b9820806 黃冠智

B9820806 黃冠智

Outline

• OLDI(Online Data Intensive) architecture• DCTCP(Datacenter TCP)• D3(Deadline Driven Delivery) Protocol• D2TCP(Deadline-Aware Datacenter TCP)– Congestion avoidance algorithm

• Simulation• Q&A

OLDI application

• Tree-based structure, divide-and-conquer algo• Soft-real-time constraints(300 ms latency)

OLDI application

• Problems：– Maintain deadline will affect respond quality– Fan-in burst cause congestive packet drops and

TCP retransmits• Sols:– Over-provision the network link bandwidths(high

cost)– increase the network’s time budget(degrades

respond quality)

OLDI application

• Datacenter network also carry background flow which push control information and data to the node– Long-lived flows cause switch buffer to high

utilization• Package drop feedback• Fair-share protocol

DCTCP

• AQM(Active Queue Management) scheme • Using CE(Congestion Encountered) bit to

control transmission rate• ECN does not have suffice to solve fan-in burst

problem– Aggregates one-bit ECN feedback and multiple

RTTs to form multiple-bit metric

D3 Protocol• The bandwidth request for the next RTT is

piggybacked on the data packets of the current RTT

• Switch grant as much of the request as possible

• Far-deadline requests arriving slightly ahead of near-deadline requests

D3 Protocol

• Challenges– Requires custom switch ASICs (high cost low vol)– Cannot coexist with legacy TCP

The DC Network Protocol Should

• Meet OLDI deadlines, especially in fan-in-burst-induced congestion

• Achieve high bandwidth for background flows• Work with existing switch hardware• Be able to coexist with legacy TCP

D2TCP

• Meet OLDI deadlines, especially in fan-in-burst-induced congestion– Deadline-aware

• Achieve high bandwidth for background flows• Work with existing switch hardware• Be able to coexist with legacy TCP

Congestion avoidance algorithm

• 𝑓 is the fraction of packets that were marked with CE bits in the most recent window

• 𝑔 is the weight given to new samples • d is the deadline imminence factor • P is the penalty function applied to the window size • W is congestion window

Congestion avoidance algorithm

• ≤ 𝛼 1 and therefore, ≤ 1𝑝• d > 1 (near-deadline flows incur lower penalty)• d < 1 (far-deadline flows incur higher penalty)• d = 1 behavior matches DCTCP

Determining d

• 𝑇𝑐 is the time needed for a flow to complete transmitting• D is the time remaining until its deadline expires

Simulation

TCP DCTCP D3 D2TCP

Background flows discuss

Missed deadline under tight deadline

b9820806 黃冠智

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