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Internet Engineering Task Force (IETF)                          M. WelzlRequest for Comments: 6297                            University of OsloCategory: Informational                                           D. RosISSN: 2070-1721                                    IT / Telecom Bretagne                                                               June 2011A Survey of Lower-than-Best-Effort Transport ProtocolsAbstract   This document provides a survey of transport protocols that are   designed to have a smaller bandwidth and/or delay impact on standard   TCP than standard TCP itself when they share a bottleneck with it.   Such protocols could be used for delay-insensitive "background"   traffic, as they provide what is sometimes called a "less than" (or   "lower than") best-effort service.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   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/rfc6297.Copyright Notice   Copyright (c) 2011 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.Welzl & Ros                   Informational                     [Page 1]

RFC 6297                  LBE Transport Survey                 June 2011Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .22.  Delay-Based Transport Protocols  . . . . . . . . . . . . . . .32.1.  Accuracy of Delay-Based Congestion Predictors  . . . . . .6     2.2.  Potential Issues with Delay-Based Congestion Control           for LBE Transport  . . . . . . . . . . . . . . . . . . . .73.  Non-Delay-Based Transport Protocols  . . . . . . . . . . . . .84.  Upper-Layer Approaches . . . . . . . . . . . . . . . . . . . .84.1.  Receiver-Oriented, Flow-Control-Based Approaches . . . . .95.  Network-Assisted Approaches  . . . . . . . . . . . . . . . . .106.  LEDBAT Considerations  . . . . . . . . . . . . . . . . . . . .127.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .128.  Security Considerations  . . . . . . . . . . . . . . . . . . .129.  Informative References . . . . . . . . . . . . . . . . . . . .121.  Introduction   This document presents a brief survey of proposals to attain a Less-   than-Best-Effort (LBE) service by means of end-host mechanisms.  We   loosely define an LBE service as a service which results in smaller   bandwidth and/or delay impact on standard TCP than standard TCP   itself, when sharing a bottleneck with it.  We refer to systems that   are designed to provide this service as LBE systems.  With the   exception of TCP Vegas, which we present for historical reasons, we   exclude systems that have been noted to exhibit LBE behavior under   some circumstances but were not designed for this purpose (e.g.,   RAPID [Kon09]).   Generally, LBE behavior can be achieved by reacting to queue growth   earlier than standard TCP would or by changing the congestion-   avoidance behavior of TCP without utilizing any additional implicit   feedback.  It is therefore assumed that readers are familiar with TCP   congestion control [RFC5681].  Some mechanisms achieve an LBE   behavior without modifying transport-protocol standards (e.g., by   changing the receiver window of standard TCP), whereas others   leverage network-level mechanisms at the transport layer for LBE   purposes.  According to this classification, solutions have been   categorized in this document as delay-based transport protocols, non-   delay-based transport protocols, upper-layer approaches, and network-   assisted approaches.  Some of the schemes in the first two categories   could be implemented using TCP without changing its header format;   this would facilitate their deployment in the Internet.  The schemes   in the third category are, by design, supposed to be especially easy   to deploy because they only describe a way in which existing   transport protocols are used.  Finally, mechanisms in the last   category require changes to equipment along the path, which can   greatly complicate their deployment.Welzl & Ros                   Informational                     [Page 2]

RFC 6297                  LBE Transport Survey                 June 2011   This document is a product of the Low Extra Delay Background   Transport (LEDBAT) working group.  It aims at putting the congestion   control algorithm that the working group has specified [Sha11] in the   context of the state of the art in LBE transport.  This survey is not   exhaustive, as this would not be possible or useful; the authors have   selected key, well-known, or otherwise interesting techniques for   inclusion at their discretion.  There is also a substantial amount of   work that is related to the LBE concept but does not present a   solution that can be installed in end-hosts or expected to work over   the Internet (e.g., there is a Diffserv-based, Lower-Effort service   [RFC3662], and the IETF Congestion Exposure (CONEX) working group is   developing a mechanism which can incentivize LEDBAT-like   applications).  Such work is outside the scope of this document.2.  Delay-Based Transport Protocols   It is wrong to generally equate "little impact on standard TCP" with   "small sending rate".  Without Explicit Congestion Notification (ECN)   support, standard TCP will normally increase its congestion window   (and effective sending rate) until a queue overflows, causing one or   more packets to be dropped and the effective rate to be reduced.  A   protocol that stops increasing the rate before this event happens   can, in principle, achieve a better performance than standard TCP.   TCP Vegas [Bra94] is one of the first protocols that was known to   have a smaller sending rate than standard TCP when both protocols   share a bottleneck [Kur00] -- yet, it was designed to achieve more,   not less, throughput than standard TCP.  Indeed, when TCP Vegas is   the only congestion control algorithm used by flows going through the   bottleneck, its throughput is greater than the throughput of standard   TCP.  Depending on the bottleneck queue length, TCP Vegas itself can   be starved by standard TCP flows.  This can be remedied to some   degree by the Random Early Detection (RED) Active Queue Management   mechanism [RFC2309].  Vegas linearly increases or decreases the   sending rate, based on the difference between the expected throughput   and the actual throughput.  The estimation is based on RTT   measurements.   The congestion-avoidance behavior is the protocol's most important   feature in terms of historical relevance as well as relevance in the   context of this document (it has been shown that other elements of   the protocol can sometimes play a greater role for its overall   behavior [Hen00]).  In congestion avoidance, once per RTT, TCP Vegas   calculates the expected throughput as WindowSize / BaseRTT, where   WindowSize is the current congestion window and BaseRTT is the   minimum of all measured RTTs.  The expected throughput is then   compared with the actual throughput, measured based on recent   acknowledgements.  If the actual throughput is smaller than theWelzl & Ros                   Informational                     [Page 3]

RFC 6297                  LBE Transport Survey                 June 2011   expected throughput minus a threshold called "beta", this is taken as   a sign of congestion, causing the protocol to linearly decrease its   rate.  If the actual throughput is greater than the expected   throughput minus a threshold called "alpha" (with alpha < beta), this   is taken as a sign that the network is underutilized, causing the   protocol to linearly increase its rate.   TCP Vegas has been analyzed extensively.  One of the most prominent   properties of TCP Vegas is its fairness between multiple flows of the   same kind, which does not penalize flows with large propagation   delays in the same way as standard TCP.  While it was not the first   protocol that uses delay as a congestion indication, its predecessors   (like CARD [Jai89], Tri-S [Wan91], or DUAL [Wan92]) are not discussed   here because of the historical "landmark" role that TCP Vegas has   taken in the literature.   Delay-based transport protocols that were designed to be non-   intrusive include TCP Nice [Ven02] and TCP Low Priority (TCP-LP)   [Kuz06].  TCP Nice [Ven02] follows the same basic approach as TCP   Vegas but improves upon it in some aspects.  Because of its moderate   linear-decrease congestion response, TCP Vegas can affect standard   TCP despite its ability to detect congestion early.  TCP Nice removes   this issue by halving the congestion window (at most once per RTT,   like standard TCP) instead of linearly reducing it.  To avoid being   too conservative, this is only done if a fixed predefined fraction of   delay-based incipient congestion signals appears within one RTT.   Otherwise, TCP Nice falls back to the congestion-avoidance rules of   TCP Vegas if no packet was lost or standard TCP if a packet was lost.   One more feature of TCP Nice is its ability to support a congestion   window of less than one packet, by clocking out single packets over   more than one RTT.  With ns-2 simulations and real-life experiments   using a Linux implementation, the authors of [Ven02] show that TCP   Nice achieves its goal of efficiently utilizing spare capacity while   being non-intrusive to standard TCP.   Other than TCP Vegas and TCP Nice, TCP-LP [Kuz06] uses only the one-   way delay (OWD) instead of the RTT as an indicator of incipient   congestion.  This is done to avoid reacting to delay fluctuations   that are caused by reverse cross-traffic.  Using the TCP Timestamps   option [RFC1323], the OWD is determined as the difference between the   receiver's Timestamp value in the ACK and the original Timestamp   value that the receiver copied into the ACK.  While the result of   this subtraction can only precisely represent the OWD if clocks are   synchronized, its absolute value is of no concern to TCP-LP, and   hence clock synchronization is unnecessary.  Using a constant   smoothing parameter, TCP-LP calculates an Exponentially Weighted   Moving Average (EWMA) of the measured OWD and checks whether the   result exceeds a threshold within the range of the minimum andWelzl & Ros                   Informational                     [Page 4]

RFC 6297                  LBE Transport Survey                 June 2011   maximum OWD that was seen during the connection's lifetime; if it   does, this condition is interpreted as an "early congestion   indication".  The minimum and maximum OWD values are initialized   during the slow-start phase.   Regarding its reaction to an early congestion indication, TCP-LP   tries to strike a middle ground between the overly conservative   choice of _immediately_ setting the congestion window to one packet,   and the presumably too aggressive choice of simply halving the   congestion window like standard TCP; TCP-LP tries to delay the former   action by an additional RTT, to see if there is persistent congestion   or not.  It does so by halving the window at first in response to an   early congestion indication, then initializing an "inference time-out   timer" and maintaining the current congestion window until this timer   fires.  If another early congestion indication appeared during this   "inference phase", the window is then set to 1; otherwise, the window   is maintained and TCP-LP continues to increase it in the standard   Additive-Increase fashion.  This method ensures that it takes at   least two RTTs for a TCP-LP flow to decrease its window to 1, and   that, like standard TCP, TCP-LP reacts to congestion at most once per   RTT.   Using a simple analytical model, the authors of TCP-LP [Kuz06]   illustrate the feasibility of a delay-based LBE transport by showing   that, due to the non-linear relationship between throughput and RTT,   it is possible to avoid interfering with standard TCP traffic even   when the flows under consideration have a larger RTT than standard   TCP flows.  With ns-2 simulations and real-life experiments using a   Linux implementation, the authors of [Kuz06] show that TCP-LP is   largely non-intrusive to TCP traffic while at the same time enabling   it to utilize a large portion of the excess network bandwidth, which   is fairly shared among competing TCP-LP flows.  They also show that   using their protocol for bulk data transfers greatly reduces file   transfer times of competing best-effort web traffic.   Sync-TCP [Wei05] follows a similar approach as TCP-LP, by adapting   its reaction to congestion according to changes in the OWD.  By   comparing the estimated (average) forward queuing delay to the   maximum observed delay, Sync-TCP adapts the Additive-Increase   Multiplicative-Decrease (AIMD) parameters depending on the trend   followed by the average delay over an observation window.  Even   though the authors of [Wei05] did not explicitly consider its use as   an LBE protocol, Sync-TCP was designed to react early to incipient   congestion, while grabbing available bandwidth more aggressively than   a standard TCP in congestion-avoidance mode.Welzl & Ros                   Informational                     [Page 5]

RFC 6297                  LBE Transport Survey                 June 2011   Delay-based congestion control is also the basis of proposals that   aim at adapting TCP's congestion avoidance to very high-speed   networks.  Some of these proposals, like Compound TCP [Tan06] [Sri08]   and TCP Illinois [Liu08], are hybrid loss- and delay-based   mechanisms, whereas others (e.g., NewVegas [Dev03], FAST TCP [Wei06],   or CODE TCP [Cha10]) are variants of Vegas based primarily on delays.2.1.  Accuracy of Delay-Based Congestion Predictors   The accuracy of delay-based congestion predictors has been the   subject of a good deal of research, see, e.g., [Bia03], [Mar03],   [Pra04], [Rew06], [McC08].  The main result of most of these studies   is that delays (or, more precisely, round-trip times) are, in   general, weakly correlated with congestion.  There are several   factors that may induce such a poor correlation:   o  Bottleneck buffer size: in principle, a delay-based mechanism      could be made "more than TCP friendly" _if_ buffers are "large      enough", so that RTT fluctuations and/or deviations from the      minimum RTT can be detected by the end-host with reasonable      accuracy.  Otherwise, it may be hard to distinguish real delay      variations from measurement noise.   o  RTT measurement issues: in principle, RTT samples may suffer from      poor resolution, due to timers which are too coarse-grained with      respect to the scale of delay fluctuations.  Also, a flow may      obtain a very noisy estimate of RTTs due to undersampling, under      some circumstances (e.g., the flow rate is much lower than the      link bandwidth).  For TCP, other potential sources of measurement      noise include TCP segmentation offloading (TSO) and the use of      delayed ACKs [Hay10].  A congested reverse path may also result in      an erroneous assessment of the congestion state of the forward      path.  Finally, in the case of fast or short-distance links, the      majority of the measured delay can in fact be due to processing in      the involved hosts; typically, this processing delay is not of      interest, and it can underlie fluctuations that are not related to      the network at all.   o  Level of statistical multiplexing and RTT sampling: it may be easy      for an individual flow to "miss" loss/queue overflow events,      especially if the number of flows sharing a bottleneck buffer is      significant.  This is nicely illustrated, e.g., in Figure 1 of      [McC08].   o  Impact of wireless links: several mechanisms that are typical of      wireless links, like link-layer scheduling and error recovery, may      induce strong delay fluctuations over short timescales [Gur04].Welzl & Ros                   Informational                     [Page 6]

RFC 6297                  LBE Transport Survey                 June 2011   Interestingly, the results of Bhandarkar et al. [Bha07] seem to paint   a slightly different picture, regarding the accuracy of delay-based   congestion prediction.  Bhandarkar et al. claim that it is possible   to significantly improve prediction accuracy by adopting some simple   techniques (smoothing of RTT samples, increasing the RTT sampling   frequency).  Nonetheless, they acknowledge that even with such   techniques, it is not possible to eradicate detection errors.  Their   proposed delay-based congestion-avoidance method, PERT (Probabilistic   Early Response TCP), mitigates the impact of residual detection   errors by means of a probabilistic response mechanism to congestion-   detection events.2.2.  Potential Issues with Delay-Based Congestion Control for LBE      Transport   Whether a delay-based protocol behaves in its intended manner (e.g.,   it is "more than TCP friendly", or it grabs available bandwidth in a   very aggressive manner) may depend on the accuracy issues listed inSection 2.1.  Moreover, protocols like Vegas need to keep an estimate   of the minimum ("base") delay; this makes such protocols highly   sensitive to eventual changes in the end-to-end route during the   lifetime of the flow [Mo99].   Regarding the issue of false positives or false negatives with a   delay-based congestion detector, most studies focus on the loss of   throughput coming from the erroneous detection of queue build-up and   of alleviation of congestion.  Arguably, for an LBE transport   protocol it's better to err on the "more-than-TCP-friendly side",   that is, to always yield to _perceived_ congestion whether it is   "real" or not; however, failure to detect congestion (due to one of   the above accuracy problems) would result in behavior that is not   LBE.  For instance, consider the case in which the bottleneck buffer   is small, so that the contribution of queueing delay at the   bottleneck to the global end-to-end delay is small.  In such a case,   a flow using a delay-based mechanism might end up consuming a good   deal of bandwidth with respect to a competing standard TCP flow,   unless it also incorporates a suitable reaction to loss.   A delay-based mechanism may also suffer from the so-called "latecomer   advantage" (or "latecomer unfairness") problem.  Consider the case in   which the bottleneck link is already (very) congested.  In such a   scenario, delay variations may be quite small; hence, it may be very   difficult to tell an empty queue from a heavily-loaded queue, in   terms of delay fluctuation.  Therefore, a newly-arriving delay-based   flow may start sending faster when there is already heavy congestion,   eventually driving away loss-based flows [Sha05] [Car10].Welzl & Ros                   Informational                     [Page 7]

RFC 6297                  LBE Transport Survey                 June 20113.  Non-Delay-Based Transport Protocols   There exist a few transport-layer proposals that achieve an LBE   service without relying on delay as an indicator of congestion.  In   the algorithms discussed below, the loss rate of the flow determines,   either implicitly or explicitly, the sending rate (which is adapted   so as to obtain a lower share of the available bandwidth than   standard TCP); such mechanisms likely cause more queuing delay and   react to congestion more slowly than delay-based ones.   4CP [Liu07], which stands for "Competitive and Considerate Congestion   Control", is a protocol that provides an LBE service by changing the   window control rules of standard TCP.  A "virtual window" is   maintained that, during a so-called "bad congestion phase", is   reduced to less than a predefined minimum value of the actual   congestion window.  The congestion window is only increased again   once the virtual window exceeds this minimum, and in this way the   virtual window controls the duration during which the sender   transmits with a fixed minimum rate.  Whether the congestion state is   "bad" or "good" depends on whether the loss event rate is above or   below a threshold (or target) value.  The 4CP congestion-avoidance   algorithm allows for setting a target average window and avoids   starvation of "background" flows while bounding the impact on   "foreground" flows.  Its performance was evaluated in ns-2   simulations and in real-life experiments with a kernel-level   implementation in Microsoft Windows Vista.   The MulTFRC [Dam09] protocol is an extension of TCP-Friendly Rate   Control (TFRC) [RFC5348] for multiple flows.  MulTFRC takes the main   idea of MulTCP [Cro98] and similar proposals (e.g., [Hac04], [Hac08],   [Kuo08]) a step further.  A single MulTCP flow tries to emulate (and   be as friendly as) a number N > 1 of parallel TCP flows.  By   supporting values of N between 0 and 1, MulTFRC can be used as a   mechanism for an LBE service.  Since it does not react to delay like   the protocols described inSection 2 but adjusts its rate like TFRC,   MulTFRC can probably be expected to be more aggressive than   mechanisms such as TCP Nice or TCP-LP.  This also means that MulTFRC   is less likely to be prone to starvation, as its aggressiveness is   tunable at a fine granularity, even when N is between 0 and 1.4.  Upper-Layer Approaches   The proposals described in this section do not require modifying   transport-protocol standards.  Most of them can be regarded as   running "on top" of an existing transport, even though they may be   implemented either at the application layer (i.e., in user-level   processes), or in the kernel of the end-hosts' operating systems.Welzl & Ros                   Informational                     [Page 8]

RFC 6297                  LBE Transport Survey                 June 2011   Such "upper-layer" mechanisms may arguably be easier to deploy than   transport-layer approaches, since they do not require any changes to   the transport itself.   A simplistic, application-level approach to a background transport   service may consist in scheduling automated transfers at times when   the network is lightly loaded, e.g., as described in [Dyk02] for   cooperative proxy caching.  An issue with such a technique is that it   may not necessarily be applicable to applications like peer-to-peer   file transfer, since the notion of an "off-peak hour" is not   meaningful when end-hosts may be located anywhere in the world.   The so-called Background Intelligent Transfer Service [BITS] is   implemented in several versions of Microsoft Windows.  BITS uses a   system of application-layer priority levels for file-transfer jobs,   together with monitoring of bandwidth usage of the network interface   (or, in more recent versions, of the network gateway connected to the   end-host), so that low-priority transfers at a given end-host give   way to both high-priority (foreground) transfers and traffic from   interactive applications at the same host.   A different approach is taken in [Egg05] -- here, the priority of a   flow is reduced via a generic idletime scheduling strategy in a   host's operating system.  While results presented in this paper show   that the new scheduler can effectively shield regular tasks from low-   priority ones (e.g., TCP from greedy UDP) with only a minor   performance impact, it is an underlying assumption that all involved   end-hosts would use the idletime scheduler.  In other words, it is   not the focus of this work to protect a standard TCP flow that   originates from any host where the presented scheduling scheme may   not be implemented.4.1.  Receiver-Oriented, Flow-Control-Based Approaches   Some proposals for achieving an LBE behavior work by exploiting   existing transport-layer features -- typically, at the "receiving"   side.  In particular, TCP's built-in flow control can be used as a   means to achieve a low-priority transport service.   The mechanism described in [Spr00] is an example of the above   technique.  Such mechanism controls the bandwidth by letting the   receiver intelligently manipulate the receiver window of standard   TCP.  This is possible because the authors assume a client-server   setting where the receiver's access link is typically the bottleneck.   The scheme incorporates a delay-based calculation of the expected   queue length at the bottleneck, which is quite similar to the   calculation in the above delay-based protocols, e.g., TCP Vegas.   Using a Linux implementation, where TCP flows are classifiedWelzl & Ros                   Informational                     [Page 9]

RFC 6297                  LBE Transport Survey                 June 2011   according to their application's needs, Spring et al. show in [Spr00]   that a significant improvement in packet latency can be attained over   an unmodified system, while maintaining good link utilization.   A similar method is employed by Mehra et al. [Meh03], where both the   advertised receiver window and the delay in sending ACK messages are   dynamically adapted to attain a given rate.  As in [Spr00], Mehra et   al. assume that the bottleneck is located at the receiver's access   link.  However, the latter also propose a bandwidth-sharing system,   allowing control of the bandwidth allocated to different flows, as   well as allotment of a minimum rate to some flows.   Receiver window tuning is also done in [Key04], where choosing the   right value for the window is phrased as an optimization problem.  On   this basis, two algorithms are presented, binary search (which is   faster than the other one at achieving a good operation point but   fluctuates) and stochastic optimization (which does not fluctuate but   converges slower than binary search).  These algorithms merely use   the previous receiver window and the amount of data received during   the previous control interval as input.  According to [Key04], the   encouraging simulation results suggest that such an application-level   mechanism can work almost as well as a transport-layer scheme like   TCP-LP.   Another way of dealing with non-interactive flows, like web   prefetching, is to rate-limit the transfer of such bursty traffic   [Cro98b].  Note that one of the techniques used in [Cro98b] is,   precisely, to have the downloading application adapt the TCP receiver   window, so as to reduce the data rate to the minimum needed (thus   disturbing other flows as little as possible while respecting a   deadline for the transfer of the data).5.  Network-Assisted Approaches   Network-layer mechanisms, like active queue management (AQM) and   packet scheduling in routers, can be exploited by a transport   protocol for achieving an LBE service.  Such approaches may result in   improved protection of non-LBE flows (e.g., when scheduling is used);   besides, approaches using an explicit, AQM-based congestion signaling   may arguably be more robust than, say, delay-based transports for   detecting impending congestion.  However, an obvious drawback of any   network-assisted approach is that, in principle, they need   modifications in both end-hosts and intermediate network nodes.   Harp [Kok04] realizes an LBE service by dissipating background   traffic to less-utilized paths of the network, based on multipath   routing and multipath congestion control.  This is achieved without   changing all routers, by using edge nodes as relays.  According toWelzl & Ros                   Informational                    [Page 10]

RFC 6297                  LBE Transport Survey                 June 2011   the authors, these edge nodes should be gateways of organizations in   order to align their scheme with usage incentives, but the technical   solution would also work if Harp was only deployed in end-hosts.  It   detects impending congestion by looking at delay, similar to TCP Nice   [Ven02], and manages to improve the utilization and fairness of TCP   over pure single-path solutions without requiring any changes to the   TCP itself.   Another technique is that used by protocols like Network-Friendly TCP   (NF-TCP) [Aru10], where a bandwidth-estimation module integrated into   the transport protocol allows to rapidly take advantage of free   capacity.  NF-TCP combines this with an early congestion detection   based on Explicit Congestion Notification (ECN) [RFC3168] and RED   [RFC2309]; when congestion starts building up, appropriate tuning of   a RED queue allows to mark low-priority (i.e., NF-TCP) packets with a   much higher probability than high-priority (i.e., standard TCP)   packets, so low-priority flows yield up bandwidth before standard TCP   flows.  NF-TCP could be implemented by adapting the congestion   control behavior of TCP without requiring to change the protocol on   the wire -- with the only exception that NF-TCP-capable routers must   be able to somehow distinguish NF-TCP traffic from other TCP traffic.   In [Ven08], Venkataraman et al. propose a transport-layer approach to   leverage an existing, network-layer LBE service based on priority   queueing.  Their transport protocol, which they call PLT (Priority-   Layer Transport), splits a layer-4 connection into two flows, a high-   priority one and a low-priority one.  The high-priority flow is sent   over the higher-priority queueing class (in principle, offering a   best-effort service) using an AIMD, TCP-like congestion control   mechanism.  The low-priority flow, which is mapped to the LBE class,   uses a non TCP-friendly congestion control algorithm.  The goal of   PLT is thus to maximize its aggregate throughput by exploiting unused   capacity in an aggressive way, while protecting standard TCP flows   carried by the best-effort class.  Similar in spirit, [Ott03]   proposes simple changes to only the AIMD parameters of TCP for use   over a network-layer LBE service, so that such "filler" traffic may   aggressively consume unused bandwidth.  Note that [Ven08] also   considers a mechanism for detecting the lack of priority queueing in   the network, so that the non-TCP friendly flow may be inhibited.  The   PLT receiver monitors the loss rate of both flows; if the high-   priority flow starts seeing losses while the low-priority one does   not experience 100% loss, this is taken as an indication of the   absence of strict priority queueing.Welzl & Ros                   Informational                    [Page 11]

RFC 6297                  LBE Transport Survey                 June 20116.  LEDBAT Considerations   The previous sections have shown that there is a large amount of work   on attaining an LBE service, and that it is quite heterogeneous in   nature.  The algorithm developed by the LEDBAT working group [Sha11]   can be classified as a delay-based mechanism; as such, it is similar   in spirit to the protocols presented inSection 2.  It is, however,   not a protocol -- how it is actually applied to the Internet, i.e.,   how to use existing or even new transport protocols together with the   LEDBAT algorithm, is not defined by the LEDBAT working group.  As it   heavily relies on delay, the discussion in Sections2.1 and2.2   applies to it.  The performance of LEDBAT has been analyzed in   comparison with some of the other work presented here in several   articles, e.g.  [Aru10], [Car10], [Sch10], but these analyses have to   be examined with care: at the time of writing, LEDBAT was still a   moving target.7.  Acknowledgements   The authors would like to thank Melissa Chavez, Dragana Damjanovic,   and Yinxia Zhao for reference pointers, as well as Jari Arkko,   Mayutan Arumaithurai, Elwyn Davies, Wesley Eddy, Stephen Farrell,   Mirja Kuehlewind, Tina Tsou, and Rolf Winter for their detailed   reviews and suggestions.8.  Security Considerations   This document introduces no new security considerations.9.  Informative References   [Aru10]    Arumaithurai, M., Fu, X., and K. Ramakrishnan, "NF-TCP: A              Network Friendly TCP Variant for Background Delay-              Insensitive Applications", Technical Report No. IFI-TB-              2010-05, Institute of Computer Science, University of              Goettingen, Germany, September 2010, <http://www.net.informatik.uni-goettingen.de/publications/1718/NF-TCP-techreport.pdf>.   [BITS]     Microsoft, "Windows Background Intelligent Transfer              Service",              <http://msdn.microsoft.com/library/bb968799(VS.85).aspx>.   [Bha07]    Bhandarkar, S., Reddy, A., Zhang, Y., and D. Loguinov,              "Emulating AQM from end hosts", Proceedings of ACM              SIGCOMM 2007, 2007.Welzl & Ros                   Informational                    [Page 12]

RFC 6297                  LBE Transport Survey                 June 2011   [Bia03]    Biaz, S. and N. Vaidya, "Is the round-trip time correlated              with the number of packets in flight?", Proceedings of the              3rd ACM SIGCOMM conference on Internet measurement (IMC              '03), pages 273-278, 2003.   [Bra94]    Brakmo, L., O'Malley, S., and L. Peterson, "TCP Vegas: New              techniques for congestion detection and avoidance",              Proceedings of SIGCOMM '94, pages 24-35, August 1994.   [Car10]    Carofiglio, G., Muscariello, L., Rossi, D., and S.              Valenti, "The quest for LEDBAT fairness", Proceedings of              IEEE GLOBECOM 2010, December 2010.   [Cha10]    Chan, Y., Lin, C., Chan, C., and C. Ho, "CODE TCP: A              competitive delay-based TCP", Computer              Communications, 33(9):1013-1029, June 2010.   [Cro98]    Crowcroft, J. and P. Oechslin, "Differentiated end-to-end              Internet services using a weighted proportional fair              sharing TCP", ACM SIGCOMM Computer Communication              Review, vol. 28, no. 3, pp. 53-69, July 1998.   [Cro98b]   Crovella, M. and P. Barford, "The network effects of              prefetching", Proceedings of IEEE INFOCOM 1998,              April 1998.   [Dam09]    Damjanovic, D. and M. Welzl, "MulTFRC: Providing Weighted              Fairness for Multimedia Applications (and others too!)",              ACM Computer Communication Review, vol. 39, no. 3,              July 2009.   [Dev03]    De Vendictis, A., Baiocchi, A., and M. Bonacci, "Analysis              and enhancement of TCP Vegas congestion control in a mixed              TCP Vegas and TCP Reno network scenario", Performance              Evaluation, 53(3-4):225-253, 2003.   [Dyk02]    Dykes, S. and K. Robbins, "Limitations and benefits of              cooperative proxy caching", IEEE Journal on Selected Areas              in Communications, 20(7):1290-1304, September 2002.   [Egg05]    Eggert, L. and J. Touch, "Idletime Scheduling with              Preemption Intervals", Proceedings of 20th ACM Symposium              on Operating Systems Principles, SOSP 2005, Brighton,              United Kingdom, pp. 249/262, October 2005.   [Gur04]    Gurtov, A. and S. Floyd, "Modeling wireless links for              transport protocols", ACM SIGCOMM Computer Communications              Review, 34(2):85-96, April 2004.Welzl & Ros                   Informational                    [Page 13]

RFC 6297                  LBE Transport Survey                 June 2011   [Hac04]    Hacker, T., Noble, B., and B. Athey, "Improving Throughput              and Maintaining Fairness using Parallel TCP", Proceedings              of IEEE INFOCOM 2004, March 2004.   [Hac08]    Hacker, T. and P. Smith, "Stochastic TCP: A Statistical              Approach to Congestion Avoidance", Proceedings of              PFLDnet 2008, March 2008.   [Hay10]    Hayes, D., "Timing enhancements to the FreeBSD kernel to              support delay and rate based TCP mechanisms", Technical              Report 100219A, Centre for Advanced Internet              Architectures, Swinburne University of Technology,              February 2010.   [Hen00]    Hengartner, U., Bolliger, J., and T. Gross, "TCP Vegas              revisited", Proceedings of IEEE INFOCOM 2000, March 2000.   [Jai89]    Jain, R., "A delay-based approach for congestion avoidance              in interconnected heterogeneous computer networks", ACM              Computer Communication Review, 19(5):56-71, October 1989.   [Key04]    Key, P., Massoulie, L., and B. Wang, "Emulating Low-              Priority Transport at the Application Layer: a Background              Transfer Service", Proceedings of ACM SIGMETRICS 2004,              January 2004.   [Kok04]    Kokku, R., Bohra, A., Ganguly, S., and A. Venkataramani,              "A Multipath Background Network Architecture", Proceedings              of IEEE INFOCOM 2007, May 2007.   [Kon09]    Konda, V. and J. Kaur, "RAPID: Shrinking the Congestion-              control Timescale", Proceedings of IEEE INFOCOM 2009,              April 2009.   [Kuo08]    Kuo, F. and X. Fu, "Probe-Aided MulTCP: an aggregate              congestion control mechanism", ACM SIGCOMM Computer              Communication Review, vol. 38, no. 1, pp. 17-28,              January 2008.   [Kur00]    Kurata, K., Hasegawa, G., and M. Murata, "Fairness              Comparisons Between TCP Reno and TCP Vegas for Future              Deployment of TCP Vegas", Proceedings of INET 2000,              July 2000.Welzl & Ros                   Informational                    [Page 14]

RFC 6297                  LBE Transport Survey                 June 2011   [Kuz06]    Kuzmanovic, A. and E. Knightly, "TCP-LP: low-priority              service via end-point congestion control", IEEE/ACM              Transactions on Networking (ToN),  Volume 14, Issue 4, pp.              739-752., August 2006,              <http://www.ece.rice.edu/networks/TCP-LP/>.   [Liu07]    Liu, S., Vojnovic, M., and D. Gunawardena, "Competitive              and Considerate Congestion Control for Bulk Data              Transfers", Proceedings of IWQoS 2007, June 2007.   [Liu08]    Liu, S., Basar, T., and R. Srikant, "TCP-Illinois: A loss-              and delay-based congestion control algorithm for high-              speed networks", Performance Evaluation, 65(6-7):417-440,              2008.   [Mar03]    Martin, J., Nilsson, A., and I. Rhee, "Delay-based              congestion avoidance for TCP", IEEE/ACM Transactions on              Networking, 11(3):356-369, June 2003.   [McC08]    McCullagh, G. and D. Leith, "Delay-based congestion              control: Sampling and correlation issues revisited",              Technical report, Hamilton Institute, 2008.   [Meh03]    Mehra, P., Zakhor, A., and C. De Vleeschouwer, "Receiver-              Driven Bandwidth Sharing for TCP", Proceedings of IEEE              INFOCOM 2003, April 2003.   [Mo99]     Mo, J., La, R., Anantharam, V., and J. Walrand, "Analysis              and Comparison of TCP Reno and TCP Vegas", Proceedings of              IEEE INFOCOM 1999, March 1999.   [Ott03]    Ott, B., Warnky, T., and V. Liberatore, "Congestion              control for low-priority filler traffic", SPIE QoS 2003              (Quality of Service over Next-Generation Internet), In              Proc. SPIE, Vol. 5245, 154, Monterey (CA), USA, July 2003.   [Pra04]    Prasad, R., Jain, M., and C. Dovrolis, "On the              effectiveness of delay-based congestion avoidance",              Proceedings of PFLDnet, 2004.   [RFC1323]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions              for High Performance",RFC 1323, May 1992.Welzl & Ros                   Informational                    [Page 15]

RFC 6297                  LBE Transport Survey                 June 2011   [RFC2309]  Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,              S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,              Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,              S., Wroclawski, J., and L. Zhang, "Recommendations on              Queue Management and Congestion Avoidance in the              Internet",RFC 2309, April 1998.   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition              of Explicit Congestion Notification (ECN) to IP",RFC 3168, September 2001.   [RFC3662]  Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort              Per-Domain Behavior (PDB) for Differentiated Services",RFC 3662, December 2003.   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP              Friendly Rate Control (TFRC): Protocol Specification",RFC 5348, September 2008.   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion              Control",RFC 5681, September 2009.   [Rew06]    Rewaskar, S., Kaur, J., and D. Smith, "Why don't delay-              based congestion estimators work in the real-world?",              Technical report TR06-001, University of North Carolina at              Chapel Hill, Dept. of Computer Science, January 2006.   [Sch10]    Schneider, J., Wagner, J., Winter, R., and H. Kolbe, "Out              of my Way -- Evaluating Low Extra Delay Background              Transport in an ADSL Access Network", Proceedings of the              22nd International Teletraffic Congress ITC22, 2010.   [Sha05]    Shalunov, S., Dunn, L., Gu, Y., Low, S., Rhee, I., Senger,              S., Wydrowski, B., and L. Xu, "Design Space for a Bulk              Transport Tool", Technical Report, Internet2 Transport              Group, May 2005.   [Sha11]    Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,              "Low Extra Delay Background Transport (LEDBAT)", Work              in Progress, May 2011.   [Spr00]    Spring, N., Chesire, M., Berryman, M., Sahasranaman, V.,              Anderson, T., and B. Bershad, "Receiver based management              of low bandwidth access links", Proceedings of IEEE              INFOCOM 2000, pp. 245-254, vol. 1, 2000.Welzl & Ros                   Informational                    [Page 16]

RFC 6297                  LBE Transport Survey                 June 2011   [Sri08]    Sridharan, M., Tan, K., Bansala, D., and D. Thaler,              "Compound TCP: A New TCP Congestion Control for High-Speed              and Long Distance Networks", Work in Progress,              November 2008.   [Tan06]    Tan, K., Song, J., Zhang, Q., and M. Sridharan, "A              Compound TCP approach for high-speed and long distance              networks", Proceedings of IEEE INFOCOM 2006, Barcelona,              Spain, April 2008.   [Ven02]    Venkataramani, A., Kokku, R., and M. Dahlin, "TCP Nice: a              mechanism for background transfers", Proceedings of              OSDI '02, 2002.   [Ven08]    Venkataraman, V., Francis, P., Kodialam, M., and T.              Lakshman, "A priority-layered approach to transport for              high bandwidth-delay product networks", Proceedings of ACM              CoNEXT, Madrid, December 2008.   [Wan91]    Wang, Z. and J. Crowcroft, "A new congestion control              scheme: slow start and search (Tri-S)", ACM Computer              Communication Review, 21(1):56-71, January 1991.   [Wan92]    Wang, Z. and J. Crowcroft, "Eliminating periodic packet              losses in the 4.3-Tahoe BSD TCP congestion control              algorithm", ACM Computer Communication Review, 22(2):9-16,              January 1992.   [Wei05]    Weigle, M., Jeffay, K., and F. Smith, "Delay-based early              congestion detection and adaptation in TCP: impact on web              performance", Computer Communications, 28(8):837-850,              May 2005.   [Wei06]    Wei, D., Jin, C., Low, S., and S. Hegde, "FAST TCP:              Motivation, architecture, algorithms, performance", IEEE/              ACM Transactions on Networking, 14(6):1246-1259,              December 2006.Welzl & Ros                   Informational                    [Page 17]

RFC 6297                  LBE Transport Survey                 June 2011Authors' Addresses   Michael Welzl   University of Oslo   Department of Informatics, PO Box 1080 Blindern   N-0316 Oslo   Norway   Phone: +47 22 85 24 20   EMail: michawe@ifi.uio.no   David Ros   Institut Telecom / Telecom Bretagne   Rue de la Chataigneraie, CS 17607   35576 Cesson Sevigne cedex   France   Phone: +33 2 99 12 70 46   EMail: david.ros@telecom-bretagne.euWelzl & Ros                   Informational                    [Page 18]