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Yao 7 Huawei Technologies 8 January 7, 2020 10 BGP-LS Extensions for Advertising Path MTU 11 draft-zhu-idr-bgp-ls-path-mtu-02 13 Abstract 15 BGP Link State (BGP-LS) describes a mechanism by which link-state and 16 TE information can be collected from networks and shared with 17 external components using the BGP routing protocol. The centralized 18 controller (PCE/SDN) completes the service path calculation based on 19 the information transmitted by the BGP-LS and delivers the result to 20 the Path Computation Client (PCC) through the PCEP or BGP protocol. 22 Segment Routing (SR) leverages the source routing paradigm, which can 23 be directly applied to the MPLS architecture with no change on the 24 forwarding plane and applied to the IPv6 architecture, with a new 25 type of routing header, called SRH. The SR uses the IGP protocol as 26 the control protocol. Compared to the MPLS tunneling technology, the 27 SR does not require additional signaling. Therefore, the SR does not 28 support the negotiation of the Path MTU. Since Multiple labels or 29 SRv6 SIDs are pushed in the packets, it is more likely that the 30 packet size exceeds the path mtu of SR tunnel. 32 This document specify the extension to BGP Link State (BGP-LS) to 33 carry maximum transmission unit (MTU) messages of link. The PCE/SDN 34 calculates the Path MTU while completing the service path calculation 35 based on the information transmitted by the BGP-LS. 37 Requirements Language 39 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 40 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 41 document are to be interpreted as described in RFC 2119 [RFC2119]. 43 Status of This Memo 45 This Internet-Draft is submitted in full conformance with the 46 provisions of BCP 78 and BCP 79. 48 Internet-Drafts are working documents of the Internet Engineering 49 Task Force (IETF). Note that other groups may also distribute 50 working documents as Internet-Drafts. The list of current Internet- 51 Drafts is at https://datatracker.ietf.org/drafts/current/. 53 Internet-Drafts are draft documents valid for a maximum of six months 54 and may be updated, replaced, or obsoleted by other documents at any 55 time. It is inappropriate to use Internet-Drafts as reference 56 material or to cite them other than as "work in progress." 58 This Internet-Draft will expire on July 10, 2020. 60 Copyright Notice 62 Copyright (c) 2020 IETF Trust and the persons identified as the 63 document authors. All rights reserved. 65 This document is subject to BCP 78 and the IETF Trust's Legal 66 Provisions Relating to IETF Documents 67 (https://trustee.ietf.org/license-info) in effect on the date of 68 publication of this document. Please review these documents 69 carefully, as they describe your rights and restrictions with respect 70 to this document. Code Components extracted from this document must 71 include Simplified BSD License text as described in Section 4.e of 72 the Trust Legal Provisions and are provided without warranty as 73 described in the Simplified BSD License. 75 Table of Contents 77 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 78 2. Deploying scenarios . . . . . . . . . . . . . . . . . . . . . 4 79 3. BGP_LS Extensions for Path MTU . . . . . . . . . . . . . . . 5 80 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 81 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5 82 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 83 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 84 7.1. Normative References . . . . . . . . . . . . . . . . . . 5 85 7.2. Informative References . . . . . . . . . . . . . . . . . 6 86 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 88 1. Introduction 90 [RFC7752]describes the implementation mechanism of BGP-LS by which 91 link-state and TE information can be collected from networks and 92 shared with external components using the BGP routing protocol 93 [RFC4271]. BGP-LS allows the necessary Link-State Database (LSDB) 94 and Traffic Engineering Database (TEDB) information to be collected 95 from the IGP within the network, filtered according to configurable 96 policy, and distributed to the PCE as necessary. 98 The appropriate MTU size guarantees efficient data transmission. If 99 the MTU size is too small and the packet size is large, fragmentation 100 may occur too much and packets are discarded by the QoS queue. If 101 the MTU configuration is too large, packet transmission may be slow. 102 PathMTU is the maximum length of a packet that can pass through a 103 path without fragmentation. [RFC1191] describes a technique for 104 dynamically discovering the maximum transmission unit (MTU) of an 105 arbitrary internet path. 107 The traditional MPLS tunneling technology has signaling for 108 establishing a path. [RFC3988] defines the mechanism for 109 automatically discovering the Path MTU of LSPs of LDP tunnels. For a 110 certain FEC, the LSR compares the MTU advertised by all downstream 111 devices with the MTU of the FEC output interface in the local device, 112 and calculates the minimum value for the upstream device. 114 [RFC3209] specify the mechanism of MTU signaling in RSVP-TE. The 115 ingress node of the RSVP-TE tunnel sends a Path message to the 116 downstream device. The Adspec object in the Path message carries the 117 MTU. Each node along the tunnel receives a Path message, compares 118 the MTU value in the Adspec object with the interface MTU value and 119 MPLS MTU configured on the physical output interface of the local 120 tunnel , obtains the minimum MTU value, and puts it into the newly 121 constructed Path message and continues to send it to the downstream 122 equipment. Thus, the MTU carried in the Path message received by the 123 Egress node is the minimum value of the path MTU. The Egress node 124 brings the negotiated Path MTU back to the Ingress node through the 125 Resv message. 127 Segment Routing (SR) described in [RFC8402] leverages the source 128 routing paradigm. Segment Routing can be directly applied to the 129 MPLS architecture with no change on the forwarding plane [RFC8660] 130 and applied to the IPv6 architecture with a new type of routing 131 header called the SR header (SRH) 132 [I-D.ietf-6man-segment-routing-header]. 133 [I-D.ietf-idr-bgp-ls-segment-routing-ext] defines SR extensions to 134 BGP-LS and specifies the TLVs and sub-TLVs for advertising SR 135 information. Based on the SR information reported by the BGP-LS, the 136 SDN can calculate the end-to-end explicit SR-TE paths or SR Policies. 138 Nevertheless, Segment Routing is a tunneling technology based on the 139 IGP protocol as the control protocol, and there is no additional 140 signaling for establishing the path. so the Segment Routing tunnel 141 cannot currently support the negotiation mechanism of the MTU. 142 Multiple labels or SRv6 SIDs are pushed in the packets. This causes 143 the length of the packets encapsulated in the Segment Routing tunnel 144 to increase during packet forwarding. This is more likely to cause 145 packet size exceed than traditional MPLS packet size. 147 This document specify the extension to BGP Link State (BGP-LS) to 148 carry link maximum transmission unit (MTU) messages. 150 2. Deploying scenarios 152 This document suggests a solution to extension to BGP Link State 153 (BGP-LS) to carry maximum transmission unit (MTU) messages. The MTU 154 information of the link is acquired through the process of collecting 155 link state and TE information by BGP-LS. Concretely, a router 156 maintains one or more databases for storing link-state information 157 about nodes and links in any given area. The router's BGP process 158 can retrieve topology from these LSDBs and distribute it to a 159 consumer, either directly or via a peer BGP speaker (typically a 160 dedicated Route Reflector). As for how IGP collects link MTU 161 information and stores it in LSDB, which is beyond the scope of this 162 article. 164 As per [RFC7752], the collection of link-state and TE information and 165 its distribution to consumers is shown in the following figure. 167 +-----------+ 168 | Consumer | 169 +-----------+ 170 ^ 171 | 172 +-----------+ 173 | BGP | +-----------+ 174 | Speaker | | Consumer | 175 +-----------+ +-----------+ 176 ^ ^ ^ ^ 177 | | | | 178 +---------------+ | +-------------------+ | 179 | | | | 180 +-----------+ +-----------+ +-----------+ 181 | BGP | | BGP | | BGP | 182 | Speaker | | Speaker | . . . | Speaker | 183 +-----------+ +-----------+ +-----------+ 184 ^ ^ ^ 185 | | | 186 IGP IGP IGP 188 Figure 1: Collection of Link-State and TE Information 190 3. BGP_LS Extensions for Path MTU 192 [RFC7752] defines the BGP-LS NLRI that can be a Node NLRI, a Link 193 NLRI or a Prefix NLRI. The corresponding BGP-LS attribute is a Node 194 Attribute, a Link Attribute or a Prefix Attribute. [RFC7752] defines 195 the TLVs that map link-state information to BGP-LS NLRI and the BGP- 196 LS attribute. Therefore, according to this document, a new sub TLV 197 is added to the Link Attribute TLV. 199 The format of the sub-TLV is as shown below. 201 x TYPE - TBD 202 x LENGTH - Total length of the value field, it should be 2 203 x VALUE - 2-byte MTU value of the link 205 No. of Octets 206 +-----------------+ 207 | MTU value | 2 208 +-----------------+ 210 Figure 2. Sub-TLV Format for MTU 212 Whenever there is a change in MTU value represented by Link Attribute 213 TLV, BGP-LS should re-originate the respective TLV with the new MTU 214 value. 216 4. IANA Considerations 218 This document requests assigning a new code-points from the BGP-LS 219 Link Descriptor and Attribute TLVs registry as specified in sections 220 3. 222 5. Security Considerations 224 This document does not introduce security issues beyond those 225 discussed in RFC7752. 227 6. Acknowledgements 229 7. References 231 7.1. Normative References 233 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 234 Requirement Levels", BCP 14, RFC 2119, 235 DOI 10.17487/RFC2119, March 1997, 236 . 238 7.2. Informative References 240 [I-D.ietf-6man-segment-routing-header] 241 Filsfils, C., Dukes, D., Previdi, S., Leddy, J., 242 Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header 243 (SRH)", draft-ietf-6man-segment-routing-header-26 (work in 244 progress), October 2019. 246 [I-D.ietf-idr-bgp-ls-segment-routing-ext] 247 Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H., 248 and M. Chen, "BGP Link-State extensions for Segment 249 Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-16 250 (work in progress), June 2019. 252 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 253 DOI 10.17487/RFC1191, November 1990, 254 . 256 [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., 257 and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP 258 Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001, 259 . 261 [RFC3988] Black, B. and K. Kompella, "Maximum Transmission Unit 262 Signalling Extensions for the Label Distribution 263 Protocol", RFC 3988, DOI 10.17487/RFC3988, January 2005, 264 . 266 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 267 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 268 DOI 10.17487/RFC4271, January 2006, 269 . 271 [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and 272 S. Ray, "North-Bound Distribution of Link-State and 273 Traffic Engineering (TE) Information Using BGP", RFC 7752, 274 DOI 10.17487/RFC7752, March 2016, 275 . 277 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 278 Decraene, B., Litkowski, S., and R. Shakir, "Segment 279 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 280 July 2018, . 282 [RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S., 283 Decraene, B., Litkowski, S., and R. Shakir, "Segment 284 Routing with the MPLS Data Plane", RFC 8660, 285 DOI 10.17487/RFC8660, December 2019, 286 . 288 Authors' Addresses 290 Yongqing Zhu 291 China Telecom 292 109, West Zhongshan Road, Tianhe District. 293 Guangzhou 510000 294 China 296 Email: zhuyq@gsta.com 298 Zhibo Hu 299 Huawei Technologies 300 Huawei Bld., No.156 Beiqing Rd. 301 Beijing 100095 302 China 304 Email: huzhibo@huawei.com 306 Gang Yan 307 Huawei Technologies 308 Huawei Bld., No.156 Beiqing Rd. 309 Beijing 100095 310 China 312 Email: yangang@huawei.com 314 Junda Yao 315 Huawei Technologies 316 Huawei Bld., No.156 Beiqing Rd. 317 Beijing 100095 318 China 320 Email: yaojunda@huawei.com