Packet Loss

Packet Loss
Packet loss may seem to be the most important issue, but that is often not the case. If packet loss
occurs in connection-oriented services, then the lost packet will be requested and retransmitted.
This may be annoying if it slows down the data transfer too much, but connection-oriented
applications are built to manage this problem. Nonetheless, this is seen by ISPs as being a large
problem, because it results in packets being retransmitted with smaller TCP windows, thus
causing a positive feedback circuit.
In a connectionless network, once lost, the data is gone forever. If the loss exceeds certain
parameters (which are different for each application), then the application will be deemed unusable
and terminated either by the user (quality too poor) or by the application itself. This may
not be a problem at lower values—voice, for example, may just sound less clear, as in a noisy
analog circuit. In either case, the user and application are exposed to the poor quality with the
resulting dissatisfaction regarding the network. Packet loss can occur in a number of places,
with each location introducing loss in a different way:
Line loss
Line loss
is usually caused by data corruption on unacknowledged links. Corrupted
packets may fail a checksum and are discarded, but are not scheduled for retransmission. In a
well-designed Ethernet network, this should be a rare occurrence.
Buffer overflows
Buffer overflows
occur when network devices are too busy internally, or
when the output network is congested. The key to managing buffer overflows lies in early detection
of the problem and careful application throttling.
Discard eligible
Discard eligible
packets are flagged to be deliberately dropped when congestion
occurs on Frame Relay and ATM networks. There is no exactly comparable process with
Ethernet LANs, but if we establish traffic classes in order to create priorities, then it follows that
those frames in the lowest priority traffic streams run the risk of being dropped more frequently
as network congestion occurs.