< draft-ietf-mpls-fr-05.txt		draft-ietf-mpls-fr-06.txt >
			A new Request for Comments is now available in online RFC libraries.
	MPLS Working Group A. Conta (3COM)		RFC 3034
	INTERNET-DRAFT P. Doolan (Ennovate)
	A. Malis (Lucent)
	13 June 2000
	Expires 13 December 2000
	Use of Label Switching on Frame Relay Networks
	Specification
	draft-ietf-mpls-fr-05.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
	This document defines the model and generic mechanisms for
	Multiprotocol Label Switching on Frame Relay networks. Furthermore,
	it extends and clarifies portions of the Multiprotocol Label
	Switching Architecture described in [ARCH] and the Label Distribution
	Protocol (LDP) described in [LDP] relative to Frame Relay Networks.
	MPLS enables the use of Frame Relay Switches as Label Switching
	Routers (LSRs).
	Table of Contents
	Status of this Memo.........................................1
	Table of Contents...........................................2
	1. Introduction................................................3
	2. Terminology.................................................3
	3. Special Characteristics of Frame Relay Switches.............5
	4. Label Encapsulation.........................................5
	5. Frame Relay Label Switching Processing......................7
	5.1 Use of DLCIs..............................................7
	5.2 Homogeneous LSPs..........................................8
	5.3 Heterogeneous LSPs........................................8
	5.4 Frame Relay Label Switching Loop Prevention and Control...8
	5.4.1 FR-LSRs Loop Control - MPLS TTL Processing.............9
	5.4.2 Performing MPLS TTL calculations......................10
	5.5 Label Processing by Ingress FR-LSRs......................13
	5.6 Label Processing by Core FR-LSRs.........................14
	5.7 Label Processing by Egress FR-LSRs.......................14
	6 Label Switching Control Component for Frame Relay..........15
	6.1 Hybrid Switches (Ships in the Night) ...................16
	7 Label Allocation and Maintenance Procedures ...............16
	7.1 Edge LSR Behavior........................................16
	7.2 Efficient use of label space-Merging FR-LSRs.............19
	7.3 LDP message fields specific to Frame Relay...............20
	8 Security Considerations ..................................22
	9 Acknowledgments ..........................................23
	10 References ...............................................23
	11 Authors' Addresses .......................................24
	Appendix A - changes since previous versions..................25
	1. Introduction
	The Multiprotocol Label Switching Architecture is described in
	[ARCH]. It is possible to use Frame Relay switches as Label Switching
	Routers. Such Frame Relay switches run network layer routing
	algorithms (such as OSPF, IS-IS, etc.), and their forwarding is based
	on the results of these routing algorithms. No specific Frame Relay
	routing is needed.
	When a Frame Relay switch is used for label switching, the top
	(current) label, on which forwarding decisions are based, is carried
	in the DLCI field of the Frame Relay data link layer header of a
	frame. Additional information carried along with the top (current)
	label, but not processed by Frame Relay switching, along with other
	labels, if the packet is multiply labeled, are carried in the generic
	MPLS encapsulation defined in [STACK].
	Frame Relay permanent virtual circuits (PVCs) could be configured to
	carry label switching based traffic. The DLCIs would be used as MPLS
	Labels and the Frame Relay switches would become Frame Relay Label
	Switching Routers, while the MPLS traffic would be encapsulated
	according to this specification, and would be forwarded based on
	network layer routing information.
	The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED,
	SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as
	defined in RFC 2119.
	This document is a companion document to [STACK] and [ATM].
	2. Terminology
	LSR
	A Label Switching Router (LSR) is a device which implements the
	label switching control and forwarding components described in
	[ARCH].
	LC-FR
	A label switching controlled Frame Relay (LC-FR) interface is a
	Frame Relay interface controlled by the label switching control
	component. Packets traversing such an interface carry labels in
	the DLCI field.
	FR-LSR
	A FR-LSR is an LSR with one or more LC-FR interfaces which
	forwards frames between two such interfaces using labels carried
	in the DLCI field.
	FR-LSR domain
	A FR-LSR domain is a set of FR-LSRs, which are mutually
	interconnected by LC-FR interfaces.
	Edge Set
	The Edge Set of an FR-LSR domain is the set of LSRs, which are
	connected to the domain by LC-FR interfaces.
	Forwarding Encapsulation
	The Forwarding Encapsulation is the type of MPLS encapsulation
	(Frame Relay, ATM, Generic) of a packet that determines the
	packet's MPLS forwarding, or the network layer encapsulation if
	that packet is forwarded based on the network layer (IP,
	etc...)header.
	Input Encapsulation
	The Input Encapsulation is the type of MPLS encapsulation (Frame
	Relay, ATM, Generic) of a packet when that packet is received on
	an LSR's interface, or the network layer (IP,
	etc...)encapsulation if that packet has no MPLS encapsulation.
	Output Encapsulation
	The Output Encapsulation is the type of MPLS encapsulation
	(Frame Relay, ATM, Generic) of a packet when that packet is
	transmitted on an LSR's interface, or the network layer (IP,
	etc...)encapsulation if that packet has no MPLS encapsulation.
	Input TTL
	The Input TTL is the MPLS TTL of the top of the stack when a
	labeled packet is received on an LSR interface, or the network
	layer (IP) TTL if the packet is not labeled.
	Output TTL
	The Output TTL is the MPLS TTL of the top of the stack when a
	labeled packet is transmitted on an LSR interface, or the
	network layer (IP) TTL if the packet is not labeled.
	Additionally, this document uses terminology from [ARCH].
	3. Special characteristics of Frame Relay Switches
	While the label switching architecture permits considerable
	flexibility in LSR implementation, a FR-LSR is constrained by the
	capabilities of the (possibly pre-existing) hardware and the
	restrictions on such matters as frame format imposed by the
	Multiprotocol Interconnect over Frame Relay [MIFR], or Frame Relay
	standards [FRF], etc.... Because of these constraints, some special
	procedures are required for FR-LSRs.
	Some of the key features of Frame Relay switches that affect their
	behavior as LSRs are:
	- the label swapping function is performed on fields (DLCI) in the
	frame's Frame Relay data link header; this dictates the size and
	placement of the label(s) in a packet. The size of the DLCI
	field can be 10 (default) or 23 bits, and it can span two or
	four bytes in the header.
	- there is generally no capability to perform a `TTL-decrement'
	function as is performed on IP headers in routers.
	- congestion control is performed by each node based on parameters
	that are passed at circuit creation. Flags in the frame headers
	may be set as a consequence of congestion, or exceeding the
	contractual parameters of the circuit.
	- although in a standard switch it may be possible to configure
	multiple input DLCIs to one output DLCI resulting in a
	multipoint-to-point circuit, multipoint-to-multipoint VCs are
	generally not fully supported.
	This document describes ways of applying label switching to Frame
	Relay switches, which work within these constraints.
	4. Label Encapsulation
	By default, all labeled packets should be transmitted with the
	generic label encapsulation as defined in [STACK], using the frame
	relay null encapsulation mechanism:
	0 1 (Octets)
	+-----------------------+-----------------------+
	(Octets)0 | |
	/ Q.922 Address /
	/ (length 'n' equals 2 or 4) /
	| |
	+-----------------------+-----------------------+
	n | . |
	/ . /
	/ MPLS packet /
	| . |
	+-----------------------+-----------------------+
	"n" is the length of the Q.922 Address which can be 2 or 4
	octets.
	The Q.922 [ITU] representation of a DLCI (in canonical order -
	the first bit is stored in the least significant, i.e., the
	right-most bit of a byte in memory) [CANON]is the following:
	7 6 5 4 3 2 1 0 (bit order)
	+-----+-----+-----+-----+-----+-----+-----+-----+
	(octet) 0 | DLCI(high order) | 0 | 0 |
	+-----+-----+-----+-----+-----+-----+-----+-----+
	1 | DLCI(low order) | 0 | 0 | 0 | 1 |
	+-----+-----+-----+-----+-----+-----+-----+-----+
	10 bits DLCI
	7 6 5 4 3 2 1 0 (bit order)
	+-----+-----+-----+-----+-----+-----+-----+-----00
	(octet) 0 | DLCI(high order) | 0 | 0 |
	+-----+-----+-----+-----+-----+-----+-----+-----
	1 | DLCI | 0 | 0 | 0 | 0 |
	+-----+-----+-----+-----+-----+-----+-----+-----+
	2 | DLCI | 0 |
	+-----+-----+-----+-----+-----+-----+-----+-----+
	3 | DLCI (low order) | 0 | 1 |
	+-----+-----+-----+-----+-----+-----+-----+-----+
	23 bits DLCI
	The use of the frame relay null encapsulation implies that labels
	implicitly encode the network protocol type.
	Rules regarding the construction of the label stack, and error
	messages returned to the frame source are also described in [STACK].
	The generic encapsulation contains "n" labels for a label stack of
	depth "n" [STACK], where the top stack entry carries significant
	values for the EXP, S , and TTL fields [STACK] but not for the label,
	which is rather carried in the DLCI field of the Frame Relay data
	link header encoded in Q.922 [ITU] address format.
	5. Frame Relay Label Switching Processing
	5.1 Use of DLCIs
	Label switching is accomplished by associating labels with routes and
	using the label value to forward packets, including determining the
	value of any replacement label. See [ARCH] for further details. In a
	FR-LSR, the top (current) MPLS label is carried in the DLCI field of
	the Frame Relay data link layer header of the frame. The top label
	carries implicitly information about the network protocol type.
	For two connected FR-LSRs, a full-duplex connection must be available
	for LDP. The DLCI for the LDP VC is assigned a value by way of
	configuration, similar to configuring the DLCI used to run IP routing
	protocols between the switches.
	With the exception of this configured value, the DLCI values used for
	MPLS in the two directions of the link may be treated as belonging to
	two independent spaces, i.e. VCs may be half-duplex, each direction
	with its own DLCI.
	The allowable ranges of DLCIs, the size of DLCIs, and the support for
	VC merging MUST be communicated through LDP messages. Note that the
	range of DLCIs used for labels depends on the size of the DLCI field.
	5.2 Homogeneous LSPs
	If <LSR1, LSR2, LSR3> is an LSP, it is possible that LSR1, LSR2, and
	LSR3 will use the same encoding of the label stack when transmitting
	packet P from LSR1, to LSR2, and then to LSR3. Such an LSP is
	homogeneous.
	5.3 Heterogeneous LSPs
	If <LSR1, LSR2, LSR3> is an LSP, it is possible that LSR1 will use
	one encoding of the label stack when transmitting packet P to LSR2,
	but LSR2 will use a different encoding when transmitting a packet P
	to LSR3. In general, the MPLS architecture supports LSPs with
	different label stack encodings on different hops. When a labeled
	packet is received, the LSR must decode it to determine the current
	value of the label stack, then must operate on the label stack to
	determine the new label value of the stack, and then encode the new
	value appropriately before transmitting the labeled packet to its
	next hop.
	Naturally there will be MPLS networks which contain a combination of
	Frame Relay switches operating as LSRs, and other LSRs, which operate
	using other MPLS encapsulations, such as the Generic (MPLS shim
	header), or ATM encapsulation. In such networks there may be some
	LSRs, which have Frame Relay interfaces as well as MPLS Generic
	("MPLS Shim") interfaces. This is one example of an LSR with
	different label stack encodings on different hops of the same LSP.
	Such an LSR may swap off a Frame Relay encoded label on an incoming
	interface and replace it with a label encoded into a Generic MPLS
	(MPLS shim) header on the outgoing interface.
	5.4 Frame Relay Label Switching Loop Prevention and Control
	FR-LSRs SHOULD operate on loop free FR-LSPs or LSP Frame Relay
	segments. Therefore, FR-LSRs SHOULD use loop detection and MAY use
	loop prevention mechanisms as described in [ARCH], and [LDP].
	5.4.1 FR-LSRs Loop Control - MPLS TTL processing
	The MPLS TTL encoded in the MPLS label stack is a mechanism used to:
	(a) suppress loops;
	(b) limit the scope of a packet.
	When a packet travels along an LSP, it should emerge with the same
	TTL value that it would have had if it had traversed the same
	sequence of routers without having been label switched. If the
	packet travels along a hierarchy of LSPs, the total number of LSR-
	hops traversed should be reflected in its TTL value when it emerges
	from the hierarchy of LSPs [ARCH].
	The initial value of the MPLS TTL is loaded into a newly pushed label
	stack entry from the previous TTL value, whether that is from the
	network layer header when no previous label stack existed, or from a
	pre-existent lower level label stack entry.
	A FR-LSR switching same level labeled packets does not decrement the
	MPLS TTL. A sequence of such FR-LSR is a "non-TTL segment".
	When a packet emerges from a "non-TTL LSP segment", it should however
	reflect in the TTL the number of LSR-hops it traversed. In the
	unicast case, this can be achieved by propagating a meaningful LSP
	length or LSP Frame Relay segment length to the FR-LSR ingress nodes,
	enabling the ingress to decrement the TTL value before forwarding
	packets into a non-TTL LSP segment [ARCH].
	When an ingress FR-LSR determines upon decrementing the MPLS TTL that
	a particular packet's TTL will expire before the packet reaches the
	egress of the "non-TTL LSP segment", the FR-LSR MUST not label switch
	the packet, but rather follow the specifications in [STACK] in an
	attempt to return an error message to the packet's source:
	- it treats the packet as an expired packet and return an ICMP
	message to its source.
	- it forwards the packet, as an unlabeled packet, with a TTL
	that reflects the IP (network layer) forwarding.
	If the incoming TTL is 1, only the first option applies.
	In the multicast case, a meaningful LSP length or LSP segment length
	is propagated to the FR-LSR egress node, enabling the egress to
	decrement the TTL value before forwarding packets out of the non-TTL
	LSP segment.
	5.4.2 Performing MPLS TTL calculations
	The calculation applied to the "input TTL" that yields the "output
	TTL" depends on (i)the "input encapsulation", (ii)the "forwarding
	encapsulation", and (iii)the "output encapsulation". The
	relationship among (i),(ii), and (iii), can be defined as a function
	"D" of "input encapsulation" (ie), "forwarding encapsulation" (fe),
	and "output encapsulation" (oe). Subsequently the calculation applied
	to the "input TTL" to yield the "output TTL" can be described as:
	output TTL = input TTL - D(ie, fe, oe)
	or in a brief notation:
	output TTL = input TTL - d
	where "d" has three possible values: "0","1", or "the number of hops
	of the LSP segment":
	For unicast transmission:
	+================+=================+=================+=================+
	| | Type of | Type of | Type of |
	| d | Input | Forwarding | Output |
	| | Encapsulation | Encapsulation | Encapsulation |
	+================+=================+=================+=================+
	| 0 | Frame Relay | Frame Relay | Frame Relay |
	+----------------+-----------------+-----------------+-----------------+
	| 1 | any | Generic MPLS | Generic MPLS |
	+----------------+-----------------+-----------------+-----------------+
	| number of hops | | Generic MPLS | |
	| of | any | or | Frame Relay |
	| LSP segment | |IP(network layer)| |
	+================+=================+=================+=================+
	The "number of hops of the LSP segment" is the value of the "hop
	count" that is attached with the label used when the packet is
	forwarded, if LDP [LDP] has provided such a "hop count" value when it
	distributed the label for the LSP, that is the LDP message had a "hop
	count object". If LDP didn't provide a "hop count", or it provided an
	"unknown" value, the default value of the "number of hops of the
	segment" is 1.
	When sending a label binding upstream, the "hop count" associated
	with the corresponding binding from downstream, if different than the
	"unknown" value, MUST be incremented by 1, and the result transmitted
	upstream as the hop count associated with the new binding (the
	"unknown" value is transmitted unchanged). If the new "hop count"
	value exceeds the "maximum" value, the FR-LSR MUST NOT pass the
	binding upstream, but instead MUST send an error upstream
	[LDP][ARCH].
	For multicast transmission:
	+================+=================+=================+=================+
	| | Type of | Type of | Type of |
	| d | Input | Forwarding | Output |
	| | Encapsulation | Encapsulation | Encapsulation |
	+================+=================+=================+=================+
	| 0 | Frame Relay | Frame Relay | Frame Relay |
	+----------------+-----------------+-----------------+-----------------+
	| | | Generic MPLS | |
	| 1 | any | or | Frame Relay |
	| | |IP(network layer)| |
	+----------------+-----------------+-----------------+-----------------+
	| number of hops | | Generic MPLS | |
	| of | Frame Relay | or | any |
	| LSP segment | |IP(network layer)| |
	+================+=================+=================+=================+
	Referring to the "forwarding encapsulation" with the abbreviation "I"
	for IP (network layer), "G" for Generic MPLS, and "F" for Frame
	Relay MPLS, referring to an LSR interface with the abbreviation "i"
	if the input or output encapsulation is IP and no MPLS encapsulation,
	"g" when the input or output MPLS encapsulation is Generic MPLS, "f"
	when it is Frame Relay, "a" when it is ATM, and furthermore
	considering the symbols "iIf", "gGf", "fFf", etc... as LSRs with
	input, forwarding and output encapsulations as referred above, the
	following describes examples of TTL calculations for the Homogeneous
	and Heterogeneous LSPs discussed in previous sections:
	Homogeneous LSP
	---------------
	IP_ttl = n IP_ttl=mpls_ttl-1 = n-6
	--------->iIf fIi--------->
	| mpls_ttl = n-5 ^
	| |
	number of hops 1| Frame Relay |5
	| |
	V 2 3 4 |
	fFf--->fFf--->fFf--->fFf
	"iIf" is "ingress LSR" in Frame Relay LSP and
	calculates: mpls_ttl = IP_TTL - number of hops = n-5
	"fIi" is "egress LSR" from Frame Relay LSP, and
	calculates: IP_ttl = mpls_ttl-1 = n-6
	Heterogeneous LSP
	-----------------
	ingress LSR egress LSR
	IP_ttl = n IP_ttl = n - 15
	links LAN PPP FR ATM PPP FR LAN
	--->iIg-->gGg-->gGf fGa aGg-->gGf fGg-->gIi--->
	hops 1 2 | 6 | | 9 | 10 | 13 ^ 14 15
	|1 4| |1 3| |1 3|
	V 2 3 | V 2 | V 2 |
	fFf-->fFf-->fFf aAa-->aAa fFf-->fFf
	mpls_ttl
	n-1 n-2 (n-2)-4=n-6 (n-6)-3=n-9 n-10 n-13 n-14
	"iIg" is "ingress LSR" in LSP; it calculates: mpls_ttl=n-1
	"gGf" is "egress LSR" from Generic MPLS segment, and
	"ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-6
	"fGa" "egress LSR" from Frame Relay segment, and
	"ingress LSR" in ATM segment and calculates: mpls_ttl=n-9
	"gGf" is "egress LSR" from Generic MPLS segment, and
	"ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-13
	"fGg" is "egress LSR" from Frame Relay segment, and
	ingress LSR" in Generic MPLS segment and calculates: mpls_ttl=n-14
	"gIi" is "egress LSR" from LSP and calculates: IP_ttl=n-15
	And further examples:
	Frame Relay Unicast -- TTL calculated at ingress
	(ingress LSR) 1 2 3 4
	x--->---+--->---+--->>--+-->>---x (egress LSR)
	o.ttl=i.ttl-4 | 2 3
	^
	hops 1|
	|
	x (ingress LSR)
	o.ttl=i.ttl-3
	Frame Relay Multicast -- TTL calculated at egress
	(egress LSR)x o.ttl=i.ttl-3
	hops |
	^3
	(ingress LSR) | o.ttl=i.ttl-4
	x--->---+--->---+--->---+--->---x (egress LSR)
	1 2 3 4
	5.5 Label Processing by Ingress FR-LSRs
	When a packet first enters an MPLS domain, the packet is forwarded by
	normal network layer forwarding operations with the exception that
	the outgoing encapsulation will include an MPLS label stack [STACK]
	with at least one entry. The frame relay null encapsulation will
	carry information about the network layer protocol implicitly in the
	label, which MUST be associated only with that network protocol. The
	TTL field in the top label stack entry is filled with the network
	layer TTL (or hop limit) resulted after network layer forwarding
	[STACK]. The further FR-LSR processing is similar in both possible
	cases:
	(a) the LSP is homogeneous -- Frame Relay only -- and the FR-LSR is
	the ingress.
	(b) the LSP is heterogeneous -- Frame Relay, PPP, Ethernet, ATM,
	etc... segments form the LSP -- and the FR-LSR is the ingress into a
	Frame Relay
	segment.
	For unicast packets, the MPLS TTL SHOULD be decremented with the
	number of hops of the Frame Relay LSP (homogeneous), or Frame Relay
	segment of the LSP (heterogeneous). An LDP constructing the LSP
	SHOULD pass meaningful information to the ingress FR-LSR regarding
	the number of hops of the "non-TTL segment".
	For multicast packets, the MPLS TTL SHOULD be decremented by 1. An
	LDP constructing the LSP SHOULD pass meaningful information to the
	egress FR-LSR regarding the number of hops of the "non-TTL segment".
	Next, the MPLS encapsulated packet is passed down to the Frame Relay
	data link driver with the top label as output DLCI. The Frame Relay
	frame carrying the MPLS encapsulated packet is forwarded onto the
	Frame Relay VC to the next LSR.
	5.6 Label Processing by Core FR-LSRs
	In a FR-LSR, the current (top) MPLS label is carried in the DLCI
	field of the Frame Relay data link layer header of the frame. Just as
	in conventional Frame Relay, for a frame arriving at an interface,
	the DLCI carried by the Frame Relay data link header is looked up in
	the DLCI Information Base, replaced with the correspondent output
	DLCI, and transmitted on the outgoing interface (forwarded to the
	next hop node).
	The current label information is also carried in the top of the label
	stack. In the top-level entry, all fields except the label
	information, which is carried and switched in the Frame Relay frame
	data link-layer header, are of current significance.
	5.7 Label Processing by Egress FR-LSRs
	When reaching the end of a Frame Relay LSP, the FR-LSR pops the label
	stack [ARCH]. If the label popped is the last label, it is necessary
	to determine the particular network layer protocol which is being
	carried. The label stack carries no explicit information to identify
	the network layer protocol. This must be inferred from the value of
	the label which is popped from the stack.
	If the label popped is not the last label, the previous top level
	MPLS TTL is propagated to the new top label stack entry.
	If the FR-LSR is the egress switch of a Frame Relay segment of a
	hybrid LSP, and the end of the Frame Relay segment is not the end of
	the LSP, the MPLS packet will be processed for forwarding onto the
	next segment of the LSP based on the information held in the Next Hop
	Label Forwarding Entry (NHLFE) [ARCH]. The output label is set to the
	value from the NHLFE, and the MPLS TTL is decremented by the
	appropriate value depending the type of the output interface and the
	type of transmit operation (see section 6.3). Further, the MPLS
	packet is forwarded according to the MPLS specifications for the
	particular link of the next segment of the LSP.
	For unicast packets, the MPLS TTL SHOULD be decremented by one if the
	output interface is a generic one, or with the number of hops of the
	next ATM segment of the LSP (heterogeneous), if the output interface
	is an ATM (non-TTL) interface.
	For multicast packets, the MPLS TTL SHOULD be decremented by the
	number of hops of the FR segment being exited. An LDP constructing
	the LSP SHOULD pass meaningful information to the egress FR-LSR
	regarding the number of hops of the FR "non-TTL segment".
	6. Label Switching Control Component for Frame Relay
	To support label switching a Frame Relay Switch MUST implement the
	control component of label switching, which consists primarily of
	label allocation and maintenance procedures. Label binding
	information MAY be communicated by several mechanisms, one of which
	is the Label Distribution Protocol (LDP) [LDP].
	Since the label switching control component uses information learned
	directly from network layer routing protocols, this implies that the
	switch MUST participate as a peer in these protocols (e.g., OSPF,
	IS-IS).
	In some cases, LSRs may use other protocols (e.g. RSVP, PIM, BGP) to
	distribute label bindings. In these cases, a Frame Relay LSR should
	participate in these protocols.
	In the case where Frame Relay circuits are established via LDP, or
	RSVP, or others, with no involvement from traditional Frame Relay
	mechanisms, it is assumed that circuit establishing contractual
	information such as input/output maximum frame size,
	incoming/outgoing requested/agreed throughput, incoming/outgoing
	acceptable throughput, incoming/outgoing burst size,
	incoming/outgoing frame rate, used in transmitting, and congestion
	control MAY be passed to the FR-LSRs through RSVP, or can be
	statically configured. It is also assumed that congestion control and
	frame header flagging as a consequence of congestion, would be done
	by the FR-LSRs in a similar fashion as for traditional Frame Relay
	circuits. With the goal of emulating a best-effort router as default,
	the default VC parameters, in the absence of LDP, RSVP, or other
	mechanisms participation to setting such parameters, should be zero
	CIR, so that input policing will set the DE bit in incoming frames,
	but no frames are dropped.
	Control and state information for the circuits based on MPLS MAY be
	communicated through LDP.
	Support of label switching on a Frame Relay switch requires
	conformance only to [FRF] (framing, bit-stuffing, headers, FCS)
	except for section 2.3 (PVC control signaling procedures, aka LMI).
	Q.933 signaling for PVCs and/or SVCs is not required. PVC and/or SVC
	signaling may be used for non-MPLS (standard Frame Relay) PVCs and/or
	SVCs when both are running on the same interface as MPLS, as
	discussed in the next section.
	6.1 Hybrid Switches (Ships in the Night)
	The existence of the label switching control component on a Frame
	Relay switch does not preclude the ability to support the Frame Relay
	control component defined by the ITU and Frame Relay Forum on the
	same switch and the same interfaces (NICs). The two control
	components, label switching and those defined by ITU/Frame Relay
	Forum, would operate independently.
	Definition of how such a device operates is beyond the scope of this
	document. However, only a small amount of information needs to be
	consistent between the two control components, such as the portions
	of the DLCI space which are available to each component.
	7. Label Allocation and Maintenance Procedures
	The mechanisms and message formats of a Label Distribution Protocol
	are documented in [ARCH] and [LDP]. The "downstream-on-demand" label
	allocation and maintenance mechanism discussed in this section MUST
	be used by FR-LSRs that do not support VC merging, and it MAY also be
	used by FR-LSRs that do support VC merging (note that this mechanism
	applies to hop-by-hop routed traffic):
	7.1 Edge LSR Behavior
	Consider a member of the Edge Set of a FR-LSR domain. Assume that, as
	a result of its routing calculations, it selects a FR-LSR as the next
	hop of a certain route (FEC), and that the next hop is reachable via
	a LC-Frame Relay interface. Assume that the next-hop FR-LSR is an
	"LDP-peer" [ARCH][LDP]. The Edge LSR sends an LDP "request" message
	for a label binding from the next hop, downstream LSR. When the Edge
	LSR receives in response from the downstream LSR the label binding
	information in an LDP "mapping" message, the label is stored in the
	Label Information Base (LIB) as an outgoing label for that FEC. The
	"mapping" message may contain the "hop count" object, which
	represents the number of hops a packet will take to cross the FR-LSR
	domain to the Egress FR-LSR when using this label. This information
	may be stored for TTL calculation. Once this is done, the LSR may use
	MPLS forwarding to transmit packets in that FEC.
	When a member of the Edge Set of the FR-LSR domain receives an LDP
	"request" message from a FR-LSR for a FEC, it means it is the
	Egress-FR-LSR. It allocates a label, creates a new entry in its Label
	Information Base (LIB), places that label in the incoming label
	component of the entry, and returns (via LDP) a "mapping" message
	containing the allocated label back upstream to the LDP peer that
	originated the request. The "mapping" message contains the "hop
	count" object value set to 1.
	When a routing calculation causes an Edge LSR to change the next hop
	for a route, and the former next hop was in the FR-LSR domain, the
	Edge LSR should notify the former next hop (via an LDP "release"
	message) that the label binding associated with the route is no
	longer needed.
	When a Frame Relay-LSR receives an LDP "request" message for a
	certain route (FEC) from an LDP peer connected to the FR-LSR over a
	LC-FR interface, the FR-LSR takes the following actions:
	- it allocates a label, creates a new entry in its Label
	Information Base (LIB), and places that label in the incoming
	label component of the entry;
	- it propagates the "request", by sending an LDP "request"
	message to the next hop LSR, downstream for that route (FEC);
	In the "ordered control" mode [ARCH], the FR-LSR will wait for its
	"request" to be responded from downstream with a "mapping" message
	before returning the "mapping" upstream in response to a "request"
	("ordered control" approach [ARCH]). In this case, the FR-LSR
	increments the hop count it received from downstream and uses this
	value in the "mapping" it returns upstream.
	Alternatively, the FR-LSR may return the binding upstream without
	waiting for a binding from downstream ("independent control" approach
	[ARCH]). In this case, it uses a reserved value for hop count in the
	"mapping", indicating that it is 'unknown'. The correct value for hop
	count will be returned later, as described below.
	Since both the "ordered" and "independent" control has advantages and
	disadvantages, this is left as an implementation, or configuration
	choice.
	Once the FR-LSR receives in response the label binding in an LDP
	"mapping" message from the next hop, it places the label into the
	outgoing label component of the LIB entry.
	Note that a FR-LSR, or a member of the edge set of a FR-LSR domain,
	may receive multiple binding requests for the same route (FEC) from
	the same FR-LSR. It must generate a new "mapping" for each "request"
	(assuming adequate resources to do so), and retain any existing
	mapping(s). For each "request" received, a FR-LSR should also
	generate a new binding "request" toward the next hop for the route
	(FEC).
	When a routing calculation causes a FR-LSR to change the next hop for
	a route (FEC), the FR-LSR should notify the former next hop (via an
	LDP "release" message) that the label binding associated with the
	route is no longer needed.
	When a LSR receives a notification that a particular label binding is
	no longer needed, the LSR may deallocate the label associated with
	the binding, and destroy the binding. This mode is the "conservative
	label retention mode" [ARCH]. In the case where a FR-LSR receives
	such notification and destroys the binding, it should notify the next
	hop for the route that the label binding is no longer needed. If a
	LSR does not destroy the binding (the FR-LSR is configured in
	"liberal label retention mode" [ARCH]), it may re-use the binding
	only if it receives a request for the same route with the same hop
	count as the request that originally caused the binding to be
	created.
	When a route changes, the label bindings are re-established from the
	point where the route diverges from the previous route. LSRs
	upstream of that point are (with one exception, noted below)
	oblivious to the change. Whenever a LSR changes its next hop for a
	particular route, if the new next hop is a FR-LSR or a member of the
	edge set reachable via a LC-FR interface, then for each entry in its
	LIB associated with the route the LSR should request (via LDP) a
	binding from the new next hop.
	When a FR-LSR receives a label binding from a downstream neighbor, it
	may already have provided a corresponding label binding for this
	route to an upstream neighbor, either because it is using
	"independent control" or because the new binding from downstream is
	the result of a routing change. In this case, it should extract the
	hop count from the new binding and increment it by one. If the new
	hop count is different from that which was previously conveyed to the
	upstream neighbor (including the case where the upstream neighbor was
	given the value 'unknown') the FR-LSR must notify the upstream
	neighbor of the change. Each FR-LSR in turn increments the hop count
	and passes it upstream until it reaches the ingress Edge LSR.
	Whenever a FR-LSR originates a label binding request to its next hop
	LSR as a result of receiving a label binding request from another
	(upstream) LSR, and the request to the next hop LSR is not satisfied,
	the FR-LSR should destroy the binding created in response to the
	received request, and notify the requester (via an LDP "withdraw"
	message).
	When an LSR determines that it has lost its LDP session with another
	LSR, the following actions are taken:
	- MUST discard any binding information learned via this
	connection;
	- For any label bindings that were created as a result of
	receiving label binding requests from the peer, the LSR may
	destroy these bindings (and deallocate labels associated
	with these binding).
	7.2 Efficient use of label space - Merging FR-LSRs
	The above discussion assumes that an edge LSR will request one label
	for each prefix in its routing table that has a next hop in the FR-
	LSR domain. In fact, it is possible to significantly reduce the
	number of labels needed by having the edge LSR request instead one
	label for several routes. Use of many-to-one mappings between routes
	(address prefixes) and labels using the notion of Forwarding
	Equivalence Classes (as described in [ARCH]) provides a mechanism to
	conserve the number of labels.
	Note that conserving label space (VC merging) may be restricted in
	case the frame traffic requires Frame Relay fragmentation. The issue
	is that Frame Relay fragments must be transmitted in sequence, i.e.
	fragments of distinct frames must not be interleaved. If the
	fragmenting FR-LSR ensures the transmission in sequence of all
	fragments of a frame, without interleaving with fragments of other
	frames, then label conservation (VC merging) can be performed.
	When label conservation is used, when a FR-LSR receives a binding
	request from an upstream LSR for a certain FEC, and it does already
	have an outgoing label binding for that FEC, it does not need to
	issue a downstream binding request. Instead, it may allocate an
	incoming label, and return that label in a binding to the upstream
	requester. Packets received from the requester, with that label as
	top label, will be forwarded after replacing the label with the
	existing outgoing label for that FEC. If the FR-LSR does not have an
	outgoing label binding for that FEC, but does have an outstanding
	request for one, it need not issue another request. This means that
	in a label conservation case, a FR-LSR must respond with a new
	binding for every upstream request, but it may need to send one
	binding request downstream.
	In case of label conservation, if a change in the routing table
	causes a FR-LSR to select a new next hop for one of its FECs, it MAY
	release the binding for that route from the former next hop. If it
	doesn't already have a corresponding binding for the new next hop, it
	must request one (note that the choice depends on the label retention
	mode [ARCH]).
	If a new binding is obtained, which contain a hop count that differs
	from that of the old binding, the FR-LSR must process the new hop
	count: increment by 1, if different than "unknown", and notify the
	upstream neighbors who have label bindings for this FEC of the new
	value. To ensure that loops will be detected, if the new hop count
	exceeds the "maximum" value, the label values for this FEC must be
	withdrawn from all upstream neighbors to whom a binding was
	previously sent.
	7.3 LDP messages specific to Frame Relay
	The Label Distribution Protocol [LDP] messages exchanged between two
	Frame Relay "LDP-peer" LSRs may contain Frame Relay specific
	information such as:
	"Frame Relay Label Range":
	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
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Reserved |Len| Minimum DLCI |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Reserved | Maximum DLCI |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	with the following fields:
	Reserved
	This fields are reserved. They must be set to zero on transmission
	and must be ignored on receipt.
	Len
	This field specifies the number of bits of the DLCI. The following
	values are supported:
	Len DLCI bits
	0 10
	1 23
	Minimum DLCI
	This 23 bit field is the binary value of the lower bound of a block
	of Data Link Connection Identifiers (DLCIs) that is supported by
	the originating FR-LSR. The Minimum DLCI should be right justified
	in this field and the preceding bits should be set to 0.
	Maximum DLCI
	This 23 bit field is the binary value of the upper bound of a block
	of Data Link Connection Identifiers (DLCIs) that is supported by
	the originating FR-LSR. The Maximum DLCI should be right justified
	in this field and the preceding bits should be set to 0.
	"Frame Relay Merge":
	0 1 2 3 4 5 6 7
	+-+-+-+-+-+-+-+-+
	| Reserved |M|
	+-+-+-+-+-+-+-+-+
	with the following fields:
	Merge
	One bit field that specifies the merge capabilities of the FR-LSR:
	Value Meaning
	0 Merge NOT supported
	1 Merge supported
	A FR-LSR that supports VC merging MUST ensure that fragmented
	frames from distinct incoming DLCIs are not interleaved on the
	outgoing DLCI.
	Reserved
	This field is reserved. It must be set to zero on transmission and
	must be ignored on receipt.
	and "Frame Relay Label":
	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
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Reserved |Len| DLCI |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	with the following fields:
	Reserved
	This field is reserved. It must be set to zero on transmission and
	must be ignored on receipt.
	Len
	This field specifies the number of bits of the DLCI. The following
	values are supported:
	Len DLCI bits
	0 10
	1 23
	DLCI
	The binary value of the Frame Relay Label. The significant number
	of bits (10 or 23) of the label value are to be encoded into the
	Data Link Connection Identifier (DLCI) field when part of the Frame
	Relay data link header (see Section 4.).
	8. Security Considerations
	This section looks at the security aspects of:
	(a) frame traffic,
	(b) label distribution.
	MPLS encapsulation has no effect on authenticated or encrypted
	network layer packets, that is IP packets that are authenticated or
	encrypted will incur no change.
	The MPLS protocol has no mechanisms of its own to protect against
	misdirection of packets or the impersonation of an LSR by accident or
	malicious intent.
	Altering by accident or forgery an existent label in the DLCI field
	of the Frame Relay data link layer header of a frame or one or more
	fields in a potentially following label stack affects the forwarding
	of that frame.
	The label distribution mechanism can be secured by applying the
	appropriate level of security to the underlying protocol carrying
	label information - authentication or encryption - see [LDP].
	9. Acknowledgments
	The initial version of this document was derived from the Label
	Switching over ATM document [ATM].
	Thanks for the extensive reviewing and constructive comments from (in
	alphabetical order) Dan Harrington, Milan Merhar, Martin Mueller,
	Eric Rosen. Also thanks to George Swallow for the suggestion to use
	null encapsulation, and to Eric Gray for his reviewing.
	Also thanks to Nancy Feldman and Bob Thomas for their collaboration
	in including the LDP messages specific to Frame Relay LSRs.
	10. References		Title: Use of Label Switching on Frame Relay Networks
			Specification
			Author(s): A. Conta, P. Doolan, A. Malis
			Status: Standards Track
			Date: January 2001
			Mailbox: aconta@txc.com, pdoolan@ennovatenetworks.com,
			Andy.Malis@vivacenetworks.com
			Pages: 24
			Characters: 53176
			Updates/Obsoletes/SeeAlso: None
	[MIFR] T. Bradley, C. Brown, A. Malis "Multiprotocol Interconnect		I-D Tag: draft-ietf-mpls-fr-06.txt
	over Frame Relay" RFC 2427, September 1998.
	[ARCH] E. Rosen, R. Callon, A. Vishwanathan, "Multi-Protocol Label		URL: ftp://ftp.isi.edu/in-notes/rfc3034.txt
	Switching Architecture", Work in Progress, July 1998.
	[LDP] L. Anderson, P. Doolan, N. Feldman, A. Fredette, R. Thomas,		This document defines the model and generic mechanisms for
	"Label Distribution Protocol", Work in Progress, August 1998.		Multiprotocol Label Switching on Frame Relay networks. Furthermore,
			it extends and clarifies portions of the Multiprotocol Label Switching
			Architecture described in [ARCH] and the Label Distribution Protocol
			(LDP) described in [LDP] relative to Frame Relay Networks. MPLS
			enables the use of Frame Relay Switches as Label Switching Routers
			(LSRs).
	[STACK] E. Rosen et al, "Label Switching: Label Stack Encodings",		This document is a product of the Multiprotocol Label Switching
	Work in Progress, September 1998		Working Group of the IETF.
	[ATM] B. Davie et al. "Use of Label Switching with ATM", Work in		This is now a Proposed Standard Protocol.
	Progress, July 1998.
	[ITU] International Telecommunications Union, "ISDN Data Link Layer		This document specifies an Internet standards track protocol for
	Specification for Frame Mode Bearer Services", ITU-T Recommendation		the Internet community, and requests discussion and suggestions
	Q.922, 1992.		for improvements. Please refer to the current edition of the
			"Internet Official Protocol Standards" (STD 1) for the
			standardization state and status of this protocol. Distribution
			of this memo is unlimited.
	[FRF] Frame Relay Forum, User-to-Network Implementation Agreement		This announcement is sent to the IETF list and the RFC-DIST list.
	(UNI), FRF 1.1, January 19, 1996		Requests to be added to or deleted from the IETF distribution list
	11.Authors' Addresses		should be sent to IETF-REQUEST@IETF.ORG. Requests to be
			added to or deleted from the RFC-DIST distribution list should
			be sent to RFC-DIST-REQUEST@RFC-EDITOR.ORG.
	Alex Conta		Details on obtaining RFCs via FTP or EMAIL may be obtained by sending
	3COM		an EMAIL message to rfc-info@RFC-EDITOR.ORG with the message body
	100 3COM Drive		help: ways_to_get_rfcs. For example:
	Marlborough, MA 01752
	+1 508 323-2297
	E-mail: Alex_Conta@ne.3com.com
	Paul Doolan		To: rfc-info@RFC-EDITOR.ORG
	Ennovate Networks		Subject: getting rfcs
	60 Codman Hill Rd
	Boxborough MA 01719
	+1 978 263-2002
	E-mail: pdoolan@ennovatenetworks.com
	Andrew Malis		help: ways_to_get_rfcs
	Lucent Technologies
	1 Robbins Rd
	Westford, MA 01886
	+1 978 952-7414
	E-mail: amalis@lucent.com
	Appendix A - Changes since previous versions
	From "version 02 to 03"		Requests for special distribution should be addressed to either the
	- Replace "cloud" with "domain",		author of the RFC in question, or to RFC-Manager@RFC-EDITOR.ORG. Unless
	- Update references to other documents,		specifically noted otherwise on the RFC itself, all RFCs are for
	- Change definitions in "Terminology" section,		unlimited distribution.echo
	- Add more definitions to "Terminology" section,		Submissions for Requests for Comments should be sent to
	- Make editorial changes to text and figures,		RFC-EDITOR@RFC-EDITOR.ORG. Please consult RFC 2223, Instructions to RFC
	- Change "Performing TTL calculations" section,		Authors, for further information.
	- Add more reviewers in "Acknowledgments" section,
	- Add Appendix A - changes.
End of changes. 14 change blocks.
	1012 lines changed or deleted		41 lines changed or added
This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/