The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

8.6. Flow Control Symmetry

Flow control can be deployed either symmetrically or asymmetrically. In certain network configurations, it may be desirable to allow either device at the ends of a full duplex link to throttle the transmissions of the other; in other configurations it may be desirable to allow flow control in one direction only.

8.6.1. Symmetric Flow Control

Symmetric flow control makes sense when:

• Both devices must deal with a high statistical variance of frame arrivals (i.e., short-term transient overloads around some lower average traffic level).

• Both devices have similar buffer memory constraints (i.e., neither is inherently capable of absorbing the transients better than the other).

• The traffic pattern is relatively uniform (i.e., the traffic flows are not weighted heavily in either direction).

• Neither device is the source or sink of much of the traffic.

One common scenario meeting all of these conditions is a switch-to-switch full duplex link, depicted in Figure 8-11.^[]

^[] A router-to-router link would also meet all of these criteria, and would be a good candidate for symmetric flow control as well. The choice of a switch in this context is purely exemplary.

8.6.2. Asymmetric Flow Control

In some circumstances, it may be better to allow one of the link partners to throttle the other, but not vice-versa. The most common scenario for this is that an end station is connected to an internetworking device (for example, switch or router) through a dedicated full duplex ...