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Internet Engineering Task Force (IETF)                         M. AllmanRequest for Comments: 5827                                          ICSICategory: Experimental                                    K. AvrachenkovISSN: 2070-1721                                                    INRIA                                                               U. Ayesta                                           BCAM-IKERBASQUE and LAAS-CNRS                                                              J. Blanton                                                         Ohio University                                                               P. Hurtig                                                     Karlstad University                                                              April 2010Early Retransmit for TCPand Stream Control Transmission Protocol (SCTP)Abstract   This document proposes a new mechanism for TCP and Stream Control   Transmission Protocol (SCTP) that can be used to recover lost   segments when a connection's congestion window is small.  The "Early   Retransmit" mechanism allows the transport to reduce, in certain   special circumstances, the number of duplicate acknowledgments   required to trigger a fast retransmission.  This allows the transport   to use fast retransmit to recover segment losses that would otherwise   require a lengthy retransmission timeout.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for examination, experimental implementation, and   evaluation.   This document defines an Experimental Protocol for the Internet   community.  This document is a product of the Internet Engineering   Task Force (IETF).  It represents the consensus of the IETF   community.  It has received public review and has been approved for   publication by the Internet Engineering Steering Group (IESG).  Not   all documents approved by the IESG are a candidate for any level of   Internet Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5827.Allman, et al.                Experimental                      [Page 1]

RFC 5827            Early Retransmit for TCP and SCTP         April 2010Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.1.  Introduction   Many researchers have studied the problems with TCP's loss recovery   [RFC793,RFC5681] when the congestion window is small, and they have   outlined possible mechanisms to mitigate these problems   [Mor97, BPS+98, Bal98, LK98,RFC3150, AA02].  SCTP's [RFC4960] loss   recovery and congestion control mechanisms are based on TCP, and   therefore the same problems impact the performance of SCTP   connections.  When the transport detects a missing segment, the   connection enters a loss recovery phase.  There are several variants   of the loss recovery phase depending on the TCP implementation.  TCP   can use slow-start-based recovery or fast recovery [RFC5681], NewReno   [RFC3782], and loss recovery, based on selective acknowledgments   (SACKs) [RFC2018, FF96,RFC3517].  SCTP's loss recovery is not as   varied due to the built-in selective acknowledgments.   All of the above variants have two methods for invoking loss   recovery.  First, if an acknowledgment (ACK) for a given segment is   not received in a certain amount of time, a retransmission timer   fires, and the segment is resent [RFC2988,RFC4960].  Second, the   "fast retransmit" algorithm resends a segment when three duplicateAllman, et al.                Experimental                      [Page 2]

RFC 5827            Early Retransmit for TCP and SCTP         April 2010   ACKs arrive at the sender [Jac88,RFC5681].  Duplicate ACKs are   triggered by out-of-order arrivals at the receiver.  However, because   duplicate ACKs from the receiver are triggered by both segment loss   and segment reordering in the network path, the sender waits for   three duplicate ACKs in an attempt to disambiguate segment loss from   segment reordering.  When the congestion window is small, it may not   be possible to generate the required number of duplicate ACKs to   trigger fast retransmit when a loss does happen.   Small congestion windows can occur in a number of situations, such   as:   (1) The connection is constrained by end-to-end congestion control       when the connection's share of the path is small, the path has a       small bandwidth-delay product, or the transport is ascertaining       the available bandwidth in the first few round-trip times of slow       start.   (2) The connection is "application limited" and has only a limited       amount of data to send.  This can happen any time the application       does not produce enough data to fill the congestion window.  A       particular case when all connections become application limited       is as the connection ends.   (3) The connection is limited by the receiver's advertised window.   The transport's retransmission timeout (RTO) is based on measured   round-trip times (RTT) between the sender and receiver, as specified   in [RFC2988] (for TCP) and [RFC4960] (for SCTP).  To prevent spurious   retransmissions of segments that are only delayed and not lost, the   minimum RTO is conservatively chosen to be 1 second.  Therefore, it   behooves TCP senders to detect and recover from as many losses as   possible without incurring a lengthy timeout during which the   connection remains idle.  However, if not enough duplicate ACKs   arrive from the receiver, the fast retransmit algorithm is never   triggered -- this situation occurs when the congestion window is   small, if a large number of segments in a window are lost, or at the   end of a transfer as data drains from the network.  For instance,   consider a congestion window of three segments' worth of data.  If   one segment is dropped by the network, then at most two duplicate   ACKs will arrive at the sender.  Since three duplicate ACKs are   required to trigger fast retransmit, a timeout will be required to   resend the dropped segment.  Note that delayed ACKs [RFC5681] may   further reduce the number of duplicate ACKs a receiver sends.   However, we assume that receivers send immediate ACKs when there is a   gap in the received sequence space per [RFC5681].Allman, et al.                Experimental                      [Page 3]

RFC 5827            Early Retransmit for TCP and SCTP         April 2010   [BPS+98] shows that roughly 56% of retransmissions sent by a busy Web   server are sent after the RTO timer expires, while only 44% are   handled by fast retransmit.  In addition, only 4% of the RTO timer-   based retransmissions could have been avoided with SACK, which has to   continue to disambiguate reordering from genuine loss.  Furthermore,   [All00] shows that for one particular Web server, the median number   of bytes carried by a connection is less than four segments,   indicating that more than half of the connections will be forced to   rely on the RTO timer to recover from any losses that occur.  Thus,   loss recovery that does not rely on the conservative RTO is likely to   be beneficial for short TCP transfers.   The limited transmit mechanism introduced in [RFC3042] and currently   codified in [RFC5681] allows a TCP sender to transmit previously   unsent data upon receipt of each of the two duplicate ACKs that   precede a fast retransmit.  SCTP [RFC4960] uses SACK information to   calculate the number of outstanding segments in the network.  Hence,   when the first two duplicate ACKs arrive at the sender, they will   indicate that data has left the network, and they will allow the   sender to transmit new data (if available), similar to TCP's limited   transmit algorithm.  In the remainder of this document, we use   "limited transmit" to include both TCP and SCTP mechanisms for   sending in response to the first two duplicate ACKs.  By sending   these two new segments, the sender is attempting to induce additional   duplicate ACKs (if appropriate), so that fast retransmit will be   triggered before the retransmission timeout expires.  The sender-side   "Early Retransmit" mechanism outlined in this document covers the   case when previously unsent data is not available for transmission   (case (2) above) or cannot be transmitted due to an advertised window   limitation (case (3) above).   Note: This document is being published as an experimental RFC, as   part of the process for the TCPM working group and the IETF to assess   whether the proposed change is useful and safe in the heterogeneous   environments, including which variants of the mechanism are the most   effective.  In the future, this specification may be updated and put   on the standards track if its safeness and efficacy can be   demonstrated.2.  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 inRFC 2119 [RFC2119].   The reader is expected to be familiar with the definitions given in   [RFC5681].Allman, et al.                Experimental                      [Page 4]

RFC 5827            Early Retransmit for TCP and SCTP         April 20103.  Early Retransmit Algorithm   The Early Retransmit algorithm calls for lowering the threshold for   triggering fast retransmit when the amount of outstanding data is   small and when no previously unsent data can be transmitted (such   that limited transmit could be used).  Duplicate ACKs are triggered   by each arriving out-of-order segment.  Therefore, fast retransmit   will not be invoked when there are less than four outstanding   segments (assuming only one segment loss in the window).  However,   TCP and SCTP are not required to track the number of outstanding   segments, but rather the number of outstanding bytes or messages.   (Note that SCTP's message boundaries do not necessarily correspond to   segment boundaries.)  Therefore, applying the intuitive notion of a   transport with less than four segments outstanding is more   complicated than it first appears.  InSection 3.1, we describe a   "byte-based" variant of Early Retransmit that attempts to roughly map   the number of outstanding bytes to a number of outstanding segments   that is then used when deciding whether to trigger Early Retransmit.   InSection 3.2, we describe a "segment-based" variant that represents   a more precise algorithm for triggering Early Retransmit.  This   precision comes at the cost of requiring additional state to be kept   by the TCP sender.  In both cases, we describe SACK-based and non-   SACK-based versions of the scheme (of course, the non-SACK version   will not apply to SCTP).  This document explicitly does not prefer   one variant over the other, but leaves the choice to the implementer.3.1.  Byte-Based Early Retransmit   A TCP or SCTP sender MAY use byte-based Early Retransmit.   Upon the arrival of an ACK, a sender employing byte-based Early   Retransmit MUST use the following two conditions to determine when an   Early Retransmit is sent:   (2.a) The amount of outstanding data (ownd) -- data sent but not yet         acknowledged -- is less than 4*SMSS bytes (as defined in         [RFC5681]).         Note that in the byte-based variant of Early Retransmit, "ownd"         is equivalent to "FlightSize" (defined in [RFC5681]).  We use         different notation, because "ownd" is not consistent with         FlightSize throughout this document.         Also note that in SCTP, messages will have to be converted to         bytes to make this variant of Early Retransmit work.Allman, et al.                Experimental                      [Page 5]

RFC 5827            Early Retransmit for TCP and SCTP         April 2010   (2.b) There is either no unsent data ready for transmission at the         sender, or the advertised receive window does not permit new         segments to be transmitted.   When the above two conditions hold and a TCP connection does not   support SACK, the duplicate ACK threshold used to trigger a   retransmission MUST be reduced to:                ER_thresh = ceiling (ownd/SMSS) - 1                 (1)   duplicate ACKs, where ownd is expressed in terms of bytes.  We call   this reduced ACK threshold enabling "Early Retransmission".   When conditions (2.a) and (2.b) hold and a TCP connection does   support SACK or SCTP is in use, Early Retransmit MUST be used only   when "ownd - SMSS" bytes have been SACKed.   If either (or both) condition (2.a) and/or (2.b) does not hold, the   transport MUST NOT use Early Retransmit, but rather prefer the   standard mechanisms, including fast retransmit and limited transmit.   As noted above, the drawback of this byte-based variant is precision   [HB08].  We illustrate this with two examples:      + Consider a non-SACK TCP sender that uses an SMSS of 1460 bytes        and transmits three segments, each with 400 bytes of payload.        This is a case where Early Retransmit could aid loss recovery if        one segment is lost.  However, in this case, ER_thresh will        become zero, per Equation (1), because the number of outstanding        bytes is a poor estimate of the number of outstanding segments.        A similar problem occurs for senders that employ SACK, as the        expression "ownd - SMSS" will become negative.      + Next, consider a non-SACK TCP sender that uses an SMSS of        1460 bytes and transmits 10 segments, each with 400 bytes of        payload.  In this case, ER_thresh will be 2 per Equation (1).        Thus, even though there are enough segments outstanding to        trigger fast retransmit with the standard duplicate ACK        threshold, Early Retransmit will be triggered.  This could cause        or exacerbate performance problems caused by segment reordering        in the network.Allman, et al.                Experimental                      [Page 6]

RFC 5827            Early Retransmit for TCP and SCTP         April 20103.2.  Segment-Based Early Retransmit   A TCP or SCTP sender MAY use segment-based Early Retransmit.   Upon the arrival of an ACK, a sender employing segment-based Early   Retransmit MUST use the following two conditions to determine when an   Early Retransmit is sent:   (3.a) The number of outstanding segments (oseg) -- segments sent but         not yet acknowledged -- is less than four.   (3.b) There is either no unsent data ready for transmission at the         sender, or the advertised receive window does not permit new         segments to be transmitted.   When the above two conditions hold and a TCP connection does not   support SACK, the duplicate ACK threshold used to trigger a   retransmission MUST be reduced to:                  ER_thresh = oseg - 1                              (2)   duplicate ACKs, where oseg represents the number of outstanding   segments.  (We discuss tracking the number of outstanding segments   below.)  We call this reduced ACK threshold enabling "Early   Retransmission".   When conditions (3.a) and (3.b) hold and a TCP connection does   support SACK or SCTP is in use, Early Retransmit MUST be used only   when "oseg - 1" segments have been SACKed.  A segment is considered   to be SACKed when all of its data bytes (TCP) or data chunks (SCTP)   have been indicated as arrived by the receiver.   If either (or both) condition (3.a) and/or (3.b) does not hold, the   transport MUST NOT use Early Retransmit, but rather prefer the   standard mechanisms, including fast retransmit and limited transmit.   This version of Early Retransmit solves the precision issues   discussed in the previous section.  As noted previously, the cost is   that the implementation will have to track segment boundaries to form   an understanding as to how many actual segments have been   transmitted, but not acknowledged.  This can be done by the sender   tracking the boundaries of the three segments on the right side of   the current window (which involves tracking four sequence numbers in   TCP).  This could be done by keeping a circular list of the segment   boundaries, for instance.  Cumulative ACKs that do not fall within   this region indicate that at least four segments are outstanding, and   therefore Early Retransmit MUST NOT be used.  When the outstanding   window becomes small enough that Early Retransmit can be invoked, aAllman, et al.                Experimental                      [Page 7]

RFC 5827            Early Retransmit for TCP and SCTP         April 2010   full understanding of the number of outstanding segments will be   available from the four sequence numbers retained.  (Note: the   implicit sequence number consumed by the TCP FIN bit can also be   included in the tracking of segment boundaries.)4.  Discussion   In this section, we discuss a number of issues surrounding the Early   Retransmit algorithm.4.1.  SACK vs. Non-SACK   The SACK variant of the Early Retransmit algorithm is preferred to   the non-SACK variant in TCP due to its robustness in the face of ACK   loss (since SACKs are sent redundantly), and due to interactions with   the delayed ACK timer (SCTP does not have a non-SACK mode and   therefore naturally supports SACK-based Early Retransmit).  Consider   a flight of three segments, S1...S3, with S2 being dropped by the   network.  When S1 arrives, it is in order, and so the receiver may or   may not delay the ACK, leading to two scenarios:   (A) The ACK for S1 is delayed: In this case, the arrival of S3 will       trigger an ACK to be transmitted, covering S1 (which was       previously unacknowledged).  In this case, Early Retransmit       without SACK will not prevent an RTO because no duplicate ACKs       will arrive.  However, with SACK, the ACK for S1 will also       include SACK information indicating that S3 has arrived at the       receiver.  The sender can then invoke Early Retransmit on this       ACK because only one segment remains outstanding.   (B) The ACK for S1 is not delayed: In this case, the arrival of S1       triggers an ACK of previously unacknowledged data.  The arrival       of S3 triggers a duplicate ACK (because it is out of order).       Both ACKs will cover the same segment (S1).  Therefore,       regardless of whether SACK is used, Early Retransmit can be       performed by the sender (assuming no ACK loss).4.2.  Segment Reordering   Early Retransmit is less robust in the face of reordered segments   than when using the standard fast retransmit threshold.  Research   shows that a general reduction in the number of duplicate ACKs   required to trigger fast retransmit to two (rather than three) leads   to a reduction in the ratio of good to bad retransmits by a factor of   three [Pax97].  However, this analysis did not include the additional   conditioning on the event that the ownd was smaller than four   segments and that no new data was available for transmission.Allman, et al.                Experimental                      [Page 8]

RFC 5827            Early Retransmit for TCP and SCTP         April 2010   A number of studies have shown that network reordering is not a rare   event across some network paths.  Various measurement studies have   shown that reordering along most paths is negligible, but along   certain paths can be quite prevalent [Pax97,BPS99,BS02,Pir05].   Evaluating Early Retransmit in the face of real segment reordering is   part of the experiment we hope to instigate with this document.4.3.  Worst Case   Next, we note two "worst case" scenarios for Early Retransmit:   (1) Persistent reordering of segments coupled with an application       that does not constantly send data can result in large numbers of       needless retransmissions when using Early Retransmit.  For       instance, consider an application that sends data two segments at       a time, followed by an idle period when no data is queued for       delivery.  If the network consistently reorders the two segments,       the sender will needlessly retransmit one out of every two unique       segments transmitted when using the above algorithm (meaning that       one-third of all segments sent are needless retransmissions).       However, this would only be a problem for long-lived connections       from applications that transmit in spurts.   (2) Similar to the above, consider the case of that consist of two       segment each and always experience reordering.  Just as in (1)       above, one out of every two unique data segments will be       retransmitted needlessly; therefore, one-third of the traffic       will be spurious.   Currently, this document offers no suggestion on how to mitigate the   above problems.  However, the worst cases are likely pathological.   Part of the experiments that this document hopes to trigger would   involve better understanding of whether such theoretical worst-case   scenarios are prevalent in the network, and in general, to explore   the trade-off between spurious fast retransmits and the delay imposed   by the RTO.Appendix A does offer a survey of possible mitigations   that call for curtailing the use of Early Retransmit when it is   making poor retransmission decisions.5.  Related Work   There are a number of similar proposals in the literature that   attempt to mitigate the same problem that Early Retransmit addresses.   Deployment of Explicit Congestion Notification (ECN) [Flo94,RFC3168]   may benefit connections with small congestion window sizes [RFC2884].   ECN provides a method for indicating congestion to the end-host   without dropping segments.  While some segment drops may still occur,Allman, et al.                Experimental                      [Page 9]

RFC 5827            Early Retransmit for TCP and SCTP         April 2010   ECN may allow a transport to perform better with small congestion   window sizes because the sender will be required to detect less   segment loss [RFC2884].   [Bal98] outlines another solution to the problem of having no new   segments to transmit into the network when the first two duplicate   ACKs arrive.  In response to these duplicate ACKs, a TCP sender   transmits zero-byte segments to induce additional duplicate ACKs.   This method preserves the robustness of the standard fast retransmit   algorithm at the cost of injecting segments into the network that do   not deliver any data, and therefore are potentially wasting network   resources (at a time when there is a reasonable chance that the   resources are scarce).   [RFC4653] also defines an orthogonal method for altering the   duplicate ACK threshold.  The mechanisms proposed in this document   decrease the duplicate ACK threshold when a small amount of data is   outstanding.  Meanwhile, the mechanisms in [RFC4653] increase the   duplicate ACK threshold (over the standard of 3) when the congestion   window is large in an effort to increase robustness to segment   reordering.6.  Security Considerations   The security considerations found in [RFC5681] apply to this   document.  No additional security problems have been identified with   Early Retransmit at this time.7.  Acknowledgments   We thank Sally Floyd for her feedback in discussions about Early   Retransmit.  The notion of Early Retransmit was originally sketched   in an Internet-Draft co-authored by Sally Floyd and Hari   Balakrishnan.  Armando Caro, Joe Touch, Alexander Zimmermann, and   many members of the TSVWG and TCPM working groups provided good   discussions that helped shape this document.  Our thanks to all!8.  References8.1.  Normative References   [RFC793]    Postel, J., "Transmission Control Protocol", STD 7,RFC 793, September 1981.   [RFC2018]   Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP               Selective Acknowledgment Options",RFC 2018,               October 1996.Allman, et al.                Experimental                     [Page 10]

RFC 5827            Early Retransmit for TCP and SCTP         April 2010   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2883]   Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An               Extension to the Selective Acknowledgement (SACK) Option               for TCP",RFC 2883, July 2000.   [RFC2988]   Paxson, V. and M. Allman, "Computing TCP's Retransmission               Timer",RFC 2988, November 2000.   [RFC3042]   Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing               TCP's Loss Recovery Using Limited Transmit",RFC 3042,               January 2001.   [RFC4960]   Stewart, R., Ed., "Stream Control Transmission Protocol",RFC 4960, September 2007.   [RFC5681]   Allman, M., Paxson, V., and E. Blanton, "TCP Congestion               Control",RFC 5681, September 2009.8.2.  Informative References   [AA02]      Urtzi Ayesta, Konstantin Avrachenkov, "The Effect of the               Initial Window Size and Limited Transmit Algorithm on the               Transient Behavior of TCP Transfers", In Proc. of the               15th ITC Internet Specialist Seminar, Wurzburg,               July 2002.   [All00]     Mark Allman.  A Web Server's View of the Transport Layer.               ACM Computer Communication Review, October 2000.   [Bal98]     Hari Balakrishnan.  Challenges to Reliable Data Transport               over Heterogeneous Wireless Networks.  Ph.D. Thesis,               University of California at Berkeley, August 1998.   [BPS+98]    Hari Balakrishnan, Venkata Padmanabhan,               Srinivasan Seshan, Mark Stemm, and Randy Katz.  TCP               Behavior of a Busy Web Server: Analysis and Improvements.               Proc. IEEE INFOCOM Conf., San Francisco, CA, March 1998.   [BPS99]     Jon Bennett, Craig Partridge, Nicholas Shectman.  Packet               Reordering is Not Pathological Network Behavior.               IEEE/ACM Transactions on Networking, December 1999.   [BS02]      John Bellardo, Stefan Savage.  Measuring Packet               Reordering, ACM/USENIX Internet Measurement Workshop,               November 2002.Allman, et al.                Experimental                     [Page 11]

RFC 5827            Early Retransmit for TCP and SCTP         April 2010   [FF96]      Kevin Fall, Sally Floyd.  Simulation-based Comparisons of               Tahoe, Reno, and SACK TCP.  ACM Computer Communication               Review, July 1996.   [Flo94]     Sally Floyd.  TCP and Explicit Congestion Notification.               ACM Computer Communication Review, October 1994.   [HB08]      Per Hurtig, Anna Brunstrom.  Enhancing SCTP Loss               Recovery: An Experimental Evaluation of Early Retransmit.               Elsevier Computer Communications, Vol. 31(16),               October 2008, pp. 3778-3788.   [Jac88]     Van Jacobson.  Congestion Avoidance and Control.  ACM               SIGCOMM 1988.   [LK98]      Dong Lin, H.T. Kung.  TCP Fast Recovery Strategies:               Analysis and Improvements.  Proc. IEEE INFOCOM Conf.,               San Francisco, CA, March 1998.   [Mor97]     Robert Morris.  TCP Behavior with Many Flows.  Proc.               Fifth IEEE International Conference on Network Protocols,               October 1997.   [Pax97]     Vern Paxson.  End-to-End Internet Packet Dynamics.  ACM               SIGCOMM, September 1997.   [Pir05]     N. M. Piratla, "A Theoretical Foundation, Metrics and               Modeling of Packet Reordering and Methodology of Delay               Modeling using Inter-packet Gaps," Ph.D. Dissertation,               Department of Electrical and Computer Engineering,               Colorado State University, Fort Collins, CO, Fall 2005.   [RFC2884]   Hadi Salim, J. and U. Ahmed, "Performance Evaluation of               Explicit Congestion Notification (ECN) in IP Networks",RFC 2884, July 2000.   [RFC3150]   Dawkins, S., Montenegro, G., Kojo, M., and V. Magret,               "End-to-end Performance Implications of Slow Links",BCP 48,RFC 3150, July 2001.   [RFC3168]   Ramakrishnan, K., Floyd, S., and D. Black, "The Addition               of Explicit Congestion Notification (ECN) to IP",RFC 3168, September 2001.   [RFC3517]   Blanton, E., Allman, M., Fall, K., and L. Wang, "A               Conservative Selective Acknowledgment (SACK)-based Loss               Recovery Algorithm for TCP",RFC 3517, April 2003.Allman, et al.                Experimental                     [Page 12]

RFC 5827            Early Retransmit for TCP and SCTP         April 2010   [RFC3522]   Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm               for TCP",RFC 3522, April 2003.   [RFC3782]   Floyd, S., Henderson, T., and A. Gurtov, "The NewReno               Modification to TCP's Fast Recovery Algorithm",RFC 3782,               April 2004.   [RFC4653]   Bhandarkar, S., Reddy, A., Allman, M., and E. Blanton,               "Improving the Robustness of TCP to Non-Congestion               Events",RFC 4653, August 2006.Allman, et al.                Experimental                     [Page 13]

RFC 5827            Early Retransmit for TCP and SCTP         April 2010Appendix A.  Research Issues in Adjusting the Duplicate ACK Threshold   Decreasing the number of duplicate ACKs required to trigger fast   retransmit, as suggested inSection 3, has the drawback of making   fast retransmit less robust in the face of minor network reordering.   Two egregious examples of problems caused by reordering are given inSection 4.  This appendix outlines several schemes that have been   suggested to mitigate the problems caused by Early Retransmit in the   face of segment reordering.  These methods need further research   before they are suggested for general use (and current consensus is   that the cases that make Early Retransmit unnecessarily retransmit a   large amount of data are pathological, and therefore, these   mitigations are not generally required).   MITIGATION A.1: Allow a connection to use Early Retransmit as long as      the algorithm is not injecting "too much" spurious data into the      network.  For instance, using the information provided by TCP's      D-SACK option [RFC2883] or SCTP's Duplicate Transmission Sequence      Number (Duplicate-TSN) notification, a sender can determine when      segments sent via Early Retransmit are needless.  Likewise, using      Eifel [RFC3522], the sender can detect spurious Early Retransmits.      Once spurious Early Retransmits are detected, the sender can      either eliminate the use of Early Retransmit, or limit the use of      the algorithm to ensure that an acceptably small fraction of the      connection's transmissions are not spurious.  For example, a      connection could stop using Early Retransmit after the first      spurious retransmit is detected.   MITIGATION A.2: If a sender cannot reliably determine whether an      Early-Retransmitted segment is spurious or not, the sender could      simply limit Early Retransmits, either to some fixed number per      connection (e.g., Early Retransmit is allowed only once per      connection), or to some small percentage of the total traffic      being transmitted.   MITIGATION A.3: Allow a connection to trigger Early Retransmit using      the criteria given inSection 3, in addition to a "small" timeout      [Pax97].  For instance, a sender may have to wait for two      duplicate ACKs and then T msec before Early Retransmit is invoked.      The added time gives reordered acknowledgments time to arrive at      the sender and avoid a needless retransmit.  Designing a method      for choosing an appropriate timeout is part of the research that      would need to be involved in this scheme.Allman, et al.                Experimental                     [Page 14]

RFC 5827            Early Retransmit for TCP and SCTP         April 2010Authors' Addresses   Mark Allman   International Computer Science Institute   1947 Center Street, Suite 600   Berkeley, CA 94704-1198   USA   Phone: 440-235-1792   EMail: mallman@icir.orghttp://www.icir.org/mallman/   Konstantin Avrachenkov   INRIA   2004 route des Lucioles, B.P.93   06902, Sophia Antipolis   France   Phone: 00 33 492 38 7751   EMail: k.avrachenkov@sophia.inria.frhttp://www-sop.inria.fr/members/Konstantin.Avratchenkov/me.html   Urtzi Ayesta   BCAM-IKERBASQUE                         LAAS-CNRS   Bizkaia Technology Park, Building 500   7 Avenue Colonel Roche   48160 Derio                             31077, Toulouse   Spain                                   France                                           EMail: urtzi@laas.frhttp://www.laas.fr/~urtzi   Josh Blanton   Ohio University   301 Stocker Center   Athens, OH  45701   USA   EMail: jblanton@irg.cs.ohiou.edu   Per Hurtig   Karlstad University   Department of Computer Science   Universitetsgatan 2 651 88   Karlstad   Sweden   EMail: per.hurtig@kau.seAllman, et al.                Experimental                     [Page 15]