Internet Engineering Task Force Sumitha Bhandarkar INTERNET DRAFT A. L. Narasimha Reddy draft-ietf-tcpm-tcp-dcr-03.txt Texas A&M University Expires : August 2005 Mark Allman ICIR Ethan Blanton Purdue University February 2005 Improving the Robustness of TCP to Non-Congestion Events Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. 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 become aware will be disclosed, in accordance with RFC 3668. 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 in August 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract: This document specifies Non-Congestion Robustness (NCR) for TCP. In the absence of explicit congestion notification from the network, TCP's loss recovery algorithms treat the receipt of three duplicate Bhandarkar/et. al. Expires August 2005 [Page 1] draft-ietf-tcpm-tcp-dcr-03 February 2005 acknowledgments as an implicit indication of congestion in the network. This is not always correct, notably in the case when network paths reorder segments (for whatever reason), resulting in degraded performance. TCP-NCR is designed to mitigate this degraded performance by increasing the number of duplicate acknowledgments required to trigger loss recovery, based on the current state of the connection, in an effort to disambiguate true segment loss from segment reordering. In addition, we specify a change to TCP's congestion reaction decision point, as well (but, do not require such a change to use NCR). This document specifies the changes to TCP, as well as the costs and benefits of these modifications. 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 [RFC2119]. Readers should be familiar with the TCP terminology given in [RFC2581] and [RFC3517]. 1. Introduction One strength of TCP [RFC793] lies in its ability to adjust its sending rate according to the perceived congestion in the network [Jac88,RFC2581]. In the absence of explicit notification of congestion from the network, TCP uses segment loss as an indication of congestion (i.e., assuming queue overflow). TCP receivers send cumulative acknowledgments (ACKs) indicating the next sequence number expected from the sender for arriving segments [RFC793]. When segments arrive out of order duplicate ACKs are generated. As specified in [RFC2581], a TCP sender uses the arrival of three duplicate ACKs as an indication of segment loss. The TCP sender retransmits the lost segment and reduces the load imposed on the network, assuming the segment loss was caused by resource contention within the network path. The TCP sender does not assume loss on the first duplicate ACK, but waits for three dupacks to account for mild reordering. However, the use of this constant number of duplicate ACKs has a number of implications that can be mitigated if the duplicate ACK requirement is changed. The following is an example of TCP's behavior: + TCP A is the data sender and TCP B is the data receiver. + TCP A sends 10 segments each consisting of a single data byte (i.e., transmits bytes 1-10 in segments 1-10). Bhandarkar/et. al. Expires August 2005 [Page 2] draft-ietf-tcpm-tcp-dcr-03 February 2005 + Assume segment 3 is dropped in the network. + TCP B cumulatively acknowledges segments 1 and 2, making the cumulative ACK transmitted to the sender 3 (the next expected sequence number). (Note: TCP B may generate one or two ACKs, depending on whether delayed ACKs [RFC1122,RFC2581] are employed.) + The arrival of segments 4-10 at TCP B will each trigger the transmission of a cumulative ACK for sequence number 3. (Note: [RFC2581] recommends that delayed ACKs not be used when the ACK is triggered by an out-of-order segment.) + When TCP A receives the third duplicate ACK (or fourth ACK overall) for sequence number 3, TCP A will retransmit segment 3 and reduce the sending rate by roughly half (see [RFC2581] for specifics on the congestion control state adjustments). Alternatively, suppose segment 3 was not dropped by the network, but rather delayed such that segment 3 arrives after segment 10. The above scenario will play out in precisely the same manner. In other words, TCP is not capable of disambiguating that level of packet reordering from loss. The following is the specific motivation behind making TCP robust to reordered segments: * A number of Internet measurement studies have shown that packet reordering is not a rare phenomenon [Pax97,BPS99,JIDKT03,GPL04]. Further, the reordering can be well beyond that which fast retransmit can cope with using the arrival of three duplicate ACKs to disambiguate loss and reordering. * [BA02,ZKFP03] show the negative performance implications that packet reordering has on current TCP. * The requirement imposed by TCP for almost in-order packet delivery places a severe constraint on the design of future technology. Novel routing algorithms, network components, link-layer retransmission mechanisms and applications could all be looked at with a fresh perspective if TCP were to be more robust to segment reordering. For instance, high speed packet switches could cause resequencing of packets if TCP were more robust. There has been work proposed in the literature explicitly to ensure that packet ordering is maintained in such switches [KM02]. Also, link-layer mechanisms that attempt to recover from packet corruption by retransmitting could be Bhandarkar/et. al. Expires August 2005 [Page 3] draft-ietf-tcpm-tcp-dcr-03 February 2005 allowed to reorder packets and, hence, increase the chances of local loss repair rather than relying on TCP to repair the loss (and, needlessly reduce its sending rate). Other examples are multi-path routing, high-delay satellite links and some of the schemes proposed for differentiated services architecture. By making TCP more robust to non-congestion events, TCP-NCR may open the design space of the future Internet components. In this document we specify a set of sender modifications to provide Non-Congestion Robustness (NCR) to TCP. In particular, these changes are built on top of TCP with selective acknowledgments (SACKs) [RFC2018] and the SACK-based loss recovery scheme given in [RFC3517], since SACK is widely deployed at this point ([MAF04] indicates that 68% of web servers and 88% of web clients utilize SACK as of spring, 2004). The remainder of this document is organized as follows. In Section 2, we specify the TCP-NCR algorithm. Section 3 provides a brief overview of the benefits of TCP-NCR, while Section 4 discusses the drawbacks of TCP-NCR. Section 5 discusses related work. Section 6 discusses security concerns. 2. Algorithm The TCP-NCR modifications make two fundamental changes to the way [RFC3517] currently operates, as follows. First, the trigger for retransmitting a segment is changed from three duplicate ACKs [RFC2581,RFC3517] to a congestion window's worth of duplicate ACKs. This provides more time for packet reordering to "work itself out" before the TCP sender infers that a segment has been lost and needs retransmitted. Setting the retransmission point is a balancing act. On the one hand, if the trigger is too aggressive (as is sometimes the situation in current TCP stacks using three duplicate acknowledgments to trigger loss recovery), the TCP sender cannot accurately disambiguate loss from reordering. On the other hand, waiting too long to decide to use fast retransmit risks relying on the costly retransmission timeout (RTO) mechanism [RFC2988]. Using a congestion window's worth of duplicate ACKs provides a reasonable tradeoff because the delay involved (roughly one RTT) is strictly less than the RTO and there is enough data in the pipe to generate the number of duplicate ACKs required to trigger a retransmission (given the extended version of Limited Transmit [RFC3042] specified below). Second, TCP-NCR decouples initial congestion control decisions from retransmission decisions, in some cases delaying congestion control changes relative to TCP's current behavior defined in [RFC2581]. The Bhandarkar/et. al. Expires August 2005 [Page 4] draft-ietf-tcpm-tcp-dcr-03 February 2005 algorithm provides two alternatives for extending Limited Transmit. The two variants of extended Limited Transmit are: Careful Limited Transmit: This variant calls for reducing the sending rate at approximately the same time [RFC2581] implementations reduce the congestion window, while at the same time withholding a retransmission (and the final congestion determination) for approximately one RTT. Aggressive Limited Transmit: This variant calls for maintaining the sending rate in the face of duplicate ACKs until TCP concludes a segment is lost and needs to be retransmitted. (which, per the above, TCP-NCR delays by one RTT when compared with current loss recovery schemes). TCP-NCR implementation MUST use either Careful Limited Transmit or Aggressive Limited Transmit. A constant MUST be set depending on which variant of extended Limited Transmit is used, as follows: Careful Limited Transmit: LT_F = 2/3 Aggressive Limited Transmit: LT_F = 1/2 This constant reflects the fraction of outstanding data that must be ACKed before a retransmission is triggered. Since NCR's goal is to wait roughly one RTT to retransmit, the fraction reflects the different number of segments that will be transmitted during extended Limited Transmit by the two schemes (and therefore their aggressiveness). The TCP-NCR modifications specified in this document lend themselves to incremental deployment. Only the TCP implementation on the sender side requires modification. The changes themselves are modest. However, as will be discussed below, availability of additional buffer space at the receiver will help maximize the benefits of using TCP-NCR but are not strictly necessary. The following algorithms depend on the notions provided by [RFC3517] Bhandarkar/et. al. Expires August 2005 [Page 5] draft-ietf-tcpm-tcp-dcr-03 February 2005 and we assume the reader is familiar with the terminology given in [RFC3517]. The TCP-NCR algorithm can be adapted to alternate SACK- based loss recovery schemes. [BR04,BSRV04] outline non-SACK-based algorithms, however, we do not specify those algorithms in this document and do not recommend them due to both the complexity and security implications of having only a gross understanding of the number of outstanding segments in the network. A TCP connection using the Nagle algorithm [RFC896,RFC1122] MAY employ the TCP-NCR algorithm. If a TCP implementation does implement TCP-NCR the implementation MUST follow the various specifications provides in sections 2.1 - 2.4. If the Nagle algorithm is not being used there is no way to accurately calculate the number of outstanding segments in the network (and, therefore, no good way to derive an appropriate duplicate ACK threshold). A TCP connection that does not employ the Nagle algorithm MAY use TCP-NCR if the TCP implementation tracks the sequence numbers transmitted in each segment and the following algorithm is carefully adapted. 2.1. Initialization When entering a period of loss / reordering detection and Extended Limited Transmit a TCP-NCR MUST initialize several state variables. A TCP MUST enter Extended Limited Transmit upon receiving the first ACK with a SACK block after the reception of an ACK that (a) did not contain SACK information and (b) did increase the connection's cumulative ACK point. The initializations are: (I.1) Save the current FlightSize. FlightSizePrev = FlightSize (I.2) Set a variable for tracking the number of segments for which an ACK does not trigger a transmission during Careful Limited Transmit. Skipped = 0 (I.3) Set DupThresh (from [RFC3517]) based on the size of the current FlightSize. DupThresh = max (LT_F * (FlightSize / SMSS),3) Note: We keep the lower bound of DupThresh = 3 from [RFC2581,RFC3517]. In addition to the above steps, the incoming ACK MUST be processed with the E series of steps in section 2.3. Bhandarkar/et. al. Expires August 2005 [Page 6] draft-ietf-tcpm-tcp-dcr-03 February 2005 2.2. Terminating Extended Limited Transmit and Preventing Bursts Extended Limited Transmit MUST be terminated at the start of loss recovery as outlined in section 2.4. The arrival of an ACK that advances the cumulative ACK point before loss recovery is triggered signals that the series of duplicate ACKs were caused by reordering and not congestion. Therefore, the receipt of an ACK that extends the cumulative ACK point MUST terminate Extended Limited Transmit. As described below, an ACK that also contains SACK information will also trigger the beginning of a new Extended Limited Transmit phase. Upon the termination of Extended Limited Transmit, and especially when using the Careful variant, TCP- NCR may be in a situation where the entire cwnd is not being utilized and therefore TCP-NCR will be prone to transmitting a burst of segments into the network. Therefore, upon exiting Extended Limited Transmit the following steps MUST be taken. When a TCP-NCR in the Extended Limited Transmit phase receives an ACK that updates the cumulative ACK point (regardless of whether the ACK contains SACK information), the following steps MUST be taken: (T.1) cwnd = min (FlightSize + SMSS,FlightSizePrev) This step ensures that cwnd is not grossly larger than the amount of data outstanding --- a situation that would cause a line rate burst. (T.2) ssthresh = FlightSizePrev This step provides TCP-NCR with a sense of "history". If step (T.1) reduces cwnd below FlightSizePrev this step ensures that TCP-NCR will slow start back to operating point in effect before Extended Limited Transmit. (T.3) Transmit previously unsent data as allowed by cwnd, FlightSize, application data availability and the receiver's advertised window. (T.4) When the cumulative ACK also contains SACK information, the initializations in steps (I.2) and (I.3) from section 2.1 MUST be taken (but, not step (I.1)) to re-start Extended Limited Transmit. In addition, the series of steps in section 2.3 (the "E" steps) MUST be taken. 2.3. Extended Limited Transmit On each ACK containing SACK information that arrives after TCP-NCR Bhandarkar/et. al. Expires August 2005 [Page 7] draft-ietf-tcpm-tcp-dcr-03 February 2005 has entered the Extended Limited Transmit phase (as outlined in section 2.1) and before Extended Limited Transmit terminates, the sender MUST use the following procedure. (E.1) Use the SetPipe () procedure from [RFC3517] to set the "pipe" variable (which represents the number of bytes still considered "in the network"). (E.2) If the following comparison holds and there are SMSS bytes of previously unsent data available for transmission then transmit one segment of SMSS bytes. (pipe + Skipped) <= (FlightSizePrev - SMSS) If the comparison does not hold or no new data can be transmitted (due to lack of data from the application or the advertised window limit), skip to step (E.6). (E.3) Increment pipe by SMSS bytes. (E.4) If using Careful Limited Transmit, increment Skipped by SMSS bytes to ensure that the next SMSS bytes of SACKed data processed do not trigger a Limted Transmit transmission (since the goal of Careful Limited Transmit is to send upon the reception of every second duplicate ACK). (E.5) Return to step (E.2) to ensure that as many bytes as appropriate are transmitted. This provides robustness to ACK loss that can be (largely) compensated for using SACK information. (E.6) Reset DupThresh via: DupThresh = max (LT_F * (FlightSize / SMSS),3) where FlightSize is the total number of bytes that have not been cumulatively acknowledged. 2.4 Entering Loss Recovery When a segment is deemed lost via the algorithms in [RFC3517], Extended Limited Transmit MUST be terminated, leaving the algoritms in [RFC3517] to govern TCP's behavior. One slight change to [RFC3517] MUST be made, however. In section 5, step (2) of [RFC3517] MUST be changed to: (2) ssthresh = cwnd = (FlightSizePrev / 2) Bhandarkar/et. al. Expires August 2005 [Page 8] draft-ietf-tcpm-tcp-dcr-03 February 2005 This ensures that the congestion control modifications are made with respect to the amount of data in the network before FlightSize was increased by Extended Limited Transmit. 3. Advantages The major advantages of TCP-NCR are two-fold. As discussed in section 1, TCP-NCR will open up the design space for network applications and components that are currently constrained by TCP's lack of robustness to packet reordering. The second advantage is in terms of an increase in TCP performance. [BR04] presents ns-2 [NS-2] simulations of a pre-cursor to the TCP- NCR algorithm specified in this document, called TCP-DCR (Delayed Congestion Response). The paper shows that TCP-DCR aids performance in comparison to unmodified TCP in the presence of packet reordering. In addition, the extended version of [BR04] presents results based on emulations involving Linux (kernel 2.4.24). These results show that the performance of TCP-DCR is similar to Linux's native implementation that seeks to "undo" wrong decisions based on DSACK [RFC2883] feedback (similar to the schemes outlined in [ZKFP03]) when packets are reordered by less than one RTT. The advantages of using TCP-DCR over the DSACK-based scheme is that the DSACK-based scheme tries to estimate the exact amount of reordering in the network using fairly complex algorithms, whereas TCP-DCR achieves similar results with less complicated modifications. In addition, [BR04,BSRV04] illustrate the ability of TCP-DCR to allow for the improvement of other parts of the system. For example, these papers show that increasing TCP's robustness to packet reordering allows for a novel wireless ARQ mechanism to be added at the link- layer. The added robustness of the link-layer to channel errors, in turn, increases TCP performance by not requiring TCP to retransmit packets that were dropped due to corruption (and, hence, also prevents TCP from needlessly reducing the sending rate when retransmitting these segments). 4. Disadvantages While we note that all of the changes outlined above are implemented in the sender, the receiver also potentially has a part to play. In particular, TCP-NCR increases the receiver's buffering requirement by up to an extra cwnd -- in the case of the TCP sender using Aggressive Limited Transmit and actual loss occurring in the network. Therefore, to maximize the benefits from TCP-NCR receivers should advertise a large window to absorb the extra out-of-order traffic. In the case that the additonal buffer requirements are not met, the use of the above algorithm takes into account the reduced advertised Bhandarkar/et. al. Expires August 2005 [Page 9] draft-ietf-tcpm-tcp-dcr-03 February 2005 window, resulting in slighlty reduced robustness to reordering. (The worst case robustness of cwnd/2 still offers an improvement over existing [RFC2581] implementations.) In addition, using TCP-NCR could delay the delivery of data to the application by up to one RTT because the fast retransmission point is delayed by roughly one RTT in TCP-NCR. Applications that are sensitive to such delays should turn off the TCP-NCR option. Finally, the use of TCP-NCR makes the recovery from congestion events sluggish. While the simulation study presented in [BR04,BSRV04] shows that this does not have a significant impact further experimentation on the real Internet is required to verify that result. 5. Related Work Over the past few years, several solutions have been proposed to improve the performance of TCP in the face of segment reordering. These schemes generally fall into one of two categories (with some overlap): mechanisms that try to prevent spurious retransmits from happening and mechanisms that try to detect spurious retransmits and "undo" the needless congestion control state changes that have been taken. [BA02,ZKFP03] attempt to prevent segment reordering from triggering spurious retransmits by using various algorithms to approximate the duplicate ACK threshold required to disambiguate loss and reordering over the given network path. TCP-NCR similarly tries to prevent spurious retransmits. However, TCP-NCR takes a simplified approach compared to those in [BA02,ZKFP03] in that TCP-NCR simply delays retransmission by a fixed amount (in comparison to standard TCP), while the other schemes use relatively complex algorithms in an attempt to derive a more precise value for DupThresh that depends on the network conditions. While TCP-NCR offers simplicity the other schemes may offer more precision such that applications would not be forced to wait as long for their retransmissions. On the other hand, several schemes have been developed to detect and mitigate needless retransmissions after the fact. [RFC3522,RFC3708,BA02,LG04,SK04] present algorithms to detect spurious retransmits and mitigate the changes these events made to the congestion control state. TCP-NCR could be used in conjunction with these algorithms, with TCP-NCR attempting to prevent spurious retransmits and some other scheme kicking in if the prevention failed. In addition, we note that TCP-NCR is concentrated on preventing spurious fast retransmits and some of the above algorithms also attempt to detect and mitigate spurious timeout-based retransmits. Bhandarkar/et. al. Expires August 2005 [Page 10] draft-ietf-tcpm-tcp-dcr-03 February 2005 6. Security Considerations We do not believe there are security implications involved with TCP- NCR over and above those for general TCP congestion control [RFC2581]. In particular, the Extended Limited Transmit algorithms have been specifically designed to not be susceptible to the sorts of ACK splitting attacks TCP's general TCP congestion control is vulnerable to (as discussed in [RFC3465]. 8. Acknowledgements Sally Floyd, Nauzad Sadry and Nitin Vaidya as well as feedback from from the TCPM working group have contributed significantly to this document. Our thanks to all! 9. Normative References [RFC793] J. Postel, "Transmission Control Protocol", RFC 793, September 1981. [RFC2018] M. Mathis, J. Mahdavi, S. Floyd and A. Romanow, "TCP selective acknowledgment options," Internet RFC 2018. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2581] M. Allman, V. Paxson, and W. Stevens, "TCP Congestion Control", RFC 2581, April 1999. [RFC3042] M. Allman, H. Balakrishnan and S. Floyd, "Enhancing TCP's Loss Recovery Using Limited Transmit", RFC 3042, January 2001. [RFC3517] E. Blanton, M. Allman, K. Fall and L. Wang, "A Conservative Selective Acknowledgment (SACK)-based Loss Recovery Algorithm for TCP", RFC 3517, April 2003. 9. Informative References [BA02] E. Blanton and M. Allman, "On Making TCP More Robust to Packet Reordering," ACM Computer Communication Review, January 2002. [BPS99] J. Bennett, C. Partridge, and N. Shectman, "Packet reordering is not pat hological network behavior," IEEE/ACM Transactions on Networking, December 1999. [BR04] Sumitha Bhandarkar and A. L. Narasimha Reddy, "TCP-DCR: Making TCP Robust to Non-Congestion Events", In the Proceedings of Networking 2004 conference, May 2004. Extended version available as Bhandarkar/et. al. Expires August 2005 [Page 11] draft-ietf-tcpm-tcp-dcr-03 February 2005 tech report TAMU-ECE-2003-04. [BSRV04] Sumitha Bhandarkar, Nauzad Sadry, A. L. Narasimha Reddy and Nitin Vaidya, "TCP-DCR: A Novel Protocol for Tolerating Wireless Channel Errors", To appear in IEEE Transactions on Mobile Computing [GPL04] Ladan Gharai, Colin Perkins and Tom Lehman, "Packet Reordering, High Speed Networks and Transport Protocol Performance", ICCCN 2004, October 2004. [Jac88] V. Jacobson, "Congestion Avoidance and Control", Computer Communication Review, vol. 18, no. 4, pp. 314-329, Aug. 1988. ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z. [JIDKT03] S. Jaiswal, G. Iannaccone, C. Diot, J. Kurose, and D. Towsley, "Measurement and Classification of Out-of-Sequence Packets in a Tier-1 IP Backbone," Proceedings of IEEE INFOCOM, 2003. [KM02] I. Keslassy and N. McKeown, "Maintaining packet order in twostage switche s," Proceedings of the IEEE Infocom, June 2002 [LG04] R. Ludwig, A. Gurtov, "The Eifel Response Algorithm for TCP", Internet-Draft draft-ietf-tsvwg-tcp-eifel-response-06.txt (work in progress). September 2004. [MAF04] A. Medina, M. Allman, S. Floyd. Measuring Interactions Between Transport Protocols and Middleboxes. ACM SIGCOMM/USENIX Internet Measurement Conference, Taormina, Sicily, Italy, October 2004. [NS-2] ns-2 Network Simulator. http://www.isi.edu/nsnam/ [Pax97] V. Paxson, "End-to-End Internet Packet Dynamics," Proceedings of ACM SIGCOMM, September 1997. [RFC896] J. Nagle, "Congestion Control in IP/TCP Internetworks", RFC 896, January 1984. [RFC1122] R. Braden, "Requirements for Internet Hosts - Communication Layers", RFC 1122, October 1989. [RFC2883] Sally Floyd, Jamshid Mahdavi, Matt Mathis and Matt Podolsky, "An Extension to the Selective Acknowledgement (SACK) Option for TCP," RFC 2883, July 2000. [RFC2988] V. Paxson and M. Allman, "Computing TCP's Retransmission Timer", RFC 2988, November 2000. Bhandarkar/et. al. Expires August 2005 [Page 12] draft-ietf-tcpm-tcp-dcr-03 February 2005 [RFC3465] M. Allman. TCP Congestion Control with Appropriate Byte Counting (ABC), February 2003. RFC 3465. [RFC3522] R. Ludwig and M. Meyer, "The Eifel Detection Algorithm for TCP," RFC 3522, April 2003. [RFC3708] E. Blanton and M. Allman, "Using TCP Duplicate Selective Acknowledgement (DSACKs) and Stream Control Transmission Protocol (SCTP) Duplicate Transmission Sequence Numbers (TSNs) to Detect Spurious Retransmissions", RFC 3708, February 2004. [SK04] P. Sarolahti, M. Kojo, "Forward RTO-Recovery (F-RTO): An Algorithm for Detecting Spurious Retransmission Timeouts with TCP and SCTP", Internet-Draft draft-ietf-tcpm-frto-02.txt (work in progress). November 2004. [ZKFP03] M. Zhang, B. Karp, S. Floyd, L. Peterson, RR-TCP: A Reordering-Robust TCP with DSACK, in Proceedings of the Eleventh IEEE International Conference on Networking Protocols (ICNP 2003), Atlanta, GA, November, 2003. 13. Author's Addresses Sumitha Bhandarkar Dept. of Elec. Engg. 214 ZACH College Station, TX 77843-3128 Phone: (512) 468-8078 Email: sumitha@tamu.edu URL : http://students.cs.tamu.edu/sumitha/ A. L. Narasimha Reddy Professor Dept. of Elec. Engg. 315C WERC College Station, TX 77843-3128 Phone : (979) 845-7598 Email : reddy@ee.tamu.edu URL : http://ee.tamu.edu/~reddy/ Mark Allman ICSI Center for Internet Research 1947 Center Street, Suite 600 Berkeley, CA 94704-1198 Phone: (216) 243-7361 Email: mallman@icir.org URL: http://www.icir.org/mallman/ Bhandarkar/et. al. Expires August 2005 [Page 13] draft-ietf-tcpm-tcp-dcr-03 February 2005 Ethan Blanton Purdue University Computer Sciences 1398 Computer Science Building West Lafayette, IN 47907 EMail: eblanton@cs.purdue.edu Intellectual Property Statement 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 found in BCP 78 and BCP 79. 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Disclaimer of Validity 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 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. Copyright Statement Copyright (C) The Internet Society (2005). 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. Acknowledgment Bhandarkar/et. al. Expires August 2005 [Page 14] draft-ietf-tcpm-tcp-dcr-03 February 2005 Funding for the RFC Editor function is currently provided by the Internet Society. Bhandarkar/et. al. Expires August 2005 [Page 15]