P802.1Qdt Status Update andNew Proposal Discussion
Lily LvPaul Congdon
Mick Seaman
James McIntosh1

P802.1Qdt Status Update2

Draft D0.2 is Available• Qdt draft has started in the Security TG

PFC headroom
Delay Value = 2 x (Cable Delay) + TXds1 + RXds2 + HDs2 + TXds2 + RXds1 + 2 x (Max Frame) + (PFC Frame)• Qdt includes 2 new functions:

Medium delay

Internal processing delay

Fixed delay• Automatic PFC headroom measurement

MACsec protection• MACsec protection on PFC frames
PFC Initiator• P802.1 Qdt draft 0.2 is available:
https://www.ieee802.org/1/files/private/dt-drafts/d0/802-1Qdt-d0-2.pdf• D0.2 builds the overall framework of specification

PFC Aware
EntityPFC CapableEntity

M_CONTROL.indication

M_CONTROL.requrest

Bridge Port Connectivity

Bridge Port Connectivity

（ISS）

（ISS）

• It specifies new functions in Clause 36 - Priority-based
Flow Control (PFC)• It updates PFC management, covering Clause 12 - Bridge
management, Clause 48 - YANG Data Models, Annex D IEEE 802.1 Organizationally Specific TLVs

PFC Reciever

M_UNIDATA.indication

M_UNIDATA.requrest（ISS）（ISS）

MAC privacy protection

MAC privacy protection

（ISS）

（ISS）

MACsec

MACsec802.1
MA_DATA.requrest

MA_CONTROL.requrest
（MCF）

（MCF）

MA_CONTROL.indication
（MCF）

（MAC）

（MAC）

MA_DATA.requrest

MA_DATA.indication

（MAC）

（MAC）

MA_DATA.indication

MAC

MAC

• It updates other relevant clauses, e.g. Clause 1 Overview, Clause 3 – Definitions, etc.

（MCF）

MAC Control

MAC Control
MA_DATA.requrest

MA_DATA.indication

Point-to-point full duplex link

MCF：Interface to MAC client
MAC：interface to subordinate sublayer3802.3

Re-thinking PFC Headroom Measurement• The current approach for PFC headroom measurement aroused muchdiscussion in the Security TG (see subsequent slides).
• The conclusion is to propose a new way to obtain the PFC headroom
measurement instead of re-using PTP.
• Present the new proposal in TSN for broader discussion.

4

PFC Headroom Measurement
Discussion

5

Take Another Look at PFC Headroom
What is actually required for PFC headroom is knowledge of the maximum amount of data that will bereceived after the decision to send a PFC has been made at PFC initiator.
PFC initiator

PFC receiverThis includes• Data that the PFC receiver has already transmitted at that decision time
• Data sent during the time from decision time to PFC transmit
including PFC transmit delay, PFC recognition and processing by PFC
receiver, and time to halt transmission at PFC receiver.
Delay Value = 2*(Cable Delay) + TXds1 + RXds2 + TXds2+ RXds1 + HDs2 + 2*(Max Frame) + (PFC Frame)

This is effectively the "round trip " from the PFC initiator’s PFC transmitting back to the PFC initiator’s
reception of data controlled by the PFC.
It is actually independent of delay symmetry on the physical communication medium.
6

Current Method of PFC Headroom Measurement
Use the existing PTP protocol and enhance the existing DCBX protocol to support automated
PFC headroom calculation.
PFC initiator

PFC receiverInterface Delay

Cut the “round-trip” into medium delay and internal processing delay.
• Medium delay• Reuse PTP protocol to measure round-trip link delay• Internal Processing delay (implementation known value shared by DCBX)• Both PFC initiator and PFC receiver know its own internal processing
delay (interface delay + higher layer delay).
• Define separate mechanism using DCBX to convey PFC receiverinternal processing delay to PFC initiator.
• PFC initiator calculates the total internal processing delay.
Finally, sum up medium delay, internal processing delay and fixed delay to get“round trip” delay as PFC headroom.
7

Concerns with the Current Method
Requiring PTP for PFC Headroom measurement increases implementation/standard
development complexity.
• PTP objectives and PFC Headroom measurement objectives are not aligned.
• PTP was not designed to support PFC. It intends to precisely measure point to point cable delay. There are discussion
on timestamp point, trying to make it as close to medium as possible.
• However, PFC headroom measurement includes roundtrip delay from the point above the MAC to another point
above the MAC on the other end.
• Re-using PTP adds implementation restrictions/limitations to DC switches.
• Hardware is required to be capable to get timestamp at lower point.

• 1588 would need to be a normative reference in .1Q.
• In most cases, 1588 is not required/supported in DC environments.
• Qdt only needs part of 1588.
• Inventing subset of 1588 is not desirable.
• Qdt becomes dependent on 1588 and must track updates.
8New PFC Headroom Measurement proposal (1/3)
StationPFC 1initiator• Specify a new request-response measurement to measure
the “round trip delay”.

t1

PFC Measurement Requestt2• Measure t1 (the timestamp of sending request) and t4 (the timestamp ofreceiving response)

PFC Measurement Responset3• Both t1 and t4 are timestamps above MAC on PFC initiator.

• No strict requirement on the hardware.

Station 2
PFC receivert4• DV = t4-t1• DV = t4 – t1 – (t3 – t2) + HD ≈ t4 – t1
• t3-t2 is the time to generate PFC headroom measurement Response.

• HD is Higher layer delay (PFC reaction delay).
• (t3 – t2) is similar as HD. But if the implementation is different, it should be accountedfor. The response message can carry the compensation value, or DCBX can carry it.

• Design both request and response measurement frames as MAC data frames• If they are designed as MAC control frame, 802.3 would need to be involved.
Better to minimize other standards dependency/involvement.
9New PFC Headroom Measurement proposal (2/3)• Accuracy analysis

StationPFC 1initiator• t4-t1 accuracy

t1• Assuming 100 meters, t4-t1 is on the micro-second level (1us + internal processing delay)PFC Hdr Measurement
Request• Clock measuring t4-t1 local to initiator independent of other clocks in the systems.
Inaccuracy within tens of nano seconds will have little impact on PFC headroom
measurement.

t2PFC Hdr Measurement
Response• 1ns inaccuracy (100Gbps) leads to 100 bits mismatch

Station 2
PFC receivert3• In practice, buffers to store the packet are usually allocated in chunks (e.g. 160 byte). Buffer t4
chunk size could accommodate the inaccuracy.

• Any t3-t2 adjustment is similarly local to PFC receiver (independent of other clocks in thesystems).
• PFC initiator can adjust for its own different reference points if necessary.

100G,500m

100G,100m

100G,20m

Fixed
Delay

Internal Processing Delay（802.3, no MACsec）

MediumDelay

Headroom（t4-t1）

t4-t1 mismatch

32992

203776500000

92KB

10 ns

0.125KB

0.1%

100 ns

1.25KB1%

10 ns

0.125KB

0.3%

100 ns

1.25KB3%

10 ns

0.125KB

0.4%

100 ns

1.25KB

4%

32992

32992

203776203776

100000

20000

42KB

32KB

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New PFC Headroom Measurement proposal (3/3)• Accuracy analysis• MAC control frame processing delay vs. MAC data frame processing

StationPFC 1initiator

t1

PFC Hdr Measurement
Requestt2• Traditional PFC is MAC control frame. The internal path of MAC control frame
is different from MAC data frame. But the processing time difference is likelytrivial.
• MAC data frame measurement is more conservative than a MAC control
frame measurement, but the difference will not waste unnecessary buffermemory.

Station 2PFC receiverPFC Hdr Measurement
Responset3

t4• PFC initiator can set its own adjustment parameter to accommodate thedifference.
• PFC receiver can convey the adjustment parameter to the PFC initiator viaDCBX.

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Benefits of the New Proposal• The "round trip delay" is a better measure of the headroom as it avoids the need to add in aworst-case estimate of the internal transmission and processing delays in the systems.
• The "round trip delay” measurement would naturally account for any additional internaldelays, such as the use of MACsec to protect the PFC frames themselves• Benefits for standards development:• .1Q does not need to refer to 1588 as a normative reference.
• .1Qdt has the flexibility to progress independently of other standards.

• Benefits for implementation:• No strict requirements on hardware.
• Data center switches do not need to comply with 1588 or decide which relevant parts of the standard touse.

12New Proposal Impact On Qdt PAR & CSD• ‘PTP’ mentioned in current PAR• ‘PTP’ mentioned in current CSD
1.2.4 Technical Feasibility

1.2.5 Economic Feasibility

PAR&CSD need to be updated if new proposal is adopted.
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Summary & Next Steps

14

Summary & Next steps• .1Qdt draft work has started.
• The development of standard draft is limited until a PFC headroom
measurement method can be decided.
• A new method of PFC headroom measurement method has been
proposed by the Security TG.
• Modify PAR&CSD if new proposal is adopted.

• Question: Shall we produce a new draft with the new method, or do weneed further discussion?
15Backup Slides

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100G,500m

100G,100m

100G,20m

Fixed
Delay

Internal Processing
Delay（802.3, noMACsec）

Medium Delay

Headroom（t4-t1）

t4-t1 mismatch

32992

203776500000

92KB

10 ns

0.125KB

0.1%

100 ns

1.25KB1%

10 ns

0.125KB

0.3%

100 ns

1.25KB3%

10 ns

0.125KB

0.4%

100 ns

1.25KB

4%

32992

32992

203776203776

100000

42KB

20000

32KB

Sublayer

25GbE(ns)

100GbE(ns)RS, MAC and MAC control

327.68245.76

BASE-R PCS

143.36353.28BASE-R PMA

163.8492.16

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