, the more powerful the language, the greater the degree of
customizability that can be implemented;
– possibility of defining a stable state on the basis of the previous
messages in the same stream, so as to increase the rate of execution
without having to redefine a state of the virtual machine;
– granularity of control, which enables us to modify the behavior
of a node for all packets that pass through it, regardless of the stream
to which that data packet belongs, or, at the other extreme, to modify
the node’s behavior only for the particular packet being processed at
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3.9. Active networks
Active networks are similar to other packet- and frame-transfer
networks. The basic unit transferred in these networks is the packet.
The role of the nodes is to examine the different fields in the packet,
which are placed in accordance with a very rigid predefined system. In
particular, the address field determines the output port. The different
fields in the packet are interpreted by a virtual machine. We can
consider that this virtual machine is a packet interface, which is
typically called a “network API”, or “NAPI” (Network Application
Programming Interface). For an IP network, the IP API is the
language defined by the syntax and semantics of the IP header. In
typical networks, virtual machines are fixed and the language used is
basic.
With regard to active networks, we can say that the nodes provide
a programmable NAPI. If we consider that, in an IP network, the
header of the packet provides the input to the virtual machine, we can
define an active network as one in which the nodes possess a virtual
machine that executes the code contained in the packet headers.
Numerous categories of active networks can be defined on the
basis of the following attributes:
– expressive power of the language, which determines the degree
with which the network will be able to be programmed. The language
can range from simple orders to very highly-evolved languages.
The simpler the language, the shorter the computation time.
Conversely, the more powerful the language, the greater the degree of
customizability that can be implemented;
– possibility of defining a stable state on the basis of the previous
messages in the same stream, so as to increase the rate of execution
without having to redefine a state of the virtual machine;
– granularity of control, which enables us to modify the behavior
of a node for all packets that pass through it, regardless of the stream
to which that data packet belongs, or, at the other extreme, to modify
the node’s behavior only for the particular packet being processed at
www.it-ebooks.info