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

Internet Engineering Task Force                                  A. Jain
INTERNET-DRAFT                                               F5 Networks
draft-ietf-tsvwg-quickstart-01.txt                              S. Floyd
Expires: April 2006                                            M. Allman
                                                                    ICIR
                                                            P. Sarolahti
                                                   Nokia Research Center
                                                         13 October 2005


                       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
    have been or will be disclosed, and any of which he or she becomes
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    This Internet-Draft will expire on April 2006.

Copyright Notice

    Copyright (C) The Internet Society (2005). All Rights Reserved.







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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
    designed to be used by a range of transport protocols, in this
    document we describe its use with TCP.  By using Quick-Start, a TCP
    host, say, host A, would indicate its desired sending rate in bytes
    per second, using a Quick Start Request 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.  If the Quick-Start
    request is not approved, then the sender would use the default
    congestion control mechanisms.  The Quick-Start mechanism can
    determine if there are routers along the path that do not understand
    the Quick-Start Request 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.  Quick-Start is designed to allow connections
    to use higher sending rates when there is significant unused
    bandwidth along the path, and all of the routers along the path
    support the Quick-Start Request.




























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    TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:
     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
         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
           multi-path 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



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       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.
       - 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].



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     * 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. . . . . . . . . . . . . . . . . . . . . . . . .   9
    2. Assumptions and General Principles. . . . . . . . . . . . . .  10
       2.1. Overview of Quick-Start. . . . . . . . . . . . . . . . .  11
    3. The Quick-Start Request in IP . . . . . . . . . . . . . . . .  14
       3.1. The Quick-Start Request Option for IPv4. . . . . . . . .  14
       3.2. The Quick-Start Request Option for IPv6. . . . . . . . .  16
       3.3. Processing the Quick-Start Request at
       Routers . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
       3.4. The QS Nonce . . . . . . . . . . . . . . . . . . . . . .  18
    4. The Quick-Start Mechanisms in TCP . . . . . . . . . . . . . .  20
       4.1. When to Use Quick-Start. . . . . . . . . . . . . . . . .  21
       4.2. The Quick-Start Response Option in the TCP
       header. . . . . . . . . . . . . . . . . . . . . . . . . . . .  23
       4.3. TCP: Sending the Quick-Start Response. . . . . . . . . .  24
       4.4. TCP: Receiving and Using the Quick-Start
       Response Packet . . . . . . . . . . . . . . . . . . . . . . .  24
       4.5. TCP: Responding to a Loss of a Quick-Start
       Packet. . . . . . . . . . . . . . . . . . . . . . . . . . . .  26
       4.6. TCP: A Quick-Start Request for a Larger Ini-
       tial Window . . . . . . . . . . . . . . . . . . . . . . . . .  27
          4.6.1. Interactions with Path MTU Discovery. . . . . . . .  27
          4.6.2. Quick-Start Request Packets that are
          Discarded by Middleboxes . . . . . . . . . . . . . . . . .  27
       4.7. TCP: A Quick-Start Request in the Middle of
       Connection. . . . . . . . . . . . . . . . . . . . . . . . . .  29
       4.8. An Example Quick-Start Scenario with TCP . . . . . . . .  29
    5. Quick-Start and IPsec AH. . . . . . . . . . . . . . . . . . .  30
    6. Quick-Start in IP Tunnels . . . . . . . . . . . . . . . . . .  31
       6.1. Simple Tunnels That Are Compatible with
       Quick-Start . . . . . . . . . . . . . . . . . . . . . . . . .  33
          6.1.1. Simple Tunnels that are Aware of Quick-
          Start. . . . . . . . . . . . . . . . . . . . . . . . . . .  33
       6.2. Simple Tunnels That Are Not Compatible with
       Quick-Start . . . . . . . . . . . . . . . . . . . . . . . . .  34
       6.3. Tunnels That Support Quick-Start . . . . . . . . . . . .  35
    7. The Quick-Start Mechanism in other Transport Pro-
    tocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  36
    8. Using Quick-Start . . . . . . . . . . . . . . . . . . . . . .  37
       8.1. Determining the Rate to Request. . . . . . . . . . . . .  37
       8.2. Deciding the Permitted Rate Request at a
       Router. . . . . . . . . . . . . . . . . . . . . . . . . . . .  37
    9. Evaluation of Quick-Start . . . . . . . . . . . . . . . . . .  38
       9.1. Benefits of Quick-Start. . . . . . . . . . . . . . . . .  39
       9.2. Costs of Quick-Start . . . . . . . . . . . . . . . . . .  39
       9.3. Quick-Start with QoS-enabled Traffic . . . . . . . . . .  41
       9.4. Protection against Misbehaving Nodes . . . . . . . . . .  41



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          9.4.1. Receivers Lying about Whether the
          Request was Approved . . . . . . . . . . . . . . . . . . .  41
          9.4.2. Receivers Lying about the Approved
          Rate . . . . . . . . . . . . . . . . . . . . . . . . . . .  42
          9.4.3. Collusion between Misbehaving Routers . . . . . . .  43
          9.4.4. Misbehaving Middleboxes and the IP
          TTL. . . . . . . . . . . . . . . . . . . . . . . . . . . .  44
       9.5. Attacks on Quick-Start . . . . . . . . . . . . . . . . .  45
       9.6. Simulations with Quick-Start . . . . . . . . . . . . . .  45
    10. Implementation and Deployment Issues . . . . . . . . . . . .  46
       10.1. Implementation Issues for Sending Quick-
       Start Requests. . . . . . . . . . . . . . . . . . . . . . . .  46
       10.2. Implementation Issues for Processing Quick-
       Start Requests. . . . . . . . . . . . . . . . . . . . . . . .  47
       10.3. Possible Deployment Scenarios . . . . . . . . . . . . .  47
       10.4. Would QuickStart packets take the slow path
       in routers? . . . . . . . . . . . . . . . . . . . . . . . . .  48
       10.5. A Comparison with the Deployment Problems
       of ECN. . . . . . . . . . . . . . . . . . . . . . . . . . . .  48
    11. Related Work . . . . . . . . . . . . . . . . . . . . . . . .  49
       11.1. Fast Start-ups without Explicit Information
       from Routers. . . . . . . . . . . . . . . . . . . . . . . . .  49
       11.2. Optimistic Sending without Explicit Infor-
       mation from Routers . . . . . . . . . . . . . . . . . . . . .  50
       11.3. Fast Start-ups with other Information from
       Routers . . . . . . . . . . . . . . . . . . . . . . . . . . .  51
       11.4. Fast Start-ups with more Fine-Grained Feed-
       back from Routers . . . . . . . . . . . . . . . . . . . . . .  52
    12. Security Considerations. . . . . . . . . . . . . . . . . . .  52
    13. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . .  53
    14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  54
    A. Design Decisions. . . . . . . . . . . . . . . . . . . . . . .  54
       A.1. Alternate Mechanisms for the Quick-Start
       Request: ICMP and RSVP. . . . . . . . . . . . . . . . . . . .  54
          A.1.1. ICMP. . . . . . . . . . . . . . . . . . . . . . . .  54
          A.1.2. RSVP. . . . . . . . . . . . . . . . . . . . . . . .  56
       A.2. Alternate Encoding Functions . . . . . . . . . . . . . .  57
       A.3. The Quick-Start Request: Packets or Bytes? . . . . . . .  58
       A.4. Quick-Start Semantics: Total Rate or Addi-
       tional Rate?. . . . . . . . . . . . . . . . . . . . . . . . .  59
       A.5. Alternate Responses to the Loss of a Quick-
       Start Packet. . . . . . . . . . . . . . . . . . . . . . . . .  60
       A.6. Why Not Include More Functionality?. . . . . . . . . . .  61
       A.7. The Earlier QuickStart Nonce . . . . . . . . . . . . . .  64
    B. Quick-Start with DCCP . . . . . . . . . . . . . . . . . . . .  65
    C. Possible Router Algorithm . . . . . . . . . . . . . . . . . .  67
    D. Possible Uses for the Reserved Fields . . . . . . . . . . . .  68
    Normative References . . . . . . . . . . . . . . . . . . . . . .  70



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    Informative References . . . . . . . . . . . . . . . . . . . . .  70
    E. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  74
       E.1. IP Option. . . . . . . . . . . . . . . . . . . . . . . .  74
       E.2. TCP Option . . . . . . . . . . . . . . . . . . . . . . .  75
    AUTHORS' ADDRESSES . . . . . . . . . . . . . . . . . . . . . . .  75
    Full Copyright Statement . . . . . . . . . . . . . . . . . . . .  75
    Intellectual Property. . . . . . . . . . . . . . . . . . . . . .  76












































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1.  Introduction

    Each TCP connection begins with a question: "What is the appropriate
    sending rate for the current network path?"  The question is not
    answered explicitly for TCP, but each TCP connection determines the
    sending rate by probing the network path and altering the congestion
    window (cwnd) based on perceived congestion.  Each 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
    appropriate in the current network conditions.

    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.

    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.



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    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 get
    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.


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.  [ZPS00]
    shows this assumption should be generally valid, although [RFC3819]
    discusses a range of Bandwidth on Demand subnets.

    * 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.

    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 request for a higher sending rate
    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.

    There are also a number of questions regarding the Quick-Start
    mechanism that are discussed later in this document.

    Questions:

    * Would the benefits of the Quick-Start mechanism be worth the added
    complexity?

    The benefits and drawbacks of Quick-Start are discussed in more
    detail in Section 9 on "Evaluation of Quick-Start".

    * One practical consideration is that packets with known and unknown
    IP options are often dropped in the current Internet [MAF04].

    This does not preclude using Quick-Start in Intranets.  Further,
    [MAF04] also shows that over time the blocking of packets
    negotiating ECN has become less common, and therefore an incremental
    deployment story for Quick-Start based on IP Options is not out of
    the question.  Appendix A.1 on "Alternate Mechanisms for the Quick-
    Start Request" discusses the possibility of using RSVP or ICMP
    instead of IP Options for carrying Quick-Start Requests to routers.

    * A second practical consideration is that packets could be dropped
    at non-IP queues along the path.

    This is discussed in more detail in Section 9.2 .

    * Apart from the merits and shortcomings of the Quick-Start
    mechanism, is there likely to be a compelling need to add explicit
    congestion-related feedback from routers over and above the one-bit
    feedback from ECN?

    If the answer to the question above is yes, should we be considering
    ways to incorporate Quick-Start in mechanisms that, while more
    complex, are also sufficiently more powerful than Quick-Start, or
    should Quick-Start be considered as orthogonal to such mechanisms?
    This is discussed further in Appendix A.6 on "Why Not Include More
    Functionality".


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



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    section is non-normative; the normative description of Quick-Start
    with IP and TCP follows in Sections 3 and 4. Quick-Start can 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
    Request (QSR) 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.  In addition, Quick-Start assumes a unicast, congestion-
    controlled transport protocol; we do not consider the use of Quick-
    Start for multicast traffic.

    The Quick-Start Request Option includes a request for a sending rate
    in bytes 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 and information about the TTL to the sender using transport-
    level mechanisms.  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 this information 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.

    Figure 1 shows a successful use of Quick-Start, with both routers
    along the path approving the Quick-Start Request.  In this example,
    Quick-Start is used by TCP to establish the initial congestion
    window.












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       Sender        Router 1       Router 2          Receiver
       ------        --------       --------          --------
     | <IP TTL: 63>
     | <QS TTL: 91>
     | <TTL Diff: 28>
     | Quick-Start Request
     | in SYN or SYN/ACK -->
     |
     |               Decrement
     |               QS TTL
     |               to approve
     |               request -->
     |
     |                              Decrement
     |                              QS TTL
     |                              to approve
     |                              request -->
     |
     |                                           <IP TTL: 61>
     |                                           <QS TTL: 89>
     |                                           <TTL Diff: 28>
     |                                           Return Quick-Start
     |                                            info to sender in
     |                                          <-- TCP ACK packet.
     |
     | <TTL Diff: 28>
     | Quick-Start approved,
     | translate to cwnd.
     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,
    including the default initial congestion window, response to idle
    periods, etc.












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       Sender        Router 1       Router 2          Receiver
       ------        --------       --------          --------
     | <IP TTL: 63>
     | <QS TTL: 91>
     | <TTL Diff: 28>
     | Quick-Start Request
     | in SYN or SYN/ACK -->
     |
     |               Decrement
     |               QS TTL
     |               to approve
     |               request -->
     |
     |                              Forward packet
     |                              without modifying
     |                              Quick-Start Option. -->
     |
     |                                           <IP TTL: 61>
     |                                           <QS TTL: 90>
     |                                           <TTL Diff: 29>
     |                                           Return Quick-Start
     |                                            info to sender in
     |                                          <-- TCP ACK packet.
     |
     | <TTL Diff: 29>
     | Quick-Start not approved.
     V Use default initial cwnd. -->

               Figure 2: An unsuccessful Quick-Start Request.



3.  The Quick-Start Request in IP


3.1.  The Quick-Start Request 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     |  QS TTL       | Resv. | Rate  |
    |               |               |               |       |Request|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        QS Nonce                           |   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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


    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 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).  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.

    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)

    The fourth byte includes a four-bit Reserved field, and a four-bit
    Rate Request field.  The second four bytes contain a 30-bit QS Nonce
    and a two-bit Reserved field.  The sender SHOULD set the reserved
    fields to zero, and routers SHOULD ignore the reserved fields.  The
    sender SHOULD set the 30-bit QS Nonce to a random value.

    The sender initializes the Rate Request to the desired sending rate,
    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 A.2.





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     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.

    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
    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.

    In IPv4, a change in IP options at routers requires recalculating
    the IP header checksum.


3.2.  The Quick-Start Request Option for IPv6

    The Quick-Start Request Option for IPv6 is placed in the Hop-by-Hop
    Options extension header that is processed at every network node
    along the communication path [RFC 2460]. The option format following
    the generic Hop-by-Hop Options header is similar to the IPv4 format
    with the exception that the Length field should exclude the common



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    type and length fields in the option format and be set to 6 bytes.


     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=6     |  QS TTL       | Resv. | Rate  |
    |               |               |               |       |Request|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        QS Nonce                           |   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 4.  The Quick-Start Request Option for IPv6.


    The transport receiver compares the Quick-Start TTL with the IPv6
    Hop Limit field in order 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 Request 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.

    Note that [RFC2460] specifies that when a specific flow label has
    been assigned to packets, the contents of the Hop-by-Hop options,
    excluding the next header field, must originate with the same
    contents throughout the IP flow lifetime.  However, the Quick-Start
    Request option would be included in only a small fraction of the
    packets during a flow lifetime.  Thus, Quick-Start SHOULD NOT be
    used in an IPv6 connection that uses flow labels unless the
    experimental specification of flow labels in Appendix A of RFC 2460
    is changed.  We note that RFC 2460 states that the use of the flow
    label field in IPv6 "is, at the time of writing, still experimental
    and subject to change as the requirements for flow support in the
    Internet become clearer" [RFC2460].


3.3.  Processing the Quick-Start Request at Routers

    Each participating router can either terminate or approve the Quick-
    Start Request.  The router terminates the Quick-Start Request if the
    router is not underutilized, and therefore has decided not to grant



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    the Quick-Start Request.

    A router that wishes to terminate the Quick-Start Request SHOULD
    delete the Quick-Start Request from the IP header.  This saves
    resources as downstream routers will have no option to process.  If
    a Quick-Start-capable router wishes to deny the request but doesn't
    delete the Quick-Start Request from the IP header, then the router
    SHOULD zero the QS TTL, QS Nonce, and Rate Request fields.  This may
    be more efficient for routers to implement than deleting the Quick-
    Start option.  A router that doesn't understand the Quick-Start
    option will simply forward the packet with the Quick-Start Request
    unchanged.

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

    * The router MUST decrement the QS TTL by 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
    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.

    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.


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



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    Nonce.

    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
    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



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    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 from
    the sender is that the original value for the QS Nonce is not easily
    guessable by the receiver.  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.

    A requirement of the routers is that the receiver can not 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.


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 bytes per second,
    appropriately formatted, in the Quick-Start Request 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.)  The Quick-Start Request also includes random values
    for the QS TTL and the QS Nonce.  When used for initial start-up,
    the Quick-Start request packet can be either the SYN or SYN/ACK
    packet, as described above.  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,



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    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 uses TCP's default congestion
    control procedure.  For initial start-up, host A uses the default
    initial congestion window.

    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 packets are defined as
    packets sent as the result of a successful Quick-Start request, up
    to the time when the first Quick-Start packet is acknowledged.  In
    order to use Quick-Start, the TCP host MUST use rate-based pacing 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.


4.1.  When to Use Quick-Start

    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, as discussed above.
        (A transport that uses TCP Control Block sharing, the Congestion
        Manager, or the like may not need Quick-Start to determine an
        appropriate rate.)





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        (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 for
        the appropriate rate at which to send.  (Alternatively,
        traditional slow-start should be used in this 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
        assess the available capacity on the network path.  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 Rate Request.

    As a general guideline, a TCP sender SHOULD NOT send a Quick-Start
    request until it has confirmed that is ready to transmit enough data
    to use the requested rate over the round-trip time of the connection
    (or over 100 ms, if the round-trip time is not known).  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 4.6 discusses some of the issues of using Quick-Start at



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    connection initiation, and Section 4.7 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

    TCP's Quick-Start Response option is defined as follows:



     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                                |   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 6.  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 four 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 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).

    The last four bytes of the TCP option contain the 30-bit QS Nonce
    and a two-bit Reserved field.

    We note that the Quick-Start Response Option for TCP contains eight
    bytes, and the length of the TCP option field is generally at most
    40 bytes.  Other TCP options that might be used include Time Stamp
    (ten bytes), Window Scale (three bytes), Maximum Segment Size (four
    bytes), Selective Acknowledgments Data (at least ten bytes), and
    Selective Acknowledgments Permitted (two bytes).




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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
    Quick-Start Response option is set to the received value of the Rate
    Request in the Quick-Start Request 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 Request option.

    The Quick-Start Response will NOT be resent if it is lost in the
    network. Packet loss is 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 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 checks of the TTL Diff and the Rate Request are successful,
    then the TCP host sets its Quick-Start congestion window (in terms
    of MSS-sized segments), QS-cwnd, as 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).




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    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.  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.  (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].)

    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 specified
    Rate Request.  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 QuickStart 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 reduces its 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 R*T/(B*2)
    packets, the actual number of packets that were needed to fill the
    pipe at a sending rate of R KBps.

    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.


4.5.  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 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, 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 A.5 discusses possible
    alternatives in responding to the loss of a Quick-Start packet.

    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.








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4.6.  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.


4.6.1.  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:

        "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.

    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.



4.6.2.  Quick-Start Request Packets that are Discarded by 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 middleboxes, where a middlebox is



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    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 connection is established if the TCP
    SYN packet contains an IP TimeStamp Option).  In both cases, this is
    presumably due to 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 recommend 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.

    In the case of a retransmission, in addition to resending the SYN or
    SYN/ACK packet without the Quick-Start Request, the TCP sender
    SHOULD use an RTO of three seconds and a different Initial Sequence
    Number.  Using this scheme the TCP sender MUST keep track of when
    each of the SYN (or SYN/ACKs) was transmitted.  In this way, an
    acknowledgement for the retransmitted SYN or SYN/ACK packet can be
    matched with the SYN or SYN/ACK being acknowledged, and the
    transmission time of the SYN (or SYN/ACK) being acknowledged can be
    used for an RTT measurement to seed the RTO.  If only the
    retransmitted SYN or SYN/ACK is acknowledged, the TCP sender can
    reasonably assume that the earlier SYN or SYN/ACK with the Quick-
    Start option was dropped by the network because of the option and
    not because of congestion.  In this case, the TCP sender can refrain
    from performing TCP's standard congestion control state changes.

    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 middlebox dropping the packet because of the IP
    Option, or because of a 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



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    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 IP Options than due to an ECN request in
    the TCP header [MAF04].


4.7.  TCP: A Quick-Start Request in the Middle of 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
    connection, for example after an idle period: (1) determining the
    rate to request; and (2) the response if Quick-Start packets are
    dropped;

    (1) Determining the rate to request:
    In the middle of connection, an easy rule of thumb would be for the
    TCP sender to determine the largest congestion window that the TCP
    connection achieved since the last packet drop, to translate this
    congestion window to a sending rate, and use this rate in the Quick-
    Start request.  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.

    In the case of an idle period, the sender SHOULD NOT use Quick-Start
    if the idle period has been less than an RTO, and the congestion
    window has not decayed down to less than half of its value at the
    start of the idle period.  Such a use of Quick-Start requires
    further investigation.

    (2) 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.

    We note that a packet in the middle of a connection carrying a
    Quick-Start Request might or might not carry a data payload.  For
    example, for TCP, the Quick-Start Request could be carried by a data
    packet, or by a pure acknowledgement packet.


4.8.  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



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    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,
    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.

    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.


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 [RFC2402,2402bis].  Changes to the
    Quick-Start option in the IP header do not affect this AH ICV.  The
    tunnel considerations in Section 3.6 below apply to all IPsec
    tunnels, regardless of what IPsec headers or processing are used in
    conjunction with the tunnel.




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    Because the contents of the Quick-Start Request 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 2402 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 Request 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 Request option data can change en
    route.  RFC 2402 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.


6.  Quick-Start in IP Tunnels

    This section considers interactions between Quick-Start and IP
    tunnels, including IPsec [RFC2401,2401bis] and IP in IP [RFC2003].

    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
    connection, 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].








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                        __ 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 5: Categories of Tunnels.


    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 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



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    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
    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



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    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 resetting the Rate Request field in the inner header, or by
    deleting the Quick-Start Request 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.


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
    [RFC2401].

    As one example, if a remote access VPN client uses a BITS structure,
    then Quick-Start obstacles between the client and the VPN gateway



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    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 [RFC2401].
    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].

    Section 6.2 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



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    associations.  We do not consider the interactions between Quick-
    Start and IPsec with IKEv1 [RFC2409] in this document.  When the RFC
    for IKEv2 [IKEv2] is published, we will 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.



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 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
    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.




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    * 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 [SAF05], 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.  [SAF05] 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.  Following
    Section 4.1, the sender SHOULD NOT request a sending rate larger
    than it is able to use over the 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.


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.  As an example, the router
    could ask the following questions:




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    * Has the router's output link been underutilized for some time
    (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 this 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], for the current time between T1 and
    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 C, and more discussion of these
    issues is available in [SAF05].)

    * If the router's output link has been underutilized and the
    aggregate 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.  [SAF05] 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, [SAF05] 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








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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
    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 has 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



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    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
    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 4.6, not all packets would carry the Quick-Start
    Request 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 9.5, routers can easily protect against this
    by enforcing a limit on the rate at which Quick-Start requests will
    be considered.

    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.  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.

    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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    As discussed in Section 6.2, Quick-Start could also 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.  However, as noted in Section 6.2, this is similar to what
    can happen without Quick-Start when a connection path is changed
    after the connection had already established a certain sending rate
    on the original path.

    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.


9.4.  Protection against Misbehaving Nodes

    In this section we discuss the protection against receivers or
    colluding middleboxes lying about the Quick-Start Request.  First,
    we note that it is not necessarily in the receiver's interest to lie
    about the Quick-Start Request.  If the sender sends at too-high of
    an initial rate, and has a packet dropped, this does not necessarily
    improve the performance of the connection, relative to the case when
    the Quick-Start Request was not approved.


9.4.1.  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



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    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, the 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.


9.4.2.  Receivers Lying about the Approved Rate

    A second form of 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



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    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.


9.4.3.  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
    Request Option from the packet, the ingress router could have sent
    the Quick-Start Request Option to the egress router out-of-band,



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    with the egress router inserting the Quick-Start Request 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 does not necessarily give an advantage to the traffic
    covered by that collusion.  If some router along the path really
    does not have enough available bandwidth to approve the Quick-Start
    request, then the Quick-Start packets sent as a result of the
    falsely-approved request could be dropped in the network, to the
    resulting 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 necessarily be to
    the connection's advantage for this to happen.  In addition, the
    router between the ingress and egress nodes that denied the request
    could be monitoring connection performance, actively penalizing
    nodes that seem to be using Quick-Start after a Quick-Start request
    was denied.

    If the congested router was ECN-capable, and the colluding ingress
    and egress routers were 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).


9.4.4.  Misbehaving Middleboxes and the IP TTL

    A separate possibility 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



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    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).


9.5.  Attacks on Quick-Start

    As discussed in [SAF05], Quick-Start is vulnerable to two kinds of
    Quick-Start 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, attacks to tie up the available Quick-
    Start bandwidth, is more difficult to defend against.  As discussed
    in [SAF05]. 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 `wasting' potential
    Quick-Start bandwidth, resulting in routers denying subsequent
    Quick-Start Requests that if approved would in fact have been used.
    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.

    [SAF05] 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-larqe Quick-Start requests.


9.6.  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



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    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 [SAF05] 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.


10.1.  Implementation Issues for Sending Quick-Start Requests

    Section 4.6 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 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.  Datagram-based real-time streaming applications, on the
    other hand, may have a specific preference on the transmission rate
    and they could indicate the required rate explicitly to the
    transport protocol to be used in the Quick-Start Request.

    We note that when Quick-Start is used, the TCP sender is required to
    implement an additional timer for the paced transmission of Quick-



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    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 network 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 send end up being dropped.


10.3.  Possible Deployment Scenarios

    Because of possible problems discussed above concerning using Quick-
    Start over some network paths, the most realistic initial deployment
    of Quick-Start would likely to 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.

    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 often have large
    network capacity and the networks are underutilized for most of the
    time.  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



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    scientific computing.

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

    * 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.


10.4.  Would QuickStart packets take the slow path in routers?

    How much delay would the slow path add to the processing time for
    this packet?  Similarly, if QuickStart packets took the slow path,
    how much stress would it add to routers for there to be many more
    packets on the slow path, because of the number of packets using
    QuickStart?  These are both questions to be explored while
    experimenting with Quick-Start in the Internet.


10.5.  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 its 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.

    However, in spite of these issues, there is some hope for the
    deployment of Quick-Start, at least in protected corners of the



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    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.


11.  Related Work

    The Quick-Start proposal, taken together with HighSpeed TCP [F03] 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.  % One of the open questions addressed
    later in this % document is whether the % limited capabilities of
    Quick-Start are sufficient to warrant % standardization and
    deployment, or whether more work is % needed first to explore the
    space of potential mechanisms.

    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 three classes of proposals (no explicit
    feedback from routers; explicit feedback about the initial rate; and
    more fine-grained feedback from routers) in the sections below.


11.1.  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 explicit feedback from routers.




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    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.  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.

    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.

    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:
    A second advantage of explicit feedback from the routers is that the
    available bandwidth along the path does not necessarily map to the
    allowed sending rate for an individual flow.  As an example, if the
    TCP sender sends four packets back-to-back in the initial window,
    and the TCP receiver reports that the data packets were received
    with roughly the same spacing as they were transmitted, does this
    mean that the flow can infer an underutilized path?  And how fast
    can the flow send in the next round-trip time?  Do the results
    depend on the level of statistical multiplexing at the congested
    link, and on the number of flows attempting a faster start-up at the
    same time?


11.2.  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



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    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 key question would be the potential role of optimistic
    senders in amplifying the damage done by a Distributed Denial of
    Service (DDoS) attack.

    (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.


11.3.  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



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    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 Explicit Transport Error Notification (ETEN),
    which includes a cumulative mechanism to notify endpoints of
    aggregate congestion statistics along the path [KAPS02].


11.4.  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 A.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.

    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.


12.  Security Considerations

    Sections 9.4 and 9.5 discuss the security considerations related to
    Quick-Start.  Section 9.4 discusses the potential abuse of Quick-



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    Start of receivers lying about whether the request was approved or
    about the approved rate; of routers in collusion to misuse Quick-
    Start; and of 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 would 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 9.5 discusses a potential attack on the routers' processing
    and state load from an attack of Quick-Start Requests.  Section 9.5
    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.

    Section 4.6.2 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 Request
    option MUST be treated as a mutable IPv4 option, and hence
    completely zeroed for AH ICV calculation purposes; this is also the
    treatment required by RFC 2402 for unrecognized IPv4 options.  The
    IPv6 Quick-Start Request option's IANA-allocated option type
    indicates that it is a mutable option, hence, according to RFC 2402,
    its option data MUST be zeroed for AH ICV computation purposes.  See
    RFC 2402 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.


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



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    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 QuickStart and IP tunnels.  We also
    thank Mohammed Ashraf, John Border, Martin Duke, Tom Dunigan, Gorry
    Fairhurst, John Heidemann, Paul Hyder, Dina Katabi and Vern Paxson,
    for feedback.  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 is a modification of a draft originally by Amit Jain for
    Initial Window Discovery.


A.  Design Decisions


A.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.


A.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; for now, we



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    don't address the mechanisms that would be needed to accomplish this
    task.  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.  If the ICMP Quick-Start Request reached the receiver, the
    receiver would use transport-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 was sent concurrently but separately from the TCP SYN
    packet.)

    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 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 tunnel mode [RFC2401].  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.





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A.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
    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
    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.





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A.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 both
    four-bit and eight-bit Rate Request fields.

    Linear functions:
    The Quick-Start Request contains an 8-bit field for the Rate
    Request.  One possible proposal would be for this field to be
    formatted in bits per second, scaled so that one unit equals 80
    Kbps.  Thus, for the value N in the Rate Request field, the
    requested rate is 80,000*N bps.  This gives a request range between
    80 Kbps and 20.48 Mbps.  For 1500-byte packets, this corresponds to
    a request range between 6 and 1706 packets per second.

    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
    K=40,000, the request range would be from 80 Kbps to 40*2^256 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 is possible 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 is not acceptable, then
    five bits could be used for the Rate Request field.

    If the granularity of factors of two is 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.

    We note that the Rate Request also has to be constrained by the
    abilities 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



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    give a maximum sending rate of 1.07 Gbps.


A.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 will 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.  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.



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    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.


A.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.

    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



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    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.

    For either of these alternatives, there would not be room to report
    the current sending rate in the Quick-Start Option using the current
    minimal format for the Quick-Start Request.  Thus, either the Quick-
    Start Option would have to take more than four bytes to include a
    report of the current sending rate, or the current sending rate
    would not be reported to the routers.


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

    Section 4.5 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,
    even though other packets from that window were dropped in the
    network.  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



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    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
    any such approaches could give the TCP receiver an incentive to lie
    about the Quick-Start request.  That is, if the sender reverts to
    slow-start when a Quick-Start packet is dropped, then it is
    generally not to the receiver's advantage to report a larger rate
    request than was actually approved if the result is going to be a
    Quick-Start packet dropped in the network.  However, if the receiver
    benefits from a larger Quick-Start window even when the larger
    window results in Quick-Start packets dropped in the network, then
    the receiver has a greater incentive to lie about the received rate
    request, in an effort to get the sender to use a larger initial
    sending rate.


A.6.  Why Not Include More Functionality?

    This proposal for Quick-Start is a rather coarse-grained mechanism
    that would allow connections to use higher sending rates along
    underutilized paths, but that does not attempt to provide a next-
    generation transport protocol, and does not attempt the goal of
    providing very high throughput with very low delay.  As Section 11.4
    discusses, there are a number of proposals such as XCP, MaxNet, and
    AntiECN for more fine-grained per-packet feedback from routers that
    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, 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 amenable to incremental deployment in



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    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 multi-path 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.

    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,
    though there has been discussion of variants of XCP with less per-



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    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.  For example, for a mechanism for requesting a
    initial sending rate in an underutilized environment, the fairness
    issues of a general congestion control mechanism go away, and 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 2 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 2: 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 of meeting some of the 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.


A.7.  The Earlier QuickStart Nonce

    An earlier version of this document included a Request-Approved
    QuickStart 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 QuickStart Nonce would have been returned by the
    transport receiver to the transport sender in the Quick-Start



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    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 QuickStart 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
    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.


B.  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
    [KHF05] 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



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    does the sender respond if a Quick-Start packet is dropped; and (2)
    when does the sender determine that there has been no feedback from
    the receiver, and reduce the sending rate?

    (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 prudent to use a smaller
    time interval, e.g., to reduce the sending rate if no feedback is
    received from the receiver in at least two round-trip times.

    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 sufficient; 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; (2) what is
    the response to a loss; and (3) when does the sender determine that
    there has been no feedback from the receiver, and the sending rate
    must be reduced?

    (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



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    allow the sender to send more that 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.


C.  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,
    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.

    [SAF05] 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
    [SAF05]).  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 [SAF05] 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



Jain/Floyd/Allman/Sarolahti                        Section C.  [Page 67]


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    Quick-Start Rate Requests (also from [SAF05]).  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 simpliest 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 [SAF05] 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 is less than the
    threshold then Rate Requests will 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 gross the
    approved rate should be rounded down when it does not fall exactly
    on one of the rates allowed by the encoding scheme.


D.  Possible Uses for the Reserved Fields

    The Quick-Start Request Option contains a four-bit Reserved field in
    the first four bytes, and a two-bit Reserved field in the last four
    bytes.  In this section we discuss some of the possible uses that



Jain/Floyd/Allman/Sarolahti                        Section D.  [Page 68]


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    have been discussed for these Reserved bits.

    Reporting Approved Rate: A Quick-Start Request with the Reporting
    Approved Rate bit set would not be requesting Quick-Start bandwidth,
    but would be reporting the approved rate for Quick-Start bandwidth
    that was approved one round-trip time earlier.  This could be used
    by routers to track which Quick-Start requests were actually
    approved and in use, along with the approved rate.

    Report of Current Sending Rate: A Quick-Start Request with the
    `Report of Current Sending Rate' bit set 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 bit for `Request to Increase
    Sending Rate' would indicate that the connection is not idle or just
    starting up, but is attemmpting to use Quick-Start to increase its
    current sending rate.  This bit would not change the semantics of
    the Rate Request field.

    RTT Estimate: 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.

    Informational Request: An Informational Request bit 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.

    Use Format X for the Rate Request Field: A Quick-Start bit 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 bit could be set in a Quick-
    Start request if 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.




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    If any of these functions were standardized for Quick-Start, the
    standardization would also have to address the issue of backwards
    compatibility with older Quick-Start routers or end-nodes that do
    not understand the newly-added function.


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.

    [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.

    [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.

    [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.




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

    [RFC2246] T. Dierks and C. Allen, The TLS Protocol, RFC 2246.

    [RFC2309] B. Braden, D. Clark, J. Crowcroft, B. Davie, S. Deering,
    D. Estrin, S. Floyd, V. Jacobson, G. Minshall, C. Partridge, L.
    Peterson, K.  Ramakrishnan, S. Shenker, J. Wroclawski, L. Zhang,
    Recommendations on Queue Management and Congestion Avoidance in the
    Internet, RFC 2309, April 1998.

    [RFC2401] S. Kent and R. Atkinson. Security Architecture for the
    Internet Protocol. RFC 2401, November 1998.

    [2401bis] S. Kent and K. Seo, Security Architecture for the Internet
    Protocol, internet-draft draft-ietf-ipsec-rfc2401bis-06.txt, work-
    in-progress, March 2005.

    [RFC2402] S. Kent and R. Atkinson. IP Authentication Header. RFC
    2402, November 1998.

    [2402bis] S. Kent, IP Authentication Header, internet-draft draft-
    ietf-ipsec-rfc2402bis-11.txt work-in-progress, March 2005.

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

    [RFC2416] T. Shepard and C. Partridge.  When TCP Starts Up With Four
    Packets Into Only Three Buffers.  RFC 2416. September 1998.

    [RFC2463] A. Conta and S. Deering. Internet Control Message Protocol
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    [RFC2488] M. Allman, D. Glover, and L. Sanchez. Enhancing TCP Over
    Satellite Channels using Standard Mechanisms. RFC 2488. January
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    [RFC2661] W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn, and
    B.  Palter, Layer Two Tunneling Protocol "L2TP", RFC 2661, August
    1999.

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

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




Jain/Floyd/Allman/Sarolahti                                    [Page 71]


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    [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.

    [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.

    [AHO98] M. Allman, C. Hayes and S. Ostermann. An evaluation of TCP
    with Larger Initial Windows. ACM Computer Communication Review, July
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    [FF99] Floyd, S., and Fall, K., Promoting the Use of End-to-End
    Congestion Control in the Internet, IEEE/ACM Transactions on
    Networking, August 1999.

    [F03] Floyd, S., HighSpeed TCP for Large Congestion Windows, RFC
    3649, December 2003.

    [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,
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    [H05] P. Hoffman, email, October 2005.  Citation for acknowledgement
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    Detection: Evasion, Traffic Normalization, and End-to-End Protocol
    Semantics, Proc. USENIX Security Symposium 2001.

    [IKEv2] Kaufman, C., (ed.), "Internet Key Exchange (IKEv2)
    Protocol", draft-ietf-ipsec-ikev2-17.txt, Internet draft (work in



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    progress), September 2004.

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

    [JD02] Manish Jain, Constantinos Dovrolis, End-to-End Available
    Bandwidth: Measurement Methodology, Dynamics, and Relation with TCP
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    [KHR02] Dina Katabi, Mark Handley, and Charles Rohrs, Internet
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    Environments. ACM Sigcomm 2002, August 2002.  URL
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    [KHF05] E. Kohler, M. Handley, and S. Floyd, Datagram Congestion
    Control Protocol (DCCP), internet draft draft-ietf-dccp-spec-11.txt,
    work in progress, March 2005.

    [K03] S. Kunniyur, "AntiECN Marking: A Marking Scheme for High
    Bandwidth Delay Connections", Proceedings, IEEE ICC '03, May 2003.
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    [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/".

    [L05] Guohan Lu, Nonce in TCP Quick Start, draft, September 2005.
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    [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.  To appear in
    Computer Communications Review, April 2004.

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    "http://netlab.caltech.edu/~bartek/maxnet.htm".

    [PK98] Venkata N. Padmanabhan and Randy H. Katz, TCP Fast Start: A
    Technique For Speeding Up Web Transfers, IEEE GLOBECOM '98, November
    1998.

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



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    acknowledgement purposes only.

    [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/".

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

    [SAF05] Pasi Sarolahti, Mark Allman, and Sally Floyd.  Evaluating
    Quick-Start for TCP.  Under submission, May 2005.  URL
    "http://www.icir.org/floyd/quickstart.html".

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

    [W00] Michael Welzl: PTP: Better Feedback for Adaptive Distributed
    Multimedia Applications on the Internet, IPCCC 2000 (19th IEEE
    International Performance, Computing, And Communications
    Conference), Phoenix, Arizona, USA, 20-22 February 2000.  URL
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    [W03] Michael Welzl, PMTU-Options: Path MTU Discovery Using Options,
    expired internet-draft draft-welzl-pmtud-options-01.txt, work-in-
    progress.  February 2003.

    [ZPS00] Y. Zhang, V. Paxson, and S. Shenker,  The Stationarity of
    Internet Path Properties: Routing, Loss, and Throughput, ACIRI
    Technical Report, May 2000.


E.  IANA Considerations

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


E.1.  IP Option

    Quick-Start requires that both an IPv4 and an IPv6 Option Number be
    allocated.  The IPv4 Option would have a copied flag of 0, a class
    field of 00, and the assigned five-bit option number.  The IPv6
    Option would have the first three bits of "001" [RFC 2460, Section
    4.2], with the first two bits indicating 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 indicating that the
    Option Data may change en-route.



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    In both cases the name of the option would be "QSR - Quick-Start
    Request", with this document as the reference document.


E.2.  TCP Option

    Quick-Start also requires that a TCP Option Number be allocated.
    The Length would be 4, and the Meaning would be "Quick-Start
    Request", with this document as the reference document.


AUTHORS' ADDRESSES


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

    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) 243-7361
    Email: mallman@icir.org
    URL: http://www.icir.org/mallman/

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


Full Copyright Statement

    Copyright (C) The Internet Society 2005.  This document is subject
    to the rights, licenses and restrictions contained in BCP 78, and
    except as set forth therein, the authors retain all their rights.

    This document and the information contained herein are provided on



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    an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
    REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
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