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Li 3 Internet-Draft Huawei Technologies 4 Intended status: Standards Track Y. Zhu 5 Expires: January 2, 2020 China Telecom 6 Z. Li 7 Huawei Technologies 8 July 1, 2019 10 Segment Routing Path MTU in BGP 11 draft-li-idr-sr-policy-path-mtu-02 13 Abstract 15 Segment Routing is a source routing paradigm that explicitly 16 indicates the forwarding path for packets at the ingress node. An SR 17 policy is a set of candidate SR paths consisting of one or more 18 segment lists with necessary path attributes. However, the path 19 maximum transmission unit (MTU) information for SR path is not 20 available in the SR policy since the SR does not require signaling. 21 This document defines extensions to BGP to distribute path MTU 22 information within SR policies. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at https://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on January 2, 2020. 41 Copyright Notice 43 Copyright (c) 2019 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (https://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 59 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 60 2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 61 3. SR Policy for Path MTU . . . . . . . . . . . . . . . . . . . 3 62 3.1. SR Path MTU Sub-TLV . . . . . . . . . . . . . . . . . . . 4 63 4. Operations . . . . . . . . . . . . . . . . . . . . . . . . . 5 64 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 65 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 66 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 67 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 68 8.1. Normative References . . . . . . . . . . . . . . . . . . 6 69 8.2. Informative References . . . . . . . . . . . . . . . . . 6 70 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 72 1. Introduction 74 Segment routing (SR) [RFC8402] is a source routing paradigm that 75 explicitly indicates the forwarding path for packets at the ingress 76 node. The ingress node steers packets into a specific path according 77 to the Segment Routing Policy ( SR Policy) as defined in 78 [I-D.ietf-spring-segment-routing-policy]. In order to distribute SR 79 policies to the headend, [I-D.ietf-idr-segment-routing-te-policy] 80 specifies a mechanism by using BGP. 82 The maximum transmission unit (MTU) is the largest size packet or 83 frame, in bytes, that can be sent in a network. An MTU that is too 84 large might cause retransmissions. Too small an MTU might cause the 85 router to send and handle relatively more header overhead and 86 acknowledgments. When an LSP is created across a set of links with 87 different MTU sizes, the ingress router needs to know what the 88 smallest MTU is on the LSP path. If this MTU is larger than the MTU 89 of one of the intermediate links, traffic might be dropped, because 90 MPLS packets cannot be fragmented. Also, the ingress router may not 91 be aware of this type of traffic loss, because the control plane for 92 the LSP would still function normally. [RFC3209] specify the 93 mechanism of MTU signaling in RSVP. Likewise, SRv6 pakcets will be 94 dropped if the packet size is larger than path MTU, since IPv6 packet 95 can not be fragmented on transmission [RFC8200] . 97 However, the path maximum transmission unit (MTU) information for SR 98 path is not available since the SR does not require signaling. This 99 document defines extensions to BGP to distribute path MTU information 100 within SR policies. The MTU information can be obtained via IGP 101 [I-D.hu-lsr-isis-path-mtu], BGP-LS [I-D.zhu-idr-bgp-ls-path-mtu] or 102 some other means. 104 2. Terminology 106 This memo makes use of the terms defined in [RFC8402] and [RFC3209]. 108 2.1. Requirements Language 110 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 111 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 112 "OPTIONAL" in this document are to be interpreted as described in BCP 113 14 [RFC2119] [RFC8174] when, and only when, they appear in all 114 capitals, as shown here. 116 3. SR Policy for Path MTU 118 As defined in [I-D.ietf-idr-segment-routing-te-policy] , the SR 119 policy encoding structure is as follows: 121 SR Policy SAFI NLRI: 122 Attributes: 123 Tunnel Encaps Attribute (23) 124 Tunnel Type: SR Policy 125 Binding SID 126 Preference 127 Priority 128 Policy Name 129 Explicit NULL Label Policy (ENLP) 130 Segment List 131 Weight 132 Segment 133 Segment 134 ... 135 ... 137 As introduced in Section 1, each SR path has it's path MTU. SR 138 policy with SR path MTU information is expressed as below: 140 SR Policy SAFI NLRI: 141 Attributes: 142 Tunnel Encaps Attribute (23) 143 Tunnel Type: SR Policy 144 Binding SID 145 Preference 146 Priority 147 Policy Name 148 Explicit NULL Label Policy (ENLP) 149 Segment List 150 Weight 151 Path MTU 152 Segment 153 Segment 154 ... 155 ... 157 3.1. SR Path MTU Sub-TLV 159 An SR Path MTU sub-TLV is associated with an SR path specified by a 160 segment list sub-TLV or path segment as defined in 161 [I-D.ietf-spring-mpls-path-segment] and 162 [I-D.li-spring-srv6-path-segment], and it MUST appear only once 163 within a Segment List sub-TLV. It has the following format: 165 0 1 2 3 166 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 1 167 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 168 | Type | Length | RESERVED | 169 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 170 | Path MTU | 171 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 172 Figure 1. Path MTU sub-TLV 174 Where: 176 Type: to be assigned by IANA. 178 Length: the total length of the value field not including Type and 179 Length fields. 181 Reserved: 16 bits reserved and MUST be set to 0 on transmission and 182 MUST be ignored on receipt. 184 Path MTU: 4 bytes value of path MTU in octets. The value can be 185 calculated by a central controller or other devices based on the 186 information that learned via IGP of BGP-LS or other means. 188 Whenever the path MTU of a physical or logical interface is changed, 189 a new SR policy with new path MTU information should be updated 190 accordingly by BGP. 192 4. Operations 194 The document does not bring new operation beyong the description of 195 operations defined in [I-D.ietf-idr-segment-routing-te-policy]. The 196 existing operations defined in 197 [I-D.ietf-idr-segment-routing-te-policy] can apply to this document 198 directly. 200 Typically but not limit to, the SR policies carrying path MTU 201 infomation are configured by a controller. 203 After configuration, the SR policies carrying path MTU infomation 204 will be advertised by BGP update messages. The operation of 205 advertisement is the same as defined in 206 [I-D.ietf-idr-segment-routing-te-policy], as well as the receiption. 208 The consumer of the SR policies is not the BGP process. The 209 operation of sending information to consumers is out of scope of this 210 document. 212 5. IANA Considerations 214 This document defines a new Sub-TLV in registries "SR Policy List 215 Sub- TLVs" [I-D.ietf-idr-segment-routing-te-policy]: 217 Value Description Reference 218 --------------------------------------------------------------------- 219 TBA Path MTU sub-TLV This document 221 6. Security Considerations 223 TBA 225 7. Acknowledgements 227 TBA 229 8. References 230 8.1. Normative References 232 [I-D.ietf-idr-segment-routing-te-policy] 233 Previdi, S., Filsfils, C., Jain, D., Mattes, P., Rosen, 234 E., and S. Lin, "Advertising Segment Routing Policies in 235 BGP", draft-ietf-idr-segment-routing-te-policy-06 (work in 236 progress), May 2019. 238 [I-D.ietf-spring-segment-routing-policy] 239 Filsfils, C., Sivabalan, S., daniel.voyer@bell.ca, d., 240 bogdanov@google.com, b., and P. Mattes, "Segment Routing 241 Policy Architecture", draft-ietf-spring-segment-routing- 242 policy-03 (work in progress), May 2019. 244 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 245 Requirement Levels", BCP 14, RFC 2119, 246 DOI 10.17487/RFC2119, March 1997, 247 . 249 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 250 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 251 May 2017, . 253 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 254 Decraene, B., Litkowski, S., and R. Shakir, "Segment 255 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 256 July 2018, . 258 8.2. Informative References 260 [I-D.hu-lsr-isis-path-mtu] 261 Hu, Z., Zhu, Y., Li, Z., and L. Dai, "IS-IS Extensions for 262 Path MTU", draft-hu-lsr-isis-path-mtu-00 (work in 263 progress), June 2018. 265 [I-D.ietf-spring-mpls-path-segment] 266 Cheng, W., Li, H., Chen, M., Gandhi, R., and R. Zigler, 267 "Path Segment in MPLS Based Segment Routing Network", 268 draft-ietf-spring-mpls-path-segment-00 (work in progress), 269 March 2019. 271 [I-D.li-spring-srv6-path-segment] 272 Li, C., Cheng, W., Chen, M., Dhody, D., Li, Z., Dong, J., 273 and R. Gandhi, "Path Segment for SRv6 (Segment Routing in 274 IPv6)", draft-li-spring-srv6-path-segment-01 (work in 275 progress), June 2019. 277 [I-D.zhu-idr-bgp-ls-path-mtu] 278 Zhu, Y., Hu, Z., Yan, G., and J. Yao, "BGP-LS Extensions 279 for Advertising Path MTU", draft-zhu-idr-bgp-ls-path- 280 mtu-00 (work in progress), June 2018. 282 [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., 283 and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP 284 Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001, 285 . 287 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 288 (IPv6) Specification", STD 86, RFC 8200, 289 DOI 10.17487/RFC8200, July 2017, 290 . 292 Authors' Addresses 294 Cheng Li 295 Huawei Technologies 296 Huawei Campus, No. 156 Beiqing Rd. 297 Beijing 100095 298 China 300 Email: chengli13@huawei.com 302 YongQing Zhu 303 China Telecom 304 109, West Zhongshan Road, Tianhe District. 305 Guangzhou 306 China 308 Email: zhuyq.gd@chinatelecom.cn 310 Zhenbin Li 311 Huawei Technologies 312 Huawei Campus, No. 156 Beiqing Rd. 313 Beijing 100095 314 China 316 Email: lizhenbin@huawei.com