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INTERNET-DRAFT                                                   F. Chen
Intended Status: Informational                                    W. Sun
Expires: Feb 09, 2019                                Huawei Technologies
                                                            Aug 10, 2018


           Problem Statement of RoCEv2 Congestion Management
            draft-chen-nfsv4-rocev2-cm-problem-statement-00

Abstract

   On IP-routed datacenter networks, RDMA is deployed using RoCEv2
   protocol. RoCEv2 specification does not define the congestion
   management 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 network. 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). There
   is a demanding for the new RoCE protocol(temporary alias RoCEv3) to
   provide stronger congestion management and load balancing mechanisms
   for RDMA deployment in modern datacenter.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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Copyright and License Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
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   Copyright (c) 2018 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  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3  Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4  Problem statement & requirements  . . . . . . . . . . . . . . .  4
   5  Current Congestion Management for RoCEv2  . . . . . . . . . . .  4
     5.1 PFC  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     5.2 ECN  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   6. Congestion Management Practice  . . . . . . . . . . . . . . . .  5
     6.1 Packet Retransmission  . . . . . . . . . . . . . . . . . . .  5
     6.2 Congestion Control Mechanisms  . . . . . . . . . . . . . . .  5
     6.4 Load Balancing . . . . . . . . . . . . . . . . . . . . . . .  6
   7  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   8  Security Considerations . . . . . . . . . . . . . . . . . . . .  7
   9  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .  7
   10  References . . . . . . . . . . . . . . . . . . . . . . . . . .  7













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1  Introduction

   With the emerging Distributed Storage, AI/HPC, Machine Learning,
   etc., modern datacenter applications demand high throughput(40Gbps
   and above) with ultra-low latency of < 10 microsecond per hop from
   the network, with low CPU overhead. Remote Direct Memory Access
   (RDMA) can meet these needs on Ethernet.

   On IP-routed datacenter networks, RDMA is deployed using RoCEv2
   protocol. 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[1].

   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) 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 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, etc. Shall we consider to develop next
   Generation RoCE protocol(alias RoCEv3) with stronger congestion
   management and load balancing mechanisms for RDMA deployment in
   modern datacenter?

2  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3  Abbreviations

   RCM - RoCEv2 Congestion Management

   PFC - Priority-based Flow Control

   ECN - Explicit Congestion Notification

   DCQCN - Data Center Quantized Congestion Notification

   AI/HPC - Artificial Intelligence/High-Performance computing

   ECMP - Equal-Cost Multipath



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4  Problem statement & requirements

   Network congestion happens in the network switches 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 and
   in the network, congestion leads to dramatic performance degradation.

   Generally, RoCEv2 relies on Link-Layer Flow-Control IEEE
   802.1Qbb(PFC) to provide a lossless underlying networks. Lossless
   networks implement mechanism of flow control, which pauses the
   traffic in the incoming link before the buffer overfills, and by that
   prevents case of dropping packets[2]. However, PFC can lead to poor
   application performance due to problems like head-of-line blocking
   and unfairness[3]. In order to avoid the problems involved by PFC,
   there is another faction research on the congestion control
   mechanisms over the lossy network.

   We need a kind of protocol with stronger capability of congestion
   management to achieve the high throughput and low latency in the
   large-scale datacenter network with more flexible requirement to the
   underlay network. The interoperability is also required among the
   industry practice.

5  Current Congestion Management for RoCEv2
5.1 PFC
   RDMA is deployed using the RoCEv2 protocol, which relies on IEEE
   802.1Qbb Priority-based Flow Control (PFC) to enable a drop-free
   network.

   PFC is a link level protocol that allows a receiver to assert flow
   control telling 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[4].

5.2 ECN

   Explicit congestion notification (ECN) enables end-to-end congestion
   notification between two endpoints on TCP/IP based networks. ECN
   notifies networks about congestion with the goal of reducing packet
   loss and delay by making the sending device decrease the transmission
   rate until the congestion clears, without dropping packets. RFC 3168,
   The Addition of Explicit Congestion Notification (ECN) to IP, defines
   ECN.




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6. Congestion Management Practice
6.1 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[5].

   If new RDMA protocol does not rely on the lossless layer 2 network,
   an efficient method of Packet Retransmission is necessary.

6.2 Congestion Control Mechanisms

6.2.1 RTT-based Congestion Control

   The typical practice of RTT based Congestion Control is TIMELY[6]. It
   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 that
   these RTTs are sufficient to estimate switch queueing. 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, so PFC is necessary because drops hurt performance badly. That
   is to say TIMELY needs PFC to provide lossless underlay network.

6.2.2 Credit-based Congestion Control
   ExpressPass[7] is an end-to-end credit-scheduled, delay-bounded
   congestion control for datacenters. ExpressPass uses credit packets
   to control congestion even before sending data packets, which enables
   to achieve bounded delay and fast convergence. It uses end-to-end
   credit transfer for bandwidth allocation and fine-grained packet
   scheduling.

6.2.3 ECN-based congestion control

   Data Center Quantized Congestion Notification (DCQCN)[3] 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.

   Although RoCEv2 standard[1] does not list DCQCN as the RCM mechanism,
   but it is widely used in the industry practice.





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   6.3 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 switch or on NIC side. Here will not go into the details.

6.4 Load Balancing
6.4.1 ECMP

   RoCEv2 packets use an opaque flow identifier in their UDP Source Port
   field for ECMP method to implement path selection mechanisms for load
   balancing and improve utilization of the fabric topology.

   Traditional ECMP can not 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.

6.4.2 Flowlet

   The typical Flowlet-based load balancing is CONGA[8]. 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.

6.4.3 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.

7  Summary
   The new emerging RoCE based applications urge the practice of
   different congestion management mechanisms to be practiced in kinds
   of modern large-scale datacenter network. In this problem statement,
   not all the mainstream mechanisms are introduced. It is still needed
   to extend when considering the future RoCE protocol(temporary alias



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   RoCEv3) with robot congestion management capability and more flexible
   requirement on layer 2 network which might be the next direction.

8  Security Considerations
   This document does not introduce any additional security constraints.

9  IANA Considerations TBD

10  References

   [1] Infiniband Trade Association. Supplement to InfiniBand
   architecture specification volume 1 release 1.2.2 annex A17: RoCEv2
   (IP routable RoCE), 2014.

   [2] Understanding RoCEv2 Congestion Management,
   https://community.mellanox.com/docs/DOC-2321

   [3] Zhu, Yibo, et al. "Congestion Control for Large-Scale RDMA
   Deployments." Acm Sigcomm Computer Communication Review
   45.5(2015):523-536.

   [4] Hu, Shuihai, et al. "Deadlocks in Datacenter Networks: Why Do
   They Form, and How to Avoid Them." The, ACM Workshop ACM, 2016:92-98.

   [5] Mittal, Radhika, et al. "Revisiting Network Support for RDMA."
   (2018).

   [6] Mittal, Radhika, et al. "TIMELY: RTT-based Congestion Control for
   the Datacenter." ACM Conference on Special Interest Group on Data
   Communication ACM, 2015:537-550.

   [7] Cho, Inho, D. Han, and K. Jang. "ExpressPass: End-to-End Credit-
   based Congestion Control for Datacenters." (2016).

   [8] Alizadeh, Mohammad, et al. "CONGA: distributed congestion-aware
   load balancing for datacenters." ACM Conference on SIGCOMM ACM,
   2014:503-514.














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   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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