Internet Engineering Task Force A. Kuzmanovic INTERNET-DRAFT A. Mondal Intended status: Proposed Standard Northwestern University Expires: 22 February 2009 S. Floyd Updates: 3168 ICIR K.K. Ramakrishnan AT&T 22 August 2008 Adding Explicit Congestion Notification (ECN) Capability to TCP's SYN/ACK Packets draft-ietf-tcpm-ecnsyn-06.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 2008. Copyright Notice Copyright (C) The IETF Trust (2008). Floyd et al. Expires: 22 February 2009 [Page 1]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 Abstract This draft specifies a modification to RFC 3168 to allow TCP SYN/ACK packets to be ECN-Capable. For TCP, RFC 3168 only specifies setting an ECN-Capable codepoint on data packets, and not on SYN and SYN/ACK packets. However, because of the high cost to the TCP transfer of having a SYN/ACK packet dropped, with the resulting retransmit timeout, this document specifies the use of ECN for the SYN/ACK packet itself, when sent in response to a SYN packet with the two ECN flags set in the TCP header, indicating a willingness to use ECN. Setting TCP SYN/ACK packets as ECN-Capable can be of great benefit to the TCP connection, avoiding the severe penalty of a retransmit timeout for a connection that has not yet started placing a load on the network. The sender of the SYN/ACK packet must respond to a report of an ECN-marked SYN/ACK packet by reducing its initial congestion window from two, three, or four segments to one segment, thereby reducing the subsequent load from that connection on the network. This document updates RFC 3168. Table of Contents 1. Introduction ....................................................5 2. Conventions and Terminology .....................................6 3. Specification ...................................................7 3.1. SYN/ACK Packets Dropped in the Network .....................7 3.2. SYN/ACK Packets ECN-Marked in the Network ..................8 3.3. Management Interface ......................................10 4. Discussion .....................................................10 4.1. Flooding Attacks ..........................................10 4.2. The TCP SYN Packet ........................................11 4.3. SYN/ACK Packets and Packet Size ...........................11 4.4. Response to ECN-marking of SYN/ACK Packets ................12 5. Related Work ...................................................13 6. Performance Evaluation .........................................14 6.1. The Costs and Benefit of Adding ECN-Capability ............14 6.2. An Evaluation of Different Responses to ECN-Marked SYN/ACK Packets ........................................................15 7. Security Considerations ........................................16 7.1. 'Bad' Routers or Middleboxes ..............................16 7.2. Congestion Collapse .......................................16 8. Conclusions ....................................................17 9. Acknowledgements ...............................................18 A. Report on Simulations ..........................................18 A.1. Simulations with RED in Packet Mode .......................19 A.2. Simulations with RED in Byte Mode .........................21 B. Issues of Incremental Deployment ...............................23 Normative References ..............................................26 Informative References ............................................26 Floyd et al. Expires: 22 February 2009 [Page 2]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 IANA Considerations ...............................................27 Full Copyright Statement ..........................................28 Intellectual Property .............................................28 NOTE TO RFC EDITOR: PLEASE DELETE THIS NOTE UPON PUBLICATION. Changes from draft-ietf-tcpm-ecnsyn-05: * Added "Updates: 3168" to the header. Added a reference to RFC 4987. Mild editing. Feedback from Lars's Area Director review. * Updated simulation results with new simulation scripts that don't require any modifications to the ns simulator, and that all use the same seed for generating traffic. The results are somewhat different for the very-high-congestion scenarios (with loss rates of 25% in the absence of ECN-capability for SYN/ACK packets). This is reflected in the simulations with a target load of 125% in Tables 1 and 2. * Added the URL for the web page that has the simulation scripts. Changes from draft-ietf-tcpm-ecnsyn-04: * Updating the copyright date. Changes from draft-ietf-tcpm-ecnsyn-03: * General editing. This includes using the terms "initiator" and "responder" for the two ends of the TCP connection. Feedback from Alfred Hoenes. * Added some text to the backwards compatibility discussion, now in Appendix B, about the pros and cons of using a TCP flag for the TCP initiator to signal that it understands ECN-Capable SYN/ACK packets. The consensus at this time is not to use such a flag. Also added a recommendation that TCP implementations include a management interface to turn off the use of ECN for SYN/ACK packets. From email from Bob Briscoe. Changes from draft-ietf-tcpm-ecnsyn-02: * Added to the discussion in the Security section of whether ECN-Capable TCP SYN packets have problems with firewalls, over and above the known problems of TCP data packets (e.g., as in the Microsoft report). From a question raised Floyd et al. Expires: 22 February 2009 [Page 3]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 at the TCPM meeting at the July 2007 IETF. * Added a sentence to the discussion of routers or middleboxes that *might* drop TCP SYN packets on the basis of IP header fields. Feedback from Remi Denis-Courmont. * General editing. Feedback from Alfred Hoenes. Changes from draft-ietf-tcpm-ecnsyn-01: * Changes in response to feedback from Anil Agarwal. * Added a look at the costs of adding ECN-Capability to SYN/ACKs in a highly-congested scenario. From feedback from Mark Allman and Janardhan Iyengar. * Added a comparative evaluation of two possible responses to an ECN-marked SYN/ACK packet. From Mark Allman. Changes from draft-ietf-tcpm-ecnsyn-00: * Only updating the revision number. Changes from draft-ietf-twvsg-ecnsyn-00: * Changed name of draft to draft-ietf-tcpm-ecnsyn. * Added a discussion in Section 3 of "Response to ECN-marking of SYN/ACK packets". Based on suggestions from Mark Allman. * Added a discussion to the Conclusions about adding ECN-capability to relevant set-up packets in other protocols. From a suggestion from Wesley Eddy. * Added a description of SYN exchanges with SYN cookies. From a suggestion from Wesley Eddy. * Added a discussion of one-way data transfers, where the host sending the SYN/ACK packet sends no data packets. * Minor editing, from feedback from Mark Allman and Janardhan Iyengar. * Future work: a look at the costs of adding ECN-Capability in a worst-case scenario. From feedback from Mark Allman and Janardhan Iyengar. Floyd et al. Expires: 22 February 2009 [Page 4]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 * Future work: a comparative evaluation of two possible responses to an ECN-marked SYN/ACK packet. Changes from draft-kuzmanovic-ecn-syn-00.txt: * Changed name of draft to draft-ietf-twvsg-ecnsyn. END OF NOTE TO RFC EDITOR. 1. Introduction TCP's congestion control mechanism has primarily used packet loss as the congestion indication, with packets dropped when buffers overflow. With such tail-drop mechanisms, the packet delay can be high, as the queue at bottleneck routers can be fairly large. Dropping packets only when the queue overflows, and having TCP react only to such losses, results in: 1) significantly higher packet delay; 2) unnecessarily many packet losses; and 3) unfairness due to synchronization effects. The adoption of Active Queue Management (AQM) mechanisms allows better control of bottleneck queues [RFC2309]. This use of AQM has the following potential benefits: 1) better control of the queue, with reduced queueing delay; 2) fewer packet drops; and 3) better fairness because of fewer synchronization effects. With the adoption of ECN, performance may be further improved. When the router detects congestion before buffer overflow, the router can provide a congestion indication either by dropping a packet, or by setting the Congestion Experienced (CE) codepoint in the Explicit Congestion Notification (ECN) field in the IP header [RFC3168]. The IETF has standardized the use of the Congestion Experienced (CE) codepoint in the IP header for routers to indicate congestion. For incremental deployment and backwards compatibility, the RFC on ECN [RFC3168] specifies that routers may mark ECN-capable packets that would otherwise have been dropped, using the Congestion Experienced codepoint in the ECN field. The use of ECN allows TCP to react to congestion while avoiding unnecessary retransmissions and, in some cases, unnecessary retransmit timeouts. Thus, using ECN has several benefits: 1) For short transfers, a TCP connection's congestion window may be small. For example, if the current window contains only one packet, and that packet is dropped, TCP will have to wait for a retransmit timeout to recover, reducing its overall throughput. Similarly, if the current window contains only a few packets and one of those Floyd et al. Expires: 22 February 2009 [Page 5]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 packets is dropped, there might not be enough duplicate acknowledgements for a fast retransmission, and the sender of the data packet might have to wait for a delay of several round-trip times using Limited Transmit [RFC3042]. With the use of ECN, short flows are less likely to have packets dropped, sometimes avoiding unnecessary delays or costly retransmit timeouts. 2) While longer flows may not see substantially improved throughput with the use of ECN, they experience lower loss. This may benefit TCP applications that are latency- and loss-sensitive, because of the avoidance of retransmissions. RFC 3168 only specifies marking the Congestion Experienced codepoint on TCP's data packets, and not on SYN and SYN/ACK packets. RFC 3168 specifies the negotiation of the use of ECN between the two TCP end- points in the TCP SYN and SYN-ACK exchange, using flags in the TCP header. Erring on the side of being conservative, RFC 3168 does not specify the use of ECN for the SYN/ACK packet itself. However, because of the high cost to the TCP transfer of having a SYN/ACK packet dropped, with the resulting retransmit timeout, this document specifies the use of ECN for the SYN/ACK packet itself. This can be of great benefit to the TCP connection, avoiding the severe penalty of a retransmit timeout for a connection that has not yet started placing a load on the network. The sender of the SYN/ACK packet must respond to a report of an ECN-marked SYN/ACK packet by reducing its initial congestion window from two, three, or four segments to one segment, reducing the subsequent load from that connection on the network. The use of ECN for SYN/ACK packets has the following potential benefits: 1) Avoidance of a retransmit timeout; 2) Improvement in the throughput of short connections. This draft specifies ECN+, a modification to RFC 3168 to allow TCP SYN/ACK packets to be ECN-Capable. Section 3 contains the specification of the change, while Section 4 discusses some of the issues, and Section 5 discusses related work. Section 6 contains an evaluation of the specified change. 2. Conventions and Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119]. We use the following terminology from RFC 3168: Floyd et al. Expires: 22 February 2009 [Page 6]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 The ECN field in the IP header: o CE: the Congestion Experienced codepoint; and o ECT: either one of the two ECN-Capable Transport codepoints. The ECN flags in the TCP header: o CWR: the Congestion Window Reduced flag; and o ECE: the ECN-Echo flag. ECN-setup packets: o ECN-setup SYN packet: a SYN packet with the ECE and CWR flags; o ECN-setup SYN-ACK packet: a SYN-ACK packet with ECE but not CWR. In this document we use the terms "initiator" and "responder" to refer to the sender of the SYN packet and of the SYN-ACK packet, respectively. 3. Specification This section specifies the modification to RFC 3168 to allow TCP SYN/ACK packets to be ECN-Capable. RFC 3168 in Section 6.1.1. states that "A host MUST NOT set ECT on SYN or SYN-ACK packets." In this section, we specify that a TCP node MAY respond to an ECN-setup SYN packet by setting ECT in the responding ECN-setup SYN/ACK packet, indicating to routers that the SYN/ACK packet is ECN-Capable. This allows a congested router along the path to mark the packet instead of dropping the packet as an indication of congestion. Assume that TCP node A transmits to TCP node B an ECN-setup SYN packet, indicating willingness to use ECN for this connection. As specified by RFC 3168, if TCP node B is willing to use ECN, node B responds with an ECN-setup SYN-ACK packet. 3.1. SYN/ACK Packets Dropped in the Network Figure 1 shows an interchange with the SYN/ACK packet dropped by a congested router. Node B waits for a retransmit timeout, and then retransmits the SYN/ACK packet. Floyd et al. Expires: 22 February 2009 [Page 7]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 --------------------------------------------------------------- TCP Node A Router TCP Node B ---------- ------ ---------- ECN-setup SYN packet ---> ECN-setup SYN packet ---> <--- ECN-setup SYN/ACK, possibly ECT 3-second timer set SYN/ACK dropped . . . 3-second timer expires <--- ECN-setup SYN/ACK, not ECT <--- ECN-setup SYN/ACK Data/ACK ---> Data/ACK ---> <--- Data (one to four segments) --------------------------------------------------------------- Figure 1: SYN exchange with the SYN/ACK packet dropped. If the SYN/ACK packet is dropped in the network, the responder (node B) responds by waiting three seconds for the retransmit timer to expire [RFC2988]. If a SYN/ACK packet with the ECT codepoint is dropped, the responder SHOULD resend the SYN/ACK packet without the ECN-Capable codepoint. (Although we are not aware of any middleboxes that drop SYN/ACK packets that contain an ECN-Capable codepoint in the IP header, we have learned to design our protocols defensively in this regard [RFC3360].) We note that if syn-cookies were used by the responder (node B) in the exchange in Figure 1, the responder wouldn't set a timer upon transmission of the SYN/ACK packet [SYN-COOK] [RFC4987]. In this case, if the SYN/ACK packet was lost, the initiator (Node A) would have to timeout and retransmit the SYN packet in order to trigger another SYN-ACK. 3.2. SYN/ACK Packets ECN-Marked in the Network Figure 2 shows an interchange with the SYN/ACK packet sent as ECN- Capable, and ECN-marked instead of dropped at the congested router. Floyd et al. Expires: 22 February 2009 [Page 8]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 --------------------------------------------------------------- TCP Node A Router TCP Node B ---------- ------ ---------- ECN-setup SYN packet ---> ECN-setup SYN packet ---> <--- ECN-setup SYN/ACK, ECT <--- Sets CE on SYN/ACK <--- ECN-setup SYN/ACK, CE Data/ACK, ECN-Echo ---> Data/ACK, ECN-Echo ---> Window reduced to one segment. <--- Data, CWR (one segment only) --------------------------------------------------------------- Figure 2: SYN exchange with the SYN/ACK packet marked. If the initiator (node A) receives a SYN/ACK packet that has been marked by the congested router, with the CE codepoint set, the initiator MUST respond by setting the ECN-Echo flag in the TCP header of the responding ACK packet. As specified in RFC 3168, the initiator continues to set the ECN-Echo flag in packets until it receives a packet with the CWR flag set. When the responder (node B) receives the ECN-Echo packet reporting the Congestion Experienced indication in the SYN/ACK packet, the responder MUST set the initial congestion window to one segment, instead of two segments as allowed by [RFC2581], or three or four segments allowed by [RFC3390]. If the responder (node B) was going to use an initial window of one segment, and receives an ECN-Echo packet informing it of a Congestion Experienced indication on its SYN/ACK packet, the responder MAY continue to send with an initial window of one segment, without waiting for a retransmit timeout. We note that this updates RFC 3168, which specifies that "the sending TCP MUST reset the retransmit timer on receiving the ECN-Echo packet when the congestion window is one." As specified by RFC 3168, the responder (node B) also sets the CWR flag in the TCP header of the next data packet sent, to acknowledge its receipt of and reaction to the ECN-Echo flag. If the data transfer in Figure 2 is entirely from Node A to Node B, then data packets from Node A continue to set the ECN-Echo flag in data packets, waiting for the CWR flag from Node B acknowledging a response to the ECN-Echo flag. Floyd et al. Expires: 22 February 2009 [Page 9]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 3.3. Management Interface The TCP implementation using ECN-Capable SYN/ACK packets SHOULD include a management interface to allow the use of ECN to be turned off for SYN/ACK packets. This is to deal with possible backwards compatibility problems such as those discussed in Appendix B. 4. Discussion The rationale for the specification in this document is the following. When node B receives a TCP SYN packet with ECN-Echo bit set in the TCP header, this indicates that node A is ECN-capable. If node B is also ECN-capable, there are no obstacles to immediately setting one of the ECN-Capable codepoints in the IP header in the responding TCP SYN/ACK packet. There can be a great benefit in setting an ECN-capable codepoint in SYN/ACK packets, as is discussed further in [ECN+], and reported briefly in Section 5 below. Congestion is most likely to occur in the server-to-client direction. As a result, setting an ECN-capable codepoint in SYN/ACK packets can reduce the occurrence of three- second retransmit timeouts resulting from the drop of SYN/ACK packets. 4.1. Flooding Attacks Setting an ECN-Capable codepoint in the responding TCP SYN/ACK packets does not raise any novel security vulnerabilities. For example, provoking servers or hosts to send SYN/ACK packets to a third party in order to perform a "SYN/ACK flood" attack would be highly inefficient. Third parties would immediately drop such packets, since they would know that they didn't generate the TCP SYN packets in the first place. Moreover, such SYN/ACK attacks would have the same signatures as the existing TCP SYN attacks. Provoking servers or hosts to reply with SYN/ACK packets in order to congest a certain link would also be highly inefficient because SYN/ACK packets are small in size. However, the addition of ECN-Capability to SYN/ACK packets could allow SYN/ACK packets to persist for more hops along a network path before being dropped, thus adding somewhat to the ability of a SYN/ACK attack to flood a network link. Floyd et al. Expires: 22 February 2009 [Page 10]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 4.2. The TCP SYN Packet There are several reasons why an ECN-Capable codepoint MUST NOT be set in the IP header of the initiating TCP SYN packet. First, when the TCP SYN packet is sent, there are no guarantees that the other TCP endpoint (node B in Figure 2) is ECN-capable, or that it would be able to understand and react if the ECN CE codepoint was set by a congested router. Second, the ECN-Capable codepoint in TCP SYN packets could be misused by malicious clients to `improve' the well-known TCP SYN attack. By setting an ECN-Capable codepoint in TCP SYN packets, a malicious host might be able to inject a large number of TCP SYN packets through a potentially congested ECN-enabled router, congesting it even further. For both these reasons, we continue the restriction that the TCP SYN packet MUST NOT have the ECN-Capable codepoint in the IP header set. 4.3. SYN/ACK Packets and Packet Size There are a number of router buffer architectures that have smaller dropping rates for small (SYN) packets than for large (data) packets. For example, for a Drop Tail queue in units of packets, where each packet takes a single slot in the buffer regardless of packet size, small and large packets are equally likely to be dropped. However, for a Drop Tail queue in units of bytes, small packets are less likely to be dropped than are large ones. Similarly, for RED in packet mode, small and large packets are equally likely to be dropped or marked, while for RED in byte mode, a packet's chance of being dropped or marked is proportional to the packet size in bytes. For a congested router with an AQM mechanism in byte mode, where a packet's chance of being dropped or marked is proportional to the packet size in bytes, the drop or marking rate for TCP SYN/ACK packets should generally be low. In this case, the benefit of making SYN/ACK packets ECN-Capable should be similarly moderate. However, for a congested router with a Drop Tail queue in units of packets or with an AQM mechanism in packet mode, and with no priority queueing for smaller packets, small and large packets should have the same probability of being dropped or marked. In such a case, making SYN/ACK packets ECN-Capable should be of significant benefit. We believe that there are a wide range of behaviors in the real world in terms of the drop or mark behavior at routers as a function of packet size [Tools] (Section 10). We note that all of these alternatives listed above are available in the NS simulator (Drop Tail queues are by default in units of packets, while the default for Floyd et al. Expires: 22 February 2009 [Page 11]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 RED queue management has been changed from packet mode to byte mode). 4.4. Response to ECN-marking of SYN/ACK Packets One question is why TCP SYN/ACK packets should be treated differently from other packets in terms of the end node's response to an ECN- marked packet. Section 5 of RFC 3168 specifies the following: "Upon the receipt by an ECN-Capable transport of a single CE packet, the congestion control algorithms followed at the end-systems MUST be essentially the same as the congestion control response to a *single* dropped packet. For example, for ECN-Capable TCP the source TCP is required to halve its congestion window for any window of data containing either a packet drop or an ECN indication." In particular, Section 6.1.2 of RFC 3168 specifies that when the TCP congestion window consists of a single packet and that packet is ECN- marked in the network, then the data sender must reduce the sending rate below one packet per round-trip time, by waiting for one RTO before sending another packet. If the RTO was set to the average round-trip time, this would result in halving the sending rate; because the RTO is in fact larger than the average round-trip time, the sending rate is reduced to less than half of its previous value. TCP's congestion control response to the *dropping* of a SYN/ACK packet is to wait a default time before sending another packet. This document argues that ECN gives end-systems a wider range of possible responses to the *marking* of a SYN/ACK packet, and that waiting a default time before sending a data packet is not the desired response. On the conservative end, one could assume an effective congestion window of one packet for the SYN/ACK packet, and respond to an ECN- marked SYN/ACK packet by reducing the sending rate to one packet every two round-trip times. As an approximation, the TCP end-node could measure the round-trip time T between the sending of the SYN/ACK packet and the receipt of the acknowledgement, and reply to the acknowledgement of the ECN-marked SYN/ACK packet by waiting T seconds before sending a data packet. However, we note that for an ECN-marked SYN/ACK packet, halving the *congestion window* is not the same as halving the *sending rate*; there is no `sending rate' associated with an ECN-Capable SYN/ACK packet, as such packets are only sent as the first packet in a connection from that host. Further, a router's marking of a SYN/ACK packet is not affected by any past history of that connection. Floyd et al. Expires: 22 February 2009 [Page 12]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 Adding ECN-Capability to SYN/ACK packets allows the simple response of the responder setting the initial congestion window to one packet, instead of its allowed default value of two, three, or four packets, with the responder proceeding with a cautious sending rate of one packet per round-trip time. If that data packet is ECN-marked or dropped, then the responder will wait an RTO before sending another packet. This document argues that this approach is useful to users, with no dangers of congestion collapse or of starvation of competing traffic. This is discussed in more detail below in Section 6.2. In particular, Section 6.2 discusses simulation results that support the responder's specified behavior of setting the initial congestion window to one packet in response to an ECN-marked SYN/ACK packet. We note that if the data transfer is entirely from Node A to Node B, then there is no effective difference between the two possible responses to an ECN-marked SYN/ACK packet outlined above. In either case, Node B sends no data packets, only sending acknowledgement packets in response to received data packets. 5. Related Work The addition of ECN-capability to TCP's SYN/ACK packets was proposed in [ECN+]. The paper includes an extensive set of simulation and testbed experiments to evaluate the effects of the proposal, using several Active Queue Management (AQM) mechanisms, including Random Early Detection (RED) [RED], Random Exponential Marking (REM) [REM], and Proportional Integrator (PI) [PI]. The performance measures were the end-to-end response times for each request/response pair, and the aggregate throughput on the bottleneck link. The end-to-end response time was computed as the time from the moment when the request for the file is sent to the server, until that file is successfully downloaded by the client. The measurements from [ECN+] show that setting an ECN-Capable codepoint in the IP packet header in TCP SYN/ACK packets systematically improves performance with all evaluated AQM schemes. When SYN/ACK packets at a congested router are ECN-marked instead of dropped, this can avoid a long initial retransmit timeout, improving the response time for the affected flow dramatically. [ECN+] shows that the impact on aggregate throughput can also be quite significant, because marking SYN ACK packets can prevent larger flows from suffering long timeouts before being "admitted" into the network. In addition, the testbed measurements from [ECN+] show that web servers setting the ECN-Capable codepoint in TCP SYN/ACK packets could serve more requests. As a final step, [ECN+] explores the co-existence of flows that do Floyd et al. Expires: 22 February 2009 [Page 13]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 and don't set the ECN-capable codepoint in TCP SYN/ACK packets. The results in [ECN+] show that both types of flows can coexist, with some performance degradation for flows that don't use ECN+. Flows that do use ECN+ improve their end-to-end performance. At the same time, the performance degradation for flows that don't use ECN+, as a result of the flows that do use ECN+, increases as a greater fraction of flows use ECN+. 6. Performance Evaluation 6.1. The Costs and Benefit of Adding ECN-Capability [ECN+] explores the costs and benefits of adding ECN-Capability to SYN/ACK packets with both simulations and experiments. The addition of ECN-capability to SYN/ACK packets could be of significant benefit for those ECN connections that would have had the SYN/ACK packet dropped in the network, and for which the ECN-Capability would allow the SYN/ACK to be marked rather than dropped. The percent of SYN/ACK packets on a link can be quite high. In particular, measurements on links dominated by web traffic indicate that 15-20% of the packets can be SYN/ACK packets [SCJO01]. The benefit of adding ECN-capability to SYN/ACK packets depends in part on the size of the data transfer. The drop of a SYN/ACK packet can increase the download time of a short file by an order of magnitude, by requiring a three-second retransmit timeout. For longer-lived flows, the effect of a dropped SYN/ACK packet on file download time is less dramatic. However, even for longer-lived flows, the addition of ECN-capability to SYN/ACK packets can improve the fairness among long-lived flows, as newly-arriving flows would be less likely to have to wait for retransmit timeouts. One question that arises is what fraction of connections would see the benefit from making SYN/ACK packets ECN-capable, in a particular scenario. Specifically: (1) What fraction of arriving SYN/ACK packets are dropped at the congested router when the SYN/ACK packets are not ECN-capable? (2) Of those SYN/ACK packets that are dropped, what fraction would have been ECN-marked instead of dropped if the SYN/ACK packets had been ECN-capable? To answer (1), it is necessary to consider not only the level of congestion but also the queue architecture at the congested link. As described in Section 4 above, for some queue architectures small packets are less likely to be dropped than large ones. In such an Floyd et al. Expires: 22 February 2009 [Page 14]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 environment, SYN/ACK packets would have lower packet drop rates; question (1) could not necessarily be inferred from the overall packet drop rate, but could be answered by measuring the drop rate for SYN/ACK packets directly. In such an environment, adding ECN- capability to SYN/ACK packets would be of less dramatic benefit than in environments where all packets are equally likely to be dropped regardless of packet size. As question (2) implies, even if all of the SYN/ACK packets were ECN- capable, there could still be some SYN/ACK packets dropped instead of marked at the congested link; the full answer to question (2) depends on the details of the queue management mechanism at the router. If congestion is sufficiently bad, and the queue management mechanism cannot prevent the buffer from overflowing, then SYN/ACK packets will be dropped rather than marked upon buffer overflow whether or not they are ECN-capable. For some AQM mechanisms, ECN-capable packets are marked instead of dropped any time this is possible, that is, any time the buffer is not yet full. For other AQM mechanisms however, such as the RED mechanism as recommended in [RED], packets are dropped rather than marked when the packet drop/mark rate exceeds a certain threshold, e.g., 10%, even if the packets are ECN-capable. For a router with such an AQM mechanism, when congestion is sufficiently severe to cause a high drop/mark rate, some SYN/ACK packets would be dropped instead of marked whether or not they were ECN-capable. Thus, the degree of benefit of adding ECN-Capability to SYN/ACK packets depends not only on the overall packet drop rate in the network, but also on the queue management architecture at the congested link. 6.2. An Evaluation of Different Responses to ECN-Marked SYN/ACK Packets This document specifies that the end-node responds to the report of an ECN-marked SYN/ACK packet by setting the initial congestion window to one segment, instead of its possible default value of two to four segments. We call this ECN+ with NoWaiting. However, Section 4 discussed another possible response to an ECN-marked SYN/ACK packet, of the end-node waiting an RTT before sending a data packet. We call this approach ECN+ with Waiting. Simulations comparing the performance with Standard ECN (without ECN- marked SYN/ACK packets), ECN+ with NoWaiting, and ECN+ with Waiting show little difference, in terms of aggregate congestion, between ECN+ with NoWaiting and ECN+ with Waiting. The details are given in Appendix A below. Our conclusions are that ECN+ with NoWaiting is Floyd et al. Expires: 22 February 2009 [Page 15]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 perfectly safe, and there are no congestion-related reasons for preferring ECN+ with Waiting over ECN+ with NoWaiting. That is, there is no need for the TCP end-node to wait a round-trip time before sending a data packet after receiving an acknowledgement of an ECN-marked SYN/ACK packet. 7. Security Considerations TCP packets carrying the ECT codepoint in IP headers can be marked rather than dropped by ECN-capable routers. This raises several security concerns that we discuss below. 7.1. 'Bad' Routers or Middleboxes There are a number of known deployment problems from using ECN with TCP traffic in the Internet. The first reported problem, dating back to 2000, is of a small but decreasing number of routers or middleboxes that reset a TCP connection in response to TCP SYN packets using flags in the TCP header to negotiate ECN-capability [Kelson00] [RFC3360] [MAF05]. Dave Thaler reported at the March 2007 IETF of new two problems encountered by TCP connections using ECN; the first of the two problems concerns routers that crash when a TCP data packet arrives with the ECN field in the IP header with the codepoint ECT(0) or ECT(1), indicating that an ECN-Capable connection has been established [SBT07]. While there is no evidence that any routers or middleboxes drop SYN/ACK packets that contain an ECN-Capable or CE codepoint in the IP header, such behavior cannot be excluded. (There seems to be a number of routers or middleboxes that drop TCP SYN packets that contain known or unknown IP options [MAF05] (Figure 1).) Thus, as specified in Section 3, if a SYN/ACK packet with the ECT or CE codepoint is dropped, the TCP node SHOULD resend the SYN/ACK packet without the ECN-Capable codepoint. There is also no evidence that any routers or middleboxes crash when a SYN/ACK arrives with an ECN- Capable or CE codepoint in the IP header (over and above the routers already known to crash when a data packet arrives with either ECT(0) or ECT(1)), but we have not conducted any measurement studies of this [F07]. 7.2. Congestion Collapse Because TCP SYN/ACK packets carrying an ECT codepoint could be ECN- marked instead of dropped at an ECN-capable router, the concern is whether this can either invoke congestion, or worsen performance in Floyd et al. Expires: 22 February 2009 [Page 16]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 highly congested scenarios. However, after learning that a SYN/ACK packet was ECN-marked, the responder will only send one data packet; if this data packet is ECN-marked, the responder will then wait for a retransmission timeout. In addition, routers are free to drop rather than mark arriving packets in times of high congestion, regardless of whether the packets are ECN-capable. When congestion is very high and a router's buffer is full, the router has no choice but to drop rather than to mark an arriving packet. The simulations reported in Appendix A show that even with demanding traffic mixes dominated by short flows and high levels of congestion, the aggregate packet dropping rates are not significantly different with Standard ECN, ECN+ with NoWaiting, or ECN+ with Waiting. In particular, the simulations show that in periods of very high congestion the packet-marking rate is low with or without ECN+, and the use of ECN+ does not significantly increase the number of dropped or marked packets. The simulations show that ECN+ is most effective in times of moderate congestion. In these moderate-congested scenarios, the use of ECN+ increases the number of ECN-marked packets, because ECN+ allows SYN/ACK packets to be ECN-marked. At the same time, in these times of moderate congestion, the use of ECN+ instead of Standard ECN does not significantly affect the overall levels of congestion. The simulations show that the use of ECN+ is less effective in times of high congestion; the simulations show that in times of high congestion more packets are dropped instead of marked, both with Standard ECN and with ECN+. In times of high congestion, the buffer can overflow, even with Active Queue Management and ECN; when the buffer is full arriving packets are dropped rather than marked, whether the packets are ECN-capable or not. Thus while ECN+ is less effective in times of high congestion, it still doesn't result in a significant increase in the level of congestion. More details are given in the appendix. 8. Conclusions This draft specifies a modification to RFC 3168 to allow TCP nodes to send SYN/ACK packets as being ECN-Capable. Making the SYN/ACK packet ECN-Capable avoids the high cost to a TCP transfer when a SYN/ACK packet is dropped by a congested router, by avoiding the resulting retransmit timeout. This improves the throughput of short connections. The sender of the SYN/ACK packet responds to an ECN mark by reducing its initial congestion window from two, three, or four segments to one segment, reducing the subsequent load from that connection on the network. The addition of ECN-capability to SYN/ACK packets is particularly beneficial in the server-to-client direction, Floyd et al. Expires: 22 February 2009 [Page 17]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 where congestion is more likely to occur. In this case, the initial information provided by the ECN marking in the SYN/ACK packet enables the server to more appropriately adjust the initial load it places on the network. 9. Acknowledgements We thank Anil Agarwal, Mark Allman, Remi Denis-Courmont, Wesley Eddy, Lars Eggert, Alfred Hoenes, Janardhan Iyengar, and Pasi Sarolahti for feedback on earlier versions of this draft. A. Report on Simulations This section reports on simulations showing the costs of adding ECN+ in highly-congested scenarios. This section also reports on simulations for a comparative evaluation between ECN+ with NoWaiting and ECN+ with Waiting. The simulations are run with a range of file-size distributions, using the PackMime traffic generator in the ns-2 simulator. They all use a heavy-tailed distribution of file sizes. The simulations reported in the tables below use a mean file size of 3 KBypes, to show the results with a traffic mix with a large number of small transfers. Othe simulations were run with mean file sizes of 5 KBytes, 7 Kbytes, 14 KBytes, and 17 Kbytes. The title of each chart gives the targeted average load from the traffic generator. Because the simulations use a heavy-tailed distribution of file sizes, and run for only 85 seconds (including ten seconds of warm-up time), the actual load is often much smaller than the targeted load. The congested link is 100 Mbps. RED is run in gentle mode, and arriving ECN-Capable packets are only dropped instead of marked if the buffer is full (and the router has no choice). We explore two alternatives for a TCP node's response to a report of an ECN-marked SYN/ACK packet. With ECN+ with NoWaiting, the TCP node sends a data packet immediately (with an initial congestion window of one segment). With the alternative ECN+ with Waiting, the TCP node waits a round-trip time before sending a data packet; the responder already has one measurement of the round-trip time when the acknowledgement for the SYN/ACK packet is received. In the tables below, ECN+ refers to ECN+ with NoWaiting, where the responder starts transmitting immediately, and ECN+/wait refers to ECN+ with Waiting, where the responder waits a round-trip time before sending a data packet into the network. The simulation scripts are available on [ECN-SYN]. Floyd et al. Expires: 22 February 2009 [Page 18]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 A.1. Simulations with RED in Packet Mode The simulations with RED in packet mode and with the queue in packets show that ECN+ is useful in times of moderate congestion, though it adds little benefit in times of high congestion. The simulations show a minimal increase in levels of congestion with either ECN+ with Waiting or ECN+ with NoWaiting, either in terms of packet dropping or marking rates or in terms of the distribution of responses times. Thus, the simulations show no problems with ECN+ in times of high congestion, and no reason to use ECN+ with Waiting instead of ECN+ with NoWaiting. Table 1 shows the congestion levels for simulations with RED in packet mode, with a queue in packets. To explore a worst-case scenario, these simulations use a traffic mix with an unrealistically small flow size distribution, with a mean flow size of 3 Kbytes. For each table showing a particular traffic load, the three rows show the number of packets dropped, the number of packets ECN-marked, and the aggregate packet drop rate, and the three columns show the simulations with Standard ECN, ECN+ (NoWaiting) and ECN+/wait. These simulations were run with RED set to mark instead of drop packets any time that the queue is not full. For the default implementation of RED in the ns-2 simulator, the router drops instead of marks arriving packets when the average queue size exceeds a configured threshold. The usefulness of ECN+: The first thing to observe is that for all of the simulations, the use of ECN+ or ECN+/wait significantly increased the number of packets marked. This indicates that with ECN+ or ECN+/wait, many SYN/ACK packets are marked instead of dropped. Little increase in congestion, sometimes: The second thing to observe is that for the simulations with low or moderate levels of congestion (that is, with packet drop rates less than 10%), the use of ECN+ or ECN+/wait decreases the aggregate packet drop rate, relative to the simulations with ECN. This makes sense, since with low or moderate levels of congestion, ECN+ allows SYN/ACK packets to be marked instead of dropped, and the use of ECN+ doesn't add to the aggregate congestion. However, for the simulations with packet drop rates of 15% or higher with ECN, the use of ECN+ or ECN+/wait increases the aggregate packet drop rate, sometimes even doubling it. Comparing ECN+ and ECN+/wait: The third thing to observe is that the aggregate packet drop rate is generally higher with ECN+/wait than with ECN+. Thus, there is no congestion-related reason to prefer ECN+/wait over ECN+. Floyd et al. Expires: 22 February 2009 [Page 19]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 Target Load = 95%: ECN ECN+ ECN+/wait ------- ------- ------- Dropped 18,512 11,244 12,135 Marked 27,026 36,977 38,743 Loss rate 1.27% 0.78% 0.84% Throughput 81% 81% 81% Target Load = 110%: ECN ECN+ ECN+/wait ------- ------- ------- Dropped 165,866 110,525 144,821 Marked 180,714 290,629 311,233 Loss rate 9.04% 6.36% 7.94% Throughput 92% 92% 92% Target Load = 125%: ECN ECN+ ECN+/wait ------- ------- ------- Dropped 574,114 1,764,677 2,229,280 Marked 409,441 1,172,550 1,181,209 Loss rate 24.55% 52.00% 57.64% Throughput 94% 98% 97% Table 1: Simulations with an average flow size of 3 Kbytes, a 100 Mbps link, RED in packet mode, queue in packets. Floyd et al. Expires: 22 February 2009 [Page 20]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 Target Load = 95%: ECN ECN+ ECN+/wait ------- ------- ------- Dropped 8,754 6,719 7,269 Marked 10,376 17,637 16,956 Loss rate 5.68% 4.50% 4.75% Throughput 78% 78% 78% Target Load = 110%: ECN ECN+ ECN+/wait ------- ------- ------- Dropped 32,110 32,014 48,838 Marked 28,476 56,550 62,252 Loss rate 15.68% 16.11% 21.92% Throughput 96% 96% 96% Target Load = 125%: ECN ECN+ ECN+/wait ------- ------- ------- Dropped 60,710 174,920 215,001 Marked 43,497 119,620 118,172 Loss rate 25.08% 51.59% 56.27% Throughput 98% 98% 98% Target Load = 150%: ECN ECN+ ECN+/wait ------- ------- ------- Dropped 133,128 250,762 327,584 Marked 63,306 146,581 147,307 Loss rate 43.34% 61.11% 67.33% Throughput 93% 100% 100% Table 2: Simulations with an average flow size of 3 Kbytes, a 10 Mbps link, RED in packet mode, queue in packets. A.2. Simulations with RED in Byte Mode Table 3 below shows simulations with RED in byte mode and the queue in bytes. There is no significant increase in aggregate congestion with the use of ECN+ or ECN+/wait, and no congestion-related reason to prefer ECN+/wait over ECN+. However, unlike the simulations with RED in packet mode, the simulations with RED in byte mode show little benefit from the use of ECN+ or ECN+/wait, in that the packet marking rate with ECN+ or ECN+/wait is not much different than the packet marking rate with Floyd et al. Expires: 22 February 2009 [Page 21]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 Standard ECN. This is because with RED in byte mode, small packets like SYN/ACK packets are rarely dropped or marked - that is, there is no drawback from the use of ECN+ in these scenarios, but not much need for ECN+ either, in a scenario where small packets are unlikely to be dropped or marked. Target Load = 95%: ECN ECN+ ECN+/wait ------- ------- ------- Dropped 739 438 442 Marked 32,405 34,357 34,000 Loss rate 0.05% 0.03% 0.03% Throughput 81% 81% 81% Target Load = 110%: ECN ECN+ ECN+/wait ------- ------- ------- Dropped 2,473 1,679 3,020 Marked 226,971 222,234 327,608 Loss rate 0.15% 0.10% 0.18% Throughput 92% 92% 91% Target Load = 125%: ECN ECN+ ECN+/wait ------- ------- ------- Dropped 19,358 14,057 14,064 Marked 717,123 728,513 729,001 Loss rate 1.07% 0.78% 0.78% Throughput 95% 95% 95% Table 3: Simulations with an average flow size of 3 Kbytes, a 100 Mbps link, RED in byte mode, queue in bytes. Floyd et al. Expires: 22 February 2009 [Page 22]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 Target Load = 95%: ECN ECN+ ECN+/wait ------- ------- ------- Dropped 142 81 78 Marked 11,694 11,812 11,964 Loss rate 0.01% 0.06% 0.05% Throughput 78% 78% 78% Target Load = 110%: ECN ECN+ ECN+/wait ------- ------- ------- Dropped 314 215 188 Marked 39,697 42,388 40,229 Loss rate 0.19% 0.13% 0.11% Throughput 95% 94% 95% Target Load = 125%: ECN ECN+ ECN+/wait ------- ------- ------- Dropped 1,599 1,011 985 Marked 74,567 75,782 75,528 Loss rate 0.87% 0.56% 0.54% Throughput 98% 98% 98% Target Load = 150%: ECN ECN+ ECN+/wait ------- ------- ------- Dropped 2,429 1,538 1,571 Marked 85,312 86,481 86,476 Loss rate 1.22% 0.78% 0.79% Throughput 98% 98% 98% Table 4: Simulations with an average flow size of 3 Kbytes, a 10 Mbps link, RED in byte mode, queue in bytes. B. Issues of Incremental Deployment In order for TCP node B to send a SYN/ACK packet as ECN-Capable, node B must have received an ECN-setup SYN packet from node A. However, it is possible that node A supports ECN, but either ignores the CE codepoint on received SYN/ACK packets, or ignores SYN/ACK packets with the ECT or CE codepoint set. If the TCP initiator ignores the CE codepoint on received SYN/ACK packets, this would mean that the TCP responder would not respond to this congestion indication. However, this seems to us an acceptable cost to pay in the incremental deployment of ECN-Capability for TCP's SYN/ACK packets. It would mean that the responder would not reduce the initial congestion window from two, three, or four segments down to one Floyd et al. Expires: 22 February 2009 [Page 23]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 segment, as it should. However, the TCP end nodes would still respond correctly to any subsequent CE indications on data packets later on in the connection. Figure 3 shows an interchange with the SYN/ACK packet ECN-marked, but with the ECN mark ignored by the TCP originator. --------------------------------------------------------------- TCP Node A Router TCP Node B ---------- ------ ---------- ECN-setup SYN packet ---> ECN-setup SYN packet ---> <--- ECN-setup SYN/ACK, ECT <--- Sets CE on SYN/ACK <--- ECN-setup SYN/ACK, CE Data/ACK, No ECN-Echo ---> Data/ACK ---> <--- Data (up to four packets) --------------------------------------------------------------- Figure 3: SYN exchange with the SYN/ACK packet marked, but with the ECN mark ignored by the TCP initiator. Thus, to be explicit, when a TCP connection includes an initiator that supports ECN but *does not* support ECN-Capability for SYN/ACK packets, in combination with a responder that *does* support ECN- Capability for SYN/ACK packets, it is possible that the ECN-Capable SYN/ACK packets will be marked rather than dropped in the network, and that the responder will not learn about the ECN mark on the SYN/ACK packet. This would not be a problem if most packets from the responder supporting ECN for SYN/ACK packets were in long-lived TCP connections, but it would be more problematic if most of the packets were from TCP connections consisting of four data packets, and the TCP responder for these connections was ready to send its data packets immediately after the SYN/ACK exchange. Of course, with *severe* congestion, the SYN/ACK packets would likely be dropped rather than ECN-marked at the congested router, preventing the TCP responder from adding to the congestion by sending its initial window of four data packets. It is also possible that in some older TCP implementation, the initiator would ignore arriving SYN/ACK packets that had the ECT or CE codepoint set. This would result in a delay in connection set-up for that TCP connection, with the initiator re-sending the SYN packet after a retransmit timeout. We are not aware of any TCP Floyd et al. Expires: 22 February 2009 [Page 24]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 implementations with this behavior. One possibility for coping with problems of backwards compatibility would be for TCP initiators to use a TCP flag that means "I understand ECN-Capable SYN/ACK packets". If this document were to standardize the use of such an "ECN-SYN" flag, then the TCP responder would only send a SYN/ACK packet as ECN-capable if the incoming SYN packet had the "ECN-SYN" flag set. An ECN-SYN flag would prevent the backwards compatibility problems described in the paragraphs above. One drawback to the use of an ECN-SYN flag is that it would use one of the four remaining reserved bits in the TCP header, for a transient backwards compatibility problem. This drawback is limited by the fact that the "ECN-SYN" flag would be defined only for use with ECN-setup SYN packets; that bit in the TCP header could be defined to have other uses for other kinds of TCP packets. Factors in deciding not to use an ECN-SYN flag include the following: (1) The limited installed base: At the time that this document was written, the TCP implementations in Microsoft Vista and Mac OS X included ECN, but ECN was not enabled by default [SBT07]. Thus, there was not a large deployed base of ECN-Capable TCP implementations. This limits the scope of any backwards compatibility problems. (2) Limits to the scope of the problem: The backwards compatibility problem would not be serious enough to cause congestion collapse; with severe congestion, the buffer at the congested router will overflow, and the congested router will drop rather than ECN-mark arriving SYN packets. Some active queue management mechanisms might switch from packet-marking to packet-dropping in times of high congestion before buffer overflow, as recommended in Section 19.1 of RFC 3168. This helps to prevent congestion collapse problems with the use of ECN. (3) Detection of and response to backwards-compatibility problems: A TCP responder such as a web server can't differentiate between a SYN/ACK packet that is not ECN-marked in the network, and a SYN/ACK packet that is ECN-marked, but where the ECN mark is ignored by the TCP initiator. However, a TCP responder *can* detect if a SYN/ACK packet is sent as ECN-capable and not reported as ECN-marked, but data packets are dropped or marked from the initial window of data. We will call this scenario "initial-window-congestion". If a web server frequently experienced initial-window congestion (without SYN/ACK congestion), then the web server *might* be experiencing backwards compatibility problems with ECN-Capable SYN/ACK packets, and could respond by not sending SYN/ACK packets as ECN-Capable. Floyd et al. Expires: 22 February 2009 [Page 25]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 Normative References [RFC 2119] S. Bradner, Key words for use in RFCs to Indicate Requirement Levels, RFC 2119, March 1997. [RFC3168] K.K. Ramakrishnan, S. Floyd, and D. Black, The Addition of Explicit Congestion Notification (ECN) to IP, RFC 3168, Proposed Standard, September 2001. Informative References [ECN+] A. Kuzmanovic, The Power of Explicit Congestion Notification, SIGCOMM 2005. [ECN-SYN] ECN-SYN web page with simulation scripts, URL "http://www.icir.org/floyd/ecn-syn". [F07] S. Floyd, "[BEHAVE] Response of firewalls and middleboxes to TCP SYN packets that are ECN-Capable?", August 2, 2007, email sent to the BEHAVE mailing list, URL "http://www1.ietf.org/mail- archive/web/behave/current/msg02644.html". [Kelson00] Dax Kelson, note sent to the Linux kernel mailing list, September 10, 2000. [MAF05] A. Medina, M. Allman, and S. Floyd. Measuring the Evolution of Transport Protocols in the Internet, ACM CCR, April 2005. [PI] C. Hollot, V. Misra, W. Gong, and D. Towsley, On Designing Improved Controllers for AQM Routers Supporting TCP Flows, April 1998. [RED] Floyd, S., and Jacobson, V. Random Early Detection gateways for Congestion Avoidance . IEEE/ACM Transactions on Networking, V.1 N.4, August 1993. [REM] S. Athuraliya, V. H. Li, S. H. Low and Q. Yin, REM: Active Queue Management, IEEE Network, May 2001. [RFC2309] B. Braden et al., Recommendations on Queue Management and Congestion Avoidance in the Internet, RFC 2309, April 1998. [RFC2581] M. Allman, V. Paxson, and W. Stevens, TCP Congestion Control, RFC 2581, April 1999. [RFC2988] V. Paxson and M. Allman, Computing TCP's Retransmission Timer, RFC 2988, November 2000. Floyd et al. Expires: 22 February 2009 [Page 26]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 [RFC3042] M. Allman, H. Balakrishnan, and S. Floyd, Enhancing TCP's Loss Recovery Using Limited Transmit, RFC 3042, Proposed Standard, January 2001. [RFC3360] S. Floyd, Inappropriate TCP Resets Considered Harmful, RFC 3360, August 2002. [RFC3390] M. Allman, S. Floyd, and C. Partridge, Increasing TCP's Initial Window, RFC 3390, October 2002. [RFC4987] W. Eddy, TCP SYN Flooding Attacks and Common Mitigations, RFC 4987, August 2007. [SCJO01] F. Smith, F. Campos, K. Jeffay, and D. Ott, What TCP/IP Protocol Headers Can Tell us about the Web, SIGMETRICS, June 2001. [SYN-COOK] Dan J. Bernstein, SYN cookies, 1997, see also <http://cr.yp.to/syncookies.html> [SBT07] M. Sridharan, D. Bansal, and D. Thaler, Implementation Report on Experiences with Various TCP RFCs, Presentation in the TSVAREA, IETF 68, March 2007. URL "http://www3.ietf.org/proceedings/07mar/slides/tsvarea-3/sld6.htm". [Tools] S. Floyd and E. Kohler, Tools for the Evaluation of Simulation and Testbed Scenarios, Internet-draft draft-irtf-tmrg- tools-05, work in progress, February 2008. IANA Considerations There are no IANA considerations regarding this document. Authors' Addresses Floyd et al. Expires: 22 February 2009 [Page 27]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 Aleksandar Kuzmanovic Phone: +1 (847) 467-5519 Northwestern University Email: akuzma at northwestern.edu URL: http://cs.northwestern.edu/~a Amit Mondal Northwestern University Email: a-mondal at northwestern.edu Sally Floyd Phone: +1 (510) 666-2989 ICIR (ICSI Center for Internet Research) Email: floyd@icir.org URL: http://www.icir.org/floyd/ K. K. Ramakrishnan Phone: +1 (973) 360-8764 AT&T Labs Research Email: kkrama at research.att.com URL: http://www.research.att.com/info/kkrama Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be Floyd et al. Expires: 22 February 2009 [Page 28]
INTERNET-DRAFT ECN and SYN/ACK Packets August 2008 found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. Floyd et al. Expires: 22 February 2009 [Page 29]
