Cisco Cisco Nexus 5010 Switch Libro bianco

 

 

White Paper 

© 2015 Cisco Systems, Inc. All rights reserved. This document is Cisco Public Information. 

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Figure 1.    IEEE 802.3x PAUSE and PFC Frame Format 

 

As Figure 1 shows, a 64-byte MAC control frame is used by both IEEE 802.3x PAUSE and PFC. In both cases, 

numeric values can be used to describe the requested duration of PAUSE. However, since PFC acts independently 

on eight different CoSs,  the frame describes the PAUSE duration for each CoS. 

The PAUSE duration for each CoS is a 2-byte value that expresses time as a number of quanta, where each 

represents the time needed to transmit 512 bits at the current network speed. A PAUSE duration of zero quanta has 

the special meaning of unpausing a CoS. Typical implementations will not try to guess a specific duration for PAUSE, 

instead relying on the X-ON and X-OFF style behavior that can be obtained by setting PAUSE for a large number of 

quanta and then explicitly resuming traffic when appropriate.. 

A receiver using PFC must predict the potential for buffer exhaustion for a CoS, and respond by generating an explicit 

PAUSE frame for that CoS when that condition arises. The PAUSE frame needs to be sent back to the other end of 

the wire early enough, so that the talkative sender has time to stop transmitting before buffers overflow on the 

receiving side. 

Obviously, since bits on a wire travel at a finite speed, the length of the wire affects how early the receiving end must 

act. The longer the wire, the earlier a receiver must send back a PAUSE frame. 

Put another way, at any point in time the receiver must have enough residual buffers available to store any packet 

that might be in flight while the PAUSE frame travels back to the sender and gets processed there. Since after the 

PAUSE request 

has been sent, the system “transmitter + wire + receiver” must drain all existing packets into receiver 

buffers, the definition of an appropriate buffer threshold on the receiver side is critical to a functioning PFC 

implementation. 

Consider the case of a stream of packets flowing from sender S to receiver R through cable C. To set up the receiver 

R thresholds for an appropriate PAUSE implementation, the following factors need to be considered: