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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group David J. Smith 3 Internet Draft John Mullooly 4 Intended status: Proposed Standard Cisco Systems, Inc. 5 Expiration Date: July 2010 6 William Jaeger 7 AT&T 9 Tom Scholl 10 AT&T Labs 12 January 6, 2010 14 Requirements for Label Edge Router Forwarding of IPv4 Option Packets 16 draft-ietf-mpls-ip-options-03.txt 18 Status of this Memo 20 This Internet-Draft is submitted to IETF in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF), its areas, and its working groups. Note that 25 other groups may also distribute working documents as Internet- 26 Drafts. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 The list of current Internet-Drafts can be accessed at 34 http://www.ietf.org/ietf/1id-abstracts.txt. 36 The list of Internet-Draft Shadow Directories can be accessed at 37 http://www.ietf.org/shadow.html. 39 This Internet-Draft will expire on July 1, 2010. 41 Copyright Notice 43 Copyright (c) 2010 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the BSD License. 56 Abstract 58 This document imposes a new requirement on Label Edge Routers (LER) 59 specifying that when determining whether to MPLS encapsulate an IP 60 packet, the determination is made independent of any IP options that 61 may be carried in the IP packet header. Lack of a formal standard 62 has resulted in different LER forwarding behaviors for IP packets 63 with header options despite being associated with a prefix-based 64 Forwarding Equivalence Class (FEC). IP option packets that belong to 65 a prefix-based FEC but fail to be MPLS encapsulated simply due to 66 their header options present a security risk against the MPLS 67 infrastructure. Further, LERs that are unable to MPLS encapsulate IP 68 packets with header options cannot operate in certain MPLS 69 environments. This new requirement will reduce the risk of IP 70 options-based security attacks against LSRs as well as assist LER 71 operation across MPLS networks which minimize the IP routing 72 information carried by LSRs. 74 Table of Contents 76 1 Specification of Requirements ...................... 3 77 2 Motivation ......................................... 3 78 3 Introduction ....................................... 3 79 4 Ingress Label Edge Router Requirement .............. 4 80 5 Security Considerations ............................ 5 81 5.1 IP Option Packets that Bypass MPLS Encapsulation ... 5 82 5.2 Router Alert Label Imposition ...................... 7 83 6 IANA Considerations ................................ 7 84 7 Conclusion ......................................... 8 85 8 Acknowledgements ................................... 8 86 9 Normative References ............................... 8 87 10 Informational References ........................... 8 88 11 Authors' Addresses ................................. 9 90 1. Specification of Requirements 92 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 93 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 94 document are to be interpreted as described in [RFC2119]. 96 2. Motivation 98 This document is motivated by the need to formalize MPLS 99 encapsulation processing of IPv4 packets with header options in order 100 to mitigate the existing risks of IP options-based security attacks 101 against MPLS infrastructures. We believe that this document adds 102 details that have not been fully addressed in [RFC3031] and 103 [RFC3032], and that the methods presented in this document update 104 [RFC3031] as well as complement [RFC3270], [RFC3443] and [RFC4950]. 106 3. Introduction 108 The IP packet header provides for various IP options as originally 109 specified in [RFC791]. IP header options are used to enable control 110 functions within the IP data forwarding plane that are required in 111 some specific situations but not necessary for most common IP 112 communications. Typical IP header options include provisions for 113 timestamps, security, and special routing. Example IP header options 114 & applications include but are not limited to: 115 o Strict & Loose Source Route Options: Used to IP route the IP 116 packet based on information supplied by the source. 117 o Record Route Option: Used to trace the route an IP packet takes. 118 o Router Alert Option: Indicates to downstream IP routers to 119 examine these IP packets more closely. 120 The list of current IP header options can be accessed at [IANA]. 122 IP packets may or may not use IP header options (they are optional) 123 but IP header option handling mechanisms must be implemented by all 124 IP protocol stacks (hosts and routers). Each IP header option has 125 distinct header fields and lengths. IP options extend the IP packet 126 header length beyond the minimum of 20 octets. As a result, IP 127 packets received with header options are typically handled as 128 exceptions and in a less efficient manner due to their variable 129 length and complex processing requirements. Many router 130 implementations, for example, punt such IP option packets from the 131 hardware forwarding (fast) path into the software forwarding (slow) 132 path. 134 Multi-Protocol Label Switching (MPLS) [RFC3031] is a technology in 135 which packets associated with a prefix-based Forwarding Equivalence 136 Class (FEC) are encapsulated with a label stack and then switched 137 along a label switched path (LSP) by a sequence of label switch 138 routers (LSRs). These intermediate LSRs do not generally perform any 139 processing of the IP header as packets are forwarded. (There are some 140 exceptions to this rule, such as ICMP processing and LSP ping, as 141 described in [RFC3032] and [RFC4379], respectively.) Many MPLS 142 deployments rely on LSRs to provide layer 3 transparency much like 143 ATM switches are transparent at layer 2. Such deployments often 144 minimize the IP routing information (e.g., no BGP transit routes) 145 carried by LSRs since not necessary for MPLS forwarding of transit 146 packets. 148 Even though MPLS encapsulation seems to offer a viable solution to 149 provide layer 3 transparency, there is currently no formal standard 150 for MPLS encapsulation of IP packets with header options that belong 151 to a prefix-based FEC. Lack of a formal standard has resulted in 152 inconsistent forwarding behaviors by ingress LERs. When MPLS 153 encapsulated by an ingress LER, for example, the IP header including 154 option fields of transit packets are transparent to downstream LSRs 155 given MPLS forwarding. Conversely, when IP routed by an ingress LER, 156 downstream LSRs must apply IP forwarding rules which may expose the 157 LSRs to IP security attacks and for which they (the LSRs) may have 158 insufficient IP routing information. 160 IP option packets that belong to a prefix-based FEC but fail to be 161 MPLS encapsulated simply due to their header options present a 162 security risk against the MPLS infrastructure. Further, LERs that 163 are unable to MPLS encapsulate IP packets with header options cannot 164 operate as an LER in certain MPLS environments. This new requirement 165 will reduce the risk of IP options-based security attacks against 166 LSRs as well as assist LER operation across MPLS networks which 167 minimize the IP routing information (e.g., no BGP transit routes) 168 carried by LSRs. 170 4. Ingress Label Edge Router Requirement 172 An ingress LER MUST implement the following policy: 174 o When determining whether to push an MPLS label stack onto an IP 175 packet, the determination is made without considering any IP 176 options that may be carried in the IP packet header. Further, 177 the label values that appear in the label stack are determined 178 without considering any such IP options. 180 This policy MAY be configurable on an ingress LER, however, it SHOULD 181 be enabled by default. When processing of signaling messages or data 182 packets with more specific forwarding rules is enabled, this policy 183 SHOULD NOT alter the specific processing rules. This applies to, but 184 is not limited to, RSVP as per [RFC2205] as well as other FEC 185 elements defined by future specifications. Further, how an ingress 186 LER processes the IP header options of packets before MPLS 187 encapsulation is out of scope since the IP packets are received as 188 they enter the MPLS domain. 190 Implementation of the above policy prevents IP packets that belong to 191 a prefix-based FEC from bypassing MPLS encapsulation due to header 192 options. The policy also prevents specific option types such as 193 Router Alert (option value 148), for example, from forcing MPLS 194 imposition of the MPLS Router Alert Label (label value 1) at ingress 195 LERs. Without this policy, the MPLS infrastructure is exposed to 196 security attacks using legitimate IP packets crafted with header 197 options. Further, LERs that are unable to MPLS encapsulate IP 198 packets with header options cannot operate as an LER in certain MPLS 199 environments as described above in Section 3. 201 5. Security Considerations 203 There are two potential categories of attacks using crafted IP option 204 packets that threaten existing MPLS infrastructures. Both are 205 described below. To mitigate the risk of these specific attacks, the 206 ingress LER policy specified above is required. 208 5.1. IP Option Packets that Bypass MPLS Encapsulation 210 Given that a router's exception handling process (i.e., CPU, 211 processor line-card bandwidth, etc.) used for IP header option 212 processing is often shared with IP control and management protocol 213 router resources, a flood of IP packets with header options may 214 adversely affect a router's control and management protocols, 215 thereby, triggering a denial-of-service (DoS) condition. Note, IP 216 packets with header options may be valid transit IP packets with 217 legitimate sources and destinations. Hence, a DoS-like condition may 218 be triggered on downstream transit IP routers that lack protection 219 mechanisms even in the case of legitimate IP option packets. 221 IP option packets that belong to a prefix-based FEC yet bypass MPLS 222 encapsulation at a ingress LER may be inadvertently IP routed 223 downstream across the MPLS core network (not label switched). This 224 allows an external attacker the opportunity to maliciously craft 225 seemingly legitimate IP packets with specific IP header options in 226 order to intentionally bypass MPLS encapsulation at the MPLS edge 227 (i.e., ingress LER) and trigger a DoS condition on downstream LSRs. 228 Some of the specific types of IP option-based security attacks that 229 may be leveraged against MPLS networks include: 230 o Crafted IP option packets that belong to a prefix-based FEC yet 231 bypass MPLS encapsulation at a ingress LER may allow an attacker 232 to DoS downstream LSRs by saturating their software forwarding 233 paths. By targeting a LSR's exception path, control and 234 management protocols may be adversely affected and, thereby, a 235 LSR's availability. This assumes, of course, that downstream 236 LSRs lack protection mechanisms. 237 o Crafted IP option packets that belong to a prefix-based FEC yet 238 bypass MPLS encapsulation at a ingress LER may allow for IP TTL 239 expiry-based DoS attacks against downstream LSRs. MPLS enables 240 IP core hiding whereby transit IP traffic flows see the MPLS 241 network as a single router hop [RFC3443]. However, MPLS core 242 hiding does not apply to packets that bypass MPLS encapsulation 243 and, therefore, IP option packets may be crafted to expire on 244 downstream LSRs which may trigger a DoS condition. Bypassing 245 MPLS core hiding is an additional security consideration since it 246 exposes the network topology. 247 o Crafted IP option packets that belong to a prefix-based FEC yet 248 bypass MPLS encapsulation at a ingress LER may allow for DoS 249 attacks against downstream LSRs that do not carry the IP routing 250 information required to forward transit IP traffic. Lack of such 251 IP routing information may prevent legitimate IP option packets 252 from transiting the MPLS network and, further, may trigger 253 generation of ICMP destination unreachable messages which could 254 lead to a DoS condition. This assumes, of course, that 255 downstream LSRs lack protection mechanisms and do not carry the 256 IP routing information required to forward transit traffic. 257 o Crafted IP option packets that belong to a prefix-based FEC yet 258 bypass MPLS encapsulation at a ingress LER may allow an attacker 259 to bypass LSP Diff-Serv tunnels [RFC3270] and any associated MPLS 260 CoS field [RFC5462] marking policies at ingress LERs and, 261 thereby, adversely affect (i.e., DoS) high-priority traffic 262 classes within the MPLS core. Further, this could also lead to 263 theft of high-priority services by unauthorized parties. This 264 assumes, of course, that the [RFC3270] Pipe model is deployed 265 within the MPLS core. 266 o Crafted IP strict and loose source route option packets that 267 belong to a prefix-based FEC yet bypass MPLS encapsulation at a 268 ingress LER may allow an attacker to specify explicit IP 269 forwarding path(s) across an MPLS network and, thereby, target 270 specific LSRs with any of the DoS attacks outlined above. This 271 assumes, of course, that the MPLS network is enabled to process 272 IP strict and loose source route options. 273 o Crafted RSVP packets that belong to a prefix-based FEC yet bypass 274 MPLS encapsulation at a ingress LER may allow an attacker to 275 build RSVP soft-states [RFC2205] on downstream LSRs which could 276 lead to theft of service by unauthorized parties or to a DoS 277 condition caused by locking up LSR resources. This assumes, of 278 course, that the MPLS network is enabled to process RSVP packets. 280 The security attacks outlined above specifically apply to IP option 281 packets that belong to a prefix-based FEC yet bypass ingress LER 282 label stack imposition. Additionally, these attacks only apply to IP 283 option packets forwarded using the global routing table (i.e., IPv4 284 address family) of a ingress LER. IP option packets associated with 285 a BGP/MPLS IP VPN service are always MPLS encapsulated by the ingress 286 LER per [RFC4364] given that packet forwarding uses a Virtual 287 Forwarding/Routing (VRF) instance. Therefore, BGP/MPLS IP VPN 288 services are not subject to the threats outlined above [RFC4381]. 289 Further, IPv6 [RFC2460] makes use of extension headers not header 290 options and is therefore outside the scope of this document. A 291 separate security threat that does apply to both BGP/MPLS IP VPNs and 292 the IPv4 address family makes use of the Router Alert Label. This is 293 described directly below. 295 5.2. Router Alert Label Imposition 297 [RFC3032] defines a "Router Alert Label" (label value of 1) which is 298 analogous to the "Router Alert" IP header option (option value of 299 148). The MPLS Router Alert Label (when exposed and processed only) 300 indicates to downstream LSRs to examine these MPLS packets more 301 closely. MPLS packets with the MPLS Router Alert Label are also 302 handled as an exception by LSRs and, again, in a less efficient 303 manner. At the time of this writing, the only legitimate use of the 304 Router Alert Label is for LSP ping/trace [RFC4379]. Since there is 305 also no formal standard for Router Alert Label imposition at ingress 306 LERs: 307 o Crafted IP packets with specific IP header options (e.g., Router 308 Alert) and that belong to a prefix-based FEC may allow an 309 attacker to force MPLS imposition of the Router Alert Label at 310 ingress LERs and, thereby, trigger a DoS condition on downstream 311 LSRs. This assumes, of course, that downstream LSRs lack 312 protection mechanisms. 314 6. IANA Considerations 316 This document has no actions for IANA. 318 7. Conclusion 320 This document imposes a new requirement on ingress LERs in order to 321 reduce the risk of IP options-based security attacks against LSRs as 322 well as to assist LER operation across MPLS networks which minimize 323 the IP routing information carried by LSRs. 325 8. Acknowledgements 327 The authors would like to thank Adrian Cepleanu, Bruce Davie, Rick 328 Huber, Chris Metz, Pradosh Mohapatra, Ashok Narayanan, Carlos 329 Pignataro, Eric Rosen, Mark Szczesniak and Yung Yu for their valuable 330 comments and suggestions. 332 9. Normative References 334 [RFC791] Postel, J., "Internet Protocol Specification," RFC791, 335 September 1981. 337 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 338 Requirement Levels," March 1997. 340 [RFC3031] Rosen, E., Viswanathan, A., and Callon, R., "MPLS Label 341 Switching Architecture," RFC3031, January 2001. 343 [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., 344 Farinacci, D., Li, T., and Conta, A., "MPLS Label Stack Encoding," 345 RFC3032, January 2001. 347 10. Informational References 349 [RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S., Jamin, S., 350 "Resource ReSerVation Protocol -- Version 1 Functional 351 Specification," RFC2205, September 1997. 353 [RFC2460] Deering, S., Hinden, R. "Internet Protocol, Version 6 354 Specification," RFC2460, December 1998. 356 [RFC3209] Awduche, D., L. Berger, D. Gan, T. Li, V. Srinivasan, G. 357 Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels," RFC3209, 358 December 2001. 360 [RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen, 361 P., Krishnan, R., Cheval, P., Heinanen, J., "Multi-Protocol Label 362 Switching Support of Differentiated Services," RFC3270, May 2002. 364 [RFC3443] Agarwal, P., Akyol, B., "Time To Live (TTL) Processing in 365 Multi-Protocol Label Switching (MPLS) Networks," RFC3443, January 366 2003. 368 [RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private 369 Networks (VPNs)," RFC4364, February 2006. 371 [RFC4379] "Kompella, K., Swallow, G., "Detecting Multi-Protocol Label 372 Switched (MPLS) Data Plane Failures," RFC4379, February 2006. 374 [RFC4381] Behringer, M., "Analysis of the Security of BGP/MPLS IP 375 Virtual Private Networks (VPNs)," RFC4381, February 2006. 377 [RFC4950] Bonica, R., Gan, D., Tappan, D., and Pignataro, C., "ICMP 378 Extensions for Multiprotocol Label Switching," RFC4950, August 2007. 380 [IANA] "IP Option Numbers," IANA, February 15, 2007, 381 . 383 [RFC5462] Andersson, L., and R. Asati, "Multiprotocol Label Switching 384 (MPLS) Label Stack Entry: EXP Field Renamed to Traffic Class Field," 385 RFC5462, February 2009. 387 11. Authors' Addresses 389 William Jaeger 390 AT&T 391 200 S. Laurel Avenue 392 Middletown, NJ 07748 393 Email: wjaeger@att.com 395 John Mullooly 396 Cisco Systems, Inc. 397 111 Wood Avenue 398 Iselin, NJ 08830 399 E-mail: jmullool@cisco.com 401 Tom Scholl 402 AT&T Labs 403 200 S. Laurel Avenue 404 Middletown, NJ 07748 405 Email: ts3127@att.com 406 David J. Smith 407 Cisco Systems, Inc. 408 111 Wood Avenue 409 Iselin, NJ 08830 410 E-mail: djsmith@cisco.com