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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) == Unused Reference: 'RFC3032' is defined on line 239, but no explicit reference was found in the text == Unused Reference: 'RFC3270' is defined on line 243, but no explicit reference was found in the text Summary: 7 errors (**), 0 flaws (~~), 4 warnings (==), 8 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group Eric C. Rosen 2 Internet Draft Cisco Systems, Inc. 3 Expiration Date: August 2005 4 Updates RFC 3032 6 February 2005 8 Removing a Restriction on the use of MPLS Explicit NULL 10 draft-ietf-mpls-explicit-null-02.txt 12 Status of this Memo 14 By submitting this Internet-Draft, I certify that any applicable 15 patent or other IPR claims of which I am aware have been disclosed, 16 or will be disclosed, and any of which I become aware will be 17 disclosed, in accordance with RFC 3668. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that other 21 groups may also distribute working documents as Internet-Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six months 24 and may be updated, replaced, or obsoleted by other documents at any 25 time. It is inappropriate to use Internet-Drafts as reference 26 material or to cite them other than as "work in progress." 28 The list of current Internet-Drafts can be accessed at 29 http://www.ietf.org/ietf/1id-abstracts.txt. 31 The list of Internet-Draft Shadow Directories can be accessed at 32 http://www.ietf.org/shadow.html. 34 Abstract 36 The label stack encoding for MPLS (Multi-protocol Label Switching) 37 defines a reserved label value known as "IPv4 Explicit NULL" and a 38 reserved label value known as "IPv6 Explicit NULL". Previously, 39 these labels were only legal when they occurred at the bottom of the 40 MPLS label stack. This restriction is now removed, so that these 41 label values may legally occur anywhere in the stack. 43 This document updates RFC 3032. 45 Contents 47 1 Introduction ......................................... 2 48 2 Detail of Change ..................................... 2 49 3 Reasons for Change ................................... 4 50 4 Deployment Considerations ............................ 5 51 5 Security Considerations .............................. 6 52 6 Acknowledgments ...................................... 6 53 7 Normative References ................................. 6 54 8 Informative References ............................... 6 55 9 Author's Address ..................................... 6 56 10 Intellectual Property Statement ...................... 6 57 11 Full Copyright Statement ............................. 7 59 1. Introduction 61 RFC 3032 defines a reserved label value known as "IPv4 Explicit NULL" 62 and a reserved label value known as "IPv6 Explicit NULL". It states 63 that these label values are only legal at the bottom of the MPLS 64 label stack. However, no reason is given for this restriction. 66 It has turned out that in practice there are some situations in which 67 it is useful to send MPLS packets which have Explicit NULL occur 68 other than at that bottom of the label stack. While the intended 69 semantics are obvious enough, the fact that such packets are 70 gratuitously declared by RFC 3032 to be illegal has made it difficult 71 to handle these situations in an interoperable manner. 73 This document updates RFC 3032 by removing the unnecessary 74 restriction, so that the two aforementioned label values are legal 75 anywhere in the label stack. 77 2. Detail of Change 79 RFC 3032 states on page 4: 81 There are several reserved label values: 83 i. A value of 0 represents the "IPv4 Explicit NULL Label". This 84 label value is only legal at the bottom of the label stack. 85 It indicates that the label stack must be popped, and the 86 forwarding of the packet must then be based on the IPv4 87 header. 89 iii. A value of 2 represents the "IPv6 Explicit NULL Label". This 90 label value is only legal at the bottom of the label stack. 91 It indicates that the label stack must be popped, and the 92 forwarding of the packet must then be based on the IPv6 93 header. 95 Paragraph i is hereby changed to read: 97 i. A value of 0 represents the "IPv4 Explicit NULL Label". 99 An IPv4 Explicit NULL at the top of the label stack means that 100 the stack must be popped. 102 If the NULL was not the only label on the stack, this will 103 cause the label beneath it to rise to the top of the stack. 104 The disposition of the packet is based on the label that has 105 now risen to the top. 107 If, on the other hand, the NULL was the only label on the 108 stack, then the stack is now empty. The resulting packet is 109 treated as an IPv4 packet, and its disposition is based on the 110 IPv4 header. 112 Paragraph iii is hereby changed to read: 114 iii. A value of 2 represents the "IPv6 Explicit NULL Label". 116 An IPv6 Explicit NULL at the top of the label stack means that 117 the stack must be popped. 119 If the NULL was not the only label on the stack, this will 120 cause the label beneath it to rise to the top of the stack. 121 The disposition of the packet is based on the label that has 122 now risen to the top. 124 If, on the other hand, the NULL was the only label on the 125 stack, then the stack is now empty. The resulting packet is 126 treated as an IPv6 packet, and its disposition is based on the 127 IPv6 header. 129 3. Reasons for Change 131 Restricting Explicit NULL to the bottom of the stack has caused some 132 problems in practice. 134 With this restriction in place, one should not distribute, to a 135 particular label distribution peer, a binding of Explicit NULL to a 136 particular FEC, unless the following condition (call it "Condition 137 L") holds: all MPLS packets received by that peer with an incoming 138 label corresponding to that FEC contain only a single label stack 139 entry. If Explicit NULL is bound to the FEC, but Condition L doesn't 140 hold, the peer is being requested to create illegal packets. None of 141 the MPLS specifications say what the peer is actually supposed to do 142 in this case. This situation is made more troublesome by the facts 143 that, in practice, Condition L rarely holds, and it is not possible 144 in general to determine whether it holds or not. 146 Further, if one is supporting the Pipe Model of RFC3270, there are 147 good reasons to create label stacks in which Explicit NULL is at the 148 top of the label stack, but a non-null label is at the bottom. 150 RFC3270 specifies the procedures for MPLS support of Differentiated 151 Services. In particular, it defines a "Pipe Model", in which 152 (quoting from RFC3270, section 2.6.2): 154 "tunneled packets must convey two meaningful pieces of Diff-Serv 155 information: 157 - the Diff-Serv information which is meaningful to intermediate 158 nodes along the LSP span including the LSP Egress (which we 159 refer to as the 'LSP Diff-Serv Information'). This LSP Diff- 160 Serv Information is not meaningful beyond the LSP Egress: 161 Whether Traffic Conditioning at intermediate nodes on the LSP 162 span affects the LSP Diff-Serv information or not, this updated 163 Diff-Serv information is not considered meaningful beyond the 164 LSP Egress and is ignored. 166 - the Diff-Serv information which is meaningful beyond the LSP 167 Egress (which we refer to as the 'Tunneled Diff-Serv 168 Information'). This information is to be conveyed by the LSP 169 Ingress to the LSP Egress. This Diff-Serv information is not 170 meaningful to the intermediate nodes on the LSP span." 172 When the Pipe Model is in use, it is common practice for the LSP 173 Egress to bind Explicit Null to the tunnel's FEC. The intention is 174 that the LSP Diff-Serv information will be carried in the EXP bits 175 of the Explicit Null label stack entry, and the tunneled Diff-Serv 176 information will be carried in whatever is "below" the Explicit Null 177 label stack entry, i.e., in the IP header DS bits or in the EXP bits 178 of the next entry on the MPLS label stack. 180 Naturally, this practice causes a problem if the Pipe Model LSP is 181 being used to tunnel MPLS packets (i.e., if Condition L does not 182 hold). With strict adherence to RFCs 3031 and 3036, this practice 183 results in an MPLS packet where Explicit NULL is at the top of the 184 label stack, even though it is not the only entry in the label 185 stack. However, RFC 3032 makes this packet illegal. 187 Some implementations simply transmit the illegal packet. Others try 188 to convert it to a legal packet by stripping off the Explicit NULL 189 before transmitting it. However, that breaks the Pipe Model by 190 discarding the LSP Diff-Serv information. It is conceivable that 191 there may be an implementation which drops the illegal packet 192 entirely; this would also break the Pipe Model, as it would lose not 193 only the LSP Diff-Serv information but the entire packet. 195 Of course the LSP egress is not compelled to bind Explicit NULL to 196 the tunnel's FEC; an ordinary label could be used instead. However, 197 using Explicit NULL enables the egress to determine immediately 198 (i.e., without need for lookup in the Label Information Base) that 199 the further forwarding of the packet is to be determined by whatever 200 is below the label. Avoiding this lookup can have favorable 201 implications on forwarding performance. 203 Removing the restriction that Explicit Null only occur at the bottom 204 of the stack is the simplest way to facilitate the proper operation 205 of the Pipe Model. 207 4. Deployment Considerations 209 Implementations which adhere to this specification will interoperate 210 correctly, and will correctly support the Pipe Model. 212 Implementations which do not adhere to this specification may not 213 interoperate. In particular if a router advertises a binding of 214 Explicit NULL, and if that router has an upstream LDP peer which will 215 not transmit a packet that has multiple label stack entries with 216 Explicit Null at top of the stack, then it will not be possible to 217 use Explicit NULL to support the Pipe Model until the upstream LDP 218 peer is brought into compliance with this specification. 220 It is possible that there may be a router implementation, preceding 221 this specification, which will discard any received packet with 222 multiple label stack entries and a top label value of Explicit Null. 223 It is advisable to configure any such routers so that they do not 224 advertise any bindings to Explicit Null. 226 5. Security Considerations 228 This document updates RFC 3032 by allowing Explicit NULL to occur at 229 any position in the label stack. This modification does not impose 230 any new security considerations beyond those discussed in RFC 3032. 232 6. Acknowledgments 234 Thanks to Rahul Aggarwal, Francois LeFaucheur, Yakov Rekhter, and Dan 235 Tappan for their helpful comments. 237 7. Normative References 239 [RFC3032] "MPLS Label Stack Encoding", Rosen, et. al., January 2001 241 8. Informative References 243 [RFC3270] "Multi-Protocol Label Switching (MPLS) Support of 244 Differentiated Services", Le Faucheur, et. al., May 2002 246 9. Author's Address 248 Eric C. Rosen 249 Cisco Systems, Inc. 250 1414 Massachusetts Avenue 251 Boxborough, MA 01719 252 Email: erosen@cisco.com 254 10. Intellectual Property Statement 256 The IETF takes no position regarding the validity or scope of any 257 Intellectual Property Rights or other rights that might be claimed to 258 pertain to the implementation or use of the technology described in 259 this document or the extent to which any license under such rights 260 might or might not be available; nor does it represent that it has 261 made any independent effort to identify any such rights. Information 262 on the procedures with respect to rights in RFC documents can be 263 found in BCP 78 and BCP 79. 265 Copies of IPR disclosures made to the IETF Secretariat and any 266 assurances of licenses to be made available, or the result of an 267 attempt made to obtain a general license or permission for the use of 268 such proprietary rights by implementers or users of this 269 specification can be obtained from the IETF on-line IPR repository at 270 http://www.ietf.org/ipr. 272 The IETF invites any interested party to bring to its attention any 273 copyrights, patents or patent applications, or other proprietary 274 rights that may cover technology that may be required to implement 275 this standard. Please address the information to the IETF at ietf- 276 ipr@ietf.org. 278 11. Full Copyright Statement 280 Copyright (C) The Internet Society (2005). This document is subject 281 to the rights, licenses and restrictions contained in BCP 78 and 282 except as set forth therein, the authors retain all their rights. 284 This document and the information contained herein are provided on an 285 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 286 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 287 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 288 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 289 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 290 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.