Network Working Group M Bocci Internet Draft Alcatel S.Bryant Cisco Systems Expires: January 2006 July 9, 2005 An Architecture for Multi-Segment Pseudo Wire Emulation Edge-to-Edge draft-bocci-bryant-pwe3-ms-pw-arch-00.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on December 1, 2005. Bocci et al Expires January 9, 2006 [Page 1] Internet-Draft Multi-Segment PWE3 Architecture July 2005 Copyright Notice Copyright (C) The Internet Society (2005). All Rights Reserved. Abstract This document describes an architecture for extending pseudo wire emulation across multiple packet switched network segments. Scenarios are discussed where each segment of a given edge-to-edge emulated service spans a different provider's PSN, and where the emulated service originates and terminates on the same providers PSN, but may pass through several PSN tunnel segments in that PSN. It presents an architectural framework for such multi-segment pseudo wires, defines terminology, and specifies the various protocol elements and their functions. Conventions used in this document 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 RFC-2119 [1]. Table of Contents 1. Introduction................................................3 1.1. Motivation.............................................3 1.2. Non-Goals of this Document..............................6 1.3. Terminology............................................6 2. Applicability...............................................7 3. Protocol Layering model......................................7 3.1. Domain of Multi-Segment PWE3............................7 3.2. Payload Types..........................................8 4. Multi-Segment PWE3 Reference Model...........................8 4.1. Intra-Provider Architecture.............................9 4.2. Inter-Provider Architecture.............................9 4.3. PW Switching Models....................................10 4.3.1. Switching using ACs...............................10 4.3.2. Switching using PWs...............................10 5. PE Reference Model.........................................10 5.1. PWE3 Pre-processing....................................10 5.1.1. Forwarding........................................11 5.1.2. Native Service Processing.........................11 6. Protocol Stack reference Model..............................11 7. Maintenance Reference Model.................................12 8. PW Demultiplexer Layer and PSN Requirements.................12 9. Control Plane..............................................12 Bocci & Bryant Expires January 9, 2006 [Page 2] Internet-Draft Multi-Segment PWE3 Architecture July 2005 10. Fragmentation.............................................13 11. Management and Monitoring..................................13 12. IANA Considerations........................................13 13. Security Considerations....................................13 14. Acknowledgments...........................................13 15. References................................................14 15.1. Normative References..................................14 Author's Addresses............................................14 Intellectual Property Statement................................14 Disclaimer of Validity........................................15 Copyright Statement...........................................15 Acknowledgment................................................15 1. Introduction RFC 3985 [2] defines the architecture for pseudo wires, where a pseudo wire (PW) both originates and terminates on the edge of the same packet switched network (PSN). The PW passes through a maximum of one PSN tunnel between the originating and terminating PEs. This document extends the architecture in RFC 3985 to enable pseudo wires to be extended through multiple PSN tunnels. Use cases for multi-segment pseudo wires, and the consequent requirements, are defined in [3]. 1.1. Motivation PWE3 aims to provide point-to-point connectivity between two edges of a provider network. Requirements for Multi-Segment Pseudo-Wires for this are specified in [3]. These requirements address three main problems: o How to scale PWE3 when the number of PEs grows to many hundreds or thousands, while minimizing the complexity of the PEs and P routers. o How to provide PWE3 across multiple PSN routing domains or areas in the same provider. o How to provide PWE3 across multiple provider domains, and different PSN types. Consider a single PWE3 domain, such as that shown in Figure 1. There are 4 PEs, and PWE3 must be provided from any PE to any other PE. Traditionally, this would be achieved by establishing a full mesh of PSN tunnels between the PEs. This would also require a full mesh of LDP signaling adjacencies between the PEs. Pseudo wires could then be Bocci & Bryant Expires January 9, 2006 [Page 3] Internet-Draft Multi-Segment PWE3 Architecture July 2005 established between any PE and any other PE via a single, direct tunnel. PEs must terminate all pseudo wires that are carried on PSN tunnels that terminate on that PE according to the architecture of RFC 3985. This solution is adequate for small numbers of PEs, but the number of PEs and signaling adjacencies will grow in proportion to the square of the number of PEs. A more efficient solution for large numbers of PEs would be to support a partial mesh of PSN tunnels between the PEs, as shown in Figure 1. For example, consider a PWE3 service whose endpoints are PE1 and PE4. Pseudo wires for this can take the path PE1->PE2->PE3, and rather than terminating at PE2, be switched between ingress and egress PSN tunnels on that PE. This requires a capability in PE2 that can concatenate PW segments PE1-PE2 to PW segments PE2-PE3. The end- to-end PW is known as a multi-segment PW. ,,..--..,,_ .-`` `'., +-----+` '+-----+ | PE1 |---------------------| PE2 | | |---------------------| | +-----+ PSN Tunnel +-----+ / || || \ / || || \ | || || | | || PSN || | | || || | \ || || / \ || || / \|| ||/ +-----+ +-----+ | PE3 |---------------------| PE4 | | |---------------------| | +-----+`'.,_ ,.'` +-----+ `'''---''`` Figure 1 Single PSN PWE3 Scaling Figure 1 shows a simple flat PSN topology. However, large provider networks are typically not flat, consisting of many domains that are connected together to provide edge-to-edge services. The elements in each domain are specialized for a particular role. An example application is shown in Figure 2. Here, the providers network is divided into three domains: Two access domains and the core domain. The access domains represent the edge of the provider's network at which services are delivered. In the access domain, simplicity is required in order to minimize the cost of the network. Bocci & Bryant Expires January 9, 2006 [Page 4] Internet-Draft Multi-Segment PWE3 Architecture July 2005 The core domain must support all of the aggregated services from the access domains, and the design requirements here are for scalability, performance, and information hiding (i.e. minimal state). The core must not be exposed to the state associated with large numbers of individual edge-to-edge flows. That is, the core must be simple and fast. In a traditional layer 2 network, the interconnection points between the domains are where services in the access domains are aggregated for transport across the core to other access domains. In an IP network, the interconnection points would also represent interworking points between different types of IP networks e.g. those with MPLS and those without, and also points where network policies can be applied. <----------------Edge to Edge Emulated Services---------> .-., ,..-.., .-., ,' . ,-` `', ,' . / \ .` `, / \ / \ / , / \ AC +----+ +----+ +----+ +----+ AC ---| PE |=====| PE |===============| PE |=======| PE |--- | 1 | | 2 | | 3 | | 4 | +----+ +----+ +----+ +----+ \ / \ / \ / \ / \ Core ` \ / `, ` '. ,` `, ` '-'` `., _.` '-'` Access 1 `''-''` Access 2 Figure 2 Multi-Domain Network Model This model can also be applied to inter-provider services, where they also rely on a number of separate provider networks to be connected together. Consider the application of this model to PWE3. PWE3 uses tunneling mechanisms such as MPLS to enable the underlying IP PSN to emulate characteristics of the native service. One solution to the multi- domain network model above is to extend PSN tunnels edge-to-edge between all of the PEs in access domain 1 and all of the PEs in access domain 2, but this runs into the scaling issues described above, and also exposes access and the core of the network to undesirable complexity. An alternative is to constrain the complexity to the network domain interconnection points (PE2 and PE3 in the Bocci & Bryant Expires January 9, 2006 [Page 5] Internet-Draft Multi-Segment PWE3 Architecture July 2005 example above). Pseudo-wires between PE1 and PE4 would then be switched between PSN tunnels at the interconnection points, enabling PWs from may PEs in the access domains to be aggregated across only a few PSN tunnels in the core of the network. PEs in the access domains would only need to maintain direct signaling sessions, and PSN tunnels, with other PEs in their own domain, thus minimizing complexity of the access domains. 1.2. Non-Goals of this Document The following are non-goals for this document: o The on-the-wire specification of PW encapsulations o Requirements on multi-segment pseudo-wires. o The detailed specification of mechanisms for establishing and maintaining multi-segment pseudo-wires. 1.3. Terminology The terminology specified in RFC 3985 applies. In addition, we define the following terms: o Ultimate PE (U-PE). A PE where the customer-facing attachment circuits (ACs) are bound to a PW forwarder. An ultimate PE is present in the first and last segments of a MS-PW. o Single-Segment PW (SS-PW). A PW setup directly between two U-PE devices. Each PW in one direction of a SS-PW traverses one PSN tunnel that connects the two U-PEs. o Multi-Segment PW (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 a MS-PW by definition MUST terminate on a U-PE. o PW Switching Provider Edge (S-PE). A PE capable of switching the control and data planes of the preceding and succeeding PW segments in a MS-PW. It is therefore a PW switching point for a MS-PW. A PW Switching Point is never the S-PE and the U-PE for the same MS-PW. A PW switching point runs necessary protocols to setup and manage PW segments with other PW switching points and ultimate PEs. o PW Segment. A part of a single-segment or multi-segment PW, which is set up between two PE devices, U-PEs and/or S-PEs. Bocci & Bryant Expires January 9, 2006 [Page 6] Internet-Draft Multi-Segment PWE3 Architecture July 2005 2. Applicability A MS-PW is a single PW that for technical or administrative reasons is segmented into a number of concatenated hops. From the perspective of a U-PE, a MS-PW is indistinguishable from a SS-PW. Thus, the following are equivalent from the perspective of the UPE +----+ +----+ |UPE1+--------------------------------------------------+UPE2| +----+ +----+ |<----------------------PW------------------------>| +----+ +---+ +---+ +----+ |UPE1+--------------+SPE+-----------+SPE+---------------+UPE2| +----+ +---+ +---+ +----+ Figure 3 MS-PW Equivalence Although a MS-PW may require services such as node discovery and path signaling to construct the PW, it should not be confused with a L2VPN system, which also requires these services. A VPWS connects its endpoints via a set of PWs. MS-PW is a mechanism that abstracts the construction of complex PWs from the construction of a L2VPN. Thus a U-PE might be an edge device optimized for simplicity and an S-PE might be an aggregation device designed to absorb the complexity of continuing the PW across the core of one or more service provider networks to another UPE located at the edge of the network. 3. Protocol Layering model The protocol-layering model specified in RFC 3985 applies to multi- segment PWE3 with the following clarification: the pseudo-wires may be considered to be a separate layer to the PSN tunnel. That is, they are independent of the PSN tunnel routing, operations, signaling and maintenance. The design of PW routing domains should not imply that the underlying PSN routing domains are the same. However, MS-PW will reuse the protocols of the PSN. 3.1. Domain of Multi-Segment PWE3 PWE3 defines the Encapsulation Layer, the method of carrying various payload types, and the interface to the PW Demultiplexer Layer. It is expected that other layers will provide the following: . PSN tunnel setup, maintenance and routing Bocci & Bryant Expires January 9, 2006 [Page 7] Internet-Draft Multi-Segment PWE3 Architecture July 2005 . U-PE discovery It is assumed that any node that is reachable via a PSN tunnel from an S-PE or U-PE is a PE, a subset of which may be capable of behaving as an S-PE. The selection of which S-PEs to use to reach a U-PE is considered to be in the domain of PWE3. 3.2. Payload Types Multi-segment PWE3 is applicable to all PWE3 payload types. The same encapsulations are used in both SS-PW and MH-PW. 4. Multi-Segment PWE3 Reference Model The PWE3 reference architecture for the single segment case is shown in [2]. This architecture applies to the case where a PSN tunnel extends between two edges of a single PSN domain to transport a PW with endpoints at these edges. Native |<-----------Pseudo Wire----------->| Native Service | | Service (AC) | |<-PSN1-->| |<-PSN2-->| | (AC) | V V V V V V | | +----+ +-----+ +----+ +----+ | |UPE1|=========|SPE1 |=========|UPE2| | +----+ | |-------|....PW.Seg't1........PW Seg't3.....|----------| | | CE1| | | | | | | | | |CE2 | | |-------|....PW.Seg't2.......|PW Seg't4.....|----------| | +----+ | | |=========| |=========| | | +----+ ^ +----+ +-----+ +----+ ^ | Provider Edge 1 ^ Provider Edge 2 | | | | | | | | PW switching point | | | |<------------------- Emulated Service ------------------>| Figure 4 PW switching Reference Model Figure 4 extends this architecture to show a multi-segment case. The PEs that provide PWE3 to CE1 and CE2 are Ultimate-PE1 (U-PE1) and Ultimate-PE2 (U-PE2) respectively. A PSN tunnel extends from U-PE1 to switching-PE1 (S-PE1) across PSN1, and a second PSN tunnel extends from S-PE1 to S-PE2 across PSN2. PWs are used to connect the attachment circuits (ACs) attached to PE1 to the corresponding ACs Bocci & Bryant Expires January 9, 2006 [Page 8] Internet-Draft Multi-Segment PWE3 Architecture July 2005 attached to PE3. Each PW segment on the tunnel across PSN1 is switched to a PW segment in the tunnel across PSN2 at S-PE1 to complete the multi-segment PW (MS-PW) between U-PE1 and U-PE2. S-PE1 is therefore the PW switching point. PW segment 1 and PW segment 3 are segments of the same MS-PW while PW segment 2 and PW segment 4 are segments of another MS-PW. PW segments of the same MS-PW (e.g., PW1 and PW3) MAY be of the same PW type or different type, and PSN tunnels (e.g., PSN1 and PSN2) can be the same or different technology. This document requires support for MS-PWs with segments of the same type. An S-PE switches an MS-PW from one segment to another based on the PW identifiers (e.g., PW label in case of MPLS PWs). Note that although Figure 4 only shows a single S-PE, a PW may transit more one S-PE along its path. This architecture is applicable when the S-PEs are statically chosen, or when they are chosen using a dynamic path selection mechanism. 4.1. Intra-Provider Architecture There is a requirement to deploy PWs edge to edge in large service provider networks [3]. Such networks typically encompass hundreds or thousands of aggregation devices at the edge, each of which would be a PE. These networks may be partitioned into separate metro and core PWE3 domains, where the PEs are interconnected by a sparse mesh of tunnels. Whether or not the network is partitioned in to separate PWE3 domains, there is a also a requirement to support a partial mesh of traffic engineered PSN tunnels. The architecture shown in Figure 4 can be used to support such cases. PSN1 and PSN2 may be in different administrative domains or access, core or metro regions within the same providers network. Alternatively, U-PE1, SPE1 and U-PE2 may reside at the edges of the same PSN. 4.2. Inter-Provider Architecture Intra-provider PWs may need to be switched between PSN tunnels at the provider boundary in order to minimize the number of tunnels required to provide PWE3 services to CEs attached to each providers network. In addition, AAA and security and mechanisms may need to be implemented on a per-PW basis at the provider boundary. Bocci & Bryant Expires January 9, 2006 [Page 9] Internet-Draft Multi-Segment PWE3 Architecture July 2005 4.3. PW Switching Models 4.3.1. Switching using ACs. In this model, the PW reverts to the native service at the provider boundary PE. This AC is then connected to a separate PW at the peer provider boundary PE. In this case, the reference models of RFC 3965 apply to each segment and to the PEs. The remaining PE architectural considerations in this document do not apply to this case. 4.3.2. Switching using PWs. In this model, PW segments are switched between PSN tunnels in each providers network, without reverting to the native service at the boundary. For example, in Figure 4, PSN 1 and PSN 2 would be different provider's networks. However, this would require that S-PE1 be a member of both provider networks. An alternative architecture is shown in Figure 5. |<--------------Pseudo Wire----------->| | Provider Provider | AC | |<----1---->| |<----2--->| | AC | V V V V V V | | +----+ +-----+ +----+ +----+ | +----+ | | |=====| |=====| |=====| | | +----+ | |-------|.....PW.1........PW.2.......PW.3......|-------| | | CE1| | | | | | | | | | | |CE2 | +----+ | | |=====| |=====| |=====| | | +----+ ^ +----+ +-----+ +----+ +----+ ^ | PE1 PE2 PE3 PE4 | | ^ ^ | | | | | | PW switching points | | | | | |<------------------- Emulated Service --------------->| Figure 5 Inter-Provider Reference Model 5. PE Reference Model 5.1. PWE3 Pre-processing PWE3 preprocessing is applied in the U-PEs as specified in RFC 3985. Processing at the S-PEs is specified in the following sections. Bocci & Bryant Expires January 9, 2006 [Page 10] Internet-Draft Multi-Segment PWE3 Architecture July 2005 5.1.1. Forwarding The forwarders in the S-PE forward packets from one or more PW segments on the ingress PSN facing interface of the S-PE to one or more PW segments on the egress PSN facing interface of the S-PE. The forwarder selects the egress segment PW based on the ingress PW label. The mapping of ingress to egress PW label may be statically or dynamically configured. Figure 5 shows how a single forwarder is associated with each PW segment at the S-PE. +------------------------------------------+ | S-PE Device | +------------------------------------------+ Ingress | | | | Egress PW instance | Single | | Single | PW Instance <==========>X PW Instance + Forwarder + PW Instance X<==========> | | | | +------------------------------------------+ Figure 6 Point-to-Point Service Other mappings of PW to forwarder are for further study. 5.1.2. Native Service Processing There is no native service processing in the S-PEs. 6. Protocol Stack reference Model Figure 7 illustrates the protocol stack reference model for multi- segment PWs. Bocci & Bryant Expires January 9, 2006 [Page 11] Internet-Draft Multi-Segment PWE3 Architecture July 2005 +----------------+ +----------------+ |Emulated Service| |Emulated Service| |(e.g., TDM, ATM)|<======= Emulated Service =======>|(e.g., TDM, ATM)| +----------------+ +----------------+ | Payload | | Payload | | Encapsulation |<== Multi-segment Pseudo Wire ===>| Encapsulation | +----------------+ +--------+ +----------------+ |PW Demultiplexer||PW Demux||PW Demultiplexer| +----------------+ +--------+ +----------------+ | PSN Tunnel, || PSN || PSN Tunnel, | | PSN & Physical | |Physical| | PSN & Physical | | Layers | | Layers | | Layers | +-------+--------+ +--------+ +----------------+ | .......... | .......... | | / \ | / \ | +==========/ PSN \===/ PSN \==========+ \ domain 1 / \ domain 2 / \__________/ \__________/ `````````` `````````` Figure 7 Multi-Segment PW Protocol Stack The MS-PW provides the CE with an emulated physical or virtual connection to its peer at the far end. Native service PDUs from the CE are passed through an Encapsulation Layer and a PW demultiplexer is added at the sending U-PE. The PDU is sent over PSN domain 1. The receiving S-PE removes the existing PW demultiplexer, adds a new demultiplexer, and then sends the PDU over PSN2. Policies may also be applied to the PW at this point. The receiving U-PE removes the PW demultiplexer and restores the payload to its native format for transmission to the destination CE. Where the encapsulation format is different e.g. MPLS and L2TPv3, the payload encapsulation may be transparently translated at the S-PE. 7. Maintenance Reference Model To be added in a future version. 8. PW Demultiplexer Layer and PSN Requirements To be added in a future version. 9. Control Plane For multi-segment pseudo wires, the intermediate PW switching points may be statically provisioned, or they may be dynamically signaled. Bocci & Bryant Expires January 9, 2006 [Page 12] Internet-Draft Multi-Segment PWE3 Architecture July 2005 For the dynamic case, there are two options for selecting the path of the PW: o U-PEs determine the full path of the PW through intermediate switching points. This may be either static or based on a dynamic PW path selection mechanism. o The each segment of the PW path is determined locally by each U-PE or S-PE, either through static configuration or based on a dynamic PW path selection mechanism. Further details of the impact of these on the control plane architecture will be provided in a future revision. 10. Fragmentation An SPE is not required to make any attempt to reassemble a fragmented PW payload. An SPE may fragment a PW payload fragment. 11. Management and Monitoring To be added in a later version. 12. IANA Considerations To be added in a future version. 13. Security Considerations To be added in a later version. 14. Acknowledgments The authors gratefully acknowledge the input of Mustapha Aissaoui, Dimitri Papadimitrou, and Luca Martini. Bocci & Bryant Expires January 9, 2006 [Page 13] Internet-Draft Multi-Segment PWE3 Architecture July 2005 15. References 15.1. Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Bryant, S. and Pate, P. (Editors), "Pseudo Wire Emulation Edge- to-Edge (PWE3) Architecture", RFC 3985, March 2005 [3] Martini, S. Bitar, N. and Bocci, M (Editors), "Requirements for inter domain Pseudo-Wires", draft-martini-pwe3-mh-pw- requirements-01.txt, internet Draft, March 2005 Author's Addresses Matthew Bocci Alcatel Voyager Place, Shoppenhangers Rd, Maidenhead, Berks, UK Email: matthew.bocci@alcatel.co.uk Stewart Bryant Cisco Systems, 250, Longwater, Green Park, Reading, RG2 6GB, United Kingdom. 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Please address the information to the IETF at ietf ipr@ietf.org Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Bocci & Bryant Expires January 9, 2006 [Page 15]