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Saad 8 Cisco Systems 9 October 15, 2018 11 Signaling RSVP-TE tunnels on a shared MPLS forwarding plane 12 draft-ietf-mpls-rsvp-shared-labels-05.txt 14 Abstract 16 As the scale of MPLS RSVP-TE networks has grown, so the number of 17 Label Switched Paths (LSPs) supported by individual network elements 18 has increased. Various implementation recommendations have been 19 proposed to manage the resulting increase in control plane state. 21 However, those changes have had no effect on the number of labels 22 that a transit Label Switching Router (LSR) has to support in the 23 forwarding plane. That number is governed by the number of LSPs 24 transiting or terminated at the LSR and is directly related to the 25 total LSP state in the control plane. 27 This document defines a mechanism to prevent the maximum size of the 28 label space limit on an LSR from being a constraint to control plane 29 scaling on that node. It introduces the notion of pre-installed 'per 30 Traffic Engineering (TE) link labels' that can be shared by MPLS 31 RSVP-TE LSPs that traverse these TE links. This approach 32 significantly reduces the forwarding plane state required to support 33 a large number of LSPs. This couples the feature benefits of the 34 RSVP-TE control plane with the simplicity of the Segment Routing MPLS 35 forwarding plane. 37 Requirements Language 39 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 40 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 41 "OPTIONAL" in this document are to be interpreted as described in BCP 42 14 [RFC2119] [RFC8174] when, and only when, they appear in all 43 capitals, as shown here. 45 Status of This Memo 47 This Internet-Draft is submitted in full conformance with the 48 provisions of BCP 78 and BCP 79. 50 Internet-Drafts are working documents of the Internet Engineering 51 Task Force (IETF). Note that other groups may also distribute 52 working documents as Internet-Drafts. The list of current Internet- 53 Drafts is at http://datatracker.ietf.org/drafts/current/. 55 Internet-Drafts are draft documents valid for a maximum of six months 56 and may be updated, replaced, or obsoleted by other documents at any 57 time. It is inappropriate to use Internet-Drafts as reference 58 material or to cite them other than as "work in progress." 60 This Internet-Draft will expire on April 18, 2019. 62 Copyright Notice 64 Copyright (c) 2018 IETF Trust and the persons identified as the 65 document authors. All rights reserved. 67 This document is subject to BCP 78 and the IETF Trust's Legal 68 Provisions Relating to IETF Documents 69 (http://trustee.ietf.org/license-info) in effect on the date of 70 publication of this document. Please review these documents 71 carefully, as they describe your rights and restrictions with respect 72 to this document. Code Components extracted from this document must 73 include Simplified BSD License text as described in Section 4.e of 74 the Trust Legal Provisions and are provided without warranty as 75 described in the Simplified BSD License. 77 Table of Contents 79 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 80 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 81 3. Allocation of TE Link Labels . . . . . . . . . . . . . . . . 5 82 4. Segment Routed RSVP-TE Tunnel Setup . . . . . . . . . . . . . 5 83 5. Delegating Label Stack Imposition . . . . . . . . . . . . . . 7 84 5.1. Stacking at the Ingress . . . . . . . . . . . . . . . . . 8 85 5.1.1. Stack to Reach Delegation Hop . . . . . . . . . . . . 8 86 5.1.2. Stack to Reach Egress . . . . . . . . . . . . . . . . 9 87 5.2. Explicit Delegation . . . . . . . . . . . . . . . . . . . 10 88 5.3. Automatic Delegation . . . . . . . . . . . . . . . . . . 10 89 5.3.1. Effective Transport Label-Stack Depth (ETLD) . . . . 10 90 6. Mixing TE Link Labels and Regular Labels in an RSVP-TE Tunnel 12 91 7. Construction of Label Stacks . . . . . . . . . . . . . . . . 12 92 8. Facility Backup Protection . . . . . . . . . . . . . . . . . 13 93 8.1. Link Protection . . . . . . . . . . . . . . . . . . . . . 13 94 9. Protocol Extensions . . . . . . . . . . . . . . . . . . . . . 14 95 9.1. Requirements . . . . . . . . . . . . . . . . . . . . . . 14 96 9.2. Attribute Flags TLV: TE Link Label . . . . . . . . . . . 15 97 9.3. RRO Label Subobject Flag: TE Link Label . . . . . . . . . 15 98 9.4. Attribute Flags TLV: LSI-D . . . . . . . . . . . . . . . 15 99 9.5. RRO Label Subobject Flag: Delegation Label . . . . . . . 16 100 9.6. Attributes Flags TLV: LSI-D-S2E . . . . . . . . . . . . . 16 101 9.7. Attributes TLV: ETLD . . . . . . . . . . . . . . . . . . 16 102 10. OAM Considerations . . . . . . . . . . . . . . . . . . . . . 17 103 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 104 12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 17 105 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 106 13.1. Attribute Flags: TE Link Label, LSI-D, LSI-D-S2E . . . . 18 107 13.2. Attribute TLV: ETLD . . . . . . . . . . . . . . . . . . 18 108 13.3. Record Route Label Sub-object Flags: TE Link Label, 109 Delegation Label . . . . . . . . . . . . . . . . . . . . 18 110 13.4. Error Codes and Error Values . . . . . . . . . . . . . . 19 111 14. Security Considerations . . . . . . . . . . . . . . . . . . . 19 112 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 113 15.1. Normative References . . . . . . . . . . . . . . . . . . 19 114 15.2. Informative References . . . . . . . . . . . . . . . . . 20 115 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 117 1. Introduction 119 The scaling of RSVP-TE [RFC3209] control plane implementations can be 120 improved by adopting the guidelines and mechanisms described in 121 [RFC2961] and [RFC8370]. These documents do not make any difference 122 to the forwarding plane state required to handle the control plane 123 state. The forwarding plane state remains unchanged and is directly 124 proportional to the total number of Label Switching Paths (LSPs) 125 supported by the control plane. 127 This document describes a mechanism that prevents the size of the 128 platform specific label space on a Label Switching Router (LSR) from 129 being a constraint to pushing the limits of control plane scaling on 130 that node. 132 This work introduces the notion of pre-installed 'per Traffic 133 Engineering (TE) link labels' that are allocated by an LSR. Each 134 such label is installed in the MPLS forwarding plane with a 'pop' 135 operation and the instruction to forward the received packet over the 136 TE link. An LSR advertises this label in the Label object of a Resv 137 message as LSPs are set up and they are recorded hop by hop in the 138 Record Route object (RRO) of the Resv message as it traverses the 139 network. To make use of this feature, the ingress Label Edge Router 140 (LER) pushes a stack of labels [RFC3031] as received in the RRO. 142 These 'TE link labels' can be shared by MPLS RSVP-TE LSPs that 143 traverse the same TE link. 145 This forwarding plane behavior fits in the MPLS architecture 146 [RFC3031] and is same as that exhibited by Segment Routing (SR) 147 [RFC8402] when using an MPLS forwarding plane and a series of 148 adjacency segments [I-D.ietf-spring-segment-routing-mpls]. This work 149 couples the feature benefits of the RSVP-TE control plane with the 150 simplicity of the Segment Routing MPLS forwarding plane. 152 RSVP-TE using a shared MPLS forwarding plane offers the following 153 benefits: 155 1. Shared Labels: The transit label on a TE link is shared among 156 RSVP-TE tunnels traversing the link and is used independent of 157 the ingress and egress of the LSPs. 159 2. Faster LSP setup time: No forwarding plane state needs to be 160 programmed during LSP setup and teardown resulting in faster time 161 for provisioning and deprovisioning LSPs. 163 3. Hitless re-routing: New transit labels are not required during 164 make-before-break (MBB) in scenarios where the new LSP instance 165 traverses the exact same path as the old LSP instance. This 166 saves the ingress LER and the services that use the tunnel from 167 needing to update the forwarding plane with new tunnel labels and 168 so makes MBB events faster. Periodic MBB events are relatively 169 common in networks that deploy the 'auto-bandwidth' feature on 170 RSVP-TE LSPs to monitor bandwidth utilization and periodically 171 adjust LSP bandwidth. 173 4. Mix and match labels: Both 'TE link labels' and regular labels 174 can be used on transit hops for a single RSVP-TE tunnel (see 175 Section 6). This allows backward compatibility with transit LSRs 176 that provide regular labels in Resv messages. 178 No additional extensions to routing protocols are required in order 179 to support key functionalities such as bandwidth admission control, 180 LSP priorities, preemption and auto-bandwidth on this shared MPLS 181 forwarding plane. This document also discusses how Fast Reroute 182 [RFC4090] via facility backup link protection using regular bypass 183 tunnels can be supported on this forwarding plane. 185 The signaling procedures and extensions discussed in this document do 186 not apply to Point to Multipoint (P2MP) RSVP-TE Tunnels. 188 2. Terminology 190 The following terms are used in this document: 192 TE link label: An incoming label at an LSR that will be popped by 193 the LSR with the packet being forwarded over a specific outgoing 194 TE link to a neighbor. 196 Shared MPLS forwarding plane: An MPLS forwarding plane where every 197 participating LSR uses TE link labels on every LSP. 199 Segment Routed RSVP-TE tunnel: An MPLS RSVP-TE tunnel that requests 200 the use of a shared MPLS forwarding plane at every hop of the LSP. 201 The corresponding LSPs are referred to as Segment Routed RSVP-TE 202 LSPs. 204 Delegation hop: A transit hop of a Segment Routed RSVP-TE LSP that 205 is selected to assist in the imposition of the label stack in 206 scenarios where the ingress LER cannot impose the full label 207 stack. There could be multiple delegation hops along the path of 208 a Segment Routed RSVP-TE LSP. 210 Delegation label: A label assigned at the delegation hop to 211 represent a set of labels that will be pushed at this hop. 213 3. Allocation of TE Link Labels 215 An LSR that participates in a shared MPLS forwarding plane MUST 216 allocate a unique TE link label for each TE link. When an LSR 217 encounters a TE link label at the top of the label stack it MUST pop 218 the label and forward the packet over the TE link to the downstream 219 neighbor on the RSVP-TE tunnel. 221 Multiple TE link labels MAY be allocated for the TE link to 222 accommodate tunnels requesting protection. 224 Implementations that maintain per label bandwidth accounting at each 225 hop must aggregate the reservations made for all the LSPs using the 226 shared TE link label. 228 4. Segment Routed RSVP-TE Tunnel Setup 230 This section provides an example of how the RSVP-TE signaling 231 procedure works to set up a tunnel utilizing a shared MPLS forwarding 232 plane. The sample topology below is used to explain the example. 233 Labels shown at each node are TE link labels that, when present at 234 the top of the label stack, indicate that they should be popped and 235 that the packet should be forwarded on the TE link to the neighbor. 237 +---+100 +---+150 +---+200 +---+250 +---+ 238 | A |-----| B |-----| C |-----| D |-----| E | 239 +---+ +---+ +---+ +---+ +---+ 240 |110 |450 |550 |650 |850 241 | | | | | 242 | |400 |500 |600 |800 243 | +---+ +---+ +---+ +---+ 244 +-------| F |-----|G |-----|H |-----|I | 245 +---+300 +---+350 +---+700 +---+ 247 Figure 1: Sample Topology - TE Link Labels 249 Consider two tunnels: 251 RSVP-TE tunnel T1: From A to E on path A-B-C-D-E 253 RSVP-TE tunnel T2: From F to E on path F-B-C-D-E 255 Both tunnels share the TE links B-C, C-D, and D-E. 257 RSVP-TE is used to signal the setup of tunnel T1 (using the TE link 258 label attributes flag defined in Section 9.2). When LSR D receives 259 the Resv message from the egress LER E, it checks the next-hop TE 260 link (D-E) and provides the TE link label (250) in the Resv message 261 for the tunnel placing the label value in the Label object and also 262 in the Label subobject carried in the RRO and setting the TE link 263 label flag as defined in Section 9.3. 265 Similarly, LSR C provides the TE link label (200) for the TE link 266 C-D, and LSR B provides the TE link label (150) for the TE link B-C. 268 For tunnel T2, the transit LSRs provide the same TE link labels as 269 described for tunnel T1 as the links B-C, C-D, and D-E are common 270 between the two LSPs. 272 The ingress LERs (A and F) will push the same stack of labels (from 273 top of stack to bottom of stack) {150, 200, 250} for tunnels T1 and 274 T2 respectively. 276 It should be noted that a transit LSR does not swap the top TE link 277 label on an incoming packet (the label that it advertised in the Resv 278 message it sent). All it has to do is pop the top label and forward 279 the packet. 281 The values in the Label subobjects in the RRO are of interest to the 282 ingress LERs in order to construct the stack of labels to impose on 283 the packets. 285 If, in this example, there was another RSVP-TE tunnel T3 from F to I 286 on path F-B-C-D-E-I, then this would also share the TE links B-C, 287 C-D, and D-E and additionally traverse link E-I. The label stack 288 used by F would be {150, 200, 250, 850}. Hence, regardless of the 289 ingress and egress LERs from where the LSPs start and end, they will 290 share LSR labels at shared hops in the shared MPLS forwarding plane. 292 There MAY be local operator policy at the ingress LER that influences 293 the maximum depth of the label stack that can be pushed for a Segment 294 Routed RSVP-TE tunnel. Prior to signaling the LSP, the ingress LER 295 may determine that it would be unable to push a label stack 296 containing one label for each hop along the path. In some scenarios, 297 the ingress LER may not have sufficient information to make that 298 determination. In these cases the LER SHOULD adopt the techniques 299 described in Section 5. 301 5. Delegating Label Stack Imposition 303 One or more transit LSRs can assist the ingress LER by imposing part 304 of the label stack required for the path. Consider the example in 305 Figure 2 with an RSVP-TE tunnel from A to L on path 306 A-B-C-D-E-F-G-H-I-J-K-L. In this case, the LSP is too long for LER A 307 to impose the full label stack, so it uses the assistance of 308 delegation hops LSR D and LSR I to impose parts of the label stack. 310 Each delegation hop allocates a delegation label to represent a set 311 of labels that will be pushed at this hop. When a packet arrives at 312 a delegation hop LSR with a delegation label, the LSR pops the label 313 and pushes a set of labels before forwarding the packet. 315 1250d 316 +---+100p +---+150p +---+200p +---+250p +---+300p +---+ 317 | A |------| B |------| C |------| D |------| E |------| F | 318 +---+ +---+ +---+ +---+ +---+ +---+ 319 |350p 320 | 321 1500d | 322 +---+ 600p+---+ 550p+---+ 500p+---+ 450p+---+ 400p+---+ 323 | L |------| K |------| J |------| I |------| H |------+ G + 324 +---+ +---+ +---+ +---+ +---+ +---+ 326 Notation :