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2 Network Working Group L. Geng
3 Internet-Draft China Mobile
4 Intended status: Informational L. Qiang
5 Expires: September 9, 2019 Huawei Technologies
6 T. Eckert
7 Huawei
8 March 8, 2019
10 Technical Requirements of Bounded Latency Forwarding
11 draft-geng-detnet-requirements-bounded-latency-01
13 Abstract
15 This document analyses the technical requirements that Layer 3
16 bounded latency forwarding scheme should satisfy.
18 Status of This Memo
20 This Internet-Draft is submitted in full conformance with the
21 provisions of BCP 78 and BCP 79.
23 Internet-Drafts are working documents of the Internet Engineering
24 Task Force (IETF). Note that other groups may also distribute
25 working documents as Internet-Drafts. The list of current Internet-
26 Drafts is at https://datatracker.ietf.org/drafts/current/.
28 Internet-Drafts are draft documents valid for a maximum of six months
29 and may be updated, replaced, or obsoleted by other documents at any
30 time. It is inappropriate to use Internet-Drafts as reference
31 material or to cite them other than as "work in progress."
33 This Internet-Draft will expire on September 9, 2019.
35 Copyright Notice
37 Copyright (c) 2019 IETF Trust and the persons identified as the
38 document authors. All rights reserved.
40 This document is subject to BCP 78 and the IETF Trust's Legal
41 Provisions Relating to IETF Documents
42 (https://trustee.ietf.org/license-info) in effect on the date of
43 publication of this document. Please review these documents
44 carefully, as they describe your rights and restrictions with respect
45 to this document. Code Components extracted from this document must
46 include Simplified BSD License text as described in Section 4.e of
47 the Trust Legal Provisions and are provided without warranty as
48 described in the Simplified BSD License.
50 Table of Contents
52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
53 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
54 1.2. Terminology & Abbreviations . . . . . . . . . . . . . . . 3
55 2. Tolerance of Time Deviation . . . . . . . . . . . . . . . . . 3
56 3. Long Link Propagation Delay . . . . . . . . . . . . . . . . . 3
57 4. Massive Dynamic Flows . . . . . . . . . . . . . . . . . . . . 4
58 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
59 6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
60 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
61 8. Normative References . . . . . . . . . . . . . . . . . . . . 5
62 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
64 1. Introduction
66 DetNet is chartered to provide deterministic forwarding over Layer 3.
67 Deterministic forwarding means packet forwarding with bounded
68 latency, loss and delay variation
69 [draft-ietf-detnet-problem-statement]. In current DetNet's
70 discussion, low packet loss is mainly achieved through PREOF (Packet
71 Replication, Elimination, and Ordering
72 Functions)[draft-ietf-detnet-architecture]. This document focuses on
73 bounded latency.
75 Common IP/MPLS forwarding has long tail effect that couldn't
76 guarantee bounded latency. DetNet has to have an approach to
77 identify DetNet flows, and divert them into a DetNet forwarding plane
78 in which some schemes are adapted to guarantee bounded latency.
79 There are several schemes are proposed for bounded latency forwarding
80 such as dedicated tunnel, light load with per-flow per-hop shaping,
81 Time Aware Shaping[IEEE802.1Qbv], Cyclic Queuing and
82 Forwarding[IEEE802.1Qch], Scalable Deterministic
83 Forwarding[draft-qiang-detnet-large-scale-detnet], and SR based
84 bounded latency[draft-chen-detnet-sr-based-bounded-latency]. This
85 document is not going to compare and analyze these schemes, but to
86 propose some factors (Layer 3 specific) worth to be considered when
87 selecting Layer 3 bounded latency forwarding scheme.
89 1.1. Requirements Language
91 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
92 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
93 "OPTIONAL" in this document are to be interpreted as described in BCP
94 14 [RFC2119][RFC8174] when, and only when, they appear in all
95 capitals, as shown here.
97 1.2. Terminology & Abbreviations
99 This document uses the terminology defined in
100 [draft-ietf-detnet-architecture].
102 TSN: Time Sensitive Network
104 2. Tolerance of Time Deviation
106 One of DetNet's objectives is to stitch TSN domains together as shown
107 in Figure 1. We know that devices inside a TSN domain are time-
108 synchronized, and most of TSN forwarding schemes rely on precise time
109 synchronization. However two TSN domains have a high probability of
110 being asynchronous, while DetNet needs to connect them together and
111 provide end-to-end deterministic forwarding. Therefore, it is worthy
112 of have a DetNet forwarding plane which can keep the bounded latency
113 even under an unsynchronized situation. Otherwise, buffer is needed
114 to absorb the time deviation, and end-users have to bear the latency
115 and cost increase introduced by buffer.
117 Moreover, there are
119 +--------------+ +--------------+
120 | | DetNet Forwarding | |
121 | TSN Domain I +-----------------------------+ TSN Domain II|
122 | | | |
123 +--------------+ +--------------+
125 Figure 1: DetNet Scenario
127 3. Long Link Propagation Delay
129 In contrast to Layer 2 TSN that deployed in LAN, Layer 3 DetNet is
130 expected to be deployed in larger scale network that has longer link
131 propagation delay. Long link propagation delay can cause some
132 troubles to simple cyclic forwarding schemes, like[IEEE802.1Qch].
133 IEEE 802.1 Qch works on the basis of time synchronization, and one
134 hop forwarding (include packet sending, packet transmission on link,
135 and packet receiving three operations) is required to be finished
136 within one cycle as shown in Figure 2. Long links whose latency
137 exceed the cycle time will make IEEE 802.1 Qch doesn't work. One
138 possible solution is to set cycle time to be a bigger value, in order
139 to absorb long link propagation delay. However, this solution will
140 lead to larger jitter. The reason is that cyclic forwarding schemes
141 only try to ensure the packet arrives at a node within a certain
142 cycle, and packet's arrival time may vary within that cycle.
144 /--\ " "
145 | | " Send "
146 \--/ " * "
147 Upstream Node " \ "
148 " \ "
149 " Short "
150 " Link\ "
151 " \ "
152 /--\ " v "
153 | | " * "
154 \--/ " Receive"
155 Downstream Node " "
156 " "
157 | | |
158 Time Line ----+-----------+-----------+---->
159 0 1 2
161 Figure 2: One Hop Forwarding in Qch
163 [Jitter on link is TBD]
165 4. Massive Dynamic Flows
167 Considering the features of TSN applications we can speculate that
168 the number of TSN flows will not dramatically change with time.
169 While DetNet targets at larger-scale deployment. There are more
170 time-sensitive applications such as VR communication, they may
171 require establishment or tear-down of the DetNet connections
172 frequently. Meanwhile, layer 3 device may serve millions of traffic
173 flows simultaneously. Hence those schemes that need complex
174 calculations may not be applicable in DetNet. More importantly,
175 forwarding schemes need to avoid impact on those in-transit flows
176 when new flows are added (or old flows are removed).
178 5. IANA Considerations
180 This document makes no request of IANA.
182 6. Security Considerations
184 This document will not introduce new security problems.
186 7. Acknowledgements
188 The Authors would like to thank David Black for his review,
189 suggestion and comments to this document.
191 8. Normative References
193 [draft-chen-detnet-sr-based-bounded-latency]
194 "SR based Bounded Latency",
195 .
198 [draft-ietf-detnet-architecture]
199 "DetNet Architecture", .
202 [draft-ietf-detnet-problem-statement]
203 "DetNet Problem Statement",
204 .
207 [draft-qiang-detnet-large-scale-detnet]
208 "Large-Scale Deterministic Network",
209 .
212 [IEEE802.1Qbv]
213 "Enhancements for Scheduled Traffic", 2016.
215 [IEEE802.1Qch]
216 "Cyclic Queuing and Forwarding", 2016.
218 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
219 Requirement Levels", BCP 14, RFC 2119,
220 DOI 10.17487/RFC2119, March 1997,
221 .
223 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
224 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
225 May 2017, .
227 Authors' Addresses
229 Liang Geng
230 China Mobile
231 Beijing
232 China
234 Email: gengliang@chinamobile.com
235 Li Qiang
236 Huawei Technologies
237 Huawei Campus, No. 156 Beiqing Rd.
238 Beijing 100095
239 China
241 Email: qiangli3@huawei.com
243 Toerless Eckert
244 Huawei USA - Futurewei Technologies Inc.
245 2330 Central Expy
246 Santa Clara 95050
247 USA
249 Email: tte+ietf@cs.fau.de