Network Working Group Luca Martini Internet Draft Level 3 Communications, LLC. Expiration Date: August 2003 Jeremy Brayley Matthew Bocci Laurel Networks, Inc. Alcatel Eric C. Rosen Ghassem Koleyni Cisco Systems, Inc. Nortel Networks. February 2003 Encapsulation Methods for Transport of ATM Cells/Frame Over IP and MPLS Networks draft-ietf-pwe3-atm-encap-01.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 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. Martini, et al. [Page 1] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 Table of Contents 1 Specification of Requirements .......................... 3 2 Introduction ........................................... 3 3 Terminology ............................................ 4 4 General encapsulation method ........................... 5 4.1 The Control Word ....................................... 5 4.1.1 Setting the sequence number ............................ 7 4.1.2 Processing the sequence number ......................... 7 4.2 MTU Requirements ....................................... 8 4.3 ATM OAM Cell Support ................................... 8 5 ATM .................................................... 9 6 ATM one-to-one Cell Mode ............................... 9 6.1 Applicability .......................................... 9 6.2 Implementation and deployment considerations ........... 10 6.3 Limitations ............................................ 11 6.4 ATM one-to-one Service Encapsulation ................... 11 6.5 Length and Sequence Number ............................. 12 6.5.1 Setting the length field ............................... 13 6.5.2 Processing the length field ............................ 13 6.6 ATM VCC Cell Transport Service ......................... 13 6.7 ATM VPC Services ....................................... 15 6.7.1 ATM VPC Cell Transport Services ........................ 15 6.7.2 OAM Cell Support ....................................... 16 7 ATM n-to-one Cell Mode ................................. 17 7.1 ATM n-to-one Service Encapsulation ..................... 17 7.2 CLP bit to Quality of Service mapping .................. 20 7.3 Applicability Statement for n-to-one mode .............. 20 7.4 Review of header information ........................... 21 7.5 MPLS Shim S Bit Value .................................. 22 7.6 MPLS Shim TTL Values ................................... 22 8 ATM AAL5 CPCS-SDU Mode ................................. 22 8.1 Applicability Statement ................................ 22 8.2 Transparent AAL5 SDU Frame Encapsulation ............... 23 9 AAL5 PDU frame mode .................................... 24 9.1 Applicability .......................................... 25 9.1.1 Implementation and deployment considerations ........... 26 9.1.2 Limitations ............................................ 26 9.2 Transparent AAL5 PDU Frame encapsulation ............... 27 9.3 Fragmentation .......................................... 28 9.3.1 Procedures in the ATM-to-PSN Direction ................. 29 9.3.2 Procedures in the PSN-to-ATM Direction ................. 29 10 Security Considerations ................................ 29 11 Intellectual Property Disclaimer ....................... 30 12 References ............................................. 30 13 Author Information ..................................... 30 Martini, et al. [Page 2] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 1. Specification of Requirements The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 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 providers 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: - Leveraging of the existing systems and services to provide increased capacity from a packet switched core. - Preserving existing network operational processes and procedures used to maintain the legacy services. - 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. [Page 3] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 |<----- Pseudo Wire ---->| | | | |<-- PSN Tunnel -->| | ATM Service V V V V ATM Service | +----+ +----+ | +----+ | | PE1|==================| PE2| | +----+ | |----------|............PW1.............|----------| | | CE1| | | | | | | |CE2 | | |----------|............PW2.............|----------| | +----+ | | |==================| | | +----+ ^ +----+ +----+ | ^ | Provider Edge 1 Provider Edge 2 | | | |<-------------- Emulated Service ---------------->| Customer Customer Edge 1 Edge 2 Figure 1: ATM Service Reference Model QoS related issues are not discussed in this draft. This draft describes two methods of ATM cell encapsulation, one-to-one mode and n-to-one mode. This draft describes two methods of AAL5 encapsulation, PDU mode and SDU mode. 3. Terminology One-to-one mode: The One-to-one mode specifies an encapsulation method which maps one ATM VCC (or one ATM VPC) to one PSN Tunnel. N-to-one mode (N >= 1): The N-to-one mode specifies an encapsulation method which maps one or more ATM VCCs (or one or more ATM VPCs) to one PSN tunnel. 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. Martini, et al. [Page 4] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 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 a PW from the PSN, re-converted back in the emulated service, and sent out to a CE. 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.) CTD Cell Transfer Delay MTU Maximum Transfer Unit OAM Operations, Administration, and Maintenance. PVC Permanent Virtual Connection. An ATM connection that is provisioned via a network management interface. The connection is not signalled. VCC Virtual Circuit Connection. An ATM connection that is switched based on the cell header's VCI. VPC Virtual Path Connection. An ATM connection that is switched based on the cell header's VPI. 4. General encapsulation method 4.1. The Control Word There are three requirements that may need to be satisfied when transporting layer 2 protocols over an IP or MPLS backbone: -i. Sequentiality may need to be preserved. -ii. 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. -iii. Control bits carried in the header of the layer 2 frame may need to be transported. The control word defined here addresses all three of these Martini, et al. [Page 5] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 requirements. For some protocols this word is REQUIRED, and for others OPTIONAL. For protocols where the control word is OPTIONAL implementations MUST support sending no control word, and MAY support sending a control word. In all cases the egress router must be aware of whether the ingress router will send a control word over a specific virtual circuit. This may be achieved by configuration of the routers, or by signaling, for example as defined in [1]. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Rsvd | Flags |Res| Length | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ In the above diagram the first 4 bits are reserved for future use. They MUST be set to 0 when transmitting, and MUST be ignored upon receipt. The next 4 bits provide space for carrying protocol specific flags. These are defined in the protocol-specific details below. These bits are reserved and MUST be set to 0 upon transmission and ignored upon reception. The next 6 bits provide a length field, which is used as follows: If the packet's length (defined as the length of the layer 2 payload plus the length of the control word) is less than 64 bytes, the length field MUST be set to the packet's length. Otherwise the length field MUST be set to zero. The value of the length field, if non- zero, can be used to remove any padding. When the packet reaches the service provider's egress router, it may be desirable to remove the padding before forwarding the packet. 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. Martini, et al. [Page 6] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 4.1.1. Setting the sequence number For a given emulated VC, and a pair of routers PE1 and PE2, if PE1 supports packet sequencing then the following procedures should be used: - the initial packet transmitted on the emulated VC MUST use sequence number 1 - subsequent packets MUST increment the sequence number by one for each packet - when the transmit sequence number reaches the maximum 16 bit value (65535) the sequence number MUST wrap to 1 If the transmitting router PE1 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 a router PE2 supports receive sequence number processing, then the following procedures should be used: When an emulated VC is initially set up, the "expected sequence number" associated with it MUST be initialized to 1. When a packet is received on that emulated VC, the sequence number should be processed as follows: - if the sequence number on the packet is 0, then the packet passes the sequence number check - otherwise if the packet sequence number >= the expected sequence number and the packet sequence number - the expected sequence number < 32768, then the packet is in order. - otherwise if the packet sequence number < the expected sequence number and the expected sequence number - the packet sequence number >= 32768, then the packet is in order. - otherwise the packet is out of order. If a packet passes the sequence number check, or is in order then, it can be delivered immediately. If the packet is in order, then the expected sequence number should be set using the algorithm: expected_sequence_number := packet_sequence_number + 1 mod 2**16 if (expected_sequence_number = 0) then expected_sequence_number := 1; Martini, et al. [Page 7] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 Packets which are received out of order MAY be dropped or reordered at the discretion of the receiver. If a router PE2 does not support receive sequence number processing, then the sequence number field MAY be ignored. 4.2. MTU Requirements The network MUST be configured with an MTU that is sufficient to transport the largest encapsulation frames. If MPLS is used as the tunneling protocol, for example, this is likely to be 12 or more bytes greater than the largest frame size. Other tunneling protocols may have longer headers and require larger MTUs. If the ingress router determines that an encapsulated layer 2 PDU exceeds the MTU of the tunnel through which it must be sent, the PDU MUST be dropped. If an egress router receives an encapsulated layer 2 PDU whose payload length (i.e., the length of the PDU itself without any of the encapsulation headers), exceeds the MTU of the destination layer 2 interface, the PDU MUST be dropped. 4.3. ATM OAM Cell Support In general when configured for ATM service, both PE's SHOULD act as a VC switch, in accordance with the OAM procedures defined in [7]. 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 PEs SHALL use the single ATM VCC cell mode encapsulation when passing an OAM cell. 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 Martini, et al. [Page 8] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 AIS. 5. ATM This Draft defines two methods for encapsulation of ATM cells, namely, One-to-one mode and N-to-one mode. The One-to-one mode specifies an encapsulation method that maps one ATM VCC or one ATM VPC to one Pseudo-Wire. For VCCs, the VPI/VCI is not included. For VPCs, the VPI is not included. Cells from one VCC or one VPC may be concatenated. The N-to-one mode (N >= 1) specifies an encapsulation method that maps one or more ATM VCCs (or one or more ATM VPCs) to one Pseudo- Wire. One format is used for both the VCC or VPC mapping to the tunnel. The 4-octet ATM header is unaltered in the encapsulation, thus the VPI/VCI is always present. Cells from one or more VCCs (or one or more VPCs) may be concatenated. Furthermore different encapsulations are supported for ATM AAL5 transport: one for ATM AAL5 SDUs, and and another for ATM AAL5 PDUs. 6. ATM one-to-one Cell Mode The One-to-one mode described in this Draft allows a service provider to offer an ATM PVC or SVC based service across a network. The encapsulation allows one ATM VCC or VPC to be carried within a single Pseudo-Wire. 6.1. Applicability The primary application of one-to-one ATM cell encapsulation over PSN is the transparent carriage of ATM layer services over a PSN. An ATM layer service is the transfer of ATM cells over a VCC or a VPC between communicating upper layer entities. The nature of the service, as defined by the ATM service category [5] or ATM transfer capability [6], should be preserved. To provide this, the basic requirement of the ATM-PSN interworking function is to map the ATM cells belonging to either VCC or VPC, together with any related OAM and protocol control information into a PW. Two network applications that utilize the cell mode encapsulation are: Martini, et al. [Page 9] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 a. The transport of multiservice ATM over a packet core network. 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 benefits of this model to a service provider are threefold: -i. Leveraging of the existing systems and services to provide increased capacity from a packet switched core. -ii. Preserving existing network operational processes and procedures used to maintain the legacy services. -iii. 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. b. L2 VPN service over a PSN infrastructure. In this case, VPN sites are connected through ATM VCCs or VPCs, as in today's L2 VPNs. The basic cell encapsulation allows the VPN service provider to transparently extend this L2 connectivity over its PSN while still providing the contracted SLA with the VPN customer. The advantage is for the service provider to combine L2 and L3 services over the same PSN. Figure 1 shows the reference model for carrying ATM services over a PSN. An ATM VCC or VPC is carried over a PW. The PW corresponding to any VCC or VPC may be further tunneled in a transport PSN tunnel to achieve multiplexing gain and bandwidth efficiency. ATM over PSN service provides end users with the same quality of service on any given VPC or VCC as per the QoS commitments in the ATM service traffic contract. Concatenation of ATM cells belonging to a VCC or a VPC provides added bandwidth efficiency while preserving the specific information (CLP/PTI) of each cell. 6.2. Implementation and deployment considerations Although the Single ATM cell encapsulation provides the simplest way for encapsulating ATM cells within a single MPLS packet, it lacks bandwidth efficiency. This can be improved substantially by the use of the procedures enabling cells from any given VCC or VPC to be concatenated within the corresponding ATM PW. Martini, et al. [Page 10] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 6.3. Limitations Cell encapsulation only supports point-to-point LSPs. Multi-point- to-point and point-to-multi-point are for further study (FFS). When PSN is MPLS network, to have bi-directional connectivity, as required in ATM, two LSPs should be configured, one for each direction (ATM-to-MPLS and MPLS-to-ATM) of the ATM connection. The number of concatenated ATM cells is limited by the MTU (Maximum Transfer Unit) size and the cell transfer delay (CTD) and cell delay variation (CDV) objectives. 6.4. ATM one-to-one 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 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Optional Length and Sequence Number | ATM Specific | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ATM Service Payload | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: General format for one-to-one mode encapsulation over PSNs 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. This field is optional. The ATM Specific Header is inserted before the ATM service payload. Martini, et al. [Page 11] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 The ATM Specific Header contains 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. 6.5. Length and Sequence Number The length and sequence number are not required for all services. Length and sequence number are 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, length of the length field, 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 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. Treatment of the sequence number is according to previous sections "Setting the sequence number", and "Processing the sequence number". Length field is not required for the cell mode. Martini, et al. [Page 12] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 6.5.1. Setting the length field All cell transport services MUST always set the length field to 0 to indicate to the remote PE that no padding was applied. 6.5.2. Processing the length field Since length field is not used for cell mode, no processing is required. 6.6. 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 single cell transport service is OPTIONAL. This service MUST use the following encapsulation format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PSN Transport Header (As Required) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Pseudo Wire Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Optional Length and Sequence Number |M|V|Res| PTI |C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | ATM Cell Payload ( 48 bytes ) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Single ATM VCC Cell Encapsulation * M (transport mode) bit 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. * 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. Martini, et al. [Page 13] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 * 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 means that 49 bytes are used to encapsulate each 53 byte ATM cell. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Optional Length and Sequence 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 Martini, et al. [Page 14] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 6.7. 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. 6.7.1. ATM VPC Cell Transport Services The ATM VPC cell transport service is OPTIONAL. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Optional Length and Sequence Number |M|V|Res| PTI |C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VCI | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | ATM Cell Payload ( 48 bytes ) | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: 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 Martini, et al. [Page 15] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 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 means that 51 bytes are used to encapsulate each 53 byte ATM cell. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Optional Length and Sequence 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 6: Multiple Cell VPC Encapsulation 6.7.2. 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 [7]. The PEs MUST be able to pass the following OAM cells transparently: Martini, et al. [Page 16] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 - 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 PEs SHALL use the ATM VPC one-to-one cell encapsulation when passing an OAM cell. The OAM cell MAY be encapsulated together with other user data cells if multiple cell encapsulation is used. The ingress PE MUST be able to generate an F4 AIS upon reception of a lower layer defect (such as LOS). 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. 7. ATM n-to-one Cell Mode The N-to-one mode (N >= 1) described in this Draft allows a service provider to offer an ATM PVC or SVC based service across a network. The encapsulation allows multiple ATM VCCs or VPCs to be carried within a single PSN tunnel. A service provider may also use N-to-one mode to provision either one VCC or one VPC on a tunnel. This section defines the VCC and VPC cell relay services over a PSN and their applicability. 7.1. ATM n-to-one Service Encapsulation This section describes the general encapsulation format for ATM over PSN pseudo wires. Martini, et al. [Page 17] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 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 7: 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. In this encapsulation mode ATM cells are transported individually. This is the only REQUIRED encapsulation for ATM. The ATM cell encapsulation consists of an OPTIONAL control word, and one or more ATM cells - each consisting of a 4 byte ATM cell header and the 48 byte ATM cell payload. This ATM cell header is defined as in the FAST encapsulation [4] section 3.1.1, but without the trailer byte. The length of each frame, without the encapsulation headers, is a multiple of 52 bytes long. The maximum number of ATM cells that can be fitted in a frame, in this fashion, is limited only by the network MTU and by the ability of the egress router to process them. The ingress router MUST NOT send more cells than the egress router is willing to receive. The number of cells that the egress router is willing to receive may either be configured in the ingress router or may be signaled, for example using the methods described in [1]. The number of cells encapsulated in a particular frame can be inferred by the frame length. The control word is OPTIONAL. If the control word is used then the flag bits in the control word are not used, and MUST be set to 0 when transmitting, and MUST be ignored upon receipt. The EFCI and CLP bits are carried across the network in the ATM cell Martini, et al. [Page 18] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 header. The edge routers that implement this document MAY, when either adding or removing the encapsulation described herein, change the EFCI bit from zero to one in order to reflect congestion in the network that is known to the edge router, and the CLP bit from zero to one to reflect marking from edge policing of the ATM Sustained Cell Rate. The EFCI and CLP bits SHOULD NOT be changed from one to zero. This diagram illustrates an encapsulation of two ATM cells: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Control word ( Optional ) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VPI | VCI | PTI |C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ATM Payload ( 48 bytes ) | | " | | " | | " | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VPI | VCI | PTI |C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ATM Payload ( 48 bytes ) | | " | | " | | " | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: Multiple Cell ATM Encapsulation * When multiple VCCs or VPCs are transported in one pseudo-wire VPI/VCI values MUST be unique. When the multiple VCCs or VPCs, are from different a physical transmission path it may be necessary to assign unique VPI/VCI values to the ATM connections. If they are from the same physical transmission path, the VPI/VCI values are unique. * VPI The ingress router MUST copy the VPI field from the incoming cell into this field. For particular emulated VCs, the egress router MAY generate a new VPI and ignore the VPI contained in this field. Martini, et al. [Page 19] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 * VCI The ingress router MUST copy the VCI field from the incoming ATM cell header into this field. For particular emulated VCs, the egress router MAY generate a new VCI. * PTI & CLP ( C bit ) The PTI and CLP fields are the PTI and CLP fields of the incoming ATM cells. The cell headers of the cells within the packet are the ATM headers (without HEC) of the incoming cell. 7.2. CLP bit to Quality of Service mapping The ingress router MAY consider the CLP bit when determining the value to be placed in the Quality of Service fields (e.g. the EXP fields of the MPLS label stack) of the encapsulating protocol. This gives the network visibility of the CLP bit. Note however that cells from the same VC MUST NOT be reordered. 7.3. Applicability Statement for n-to-one mode The N-to-one cell relay encapsulation described in this document allows a service provider to offer a PVC/PVP or SVC/SVP based VCC/VPC cell relay service across an IP or MPLS PSN. The encapsulation allows multiple VCCs/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 VCC to a PSN tunnel in order to satisfy QoS or restoration requirements. The encapsulation also supports the binding of multiple VCCs/VPCs 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/VPC services. The VCC/VPC cell relay service has the following attributes: -i. Supports all ATM Adaptation Layers Types. -ii. 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. -iii. In order to gain transport efficiency on the PSN, multiple cells may be encapsulated in a single PW PDU. This process is called cell concatenation . How many cells to insert or how long to wait for cell arrival before sending a PW PDU is Martini, et al. [Page 20] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 an implementation decision. Cell concatenation introduces latency to a cell relay service. -iv. 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: -i. 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. -ii. 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. -iii. There is no currently defined method to support EPD/PPD on the PSN. -iv. 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. 7.4. 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, 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. Similar mechanisms can be used to map ATM header information to other type of PSN tunnel PDU headers. Martini, et al. [Page 21] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 7.5. MPLS Shim S Bit Value The ingress LSR, PE1, MUST set the S bit of the VC label to a value of 1 to denote that the VC label is at the bottom of the stack. 7.6. MPLS Shim TTL Values The ingress LSR, PE1, SHOULD set the TTL field of the VC label to a value of 2. 8. ATM AAL5 CPCS-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 CPCS-SDU 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). 8.1. 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 Martini, et al. [Page 22] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 between ATM attached routers. 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. 8.2. Transparent AAL5 SDU Frame Encapsulation The AAL5 CPCS-SDU is prepended by the following header: 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 9: AAL5 CPCS-SDU 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 Martini, et al. [Page 23] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 cell relay encapsulation, figure 8. If not set, the PDU contains an AAL5 payload. The ability to transport an ATM cell in the AAL5 SDU 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 router, PE1, SHOULD set this bit to 1 if the EFCI bit of the final cell of those that transported the AAL5 CPCS-SDU 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 router, PE2, SHOULD set the EFCI bit of all cells that transport the AAL5 CPCS-SDU to the value contained in this field. * C ( CLP ) Bit The ingress router, PE1, SHOULD set this bit to 1 if the CLP bit of any of the ATM cells that transported the AAL5 CPCS-SDU is set to 1, or if the CLP 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 router, PE2, SHOULD set the CLP bit of all cells that transport the AAL5 CPCS-SDU to the value contained in this field. * U ( Command / Response Field ) Bit When FRF.8.1 Frame Relay / ATM PVC Service Interworking [3] 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 router, PE1, SHOULD copy this bit to the U bit of the control word. The egress router, PE2, SHOULD copy the U bit to the CPCS-UU Least Significant Bit (LSB) of the AAL5 CPCS PDU. 9. AAL5 PDU frame mode The AAL5 payload PDU service is OPTIONAL. Martini, et al. [Page 24] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 9.1. Applicability The primary application supported by AAL5 PDU frame encapsulation over PSN is the transparent carriage of ATM layer services that use AAL5 to carry higher layer frames. The PDU frame mode takes advantage of the delineation of higher layer frames in the ATM layer to provide increased bandwidth efficiency compared with the basic cell encapsulation mode. The nature of the service, as defined by the ATM service category or the ATM transfer capability should be preserved. To provide this, the basic requirement of the ATM-PSN interworking function is to map the AAL5 PDU frames belonging to a VCC, together with any related OAM and protocol control information, into a PW. Two network applications that utilize the PDU frame mode encapsulation are: a The transport of multi-service ATM over a packet core network where AAL5 is used as the adaptation layer. 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 benefits of this model to a service provider are threefold: -i. Leveraging of the existing systems and services to provide increased capacity from a packet switched core. -ii. Preserving existing network operational processes and procedures used to maintain the legacy services e.g. ATM OAM and ATM security. -iii. 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. b L2 VPN service over a PSN infrastructure. In this case, VPN sites are connected through ATM VCCs, as in today's L2 VPNs. The transparent PDU frame mode encapsulation allows the VPN service provider to transparently extend this L2 connectivity over its PSN while achieving bandwidth efficiency gains over the basic cell mode and supporting ATM layer applications of the VPN customer, such as ATM security. The advantage is for the service provider to combine L2 and L3 services over the same PSN. One important consideration to make when interworking is to allow OAM information to be treated as in the original network. The interworking function allows this transparency while performing Martini, et al. [Page 25] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 AAL5 frame encapsulation. Fragmentation may be performed in order to maintain the position of the OAM cells with respect to the user cells. Fragmentation may also be performed to maintain the size of the packet carrying the AAL5 PDU within the MTU of the link. Cell Loss priority (CLP) field conveys the priority of the cell in the connection. The Explicit Forward Congestion Indicator (EFCI) field conveys the congestion state of ATM network. Information on both of these fields is obtained from the ATM cell header. CLP and EFCI fields are both part of the ATM service specific information header. The whole AAL5-PDU is encapsulated. In this case all necessary parameters such as CPCS-UU (CPCS User-to-User indicator), CPI (Common Part Indicator), Length (Length of the CPCS-SDU) and CRC (Cyclic Redundancy Check) are transported as part of the payload. Note that carrying of the full PDU also allows the simplification of the fragmentation operation since it is performed at cell boundaries and the CRC in the trailer of the AAL5 PDU can be used to check the integrity of the PDU. 9.1.1. Implementation and deployment considerations AAL5 transparent mode is only applicable to services that use AAL5 to carry higher layer frames over ATM VCCs. 9.1.2. Limitations AAL5 frame encapsulation only supports point-to-point LSPs. Multi- point-to-point and point-to-multi-point are for further study (FFS). Length of AAL5 frame may exceed the MTU of the PSN. This requires fragmentation, which may not be available to all nodes at the PW endpoint. The maximum number of cells of an AAL5 PDU that may be reassembled before transport across the PSN may be limited by the cell transfers delay (CTD) and cell delay variation (CDV) objectives of the connection. This mode does not preserve the value of the CLP bit for every ATM cell within an AAL5 PDU. Therefore, transparency of the CLP setting may be violated. Additionally, tagging of some cells may occur when tagging is not allowed by the conformance definition. Martini, et al. [Page 26] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 This mode does not preserve the EFCI state for every ATM cell within an AAL5 PDU. Therefore, transparency of the EFCI state may be violated. 9.2. Transparent AAL5 PDU Frame encapsulation 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 transport service. However, the AAL5 transparent VCC service carries the entire AAL5 CPCS-PDU, including the PAD and trailer. Note that the AAL5 CPCS-PDU is not processed _ i.e. an AAL5 frame with an invalid CRC or length field will be transported. One reason for this is that there may be a security agent that has scrambled the ATM cell payloads that form the AAL5 CPCS-PDU. 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. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Optional Length and Sequence Number |M|V| Res |U|E|C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + " | | AAL5 CPCS-PDU | | (n * 48 bytes) | | " | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: 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 one-to-one cell mode. Martini, et al. [Page 27] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 * U Bit This field indicates whether this frame contains the last cell of an AAL5 PDU and represents the value of the ATM User-to-User bit for the last ATM cell of the PSN frame. Note: The ATM User-to- User bit is the least significant bit of the PTI field in the ATM header. This field is used to support the fragmentation functionality described later in this section. + * 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 (ingress): 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 (egress): 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. * C (CLP) bit This field is used to convey the cell loss priority of the ATM cells. ATM-to-PSN direction (ingress): The CLP field of the control byte is set to 1 if any of the constituent cells of the AAL5 PDU or AAL5 fragment has its CLP bit set to 1; otherwise this field is set to 0. PSN-to-ATM direction (egress): The CLP bit of all constituent cells for an AAL5 PDU or AAL5 fragment is set to the value of the CLP 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. 9.3. 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. Martini, et al. [Page 28] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 When fragmentation occurs, the procedures described in the following subsections shall be followed. 9.3.1. Procedures in the ATM-to-PSN Direction The following procedures shall apply while fragmenting AAL5 PDUs: - Fragmentation shall always occur at cell boundaries within the AAL5 PDU. - Set the UU bit to the value of the ATM User-to-User bit in the cell header of the most recently received ATM cell. - The E and C bits of the fragment shall be set as defined earlier in section 9. - If the arriving cell is an OAM or an RM cell, send the current PSN frame and then send the OAM or RM cell using one-to-one single cell mode. 9.3.2. Procedures in the PSN-to-ATM Direction The following procedures shall apply: - The 3-bit PTI field of each ATM cell header is constructed as follows: -i. The most significant bit is set to 0, indicating a user data cell. -ii. The middle bit is set to the E bit value of the fragment. -iii. The least significant bit for the last ATM cell in the PSN frame is set to the value of the UU bit of Figure 10. -iv. The least significant PTI bit is set to 0 for all other cells in the PSN frame. - The CLP bit of each ATM cell header is set to the value of the C bit of the control byte in Figure 10. - When a fragment is received, each constituent ATM cell is sent in correct order. 10. Security Considerations This document specifies only encapsulations, and not the protocols used to carry the encapsulated packets across the network. Each such protocol may have its own set of security issues, but those issues are not affected by the encapsulations specified herein. Martini, et al. [Page 29] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 11. Intellectual Property Disclaimer This document is being submitted for use in IETF standards discussions. 12. References [1] "Transport of Layer 2 Frames Over MPLS", draft-ietf-pwe3- control-protocol-00.txt. ( work in progress ) [2] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D. Tappan, G. Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032 [3] "Requirements for Peudo Wire Emulation Edge-to-Edge (PWE3", draft-ietf-pwe3-requirements-03.txt. ( work in Progress ) [4] ATM Forum Specification fb-fbatm-0151.000 (2000) ,Frame Based ATM over SONET/SDH Transport (FAST) [5] ATM Forum Specification af-tm-0121.000 (1999), Traffic Management Specification Version 4.1. [6] ITU-T Recommendation I.371 (2000), Traffic control and congestion control in B-ISDN. [7] ITU-T Recommendation I.610, (1999), B-ISDN operation and maintenance principles and functions. 13. Author Information Luca Martini Level 3 Communications, LLC. 1025 Eldorado Blvd. Broomfield, CO, 80021 e-mail: luca@level3.net Nasser El-Aawar Level 3 Communications, LLC. 1025 Eldorado Blvd. Broomfield, CO, 80021 e-mail: nna@level3.net Martini, et al. [Page 30] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 Giles Heron PacketExchange Ltd. The Truman Brewery 91 Brick Lane LONDON E1 6QL United Kingdom e-mail: giles@packetexchange.net Dimitri Stratton Vlachos Mazu Networks, Inc. 125 Cambridgepark Drive Cambridge, MA 02140 e-mail: d@mazunetworks.com Dan Tappan Cisco Systems, Inc. 250 Apollo Drive Chelmsford, MA, 01824 e-mail: tappan@cisco.com Jayakumar Jayakumar, Cisco Systems Inc. 225, E.Tasman, MS-SJ3/3, San Jose , CA, 95134 e-mail: jjayakum@cisco.com Eric Rosen Cisco Systems, Inc. 250 Apollo Drive Chelmsford, MA, 01824 e-mail: erosen@cisco.com Steve Vogelsang Laurel Networks, Inc. Omega Corporate Center 1300 Omega Drive Pittsburgh, PA 15205 e-mail: sjv@laurelnetworks.com Martini, et al. [Page 31] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 Jeremy Brayley Laurel Networks, Inc. Omega Corporate Center 1300 Omega Drive Pittsburgh, PA 15205 e-mail: jbrayley@laurelnetworks.com Gerald de Grace Laurel Networks, Inc. Omega Corporate Center 1300 Omega Drive Pittsburgh, PA 15205 e-mail: gdegrace@laurelnetworks.com John Shirron Laurel Networks, Inc. Omega Corporate Center 1300 Omega Drive Pittsburgh, PA 15205 e-mail: jshirron@laurelnetworks.com Andrew G. Malis Vivace Networks, Inc. 2730 Orchard Parkway San Jose, CA 95134 e-mail: Andy.Malis@vivacenetworks.com Vinai Sirkay Vivace Networks, Inc. 2730 Orchard Parkway San Jose, CA 95134 e-mail: sirkay@technologist.com Chris Liljenstolpe Cable & Wireless 11700 Plaza America Drive Reston, VA 20190 e-mail: chris@cw.net Martini, et al. [Page 32] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 Kireeti Kompella Juniper Networks 1194 N. Mathilda Ave Sunnyvale, CA 94089 e-mail: kireeti@juniper.net Ghassem Koleyni Nortel Networks P O Box 3511, Station C Ottawa, Ontario, K1Y 4H7 Canada e-mail: ghassem@nortelnetworks.com John Fischer Alcatel 600 March Rd Kanata, ON, Canada. K2K 2E6 e-mail: john.fischer@alcatel.com Matthew Bocci Alcatel Grove House, Waltham Road Rd White Waltham, Berks, UK. SL6 3TN e-mail: matthew.bocci@alcatel.co.uk Mustapha Aissaoui Alcatel 600 March Rd Kanata, ON, Canada. K2K 2E6 e-mail: mustapha.aissaoui@alcatel.com Tom Walsh Lucent Technologies 1 Robbins Road Westford, MA 01886 USA e-mail: tdwalsh@lucent.com John Rutemiller Marconi Networks 1000 Marconi Drive Warrendale, PA 15086 e-mail: John.Rutemiller@marconi.com Martini, et al. [Page 33] Internet Draft draft-ietf-pwe3-atm-encap-01.txt February 2003 Rick Wilder Masergy Communications 2901 Telestar Ct. Falls Church, VA 22042 e-mail: rwilder@masergy.com Laura Dominik Qwest Communications, Inc. 600 Stinson Blvd. Minneapolis, MN, 55413 Email: ldomini@qwest.com Martini, et al. [Page 34]