draft-ietf-mpls-rfc3036bis-04.txt		rfc5036.txt
	Network Working Group Loa Andersson
	Internet Draft Ina Minei
	Expiration Date: March 2007 Bob Thomas
	Editors
	September 2006
	LDP Specification
	draft-ietf-mpls-rfc3036bis-04.txt
	Status of this Memo
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	Source of this document
	This document is produced as part of advancing the LDP specification
	to draft standard status. The LDP specification was originally
	published as RFC 3036 in January 2001. It was produced by the MPLS
	working of the IETF and was jointly authored by Loa Andersson, Paul
	Doolan, Nancy Feldman, Andre Fredette and Bob Thomas.
	Abstract
	The architecture for Multi Protocol Label Switching (MPLS) is
	described in RFC 3031 [RFC3031]. A fundamental concept in MPLS is
	that two Label Switching Routers (LSRs) must agree on the meaning of
	the labels used to forward traffic between and through them. This
	common understanding is achieved by using a set of procedures, called
	a label distribution protocol, by which one LSR informs another of
	label bindings it has made. This document defines a set of such
	procedures called LDP (for Label Distribution Protocol) by which LSRs
	distribute labels to support MPLS forwarding along normally routed
	paths.
	Table of Contents
	Source of this document ............................... 1
	1 LDP Overview .......................................... 7
	1.1 LDP Peers ............................................. 8
	1.2 LDP Message Exchange .................................. 8
	1.3 LDP Message Structure ................................. 9
	1.4 LDP Error Handling .................................... 9
	1.5 LDP Extensibility and Future Compatibility ............ 9
	1.6 Specification Language ................................ 9
	2 LDP Operation ......................................... 10
	2.1 FECs .................................................. 10
	2.2 Label Spaces, Identifiers, Sessions and Transport ..... 11
	2.2.1 Label Spaces .......................................... 11
	2.2.2 LDP Identifiers ....................................... 11
	2.2.3 LDP Sessions .......................................... 12
	2.2.4 LDP Transport ......................................... 12
	2.3 LDP Sessions between non-Directly Connected LSRs ...... 12
	2.4 LDP Discovery ......................................... 13
	2.4.1 Basic Discovery Mechanism ............................. 13
	2.4.2 Extended Discovery Mechanism .......................... 13
	2.5 Establishing and Maintaining LDP Sessions ............ 14
	2.5.1 LDP Session Establishment ............................. 14
	2.5.2 Transport Connection Establishment .................... 15
	2.5.3 Session Initialization ................................ 16
	2.5.4 Initialization State Machine .......................... 19
	2.5.5 Maintaining Hello Adjacencies ......................... 21
	2.5.6 Maintaining LDP Sessions .............................. 21
	2.6 Label Distribution and Management ..................... 22
	2.6.1 Label Distribution Control Mode ....................... 22
	2.6.1.1 Independent Label Distribution Control ................ 22
	2.6.1.2 Ordered Label Distribution Control .................... 22
	2.6.2 Label Retention Mode .................................. 23
	2.6.2.1 Conservative Label Retention Mode ..................... 23
	2.6.2.2 Liberal Label Retention Mode .......................... 24
	2.6.3 Label Advertisement Mode .............................. 24
	2.7 LDP Identifiers and Next Hop Addresses ................ 24
	2.8 Loop Detection ........................................ 25
	2.8.1 Label Request Message ................................. 26
	2.8.2 Label Mapping Message ................................. 27
	2.8.3 Discussion ............................................ 29
	2.9 Authenticity and Integrity of LDP Messages ............ 29
	2.9.1 TCP MD5 Signature Option .............................. 30
	2.9.2 LDP Use of TCP MD5 Signature Option ................... 31
	2.10 Label Distribution for Explicitly Routed LSPs ......... 32
	3 Protocol Specification ................................ 32
	3.1 LDP PDUs .............................................. 32
	3.2 LDP Procedures ........................................ 33
	3.3 Type-Length-Value Encoding ............................ 34
	3.4 TLV Encodings for Commonly Used Parameters ............ 35
	3.4.1 FEC TLV ............................................... 36
	3.4.1.1 FEC Procedures ........................................ 37
	3.4.2 Label TLVs ............................................ 38
	3.4.2.1 Generic Label TLV ..................................... 38
	3.4.2.2 ATM Label TLV ......................................... 38
	3.4.2.3 Frame Relay Label TLV ................................. 39
	3.4.3 Address List TLV ...................................... 40
	3.4.4 Hop Count TLV ......................................... 41
	3.4.4.1 Hop Count Procedures .................................. 41
	3.4.5 Path Vector TLV ....................................... 43
	3.4.5.1 Path Vector Procedures ................................ 43
	3.4.5.1.1 Label Request Path Vector ............................. 43
	3.4.5.1.2 Label Mapping Path Vector ............................. 44
	3.4.6 Status TLV ............................................ 45
	3.5 LDP Messages .......................................... 46
	3.5.1 Notification Message .................................. 49
	3.5.1.1 Notification Message Procedures ....................... 50
	3.5.1.2 Events Signaled by Notification Messages .............. 50
	3.5.1.2.1 Malformed PDU or Message .............................. 51
	3.5.1.2.2 Unknown or Malformed TLV .............................. 52
	3.5.1.2.3 Session KeepAlive Timer Expiration .................... 52
	3.5.1.2.4 Unilateral Session Shutdown ........................... 52
	3.5.1.2.5 Initialization Message Events ......................... 52
	3.5.1.2.6 Events Resulting From Other Messages .................. 53
	3.5.1.2.7 Internal Errors ....................................... 53
	3.5.1.2.8 Miscellaneous Events .................................. 53
	3.5.2 Hello Message ......................................... 53
	3.5.2.1 Hello Message Procedures .............................. 55
	3.5.3 Initialization Message ................................ 57
	3.5.3.1 Initialization Message Procedures ..................... 65
	3.5.4 KeepAlive Message ..................................... 65
	3.5.4.1 KeepAlive Message Procedures .......................... 65
	3.5.5 Address Message ....................................... 66
	3.5.5.1 Address Message Procedures ............................ 66
	3.5.6 Address Withdraw Message .............................. 67
	3.5.6.1 Address Withdraw Message Procedures ................... 68
	3.5.7 Label Mapping Message ................................. 68
	3.5.7.1 Label Mapping Message Procedures ...................... 69
	3.5.7.1.1 Independent Control Mapping ........................... 70
	3.5.7.1.2 Ordered Control Mapping ............................... 70
	3.5.7.1.3 Downstream on Demand Label Advertisement .............. 71
	3.5.7.1.4 Downstream Unsolicited Label Advertisement ............ 71
	3.5.8 Label Request Message ................................. 72
	3.5.8.1 Label Request Message Procedures ...................... 73
	3.5.9 Label Abort Request Message ........................... 74
	3.5.9.1 Label Abort Request Message Procedures ................ 75
	3.5.10 Label Withdraw Message ................................ 77
	3.5.10.1 Label Withdraw Message Procedures ..................... 78
	3.5.11 Label Release Message ................................. 78
	3.5.11.1 Label Release Message Procedures ...................... 79
	3.6 Messages and TLVs for Extensibility ................... 80
	3.6.1 LDP Vendor-private Extensions ......................... 80
	3.6.1.1 LDP Vendor-private TLVs ............................... 80
	3.6.1.2 LDP Vendor-private Messages ........................... 82
	3.6.2 LDP Experimental Extensions ........................... 83
	3.7 Message Summary ....................................... 84
	3.8 TLV Summary ........................................... 84
	3.9 Status Code Summary ................................... 85
	3.10 Well-known Numbers .................................... 86
	3.10.1 UDP and TCP Ports ..................................... 86
	3.10.2 Implicit NULL Label ................................... 86
	4 IANA Considerations ................................... 87
	4.1 Message Type Name Space ............................... 87
	4.2 TLV Type Name Space ................................... 87
	4.3 FEC Type Name Space ................................... 88
	4.4 Status Code Name Space ................................ 88
	4.5 Experiment ID Name Space .............................. 88
	5 Security Considerations ............................... 89
	5.1 Spoofing .............................................. 89
	5.2 Privacy ............................................... 90
	5.3 Denial of Service ..................................... 90
	6 Areas for Future Study ................................ 92
	7 Changes from RFC3036 .................................. 93
	8 Acknowledgments ....................................... 95
	9 References ............................................ 96
	9.1 Normative references .................................. 96
	9.2 Non-normative references .............................. 96
	10 Intellectual Property Statement ....................... 97
	11 Editors' Addresses .................................... 98
	Appendix A LDP Label Distribution Procedures ..................... 99
	A.1 Handling Label Distribution Events .................... 101
	A.1.1 Receive Label Request ................................. 102
	A.1.2 Receive Label Mapping ................................. 105
	A.1.3 Receive Label Abort Request ........................... 111
	A.1.4 Receive Label Release ................................. 113
	A.1.5 Receive Label Withdraw ................................ 115
	A.1.6 Recognize New FEC ..................................... 117
	A.1.7 Detect Change in FEC Next Hop ......................... 119
	A.1.8 Receive Notification / Label Request Aborted .......... 122
	A.1.9 Receive Notification / No Label Resources ............. 123
	A.1.10 Receive Notification / No Route ....................... 123
	A.1.11 Receive Notification / Loop Detected .................. 124
	A.1.12 Receive Notification / Label Resources Available ...... 125
	A.1.13 Detect local label resources have become available .... 126
	A.1.14 LSR decides to no longer label switch a FEC ........... 127
	A.1.15 Timeout of deferred label request ..................... 127
	A.2 Common Label Distribution Procedures .................. 128
	A.2.1 Send_Label ............................................ 128
	A.2.2 Send_Label_Request .................................... 130
	A.2.3 Send_Label_Withdraw ................................... 131
	A.2.4 Send_Notification ..................................... 131
	A.2.5 Send_Message .......................................... 132
	A.2.6 Check_Received_Attributes ............................. 132
	A.2.7 Prepare_Label_Request_Attributes ...................... 133
	A.2.8 Prepare_Label_Mapping_Attributes ...................... 135
	Full Copyright Statement .............................. 138
	1. LDP Overview
	The MPLS architecture [RFC3031] defines a label distribution protocol
	as a set of procedures by which one Label Switched Router (LSR)
	informs another of the meaning of labels used to forward traffic
	between and through them.
	The MPLS architecture does not assume a single label distribution
	protocol. In fact, a number of different label distribution proto-
	cols are being standardized. Existing protocols have been extended
	so that label distribution can be piggybacked on them. New protocols
	have also been defined for the explicit purpose of distributing
	labels. The MPLS architecture discusses some of the considerations
	when choosing a label distribution protocol for use in particular
	MPLS applications such as Traffic Engineering [RFC2702].
	The Label Distribution Protocol (LDP) is a protocol defined for dis-
	tributing labels. It was originally published as RFC3036 in January
	2001. It was produced by the MPLS working of the IETF and was jointly
	authored by Loa Andersson, Paul Doolan, Nancy Feldman, Andre Fredette
	and Bob Thomas.
	LDP is a protocol defined for distributing labels. It is the set of
	procedures and messages by which Label Switched Routers (LSRs) estab-
	lish Label Switched Paths (LSPs) through a network by mapping net-
	work-layer routing information directly to data-link layer switched
	paths. These LSPs may have an endpoint at a directly attached neigh-
	bor (comparable to IP hop-by-hop forwarding), or may have an endpoint
	at a network egress node, enabling switching via all intermediary
	nodes.
	LDP associates a Forwarding Equivalence Class (FEC) [RFC3031] with
	each LSP it creates. The FEC associated with an LSP specifies which
	packets are "mapped" to that LSP. LSPs are extended through a net-
	work as each LSR "splices" incoming labels for a FEC to the outgoing
	label assigned to the next hop for the given FEC.
	More information about the applicability of LDP can be found in
	[RFC3037].
	This document assumes (but does not require) familiarity with the
	MPLS architecture [RFC3031]. Note that [RFC3031] includes a glossary
	of MPLS terminology, such as ingress, label switched path, etc.
	1.1. LDP Peers
	Two LSRs which use LDP to exchange label/FEC mapping information are
	known as "LDP Peers" with respect to that information and we speak of
	there being an "LDP Session" between them. A single LDP session
	allows each peer to learn the other's label mappings; i.e., the pro-
	tocol is bi-directional.
	1.2. LDP Message Exchange
	There are four categories of LDP messages:
	1. Discovery messages, used to announce and maintain the presence
	of an LSR in a network.
	2. Session messages, used to establish, maintain, and terminate
	sessions between LDP peers.
	3. Advertisement messages, used to create, change, and delete
	label mappings for FECs.
	4. to signal error information.
	Discovery messages provide a mechanism whereby LSRs indicate their
	presence in a network by sending a Hello message periodically. This
	is transmitted as a UDP packet to the LDP port at the `all routers on
	this subnet' group multicast address. When an LSR chooses to estab-
	lish a session with another LSR learned via the Hello message, it
	uses the LDP initialization procedure over TCP transport. Upon suc-
	cessful completion of the initialization procedure, the two LSRs are
	LDP peers, and may exchange advertisement messages.
	When to request a label or advertise a label mapping to a peer is
	largely a local decision made by an LSR. In general, the LSR
	requests a label mapping from a neighboring LSR when it needs one,
	and advertises a label mapping to a neighboring LSR when it wishes
	the neighbor to use a label.
	Correct operation of LDP requires reliable and in order delivery of
	messages. To satisfy these requirements LDP uses the TCP transport
	for session, advertisement and notification messages; i.e., for
	everything but the UDP-based discovery mechanism.
	1.3. LDP Message Structure
	All LDP messages have a common structure that uses a Type-Length-
	Value (TLV) encoding scheme; see Section "Type-Length-Value" encod-
	ing. The Value part of a TLV-encoded object, or TLV for short, may
	itself contain one or more TLVs.
	1.4. LDP Error Handling
	LDP errors and other events of interest are signaled to an LDP peer
	by notification messages.
	There are two kinds of LDP notification messages:
	1. Error notifications, used to signal fatal errors. If an LSR
	receives an error notification from a peer for an LDP session,
	it terminates the LDP session by closing the TCP transport con-
	nection for the session and discarding all label mappings
	learned via the session.
	2. Advisory notifications, used to pass an LSR information about
	the LDP session or the status of some previous message received
	from the peer.
	1.5. LDP Extensibility and Future Compatibility
	Functionality may be added to LDP in the future. It is likely that
	future functionality will utilize new messages and object types
	(TLVs). It may be desirable to employ such new messages and TLVs
	within a network using older implementations that do not recognize
	them. While it is not possible to make every future enhancement
	backwards compatible, some prior planning can ease the introduction
	of new capabilities. This specification defines rules for handling
	unknown message types and unknown TLVs for this purpose.
	1.6. Specification Language
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	document are to be interpreted as described in [RFC2119].
	2. LDP Operation
	2.1. FECs
	It is necessary to precisely specify which packets may be mapped to
	each LSP. This is done by providing a FEC specification for each
	LSP. The FEC identifies the set of IP packets which may be mapped to
	that LSP.
	Each FEC is specified as a set of one or more FEC elements. Each FEC
	element identifies a set of packets which may be mapped to the corre-
	sponding LSP. When an LSP is shared by multiple FEC elements, that
	LSP is terminated at (or before) the node where the FEC elements can
	no longer share the same path.
	This specification defines a single type of FEC element, the "Address
	Prefix FEC element". This element is an address prefix of any length
	from 0 to a full address, inclusive.
	Additional FEC elements may be defined, as needed, by other specifi-
	cations.
	In the remainder of this section we give the rules to be used for
	mapping packets to LSPs that have been set up using an Address Prefix
	FEC element.
	We say that a particular address "matches" a particular address pre-
	fix if and only if that address begins with that prefix. We also say
	that a particular packet matches a particular LSP if and only if that
	LSP has an Address Prefix FEC element which matches the packet's des-
	tination address. With respect to a particular packet and a particu-
	lar LSP, we refer to any Address Prefix FEC element which matches the
	packet as the "matching prefix".
	The procedure for mapping a particular packet to a particular LSP
	uses the following rules. Each rule is applied in turn until the
	packet can be mapped to an LSP.
	- If a packet matches exactly one LSP, the packet is mapped to that
	LSP.
	- If a packet matches multiple LSPs, it is mapped to the LSP whose
	matching prefix is the longest. If there is no one LSP whose
	matching prefix is longest, the packet is mapped to one from the
	set of LSPs whose matching prefix is longer than the others. The
	procedure for selecting one of those LSPs is beyond the scope of
	this document.
	- If it is known that a packet must traverse a particular egress
	router, and there is an LSP which has an Address Prefix FEC ele-
	ment which is a /32 address of that router, then the packet is
	mapped to that LSP. The procedure for obtaining this knowledge
	is beyond the scope of this document.
	The procedure for determining that a packet must traverse a particu-
	lar egress router is beyond the scope of this document. (As an exam-
	ple, if one is running a link state routing algorithm, it may be pos-
	sible to obtain this information from the link state data base. As
	another example, if one is running BGP, it may be possible to obtain
	this information from the BGP next hop attribute of the packet's
	route.)
	2.2. Label Spaces, Identifiers, Sessions and Transport
	2.2.1. Label Spaces
	The notion of "label space" is useful for discussing the assignment
	and distribution of labels. There are two types of label spaces:
	- Per interface label space. Interface-specific incoming labels
	are used for interfaces that use interface resources for labels.
	An example of such an interface is a label-controlled ATM inter-
	face that uses VCIs as labels, or a Frame Relay interface that
	uses DLCIs (Data Link Connection Identifiers) as labels.
	Note that the use of a per interface label space only makes sense
	when the LDP peers are "directly connected" over an interface,
	and the label is only going to be used for traffic sent over that
	interface.
	- Per platform label space. Platform-wide incoming labels are used
	for interfaces that can share the same labels.
	2.2.2. LDP Identifiers
	An LDP identifier is a six octet quantity used to identify an LSR
	label space. The first four octets identify the LSR and must be a
	globally unique value, such as a 32-bit router Id assigned to the
	LSR. The last two octets identify a specific label space within the
	LSR. The last two octets of LDP Identifiers for platform-wide label
	spaces are always both zero. This document uses the following print
	representation for LDP Identifiers:
	<LSR Id> : <label space id>
	e.g., lsr171:0, lsr19:2.
	Note that an LSR that manages and advertises multiple label spaces
	uses a different LDP Identifier for each such label space.
	A situation where an LSR would need to advertise more than one label
	space to a peer and hence use more than one LDP Identifier occurs
	when the LSR has two links to the peer and both are ATM (and use per
	interface labels). Another situation would be where the LSR had two
	links to the peer, one of which is ethernet (and uses per platform
	labels) and the other of which is ATM.
	2.2.3. LDP Sessions
	LDP sessions exist between LSRs to support label exchange between
	them.
	When an LSR uses LDP to advertise more than one label space to
	another LSR it uses a separate LDP session for each label space.
	2.2.4. LDP Transport
	LDP uses TCP as a reliable transport for sessions.
	When multiple LDP sessions are required between two LSRs there is
	one TCP session for each LDP session.
	2.3. LDP Sessions between non-Directly Connected LSRs
	LDP sessions between LSRs that are not directly connected at the link
	level may be desirable in some situations.
	For example, consider a "traffic engineering" application where LSRa
	sends traffic matching some criteria via an LSP to non-directly con-
	nected LSRb rather than forwarding the traffic along its normally
	routed path.
	The path between LSRa and LSRb would include one or more intermediate
	LSRs (LSR1,...LSRn). An LDP session between LSRa and LSRb would
	enable LSRb to label switch traffic arriving on the LSP from LSRa by
	providing LSRb means to advertise labels for this purpose to LSRa.
	In this situation LSRa would apply two labels to traffic it forwards
	on the LSP to LSRb: a label learned from LSR1 to forward traffic
	along the LSP path from LSRa to LSRb; and a label learned from LSRb
	to enable LSRb to label switch traffic arriving on the LSP.
	LSRa first adds the label learned via its LDP session with LSRb to
	the packet label stack (either by replacing the label on top of the
	packet label stack with it if the packet arrives labeled or by push-
	ing it if the packet arrives unlabeled). Next, it pushes the label
	for the LSP learned from LSR1 onto the label stack.
	2.4. LDP Discovery
	LDP discovery is a mechanism that enables an LSR to discover poten-
	tial LDP peers. Discovery makes it unnecessary to explicitly config-
	ure an LSR's label switching peers.
	There are two variants of the discovery mechanism:
	- A basic discovery mechanism used to discover LSR neighbors that
	are directly connected at the link level.
	- An extended discovery mechanism used to locate LSRs that are not
	directly connected at the link level.
	2.4.1. Basic Discovery Mechanism
	To engage in LDP Basic Discovery on an interface an LSR periodically
	sends LDP Link Hellos out the interface. LDP Link Hellos are sent as
	UDP packets addressed to the well-known LDP discovery port for the
	"all routers on this subnet" group multicast address.
	An LDP Link Hello sent by an LSR carries the LDP Identifier for the
	label space the LSR intends to use for the interface and possibly
	additional information.
	Receipt of an LDP Link Hello on an interface identifies a "Hello
	adjacency" with a potential LDP peer reachable at the link level on
	the interface as well as the label space the peer intends to use for
	the interface.
	2.4.2. Extended Discovery Mechanism
	LDP sessions between non-directly connected LSRs are supported by LDP
	Extended Discovery.
	To engage in LDP Extended Discovery an LSR periodically sends LDP
	Targeted Hellos to a specific address. LDP Targeted Hellos are sent
	as UDP packets addressed to the well-known LDP discovery port at the
	specific address.
	An LDP Targeted Hello sent by an LSR carries the LDP Identifier for
	the label space the LSR intends to use and possibly additional
	optional information.
	Extended Discovery differs from Basic Discovery in the following
	ways:
	- A Targeted Hello is sent to a specific address rather than to the
	"all routers" group multicast address for the outgoing interface.
	- Unlike Basic Discovery, which is symmetric, Extended Discovery is
	asymmetric.
	One LSR initiates Extended Discovery with another targeted LSR,
	and the targeted LSR decides whether to respond to or ignore the
	Targeted Hello. A targeted LSR that chooses to respond does so
	by periodically sending Targeted Hellos to the initiating LSR.
	Receipt of an LDP Targeted Hello identifies a "Hello adjacency" with
	a potential LDP peer reachable at the network level and the label
	space the peer intends to use.
	2.5. Establishing and Maintaining LDP Sessions
	2.5.1. LDP Session Establishment
	The exchange of LDP Discovery Hellos between two LSRs triggers LDP
	session establishment. Session establishment is a two step process:
	- Transport connection establishment.
	- Session initialization
	The following describes establishment of an LDP session between LSRs
	LSR1 and LSR2 from LSR1's point of view. It assumes the exchange of
	Hellos specifying label space LSR1:a for LSR1 and label space LSR2:b
	for LSR2.
	2.5.2. Transport Connection Establishment
	The exchange of Hellos results in the creation of a Hello adjacency
	at LSR1 that serves to bind the link (L) and the label spaces LSR1:a
	and LSR2:b.
	1. If LSR1 does not already have an LDP session for the exchange of
	label spaces LSR1:a and LSR2:b it attempts to open a TCP con-
	nection for a new LDP session with LSR2.
	LSR1 determines the transport addresses to be used at its end
	(A1) and LSR2's end (A2) of the LDP TCP connection. Address A1
	is determined as follows:
	a. If LSR1 uses the Transport Address optional object (TLV) in
	Hello's it sends to LSR2 to advertise an address, A1 is the
	address LSR1 advertises via the optional object;
	b. If LSR1 does not use the Transport Address optional object,
	A1 is the source address used in Hellos it sends to LSR2.
	Similarly, address A2 is determined as follows:
	a. If LSR2 uses the Transport Address optional object, A2 is
	the address LSR2 advertises via the optional object;
	b. If LSR2 does not use the Transport Address optional object,
	A2 is the source address in Hellos received from LSR2.
	2. LSR1 determines whether it will play the active or passive role
	in session establishment by comparing addresses A1 and A2 as
	unsigned integers. If A1 > A2, LSR1 plays the active role;
	otherwise it is passive.
	The procedure for comparing A1 and A2 as unsigned integers is:
	- If A1 and A2 are not in the same address family, they are
	incomparable, and no session can be established.
	- Let U1 be the abstract unsigned integer obtained by treat-
	ing A1 as a sequence of bytes, where the byte which appears
	earliest in the message is the most significant byte of the
	integer and the byte which appears latest in the message is
	the least significant byte of the integer.
	Let U2 be the abstract unsigned integer obtained from A2 in
	a similar manner.
	- Compare U1 with U2. If U1 > U2, then A1 > A2; if U1 < U2,
	then A1 < A2.
	3. If LSR1 is active, it attempts to establish the LDP TCP connec-
	tion by connecting to the well-known LDP port at address A2.
	If LSR1 is passive, it waits for LSR2 to establish the LDP TCP
	connection to its well-known LDP port.
	Note that when an LSR sends a Hello it selects the transport address
	for its end of the session connection and uses the Hello to advertise
	the address, either explicitly by including it in an optional Trans-
	port Address TLV or implicitly by omitting the TLV and using it as
	the Hello source address.
	An LSR MUST advertise the same transport address in all Hellos that
	advertise the same label space. This requirement ensures that two
	LSRs linked by multiple Hello adjacencies using the same label spaces
	play the same connection establishment role for each adjacency.
	2.5.3. Session Initialization
	After LSR1 and LSR2 establish a transport connection they negotiate
	session parameters by exchanging LDP Initialization messages. The
	parameters negotiated include LDP protocol version, label distribu-
	tion method, timer values, VPI/VCI (Virtual Path Identifier/ Virtual
	Channel Identifier) ranges for label controlled ATM, DLCI (Data Link
	Connection Identifier) ranges for label controlled Frame Relay, etc.
	Successful negotiation completes establishment of an LDP session
	between LSR1 and LSR2 for the advertisement of label spaces LSR1:a
	and LSR2:b.
	The following describes the session initialization from LSR1's point
	of view.
	After the connection is established, if LSR1 is playing the active
	role, it initiates negotiation of session parameters by sending an
	Initialization message to LSR2. If LSR1 is passive, it waits for
	LSR2 to initiate the parameter negotiation.
	In general when there are multiple links between LSR1 and LSR2 and
	multiple label spaces to be advertised by each, the passive LSR can-
	not know which label space to advertise over a newly established TCP
	connection until it receives the LDP Initialization message on the
	connection. The Initialization message carries both the LDP Identi-
	fier for the sender's (active LSR's) label space and the LDP Identi-
	fier for the receiver's (passive LSR's) label space.
	By waiting for the Initialization message from its peer the passive
	LSR can match the label space to be advertised by the peer (as deter-
	mined from the LDP Identifier in the PDU header for the Initializa-
	tion message) with a Hello adjacency previously created when Hellos
	were exchanged.
	1. When LSR1 plays the passive role:
	a. If LSR1 receives an Initialization message it attempts to
	match the LDP Identifier carried by the message PDU with a
	Hello adjacency.
	b. If there is a matching Hello adjacency, the adjacency speci-
	fies the local label space for the session.
	Next LSR1 checks whether the session parameters proposed in
	the message are acceptable. If they are, LSR1 replies with
	an Initialization message of its own to propose the parame-
	ters it wishes to use and a KeepAlive message to signal
	acceptance of LSR2's parameters. If the parameters are not
	acceptable, LSR1 responds by sending a Session
	Rejected/Parameters Error Notification message and closing
	the TCP connection.
	c. If LSR1 cannot find a matching Hello adjacency it sends a
	Session Rejected/No Hello Error Notification message and
	closes the TCP connection.
	d. If LSR1 receives a KeepAlive in response to its Initializa-
	tion message, the session is operational from LSR1's point
	of view.
	e. If LSR1 receives an Error Notification message, LSR2 has
	rejected its proposed session and LSR1 closes the TCP con-
	nection.
	2. When LSR1 plays the active role:
	a. If LSR1 receives an Error Notification message, LSR2 has
	rejected its proposed session and LSR1 closes the TCP con-
	nection.
	b. If LSR1 receives an Initialization message, it checks
	whether the session parameters are acceptable. If so, it
	replies with a KeepAlive message. If the session parame-
	ters are unacceptable, LSR1 sends a Session Rejected/Param-
	eters Error Notification message and closes the connection.
	c. If LSR1 receives a KeepAlive message, LSR2 has accepted its
	proposed session parameters.
	d. When LSR1 has received both an acceptable Initialization
	message and a KeepAlive message the session is operational
	from LSR1's point of view.
	Until the LDP session is established, no other messages
	except those listed in the procedures above may be
	exchanged, and the rules for processing the U-bit in LDP
	messages are overridden. If a message other than those
	listed in the procedures above is received, a Shutdown msg
	MUST be transmitted and the transport connection MUST be
	closed.
	It is possible for a pair of incompatibly configured LSRs that
	disagree on session parameters to engage in an endless sequence
	of messages as each NAKs the other's Initialization messages with
	Error Notification messages.
	An LSR MUST throttle its session setup retry attempts with an
	exponential backoff in situations where Initialization messages
	are being NAK'd. It is also recommended that an LSR detecting
	such a situation take action to notify an operator.
	The session establishment setup attempt following a NAK'd Ini-
	tialization message MUST be delayed no less than 15 seconds, and
	subsequent delays MUST grow to a maximum delay of no less than 2
	minutes. The specific session establishment action that must be
	delayed is the attempt to open the session transport connection
	by the LSR playing the active role.
	The throttled sequence of Initialization NAKs is unlikely to
	cease until operator intervention reconfigures one of the LSRs.
	After such a configuration action there is no further need to
	throttle subsequent session establishment attempts (until their
	initialization messages are NAK'd).
	Due to the asymmetric nature of session establishment, reconfigu-
	ration of the passive LSR will go unnoticed by the active LSR
	without some further action. Section "Hello Message" describes
	an optional mechanism an LSR can use to signal potential LDP
	peers that it has been reconfigured.
	2.5.4. Initialization State Machine
	It is convenient to describe LDP session negotiation behavior in
	terms of a state machine. We define the LDP state machine to have
	five possible states and present the behavior as a state transition
	table and as a state transition diagram. Note that a shutdown message
	is implemented as a notification message with a status TLV indicating
	a fatal error.
	Session Initialization State Transition Table
	STATE EVENT NEW STATE
	NON EXISTENT Session TCP connection established INITIALIZED
	established
	INITIALIZED Transmit Initialization msg OPENSENT
	(Active Role)
	Receive acceptable OPENREC
	Initialization msg
	(Passive Role )
	Action: Transmit Initialization
	msg and KeepAlive msg
	Receive Any other LDP msg NON EXISTENT
	Action: Transmit Error Notification msg
	(NAK) and close transport connection
	OPENREC Receive KeepAlive msg OPERATIONAL
	Receive Any other LDP msg NON EXISTENT
	Action: Transmit Error Notification msg
	(NAK) and close transport connection
	OPENSENT Receive acceptable OPENREC
	Initialization msg
	Action: Transmit KeepAlive msg
	Receive Any other LDP msg NON EXISTENT
	Action: Transmit Error Notification msg
	(NAK) and close transport connection
	OPERATIONAL Receive Shutdown msg NON EXISTENT
	Action: Transmit Shutdown msg and
	close transport connection
	Receive other LDP msgs OPERATIONAL
	Timeout NON EXISTENT
	Action: Transmit Shutdown msg and
	close transport connection
	Session Initialization State Transition Diagram
	+------------+
	| |
	+------------>|NON EXISTENT|<--------------------+
	| | | |
	| +------------+ |
	| Session | ^ |
	| connection | | |
	| established | | Rx any LDP msg except |
	| V | Init msg or Timeout |
	| +-----------+ |
	Rx Any other | | | |
	msg or | |INITIALIZED| |
	Timeout / | +---| |-+ |
	Tx NAK msg | | +-----------+ | |
	| | (Passive Role) | (Active Role) |
	| | Rx Acceptable | Tx Init msg |
	| | Init msg / | |
	| | Tx Init msg | |
	| | Tx KeepAlive | |
	| V msg V |
	| +-------+ +--------+ |
	| | | | | |
	+---|OPENREC| |OPENSENT|----------------->|
	+---| | | | Rx Any other msg |
	| +-------+ +--------+ or Timeout |
	Rx KeepAlive | ^ | Tx NAK msg |
	msg | | | |
	| | | Rx Acceptable |
	| | | Init msg / |
	| +----------------+ Tx KeepAlive msg |
	| |
	| +-----------+ |
	+----->| | |
	|OPERATIONAL| |
	| |---------------------------->+
	+-----------+ Rx Shutdown msg
	All other | ^ or Timeout /
	LDP msgs | | Tx Shutdown msg
	| |
	+---+
	2.5.5. Maintaining Hello Adjacencies
	An LDP session with a peer has one or more Hello adjacencies.
	An LDP session has multiple Hello adjacencies when a pair of LSRs is
	connected by multiple links that share the same label space; for
	example, multiple PPP links between a pair of routers. In this situ-
	ation the Hellos an LSR sends on each such link carry the same LDP
	Identifier.
	LDP includes mechanisms to monitor the necessity of an LDP session
	and its Hello adjacencies.
	LDP uses the regular receipt of LDP Discovery Hellos to indicate a
	peer's intent to use the label space identified by the Hello. An LSR
	maintains a hold timer with each Hello adjacency which it restarts
	when it receives a Hello that matches the adjacency. If the timer
	expires without receipt of a matching Hello from the peer, LDP con-
	cludes that the peer no longer wishes to label switch using that
	label space for that link (or target, in the case of Targeted Hellos)
	or that the peer has failed. The LSR then deletes the Hello adja-
	cency. When the last Hello adjacency for a LDP session is deleted,
	the LSR terminates the LDP session by sending a Notification message
	and closing the transport connection.
	2.5.6. Maintaining LDP Sessions
	LDP includes mechanisms to monitor the integrity of the LDP session.
	LDP uses the regular receipt of LDP PDUs on the session transport
	connection to monitor the integrity of the session. An LSR maintains
	a KeepAlive timer for each peer session which it resets whenever it
	receives an LDP PDU from the session peer. If the KeepAlive timer
	expires without receipt of an LDP PDU from the peer the LSR concludes
	that the transport connection is bad or that the peer has failed, and
	it terminates the LDP session by closing the transport connection.
	After an LDP session has been established, an LSR must arrange that
	its peer receive an LDP PDU from it at least every KeepAlive time
	period to ensure the peer restarts the session KeepAlive timer. The
	LSR may send any protocol message to meet this requirement. In cir-
	cumstances where an LSR has no other information to communicate to
	its peer, it sends a KeepAlive message.
	An LSR may choose to terminate an LDP session with a peer at any
	time. Should it choose to do so, it informs the peer with a Shutdown
	message.
	2.6. Label Distribution and Management
	The MPLS architecture [RFC3031] allows an LSR to distribute a FEC
	label binding in response to an explicit request from another LSR.
	This is known as Downstream On Demand label distribution. It also
	allows an LSR to distribute label bindings to LSRs that have not
	explicitly requested them. [RFC3031] calls this method of label dis-
	tribution Unsolicited Downstream; this document uses the term Down-
	stream Unsolicited.
	Both of these label distribution techniques may be used in the same
	network at the same time. However, for any given LDP session, each
	LSR must be aware of the label distribution method used by its peer
	in order to avoid situations where one peer using Downstream Unso-
	licited label distribution assumes its peer is also. See Section
	"Downstream on Demand label Advertisement".
	2.6.1. Label Distribution Control Mode
	The behavior of the initial setup of LSPs is determined by whether
	the LSR is operating with independent or ordered LSP control. An LSR
	may support both types of control as a configurable option.
	2.6.1.1. Independent Label Distribution Control
	When using independent LSP control, each LSR may advertise label map-
	pings to its neighbors at any time it desires. For example, when
	operating in independent Downstream on Demand mode, an LSR may answer
	requests for label mappings immediately, without waiting for a label
	mapping from the next hop. When operating in independent Downstream
	Unsolicited mode, an LSR may advertise a label mapping for a FEC to
	its neighbors whenever it is prepared to label-switch that FEC.
	A consequence of using independent mode is that an upstream label can
	be advertised before a downstream label is received.
	2.6.1.2. Ordered Label Distribution Control
	When using LSP ordered control, an LSR may initiate the transmission
	of a label mapping only for a FEC for which it has a label mapping
	for the FEC next hop, or for which the LSR is the egress. For each
	FEC for which the LSR is not the egress and no mapping exists, the
	LSR MUST wait until a label from a downstream LSR is received before
	mapping the FEC and passing corresponding labels to upstream LSRs.
	An LSR may be an egress for some FECs and a non-egress for others.
	An LSR may act as an egress LSR, with respect to a particular FEC,
	under any of the following conditions:
	1. The FEC refers to the LSR itself (including one of its directly
	attached interfaces).
	2. The next hop router for the FEC is outside of the Label Switch-
	ing Network.
	3. FEC elements are reachable by crossing a routing domain bound-
	ary, such as another area for OSPF summary networks, or another
	autonomous system for OSPF AS externals and BGP routes
	[RFC2328] [RFC4271].
	Note that whether an LSR is an egress for a given FEC may change over
	time, depending on the state of the network and LSR configuration
	settings.
	2.6.2. Label Retention Mode
	The MPLS architecture [RFC3031] introduces the notion of label reten-
	tion mode which specifies whether an LSR maintains a label binding
	for a FEC learned from a neighbor that is not its next hop for the
	FEC.
	2.6.2.1. Conservative Label Retention Mode
	In Downstream Unsolicited advertisement mode, label mapping adver-
	tisements for all routes may be received from all peer LSRs. When
	using conservative label retention, advertised label mappings are
	retained only if they will be used to forward packets (i.e., if they
	are received from a valid next hop according to routing). If operat-
	ing in Downstream on Demand mode, an LSR will request label mappings
	only from the next hop LSR according to routing. Since Downstream on
	Demand mode is primarily used when label conservation is desired
	(e.g., an ATM switch with limited cross connect space), it is typi-
	cally used with the conservative label retention mode.
	The main advantage of the conservative mode is that only the labels
	that are required for the forwarding of data are allocated and main-
	tained. This is particularly important in LSRs where the label space
	is inherently limited, such as in an ATM switch. A disadvantage of
	the conservative mode is that if routing changes the next hop for a
	given destination, a new label must be obtained from the new next hop
	before labeled packets can be forwarded.
	2.6.2.2. Liberal Label Retention Mode
	In Downstream Unsolicited advertisement mode, label mapping adver-
	tisements for all routes may be received from all LDP peers. When
	using liberal label retention, every label mappings received from a
	peer LSR is retained regardless of whether the LSR is the next hop
	for the advertised mapping. When operating in Downstream on Demand
	mode with liberal label retention, an LSR might choose to request
	label mappings for all known prefixes from all peer LSRs. Note, how-
	ever, that Downstream on Demand mode is typically used by devices
	such as ATM switch-based LSRs for which the conservative approach is
	recommended.
	The main advantage of the liberal label retention mode is that reac-
	tion to routing changes can be quick because labels already exist.
	The main disadvantage of the liberal mode is that unneeded label map-
	pings are distributed and maintained.
	2.6.3. Label Advertisement Mode
	Each interface on an LSR is configured to operate in either Down-
	stream Unsolicited or Downstream on Demand advertisement mode. LSRs
	exchange advertisement modes during initialization. The major dif-
	ference between Downstream Unsolicited and Downstream on Demand modes
	is in which LSR takes responsibility for initiating mapping requests
	and mapping advertisements.
	2.7. LDP Identifiers and Next Hop Addresses
	An LSR maintains learned labels in a Label Information Base (LIB).
	When operating in Downstream Unsolicited mode, the LIB entry for an
	address prefix associates a collection of (LDP Identifier, label)
	pairs with the prefix, one such pair for each peer advertising a
	label for the prefix.
	When the next hop for a prefix changes the LSR must retrieve the
	label advertised by the new next hop from the LIB for use in forward-
	ing. To retrieve the label the LSR must be able to map the next hop
	address for the prefix to an LDP Identifier.
	Similarly, when the LSR learns a label for a prefix from an LDP peer,
	it must be able to determine whether that peer is currently a next
	hop for the prefix to determine whether it needs to start using the
	newly learned label when forwarding packets that match the prefix.
	To make that decision the LSR must be able to map an LDP Identifier
	to the peer's addresses to check whether any are a next hop for the
	prefix.
	To enable LSRs to map between a peer LDP identifier and the peer's
	addresses, LSRs advertise their addresses using LDP Address and With-
	draw Address messages.
	An LSR sends an Address message to advertise its addresses to a peer.
	An LSR sends a Withdraw Address message to withdraw previously adver-
	tised addresses from a peer
	2.8. Loop Detection
	Loop detection is a configurable option which provides a mechanism
	for finding looping LSPs and for preventing Label Request messages
	from looping in the presence of non-merge capable LSRs.
	The mechanism makes use of Path Vector and Hop Count TLVs carried by
	Label Request and Label Mapping messages. It builds on the following
	basic properties of these TLVs:
	- A Path Vector TLV contains a list of the LSRs that its containing
	message has traversed. An LSR is identified in a Path Vector
	list by its unique LSR Identifier (Id), which is the first four
	octets of its LDP Identifier. When an LSR propagates a message
	containing a Path Vector TLV it adds its LSR Id to the Path Vec-
	tor list. An LSR that receives a message with a Path Vector that
	contains its LSR Id detects that the message has traversed a
	loop. LDP supports the notion of a maximum allowable Path Vector
	length; an LSR that detects a Path Vector has reached the maximum
	length behaves as if the containing message has traversed a loop.
	- A Hop Count TLV contains a count of the LSRS that the containing
	message has traversed. When an LSR propagates a message contain-
	ing a Hop Count TLV it increments the count. An LSR that detects
	a Hop Count has reached a configured maximum value behaves as if
	the containing message has traversed a loop. By convention a
	count of 0 is interpreted to mean the hop count is unknown.
	Incrementing an unknown hop count value results in an unknown hop
	count value (0).
	The following paragraphs describes LDP loop detection procedures.
	For these paragraphs, and only these paragraphs, "MUST" is redefined
	to mean "MUST if configured for loop detection". The paragraphs
	specify messages that MUST carry Path Vector and Hop Count TLVs.
	Note that the Hop Count TLV and its procedures are used without the
	Path Vector TLV in situations when loop detection is not configured
	(see [RFC3035] and [RFC3034]).
	2.8.1. Label Request Message
	The use of the Path Vector TLV and Hop Count TLV prevent Label
	Request messages from looping in environments that include non-merge
	capable LSRs.
	The rules that govern use of the Hop Count TLV in Label Request mes-
	sages by LSR R when Loop Detection is enabled are the following:
	- The Label Request message MUST include a Hop Count TLV.
	- If R is sending the Label Request because it is a FEC ingress, it
	MUST include a Hop Count TLV with hop count value 1.
	- If R is sending the Label Request as a result of having received a
	Label Request from an upstream LSR, and if the received Label
	Request contains a Hop Count TLV, R MUST increment the received hop
	count value by 1 and MUST pass the resulting value in a Hop Count
	TLV to its next hop along with the Label Request message;
	The rules that govern use of the Path Vector TLV in Label Request
	messages by LSR R when Loop Detection is enabled are the following:
	- If R is sending the Label Request because it is a FEC ingress,
	then if R is non-merge capable, it MUST include a Path Vector TLV
	of length 1 containing its own LSR Id.
	- If R is sending the Label Request as a result of having received
	a Label Request from an upstream LSR, then if the received Label
	Request contains a Path Vector TLV or if R is non-merge capable:
	R MUST add its own LSR Id to the Path Vector, and MUST pass
	the resulting Path Vector to its next hop along with the
	Label Request message. If the Label Request contains no Path
	Vector TLV, R MUST include a Path Vector TLV of length 1 con-
	taining its own LSR Id.
	Note that if R receives a Label Request message for a particular
	FEC, and R has previously sent a Label Request message for that FEC
	to its next hop and has not yet received a reply, and if R intends
	to merge the newly received Label Request with the existing out-
	standing Label Request, then R does not propagate the Label Request
	to the next hop.
	If R receives a Label Request message from its next hop with a Hop
	Count TLV which exceeds the configured maximum value, or with a
	Path Vector TLV containing its own LSR Id or which exceeds the max-
	imum allowable length, then R detects that the Label Request
	message has traveled in a loop.
	When R detects a loop, it MUST send a Loop Detected Notification
	message to the source of the Label Request message and drop the
	Label Request message.
	2.8.2. Label Mapping Message
	The use of the Path Vector TLV and Hop Count TLV in the Label Mapping
	message provide a mechanism to find and terminate looping LSPs. When
	an LSR receives a Label Mapping message from a next hop, the message
	is propagated upstream as specified below until an ingress LSR is
	reached or a loop is found.
	The rules that govern the use of the Hop Count TLV in Label Mapping
	messages sent by an LSR R when Loop Detection is enabled are the fol-
	lowing:
	- R MUST include a Hop Count TLV.
	- If R is the egress, the hop count value MUST be 1.
	- If the Label Mapping message is being sent to propagate a Label
	Mapping message received from the next hop to an upstream peer, the
	hop count value MUST be determined as follows:
	o If R is a member of the edge set of an LSR domain whose LSRs do
	not perform 'TTL-decrement' (e.g., an ATM LSR domain or a Frame
	Relay LSR domain) and the upstream peer is within that domain, R
	MUST reset the hop count to 1 before propagating the message.
	o Otherwise, R MUST increment the hop count received from the next
	hop before propagating the message.
	- If the Label Mapping message is not being sent to propagate a
	Label Mapping message, the hop count value MUST be the result of
	incrementing R's current knowledge of the hop count learned from
	previous Label Mapping messages. Note that this hop count value
	will be unknown if R has not received a Label Mapping message from
	the next hop.
	Any Label Mapping message MAY contain a Path Vector TLV. The rules
	that govern the mandatory use of the Path Vector TLV in Label Mapping
	messages sent by LSR R when Loop Detection is enabled are the follow-
	ing:
	- If R is the egress, the Label Mapping message need not include a
	Path Vector TLV.
	- If R is sending the Label Mapping message to propagate a Label
	Mapping message received from the next hop to an upstream peer,
	then:
	o If R is merge capable and if R has not previously sent a Label
	Mapping message to the upstream peer, then it MUST include a
	Path Vector TLV.
	o If the received message contains an unknown hop count, then R
	MUST include a Path Vector TLV.
	o If R has previously sent a Label Mapping message to the
	upstream peer, then it MUST include a Path Vector TLV if the
	received message reports an LSP hop count increase, a change in
	hop count from unknown to known, or a change from known to
	unknown.
	If the above rules require R include a Path Vector TLV in the Label
	Mapping message, R computes it as follows:
	o If the received Label Mapping message included a Path Vector,
	the Path Vector sent upstream MUST be the result of adding R's
	LSR Id to the received Path Vector.
	o If the received message had no Path Vector, the Path Vector
	sent upstream MUST be a path vector of length 1 containing R's
	LSR Id.
	- If the Label Mapping message is not being sent to propagate a
	received message upstream, the Label Mapping message MUST
	include a Path Vector of length 1 containing R's LSR Id.
	If R receives a Label Mapping message from its next hop with a
	Hop Count TLV which exceeds the configured maximum value, or with
	a Path Vector TLV containing its own LSR Id or which exceeds the
	maximum allowable length, then R detects that the corresponding
	LSP contains a loop.
	When R detects a loop, it MUST stop using the label for forward-
	ing, drop the Label Mapping message, and signal Loop Detected
	status to the source of the Label Mapping message.
	2.8.3. Discussion
	If loop detection is desired in an MPLS domain, then it should be
	turned on in ALL LSRs within that MPLS domain, else loop detection
	will not operate properly and may result in undetected loops or in
	falsely detected loops.
	LSRs which are configured for loop detection are NOT expected to
	store the path vectors as part of the LSP state.
	Note that in a network where only non-merge capable LSRs are present,
	Path Vectors are passed downstream from ingress to egress, and are
	not passed upstream. Even when merge is supported, Path Vectors need
	not be passed upstream along an LSP which is known to reach the
	egress. When an LSR experiences a change of next hop, it need pass
	Path Vectors upstream only when it cannot tell from the hop count
	that the change of next hop does not result in a loop.
	In the case of ordered label distribution, Label Mapping messages are
	propagated from egress toward ingress, naturally creating the Path
	Vector along the way. In the case of independent label distribution,
	an LSR may originate a Label Mapping message for an FEC before
	receiving a Label Mapping message from its downstream peer for that
	FEC. In this case, the subsequent Label Mapping message for the FEC
	received from the downstream peer is treated as an update to LSP
	attributes, and the Label Mapping message must be propagated
	upstream. Thus, it is recommended that loop detection be configured
	in conjunction with ordered label distribution, to minimize the num-
	ber of Label Mapping update messages.
	2.9. Authenticity and Integrity of LDP Messages
	This section specifies a mechanism to protect against the introduc-
	tion of spoofed TCP segments into LDP session connection streams.
	The use of this mechanism MUST be supported as a configurable option.
	The mechanism is based on use of the TCP MD5 Signature Option speci-
	fied in [RFC2385] for use by BGP [RFC4271]. See [RFC1321] for a
	specification of the MD5 hash function. From a standards maturity
	point of view, the current document relates to [RFC2385] the same way
	as [RFC4271] relates to [RFC2385]. This is explained in [RFC4278].
	2.9.1. TCP MD5 Signature Option
	The following quotes from [RFC2385] outline the security properties
	achieved by using the TCP MD5 Signature Option and summarizes its
	operation:
	"IESG Note
	This document describes current existing practice for securing
	BGP against certain simple attacks. It is understood to have
	security weaknesses against concerted attacks."
	"Abstract
	This memo describes a TCP extension to enhance security for
	BGP. It defines a new TCP option for carrying an MD5 [RFC1321]
	digest in a TCP segment. This digest acts like a signature for
	that segment, incorporating information known only to the con-
	nection end points. Since BGP uses TCP as its transport, using
	this option in the way described in this paper significantly
	reduces the danger from certain security attacks on BGP."
	"Introduction
	The primary motivation for this option is to allow BGP to pro-
	tect itself against the introduction of spoofed TCP segments
	into the connection stream. Of particular concern are TCP
	resets.
	To spoof a connection using the scheme described in this paper,
	an attacker would not only have to guess TCP sequence numbers,
	but would also have had to obtain the password included in the
	MD5 digest. This password never appears in the connection
	stream, and the actual form of the password is up to the appli-
	cation. It could even change during the lifetime of a particu-
	lar connection so long as this change was synchronized on both
	ends (although retransmission can become problematical in some
	TCP implementations with changing passwords).
	Finally, there is no negotiation for the use of this option in
	a connection, rather it is purely a matter of site policy
	whether or not its connections use the option."
	"MD5 as a Hashing Algorithm
	Since this memo was first issued (under a different title), the
	MD5 algorithm has been found to be vulnerable to collision
	search attacks [Dobb], and is considered by some to be
	insufficiently strong for this type of application.
	This memo still specifies the MD5 algorithm, however, since the
	option has already been deployed operationally, and there was
	no "algorithm type" field defined to allow an upgrade using the
	same option number. The original document did not specify a
	type field since this would require at least one more byte, and
	it was felt at the time that taking 19 bytes for the complete
	option (which would probably be padded to 20 bytes in TCP
	implementations) would be too much of a waste of the already
	limited option space.
	This does not prevent the deployment of another similar option
	which uses another hashing algorithm (like SHA-1). Also, if
	most implementations pad the 18 byte option as defined to 20
	bytes anyway, it would be just as well to define a new option
	which contains an algorithm type field.
	This would need to be addressed in another document, however."
	End of quotes from [RFC2385].
	2.9.2. LDP Use of TCP MD5 Signature Option
	LDP uses the TCP MD5 Signature Option as follows:
	- Use of the MD5 Signature Option for LDP TCP connections is a con-
	figurable LSR option.
	- An LSR that uses the MD5 Signature Option is configured with a
	password (shared secret) for each potential LDP peer.
	- The LSR applies the MD5 algorithm as specified in [RFC2385] to
	compute the MD5 digest for a TCP segment to be sent to a peer.
	This computation makes use of the peer password as well as the
	TCP segment.
	- When the LSR receives a TCP segment with an MD5 digest, it vali-
	dates the segment by calculating the MD5 digest (using its own
	record of the password) and compares the computed digest with the
	received digest. If the comparison fails, the segment is dropped
	without any response to the sender.
	- The LSR ignores LDP Hellos from any LSR for which a password has
	not been configured. This ensures that the LSR establishes LDP
	TCP connections only with LSRs for which a password has been con-
	figured.
	2.10. Label Distribution for Explicitly Routed LSPs
	Traffic Engineering [RFC2702] is expected to be an important MPLS
	application. MPLS support for Traffic Engineering uses explicitly
	routed LSPs, which need not follow normally-routed (hop-by-hop) paths
	as determined by destination-based routing protocols. CR-LDP [CRLDP]
	defines extensions to LDP to use LDP to set up explicitly routed
	LSPs.
	3. Protocol Specification
	Previous sections that describe LDP operation have discussed scenar-
	ios that involve the exchange of messages among LDP peers. This sec-
	tion specifies the message encodings and procedures for processing
	the messages.
	LDP message exchanges are accomplished by sending LDP protocol data
	units (PDUs) over LDP session TCP connections.
	Each LDP PDU can carry one or more LDP messages. Note that the mes-
	sages in an LDP PDU need not be related to one another. For example,
	a single PDU could carry a message advertising FEC-label bindings for
	several FECs, another message requesting label bindings for several
	other FECs, and a third notification message signaling some event.
	3.1. LDP PDUs
	Each LDP PDU is an LDP header followed by one or more LDP messages.
	The LDP header is:
	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
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Version | PDU Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| LDP Identifier |
	+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Version
	Two octet unsigned integer containing the version number of the proto-
	col. This version of the specification specifies LDP protocol version
	1.
	PDU Length
	Two octet integer specifying the total length of this PDU in octets,
	excluding the Version and PDU Length fields.
	The maximum allowable PDU Length is negotiable when an LDP session is
	initialized. Prior to completion of the negotiation the maximum
	allowable length is 4096 bytes.
	LDP Identifier
	Six octet field that uniquely identifies the label space of the send-
	ing LSR for which this PDU applies. The first four octets identify
	the LSR and MUST be a globally unique value. It SHOULD be a 32-bit
	router Id assigned to the LSR and also used to identify it in loop
	detection Path Vectors. The last two octets identify a label space
	within the LSR. For a platform-wide label space, these SHOULD both be
	zero.
	Note that there is no alignment requirement for the first octet of an
	LDP PDU.
	3.2. LDP Procedures
	LDP defines messages, TLVs and procedures in the following areas:
	- Peer discovery;
	- Session management;
	- Label distribution;
	- Notification of errors and advisory information.
	The sections that follow describe the message and TLV encodings for
	these areas and the procedures that apply to them.
	The label distribution procedures are complex and are difficult to
	describe fully, coherently and unambiguously as a collection of sepa-
	rate message and TLV specifications.
	Appendix A, "LDP Label Distribution Procedures", describes the label
	distribution procedures in terms of label distribution events that
	may occur at an LSR and how the LSR must respond. Appendix A is the
	specification of LDP label distribution procedures. If a procedure
	described elsewhere in this document conflicts with Appendix A,
	Appendix A specifies LDP behavior.
	3.3. Type-Length-Value Encoding
	LDP uses a Type-Length-Value (TLV) encoding scheme to encode much of
	the information carried in LDP messages.
	An LDP TLV is encoded as a 2 octet field that uses 14 bits to specify
	a Type and 2 bits to specify behavior when an LSR doesn't recognize
	the Type, followed by a 2 octet Length Field, followed by a variable
	length Value field.
	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
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	|U|F| Type | Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	| Value |
	~ ~
	| |
	| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	U bit
	Unknown TLV bit. Upon receipt of an unknown TLV, if U is clear
	(=0), a notification MUST be returned to the message originator
	and the entire message MUST be ignored; if U is set (=1), the
	unknown TLV MUST be silently ignored and the rest of the message
	processed as if the unknown TLV did not exist. The sections fol-
	lowing that define TLVs specify a value for the U-bit.
	F bit
	Forward unknown TLV bit. This bit applies only when the U bit is
	set and the LDP message containing the unknown TLV is to be for-
	warded. If F is clear (=0), the unknown TLV is not forwarded with
	the containing message; if F is set (=1), the unknown TLV is for-
	warded with the containing message. The sections following that
	define TLVs specify a value for the F-bit. By setting both the U
	and F bits, a TLV can be propagated as opaque data through nodes
	that do not recognize the TLV.
	Type
	Encodes how the Value field is to be interpreted.
	Length
	Specifies the length of the Value field in octets.
	Value
	Octet string of Length octets that encodes information to be
	interpreted as specified by the Type field.
	Note that there is no alignment requirement for the first octet of a
	TLV.
	Note that the Value field itself may contain TLV encodings. That is,
	TLVs may be nested.
	The TLV encoding scheme is very general. In principle, everything
	appearing in an LDP PDU could be encoded as a TLV. This specifica-
	tion does not use the TLV scheme to its full generality. It is not
	used where its generality is unnecessary and its use would waste
	space unnecessarily. These are usually places where the type of a
	value to be encoded is known, for example by its position in a mes-
	sage or an enclosing TLV, and the length of the value is fixed or
	readily derivable from the value encoding itself.
	Some of the TLVs defined for LDP are similar to one another. For
	example, there is a Generic Label TLV, an ATM Label TLV, and a Frame
	Relay TLV; see Sections "Generic Label TLV", "ATM Label TLV", and
	"Frame Relay TLV".
	While it is possible to think about TLVs related in this way in terms
	of a TLV type that specifies a TLV class and a TLV subtype that spec-
	ifies a particular kind of TLV within that class, this specification
	does not formalize the notion of a TLV subtype.
	The specification assigns type values for related TLVs, such as the
	label TLVs, from a contiguous block in the 16-bit TLV type number
	space.
	Section "TLV Summary" lists the TLVs defined in this version of the
	protocol and the section in this document that describes each.
	3.4. TLV Encodings for Commonly Used Parameters
	There are several parameters used by more than one LDP message. The
	TLV encodings for these commonly used parameters are specified in
	this section.
	3.4.1. FEC TLV
	Labels are bound to Forwarding Equivalence Classes (FECs). A FEC is
	a list of one or more FEC elements. The FEC TLV encodes FEC items.
	Its encoding is:
	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|0| FEC (0x0100) | Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| FEC Element 1 |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	~ ~
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| FEC Element n |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	FEC Element 1 to FEC Element n
	There are several types of FEC elements; see Section "FECs". The
	FEC element encoding depends on the type of FEC element.
	A FEC Element value is encoded as a 1 octet field that specifies
	the element type, and a variable length field that is the type-
	dependent element value. Note that while the representation of
	the FEC element value is type-dependent, the FEC element encoding
	itself is one where standard LDP TLV encoding is not used.
	The FEC Element value encoding is:
	FEC Element Type Value
	type name
	Wildcard 0x01 No value; i.e., 0 value octets;
	see below.
	Prefix 0x02 See below.
	Note that this version of LDP supports the use of multiple FEC
	Elements per FEC for the Label Mapping message only. The use of
	multiple FEC Elements in other messages is not permitted in this
	version, and is a subject for future study.
	Wildcard FEC Element
	To be used only in the Label Withdraw and Label Release
	Messages. Indicates the withdraw/release is to be applied to
	all FECs associated with the label within the following label
	TLV. Must be the only FEC Element in the FEC TLV.
	Prefix FEC Element value encoding:
	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
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Prefix (2) | Address Family | PreLen |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Prefix |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Address Family
	Two octet quantity containing a value from ADDRESS FAMILY NUM-
	BERS in [ASSIGNED_AF] that encodes the address family for the
	address prefix in the Prefix field.
	PreLen
	One octet unsigned integer containing the length in bits of the
	address prefix that follows. A length of zero indicates a pre-
	fix that matches all addresses (the default destination); in
	this case the Prefix itself is zero octets).
	Prefix
	An address prefix encoded according to the Address Family
	field, whose length, in bits, was specified in the PreLen
	field, padded to a byte boundary.
	3.4.1.1. FEC Procedures
	If in decoding a FEC TLV an LSR encounters a FEC Element with an
	Address Family it does not support, it SHOULD stop decoding the FEC
	TLV, abort processing the message containing the TLV, and send an
	"Unsupported Address Family" Notification message to its LDP peer
	signaling an error.
	If it encounters a FEC Element type it cannot decode, it SHOULD stop
	decoding the FEC TLV, abort processing the message containing the
	TLV, and send an "Unknown FEC" Notification message to its LDP peer
	signaling an error.
	3.4.2. Label TLVs
	Label TLVs encode labels. Label TLVs are carried by the messages
	used to advertise, request, release and withdraw label mappings.
	There are several different kinds of Label TLVs which can appear in
	situations that require a Label TLV.
	3.4.2.1. Generic Label TLV
	An LSR uses Generic Label TLVs to encode labels for use on links for
	which label values are independent of the underlying link technology.
	Examples of such links are PPP and Ethernet.
	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|0| Generic Label (0x0200) | Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Label |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Label
	This is a 20-bit label value represented as a 20-bit number in a 4
	octet field as follows:
	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
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Label | |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	For further information, see [RFC3032].
	3.4.2.2. ATM Label TLV
	An LSR uses ATM Label TLVs to encode labels for use on ATM links.
	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|0| ATM Label (0x0201) | Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	|Res| V | VPI | VCI |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Res
	This field is reserved. It MUST be set to zero on transmission
	and MUST be ignored on receipt.
	V-bits
	Two-bit switching indicator. If V-bits is 00, both the VPI and
	VCI are significant. If V-bits is 01, only the VPI field is sig-
	nificant. If V-bit is 10, only the VCI is significant.
	VPI
	Virtual Path Identifier. If VPI is less than 12-bits it SHOULD be
	right justified in this field and preceding bits SHOULD be set to
	0.
	VCI
	Virtual Channel Identifier. If the VCI is less than 16- bits, it
	SHOULD be right justified in the field and the preceding bits MUST
	be set to 0. If Virtual Path switching is indicated in the V-bits
	field, then this field MUST be ignored by the receiver and set to
	0 by the sender.
	3.4.2.3. Frame Relay Label TLV
	An LSR uses Frame Relay Label TLVs to encode labels for use on Frame
	Relay links.
	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|0| Frame Relay Label (0x0202)| Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Reserved |Len| DLCI |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Res
	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 follow-
	ing values are supported:
	0 = 10 bits DLCI
	2 = 23 bits DLCI
	Len values 1 and 3 are reserved.
	DLCI
	The Data Link Connection Identifier
	For a 10bit DLCI, the encoding is:
	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|0| Frame Relay Label (0x0202)| Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Reserved |Len| 0 | 10 bit DLCI |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	For a 23bit DLCI, the encoding is:
	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|0| Frame Relay Label (0x0202)| Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Reserved |Len| 23 bit DLCI |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	For further information, see [RFC3034].
	3.4.3. Address List TLV
	The Address List TLV appears in Address and Address Withdraw mes-
	sages.
	Its encoding is:
	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|0| Address List (0x0101) | Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Address Family | |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
	| |
	| Addresses |
	~ ~
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Address Family
	Two octet quantity containing a value from ADDRESS FAMILY NUMBERS
	in [ASSIGNED_AF] that encodes the addresses contained in the
	Addresses field.
	Addresses
	A list of addresses from the specified Address Family. The
	encoding of the individual addresses depends on the Address
	Family.
	The following address encodings are defined by this version of the
	protocol:
	Address Family Address Encoding
	IPv4 4 octet full IPv4 address
	IPv6 16 octet full IPv6 address
	3.4.4. Hop Count TLV
	The Hop Count TLV appears as an optional field in messages that set
	up LSPs. It calculates the number of LSR hops along an LSP as the
	LSP is being setup.
	Note that setup procedures for LSPs that traverse ATM and Frame Relay
	links require use of the Hop Count TLV (see [RFC3035] and [RFC3034]).
	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|0| Hop Count (0x0103) | Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| HC Value |
	+-+-+-+-+-+-+-+-+
	HC Value
	1 octet unsigned integer hop count value.
	3.4.4.1. Hop Count Procedures
	During setup of an LSP an LSR R may receive a Label Mapping or Label
	Request message for the LSP that contains the Hop Count TLV. If it
	does, it SHOULD record the hop count value.
	If LSR R then propagates the Label Mapping message for the LSP to an
	upstream peer or the Label Request message to a downstream peer to
	continue the LSP setup, it must determine a hop count to include in
	the propagated message as follows:
	- If the message is a Label Request message, R MUST increment the
	received hop count;
	- If the message is a Label Mapping message, R determines the hop
	count as follows:
	o If R is a member of the edge set of an LSR domain whose LSRs do
	not perform 'TTL-decrement' and the upstream peer is within
	that domain, R MUST reset the hop count to 1 before propagating
	the message.
	o Otherwise, R MUST increment the received hop count.
	The first LSR in the LSP (ingress for a Label Request message, egress
	for a Label Mapping message) SHOULD set the hop count value to 1.
	By convention a value of 0 indicates an unknown hop count. The
	result of incrementing an unknown hop count is itself an unknown hop
	count (0).
	Use of the unknown hop count value greatly reduces the signaling
	overhead when independent control is used. When a new LSP is estab-
	lished, each LSR starts with unknown hop count. Addition of a new
	LSR whose hop count is also unknown does not cause a hop count update
	to be propagated upstream since the hop count remains unknown. When
	the egress is finally added to the LSP, then the LSRs propagate hop
	count updates upstream via Label Mapping messages.
	Without use of the unknown hop count, each time a new LSR is added to
	the LSP a hop count update would need to be propagated upstream if
	the new LSR is closer to the egress than any of the other LSRs.
	These updates are useless overhead since they don't reflect the hop
	count to the egress.
	>From the perspective of the ingress node, the fact that the hop
	count is unknown implies nothing about whether a packet sent on the
	LSP will actually make it to the egress. All it implies is that the
	hop count update from the egress has not yet reached the ingress.
	If an LSR receives a message containing a Hop Count TLV, it MUST
	check the hop count value to determine whether the hop count has
	exceeded its configured maximum allowable value. If so, it MUST
	behave as if the containing message has traversed a loop by sending a
	Notification message signaling Loop Detected in reply to the sender
	of the message.
	If Loop Detection is configured, the LSR MUST follow the procedures
	specified in Section "Loop Detection".
	3.4.5. Path Vector TLV
	The Path Vector TLV is used with the Hop Count TLV in Label Request
	and Label Mapping messages to implement the optional LDP loop detec-
	tion mechanism. See Section "Loop Detection". Its use in the Label
	Request message records the path of LSRs the request has traversed.
	Its use in the Label Mapping message records the path of LSRs a label
	advertisement has traversed to setup an LSP. Its encoding is:
	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|0| Path Vector (0x0104) | Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| LSR Id 1 |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	~ ~
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| LSR Id n |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	One or more LSR Ids
	A list of router-ids indicating the path of LSRs the message has
	traversed. Each LSR Id is the first four octets (router-id) of
	the LDP identifier for the corresponding LSR. This ensures it is
	unique within the LSR network.
	3.4.5.1. Path Vector Procedures
	The Path Vector TLV is carried in Label Mapping and Label Request
	messages when loop detection is configured.
	3.4.5.1.1. Label Request Path Vector
	Section "Loop Detection" specifies situations when an LSR must
	include a Path Vector TLV in a Label Request message.
	An LSR that receives a Path Vector in a Label Request message MUST
	perform the procedures described in Section "Loop Detection".
	If the LSR detects a loop, it MUST reject the Label Request message.
	The LSR MUST:
	1. Transmit a Notification message to the sending LSR signaling
	"Loop Detected".
	2. Not propagate the Label Request message further.
	Note that a Label Request message with Path Vector TLV is forwarded
	until:
	1. A loop is found,
	2. The LSP egress is reached,
	3. The maximum Path Vector limit or maximum Hop Count limit is
	reached. This is treated as if a loop had been detected.
	3.4.5.1.2. Label Mapping Path Vector
	Section "Loop Detection" specifies the situations when an LSR must
	include a Path Vector TLV in a Label Mapping message.
	An LSR that receives a Path Vector in a Label Mapping message MUST
	perform the procedures described in Section "Loop Detection".
	If the LSR detects a loop, it MUST reject the Label Mapping message
	in order to prevent a forwarding loop. The LSR MUST:
	1. Transmit a Label Release message carrying a Status TLV to the
	sending LSR to signal "Loop Detected".
	2. Not propagate the message further.
	3. Check whether the Label Mapping message is for an existing LSP.
	If so, the LSR must unsplice any upstream labels which are
	spliced to the downstream label for the FEC.
	Note that a Label Mapping message with a Path Vector TLV is forwarded
	until:
	1. A loop is found,
	2. An LSP ingress is reached, or
	3. The maximum Path Vector or maximum Hop Count limit is reached.
	This is treated as if a loop had been detected.
	3.4.6. Status TLV
	Notification messages carry Status TLVs to specify events being sig-
	naled.
	The encoding for the Status TLV is:
	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
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	|U|F| Status (0x0300) | Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Status Code |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message Type |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	U bit
	SHOULD be 0 when the Status TLV is sent in a Notification message.
	SHOULD be 1 when the Status TLV is sent in some other message.
	F bit
	SHOULD be the same as the setting of the F-bit in the Status Code
	field.
	Status Code
	32-bit unsigned integer encoding the event being signaled. The
	structure of a Status Code is:
	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
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	|E|F| Status Data |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	E bit
	Fatal error bit. If set (=1), this is a fatal error notifica-
	tion. If clear (=0), this is an advisory notification.
	F bit
	Forward bit. If set (=1), the notification SHOULD be forwarded
	to the LSR for the next-hop or previous-hop for the LSP, if
	any, associated with the event being signaled. If clear (=0),
	the notification SHOULD NOT be forwarded.
	Status Data
	30-bit unsigned integer which specifies the status information.
	This specification defines Status Codes (32-bit unsigned integers
	with the above encoding).
	A Status Code of 0 signals success.
	Message ID
	If non-zero, 32-bit value that identifies the peer message to
	which the Status TLV refers. If zero, no specific peer message is
	being identified.
	Message Type
	If non-zero, the type of the peer message to which the Status TLV
	refers. If zero, the Status TLV does not refer to any specific
	message type.
	Note that use of the Status TLV is not limited to Notification mes-
	sages. A message other than a Notification message may carry a Sta-
	tus TLV as an Optional Parameter. When a message other than a Noti-
	fication carries a Status TLV the U-bit of the Status TLV SHOULD be
	set to 1 to indicate that the receiver SHOULD silently discard the
	TLV if unprepared to handle it.
	3.5. LDP Messages
	All LDP messages have the following format:
	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
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	|U| Message Type | Message Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	+ +
	| Mandatory Parameters |
	+ +
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	+ +
	| Optional Parameters |
	+ +
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	U bit
	Unknown message bit. Upon receipt of an unknown message, if U is
	clear (=0), a notification is returned to the message originator;
	if U is set (=1), the unknown message is silently ignored. The
	sections following that define messages specify a value for the U-
	bit.
	Message Type
	Identifies the type of message
	Message Length
	Specifies the cumulative length in octets of the Message ID,
	Mandatory Parameters, and Optional Parameters.
	Message ID
	32-bit value used to identify this message. Used by the sending
	LSR to facilitate identifying notification messages that may apply
	to this message. An LSR sending a notification message in
	response to this message SHOULD include this Message Id in the
	Status TLV carried by the notification message; see Section "Noti-
	fication Message".
	Mandatory Parameters
	Variable length set of required message parameters. Some messages
	have no required parameters.
	For messages that have required parameters, the required parame-
	ters MUST appear in the order specified by the individual message
	specifications in the sections that follow.
	Optional Parameters
	Variable length set of optional message parameters. Many messages
	have no optional parameters.
	For messages that have optional parameters, the optional parame-
	ters may appear in any order.
	Note that there is no alignment requirement for the first octet of an
	LDP message and that there is no padding at the end of a message;
	that is, parameters can end at odd-byte boundaries.
	The following message types are defined in this version of LDP:
	Message Name Section Title
	Notification "Notification Message"
	Hello "Hello Message"
	Initialization "Initialization Message"
	KeepAlive "KeepAlive Message"
	Address "Address Message"
	Address Withdraw "Address Withdraw Message"
	Label Mapping "Label Mapping Message"
	Label Request "Label Request Message"
	Label Abort Request "Label Abort Request Message"
	Label Withdraw "Label Withdraw Message"
	Label Release "Label Release Message"
	The sections that follow specify the encodings and procedures for
	these messages.
	Some of the above messages are related to one another, for example
	the Label Mapping, Label Request, Label Withdraw, and Label Release
	messages.
	While it is possible to think about messages related in this way in
	terms of a message type that specifies a message class and a message
	subtype that specifies a particular kind of message within that
	class, this specification does not formalize the notion of a message
	subtype.
	The specification assigns type values for related messages, such as
	the label messages, from of a contiguous block in the 16-bit message
	type number space.
	3.5.1. Notification Message
	An LSR sends a Notification message to inform an LDP peer of a sig-
	nificant event. A Notification message signals a fatal error or pro-
	vides advisory information such as the outcome of processing an LDP
	message or the state of the LDP session.
	The encoding for the Notification Message is:
	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| Notification (0x0001) | Message Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Status (TLV) |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Optional Parameters |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Message ID
	32-bit value used to identify this message.
	Status TLV
	Indicates the event being signaled. The encoding for the Status
	TLV is specified in Section "Status TLV".
	Optional Parameters
	This variable length field contains 0 or more parameters, each
	encoded as a TLV. The following Optional Parameters are generic
	and may appear in any Notification Message:
	Optional Parameter Type Length Value
	Extended Status 0x0301 4 See below
	Returned PDU 0x0302 var See below
	Returned Message 0x0303 var See below
	Other Optional Parameters, specific to the particular event being
	signaled by the Notification Messages may appear. These are
	described elsewhere.
	Extended Status
	The 4 octet value is an Extended Status Code that encodes addi-
	tional information that supplements the status information con-
	tained in the Notification Status Code.
	Returned PDU
	An LSR uses this parameter to return part of an LDP PDU to the
	LSR that sent it. The value of this TLV is the PDU header and
	as much PDU data following the header as appropriate for the
	condition being signaled by the Notification message.
	Returned Message
	An LSR uses this parameter to return part of an LDP message to
	the LSR that sent it. The value of this TLV is the message
	type and length fields and as much message data following the
	type and length fields as appropriate for the condition being
	signaled by the Notification message.
	3.5.1.1. Notification Message Procedures
	If an LSR encounters a condition requiring it to notify its peer with
	advisory or error information it sends the peer a Notification mes-
	sage containing a Status TLV that encodes the information and option-
	ally additional TLVs that provide more information about the condi-
	tion.
	If the condition is one that is a fatal error the Status Code carried
	in the notification will indicate that. In this case, after sending
	the Notification message the LSR SHOULD terminate the LDP session by
	closing the session TCP connection and discard all state associated
	with the session, including all label-FEC bindings learned via the
	session.
	When an LSR receives a Notification message that carries a Status
	Code that indicates a fatal error, it SHOULD terminate the LDP ses-
	sion immediately by closing the session TCP connection and discard
	all state associated with the session, including all label-FEC bind-
	ings learned via the session.
	The above statement does not apply to the processing of the Shutdown
	message in the session initialization procedure. When an LSR receives
	a Shutdown message during session initialization, it SHOULD transmit
	a Shutdown message and then close the transport connection.
	3.5.1.2. Events Signaled by Notification Messages
	It is useful for descriptive purpose to classify events signaled by
	Notification Messages into the following categories.
	3.5.1.2.1. Malformed PDU or Message
	Malformed LDP PDUs or Messages that are part of the LDP Discovery
	mechanism are handled by silently discarding them.
	An LDP PDU received on a TCP connection for an LDP session is mal-
	formed if:
	- The LDP Identifier in the PDU header is unknown to the
	receiver, or it is known but is not the LDP Identifier associ-
	ated by the receiver with the LDP peer for this LDP session.
	This is a fatal error signaled by the Bad LDP Identifier Status
	Code.
	- The LDP protocol version is not supported by the receiver, or
	it is supported but is not the version negotiated for the ses-
	sion during session establishment. This is a fatal error sig-
	naled by the Bad Protocol Version Status Code.
	- The PDU Length field is too small (< 14) or too large
	(> maximum PDU length). This is a fatal error signaled by the
	Bad PDU Length Status Code. Section "Initialization Message"
	describes how the maximum PDU length for a session is deter-
	mined.
	An LDP Message is malformed if:
	- The Message Type is unknown.
	If the Message Type is < 0x8000 (high order bit = 0) it is an
	error signaled by the Unknown Message Type Status Code.
	If the Message Type is >= 0x8000 (high order bit = 1) it is
	silently discarded.
	- The Message Length is too large, that is, indicates that the
	message extends beyond the end of the containing LDP PDU. This
	is a fatal error signaled by the Bad Message Length Status
	Code.
	- The Message Length is too small, that is, smaller than the
	smallest possible value component. This is a fatal error sig-
	naled by the Bad Message Length Status Code.
	- The message is missing one or more Mandatory Parameters. This
	is a non-fatal error signaled by the Missing Message Parameters
	Status Code.
	3.5.1.2.2. Unknown or Malformed TLV
	Malformed TLVs contained in LDP messages that are part of the LDP
	Discovery mechanism are handled by silently discarding the containing
	message.
	A TLV contained in an LDP message received on a TCP connection of an
	LDP is malformed if:
	- The TLV Length is too large, that is, indicates that the TLV
	extends beyond the end of the containing message. This is a
	fatal error signaled by the Bad TLV Length Status Code.
	- The TLV type is unknown.
	If the TLV type is < 0x8000 (high order bit 0) it is an error
	signaled by the Unknown TLV Status Code.
	If the TLV type is >= 0x8000 (high order bit 1) the TLV is
	silently dropped.
	- The TLV Value is malformed. This occurs when the receiver han-
	dles the TLV but cannot decode the TLV Value. This is inter-
	preted as indicative of a bug in either the sending or receiv-
	ing LSR. It is a fatal error signaled by the Malformed TLV
	Value Status Code.
	3.5.1.2.3. Session KeepAlive Timer Expiration
	This is a fatal error signaled by the KeepAlive Timer Expired Status
	Code.
	3.5.1.2.4. Unilateral Session Shutdown
	This is a fatal event signaled by the Shutdown Status Code. The
	Notification Message may optionally include an Extended Status TLV to
	provide a reason for the Shutdown. The sending LSR terminates the
	session immediately after sending the Notification.
	3.5.1.2.5. Initialization Message Events
	The session initialization negotiation (see Section "Session Initial-
	ization") may fail if the session parameters received in the Initial-
	ization Message are unacceptable. This is a fatal error. The spe-
	cific Status Code depends on the parameter deemed unacceptable, and
	is defined in Sections "Initialization Message".
	3.5.1.2.6. Events Resulting From Other Messages
	Messages other than the Initialization message may result in events
	that must be signaled to LDP peers via Notification Messages. These
	events and the Status Codes used in the Notification Messages to sig-
	nal them are described in the sections that describe these messages.
	3.5.1.2.7. Internal Errors
	An LDP implementation may be capable of detecting problem conditions
	specific to its implementation. When such a condition prevents an
	implementation from interacting correctly with a peer, the implemen-
	tation should, when capable of doing so, use the Internal Error Sta-
	tus Code to signal the peer. This is a fatal error.
	3.5.1.2.8. Miscellaneous Events
	These are events that fall into none of the categories above. There
	are no miscellaneous events defined in this version of the protocol.
	3.5.2. Hello Message
	LDP Hello Messages are exchanged as part of the LDP Discovery Mecha-
	nism; see Section "LDP Discovery".
	The encoding for the Hello Message is:
	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| Hello (0x0100) | Message Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Common Hello Parameters TLV |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Optional Parameters |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Message ID
	32-bit value used to identify this message.
	Common Hello Parameters TLV
	Specifies parameters common to all Hello messages. The encoding
	for the Common Hello Parameters TLV is:
	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|0| Common Hello Parms(0x0400)| Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Hold Time |T|R| Reserved |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Hold Time,
	Hello hold time in seconds. An LSR maintains a record of Hel-
	los received from potential peers (see Section "Hello Message
	Procedures"). Hello Hold Time specifies the time the sending
	LSR will maintain its record of Hellos from the receiving LSR
	without receipt of another Hello.
	A pair of LSRs negotiates the hold times they use for Hellos
	from each other. Each proposes a hold time. The hold time
	used is the minimum of the hold times proposed in their Hellos.
	A value of 0 means use the default, which is 15 seconds for
	Link Hellos and 45 seconds for Targeted Hellos. A value of
	0xffff means infinite.
	T, Targeted Hello
	A value of 1 specifies that this Hello is a Targeted Hello. A
	value of 0 specifies that this Hello is a Link Hello.
	R, Request Send Targeted Hellos
	A value of 1 requests the receiver to send periodic Targeted
	Hellos to the source of this Hello. A value of 0 makes no
	request.
	An LSR initiating Extended Discovery sets R to 1. If R is 1,
	the receiving LSR checks whether it has been configured to send
	Targeted Hellos to the Hello source in response to Hellos with
	this request. If not, it ignores the request. If so, it ini-
	tiates periodic transmission of Targeted Hellos to the Hello
	source.
	Reserved
	This field is reserved. It MUST be set to zero on transmission
	and ignored on receipt.
	Optional Parameters
	This variable length field contains 0 or more parameters, each
	encoded as a TLV. The optional parameters defined by this ver-
	sion of the protocol are
	Optional Parameter Type Length Value
	IPv4 Transport Address 0x0401 4 See below
	Configuration 0x0402 4 See below
	Sequence Number
	IPv6 Transport Address 0x0403 16 See below
	IPv4 Transport Address
	Specifies the IPv4 address to be used for the sending LSR when
	opening the LDP session TCP connection. If this optional TLV
	is not present the IPv4 source address for the UDP packet car-
	rying the Hello SHOULD be used.
	Configuration Sequence Number
	Specifies a 4 octet unsigned configuration sequence number that
	identifies the configuration state of the sending LSR. Used by
	the receiving LSR to detect configuration changes on the send-
	ing LSR.
	IPv6 Transport Address
	Specifies the IPv6 address to be used for the sending LSR when
	opening the LDP session TCP connection. If this optional TLV
	is not present the IPv6 source address for the UDP packet car-
	rying the Hello SHOULD be used.
	3.5.2.1. Hello Message Procedures
	An LSR receiving Hellos from another LSR maintains a Hello adjacency
	corresponding to the Hellos. The LSR maintains a hold timer with the
	Hello adjacency which it restarts whenever it receives a Hello that
	matches the Hello adjacency. If the hold timer for a Hello adjacency
	expires the LSR discards the Hello adjacency: see sections "Maintain-
	ing Hello Adjacencies" and "Maintaining LDP Sessions".
	We recommend that the interval between Hello transmissions be at most
	one third of the Hello hold time.
	An LSR processes a received LDP Hello as follows:
	1. The LSR checks whether the Hello is acceptable. The criteria
	for determining whether a Hello is acceptable are implementa-
	tion dependent (see below for example criteria).
	2. If the Hello is not acceptable, the LSR ignores it.
	3. If the Hello is acceptable, the LSR checks whether it has a
	Hello adjacency for the Hello source. If so, it restarts the
	hold timer for the Hello adjacency. If not it creates a Hello
	adjacency for the Hello source and starts its hold timer.
	4. If the Hello carries any optional TLVs the LSR processes them
	(see below).
	5. Finally, if the LSR has no LDP session for the label space
	specified by the LDP identifier in the PDU header for the
	Hello, it follows the procedures of Section "LDP Session Estab-
	lishment".
	The following are examples of acceptability criteria for Link and
	Targeted Hellos:
	A Link Hello is acceptable if the interface on which it was
	received has been configured for label switching.
	A Targeted Hello from source address A is acceptable if either:
	- The LSR has been configured to accept Targeted Hellos, or
	- The LSR has been configured to send Targeted Hellos to A.
	The following describes how an LSR processes Hello optional TLVs:
	Transport Address
	The LSR associates the specified transport address with the
	Hello adjacency.
	Configuration Sequence Number
	The Configuration Sequence Number optional parameter is used by
	the sending LSR to signal configuration changes to the receiv-
	ing LSR. When a receiving LSR playing the active role in LDP
	session establishment detects a change in the sending LSR con-
	figuration, it may clear the session setup backoff delay, if
	any, associated with the sending LSR (see Section "Session Ini-
	tialization").
	A sending LSR using this optional parameter is responsible for
	maintaining the configuration sequence number it transmits in
	Hello messages. Whenever there is a configuration change on
	the sending LSR, it increments the configuration sequence num-
	ber.
	3.5.3. Initialization Message
	The LDP Initialization Message is exchanged as part of the LDP ses-
	sion establishment procedure; see Section "LDP Session Establish-
	ment".
	The encoding for the Initialization Message is:
	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| Initialization (0x0200) | Message Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Common Session Parameters TLV |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Optional Parameters |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Message ID
	32-bit value used to identify this message.
	Common Session Parameters TLV
	Specifies values proposed by the sending LSR for parameters that
	must be negotiated for every LDP session.
	The encoding for the Common Session Parameters TLV is:
	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|0| Common Sess Parms (0x0500)| Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Protocol Version | KeepAlive Time |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	|A|D| Reserved | PVLim | Max PDU Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Receiver LDP Identifier |
	+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++
	Protocol Version
	Two octet unsigned integer containing the version number of the
	protocol. This version of the specification specifies LDP pro-
	tocol version 1.
	KeepAlive Time
	Two octet unsigned non zero integer that indicates the number
	of seconds that the sending LSR proposes for the value of the
	KeepAlive Time. The receiving LSR MUST calculate the value of
	the KeepAlive Timer by using the smaller of its proposed
	KeepAlive Time and the KeepAlive Time received in the PDU. The
	value chosen for KeepAlive Time indicates the maximum number of
	seconds that may elapse between the receipt of successive PDUs
	from the LDP peer on the session TCP connection. The KeepAlive
	Timer is reset each time a PDU arrives.
	A, Label Advertisement Discipline
	Indicates the type of Label advertisement. A value of 0 means
	Downstream Unsolicited advertisement; a value of 1 means Down-
	stream On Demand.
	If one LSR proposes Downstream Unsolicited and the other pro-
	poses Downstream on Demand, the rules for resolving this dif-
	ference is:
	- If the session is for a label-controlled ATM link or a
	label-controlled Frame Relay link, then Downstream on Demand
	MUST be used.
	- Otherwise, Downstream Unsolicited MUST be used.
	If the label advertisement discipline determined in this way is
	unacceptable to an LSR, it MUST send a Session Rejected/Parame-
	ters Advertisement Mode Notification message in response to the
	Initialization message and not establish the session.
	D, Loop Detection
	Indicates whether loop detection based on path vectors is
	enabled. A value of 0 means loop detection is disabled; a
	value of 1 means that loop detection is enabled.
	PVLim, Path Vector Limit
	The configured maximum path vector length. MUST be 0 if loop
	detection is disabled (D = 0). If the loop detection proce-
	dures would require the LSR to send a path vector that exceeds
	this limit, the LSR will behave as if a loop had been detected
	for the FEC in question.
	When Loop Detection is enabled in a portion of a network, it is
	recommended that all LSRs in that portion of the network be
	configured with the same path vector limit. Although knowledge
	of a peer's path vector limit will not change an LSR's behav-
	ior, it does enable the LSR to alert an operator to a possible
	misconfiguration.
	Reserved
	This field is reserved. It MUST be set to zero on transmission
	and ignored on receipt.
	Max PDU Length
	Two octet unsigned integer that proposes the maximum allowable
	length for LDP PDUs for the session. A value of 255 or less
	specifies the default maximum length of 4096 octets.
	The receiving LSR MUST calculate the maximum PDU length for the
	session by using the smaller of its and its peer's proposals
	for Max PDU Length. The default maximum PDU length applies
	before session initialization completes.
	If the maximum PDU length determined this way is unacceptable
	to an LSR, it MUST send a Session Rejected/Parameters Max PDU
	Length Notification message in response to the Initialization
	message and not establish the session.
	Receiver LDP Identifier
	Identifies the receiver's label space. This LDP Identifier,
	together with the sender's LDP Identifier in the PDU header
	enables the receiver to match the Initialization message with
	one of its Hello adjacencies; see Section "Hello Message Proce-
	dures".
	If there is no matching Hello adjacency, the LSR MUST send a
	Session Rejected/No Hello Notification message in response to
	the Initialization message and not establish the session.
	Optional Parameters
	This variable length field contains 0 or more parameters, each
	encoded as a TLV. The optional parameters are:
	Optional Parameter Type Length Value
	ATM Session Parameters 0x0501 var See below
	Frame Relay Session 0x0502 var See below
	Parameters
	ATM Session Parameters
	Used when an LDP session manages label exchange for an ATM link
	to specify ATM-specific session parameters.
	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|0| ATM Sess Parms (0x0501) | Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| M | N |D| Reserved |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| ATM Label Range Component 1 |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	~ ~
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| ATM Label Range Component N |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	M, ATM Merge Capabilities
	Specifies the merge capabilities of an ATM switch. The
	following values are supported in this version of the
	specification:
	Value Meaning
	0 Merge not supported
	1 VP Merge supported
	2 VC Merge supported
	3 VP & VC Merge supported
	If the merge capabilities of the LSRs differ, then:
	- Non-merge and VC-merge LSRs may freely interoperate.
	- The interoperability of VP-merge-capable switches with non-
	VP-merge-capable switches is a subject for future study.
	When the LSRs differ on the use of VP-merge, the session is
	established, but VP merge is not used.
	Note that if VP merge is used, it is the responsibility of the
	ingress node to ensure that the chosen VCI is unique within the
	LSR domain.
	N, Number of label range components
	Specifies the number of ATM Label Range Components included in
	the TLV.
	D, VC Directionality
	A value of 0 specifies bidirectional VC capability, meaning the
	LSR can (within a given VPI) support the use of a given VCI as
	a label for both link directions independently. A value of 1
	specifies unidirectional VC capability, meaning (within a given
	VPI) a given VCI may appear in a label mapping for one direc-
	tion on the link only. When either or both of the peers speci-
	fies unidirectional VC capability, both LSRs use unidirectional
	VC label assignment for the link as follows. The LSRs compare
	their LDP Identifiers as unsigned integers. The LSR with the
	larger LDP Identifier may assign only odd-numbered VCIs in the
	VPI/VCI range as labels. The system with the smaller LDP Iden-
	tifier may assign only even-numbered VCIs in the VPI/VCI range
	as labels.
	Reserved
	This field is reserved. It MUST be set to zero on transmission
	and ignored on receipt.
	One or more ATM Label Range Components
	A list of ATM Label Range Components which together specify the
	Label range supported by the transmitting LSR.
	A receiving LSR MUST calculate the intersection between the
	received range and its own supported label range. The inter-
	section is the range in which the LSR may allocate and accept
	labels. LSRs MUST NOT establish a session with neighbors for
	which the intersection of ranges is NULL. In this case, the
	LSR MUST send a Session Rejected/Parameters Label Range Notifi-
	cation message in response to the Initialization message and
	not establish the session.
	The encoding for an ATM Label Range Component is:
	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
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Res | Minimum VPI | Minimum VCI |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Res | Maximum VPI | Maximum VCI |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Res
	This field is reserved. It MUST be set to zero on transmis-
	sion and MUST be ignored on receipt.
	Minimum VPI (12 bits)
	This 12 bit field specifies the lower bound of a block of
	Virtual Path Identifiers that is supported on the originat-
	ing switch. If the VPI is less than 12-bits it SHOULD be
	right justified in this field and preceding bits SHOULD be
	set to 0.
	Minimum VCI (16 bits)
	This 16 bit field specifies the lower bound of a block of
	Virtual Connection Identifiers that is supported on the
	originating switch. If the VCI is less than 16-bits it
	SHOULD be right justified in this field and preceding bits
	SHOULD be set to 0.
	Maximum VPI (12 bits)
	This 12 bit field specifies the upper bound of a block of
	Virtual Path Identifiers that is supported on the originat-
	ing switch. If the VPI is less than 12-bits it SHOULD be
	right justified in this field and preceding bits SHOULD be
	set to 0.
	Maximum VCI (16 bits)
	This 16 bit field specifies the upper bound of a block of
	Virtual Connection Identifiers that is supported on the
	originating switch. If the VCI is less than 16-bits it
	SHOULD be right justified in this field and preceding bits
	SHOULD be set to 0.
	When peer LSRs are connected indirectly by means of an ATM VP, the
	sending LSR SHOULD set the Minimum and Maximum VPI fields to 0,
	and the receiving LSR MUST ignore the Minimum and Maximum VPI
	fields.
	Frame Relay Session Parameters
	Used when an LDP session manages label exchange for a Frame
	Relay link to specify Frame Relay-specific session parameters.
	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|0| FR Sess Parms (0x0502) | Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| M | N |D| Reserved |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Frame Relay Label Range Component 1 |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	~ ~
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Frame Relay Label Range Component N |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	M, Frame Relay Merge Capabilities
	Specifies the merge capabilities of a Frame Relay switch. The
	following values are supported in this version of the specifi-
	cation:
	Value Meaning
	0 Merge not supported
	1 Merge supported
	Non-merge and merge Frame Relay LSRs may freely interoperate.
	N, Number of label range components Specifies the number of Frame
	Relay Label Range Components included in the TLV.
	D, VC Directionality
	A value of 0 specifies bidirectional VC capability, meaning the
	LSR can support the use of a given DLCI as a label for both
	link directions independently. A value of 1 specifies unidi-
	rectional VC capability, meaning a given DLCI may appear in a
	label mapping for one direction on the link only. When either
	or both of the peers specifies unidirectional VC capability,
	both LSRs use unidirectional VC label assignment for the link
	as follows. The LSRs compare their LDP Identifiers as unsigned
	integers. The LSR with the larger LDP Identifier may assign
	only odd-numbered DLCIs in the range as labels. The system
	with the smaller LDP Identifier may assign only even-numbered
	DLCIs in the range as labels.
	Reserved
	This field is reserved. It MUST be set to zero on transmission
	and ignored on receipt.
	One or more Frame Relay Label Range Components
	A list of Frame Relay Label Range Components which together
	specify the Label range supported by the transmitting LSR.
	A receiving LSR MUST calculate the intersection between the
	received range and its own supported label range. The inter-
	section is the range in which the LSR may allocate and accept
	labels. LSRs MUST NOT establish a session with neighbors for
	which the intersection of ranges is NULL. In this case, the
	LSR MUST send a Session Rejected/Parameters Label Range Notifi-
	cation message in response to the Initialization message and
	not establish the session.
	The encoding for a Frame Relay Label Range Component is:
	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 |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Reserved
	This field is reserved. It MUST be set to zero on transmis-
	sion and ignored on receipt.
	Len
	This field specifies the number of bits of the DLCI. The
	following values are supported:
	Len DLCI bits
	0 10
	2 23
	Len values 1 and 3 are reserved.
	Minimum DLCI
	This 23-bit field specifies the lower bound of a block of
	Data Link Connection Identifiers (DLCIs) that is supported
	on the originating switch. The DLCI SHOULD be right justi-
	fied in this field and unused bits SHOULD be set to 0.
	Maximum DLCI
	This 23-bit field specifies the upper bound of a block of
	Data Link Connection Identifiers (DLCIs) that is supported
	on the originating switch. The DLCI SHOULD be right
	justified in this field and unused bits SHOULD be set to 0.
	Note that there is no Generic Session Parameters TLV for sessions
	which advertise Generic Labels.
	3.5.3.1. Initialization Message Procedures
	See Section "LDP Session Establishment" and particularly Section
	"Session Initialization" for general procedures for handling the Ini-
	tialization Message.
	3.5.4. KeepAlive Message
	An LSR sends KeepAlive Messages as part of a mechanism that monitors
	the integrity of the LDP session transport connection.
	The encoding for the KeepAlive Message is:
	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| KeepAlive (0x0201) | Message Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Optional Parameters |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Message ID
	32-bit value used to identify this message.
	Optional Parameters
	No optional parameters are defined for the KeepAlive message.
	3.5.4.1. KeepAlive Message Procedures
	The KeepAlive Timer mechanism described in Section "Maintaining LDP
	Sessions" resets a session KeepAlive timer every time an LDP PDU is
	received on the session TCP connection. The KeepAlive Message is
	provided to allow reset of the KeepAlive Timer in circumstances where
	an LSR has no other information to communicate to an LDP peer.
	An LSR MUST arrange that its peer receive an LDP Message from it at
	least every KeepAlive Time period. Any LDP protocol message will do
	but, in circumstances where no other LDP protocol messages have been
	sent within the period, a KeepAlive message MUST be sent.
	3.5.5. Address Message
	An LSR sends the Address Message to an LDP peer to advertise its
	interface addresses.
	The encoding for the Address Message is:
	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| Address (0x0300) | Message Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	| Address List TLV |
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Optional Parameters |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Message ID
	32-bit value used to identify this message.
	Address List TLV
	The list of interface addresses being advertised by the sending
	LSR. The encoding for the Address List TLV is specified in
	Section "Address List TLV".
	Optional Parameters
	No optional parameters are defined for the Address message.
	3.5.5.1. Address Message Procedures
	An LSR that receives an Address Message message uses the addresses it
	learns to maintain a database for mapping between peer LDP Identi-
	fiers and next hop addresses; see Section "LDP Identifiers and Next
	Hop Addresses".
	When a new LDP session is initialized and before sending Label Map-
	ping or Label Request messages an LSR SHOULD advertise its interface
	addresses with one or more Address messages.
	Whenever an LSR "activates" a new interface address, it SHOULD
	advertise the new address with an Address message.
	Whenever an LSR "de-activates" a previously advertised address, it
	SHOULD withdraw the address with an Address Withdraw message; see
	Section "Address Withdraw Message".
	If an LSR does not support the Address Family specified in the
	Address List TLV, it SHOULD send an "Unsupported Address Family"
	Notification to its LDP signalling an error and abort processing the
	message.
	An LSR may re-advertise an address (A) that it has previously adver-
	tised without explicitly withdrawing the address. If the receiver
	already has address binding (LSR, A) it SHOULD take no further
	action.
	An LSR may withdraw an address (A) without having previously adver-
	tised it. If the receiver has no address binding (LSR, A), it SHOULD
	take no further action.
	3.5.6. Address Withdraw Message
	An LSR sends the Address Withdraw Message to an LDP peer to withdraw
	previously advertised interface addresses.
	The encoding for the Address Withdraw Message is:
	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| Address Withdraw (0x0301) | Message Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	| Address List TLV |
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Optional Parameters |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Message ID
	32-bit value used to identify this message.
	Address list TLV
	The list of interface addresses being withdrawn by the sending
	LSR. The encoding for the Address list TLV is specified in
	Section "Address List TLV".
	Optional Parameters
	No optional parameters are defined for the Address Withdraw mes-
	sage.
	3.5.6.1. Address Withdraw Message Procedures
	See Section "Address Message Procedures"
	3.5.7. Label Mapping Message
	An LSR sends a Label Mapping message to an LDP peer to advertise FEC-
	label bindings to the peer.
	The encoding for the Label Mapping Message is:
	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| Label Mapping (0x0400) | Message Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| FEC TLV |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Label TLV |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Optional Parameters |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Message ID
	32-bit value used to identify this message.
	FEC TLV
	Specifies the FEC component of the FEC-Label mapping being adver-
	tised. See Section "FEC TLV" for encoding.
	Label TLV
	Specifies the Label component of the FEC-Label mapping. See Sec-
	tion "Label TLV" for encoding.
	Optional Parameters
	This variable length field contains 0 or more parameters, each
	encoded as a TLV. The optional parameters are:
	Optional Parameter Length Value
	Label Request 4 See below
	Message ID TLV
	Hop Count TLV 1 See below
	Path Vector TLV variable See below
	The encodings for the Hop Count, and Path Vector TLVs can be found
	in Section "TLV Encodings for Commonly Used Parameters".
	Label Request Message ID
	If this Label Mapping message is a response to a Label Request
	message it MUST include the Request Message Id optional parame-
	ter. The value of this optional parameter is the Message Id of
	the corresponding Label Request Message.
	Hop Count
	Specifies the running total of the number of LSR hops along the
	LSP being setup by the Label Message. Section "Hop Count Pro-
	cedures" describes how to handle this TLV.
	Path Vector
	Specifies the LSRs along the LSP being setup by the Label Mes-
	sage. Section "Path Vector Procedures" describes how to handle
	this TLV.
	3.5.7.1. Label Mapping Message Procedures
	The Mapping message is used by an LSR to distribute a label mapping
	for a FEC to an LDP peer. If an LSR distributes a mapping for a FEC
	to multiple LDP peers, it is a local matter whether it maps a single
	label to the FEC, and distributes that mapping to all its peers, or
	whether it uses a different mapping for each of its peers.
	An LSR is responsible for the consistency of the label mappings it
	has distributed, and that its peers have these mappings.
	An LSR receiving a Label Mapping message from a downstream LSR for a
	Prefix SHOULD NOT use the label for forwarding unless its routing ta-
	ble contains an entry that exactly matches the FEC Element.
	See Appendix A "LDP Label Distribution Procedures" for more details.
	3.5.7.1.1. Independent Control Mapping
	If an LSR is configured for independent control, a mapping message is
	transmitted by the LSR upon any of the following conditions:
	1. The LSR recognizes a new FEC via the forwarding table, and the
	label advertisement mode is Downstream Unsolicited advertise-
	ment.
	2. The LSR receives a Request message from an upstream peer for a
	FEC present in the LSR's forwarding table.
	3. The next hop for a FEC changes to another LDP peer, and loop
	detection is configured.
	4. The attributes of a mapping change.
	5. The receipt of a mapping from the downstream next hop AND
	a) no upstream mapping has been created OR
	b) loop detection is configured OR
	c) the attributes of the mapping have changed.
	3.5.7.1.2. Ordered Control Mapping
	If an LSR is doing ordered control, a Mapping message is transmitted
	by downstream LSRs upon any of the following conditions:
	1. The LSR recognizes a new FEC via the forwarding table, and is
	the egress for that FEC.
	2. The LSR receives a Request message from an upstream peer for a
	FEC present in the LSR's forwarding table, and the LSR is the
	egress for that FEC OR has a downstream mapping for that FEC.
	3. The next hop for a FEC changes to another LDP peer, and loop
	detection is configured.
	4. The attributes of a mapping change.
	5. The receipt of a mapping from the downstream next hop AND
	a) no upstream mapping has been created OR
	b) loop detection is configured OR
	c) the attributes of the mapping have changed.
	3.5.7.1.3. Downstream on Demand Label Advertisement
	In general, the upstream LSR is responsible for requesting label map-
	pings when operating in Downstream on Demand mode. However, unless
	some rules are followed, it is possible for neighboring LSRs with
	different advertisement modes to get into a livelock situation where
	everything is functioning properly, but no labels are distributed.
	For example, consider two LSRs Ru and Rd where Ru is the upstream LSR
	and Rd is the downstream LSR for a particular FEC. In this example,
	Ru is using Downstream Unsolicited advertisement mode and Rd is using
	Downstream on Demand mode. In this case, Rd may assume that Ru will
	request a label mapping when it wants one and Ru may assume that Rd
	will advertise a label if it wants Ru to use one. If Rd and Ru oper-
	ate as suggested, no labels will be distributed from Rd to Ru.
	This livelock situation can be avoided if the following rule is
	observed: an LSR operating in Downstream on Demand mode SHOULD NOT be
	expected to send unsolicited mapping advertisements. Therefore, if
	the downstream LSR is operating in Downstream on Demand mode, the
	upstream LSR is responsible for requesting label mappings as needed.
	3.5.7.1.4. Downstream Unsolicited Label Advertisement
	In general, the downstream LSR is responsible for advertising a label
	mapping when it wants an upstream LSR to use the label. An upstream
	LSR may issue a mapping request if it so desires.
	The combination of Downstream Unsolicited mode and conservative label
	retention can lead to a situation where an LSR releases the label for
	a FEC that it later needs. For example, if LSR Rd advertises to LSR
	Ru the label for a FEC for which it is not Ru's next hop, Ru will
	release the label. If Ru's next hop for the FEC later changes to Rd,
	it needs the previously released label.
	To deal with this situation either Ru can explicitly request the
	label when it needs it, or Rd can periodically readvertise it to Ru.
	In many situations Ru will know when it needs the label from Rd. For
	example, when its next hop for the FEC changes to Rd. However, there
	could be situations when Ru does not. For example, Rd may be
	attempting to establish an LSP with non-standard properties. Forcing
	Ru to explicitly request the label in this situation would require it
	to maintain state about a potential LSP with non-standard properties.
	In situations where Ru knows it needs the label, it is responsible
	for explicitly requesting the label by means of a Label Request mes-
	sage. In situations where Ru may not know that it needs the label,
	Rd is responsible for periodically readvertising the label to Ru.
	For this version of LDP, the only situation where Ru knows it needs a
	label for a FEC from Rd is when Rd is its next hop for the FEC, Ru
	does not have a label from Rd, and the LSP for the FEC is one that
	can be established with TLVs defined in this document.
	3.5.8. Label Request Message
	An LSR sends the Label Request Message to an LDP peer to request a
	binding (mapping) for a FEC.
	The encoding for the Label Request Message is:
	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| Label Request (0x0401) | Message Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| FEC TLV |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Optional Parameters |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Message ID
	32-bit value used to identify this message.
	FEC TLV
	The FEC for which a label is being requested. See Section "FEC
	TLV" for encoding.
	Optional Parameters
	This variable length field contains 0 or more parameters, each
	encoded as a TLV. The optional parameters are:
	Optional Parameter Length Value
	Hop Count TLV 1 See below
	Path Vector TLV variable See below
	The encodings for the Hop Count, and Path Vector TLVs can be found
	in Section "TLV Encodings for Commonly Used Parameters".
	Hop Count
	Specifies the running total of the number of LSR hops along the
	LSP being setup by the Label Request Message. Section "Hop
	Count Procedures" describes how to handle this TLV.
	Path Vector
	Specifies the LSRs along the LSR being setup by the Label
	Request Message. Section "Path Vector Procedures" describes
	how to handle this TLV.
	3.5.8.1. Label Request Message Procedures
	The Request message is used by an upstream LSR to explicitly request
	that the downstream LSR assign and advertise a label for a FEC.
	An LSR may transmit a Request message under any of the following con-
	ditions:
	1. The LSR recognizes a new FEC via the forwarding table, and the
	next hop is an LDP peer, and the LSR doesn't already have a
	mapping from the next hop for the given FEC.
	2. The next hop to the FEC changes, and the LSR doesn't already
	have a mapping from that next hop for the given FEC.
	Note that if the LSR already has a pending Label Request mes-
	sage for the new next hop it SHOULD NOT issue an additional
	Label Request in response to the next hop change.
	3. The LSR receives a Label Request for a FEC from an upstream LDP
	peer, the FEC next hop is an LDP peer, and the LSR doesn't
	already have a mapping from the next hop.
	Note that since a non-merge LSR must setup a separate LSP for
	each upstream peer requesting a label, it must send a separate
	Label Request for each such peer. A consequence of this is
	that a non-merge LSR may have multiple Label Request messages
	for a given FEC outstanding at the same time.
	The receiving LSR SHOULD respond to a Label Request message with a
	Label Mapping for the requested label or with a Notification message
	indicating why it cannot satisfy the request.
	When the FEC for which a label is requested is a Prefix FEC Element,
	the receiving LSR uses its routing table to determine its response.
	Unless its routing table includes an entry that exactly matches the
	requested Prefix, the LSR MUST respond with a No Route Notification
	message.
	The message ID of the Label Request message serves as an identifier
	for the Label Request transaction. When the receiving LSR responds
	with a Label Mapping message, the mapping message MUST include a
	Label Request/Returned Message ID TLV optional parameter which
	includes the message ID of the Label Request message. Note that
	since LSRs use Label Request message IDs as transaction identifiers
	an LSR SHOULD NOT reuse the message ID of a Label Request message
	until the corresponding transaction completes.
	This version of the protocol defines the following Status Codes for
	the Notification message that signals a request cannot be satisfied:
	No Route
	The FEC for which a label was requested includes a FEC Element
	for which the LSR does not have a route.
	No Label Resources
	The LSR cannot provide a label because of resource limitations.
	When resources become available the LSR MUST notify the
	requesting LSR by sending a Notification message with the Label
	Resources Available Status Code.
	An LSR that receives a No Label Resources response to a Label
	Request message MUST NOT issue further Label Request messages
	until it receives a Notification message with the Label
	Resources Available Status code.
	Loop Detected
	The LSR has detected a looping Label Request message.
	See Appendix A "LDP Label Distribution Procedures" for more details.
	3.5.9. Label Abort Request Message
	The Label Abort Request message may be used to abort an outstanding
	Label Request message.
	The encoding for the Label Abort Request Message is:
	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| Label Abort Req (0x0404) | Message Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| FEC TLV |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Label Request Message ID TLV |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Optional Parameters |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Message ID
	32-bit value used to identify this message.
	FEC TLV
	Identifies the FEC for which the Label Request is being aborted.
	Label Request Message ID TLV
	Specifies the message ID of the Label Request message to be
	aborted.
	Optional Parameters
	No optional parameters are defined for the Label Abort Req mes-
	sage.
	3.5.9.1. Label Abort Request Message Procedures
	An LSR Ru may send a Label Abort Request message to abort an out-
	standing Label Request message for FEC sent to LSR Rd in the follow-
	ing circumstances:
	1. Ru's next hop for FEC has changed from LSR Rd to LSR X; or
	2. Ru is a non-merge, non-ingress LSR and has received a Label
	Abort Request for FEC from an upstream peer Y.
	3. Ru is a merge, non-ingress LSR and has received a Label Abort
	Request for FEC from an upstream peer Y and Y is the only
	(last) upstream LSR requesting a label for FEC.
	There may be other situations where an LSR may choose to abort an
	outstanding Label Request message in order to reclaim resource asso-
	ciated with the pending LSP. However, specification of general
	strategies for using the abort mechanism is beyond the scope of LDP.
	When an LSR receives a Label Abort Request message, if it has not
	previously responded to the Label Request being aborted with a Label
	Mapping message or some other Notification message, it MUST acknowl-
	edge the abort by responding with a Label Request Aborted Notifica-
	tion message. The Notification MUST include a Label Request Message
	ID TLV that carries the message ID of the aborted Label Request mes-
	sage.
	If an LSR receives a Label Abort Request Message after it has
	responded to the Label Request in question with a Label Mapping mes-
	sage or a Notification message, it ignores the abort request.
	If an LSR receives a Label Mapping message in response to a Label
	Request message after it has sent a Label Abort Request message to
	abort the Label Request, the label in the Label Mapping message is
	valid. The LSR may choose to use the label or to release it with a
	Label Release message.
	An LSR aborting a Label Request message may not reuse the Message ID
	for the Label Request message until it receives one of the following
	from its peer:
	- A Label Request Aborted Notification message acknowledging the
	abort;
	- A Label Mapping message in response to the Label Request mes-
	sage being aborted;
	- A Notification message in response to the Label Request message
	being aborted (e.g., Loop Detected, No Label Resources, etc.).
	To protect itself against tardy peers or faulty peer implementations
	an LSR may choose to time out receipt of the above. The time out
	period should be relatively long (several minutes). If the time out
	period elapses with no reply from the peer the LSR may reuse the Mes-
	sage Id of the Label Request message; if it does so, it should also
	discard any record of the outstanding Label Request and Label Abort
	messages.
	Note that the response to a Label Abort Request message is never
	"ordered". That is, the response does not depend on the downstream
	state of the LSP setup being aborted. An LSR receiving a Label Abort
	Request message MUST process it immediately, regardless of the down-
	stream state of the LSP, responding with a Label Request Aborted
	Notification or ignoring it, as appropriate.
	3.5.10. Label Withdraw Message
	An LSR sends a Label Withdraw Message to an LDP peer to signal the
	peer that the peer may not continue to use specific FEC-label map-
	pings the LSR had previously advertised. This breaks the mapping
	between the FECs and the labels.
	The encoding for the Label Withdraw Message is:
	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| Label Withdraw (0x0402) | Message Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| FEC TLV |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Label TLV (optional) |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Optional Parameters |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Message ID
	32-bit value used to identify this message.
	FEC TLV
	Identifies the FEC for which the FEC-label mapping is being with-
	drawn.
	Optional Parameters
	This variable length field contains 0 or more parameters, each
	encoded as a TLV. The optional parameters are:
	Optional Parameter Length Value
	Label TLV variable See below
	The encoding for Label TLVs are found in Section "Label TLVs".
	Label
	If present, specifies the label being withdrawn (see procedures
	below).
	3.5.10.1. Label Withdraw Message Procedures
	An LSR transmits a Label Withdraw message under the following condi-
	tions:
	1. The LSR no longer recognizes a previously known FEC for which
	it has advertised a label.
	2. The LSR has decided unilaterally (e.g., via configuration) to
	no longer label switch a FEC (or FECs) with the label mapping
	being withdrawn.
	The FEC TLV specifies the FEC for which labels are to be withdrawn.
	If no Label TLV follows the FEC, all labels associated with the FEC
	are to be withdrawn; otherwise only the label specified in the
	optional Label TLV is to be withdrawn.
	The FEC TLV may contain the Wildcard FEC Element; if so, it may con-
	tain no other FEC Elements. In this case, if the Label Withdraw mes-
	sage contains an optional Label TLV, then the label is to be with-
	drawn from all FECs to which it is bound. If there is not an
	optional Label TLV in the Label Withdraw message, then the sending
	LSR is withdrawing all label mappings previously advertised to the
	receiving LSR.
	An LSR that receives a Label Withdraw message MUST respond with a
	Label Release message.
	See Appendix A "LDP Label Distribution Procedures" for more details.
	3.5.11. Label Release Message
	An LSR sends a Label Release message to an LDP peer to signal the
	peer that the LSR no longer needs specific FEC-label mappings previ-
	ously requested of and/or advertised by the peer.
	The encoding for the Label Release Message is:
	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| Label Release (0x0403) | Message Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| FEC TLV |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Label TLV (optional) |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Optional Parameters |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Message ID
	32-bit value used to identify this message.
	FEC TLV
	Identifies the FEC for which the FEC-label mapping is being
	released.
	Optional Parameters
	This variable length field contains 0 or more parameters, each
	encoded as a TLV. The optional parameters are:
	Optional Parameter Length Value
	Label TLV variable See below
	The encodings for Label TLVs are found in Section "Label TLVs".
	Label
	If present, the label being released (see procedures below).
	3.5.11.1. Label Release Message Procedures
	An LSR transmits a Label Release message to a peer when it is no
	longer needs a label previously received from or requested of that
	peer.
	An LSR MUST transmit a Label Release message under any of the follow-
	ing conditions:
	1. The LSR which sent the label mapping is no longer the next hop
	for the mapped FEC, and the LSR is configured for conservative
	operation.
	2. The LSR receives a label mapping from an LSR which is not the
	next hop for the FEC, and the LSR is configured for conserva-
	tive operation.
	3. The LSR receives a Label Withdraw message.
	Note that if an LSR is configured for "liberal mode", a release mes-
	sage will never be transmitted in the case of conditions (1) and (2)
	as specified above. In this case, the upstream LSR keeps each unused
	label, so that it can immediately be used later if the downstream
	peer becomes the next hop for the FEC.
	The FEC TLV specifies the FEC for which labels are to be released.
	If no Label TLV follows the FEC, all labels associated with the FEC
	are to be released; otherwise only the label specified in the
	optional Label TLV is to be released.
	The FEC TLV may contain the Wildcard FEC Element; if so, it may con-
	tain no other FEC Elements. In this case, if the Label Release mes-
	sage contains an optional Label TLV, then the label is to be released
	for all FECs to which it is bound. If there is not an
	optional Label TLV in the Label Release message, then the sending LSR
	is releasing all label mappings previously learned from the receiving
	LSR.
	See Appendix A "LDP Label Distribution Procedures" for more details.
	3.6. Messages and TLVs for Extensibility
	Support for LDP extensibility includes the rules for the U and F bits
	that specify how an LSR handles unknown TLVs and messages.
	This section specifies TLVs and messages for vendor-private and
	experimental use.
	3.6.1. LDP Vendor-private Extensions
	Vendor-private TLVs and messages are used to convey vendor-private
	information between LSRs.
	3.6.1.1. LDP Vendor-private TLVs
	The Type range 0x3E00 through 0x3EFF is reserved for vendor-private
	TLVs.
	The encoding for a vendor-private TLV is:
	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
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	|U|F| Type (0x3E00-0x3EFF) | Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Vendor ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	| Data.... |
	~ ~
	| |
	| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	U bit
	Unknown TLV bit. Upon receipt of an unknown TLV, if U is clear
	(=0), a notification MUST be returned to the message originator
	and the entire message MUST be ignored; if U is set (=1), the
	unknown TLV is silently ignored and the rest of the message is
	processed as if the unknown TLV did not exist.
	The determination as to whether a vendor-private message is under-
	stood is based on the Type and the mandatory Vendor ID field.
	Implementations that support vendor-private TLVs MUST support a
	user-accessible configuration interface that causes the U-bit to
	be set on all transmitted vendor-private TLVs; this requirement
	MAY be satisfied by a user-accessible configuration interface that
	prevents transmission of all vendor-private TLVs for which the U-
	bit is clear.
	F bit
	Forward unknown TLV bit. This bit only applies when the U bit is
	set and the LDP message containing the unknown TLV is is to be
	forwarded. If F is clear (=0), the unknown TLV is not forwarded
	with the containing message; if F is set (=1), the unknown TLV is
	forwarded with the containing message.
	Type
	Type value in the range 0x3E00 through 0x3EFF. Together, the Type
	and Vendor Id field specify how the Data field is to be inter-
	preted.
	Length
	Specifies the cumulative length in octets of the Vendor ID and
	Data fields.
	Vendor Id
	802 Vendor ID as assigned by the IEEE.
	Data
	The remaining octets after the Vendor ID in the Value field are
	optional vendor-dependent data.
	3.6.1.2. LDP Vendor-private Messages
	The Message Type range 0x3E00 through 0x3EFF is reserved for vendor-
	private Messages.
	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
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	|U| Msg Type (0x3E00-0x3EFF) | Message Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Vendor ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	+ +
	| Remaining Mandatory Parameters |
	+ +
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	+ +
	| Optional Parameters |
	+ +
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	U bit
	Unknown message bit. Upon receipt of an unknown message, if U is
	clear (=0), a notification is returned to the message originator;
	if U is set (=1), the unknown message is silently ignored.
	The determination as to whether a vendor-private message is under-
	stood is based on the Msg Type and the Vendor ID parameter.
	Implementations that support vendor-private messages MUST support
	a user-accessible configuration interface that causes the U-bit to
	be set on all transmitted vendor-private messages; this require-
	ment MAY be satisfied by a user-accessible configuration interface
	that prevents transmission of all vendor-private messages for
	which the U-bit is clear.
	Msg Type
	Message type value in the range 0x3E00 through 0x3EFF. Together,
	the Msg Type and the Vendor ID specify how the message is to be
	interpreted.
	Message Length
	Specifies the cumulative length in octets of the Message ID, Ven-
	dor ID, Remaining Mandatory Parameters and Optional Parameters.
	Message ID
	32-bit integer used to identify this message. Used by the sending
	LSR to facilitate identifying notification messages that may apply
	to this message. An LSR sending a notification message in
	response to this message will include this Message Id in the noti-
	fication message; see Section "Notification Message".
	Vendor ID
	802 Vendor ID as assigned by the IEEE.
	Remaining Mandatory Parameters
	Variable length set of remaining required message parameters.
	Optional Parameters
	Variable length set of optional message parameters.
	3.6.2. LDP Experimental Extensions
	LDP support for experimentation is similar to support for vendor-
	private extensions with the following differences:
	- The Type range 0x3F00 through 0x3FFF is reserved for experimen-
	tal TLVs.
	- The Message Type range 0x3F00 through 0x3FFF is reserved for
	experimental messages.
	- The encodings for experimental TLVs and messages are similar to
	the vendor-private encodings with the following difference.
	Experimental TLVs and messages use an Experiment ID field in
	place of a Vendor ID field. The Experiment ID field is used
	with the Type or Message Type field to specify the interpreta-
	tion of the experimental TLV or Message.
	Administration of Experiment IDs is the responsibility of the
	experimenters.
	3.7. Message Summary
	The following are the LDP messages defined in this version of the
	protocol.
	Message Name Type Section Title
	Notification 0x0001 "Notification Message"
	Hello 0x0100 "Hello Message"
	Initialization 0x0200 "Initialization Message"
	KeepAlive 0x0201 "KeepAlive Message"
	Address 0x0300 "Address Message"
	Address Withdraw 0x0301 "Address Withdraw Message"
	Label Mapping 0x0400 "Label Mapping Message"
	Label Request 0x0401 "Label Request Message"
	Label Withdraw 0x0402 "Label Withdraw Message"
	Label Release 0x0403 "Label Release Message"
	Label Abort Request 0x0404 "Label Abort Request Message"
	Vendor-Private 0x3E00- "LDP Vendor-private Extensions"
	0x3EFF
	Experimental 0x3F00- "LDP Experimental Extensions"
	0x3FFF
	3.8. TLV Summary
	The following are the TLVs defined in this version of the protocol.
	TLV Type Section Title
	FEC 0x0100 "FEC TLV"
	Address List 0x0101 "Address List TLV"
	Hop Count 0x0103 "Hop Count TLV"
	Path Vector 0x0104 "Path Vector TLV"
	Generic Label 0x0200 "Generic Label TLV"
	ATM Label 0x0201 "ATM Label TLV"
	Frame Relay Label 0x0202 "Frame Relay Label TLV"
	Status 0x0300 "Status TLV"
	Extended Status 0x0301 "Notification Message"
	Returned PDU 0x0302 "Notification Message"
	Returned Message 0x0303 "Notification Message"
	Common Hello 0x0400 "Hello Message"
	Parameters
	IPv4 Transport Address 0x0401 "Hello Message"
	Configuration 0x0402 "Hello Message"
	Sequence Number
	IPv6 Transport Address 0x0403 "Hello Message"
	Common Session 0x0500 "Initialization Message"
	Parameters
	ATM Session Parameters 0x0501 "Initialization Message"
	Frame Relay Session 0x0502 "Initialization Message"
	Parameters
	Label Request 0x0600 "Label Mapping Message"
	Message ID
	Vendor-Private 0x3E00- "LDP Vendor-private Extensions"
	0x3EFF
	Experimental 0x3F00- "LDP Experimental Extensions"
	0x3FFF
	3.9. Status Code Summary
	The following are the Status Codes defined in this version of the
	protocol.
	The "E" column is the required setting of the Status Code E-bit; the
	"Status Data" column is the value of the 30-bit Status Data field in
	the Status Code TLV. Note that the setting of the Status Code F-bit
	is at the discretion of the LSR originating the Status TLV.
	Status Code E Status Data Section Title
	Success 0 0x00000000 "Status TLV"
	Bad LDP Identifier 1 0x00000001 "Events Signaled by ..."
	Bad Protocol Version 1 0x00000002 "Events Signaled by ..."
	Bad PDU Length 1 0x00000003 "Events Signaled by ..."
	Unknown Message Type 0 0x00000004 "Events Signaled by ..."
	Bad Message Length 1 0x00000005 "Events Signaled by ..."
	Unknown TLV 0 0x00000006 "Events Signaled by ..."
	Bad TLV length 1 0x00000007 "Events Signaled by ..."
	Malformed TLV Value 1 0x00000008 "Events Signaled by ..."
	Hold Timer Expired 1 0x00000009 "Events Signaled by ..."
	Shutdown 1 0x0000000A "Events Signaled by ..."
	Loop Detected 0 0x0000000B "Loop Detection"
	Unknown FEC 0 0x0000000C "FEC Procedures"
	No Route 0 0x0000000D "Label Request Mess ..."
	No Label Resources 0 0x0000000E "Label Request Mess ..."
	Label Resources / 0 0x0000000F "Label Request Mess ..."
	Available
	Session Rejected/ 1 0x00000010 "Session Initialization"
	No Hello
	Session Rejected/ 1 0x00000011 "Session Initialization"
	Parameters Advertisement Mode
	Session Rejected/ 1 0x00000012 "Session Initialization"
	Parameters Max PDU Length
	Session Rejected/ 1 0x00000013 "Session Initialization"
	Parameters Label Range
	KeepAlive Timer 1 0x00000014 "Events Signaled by ..."
	Expired
	Label Request Aborted 0 0x00000015 "Label Request Abort ..."
	Missing Message 0 0x00000016 "Events Signaled by ..."
	Parameters
	Unsupported Address 0 0x00000017 "FEC Procedures"
	Family "Address Message Proc ..."
	Session Rejected/ 1 0x00000018 "Session Initialization"
	Bad KeepAlive Time
	Internal Error 1 0x00000019 "Events Signaled by ..."
	3.10. Well-known Numbers
	3.10.1. UDP and TCP Ports
	The UDP port for LDP Hello messages is 646.
	The TCP port for establishing LDP session connections is 646.
	3.10.2. Implicit NULL Label
	The Implicit NULL label is defined in [RFC3031] as follows:
	"The Implicit NULL label is a label with special semantics which an
	LSR can bind to an address prefix. If LSR Ru, by consulting its ILM
	(Incoming Label Map) sees that labeled packet P must be forwarded
	next to Rd, but that Rd has distributed a binding of Implicit NULL to
	the corresponding address prefix, then instead of replacing the value
	of the label on top of the label stack, Ru pops the label stack, and
	then forwards the resulting packet to Rd."
	The implicit NULL label is represented in LDP as a Generic Label TLV
	with a Label field value of 3, as defined in [RFC3032].
	4. IANA Considerations
	LDP defines the following name spaces which require management:
	- Message Type Name Space.
	- TLV Type Name Space.
	- FEC Type Name Space.
	- Status Code Name Space.
	- Experiment ID Name Space.
	The following sections provide guidelines for managing these name
	spaces.
	4.1. Message Type Name Space
	LDP divides the name space for message types into three ranges. The
	following are the guidelines for managing these ranges:
	- Message Types 0x0000 - 0x3DFF. Message types in this range are
	part of the LDP base protocol. Following the policies outlined
	in [IANA], Message types in this range are allocated through an
	IETF Consensus action.
	- Message Types 0x3E00 - 0x3EFF. Message types in this range are
	reserved for Vendor Private extensions and are the responsibil-
	ity of the individual vendors (see Section "LDP Vendor-private
	Messages"). IANA management of this range of the Message Type
	Name Space is unnecessary.
	- Message Types 0x3F00 - 0x3FFF. Message types in this range are
	reserved for Experimental extensions and are the responsibility
	of the individual experimenters (see Sections "LDP Experimental
	Extensions" and "Experiment ID Name Space"). IANA management
	of this range of the Message Type Name Space is unnecessary;
	however, IANA is responsible for managing part of the Experi-
	ment ID Name Space (see below).
	4.2. TLV Type Name Space
	LDP divides the name space for TLV types into three ranges. The fol-
	lowing are the guidelines for managing these ranges:
	- TLV Types 0x0000 - 0x3DFF. TLV types in this range are part of
	the LDP base protocol. Following the policies outlined in
	[IANA], TLV types in this range are allocated through an IETF
	Consensus action.
	- TLV Types 0x3E00 - 0x3EFF. TLV types in this range are
	reserved for Vendor Private extensions and are the responsibil-
	ity of the individual vendors (see Section "LDP Vendor-private
	TLVs"). IANA management of this range of the TLV Type Name
	Space is unnecessary.
	- TLV Types 0x3F00 - 0x3FFF. TLV types in this range are
	reserved for Experimental extensions and are the responsibility
	of the individual experimenters (see Sections "LDP Experimental
	Extensions" and "Experiment ID Name Space"). IANA management
	of this range of the TLV Name Space is unnecessary; however,
	IANA is responsible for managing part of the Experiment ID Name
	Space (see below).
	4.3. FEC Type Name Space
	The range for FEC types is 0 - 255.
	Following the policies outlined in [IANA], FEC types in the range 0 -
	127 are allocated through an IETF Consensus action, types in the
	range 128 - 191 are allocated as First Come First Served, and types
	in the range 192 - 255 are reserved for Private Use.
	4.4. Status Code Name Space
	The range for Status Codes is 0x00000000 - 0x3FFFFFFF.
	Following the policies outlined in [IANA], Status Codes in the range
	0x00000000 - 0x1FFFFFFF are allocated through an IETF Consensus
	action, codes in the range 0x20000000 - 0x3EFFFFFF are allocated as
	First Come First Served, and codes in the range 0x3F000000 -
	0x3FFFFFFF are reserved for Private Use.
	4.5. Experiment ID Name Space
	The range for Experiment Ids is 0x00000000 - 0xffffffff.
	Following the policies outlined in [IANA], Experiment Ids in the
	range 0x00000000 - 0xefffffff are allocated as First Come First
	Served and Experiment Ids in the range 0xf0000000 - 0xffffffff are
	reserved for Private Use.
	5. Security Considerations
	This section identifies threats to which LDP may be vulnerable and
	discusses means by which those threats might be mitigated.
	5.1. Spoofing
	There are two types of LDP communication that could be the target of
	a spoofing attack.
	1. Discovery exchanges carried by UDP.
	LSRs indicate their willingness to establish and maintain LDP ses-
	sions by periodically sending Hello messages. Receipt of a Hello
	serves to create a new "Hello adjacency", if one does not already
	exist, or to refresh an existing one. Spoofing a Hello packet for
	an existing adjacency can cause the adjacency to time out and that
	can result in termination of the associated session. This can
	occur when the spoofed Hello specifies a small Hold Time, causing
	the receiver to expect Hellos within this interval, while the true
	neighbor continues sending Hellos at the lower, previously agreed
	to, frequency.
	LSRs directly connected at the link level exchange Basic Hello
	messages over the link. The threat of spoofed Basic Hellos can be
	reduced by:
	o Accepting Basic Hellos only on interfaces to which LSRs that
	can be trusted are directly connected.
	o Ignoring Basic Hellos not addressed to the All Routers on
	this Subnet multicast group.
	LSRs not directly connected at the link level may use Extended
	Hello messages to indicate willingness to establish an LDP ses-
	sion. An LSR can reduce the threat of spoofed Extended Hellos by
	filtering them and accepting only those originating at sources
	permitted by an access list.
	2. Session communication carried by TCP.
	LDP specifies use of the TCP MD5 Signature Option to provide for
	the authenticity and integrity of session messages.
	[RFC2385] asserts that MD5 authentication is now considered by
	some to be too weak for this application. It also points out that
	a similar TCP option with a stronger hashing algorithm (it cites
	SHA-1 as an example) could be deployed. To our knowledge no such
	TCP option has been defined and deployed. However, we note that
	LDP can use whatever TCP message digest techniques are available,
	and when one stronger than MD5 is specified and implemented,
	upgrading LDP to use it would be relatively straightforward.
	5.2. Privacy
	LDP provides no mechanism for protecting the privacy of label distri-
	bution.
	The security requirements of label distribution protocols are essen-
	tially identical to those of the protocols which distribute routing
	information. By providing a mechanism to ensure the authenticity and
	integrity of its messages LDP provides a level of security which is
	at least as good as, though no better than, that which can be pro-
	vided by the routing protocols themselves. The more general issue of
	whether privacy should be required for routing protocols is beyond
	the scope of this document.
	One might argue that label distribution requires privacy to address
	the threat of label spoofing. However, that privacy would not pro-
	tect against label spoofing attacks since data packets carry labels
	in the clear. Furthermore, label spoofing attacks can be made with-
	out knowledge of the FEC bound to a label.
	To avoid label spoofing attacks, it is necessary to ensure that
	labeled data packets are labeled by trusted LSRs and that the labels
	placed on the packets are properly learned by the labeling LSRs.
	5.3. Denial of Service
	LDP provides two potential targets for denial of service (DoS)
	attacks:
	1. Well known UDP Port for LDP Discovery
	An LSR administrator can address the threat of DoS attacks via
	Basic Hellos by ensuring that the LSR is directly connected only
	to peers which can be trusted to not initiate such an attack.
	Interfaces to peers interior to the administrator's domain should
	not represent a threat since interior peers are under the adminis-
	trator's control. Interfaces to peers exterior to the domain rep-
	resent a potential threat since exterior peers are not. An admin-
	istrator can reduce that threat by connecting the LSR only to
	exterior peers that can be trusted to not initiate a Basic Hello
	attack.
	DoS attacks via Extended Hellos are potentially a more serious
	threat. This threat can be addressed by filtering Extended Hellos
	using access lists that define addresses with which extended dis-
	covery is permitted. However, performing the filtering requires
	LSR resource.
	In an environment where a trusted MPLS cloud can be identified,
	LSRs at the edge of the cloud can be used to protect interior LSRs
	against DoS attacks via Extended Hellos by filtering out Extended
	Hellos originating outside of the trusted MPLS cloud, accepting
	only those originating at addresses permitted by access lists.
	This filtering protects LSRs in the interior of the cloud but con-
	sumes resources at the edges.
	2. Well known TCP port for LDP Session Establishment
	Like other control plane protocols that use TCP, LDP may be the
	target of DoS attacks, such a SYN attacks. LDP is no more or less
	vulnerable to such attacks than other control plane protocols that
	use TCP.
	The threat of such attacks can be mitigated somewhat by the fol-
	lowing:
	o An LSR SHOULD avoid promiscuous TCP listens for LDP session
	establishment. It SHOULD use only listens that are specific
	to discovered peers. This enables it to drop attack packets
	early in their processing since they are less likely to
	match existing or in-progress connections.
	o The use of the MD5 option helps somewhat since it prevents a
	SYN from being accepted unless the MD5 segment checksum is
	valid. However, the receiver must compute the checksum
	before it can decide to discard an otherwise acceptable SYN
	segment.
	o The use of access list mechanisms applied at the boundary of
	the MPLS cloud in a manner similar to that suggested above
	for Extended Hellos can protect the interior against attacks
	originating from outside the cloud.
	6. Areas for Future Study
	The following topics not addressed in this version of LDP are possi-
	ble areas for future study:
	- Section 2.16 of the MPLS architecture [RFC3031] requires that
	the initial label distribution protocol negotiation between
	peer LSRs enable each LSR to determine whether its peer is
	capable of popping the label stack. This version of LDP
	assumes that LSRs support label popping for all link types
	except ATM and Frame Relay. A future version may specify means
	to make this determination part of the session initiation nego-
	tiation.
	- LDP support for CoS (class of service) is not specified in this
	version. CoS support may be addressed in a future version.
	- LDP support for multicast is not specified in this version.
	Multicast support may be addressed in a future version.
	- LDP support for multipath label switching is not specified in
	this version. Multipath support may be addressed in a future
	version.
	- LDP support for signalling the maximum transmission unit is not
	specified in this version. It is discussed in the experimental
	document [LDP-MTU].
	- The current specification does not address basic peer discovery
	on non-broadcast multi access (NBMA) media. The solution avail-
	able in the current specification is to use extended peer dis-
	covery in such setups. The issue of defining a mechanism seman-
	tically similar to basic discovery (1 hop limit, bind the hello
	adjacency to an interface) that uses a preconfigured neighbor
	addresses, is left for further study.
	- The current specification does not support shutting down an
	adjacency. The motivation for doing it, and the mechanisms for
	achieving it are left for further study.
	- The current specification does not include a method for secur-
	ing Hello messages, to detect spoofing of Hellos. The scenarios
	where this is necessary, as well as the mechanism for achieving
	it are left for future study.
	- The current specification does not have the ability to detect a
	stateless fast control plane restart. The method for achieving
	this, possibly through an "incarnation/instance" number carried
	in the Hello message is left for future study.
	- The current specification does not support an "end of LIB" mes-
	sage, analogous to BGP's end of RIB message which an LDP LSR
	(operating in DU mode) would use following session establish-
	ment. The discussion on the need for such a mechanism and its
	implementation are left for future study.
	- The current specification does not deal with situations where
	different LSRs advertise the same address. Such situations
	typically occur as the result of configuration errors, and the
	goal in this case is to provide the LSRs advertising the same
	address with enough information to enable operators to take
	corrective action. The specification of this mechanism is left
	for a separate document.
	7. Changes from RFC3036
	Here is a list of changes from RFC3036
	1. Removed the Host Address FEC and references to it, since it is
	not used by any implementation.
	2. Split the reference list into normative and non-normative ref-
	erences
	3. Removed "MPLS using ATM VP Switching" from the list of norma-
	tive references, and references to it.
	4. Removed reference to RFC 1700 and replaced it with a link to
	http://www.iana.org/assignments/address-family-numbers.
	5. Removed reference to RFC 1771 and replaced it with a reference
	to RFC 4271.
	6. Clarified the use of the F bit.
	7. Added option to allow split horizon when doing ordered control.
	8. Clarified the processing of messages with the U-bit set during
	the session initialization procedures
	9. Clarified the processing of the E-bit during session initial-
	ization procedures.
	10. Added text explaining that the shutdown message in the state
	transition diagram is implemented as as a notification message
	with a status TLV indicating a fatal error.
	11. Added case for TLV length too short in the specification for
	handling malformed TLVs.
	12. Explained the security threat posed by hello spoofing.
	13. Added reference to 4271 and 4278 and text for standards matu-
	rity variance with regards to the MD5 option.
	14. Added text from 3031 explaining the handling of implicit NULL
	label.
	15. Included the encoding of DLCIs to remove normative reference to
	3034.
	16. Moved references to 3031, 3032 and 3034 to informative.
	17. In the section describing handling of unknown TLV, removed ref-
	erence to inexistent section (errata in original document).
	18. Added text clarifying how to achieve interoperability when
	sending vendor-private TLVs and messages.
	19. In the "receive label request" procedures, if a loop is
	detected, changed the procedure to send a notification before
	aborting the rest of the processing.
	20. In "receive label release" procedures, clarified the behavior
	for merge-capable LSRs.
	21. In "receive label release" procedures, clarified the behavior
	for receipt of an unknown FEC.
	22. In note 4 of "Detect Change in FEC Next Hop", modified the text
	to reference the correct set of conditions for sending a label
	request procedure (typo in the original document).
	23. In the procedures for "LSR decides to no longer label switch a
	FEC", clarified the fact that the label must not be reused
	until a label release is received.
	24. In the routine "Prepare_Label_Mapping_Attributes" added a note
	regarding the treatment of unknown TLVs according to their U
	and F bits.
	25. In the address message processing procedures, clarified the
	behavior for the case where an LSR receives re-advertisement of
	an address previously advertised it, or withdrawal of an
	address from an LSR that has not previously advertised that
	address.
	26. In the routine "Receive Label Mapping" clarified the meaning of
	PrevAdvLabel when no label advertisement message has been sent
	previously.
	27. In the "Receive Label Mapping" procedures, if a loop is
	detected, modified the procedure to send a notification before
	aborting the rest of the processing.
	28. In the "Receive Label Mapping" procedures, corrected step
	LMp.10 to handle label mapping messages for additional (non-
	merged) LSPs for the FEC.
	29. In the "Receive Label Mapping" procedures, clarified behavior
	when receiving a duplicate label for the same FEC.
	30. In the routine "Receive Label Abort Request" clarified the
	behavior for non-merging LSRs.
	31. Added the following items to the section discussing areas for
	future study:
	o extensions for communicating the maximum transmission unit
	o basic peer discovery on NBMA media
	o option of shutting down an adjacency
	o mechanisms for securing Hello messages
	o detection of a stateless fast control plane restart
	o support of "end of LIB" message
	o mechanisms for dealing with the case where different LSRs
	advertise the same address.
	8. Acknowledgments
	This document was originally published as RFC 3036 in January 2001.
	It was produced by the MPLS working of the IETF and was jointly
	authored by Loa Andersson, Paul Doolan, Nancy Feldman, Andre Fredette
	and Bob Thomas.
	The ideas and text in RFC3036 were collected from a number of
	sources. We would like to thank Rick Boivie, Ross Callon, Alex
	Conta, Eric Gray, Yoshihiro Ohba, Eric Rosen, Bernard Suter, Yakov
	Rekhter, and Arun Viswanathan for their input for RFC3036.
	The editors would like to thank Eric Gray, David Black and Sam Hart-
	man for their input to and review of the current document.
	In addition, the editors would like to thank the members of the mpls
	working group for their ideas and the support they have given to this
	document, and in particular to Eric Rosen, Luca Martini, Markus Jork,
	Mark Duffy, Vach Kompella, Kishore Tiruveedhula. Rama Ramakrishnan,
	Nick Weeds, Adrian Farrel and Andy Malis.
	9. References
	9.1. Normative references
	[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing an
	IANA Considerations Section in RFCs", BCP 26, RFC 2434,
	October 1998.
	[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm," RFC 1321,
	April 1992.
	[RFC1483] Heinanen, J., "Multiprotocol Encapsulation over ATM Adap-
	tation Layer 5", RFC 1483, July 1993.
	[ASSIGNED_AF] http://www.iana.org/assignments/address-family-numbers
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119, March 1997.
	[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP
	MD5 Signature Option", RFC 2385, August 1998.
	[RFC3035] Davie, B., Lawrence, J., McCloghrie, K., Rekhter, Y.,
	Rosen, E., Swallow, G. and P. Doolan, "MPLS using LDP and
	ATM VC Switching", RFC 3035, January 2001.
	[RFC3037] Thomas, B. and E. Gray, "LDP Applicability", RFC 3037,
	January 2001.
	9.2. Non-normative references
	[CRLDP] L. Andersson, A. Fredette, B. Jamoussi, R. Callon, P.
	Doolan, N. Feldman, E. Gray, J. Halpern, J. Heinanen T.
	E. Kilty, A. G. Malis, M. Girish, K. Sundell, P. Vaana-
	nen, T. Worster, L. Wu, R. Dantu, "Constraint-Based LSP
	Setup using LDP", RFC 3212, January 2002
	[DIFFSERV] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
	and W. Weiss, "An Architecture for Differentiated Ser-
	vices", RFC 2475, December 1998.
	[LDP-MTU] Black, B. and Kompella, K. "Maximum Transmission Unit
	Signalling Extensions for the Label Distribution Proto-
	col", draft-ietf-mpls-ldp-mtu-extensions-03.txt, April
	2004.
	[RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S. and S.
	Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
	Functional Specification", RFC 2205, September 1997.
	[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
	[RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J.
	McManus, "Requirements for Traffic Engineering over
	MPLS", RFC 2702, September 1999.
	[RFC3031] Rosen, E., Viswanathan, A. and R. Callon, "Multiprotocol
	Label Switching Architecture", RFC 3031, January 2001.
	[RFC3032] Rosen, E., Rekhter, Y., Tappan, D., Farinacci, D.,
	Fedorkow, G., Li, T. and A. Conta, "MPLS Label Stack
	Encoding", RFC 3032, January 2001.
	[RFC3034] Conta, A., Doolan, P. and A. Malis, "Use of Label Switch-
	ing on Frame Relay Networks Specification", RFC 3034,
	January 2001.
	[RFC4271] Rekhter, Y., Li, T. and Hares, S. "A Border Gateway Pro-
	tocol 4 (BGP-4)", RFC 4271, January 2006.
	[RFC4278] Bellovin, S., Zinin, A., "Standards Maturity Variance
	Regarding the TCP MD5 Signature Option (RFC 2385) and the
	BGP-4 Specification", RFC 4278, January 2006.
	10. Intellectual Property Statement
	The IETF takes no position regarding the validity or scope of any
	Intellectual Property Rights or other rights that might be claimed to
	pertain to the implementation or use of the technology described in
	this document or the extent to which any license under such rights
	might or might not be available; nor does it represent that it has
	made any independent effort to identify any such rights. Information
	on the procedures with respect to rights in RFC documents can be
	found in BCP 78 and BCP 79.
	Copies of IPR disclosures made to the IETF Secretariat and any assur-
	ances of licenses to be made available, or the result of an attempt
	made to obtain a general license or permission for the use of such
	proprietary rights by implementers or users of this specification can
	be obtained from the IETF on-line IPR repository at
	http://www.ietf.org/ipr.
	The IETF invites any interested party to bring to its attention any
	copyrights, patents or patent applications, or other proprietary
	rights that may cover technology that may be required to implement
	this standard. Please address the information to the IETF at ietf-
	ipr@ietf.org.
	11. Editors' Addresses
	Loa Andersson
	Acreo AB
	Isafjordsgatan 22
	Kista, Sweden
	email: loa.andersson@acreo.se
	loa@pi.se
	Ina Minei
	Juniper Networks
	1194 N. Mathilda Ave.
	Sunnyvale, CA 94089
	email: ina@juniper.net
	Bob Thomas
	Cisco Systems, Inc.
	1414 Massachusetts Ave
	Boxborough, MA 01719
	email: rhthomas@cisco.com
	Appendix A. LDP Label Distribution Procedures
	This section specifies label distribution behavior in terms of LSR
	response to the following events:
	- Receive Label Request Message;
	- Receive Label Mapping Message;
	- Receive Label Abort Request Message;
	- Receive Label Release Message;
	- Receive Label Withdraw Message;
	- Recognize new FEC;
	- Detect change in FEC next hop;
	- Receive Notification Message / Label Request Aborted;
	- Receive Notification Message / No Label Resources;
	- Receive Notification Message / No Route;
	- Receive Notification Message / Loop Detected;
	- Receive Notification Message / Label Resources Available;
	- Detect local label resources have become available;
	- LSR decides to no longer label switch a FEC;
	- Timeout of deferred label request.
	The specification of LSR behavior in response to an event has three
	parts:
	1. Summary. Prose that describes LSR response to the event in
	overview.
	2. Context. A list of elements referred to by the Algorithm part
	of the specification. (See 3.)
	3. Algorithm. An algorithm for LSR response to the event.
	The Summary may omit details of the LSR response, such as bookkeeping
	action or behavior dependent on the LSR label advertisement mode,
	control mode, or label retention mode in use. The intent is that the
	Algorithm fully and unambiguously specify the LSR response.
	The algorithms in this section use procedures defined in the MPLS
	architecture specification [RFC3031] for hop-by-hop routed traffic.
	These procedures are:
	- Label Distribution procedure, which is performed by a down-
	stream LSR to determine when to distribute a label for a FEC to
	LDP peers. The architecture defines four Label Distribution
	procedures:
	. Downstream Unsolicited Independent Control, called
	PushUnconditional in [RFC3031].
	. Downstream Unsolicited Ordered Control, called
	PushConditional in [RFC3031].
	. Downstream On Demand Independent Control, called
	PulledUnconditional in [RFC3031].
	. Downstream On Demand Ordered Control, called
	PulledConditional in [RFC3031].
	- Label Withdrawal procedure, which is performed by a downstream
	LSR to determine when to withdraw a FEC label mapping previ-
	ously distributed to LDP peers. The architecture defines a
	single Label Withdrawal procedure. Whenever an LSR breaks the
	binding between a label and a FEC, it MUST withdraw the FEC
	label mapping from all LDP peers to which it has previously
	sent the mapping.
	- Label Request procedure, which is performed by an upstream LSR
	to determine when to explicitly request that a downstream LSR
	bind a label to a FEC and send it the corresponding label map-
	ping. The architecture defines three Label Request procedures:
	. Request Never. The LSR never requests a label.
	. Request When Needed. The LSR requests a label whenever
	it needs one.
	. Request On Request. This procedure is used by
	non-label merging LSRs. The LSR requests a label
	when it receives a request for one, in addition
	to whenever it needs one.
	- Label Release procedure, which is performed by an upstream LSR
	to determine when to release a previously received label map-
	ping for a FEC. The architecture defines two Label Release
	procedures:
	. Conservative label retention, called Release On Change in
	[RFC3031].
	. Liberal label retention, called No Release On Change in
	[RFC3031].
	- Label Use procedure, which is performed by an LSR to determine
	when to start using a FEC label for forwarding/switching. The
	architecture defines three Label Use procedures:
	. Use Immediate. The LSR immediately uses a label received
	from a FEC next hop for forwarding/switching.
	. Use If Loop Free. The LSR uses a FEC label received from a
	FEC next hop for forwarding/switching only if it has
	determined that by doing so it will not cause a forwarding
	loop.
	. Use If Loop Not Detected. This procedure is the same as Use
	Immediate unless the LSR has detected a loop in the FEC LSP.
	Use of the FEC label for forwarding/switching will continue
	until the next hop for the FEC changes or the loop is no
	longer detected.
	This version of LDP does not include a loop prevention mecha-
	nism; therefore, the procedures below do not make use of the
	Use If Loop Free procedure.
	- Label No Route procedure (called Label Not Available procedure
	in [RFC3031]), which is performed by an upstream LSR to deter-
	mine how to respond to a No Route notification from a down-
	stream LSR in response to a request for a FEC label mapping.
	The architecture specification defines two Label No Route pro-
	cedures:
	. Request Retry. The LSR should issue the label request at a
	later time.
	. No Request Retry. The LSR should assume the downstream LSR
	will provide a label mapping when the downstream LSR has a
	next hop and it should not reissue the request.
	A.1. Handling Label Distribution Events
	This section defines LDP label distribution procedures by specifying
	an algorithm for each label distribution event. The requirement on
	an LDP implementation is that its event handling must have the effect
	specified by the algorithms. That is, an implementation need not
	follow exactly the steps specified by the algorithms as long as the
	effect is identical.
	The algorithms for handling label distribution events share common
	actions. The specifications below package these common actions into
	procedure units. Specifications for these common procedures are in
	their own section "Common Label Distribution Procedures", which fol-
	lows this.
	An implementation would use data structures to store information
	about protocol activity. This appendix specifies the information to
	be stored in sufficient detail to describe the algorithms, and
	assumes the ability to retrieve the information as needed. It does
	not specify the details of the data structures.
	A.1.1. Receive Label Request
	Summary:
	The response by an LSR to receipt of a FEC label request from an
	LDP peer may involve one or more of the following actions:
	- Transmission of a notification message to the requesting LSR
	indicating why a label mapping for the FEC cannot be provided;
	- Transmission of a FEC label mapping to the requesting LSR;
	- Transmission of a FEC label request to the FEC next hop;
	- Installation of labels for forwarding/switching use by the LSR.
	Context:
	- LSR. The LSR handling the event.
	- MsgSource. The LDP peer that sent the message.
	- FEC. The FEC specified in the message.
	- RAttributes. Attributes received with the message. E.g., Hop
	Count, Path Vector.
	- SAttributes. Attributes to be included in Label Request mes-
	sage, if any, propagated to FEC Next Hop.
	- StoredHopCount. The hop count, if any, previously recorded for
	the FEC.
	Algorithm:
	LRq.1 Execute procedure Check_Received_Attributes (MsgSource,
	LabelRequest, RAttributes).
	If Loop Detected, goto LRq.4.
	LRq.2 Is there a Next Hop for FEC?
	If not, goto LRq.5.
	LRq.3 Is MsgSource the Next Hop?
	Ifnot, goto LRq.6.
	LRq.4 Execute procedure Send_Notification (MsgSource, Loop
	Detected).
	Goto LRq.13
	LRq.5 Execute procedure Send_Notification (MsgSource, No Route).
	Goto LRq.13.
	LRq.6 Has LSR previously received a label request for FEC from
	MsgSource?
	If not, goto LRq.8. (See Note 1.)
	LRq.7 Is the label request a duplicate request?
	If so, Goto LRq.13. (See Note 2.)
	LRq.8 Record label request for FEC received from MsgSource and
	mark it pending.
	LRq.9 Perform LSR Label Distribution procedure:
	For Downstream Unsolicited Independent Control OR
	For Downstream On Demand Independent Control
	1. Has LSR previously received and retained a label map-
	ping for FEC from Next Hop?.
	Is so, set Propagating to IsPropagating.
	If not, set Propagating to NotPropagating.
	2. Execute procedure Prepare_Label_Map-
	ping_Attributes(MsgSource, FEC, RAttributes, SAt-
	tributes, Propagating, StoredHopCount).
	3. Execute procedure Send_Label (MsgSource, FEC, SAt-
	tributes).
	4. Is LSR egress for FEC? OR
	Has LSR previously received and retained a label map-
	ping for FEC from Next Hop?
	If so, goto LRq.11.
	If not, goto LRq.10.
	For Downstream Unsolicited Ordered Control OR
	For Downstream On Demand Ordered Control
	1. Is LSR egress for FEC? OR
	Has LSR previously received and retained a label map-
	ping for FEC from Next Hop? (See Note 3.)
	If not, goto LRq.10.
	2. Execute procedure Prepare_Label_Map-
	ping_Attributes(MsgSource, FEC, RAttributes, SAt-
	tributes, IsPropagating, StoredHopCount)
	3. Execute procedure Send_Label (MsgSource, FEC, SAt-
	tributes).
	Goto LRq.11.
	LRq.10 Perform LSR Label Request procedure:
	For Request Never
	1. Goto LRq.13.
	For Request When Needed OR
	For Request On Request
	1. Execute procedure Prepare_Label_Request_Attributes
	(Next Hop, FEC, RAttributes, SAttributes);
	2. Execute procedure Send_Label_Request (Next Hop, FEC,
	SAttributes).
	Goto LRq.13.
	LRq.11 Has LSR successfully sent a label for FEC to MsgSource?
	If not, goto LRq.13. (See Note 4.)
	LRq.12 Perform LSR Label Use procedure.
	For Use Immediate OR
	For Use If Loop Not Detected
	1. Install label sent to MsgSource and label from Next
	Hop (if LSR is not egress) for forwarding/switching
	use.
	LRq.13 DONE
	Notes:
	1. In the case where MsgSource is a non-label merging LSR it will
	send a label request for each upstream LDP peer that has
	requested a label for FEC from it. The LSR must be able to
	distinguish such requests from a non-label merging MsgSource
	from duplicate label requests.
	The LSR uses the message ID of received Label Request messages
	to detect duplicate requests. This means that an LSR (the
	upstream peer) may not reuse the message ID used for a Label
	Request until the Label Request transaction has completed.
	2. When an LSR sends a label request to a peer it records that the
	request has been sent and marks it as outstanding. As long as
	the request is marked outstanding the LSR SHOULD NOT send
	another request for the same label to the peer. Such a second
	request would be a duplicate. The Send_Label_Request procedure
	described below obeys this rule.
	A duplicate label request is considered a protocol error and
	SHOULD be dropped by the receiving LSR (perhaps with a suitable
	notification returned to MsgSource).
	3. If LSR is not merge-capable, this test will fail.
	4. The Send_Label procedure may fail due to lack of label
	resources, in which case the LSR SHOULD NOT perform the Label
	Use procedure.
	A.1.2. Receive Label Mapping
	Summary:
	The response by an LSR to receipt of a FEC label mapping from an
	LDP peer may involve one or more of the following actions:
	- Transmission of a label release message for the FEC label to
	the LDP peer;
	- Transmission of label mapping messages for the FEC to one or
	more LDP peers,
	- Installation of the newly learned label for forwarding/switch-
	ing use by the LSR.
	Context:
	- LSR. The LSR handling the event.
	- MsgSource. The LDP peer that sent the message.
	- FEC. The FEC specified in the message.
	- Label. The label specified in the message.
	- PrevAdvLabel. The label for FEC, if any, previously advertised
	to an upstream peer. Assuming no label was previously adver-
	tised, this is the same label as the one in the Label Mapping
	Message being processed.
	- StoredHopCount. The hop count previously recorded for the FEC.
	- RAttributes. Attributes received with the message. E.g., Hop
	Count, Path Vector.
	- SAttributes to be included in Label Mapping message, if any,
	propagated to upstream peers.
	Algorithm:
	LMp.1 Does the received label mapping match an outstanding
	label request for FEC previously sent to MsgSource.
	If not, goto LMp.3.
	LMp.2 Delete record of outstanding FEC label request.
	LMp.3 Execute procedure Check_Received_Attributes (MsgSource,
	LabelMapping, RAttributes).
	If No Loop Detected, goto LMp.9.
	LMp.4 Does the LSR have a previously received label mapping for
	FEC from MsgSource? (See Note 1.)
	If not, goto LMp.8.