Packet Loss Issues

Packet Loss Issues
Packet loss can cause jerky transmission of voice or video, slow application
performance, or corrupt data. By default, when a software queue is full
(congested), the switch or router drops all other traffic bound for that queue.
This is called tail drop. It can cause some problems:
¦ TCP global synchronization.
¦ TCP buffer starvation.
¦ Delay and jitter.
¦ High-priority traffic is dropped, whereas low-priority traffic is sent.
Congestion avoidance attempts to prevent tail drop. To accomplish this,
increase link bandwidth, use queuing to guarantee a certain amount of traffic
to each application, or use Weighted Random Early Detection (WRED).
WRED drops lower-priority traffic (based on Differentiated Services Code
Point [DSCP] or IP Precedence values) as a queue starts to fill and drops
high-priority traffic only when the queue is almost full. If the queue fills
completely, however, tail drop is used. The drop thresholds and the drop
ratios are configurable. WRED works best with TCP traffic, because TCP
dynamically adjusts its sending rate when packets are dropped. Do not use
WRED for voice traffic. The “Congestion Avoidance” section describes this
more completely.
Four other causes of packet drop are: frame errors, lack of buffer space
(called an ignore), a CPU that is unable to assign a free buffer to it (called
an overrun), or a CPU that is too busy to process inbound packets so the
inbound queue fills.