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Chapter 6
Meters, queues and IPQoS
E-NIT-CTC-20041213-0013 v0.5
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MTU explained. In this section we will have a closer look at the MTU values and what exactly does it
do.
Sometimes it might be usefull to lower the MTU of a link when EF data is to be sent.
The reason is that, even if an EF packet gets top priority, it might still get stuck
behind a large data packet that has just started to go out.
The MTU typically needs to be changed on links with a slow uplink (<128Kb/s). The
MTU is set to 1500 bytes by default.
If a default packet of 1500 bytes, is send over a 64Kb link, it takes 18ms before it is
send completly. This could cause delay/jitter for time sensitive data like voice. This is
called serialization delay. By decreasing the MTU, IP packets (with a normal lenght
of 1500 bytes) will be fragmented in smaller packets to meet the defined MTU size.
The example below can illustrate this:
Real time MTU
Elastic Traffic MTU
Elastic MTU Real time MTU Real time MTUElastic MTU
214 ms transfer time for 1500 byte frame
at 56kbps
The problem : A voice-packet gets highest priority but gets stuck
behind a large data-packet that is being sent out.
The solution: fragment packets when EF exists