Quality of Service
312
9.2
Which data packets to prefer?
BAT54-Rail/F..
Release
7.54
06/08
9.2 Which data packets to
prefer?
The necessity of a QoS concept results only from the fact that the available
bandwidth is not always sufficient for transferring all pending data packets re-
liably and on time. Load peaks result easily from running simultaneously
large FTP downloads, while exchanging e-mails and using IP telephones
over the data line. In order to meet also in these situations the demands of
the desired data transfer, certain data packets must be treated preferentially.
It is necessary for this, that at first a BAT recognizes which data packets
should be preferred at all.
There are two possibilities to signal the need for a preferential treatment of
data packets in the BAT:
D The application, as e.g. the software of certain IP telephones, is itself able
to mark the data packets appropriately. This marking, the “tag”, is set with-
in the header of the IP packets. The two different variants of this marking
“ToS” and “DiffServ” can simply described assume the following states:
D ToS “Low Delay“
D ToS “High Reliability“
D DiffServ “Expedited Forwarding“
D DiffServ “Assured Forwarding“
Note: The IP header bits of the ToS resp. DiffServ field are copied in case of
a VPN route also into the enclosing IP header of the IPSec VPN packet.
Thus QoS is available also for VPN routes over the Internet, as long as
your provider treats according packets preferentially also in the WAN.
D When the application itself has no possibility to mark the data packets ap-
propriately, the BAT can ensure the correct treatment. For this, it uses the
existing functions of the firewall, which can classify e.g. data packets ac-
cording to subnets or services (applications). Due to these functions it is
e. g. possible to treat individually data packets of a FTP connection or
those of a certain department (in a separate subnet).
For treatment of data packets classified by the firewall the following two
possibilities can be chosen:
D Guaranteed minimum bandwidth
D Limited maximum bandwidth
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