An Investigation into Transport Protocols and Data Transport ...

3.2. Network Monitoring 43
Whilst latency is an important metric, especially when applied to applications
such as voice or video conferencing, it is essentially constrained by
the physical (relative) location of the two hosts 1 .
As the majority of the
Internet is a shared resource with Best Effort [CF98] scheduling of resources,
consecutive pings may experience different latencies and hence cause jitter.
Similarly, the traversal of packets through different paths 2 may also increase
the jitter, so much so that packets may arrive ‘out-of-order’.
The transfer of bulk amounts of data involve the movement of many
packets of data. As such the microscopic effects of jitter and delay will also
affect the macroscopic effects of bulk data transport. Also, as the raw rate at
which data can be transfered is often a useful and readily measurable metric,
the issue of bandwidth monitoring has become an important indication of the
performance of many Internet applications. Unlike latency, which is limited
by physical constraints, such as the speed of light, the capacity (optimal
throughput) of hardware is limited by the clock frequency and protocols
used to control the medium - as defined by OSI Layer 2.
Two bandwidth metrics that are commonly associated with a path are
the capacity and the available throughput [LTC + 04]. The capacity is the maximum
throughput that the Internet path can provide to an application when
there is no competing traffic load (cross traffic). The available throughput,
on the other hand, is the maximum throughput that the path can provide to
an application, given the path’s current cross traffic load. Measuring the capacity
is crucial in calibrating and managing links. Measuring the available
bandwidth, on the other hand, is of great importance for predicting the end-
1 Delays caused by queuing at router and switches may also affect latency.
2 Both between routers and within routers themselves depending on router design/configuration.