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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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-13) exists of draft-ietf-bier-ping-02 == Outdated reference: A later version (-15) exists of draft-ietf-bier-oam-requirements-04 Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 1 comment (--).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: July 21, 2018 Juniper Networks 6 A. Dolganow 7 Nokia 8 January 17, 2018 10 Path Maximum Transmission Unit Discovery (PMTUD) for Bit Index Explicit 11 Replication (BIER) Layer 12 draft-ietf-bier-path-mtu-discovery-03 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 July 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. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 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 a 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 as BFR, forwards BIER encapsulated 93 packets to BFERs, identified by respective BMPs, according to a Bit 94 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 acknowledgement. 139 Thus, possibly through a series of iterations, varying the size of 140 the probe packet, the packet source discovers the PMTU of the 141 particular 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 acknowledgement. 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 link (B,D) is smaller 150 than the MTU on the other links, using existing PMTUD mechanism 151 probes will unnecessary flood to leaf nodes E, F, and G for the 152 second and consecutive times and positive responses will be generated 153 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 176 subset of targeted BFERs and transmitting series of probe packets, 177 the MTU of that multicast distribution tree. The critical step is 178 that in case of failure at an intermediate BFR to forward towards the 179 subset of targeted downstream BFERs, the BFR responds with a partial 180 (compared to the one it received in the request) bitmask towards the 181 originating BFIR in error notification. That allows for 182 retransmission of the next probe with smaller MTU address only 183 towards the failed downstream BFERs instead of all BFERs addressed in 184 the previous probe. In the scenario discussed in Section 2 the 185 second and all following (if needed) probes will be sent only to the 186 node D since MTU discovery of E, F, and G has been completed already 187 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 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 link (B,D) are 200 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 o 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 o 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 o I flag MUST be cleared; 218 o 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 o 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 be 227 cleared from the BMS; 229 o 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 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, if upon expiration of the Echo 237 Request timer BFIR didn't receive any Echo Reply, then BFIR MAY 238 continue to retransmit the probe using the initial size and MAY apply 239 probe delay retransmission procedures. The algorithm used to delay 240 retransmission procedures on BFIR is outside the scope of this 241 specification. The BFIR sends probes using BMS and locally defined 242 retransmission procedures until either the bit string is clear, i.e. 243 contains no set bits, or until the BFIR retransmission procedure 244 terminates and PMTU discovery is declared unsuccessful. In case of 245 convergence of the procedure, the size of the last probe indicates 246 the PMTU size that can be used for all BFERs in the initial BMS 247 without incurring fragmentation. 249 Thus we conclude that in order to comply with the requirement in 250 [I-D.ietf-bier-oam-requirements]: 252 o a BFR SHOULD support PMTUD; 254 o 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 o a BFIR MUST have a locally defined of 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 o Type: indicates Data TLV, to be allocated by IANA Section 4. 280 o Length: the length of the Data field in octets. 282 o Data: n octets (n = Length) of arbitrary data. The receiver 283 SHOULD ignore it. 285 4. IANA Considerations 287 IANA is requested to assign new Type value for Data TLV Type from its 288 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. Acknowledgement 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", draft-ietf-bier- 315 ping-02 (work in progress), July 2017. 317 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 318 DOI 10.17487/RFC1191, November 1990, 319 . 321 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 322 Requirement Levels", BCP 14, RFC 2119, 323 DOI 10.17487/RFC2119, March 1997, 324 . 326 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 327 Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, 328 . 330 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 331 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 332 May 2017, . 334 [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., 335 "Path MTU Discovery for IP version 6", STD 87, RFC 8201, 336 DOI 10.17487/RFC8201, July 2017, 337 . 339 7.2. Informative References 341 [I-D.ietf-bier-oam-requirements] 342 Mirsky, G., Nordmark, E., Pignataro, C., Kumar, N., 343 Aldrin, S., Zheng, L., Chen, M., Akiya, N., and S. 344 Pallagatti, "Operations, Administration and Maintenance 345 (OAM) Requirements for Bit Index Explicit Replication 346 (BIER) Layer", draft-ietf-bier-oam-requirements-04 (work 347 in progress), July 2017. 349 [RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A., 350 Przygienda, T., and S. Aldrin, "Multicast Using Bit Index 351 Explicit Replication (BIER)", RFC 8279, 352 DOI 10.17487/RFC8279, November 2017, 353 . 355 Authors' Addresses 357 Greg Mirsky 358 ZTE Corp. 360 Email: gregimirsky@gmail.com 362 Tony Przygienda 363 Juniper Networks 365 Email: prz@juniper.net 367 Andrew Dolganow 368 Nokia 370 Email: andrew.dolganow@nokia.com