Network Working Group L. Geng Internet-Draft China Mobile Intended status: Informational L. Qiang Expires: September 9, 2019 Huawei Technologies T. Eckert Huawei March 8, 2019 Technical Requirements of Bounded Latency Forwarding draft-geng-detnet-requirements-bounded-latency-01 Abstract This document analyses the technical requirements that Layer 3 bounded latency forwarding scheme should satisfy. 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 September 9, 2019. Copyright Notice Copyright (c) 2019 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. Geng, et al. Expires September 9, 2019 [Page 1] Internet-Draft Requirements of Bounded Latency March 2019 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2 1.2. Terminology & Abbreviations . . . . . . . . . . . . . . . 3 2. Tolerance of Time Deviation . . . . . . . . . . . . . . . . . 3 3. Long Link Propagation Delay . . . . . . . . . . . . . . . . . 3 4. Massive Dynamic Flows . . . . . . . . . . . . . . . . . . . . 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 6. Security Considerations . . . . . . . . . . . . . . . . . . . 4 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4 8. Normative References . . . . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction DetNet is chartered to provide deterministic forwarding over Layer 3. Deterministic forwarding means packet forwarding with bounded latency, loss and delay variation [draft-ietf-detnet-problem-statement]. In current DetNet's discussion, low packet loss is mainly achieved through PREOF (Packet Replication, Elimination, and Ordering Functions)[draft-ietf-detnet-architecture]. This document focuses on bounded latency. Common IP/MPLS forwarding has long tail effect that couldn't guarantee bounded latency. DetNet has to have an approach to identify DetNet flows, and divert them into a DetNet forwarding plane in which some schemes are adapted to guarantee bounded latency. There are several schemes are proposed for bounded latency forwarding such as dedicated tunnel, light load with per-flow per-hop shaping, Time Aware Shaping[IEEE802.1Qbv], Cyclic Queuing and Forwarding[IEEE802.1Qch], Scalable Deterministic Forwarding[draft-qiang-detnet-large-scale-detnet], and SR based bounded latency[draft-chen-detnet-sr-based-bounded-latency]. This document is not going to compare and analyze these schemes, but to propose some factors (Layer 3 specific) worth to be considered when selecting Layer 3 bounded latency forwarding scheme. 1.1. Requirements Language 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. Geng, et al. Expires September 9, 2019 [Page 2] Internet-Draft Requirements of Bounded Latency March 2019 1.2. Terminology & Abbreviations This document uses the terminology defined in [draft-ietf-detnet-architecture]. TSN: Time Sensitive Network 2. Tolerance of Time Deviation One of DetNet's objectives is to stitch TSN domains together as shown in Figure 1. We know that devices inside a TSN domain are time- synchronized, and most of TSN forwarding schemes rely on precise time synchronization. However two TSN domains have a high probability of being asynchronous, while DetNet needs to connect them together and provide end-to-end deterministic forwarding. Therefore, it is worthy of have a DetNet forwarding plane which can keep the bounded latency even under an unsynchronized situation. Otherwise, buffer is needed to absorb the time deviation, and end-users have to bear the latency and cost increase introduced by buffer. Moreover, there are +--------------+ +--------------+ | | DetNet Forwarding | | | TSN Domain I +-----------------------------+ TSN Domain II| | | | | +--------------+ +--------------+ Figure 1: DetNet Scenario 3. Long Link Propagation Delay In contrast to Layer 2 TSN that deployed in LAN, Layer 3 DetNet is expected to be deployed in larger scale network that has longer link propagation delay. Long link propagation delay can cause some troubles to simple cyclic forwarding schemes, like[IEEE802.1Qch]. IEEE 802.1 Qch works on the basis of time synchronization, and one hop forwarding (include packet sending, packet transmission on link, and packet receiving three operations) is required to be finished within one cycle as shown in Figure 2. Long links whose latency exceed the cycle time will make IEEE 802.1 Qch doesn't work. One possible solution is to set cycle time to be a bigger value, in order to absorb long link propagation delay. However, this solution will lead to larger jitter. The reason is that cyclic forwarding schemes only try to ensure the packet arrives at a node within a certain cycle, and packet's arrival time may vary within that cycle. Geng, et al. Expires September 9, 2019 [Page 3] Internet-Draft Requirements of Bounded Latency March 2019 /--\ " " | | " Send " \--/ " * " Upstream Node " \ " " \ " " Short " " Link\ " " \ " /--\ " v " | | " * " \--/ " Receive" Downstream Node " " " " | | | Time Line ----+-----------+-----------+----> 0 1 2 Figure 2: One Hop Forwarding in Qch [Jitter on link is TBD] 4. Massive Dynamic Flows Considering the features of TSN applications we can speculate that the number of TSN flows will not dramatically change with time. While DetNet targets at larger-scale deployment. There are more time-sensitive applications such as VR communication, they may require establishment or tear-down of the DetNet connections frequently. Meanwhile, layer 3 device may serve millions of traffic flows simultaneously. Hence those schemes that need complex calculations may not be applicable in DetNet. More importantly, forwarding schemes need to avoid impact on those in-transit flows when new flows are added (or old flows are removed). 5. IANA Considerations This document makes no request of IANA. 6. Security Considerations This document will not introduce new security problems. 7. Acknowledgements The Authors would like to thank David Black for his review, suggestion and comments to this document. Geng, et al. Expires September 9, 2019 [Page 4] Internet-Draft Requirements of Bounded Latency March 2019 8. Normative References [draft-chen-detnet-sr-based-bounded-latency] "SR based Bounded Latency", . [draft-ietf-detnet-architecture] "DetNet Architecture", . [draft-ietf-detnet-problem-statement] "DetNet Problem Statement", . [draft-qiang-detnet-large-scale-detnet] "Large-Scale Deterministic Network", . [IEEE802.1Qbv] "Enhancements for Scheduled Traffic", 2016. [IEEE802.1Qch] "Cyclic Queuing and Forwarding", 2016. [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, . Authors' Addresses Liang Geng China Mobile Beijing China Email: gengliang@chinamobile.com Geng, et al. Expires September 9, 2019 [Page 5] Internet-Draft Requirements of Bounded Latency March 2019 Li Qiang Huawei Technologies Huawei Campus, No. 156 Beiqing Rd. Beijing 100095 China Email: qiangli3@huawei.com Toerless Eckert Huawei USA - Futurewei Technologies Inc. 2330 Central Expy Santa Clara 95050 USA Email: tte+ietf@cs.fau.de Geng, et al. Expires September 9, 2019 [Page 6]