Annex N Updates
Lily Lyu
March Plenary

Contents• N.1 Overview• N.2 Delay model of PFC headroom• N.3 Internal processing delay• N.4 Link delay• N.5 Worst-case delay• N.6 Buffer allocation example

N.1 Overview• What is PFC headroom
When activate PFC operation, “the PFC headroom (see 36.1.1) is theminimum buffer size that needs to remain available on receiver. It helps
implementation to allocate buffer for PFC-enabled priorities. ”• Introduce Annex N content“This annex explains delay model of PFC headroom, and provides anexample of buffer allocation based on the PFC headroom calculation.”N.2 Delay model of PFC headroom
PFC headroom calculation considers various delays accumulated
36.1.1

Update of 36.1.1a) B’s reception processing to calculate the remaining buffering
following frame receipt.
b) B’s PFC Initiator to initiate PFC following that buffering
calculation.
c) Encoding of the PFC frame and any other transmission delays
associated with B’s interface stack.
d) Any prior in-progress frame transmission by B (possibly of amaximum sized frame) to complete.
e) PFC frame transmission on the physical link.
f) The link delay for transmission from B to A.
g) PFC frame reception, including frame validation, by A’s interfacestack.
h) A’s PFC Receiver to decode the PFC frame and halt transmission
selection for specified priorities.
i) Any in-progress frame transmission by A (possibly of a maximumsized frame) to complete.
j) The link delay for transmission from A to B.
k) Reception delays associated with B’s interface stack, reception
processing, and buffering.

PFC frame transmission in PFC initiator station B:a) B’s reception processing to calculate the remaining buffering following frame receipt.
b) B’s PFC Initiator to initiate PFC following that buffering calculation and PFC frame encoded
ready for transmission.
c) Any prior in-progress frame transmission by B (possibly of a maximum sized frame) tocomplete.
d) First bit of PFC frame sent to MAC service.
e) Last bit of PFC frame sent on the physical link.
PFC frame transmission across link from B to A:f) The link delay for transmission from B to A.
PFC frame reception in PFC receiver station A (including PFC taking action):g) PFC frame reception since the last bit of PFC frame received on link, including frame
validation, by A’s interface stack.
h) A’s PFC Receiver to decode the PFC frame and halt transmission selection for specified
priorities.
User data transmission in PFC receiver station A:i) Any in-progress frame transmission by A (possibly of a maximum sized frame) to complete.
j) Last bit of last frame sent on the physical link.

User data transmission across link from A to B:k) The link delay for transmission from A to B.
User data reception in PFC initiator station A:l) Reception delays associated with B’s interface stack, reception processing, and buffering.

N.2 Delay model of PFC headroom
Internal processing delay(ID): the time spent on frame processing within PFC initiator station and PFC receiver station, includinginterface stack delay, queue status change delay, and buffering delay etc., assuming no prior in-progress frame transmission
Link delay(LD): the time spent on physical link between PFC initiator station and PFC receiver station
Worst-case delay(WD): the additional time needed for amaximum sized frame transmission before the PFC
frame transmission at PFC initiator station, and the
additional time needed for a maximum sized frametransmission after the PFC frame taking effect at PFC
receiver station

PFC frame transmission in PFC initiator station B:a) B’s reception processing to calculate the remaining buffering following frame receipt.
b) B’s PFC Initiator to initiate PFC following that buffering calculation and PFC frame encoded
ready for transmission.
c) Any prior in-progress frame transmission by B (possibly of a maximum sized frame) tocomplete.
d) First bit of PFC frame sent to MAC service.
e) Last bit of PFC frame sent on the physical link.
PFC frame transmission across link from B to A:f) The link delay for transmission from B to A.
PFC frame reception in PFC receiver station A (including PFC taking action):g) PFC frame reception since the last bit of PFC frame received on link, including frame
validation, by A’s interface stack.
h) A’s PFC Receiver to decode the PFC frame and halt transmission selection for specified
priorities.
User data transmission in PFC receiver station A:i) Any in-progress frame transmission by A (possibly of a maximum sized frame) to complete.
j) Last bit of last frame sent on the physical link.

User data transmission across link from A to B:k) The link delay for transmission from A to B.
User data reception in PFC initiator station A:l) Reception delays associated with B’s interface stack, reception processing, and buffering.

N.2 Delay model of PFC headroom
Figure shows a possible worst-case delay example where MACsec is disabled for PFC frame and user data.

①••DV = (ID1-1+ID1-2+ID1-3) + WD1 + LD2 + ID3 + WD4 + ID4 + LD5 + ID6
It is the round-trip delay from PFC initiation to last frame reception and buffered, plus 2 maximum
sized frames represented by bit times.

①②③

④

②
③

④
⑤
⑥

⑤
⑥

N.3 Internal processing delay
The Internal Processing Delay is implementation dependent.
It comprises• frame processing delays above MAC service which is between MAC control client and transmission selection• MAC and PHY layer interface delays.

Example of processing delays above MAC service areMACsec and entering pause state delays.
• MACsec delay: For link speeds of up to 10Gb/s,
MACsec constrains each of the transmit delay and thereceive delay to a maximum of 19 360 bit times (see36.1.3.3).
• Entering pause state delay: This standard defines aqueue shall go into paused state in no more than614.4 ns (see 36.1.3.3). This delay is equivalent to 6144
bit times at the speed of 10Gb/s.
IEEE 802.3 Interface Delays

N.4 Link delay
The Link Delay is the propagation delay over the transmission medium and can be approximated by thefollowing equation:

where u is the signal propagation speed in the medium and BT is the bit time of the medium.

N.5 Worst-case delay
The Worst-case Delay comprises 2 parts.
• At PFC initiator station, it is assumed amaximum sized frame just starttransmission from Transmission
Selection when PFC is invoked. PFC
frame has to wait until this in-progress
frame complete transmission.
• At PFC receiver station, it is assumed
queue is paused but a maximum sizedframe just starts transmission. Thus, bittimes of the maximum sized frame isadded into the total delay.

Note:
Enabling MACsec impacts worst-case delay model.
That is implementation dependent.

N.6 Buffer allocation example
10GBASE-T PHY example:–––––––

PFC frame generation: 200 bit times;
Maximum envelope frame size: 2000 octets, 16 160 bit times;
PFC frame size: 64 octets, 672 bit times;
XGMII MAC/RS and XAUI interface: 8192 + 2 × 2048 = 12 288
bit times;
10GBASE-T Delay: 25 600 bit times;
100 meters Cat6 cable: 5556 bit times (computed assuming u =0.6 × c, where c is the speed of the light in meters per second);
Entering paused state = 6144 bit times

MACsec not supported:
DV = (ID1-1+ID1-2+ID1-3) + WD1 + LD2 + ID3 + WD4 + ID4 +LD5 + ID6
DV =( (200) + (672 + (12 288 + 25 600) /2))+ (16 160) + (5556) +((12 288 + 25 600) /2 + 6144) + (16 160) + ((12 288 + 25 600) /2)+ (5556) + ((12 288 + 25 600) /2) = 126 224 bit times---> PFC headroom is 15.4kB

MACsec supported (data frame):
WD1 and ID4, each is incremented by 19 360 bit times
DV = 126 224 + 19 360 + 19 360 = 164 944 bit times---> PFC headroom is 20kB

Buffer allocation strategy:
2 times PFC headroom is allocated to PFC enabled priority queue. XON/XOFF threshold is set to the size of PFC headroom.
Buffer 1

15.4kBPFC guarantees no frame loss and no throughput loss.

20kB
XON/XOFF

Buffer 2

15.4kB

20kB

30.8kB

40kB

NO MACsec

MACsecWhen priority queue is increased to XOFF, PFC pause is invoked. Buffer 1 is used to absorb inflight frames.
When priority queue is decreased to XOFF, PFC resume is invoked. Buffer 2 is used to prevent traffic stop.