Multimedia
Distribution (3)
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The SOMA VoD service requires the presence of the just described infrastructure
composed by distributed and coordinated agents. Therefore, before the multimedia
stream can start to flow, users have to wait for the completion of a setup
phase in which the service provider determines the path between
the source and the target, and distributes all ACs and QoSNs needed on participating
nodes. After the setup phase has negotiated the service levels, the stream
can flow from source to target. During service provision, the flow can also
be dynamically adapted,
to adjust the required QoS level at runtime with a best-effort approach, or
to dynamically organize corouted paths, with a further distribution of agents
on new nodes. Dynamic
adaptations introduce overhead, that is only a percentage of
the one required by the setup phase. For that reason, we report about the
setup costs in a normal usage scenario.
In this phase, one lightweight
agent (about 1KB-sized) is sent from the source to the target to identify the
path for the multimedia stream. This agent reports back to the source the information
about how many ACs (about 6KB-sized) and QoSNs (about 4KB-sized) have to be
instantiated and to be sent in parallel to interested nodes. We have considered
the worst case when none of the intermediate nodes has neither
the AC nor the QoSN agent. In more realistic scenarios, hosts may have already
the AC agent running for purposes of remote monitoring and diagnosis of network
resources. In addition, the VoD service should be typically carried out in untrusted
environments, where cooperating agents have to pass integrity
and authentication checks before being allowed to operate
to local resources.
The SOMA architecture permits
users to choose which subset of functionality are used by specific
services. In that way, services can obtain the most suitable
trade-off between performance and security,
depending on the level of trust of the environment and on the criticality of
the application domain. In particular, the VoD service provided in a trusted
environment may omit authentication and integrity checks, with considerable
time saving on the setup overhead.
We report the setup time
for an application case where the source is 8 hop far from the target, i.e.
the multimedia flow has to pass through 7 intermediate non-tunneled nodes to
reach its target. The intermediate nodes hosts the default places of the different
domains they belong to, and the different domains model a networked system that
consists of several departmental LANs. The LANs are heterogeneous and composed
by PentiumII PCs with Windows NT and Sun SPARCstation with Solaris 2.5. In the
case of untrusted environments, the multimedia source has to calculate a 1024-bytes
blocksize MD5 hash of all the sent agents and to sign them with its 1024-bits
RSA private key. Any intermediate node must perform the security checks to verify
the signature and the hash. The average setup time measured in this scenario
is 10907 ms. In the case of trusted environments,
SOMA gives the possibility of providing the VoD service with no security checks,
with a considerably reduced average setup time (7634 ms
that is 30% less of the first case).
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