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Network Working Group                                          E. Lochin
Internet-Draft                                    National ICT Australia
Expires: February 2, 2007                                    L. Dairaine
                                                      ENSICA - LAAS/CNRS
                                                              G. Jourjon
                                                  National ICT Australia
                                                             August 2006


  Guaranteed TCP Friendly Rate Control (gTFRC) for DiffServ/AF Network
                    draft-lochin-ietf-tsvwg-gtfrc-02

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   This Internet-Draft will expire on February 2, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This memo introduces gTFRC, a TCP-Friendly Rate Control providing
   throughput guarantee over the DiffServ/AF class. gTFRC is largely
   based on TFRC [2].  It provides a mean to take into account the
   quality of service negotiated with the network.  As a result, the
   mechanism is able to reach a minimum throughput guarantee whatever



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   the flow's RTT and target rate.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Guaranteed TCP Friendly Rate Control . . . . . . . . . . . . .  4
     2.1.  Transmit rate equation . . . . . . . . . . . . . . . . . .  4
     2.2.  Target rate default value  . . . . . . . . . . . . . . . .  4
     2.3.  Target rate setting  . . . . . . . . . . . . . . . . . . .  4
   3.  Simulation of gTFRC  . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Model and hypothesis . . . . . . . . . . . . . . . . . . .  5
     3.2.  Results  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Analysis . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.4.  Discussion . . . . . . . . . . . . . . . . . . . . . . . .  7
       3.4.1.  Security concern . . . . . . . . . . . . . . . . . . .  7
       3.4.2.  Case of under-provisioned network  . . . . . . . . . .  8
   4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11






























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1.  Introduction

   This memo introduces gTFRC, a TCP-Friendly rate control providing
   throughput guarantee for unicast flows over the DiffServ/AF class.
   gTFRC is an adaptation of the TCP-friendly Rate Control (TFRC) [2].
   This document only specifies the modification of TFRC and do not
   present the core TFRC mechanism that remains unchanged.

   TFRC is a congestion control mechanism for unicast flows operating in
   a best-effort Internet environment [2].  Based on the TCP throughput
   equation, it is designed to be reasonably fair when competing for
   bandwidth with TCP flow.  It generates a flow with a much lower
   variation of throughput over time than TCP.  As a result, it is
   particularly suitable for multimedia application such as video
   streaming or telephony over Internet.

   The DiffServ Assured Forwarding Class [1] has been designed to
   provide a guaranteed minimal throughput that many multimedia
   applications can take advantage of.  The service offers is called
   Assured Service (AS) and built over the AF PHB.  The minimum
   guaranteed throughput (also called target rate) is supposed to be
   known after a negotiation phase involving application level software.
   Adaptive application can make use of this guarantee, allowing to rely
   on a minimum rate when the network is congested, and possibly using
   higher rate otherwise.  In this service class, a congestion control
   is required in such a way to discover the current available bandwidth
   and share it fairly with other competing flows.  Nevertheless, due to
   the minimum bandwidth guarantee, the congestion control mechanism
   should never reduce the flow throughput at a value less than the
   negotiated guaranty.

   When TFRC is used over a DiffServ/AF network, in spite of a good
   behavior in term of available bandwidth sharing, it not always reach
   the target rate.  Even if the target rate is finally reached, a long
   time can happened (several tens of seconds) before the flow rate
   converges to this value.  Then, depending on end-to-end delay and the
   loss probability of the various connections, the application does not
   obtained the requested target rate it should, even if the underlying
   network provides an adequate throughput guarantee.

   This document suggests a simple approach to solve this problem.  A
   minimal adaption of TFRC allows the application to quickly reach its
   target rate whatever the RTT value of the application's flow, while
   still sharing fairly the available bandwidth over the various TCP-
   friendly connections.






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2.  Guaranteed TCP Friendly Rate Control

   In the context of the Additive Increase Multiplicative Decrease
   approaches like TCP, the only way to obtain a service differentiation
   with TCP protocol is to use DiffServ traffic conditioners.  Indeed,
   the AIMD principle do not use the instantaneous TCP throughput as an
   input value for its congestion control and then can not make direct
   use of the target rate value.  On the contrary, to compute the actual
   sending rate, TFRC uses the current rate in conjunction with the RTT
   and the loss event of flow.  Nevertheless, the TCP equation that
   drives TFRC does not take into account the minimum guaranteed part of
   the network capacity.

   gTFRC is made aware of the target rate value which is integrated into
   the transmit rate equation.  Thanks to this knowledge, the
   application's flow is sent in conformance with the negotiated QoS
   while staying TCP-friendly in its out-profile part.

2.1.  Transmit rate equation

   The transmit rate is computed at sender side as the maximum between
   the TFRC rate estimation and the target rate.  The throughput
   equation used in gTFRC is:

   G = max(g, X)

   Where:

      G is the transmit rate in bytes/second.

      g is the target rate in bytes/second.

      X is the transmit rate in bytes/second computed by the TCP
      throughput algorithm specified in RFC 3448 [2].

   The rest of the gTFRC mechanism follows entirely the TFRC
   specification given in RFC 3448 [2].

2.2.  Target rate default value

   The target rate g MUST have a default value of zero byte/second.  In
   this case, the default behavior of gTFRC corresponds to TFRC.

2.3.  Target rate setting

   gTFRC requires the knowledge of the target rate the DiffServ/AF
   network service provides to the session.  This knowledge MAY be
   achieved by the use of a new socket option.



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3.  Simulation of gTFRC

3.1.  Model and hypothesis

   gTFRC has been evaluated over a DiffServ network using ns-2.28
   simulator. gTFRC has been implemented from the TFRC ns-2 code base.
   The Nortel DiffServ model [3] has been used as QoS testbed.

   The network architecture is shown in the following figure.

   ----                                                        ----
   |s1|--------                                      --------- |r1|
   ----  10 Mb \                                    /  10 Mb   ----
         5 ms   \                                  /    x ms
                 \------       ------       ------/
                  |edge|-------|core|-------|edge|
                 /------       ------       ------\
                /        10 Mb          1 Mb       \
   ----        /          5 ms          10ms        \          ----
   |s2|--------                                      --------- |r2|
   ----  10 Mb                                         10 Mb   ----
          5 ms                                          y ms

   where x and y take different RTT values in function of the
   experiment.

   Figure 1

   In these experiments, the objective was to compare the performance of
   TFRC and gTFRC.

   The simulation has been achieved with the two following scenarios:

   1.  the network is exactly-provisioned (it means there is no excess
       bandwidth for the out-profile traffic).

   2.  network is over-provisioned (when there is excess bandwidth).

   A network is under-provisioned when the amount of in-profile traffic
   is higher than the resource allocated to the AF class.  This case is
   considered as a bad network provision and then is excluded from the
   field of this study.

   In the simulations:

   o  packet size is fixed to 1500 bytes;





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   o  we use a two colors token bucket marker with a bucket size of 10^4
      bytes defined in RFC 2697 [5];

   o  the queues size are 50 packets and RIO parameters are:
      (MIN_out,MAX_out,P_out, MIN_in,MAX_in,P_in) =
      (10,20,0.1,20,40,0.02);

   o  the bottleneck between the core and the egress router is
      1000Kbits/s;

   o  measurements are carried during 100 seconds.

   For each experiments, we evaluate the throughput at the server side
   and compute the instantaneous throughput and the average throughput
   for the experiment.  We resport the instantaneous throughput values
   at 20s, 50s and 100s.  Because some flow can cross one or several
   DiffServ domains and then, obtain a very large RTT difference, we
   compare flows with a high RTT difference (i.e., 600ms).

3.2.  Results

   The following table presents the comparative results between TFRC and
   gTFRC for an exactly provisioned network.

   +========+=======+========+=======+=======+=======+=======+
   |Protocol|  RTT  | Target | After | After | After |       |
   | #flow  |  (ms) | (Kb/s) |  20s  |  50s  |  100s |Average|
   +========+=======+========+=======+=======+=======+=======+
   | TFRC#1 | 640ms |  800   |  376  |  584  |  784  |  571  |
   | TFRC#2 |  40ms |  200   |  584  |  416  |  232  |  419  |
   +--------+-------+--------+-------+-------+-------+-------+
   |gTFRC#1 | 640ms |  800   |  376  |  784  |  800  |  722  |
   |gTFRC#2 |  40ms |  200   |  584  |  224  |  200  |  271  |
   +========+=======+========+=======+=======+=======+=======+

   Figure 2















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   The following table presents the comparative results between TFRC and
   gTFRC for an over-provisioned network with either same or different
   RTT values for the competing flows.

   +========+=======+========+=======+=======+=======+=======+
   |Protocol|  RTT  | Target | After | After | After |       |
   |Protocol|  (ms) | (Kb/s) |  20s  |  50s  |  100s |Average|
   +========+=======+========+=======+=======+=======+=======+
   | TFRC#1 | 250ms |  700   |  296  |  744  |  744  |  654  |
   | TFRC#2 | 250ms |  100   |  704  |  256  |  248  |  319  |
   +--------+-------+--------+-------+-------+-------+-------+
   |gTFRC#1 | 250ms |  700   |  744  |  800  |  696  |  727  |
   |gTFRC#2 | 250ms |  100   |  256  |  200  |  304  |  254  |
   +========+=======+========+=======+=======+=======+=======+
   | TFRC#1 | 640ms |  600   |  376  |  520  |  608  |  504  |
   | TFRC#2 |  40ms |  200   |  584  |  480  |  400  |  489  |
   +--------+-------+--------|-------+-------+-------+-------+
   |gTFRC#1 | 640ms |  600   |  376  |  600  |  600  |  554  |
   |gTFRC#2 |  40ms |  200   |  584  |  408  |  400  |  439  |
   +========+=======+========+=======+=======+=======+=======+

   Figure 3

   Extended results of this simulation campaign are available in [6]

3.3.  Analysis

   From these simulations, we see that gTFRC allows to reach a target
   rate more quickly than TFRC.  This is true whatever the RTT or the
   target rate of the flow.  The reason is obvious since at the first
   rate decrease evaluation of the algorithm, gTFRC returns a rate equal
   to the target rate.  If the evaluated rate is higher than the target
   rate, the classical TFRC algorithm is applied.  Concerning the
   DiffServ network behavior, the use of gTFRC raises the number of in-
   profile packets in the network and avoid the problem of the bandwidth
   sharing of the out-profile traffic.  For information purpose,
   concerning the Figure 2, between TFRC and gTFRC, the number of in-
   profile traffic raises from 73.7% to 90.16%.

3.4.  Discussion

3.4.1.  Security concern

   As we give the possibility to the application to instantiate through
   a setsockopt() function the target rate negotiated between the QoS
   network and the application, we can imagine that a misbehaving person
   could abuse of this functionality by giving an higher value to the
   guarantee g.  In this case, the misbehaving person sends an UDP-like



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   traffic and increases its out-profile traffic.  In the context of a
   DiffServ/AF class, the edge router will still mark in-profile the
   packets in respect with the negotiated profile and out-profile the
   excess part.  As as result, in case of network congestion, the
   dropping precedence set in the core router will drop this excess
   traffic.  The misbehaving person will not take advantage of the
   situation as the number of losses of its own flow increases as well.
   Then, the in-profile traffic remains protected in the network and the
   out-profile traffic perceives a kind of flooding attack.  As the out-
   profile traffic is a best-effort traffic, the use of gTFRC does not
   disturb the DiffServ network.

3.4.2.  Case of under-provisioned network

   In the context of a DiffServ/AF class, a network is under-provisioned
   when the amount of in-profile traffic is higher than the resource
   allocated to the AF class.  This case could occur if the Bandwidth
   Broker [4] of a DiffServ network sends or receives false information.
   In a DiffServ context, if the gTFRC source emits below its target
   rate and if the gTFRC flow gets losses, it means that the in-profile
   traffic is no guaranteed anymore in the network.  In order to tackle
   this problem, two approaches are possible.  The first one is to
   pursue to emit at the guarantee g.  This behaviour is legitimate
   since the service provider must provide to the client the service
   that it paid for.  The second one is to react to this congestion.
   This can be done by adding a second threshold (y) to gTFRC .  This
   threshold can be applied as following: if the emission rate X
   returned by the receiver is y times below the target rate g, the
   sender MUST emit to X. In the case where X < g/y, it means that a
   bunch of losses has occurred in the in-profile part and that the
   congestion could be due to a wrong setting.  We believe that this
   problem should not be solved inside gTFRC itself and should remain
   under the responsibility of the service provider.


















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4.  Acknowledgements

   This research work has been conducted in the framework of the EuQoS
   European project (http://www.euqos.org).  The authors has been
   supported by funding from National ICT Australia (NICTA).  The
   authors are also grateful Sally Floyd and Mark Allman and James M.
   Polk for useful discussions.

5.  References

   [1]  Heinanen, J., Weiss, W., Wroclawski, J., and J. Heinanen,
        "Assured Forwarding PHB Group", RFC 2597, STD 1, June 1999.

   [2]  Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP Friendly
        Rate Control (TFRC): Protocol Specification", RFC 3448, STD 1,
        January 2003.

   [3]  Pieda, P., Ethridge, J., Baines, M., and F. Shallwani, "A
        Network Simulator Differentiated Services Implementation", Open
        IP , Nortel Networks, available at http://www.isi.edu/nsman/ns,
        July 2000.

   [4]  Nichols, K., Jacobson, V., and L. Zhang, "A two-bit
        differentiated services architecture for the internet",
        RFC 2638, STD 1, July 1999.

   [5]  Heinanen, J. and R. Guerin, "A Single Rate Three Color Marker",
        RFC 2697, STD 1, September 1999.

   [6]  Lochin, E., Dairaine, L., and G. Jourjon, "gTFRC: a QoS-aware
        congestion Control Algorithm", 5th International Conference on
        Networking (ICN'2006) , October 2005.



















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Authors' Addresses

   Emmanuel Lochin
   National ICT Australia
   Australia Technology Park
   Eveleigh, NSW  1430
   Australia

   Phone: +61 8374 5541
   Email: Emmanuel.Lochin@nicta.com.au
   URI:   http://www.nicta.com.au/


   Laurent Dairaine
   ENSICA - LAAS/CNRS
   1, place Emile Blouin
   Toulouse, Cedex 5  31056
   France

   Phone: +33 5 61 61 85 00
   Email: Laurent.Dairaine@ensica.fr
   URI:   http://www.ensica.fr/


   Guillaume Jourjon
   National ICT Australia
   Australia Technology Park
   Eveleigh, NSW  1430
   Australia

   Phone: +61 8374 5206
   Email: Guillaume.Jourjon@nicta.com.au
   URI:   http://www.nicta.com.au/


















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