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Dolganow 7 Individual contributor 8 4 October 2021 10 Path Maximum Transmission Unit Discovery (PMTUD) for Bit Index Explicit 11 Replication (BIER) Layer 12 draft-ietf-bier-path-mtu-discovery-11 14 Abstract 16 This document describes Path Maximum Transmission Unit Discovery 17 (PMTUD) in Bit Indexed Explicit Replication (BIER) layer. 19 Status of This Memo 21 This Internet-Draft is submitted in full conformance with the 22 provisions of BCP 78 and BCP 79. 24 Internet-Drafts are working documents of the Internet Engineering 25 Task Force (IETF). Note that other groups may also distribute 26 working documents as Internet-Drafts. The list of current Internet- 27 Drafts is at https://datatracker.ietf.org/drafts/current/. 29 Internet-Drafts are draft documents valid for a maximum of six months 30 and may be updated, replaced, or obsoleted by other documents at any 31 time. It is inappropriate to use Internet-Drafts as reference 32 material or to cite them other than as "work in progress." 34 This Internet-Draft will expire on 7 April 2022. 36 Copyright Notice 38 Copyright (c) 2021 IETF Trust and the persons identified as the 39 document authors. All rights reserved. 41 This document is subject to BCP 78 and the IETF Trust's Legal 42 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 43 license-info) in effect on the date of publication of this document. 44 Please review these documents carefully, as they describe your rights 45 and restrictions with respect to this document. Code Components 46 extracted from this document must include Simplified BSD License text 47 as described in Section 4.e of the Trust Legal Provisions and are 48 provided without warranty as described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 53 1.1. Conventions used in this document . . . . . . . . . . . . 3 54 1.1.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . 3 55 1.1.2. Requirements Language . . . . . . . . . . . . . . . . 3 56 2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3 57 3. PMTUD Mechanism for BIER . . . . . . . . . . . . . . . . . . 4 58 3.1. Data TLV for BIER Ping . . . . . . . . . . . . . . . . . 6 59 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 60 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 61 6. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 7 62 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 63 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 64 7.2. Informative References . . . . . . . . . . . . . . . . . 8 65 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 67 1. Introduction 69 In packet switched networks, when a host seeks to transmit data to a 70 target destination, the data is transmitted as a set of packets. In 71 many cases, it is more efficient to use the largest size packets that 72 are less than or equal to the least Maximum Transmission Unit (MTU) 73 for any forwarding device along the routed path to the IP destination 74 for these packets. Such "least MTU" is known as Path MTU (PMTU). 75 Fragmentation or packet drop, silent or not, may occur on hops along 76 the route where an MTU is smaller than the size of the datagram. To 77 avoid any of the listed above behaviors, the packet source must find 78 the value of the least MTU, i.e., PMTU, that will be encountered 79 along the route that a set of packets will follow to reach the given 80 set of destinations. Such MTU determination along a specific path is 81 referred to as path MTU discovery (PMTUD). 83 [RFC8279] introduces and explains Bit Index Explicit Replication 84 (BIER) architecture and how it supports the forwarding of multicast 85 data packets. A BIER domain consists of Bit-Forwarding Routers 86 (BFRs) that are uniquely identified by their respective BFR-ids. An 87 ingress border router (acting as a Bit Forwarding Ingress Router 88 (BFIR)) inserts a Forwarding Bit Mask (F-BM) into a packet. Each 89 targeted egress node (referred to as a Bit Forwarding Egress Router 90 (BFER)) is represented by Bit Mask Position (BMP) in the BMS. A 91 transit or intermediate BIER node, referred to as BFR, forwards BIER 92 encapsulated packets to BFERs, identified by respective BMPs, 93 according to a Bit Index Forwarding Table (BIFT). 95 1.1. Conventions used in this document 97 1.1.1. Acronyms 99 BFR: Bit-Forwarding Router 101 BFER: Bit-Forwarding Egress Router 103 BFIR: Bit-Forwarding Ingress Router 105 BIER: Bit Index Explicit Replication 107 BIFT: Bit Index Forwarding Tree 109 F-BM: Forwarding Bit Mask 111 MTU: Maximum Transmission Unit 113 OAM: Operations, Administration and Maintenance 115 PMTUD: Path MTU Discovery 117 1.1.2. Requirements Language 119 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 120 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 121 "OPTIONAL" in this document are to be interpreted as described in BCP 122 14 [RFC2119] [RFC8174] when, and only when, they appear in all 123 capitals, as shown here. 125 2. Problem Statement 127 [I-D.ietf-bier-oam-requirements] sets forth the requirement to define 128 PMTUD protocol for BIER domain. This document describes the 129 extension to [I-D.ietf-bier-ping] for use in the BIER PMTUD solution. 131 Current PMTUD mechanisms ([RFC1191], [RFC8201], and [RFC4821]) are 132 primarily targeted to work on point-to-point, i.e. unicast paths. 133 These mechanisms use packet fragmentation control by disabling 134 fragmentation of the probe packet. As a result, a transient node 135 that cannot forward a probe packet that is bigger than its link MTU 136 sends to the packet source an error notification, otherwise the 137 packet destination may respond with a positive acknowledgment. Thus, 138 possibly through a series of iterations, varying the size of the 139 probe packet, the packet source discovers the PMTU of the particular 140 path. 142 Thus applied such existing PMTUD solutions are inefficient for point- 143 to-multipoint paths constructed for multicast traffic. Probe packets 144 must be flooded through the whole set of multicast distribution paths 145 over and over again until the very last egress responds with a 146 positive acknowledgment. Consider without loss of generality an 147 example multicast network presented in Figure 1, where MTU on all 148 links but one (B, D) is the same. If MTU on the link (B, D) is 149 smaller than the MTU on the other links, using existing PMTUD 150 mechanism probes will unnecessary flood to leaf nodes E, F, and G for 151 the second and consecutive times and positive responses will be 152 generated and received by root A repeatedly. 154 ----- 155 --| D | 156 ----- / ----- 157 --| B |-- 158 / ----- \ ----- 159 / --| E | 160 ----- / ----- 161 | A |--- ----- 162 ----- \ --| F | 163 \ ----- / ----- 164 --| C |-- 165 ----- \ ----- 166 --| G | 167 ----- 169 Figure 1: Multicast network 171 3. PMTUD Mechanism for BIER 173 A BFIR selects a set of BFERs for the specific multicast 174 distribution. Such a BFIR determines, by explicitly controlling a 175 subset of targeted BFERs and transmitting a series of probe packets, 176 the MTU of that multicast distribution tree. In the case of ECMP, 177 BFIR MAY test each path by variating the value in the Entropy field. 178 The critical step is that in case of failure at an intermediate BFR 179 to forward towards the subset of targeted downstream BFERs, the BFR 180 responds with a partial (compared to the one it received in the 181 request) bitmask towards the originating BFIR in error notification. 182 That allows for retransmission of the next probe with a smaller MTU 183 address only towards the failed downstream BFERs instead of all BFERs 184 addressed in the previous probe. In the scenario discussed in 185 Section 2 the second and all following (if needed) probes will be 186 sent only to the node D since MTU discovery of E, F, and G has been 187 completed already by the first probe successfully. 189 [I-D.ietf-bier-ping] introduced BIER Ping as a transport-independent 190 OAM mechanism to detect and localize failures in the BIER data plane. 191 This document specifies how BIER Ping can be used to perform 192 efficient PMTUD in the BIER domain. 194 Consider the network displayed in Figure 1 to be a presentation of a 195 BIER domain and all nodes to be BFRs. To discover MTU over BIER 196 domain to BFERs D, F, E, and G BFIR A will use BIER Ping with Data 197 TLV, defined in Section 3.1. Size of the first probe set to M_max 198 determined as minimal MTU value of BFIR's links to BIER domain. As 199 has been assumed in Section 2, MTUs of all links but the link (B, D) 200 are the same. Thus BFERs E, F, and G would receive BIER Echo Request 201 and will send their respective replies to BFIR A. BFR B may pass the 202 packet which is too large to forward over egress link (B, D) to the 203 appropriate network layer for error processing where it would be 204 recognized as a BIER Echo Request packet. BFR B MUST send BIER Echo 205 Reply to BFIR A and MUST include Downstream Mapping TLV, defined in 206 [I-D.ietf-bier-ping] setting its fields in the following fashion: 208 * MTU SHOULD be set to the minimal MTU value among all egress BIER 209 links, logical links between this and downstream BFRs, that could 210 be used to reach B's downstream BFERs; 212 * Address Type MUST be set to 0 [Ed.note: we need to define 0 as 213 valid value for the Address Type field with the specific semantics 214 to "Ignore" it.] 216 * I flag MUST be cleared; 218 * Downstream Interface Address field (4 octets) MUST be zeroed and 219 MUST include in the Egress Bitstring sub-TLV the list of all BFERs 220 that cannot be reached because the attempted MTU turned out to be 221 too small. 223 The BFIR will receive either of the two types of packets: 225 * a positive Echo Reply from one of BFERs to which the probe has 226 been sent. In this case, the bit corresponding to the BFER MUST 227 be cleared from the BMS; 229 * a negative Echo Reply with bit string listing unreached BFERs and 230 recommended MTU value MTU'. The BFIR MUST add the bit string to 231 its BMS and set the size of the next probe as min(MTU, MTU') 233 If upon expiration of the Echo Request timer BFIR didn't receive any 234 Echo Replies, then the size of the probe SHOULD be decreased. There 235 are scenarios when an implementation of the PMTUD would not decrease 236 the size of the probe. For example, suppose upon expiration of the 237 Echo Request timer BFIR didn't receive any Echo Reply. In that case, 238 BFIR MAY continue to retransmit the probe using the initial size and 239 MAY apply probe delay retransmission procedures. The algorithm used 240 to delay retransmission procedures on BFIR is outside the scope of 241 this specification. The BFIR sends probes using BMS and locally 242 defined retransmission procedures until either the bit string is 243 clear, i.e., contains no set bits, or until the BFIR retransmission 244 procedure terminates and PMTU discovery is declared unsuccessful. In 245 the case of convergence of the procedure, the size of the last probe 246 indicates the PMTU size that can be used for all BFERs in the initial 247 BMS without incurring fragmentation. 249 Thus we conclude that in order to comply with the requirement in 250 [I-D.ietf-bier-oam-requirements]: 252 * a BFR SHOULD support PMTUD; 254 * a BFR MAY use defined per BIER sub-domain MTU value as initial MTU 255 value for discovery or use it as MTU for this BIER sub-domain to 256 reach BFERs; 258 * a BFIR MUST have a locally defined PMTUD probe retransmission 259 procedure. 261 3.1. Data TLV for BIER Ping 263 There needs to be a control for probe size in order to support the 264 BIER PMTUD. Data TLV format is presented in Figure 2. 266 0 1 2 3 267 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 1 268 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 269 | Type (TBA1) | Length | 270 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 271 | Data | 272 ~ ~ 273 | | 274 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 276 Figure 2: Data TLV format 278 * Type: indicates Data TLV, to be allocated by IANA Section 4. 280 * Length: the length of the Data field in octets. 282 * Data: n octets (n = Length) of arbitrary data. The receiver 283 SHOULD ignore it. 285 4. IANA Considerations 287 IANA is requested to assign a new Type value for Data TLV Type from 288 its registry of TLV and sub-TLV Types of BIER Ping as follows: 290 +=======+=============+===============+ 291 | Value | Description | Reference | 292 +=======+=============+===============+ 293 | TBA1 | Data | This document | 294 +-------+-------------+---------------+ 296 Table 1: Data TLV Type 298 5. Security Considerations 300 Routers that support PMTUD based on this document are subject to the 301 same security considerations as defined in [I-D.ietf-bier-ping] 303 6. Acknowledgment 305 Authors greatly appreciate thorough review and the most detailed 306 comments by Eric Gray. 308 7. References 310 7.1. Normative References 312 [I-D.ietf-bier-ping] 313 Kumar, N., Pignataro, C., Akiya, N., Zheng, L., Chen, M., 314 and G. Mirsky, "BIER Ping and Trace", Work in Progress, 315 Internet-Draft, draft-ietf-bier-ping-07, 11 May 2020, 316 . 319 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 320 DOI 10.17487/RFC1191, November 1990, 321 . 323 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 324 Requirement Levels", BCP 14, RFC 2119, 325 DOI 10.17487/RFC2119, March 1997, 326 . 328 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 329 Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, 330 . 332 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 333 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 334 May 2017, . 336 [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., 337 "Path MTU Discovery for IP version 6", STD 87, RFC 8201, 338 DOI 10.17487/RFC8201, July 2017, 339 . 341 7.2. Informative References 343 [I-D.ietf-bier-oam-requirements] 344 Mirsky, G., Kumar, N., Chen, M., and S. Pallagatti, 345 "Operations, Administration and Maintenance (OAM) 346 Requirements for Bit Index Explicit Replication (BIER) 347 Layer", Work in Progress, Internet-Draft, draft-ietf-bier- 348 oam-requirements-11, 15 November 2020, 349 . 352 [RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A., 353 Przygienda, T., and S. Aldrin, "Multicast Using Bit Index 354 Explicit Replication (BIER)", RFC 8279, 355 DOI 10.17487/RFC8279, November 2017, 356 . 358 Authors' Addresses 360 Greg Mirsky 361 Ericsson 363 Email: gregimirsky@gmail.com 365 Tony Przygienda 366 Juniper Networks 368 Email: prz@juniper.net 370 Andrew Dolganow 371 Individual contributor 373 Email: adolgano@gmail.com