Network Working Group Eric C. Rosen Internet Draft Cisco Systems, Inc. Expiration Date:
SeptemberOctober 2004 Updates RFC 3032 MarchApril 2004 Removing a Restriction on the use of MPLS Explicit NULL draft-ietf-mpls-explicit-null-00.txtdraft-ietf-mpls-explicit-null-01.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract The label stack encoding for MPLS (Multi-protocol Label Switching) defines a reserved label value known as "IPv4 Explicit NULL" and a reserved label value known as "IPv6 Explicit NULL". Previously, these labels were only legal when they occurred at the bottom of the MPLS label stack. This restriction is now removed, so that these label values may legally occur anywhere in the stack. Contents 1 Introduction ......................................... 2 2 Detail of Change ..................................... 2 3 Reasons for Change ................................... 4 4 Security Considerations .............................. 5 5 Acknowledgments ...................................... 5 56 Normative References ................................. 56 7 Informative References ............................... 5 76 8 Author's Address ..................................... 6 89 Intellectual Property Notice ......................... 6 910 Copyright Notice ..................................... 67 1. Introduction RFC 3032 defines a reserved label value known as "IPv4 Explicit NULL" and a reserved label value known as "IPv6 Explicit NULL". It states that these label values are only legal at the bottom of the MPLS label stack. However, no reason is given for this restriction. It has turned out that in practice there are some situations in which it is useful to send MPLS packets which have Explicit NULL occur other than at that bottom of the label stack. While the intended semantics are obvious enough, the fact that such packets are gratuitously declared by RFC 3032 to be illegal has made it difficult to handle these situations in an interoperable manner. This document updates RFC 3032 by removing the unnecessary restriction, so that the two aforementioned label values are legal anywhere in the label stack. 2. Detail of Change RFC 3032 states on page 4: There are several reserved label values: i. A value of 0 represents the "IPv4 Explicit NULL Label". This label value is only legal at the bottom of the label stack. It indicates that the label stack must be popped, and the forwarding of the packet must then be based on the IPv4 header. iii. A value of 2 represents the "IPv6 Explicit NULL Label". This label value is only legal at the bottom of the label stack. It indicates that the label stack must be popped, and the forwarding of the packet must then be based on the IPv6 header. Paragraph i is hereby changed to read: i. A value of 0 represents the "IPv4 Explicit NULL Label". An IPv4 Explicit NULL at the top of the label stack means that the stack must be popped. If the NULL was not the only label on the stack, this will cause the label beneath it to rise to the top of the stack. The disposition of the packet is based on the label that has now risen to the top. If, on the other hand, the NULL was the only label on the stack, then the stack is now empty. The resulting packet is treated as an IPv4 packet, and its disposition is based on the IPv4 header. Paragraph iii is hereby changed to read: iii. A value of 2 represents the "IPv6 Explicit NULL Label". An IPv6 Explicit NULL at the top of the label stack means that the stack must be popped. If the NULL was not the only label on the stack, this will cause the label beneath it to rise to the top of the stack. The disposition of the packet is based on the label that has now risen to the top. If, on the other hand, the NULL was the only label on the stack, then the stack is now empty. The resulting packet is treated as an IPv6 packet, and its disposition is based on the IPv6 header. 3. Reasons for Change Restricting Explicit NULL to the bottom of the stack has caused some problems in practice. With this restriction in place, one should not distribute, to a particular label distribution peer, a binding of Explicit NULL to a particular FEC, unless the following condition (call it "Condition L") holds: all MPLS packets received by that peer with an incoming label corresponding to that FEC contain only a single label stack entry. If Explicit NULL is bound to the FEC, but Condition L doesn't hold, the peer is being requested to create illegal packets. None of the MPLS specifications say what the peer is actually supposed to do in this case. This situation is made more troublesome by the facts that, in practice, Condition L rarely holds, and it is not possible in general to determine whether it holds or not. Further, if one is supporting the Pipe Model of RFC3270, there are good reasons to create label stacks in which Explicit NULL is at the top of the label stack, but a non-null label is at the bottom. RFC3270 specifies the procedures for MPLS support of Differentiated Services. In particular, it defines a "Pipe Model", in which (quoting from RFC3270, section 2.6.2): "tunneled packets must convey two meaningful pieces of Diff-Serv information: - the Diff-Serv information which is meaningful to intermediate nodes along the LSP span including the LSP Egress (which we refer to as the 'LSP Diff-Serv Information'). This LSP Diff- Serv Information is not meaningful beyond the LSP Egress: Whether Traffic Conditioning at intermediate nodes on the LSP span affects the LSP Diff-Serv information or not, this updated Diff-Serv information is not considered meaningful beyond the LSP Egress and is ignored. - the Diff-Serv information which is meaningful beyond the LSP Egress (which we refer to as the 'Tunneled Diff-Serv Information'). This information is to be conveyed by the LSP Ingress to the LSP Egress. This Diff-Serv information is not meaningful to the intermediate nodes on the LSP span." When the Pipe Model is in use, it is common practice for the LSP Egress to bind Explicit Null to the tunnel's FEC. The intention is that the LSP Diff-Serv information will be carried in the EXP bits of the Explicit Null label stack entry, and the tunneled Diff-Serv information will be carried in whatever is "below" the Explicit Null label stack entry, i.e., in the IP header DS bits or in the EXP bits of the next entry on the MPLS label stack. Naturally, this practice causes a problem if the Pipe Model LSP is being used to tunnel MPLS packets (i.e., if Condition L does not hold). With strict adherence to RFCs 3031 and 3036, this practice results in an MPLS packet where Explicit NULL is at the top of the label stack, even though it is not the only entry in the label stack. However, RFC 3032 makes this packet illegal. Some implementations simply transmit the illegal packet. Others try to convert it to a legal packet by stripping off the Explicit NULL before transmitting it. However, that breaks the Pipe Model by discarding the LSP Diff-Serv information. Of course the LSP egress is not compelled to bind Explicit NULL to the tunnel's FEC; an ordinary label could be used instead. However, using Explicit NULL enables the egress to determine immediately (i.e., without need for lookup in the Label Information Base) that the further forwarding of the packet is to be determined by whatever is below the label. Avoiding this lookup can have favorable implications on forwarding performance. Removing the restriction that Explicit Null only occur at the bottom of the stack is the simplest way to facilitate the proper operation of the Pipe Model. 4. Security Considerations This document updates RFC 3032 by allowing Explicit NULL to occur at any position in the label stack. This modification does not impose any new security considerations beyond those discussed in RFC 3032. 5. Acknowledgments Thanks to Rahul Aggarwal, Francois LeFaucheur, Yakov Rekhter, and Dan Tappan for their helpful comments. 5.6. Normative References [RFC3032] "MPLS Label Stack Encoding", Rosen, et. al., January 2001 6.7. Informative References [RFC3270] "Multi-Protocol Label Switching (MPLS) Support of Differentiated Services", Le Faucheur, et. al., May 2002 7.8. Author's Address Eric C. Rosen Cisco Systems, Inc. 1414 Massachusetts Avenue Boxborough, MA 01719 Email: firstname.lastname@example.org 8.9. Intellectual Property Notice The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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