[RFC1633] defines a flow as a distinguishable stream of related datagrams that
result from a
single
user activity and require the same QoS.
A flow is considered simplex, which means that it has one source and---in a multicasting-capable network---n destinations.
Flows define the finest granularity of packet streams distinguishable by the IS architecture.
A network element uses filter specs to identify the flow that a packet belongs to.
These filter specifications contain the necessary information for performing the identification;
for example, the source and destination host address and port number together with the protocol ID,
or flow IDs as proposed for IPv6.
The allowed traffic pattern of a flow is described by means of a traffic specification or TSpec. Note that the TSpec describes the worst-case behaviour of the sender, not the average traffic generation. As long as a source conforms to its TSpec, it can expect that contracted QoS guarantees are met by the network. A TSpec might, for example, take the form of a token-bucket specification, or an upper bound on the peak sending rate.
The QoS requests of a flow are expressed by a service request specification or RSpec. Such specifications might contain bandwidth requirements, acceptable maximum delays or packet loss rates.
The combination of the information contained in an RSpec and a TSpec is sometimes called a
flow specification or
flowspec.
A flowspec and a filter spec together constitute a flow descriptor.
A service or QoS control service describes a named, coordinated set of QoS control capabilities that are provided by a single network element. A service definition must comprise a specification of the functions to be performed by the element, the information necessary to perform these functions, the information that the element exports to other entities in the system, as well as relevant information.
The IS working group has defined a framework [SW96a] for specifying services provided
by network elements within an internetwork. This draft lays down the service specification document format
as well as naming conventions for services and parameters.
Each service parameter appears in two forms: local and composed.
Local values describe the conditions at a particular network element,
whereas composed values reflect the composition of local values along a path through the network.
Hence, the definition of a parameter must include a corresponding composition rule which defines
how to combine an incoming composed value with the local value; the result of this composition may be
forwarded to the next element along the path.
For example, to characterise the maximum transfer unit that all elements along a data path up
to the local system can handle, the composition rule would be the minimum function applied
to the incoming composed value and the local value.
A set of general control and characterisation parameter definitions common and meaningful to all services is collected in [SW96b].