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rfc:rfc4782

Network Working Group S. Floyd Request for Comments: 4782 M. Allman Category: Experimental ICIR

                                                               A. Jain
                                                           F5 Networks
                                                          P. Sarolahti
                                                 Nokia Research Center
                                                          January 2007
                     Quick-Start for TCP and IP

Status of This Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The IETF Trust (2007).

Abstract

 This document specifies an optional Quick-Start mechanism for
 transport protocols, in cooperation with routers, to determine an
 allowed sending rate at the start and, at times, in the middle of a
 data transfer (e.g., after an idle period).  While Quick-Start is
 designed to be used by a range of transport protocols, in this
 document we only specify its use with TCP.  Quick-Start is designed
 to allow connections to use higher sending rates when there is
 significant unused bandwidth along the path, and the sender and all
 of the routers along the path approve the Quick-Start Request.
 This document describes many paths where Quick-Start Requests would
 not be approved.  These paths include all paths containing routers,
 IP tunnels, MPLS paths, and the like that do not support Quick-
 Start.  These paths also include paths with routers or middleboxes
 that drop packets containing IP options.  Quick-Start Requests could
 be difficult to approve over paths that include multi-access layer-
 two networks.  This document also describes environments where the
 Quick-Start process could fail with false positives, with the sender
 incorrectly assuming that the Quick-Start Request had been approved
 by all of the routers along the path.  As a result of these concerns,
 and as a result of the difficulties and seeming absence of motivation
 for routers, such as core routers to deploy Quick-Start, Quick-Start
 is being proposed as a mechanism that could be of use in controlled

Floyd, et al. Experimental [Page 1] RFC 4782 Quick-Start for TCP and IP January 2007

 environments, and not as a mechanism that would be intended or
 appropriate for ubiquitous deployment in the global Internet.

Table of Contents

 1. Introduction ....................................................4
    1.1. Conventions and Terminology ................................5
 2. Assumptions and General Principles ..............................6
    2.1. Overview of Quick-Start ....................................7
 3. The Quick-Start Option in IP ...................................10
    3.1. The Quick-Start Option for IPv4 ...........................10
    3.2. The Quick-Start Option for IPv6 ...........................13
    3.3. Processing the Quick-Start Request at Routers .............14
         3.3.1. Processing the Report of Approved Rate .............15
    3.4. The QS Nonce ..............................................16
 4. The Quick-Start Mechanisms in TCP ..............................18
    4.1. Sending the Quick-Start Request ...........................19
    4.2. The Quick-Start Response Option in the TCP header .........20
    4.3. TCP: Sending the Quick-Start Response .....................21
    4.4. TCP: Receiving and Using the Quick-Start Response Packet ..22
    4.5. TCP: Controlling Acknowledgement Traffic on the
         Reverse Path ..............................................24
    4.6. TCP: Responding to a Loss of a Quick-Start Packet .........26
    4.7. TCP: A Quick-Start Request for a Larger Initial Window ....26
         4.7.1. Interactions with Path MTU Discovery ...............26
         4.7.2. Quick-Start Request Packets that are
                Discarded by Routers or Middleboxes ................27
    4.8. TCP: A Quick-Start Request in the Middle of a Connection ..28
    4.9. An Example Quick-Start Scenario with TCP ..................29
 5. Quick-Start and IPsec AH .......................................30
 6. Quick-Start in IP Tunnels and MPLS .............................31
    6.1. Simple Tunnels that Are Compatible with Quick-Start .......33
         6.1.1. Simple Tunnels that Are Aware of Quick-Start .......33
    6.2. Simple Tunnels that Are Not Compatible with Quick-Start ...34
    6.3. Tunnels That Support Quick-Start ..........................35
    6.4. Quick-Start and MPLS ......................................35
 7. The Quick-Start Mechanism in Other Transport Protocols .........36
 8. Using Quick-Start ..............................................37
    8.1. Determining the Rate to Request ...........................37
    8.2. Deciding the Permitted Rate Request at a Router ...........37
 9. Evaluation of Quick-Start ......................................38
    9.1. Benefits of Quick-Start ...................................38
    9.2. Costs of Quick-Start ......................................39
    9.3. Quick-Start with QoS-Enabled Traffic ......................41
    9.4. Protection against Misbehaving Nodes ......................41
         9.4.1. Misbehaving Senders ................................41
         9.4.2. Receivers Lying about Whether the Request
                was Approved .......................................43

Floyd, et al. Experimental [Page 2] RFC 4782 Quick-Start for TCP and IP January 2007

         9.4.3. Receivers Lying about the Approved Rate ............43
         9.4.4. Collusion between Misbehaving Routers ..............44
    9.5. Misbehaving Middleboxes and the IP TTL ....................46
    9.6. Attacks on Quick-Start ....................................46
    9.7. Simulations with Quick-Start ..............................47
 10. Implementation and Deployment Issues ..........................47
    10.1. Implementation Issues for Sending Quick-Start Requests ...47
    10.2. Implementation Issues for Processing Quick-Start
          Requests .................................................48
    10.3. Possible Deployment Scenarios ............................48
    10.4. A Comparison with the Deployment Problems of ECN .........50
 11. Security Considerations .......................................50
 12. IANA Considerations ...........................................52
    12.1. IP Option ................................................52
    12.2. TCP Option ...............................................52
 13. Conclusions ...................................................53
 14. Acknowledgements ..............................................53
 Appendix A. Related Work ..........................................54
    A.1. Fast Start-Ups without Explicit Information from Routers ..54
    A.2. Optimistic Sending without Explicit Information from
         Routers ...................................................56
    A.3. Fast Start-Ups with Other Information from Routers ........56
    A.4. Fast Start-Ups with More Fine-Grained Feedback from
         Routers ...................................................57
    A.5. Fast Start-ups with Lower-Than-Best-Effort Service ........58
 Appendix B. Design Decisions ......................................59
    B.1. Alternate Mechanisms for the Quick-Start Request:
         ICMP and RSVP .............................................59
         B.1.1. ICMP ...............................................59
         B.1.2. RSVP ...............................................60
    B.2. Alternate Encoding Functions ..............................61
    B.3. The Quick-Start Request: Packets or Bytes? ................63
    B.4. Quick-Start Semantics: Total Rate or Additional Rate? .....64
    B.5. Alternate Responses to the Loss of a Quick-Start Packet ...65
    B.6. Why Not Include More Functionality? .......................66
    B.7. Alternate Implementations for a Quick-Start Nonce .........69
         B.7.1. An Alternate Proposal for the Quick-Start Nonce ....69
         B.7.2. The Earlier Request-Approved Quick-Start Nonce .....69
 Appendix C. Quick-Start with DCCP .................................70
 Appendix D. Possible Router Algorithm .............................72
 Appendix E. Possible Additional Uses for the Quick-Start ..........74
 Normative References ..............................................75
 Informative References ............................................75

Floyd, et al. Experimental [Page 3] RFC 4782 Quick-Start for TCP and IP January 2007

1. Introduction

 Each connection begins with a question: "What is the appropriate
 sending rate for the current network path?"  The question is not
 answered explicitly, but each TCP connection determines the sending
 rate by probing the network path and altering the congestion window
 (cwnd) based on perceived congestion.  Each TCP connection starts
 with a pre-configured initial congestion window (ICW).  Currently,
 TCP allows an initial window of between one and four segments of
 maximum segment size (MSS) ([RFC2581], [RFC3390]).  The TCP
 connection then probes the network for available bandwidth using the
 slow-start procedure ([Jac88], [RFC2581]), doubling cwnd during each
 congestion-free round-trip time (RTT).
 The slow-start algorithm can be time-consuming --- especially over
 networks with large bandwidth or long delays.  It may take a number
 of RTTs in slow-start before the TCP connection begins to fully use
 the available bandwidth of the network.  For instance, it takes
 log_2(N) - 2 round-trip times to build cwnd up to N segments,
 assuming an initial congestion window of 4 segments.  This time in
 slow-start is not a problem for large file transfers, where the
 slow-start stage is only a fraction of the total transfer time.
 However, in the case of moderate-sized transfers, the connection
 might carry out its entire transfer in the slow-start phase, taking
 many round-trip times, where one or two RTTs might have been
 sufficient when using the currently available bandwidth along the
 path.
 A fair amount of work has already been done to address the issue of
 choosing the initial congestion window for TCP, with RFC 3390
 allowing an initial window of up to four segments based on the MSS
 used by the connection [RFC3390].  Our underlying premise is that
 explicit feedback from all the routers along the path would be
 required, in the current architecture, for best-effort connections to
 use initial windows significantly larger than those allowed by
 [RFC3390], in the absence of other information about the path.
 In using Quick-Start, a TCP host (say, host A) would indicate its
 desired sending rate in bytes per second, using a Quick-Start Option
 in the IP header of a TCP packet.  Each router along the path could,
 in turn, either approve the requested rate, reduce the requested
 rate, or indicate that the Quick-Start Request is not approved.  (We
 note that the `routers' referred to in this document also include the
 IP-layer processing of the Quick-Start Request at the sender.)  In
 approving a Quick-Start Request, a router does not give preferential
 treatment to subsequent packets from that connection; the router is
 only asserting that it is currently underutilized and believes there
 is sufficient available bandwidth to accommodate the sender's

Floyd, et al. Experimental [Page 4] RFC 4782 Quick-Start for TCP and IP January 2007

 requested rate.  The Quick-Start mechanism can determine if there are
 routers along the path that do not understand the Quick-Start Option,
 or have not agreed to the Quick-Start rate request.  TCP host B
 communicates the final rate request to TCP host A in a transport-
 level Quick-Start Response in an answering TCP packet.
 If the Quick-Start Request is approved by all routers along the path,
 then the TCP host can send at up to the approved rate for a window of
 data.  Subsequent transmissions will be governed by the default TCP
 congestion control mechanisms of that connection.  If the Quick-Start
 Request is not approved, then the sender would use the default
 congestion control mechanisms.
 Quick-Start would not be the first mechanism for explicit
 communication from routers to transport protocols about sending
 rates.  Explicit Congestion Notification (ECN) gives explicit
 congestion control feedback from routers to transport protocols,
 based on the router detecting congestion before buffer overflow
 [RFC3168].  In contrast, routers would not use Quick-Start to give
 congestion information, but instead would use Quick-Start as an
 optional mechanism to give permission to transport protocols to use
 higher sending rates, based on the ability of all the routers along
 the path to determine if their respective output links are
 significantly underutilized.
 Section 2 gives an overview of Quick-Start.  The formal
 specifications for Quick-Start are contained in Sections 3, 4, 6.1.1,
 and 6.3.  In particular, Quick-Start is specified for IPv4 and for
 IPv6 in Section 3, and is specified for TCP in Section 4.  Section 6
 consists mostly of a non-normative discussion of interactions of
 Quick-Start with IP tunnels and MPLS; however, Section 6.1.1 and 6.3
 specify behavior for IP tunnels that are aware of Quick-Start.
 The rest of the document is non-normative, as follows.  Section 5
 shows that Quick-Start is compatible with IPsec AH (Authentication
 Header).  Section 7 gives a non-normative set of guidelines for
 specifying Quick-Start in other transport protocols, and Section 8
 discusses using Quick-Start in transport end-nodes and routers.
 Section 9 gives an evaluation of the costs and benefits of Quick-
 Start, and Section 10 discusses implementation and deployment issues.
 The appendices discuss related work, Quick-Start design decisions,
 and possible router algorithms.

1.1. Conventions and Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

Floyd, et al. Experimental [Page 5] RFC 4782 Quick-Start for TCP and IP January 2007

2. Assumptions and General Principles

 This section describes the assumptions and general principles behind
 the design of the Quick-Start mechanism.
 Assumptions:
  • The data transfer in the two directions of a connection traverses

different queues, and possibly even different routers. Thus, any

   mechanism for determining the allowed sending rate would have to be
   used independently for each direction.
  • The path between the two endpoints is relatively stable, such that

the path used by the Quick-Start Request is generally the same path

   used by the Quick-Start packets one round-trip time later.
   [ZDPS01] shows this assumption should be generally valid.  However,
   [RFC3819] discusses a range of Bandwidth on Demand subnets that
   could cause the characteristics of the path to change over time.
  • Any new mechanism must be incrementally deployable and might not be

supported by all the routers and/or end-hosts. Thus, any new

   mechanism must be able to accommodate non-supporting routers or
   end-hosts without disturbing the current Internet semantics.  We
   note that, while Quick-Start is incrementally deployable in this
   sense, a Quick-Start Request cannot be approved for a particular
   connection unless both end-nodes and all the routers along the path
   have been configured to support Quick-Start.
 General Principles:
  • Our underlying premise is that explicit feedback from all the

routers along the path would be required, in the current

   architecture, for best-effort connections to use initial windows
   significantly larger than those allowed by [RFC3390], in the
   absence of other information about the path.
  • A router should only approve a Quick-Start Request if the output

link is underutilized. Any other approach will result in either

   per-flow state at the router, or the possibility of a (possibly
   transient) queue at the router.
  • No per-flow state should be required at the router. Note that,

while per-flow state is not required, we also do not preclude a

   router from storing per-flow state for making Quick-Start decisions
   or for checking for misbehaving nodes.

Floyd, et al. Experimental [Page 6] RFC 4782 Quick-Start for TCP and IP January 2007

2.1. Overview of Quick-Start

 In this section, we give an overview of the use of Quick-Start with
 TCP to request a higher congestion window.  The description in this
 section is non-normative; the normative description of Quick-Start
 with IP and TCP follows in Sections 3 and 4.  Quick-Start could be
 used in the middle of a connection, e.g., after an idle or
 underutilized period, as well as for the initial sending rate; these
 uses of Quick-Start are discussed later in the document.
 Quick-Start requires end-points and routers to work together, with
 end-points requesting a higher sending rate in the Quick-Start (QS)
 option in IP, and routers along the path approving, modifying,
 discarding, or ignoring (and therefore disallowing) the Quick-Start
 Request.  The receiver uses reliable, transport-level mechanisms to
 inform the sender of the status of the Quick-Start Request.  For
 example, when TCP is used, the TCP receiver sends feedback to the
 sender using a Quick-Start Response option in the TCP header.  In
 addition, Quick-Start assumes a unicast, congestion-controlled
 transport protocol; we do not consider the use of Quick-Start for
 multicast traffic.
 When sent as a request, the Quick-Start Option includes a request for
 a sending rate in bits per second, and a Quick-Start Time to Live (QS
 TTL) to be decremented by every router along the path that
 understands the option and approves the request.  The Quick-Start TTL
 is initialized by the sender to a random value.  The transport
 receiver returns the rate, information about the TTL, and the Quick-
 Start Nonce to the sender using transport-level mechanisms; for TCP,
 the receiver sends this information in the Quick-Start Response in
 the TCP header.  In particular, the receiver computes the difference
 between the Quick-Start TTL and the IP TTL (the TTL in the IP header)
 of the Quick-Start Request packet, and returns this in the Quick-
 Start Response.  The sender uses the TTL difference to determine if
 all the routers along the path decremented the Quick-Start TTL,
 approving the Quick-Start Request.
 If the request is approved by all 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 the sender's IP
 layer and both routers along the path approving the Quick-Start
 Request, and the TCP receiver using the Quick-Start Response to
 return information to the TCP sender.  In this example, Quick-Start
 is used by TCP to establish the initial congestion window.

Floyd, et al. Experimental [Page 7] RFC 4782 Quick-Start for TCP and IP January 2007

 Sender        Router 1       Router 2          Receiver
 ------        --------       --------          --------
 | <IP TTL: 63>
 | <QS TTL: 91>
 | <TTL Diff: 28>
 | Quick-Start Request
 | in SYN or SYN/ACK.
 | IP: Decrement QS TTL
 | to approve request -->
 |
 |               Decrement
 |               QS TTL
 |               to approve
 |               request -->
 |
 |                              Decrement
 |                              QS TTL
 |                              to approve
 |                              request -->
 |
 |                                           <IP TTL: 60>
 |                                           <QS TTL: 88>
 |                                           <TTL Diff: 28>
 |                                           Return Quick-Start
 |                                            info to sender in
 |                                           Quick-Start Response
 |                                          <-- in TCP ACK packet.
 |
 | <TTL Diff: 28>
 | Quick-Start approved,
 | translate to cwnd.
 | Report Approved Rate.
 V Send cwnd paced over one RTT. -->
              Figure 1: A Successful Quick-Start Request.
 Figure 2 shows an unsuccessful use of Quick-Start, with one of the
 routers along the path not approving the Quick-Start Request.  If the
 Quick-Start Request is not approved, then the sender uses the default
 congestion control mechanisms for that transport protocol, including
 the default initial congestion window, response to idle periods, etc.

Floyd, et al. Experimental [Page 8] RFC 4782 Quick-Start for TCP and IP January 2007

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

Floyd, et al. Experimental [Page 9] RFC 4782 Quick-Start for TCP and IP January 2007

3. The Quick-Start Option in IP

3.1. The Quick-Start Option for IPv4

 The Quick-Start Request for IPv4 is defined as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Option      |  Length=8     | Func. | Rate  |   QS TTL      |
 |               |               | 0000  |Request|               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        QS Nonce                           | R |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 3: The Quick-Start Option for IPv4.
                        A Quick-Start Request.
 The first byte contains the option field, which includes the one-bit
 copy flag, the 2-bit class field, and the 5-bit option number.
 The second byte contains the length field, indicating an option
 length of eight bytes.
 The third byte includes a four-bit Function field.  If the Function
 field is set to "0000", then the Quick-Start Option is a Rate
 Request.  In this case, the second half of the third byte is a four-
 bit Rate Request field.
 For a Rate Request, the fourth byte contains the Quick-Start TTL (QS
 TTL) field.  The sender MUST set the QS TTL field to a random value.
 Routers that approve the Quick-Start Request decrement the QS TTL
 (mod 256) by the same amount that they decrement the IP TTL.  (As
 elsewhere in this document, we use the term `router' imprecisely to
 also include the Quick-Start functionality at the IP layer at the
 sender.)  The QS TTL is used by the sender to detect if all the
 routers along the path understood and approved the Quick-Start
 option.
 For a Rate Request, the transport sender MUST calculate and store the
 TTL Diff, the difference between the IP TTL value, and the QS TTL
 value in the Quick-Start Request packet, as follows:
 TTL Diff = ( IP TTL - QS TTL ) mod 256                         (1)

Floyd, et al. Experimental [Page 10] RFC 4782 Quick-Start for TCP and IP January 2007

 For a Rate Request, bytes 5-8 contain a 30-bit QS Nonce, discussed in
 Section 3.4, and a two-bit Reserved field.  The sender SHOULD set the
 reserved field to zero, and routers and receivers SHOULD ignore the
 reserved field.  The sender SHOULD set the 30-bit QS Nonce to a
 random value.
 The sender initializes the Rate Request to the desired sending rate,
 including an estimate of the transport and IP header overhead.  The
 encoding function for the Rate Request sets the request rate to K*2^N
 bps (bits per second), for N the value in the Rate Request field, and
 for K set to 40,000.  For N=0, the rate request would be set to zero,
 regardless of the encoding function.  This is illustrated in Table 1
 below.  For the four-bit Rate Request field, the request range is
 from 80 Kbps to 1.3 Gbps.  Alternate encodings that were considered
 for the Rate Request are given in Appendix B.2.
  N     Rate Request (in Kbps)
 ---    ----------------------
  0            0
  1           80
  2          160
  3          320
  4          640
  5        1,280
  6        2,560
  7        5,120
  8       10,240
  9       20,480
 10       40,960
 11       81,920
 12      163,840
 13      327,680
 14      655,360
 15    1,310,720
 Table 1: Mapping from Rate Request Field to Rate Request in Kbps.
 Routers can approve the Quick-Start Request for a lower rate by
 decreasing the Rate Request in the Quick-Start Request.  Section 4.2
 discusses the Quick-Start Response from the TCP receiver to the TCP
 sender, and Section 4.4 discusses the TCP sender's mechanism for
 determining if a Quick-Start Request has been approved.

Floyd, et al. Experimental [Page 11] RFC 4782 Quick-Start for TCP and IP January 2007

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Option      |  Length=8     | Func. | Rate  |   Not Used    |
 |               |               | 1000  | Report|               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        QS Nonce                           | R |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 4: The Quick-Start Option for IPv4.
                       Report of Approved Rate.
 If the Function field in the third byte of the Quick-Start Option is
 set to "1000", then the Quick-Start Option is a Report of Approved
 Rate.  In this case, the second four bits in the third byte are the
 Rate Report field, formatted exactly as in the Rate Request field in
 a Rate Request.  For a Report of Approved Rate, the fourth byte of
 the Quick-Start Option is not used.  Bytes 5-8 contain a 30-bit QS
 Nonce and a 2-bit Reserved field.
 After an approved Rate Request, the sender MUST report the Approved
 Rate, using a Quick-Start Option configured as a Report of Approved
 Rate with the Rate Report field set to the approved rate, and the QS
 Nonce set to the QS Nonce sent in the Quick-Start Request.  The
 packet containing the Report of Approved Rate MUST be either a
 control packet sent before the first Quick-Start data packet, or a
 Quick-Start Option in the first data packet itself.  The Report of
 Approved Rate does not have to be sent reliably; for example, if the
 approved rate is reported in a separate control packet, the sender
 does not necessarily know if the control packet has been dropped in
 the network.  If the packet containing the Quick-Start Request is
 acknowledged, but the acknowledgement packet does not contain a
 Quick-Start Response, then the sender MUST assume that the Quick-
 Start Request was denied, and set a Report of Approved Rate with a
 rate of zero.  Routers may choose to ignore the Report of Approved
 Rate, or to use the Report of Approved Rate but ignore the QS Nonce.
 Alternately, some routers that use the Report of Approved Rate may
 choose to match the QS Nonce, masked by the approved rate, with the
 QS Nonce seen in the original request.
 If the Rate Request is denied, the sender MUST send a Report of
 Approved Rate with the Rate Report field set to zero.
 We note that, unlike a Quick-Start Request sent at the beginning of a
 connection, when a Quick-Start Request is sent in the middle of a
 connection, the connection could already have an established
 congestion window or sending rate.  The Rate Request is the requested
 total rate for the connection, including the current rate of the

Floyd, et al. Experimental [Page 12] RFC 4782 Quick-Start for TCP and IP January 2007

 connection; the Rate Request is *not* a request for an additional
 sending rate over and above the current sending rate.  If the Rate
 Request is denied, or lowered to a value below the connection's
 current sending rate, then the sender ignores the request, and
 reverts to the default congestion control mechanisms of the transport
 protocol.
 The use of the Quick-Start Option does not require the additional use
 of the Router Alert Option [RFC2113].
 We note that in IPv4, a change in IP options at routers requires
 recalculating the IP header checksum.

3.2. The Quick-Start Option for IPv6

 The Quick-Start Option for IPv6 is placed in the Hop-by-Hop Options
 extension header that is processed at every network node along the
 communication path [RFC2460].  The option format following the
 generic Hop-by-Hop Options header is identical to the IPv4 format,
 with the exception that the Length field should exclude the common
 type and length fields in the option format and be set to 6 bytes
 instead of 8 bytes.
 For a Quick-Start Request, the transport receiver compares the
 Quick-Start TTL with the IPv6 Hop Limit field to calculate the TTL
 Diff.  (The Hop Limit in IPv6 is the equivalent of the TTL in IPv4.)
 That is, TTL Diff MUST be calculated and stored as follows:
 TTL Diff = ( IPv6 Hop Limit - QS TTL ) mod 256                  (2)
 Unlike IPv4, modifying or deleting the Quick-Start IPv6 Option does
 not require checksum re-calculation, because the IPv6 header does not
 have a checksum field, and modifying the Quick-Start Request in the
 IPv6 Hop-by-Hop options header does not affect the IPv6 pseudo-
 header checksum used in upper-layer checksum calculations.
 Appendix A of RFC 2460 requires that all packets with the same flow
 label must be originated with the same hop-by-hop header contents,
 which would be incompatible with Quick-Start.  However, a later IPv6
 flow label specification [RFC3697] updates the use of flow labels in
 IPv6 and removes this restriction.  Therefore, Quick-Start is
 compatible with the current IPv6 specifications.

Floyd, et al. Experimental [Page 13] RFC 4782 Quick-Start for TCP and IP January 2007

3.3. Processing the Quick-Start Request at Routers

 The Quick-Start Request does not report the current sending rate of
 the connection sending the request; in the default case of a router
 (or IP-layer implementation at an end-node) that does not maintain
 per-flow state, a router makes the conservative assumption that the
 flow's current sending rate is zero.  Each participating router can
 either terminate or approve the Quick-Start Request.  A router MUST
 only approve a Quick-Start Request if the output link is
 underutilized, and if the router judges that the output link will
 continue to be underutilized if this and earlier approved requests
 are used by the senders.  Otherwise, the router reduces or terminates
 the Quick-Start Request.
 While the paragraph above defines the *semantics* of approving a
 Quick-Start Request, this document does not specify the specific
 algorithms to be used by routers in processing Quick-Start Requests
 or Reports.  This is similar to RFC 3168, which specifics the
 semantics of the ECN codepoints in the IP header, but does not
 specify specific algorithms for routers to use in deciding when to
 drop or mark packets before buffer overflow.
 A router that wishes to terminate the Quick-Start Request SHOULD
 either delete the Quick-Start Request from the IP header or zero the
 QS TTL, QS Nonce, and Rate Request fields.  Deleting the Quick-Start
 Request saves resources because downstream routers will have no
 option to process, but zeroing the Rate Request field may be more
 efficient for routers to implement.  As suggested in [B05], future
 additions to Quick-Start could define error codes for routers to
 insert into the QS Nonce field to report back to the sender the
 reason that the Quick-Start Request was denied, e.g., that the router
 is denying all Quick-Start Requests at this time, or that this
 router, as a matter of policy, does not grant Quick-Start requests.
 A router that doesn't understand the Quick-Start Option will simply
 forward the packet with the Quick-Start Request unchanged (ensuring
 that the TTL Diff will not match and Quick-Start will not be used).
 If the participating router has decided to approve the Quick-Start
 Request, it does the following:
  • The router MUST decrement the QS TTL by the amount the IP TTL is

decremented (usually one).

  • If the router is only willing to approve a Rate Request less than

that in the Quick-Start Request, then the router replaces the Rate

   Request with a smaller value.  The router MUST NOT increase the
   Rate Request in the Quick-Start Request.  If the router decreases

Floyd, et al. Experimental [Page 14] RFC 4782 Quick-Start for TCP and IP January 2007

   the Rate Request, the router MUST also modify the QS Nonce, as
   described in Section 3.4.
  • In IPv4, the router MUST update the IP header checksum.
 If the router approves the Quick-Start Request, this approval SHOULD
 be taken into account in the router's decision to accept or reject
 subsequent Quick-Start Requests (e.g., using a variable that tracks
 the recent aggregate of accepted Quick-Start Requests).  This
 consideration of earlier approved Quick-Start Requests is necessary
 to ensure that the router only approves a Quick-Start Request when
 the router judges that the output link will remain underutilized if
 this and earlier Quick-Start Requests are used by the senders.
 In addition, the approval of a Quick-Start Request SHOULD NOT be used
 by the router to affect the treatment of the data packets that arrive
 from this connection in the next few round-trip times.  That is, the
 approval by the router of a Quick-Start Request does not give
 differential treatment for Quick-Start data packets at that router;
 it is only a statement from the router that the router believes that
 the subsequent Quick-Start data packets from this connection will not
 change the current underutilized state of the router.
 A non-participating router forwards the Quick-Start Request
 unchanged, without decrementing the QS TTL.  The non-participating
 router still decrements the TTL field in the IP header, as is
 required for all routers [RFC1812].  As a result, the sender will be
 able to detect that the Quick-Start Request had not been understood
 or approved by all of the routers along the path.
 A router that uses multipath routing for packets within a single
 connection MUST NOT approve a Quick-Start Request.  This is discussed
 in more detail in Section 9.2.

3.3.1. Processing the Report of Approved Rate

 If the Quick-Start Option has the Function field set to "1000", then
 this is a Report of Approved Rate, rather than a Rate Request.  The
 router MAY ignore such an option, and, in any case, it MUST NOT
 modify the contents of the option for a Report of Approved Rate.
 However, the router MAY use the Approved Rate report to check that
 the sender is not lying about the approved rate.  If the reported
 Approved Rate is higher than the rate that the router actually
 approved for this connection in the previous round-trip time, then
 the router may implement some policy for cheaters.  For instance, the
 router MAY decide to deny future Quick-Start Requests from this
 sender, including, if desired, deleting Quick-Start Requests from
 future packets from this sender.  Section 9.4.1 discusses misbehaving

Floyd, et al. Experimental [Page 15] RFC 4782 Quick-Start for TCP and IP January 2007

 senders in more detail.  From the Report of Approved Rate, the router
 can also learn if some of the downstream routers have approved the
 Quick-Start Request for a smaller rate or denied the use of Quick-
 Start, and adjust its bandwidth allocations accordingly.

3.4. The QS Nonce

 The QS Nonce gives the Quick-Start sender some protection against
 receivers lying about the value of the received Rate Request.  This
 is particularly important if the receiver knows the original value of
 the Rate Request (e.g., when the sender always requests the same
 value, and the receiver has a long history of communication with that
 sender).  Without the QS Nonce, there is nothing to prevent the
 receiver from reporting back to the sender a Rate Request of K, when
 the received Rate Request was, in fact, less than K.
 Table 2 gives the format for the 30-bit QS Nonce.
 Bits         Purpose
 ---------    ------------------
 Bits 0-1:    Rate 15 -> Rate 14
 Bits 2-3:    Rate 14 -> Rate 13
 Bits 4-5:    Rate 13 -> Rate 12
 Bits 6-7:    Rate 12 -> Rate 11
 Bits 8-9:    Rate 11 -> Rate 10
 Bits 10-11:  Rate 10 -> Rate 9
 Bits 12-13:  Rate 9 -> Rate 8
 Bits 14-15:  Rate 8 -> Rate 7
 Bits 16-17:  Rate 7 -> Rate 6
 Bits 18-19:  Rate 6 -> Rate 5
 Bits 20-21:  Rate 5 -> Rate 4
 Bits 22-23:  Rate 4 -> Rate 3
 Bits 24-25:  Rate 3 -> Rate 2
 Bits 26-27:  Rate 2 -> Rate 1
 Bits 28-29:  Rate 1 -> Rate 0
 Table 2: The QS Nonce.
 The transport sender MUST initialize the QS Nonce to a random value.
 If the router reduces the Rate Request from rate K to rate K-1, then
 the router MUST set the field in the QS Nonce for "Rate K -> Rate
 K-1" to a new random value.  Similarly, if the router reduces the
 Rate Request by N steps, the router MUST set the 2N bits in the
 relevant fields in the QS Nonce to a new random value.  The receiver
 MUST report the QS Nonce back to the sender.

Floyd, et al. Experimental [Page 16] RFC 4782 Quick-Start for TCP and IP January 2007

 If the Rate Request was not decremented in the network, then the QS
 Nonce should have its original value.  Similarly, if the Rate Request
 was decremented by N steps in the network, and the receiver reports
 back a Rate Request of K, then the last 2K bits of the QS Nonce
 should have their original value.
 With the QS Nonce, the receiver has a 1/4 chance of cheating about
 each step change in the rate request.  Thus, if the rate request is
 reduced by two steps in the network, the receiver has a 1/16 chance
 of successfully reporting that the original request was approved, as
 this requires reporting the original value for the QS nonce.
 Similarly, if the rate request is reduced many steps in the network,
 and the receiver receives a QS Option with a rate request of K, the
 receiver has a 1/16 chance of guessing the original values for the
 fields in the QS nonce for "Rate K+2 -> Rate K+1" and "Rate K+1 ->
 Rate K".  Thus, the receiver has a 1/16 chance of successfully lying
 and saying that the received rate request was K+2 instead of K.
 We note that the protection offered by the QS Nonce is the same
 whether one router makes all the decrements in the rate request, or
 whether they are made at different routers along the path.
 The requirements for randomization for the sender and routers in
 setting `random' values in the QS Nonce are not stringent -- almost
 any form of pseudo-random numbers will do.  The requirement is that
 the original value for the QS Nonce is not easily predictable by the
 receiver, and in particular, the nonce MUST NOT be easily determined
 from inspection of the rest of the packet or from previous packets.
 In particular, the nonce MUST NOT be based only on a combination of
 specific packet header fields.  Thus, if two bits of the QS Nonce are
 changed by a router along the path, the receiver should not be able
 to guess those two bits from the other 28 bits in the QS Nonce.
 An additional requirement is that the receiver cannot be able to
 tell, from the QS Nonce itself, which numbers in the QS Nonce were
 generated by the sender, and which were generated by routers along
 the path.  This makes it harder for the receiver to infer the value
 of the original rate request, making it one step harder for the
 receiver to cheat.
 Section 9.4 also considers issues of receiver cheating in more
 detail.

Floyd, et al. Experimental [Page 17] RFC 4782 Quick-Start for TCP and IP January 2007

4. The Quick-Start Mechanisms in TCP

 This section describes how the Quick-Start mechanism would be used in
 TCP.  We first sketch the procedure and then tightly define it in the
 subsequent subsections.
 If a TCP sender (say, host A) would like to use Quick-Start, the TCP
 sender puts the requested sending rate in bits per second,
 appropriately formatted, in the Quick-Start Option in the IP header
 of the TCP packet, called the Quick-Start Request packet.  (We will
 be somewhat loose in our use of "packet" vs. "segment" in this
 section.)  When used for initial start-up, the Quick-Start Request
 packet can be either the SYN or SYN/ACK packet, as illustrated in
 Figure 1.  The requested rate includes an estimate for the transport
 and IP header overhead.  The TCP receiver (say, host B) returns the
 Quick-Start Response option in the TCP header in the responding
 SYN/ACK packet or ACK packet, called the Quick-Start Response packet,
 informing host A of the results of their request.
 If the acknowledging packet does not contain a Quick-Start Response,
 or contains a Quick-Start Response with the wrong value for the TTL
 Diff or the QS Nonce, then host A MUST assume that its Quick-Start
 request failed.  In this case, host A sends a Report of Approved Rate
 with a Rate Report of zero, and uses TCP's default congestion control
 procedure.  For initial start-up, host A uses the default initial
 congestion window ([RFC2581], [RFC3390]).
 If the returning packet contains a valid Quick-Start Response, then
 host A uses the information in the response, along with its
 measurement of the round-trip time, to determine the Quick-Start
 congestion window (QS-cwnd).  Quick-Start data packets are defined as
 data packets sent as the result of a successful Quick-Start request,
 up to the time when the first Quick-Start packet is acknowledged.
 The sender also sends a Report of Approved Rate.  In order to use
 Quick-Start, the TCP host MUST use rate-based pacing [VH97] to
 transmit Quick-Start packets at the rate indicated in the Quick-Start
 Response, at the level of granularity possible by the sending host.
 We note that the limitations of interrupt timing on computers can
 limit the ability of the TCP host in rate-pacing the outgoing
 packets.
 The two TCP end-hosts can independently decide whether to request
 Quick-Start.  For example, host A could send a Quick-Start Request in
 the SYN packet, and host B could also send a Quick-Start Request in
 the SYN/ACK packet.

Floyd, et al. Experimental [Page 18] RFC 4782 Quick-Start for TCP and IP January 2007

4.1. Sending the Quick-Start Request

 When sending a Quick-Start Request, the TCP sender SHOULD send the
 request on a packet that requires an acknowledgement, such as a SYN,
 SYN/ACK, or data packet.  In this case, if the packet is acknowledged
 but no Quick-Start Response is included, then the sender knows that
 the Quick-Start Request has been denied, and can send a Report of
 Approved Rate.
 In addition to the use of Quick-Start when a connection is
 established, there are several additional points in a connection when
 a transport protocol may want to issue a Rate Request.  We first
 reiterate the notion that Quick-Start is a coarse-grained mechanism.
 That is, Quick-Start's Rate Requests are not meant to be used for
 fine-grained control of the transport's sending rate.  Rather, the
 transport MAY issue a Rate Request when no information about the
 appropriate sending rate is available, and the default congestion
 control mechanisms might be significantly underestimating the
 appropriate sending rate.
 The following are potential points where Quick-Start may be useful:
 (1) At or soon after connection initiation, when the transport has no
     idea of the capacity of the network path, as discussed above.  (A
     transport that uses TCP Control Block sharing [RFC2140], the
     Congestion Manager [RFC3124], or other mechanisms for sharing
     congestion information may not need Quick-Start to determine an
     appropriate rate.)
 (2) After an idle period when the transport no longer has a validated
     estimate of the available bandwidth for this flow.  (An example
     could be a persistent-HTTP connection when a new HTTP request is
     received after an idle period.)
 (3) After a host has received explicit indications that one of the
     endpoints has moved its point of network attachment.  This can
     happen due to some underlying mobility mechanism like Mobile IP
     ([RFC3344], [RFC3775]).  Some transports, such as Steam Control
     Transmission Protocol (SCTP) [RFC2960], may associate with
     multiple IP addresses and can switch addresses (and therefore
     network paths) in mid-connection.  If the transport has concrete
     knowledge of a changing network path, then the current sending
     rate may not be appropriate, and the transport sender may use
     Quick-Start to probe the network to see if it can send at a
     higher rate.  (Alternatively, traditional slow-start should be
     used in this case when Quick-Start is not available.)

Floyd, et al. Experimental [Page 19] RFC 4782 Quick-Start for TCP and IP January 2007

 (4) After an application-limited period, when the sender has been
     using only a small amount of its appropriate share of the network
     capacity and has no valid estimate for its fair share.  In this
     case, Quick-Start may be an appropriate mechanism to determine if
     the sender can send at a higher rate.  For instance, consider an
     application that steadily exchanges low- rate control messages
     and suddenly needs to transmit a large amount of data.
 Of the above, this document recommends that a TCP sender MAY attempt
 to use Quick-Start in cases (1) and (2).  It is NOT RECOMMENDED that
 a TCP sender use Quick-Start for case (3) at the current time.  Case
 (3) requires external notifications not presently defined for TCP or
 other transport protocols.  Finally, a TCP SHOULD NOT use Quick-
 Start for case (4) at the current time.  Case (4) requires further
 thought and investigation with regard to how the transport protocol
 could determine it was in a situation that would warrant transmitting
 a Quick-Start Request.
 As a general guideline, a TCP sender SHOULD NOT request a sending
 rate larger than it is able to use over the next round-trip time of
 the connection (or over 100 ms, if the round-trip time is not known),
 except as required to round up the desired sending rate to the next-
 highest allowable request.
 In any circumstances, the sender MUST NOT make a QS request if it has
 made a QS request within the most recent round-trip time.
 Section 4.7 discusses some of the issues of using Quick-Start at
 connection initiation, and Section 4.8 discusses issues that arise
 when Quick-Start is used to request a larger sending rate after an
 idle period.

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

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

Floyd, et al. Experimental [Page 20] RFC 4782 Quick-Start for TCP and IP January 2007

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Kind      |  Length=8     | Resv. | Rate  |   TTL Diff    |
 |               |               |       |Request|               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   QS Nonce                                | R |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Figure 5: The Quick-Start Response Option in the TCP Header.
 The first byte of the Quick-Start Response option contains the option
 kind, identifying the TCP option.
 The second byte of the Quick-Start Response option contains the
 option length in bytes.  The length field MUST be set to 8 bytes.
 The third byte of the Quick-Start Response option contains a four-
 bit Reserved field, and the four-bit allowed Rate Request, formatted
 as in the Quick-Start Rate Request option.
 The fourth byte of the TCP option contains the TTL Diff.  The TTL
 Diff contains the difference between the IP TTL and QS TTL fields in
 the received Quick-Start Request packet, as calculated in equations
 (1) or (2) (depending on whether IPv4 or IPv6 is used).
 Bytes 5-8 of the TCP option contain the 30-bit QS Nonce and a two-
 bit Reserved field.
 We note that, while there are limitations on the potential size of
 the Quick-Start Response Option, a Quick-Start Response Option of
 eight bytes should not be a problem.  The TCP Options field can
 contain up to 40 bytes.  Other TCP options that might be used in a
 SYN or SYN/ACK packet include Maximum Segment Size (four bytes), Time
 Stamp (ten bytes), Window Scale (three bytes), and Selective
 Acknowledgments Permitted (two bytes).

4.3. TCP: Sending the Quick-Start Response

 An end host (say, host B) that receives an IP packet containing a
 Quick-Start Request passes the Quick-Start Request, along with the
 value in the IP TTL field, to the receiving TCP layer.
 If the TCP host is willing to permit the Quick-Start Request, then a
 Quick-Start Response option is included in the TCP header of the
 corresponding acknowledgement packet.  The Rate Request in the
 Quick-Start Response option is set to the received value of the Rate
 Request in the Quick-Start Option, or to a lower value if the TCP

Floyd, et al. Experimental [Page 21] RFC 4782 Quick-Start for TCP and IP January 2007

 receiver is only willing to allow a lower Rate Request.  The TTL Diff
 in the Quick-Start Response is set to the difference between the IP
 TTL value and the QS TTL value as given in equation (1) or (2)
 (depending on whether IPv4 or IPv6 is used).  The QS Nonce in the
 Response is set to the received value of the QS Nonce in the Quick-
 Start Option.
 If an end host receives an IP packet with a Quick-Start Request with
 a rate request of zero, then that host SHOULD NOT send a Quick-Start
 Response.
 The Quick-Start Response MUST NOT be resent if it is lost in the
 network.  Packet loss could be an indication of congestion on the
 return path, in which case it is better not to approve the Quick-
 Start Request.

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

 A TCP host (say, TCP host A) that sent a Quick-Start Request and
 receives a Quick-Start Response in an acknowledgement first checks
 that the Quick-Start Response is valid.  The Quick-Start Response is
 valid if it contains the correct value for the TTL Diff, and an equal
 or lesser value for the Rate Request than that transmitted in the
 Quick-Start Request.  In addition, if the received Rate Request is K,
 then the rightmost 2K bits of the QS Nonce must match those bits in
 the QS Nonce sent in the Quick-Start Request.  If these checks are
 not successful, then the Quick-Start Request failed, and the TCP host
 MUST use the default TCP congestion window that it would have used
 without Quick-Start.  If the rightmost 2K bits of the QS Nonce do not
 match those bits in the QS Nonce sent in the Quick-Start Request, for
 a received Rate Request of K, then the TCP host MUST NOT send
 additional Quick-Start Requests during the life of the connection.
 Whether or not the Quick-Start Request was successful, the host
 receiving the Quick-Start Response MUST send a Report of Approved
 Rate.  Similarly, if the packet containing the Quick-Start Request is
 acknowledged, but the acknowledgement does not include a Quick-Start
 Response, then the sender MUST send a Report of Approved Rate.
 If the checks of the TTL Diff and the Rate Request are successful,
 and the TCP host is going to use the Quick-Start Request, it MUST
 start using it within one round-trip time of receiving the Quick-
 Start Response, or within three seconds, whichever is smaller.  To
 use the Quick-Start Request, the host sets its Quick-Start congestion
 window (in terms of MSS-sized segments), QS-cwnd, as follows:
 QS-cwnd = (R * T) / (MSS + H)                                (3)

Floyd, et al. Experimental [Page 22] RFC 4782 Quick-Start for TCP and IP January 2007

 where R is the Rate Request in bytes per second, T is the measured
 round-trip time in seconds, and H is the estimated TCP/IP header size
 in bytes (e.g., 40 bytes).
 Derivation: the sender is allowed to transmit at R bytes per second
 including packet headers, but only R*MSS/(MSS+H) bytes per second, or
 equivalently R*T*MSS/(MSS+H) bytes per round-trip time, of
 application data.
 The TCP host SHOULD set its congestion window cwnd to QS-cwnd only if
 QS-cwnd is greater than cwnd; otherwise, QS-cwnd is ignored.  If
 QS-cwnd is used, the TCP host sets a flag that it is in Quick-Start
 mode, and while in Quick-Start mode, the TCP sender MUST use rate-
 based pacing to pace out Quick-Start packets at the approved rate.
 If, during Quick-Start mode, the TCP sender receives ACKs for packets
 sent before this Quick-Start mode was entered, these ACKs are
 processed as usual, following the default congestion control
 mechanisms.  Quick-Start mode ends when the TCP host receives an ACK
 for one of the Quick-Start packets.
 If the congestion window has not been fully used when the first ack
 arrives ending the Quick-Start mode, then the congestion window is
 decreased to the amount that has actually been used so far.  This is
 necessary because when the Quick-Start Response is received, the TCP
 sender's round-trip-time estimate might be longer than for succeeding
 round-trip times, e.g., because of delays at routers processing the
 IP Quick-Start Option, or because of delays at the receiver in
 responding to the Quick-Start Request packet.  In this case, an
 overly large round-trip-time estimate could have caused the TCP
 sender to translate the approved Quick-Start sending rate in bytes
 per second into a congestion window that is larger than needed, with
 the TCP sender receiving an ACK for the first Quick- Start packet
 before the entire congestion window has been used.  Thus, when the
 TCP sender receives the first ACK for a Quick-Start packet, the
 sender MUST reduce the congestion window to the amount that has
 actually been used.
 As an example, a TCP sender with an approved Quick-Start Request of R
 KBps, B-byte packets including headers, and an RTT estimate of T
 seconds, would translate the Rate Request of R KBps to a congestion
 window of R*T/B packets.  The TCP sender would send the Quick-Start
 packets rate-paced at R KBps.  However, if the actual current round-
 trip time was T/2 seconds instead of T seconds, then the sender would
 begin to receive acknowledgements for Quick-Start packets after T/2
 seconds.  Following the paragraph above, the TCP sender would then
 reduce its congestion window from R*T/B to approximately R*T/(B*2)
 packets, the actual number of packets that were needed to fill the
 pipe at a sending rate of R KBps.  (Note: this is just an

Floyd, et al. Experimental [Page 23] RFC 4782 Quick-Start for TCP and IP January 2007

 illustration; the congestion window is actually set to the amount of
 data sent before the ACK arrives and not based on equations above.)
 After Quick-Start mode is exited and the congestion window adjusted
 if necessary, the TCP sender returns to using the default congestion-
 control mechanisms, processing further incoming ACK packets as
 specified by those congestion control mechanisms.  For example, if
 the TCP sender was in slow-start prior to the Quick-Start Request,
 and no packets were lost or marked since that time, then the sender
 continues in slow-start after exiting Quick-Start mode, as allowed by
 ssthresh.
 To add robustness, the TCP sender MUST use Limited Slow-Start
 [RFC3742] along with Quick-Start.  With Limited Slow-Start, the TCP
 sender limits the number of packets by which the congestion window is
 increased for one window of data during slow-start.
 When Quick-Start is used at the beginning of a connection, before any
 packet marks or losses have been reported, the TCP host MAY use the
 reported Rate Request to set the slow-start threshold to a desired
 value, e.g., to some small multiple of the congestion window.  A
 possible future research topic is how the sender might modify the
 slow-start threshold at the beginning of a connection to avoid
 overshooting the path capacity.  (The initial value of ssthresh is
 allowed to be arbitrarily high, and some TCP implementations use the
 size of the advertised window for ssthresh [RFC2581].)

4.5. TCP: Controlling Acknowledgement Traffic on the Reverse Path

 When a Quick-Start Request is approved for a TCP sender, the
 resulting Quick-Start data traffic can result in a sudden increase in
 traffic for pure ACK packets on the reverse path.  For example, for
 the largest Quick-Start Request of 1.3 Gbps, given a TCP sender with
 1500-byte packets and a TCP receiver with delayed acknowledgements
 acking every other packet, this could result in 17.3 Mbps of
 acknowledgement traffic on the reverse path.
 One possibility, in cases with large Quick-Start Requests, would be
 for TCP receivers to send Quick-Start Requests to request bandwidth
 for the acknowledgement traffic on the reverse path.  However, in our
 view, a better approach would be for TCP receivers to simply control
 the rate of sending acknowledgement traffic.  The optimal future
 solution would involve the explicit use of congestion control for TCP
 acknowledgement traffic, as is done now for the acknowledgement
 traffic in DCCP's CCID2 [RFC4341].

Floyd, et al. Experimental [Page 24] RFC 4782 Quick-Start for TCP and IP January 2007

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

should have a sample of the RTT. If the TCP receiver does not have

   a measurement of the round-trip time, it can use the time between
   the receipt of the Quick-Start Request and the Report of Approved
   Rate.
  • The Approved Rate Request (R): When the TCP receiver receives the

Quick-Start Response packet, the receiver knows the value of the

   approved Rate Request.
  • The MSS: TCP advertises the MSS during the initial three-way

handshake; therefore, the receiver should have an understanding of

   the packet size the sender will be using.  If the receiver does not
   have such an understanding or wishes to confirm the negotiated MSS,
   the size of the first data packet can be used.
 With this set of information, the TCP receiver can restrict its
 sending rate for pure acknowledgment traffic to at most 100 pure ACK
 packets per RTT by sending at most one ACK for every K data packets,
 for the ACK Ratio K set to R*RTT/(100*8*MSS).  The receiver would
 acknowledge the first Quick-Start data packet, and every succeeding K
 data packets.  Thus, for a somewhat extreme case of R=1.3 Gbps,
 RTT=0.2 seconds, and MSS=1500 bytes, K would be set to 216, and the
 receiver would acknowledge every 216 data packets.  From [RFC2581],
 the ACK Ratio K should have a minimum value of two.  When the ACK
 Ratio is greater than two, and the TCP sender receives
 acknowledgements each acknowledging more than two data packets, the
 TCP sender may want to use rate-based pacing to control the
 burstiness of its outgoing data traffic.
 In the absence of explicit congestion control mechanisms, the TCP end
 nodes cannot determine the packet drop rate for pure acknowledgement
 traffic.  This is true with or without Quick-Start.  However, the TCP
 receiver could limit its increase in the sending rate for pure ACK
 packets by at most doubling the sending rate for pure ACK packets
 from one round-trip time to the next.  The TCP receiver would do this
 by halving the ACK Ratio each round-trip time.
 Note that the above is one particular mechanism that could be used to
 control the ACK stream.  Future work that investigates this scheme
 and others in detail is encouraged.

Floyd, et al. Experimental [Page 25] RFC 4782 Quick-Start for TCP and IP January 2007

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

 For TCP, we have defined a "Quick-Start packet" as one of the packets
 sent in the window immediately following a successful Quick-Start
 Request.  After detecting the loss or ECN-marking of a Quick-Start
 packet, TCP MUST revert to the default congestion control procedures
 that would have been used if the Quick-Start Request had not been
 approved.  For example, if Quick-Start is used for setting the
 initial window, and a packet from the initial window is lost or
 marked, then the TCP sender MUST then slow-start with the default
 initial window that would have been used if Quick-Start had not been
 used.  In addition to reverting to the default congestion control
 mechanisms, the sender MUST take into account that the Quick-Start
 congestion window was too large.  Thus, the sender SHOULD decrease
 ssthresh to, at most, half the number of Quick-Start packets that
 were successfully transmitted.  Appendix B.5 discusses possible
 alternatives in responding to the loss of a Quick-Start packet.
 If a Quick-Start packet is lost or ECN-marked, then the sender SHOULD
 NOT make future Quick-Start Requests for this connection.
 We note that ECN [RFC3168] MAY be used with Quick-Start.  As is
 always the case with ECN, the sender's congestion control response to
 an ECN-marked Quick-Start packet is the same as the response to a
 dropped Quick-Start packet, thus reverting to slow start in the case
 of Quick-Start packets marked as experiencing congestion.

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

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

4.7.1. Interactions with Path MTU Discovery

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

Floyd, et al. Experimental [Page 26] RFC 4782 Quick-Start for TCP and IP January 2007

 If the sender knows the Path MTU when the initial window is sent
 (e.g., from a PMTUD cache or from some other IETF-approved method),
 then the sender SHOULD use that MTU for segments in the initial
 window.  Unfortunately, the sender doesn't necessarily know the Path
 MTU when it sends packets in the initial window.  In this case, the
 sender should be conservative in the packet size used.  Sending a
 large number of overly large packets with the DF bit set is not
 desirable, but sending a large number of packets that are fragmented
 in the network can be equally undesirable.
 If the sender doesn't know the Path MTU when the initial window is
 sent, the sender SHOULD send one large packet in the initial window
 with the DF bit set, and send the remaining packets in the initial
 window with a smaller MTU of 576 bytes (or 1280 bytes with IPv6).
 A second possibility would be for the sender to delay sending the
 Quick-Start Request for one round-trip time by sending the Quick-
 Start Request with the first window of data, while also doing Path
 MTU Discovery.
 The sender may be using an iterative approach such as Packetization
 Layer Path MTU Discovery (PLPMTUD) [MH06] for Path MTU Discovery,
 where the sender tests successively larger MTUs.  If a probe is
 successfully delivered, then the MTU can be raised to reflect the
 value used in that probe.  We would note that PLPMTUD does not allow
 the sender to determine the Path MTU before sending the initial
 window of data.

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

      Middleboxes
 It is always possible for a TCP SYN packet carrying a Quick-Start
 request to be dropped in the network due to congestion, or to be
 blocked due to interactions with routers or middleboxes, where a
 middlebox is defined as any intermediary box performing functions
 apart from normal, standard functions of an IP router on the data
 path between a source host and destination host [RFC3234].
 Measurement studies of interactions between transport protocols and
 middleboxes [MAF04] show that for 70% of the Web servers
 investigated, no connection is established if the TCP SYN packet
 contains an unknown IP option (and for 43% of the Web servers, no
 connection is established if the TCP SYN packet contains an IP
 TimeStamp Option).  In both cases, this is presumably due to routers
 or middleboxes along that path.
 If the TCP sender doesn't receive a response to the SYN or SYN/ACK
 packet containing the Quick-Start Request, then the TCP sender SHOULD
 resend the SYN or SYN/ACK packet without the Quick-Start Request.

Floyd, et al. Experimental [Page 27] RFC 4782 Quick-Start for TCP and IP January 2007

 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].
 RFCs 1122 and 2988 specify that the sender should set the initial RTO
 (retransmission timeout) to three seconds, though many TCP
 implementations set the initial RTO to one second.  For a TCP SYN
 packet sent with a Quick-Start request, the TCP sender SHOULD use an
 initial RTO of three seconds.
 We note that if the TCP SYN packet is using the IP Quick-Start Option
 for a Quick-Start Request, and it is also using bits in the TCP
 header to negotiate ECN-capability with the TCP host at the other
 end, then the drop of a TCP SYN packet could be due to congestion, a
 router or middlebox dropping the packet because of the IP Option, or
 a router or middlebox dropping the packet because of the information
 in the TCP header negotiating ECN.  In this case, the sender could
 resend the dropped packet without either the Quick-Start or the ECN
 requests.  Alternately, the sender could resend the dropped packet
 with only the ECN request in the TCP header, resending the TCP SYN
 packet without either the Quick-Start or the ECN requests if the
 second TCP SYN packet is dropped.  The second choice seems
 reasonable, given that a TCP SYN packet today is more likely to be
 blocked due to policies that discard packets with IP Options than due
 to policies that discard packets with ECN requests in the TCP header
 [MAF04].

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

 This section discusses the following issues that arise when Quick-
 Start is used by TCP to request a larger window in the middle of a
 connection, such as after an idle period: (1) determining the rate to
 request; (2) when to make a request; and (3) the response if Quick-
 Start packets are dropped.
 (1) Determining the rate to request:
     For a connection that has not yet had a congestion event (that
     is, a marked or dropped packet), the TCP sender is not restricted
     in the rate that it requests.  As an example, a server might wait
     and send a Quick-Start Request after a short interaction with the
     client.
     To use a Quick-Start Request in a connection that has already
     experienced a congestion event, and that has not had a recent
     mobility event, the TCP sender can determine the largest
     congestion window that the TCP connection achieved since the last
     packet drop and translate this to a sending rate to get the

Floyd, et al. Experimental [Page 28] RFC 4782 Quick-Start for TCP and IP January 2007

     maximum allowed request rate.  If the request is granted, then
     the sender essentially restarts with its old congestion window
     from before it was reduced, for example, during an idle period.
     A Quick-Start Request sent in the middle of a TCP connection
     SHOULD be sent on a data packet.
 (2) When to make a request:
     A TCP connection MAY make a Quick-Start Request before the
     connection has experienced a congestion event, or after an idle
     period of at least one RTO.
 (3) Response if Quick-Start packets are dropped:
     If Quick-Start packets are dropped in the middle of connection,
     then the sender MUST revert to half the Quick-Start window, or to
     the congestion window that the sender would have used if the
     Quick-Start request had not been approved, whichever is smaller.

4.9. An Example Quick-Start Scenario with TCP

 The following is an example scenario of when both hosts request
 Quick-Start for setting their initial windows.  This is similar to
 Figures 1 and 2 in Section 2.1, except that it illustrates a TCP
 connection with both TCP hosts sending Quick-Start Requests.
  • The TCP SYN packet from Host A contains a Quick-Start Request in

the IP header.

  • Routers along the forward path modify the Quick-Start Request as

appropriate.

  • Host B receives the Quick-Start Request in the SYN packet, and

calculates the TTL Diff. If Host B approves the Quick-Start

   Request, then Host B sends a Quick-Start Response in the TCP header
   of the SYN/ACK packet.  Host B also sends a Quick-Start Request in
   the IP header of the SYN/ACK packet.
  • Routers along the reverse path modify the Quick-Start Request as

appropriate.

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

checks the TTL Diff, Rate Request, and QS Nonce for validity. If

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

Floyd, et al. Experimental [Page 29] RFC 4782 Quick-Start for TCP and IP January 2007

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

checks the TTL Diff, Rate Request, and QS Nonce for validity. If

   the Quick-Start Response is valid, then Host B sets its initial
   congestion window appropriately, and sets up rate-based pacing to
   be used with its initial window.  If the Quick-Start Response is
   not valid, then Host B uses TCP's default initial window.  In
   either case, Host B sends a Report of Approved Rate.

5. Quick-Start and IPsec AH

 This section shows that Quick-Start is compatible with IPsec
 Authentication Header (AH).  AH uses an Integrity Check Value (ICV)
 in the IPsec Authentication Header to verify both message
 authentication and integrity [RFC4302].  Changes to the Quick-Start
 Option in the IP header do not affect this AH ICV.  The tunnel
 considerations in Section 6 below apply to all IPsec tunnels,
 regardless of what IPsec headers or processing are used in
 conjunction with the tunnel.
 Because the contents of the Quick-Start Option can change along the
 path, it is important that these changes not affect the IPsec
 Authentication Header Integrity Check Value (AH ICV).  For IPv4, RFC
 4302 requires that unrecognized IPv4 options be zeroed for AH ICV
 computation purposes, so Quick-Start IP Option data changing en route
 does not cause problems with existing IPsec AH implementations for
 IPv4.  If the Quick-Start Option is recognized, it MUST be treated as
 a mutable IPv4 option, and hence be completely zeroed for AH ICV
 calculation purposes.  IPv6 option numbers explicitly indicate
 whether the option is mutable; the third-highest order bit in the
 IANA-allocated option type has the value 1 to indicate that the
 Quick-Start Option data can change en route.  RFC 4302 requires that
 the option data of any such option be zeroed for AH ICV computation
 purposes.  Therefore, changes to the Quick-Start Option in the IP
 header do not affect the calculation of the AH ICV.

Floyd, et al. Experimental [Page 30] RFC 4782 Quick-Start for TCP and IP January 2007

6. Quick-Start in IP Tunnels and MPLS

 This section considers interactions between Quick-Start and IP
 tunnels, including IPsec ([RFC4301]), IP in IP [RFC2003], GRE
 [RFC2784], and others.  This section also considers interactions
 between Quick-Start and MPLS [RFC3031].
 In the discussion, we use TTL Diff, defined earlier as the difference
 between the IP TTL and the Quick-Start TTL, mod 256.  Recall that the
 sender considers the Quick-Start Request approved only if the value
 of TTL Diff for the packet entering the network is the same as the
 value of TTL Diff for the packet exiting the network.
 Simple tunnels: IP tunnel modes are generally based on adding a new
 "outer" IP header that encapsulates the original or "inner" IP header
 and its associated packet.  In many cases, the new "outer" IP header
 may be added and removed at intermediate points along a path,
 enabling the network to establish a tunnel without requiring endpoint
 participation.  We denote tunnels that specify that the outer header
 be discarded at tunnel egress as "simple tunnels", and we denote
 tunnels where the egress saves and uses information from the outer
 header before discarding it as "non-simple tunnels".  An example of a
 "non-simple tunnel" would be a tunnel configured to support ECN,
 where the egress router might copy the ECN codepoint in the outer
 header to the inner header before discarding the outer header
 [RFC3168].
                     __ Tunnels Compatible with Quick-Start
                    /
 Simple Tunnels  __/
                   \
                    \__ Tunnels Not Compatible with Quick-Start
                                  (False Positives!)
                         __ Tunnels Supporting Quick-Start
                        /
                       /
 Non-Simple Tunnels __/_____ Tunnels Compatible with Quick-Start,
                      \          but Not Supporting Quick-Start
                       \
                        \__ Tunnels Not Compatible with Quick-Start?
                   Figure 6: Categories of Tunnels.

Floyd, et al. Experimental [Page 31] RFC 4782 Quick-Start for TCP and IP January 2007

 Tunnels that are compatible with Quick-Start: We say that an IP
 tunnel `is not compatible with Quick-Start' if the use of a Quick-
 Start Request over such a tunnel allows false positives, where the
 TCP sender incorrectly believes that the Quick-Start Request was
 approved by all routers along the path.  If the use of Quick-Start
 over the tunnel does not cause false positives, we say that the IP
 tunnel `is compatible with Quick-Start'.
 If the IP TTL of the inner header is decremented during forwarding
 before tunnel encapsulation takes place, then the simple tunnel is
 compatible with Quick-Start, with Quick-Start Requests being
 rejected.  Section 6.1 describes in more detail the ways that a
 simple tunnel can be compatible with Quick-Start.
 There are some simple tunnels that are not compatible with Quick-
 Start, allowing `false positives' where the TCP sender incorrectly
 believes that the Quick-Start Request was approved by all routers
 along the path.  This is discussed in Section 6.2 below.
 One of our tasks in the future will be to investigate the occurrence
 of tunnels that are not compatible with Quick-Start, and to track the
 extent to which such tunnels are modified over time.  The evaluation
 of the problem of false positives from tunnels that are not
 compatible with Quick-Start will affect the progression of Quick-
 Start from Experimental to Proposed Standard, and will affect the
 degree of deployment of Quick-Start while in Experimental mode.
 Tunnels that support Quick-Start: We say that an IP tunnel `supports
 Quick-Start' if it allows routers along the tunnel path to process
 the Quick-Start Request and give feedback, resulting in the
 appropriate possible acceptance of the Quick-Start Request.  Some
 tunnels that are compatible with Quick-Start support Quick-Start,
 while others do not.  We note that a simple tunnel is not able to
 support Quick-Start.
 From a security point of view, the use of Quick-Start in the outer
 header of an IP tunnel might raise security concerns because an
 adversary could tamper with the Quick-Start information that
 propagates beyond the tunnel endpoint, or because the Quick-Start
 Option exposes information to network scanners.  Our approach is to
 make supporting Quick-Start an option for IP tunnels.  That is, in
 environments or tunneling protocols where the risks of using Quick-
 Start are judged to outweigh its benefits, the tunnel can simply
 delete the Quick-Start Option or zero the Quick-Start rate request
 and QS TTL fields before encapsulation.  The result is that there are
 two viable options for IP tunnels to be compatible with Quick-Start.
 The first option is the simple tunnel described above and in Section
 6.1, where the tunnel is compatible with Quick-Start but does not

Floyd, et al. Experimental [Page 32] RFC 4782 Quick-Start for TCP and IP January 2007

 support Quick-Start, where all Quick-Start Requests along the path
 will be rejected.  The second approach is a Quick-Start-capable mode,
 described in Section 6.3, where the tunnel actively supports Quick-
 Start.

6.1. Simple Tunnels that Are Compatible with Quick-Start

 This section describes the ways that a simple tunnel can be
 compatible with Quick-Start but not support Quick-Start, resulting in
 the rejection of all Quick-Start Requests that traverse the tunnel.
 If the tunnel ingress for the simple tunnel is at a router, the IP
 TTL of the inner header is generally decremented during forwarding
 before tunnel encapsulation takes place.  In this case, TTL Diff will
 be changed, correctly causing the Quick-Start Request to be rejected.
 For a simple tunnel, it is preferable if the Quick-Start Request is
 not copied to the outer header, saving the routers within the tunnel
 from unnecessarily processing the Quick-Start Request.  However, the
 Quick-Start Request will be rejected correctly in this case whether
 or not the Quick-Start Request is copied to the outer header.

6.1.1. Simple Tunnels that Are Aware of Quick-Start

 If a tunnel ingress is aware of Quick-Start, but does not want to
 support Quick-Start, then the tunnel ingress MUST either zero the
 Quick-Start rate request, QS TTL, and QS Nonce fields, or remove the
 Quick-Start Option from the inner header before encapsulation.
 Section 6.3 describes the procedures for a tunnel that does want to
 support Quick-Start.
 Deleting the Quick-Start Option or zeroing the Quick-Start rate
 request *after decapsulation* also serves to prevent the propagation
 of Quick-Start information, and is compatible with Quick-Start.  If
 the outer header does not contain a Quick-Start Request, a Quick-
 Start-aware tunnel egress MUST reject the inner Quick-Start Request
 by zeroing the Rate Request field in the inner header, or by deleting
 the Quick-Start Option.
 If the tunnel ingress is at a sending host or router where the IP TTL
 is not decremented prior to encapsulation, and neither tunnel
 endpoint is aware of Quick-Start, then this allows false positives,
 described in the section below.

Floyd, et al. Experimental [Page 33] RFC 4782 Quick-Start for TCP and IP January 2007

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

 Sometimes a tunnel implementation that does not support Quick-Start
 is independent of the TCP sender or a router implementation that
 supports Quick-Start.  In these cases, it is possible that a Quick-
 Start Request gets erroneously approved without the routers in the
 tunnel having individually approved the request, causing a false
 positive.
 If a tunnel ingress is a separate component from the TCP sender or IP
 forwarding, it is possible that a packet with a Quick-Start option is
 encapsulated without the IP TTL being decremented first, or with both
 IP TTL and QS TTL being decremented before the tunnel encapsulation
 takes place.  If the tunnel ingress does not know about Quick-Start,
 a valid Quick-Start Request with unchanged TTL Diff traverses in the
 inner header, while the outer header most likely does not carry a
 Quick-Start Request.  If the tunnel egress also does not support
 Quick-Start, it remains possible that the Quick-Start Request would
 be falsely approved, because the packet is decapsulated using the
 Quick-Start Request from the inner header, and the value of TTL Diff
 echoed to the sender remains unchanged.  For example, such a scenario
 can occur with a Bump-In-The-Stack (BITS), an IPsec encryption
 implementation where the data encryption occurs between the network
 drivers and the TCP/IP protocol stack [RFC4301].
 As one example, if a remote access VPN client uses a BITS structure,
 then Quick-Start obstacles between the client and the VPN gateway
 won't be seen.  This is a particular problem because the path between
 the client and the VPN gateway is likely to contain the most
 congested part of the path.  Because most VPN clients are reported to
 use BITS [H05], we will explore this in more detail.
 A Bump-In-The-Wire (BITW) is an IPsec encryption implementation where
 the encryption occurs on an outboard processor, offloading the
 encryption processing overhead from the host or router [RFC4301].
 The BITW device is usually IP addressable, which means that the IP
 TTL is decremented before the packet is passed to the BITW.  If the
 QS TTL is not decremented, then the value of TTL Diff is changed, and
 the Quick-Start Request will be denied.  However, if the BITW
 supports a host and does not have its own IP address, then the IP TTL
 is not decremented before the packet is passed from the host to the
 BITW, and a false positive could occur.
 Other tunnels that need to be looked at are IP tunnels over non-
 network protocols, such as IP over TCP and IP over UDP [RFC3948], and
 tunnels using the Layer Two Tunneling Protocol [RFC2661].

Floyd, et al. Experimental [Page 34] RFC 4782 Quick-Start for TCP and IP January 2007

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

6.3. Tunnels That Support Quick-Start

 This section discusses tunnels configured to support Quick-Start.
 If the tunnel ingress node chooses to locally approve the Quick-
 Start Request, then the ingress node MUST decrement the Quick-Start
 TTL at the same time it decrements the IP TTL, and MUST copy IP TTL
 and the Quick-Start Option from the inner IP header to the outer
 header.  During encapsulation, the tunnel ingress MUST zero the
 Quick-Start rate request field in the inner header to ensure that the
 Quick-Start Request will be rejected if the tunnel egress does not
 support Quick-Start.
 If the tunnel ingress node does not choose to locally approve the
 Quick-Start Request, then it MUST either delete the Quick-Start
 option from the inner header before encapsulation, or zero the QS TTL
 and the Rate Request fields before encapsulation.
 Upon decapsulation, if the outer header contains a Quick-Start
 option, the tunnel egress MUST copy the IP TTL and the Quick-Start
 option from the outer IP header to the inner header.
 IPsec uses the IKE (Internet Key Exchange) Protocol for security
 associations.  We do not consider the interactions between Quick-
 Start and IPsec with IKEv1 [RFC2409] in this document.  Now that the
 RFC for IKEv2 [RFC4306] is published, we plan to specify a
 modification of IPsec to allow the support of Quick-Start to be
 negotiated; this modification will specify the negotiation between
 tunnel endpoints to allow or forbid support for Quick-Start within
 the tunnel.  This was done for ECN for IPsec tunnels, with IKEv1
 [RFC3168, Section 9.2].  This negotiation of Quick-Start capability
 in an IPsec tunnel will be specified in a separate IPsec document.
 This document will also include a discussion of the potential effects
 of an adversary's modifications of the Quick-Start field (as in
 Sections 18 and 19 of RFC 3168), and of the security considerations
 of exposing the Quick-Start rate request to network scanners.

6.4. Quick-Start and MPLS

 The behavior of Quick-Start with MPLS is similar to the behavior of
 Quick-Start with IP Tunnels.  For those MPLS paths where the IP TTL
 is decremented as part of traversing the MPLS path, these paths are
 compatible with Quick-Start, but do not support Quick-Start; Quick-

Floyd, et al. Experimental [Page 35] RFC 4782 Quick-Start for TCP and IP January 2007

 Start Requests that are traversing these paths will be correctly
 understood by the transport sender as having been denied.  Any MPLS
 paths where the IP TTL is not decremented as part of traversing the
 MPLS path would be not compatible with Quick-Start; such paths would
 result in false positives, where the TCP sender incorrectly believes
 that the Quick-Start Request was approved by all routers along the
 path.
 For cases where the ingress node to the MPLS path is aware of Quick-
 Start, this node should either zero the Quick-Start rate request, QS
 TTL, and QS Nonce fields, or remove the Quick-Start Option from the
 IP header.

7. The Quick-Start Mechanism in Other Transport Protocols

 The section earlier specified the use of Quick-Start in TCP.  In this
 section, we generalize this to give guidelines for the use of Quick-
 Start with other transport protocols.  We also discuss briefly how
 Quick-Start could be specified for other transport protocols.
 The general guidelines for Quick-Start in transport protocols are as
 follows:
  • Quick-Start is only specified for unicast transport protocols with

appropriate congestion control mechanisms. Note: Quick-Start is

   not a replacement for standard congestion control techniques, but
   meant to augment their operation.
  • A transport-level mechanism is needed for the Quick-Start Response

from the receiver to the sender. This response contains the Rate

   Request, TTL Diff, and QS Nonce.
  • The sender checks the validity of the Quick-Start Response.
  • The sender has an estimate of the round-trip time, and translates

the Quick-Start Response into an allowed window or allowed sending

   rate.  The sender sends a Report of the Approved Rate.  The sender
   starts sending Quick-Start packets, rate-paced out at the approved
   sending rate.
  • After the sender receives the first acknowledgement packet for a

Quick-Start packet, no more Quick-Start packets are sent. The

   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.

Floyd, et al. Experimental [Page 36] RFC 4782 Quick-Start for TCP and IP January 2007

  • When the last Quick-Start packet is acknowledged, the sender

continues using the standard congestion control mechanisms of that

   protocol.
  • If one of the Quick-Start packets is lost, then the sender reverts

to the standard congestion control method of that protocol that

   would have been used if the Quick-Start Request had not been
   approved.  In addition, the sender takes into account the
   information that the Quick-Start congestion window was too large
   (e.g., by decreasing ssthresh in TCP).

8. Using Quick-Start

8.1. Determining the Rate to Request

 As discussed in [SAF06], the data sender does not necessarily have
 information about the size of the data transfer at connection
 initiation; for example, in request-response protocols such as HTTP,
 the server doesn't know the size or name of the requested object
 during connection initiation.  [SAF06] explores some of the
 performance implications of overly large Quick-Start Requests, and
 discusses heuristics that end-nodes could use to size their requests
 appropriately.  For example, the sender might have information about
 the bandwidth of the last-mile hop, the size of the local socket
 buffer, or of the TCP receive window, and could use this information
 in determining the rate to request.  Web servers that mostly have
 small objects to transfer might decide not to use Quick-Start at all,
 since Quick-Start would be of little benefit to them.
 Quick-Start will be more effective if Quick-Start Requests are not
 larger than necessary; every Quick-Start Request that is approved but
 not used (or not fully used) takes away from the bandwidth pool
 available for granting successive Quick-Start Requests.

8.2. Deciding the Permitted Rate Request at a Router

 In this section, we briefly outline how a router might decide whether
 or not to approve a Quick-Start Request.  The router should ask the
 following questions:
  • Has the router's output link been underutilized for some time

(e.g., several seconds)?

  • Would the output link remain underutilized if the arrival rate were

to increase by the aggregate rate requests that the router has

   approved over the last fraction of a second?

Floyd, et al. Experimental [Page 37] RFC 4782 Quick-Start for TCP and IP January 2007

 In order to answer the last question, the router must have some
 knowledge of the available bandwidth on the output link and of the
 Quick-Start bandwidth that could arrive due to recently approved
 Quick-Start Requests.  In this way, if an underutilized router
 experiences a flood of Quick-Start Requests, the router can begin to
 deny Quick-Start Requests while the output link is still
 underutilized.
 A simple way for the router to keep track of the potential bandwidth
 from recently approved requests is to maintain two counters: one for
 the total aggregate Rate Requests that have been approved in the
 current time interval [T1, T2], and one for the total aggregate Rate
 Requests approved over a previous time interval [T0, T1].  However,
 this document doesn't specify router algorithms for approving Quick-
 Start Requests, or make requirements for the appropriate time
 intervals for remembering the aggregate approved Quick-Start
 bandwidth.  A possible router algorithm is given in Appendix E, and
 more discussion of these issues is available in [SAF06].
  • If the router's output link has been underutilized and the

aggregate of the Quick-Start Request Rate options granted is low

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

9. Evaluation of Quick-Start

9.1. Benefits of Quick-Start

 The main benefit of Quick-Start is the faster start-up for the
 transport connection itself.  For a small TCP transfer of one to five
 packets, Quick-Start is probably of very little benefit;  at best, it
 might shorten the connection lifetime from three to two round-trip
 times (including the round-trip time for connection establishment).
 Similarly, for a very large transfer, where the slow-start phase
 would have been only a small fraction of the connection lifetime,
 Quick-Start would be of limited benefit.  Quick-Start would not
 significantly shorten the connection lifetime, but it might eliminate
 or at least shorten the start-up phase.  However, for moderate-sized
 connections in a well-provisioned environment, Quick-Start could
 possibly allow the entire transfer of M packets to be completed in

Floyd, et al. Experimental [Page 38] RFC 4782 Quick-Start for TCP and IP January 2007

 one round-trip time (after the initial round-trip time for the SYN
 exchange), instead of the log_2(M)-2 round-trip times that it would
 normally take for the data transfer, in an uncongested environments
 (assuming an initial window of four packets).

9.2. Costs of Quick-Start

 This section discusses the costs of Quick-Start for the connection
 and for the routers along the path.
 The cost of having a Quick-Start Request packet dropped:
 Measurement studies cited earlier [MAF04] suggest that on a wide
 range of paths in the Internet, TCP SYN packets containing unknown IP
 options will be dropped.  Thus, for the sender one risk in using
 Quick-Start is that the packet carrying the Quick-Start Request could
 be dropped in the network.  It is particularly costly to the sender
 when a TCP SYN packet is dropped, because in this case the sender
 should wait for an RTO of three seconds before re-sending the SYN
 packet, as specified in Section 4.7.2.
 The cost of having a Quick-Start data packet dropped:
 Another risk for the sender in using Quick-Start lies in the
 possibility of suffering from congestion-related losses of the
 Quick-Start data packets.  This should be an unlikely situation
 because routers are expected to approve Quick-Start Requests only
 when they are significantly underutilized.  However, a transient
 increase in cross-traffic in one of the routers, a sudden decrease in
 available bandwidth on one of the links, or congestion at a non-IP
 queue could result in packet losses even when the Quick-Start Request
 was approved by all of the routers along the path.  If a Quick-Start
 packet is dropped, then the sender reverts to the congestion control
 mechanisms it would have used if the Quick-Start Request had not been
 approved, so the performance cost to the connection of having a
 Quick-Start packet dropped is small, compared to the performance
 without Quick-Start.  (On the other hand, the performance difference
 between Quick-Start with a Quick-Start packet dropped and Quick-
 Start with no Quick-Start packet dropped can be considerable.)
 Added complexity at routers:
 The main cost of Quick-Start at routers concerns the costs of added
 complexity.  The added complexity at the end-points is moderate, and
 might easily be outweighed by the benefit of Quick-Start to the end
 hosts.  The added complexity at the routers is also somewhat
 moderate; it involves estimating the unused bandwidth on the output
 link over the last several seconds, processing the Quick-Start
 request, and keeping a counter of the aggregate Quick-Start rate
 approved over the last fraction of a second.  However, this added
 complexity at routers adds to the development cycle, and could

Floyd, et al. Experimental [Page 39] RFC 4782 Quick-Start for TCP and IP January 2007

 prevent the addition of other competing functionality to routers.
 Thus, careful thought would have to be given to the addition of
 Quick-Start to IP.
 The slow path in routers:
 Another drawback of Quick-Start is that packets containing the
 Quick-Start Request message might not take the fast path in routers,
 particularly in the beginning of Quick-Start's deployment in the
 Internet.  This would mean some extra delay for the end hosts, and
 extra processing burden for the routers.  However, as discussed in
 Sections 4.1 and 4.7, not all packets would carry the Quick-Start
 option.  In addition, for the underutilized links where Quick-Start
 Requests could actually be approved, or in typical environments where
 most of the packets belong to large flows, the burden of the Quick-
 Start Option on routers would be considerably reduced.  Nevertheless,
 it is still conceivable, in the worst case, that many packets would
 carry Quick-Start Requests; this could slow down the processing of
 Quick-Start packets in routers considerably.  As discussed in Section
 9.6, routers can easily protect against this by enforcing a limit on
 the rate at which Quick-Start Requests will be considered.  [RW03]
 and [RW04] contain measurements of the impact of IP Option Processing
 on packet round-trip times.
 Multiple paths:
 One limitation of Quick-Start is that it presumes that the data
 packets of a connection will follow the same path as the Quick-Start
 request packet.  If this is not the case, then the connection could
 be sending the Quick-Start packets, at the approved rate, along a
 path that was already congested, or that became congested as a result
 of this connection.  Thus, Quick-Start could give poor performance
 when there is a routing change immediately after the Quick-Start
 Request is approved, and the Quick-Start data packets follow a
 different path from that of the original Quick-Start Request.  This
 is, however, similar to what would happen for a connection with
 sufficient data, if the connection's path was changed in the middle
 of the connection, which had already established the allowed initial
 rate.
 As specified in Section 3.3, a router that uses multipath routing for
 packets within a single connection must not approve a Quick-Start
 Request.  Quick-Start would not perform robustly in an environment
 with multipath routing, where different packets in a connection
 routinely follow different paths.  In such an environment, the
 Quick-Start Request and some fraction of the packets in the
 connection might take an underutilized path, while the rest of the
 packets take an alternate, congested path.

Floyd, et al. Experimental [Page 40] RFC 4782 Quick-Start for TCP and IP January 2007

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

9.3. Quick-Start with QoS-Enabled Traffic

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

9.4. Protection against Misbehaving Nodes

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

9.4.1. Misbehaving Senders

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

Floyd, et al. Experimental [Page 41] RFC 4782 Quick-Start for TCP and IP January 2007

 that sender, e.g., by deleting the Quick-Start Request from future
 packets from that sender.  We note that routers are not required to
 use Approved Rate reports to check if senders are cheating; this is
 at the discretion of the router.
 If a router sees a Report of Approved Rate, and did not see an
 earlier Quick-Start Request, then either the sender could be
 cheating, or the connection's path could have changed since the
 Quick-Start Request was sent.  In either case, the router could
 decide to deny future Quick-Start Requests for this connection.  In
 particular, it is reasonable for the router to deny a Quick-Start
 request if either the sender is cheating, or if the connection path
 suffers from path changes or multipathing.
 If a router approved a Quick-Start Request, but does not see a
 subsequent Approved Rate report, then there are several
 possibilities: (1) the request was denied and/or dropped downstream,
 and the sender did not send a Report of Approved Rate; (2) the
 request was approved, but the sender did not send a Report of
 Approved Rate; (3) the Approved Rate report was dropped in the
 network; or (4) the Approved Rate report took a different path from
 the Quick-Start Request.  In any of these cases, the router would be
 justified in denying future Quick-Start Requests for this connection.
 In any of the cases mentioned in the three paragraphs above (i.e., an
 Approved Rate report that is larger than the Rate Request in the
 earlier Quick-Start Request, a Report of Approved Rate with no
 preceding Rate Request, or a Rate Request with no Report of Approved
 Rate), a traffic policer may assume that Quick-Start is not being
 used appropriately, or is being used in an unsuitable environment
 (e.g., with multiple paths), and take some corresponding action.
 What are the incentives for a sender to cheat by over-sending after a
 Quick-Start Request?  Assuming that the sender's interests are
 measured by a performance metric such as the completion time for its
 connections, sometimes it might be in the sender's interests to
 cheat, and sometimes it might not;  in some cases, it could be
 difficult for the sender to judge whether it would be in its
 interests to cheat.  The incentives for a sender to cheat by over-
 sending after a Quick-Start Request are not that different from the
 incentives for a sender to cheat by over-sending even in the absence
 of Quick-Start, with one difference: the use of Quick-Start could
 help a sender evade policing actions from policers in the network.
 The Report of Approved Rate is designed to address this and to make
 it harder for senders to use Quick-Start to `cover' their cheating.

Floyd, et al. Experimental [Page 42] RFC 4782 Quick-Start for TCP and IP January 2007

9.4.2. Receivers Lying about Whether the Request was Approved

 One form of misbehavior would be for the receiver to lie to the
 sender about whether the Quick-Start Request was approved, by falsely
 reporting the TTL Diff and QS Nonce.  If a router that understands
 the Quick-Start Request denies the request by deleting the request or
 by zeroing the QS TTL and QS Nonce, then the receiver can "lie" about
 whether the request was approved only by successfully guessing the
 value of the TTL Diff and QS Nonce to report.  The chance of the
 receiver successfully guessing the correct value for the TTL Diff is
 1/256, and the chance of the receiver successfully guessing the QS
 nonce for a reported rate request of K is 1/(2K).
 However, if the Quick-Start Request is denied only by a non-Quick-
 Start-capable router, or by a router that is unable to zero the QS
 TTL and QS Nonce fields, then the receiver could lie about whether
 the Quick-Start Requests were approved by modifying the QS TTL in
 successive requests received from the same host.  In particular, if
 the sender does not act on a Quick-Start Request, then the receiver
 could decrement the QS TTL by one in the next request received from
 that host before calculating the TTL Diff, and decrement the QS TTL
 by two in the following received request, until the sender acts on
 one of the Quick-Start Requests.
 Unfortunately, if a router doesn't understand Quick-Start, then it is
 not possible for that router to take an active step such as zeroing
 the QS TTL and QS Nonce to deny a request.  As a result, the QS TTL
 is not a fail-safe mechanism for preventing lying by receivers in the
 case of non-Quick-Start-capable routers.
 What would be the incentives for a receiver to cheat in reporting on
 a Quick-Start Request, in the absence of a mechanism such as the QS
 Nonce?  In some cases, cheating would be of clear benefit to the
 receiver, resulting in a faster completion time for the transfer.  In
 other cases, where cheating would result in Quick-Start packets being
 dropped in the network, cheating might or might not improve the
 receiver's performance metric, depending on the details of that
 particular scenario.

9.4.3. Receivers Lying about the Approved Rate

 A second form of receiver misbehavior would be for the receiver to
 lie to the sender about the Rate Request for an approved Quick-Start
 Request, by increasing the value of the Rate Request field.  However,
 the receiver doesn't necessarily know the Rate Request in the
 original Quick-Start Request sent by the sender, and a higher Rate
 Request reported by the receiver will only be considered valid by the
 sender if it is no higher than the Rate Request originally requested

Floyd, et al. Experimental [Page 43] RFC 4782 Quick-Start for TCP and IP January 2007

 by the sender.  For example, if the sender sends a Quick-Start
 Request with a Rate Request of X, and the receiver reports receiving
 a Quick-Start Request with a Rate Request of Y > X, then the sender
 knows that either some router along the path malfunctioned
 (increasing the Rate Request inappropriately), or the receiver is
 lying about the Rate Request in the received packet.
 If the sender sends a Quick-Start Request with a Rate Request of Z,
 the receiver receives the Quick-Start Request with an approved Rate
 Request of X, and reports a Rate Request of Y, for X < Y <= Z, then
 the receiver only succeeds in lying to the sender about the approved
 rate if the receiver successfully reports the rightmost 2Y bits in
 the QS nonce.
 If senders often use a configured default value for the Rate Request,
 then receivers would often be able to guess the original Rate
 Request, and this would make it easier for the receiver to lie about
 the value of the Rate Request field.  Similarly, if the receiver
 often communicates with a particular sender, and the sender always
 uses the same Rate Request for that receiver, then the receiver might
 over time be able to infer the original Rate Request used by the
 sender.
 There are several possible additional forms of protection against
 receivers lying about the value of the Rate Request.  One possible
 additional protection would be for a router that decreases a Rate
 Request in a Quick-Start Request to report the decrease directly to
 the sender.  However, this could lead to many reports back to the
 sender for a single request, and could also be used in address-
 spoofing attacks.
 A second limited form of protection would be for senders to use some
 degree of randomization in the requested Rate Request, so that it is
 difficult for receivers to guess the original value for the Rate
 Request.  However, this is difficult because there is a fairly coarse
 granularity in the set of rate requests available to the sender, and
 randomizing the initial request only offers limited protection, in
 any case.

9.4.4. Collusion between Misbehaving Routers

 In addition to protecting against misbehaving receivers, it is
 necessary 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

Floyd, et al. Experimental [Page 44] RFC 4782 Quick-Start for TCP and IP January 2007

 decremented rate request might not have been willing to approve the
 larger, original request.
 Another form of collusion would be for the ingress router to inform
 the egress router out-of-band of the TTL Diff and QS Nonce for the
 request packet at the ingress.  This would enable the egress router
 to modify the QS TTL and QS Nonce so that it appeared that all the
 routers along the path had approved the request.  There does not
 appear to be any protection against a colluding ingress and egress
 router.  Even if an intermediate router had deleted the Quick-Start
 Option from the packet, the ingress router could have sent the
 Quick-Start Option to the egress router out-of-band, with the egress
 router inserting the Quick-Start Option, with a modified QS TTL
 field, back in the packet.
 However, unlike ECN, there is somewhat less of an 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 the routers along the path.  With ECN, a colluding ingress router
 could falsely mark a packet as ECN-capable, with the colluding egress
 router returning the ECN field in the IP header to its original non-
 ECN-capable codepoint, and congested routers along the path could
 have been fooled into not dropping that packet.  This collusion would
 give an unfair competitive advantage to the traffic protected by the
 colluding ingress and egress routers.
 In contrast, with Quick-Start, the collusion of the ingress and
 egress routers to make it falsely appear that a Quick-Start Request
 was approved sometimes would give an advantage to the traffic covered
 by that collusion, and sometimes would give a disadvantage, depending
 on the details of the scenario.  If some router along the path really
 does not have enough available bandwidth to approve the Quick-Start
 Request, then Quick-Start packets sent as a result of the falsely
 approved request could be dropped in the network, to the possible
 disadvantage of the connection.  Thus, while the ingress and egress
 routers could collude to prevent intermediate routers from denying a
 Quick-Start Request, it would not always be to the connection's
 advantage for this to happen.  One defense against such a collusion
 would be for some router between the ingress and egress nodes that
 denied the request to monitor connection performance, penalizing
 connections that seem to be using Quick-Start after a Quick-Start
 Request was denied, or that are reporting an Approved Rate higher
 than that actually approved by that router.
 If the congested router is ECN-capable, and the colluding ingress and
 egress routers are lying about ECN-capability as well as about
 Quick-Start, then the result could be that the Quick-Start Request
 falsely appears to the sender to have been approved, and the Quick-

Floyd, et al. Experimental [Page 45] RFC 4782 Quick-Start for TCP and IP January 2007

 Start packets falsely appear to the congested router to be ECN-
 capable.  In this case, the colluding routers might succeed in giving
 a competitive advantage to the traffic protected by their collusion
 (if no intermediate router is monitoring to catch such misbehavior).

9.5. Misbehaving Middleboxes and the IP TTL

 One possible difficulty is that of traffic normalizers [HKP01], or
 other middleboxes along that path, that rewrite IP TTLs in order to
 foil other kinds of attacks in the network.  If such a traffic
 normalizer rewrote 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.  Rewriting the IP TTL could result in false
 positives (with the sender incorrectly believing that the Quick-
 Start Request was approved) as well as false negatives (with the
 sender incorrectly believing that the Quick-Start Request was
 denied).

9.6. Attacks on Quick-Start

 As discussed in [SAF06], Quick-Start is vulnerable to two kinds of
 attacks: (1) attacks to increase the routers' processing and state
 load and (2) attacks with bogus Quick-Start Requests to temporarily
 tie up available Quick-Start bandwidth, preventing routers from
 approving Quick-Start Requests from other connections.  Routers can
 protect against the first kind of attack by applying a simple limit
 on the rate at which Quick-Start Requests will be considered by the
 router.
 The second kind of attack, to tie up the available Quick-Start
 bandwidth, is more difficult to defend against.  As discussed in
 [SAF06], Quick-Start Requests that are not going to be used, either
 because they are from malicious attackers or because they are denied
 by routers downstream, can result in short-term `wasting' of
 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.

Floyd, et al. Experimental [Page 46] RFC 4782 Quick-Start for TCP and IP January 2007

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

9.7. Simulations with Quick-Start

 Quick-Start was added to the NS simulator [SH02] by Srikanth
 Sundarrajan, and additional functionality was added by Pasi
 Sarolahti.  The validation test is at `test-all-quickstart' in the
 `tcl/test' directory in NS.  The initial simulation studies from
 [SH02] show a significant performance improvement using Quick-Start
 for moderate-sized flows (between 4 KB and 128 KB) in underutilized
 environments.  These studies are of file transfers, with the
 improvement measured as the relative increase in the overall
 throughput for the file transfer.  The study shows that potential
 improvement from Quick-Start is proportional to the delay-bandwidth
 product of the path.
 The Quick-Start simulations in [SAF06] explore the following: the
 potential benefit of Quick-Start for the connection, the relative
 benefits of different router-based algorithms for approving Quick-
 Start Requests, and the effectiveness of Quick-Start as a function of
 the senders' algorithms for choosing the size of the rate request.

10. Implementation and Deployment Issues

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

10.1. Implementation Issues for Sending Quick-Start Requests

 Section 4.7 discusses some of the issues with deciding the initial
 sending rate to request.  Quick-Start raises additional issues about
 the communication between the transport protocol and the application,
 and about the use of past history with Quick-Start in the end node.
 One possibility is that a protocol implementation could provide an
 API for applications to indicate when they want to request Quick-
 Start, and what rate they would like to request.  In the conventional
 socket API, this could be a socket option that is set before a
 connection is established.  Some applications, such as those that use
 TCP for bulk transfers, do not have interest in the transmission
 rate, but they might know the amount of data that can be sent

Floyd, et al. Experimental [Page 47] RFC 4782 Quick-Start for TCP and IP January 2007

 immediately.  Based on this, the sender implementation could decide
 whether Quick-Start would be useful, and what rate should be
 requested.
 We note that when Quick-Start is used, the TCP sender is required to
 save the QS Nonce and the TTL Diff when the Quick-Start Request is
 sent, and to implement an additional timer for the paced transmission
 of Quick-Start packets.

10.2. Implementation Issues for Processing Quick-Start Requests

 A router or other network host must be able to determine the
 approximate bandwidth of its outbound network interfaces in order to
 process incoming Quick-Start rate requests, including those that
 originate from the host itself.  One possibility would be for hosts
 to rely on configuration information to determine link bandwidths;
 this has the drawback of not being robust to errors in configuration.
 Another possibility would be for network device drivers to infer the
 bandwidth for the interface and to communicate this to the IP layer.
 Particular issues will arise for wireless links with variable
 bandwidth, where decisions will have to be made about how frequently
 the host gets updates of the changing bandwidth.  It seems
 appropriate that Quick-Start Requests would be handled particularly
 conservatively for links with variable bandwidth; to avoid cases
 where Quick-Start Requests are approved, the link bandwidth is
 reduced, and the data packets that are sent end up being dropped.
 Difficult issues also arise for paths with multi-access links (e.g.,
 Ethernet).  Routers or end-nodes with multi-access links should be
 particularly conservative in granting Quick-Start Requests.  In
 particular, for some multi-access links, there may be no procedure
 for an attached node to use to determine whether all parts of the
 multi-access link have been underutilized in the recent past.

10.3. Possible Deployment Scenarios

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

Floyd, et al. Experimental [Page 48] RFC 4782 Quick-Start for TCP and IP January 2007

 Quick-Start is not currently intended for ubiquitous deployment in
 the global Internet.  In particular, Quick-Start should not be
 enabled by default in end-nodes or in routers; instead, when Quick-
 Start is used, it should be explicitly enabled by users or system
 administrators.
 Below are a few examples of networking environments where Quick-
 Start would potentially be useful.  These are the environments that
 might consider an initial deployment of Quick-Start in the routers
 and end-nodes, where the incentives for routers to deploy Quick-
 Start might be the most clear.
  • Centrally administrated organizational intranets: These intranets

often have large network capacity, with networks that are

   underutilized for much of the time [PABL+05].  Such intranets might
   also include high-bandwidth and high-delay paths to remote sites.
   In such an environment, Quick-Start would be of benefit to users,
   and there would be a clear incentive for the deployment of Quick-
   Start in routers.  For example, Quick-Start could be quite useful
   in high-bandwidth networks used for scientific computing.
  • Wireless networks: Quick-Start could also be useful in high-delay

environments of Cellular Wide-Area Wireless Networks, such as the

   GPRS [BW97] and their enhancements and next generations.  For
   example, GPRS EDGE (Enhanced Data for GSM Evolution) is expected to
   provide wireless bandwidth of up to 384 Kbps (roughly 32 1500-byte
   packets per second) while the GPRS round-trip times range typically
   from a few hundred milliseconds to over a second, excluding any
   possible queueing delays in the network [GPAR02].  In addition,
   these networks sometimes have variable additional delays due to
   resource allocation that could be avoided by keeping the connection
   path constantly utilized, starting from initial slow-start.  Thus,
   Quick-Start could be of significant benefit to users in these
   environments.
  • Paths over satellite links: Geostationary Orbit (GEO) satellite

links have one-way propagation delays on the order of 250 ms while

   the bandwidth can be measured in megabits per second [RFC2488].
   Because of the considerable bandwidth-delay product on the link,
   TCP's slow-start is a major performance limitation in the beginning
   of the connection.  A large initial congestion window would be
   useful to users of such satellite links.
  • Single-hop paths: Quick-Start should work well over point-to-point

single-hop paths, e.g., from a host to an adjacent server. Quick-

   Start would work over a single-hop IP path consisting of a multi-
   access link only if the host was able to determine if the path to
   the next IP hop has been significantly underutilized over the

Floyd, et al. Experimental [Page 49] RFC 4782 Quick-Start for TCP and IP January 2007

   recent past.  If the multi-access link includes a layer-2 switch,
   then the attached host cannot necessarily determine the status of
   the other links in the layer-2 network.

10.4. A Comparison with the Deployment Problems of ECN

 Given the glacially slow rate of deployment of ECN in the Internet to
 date [MAF05], it is disconcerting to note that some of the deployment
 problems of Quick-Start are even greater than those of ECN.  First,
 unlike ECN, which can be of benefit even if it is only deployed on
 one of the routers along the end-to-end path, a connection's use of
 Quick-Start requires Quick-Start deployment on all of the routers
 along the end-to-end path.  Second, unlike ECN, which uses an
 allocated field in the IP header, Quick-Start requires the extra
 complications of an IP Option, which can be difficult to pass through
 the current Internet [MAF05].
 However, in spite of these issues, there is some hope for the
 deployment of Quick-Start, at least in protected corners of the
 Internet, because the potential benefits of Quick-Start to the user
 are considerably more dramatic than those of ECN.  Rather than simply
 replacing the occasional dropped packet by an ECN-marked packet,
 Quick-Start is capable of dramatically increasing the throughput of
 connections in underutilized environments [SAF06].

11. Security Considerations

 Sections 9.4 and 9.6 discuss the security considerations related to
 Quick-Start.  Section 9.4 discusses the potential abuse of Quick-
 Start by senders or receivers lying about whether the request was
 approved or about the approved rate, and of routers in collusion to
 misuse Quick-Start.  Section 9.5 discusses potential problems with
 traffic normalizers that rewrite IP TTLs in packet headers.  All
 these problems could result in the sender using a Rate Request that
 was inappropriately large, or thinking that a request was approved
 when it was in fact denied by at least one router along the path.
 This inappropriate use of Quick-Start could result in congestion and
 an unacceptable level of packet drops along the path.  Such
 congestion could also be part of a Denial of Service attack.
 Section 9.6 discusses a potential attack on the routers' processing
 and state load from an attack of Quick-Start Requests.  Section 9.6
 also discusses a potential attack on the available Quick-Start
 bandwidth by sending bogus Quick-Start Requests for bandwidth that
 will not, in fact, be used.  While this impacts the global usability
 of Quick-Start, it does not endanger the network as a whole since TCP
 uses standard congestion control if Quick-Start is not available.

Floyd, et al. Experimental [Page 50] RFC 4782 Quick-Start for TCP and IP January 2007

 Section 4.7.2 discusses the potential problem of packets with Quick-
 Start Requests dropped by middleboxes along the path.
 As discussed in Section 5, for IPv4 IPsec Authentication Header
 Integrity Check Value (AH ICV) calculation, the Quick-Start Option is
 a mutable IPv4 option and hence completely zeroed for AH ICV
 calculation purposes.  This is also the treatment required by RFC
 4302 for unrecognized IPv4 options.  The IPv6 Quick-Start Option's
 IANA-allocated option type indicates that it is a mutable option;
 hence, according to RFC 4302, its option data is required to be
 zeroed for AH ICV computation purposes.  See RFC 4302 for further
 explanation.
 Section 6.2 discusses possible problems of Quick-Start used by
 connections carried over simple tunnels that are not compatible with
 Quick-Start.  In this case, it is possible that a Quick-Start Request
 is erroneously considered approved by the sender without the routers
 in the tunnel having individually approved the request, causing a
 false positive.
 We note two high-order points here.  First, the Quick-Start Nonce
 goes a long way towards preventing large-scale cheating.  Second,
 even if a host occasionally uses Quick-Start when it is not approved
 by the entire network path, the network will not collapse.  Quick-
 Start does not remove TCP's basic congestion control mechanisms;
 these will kick in when the network is heavily loaded, relegating any
 Quick-Start mistake to a transient.

Floyd, et al. Experimental [Page 51] RFC 4782 Quick-Start for TCP and IP January 2007

12. IANA Considerations

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

12.1. IP Option

 Quick-Start requires both an IPv4 Option Number (Section 3.1) and an
 IPv6 Option Number (Section 3.2).
 IPv4 Option Number:
 Copy Class Number Value Name
 ---- ----- ------ ----- ----
    0    00     25    25   QS    - Quick-Start
 IPv6 Option Number [RFC2460]:
 HEX         act  chg  rest
 ---         ---  ---  -----
   6          00   1   00110     Quick-Start
 For the IPv6 Option Number, the first two bits indicate that the IPv6
 node may skip over this option and continue processing the header if
 it doesn't recognize the option type, and the third bit indicates
 that the Option Data may change en-route.
 In both cases, this document should be listed as the reference
 document.

12.2. TCP Option

 Quick-Start requires a TCP Option Number (Section 4.2).
 TCP Option Number:
 Kind Length Meaning
 ---- ------ ------------------------------
   27 8      Quick-Start Response
 This document should be listed as the reference document.

Floyd, et al. Experimental [Page 52] RFC 4782 Quick-Start for TCP and IP January 2007

13. Conclusions

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

14. Acknowledgements

 The authors wish to thank Mark Handley for discussions of these
 issues.  The authors also thank the End-to-End Research Group, the
 Transport Services Working Group, and members of IPAM's program on
 Large-Scale Communication Networks for both positive and negative
 feedback on this proposal.  We thank Srikanth Sundarrajan for the
 initial implementation of Quick-Start in the NS simulator, and for
 the initial simulation study.  Many thanks to David Black and Joe
 Touch for extensive feedback on Quick-Start and IP tunnels.  We also
 thank Mohammed Ashraf, John Border, Bob Briscoe, Martin Duke, Tom
 Dunigan, Mitchell Erblich, Gorry Fairhurst, John Heidemann, Paul
 Hyder, Dina Katabi, and Vern Paxson for feedback.  Thanks also to
 Gorry Fairhurst for the suggestion of adding the QS Nonce to the
 Report of Approved Rate.
 The version of the QS Nonce in this document is based on a proposal
 from Guohan Lu [L05].  Earlier versions of this document contained an
 eight-bit QS Nonce, and subsequent versions discussed the possibility
 of a four-bit QS Nonce.
 This document builds upon the concepts described in [RFC3390],
 [AHO98], [RFC2415], and [RFC3168].  Some of the text on Quick-Start
 in tunnels was borrowed directly from RFC 3168.
 This document is the development of a proposal originally by Amit
 Jain for Initial Window Discovery.

Floyd, et al. Experimental [Page 53] RFC 4782 Quick-Start for TCP and IP January 2007

Appendix A. Related Work

 The Quick-Start proposal, taken together with HighSpeed TCP [RFC3649]
 or other transport protocols for high-bandwidth transfers, could go a
 significant way towards extending the range of performance for best-
 effort traffic in the Internet.  However, there are many things that
 the Quick-Start proposal would not accomplish.  Quick-Start is not a
 congestion control mechanism, and would not help in making more
 precise use of the available bandwidth -- that is, of achieving the
 goal of high throughput with low delay and low packet-loss rates.
 Quick-Start would not give routers more control over the decrease
 rates of active connections.
 In addition, any evaluation of Quick-Start must include a discussion
 of the relative benefits of approaches that use no explicit
 information from routers, and of approaches that use more fine-
 grained feedback from routers as part of a larger congestion control
 mechanism.  We discuss several classes of proposals in the sections
 below.

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

 One possibility would be for senders to use information from the
 packet streams to learn about the available bandwidth, without
 explicit information from routers.  These techniques would not allow
 a start-up as fast as that available from Quick-Start in an
 underutilized environment; one already has to have sent some packets
 in order 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 than is possible *with* explicit feedback, it is also
 possible that approaches that are more aggressive than Slow-Start are
 possible without the complexity involved in obtaining explicit
 feedback from routers.
 Periodic packet streams:
 [JD02] explores the use of periodic packet streams to estimate the
 available bandwidth along a path.  The idea is that the one-way
 delays of a periodic packet stream show an increasing trend when the
 stream's rate is higher than the available bandwidth (due to an
 increasing queue).  While [JD02] states that the proposed mechanism
 does not cause significant increases in network utilization, losses,
 or delays when done by one flow at a time, the approach could be
 problematic if conducted concurrently by a number of flows.  [JD02]
 also gives an overview of some of the earlier work on inferring the
 available bandwidth from packet trains.

Floyd, et al. Experimental [Page 54] RFC 4782 Quick-Start for TCP and IP January 2007

 Swift-Start:
 The Swift Start proposal from [PRAKS02] combines packet-pair and
 packet-pacing techniques.  An initial congestion window of four
 segments is used to estimate the available bandwidth along the path.
 This estimate is then used to dramatically increase the congestion
 window during the second RTT of data transmission.
 SPAND:
 In the TCP/SPAND proposal from [ZQK00] for speeding up short data
 transfers, network performance information would be shared among many
 co-located hosts to estimate each connection's fair share of the
 network resources.  Based on such estimation and the transfer size,
 the TCP sender would determine the optimal initial congestion window
 size.  The design for TCP/SPAND uses a performance gateway that
 monitors all traffic entering and leaving an organization's network.
 Sharing information among TCP connections:
 The Congestion Manager [RFC3124] and TCP control block sharing
 [RFC2140] both propose sharing congestion information among multiple
 TCP connections with the same endpoints.  With the Congestion
 Manager, a new TCP connection could start with a high initial cwnd,
 if it was sharing the path and the cwnd with a pre-existing TCP
 connection to the same destination that had already obtained a high
 congestion window.  RFC 2140 discusses ensemble sharing, where an
 established connection's congestion window could be `divided up' to
 be shared with a new connection to the same host.  However, neither
 of these approaches addresses the case of a connection to a new
 destination, with no existing or recent connection (and therefore
 congestion control state) to that destination.
 While continued research on the limits of the ability of TCP and
 other transport protocols to learn of available bandwidth without
 explicit feedback from the router seems useful, we note that there
 are several fundamental advantages of explicit feedback from routers.
 (1) Explicit feedback is faster than implicit feedback:
     One advantage of explicit feedback from the routers is that it
     allows the transport sender to reliably learn of available
     bandwidth in one round-trip time.
 (2) Explicit feedback is more reliable than implicit feedback:
     Techniques that attempt to assess the available bandwidth at
     connection start-up using implicit techniques are more error-
     prone than techniques that involve every element in the network
     path.  While explicit information from the network can be wrong,
     it has a much better chance of being appropriate than an end-host
     trying to *estimate* an appropriate sending rate using "block
     box" probing techniques of the entire path.

Floyd, et al. Experimental [Page 55] RFC 4782 Quick-Start for TCP and IP January 2007

A.2. Optimistic Sending without Explicit Information from Routers

 Another possibility that has been suggested [S02] is for the sender
 to start with a large initial window without explicit permission from
 the routers and without bandwidth estimation techniques and for the
 first packet of the initial window to contain information, such as
 the size or sending rate of the initial window.  The proposal would
 be that congested routers would use this information in the first
 data packet to drop or delay many or all of the packets from that
 initial window.  In this way, a flow's optimistically large initial
 window would not force the router to drop packets from competing
 flows in the network.  Such an approach would seem to require some
 mechanism for the sender to ensure that the routers along the path
 understood the mechanism for marking the first packet of a large
 initial window.
 Obviously, there would be a number of questions to consider about an
 approach of optimistic sending.
 (1) Incremental deployment:
     One question would be the potential complications of incremental
     deployment, where some of the routers along the path might not
     understand the packet information describing the initial window.
 (2) Congestion collapse:
     There could also be concerns about congestion collapse if many
     flows used large initial windows, many packets were dropped from
     optimistic initial windows, and many congested links ended up
     carrying packets that are only going to be dropped downstream.
 (3) Distributed Denial of Service attacks:
     A third question would be the potential role of optimistic
     senders in amplifying the damage done by a Distributed Denial of
     Service (DDoS) attack (assuming attackers use compliant
     congestion control in the hopes of "flying under the radar").
 (4) Performance hits if a packet is dropped:
     A fourth issue would be to quantify the performance hit to the
     connection when a packet is dropped from one of the initial
     windows.

A.3. Fast Start-Ups with Other Information from Routers

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

Floyd, et al. Experimental [Page 56] RFC 4782 Quick-Start for TCP and IP January 2007

 An IP Option about the free buffer size:
 In related work, [P00] investigates the use of a slightly different
 IP option for TCP connections to discover the available bandwidth
 along the path.  In that proposal, the IP option would query the
 routers along the path about the smallest available free buffer size.
 Also, the IP option would have been sent after the initial SYN
 exchange, when the TCP sender already had an estimate of the round-
 trip time.
 The Performance Transparency Protocol:
 The Performance Transparency Protocol (PTP) includes a proposal for a
 single PTP packet that would collect information from routers along
 the path from the sender to the receiver [W00].  For example, a
 single PTP packet could be used to determine the bottleneck bandwidth
 along a path.
 ETEN:
 Additional proposals for end nodes to collect explicit information
 from routers include one variant of Explicit Transport Error
 Notification (ETEN), which includes a cumulative mechanism to notify
 endpoints of aggregate congestion statistics along the path [KAPS02].
 (A second variant in [KSEPA04] does not depend on cumulative
 congestion statistics from the network.)

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

 Proposals for more fine-grained, congestion-related feedback from
 routers include XCP [KHR02], MaxNet [MaxNet], and AntiECN marking
 [K03].  Appendix B.6 discusses in more detail the relationship
 between Quick-Start and proposals for more fine-grained per-packet
 feedback from routers.
 XCP:
 Proposals, such as XCP for new congestion control mechanisms based on
 more feedback from routers, are more powerful than Quick-Start, but
 also are more complex to understand and more difficult to deploy.
 XCP routers maintain no per-flow state, but provide more fine-
 grained feedback to end-nodes than the one-bit congestion feedback of
 ECN.  The per-packet feedback from XCP can be positive or negative,
 and specifies the increase or decrease in the sender's congestion
 window when this packet is acknowledged.  XCP is a full-fledge
 congestion control scheme, whereas Quick-Start represents a quick
 check to determine if the network path is significantly underutilized
 such that a connection can start faster and then fall back to TCP's
 standard congestion control algorithms.

Floyd, et al. Experimental [Page 57] RFC 4782 Quick-Start for TCP and IP January 2007

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

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

 There have been proposals for routers to provide a Lower Effort
 differentiated service that would be lower than best effort
 [RFC3662].  Such a service could carry traffic for which delivery is
 strictly optional, or could carry traffic that is important but that
 has low priority in terms of time.  Because it does not interfere
 with best-effort traffic, Lower Effort services could be used by
 transport protocols that start up faster than slow-start.  For
 example, [SGF05] is a proposal for the transport sender to use low-
 priority traffic for much of the initial traffic, with routers
 configured to use strict priority queueing.
 A separate but related issue is that of below-best-effort TCP,
 variants of TCP that would not rely on Lower Effort services in the
 network, but would approximate below-best-effort traffic by detecting
 and responding to congestion sooner than standard TCP.  TCP Nice
 [V02] and TCP Low Priority (TCP-LP) [KK03] are two such proposals for
 below-best-effort TCP, with the purpose of allowing TCP connections
 to use the bandwidth unused by TCP and other traffic in a non-
 intrusive fashion.  Both TCP Nice and TCP Low Priority use the
 default slow-start mechanisms of TCP.
 We note that Quick-Start is quite different from either a Lower-
 Effort service or a below-best-effort variant of TCP.  Unlike these
 proposals, Quick-Start is intended to be useful for best-effort
 traffic that wishes to receive at least as much bandwidth as
 competing best-effort connections.

Floyd, et al. Experimental [Page 58] RFC 4782 Quick-Start for TCP and IP January 2007

Appendix B. Design Decisions

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

 This document has proposed using an IP Option for the Quick-Start
 Request from the sender to the receiver, and using transport
 mechanisms for the Quick-Start Response from the receiver back to the
 sender.  In this section, we discuss alternate mechanisms, and
 consider whether ICMP ([RFC792], [RFC4443]) or RSVP [RFC2205]
 protocols could be used for delivering the Quick-Start Request.

B.1.1. ICMP

 Being a control protocol used between Internet nodes, one could argue
 that ICMP is the ideal method for requesting permission for faster
 start-up from routers.  The ICMP header is above the IP header.
 Quick-Start could be accomplished with ICMP as follows: If the ICMP
 protocol is used to implement Quick-Start, the equivalent of the
 Quick-Start IP option would be carried in the ICMP header of the ICMP
 Quick-Start Request.  The ICMP Quick-Start Request would have to pass
 by the routers on the path to the receiver, possibly using the IP
 Router Alert option [RFC2113].  A router that approves the Quick-
 Start Request would take the same actions as in the case with the
 Quick-Start IP Option, and forward the packet to the next router
 along the path.  A router that does not approve the Quick-Start
 Request, even with a decreased value for the Requested Rate, would
 delete the ICMP Quick-Start Request, and send an ICMP Reply to the
 sender that the request was not approved.  If the ICMP Reply was
 dropped in the network, and did not reach the receiver, the sender
 would still know that the request was not approved from the absence
 of feedback from the receiver.  If the ICMP Quick-Start Request was
 dropped in the network due to congestion, the sender would assume
 that the request was not approved.  The ICMP message would need the
 source and destination port numbers for demultiplexing at the end
 nodes.  If the ICMP Quick-Start Request reached the receiver, the
 receiver would use transport-level or application-level mechanisms to
 send a response to the sender, exactly as with the IP Option.
 One benefit of using ICMP would be that the delivery of the TCP SYN
 packet or other initial packet would not be delayed by IP option
 processing at routers.  A greater advantage is that if middleboxes
 were blocking packets with Quick-Start Requests, using the Quick-
 Start Request in a separate ICMP packet would mean that the middlebox
 behavior would not affect the connection as a whole.  (To get this
 robustness to middleboxes with TCP using an IP Quick-Start Option,
 one would have to have a TCP-level Quick-Start Request packet that
 could be sent concurrently with, but separately from, the TCP SYN
 packet.)

Floyd, et al. Experimental [Page 59] RFC 4782 Quick-Start for TCP and IP January 2007

 However, there are a number of disadvantages to using ICMP.  Some
 firewalls and middleboxes may not forward the ICMP Quick-Start
 Request packets.  (If an ICMP Reply packet from a router to the
 sender is dropped in the network, the sender would still know that
 the request was not approved, as stated earlier, so this would not be
 as serious of a problem.)  In addition, it would be difficult, if not
 impossible, for a router in the middle of an IP tunnel to deliver an
 ICMP Reply packet to the actual source, for example, when the inner
 IP header is encrypted, as in IPsec ESP tunnel mode [RFC4301].
 Again, however, the ICMP Reply packet would not be essential to the
 correct operation of ICMP Quick-Start.
 Unauthenticated out-of-band ICMP messages could enable some types of
 attacks by third-party malicious hosts that are not possible when the
 control information is carried in-band with the IP packets that can
 only be altered by the routers on the connection path.  Finally, as a
 minor concern, using ICMP would cause a small amount of additional
 traffic in the network, which is not the case when using IP options.

B.1.2. RSVP

 With some modifications, RSVP [RFC2205] could be used as a bearer
 protocol for carrying the Quick-Start Requests.  Because routers are
 expected to process RSVP packets more extensively than the normal
 transport protocol IP packets, delivering a Quick-Start rate request
 using an RSVP packet would seem an appealing choice.  However, Quick-
 Start with RSVP would require a few differences from the conventional
 usage of RSVP.  Quick-Start would not require periodical refreshing
 of soft state, because Quick-Start does not require per-connection
 state in routers.  Quick-Start Requests would be transmitted
 downstream from the sender to receiver in the RSVP Path messages,
 which is different from the conventional RSVP model where the
 reservations originate from the receiver.  Furthermore, the Quick-
 Start Response would be sent using the transport-level or
 application-level mechanisms, instead of using the RSVP Resv message.
 If RSVP was used for carrying a Quick-Start Request, a new "Quick-
 Start Request" class object would be included in the RSVP Path
 message that is sent from the sender to receiver.  The object would
 contain the rate request field in addition to the common length and
 type fields.  The Send_TTL field in the RSVP common header could be
 used as the equivalent of the QS TTL field.  The Quick-Start capable
 routers along the path would inspect the Quick-Start Request object
 in the RSVP Path message, decrement Send_TTL, and adjust the rate
 request field if needed.  If an RSVP router did not understand the
 Quick-Start Request object, it would reject the entire RSVP message
 and send an RSVP PathErr message back to the sender.  When an RSVP
 message with the Quick-Start Request object reaches the receiver, the

Floyd, et al. Experimental [Page 60] RFC 4782 Quick-Start for TCP and IP January 2007

 receiver sends a Quick-Start Reply message in the corresponding
 transport protocol header in the same way as described in the context
 of IP options earlier.  If the RSVP message with the Quick-Start
 Request object was dropped along the path, the transport sender would
 simply proceed with the normal congestion control procedures.
 Much of the discussion about benefits and drawbacks of using ICMP for
 making the Quick-Start Request also applies to the RSVP case.  If the
 Quick-Start Request was transmitted in a separate packet instead of
 as an IP option, the transport protocol packet delivery would not be
 delayed due to IP option processing at the routers, and the initial
 transport packets would reach their destination more reliably.  The
 possible disadvantages of using ICMP and RSVP are also expected to be
 similar: middleboxes in the network may not be able to forward the
 Quick-Start Request messages, and the IP tunnels might cause problems
 for processing the Quick-Start Requests.

B.2. Alternate Encoding Functions

 In this section, we look at alternate encoding functions for the Rate
 Request field in the Quick-Start Request.  The main requirements for
 this function is that it should have a sufficiently wide range for
 the requested rate.  There is no need for overly fine-grained
 precision in the requested rate.  Similarly, while it would be
 attractive for the encoding function to be easily computable, it is
 also possible for end-nodes and routers to simply store the table
 giving the mapping between the value N in the Rate Request field, and
 the actual rate request f(N).  In this section, we consider possible
 encoding methods for Rate Request fields of different sizes,
 including four-bit, eight-bit, and larger Rate Request fields.
 Linear functions:
 One possible proposal would be for the Rate Request field to be
 formatted in bits per second, scaled so that one unit equals M Kbps,
 for some fixed value of M.  Thus, for the value N in the Rate Request
 field, the requested rate would be M*N Kbps.
 Powers of two:
 If a granularity of factors of two is sufficient for the Rate
 Request, then the encoding function with the most range would be for
 the requested rate to be K*2^N; for N, the value in the Rate Request
 field; and for K, some constant.  For N=0, the rate request would be
 set to zero, regardless of the encoding function.  For example, for
 K=40,000 and an eight-bit Rate Request field, the request range would
 be from 80 Kbps to 40*2^255 Kbps.  This clearly would be an
 unnecessarily large request range.

Floyd, et al. Experimental [Page 61] RFC 4782 Quick-Start for TCP and IP January 2007

 For a four-bit Rate Request field, the upper limit on the rate
 request is 1.3 Gbps.  It seems to us that an upper limit of 1.3 Gbps
 would be fine for the Quick-Start rate request, and that connections
 wishing to start up with a higher initial sending rate should be
 encouraged to use other mechanisms, such as the explicit reservation
 of bandwidth.  If an upper limit of 1.3 Gbps was not acceptable, then
 five or six bits could be used for the Rate Request field.
 The lower limit of 80 Kbps could be useful for flows with round-trip
 times of a second or more.  For a flow with a round-trip time of one
 second, as is typical in some wireless networks, the TCP initial
 window of 4380 bytes allowed by [RFC3390] (given appropriate packet
 sizes) would translate to an initial sending rate of 35 Kbps.  Thus,
 for TCP flows, a rate request of 80 Kbps could be useful for some
 flows with large round-trip times.
 The lower limit of 80 Kbps could also be useful for some non-TCP
 flows that send small packets, with at most one small packet every 10
 ms.  A rate request of 80 Kbps would translate to a rate of a hundred
 100-byte packets per second (including packet headers).  While some
 small-packet flows with large round-trip times might find a smaller
 rate request of 40 Kbps to be useful, our assumption is that a lower
 limit of 80 Kbps on the rate request will be generally sufficient.
 Again, if the lower limit of 80 kbps was not acceptable, then extra
 bits could be used for the Rate Request field.
 If the granularity of factors of two was too coarse, then the
 encoding function could use a base less than two.  An alternate form
 for the encoding function would be to use a hybrid of linear and
 exponential functions.
 A mantissa and exponent representation:
 Section 4.4 of [B05] suggests a mantissa and exponent representation
 for the Quick-Start encoding function.  With e and f as the binary
 numbers in the exponent and mantissa fields, and with 0 <= f < 1,
 this would represent the rate (1+f)*2^e.  [B05] suggests a mantissa
 field for f of 8, 16, or 24 bits, with an exponent field for e of 8
 bits.  This representation would allow larger rate requests, with an
 encoding that is less coarse than the powers-of-two encoding used in
 this document.
 Constraints of the transport protocol:
 We note that the Rate Request is also constrained by the abilities 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.

Floyd, et al. Experimental [Page 62] RFC 4782 Quick-Start for TCP and IP January 2007

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

 One of the design questions is whether the Rate Request field should
 be in bytes per second or in packets per second.  We discuss this
 separately from the perspective of the transport, and from the
 perspective of the router.
 For TCP, the results from the Quick-Start Request are translated into
 a congestion window in bytes, using the measured round-trip time and
 the MSS.  This window applies only to the bytes of data payload, and
 does not include the bytes in the TCP or IP packet headers.  Other
 transport protocols would conceivably use the Quick-Start Request
 directly in packets per second, or could translate the Quick-Start
 Request to a congestion window in packets.
 The assumption of this document is that the router only approves the
 Quick-Start Request when the output link is significantly
 underutilized.  For this, the router could measure the available
 bandwidth in bytes per second, or could convert between packets and
 bytes by some mechanism.
 If the Quick-Start Request was in bytes per second, and applied only
 to the data payload, then the router would have to convert from bytes
 per second of data payload, to bytes per second of packets on the
 wire.  If the Rate Request field was in bytes per second, and the
 sender ended up using very small packets, this could translate to a
 significantly larger number in terms of bytes per second on the wire.
 Therefore, for a Quick-Start Request in bytes per second, it makes
 most sense for this to include the transport and IP headers as well
 as the data payload.  Of course, this will be, at best, a rough
 approximation on the part of the sender; the transport-level sender
 might not know the size of the transport and IP headers in bytes, and
 might know nothing at all about the separate headers added in IP
 tunnels downstream.  This rough estimate seems sufficient, however,
 given the overall lack of fine precision in Quick-Start
 functionality.
 It has been suggested that the router could possibly use information
 from the MSS option in the TCP packet header of the SYN packet to
 convert the Quick-Start Request from packets per second to bytes per
 second, or vice versa.  This would be problematic for several
 reasons.  First, if IPsec is used, the TCP header will be encrypted.
 Second, the MSS option is defined as the maximum MSS that the TCP
 sender expects to receive, not the maximum MSS that the TCP sender
 plans to send [RFC793].  However, it is probably often the case that
 this MSS also applies as an upper bound on the MSS used by the TCP
 sender in sending.

Floyd, et al. Experimental [Page 63] RFC 4782 Quick-Start for TCP and IP January 2007

 We note that the sender does not necessarily know the Path MTU when
 the Quick-Start Request is sent, or when the initial window of data
 is sent.  Thus, with IPv4, packets from the initial window could end
 up being fragmented in the network if the "Don't Fragment" (DF) bit
 is not set [RFC1191].  A Rate Request in bytes per second is
 reasonably robust to fragmentation.  Clearly, a Rate Request in
 packets per second is less robust in the presence of fragmentation.
 Interactions between larger initial windows and Path MTU Discovery
 are discussed in more detail in RFC 3390 [RFC3390].
 For a Quick-Start Request in bytes per second, the transport senders
 would have the additional complication of estimating the bandwidth
 usage added by the packet headers.
 We have chosen a Rate Request field in bytes per second rather than
 in packets per second because it seems somewhat more robust,
 particularly to routers.

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

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

Floyd, et al. Experimental [Page 64] RFC 4782 Quick-Start for TCP and IP January 2007

 fairness is automatic; the router is not granting rate requests for
 *additional* bandwidth without knowing the current sending rates of
 the different flows.
 The Additional Rate semantics also lends itself to gaming by the
 connection, with senders sending frequent Quick-Start Requests in the
 hope of gaining a higher rate.  If the router is granting the same
 maximum rate for all rate requests, then there is little benefit to a
 connection of sending a rate request over and over again.  However,
 if the router is granting an *additional* rate with each rate
 request, over and above the current sending rate, then it is in a
 connection's interest to send as many rate requests as possible, even
 if very few of them are, in fact, granted.
 Appendix E discusses a Report of Current Sending Rate as one possible
 function in the Quick-Start Option.  However, we have not
 standardized this possible use at this time.

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

 Section 4.6 discusses TCP's response to the loss of a Quick-Start
 packet in the initial window.  This section discusses several
 alternate responses.
 One possible alternative to reverting to the default Slow-Start after
 the loss of a Quick-Start packet from the initial window would have
 been to halve the congestion window and continue in congestion
 avoidance.  However, we note that this would not have been a
 desirable response for either the connection or for the network as a
 whole.  The packet loss in the initial window indicates that Quick-
 Start failed in finding an appropriate congestion window, meaning
 that the congestion window after halving may easily also be wrong.
 A more moderate alternate would be to continue in congestion
 avoidance from a window of (W-D)/2, where W is the Quick-Start
 congestion window, and D is the number of packets dropped or marked
 from that window.  However, such an approach would implicitly assume
 that the number of Quick-Start packets delivered is a good indication
 of the appropriate available bandwidth for that flow, even though
 other packets from that window were dropped in the network.  And,
 further, that using half the number of segments that were
 successfully transmitted is conservative enough to account for the
 possibly inaccurate congestion window indication.  We believe that
 such an assumption would require more analysis at this point,
 particularly in a network with a range of packet dropping mechanisms
 at the router, and we cannot recommend it at this time.

Floyd, et al. Experimental [Page 65] RFC 4782 Quick-Start for TCP and IP January 2007

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

B.6. Why Not Include More Functionality?

 This proposal for Quick-Start is a rather coarse-grained mechanism
 that would allow a connection to use a higher sending rate along
 underutilized paths, but that does not attempt to provide a next-
 generation transport protocol or congestion control mechanism, and
 does not attempt the goal of providing very high throughput with very
 low delay.  Appendix A.4 discusses a number of proposals (such as
 XCP, MaxNet, and AntiECN) that provide more fine-grained per-packet
 feedback from routers than the current congestion control mechanisms
 and that attempt these more ambitious goals.
 Compared to proposals such as XCP and AntiECN, Quick-Start offers
 much less control.  Quick-Start does not attempt to provide a new
 congestion control mechanism, but simply to get permission from
 routers for a higher sending rate at start-up, or after an idle
 period.  Quick-Start can be thought of as an "anti-congestion-
 control" mechanism that is only of any use when all the routers along
 the path are significantly underutilized.  Thus, Quick-Start is of no
 use towards a target of high link utilization, or fairness in a
 high-utilization scenario, or controlling queueing delay during high
 utilization, or anything of the like.
 At the same time, Quick-Start would allow larger initial windows than
 would proposals such as AntiECN, requires less input to routers than
 XCP (e.g., XCP's cwnd and RTT fields), and would require less
 frequent feedback from routers than any new congestion control
 mechanism.  Thus, Quick-Start is significantly less powerful than
 proposals for new congestion control mechanisms, such as XCP and
 AntiECN, but as powerful or more powerful in terms of the specific
 issue of allowing larger initial windows.  Also, (we think) it is
 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

Floyd, et al. Experimental [Page 66] RFC 4782 Quick-Start for TCP and IP January 2007

 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 start-ups even
 in environments with rather high link utilization.  Is this a
 pressing requirement?  Are the other benefits of more fine-grained
 congestion control feedback from routers a pressing requirement?
 We would argue that even if more fine-grained per-packet feedback
 from routers was implemented, it is reasonable to have a separate
 mechanism, such as Quick-Start, for indicating an allowed initial
 sending rate, or an allowed total sending rate after an idle or
 underutilized period.
 One difference between Quick-Start and current proposals for fine-
 grained per-packet feedback, such as XCP, is that XCP is designed to
 give robust performance even in the case where different packets
 within a connection routinely follow different paths.  XCP achieves
 relatively robust performance in the presence of multipath routing by
 using per-packet feedback, where the feedback carried in a single
 packet is about the relative increase or decrease in the rate or
 window to take effect when that particular packet is acknowledged,
 not about the allowed sending rate for the connection as a whole.
 In contrast, Quick-Start sends a single Quick-Start Request, and the
 answer to that request gives the allowed sending rate for an entire
 window of data.  As a result, Quick-Start could be problematic in an
 environment where some fraction of the packets in a window of data
 take path A, and the rest of the packets take path B; for example,
 the Quick-Start Request could have traveled on path A, while half the
 Quick-Start packets sent in the succeeding round-trip time are routed
 on path B.  We note that [ZDPS01] shows Internet paths to be stable
 on the order of RTTs.
 There are also differences between Quick-Start and some of the
 proposals for per-packet feedback in terms of the number of bits of
 feedback required from the routers to the end-nodes.  Quick-Start
 uses four bits of feedback in the rate request field to indicate the
 allowed sending rate.  XCP allocates a byte for per-packet feedback,
 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

Floyd, et al. Experimental [Page 67] RFC 4782 Quick-Start for TCP and IP January 2007

 feedback; it seems likely that determining the initial sending rate
 fundamentally requires more bits of feedback from routers than does
 the steady-state, per-packet feedback to increase or decrease the
 sending rate.
 On a more practical level, one difference between Quick-Start and
 proposals for per-packet feedback is that there are fewer open issues
 with Quick-Start than there would be with a new congestion control
 mechanism.  Because Quick-Start is a mechanism for requesting an
 initial sending rate in an underutilized environment, its fairness
 issues are less severe than those of a general congestion control
 mechanism.  With Quick-Start, there is no need for the end nodes to
 tell the routers the round-trip time and congestion window, as is
 done in XCP; all that is needed is for the end nodes to report the
 requested sending rate.
 Table 3 provides a summary of the differences between Quick-Start and
 proposals for per-packet congestion control feedback.
                                             Proposals for
                       Quick-Start           Per-Packet Feedback
 +------------------+----------------------+----------------------+
  Semantics:        | Allowed sending rate | Change in rate/window,
                    |  per connection.     |  per-packet.
 +------------------+----------------------+----------------------+
  Relationship to   | In addition.         | Replacement.
  congestion ctrl:  |                      |
 +------------------+----------------------+----------------------+
  Frequency:        | Start-up, or after   | Every packet.
                    |  an idle period.     |
 +------------------+----------------------+----------------------+
  Limitations:      | Only useful on       | General congestion
                    |  underutilized paths.|  control mechanism.
 +------------------+----------------------+----------------------+
  Input to routers: | Rate request.        |RTT, cwnd, request (XCP)
                    |                      | None (Anti-ECN).
 +------------------+----------------------+----------------------+
  Bits of feedback: | Four bits for        | A few bits would
                    |   rate request.      |  suffice?
 +------------------+----------------------+----------------------+
      Table 3: Differences between Quick-Start and Proposals for
                   Fine-Grained Per-Packet Feedback.
 A separate question concerns whether mechanisms, such as Quick-Start,
 in combination with HighSpeed TCP and other changes in progress,
 would make a significant contribution towards meeting some of these
 needs for new congestion control mechanisms.  This could be viewed as

Floyd, et al. Experimental [Page 68] RFC 4782 Quick-Start for TCP and IP January 2007

 a positive step towards meeting some of the more pressing current
 needs with a simple and reasonably deployable mechanism, or
 alternately, as a negative step of unnecessarily delaying more
 fundamental changes.  Without answering this question, we would note
 that our own approach tends to favor the incremental deployment of
 relatively simple mechanisms, as long as the simple mechanisms are
 not short-term hacks, but mechanisms that lead the overall
 architecture in the fundamentally correct direction.

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

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

 An alternate proposal for the Quick-Start Nonce from [B05] would be
 for an n-bit field for the QS Nonce, with the sender generating a
 random nonce when it generates a Quick-Start Request.  Each router
 that reduces the Rate Request by r would hash the QS nonce r times,
 using a one-way hash function such as MD5 [RFC1321] or the secure
 hash 1 [SHA1].  The receiver returns the QS nonce to the sender.
 Because the sender knows the original value for the nonce, and the
 original rate request, the sender knows the total number of steps s
 that the rate has been reduced.  The sender then hashes the original
 nonce s times to check whether the result is the same as the nonce
 returned by the receiver.
 This alternate proposal for the nonce would be considerably more
 powerful than the QS nonce described in Section 3.4, but it would
 also require more CPU cycles from the routers when they reduce a
 Quick-Start Request, and from the sender in verifying the nonce
 returned by the receiver.  As reported in [B05], routers could
 protect themselves from processor exhaustion attacks by limiting the
 rate at which they will approve reductions of Quick-Start Requests.
 Both the Function field and the Reserved field in the Quick-Start
 Option would allow the extension of Quick-Start to use Quick-Start
 requests with the alternate proposal for the Quick-Start Nonce, if it
 was ever desired.

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

 An earlier version of this document included a Request-Approved
 Quick-Start Nonce (QS Nonce) that was initialized by the sender to a
 non-zero, `random' eight-bit number, along with a QS TTL that was
 initialized to the same value as the TTL in the IP header.  The
 Request-Approved Quick-Start Nonce would have been returned by the
 transport receiver to the transport sender in the Quick-Start
 Response.  A router could deny the Quick-Start Request by failing to
 decrement the QS TTL field, by zeroing the QS Nonce field, or by

Floyd, et al. Experimental [Page 69] RFC 4782 Quick-Start for TCP and IP January 2007

 deleting the Quick-Start Request from the packet header.  The QS
 Nonce was included to provide some protection against broken
 downstream routers, or against misbehaving TCP receivers that might
 be inclined to lie about whether the Rate Request was approved.  This
 protection is now provided by the QS Nonce, by the use of a random
 initial value for the QS TTL field, and by Quick-Start-capable
 routers hopefully either deleting the Quick-Start Option or zeroing
 the QS TTL and QS Nonce fields when they deny a request.
 With the old Request-Approved Quick-Start Nonce, along with the QS
 TTL field set to the same value as the TTL field in the IP header,
 the Quick-Start Request mechanism would have been self-terminating;
 the Quick-Start Request would terminate at the first participating
 router after a non-participating router had been encountered on the
 path.  This minimizes unnecessary overhead incurred by routers
 because of option processing for the Quick-Start Request.  In the
 current specification, this "self-terminating" property is provided
 by Quick-Start-capable routers hopefully either deleting the Quick-
 Start Option or zeroing the Rate Request field when they deny a
 request.  Because the current specification uses a random initial
 value for the QS TTL, Quick-Start-capable routers can't tell if the
 Quick-Start Request is invalid because of non-Quick-Start-capable
 routers upstream.  This is the cost of using a design that makes it
 difficult for the receiver to cheat about the value of the QS TTL.

Appendix C. Quick-Start with DCCP

 DCCP is a new transport protocol for congestion-controlled,
 unreliable datagrams, intended for applications such as streaming
 media, Internet telephony, and online 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 TCP Friendly Rate
 Control (TFRC), an equation-based form of congestion control.  We
 refer the reader to [RFC4340] for a more detailed description of DCCP
 and 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:

Floyd, et al. Experimental [Page 70] RFC 4782 Quick-Start for TCP and IP January 2007

 (1) How does the sender respond if a Quick-Start packet is dropped?
     As in TCP, if an initial Quick-Start packet is dropped, the CCID
     3 sender should revert to the congestion control mechanisms it
     would have used if the Quick-Start Request had not been approved.
 (2) When does the sender decide there has been no feedback from the
     receiver?
     Unlike TCP, CCID 3 does not use acknowledgements for every
     packet, or for every other packet.  In contrast, the CCID 3
     receiver sends feedback to the sender roughly once per round-trip
     time.  In CCID 3, the allowed sending rate is halved if no
     feedback is received from the receiver in at least four round-
     trip times (when the sender is sending at least one packet every
     two round-trip times).  When a Quick-Start Request is used, it
     would seem necessary to use a smaller time interval, e.g., to
     reduce the sending rate if no feedback arrives from the receiver
     in at least two round-trip times.
 The question also arises of how the sending rate should be reduced
 after a period of no feedback from the receiver.  As with TCP, the
 default CCID 3 response of halving the sending rate is not
 necessarily a sufficient response to the absence of feedback; an
 alternative is to reduce the sending rate to the sending rate that
 would have been used if no Quick-Start Request had been approved.
 That is, if a CCID 3 sender uses a Quick-Start Request, special rules
 might be required to handle the sender's response to a period of no
 feedback from the receiver regarding the Quick-Start packets.
 Similarly, in considering the use of Quick-Start with CCID 3 for
 requesting a higher sending rate after an idle period, the following
 questions arise:
 (1) What rate does the sender request?
     As in TCP, there is a straightforward answer to the rate request
     that the CCID 3 sender should use in requesting a higher sending
     rate after an idle period.  The sender knows the current loss
     event rate, either from its own calculations or from feedback
     from the receiver, and can determine the sending rate allowed by
     that loss event rate.  This is the upper bound on the sending
     rate that should be requested by the CCID 3 sender.  A Quick-
     Start Request is useful with CCID 3 when the sender is coming out
     of an idle or underutilized period, because in standard
     operation, CCID 3 does not allow the sender to send more than
     twice as fast as the receiver has reported received in the most
     recent feedback message.

Floyd, et al. Experimental [Page 71] RFC 4782 Quick-Start for TCP and IP January 2007

 (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.

Appendix D. Possible Router Algorithm

 This specification does not tightly define the algorithm a router
 uses when deciding whether to approve a Quick-Start Rate Request or
 whether and how to reduce a Rate Request.  A range of algorithms is
 likely useful in this space and we consider the algorithm a
 particular router uses to be a local policy decision.  In addition,
 we believe that additional experimentation with router algorithms is
 necessary to have a solid understanding of the dynamics various
 algorithms impose.  However, we provide one particular algorithm in
 this appendix as an example and as a framework for thinking about
 additional mechanisms.
 [SAF06] provides several algorithms routers can use to consider
 incoming Rate Requests.  The decision process involves two
 algorithms.  First, the router needs to track the link utilization
 over the recent past.  Second, this utilization needs to be updated
 by the potential new bandwidth from recent Quick-Start approvals, and
 then compared with the router's notion of when it is underutilized
 enough to approve Quick-Start Requests (of some size).
 First, we define the "peak utilization" estimation technique (from
 [SAF06]).  This mechanism records the utilization of the link every S
 seconds and stores the most recent N of these measurements.  The
 utilization is then taken as the highest utilization of the N
 samples.  This method, therefore, keeps N*S seconds of history.  This
 algorithm reacts rapidly to increases in the link utilization.  In
 [SAF06], S is set to 0.15 seconds, and experiments use values for N
 ranging from 3 to 20.
 Second, we define the "target" algorithm for processing incoming
 Quick-Start Rate Requests (also from [SAF06]).  The algorithm relies

Floyd, et al. Experimental [Page 72] RFC 4782 Quick-Start for TCP and IP January 2007

 on knowing the bandwidth of the outgoing link (which, in many cases,
 can be easily configured), the utilization of the outgoing link (from
 an estimation technique such as given above), and an estimate of the
 potential bandwidth from recent Quick-Start approvals.
 Tracking the potential bandwidth from recent Quick-Start approvals is
 another case where local policy dictates how it should be done.  The
 simplest method, outlined in Section 8.2, is for the router to keep
 track of the aggregate Quick-Start rate requests approved in the most
 recent two or more time intervals, including the current time
 interval, and to use the sum of the aggregate rate requests over
 these time intervals as the estimate of the approved Rate Requests.
 The experiments in [SAF06] keep track of the aggregate approved Rate
 Requests over the most recent two time intervals, for intervals of
 150 msec.
 The target algorithm also depends on a threshold (qs_thresh) that is
 the fraction of the outgoing link bandwidth that represents the
 router's notion of "significantly underutilized".  If the
 utilization, augmented by the potential bandwidth from recent Quick-
 Start approvals, is above this threshold, then no Quick-Start Rate
 Requests will be approved.  If the utilization, again augmented by
 the potential bandwidth from recent Quick-Start approvals, is less
 than the threshold, then Rate Requests can be approved.  The Rate
 Requests will be reduced such that the bandwidth allocated would not
 drive the utilization to more than the given threshold.  The
 algorithm is:
   util_bw = bandwidth * utilization;
   util_bw = util_bw + recent_qs_approvals;
   if (util_bw < (qs_thresh * bandwidth))
   {
       approved = (qs_thresh * bandwidth) - util_bw;
       if (rate_request < approved)
           approved = rate_request;
       approved = round_down (approved);
       recent_qs_approvals += approved;
   }
 Also note that, given that Rate Requests are fairly coarse, the
 approved rate should be rounded down when it does not fall exactly on
 one of the rates allowed by the encoding scheme.
 Routers that wish to keep close track of the allocated Quick-Start
 bandwidth could use Approved Rate reports to learn when rate requests
 had been decremented downstream in the network, and also to learn
 when a sender begins to use the approved Quick-Start Request.

Floyd, et al. Experimental [Page 73] RFC 4782 Quick-Start for TCP and IP January 2007

Appendix E. Possible Additional Uses for the Quick-Start Option

 The Quick-Start Option contains a four-bit Function field in the
 third byte, enabling additional uses to be defined for the Quick-
 Start Option.  In this section, we discuss some of the possible
 additional uses that have been discussed for Quick-Start.  The
 Function field makes it easy to add new functions for the Quick-
 Start Option.
 Report of Current Sending Rate: A Quick-Start Request with the
 `Report of Current Sending Rate' codepoint set in the Function field
 would be using the Rate Request field to report the current estimated
 sending rate for that connection.  This could accompany a second
 Quick-Start Request in the same packet containing a standard rate
 request, for a connection that is using Quick-Start to increase its
 current sending rate.
 Request to Increase Sending Rate: A codepoint for `Request to
 Increase Sending Rate' in the Function field would indicate that the
 connection is not idle or just starting up, but is attempting to use
 Quick-Start to increase its current sending rate.  This codepoint
 would not change the semantics of the Rate Request field.
 RTT Estimate: If a codepoint for `RTT Estimate' was used, a field for
 the RTT Estimate would contain one or more bits giving the sender's
 rough estimate of the round-trip time, if known.  E.g., the sender
 could estimate whether the RTT was greater or less than 200 ms.
 Alternately, if the sender had an estimate of the RTT when it sends
 the Rate Request, the two-bit Reserved field at the end of the
 Quick-Start Option could be used for a coarse-grained encoding of the
 RTT.
 Informational Request: An Informational Request codepoint in the
 Function field would indicate that a request is purely informational,
 for the sender to find out if a rate request would be approved, and
 what size rate request would be approved when the Informational
 Request is sent.  For example, an Informational Request could be
 followed one round-trip time later by a standard Quick-Start Request.
 A router approving an Informational Request would not consider this
 as an approval for Quick-Start bandwidth to be used, and would not be
 under any obligation to approve a similar standard Quick-Start
 Request one round-trip time later.  An Informational Request with a
 rate request of zero could be used by the sender to find out if all
 of the routers along the path supported Quick-Start.
 Use Format X for the Rate Request Field: A Quick-Start codepoint for
 `Use Format X for the Rate Request Field' would indicate that an
 alternate format was being used for the Rate Request field.

Floyd, et al. Experimental [Page 74] RFC 4782 Quick-Start for TCP and IP January 2007

 Do Not Decrement: A Do Not Decrement codepoint could be used for a
 Quick-Start Request where the sender would rather have the request
 denied than to have the rate request decremented in the network.
 This could be used if the sender was only interested in using Quick-
 Start if the original rate request was approved.
 Temporary Bandwidth Use: A Temporary codepoint has been proposed to
 indicate that a connection would only use the requested bandwidth for
 a single time interval.

Normative References

 [RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
           793, September 1981.
 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
           November 1990.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
           (IPv6) Specification", RFC 2460, December 1998.
 [RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
           Control", RFC 2581, April 1999.
 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of
           Explicit Congestion Notification (ECN) to IP", RFC 3168,
           September 2001.
 [RFC3390] Allman, M., Floyd, S., 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, March 2004.

Informative References

 [RFC792]  Postel, J., "Internet Control Message Protocol", STD 5, RFC
           792, September 1981.
 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
           April 1992.
 [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
           RFC 1812, June 1995.

Floyd, et al. Experimental [Page 75] RFC 4782 Quick-Start for TCP and IP January 2007

 [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
           October 1996.
 [RFC2113] Katz, D., "IP Router Alert Option", RFC 2113, February
           1997.
 [RFC2140] Touch, J., "TCP Control Block Interdependence", RFC 2140,
           April 1997.
 [RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S., and S.
           Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
           Functional Specification", RFC 2205, September 1997.
 [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
           (IKE)", RFC 2409, November 1998.
 [RFC2415] Poduri, K. and K. Nichols, "Simulation Studies of Increased
           Initial TCP Window Size", RFC 2415, September 1998.
 [RFC2488] Allman, M., Glover, D., and L. Sanchez, "Enhancing TCP Over
           Satellite Channels using Standard Mechanisms", BCP 28, RFC
           2488, January 1999.
 [RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G.,
           and B. Palter, "Layer Two Tunneling Protocol 'L2TP'", RFC
           2661, August 1999.
 [RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,
           "Generic Routing Encapsulation (GRE)", RFC 2784, March
           2000.
 [RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
           Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang,
           L., and V. Paxson, "Stream Control Transmission Protocol",
           RFC 2960, October 2000.
 [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
           Label Switching Architecture", RFC 3031, January 2001.
 [RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion Manager",
           RFC 3124, June 2001.
 [RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
           Issues", RFC 3234, February 2002.
 [RFC3344] Perkins, C., Ed., "IP Mobility Support for IPv4", RFC 3344,
           August 2002.

Floyd, et al. Experimental [Page 76] RFC 4782 Quick-Start for TCP and IP January 2007

 [RFC3360] Floyd, S., "Inappropriate TCP Resets Considered Harmful",
           BCP 60, RFC 3360, August 2002.
 [RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
           RFC 3649, December 2003.
 [RFC3662] Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort
           Per-Domain Behavior (PDB) for Differentiated Services", RFC
           3662, December 2003.
 [RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering,
           "IPv6 Flow Label Specification", RFC 3697, March 2004.
 [RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
           in IPv6", RFC 3775, June 2004.
 [RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig,
           R., Mahdavi, J., Montenegro, G., Touch, J., and L. Wood,
           "Advice for Internet Subnetwork Designers", BCP 89, RFC
           3819, July 2004.
 [RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
           Stenberg, "UDP Encapsulation of IPsec ESP Packets", RFC
           3948, January 2005.
 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
           Internet Protocol", RFC 4301, December 2005.
 [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December
           2005.
 [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
           4306, December 2005.
 [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion
           Control Protocol (DCCP)", RFC 4340, March 2006.
 [RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
           Control Protocol (DCCP) Congestion Control ID 2: TCP-like
           Congestion Control", RFC 4341, March 2006.
 [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
           Message Protocol (ICMPv6) for the Internet Protocol Version
           6 (IPv6) Specification", RFC 4443, March 2006.
 [AHO98]   M. Allman, C. Hayes and S. Ostermann. An evaluation of TCP
           with Larger Initial Windows. ACM Computer Communication
           Review, July 1998.

Floyd, et al. Experimental [Page 77] RFC 4782 Quick-Start for TCP and IP January 2007

 [B05]     Briscoe, B., "Review: Quick-Start for TCP and IP",
           <http://www.cs.ucl.ac.uk/staff/B.Briscoe/pubs.html>,
           November 2005.
 [BW97]    G. Brasche and B. Walke. Concepts, Services and Protocols
           of the new GSM Phase 2+ General Packet Radio Service. IEEE
           Communications Magazine, pages 94--104, August 1997.
 [GPAR02]  A. Gurtov, M. Passoja, O. Aalto, and M. Raitola. Multi-
           Layer Protocol Tracing in a GPRS Network. In Proceedings of
           the IEEE Vehicular Technology Conference (Fall VTC2002),
           Vancouver, Canada, September 2002.
 [H05]     P. Hoffman, email, October 2005.  Citation for
           acknowledgement purposes only.
 [HKP01]   M. Handley, C. Kreibich and V. Paxson, Network Intrusion
           Detection: Evasion, Traffic Normalization, and End-to-End
           Protocol Semantics, Proc. USENIX Security Symposium 2001.
 [Jac88]   V. Jacobson, Congestion Avoidance and Control, ACM SIGCOMM.
 [JD02]    Manish Jain, Constantinos Dovrolis, End-to-End Available
           Bandwidth: Measurement Methodology, Dynamics, and Relation
           with TCP Throughput, SIGCOMM 2002.
 [K03]     S. Kunniyur, "AntiECN Marking: A Marking Scheme for High
           Bandwidth Delay Connections", Proceedings, IEEE ICC '03,
           May 2003.  <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.
           <http://www.icir.org/mallman/papers/>.
 [KHR02]   Dina Katabi, Mark Handley, and Charles Rohrs, Internet
           Congestion Control for Future High Bandwidth-Delay Product
           Environments. ACM Sigcomm 2002, August 2002.
           <http://ana.lcs.mit.edu/dina/XCP/>.
 [KK03]    A. Kuzmanovic and E. W. Knightly.  TCP-LP: A Distributed
           Algorithm for Low Priority Data Transfer.  INFOCOM 2003,
           April 2003.

Floyd, et al. Experimental [Page 78] RFC 4782 Quick-Start for TCP and IP January 2007

 [KSEPA04] Rajesh Krishnan, James Sterbenz, Wesley Eddy, Craig
           Partridge, Mark Allman. Explicit Transport Error
           Notification (ETEN) for Error-Prone Wireless and Satellite
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 [L05]     Guohan Lu, Nonce in TCP Quick Start, September 2005.
           <http://www.net-glyph.org/~lgh/nonce-usage.pdf>.
 [MH06]    Mathis, M. and J. Heffner, "Packetization Layer Path MTU
           Discovery", Work in Progress, December 2006.
 [MAF04]   Alberto Medina, Mark Allman, and Sally Floyd, Measuring
           Interactions Between Transport Protocols and Middleboxes,
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 [MAF05]   Alberto Medina, Mark Allman, and Sally Floyd.  Measuring
           the Evolution of Transport Protocols in the Internet.
           Computer Communications Review, April 2005.
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           <http://netlab.caltech.edu/~bartek/maxnet.htm>.
 [P00]     Joon-Sang Park, Bandwidth Discovery of a TCP Connection,
           report to John Heidemann, 2000, private communication.
           Citation for acknowledgement purposes only.
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           Paxson, Brian Tierney.  A First Look at Modern Enterprise
           Traffic.  ACM SIGCOMM/USENIX Internet Measurement
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           Krishnan, James P.G. Sterbenz. A Swifter Start for TCP.
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 [RW04]    Mattia Rossi and Michael Welzl, On the Impact of IP Option
           Processing -   Part 2, Preprint-Reihe des Fachbereichs
           Mathematik - Informatik, No. 26, Institute of Computer
           Science, University of Innsbruck, Austria, July 2004.

Floyd, et al. Experimental [Page 79] RFC 4782 Quick-Start for TCP and IP January 2007

 [S02]     Ion Stoica, private communication, 2002.  Citation for
           acknowledgement purposes only.
 [SAF06]   Pasi Sarolahti, Mark Allman, and Sally Floyd.  Determining
           an Appropriate Sending Rate Over an Underutilized Network
           Path.  February 2006.
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           network bandwidth to improve TCP performance", ACM SIGCOMM
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           Washington, D.C., publication 180-1, April 1995.
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           Mechanism for Background Transfers.  OSDI 2002.
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           Idle TCP Connections, Technical Report 97-661, University
           of Southern California, November 1997.
 [W00]     Michael Welzl: PTP: Better Feedback for Adaptive
           Distributed Multimedia Applications on the Internet, IPCCC
           2000 (19th IEEE International Performance, Computing, And
           Communications Conference), Phoenix, Arizona, USA, 20-22
           February 2000.
           <http://www.welzl.at/research/publications/>.
 [ZDPS01]  Y. Zhang, N. Duffield, V. Paxson, and S. Shenker,  On the
           Constancy of Internet Path Properties, Proc. ACM SIGCOMM
           Internet Measurement Workshop, November 2001.
 [ZQK00]   Y. Zhang, L. Qiu, and S. Keshav, Speeding Up Short Data
           Transfers: Theory, Architectural Support, and Simulation
           Results, NOSSDAV 2000, June 2000.

Floyd, et al. Experimental [Page 80] RFC 4782 Quick-Start for TCP and IP January 2007

Authors' Addresses

 Sally Floyd
 Phone: +1 (510) 666-2989
 ICIR (ICSI Center for Internet Research)
 EMail: floyd@icir.org
 URL: http://www.icir.org/floyd/
 Mark Allman
 ICSI Center for Internet Research
 1947 Center Street, Suite 600
 Berkeley, CA 94704-1198
 Phone: (440) 235-1792
 EMail: mallman@icir.org
 URL: http://www.icir.org/mallman/
 Amit Jain
 F5 Networks
 EMail: a.jain@f5.com
 Pasi Sarolahti
 Nokia Research Center
 P.O. Box 407
 FI-00045 NOKIA GROUP
 Finland
 Phone: +358 50 4876607
 EMail: pasi.sarolahti@iki.fi

Floyd, et al. Experimental [Page 81] RFC 4782 Quick-Start for TCP and IP January 2007

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Floyd, et al. Experimental [Page 82]

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