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Versions: (draft-renker-dccp-tfrc-rtt-option) 00 01 02 03 04 05 06 RFC 6323

DCCP Working Group                                             G. Renker
Internet-Draft                                              G. Fairhurst
Updates: 4342, 5622                               University of Aberdeen
(if approved)                                           December 9, 2010
Intended status: Standards Track
Expires: June 12, 2011


                  Sender RTT Estimate Option for DCCP
                 draft-ietf-dccp-tfrc-rtt-option-01.txt

Abstract

   This document describes an update to CCID-3/4 that addresses
   parameter-estimation problems occurring with TFRC-based DCCP
   congestion control.

   The fix uses a recommendation made in the original TFRC
   specification.  It avoids the inherent problems of receiver-based RTT
   sampling, by utilising higher-accuracy RTT samples already available
   at the sender.  It is integrated into the feature set of DCCP as an
   end-to-end negotiable extension, upward and downward compatible.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on June 12, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Problems caused by sampling the RTT at the receiver  . . . . .  4
     2.1.  List of problems encountered with a real implementation  .  4
     2.2.  Other areas affected by the RTT sampling problems  . . . .  6
       2.2.1.  Measured Receive Rate X_recv . . . . . . . . . . . . .  6
       2.2.2.  Disambiguation and Accuracy of Loss Intervals  . . . .  7
       2.2.3.  Determining Quiescence . . . . . . . . . . . . . . . .  7
       2.2.4.  Practical Considerations . . . . . . . . . . . . . . .  7
   3.  Specification  . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Conventions  . . . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Options and Features . . . . . . . . . . . . . . . . . . .  8
       3.2.1.  RTT Estimate Option  . . . . . . . . . . . . . . . . .  8
       3.2.2.  Send RTT Estimate Feature  . . . . . . . . . . . . . . 10
     3.3.  Basic Usage  . . . . . . . . . . . . . . . . . . . . . . . 11
     3.4.  Receiver Robustness Measures . . . . . . . . . . . . . . . 12
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Option Types . . . . . . . . . . . . . . . . . . . . . . . 14
     5.2.  Feature Numbers  . . . . . . . . . . . . . . . . . . . . . 15
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21


















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1.  Introduction

   This document defines a Standards Track update to both a sender and
   receiver that implement DCCP CCID-3/4, addressing RTT estimation
   problems that were observed when using a real implementation
   [RFC4342], [RFC5622].

   To fix these problems, this document presents a solution based on a
   concept first recommended in [RFC5348], 3.2.1; i.e. to measure the
   RTT at the sender.  This results in a higher reliability and
   frequency of samples, and avoids the inherent problems of receiver-
   based RTT sampling discussed below.

   We begin by analysing the encountered problems in the next section.
   The update is presented in Section 3.  We then discuss security
   considerations in Section 4, and list the resulting IANA
   considerations in Section 5.


































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2.  Problems caused by sampling the RTT at the receiver

   There are at least six areas that make a TFRC receiver vulnerable to
   inaccuracies or absence of (receiver-based) RTT samples:

   o  the measured sending rate, X_recv ([RFC5348], 6.2);

   o  synthesis of the first loss interval ([RFC5348], 6.3.1);

   o  disambiguation of loss events ([RFC4342], 10.2);

   o  validation of loss intervals ([RFC4342], 6.1);

   o  ensuring that at least one feedback packet is sent per RTT
      ([RFC4342], 10.3);

   o  determining quiescence periods ([RFC4342], 6.4).

2.1.  List of problems encountered with a real implementation

   This section summarizes several years of experience using the Linux
   implementation of CCID-3 and CCID-4.  It lists the problems
   encountered with receiver-based RTT sampling over real networks, in a
   variety of wired and wireless environments and under different link-
   layer conditions.

   The Linux DCCP/TFRC implementation is based on the RTT-sampling
   algorithm specified in [RFC4342], 8.1.  This algorithm relies on a
   coarse-grained window-counter (units of RTT/4), and uses packet
   inter-arrival times to estimate the current RTT of the network.

   The algorithm is effective only for packets with modulo-16 CCVal
   differences less than 5, due to limitations noted in sections 8.1 and
   10.3 of [RFC4342].  A CCVal difference less than 4 means sampling at
   sub-RTT scale; [RFC4342], 8.1 thus suggests differences between 2 and
   4, the latter being preferable (equivalent to a full RTT).  The same
   section limits the maximum CCVal difference between data-carrying
   packets to 5, in order to avoid wrap-around.  As a consequence, the
   timing interval for adjacent packets with a CCVal difference greater
   than 4 can not be determined: such samples have to be discarded.

   A second problem arises when there are holes in the sequence space.
   Because the 4-bit CCVal counter may cycle around multiple times, it
   is not possible to determine window-counter wrap-around whenever
   sequence numbers of subsequent packets are not immediately adjacent.
   This problem occurs when packets are delayed, reordered, or lost in
   the network.




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   As a consequence, RTT sampling has to be paused during times of loss.
   This however aggravates the problem, since the sender now requires
   new feedback from the receiver, but the receiver is unable to provide
   accurate and up-to-date information: the receiver is unable to sample
   the RTT, accordingly also not able to estimate X_recv correctly,
   which then in turn affects X_Bps at the sender.

   The third limitation arises from using inter-arrival times as
   representatives of network inter-packet gaps.  It is well known that
   the inter-packet gap of packets is not constant along a network path.
   Furthermore, modern network interface cards do not necessarily
   deliver each packet at the time it is received, but rather in a
   bunch, to avoid overly frequent interrupts [MR97].  As a result,
   inter-packet arrival times may converge to zero, when subsequent
   packets are being delivered at virtually the same time.

   The fourth problem is that of under-sampling and thus related to the
   first limitation.  If loss occurs while the receiver has not yet had
   a chance to sample the RTT, it needs to fall back to some fixed RTT
   constant to plug into the equation of [RFC5348], 6.3.1.  (The sender,
   for example, uses a fixed value of 1 second when it can not obtain an
   initial RTT sample, see [RFC5348], 4.2).

   In particular, if the loss is caused by a transient condition, this
   fourth problem causes a subsequent deterioration of the connection
   (rate reduction), further aggravated by the fact that TFRC takes
   longer than common window-based protocols to recover from a reduction
   of its allowed sending rate.

   The fifth and last problem is an unfortunate interaction between
   nofeedback timer and sampling packet inter-arrivals at the receiver.

   Consider an example traffic channel that is temporarily unavailable
   for longer than 4 RTTs.  The nofeedback timer at the sender will
   halve X after 4*RTT.  At the same time, the RTT average at the sender
   will either remain unchanged (if no new sample arrived); or it will
   change at a much slower rate, due to the averaging described in point
   (2) of [RFC5348], 4.3.  The value of RTT/4 used by the CCVal
   algorithm ([RFC4342], 8.1) therefore changes much slower than X.

   To the receiver this condition will look as if the inter-packet gap
   suddenly doubled, meaning it will use samples of twice the actual
   RTT.  Thus the RTT averaged at the receiver (which typically also has
   fewer usable samples than the sender) increases accordingly.  This in
   turn leads to a lower reported value of X_recv, which then further
   reduces the sending rate X allowed at the sender (in the worst case
   doubling the perceived RTT translates into halving X_recv).  The
   consequence is that the inter-packet gap increases even further,



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   causing the process to repeat itself, until the sending rate
   converges to 1 packet per t_mbi=64 seconds.

   The above behaviour was observed on an 802.11 link where short holes
   of interference triggered repeated halving of the sending rate, until
   it terminated at 1 packet per 64 seconds.  Under these conditions the
   connection is no longer in a usable state, unless buffering of more
   than 64 seconds is applied - which is impossible for interactive
   applications, and unacceptable for many audio/video applications.

   Trying to smooth over these effects by imposing heavy filtering on
   the RTT samples did not substantially improve the situation, nor does
   it solve the problem of under-sampling.

   The TFRC sender, on the other hand, is much better equipped to
   estimate the RTT and can do this more accurately.  This is in
   particular due to the use of timestamps and elapsed time information
   ([RFC5348], 3.2.2), which are mandatory in CCID-3 (sections 6 and 8.2
   of [RFC4342]).

2.2.  Other areas affected by the RTT sampling problems

   We here analyse the impact that unreliability of receiver-based RTT
   sampling has on the areas listed at the begin of this section.

   In addition, benefits of sender-based RTT sampling have already been
   pointed out in [RFC5348], and in the specification of CCID-3
   [RFC4342], at the end of section 10.2.

2.2.1.  Measured Receive Rate X_recv

   A key problem is that the reliability of X_recv [RFC4342] depends
   directly upon the reliability and accuracy of RTT samples.  This
   means that failures propagate from one parameter to another.

   Errata IDs 610 and 611 update [RFC4342] to use the definition of the
   receive rate as specified in [RFC5348].

   Having an explicit (rather than a coarse-grained) RTT estimate allows
   measurement of X_recv with greater accuracy, and isolates failure.

   An explicit RTT estimate also enables the receiver to more accurately
   perform the test in step (2) of [RFC4342], 6.2, i.e. to check whether
   less or more than one RTT has passed since the last feedback.







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2.2.2.  Disambiguation and Accuracy of Loss Intervals

   Since a loss event is defined as one or more lost (ECN-marked) data
   packets in one RTT ([RFC5348], 5.2), the receiver needs accurate RTT
   estimates to validate and accurately separate loss events.  Moreover,
   [RFC5348], 5.2 expressly points out the sender RTT estimate as
   RECOMMENDED for this purpose.

   Having the sender RTT Estimate available further increases the
   accuracy of the information reported by the receiver.  The definition
   of Loss Intervals in [RFC4342], 6.1 needs the RTT to separate the
   lossy parts; in particular, lossy parts spanning a period of more
   than one RTT are invalid.

   A similar benefit arises in the computation of the loss event rate:
   as discussed in section 9.2 of [RFC4342], it may happen that sender
   and receiver compute different loss event rates, due to differences
   in the available timing information.  An explicit RTT estimate
   increases the accuracy of information available at the receiver, thus
   the sender may not need to recompute the (less reliable) loss event
   rate reported by the receiver.

2.2.3.  Determining Quiescence

   The quiescence period is defined as max(2 * RTT, 0.2 sec) in section
   6.4 of [RFC4342].  An explicit RTT estimate avoids under- and over-
   estimating quiescence periods.

2.2.4.  Practical Considerations

   Using explicit RTT estimates contributes to greater robustness and
   can also result in simpler implementation.

   First, it becomes easier to separate adjacent loss events.  The 4-bit
   counter value wraps relatively frequently, which requires additional
   procedures to avoid aliasing effects.

   Second, the receiver is better able to determine when to send
   feedback packets.  It can perform the test described in step (2) of
   [RFC5348], 6.2 more accurately.  Moreover, unnecessary expiration of
   the nofeedback timer (as described in [RFC4342], 10.3) can be
   avoided.

   Lastly, a sender-based RTT estimate option can be used by middleboxes
   to verify that a flow uses conforming end-to-end congestion control
   ([RFC4342], 10.2).





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3.  Specification

3.1.  Conventions

   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].

   This document uses the conventions of [RFC5348], [RFC4340],
   [RFC4342], and [RFC5622].

   All multi-byte field descriptions presented in this documented are in
   network byte order (most significant byte first).

3.2.  Options and Features

   This document defines a single TFRC-specific option, RTT Estimate,
   described in the next subsection.

   Following the guidelines in [RFC4340], section 15, the use of the RTT
   Estimate Option is governed by an associated feature, Send RTT
   Estimate Feature.  This feature is described in the second
   subsection.

3.2.1.  RTT Estimate Option

   The sender communicates its current RTT estimate to the receiver
   using a RTT Estimate Option.























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==> RFC Editor's Note:

   Please replace 'XX' with IANA value when published and delete this
   note.

           +------+---------------+--------------+------------+
           | Type | Option Length |    Meaning   | DCCP Data? |
           +------+---------------+--------------+------------+
           |  XX  |     3/4/5     | RTT Estimate |      Y     |
           +------+---------------+--------------+------------+

         Table 1: The RTT Estimate Option defined by this document

   Column meanings are as per [RFC4340], section 5.8 (table 3).  This
   option is permitted in any DCCP packet, has option number XX and a
   length of 3-5 bytes.

   A Sender RTT Estimate Option is valid if it satisfies one of the
   three following formats:

      +--------+--------+--------+
      |xxxxxxxx|00000011|  RTT   |
      +--------+--------+--------+
       Type=XX  Length=3  Estimate


      +--------+--------+--------+--------+
      |xxxxxxxx|00000100|       RTT       |
      +--------+--------+--------+--------+
       Type=XX  Length=4      Estimate


      +--------+--------+--------+--------+--------+
      |xxxxxxxx|00000101|           RTT            |
      +--------+--------+--------+--------+--------+
       Type=XX  Length=5          Estimate

   The 1..3 value bytes of the option data carry the current RTT
   estimate of the sender, using a granularity of 1 microsecond.  This
   allows values up to 16.7 seconds (corresponding to 0xFFFFFE) to be
   communicated.

   The value 0xFFFFFF is reserved to indicate significant delay spikes,
   larger than 16.7 seconds.  This is qualitative rather than
   quantitative information, to alert the receiver that there is a
   network problem (for instance jamming on a wireless channel).

   The use of the RTT Estimate Option on networks with RTTs larger than



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   16.7 seconds is not specified by this document.

   A value of 0 indicates the absence of a valid RTT sample.  The sender
   MUST set the value to 0 if it does not yet have an RTT estimate.

   The sender SHOULD select the smallest format suitable to carry the
   RTT estimate (i.e., less than 1 byte of leading zeroes).

3.2.2.  Send RTT Estimate Feature

   The Send RTT Estimate feature lets endpoints negotiate whether the
   sender MUST provide RTT Estimate options on its data packets.







































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==> RFC Editor's Note:

   Please replace 'YY' with IANA value when published and delete this
   note.

   Send RTT Estimate has feature number YY and is server-priority.  It
   takes one-byte Boolean values.  Values greater than 1 are invalid and
   MUST be ignored.

    +--------+-------------------+------------+---------------+-------+
    | Number |      Meaning      | Rec'n Rule | Initial Value | Req'd |
    +--------+-------------------+------------+---------------+-------+
    |   YY   | Send RTT Estimate |     SP     |       0       |   N   |
    +--------+-------------------+------------+---------------+-------+

      Table 2: The Send RTT Estimate feature defined by this document

   The column meanings are described in [RFC4340], section 6.4.  In
   particular, the feature is by default off (initial value of 0), and
   the extension is not required to be understood by every DCCP
   implementation (cf. [RFC4340], section 15).

   DCCP B sends a "Change R(Send RTT Estimate, 1)" to ask DCCP A to send
   RTT Estimate options as part of its data traffic.

3.3.  Basic Usage

   When the Send RTT Estimate Feature is enabled, the sender MUST
   provide an RTT Estimate Option on all of its Data, DataAck, Sync, and
   SyncAck packets.  It MAY in addition provide the RTT Estimate Option
   on other packet types, such as DCCP-Ack.

   The sender MUST implement and continue to update the CCVal window
   counter as specified in [RFC4342], section 8.1, even when the Send
   RTT Estimate Feature is on.

   If the receiver has requested the use of the RTT Estimate Option, it
   MUST use the RTT value reported by that option in all places that
   require a RTT (listed at the begin of Section 2).  If the receiver
   encounters an invalid RTT Estimate Option (Section 3.2.1), it MUST
   reset the connection with Reset Code 5, "Option Error", where the
   Data 1..3 fields are set to the first 3 bytes of the offending RTT
   Estimate Option.

   The receiver SHOULD track the long-term RTT estimate using a moving
   average, such as the one specified in [RFC5348], 4.3.  This long-term
   estimate is referred to as "receiver_RTT" below.




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   When the Send RTT Estimate Feature is disabled, the receiver MUST
   estimate the RTT as previously specified in [RFC4340], [RFC4342], and
   [RFC5622].

3.4.  Receiver Robustness Measures

   This subsection specifies robustness measures for the receiver when
   the Send RTT Estimate Feature is on.

   The 0-valued and 0xFFFFFF-valued RTT Estimate Options are both
   referred to as "no-number RTT options".  RTT Estimate Options with
   values in the range of 1..0xFFFFFE are analogously called "numeric
   RTT options".

   Until the first numeric RTT option arrives, the receiver uses a value
   of 0.5 seconds for receiver_RTT (to match the initial 2 second
   timeout of the TFRC nofeedback timer, [RFC5348], 4.2).

   If the path RTT is known, e.g. from a previous connection [RFC2140],
   the receiver MAY reuse the previously known path RTT value to seed
   its long-term RTT estimate.

   The sender is permitted to occasionally send no-number RTT options,
   covering for transient changes and spurious disruptions.  During
   these times, the receiver continues to use its long-term receiver_RTT
   value.

   To avoid that the long-term estimate at the receiver drifts in such a
   way that it under-estimates the RTT, a simple back-off scheme is
   employed: if the sender supplies no-number RTT options for longer
   than receiver_RTT units of time, the receiver sets

             receiver_RTT = MIN(2 * receiver_RTT, t_mbi)

   where t_mbi = 64 seconds is the maximum backoff interval ([RFC5348],
   Appendix A).  For the next round of no-number RTT options, the
   updated value of receiver_RTT applies.

   This back-off mechanism ensures that short-term disruptions do not
   have a lasting impact, whereas long-term problems will result in
   asymptotically high receiver_RTT values.

   To bail out from a hanging session, the receiver MAY close the
   connection when receiver_RTT has reached the value MAX_RTT.







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4.  Security Considerations

   Security considerations for CCID-3 have been discussed in section 11
   of [RFC4342]; for CCID-4 these have been discussed in section 13 of
   [RFC5622], referring back to the same section of [RFC4342].

   This document introduces an extension to communicate the current RTT
   estimate of the sender to the receiver of a TFRC communication.

   By altering the value of the RTT Estimate Option, it is possible to
   interfere with the behaviour of a flow using TFRC.  In particular,
   since accuracy of the RTT estimate directly influences the accuracy
   of the measured sending rate X_recv, it would be possible to obtain
   either higher or lower sending rates than are warranted by the
   current network conditions.

   This is only possible if an attacker is on the same path as the DCCP
   sender and receiver, and is able to guess valid sequence numbers.
   Therefore the considerations of section 18 in [RFC4340] apply.
































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5.  IANA Considerations

   This document requests identical allocation in the dccp-ccid3-
   parameters and the dccp-ccid4-parameters registries.

5.1.  Option Types

   This document defines a single CCID-specific option for communicating
   RTT estimates from the HC-sender to the HC-receiver.  Following
   [RFC4340], 10.3, this requires an option number for the RTT Estimate
   Option in the range 128...191.








































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Note to IANA and the RFC editor

   When the IANA has allocated an option number for the `RTT Estimate'
   option, please replace all occurrences of the placeholder `XX' in
   this text with that number and delete this note.  (Due to [RFC4340],
   19.3 and [RFC4342], 12.2, the option number would be allocated in the
   range 128...183/191.)

5.2.  Feature Numbers

   This document defines a single CCID-specific feature number for the
   Send RTT Estimate feature which is located at the HC-sender.
   Following [RFC4340], 10.3, a feature number in the range 128...191 is
   required.





































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Note to IANA and the RFC editor

   When the IANA has allocated an option number for the `Send RTT
   Estimate' feature, please replace all occurrences of the placeholder
   `YY' in this text with that number and delete this note.  (Due to
   [RFC4340], 19.4 and [RFC4342], 12.3, the feature number would be
   allocated in the range 128...183/191.)












































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6.  References

6.1.  Normative References

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

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

   [RFC4342]  Floyd, S., Kohler, E., and J. Padhye, "Profile for
              Datagram Congestion Control Protocol (DCCP) Congestion
              Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
              March 2006.

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 5348, September 2008.

6.2.  Informative References

   [MR97]     Mogul, J. and K. Ramakrishnan, "Eliminating Receive
              Livelock in an Interrupt-Driven Kernel", ACM Transactions
              on Computer Systems (TOCS), 15(3):217-252, August 1997.

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

   [RFC5622]  Floyd, S. and E. Kohler, "Profile for Datagram Congestion
              Control Protocol (DCCP) Congestion ID 4: TCP-Friendly Rate
              Control for Small Packets (TFRC-SP)", RFC 5622,
              August 2009.



















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==> NOTE TO THE RFC EDITOR: PLEASE REMOVE THIS LOG PRIOR TO PUBLICATION

   The following changelog lists the changes since revision 01 of the
   preceding individual submissions
   draft-renker-dccp-tfrc-rtt-option-00/01.

   o  General:

         - added detailed changelog to track comments

         - changed document name to reflect working group

         - updated date to October

         - made spelling of RTT Estimate Option (singular) consistent

         - moved reference to RFC 5622 from normative to informative,
         since document status is Experimental

   o  Section 2.1:

         - clarified problematic cases of too small CCVal differences
         and CCVal differences > 4, feedback by Pasi

   o  Section 3.1:

         - clarified the byte ordering used by this document, feedback
         by Pasi

   o  Section 3.2:

         - corrected naming of Send RTT Estimate Feature, feedback by
         Eddie

         - removed superfluous remark regarding scaling to microsecond
         granularity in 3.2.1, feedback by Pasi

         - removed recommendation of preferring long-term RTT stamples,
         since this can not be generalized (connection may be short or
         path RTT may change, in both cases a long-term sample would not
         be useful), feedback by Pasi

         - made option variable-length (3/4/5 bytes), feedback by Eddie

         - specified condition for syntactic option validity

         - limited the maximum option size to 3 bytes and justified
         decision why not to support RTTs greater than 16 seconds, in



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         reply to feedback by Eddie

         - clarified that the sender MUST use 0 to indicate absence of a
         valid RTT estimate

         - clarified the highest path RTT value supported by this
         document (16.7 sec)

         - reserved 0xFFFFFF as special value to communicate out-of-
         bounds exceptions, network problems resulting in
         disproportionately high delay spikes (> 16.7 seconds)

   o  Section 3.3:

         - corrected naming of Send RTT Estimate Feature, feedback by
         Eddie

         - specified what happens if invalid RTT Estimate options are
         received

         - specified what happens if the sender persistently sends
         0-valued RTT Estimate options, feedback by Eddie

         - specified how the exceptional value 0xFFFFFF should be
         handled

         - added reference for reusing previously known path RTT value

   Changes between revision 00 and 01 of this draft:

   o  General changes:

         - incremented date and revision number

         - various minor changes of syntax, typos, and paragraph
         formatting

   o  Section 2.1:

         - completely rewrote the description of the fifth problem in
         order to more clearly/precisely identify problem causes,
         following feedback from Michael

   o  Section 2.2.4:

         - simplified sentence referring to aliasing effects (implicitly
         referencing section 10.2 of RFC4342)




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         - clarified how middleboxes might use a sender-based RTT
         estimate option to verify end-to-end congestion control
         (suggestion and quote taken from RFC4342, section 10.2)

   o  Section 3.3:

         - clarified how the receiver must behave if the Send RTT
         Estimate Feature is disabled, following feedback from Eddie

         - removed the requirement that the receiver should additionally
         track CCVal window counter values when the Send RTT Estimate
         Feature is on

         - removed suggestion that the receiver should take measures to
         improve the quality of the connection, feedback by Michael

         - moved all receiver robustness measures to the new section 3.4

         - changed section title to reflect restructuring of content

   o  Section 3.4:

         - new section, written from scratch, to address the
         shortcomings of the previous scheme, which were identified by
         Michael Welzl

         - specifies what to do when the sender supplies no-number RTT
         options for short and extended periods of time

   ====> END OF NOTE TO THE RFC EDITOR <====





















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Authors' Addresses

   Gerrit Renker
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen  AB24 3UE
   Scotland

   Email: gerrit@erg.abdn.ac.uk
   URI:   http://www.erg.abdn.ac.uk


   Godred Fairhurst
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen  AB24 3UE
   Scotland

   Email: gorry@erg.abdn.ac.uk
   URI:   http://www.erg.abdn.ac.uk





























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