PWE3 Working Group Luca Martini Internet Draft Nasser El-Aawar Expiration Date: December 2002 Level 3 Communications, LLC. Jeremy Brayley Rick Wilder Gerald de Grace Masergy Communications John Shirron Laurel Networks, Inc. Dimitri Stratton Vlachos Mazu Networks, Inc. Andrew G. Malis Vivace Networks, Inc. Tom Johnson Litchfield Communications, Inc. Jayakumar Jayakumar Durai Chinnaiah Chris Liljenstolpe Dan Tappan Cable & Wireless Eric Rosen Cisco Systems, Inc. John Rutemiller Marconi Networks Laura Dominik Qwest Communications, Inc. Giles Heron PacketExchange Ltd. Kireeti Kompella Juniper Networks Neil Harrison British Telecom Tom Walsh Lucent Technologies June 2002 Encapsulation Methods for Transport of ATM Cells/Frame Over IP and MPLS Networks draft-martini-atm-encap-mpls-01.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 [1]. 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. Martini, et al. Expires December 2002 [Page 1] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract A framework for providing various Layer 1 and Layer 2 services over a Packet Switched Network has been described in [3]. This draft provides encapsulation formats and guidelines for transporting a variety of ATM services over a PSN. This draft includes refinements to draft-brayley-pwe3-atm-service that was previously submitted to the PWE3 working group. It reuses the ATM cell and AAL5 encapsulation defined in the original martini encapsulation draft [7], but includes an applicability statement for each service along with ATM OAM handling and QoS guidelines. Table of Contents 1 Conventions used in this document..............................3 2 Introduction...................................................3 3 Terminology....................................................4 4 ATM Service Encapsulation......................................5 4.1 ATM control Word............................................5 4.1.1 Setting the sequence number............................6 4.1.2 Processing the sequence number.........................7 5 ATM Cell Relay Services........................................8 5.1 VCC Cell Relay Service......................................8 5.1.1 Applicability Statement................................8 5.1.2 ATM OAM Cell Support...................................9 5.2 VPC Cell Relay Service.....................................10 5.2.1 Applicability Statement...............................10 5.2.2 ATM OAM Cell Support..................................11 5.3 Cell Relay encapsulation format............................12 5.4 Review of header information...............................12 6 AAL5 Payload VCC Service (AAL5-SDU mode)......................13 6.1 Applicability Statement....................................13 6.2 Encapsulation..............................................14 6.3 ATM OAM Cell Support.......................................15 7 ILMI support..................................................16 8 QoS considerations............................................16 9 Security Considerations.......................................17 10 Intellectual Property Disclaimer.............................17 11 References...................................................18 12 Authors' Addresses...........................................18 13 Appendix A: ATM transparent port service.....................21 13.1 Defect Handling...........................................21 Martini, et al. Expires December 2002 [Page 2] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 13.1.1 Defect Indication using ATM Physical Layer OAM messages ............................................................22 13.1.2 Defect Indication using Loss of Cell Delineation (LCD)23 13.1.3 Comparison............................................24 1 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 [2]. 2 Introduction Many service providers have multiple service networks and the Operational Support System capabilities needed to support these existing service offerings. Packet Switched Networks (PSNs) have the potential to reduce the complexity of a service providerÆs infrastructure by allowing virtually any existing digital service to be supported over a single networking infrastructure. The benefit of this model to a service provider is threefold: 1. Leveraging of the existing systems and services to provide increased capacity from a packet switched core. 2. Preserving existing network operational processes and procedures used to maintain the legacy services. 3. Using the common packet switched network infrastructure to support both the core capacity requirements of existing services and the requirements of new services supported natively over the packet switched network. This draft describes a method to carry ATM services over IP, L2TP and MPLS. It lists ATM specific requirements and provides encapsulation formats and semantics for connecting ATM edge networks through a core packet network using IP, L2TP or MPLS. The techniques described in this draft will allow ATM service providers to take advantage of new technologies in the core in order to provide ATM multi-services. Figure 1, below displays the ATM services reference model. This model is adapted from [3]. Martini, et al. Expires December 2002 [Page 3] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 |<------- Pseudo Wire ------>| | | | |<-- PSN Tunnel -->| | V V V V ATM Service+----+ +----+ ATM Service +-----+ | | PE1|==================| PE2| | +-----+ | |----------|............PW1.............|----------| | | CE1 | | | | | | | | CE2 | | |----------|............PW2.............|----------| | +-----+ | | |==================| | | +-----+ ^ | +----+ +----+ | ^ | | Provider Provider | | | | Edge 1 Edge 2 | | | | |<-------------- Emulated Service ---------------->| Customer Customer Edge 1 Edge 2 Figure 1: ATM Service Reference Model 3 Terminology Packet Switched Network - A Packet Switched Network (PSN) is an IP or MPLS network. 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. 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. 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 PSN devices. PSN Bound - The traffic direction where information from a CE is adapted to a PW, and PW-PDUs are sent into the PSN. CE Bound - The traffic direction where PW-PDUs are received on Martini, et al. Expires December 2002 [Page 4] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 a PW from the PSN, re-converted back in the emulated service, and sent out to a CE. Ingress û The point where the ATM service is encapsulated into a Pseudo Wire PDU (ATM to PSN direction.) Egress - The point where the ATM service is decapsulated from a Pseudo Wire PDU (PSN to ATM direction.) 4 ATM Service Encapsulation This section describes the general encapsulation format for ATM over PSN pseudo wires. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PSN Transport Header (As Required) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Pseudo Wire Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ATM Control Word | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ATM Service Payload | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: General format for ATM encapsulation over PSNs The PSN Transport Header depends on the particular tunneling technology in use (L2TP or MPLS). This header is used to transport the encapsulated ATM information through the packet switched core. The Pseudo Wire Header identifies a particular ATM service on a tunnel. Non-ATM services may also be carried on the PSN tunnel. The ATM Control Word is inserted before the ATM service payload. It may contain a length and sequence number in addition to certain control bits needed to carry the service. The ATM Service Payload is specific to the service being offered via the Pseudo Wire. It is defined in the following sections. 4.1 ATM control Word The ATM control word is part of the ATM specific header. It is not required for all services. The control word is designed to satisfy three requirements. Martini, et al. Expires December 2002 [Page 5] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 - Ability to detect out of order delivery of PDUs. - Ability to detect padding added by certain link technologies. - Control bits for ATM services. In all cases the egress PE MUST be aware of whether the ingress PE will send a control word over a specific Pseudo Wire. This may be achieved using static configuration or using Pseudo Wire specific signaling. The control word is defined as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | Length | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: ATM control word The first 10 bits provide space for carrying ATM service specific flags. These are defined in the ATM service descriptions below. If a particular service does not require the use of these flags, these bits MUST be set to 0 upon transmission and ignored upon receipt. The next 6 bits provide a length field, which is used as follows: The Pseudo Wire may traverse a network link that requires a minimum frame size. (Ethernet is a practical example with a minimum frame size of 64 octets.) Such links will apply padding to the Pseudo Wire PDU to reach its minimum frame size. A mechanism is required for the egress PE to detect and remove such padding. If the total length of the Pseudo Wire PDU - including the control word - is less than 64 octets, the length field MUST be set to the PDU length. Otherwise the length field MUST be set to zero. The next 16 bits provide a sequence number that can be used to guarantee ordered packet delivery. The processing of the sequence number field is OPTIONAL. The sequence number space is a 16 bit, unsigned circular space. The sequence number value 0 is used to indicate an unsequenced packet. 4.1.1 Setting the sequence number The following procedures MUST be used by the ingress PE if sequencing is desired for a given ATM service: Martini, et al. Expires December 2002 [Page 6] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 - the initial PDU transmitted on the Pseudo Wire MUST use sequence number 1 - the PE MUST increment the sequence number by one for each subsequent PDU - when the transmit sequence number reaches the maximum 16 bit value (65535) the sequence number MUST wrap to 1 If the ingress PE does not support sequence number processing, then the sequence number field in the control word MUST be set to 0. 4.1.2 Processing the sequence number If the egress PE supports receive sequence number processing, then the following procedures MUST be used: When an ATM service is initially created, the "expected sequence number" associated with it MUST be initialized to 1. When a PDU is received on the Pseudo Wire associated with the ATM service, the sequence number MUST be processed as follows: - if the sequence number on the packet is 0, then the PDU passes the sequence number check - otherwise if the PDU sequence number >= the expected sequence number and the PDU sequence number - the expected sequence number < 32768, then the PDU is in order. - otherwise if the PDU sequence number < the expected sequence number and the expected sequence number - the PDU sequence number >= 32768, then the PDU is in order. - otherwise the PDU is out of order. If a PDU passes the sequence number check, or is in order then, it can be delivered immediately. If the PDU is in order, then the expected sequence number MUST be set using the algorithm: expected_sequence_number := PDU_sequence_number + 1 mod 2**16 if (expected_sequence_number = 0) then expected_sequence_number := 1; Pseudo Wire PDUs that are received out of order MAY be dropped or reordered at the discretion of the egress PE. If the egress PE does not support receive sequence number processing, then the sequence number field MAY be ignored. Martini, et al. Expires December 2002 [Page 7] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 5 ATM Cell Relay Services This section defines the VCC and VPC cell relay services over a PSN and their applicability. 5.1 VCC Cell Relay Service A VCC cell relay service may be offered by mapping an ATM Virtual Channel Connection to a single Pseudo Wire. The Pseudo Wire is identified by a unique value in the PW Header. The ingress PE may map one or more VCCs to a single Pseudo Wire. The VCC cell relay service assumes that a PE has the ability to change the VPI/VCI. The egress PE may make its forwarding decision based on a combination of the incoming PW header and VPI/VCI within the PW PDU. 5.1.1 Applicability Statement The VCC cell relay encapsulation described in this document allows a service provider to offer a PVC or SVC based VCC cell relay service across an IP or MPLS PSN. The encapsulation allows multiple VCCs to be carried within a single PSN tunnel. This does not preclude the possibility that a service provider may wish to provision a single VCC to a PSN tunnel in order to satisfy QoS or restoration requirements. The encapsulation also supports the binding of multiple VCCs to a single Pseudo Wire. This capability is useful in order to make more efficient use of the PW demultiplexing header space as well as to ease provisioning of the VCC services. The VCC cell relay service has the following attributes: 1. Supports all ATM Adaptation Layers. 2. Non-terminating OAM/Admin cells are transported among the user cells in the same order as they are received. This requirement enables the use of various performance management and security applications. 3. In order to gain transport efficiency on the PSN, multiple cells may be encapsulated in a single PW PDU. This process is called ôcell concatenationö. How many cells to insert or how long to wait for cell arrival before sending a PW PDU is an implementation decision. Like any SAR scheme, cell concatenation introduces latency to a cell relay service. Martini, et al. Expires December 2002 [Page 8] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 4. The CLP bit from each cell may be mapped to a corresponding marking on the PW PDU. This allows the drop precedence to be preserved across the PSN. The VCC cell relay service encapsulation has the following drawbacks: 1. There is no currently defined method to translate the forward congestion indication (EFCI) to a corresponding function in the PSN. Nor is there a way to translate PSN congestion to the EFCI upon transmission by the egress PE. 2. The ATM cell header checksum can correct a single bit error in the cell header. Analogous functionality does not exist in most PSNs. A single bit error in a PW PDU will most likely cause the packet to be dropped due to a L2 FCS failure. 3. There is no currently defined method to support EPD/PPD on the PSN. 4. There are currently no OAM mechanisms defined for the PSN like those defined for ATM. Therefore the methods for the detection/consequent-actions of failures in the PSN are not specified. This also means that QoS/availability metrics cannot be specified for the PSN. 5.1.2 ATM OAM Cell Support When configured for a VCC cell relay service, both PEÆs SHOULD act as a VC switch in accordance with the procedures defined in [4]. The PEs SHOULD be able to pass the following OAM cells transparently: - F5 AIS (segment and end-to-end) - F5 RDI (segment and end-to-end) - F5 loopback (segment and end-to-end) - Resource Management - Performance Management - Continuity Check (segment and end-to-end) - Security The ingress PE SHOULD be able to generate an F5 AIS upon reception of a corresponding F4 AIS from the CE or due to a lower layer defect (such as LOS) on the ingress PE port. The egress PE SHOULD be able to generate an F5 AIS for the VCC due to a PSN failure. A method to reliably detect a PSN tunnel failure is required but not specified in this draft. If the ingress PE cannot support the generation of OAM cells, it MAY notify the egress PE using a Pseudo Wire specific maintenance Martini, et al. Expires December 2002 [Page 9] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 mechanism to be defined. For example, the ingress PE MAY withdraw the Pseudo Wire (VC label) associated with the service. Upon receiving such a notification, the egress PE SHOULD generate the appropriate F5 AIS. 5.2 VPC Cell Relay Service A Virtual Path Connection cell relay service may be offered by mapping an ATM Virtual Path Connection to a single Pseudo Wire. Like the VCC cell relay service, the Pseudo Wire is identified by a unique value in the PW Header. The ingress PE may map one or more VPCs to a single Pseudo Wire. The VPC cell relay service assumes that a PE has the ability to change the VPI. As befitting a VP level service, the VCI must remain constant. The egress PE may make its forwarding decision based on a combination of the incoming PW header and VPI within the PW PDU. 5.2.1 Applicability Statement The VPC cell relay encapsulation described in this document allows a service provider to offer a PVC or SVC based VPC cell relay service across an IP or MPLS PSN. The encapsulation allows multiple VPCs to be carried within a single PSN tunnel. This does not preclude the possibility that a service provider may wish to provision a single VPC to a PSN tunnel in order to satisfy QoS or restoration requirements. The encapsulation also supports the binding of multiple VPCs to a single Pseudo Wire. This capability is useful to ease provisioning of several VPCs. The VPC cell relay service shares many of the same attributes of the VCC cell relay service as defined above: 1. Non-terminating OAM/Admin cells are transported among the user cells in the same order as they are received. 2. The encapsulation also allows cell concatenation. If enough cells are inserted into a single PW PDU the resulting byte stream can become more efficient than a native ATM service. 3. The CLP bit from each cell may be mapped to a corresponding marking on the PW PDU. This allows the drop precedence to be preserved across the PSN. Martini, et al. Expires December 2002 [Page 10] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 The VPC cell relay service encapsulation has the following drawbacks: 1. There is no currently defined method to translate the forward congestion indication (EFCI) to a corresponding function in the PSN. Nor is there a way to translate PSN congestion to the EFCI upon transmission by the egress PE. 2. The ATM cell header checksum can correct a single bit error in the cell header. Analogous functionality does not exist in most PSNs. A single bit error in a PW PDU will most likely cause the packet to be dropped due to a L2 FCS failure. 3. There are currently no OAM mechanisms defined for the PSN like those defined for ATM. 5.2.2 ATM OAM Cell Support When configured for a VPC cell relay service, both PEs SHOULD act as a VP cross-connect in accordance with the OAM procedures defined in [4]. The PEs SHOULD be able to pass the following ATM OAM cells transparently: - F4 AIS (segment and end-to-end) - F4 RDI (segment and end-to-end) - F4 loopback (segment and end-to-end) - F5 AIS (segment and end-to-end) - F5 RDI (segment and end-to-end) - F5 loopback (segment and end-to-end) - Resource Management - Performance Management - Continuity Check (segment and end-to-end) - Security The ingress PE SHOULD be able to generate an F4 AIS due to a lower layer defect (such as LOS) on the ingress PE port. The egress PE SHOULD be able to generate an F4 AIS for the VPC due to a PSN failure. If the ingress PE cannot support the generation of OAM cells, it MAY notify the egress PE using a Pseudo Wire specific maintenance mechanism to be defined. For example, the ingress PE MAY withdraw the Pseudo Wire (VC label) associated with the service. Upon receiving such a notification, the egress PE SHOULD generate the appropriate F4 AIS. Martini, et al. Expires December 2002 [Page 11] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 5.3 Cell Relay encapsulation format The VCC and VPC cell relay services use a common encapsulation format. The ATM control word as defined in section 4.1. Its use is OPTIONAL and necessary only if sequencing is desired. If the ATM control word is used, then the Flag and Length fields should be set to 0 upon transmission and ignored upon receipt. If sequencing is not used, the control word must not be present. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ATM control word (optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VPI | VCI | PTI |C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | " | | ATM Payload (48 octets) | | " | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VPI | VCI | PTI |C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | " | | ATM Payload (48 octets) | | " | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: VCC and VPC cell relay encapsulation 5.4 Review of header information The review of the ATM header at PE devices is OPTIONAL. While information carried in the cell encapsulation is carried transparently through the PSN, and does not require a SAR function, inspection of the header information provides a mechanism to map characteristics of the transported information to the PSN. Each cell is inspected at the PE device and service requirements are mapped accordingly in the packet based network. It is through this examination that control mechanisms such as congestion management can be translated for transport in the PSN. This capability could also be used to support the mapping of ATM QoS to CoS. Direct examination of the header provides a view of the CLP field and the payload type indicator (PTI) field on a per cell basis. The PTI field provides the ATM User-to-User indication (used by Martini, et al. Expires December 2002 [Page 12] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 some AALs), upstream congestion, and discrimination between user and admin cells. Payload types carrying user information can also indicate (i) whether congestion was experienced (by EFCI) or (ii) whether the cell contains an indication of the last cell of an AAL5 PDU. A specific implementation is the mapping of the CLP bit into a MPLS based core network. In order to emulate drop precedence on a MPLS tunnel, the CLP bit is associated with a pair of configurable MPLS EXP values. Cells with CLP = 0 are encapsulated into a MPLS packet with EXP = 000. Cells with CLP = 1 are encapsulated with EXP = 001. This information is carried in all levels of labels as necessary. 6 AAL5 Payload VCC Service (AAL5-SDU mode) The AAL5 payload VCC service defines a mapping between the payload of an AAL5 VCC and a single Pseudo Wire. The AAL5 payload VCC service requires ATM segmentation and reassembly support on the PE. The AAL5 payload VCC service is OPTIONAL. Even the smallest TCP packet requires two ATM cells when sent over AAL5 on a native ATM device. It is desirable to avoid this padding on the Pseudo Wire. Therefore, once the ingress PE reassembles the AAL5 CPCS-PDU, the PE discards the PAD and CPCS-PDU trailer then inserts the resulting payload into a Pseudo Wire PDU. The egress PE MUST regenerate the PAD and trailer before transmitting the AAL5 frame on the egress ATM port. This service does allow the transport of OAM and RM cells, but does not attempt to maintain the relative order of these cells with respect to the cells that comprise the AAL5 CPCS-PDU. OAM cells that arrive during the reassembly of a single AAL5 CPCS-PDU are sent immediately on the Pseudo Wire, followed by the AAL5 payload. Therefore, the AAL5 payload VCC service will not be suitable for ATM applications that require strict ordering of OAM cells (such as performance monitoring and security applications). 6.1 Applicability Statement It is possible to carry any ATM service using the VCC and VPC cell relay encapsulations defined in the previous section. After all, ATM is inherently a cell-based technology. However, a vast majority of the data carried on ATM networks is frame based and therefore uses AAL5. For example, most Frame Relay services are provided on an ATM backbone using AAL5 and of course AAL5 is used to carry IP PDUs between ATM attached routers. Martini, et al. Expires December 2002 [Page 13] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 The AAL5-SDU service is designed with this reality in mind. The encapsulation defined below is more efficient for small AAL5 SDUs than the VCC cell relay service. In turn it presents a more efficient alternative to the cell relay service when carrying RFC 2684 encapsulated IP PDUs across a PSN. The AAL5-SDU encapsulation requires Segmentation and Reassembly on the PE-CE ATM interface. This SAR function is provided by common off-the-shelf hardware components. Once reassembled, the AAL5-SDU is carried via a Pseudo Wire to the egress PE. Herein lies another advantage of the AAL5-SDU encapsulation. Using the AAL5-SDU mode the egress PE does not have to perform reassembly itself on the PSN facing interface when converting to a frame based medium. For example, the AAL5-SDU mode allows easier extraction of an IP PDU for processing, or conversion to a different frame technology such as Frame Relay or Ethernet. When using the cell relay service to provide this same functionality, the egress PE must reassemble cells arriving over a PSN tunnel. 6.2 Encapsulation The AAL5 payload service encapsulation is shown below. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Res |T|E|C|U|Res| Length | Sequence Number (Optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | " | | ATM cell or AAL5 CPCS-SDU | | " | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: AAL5 payload service encapsulation The AAL5 payload service encapsulation requires the ATM control word. The Flag bits are described below. * Res (Reserved) These bits are reserved and MUST be set to 0 upon transmission and ignored upon reception. * T (Transport type) bit Bit (T) of the control word indicates whether the packet contains an ATM admin cell or an AAL5 payload. If T = 1, the packet contains an ATM admin cell, encapsulated according to the VCC cell relay encapsulation of section Error! Reference source not found.. If not set, the PDU contains an AAL5 payload. The ability to transport an ATM cell in the AAL5 SDU Martini, et al. Expires December 2002 [Page 14] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 mode is intended to provide a means of enabling administrative functionality over the AAL5 VCC (though it does not endeavor to preserve user-cell and admin-cell arrival/transport ordering). * E (EFCI) Bit The ingress PE device SHOULD set this bit to 1 if the EFCI bit of the final cell of the incoming AAL5 payload is set to 1, or if the EFCI bit of the single ATM cell to be transported in the packet is set to 1. Otherwise this bit SHOULD be set to 0. The egress PE device SHOULD set the EFCI bit of all the outgoing cells that transport the AAL5 payload to the value contained in this field. * C (CLP) Bit The ingress PE device SHOULD set this bit to 1 if the CLP bit of any of the incoming ATM cells of the AAL5 payload are set to 1, or if the CLP bit of the single ATM cell that is to be transported in the packet is set to 1. Otherwise this bit SHOULD be set to 0. The egress PE device SHOULD set the CLP bit of all outgoing cells that transport the AAL5 CPCS-PDU to the value contained in this field. * U (Command/Response) Bit When FRF.8.1 Frame Relay / ATM PVC Service Interworking (see [5]) traffic is being transported, the CPCS-UU Least Significant Bit (LSB) of the AAL5 CPCS-PDU may contain the Frame Relay C/R bit. The ingress PE device SHOULD copy this bit to the U bit of the control word. The egress PE device SHOULD copy the U bit to the CPCS-UU Least Significant Bit (LSB) of the AAL5 payload. The Length and Sequence Number fields are described in section 4.1. In case of a reassembly timeout, the encapsulating PE should discard all component cells of the AAL5 frame. 6.3 ATM OAM Cell Support Similar to the VCC cell relay service, both PEs SHOULD act as a VC switch in accordance with the OAM procedures defined in [4]. The PEs SHOULD be able to pass the following OAM cells transparently: - F5 AIS (segment and end-to-end) Martini, et al. Expires December 2002 [Page 15] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 - F5 RDI (segment and end-to-end) - F5 loopback (segment and end-to-end) - Resource Management - Continuity Check (segment and end-to-end) Because this service does not guarantee the original OAM cell position within the AAL5 composite cells, the following cell types are discarded by the ingress PE: - Performance Management - Security The ingress PE SHOULD be able to generate an F5 AIS upon reception of a corresponding F4 AIS from the CE or due to a lower layer defect (such as LOS) on the ingress PE port. The egress PE SHOULD be able to generate an F5 AIS for the VCC due to a PSN failure. A method to reliably detect a PSN tunnel failure is required but not specified in this draft. If the ingress PE cannot support the generation of OAM cells, it MAY notify the egress PE using a Pseudo Wire specific maintenance mechanism to be defined. For example, the ingress PE MAY withdraw the Pseudo Wire (VC label) associated with the service. Upon receiving such a notification, the egress PE SHOULD generate the appropriate F5 AIS. 7 ILMI support Integrated Local Management Interface (ILMI) typically is used in ATM networks for neighbor resource availability detection, address registration, auto-configuration, and loss of connectivity detection. ILMI messages are sent as SNMP PDUs within ATM AAL5 cells. A PE MAY provide an ATM ILMI to its attached CE. If the ingress PE receives an ILMI message indicating that the ATM service (VCC or VPC) is down, it MAY use a Pseudo Wire specific mechanism to notify the egress PE of the ATM service status. For example, a PE using an MPLS based Pseudo Wire may withdraw its advertised VC label. When receiving such a notification, the egress PE MAY use ILMI to signal the ATM service status to its attached CE. 8 QoS considerations The ingress PE should have the ability to maintain separation of ATM traffic classes (i.e. CBR, rt-VBR, nrt-VBR, ABR, and UBR) for each of the services transported across the PSN. The mechanism used to maintain these traffic classes depends upon the PSN in use. Martini, et al. Expires December 2002 [Page 16] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 For example, does the PSN support resource assignments per PSN tunnel? Can it support per PSN tunnel queuing? The actual mechanisms to support the ATM traffic classes should be left up to the operator. This section offers some suggestions. QoS assignment on the PSN requires close inspection of incoming cell headers. This includes mapping the VPI/VCI to a specific PSN traffic class and using the CLP bit to determine the PSN drop precedence. For example, when processing incoming cells for a CBR VCC service, the ingress PE may mark the outgoing Pseudo Wire PDUs with a particular DSCP or MPLS EXP. (Marking depends upon the PSN in use.) Downstream PSN devices should use this marking to map these PW PDU's to queuing and scheduling resources that emulate an ATM CBR service (i.e. low latency, guaranteed bandwidth). If the PSN is MPLS based, the ingress PE may associate ATM services with E-LSPs or L-LSPs as defined in [8]. The PSN should also have the ability to maintain the ATM cell loss priority (CLP) of incoming cells. For example, in case of an MPLS based PSN, the ingress PE may mark both the PSN transport and Pseudo Wire labels with EXP = 010 or EXP = 011 depending upon the incoming cell's CLP value. (If the PW PDU contains multiple ATM cells the ingress PE should not mark the PW PDU to convey a single drop precedence.) For AAL5 services, the ingress PE should mark the PW PDU using the same algorithm that determines the C (CLP) bit (i.e if any cell has CLP = 1 then the C bit should be set to 1.) The following is an example of mapping ATM service classes and CLP to a Diff-Serv capable PSN. ATM traffic class CLP PSN marking --------------------------------------------------- CBR 0 DSCP=000110 or EXP=110 CBR 1 DSCP=000111 or EXP=111 rt-VBR 0 DSCP=000100 or EXP=100 rt-VBR 1 DSCP=000101 or EXP=101 nrt-VBR 0 DSCP=000010 or EXP=010 nrt-VBR 1 DSCP=000011 or EXP=011 UBR 0 DSCP=000000 or EXP=000 UBR 1 DSCP=000001 or EXP=001 9 Security Considerations This draft does not introduce any new security considerations to IP or MPLS. 10 Intellectual Property Disclaimer Martini, et al. Expires December 2002 [Page 17] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 This document is being submitted for use in IETF standards discussions. 11 References [1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 [3] "Requirements for Pseudo Wire Emulation Edge-to-Edge (PWE3)", draft-ietf-pwe3-requirements-01.txt, Work in Progress [4] ITU-T Recommendation I.610, "B-ISDN operation and maintenance principles and functions", February 1999 [5] "Frame Relay / ATM PVC Service Interworking Implementation Agreement (FRF.8.1)", Frame Relay Forum 2000. [6] "Frame Based ATM over SONET/SDH Transport (FAST)," ATM Forum 2000. [7] "Encapsulation Methods for Transport of Layer 2 Frames Over MPLS", draft-martini-l2circuit-encap-mpls-04.txt, Work in Progress [8] "MPLS Support of Differentiated Services", draft-ietf-mpls- diff-ext-09.txt, Work in Progress [9] ITU-T Recommendation I.432.2, "B-ISDN User-Network Interface Physical Layer specification: 155 520 kbit/s and 622 080 kbit/s operation", February 1999 12 Authors' Addresses Luca Martini Level 3 Communications, LLC. 1025 Eldorado Blvd. Broomfield, CO, 80021 Email: luca@level3.net Nasser El-Aawar Level 3 Communications, LLC. 1025 Eldorado Blvd. Broomfield, CO, 80021 Email: nna@level3.net Jeremy Brayley Laurel Networks, Inc. 1300 Omega Drive Martini, et al. Expires December 2002 [Page 18] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 Pittsburgh, PA 15205 Email: jbrayley@laurelnetworks.com Gerald de Grace Laurel Networks, Inc. 1300 Omega Drive Pittsburgh, PA 15205 Email: gdegrace@laurelnetworks.com John Shirron Laurel Networks, Inc. 1300 Omega Drive Pittsburgh, PA 15205 Email: jshirron@laurelnetworks.com Andrew G. Malis Vivace Networks, Inc. 2730 Orchard Parkway San Jose, CA 95134 Email: Andy.Malis@vivacenetworks.com Jayakumar Jayakumar Cisco Systems, Inc. 225, E. Tasman, MS-SJ3/3, San Jose, CA, 95134 Email: jjayakum@cisco.com Durai Chinnaiah Cisco Systems, Inc. 225, E. Tasman, MS-SJ3/3, San Jose, CA, 95134 Email: dchinnai@cisco.com Dan Tappan Cisco Systems, Inc. 50 Apollo Drive Chelmsford, MA, 01824 Email: tappan@cisco.com Eric Rosen Cisco Systems, Inc. 250 Apollo Drive Chelmsford, MA, 01824 Email: erosen@cisco.com Rick Wilder Masergy Communications 2901 Telestar Ct. Falls Church, VA 22042 Email: rwilder@masergy.com Dimitri Stratton Vlachos Martini, et al. Expires December 2002 [Page 19] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 Mazu Networks, Inc. 125 Cambridgepark Drive Cambridge, MA 02140 Email: d@mazunetworks.com Thomas K. Johnson Litchfield Communications, Inc. 76 Westbury Park Rd. Watertown, CT 06795 Email: tom_johnson@litchfieldcomm.com Chris Liljenstolpe Cable & Wireless 11700 Plaza America Drive Reston, VA 20190 Email: chris@cw.net John Rutemiller Marconi Networks 1000 Marconi Drive Warrendale, PA 15086 Email: John.Rutemiller@marconi.com Giles Heron PacketExchange Ltd. The Truman Brewery 91 Brick Lane LONDON E1 6QL United Kingdom Tel.: +44 7880 506185 Email: giles@packetexchange.net Laura Dominik Qwest Communications, Inc. 600 Stinson Blvd. Minneapolis, MN, 55413 Email: ldomini@qwest.com Kireeti Kompella Juniper Networks 1194 N. Mathilda Ave Sunnyvale, CA 94089 Email: kireeti@juniper.net Tom Walsh Lucent Technologies 1 Robbins Road Westford, MA 01886 USA Email: tdwalsh@lucent.com Neil Harrison British Telecom Martini, et al. Expires December 2002 [Page 20] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 Email: neil.2.Harrison@bt.com 13 Appendix A: ATM transparent port service The main application of the transparent port service is to migrate ATM services to a PSN without having to provision the ATM CE devices. The ATM CEs will view the ATM transparent port service as if they were directly connected via a TDM leased line. This ôserviceö is most likely to be used as an internal function in a ATM service providerÆs network as a way to connect existing ATM switches via a higher speed PSN. The transparent port service is a natural benefit of the cell relay encapsulation specified above. Previous versions of this document included the transparent port service as just another cell relay service. However, although this service is very useful for the reasons above, there are concerns about defect handling. Therefore weÆve moved the transparent port service to this appendix as a application of the cell relay encapsulations. The ATM transparent port service emulates connectivity between two remote ATM ports. This service is useful when one desires to connect two CEs without interfering at the VPC or VCC layer. The ingress PE MUST discard any idle/unassigned cells received from the ingress ATM port, and map all other received cells to a single Pseudo Wire. The egress PE SHOULD not change the VPI, VCI, PTI, or CLP bits when it sends these cells on the egress ATM port. Therefore the transparent port service appears to emulate an ATM transmission convergence layer connection between two ports. However, since the ingress PE discards idle/unassigned cells, this service benefits from statistical multiplexing. This service MUST use the VCC cell relay encapsulation defined in section 5.3. 13.1 Defect Handling Transparent port service provides a capability similar to a cell- based transmission repeater service. The ATM cell stream is transparently carried from the source to the destination on a per- port basis. The interworking function (PE device) acts as a repeater for this service. For purposes of defect handling, transparent port service is modeled as an F3 layer capability. The interworking function acts as a cell repeater between the SONET/SDH link and the MPLS Pseudo- Martini, et al. Expires December 2002 [Page 21] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 Wire. Procedures are based loosely on I.432.2 [9], section 7.2.2.4. Only maintenance signals are supported. Performance monitoring is not supported. It is assumed that the SONET/SDH link and the PSN Pseudo-wire provide this function. ATM switches may implement F3 level cell-based services to achieve end-to-end performance monitoring. The procedures for applying this capability to PSN cell-based transmission are outside the scope of this document. This service is completely transparent to the ATM F4 and F5 fault management layer. The PEÆs MUST not terminate ATM F4/F5 OAM or admin cells. Transparent port service must provide indications to the egress ATM device regarding the presence of an upstream failure. There are two types of upstream failures. 1) Failure of the Pseudo wire. 2) Failure of the transmission path from the ingress ATM device. The mechanism used by the Pseudo wire to indicate a failure is outside the scope of this document. Possible mechanisms are to withdraw the PW, indicate a change of status using signaling, or some form of OAM mechanism. Upon indication of an upstream failure, an indication MUST be sent to the egress ATM device. A fault is indicated either by sending an ATM physical layer OAM message or disrupting the cell stream forcing a downstream Loss of Cell Delineation (LCD). Both procedures MUST be supported and MUST be configurable per link. 13.1.1 Defect Indication using ATM Physical Layer OAM messages Physical Layer OAM cells are used by this document to provide an additional F3 transmission path layer above the F3 transmission path layer provided by SONET/SDH, or PDH interfaces. The use of this capability requires support from the ATM devices at both ends of the PSN transparent port service. The Physical Layer OAM cell is adapted from I.432.2. Only the IAS and RDI capabilities are supported. The format of the ATM cell is shown in Figure 6: Martini, et al. Expires December 2002 [Page 22] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VPI=0 | VCI=0 |0|1|1|1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HEC | 6 A | AIS | 6 A | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6 A | 6 A | 6 A | 6 A | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6 A | 6 A | 6 A | 6 A | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6 A | 6 A | 6 A | 6 A | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6 A | 6 A | 6 A | 6 A | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6 A | 6 A | 6 A | 6 A | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6 A | 6 A | 6 A | 6 A | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6 A | 6 A | RDI | 6 A | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6 A | 6 A | 6 A | 6 A | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6 A | 6 A | 6 A | 6 A | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6 A | 6 A | 6 A | 6 A | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6 A | 6 A | 6 A |0 0 0 0 0 0|CEC| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | C E C | +-+-+-+-+-+-+-+-+ Figure 6: Cell format for Transparent Port Service Maintenance Upon detection of a failure in the PSN Pseudo-wire, the interworking function (egress PE device) must send a Transmission Path Alarm Indication Signal (TP-AIS) to the egress ATM device by placing the value 0x01 in the second octet of the ATM cell payload. ATM devices implementing this capability must send a Transmission Path Remote Defect Indication (TP-RDI) by placing the value 0x01 in the 30th octet of the ATM cell payload. 13.1.2 Defect Indication using Loss of Cell Delineation (LCD) Defect indication using Loss of Cell Delineation (LCD) is accomplished by disrupting the cell stream without affecting the lower transmission layers. Martini, et al. Expires December 2002 [Page 23] Internet Draft draft-martini-atm-encap-mpls-01.txt June 2002 The LCD condition will occur if the state of the cell scrambling is changed. With cell scrambling disabled, the egress ATM device will not be able to locate a valid HEC value. Disabling the cell scrambler has the potential of causing faults in the lower layer, or false cell delineation. This can happen if the cell payload contains certain patterns. This is not a problem during the failure because upstream ATM cells will not be arriving. The only cells sent to the egress ATM device will be Idle/Unassigned cells. The potential vulnerability is during the time when the upstream service is restored and the cell scrambler is re-enabled. To prevent this vulnerability, the transparent port service MUST disconnect the Pseudo Wire from the egress path prior to disabling cell scrambling. After the upstream failure is cleared, the transparent port service MUST re-enable the cell scrambler prior to re-connecting the Pseudo Wire to the egress path. 13.1.3 Comparison The advantage of using physical layer OAM messages is it provides proper defect notification and alarm management. The disadvantage of using physical layer OAM messages is that the ATM Physical Layer OAM cells is not currently supported for non- cell based interface types, and therefore is not supported by existing ATM equipment. The advantage of using Loss of Cell Delineation is it will work with existing ATM devices. The disadvantage of using Loss of Cell Delineation is that is will cause an improper alarm condition at the egress CE device. However, the alarm condition will not propagate beyond the egress CE device. Martini, et al. Expires December 2002 [Page 24]