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Internet Engineering Task Force (IETF)                         P. HurtigRequest for Comments: 7765                                  A. BrunstromCategory: Experimental                               Karlstad UniversityISSN: 2070-1721                                               A. Petlund                                           Simula Research Laboratory AS                                                                M. Welzl                                                      University of Oslo                                                           February 2016TCP and Stream Control Transmission Protocol (SCTP) RTO RestartAbstract   This document describes a modified sender-side algorithm for managing   the TCP and Stream Control Transmission Protocol (SCTP)   retransmission timers that provides faster loss recovery when there   is a small amount of outstanding data for a connection.  The   modification, RTO Restart (RTOR), allows the transport to restart its   retransmission timer using a smaller timeout duration, so that the   effective retransmission timeout (RTO) becomes more aggressive in   situations where fast retransmit cannot be used.  This enables faster   loss detection and recovery for connections that are short lived or   application limited.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for examination, experimental implementation, and   evaluation.   This document defines an Experimental Protocol for the Internet   community.  This document is a product of the Internet Engineering   Task Force (IETF).  It represents the consensus of the IETF   community.  It has received public review and has been approved for   publication by the Internet Engineering Steering Group (IESG).  Not   all documents approved by the IESG are a candidate for any level of   Internet Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7765.Hurtig, et al.                Experimental                      [Page 1]

RFC 7765                TCP and SCTP RTO Restart           February 2016Copyright Notice   Copyright (c) 2016 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.Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .32.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .43.  RTO Overview and Rationale for RTOR . . . . . . . . . . . . .44.  RTOR Algorithm  . . . . . . . . . . . . . . . . . . . . . . .65.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .75.1.  Applicability . . . . . . . . . . . . . . . . . . . . . .75.2.  Spurious Timeouts . . . . . . . . . . . . . . . . . . . .75.3.  Tracking Outstanding and Previously Unsent Segments . . .86.  Related Work  . . . . . . . . . . . . . . . . . . . . . . . .97.  SCTP Socket API Considerations  . . . . . . . . . . . . . . .107.1.  Data Types  . . . . . . . . . . . . . . . . . . . . . . .10     7.2.  Socket Option for Controlling the RTO Restart Support           (SCTP_RTO_RESTART)  . . . . . . . . . . . . . . . . . . .108.  Security Considerations . . . . . . . . . . . . . . . . . . .119.  References  . . . . . . . . . . . . . . . . . . . . . . . . .119.1.  Normative References  . . . . . . . . . . . . . . . . . .119.2.  Informative References  . . . . . . . . . . . . . . . . .13   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .14   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .15Hurtig, et al.                Experimental                      [Page 2]

RFC 7765                TCP and SCTP RTO Restart           February 20161.  Introduction   TCP and SCTP use two almost identical mechanisms to detect and   recover from data loss, specified in [RFC6298] and [RFC5681] for TCP   and [RFC4960] for SCTP.  First, if transmitted data is not   acknowledged within a certain amount of time, a retransmission   timeout (RTO) occurs and the data is retransmitted.  While the RTO is   based on measured round-trip times (RTTs) between the sender and   receiver, it also has a conservative lower bound of 1 second to   ensure that delayed data are not mistaken as lost.  Second, when a   sender receives duplicate acknowledgments or similar information via   selective acknowledgments, the fast retransmit algorithm suspects   data loss and can trigger a retransmission.  Duplicate (and   selective) acknowledgments are generated by a receiver when data   arrives out of order.  As both data loss and data reordering cause   out-of-order arrival, fast retransmit waits for three out-of-order   notifications before considering the corresponding data as lost.  In   some situations, however, the amount of outstanding data is not   enough to trigger three such acknowledgments, and the sender must   rely on lengthy RTOs for loss recovery.   The amount of outstanding data can be small for several reasons:   (1)  The connection is limited by congestion control when the path        has a low total capacity (bandwidth-delay product) or the        connection's share of the capacity is small.  It is also limited        by congestion control in the first few RTTs of a connection or        after an RTO when the available capacity is probed using        slow-start.   (2)  The connection is limited by the receiver's available buffer        space.   (3)  The connection is limited by the application if the available        capacity of the path is not fully utilized (e.g., interactive        applications) or is at the end of a transfer.   While the reasons listed above are valid for any flow, the third   reason is most common for applications that transmit short flows or   use a bursty transmission pattern.  A typical example of applications   that produce short flows are web-based applications.  [RJ10] shows   that 70% of all web objects, found at the top 500 sites, are too   small for fast retransmit to work.  [FDT13] shows that about 77% of   all retransmissions sent by a major web service are sent after RTO   expiry.  Applications with bursty transmission patterns often send   data in response to actions or as a reaction to real life events.   Typical examples of such applications are stock-trading systems,   remote computer operations, online games, and web-based applicationsHurtig, et al.                Experimental                      [Page 3]

RFC 7765                TCP and SCTP RTO Restart           February 2016   using persistent connections.  What is special about this class of   applications is that they are often time dependent, and extra latency   can reduce the application service level [P09].   The RTO Restart (RTOR) mechanism described in this document makes the   effective RTO slightly more aggressive when the amount of outstanding   data is too small for fast retransmit to work, in an attempt to   enable faster loss recovery while being robust to reordering.  While   RTOR still conforms to the requirement for when a segment can be   retransmitted, specified in [RFC6298] for TCP and [RFC4960] for SCTP,   it could increase the risk of spurious timeouts.  To determine   whether this modification is safe to deploy and enable by default,   further experimentation is required.Section 5 discusses experiments   still needed, including evaluations in environments where the risk of   spurious retransmissions are increased, e.g., mobile networks with   highly varying RTTs.   The remainder of this document describes RTOR and its implementation   for TCP only, to make the document easier to read.  However, the RTOR   algorithm described inSection 4 is applicable also for SCTP.   Furthermore,Section 7 details the SCTP socket API needed to control   RTOR.2.  Terminology   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 2119 [RFC2119].   This document introduces the following variables:   o  The number of previously unsent segments (prevunsnt): The number      of segments that a sender has queued for transmission, but has not      yet sent.   o  RTO Restart threshold (rrthresh): RTOR is enabled whenever the sum      of the number of outstanding and previously unsent segments      (prevunsnt) is below this threshold.3.  RTO Overview and Rationale for RTOR   The RTO management algorithm described in [RFC6298] recommends that   the retransmission timer be restarted when an acknowledgment (ACK)   that acknowledges new data is received and there is still outstanding   data.  The restart is conducted to guarantee that unacknowledged   segments will be retransmitted after approximately RTO seconds.  The   standardized RTO timer management is illustrated in Figure 1, where a   TCP sender transmits three segments to a receiver.  The arrival ofHurtig, et al.                Experimental                      [Page 4]

RFC 7765                TCP and SCTP RTO Restart           February 2016   the first and second segment triggers a delayed ACK (delACK)   [RFC1122], which restarts the RTO timer at the sender.  The RTO is   restarted approximately one RTT after the transmission of the third   segment.  Thus, if the third segment is lost, as indicated in   Figure 1, the effective loss detection time becomes "RTO + RTT"   seconds.  In some situations, the effective loss detection time   becomes even longer.  Consider a scenario where only two segments are   outstanding.  If the second segment is lost, the time to expire the   delACK timer will also be included in the effective loss detection   time.            Sender                               Receiver                          ...            DATA [SEG 1] ----------------------> (ack delayed)            DATA [SEG 2] ----------------------> (send ack)            DATA [SEG 3] ----X         /-------- ACK            (restart RTO)  <----------/                          ...            (RTO expiry)            DATA [SEG 3] ---------------------->                       Figure 1: RTO Restart Example   For bulk traffic, the current approach is beneficial -- it is   described in [EL04] to act as a "safety margin" that compensates for   some of the problems that the authors have identified with the   standard RTO calculation.  Notably, the authors of [EL04] also state   that "this safety margin does not exist for highly interactive   applications where often only a single packet is in flight."  In   general, however, as long as enough segments arrive at a receiver to   enable fast retransmit, RTO-based loss recovery should be avoided.   RTOs should only be used as a last resort, as they drastically lower   the congestion window as compared to fast retransmit.   Although fast retransmit is preferable, there are situations where   timeouts are appropriate or are the only choice.  For example, if the   network is severely congested and no segments arrive, RTO-based   recovery should be used.  In this situation, the time to recover from   the loss(es) will not be the performance bottleneck.  However, for   connections that do not utilize enough capacity to enable fast   retransmit, RTO-based loss detection is the only choice, and the time   required for this can become a performance bottleneck.Hurtig, et al.                Experimental                      [Page 5]

RFC 7765                TCP and SCTP RTO Restart           February 20164.  RTOR Algorithm   To enable faster loss recovery for connections that are unable to use   fast retransmit, RTOR can be used.  This section specifies the   modifications required to use RTOR.  By resetting the timer to "RTO -   T_earliest", where T_earliest is the time elapsed since the earliest   outstanding segment was transmitted, retransmissions will always   occur after exactly RTO seconds.   This document specifies an OPTIONAL sender-only modification to TCP   and SCTP, which updates step 5.3 inSection 5 of [RFC6298] (and a   similar update inSection 6.3.2 of [RFC4960] for SCTP).  A sender   that implements this method MUST follow the algorithm below:      When an ACK is received that acknowledges new data:      (1)  Set T_earliest = 0.      (2)  If the sum of the number of outstanding and previously unsent           segments (prevunsnt) is less than an RTOR threshold           (rrthresh), set T_earliest to the time elapsed since the           earliest outstanding segment was sent.      (3)  Restart the retransmission timer so that it will expire after           (for the current value of RTO):           (a)  RTO - T_earliest, if RTO - T_earliest > 0.           (b)  RTO, otherwise.   The RECOMMENDED value of rrthresh is four, as this value will ensure   that RTOR is only used when fast retransmit cannot be triggered.   With this update, TCP implementations MUST track the time elapsed   since the transmission of the earliest outstanding segment   (T_earliest).  As RTOR is only used when the amount of outstanding   and previously unsent data is less than rrthresh segments, TCP   implementations also need to track whether the amount of outstanding   and previously unsent data is more, equal, or less than rrthresh   segments.  Although some packet-based TCP implementations (e.g.,   Linux TCP) already track both the transmission times of all segments   and also the number of outstanding segments, not all implementations   do.Section 5.3 describes how to implement segment tracking for a   general TCP implementation.  To use RTOR, the calculated expiration   time MUST be positive (step 3(a) in the list above); this is required   to ensure that RTOR does not trigger retransmissions prematurely when   previously retransmitted segments are acknowledged.Hurtig, et al.                Experimental                      [Page 6]

RFC 7765                TCP and SCTP RTO Restart           February 20165.  Discussion   Although RTOR conforms to the requirement in [RFC6298] that segments   must not be retransmitted earlier than RTO seconds after their   original transmission, RTOR makes the effective RTO more aggressive.   In this section, we discuss the applicability and the issues related   to RTOR.5.1.  Applicability   The currently standardized algorithm has been shown to add at least   one RTT to the loss recovery process in TCP [LS00] and SCTP [HB11]   [PBP09].  For applications that have strict timing requirements   (e.g., interactive web) rather than throughput requirements, using   RTOR could be beneficial because the RTT and the delACK timer of   receivers are often large components of the effective loss recovery   time.  Measurements in [HB11] have shown that the total transfer time   of a lost segment (including the original transmission time and the   loss recovery time) can be reduced by 35% using RTOR.  These results   match those presented in [PGH06] and [PBP09], where RTOR is shown to   significantly reduce retransmission latency.   There are also traffic types that do not benefit from RTOR.  One   example of such traffic is bulk transmission.  The reason why bulk   traffic does not benefit from RTOR is that such traffic flows mostly   have four or more segments outstanding, allowing loss recovery by   fast retransmit.  However, there is no harm in using RTOR for such   traffic as the algorithm is only active when the amount of   outstanding and unsent segments are less than rrthresh (default 4).   Given that RTOR is a mostly conservative algorithm, it is suitable   for experimentation as a system-wide default for TCP traffic.5.2.  Spurious Timeouts   RTOR can in some situations reduce the loss detection time and   thereby increase the risk of spurious timeouts.  In theory, the   retransmission timer has a lower bound of 1 second [RFC6298], which   limits the risk of having spurious timeouts.  However, in practice,   most implementations use a significantly lower value.  Initial   measurements show slight increases in the number of spurious timeouts   when such lower values are used [RHB15].  However, further   experiments, in different environments and with different types of   traffic, are encouraged to quantify such increases more reliably.   Does a slightly increased risk matter?  Generally, spurious timeouts   have a negative effect on the network as segments are transmitted   needlessly.  However, recent experiments do not show a significantHurtig, et al.                Experimental                      [Page 7]

RFC 7765                TCP and SCTP RTO Restart           February 2016   increase in network load for a number of realistic scenarios [RHB15].   Another problem with spurious retransmissions is related to the   performance of TCP/SCTP, as the congestion window is reduced to one   segment when timeouts occur [RFC5681].  This could be a potential   problem for applications transmitting multiple bursts of data within   a single flow, e.g., web-based HTTP/1.1 and HTTP/2.0 applications.   However, results from recent experiments involving persistent web   traffic [RHB15] revealed a net gain using RTOR.  Other types of   flows, e.g., long-lived bulk flows, are not affected as the algorithm   is only applied when the amount of outstanding and unsent segments is   less than rrthresh.  Furthermore, short-lived and application-limited   flows are typically not affected as they are too short to experience   the effect of congestion control or have a transmission rate that is   quickly attainable.   While a slight increase in spurious timeouts has been observed using   RTOR, it is not clear whether or not the effects of this increase   mandate any future algorithmic changes -- especially since most   modern operating systems already include mechanisms to detect   [RFC3522] [RFC3708] [RFC5682] and resolve [RFC4015] possible problems   with spurious retransmissions.  Further experimentation is needed to   determine this and thereby move this specification from Experimental   to the Standards Track.  For instance, RTOR has not been evaluated in   the context of mobile networks.  Mobile networks often incur highly   variable RTTs (delay spikes), due to e.g., handovers, and would   therefore be a useful scenario for further experimentation.5.3.  Tracking Outstanding and Previously Unsent Segments   The method of tracking outstanding and previously unsent segments   will probably differ depending on the actual TCP implementation.  For   packet-based TCP implementations, tracking outstanding segments is   often straightforward and can be implemented using a simple counter.   For byte-based TCP stacks, it is a more complex task.Section 3.2 of   [RFC5827] outlines a general method of tracking the number of   outstanding segments.  The same method can be used for RTOR.  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 rrthresh segments on the right side of the current   window (which involves tracking rrthresh + 1 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 rrthresh segments are outstanding,   and therefore RTOR is not enabled.  When the outstanding window   becomes small enough that RTOR can be invoked, a full understanding   of the number of outstanding segments will be available from the   rrthresh + 1 sequence numbers retained.  (Note: the implicit sequenceHurtig, et al.                Experimental                      [Page 8]

RFC 7765                TCP and SCTP RTO Restart           February 2016   number consumed by the TCP FIN bit can also be included in the   tracking of segment boundaries.)   Tracking the number of previously unsent segments depends on the   segmentation strategy used by the TCP implementation, not whether it   is packet based or byte based.  In the case where segments are formed   directly on socket writes, the process of determining the number of   previously unsent segments should be trivial.  In the case that   unsent data can be segmented (or resegmented) as long as it is still   unsent, a straightforward strategy could be to divide the amount of   unsent data (in bytes) with the Sender Maximum Segment Size (SMSS) to   obtain an estimate.  In some cases, such an estimation could be too   simplistic, depending on the segmentation strategy of the TCP   implementation.  However, this estimation is not critical to RTOR.   The tracking of prevunsnt is only made to optimize a corner case in   which RTOR was unnecessarily disabled.  Implementations can use a   simplified method by setting prevunsnt to rrthresh whenever   previously unsent data is available, and set prevunsnt to zero when   no new data is available.  This will disable RTOR in the presence of   unsent data and only use the number of outstanding segments to   enable/disable RTOR.6.  Related Work   There are several proposals that address the problem of not having   enough ACKs for loss recovery.  In what follows, we explain why the   mechanism described here is complementary to these approaches:   The limited transmit mechanism [RFC3042] allows a TCP sender to   transmit a previously unsent segment for each of the first two   duplicate acknowledgements (dupACKs).  By transmitting new segments,   the sender attempts to generate additional dupACKs to enable fast   retransmit.  However, limited transmit does not help if no previously   unsent data is ready for transmission.  [RFC5827] specifies an early   retransmit algorithm to enable fast loss recovery in such situations.   By dynamically lowering the number of dupACKs needed for fast   retransmit (dupthresh), based on the number of outstanding segments,   a smaller number of dupACKs is needed to trigger a retransmission.   In some situations, however, the algorithm is of no use or might not   work properly.  First, if a single segment is outstanding and lost,   it is impossible to use early retransmit.  Second, if ACKs are lost,   early retransmit cannot help.  Third, if the network path reorders   segments, the algorithm might cause more spurious retransmissions   than fast retransmit.  The recommended value of RTOR's rrthresh   variable is based on the dupthresh, but it is possible to adapt to   allow tighter integration with other experimental algorithms such as   early retransmit.Hurtig, et al.                Experimental                      [Page 9]

RFC 7765                TCP and SCTP RTO Restart           February 2016   Tail Loss Probe [TLP] is a proposal to send up to two "probe   segments" when a timer fires that is set to a value smaller than the   RTO.  A "probe segment" is a new segment if new data is available,   else it is a retransmission.  The intention is to compensate for   sluggish RTO behavior in situations where the RTO greatly exceeds the   RTT, which, according to measurements reported in [TLP], is not   uncommon.  Furthermore, TLP also tries to circumvent the congestion   window reset to one segment by instead enabling fast recovery.  The   probe timeout (PTO) is normally two RTTs, and a spurious PTO is less   risky than a spurious RTO because it would not have the same negative   effects (clearing the scoreboard and restarting with slow-start).   TLP is a more advanced mechanism than RTOR, requiring e.g., SACK to   work, and is often able to further reduce loss recovery times.   However, it also noticeably increases the amount of spurious   retransmissions, as compared to RTOR [RHB15].   TLP is applicable in situations where RTOR does not apply, and it   could overrule (yielding a similar general behavior, but with a lower   timeout) RTOR in cases where the number of outstanding segments is   smaller than four and no new segments are available for transmission.   The PTO has the same inherent problem of restarting the timer on an   incoming ACK and could be combined with a strategy similar to RTOR's   to offer more consistent timeouts.7.  SCTP Socket API Considerations   This section describes how the socket API for SCTP defined in   [RFC6458] is extended to control the usage of RTO restart for SCTP.   Please note that this section is informational only.7.1.  Data Types   This section uses data types from [IEEE.9945]: uintN_t means an   unsigned integer of exactly N bits (e.g., uint16_t).  This is the   same as in [RFC6458].7.2.  Socket Option for Controlling the RTO Restart Support      (SCTP_RTO_RESTART)   This socket option allows the enabling or disabling of RTO Restart   for SCTP associations.   Whether or not RTO restart is enabled per default is implementation   specific.Hurtig, et al.                Experimental                     [Page 10]

RFC 7765                TCP and SCTP RTO Restart           February 2016   This socket option uses IPPROTO_SCTP as its level and   SCTP_RTO_RESTART as its name.  It can be used with getsockopt() and   setsockopt().  The socket option value uses the following structure   defined in [RFC6458]:   struct sctp_assoc_value {     sctp_assoc_t assoc_id;     uint32_t assoc_value;   };   assoc_id:  This parameter is ignored for one-to-one style sockets.      For one-to-many style sockets, this parameter indicates upon which      association the user is performing an action.  The special      sctp_assoc_t SCTP_{FUTURE|CURRENT|ALL}_ASSOC can also be used in      assoc_id for setsockopt().  For getsockopt(), the special value      SCTP_FUTURE_ASSOC can be used in assoc_id, but it is an error to      use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.   assoc_value:  A non-zero value encodes the enabling of RTO restart      whereas a value of 0 encodes the disabling of RTO restart.   sctp_opt_info() needs to be extended to support SCTP_RTO_RESTART.8.  Security Considerations   This document specifies an experimental sender-only modification to   TCP and SCTP.  The modification introduces a change in how to set the   retransmission timer's value when restarted.  Therefore, the security   considerations found in [RFC6298] apply to this document.  No   additional security problems have been identified with RTO Restart at   this time.9.  References9.1.  Normative References   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -              Communication Layers", STD 3,RFC 1122,              DOI 10.17487/RFC1122, October 1989,              <http://www.rfc-editor.org/info/rfc1122>.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <http://www.rfc-editor.org/info/rfc2119>.Hurtig, et al.                Experimental                     [Page 11]

RFC 7765                TCP and SCTP RTO Restart           February 2016   [RFC3042]  Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing              TCP's Loss Recovery Using Limited Transmit",RFC 3042,              DOI 10.17487/RFC3042, January 2001,              <http://www.rfc-editor.org/info/rfc3042>.   [RFC3522]  Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm              for TCP",RFC 3522, DOI 10.17487/RFC3522, April 2003,              <http://www.rfc-editor.org/info/rfc3522>.   [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,              DOI 10.17487/RFC3708, February 2004,              <http://www.rfc-editor.org/info/rfc3708>.   [RFC4015]  Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm              for TCP",RFC 4015, DOI 10.17487/RFC4015, February 2005,              <http://www.rfc-editor.org/info/rfc4015>.   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",RFC 4960, DOI 10.17487/RFC4960, September 2007,              <http://www.rfc-editor.org/info/rfc4960>.   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion              Control",RFC 5681, DOI 10.17487/RFC5681, September 2009,              <http://www.rfc-editor.org/info/rfc5681>.   [RFC5682]  Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,              "Forward RTO-Recovery (F-RTO): An Algorithm for Detecting              Spurious Retransmission Timeouts with TCP",RFC 5682,              DOI 10.17487/RFC5682, September 2009,              <http://www.rfc-editor.org/info/rfc5682>.   [RFC5827]  Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J., and              P. Hurtig, "Early Retransmit for TCP and Stream Control              Transmission Protocol (SCTP)",RFC 5827,              DOI 10.17487/RFC5827, May 2010,              <http://www.rfc-editor.org/info/rfc5827>.   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,              "Computing TCP's Retransmission Timer",RFC 6298,              DOI 10.17487/RFC6298, June 2011,              <http://www.rfc-editor.org/info/rfc6298>.Hurtig, et al.                Experimental                     [Page 12]

RFC 7765                TCP and SCTP RTO Restart           February 20169.2.  Informative References   [EL04]     Ekstroem, H. and R. Ludwig, "The Peak-Hopper: A New End-              to-End Retransmission Timer for Reliable Unicast              Transport", IEEE INFOCOM 2004,              DOI 10.1109/INFCOM.2004.1354671, March 2004.   [FDT13]    Flach, T., Dukkipati, N., Terzis, A., Raghavan, B.,              Cardwell, N., Cheng, Y., Jain, A., Hao, S., Katz-Bassett,              E., and R. Govindan, "Reducing Web Latency: the Virtue of              Gentle Aggression", Proc. ACM SIGCOMM Conf.,              DOI 10.1145/2486001.2486014, August 2013.   [HB11]     Hurtig, P. and A. Brunstrom, "SCTP: designed for timely              message delivery?", Springer Telecommunication Systems 47              (3-4), DOI 10.1007/s11235-010-9321-3, August 2011.   [IEEE.9945]              IEEE/ISO/IEC, "International Standard - Information              technology Portable Operating System Interface (POSIX)              Base Specifications, Issue 7", IEEE 9945-2009,              <http://standards.ieee.org/findstds/standard/9945-2009.html>.   [LS00]     Ludwig, R. and K. Sklower, "The Eifel retransmission              timer", ACM SIGCOMM Comput. Commun. Rev., 30(3),              DOI 10.1145/382179.383014, July 2000.   [P09]      Petlund, A., "Improving latency for interactive, thin-              stream applications over reliable transport", Unipub PhD              Thesis, Oct 2009.   [PBP09]    Petlund, A., Beskow, P., Pedersen, J., Paaby, E., Griwodz,              C., and P. Halvorsen, "Improving SCTP retransmission              delays for time-dependent thin streams", Springer              Multimedia Tools and Applications, 45(1-3),              DOI 10.1007/s11042-009-0286-8, October 2009.   [PGH06]    Pedersen, J., Griwodz, C., and P. Halvorsen,              "Considerations of SCTP Retransmission Delays for Thin              Streams", IEEE LCN 2006, DOI 10.1109/LCN.2006.322082,              November 2006.   [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.              Yasevich, "Sockets API Extensions for the Stream Control              Transmission Protocol (SCTP)",RFC 6458,              DOI 10.17487/RFC6458, December 2011,              <http://www.rfc-editor.org/info/rfc6458>.Hurtig, et al.                Experimental                     [Page 13]

RFC 7765                TCP and SCTP RTO Restart           February 2016   [RHB15]    Rajiullah, M., Hurtig, P., Brunstrom, A., Petlund, A., and              M. Welzl, "An Evaluation of Tail Loss Recovery Mechanisms              for TCP", ACM SIGCOMM CCR 45 (1),              DOI 10.1145/2717646.2717648, January 2015.   [RJ10]     Ramachandran, S., "Web metrics: Size and number of              resources", May 2010, <https://goo.gl/0a6Q9A>.   [TLP]      Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,              "Tail Loss Probe (TLP): An Algorithm for Fast Recovery of              Tail Losses", Work in Progress,draft-dukkipati-tcpm-tcp-loss-probe-01, February 2013.Acknowledgements   The authors wish to thank Michael Tuexen for contributing the SCTP   Socket API considerations and Godred Fairhurst, Yuchung Cheng, Mark   Allman, Anantha Ramaiah, Richard Scheffenegger, Nicolas Kuhn,   Alexander Zimmermann, and Michael Scharf for commenting on the   document and the ideas behind it.   All the authors are supported by RITE (http://riteproject.eu/), a   research project (ICT-317700) funded by the European Community under   its Seventh Framework Program.  The views expressed here are those of   the author(s) only.  The European Commission is not liable for any   use that may be made of the information in this document.Hurtig, et al.                Experimental                     [Page 14]

RFC 7765                TCP and SCTP RTO Restart           February 2016Authors' Addresses   Per Hurtig   Karlstad University   Universitetsgatan 2   Karlstad  651 88   Sweden   Phone: +46 54 700 23 35   Email: per.hurtig@kau.se   Anna Brunstrom   Karlstad University   Universitetsgatan 2   Karlstad  651 88   Sweden   Phone: +46 54 700 17 95   Email: anna.brunstrom@kau.se   Andreas Petlund   Simula Research Laboratory AS   P.O. Box 134   Lysaker  1325   Norway   Phone: +47 67 82 82 00   Email: apetlund@simula.no   Michael Welzl   University of Oslo   PO Box 1080 Blindern   Oslo  N-0316   Norway   Phone: +47 22 85 24 20   Email: michawe@ifi.uio.noHurtig, et al.                Experimental                     [Page 15]