Internet Draft Prayson Pate, Editor Document: draft-pate-pwe3-framework-03.txt Overture Networks, Inc. XiPeng Xiao Photuris Inc. Craig White Tricci So Level 3 Communications, LLC. Caspian Networks Kireeti Kompella Andrew G. Malis Juniper Networks, Inc. Vivace Networks Thomas K. Johnson Thomas D. Nadeau Litchfield Communications Stewart Bryant Cisco Systems Framework for Pseudo Wire Emulation Edge-to-Edge (PWE3) draft-pate-pwe3-framework-03.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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. Abstract This document describes a framework for Pseudo Wire Emulation Edge-to-Edge (PWE3). It discusses the emulation of circuits (such as T1, E1, T3, E3 and SONET/SDH) and services (such as ATM and Frame Relay) over packet switched networks (PSNs) using IP or MPLS. It presents an architectural framework for pseudo wires (PWs), defines terminology, specifies the various protocol elements and their functions, overviews some of the services that will be supported and discusses how PWs fit into the broader context of protocols. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Internet Draft draft-pate-pwe3-framework-03 January, 2002 Table of Contents 1 Introduction ................................................. 3 1.1 What Are Pseudo Wires? .................................... 3 1.2 Goals of This Document ..................................... 4 1.3 Non-Goals .................................................. 4 1.4 Terminology ................................................ 4 2 Background and Motivation .................................... 7 2.1 Current Network Architecture ............................... 7 2.2 PWE3 as a Path to Convergence .............................. 9 2.3 Suitable Applications for PWE3 ............................. 10 2.4 Summary .................................................... 10 3 Architecture of Pseudo Wires ................................. 10 3.1 Network Reference Model .................................... 11 3.2 Maintenance Reference Model ................................ 11 3.3 Maintenance Architecture ................................... 12 3.4 Protocol Stack Reference Model ............................. 12 3.5 Logical Protocol Layering Model ............................ 13 3.6 Architecture Assumptions ................................... 16 4 Design Considerations ........................................ 16 4.1 PW-PDU Validation .......................................... 16 4.2 PW-PDU Sequencing .......................................... 17 4.3 Session Multiplexing ....................................... 17 4.4 Security ................................................... 17 4.5 Encapsulation Control ...................................... 17 4.6 Statistics ................................................. 18 4.7 Traceroute ................................................. 18 4.8 Congestion Considerations .................................. 19 4.9 PW SNMP MIB Architecture ................................... 19 5 Acknowledgments .............................................. 21 6 References ................................................... 21 7 Security Considerations ...................................... 23 8 IANA Considerations .......................................... 23 9 Authors' Addresses ........................................... 23 10 Full Copyright Section ...................................... 24 Pate et al. Expires July 2002 [Page 2] Internet Draft draft-pate-pwe3-framework-03 January, 2002 1. Introduction This document describes a framework for Pseudo Wire Emulation Edge-to-Edge (PWE3). It discusses the emulation of circuits (such as T1, E1, T3, E3 and SONET/SDH) and services (such as ATM and Frame Relay) over packet switched networks (PSNs) using IP or MPLS. It presents an architectural framework for pseudo wires (PWs), defines terminology, specifies the various protocol elements and their functions, overviews the services supported and discusses how PWs fit into the broader context of protocols. See [XIAO] for the requirements for PWs. 1.1. What Are Pseudo Wires? 1.1.1. Definition PWE3 is a mechanism that emulates the essential attributes of a service (such as a T1 leased line or Frame Relay) over a PSN. The required functions of PWs include encapsulating service-specific bit-streams or PDUs arriving at an ingress port, and carrying them across a path or tunnel, managing their timing and order, and any other operations required to emulate the behavior and characteristics of the service as faithfully as possible. From the customer perspective, the PW is perceived as an unshared link or circuit of the chosen service. However, there may be deficiencies that impede some applications from being carried on a PW. These limitations should be fully described in the appropriate service-specific Encapsulation, Emulation and Maintenance Documents (EEMDs) and Applicability Statements (ASes). 1.1.2. Functions PWs provide the following functions in order to emulate the behavior and characteristics of the desired service. - Encapsulation of service-specific PDUs or circuit data arriving at an ingress port (logical or physical). - Carrying the encapsulated data across a tunnel. - Managing the signaling, timing, order or other aspects of the service at the boundaries of the PW. - Service-specific status and alarm management. EEMDs and/or ASes for each service will describe any shortfalls of the emulation's faithfulness. Pate et al. Expires July 2002 [Page 3] Internet Draft draft-pate-pwe3-framework-03 January, 2002 1.2. Goals of This Document - Description of the motivation for creating PWs, and some background on how they may be deployed. - Description of an architecture and terminology for PWs. - Description of the statistics and other network management information needed for tunnel operation and management. - Whenever possible, relevant requirements from existing IETF documents and other sources will be incorporated by reference. 1.3. Non-Goals The following are non-goals for this document: - The detailed specification of the bits and bytes of the encapsulations of the various services. This description is contained in an EEMD and/or ES. - The detailed definition of the protocols involved in PW setup and maintenance. The following are outside the scope of PWE3: - Discussion of the protection of the encapsulated content of the PW. - Any multicast service not native to the emulated medium. Thus, Ethernet transmission to a "multicast" IEEE-48 address is in scope, while multicast services like MARS that are implemented on top of the medium are out of scope. - Methods to signal or control the underlying PSN. 1.4. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALLNOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. Below are the definitions for the terms used throughout the document. Packet Switched Network A Packet Switched Network (PSN) is a network using IP or MPLS as the unit of switching. Pseudo Wire Emulation Edge to Edge Pseudo Wire Emulation Edge to Edge (PWE3) is a mechanism that emulates the essential attributes of a service (such as a T1 leased line or Frame Relay) over a PSN. Pate et al. Expires July 2002 [Page 4] Internet Draft draft-pate-pwe3-framework-03 January, 2002 Customer Edge A Customer Edge (CE) is a device where one end of an emulated service originates and terminates. The CE is not aware that it is using an emulated service rather than a "real" service. Provider Edge A Provider Edge (PE) is a device that provides PWE3 to a CE. Pseudo Wire A Pseudo Wire (PW) is a connection between two PEs carried over a PSN. The PE provides the adaptation between the CE and the PW. PW End Service A Pseudo Wire End Service (PWES) is the interface between a PE and a CE. This can be a physical interface like a T1 or Ethernet, or a virtual interface like a VC or VLAN. Pseudo Wire PDU A Pseudo Wire PDU is a PDU sent on the PW that contains all of the data and control information necessary to provide the desired service. PSN Tunnel A PSN Tunnel is a tunnel inside which multiple PWs can be nested so that they are transparent to core network devices. PW Domain A PW Domain (PWD) is a collection of instances of PWs that are within the scope of a single homogeneous administrative domain (e.g. PW over MPLS network or PW over IP network etc.). Path-oriented PW A Path-oriented PW is a PW for which the network devices of the underlying PSN must maintain path state information. Non-path-oriented PW A Non-path-oriented PW is a PW for which the network devices of the underlying PSN need not maintain path state information. Interworking Interworking is used to express interactions between networks, between end systems, or between parts thereof, with the aim of providing a functional entity capable of supporting an end-to-end communication. The interactions required to provide a functional entity rely on functions and on the means to select these functions. Interworking Function An Interworking Function (IWF) is a functional entity that facilitates interworking between two dissimilar networks (e.g., ATM & MPLS, ATM & L2TP, etc.). A PE performs the IWF function. Pate et al. Expires July 2002 [Page 5] Internet Draft draft-pate-pwe3-framework-03 January, 2002 Service Interworking In Service Interworking, the IWF (Interworking Function) between two dissimilar protocols (e.g., ATM & MPLS, Frame Relay & ATM, ATM & IP, ATM & L2TP, etc.) terminates the protocol used in one network and translates (i.e. maps) its Protocol Control Information (PCI) to the PCI of the protocol used in other network for User, Control and Management Plane functions to the extent possible. In general, since not all functions may be supported in one or other of the networks, the translation of PCI may be partial or non-existent. However, this should not result in any loss of user data since the payload is not affected by PCI conversion at the service interworking IWF. Network Interworking In Network Interworking, the PCI (Protocol Control Information) of the protocol and the payload information used in two similar networks are transferred transparently by an IWF of the PE across the PSN. Typically the IWF of the PE encapsulates the information which is transmitted by means of an adaptation function and transfers it transparently to the other network. Applicability Statement Each PW service will have an Applicability Statement (AS) that describes the applicability of PWs for that service. Encapsulation, Emulation and Maintenance Documents Each PW service will have an Encapsulation, Emulation and Maintenance Document (EEMDs) that described the particulars of PWs for that service, as well as the degree of faithfulness to that service. Inbound The traffic direction where information from a CE is adapted to a PW, and PW-PDUs are sent into the PSN. Outbound The traffic direction where PW-PDUs are received on a PW from the PSN, re-converted back in the emulated service, and sent out to a CE. CE Signaling CE (end-to-end) Signaling refers to messages sent and received by the CEs. It may be desirable or even necessary for the PE to participate in or monitor this signaling in order to effectively emulate the service. Pate et al. Expires July 2002 [Page 6] Internet Draft draft-pate-pwe3-framework-03 January, 2002 PE/PW Maintenance PE/PW Maintenance is used by the PEs to set up, maintain and tear down the PW. It may be coupled with CE signaling in order to effectively manage the PW. PSN Tunnel Signaling PSN Tunnel Signaling is used to set up, maintain and remove the underlying PSN tunnel. An example would be LDP in MPLS for maintaining LSPs. 2. Background and Motivation Why is anyone interested in PWs? This section gives some background on where networks are today and why they are changing. It also talks about the motivation to provide converged networks while continuing to support existing services. Finally it discusses how PWs can be a solution for this dilemma. 2.1. Current Network Architecture 2.1.1. Multiple Networks For any given service provider delivering multiple services, the current "network" usually consists of parallel or "overlay" networks. Each of these networks implements a specific service, such as voice, Frame Relay, Internet access, etc. This is quite expensive, both in terms of capital expense as well as in operational costs. Furthermore, the presence of multiple networks complicates planning. Service providers wind up asking themselves these questions: - Which of my networks do I build out? - How many fibers do I need for each network? - How do I efficiently manage multiple networks? A converged network helps service providers answer these questions in a consistent and economical fashion. 2.1.2. Convergence Today There are some examples of convergence in today's network: - Frame Relay is frequently carried over ATM networks using [FRF.5] interworking. - T1, E1 and T3 circuits are sometimes carried over ATM networks using [ATMCES]. - Voice is carried over ATM (using AAL2), Frame Relay (using FRF.11 VoFR), IP (using VoIP) and MPLS (using VoMPLS) networks. Pate et al. Expires July 2002 [Page 7] Internet Draft draft-pate-pwe3-framework-03 January, 2002 Deployment of these examples range from limited (ATM CES) to fairly common (FRF.5 interworking) to rapidly growing (VoIP). 2.1.3. The Emerging Converged Network Many service providers are finding that the new IP-based and MPLS-based switching systems are much less costly to acquire, deploy and maintain than the systems that they replace. The new systems take advantage of advances in technology in these ways: - The newer systems leverage mass production of ASICs and optical interfaces to reduce capital expense. - The bulk of the traffic in the network today originates from packet sources. Packet switches can economically switch and deliver this traffic natively. - Variable-length switches have lower system costs than ATM due to simpler switching mechanisms as well as elimination of segmentation and reassembly (SAR) at the edges of the network. - Deployment of services is simpler due to the connectionless nature of IP services or the rapid provisioning of MPLS applications. 2.1.4. Transition to a Packet-Optimized Converged Network The greatest assets for many service providers are the physical communications links that they own. The time and costs associated with acquiring the necessary rights of way, getting the required governmental approvals, and physically installing the cabling over a variety of terrains and obstacles represents a significant asset that is difficult to replace. Their greatest on-going costs are the operational expenses associated with maintaining and operating their networks. In order to maximize the return on their assets and minimize their operating costs, service providers often look to consolidate the delivery of multiple service types onto a single networking technology. The first generation converged network was based on TDM (time-division multiplexing) technology. Voice, video, and data traffic have been carried successfully across TDM/DACS-based networks for decades. TDM technology has some significant drawbacks as a converged networking technology. Operational costs for TDM networks remain relatively high because the provisioning of end-to-end TDM circuits is typically a tedious and labor-intensive task. In addition, TDM switching does not make the best use of the communications links. This is because fixed assignment of timeslots does not allow for the statistical multiplexing of bursty data traffic (i.e. temporarily unused bandwidth on one timeslot cannot be dynamically re-allocated to another service). Pate et al. Expires July 2002 [Page 8] Internet Draft draft-pate-pwe3-framework-03 January, 2002 The second generation of converged network was based on ATM technology. Today many service providers convert voice, video, and data traffic into fixed-length cells for carriage across ATM-based networks. ATM improves upon TDM technology by providing the ability to statistically multiplex different types of traffic onto communications links. In addition, ATM SPVC technology is often used to automatically provision end-to-end services, providing an additional advantage over traditional TDM networks. However, ATM has several significant drawbacks. One of the most frequently cited problems with ATM is the so-called cell-tax, which refers to the 5 bytes out of 53 used as an ATM cell header. Another significant problem with ATM is the AAL5 SAR, which becomes extremely difficult to implement above 1 Gbps. There are also issues with the long-term scalability of ATM, especially as a switching layer beneath IP. As packet traffic takes up a larger and larger portion of the available network bandwidth, it becomes increasingly useful to optimize public networks for the Internet Protocol. However, many service providers are confronting several obstacles in engineering packet-optimized networks. Although Internet traffic is the fastest growing traffic segment, it does not generate the highest revenue per bit. For example, Frame Relay traffic currently generates a higher revenue per bit than do native IP services. Private line TDM services still generate even more revenue per bit than does Frame Relay. In addition, there is a tremendous amount of legacy equipment deployed within public networks that does not communicate using the Internet Protocol. Service providers continue to utilize non-IP equipment to deploy a variety of services, and see a need to interconnect this legacy equipment over their IP-optimized core networks. 2.2. PWE3 as a Path to Convergence How do service providers realize the capital and operational benefits of a new packet-based infrastructure, while leveraging the existing base of SONET (Synchronous Optical Network) gear, and while also protecting the large revenue stream associated with this equipment? How do they move from mature Frame Relay or ATM networks, while still being able to provide these lucrative services? One possibility is the emulation of circuits or services via PWs. Circuit emulation over ATM and interworking of Frame Relay and ATM have already been standardized. Emulation allows existing circuits and/or services to be carried across the new infrastructure, and thus enables the interworking of disparate networks. [ATMCES] provides some insight into the requirements for such a service: There is a user demand for carrying certain types of constant bit rate (CBR) or "circuit" traffic over Asynchronous Transfer Mode (ATM) networks. As ATM is essentially a packet- rather than circuit-oriented transmission technology, it must emulate circuit Pate et al. Expires July 2002 [Page 9] Internet Draft draft-pate-pwe3-framework-03 January, 2002 characteristics in order to provide good support for CBR traffic. A critical attribute of a Circuit Emulation Service (CES) is that the performance realized over ATM should be comparable to that experienced with the current PDH/SDH technology. Implemented correctly, PWE3 can provide a means for supporting today's services over a new network. 2.3. Suitable Applications for PWE3 What makes an application suitable (or not) for PWE3 emulation? When considering PWs as a means of providing an application, the following questions must be considered: - Is the application sufficiently deployed to warrant emulation? - Is there interest on the part of service providers in providing an emulation for the given application? - Is there interest on the part of equipment manufacturers in providing products for the emulation of a given application? - Are the complexities and limitations of providing an emulation worth the savings in capital and operational expenses? If the answer to all four questions is "yes", then the application is likely to be a good candidate for PWE3. Otherwise, there may not be sufficient overlap between the customers, service providers, equipment manufacturers and technology to warrant providing such an emulation. 2.4. Summary To maximize the return on their assets and minimize their operational costs, many service providers are looking to consolidate the delivery of multiple service offerings and traffic types onto a single IP-optimized network. In order to create this next-generation converged network, standard methods must be developed to emulate existing telecommunications formats such as Ethernet, Frame Relay, ATM, and TDM over IP-optimized core networks. This document describes a framework accomplishing this goal. 3. Architecture of Pseudo Wires Pate et al. Expires July 2002 [Page 10] Internet Draft draft-pate-pwe3-framework-03 January, 2002 3.1. Network Reference Model Figure 1 below shows the network reference model for PWs. As shown, the PW provides an emulated service between the customer edges (CEs). |<------- Pseudo Wire ------>| | |<-- PSN Tunnel -->| | PW V V V V PW End Service +----+ +----+ End Service +-----+ | | PE1|==================| PE2| | +-----+ | |----------|............PW1.............|----------| | | CE1 | | | | | | | | CE2 | | |----------|............PW2.............|----------| | +-----+ | | |==================| | | +-----+ Customer^ +----+ +----+ | ^Customer Edge 1 | Provider Edge 1 Provider Edge 2 | Edge 1 |<-------------- Emulated Service ---------------->| Figure 1: PWE3 Network Reference Model Any bits or packets presented at the PW End Service (PWES) are encapsulated in a PW-PDU and carried across the underlying network. The PEs perform the encapsulation, decapsulation, order management, timing and any other functions required by the service. In some cases the PWES can be treated as a virtual interfaces into a further processing (like switching or bridging) of the original service before the physical connection to the CE. Examples include Ethernet bridging, SONET cross-connect, translation of locally-significant identifiers such as VCI/VPI, etc. to other service type, etc. The underlying PSN is not involved in any of these service-specific operations. 3.2. Maintenance Reference Model Figure 2 below shows the maintenance reference model for PWs. |<------- CE (end-to-end) Signaling ------>| | |<---- PW/PE Maintenance ----->| | | | |<-- PSN Tunnel -->| | | | | | Signaling | | | | V V (out of scope) V V | v +-----+ +-----+ v +-----+ | PE1 |==================| PE2 | +-----+ | |-----|.............PW1..............|-----| | | CE1 | | | | | | CE2 | | |-----|.............PW2..............|-----| | +-----+ | |==================| | +-----+ +-----+ +-----+ Customer Provider Provider Customer Edge 1 Edge 1 Edge 2 Edge 2 Figure 2: PWE3 Maintenance Reference Model Pate et al. Expires July 2002 [Page 11] Internet Draft draft-pate-pwe3-framework-03 January, 2002 - The CE (end-to-end) signaling is between the CEs. This signaling includes Frame Relay PVC status signaling, ATM SVC signaling, etc. - The PW/PE Maintenance is used between the PEs to set up, maintain and tear down PWs, including any required coordination of parameters between the two ends. - The PSN Tunnel signaling controls the underlying PSN. An example would be LDP in MPLS for maintaining LSPs. This type of signaling is not within the scope of PWE3. 3.3. Maintenance Architecture 3.3.1. PW Setup How are PWs set up? 3.3.2. PW Integrity How do we ensure the sanity or connectivity of the PW over the PSN? 3.3.3. Service Maintenance How does CE maintenance behavior e.g. FR LMI, ATM AIS/RDI, etc. affect the PW? 3.4. Protocol Stack Reference Model Figure 3 below shows the protocol stack reference model for PWs. +----------------+ +----------------+ |Emulated Service| Emulated Service |Emulated Service| |(TDM, ATM, etc).|<===========================>|(TDM, ATM, etc.)| +----------------+ Pseudo Wire +----------------+ | Encapsulation |<===========================>| Encapsulation | +----------------+ PSN Tunnel +----------------+ | IP/MPLS |<===========================>| IP/MPLS | +----------------+ +----------------+ | Physical | | Physical | +-------+--------+ _____________ +--------+-------+ | / \ | +===============/ PSN \===============+ \ / \_____________/ Figure 3: PWE3 Protocol Stack Reference Model Pate et al. Expires July 2002 [Page 12] Internet Draft draft-pate-pwe3-framework-03 January, 2002 The PW provides the CE with what appears to be a connection to its peer at the far end. Bits or PDUs from the CE are passed through an encapsulation layer. 3.5. Logical Protocol Layering Model 3.5.1. Protocol Layers The logical protocol-layering model needed to support a PW is shown in Figure 4 below. +---------------------------+ | Payload | +---------------------------+ | Encapsulation Layer | +---------------------------+ | Multiplexing Layer | +---------------------------+ | PSN Header | +---------------------------+ | MAC/Datalink | +---------------------------+ | Physical | +---------------------------+ Figure 4: Logical Protocol Layering Model The logical protocol-layering model shown in Figure 4 above minimizes the differences between the PWs operating over different PSN types. The payload is transported over the Encapsulation Layer that carries any information that is not available in the payload itself and which is needed by the PW outbound interface to send the reconstructed service to the CE. If needed, this layer also provides support for real-time processing, sequencing and indication of length. The Multiplexing Layer provides the ability to deliver multiple PWs over a single PSN tunnel. The PSN header, MAC/datalink and physical layer definitions are outside the scope of this framework. 3.5.2. Instantiation of the Protocol Layers The instantiation of the logical protocol-layering model of Figure 4 is shown in Figure 5 below. Where possible, the components shown below use existing IETF standards and common work in progress. Otherwise, the goal was to call for the design of components that have the wider application within the IETF. Pate et al. Expires July 2002 [Page 13] Internet Draft draft-pate-pwe3-framework-03 January, 2002 +=========================================+ - - - - - - - - - - | Payload (circuit/cell/packet) | Payload +=========================================+ - - - - - - - - - - | Encapsulation-specific information | | Sequencing (optional) | Encapsulation | Length (payload/PSN-specific) | Layer | Timing Information (payload-specific) | +============+============================+ - - - - - - - - - - | L2TPv3 | MPLS shim | Multiplexing Layer +------------+-------+--------------------+ - - - - - - - - - - | IPv4/v6 | MPLS | PSN Header +====================+====================+ - - - - - - - - - - | MAC/Data-link | +=========================================+ | Physical | +=========================================+ Figure 5: Instantiated Protocol Layering 3.5.2.1. Payload The payload is generically classified into three types: circuit, cell and packet. Within these generic types there will be specific service types. For example, the generic packet type includes Frame Relay and Ethernet. The design of the encapsulation layer, and the choice between transporting the payload in a native or intermediate format, will be defined in the service-specific EEMD and/or AS. 3.5.2.1.1. Circuit Payload A circuit payload is a sampled set of bits relayed across the PW. This service will need sequencing and real-time support. 3.5.2.1.2. Cell Payload A cell payload is one, or more, ATM cells relayed across the PW. This service will need sequence number support, and may also need real-time support. The payload may consist of a single cell or a cluster of cells. The cells to be incorporated in the payload may be selected by filtering on VCI/VPI. In the case of a trunked interface the payload may be considered complete on the expiry of a timer or when a fixed number of cells have been received or both. 3.5.2.1.3. Packet Payload A packet payload would normally be relayed across the PW as a single unit. A packet payload may need sequencing or sequencing and real-time support. A packet payload may additionally require fragmentation Pate et al. Expires July 2002 [Page 14] Internet Draft draft-pate-pwe3-framework-03 January, 2002 3.5.2.2. Sequencing Support for sequencing depends on the payload type, and may be omitted if not needed. For example, Frame Relay always requires the preservation of packet order. However, where the Frame Relay service is only being used to carry IP, it may be desirable to relax that constraint in return for reduced per-packet processing cost. [RTP] or encapsulation-specific sequence numbers may be used for sequencing. 3.5.2.3. Timing A suitable real-time protocol is RTP [RTP]. This is an extensible protocol designed to be extended to carry new payload types over a transport layer running over an IP network. It includes a control protocol for managing the timing service. RTP is not currently defined for operation over L2TP. A short document describing the correct method of multiplexing the RTP control and data over LT2Pv3 is required. 3.5.2.4. Multiplexing Layer Suitable protocols for use as a multiplexing layer are L2TPv3 [LAU] and MPLS [MPLS]. - The updated definition of L2TP in [LAU] is specified to operate directly over a PSN, and in the limiting case the L2TP data encapsulation reduces to a four-octet opaque data field. L2TPv3 is specified for operation in both manually configured and negotiated mode. The associated control protocol is extensible and may be used to signal PW-specific configuration or run-time information. - MPLS may also be used in the multiplexing layer. In this case, an MPLS tag is used as a "shim" to identify the particular PW of interest. 3.5.2.5. PSN Layer The three PSN types within the scope of the IETF are IPv4, IPv6 and MPLS. IPv4 and IPv6 both provide the necessary switching, length and fragmentation services needed to support all IETF specified transport protocols. L2TPv3 is specified to run directly over IPv4 and IPv6. When the PSN is IPv4 or IPv6 no PSN convergence layer is needed. MPLS provides switching service, but no length or fragmentation service. When MPLS is used as the PSN, the encapsulation must provide length and fragmentation services, if needed. Pate et al. Expires July 2002 [Page 15] Internet Draft draft-pate-pwe3-framework-03 January, 2002 3.6. Architecture Assumptions 1) The current design is focused on a point-to-point and same-to-same service interface at both end of the PW. Only network interworking will be performed at the edge or the PW. Support for service interworking is for out of scope. 2) The initial design of PWE3 is focused on a single homogeneous administrative PWD (e.g. PW over MPLS or PW over IP etc. ONLY). Interworking between different PW types and the support of inter-domain PWs are for further study. 3) The design of PW will not perfectly emulate the characteristics of the native service. It will be dependent on both the emulated service, as well as on the network implementation. An EEMD and AS shall be created for each service to describe the degree of faithfulness of a PW to the native service. 4) Only the permanent emulated circuit type (e.g. PVC/PVP) is considered initially. The switched emulated circuit type (e.g. SVC/SVP) will be for further study. 5) The creation and placement of the PSN tunnel to support the PW is not within the scope. 6) The current PW encapsulation approach considerations are focused on IPv4, IPv6 and MPLS. Other encapsulation approaches are for further study. 7) Current PW service applications are focused on Ethernet (i.e. Ethernet II (DIX), 802.3 "raw", Ethernet 802.2, Ethernet SNAP, 802.3ac VLAN, 802.1Q), Frame Relay, ATM and TDM (e.g. DS1, DS3, E1, SONET/SDH etc.). 8) Within the single administrative PWD, the design of the PW assumes the inheritance of the security mechanism that has been applied to the emulated services. The PW also inherits any security features from the PSN e.g. IPsec for an IP PSN. No PW-specific security mechanism will be specified. 4. Design Considerations 4.1. PW-PDU Validation It is a common practice to use a checksum, CRC or FCS to assure end-to-end integrity of frames. The PW service-specific mechanisms must define whether the packet's checksum shall be preserved across the PWD or be removed at the ingress PE and then be re-calculated at the egress PE. The former approach saves work, while the later saves bandwidth. Pate et al. Expires July 2002 [Page 16] Internet Draft draft-pate-pwe3-framework-03 January, 2002 For protocols like ATM and Frame Relay, the checksum is only applicable to a single link. This is because the circuit identifiers (e.g. Frame Relay DLCI or ATM VPI/VCI) have only local significance and are changed on each hop or span. If the circuit identifier (and thus checksum) is going to change as a part of the PW emulation, it would be more efficient to strip and re-calculate the checksum. Other PDU headers (e.g. UDP in IP) do not change during transit. It would make sense to preserve these types of checksums. The EEMD for each protocol must describe the validation scheme to be used. 4.2. PW-PDU Sequencing One major consideration of PW design is how to ensure in-sequence delivery of packets, if needed. The design of the PW for each protocol must consider the support of the PSN for in-order delivery as well as the requirements of the particular application. for example, IP is connectionless and does not guarantee in-order delivery. When using IP, a PW sequence number may be needed for some applications (such as TDM). 4.3. Session Multiplexing One way to facilitate scaling is to increase the number of PWs per underlying tunnel. There are two ways to achieve this: - For a service like Relay or ATM, all of the VCs on a given port could be lumped together. VCs would not be distinguishable within the PWD. - Service SDUs could be distinguished within a PW-PDU by port, channel or VC identifiers. This approach would allow for switching or grooming in the PWD. 4.4. Security Each EEMD must specify a means to protect the control of the PWE and the PE/PW signaling. The security-related protection of the encapsulated content of the PW is outside of scope. 4.5. Encapsulation Control 4.5.1. Scalability Different service types may be required between CEs. Support of multiple services implies that a range of PWD label spaces may be needed. If the PWD spans a PSN supporting traffic engineering, then the ability to supporting label stacking would be desirable. Pate et al. Expires July 2002 [Page 17] Internet Draft draft-pate-pwe3-framework-03 January, 2002 4.5.2. Service Integration It may be desirable to design a PW to transport a variety of services which have different transport characteristics. To achieve this integration it may be useful to allow the service requirements to be mapped to the tunneling label in such a way that the PWD can apply the appropriate service and transport management to the PW. 4.6. Statistics The PE can tabulate statistics that help monitor the state of the network, and to help with measurement of SLAs. Typical counters include: - Counts of PW-PDUs sent and received, with and without errors. - Counts of PW-PDUs lost (TDM only). - Counts of service PDUs sent and received, with and without errors (non-TDM). - Service-specific interface counts. These counters would be contained in a MIB, they should not replicate existing MIB counters. 4.7. Traceroute Tracing functionality is desirable for emulated circuits and services, because it allows verification and remediation of the operation and configuration of the forwarding plane. [BONICA] describes the requirements for a generic route tracing application. Applicability of these requirements to PWE3 is an interesting problem, as many of the emulated services have no equivalent function. In general, there is not a way to trace the forwarding plane of an TDM or Frame Relay PVC. ATM does provide an option in the loopback OAM cell to return each intermediate hop (see [I.610]). There needs to be a mechanism through which upper layers can ask emulated services to reveal their internal forwarding details. A common mechanism for all emulated services over a particular PSN may be possible. For example, if MPLS is the PSN, the path for a VC LSP could be revealed via the signaling from the underlying TE tunnel LSP, or perhaps via the proposed MPLS OAM. However, when we are trying to trace the entire emulated service, starting from the CE (e.g. an ATM VCC), then a uniform approach probably will not work and different approaches would be required for different emulated services. Pate et al. Expires July 2002 [Page 18] Internet Draft draft-pate-pwe3-framework-03 January, 2002 4.8. Congestion Considerations The PSN carrying the PW may be subject to congestion. The congestion characteristics will vary with the PSN type, the network architecture and configuration, and the loading of the PSN. Each PW EEMD and/or AS will have to specify whether it needs an appropriate mechanism for operating in the presence of this congestion, including methods of mapping between its native congestion reporting and avoidance mechanisms, and those provided by the PW. 4.9. PW SNMP MIB Architecture This section describes the general architecture for SNMP MIBs used to manage PW services and the underlying PSN. The intent here is to provides a clear picture of how all of the pertinent MIBs fit together to form a cohesive management framework for deploying PWE3 services. 4.9.1. MIB Layering The SNMP MIBs created for PWE3 should fit the architecture shown in Figure 6. +-----------+ +-----------+ +-----------+ Service | CEM | | Ethernet | | ATM | Layer |Service MIB| |Service MIB| ... |Service MIB| +-----------+ +-----------+ +-----------+ \ | / \ | / - - - - - - - - - - - - \ - - - | - - - - / - - - - - - - \ | / +-------------------------------------------+ Generic PW | Generic PW MIBs | Layer +-------------------------------------------+ / | \ - - - - - - - - - - - - / - - - | - - - - \ - - - - - - - / | \ / | \ +-----------+ +-----------+ +-----------+ PSN VC |GRE VC MIB | |L2TP VC MIB| |MPLS VC MIB| Layer +-----------+ +-----------+ +-----------+ | | | - - - - - - - - - | - - - - - - | - - - - - - - - | - - - | | | +-----------+ +-----------+ +-----------+ PSN |GRE MIB(s) | |L2TP MIB(s)| |MPLS MIB(s)| Layer +-----------+ +-----------+ +-----------+ Figure 6: Relationship of SNMP MIBs Pate et al. Expires July 2002 [Page 19] Internet Draft draft-pate-pwe3-framework-03 January, 2002 Figure 7 shows an example for a TDM PW carried over MPLS. +-----------------+ | SONET MIB | RFC2558 +-----------------+ | +-----------------+ Service |SONET Service MIB| pw-cem-mib Layer +-----------------+ - - - - - - - - - - | - - - - - - - - - - - - - - - +-----------------+ Generic PW | Generic PW MIBS | pw-tc-mib Layer +-----------------+ pw-mib - - - - - - - - - - | - - - - - - - - - - - - - - - +-----------------+ PSN VC | MPLS VC MIBS | pw-mpls-mib Layer +-----------------+ - - - - - - - - - - | - - - - - - - - - - - - - - - +-----------------+ PSN | MPLS MIBs | mpls-te-mib Layer +-----------------+ mpls-lsr-mib Figure 7: Service-specific Example for MIBs Note that there is a separate MIB for each emulated service as well as one for each underlying PSN. These MIBs may be used in various combinations as needed. 4.9.2. Service Layer MIBs The first layer is referred to as the Service Layer. It contains MIBs for PWE3 services such as Ethernet, ATM, circuits and Frame Relay. This layer contains those corresponding MIBs used to mate or adapt those emulated services to the underlying services. This working group should not produce any MIBs for managing the general service; rather, it should produce just those MIBs that are used to interface or adapt the emulated service onto the PWE3 management framework. For example, the standard SONET MIB [SONETMIB] is designed and maintained by another working group. Also, the SONET MIB is designed to manage the native service without PW emulation. Since the PWE3 working group is chartered to produce the corresponding adaptation MIB, in this case, it would produce the PW-CEM-MIB [PWMPLSMIB] that would be used to adapt SONET services to the underlying PSN that carries the PWE3 service. 4.9.3. Generic PW MIBs The second layer is referred to as the Generic PW Layer. This layer is composed of two MIBs: the PWE-TC-MIB [PWTCMIB] and the PWE-MIB [PWMIB]. These MIBs are responsible for providing general PWE3 counters and service models used for monitoring and configuration of PWE3 services over any supported PSN service. That is, this MIB Pate et al. Expires July 2002 [Page 20] Internet Draft draft-pate-pwe3-framework-03 January, 2002 provides a general model of PWE3 abstraction for management purposes. This MIB is used to interconnect the Service Layer MIBs to the PSN VC Layer MIBs. The latter will be described in the next section. This layer also provides the PW-TC-MIB [PWTCMIB]. This MIB contains common SMI textual conventions [SMIv2] that may be used by any PW MIB. 4.9.4. PSN VC Layer MIBs The third layer in the PWE3 management architecture is referred to as the PSN VC layer. This layer is comprised of MIBs that are specifically designed to interface general PWE3 services (VCs) onto those underlying PSN services. In general this means that the MIB provides a means with which an operator can map the PW service onto the native PSN service. For example, in the case of MPLS, it is required that the general VC service be layered onto MPLS LSPs or Traffic Engineered (TE) Tunnels [MPLS]. In this case, the PW-MPLS-MIB [PWMPLSMIB] was created to adapt the general PWE3 circuit services onto MPLS. Like the Service Layer described above the PWE3 working group should produce these MIBs. 4.9.5. PSN Layer MIBs The fourth and final layer in the PWE3 management architecture is referred to as the PSN layer. This layer is comprised of those MIBs that control the PSN service-specific services. For example, in the case of the MPLS [MPLS] PSN service, the MPLS-LSR-MIB [LSRMIB] and the MPLS-TE-MIB [TEMIB] are used to interface the general PWE3 VC services onto native MPLS LSPs and/or TE tunnels to carry the emulated services. In addition, the MPLS-LDP-MIB [LDPMIB] may be used to reveal the MPLS labels that are distributed over the MPLS PSN in order to maintain the PW service. The MIBs in this layer are produced by other working groups that design and specify the native PSN services. These MIBs should contain the appropriate mechanisms for monitoring and configuring the PSN service such that the emulated PWE3 service will function correctly. 5. Acknowledgments This document was created by the PWE3 Framework design team. Valuable input was also provided by John Rutemiller, David Zelig, Durai Chinnaiah, Jayakumar Jayakumar, Ron Bonica, Ghassem Koleyni, and Eric Rosen. 6. References [L2TP] W.M. Townsley, A. Valencia, A. Rubens, G. Singh Pall, G. Zorn, B. Palter, "Layer Two Tunneling Protocol (L2TP)", RFC 2661, August 1999. [RTP] H. Schulzrinne et al, "RTP: A Transport Protocol for Real-Time Applications", RFC1889, January 1996. Pate et al. Expires July 2002 [Page 21] Internet Draft draft-pate-pwe3-framework-03 January, 2002 [MPLS] E. Rosen, "Multiprotocol Label Switching Architecture", RFC3031, January 2001. [IP] DARPA, "Internet Protocol", RFC791, September 1981. [CONGEST] S. Floyd, "Congestion Control Principles", RFC2914, September 2000. [SONETMIB] K. Tesink, "Definitions of Managed Objects for the SONET/SDH Interface Type", RFC2558, March 1999. [SMIv2] Case et al, "Structure of Management Information for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC1902, January 1996. [MALIS] Malis et al, "SONET/SDH Circuit Emulation over Packet (CEP)" (draft-malis-pwe3-sonet-01.txt), work in progress, November 2001. [XIAO] Xiao et al, "Requirements for Pseudo Wire Emulation Edge-to-Edge (PWE3)" (draft-pwe3-requirements-02.txt), work in progress, November 2001. [BONICA] Bonica et al, "Tracing Requirements for Generic Tunnels" (draft-bonica-tunneltrace-01.txt), work in progress, February 2001. [LAU] J Lau et al, Layer Two Tunneling Protocol "L2TP", (draft-ietf-l2tpext-l2tp-base-01.txt) work in progress, October 2001. [PWMIB] Zelig et al, "Pseudo Wire (PW) Management Information Base Using SMIv2", (draft-zelig-pw-mib-00.txt), work in progress, July 2001. [PWTCMIB] Nadeau et al, "Definitions for Textual Conventions and OBJECT-IDENTITIES for Pseudo-Wires Management" (draft-nadeau-pw-tc-mib-00.txt), work in progress, July 2001. [PWMPLSMIB] Danenberg et al, "SONET/SDH Circuit Emulation Service Over MPLS (CEM) Management Information Base Using SMIv2", (draft-danenberg-pw-cem-mib-00.txt), work in progress, July 2001. [LSRMIB] Srinivasan et al, "MPLS Label Switch Router Management Information Base Using SMIv2", draft-ietf-mpls-lsr-mib-07.txt, work in progress, January 2001. [TEMIB] Srinivasan et al, "Traffic Engineering Management Information Base Using SMIv2", Pate et al. Expires July 2002 [Page 22] Internet Draft draft-pate-pwe3-framework-03 January, 2002 , work in progress, November 2000. [LDP-MIB] Cucchiara, J., Sjostrand, H., and Luciani, J., "Definitions of Managed Objects for the Multiprotocol Label Switching, Label Distribution Protocol (LDP)", , work in progress, August 2001. [ATMCES] ATM Forum, "Circuit Emulation Service Interoperability Specification Version 2.0" (af-vtoa-0078-000), January 1997. [FRF.5] O'Leary et al, "Frame Relay/ATM PVC Network Interworking Implementation Agreement", Frame Relay Forum FRF.5, December 20, 1994. ITU Recommendation Q.933, Annex A, Geneva, 1995. [I.363.1] ITU, "B-ISDN ATM Adaptation Layer specification: Type 1 AAL", Recommendation I.363.1, August, 1996. [I.610] ITU, "B-ISDN Operation and Maintenance Principles and Functions", ITU Recommendation I.610, February, 1999. [802.3] IEEE, "ISO/IEC 8802-3: 2000 (E), Information technology--Telecommunications and information exchange between systems --Local and metropolitan area networks --Specific requirements --Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications", 2000. 7. Security Considerations It may be desirable to define methods for ensuring security during exchange of encapsulation control information at an administrative boundary of the PSN. 8. IANA Considerations There are no IANA considerations for this document. 9. Authors' Addresses Prayson Pate XiPeng Xiao Overture Networks, Inc. Photuris, Inc. P. O. Box 14864 2025 Stierlin Court RTP, NC, USA 27709 Mountain View, CA 94043 prayson.pate@overturenetworks.com xiaoxipe@cse.msu.edu Tricci So Craig White Caspian Networks Level 3 Communications, LLC. 170 Baytech Dr. 1025 Eldorado Blvd. San Jose, CA 95134 Broomfield, CO, 80021 tso@caspiannetworks.com Craig.White@Level3.com Pate et al. Expires July 2002 [Page 23] Internet Draft draft-pate-pwe3-framework-03 January, 2002 Kireeti Kompella Andrew G. Malis Juniper Networks, Inc. Vivace Networks, Inc. 1194 N. Mathilda Ave. 2730 Orchard Parkway Sunnyvale, CA 94089 San Jose, CA 95134 kireeti@juniper.net Andy.Malis@vivacenetworks.com Thomas K. Johnson Thomas D. Nadeau Litchfield Communications Cisco Systems, Inc. 76 Westbury Park Rd. 250 Apollo Drive Watertown, CT 06795 Chelmsford, MA 01824 tom_johnson@litchfieldcomm.com tnadeau@cisco.com Stewart Bryant Cisco Systems Ltd 4, The Square Stockley Park Uxbridge UB11 1BL UK stbryant@cisco.com 10. Full Copyright Section Copyright (C) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. 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