draft-ietf-mpls-explicit-null-02.txt		rfc4182.txt
	Network Working Group Eric C. Rosen
	Internet Draft Cisco Systems, Inc.
	Expiration Date: August 2005
	Updates RFC 3032
	February 2005		Network Working Group E. Rosen
			Request for Comments: 4182 Cisco Systems, Inc.
			Updates: 3032 September 2005
			Category: Standards Track
	Removing a Restriction on the use of MPLS Explicit NULL		Removing a Restriction on the use of MPLS Explicit NULL
	draft-ietf-mpls-explicit-null-02.txt		Status of This Memo
	Status of this Memo
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	Abstract		Abstract
	The label stack encoding for MPLS (Multi-protocol Label Switching)		The label stack encoding for Multi-protocol Label Switching (MPLS)
	defines a reserved label value known as "IPv4 Explicit NULL" and a		defines a reserved label value known as "IPv4 Explicit NULL" and a
	reserved label value known as "IPv6 Explicit NULL". Previously,		reserved label value known as "IPv6 Explicit NULL". Previously,
	these labels were only legal when they occurred at the bottom of the		these labels were only legal when they occurred at the bottom of the
	MPLS label stack. This restriction is now removed, so that these		MPLS label stack. This restriction is now removed, so that these
	label values may legally occur anywhere in the stack.		label values may legally occur anywhere in the stack.
	This document updates RFC 3032.		This document updates RFC 3032.
	Contents		Table of Contents
	1 Introduction ......................................... 2		1. Introduction ....................................................2
	2 Detail of Change ..................................... 2		2. Detail of Change ................................................2
	3 Reasons for Change ................................... 4		3. Reasons for Change ..............................................3
	4 Deployment Considerations ............................ 5		4. Deployment Considerations .......................................5
	5 Security Considerations .............................. 6		5. Security Considerations .........................................5
	6 Acknowledgments ...................................... 6		6. Acknowledgments .................................................5
	7 Normative References ................................. 6		7. Normative References ............................................5
	8 Informative References ............................... 6		8. Informative References ..........................................5
	9 Author's Address ..................................... 6
	10 Intellectual Property Statement ...................... 6
	11 Full Copyright Statement ............................. 7
	1. Introduction		1. Introduction
	RFC 3032 defines a reserved label value known as "IPv4 Explicit NULL"		RFC 3032 defines a reserved label value known as "IPv4 Explicit NULL"
	and a reserved label value known as "IPv6 Explicit NULL". It states		and a reserved label value known as "IPv6 Explicit NULL" [RFC3032].
	that these label values are only legal at the bottom of the MPLS		It states that these label values are only legal at the bottom of the
	label stack. However, no reason is given for this restriction.		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 has turned out that in practice there are some situations in which
	it is useful to send MPLS packets which have Explicit NULL occur		it is useful to send MPLS packets that have Explicit NULL occur
	other than at that bottom of the label stack. While the intended		somewhere other than at that bottom of the label stack. While the
	semantics are obvious enough, the fact that such packets are		intended semantics are obvious enough, the fact that such packets are
	gratuitously declared by RFC 3032 to be illegal has made it difficult		gratuitously declared by RFC 3032 to be illegal has made it difficult
	to handle these situations in an interoperable manner.		to handle these situations in an interoperable manner.
	This document updates RFC 3032 by removing the unnecessary		This document updates RFC 3032 by removing the unnecessary
	restriction, so that the two aforementioned label values are legal		restriction, so that the two aforementioned label values are legal
	anywhere in the label stack.		anywhere in the label stack.
	2. Detail of Change		2. Detail of Change
	RFC 3032 states on page 4:		RFC 3032 states on page 4:
	skipping to change at page 4, line 12		skipping to change at page 3, line 34
	treated as an IPv6 packet, and its disposition is based on the		treated as an IPv6 packet, and its disposition is based on the
	IPv6 header.		IPv6 header.
	3. Reasons for Change		3. Reasons for Change
	Restricting Explicit NULL to the bottom of the stack has caused some		Restricting Explicit NULL to the bottom of the stack has caused some
	problems in practice.		problems in practice.
	With this restriction in place, one should not distribute, to a		With this restriction in place, one should not distribute, to a
	particular label distribution peer, a binding of Explicit NULL to a		particular label distribution peer, a binding of Explicit NULL to a
	particular FEC, unless the following condition (call it "Condition		particular Forwarding Equivalence Class (FEC), unless the following
	L") holds: all MPLS packets received by that peer with an incoming		condition (call it "Condition L") holds: all MPLS packets received by
	label corresponding to that FEC contain only a single label stack		that peer with an incoming label corresponding to that FEC contain
	entry. If Explicit NULL is bound to the FEC, but Condition L doesn't		only a single label stack entry. If Explicit NULL is bound to the
	hold, the peer is being requested to create illegal packets. None of		FEC, but Condition L doesn't hold, the peer is being requested to
	the MPLS specifications say what the peer is actually supposed to do		create illegal packets. None of the MPLS specifications say what the
	in this case. This situation is made more troublesome by the facts		peer is actually supposed to do in this case. This situation is made
	that, in practice, Condition L rarely holds, and it is not possible		more troublesome by the facts that, in practice, Condition L rarely
	in general to determine whether it holds or not.		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		Further, if one is supporting the Pipe Model of RFC 3270 [RFC3270],
	good reasons to create label stacks in which Explicit NULL is at the		there are good reasons to create label stacks in which Explicit NULL
	top of the label stack, but a non-null label is at the bottom.		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		RFC3270 specifies the procedures for MPLS support of Differentiated
	Services. In particular, it defines a "Pipe Model", in which		Services. In particular, it defines a "Pipe Model" in which (quoting
	(quoting from RFC3270, section 2.6.2):		from RFC 3270, Section 2.6.2):
	"tunneled packets must convey two meaningful pieces of Diff-Serv		"tunneled packets must convey two meaningful pieces of Diff-Serv
	information:		information:
	- the Diff-Serv information which is meaningful to intermediate		- the Diff-Serv information which is meaningful to intermediate
	nodes along the LSP span including the LSP Egress (which we		nodes along the LSP span including the LSP Egress (which we refer
	refer to as the 'LSP Diff-Serv Information'). This LSP Diff-		to as the 'LSP Diff-Serv Information'). This LSP Diff-Serv
	Serv Information is not meaningful beyond the LSP Egress:		Information is not meaningful beyond the LSP Egress: Whether
	Whether Traffic Conditioning at intermediate nodes on the LSP		Traffic Conditioning at intermediate nodes on the LSP span
	span affects the LSP Diff-Serv information or not, this updated		affects the LSP Diff-Serv information or not, this updated Diff-
	Diff-Serv information is not considered meaningful beyond the		Serv information is not considered meaningful beyond the LSP
	LSP Egress and is ignored.		Egress and is ignored.
	- the Diff-Serv information which is meaningful beyond the LSP		- the Diff-Serv information which is meaningful beyond the LSP
	Egress (which we refer to as the 'Tunneled Diff-Serv		Egress (which we refer to as the 'Tunneled Diff-Serv
	Information'). This information is to be conveyed by the LSP		Information'). This information is to be conveyed by the LSP
	Ingress to the LSP Egress. This Diff-Serv information is not		Ingress to the LSP Egress. This Diff-Serv information is not
	meaningful to the intermediate nodes on the LSP span."		meaningful to the intermediate nodes on the LSP span."
	When the Pipe Model is in use, it is common practice for the LSP		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		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		that the LSP Diff-Serv information will be carried in the EXP bits of
	of the Explicit Null label stack entry, and the tunneled Diff-Serv		the Explicit Null label stack entry, and the tunneled Diff-Serv
	information will be carried in whatever is "below" the Explicit Null		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		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.		of the next entry on the MPLS label stack.
	Naturally, this practice causes a problem if the Pipe Model LSP is		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		being used to tunnel MPLS packets (i.e., if Condition L does not
	hold). With strict adherence to RFCs 3031 and 3036, this practice		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		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		label stack, even though it is not the only entry in the label stack.
	stack. However, RFC 3032 makes this packet illegal.		However, RFC 3032 makes this packet illegal.
	Some implementations simply transmit the illegal packet. Others try		Some implementations simply transmit the illegal packet. Others try
	to convert it to a legal packet by stripping off the Explicit NULL		to convert it to a legal packet by stripping off the Explicit NULL
	before transmitting it. However, that breaks the Pipe Model by		before transmitting it. However, that breaks the Pipe Model by
	discarding the LSP Diff-Serv information. It is conceivable that		discarding the LSP Diff-Serv information. It is conceivable that
	there may be an implementation which drops the illegal packet		there may be an implementation that drops the illegal packet
	entirely; this would also break the Pipe Model, as it would lose not		entirely; this would also break the Pipe Model, as it would lose not
	only the LSP Diff-Serv information but the entire packet.		only the LSP Diff-Serv information, but the entire packet.
	Of course the LSP egress is not compelled to bind Explicit NULL to		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,		the tunnel's FEC; an ordinary label could be used instead. However,
	using Explicit NULL enables the egress to determine immediately		using Explicit NULL enables the egress to determine immediately
	(i.e., without need for lookup in the Label Information Base) that		(i.e., without need for lookup in the Label Information Base) that
	the further forwarding of the packet is to be determined by whatever		the further forwarding of the packet is to be determined by whatever
	is below the label. Avoiding this lookup can have favorable		is below the label. Avoiding this lookup can have favorable
	implications on forwarding performance.		implications on forwarding performance.
	Removing the restriction that Explicit Null only occur at the bottom		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 stack is the simplest way to facilitate the proper operation
	of the Pipe Model.		of the Pipe Model.
	4. Deployment Considerations		4. Deployment Considerations
	Implementations which adhere to this specification will interoperate		Implementations that adhere to this specification will interoperate
	correctly, and will correctly support the Pipe Model.		correctly, and will correctly support the Pipe Model.
	Implementations which do not adhere to this specification may not		Implementations that do not adhere to this specification may not
	interoperate. In particular if a router advertises a binding of		interoperate. In particular, if a router advertises a binding of
	Explicit NULL, and if that router has an upstream LDP peer which will		Explicit NULL, and if that router has an upstream LDP peer that will
	not transmit a packet that has multiple label stack entries with		not transmit a packet that has multiple label stack entries with
	Explicit Null at top of the stack, then it will not be possible to		Explicit Null at top of the stack, then it will not be possible to
	use Explicit NULL to support the Pipe Model until the upstream LDP		use Explicit NULL to support the Pipe Model until the upstream LDP
	peer is brought into compliance with this specification.		peer is brought into compliance with this specification.
	It is possible that there may be a router implementation, preceding		It is possible that there may be a router implementation, preceding
	this specification, which will discard any received packet with		this specification, which will discard any received packet with
	multiple label stack entries and a top label value of Explicit Null.		multiple label stack entries and a top label value of Explicit Null.
	It is advisable to configure any such routers so that they do not		It is advisable to configure any such routers so that they do not
	advertise any bindings to Explicit Null.		advertise any bindings to Explicit Null.
	skipping to change at page 6, line 19		skipping to change at page 5, line 41
	any position in the label stack. This modification does not impose		any position in the label stack. This modification does not impose
	any new security considerations beyond those discussed in RFC 3032.		any new security considerations beyond those discussed in RFC 3032.
	6. Acknowledgments		6. Acknowledgments
	Thanks to Rahul Aggarwal, Francois LeFaucheur, Yakov Rekhter, and Dan		Thanks to Rahul Aggarwal, Francois LeFaucheur, Yakov Rekhter, and Dan
	Tappan for their helpful comments.		Tappan for their helpful comments.
	7. Normative References		7. Normative References
	[RFC3032] "MPLS Label Stack Encoding", Rosen, et. al., January 2001		[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
			Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
			Encoding", RFC 3032, January 2001.
	8. Informative References		8. Informative References
	[RFC3270] "Multi-Protocol Label Switching (MPLS) Support of		[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
	Differentiated Services", Le Faucheur, et. al., May 2002		P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
			Protocol Label Switching (MPLS) Support of Differentiated
			Services", RFC 3270, May 2002.
	9. Author's Address		Author's Address
	Eric C. Rosen		Eric C. Rosen
	Cisco Systems, Inc.		Cisco Systems, Inc.
	1414 Massachusetts Avenue		1414 Massachusetts Avenue
	Boxborough, MA 01719		Boxborough, MA 01719
	Email: erosen@cisco.com
	10. Intellectual Property Statement		EMail: erosen@cisco.com
			Full Copyright Statement
			Copyright (C) The Internet Society (2005).
			This document is subject to the rights, licenses and restrictions
			contained in BCP 78, and except as set forth therein, the authors
			retain all their rights.
			This document and the information contained herein are provided on an
			"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
			OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
			ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
			INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
			INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
			WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
			Intellectual Property
	The IETF takes no position regarding the validity or scope of any		The IETF takes no position regarding the validity or scope of any
	Intellectual Property Rights or other rights that might be claimed to		Intellectual Property Rights or other rights that might be claimed to
	pertain to the implementation or use of the technology described in		pertain to the implementation or use of the technology described in
	this document or the extent to which any license under such rights		this document or the extent to which any license under such rights
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	found in BCP 78 and BCP 79.		found in BCP 78 and BCP 79.
	skipping to change at page 7, line 19		skipping to change at page 7, line 45
	such proprietary rights by implementers or users of this		such proprietary rights by implementers or users of this
	specification can be obtained from the IETF on-line IPR repository at		specification can be obtained from the IETF on-line IPR repository at
	http://www.ietf.org/ipr.		http://www.ietf.org/ipr.
	The IETF invites any interested party to bring to its attention any		The IETF invites any interested party to bring to its attention any
	copyrights, patents or patent applications, or other proprietary		copyrights, patents or patent applications, or other proprietary
	rights that may cover technology that may be required to implement		rights that may cover technology that may be required to implement
	this standard. Please address the information to the IETF at ietf-		this standard. Please address the information to the IETF at ietf-
	ipr@ietf.org.		ipr@ietf.org.
	11. Full Copyright Statement		Acknowledgement
	Copyright (C) The Internet Society (2005). This document is subject
	to the rights, licenses and restrictions contained in BCP 78 and
	except as set forth therein, the authors retain all their rights.
	This document and the information contained herein are provided on an		Funding for the RFC Editor function is currently provided by the
	"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS		Internet Society.
	OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
	ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
	INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
	INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
	WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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