Explicit Congestion Notification ECN

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Explicit Congestion NotificationECN

• Tilo Hamann
• Technical University Hamburg-Harburg, Germany

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Outline

• Motivation for ECN
• How ECN works
• Benefits of ECN
• New-ECN TCP
• Algorithm
• Simulation results
• Some numbers
• Conclusions

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Motivation for ECN

• TCP relies on packets drops to detect congestion
• usually caused by queue overflow at a gateway
• many connections see packet drops
• synchronization of loss
• greater variance in queueing delay
• Solution active queue management (e.g. RED)
• avoids synchronization of loss across multiple
  flows
• tries to maintain a smaller average queue size
• attempts to increase fairness
• but packet drops are still the indication of
  congestion

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Motivation for ECN

• ECN uses marked instead of dropped packets for
  congestion indication
• ECN can react to congestion before packets get
  lost
• ECN avoids transmission delay due to unnecessary
  packet drops
• TCP sender does not need to wait for timeouts or
  duplicate ACKs
• currently ECN is only defined for data packets
• congestion in the ACK path is open for future
  research

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How ECN works TCP header

• ECN makes use of four bits in the TCP header
• ECN Capable Transmission (ECT) bit DS field, bit
  6
• Congestion Experienced (CE) bit DS field, bit
  7(these two bits are currently listed unused in
  RFC2474 (DiffServ))
• ECN echo flag reserved field, bit 9
• Congestion Window Reduced (CWR) flag reserved
  field, bit 8
• Definitions
• ECT packet a data packet of a ECN capable
  connection
• CE packet a marked ECT packet (ECT bit 1, CE
  bit 1)
• ECN echo packet an ACK with ECN echo flag set to
  1

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How ECN works connection setup

• Connection setup phase
• TCP sender and receiver negotiate their ECN
  capability during connection setup
• TCP sender sets the ECN echo flag and CWR in the
  SYN packet
• TCP receiver sets, if also ECN capable, only the
  ECN echo flag in the SYN-ACK packet
• If and only if both are ECN capable, the ECT bit
  gets set in every data packet

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How ECN works Gateway, TCP Receiver

• If a ECT packet arrives at a RED gateway
• it gets marked randomly (RED algorithm) if
• the average queue size is greater than minth and
• less than maxth
• it gets always dropped if
• average queue size is greater than maxth
• the queue is full
• After the receiver gets a CE packet, the ECN echo
  flag gets set in the next outgoing ACK
• The receiver continues to set the echo bit until
  it receives a data packet with the CWR flag set

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How ECN works TCP sender

• TCP should react to a ECN echo packet like it
  would to a lost packet
• cut the congestion window cwnd_ ½ cwnd_
• and reduce the slow start threshold
• But TCP should not
• increase cwnd_ for a ECN echo ACK
• trigger an slow start in TCP-Tahoe
• wait for roughly a half RTT in the fast recovery
  phase of TCP-Reno

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How ECN works TCP sender

• TCP should react at most once per RTT to any
  congestion indication, including
• timeout of the retransmit timer
• duplicate ACKs
• ECN echo ACK
• TCP should set the CWR flag in the next outgoing
  packet after it has reduced its congestion window
  for any reason

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Benefits of ECN

• Benefits for short web transfers
• most of TCP loss recovery is expected to be with
  timeouts
• worst case first packet gets dropped
• ECN reduces packets drops
• Bulk data transfers
• Real time flows (Non-TCP)

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New-ECN

• ECN-TCP is biased against connections with longer
  RTTs
• Question Can we use the marks to achieve a fair
  sharing of the available bandwidth?
• Approach
• reduce window and
• reduce the slope of the window-increase for each
  marked packet
• increase window for each regular ACK and
• increase slope every RTT while receiving regular
  ACKs

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New-ECN Algorithm

• Regular ACK received
• increase window
• if ((First_Mark_Received_ 1)
  (Not_Increased_within_the_last_RTT))
• TCP initialization
• First_Mark_Received_ 0

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New-ECN Algorithm

• Marked packet received
• First_Mark_Received_ 1
• if (seqno_Ack_ ? recover_ ) recover_
  maxseq_

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New-ECN ns simulation parameters

• RED
• pmax 0.1
• maxth 20
• minth 10
• wq 0.002
• Network topology

• New-ECN
• ?1 0.9
• ?2 0.9
• ? 16
• tcpTick_ 0.01
• packet size 512byte

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4 New-ECN connections (Test 10)
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4 New-ECN connections (Test 10)
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4 New-ECN vs. 4 ECN connections (Test 7)
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TCP-Friendliness (1) (Test 8)
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TCP-Friendliness (2) (Test 1)
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TCP-Friendliness (3) (Test 2)
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4 New-ECN connections (2) (Test 9)
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4 New-ECN connections (3) (Test 9)
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New-ECN some numbers

• 4 New-ECN TCP connections
• Fairnessindex
• TCP-Friendliness (Test 8)

• 4 ECN TCP connections
• Fairnessindex

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New-ECN some numbers (2)

• seems to be proportional to RTT? with 1lt ?
  lt2
• where and
• ? is ranges over 1 to a few RTT

?
t
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Conclusions

• It is possible to achieve fairness by using the
  congestion feedback provided by ECN
• New-ECN seems to be TCP friendly
• Some potential problems
• How to choose the design parameters?
• Interaction between TCP-Reno and New-ECN TCP
• Future work
• multiple congested gateways
• more congestion indication bits?
• ECN Home-Page
• http//www.aciri.org/floyd/ecn.html