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Internet Engineering Task Force                      Amit K. Jain
INTERNET DRAFT                                     Array Networks
draft-amit-quick-start-02.txt                         Sally Floyd
                                                             ICIR
                                                    October, 2002



                       Quick-Start for TCP and IP



                          Status of this Memo


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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

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

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

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

Abstract

   This draft outlines an optional Quick-Start mechanism for transport
   protocols to determine an optional allowed initial congestion window
   or initial sending rate at the start of a data transfer.  This
   mechanism is designed to be used by a range of transport protocols;
   however, in this document we only describe its use with TCP and IPv4.
   By using Quick-Start, a TCP host, say, host A, would indicate its
   desired initial sending rate in packets per second in a Quick Start
   Request option in the IP header of the initial TCP SYN or SYN/ACK
   packet. Each router in turn could either approve the specified
   initial rate, reduce the specified initial rate, or indicate that
   nothing above the default initial rate for that protocol would be
   allowed.  The Quick-Start mechanism also can determine if there are
   routers along the path that do not understand the Quick Start Request



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   option, or have not agreed to the initial rate described in the
   option.  TCP host B communicates the final initial rate to TCP host A
   in a transport-level Quick-Start Response in the answering SYN/ACK or
   ACK packet.  Quick-Start is designed to allow TCP connections to use
   high initial windows in circumstances when there is significant
   unused bandwidth along the path, and all of the routers along the
   path support the Quick-Start Request.  This proposal is a request for
   feedback from the community.

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

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

1. Introduction

   The life of a TCP connection begins with a question, that is, "With
   what rate I can transfer the data?" The question is not answered
   explicitly for TCP, but each TCP connection figures out the answer
   for itself. This is done by each connection starting with an initial
   congestion window (called cwnd) of from one to four MSS-sized
   segments, for an MSS the Maximum Segment Size in bytes. Currently,
   the TCP protocol allows an initial window of three or four segments
   [RFC3390]. The TCP connection then probes the network for available
   bandwidth using the slow-start procedure, essentially doubling its
   congestion window each round-trip time.

   The probing mechanism of slow-start is time-consuming and causes an
   overhead in terms of queueing delay.  It may take a number of round-
   trip times in slow-start before the TCP connection begins to fully
   use the available bandwidth of the network; it takes log N round-trip
   times to build up to a congestion window of N 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 web transfers the connection might
   carry out its entire transfer in the slow-start phase.  In an
   underutilized, high-bandwidth network, such a transfer can end up
   taking log N round-trip times to transfer the data, where one or two
   round-trip times might have sufficed.



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   A fair amount of work has already been done to address the issue of
   choosing the initial window for TCP, with RFC 3390 allowing an
   initial window of up to four segments [RFC3390].  Our underlying
   premise is that explicit feedback from all of the routers along the
   path would be required for best-effort connections to use initial
   windows higher than four segments.

   The Congestion Manager proposes sharing congestion information among
   multiple TCP connections with the same endpoints [RFC2140].  With the
   Congestion Manager, a new TCP connection could start with a high
   initial window if it was sharing the path with a pre-existing TCP
   connection, with a high congestion window, to the same destination.
   It is also possible that a newly-starting TCP connection could make
   use of congestion information from a recently-terminated TCP
   connection to the same destination.  However, neither of these
   approaches are of any use for a connection to a new destination when
   there is no existing or recent connection to that destination.

   Active Queue Management and Explicit Congestion Notification (ECN)
   are both based on the router detecting congestion before buffer
   overflow [RFC3168].  In a similar but somewhat simpler fashion,
   Quick-Start is based on the ability of the router to determine
   whether or not the output link is significantly underutilized.

2. Assumptions, General Principles, and Open Questions

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

   Assumptions:

   * A router can determine reasonably easily if the output link has
   been significantly underutilized over a period of time.

   * 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 initial window or initial
   sending rate would have to be used independently for each direction.

   * Any new mechanism must be incrementally deployed, and may not be
   supported by all of the routers and/or end-hosts.  Thus, any new
   mechanism must be able to accommodate non-supporting routers or end-
   hosts without disturbing the current Internet semantics.

   * After the initial SYN exchange, a TCP data sender would be able to
   translate an initial sending rate in packets per second into an
   initial congestion window of MSS-sized segments.




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   General Principles:

   * In order for best-effort connections to use initial windows higher
   than four segments, explicit feedback from all of the routers along
   the path would be required.

   * A router should only allow an initial sending rate higher than the
   transport protocol's default initial rate if the router is
   significantly 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 is kept at the router for this mechanism.  In
   Quick-Start, the only state kept at the router is the aggregate
   initial sending rate authorized over the most recent interval of time
   (e.g., quarter of a second).

   There are also a number of open questions regarding the Quick-Start
   mechanism outlined in this draft.

   Open Questions:

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

   One drawback of the Quick-Start mechanism is that the SYN and SYN/ACK
   packets containing the Quick-Start option would presumeably be
   processed in the slow path in routers.  This would reduce the
   capacity of routers to handle Quick-Start requests, and delay the
   initial SYN exchange for the connection.

   Another drawback is that Quick-Start would require functionality in
   the router to estimate the current link utilization, and to keep
   track of the aggregate Quick-Start rate authorized over the last
   interval of time.  Adding new functionality to routers should not be
   done lightly, and any mechanisms that would require new functionality
   in routers would have to be carefully considered.

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

   * If the answer to the question above is yes, should we be
   considering mechanisms that, while more complex, are also
   sufficiently more powerful than Quick-Start?

   There are a number of such mechanisms that have been proposed in the



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   literature for more fine-grained congestion-related feedback from
   routers [KHR02].  Quick-Start is extremely coarse-grained, in that in
   its current form it only applies to the initial window of the
   connection.  Quick-Start also focuses in on the specific issue of
   allowing very high initial sending rates for connections over
   underutilized, high-bandwidth paths.  Quick-Start might first be
   deployed, for example, in an overprovisioned high-bandwidth Intranet,
   to allow much quicker transfers of best-effort traffic.

3. The Quick-Start Request in IPv4.

   Quick-Start would require end-points and routers to work together,
   with end-points requesting a higher initial sending rate in the
   Quick-Start Request (QSR) option in IP, and routers along the path
   approving, modifying, or discarding or ignoring (and therefore
   disallowing) the Quick-Start Request.  The receiver would use
   transport-level mechanisms to inform the sender of the status of the
   Quick-Start Request.  We note that the Quick-Start Request implicitly
   assumes a unicast transport protocol; we have not considered the use
   of the Quick-Start Request for multicast traffic.

3.1. The Quick-Start Request Option for IPv4

   The Quick-Start Request for IPv4 would be defined as follows:

           0          1          2          3
      +----------+----------+----------+----------+
      | Option   | Length=4 |  QS TTL  | Initial  |
      |          |          |          | Rate     |
      +----------+----------+----------+----------+

      Figure 1.  The Quick-Start Request Option for IPv4.

   The first byte contains the option field, which includes the one-bit
   copy flag, the 2-bit class field, and the 5-bit option number.

   The second byte contains the length field, which indicates an option
   length of four bytes.

   The third byte contains the Quick-Start TTL (QS TTL) field.  If the
   sender decides to use Quick-Start, then the sender sets the QS TTL
   field to a random value.  Routers that approve the Quick-Start
   Request decrement the QS TTL (mod 256).  The QS TTL is used by the
   sender to detect if all of the routers along the path understood and
   approved the Quick-Start option.

   The TCP sender also calculates and remembers the TTL Diff, the
   difference between the TTL value and the QS TTL value in the



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   transmitted SYN packet, as follows:

   TTL Diff = ( TTL - QS TTL ) mod 256

   The fourth byte is the requested Initial Rate (IR) field.  The sender
   initializes the Initial Rate to the desired initial sending rate.
   The current proposal is for this field to be formatted in packets per
   0.1 sec.  Routers can approve the Quick-Start Request for a lower
   Initial Rate by decreasing the Initial Rate in the Quick-Start
   Request.

   Note that the one-byte Initial Rate field, formatted in packets per
   0.1 sec, limits the Initial Rate to at most 2560 packets/sec.  For
   1500-byte packets, this corresponds to an initial rate of 30 Mbps.  A
   larger option field, or a different encoding for the one-byte
   requested Initial Rate option, would be needed to allow a higher
   range for the requested initial rate.  One suggestion has been for an
   Initial Rate field encoded on a logarithmic scale, to allow a wider
   range of Initial Rates.

   If the Quick-Start Request is not approved, then the sender uses the
   default initial rate or initial window for that transport protocol.

3.1.1.  The Quick-Start Request: Packets or Bytes?

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

   For TCP, the results from the Quick-Start Request are translated into
   an initial window in bytes, using the measured round-trip time and
   the MSS.  This initial 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 an initial window in packets.

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

   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 Initial Rate field was in bytes per second and the



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   sender ended up using very small packets, this could translate to a
   significantly larger number in terms of bytes per second on the wire.

   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 serva.  One problem is that 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].

   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, packets from the initial window could end up being
   fragmented in the network if the "Don't Fragment" (DF) bit is not set
   [RFC1191].  Interactions between larger initial windows and Path MTU
   Discovery are discussed in more detail in RFC 3390 [RFC3390].

   We have chosen an Initial Rate field in packets per second rather
   than in bytes per second because it seems somewhat more robust
   (avoiding big surprises if the sender ends up using small packets).
   However, we note that more consideration of this issue is probably
   needed.

3.2.  Processing the Quick-Start Request at Routers

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

   The preferable method for a router to terminate the Quick-Start
   Request is to delete the Quick-Start Request from the IP header.  A
   less preferable but possibly more efficient method is to simply
   forward the packet with the Quick-Start Request unchanged, or with
   the Initial Rate set to zero.

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

   * It decrements the QS TTL by one.

   * If the router is only willing to approve an Initial Rate less than
   that in the Quick-Start Request, then the router puts the new Initial
   Rate in that field of the Quick-Start Request.

   A non-participating router forwards the Quick-Start Request
   unchanged, without decrementing the QS TTL.  Of course, the non-
   participating router still decrements the TTL field in the IP header,



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   as is required for all routers.  As a result, the TCP sender will be
   able to detect that the Quick-Start Request is not valid.

   A router that modifies or deletes the Quick-Start Request in the IPv4
   header also has to update the IPv4 Header checksum.

3.3. Deciding the Permitted Initial Rate at a Router

   In this section we briefly outline how a router might decide whether
   or not to approve a Quick-Start Request.  As an example, the router
   could ask the following questions:

   * Has the router's output link been underutilized for some time
   (e.g., several seconds).

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

   Answering this question requires that the router have some knowledge
   of the available bandwidth on the output link for that output queue.
   It also requires that the router keep two counters, one indicating
   the total aggregate Initial Rates that have been approved over the
   recent interval of time, and one for the total aggregate Initial
   Rates approved over the previous interval of time.  Thus, if an
   underutilized router experiences a SYN flood, then the router would
   begin to deny Initial Rate requests, even if the router remains
   underutilized.

   * If the answer to both of the previous questions is Yes, then the
   router could approve the Quick-Start Request.  The router could allow
   an Initial Rate that was a small fraction of the available unused
   bandwidth of the output link.

4. The Quick-Start Mechanisms in TCP.

   This section describes how the Quick-Start mechanism would be used in
   TCP.  If a TCP sender, say host A, would like to request Quick-Start,
   the TCP sender puts the requested initial sending rate in packets per
   second in the Quick-Start Request option in the IP header of the SYN
   or SYN/ACK packet, as described above.  The TCP host B returns the
   Quick-Start Response option in the TCP header in the responding
   SYN/ACK packet or ACK packet, respectively, informing host A of the
   results of their request.

   If the returning packet does not contain a Quick-Start Response, or
   contains a Quick-Start Response with the wrong value for the TTL
   Diff, then host A has to assume that its Quick-Start request failed.



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   In this case, host A uses TCP's default initial window.

   If the returning packet does contain 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 initial
   congestion window.  In order to use Quick-Start, the TCP host would
   also be required to use rate-based pacing to pace out the packets in
   the initial window at the rate indicated in the Quick-Start Response.

   In a similar manner, if TCP host B requests Quick-Start in the IP
   header of the TCP SYN/ACK packet, then TCP host A returns the Quick-
   Start Response in the TCP header of the answering ACK packet.  The
   two TCP end-hosts can independently decide whether to request Quick-
   Start.

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

   The Quick-Start Response option is as follows:

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

      Figure 2.  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
   length field, specifying the option length in bytes.  The length
   field is set to four bytes.

   The third byte of the Quick-Start Response option contains the
   allowed Initial Rate, formatted as in the Quick-Start Request option.

   The fourth byte of the TCP option contains the TTL Diff.  The TTL
   Diff contains the difference, in the received SYN or SYN/ACK packet,
   between the TTL field in the IP header and the QS TTL field in the
   Quick-Start Request Option.

4.2.  Sending the Quick-Start Response

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



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   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
   answering acknowledgement packet.  The Initial Rate in the Quick-
   Start Response option is set to the received value of the Initial
   Rate in the Quick-Start Request option, or to a lower value if the
   TCP receiver is only willing to allow a lower Initial Rate.  The TTL
   Diff in the Quick-Start Response is set to the difference between the
   TTL value and the QS TTL value as follows:

   TTL Diff = ( TTL - QS TTL ) mod 256

   If the Quick-Start Response is being sent on the SYN/ACK, in response
   to a Quick-Start Request on the SYN, then the Quick-Start Response
   will be resent if the SYN/ACK has to be retransmitted.  If the Quick-
   Start Response is being sent on the ACK, in response to the Quick-
   Start Request on the SYN/ACK, then the Quick-Start Response has to be
   resent on data packets until that TCP host receives an
   acknowledgement from the other end.

4.3.  Receiving and Using the Quick-Start Response

   A TCP host, say TCP host A, that sent a Quick-Start Request in a SYN
   or SYN/ACK, and receives an answering Quick-Start Response in the
   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 Initial Rate
   than that transmitted in the Quick-Start Request.  If this check is
   not successful, then the Quick-Start request failed, and the TCP host
   uses the default TCP initial window.

   If the checks of the TTL Diff and the Initial Rate are successful,
   then the TCP host sets its initial congestion window to R*T*MSS
   bytes, for R the Initial Rate in packets per second and T the
   measured round-trip time in seconds.  The TCP host sets a flag that
   it is in Quick-Start mode, and while in Quick-Start mode the TCP
   sender uses rate-based pacing, pacing out packets at the specified
   Initial Rate.

   Because the initial SYN packet with the Quick-Start Request
   presumeably was not able to use the fast path in routers, the initial
   round-trip time measurement might be unnecessarily large.  The Quick-
   Start mode ends when the TCP host first receives an ACK for one of
   the data packets.  If the initial congestion window has not been
   fully used, then the initial congestion window is decreased to the
   amount that has actually been used so far.  This should address the
   problem of an overly-large congestion window from an overly-large
   initial measurement of the round-trip time.




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   After sending the initial window, the TCP sender remains in slow-
   start, and continues to increase its congestion window rather
   aggressively from one round-trip time to the next.  To add
   robustness, the TCP sender would be required to use Limited Slow-
   Start along with Quick-Start.  With Limited Slow-Start, the TCP
   sender limits the number of segments by which the congestion window
   is increased for one window of data during slow-start [F02a].

4.4.  An Example Quick-Start Scenario with TCP

   The following is an example scenario in the case when both hosts
   request Quick-Start:

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

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

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

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

   * Host A receives the Quick-Start Response in the SYN/ACK packet, and
   checks the TTL Diff and Initial Rate for validity.  If they are
   valid, then Host A sets its initial congestion window appropriately,
   and sets up rate-based pacing to be used with the initial window.  If
   the Quick-Start Response is not valid, then Host A uses TCP's default
   initial window.

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

   * Host A repeats sending the Quick-Start Response in data packets at
   least once per round-trip time until it receives an acknowledgement
   from Host B for one of those data packets.

   * Host B receives the Quick-Start Response in an ACK packet, and
   checks the TTL Diff and Initial Rate 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
   the initial window.  If the Quick-Start Response is not valid, then



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   Host B uses TCP's default initial window.

5.  Evaluation of Quick-Start

   The main benefit of Quick-Start is to 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 of N packets
   in well-provisioned environments that allow Quick-Start requests of M
   packets per second, the use of Quick-Start could shorten the
   connection lifetime from log N round-trip times to at most N/M + 1
   round-trip times.  For large values of N and M, this would be a
   dramatic shortening of the connection lifetime.

   The main cost of Quick-Start concerns the costs of added complexity
   at the routers.  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, in that it involves estimating the unused bandwidth on the
   output link over the last several seconds, and keeping a counter of
   the aggregate Quick-Start rate approved over the last fraction of a
   second.  However, this added complexity at the routers adds to the
   development cycle, and could prevent the addition of other competing
   functionality to routers.  Thus, careful thought would have to be
   given to the addition of Quick-Start to IP.

   Another drawback of Quick-Start is that packets containing the Quick-
   Start Request message presumeably would not take the fast path in
   routers.  This would mean extra delay for the end hosts, and extra
   processing burden for the routers.  This extra burden is mitigated
   somewhat by the following factors: only SYN and SYN-ACK packets would
   carry the Quick-Start Request option;  very small flows of, say, one
   to five packets would receive little benefit from Quick-Start, and
   presumeably would not use the Quick-Start Request;  flows from end
   hosts with low-bandwidth access links would receive little benefit
   from Quick-Start, and hopefully could be configured not to use the
   Quick-Start Request.  In addition, 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 conceiveable, in the worst case, that up to 10% of the packets
   were SYN or SYN/ACK packets using a Quick-Start Request, and this
   could slow down the processing of SYN or SYN/ACK packets in routers



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   considerably.

   One limitation of Quick-Start is that it presumes that the data
   packets of a connection will follow the same path as the SYN or
   SYN/ACK packet.  If this is not the case, then the connection could
   be sending the initial window, at the permitted initial rate, along a
   path that was already congested, or that became congested as a result
   of this connection.  This is, however, similar to what would happen
   if the connection's path was changed in the middle of the connection,
   when the connection had already established the allowed initial rate.

   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.  The hope 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.

   The discussion in this paper has largely been of the Quick-Start
   mechanism 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.  However,
   we would note that routers should be discouraged from granting Quick-
   Start requests for higher-priority traffic when this is likely to
   result in significant packet loss for lower-priority traffic.

   The Quick-Start proposal, taken together with the recent proposal for
   HighSpeed TCP [F02], would 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.  For example, Quick-Start as it is currently
   specified would not allow flows to ramp-up quickly in the middle of
   the connection.  Quick-Start would not help in making more precise
   use of the available bandwidth, that is, of achieving the goal of
   very high throughput with very low delay and very low packet loss
   rates.  Quick-Start would not give routers any additional power in
   allocating bandwidth in the interests of greater fairness, or in
   having more control over slow decrease rates of active connections.
   One of the open questions is whether the limited capabilities of
   Quick-Start are sufficient to warrant standardization and deployment,
   or whether more work is needed to explore the space of potential
   mechanisms.







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6.  Other Mechanisms for Fast Start-ups.

   Any evaluation of Quick-Start must include evaluating the relative
   benefits of approachs that use no explicit information from routers,
   and of approaches that use more fine-grained feedback from routers as
   part of a larger congestion control mechanism.  We discuss three
   classes of proposals (no explicit feedback from routers; explicit
   feedback about the initial rate; and more fine-grained feedback from
   routers) in the sections below.

6.1  Faster 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.

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

   One possible path for future research would be to explore the limits
   of the ability of TCP flows to infer the available bandwidth using
   their own packet stream, without explicit feedback from the router.
   One advantage of explicit feedback from the router is that it allows
   the TCP sender to discover the available bandwidth immediately after
   the initial SYN exchange, possibly allowing a very large initial
   window.  A second advantage of explicit feedback from the router is
   that the available bandwidth along the path does not necessarily map
   to the allowed sending rate for an individual flow; when multiple
   flows are trying to infer their allowed sending rate, the use of
   explicit feedback from the router adds considerable robustness.
   Nevertheless, it would also be useful to explore the limits of start-
   up behavior without explicit feedback from the router.

   As an example, if the TCP sender sends four packets back-to-back in
   the initial window, and the TCP receiver reports the timing of the
   receipt of the data packets, and the data packets were received with
   roughly the same spacing as they were transmitted, does this mean
   that the flow can infer an underutilized path?  What if the round-
   trip time is also considerably larger than the transmission time of
   the four packets?  And if the sender can infer an underutilized path,



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   can the sender increase faster than slow-start for the next window of
   data?  While it seems clear that approaches *without* explicit
   feedback from the routers will be strictly less powerful that is
   possible *with* explicit feedback, it is also possible that some
   approaches that are more aggressive than slow-start are possible
   without explicit feedback from routers.  Proposals for the TCP sender
   to infer about available bandwidth along the path without explicit
   feedback from routers include the Swift Start proposal in [PRAKS02].
   Swift Start combines packet-pair and packet-pacing techniques,
   beginning with a four-segment burst of packets to estimate the
   available bandwidth along the path.

   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 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 routers under congestion 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.

   Obviously there would be a number of questions to consider about an
   approach of optimistic sending.  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.  There could also be concerns about congestion
   collapse if many flows used large initial windows, many complete sets
   of initial windows were dropped, and many congested links ended up
   carrying packets that are only going to be dropped downstream.  A
   more thorough understanding of the dangers (or absence of dangers) of
   such optimistic larger initial windows would be useful.

6.2.  Faster Start-ups with other Information from Routers

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

   In related work, Joon-Sang Park and John Heidemann investigated the
   use of a slightly different IP option for TCP connections to discover
   the available bandwidth along the path.  In that variant, the IP
   option would query the routers along the path about the smallest
   available free buffer size. Also, the IP option was sent after the
   initial SYN exchange, when the TCP sender already had an estimate of
   the round-trip time.

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



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   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.

   Additional proposals for end nodes to collect explicit information
   from routers include Explicit Transport Error Notification (ETEN),
   which includes a cumulative mechanism to notify endpoints of
   aggregate congestion statistics along the path [KAPS02].

6.3.  Faster Start-ups with more Fine-Grained Feedback from Routers

   Proposals for more fine-grained congestion-related feedback from
   routers include XCP [KHR02].  Proposals such as XCP are more powerful
   than Quick-Start, involving significant changes to the congestion
   control mechanisms of the Internet, but also are more complex to
   understand and more difficult to deploy.

   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, for
   example;  Quick-Start is designed for use in environments that are
   significantly underutilized.  More powerful mechanisms with more
   fine-grained feedback from routers can allow faster startups even in
   environments with rather high link utilization.  Is this a pressing
   requirement?

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

7.  Conclusions

   We are presenting the Quick-Start mechanism not as a mechanism that
   we believe is urgently needed in the current Internet, but rather as
   a proposal for a simple, understandable, and incrementally-deployable
   mechanism that would be sufficient to allow connections to start up



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   with large initial rates, or large initial congestion windows, in
   overprovisioned, high-bandwidth environments.  We are not making any
   predictions about what it likely to be a typical environment in the
   future Internet.  However, we expect there will be an increasing
   number of overprovisioned, high-bandwidth environments where the
   Quick-Start mechanism, or another mechanism of similar power, could
   be of significant benefit to a wide range of traffic.  We are
   presenting the Quick-Start mechanism as a request for feedback from
   the Internet community in considering these issues.

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

   This is a modification of a draft originally by Amit Jain for Initial
   Window Discovery.

9. Normative References

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

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

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

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

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

10. Informative References

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




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   [FF99] Floyd, S., and Fall, K., Promoting the Use of End-to-End
   Congestion Control in the Internet, IEEE/ACM Transactions on
   Networking, August 1999.

   [F02] Floyd, S., HighSpeed TCP for Large Congestion Windows,
   internet-draft draft-floyd-tcp-highspeed-01.txt, work in progress,
   August 2002.

   [F02a] Floyd, S., Limited Slow-Start for TCP with Large Congestion
   Windows, internet-draft draft draft-floyd-tcp-slowstart-01.txt, work
   in progress, August 2002.

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

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

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

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

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

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

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

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




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

   [W00] Michael Welzl: PTP: Better Feedback for Adaptive Distributed
   Multimedia Applications on the Internet, IPCCC 2000 (19th IEEE
   International Performance, Computing, And Communications Conference),
   Phoenix, Arizona, USA, 20-22 February 2000.  URL
   "http://informatik.uibk.ac.at/users/c70370/research/publications/".

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

11. Security Considerations

   The only security consideration would be if the use of Quick-Start
   resulted in the sender using an Initial Rate that was inappropriately
   large, resulting in congestion along the path.  Such congestion could
   result in an unacceptable level of packet drops along the path.  Such
   congestion could also be part of a Denial of Service attack.

   A misbehaving TCP sender could use a non-conformant initial
   congestion window even without the use of Quick-Start, so we restrict
   our attention to problems with Quick-Start with conformant TCP
   senders.  (We also note that if the TCP sender is a busy web server,
   then the TCP sender has some incentive to be conformant in this
   regard.)

   If a router that understands the Quick-Start Request deletes the
   Request, or zeroes the QS TTL in the request, then the chances of a
   downstream router or misbehaving receiver guessing the value of the
   QS TTL is at most 1/256.  Thus, deleting the Quick-Start Request
   makes it unlikely that the receiver would be able to send a valid
   Quick-Start Response back to the sender.

   If there are routers along the path that do not understand or approve
   of the Quick-Start Request, and that forward the Quick-Start Request
   unchanged, it would be not be easy for a downstream router or the
   receiver to cheat and modify the QS TTL field so that the request was
   considered valid, because the downstream routers do not know the
   initial value for the QS TTL.

   It would be easy for routers or for the receiver to increase the
   Initial Rate, making the Initial Rate higher than that approved by
   upstream routers.  Routers could only effectively cheat in this
   manner if there were no downstream routers that objected to that
   Initial Rate.  Receivers, however, would easily increase the Initial
   Rate returned in the Quick-Start Response, causing unnecessary
   congestion for the next round-trip time along the path.  However,



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   this higher Initial Rate will only be considered valid by the TCP
   sender if it is no higher than the Initial Rate originally requested
   by the sender.  Thus, this limits the ability of the TCP receiver to
   cheat in this regard.

12. IANA Considerations

   The only IANA Considerations would be the addition of an IP option to
   the list of IP options, and the addition of a TCP option to the list
   of TCP options.

13. Discussion of a QuickStart Nonce

   An earlier version of this document included a QuickStart 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 at
   the TTL in the IP header.  The QuickStart Nonce would have been
   returned by the TCP receiver to the TCP sender in the Quick-Start
   Response.  A router could deny the Quick-Start request by failing to
   decrement the QS TTL field, by zeroing the QS Nonce field, or by
   deleting the Quick-Start Request from the packet header.  The QS
   Nonce was included to provide some protection against broken
   downstream routers, or against misbehaving TCP receivers who might be
   inclined to lie about the Initial Rate.  This protection is now
   provided by the use of a random initial value for the QS TTL field.

   With the old QuickStart Nonce, along with the QS TTL field set to the
   same value as the TTL field in the IP header, the Quick-Start Request
   mechanism would have been self-terminating; the Quick-Start Request
   would terminate at the first participating router after a non-
   participating router has been encountered on the path.  This would
   have minimized unnecessary overhead incurred by routers because of
   option processing for the Quick-Start Request.  Thus, one
   disadvantage of the new approach with a random initial value for the
   QS TTL field is that intermediate routers can no longer determine
   when some upstream router has not understood the QuickStart option.
   However, a disadvantage of the old approach was that it offered no
   protection against downstream routers or the TCP receiver hiding
   evidence of upstream routers that do not understand the QuickStart
   option.

   AUTHORS' ADDRESSES


      Amit Jain
      Array Networks
      Email : ajain@arraynetworks.net




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














































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