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

8.5. IEEE 802.3x Flow Control Implementation Issues

In this section we discuss a number of issues related to the implementation of Ethernet systems that support and use the PAUSE mechanism just described. Readers not interested in the low-level design of hardware and software for Ethernet interfaces can safely skip this section.

8.5.1. Design Implications of PAUSE Function

Flow control (specifically, the transmission and reception of PAUSE frames) imposes some new twists that did not previously exist for the design of Ethernet devices. Prior to flow control, all Ethernet frames transmitted by a given interface were submitted by a higher layer protocol or application. Similarly, all received frames were checked for validity and then passed up to the higher-layer entity without the contents being inspected, interpreted, and acted upon by the Ethernet interface. This changes with the implementation of flow control, as the link interface can now generate PAUSE frames, and must inspect each incoming frame to determine if it is a PAUSE request from its link partner and act upon PAUSE requests. This raises some important issues in the design of Ethernet interfaces supporting the PAUSE function.

8.5.1.1. Inserting PAUSE Frames in the Transmit Queue

Without the PAUSE function, an Ethernet interface simply transmits frames in the order presented by the device driver. Because Ethernet has no concept of priority access (or user priority), a single-queue model can be used for the transmitter. ...