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Network Working Group                                            R. Miao
Internet-Draft                                                    H. Liu
Intended status: Experimental                              Alibaba Group
Expires: January 30, 2021                                         R. Pan
                                                                  J. Lee
                                                                  C. Kim
                                                       Intel Corporation
                                                                B. Gafni
                                                           Y. Shpigelman
                                             Mellanox Technologies, Inc.
                                                           July 29, 2020


           HPCC++: Enhanced High Precision Congestion Control
                      draft-pan-tsvwg-hpccplus-01

Abstract

   Congestion control (CC) is the key to achieving ultra-low latency,
   high bandwidth and network stability in high-speed networks.
   However, the existing high-speed CC schemes have inherent limitations
   for reaching these goals.

   In this document, we describe HPCC++ (High Precision Congestion
   Control), a new high-speed CC mechanism which achieves the three
   goals simultaneously.  HPCC++ leverages inband telemetry to obtain
   precise link load information and controls traffic precisely.  By
   addressing challenges such as delayed inband telemetry information
   during congestion and overreaction to inband telemetry information,
   HPCC++ can quickly converge to utilize free bandwidth while avoiding
   congestion, and can maintain near-zero in-network queues for ultra-
   low latency.  HPCC++ is also fair and easy to deploy in hardware,
   implementable with commodity NICs and switches.

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 https://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."



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   This Internet-Draft will expire on January 30, 2021.

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  System Overview . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  HPCC++ Algorithm  . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Notations . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Design Functions and Procedures . . . . . . . . . . . . .   6
   5.  Configuration Parameters  . . . . . . . . . . . . . . . . . .   8
   6.  Design Enhancement and Implementation . . . . . . . . . . . .   8
     6.1.  HPCC++ Guidelines . . . . . . . . . . . . . . . . . . . .   9
     6.2.  Receiver-based HPCC . . . . . . . . . . . . . . . . . . .   9
     6.3.  Switch-side Optimizations . . . . . . . . . . . . . . . .  10
   7.  Reference Implementations . . . . . . . . . . . . . . . . . .  11
     7.1.  Inband telemetry padding at the network elements  . . . .  11
     7.2.  Congestion control at NICs  . . . . . . . . . . . . . . .  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  13
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  13
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     12.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   The link speed in data center networks has grown from 1Gbps to
   100Gbps in the past decade, and this growth is continuing.  Ultralow
   latency and high bandwidth, which are demanded by more and more




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   applications, are two critical requirements in today's and future
   high-speed networks.

   Given that traditional software-based network stacks in hosts can no
   longer sustain the critical latency and bandwidth requirements
   [Zhu-SIGCOMM2015], offloading network stacks into hardware is an
   inevitable direction in high-speed networks.  Large-scale networks
   with RDMA (remote direct memory access) over Converged Ethernet
   Version 2 (RoCEv2) often uses hardware-offloading solutions.  In some
   cases, the RDMA networks still face fundamental challenges to
   reconcile low latency, high bandwidth utilization, and high
   stability.

   This document describes a new CC mechanism, HPCC++ (Enhanced High
   Precision Congestion Control), for large-scale, high-speed networks.
   The key idea behind HPCC++ is to leverage the precise link load
   information from inband telemetry to compute accurate flow rate
   updates.  Unlike existing approaches that often require a large
   number of iterations to find the proper flow rates, HPCC++ requires
   only one rate update step in most cases.  Using precise information
   from inband telemetry enables HPCC++ to address the limitations in
   current CC schemes.  First, HPCC++ senders can quickly ramp up flow
   rates for high utilization and ramp down flow rates for congestion
   avoidance.  Second, HPCC++ senders can quickly adjust the flow rates
   to keep each link's output rate slightly lower than the link's
   capacity, preventing queues from being built-up as well as preserving
   high link utilization.  Finally, since sending rates are computed
   precisely based on direct measurements at switches, HPCC++ requires
   merely three independent parameters that are used to tune fairness
   and efficiency.

   The base form of HPCC++ is the original HPCC algorithm and its full
   description can be found in [SIGCOMM-HPCC].  While the original
   design lays the foundation for inband telemetry based precision
   congestion control, HPCC++ is an enhanced version which takes into
   account system constraints and aims to reduce the design overhead and
   further improves the performance.  Section 6 describes these detailed
   proposed design enhancements and guidelines.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.





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3.  System Overview

   Figure 1 shows the end-to-end system that HPCC++ operates in.  During
   the traverse of the packet from the sender to the receiver, each
   switch along the path inserts inband telemetry that reports the
   current state of the packet's egress port, including timestamp (ts),
   queue length (qLen), transmitted bytes (txBytes), and the link
   bandwidth capacity (B), together with switch_ID and port_ID.  When
   the receiver gets the packet, it may copy all the inband telemetry
   recorded from the network to the ACK message it sends back to the
   sender, and then the sender decides how to adjust its flow rate each
   time it receives an ACK with network load information.
   Alternatively, the receiver may calculate the flow rate based on the
   inband telemetry information and feedback the calculated rate back to
   the sender.  The notification packets would include delayed ack
   information as well.

   Note that there also exist network nodes along the reverse
   (potentially uncongested) path that the RTCP feedback reports
   traverse.  Those network nodes are not shown in the figure for sake
   of brevity.



    +---------+  pkt    +-------+ pkt+tlm +-------+ pkt+tlm +----------+
    |  Data   |-------->|       |-------->|       |-------->| Data     |
    |  Sender |=========|Switch1|=========|Switch2|=========| Receiver |
    +---------+ Link-0  +-------+  Link-1 +-------+  Link-2 +----------+
        /|\                                                        |
         |                                                         |
         +---------------------------------------------------------+
                         Notification Packets/ACKs

              Figure 1: System Overview (tlm=inband telemtry)

   o  Data sender: responsible for controlling inflight bytes.  HPCC++
      is a window-based CC scheme that controls the number of inflight
      bytes.  The inflight bytes mean the amount of data that have been
      sent, but not acknowledged at the sender yet.  Controlling
      inflight bytes has an important advantage compared to controlling
      rates.  In the absence of congestion, the inflight bytes and rate
      are interchangeable with equation inflight = rate * T where T is
      the base propagation RTT.  The rate can be calculated locally or
      obtained from the notification packet.  The sender may further use
      the data pacing mechanism in hardware to limit the rate
      accordingly.





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   o  Network nodes: responsible of inserting the inband telemetry
      information to the data packet.  The inband telemetry information
      reports the current load of the packet's egress port, including
      timestamp (ts), queue length (qLen), transmitted bytes (txBytes),
      and the link bandwidth capacity (B).  Besides, the inband
      telemetry contains switch_ID and port_ID to identify a link.

   o  Data receiver: responsible for either reflecting back the inband
      telemetry information in the data packet or calculating the proper
      flow rate based on network congestion information in inband
      telemetry and sending notification packets back to the sender.

4.  HPCC++ Algorithm

   HPCC++ is a window-based congestion control algorithm.  The key
   design choice of HPCC++ is to rely on network nodes to provide fine-
   grained load information, such as queue size and accumulated tx/rx
   traffic to compute precise flow rates.  This has two major benefits:
   (i) HPCC++ can quickly converge to proper flow rates to highly
   utilize bandwidth while avoiding congestion; and (ii) HPCC++ can
   consistently maintain a close-to-zero queue for low latency.

   This section introduces the list of notations and describes the core
   congestion control algorithm.

4.1.  Notations

   This section summarizes the list of variables and parameters used in
   the HPCC++ algorithm.  Figure 3 also includes the default values for
   choosing the algorithm parameters either to represent a typical
   setting in practical applications or based on theoretical and
   simulation studies.



















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     +--------------+-------------------------------------------------+
     | Notation     | Variable Name                                   |
     +--------------+-------------------------------------------------+
     | W_i          | Window for flow i                               |
     | Wc_i         | Reference window for flow i                     |
     | B_j          | Bandwidth for Link j                            |
     | I_j          | Estimated inflight bytes for Link j             |
     | U_j          | Normalized inflight bytes for Link j            |
     | qlen         | Telemetry info: link j queue length             |
     | txRate       | Telemetry info: link j output rate              |
     | ts           | Telemetry info: timestamp                       |
     | txBytes      | Telemetry info: link j total transmitted bytes  |
     |              |                  associated with timestamp ts   |
     +--------------+-------------------------------------------------+

                       Figure 2: List of variables.

    +--------------+----------------------------------+----------------+
    | Notation     | Parameter Name                   | Default Value  |
    +--------------+----------------------------------+----------------+
    | T            | Known baseline RTT               |    5us         |
    | eta          | Target link utilization          |    95%         |
    | maxStage     | Maximum stages for additive      |                |
    |              | increases                        |    5           |
    | N            | Maximum number of flows          |    ...         |
    | W_ai         | Additive increase amount         |    ...         |
    +--------------+----------------------------------+----------------+

     Figure 3: List of algorithm parameters and their default values.

4.2.  Design Functions and Procedures

   The HPCC++ algorithm can be outlined as below:

   1:    Function MeasureInflight(ack)
   2:    u = 0;
   3:    for each link i on the path do
   4:              ack.L[i].txBytes-L[i].txBytes
         txRate =  ----------------------------- ;
                       ack.L[i].ts-L[i].ts
   5:          min(ack.L[i].qlen,L[i].qlen)      txRate
         u' = ----------------------------- +  ---------- ;
                   ack.L[i].B*T                ack.L[i].B
   6:    if u' > u then
   7:       u = u'; tau = ack.L[i].ts -  L[i].ts;
   8:    tau = min(tau, T);
   9:    U = (1 - tau/T)*U + tau/T*u;
   10:   return U;



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   11:   Function ComputeWind(U, updateWc)
   12:   if U >= eta or incStage >= maxStagee then
   13:           Wc
            W = ----- + W_ai;
                U/eta
   14:   if updateWc then
   15:      incStagee = 0; Wc = W ;
   16:   else
   17:      W = Wc + W_ai ;
   18:      if updateWc then
   19:         incStage++; Wc = W ;
   20:   return W

   21:   Procedure NewAck(ack)
   22:   if ack.seq > lastUpdateSeq then
   23:      W = ComputeWind(MeasureInflight(ack), True);
   24:      lastUpdateSeq = snd_nxt;
   25:   else
   26:      W = ComputeWind(MeasureInflight(ack), False);
   27:   R = W/T; L = ack.L;

   The above illustrates the overall process of CC at the sender side
   for a single flow.  Each newly received ACK message triggers the
   procedure NewACK at Line 21.  At Line 22, the variable lastUpdateSeq
   is used to remember the first packet sent with a new W c , and the
   sequence number in the incoming ACK should be larger than
   lastUpdateSeq to trigger a new sync betweenW c andW (Line 14-15 and
   18-19).  The sender also remembers the pacing rate and current inband
   telemetry information at Line 27.  The sender computes a new window
   size W at Line 23 or Line 26, depending on whether to update W c ,
   with function MeasureInflight and ComputeWind.  Function
   MeasureInflight estimates normalized inflight bytes with Eqn (2) at
   Line 5.  First, it computes txRate of each link from the current and
   last accumulated transferred bytes txBytes and timestamp ts (Line 4).
   It also uses the minimum of the current and last qlen to filter out
   noises in qlen (Line 5).  The loop from Line 3 to 7 selects maxi(Ui)
   in Eqn. (3).  Instead of directly using maxi(Ui), we use an EWMA
   (Exponentially Weighted Moving Average) to filter the noises from
   timer inaccuracy and transient queues.  (Line 9).  Function
   ComputeWind combines multiplicative increase/ decrease (MI/MD) and
   additive increase (AI) to balance the reaction speed and fairness.
   If a sender finds it should increase the window size, it first tries
   AI for maxStage times with the stepWAI (Line 17).  If it still finds
   room to increase after maxStage times of AI or the normalized
   inflight bytes is above, it calls Eqn (4) once to quickly ramp up or
   ramp down the window size (Line 12-13).





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5.  Configuration Parameters

   HPCC++ has three easy-to-set parameters: eta, maxStagee, and W_ai.
   eta controls a simple tradeoff between utilization and transient
   queue length (due to the temporary collision of packets caused by
   their random arrivals, so we set it to 95% by default, which only
   loses 5% bandwidth but achieves almost zero queue.  maxStage controls
   a simple tradeoff between steady state stability and the speed to
   reclaim free bandwidth.  We find maxStage = 5 is conservatively large
   for stability, while the speed of reclaiming free bandwidth is still
   much faster than traditional additive increase, especially in high
   bandwidth networks.  W_ai controls the tradeoff between the maximum
   number of concurrent flows on a link that can sustain near-zero
   queues and the speed of convergence to fairness.  Note that none of
   the three parameters are reliability-critical.

   HPCC++'s design brings advantages to short-lived flows, by allowing
   flows starting at line-rate and the separation of utilization
   convergence and fairness convergence.  HPCC++ achieves fast
   utilization convergence to mitigate congestion in almost one round-
   trip time, while allows flows to gradually converge to fairness.
   This design feature of HPCC++ is especially helpful for the workload
   of datacenter applications, where flows are usually short and
   latency-sensitive.  Normally we set a very small W_ai to support a
   large number of concurrent flows on a link, because slower fairness
   is not critical.  A rule of thumb is to set W_ai = W_init*(1-eta) / N
   where N is the expected or receiver reported maximum number of
   concurrent flows on a link.  The intuition is that the total additive
   increase every round (N*W_ai ) should not exceed the bandwidth
   headroom, and thus no queue forms.  Even if the actual number of
   concurrent flows on a link exceeds N, the CC is still stable and
   achieves full utilization, but just cannot maintain zero queues.

6.  Design Enhancement and Implementation

   The basic design of HPCC++, i.e. HPCC, as described above is to add
   inband telemetry information into every data packet to response
   congestion as soon as the very first packet observing the network
   congestion.  This is especially helpful to reduce the risk of severe
   congestion in incast scenario at the first round-trip time.  In
   addition, original HPCC's algorithm introduction of Wc is for the
   purpose of solving the over-reaction issue from using this per-packet
   response.

   Alternatively, the inband telemetry information needs not to be added
   to every data packet to reduce the overhead.  Switches can generate
   inband telemetry less frequently, e.g., once per RTT or upon
   congestion happening.



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6.1.  HPCC++ Guidelines

   To ensure network stability, HPCC++ establishes a few guidelines for
   different implementations:

   o  The algorithm should commit the window/rate update at most once
      per round-trip time, similar to the procedure of updating Wc.

   o  To support different workloads and to properly set W_ai, HPCC++
      allows the option to incorporate mechanisms to speed up the
      fairness convergence.

   o  The switch should capture inband telemetry information that
      includes link load (txBytes, qlen, ts) and link spec (switch_ID,
      port_ID, B) at the egress port.  Note, each switch should record
      all those information at the single snapshot to achieve a precise
      link load estimate.

   o  HPCC++ can use a probe packet to query the inband telemetry
      information.  Thereby, the probe packets should take the same
      routing path and QoS queueing with the data packets.

   As long the above guidelines are met, this document does not mandate
   a particular inband telemetry header format or encapsulation, which
   are orthogonal to the HPCC++ algorithms described in this document.
   The algorithm can be implemented with a choice of inband telemetry
   protocols, such as in-band network telemetry [P4-INT], IOAM
   [I-D.ietf-ippm-ioam-data], IFA [I-D.ietf-kumar-ippm-ifa] and others.

6.2.  Receiver-based HPCC

   Note that the window/rate calculation can be implemented at either
   the data sender or the data receiver.  If the ACK packets already
   exist for reliability purpose, the inband telemetry information can
   be echoed back to the sender via ACK self-clocking.  Not all ACK
   packets need to carry the inband telemetry information.  To reduce
   the Packet Per Second (PPS) overhead, the receiver may examine the
   inband telemetry information and adopt the technique of delayed ACKs
   that only sends out an ACK for a few of received packets.  In order
   to reduce PPS even further, one may implement the algorithm at the
   receiver and feedback the calculated window in the ACK packet once
   every RTT.

   The receiver-based algorithm, Rx-HPCC, is based on int.L, which is
   the inband telemetry information in the packet header.  The receiver
   performs the same functions except using int.L instead of ack.L.  The
   new function NewINT(int.L) is to replace NewACK(int.L)




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   28:   Procedure NewINT(int.L)
   29:   if now > (lastUpdateTime + T) then
   30:      W = ComputeWind(MeasureInflight(int), True);
   31:      send_ack(W)
   32:      lastUpdateTime = now;
   33:   else
   34:      W = ComputeWind(MeasureInflight(int), False);

   Here, since the receiver does not know the starting sequence number
   of a burst, it simply records the lastUpdateTime.  If time T has
   passed since lastUpdateTime, the algorithm would recalcuate Wc as in
   Line 30 and send out the ACK packet which would include W informtion.
   Otherwise, it would just update W information locally.  This would
   reduce the amount of traffic that needs to be feedback to the data
   sender.

   Note that the receiver can also measure the number of outstanding
   flows, N, if the last hop is the congestion point and use this
   information to dynamically adjust W_ai to achieve better fairness.
   The improvement would allow flows to quickly converge to fairness
   without causing large swings under heavy load.

6.3.  Switch-side Optimizations

   Switches can potentially generate and send separate packets
   containing inband telemetry information (aka inband telemetry
   response packets) directly back to the data senders so that they can
   slow down as soon as possible.  This fast feedback and reaction can
   further reduce buffer size consumption upon heavy incast.  Switches
   can consider the level of congestion to decide when to trigger direct
   inband telemetry responses.  A simple bloom-filter and timer can be
   used at switches to avoid sending a burst of inband telemetry
   responses to the same sender.  An inband telemetry response packet
   must carry the sequence number of the original data packet, so that
   the sender can correctly correlate the inband telemetry response with
   the data packet triggered the inband telemetry response.

   One may optimize the inband telemetry header overhead by implementing
   a simple subscription-based inband telemetry.  The data senders may
   use a different DSCP codepoint or a flag bit in the inband telemetry
   instruction header to indicate inband telemetry subscription.  (We
   expect future inband telemetry specs to support such a subscription
   service.)  The senders can selectively subscribe to inband telemetry
   on a per-packet basis to control the inband telemetry data overhead.
   While forwarding inband telemetry-subscribed data packets, the
   switches can monitor the level of congestion and conditionally
   generate separate inband telemetry responses as described above.  The
   inband telemetry responses can be directly sent back to the senders



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   or to the receivers depending on which version of HPCC++ algorithm
   (sender-based or receiver-based) is used in the network.

7.  Reference Implementations

   A prototype of HPCC++ in NICs is implemented to realize the CC
   algorithm and switches to realize the inband telemetry feature.

7.1.  Inband telemetry padding at the network elements

   HPCC++ only relies on packets to share information across senders,
   receivers, and switches.  HPCC++ is open to a variety of inband
   telemetry format standards.  Inside a data center, the path length is
   often no more than 5 hops.  The overhead of the inband telemetry
   padding for HPCC++ is considered to be low.

7.2.  Congestion control at NICs

   Figure 4 shows HPCC++ implementation on a NIC.  The NIC provides an
   HPCC++ module that resides on the data path of the NIC, HPCC++
   modules realize both sender and receiver roles.


  +------------------------------------------------------------------+
  |  +---------+ window update +-----------+ PktSend +-----------+   |
  |  |         |-------------->| Scheduler |-------> |Tx pipeline|---+->
  |  |         | rate update   +-----------+         +-----------+   |
  |  |  HPCC++ |                                           ^         |
  |  |         |                           inband telemetry|         |
  |  |  module |                                           |         |
  |  |         |                                     +-----+-----+   |
  |  |         |<----------------------------------- |Rx pipeline| <-+--
  |  +---------+      telemetry response event       +-----------+   |
  +------------------------------------------------------------------+


                 Figure 4: Overview of NIC Implementation

   1.  Sender side flow

   The HPCC++ module running the HPCC CC algorithm in the sender side
   for every flow in the NIC.  Flow can be defined by some transport
   parameters including 5-tuples, destination QP (queue pair), etc.  It
   receives inband telemetry response events per flow which are
   generated from the RX pipeline, adjusts the sending window and rate,
   and update the scheduler on the rate and window of the flow.





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   The scheduler contains a pacing mechanism that determine the flow
   rate by the value it got from the algorithm.  It also maintains the
   current sending window size for active flows.  If the pacing
   mechanism and the flow's sending window permits, the scheduler
   invokes for the flow a PktSend command to TX pipeline.

   The TX pipeline implements RoCEv2 processing.  Once it receives the
   PktSend event with flow ID from the scheduler, it generates the
   corresponding packet and delivers to the Network.  If a sent packet
   should collect telemetry on its way the TX pipeline may add
   indications/headers that triggers the network elements to add
   telemetry data according to the inband telemetry protocol in use.
   The telemetry can be collected by the data packet or by dedicated
   prob packets generated in the TX pipeline.

   The RX pipe parses the incoming packets from the network and
   identifies whether telemetry is embedded in the parsed packet.  On
   receiving a telemetry response packet, the RX pipeline extracts the
   network status from the packet and passes it to the HPCC++ module for
   processing.  A telemetry response packet can be an ACK containing
   inband telemetry, or a dedicated telemetry response prob packet.

   2.  Receiver side flow

   On receiving a packet containing inband telemetry, the RX pipeline
   extracts the network status, and the flow parameters from the packet
   and passes it to the TX pipeline.  The packet can be a data packet
   containing inband telemetry, or a dedicated telemetry request prob
   packet.  The Tx pipeline may process and edit the telemetry data, and
   then sends back to the sender the data using either an ACK packet of
   the flow or a dedicated telemetry response packet.

8.  IANA Considerations

   This document makes no request of IANA.

9.  Security Considerations

   The rate adaptation mechanism in HPCC++ relies on feedback from the
   network.  As such, it is vulnerable to attacks where feedback
   messages are hijacked, replaced, or intentionally injected with
   misleading information resulting in denial of service, similar to
   those that can affect TCP.  It is therefore RECOMMENDED that the
   notification feedback message is at least integrity checked.  In
   addition, [I-D.ietf-avtcore-cc-feedback-message] discusses the
   potential risk of a receiver providing misleading congestion feedback
   information and the mechanisms for mitigating such risks.




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10.  Acknowledgments

   The authors would like to thank ... for their valuable review
   comments and helpful input to this specification.

11.  Contributors

   The following individuals have contributed to the implementation and
   evaluation of the proposed scheme, and therefore have helped to
   validate and substantially improve this specification: Pedro Y.
   Segura, Roberto P.  Cebrian, Robert Southworth and Malek Musleh.

12.  References

12.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

12.2.  Informative References

   [I-D.ietf-avtcore-cc-feedback-message]
              Sarker, Z., Perkins, C., Singh, V., and M. Ramalho, "RTP
              Control Protocol (RTCP) Feedback for Congestion Control",
              draft-ietf-avtcore-cc-feedback-message-07 (work in
              progress), June 2020.

   [I-D.ietf-ippm-ioam-data]
              "Data Fields for In-situ OAM", March 2020,
              <https://tools.ietf.org/html/draft-ietf-ippm-ioam-data-
              09>.

   [I-D.ietf-kumar-ippm-ifa]
              "Inband Flow Analyzer", February 2019,
              <https://tools.ietf.org/html/draft-kumar-ippm-ifa-01>.

   [P4-INT]   "In-band Network Telemetry (INT) Dataplane Specification,
              v2.0", February 2020, <https://github.com/p4lang/p4-
              applications/blob/master/docs/INT_v2_0.pdf>.






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   [SIGCOMM-HPCC]
              Li, Y., Miao, R., Liu, H., Zhuang, Y., Fei Feng, F., Tang,
              L., Cao, Z., and M. Zhang, "HPCC: High Precision
              Congestion Control", ACM SIGCOMM Beijing, China, August
              2019.

   [Zhu-SIGCOMM2015]
              Zhu, Y., Eran, H., Firestone, D., Guo, C., Lipshteyn, M.,
              Liron, Y., Padhye, J., Raindel, S., Yahia, M., and M.
              Zhang, "Congestion Control for Large-Scale RDMA
              Deployments", ACM SIGCOMM London, United Kingdom, August
              2015.

Authors' Addresses

   Rui Miao
   Alibaba Group
   525 Almanor Ave, 4th Floor
   Sunnyvale, CA  94085
   USA

   Email: miao.rui@alibaba-inc.com


   Hongqiang H. Liu
   Alibaba Group
   108th Ave NE, Suite 800
   Bellevue, WA  98004
   USA

   Email: hongqiang.liu@alibaba-inc.com


   Rong Pan
   Intel, Corp.
   2200 Mission College Blvd.
   Santa Clara, CA  95054
   USA

   Email: rong.pan@intel.com











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   Jeongkeun Lee
   Intel, Corp.
   4750 Patrick Henry Dr.
   Santa Clara, CA  95054
   USA

   Email: jk.lee@intel.com


   Changhoon Kim
   Intel Corporation
   4750 Patrick Henry Dr.
   Santa Clara, CA  95054
   USA

   Email: chang.kim@intel.com


   Barak Gafni
   Mellanox Technologies, Inc.
   350 Oakmead Parkway, Suite 100
   Sunnyvale, CA  94085
   USA

   Email: gbarak@mellanox.com


   Yuval Shpigelman
   Mellanox Technologies, Inc.
   Haim Hazaz 3A
   Netanya  4247417
   Israel

   Email: yuvals@nvidia.com

















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