rfc2963.txt

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Network Working Group                                      O. Bonaventure
Request for Comments: 2963                                          FUNDP
Category: Informational                                     S. De Cnodder
                                                                  Alcatel
                                                             October 2000


           A Rate Adaptive Shaper for Differentiated Services

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This memo describes several Rate Adaptive Shapers (RAS) that can be
   used in combination with the single rate Three Color Markers (srTCM)
   and the two rate Three Color Marker (trTCM) described in RFC2697 and
   RFC2698, respectively.  These RAS improve the performance of TCP when
   a TCM is used at the ingress of a diffserv network by reducing the
   burstiness of the traffic.  With TCP traffic, this reduction of the
   burstiness is accompanied by a reduction of the number of marked
   packets and by an improved TCP goodput.  The proposed RAS can be used
   at the ingress of Diffserv networks providing the Assured Forwarding
   Per Hop Behavior (AF PHB).  They are especially useful when a TCM is
   used to mark traffic composed of a small number of TCP connections.

1. Introduction

   In DiffServ networks [RFC2475], the incoming data traffic, with the
   AF PHB in particular, could be subject to marking where the purpose
   of this marking is to provide a low drop probability to a minimum
   part of the traffic whereas the excess will have a larger drop
   probability.  Such markers are mainly token bucket based such as the
   single rate Three Color Marker (srTCM) and two rate Three Color
   Marker (trTCM) described in [RFC2697] and [RFC2698], respectively.

   Similar markers were proposed for ATM networks and simulations have
   shown that their performance with TCP traffic was not always
   satisfactory and several researchers have shown that these
   performance problems could be solved in two ways:




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   1. increasing the burst size, i.e. increasing the Committed Burst
      Size (CBS) and the Peak Burst Size (PBS) in case of the trTCM, or

   2. shaping the traffic such that a part of the burstiness is removed.

   The first solution has as major disadvantage that the traffic sent to
   the network can be very bursty and thus engineering the network to
   provide a low packet loss ratio can become difficult.  To efficiently
   support bursty traffic, additional resources such as buffer space are
   needed.  Conversely, the major disadvantage of shaping is that the
   traffic encounters additional delay in the shaper's buffer.

   In this document, we propose two shapers that can reduce the
   burstiness of the traffic upstream of a TCM.  By reducing the
   burstiness of the traffic, the adaptive shapers increase the
   percentage of packets marked as green by the TCM and thus the overall
   goodput of the users attached to such a shaper.

   Such rate adaptive shapers will probably be useful at the edge of the
   network (i.e. inside access routers or even network adapters).  The
   simulation results in [Cnodder] show that these shapers are
   particularly useful when a small number of TCP connections are
   processed by a TCM.

   The structure of this document follows the structure proposed in
   [Nichols].  We first describe two types of rate adaptive shapers in
   section two.  These shapers correspond to respectively the srTCM and
   the trTCM.  In section 3, we describe an extension to the simple
   shapers that can provide a better performance. We briefly discuss
   simulation results in the appendix.

2. Description of the rate adaptive shapers

2.1. Rate adaptive shaper

   The rate adaptive shaper is based on a similar shaper proposed in
   [Bonaventure] to improve the performance of TCP with the Guaranteed
   Frame Rate [TM41] service category in ATM networks.  Another type of
   rate adaptive shaper suitable for differentiated services was briefly
   discussed in [Azeem].  A RAS will typically be used as shown in
   figure 1 where the meter and the marker are the TCMs proposed in
   [RFC2697] and [RFC2698].









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                                     Result
                                  +----------+
                                  |          |
                                  |          V
                 +--------+   +-------+   +--------+
      Incoming   |        |   |       |   |        |   Outgoing
      Packet  ==>|  RAS   |==>| Meter |==>| Marker |==>Packet
      Stream     |        |   |       |   |        |   Stream
                 +--------+   +-------+   +--------+

                        Figure 1. Rate adaptive shaper

   The presentation of the rate adaptive shapers in Figure 1 is somewhat
   different as described in [RFC2475] where the shaper is placed after
   the meter.  The main objective of the shaper is to produce at its
   output a traffic that is less bursty than the input traffic, but the
   shaper avoids to discard packets in contrast with classical token
   bucket based shapers.  The shaper itself consists of a tail-drop FIFO
   queue which is emptied at a variable rate.  The shaping rate, i.e.
   the rate at which the queue is emptied, is a function of the
   occupancy of the FIFO queue.  If the queue occupancy increases, the
   shaping rate will also increase in order to prevent loss and too
   large delays through the shaper.  The shaping rate is also a function
   of the average rate of the incoming traffic.  The shaper was designed
   to be used in conjunction with meters such as the TCMs proposed in
   [RFC2697] and [RFC2698].

   There are two types of rate adaptive shapers.  The single rate rate
   adaptive shaper (srRAS) will typically be used upstream of a srTCM
   while the two rates rate adaptive shaper (trRAS) will usually be used
   upstream of a trTCM.

2.2. Configuration of the srRAS

   The srRAS is configured by specifying four parameters: the Committed
   Information Rate (CIR), the Maximum Information Rate (MIR) and two
   buffer thresholds: CIR_th (Committed Information Rate threshold) and
   MIR_th (Maximum Information Rate threshold).  The CIR shall be
   specified in bytes per second and MUST be configurable.  The MIR
   shall be specified in the same unit as the CIR and SHOULD be
   configurable.  To achieve a good performance, the CIR of a srRAS will
   usually be set to the same value as the CIR of the downstream srTCM.
   A typical value for the MIR would be the line rate of the output link
   of the shaper.  When the CIR and optionally the MIR are configured,
   the srRAS MUST ensure that the following relation is verified:






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               CIR <= MIR <= line rate

   The two buffer thresholds, CIR_th and MIR_th shall be specified in
   bytes and SHOULD be configurable.  If these thresholds are
   configured, then the srRAS MUST ensure that the following relation
   holds:

               CIR_th <= MIR_th <= buffer size of the shaper

   The chosen values for CIR_th and MIR_th will usually depend on the
   values chosen for CBS and PBS in the downstream srTCM.  However, this
   dependency does not need to be standardized.

2.3. Behavior of the srRAS

   The output rate of the shaper is based on two factors.  The first one
   is the (long term) average rate of the incoming traffic.  This
   average rate can be computed by several means.  For example, the
   function proposed in [Stoica] can be used (i.e. EARnew = [(1-exp(-
   T/K))*L/T] + exp(-T/K)*EARold where EARold is the previous value of
   the Estimated Average Rate, EARnew is the updated value, K a
   constant, L the size of the arriving packet and T the amount of time
   since the arrival of the previous packet).  Other averaging functions
   can be used as well.

   The second factor is the instantaneous occupancy of the FIFO buffer
   of the shaper.  When the buffer occupancy is below CIR_th, the output
   rate of the shaper is set to the maximum of the estimated average
   rate (EAR(t)) and the CIR.  This ensures that the shaper buffer will
   be emptied at least at a rate equal to CIR.  When the buffer
   occupancy increases above CIR_th, the output rate of the shaper is
   computed as the maximum of the EAR(t) and a linear function F of the
   buffer occupancy for which F(CIR_th)=CIR and F(MIR_th)=MIR.  When the
   buffer occupancy reaches the MIR_th threshold, the output rate of the
   shaper is set to the maximum information rate.  The computation of
   the shaping rate is illustrated in figure 2.  We expect that real
   implementations will only use an approximate function to compute the
   shaping rate.













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                   ^
     Shaping rate  |
                   |
                   |
              MIR  |                      =========
                   |                    //
                   |                  //
           EAR(t)  |----------------//
                   |              //
                   |            //
             CIR   |============
                   |
                   |
                   |
                   |------------+---------+----------------------->
                             CIR_th      MIR_th Buffer occupancy

              Figure 2. Computation of shaping rate for srRAS

2.4. Configuration of the trRAS

   The trRAS is configured by specifying six parameters: the Committed
   Information Rate (CIR), the Peak Information Rate (PIR), the Maximum
   Information Rate (MIR) and three buffer thresholds: CIR_th, PIR_th
   and MIR_th.  The CIR shall be specified in bytes per second and MUST
   be configurable.  To achieve a good performance, the CIR of a trRAS
   will usually be set at the same value as the CIR of the downstream
   trTCM.  The PIR shall be specified in the same unit as the CIR and
   MUST be configurable.  To achieve a good performance, the PIR of a
   trRAS will usually be set at the same value as the PIR of the
   downstream trRAS.  The MIR SHOULD be configurable and shall be
   specified in the same unit as the CIR.  A typical value for the MIR
   will be the line rate of the output link of the shaper.  When the
   values for CIR, PIR and optionally MIR are configured, the trRAS MUST
   ensure that the following relation is verified:

               CIR <= PIR <= MIR <= line rate

   The three buffer thresholds, CIR_th, PIR_th and MIR_th shall be
   specified in bytes and SHOULD be configurable.  If these thresholds
   are configured, then the trRAS MUST ensure that the following
   relation is verified:

               CIR_th <= PIR_th <= MIR_th <= buffer size of the shaper

   The CIR_th, PIR_th and MIR_th will usually depend on the values
   chosen for the CBS and the PBS in the downstream trTCM.  However,
   this dependency does not need to be standardized.



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2.5. Behavior of the trRAS

   The output rate of the trRAS is based on two factors.  The first is
   the (long term) average rate of the incoming traffic.  This average
   rate can be computed as for the srRAS.

   The second factor is the instantaneous occupancy of the FIFO buffer
   of the shaper.  When the buffer occupancy is below CIR_th, the output
   rate of the shaper is set to the maximum of the estimated average
   rate (EAR(t)) and the CIR.  This ensures that the shaper will always
   send traffic at least at the CIR.  When the buffer occupancy
   increases above CIR_th, the output rate of the shaper is computed as
   the maximum of the EAR(t) and a piecewise linear function F of the
   buffer occupancy.  This piecewise function can be defined as follows.
   The first piece is between zero and CIR_th where F is equal to CIR.
   This means that when the buffer occupancy is below a certain
   threshold CIR_th, the shaping rate is at least CIR.  The second piece
   is between CIR_th and PIR_th where F increases linearly from CIR to
   PIR.  The third part is from PIR_th to MIR_th where F increases
   linearly from PIR to the MIR and finally when the buffer occupancy is
   above MIR_th, the shaping rate remains constant at the MIR.  The
   computation of the shaping rate is illustrated in figure 3.  We
   expect that real implementations will use an approximation of the
   function shown in this figure to compute the shaping rate.

                 ^
   Shaping rate  |
                 |
           MIR   |                               ======
                 |                            ///
                 |                         ///
           PIR   |                      ///
                 |                    //
                 |                  //
         EAR(t)  |----------------//
                 |              //
                 |            //
           CIR   |============
                 |
                 |
                 |
                 |------------+---------+--------+-------------------->
                         CIR_th      PIR_th    MIR_th  Buffer occupancy

            Figure 3. Computation of shaping rate for trRAS






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3. Description of the green RAS.

3.1. The green rate adaptive shapers

   The srRAS and the trRAS described in the previous section are not
   aware of the status of the meter.  This entails that a RAS could
   unnecessarily delay a packet although there are sufficient tokens
   available to color the packet green.  This delay could mean that TCP
   takes more time to increase its congestion window and this may lower
   the performance with TCP traffic.  The green RAS shown in figure 4
   solves this problem by coupling the shaper with the meter.

                         Status       Result
                      +----------+ +----------+