< draft-christian-gre-over-clnp-01.txt		draft-christian-gre-over-clnp-02.txt >
	Internet Engineering Task Force P. Christian,		A new Request for Comments is now available in online RFC libraries.
	INTERNET-DRAFT Nortel Networks
	Category: Informational May 2001
	Expires: November 2001
	Generic Routing Encapsulation over CLNS networks
	<draft-christian-gre-over-clnp-01.txt>
	Status of this Memo
	This document is an Internet-Draft and is in full conformance with
	all provisions of Section 10 of RFC 2026.
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	This memo provides information for the Internet community. This memo
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	Distribution of this draft is unlimited.
	Christian Expires November 2001 1
	Generic Routing Encapsulation over CLNS Networks
	Abstract
	RFC 2784 Generic Routing Encapsulation (GRE) [1] provides a standard
	method for transporting one arbitrary network layer protocol over
	another arbitrary network layer protocol.
	RFC 1702 Generic Routing Encapsulation over IPv4 networks [2]
	provides a standard method for transporting an arbitrary network
	layer protocol over IPv4 using GRE.
	However no standard method exists for transporting other network
	layer protocols over CLNS. This causes lack of interoperability
	between different vendors' products as they provide solutions to
	migrate from CLNS networks to IP networks. This is a problem
	specifically in, but not limited to, the context of management
	networks for SONET and SDH networks elements.
	This document proposes a method for transporting an arbitrary
	protocol over a CLNS network using GRE.
	This may then be used as a method to tunnel IPv4 or IPv6 over CLNS.
	This document is an Independent Submission. Comments should be
	submitted to christi@nortelnetworks.com.
	1. Introduction
	Large networks exist for the purpose of providing management
	communications for SONET and SDH network elements. Standards
	Bellcore GR-253-CORE [3] and ITU-T G.784 [4] mandate that these
	networks are based on CLNS.
	Many vendors have already started to offer SONET and SDH products
	that are managed by IP instead of CLNS and a general migration from
	CLNS towards IP is anticipated within the industry.
	Part of any migration strategy from CLNS to IP should provide for
	the co-existence of both CLNS managed and IP managed network
	elements in the same network.
	Such a migration strategy should foresee the need to manage existing
	CLNS managed network elements that become isolated by a new IP
	network. Such a scenario may be tackled by tunnelling CLNP PDUs over
	IP using the existing GRE standard RFC 2784 [1] and informational
	RFC 1702 [2]. Networks have already been deployed that use this
	method.
	Such a migration strategy should also foresee the need to manage new
	IP managed network elements that are installed on the far side of
	existing CLNS managed network. Such a scenario requires a method for
	tunnelling IP over CLNS.
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	Generic Routing Encapsulation over CLNS Networks
	2. GRE over CLNS advantages
	Using GRE to tunnel IP over CLNS offers some advantages.
	In the absence of a standard for tunnelling IP over CLNS, GRE as
	specified in RFC 2784 [1] is the most applicable standard that
	exists.
	The move from CLNS to IP comes at a time when IP is itself
	migrating from IPv4 to IPv6. GRE defines a method to tunnel any
	protocol that has an Ethernet Protocol Type. Therefore by defining
	a method for CLNS to transport GRE, a method will then exist for
	CLNS to transport any other protocol that has an Ethernet Protocol
	Type defined in RFC 1700 [5]. Thus GRE over CLNS can be used to
	tunnel both IPv4 and IPv6.
	GRE is already commonly used to tunnel CLNP PDUs over IP and so
	using GRE to tunnel IP over CLNS gives a common approach to
	tunnelling and may simplify software within network elements that
	initiate and terminate tunnels.
	The only disadvantage of using GRE is the extra minimum of four
	bytes that will be used between CLNP header and IP payload packet.
	Given the large size of CLNP headers this will not make a
	significant difference to the performance of any network that has IP
	over CLNP PDUs present on it.
	3. Transporting GRE packets over CLNS.
	It is suggested that GRE should be transported over CLNS at the
	lowest layer possible, which is as a transport layer protocol over
	the network layer. This can be achieved by placing the entire GRE
	packet inside a CLNP Data Type PDU (DT PDU) as data payload.
	The GRE packet is a GRE packet as defined in RFC 2784 [1], in other
	words GRE header plus payload packet.
	Data payload is the part of a Data PDU that is described as "Data"
	in the structure of a Data PDU in ISO/IEC 8473-1 [6].
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	Generic Routing Encapsulation over CLNS Networks
	For convenience the structure of a Data PDU is reproduced from
	ISO/IEC 8473-1 [6] below:-
	Octet
	----------------------------------------
	| Network Layer Protocol Identifier | 1
	----------------------------------------
	| Length Indicator | 2
	----------------------------------------
	| Version/Protocol Id Extension | 3
	----------------------------------------
	| Lifetime | 4
	----------------------------------------
	| SP | MS | E/R | Type | 5
	----------------------------------------
	| Segment Length | 6,7
	----------------------------------------
	| Checksum | 8,9
	----------------------------------------
	| Destination Address Length Indicator | 10
	----------------------------------------
	| | 11
	| Destination Address |
	| | m-1
	----------------------------------------
	| Source Address Length Indicator | m
	----------------------------------------
	| | m+1
	| Source Address |
	| | n-1
	----------------------------------------
	| Data Unit Identifier | n,n+1
	----------------------------------------
	| Segment Offset | n+2,n+3
	----------------------------------------
	| Total Length | n+4,n+5
	----------------------------------------
	| | n+6
	| Options |
	| | p
	----------------------------------------
	| | p+1
	| Data ( GRE packet ) |
	| | z
	----------------------------------------
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	Generic Routing Encapsulation over CLNS Networks
	4. NSAP selector (N-SEL) value.
	Transport of GRE packets is a new type of Network Service (NS) user.
	Different Network Service users are identified by using different
	NSAP selector bytes also known as N-SEL bytes.
	This is a similar concept to the use of the IP Protocol Type used in
	IP packets.
	Whilst it is not strictly necessary for all vendors to use the same
	N-SEL values, they must use the same N-SEL value for it to be
	possible for one vendor's CLNS device or network element to initiate
	a GRE tunnel which is then terminated on a different vendor's CLNS
	device.
	Although N-SEL values (other than zero) are not defined in CLNS/CLNP
	standards, some are defined when CLNS is used in SONET networks by
	Bellcore GR-253-CORE [3] whilst others are in common use.
	As the IP protocol number for GRE is 47, as defined in RFC 1702 [2],
	and as 47 is not commonly used as an N-SEL value, it is suggested
	that 47 (decimal) should be used as an N-SEL value to indicate to
	the CLNS stack that the Data portion of the Data Type PDU contains a
	GRE packet.
	The N-SEL byte should be set to 47 (decimal) in both the source
	address and the destination address of the CLNP PDU.
	The N-SEL value of 47 should indicate only that the payload is GRE,
	and the device or network element that transmits the PDU should use
	the GRE header to indicate what protocol (for example IPv4 or IPv6)
	is encapsulated within the GRE packet in conformance with RFC 2784
	[1]. Similarly the device or network element that receives the PDU
	should then inspect the GRE header to ascertain what protocol is
	contained within the GRE packet in conformance with RFC 2784 [1].
	5. Segmentation Permitted (SP) value.
	It is recommended that the SP flag in all CLNP PDUs containing GRE
	packets should be set.
	If the SP flag is not set, and a CLNP PDU is too large for a
	particular link, then a CLNS device or network element will drop the
	PDU. The originator of the packet that is inside the GRE packet will
	not have visibility of the packet loss or the reason for the packet
	loss, and a black hole may form.
	6. Interaction with Path MTU Discovery (PMTU), RFC 1191 [7].
	A tunnel entry point for a GRE tunnel should treat IP packets that
	are bigger than the MTU size of the GRE tunnel as per RFC 1191 [7].
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	Generic Routing Encapsulation over CLNS Networks
	If the oversize IP packet that is about to enter the GRE tunnel does
	not have its Don't Fragment (DF) bit set then it should be
	fragmented before entering the tunnel.
	If the oversize IP packet that is about to enter the GRE tunnel has
	its DF bit set then the packet should be discarded, and an ICMP
	unreachable error message (in particular the "fragmentation needed
	and DF set" code) should be sent back to the originator of the
	packet as described in RFC 1191 [7].
	7. Security Considerations
	CLNS and GRE do not provide any security when employed in the way
	recommended in this document.
	If security is required, then it must be provided by other methods
	and applied to the payload protocol before it is transported by GRE
	over CLNS.
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	Generic Routing Encapsulation over CLNS Networks
	8. References
	[1] Farinacci, D., Li, T., Hanks, S., Meyer, D. and P. Traina,
	"Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.
	[2] Hanks, S., Li, T., Farinacci, D. and P.Traina, "Generic
	Routing Encapsulation over IPv4", RFC 1702, October 1994.
	[3] Bellcore Publication GR-253-Core "Synchronous Optical Network
	(SONET) Transport Systems: Common Generic Criteria", January 1999
	[4] ITU-T Recommendation G.784 "Synchronous Digital Hierarchy
	(SDH) management", June 1999
	[5] J. Reynolds, J., Editor, "Assigned Numbers", RFC 1700, October
	1994
	[6] "Information technology - Protocol for providing the
	connectionless-mode network service", ISO/IEC 8473-1, 1994
	[7] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
	November 1990.
	9. Acknowledgments		RFC 3147
	Chris Murton, Paul Fee, Mike Tate for their contribution in writing		Title: Generic Routing Encapsulation over CLNS Networks
	this document.		Author(s): P. Christian
			Status: Informational
			Date: July 2001
			Mailbox: christi@nortelnetworks.com
			Pages: 8
			Characters: 14125
			Updates/Obsoletes/SeeAlso: None
	10. Author's Address		I-D Tag: draft-christian-gre-over-clnp-02.txt
	Philip Christian		URL: ftp://ftp.rfc-editor.org/in-notes/rfc3147.txt
	Nortel Networks Harlow Laboratories
	London Road, Harlow,
	Essex, CM17 9NA UK
	Email: christi@nortelnetworks.com
	Christian Expires November 2001 7		This document proposes a method for transporting an arbitrary protocol
	Generic Routing Encapsulation over CLNS Networks		over a CLNS (Connectionless Network Service) network using GRE
			(Generic Routing Encapsulation). This may then be used as a method to
			tunnel IPv4 or IPv6 over CLNS.
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