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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-13) exists of draft-ietf-bier-ping-03 == Outdated reference: A later version (-14) exists of draft-ietf-bier-oam-requirements-05 Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 BIER Working Group G. Mirsky 3 Internet-Draft ZTE Corp. 4 Intended status: Standards Track T. Przygienda 5 Expires: December 21, 2018 Juniper Networks 6 A. Dolganow 7 Nokia 8 June 19, 2018 10 Path Maximum Transmission Unit Discovery (PMTUD) for Bit Index Explicit 11 Replication (BIER) Layer 12 draft-ietf-bier-path-mtu-discovery-04 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 December 21, 2018. 36 Copyright Notice 38 Copyright (c) 2018 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 43 (https://trustee.ietf.org/license-info) in effect on the date of 44 publication of this document. Please review these documents 45 carefully, as they describe your rights and restrictions with respect 46 to this document. Code Components extracted from this document must 47 include Simplified BSD License text as described in Section 4.e of 48 the Trust Legal Provisions and are provided without warranty as 49 described in the Simplified BSD License. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 54 1.1. Conventions used in this document . . . . . . . . . . . . 3 55 1.1.1. Terminology . . . . . . . . . . . . . . . . . . . . . 3 56 1.1.2. Requirements Language . . . . . . . . . . . . . . . . 3 57 2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3 58 3. PMTUD Mechanism for BIER . . . . . . . . . . . . . . . . . . 4 59 3.1. Data TLV for BIER Ping . . . . . . . . . . . . . . . . . 6 60 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 61 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 62 6. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 7 63 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 64 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 65 7.2. Informative References . . . . . . . . . . . . . . . . . 8 66 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 68 1. Introduction 70 In packet switched networks, when a host seeks to transmit data to a 71 target destination, the data is transmitted as a set of packets. In 72 many cases, it is more efficient to use the largest size packets that 73 are less than or equal to the least Maximum Transmission Unit (MTU) 74 for any forwarding device along the routed path to the IP destination 75 for these packets. Such "least MTU" is known as Path MTU (PMTU). 76 Fragmentation or packet drop, silent or not, may occur on hops along 77 the route where an MTU is smaller than the size of the datagram. To 78 avoid any of the listed above behaviors, the packet source must find 79 the value of the least MTU, i.e. PMTU, that will be encountered along 80 the route that a set of packets will follow to reach the given set of 81 destinations. Such MTU determination along a specific path is 82 referred to as path MTU discovery (PMTUD). 84 [RFC8279] introduces and explains Bit Index Explicit Replication 85 (BIER) architecture and how it supports forwarding of multicast data 86 packets. A BIER domain consists of Bit-Forwarding Routers (BFRs) 87 that are uniquely identified by their respective BFR-ids. An ingress 88 border router (acting as a Bit Forwarding Ingress Router (BFIR)) 89 inserts a Forwarding Bit Mask (F-BM) into a packet. Each targeted 90 egress node (referred to as a Bit Forwarding Egress Router (BFER)) is 91 represented by Bit Mask Position (BMP) in the BMS. A transit or 92 intermediate BIER node, referred to as BFR, forwards BIER 93 encapsulated packets to BFERs, identified by respective BMPs, 94 according to a Bit Index Forwarding Table (BIFT). 96 1.1. Conventions used in this document 98 1.1.1. Terminology 100 BFR: Bit-Forwarding Router 102 BFER: Bit-Forwarding Egress Router 104 BFIR: Bit-Forwarding Ingress Router 106 BIER: Bit Index Explicit Replication 108 BIFT: Bit Index Forwarding Tree 110 F-BM: Forwarding Bit Mask 112 MTU: Maximum Transmission Unit 114 OAM: Operations, Administration and Maintenance 116 PMTUD: Path MTU Discovery 118 1.1.2. Requirements Language 120 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 121 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 122 "OPTIONAL" in this document are to be interpreted as described in BCP 123 14 [RFC2119] [RFC8174] when, and only when, they appear in all 124 capitals, as shown here. 126 2. Problem Statement 128 [I-D.ietf-bier-oam-requirements] sets forth the requirement to define 129 PMTUD protocol for BIER domain. This document describes the 130 extension to [I-D.ietf-bier-ping] for use in BIER PMTUD solution. 132 Current PMTUD mechanisms ([RFC1191], [RFC8201], and [RFC4821]) are 133 primarily targeted to work on point-to-point, i.e. unicast paths. 134 These mechanisms use packet fragmentation control by disabling 135 fragmentation of the probe packet. As a result, a transient node 136 that cannot forward a probe packet that is bigger than its link MTU 137 sends to the packet source an error notification, otherwise the 138 packet destination may respond with a positive acknowledgment. Thus, 139 possibly through a series of iterations, varying the size of the 140 probe packet, the packet source discovers the PMTU of the particular 141 path. 143 Thus applied such existing PMTUD solutions are inefficient for point- 144 to-multipoint paths constructed for multicast traffic. Probe packets 145 must be flooded through the whole set of multicast distribution paths 146 over and over again until the very last egress responds with a 147 positive acknowledgment. Consider without loss of generality an 148 example multicast network presented in Figure 1, where MTU on all 149 links but one (B, D) is the same. If MTU on the link (B, D) is 150 smaller than the MTU on the other links, using existing PMTUD 151 mechanism probes will unnecessary flood to leaf nodes E, F, and G for 152 the second and consecutive times and positive responses will be 153 generated and received by root A repeatedly. 155 ----- 156 --| D | 157 ----- / ----- 158 --| B |-- 159 / ----- \ ----- 160 / --| E | 161 ----- / ----- 162 | A |--- ----- 163 ----- \ --| F | 164 \ ----- / ----- 165 --| C |-- 166 ----- \ ----- 167 --| G | 168 ----- 170 Figure 1: Multicast network 172 3. PMTUD Mechanism for BIER 174 A BFIR selects a set of BFERs for the specific multicast 175 distribution. Such a BFIR determines, by explicitly controlling a 176 subset of targeted BFERs and transmitting series of probe packets, 177 the MTU of that multicast distribution tree. In case of ECMP, BFIR 178 MAY test each path by variating the value in Entropy field. The 179 critical step is that in case of failure at an intermediate BFR to 180 forward towards the subset of targeted downstream BFERs, the BFR 181 responds with a partial (compared to the one it received in the 182 request) bitmask towards the originating BFIR in error notification. 183 That allows for retransmission of the next probe with smaller MTU 184 address only towards the failed downstream BFERs instead of all BFERs 185 addressed in the previous probe. In the scenario discussed in 186 Section 2 the second and all following (if needed) probes will be 187 sent only to the node D since MTU discovery of E, F, and G has been 188 completed already by the first probe successfully. 190 [I-D.ietf-bier-ping] introduced BIER Ping as a transport-independent 191 OAM mechanism to detect and localize failures in the BIER data plane. 192 This document specifies how BIER Ping can be used to perform 193 efficient PMTUD in the BIER domain. 195 Consider the network displayed in Figure 1 to be a presentation of a 196 BIER domain and all nodes to be BFRs. To discover MTU over BIER 197 domain to BFERs D, F, E, and G BFIR A will use BIER Ping with Data 198 TLV, defined in Section 3.1. Size of the first probe set to M_max 199 determined as minimal MTU value of BFIR's links to BIER domain. As 200 has been assumed in Section 2, MTUs of all links but the link (B, D) 201 are the same. Thus BFERs E, F, and G would receive BIER Echo Request 202 and will send their respective replies to BFIR A. BFR B may pass the 203 packet which is too large to forward over egress link (B, D) to the 204 appropriate network layer for error processing where it would be 205 recognized as a BIER Echo Request packet. BFR B MUST send BIER Echo 206 Reply to BFIR A and MUST include Downstream Mapping TLV, defined in 207 [I-D.ietf-bier-ping] setting its fields in the following fashion: 209 o MTU SHOULD be set to the minimal MTU value among all egress BIER 210 links, logical links between this and downstream BFRs, that could 211 be used to reach B's downstream BFERs; 213 o Address Type MUST be set to 0 [Ed.note: we need to define 0 as 214 valid value for the Address Type field with the specific semantics 215 to "Ignore" it.] 217 o I flag MUST be cleared; 219 o Downstream Interface Address field (4 octets) MUST be zeroed and 220 MUST include in the Egress Bitstring sub-TLV the list of all BFERs 221 that cannot be reached because the attempted MTU turned out to be 222 too small. 224 The BFIR will receive either of the two types of packets: 226 o a positive Echo Reply from one of BFERs to which the probe has 227 been sent. In this case, the bit corresponding to the BFER MUST 228 be cleared from the BMS; 230 o a negative Echo Reply with bit string listing unreached BFERs and 231 recommended MTU value MTU'. The BFIR MUST add the bit string to 232 its BMS and set the size of the next probe as min(MTU, MTU') 234 If upon expiration of the Echo Request timer BFIR didn't receive any 235 Echo Replies, then the size of the probe SHOULD be decreased. There 236 are scenarios when an implementation of the PMTUD would not decrease 237 the size of the probe. For example, if upon expiration of the Echo 238 Request timer BFIR didn't receive any Echo Reply, then BFIR MAY 239 continue to retransmit the probe using the initial size and MAY apply 240 probe delay retransmission procedures. The algorithm used to delay 241 retransmission procedures on BFIR is outside the scope of this 242 specification. The BFIR sends probes using BMS and locally defined 243 retransmission procedures until either the bit string is clear, i.e. 244 contains no set bits, or until the BFIR retransmission procedure 245 terminates and PMTU discovery is declared unsuccessful. In case of 246 convergence of the procedure, the size of the last probe indicates 247 the PMTU size that can be used for all BFERs in the initial BMS 248 without incurring fragmentation. 250 Thus we conclude that in order to comply with the requirement in 251 [I-D.ietf-bier-oam-requirements]: 253 o a BFR SHOULD support PMTUD; 255 o a BFR MAY use defined per BIER sub-domain MTU value as initial MTU 256 value for discovery or use it as MTU for this BIER sub-domain to 257 reach BFERs; 259 o a BFIR MUST have a locally defined of PMTUD probe retransmission 260 procedure. 262 3.1. Data TLV for BIER Ping 264 There needs to be a control for probe size in order to support the 265 BIER PMTUD. Data TLV format is presented in Figure 2. 267 0 1 2 3 268 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 1 269 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 270 | Type (TBA1) | Length | 271 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 272 | Data | 273 ~ ~ 274 | | 275 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 277 Figure 2: Data TLV format 279 o Type: indicates Data TLV, to be allocated by IANA Section 4. 281 o Length: the length of the Data field in octets. 283 o Data: n octets (n = Length) of arbitrary data. The receiver 284 SHOULD ignore it. 286 4. IANA Considerations 288 IANA is requested to assign new Type value for Data TLV Type from its 289 registry of TLV and sub-TLV Types of BIER Ping as follows: 291 +-------+-------------+---------------+ 292 | Value | Description | Reference | 293 +-------+-------------+---------------+ 294 | TBA1 | Data | This document | 295 +-------+-------------+---------------+ 297 Table 1: Data TLV Type 299 5. Security Considerations 301 Routers that support PMTUD based on this document are subject to the 302 same security considerations as defined in [I-D.ietf-bier-ping] 304 6. Acknowledgment 306 Authors greatly appreciate thorough review and the most detailed 307 comments by Eric Gray. 309 7. References 311 7.1. Normative References 313 [I-D.ietf-bier-ping] 314 Kumar, N., Pignataro, C., Akiya, N., Zheng, L., Chen, M., 315 and G. Mirsky, "BIER Ping and Trace", draft-ietf-bier- 316 ping-03 (work in progress), January 2018. 318 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 319 DOI 10.17487/RFC1191, November 1990, 320 . 322 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 323 Requirement Levels", BCP 14, RFC 2119, 324 DOI 10.17487/RFC2119, March 1997, 325 . 327 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 328 Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, 329 . 331 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 332 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 333 May 2017, . 335 [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., 336 "Path MTU Discovery for IP version 6", STD 87, RFC 8201, 337 DOI 10.17487/RFC8201, July 2017, 338 . 340 7.2. Informative References 342 [I-D.ietf-bier-oam-requirements] 343 Mirsky, G., Nordmark, E., Pignataro, C., Kumar, N., 344 Aldrin, S., Zheng, L., Chen, M., Akiya, N., and S. 345 Pallagatti, "Operations, Administration and Maintenance 346 (OAM) Requirements for Bit Index Explicit Replication 347 (BIER) Layer", draft-ietf-bier-oam-requirements-05 (work 348 in progress), January 2018. 350 [RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A., 351 Przygienda, T., and S. Aldrin, "Multicast Using Bit Index 352 Explicit Replication (BIER)", RFC 8279, 353 DOI 10.17487/RFC8279, November 2017, 354 . 356 Authors' Addresses 358 Greg Mirsky 359 ZTE Corp. 361 Email: gregimirsky@gmail.com 363 Tony Przygienda 364 Juniper Networks 366 Email: prz@juniper.net 368 Andrew Dolganow 369 Nokia 371 Email: andrew.dolganow@nokia.com