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Versions: 00 01

TSVWG                                                            F. Chen
Internet-Draft                                                    W. Sun
Intended status: Informational                                     X. Yu
Expires: December 27, 2019                 Huawei Technologies Co., Ltd.
                                                            R. Even, Ed.
                                                                  Huawei
                                                           June 25, 2019


             Data Center Congestion Management requirements
                draft-yueven-tsvwg-dccm-requirements-00

Abstract

   On IP-routed datacenter networks, RDMA is deployed using RoCEv2
   protocol.  RoCEv2 specification does not define a strong congestion
   management mechanisms and load balancing methods.  RoCEv2 relies on
   the existing Link-Layer Flow-Control IEEE 802.1Qbb(Priority-based
   Flow Control, PFC) to provide a lossless fabric.  RoCEv2 Congestion
   Management(RCM) use ECN(Explicit Congestion Notification, defined in
   RFC3168) to signal the congestion to the destination and use the
   congestion notification to reduce the rate of injection and increase
   the injection rate when the extent of congestion decreases.  More and
   more practice of congestion management for RoCEv2 appear in the
   industry, such as DCQCN(Data Center Quantized Congestion
   Notification).  This document describes the current state of flow
   control and congestion handling in the DC using RoCEv2 and provides
   requirements for new directions for better congestion control.

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."

   This Internet-Draft will expire on December 27, 2019.






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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Abbreviations . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Current Congestion Management mechanisms  . . . . . . . . . .   4
     4.1.  Priority-based Flow Control (PFC) . . . . . . . . . . . .   4
     4.2.  Explicit Congestion Notification  . . . . . . . . . . . .   4
   5.  Congestion Management Practice  . . . . . . . . . . . . . . .   4
     5.1.  Packet Retransmission . . . . . . . . . . . . . . . . . .   5
     5.2.  Congestion Control Mechanisms . . . . . . . . . . . . . .   5
       5.2.1.  RTT-based Congestion Control  . . . . . . . . . . . .   5
       5.2.2.  Credit-based Congestion Control . . . . . . . . . . .   5
       5.2.3.  ECN-based Congestion Control  . . . . . . . . . . . .   6
     5.3.  Re-ordering . . . . . . . . . . . . . . . . . . . . . . .   6
     5.4.  Load Balancing  . . . . . . . . . . . . . . . . . . . . .   6
       5.4.1.  Equal-cost multi-path routing (ECMP)  . . . . . . . .   6
       5.4.2.  Flowlet . . . . . . . . . . . . . . . . . . . . . . .   6
       5.4.3.  Per-packet  . . . . . . . . . . . . . . . . . . . . .   7
   6.  Data Center Congestion Management requirements  . . . . . . .   7
   7.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   With the emerging Distributed Storage, AI/HPC(High Performance
   Computing), Machine Learning, etc., modern datacenter applications
   demand high throughput(40Gbps and above) with ultra-low latency of



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   less than 10 microsecond per hop from the network, with low CPU
   overhead.  The high link speed (>40Gb/s) in Data Centers (DC) are
   making network transfers complete faster and in fewer RTTs.  Network
   traffic in a data center is often a mix of short and long flows,
   where the short flows require low latencies and the long flows
   require high throughputs.

   On IP-routed datacenter networks, RDMA is deployed using RoCEv2
   protocol.  RoCEv2 [RoCEv2] is a straightforward extension of the RoCE
   protocol that involves a simple modification of the RoCE packet
   format.  RoCEv2 packets carry an IP header which allows traversal of
   IP L3 Routers and a UDP header that serves as a stateless
   encapsulation layer for the RDMA Transport Protocol Packets over IP.

   RoCEv2 Congestion Management (RCM) provides the capability to avoid
   congestion hot spots and optimize the throughput of the fabric.  RCM
   relies on the existing Link-Layer Flow-Control IEEE 802.1Qbb(PFC)
   [IEEE.802.1QBB_2011] to provide a drop free network.  RoCEv2
   Congestion Management(RCM) also use ECN [RFC3168] to signal the
   congestion to the destination and use the congestion notification as
   an input to the sender to reduce the rate of injection and increase
   the injection rate when the extent of congestion decreases.  The rate
   reduction by the sender as well as the increase in data injection is
   left to the implementation.

   An enhancement to the congestion handling for ROCEv2 is the DCQCN
   [DCQCN] providing similar functionality to QCN and DCTCP, it is
   implemented in some of the ROCEv2 NICs but is not part of the ROCEv2
   specification.  As such, vendors have their own implementations which
   makes it difficult to interoperate with each other efficiently.

   iWARP [RFC5040] provides a TCP based transport of RDMA, it is
   implemented in the NIC and is leveraging TCP retransmission and does
   not require a lossless fabric

2.  Conventions

   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.

3.  Abbreviations

      RCM - RoCEv2 Congestion Management

      PFC - Priority-based Flow Control



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      ECN - Explicit Congestion Notification

      DCQCN - Data Center Quantized Congestion Notification

      AI/HPC - Artificial Intelligence/High-Performance computing

      ECMP - Equal-Cost Multipath

      NIC - Network Interface Card

4.  Current Congestion Management mechanisms

4.1.  Priority-based Flow Control (PFC)

   RDMA can be deployed using the RoCEv2 protocol [RoCEv2], and relies
   on IEEE 802.1Qbb Priority-based Flow Control (PFC)
   [IEEE.802.1QBB_2011] to enable a drop-free network.

   PFC is a link level protocol that allows a receiver to assert flow
   control by requesting the transmitter to pause sending traffic for a
   specified priority.  However, because PFC will stop all traffic in a
   particular traffic class at the ingress port, the flows destined to
   other ports will also be blocked.

   The known problems of PFC are head-of-line blocking, unfairness,
   deadlock [deadlocks]

4.2.  Explicit Congestion Notification

   Explicit Congestion Notification (ECN) [RFC3168] is used by the
   network to notify that congestion is detected before actually
   removing packets.  Data Center TCP (DCTCP) [RFC8257]: TCP Congestion
   Control for Data Centers is an Informational RFC that extends the
   Explicit Congestion Notification (ECN) processing to estimate the
   fraction of bytes that encounter congestion, DCTCP then scales the
   TCP congestion window based on this estimate.  DCTCP does not change
   the ECN reporting in TCP.  Other ECN notification mechanisms for UDP
   based transports are specified for RTP in [RFC6679] and for QUIC
   [I-D.ietf-quic-transport].  The ECN notification are reported from
   the end receiver to the sender and the notification includes only the
   occurrence of ECN in the TCP case and the number of ECN marked packet
   for RTP and QUIC.

5.  Congestion Management Practice







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5.1.  Packet Retransmission

   NICs were not designed to deal with losses efficiently.  Receiver
   discards out-of-order packets.  Sender does go-back-N on detecting
   packet loss.  RoCEv2 adopt Go-back-N loss recovery and needs lossless
   layer 2 (by using PFC) for good performance.

   iWARP [RFC5040] provides a TCP based transport of RDMA, it is
   implemented in the NIC and is leveraging TCP retransmission and does
   not require a lossless fabric.

   Based on iWARP congestion and packet loss handling an experiment to
   optimize the congestion control is in the improved RoCE NIC design
   [IRN] that makes two key changes to current RoCE NICs: (1) improving
   the loss recovery mechanism (similar to TCP with SACK), and (2) basic
   end-to-end flow control (termed BDP-FC) which bounds the number of
   in-flight packets by the bandwidth-delay product of the network, BDP-
   FC is a static value that is calculated based on the number of hops
   between the sender and the receiver.  The tests results show that it
   provides better congestion handling comparing to DCQCN [DCQCN].  IRN
   work without PFC which is one of the concerns when using DCQCN.

   Enhancements such as selective retransmission can be considered to
   not rely on a lossless network.

5.2.  Congestion Control Mechanisms

5.2.1.  RTT-based Congestion Control

   The typical practice of RTT based Congestion Control is TIMELY
   [TIMELY].  TIMELY introduces the simple packet delay, measured as
   round-trip times at hosts, is an effective congestion signal without
   the need for switch feedback.  TIMELY measures RTT with microsecond
   accuracy, and these RTTs are sufficient to estimate switch queuing.
   TIMELY can adjust transmission rates using RTT gradients to keep
   packet latency low while delivering high bandwidth.  TIMELY is a
   delay-based congestion control protocol for use in the datacenter.

   Because the RDMA transport is in the NIC and sensitive to packet
   drops, PFC is necessary because packets drops hurt performance badly.
   TIMELY needs PFC to provide lossless underlay network.

5.2.2.  Credit-based Congestion Control

   ExpressPass [ExpressPass] is an end-to-end credit-scheduled, delay-
   bounded congestion control for data centers.  ExpressPass uses credit
   packets to control congestion even before sending data packets, which
   enables to achieve bounded delay and fast convergence.  It uses end-



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   to-end credit transfer for bandwidth allocation and fine-grained
   packet scheduling.

5.2.3.  ECN-based Congestion Control

   Data Center Quantized Congestion Notification (DCQCN) [DCQCN] is an
   end-to-end congestion control scheme for RoCEv2.  DCQCN is a
   combination of ECN and PFC to support end-to-end lossless Ethernet.
   The idea behind DCQCN is to allow ECN to do flow control by
   decreasing the transmission rate at the sender when congestion
   starts, thereby minimizing the time PFC is triggered.  Configuring
   the ECN and PFC timeouts is challenging when there are more routers
   in the DC.

5.3.  Re-ordering

   When the packets arrive at the destination out-of-order, the
   destination should store the packets to restore the order.
   Destination should assign special buffer resource to perform re-
   ordering.  There are many methods to implement the re-ordering either
   on the switches or on the NIC side.

5.4.  Load Balancing

5.4.1.  Equal-cost multi-path routing (ECMP)

   RoCEv2 packets use an opaque flow identifier in the UDP Source Port
   field for ECMP method to implement path selection mechanisms for load
   balancing and improve utilization of the fabric topology.
   Traditional ECMP cannot balance loads well in the data center network
   because it splits loads at the granularity of flow.  The finer the
   granularity of load balancing, the more effective the load balancing
   is and the higher the utilization of network bandwidth can be
   achieved.

5.4.2.  Flowlet

   The typical Flowlet-based load balancing is CONGA [CONGA].  CONGA is
   a network-based distributed congestion-aware load balancing mechanism
   for datacenters.  It splits TCP flows into flowlets, estimates real-
   time congestion on fabric paths, and allocates flowlets to paths
   based on feedback from remote switches.

   Flowlets are bursts of packets from a flow.  The idle interval
   between two bursts of packets is larger than the maximum difference
   in latency among the paths.  So the second burst can be sent along a
   different path than the first without reordering packets.




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5.4.3.  Per-packet

   The effect of packet-based load balancing is the best because the
   corresponding granularity is the smallest.  The consequence is that
   packets belonging to the same flow will be allocated to different
   paths.  When the forwarding delays of paths are different, it is
   possible that packets may arrive at the receiver out-of-order.

6.  Data Center Congestion Management requirements

   The first issue is with incast traffic.  Network congestion happens
   in the network routers when the incoming traffic is larger than the
   bandwidth of the outgoing link on which it has to be transmitted.
   Congestion is the primary source of loss in the network, congestion
   leads to performance degradation.

   Another issue to address is packet loss due to out-of-order packets
   which may happen when load balancing is used.  RoCEv2 adopt Go-back-N
   loss recovery and requires lossless fabric to prevent retransmission
   but is not addressing the packet loss due to re-ordering.

   RoCEv2 relies on Link-Layer Flow-Control IEEE 802.1Qbb(PFC)
   [IEEE.802.1QBB_2011] to provide a lossless underlay networks.
   Lossless networks is implement by a mechanism of flow control, which
   pauses the traffic with priority granularity in the incoming link
   before the buffer overfills, and by that prevents the case of
   dropping packets [CongestionManagment].  However, PFC can lead to
   poor application performance due to problems like head-of-line
   blocking and unfairness [DCQCN].

   Although DCQCN is widely deployed, due to the lack of formal
   specification, vendors have their own implementations which make it
   difficult to interoperate with each other efficiently.  Moreover, the
   potential new congestion control mechanisms should also be considered
   to be compatible with existing ones.

   Besides, with the development of RDMA fabric, the mixture of RDMA
   traffic and normal TCP traffic might also bring issues due to their
   employed different flow control and congestion control mechanisms.

   In order to achieve the high throughput and low latency in the large-
   scale datacenter network, the following requirements for datacenter
   network congestion management are suggested:

   o  Resolve incast traffic in the network.

   o  Provide more efficient network congestion management for RDMA
      traffic to avoid retransmission.



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   o  Provide better interoperability between vendors.

   o  Provide fairness mixture of RDMA traffic and normal TCP traffics.

   o  Provide compatibility when more than one congestion control
      mechanism is used.

7.  Summary

   As discussed in Section 6, we need an enhancement to current RDMA
   transport protocols with stronger capability of congestion management
   to achieve the high throughput and low latency in the large-scale
   datacenter network.  The solution should also have more flexible
   requirement from the underlay network.  The solution should work with
   ROCEv2 but should be more general so it can be used with iWARP as
   well.

8.  Security Considerations

   TBD

9.  IANA Considerations

   No IANA action

10.  References

10.1.  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>.

10.2.  Informative References

   [CONGA]    Alizadeh, M., Edsall, T., Dharmapurikar, S., Vaidyanathan,
              R., Chu, K., Lam, V. T., Matus, F., Pan, R., Yadav, N.,
              and G. Varghese, "CONGA: Distributed Congestion-Aware Load
              Balancing for Datacenters", 2 2015,
              <https://people.csail.mit.edu/alizadeh/papers/
              conga-sigcomm14.pdf>.





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   [CongestionManagment]
              "Understanding RoCEv2 Congestion Management", 12 2018,
              <https://community.mellanox.com/s/article/
              understanding-rocev2-congestion-management>.

   [DCQCN]    Zhu, Y., Eran, H., Firestone, D., Guo, C., Lipshteyn, M.,
              Liron, Y., Padhye, J., Raindel, S., Yahia, M. H., and M.
              Zhang, "Congestion control for large-scale RDMA
              deployments. In ACM SIGCOMM Computer Communication Review,
              Vol. 45. ACM, 523-536.", 8 2015,
              <https://conferences.sigcomm.org/sigcomm/2015/pdf/papers/
              p523.pdf >.

   [deadlocks]
              Hu, S., Zhu, Y., Cheng, P., Guo, C., Tan, K., Padhye, J.,
              and K. Chen, "Deadlocks in Datacenter Networks: Why Do
              They Form, and How to Avoid Them", 11 2016,
              <https://www.microsoft.com/en-us/research/wp-
              content/uploads/2016/10/hotnets16-final67.pdf>.

   [ExpressPass]
              Cho, I., Han, D., and K. Jang, "ExpressPass: End-to-End
              Credit-based Congestion Control for Datacenters", 10 2016,
              <https://arxiv.org/pdf/1610.04688.pdf>.

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-20 (work
              in progress), April 2019.

   [IEEE.802.1QBB_2011]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks--Media Access Control (MAC) Bridges and Virtual
              Bridged Local Area Networks--Amendment 17: Priority-based
              Flow Control", IEEE 802.1Qbb-2011,
              DOI 10.1109/ieeestd.2011.6032693, September 2011,
              <http://ieeexplore.ieee.org/servlet/
              opac?punumber=6032691>.

   [IRN]      Mittal, R., Shpiner, A., Panda, A., Zahavi, E.,
              Krishnamurthy, A., Ratnasamy, S., and S. Shenker,
              "Revisiting Network Support for RDMA. In Proceedings of
              the 2018 Conference of the ACM Special Interest Group on
              Data Communication (SIGCOMM '18)", 8 2018,
              <https://dl.acm.org/citation.cfm?id=3230557>.






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   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC5040]  Recio, R., Metzler, B., Culley, P., Hilland, J., and D.
              Garcia, "A Remote Direct Memory Access Protocol
              Specification", RFC 5040, DOI 10.17487/RFC5040, October
              2007, <https://www.rfc-editor.org/info/rfc5040>.

   [RFC6679]  Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
              and K. Carlberg, "Explicit Congestion Notification (ECN)
              for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
              2012, <https://www.rfc-editor.org/info/rfc6679>.

   [RFC8257]  Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
              and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
              Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
              October 2017, <https://www.rfc-editor.org/info/rfc8257>.

   [RoCEv2]   "Infiniband Trade Association. Supplement to InfiniBand
              architecture specification volume 1 release 1.2.2 annex
              A17: RoCEv2 (IP routable RoCE).",
              <https://cw.infinibandta.org/document/dl/7781>.

   [TIMELY]   Mittal, R., Lam, T., Dukkipati, N., Blem, E., Wassel, H.,
              Ghobadi, M., Vahdat, A., Wang, Y., Wetherall, D., and D.
              Zats, "RTT-based Congestion Control for the Datacenter", 8
              2015,
              <https://conferences.sigcomm.org/sigcomm/2015/pdf/papers/
              p537.pdf>.

Authors' Addresses

   Fei Chen
   Huawei Technologies Co., Ltd.

   Email: chenfei57@huawei.com


   Wenhao Sun
   Huawei Technologies Co., Ltd.

   Email: sam.sunwenhao@huawei.com







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   Xiang Yu
   Huawei Technologies Co., Ltd.

   Email: yolanda.yu@huawei.com


   Roni Even (editor)
   Huawei

   Email: roni.even@huawei.com









































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