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IETF87 Berlin DCTCP implementation in FreeBSD. Midori Kato Richard Scheffenegger Lars Eggert Alexander Zimmermann. DCTCP ( Data Center TCP) overview. Problem statement Large FIFO buffers in the presence of bulk transfers impacts latency-sensitive flows DCTCP

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ietf87 berlin dctcp implementation in freebsd

IETF87 BerlinDCTCP implementation in FreeBSD

Midori Kato

Richard Scheffenegger

Lars Eggert

Alexander Zimmermann

dctcp data center tcp overview
DCTCP (Data Center TCP) overview
  • Problem statement
    • Large FIFO buffers in the presence of bulk transfers impacts latency-sensitive flows
  • DCTCP
    • Maintain the queue size short using ECN (Explicit Congestion Notification)
    • Endpoint-only mechanism
      • Switches/routers’ mechanism is same as standard ECN

throughput-sensitive flows

High

throughput

High

throughput

Short delay

Long delay

latency-sensitive flows

ECN threshold

dctcp algorithm
DCTCP algorithm

Sender side

Receiver side

Mark ECE only when CE packet is received

send immediate ACK when CE state is changed (regardless of delayed ACK)

  • Maintain the fraction of ECN marked seg.for each RTT

and update average fraction

of marked seg. ( )

  • Adopt alpha to cwnd decrease

wDelayed ACK

wo Delayed ACK

S

S

R

R

Immediate

ACK

** reaction to packet loss (e.g., dup ack and timeout) is same with NewReno

CE (Congestion Experience)

ECE (ECN Echo)

freebsd implementation status 1 2
FreeBSD implementation status [1/2]
  • Total # of lines: 479 lines
    • tcp_input.c (24 lines), tcp_output.c (12 lines),

cc.h (4 lines), cc_dctcp.c (439 lines)

    • AddECN handling functions to CC module
    • Add DCTCP as one of CC algorithms
  • References for FreeBSD DCTCP
    • Released Linux patch [1]
    • DCTCP paper “Data Center TCP (DCTCP) [SIGCOMM’10]”

[1] http://simula.stanford.edu/~alizade/Site/DCTCP.html

freebsd implementation status 2 2
FreeBSD implementation status [2/2]
  • Design issues
    • Cwnd behavior on first ECN
      • Halve or Ignore cwnd
    • Reconsideration of alpha update interval
      • every RTT or more frequent (e.g., every ack)
    • Alpha value handling after idle time
      • Discardor keep alpha
    • Alpha calculation affected by receive buffer control
      • Change nothing or handle alpha as special case

Topic in this talk

In progress

open issue cwnd behavior on first ecn
Open issue: cwndbehavior on first ECN

Option 1: DCTCP never reduces cwnd at first congestion recovery

Option 2: halves cwnd at first congestion recovery

0

0.5

Linux DCTCP

Linux DCTCP

[KBytes]

[ms]

150

100

50

40

20

Benefit:

Delay converges

quickly

150

100

50

FreeBSD DCTCP

FreeBSD DCTCP

40

20

0.4 0.8 1.2 1.6

Time[s]

0.4 0.8 1.2 1.6

Time[s]

performance with small queue size
Performance with small queue size

Experiment

- Measure transfer time for XMB and SRTT (Smoothed RTT) using flowgrind

sender

receiver

dummynet

  • BW: 2Mbps, delay: 20ms
  • qlen: 7, ecn_thresh: 5
performance with large queue size
Performance with large queue size

Experiment

- Measure transfer time for XMB and SRTT (Smoothed RTT) using flowgrind

sender

receiver

dummynet

  • BW: 2Mbps, delay: 20ms
  • qlen: 30, ecn_thresh: 10
summary
Summary
  • Introduce DCTCP implementation in FreeBSD and design issues
    • Plan to submit our code to FreeBSD
  • Future work
    • Interaction with RFC3168
    • CUBIC and DCTCP
    • Change architecture for coexist of DCTCP and other CC algorithms
performance evaluation
Performance evaluation

Experiment

- Measure transfer time for X KBand SRTT (Smoothed RTT) using flowgrind

sender

receiver

dummynet

  • BW: 2Mbps, delay: 20ms
  • qlen: 14, ecn_thresh: 10