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| author | Thomas Voss <mail@thomasvoss.com> | 2024-11-27 20:54:24 +0100 |
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| committer | Thomas Voss <mail@thomasvoss.com> | 2024-11-27 20:54:24 +0100 |
| commit | 4bfd864f10b68b71482b35c818559068ef8d5797 (patch) | |
| tree | e3989f47a7994642eb325063d46e8f08ffa681dc /doc/rfc/rfc6073.txt | |
| parent | ea76e11061bda059ae9f9ad130a9895cc85607db (diff) | |
doc: Add RFC documents
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diff --git a/doc/rfc/rfc6073.txt b/doc/rfc/rfc6073.txt new file mode 100644 index 0000000..0147708 --- /dev/null +++ b/doc/rfc/rfc6073.txt @@ -0,0 +1,2411 @@ + + + + + + +Internet Engineering Task Force (IETF) L. Martini +Request for Comments: 6073 C. Metz +Category: Standards Track Cisco Systems, Inc. +ISSN: 2070-1721 T. Nadeau + LucidVision + M. Bocci + M. Aissaoui + Alcatel-Lucent + January 2011 + + + Segmented Pseudowire + +Abstract + + This document describes how to connect pseudowires (PWs) between + different Packet Switched Network (PSN) domains or between two or + more distinct PW control plane domains, where a control plane domain + uses a common control plane protocol or instance of that protocol for + a given PW. The different PW control plane domains may belong to + independent autonomous systems, or the PSN technology is + heterogeneous, or a PW might need to be aggregated at a specific PSN + point. The PW packet data units are simply switched from one PW to + another without changing the PW payload. + +Status of This Memo + + This is an Internet Standards Track document. + + This document is a product of the Internet Engineering Task Force + (IETF). It represents the consensus of the IETF community. It has + received public review and has been approved for publication by the + Internet Engineering Steering Group (IESG). Further information on + Internet Standards is available in Section 2 of RFC 5741. + + Information about the current status of this document, any errata, + and how to provide feedback on it may be obtained at + http://www.rfc-editor.org/info/rfc6073. + + + + + + + + + + + + + +Martini, et al. Standards Track [Page 1] + +RFC 6073 Segmented Pseudowire January 2011 + + +Copyright Notice + + Copyright (c) 2011 IETF Trust and the persons identified as the + document authors. All rights reserved. + + This document is subject to BCP 78 and the IETF Trust's Legal + Provisions Relating to IETF Documents + (http://trustee.ietf.org/license-info) in effect on the date of + publication of this document. Please review these documents + carefully, as they describe your rights and restrictions with respect + to this document. Code Components extracted from this document must + include Simplified BSD License text as described in Section 4.e of + the Trust Legal Provisions and are provided without warranty as + described in the Simplified BSD License. + + This document may contain material from IETF Documents or IETF + Contributions published or made publicly available before November + 10, 2008. The person(s) controlling the copyright in some of this + material may not have granted the IETF Trust the right to allow + modifications of such material outside the IETF Standards Process. + Without obtaining an adequate license from the person(s) controlling + the copyright in such materials, this document may not be modified + outside the IETF Standards Process, and derivative works of it may + not be created outside the IETF Standards Process, except to format + it for publication as an RFC or to translate it into languages other + than English. + +Table of Contents + + 1. Introduction ....................................................4 + 2. Specification of Requirements ...................................5 + 3. Terminology .....................................................5 + 4. General Description .............................................6 + 5. PW Switching and Attachment Circuit Type ........................9 + 6. Applicability ...................................................9 + 7. MPLS-PW to MPLS-PW Switching ...................................10 + 7.1. Static Control Plane Switching ............................10 + 7.2. Two LDP Control Planes Using the Same FEC Type ............11 + 7.2.1. FEC 129 Active/Passive T-PE Election Procedure .....11 + 7.3. LDP Using FEC 128 to LDP Using the Generalized FEC 129 ....12 + 7.4. LDP SP-PE TLV .............................................12 + 7.4.1. PW Switching Point PE Sub-TLVs .....................14 + 7.4.2. Adaptation of Interface Parameters .................15 + 7.5. Group ID ..................................................16 + 7.6. PW Loop Detection .........................................16 + 8. MPLS-PW to L2TPv3-PW Control Plane Switching ...................16 + 8.1. Static MPLS and L2TPv3 PWs ................................17 + 8.2. Static MPLS PW and Dynamic L2TPv3 PW ......................17 + + + +Martini, et al. Standards Track [Page 2] + +RFC 6073 Segmented Pseudowire January 2011 + + + 8.3. Static L2TPv3 PW and Dynamic LDP/MPLS PW ..................17 + 8.4. Dynamic LDP/MPLS and L2TPv3 PWs ...........................17 + 8.4.1. Session Establishment ..............................18 + 8.4.2. Adaptation of PW Status message ....................18 + 8.4.3. Session Tear Down ..................................18 + 8.5. Adaptation of L2TPv3 AVPs to Interface Parameters .........19 + 8.6. Switching Point TLV in L2TPv3 .............................20 + 8.7. L2TPv3 and MPLS PW Data Plane .............................20 + 8.7.1. Mapping the MPLS Control Word to L2TP ..............21 + 9. Operations, Administration, and Maintenance (OAM) ..............22 + 9.1. Extensions to VCCV to Support MS-PWs ......................22 + 9.2. OAM from MPLS PW to L2TPv3 PW .............................22 + 9.3. OAM Data Plane Indication from MPLS PW to MPLS PW .........22 + 9.4. Signaling OAM Capabilities for Switched Pseudowires .......23 + 9.5. OAM Capability for MS-PWs Demultiplexed Using MPLS ........23 + 9.5.1. MS-PW and VCCV CC Type 1 ...........................24 + 9.5.2. MS-PW and VCCV CC Type 2 ...........................24 + 9.5.3. MS-PW and VCCV CC Type 3 ...........................24 + 9.6. MS-PW VCCV Operations .....................................24 + 9.6.1. VCCV Echo Message Processing .......................25 + 9.6.2. Detailed VCCV Procedures ...........................27 + 10. Mapping Switched Pseudowire Status ............................31 + 10.1. PW Status Messages Initiated by the S-PE .................32 + 10.1.1. Local PW2 Transmit Direction Fault ................33 + 10.1.2. Local PW1 Transmit Direction Fault ................34 + 10.1.3. Local PW2 Receive Direction Fault .................34 + 10.1.4. Local PW1 Receive Direction Fault .................34 + 10.1.5. Clearing Faults ...................................34 + 10.2. PW Status Messages and SP-PE TLV Processing ..............35 + 10.3. T-PE Processing of PW Status Messages ....................35 + 10.4. Pseudowire Status Negotiation Procedures .................35 + 10.5. Status Dampening .........................................35 + 11. Peering between Autonomous Systems ............................35 + 12. Congestion Considerations .....................................36 + 13. Security Considerations .......................................36 + 13.1. Data Plane Security ......................................36 + 13.1.1. VCCV Security Considerations ......................36 + 13.2. Control Protocol Security ................................37 + 14. IANA Considerations ...........................................38 + 14.1. L2TPv3 AVP ...............................................38 + 14.2. LDP TLV TYPE .............................................38 + 14.3. LDP Status Codes .........................................38 + 14.4. L2TPv3 Result Codes ......................................38 + 14.5. New IANA Registries ......................................39 + 15. Normative References ..........................................39 + 16. Informative References ........................................40 + 17. Acknowledgments ...............................................42 + 18. Contributors ..................................................42 + + + +Martini, et al. Standards Track [Page 3] + +RFC 6073 Segmented Pseudowire January 2011 + + +1. Introduction + + The PWE3 Architecture [RFC3985] defines the signaling and + encapsulation techniques for establishing Single-Segment Pseudowires + (SS-PWs) between a pair of terminating PEs. Multi-Segment + Pseudowires (MS-PWs) are most useful in two general cases: + + -i. In some cases it is not possible, desirable, or feasible to + establish a PW control channel between the terminating source + and destination PEs. At a minimum, PW control channel + establishment requires knowledge of and reachability to the + remote (terminating) PE IP address. The local (terminating) + PE may not have access to this information because of + topology, operational, or security constraints. + + An example is the inter-AS L2VPN scenario where the + terminating PEs reside in different provider networks (ASes) + and it is the practice to cryptographically sign all control + traffic exchanged between two networks. Technically, an + SS-PW could be used but this would require cryptographic + signatures on ALL terminating source and destination PE + nodes. An MS-PW allows the providers to confine key + administration to just the PW switching points connecting the + two domains. + + A second example might involve a single AS where the PW setup + path between the terminating PEs is computed by an external + entity. Assume that a full mesh of PWE3 control channels is + established between PE-A, PE-B, and PE-C. A client-layer L2 + connection tunneled through a PW is required between + terminating PE-A and PE-C. The external entity computes a PW + setup path that passes through PE-B. This results in two + discrete PW segments being built: one between PE-A and PE-B + and one between PE-B and PE-C. The successful client-layer + L2 connection between terminating PE-A and terminating PE-C + requires that PE-B performs the PWE3 switching process. + + A third example involves the use of PWs in hierarchical + IP/MPLS networks. Access networks connected to a backbone + use PWs to transport customer payloads between customer sites + serviced by the same access network and up to the edge of the + backbone where they can be terminated or switched onto a + succeeding PW segment crossing the backbone. The use of PWE3 + switching between the access and backbone networks can + potentially reduce the PWE3 control channels and routing + information processed by the access network T-PEs. + + + + + +Martini, et al. Standards Track [Page 4] + +RFC 6073 Segmented Pseudowire January 2011 + + + It should be noted that PWE3 switching does not help in any + way to reduce the amount of PW state supported by each access + network T-PE. + + -ii. In some applications, the signaling protocol and + encapsulation on each segment of the PW are different. The + terminating PEs are connected to networks employing different + PW signaling and encapsulation protocols. In this case, it + is not possible to use an SS-PW. An MS-PW with the + appropriate signaling protocol interworking performed at the + PW switching points can enable PW connectivity between the + terminating PEs in this scenario. + + A more detailed discussion of the requirements pertaining to MS-PWs + can be found in [RFC5254]. + + There are four different mechanisms to establish PWs: + + -i. Static configuration of the PW (MPLS or Layer 2 Tunneling + Protocol version 3 (L2TPv3)) + -ii. LDP using FEC 128 (PWid FEC Element) + -iii. LDP using FEC 129 (Generalized PWid FEC Element) + -iv. L2TPv3 + + While MS-PWs are composed of PW segments, each PW segment cannot + function independently, as the PW service is always instantiated + across the complete MS-PW. Hence, no PW segments can be signaled or + be operational without the complete MS-PW being signaled at once. + +2. Specification of Requirements + + 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]. + +3. Terminology + + - PW Terminating Provider Edge (T-PE). A PE where the customer- + facing attachment circuits (ACs) are bound to a PW forwarder. A + Terminating PE is present in the first and last segments of a + MS-PW. This incorporates the functionality of a PE as defined in + [RFC3985]. + + - Single-Segment Pseudowire (SS-PW). A PW set up directly between + two T-PE devices. The PW label is unchanged between the + originating and terminating T-PEs. + + + + + +Martini, et al. Standards Track [Page 5] + +RFC 6073 Segmented Pseudowire January 2011 + + + - Multi-Segment Pseudowire (MS-PW). A static or dynamically + configured set of two or more contiguous PW segments that behave + and function as a single point-to-point PW. Each end of an MS-PW + by definition MUST terminate on a T-PE. + + - PW Segment. A part of a single-segment or multi-segment PW, which + traverses one PSN tunnel in each direction between two PE devices, + T-PEs and/or S-PEs (switching PE). + + - PW Switching Provider Edge (S-PE). A PE capable of switching the + control and data planes of the preceding and succeeding PW segments + in an MS-PW. The S-PE terminates the PSN tunnels of the preceding + and succeeding segments of the MS-PW. It therefore includes a PW + switching point for an MS-PW. A PW switching point is never the + S-PE and the T-PE for the same MS-PW. A PW switching point runs + necessary protocols to set up and manage PW segments with other PW + switching points and terminating PEs. An S-PE can exist anywhere a + PW must be processed or policy applied. It is therefore not + limited to the edge of a provider network. + + - MS-PW path. The set of S-PEs that will be traversed in sequence to + form the MS-PW. + +4. General Description + + A pseudowire (PW) is a mechanism that carries the essential elements + of an emulated service from one PE to one or more other PEs over a + PSN as described in Figure 1 and in [RFC3985]. Many providers have + deployed PWs as a means of migrating existing (or building new) L2VPN + services (e.g., Frame Relay, ATM, or Ethernet) onto a PSN. + + PWs may span multiple domains of the same or different provider + networks. In these scenarios, PW control channels (i.e., targeted + LDP, L2TPv3) and PWs will cross AS boundaries. + + Inter-AS L2VPN functionality is currently supported, and several + techniques employing MPLS encapsulation and LDP signaling have been + documented [RFC4364]. It is also straightforward to support the same + inter-AS L2VPN functionality employing L2TPv3. In this document, we + define a methodology to switch a PW between different Packet Switched + Network (PSN) domains or between two or more distinct PW control + plane domains. + + + + + + + + + +Martini, et al. Standards Track [Page 6] + +RFC 6073 Segmented Pseudowire January 2011 + + + |<-------------- Emulated Service ---------------->| + | | + | |<-------- Pseudowire ------>| | + | | | | + | | |<-- PSN Tunnel -->| | | + | V V V V | + 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 + + There are two methods for switching a PW between two PW domains. In + the first method (Figure 2), the two separate control plane domains + terminate on different PEs. + + |<-------Multi-Segment Pseudowire------->| + | PSN PSN | + AC | |<-1->| |<-2->| | AC + | V V V V V V | + | +----+ +-----+ +----+ +----+ | + +----+ | | |=====| | | |=====| | | +----+ + | |-------|......PW1.......|--AC1--|......PW2......|-------| | + | CE1| | | | | | | | | | | |CE2 | + | |-------|......PW3.......|--AC2--|......PW4......|-------| | + +----+ | | |=====| | | |=====| | | +----+ + ^ +----+ +-----+ +----+ +----+ ^ + | PE1 PE2 PE3 PE4 | + | ^ ^ | + | | | | + | PW switching points | + | | + | | + |<-------------------- Emulated Service ---------------->| + + Figure 2: PW Switching Using AC Reference Model + + + + + +Martini, et al. Standards Track [Page 7] + +RFC 6073 Segmented Pseudowire January 2011 + + + In Figure 2, pseudowires in two separate PSNs are stitched together + using native service attachment circuits. PE2 and PE3 only run the + control plane for the PSN to which they are directly attached. At + PE2 and PE3, PW1 and PW2 are connected using attachment circuit AC1, + while PW3 and PW4 are connected using attachment circuit AC2. + + Native |<-----Multi-Segment Pseudowire------>| Native + Service | PSN PSN | Service + (AC) | |<-Tunnel->| |<-Tunnel->| | (AC) + | V V 1 V V 2 V V | + | +----+ +-----+ +----+ | + +----+ | |TPE1|==========|SPE1 |==========|TPE2| | +----+ + | |------|.....PW.Seg't1....X....PW.Seg't3.....|-------| | + | CE1| | | | | | | | | |CE2 | + | |------|.....PW.Seg't2....X....PW.Seg't4.....|-------| | + +----+ | | |==========| |==========| | | +----+ + ^ +----+ +-----+ +----+ ^ + | Provider Edge 1 ^ Provider Edge 2 | + | | | + | | | + | PW switching point | + | | + |<----------------- Emulated Service --------------->| + + Figure 3: MS-PW Reference Model + + In Figure 3, SPE1 runs two separate control planes: one toward TPE1, + and one toward TPE2. The PW switching point (S-PE) is configured to + connect PW Segment 1 and PW Segment 3 together to complete the multi- + segment PW between TPE1 and TPE2. PW Segment 1 and PW Segment 3 MUST + be of the same PW type, but PSN Tunnel 1 and PSN Tunnel 2 need not be + the same technology. In the latter case, if the PW is switched to a + different technology, the PEs must adapt the PDU encapsulation + between the different PSN technologies. In the case where PSN Tunnel + 1 and PSN Tunnel 2 are the same technology, the PW PDU does not need + to be modified, and PDUs are then switched between the pseudowires at + the PW label level. + + It should be noted that it is possible to adapt one PSN technology to + a different one, for example, MPLS over an IP encapsulation or + Generic Routing Encapsulation (GRE) [RFC4023], but this is outside + the scope of this document. Further, one could perform an + interworking function on the PWs themselves at the S-PE, allowing + conversion from one PW type to another, but this is also outside the + scope of this document. + + This document describes procedures for building multi-segment + pseudowires using manual configuration of the switching point PE1. + + + +Martini, et al. Standards Track [Page 8] + +RFC 6073 Segmented Pseudowire January 2011 + + + Other documents may build on this base specification to automate the + configuration and selection of S-PE1. All elements of the + establishment of end-to-end MS-PWs including routing and signaling + are out of scope of this document, and any discussion in this + document serves purely as examples. It should also be noted that a + PW can traverse multiple PW switching points along it's path, and the + edge PEs will not require any specific knowledge of how many S-PEs + the PW has traversed (though this may be reported for troubleshooting + purposes). + + The general approach taken for MS-PWs is to connect the individual + control planes by passing along any signaling information immediately + upon reception. First, the S-PE is configured to switch a PW segment + from a specific peer to another PW segment destined for a different + peer. No control messages are exchanged yet, as the S-PE does not + have enough information to actually initiate the PW setup messages. + However, if a session does not already exist, a control protocol + (LDP/L2TP) session MAY be setup. In this model, the MS-PW setup is + starting from the T-PE devices. Once the T-PE is configured, it + sends the PW control setup messages. These messages are received by + the S-PE, and immediately used to form the PW setup messages for the + next SS-PW of the MS-PW. + +5. PW Switching and Attachment Circuit Type + + The PWs in each PSN are established independently, with each PSN + being treated as a separate PW domain. For example, in Figure 2 for + the case of MPLS PSNs, PW1 is setup between PE1 and PE2 using the LDP + targeted session as described in [RFC4447], and at the same time a + separate pseudowire, PW2, is setup between PE3 and PE4. The ACs for + PW1 and PW2 at PE2 and PE3 MUST be configured such that they are the + same PW type, e.g., ATM Virtual Channel Connection (VCC), Ethernet + VLAN, etc. + +6. Applicability + + The general applicability of MS-PWs and their relationship to L2VPNs + are described in [RFC5659]. The applicability of a PW type, as + specified in the relevant RFC for that encapsulation (e.g., [RFC4717] + for ATM), applies to each segment. This section describes further + applicability considerations. + + As with SS-PWs, the performance of any segment will be limited by the + performance of the underlying PSN. The performance may be further + degraded by the emulation process, and performance degradation may be + further increased by traversing multiple PW segments. Furthermore, + the overall performance of an MS-PW is no better than the worst- + performing segment of that MS-PW. + + + +Martini, et al. Standards Track [Page 9] + +RFC 6073 Segmented Pseudowire January 2011 + + + Since different PSN types may be able to achieve different maximum + performance objectives, it is necessary to carefully consider which + PSN types are used along the path of an MS-PW. + +7. MPLS-PW to MPLS-PW Switching + + Referencing Figure 3, T-PE1 set up PW Segment 1 using the LDP + targeted session as described in [RFC4447], at the same time a + separate pseudowire, PW Segment 3, is setup to T-PE2. Each PW is + configured independently on the PEs, but on S-PE1, PW Segment 1 is + connected to PW Segment 3. PDUs are then switched between the + pseudowires at the PW label level. Hence, the data plane does not + need any special knowledge of the specific pseudowire type. A simple + standard MPLS label swap operation is sufficient to connect the two + PWs, and in this case the PW adaptation function cannot be used. + However, when pushing a new PSN label, the Time to Live (TTL) SHOULD + be set to 255, or some other locally configured fixed value. + + This process can be repeated as many times as necessary; the only + limitation to the number of S-PEs traversed is imposed by the TTL + field of the PW MPLS label. The setting of the TTL of the PW MPLS + label is a matter of local policy on the originating PE, but SHOULD + be set to 255. However, if the PW PDU contains an Operations, + Administration, and Maintenance (OAM) packet, then the TTL can be set + to the required value as explained later in this document. + + There are three different mechanisms for MPLS-to-MPLS PW setup: + + -i. Static configuration of the PW + -ii. LDP using FEC 128 + -iii. LDP using the generalized FEC 129 + + This results in four distinct PW switching situations that are + significantly different and must be considered in detail: + + -i. Switching between two static control planes + -ii. Switching between a static and a dynamic LDP control plane + -iii. Switching between two LDP control planes using the same FEC + type + -iv. Switching between LDP using FEC 128 and LDP using the + generalized FEC 129 + +7.1. Static Control Plane Switching + + In the case of two static control planes, the S-PE MUST be configured + to direct the MPLS packets from one PW into the other. There is no + control protocol involved in this case. When one of the control + planes is a simple static PW configuration and the other control + + + +Martini, et al. Standards Track [Page 10] + +RFC 6073 Segmented Pseudowire January 2011 + + + plane is a dynamic LDP FEC 128 or generalized PW FEC, then the static + control plane should be considered similar to an attachment circuit + (AC) in the reference model of Figure 1. The switching point PE + SHOULD signal the appropriate PW status if it detects a failure in + sending or receiving packets over the static PW segment. In the + absence of a PW status communication mechanism when the PW is + statically configured, the status communicated to the dynamic LDP PW + will be limited to local interface failures. In this case, the S-PE + PE behaves in a very similar manner to a T-PE, assuming an active + signaling role. This means that the S-PE will immediately send the + LDP Label Mapping message if the static PW is deemed to be UP. + +7.2. Two LDP Control Planes Using the Same FEC Type + + The S-PE SHOULD assume an initial passive role. This means that when + independent PWs are configured on the switching point, the Label + Switching Router (LSR) does not advertise the LDP PW FEC mapping + until it has received at least one of the two PW LDP FECs from a + remote PE. This is necessary because the switching point LSR does + not know a priori what the interface parameter field in the initial + FEC advertisement will contain. + + If one of the S-PEs doesn't accept an LDP Label Mapping message, then + a Label Release message may be sent back to the originator T-PE + depending on the cause of the error. LDP liberal label retention + mode still applies; hence, if a PE is simply not configured yet, the + label mapping is stored for future use. An MS-PW is declared UP only + when all the constituent SS-PWs are UP. + + The Pseudowire Identifier (PWid), as defined in [RFC4447], is a + unique number between each pair of PEs. Hence, each SS-PW that forms + an MS-PW may have a different PWid. In the case of the generalized + PW FEC, the Attachment Group Identifier (AGI) / Source Attachment + Identifier (SAI) / Target Attachment Identifier (TAI) may have to + also be different for some, or sometimes all, SS-PWs. + +7.2.1. FEC 129 Active/Passive T-PE Election Procedure + + When an MS-PW is signaled using FEC 129, each T-PE might + independently start signaling the MS-PW. If the MS-PW path is not + statically configured, in certain cases the signaling procedure could + result in an attempt to set up each direction of the MS-PW through + different S-PEs. If an operator wishes to avoid this situation, one + of the T-PEs MUST start the PW signaling (active role), while the + other waits to receive the LDP label mapping before sending the + respective PW LDP Label Mapping message (passive role). When the + MS-PW path is not statically configured, the active T-PE (the Source + + + + +Martini, et al. Standards Track [Page 11] + +RFC 6073 Segmented Pseudowire January 2011 + + + T-PE) and the passive T-PE (the Target T-PE) MUST be identified + before signaling is initiated for a given MS-PW. + + The determination of which T-PE assumes the active role SHOULD be + done as follows: + + The SAII and TAII are compared as unsigned integers; if the SAII is + larger, then the T-PE assumes the active role. + + The selection process to determine which T-PE assumes the active role + MAY be superseded by manual provisioning. In this case, one of the + T-PEs MUST be set to the active role, and the other one MUST be set + to the passive role. + +7.3. LDP Using FEC 128 to LDP Using the Generalized FEC 129 + + When a PE is using the generalized FEC 129, there are two distinct + roles that a PE can assume: active and passive. A PE that assumes + the active role will send the LDP PW setup message, while a passive + role PE will simply reply to an incoming LDP PW setup message. The + S-PE will always remain passive until a PWid FEC 128 LDP message is + received, which will cause the corresponding generalized PW FEC LDP + message to be formed and sent. If a generalized FEC PW LDP message + is received while the switching point PE is in a passive role, the + corresponding PW FEC 128 LDP message will be formed and sent. + + PWids need to be mapped to the corresponding AGI/TAI/SAI and vice + versa. This can be accomplished by local S-PE configuration, or by + some other means, such as some form of auto discovery. Such other + means are outside the scope of this document. + +7.4. LDP SP-PE TLV + + The edge-to-edge PW might traverse several switching points, in + separate administrative domains. For management and troubleshooting + reasons, it is useful to record information about the switching + points at the S-PEs that the PW traverses. This is accomplished by + using a PW Switching Point PE TLV (SP-PE TLV). + + Sending the SP-PE TLV is OPTIONAL; however, the PE or S-PE MUST + process the TLV upon reception. The "U" bit MUST be set for backward + compatibility with T-PEs that do not support the MS-PW extensions + described in the document. The SP-PE TLV MAY appear only once for + each switching point traversed, and it cannot be of length zero. The + SP-PE TLV is appended to the PW FEC at each S-PE, and the order of + the SP-PE TLVs in the LDP message MUST be preserved. The SP-PE TLV + + + + + +Martini, et al. Standards Track [Page 12] + +RFC 6073 Segmented Pseudowire January 2011 + + + is necessary to support some of the Virtual Circuit Connectivity + Verification (VCCV) functions for MS-PWs. See Section 9.5 for more + details. The SP-PE TLV is encoded 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 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |1|0| SP-PE TLV (0x096D) | SP-PE TLV Length | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Sub-TLV Type | Length | Variable Length Value | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Variable Length Value | + | " " " | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + - SP-PE TLV Length + + Specifies the total length of all the following SP-PE TLV fields + in octets. + + - Sub-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. + + PW Switching Point PE sub-TLV Types are assigned by IANA according to + the process defined in Section 14 (IANA Considerations) below. + + For local policy reasons, a particular S-PE can filter out all + SP-PE TLVs in a Label Mapping message that traverses it and not + include its own SP-PE TLV. In this case, from any upstream PE, + it will appear as if this particular S-PE is the T-PE. This + might be necessary, depending on local policy, if the S-PE is at + the service provider administrative boundary. It should also be + noted that because there are no SP-PE TLVs describing the path + beyond the S-PE that removed them, VCCV will only work as far as + that S-PE. + + + + + + +Martini, et al. Standards Track [Page 13] + +RFC 6073 Segmented Pseudowire January 2011 + + +7.4.1. PW Switching Point PE Sub-TLVs + + The SP-PE TLV contains sub-TLVs that describe various characteristics + of the S-PE traversed. The SP-PE TLV MUST contain the appropriate + mandatory sub-TLVs specified below. The definitions of the PW + Switching Point PE sub-TLVs are as follows: + + - PWid of last PW segment traversed. + + This is only applicable if the last PW segment traversed used + LDP FEC 128 to signal the PW. This sub-TLV type contains a PWid + in the format of the PWid described in [RFC4447]. This is just + a 32-bit unsigned integer number. + + - PW Switching Point description string. + + An OPTIONAL description string of text up to 80 characters long. + Human-readable text MUST be provided in the UTF-8 character set + using the Default Language [RFC2277]. + + - Local IP address of PW Switching Point. + + The local IPv4 or IPv6 address of the PW Switching Point. This + is an OPTIONAL Sub-TLV. In most cases, this will be the local + LDP session IP address of the S-PE. + + - Remote IP address of the last PW Switching Point traversed or of + the T-PE. + + The IPv4 or IPv6 address of the last PW Switching Point + traversed or of the T-PE. This is an OPTIONAL Sub-TLV. In most + cases, this will be the remote IP address of the LDP session. + This Sub-TLV SHOULD only be included if there are no other SP-PE + TLVs present from other S-PEs, or if the remote IP address of + the LDP session does not correspond to the "Local IP address of + PW Switching Point" TLV value contained in the last SP-PE TLV. + + - The FEC element of last PW segment traversed. + + This is only applicable if the last PW segment traversed used + LDP FEC 129 to signal the PW. + + + + + + + + + + +Martini, et al. Standards Track [Page 14] + +RFC 6073 Segmented Pseudowire January 2011 + + + The FEC element of the last PW segment traversed. This is encoded in + 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 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | AGI Type | Length | Value | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + ~ AGI Value (contd.) ~ + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | AII Type | Length | Value | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + ~ SAII Value (contd.) ~ + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | AII Type | Length | Value | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + ~ TAII Value (contd.) ~ + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + - L2 PW address of the PW Switching Point (recommended format). + + This sub-TLV type contains an L2 PW address of PW Switching + Point in the format described in Section 3.2 of [RFC5003]. This + includes the AII type field and length, as well as the L2 PW + address with the AC ID field set to zero. + +7.4.2. Adaptation of Interface Parameters + + [RFC4447] defines several interface parameters, which are used by the + Network Service Processing (NSP) to adapt the PW to the attachment + circuit (AC). The interface parameters are only used at the + endpoints, and MUST be passed unchanged across the S-PE. However, + the following interface parameters MAY be modified as follows: + + - 0x03 Optional Interface Description string + This Interface parameter MAY be modified or altogether removed + from the FEC element depending on local configuration policies. + + - 0x09 Fragmentation indicator + This parameter MAY be inserted in the FEC by the switching point + if it is capable of re-assembly of fragmented PW frames + according to [RFC4623]. + + + + + + +Martini, et al. Standards Track [Page 15] + +RFC 6073 Segmented Pseudowire January 2011 + + + - 0x0C VCCV parameter + This Parameter contains the Control Channel (CC) type and + Connectivity Verification (CV) type bit fields. The CV type bit + field MUST be reset to reflect the CV type supported by the + S-PE. The CC type bit field MUST have bit 1 "Type 2: MPLS + Router Alert Label" set to 0. The other bit fields MUST be + reset to reflect the CC type supported by the S-PE. + +7.5. Group ID + + The Group ID (GR ID) is used to reduce the number of status messages + that need to be sent by the PE advertising the PW FEC. The GR ID has + local significance only, and therefore MUST be mapped to a unique GR + ID allocated by the S-PE. + +7.6. PW Loop Detection + + A switching point PE SHOULD inspect the PW Switching Point PE TLV, to + verify that its own IP address does not appear in it. If the PE's IP + address appears in a received PW Switching Point PE TLV, the PE + SHOULD break the loop and send a label release message with the + following error code: + + Value E Description + 0x0000003A 0 PW Loop Detected + + If an S-PE along the MS-PW removed all SP-PE TLVs, as mentioned + above, this loop detection method will fail. + +8. MPLS-PW to L2TPv3-PW Control Plane Switching + + Both MPLS and L2TPv3 PWs may be static or dynamic. This results in + four possibilities when switching between L2TPv3 and MPLS. + + -i. Switching between static MPLS and L2TPv3 PWs + -ii. Switching between a static MPLS PW and a dynamic L2TPv3 PW + -iii. Switching between a static L2TPv3 PW and a dynamic LDP/MPLS + PW + -iv. Switching between a dynamic LDP/MPLS PW and a dynamic L2TPv3 + PW + + + + + + + + + + + +Martini, et al. Standards Track [Page 16] + +RFC 6073 Segmented Pseudowire January 2011 + + +8.1. Static MPLS and L2TPv3 PWs + + In the case of two static control planes, the S-PE MUST be configured + to direct packets from one PW into the other. There is no control + protocol involved in this case. The configuration MUST include which + MPLS PW Label maps to which L2TPv3 Session ID (and associated Cookie, + if present) as well as which MPLS Tunnel Label maps to which PE + destination IP address. + +8.2. Static MPLS PW and Dynamic L2TPv3 PW + + When a statically configured MPLS PW is switched to a dynamic L2TPv3 + PW, the static control plane should be considered identical to an + attachment circuit (AC) in the reference model of Figure 1. The + switching point PE SHOULD signal the appropriate PW status if it + detects a failure in sending or receiving packets over the static PW. + Because the PW is statically configured, the status communicated to + the dynamic L2TPv3 PW will be limited to local interface failures. + In this case, the S-PE behaves in a very similar manner to a T-PE, + assuming an active role. + +8.3. Static L2TPv3 PW and Dynamic LDP/MPLS PW + + When a statically configured L2TPv3 PW is switched to a dynamic + LDP/MPLS PW, then the static control plane should be considered + identical to an attachment circuit (AC) in the reference model of + Figure 1. The switching point PE SHOULD signal the appropriate PW + status (via an L2TPv3 Set-Link-Info (SLI) message) if it detects a + failure in sending or receiving packets over the static PW. Because + the PW is statically configured, the status communicated to the + dynamic LDP/MPLS PW will be limited to local interface failures. In + this case, the S-PE behaves in a very similar manner to a T-PE, + assuming an active role. + +8.4. Dynamic LDP/MPLS and L2TPv3 PWs + + When switching between dynamic PWs, the switching point always + assumes an initial passive role. Thus, it does not initiate an + LDP/MPLS or L2TPv3 PW until it has received a connection request + (Label Mapping or Incoming-Call-Request (ICRQ)) from one side of the + node. Note that while MPLS PWs are made up of two unidirectional + Label Switched Paths (LSPs) bonded together by FEC identifiers, + L2TPv3 PWs are bidirectional in nature, setup via a three-message + exchange (ICRQ, Incoming-Call-Reply (ICRP), and Incoming-Call- + Connected (ICCN)). Details of Session Establishment, Tear Down, and + PW Status signaling are detailed below. + + + + + +Martini, et al. Standards Track [Page 17] + +RFC 6073 Segmented Pseudowire January 2011 + + +8.4.1. Session Establishment + + When the S-PE receives an L2TPv3 ICRQ message, the identifying AVPs + included in the message are mapped to FEC identifiers and sent in an + LDP Label Mapping message. Conversely, if an LDP Label Mapping + message is received, it is either mapped to an ICRP message or causes + an L2TPv3 session to be initiated by sending an ICRQ. + + Following are two example exchanges of messages between LDP and + L2TPv3. The first is a case where an L2TPv3 T-PE initiates an MS-PW; + the second is a case where an MPLS T-PE initiates an MS-PW. + + PE 1 (L2TPv3) PW Switching Node PE3 (MPLS/LDP) + + AC "Up" + L2TPv3 ICRQ ---> + LDP Label Mapping ---> + AC "Up" + <--- LDP Label Mapping + <--- L2TPv3 ICRP + L2TPv3 ICCN ---> + <-------------------- MS-PW Established ------------------> + PE 1 (MPLS/LDP) PW Switching Node PE3 (L2TPv3) + + AC "Up" + LDP Label Mapping ---> + L2TPv3 ICRQ ---> + <--- L2TPv3 ICRP + <--- LDP Label Mapping + L2TPv3 ICCN ---> + AC "Up" + <-------------------- MS-PW Established ------------------> + +8.4.2. Adaptation of PW Status Message + + L2TPv3 uses the SLI message to indicate an interface status change + (such as the interface transitioning from "Up" or "Down"). MPLS/LDP + PWs either signal this via an LDP Label Withdraw or the PW Status + Notification message defined in Section 4.4 of [RFC4447]. The LDP + status TLV bit SHOULD be mapped to the L2TPv3 equivalent Extended + Circuit Status Values TLV specified in [RFC5641]. + +8.4.3. Session Tear Down + + L2TPv3 uses a single message, Call-Disconnect-Notify (CDN), to tear + down a pseudowire. The CDN message translates to a Label Withdraw + message in LDP. Following are two example exchanges of messages + + + + +Martini, et al. Standards Track [Page 18] + +RFC 6073 Segmented Pseudowire January 2011 + + + between LDP and L2TPv3. The first is a case where an L2TPv3 T-PE + initiates the termination of an MS-PW; the second is a case where an + MPLS T-PE initiates the termination of an MS-PW. + + PE 1 (L2TPv3) PW Switching Node PE3 (MPLS/LDP) + + AC "Down" + L2TPv3 CDN ---> + LDP Label Withdraw ---> + AC "Down" + <-- LDP Label Release + + <--------------- MS-PW Data Path Down ------------------> + PE 1 (MPLS LDP) PW Switching Node PE3 (L2TPv3) + + AC "Down" + LDP Label Withdraw ---> + L2TPv3 CDN --> + <-- LDP Label Release + AC "Down" + + <---------------- MS-PW Data Path Down ------------------> + +8.5. Adaptation of L2TPv3 AVPs to Interface Parameters + + [RFC4447] defines several interface parameters that MUST be mapped to + the equivalent AVPs in L2TPv3 setup messages. + + * Interface MTU + + The Interface MTU parameter is mapped directly to the L2TP + "Interface Maximum Transmission Unit" AVP defined in [RFC4667]. + + * Max Number of Concatenated ATM cells + + This interface parameter is mapped directly to the L2TP "ATM + Maximum Concatenated Cells AVP" described in Section 6 of + [RFC4454]. + + * PW Type + + The PW Type defined in [RFC4446] is mapped to the L2TPv3 + "Pseudowire Type" AVP defined in [RFC3931]. + + * PWid (FEC 128) + + For FEC 128, the PWid is mapped directly to the L2TPv3 "Remote + End ID" AVP defined in [RFC3931]. + + + +Martini, et al. Standards Track [Page 19] + +RFC 6073 Segmented Pseudowire January 2011 + + + * Generalized FEC 129 SAI/TAI + + Section 4.3 of [RFC4667] defines how to encode the SAI and TAI + parameters. These can be mapped directly. + + Other interface parameter mappings are unsupported when switching + between LDP/MPLS and L2TPv3 PWs. + +8.6. PW Switching Point PE TLV in L2TPv3 + + When translating between LDP and L2TPv3 control messages, the PW + Switching Point PE TLV described earlier in this document is carried + in a single variable-length L2TP AVP present in the ICRQ and ICRP + messages, and optionally in the ICCN message. + + The L2TP "PW Switching Point AVP" is Attribute Type 101. The AVP MAY + be hidden (the L2TP AVP H-bit may be 0 or 1), the length of the AVP + is 6 plus the length of the series of Switching Point PE sub-TLVs + included in the AVP, and the AVP MUST NOT be marked Mandatory (the + L2TP AVP M-bit MUST be 0). + +8.7. L2TPv3 and MPLS PW Data Plane + + When switching between an MPLS and L2TP PW, packets are sent in their + entirety from one PW to the other, replacing the MPLS label stack + with the L2TPv3 and IP header or vice versa. + + Section 5.4 of [RFC3985] discusses the purpose of the various shim + headers necessary for enabling a pseudowire over an IP or MPLS PSN. + For L2TPv3, the Payload Convergence and Sequencing function is + carried out via the Default L2-Specific Sublayer defined in + [RFC3931]. For MPLS, these two functions (together with PSN + Convergence) are carried out via the MPLS Control Word. Since these + functions are different between MPLS and L2TPv3, interworking between + the two may be necessary. + + The L2TP L2-Specific Sublayer and MPLS Control Word are shim headers, + which in some cases are not necessary to be present at all. For + example, an Ethernet PW with sequencing disabled will generally not + require an MPLS Control Word or L2TP Default L2-Specific Sublayer to + be present at all. In this case, Ethernet frames are simply sent + from one PW to the other without any modification beyond the MPLS and + L2TP/IP encapsulation and decapsulation. + + The following section offers guidelines for how to interwork between + L2TP and MPLS for those cases where the Payload Convergence, + Sequencing, or PSN Convergence functions are necessary on one or both + sides of the switching node. + + + +Martini, et al. Standards Track [Page 20] + +RFC 6073 Segmented Pseudowire January 2011 + + +8.7.1. Mapping the MPLS Control Word to L2TP + + The MPLS Control Word consists of (from left to right): + + 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 0 0| Reserved | Length | Sequence Number | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + -i. These bits are always zero in an MPLS PW PDU. It is not + necessary to map them to L2TP. + + -ii. These six bits may be used for Payload Convergence depending + on the PW type. For ATM, the first four of these bits are + defined in [RFC4717]. These map directly to the bits + defined in [RFC4454]. For Frame Relay, these bits indicate + how to set the bits in the Frame Relay header that must be + regenerated for L2TP as it carries the Frame Relay header + intact. + + -iii. L2TP determines its payload length from IP. Thus, this + Length field need not be carried directly to L2TP. This + Length field will have to be calculated and inserted for + MPLS when necessary. + + -iv. The Default L2-Specific Sublayer has a sequence number with + different semantics than that of the MPLS Control Word. + This difference eliminates the possibility of supporting + sequencing across the MS-PW by simply carrying the sequence + number through the switching point transparently. As such, + sequence numbers MAY be supported by checking the sequence + numbers of packets arriving at the switching point and + regenerating a new sequence number in the appropriate format + for the PW on egress. If this type of sequence interworking + at the switching node is not supported, and a T-PE requests + sequencing of all packets via the L2TP control channel + during session setup, the switching node SHOULD NOT allow + the session to be established by sending a CDN message with + Result Code set to 31 "Sequencing not supported". + + + + + + + + + + + +Martini, et al. Standards Track [Page 21] + +RFC 6073 Segmented Pseudowire January 2011 + + +9. Operations, Administration, and Maintenance (OAM) + +9.1. Extensions to VCCV to Support MS-PWs + + Single-segment pseudowires are signaled using the Virtual Circuit + Connectivity Verification (VCCV) parameter included in the interface + parameter field of the PWid FEC TLV or the interface parameter sub- + TLV of the Generalized PWid FEC TLV as described in [RFC5085]. When + a switching point exists between PE nodes, it is required to be able + to continue operating VCCV end-to-end across a switching point and to + provide the ability to trace the path of the MS-PW over any number of + segments. + + This document provides a method for achieving these two objectives. + This method is based on reusing the existing VCCV Control Word (CW) + and decrementing the TTL of the PW label at each S-PE in the path of + the MS-PW. + +9.2. OAM from MPLS PW to L2TPv3 PW + + When an MS-PW includes SS-PWs that use the L2TPv3, the MPLS PW OAM + MUST be terminated at the S-PE connecting the L2TPv3 and MPLS + segments. Status information received in a particular PW segment can + then be used to generate the appropriate status messages on the + following PW segment. In the case of L2TPV3, the status bits in the + circuit status AVP defined in Section 5.4.5 of [RFC3931] and Extended + Circuit Status Values defined in [RFC5641] can be mapped directly to + the PW status bits defined in Section 5.4.3 of [RFC4447]. + + VCCV messages are specific to the MPLS data plane and cannot be used + for an L2TPv3 PW segment. Therefore, the S-PE MUST NOT send the VCCV + parameter included in the interface parameter field of the PWid FEC + TLV or the sub-TLV interface parameter of the Generalized PWid FEC + TLV. It might be possible to translate VCCV messages from L2TPv3 PW + segments to MPLS PW segments and vice versa; however, this topic is + left for further study. + +9.3. OAM Data Plane Indication from MPLS PW to MPLS PW + + As stated above, the S-PE MUST perform a standard MPLS label swap + operation on the MPLS PW label. By the rules defined in [RFC3032], + the PW label TTL MUST be decreased at every S-PE. Once the PW label + TTL reaches the value of 0, the packet is sent to the control plane + to be processed. Hence, by controlling the PW TTL value of the PW + label, it is possible to select exactly which S-PE will respond to + the VCCV packet. + + + + + +Martini, et al. Standards Track [Page 22] + +RFC 6073 Segmented Pseudowire January 2011 + + +9.4. Signaling OAM Capabilities for Switched Pseudowires + + Similarly to SS-PW, MS-PW VCCV capabilities are signaled using the + VCCV parameter included in the interface parameter field of the PWid + FEC TLV or the sub-TLV interface parameter of the Generalized PWid + FEC TLV as described in [RFC5085]. + + In Figure 3, T-PE1 uses the VCCV parameter included in the interface + parameter field of the PWid FEC TLV or the sub-TLV interface + parameter of the Generalized PWid FEC TLV to indicate to the far-end + T-PE2 what VCCV capabilities T-PE1 supports. This is the same VCCV + parameter as would be used if T-PE1 and T-PE2 were connected + directly. S-PE2, which is a PW switching point, as part of the + adaptation function for interface parameters, processes locally the + VCCV parameter then passes it to T-PE2. If there were multiple S-PEs + on the path between T-PE1 and T-PE2, each would carry out the same + processing, passing along the VCCV parameter. The local processing + of the VCCV parameter removes CC Types specified by the originating + T-PE that are not supported on the S-PE. For example, if T-PE1 + indicates that it supports CC Types 1, 2, and 3, then the S-PE + removes the Router Alert CC Type 2, leaving the rest of the TLV + unchanged, and passes the modified VCCV parameter to the next S-PE + along the path. + + The far end T-PE (T-PE2) receives the VCCV parameter indicating only + the CC Types that are supported by the initial T-PE (T-PE1) and all + S-PEs along the PW path. + +9.5. OAM Capability for MS-PWs Demultiplexed Using MPLS + + The VCCV parameter ID is defined as follows in [RFC4446]: + + Parameter ID Length Description + 0x0c 4 VCCV + + The format of the VCCV parameter field is 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 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | 0x0c | 0x04 | CC Types | CV Types | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + Bit 0 (0x01) - Type 1: PWE3 Control Word with 0001b as + first nibble as defined in [RFC4385] + Bit 1 (0x02) - Type 2: MPLS Router Alert Label + Bit 2 (0x04) - Type 3: MPLS Demultiplexor PW Label with + TTL == 1 (Type 3). + + + +Martini, et al. Standards Track [Page 23] + +RFC 6073 Segmented Pseudowire January 2011 + + +9.5.1. MS-PW and VCCV CC Type 1 + + VCCV CC Type 1 can be used for MS-PWs. However, if the CW is enabled + on user packets, VCCV CC Type 1 MUST be used according to the rules + in [RFC5085]. When using CC Type 1 for MS-PWs, the PE transmitting + the VCCV packet MUST set the TTL to the appropriate value to reach + the destination S-PE. However, if the packet is destined for the + T-PE, the TTL can be set to any value that is sufficient for the + packet to reach the T-PE. + +9.5.2. MS-PW and VCCV CC Type 2 + + VCCV CC Type 2 is not supported for MS-PWs and MUST be removed from a + VCCV parameter field by the S-PE. + +9.5.3. MS-PW and VCCV CC Type 3 + + VCCV CC Type 3 can be used for MS-PWs; however, if the CW is enabled, + VCCV Type 1 is preferred according to the rules in [RFC5085]. Note + that for using the VCCV Type 3, TTL method, the PE will set the PW + label TTL to the appropriate value necessary to reach the target PE; + otherwise, the VCCV packet might be forwarded over the AC to the + Customer Premise Equipment (CPE). + +9.6. MS-PW VCCV Operations + + This document specifies four VCCV operations: + + -i. End-to-end MS-PW connectivity verification. This operation + enables the connectivity of the MS-PW to be tested from + source T-PE to destination T-PE. In order to do this, the + sending T-PE must include the FEC used in the last segment + of the MS-PW to the destination T-PE in the VCCV-Ping echo + request. This information is either configured at the + sending T-PE or is obtained by processing the corresponding + sub-TLVs of the optional SP-PE TLV, as described below. + + -ii. Partial MS-PW connectivity verification. This operation + enables the connectivity of any contiguous subset of the + segments of an MS-PW to be tested from the source T-PE or a + source S-PE to a destination S-PE or T-PE. Again, the FEC + used on the last segment to be tested must be included in + the VCCV-Ping echo request message. This information is + determined by the sending T-PE or S-PE as in (i) above. + + -iii. MS-PW path verification. This operation verifies the path + of the MS-PW, as returned by the SP-PE TLV, against the + actual data path of the MS-PW. The sending T-PE or S-PE + + + +Martini, et al. Standards Track [Page 24] + +RFC 6073 Segmented Pseudowire January 2011 + + + iteratively sends a VCCV echo request to each S-PE along the + MS-PW path, using the FEC for the corresponding MS-PW + segment in the SP-PE TLV. If the SP-PE TLV information is + correct, then a VCCV echo reply showing that this is a valid + router for the FEC will be received. However, if the SP-PE + TLV information is incorrect, then this operation enables + the first incorrect switching point to be determined, but + not the actual path of the MS-PW beyond that. This + operation cannot be used when the MS-PW is statically + configured or when the SP-PE TLV is not supported. The + processing of the PW Switching Point PE TLV used for this + operation is described below. This operation is OPTIONAL. + + -iv. MS-PW path trace. This operation traces the data path of + the MS-PW using FECs included in the Target FEC stack TLV + [RFC4379] returned by S-PEs or T-PEs in an echo reply + message. The sending T-PE or S-PE uses this information to + recursively test each S-PE along the path of the MS-PW in a + single operation in a similar manner to LSP trace. This + operation is able to determine the actual data path of the + MS-PW, and can be used for both statically configured and + signaled MS-PWs. Support for this operation is OPTIONAL. + + Note that the above operations rely on intermediate S-PEs and/or the + destination T-PE to include the PW Switching Point PE TLV as a part + of the MS-PW setup process, or to include the Target FEC stack TLV in + the VCCV echo reply message. For various reasons, e.g., privacy or + security of the S-PE/T-PE, this information may not be available to + the source T-PE. In these cases, manual configuration of the FEC MAY + still be used. + +9.6.1. VCCV Echo Message Processing + + The challenge for the control plane is to be able to build the VCCV + echo request packet with the necessary information to reach the + desired S-PE or T-PE, for example, the target FEC 128 PW sub-TLV of + the downstream PW segment that the packet is destined for. This + could be even more difficult in situations in which the MS-PW spans + different providers and Autonomous Systems. + + For example, in Figure 3, T-PE1 has the FEC 128 of the segment (PW + segment 1), but it does not readily have the information required to + compose the FEC 128 of the following segment (PW segment 3), if a + VCCV echo request is to be sent to T-PE2. This can be achieved by + the methods described in the following subsections. + + + + + + +Martini, et al. Standards Track [Page 25] + +RFC 6073 Segmented Pseudowire January 2011 + + +9.6.1.1. Sending a VCCV Echo Request + + When performing a partial or end-to-end connectivity or path + verification, the sender of the echo request message requires the FEC + of the last segment to the target S-PE/T-PE node. This information + can either be configured manually or be obtained by inspecting the + corresponding sub-TLVs of the PW Switching Point PE TLV. + + The necessary SP-PE sub-TLVs are: + + Type Description + 0x01 PWid of last PW segment traversed + 0x03 Local IP address of PW Switching Point + 0x04 Remote IP address of last PW Switching Point traversed or + of the T-PE + + When performing an OPTIONAL MS-PW path trace operation, the T-PE will + automatically learn the target FEC by probing, one by one, the S-PEs + of the MS-PW path, using the FEC returned in the Target FEC stack of + the previous VCCV echo reply. + +9.6.1.2. Receiving a VCCV Echo Request + + Upon receiving a VCCV echo request, the control plane on S-PEs (or + the target node of each segment of the MS-PW) validates the request + and responds to the request with an echo reply consisting of a return + code of 8 (label switched at stack depth) indicating that it is an + S-PE and not the egress router for the MS-PW. + + S-PEs that wish to reveal their downstream next-hop in a trace + operation should include the FEC of the downstream PW segment in the + Target FEC stack (as per Sections 3.2 and 4.5 of [RFC4379]) of the + echo reply message. FEC 128 PWs MUST use the format shown in Section + 3.2.9 of [RFC4379] for the sub-TLV in the Target FEC stack, while FEC + 129 PWs MUST use the format shown in Section 3.2.10 of [RFC4379] for + the sub-TLV in the Target FEC stack. Note that an S-PE MUST NOT + include this FEC information in the reply if it has been configured + not to do so for administrative reasons or for reasons explained + previously. + + If the node is the T-PE or the egress node of the MS-PW, it responds + to the echo request with an echo reply with a return code of 3 + (Egress Router). + + + + + + + + +Martini, et al. Standards Track [Page 26] + +RFC 6073 Segmented Pseudowire January 2011 + + +9.6.1.3. Receiving a VCCV Echo Reply + + The operation to be taken by the node receiving the echo reply in + response to an echo request depends on the VCCV mode of operation + described above. See Section 9.5.2 for detailed procedures. + +9.6.2. Detailed VCCV Procedures + + There are two similar methods of verifying the MS-PW path: Path Trace + and Path Verification. Path Trace does not use the LDP control plane + to obtain information on the path to verify, so this method is well + suited if portions of the MS-PW are statically configured SS-PWs. + The Path Verification method relies on information obtained from the + LDP control plane, and hence offers better verification of the + current forwarding behavior compared to the LDP signaled forwarding + information of the MS-PW path. However, in the case where there are + statically signaled SS-PWs in the MS-PW path, the path information is + unavailable and must be programmed manually. + +9.6.2.1. End-to-End Connectivity Verification between T-PEs + + In Figure 3, if T-PE1, S-PE, and T-PE2 support Control Word, the PW + control plane will automatically negotiate the use of the CW. VCCV + CC Type 3 will function correctly whether or not the CW is enabled on + the PW. However, VCCV Type 1 (which can be use for end-to-end + verification only) is only supported if the CW is enabled. + + At the S-PE, the data path operations include an outer label pop, + inner label swap, and new outer label push. Note that there is no + requirement for the S-PE to inspect the CW. Thus, the end-to-end + connectivity of the multi-segment pseudowire can be verified by + performing all of the following steps: + + -i. The T-PE forms a VCCV-Ping echo request message with the FEC + matching that of the last PW segment to the destination + T-PE. + + -ii. The T-PE sets the inner PW label TTL to the exact value to + allow the packet to reach the far-end T-PE. (The value is + determined by counting the number of S-PEs from the control + plane information.) Alternatively, if CC Type 1 is + supported, the packet can be encapsulated according to CC + Type 1 in [RFC5085]. + + -iii. The T-PE sends a VCCV packet that will follow the exact same + data path at each S-PE as that taken by data packets. + + + + + +Martini, et al. Standards Track [Page 27] + +RFC 6073 Segmented Pseudowire January 2011 + + + -iv. The S-PE may perform an outer label pop, if Penultimate Hop + Popping (PHP) is disabled, and will perform an inner label + swap with TTL decrement and a new outer label push. + + -v. There is no requirement for the S-PE to inspect the CW. + + -vi. The VCCV packet is diverted to VCCV control processing at + the destination T-PE. + + -vii. The destination T-PE replies using the specified reply mode, + i.e., reverse PW path or IP path. + +9.6.2.2. Partial Connectivity Verification from T-PE + + In order to trace part of the multi-segment pseudowire, the TTL of + the PW label may be used to force the VCCV message to 'pop out' at an + intermediate node. When the TTL expires, the S-PE can determine that + the packet is a VCCV packet either by checking the CW or (if the CW + is not in use) by checking for a valid IP header with UDP destination + port 3503. The packet should then be diverted to VCCV processing. + + In Figure 3, if T-PE1 sends a VCCV message with the TTL of the PW + label equal to 1, the TTL will expire at the S-PE. T-PE1 can thus + verify the first segment of the pseudowire. + + The VCCV packet is built according to [RFC4379], Section 3.2.9 for + FEC 128, or Section 3.2.10 for FEC 129. All the information + necessary to build the VCCV LSP ping packet is collected by + inspecting the S-PE TLVs. + + Note that this use of the TTL is subject to the caution expressed in + [RFC5085]. If a penultimate LSR between S-PEs or between an S-PE and + a T-PE manipulates the PW label TTL, the VCCV message may not emerge + from the MS-PW at the correct S-PE. + +9.6.2.3. Partial Connectivity Verification between S-PEs + + Assuming that all nodes along an MS-PW support the Control Word CC + Type 3, VCCV between S-PEs may be accomplished using the PW label TTL + as described above. In Figure 3, the S-PE may verify the path + between it and T-PE2 by sending a VCCV message with the PW label TTL + set to 1. Given a more complex network with multiple S-PEs, an S-PE + may verify the connectivity between it and an S-PE two segments away + by sending a VCCV message with the PW label TTL set to 2. Thus, an + S-PE can diagnose connectivity problems by successively increasing + the TTL. All the information needed to build the proper VCCV echo + + + + + +Martini, et al. Standards Track [Page 28] + +RFC 6073 Segmented Pseudowire January 2011 + + + request packet (as described in [RFC4379], Sections 3.2.9 or 3.2.10) + is obtained automatically from the LDP label mapping that contains + S-PE TLVs. + +9.6.2.4. MS-PW Path Verification + + As an example, in Figure 3, VCCV trace can be performed on the MS-PW + originating from T-PE1 by a single operational command. The + following process ensues: + + -i. T-PE1 sends a VCCV echo request with TTL set to 1 and a FEC + containing the pseudowire information of the first segment + (PW1 between T-PE1 and S-PE) to S-PE for validation. If FEC + Stack Validation is enabled, the request may also include an + additional sub-TLV such as LDP Prefix and/or RSVP LSP, + dependent on the type of transport tunnel the segmented PW + is riding on. + + -ii. S-PE validates the echo request with the FEC. Since it is a + switching point between the first and second segment, it + builds an echo reply with a return code of 8 and sends the + echo reply back to T-PE1. + + -iii. T-PE1 builds a second VCCV echo request based on the + information obtained from the control plane (SP-PE TLV). It + then increments the TTL and sends it out to T-PE2. Note + that the VCCV echo request packet is switched at the S-PE + data path and forwarded to the next downstream segment + without any involvement from the control plane. + + -iv. T-PE2 receives and validates the echo request with the FEC. + Since T-PE2 is the destination node or the egress node of + the MS-PW, it replies to T-PE1 with an echo reply with a + return code of 3 (Egress Router). + + -v. T-PE1 receives the echo reply from T-PE2. T-PE1 is made + aware that T-PE2 is the destination of the MS-PW because the + echo reply has a return code of 3. The trace process is + completed. + + If no echo reply is received, or an error code is received from a + particular PE, the trace process MUST stop immediately, and packets + MUST NOT be sent further along the MS-PW. + + For more detail on the format of the VCCV echo packet, refer to + [RFC5085] and [RFC4379]. The TTL here refers to that of the inner + (PW) label TTL. + + + + +Martini, et al. Standards Track [Page 29] + +RFC 6073 Segmented Pseudowire January 2011 + + +9.6.2.5. MS-PW Path Trace + + As an example, in Figure 3, VCCV trace can be performed on the MS-PW + originating from T-PE1 by a single operational command. The + following OPTIONAL process ensues: + + -i. T-PE1 sends a VCCV echo request with TTL set to 1 and a FEC + containing the pseudowire information of the first segment + (PW1 between T-PE1 and S-PE) to S-PE for validation. If FEC + Stack Validation is enabled, the request may also include an + additional sub-TLV such as LDP Prefix and/or RSVP LSP, + dependent on the type of transport tunnel the segmented PW + is riding on. + + -ii. The S-PE validates the echo request with the FEC. + + -iii. The S-PE builds an echo reply with a return code of 8 and + sends the echo reply back to T-PE1, appending the FEC 128 + information for the next segment along the MS-PW to the VCCV + echo reply packet using the Target FEC stack TLV (as per + Sections 3.2 and 4.5 of [RFC4379]). + + -iv. T-PE1 builds a second VCCV echo request based on the + information obtained from the FEC stack TLV received in the + previous VCCV echo reply. It then increments the TTL and + sends it out to T-PE2. Note that the VCCV echo request + packet is switched at the S-PE data path and forwarded to + the next downstream segment without any involvement from the + control plane. + + -v. T-PE2 receives and validates the echo request with the FEC. + Since T-PE2 is the destination node or the egress node of + the MS-PW, it replies to T-PE1 with an echo reply with a + return code of 3 (Egress Router). + + -vi. T-PE1 receives the echo reply from T-PE2. T-PE1 is made + aware that T-PE2 is the destination of the MS-PW because the + echo reply has a return code of 3. The trace process is + completed. + + If no echo reply is received, or an error code is received from a + particular PE, the trace process MUST stop immediately, and packets + MUST NOT be sent further along the MS-PW. + + For more detail on the format of the VCCV echo packet, refer to + [RFC5085] and [RFC4379]. The TTL here refers to that of the inner + (PW) label TTL. + + + + +Martini, et al. Standards Track [Page 30] + +RFC 6073 Segmented Pseudowire January 2011 + + +10. Mapping Switched Pseudowire Status + + In the PW switching with attachment circuits case (Figure 2), PW + status messages indicating PW or attachment circuit faults MUST be + mapped to fault indications or OAM messages on the connecting AC as + defined in [PW-MSG-MAP]. + + In the PW control plane switching case (Figure 3), there is no + attachment circuit at the S-PE, but the two PWs are connected + together. Similarly, the status of the PWs is forwarded unchanged + from one PW to the other by the control plane switching function. + However, it may sometimes be necessary to communicate fault status of + one of the locally attached PW segments at an S-PE. For LDP, this + can be accomplished by sending an LDP notification message containing + the PW status TLV, as well as an OPTIONAL PW Switching Point PE TLV + 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 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |0| Notification (0x0001) | Message Length | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Message ID | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |0|1| Status (0x0300) | Length | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |0|1| Status Code=0x00000028 | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Message ID=0 | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Message Type=0 | PW Status TLV | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | PW Status TLV | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | PW Status TLV | PWid FEC or Generalized ID FEC| + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + ~ ~ + | PWid FEC or Generalized ID FEC (contd.) | + | | + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |1|0| SP-PE TLV (0x096D) | SP-PE TLV Length | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Type | Length | Variable Length Value | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + + + + + +Martini, et al. Standards Track [Page 31] + +RFC 6073 Segmented Pseudowire January 2011 + + + Only one SP-PE TLV can be present in this message. This message is + then relayed by each S-PE unchanged. The T-PE decodes the status + message and the included SP-PE TLV to detect exactly where the fault + occurred. At the T-PE, if there is no SP-PE TLV included in the LDP + status notification, then the status message can be assumed to have + originated at the remote T-PE. + + The merging of the received LDP status and the local status for the + PW segments at an S-PE can be summarized as follows: + + -i. When the local status for both PW segments is UP, the S-PE + passes any received AC or PW status bits unchanged, i.e., + the status notification TLV is unchanged, but the PWid in + the case of a FEC 128 TLV is set to the value of the PW + segment of the next hop. + + -ii. When the local status for any of the PW segments is at + fault, the S-PE always sends the local status bits + regardless of whether the received status bits from the + remote node indicated that an upstream fault has cleared. + AC status bits are passed along unchanged. + +10.1. PW Status Messages Initiated by the S-PE + + The PW fault directions are defined as follows: + + +-------+ + ---PW1 Receive---->| |-----PW2 Transmit----> + S-PE1 | S-PE2 | S-PE3 + <--PW1 Transmit----| |<----PW2 Receive------ + +-------+ + + Figure 4: S-PE and PW Transmission/Reception Directions + + When a local fault is detected by the S-PE, a PW status message is + sent in both directions along the PW. Since there are no attachment + circuits on an S-PE, only the following status messages are relevant: + + 0x00000008 - Local PSN-facing PW (ingress) Receive Fault + 0x00000010 - Local PSN-facing PW (egress) Transmit Fault + + Each S-PE needs to store only two 32-bit PW status words for each PW + segment: one for local failures and one for remote failures (normally + received from another PE). The first failure will set the + appropriate bit in the 32-bit status word, and each subsequent + failure will be ORed to the appropriate PW status word. In the case + + + + + +Martini, et al. Standards Track [Page 32] + +RFC 6073 Segmented Pseudowire January 2011 + + + that the PW status word stores remote failures, this rule has the + effect of a logical OR operation with the first failure received on + the particular PW segment. + + It should be noted that remote failures received on an S-PE are just + passed along the MS-PW unchanged, while local failures detected an + S-PE are signaled on both PW segments. + + A T-PE can receive multiple failures from S-PEs along the MS-PW; + however, only the failure from the remote closest S-PE will be stored + (last PW status message received). The PW status word received is + just ORed to any existing remote PW status already stored on the + T-PE. + + Given that there are two PW segments at a particular S-PE for a + particular MS-PW (referring to Figure 4), there are four possible + failure cases as follows: + + -i. PW2 Transmit direction fault + -ii. PW1 Transmit direction fault + -iii. PW2 Receive direction fault + -iv. PW1 Receive direction fault + + Once a PW status notification message is initiated at an S-PE for a + particular PW status bit, any further status message for the same + status bit (and received from an upstream neighbor) is processed + locally and not forwarded until the S-PE original status error state + is cleared. + + Each S-PE along the MS-PW MUST store any PW status messages + transiting it. If more than one status message with the same PW + status bit set is received by a T-PE or S-PE, only the last PW status + message is stored. + +10.1.1. Local PW2 Transmit Direction Fault + + When this failure occurs, the S-PE will take the following actions: + + * Send a PW status message to S-PE3 containing "0x00000010 - Local + PSN-facing PW (egress) Transmit Fault". + + * Send a PW status message to S-PE1 containing "0x00000008 - Local + PSN-facing PW (ingress) Receive Fault". + + * Store 0x00000010 in the local PW status word for the PW segment + toward S-PE3. + + + + + +Martini, et al. Standards Track [Page 33] + +RFC 6073 Segmented Pseudowire January 2011 + + +10.1.2. Local PW1 Transmit Direction Fault + + When this failure occurs, the S-PE will take the following actions: + + * Send a PW status message to S-PE1 containing "0x00000010 - Local + PSN-facing PW (egress) Transmit Fault". + + * Send a PW status message to S-PE3 containing "0x00000008 - Local + PSN-facing PW (ingress) Receive Fault". + + * Store 0x00000010 in the local PW status word for the PW segment + toward S-PE1. + +10.1.3. Local PW2 Receive Direction Fault + + When this failure occurs, the S-PE will take the following actions: + + * Send a PW status message to S-PE3 containing "0x00000008 - Local + PSN-facing PW (ingress) Receive Fault". + + * Send a PW status message to S-PE1 containing "0x00000010 - Local + PSN-facing PW (egress) Transmit Fault". + + * Store 0x00000008 in the local PW status word for the PW segment + toward S-PE3. + +10.1.4. Local PW1 Receive Direction Fault + + When this failure occurs, the S-PE will take the following actions: + + * Send a PW status message to S-PE1 containing "0x00000008 - Local + PSN-facing PW (ingress) Receive Fault". + + * Send a PW status message to S-PE3 containing "0x00000010 - Local + PSN-facing PW (egress) Transmit Fault". + + * Store 0x00000008 in the local PW status word for the PW segment + toward S-PE1. + +10.1.5. Clearing Faults + + Remote PW status fault clearing messages received by an S-PE will + only be forwarded if there are no corresponding local faults on the + S-PE. (Local faults always supersede remote faults.) + + Once the local fault has cleared, and there is no corresponding (same + PW status bit set) remote fault, a PW status message is sent out to + the adjacent PEs, clearing the fault. + + + +Martini, et al. Standards Track [Page 34] + +RFC 6073 Segmented Pseudowire January 2011 + + + When a PW status fault clearing message is forwarded, the S-PE will + always send the SP-PE TLV associated with the PE that cleared the + fault. + +10.2. PW Status Messages and SP-PE TLV Processing + + When a PW status message is received that includes an SP-PE TLV, the + SP-PE TLV information MAY be stored, along with the contents of the + PW status Word according to the procedures described above. The + SP-PE TLV stored is always the SP-PE TLV that is associated with the + PE that set that particular last fault. If subsequent PW status + messages for the same PW status bit are received, the SP-PE TLV will + overwrite the previously stored SP-PE TLV. + +10.3. T-PE Processing of PW Status Messages + + The PW switching architecture is based on the concept that the T-PE + should process the PW LDP messages in the same manner as if it were + participating in the setup of a PW segment. However, a T-PE + participating in an MS-PW SHOULD be able to process the SP-PE TLV. + Otherwise, the processing of PW status messages and other PW setup + messages is exactly as described in [RFC4447]. + +10.4. Pseudowire Status Negotiation Procedures + + Pseudowire status signaling methodology, defined in [RFC4447], SHOULD + be transparent to the switching point. + +10.5. Status Dampening + + When the PW control plane switching methodology is used to cross an + administrative boundary, it might be necessary to prevent excessive + status signaling changes from being propagated across the + administrative boundary. This can be achieved by using a similar + method as commonly employed for the BGP route advertisement + dampening. The details of this OPTIONAL algorithm are a matter of + implementation and are outside the scope of this document. + +11. Peering between Autonomous Systems + + The procedures outlined in this document can be employed to provision + and manage MS-PWs crossing AS boundaries. The use of more advanced + mechanisms involving auto-discovery and ordered PWE3 MS-PW signaling + will be covered in a separate document. + + + + + + + +Martini, et al. Standards Track [Page 35] + +RFC 6073 Segmented Pseudowire January 2011 + + +12. Congestion Considerations + + Each PSN carrying the PW may be subject to congestion. The + congestion considerations in [RFC3985] apply to PW segments as well. + Each PW segment will handle any congestion experienced by the PW + traffic independently of the other MS-PW segments. It is possible + that passing knowledge of congestion between segments and to the + T-PEs can result in more efficient edge-to-edge congestion mitigation + systems. However, any specific methods of congestion mitigation are + outside the scope of this document and left for further study. + +13. Security Considerations + + This document specifies the LDP, L2TPv3, and VCCV extensions that are + needed for setting up and maintaining pseudowires. The purpose of + setting up pseudowires is to enable Layer 2 frames to be encapsulated + and transmitted from one end of a pseudowire to the other. + Therefore, we discuss the security considerations for both the data + plane and the control plane in the following sections. The + guidelines and security considerations specified in [RFC5920] also + apply to MS-PW when the PSN is MPLS. + +13.1. Data Plane Security + + Data plane security considerations as discussed in [RFC4447], + [RFC3931], and [RFC3985] apply to this extension without any changes. + +13.1.1. VCCV Security Considerations + + The VCCV technology for MS-PW offers a method for the service + provider to verify the data path of a specific PW. This involves + sending a packet to a specific PE and receiving an answer that either + confirms the information contained in the packet or indicates that it + is incorrect. This is a very similar process to the commonly used IP + ICMP ping and TTL expired methods for IP networks. It should be + noted that when using VCCV Type 3 for PW when the CW is not enabled, + if a packet is crafted with a TTL greater than the number of hops + along the MS-PW path, or an S-PE along the path mis-processes the + TTL, the packet could mistakenly be forwarded out of the attachment + circuit as a native PW packet. This packet would most likely be + treated as an error packet by the CE. However, if this possibility + is not acceptable, the CW should be enabled to guarantee that a VCCV + packet will never be mistakenly forwarded to the AC. + + + + + + + + +Martini, et al. Standards Track [Page 36] + +RFC 6073 Segmented Pseudowire January 2011 + + +13.2. Control Protocol Security + + General security considerations with regard to the use of LDP are + specified in Section 5 of RFC 5036. Security considerations with + regard to the L2TPv3 control plane are specified in [RFC3931]. These + considerations apply as well to the case where LDP or L2TPv3 is used + to set up PWs. + + 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 + session request MUST NOT be accepted unless its IP source address is + known to be the source of an "eligible" 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. + (Note that if the auto-discovery protocol were not trusted, the set + of "eligible peers" it produces could not be trusted.) + + Even if a connection request appears to come from an eligible peer, + its source address may have been spoofed. So 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. + + For a greater degree of security, the LDP authentication option, as + described in Section 2.9 of [RFC5036], or the Control Message + Authentication option of [RFC3931], MAY be used. This provides + integrity and authentication for the control messages, and eliminates + the possibility of source address spoofing. Use of the message + authentication option does not provide privacy, but privacy of + control messages is not usually considered to be highly important. + Both the LDP and L2TPv3 message authentication options rely on the + configuration of pre-shared keys, making it difficult to deploy when + the set of eligible neighbors is determined by an auto-configuration + protocol. + + The protocol described in this document relies on the LDP MD5 + authentication key option, as described in Section 2.9 of [RFC5036], + to provide integrity and authentication for the LDP messages and + protect against source address spoofing. This mechanism relies on + the configuration of pre-shared keys, which typically introduces some + fragility. In the specific case of MS-PW, the number of links that + leave an organization will be limited in practice, so the reliance on + pre-shared keys should be manageable. + + + + +Martini, et al. Standards Track [Page 37] + +RFC 6073 Segmented Pseudowire January 2011 + + + When the Generalized PWid FEC Element is used, it is possible that a + particular peer may be one of the eligible 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. + +14. IANA Considerations + +14.1. L2TPv3 AVP + + This document uses a new L2TP parameter; IANA already maintains the + registry "Control Message Attribute Value Pairs" defined by + [RFC3438]. The following new value has been assigned: + + 101 PW Switching Point AVP + +14.2. LDP TLV TYPE + + This document uses a new LDP TLV type; IANA already maintains the + registry "TLV TYPE NAME SPACE" defined by RFC 5036. The following + value has been assigned: + + TLV type Description + 0x096D Pseudowire Switching Point PE TLV + +14.3. LDP Status Codes + + This document uses a new LDP status code; IANA already maintains the + registry "STATUS CODE NAME SPACE" defined by RFC 5036. The following + value has been assigned: + + Assignment E Description + 0x0000003A 0 PW Loop Detected + +14.4. L2TPv3 Result Codes + + This document uses a new L2TPv3 Result Code for the CDN message, as + assigned by IANA in the "Result Code AVP (Attribute Type 1) Values" + registry. + + + + + + + +Martini, et al. Standards Track [Page 38] + +RFC 6073 Segmented Pseudowire January 2011 + + + Registry Name: Result Code AVP (Attribute Type 1) Values Defined + Result Code values for the CDN message are: + + Assignment Description + 31 Sequencing not supported + +14.5. New IANA Registries + + IANA has set up a registry named "Pseudowire Switching Point PE sub- + TLV Type". These are 8-bit values. Type values 1 through 6 are + defined in this document. Type values 7 through 64 are to be + assigned by IANA using the "Expert Review" policy defined in + [RFC5226]. Type values 65 through 127, as well as 0 and 255, are to + be allocated using the IETF consensus policy defined in RFC 5226. + Type values 128 through 254 are reserved for vendor proprietary + extensions and are to be assigned by IANA, using the "First Come + First Served" policy defined in RFC 5226. + + The Type Values are assigned as follows: + + Type Length Description + + 0x01 4 PWid of last PW segment traversed + 0x02 variable PW Switching Point description string + 0x03 4/16 Local IP address of PW Switching Point + 0x04 4/16 Remote IP address of last PW Switching Point + traversed or of the T-PE + 0x05 variable FEC Element of last PW segment traversed + 0x06 12 L2 PW address of PW Switching Point + +15. Normative References + + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, March 1997. + + [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and + Languages", BCP 18, RFC 2277, January 1998. + + [RFC3931] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed., + "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", RFC + 3931, March 2005. + + [RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private + Networks (VPNs)", RFC 4364, February 2006. + + [RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol + Label Switched (MPLS) Data Plane Failures", RFC 4379, + February 2006. + + + +Martini, et al. Standards Track [Page 39] + +RFC 6073 Segmented Pseudowire January 2011 + + + [RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson, + "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word + for Use over an MPLS PSN", RFC 4385, February 2006. + + [RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to + Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006. + + [RFC4447] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., + and G. Heron, "Pseudowire Setup and Maintenance Using + the Label Distribution Protocol (LDP)", RFC 4447, April + 2006. + + [RFC5003] Metz, C., Martini, L., Balus, F., and J. Sugimoto, + "Attachment Individual Identifier (AII) Types for + Aggregation", RFC 5003, September 2007. + + [RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed., + "LDP Specification", RFC 5036, October 2007. + + [RFC5085] Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire + Virtual Circuit Connectivity Verification (VCCV): A + Control Channel for Pseudowires", RFC 5085, December + 2007. + + [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an + IANA Considerations Section in RFCs", BCP 26, RFC 5226, + May 2008. + + [RFC5641] McGill, N. and C. Pignataro, "Layer 2 Tunneling Protocol + Version 3 (L2TPv3) Extended Circuit Status Values", RFC + 5641, August 2009. + +16. Informative References + + [PW-MSG-MAP] Aissaoui, M., Busschbach, P., Morrow, M., Martini, L., + Stein, Y(J)., Allan, D., and T. Nadeau, "Pseudowire (PW) + OAM Message Mapping", Work in Progress, October 2010. + + [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., + Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack + Encoding", RFC 3032, January 2001. + + [RFC3438] Townsley, W., "Layer Two Tunneling Protocol (L2TP) + Internet Assigned Numbers Authority (IANA) + Considerations Update", BCP 68, RFC 3438, December 2002. + + + + + + +Martini, et al. Standards Track [Page 40] + +RFC 6073 Segmented Pseudowire January 2011 + + + [RFC3985] Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire + Emulation Edge-to-Edge (PWE3) Architecture", RFC 3985, + March 2005. + + [RFC4023] Worster, T., Rekhter, Y., and E. Rosen, Ed., + "Encapsulating MPLS in IP or Generic Routing + Encapsulation (GRE)", RFC 4023, March 2005. + + [RFC4454] Singh, S., Townsley, M., and C. Pignataro, "Asynchronous + Transfer Mode (ATM) over Layer 2 Tunneling Protocol + Version 3 (L2TPv3)", RFC 4454, May 2006. + + [RFC4623] Malis, A. and M. Townsley, "Pseudowire Emulation Edge- + to-Edge (PWE3) Fragmentation and Reassembly", RFC 4623, + August 2006. + + [RFC4667] Luo, W., "Layer 2 Virtual Private Network (L2VPN) + Extensions for Layer 2 Tunneling Protocol (L2TP)", RFC + 4667, September 2006. + + [RFC4717] Martini, L., Jayakumar, J., Bocci, M., El-Aawar, N., + Brayley, J., and G. Koleyni, "Encapsulation Methods for + Transport of Asynchronous Transfer Mode (ATM) over MPLS + Networks", RFC 4717, December 2006. + + [RFC5254] Bitar, N., Ed., Bocci, M., Ed., and L. Martini, Ed., + "Requirements for Multi-Segment Pseudowire Emulation + Edge-to-Edge (PWE3)", RFC 5254, October 2008. + + [RFC5659] Bocci, M. and S. Bryant, "An Architecture for Multi- + Segment Pseudowire Emulation Edge-to-Edge", RFC 5659, + October 2009. + + [RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS + Networks", RFC 5920, July 2010. + + + + + + + + + + + + + + + + +Martini, et al. Standards Track [Page 41] + +RFC 6073 Segmented Pseudowire January 2011 + + +17. Acknowledgments + + The authors wish to acknowledge the contributions of Satoru + Matsushima, Wei Luo, Neil Mcgill, Skip Booth, Neil Hart, Michael Hua, + and Tiberiu Grigoriu. + +18. Contributors + + The following people also contributed text to this document: + + Florin Balus + Alcatel-Lucent + 701 East Middlefield Rd. + Mountain View, CA 94043 + US + EMail: florin.balus@alcatel-lucent.com + + + Mike Duckett + Bellsouth + Lindbergh Center, D481 + 575 Morosgo Dr + Atlanta, GA 30324 + US + EMail: mduckett@bellsouth.net + + + + + + + + + + + + + + + + + + + + + + + + + + +Martini, et al. Standards Track [Page 42] + +RFC 6073 Segmented Pseudowire January 2011 + + +Authors' Addresses + + Luca Martini + Cisco Systems, Inc. + 9155 East Nichols Avenue, Suite 400 + Englewood, CO 80112 + US + EMail: lmartini@cisco.com + + + Chris Metz + Cisco Systems, Inc. + EMail: chmetz@cisco.com + + + Thomas D. Nadeau + EMail: tnadeau@lucidvision.com + + + Matthew Bocci + Alcatel-Lucent + Grove House, Waltham Road Rd + White Waltham, Berks SL6 3TN + UK + EMail: matthew.bocci@alcatel-lucent.co.uk + + + Mustapha Aissaoui + Alcatel-Lucent + 600, March Road, + Kanata, ON + Canada + EMail: mustapha.aissaoui@alcatel-lucent.com + + + + + + + + + + + + + + + + + + +Martini, et al. Standards Track [Page 43] + |