PWE3 Working Group John Fischer Internet Draft Matthew Bocci Expiration Date: February 2002 Alcatel A. Siddiqui Mina Azad Cable & Wireless Ghassem Koleyni Nortel Networks September 2001 PWE3: ATM service description draft-fischer-pwe3-atm-service-00.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. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract Generic requirements for Pseudo Wire Emulation Edge-to-Edge (PWE3) have been described in [6]. This draft 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. This basic application of ATM interworking will allow ATM service providers to take advantage of new technologies in the core in order to provide ATM multi-services. Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 Table of Contents 1 Conventions used in this document..............................3 2 Introduction...................................................3 3 Terminology....................................................4 4 General Requirements...........................................5 5 ATM Service Encapsulation......................................5 5.1 Length and Sequence Number ..................................6 5.1.1 Setting the sequence number ............................7 5.1.2 Processing the sequence number .........................7 6 ATM VCC Services...............................................8 6.1 ATM VCC Cell Transport Service ..............................8 6.1.1 ATM OAM Cell Support ..................................10 6.2 ATM VCC Frame Transport Service ............................11 6.2.1 Transparent AAL5 PDU Frame Service ....................11 6.2.1.1 OAM Cell Support ..................................13 6.2.1.2 Fragmentation .....................................13 6.2.1.2.1 Procedures in the ATM-to-MPLS Direction ......13 6.2.1.2.2 Procedures in the MPLS-to-ATM Direction ......14 6.2.2 AAL5 SDU Frame Service ................................14 6.2.3 OAM Cell Support ......................................15 7 ATM VPC Services..............................................16 7.1 ATM VPC Cell Transport Services ............................16 7.1.1 OAM Cell Support ......................................18 8 ATM Port Services.............................................19 8.1 OAM Cell Support ...........................................20 9 ILMI support..................................................20 10 QoS considerations...........................................21 11 Pseudo-Wire specific requirements............................21 11.1 MPLS requirements .........................................21 11.1.1 MPLS Transport Label .................................22 11.1.2 MPLS Pseudo Wire Label ...............................22 11.2 L2TP requirements .........................................23 11.3 IP requirements ...........................................23 12 Security Considerations......................................23 13 Intellectual Property Disclaimer.............................23 14 References...................................................23 15 Acknowledgments..............................................24 16 Authors' Addresses...........................................24 Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 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 provider's 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 [6]. Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 |<------- 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 a network using IP, MPLS or L2TP 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. 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 network devices. Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 Ingress _ 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 General Requirements For transport of an ATM service across a PSN, the PSN SHOULD be able to: 1. Carry all AAL types transparently. 2. Carry multiple ATM connections (VPCs and/or VCCs). 3. Support ATM OAM applications. 4. Transport Cell Loss Priority (CLP) and Payload Type Indicator (PTI) information from the ATM cell header. 5. Provide a mechanism to detect mis-ordering of ATM cells over the PSN. 6. Support traffic contracts and the QoS commitments made to the ATM connections (through the use of existing IETF defined Diff-Serv techniques). 5 ATM Service Encapsulation This section describes the general encapsulation format for ATM over PSN pseudo wires, such as IP, L2TP, or MPLS. The specifics pertaining to each packet technology are covered in later sections. Figure 2 provides a general format for encapsulation of ATM cells (or frames) into packets. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | [ Length and Sequence Number ] | ATM Specific | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ATM Service Payload | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: General format for ATM encapsulation over PSNs Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 The PSN Transport Header depends on the packet technology: IP, L2TP or MPLS. This header is used to transport the encapsulated ATM information through the packet switched core. This header is always present if the Pseudo Wire is MPLS. The Pseudo Wire Header depends on the packet technology: IP, L2TP or MPLS. It identifies a particular ATM service within the PSN tunnel. The Length and Sequence Number is inserted after the Pseudo Wire Header. These fields are optional. The ATM Specific Header is inserted before the ATM service payload. The ATM Specific Header certain control bits needed to carry the service. These are defined in the ATM service descriptions below. The length of ATM specific header may not always be one octet. It depends on the service type. The ATM payload octet group is the payload of the service that is being encapsulated. 5.1 Length and Sequence Number The length and sequence is not required for all services. The control word is designed to satisfy these requirements. - Sequentiality may need to be preserved. - Small packets may need to be padded in order to be transmitted on a medium where the minimum transport unit is larger than the actual packet size. The one-octet Length indicates length of the packet payload that includes Sequence number length, the ATM specific header length and the payload length (i.e., Pseudo Wire PDU). The Length field is set to 0 by the ingress PE if not used and is ignored by the egress PE. If the Pseudo Wire traverses a network link that requires a minimum frame size such as Ethernet as a practical example, with a minimum frame size of 64 octets, then such links will apply padding to the Pseudo Wire PDU to reach its minimum frame size. In this case the length field MUST be set to the PDU length. A mechanism is required for the egress PE to detect and remove such padding. The Sequence Number is a 2-octet field that may be used to track packet order delivery. This field is set to 0 by the ingress PE if not used and is ignored by the egress PE. The sequence number Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 space is a 16-bit, unsigned circular space. Processing of the sequence number field is OPTIONAL. In all cases the egress PE MUST be aware of whether the ingress PE will send the length and sequence number over a specific Pseudo Wire. This may be achieved using static configuration or using Pseudo Wire specific signaling. 5.1.1 Setting the sequence number The following procedures SHOULD be used by the ingress PE if sequencing is desired for a given ATM service: - 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. 5.1.2 Processing the sequence number If the egress PE supports receive sequence number processing, then the following procedures SHOULD 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 SHOULD 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. Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 - 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 SHOULD 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. 6 ATM VCC Services This section defines three types of ATM VCC services that may be supported over the PSN: ATM cell, ATM AAL5 PDU, and ATM AAL5 SDU. 6.1 ATM VCC Cell Transport Service The VCC cell transport service is characterized by the mapping of a single ATM VCC (VPI/VCI) to a Pseudo Wire. This service is fully transparent to the ATM Adaptation Layer. The VCC cell relay service is MANDATORY. This service MUST use the following cell mode encapsulation: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | [ Length and Sequencing Number ] |M|V|Res| PTI |C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | ATM Cell Payload ( 48 bytes ) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Single ATM VCC Cell Encapsulation * M (transport mode) bit Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 Bit (M) of the control byte indicates whether the packet contains an ATM cell or a frame payload. If set to 0, the packet contains an ATM cell. If set to 1, the PDU contains an AAL5 payload. The ability to transport an ATM cell in the AAL5 mode is intended to provide a means of enabling OAM functionality over the AAL5 VCC. * V (VCI present) bit Bit (V) of the control byte indicates whether the VCI field is present in the packet. If set to 1, the VCI field is present for the cell. If set to 0, no VCI field is present. In the case of a VCC, the VCI field is not required. For VPC, the VCI field is required and is transmitted with each cell. * Reserved bits The reserved bits should be set to 0 at the transmitter and ignored upon reception. * PTI Bits The 3-bit Payload Type Identifier (PTI) incorporates ATM Layer PTI coding of the cell. These bits are set to the value of the PTI of the encapsulated ATM cell. * C (CLP) Bit The Cell Loss Priority (CLP) field indicates CLP value of the encapsulated cell. For increased transport efficiency, the ingress PE SHOULD be able to encapsulate multiple ATM cells into a Pseudo Wire PDU. The ingress and egress PE SHOULD agree to a maximum number of cells in a single Pseudo Wire PDU. This agreement may be accomplished via a Pseudo Wire specific signaling mechanism or via static configuration. When multiple cells are encapsulated in the same PSN packet, the ATM control byte must be repeated for each cell. This implies that 49 bytes are used to encapsulate each 53 byte ATM cell. Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | [ Length and Sequencing Number ] |M|V|Res| PTI |C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | ATM Cell Payload ( 48 bytes ) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|V|Res| PTI |C| | +-+-+-+-+-+-+-+-+ | | ATM Cell Payload ( 48 bytes ) | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+ Figure 4: Multiple ATM VCC Cell Encapsulation 6.1.1 ATM OAM Cell Support When configured for a VCC cell relay service, both PE's SHOULD act as a VC switch in accordance with the OAM procedures defined in [5]. 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 - Security The ingress PE SHOULD be able to generate an F5 AIS upon reception of a corresponding F4 AIS or lower layer defect (such as LOS). The egress PE SHOULD be able to generate an F5 AIS based on a PSN failure (such as a PSN tunnel failure or LOS on the PSN port). 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. Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 Upon receiving such a notification, the egress PE SHOULD generate the appropriate F5 AIS. 6.2 ATM VCC Frame Transport Service The frame mode services were designed specifically for better transport efficiency than the cell mode. Two modes of AAL5 frame transport are available. The transparent AAL5 PDU mode is intended to be more efficient than cell mode, yet still provide full ATM transparency including the correct sequencing of OAM cells on an AAL5 flow. The payload AAL5 SDU mode is intended to provide transport efficiency than the PDU mode while foregoing some ATM transparency. It is important that the PEs be able to efficiently switch between the frame and cell modes in order to support ATM OAM functions. The agreement to transport transparent AAL5 PDUs or payload AAL5 SDUs may be accomplished via a Pseudo Wire specific signaling mechanism or via static configuration. 6.2.1 Transparent AAL5 PDU Frame Service In this mode, the ingress PE encapsulates the entire CPCS-PDU including the PAD and trailer. This mode MAY support fragmentation in order to maintain OAM cell sequencing. Like the ATM AAL5 payload VCC service, the AAL5 transparent VCC service is intended to be more efficient than the VCC cell relay service. However, the AAL5 transparent VCC service carries the entire AAL5 CPCS-PDU, including the PAD and trailer. This service supports all OAM cell flows by using a fragmentation procedure that ensures that OAM cells are not repositioned in respect to AAL5 composite cells. The AAL5 transparent VCC service is OPTIONAL. Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | [ Length and Sequencing Number ] |M|V|Res|Frg|E|C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + " | | AAL5 CPCS-PDU | | (n * 48 bytes) | | " | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: AAL5 transparent service encapsulation The first octet following the Pseudo Wire Header carries control information. The M, V, Res, and C bits are as defined earlier for VCC cell mode. * Frg (Fragmentation) Bits This field is used to support the fragmentation functionality described later in this section. - 11 Single Segment Message (Beginning and End of Message) - 10 Beginning of Message - 00 Continuation of Message - 01 End of Message * E (EFCI) bit This field is used to convey the EFCI state of the ATM cells. The EFCI state is indicated in the middle bit of each ATM cell's PTI field. ATM-to-PSN direction: The EFCI field of the control byte is set to the EFCI state of the last cell of the AAL5 PDU or AAL5 fragment. PSN-to-ATM direction: The EFCI state of all constituent cells of the AAL5 PDU or AAL5 fragment is set to the value of the EFCI field in the control byte. The payload consists of the re-assembled AAL5 CPCS-PDU, including the AAL5 padding and trailer or the AAL5 fragment. Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 6.2.1.1 OAM Cell Support When configured for the AAL5 transparent VCC service, both PE's SHOULD act as a VC switch, in accordance with the OAM procedures defined in [5]. 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 - Security The ingress PE SHOULD be able to generate an F5 AIS upon reception of a corresponding F4 AIS or lower layer defect (such as LOS). The egress PE SHOULD be able to generate an F5 AIS based on a PSN failure (such as a PSN tunnel failure or LOS on the PSN port). 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. 6.2.1.2 Fragmentation The ingress PE may not always be able to reassemble a full AAL5 frame. This may be due to the AAL5 PDU exceeding the Pseudo Wire MTU or when OAM cells arrive during reassembly of the AAL5 PDU. In these cases, the AAL5 PDU shall be fragmented. In addition, fragmentation may be desirable to bound ATM cell delay. If no fragmentation occurs, then the fragmentation bits are set to 11 (SSM, Single Segment Message). When fragmentation occurs, the procedures described in the following subsections shall be followed. 6.2.1.2.1 Procedures in the ATM-to-MPLS Direction The following procedures shall apply while fragmenting AAL5 PDUs: Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 - Fragmentation shall always occur at cell boundaries within the AAL5 PDU. - For the first fragment, the FRG bits shall be set to 10 (BOM, Beginning Of Message). - For the last fragment, the FRG bits shall be set to 01 (EOM, End Of Message). - For all other fragments, the FRG bits shall be set to 00 (COM, Continuation Of Message). - The E and C bits shall be set as defined easrlier in section 6. 6.2.1.2.2 Procedures in the MPLS-to-ATM Direction The following procedures shall apply: - The 3-bit PTI field of each ATM cell header is constructed as follows: + The most significant bit is set to 0, indicating a user data cell. + The middle bit is set to the E bit value of the fragment. + The least significant bit is set to 1 for the last ATM cell of a fragment where the FRG bits are 01 (EOM) or 11(SSM); otherwise this bit is set to 0. - The C bit of each ATM cell header is set to the value of C bit the control byte. 6.2.2 AAL5 SDU Frame Service The AAL5 payload VCC service defines a mapping between the payload of an AAL5 VCC and a single Pseudo Wire. This service is OPTIONAL. The AAL5 payload VCC service requires ATM segmentation and reassembly support on the PE. Once 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. Although any AAL5 PDU may be transported using the VCC cell relay service and cell mode encapsulation, the AAL5 payload VCC service is designed for better transport efficiency. 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 Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 are sent immediately on the Pseudo Wire, followed by the AAL5 payload. Therefore, the AAL5 payload VCC service may not be suitable for some ATM applications that require strict ordering of OAM cells (such as performance monitoring and security applications). 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PSN Transport Header (As Required) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Pseudo Wire Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | [ Length and Sequencing Number ] |M|V|R|U|Frg|E|C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | " | | AAL5 SDU payload | | " | | " | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: AAL5 payload service encapsulation The first octet following the Pseudo Wire Header carries control information. The M, V, R (reserved), and C bits are as defined earlier for VCC cell mode. Since fragmentation is not required, the fragmentation bits are set to 11 to indicate a complete frame. * U (UU Octet Command/Response) Bit When FRF.8.1 Frame Relay / ATM PVC Service Interworking 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 C bit of the control byte. The egress PE device SHOULD copy the C bit to the CPCS-UU Least Significant Bit (LSB) of the AAL5 payload. 6.2.3 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 [5]. Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 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 - Continuity Check 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 or lower layer defect (such as LOS). The egress PE SHOULD be able to generate an F5 AIS based on a PSN failure (such as a PSN tunnel failure or LOS on the PSN port). 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 ATM VPC Services The VPC service is defined by mapping a single VPC (VPI) to a Pseudo Wire. As such it emulates as Virtual Path cross-connect across the PSN. All VCCs belonging to the VPC are carried transparently by the VPC service. The egress PE may choose to apply a different VPI other than the one that arrived at the ingress PE. The egress PE MUST choose the outgoing VPI based solely upon the Pseudo Wire header. As a VPC service, the egress PE MUST NOT change the VCI field. 7.1 ATM VPC Cell Transport Services The ATM VPC cell transport service is OPTIONAL. This service MUST use the following cell mode encapsulation: Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | [ Length and Sequencing Number ] |M|V|Res| PTI |C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VCI | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | ATM Cell Payload ( 48 bytes ) | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: Single Cell VPC Encapsulation The ATM control byte contains the same information as in the VCC encapsulation except for the VCI field. * VCI Bits The 16-bit Virtual Circuit Identifier (VCI) incorporates ATM Layer VCI value of the cell. For increased transport efficiency, the ingress PE SHOULD be able to encapsulate multiple ATM cells into a Pseudo Wire PDU. The ingress and egress PE SHOULD agree to a maximum number of cells in a single Pseudo Wire PDU. This agreement may be accomplished via a Pseudo Wire specific signaling mechanism or via static configuration. When multiple ATM cells are encapsulated in the same PSN packet, the ATM control byte must be repeated for each cell. This implies that 51 bytes are used to encapsulate each 53 byte ATM cell. Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | [ Length and Sequencing Number ] |M|V|Res| PTI |C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VCI | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | ATM Cell Payload ( 48 bytes ) | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |M|V|Res| PTI |C| VCI | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VCI | | +-+-+-+-+-+-+-+-+ | | ATM Cell Payload ( 48 bytes ) | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+ Figure 8: Multiple Cell VPC Encapsulation 7.1.1 OAM Cell Support When configured for a VPC cell relay service, both PE's SHOULD act as a VP cross-connect in accordance with the OAM procedures defined in [5]. The PEs SHOULD be able to pass the following 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 - Security The ingress PE MUST be able to generate an F4 AIS upon reception of a lower layer defect (such as LOS). Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 The egress PE SHOULD be able to generate an F4 AIS based on a PSN failure (such as a PSN tunnel failure or LOS on the PSN port). 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. 8 ATM Port Services This mode is not required based on the PWE3 requirements draft [6]. Other design teams (eg Frame Relay) are not considering this mode of operation. If it is required, the Requirements Draft should be updated and the following encapsulation may be considered. Transparent port encapsulation is used to emulate an ATM Port-to- Port connection over a PSN. This service is useful when one desires to connect two CEs without interfering at the VPC or VCC layer. The ingress PE SHOULD discard any idle/unassigned cells received from the ingress ATM port, and map all other received cells to a single Pseudo Wire. A mechanism to discard idle/unassigned cells received from the ATM port by PE is for further study. The egress PE SHOULD not change the VPI, VCI, PTI, or CLP bits when it sends these cells on the egress ATM port. This service SHOULD appear as a Layer 1 service (such as SONET/SDH) to CE devices. However, the service provider may benefit from increased transport efficiency due to statistical multiplexing. The transparent port cell relay service is OPTIONAL. It uses the encapsulation defined below. The agreement to transport this type of packet may be accomplished via a Pseudo Wire specific signaling mechanism or via static configuration For increase transport efficiency, the ingress PE SHOULD be able to encapsulate multiple ATM cells into a Pseudo Wire PDU. The ingress and egress PE SHOULD agree to a maximum number of cells in a single Pseudo Wire PDU. This agreement may be accomplished via a Pseudo Wire specific signaling mechanism or via static configuration. Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | [ Length and Sequencing Number ] | Rsvd | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VPI | VCI | PTI |C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | " | | ATM Payload ( 48 bytes ) | | " | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VPI | VCI | PTI |C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | " | | ATM Payload ( 48 bytes ) | | " | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: Port-to-Port mode encapsulation 8.1 OAM Cell Support This service is completely transparent to the F4 and F5 OAM layer. The PEs MUST pass all OAM and resource management cells. If the ingress PE detects a physical layer defect (such as LOS) it SHOULD be able to notify the egress PE via a mechanism specific to the Pseudo Wire in use. When it receives such a notification, the egress PE SHOULD propagate the failure (such as sending a SONET Line AIS). 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 physical layer AIS. 9 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 Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 detection. ILMI messages are sent as SNMP PDU's 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. 10 QoS considerations TBD. 11 Pseudo-Wire specific requirements 11.1 MPLS requirements Figure 10 provides a general format for interworking between ATM and MPLS. The Pseudo Wire Endpoint uses a unique MPLS label, the pseudo wire label, to identify each direction of an ATM connection. This label allows the PWE to access context information for the connection. As an example, the context may contain the information regarding connection type such as VCC or VPC or VPI/VCI value that need to be inserted into the ATM cell header in the MPLS-to-ATM direction. Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MPLS Transport Label | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MPLS Pseudo Wire Label | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | [ Length and Sequence Number ] | ATM Specific | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ATM Service Payload | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: Format for MPLS PSNs 11.1.1 MPLS Transport Label The 4-octet MPLS transport label identifies a LSP used to transport traffic between two ATM-MPLS pseudo wire endpoints. This label is used to switch the transport LSP between core LSRs. 11.1.2 MPLS Pseudo Wire Label The 4-octet interworking label uniquely identifies one pseudo wire LSP, carried inside a MPLS transport LSP. The pseudo wire label has the structure of a standard MPLS shim header. More than one pseudo wire LSP may be supported by one MPLS transport LSP. The pseudo wire endpoint provides the association between the ATM connection and MPLS LSP by means of the 20-bit field of the pseudo wire LSP. In this association, in the ATM-to-MPLS direction a translation of VCI/VPI to the 20-bit label is performed, while in the MPLS-to-ATM direction the 20-bit label is translated to VCI/VPI of the ATM connection. This association is signalled or provisioned between the two pseudo wire endpoints. Since MPLS LSP is unidirectional, for the case of bi-directional ATM VCCs, there will be two different pseudo wire LSPs, one for each direction of the connection. These may have different label values. Setting of the EXP and TTL is for further study. The S bit is set to 1 since this is the last label in the bottom of the MPLS stack. The pseudo wire label is not visible to the LSRs within the MPLS core network. Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 11.2 L2TP requirements TBD. 11.3 IP requirements TBD. 12 Security Considerations This draft does not introduce any new security considerations to IP, L2TP or MPLS. 13 Intellectual Property Disclaimer This document is being submitted for use in IETF standards discussions. 14 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] "Frame Relay / ATM PVC Service Interworking Implementation Agreement (FRF.8.1)", Frame Relay Forum 2000. [4] "Frame Based ATM over SONET/SDH Transport (FAST)," ATM Forum 2000. [5] ITU-T Recommendation I.610 (February 1999): B-ISDN operation and maintenance principles and functions [6] IETF draft-ietf-pwe3-requirements-00.txt, work in progress, (17 May 2001): Requirements for pseudo Wire Emulation Edge-to- Edge (PWE3) [7] "Encapsulation Methods for Transport of Layer 2 Frames Over MPLS", draft-martini-l2circuit-encap-mpls-03.txt, Work in Progress [8] "Requirements and framework for ATM network interworking over MPLS", draft-koleyni-pwe3-atm-over-mpls-02.txt, Work in Progress Internet Draft draft-fischer-pwe3-atm-service-00.txt Sept 2001 15 Acknowledgments 16 Authors' Addresses John Fischer Alcatel 600 March Rd Kanata, ON, Canada. K2K 2E6 Email: john.fischer@alcatel.com Matthew Bocci Alcatel Voyager Place, Shoppenhangers Rd Maidenhead, Berks, UK SL6 2PJ Email: matthew.bocci@alcatel.co.uk Mina Azad Nortel Networks P O Box 3511, Station C Ottawa, ON, Canada K1Y 4H7 Email: mazad@nortelnetworks.com Ghassem Koleyni Nortel Networks P O Box 3511, Station C Ottawa, Ontario, K1Y 4H7 Canada Email: ghassem@nortelnetworks.com Adeel A. Siddiqui Cable & Wireless 11700 Plaza America Drive Reston, Virginia 20190, USA Email: adeel.siddiqui@cwusa.com