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Jork 3 Internet Draft NextPoint Networks 4 Category: Informational Alia Atlas 5 Expires: August 2008 British Telecom 6 L. Fang 7 Cisco Systems, Inc. 9 February 2008 11 LDP IGP Synchronization 12 draft-ietf-mpls-ldp-igp-sync-01.txt 14 Status of this Memo 16 By submitting this Internet-Draft, each author represents that any 17 applicable patent or other IPR claims of which he or she is aware 18 have been or will be disclosed, and any of which he or she becomes 19 aware will be disclosed, in accordance with Section 6 of BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF), its areas, and its working groups. Note that 23 other groups may also distribute working documents as Internet- 24 Drafts. 26 Internet-Drafts are draft documents valid for a maximum of six 27 months and may be updated, replaced, or obsoleted by other 28 documents at any time. It is inappropriate to use Internet-Drafts 29 as reference material or to cite them other than as "work in 30 progress." 32 The list of current Internet-Drafts can be accessed at 33 http://www.ietf.org/ietf/1id-abstracts.txt 34 The list of Internet-Draft Shadow Directories can be accessed at 35 http://www.ietf.org/shadow.html. 37 Copyright Notice 38 Copyright (C) The IETF Trust (2008). 40 Abstract 42 In certain networks there is a dependency on edge-to-edge LSPs setup 43 by LDP, e.g. networks that are used for MPLS VPN applications. For 44 such applications it is not possible to rely on IP forwarding if the 45 MPLS LSP is not operating appropriately. Blackholing of labeled 47 M. Jork, A. Atlas, and L. Fang 48 1 49 LDP IGP Synchronization February 2008 51 traffic can occur in situations where the IGP is operational on a 52 link but LDP is not operational on that link. While the link could 53 still be used for IP forwarding, it is not useful for traffic with 54 packets carrying a label stack of more than one label or when the IP 55 address carried in the packet is out of the RFC1918 space. This 56 document describes a mechanism to avoid traffic loss due to this 57 condition without introducing any protocol changes. 59 Table of Contents 61 1. Introduction..................................................2 62 2. Proposed Solution.............................................3 63 3. Applicability.................................................4 64 4. Interaction With TE Tunnels...................................5 65 5. Security Considerations.......................................5 66 6. IANA Considerations...........................................5 67 7. Normative References..........................................6 68 8. Informational References......................................6 69 9. Author's Addresses............................................6 70 10. Acknowledgements............................................8 72 Conventions used in this document 74 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 75 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in 76 this document are to be interpreted as described in RFC2119 [RFC 77 2119]. 79 1. Introduction 81 LDP [RFC5036] establishes MPLS LSPs along the shortest path to a 82 destination as determined by IP forwarding. In a common network 83 design, LDP is used to provide label switched paths throughout the 84 complete network domain covered by an IGP such as OSPF [RFC2328] or 85 IS-IS [ISO.10589.1992], i.e. all links in the domain have IGP as 86 well as LDP adjacencies. 88 A variety of services a network provider may want to deploy over an 89 LDP enabled network depend on the availability of edge to edge 90 label switched paths. In a L2 or L3 VPN scenario for example, a 91 given PE router relies on the availability of a complete MPLS 92 forwarding path to the other PE routers for the VPNs it serves. 94 M. Jork, Alia Atlas, and L. Fang 2 95 LDP IGP Synchronization February 2008 97 This means that along the IP shortest path from one PE router to 98 the other, all the links need to have operational LDP sessions and 99 the necessary label binding must have been exchanged over those 100 sessions. If only one link along the IP shortest path is not 101 covered by an LDP session, a blackhole exists and services 102 depending on MPLS forwarding will fail. This might be a transient 103 or a persistent error condition. Some of the reasons for it could 104 be 106 - A configuration error 108 - An implementation bug 110 - The link has just come up and has an IGP adjacency but LDP has 111 either not yet established an adjacency or session or 112 distributed all the label bindings. 114 The LDP protocol itself has currently no means to indicate to a 115 service depending on it whether there is an uninterrupted label 116 switched path available to the desired destination or not. 118 2. Proposed Solution 120 The problem described above exists because LDP is tied to IP 121 forwarding decisions but no coupling between the IGP and LDP 122 operational state on a given link exists. If IGP is operational on 123 a link but LDP is not, a potential network problem exists. So the 124 solution described by this document is to discourage a link from 125 being used for IP forwarding as long as LDP is not fully 126 operational. 128 This has some similarity to the mechanism specified in [RFC3137] 129 which allows an OSPF router to advertise that it should not be used 130 as a transit router. One difference is that [RFC3137] raises the 131 link costs on all (stub) router links, while the mechanism 132 described in here applies on a per-link basis. 134 In detail: when LDP is not "fully operational" (see below) on a 135 given link, the IGP will advertise the link with maximum cost to 136 avoid any transit traffic over it if possible. In the case of OSPF 137 this cost is LSInfinity (16-bit value 0xFFFF) as proposed in 138 [RFC3137]. Note that the link is not just simply removed from the 139 topology because LDP depends on the IP reachability to establish 140 its adjacency and session. Also, if there is no other link in the 141 network to reach a particular destination, no additional harm is 142 done by making this link available for IP forwarding at maximum 143 cost. 145 M. Jork, Alia Atlas, and L. Fang 3 146 LDP IGP Synchronization February 2008 148 LDP is considered fully operational on a link when an LDP hello 149 adjacency exists on it, a suitable associated LDP session (matching 150 the LDP Identifier of the hello adjacency) is established to the 151 peer at the other end of the link and all label bindings have been 152 exchanged over the session. The latter condition can not generally 153 be verified by a router and some heuristics may have to be used. A 154 simple implementation strategy is to wait some time after LDP 155 session establishment before declaring LDP fully operational in 156 order to allow for the exchange of label bindings. This is 157 typically sufficient to deal with the link when it is being brought 158 up. LDP protocol extensions to indicate the complete transmission of 159 all currently available label bindings after a session has come up 160 are conceivable but not addressed in this document. 162 The mechanism described in this document does not entail any 163 protocol changes and is a local implementation issue. However, it 164 is recommended that both sides of a link implement this mechanism 165 to be effective and to avoid asymmetric link costs which could 166 cause problems with IP multicast forwarding. 168 The problem space and solution specified in this document have also 169 been discussed in an IEEE Communications Magazine paper [LDP-Fail]. 171 3. Applicability 173 In general, the proposed procedure is applicable in networks where 174 the availability of LDP signaled MPLS LSPs and avoidance of 175 blackholes for MPLS traffic is more important than always choosing 176 an optimal path for IP forwarded traffic. Note however that non- 177 optimal IP forwarding only occurs for a short time after a link 178 comes up or when there is a genuine problem on a link. In the 179 latter case an implementation should issue network management alerts 180 to report the error condition and enable the operator to address it. 182 Example network scenarios that benefit from the mechanism described 183 here are MPLS VPNs and BGP-free core network designs where traffic 184 can only be forwarded through the core when LDP forwarding state is 185 available throughout. 187 The usefulness of this mechanism also depends on the availability 188 of alternate paths with sufficient bandwidth in the network should 189 one link be assigned to the maximum cost due to unavailability of 190 LDP service over it. 192 On broadcast links with more than one IGP/LDP peer, the cost-out 193 procedure can only be applied to the link as a whole and not an 194 individual peer. So a policy decision has to be made whether the 196 M. Jork, Alia Atlas, and L. Fang 4 197 LDP IGP Synchronization February 2008 199 unavailability of LDP service to one peer should result in the 200 traffic being diverted away from all the peers on the link. 202 4. Interaction With TE Tunnels 204 In some networks, LDP is used in conjunction with RSVP-TE which sets 205 up traffic-engineered tunnels. The path computation for the TE 206 tunnels is based on the TE link cost which is flooded by the IGP in 207 addition to the regular IP link cost. The mechanism described in 208 this document should only be applied to the IP link cost to prevent 209 any unnecessary TE tunnel reroutes. 211 In order to establish LDP LSPs across a TE tunnel, a targeted LDP 212 session between the tunnel endpoints needs to exist. This presents 213 a problem very similar to the case of a regular LDP session over a 214 link (the case discussed so far): when the TE tunnel is used for IP 215 forwarding, the targeted LDP session needs to be operational to 216 avoid LDP connectivity problems. Again, raising the IP cost of the 217 tunnel while there is no operational LDP session will solve the 218 problem. When there is no IGP adjacency over the tunnel and the 219 tunnel is not advertised as link into the IGP, this becomes a local 220 issue of the tunnel headend router. 222 5. Security Considerations 224 A DoS attack brings down LDP service on a link or prevents it from 225 becoming operational on a link could be one of the possibilities 226 that causes LDP related traffic blackholing. This document does not 227 address how to prevent LDP session failure. The mechanism described 228 here is to prevent the link to be used when LDP is not operational 229 while IGP is. Assigning the IGP cost to maximum on the link where 230 LDP is failed and IGP is not should not introduce new security 231 threats. The operation is internal in the router to allow LDP and 232 IGP to communicate and react. Making many LDP links unavailable, 233 however, is a security threat which can cause traffic being dropped 234 due to limited available network capacity. This may be trigged by 235 operational error or implementation error. They are considered as 236 general Security issues and should follow the current best security 237 practice. 239 6. IANA Considerations 241 This document has no actions for IANA. 243 M. Jork, Alia Atlas, and L. Fang 5 244 LDP IGP Synchronization February 2008 246 7. Normative References 248 [RFC5036] Andersson, L., Doolan, P., Feldman, N., Fredette, A., 249 and B. Thomas, "LDP Specification", RFC 5036, October 2007. 251 [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. 253 8. Informational References 255 [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate 256 Requirement Levels", BCP 14, RFC 2119, March 1997 258 [RFC3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D. 259 McPherson, "OSPF Stub Router Advertisement", RFC 3137, June 2001. 261 [ISO.10589.1992]International Organization for 262 Standardization,"Intermediate system to intermediate system intra- 263 domain-routing routine information exchange protocol for use in 264 conjunction with the protocol for providing the connectionless-mode 265 Network Service (ISO 8473)", ISO Standard 10589, 1992. 267 [LDP-Fail] Fang, L., Atlas, A., Chiussi, F., Kompella, K., and 268 Swallow, G., "LDP Failure Detection and Recovery", IEEE 269 Communications Magazine, Vol.42, No.10, October 2004. 271 9. Author's Addresses 273 Markus Jork 274 NextPoint Networks 275 3 Fedral St. 276 Billerica, MA 01821 277 USA 278 Email: mjork@nextpointnetworks.com 280 Alia Atlas 281 British Telecom 282 Email: alia.atlas@bt.com 284 Luyuan Fang 285 Cisco Systems, Inc. 286 300 Beaver Brook Road 287 Boxborough, MA 01719 288 USA 289 Email: lufang@cisco.com 291 Intellectual Property 293 M. Jork, Alia Atlas, and L. Fang 6 294 LDP IGP Synchronization February 2008 296 The IETF takes no position regarding the validity or scope of any 297 Intellectual Property Rights or other rights that might be claimed 298 to pertain to the implementation or use of the technology described 299 in this document or the extent to which any license under such 300 rights might or might not be available; nor does it represent that 301 it has made any independent effort to identify any such rights. 302 Information on the procedures with respect to rights in RFC 303 documents can be found in BCP 78 and BCP 79. 305 Copies of IPR disclosures made to the IETF Secretariat and any 306 assurances of licenses to be made available, or the result of an 307 attempt made to obtain a general license or permission for the use 308 of such proprietary rights by implementers or users of this 309 specification can be obtained from the IETF on-line IPR repository 310 at http://www.ietf.org/ipr. 312 The IETF invites any interested party to bring to its attention any 313 copyrights, patents or patent applications, or other proprietary 314 rights that may cover technology that may be required to implement 315 this standard. Please address the information to the IETF at ietf- 316 ipr@ietf.org. 318 Full Copyright Statement 320 Copyright (C) The IETF Trust (2008). 322 This document is subject to the rights, licenses and restrictions 323 contained in BCP 78, and except as set forth therein, the authors 324 retain all their rights. 326 This document and the information contained herein are provided on 327 an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE 328 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE 329 IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL 330 WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY 331 WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE 332 ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS 333 FOR A PARTICULAR PURPOSE. 335 Intellectual Property 337 The IETF takes no position regarding the validity or scope of any 338 Intellectual Property Rights or other rights that might be claimed 339 to pertain to the implementation or use of the technology described 340 in this document or the extent to which any license under such 341 rights might or might not be available; nor does it represent that 343 M. Jork, Alia Atlas, and L. Fang 7 344 LDP IGP Synchronization February 2008 346 it has made any independent effort to identify any such rights. 347 Information on the procedures with respect to rights in RFC 348 documents can be found in BCP 78 and BCP 79. 350 Copies of IPR disclosures made to the IETF Secretariat and any 351 assurances of licenses to be made available, or the result of an 352 attempt made to obtain a general license or permission for the use 353 of such proprietary rights by implementers or users of this 354 specification can be obtained from the IETF on-line IPR repository 355 at http://www.ietf.org/ipr. 357 The IETF invites any interested party to bring to its attention any 358 copyrights, patents or patent applications, or other proprietary 359 rights that may cover technology that may be required to implement 360 this standard. Please address the information to the IETF at ietf- 361 ipr@ietf.org. 363 10. Acknowledgements 365 Funding for the RFC Editor function is provided by the IETF 366 Administrative Support Activity (IASA). 368 The authors would like to thank Loa Andersson for his review and 369 comments. 371 M. Jork, Alia Atlas, and L. Fang 8