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Versions: 00                                                            
Network Working Group                                            R. Miao
Internet-Draft                                                    H. Liu
Intended status: Experimental                              Alibaba Group
Expires: September 8, 2022                                        R. Pan
                                                                  J. Lee
                                                                  C. Kim
                                                       Intel Corporation
                                                                B. Gafni
                                                           Y. Shpigelman
                                             Mellanox Technologies, Inc.
                                                             J. Tantsura
                                                   Microsoft Corporation
                                                           March 7, 2021

           HPCC++: Enhanced High Precision Congestion Control


   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 signaling during congestion and
   overreaction to the congestion signaling using inband and granular
   telemetry, HPCC++ can quickly converge to utilize all the available
   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

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/.

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   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 September 8, 2022.

Copyright Notice

   Copyright (c) 2021 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
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  System Overview . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  HPCC++ Algorithm  . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Notations . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Design Functions and Procedures . . . . . . . . . . . . .   6
   5.  Configuration Parameters  . . . . . . . . . . . . . . . . . .   8
   6.  Design enhancement and implementation . . . . . . . . . . . .   9
     6.1.  Inband telemetry padding at the network switches  . . . .   9
       6.1.1.  Inband telemetry on IFA2.0  . . . . . . . . . . . . .   9
       6.1.2.  Inband telemetry on IOAM  . . . . . . . . . . . . . .   9
       6.1.3.  Inband telemetry on P4  . . . . . . . . . . . . . . .   9
     6.2.  Congestion Notification . . . . . . . . . . . . . . . . .  10
       6.2.1.  Forward direction Congestion detection  . . . . . . .  11
       6.2.2.  Reverse direction . . . . . . . . . . . . . . . . . .  11
     6.3.  Congestion control at NICs  . . . . . . . . . . . . . . .  12
       6.3.1.  Sender-based HPCC . . . . . . . . . . . . . . . . . .  12
       6.3.2.  Receiver-based HPCC . . . . . . . . . . . . . . . . .  13
   7.  Reference Implementation  . . . . . . . . . . . . . . . . . .  14
     7.1.  Implementation on RDMA RoCEv2 . . . . . . . . . . . . . .  14
     7.2.  Implementation on TCP . . . . . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   9.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     9.1.  Internet Deployment . . . . . . . . . . . . . . . . . . .  15

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     9.2.  Switch-assisted congestion control  . . . . . . . . . . .  16
     9.3.  Work with QoS queuing . . . . . . . . . . . . . . . . . .  16
     9.4.  Path migration  . . . . . . . . . . . . . . . . . . . . .  17
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  17
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     12.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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
   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 as
   described in [Zhu-SIGCOMM2015], offloading network stacks into
   hardware is an inevitable direction in high-speed networks.  As an
   example, large-scale networks with RDMA (remote direct memory access)
   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 congestion control 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 signaled through 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 congestion control 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

   The base form of HPCC++ is the original HPCC algorithm and its full
   description can be found in [SIGCOMM-HPCC].  While the original

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   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.

   This document describes the architecture changes in switches and end-
   hosts to support the needed tranmission of inband telemetry and its
   consumption, that imporves the efficiency in handling network

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

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 feedback reports traverse.
   Those network nodes are not shown in the figure for sake of brevity.

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    +---------+  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 congestion control scheme that controls the
      number of inflight bytes.  The inflight bytes mean the amount of
      data that have been sent, but not acknowledged by 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, potentially implemented in
      hardware, to limit the rate accordingly.

   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 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.

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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.

     | 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:

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   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 =  ----------------------------- ;
   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;

   11: Function ComputeWind(U, updateWc)
   12:    if U >= eta or incStage >= maxStagee then
   13:             Wc
              W = ----- + W_ai;
   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

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   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).

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.

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6.  Design enhancement and implementation

   There are three compoments HPCC++ needs to implement: telementry
   padding, congestion notification, and rate update.

6.1.  Inband telemetry padding at the network switches

   HPCC++ only relies on packets to share information across senders,
   receivers, and switches.  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.  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.

   As long the above algorithm is met, HPCC++ is open to a variety of
   inband telemetry format standards, which are orthogonal to the HPCC++
   algorithm.  Although this document does not mandate a particular
   inband telemetry header format or encapsulation, we provide concrete
   implementation specifications using strandard inband telemetry
   protocols, including IFA [I-D.ietf-kumar-ippm-ifa], IETF IOAM
   [I-D.ietf-ippm-ioam-data], and P4.org INT [P4-INT].  In fact, the
   emerging inband telemetry protocols inform the evolution for a
   broader range of protocols and network functions, where this document
   leverages the trend to propose the architecture change to support in-
   network functions like congestion control with high efficiency.

6.1.1.  Inband telemetry on IFA2.0

   For more details, please refer to IFA [I-D.ietf-kumar-ippm-ifa]

6.1.2.  Inband telemetry on IOAM

   Please refer to IETF IOAM [I-D.ietf-ippm-ioam-data]

6.1.3.  Inband telemetry on P4

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |  nHop |        pathID         |          Padding              |
      | Speed |                    Timestamp                  |txBytes|
      |      txBytes(lower)           |         Queue Length          |
      |                            2nd Hop                            |
      |                            2nd Hop(lower)                     |
      |                    Options                    |    Padding    |

                    Figure 4: Example P4.org INT header

   Figure 4 shows the packet format of the INT padding after UDP header.
   The field nHop is the hop count of the packet's path.  The field
   pathID is the XOR of all the switch IDs (which are 12 bits) along the
   path.  The sender sets nHop and pathID to 0.  Each switch along the
   path adds nHop by 1, and XORs its own switch ID to the pathID.  The
   sender uses pathID to judge whether the path of the flow has been
   changed.  If so, it throws away the existing status records of the
   flow and builds up new records.  Each switch has an 8-byte field to
   record the status of the egress port of the packet when the packet is
   emitted.  B is a enum type which indicates the speed type of the
   port(e.g. 40Gbps, 100Gbps, etc.).  Timestamp (24 bits) is when the
   packet is emitted from its egress port, txBytes (20 bits) is the
   accumulative total bytes sent from the egress port, and Queue length
   (16 bits) is the current queue length of the egress port.

6.2.  Congestion Notification

   HPCC++ uses congestion notification to fetch network congestion
   information from switches for proper rate updates at end-hosts.
   Although the basic algorithm described in Section 4 is to add inband
   telemetry information into every data packet for optimal performance,
   HPCC++ supports flexible implementation choices to work seamly with
   transport protocol stacks.  We consider congestion nofication choices
   in both forward and reverse directions of the traffic.

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6.2.1.  Forward direction Congestion detection

   Forward direction is the traffic direction of data packets that
   experience bandwidth contention and possible network congestion.  The
   function of congestion notification in forward direction is to fetch
   inband telemetry from switches.  HPCC++ defines two approaches of
   doing this.

   1.  Inband with data packet.

   This is basic algorithm setting described in Section 4, where the
   end-host inserts inband telemetry header into data packets.  Switches
   along the path detect the inband telemetry header and correspondingly
   add inband telemetry information into data packet to react to
   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 scenarios 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.  Different with in Section 4, end-host can choice uses
   every data packet or only a subset of data packets to reduce the
   overhead.  To insert telemetry header, differet telemetry protocols
   have specific settings for IFA, IETF IOAM, and P4.org INT as

   2.  Probe packet.

   Switches touching every data packet for inband telemetry inserting
   may lead to security or performance concerns, HPCC++ supports the
   ``out-of-band'' approach that uses special-generated probe packets at
   end-hosts to fetch inband telemetry from switches.  Thereby, the
   probe packets should take the same routing path and QoS queueing with
   the data packets.  End-hosts can generate probe packets less
   frequently and we recommend once per round trip time.  In addition,
   the end-host issues probe packets only when it has data packet in the

6.2.2.  Reverse direction

   Reverse direction is the receiver conveying inband telemetry back to
   traffic sender for rate updates.  Similar to forward direction, there
   are also inband and out-of-band approaches.

   1.  Inband with ACK packet.

   HPCC++ supports to use the ACK packet in transport protocols to
   convey the inband telemetry.  TCP generates ACK packet once per every

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   data packet or per a few data packets.  With ACK packet, the receive
   sends accumulated inband telemetry back to sender for rate updates.

   2.  Notification packet.

   Using ACK packet for inband telemetry notification requires transport
   stack modification and sometimes leads to delay in notification when
   certain delayed acknowledged mechanism is used.  Hence, HPCC++ allows
   the receiver to use special-generated notification packets to deliver
   inband telemetry.  The nofication packet is generated per each probe
   packet or data packet with inband telemetry.

6.3.  Congestion control at NICs

6.3.1.  Sender-based HPCC

   Figure 5 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 5: 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.

   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

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   mechanism and the flow's sending window permits, the scheduler
   invokes for the flow a PktSend command to TX pipeline.

   The TX pipeline implements packet 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.

6.3.2.  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
   information.  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.

7.  Reference Implementation

   HPCC++ can be adopted as the CC algorithm by a wide range of
   transport protocols such as TCP and UDP, as well as others that may
   run on top of them, such as iWARP, RoCE etc.  It requires to have the
   window limit and congestion feedback through ACK self-clocking, which
   naturally conforms to the paradigm of TCP design.  With that, HPCC++
   introduces a scheme to measure the total inflight bytes for more
   precise congestion control.  To run in UDP, some modifications need
   to be done to enforce the window limit and collect congestion
   feedback via probing packets, which is incremental.

7.1.  Implementation on RDMA RoCEv2

   We describe reference implementation on RDMA RoCEv2.  This is an
   implementation for ``Sender-based HPCC++'' (see section 6.3.1.) using
   dedicated probe packets to collect the telemetry.  HPCC++ module in
   the sender triggers the sending of ``telemetry request packet'' for a
   given flow.  The NIC then sends the probe packet.  The packet will
   have the same IP and UDP headers as the data packets of the given
   flow.  Such packet is expected to be sent every RTT, see section 6
   for more details.  On receiving of telemetry request packet, the NIC
   extracts the telemetry from all the links along the path from the
   sender.  HPCC++ module chooses the link with the highest inflight
   bytes and sends its telemetry (queue length, timestamp and tx bytes)
   back to the receiver on top of dedicated ``telemetry response

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   packet''.  On receiving of telemetry response packet, the NIC
   extracts the telemetry and pass it to the HPCC++ module which using
   this info to implement the rate update scheme.

7.2.  Implementation on TCP

   Taking the benefit of precise congestion control for TCP is a natural
   next step.  Since TCP segmentation at TX side (e.g., TSO) and
   coalescing at RX side (e.g., GRO) happen at the NIC HW or low-layer
   of TCP/IP stack, carrying per-pkt inband telemetry info between the
   TCP congestion control engine and network fabric has to work with the
   TSO and GRO.  Instead, one way to adopt HPCC++ for TCP is using the
   special probe and notification packets to retrieve inband telemetry
   information.  The sender generates a probe packet when it is actively
   sending data.  The probe packet has the same 5-tuples (source and
   destination addresses, source and destination ports and protocol
   number) with the data packets and the inband telemetry header.  The
   switches along the path identify the probe packet by its inband
   telemetry header and insert the inband telemetry.  Once received the
   probe packet with inband telemetry, the receiver replies with a
   response packet piggybacking the inband telemetry to the sender.
   Note, both probe and response packets use a special DSCP number so
   that it can bypass the TSO and GRO in each side.

8.  IANA Considerations

   This document makes no request of IANA.

9.  Discussion

9.1.  Internet Deployment

   Although the discussion above mainly focuses on the data center
   environment, HPCC++ can be adopted at Internet at large.  There are
   several security considerations one should be aware of.

   There may rise privacy concern when the telemetry information is
   conveyed across Autonomous Systems (ASes) and back to end-users.  The
   link load information captured in telemetry can potentially reveal
   the provider's network capacity, route utilization, scheduling
   policy, etc.  Those usually are considered to be sensitive data of
   the network providers.  Hence, certain action may take to anonymize
   the telemetry data and only convey the relative ratio in rate
   adaptation across ASes without revealing the actual network load.

   Another consideration is the security of receiving telemetry
   information.  The rate adaptation mechanism in HPCC++ relies on
   feedback from the network.  As such, it is vulnerable to attacks

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   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.

9.2.  Switch-assisted congestion control

   HPCC++ falls in the general category of switch-assisted congestion
   control.  However, HPCC++ includes a few unique design choices that
   are different from other switch-assisted approaches.

   o  First, HPCC++ implements a primal-mode algorithm that requires
      only the ``write-to-packet'' operation from switches, which has
      already been supported by telemetry protocols like INT [P4-INT] or
      IOAM [I-D.ietf-ippm-ioam-data].  Please note that this is very
      different from dual-mode algorithms such as XCP
      [Katabi-SIGCOMM2002] and RCP [Dukkipati-RCP], where switches take
      an actively role in determining flows' rates.

   o  Second, HPCC++ achieves a fast utilization convergence by
      decoupling it from fairness convergence, which is inspired by XCP.

   o  Third, HPCC++ enables the switch-guided multiplicative increase
      (MI) by defining the ``inflight byte'' to quantify the link load.
      The inflight byte tells both the underload and overload of the
      link precisely and thus it allows the flow to increase/decrease
      the rate multiplicatively and safely.  By contrast, traditional
      approaches of using the queue length or RTT as the feedback cannot
      guide the rate increase and instead have to rely on additive
      increase (AI) with heuristics.  As the link speed contines to
      grow, this becomes increasingly slow in reclaiming the unused
      bandwidth.  Besides, queue-based feedback mechanisms subject to
      latency inflation.

   o  Last, HPCC++ uses TX rate instead of RX rate used by XCP and RCP.
      As detailed in [SIGCOMM-HPCC], we view the TX rate is more precise
      because RX rate and queue length are overlapped and thus it causes

9.3.  Work with QoS queuing

   Under the use of QoS (Quality of service) priority queuing in
   switches, the length of flow's own queue cannot tell the actual
   queuing time and the exact extent of congestion.  Although general
   approaches for running congestion control with QoS queuing are out of

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   the scope of this document, we provide a few hints for HPCC++ running
   friendly with QoS queuing.  In this case, HPCC++ can leverage the
   packet sojourn time (the egress timestamp minus the ingress
   timestamp) instead of the queue length to quantify the packet's
   actual queuing delay.  In addition, the operators typically use the
   Deficit Weighted Round Robin (DWRR) instead of the strict priority
   (SP) as their QoS scheduling to prevent traffic starvation.  DWRR
   provides a minimum bandwdith guarantee for each queue so that HPCC++
   can leverage it for precise rate update to avoid congestion.

9.4.  Path migration

   HPCC++ allows switches and end-hosts to share precise information of
   network utilization, which suggests a framework for path selection
   and rate control at end-hosts.  The framework HPCC++ enabled is to
   leverage each switch to report its link load information via inband
   telemetry.  The end-host fetches inband telemetry along the traffic
   routes and makes a timely and accurate decision on path selection and
   traffic admission.

10.  Acknowledgments

   The authors would like to thank RTGWG members 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,

   [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>.

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12.2.  Informative References

              Dukkipati, N., "Rate Control Protocol (RCP): Congestion
              control to make flows complete quickly.", Stanford
              University , 2008.

              Sarker, Z., Perkins, C., Singh, V., and M. A. Ramalho,
              "RTP Control Protocol (RTCP) Feedback for Congestion
              Control", draft-ietf-avtcore-cc-feedback-message-09 (work
              in progress), November 2020.

              "Data Fields for In-situ OAM", March 2020,

              "Inband Flow Analyzer", February 2019,

              Katabi, D., Handley, M., and C. Rohrs, "Congestion Control
              for High Bandwidth-Delay Product Networks", ACM
              SIGCOMM Pittsburgh, Pennsylvania, USA, October 2002.

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

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

              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

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Authors' Addresses

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

   Email: miao.rui@alibaba-inc.com

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

   Email: hongqiang.liu@alibaba-inc.com

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

   Email: rong.pan@intel.com

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

   Email: jk.lee@intel.com

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

   Email: chang.kim@intel.com

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

   Email: gbarak@mellanox.com

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

   Email: yuvals@nvidia.com

   Jeff Tantsura
   Microsoft Corporation
   One Microsoft Way
   Redmond, Washington  98052-6399

   Email: jefftantsura@microsoft.com

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