Recommendations - D-Link NetDefend DFL-210 User Manual

10.1.10. Recommendations

Group Limits and Guarantees
In addition to specifying a total limit for group users, limits can be specified for each preference. If
we specify a group user limit of 30 bps for precedence 2 then this means that users assigned a
precedence of 2 by a Pipe Rule will be guaranteed 30 bps no matter how many users are using the
pipe. Just as with normal pipe precedences, traffic in excess of 30 bps for users at precedence 2 is
moved down to the Best Effort precedence.
Continuing with the previous example, we could limit how much guaranteed bandwidth each inside
user gets for inbound SSH traffic. This prevents a single user from using up all available
high-priority bandwidth.
First we group the users of the ssh-in pipe so limits will apply to each user on the internal network.
Since the packets are inbound, we select the grouping for the ssh-in pipe to be "Per DestIP".
Now we specify per-user limits by setting the precedence 2 limit to 16 kbps per user. This means
that each user will get no more than a 16 kbps guarantee for their SSH traffic. If desired, we could
also limit the group total bandwidth for each user to some value, such as 40 kbps.
There will be a problem if there are more than 5 users utilizing SSH simultaneously: 16 kbps times
5 is more than 64 kbps. The total limit for the pipe will still be in effect, and each user will have to
compete for the available precedence 2 bandwidth the same way they have to compete for the lowest
precedence bandwidth. Some users will still get their 16 kbps, some will not.
Dynamic balancing can be enabled to improve this situation by making sure all of the 5 users get the
same amount of limited bandwidth. When the 5th user begins to generate SSH traffic, balancing
lowers the limit per user to about 13 kbps (64 kbps divided by 5 users).
Dynamic Balancing takes place within each precedence of a pipe individually. This means that if
users are allotted a certain small amount of high priority traffic, and a larger chunk of best-effort
traffic, all users will get their share of the high-precedence traffic as well as their fair share of the
best-effort traffic.
10.1.10. Recommendations
The importance of setting a pipe limit
Traffic shaping only comes into effect when a NetDefendOS pipe is full. That is to say, it is passing
as much traffic as the total limit allows. If a 500 kbps pipe is carrying 400 kbps of low priority
traffic and 90 kbps of high priority traffic then there is 10 kbps of bandwidth left and there is no
reason to throttle back anything. It is therefore important to specify a total limit for a pipe so that it
knows what its capacity is and the precedence mechanism is totally dependent on this.
Pipe limits for VPN
Traffic shaping measures the traffic inside VPN tunnels. This is the raw unencrypted data without
any protocol overhead so it will be less than the actual VPN traffic. VPN protocols such as IPsec
can add significant overhead to the data and for this reason it is recommended that the limits
specified in the traffic shaping pipes for VPN traffic are set at around 20% below the actual
available bandwidth.
Relying on the group limit
A special case when a total pipe limit is not specified is when a group limit is used instead. The
bandwidth limit is then placed on, for example, each user of a network where the users must share a
fixed bandwidth resource. An ISP might use this approach to limit individual user bandwidth by
specifying a "Per DestinationIP" grouping. Knowing when the pipe is full is not important since the
only constraint is on each user. If precedences were used the pipe maximum would have to be used.
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Chapter 10. Traffic Management