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Versions: 00 01 02                                                      
Network Working Group                                       I. Johansson
Internet-Draft                                               Ericsson AB
Intended status: Informational                             July 20, 2016
Expires: January 21, 2017


                Congestion control for 4G and 5G access
                    draft-johansson-cc-for-4g-5g-02

Abstract

   This memo outlines the challenge that 4G and 5G access brings for
   transport protocol congestion control and also outlines a few simple
   examples that can improve transport protocol congestion control
   performance in 4G and 5G access.

Requirements Language

   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 RFC 2119 [RFC2119].

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 January 21, 2017.

Copyright Notice

   Copyright (c) 2016 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
   publication of this document.  Please review these documents



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The 4G protocol stack impact on transport protocols . . . . .   3
     2.1.  PDCP layer  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  RLC layer . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  MAC layer . . . . . . . . . . . . . . . . . . . . . . . .   5
       2.3.1.  Downlink scheduling . . . . . . . . . . . . . . . . .   6
       2.3.2.  Uplink scheduling . . . . . . . . . . . . . . . . . .   6
   3.  4G and 5G evolution . . . . . . . . . . . . . . . . . . . . .   7
   4.  Requirements for improved performance . . . . . . . . . . . .   8
   5.  Congestion control examples . . . . . . . . . . . . . . . . .   9
     5.1.  Fast Increase . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Hybrid Cubic  . . . . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Wireless access is becoming more and more widely used, 4G (LTE)
   access is one wireless access technology that has built in support
   for seamless mobility that gives the end user a feeling of being
   always connected.  Transport endpoints may even be unaware of the
   existence of the 4G access.  Everyday use for 4G access includes web,
   chat, streaming video and lately also WebRTC.  These use cases pose
   different requirements on the underlying access.  Evolving existing
   radio-access technologies like LTE, and new 5G technologies will all
   be part of a future flexible and dynamic 5G system. 5G has potential
   to offer lower latency and higher peak throughput.  The goal of this
   document is to provide sufficient input to guide the development of
   congestion control that is better suited for 4G and 5G access,
   without an explicit need to know about the presence of 4G or 5G
   access along the transmission path.







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2.  The 4G protocol stack impact on transport protocols

   This section will go into the different layers in the 4G protocol
   stack.  It will not delve in the very details, recommended reading
   for more details is found in [LTE].  Rather this section will
   illustrate what effect each layer can have on the transport protocol
   efficiency.  The description is focused mainly on default radio
   access bearers, which are commonly used for OTT (Over The Top)
   services, these bearers typically use Acknowledged Mode (AM), which
   means that packet loss only occurs as the result of packet drops in
   AQM (Active Queue manager).  Specialized services like VoLTE (Voice
   over LTE) use different bearer configurations, this is however
   outside the scope of this document.  The concept of bearer is
   mentioned throughout the document, a bearer is to be seen as a data
   channel for a given terminal or UE (User Equipment), there could be
   many bearers with different priorities for a given terminal.

                               ------------
                               |          |
                               |    IP    |
                               |          |
                               ------------
                               |   PDCP   |
                               ------------
                               |   RLC    |
                               ------------
                               |   MAC    |
                               ------------
                               |   PHY    |
                               ------------

                            LTE protocol stack

2.1.  PDCP layer

   The PDCP (Packet Data Convergence Protocol Layer), ensures that
   intra-RAT (Radio Access Type) handover, i.e. LTE to LTE is reasonably
   seamless.  The PDCP layer makes sure that packets pending
   transmission in one cell to terminal connection, are transmitted in
   the new cell to terminal connection.  This way all packets will be
   ensured to be delivered to the endpoint.  PDCP also ensures that all
   packets are delivered in order up to higher layers.

   Packet retransmission typically means that the amount of data to
   transmit increases immediately after the handover, first the
   retransmission data needs to be transmitted, then the incoming data
   needs to be transmitted.  Depending on the available link throughput
   after the handover, an increased RTT may be experienced at a handover



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   event.  In addition, a small temporal delay increase can occur as
   packet transmission is inhibited at the handover event.  Unnecessary,
   retransmission at handover can be minimized by means of PDCP status
   reports, but this is not always implemented.  Because of the above it
   is a good practice to keep the amount of data in flight as small as
   possible, without sacrificing throughput.  Excessive amounts of data
   in flight means potentially more data to retransmit at handover and
   thus an even more increased RTT.

   Packets are typically not lost at handover in a 4G/LTE system.
   Reliable delivery at handover may however be turned off or is simply
   not implemented, this means that packets may be lost at handover.
   The amount of lost packets is then proportional to the number of
   packets in flight, it is therefore instrumental that the amount of
   packets in flight is as small as possible, with the objective to
   reach full link utilization, nothing more.  Bufferbloat [REF] can for
   instance lead to that 1000s of packets are lost at handover, which is
   of course undesired as it can affect media quality quite
   considerably.

2.2.  RLC layer

   The role of the RLC (Radio Link Control) layer is to ensure that
   packets are delivered in order up to the higher layers, in addition
   the RLC layer corrects errors that can occur on the MAC layer.  The
   in order delivery constraint means that additional HoL (Head of Line)
   blocking delay can occur due to errors on the MAC layer.

   The throughput on lower layers can vary quite considerably, this
   manifests itself as a varying size of the available transport.  The
   transport block size depends on how much of the available resources
   is allocated to a given bearer at a given time instant and also on
   the channel quality.  Because of this the size of the transport
   blocks can vary from tens of bytes, up to more than 10000 bytes.  The
   rate of change in transport block size also varies with terminal
   mobility as higher terminal mobility means faster changing channel
   fading and thus a faster changing channel quality.

   For optimal spectrum efficiency, it is important that a sufficient
   amount of data is available to fill the transport blocks, this data
   needs to be available already when a bearer is scheduled, in practice
   within a fraction of a millisecond.  To satisfy this requirement,
   packets need to be queued up and ready for immediate transmission,
   either on the RLC or the PDCP layer.  The transport protocol server
   (TCP, QUIC) is typically too far away to satisfy this need.

   The need to instantly provide sufficient data for optimal spectrum
   efficiency, given the variability in transport block sizes and



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   scheduling opportunities for a given bearer, quite naturally leads to
   a variation in queuing delay and this variation can be larger than
   what is to be expected from e.g. fixed line access.

   The requirement above to have data available can be seen something
   that contradicts the strive for low latency, and there is indeed a
   balance to be struck here.  What to aim for, maximum throughput or
   low latency, is something that depends on the requirements from the
   application.  A desire for very low latency comes generally at the
   cost of reduced peak throughput, this applies to default radio access
   bearers.  QoS classed bearers can have different characteristics and
   may well be able to deliver both very low latency and high
   throughput.

2.3.  MAC layer

   The MAC (Media Access Control) layer handles transmission of
   transport blocks, the outcome of a transmission attempt can be either
   success or failure.  The signaling of the success is handled with a
   single ACK/NAK bit.  Upon indicated failure, the transport block is
   retransmitted (with a different channel coding), soft decoding is
   utilized and the softbits of the first transmission and the
   retransmission are combined, hopefully with a successful result, if
   not the case a 3rd retransmission can occur and so on.  This is
   referred to as HARQ (Hybrid Automatic Repeat Request).  The maximum
   number of retransmissions is configurable, if the maximum number of
   retransmission is reached without successful transmission then the
   RLC layer will have to handle the retransmission instead.

   Errors may also happen in the transmission of the ACK/NAK bit.  The
   event that ACK is decoded as NAK will only lead to that an extra
   superfluous retransmission occur.  The event that a NAK is decoded as
   an ACK will be handled by the RLC layer as the result of a detected
   RLC checksum error.

   MAC layer retransmissions naturally lead to out of order delivery up
   to higher layers as some transport blocks are transmitted error free
   while others need retransmission.  The role of the RLC layer is to
   ensure in order delivery, the effect of this is that HARQ
   retransmissions and HARQ failures lead to additional delays.

   Scheduling of many bearers has the effect that available resources
   have to be shared between two or more bearers.  When new bearers with
   data to transmit are added in a cell (either handover, or new
   traffic), it means that the amount of resources need to be shared
   with an additional party.  This can give a large drop in available
   throughput for already existing users, with the effect of an




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   increased queuing delay that decreased only when the transmission
   rate over the bearer is reduced.

   The downlink and uplink scheduling differs in some details which are
   described in the following sub-sections.

2.3.1.  Downlink scheduling

   Downlink scheduling, or scheduling of packets to terminals, is
   controlled by the base station.  For each TTI (1ms interval) a
   decision is made on which bearer is to become scheduled, i.e.
   packets are forwarded from the queue to the terminal in question.
   The scheduling decision is based on channel quality, and possibly
   historic bitrate for the given bearers, or it may be just a simple
   round robin scheduler.  The very details of the scheduling algorithms
   are vendor specific.
   DRX (Discontinuous Reception) is a feature implemented to save
   battery power, in which the terminal sleeps and only checks for the
   presence on downlink data only at regular intervals.
   Given the facts above, downlink data cannot always be transmitted
   immediately, this has the effect that additional jitter may be added
   (in the order of 10-20ms).  Congestion control algorithms that are
   tuned with a high sensitivity to delay can by mistake treat this
   jitter as congestion.

2.3.2.  Uplink scheduling

   As is the case with downlink scheduling, uplink scheduling is
   controlled by the base station.  A terminal that has data to transmit
   will first transmit a scheduling request to the base station.  The
   scheduling request does not indicate how many bytes that are in the
   uplink queue.  The base station transmits a scheduling grant back,
   with a delay that depends on the overall load level.  The scheduling
   grant indicates how many bytes that can be transmitted by the
   terminal.  After this the terminal can transmit the allowed number of
   bytes, if there is still data in the queue, then a BSR (Buffer Status
   Report) is attached to the uplink transmission which will in turn
   trigger an additional scheduling grant from the base station to the
   terminal and so on until all the data in the queue is transmitted.
   The uplink scheduling regime outlined above can break up packet
   trains, for instance a set of 10 ACKs in the uplink may be broken up
   to an initial transmission of 2 ACKs followed by the transmission of
   the remaining 8 ACKs, the HARQ RTT is typically 8ms, which means that
   the remaining 8 ACKs are delivered upstream 8ms later.  This can
   cause problems for congestion control algorithms that depend on e.g.
   packet train based estimation of throughput.  Also, algorithms that
   depend on precise RTT estimates may by mistake treat the occurrence
   above as emerging congestion in the downlink.



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   This behavior can also trigger coalescing issues similar to those
   experienced when ACK compression occur.
   Worth notice is also that the above effects can occur at low as well
   as high network load levels.

   ACK traffic in uplink can also be delayed due to for instance lack of
   signaling channel resources for instance if many terminals generate
   ACK traffic that is so sparse that scheduling requests need to be
   generated with high frequency.  A transport protocol design that
   tries to limit the amount of ACK traffic can have a performance
   benefit under such circumstances as the limitation is then more
   controlled and the protocol can be optimized for this.  Reduced ACKs
   can unfortunately cause coalescing, something that may be mitigated
   to some extent by means of packet pacing.

3.  4G and 5G evolution

   It is currently unclear in what aspect a 5G protocol stack will
   affect transport protocol performance.  Listed below are however some
   features of evolved 4G and 5G that have relevance in this context:

   o  Shorter TTIs (Transmission Time Interval) has the potential to
      reduce the latency.  Given that shorter TTIs have impact on the
      scheduling and also the retransmissions, the impact of shorter
      TTIs is that errors on the MAC layer will cause less jitter.

   o  Larger throughput variations can occur as a result of techniques
      like carrier aggregation and dual connectivity.  Carrier
      aggregation means that additional carriers (possibly in very high
      frequency bands) are added.  Dual connectivity can combine two
      similar or different radio access technologies on lower layers
      (below IP).  Both technologies mentioned above can lead to large
      variations in available throughput.

   o  ECN is specified in 3GPP 36.300 [TS_36300].  ECN can provide with
      prompt indication of congestion without the need for packet drop
      caused by normal AQM operation, this can provide with a benefit
      for e.g. latency sensitive traffic.  ECN also gives a explicit
      indication of congestion, opposed to the implicit congestion
      indications that loss and delay gives.

   The shorter TTI feature is part of the 5G standardization, it should
   however be stated that the








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4.  Requirements for improved performance

   The above considerations lead to a few things to consider when
   congestion control is designed for optimal performance in 4G and 5G
   networks:

   o  Avoid dependencies on precise RTT estimates: A typical real life
      case is the Hybrid Slowstart algorithm bundled with the Cubic
      congestion control.  Uplink scheduling can break up transmission
      of ACKs which will in turn lead to increased RTTs that can falsely
      be interpreted as congestion.

   o  Use timestamps: Related to RTT estimate issue above.  For instance
      a modified Hybrid Slowstart algorithm can take timestamp values
      into account and thus limit the effect of uplink scheduling
      effects that can distort the transmission of ACKs.

   o  Minimize latency under load: The quickly changing throughput in
      4G/5G calls for a sensible balance between latency and throughput.
      Some amount of bufferbloat needs to be accepted in order to have
      enough data to utilize the radio resources optimally and get a
      high spectrum efficiency, this can however make the reaction to
      reduced throughput more sluggish.  Hybrid loss/delay based
      congestion control can here be used to find a good balance between
      latency and throughput.

   o  ECN support: The transport protocols should support negotiation
      and use of ECN.

   o  Faster congestion window increase: Traditional AIMD (Additive
      Increase Multiplicative Decrease) based congestion avoidance
      algorithms are too slow to gain the benefits of e.g added
      carriers, therefore, more high speed alternatives should be
      considered, that are still reactive to congestion.

   o  Packet pacing: ACK compression effects can easily occur in 4G/5G
      networks, packet pacing should be encouraged to mitigate the
      coalescing effects caused by ACK compression, and will at the same
      time make ECN detection algorithms more robust.

   o  ACK reduction: Consider if it is possible to reduce the intensity
      of acknowledgements, especially in the uplink.  Packet pacing may
      here be beneficial as it can mitigate the coalescing effects that
      can occur due to reduced ACK intensity.







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5.  Congestion control examples

   This section lists a few examples of algorithm that can be useful

   o  Default slowstart algorithms generally only operates at flow
      start-up and after a retransmission timeout.  The drawback with
      this approach is that the congestion control cannot quickly grab
      new available capacity due to e.g the addition of an extra
      carrier.  Fast Increase is a simple add-on to Cubic that resumes
      slowstart if it is detected that the bottleneck is underutilized
      for a while.  A similar approach can be used for other congestion
      control algorithms.

   o  Hybrid Cubic borrows the queue delay estimation from LEDBAT
      [RFC6817].  With this addition it is possible to set a target
      queuing delay that adds a limit to the congestion window based on
      network queuing delay in addition to the already existing loss
      based control of the congestion window.

   o  Various High Speed congestion control algorithms such as SIAD
      (Scalable Increase Adaptive Decrease) [TCP_SIAD] can provide with
      improved performance in the presence of large changes in available
      throughput resulting from e.g added carriers.

   The Fast Increase and Hybrid Cubic algorithms are described in more
   detail below.  The code samples are shown with the Linux kernel 4.4
   version of tcp_cubic.c as basis.

5.1.  Fast Increase

   The idea behind Fast Increase is simply to allow the Cubic congestion
   control to rapidly increase the congestion window if the RTT has been
   only marginally higher than the min RTT for a number of round trips,
   this is realised by forcing the ca->cnt variable to a low value.  The
   HyStart delay algorithm used for this purpose.  Code for this is
   shown below with the code from Linux tcp_cubic.c as basis

   =======================
   Function bictcp_acked(..) is modified according
   to the code snippet below
   New state variables are added, all initialized to 0
     u32 last_rtt_high
     u32 last_fast_increase
     u8 do_fast_increase
   New constants
     #define N_RTT_LOW 5
     #define N_RTT_FAST_INCREASE 20




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   /** Old code **/
   /* first time call or link delay decreases */
   if (ca->delay_min == 0 || ca->delay_min > delay)
      ca->delay_min = delay;


   /** New code **/
   /*
    * Function bictcp_acked is modified to initiate fast increase when
    * RTT is lower than a given value for a given number of RTTs
    * srtt_l is a long term average of srtt
    */
   if (ca->curr_rtt - ca->delay_min > (srtt_l-ca->delay_min)/2) {
      ca->last_rtt_high = bictcp_clock();
   } else {
      u32 now = bictcp_clock();
      if (now - ca->last_rtt_high > N_RTT_LOW*ca->delay_min &&
          now - ca->last_fast_increase > N_RTT_FAST_INCREASE*ca->delay_min) {
        ca->do_fast_increase = 1;
        ca->last_fast_increase = now;
      }
   }
   /** End of new code **/


   =======================
   Function bictcp_update(...) is updated with a code snippet after
   tcp_friendliness
   /*
   * Enable fast increase of the CWND
   */
   if (ca->do_fast_increase)
     ca->cnt = min(ca->cnt,2);


   =======================
   Function bictcp_recalc_ssthresh(...) is modified
   /*
   * Resent do_fast_increase flag
   */
   ca->do_fast_increase = 0;



                            Fast Increase code

   ca->last_fast_increase should be updated to current time whenever a
   loss or ECN event is detected.



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5.2.  Hybrid Cubic

   The Hybrid Cubic algorithm adds delay sensitivity to the Cubic
   congestion avoidance algorithm.  It is, in the description below
   assumed that the timestamp option is enabled and that queue delay
   samples are computed, according to the description in LEDBAT
   [RFC6817].  Furthermore it is assumed than the timestamp clock
   frequency in sender and receiver are identical or that the sender can
   infer the timestamp clock frequency of the receiver and recompute
   timestamp values based on this information.

   The function bictcp_update is updated according to the
   code snippet below.
   New state variables
     float queue_delay
     u32 last_hycubic_cwnd_reduced
   New constants
     #define QUEUE_DELAY_TARGET 0.1
     #define QUEUE_DELAY_GAIN_UP 100.0
     #define QUEUE_DELAY_GAIN_DOWN 0.2



   /** New code, inserted before tcp_friendlness: **/
   /*
    * The cnt variable is modified depending on the
    * relation between the OWD and the OWD target
    */
   if (ca->owd < QUEUE_DELAY_TARGET) {
      float tmp = ca->queue_delay/QUEUE_DELAY_TARGET;
      int cnt_d = (int) (tmp*QUEUE_DELAY_GAIN_UP);
      /*
       * q is less than QUEUE_DELAY target
       * Increase cnt as OWD is approaching target
       * This will slow down congestion window growth
       * when owd increases
       */
      ca->cnt += cnt_d;
   } else {
      /*
       * Set cnt to a high value,to prevent further growth
       */
      ca->cnt = 1000;
   }
   /** End of new code **/






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   /* TCP Friendly */
   if (tcp_friendliness) {
      u32 scale = beta_scale;

      delta = (cwnd * scale) >> 3;
      /** New code **/
      if (ca->queue_delay < QUEUE_DELAY_TARGET) {
      /** End of new code **/
         while (ca->ack_cnt > delta) {  /* update tcp cwnd */
            ca->ack_cnt -= delta;
            ca->tcp_cwnd++;
         }
      /** New code **/
      } else {
         u32 now = bictcp_clock();
         if (now-ca->last_hycubic_cwnd_reduced > delay) {
            /* At most one reduction per RTT
            float overshoot = (ca->queue_delay-QUEUE_DELAY_TARGET)/delay;
            float alpha = min(0.5,overshoot*QUEUE_DELAY_GAIN_DOWN);
            ca->tcp_cwnd = (int)(ca->tcp_cwnd*(1.0-alpha));
            ca->ssthresh = ca->tcp_cwnd;
            ca->epoch_start = 0;
            ca->last_hycubic_cwnd_reduced = now;
         }

      }
      /** End of new code **/

      if (ca->tcp_cwnd > cwnd) { /* if bic is slower than tcp */
         delta = ca->tcp_cwnd - cwnd;
         max_cnt = cwnd / delta;
         if (ca->cnt > max_cnt)
            ca->cnt = max_cnt;
      }
   }



                               Hybrid Cubic

   Note that the code is not fully functional, for instance the floating
   point arithmetic need to be converted to fixed point ditto.

6.  IANA Considerations

   This document makes no request of IANA.





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7.  Security Considerations

   The possible outcome of this work has the same possible security
   considerations as other work around congestion control.

8.  Acknowledgements

   The following persons have contributed with comments and suggestions
   for improvements: Kristofer Sandlund, Mats Nordberg, Hans Hannu,
   Torsten Dudda and Szilveszter Nadas.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

9.2.  Informative References

   [LTE]      "4G: LTE/LTE-Advanced for Mobile Broadband, Second
              Edition", 2013.

   [RFC6817]  Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
              DOI 10.17487/RFC6817, December 2012,
              <http://www.rfc-editor.org/info/rfc6817>.

   [RFC7567]  Baker, F., Ed. and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management",
              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
              <http://www.rfc-editor.org/info/rfc7567>.

   [TCP_SIAD]
              "TCP SIAD: Congestion Control
              https://www.ietf.org/proceedings/91/slides/slides-91-
              iccrg-5.pdf", 2015.

   [TS_36300]
              "3GPP TS 36.300", 2015.

Author's Address







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   Ingemar Johansson
   Ericsson AB
   Laboratoriegraend 11
   Luleaa  977 53
   Sweden

   Phone: +46 730783289
   Email: ingemar.s.johansson@ericsson.com











































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