Network Working Group L. Martini, Ed. Request for Comments: 4447 E. Rosen Category: Standards Track Cisco Systems, Inc. N. El-Aawar Level 3 Communications, LLC. T. Smith Network Appliance, Inc. G. Heron Tellabs April 2006 Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP) 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. Copyright Notice Copyright (C) The Internet Society (2006).
AbstractLayer 2 services (such as Frame Relay, Asynchronous Transfer Mode, and Ethernet) can be "emulated" over an MPLS backbone by encapsulating the Layer 2 Protocol Data Units (PDU) and transmitting them over "pseudowires". It is also possible to use pseudowires to provide low-rate Time Division Multiplexed and a Synchronous Optical NETworking circuit emulation over an MPLS-enabled network. This document specifies a protocol for establishing and maintaining the pseudowires, using extensions to Label Distribution Protocol (LDP). Procedures for encapsulating Layer 2 PDUs are specified in a set of companion documents.
1. Introduction ....................................................3 2. Specification of Requirements ...................................5 3. The Pseudowire Label ............................................5 4. Details Specific to Particular Emulated Services ................7 4.1. IP Layer 2 Transport .......................................7 5. LDP .............................................................7 5.1. LDP Extensions .............................................8 5.2. The PWid FEC Element .......................................8 5.3. The Generalized PWid FEC Element ..........................10 5.3.1. Attachment Identifiers .............................11 5.3.2. Encoding the Generalized ID FEC Element ............13 184.108.40.206. Interface Parameters TLV ..................14 220.127.116.11. PW Grouping TLV ...........................14 5.3.3. Signaling Procedures ...............................15 5.4. Signaling of Pseudowire Status ............................16 5.4.1. Use of Label Mappings Messages .....................16 5.4.2. Signaling PW Status ................................17 5.4.3. Pseudowire Status Negotiation Procedures ...........18 5.5. Interface Parameters Sub-TLV ..............................19 6. Control Word ...................................................20 6.1. PW Types for Which the Control Word is REQUIRED ...........20 6.2. PW Types for Which the Control Word is NOT Mandatory ......21 6.3. LDP Label Withdrawal Procedures ...........................22 6.4. Sequencing Considerations .................................23 6.4.1. Label Advertisements ...............................23 6.4.2. Label Release ......................................24 7. IANA Considerations ............................................24 7.1. LDP TLV TYPE ..............................................24 7.2. LDP Status Codes ..........................................24 7.3. FEC Type Name Space .......................................25 8. Security Considerations ........................................25 8.1. Data-Plane Security .......................................25 8.2. Control-Plane Security ....................................26 9. Acknowledgements ...............................................27 10. Normative References ..........................................27 11. Informative References ........................................27 12. Additional Contributing Authors ...............................28 Appendix A. C-bit Handling Procedures Diagram .....................31
FRAME], [ATM], [PPPHDLC], and [ETH], it is explained how to encapsulate a Layer 2 Protocol Data Unit (PDU) for transmission over an MPLS-enabled network. Those documents specify that a "pseudowire header", consisting of a demultiplexor field, will be prepended to the encapsulated PDU. The pseudowire demultiplexor field is prepended before transmitting a packet on a pseudowire. When the packet arrives at the remote endpoint of the pseudowire, the demultiplexor is what enables the receiver to identify the particular pseudowire on which the packet has arrived. To transmit the packet from one pseudowire endpoint to another, the packet may need to travel through a "Packet Switched Network (PSN) tunnel"; this will require that an additional header be prepended to the packet. Accompanying documents [CEP, SAToP] specify methods for transporting time-division multiplexing (TDM) digital signals (TDM circuit emulation) over a packet-oriented MPLS-enabled network. The transmission system for circuit-oriented TDM signals is the Synchronous Optical Network (SONET)[SDH]/Synchronous Digital Hierarchy (SDH) [ITUG]. To support TDM traffic, which includes voice, data, and private leased-line service, the pseudowires must emulate the circuit characteristics of SONET/SDH payloads. The TDM signals and payloads are encapsulated for transmission over pseudowires. A pseudowire demultiplexor and a PSN tunnel header is prepended to this encapsulation. [SAToP] describes methods for transporting low-rate time-division multiplexing (TDM) digital signals (TDM circuit emulation) over PSNs, while [CEP] similarly describes transport of high-rate TDM (SONET/SDH). To support TDM traffic, the pseudowires must emulate the circuit characteristics of the original T1, E1, T3, E3, SONET, or SDH signals. [SAToP] does this by encapsulating an arbitrary but constant amount of the TDM data in each packet, and the other methods encapsulate TDM structures. In this document, we specify the use of the MPLS Label Distribution Protocol, LDP [RFC3036], as a protocol for setting up and maintaining the pseudowires. In particular, we define new TLVs, FEC elements, parameters, and codes for LDP, which enable LDP to identify pseudowires and to signal attributes of pseudowires. We specify how a pseudowire endpoint uses these TLVs in LDP to bind a demultiplexor field value to a pseudowire, and how it informs the remote endpoint of the binding. We also specify procedures for reporting pseudowire status changes, for passing additional information about the pseudowire as needed, and for releasing the bindings.
In the protocol specified herein, the pseudowire demultiplexor field is an MPLS label. Thus, the packets that are transmitted from one end of the pseudowire to the other are MPLS packets, which must be transmitted through an MPLS tunnel. However, if the pseudowire endpoints are immediately adjacent and penultimate hop popping behavior is in use, the MPLS tunnel may not be necessary. Any sort of PSN tunnel can be used, as long as it is possible to transmit MPLS packets through it. The PSN tunnel can itself be an MPLS LSP, or any other sort of tunnel that can carry MPLS packets. Procedures for setting up and maintaining the MPLS tunnels are outside the scope of this document. This document deals only with the setup and maintenance of point-to- point pseudowires. Neither point-to-multipoint nor multipoint-to- point pseudowires are discussed. QoS-related issues are not discussed in this document. The following two figures describe the reference models that are derived from [RFC3985] to support the PW emulated services. |<-------------- Emulated Service ---------------->| | | | |<------- Pseudowire ------->| | | | | | |Attachment| |<-- PSN Tunnel -->| |Attachment| | Circuit V V V V Circuit | V (AC) +----+ +----+ (AC) V +-----+ | | PE1|==================| PE2| | +-----+ | |----------|............PW1.............|----------| | | CE1 | | | | | | | | CE2 | | |----------|............PW2.............|----------| | +-----+ ^ | | |==================| | | ^ +-----+ ^ | +----+ +----+ | | ^ | | Provider Edge 1 Provider Edge 2 | | | | | | Customer | | Customer Edge 1 | | Edge 2 | | native service native service Figure 1: PWE3 Reference Model
+-----------------+ +-----------------+ |Emulated Service | |Emulated Service | |(e.g., TDM, ATM) |<==== Emulated Service ===>|(e.g., TDM, ATM) | +-----------------+ +-----------------+ | Payload | | Payload | | Encapsulation |<====== Pseudowire =======>| Encapsulation | +-----------------+ +-----------------+ |PW Demultiplexer | |PW Demultiplexer | | PSN Tunnel, |<======= PSN Tunnel ======>| PSN Tunnel, | | PSN & Physical | | PSN & Physical | | Layers | | Layers | +-------+---------+ __________ +---------+-------+ | / \ | +===============/ PSN \================+ \ / \____________/ Figure 2: PWE3 Protocol Stack Reference Model For the purpose of this document, PE1 will be defined as the ingress router, and PE2 as the egress router. A layer 2 PDU will be received at PE1, encapsulated at PE1, transported and decapsulated at PE2, and transmitted out of PE2. RFC2119].
(For example, if the MPLS tunnel is an LSP and penultimate hop popping is used, when the packet arrives at PE2, it will contain no information identifying the tunnel.) When PE2 receives a packet over a pseudowire, it must be able to determine that the packet was in fact received over a pseudowire, and it must be able to associate that packet with a particular pseudowire. PE2 is able to do this by examining the MPLS label that serves as the pseudowire demultiplexor field shown in Figure 2. Call this label the "PW label". When PE1 sends a Layer 2 PDU to PE2, it creates an MPLS packet by adding the PW label to the packet, thus creating the first entry of the label stack. If the PSN tunnel is an MPLS LSP, the PE1 pushes another label (the tunnel label) onto the packet as the second entry of the label stack. The PW label is not visible again until the MPLS packet reaches PE2. PE2's disposition of the packet is based on the PW label. If the payload of the MPLS packet is, for example, an ATM AAL5 PDU, the PW label will generally correspond to a particular ATM VC at PE2. That is, PE2 needs to be able to infer from the PW label the outgoing interface and the VPI/VCI value for the AAL5 PDU. If the payload is a Frame Relay PDU, then PE2 needs to be able to infer from the PW label the outgoing interface and the DLCI value. If the payload is an Ethernet frame, then PE2 needs to be able to infer from the PW label the outgoing interface, and perhaps the VLAN identifier. This process is uni-directional and will be repeated independently for bi-directional operation. It is REQUIRED that the same PW ID and PW type be assigned for a given circuit in both directions. The group ID (see below) MUST NOT be required to match in both directions. The transported frame MAY be modified when it reaches the egress router. If the header of the transported Layer 2 frame is modified, this MUST be done at the egress LSR only. Note that the PW label must always be at the bottom of the packet's label stack, and labels MUST be allocated from the per-platform label space. This document does not specify a method for distributing the MPLS tunnel label or any other labels that may appear above the PW label on the stack. Any acceptable method of MPLS label distribution will do. This document specifies a protocol for assigning and distributing the PW label. This protocol is LDP, extended as specified in the remainder of this document. An LDP session must be set up between the pseudowire endpoints. LDP MUST be used in its "downstream unsolicited" mode. LDP's "liberal label retention" mode SHOULD be used.
In addition to the protocol specified herein, static assignment of PW labels may be used, and implementations of this protocol SHOULD provide support for static assignment. This document specifies all the procedures necessary to set up and maintain the pseudowires needed to support "unswitched" point-to- point services, where each endpoint of the pseudowire is provisioned with the identify of the other endpoint. There are also protocol mechanisms specified herein that can be used to support switched services and other provisioning models. However, the use of the protocol mechanisms to support those other models and services is not described in this document. RFC3032]. The PW control word MAY be inserted between the MPLS label stack and the IP payload. The encapsulation of the IP packets for forwarding on the attachment circuit is implementation specific, is part of the native service processing (NSP) function [RFC3985], and is outside the scope of this document. RFC3036]. The PEs will establish an LDP session using the Extended Discovery mechanism described in [LDP, sections 2.4.2 and 2.5]. An LDP Label Mapping message contains an FEC TLV, a Label TLV, and zero or more optional parameter TLVs. The FEC TLV is used to indicate the meaning of the label. In the current context, the FEC TLV would be used to identify the particular pseudowire that a particular label is bound to. In this specification, we define two new FEC TLVs to be used for identifying pseudowires. When setting up a particular pseudowire, only one of these FEC TLVs is used. The one to be used will depend on the particular service being emulated and on the particular provisioning model being supported. LDP allows each FEC TLV to consist of a set of FEC elements. For setting up and maintaining pseudowires, however, each FEC TLV MUST contain exactly one FEC element.
The LDP base specification has several kinds of label TLVs, including the Generic Label TLV, as specified in [RFC3036], section 18.104.22.168. For setting up and maintaining pseudowires, the Generic Label TLV MUST be used. 5.4.2 Notification PW Interface Parameters TLV 22.214.171.124 FEC PW Grouping ID TLV 126.96.36.199 FEC Additionally, the following new FEC element types are defined: FEC Element Type Specified in Section Defined for Message =================================================================== 0x80 5.2 FEC 0x81 5.3 FEC The following new LDP error codes are also defined: Status Code Specified in Section ==================================================================== "Illegal C-Bit" 6.1 "Wrong C-Bit" 6.2 "Incompatible bit-rate" [CEP] "CEP/TDM mis-configuration" [CEP] "PW status" 5.4.2 "Unassigned/Unrecognized TAI" 5.3.3 "Generic Misconfiguration Error" [SAToP] "Label Withdraw PW Status Method Not Supported" 5.4.1
For this purpose, a new type of FEC element is defined. The FEC element type is 0x80 and is defined 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PWid (0x80) |C| PW type |PW info Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Group ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PW ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface Parameter Sub-TLV | | " | | " | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - PW type A 15-bit quantity containing a value that represents the type of PW. Assigned values are specified in "IANA Allocations for Pseudowire Edge to Edge Emulation (PWE3)" [IANA]. - Control word bit (C) The bit (C) is used to flag the presence of a control word as follows: C = 1 Control word present on this PW. C = 0 No control word present on this PW. Please see the section "C-Bit Handling Procedures" for further explanation. - PW information length Length of the PW ID field and the interface parameters sub-TLV in octets. If this value is 0, then it references all PWs using the specified group ID, and there is no PW ID present; nor are there any interface parameter sub-TLVs. - Group ID An arbitrary 32-bit value that represents a group of PWs that is used to create groups in the PW space. The group ID is intended to be used as a port index, or a virtual tunnel index. To simplify configuration, a particular PW ID at ingress could be part of the virtual tunnel for transport to the egress router.
The Group ID is very useful for sending wild card label withdrawals, or PW wild card status notification messages to remote PEs upon physical port failure. - PW ID A non-zero 32-bit connection ID that, together with the PW type, identifies a particular PW. Note that the PW ID and the PW type MUST be the same at both endpoints. - Interface Parameter Sub-TLV This variable-length TLV is used to provide interface-specific parameters, such as attachment circuit MTU. Note that as the "interface parameter sub-TLV" is part of the FEC, the rules of LDP make it impossible to change the interface parameters once the pseudowire has been set up. Thus, the interface parameters field must not be used to pass information, such as status information, that may change during the life of the pseudowire. Optional parameter TLVs should be used for that purpose. Using the PWid FEC, each of the two pseudowire endpoints independently initiates the setup of a unidirectional LSP. An outgoing LSP and an incoming LSP are bound together into a single pseudowire if they have the same PW ID and PW type.
RFC3985], a pseudowire can be thought of as connecting two "forwarders". The protocol used to set up a pseudowire must allow the forwarder at one end of a pseudowire to identify the forwarder at the other end. We use the term "attachment identifier", or "AI", to refer to the field that the protocol uses to identify the forwarders. In the PWid FEC, the PWid field serves as the AI. In this section, we specify a more general form of AI that is structured and of variable length. Every Forwarder in a PE must be associated with an Attachment Identifier (AI), either through configuration or through some algorithm. The Attachment Identifier must be unique in the context of the PE router in which the Forwarder resides. The combination <PE router IP address, AI> must be globally unique. It is frequently convenient to regard a set of Forwarders as being members of a particular "group", where PWs may only be set up among members of a group. In such cases, it is convenient to identify the Forwarders relative to the group, so that an Attachment Identifier would consist of an Attachment Group Identifier (AGI) plus an Attachment Individual Identifier (AII). An Attachment Group Identifier may be thought of as a VPN-id, or a VLAN identifier, some attribute that is shared by all the Attachment PWs (or pools thereof) that are allowed to be connected. The details of how to construct the AGI and AII fields identifying the pseudowire endpoints are outside the scope of this specification. Different pseudowire applications, and different provisioning models, will require different sorts of AGI and AII fields. The specification of each such application and/or model must include the rules for constructing the AGI and AII fields. As previously discussed, a (bidirectional) pseudowire consists of a pair of unidirectional LSPs, one in each direction. If a particular pseudowire connects PE1 with PE2, the PW direction from PE1 to PE2 can be identified as: <PE1, <AGI, AII1>, PE2, <AGI, AII2>>, The PW direction from PE2 to PE1 can be identified by: <PE2, <AGI, AII2>, PE1, <AGI, AII1>>.
Note that the AGI must be the same at both endpoints, but the AII will in general be different at each endpoint. Thus, from the perspective of a particular PE, each pseudowire has a local or "Source AII", and a remote or "Target AII". The pseudowire setup protocol can carry all three of these quantities: - Attachment Group Identifier (AGI) - Source Attachment Individual Identifier (SAII) - Target Attachment Individual Identifier (TAII) If the AGI is non-null, then the Source AI (SAI) consists of the AGI together with the SAII, and the Target AI (TAI) consists of the TAII together with the AGI. If the AGI is null, then the SAII and TAII are the SAI and TAI, respectively. The interpretation of the SAI and TAI is a local matter at the respective endpoint. The association of two unidirectional LSPs into a single bidirectional pseudowire depends on the SAI and the TAI. Each application and/or provisioning model that uses the Generalized ID FEC element must specify the rules for performing this association.
IANA] document. The SAII, TAII, and AGI are simply carried as octet strings. The length byte specifies the size of the Value field. The null string can be sent by setting the length byte to 0. If a particular application does not need all three of these sub-elements, it MUST send all the sub-elements but set the length to 0 for the unused sub-elements. The PW information length field contains the length of the SAII, TAII, and AGI, combined in octets. If this value is 0, then it references all PWs using the specified grouping ID. In this case, there are no other FEC element fields (AGI, SAII, etc.) present, nor any interface parameters TLVs. Note that the interpretation of a particular field as AGI, SAII, or TAII depends on the order of its occurrence. The type field identifies the type of the AGI, SAII, or TAII. When comparing two
occurrences of an AGI (or SAII or TAII), the two occurrences are considered identical if the type, length, and value fields of one are identical, respectively, to those of the other. "Interface Parameters Sub-TLV", below.
MAY use the wild card withdraw semantics, but the remote PEs MUST implement support for wild card messages. This TLV MUST only be used when sending the Generalized PW ID FEC. To issue a wildcard command (status or withdraw): - Set the PW Info Length to 0 in the Generalized ID FEC Element. - Send only the PW Grouping ID TLV with the FEC (no AGI/SAII/TAII is sent).
If the Label Mapping Message has a valid TAI, PE2 must decide whether to accept it. The procedures for so deciding will depend on the particular type of Forwarder identified by the TAI. Of course, the Label Mapping message may be rejected due to standard LDP error conditions as detailed in [RFC3036]. If PE2 decides to accept the Label Mapping message, then it has to make sure that a PW LSP is set up in the opposite (PE1-->PE2) direction. If it has already signaled for the corresponding PW LSP in that direction, nothing more needs to be done. Otherwise, it must initiate such signaling by sending a Label Mapping message to PE1. This is very similar to the Label Mapping message PE2 received, but the SAI and TAI are reversed. Thus, a bidirectional PW consists of two LSPs, where the FEC of one has the SAII and TAII reversed with respect to the FEC of the other.
IANA]. The length specifies the length of the Status Code field in octets (equal to 4). Each bit in the status code field can be set individually to indicate more than a single failure at once. Each fault can be cleared by sending an appropriate Notification message in which the respective bit is cleared. The presence of the lowest bit (PW Not Forwarding) acts only as a generic failure indication when there is a link-down event for which none of the other bits apply. The Status TLV is transported to the remote PW peer via the LDP Notification message. The general format of 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) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PW Status TLV | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PWId FEC TLV or Generalized ID FEC TLV | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Status TLV status code is set to 0x00000028, "PW status", to indicate that PW status follows. Since this notification does not refer to any particular message, the Message Id and Message Type fields are set to 0. The PW FEC TLV SHOULD not include the interface parameter sub-TLVs, as they are ignored in the context of this message. When a PE's attachment circuit encounters an error, use of the PW Notification Message allows the PE to send a single "wild card" status message, using a PW FEC TLV with only the group ID set, to denote this change in status for all affected PW connections. This status message contains either the PW FEC TLV with only the group ID set, or else it contains the Generalized FEC TLV with only the PW Grouping ID TLV. As mentioned above, the Group ID field of the PWid FEC element, or the PW Grouping ID TLV used with the Generalized ID FEC element, can be used to send a status notification for all arbitrary sets of PWs. This procedure is OPTIONAL, and if it is implemented, the LDP Notification message should be as follows: If the PWid FEC element is used, the PW information length field is set to 0, the PW ID field is not present, and the interface parameter sub-TLVs are not present. If the Generalized FEC element is used, the AGI, SAII, and TAII are not present, the PW information length field is set to 0, the PW Grouping ID TLV is included, and the Interface Parameters TLV is omitted. For the purpose of this document, this is called the "wild card PW status notification procedure", and all PEs implementing this design are REQUIRED to accept such a notification message but are not required to send it.
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + PWId FEC or Generalized ID FEC + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface Parameters | | " | | " | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0| Generic Label (0x0200) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Label | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0| PW Status (0x096A) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Status Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ If a PW Status TLV is included in the initial Label Mapping message for a PW, then if the Label Mapping message from the remote PE for that PW does not include a PW status TLV, or if the remote PE does not support the PW Status TLV, the PW will revert to the label withdraw method of signaling PW status. Note that if the PW Status TLV is not supported by the remote peer, the peer will automatically ignore it, since the I (ignore) bit is set in the TLV. The PW Status TLV, therefore, will not be present in the corresponding FEC advertisement from the remote LDP peer, which results in exactly the above behavior. If the PW Status TLV is not present following the FEC TLV in the initial PW Label Mapping message received by a PE, then the PW Status TLV will not be used, and both PEs supporting the pseudowire will revert to the label withdraw procedure for signaling status changes. If the negotiation process results in the usage of the PW status TLV, then the actual PW status is determined by the PW status TLV that was sent within the initial PW Label Mapping message. Subsequent updates of PW status are conveyed through the notification message.
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-TLV Type | Length | Variable Length Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Variable Length Value | | " | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The interface parameter sub-TLV type values are specified in "IANA Allocations for Pseudowire Edge to Edge Emulation (PWE3)" [IANA]. The Length field is defined as the length of the interface parameter including the parameter id and length field itself. Processing of the interface parameters should continue when unknown interface parameters are encountered, and they MUST be silently ignored. - Interface MTU sub-TLV type A 2-octet value indicating the MTU in octets. This is the Maximum Transmission Unit, excluding encapsulation overhead, of the egress packet interface that will be transmitting the decapsulated PDU that is received from the MPLS-enabled network. This parameter is applicable only to PWs transporting packets and is REQUIRED for these PW types. If this parameter does not match in both directions of a specific PW, that PW MUST NOT be enabled. - Optional Interface Description string sub-TLV type This arbitrary, and OPTIONAL, interface description string is used to send a human-readable administrative string describing the interface to the remote. This parameter is OPTIONAL and is applicable to all PW types. The interface description parameter string length is variable and can be from 0 to 80 octets. Human- readable text MUST be provided in the UTF-8 charset using the Default Language [RFC2277].
The next action depends on what control message is next received for that PW. The possibilities are as follows: -i. A Label Mapping message with the same c bit value as specified in the Label Mapping message that was sent. PW setup is now complete, and the control word is used if c=1 but is not used if c=0. -ii. A Label Mapping message with c=1, but the Label Mapping message that was sent has c=0. In this case, ignore the received Label Mapping message and continue to wait for the next control message for the PW. -iii. A Label Mapping message with c=0, but the Label Mapping message that was sent has c=1. In this case, send a Label Withdraw message with a "Wrong C-bit" status code, followed by a Label Mapping message that has c=0. PW setup is now complete, and the control word is not used. -iv. A Label Withdraw message with the "Wrong c-bit" status code. Treat as a normal Label Withdraw, but do not respond. Continue to wait for the next control message for the PW. If at any time after a Label Mapping message has been received a corresponding Label Withdraw or Release is received, the action taken is the same as for any Label Withdraw or Release that might be received at any time. If both endpoints prefer the use of the control word, this procedure will cause it to be used. If either endpoint prefers not to use the control word or does not support the control word, this procedure will cause it not to be used. If one endpoint prefers to use the control word but the other does not, the one that prefers not to use it is has no extra protocol to execute; it just waits for a Label Mapping message that has c=0. The diagram in Appendix A illustrates the above procedure.
If the Generalized FEC element is used, the AGI, SAII, and TAII are not present, the PW information length field is set to 0, the PW Grouping ID TLV is included, the Interface Parameters TLV is not present, and the Label TLV is not present. For the purpose of this document, this is called the "wild card withdraw procedure", and all PEs implementing this design are REQUIRED to accept such withdrawn message but are not required to send it. Note that the PW Grouping ID TLV only applies to PWs using the Generalized ID FEC element, while the Group ID only applies to PWid FEC element. The interface parameter sub-TLVs, or TLV, MUST NOT be present in any LDP PW Label Withdraw or Label Release message. A wild card Label Release message MUST include only the group ID, or Grouping ID TLV. A Label Release message initiated by a PE router must always include the PW ID.
attempting to re-advertise a recently released label. This is only an issue when sequence number processing is enabled at the disposition router. RFC 3036. The following values are suggested for assignment: TLV type Description ===================================== 0x096A PW Status TLV 0x096B PW Interface Parameters TLV 0x096C Group ID TLV RFC 3036. The following values are suggested for assignment: Range/Value E Description Reference ------------- ----- ---------------------- --------- 0x00000024 0 Illegal C-Bit [RFC4447] 0x00000025 0 Wrong C-Bit [RFC4447] 0x00000026 0 Incompatible bit-rate [RFC4447] 0x00000027 0 CEP-TDM mis-configuration [RFC4447] 0x00000028 0 PW Status [RFC4447] 0x00000029 0 Unassigned/Unrecognized TAI [RFC4447] 0x0000002A 0 Generic Misconfiguration Error [RFC4447] 0x0000002B 0 Label Withdraw PW Status Method [RFC4447]
First, there is a possibility that the PE receiving PW PDUs will get a PDU that appears to be from the PE transmitting the PW into the PSN, but that was not actually transmitted by the PE originating the PW. (That is, the specified encapsulations do not by themselves enable the decapsulator to authenticate the encapsulator.) A second problem is the possibility that the PW PDU will be altered between the time it enters the PSN and the time it leaves the PSN (i.e., the specified encapsulations do not by themselves assure the decapsulator of the packet's integrity.) A third problem is the possibility that the PDU's contents will be seen while the PDU is in transit through the PSN (i.e., the specification encapsulations do not ensure privacy.) How significant these issues are in practice depends on the security requirements of the applications whose traffic is being sent through the tunnel, and how secure the PSN itself is. section 5 of RFC 3036. Those considerations also apply to the case where LDP is used to set up pseudowires. A pseudowire connects two attachment circuits. It is important to make sure that LDP connections are not arbitrarily accepted from anywhere, or else a local attachment circuit might get connected to an arbitrary remote attachment circuit. Therefore, an incoming LDP session request MUST NOT be accepted unless its IP source address is known to be the source of an "eligible" LDP peer. The set of eligible peers could be pre-configured (either as a list of IP addresses, or as a list of address/mask combinations), or it could be discovered dynamically via an auto-discovery protocol that is itself trusted. (Obviously, if the auto-discovery protocol were not trusted, the set of "eligible peers" it produces could not be trusted.) Even if an LDP connection request appears to come from an eligible peer, its source address may have been spoofed. Therefore, some means of preventing source address spoofing must be in place. For example, if all the eligible peers are in the same network, source address filtering at the border routers of that network could eliminate the possibility of source address spoofing. The LDP MD5 authentication key option, as described in section 2.9 of RFC 3036, MUST be implemented, and for a greater degree of security, it must be used. This provides integrity and authentication for the LDP messages and eliminates the possibility of source address spoofing. Use of the MD5 option does not provide privacy, but privacy of the LDP control messages is not usually considered important. As the MD5 option relies on the configuration of pre-
shared keys, it does not provide much protection against replay attacks. In addition, its reliance on pre-shared keys may make it very difficult to deploy when the set of eligible neighbors is determined by an auto-configuration protocol. When the Generalized ID FEC Element is used, it is possible that a particular LDP peer may be one of the eligible LDP peers but may not be the right one to connect to the particular attachment circuit identified by the particular instance of the Generalized ID FEC element. However, given that the peer is known to be one of the eligible peers (as discussed above), this would be the result of a configuration error, rather than a security problem. Nevertheless, it may be advisable for a PE to associate each of its local attachment circuits with a set of eligible peers rather than have just a single set of eligible peers associated with the PE as a whole. [RFC2119] Bradner S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997 [RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and B. Thomas, "LDP Specification", RFC 3036, January 2001. [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack Encoding", RFC 3032, January 2001. [IANA] Martini, L., "IANA Allocations for Pseudowire Edge to Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006. [CEP] Malis, A., Pate, P., Cohen, R., Ed., and D. Zelig, "SONET/SDH Circuit Emulation Service Over Packet (CEP)", Work in Progress. [SAToP] Vainshtein, A., Ed. and Y. Stein, Ed., "Structure-Agnostic TDM over Packet (SAToP)", Work in Progress.
[FRAME] Martini, L., Ed. and C. Kawa, Ed., "Encapsulation Methods for Transport of Frame Relay Over MPLS Networks", Work in Progress. [ATM] Martini, L., Ed., El-Aawar, N., and M. Bocci, Ed., "Encapsulation Methods for Transport of ATM Over MPLS Networks", Work in Progress. [PPPHDLC] Martini, L., Rosen, E., Heron, G., and A. Malis, "Encapsulation Methods for Transport of PPP/HDLC Frames Over IP and MPLS Networks", Work in Progress. [ETH] Martini, L., Rosen, E., El-Aawar, N., and G. Heron, "Encapsulation Methods for Transport of Ethernet Over MPLS Networks", RFC 4448, April 2006. [SDH] American National Standards Institute, "Synchronous Optical Network Formats," ANSI T1.105-1995. [ITUG] ITU Recommendation G.707, "Network Node Interface For The Synchronous Digital Hierarchy", 1996. [RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture", RFC 3985, March 2005. [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998.
Alex Hamilton, Cisco Systems Inc. 285 W. Tasman, MS-SJCI/3/4, San Jose, CA, 95134 EMail: firstname.lastname@example.org Steve Vogelsang ECI Telecom Omega Corporate Center 1300 Omega Drive Pittsburgh, PA 15205 EMail: email@example.com John Shirron ECI Telecom Omega Corporate Center 1300 Omega Drive Pittsburgh, PA 15205 EMail: firstname.lastname@example.org Andrew G. Malis Tellabs 90 Rio Robles Dr. San Jose, CA 95134 EMail: Andy.Malis@tellabs.com Vinai Sirkay Redback Networks 300 Holger Way San Jose, CA 95134 EMail: email@example.com Vasile Radoaca Nortel Networks 600 Technology Park Billerica MA 01821 EMail: firstname.lastname@example.org
Chris Liljenstolpe Alcatel 11600 Sallie Mae Dr. 9th Floor Reston, VA 20193 EMail: email@example.com Dave Cooper Global Crossing 960 Hamlin Court Sunnyvale, CA 94089 EMail: firstname.lastname@example.org Kireeti Kompella Juniper Networks 1194 N. Mathilda Ave Sunnyvale, CA 94089 EMail: email@example.com Dan Tappan Cisco Systems, Inc. 1414 Massachusetts Avenue Boxborough, MA 01719 EMail: firstname.lastname@example.org
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