1. Explicit Congestion Notification (ECN) RFC 3168 Justin Yackoski DEGAS Networking Group yackoski@cis.udel.edu CISC856 – TCP/IP Thanks to Namratha Hundigopal and Preethi Natarajan for slides

2. 2 10/3/2005 Overview The Problem – Congestion The other solution(s) TCP's basic congestion mechanisms Active Queue Management Description of ECN How ECN works Why ECN is better Performance evaluation Status of ECN

3. 3 10/3/2005 Congestion Router must buffer packets because input > output End-to-end delay increases as buffer fills When buffer is full, “tail drop” occurs

4. 4 10/3/2005 TCP without ECN Congestion Detection Retransmit Timeout 3 duplicate ACKs “Congestion Avoidance” Happens after congestion has already occurred (Multiplicative decrease of cwnd AFTER loss) Current TCP does something like congestion ‘recovery’ Network is treated as a black box, no way to know of impending doom

5. 5 What can sometimes happen? Global Synchronization – many connections reduce sending rate at same time, channel is under-utilized Lock Out – queue space is monopolized by a few connections Full Buffers – If network is operating at capacity, buffers stay full No spare room for “bursts” of traffic End-to-end delay is increased due to queuing delays

6. 6 Active Queue Management Detect “incipient” (early) congestion Try to keep average queue size in “good” range Randomly choose IP-PDUs to notify about congestion (how?) Average queue size lies in-between the thresholds minmax

7. 7 10/3/2005 What always happens? Packet drops each time congestion is detected, very inefficient BAD for real-time applications

8. 8 10/3/2005 Explicit Congestion Notification ECN is an AQM mechanism Routers notify TCP about incipient congestion Use TCP/IP headers to send ECN signals TCP treats ECN signals exactly the same as when a single dropped packet is detected BUT – Packets are NOT actually dropped

9. 9 10/3/2005 2 bits => 4 ECN Codepoints ECT(0) (ECN Capable Transport (0) )10 CE (Congestion Experienced)11 ECT(1) (ECN Capable Transport(1) )01 Not-ECT (Not ECN Capable Transport) 00 NameValue ECN Bits in IP Header

10. 10 10/3/2005 CWR flag - Congestion Window Reduced flag ECE flag - ECN-Echo flag ECN Bits in TCP Header

11. 11 10/3/2005 Negotiation between TCP transport entities senderreceiver ECN-setup SYN ECN-setup SYN-ACK ACK TCP Sender – sets both ECE and CWR in SYN TCP Receiver – sets only ECE in SYN-ACK A host must not set ECT in SYN or SYN-ACK Some faulty firewalls either drop an ECN-setup SYN packet or respond with an RST TCP-PDU

12. 12 10/3/2005 Typical sequence of events(1) ECT is set in IP-PDU’s carrying data transmitted by the sender to indicate that ECN is supported by transport entities for this PDU  ECN enabled sender ECN enabled receiver ECN enabled router ECT set ECN Negotiated during connection establishment

13. 13 10/3/2005 ECN-capable router detects incipient congestion, and sees that ECT is set in the IP-PDU The router sets CE in the IP-PDU ECT set Incipient Congestion, set CE th max th min CE set Typical sequence of events(2)

14. 14 10/3/2005 ECN enabled receiver receives the IP-PDU with CE set. Receiver conveys the congestion information to the transport sender by setting ECE in the ACK TCP-PDU ECN enabled sender ECN enabled receiver CE set ECE set in ACK Congestion!!! Let me inform the sender Typical sequence of events(3)

15. 15 10/3/2005 TCP sender receives the TCP-PDU with ECE set Sender becomes aware of incipient congestion in network Sender reacts as if a TCP-PDU was dropped (sender’s cwnd reduced). ECE set ACK  ECN enabled sender ECN enabled receiver Incipient Congestion, reduce cwnd Typical sequence of events(4)

16. 16 10/3/2005 TCP sender sets CWR in the next new TCP-PDU to the receiver Indicates that the sender has reacted to congestion by reducing the cwnd ECN enabled sender ECN enabled receiver Incipient Congestion, reduce cwnd, Set CWR CWR set Typical sequence of events(5)

17. 17 10/3/2005 Receiver stops sending ACKs with ECE set after getting a TCP-PDU with CWR set if there is no new congestion in the network Sender has reduced cwnd, stop setting ECE flag ECN enabled sender ECN enabled receiver ECN enabled router CWR set ECE set ACK Typical sequence of events(6)

18. 18 10/3/2005 Rules of the Game - Sender On receipt of ECE ACK packet, TCP sender SHOULD react in the same way as it would for a congestion loss in non-ECN-capable TCP Sender TCP SHOULD NOT react more than once every RTT to the ECE ACK packet Why? – We saw that receiver keeps sending ECE set ACKs until Receiver gets a TCP-PDU with CWR set from the sender For CWR set TCP-PDU to reach the receiver and get acked takes at least 1 RTT. So any more ECEs received in this time span is for the same instance of congestion

19. 19 10/3/2005 Rules of the Game - Sender TCP sender should set CWR in the first new TCP-PDU the sender transmits after receiving an ECE set ACK What if a CWR set TCP-PDU is lost? Sender TCP detects the loss The loss is treated as a new instance of congestion in network Sender will have to again reduce its cwnd and retransmits the lost TCP-PDU without CWR set

20. 20 10/3/2005 Rules of the Game - Receiver To overcome dropped ECE ACK packets, receiver MUST keep sending ECE ACKs until it gets a TCP-PDU with CWR set Any more IP-PDUs with CE set are treated as new instances of congestion in the network In delayed ACKs, ECE in ACK is set if CE is set for any of the IP-PDUs being acknowledged What does the receipt of CWR guarantee? The sender received the ECE message ? The sender reduced its congestion window ? NO YES

21. 21 10/3/2005 Advantages of ECN Prevents unnecessary packet drops at routers  less retransmissions  improvement in the “GOODPUT” Avoids timeouts by getting faster notification to end hosts Less retransmissions also means less traffic on the network

22. 22 10/3/2005 ECN Performance Improvements ECN+ - allow SYN ACKs to be marked Internet draft currently RED* - mark packets using ECN, don’t drop

23. 23 10/3/2005 Is ECN Secure? Does ECN add any insecurities to TCP and/or IP or make any problems worse? Can a malicious router: Interfere with handshake? Falsely report congestion? Fail to report congestion? Disable ECN?

24. 24 10/3/2005 Is ECN used? Implemented in linux 2.4+, Solaris 9+, and Cisco routers since 12.2(8)T

25. 25 10/3/2005 References RFC 3168 – ECN RFC 2309 – AQM A. Kuzmanovic. The Power of Explicit Congestion Notification. SIGCOMM ’05 A. Medina, M. Allman, and S. Floyd. Measuring the Evolution of Transport Protocols in the Internet. ACM CCR. 2005 http://www.cs.ucla.edu/NRL/hpi/tcpw/tcpw_sample/sample.html http://www.icir.org/floyd/ecn.html http://www.icir.org/floyd/ecn/ecn_security.txt Slides from Namratha Hundigopal and Preethi Natarajan