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Versions: 00 01 02 03 04 05 06 07 RFC 4782

Internet Engineering Task Force                                 S. Floyd
INTERNET-DRAFT                                                 M. Allman
draft-ietf-tsvwg-quickstart-07.txt                                  ICIR
Expires: April 2007                                              A. Jain
                                                             F5 Networks
                                                            P. Sarolahti
                                                   Nokia Research Center
                                                          9 October 2006



                       Quick-Start for TCP and IP


Status of this Memo

    By submitting this Internet-Draft, each author represents that any
    applicable patent or other IPR claims of which he or she is aware
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    This Internet-Draft will expire on April 2007.

Abstract

    This document specifies an optional Quick-Start mechanism for
    transport protocols, in cooperation with routers, to determine an
    allowed sending rate at the start and at times in the middle of a
    data transfer (e.g., after an idle period).  While Quick-Start is



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    designed to be used by a range of transport protocols, in this
    document we only specify its use with TCP.  Quick-Start is designed
    to allow connections to use higher sending rates when there is
    significant unused bandwidth along the path and the sender and all
    of the routers along the path approve the Quick-Start Request.

    This document describes many paths where Quick-Start requests would
    not be approved.  These paths include all paths containing routers,
    IP tunnels, MPLS paths, and the like that do not support Quick-
    Start.  These paths also include paths with routers or middleboxes
    that drop packets containing IP options.  Quick-Start requests could
    be difficult to approve over paths that include multi-access layer-
    two networks.  This document also describes environments where the
    Quick-Start process could fail with false positives, with the sender
    incorrectly assuming that the Quick-Start request had been approved
    by all of the routers along the path.  As a result of these
    concerns, and as a result of the difficulties and seeming absence of
    motivation for routers such as core routers to deploy Quick-Start,
    Quick-Start is being proposed as a mechanism that could be of use in
    controlled environments, and not as a mechanism that would be
    intended or appropriate for ubiquitous deployment in the global
    Internet.





























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    TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

     Changes from draft-ietf-tsvwg-quickstart-06:

     * Changes in reponse to the review from the
       General Area Review Team:
       - Added text to Overview about the role of TCP feedback.
       - Updated the flow label discussion from RFC2460 to RFC3697.
       - Instead of saying that the router SHOULD remove the QS Option
         when denying a request, but MAY zero fields instead,
         said that the router SHOULD either remove the QS option
         or zero the fields.
       - Fixed typos and clarified some text.

     * Still needs feedback from the ipv6 or v6ops community;
       - perhaps also have IPv6 people read the discussion of
         end-point address changes in Section 4.1.

     Changes from draft-ietf-tsvwg-quickstart-05:

     * Minor editing in response to AD feedback from
       Lars Eggert.
       This includes changing one "should" to "SHOULD",
       and changing formatting of the IANA Considerations
       section.

     * Clarifying in the Introduction that the QS router
       does not give preferential treatment to QS packets.
       In response to email from Fil Dickinson.

     * Added a discussion of interactions between
       Quick-Start and draft-ietf-pmtud-method.  In
       response to AD Feedback from Lars Eggert.

     * Deleted Appendix F on "Feedback from Bob Briscoe".
       From AD feedback about deleting unnecessary
       appendices.

     * Added a paragraph to the Introduction about which
       sections contain normative references, and which
       sections are general discussion.  From AD feedback.

     * Added a discussion about congestion control for
       TCP's reverse-path traffic.  From feedback from
       Mitchell Erblich.

     Changes from draft-ietf-tsvwg-quickstart-04:




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     * Reformatted references so that "[RFC2581, RFC3390]"
       is instead "([RFC2581], [RFC3390])", to eliminate
       bug reports from the idnits tool.  From feedback
       from Dan Romascanu.

    * Rephrased beginning of second paragraph in the
      Abstract.  From feedback from James Polk.

     Changes from draft-ietf-tsvwg-quickstart-03:

     * Added a discussion of the lower limit of the rate request
       of 80 kbps, from feedback from Gorry Fairhurst.

     * Added the QS Nonce to the QS Approved Rate.
       From feedback from Gorry Fairhurst.

     * Moved the Related Work section to the appendix.
       From feedback from Gorry Fairhurst.

     Changes from draft-ietf-tsvwg-quickstart-02:

     * Some general editing.

     * Said that if the receiver receives a Quick-Start Request
       with a rate of zero, then the receiver SHOULD NOT send
       a Quick-Start response.  And that if the sender
       receives an acknowledgement of its packet with no
       Quick-Start response, then the sender MUST assume that the
       request was denied, and send a Report of Approved Rate
       with a rate of zero.

     * Said that if a Quick-Start packet is dropped or marked,
       the sender should not make more Quick-Start requests in this
       connection.

     * Said that the Quick-Start Request SHOULD be sent on a packet
       that requires an acknowledgement, e.g., a SYN, SYN/ACK, or data
       packet.

     * Made changes to the section on "TCP: A Quick-Start Request in the
       Middle of a Connection".

     * Added that if the TCP host is going to use the successful
       Quick-Start Request, it MUST start using it within one
       round-trip time of receiving the Quick-Start Response,
       or within three seconds, whichever is smaller.

     * Added a stronger applicability statement, in the abstract



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       and in Section 10 on "Implementation and Deployment Issues".
       From feedback from the working group.

     * Added a section about MPLS.  From feedback from Mitchell
       Erblichs.

     * Strengthened the language of the difficulties presented by
       multi-access links.

     * Added a discussion in Section 10.3 about the deployment of
       Quick-Start on single-hop paths.  From feedback from
       Mitchell Erblichs.

     * Clarified that the "router" function of approving
       Quick-Start requests includes the IP-layer processing
       at the sender.

     * Clarified in Section 3.3 on "Processing the Quick-Start
       Request at Routers" that this document standardizes only
       the semantics of Quick-Start, and not the specific
       algorithms for processing Quick-Start requests at routers.

     * Clarified in Section 3.3 on "Processing the Quick-Start
       Request at Routers" that a router will have to consider
       the previous Quick-Start requests in approving a new one.

     * In Section 9.3 on "Quick-Start with QoS-enabled Traffic",
       which says that routers are free to take into account
       the diff-serv codepoint in considering QS requests, clarified
       that routers are also free to take into account their own
       understanding of priorities.

     * Added the Temporary bit to Appendix on "Possible Additional
       Uses for the Quick-Start Option".  Clarified that the Quick-Start
       mechanism is not designed to help routers achieve full link
       utilization.

     * Editing from feedback from Gorry Fairhurst.

     Changes from draft-ietf-tsvwg-quickstart-01:

     * Added a citation to SPAND: Speeding Up Short Data Transfers.
     * Added a sentence in Section 8.1 on "Implementation Issues for
       Processing Quick-Start Requests" about multi-access links.
     * Mentioned the IP Router Alert option, RFC 2113, in Appendix.
     * Added a discussion of lower-than-best-effort service.
     * Added a few sentences about the requirements for
       randomness in the nonce.



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     * Changed the name of the option from the Quick-Start Request
       Option to the Quick-Start Option.
     * Changed the semantics of the Reserved field to the Function
       field, adding that a Quick-Start option is only interpreted
       as a request if this field is zero.
     * Changed the "Reporting Approved Rate" option from a
       "Possible Use" in Appendix to a required use in Section 3.1,
       to allow routers and receivers some protection against
       misbehaving senders.
     * Changes from feedback from Bob Briscoe:
       - Added Appendix about Sections 1-3 of
         Bob Briscoe's document.
       - Added a clarification that the approval of a
         Quick-Start request at a router does not affect
         the treatment of the subsequent arriving
         Quick-Start data packets.
       - Added the one-way hash function as an alternate
         implementation for the QS Nonce.
       - Clarified the phrase "incrementally deployable", adding
         the following:
         "We note that while Quick-Start is incrementally deployable
         in this sense, a Quick-Start request cannot be approved
         for a particular connection unless both end-nodes and all
         of the routers along the path have been configured to
         support Quick-Start."
       - Clarified semantics about additional rate.
       - Said that when denying a rate request, the router
         may in the future use the QS Nonce field to report
         an error code.
       - Add Bob's suggestion from Section 4.4 as an alternate
         possible rate encoding.
       - Made changes suggested in Section 5.1.3 of Bob's paper,
         including saying that the router should decrement the QS TTL
         by the same amount that it decrements the IP TTL (on the
         off chance that it decrements the IP TTL by more than one).
       - Fixed nits.

     Changes from draft-ietf-tsvwg-quickstart-00:
     * Added a 30-bit QS Nonce.  Based on feedback from Guohan Lu
       and Gorry Fairhurst (and deleted the text about a possible
       four-bit QS nonce).
     * Added a new section "Quick-Start and IPsec AH", based on feedback
         from Joe Touch and David Black.
     * Revised "Quick-Start in IP Tunnels" Section, based on feedback
       from Joe Touch and David Black.
     * Added a section about "Possible Uses for the Reserved Fields".
     * Changes from feedback from Gorry Fairhurst:
       - Section 4.4, revised the explanation for reducing the



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         congestion window when the first ACK for a Quick-Start
         packet is received.
       - Section 6.4, deleted the last sentence.
       - Minor editing changes.
       - Revised Section 4.6.2 to say that sender SHOULD send one packet
         with an initial RTO of three seconds.
       - Revised Section 4.6.3 to say that the TCP sender SHOULD use an
         initial RTO setting of three seconds.
       - Added text to Section 6.2 on Multiple Paths, discussing
           multipath routing.
       - Clarified about GPRS round-trip times.
       - Clarified about PMTUD and the first window of data.
       - A small reorganization, rearranging sections.
     * Changes from feedback from Martin Duke:
       - Revised text about the size of QS requests.
       - Added some text to Section 4.1, about when to send QS requests.

     Changes from draft-amit-quick-start-04.txt:
     * A significant amount of general editing.
     * Because the Rate Request field only uses four bits, specified
       that the other four bits are reserved, and talked about a
       possible use for them.  This is discussed in a new section on
       "A Rate-Reduced Nonce?"
     * Specified that a Quick-Start-capable router denying a request
       SHOULD delete the Quick-Start option, and if this is not
       possible, SHOULD zero the QS TTL and the Rate Request fields.
     * Made the following change:  If the Quick-Start Response is lost
       in the network, it is not retransmitted.
     * For PMTUD, in Section 4.6, added a suggestion to send one large
       packet in the initial window for PMTUD, and to send the other
       packets at 576 bytes.
     * Added a paragraph to Section 4.6.3 on retransmitted SYN packets,
       saying they should use an RTO of three seconds and a new ISN
       on the retransmitted SYN packet.
     * Added that "TCP SHOULD NOT use Quick-Start" after an
       application-limited period at this time, in Section 4.1, in
       addition to the old sentence that this "requires further thought
       and investigation".
     * Added an appendix on "Possible Router Algorithm".
     * Moved the section on "Quick-Start with DCCP" to the appendix.
     * Name changed from draft-amit-quick-start-04.txt to
       draft-tsvwg-quickstart-00.txt.

     Changes from draft-amit-quick-start-03.txt:
     * Added a citation to the paper on "Evaluating Quick-Start for
       TCP", and added pointers to the work in that paper.
       This work includes:
       - Discussions of router algorithms.



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       - Discussions of sizing Quick-Start requests.
     * Added sections on "Misbehaving Middleboxes", and on "Attacks on
       Quick-Start".

     Changes from draft-amit-quick-start-02.txt:
     * Added a discussion on Using Quick-Start in the Middle of a
       Connection.  The request would be on the total rate,
       not on the additional rate.
     * Changed name "Initial Rate" to "Rate Request", and changed
       the units from packets per second to bytes per second.
     * The following sections are new:
       - The Quick-Start Request Option for IPv6
       - Quick-Start in IP Tunnels
       - When to Use Quick-Start
       - TCP: Responding to a Loss of a Quick-Start Packet
       - TCP: A Quick-Start Request for a Larger Initial Window
       - TCP: A Quick-Start Request after an Idle Period
       - The Quick-Start Mechanisms in DCCP and other Transport
         Protocols
       - Quick-Start with DCCP
       - Implementation and Deployment Issues
       - Design Decisions
     * Added a discussion of Kunniyur's Anti-ECN proposal.
     * Added a section on simulations, with a brief discussion of the
       simulations by Srikanth Sundarrajan.

     Changes from draft-amit-quick-start-01.txt:
     * Added a discussion in the related work section about the
       possibility of optimistically sending a large initial window,
       without explicit permission of routers.
     * Added a discussion in the related work section about the
       tradeoffs of XCP vs. Quick-Start.
     * Added a section on "The Quick-Start Request: Packets or Bytes?"

     Changes from draft-amit-quick-start-00.txt:
     * The addition of a citation to [KHR02].
     * The addition of a Related Work section.
     * Deleted the QS Nonce, in favor of a random initial value for the
       QS TTL.












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                             Table of Contents

    1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  12
       1.1. Conventions and Terminology. . . . . . . . . . . . . . .  14
    2. Assumptions and General Principles. . . . . . . . . . . . . .  14
       2.1. Overview of Quick-Start. . . . . . . . . . . . . . . . .  15
    3. The Quick-Start Option in IP. . . . . . . . . . . . . . . . .  17
       3.1. The Quick-Start Option for IPv4. . . . . . . . . . . . .  17
       3.2. The Quick-Start Option for IPv6. . . . . . . . . . . . .  21
       3.3. Processing the Quick-Start Request at
       Routers . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
          3.3.1. Processing the Report of Approved
          Rate . . . . . . . . . . . . . . . . . . . . . . . . . . .  23
       3.4. The QS Nonce . . . . . . . . . . . . . . . . . . . . . .  24
    4. The Quick-Start Mechanisms in TCP . . . . . . . . . . . . . .  26
       4.1. Sending the Quick-Start Request. . . . . . . . . . . . .  27
       4.2. The Quick-Start Response Option in the TCP
       header. . . . . . . . . . . . . . . . . . . . . . . . . . . .  28
       4.3. TCP: Sending the Quick-Start Response. . . . . . . . . .  29
       4.4. TCP: Receiving and Using the Quick-Start
       Response Packet . . . . . . . . . . . . . . . . . . . . . . .  30
       4.5. TCP: Controlling Acknowledgement Traffic on
       the Reverse Path  . . . . . . . . . . . . . . . . . . . . . .  32
       4.6. TCP: Responding to a Loss of a Quick-Start
       Packet. . . . . . . . . . . . . . . . . . . . . . . . . . . .  34
       4.7. TCP: A Quick-Start Request for a Larger Ini-
       tial Window . . . . . . . . . . . . . . . . . . . . . . . . .  34
          4.7.1. Interactions with Path MTU Discovery. . . . . . . .  34
          4.7.2. Quick-Start Request Packets that are
          Discarded by Routers or Middleboxes. . . . . . . . . . . .  35
       4.8. TCP: A Quick-Start Request in the Middle of
       a Connection. . . . . . . . . . . . . . . . . . . . . . . . .  36
       4.9. An Example Quick-Start Scenario with TCP . . . . . . . .  37
    5. Quick-Start and IPsec AH. . . . . . . . . . . . . . . . . . .  38
    6. Quick-Start in IP Tunnels and MPLS. . . . . . . . . . . . . .  39
       6.1. Simple Tunnels That Are Compatible with
       Quick-Start . . . . . . . . . . . . . . . . . . . . . . . . .  41
          6.1.1. Simple Tunnels that are Aware of Quick-
          Start. . . . . . . . . . . . . . . . . . . . . . . . . . .  41
       6.2. Simple Tunnels That Are Not Compatible with
       Quick-Start . . . . . . . . . . . . . . . . . . . . . . . . .  42
       6.3. Tunnels That Support Quick-Start . . . . . . . . . . . .  43
       6.4. Quick-Start and MPLS . . . . . . . . . . . . . . . . . .  44
    7. The Quick-Start Mechanism in other Transport Pro-
    tocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  44
    8. Using Quick-Start . . . . . . . . . . . . . . . . . . . . . .  45
       8.1. Determining the Rate to Request. . . . . . . . . . . . .  45
       8.2. Deciding the Permitted Rate Request at a



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       Router. . . . . . . . . . . . . . . . . . . . . . . . . . . .  45
    9. Evaluation of Quick-Start . . . . . . . . . . . . . . . . . .  46
       9.1. Benefits of Quick-Start. . . . . . . . . . . . . . . . .  46
       9.2. Costs of Quick-Start . . . . . . . . . . . . . . . . . .  47
       9.3. Quick-Start with QoS-enabled Traffic . . . . . . . . . .  49
       9.4. Protection against Misbehaving Nodes . . . . . . . . . .  49
          9.4.1. Misbehaving Senders . . . . . . . . . . . . . . . .  49
          9.4.2. Receivers Lying about Whether the
          Request was Approved . . . . . . . . . . . . . . . . . . .  51
          9.4.3. Receivers Lying about the Approved
          Rate . . . . . . . . . . . . . . . . . . . . . . . . . . .  52
          9.4.4. Collusion between Misbehaving Routers . . . . . . .  53
       9.5. Misbehaving Middleboxes and the IP TTL . . . . . . . . .  54
       9.6. Attacks on Quick-Start . . . . . . . . . . . . . . . . .  54
       9.7. Simulations with Quick-Start . . . . . . . . . . . . . .  55
    10. Implementation and Deployment Issues . . . . . . . . . . . .  55
       10.1. Implementation Issues for Sending Quick-
       Start Requests. . . . . . . . . . . . . . . . . . . . . . . .  56
       10.2. Implementation Issues for Processing Quick-
       Start Requests. . . . . . . . . . . . . . . . . . . . . . . .  56
       10.3. Possible Deployment Scenarios . . . . . . . . . . . . .  57
       10.4. A Comparison with the Deployment Problems
       of ECN. . . . . . . . . . . . . . . . . . . . . . . . . . . .  58
    11. Security Considerations. . . . . . . . . . . . . . . . . . .  58
    12. IANA Considerations. . . . . . . . . . . . . . . . . . . . .  60
       12.1. IP Option . . . . . . . . . . . . . . . . . . . . . . .  60
       12.2. TCP Option. . . . . . . . . . . . . . . . . . . . . . .  60
    13. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . .  61
    14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  61
    A. Related Work. . . . . . . . . . . . . . . . . . . . . . . . .  62
       A.1. Fast Start-ups without Explicit Information
       from Routers. . . . . . . . . . . . . . . . . . . . . . . . .  62
       A.2. Optimistic Sending without Explicit Informa-
       tion from Routers . . . . . . . . . . . . . . . . . . . . . .  64
       A.3. Fast Start-ups with other Information from
       Routers . . . . . . . . . . . . . . . . . . . . . . . . . . .  65
       A.4. Fast Start-ups with more Fine-Grained Feed-
       back from Routers . . . . . . . . . . . . . . . . . . . . . .  65
       A.5. Fast Start-ups with Lower-Than-Best-Effort
       Service . . . . . . . . . . . . . . . . . . . . . . . . . . .  66
    B. Design Decisions. . . . . . . . . . . . . . . . . . . . . . .  66
       B.1. Alternate Mechanisms for the Quick-Start
       Request: ICMP and RSVP. . . . . . . . . . . . . . . . . . . .  67
          B.1.1. ICMP. . . . . . . . . . . . . . . . . . . . . . . .  67
          B.1.2. RSVP. . . . . . . . . . . . . . . . . . . . . . . .  68
       B.2. Alternate Encoding Functions . . . . . . . . . . . . . .  69
       B.3. The Quick-Start Request: Packets or Bytes? . . . . . . .  71
       B.4. Quick-Start Semantics: Total Rate or Addi-



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       tional Rate?. . . . . . . . . . . . . . . . . . . . . . . . .  72
       B.5. Alternate Responses to the Loss of a Quick-
       Start Packet. . . . . . . . . . . . . . . . . . . . . . . . .  73
       B.6. Why Not Include More Functionality?. . . . . . . . . . .  74
       B.7. Alternate Implementations for a Quick-Start
       Nonce . . . . . . . . . . . . . . . . . . . . . . . . . . . .  77
          B.7.1. An Alternate Proposal for the Quick-
          Start Nonce. . . . . . . . . . . . . . . . . . . . . . . .  77
          B.7.2. The Earlier Request-Approved Quick-
          Start Nonce. . . . . . . . . . . . . . . . . . . . . . . .  78
    C. Quick-Start with DCCP . . . . . . . . . . . . . . . . . . . .  79
    D. Possible Router Algorithm . . . . . . . . . . . . . . . . . .  80
    E. Possible Additional Uses for the Quick-Start
    Option . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  82
    Normative References . . . . . . . . . . . . . . . . . . . . . .  83
    Informative References . . . . . . . . . . . . . . . . . . . . .  84
    AUTHORS' ADDRESSES . . . . . . . . . . . . . . . . . . . . . . .  89
    Full Copyright Statement . . . . . . . . . . . . . . . . . . . .  90
    Intellectual Property. . . . . . . . . . . . . . . . . . . . . .  90
































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1.  Introduction

    Each connection begins with a question: "What is the appropriate
    sending rate for the current network path?"  The question is not
    answered explicitly, but each TCP connection determines the sending
    rate by probing the network path and altering the congestion window
    (cwnd) based on perceived congestion.  Each TCP connection starts
    with a pre-configured initial congestion window (ICW).  Currently,
    TCP allows an initial window of between one and four MSS-sized
    segments ([RFC2581], [RFC3390]).  The TCP connection then probes the
    network for available bandwidth using the slow-start procedure
    ([Jac88], [RFC2581]), doubling cwnd during each congestion-free
    round-trip time (RTT).

    The slow-start algorithm can be time-consuming --- especially over
    networks with large bandwidth or long delays.  It may take a number
    of RTTs in slow-start before the TCP connection begins to fully use
    the available bandwidth of the network.  For instance, it takes
    log_2(N) - 2 round-trip times to build cwnd up to N segments,
    assuming an initial congestion window of 4 segments.  This time in
    slow-start is not a problem for large file transfers, where the
    slow-start stage is only a fraction of the total transfer time.
    However, in the case of moderate-sized transfers the connection
    might carry out its entire transfer in the slow-start phase, taking
    many round-trip times, where one or two RTTs might have been
    sufficient when using the currently available bandwidth along the
    path.

    A fair amount of work has already been done to address the issue of
    choosing the initial congestion window for TCP, with RFC 3390
    allowing an initial window of up to four segments based on the MSS
    used by the connection [RFC3390].  Our underlying premise is that
    explicit feedback from all of the routers along the path would be
    required, in the current architecture, for best-effort connections
    to use initial windows significantly larger than those allowed by
    [RFC3390], in the absence of other information about the path.

    In using Quick-Start, a TCP host, say, host A, would indicate its
    desired sending rate in bytes per second, using a Quick-Start option
    in the IP header of a TCP packet.  Each router along the path could,
    in turn, either approve the requested rate, reduce the requested
    rate, or indicate that the Quick-Start request is not approved.  (We
    note that the `routers' referred to in this document also include
    the IP-layer processing of the Quick-Start request at the sender.)
    In approving a Quick-Start request, a router does not give
    preferential treatment to subsequent packets from that connection;
    the router is only asserting that it is currently underutilized and
    believes there is sufficient available bandwidth to accommodate the



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    sender's requested rate.  The Quick-Start mechanism can determine if
    there are routers along the path that do not understand the Quick-
    Start option, or have not agreed to the Quick-Start rate request.
    TCP host B communicates the final rate request to TCP host A in a
    transport-level Quick-Start Response in an answering TCP packet.

    If the Quick-Start request is approved by all routers along the
    path, then the TCP host can send at up to the approved rate for a
    window of data.  Subsequent transmissions will be governed by the
    default TCP congestion control mechanisms of that connection.  If
    the Quick-Start request is not approved, then the sender would use
    the default congestion control mechanisms.

    Quick-Start would not be the first mechanism for explicit
    communication from routers to transport protocols about sending
    rates.  Explicit Congestion Notification (ECN) gives explicit
    congestion control feedback from routers to transport protocols,
    based on the router detecting congestion before buffer overflow
    [RFC3168].  In contrast, routers would not use Quick-Start to give
    congestion information, but instead would use Quick-Start as an
    optional mechanism to give permission to transport protocols to use
    higher sending rates, based on the ability of all the routers along
    the path to determine if their respective output links are
    significantly underutilized.

    Section 2 gives an overview of Quick-Start.  The formal
    specifications for Quick-Start are contained in Sections 3, 4,
    6.1.1, and 6.3.  In particular, Quick-Start is specified for IPv4
    and for IPv6 in Section 3, and is specified for TCP in Section 4.
    Section 6 consists mostly of a non-normative discussion of
    interactions of Quick-Start with IP tunnels and MPLS; however,
    Section 6.1.1 and 6.3 specify behavior for IP tunnels that are aware
    of Quick-Start.

    The rest of the document is non-normative, as follows.  Section 5
    shows that Quick-Start is compatible with IPsec AH (Authentication
    Header).  Section 7 gives a non-normative set of guidelines for
    specifying Quick-Start in other transport protocols, and Section 8
    discusses using Quick-Start in transport end-nodes and in routers.
    Section 9 gives an evaluation of the costs and benefits of Quick-
    Start, and Section 10 discusses implementation and deployment
    issues.  The appendices discuss related work, Quick-Start design
    decisions, and possible router algorithms.








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1.1.  Conventions and 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 in [RFC2119].


2.  Assumptions and General Principles

    This section describes the assumptions and general principles behind
    the design of the Quick-Start mechanism.

    Assumptions:

    * The data transfer in the two directions of a connection traverses
    different queues, and possibly even different routers.  Thus, any
    mechanism for determining the allowed sending rate would have to be
    used independently for each direction.

    * The path between the two endpoints is relatively stable, such that
    the path used by the Quick-Start request is generally the same path
    used by the Quick-Start packets one round-trip time later.  [ZDPS01]
    shows this assumption should be generally valid.  However, [RFC3819]
    discusses a range of Bandwidth on Demand subnets that could cause
    the characteristics of the path to change over time.

    * Any new mechanism must be incrementally deployable, and might not
    be supported by all of the routers and/or end-hosts.  Thus, any new
    mechanism must be able to accommodate non-supporting routers or end-
    hosts without disturbing the current Internet semantics.  We note
    that while Quick-Start is incrementally deployable in this sense, a
    Quick-Start request cannot be approved for a particular connection
    unless both end-nodes and all of the routers along the path have
    been configured to support Quick-Start.

    General Principles:

    * Our underlying premise is that explicit feedback from all of the
    routers along the path would be required, in the current
    architecture, for best-effort connections to use initial windows
    significantly larger than those allowed by [RFC3390], in the absence
    of other information about the path.

    * A router should only approve a Quick-Start request if the output
    link is underutilized.  Any other approach will result in either
    per-flow state at the router, or the possibility of a (possibly
    transient) queue at the router.




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    * No per-flow state should be required at the router.  Note that
    while per-flow state is not required, we also do not preclude a
    router from storing per-flow state for making Quick-Start decisions
    or for checking for misbehaving nodes.


2.1.  Overview of Quick-Start

    In this section we give an overview of the use of Quick-Start with
    TCP to request a higher congestion window.  The description in this
    section is non-normative; the normative description of Quick-Start
    with IP and TCP follows in Sections 3 and 4.  Quick-Start could be
    used in the middle of a connection, e.g., after an idle or
    underutilized period, as well as for the initial sending rate; these
    uses of Quick-Start are discussed later in the document.

    Quick-Start requires end-points and routers to work together, with
    end-points requesting a higher sending rate in the Quick-Start (QS)
    option in IP, and routers along the path approving, modifying,
    discarding or ignoring (and therefore disallowing) the Quick-Start
    Request.  The receiver uses reliable, transport-level mechanisms to
    inform the sender of the status of the Quick-Start Request.  For
    example, when TCP is used, the TCP receiver sends feedback to the
    sender using a Quick-Start Response option in the TCP header.  In
    addition, Quick-Start assumes a unicast, congestion-controlled
    transport protocol; we do not consider the use of Quick-Start for
    multicast traffic.

    When sent as a request, the Quick-Start Option includes a request
    for a sending rate in bits per second, and a Quick-Start TTL (QS
    TTL) to be decremented by every router along the path that
    understands the option and approves the request.  The Quick-Start
    TTL is initialized by the sender to a random value.  The transport
    receiver returns the rate, information about the TTL and the Quick-
    Start Nonce to the sender using transport-level mechanisms; for TCP,
    the receiver sends this information in the Quick-Start Response in
    the TCP header.  In particular, the receiver computes the difference
    between the Quick-Start TTL and the IP TTL (the TTL in the IP
    header) of the Quick-Start request packet, and returns this in the
    Quick-Start response.  The sender uses the TTL difference to
    determine if all of the routers along the path decremented the
    Quick-Start TTL, approving the Quick-Start Request.

    If the request is approved by all of the routers along the path,
    then the TCP sender combines this allowed rate with the measurement
    of the round-trip time, and ends up with an allowed TCP congestion
    window.  This window is sent rate-paced over the next round-trip
    time, or until an ACK packet is received.



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    Figure 1 shows a successful use of Quick-Start, with the sender's IP
    layer and both routers along the path approving the Quick-Start
    Request, and the TCP receiver using the Quick-Start Response to
    return information to the TCP sender.  In this example, Quick-Start
    is used by TCP to establish the initial congestion window.


       Sender        Router 1       Router 2          Receiver
       ------        --------       --------          --------
     | <IP TTL: 63>
     | <QS TTL: 91>
     | <TTL Diff: 28>
     | Quick-Start Request
     | in SYN or SYN/ACK.
     | IP: Decrement QS TTL
     | to approve request -->
     |
     |               Decrement
     |               QS TTL
     |               to approve
     |               request -->
     |
     |                              Decrement
     |                              QS TTL
     |                              to approve
     |                              request -->
     |
     |                                           <IP TTL: 60>
     |                                           <QS TTL: 88>
     |                                           <TTL Diff: 28>
     |                                           Return Quick-Start
     |                                            info to sender in
     |                                           Quick-Start Response
     |                                          <-- in TCP ACK packet.
     |
     | <TTL Diff: 28>
     | Quick-Start approved,
     | translate to cwnd.
     | Report Approved Rate.
     V Send cwnd paced over one RTT. -->

               Figure 1: A successful Quick-Start Request.


    Figure 2 shows an unsuccessful use of Quick-Start, with one of the
    routers along the path not approving the Quick-Start Request.  If
    the Quick-Start Request is not approved, then the sender uses the
    default congestion control mechanisms for that transport protocol,



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    including the default initial congestion window, response to idle
    periods, etc.


       Sender        Router 1       Router 2          Receiver
       ------        --------       --------          --------
     | <IP TTL: 63>
     | <QS TTL: 91>
     | <TTL Diff: 28>
     | Quick-Start Request
     | in SYN or SYN/ACK.
     | IP: Decrement QS TTL
     | to approve request -->
     |
     |               Decrement
     |               QS TTL
     |               to approve
     |               request -->
     |
     |                              Forward packet
     |                              without modifying
     |                              Quick-Start Option. -->
     |
     |                                           <IP TTL: 60>
     |                                           <QS TTL: 89>
     |                                           <TTL Diff: 29>
     |                                           Return Quick-Start
     |                                            info to sender in
     |                                           Quick-Start Response
     |                                          <-- in TCP ACK packet.
     |
     | <TTL Diff: 29>
     | Quick-Start not approved.
     | Report Approved Rate.
     V Use default initial cwnd. -->

               Figure 2: An unsuccessful Quick-Start Request.



3.  The Quick-Start Option in IP


3.1.  The Quick-Start Option for IPv4

    The Quick-Start Request for IPv4 is defined as follows:





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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Option      |  Length=8     | Func. | Rate  |   QS TTL      |
    |               |               | 0000  |Request|               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        QS Nonce                           | R |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 3.  The Quick-Start Option for IPv4.
                      A Quick-Start Request.


    The first byte contains the option field, which includes the one-bit
    copy flag, the 2-bit class field, and the 5-bit option number (to be
    assigned by IANA).

    The second byte contains the length field, indicating an option
    length of eight bytes.

    The third byte includes a four-bit Function field.  If the Function
    field is set to "0000", then the Quick-Start Option is a Rate
    Request.  In this case, the second half of the third byte is a four-
    bit Rate Request field.

    For a Rate Request, the fourth byte contains the Quick-Start TTL (QS
    TTL) field.  The sender MUST set the QS TTL field to a random value.
    Routers that approve the Quick-Start Request decrement the QS TTL
    (mod 256) by the same amount that they decrement the IP TTL.  (As
    elsewhere in this document, we use the term `router' imprecisely to
    also include the Quick-Start functionality at the IP layer at the
    sender.)  The QS TTL is used by the sender to detect if all of the
    routers along the path understood and approved the Quick-Start
    option.

    For a Rate Request, the transport sender MUST calculate and store
    the TTL Diff, the difference between the IP TTL value and the QS TTL
    value in the Quick-Start request packet, as follows:

    TTL Diff = ( IP TTL - QS TTL ) mod 256                         (1)

    For a Rate Request, bytes 5-8 contain a 30-bit QS Nonce, discussed
    in Section 3.4, and a two-bit Reserved field.  The sender SHOULD set
    the reserved field to zero, and routers and receivers SHOULD ignore
    the reserved field.  The sender SHOULD set the 30-bit QS Nonce to a
    random value.

    The sender initializes the Rate Request to the desired sending rate,



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    including an estimate of the transport and IP header overhead.  The
    encoding function for the Rate Request sets the request rate to
    K*2^N bps (bits per second), for N the value in the Rate Request
    field, and for K set to 40,000.  For N=0, the rate request would be
    set to zero, regardless of the encoding function.  This is
    illustrated in Table 1 below.  For the four-bit Rate Request field,
    the request range is from 80 Kbps to 1.3 Gbps.  Alternate encodings
    that were considered for the Rate Request are given in Appendix B.2.


     N     Rate Request (in Kbps)
    ---    -------------------
     0            0
     1           80
     2          160
     3          320
     4          640
     5        1,280
     6        2,560
     7        5,120
     8       10,240
     9       20,480
    10       40,960
    11       81,920
    12      163,840
    13      327,680
    14      655,360
    15    1,310,720

    Table 1: Mapping from Rate Request field to rate request in Kbps.


    Routers can approve the Quick-Start Request for a lower rate by
    decreasing the Rate Request in the Quick-Start Request.  Section 4.2
    discusses the Quick-Start Response from the TCP receiver to the TCP
    sender, and Section 4.4 discusses the TCP sender's mechanism for
    determining if a Quick-Start Request has been approved.














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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Option      |  Length=8     | Func. | Rate  |   Not Used    |
    |               |               | 1000  | Report|               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        QS Nonce                           | R |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 4.  The Quick-Start Option for IPv4.
                      Report of Approved Rate.


    If the Function field in the third byte of the Quick-Start Option is
    set to "1000", then the Quick-Start Option is a Report of Approved
    Rate.  In this case the second four bits in the third byte are the
    Rate Report field, formatted exactly as in the Rate Request field in
    a Rate Request.  For a Report of Approved Rate, the fourth byte of
    the Quick-Start Option are not used.  Bytes 5-8 contain a 30-bit QS
    Nonce and a two- bit Reserved field.

    After an approved Rate Request, the sender MUST report the Approved
    Rate, using a Quick-Start Option configured as a Report of Approved
    Rate with the Rate Report field set to the approved rate, and the QS
    Nonce set to the QS Nonce sent in the Quick-Start Request.  The
    packet containing the Report of Approved Rate MUST be either a
    control packet sent before the first Quick-Start data packet, or a
    Quick-Start Option in the first data packet itself.  The Report of
    Approved Rate does not have to be sent reliably; for example, if the
    approved rate is reported in a separate control packet, the sender
    does not necessarily know if the control packet has been dropped in
    the network.  If the packet contained the Quick-Start Request is
    acknowledged, but the acknowledgement packet does not contain a
    Quick-Start Response, then the sender MUST assume that the Quick-
    Start Request was denied, and set a Report of Approved Rate with a
    rate of zero.  Routers may choose to ignore the Report of Approved
    Rate, or to use the Report of Approved Rate but ignore the QS Nonce.
    Alternately, some routers that use the Report of Approved Rate may
    choose to match the QS Nonce, masked by the approved rate, with the
    QS Nonce seen in the original request.

    If the Rate Request is denied, the sender MUST send a Report of
    Approved Rate with the Rate Report field set to zero.

    We note that unlike a Quick-Start Request sent at the beginning of a
    connection, when a Quick-Start Request is sent in the middle of a
    connection, the connection could already have an established
    congestion window or sending rate.  The Rate Request is the



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    requested total rate for the connection, including the current rate
    of the connection; the Rate Request is *not* a request for an
    additional sending rate over and above the current sending rate.  If
    the Rate Request is denied, or lowered to a value below the
    connection's current sending rate, then the sender ignores the
    request, and reverts to the default congestion control mechanisms of
    the transport protocol.

    The use of the Quick-Start Option does not require the additional
    use of the Router Alert Option [RFC2113].

    We note that in IPv4, a change in IP options at routers requires
    recalculating the IP header checksum.


3.2.  The Quick-Start Option for IPv6

    The Quick-Start Option for IPv6 is placed in the Hop-by-Hop Options
    extension header that is processed at every network node along the
    communication path [RFC2460]. The option format following the
    generic Hop-by-Hop Options header is identical to the IPv4 format,
    with the exception that the Length field should exclude the common
    type and length fields in the option format and be set to 6 bytes
    instead of 8 bytes.

    For a Quick-Start Request, the transport receiver compares the
    Quick-Start TTL with the IPv6 Hop Limit field to calculate the TTL
    Diff.  (The Hop Limit in IPv6 is the equivalent of the TTL in IPv4.)
    That is, TTL Diff MUST be calculated and stored as follows:

    TTL Diff = ( IPv6 Hop Limit - QS TTL ) mod 256                  (2)

    Unlike IPv4, modifying or deleting the Quick-Start IPv6 Option does
    not require checksum re-calculation, because the IPv6 header does
    not have a checksum field, and modifying the Quick-Start Request in
    the IPv6 Hop-by-Hop options header does not affect the IPv6 pseudo-
    header checksum used in upper-layer checksum calculations.

    Appendix A of RFC 2460 requires that all packets with the same flow
    label must be originated with the same hop-by-hop header contents,
    which would be incompatible with Quick-Start. However, a later IPv6
    flow label specification [RFC3697] updates the use of flow labels in
    IPv6 and removes this restriction. Therefore Quick-Start is
    compatible with the current IPv6 specifications.







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3.3.  Processing the Quick-Start Request at Routers

    The Quick-Start Request does not report the current sending rate of
    the connection sending the request; in the default case of a router
    (or IP layer implementation at an end-node) that does not maintain
    per-flow state, a router makes the conservative assumption that the
    flow's current sending rate is zero.  Each participating router can
    either terminate or approve the Quick-Start Request.  A router MUST
    only approve a Quick-Start request if the output link is
    underutilized, and if the router judges that the output link will
    continue to be underutilized if this and earlier approved requests
    are used by the senders.  Otherwise, the router reduces or
    terminates the Quick-Start Request.

    While the paragraph above defines the *semantics* of approving a
    Quick-Start request, this document does not specify the specific
    algorithms to be used by routers in processing Quick-Start Requests
    or Reports.  This is similar to RFC 3168, which specifics the
    semantics of the ECN codepoints in the IP header, but does not
    specify specific algorithms for routers to use in deciding when to
    drop or mark packets before buffer overflow.

    A router that wishes to terminate the Quick-Start Request SHOULD
    either delete the Quick-Start Request from the IP header or zero the
    QS TTL, QS Nonce, and Rate Request fields.  Deleting the Quick-Start
    Request saves resources because downstream routers will have no
    option to process, but zeroing the Rate Request field may be more
    efficient for routers to implement.  As suggested in [B05], future
    additions to Quick-Start could define error codes for routers to
    insert into the QS Nonce field to report back to the sender the
    reason that the Quick-Start request was denied, e.g., that the
    router is denying all Quick-Start requests at this time, or that
    this router as a matter of policy does not grant Quick-Start
    requests.  A router that doesn't understand the Quick-Start option
    will simply forward the packet with the Quick-Start Request
    unchanged (ensuring that the TTL Diff will not match and Quick-Start
    will not be used).

    If the participating router has decided to approve the Quick-Start
    Request, it does the following:

    * The router MUST decrement the QS TTL by the amount the IP TTL is
    decremented (usually one).

    * If the router is only willing to approve a Rate Request less than
    that in the Quick-Start Request, then the router replaces the Rate
    Request with a smaller value.  The router MUST NOT increase the Rate
    Request in the Quick-Start Request.  If the router decreases the



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    Rate Request, the router MUST also modify the QS Nonce, as described
    in Section 3.4.

    * In IPv4, the router MUST update the IP header checksum.

    If the router approves the Quick-Start request, this approval SHOULD
    be taken into account in the router's decision to accept or reject
    subsequent Quick-Start requests (e.g., using a variable that tracks
    the recent aggregate of accepted Quick-Start requests).  This
    consideration of earlier approved Quick-Start request is necessary
    to ensure that the router only approves a Quick-Start request when
    the router judges that the output link will remain underutilized if
    this and earlier Quick-Start requests are used by the senders.

    In addition, the approval of a Quick-Start request SHOULD NOT be
    used by the router to affect the treatment of the data packets that
    arrive from this connection in the next few round-trip times.  That
    is, the approval by the router of a Quick-Start request does not
    give differential treatment for Quick-Start data packets at that
    router; it is only a statement from the router that the router
    believes that the subsequent Quick-Start data packets from this
    connection will not change the current under-utilized state of the
    router.

    A non-participating router forwards the Quick-Start Request
    unchanged, without decrementing the QS TTL.  The non-participating
    router still decrements the TTL field in the IP header, as is
    required for all routers [RFC1812].  As a result, the sender will be
    able to detect that the Quick-Start Request had not been understood
    or approved by all of the routers along the path.

    A router that uses multipath routing for packets within a single
    connection MUST NOT approve a Quick-Start request.  This is
    discussed in more detail in Section 9.2.


3.3.1.  Processing the Report of Approved Rate

    If the Quick-Start Option has the Function field set to "1000", then
    this is a Report of Approved Rate, rather than a Rate Request.  The
    router MAY ignore such an option, and in any case it MUST NOT modify
    the contents of the option for a Report of Approved Rate.  However,
    the router MAY use the Approved Rate report to check that the sender
    is not lying about the approved rate.  If the reported Approved Rate
    is higher than the rate that the router actually approved for this
    connection in the previous round-trip time, then the router may
    implement some policy for cheaters.  For instance, the router MAY
    decide to deny future Quick-Start requests from this sender,



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    including, if desired, deleting Quick-Start requests from future
    packets from this sender.  Section 9.4.1 discusses misbehaving
    senders in more detail.  From the Report of Approved Rate, the
    router can also learn if some of the downstream routers have
    approved the Quick-Start request for a smaller rate or denied the
    use of Quick-Start, and adjust its bandwidth allocations
    accordingly.


3.4.  The QS Nonce

    The QS Nonce gives the Quick-Start sender some protection against
    receivers lying about the value of the received Rate Request.  This
    is particularly important if the receiver knows the original value
    of the Rate Request (e.g., when the sender always requests the same
    value, and the receiver has a long history of communication with
    that sender).  Without the QS Nonce, there is nothing to prevent the
    receiver from reporting back to the sender a Rate Request of K, when
    the received Rate Request was in fact less than K.

    Table 2 gives the format for the 30-bit QS Nonce.


    Bits         Purpose
    ---------    ------------------
    Bits 0-1:    Rate 15 -> Rate 14
    Bits 2-3:    Rate 14 -> Rate 13
    Bits 4-5:    Rate 13 -> Rate 12
    Bits 6-7:    Rate 12 -> Rate 11
    Bits 8-9:    Rate 11 -> Rate 10
    Bits 10-11:  Rate 10 -> Rate 9
    Bits 12-13:  Rate 9 -> Rate 8
    Bits 14-15:  Rate 8 -> Rate 7
    Bits 16-17:  Rate 7 -> Rate 6
    Bits 18-19:  Rate 6 -> Rate 5
    Bits 20-21:  Rate 5 -> Rate 4
    Bits 22-23:  Rate 4 -> Rate 3
    Bits 24-25:  Rate 3 -> Rate 2
    Bits 26-27:  Rate 2 -> Rate 1
    Bits 28-29:  Rate 1 -> Rate 0

    Table 2: The QS Nonce.


    The transport sender MUST initialize the QS Nonce to a random value.
    If the router reduces the Rate Request from rate K to rate K-1, then
    the router MUST set the field in the QS Nonce for "Rate K -> Rate
    K-1" to a new random value.  Similarly, if the router reduces the



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    Rate Request by N steps, the router MUST set the 2N bits in the
    relevant fields in the QS Nonce to a new random value.  The receiver
    MUST report the QS Nonce back to the sender.

    If the Rate Request was not decremented in the network, then the QS
    Nonce should have its original value.  Similarly, if the Rate
    Request was decremented by N steps in the network, and the receiver
    reports back a Rate Request of K, then the last 2K bits of the QS
    Nonce should have their original value.

    With the QS Nonce, the receiver has a 1/4 chance of cheating about
    each step change in the rate request.  Thus, if the rate request was
    reduced by two steps in the network, the receiver has a 1/16 chance
    of successfully reporting that the original request was approved, as
    this requires reporting the original value for the QS nonce.
    Similarly, if the rate request is reduced many steps in the network,
    and the receiver receives a QS Option with a rate request of K, the
    receiver has a 1/16 chance of guessing the original values for the
    fields in the QS nonce for "Rate K+2 -> Rate K+1" and "Rate K+1 ->
    Rate K".  Thus, the receiver has a 1/16 chance in successfully lying
    and saying that the received rate request was K+2 instead of K.

    We note that the protection offered by the QS Nonce is the same
    whether one router makes all of the decrements in the rate request,
    or whether they are made at different routers along the path.

    The requirements for randomization for the sender and routers in
    setting `random' values in the QS Nonce are not stringent - almost
    any form of pseudo-random numbers would do.  The requirement is that
    the original value for the QS Nonce is not easily predictable by the
    receiver, and in particular, the nonce MUST NOT be easily determined
    from inspection of the rest of the packet or from previous packets.
    In particular, the nonce MUST NOT be based only on a combination of
    specific packet header fields.  Thus, if two bits of the QS Nonce
    are changed by a router along the path, the receiver should not be
    able to guess those two bits from the other 28 bits in the QS Nonce.

    An additional requirement is that the receiver cannot be able to
    tell, from the QS Nonce itself, which numbers in the QS Nonce were
    generated by the sender, and which were generated by routers along
    the path.  This makes it harder for the receiver to infer the value
    of the original rate request, making it one step harder for the
    receiver to cheat.

    Section 9.4 also considers issues of receiver cheating in more
    detail.





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4.  The Quick-Start Mechanisms in TCP

    This section describes how the Quick-Start mechanism would be used
    in TCP.  We first sketch the procedure and then tightly define it in
    the subsequent subsections.

    If a TCP sender, say host A, would like to use Quick-Start, the TCP
    sender puts the requested sending rate in bits per second,
    appropriately formatted, in the Quick-Start option in the IP header
    of the TCP packet, called the Quick-Start request packet.  (We will
    be somewhat loose in our use of "packet" vs. "segment" in this
    section.)  When used for initial start-up, the Quick-Start request
    packet can be either the SYN or SYN/ACK packet, as illustrated in
    Figure 1.  The requested rate includes an estimate for the transport
    and IP header overhead.  The TCP receiver, say host B, returns the
    Quick-Start Response option in the TCP header in the responding
    SYN/ACK packet or ACK packet, called the Quick-Start response
    packet, informing host A of the results of their request.

    If the acknowledging packet does not contain a Quick-Start Response,
    or contains a Quick-Start Response with the wrong value for the TTL
    Diff or the QS Nonce, then host A MUST assume that its Quick-Start
    request failed.  In this case, host A sends a Report of Approved
    Rate with a Rate Report of zero, and uses TCP's default congestion
    control procedure.  For initial start-up, host A uses the default
    initial congestion window ([RFC2581], [RFC3390]).

    If the returning packet contains a valid Quick-Start Response, then
    host A uses the information in the response, along with its
    measurement of the round-trip time, to determine the Quick-Start
    congestion window (QS-cwnd).  Quick-Start data packets are defined
    as data packets sent as the result of a successful Quick-Start
    request, up to the time when the first Quick-Start packet is
    acknowledged.  The sender also sends a Report of Approved Rate.  In
    order to use Quick-Start, the TCP host MUST use rate-based pacing
    [VH97] to transmit Quick-Start packets at the rate indicated in the
    Quick-Start Response, at the level of granularity possible by the
    sending host.  We note that the limitations of interrupt timing on
    computers can limit the ability of the TCP host in rate-pacing the
    outgoing packets.

    The two TCP end-hosts can independently decide whether to request
    Quick-Start.  For example, host A could sent a Quick-Start Request
    in the SYN packet, and host B could also send a Quick-Start Request
    in the SYN/ACK packet.






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4.1.  Sending the Quick-Start Request

    When sending a Quick-Start Request, the TCP sender SHOULD send the
    request on a packet that requires an acknowledgement, such as a SYN,
    SYN/ACK, or data packet.  In this case, if the packet is
    acknowledged but no Quick-Start Response is included, then the
    sender knows that the Quick-Start request has been denied, and can
    send a Report of Approved Rate.

    In addition to the use of Quick-Start when a connection is
    established, there are several additional points in a connection
    when a transport protocol may want to issue a Rate Request.  We
    first re-iterate the notion that Quick-Start is a coarse-grained
    mechanism.  That is, Quick-Start's Rate Requests are not meant to be
    used for fine-grained control of the transport's sending rate.
    Rather, the transport MAY issue a Rate Request when no information
    about the appropriate sending rate is available and the default
    congestion control mechanisms might be significantly underestimating
    the appropriate sending rate.

    The following are potential points where Quick-Start may be useful:


        (1) At or soon after connection initiation, when the transport
        has no idea of the capacity of the network path, as discussed
        above.  (A transport that uses TCP Control Block sharing
        [RFC2140], the Congestion Manager [RFC3124], or other mechanisms
        for sharing congestion information may not need Quick-Start to
        determine an appropriate rate.)


        (2) After an idle period when the transport no longer has a
        validated estimate of the available bandwidth for this flow.
        (An example could be a persistent-HTTP connection when a new
        HTTP request is received after an idle period.)


        (3) After a host has received explicit indications that one of
        the endpoints has moved its point of network attachment.  This
        can happen due to some underlying mobility mechanism like Mobile
        IP ([RFC3344], [RFC3775]).  Some transports, such as SCTP
        [RFC2960], may associate with multiple IP addresses and can
        switch addresses (and, therefore network paths) in mid-
        connection.  If the transport has concrete knowledge of a
        changing network path then the current sending rate may not be
        appropriate and the transport sender may use Quick-Start to
        probe the network to see if it can send at a higher rate.
        (Alternatively, traditional slow-start should be used in this



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        case when Quick-Start is not available.)


        (4) After an application-limited period when the sender has been
        using only a small amount of its appropriate share of the
        network capacity, and has no valid estimate for its fair share.
        In this case, Quick-Start may be an appropriate mechanism to
        determine if the sender can send at a higher rate.  For
        instance, consider an application that steadily exchanges low-
        rate control messages and suddenly needs to transmit a large
        amount of data.


    Of the above, this document recommends that a TCP sender MAY attempt
    to use Quick-Start in cases (1) and (2).  It is NOT RECOMMENDED that
    a TCP sender use Quick-Start for case (3) at the current time.  Case
    (3) requires external notifications not presently defined for TCP or
    other transport protocols.  Finally, a TCP SHOULD NOT use Quick-
    Start for case (4) at the current time.  Case (4) requires further
    thought and investigation with regard to how the transport protocol
    could determine it was in a situation that would warrant
    transmitting a Quick-Start Request.

    As a general guideline, a TCP sender SHOULD NOT request a sending
    rate larger than it is able to use over the next round-trip time of
    the connection (or over 100 ms, if the round-trip time is not
    known), except as required to round up the desired sending rate to
    the next-highest allowable request.

    In any circumstances, the sender MUST NOT make a QS request if it
    has made a QS request within the most recent round-trip time.

    Section 4.7 discusses some of the issues of using Quick-Start at
    connection initiation, and Section 4.8 discusses issues that arise
    when Quick-Start is used to request a larger sending rate after an
    idle period.


4.2.  The Quick-Start Response Option in the TCP header

    In order to approve the use of Quick-Start, the TCP receiver
    responds to the receipt of a Quick-Start Request with a Quick-Start
    Response, using the Quick-Start Response Option in the TCP header.
    TCP's Quick-Start Response option is defined as follows:







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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Kind      |  Length=8     | Resv. | Rate  |   TTL Diff    |
    |               |               |       |Request|               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   QS Nonce                                | R |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 5.  The Quick-Start Response option in the TCP header.


    The first byte of the Quick-Start Response option contains the
    option kind, identifying the TCP option (to be assigned by IANA).

    The second byte of the Quick-Start Response option contains the
    option length in bytes.  The length field MUST be set to 8 bytes.

    The third byte of the Quick-Start Response option contains a four-
    bit Reserved field, and the four-bit allowed Rate Request, formatted
    as in the Quick-Start Rate Request option.

    The fourth byte of the TCP option contains the TTL Diff.  The TTL
    Diff contains the difference between the IP TTL and QS TTL fields in
    the received Quick-Start request packet, as calculated in equations
    (1) or (2) (depending on whether IPv4 or IPv6 is used).

    Bytes 5-8 of the TCP option contain the 30-bit QS Nonce and a two-
    bit Reserved field.

    We note that while there are limitations on the potential size of
    the Quick-Start Response Option, a Quick-Start Response Option of
    eight bytes should not be a problem.  The TCP Options field can
    contain up to 40 bytes.  Other TCP options that might be used in a
    SYN or SYN/ACK packet include Maximum Segment Size (four bytes),
    Time Stamp (ten bytes), Window Scale (three bytes), and Selective
    Acknowledgments Permitted (two bytes).


4.3.  TCP: Sending the Quick-Start Response

    An end host, say host B, that receives an IP packet containing a
    Quick-Start Request passes the Quick-Start Request, along with the
    value in the IP TTL field, to the receiving TCP layer.

    If the TCP host is willing to permit the Quick-Start Request, then a
    Quick-Start Response option is included in the TCP header of the
    corresponding acknowledgement packet.  The Rate Request in the



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    Quick-Start Response option is set to the received value of the Rate
    Request in the Quick-Start option, or to a lower value if the TCP
    receiver is only willing to allow a lower Rate Request.  The TTL
    Diff in the Quick-Start Response is set to the difference between
    the IP TTL value and the QS TTL value as given in equation (1) or
    (2) (depending on whether IPv4 or IPv6 is used).  The QS Nonce in
    the Response is set to the received value of the QS Nonce in the
    Quick-Start option.

    If an end host receives an IP packet with a Quick-Start Request with
    a rate request of zero, then that host SHOULD NOT send a Quick-Start
    Response.

    The Quick-Start Response MUST NOT be resent if it is lost in the
    network. Packet loss could be an indication of congestion on the
    return path, in which case it is better not to approve the Quick-
    Start Request.


4.4.  TCP: Receiving and Using the Quick-Start Response Packet

    A TCP host, say TCP host A, that sent a Quick-Start Request and
    receives a Quick-Start Response in an acknowledgement first checks
    that the Quick-Start Response is valid.  The Quick-Start Response is
    valid if it contains the correct value for the TTL Diff, and an
    equal or lesser value for the Rate Request than that transmitted in
    the Quick-Start Request.  In addition, if the received Rate Request
    is K, then the rightmost 2K bits of the QS Nonce must match those
    bits in the QS Nonce sent in the Quick-Start Request.  If these
    checks are not successful, then the Quick-Start request failed, and
    the TCP host MUST use the default TCP congestion window that it
    would have used without Quick-Start.  If the rightmost 2K bits of
    the QS Nonce do not match those bits in the QS Nonce sent in the
    Quick-Start Request, for a received Rate Request of K, then the TCP
    host MUST NOT send additional Quick-Start requests during the life
    of the connection.  Whether the Quick-Start request was successful
    or not, the host receiving the Quick-Start Response MUST send a
    Report of Approved Rate.  Similarly, if the packet containing the
    Quick-Start Request is acknowledged, but the acknowledgement does
    not include a Quick-Start Response, then the sender MUST send a
    Report of Approved Rate.

    If the checks of the TTL Diff and the Rate Request are successful,
    and the TCP host is going to use the Quick-Start Request, it MUST
    start using it within one round-trip time of receiving the Quick-
    Start Response, or within three seconds, whichever is smaller.  To
    use the Quick-Start Request, the host sets its Quick-Start
    congestion window (in terms of MSS-sized segments), QS-cwnd, as



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    follows:

    QS-cwnd = (R * T) / (MSS + H)                                (3)

    where R the Rate Request in bytes per second, T the measured round-
    trip time in seconds, and H the estimated TCP/IP header size in
    bytes (e.g., 40 bytes).

    Derivation: the sender is allowed to transmit at R bytes per second
    including packet headers, but only R*MSS/(MSS+H) bytes per second,
    or equivalently R*T*MSS/(MSS+H) bytes per round-trip time, of
    application data.

    The TCP host SHOULD set its congestion window cwnd to QS-cwnd only
    if QS-cwnd is greater than cwnd; otherwise QS-cwnd is ignored.  If
    QS-cwnd is used, the TCP host sets a flag that it is in Quick-Start
    mode, and while in Quick-Start mode the TCP sender MUST use rate-
    based pacing to pace out Quick-Start packets at the approved rate.
    If, during Quick-Start mode, the TCP sender receives ACKs for
    packets sent before this Quick-Start mode was entered, these ACKs
    are processed as usual, following the default congestion control
    mechanisms.  Quick-Start mode ends when the TCP host receives an ACK
    for one of the Quick-Start packets.

    If the congestion window has not been fully used when the first ack
    arrives ending the Quick-Start mode, then the congestion window is
    decreased to the amount that has actually been used so far.  This is
    necessary because when the Quick-Start Response is received, the TCP
    sender's round-trip-time estimate might be longer than for
    succeeding round-trip times, e.g., because of delays at routers
    processing the IP Quick-Start option, or because of delays at the
    receiver in responding to the Quick-Start Request packet.  In this
    case, an overly-large round-trip-time estimate could have caused the
    TCP sender to translate the approved Quick-Start sending rate in
    bytes per second into a congestion window that is larger than
    needed, with the TCP sender receiving an ACK for the first Quick-
    Start packet before the entire congestion window has been used.
    Thus, when the TCP sender receives the first ACK for a Quick-Start
    packet, the sender MUST reduce the congestion window to the amount
    that has actually been used.

    As an example, a TCP sender with an approved Quick-Start request of
    R KBps, B-byte packets including headers, and an RTT estimate of T
    seconds, would translate the Rate Request of R KBps to a congestion
    window of R*T/B packets.  The TCP sender would send the Quick-Start
    packets rate-paced at R KBps.  However, if the actual current round-
    trip time was T/2 seconds instead of T seconds, then the sender
    would begin to receive acknowledgements for Quick-Start packets



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    after T/2 seconds.  Following the paragraph above, the TCP sender
    would then reduce its congestion window from R*T/B to approximately
    R*T/(B*2) packets, the actual number of packets that were needed to
    fill the pipe at a sending rate of R KBps.  (Note: this is just an
    illustration and that the congestion window is actually set to the
    amount of data sent before the ACK arrives and not based on
    equations above.)

    After Quick-Start mode is exited and the congestion window adjusted
    if necessary, the TCP sender returns to using the default congestion
    control mechanisms, processing further incoming ACK packets as
    specified by those congestion control mechanisms.  For example, if
    the TCP sender was in slow-start prior to the Quick-Start request,
    and no packets were lost or marked since that time, then the sender
    continues in slow-start after exiting Quick-Start mode, as allowed
    by ssthresh.

    To add robustness, the TCP sender MUST use Limited Slow-Start
    [RFC3742] along with Quick-Start.  With Limited Slow-Start, the TCP
    sender limits the number of packets by which the congestion window
    is increased for one window of data during slow-start.

    When Quick-Start is used at the beginning of a connection, before
    any packet marks or losses have been reported, the TCP host MAY use
    the reported Rate Request to set the slow-start threshold to a
    desired value, e.g., to some small multiple of the congestion
    window.  A possible future research topic is how the sender might
    modify the show-start threshold at the beginning of a connection to
    avoid overshooting the path capacity.  (The initial value of
    ssthresh is allowed to be arbitrarily high, and some TCP
    implementations use the size of the advertised window for ssthresh
    [RFC2581].)


4.5.  TCP: Controlling Acknowledgement Traffic on the Reverse Path

    When a Quick-Start Request is approved for a TCP sender, the
    resulting Quick-Start data traffic can result in a sudden increase
    in traffic for pure ACK packets on the reverse path.  For example,
    for the largest Quick-Start request of 1.3 Gbps, given a TCP sender
    with 1500-byte packets and a TCP receiver with delayed
    acknowledgements acking every other packet, this could result in
    17.3 Mbps of acknowledgement traffic on the reverse path.

    One possibility, in cases with large Quick-Start requests, would be
    for TCP receivers to send Quick-Start requests to request bandwidth
    for the acknowledgement traffic on the reverse path.  However, in
    our view, a better approach would be for TCP receivers to simply



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    control the rate of sending acknowledgement traffic.  The optimal
    future solution would involve the explicit use of congestion control
    for TCP acknowledgement traffic, as is done now for the
    acknowledgement traffic in DCCP's CCID2 [RFC4341].

    In the absence of congestion control for acknowledgement traffic,
    the TCP receiver could limit its sending rate for ACK packets sent
    in response to Quick-Start data packets.  The following information
    is needed by the TCP receiver:

    * The RTT: TCP naturally measures the RTT of the path and therefore
      should have a sample of the RTT.  If the TCP receiver does not
      have a measurement of the round-trip time, it can use the time
      between the receipt of the Quick-Start Request and the Report
      of Approved Rate.

    * The Approved Rate Request (R): When the TCP receiver receives the
      Quick-Start Response packet, the receiver knows the value of the
      approved Rate Request.

    * The MSS: TCP advertises the MSS during the initial three-way
      handshake and therefore the receiver should have an understanding
      of the packet size the sender will be using.  If the receiver does
      not have such an understanding or wishes to confirm the negotiated
      MSS, the size of the first data packet can be used.

    With this set of information the TCP receiver can restrict its
    sending rate for pure acknowledgment traffic to at most 100 pure ACK
    packets per RTT by sending at most one ACK for every K data packets,
    for the ACK Ratio K set to R*RTT/(100*8*MSS).  The receiver would
    acknowledge the first Quick-Start data packet, and every succeeding
    K data packets.  Thus, for a somewhat extreme case of R=1.3 Gbps,
    RTT=0.2 seconds, and MSS=1500 bytes, K would be set to 216, and the
    receiver would acknowledge every 216 data packets.  From [RFC2581],
    the ACK Ratio K should have a minimum value of two.  When the ACK
    Ratio is greater than two, and the TCP sender receives
    acknowledgements each acknowledging more than two data packets, the
    TCP sender may want to use rate-based pacing to control the
    burstiness of its outgoing data traffic.

    In the absence of explicit congestion control mechanisms, the TCP
    end nodes cannot determine the packet drop rate for pure
    acknowledgement traffic.  This is true with or without Quick-Start.
    However, the TCP receiver could limit its increase in the sending
    rate for pure ACK packets by at most doubling the sending rate for
    pure ACK packets from one round-trip time to the next.  The TCP
    receiver would do this by halving the ACK Ratio each round-trip
    time.



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    Note that the above is one particular mechanism that could be used
    to control the ACK stream.  Future work that investigates this
    scheme and others in detail is encouraged.


4.6.  TCP: Responding to a Loss of a Quick-Start Packet

    For TCP, we have defined a ``Quick-Start packet'' as one of the
    packets sent in the window immediately following a successful Quick-
    Start request.  After detecting the loss or ECN-marking of a Quick-
    Start packet, TCP MUST revert to the default congestion control
    procedures that would have been used if the Quick-Start request had
    not been approved.  For example, if Quick-Start is used for setting
    the initial window, and a packet from the initial window is lost or
    marked, then the TCP sender MUST then slow-start with the default
    initial window that would have been used if Quick-Start had not been
    used.  In addition to reverting to the default congestion control
    mechanisms, the sender MUST take into account that the Quick-Start
    congestion window was too large.  Thus, the sender SHOULD decrease
    ssthresh to at most half the number of Quick-Start packets that were
    successfully transmitted.  Section B.5 discusses possible
    alternatives in responding to the loss of a Quick-Start packet.

    If a Quick-Start packet is lost or ECN-marked, then the sender
    SHOULD NOT make future Quick-Start requests for this connection.

    We note that ECN [RFC3168] MAY be used with Quick-Start.  As is
    always the case with ECN, the sender's congestion control response
    to an ECN-marked Quick-Start packet is the same as the response to a
    dropped Quick-Start packet, thus reverting to slow start in the case
    of Quick-Start packets marked as experiencing congestion.


4.7.  TCP: A Quick-Start Request for a Larger Initial Window

    Some of the issues of using Quick-Start are related to the specific
    scenario in which Quick-Start is used.  This section discusses the
    following issues that arise when Quick-Start is used by TCP to
    request a larger initial window: (1) interactions with Path MTU
    Discovery (PMTUD); and (2) Quick-Start request packets that are
    discarded by middleboxes.


4.7.1.  Interactions with Path MTU Discovery

    One issue when Quick-Start is used to request a large initial window
    concerns the interactions between the large initial window and Path
    MTU Discovery.  Some of the issues are discussed in RFC 3390:



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        "When larger initial windows are implemented along with Path MTU
        Discovery [RFC1191], alternatives are to set the "Don't
        Fragment" (DF) bit in all segments in the initial window, or to
        set the "Don't Fragment" (DF) bit in one of the segments.  It is
        an open question as to which of these two alternatives is best."

    If the sender knows the Path MTU when the initial window is sent
    (e.g., from a PMTUD cache or from some other IETF-approved method),
    then the sender SHOULD use that MTU for segments in the initial
    window.  Unfortunately, the sender doesn't necessarily know the Path
    MTU when it sends packets in the initial window.  In this case, the
    sender should be conservative in the packet size used.  Sending a
    large number of overly-large packets with the DF bit set is not
    desirable, but sending a large number of packets that are fragmented
    in the network can be equally undesirable.

    If the sender doesn't know the Path MTU when the initial window is
    sent, the sender SHOULD send one large packet in the initial window
    with the DF bit set, and send the remaining packets in the initial
    window with a smaller MTU of 576 bytes (or 1280 bytes with IPv6).

    A second possibility would be for the sender to delay sending the
    Quick-Start Request for one round-trip time, sending the Quick-Start
    Request with the first window of data while also doing Path MTU
    Discovery.

    The sender may be using an iterative approach such as Packetization
    Layer Path MTU Discovery (PLPMTUD) [MH06] for Path MTU Discovery,
    where the sender tests successively larger MTUs.  If a probe is
    successfully delivered then the MTU can be raised to reflect the
    value used in that probe.  We would note that PLPMTUD does not allow
    the sender to determine the Path MTU before sending the initial
    window of data.


4.7.2.  Quick-Start Request Packets that are Discarded by Routers or
Middleboxes

    It is always possible for a TCP SYN packet carrying a Quick-Start
    request to be dropped in the network due to congestion, or to be
    blocked due to interactions with routers or middleboxes, where a
    middlebox is defined as any intermediary box performing functions
    apart from normal, standard functions of an IP router on the data
    path between a source host and destination host [RFC3234].
    Measurement studies of interactions between transport protocols and
    middleboxes [MAF04] show that for 70% of the web servers
    investigated, no connection is established if the TCP SYN packet
    contains an unknown IP option (and for 43% of the web servers, no



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    connection is established if the TCP SYN packet contains an IP
    TimeStamp Option).  In both cases, this is presumably due to routers
    or middleboxes along that path.

    If the TCP sender doesn't receive a response to the SYN or SYN/ACK
    packet containing the Quick-Start Request, then the TCP sender
    SHOULD resend the SYN or SYN/ACK packet without the Quick-Start
    Request.  Similarly, if the TCP sender receives a TCP reset in
    response to the SYN or SYN/ACK packet containing the Quick-Start
    Request, then the TCP sender SHOULD resend the SYN or SYN/ACK packet
    without the Quick-Start Request [RFC3360].

    RFC 1122 and 2988 specify that the sender should set the initial RTO
    to three seconds, though many TCP implementations set the initial
    RTO to one second.  For a TCP SYN packet sent with a Quick-Start
    request, the TCP sender SHOULD use an initial RTO of three seconds.

    We note that if the TCP SYN packet is using the IP Quick-Start
    Option for a Quick-Start request, and it is also using bits in the
    TCP header to negotiate ECN-capability with the TCP host at the
    other end, then the drop of a TCP SYN packet could be due to
    congestion, to a router or middlebox dropping the packet because of
    the IP Option, or because of a router or middlebox dropping the
    packet because of the information in the TCP header negotiating ECN.
    In this case, the sender could resend the dropped packet without
    either the Quick-Start or the ECN requests.  Alternately, the sender
    could resend the dropped packet with only the ECN request in the TCP
    header, resending the TCP SYN packet without either the Quick-Start
    or the ECN requests if the second TCP SYN packet is dropped.  The
    second choice seems reasonable, given that a TCP SYN packet today is
    more likely to be blocked due to policies that discard packets with
    IP Options than due to policies that discard packets with ECN
    requests in the TCP header [MAF04].


4.8.  TCP: A Quick-Start Request in the Middle of a Connection

    This section discusses the following issues that arise when Quick-
    Start is used by TCP to request a larger window in the middle of a
    connection, such as after an idle period: (1) determining the rate
    to request; (2) when to make a request; and (3) the response if
    Quick-Start packets are dropped;

    (1) Determining the rate to request:
    For a connection that has not yet had a congestion event (that is, a
    marked or dropped packet), the TCP sender is not restricted in the
    rate that it requests.  As an example, a server might wait and send
    a Quick-Start request after a short interaction with the client.



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    To use a Quick-Start Request in a connection that has already
    experienced a congestion event, and that has not had a recent
    mobility event, the TCP sender can determine the largest congestion
    window that the TCP connection achieved since the last packet drop
    and translate this to a sending rate to get the maximum allowed
    request rate.  If the request is granted, then the sender
    essentially restarts with its old congestion window from before it
    was reduced, for example during an idle period.

    A Quick-Start Request sent in the middle of a TCP connection SHOULD
    be sent on a data packet.

    (2) When to make a request:
    A TCP connection MAY make a Quick-Start request before the
    connection has experienced a congestion event, or after an idle
    period of at least one RTO.

    (3) Response if Quick-Start packets are dropped:
    If Quick-Start packets are dropped in the middle of connection, then
    the sender MUST revert to half of the Quick-Start window, or to the
    congestion window that the sender would have used if the Quick-Start
    request had not been approved, whichever is smaller.


4.9.  An Example Quick-Start Scenario with TCP

    The following is an example scenario in the case when both hosts
    request Quick-Start for setting their initial windows.  This is
    similar to Figures 1 and 2 in Section 2.1, except that it
    illustrates a TCP connection with both TCP hosts sending Quick-Start
    Requests.

    * The TCP SYN packet from Host A contains a Quick-Start Request in
    the IP header.

    * Routers along the forward path modify the Quick-Start Request as
    appropriate.

    * Host B receives the Quick-Start Request in the SYN packet, and
    calculates the TTL Diff.  If Host B approves the Quick-Start
    Request, then Host B sends a Quick-Start Response in the TCP header
    of the SYN/ACK packet.  Host B also sends a Quick-Start Request in
    the IP header of the SYN/ACK packet.

    * Routers along the reverse path modify the Quick-Start Request as
    appropriate.

    * Host A receives the Quick-Start Response in the SYN/ACK packet,



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    and checks the TTL Diff, Rate Request, and QS Nonce for validity.
    If they are valid, then Host A sets its initial congestion window
    appropriately, and sets up rate-based pacing to be used with the
    initial window.  If the Quick-Start Response is not valid, then Host
    A uses TCP's default initial window.  In either case, Host A sends a
    Report of Approved Rate.

    Host A also calculates the TTL Diff for the Quick-Start Request in
    the incoming SYN/ACK packet, and sends a Quick-Start Response in the
    TCP header of the ACK packet.

    * Host B receives the Quick-Start Response in an ACK packet, and
    checks the TTL Diff, Rate Request, and QS Nonce for validity.  If
    the Quick-Start Response is valid, then Host B sets its initial
    congestion window appropriately, and sets up rate-based pacing to be
    used with its initial window.  If the Quick-Start Response is not
    valid, then Host B uses TCP's default initial window.  In either
    case, Host B sends a Report of Approved Rate.


5.  Quick-Start and IPsec AH

    This section shows that Quick-Start is compatible with IPsec AH
    (Authentication Header).  AH uses an Integrity Check Value (ICV) in
    the IPsec Authentication Header to verify both message
    authentication and integrity ([RFC4302], page 85).  Changes to the
    Quick-Start option in the IP header do not affect this AH ICV.  The
    tunnel considerations in Section 6 below apply to all IPsec tunnels,
    regardless of what IPsec headers or processing are used in
    conjunction with the tunnel.

    Because the contents of the Quick-Start option can change along the
    path, it is important that these changes not affect the IPsec
    Authentication Header Integrity Check Value (AH ICV).  For IPv4, RFC
    4302 requires that unrecognized IPv4 options be zeroed for AH ICV
    computation purposes, so Quick-Start IP Option data changing en
    route does not cause problems with existing IPsec AH implementations
    for IPv4.  If the Quick-Start option is recognized, it MUST be
    treated as a mutable IPv4 option, and hence be completely zeroed for
    AH ICV calculation purposes.  IPv6 option numbers explicitly
    indicate whether the option is mutable; the 3rd highest order bit in
    the IANA-allocated option type has the value 1 to indicate that the
    Quick-Start option data can change en route.  RFC 4302 requires that
    the option data of any such option be zeroed for AH ICV computation
    purposes.  Therefore changes to the Quick-Start option in the IP
    header do not affect the calculation of the AH ICV.





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6.  Quick-Start in IP Tunnels and MPLS

    This section considers interactions between Quick-Start and IP
    tunnels, including IPsec ([RFC4301]), IP in IP [RFC2003], GRE
    [RFC2784], and others.  This section also considers interactions
    between Quick-Start and MPLS [RFC3031].

    In the discussion, we use TTL Diff, defined earlier as the
    difference between the IP TTL and the Quick-Start TTL, mod 256.
    Recall that the sender considers the Quick-Start request approved
    only if the value of TTL Diff for the packet entering the network is
    the same as the value of TTL Diff for the packet exiting the
    network.

    Simple tunnels: IP tunnel modes are generally based on adding a new
    "outer" IP header that encapsulates the original or "inner" IP
    header and its associated packet.  In many cases, the new "outer" IP
    header may be added and removed at intermediate points along a path,
    enabling the network to establish a tunnel without requiring
    endpoint participation.  We denote tunnels that specify that the
    outer header be discarded at tunnel egress as "simple tunnels", and
    we denote tunnels where the egress saves and uses information from
    the outer header before discarding it as "non-simple tunnels".  An
    example of a "non-simple tunnel" would be a tunnel configured to
    support ECN, where the egress router might copy the ECN codepoint in
    the outer header to the inner header before discarding the outer
    header [RFC3168].


                        __ Tunnels Compatible with Quick-Start
                       /
    Simple Tunnels  __/
                      \
                       \__ Tunnels Not Compatible with Quick-Start
                                     (False Positives!)


                            __ Tunnels Supporting Quick-Start
                           /
                          /
    Non-Simple Tunnels __/_____ Tunnels Compatible with Quick-Start,
                         \          but Not Supporting Quick-Start
                          \
                           \__ Tunnels Not Compatible with Quick-Start?

    Figure 6: Categories of Tunnels.





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    Tunnels that are compatible with Quick-Start: We say that an IP
    tunnel `is not compatible with Quick-Start' if the use of a Quick-
    Start Request over such a tunnel allows false positives, where the
    TCP sender incorrectly believes that the Quick-Start Request was
    approved by all routers along the path.  If the use of Quick-Start
    over the tunnel does not cause false positives, we say that the IP
    tunnel `is compatible with Quick-Start'.

    If the IP TTL of the inner header is decremented during forwarding
    before tunnel encapsulation takes place, then the simple tunnel is
    compatible with Quick-Start, with Quick-Start requests being
    rejected.  Section 6.1 describes in more detail the ways that a
    simple tunnel can be compatible with Quick-Start.

    There are some simple tunnels that are not compatible with Quick-
    Start, allowing `false positives' where the TCP sender incorrectly
    believes that the Quick-Start Request was approved by all routers
    along the path.  This is discussed in Section 6.2 below.

    One of our tasks in the future will be to investigate the occurrence
    of tunnels that are not compatible with Quick-Start, and to track
    the extent to which such tunnels are modified over time.  The
    evaluation of the problem of false positives from tunnels that are
    not compatible with Quick-Start will affect the progression of
    Quick-Start from Experimental to Proposed Standard, and will affect
    the degree of deployment of Quick-Start while in Experimental mode.

    Tunnels that support Quick-Start: We say that an IP tunnel `supports
    Quick-Start' if it allows routers along the tunnel path to process
    the Quick-Start Request and give feedback, resulting in the
    appropriate possible acceptance of the Quick-Start request.  Some
    tunnels that are compatible with Quick-Start support Quick-Start,
    while others do not.  We note that a simple tunnel is not able to
    support Quick-Start.

    From a security point of view, the use of Quick-Start in the outer
    header of an IP tunnel might raise security concerns because an
    adversary could tamper with the Quick-Start information that
    propagates beyond the tunnel endpoint, or because the Quick-Start
    Option exposes information to network scanners.  Our approach is to
    make supporting Quick-Start an option for IP tunnels.  That is, in
    environments or tunneling protocols where the risks of using Quick-
    Start are judged to outweigh its benefits, the tunnel can simply
    delete the Quick-Start option or zero the Quick-Start rate request
    and QS TTL fields before encapsulation.  The result is that there
    are two viable options for IP tunnels to be compatible with Quick-
    Start.  The first option is the simple tunnel described above and in
    Section 6.1, where the tunnel is compatible with Quick-Start but



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    does not support Quick-Start, where all Quick-Start requests along
    the path will be rejected.  The second approach is a Quick-Start-
    capable mode, described in Section 6.3, where the tunnel actively
    supports Quick-Start.


6.1.  Simple Tunnels That Are Compatible with Quick-Start

    This section describes the ways that a simple tunnel can be
    compatible with Quick-Start but not support Quick-Start, resulting
    in the rejection of all Quick-Start requests that traverse the
    tunnel.

    If the tunnel ingress for the simple tunnel is at a router, the IP
    TTL of the inner header is generally decremented during forwarding
    before tunnel encapsulation takes place.  In this case TTL Diff will
    be changed, correctly causing the Quick-Start request to be
    rejected.  For a simple tunnel it is preferable if the Quick-Start
    Request is not copied to the outer header, saving the routers within
    the tunnel from unnecessarily processing the Quick-Start request.
    However, the Quick-Start request will be rejected correctly in this
    case whether or not the Quick-Start Request is copied to the outer
    header.


6.1.1.  Simple Tunnels that are Aware of Quick-Start

    If a tunnel ingress is aware of Quick-Start, but does not want to
    support Quick-Start, then the tunnel ingress MUST either zero the
    Quick-Start rate request, QS TTL, and QS Nonce fields or remove the
    Quick-Start option from the inner header before encapsulation.
    Section 6.3 describes the procedures for a tunnel that does want to
    support Quick-Start.

    Deleting the Quick-Start option or zeroing the Quick-Start rate
    request *after decapsulation* also serves to prevent the propagation
    of Quick-Start information, and is compatible with Quick-Start.  If
    the outer header does not contain a Quick-Start Request, a Quick-
    Start-aware tunnel egress MUST reject the inner Quick-Start Request
    by zeroing the Rate Request field in the inner header, or by
    deleting the Quick-Start option.

    If the tunnel ingress is at a sending host or router where the IP
    TTL is not decremented prior to encapsulation, and neither tunnel
    endpoint is aware of Quick-Start, then this allows false positives,
    described in the section below.





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6.2.  Simple Tunnels That Are Not Compatible with Quick-Start


    Sometimes a tunnel implementation that does not support Quick-Start
    is independent of the TCP sender or a router implementation that
    supports Quick-Start.  In these cases it is possible that a Quick-
    Start Request gets erroneously approved without the routers in the
    tunnel having individually approved the request, causing a false
    positive.

    If a tunnel ingress is a separate component from the TCP sender or
    IP forwarding, it is possible that a packet with a Quick-Start
    option is encapsulated without the IP TTL being decremented first,
    or with both IP TTL and QS TTL being decremented before the tunnel
    encapsulation takes place. If the tunnel ingress does not know about
    Quick-Start, a valid Quick-Start Request with unchanged TTL Diff
    traverses in the inner header, while the outer header most likely
    does not carry a Quick-Start Request.  If the tunnel egress also
    does not support Quick-Start, it remains possible that the Quick-
    Start Request would be falsely approved, because the packet is
    decapsulated using the Quick-Start request from the inner header,
    and the value of TTL Diff echoed to the sender remains unchanged.
    For example, such a scenario can occur with a Bump-In-The-Stack
    (BITS), an IPSec encryption implementation where the data encryption
    occurs between the network drivers and the TCP/IP protocol stack
    [RFC4301].

    As one example, if a remote access VPN client uses a BITS structure,
    then Quick-Start obstacles between the client and the VPN gateway
    won't be seen.   This is a particular problem because the path
    between the client and the VPN gateway is likely to contain the most
    congested part of the path.  Because most VPN clients are reported
    to use BITS [H05], we will explore this in more detail.

    A Bump-In-The-Wire (BITW) is an IPSec encryption implementation
    where the encryption occurs on an outboard processor, offloading the
    encryption processing overhead from the host or router [RFC4301].
    The BITW device is usually IP addressable, which means that the IP
    TTL is decremented before the packet is passed to the BITW.  If the
    QS TTL is not decremented, then the value of TTL Diff is changed,
    and the Quick-Start request will be denied.  However, if the BITW
    supports a host and does not have its own IP address, then the IP
    TTL is not decremented before the packet is passed from the host to
    the BITW, and a false positive could occur.

    Other tunnels that need to be looked at are IP tunnels over non-
    network protocols, such as IP over TCP and IP over UDP [RFC3948],
    and tunnels using the Layer Two Tunneling Protocol [RFC2661].



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    Section 9.2 discusses the related issue of non-IP queues, such as
    layer-two Ethernet or ATM networks, as another instance of possible
    bottlenecks that do not participate in the Quick-Start feedback.


6.3.  Tunnels That Support Quick-Start

    This section discusses tunnels configured to support Quick-Start.

    If the tunnel ingress node chooses to locally approve the Quick-
    Start request, then the ingress node MUST decrement the Quick-Start
    TTL at the same time it decrements the IP TTL, and MUST copy IP TTL
    and the Quick-Start option from the inner IP header to the outer
    header.  During encapsulation, the tunnel ingress MUST zero the
    Quick-Start rate request field in the inner header to ensure that
    the Quick-Start request will be rejected if the tunnel egress does
    not support Quick-Start.

    If the tunnel ingress node does not choose to locally approve the
    Quick-Start request, then it MUST either delete the Quick-Start
    option from the inner header before encapsulation, or zero the QS
    TTL and the Rate Request fields before encapsulation.

    Upon decapsulation, if the outer header contains a Quick-Start
    option, the tunnel egress MUST copy the IP TTL and the Quick-Start
    option from the outer IP header to the inner header.

    IPsec uses the IKE (Internet Key Exchange) Protocol for security
    associations.  We do not consider the interactions between Quick-
    Start and IPsec with IKEv1 [RFC2409] in this document.  Now that the
    RFC for IKEv2 [RFC4306] is published, we plan to specify a
    modification of IPsec to allow the support of Quick-Start to be
    negotiated; this modification will specify the negotiation between
    tunnel endpoints to allow or forbid support for Quick-Start within
    the tunnel.  This was done for ECN for IPsec tunnels, with IKEv1
    [RFC3168, Section 9.2].  This negotiation of Quick-Start capability
    in an IPsec tunnel will be specified in a separate IPsec document.
    This document will also include a discussion of the potential
    effects of an adversary's modifications of the Quick-Start field (as
    in Sections 18 and 19 of RFC 3168), and of the security
    considerations of exposing the Quick-Start rate request to network
    scanners.









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6.4.  Quick-Start and MPLS

    The behavior of Quick-Start with MPLS is similar to the behavior of
    Quick-Start with IP Tunnels.  For those MPLS paths where the IP TTL
    is decremented as part of traversing the MPLS path, these paths are
    compatible with Quick-Start, but do not support Quick-Start; Quick-
    Start requests traversing these paths will be correctly understood
    by the transport sender as having been denied.  Any MPLS paths where
    the IP TTL is not decremented as part of traversing the MPLS path
    would be not compatible with Quick-Start; such paths would result in
    false positives, where the TCP sender incorrectly believes that the
    Quick-Start Request was approved by all routers along the path.

    For cases where the ingress node to the MPLS path is aware of Quick-
    Start, this node should either zero the Quick-Start rate request, QS
    TTL, and QS Nonce fields or remove the Quick-Start option from the
    IP header.


7.  The Quick-Start Mechanism in other Transport Protocols

    The section earlier specified the use of Quick-Start in TCP.  In
    this section, we generalize this to give guidelines for the use of
    Quick-Start with other transport protocols.  We also discuss briefly
    how Quick-Start could be specified for other transport protocols.

    The general guidelines for Quick-Start in transport protocols are as
    follows:

    * Quick-Start is only specified for unicast transport protocols with
    appropriate congestion control mechanisms.  Note: Quick-Start is not
    a replacement for standard congestion control techniques, but meant
    to augment their operation.

    * A transport-level mechanism is needed for the Quick-Start response
    from the receiver to the sender.  This response contains the Rate
    Request, TTL Diff, and QS Nonce.

    * The sender checks the validity of the Quick-Start response.

    * The sender has an estimate of the round-trip time, and translates
    the Quick-Start response into an allowed window or allowed sending
    rate.  The sender sends a Report of the Approved Rate.  The sender
    starts sending Quick-Start packets, rate-paced out at the approved
    sending rate.

    * After the sender receives the first acknowledgement packet for a
    Quick-Start packet, no more Quick-Start packets are sent.  The



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    sender adjusts its current congestion window or sending rate to be
    consistent with the actual amount of data that was transmitted in
    that round-trip time.

    * When the last Quick-Start packet is acknowledged, the sender
    continues using the standard congestion control mechanisms of that
    protocol.

    * If one of the Quick-Start packets is lost, then the sender reverts
    to the standard congestion control method of that protocol that
    would have been used if the Quick-Start request had not been
    approved.  In addition, the sender takes into account the
    information that the Quick-Start congestion window was too large
    (e.g., by decreasing ssthresh in TCP).


8.  Using Quick-Start


8.1.  Determining the Rate to Request

    As discussed in [SAF06], the data sender does not necessarily have
    information about the size of the data transfer at connection
    initiation; for example, in request-response protocols such as HTTP,
    the server doesn't know the size or name of the requested object
    during connection initiation.  [SAF06] explores some of the
    performance implications of overly-large Quick-Start requests, and
    discusses heuristics that end-nodes could use to size their requests
    appropriately.  For example, the sender might have information about
    the bandwidth of the last-mile hop, the size of the local socket
    buffer, or of the TCP receive window, and could use this information
    in determining the rate to request.  Web servers that mostly have
    small objects to transfer might decide not to use Quick-Start at
    all, since Quick-Start would be of little benefit to them.

    Quick-Start will be more effective if Quick-Start requests are not
    larger than necessary;  every Quick-Start request that is approved
    but not used (or not fully used) takes away from the bandwidth pool
    available for granting successive Quick-Start requests.


8.2.  Deciding the Permitted Rate Request at a Router

    In this section we briefly outline how a router might decide whether
    or not to approve a Quick-Start Request.  The router should ask the
    following questions:

    * Has the router's output link been underutilized for some time



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    (e.g., several seconds).

    * Would the output link remain underutilized if the arrival rate was
    to increase by the aggregate rate requests that the router has
    approved over the last fraction of a second?

    In order to answer the last question, the router must have some
    knowledge of the available bandwidth on the output link and of the
    Quick-Start bandwidth that could arrive due to recently-approved
    Quick-Start Requests.  In this way, if an underutilized router
    experiences a flood of Quick-Start requests, the router can begin to
    deny Quick-Start requests while the output link is still
    underutilized.

    A simple way for the router to keep track of the potential bandwidth
    from recently-approved requests is to maintain two counters, one for
    the total aggregate Rate Requests that have been approved in the
    current time interval [T1, T2], and one for the total aggregate Rate
    Requests approved over a previous time interval [T0, T1].  However,
    this document doesn't specify router algorithms for approving Quick-
    Start requests, or make requirements for the appropriate time
    intervals for remembering the aggregate approved Quick-Start
    bandwidth.  A possible router algorithm is given in Appendix E, and
    more discussion of these issues is available in [SAF06].)

    * If the router's output link has been underutilized and the
    aggregate of the Quick-Start Request Rate options granted is low
    enough to prevent a near-term bandwidth shortage, then the router
    could approve the Quick-Start Request.

    Section 10.2 discusses some of the implementation issues in
    processing Quick-Start requests at routers.  [SAF06] discusses the
    range of possible Quick-Start algorithms at the router for deciding
    whether to approve a Quick-Start request.  In order to explore the
    limits of the possible functionality at routers, [SAF06] also
    discusses Extreme Quick-Start mechanisms at routers, where the
    router would keep per-flow state concerning approved Quick-Start
    requests.


9.  Evaluation of Quick-Start


9.1.  Benefits of Quick-Start

    The main benefit of Quick-Start is the faster start-up for the
    transport connection itself.  For a small TCP transfer of one to
    five packets, Quick-Start is probably of very little benefit;  at



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    best, it might shorten the connection lifetime from three to two
    round-trip times (including the round-trip time for connection
    establishment).  Similarly, for a very large transfer, where the
    slow-start phase would have been only a small fraction of the
    connection lifetime, Quick-Start would be of limited benefit.
    Quick-Start would not significantly shorten the connection lifetime,
    but it might eliminate or at least shorten the start-up phase.
    However, for moderate-sized connections in a well-provisioned
    environment, Quick-Start could possibly allow the entire transfer of
    M packets to be completed in one round-trip time (after the initial
    round-trip time for the SYN exchange), instead of the log_2(M)-2
    round-trip times that it would normally take for the data transfer,
    in an uncongested environments (assuming an initial window of four
    packets).


9.2.  Costs of Quick-Start

    This section discusses the costs of Quick-Start for the connection
    and for the routers along the path.

    The cost of having a Quick-Start packet dropped:
    For the sender the biggest risk in using Quick-Start lies in the
    possibility of suffering from congestion-related losses of the
    Quick-Start packets.  This should be an unlikely situation because
    routers are expected to approve Quick-Start Requests only when they
    are significantly underutilized. However, a transient increase in
    cross-traffic in one of the routers, a sudden decrease in available
    bandwidth on one of the links, or congestion at a non-IP queue could
    result in packet losses even when the Quick-Start Request was
    approved by all of the routers along the path.  If a Quick-Start
    packet is dropped, then the sender reverts to the congestion control
    mechanisms it would have used if the Quick-Start request had not
    been approved, so the performance cost to the connection of having a
    Quick-Start packet dropped is small, compared to the performance
    without Quick-Start.  (On the other hand, the performance difference
    between Quick-Start with a Quick-Start packet dropped and Quick-
    Start with no Quick-Start packet dropped can be considerable.)

    Added complexity at routers:
    The main cost of Quick-Start at routers concerns the costs of added
    complexity.  The added complexity at the end-points is moderate, and
    might easily be outweighed by the benefit of Quick-Start to the end
    hosts.  The added complexity at the routers is also somewhat
    moderate; it involves estimating the unused bandwidth on the output
    link over the last several seconds, processing the Quick-Start
    request, and keeping a counter of the aggregate Quick-Start rate
    approved over the last fraction of a second.  However, this added



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    complexity at routers adds to the development cycle, and could
    prevent the addition of other competing functionality to routers.
    Thus, careful thought would have to be given to the addition of
    Quick-Start to IP.

    The slow path in routers:
    Another drawback of Quick-Start is that packets containing the
    Quick-Start Request message might not take the fast path in routers,
    particularly in the beginning of Quick-Start's deployment in the
    Internet.  This would mean some extra delay for the end hosts, and
    extra processing burden for the routers.  However, as discussed in
    Sections 4.1 and 4.7, not all packets would carry the Quick-Start
    option.  In addition, for the underutilized links where Quick-Start
    Requests could actually be approved, or in typical environments
    where most of the packets belong to large flows, the burden of the
    Quick-Start Option on routers would be considerably reduced.
    Nevertheless, it is still conceivable, in the worst case, that many
    packets would carry Quick-Start requests; this could slow down the
    processing of Quick-Start packets in routers considerably.  As
    discussed in Section 9.6, routers can easily protect against this by
    enforcing a limit on the rate at which Quick-Start requests will be
    considered.  [RW03] and [RW04] contain measurements of the impact of
    IP Option Processing on packet round-trip times.

    Multiple paths:
    One limitation of Quick-Start is that it presumes that the data
    packets of a connection will follow the same path as the Quick-Start
    request packet.  If this is not the case, then the connection could
    be sending the Quick-Start packets, at the approved rate, along a
    path that was already congested, or that became congested as a
    result of this connection.  Thus, Quick-Start could give poor
    performance when there is a routing change immediately after the
    Quick-Start request is approved, and the Quick-Start data packets
    follow a different path from that of the original Quick-Start
    Request.  This is, however, similar to what would happen, for a
    connection with sufficient data, if the connection's path was
    changed in the middle of the connection, when the connection had
    already established the allowed initial rate.

    As specified in Section 3.3, a router that uses multipath routing
    for packets within a single connection must not approve a Quick-
    Start request.  Quick-Start would not perform robustly in an
    environment with multipath routing, where different packets in a
    connection routinely follow different paths.  In such an
    environment, the Quick-Start request and some fraction of the
    packets in the connection might take an underutilized path, while
    the rest of the packets take an alternate, congested path.




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    Non-IP queues:
    A problem of any mechanism for feedback from routers at the IP level
    is that there can be queues and bottlenecks in the end-to-end path
    that are not in IP-level routers.  As an example, these include
    queues in layer-two Ethernet or ATM networks.  One possibility would
    be that an IP-level router adjacent to such a non-IP queue or
    bottleneck would be configured to reject Quick-Start requests if
    that was appropriate.  One would hope that in general, IP networks
    are configured so that non-IP queues between IP routers do not end
    up being the congested bottlenecks.


9.3.  Quick-Start with QoS-enabled Traffic

    The discussion in this document has largely been of Quick-Start with
    default, best-effort traffic.  However, Quick-Start could also be
    used by traffic using some form of differentiated services, and
    routers could take the traffic class into account when deciding
    whether or not to grant the Quick-Start request.  We don't address
    this context further in this paper, since it is orthogonal to the
    specification of Quick-Start.

    Routers are also free to take into account their own priority
    classifications in processing Quick-Start requests.


9.4.  Protection against Misbehaving Nodes

    In this section we discuss the protection against senders,
    receivers, or colluding routers or middleboxes lying about the
    Quick-Start Request.


9.4.1.  Misbehaving Senders

    A transport sender could try to transmit data at a higher rate than
    that approved in the Quick-Start Request.  The network could use a
    traffic policer to protect against misbehaving senders that exceed
    the approved rate, for example by dropping packets that exceed the
    allowed transmission rate. The required Report of Approved Rate
    allows traffic policers to check that the Report of Approved Rate
    does not exceed the Rate Request actually approved at that point in
    the network in the previous Quick-Start Request from that
    connection.  The required Approved Rate report also allows traffic
    policers to check that the sender's sending rate does not exceed the
    rate in the Report of Approved Rate.

    If a router or receiver receives an Approved Rate report that is



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    larger than the Rate Request in the Quick-Start request approved for
    that sender for that connection in the previous round-trip time,
    then the router or receiver could deny future Quick-Start requests
    from that sender, e.g., by deleting the Quick-Start Request from
    future packets from that sender.  We note that routers are not
    required to use Approved Rate reports to check if senders are
    cheating; this is at the discretion of the router.

    If a router sees a Report of Approved Rate, and did not see an
    earlier Quick-Start request, then either the sender could be
    cheating, or the connection's path could have changed since the
    Quick-Start request was sent.  In either case, the router could
    decide to deny future Quick-Start requests for this connection.  In
    particular, it is reasonable for the router to deny a Quick-Start
    request if either the sender is cheating, or if the connection path
    suffers from path changes or multipathing.

    If a router approved a Quick-Start Request, but does not see a
    subsequent Approved Rate report, then there are several
    possibilities: (1) the request was denied and/or dropped downstream
    and the sender did not send a Report of Approved Rate; (2) the
    request was approved but the sender did not send a Report of
    Approved Rate; (3) the Approved Rate report was dropped in the
    network; or (4) the Approved Rate report took a different path from
    the Quick-Start Request.  In any of these cases, the router would be
    justified in denying future Quick-Start Requests for this
    connection.

    In any of the cases mentioned in the three paragraphs above (i.e.,
    an Approved Rate report that is larger than the Rate Request in the
    earlier Quick-Start request; a Report of Approved Rate with no
    preceding Rate Request, or a Rate Request with no Report of Approved
    Rate), a traffic policer may assume that Quick-Start is not being
    used appropriately, or is being used in an unsuitable environment
    (e.g., with multiple paths), and take some corresponding action.

    What are the incentives for a sender to cheat by over-sending after
    a Quick-Start request?  Assuming that the sender's interests are
    measured by a performance metric such as the completion time for its
    connections, sometimes it might be in the sender's interests to
    cheat, and sometimes it might not;  in some cases it could be
    difficult for the sender to judge whether it would be in its
    interests to cheat.  The incentives for a sender to cheat by over-
    sending after a Quick-Start request are not that different from the
    incentives for a sender to cheat by over-sending even in the absence
    of Quick-Start, with one difference:  the use of Quick-Start could
    help a sender to evade policing actions from policers in the
    network.  The Report of Approved Rate is designed to address this,



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    to make it harder to senders to use Quick-Start to `cover' their
    cheating.


9.4.2.  Receivers Lying about Whether the Request was Approved

    One form of misbehavior would be for the receiver to lie to the
    sender about whether the Quick-Start Request was approved, by
    falsely reporting the TTL Diff and QS Nonce.  If a router that
    understands the Quick-Start Request denies the request by deleting
    the request or by zeroing the QS TTL and QS Nonce, then the receiver
    can ``lie" about whether the request was approved only by
    successfully guessing the value of the TTL Diff and QS Nonce to
    report.  The chance of the receiver successfully guessing the
    correct value for the TTL Diff is 1/256, and the chance of the
    receiver successfully guessing the QS nonce for a reported rate
    request of K is 1/(2K).

    However, if the Quick-Start request is denied only by a non-Quick-
    Start-capable router, or by a router that is unable to zero the QS
    TTL and QS Nonce fields, then the receiver could lie about whether
    the Quick-Start Requests were approved by modifying the QS TTL in
    successive requests received from the same host.  In particular, if
    the sender does not act on a Quick-Start Request, then the receiver
    could decrement the QS TTL by one in the next request received from
    that host before calculating the TTL Diff, and decrement the QS TTL
    by two in the following received request, until the sender acts on
    one of the Quick-Start Requests.

    Unfortunately, if a router doesn't understand Quick-Start, then it
    is not possible for that router to take an active step such as
    zeroing the QS TTL and QS Nonce to deny a request.  As a result, the
    QS TTL is not a fail-safe mechanism for preventing lying by
    receivers in the case of non-Quick-Start-capable routers.

    What would be the incentives for a receiver to cheat in reporting on
    a Quick-Start request, in the absence of a mechanism such as the QS
    Nonce?  In some cases, cheating would have been of clear benefit to
    the receiver, resulting in a faster completion time for the
    transfer.  In other cases, where cheating would have resulted in
    Quick-Start packets being dropped in the network, cheating might or
    might not have improved the receiver's performance metric, depending
    on the details of that particular scenario.








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9.4.3.  Receivers Lying about the Approved Rate

    A second form of receiver misbehavior would be for the receiver to
    lie to the sender about the Rate Request for an approved Quick-Start
    Request, by increasing the value of the Rate Request field.
    However, the receiver doesn't necessarily know the Rate Request in
    the original Quick-Start Request sent by the sender, and a higher
    Rate Request reported by the receiver will only be considered valid
    by the sender if it is no higher than the Rate Request originally
    requested by the sender.  For example, if the sender sends a Quick-
    Start Request with a Rate Request of X, and the receiver reports
    receiving a Quick-Start Request with a Rate Request of Y > X, then
    the sender knows that either some router along the path
    malfunctioned (increasing the Rate Request inappropriately), or the
    receiver is lying about the Rate Request in the received packet.

    If the sender sends a Quick-Start Request with a Rate Request of Z,
    the receiver receives the Quick-Start Request with an approved Rate
    Request of X, and reports a Rate Request of Y, for X < Y <= Z, then
    the receiver only succeeds in lying to the sender about the approved
    rate if the receiver successfully reports the rightmost 2Y bits in
    the QS nonce.

    If senders often use a configured default value for the Rate
    Request, then receivers would often be able to guess the original
    Rate Request, and this would make it easier for the receiver to lie
    about the value of the Rate Request field.  Similarly, if the
    receiver often communicates with a particular sender, and the sender
    always uses the same Rate Request for that receiver, then the
    receiver might over time be able to infer the original Rate Request
    used by the sender.

    There are several possible additional forms of protection against
    receivers lying about the value of the Rate Request.  One possible
    additional protection would be for a router that decreases a Rate
    Request in a Quick-Start Request to report the decrease directly to
    the sender.  However, this could lead to many reports back to the
    sender for a single request, and could also be used in address-
    spoofing attacks.

    A second limited form of protection would be for senders to use some
    degree of randomization in the requested Rate Request, so that it is
    difficult for receivers to guess the original value for the Rate
    Request.  However, this is difficult because there is a fairly
    coarse granularity in the set of rate requests available to the
    sender, and randomizing the initial request only offers limited
    protection in any case.




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9.4.4.  Collusion between Misbehaving Routers

    In addition to protecting against misbehaving receivers, it is
    necessary also to protect against misbehaving routers.  Consider
    collusion between an ingress router and an egress router belonging
    to the same Intranet.  The ingress router could decrement the Rate
    Request at the ingress, with the egress router increasing it again
    at the egress.  The routers between the ingress and egress that
    approved the decremented rate request might not have been willing to
    approve the larger, original request.

    Another form of collusion would be for the ingress router to inform
    the egress router out-of-band of the TTL Diff and QS Nonce for the
    request packet at the ingress.  This would enable the egress router
    to modify the QS TTL and QS Nonce so that it appeared that all of
    the routers along the path had approved the request.  There does not
    appear to be any protection against a colluding ingress and egress
    router.  Even if an intermediate router had deleted the Quick-Start
    Option from the packet, the ingress router could have sent the
    Quick-Start Option to the egress router out-of-band, with the egress
    router inserting the Quick-Start Option, with a modified QS TTL
    field, back in the packet.

    However, unlike ECN, there is somewhat less incentive for
    cooperating ingress and egress routers to collude to falsely modify
    the Quick-Start Request so that it appears to have been approved by
    all of the routers along the path.  With ECN, a colluding ingress
    router could falsely mark a packet as ECN-capable, with the
    colluding egress router returning the ECN field in the IP header to
    its original non-ECN-capable codepoint, and congested routers along
    the path could have been fooled into not dropping that packet.  This
    collusion would give an unfair competitive advantage to the traffic
    protected by the colluding ingress and egress routers.

    In contrast, with Quick-Start, the collusion of the ingress and
    egress routers to make it falsely appear that a Quick-Start request
    was approved sometimes could give an advantage to the traffic
    covered by that collusion, and sometimes would give a disadvantage,
    depending on the details of the scenario.  If some router along the
    path really does not have enough available bandwidth to approve the
    Quick-Start request, then Quick-Start packets sent as a result of
    the falsely-approved request could be dropped in the network, to the
    possible disadvantage of the connection.  Thus, while the ingress
    and egress routers could collude to prevent intermediate routers
    from denying a Quick-Start request, it would not always be to the
    connection's advantage for this to happen.  One defense against such
    a collusion would be for some router between the ingress and egress
    nodes that denied the request to monitor connection performance,



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    penalizing connections that seeem to be using Quick-Start after a
    Quick-Start request was denied, or that are reporting an Approved
    Rate higher than that actually approved by that router.

    If the congested router is ECN-capable, and the colluding ingress
    and egress routers are lying about ECN-capability as well as about
    Quick-Start, then the result could be that the Quick-Start request
    falsely appears to the sender to have been approved, and the Quick-
    Start packets falsely appear to the congested router to be ECN-
    capable.  In this case, the colluding routers might succeed in
    giving a competitive advantage to the traffic protected by their
    collusion (if no intermediate router is monitoring to catch such
    misbehavior).


9.5.  Misbehaving Middleboxes and the IP TTL

    One possible difficulty is that of traffic normalizers [HKP01] or
    other middleboxes along that path that re-write IP TTLs in order to
    foil other kinds of attacks in the network.  If such a traffic
    normalizer re-wrote the IP TTL, but did not adjust the Quick-Start
    TTL by the same amount, then the sender's mechanism for determining
    if the request was approved by all routers along the path would no
    longer be reliable.  Re-writing the IP TTL could result in false
    positives (with the sender incorrectly believing that the Quick-
    Start request was approved) as well as false negatives (with the
    sender incorrectly believing that the Quick-Start request was
    denied).


9.6.  Attacks on Quick-Start

    As discussed in [SAF06], Quick-Start is vulnerable to two kinds of
    attacks:  (1) attacks to increase the routers' processing and state
    load; and (2) attacks with bogus Quick-Start requests to temporarily
    tie up available Quick-Start bandwidth, preventing routers from
    approving Quick-Start requests from other connections.  Routers can
    protect against the first kind of attack by applying a simple limit
    on the rate at which Quick-Start requests will be considered by the
    router.

    The second kind of attack, to tie up the available Quick-Start
    bandwidth, is more difficult to defend against.  As discussed in
    [SAF06]. Quick-Start Requests that are not going to be used, either
    because they are from malicious attackers or because they are denied
    by routers downstream, can result in short-term `wasting' potential
    Quick-Start bandwidth, resulting in routers denying subsequent
    Quick-Start Requests that if approved would in fact have been used.



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    We note that the likelihood of malicious attacks would be minimized
    significantly when Quick-Start was deployed in a controlled
    environment such as an Intranet, where there was some form of
    centralized control over the users in the system.  We also note that
    this form of attack could potentially make Quick-Start unusable, but
    it would not do any further damage; in the worst case, the network
    would function as a network without Quick-Start.

    [SAF06] considers the potential of Extreme Quick-Start algorithms at
    routers, which keep per-flow state for Quick-Start connections, in
    protecting the availability of Quick-Start bandwidth in the face of
    frequent overly-large Quick-Start requests.


9.7.  Simulations with Quick-Start

    Quick-Start was added to the NS simulator [SH02] by Srikanth
    Sundarrajan, and additional functionality was added by Pasi
    Sarolahti.  The validation test is at `test-all-quickstart' in the
    `tcl/test' directory in NS.  The initial simulation studies from
    [SH02] show a significant performance improvement using Quick-Start
    for moderate-sized flows (between 4KB and 128KB) in under-utilized
    environments.  These studies are of file transfers, with the
    improvement measured as the relative increase in the overall
    throughput for the file transfer.  The study shows that potential
    improvement from Quick-Start is proportional to the delay-bandwidth
    product of the path.

    The Quick-Start simulations in [SAF06] explore the following: the
    potential benefit of Quick-Start for the connection; the relative
    benefits of different router-based algorithms for approving Quick-
    Start requests; and the effectiveness of Quick-Start as a function
    of the senders' algorithms for choosing the size of the rate
    request.


10.  Implementation and Deployment Issues

    This section discusses some of the implementation issues with Quick-
    Start.   This section also discusses some of the key deployment
    issues, such as the chicken-and-egg deployment problems of
    mechanisms that have to be deployed in both routers and end nodes in
    order to work, and the problems posed by the wide deployment of
    middleboxes today that block the use of known or unknown IP Options.







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10.1.  Implementation Issues for Sending Quick-Start Requests

    Section 4.7 discusses some of the issues with deciding the initial
    sending rate to request.  Quick-Start raises additional issues about
    the communication between the transport protocol and the
    application, and about the use of the past history with Quick-Start
    in the end node.

    One possibility is that a protocol implementation could provide an
    API for applications to indicate when they want to request Quick-
    Start, and what rate they would like to request.  In the
    conventional socket API this could be a socket option that is set
    before a connection is established.  Some applications, such as
    those that use TCP for bulk transfers, do not have interest in the
    transmission rate, but they might know the amount of data that can
    be sent immediately. Based on this, the sender implementation could
    decide whether Quick-Start would be useful, and what rate should be
    requested.

    We note that when Quick-Start is used, the TCP sender is required to
    save the QS Nonce and the TTL Diff when the Quick-Start request is
    sent, and to implement an additional timer for the paced
    transmission of Quick-Start packets.


10.2.  Implementation Issues for Processing Quick-Start Requests

    A router or other network host must be able to determine the
    approximate bandwidth of its outbound network interfaces in order to
    process incoming Quick-Start rate requests, including those that
    originate from the host itself.  One possibility would be for hosts
    to rely on configuration information to determine link bandwidths;
    this has the drawback of not being robust to errors in
    configuration.  Another possibility would be for network device
    drivers to infer the bandwidth for the interface and to communicate
    this to the IP layer.

    Particular issues will arise for wireless links with variable
    bandwidth, where decisions will have to be made about how frequently
    the host gets updates of the changing bandwidth.  It seems
    appropriate that Quick-Start Requests would be handled particularly
    conservatively for links with variable bandwidth, to avoid cases
    where Quick-Start Requests are approved, the link bandwidth is
    reduced, and the data packets that are sent end up being dropped.

    Difficult issues also arise for paths with multi-access links (e.g.,
    Ethernet).  Routers or end-nodes with multi-access links should be
    particularly conservative in granting Quick-Start requests.  In



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    particular, for some multi-access links there may be no procedure
    for an attached node to use to determine whether all parts of the
    multi-access link have been underutilized in the recent past.


10.3.  Possible Deployment Scenarios

    Because of possible problems discussed above concerning using Quick-
    Start over some network paths and the security issues discussed in
    section 11, the most realistic initial deployment of Quick-Start
    would most likely take place in Intranets and other controlled
    environments.  Quick-Start is most useful on high bandwidth-delay
    paths that are significantly underutilized. The primary initial
    users of Quick-Start would likely be in organizations that provide
    network services to their users and also have control over a large
    portion of the network path.

    Quick-Start is not currently intended for ubiquitous deployment in
    the global Internet.  In particular, Quick-Start should not be
    enabled by default in end-nodes or in routers; instead, when Quick-
    Start is used, it should be explicitly enabled by users or system
    administrators.

    Below are a few examples of networking environments where Quick-
    Start would potentially be useful.  These are the environments that
    might consider an initial deployment of Quick-Start in the routers
    and end-nodes, where the incentives for routers to deploy Quick-
    Start might be the most clear.

    * Centrally-administrated organizational Intranets: These intranets
    often have large network capacity, with networks that are
    underutilized for much of the time [PABL+05].  Such Intranets might
    also include high-bandwidth and high-delay paths to remote sites.
    In such an environment, Quick-Start would be of benefit to users,
    and there would be a clear incentive for the deployment of Quick-
    Start in routers.  For example, Quick-Start could be quite useful in
    high-bandwidth networks used for scientific computing.

    * Wireless networks: Quick-Start could also be useful in high-delay
    environments of Cellular Wide-Area Wireless Networks such as the
    GPRS [BW97] and their enhancements and next generations. For
    example, GPRS EDGE (Enhanced Data for GSM Evolution) is expected to
    provide wireless bandwidth of up to 384 Kbps (roughly 32 1500-byte
    packets per second) while the GPRS round-trip times range typically
    from few hundred milliseconds to over a second excluding any
    possible queueing delays in the network [GPAR02]. In addition, these
    networks sometimes have variable additional delays due to resource
    allocation that could be avoided by keeping the connection path



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    constantly utilized, starting from initial slow-start.  Thus, Quick-
    Start could be of significant benefit to users in these
    environments.

    * Paths over satellite links: Geostationary Orbit (GEO) satellite
    links have one-way propagation delays on the order of 250 ms while
    the bandwidth can be measured in megabits per second [RFC2488].
    Because of the considerable bandwidth-delay product on the link,
    TCP's slow-start is a major performance limitation in the beginning
    of the connection.  A large initial congestion window would be
    useful to users of such satellite links.

    * Single-hop paths: Quick-Start should work well over point-to-point
    single-hop paths, e.g., from a host to an adjacent server.  Quick-
    Start would work over a single-hop IP path consisting of a multi-
    access link only if the host was able to determine if the path to
    the next IP hop has been significantly underutilized over the recent
    past.  If the multi-access link includes a layer-2 switch, then the
    attached host cannot necessarily determine the status of the other
    links in the layer-2 network.


10.4.  A Comparison with the Deployment Problems of ECN

    Given the glacially slow rate of deployment of ECN in the Internet
    to date [MAF05], it is disconcerting to note that some of the
    deployment problems of Quick-Start are even greater than those of
    ECN.  First, unlike ECN, which can be of benefit even if it is only
    deployed on one of the routers along the end-to-end path, a
    connection's use of Quick-Start requires Quick-Start deployment on
    all of the routers along the end-to-end path.  Second, unlike ECN,
    which uses an allocated field in the IP header, Quick-Start requires
    the extra complications of an IP Option, which can be difficult to
    pass through the current Internet [MAF05].

    However, in spite of these issues, there is some hope for the
    deployment of Quick-Start, at least in protected corners of the
    Internet, because the potential benefits of Quick-Start to the user
    are considerably more dramatic than those of ECN.  Rather than
    simply replacing the occasional dropped packet by an ECN-marked
    packet, Quick-Start is capable of dramatically increasing the
    throughput of connections in underutilized environments [SAF06].


11.  Security Considerations

    Sections 9.4 and 9.6 discuss the security considerations related to
    Quick-Start.  Section 9.4 discusses the potential abuse of Quick-



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    Start by senders or receivers lying about whether the request was
    approved or about the approved rate, and of routers in collusion to
    misuse Quick-Start.  Section 9.5 discusses potential problems with
    traffic normalizers that rewrite IP TTLs in packet headers.  All of
    these problems could result in the sender using a Rate Request that
    was inappropriately large, or thinking that a request was approved
    when it was in fact denied by at least one router along the path.
    This inappropriate use of Quick-Start could result in congestion and
    an unacceptable level of packet drops along the path, Such
    congestion could also be part of a Denial of Service attack.

    Section 9.6 discusses a potential attack on the routers' processing
    and state load from an attack of Quick-Start Requests.  Section 9.6
    also discusses a potential attack on the available Quick-Start
    bandwidth by sending bogus Quick-Start requests for bandwidth that
    will not in fact be used.  While this impacts the global usability
    of Quick-Start it does not endanger the network as a whole since TCP
    uses standard congestion control if Quick-Start is not available.

    Section 4.7.2 discusses the potential problem of packets with Quick-
    Start Requests dropped by middleboxes along the path.

    As discussed in Section 5, for IPv4 IPsec Authentication Header
    Integrity Check Value (AH ICV) calculation, the Quick-Start option
    is a mutable IPv4 option, and hence completely zeroed for AH ICV
    calculation purposes; this is also the treatment required by RFC
    4302 for unrecognized IPv4 options.  The IPv6 Quick-Start option's
    IANA-allocated option type indicates that it is a mutable option,
    hence, according to RFC 4302, its option data is required to be
    zeroed for AH ICV computation purposes.  See RFC 4302 for further
    explanation.

    Section 6.2 discusses possible problems of Quick-Start used by
    connections carried over simple tunnels that are not compatible with
    Quick-Start.   In this case it is possible that a Quick-Start
    Request is erroneously considered approved by the sender without the
    routers in the tunnel having individually approved the request,
    causing a false positive.

    We note two high-order points here.  First, the Quick-Start Nonce
    goes a long way towards preventing large scale cheating.  And,
    second, even if a host occasionally uses Quick-Start when it is not
    approved by the entire network path the network will not collapse.
    Quick-Start does not remove TCP's basic congestion control
    mechanisms and these will kick in when the network is heavily
    loaded, relegating any Quick-Start mistake to a transient.





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12.  IANA Considerations

    Quick-Start requires an IP Option and a TCP Option.


12.1.  IP Option

    Quick-Start requires both an IPv4 Option Number (Section 3.1) and an
    IPv6 Option Number (Section 3.2).

    IPv4 Option Number:


    Copy Class Number Value Name
    ---- ----- ------ ----- ----
       0    00   TBD1  TBD2   QS    - Quick-Start


    IPv6 Option Number [RFC2460]:


    HEX         act  chg  rest
    ---         ---  ---  -----
    TBD3         00   1   TBD4       Quick-Start


    For the IPv6 Option Number, the first two bits indicate that the
    IPv6 node skip over this option and continue processing the header
    if it doesn't recognize the option type, and the third bit indicates
    that the Option Data may change en-route.

    In both cases this document should be listed as the reference
    document.


12.2.  TCP Option

    Quick-Start requires a TCP Option Number (Section 4.2).

    TCP Option Number:

       Kind Length Meaning
       ---- ------ ------------------------------
       TBD5 8      Quick-Start Response



    This document should be listed as the reference document.



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13.  Conclusions

    We are presenting the Quick-Start mechanism as a simple,
    understandable, and incrementally-deployable mechanism that would be
    sufficient to allow some connections to start up with large initial
    rates, or large initial congestion windows, in overprovisioned,
    high-bandwidth environments.  We expect there will be an increasing
    number of overprovisioned, high-bandwidth environments where the
    Quick-Start mechanism, or another mechanism of similar power, could
    be of significant benefit to a wide range of traffic.  We are
    presenting the Quick-Start mechanism as a request for the community
    to provide feedback and experimentation on issues relating to Quick-
    Start.


14.  Acknowledgements

    The authors wish to thank Mark Handley for discussions of these
    issues.  The authors also thank the End-to-End Research Group, the
    Transport Services Working Group, and members of IPAM's program on
    Large Scale Communication Networks for both positive and negative
    feedback on this proposal.  We thank Srikanth Sundarrajan for the
    initial implementation of Quick-Start in the NS simulator, and for
    the initial simulation study.  Many thanks to David Black and Joe
    Touch for extensive feedback on Quick-Start and IP tunnels.  We also
    thank Mohammed Ashraf, John Border, Bob Briscoe, Martin Duke, Tom
    Dunigan, Mitchell Erblich, Gorry Fairhurst, John Heidemann, Paul
    Hyder, Dina Katabi and Vern Paxson for feedback.  Thanks also to
    Gorry Fairhurst for the suggestion of adding the QS Nonce to the
    Report of Approved Rate.

    The version of the QS Nonce in this document is based on a proposal
    from Guohan Lu [L05].  Earlier versions of this document contained
    an eight-bit QS Nonce, and subsequent versions discussed the
    possibility of a four-bit QS Nonce.

    This draft builds upon the concepts described in [RFC3390], [AHO98],
    [RFC2415], and [RFC3168].  Some of the text on Quick-Start in
    tunnels was borrowed directly from RFC 3168.

    This document is the development of a proposal originally by Amit
    Jain for Initial Window Discovery.









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A.  Related Work

    The Quick-Start proposal, taken together with HighSpeed TCP
    [RFC3649] or other transport protocols for high-bandwidth transfers,
    could go a significant way towards extending the range of
    performance for best-effort traffic in the Internet.  However, there
    are many things that the Quick-Start proposal would not accomplish.
    Quick-Start is not a congestion control mechanism, and would not
    help in making more precise use of the available bandwidth, that is,
    of achieving the goal of high throughput with low delay and low
    packet loss rates.  Quick-Start would not give routers more control
    over the decrease rates of active connections.

    In addition, any evaluation of Quick-Start must include a discussion
    of the relative benefits of approaches that use no explicit
    information from routers, and of approaches that use more fine-
    grained feedback from routers as part of a larger congestion control
    mechanism.  We discuss several classes of proposals in the sections
    below.


A.1.  Fast Start-ups without Explicit Information from Routers

    One possibility would be for senders to use information from the
    packet streams to learn about the available bandwidth, without
    explicit information from routers.  These techniques would not allow
    a start-up as fast as that available from Quick-Start in an
    underutilized environment;  one has to have sent some packets
    already to use the packet stream to learn about available bandwidth.
    However, these techniques could allow a start-up considerably faster
    than the current slow-start.  While it seems clear that approaches
    *without* explicit feedback from the routers will be strictly less
    powerful that is possible *with* explicit feedback, it is also
    possible that approaches that are more aggressive than slow-start
    are possible without the complexity involved in obtaining explicit
    feedback from routers.

    Periodic packet streams:
    [JD02] explores the use of periodic packet streams to estimate the
    available bandwidth along a path.  The idea is that the one-way
    delays of a periodic packet stream show an increasing trend when the
    stream's rate is higher than the available bandwidth (due to an
    increasing queue).  While [JD02] states that the proposed mechanism
    does not cause significant increases in network utilization, losses,
    or delays when done by one flow at a time, the approach could be
    problematic if conducted concurrently by a number of flows.  [JD02]
    also gives an overview of some of the earlier work on inferring the
    available bandwidth from packet trains.



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    Swift-Start:
    The Swift Start proposal from [PRAKS02] combines packet-pair and
    packet-pacing techniques.  An initial congestion window of four
    segments is used to estimate the available bandwidth along the path.
    This estimate is then used to dramatically increase the congestion
    window during the second RTT of data transmission.

    SPAND:
    In the TCP/SPAND proposal from [ZQK00] for speeding up short data
    transfers, network performance information would be shared among
    many co-located hosts to estimate each connection's fair share of
    the network resources.  Based on such estimation and the transfer
    size, the TCP sender would determine the optimal initial congestion
    window size.  The design for TCP/SPAND uses a performance gateway
    that monitors all traffic entering and leaving an organization's
    network.

    Sharing information among TCP connections:
    The Congestion Manager [RFC3124] and TCP control block sharing
    [RFC2140] both propose sharing congestion information among multiple
    TCP connections with the same endpoints.  With the Congestion
    Manager, a new TCP connection could start with a high initial cwnd
    if it was sharing the path and the cwnd with a pre-existing TCP
    connection to the same destination that had already obtained a high
    congestion window.  RFC 2140 discusses ensemble sharing, where an
    established connection's congestion window could be `divided up' to
    be shared with a new connection to the same host.  However, neither
    of these approaches addresses the case of a connection to a new
    destination, with no existing or recent connection (and therefore
    congestion control state) to that destination.

    While continued research on the limits of the ability of TCP and
    other transport protocols to learn of available bandwidth without
    explicit feedback from the router seems useful, we note that there
    are several fundamental advantages of explicit feedback from
    routers.

    (1) Explicit feedback is faster than implicit feedback:
    One advantage of explicit feedback from the routers is that it
    allows the transport sender to reliably learn of available bandwidth
    in one round-trip time.

    (2) Explicit feedback is more reliable than implicit feedback:
    Techniques that attempt to assess the available bandwidth at
    connection startup using implicit techniques are more error-prone
    than techniques that involve every element in the network path.
    While explicit information from the network can be wrong, it has a
    much better chance of being appropriate than an end-host trying to



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    *estimate* an appropriate sending rate using "block box" probing
    techniques of the entire path.


A.2.  Optimistic Sending without Explicit Information from Routers

    Another possibility that has been suggested [S02] is for the sender
    to start with a large initial window without explicit permission
    from the routers and without bandwidth estimation techniques, and
    for the first packet of the initial window to contain information
    such as the size or sending rate of the initial window.  The
    proposal would be that congested routers would use this information
    in the first data packet to drop or delay many or all of the packets
    from that initial window.  In this way a flow's optimistically-large
    initial window would not force the router to drop packets from
    competing flows in the network.  Such an approach would seem to
    require some mechanism for the sender to ensure that the routers
    along the path understood the mechanism for marking the first packet
    of a large initial window.

    Obviously there would be a number of questions to consider about an
    approach of optimistic sending.

    (1) Incremental deployment:
    One question would be the potential complications of incremental
    deployment, where some of the routers along the path might not
    understand the packet information describing the initial window.

    (2) Congestion collapse:
    There could also be concerns about congestion collapse if many flows
    used large initial windows, many packets were dropped from
    optimistic initial windows, and many congested links ended up
    carrying packets that are only going to be dropped downstream.

    (3) Distributed Denial of Service attacks:
    A third question would be the potential role of optimistic senders
    in amplifying the damage done by a Distributed Denial of Service
    (DDoS) attack (assuming attackers use compliant congestion control
    in the hopes of "flying under the radar").

    (4) Performance hits if a packet is dropped:
    A fourth issue would be to quantify the performance hit to the
    connection when a packet is dropped from one of the initial windows.








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A.3.  Fast Start-ups with other Information from Routers

    There have been several proposals somewhat similar to Quick-Start,
    where the transport protocol collects explicit information from the
    routers along the path.

    An IP Option about the free buffer size:
    In related work, [P00] investigates the use of a slightly different
    IP option for TCP connections to discover the available bandwidth
    along the path.  In that proposal, the IP option would query the
    routers along the path about the smallest available free buffer
    size. Also, the IP option would have been sent after the initial SYN
    exchange, when the TCP sender already had an estimate of the round-
    trip time.

    The Performance Transparency Protocol:
    The Performance Transparency Protocol (PTP) includes a proposal for
    a single PTP packet that would collect information from routers
    along the path from the sender to the receiver [W00].  For example,
    a single PTP packet could be used to determine the bottleneck
    bandwidth along a path.

    ETEN:
    Additional proposals for end nodes to collect explicit information
    from routers include one variant of Explicit Transport Error
    Notification (ETEN), which includes a cumulative mechanism to notify
    endpoints of aggregate congestion statistics along the path
    [KAPS02].  (A second variant in [KSEPA04] does not depend on
    cumulative congestion statistics from the network.)


A.4.  Fast Start-ups with more Fine-Grained Feedback from Routers

    Proposals for more fine-grained congestion-related feedback from
    routers include XCP [KHR02], MaxNet [MaxNet], and AntiECN marking
    [K03].  Section B.6 discusses in more detail the relationship
    between Quick-Start and proposals for more fine-grained per-packet
    feedback from routers.

    XCP:
    Proposals such as XCP for new congestion control mechanisms based on
    more feedback from routers are more powerful than Quick-Start, but
    also are more complex to understand and more difficult to deploy.
    XCP routers maintain no per-flow state, but provide more fine-
    grained feedback to end-nodes than the one-bit congestion feedback
    of ECN.  The per-packet feedback from XCP can be positive or
    negative, and specifies the increase or decrease in the sender's
    congestion window when this packet is acknowledged.  XCP is a full-



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    fledge congestion control scheme, whereas Quick-Start represents a
    quick check to determine if the network path is significantly
    underutilized such that a connection can start faster and then fall
    back to TCP's standard congestion control algorithms.

    AntiECN:
    The AntiECN proposal is for a single bit in the packet header that
    routers could set to indicate that they are underutilized.  For each
    TCP ACK arriving at the sender indicating that a packet has been
    received with the Anti-ECN bit set, the sender would be able to
    increase its congestion window by one packet, as it would during
    slow-start.


A.5.  Fast Start-ups with Lower-Than-Best-Effort Service

    There have been proposals for routers to provide a Lower Effort
    differentiated service that would be lower than best effort
    [RFC3662].  Such a service could carry traffic for which delivery is
    strictly optional, or could carry traffic that is important but that
    has low priority in terms of time.  Because it does not interfere
    with best-effort traffic, Lower Effort services could be used by
    transport protocols that start-up faster than slow-start.  For
    example, [SGF05] is a proposal for the transport sender to use low-
    priority traffic for much of the initial traffic, with routers
    configured to use strict priority queueing.

    A separate but related issue is that of below-best-effort TCP,
    variants of TCP that would not rely on Lower Effort services in the
    network, but would approximate below-best-effort traffic by
    detecting and responding to congestion sooner that standard TCP.
    TCP Nice [V02] and TCP Low Priority (TCP-LP) [KK03] are two such
    proposals for below-best-effort TCP, with the purpose of allowing
    TCP connections to use the bandwidth unused by TCP and other traffic
    in a non-intrusive fashion.  Both TCP Nice and TCP Low Priority use
    the default slow-start mechanisms of TCP.

    We note that Quick-Start is quite different from either a Lower
    Effort service or a below-best-effort variant of TCP.  Unlike these
    proposals, Quick-Start is intended to be useful for best-effort
    traffic that wishes to receive at least as much bandwidth as
    competing best-effort connections.


B.  Design Decisions






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B.1.  Alternate Mechanisms for the Quick-Start Request: ICMP and RSVP

    This document has proposed using an IP Option for the Quick-Start
    Request from the sender to the receiver, and using transport
    mechanisms for the Quick-Start Response from the receiver back to
    the sender.  In this section we discuss alternate mechanisms, and
    consider whether ICMP ([RFC792], [RFC2463]) or RSVP [RFC2205]
    protocols could be used for delivering the Quick-Start Request.


B.1.1.  ICMP

    Being a control protocol used between Internet nodes, one could
    argue that ICMP is the ideal method for requesting a permission for
    faster startup from routers.  The ICMP header is above the IP
    header.  Quick-Start could be accomplished with ICMP as follows: If
    the ICMP protocol is used to implement Quick-Start, the equivalent
    of the Quick-Start IP option would be carried in the ICMP header of
    the ICMP Quick-Start Request.  The ICMP Quick-Start Request would
    have to pass by the routers on the path to the receiver, possibly
    using the IP Router Alert option [RFC2113].  A router that approves
    the Quick-Start Request would take the same actions as in the case
    with the Quick-Start IP Option, and forward the packet to the next
    router along the path.  A router that does not approve the Quick-
    Start Request, even with a decreased value for the Requested Rate,
    would delete the ICMP Quick-Start Request, and send an ICMP Reply to
    the sender that the request was not approved.  If the ICMP Reply was
    dropped in the network, and did not reach the receiver, the sender
    would still know that the request was not approved from the absence
    of feedback from the receiver.  If the ICMP Quick-Start request was
    dropped in the network due to congestion, the sender would assume
    that the request was not approved.  The ICMP message would need the
    source and destination port numbers for demultiplexing at the end
    nodes.  If the ICMP Quick-Start Request reached the receiver, the
    receiver would use transport-level or application-level mechanisms
    to send a response to the sender, exactly as with the IP Option.

    One benefit of using ICMP would be that the delivery of the TCP SYN
    packet or other initial packet would not be delayed by IP option
    processing at routers.  A greater advantage is that if middleboxes
    were blocking packets with Quick-Start Requests, using the Quick-
    Start Request in a separate ICMP packet would mean that the
    middlebox behavior would not affect the connection as a whole.  (To
    get this robustness to middleboxes with TCP using an IP Quick-Start
    Option, one would have to have a TCP-level Quick-Start Request
    packet that could be sent concurrently but separately from the TCP
    SYN packet.)




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    However, there are a number of disadvantages to using ICMP.  Some
    firewalls and middleboxes may not forward the ICMP Quick-Start
    Request packets.  (If an ICMP Reply packet from a router to the
    sender is dropped in the network, the sender would still know that
    the request was not approved, as stated earlier, so this would not
    be as serious of a problem.)  In addition, it would be difficult, if
    not impossible, for a router in the middle of an IP tunnel to
    deliver an ICMP Reply packet to the actual source, for example when
    the inner IP header is encrypted as in IPsec ESP tunnel mode
    [RFC4301].  Again, however, the ICMP Reply packet would not be
    essential to the correct operation of ICMP Quick-Start.

    Unauthenticated out-of-band ICMP messages could enable some types of
    attacks by third-party malicious hosts that are not possible when
    the control information is carried in-band with the IP packets that
    can only be altered by the routers on the connection path. Finally,
    as a minor concern, using ICMP would cause a small amount of
    additional traffic in the network, which is not the case when using
    IP options.


B.1.2.  RSVP

    With some modifications RSVP [RFC2205] could be used as a bearer
    protocol for carrying the Quick-Start Requests. Because routers are
    expected to process RSVP packets more extensively than the normal
    transport protocol IP packets, delivering a Quick-Start rate request
    using an RSVP packet would seem an appealing choice. However, Quick-
    Start with RSVP would require a few differences from the
    conventional usage of RSVP. Quick-Start would not require periodical
    refreshing of soft state, because Quick-Start does not require per-
    connection state in routers.  Quick-Start Requests would be
    transmitted downstream from the sender to receiver in the RSVP Path
    messages, which is different from the conventional RSVP model where
    the reservations originate from the receiver. Furthermore, the
    Quick-Start Response would be sent using the transport-level or
    application-level mechanisms instead of using the RSVP Resv message.

    If RSVP was used for carrying a Quick-Start Request, a new "Quick-
    Start Request" class object would be included in the RSVP Path
    message that is sent from the sender to receiver. The object would
    contain the rate request field in addition to the common length and
    type fields. The Send_TTL field in the RSVP common header could be
    used as the equivalent of the QS TTL field.  The Quick-Start capable
    routers along the path would inspect the Quick-Start Request object
    in the RSVP Path message, decrement Send_TTL and adjust the rate
    request field if needed. If an RSVP router did not understand the
    Quick-Start Request object, it would reject the entire RSVP message



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    and send an RSVP PathErr message back to the sender.  When an RSVP
    message with the Quick-Start Request object reaches the receiver,
    the receiver sends a Quick-Start Reply message in the corresponding
    transport protocol header in the same way as described in the
    context of IP options earlier. If the RSVP message with the Quick-
    Start Request object was dropped along the path, the transport
    sender would simply proceed with the normal congestion control
    procedures.

    Much of the discussion about benefits and drawbacks of using ICMP
    for making the Quick-Start Request also applies to the RSVP case. If
    the Quick-Start Request was transmitted in a separate packet instead
    of as an IP option, the transport protocol packet delivery would not
    be delayed due to IP option processing at the routers, and the
    initial transport packets would reach their destination more
    reliably. The possible disadvantages of using ICMP and RSVP are also
    expected to be similar: middleboxes in the network may not be able
    to forward the Quick-Start Request messages, and the IP tunnels
    might cause problems for processing the Quick-Start Requests.


B.2.  Alternate Encoding Functions

    In this section we look at alternate encoding functions for the Rate
    Request field in the Quick-Start Request.  The main requirements for
    this function is that it should have a sufficiently wide range for
    the requested rate.  There is no need for overly-fine-grained
    precision in the requested rate.  Similarly, while it would be
    attractive for the encoding function to be easily computable, it is
    also possible for end-nodes and routers to simply store the table
    giving the mapping between the value N in the Rate Request field,
    and the actual rate request f(N).  In this section we consider
    possible encoding methods for Rate Request fields of different
    sizes, including four-bit, eight-bit, and larger Rate Request
    fields.

    Linear functions:
    One possible proposal would be for the Rate Request field to be
    formatted in bits per second, scaled so that one unit equals M Kbps,
    for some fixed value of M.  Thus, for the value N in the Rate
    Request field, the requested rate would be M*N Kbps.

    Powers of two:
    If a granularity of factors of two is sufficient for the Rate
    Request, then the encoding function with the most range would be for
    the requested rate to be K*2^N, for N the value in the Rate Request
    field, and for K some constant.  For N=0, the rate request would be
    set to zero, regardless of the encoding function.  For example, for



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    K=40,000 and an eight-bit Rate Request field, the request range
    would be from 80 Kbps to 40*2^255 Kbps.  This clearly would be an
    unnecessarily large request range.

    For a four-bit Rate Request field, the upper limit on the rate
    request is 1.3 Gbps.  It seems to us that an upper limit of 1.3 Gbps
    would be fine for the Quick-Start rate request, and that connections
    wishing to start up with a higher initial sending rate should be
    encouraged to use other mechanisms, such as the explicit reservation
    of bandwidth.  If an upper limit of 1.3 Gbps was not acceptable,
    then five or six bits could be used for the Rate Request field.

    The lower limit of 80 Kbps could be useful for flows with round-trip
    times of a second or more.  For a flow with a round-trip time of one
    second, as is typical in some wireless networks, the TCP initial
    window of 4380 bytes allowed by [RFC3390] (given appropriate packet
    sizes) would translate to an initial sending rate of 35 Kbps.  Thus,
    for TCP flows, a rate request of 80 Kbps could be useful for some
    flows with large round-trip times.

    The lower limit of 80 Kbps could also be useful for some non-TCP
    flows that send small packets, with at most one small packet every
    10 ms.  A rate request of 80 Kbps would translate to a rate of a
    hundred 100-byte packets per second (including packet headers).
    While some small-packet flows with large round-trip times might find
    a smaller rate request of 40 Kbps to be useful, our assumption is
    that a lower limit of 80 Kbps on the rate request will be generally
    sufficient.  Again, if the lower limit of 80 kbps was not
    acceptable, then extra bits could be used for the Rate Request
    field.

    If the granularity of factors of two was too coarse, then the
    encoding function could use a base less than two.  An alternate form
    for the encoding function would be to use a hybrid of linear and
    exponential functions.

    A mantissa and exponent representation:
    Section 4.4 of [B05] suggests a mantissa and exponent representation
    for the Quick-Start encoding function.  With e and f as the binary
    numbers in the exponent and mantissa fields, and with 0 <= f < 1,
    this would represent the rate (1+f)*2^e.  [B05] suggests a mantissa
    field for f of 8, 16, or 24 bits, with an exponent field for e of 8
    bits.  This representation would allow larger rate requests, with an
    encoding that is less coarse than the powers-of-two encoding used in
    this document.

    Constraints of the transport protocol:
    We note that the Rate Request is also constrained by the abilities



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    of the transport protocol.  For example, for TCP with Window
    Scaling, the maximum window is at most 2**30 bytes.  For a TCP
    connection with a long, 1 second round-trip time, this would give a
    maximum sending rate of 1.07 Gbps.


B.3.  The Quick-Start Request: Packets or Bytes?

    One of the design questions is whether the Rate Request field should
    be in bytes per second or in packets per second.  We discuss this
    separately from the perspective of the transport, and from the
    perspective of the router.

    For TCP, the results from the Quick-Start Request are translated
    into a congestion window in bytes, using the measured round-trip
    time and the MSS.  This window applies only to the bytes of data
    payload, and does not include the bytes in the TCP or IP packet
    headers.  Other transport protocols would conceivably use the Quick-
    Start Request directly in packets per second, or could translate the
    Quick-Start Request to a congestion window in packets.

    The assumption of this draft is that the router only approves the
    Quick-Start Request when the output link is significantly
    underutilized.  For this, the router could measure the available
    bandwidth in bytes per second, or could convert between packets and
    bytes by some mechanism.

    If the Quick-Start Request was in bytes per second, and applied only
    to the data payload, then the router would have to convert from
    bytes per second of data payload, to bytes per second of packets on
    the wire.  If the Rate Request field was in bytes per second and the
    sender ended up using very small packets, this could translate to a
    significantly larger number in terms of bytes per second on the
    wire.  Therefore, for a Quick-Start Request in bytes per second, it
    makes most sense for this to include the transport and IP headers as
    well as the data payload.  Of course, this will be at best a rough
    approximation on the part of the sender; the transport-level sender
    might not know the size of the transport and IP headers in bytes,
    and might know nothing at all about the separate headers added in IP
    tunnels downstream.  This rough estimate seems sufficient, however,
    given the overall lack of fine precision in Quick-Start
    functionality.

    It has been suggested that the router could possibly use information
    from the MSS option in the TCP packet header of the SYN packet to
    convert the Quick-Start Request from packets per second to bytes per
    second, or vice versa.  This would be problematic for several
    reasons.  First, if IPsec is used, the TCP header will be encrypted.



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    Second, the MSS option is defined as the maximum MSS that the TCP
    sender expects to receive, not the maximum MSS that the TCP sender
    plans to send [RFC793].  However, it is probably often the case that
    this MSS also applies as an upper bound on the MSS used by the TCP
    sender in sending.

    We note that the sender does not necessarily know the Path MTU when
    the Quick-Start Request is sent, or when the initial window of data
    is sent.  Thus, with IPv4, packets from the initial window could end
    up being fragmented in the network if the "Don't Fragment" (DF) bit
    is not set [RFC1191].  A Rate Request in bytes per second is
    reasonably robust to fragmentation.  Clearly a Rate Request in
    packets per second is less robust in the presence of fragmentation.
    Interactions between larger initial windows and Path MTU Discovery
    are discussed in more detail in RFC 3390 [RFC3390].

    For a Quick-Start Request in bytes per second, the transport senders
    would have the additional complication of estimating the bandwidth
    usage added by the packet headers.

    We have chosen a Rate Request field in bytes per second rather than
    in packets per second because it seems somewhat more robust,
    particularly to routers.


B.4.  Quick-Start Semantics: Total Rate or Additional Rate?

    For a Quick-Start Request sent in the middle of a connection, there
    are two possible semantics for the Rate Request field, as follows:

    (1) Total Rate: The requested Rate Request is the requested total
    rate for the connection, including the current rate; or

    (2) Additional Rate: The requested Rate Request is the requested
    increase in the total rate for that connection, over and above the
    current sending rate.

    When the Quick-Start Request is sent after an idle period, the
    current sending rate is zero, and there is no difference between (1)
    and (2) above.  However, a Quick-Start Request can also be sent in
    the middle of a connection that has not been idle, e.g., after a
    mobility event, or after an application-limited period when the
    sender is suddenly ready to send at a much higher rate.  In this
    case, there can be a significant difference between (1) and (2)
    above.  In this section we consider briefly the tradeoffs between
    these two options, and explain why we have chosen the `Total Rate'
    semantics.




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    The Total Rate semantics makes it easier for routers to ``allocate''
    the same rate to all connections.  This lends itself to fairness,
    and improves convergence times between old and new connections.
    With the Additional Rate semantics, the router would not necessarily
    know the current sending rates of the flows requesting additional
    rates, and therefore would not have sufficient information to use
    fairness as a metric in granting rate requests.  With the Total Rate
    semantics, the fairness is automatic; the router is not granting
    rate requests for *additional* bandwidth without knowing the current
    sending rates of the different flows.

    The Additional Rate semantics also lends itself to gaming by the
    connection, with senders sending frequent Quick-Start Requests in
    the hope of gaining a higher rate.  If the router is granting the
    same maximum rate for all rate requests, then there is little
    benefit to a connection of sending a rate request over and over
    again.  However, if the router is granting an *additional* rate with
    each rate request, over and above the current sending rate, then it
    is in a connection's interest to send as many rate requests as
    possible, even if very few of them are in fact granted.

    Appendix E discusses a Report of Current Sending Rate as one
    possible function in the Quick-Start Option.  However, we have not
    standardized this possible use at this time.



B.5.  Alternate Responses to the Loss of a Quick-Start Packet

    Section 4.6 discusses TCP's response to the loss of a Quick-Start
    packet in the initial window.  This section discusses several
    alternate responses.

    One possible alternative to reverting to the default slow-start
    after the loss of a Quick-Start packet from the initial window would
    have been to halve the congestion window and continue in congestion
    avoidance.  However, we note that this would not have been a
    desirable response for either the connection or for the network as a
    whole.  The packet loss in the initial window indicates that Quick-
    Start failed in finding an appropriate congestion window, meaning
    that the congestion window after halving may easily also be wrong.

    A more moderate alternate would be to continue in congestion
    avoidance from a window of (W-D)/2, where W is the Quick-Start
    congestion window, and D is the number of packets dropped or marked
    from that window.  However, such an approach would implicitly assume
    that the number of Quick-Start packets delivered is a good
    indication of the appropriate available bandwidth for that flow,



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    even though other packets from that window were dropped in the
    network.  And, further, that using half the number of segments that
    were successfully transmitted is conservative enough to account for
    the possibly inaccurate congestion window indication.  We believe
    that such an assumption would require more analysis at this point,
    particularly in a network with a range of packet dropping mechanisms
    at the router, and we cannot recommend it at this time.

    Another drawback of approaches that don't revert back to slow-start
    when a Quick-Start packet in the initial window is dropped is that
    such approaches could give the TCP receiver a greater incentive to
    lie about the Quick-Start request.  If the sender reverts to slow-
    start when a Quick-Start packet in the initial window is dropped,
    this diminishes the benefit a receiver would get from a Quick-Start
    request that resulted in a dropped or ECN-marked packet.


B.6.  Why Not Include More Functionality?

    This proposal for Quick-Start is a rather coarse-grained mechanism
    that would allow a connection to use a higher sending rate along
    underutilized paths, but that does not attempt to provide a next-
    generation transport protocol or congestion control mechanism, and
    does not attempt the goal of providing very high throughput with
    very low delay.  As Section A.4 discusses, there are a number of
    proposals such as XCP, MaxNet, and AntiECN for more fine-grained
    per-packet feedback from routers than the current congestion control
    mechanisms, that do attempt these more ambitious goals.

    Compared to proposals such as XCP and AntiECN, Quick-Start offers
    much less control.  Quick-Start does not attempt to provide a new
    congestion control mechanism, but simply to get permission from
    routers for a higher sending rate at start-up, or after an idle
    period.  Quick-Start can be thought of as an "anti-congestion-
    control" mechanism, that is only of any use when all of the routers
    along the path are significantly under-utilized.  Thus, Quick-Start
    is of no use towards a target of high link utilization, or fairness
    in a high-utilization scenario, or controlling queueing delay during
    high-utilization, or anything of the like.

    At the same time, Quick-Start would allow larger initial windows
    than would proposals such as AntiECN, requires less input to routers
    than XCP (e.g., XCP's cwnd and RTT fields), and would require less
    frequent feedback from routers than any new congestion control
    mechanism.  Thus, Quick-Start is significantly less powerful than
    proposals for new congestion control mechanisms such as XCP and
    AntiECN, but as powerful or more powerful in terms of the specific
    issue of allowing larger initial windows, and (we think) more



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    amenable to incremental deployment in the current Internet.

    We do not discuss proposals such as XCP in detail, but simply note
    that there are a number of open questions.  One question concerns
    whether there is a pressing need for more sophisticated congestion
    control mechanisms such as XCP in the Internet.  Quick-Start is
    inherently a rather crude tool that does not deliver assurances
    about maintaining high link utilization and low queueing delay;
    Quick-Start is designed for use in environments that are
    significantly underutilized, and addresses the single question of
    whether a higher sending rate is allowed.  New congestion control
    mechanisms with more fine-grained feedback from routers could allow
    faster startups even in environments with rather high link
    utilization.  Is this a pressing requirement?  Are the other
    benefits of more fine-grained congestion control feedback from
    routers a pressing requirement?

    We would argue that even if more fine-grained per-packet feedback
    from routers was implemented, it is reasonable to have a separate
    mechanism such as Quick-Start for indicating an allowed initial
    sending rate, or an allowed total sending rate after an idle or
    underutilized period.

    One difference between Quick-Start and current proposals for fine-
    grained per-packet feedback such as XCP is that XCP is designed to
    give robust performance even in the case where different packets
    within a connection routinely follow different paths.  XCP achieves
    relatively robust performance in the presence of multipath routing
    by using per-packet feedback, where the feedback carried in a single
    packet is about the relative increase or decrease in the rate or
    window to take effect when that particular packet is acknowledged,
    not about the allowed sending rate for the connection as a whole.

    In contrast, Quick-Start sends a single Quick-Start request, and the
    answer to that request gives the allowed sending rate for an entire
    window of data.  As a result, Quick-Start could be problematic in an
    environment where some fraction of the packets in a window of data
    take path A, and the rest of the packets take path B; for example,
    the Quick-Start Request could have travelled on path A, while half
    of the Quick-Start packets sent in the succeeding round-trip time
    are routed on path B.  We note that [ZDPS01] shows Internet paths to
    be stable on the order of RTTs.

    There are also differences between Quick-Start and some of the
    proposals for per-packet feedback in terms of the number of bits of
    feedback required from the routers to the end-nodes.  Quick-Start
    uses four bits of feedback in the rate request field to indicate the
    allowed sending rate.  XCP allocates a byte for per-packet feedback,



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    though there has been discussion of variants of XCP with less per-
    packet feedback.  This would be more like other proposals such as
    anti-ECN that use a single bit of feedback from routers to indicate
    that the sender can increase as fast as slow-starting, in response
    to this particular packet acknowledgement.  In general, there is
    probably considerable power in fine-grained proposals with only two
    bits of feedback, indicating that the sender should decrease,
    maintain, or increase the sending rate or window when this packet is
    acknowledged.  However, the power of Quick-Start would be
    considerably limited if it was restricted to only two bits of
    feedback; it seems likely that determining the initial sending rate
    fundamentally requires more bits of feedback from routers than does
    the steady-state, per-packet feedback to increase or decrease the
    sending rate.

    On a more practical level, one difference between Quick-Start and
    proposals for per-packet feedback is that there are fewer open
    issues with Quick-Start than there would be with a new congestion
    control mechanism.  Because Quick-Start is a mechanism for
    requesting an initial sending rate in an underutilized environment,
    its fairness issues are less severe than those of a general
    congestion control mechanism.  With Quick-Start, there is no need
    for the end nodes to tell the routers the round-trip time and
    congestion window, as is done in XCP; all that is needed is for the
    end nodes to report the requested sending rate.

    Table 3 provides a summary of the differences between Quick-Start
    and proposals for per-packet congestion control feedback.























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                                                Proposals for
                          Quick-Start           Per-Packet Feedback
    +------------------+----------------------+----------------------+
     Semantics:        | Allowed sending rate | Change in rate/window,
                       |  per connection.     |  per-packet.
    +------------------+----------------------+----------------------+
     Relationship to   | In addition.         | Replacement.
     congestion ctrl:  |                      |
    +------------------+----------------------+----------------------+
     Frequency:        | Start-up, or after   | Every packet.
                       |  an idle period.     |
    +------------------+----------------------+----------------------+
     Limitations:      | Only useful on       | General congestion
                       |  underutilized paths.|  control mechanism.
    +------------------+----------------------+----------------------+
     Input to routers: | Rate request.        |RTT, cwnd, request (XCP)
                       |                      | None (Anti-ECN).
    +------------------+----------------------+----------------------+
     Bits of feedback: | Four bits for        | A few bits would
                       |   rate request.      |  suffice?
    +------------------+----------------------+----------------------+

      Table 3: Differences between Quick-Start and Proposals for
        Fine-Grained Per-Packet Feedback.


    A separate question concerns whether mechanisms such as Quick-Start,
    in combination with HighSpeed TCP and other changes in progress,
    would make a significant contribution towards meeting some of these
    needs for new congestion control mechanisms.  This could be viewed
    as a positive step towards meeting some of the more pressing current
    needs with a simple and reasonably deployable mechanism, or
    alternately, as a negative step of unnecessarily delaying more
    fundamental changes.  Without answering this question, we would note
    that our own approach tends to favor the incremental deployment of
    relatively simple mechanisms, as long as the simple mechanisms are
    not short-term hacks but mechanisms that lead the overall
    architecture in the fundamentally correct direction.


B.7.  Alternate Implementations for a Quick-Start Nonce


B.7.1.  An Alternate Proposal for the Quick-Start Nonce

    An alternate proposal for the Quick-Start Nonce from [B05] would be
    for an n-bit field for the QS Nonce, with the sender generating a
    random nonce when it generates a Quick-Start Request.  Each router



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    that reduces the Rate Request by r would hash the QS nonce r times,
    using a one-way hash function such as MD5 [RFC1321] or the secure
    hash 1 [SHA1].  The receiver returns the QS nonce to the sender.
    Because the sender knows the original value for the nonce, and the
    original rate request, the sender knows the total number of steps s
    that the rate has been reduced.  The sender then hashes the original
    nonce s times, to check whether the result is the same as the nonce
    returned by the receiver.

    This alternate proposal for the nonce would be considerably more
    powerful than the QS nonce described in Section 3.4, but it would
    also require more CPU cycles from the routers when they reduce a
    Quick-Start request, and from the sender in verifying the nonce
    returned by the receiver.  As reported in [B05], routers could
    protect themselves from processor exhaustion attacks by limiting the
    rate at which they will approve reductions of Quick-Start requests.

    Both the Function field and the Reserved field in the Quick-Start
    Option would allow the extension of Quick-Start to use Quick-Start
    requests with the alternate proposal for the Quick-Start Nonce, if
    it was ever desired.


B.7.2.  The Earlier Request-Approved Quick-Start Nonce

    An earlier version of this document included a Request-Approved
    Quick-Start Nonce (QS Nonce) that was initialized by the sender to a
    non-zero, `random' eight-bit number, along with a QS TTL that was
    initialized to the same value as the TTL in the IP header.  The
    Request-Approved Quick-Start Nonce would have been returned by the
    transport receiver to the transport sender in the Quick-Start
    Response.  A router could deny the Quick-Start request by failing to
    decrement the QS TTL field, by zeroing the QS Nonce field, or by
    deleting the Quick-Start Request from the packet header.  The QS
    Nonce was included to provide some protection against broken
    downstream routers, or against misbehaving TCP receivers that might
    be inclined to lie about whether the Rate Request was approved.
    This protection is now provided by the QS Nonce, by the use of a
    random initial value for the QS TTL field, and by Quick-Start-
    capable routers hopefully either deleting the Quick-Start Option or
    zeroing the QS TTL and QS Nonce fields when they deny a request.

    With the old Request-Approved Quick-Start Nonce, along with the QS
    TTL field set to the same value as the TTL field in the IP header,
    the Quick-Start Request mechanism would have been self-terminating;
    the Quick-Start Request would terminate at the first participating
    router after a non-participating router had been encountered on the
    path.  This minimizes unnecessary overhead incurred by routers



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    because of option processing for the Quick-Start Request.  In the
    current specification, this "self-terminating" property is provided
    by Quick-Start-capable routers hopefully either deleting the Quick-
    Start Option or zeroing the Rate Request field when they deny a
    request.  Because the current specification uses a random initial
    value for the QS TTL, Quick-Start-capable routers can't tell if the
    Quick-Start Request is invalid because of non-Quick-Start-capable
    routers upstream.  This is the cost of using a design that makes it
    difficult for the receiver to cheat about the value of the QS TTL.


C.  Quick-Start with DCCP

    DCCP is a new transport protocol for congestion-controlled,
    unreliable datagrams, intended for applications such as streaming
    media, Internet telephony, and on-line games.  In DCCP, the
    application has a choice of congestion control mechanisms, with the
    currently-specified Congestion Control Identifiers (CCIDs) being
    CCID 2 for TCP-like congestion control, and CCID 3 for TFRC, an
    equation-based form of congestion control. We refer the reader to
    [RFC4340] for a more detailed description of DCCP, and of the
    congestion control mechanisms.

    Because CCID 3 uses a rate-based congestion control mechanism, it
    raises some new issues about the use of Quick-Start with transport
    protocols.  In this document we don't attempt to specify the use of
    Quick-Start with DCCP.  However, we do discuss some of the issues
    that might arise.

    In considering the use of Quick-Start with CCID 3 for requesting a
    higher initial sending rate, the following questions arise:

    (1) How does the sender respond if a Quick-Start packet is dropped?
    As in TCP, if an initial Quick-Start packet is dropped, the CCID 3
    sender should revert to the congestion control mechanisms it would
    have used if the Quick-Start request had not been approved.

    (2) When does the sender decide there has been no feedback from the
    receiver?
    Unlike TCP, CCID 3 does not use acknowledgements for every packet,
    or for every other packet.  In contrast, the CCID 3 receiver sends
    feedback to the sender roughly once per round-trip time.  In CCID 3,
    the allowed sending rate is halved if no feedback is received from
    the receiver in at least four round-trip times (when the sender is
    sending at least one packet every two round-trip times).  When a
    Quick-Start request is used, it would seem necessary to use a
    smaller time interval, e.g., to reduce the sending rate if no
    feedback arrives from the receiver in at least two round-trip times.



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    The question also arises of how the sending rate should be reduced
    after a period of no feedback from the receiver.  As with TCP, the
    default CCID 3 response of halving the sending rate is not
    necessarily a sufficient response to the absence of feedback; an
    alternative is to reduce the sending rate to the sending rate that
    would have been used if no Quick-Start request had been approved.
    That is, if a CCID 3 sender uses a Quick-Start request, special
    rules might be required to handle the sender's response to a period
    of no feedback from the receiver regarding the Quick-Start packets.

    Similarly, in considering the use of Quick-Start with CCID 3 for
    requesting a higher sending rate after an idle period, the following
    questions arise:

    (1) What rate does the sender request?
    As in TCP, there is a straightforward answer to the rate request
    that the CCID 3 sender should use in requesting a higher sending
    rate after an idle period.  The sender knows the current loss event
    rate, either from its own calculations or from feedback from the
    receiver, and can determine the sending rate allowed by that loss
    event rate.  This is the upper bound on the sending rate that should
    be requested by the CCID 3 sender.  A Quick-Start request is useful
    with CCID 3 when the sender is coming out of an idle or
    underutilized period, because in standard operation CCID 3 does not
    allow the sender to send more than twice as fast as the receiver has
    reported received in the most recent feedback message.

    (2) What is the response to loss?
    The response to the loss of Quick-Start packets should be to return
    to the sending rate that would have been used if Quick-Start had not
    been requested.

    (3) When does the sender decide there has been no feedback from the
    receiver?
    As in the case of the initial sending rate, it would seem prudent to
    reduce the sending rate if no feedback is received from the receiver
    in at least two round-trip times.  It seems likely that in this
    case, the sending rate should be reduced to the sending rate that
    would have been used if no Quick-Start request had been approved.


D.  Possible Router Algorithm

    This specification does not tightly define the algorithm a router
    uses when deciding whether to approve a Quick-Start Rate Request or
    whether and how to reduce a Rate Request.  A range of algorithms is
    likely useful in this space and we consider the algorithm a
    particular router uses to be a local policy decision.  In addition,



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    we believe that additional experimentation with router algorithms is
    necessary to have a solid understanding of the dynamics various
    algorithms impose.  However, we provide one particular algorithm in
    this appendix as an example and as a framework for thinking about
    additional mechanisms.

    [SAF06] provides several algorithms routers can use to consider
    incoming Rate Requests.  The decision process involves two
    algorithms.  First, the router needs to track the link utilization
    over the recent past.  Second, this utilization needs to be updated
    by the potential new bandwidth from recent Quick-Start approvals,
    and then compared with the router's notion of when it is
    underutilized enough to approve Quick-Start requests (of some size).

    First, we define the "peak utilization" estimation technique (from
    [SAF06]).  This mechanism records the utilization of the link every
    S seconds and stores the most recent N of these measurements.  The
    utilization is then taken as the highest utilization of the N
    samples.  This method, therefore, keeps N*S seconds of history.
    This algorithm reacts rapidly to increases in the link utilization.
    In [SAF06] S is set to 0.15 seconds, and experiments use values for
    N ranging from 3 to 20.

    Second, we define the "target" algorithm for processing incoming
    Quick-Start Rate Requests (also from [SAF06]).  The algorithm relies
    on knowing the bandwidth of the outgoing link (which in many cases
    can be easily configured), the utilization of the outgoing link
    (from an estimation technique such as given above) and an estimate
    of the potential bandwidth from recent Quick-Start approvals.

    Tracking the potential bandwidth from recent Quick-Start approvals
    is another case where local policy dictates how it should be done.
    The simplest method, outlined in Section 8.2, is for the router to
    keep track of the aggregate Quick-Start rate requests approved in
    the most recent two or more time intervals, including the current
    time interval, and to use the sum of the aggregate rate requests
    over these time intervals as the estimate of the approved Rate
    Requests.  The experiments in [SAF06] keep track of the aggregate
    approved Rate Requests over the most recent two time intervals, for
    intervals of 150~msec.

    The target algorithm also depends on a threshold (qs_thresh) that is
    the fraction of the outgoing link bandwidth that represents the
    router's notion of "significantly underutilized".  If the
    utilization, augmented by the potential bandwidth from recent Quick-
    Start approvals, is above this threshold then no Quick-Start Rate
    Requests will be approved.  If the utilization, again augmented by
    the potential bandwidth from recent Quick-Start approvals, is less



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    than the threshold then Rate Requests can be approved.  The Rate
    Requests will be reduced such that the bandwidth allocated would not
    drive the utilization to more than the given threshold.  The
    algorithm is:

      util_bw = bandwidth * utilization;
      util_bw = util_bw + recent_qs_approvals;
      if (util_bw < (qs_thresh * bandwidth))
      {
          approved = (qs_thresh * bandwidth) - util_bw;
          if (rate_request < approved)
              approved = rate_request;
          approved = round_down (approved);
          recent_qs_approvals += approved;
      }

    Also note that given that Rate Requests are fairly coarse, the
    approved rate should be rounded down when it does not fall exactly
    on one of the rates allowed by the encoding scheme.

    Routers that wish to keep close track of the allocated Quick-Start
    bandwidth could use Approved Rate reports to learn when rate
    requests had been decremented downstream in the network, and also to
    learn when a sender begins to use the approved Quick-Start request.


E.  Possible Additional Uses for the Quick-Start Option

    The Quick-Start Option contains a four-bit Function field in the
    third byte, enabling additional uses to be defined for the Quick-
    Start Option.  In this section we discuss some of the possible
    additional uses that have been discussed for Quick-Start.  The
    Function field makes it easy to add new functions for the Quick-
    Start Option.

    Report of Current Sending Rate: A Quick-Start Request with the
    `Report of Current Sending Rate' codepoint set in the Function field
    would be using the Rate Request field to report the current
    estimated sending rate for that connection.  This could accompany a
    second Quick-Start Request in the same packet containing a standard
    rate request, for a connection that is using Quick-Start to increase
    its current sending rate.

    Request to Increase Sending Rate: A codepoint for `Request to
    Increase Sending Rate' in the Function field would indicate that the
    connection is not idle or just starting up, but is attempting to use
    Quick-Start to increase its current sending rate.  This codepoint
    would not change the semantics of the Rate Request field.



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    RTT Estimate: If a codepoint for `RTT Estimate' was used, a field
    for the RTT Estimate would contain one or more bits giving the
    sender's rough estimate of the round-trip time, if known.  E.g., the
    sender could estimate whether the RTT was greater or less than 200
    ms.  Alternately, if the sender had an estimate of the RTT when it
    sends the Rate Request, the two-bit Reserved field at the end of the
    Quick-Start Option could be used for a coarse-grained encoding of
    the RTT.

    Informational Request: An Informational Request codepoint in the
    Function field would indicate that a request is purely
    informational, for the sender to find out if a rate request would be
    approved, and what size rate request would be approved, when the
    Informational Request is sent.  For example, an Informational
    Request could be followed one round-trip time later by a standard
    Quick-Start Request.  A router approving an Informational Request
    would not consider this as an approval for Quick-Start bandwidth to
    be used, and would not be under any obligation to approve a similar
    standard Quick-Start Request one round-trip time later.  An
    Informational Request with a rate request of zero could be used by
    the sender to find out if all of the routers along the path
    supported Quick-Start.

    Use Format X for the Rate Request Field: A Quick-Start codepoint for
    `Use Format X for the Rate Request Field' would indicate that an
    alternate format was being used for the Rate Request field.

    Do Not Decrement: A Do Not Decrement codepoint could be used for a
    Quick-Start request where the sender would rather have the request
    denied than to have the rate request decremented in the network.
    This could be used if the sender was only interested in using Quick-
    Start if the original rate request was approved.

    Temporary Bandwidth Use: A Temporary codepoint has been proposed to
    indicate that a connection would only use the requested bandwidth
    for a single time interval.


Normative References

    [RFC793] J. Postel, Transmission Control Protocol, RFC 793,
    September 1981.

    [RFC1191] Mogul, J. and S. Deering, Path MTU Discovery, RFC 1191,
    November 1990.

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



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    [RFC2460] S. Deering and R. Hinden. Internet Protocol, Version 6
    (IPv6) Specification. RFC 2460, December 1998.

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

    [RFC3168] Ramakrishnan, K.K., Floyd, S., and Black, D.  The Addition
    of Explicit Congestion Notification (ECN) to IP.  RFC 3168, Proposed
    Standard, September 2001.

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

    [RFC3742] Floyd, S., Limited Slow-Start for TCP with Large
    Congestion Windows, RFC 3742, Experimental, March 2004.


Informative References

    [RFC792] J. Postel. Internet Control Message Protocol. RFC 792,
    September 1981.

    [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
    April 1992.

    [RFC1812] F. Baker (ed.). Requirements for IP Version 4 Routers. RFC
    1812, June 1995.

    [RFC2003]  Perkins, C., IP Encapsulation within IP, RFC 2003,
    October 1996.

    [RFC2113] D. Katz. P Router Alert Option. RFC 2113, February 1997.

    [RFC2140] J. Touch. TCP Control Block Interdependence.  RFC 2140.
    April 1997.

    [RFC2205] R. Braden, et al. Resource ReSerVation Protocol (RSVP) --
    Version 1 Functional Specification. RFC 2205, September 1997.

    [RFC2409] D. Harkins and D. Carrel, The Internet Key Exchange (IKE),
    RFC 2409, November 1998.

    [RFC2415] K. Poduri and K. Nichols. Simulation Studies of Increased
    Initial TCP Window Size. RFC 2415. September 1998.

    [RFC2463] A. Conta and S. Deering. Internet Control Message Protocol
    (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification.
    RFC 2463, December 1998.



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

    [RFC2661] W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn, and
    B.  Palter, Layer Two Tunneling Protocol "L2TP", RFC 2661, August
    1999.

    [RFC2784]  D. Farinacci, T. Li, S. Hanks, D. Meyer, and P. Traina,
    Generic Routing Encapsulation (GRE), RFC 2784, March 2000.

    [RFC2960] R. Stewart, et al. Stream Control Transmission Protocol.
    RFC 2960, October 2000.

    [RFC3031] E. Rosen, A. Viswanathan, and R. Callon.  Multiprotocol
    Label Switching Architecture.  RFC 3031.  January 2001.

    [RFC3124] H. Balakrishnan and S. Seshan. The Congestion Manager. RFC
    3124. June 2001.

    [RFC3234] B. Carpenter and S. Brim, Middleboxes: Taxonomy and
    Issues, RFC 3234, February 2002.

    [RFC3344] C. Perkins (ed.). IP Mobility Support for IPv4. RFC 3344,
    August 2002.

    [RFC3360] S. Floyd.  Inappropriate TCP Resets Considered Harmful.
    RFC 3360, August 2002.

    [RFC3649] Floyd, S., HighSpeed TCP for Large Congestion Windows, RFC
    3649, December 2003.

    [RFC3662] R. Bless, K. Nichols, and K. Wehrle.  A Lower Effort Per-
    Domain Behavior (PDB) for Differentiated Services.  RFC 3662,
    December 2003.

    [RFC3697] J. Rajahalme, A. Conta, B. Carpenter, and S. Deering. IPv6
    Flow Label Specification. RFC 3697, March 2004.

    [RFC3775] D. Johnson, C. Perkins, and J. Arkko. Mobility Support in
    IPv6. RFC 3775, June 2004.

    [RFC3819] P. Karn et al., "Advice for Internet Subnetwork
    Designers", July 2004.

    [RFC3948] A. Huttunen, B. Swander, V. Volpe, L. DiBurro, and M.
    Stenberg, UDP Encapsulation of IPsec ESP Packets, RFC 3948, January
    2005.



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    [RFC4301] S. Kent and K. Seo, Security Architecture for the Internet
    Protocol, RFC 4301, December 2005.

    [RFC4302] S. Kent, IP Authentication Header, RFC 4302, December
    2005.

    [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
    4306, December 2005.

    [RFC4340] E. Kohler, M. Handley, and S. Floyd, Datagram Congestion
    Control Protocol (DCCP), RFC 4340, March 2006.

    [RFC4341] S. Floyd and E. Kohler, Profile for Datagram Congestion
    Control Protocol (DCCP) Congestion Control ID 2: TCP-like Congestion
    Control, RFC 4341, March 2006.

    [AHO98] M. Allman, C. Hayes and S. Ostermann. An evaluation of TCP
    with Larger Initial Windows. ACM Computer Communication Review, July
    1998.

    [B05] B. Briscoe, Review: Quick-Start for TCP and IP, internet-draft
    draft-briscoe-tsvwg-quickstart-rvw-00, work-in-progress, URL
    "http://www.cs.ucl.ac.uk/staff/B.Briscoe/pubs.html", November 2005.

    [BJCG+05] Briscoe, B., Jacquet, A., Di Cairano-Gilfedder, C.,
    Salvatori, A., Soppera, A., and M. Koyabe, "Policing Congestion
    Response in an Internetwork Using Re-Feedback", SIGCOMM, August
    2005.

    [BW97] G. Brasche and B. Walke. Concepts, Services and Protocols of
    the new GSM Phase 2+ General Packet Radio Service. IEEE
    Communications Magazine, pages 94--104, August 1997.

    [GPAR02] A. Gurtov, M. Passoja, O. Aalto, and M. Raitola. Multi-
    Layer Protocol Tracing in a GPRS Network. In Proceedings of the IEEE
    Vehicular Technology Conference (Fall VTC2002), Vancouver, Canada,
    September 2002.

    [H05] P. Hoffman, email, October 2005.  Citation for acknowledgement
    purposes only.

    [HKP01] M. Handley, C. Kreibich and V. Paxson, Network Intrusion
    Detection: Evasion, Traffic Normalization, and End-to-End Protocol
    Semantics, Proc. USENIX Security Symposium 2001.

    [Jac88] V. Jacobson, Congestion Avoidance and Control, ACM SIGCOMM

    [JD02] Manish Jain, Constantinos Dovrolis, End-to-End Available



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    Bandwidth: Measurement Methodology, Dynamics, and Relation with TCP
    Throughput, SIGCOMM 2002.

    [K03] S. Kunniyur, "AntiECN Marking: A Marking Scheme for High
    Bandwidth Delay Connections", Proceedings, IEEE ICC '03, May 2003.
    URL "http://www.seas.upenn.edu/~kunniyur/".

    [KAPS02] Rajesh Krishnan, Mark Allman, Craig Partridge, James P.G.
    Sterbenz. Explicit Transport Error Notification (ETEN) for Error-
    Prone Wireless and Satellite Networks. Technical Report No. 8333,
    BBN Technologies, March 2002.  URL
    "http://www.icir.org/mallman/papers/".

    [KHR02] Dina Katabi, Mark Handley, and Charles Rohrs, Internet
    Congestion Control for Future High Bandwidth-Delay Product
    Environments. ACM Sigcomm 2002, August 2002.  URL
    "http://ana.lcs.mit.edu/dina/XCP/".

    [KK03] A. Kuzmanovic and E. W. Knightly.  TCP-LP: A Distributed
    Algorithm for Low Priority Data Transfer.  INFOCOM 2003, April 2003.

    [KSEPA04] Rajesh Krishnan, James Sterbenz, Wesley Eddy, Craig
    Partridge, Mark Allman. Explicit Transport Error Notification (ETEN)
    for Error-Prone Wireless and Satellite Networks. Computer Networks,
    46(3), October 2004.

    [L05] Guohan Lu, Nonce in TCP Quick Start, draft, September 2005.
    URL "http://www.net-glyph.org/~lgh/nonce-usage.pdf".

    [MH06] M. Mathis and J. Heffner, Packetization Layer Path MTU
    Discovery, internet-draft draft-ietf-pmtud-method-07.txt, work in
    progress, June 2006.

    [MAF04] Alberto Medina, Mark Allman, and Sally Floyd, Measuring
    Interactions Between Transport Protocols and Middleboxes, Internet
    Measurement Conference 2004, August 2004.  URL
    "http://www.icir.org/tbit/".

    [MAF05] Alberto Medina, Mark Allman, and Sally Floyd.  Measuring the
    Evolution of Transport Protocols in the Internet.  Computer
    Communications Review, April 2005.

    [MaxNet] MaxNet Home Page, URL
    "http://netlab.caltech.edu/~bartek/maxnet.htm".

    [P00] Joon-Sang Park, Bandwidth Discovery of a TCP Connection,
    report to John Jeidemann, 2000, private communication.  Citation for
    acknowledgement purposes only.



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    [PABL+05] Ruoming Pang, Mark Allman, Mike Bennett, Jason Lee, Vern
    Paxson, Brian Tierney.  A First Look at Modern Enterprise Traffic.
    ACM SIGCOMM/USENIX Internet Measurement Conference, October 2005.

    [PRAKS02] Craig Partridge, Dennis Rockwell, Mark Allman, Rajesh
    Krishnan, James P.G. Sterbenz. A Swifter Start for TCP. Technical
    Report No. 8339, BBN Technologies, March 2002.  URL
    "http://www.icir.org/mallman/papers/".

    [RW03] Mattia Rossi and Michael Welzl, On the Impact of IP Option
    Processing, Preprint-Reihe des Fachbereichs Mathematik - Informatik,
    No. 15, Institute of Computer Science, University of Innsbruck,
    Austria, October 2003.

    [RW04] Mattia Rossi and Michael Welzl, On the Impact of IP Option
    Processing -   Part 2, Preprint-Reihe des Fachbereichs Mathematik -
    Informatik, No. 26, Institute of Computer Science, University of
    Innsbruck, Austria, July 2004.

    [S02] Ion Stoica, private communication, 2002.  Citation for
    acknowledgement purposes only.

    [SAF06] Pasi Sarolahti, Mark Allman, and Sally Floyd.  Determining
    an Appropriate Sending Rate Over an Underutilized Network Path.
    February 2006.  URL "http://www.icir.org/floyd/quickstart.html".

    [SGF05]   Singh, M., Guha, S., and P. Francis, "Utilizing spare
    network bandwidth to improve TCP performance", ACM SIGCOMM 2005 Work
    in Progress session, August 2005,
    https://www.guha.cc/saikat/pub/sigcomm05-lowtcp.pdf.

    [SHA1] "Secure Hash Standard", FIPS, U.S. Department of Commerce,
    Washington, D.C. publication 180-1, April 1995.

    [SH02] Srikanth Sundarrajan and John Heidemann.  Study of TCP Quick
    Start with NS-2.  Class Project, December 2002.  Not publicly
    available; citation for acknowledgement purposes only.

    [V02] A. Venkataramani, R. Kokku, and M. Dahlin.  TCP Nice: A
    Mechanism for Background Transfers.  OSDI 2002.

    [VH97] V. Visweswaraiah and J. Heidemann, Improving Restart of Idle
    TCP Connections, Technical Report 97-661, University of Southern
    California, November 1997.

    [W00] Michael Welzl: PTP: Better Feedback for Adaptive Distributed
    Multimedia Applications on the Internet, IPCCC 2000 (19th IEEE
    International Performance, Computing, And Communications



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    Conference), Phoenix, Arizona, USA, 20-22 February 2000.  URL
    "http://www.welzl.at/research/publications/".

    [ZDPS01] Y. Zhang, N. Duffield, V. Paxson, and S. Shenker,  On the
    Constancy of Internet Path Properties, Proc. ACM SIGCOMM Internet
    Measurement Workshop, November 2001.

    [ZQK00] Y. Zhang, L. Qiu, and S. Keshav, Speeding Up Short Data
    Transfers: Theory, Architectural Support, and Simulation Results,
    NOSSDAV 2000, June 2000.


AUTHORS' ADDRESSES


    Sally Floyd
    Phone: +1 (510) 666-2989
    ICIR (ICSI Center for Internet Research)
    Email: floyd@icir.org
    URL: http://www.icir.org/floyd/

    Mark Allman
    ICSI Center for Internet Research
    1947 Center Street, Suite 600
    Berkeley, CA 94704-1198
    Phone: (440) 235-1792
    Email: mallman@icir.org
    URL: http://www.icir.org/mallman/

    Amit Jain
    F5 Networks
    Email : a.jain@f5.com

    Pasi Sarolahti
    Nokia Research Center
    P.O. Box 407
    FI-00045 NOKIA GROUP
    Finland
    Phone: +358 50 4876607
    Email: pasi.sarolahti@iki.fi











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