[Docs] [txt|pdf] [Tracker] [WG] [Email] [Nits]

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-00.txt                              S. Floyd
Expires: November 2005                                         M. Allman
                                                                    ICIR
                                                            P. Sarolahti
                                                   Nokia Research Center
                                                             31 May 2005


                       Quick-Start for TCP and IP


Status of this Memo

    This document is an Internet-Draft and is subject to all provisions
    of section 3 of RFC 3667.  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 aware will be disclosed, in accordance with
    Section 6 of BCP 79.

    Internet-Drafts are working documents of the Internet Engineering
    Task Force (IETF), its areas, and its working groups.  Note that
    other groups may also distribute working documents as Internet-
    Drafts.

    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other documents
    at any time.  It is inappropriate to use Internet-Drafts as
    reference material or to cite them other than as "work in progress."

    The list of current Internet-Drafts can be accessed at
    http://www.ietf.org/ietf/1id-abstracts.txt.

    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html.

    This Internet-Draft will expire on November 2005.

Copyright Notice

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





Jain/Floyd/Allman/Sarolahti                                     [Page 1]


INTERNET-DRAFT           Expires: November 2005                 May 2005


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.  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.





























Jain/Floyd/Allman/Sarolahti                                     [Page 2]


INTERNET-DRAFT           Expires: November 2005                 May 2005


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

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



Jain/Floyd/Allman/Sarolahti                                     [Page 3]


INTERNET-DRAFT           Expires: November 2005                 May 2005


       - TCP: A Quick-Start Request after an Idle Period
       - The Quick-Start Mechanisms in DCCP and other Transport
         Protocols
       - Quick-Start with DCCP
       - Implementation and Deployment Issues
       - Design Decisions
     * Added a discussion of Kunniyur's Anti-ECN proposal.
     * Added a section on simulations, with a brief discussion of the
       simulations by Srikanth Sundarrajan.

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

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




























Jain/Floyd/Allman/Sarolahti                                     [Page 4]


INTERNET-DRAFT           Expires: November 2005                 May 2005


                             Table of Contents

    1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   7
    2. Assumptions and General Principles. . . . . . . . . . . . . .   8
       2.1. Overview of Quick-Start. . . . . . . . . . . . . . . . .   9
    3. The Quick-Start Request in IP . . . . . . . . . . . . . . . .  12
       3.1. The Quick-Start Request Option for IPv4. . . . . . . . .  12
       3.2. The Quick-Start Request Option for IPv6. . . . . . . . .  14
       3.3. Processing the Quick-Start Request at
       Routers . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
       3.4. Deciding the Permitted Rate Request at a
       Router. . . . . . . . . . . . . . . . . . . . . . . . . . . .  16
       3.5. Quick-Start in IP Tunnels. . . . . . . . . . . . . . . .  17
       3.6. A Rate-Reduced Nonce?. . . . . . . . . . . . . . . . . .  19
    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. . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
       4.3. TCP: Sending the Quick-Start Response. . . . . . . . . .  23
       4.4. TCP: Receiving and Using the Quick-Start
       Response Packet . . . . . . . . . . . . . . . . . . . . . . .  24
       4.5. TCP: Responding to a Loss of a Quick-Start
       Packet. . . . . . . . . . . . . . . . . . . . . . . . . . . .  25
       4.6. TCP: A Quick-Start Request for a Larger Ini-
       tial Window . . . . . . . . . . . . . . . . . . . . . . . . .  25
          4.6.1. Determining the Rate to Request . . . . . . . . . .  25
          4.6.2. Interactions with Path MTU Discovery. . . . . . . .  26
          4.6.3. Quick-Start Request Packets that are
          Discarded by Middleboxes . . . . . . . . . . . . . . . . .  27
       4.7. TCP: A Quick-Start Request in the Middle of
       Connection. . . . . . . . . . . . . . . . . . . . . . . . . .  28
       4.8. An Example Quick-Start Scenario with TCP . . . . . . . .  29
    5. The Quick-Start Mechanism in other Transport Pro-
    tocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  29
    6. Evaluation of Quick-Start . . . . . . . . . . . . . . . . . .  30
       6.1. Benefits of Quick-Start. . . . . . . . . . . . . . . . .  30
       6.2. Costs of Quick-Start . . . . . . . . . . . . . . . . . .  31
       6.3. Protection against Misbehaving Nodes . . . . . . . . . .  33
          6.3.1. Receivers Lying about Whether the
          Request was Approved . . . . . . . . . . . . . . . . . . .  33
          6.3.2. Receivers Lying about the Approved
          Rate . . . . . . . . . . . . . . . . . . . . . . . . . . .  33
          6.3.3. Collusion between Misbehaving Routers . . . . . . .  35
          6.3.4. Misbehaving Middleboxes and the IP
          TTL. . . . . . . . . . . . . . . . . . . . . . . . . . . .  36
       6.4. Quick-Start with QoS-enabled Traffic . . . . . . . . . .  36
       6.5. Limitations of Quick-Start . . . . . . . . . . . . . . .  36
       6.6. Attacks on Quick-Start . . . . . . . . . . . . . . . . .  37



Jain/Floyd/Allman/Sarolahti                                     [Page 5]


INTERNET-DRAFT           Expires: November 2005                 May 2005


       6.7. Simulations with Quick-Start . . . . . . . . . . . . . .  37
    7. Related Work. . . . . . . . . . . . . . . . . . . . . . . . .  38
       7.1. Fast Start-ups without Explicit Information
       from Routers. . . . . . . . . . . . . . . . . . . . . . . . .  38
       7.2. Optimistic Sending without Explicit Informa-
       tion from Routers . . . . . . . . . . . . . . . . . . . . . .  39
       7.3. Fast Start-ups with other Information from
       Routers . . . . . . . . . . . . . . . . . . . . . . . . . . .  40
       7.4. Fast Start-ups with more Fine-Grained Feed-
       back from Routers . . . . . . . . . . . . . . . . . . . . . .  41
    8. Implementation and Deployment Issues. . . . . . . . . . . . .  41
       8.1. Implementation Issues for Sending Quick-
       Start Requests. . . . . . . . . . . . . . . . . . . . . . . .  42
       8.2. Implementation Issues for Processing Quick-
       Start Requests. . . . . . . . . . . . . . . . . . . . . . . .  42
       8.3. Possible Deployment Scenarios. . . . . . . . . . . . . .  43
       8.4. Would QuickStart packets take the slow path
       in routers? . . . . . . . . . . . . . . . . . . . . . . . . .  44
       8.5. A Comparison with the Deployment Problems of
       ECN . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  44
    9. Security Considerations . . . . . . . . . . . . . . . . . . .  44
    10. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . .  45
    11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  45
    A. Design Decisions. . . . . . . . . . . . . . . . . . . . . . .  45
       A.1. Alternate Mechanisms for the Quick-Start
       Request: ICMP and RSVP. . . . . . . . . . . . . . . . . . . .  45
          A.1.1. ICMP. . . . . . . . . . . . . . . . . . . . . . . .  46
          A.1.2. RSVP. . . . . . . . . . . . . . . . . . . . . . . .  47
       A.2. Alternate Encoding Functions . . . . . . . . . . . . . .  48
       A.3. The Quick-Start Request: Packets or Bytes? . . . . . . .  49
       A.4. Quick-Start Semantics: Total Rate or Addi-
       tional Rate?. . . . . . . . . . . . . . . . . . . . . . . . .  50
       A.5. Alternate Responses to the Loss of a Quick-
       Start Packet. . . . . . . . . . . . . . . . . . . . . . . . .  51
       A.6. Why Not Include More Functionality?. . . . . . . . . . .  52
       A.7. The Earlier QuickStart Nonce . . . . . . . . . . . . . .  55
    B. Quick-Start with DCCP . . . . . . . . . . . . . . . . . . . .  56
    C. Possible Router Algorithm . . . . . . . . . . . . . . . . . .  58
    Normative References . . . . . . . . . . . . . . . . . . . . . .  60
    Informative References . . . . . . . . . . . . . . . . . . . . .  60
    IANA Considerations. . . . . . . . . . . . . . . . . . . . . . .  63
    IP Option. . . . . . . . . . . . . . . . . . . . . . . . . . . .  63
    TCP Option . . . . . . . . . . . . . . . . . . . . . . . . . . .  64
    AUTHORS' ADDRESSES . . . . . . . . . . . . . . . . . . . . . . .  64
    Full Copyright Statement . . . . . . . . . . . . . . . . . . . .  64
    Intellectual Property. . . . . . . . . . . . . . . . . . . . . .  65





Jain/Floyd/Allman/Sarolahti                                     [Page 6]


INTERNET-DRAFT           Expires: November 2005                 May 2005


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.



Jain/Floyd/Allman/Sarolahti                         Section 1.  [Page 7]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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.

    * 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.




Jain/Floyd/Allman/Sarolahti                         Section 2.  [Page 8]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    * 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.

    Open 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 6 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 6.2.  * 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
    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



Jain/Floyd/Allman/Sarolahti                       Section 2.1.  [Page 9]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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.














Jain/Floyd/Allman/Sarolahti                      Section 2.1.  [Page 10]


INTERNET-DRAFT           Expires: November 2005                 May 2005


       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.












Jain/Floyd/Allman/Sarolahti                      Section 2.1.  [Page 11]


INTERNET-DRAFT           Expires: November 2005                 May 2005


       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:


              0              1              2              3
       +--------------+--------------+--------------+--------------+
       | Option       | Length=4     |  QS TTL      |Resv. |Rate   |
       |              |              |              |      |Request|
       +--------------+--------------+--------------+--------------+

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




Jain/Floyd/Allman/Sarolahti                      Section 3.1.  [Page 12]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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 four 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 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, 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.




















Jain/Floyd/Allman/Sarolahti                      Section 3.1.  [Page 13]


INTERNET-DRAFT           Expires: November 2005                 May 2005


     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 the Rate Request field to the 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



Jain/Floyd/Allman/Sarolahti                      Section 3.2.  [Page 14]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    type and length fields in the option format and be set to 2.


              0              1              2              3
       +--------------+--------------+--------------+--------------+
       | Option       | Length=2     |  QS TTL      |Resv. |Rate   |
       |              |              |              |      |Request|
       +--------------+--------------+--------------+--------------+

       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.  This requirement would
    have to be modified to implement Quick-Start on an IPv6
    implementation that uses flow labels, because the Quick-Start
    Request option would be included in only a small fraction of the
    packets during a flow lifetime.


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
    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



Jain/Floyd/Allman/Sarolahti                      Section 3.3.  [Page 15]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    SHOULD zero the QS TTL and the 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 of course 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 decrements the QS TTL by one.

    * If the router is only willing to approve an 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.

    * 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.  Of course, the non-
    participating router still decrements the TTL field in the IP
    header, as is required for all routers [RFC1812].  As a result, the
    sender will be able to detect that the Quick-Start Request had not
    been understood or approved by all of the routers along the path.

    A router that modifies or deletes the Quick-Start Request in the
    IPv4 header also MUST update the IPv4 Header checksum.  For IPv6, no
    checksum updates are needed.


3.4.  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:

    * 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



Jain/Floyd/Allman/Sarolahti                      Section 3.4.  [Page 16]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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 8.2 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.


3.5.  Quick-Start in IP Tunnels

    In this section we consider the effect of IP tunnels on Quick-Start.
    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 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.

    There are two legitimate ways for handling the Quick-Start Request
    with IP tunnels:

    (1) The tunnel ingress node does not support Quick-Start, or does
    not approve the Quick-Start request. The node could strip the Quick-
    Start Request option from the IP header before encapsulation.
    Alternately, the ingress node can decrement the IP TTL before
    encapsulation, while leaving the Quick-Start TTL unchanged, thereby
    changing TTL Diff.  This is the assumed behavior of current IP



Jain/Floyd/Allman/Sarolahti                      Section 3.5.  [Page 17]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    tunnels that are not aware of Quick-Start.

    For a tunnel ingress node that does not support Quick-Start,
    problems with a Quick-Start Request could still occur if a tunnel
    discards the outer header at egress and does not decrement the inner
    IP TTL at the ingress.  In this case, if both the inner IP TTL and
    the Quick-Start TTL are decremented after decapsulation at a Quick-
    Start-aware egress, or if neither is decremented at the egress, then
    TTL Diff would be the same after egress as it was before ingress, so
    that it would wrongly appear that all the routers in the tunnel had
    approved the Quick-Start request.  Fortunately, we are not aware of
    tunnel technologies that operate this way; to the best of our
    knowledge, all tunnels decrement the IP TTL either at the ingress
    before encapsulation, or at the egress router after decapsulation,
    thus changing TTL Diff.

    Even the extreme case when the tunnel ingress is at the TCP sender
    and the tunnel egress is at the TCP receiver, our assumption is that
    the IP TTL will be decremented either at the tunnel ingress or at
    the tunnel egress, changing TTL Diff and preventing the end-nodes
    from wrongly inferring that the Quick-Start Request was approved by
    all of the routers along the path.  If there are tunnels where the
    IP TTL in not decremented, perhaps for PPP over SSH, then additional
    attention will have to be paid to the robustness of Quick-Start in
    these environments.

    A Quick-Start-aware egress must also make sure that the Quick-Start
    Request is not approved if for some reason the inner header includes
    the Quick-Start Request option, the outer header does not, and the
    Quick-Start TTL and IP TTL have been decremented in a fashion that
    makes it appear as if the request has been approved.  If the Quick-
    Start Request doesn't appear in the outer header, then the egress
    node should remove the Quick-Start Request option from the inner
    header after decapsulation.  Alternately, the egress node could
    decrement the Rate Request in the Quick-Start Request option to
    zero.

    (2) The tunnel ingress node may choose to support Quick-Start, and
    locally approve the Quick-Start Request.  In this case the IP TTL
    and Quick-Start option MUST be copied from the inner IP header to
    the outer header at the tunnel ingress. Upon decapsulation, the IP
    TTL and the Quick-Start option in the outer IP header MUST be copied
    back to the inner header.  If the ingress router decrements the IP
    TTL in the inner header before encapsulation, or in the outer header
    after encapsulation, then if the ingress router wishes to approve
    the Quick-Start request, it MUST decrement the Quick-Start TTL at
    the same time, so as not to change TTL Diff.  Similarly, if the
    egress router wishes to approve the Quick-Start request, then when



Jain/Floyd/Allman/Sarolahti                      Section 3.5.  [Page 18]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    it decrements the IP TTL in the outer header before decapsulation,
    or in the inner header after decapsulation, it MUST decrement the
    Quick-Start TTL at the same time.

    A tunnel ingress node can support a Quick-Start request without
    explicitly verifying that the tunnel egress also supports Quick-
    Start.  All that the ingress node has to do is to decrement the IP
    TTL, but not the Quick-Start TTL, in the inner header after
    encapsulation.  In this case, if the egress node simply discards the
    outer header at the egress point, TTL Diff will be different after
    the tunnel egress than it was at the tunnel ingress, and the Quick-
    Start will not be considered by the end-nodes as having been
    approved in the network.  Thus, the tunnel ingress node on its own
    can provide protection against egress nodes that might discard the
    outer header at the egress point.


3.6.  A Rate-Reduced Nonce?

    One possibility for the Reserved Field, for further investigation,
    is to use the four bits for a four-bit Rate-Reduced Nonce.  The goal
    of the Rate-Reduced Nonce would be to give the Quick-Start sender
    some protection against receivers lying about the value of the
    received Rate Request.  The Rate-Reduced Nonce would be initialized
    by the sender to a random value.  When a router approves the Quick-
    Start request but reduces the Rate Request field, the router resets
    the Rate-Reduced Nonce to a new random value.  When a Quick-Start-
    capable router denies the Quick-Start request, the router either
    deletes the Quick-Start Option, or zeroes the Rate-Reduced Nonce
    when zeroing the Rate Request and the QS TTL.  The receiver reports
    the value of the Rate-Reduced Nonce back to the sender.

    The Rate-Reduced Nonce would be of use in cases where the receiver
    knows the original Rate Request R sent by the sender (e.g., because
    the sender always uses the same Rate Request), but the Rate Request
    has been decremented by routers along the path.  What prevents the
    receiver from reporting back to the sender a Rate Request of R, when
    the received Rate Request was in fact less than R?  If the Rate
    Request was not decremented in the network, then the Rate-Reduced
    Nonce should have its original value.  If the Rate Request *was*
    decremented in the network, then the probability that the Rate-
    Reduced Nonce still has its original value is 1/16.  Similarly, if
    the Rate Request was decremented in the network, the chance that the
    receiver can guess the original value of the Rate-Reduced Nonce is
    1/16.

    Thus, if the receiver reports back to the sender the original values
    for the Rate Request and the Rate-Reduced Nonce, and the correct



Jain/Floyd/Allman/Sarolahti                      Section 3.6.  [Page 19]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    value for the TTL Diff, then it is likely that the Quick-Start
    Request was in fact approved at its original value by the routers
    along the path, in particular by all of the Quick-Start-capable
    routers.  The Rate-Reduced Nonce would make it more difficult for
    the receiver to report that the Rate Request was received at its
    original value, when in fact the received Rate Request was less than
    its original value.

    We note, however, that the Rate-Reduced Nonce doesn't provide
    protection against receivers reporting that the Rate Request was
    decremented by only one step, when it fact it was decremented by
    many steps in the network.  This, if the receiver knows the original
    Rate Request from the sender, and the received rate request is
    considerably less than the original request, then the receiver could
    report a received rate request just one step smaller than the
    original request, and the Rate-Reduced Nonce wouldn't provide any
    protection against this.

    Section 6.3 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.)  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,
    or contains a Quick-Start Response with the wrong value for the TTL
    Diff, 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.




Jain/Floyd/Allman/Sarolahti                        Section 4.  [Page 20]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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 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.)


        (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.)





Jain/Floyd/Allman/Sarolahti                      Section 4.1.  [Page 21]


INTERNET-DRAFT           Expires: November 2005                 May 2005


        (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.

    Section 4.6 discusses some of the issues of using Quick-Start at
    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:








Jain/Floyd/Allman/Sarolahti                      Section 4.2.  [Page 22]


INTERNET-DRAFT           Expires: November 2005                 May 2005


            0          1          2          3
       +----------+----------+----------+----------+
       |  Kind    | Length=4 |  Rate    |   TTL    |
       |          |          | Request  |   Diff   |
       +----------+----------+----------+----------+

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


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

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

    The third byte of the Quick-Start Response option contains the
    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).


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 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.






Jain/Floyd/Allman/Sarolahti                      Section 4.3.  [Page 23]


INTERNET-DRAFT           Expires: November 2005                 May 2005


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.  If this check is 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).

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

    The TCP host SHOULD set its congestion window cwnd to QS-cwnd only
    if QS-cwnd is greater than cwnd; otherwise QS-cwnd is ignored.  If
    QS-cwnd is used, the TCP host sets a flag that it is in Quick-Start
    mode, and while in Quick-Start mode the TCP sender MUST use rate-
    based pacing to pace out Quick-Start packets at the specified Rate
    Request.  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
    addresses the problem of an overly-large congestion window from an
    overly-large measurement of the round-trip time.

    If the Quick-Start mode ends with all Quick-Start packets being
    successfully acknowledged, the TCP sender returns to using the
    default congestion control mechanisms.  After all the packets are
    acknowledged from a Quick-Start request for an initial window, for
    example, the TCP sender remains in slow-start, if permitted by
    ssthresh, continuing to increase its congestion window rather
    aggressively from one round-trip time to the next.  To add
    robustness, the TCP sender MUST use Limited Slow-Start [RFC3742]



Jain/Floyd/Allman/Sarolahti                      Section 4.4.  [Page 24]


INTERNET-DRAFT           Expires: November 2005                 May 2005


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


4.6.  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) determining the rate to
    request; (2) interactions with Path MTU Discovery; and (3) Quick-
    Start request packets that are discarded by middleboxes.


4.6.1.  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



Jain/Floyd/Allman/Sarolahti                    Section 4.6.1.  [Page 25]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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.

    In the absence of other information, there could be a configured
    value for the Quick-Start Rate Request.  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.  Therefore, it is
    recommended that the request for the initial sending rate be
    somewhat conservative, in order to improve the chances for more
    Quick-Start requests to be approved.


4.6.2.  Interactions with Path MTU Discovery

    A second 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."

    Unfortunately, the sender doesn't necessarily know the Path MTU when
    it sends packets in the initial window.  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.

    One possibility would be for the sender to send one large packet in
    the initial window with the DF bit set, and to 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.



Jain/Floyd/Allman/Sarolahti                    Section 4.6.2.  [Page 26]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    In the future, it might be possible for the TCP SYN packet to do a
    probe about the Path MTU.  For example, [W03] has proposed an IP
    Option that queries routers for their MTU before starting a Path MTU
    Discovery process.


4.6.3.  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.  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].

    While RFC 1122 and 2988 recommend that the sender should set the
    initial RTO to three seconds, many TCP implementations set the
    initial RTO to one second.  For a TCP SYN packet sent with a Quick-
    Start request, we RECOMMEND an RTO of one second, so that the sender
    can retransmit the SYN packet reasonably promptly if the original
    TCP SYN packet is dropped by a middlebox in the network.

    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.




Jain/Floyd/Allman/Sarolahti                    Section 4.6.3.  [Page 27]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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
    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



Jain/Floyd/Allman/Sarolahti                      Section 4.7.  [Page 28]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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:

    * 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 and Rate Request 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 and Rate Request 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.  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



Jain/Floyd/Allman/Sarolahti                        Section 5.  [Page 29]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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 and the TTL Diff.

    * 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.

    * 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).


6.  Evaluation of Quick-Start


6.1.  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



Jain/Floyd/Allman/Sarolahti                      Section 6.1.  [Page 30]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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).


6.2.  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
    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



Jain/Floyd/Allman/Sarolahti                      Section 6.2.  [Page 31]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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 6.6, 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 if the connection's path was changed in the middle of the
    connection, when the connection had already established the allowed
    initial rate.

    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.








Jain/Floyd/Allman/Sarolahti                      Section 6.2.  [Page 32]


INTERNET-DRAFT           Expires: November 2005                 May 2005


6.3.  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.


6.3.1.  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.  If a router that understands the
    Quick-Start Request denies the request by deleting the request or by
    zeroing the QS TTL, then the receiver can ``lie" about whether the
    request was approved only by successfully guessing the value of the
    TTL Diff to report.  The chance of the receiver successfully
    guessing the correct value for the TTL Diff is 1/256.

    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 field, 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 a TTL field 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.


6.3.2.  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 generally doesn't 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



Jain/Floyd/Allman/Sarolahti                    Section 6.3.2.  [Page 33]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    if it is no higher than the Rate Request originally requested by the
    sender.  This limits the ability of the receiver to cheat.  For
    example, if the sender sends a Quick-Start Request with an Rate
    Request of X, and the receiver reports receiving a Quick-Start
    Request with an 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.

    However, if the sender sends a Quick-Start Request with an Rate
    Request of Z, the receiver receives the Quick-Start Request with an
    approved Rate Request of X, and reports an Rate Request of Y, for X
    < Y <= Z, then the receiver succeeds in lying to the sender about
    the approved rate.

    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 forms of protection against receivers
    lying about the value of the Rate Request.  One form of protection
    would be the Rate-Reduced Nonce discussed earlier, where the
    receiver would have to report the original value of the nonce if the
    receiver reported that the original rate request was approved.

    A second possible protection would be for a router decreasing 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 third 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.








Jain/Floyd/Allman/Sarolahti                    Section 6.3.2.  [Page 34]


INTERNET-DRAFT           Expires: November 2005                 May 2005


6.3.3.  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 for the request packet
    at the ingress.  This would enable the egress router to modify the
    QS TTL 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, 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



Jain/Floyd/Allman/Sarolahti                    Section 6.3.3.  [Page 35]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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).


6.3.4.  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
    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).


6.4.  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.  However, we note that routers should
    be discouraged from granting Quick-Start requests for higher-
    priority traffic when this is likely to result in significant packet
    loss for lower-priority traffic.


6.5.  Limitations of Quick-Start

    The Quick-Start proposal, taken together with HighSpeed TCP [F03],
    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.



Jain/Floyd/Allman/Sarolahti                      Section 6.5.  [Page 36]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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.


6.6.  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.


6.7.  Simulations with Quick-Start

    Quick-Start was added to the NS simulator [SH02] by Srikanth
    Sundarrajan, and additional functionality was added by Pasi
    Sarolahti.  The validation test is at `test-all-quickstart' in the



Jain/Floyd/Allman/Sarolahti                      Section 6.7.  [Page 37]


INTERNET-DRAFT           Expires: November 2005                 May 2005


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

    The Quick-Start simulations in [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.


7.  Related Work

    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.


7.1.  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.

    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]



Jain/Floyd/Allman/Sarolahti                      Section 7.1.  [Page 38]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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?


7.2.  Optimistic Sending without Explicit Information from Routers

    Another possibility that has been suggested [S02] is for the sender
    to start with a large initial window without explicit permission
    from the routers and without bandwidth estimation techniques, and
    for the first packet of the initial window to contain information
    such as the size or sending rate of the initial window.  The
    proposal would be that congested routers would use this information
    in the first data packet to drop or delay many or all of the packets



Jain/Floyd/Allman/Sarolahti                      Section 7.2.  [Page 39]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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.


7.3.  Fast Start-ups with other Information from Routers

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

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

    The Performance Transparency Protocol:
    The Performance Transparency Protocol (PTP) includes a proposal for



Jain/Floyd/Allman/Sarolahti                      Section 7.3.  [Page 40]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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].


7.4.  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.


8.  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.




Jain/Floyd/Allman/Sarolahti                        Section 8.  [Page 41]


INTERNET-DRAFT           Expires: November 2005                 May 2005


8.1.  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-
    Start packets.


8.2.  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.





Jain/Floyd/Allman/Sarolahti                      Section 8.2.  [Page 42]


INTERNET-DRAFT           Expires: November 2005                 May 2005


8.3.  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
    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 are typically up to one
    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.





Jain/Floyd/Allman/Sarolahti                      Section 8.3.  [Page 43]


INTERNET-DRAFT           Expires: November 2005                 May 2005


8.4.  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.


8.5.  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
    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.


9.  Security Considerations

    Sections 6.3 and 6.6 discuss the security considerations related to
    Quick-Start.  Section 6.3 discusses the potential abuse of Quick-
    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 an 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 6.6 discusses a potential attack on the routers' processing
    and state load from an attack of Quick-Start Requests.  Section 6.6



Jain/Floyd/Allman/Sarolahti                        Section 9.  [Page 44]


INTERNET-DRAFT           Expires: November 2005                 May 2005


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


10.  Conclusions

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


11.  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.  We also thank Mohammed Ashraf, John
    Border, Tom Dunigan, John Heidemann, Paul Hyder, Dina Katabi, and
    Vern Paxson for feedback.  This draft builds upon the concepts
    described in [RFC3390], [AHO98], [RFC2415], and [RFC3168].

    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



Jain/Floyd/Allman/Sarolahti                      Section A.1.  [Page 45]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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
    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



Jain/Floyd/Allman/Sarolahti                    Section A.1.1.  [Page 46]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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.


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



Jain/Floyd/Allman/Sarolahti                    Section A.1.2.  [Page 47]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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.


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



Jain/Floyd/Allman/Sarolahti                      Section A.2.  [Page 48]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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
    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



Jain/Floyd/Allman/Sarolahti                      Section A.3.  [Page 49]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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.

    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 an 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



Jain/Floyd/Allman/Sarolahti                      Section A.4.  [Page 50]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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
    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



Jain/Floyd/Allman/Sarolahti                      Section A.5.  [Page 51]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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
    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?

    As Section 6.5 discussed, 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 7.4 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



Jain/Floyd/Allman/Sarolahti                      Section A.6.  [Page 52]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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
    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.



Jain/Floyd/Allman/Sarolahti                      Section A.6.  [Page 53]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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-
    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.










Jain/Floyd/Allman/Sarolahti                      Section A.6.  [Page 54]


INTERNET-DRAFT           Expires: November 2005                 May 2005


                                                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
    TCP receiver to the TCP sender in the Quick-Start Response.  A



Jain/Floyd/Allman/Sarolahti                      Section A.7.  [Page 55]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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 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 field 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 single field for the
    QS TTL, instead of two fields, one for the QS TTL and another for a
    QS-Approved Nonce.

    The Quick-Start Nonce has been resurrected in the current proposal
    for a Rate-Reduced Nonce given in Section 3.6. This proposal would
    provide specific protection against receivers lying about whether
    the rate request was decremented by routers along the path.  In this
    proposal, the Rate-Reduced Nonce would be reset to a new random
    value by routers that approve the request but decrement the value of
    the Rate Request.  Thus, if the original value for the Rate-Reduced
    Nonce is reported back by the receiver to the sender, then it is
    likely that the Rate Request was not decremented or denied by Quick-
    Start-capable routers along the path.


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



Jain/Floyd/Allman/Sarolahti                        Section B.  [Page 56]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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
    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



Jain/Floyd/Allman/Sarolahti                        Section B.  [Page 57]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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
    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



Jain/Floyd/Allman/Sarolahti                        Section C.  [Page 58]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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
    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 3.4, 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;



Jain/Floyd/Allman/Sarolahti                        Section C.  [Page 59]


INTERNET-DRAFT           Expires: November 2005                 May 2005


      }

    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.


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.

    [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.

    [RFC2309] B. Braden, D. Clark, J. Crowcroft, B. Davie, S. Deering,
    D. Estrin, S. Floyd, V. Jacobson, G. Minshall, C. Partridge, L.



Jain/Floyd/Allman/Sarolahti                                    [Page 60]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    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.

    [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
    (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification.
    RFC 2463, December 1998.

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

    [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.

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

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

    [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.




Jain/Floyd/Allman/Sarolahti                                    [Page 61]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    [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,
    September 2002.

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

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

    [JD02] Manish Jain, Constantinos Dovrolis, End-to-End Available
    Bandwidth: Measurement Methodology, Dynamics, and Relation with TCP
    Throughput, SIGCOMM 2002.

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

    [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.
    URL "http://www.seas.upenn.edu/~kunniyur/".

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

    [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.

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



Jain/Floyd/Allman/Sarolahti                                    [Page 62]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    [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
    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
    "http://informatik.uibk.ac.at/users/c70370/research/publications/".

    [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.


IANA Considerations

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


IP Option

    Quick-Start requires an IP Option Number to be allocated.  The IP
    Option would have a copied flag of 0, a class field of 00, and the



Jain/Floyd/Allman/Sarolahti                                    [Page 63]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    assigned five-bit option number.  The name of the option would be
    "QSR - Quick-Start Request", with this document as the reference
    document.


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.




Jain/Floyd/Allman/Sarolahti                                    [Page 64]


INTERNET-DRAFT           Expires: November 2005                 May 2005


    This document and the information contained herein are provided on
    an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
    REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
    INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
    IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
    THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
    WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Intellectual Property

    The IETF takes no position regarding the validity or scope of any
    Intellectual Property Rights or other rights that might be claimed
    to pertain to the implementation or use of the technology described
    in this document or the extent to which any license under such
    rights might or might not be available; nor does it represent that
    it has made any independent effort to identify any such rights.
    Information on the procedures with respect to rights in RFC
    documents can be found in BCP 78 and BCP 79.

    Copies of IPR disclosures made to the IETF Secretariat and any
    assurances of licenses to be made available, or the result of an
    attempt made to obtain a general license or permission for the use
    of such proprietary rights by implementers or users of this
    specification can be obtained from the IETF on-line IPR repository
    at http://www.ietf.org/ipr.

    The IETF invites any interested party to bring to its attention any
    copyrights, patents or patent applications, or other proprietary
    rights that may cover technology that may be required to implement
    this standard.  Please address the information to the IETF at ietf-
    ipr@ietf.org.



















Jain/Floyd/Allman/Sarolahti                                    [Page 65]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/