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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group Y. Zhu 3 Internet-Draft China Telecom 4 Intended status: Standards Track Z. Hu 5 Expires: February 11, 2021 S. Peng 6 Huawei Technologies 7 R. Mwehaire 8 MTN Uganda Ltd. 9 August 10, 2020 11 BGP-LS Extensions for Advertising Path MTU 12 draft-zhu-idr-bgp-ls-path-mtu-04 14 Abstract 16 BGP Link State (BGP-LS) describes a mechanism by which link-state and 17 TE information can be collected from networks and shared with 18 external components using the BGP routing protocol. The centralized 19 controller (PCE/SDN) completes the service path calculation based on 20 the information transmitted by the BGP-LS and delivers the result to 21 the Path Computation Client (PCC) through the PCEP or BGP protocol. 23 Segment Routing (SR) leverages the source routing paradigm, which can 24 be directly applied to the MPLS architecture with no change on the 25 forwarding plane and applied to the IPv6 architecture, with a new 26 type of routing header, called SRH. The SR uses the IGP protocol as 27 the control protocol. Compared to the MPLS tunneling technology, the 28 SR does not require additional signaling. Therefore, the SR does not 29 support the negotiation of the Path MTU. Since multiple labels or 30 SRv6 SIDs are pushed in the packets, it is more likely that the 31 packet size exceeds the path mtu of SR tunnel. 33 This document specifies the extensions to BGP Link State (BGP-LS) to 34 carry maximum transmission unit (MTU) messages of link. The PCE/SDN 35 calculates the Path MTU while completing the service path calculation 36 based on the information transmitted by the BGP-LS. 38 Requirements Language 40 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 41 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 42 document are to be interpreted as described in RFC 2119 [RFC2119]. 44 Status of This Memo 46 This Internet-Draft is submitted in full conformance with the 47 provisions of BCP 78 and BCP 79. 49 Internet-Drafts are working documents of the Internet Engineering 50 Task Force (IETF). Note that other groups may also distribute 51 working documents as Internet-Drafts. The list of current Internet- 52 Drafts is at https://datatracker.ietf.org/drafts/current/. 54 Internet-Drafts are draft documents valid for a maximum of six months 55 and may be updated, replaced, or obsoleted by other documents at any 56 time. It is inappropriate to use Internet-Drafts as reference 57 material or to cite them other than as "work in progress." 59 This Internet-Draft will expire on February 11, 2021. 61 Copyright Notice 63 Copyright (c) 2020 IETF Trust and the persons identified as the 64 document authors. All rights reserved. 66 This document is subject to BCP 78 and the IETF Trust's Legal 67 Provisions Relating to IETF Documents 68 (https://trustee.ietf.org/license-info) in effect on the date of 69 publication of this document. Please review these documents 70 carefully, as they describe your rights and restrictions with respect 71 to this document. Code Components extracted from this document must 72 include Simplified BSD License text as described in Section 4.e of 73 the Trust Legal Provisions and are provided without warranty as 74 described in the Simplified BSD License. 76 Table of Contents 78 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 79 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 80 3. Deploying scenarios . . . . . . . . . . . . . . . . . . . . . 5 81 4. BGP_LS Extensions for Link MTU . . . . . . . . . . . . . . . 5 82 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 83 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 84 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 85 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 6 86 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 87 9.1. Normative References . . . . . . . . . . . . . . . . . . 7 88 9.2. Informative References . . . . . . . . . . . . . . . . . 7 89 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 91 1. Introduction 93 [RFC7752]describes the implementation mechanism of BGP-LS by which 94 link-state and TE information can be collected from networks and 95 shared with external components using the BGP routing protocol 96 [RFC4271]. BGP-LS allows the necessary Link-State Database (LSDB) 97 and Traffic Engineering Database (TEDB) information to be collected 98 from the IGP within the network, filtered according to configurable 99 policy, and distributed to the PCE as necessary. 101 The appropriate MTU size guarantees efficient data transmission. If 102 the MTU size is too small and the packet size is large, fragmentation 103 may occur too much and packets are discarded by the QoS queue. If 104 the MTU configuration is too large, packet transmission may be slow. 105 Path MTU is the maximum length of a packet that can pass through a 106 path without fragmentation. [RFC1191] describes a technique for 107 dynamically discovering the maximum transmission unit (MTU) of an 108 arbitrary internet path. 110 The traditional MPLS tunneling technology has signaling for 111 establishing a path. [RFC3988] defines the mechanism for 112 automatically discovering the Path MTU of LSPs. For a certain FEC, 113 the LSR compares the MTU advertised by all downstream devices with 114 the MTU of the FEC output interface in the local device, and 115 calculates the minimum value for the upstream device. 117 [RFC3209] specify the mechanism of MTU signaling in RSVP-TE. The 118 ingress node of the RSVP-TE tunnel sends a Path message to the 119 downstream device. The Adspec object in the Path message carries the 120 MTU. Each node along the tunnel receives a Path message, compares 121 the MTU value in the Adspec object with the interface MTU value and 122 MPLS MTU configured on the physical output interface of the local 123 tunnel, obtains the minimum MTU value, and puts it into the newly 124 constructed Path message and continues to send it to the downstream 125 equipment. Thus, the MTU carried in the Path message received by the 126 Egress node is the minimum value of the path MTU. The Egress node 127 brings the negotiated Path MTU back to the Ingress node through the 128 Resv message. 130 Segment Routing (SR) described in [RFC8402] leverages the source 131 routing paradigm. Segment Routing can be directly applied to the 132 MPLS architecture with no change on the forwarding plane [RFC8660] 133 and applied to the IPv6 architecture with a new type of routing 134 header called the SR header (SRH) [RFC8754]. 135 [I-D.ietf-idr-bgp-ls-segment-routing-ext] defines SR extensions to 136 BGP-LS and specifies the TLVs and sub-TLVs for advertising SR 137 information. Based on the SR information reported by the BGP-LS, the 138 SDN can calculate the end-to-end explicit SR-TE paths or SR Policies. 140 Nevertheless, Segment Routing is a tunneling technology based on the 141 IGP protocol as the control protocol, and there is no additional 142 signaling for establishing the path. so the Segment Routing tunnel 143 cannot currently support the negotiation mechanism of the MTU. 144 Multiple labels or SRv6 SIDs are pushed in the packets. This causes 145 the length of the packets encapsulated in the Segment Routing tunnel 146 to increase during packet forwarding. This is more likely to cause 147 packet size exceed the traditional MPLS packet size. 149 This document specify the extension to BGP Link State (BGP-LS) to 150 carry link maximum transmission unit (MTU) messages. 152 2. Terminology 154 This draft refers to the terms defined in [RFC8201], [RFC4821] and 155 [RFC3988]. 157 MTU: Maximum Transmission Unit, the size in bytes of the largest IP 158 packet, including the IP header and payload, that can be 159 transmitted on a link or path. Note that this could more properly 160 be called the IP MTU, to be consistent with how other standards 161 organizations use the acronym MTU. 163 Link MTU: The Maximum Transmission Unit, i.e., maximum IP packet 164 size in bytes, that can be conveyed in one piece over a link. Be 165 aware that this definition is different from the definition used 166 by other standards organizations. 168 For IETF documents, link MTU is uniformly defined as the IP MTU 169 over the link. This includes the IP header, but excludes link 170 layer headers and other framing that is not part of IP or the IP 171 payload. 173 Be aware that other standards organizations generally define link 174 MTU to include the link layer headers. 176 For the MPLS data plane, this size includes the IP header and data (or 177 other payload) and the label stack but does not include any lower-layer 178 headers. A link may be an interface (such as Ethernet or Packet-over- 179 SONET), a tunnel (such as GRE or IPsec), or an LSP. 181 Path: The set of links traversed by a packet between a source node 182 and a destination node. 184 Path MTU, or PMTU: The minimum link MTU of all the links in a path 185 between a source node and a destination node. 187 3. Deploying scenarios 189 This document suggests a solution to extension to BGP Link State 190 (BGP-LS) to carry maximum transmission unit (MTU) messages. The MTU 191 information of the link is acquired through the process of collecting 192 link state and TE information by BGP-LS. Concretely, a router 193 maintains one or more databases for storing link-state information 194 about nodes and links in any given area. The router's BGP process 195 can retrieve topology from these LSDBs and distribute it to a 196 consumer, either directly or via a peer BGP speaker (typically a 197 dedicated Route Reflector). [RFC7176] specifies the ISIS mechanism 198 and extensions for link MTU Sub-TLV. 200 As per [RFC7752], the collection of link-state and TE information and 201 its distribution to consumers is shown in the following figure. 203 +-----------+ 204 | Consumer | 205 +-----------+ 206 ^ 207 | 208 +-----------+ 209 | BGP | +-----------+ 210 | Speaker | | Consumer | 211 +-----------+ +-----------+ 212 ^ ^ ^ ^ 213 | | | | 214 +---------------+ | +-------------------+ | 215 | | | | 216 +-----------+ +-----------+ +-----------+ 217 | BGP | | BGP | | BGP | 218 | Speaker | | Speaker | . . . | Speaker | 219 +-----------+ +-----------+ +-----------+ 220 ^ ^ ^ 221 | | | 222 IGP IGP IGP 224 Figure 1: Collection of Link-State and TE Information 226 Please note that this signaled MTU may be different from the actual 227 MTU, which is usually from configuration mismatches in a control 228 plane and a data plane component. 230 4. BGP_LS Extensions for Link MTU 232 [RFC7752] defines the BGP-LS NLRI that can be a Node NLRI, a Link 233 NLRI or a Prefix NLRI. The corresponding BGP-LS attribute is a Node 234 Attribute, a Link Attribute or a Prefix Attribute. [RFC7752] defines 235 the TLVs that map link-state information to BGP-LS NLRI and the BGP- 236 LS attribute. Therefore, according to this document, a new sub-TLV 237 is added to the Link Attribute TLV. 239 The format of the sub-TLV is as shown below. 241 x TYPE - TBD 242 x LENGTH - Total length of the value field, it should be 2 243 x VALUE - 2-byte MTU value of the link 245 No. of Octets 246 +-----------------+ 247 | MTU value | 2 248 +-----------------+ 250 Figure 2. Sub-TLV Format for Link MTU 252 Whenever there is a change in MTU value represented by Link Attribute 253 TLV, BGP-LS should re-originate the respective TLV with the new MTU 254 value. 256 5. IANA Considerations 258 This document requests assigning a new code-point from the BGP-LS 259 Link Descriptor and Attribute TLVs registry as specified in section 260 4. 262 Value Description Reference 263 ---------------------- ---------------------------- -------------- 264 TBD Link MTU This document 266 6. Security Considerations 268 This document does not introduce security issues beyond those 269 discussed in RFC7752. 271 7. Acknowledgements 273 8. Contributors 275 Gang Yan 276 Huawei 277 China 279 Email:yangang@huawei.com 280 Junda Yao 281 Huawei 282 China 284 Email:yaojunda@huawei.com 286 9. References 288 9.1. Normative References 290 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 291 Requirement Levels", BCP 14, RFC 2119, 292 DOI 10.17487/RFC2119, March 1997, 293 . 295 9.2. Informative References 297 [I-D.ietf-idr-bgp-ls-segment-routing-ext] 298 Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H., 299 and M. Chen, "BGP Link-State extensions for Segment 300 Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-16 301 (work in progress), June 2019. 303 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 304 DOI 10.17487/RFC1191, November 1990, 305 . 307 [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., 308 and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP 309 Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001, 310 . 312 [RFC3988] Black, B. and K. Kompella, "Maximum Transmission Unit 313 Signalling Extensions for the Label Distribution 314 Protocol", RFC 3988, DOI 10.17487/RFC3988, January 2005, 315 . 317 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 318 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 319 DOI 10.17487/RFC4271, January 2006, 320 . 322 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 323 Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, 324 . 326 [RFC7176] Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt, 327 D., and A. Banerjee, "Transparent Interconnection of Lots 328 of Links (TRILL) Use of IS-IS", RFC 7176, 329 DOI 10.17487/RFC7176, May 2014, 330 . 332 [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and 333 S. Ray, "North-Bound Distribution of Link-State and 334 Traffic Engineering (TE) Information Using BGP", RFC 7752, 335 DOI 10.17487/RFC7752, March 2016, 336 . 338 [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., 339 "Path MTU Discovery for IP version 6", STD 87, RFC 8201, 340 DOI 10.17487/RFC8201, July 2017, 341 . 343 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 344 Decraene, B., Litkowski, S., and R. Shakir, "Segment 345 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 346 July 2018, . 348 [RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S., 349 Decraene, B., Litkowski, S., and R. Shakir, "Segment 350 Routing with the MPLS Data Plane", RFC 8660, 351 DOI 10.17487/RFC8660, December 2019, 352 . 354 [RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J., 355 Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header 356 (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020, 357 . 359 Authors' Addresses 361 Yongqing Zhu 362 China Telecom 363 109, West Zhongshan Road, Tianhe District. 364 Guangzhou 510000 365 China 367 Email: zhuyq8@chinatelecom.cn 368 Zhibo Hu 369 Huawei Technologies 370 Huawei Bld., No.156 Beiqing Rd. 371 Beijing 100095 372 China 374 Email: huzhibo@huawei.com 376 Shuping Peng 377 Huawei Technologies 378 Huawei Bld., No.156 Beiqing Rd. 379 Beijing 100095 380 China 382 Email: pengshuping@huawei.com 384 Robbins Mwehaire 385 MTN Uganda Ltd. 386 Uganda 388 Email: Robbins.Mwehair@mtn.com