< draft-martini-l2circuit-encap-mpls-00.txt		draft-martini-l2circuit-encap-mpls-01.txt >
	Network Working Group Luca Martini		Network Working Group Luca Martini
	Internet Draft Nasser El-Aawar		Internet Draft Nasser El-Aawar
	Expiration Date: May 2001 Giles Heron		Expiration Date: August 2001 Giles Heron
	Level 3 Communications, LLC.		Level 3 Communications, LLC.
	Dimitri Stratton Vlachos
	Daniel Tappan		Daniel Tappan
	Eric C. Rosen		Eric C. Rosen
			Alex Hamilton
			Jayakumar Jayakumar
	Cisco Systems, Inc.		Cisco Systems, Inc.
	Steve Vogelsang		Steve Vogelsang
	John Shirron		John Shirron
			Toby Smith
	Laurel Networks, Inc.		Laurel Networks, Inc.
	Andrew G. Malis		Andrew G. Malis
			Vinai Sirkay
	Vivace Networks, Inc.		Vivace Networks, Inc.
	November 2000		Dimitri Stratton Vlachos
			Mazu Networks, Inc.
			Kireeti Kompella
			Juniper Networks
			February 2001
	Encapsulation Methods for Transport of Layer 2 Frames Over MPLS		Encapsulation Methods for Transport of Layer 2 Frames Over MPLS
	draft-martini-l2circuit-encap-mpls-00.txt		draft-martini-l2circuit-encap-mpls-01.txt
	Status of this Memo		Status of this Memo
	This document is an Internet-Draft and is in full conformance with		This document is an Internet-Draft and is in full conformance with
	all provisions of Section 10 of RFC2026.		all provisions of Section 10 of RFC2026.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
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	skipping to change at page 1, line 39 ¶		skipping to change at page 2, line 4 ¶
	all provisions of Section 10 of RFC2026.		all provisions of Section 10 of RFC2026.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF), its areas, and its working groups. Note that other		Task Force (IETF), its areas, and its working groups. Note that other
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	Abstract		Abstract
	This document describes methods for encapsulating the Protocol Data		This document describes methods for encapsulating the Protocol Data
	Units (PDUs) of layer 2 protocols such as Frame Relay, ATM AAL5,		Units (PDUs) of layer 2 protocols such as Frame Relay, ATM AAL5, or
	Ethernet for transport across an MPLS network.		Ethernet for transport across an MPLS network.
	Table of Contents		Table of Contents
	1 Specification of Requirements .......................... 2		1 Specification of Requirements .......................... 3
	2 Introduction ........................................... 2		2 Introduction ........................................... 3
	3 Optional Sequencing and/or Padding ..................... 3		3 General encapsulation method ........................... 3
	4 MTU Requirements ....................................... 4		3.1 The Control Word ....................................... 3
	5 Protocol-Specific Issues ............................... 4		3.1.1 Setting the sequence number ............................ 4
	5.1 Frame Relay ............................................ 4		3.1.2 Processing the sequence number ......................... 5
	5.2 ATM .................................................... 4		3.2 MTU Requirements ....................................... 5
	5.2.1 OAM Cell Support ....................................... 6		3.3 MPLS Shim EXP Bit Values ............................... 6
	5.2.2 CLP Bit to EXP Bit Mapping ............................. 7		3.4 MPLS Shim TTL Values ................................... 6
	5.3 Ethernet VLAN .......................................... 7		4 Protocol-Specific Details .............................. 6
	5.4 Ethernet ............................................... 7		4.1 Frame Relay ............................................ 6
	5.5 HDLC ( Cisco ) ......................................... 7		4.2 ATM .................................................... 8
	5.6 PPP .................................................... 8		4.2.1 ATM AAL5 CPCS-PDU Mode ................................. 8
	6 Security Considerations ................................ 8		4.2.2 ATM Cell Mode .......................................... 10
	7 Intellectual Property Disclaimer ....................... 8		4.2.3 OAM Cell Support ....................................... 12
	8 References ............................................. 8		4.2.4 CLP bit to MPLS label stack EXP bit mapping ............ 12
	9 Author Information ..................................... 9		4.3 Ethernet VLAN .......................................... 12
			4.4 Ethernet ............................................... 12
			4.5 HDLC ( Cisco ) ......................................... 13
			4.6 PPP .................................................... 13
			5 Security Considerations ................................ 13
			6 Intellectual Property Disclaimer ....................... 13
			7 References ............................................. 13
			8 Author Information ..................................... 14
	1. Specification of Requirements		1. Specification of Requirements
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this		"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	document are to be interpreted as described in RFC 2119		document are to be interpreted as described in RFC 2119
	2. Introduction		2. Introduction
	In an MPLS network, it is possible to carry the Protocol Data Units		In an MPLS network, it is possible to carry the Protocol Data Units
	(PDUs) of layer 2 protocols by prepending an MPLS label stack to		(PDUs) of layer 2 protocols by prepending an MPLS label stack to
	these PDUs. This document specifies the necessary encapsulation		these PDUs. This document specifies the necessary encapsulation
	procedures for accomplishing this. The control protocol methods are		procedures for accomplishing this. One possible control protocol
	described in [5]. QoS related issues are not discussed in this draft.		method is described in [1]. QoS related issues are not discussed in
			this draft. For the purpose of this document R1 will be defined as
			the ingress LSR, and R2 as the egress LSR. A layer 2 PDU will be
			received at R1, encapsulated at R1, transported, decapsulated at R2,
			and transmitted out of R2. In a similar way, the "VC label" is
			defined as the label at the bottom of the label stack used to
			transmit the layer 2 PDU.
	3. Optional Sequencing and/or Padding		3. General encapsulation method
	Sometimes it is important to guarantee that sequentiality is		When transporting layer 2 protocols over MPLS it is, in most cases,
	preserved on a layer 2 virtual circuit. To accommodate this		not necessary to transport the layer 2 encapsulation across the MPLS
	requirement, we provide an optional control word which may appear		network. In most cases the layer 2 header can be stripped at R1, and
	immediately after the label stack and immediately before the layer 2		reproduced at R2 with the help of some extra encapsulation
	PDU. This control word contains a sequence number. R1 and R2 both		information, some of which is a priori signaled, and some of which
	need to be configured with the knowledge of whether a control word		may be carried in the control word described below.
	will be used for a specific virtual circuit.
	Sometimes it is necessary to transmit a small packet on a medium		3.1. The Control Word
	where there is a minimum transport unit larger than the actual packet
	size. In this case, padding is appended to the packet. When the VC
	label is popped, it may be desirable to remove the padding before
	forwarding the packet.
	To facilitate this, the control word has a length field. If the		There are three requirements that may need to be satisfied when
	packet's length (without any padding) is less than 256 bytes, the		transporting layer 2 protocols over MPLS:
	length field MUST be set to the packet's length (without padding).		-i. Sequentiality may need to be preserved.
	Otherwise the length field MUST be set to zero. The value of the		-ii. Small packets may need to be padded in order to be
	length field, if non-zero, can be used to remove any padding.		transmitted on a medium where the minimum transport unit is
			larger than the actual packet size.
			-iii. Control bits carried in the header of the layer 2 frame may
			need to be transported.
	The generic control word is defined as follows:		The control word defined here addresses all three of these
			requirements. For some protocols this word is REQUIRED, and for
			others OPTIONAL.
			In all cases the the egress LSR must be aware of whether the ingress
			LSR will send a control word over a specific virtual circuit. This
			may be achived by configuration of the LSRs, or by signaling, for
			example as defined in [1].
			The control word is defined as follows:
	0 1 2 3		0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Reserved |T| Length | Sequence Number |		| Rsvd | Flags | Length | Sequence Number |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	In the above diagram the first 7 bits are reserved for future use.		In the above diagram the first 4 bits are reserved for future use.
	They MUST be set to 0 when transmitting, and MUST be ignored upon		They MUST be set to 0 when transmitting, and MUST be ignored upon
	receipt. The T bit is used in ATM encapsulations only, and MUST be		receipt.
	set to zero for other encapsulations. The length byte is set as
	specified above.
	The next 16 bits are the sequence number that is used to guarantee		The next 4 bits provide space for carrying protocol specific flags.
	ordered packet delivery. For a given VC label, and a given pair of		These are defined in the protocol-specific details below.
	LSRs, R1 and R2, where R2 has distributed that VC label to R1, the
	sequence number is initialized to 0. This is incremented by one for
	each successive packet carrying that VC label which R1 transmits to
	R2.
	The sequence number space is a 16 bit unsigned circular space. PDUs		The next 8 bits provide a length field, which is used as follows: If
	carrying the control word MUST NOT be delivered out of order. They		the packet's length (defined as the length of the layer 2 payload
	may be discarded or reordered.		plus the length of the control word) is less than 256 bytes, the
			length field MUST be set to the packet's length. Otherwise the length
			field MUST be set to zero. The value of the length field, if non-
			zero, can be used to remove any padding. When the packet reaches the
			service provider's egress LSR, it may be desirable to remove the
			padding before forwarding the packet.
	4. MTU Requirements		The next 16 bits provide a sequence number that can be used to
			guarantee ordered packet delivery. The processing of the sequence
			number field is OPTIONAL.
	The MPLS network should be configured with an MTU that is at least 12		The sequence number space is a 16 bit, unsigned circular space. The
	bytes larger then the largest frame size that will be transported in		sequence number value 0 is used to indicate an unsequenced packet.
	the LSPs. If a packet length, once it has been encapsulated on the
	ingress LSR, exceeds the LSP MTU, it MUST be dropped. If an egress
	LSR receives a packet on a VC LSP with a length, once the label stack
	and sequencing control word have been popped, that exceeds the MTU of
	the destination layer 2 interface it MUST be dropped.
	5. Protocol-Specific Issues		3.1.1. Setting the sequence number
	5.1. Frame Relay		Given a VC label V and a pair of LSRs R1 and R2, where R2 has
			distributed V to R1. If R1 supports packet sequencing then the
			following procedures should be used:
	A Frame Relay PDU is transported in its entirety, including the Frame		- the initial packet transmitted to label V MUST use sequence
	Relay header. The sequencing control word is OPTIONAL.		number 1
			- subsequent packets MUST increment the sequence number by one for
			each packet
	The BECN and FECN signals are carried unchanged across the network in		- when the transmit sequence number reaches the maximum 16 bit
	the Frame Relay header. These signals do not appear in the MPLS		value (65535) the sequence number MUST wrap to 1
	header, and are unseen by the MPLS network. The Label Edge Routers
	that implement this document MAY, when either adding or removing the
	encapsulation described herein, change a zero to a one in either or
	both of these bits in order to reflect congestion in the MPLS network
	that is known to the LERs. The BECN and FECN bits MUST NEVER be
	changed from one to zero.
	The ingress LSR MAY consider the DE bit of the Frame Relay header		If the transmitting LSR R1 does not support sequence number
	when determining the value to be placed in the EXP fields of the MPLS		processing, then the sequence number field in the control word MUST
	label stack. In a similar way, the egress LSR MAY consider the EXP		be set to 0.
	field of the VC label when queuing the packet for egress.
	5.2. ATM		3.1.2. Processing the sequence number
	Two encapsulations are supported for ATM transport: one for AAL5		If an LSR R2 supports receive sequence number processing, then the
	CPCS-PDUs and another for ATM cells. The AAL5 CPCS-PDU encapsulation		following procedures should be used:
	consists of the MPLS label stack, an optional sequencing control
	word, and the AAL5 CPCS-PDU. The ATM cell encapsulation consists of		When a VC label V is first distributed, the "expected sequence
	an MPLS label stack, a required generic sequencing control word, a 4		number" associated with V MUST be initialized to 1
	byte ATM cell header, and the ATM cell payload as shown below:
			When a packet is received with label V the sequence number should be
			processed as follows:
			- if the sequence number on the packet is 0, then the packet passes
			the sequence number check
			- Otherwise if the packet sequence number >= the expected sequence
			number (using an unsigned comparison, modulo 2**16), then the
			packet is in order.
			- otherwise the packet is out of order.
			If a packet passes the sequence number check, or is in order then, it
			can be delivered immediately. If the packet is in order, then the
			expected sequence number should be set using the algorithm:
			expected_sequence_number := packet_sequence_number + 1 mod 2**16
			if (expected_sequence_number = 0) then expected_sequence_number := 1;
			Packets which are received out of order MAY be dropped or reordered
			at the discretion of the receiver.
			If an LSR R2 does not support receive sequence number processing,
			then the sequence number field MAY be ignored.
			3.2. MTU Requirements
			The MPLS network MUST be configured with an MTU that is sufficient to
			transport the largest frame size that will be transported in the
			LSPs. Note that this is likely to be 12 or more bytes greater than
			the largest frame size. If a packet length, once it has been
			encapsulated on the ingress LSR, exceeds the LSP MTU, it MUST be
			dropped. If an egress LSR receives a packet on a VC LSP with a
			length, once the label stack and control word have been popped, that
			exceeds the MTU of the destination layer 2 interface, it MUST be
			dropped.
			3.3. MPLS Shim EXP Bit Values
			The ingress LSR, R1, SHOULD set the EXP field of the VC label to the
			same value as the EXP field of the previous label in the stack (if in
			fact a stack of more than one label is imposed at the ingress.) This
			will ensure that the EXP field will be visible to the egress LSR, R2,
			in the event of the packet having been penultimate hop popped.
			3.4. MPLS Shim TTL Values
			The ingress LSP, R1, MAY set the TTL field of the VC label to a value
			of 2.
			4. Protocol-Specific Details
			4.1. Frame Relay
			A Frame Relay PDU is transported without the Frame Relay header or
			the FCS. The sequencing control word is REQUIRED.
			The BECN, FECN, DE and C/R bits are carried across the network in the
			control word. The edge LSRs that implement this document MAY, when
			either adding or removing the encapsulation described herein, change
			the BECN and/or FECN bits from zero to one in order to reflect
			congestion in the MPLS network that is known to the edge LSRs, and
			the D/E bit from zero to one to reflect marking from edge policing of
			the Frame Relay Committed Information Rate. The BECN, FECN, and D/E
			bits MUST NOT be changed from one to zero.
			The following is an example of a Frame Relay packet:
	0 1 2 3		0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Reserved |T| Length | Sequence Number |		| VC Label | EXP |S| TTL |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| VPI | VCI | PTI |C|		| Rsvd |B|F|D|C| Length | Sequence Number |
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			| Frame Relay PDU |
			| " |
			| " |
			| " |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Two transport modes are supported for ATM transport, VCC transport		* B ( BECN ) Bit
	and VPC transport.
	VCC transport mode may be used to transport ATM Adaptation Layer 5		The ingress LSR, R1, MUST copy the BECN field from the incoming
	(AAL5) CPCS-PDUs, ATM cells, or both, across the VC LSP. The		Frame Relay header into this field. The egress LSR, R2, MUST
	sequencing control word is optional for VCC transport if only AAL5		generate a new BECN field based on the value of the B bit.
	CPCS-PDUs are to be transported. If ATM cells, or a combination of
	ATM cells and AAL5 CPCS-PDUs, are to be transported the sequencing
	control word is required.
	VPC transport mode may only be used to transport ATM cells. The		* F ( FECN ) Bit
	sequencing control word is optional in VPC transport mode.
			The ingress LSR, R1, MUST copy the FECN field from the incoming
			Frame Relay header into this field. The egress LSR, R2, MUST
			generate a new FECN field based on the value of the F bit.
			* D ( DE ) Bit
			The ingress LSR, R1, MUST copy the DE field from the incoming
			Frame Relay header into this field. The egress LSR, R2, MUST
			generate a new DE field based on the value of the D bit.
			The ingress LSR, R1, MAY consider the DE bit of the Frame Relay
			header when determining the value to be placed in the EXP field
			of the MPLS label stack. In a similar way, the egress LSR, R2,
			MAY consider the EXP field of the VC label when queuing the
			packet for egress. Note however that frames from the same VC MUST
			NOT be reordered by the MPLS network.
			* C ( C/R ) Bit
			The ingress LSR, R1, MUST copy the C/R bit from the received
			Frame Relay PDU to the C bit of the control word. The egress
			LSR, R2, MUST copy the C bit into the output frame.
			The Label, EXP, S, and TTL fields are described in [2].
			4.2. ATM
			Two encapsulations are supported for ATM transport: one for ATM AAL5
			and another for ATM cells.
			The AAL5 CPCS-PDU encapsulation consists of the MPLS label stack, a
			REQUIRED control word, and the AAL5 CPCS-PDU.
			The ATM cell encapsulation consists of an MPLS label stack, an
			OPTIONAL sequencing control word, a 4 byte ATM cell header, and the
			ATM cell payload.
			4.2.1. ATM AAL5 CPCS-PDU Mode
			In ATM AAL5 mode the ingress LSR is required to reassemble AAL5
			CPCS-PDUs from the incoming VC and transport each CPCS-PDU as a
			single packet. No AAL5 trailer is transported. The sequencing control
			word is REQUIRED.
			The EFCI and CLP bits are carried across the network in the control
			word. The edge LSRs that implement this document MAY, when either
			adding or removing the encapsulation described herein, change the
			EFCI bit from zero to one in order to reflect congestion in the MPLS
			network that is known to the edge LSRs, and the CLP bit from zero to
			one to reflect marking from edge policing of the ATM Sustained Cell
			Rate. The EFCI and CLP bits MUST NOT be changed from one to zero.
			The AAL5 CPCS-PDU is prepended by the following header:
			0 1 2 3
			0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			| VC Label | EXP |S| TTL |
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			| Rsvd |T|E|L|C| Length | Sequence Number |
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			| ATM AAL5 CPCS-PDU |
			| " |
			| " |
			| " |
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	* T (transport type) bit		* T (transport type) bit
	In VCC transport mode, bit (T) of the sequencing control word		Bit (T) of the control word indicates whether the MPLS packet
	indicates whether the MPLS packet contains an ATM cell or an AAL5		contains an ATM cell or an AAL5 CPCS-PDU. If set the packet
	CPCS-PDU. If set the packet contains an ATM cell, otherwise it		contains an ATM cell, encapsulated according to the ATM cell mode
	contains an AAL5 CPCS-PDU. In VPC transport mode this bit MUST be		section below, otherwise it contains an AAL5 CPCS-PDU. The
	set to 1.		ability to transportan ATM cell in the AAL5 mode is intended to
			provide a means of enabling OAM functionality over the AAL5 VC.
			* E ( EFCI ) Bit
			The ingress LSR, R1, SHOULD set this bit to 1 if the EFCI bit of
			the final cell of those that transported the AAL5 CPCS-PDU is set
			to 1, or if the EFCI bit of the single ATM cell to be transported
			in the MPLS packet is set to 1. Otherwise this bit SHOULD be set
			to 0. The egress LSR, R2, SHOULD set the EFCI bit of all cells
			that transport the AAL5 CPCS-PDU to the value contained in this
			field.
			* L ( CLP ) Bit
			The ingress LSR, R1, SHOULD set this bit to 1 if the CLP bit of
			any of the ATM cells that transported the AAL5 CPCS-PDU is set to
			1, or if the CLP bit of the single ATM cell to be transported in
			the MPLS packet is set to 1. Otherwise this bit SHOULD be set to
			0. The egress LSR, R2, SHOULD set the CLP bit of all cells that
			transport the AAL5 CPCS-PDU to the value contained in this field.
			The ingress LSR, R1, MAY consider the CLP bit of the ATM cell
			header when determining the value to be placed in the EXP fields
			of the MPLS label stack. In a similar way, the egress LSR, R2,
			MAY consider the EXP field of the VC label when queuing the
			packet for egress. Note however that frames from the same VC MUST
			NOT be reordered by the MPLS network.
			* C ( Command / Response Field ) Bit
			When FRF.8.1 Frame Relay / ATM PVC Service Interworking [3]
			traffic is being transported, the CPCS-UU Least Significant Bit
			(LSB) of the AAL5 CPCS-PDU may contain the Frame Relay C/R bit.
			The ingress LSR, R1, SHOULD copy this bit to the C bit of the
			control word. The egress LSR, R2, SHOULD copy the C bit to the
			CPCS-UU Least Significant Bit (LSB) of the AAL5 CPCS PDU.
			The Label, EXP, S, and TTL fields are described in [2].
			4.2.2. ATM Cell Mode
			In this encapsulation mode ATM cells are transported individually
			without a SAR process. Each ATM cell payload is prepended by a 4 byte
			header and concatenated to form the MPLS frame. This ATM cell header
			is defined as in the FAST encapsulation [4] section 3.1.1, but
			without the trailer byte. The length of each frame, without the MPLS
			header and the control word, is a multiple of 52 bytes long. The
			maximum number of ATM cells that can be fitted in an MPLS frame, in
			this fashion, is limited only by the MPLS network MTU and by the
			ability of the egress LSR to process them. The ingress LSR MUST NOT
			send more cells than the egress LSR is willing to receive. The number
			of cells that the egress LSR is willing to receive may either be
			configured in the ingress LSR or may be signaled, for example using
			the methods described in [1]. The number of cells encapsulated in a
			particular frame can be inferred by the frame length. The sequencing
			control word is OPTIONAL. If the control word is used then the flag
			bits in the control word are not used, and MUST be set to 0 when
			transmitting, and MUST be ignored upon receipt.
			The EFCI and CLP bits are carried across the network in the ATM cell
			header. The edge LSRs that implement this document MAY, when either
			adding or removing the encapsulation described herein, change the
			EFCI bit from zero to one in order to reflect congestion in the MPLS
			network that is known to the edge LSRs, and the CLP bit from zero to
			one to reflect marking from edge policing of the ATM Sustained Cell
			Rate. The EFCI and CLP bits MUST NOT be changed from one to zero.
			This diagram illustrates an encapsulation of two ATM cells:
			0 1 2 3
			0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			| VC Label | EXP |S| TTL |
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			| Control word ( Optional ) |
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			| VPI | VCI | PTI |C|
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			| ATM Payload ( 48 bytes ) |
			| " |
			| " |
			| " |
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			| VPI | VCI | PTI |C|
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			| ATM Payload ( 48 bytes ) |
			| " |
			| " |
			| " |
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	* VPI		* VPI
	In both modes the ingress router MAY copy the VPI field from the		The ingress router MUST copy the VPI field from the incoming cell
	incoming cell into this field. The egress router MAY generate a		into this field. The egress router MAY generate a new VPI based
	new VPI based on the value of the VC label and ignore the VPI		on the value of the VC label and ignore the VPI contained in this
	contained in this field.		field.
	* VCI		* VCI
	In VCC transport mode the ingress router MAY copy the VCI field		The ingress router MUST copy the VCI field from the incoming ATM
	from the incoming ATM cell header into this field and the egress		cell header into this field. The egress router MAY generate a
	router MAY generate a new VCI based on the value of the VC label.		new VCI based on the value of the VC label.
	When in VPC transport mode the ingress LSR MUST copy the VCI
	field from the incoming cell into this field and the egress LSR
	MUST copy the VCI from this field into the outgoing ATM cell
	header.
	* PTI & CLP		* PTI & CLP ( C bit )
	When present the ingress router SHOULD copy the PTI and CLP		The PTI and CLP fields are the PTI and CLP fields of the incoming
	fields of the outgoing frame from the ATM cell header and the		ATM cells. The cell headers of the cells within the packet are
	egress router SHOULD set the left-most and EFCI bits of the PTI		the ATM headers (without HEC) of the incoming cell.
	in all outgoing cells to that contained in the PTI field of the
	FAST header and set the CLP bit of outgoing cells to the CLP bit
	contained in the FAST header. [7]
	5.2.1. OAM Cell Support		4.2.3. OAM Cell Support
	OAM cells MAY be transported on the VC LSP. A router that does not		OAM cells MAY be transported on the VC LSP. A router that does not
	support transport of OAM cells MUST discard incoming MPLS frames on		support transport of OAM cells MUST discard incoming MPLS frames on
	an ATM VC LSP that contain an ATM cell with the high-order bit of the		an ATM VC LSP that contain an ATM cell with the high-order bit of the
	PTI field set to 1. A router that supports transport of OAM cells		PTI field set to 1. A router that supports transport of OAM cells
	MUST follow the procedures outlined in [7] section 8 for mode 0 only,		MUST follow the procedures outlined in [4] section 8 for mode 0 only,
	in addition to the applicable procedures specified in [5].		in addition to the applicable procedures specified in [1].
	A router that does not support transport of OAM cells across an LSP
	may provide OAM support on ATM PVCs using the following procedures:
	If an F5 end-to-end OAM cell is received from a VC by an LSR with a
	loopback indication value of 1 and the LSR has a label mapping for
	the VC, the LSR must decrement the loopback indication value and loop
	back the cell on the VC. Otherwise the loopback cell must be
	discarded by the LSR.
	The LSR may optionally be configured to periodically generate F5
	end-to-end loopback OAM cells on a VC. In this case, the LSR MUST
	only generate F5 end-to-end loopback cells while a label mapping
	exists for the VC. If the VC label mapping is withdrawn the LSR MUST
	cease generation of F5 end-to-end loopback OAM cells. If the LSR
	fails to receive a response to an F5 end-to-end loopback OAM cell for
	a pre-defined period of time it must withdraw the label mapping for
	the VC.
	If an ingress LSR receives an AIS F5 OAM cell, fails to receive a
	pre-defined number of the End-to-End loop OAM cells, or a physical
	interface goes down, it MUST withdraw the label mappings for all VCs
	associated with the failure. When a VC label mapping is withdrawn,
	the egress LSR MUST generate AIS F5 OAM cells on the VC associated
	with the withdrawn label mapping.
	5.2.2. CLP Bit to EXP Bit Mapping		4.2.4. CLP bit to MPLS label stack EXP bit mapping
	The ingress LSR MAY consider the CLP bit when determining the value		The ingress LSR MAY consider the CLP bit when determining the value
	to be placed in the EXP fields of the MPLS label stack.		to be placed in the EXP fields of the MPLS label stack. This will
			give the MPLS network visibility of the CLP bit. Note however that
	The egress LSR MAY consider the value of the EXP field of the VC		cells from the same VC MUST NOT be reordered by the MPLS network.
	label when determining the value of the ATM CLP bit.
	5.3. Ethernet VLAN		4.3. Ethernet VLAN
	For an ethernet 802.1q VLAN the entire ethernet frame without the		For an Ethernet 802.1q VLAN the entire Ethernet frame without the
	preamble or FCS is transported as a single packet. The sequencing		preamble or FCS is transported as a single packet. The sequencing
	control word is OPTIONAL. If a packet is received out of sequence it		control word is OPTIONAL. If the control word is used then the flag
	MUST be dropped. The 4 byte VLAN tag is transported as is, and MAY be		bits in the control word are not used, and MUST be set to 0 when
	overwritten by the egress LSR.		transmitting, and MUST be ignored upon receipt. The 4 byte VLAN tag
			is transported as is, and MAY be overwritten by the egress LSR.
	The ingress LSR MAY consider the user priority field [4] of the VLAN		The ingress LSR MAY consider the user priority field [5] of the VLAN
	tag header when determining the value to be placed in the EXP fields		tag header when determining the value to be placed in the EXP fields
	of the MPLS label stack. In a similar way, the egress LSR MAY		of the MPLS label stack. In a similar way, the egress LSR MAY
	consider the EXP field of the VC label when queuing the packet for		consider the EXP field of the VC label when queuing the packet for
	egress. Ethernet packets containing hardware level CRC, Framing		egress. Ethernet packets containing hardware level CRC errors,
	errors, or runt packets MUST be discarded on input.		framing errors, or runt packets MUST be discarded on input.
	5.4. Ethernet		4.4. Ethernet
	For simple ethernet port to port transport, the entire ethernet frame		For simple Ethernet port to port transport, the entire Ethernet frame
	without the preamble or FCS is transported as a single packet. The		without the preamble or FCS is transported as a single packet. The
	sequencing control word is OPTIONAL. If a packet is received out of		sequencing control word is OPTIONAL. If the control word is used then
	sequence it MUST be dropped. As in the Ethernet VLAN case, ethernet		the flag bits in the control word are not used, and MUST be set to 0
	packets with hardware level CRC, framing, and runt packets MUST be		when transmitting, and MUST be ignored upon receipt. As in the
	discarded on input.		Ethernet VLAN case, Ethernet packets with hardware level CRC errors,
			framing errors, and runt packets MUST be discarded on input.
	5.5. HDLC ( Cisco )		4.5. HDLC ( Cisco )
	HDLC (Cisco) mode provides port to port transport of Cisco HDLC		HDLC (Cisco) mode provides port to port transport of Cisco HDLC
	encapsulated traffic. The HDLC PDU is transported in its entirety,		encapsulated traffic. The HDLC PDU is transported in its entirety,
	including the HDLC address, control and protocol fields, but		including the HDLC address, control and protocol fields, but
	excluding HDLC flags and the FCS. Bit stuffing is undone. The		excluding HDLC flags and the FCS. Bit stuffing is undone. The
	sequencing control word is OPTIONAL.		sequencing control word is OPTIONAL.
	5.6. PPP		4.6. PPP
	PPP mode provides point to point transport of PPP encapsulated		PPP mode provides point to point transport of PPP encapsulated
	traffic, as specified in [8]. The PPP PDU is transported in its		traffic, as specified in [6]. The PPP PDU is transported in its
	entirety, including the protocol field, but excluding any media-		entirety, including the protocol field, but excluding any media-
	specific framing information, such as HDLC address and control fields		specific framing information, such as HDLC address and control fields
	or FCS. The sequencing control word is OPTIONAL.		or FCS. The sequencing control word is OPTIONAL.
	6. Security Considerations		5. Security Considerations
	This document does not affect the underlying security issues of MPLS.		This document does not affect the underlying security issues of MPLS.
	7. Intellectual Property Disclaimer		6. Intellectual Property Disclaimer
	This document is being submitted for use in IETF standards		This document is being submitted for use in IETF standards
	discussions.		discussions.
	8. References		7. References
	[1] "LDP Specification", draft-ietf-mpls-ldp-11.txt ( work in
	progress )
	[2] ITU-T Recommendation Q.933, and Q.922 Specification for Frame
	Mode Basic call control, ITU Geneva 1995
	[3] "MPLS Label Stack Encoding", draft-ietf-mpls-label-encaps-08.txt
	( work in progress )
	[4] "IEEE 802.3ac-1998" IEEE standard specification.		[1] "Transport of Layer 2 Frames Over MPLS", draft-martini-
			l2circuit-trans-mpls-05.txt. ( work in progress )
	[5] "Transport of Layer 2 Frames Over MPLS", draft-martini-		[2] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D. Tappan, G.
	l2circuit-trans-mpls-04.txt. ( work in progress )		Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032
	[6] ITU Recommendation I.610, "B-ISDN operation and maintenance		[3] "Frame Relay / ATM PVC Service Interworking Implementation
	principles and functions", 1999.		Agreement", Frame Relay Forum 2000.
	[7] "Frame Based ATM over SONET/SDH Transport (FAST)," 2000.		[4] "Frame Based ATM over SONET/SDH Transport (FAST)," 2000.
	[8] "The Point-to-Point Protocol (PPP)", RFC 1661.		[5] "IEEE 802.3ac-1998" IEEE standard specification.
	[note1] FEC element type 128 is pending IANA approval.		[6] "The Point-to-Point Protocol (PPP)", RFC 1661.
	9. Author Information		8. Author Information
	Luca Martini		Luca Martini
	Level 3 Communications, LLC.		Level 3 Communications, LLC.
	1025 Eldorado Blvd.		1025 Eldorado Blvd.
	Broomfield, CO, 80021		Broomfield, CO, 80021
	e-mail: luca@level3.net		e-mail: luca@level3.net
	Nasser El-Aawar		Nasser El-Aawar
	Level 3 Communications, LLC.		Level 3 Communications, LLC.
	1025 Eldorado Blvd.		1025 Eldorado Blvd.
	skipping to change at page 9, line 28 ¶		skipping to change at page 14, line 28 ¶
	Giles Heron		Giles Heron
	Level 3 Communications		Level 3 Communications
	66 Prescot Street		66 Prescot Street
	London		London
	E1 8HG		E1 8HG
	United Kingdom		United Kingdom
	e-mail: giles@level3.net		e-mail: giles@level3.net
	Dimitri Stratton Vlachos		Dimitri Stratton Vlachos
	Cisco Systems, Inc.		Mazu Networks, Inc.
	250 Apollo Drive		125 Cambridgepark Drive
	Chelmsford, MA, 01824		Cambridge, MA 02140
	e-mail: dvlachos@cisco.com		e-mail: d@mazunetworks.com
	Dan Tappan		Dan Tappan
	Cisco Systems, Inc.		Cisco Systems, Inc.
	250 Apollo Drive		250 Apollo Drive
	Chelmsford, MA, 01824		Chelmsford, MA, 01824
	e-mail: tappan@cisco.com		e-mail: tappan@cisco.com
			Jayakumar Jayakumar,
			Cisco Systems Inc.
			225, E.Tasman, MS-SJ3/3,
			San Jose , CA, 95134
			e-mail: jjayakum@cisco.com
			Alex Hamilton,
			Cisco Systems Inc.
			285 W. Tasman , MS-SJCI/3/4,
			San Jose, CA, 95134
			e-mail: tahamilt@cisco.com
	Eric Rosen		Eric Rosen
	Cisco Systems, Inc.		Cisco Systems, Inc.
	250 Apollo Drive		250 Apollo Drive
	Chelmsford, MA, 01824		Chelmsford, MA, 01824
	e-mail: erosen@cisco.com		e-mail: erosen@cisco.com
	Steve Vogelsang		Steve Vogelsang
	Laurel Networks, Inc.		Laurel Networks, Inc.
	2607 Nicholson Rd.		2607 Nicholson Rd.
	Sewickley, PA 15143		Sewickley, PA 15143
	e-mail: sjv@laurelnetworks.com		e-mail: sjv@laurelnetworks.com
	John Shirron		John Shirron
	Laurel Networks, Inc.		Laurel Networks, Inc.
	2607 Nicholson Rd.		2607 Nicholson Rd.
	Sewickley, PA 15143		Sewickley, PA 15143
	e-mail: jshirron@laurelnetworks.com		e-mail: jshirron@laurelnetworks.com
			Toby Smith
			Laurel Networks, Inc.
			2607 Nicholson Rd.
			Sewickley, PA 15143
			e-mail: tob@laurelnetworks.com
	Andrew G. Malis		Andrew G. Malis
	Vivace Networks, Inc.		Vivace Networks, Inc.
	2730 Orchard Parkway		2730 Orchard Parkway
	San Jose, CA 95134		San Jose, CA 95134
	Phone: +1 408 383 7223		e-mail: Andy.Malis@vivacenetworks.com
	Email: Andy.Malis@vivacenetworks.com
			Vinai Sirkay
			Vivace Networks, Inc.
			2730 Orchard Parkway
			San Jose, CA 95134
			e-mail: vinai.sirkay@vivacenetworks.com
			Kireeti Kompella
			Juniper Networks
			1194 N. Mathilda Ave
			Sunnyvale, CA 94089
			e-mail: kireeti@juniper.net
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