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rfc:rfc5827

Internet Engineering Task Force (IETF) M. Allman Request for Comments: 5827 ICSI Category: Experimental K. Avrachenkov ISSN: 2070-1721 INRIA

                                                             U. Ayesta
                                         BCAM-IKERBASQUE and LAAS-CNRS
                                                            J. Blanton
                                                       Ohio University
                                                             P. Hurtig
                                                   Karlstad University
                                                            April 2010
                      Early Retransmit for TCP
          and 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; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5827.

Allman, et al. Experimental [Page 1] RFC 5827 Early Retransmit for TCP and SCTP April 2010

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 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 duplicate

Allman, 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 in RFC 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 2010

3. 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.  In Section 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.
 In Section 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 2010

3.2. 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, a

Allman, 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. References

8.1. 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 2010

Appendix A. Research Issues in Adjusting the Duplicate ACK Threshold

 Decreasing the number of duplicate ACKs required to trigger fast
 retransmit, as suggested in Section 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 in
 Section 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 in Section 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 2010

Authors' Addresses

 Mark Allman
 International Computer Science Institute
 1947 Center Street, Suite 600
 Berkeley, CA 94704-1198
 USA
 Phone: 440-235-1792
 EMail: mallman@icir.org
 http://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.fr
 http://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.fr
                                         http://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.se

Allman, et al. Experimental [Page 15]

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