Network Working Group                                          M. Scharf
Internet-Draft                                   University of Stuttgart
Intended status: Experimental                                   S. Floyd
Expires: January 3, 2008                                            ICIR
                                                            P. Sarolahti
                                                   Nokia Research Center
                                                            July 2, 2007

       Avoiding Interactions of Quick-Start TCP and Flow Control

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document describes methods to avoid interactions between the
   flow control of the Transmission Control Protocol (TCP) and the
   Quick-Start TCP mechanism.  Quick-Start is an optional TCP congestion
   control extension that allows hosts to determine an allowed sending
   rate from feedback of routers along the path.  With Quick-Start, data

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   transfers can start with a potentially large congestion window and
   avoid the time-consuming slow-start.  In order to fully utilize the
   data rate determined by Quick-Start, the sending host must not be
   limited by the TCP flow control, i. e., the amount of free buffer
   space advertised by the receive window.

   There are two potential interactions between Quick-Start and the TCP
   flow control: First, receivers might not provide sufficiently large
   buffer space after connection setup, or they may implement buffer
   allocation strategies that implicitly assume the slow-start behavior
   on the sender side.  This document therefore provides guidelines for
   buffer allocation in hosts supporting the Quick-Start extension.
   Second, the TCP receive window scaling mechanism interferes with
   Quick-Start when being used in the initial three-way handshake
   connection setup.  This document describes a simple solution to
   overcome this problem.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  4
   3.  Quick-Start TCP and Receive Buffer Dimensioning  . . . . . . .  5
     3.1.  Receiver Buffer Allocation Strategies  . . . . . . . . . .  5
     3.2.  Recommendations for Buffer Dimensioning in Presence of
           Quick-Start Requests . . . . . . . . . . . . . . . . . . .  5
   4.  Quick-Start TCP and Receive Window Scaling . . . . . . . . . .  6
     4.1.  Receive Window Scaling . . . . . . . . . . . . . . . . . .  6
     4.2.  Problem Within the Three-way Handshake . . . . . . . . . .  6
     4.3.  Proposed Solution  . . . . . . . . . . . . . . . . . . . .  7
     4.4.  Discussion and Deployment Considerations . . . . . . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Appendix A.  Applicability to Other Proposals  . . . . . . . . . . 12
   Appendix B.  Alternative Solutions . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14

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1.  Introduction

   Quick-Start is an experimental extension for the Transmission Control
   Protocol (TCP) [RFC0793] that allows to speed up best effort data
   transfers.  The Quick-Start TCP extension is specified in [RFC4782].
   With Quick-Start, TCP hosts can request permission from the routers
   along a network path to send at a higher rate than allowed by the
   default TCP congestion control, in particular during connection setup
   or after longer idle periods.  The explicit router feedback avoids
   the time-consuming capacity probing by the TCP slow-start and can
   significantly improve transfer times over paths with a high
   bandwidth-delay product [SAF07].

   The usage of Quick-Start significantly changes the TCP behavior
   during connection setup.  This is why special care is needed in order
   to prevent interactions between Quick-Start and other TCP mechanisms.
   Specifically, TCP flow control mechanisms have to be optimized for
   the usage of Quick-Start, in particular when the TCP connection spans
   a path with a large bandwidth-delay product (BDP).  In such cases
   both congestion and receive window should have large values in order
   to achieve good TCP performance (see [RFC2488],[RFC3481]).

   Unlike the standard slow-start mechanism, the Quick-Start TCP
   extension allows the sender to use large congestion windows
   immediately after connection setup.  The usage of such large windows
   raises two questions: First, what receiver buffer allocation
   strategies should be used in combination with Quick-Start?  And
   second, how to appropriately signal these large windows?  This
   document addresses these issues and shows that Quick-Start requires
   special mechanisms in both cases.  The document thereby supplements
   the Quick-Start TCP specification [RFC4782], where flow control
   issues have not been addressed in detail.

   The rest of this document is structured as follows: First, the
   question of receive buffer allocation in combination with Quick-Start
   is addressed and dimensioning guidelines are provided.  Second, a
   modification of the receive window scaling mechanism [RFC1323] is
   specified, which is required to fully benefit from Quick-Start when
   the Quick-Start request is used in the initial <SYN> segment.

2.  Requirements Notation

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

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3.  Quick-Start TCP and Receive Buffer Dimensioning

3.1.  Receiver Buffer Allocation Strategies

   A sender can transmit up to the minimum of the congestion window and
   the receive window (also called receiver's advertised window)
   [RFC2581].  A small receive window prevents the TCP connection from
   fully utilizing paths with a larger bandwidth-delay product.  As a
   consequence, on the one hand, a TCP receiver should advertise a
   receive window that is big enough to allow an efficient utilization
   of the connection path.  On the other hand, hosts with a potentially
   high number of TCP connections need to optimize the buffer and memory
   usage to be able to serve a maximum possible number of TCP
   connections.  Finding a fixed receive buffer size that is optimal
   between these two goals is difficult.

   This is why many modern TCP implementations use an intelligent
   dynamic buffer management.  There are different auto-tuning
   techniques and heuristics [Dun06] designed to prevent the receive
   window from limiting the data rate at the sender.  An implementation
   using buffer size auto-tuning is described for instance in [SB05].  A
   common characteristic of most of these buffer allocation strategies
   is that they initially start with a rather small receive window.  The
   more data arrives, the more buffer is allocated to the corresponding
   connection.  This behavior is reasonable if the sender uses the
   standard slow-start algorithm and thus starts with a small congestion
   window anyway.  However, when using Quick-Start, a large receive
   buffer may be required immediately after connection setup.

3.2.  Recommendations for Buffer Dimensioning in Presence of Quick-Start

   When a host receives and approves a Quick-Start request, in
   particular during the connection setup, it SHOULD announce a receive
   window that is large enough so that a potential Quick-Start data
   transfer can start with a high sending window.  If buffer size auto-
   tuning is used, it SHOULD be ensured that a sufficiently high initial
   receive window is announced.  The handling of buffer space upon
   arrival of a Quick-Start request SHOULD be configurable by the
   corresponding application.

   If the TCP host has sufficient receive buffer space, it could
   estimate the required buffer space as the product of the approved
   Quick-Start rate and the round-trip time, and advertise a receive
   window based on this required buffer space.  This receive window
   should allow the other TCP host to fully use the approved Quick-Start

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   If the TCP host doesn't know the round-trip time, the TCP host could
   use an estimate of the round-trip time in calculating the required
   buffer space.  For instance, the buffer dimension could be done for a
   configurable "worst-case" RTT such as 500 ms.  Alternately, the TCP
   host could base the advertised receive window on the available buffer
   space, without calculating the buffer space required for the other
   TCP host to fully use the approved Quick-Start Request.

4.  Quick-Start TCP and Receive Window Scaling

4.1.  Receive Window Scaling

   The TCP header specified in [RFC0793] uses a 16 bit field to report
   the receive window size to the sender.  This effectively limits the
   sending window to 64 KB.  To circumvent this problem, the "Window
   Scale" TCP extension [RFC1323] defines an implicit scale factor,
   which is used to multiply the window size value found in a TCP header
   to obtain a 32 bit window size.  If enabled, the scale factor is
   announced during connection setup by the "Window Scale" TCP option in
   <SYN> and <SYN,ACK> segments.

   In general, using receive window scaling is highly beneficial for TCP
   connections over path with a large bandwidth-delay product
   [RFC2488],[RFC3481].  Otherwise, the path capacity cannot fully be
   utilized by TCP.  Quick-Start TCP can significantly speed up data
   transfers over such paths [RFC4782],[SAF07].  As a consequence, a
   host supporting Quick-Start SHOULD enable receive window scaling
   according to [RFC1323].  If Quick-Start is used in the initial three-
   way handshake, the minimum required scaling factor MAY be obtained
   from the required receive buffer space, which can be approximated as
   described in the previous section.

4.2.  Problem Within the Three-way Handshake

   A problem arises when the Quick-Start mechanism is used within the
   three-way handshake, and the Quick-Start request is added to the
   initial <SYN> segment: In this scenario, if the Quick-Start request
   is approved by the routers along the path, the receiver echoes back
   the Quick-Start response in the <SYN,ACK> segment.  This process is
   illustrated in [RFC4782].  Upon reception of the <SYN,ACK> with the
   Quick-Start response, the sender can set the congestion window to the
   determined value so that it can immediately start to send with the
   approved data rate.

   However, [RFC1323] defines that the "Window field in a SYN (i.e., a
   <SYN> or <SYN,ACK>) segment itself is never scaled."  This means that
   the maximum receive window that can be signaled to the sender in the

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   <SYN,ACK> is 64 KB.  As a consequence, the TCP flow control will
   prevent the TCP sender from having more than 64 KB of outstanding
   data, even if the receiver has much more free buffer, and the Quick-
   Start feedback allows a much larger congestion window.

   This effect essentially limits the maximum amount of data sent by
   Quick-Start to 64 KB, when the sender sends the Quick-Start request
   in the initial <SYN> segment.  Also, the congestion window after
   quiting the Quick-Start rate pacing phase is at most 64 KB, as the
   congestion window is set to the amount of data that has actually been
   sent during the rate pacing phase.  This is an undesirable
   restriction for the Quick-Start mechanism, even if 64 KB is still
   much more than the initial congestion window in slow-start that is
   allowed by [RFC3390].

   This issue only occurs when Quick-Start is used in the three-way TCP
   connection setup procedure, and only in the direction of the client
   (connection originator) to the server.  Still, this case is one of
   the planned usage scenarios for the Quick-Start TCP extension.

4.3.  Proposed Solution

   The limitation imposed by the window scaling could be addressed in
   different ways.  This document proposes the following solution: If
   necessary, the TCP host SHOULD send a scaled receive window in a
   separate <ACK> packet following the <SYN,ACK> packet.

   This means that when a host receives a <SYN> segment with a Quick-
   Start option, it processes the option as described in [RFC4782].
   Provided that the host has Quick-Start support enabled, the Quick-
   Start response is echoed back in the <SYN,ACK> segment.  As
   explained, this segment cannot announce receive windows larger than
   64 KB.  If the receiver allocates a buffer space larger than 64 KB,
   an additional empty segment (without <SYN> flag) SHOULD be sent after
   the <SYN,ACK> segment, in order to announce the true receive window.
   The resulting message flow is depicted in Figure 1.

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      Sender       Routers (approving QS request)        Receiver
      ------       -------                               --------
        |                                                  |
        | ------------------------------------------------>|
        |  QS request                                      |
        |  TCP <SYN>, unscaled receive window              |
        |      window scaling and other options            |
        |                                                  |
        | <------------------------------------------------|
        |  QS response                                     |
        |  TCP <SYN,ACK>, unscaled receive window          |
        |      window scaling and other options            |
        |                                                  |
        | <------------------------------------------------|
        |  Additional acknowledgment                       |
        |  TCP <ACK>, scaled receive window                |
        |                                                  |
        | ------------------------------------------------>|
        |  QS report                                       |
        |  TCP <ACK>                                       |
        |                                                  |
        | ================================================>|
        | ================================================>|
        |  Rate paced data transfer                        |
        |                                                  |
        | <------------------------------------------------|
        |  First new acknowledgment                        |
        V                                                  V

        Figure 1: Message sequence chart of the proposed mechanism

   After having received this additional acknowledgment, the sender is
   aware of the true available receive buffer.  Provided that the Quick-
   Start request is approved on the path and that the receive window is
   sufficiently large, this allows the sender to send more than 64 KB
   during the Quick-Start rate pacing phase.

   We note that there is some degree of freedom as to when to send the
   additional acknowledgment.  The straightforward solution is to send
   it immediately after the <SYN,ACK> segment.  But this is not
   required: It is sufficient if the sender receives this segment before
   reaching the limit of the unscaled receive window.  As a consequence,
   receivers could also delay the sending of this segment for some small
   amount of time.

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4.4.  Discussion and Deployment Considerations

   The method proposed in this document is compliant with the TCP
   specifications: Sending empty segments to increase the receive window
   is implicitly allowed by [RFC0793], and in [RFC2581] it is clearly
   stated that sending an acknowledgment is allowed to update the
   receive window.  For standard-compliant TCP stacks, implementing the
   method thus should require changes in the receiver TCP implementation

   However, sending an empty acknowledgment shortly after a <SYN,ACK>
   segment is an atypical TCP communication event.  The <SYN,ACK> and
   the additional segment could get reordered in the network.  In this
   case, the sending host will typically ignore the additional segment,
   as it is still awaiting the <SYN,ACK>.  Furthermore, middleboxes such
   as state-full firewalls might drop the additional acknowledgment.
   Even worse, this segment might also be dropped if a middlebox
   receives it earlier than the <ACK> segment from the sender.  At this
   point in time, from the viewpoint of the middlebox, the bi-
   directional end-to-end TCP connection is not yet established.  If the
   additional segment gets dropped, the sender gets informed about the
   unscaled receive window when the next new acknowledgment arrives,
   which may limit the benefit of Quick-Start.  Delaying the additional
   acknowledgment for a short period of time could help to avoid such
   problems.  Further investigation is needed to analyze whether such a
   delay is required.

   A possible alternative to the message flow in Figure 1 would be to
   piggyback the Quick-Start response on the additional acknowledgment
   segment instead of the <SYN,ACK>.  However, this approach has several
   drawbacks and is therefore not recommended: First, the Quick-Start
   response would be received later, which could cause additional
   delays.  Second, the <SYN,ACK> is immediately acknowledged by the
   <ACK> segment.  The Quick-Start rate report can thus be piggybacked
   on this <ACK>.  In contrast, if the Quick-Start response is included
   in the additional acknowledgment, the Quick-Start report has to be
   piggybacked to a data segment, i. e., it depends on the availability
   of application data whether and when the Quick-Start report is sent.

   The additional segment mandated by this document results in a network
   overhead of one segment.  In many potential usage scenarios this
   overhead will be small compared to the network load caused by the
   acknowledgments of a starting high-speed Quick-Start data transfer.

   Instead of sending one additional acknowledgment, a host could also
   send a small number of copies in order to improve robustness.  This
   could help to reduce the risk of reordering with the <SYN,ACK>
   segment.  However, given the additional overhead, it is recommended

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   to send only one acknowlegdment unless there are indications that the
   path suffers from frequent packet reordering.

5.  Security Considerations

   Quick-Start TCP imposes a number of security challenges.  Known
   security threats as well as counter-measures are discussed in the
   section "Security Considerations" of [RFC4782].  Since this document
   describes extensions to Quick-Start TCP, the security issues and
   solutions identified in [RFC4782] apply here, too.

   If a host allocates large amounts of buffer space during the three-
   way handshake, this could increase the vulnerability to "syn
   flooding" attacks: An attacker sending many Quick-Start requests
   could try to allocate much buffer space at a host, which is then not
   available any more for other TCP connections.  If most involved
   routers support Quick-Start, this type of attack is difficult to
   realize, since the routers may reject many requests before they reach
   a host.  However, an attack could be possible if some routers on the
   path do not support Quick-Start.  A simple countermeasure would be to
   set an upper limit on the total amount of buffer space granted to
   connections with Quick-Start, and possibly to deny requests if they
   arrive at a host with too high a frequency.  The main impact of this
   abuse is that Quick-Start may be rendered useless for other
   connections.  This can result in some performance degradation,
   because the default slow-start must be used instead.  In general, it
   is an inherent weak point of Quick-Start that one can send much more
   requests than required, which temporarily can block resources for
   other earnest Quick-Start requests [RFC4782].

   It is an allowed behavior for a TCP connection endpoint to send an
   additional acknowledgment segment in order to update the receive
   window.  The usage of the proposed mechanism causes some limited
   network overhead, but it does not result in additional security

6.  IANA Considerations

   This document has no actions for IANA.

7.  Acknowledgments

   Special thanks to Haiko Strotbek, Martin Koehn, Simon Hauger,
   Christian Mueller, and Gorry Fairhurst for suggestions and comments.

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8.  References

8.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC1323]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, May 1992.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2581]  Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
              Control", RFC 2581, April 1999.

   [RFC3390]  Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
              Initial Window", RFC 3390, October 2002.

   [RFC4782]  Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick-
              Start for TCP and IP", RFC 4782, January 2007.

8.2.  Informative References

   [Dun06]    Dunigan, T., "TCP auto-tuning zoo", available
              February 2006.

   [FPK07]    Falk, A., Pryadkin, Y., and D. Katabi, "Specification for
              the Explicit Control Protocol (XCP)", Internet Draft, work
              in progress, June 2007.

   [LAJ+07]   Liu, D., Allman, M., Jin, S., and L. Wang, "Congestion
              Control Without a Startup Phase", Proc. PFLDnet2007,
              February 2007.

   [RFC2488]  Allman, M., Glover, D., and L. Sanchez, "Enhancing TCP
              Over Satellite Channels using Standard Mechanisms",
              BCP 28, RFC 2488, January 1999.

   [RFC3481]  Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A., and
              F. Khafizov, "TCP over Second (2.5G) and Third (3G)
              Generation Wireless Networks", BCP 71, RFC 3481,
              February 2003.

   [SAF07]    Sarolahti, P., Allman, M., and S. Floyd, "Determining an
              Appropriate Sending Rate Over an Underutilized Network
              Path", Computer Networks, vol. 51, no. 7, 2007.

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   [SB05]     Smith, M. and S. Bishop, "Flow Control in the Linux
              Network Stack", available
              February 2005.

Appendix A.  Applicability to Other Proposals

   Besides Quick-Start, there are some other related proposals for
   behavior more aggressive than the standard slow-start.  A
   comprehensive survey of this related work can be found in [RFC4782].
   For instance, the Explicit Control Protocol (XCP) [FPK07] proposes a
   new congestion control based on explicit router feedback.
   Furthermore, there are discussions in the research community whether
   a host could start to send with an arbitrarily high data rate,
   combined with a conservative reaction in case of congestion [LAJ+07].

   Basically, the effects discussed in this document are not specific to
   Quick-Start.  An interaction with the TCP flow control could also
   occur with other congestion control mechanisms that avoid the
   standard TCP slow-start.  Receive buffer dimensioning will be a non-
   trivial task in all these cases.  The amount of information that a
   receiver can gain during a connection setup procedure differs from
   proposal to proposal.  However, the basic guideline to use a larger
   inital receive buffer allocation applies to all proposals similar to

   If the TCP header semantics apply, the interaction with receive
   window scaling mechanism could also be a problem for other
   approaches.  In this case, the workaround of sending an additional
   acknowledgment can be helpful, too.

Appendix B.  Alternative Solutions

   The limitation imposed by the window scaling could be addressed in
   several ways.  This document proposes to send an additional
   acknowledgment to announce the true receive window, if needed.  This
   method is compliant with the current TCP standards.

   Alternatively, one could circumvent [RFC1323] in several ways.  For
   instance, one could use a scaled receive window in <SYN> and
   <SYN,ACK> segments, if they include Quick-Start options.  The usage
   of a scaled window could also be indicated by some other means (e.
   g., a new TCP option).  Still, such alternative solutions would
   require changes in the TCP header semantics and might cause
   interworking problems with currently deployed TCP implementations.

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Authors' Addresses

   Michael Scharf
   University of Stuttgart
   Pfaffenwaldring 47
   D-70569 Stuttgart

   Phone: +49 711 685 69006

   Sally Floyd
   ICIR (ICSI Center for Internet Research)

   Phone: +1 (510) 666-2989

   Pasi Sarolahti
   Nokia Research Center
   P.O. Box 407

   Phone: +358 50 4876607

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Full Copyright Statement

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