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Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (July 02, 2013) is 3945 days in the past. Is this intentional? Checking references for intended status: Experimental ---------------------------------------------------------------------------- No issues found here. Summary: 1 error (**), 0 flaws (~~), 1 warning (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 TCPM Working Group M. Kuehlewind 3 Internet-Draft University of Stuttgart 4 Intended status: Experimental B. Trammell 5 Expires: January 03, 2014 ETH Zurich 6 July 02, 2013 8 A Mechanism for ECN Path Probing and Fallback 9 draft-kuehlewind-tcpm-ecn-fallback-00.txt 11 Abstract 13 Explicit Congestion Notification (ECN) is a TCP/IP extension that is 14 widely implemented but hardly used due to the perceived unusablilty 15 of ECN on many paths through the Internet caused by ECN-ignorant 16 routers and middleboxes. This document specifies an ECN probing and 17 fall-back mechanism in case ECN has be successfully negotiated 18 between two connection endpoints, but might not be usable on the 19 path. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at http://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on January 03, 2014. 38 Copyright Notice 40 Copyright (c) 2013 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 1. Introduction 55 The deployment of Explicit Congestion Notification (ECN) [RFC3168] 56 and AQM would arguably improve end-to-end performance in the 57 Internet, by providing a congestion signal to the transport layer 58 without relying on queue tail drop and packet loss. However, though 59 ECN has been standardized since 2000, implementation and deployment 60 have lagged significantly, in part due to the perceived unusablilty 61 of ECN on many paths through the Internet caused by ECN-ignorant 62 routers and middleboxes. 64 Recent research by the authors [KuNeTr13] has shown accelerating 65 deployment of ECN-capable servers in the Internet, due to the 66 deployment of TCP stacks for which ECN is enabled by default. In 67 addition, ECN is usable end-to-end on the vast majority of paths 68 measured in this study: that is, a Congestion Experienced mark will 69 cause a ECN Echo on the associated ACK. However, there still exist a 70 non-negligible number of paths on which a successfully negotiated 71 usage of ECN will not result in a connection on which congestion will 72 be correctly echoed, or worse, leads to the loss of packets with CE 73 or ECE set. 75 This document presents an experimental, in-band, runtime method for 76 determining the usability of ECN by a given traffic flow, based on 77 the active measurement method described in [KuNeTr13]. If ECN is 78 successfully negotiated but found by this method to be unusable, it 79 can be disabled on subsequent packets in the flow in order to avoid 80 connectivity problems caused by ECN-unusability on the path. 82 2. ECN Path Capability Probing 84 A TCP sender can determine whether or not its path to the receiver is 85 usable for ECN using the procedure detailed below. 87 1. The sender attempts to negotiate ECN usage as per Section 6.1.1 88 of [RFC3168]. If ECN is not successfully negotiated, the 89 procedure ends, and ECN is not used for the duration of the 90 connection. 92 2. The sender disables the normal usage of ECN for the duration of 93 the procedure, as the ECN codepoints are used for path probing. 94 This means all segments are sent with the Non-ECN-Capable 95 codepoint during this procedure unless otherwise stated. 96 Moreover, the sender will only take loss as a congestion signal 97 and will not react with window reductions to the ECN-Echo (ECE) 98 feedback signal from the receiver during this procedure. 100 3. The sender sets the Non-ECN-Capable codepoint in the IP header 101 until it has completed sending the first N segments, where N is 102 the size of the initial congestion window. Loss is used to 103 discover congestion for these segments. 105 4. The next three segments sent consist of the "CE probe": three 106 segments are sent with the Congestion Experienced codepoint set. 108 5. If all three of the CE probe segments are lost and must be 109 retransmitted, the path is deemed not ECN-usable and the sender 110 falls back as in Section 3. 112 6. If the ECE flag is not set on the ACK segment(s) sent by the 113 receiver acknowledging the CE probe segments, the path may or may 114 not be usable, as that there might be middleboxes/gateways that 115 (arguably correctly) clear CE on segments from end hosts, because 116 they assume that congestion can not have occurred up to this 117 point on the path. In this case, the sender may continue using 118 ECN, because while it may not work for detecting congestion, the 119 use of ECN does not negatively affect connectivity. Note that 120 this behavior can be more precisely detected using ECN Nonce 121 [RFC3540]. 123 7. While the sender does not reduce the congestion window for the 124 ECE segment(s) sent for the CE probe segments, it does set CWR on 125 the subsequent segment sent. 127 8. If no fallback has occurred by the time the ACK of the final CE 128 probe segment is received, the path is deemed ECN usable, and the 129 sender ends the probing procedure and proceeds to use ECN 130 normally as in [RFC3168]. 132 As the probing begins after all the segments in the initial 133 congestion window have been sent, it requires more than an initial 134 congestion window plus 6 segments (3 CE probe + 3 duplicated ACKs) of 135 available data to send. As this information is only available at the 136 higher layer, a configuration option per connection should be 137 provided to dis/enable ECN as well as ECN probing. Otherwise, ECN 138 should not be enabled for such short flows while using this 139 procedure. 141 3. ECN Fallback 143 If ECN is found to be unusable on a given flow by path capability 144 probing as in Section 2 above, the sender simply stops setting any 145 ECN-Capable-Transport codepoint on subsequent packets in the flow. 146 The receiver MUST, however, still set ECE on any ACK for a packet 147 with CE set. Note that this behavior is consistent with section 148 6.1.1 of [RFC3168]. 150 A sender may keep a cache of paths found to be unusable for ECN and 151 disable ECN for subsequent connections on a per-destination basis. 152 In this case, the reciever should periodically (i.e., on the order of 153 hours or days) expire these cache entries to cause re-probing to 154 occur in order to account for routing changes in the network. 156 [EDITOR'S NOTE: what to do on RTO?] 158 4. Discussion 160 [EDITOR'S NOTE: need to think about how this would interact with 161 conex; an analysis comparing the delay caused by path probing as 162 opposed to the delay caused by ECN failure would be interesting.] 164 [EDITOR'S NOTE: initial implementation results go here?.] 166 5. Security Considerations 168 [FIXME: we'll have to explore attacks against this mechanism which 169 could affect network or connection stability, so the following is 170 wrong...] 172 This document has no security considerations. 174 6. IANA Considerations 176 This document has no IANA considerations. 178 7. References 180 7.1. Normative References 182 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition 183 of Explicit Congestion Notification (ECN) to IP", RFC 184 3168, September 2001. 186 7.2. Informative References 188 [RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit 189 Congestion Notification (ECN) Signaling with Nonces", RFC 190 3540, June 2003. 192 [KuNeTr13] 193 Kuehlewind, M., Neuner, S., and B. Trammell, "On the state 194 of ECN and TCP Options on the Internet", Mar 2013. 196 (In LNCS 7799, Proceedings of PAM 2013, Hong Kong) 198 Authors' Addresses 200 Mirja Kuehlewind 201 University of Stuttgart 202 Pfaffenwaldring 47 203 70569 Stuttgart 204 Germany 206 Email: mirja.kuehlewind@ikr.uni-stuttgart.de 208 Brian Trammell 209 Swiss Federal Institute of Technology Zurich 210 Gloriastrasse 35 211 8092 Zurich 212 Switzerland 214 Phone: +41 44 632 70 13 215 Email: trammell@tik.ee.ethz.ch