INTERNET-DRAFT                                               R.Sallantin
Intended Status: Proposed Standard                         CNES/TAS/TESA
Expires: July 19, 2014                                         C.Baudoin
                                                     Thales Alenia Space
                                                        January 15, 2014

               Safe increase of the TCP's Initial Window
                        Using Initial Spreading


   This document proposes a new fast start-up mechanism for TCP that can
   be used to speed the beginning of an Internet connection and then
   improved the short-lived TCP connections performance.

   Initial Spreading allows to safely increase the Initial Window size
   in any cases, and notably in congested networks.

   Merging the increase in the IW with the spacing of the segments
   belonging to the Initial Window (IW), Initial Spreading is a very
   simple mechanism that improves short-lived TCP flows performance and
   do not deteriorate long-lived TCP flows performance.

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Table of Contents

   1  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3  Initial Spreading mechanism . . . . . . . . . . . . . . . . . .  4
   4  Implementation considerations . . . . . . . . . . . . . . . . .  4
     4.1   Short Round Trip Time  . . . . . . . . . . . . . . . . . .  5
     4.2  Delayed Ack . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.3  TSO/GSO . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.4 RTT measure  . . . . . . . . . . . . . . . . . . . . . . . .  6
   5  Open discussions  . . . . . . . . . . . . . . . . . . . . . . .  6
     5.1 Spacing Interval . . . . . . . . . . . . . . . . . . . . . .  6
     5.2  Increasing the upper bound TCP's IW to more than 10
          segments  . . . . . . . . . . . . . . . . . . . . . . . . .  7
     5.3  Initial Spreading and LFN . . . . . . . . . . . . . . . . .  7
   6  Security Considerations . . . . . . . . . . . . . . . . . . . .  7
   7  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .  8
   8  References  . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     8.1  Normative References  . . . . . . . . . . . . . . . . . . .  8
     8.2  Informative References  . . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9

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1  Introduction

   The long Round Trip Time is probably the most detrimental constraint
   in Long Fat Networks (LFN), such as satellite networks, and notably
   for short-lived connections when the long delay significantly
   downgrades regular slow-start performance [FA11]. Several protocols
   and even new network architectures have been proposed to deal with
   this issue. The original idea of Initial Spreading [SB13] was to
   consider a long RTT as a resource to exploit, rather than as a
   constant to bypass. The long RTT can therefore be used as an
   opportunity to safely send a large amount of data during the first
   RTT after the connection has opened. Spacing the data along the whole
   RTT would in fact hopefully guarantee high independent probability
   that each segment is successfully received.

   This approach resembles a combination of 2 TCP mechanisms: Pacing and
   Increase in the Initial Window. Both mechanisms have then been
   studied in depth to design Initial Spreading as an efficient fast
   start-up TCP mechanism, and notably avoid their respective flaws or

   The original Pacing idea is to space the segments of a same window
   along an RTT to prevent generating bursts as far as possible. Hence,
   each segment arrives separately at the buffer and the impact on its
   queue is minimized. The bit rate can then reach its maximum. However,
   [AS00] has pointed out that this lack of bursts is responsible for
   poor performance. Pacing has a tendency to overload the network, and
   then cause a synchronization of the flows, that seriously damages
   both individual and global performance.

   RFC 6928 [RFC6928] suggests to enlarge the IW size up to ten
   segments. Several articles and studies demonstrated that this would
   allow transmission of 90% of the connections in one RTT [DR10]. In
   most cases, and when the network is not congested in particular, this
   solution is probably the best one for dealing with short-lived TCP
   flows. However, in a congested environment, sending a large IW in one
   burst is likely to impact the buffers and then deteriorate the
   individual connection. Correlation between the segments of a same
   burst is responsible for major impairments when regarding the short-
   lived connections, and in particular for the connections that can be
   sent in one RTT (number of segments to be transmitted inferior to the
   upper bound value of the TCP's IW):

   o a decrease of the probability to successfully transmit the entire

   o an increase of the probability of successive segment losses.

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   o a significant reduction of the number of potential Duplicated
      Acknowledgements that are necessary to trigger fast loss recovery
      mechanisms and avoid to wait for an Retransmission Time Out.

   In favor of a conservative approach, [RFC3390] recommended the use of
   an IW equal to 3.

   Both mechanisms therefore suffer from a burst-related phenomenon, but
   in opposite ways.

   Initial Spreading has been designed to tackle previous burst issues.
   Simulations and experimentations show that Initial Spreading is not
   only efficient in case of LFNs but also for other networks with small

2  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3  Initial Spreading mechanism

   Initial Spreading [SB13] mechanism uses  the permitted upper bound
   value of the TCP's IW (e.g; RFC 6928 [RFC6928] suggests to use 10 for
   this value). Initial Spreading spaces out a number of segments
   inferior or equal to this value across the first RTT before letting
   the TCP algorithm continue conventionally:

   (1) The RTT is measured during the SYN-SYN/ACK exchange.

   (2) The first RTT is split into n spaces with n the permitted upper
       bound value of the TCP's IW. Depending on the number of segments
       to be sent, until n segments are sent every RTT/n.

   (3) After the transmission of the IW, the regular TCP algorithm is

   Thus, bursts do not downgrade the transmission of short-lived
   connections, but continue to prevent an overload of the network in
   the case of long-lived connections.

4  Implementation considerations

   In this section, we discuss a number of aspects surrounding the

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   Initial Spreading implementations.

4.1   Short Round Trip Time

   Whatever the different timer implementations, 2 segments can not be
   spaced of less than 1 Kernel timer (e.g: jiffy  for Linux kernel).

   In case where the time resulting of the division of 1 RTT by the
   upper bound value of  the TCP's IW is inferior to 1 Kernel timer,
   Initial Spreading is not activated and TCP uses a regular slow start
   with a large IW.

4.2  Delayed Ack

   The use of Delayed Ack (Del Ack) does not downgrade Initial Spreading

   Regarding long-lived connections and notably TCP's steady state, the
   effects of Del Ack are lessened by new TCP's flavors (such as TCP
   Cubic or Compound TCP [HR08][TS06]) which tend to adapt their
   congestion algorithm to take into account whether the receiver uses
   the Del Ack option or not. In doing so, they can prevent the
   connection from being too slow, and still continue to reduce
   acknowledgments traffic. In the event of short-lived connections, the
   use of Del Ack does not modify the transmission of the IW. There is
   then no change in the burst propagation.

4.3  TSO/GSO

   TSO/GSO is used to reduce the CPU overhead of TCP/IP on fast
   networks. Instead of doing the segmentation in the kernel, large
   packets are sent to the Network Interface Card (NIC). The
   segmentation is then achieved by the NIC or just before the entry
   into the driver's xmit routine.

   In its current design, Initial Spreading is not working when TSO or
   GSO are activated, but using Initial Spreading with an inactive
   TSO/GSO still enables better performance.

   Two options can be foreseen for the joint use of Initial Spreading
   and TSO/GSO:

   (1) disable TSO/GSO for the first RTT, with no impact on performance
       since the throughput is limited by the IW.

   (2) implement Initial Spreading using the TCP Offload Engine (TOE)

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4.4 RTT measure

   Initial spreading uses the SYN-SYN/ACK exchange to calculate the
   space between two segments. This measurement may not be perfectly
   accurate in congested networks when the RTT varies. Two different
   scenarios can then occur:

   o  The measured RTT is superior to the RTT of the first segment of
      the IW, and an ACK arrives before that all the segments of the IW
      have been sent. Then, Initial Spreading MUST be stopped and only
      the segments transmission that is triggered by the received ACK is

   o  The measured RTT is inferior to the RTT of the first segment of
      the IW. Consequences are negligible.

5  Open discussions

   In this section, we introduce possible improvements for Initial
   Spreading and new perspectives.

5.1 Spacing Interval

   It has been observed that most of the savings enabled by the Initial
   Spreading in congested environments comes from the independence of
   the segments sent during the first RTT. Indeed, experimentations have
   shown that preventing the bursts, Initial Spreading enables each
   segment of the IW to have an independent loss probability.

   Currently, Initial Spreading waits RTT/n seconds before transmitting
   two segments of the IW, with n the permitted upper bound value of the
   TCP's IW.

   This simple mechanism offers very good results but has two minor

   (1) An inaccuracy of the space measure (cf section 4.4).

   (2) In uncongested networks, Initial Spreading adds an extra delay
       that is equal to (IW-1) * RTT/n, with IW the number of segments
       (<= n) that can be sent during the first RTT.

   A solution could be to set the space not as a ratio of the measured
   RTT but as a minimal space that preserves the independence between
   the sent segments. Preliminary results show that smaller spacing
   interval may allow to maintain the independence of the segment loss
   probability. This may provide the same performances in congested

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   networks and improve the average delay in uncongested networks.

5.2  Increasing the upper bound TCP's IW to more than 10 segments

   [DR10] have shown that an IW of 10 segments enables to send more than
   90% of the web objects in one RTT. So the authors recommend to use
   Initial Spreading as a complement to [RFC6928].

   If the average size of the web objects continues to evolve, Initial
   Spreading can be used to raise the IW size. Simulations and
   experiments showed even better results with an IW equal to 12.

   Thus, Initial Spreading paves the way for larger IW. Further studies
   are  needed to assess the impact on the networks, notably in terms of
   individual performance, fairness, friendliness and global

5.3  Initial Spreading and LFN

   The space community designed middleboxes to mitigate poor TCP
   performance for network with large RTT [FA11]. Proxy Enhancement
   Performance (PEP) are generally used in LFN and in particular in
   satellite communication systems [RFC3135] and offer very good TCP

   Nevertheless, some recent studies have emphasized major impairments
   occasioned by the use of satellite-specific transport solutions, and
   notably TCP-PEPs, in a global context. The break of the end-to-end
   TCP semantic, which is required to isolate the satellite segment, is
   notably responsible for an increased complexity in case of mobility
   scenarios or security context. This strongly mitigates PEPs benefits
   and reopens the debate on their relevance[DC10].

   Many researchers have outlined that new TCP releases perform well for
   long-lived TCP connections, even in satellite environment [SC12], but
   continue to suffer from very poor performance in case of short-lived
   TCP connections.

   Initial Spreading enables to reduce the RTT consequences for short-
   lived TCP connections and could be an end-to-end alternative to PEP.

6  Security Considerations

   The security considerations found in [RFC5681] apply to this
   document.  No additional security problems have been identified with
   Initial Spreading at this time.

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7  IANA Considerations

   This document contains no IANA considerations.

8  References

8.1  Normative References

   [RFC3390] A. Allman and S. Floyd, "Increasing tcp's initial window,"
              RFC 3390, IETF, Proposed Standard, 2002.

   [RFC5532] T. Talpey, C. Juszczak, "Network File System (NFS) Remote
              Direct Memory Access (RDMA) Problem Statement," RFC 5532,
              IETF, Informational, May 2009.

   [RFC6928] J. Chu, N. Dukkipati, Y. Cheng, and M. Mathis, "Increasing
              tcp's initial window," RFC 6928, IETF, Experimental, Jan.

   [AH98] A. Allman, C. Hayes, and S. Ostermann, "An evaluation of TCP
              with Larger Initial Windows," ACM Computer Communication
              Review, 1998.

   [AS00] A. Aggarwal, S. Savage, and T. Anderson, "Understanding the
              performance of TCP pacing," in INFOCOM, vol. 3, mar 2000,
              pp. 1157-1165.

   [DR10] N. Dukkipati, T. Refice, Y. Cheng, J. Chu, T. Herbert, A.
              Agarwal, A. Jain, and N. Sutin, "An Argument for
              Increasing TCP's Initial Congestion Window," SIGCOMM
              Comput. Commun. Rev., vol. 40, no. 3, pp. 26-33, Jun.

   [SB13] R. Sallantin, C. Baudoin, E. Chaput, E. Dubois, F. Arnal, and
              A. Beylot, "Initial spreading: a fast start-up tcp
              mechanism," proceedings of LCN, 2013.

8.2  Informative References

   [RFC3135] J. Border, M. Kojo, J. Griner, G. Montenegro, Z. Shelby,
              "Performance Enhancing Proxies Intended to Mitigate Link-
              Related Degradations," RFC 3135, IETF, Informational, June

   [DF10] E. Dubois, J. Fasson, C. Donny, and E. Chaput, "Enhancing tcp
              based communications in mobile satellite scenarios: Tcp
              peps issues and solutions," in Proc. 5th Advanced

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              satellite multimedia systems conference (asma) and the
              11th signal processing for space communications workshop
              (spsc), pages 476-483, 2010.

   [FA11] A. Fairhurst, G. Arjuna, H. Cruickshank, and C. Baudoin,
              "Transport challenges facing a next generation hybrid
              satellite internet," in International Journal of Satellite
              Communications and networking,  2011.

   [HR08] S. Ha, I. Rhee, and L. Xu, "CUBIC: A New TCP-Friendly High-
              Speed TCP Variant," SIGOPS Oper. Syst. Rev., vol. 42, no.
              5, pp. 64-74, Jul. 2008.

   [LC09] R. Lacamera, D. Caini, C. Firrincieli, "Comparative
              performance evaluation of tcp variants on satellite
              environments," in ICC'09 Proceedings of the 2009 IEEE
              international conference on Communications, pages Pages
              5161-5165, 2009.

   [SC12]  R. Sallantin, E. Chaput, E. P. Dubois, C. Baudoin, F. Arnal,
              and A.-L.Beylot, "On the sustainability of PEPs for
              satellite Internet access," in ICSSC. AIAA, 2012.

   [TS06] K. Tan, J. Song, Q. Zhang, and M. Sridharan, "Compound TCP: A
              Scalable and TCP-friendly Congestion Control for High-
              speed Networks,"  in 4th International workshop on
              Protocols for Fast Long-Distance Networks (PFLDNet), 2006.

Authors' Addresses

   Comments are solicited and should be addressed to the working group's
   mailing list at and/or the authors:

   Renaud Sallantin
   IRIT/ENSEEIHT 2, rue Charles Camichel BP 7122
   31071 Toulouse Cedex 7
   Phone: +33 6 48 07 86 44

   Cedric Baudoin
   Thales Alenia Space (TAS)
   26 Avenue Jean Francois Champollion,
   31100 Toulouse

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   Fabrice Arnal
   Thales Alenia Space

   Emmanuel Dubois
   Centre National des Etudes Spatiales (CNES)
   18 Avenue Edouard Belin
   31400 Toulouse
   Email: emmanuel.Dubois@cnes.Fr

   Emmanuel Chaput
   IRIT / ENSEEIHT 2, rue Charles Camichel BP 7122
   31071 Toulouse Cedex 7

   Andre-Luc Beylot

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