Westermo U200 Operator manal - Manual (Page 18)

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U/R/T200 series
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6.3 Layer 3 priority
Each IPv4 header contains a ToS field, see figure below. The switch is configured to put IP
packets with the following ToS values in the high priority queue:
- 0x04 (IPTOS_RELIABILITY)
- 0x08 (IPTOS_THROUGHPUT)
- 0x10 (IPTOS_LOWDELAY)
- 0xF8
- 0xFC
High priority setting of the IP ToS field of real time critical packets must be set in the IP
protocol of the sending station. This can be done on TCP/UDP socket level by a setsockopt( )
command both on the client and server socket side in most Operating Systems (OS). E.g.:
tos = 0xFC;
setsockopt( .., IP_TOS, &tos,...)
Figure 9, IP header
6.4 Flow control
By default the switch is disabled for flow control (IEEE 803.3x), since flow control is a not a
good real time property.
6.5 Head of Line Blocking Prevention
The switch supports head of line blocking prevention for low priority packets only. This means
that low priority packets received on any port will not be forwarded to ports that are
congested. This will reduce the amount of packets in the output buffer. This function is
particular useful when high amounts of multicast, unknown unicast and broadcast traffic are
available in large networks where both 10BASE and 100BASE ports are available. High
priority packets will always be forwarded.