Explicit Congestion Notification

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Explicit CongestionNotification

• 
  
• 
  
• 
  Patrick Allison
• 
  Surren
  Deorajsingh
• 
  cse 4346
  

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ECN

• What it stands for?
• ECN Explicit Congestion notification
• Why it was started?
• New method to indicate congestion.
• To help control congestion problems in the
  network.

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ECN How it Works

• ECT- ECN Capable Transport
• End nodes are capable of ECN.
• AQM Active Queue Management.
• Before buffers are filled, router makes packet
  dropping decisions. (e.g. RED)
• CE Congestion Experienced
• Router has detected congestion using AQM.

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ECN Field - IP Header
00 Non-ECT 01 ECT capable 10 ECT capable 11
CE

• ECT 01 and 10 are synonymous. Treated as the
  same.
• End systems set ECT capable
• Router sets CE

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ECN TCP Header

• Two new fields added to bit 8 and 9 of Reserved.
• ECE ECN-Echo Let sender know that receiver
  got a CE packet.
• CWR Congestion Window Reduced Let receiver
  know the window is reduced

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Scenario
sender
receiver
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TCP Sender

• Establish connection
• Sends SYN Packet with ECE and CWR set.
• Waits for SYN-ACK Packet with ECE set.
• Transmit Data
• Sends packet with ECT field set
• If receives ACK packet with ECE congestion
  occurred.
• Reduce congestion window set CWR flag in next
  packet.

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Routers

• Receive ECT packet.
• If congestion occurs at router, manage buffers
  using AQM
• If packet would have been dropped, but room in
  buffer, check if ECN-capable.
• If ECN-capable, set CE in packet and send to
  destination.
• If packet is dropped, regular TCP congestion
  control occurs.

9
TCP Receiver

• Establish connection
• Waits for SYN Packet with ECE and CWR set
• Sends SYN-ACK Packet with ECE set
• Transmit Data
• Receives packet with CE field set
• Sends ECE-ACK packets until receives a CWR packet
  

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Negotiating ECN capabilities
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Summary

• ECN is a way for the network to handle
  congestion.
• End systems negotiate a connection and leave it
  up to routers to notify them of congestion.
• They respond to congestion by reducing
  transmission rates.
• If all systems used ECN, theoretically there
  would be no dropped packets.

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Performance Issues

• ECN Resource Efficiency
• Pentikousis and Badr
• LAN Simulation Comparison
• Drop Tail, RED w/o ECN, RED w/ECN
• Dr. Sally Floyd

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NS-2 Simulation Topology
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TCP/DT vs. TCP/ECN

• TCP/ECN aware of congestion sooner
• 10 Active Clients
• TCP/DT transmits 445 packets above payload
• TCP/ECN transmits 149 above payload
• 25 Active Clients
• TCP/ECN 535 packets above payload
• TCP/DT 1960 packets above payload
• TCP/ECN 72 reduction

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LAN SIMULATION SCENARIO
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LAN Simulation
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LAN Simulation
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LAN Simulation
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LAN Simulation TCP clock 0.1msBulk Data
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LAN Simulation TCP clock 0.1msBulk Data
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LAN Simulation TCP clock 0.1msSmallest Bulk
Data throughput
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LAN Simulation TCP clock 0.1msSmallest Bulk
Data throughput
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LAN Simulation TCP clock 0.1msHighest Telnet
Delay
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LAN Simulation TCP clock 0.1msHighest Telnet
Delay
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ECN ISSUES

• Cost and Complexity of implementation.
• A non-complaint TCP connection could set the ECN
  field to indicate that it was ECN-capable, and
  then ignore ECN notifications.
• Potential loss of ECN messages in the network. In
  this case the congestion notification message
  could fail to reach the end node.

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ECN ISSUES

• Many Firewalls and Network Address Translators
  drop all ECN packets or clear ECN bits.
• http//urchin.earth.li/cgi-bin/ecn.pl

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ECN Problem Spots
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REFERENCES

• Performance Evaluation of Explicit Congestion
  Notification (ECN) in IP Networks. (RFC 2884)
• The Addition of Explicit Congestion Notification
  (ECN) to IP. (RFC 3168)
• TCP and Explicit Congestion Notification
• Sally Floyd, Lawrence Berkeley Laboratory
• On The Resource Efficiency of Explicit Congestion
  Notification
• Pentikousis and Badr, Stony Brook University