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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (November 3, 2008) is 4946 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- -- Obsolete informational reference (is this intentional?): RFC 3137 (Obsoleted by RFC 6987) -- Obsolete informational reference (is this intentional?): RFC 3784 (Obsoleted by RFC 5305) Summary: 1 error (**), 0 flaws (~~), 1 warning (==), 9 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group M. Jork 3 Internet Draft NextPoint Networks 4 Category: Informational Alia Atlas 5 Expires: May 2008 British Telecom 6 L. Fang 7 Cisco Systems, Inc. 9 November 3, 2008 11 LDP IGP Synchronization 12 draft-ietf-mpls-ldp-igp-sync-03.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 46 traffic can occur in situations where the IGP is operational on a 47 link but LDP is not operational on that link. While the link could 48 still be used for IP forwarding, it is not useful for MPLS 49 LDP IGP Synchronization November 2008 51 forwarding, for example, MPLS VPN; BGP route free core; or IP 52 address carried in the packet is out of the RFC1918 space. This 53 document describes a mechanism to avoid traffic loss due to this 54 condition without introducing any protocol changes. 56 Table of Contents 58 1. Introduction..................................................2 59 2. Proposed Solution.............................................3 60 3. Applicability.................................................4 61 4. Interaction With TE Tunnels...................................5 62 5. Security Considerations.......................................5 63 6. IANA Considerations...........................................5 64 7. Normative References..........................................6 65 8. Informational References......................................6 66 9. Author's Addresses............................................6 67 10. Acknowledgements.............................................8 69 Conventions used in this document 71 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 72 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in 73 this document are to be interpreted as described in RFC2119 [RFC 74 2119]. 76 1. Introduction 78 LDP [RFC5036] establishes MPLS LSPs along the shortest path to a 79 destination as determined by IP forwarding. In a common network 80 design, LDP is used to provide label switched paths throughout the 81 complete network domain covered by an IGP such as OSPF [RFC2328] or 82 IS-IS [ISO.10589.1992], i.e. all links in the domain MAY have IGP 83 as well as LDP adjacencies. 85 A variety of services a network provider may want to deploy over an 86 LDP enabled network depend on the availability of edge to edge 87 label switched paths. In a L2 or L3 VPN scenario for example, a 88 given PE router relies on the availability of a complete MPLS 89 forwarding path to the other PE routers for the VPNs it serves. 90 This means that along the IP shortest path from one PE router to 91 the other, all the links need to have operational LDP sessions and 92 the necessary label binding must have been exchanged over those 93 sessions. If only one link along the IP shortest path is not 94 covered by an LDP session, a blackhole exists and services 95 depending on MPLS forwarding will fail. This might be a transient 96 LDP IGP Synchronization November 2008 98 or a persistent error condition. Some of the reasons for it could 99 be 101 - A configuration error 103 - An implementation bug 105 - The link has just come up and has an IGP adjacency but LDP has 106 either not yet established an adjacency or session or 107 distributed all the label bindings. 109 LDP protocol has currently no way to correct the issue, LDP is not 110 a routing protocol; it can not re-direct traffic to an alternate 111 IGP path. 113 2. Proposed Solution 115 The problem described above exists because LDP is tied to IP 116 forwarding decisions but no coupling between the IGP and LDP 117 operational state on a given link exists. If IGP is operational on 118 a link but LDP is not, a potential network problem exists. So the 119 solution described by this document is to discourage a link from 120 being used for IP forwarding as long as LDP is not fully 121 operational. 123 This has some similarity to the mechanism specified in [RFC3137] 124 which allows an OSPF router to advertise that it should not be used 125 as a transit router. One difference is that [RFC3137] raises the 126 link costs on all (stub) router links, while the mechanism 127 described in here applies on a per-link basis. 129 In detail: when LDP is not "fully operational" (see below) on a 130 given link, the IGP will advertise the link with maximum cost to 131 avoid any transit traffic over it if possible. In the case of 132 OSPF, this cost is LSInfinity (16-bit value 0xFFFF) as proposed in 133 [RFC3137]. In the case of ISIS, the max metric value is 2^24-2 134 (0xFFFFFE). Indeed, if a link is configured with 2^24-1 (the 135 maximum link metric per [RFC3784]) then this link is not advertised 136 in the topology. It is important to keep the link in the topology 137 to allow for IP traffic to use the link as a last resort in case of 138 massive failure. 140 LDP is considered fully operational on a link when an LDP hello 141 adjacency exists on it, a suitable associated LDP session (matching 142 the LDP Identifier of the hello adjacency) is established to the 143 peer at the other end of the link and all label bindings have been 144 exchanged over the session. At the present time, the latter 145 LDP IGP Synchronization November 2008 147 condition can not generally be verified by a router and some 148 estimated may have to be used. A simple implementation strategy is 149 to use a configurable hold down timer to allow LDP session 150 establishment before declaring LDP fully operational. The default 151 timer is not defined in this document due to the concerns of the 152 large variations of the LIB table size and the equipment 153 capabilities. In addition, this is a current work in progress on LDP 154 End-of-LIB as specified in [LDP End-of-LIB], it enables the LDP 155 speaker to signal the completion of its initial advertisement 156 following session establish. When LDP End-of-LIB is implemented, the 157 configurable hold down timer is no longer needed. The neighbor LDP 158 session is considered fully operational when the End-of-LIB 159 notification message is received. 161 This is typically sufficient to deal with the link when it is being 162 brought up. LDP protocol extensions to indicate the complete 163 transmission of all currently available label bindings after a 164 session has come up are conceivable but not addressed in this 165 document. 167 The mechanism described in this document does not entail any 168 protocol changes and is a local implementation issue. 170 The problem space and solution specified in this document have also 171 been discussed in an IEEE Communications Magazine paper [LDP-Fail]. 173 3. Applicability 175 In general, the proposed procedure is applicable in networks where 176 the availability of LDP signaled MPLS LSPs and avoidance of 177 blackholes for MPLS traffic is more important than always choosing 178 an optimal path for IP forwarded traffic. Note however that non- 179 optimal IP forwarding only occurs for a short time after a link 180 comes up or when there is a genuine problem on a link. In the 181 latter case an implementation should issue network management alerts 182 to report the error condition and enable the operator to address it. 184 Example network scenarios that benefit from the mechanism described 185 here are MPLS VPNs and BGP-free core network designs where traffic 186 can only be forwarded through the core when LDP forwarding state is 187 available throughout. 189 The usefulness of this mechanism also depends on the availability 190 of alternate paths with sufficient bandwidth in the network should 191 one link be assigned to the maximum cost due to unavailability of 192 LDP service over it. 194 LDP IGP Synchronization November 2008 196 On broadcast links with more than one IGP/LDP peer, the cost-out 197 procedure can only be applied to the link as a whole and not an 198 individual peer. So a policy decision has to be made whether the 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 240 LDP IGP Synchronization November 2008 242 This document has no actions for IANA. 244 7. Normative References 246 [RFC5036] Andersson, L., Doolan, P., Feldman, N., Fredette, A., 247 and B. Thomas, "LDP Specification", RFC 5036, October 2007. 249 [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. 251 8. Informational References 253 [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate 254 Requirement Levels", BCP 14, RFC 2119, March 1997 256 [RFC3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D. 257 McPherson, "OSPF Stub Router Advertisement", RFC 3137, June 2001. 259 [RFC3784] Smit, H., Li, T., Intermediate System to Intermediate 260 System (IS-IS) Extension for Traffic Engineering, June 2004. 262 [ISO.10589.1992]International Organization for 263 Standardization,"Intermediate system to intermediate system intra- 264 domain-routing routine information exchange protocol for use in 265 conjunction with the protocol for providing the connectionless-mode 266 Network Service (ISO 8473)", ISO Standard 10589, 1992. 268 [LDP-Fail] Fang, L., Atlas, A., Chiussi, F., Kompella, K., and 269 Swallow, G., "LDP Failure Detection and Recovery", IEEE 270 Communications Magazine, Vol.42, No.10, October 2004. 272 [LDP End-of-LIB] Asati, R., LDP End-of-LIB, draft-ietf-mpls-ldp- 273 end-of-lib-01.txt, September 2008. 275 9. Author's Addresses 277 Markus Jork 278 NextPoint Networks 279 3 Fedral St. 280 Billerica, MA 01821 281 USA 282 Email: mjork@nextpointnetworks.com 284 Alia Atlas 285 British Telecom 286 Email: alia.atlas@bt.com 287 LDP IGP Synchronization November 2008 289 Luyuan Fang 290 Cisco Systems, Inc. 291 300 Beaver Brook Road 292 Boxborough, MA 01719 293 USA 294 Email: lufang@cisco.com 296 Full Copyright Statement 298 Copyright (C) The IETF Trust (2008). 300 This document is subject to the rights, licenses and restrictions 301 contained in BCP 78, and except as set forth therein, the authors 302 retain all their rights. 304 This document and the information contained herein are provided on 305 an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE 306 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE 307 IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL 308 WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY 309 WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE 310 ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS 311 FOR A PARTICULAR PURPOSE. 313 Intellectual Property 315 The IETF takes no position regarding the validity or scope of any 316 Intellectual Property Rights or other rights that might be claimed 317 to pertain to the implementation or use of the technology described 318 in this document or the extent to which any license under such 319 rights might or might not be available; nor does it represent that 320 it has made any independent effort to identify any such rights. 321 Information on the procedures with respect to rights in RFC 322 documents can be found in BCP 78 and BCP 79. 324 Copies of IPR disclosures made to the IETF Secretariat and any 325 assurances of licenses to be made available, or the result of an 326 attempt made to obtain a general license or permission for the use 327 of such proprietary rights by implementers or users of this 328 specification can be obtained from the IETF on-line IPR repository 329 at http://www.ietf.org/ipr. 331 The IETF invites any interested party to bring to its attention any 332 copyrights, patents or patent applications, or other proprietary 333 rights that may cover technology that may be required to implement 334 this standard. Please address the information to the IETF at ietf- 335 ipr@ietf.org. 337 LDP IGP Synchronization November 2008 339 10. Acknowledgements 341 Funding for the RFC Editor function is provided by the IETF 342 Administrative Support Activity (IASA). 344 The authors would like to thank Loa Andersson for his review and 345 comments, thank Bruno Decraene for his in depth discussion, 346 comments and helpful suggestions, and thank Dave Ward for his AD 347 review comments and suggestions.