Network Working Group L. Andersson, Ed. Request for Comments: 5036 Acreo AB Obsoletes: 3036 I. Minei, Ed. Category: Standards Track Juniper Networks B. Thomas, Ed. Cisco Systems, Inc. October 2007 LDP Specification Status of This Memo 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. Abstract 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. Table of Contents 1. LDP Overview ....................................................5 1.1. LDP Peers ..................................................6 1.2. LDP Message Exchange .......................................6 1.3. LDP Message Structure ......................................7 1.4. LDP Error Handling .........................................7 1.5. LDP Extensibility and Future Compatibility .................7 1.6. Specification Language .....................................7 2. LDP Operation ...................................................8 2.1. FECs .......................................................8 2.2. Label Spaces, Identifiers, Sessions, and Transport .........9 2.2.1. Label Spaces ........................................9 2.2.2. LDP Identifiers .....................................9 2.2.3. LDP Sessions .......................................10 2.2.4. LDP Transport ......................................10 Andersson, et al. Standards Track [Page 1] RFC 5036 LDP Specification October 2007 2.3. LDP Sessions between Non-Directly Connected LSRs ..........10 2.4. LDP Discovery .............................................11 2.4.1. Basic Discovery Mechanism ..........................11 2.4.2. Extended Discovery Mechanism .......................11 2.5. Establishing and Maintaining LDP Sessions .................12 2.5.1. LDP Session Establishment ..........................12 2.5.2. Transport Connection Establishment .................12 2.5.3. Session Initialization .............................14 2.5.4. Initialization State Machine .......................16 2.5.5. Maintaining Hello Adjacencies ......................19 2.5.6. Maintaining LDP Sessions ...........................19 2.6. Label Distribution and Management .........................20 2.6.1. Label Distribution Control Mode ....................20 2.6.1.1. Independent Label Distribution Control ....20 2.6.1.2. Ordered Label Distribution Control ........20 2.6.2. Label Retention Mode ...............................21 2.6.2.1. Conservative Label Retention Mode .........21 2.6.2.2. Liberal Label Retention Mode ..............21 2.6.3. Label Advertisement Mode ...........................22 2.7. LDP Identifiers and Next Hop Addresses ....................22 2.8. Loop Detection ............................................23 2.8.1. Label Request Message ..............................23 2.8.2. Label Mapping Message ..............................25 2.8.3. Discussion .........................................26 2.9. Authenticity and Integrity of LDP Messages ................27 2.9.1. TCP MD5 Signature Option ...........................27 2.9.2. LDP Use of TCP MD5 Signature Option ................29 2.10. Label Distribution for Explicitly Routed LSPs ............29 3. Protocol Specification .........................................30 3.1. LDP PDUs ..................................................30 3.2. LDP Procedures ............................................31 3.3. Type-Length-Value Encoding ................................31 3.4. TLV Encodings for Commonly Used Parameters ................33 3.4.1. FEC TLV ............................................33 3.4.1.1. FEC Procedures ............................35 3.4.2. Label TLVs .........................................35 3.4.2.1. Generic Label TLV .........................36 3.4.2.2. ATM Label TLV .............................36 3.4.2.3. Frame Relay Label TLV .....................37 3.4.3. Address List TLV ...................................38 3.4.4. Hop Count TLV ......................................39 3.4.4.1. Hop Count Procedures ......................39 3.4.5. Path Vector TLV ....................................41 3.4.5.1. Path Vector Procedures ....................41 3.4.5.1.1. Label Request Path Vector ......41 3.4.5.1.2. Label Mapping Path Vector ......42 3.4.6. Status TLV .........................................43 Andersson, et al. Standards Track [Page 2] RFC 5036 LDP Specification October 2007 3.5. LDP Messages ..............................................44 3.5.1. Notification Message ...............................46 3.5.1.1. Notification Message Procedures ...........48 3.5.1.2. Events Signaled by Notification Messages ..48 3.5.1.2.1. Malformed PDU or Message .......48 3.5.1.2.2. Unknown or Malformed TLV .......49 3.5.1.2.3. Session KeepAlive Timer Expiration .....................50 3.5.1.2.4. Unilateral Session Shutdown ....50 3.5.1.2.5. Initialization Message Events ..50 3.5.1.2.6. Events Resulting from Other Messages .................50 3.5.1.2.7. Internal Errors ................51 3.5.1.2.8. Miscellaneous Events ...........51 3.5.2. Hello Message ......................................51 3.5.2.1. Hello Message Procedures ..................53 3.5.3. Initialization Message .............................54 3.5.3.1. Initialization Message Procedures .........63 3.5.4. KeepAlive Message ..................................63 3.5.4.1. KeepAlive Message Procedures ..............63 3.5.5. Address Message ....................................64 3.5.5.1. Address Message Procedures ................64 3.5.6. Address Withdraw Message ...........................65 3.5.6.1. Address Withdraw Message Procedures .......66 3.5.7. Label Mapping Message ..............................66 3.5.7.1. Label Mapping Message Procedures ..........67 3.5.7.1.1. Independent Control Mapping ....67 3.5.7.1.2. Ordered Control Mapping ........68 3.5.7.1.3. Downstream on Demand Label Advertisement ............68 3.5.7.1.4. Downstream Unsolicited Label Advertisement ............69 3.5.8. Label Request Message ..............................70 3.5.8.1. Label Request Message Procedures ..........71 3.5.9. Label Abort Request Message ........................72 3.5.9.1. Label Abort Request Message Procedures ....73 3.5.10. Label Withdraw Message ............................74 3.5.10.1. Label Withdraw Message Procedures ........75 3.5.11. Label Release Message .............................76 3.5.11.1. Label Release Message Procedures .........77 3.6. Messages and TLVs for Extensibility .......................78 3.6.1. LDP Vendor-Private Extensions ......................78 3.6.1.1. LDP Vendor-Private TLVs ...................78 3.6.1.2. LDP Vendor-Private Messages ...............80 3.6.2. LDP Experimental Extensions ........................81 3.7. Message Summary ...........................................81 3.8. TLV Summary ...............................................82 3.9. Status Code Summary .......................................83 Andersson, et al. Standards Track [Page 3] RFC 5036 LDP Specification October 2007 3.10. Well-Known Numbers .......................................84 3.10.1. UDP and TCP Ports .................................84 3.10.2. Implicit NULL Label ...............................84 4. IANA Considerations ............................................84 4.1. Message Type Name Space ...................................84 4.2. TLV Type Name Space .......................................85 4.3. FEC Type Name Space .......................................85 4.4. Status Code Name Space ....................................86 4.5. Experiment ID Name Space ..................................86 5. Security Considerations ........................................86 5.1. Spoofing ..................................................86 5.2. Privacy ...................................................87 5.3. Denial of Service .........................................88 6. Areas for Future Study .........................................89 7. Changes from RFC 3036 ..........................................90 8. Acknowledgments ................................................93 9. References .....................................................93 9.1. Normative References ......................................93 9.2. Informative References ....................................94 Appendix A. LDP Label Distribution Procedures ....................95 A.1. Handling Label Distribution Events .......................97 A.1.1. Receive Label Request .............................98 A.1.2. Receive Label Mapping ...........................101 A.1.3. Receive Label Abort Request .....................107 A.1.4. Receive Label Release ...........................109 A.1.5. Receive Label Withdraw ..........................111 A.1.6. Recognize New FEC ...............................113 A.1.7. Detect Change in FEC Next Hop ...................115 A.1.8. Receive Notification / Label Request Aborted ....118 A.1.9. Receive Notification / No Label Resources .......119 A.1.10. Receive Notification / No Route ................119 A.1.11. Receive Notification / Loop Detected ...........120 A.1.12. Receive Notification / Label Resources Available 121 A.1.13. Detect Local Label Resources Have Become Available ......................................122 A.1.14. LSR Decides to No Longer Label Switch a FEC ....123 A.1.15. Timeout of Deferred Label Request ..............123 A.2. Common Label Distribution Procedures ....................124 A.2.1. Send_Label ......................................124 A.2.2. Send_Label_Request ..............................125 A.2.3. Send_Label_Withdraw .............................127 A.2.4. Send_Notification ...............................127 A.2.5. Send_Message ....................................128 A.2.6. Check_Received_Attributes .......................128 A.2.7. Prepare_Label_Request_Attributes ................129 A.2.8. Prepare_Label_Mapping_Attributes ................131 Andersson, et al. Standards Track [Page 4] RFC 5036 LDP Specification October 2007 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 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. Andersson, et al. Standards Track [Page 5] RFC 5036 LDP Specification October 2007 1.1. LDP Peers 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. 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. Notification messages, used to provide advisory information and 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 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. Andersson, et al. Standards Track [Page 6] RFC 5036 LDP Specification October 2007 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 Encoding". 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 connection for the session and discarding all label mappings learned via the session. 2. Advisory Notifications, used to pass on 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]. Andersson, et al. Standards Track [Page 7] RFC 5036 LDP Specification October 2007 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 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. Andersson, et al. Standards Track [Page 8] RFC 5036 LDP Specification October 2007 - 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.) 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 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. 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: Andersson, et al. Standards Track [Page 9] RFC 5036 LDP Specification October 2007 :