Headroom Measurement
Protocol Design
Lily Lv (Huawei)
Fei Chen (Huawei)1

To-Do List
✓ Timestamp point clarification✓ DCBX：PFC Configuration TLV format design➢ PFC configuration TLV defines Capability (round-trip, PTP-based)➢ PFC informational TLV defines compensation value of PTP-based method• Protocol design of request-response measurement• Managed objects➢ The effort, implementation cost, and purpose of statistic gathering and retention requires careful consideration

Conclusions:✓ Ethertype for Qdt➢ Reuse Qcz (CI) Ethertype 89-A2✓ Timestamp accuracy➢ Describe accuracy by number of pause quantas or number of maximum length frames, instead of numberof nans seconds.

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Timestamp Points• Specify measurement timestamp points• Non-MACsec and MACsec use the same definition of measurement timestamp points• Headroom calculation for Non-MACsec and MACsec are different

3Timestamp Points
Measurement timestamp points

PFC timestamp points

PFC Headroom = (t2’+ PFC reaction delay + r_tx_shim layer delay ) –(t1’-PFC invocation delay-PFC frame - l_tx_shim layer delay ) +t4’-t3’ +2*(Max Frame)

PFC Headroom = t2-t1+ t4-t3 + 2*(Max Frame)•t1: RX queue is above threshold and invokes signal to PFC module•

t2: PFC M_CONTROL.indicaton generated. Priority is paused, butmax length frame just started transmission•

t3: last bit of maximum length frame processed by transmission
selection•

t4: last bit of frame received and queued•

t1’: last bit of measurement req frame passed to MAC service•t2’: last bit of req frame is passed from MAC service•t3’: last bit of measurement resp frame processed by transmission
selection•t4’: last bit of measurement resp frame received and queued

This is to be specified in Qdt.
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DCBX Design• PTP-based measurement requires new informational TLV• Measurement capability is reflected in PFC configuration TLV

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PFC Configuration TLV format design• Proposal :➢ PFC configuration TLV only includes ‘capability’

Each bit indicates one
capability.

If non-PTP and PTP-based are supported on both sides, each node choose its own preference.

➢ ‘PTP comp’ for PTP-based measurement passes to peer separately.
Define a new informational TLV - PFC informational TLV

Compensation value for
PTP-based measurement

DCBX informational attributes:“Informational attributes areexchanged via LLDP without
any participation in a DCBX
state machine.”
4Measurement Protocol Design7Benefit of Piggybacking Roundtrip Measurement
Piggybacking for TCP acknowledgement

Piggybacking for roundtrip measurement

BA
Data

Req

Ack + Data

Resp + Req

AckAdvantage：• Better utilize network bandwidth for full-duplex
communication

Disadvantage:• Delay in the transfer of the ACK

Set a counter on host ‘B’ to control the waiting time for dataBA

RespAdvantage：• Complete the measurement faster in query lostcase

Disadvantage:• Waste network bandwidth in some cases

Minimize bandwidth waste by optimizing themechanism?

Piggybacking Roundtrip Measurement Mechanism（1/2）
BA
Req
Resp + Req

Resp

Assumption:• Auto calculated headroom --- successfully take roundtrip measurement
at N times, and take the average value as headroomn: the number of roundtrip measurement• Request message sending interval is no more than T, but no less than t
req_timer: timer for request message sending interval• req(i): the 𝑖 𝑡ℎ request message• resp(j): response message corresponding to the 𝑗𝑡ℎ request message

Processing:• Initial stage：set n=0, req_timer=0, i=0; send req(i)Cases entering initial stage:• Node just started• Link port status changed• Manually trigger the auto headroom calculation• Vendor specified measurement cycle• ……

Piggybacking Roundtrip Measurement Mechanism（2/2）
Processing:BA••

ReqResp + ReqResp••

Exception handling:•In order to avoid request message
flooding the network, define a boundaryof sent request messages M.
• If i>M but n<N, system should stop
the measurement, and report auto
headroom calculation failed.

If req_timer increases to T, send req(i+1); set req_timer=0
If receive req(k),• If n==N, send resp(k)• If n<N,• If req_timer<t, send resp(k)• If req_timer>=t, send resp(k)+req(i+1); set req_timer=0
If receive resp(j),• Finish a single time roundtrip measurement; set n=n+1• If n<N, continue increasing req_timer (until req_timer=T to send req(i+1))• If n=N, calculate headroom by averaging N times roundtrip measurement
results, auto calculated headroom successful.
If receive resp(j)+req(k)，• Finish a single time roundtrip measurement; set n=n+1• If n=N, calculate headroom by averaging N times roundtrip measurement
results, auto calculated headroom successful; send resp(k)• If n<N,• If req_timer<t, send resp(k)• If req_timer>=t, send resp(k)+req(i+1); set req_timer=0

Measurement Message Format
PDU Ethertype
VersionSubtype
Headroom Measurement
PDU

Octet
133

Length24 bits
4 bits465-529

Measurement PDU

Re-use CI Ethertype 89-A2
Subtype 0, CIM
Subtype 1, Headroom Measurement Message

RequestOctet

LengthVersionReserved
Req/Resp/Resp+Req
Length
t1
t2
t3
t4
PSN

1
1
1
2
3
11
19
27
35

4 bits2 bits2 bits188881

p_t1(optional)
p_PSN(optional)

36
44

81Reduce

VersionReserved
036t1
t2
t3t4
PSN

ResponseVersion
Reserved
136
t1
t2
t3
t4
PSN

Resp+Req
VersionReserved
245t1
t2
t3
t4
PSN
p_t1
p_PSN

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Other Explanation for the Mechanism• Measurement time will not exceed T*M• Node only maintains PSN(i) for req message, no additional status need to be stored.
• Do not wait for a response message to be received before sending the next request

message. Do not set timer for response message timeout.
• It may send redundant request messages, but the effect can be controlled by t, Tand M.

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