Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


Network Working Group S. Bhandarkar Request for Comments: 4653 A. L. N. Reddy Category: Experimental Texas A&M University

                                                             M. Allman
                                                            E. Blanton
                                                     Purdue University
                                                           August 2006
      Improving the Robustness of TCP to Non-Congestion Events

Status of This Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).


 This document specifies Non-Congestion Robustness (NCR) for TCP.  In
 the absence of explicit congestion notification from the network, TCP
 uses loss as an indication of congestion.  One of the ways TCP
 detects loss is using the arrival of three duplicate acknowledgments.
 However, this heuristic 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 better disambiguate
 true segment loss from segment reordering.  This document specifies
 the changes to TCP, as well as the costs and benefits of these

Bhandarkar, et al. Experimental [Page 1] RFC 4653 Improving the Robustness of TCP August 2006

Table of Contents

 1. Introduction ....................................................2
    1.1. Terminology ................................................4
 2. NCR Description .................................................5
 3. Algorithm .......................................................6
    3.1. Initialization .............................................8
    3.2. Terminating Extended Limited Transmit and
         Preventing Bursts ..........................................9
    3.3. Extended Limited Transmit .................................10
    3.4. Entering Loss Recovery ....................................11
 4. Advantages .....................................................12
 5. Disadvantages ..................................................12
 6. Related Work ...................................................13
 7. Security Considerations ........................................14
 8. Acknowledgments ................................................14
 9. IANA Considerations ............................................14
 10. References ....................................................14
    10.1. Normative References .....................................14
    10.2. Informative References ...................................15

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 or second duplicate ACK, but waits for three duplicate ACKs to
 account for minor packet reordering.  However, the use of this
 constant threshold of duplicate ACKs has several problems that can be
 mitigated with a dynamic threshold.
 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. Experimental [Page 2] RFC 4653 Improving the Robustness of TCP August 2006

   + 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
   + 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
 Alternatively, suppose segment 3 was not dropped by the network, but
 rather delayed such that segment 3 arrives at TCP B after segment 10.
 The above scenario will play out in precisely the same manner
 insomuch as a retransmission of segment 3 will be triggered.  In
 other words, TCP is not capable of disambiguating this reordering
 event from a segment loss, resulting in an unnecessary retransmission
 and rate reduction.
 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 required
     for fast retransmit to be falsely triggered.
  • [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 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 (e.g.,
     [KM02]).  Also, link-layer mechanisms that attempt to recover

Bhandarkar, et al. Experimental [Page 3] RFC 4653 Improving the Robustness of TCP August 2006

     from packet corruption by retransmitting could be allowed to
     reorder packets, and thus increase the chances of local loss
     repair rather than rely on TCP to repair the loss (and,
     needlessly reduce its sending rate).  Additional examples include
     multi-path routing, high-delay satellite links, and some of the
     schemes proposed for a 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 TCP 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 ([MAF05]
 indicates that 68% of web servers and 88% of web clients utilize SACK
 as of spring 2004).
 Note that the TCP-NCR algorithm provided in this document could be
 easily adapted to SCTP [RFC2960] since SCTP uses congestion control
 algorithms similar to TCP's (and thus has the same reordering
 robustness issues).
 As noted in several places in the remainder of this document, we
 consider TCP-NCR experimental in that more experience with the
 techniques is required before TCP-NCR should be used on a large scale
 on the Internet.  We encourage implementation and experimentation
 with TCP-NCR in the hopes of gaining an understanding of its
 suitability for wide-scale deployment.
 The remainder of this document is organized as follows.  Section 2
 provides a high-level description of the TCP-NCR mechanisms.  In
 Section 3, we specify the TCP-NCR algorithm.  Section 4 provides a
 brief overview of the benefits of TCP-NCR, while Section 5 discusses
 the drawbacks.  Section 6 discusses related work.  Section 7
 discusses security concerns.

1.1. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in [RFC2119].
 Readers should be familiar with the TCP terminology (e.g.,
 FlightSize, Pipe) given in [RFC2581] and [RFC3517].

Bhandarkar, et al. Experimental [Page 4] RFC 4653 Improving the Robustness of TCP August 2006

2. NCR Description

 As discussed above, in the face of packet reordering, three duplicate
 ACKs may not be enough to disambiguate loss from reordering.  In this
 section we provide a non-normative sketch of TCP-NCR.  The detailed
 algorithms for implementing Non-Congestion Robustness for TCP are
 presented in the next section.
 The general idea behind TCP-NCR is to increase the threshold used to
 trigger a fast retransmission from the current fixed value of three
 duplicate ACKs [RFC2581] to approximately a congestion window of data
 having left the network (but not less than the currently standardized
 value of three duplicate ACKs).  Since cwnd represents the amount of
 data a TCP flow can transmit in one round-trip time (RTT), waiting to
 receive notice that cwnd bytes have left the network before deciding
 whether the root cause is loss or reordering imposes a delay of
 roughly one RTT on both the retransmission and the congestion control
 response.  The appropriate choice for a new value of the threshold is
 essentially a trade-off between making the best decision regarding
 the cause of the duplicate ACKs and responsiveness.  The choice to
 trigger a retransmission only after a cwnd's worth of data is known
 to have left the network represents roughly the largest amount of
 time a TCP can wait before the (often costly) retransmission timeout
 may be triggered.  Therefore, the algorithm described in this
 document attempts to make the best decision possible at the expense
 of timeliness.
 Simply increasing the threshold before retransmitting a segment can
 make TCP brittle to packet loss or ACK loss since such loss reduces
 the number of duplicate ACKs that will arrive at the sender from the
 receiver.  For instance, if the cwnd is 10 segments and one segment
 is lost, a duplicate ACK threshold of 10 will never be met because
 duplicate ACKs corresponding to at most 9 segments will arrive at the
 sender.  To offset the issue of loss, we extend TCP's Limited
 Transmit [RFC3042] scheme to allow for the sending of new data during
 the period when the TCP sender is disambiguating loss and reordering.
 This new data serves to increase the likelihood that enough duplicate
 ACKs arrive at the sender to trigger loss recovery if it is
 Note that TCP tightly couples reliability and congestion control:
 when a segment is declared lost, a retransmission is triggered, and a
 change to the sending rate is also made on the assumption that the
 drop is due to resource contention [RFC2581].  Therefore, simply by
 changing the retransmission trigger, the congestion control response
 is also changed.  However, we lack experience on the Internet as to
 whether delaying the point that a rate reduction takes place is

Bhandarkar, et al. Experimental [Page 5] RFC 4653 Improving the Robustness of TCP August 2006

 appropriate for wide-scale deployment.  Therefore, the Extended
 Limited Transmit mechanism proposed in this document offers two
 variants for experimentation.
 The first Extended Limited Transmit variant, Careful Limited
 Transmit, calls for the transmission of one previously unsent
 segment, in response to duplicate acknowledgments, for every two
 segments that are known to have left the network.  This effectively
 halves the sending rate, since normal TCP operation calls for the
 sending of one segment for every segment that has left the network.
 Further, the halving starts immediately and is not delayed until a
 retransmission is triggered.  In the case of packet reordering (i.e.,
 not segment loss), the congestion control state is restored to its
 previous state when reordering is determined.
 The second variant, Aggressive Limited Transmit, calls for
 transmitting one previously unsent data segment, in response to
 duplicate acknowledgments, for every segment known to have left the
 network.  With this variant, while waiting to disambiguate the loss
 from a reordering event, ACK-clocked transmission continues at
 roughly the same rate as before the event started.  Retransmission
 and the sending rate reduction happen per [RFC2581, RFC3517], albeit
 with the delayed threshold described above.  Although this approach
 delays legitimate rate reductions (possibly slightly and temporarily
 aggravating overall congestion on the network), the scheme has the
 advantage of not reducing the transmission rate in the face of
 segment reordering.
 Which of the two Extended Limited Transmit variants is best for use
 on the Internet is an open question.

3. 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 indications that a congestion
 window's worth of data has left the network.  Second, TCP-NCR
 decouples initial congestion control decisions from retransmission
 decisions, in some cases delaying congestion control changes relative
 to TCP's current behavior as defined in [RFC2581].  The algorithm
 provides two alternatives for extending Limited Transmit.  The two
 variants of extended Limited Transmit are:

Bhandarkar, et al. Experimental [Page 6] RFC 4653 Improving the Robustness of TCP August 2006

     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 that a segment is
      lost and needs to be retransmitted (which TCP-NCR delays by
      one RTT when compared with current loss recovery schemes).
 A 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 (including
 data sent during Extended Limited Transmit) that must be SACKed
 before a retransmission is triggered.  Since Aggressive Limited
 Transmit sends a new segment for every segment known to have left the
 network, a total of roughly cwnd segments will be sent during
 Aggressive Limited Transmit, and therefore ideally a total of roughly
 2*cwnd segments will be outstanding when a retransmission is
 triggered.  The duplicate ACK threshold is then set to LT_F = 1/2 of
 2*cwnd (or about 1 RTT worth of data).  The factor is different for
 Careful Limited Transmit because the sender only transmits one new
 segment for every two segments that are SACKed and therefore will
 ideally have a total of 1.5*cwnd segments outstanding when the
 retransmission is to be triggered.  Hence, the required threshold is
 LT_F=2/3 of 1.5*cwnd to delay the retransmission by roughly 1 RTT.
 There are situations whereby the sender cannot transmit new data
 during Extended Limited Transmit (e.g., lack of data from the
 application, receiver's advertised window limit).  These situations
 can lead to the problems discussed in the last section when a TCP

Bhandarkar, et al. Experimental [Page 7] RFC 4653 Improving the Robustness of TCP August 2006

 does not employ Extended Limited Transmit and is starved for ACKs.
 Therefore, TCP-NCR adapts the duplicate ACK threshold on each SACK
 arrival to be as robust as possible given the actual amount of data
 that has been transmitted, or roughly LT_F times the number of
 outstanding segments.
 The TCP-NCR modifications specified in this document lend themselves
 to incremental deployment.  Only the TCP implementation on the sender
 side requires modification (assuming both hosts support SACK).  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 is not strictly necessary.
 The following algorithms depend on the notions provided by [RFC3517],
 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
 provided in Sections 3.1 - 3.4.  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) without adding state
 to the TCP sender.  A TCP connection that does not employ the Nagle
 algorithm SHOULD NOT use TCP-NCR.  We envision that NCR could be
 adapted to an implementation that carefully tracks the sequence
 numbers transmitted in each segment.  However, we leave this as
 future work.

3.1. 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) The TCP MUST save the current FlightSize.
       FlightSizePrev = FlightSize

Bhandarkar, et al. Experimental [Page 8] RFC 4653 Improving the Robustness of TCP August 2006

 (I.2) The TCP MUST set a variable for tracking the number of
       segments for which an ACK does not trigger a transmission
       during Careful Limited Transmit.
       Skipped = 0
       (Note: Skipped is not used during Aggressive Limited
 (I.3) The TCP MUST set DupThresh (from [RFC3517]) based on 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 3.3.

3.2. Terminating Extended Limited Transmit and Preventing Bursts

 Extended Limited Transmit MUST be terminated at the start of loss
 recovery as outlined in Section 3.4.
 The arrival of an ACK that advances the cumulative ACK point while in
 Extended Limited Transmit, but before loss recovery is triggered,
 signals that a series of duplicate ACKs was 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 (in (T.4)), an ACK that extends the cumulative ACK
 point and *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, to mitigate this bursting 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:

Bhandarkar, et al. Experimental [Page 9] RFC 4653 Improving the Robustness of TCP August 2006

 (T.1) A TCP MUST reset cwnd to:
       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) A TCP MUST set ssthresh to:
       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 the operating point in effect
       before Extended Limited Transmit.
 (T.3) A TCP is now permitted to transmit previously unsent data as
       allowed by cwnd, FlightSize, application data availability, and
       the receiver's advertised window.
 (T.4) When an incoming ACK extends the cumulative ACK point and also
       contains SACK information, the initializations in steps (I.2)
       and (I.3) from Section 3.1 MUST be taken (but step (I.1) MUST
       NOT be executed) to re-start Extended Limited Transmit.  In
       addition, the series of steps in Section 3.3 (the "E" steps)
       MUST be taken.

3.3. Extended Limited Transmit

 On each ACK containing SACK information that arrives after TCP-NCR
 has entered the Extended Limited Transmit phase (as outlined in
 Section 3.1) and before Extended Limited Transmit terminates, the
 sender MUST use the following procedure.
 (E.1) The SetPipe () procedure from [RFC3517] MUST be used to set
       the "pipe" variable (which represents the number of bytes
       still considered "in the network").  Note: the current value
       of DupThresh MUST be used by SetPipe () to produce an accurate
       assessment of the amount of data still considered in the
 (E.2) If the comparison in equation (1), below, holds and there are
       SMSS bytes of previously unsent data available for
       transmission, then the sender MUST transmit one segment of SMSS
         (pipe + Skipped) <= (FlightSizePrev - SMSS)              (1)

Bhandarkar, et al. Experimental [Page 10] RFC 4653 Improving the Robustness of TCP August 2006

       If the comparison in equation (1) 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) Pipe MUST be incremented by SMSS bytes.
 (E.4) If using Careful Limited Transmit, Skipped MUST be incremented
       by SMSS bytes to ensure that the next SMSS bytes of SACKed data
       processed does not trigger a Limited Transmit transmission
       (since the goal of Careful Limited Transmit is to send upon
       receipt of every second duplicate ACK).
 (E.5) A TCP MUST return to step (E.2) to ensure that as many bytes
       as are appropriate are transmitted.  This provides robustness
       to ACK loss that can be (largely) compensated for using SACK
 (E.6) DupThresh MUST be reset via:
         DupThresh = max (LT_F * (FlightSize / SMSS),3)
       where FlightSize is the total number of bytes that have not
       been cumulatively acknowledged (which is different from

3.4. Entering Loss Recovery

 When a segment is deemed lost via the algorithms in [RFC3517],
 Extended Limited Transmit MUST be terminated, leaving the algorithms
 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)
 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.
 Note: Once the algorithm in [RFC3517] takes over from Extended
 Limited Transmit, the DupThresh value MUST be held constant until the
 loss recovery phase is terminated.

Bhandarkar, et al. Experimental [Page 11] RFC 4653 Improving the Robustness of TCP August 2006

4. Advantages

 The major advantages of TCP-NCR are twofold.  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 according to
 duplicate-SACK (DSACK) [RFC2883] feedback (similar to the schemes
 outlined in [ZKFP03]), when packets are reordered by less than one
 RTT.  The advantage 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 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 thus also prevents TCP from
 needlessly reducing the sending rate when retransmitting these

5. Disadvantages

 Although all 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 additional buffer requirements are not met, the
 use of the above algorithm takes into account the reduced advertised
 window -- with a corresponding loss in robustness to packet

Bhandarkar, et al. Experimental [Page 12] RFC 4653 Improving the Robustness of TCP August 2006

 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.  For
 instance, a socket option could be introduced to allow applications
 to control whether NCR would be used for a particular connection.
 Finally, the use of TCP-NCR makes the recovery from congestion events
 sluggish in comparison to the standard reaction in [RFC2581].  [BR04,
 BSRV04] show (via simulation) that the delay in congestion response
 has minimal impact on the connection itself and the traffic sharing a
 bottleneck.  [BBFS01] also indicates (again, via simulation) that
 "slowly responsive" congestion control may be safe for deployment in
 the Internet.  These studies suggest that schemes that slightly delay
 congestion control decisions may be reasonable; however, further
 experimentation on the Internet is required to verify these results.

6. 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
 [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 a given network path at a given time.  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 an amount based on the current cwnd (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 current patterns of packet reordering.
 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.  Future work could be undertaken to
 achieve robustness without needless delay.
 On the other hand, several schemes have been developed to detect and
 mitigate needless retransmissions after the fact.  [RFC3522, RFC3708,
 BA02, RFC4015, RFC4138] 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

Bhandarkar, et al. Experimental [Page 13] RFC 4653 Improving the Robustness of TCP August 2006

 retransmits and some other scheme kicking in if the prevention
 failed.  In addition, note that TCP-NCR is concentrated on preventing
 spurious fast retransmits; some of the above algorithms also attempt
 to detect and mitigate spurious timeout-based retransmits.

7. Security Considerations

 General attacks against the congestion control of TCP are described
 in [RFC2581].  SACK-based loss recovery for TCP [RFC3517] mitigates
 some of the duplicate ACK attacks against TCP's congestion control.
 This document builds upon that work, and the Extended Limited
 Transmit algorithms specified in this document have been designed to
 thwart the ACK division problems that are described in [RFC3465].

8. Acknowledgments

 Feedback from Lars Eggert, Ted Faber, Wesley Eddy, Gorry Fairhurst,
 Sally Floyd, Sara Landstrom, Nauzad Sadry, Pasi Sarolahti, Joe Touch,
 Nitin Vaidya, and the TCPM working group have contributed
 significantly to this document.  Our thanks to all!

9. References

9.1. 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 Acknowledgement Options", RFC 2018, October 1996.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
           Control", RFC 2581, April 1999.
 [RFC3042] Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing
           TCP's Loss Recovery Using Limited Transmit", RFC 3042,
           January 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.

Bhandarkar, et al. Experimental [Page 14] RFC 4653 Improving the Robustness of TCP August 2006

9.2. Informative References

 [BA02]    E. Blanton and M. Allman, "On Making TCP More Robust to
           Packet Reordering," ACM Computer Communication Review,
           January 2002.
 [BBFS01]  D. Bansal, H. Balakrishnan, S. Floyd and S. Shenker,
           "Dynamic Behavior of Slowly Responsive Congestion Control
           Algorithms", Proceedings of ACM SIGCOMM, Sep. 2001.
 [BPS99]   J. Bennett, C. Partridge, and N. Shectman, "Packet
           reordering is not pathological 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 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.
 [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 switches," Proceedings of the IEEE Infocom, June
 [MAF05]   A. Medina, M. Allman, S. Floyd.  Measuring the Evolution of
           Transport Protocols in the Internet.  ACM Computer
           Communication Review, 35(2), April 2005.
 [NS-2]    ns-2 Network Simulator.

Bhandarkar, et al. Experimental [Page 15] RFC 4653 Improving the Robustness of TCP August 2006

 [Pax97]   V. Paxson, "End-to-End Internet Packet Dynamics,"
           Proceedings of ACM SIGCOMM, September 1997.
 [RFC896]  Nagle, J., "Congestion control in IP/TCP internetworks",
           RFC 896, January 1984.
 [RFC1122] Braden, R., "Requirements for Internet Hosts -
           Communication Layers", STD 3, RFC 1122, October 1989.
 [RFC2883] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
           Extension to the Selective Acknowledgement (SACK) Option
           for TCP", RFC 2883, July 2000.
 [RFC2960] R. Stewart, Q. Xie, K. Morneault, C. Sharp, H.
           Schwarzbauer, T. Taylor, I. Rytina, M. Kalla, L. Zhang, V.
           Paxson.  Stream Control Transmission Protocol.  October
 [RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte
           Counting (ABC)", RFC 3465, February 2003.
 [RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for
           TCP", RFC 3522, April 2003.
 [RFC3708] Blanton, E. 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.
 [RFC4015] Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm for
           TCP", RFC 4015, February 2005.
 [RFC4138] Sarolahti, P. and M. Kojo, "Forward RTO-Recovery (F-RTO):
           An Algorithm for Detecting Spurious Retransmission Timeouts
           with TCP and the Stream Control Transmission Protocol
           (SCTP)", RFC 4138, August 2005.
 [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.

Bhandarkar, et al. Experimental [Page 16] RFC 4653 Improving the Robustness of TCP August 2006

Authors' Addresses

 Sumitha Bhandarkar
 Dept. of Elec. Engg.
 214 ZACH
 College Station, TX 77843-3128
 Phone: (512) 468-8078
 A. L. Narasimha Reddy
 Dept. of Elec. Engg.
 315C WERC
 College Station, TX 77843-3128
 Phone: (979) 845-7598
 Mark Allman
 ICSI Center for Internet Research
 1947 Center Street, Suite 600
 Berkeley, CA 94704-1198
 Phone: (440) 235-1792
 Ethan Blanton
 Purdue University Computer Science
 305 North University Street
 West Lafayette, IN  47907

Bhandarkar, et al. Experimental [Page 17] RFC 4653 Improving the Robustness of TCP August 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at


 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Bhandarkar, et al. Experimental [Page 18]

/data/webs/external/dokuwiki/data/pages/rfc/rfc4653.txt · Last modified: 2006/08/28 18:03 by

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki