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Network Working Group Request for Comments: 5036 Obsoletes: 3036 Category: Standards Track |
L. Andersson, Ed. Acreo AB I. Minei, Ed. Juniper Networks B. Thomas, Ed. Cisco Systems, Inc. October 2007 |
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
The architecture for Multiprotocol Label Switching (MPLS) is described in RFC 3031. 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.
1. LDP Overview
1.1. LDP Peers
1.2. LDP Message Exchange
1.3. LDP Message Structure
1.4. LDP Error Handling
1.5. LDP Extensibility and Future Compatibility
1.6. Specification Language
2. LDP Operation
2.1. FECs
2.2. Label Spaces, Identifiers, Sessions, and Transport
2.2.1. Label Spaces
2.2.2. LDP Identifiers
2.2.3. LDP Sessions
2.2.4. LDP Transport
2.3. LDP Sessions between Non-Directly Connected LSRs
2.4. LDP Discovery
2.4.1. Basic Discovery Mechanism
2.4.2. Extended Discovery Mechanism
2.5. Establishing and Maintaining LDP Sessions
2.5.1. LDP Session Establishment
2.5.2. Transport Connection Establishment
2.5.3. Session Initialization
2.5.4. Initialization State Machine
2.5.5. Maintaining Hello Adjacencies
2.5.6. Maintaining LDP Sessions
2.6. Label Distribution and Management
2.6.1. Label Distribution Control Mode
2.6.1.1. Independent Label Distribution Control
2.6.1.2. Ordered Label Distribution Control
2.6.2. Label Retention Mode
2.6.2.1. Conservative Label Retention Mode
2.6.2.2. Liberal Label Retention Mode
2.6.3. Label Advertisement Mode
2.7. LDP Identifiers and Next Hop Addresses
2.8. Loop Detection
2.8.1. Label Request Message
2.8.2. Label Mapping Message
2.8.3. Discussion
2.9. Authenticity and Integrity of LDP Messages
2.9.1. TCP MD5 Signature Option
2.9.2. LDP Use of TCP MD5 Signature Option
2.10. Label Distribution for Explicitly Routed LSPs
3. Protocol Specification
3.1. LDP PDUs
3.2. LDP Procedures
3.3. Type-Length-Value Encoding
3.4. TLV Encodings for Commonly Used Parameters
3.4.1. FEC TLV
3.4.1.1. FEC Procedures
3.4.2. Label TLVs
3.4.2.1. Generic Label TLV
3.4.2.2. ATM Label TLV
3.4.2.3. Frame Relay Label TLV
3.4.3. Address List TLV
3.4.4. Hop Count TLV
3.4.4.1. Hop Count Procedures
3.4.5. Path Vector TLV
3.4.5.1. Path Vector Procedures
3.4.5.1.1. Label Request Path Vector
3.4.5.1.2. Label Mapping Path Vector
3.4.6. Status TLV
3.5. LDP Messages
3.5.1. Notification Message
3.5.1.1. Notification Message Procedures
3.5.1.2. Events Signaled by Notification Messages ..48
3.5.1.2.1. Malformed PDU or Message
3.5.1.2.2. Unknown or Malformed TLV
3.5.1.2.3. Session KeepAlive Timer
Expiration
3.5.1.2.4. Unilateral Session Shutdown
3.5.1.2.5. Initialization Message Events ..50
3.5.1.2.6. Events Resulting from
Other Messages
3.5.1.2.7. Internal Errors
3.5.1.2.8. Miscellaneous Events
3.5.2. Hello Message
3.5.2.1. Hello Message Procedures
3.5.3. Initialization Message
3.5.3.1. Initialization Message Procedures
3.5.4. KeepAlive Message
3.5.4.1. KeepAlive Message Procedures
3.5.5. Address Message
3.5.5.1. Address Message Procedures
3.5.6. Address Withdraw Message
3.5.6.1. Address Withdraw Message Procedures
3.5.7. Label Mapping Message
3.5.7.1. Label Mapping Message Procedures
3.5.7.1.1. Independent Control Mapping
3.5.7.1.2. Ordered Control Mapping
3.5.7.1.3. Downstream on Demand
Label Advertisement
3.5.7.1.4. Downstream Unsolicited
Label Advertisement
3.5.8. Label Request Message
3.5.8.1. Label Request Message Procedures
3.5.9. Label Abort Request Message
3.5.9.1. Label Abort Request Message Procedures
3.5.10. Label Withdraw Message
3.5.10.1. Label Withdraw Message Procedures
3.5.11. Label Release Message
3.5.11.1. Label Release Message Procedures
3.6. Messages and TLVs for Extensibility
3.6.1. LDP Vendor-Private Extensions
3.6.1.1. LDP Vendor-Private TLVs
3.6.1.2. LDP Vendor-Private Messages
3.6.2. LDP Experimental Extensions
3.7. Message Summary
3.8. TLV Summary
3.9. Status Code Summary
3.10. Well-Known Numbers
3.10.1. UDP and TCP Ports
3.10.2. Implicit NULL Label
4. IANA Considerations
4.1. Message Type Name Space
4.2. TLV Type Name Space
4.3. FEC Type Name Space
4.4. Status Code Name Space
4.5. Experiment ID Name Space
5. Security Considerations
5.1. Spoofing
5.2. Privacy
5.3. Denial of Service
6. Areas for Future Study
7. Changes from RFC 3036
8. Acknowledgments
9. References
9.1. Normative References
9.2. Informative References
Appendix A. LDP Label Distribution Procedures
A.1. Handling Label Distribution Events
A.1.1. Receive Label Request
A.1.2. Receive Label Mapping
A.1.3. Receive Label Abort Request
A.1.4. Receive Label Release
A.1.5. Receive Label Withdraw
A.1.6. Recognize New FEC
A.1.7. Detect Change in FEC Next Hop
A.1.8. Receive Notification / Label Request Aborted
A.1.9. Receive Notification / No Label Resources
A.1.10. Receive Notification / No Route
A.1.11. Receive Notification / Loop Detected
A.1.12. Receive Notification / Label Resources Available 121
A.1.13. Detect Local Label Resources Have Become
Available
A.1.14. LSR Decides to No Longer Label Switch a FEC
A.1.15. Timeout of Deferred Label Request
A.2. Common Label Distribution Procedures
A.2.1. Send_Label
A.2.2. Send_Label_Request
A.2.3. Send_Label_Withdraw
A.2.4. Send_Notification
A.2.5. Send_Message
A.2.6. Check_Received_Attributes
A.2.7. Prepare_Label_Request_Attributes
A.2.8. Prepare_Label_Mapping_Attributes
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 protocols 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 distributing labels. It was originally published as RFC 3036 in January 2001. It was produced by the MPLS Working Group 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) establish Label Switched Paths (LSPs) through a network by mapping network-layer routing information directly to data-link layer switched paths. These LSPs may have an endpoint at a directly attached neighbor (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 network 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.
Two LSRs that 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 protocol is bidirectional.
There are four categories of LDP messages:
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 establish a session with another LSR learned via the Hello message, it uses the LDP initialization procedure over TCP transport. Upon successful 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.
All LDP messages have a common structure that uses a Type-Length- Value (TLV) encoding scheme; see Section "Type-Length-Value Encoding". The Value part of a TLV-encoded object, or TLV for short, may itself contain one or more TLVs.
LDP errors and other events of interest are signaled to an LDP peer by Notification messages.
There are two kinds of LDP Notification messages:
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.
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].
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 that 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 that may be mapped to the corresponding 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 specifications.
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 prefix 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 that matches the packet's destination address. With respect to a particular packet and a particular LSP, we refer to any Address Prefix FEC element that 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 that has an Address Prefix FEC
element that 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 particular egress router is beyond the scope of this document. (As an example, if one is running a link state routing algorithm, it may be possible 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.)
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 interface that uses VCIs (Virtual Channel Identifiers) 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.
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>
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.
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.
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.
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 connected 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 pushing it if the packet arrives unlabeled). Next, it pushes the label for the LSP learned from LSR1 onto the label stack.
LDP discovery is a mechanism that enables an LSR to discover potential LDP peers. Discovery makes it unnecessary to explicitly configure 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.
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.
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.
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.
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.
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:
Similarly, address A2 is determined as follows:
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 treating
A1 as a sequence of bytes, where the byte that appears
earliest in the message is the most significant byte of the
integer and the byte that 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.
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 Transport 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.
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 distribution 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 cannot 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 Identifier for the sender's (active LSR's) label space and the LDP Identifier 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 determined from the LDP Identifier in the PDU header for the Initialization message) with a Hello adjacency previously created when Hellos were exchanged.
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 parameters 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.
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 Initialization 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,
reconfiguration 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.
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
| |
+---+
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 situation, 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 that 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 concludes 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 adjacency. When the last Hello adjacency for an LDP session is deleted, the LSR terminates the LDP session by sending a Notification message and closing the transport connection.
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 that 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 circumstances 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.
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 distribution Unsolicited Downstream; this document uses the term Downstream 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 Unsolicited label distribution assumes its peer is also. See Section "Downstream on Demand Label Advertisement".
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.
When using independent LSP control, each LSR may advertise label mappings 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.
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:
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.
The MPLS architecture [RFC3031] introduces the notion of label retention 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.
In Downstream Unsolicited advertisement mode, label mapping advertisements 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 operating 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 typically 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 maintained. 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.
In Downstream Unsolicited advertisement mode, label mapping advertisements 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, however, 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 reaction to routing changes can be quick because labels already exist. The main disadvantage of the liberal mode is that unneeded label mappings are distributed and maintained.
Each interface on an LSR is configured to operate in either Downstream Unsolicited or Downstream on Demand advertisement mode. LSRs exchange advertisement modes during initialization. The major difference between Downstream Unsolicited and Downstream on Demand modes is in which LSR takes responsibility for initiating mapping requests and mapping advertisements.
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 forwarding. 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 Withdraw 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 advertised addresses from a peer.
Loop Detection is a configurable option that 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 Vector 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 containing 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 describe 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]).
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 messages 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 containing 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 outstanding 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 that 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 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.
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 following:
- 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:
- 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 following:
- 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:
If the above rules require R include a Path Vector TLV in the Label Mapping message, R computes it as follows:
- 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 that exceeds the configured maximum value, or with a Path Vector TLV containing its own LSR Id or that 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 forwarding, drop the Label Mapping message, and signal Loop Detected status to the source of the Label Mapping message.
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 that 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 that 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 a 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 number of Label Mapping update messages.
This section specifies a mechanism to protect against the introduction 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 specified 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].
The following quotes from [RFC2385] outline the security properties achieved by using the TCP MD5 Signature Option and summarize 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 connection 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 protect 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 application. It could even change during the lifetime of a particular 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].
LDP uses the TCP MD5 Signature Option as follows:
- Use of the MD5 Signature Option for LDP TCP connections is a
configurable 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
validates 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 configured.
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.
Previous sections that describe LDP operation have discussed scenarios that involve the exchange of messages among LDP peers. This section 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 messages 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.
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
protocol. 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
sending 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.
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 separate 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.
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
following 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
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. 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
specification 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
message 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 specifies 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.
There are several parameters used by more than one LDP message. The TLV encodings for these commonly used parameters are specified in this section.
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
NUMBERS 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 prefix 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.
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.
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 that can appear in situations that require a 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].
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
significant. 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.
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
following values are supported:
0 = 10 bits of DLCI
2 = 23 bits of DLCI
Len values 1 and 3 are reserved.
DLCI
The Data Link Connection Identifier
For a 10-bit 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 23-bit 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].
The Address List TLV appears in Address and Address Withdraw messages.
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
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 set up.
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.
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:
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 established, each LSR starts with an 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".
The Path Vector TLV is used with the Hop Count TLV in Label Request and Label Mapping messages to implement the optional LDP Loop Detection 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 set up 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.
The Path Vector TLV is carried in Label Mapping and Label Request messages when Loop Detection is configured.
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:
Note that a Label Request message with a Path Vector TLV is forwarded until:
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:
Note that a Label Mapping message with a Path Vector TLV is forwarded until:
Notification messages carry Status TLVs to specify events being signaled.
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
Notification. 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 that 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 messages. A message other than a Notification message may carry a Status TLV as an Optional Parameter. When a message other than a Notification 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.
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
"Notification Message".
Mandatory Parameters
Variable length set of required message parameters. Some messages
have no required parameters.
For messages that have required parameters, the required parameters 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 parameters 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.
An LSR sends a Notification message to inform an LDP peer of a significant event. A Notification message signals a fatal error or provides 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
additional information that supplements the status information
contained 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.
If an LSR encounters a condition requiring it to notify its peer with advisory or error information, it sends the peer a Notification message containing a Status TLV that encodes the information and optionally additional TLVs that provide more information about the condition.
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 session immediately by closing the session TCP connection and discard all state associated with the session, including all label-FEC bindings 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.
It is useful for descriptive purpose to classify events signaled by Notification messages into the following categories.
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 malformed if:
- The LDP Identifier in the PDU header is unknown to the
receiver, or it is known but is not the LDP Identifier
associated 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, d or
it is supported but is not the version negotiated for the
session during session establishment. This is a fatal error
signaled 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
determined.
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
signaled 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.
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
handles the TLV but cannot decode the TLV Value. This is
interpreted as indicative of a bug in either the sending or
receiving LSR. It is a fatal error signaled by the Malformed
TLV Value Status Code.
This is a fatal error signaled by the KeepAlive Timer Expired Status Code.
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.
The session initialization negotiation (see Section "Session Initialization") may fail if the session parameters received in the Initialization message are unacceptable. This is a fatal error. The specific Status Code depends on the parameter deemed unacceptable, and is defined in Sections "Initialization Message".
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 signal them are described in the sections that describe these messages.
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 implementation should, when capable of doing so, use the Internal Error Status Code to signal the peer. This is a fatal error.
These are events that fall into none of the categories above. There are no miscellaneous events defined in this version of the protocol.
LDP Hello messages are exchanged as part of the LDP Discovery Mechanism; 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
Hellos 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 initiates 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 of the Hello message contains 0 or more
parameters, each encoded as a TLV. The optional parameters
defined by this version 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
carrying 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
sending 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
carrying the Hello SHOULD be used.
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 "Maintaining 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:
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
receiving LSR. When a receiving LSR playing the active role in
LDP session establishment detects a change in the sending LSR
configuration, it may clear the session setup backoff delay, if
any, associated with the sending LSR (see Section "Session
Initialization").
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 number.
The LDP Initialization message is exchanged as part of the LDP session establishment procedure; see Section "LDP Session Establishment".
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