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Modifying Link Rate

By default, the capacity of links leaving an egress (output) port are set to 600 Mbps. This link rate can be modified through the Queue Tables menu item at each port (Fig. 1).

Fig. 1. Queue Tables Menu Item.

In this example, we want to set the link rate to 300 Mbps instead of its default 600 Mbps.

Fig. 2. Setting Link Rate to 300 Mbps.

Fig. 2 shows three versions of the Queue Table panel for port 2.6. The leftmost panel shows that the default link bandwidth is 600 Mbps. We have set the desired link bandwidth to 300 Mbps in the middle panel. The actual link bandwidth is 299.987 Mbps (shown in the rightmost panel) after committing. Three comments are in order:

• The bandwidth is controlled by a token bucket traffic regulator which is described in the page Tutorial => Filters, Queues and Bandwidth => NSP Architecture => Link Rate.
• The difference between the desired and actual link rate is due to the precision of the token bucket implementation which is 61 Kbps; the actual rate is an integer multiple of 61 Kbps.
• The traffic rate is the IP packet traffic rate and does not any overheads except the IP header.

The Port 6 Queues panel in the Tables window shown in Fig. 2 also contains a number of other parameters related to the ingress and egress queues (these parameters will be described later):

• The 8 VOQs at ingress port 6 have threshold and quantum parameters associated with the scheduling of packets going from input port 6 to all of the output ports (0 to 7). The quantum values are only user definable if the Distributed Queueing (DQ) algorithm is disabled. If DQ is enabled, the fields will appear grey, and the user will not be able to modify them.
• The datagram queues have threshold and quantum parameters associated with the scheduling of packets that exiting output port 6

The threshold parameter indicates the queue length. For example, the threshold for each of the VOQs is 1,000,000 bytes. All queues use a tail drop discard policy. So, an arriving packet will be dropped if adding it to the queue will exceed the queue threshold.

We set the link rate at port 2/6 to 300 Mbps using the following recipe (Fig. 2):

Fig. 3. Traffic Chart With a 300 Mbps Link Rate.

Fig. 3 shows the effect of reducing the capacity of the 2.6-to-1.7 link rate from 600 Mbps to 300 Mbps. The solid lines show the traffic rates of the UDP flows measured at input ports 2.2 through 2.4. The dashed lines show the traffic rates of the flows at port 1.7 have been reduced to the point where the aggregate rate (the +1.7 to 1.4 curve) is 300 Mbps, the capacity of the bottleneck link (NSP 2's egress port 6).

Fig. 3 shows that at around T = 259, the traffic from port 2.3 is competing with the one from port 2.2, and the total traffic exceeds 300 Mbps. At T = 267, the traffic from port 2.4 starts, and the three flows now compete for the limited bandwidth at port 2.6. But at T = 282, the traffic from port 2.2 stops leaving only two flows to compete for the link.

Tutorial >> Filters, Queues and Bandwidth	TOC