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rfc:rfc2684

Network Working Group D. Grossman Request for Comments: 2684 Motorola, Inc. Obsoletes: 1483 J. Heinanen Category: Standards Track Telia

                                                       September 1999
      Multiprotocol Encapsulation over ATM Adaptation Layer 5

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

 This memo replaces RFC 1483.  It describes two encapsulations methods
 for carrying network interconnect traffic over AAL type 5 over  ATM.
 The first method allows multiplexing of multiple protocols over a
 single ATM virtual connection whereas the second method assumes that
 each protocol is carried over a separate ATM virtual connection.

Applicability

 This specification is intended to be used in implementations which
 use ATM networks to carry multiprotocol traffic among hosts, routers
 and bridges which are ATM end systems.

1. Introduction

 Asynchronous Transfer Mode (ATM) wide area, campus and local area
 networks are used to transport IP datagrams and other connectionless
 traffic between hosts, routers, bridges and other networking devices.
 This memo describes two methods for carrying connectionless routed
 and bridged Protocol Data Units (PDUs) over an ATM network.  The "LLC
 Encapsulation" method allows multiplexing of multiple protocols over
 a single ATM virtual connection (VC).  The protocol type of each PDU
 is identified by a prefixed IEEE 802.2 Logical Link Control (LLC)
 header. In the "VC Multiplexing" method, each ATM VC carries PDUs of
 exactly one protocol type.  When multiple protocols need to be
 transported, there is a separate VC for each.

Grossman & Heinanen Standards Track [Page 1] RFC 2684 Multiprotocol Over AALS September 1999

 The unit of transport in ATM is a 53 octet fixed length PDU called a
 cell.  A cell consists of a 5 octet header and a 48 byte payload.
 Variable length PDUs, including those addressed in this memo, must be
 segmented by the transmitter to fit into the 48 octet ATM cell
 payload, and reassembled by the receiver.  This memo specifies the
 use of the ATM Adaptation Layer type 5 (AAL5), as defined in ITU-T
 Recommendation I.363.5 [2] for this purpose. Variable length PDUs are
 carried in the Payload field of the AAL5 Common Part Convergence
 Sublayer (CPCS) PDU.
 This memo only describes how routed and bridged PDUs are carried
 directly over the AAL5  CPCS, i.e., when the Service Specific
 Convergence Sublayer (SSCS) of AAL5 is absent.  If Frame Relay
 Service Specific Convergence Sublayer (FR-SSCS), as defined in ITU-T
 Recommendation I.365.1 [3], is used over the CPCS, then routed and
 bridged PDUs are carried using the NLPID multiplexing method
 described in RFC 2427 [4]. The RFC 2427 encapsulation MUST be used in
 the special case that Frame Relay Network Interworking or transparent
 mode Service Interworking [9] are used, but is NOT RECOMMENDED for
 other applications.  Appendix A (which is for information only) shows
 the format of the FR-SSCS-PDU as well as how IP and CLNP PDUs are
 encapsulated over FR-SSCS according to RFC 2427.
 This memo also includes an optional encapsulation for use with
 Virtual Private Networks that operate over an ATM subnet.
 If it is desired to use the facilities which are designed for the
 Point-to-Point Protocol (PPP), and there exists a point-to-point
 relationship between peer systems, then RFC 2364, rather than this
 memo, applies.

2. Conventions

 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
 SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
 they appear in this document, are to be interpreted as described in
 RFC 2119 [10].

3. Selection of the Multiplexing Method

 The decision as to whether to use LLC encapsulation or VC-
 multiplexing depends on implementation and system requirements.  In
 general, LLC encapsulation tends to require fewer VCs in a
 multiprotocol environment.  VC multiplexing tends to reduce
 fragmentation overhead (e.g., an IPV4 datagram containing a TCP
 control packet with neither IP nor TCP options exactly fits into a
 single cell).

Grossman & Heinanen Standards Track [Page 2] RFC 2684 Multiprotocol Over AALS September 1999

 When two ATM end systems wish to exchange connectionless PDUs across
 an ATM Permanent Virtual Connection (PVC), selection of the
 multiplexing method is done by configuration.  ATM connection control
 signalling procedures are used to negotiate the encapsulation method
 when ATM Switched Virtual Connections (SVCs) are to be used.  [5] and
 [8] specify how this negotiation is done.

4. AAL5 PDU Format

 For both multiplexing methods, routed and bridged PDUs MUST be
 encapsulated within the Payload field of an AAL5 CPCS-PDU.
 ITU-T Recomendation I.363.5 [2] provides the complete definition of
 the AAL5 PDU format and procedures at the sender and receiver. The
 AAL5 message mode service, in the non-assured mode of operation MUST
 be used. The corrupted delivery option MUST NOT be used.  A
 reassembly timer MAY be used. The following description is provided
 for information.
 The format of the AAL5 CPCS-PDU is shown below:
                   AAL5 CPCS-PDU Format
             +-------------------------------+
             |             .                 |
             |             .                 |
             |        CPCS-PDU Payload       |
             |     up to 2^16 - 1 octets)    |
             |             .                 |
             |             .                 |
             +-------------------------------+
             |      PAD ( 0 - 47 octets)     |
             +-------------------------------+ -------
             |       CPCS-UU (1 octet )      |
             +-------------------------------+
             |         CPI (1 octet )        |
             +-------------------------------+CPCS-PDU Trailer
             |        Length (2 octets)      |
             +-------------------------------|
             |         CRC (4 octets)        |
             +-------------------------------+ -------
 The Payload field contains user information up to 2^16 - 1 octets.
 The PAD field pads the CPCS-PDU to fit exactly into the ATM cells
 such that the last 48 octet cell payload created by the SAR sublayer
 will have the CPCS-PDU Trailer right justified in the cell.

Grossman & Heinanen Standards Track [Page 3] RFC 2684 Multiprotocol Over AALS September 1999

 The CPCS-UU (User-to-User indication) field is used to transparently
 transfer CPCS user to user information.  The field is not used by the
 multiprotocol ATM encapsulation described in this memo and MAY be set
 to any value.
 The CPI (Common Part Indicator) field aligns the CPCS-PDU trailer to
 64 bits.  This field MUST be coded as 0x00.
 The Length field indicates the length, in octets, of the Payload
 field.  The maximum value for the Length field is 65535 octets.  A
 Length field coded as 0x00 is used for the abort function.
 The CRC field is used to detect bit errors in the CPCS-PDU.  A CRC-32
 is used.

5. LLC Encapsulation

 LLC Encapsulation is needed when more than one protocol might be
 carried over the same VC.  In order to allow the receiver to properly
 process the incoming AAL5 CPCS-PDU, the Payload Field contains
 information necessary to identify the protocol of the routed or
 bridged PDU.  In LLC Encapsulation, this information MUST be encoded
 in an LLC header placed in front of the carried PDU.
 Although this memo only deals with protocols that operate over LLC
 Type 1 (unacknowledged connectionless mode) service, the same
 encapsulation principle also applies to protocols operating over LLC
 Type 2 (connection-mode) service.  In the latter case the format and
 contents of the LLC header would be as described in IEEE 802.1 and
 IEEE 802.2.

5.1. LLC Encapsulation for Routed Protocols

 In LLC Encapsulation, the protocol type of routed PDUs MUST be
 identified by prefixing an IEEE 802.2 LLC header to each PDU.  In
 some cases, the LLC header MUST be followed by an IEEE 802.1a
 SubNetwork Attachment Point (SNAP) header.  In LLC Type 1 operation,
 the LLC header MUST consist of three one octet fields:
                  +------+------+------+
                  | DSAP | SSAP | Ctrl |
                  +------+------+------+
 In LLC Encapsulation for routed protocols, the Control field MUST be
 set to 0x03, specifying a Unnumbered Information (UI) Command PDU.

Grossman & Heinanen Standards Track [Page 4] RFC 2684 Multiprotocol Over AALS September 1999

 The LLC header value 0xFE-FE-03 MUST be used to identify a routed PDU
 in the ISO NLPID format (see [6] and Appendix B). For NLPID-formatted
 routed PDUs,  the content of the AAL5 CPCS-PDU Payload field MUST be
 as follows:
          Payload Format for Routed NLPID-formatted PDUs
               +-------------------------------+
               |       LLC  0xFE-FE-03         |
               +-------------------------------+
               |     NLPID (1 octet)           |
               +-------------------------------+
               |             .                 |
               |            PDU                |
               |     (up to 2^16 - 4 octets)   |
               |             .                 |
               +-------------------------------+
 The routed protocol MUST be identified by a one octet NLPID field
 that is part of Protocol Data.  NLPID values are administered by ISO
 and ITU-T.  They are defined in ISO/IEC TR 9577 [6] and some of the
 currently defined ones are listed in Appendix C.
 An NLPID value of 0x00 is defined in ISO/IEC TR 9577 as the Null
 Network Layer or Inactive Set.  Since it has no significance within
 the context of this encapsulation scheme, a NLPID value of 0x00 MUST
 NOT be used.
 Although there is a NLPID value (0xCC) that indicates IP, the NLPID
 format MUST NOT be used for IP.  Instead, IP datagrams MUST be
 identified by a SNAP header, as defined below.
 The presence of am IEEE 802.1a SNAP header is indicated by the LLC
 header value 0xAA-AA-03. A SNAP header is of the form
              +------+------+------+------+------+
              |         OUI        |     PID     |
              +------+------+------+------+------+
 The SNAP header consists of a three octet Organizationally Unique
 Identifier (OUI) and a two octet Protocol Identifier (PID).  The OUI
 is administered by IEEE and  identifies an organization which
 administers the values which might be assigned to the PID.  The SNAP
 header thus uniquely identifies a routed or bridged protocol.  The
 OUI value 0x00-00-00 indicates that the PID is an EtherType.

Grossman & Heinanen Standards Track [Page 5] RFC 2684 Multiprotocol Over AALS September 1999

 The format of the AAL5 CPCS-PDU Payload field for routed non-NLPID
 Formatted PDUs MUST be as follows:
       Payload Format for Routed non-NLPID formatted PDUs
              +-------------------------------+
              |       LLC  0xAA-AA-03         |
              +-------------------------------+
              |        OUI 0x00-00-00         |
              +-------------------------------+
              |     EtherType (2 octets)      |
              +-------------------------------+
              |             .                 |
              |    Non-NLPID formatted PDU    |
              |     (up to 2^16 - 9 octets)   |
              |             .                 |
              +-------------------------------+
 In the particular case of an IPv4 PDU, the Ethertype value is 0x08-
 00, and the payload format MUST be:
              Payload Format for Routed IPv4 PDUs
              +-------------------------------+
              |       LLC  0xAA-AA-03         |
              +-------------------------------+
              |        OUI 0x00-00-00         |
              +-------------------------------+
              |       EtherType 0x08-00       |
              +-------------------------------+
              |             .                 |
              |          IPv4 PDU             |
              |     (up to 2^16 - 9 octets)   |
              |             .                 |
              +-------------------------------+
 This format is consistent with that defined in RFC 1042 [7].

5.2. LLC Encapsulation for Bridged Protocols

 In LLC Encapsulation, bridged PDUs are encapsulated by identifying
 the type of the bridged media in the SNAP header.  The presence of
 the SNAP header MUST be indicated by the LLC header value 0xAA-AA-03.
 The OUI value in the SNAP header MUST be the 802.1 organization code
 0x00-80-C2. The type of the bridged media MUST be specified by the
 two octet PID. The PID MUST also indicate whether the original Frame
 Check Sequence (FCS) is preserved within the bridged PDU. Appendix B
 provides a list of media type (PID) values that can be used in ATM
 encapsulation.

Grossman & Heinanen Standards Track [Page 6] RFC 2684 Multiprotocol Over AALS September 1999

 The AAL5 CPCS-PDU Payload field carrying a bridged PDU MUST have one
 of the following formats.  The necessary number of padding octets
 MUST be added after the PID field in order to align the
 Ethernet/802.3 LLC Data field, 802.4 Data Unit field, 802.5 Info
 field, FDDI Info field or 802.6 Info field (respectively) of the
 bridged PDU to begin at a four octet boundary.  The bit ordering of
 the MAC address MUST be the same as it would be on the LAN or MAN
 (e.g., in canoncial form for bridged Ethernet/IEEE 802.3 PDUs, but in
 802.5/FDDI format for bridged 802.5 PDUs).
        Payload Format for Bridged Ethernet/802.3 PDUs
              +-------------------------------+
              |       LLC  0xAA-AA-03         |
              +-------------------------------+
              |        OUI 0x00-80-C2         |
              +-------------------------------+
              |    PID 0x00-01 or 0x00-07     |
              +-------------------------------+
              |         PAD 0x00-00           |
              +-------------------------------+
              |    MAC destination address    |
              +-------------------------------+
              |                               |
              |   (remainder of MAC frame)    |
              |                               |
              +-------------------------------+
              |  LAN FCS (if PID is 0x00-01)  |
              +-------------------------------+
 The Ethernet/802.3 physical layer requires padding of frames to a
 minimum size. A bridge that uses uses the Bridged Ethernet/802.3
 encapsulation format with the preserved LAN FCS MUST include padding.
 A bridge that uses the Bridged Ethernet/802.3 encapsulation format
 without the preserved LAN FCS MAY either include padding, or omit it.
 When a bridge receives a frame in this format without the LAN FCS, it
 MUST be able to insert the necessary padding (if none is already
 present) before forwarding to an Ethernet/802.3 subnetwork.

Grossman & Heinanen Standards Track [Page 7] RFC 2684 Multiprotocol Over AALS September 1999

              Payload Format for Bridged 802.4 PDUs
                +-------------------------------+
                |       LLC  0xAA-AA-03         |
                +-------------------------------+
                |        OUI 0x00-80-C2         |
                +-------------------------------+
                |    PID 0x00-02 or 0x00-08     |
                +-------------------------------+
                |        PAD 0x00-00-00         |
                +-------------------------------+
                |    Frame Control (1 octet)    |
                +-------------------------------+
                |    MAC destination address    |
                +-------------------------------+
                |                               |
                |   (remainder of MAC frame)    |
                |                               |
                +-------------------------------+
                |  LAN FCS (if PID is 0x00-02)  |
                +-------------------------------+
              Payload Format for Bridged 802.5 PDUs
                +-------------------------------+
                |       LLC  0xAA-AA-03         |
                +-------------------------------+
                |        OUI 0x00-80-C2         |
                +-------------------------------+
                |    PID 0x00-03 or 0x00-09     |
                +-------------------------------+
                |        PAD 0x00-00-XX         |
                +-------------------------------+
                |    Frame Control (1 octet)    |
                +-------------------------------+
                |    MAC destination address    |
                +-------------------------------+
                |                               |
                |   (remainder of MAC frame)    |
                |                               |
                +-------------------------------+
                |  LAN FCS (if PID is 0x00-03)  |
                +-------------------------------+
 Since the 802.5 Access Control (AC) field has no significance outside
 the local 802.5 subnetwork, it is treated by this encapsulation as
 the last octet of the three octet PAD field.   It MAY be set to any
 value by the sending bridge and MUST be ignored by the receiving
 bridge.

Grossman & Heinanen Standards Track [Page 8] RFC 2684 Multiprotocol Over AALS September 1999

               Payload Format for Bridged FDDI PDUs
                +-------------------------------+
                |       LLC  0xAA-AA-03         |
                +-------------------------------+
                |        OUI 0x00-80-C2         |
                +-------------------------------+
                |    PID 0x00-04 or 0x00-0A     |
                +-------------------------------+
                |        PAD 0x00-00-00         |
                +-------------------------------+
                |    Frame Control (1 octet)    |
                +-------------------------------+
                |    MAC destination address    |
                +-------------------------------+
                |                               |
                |   (remainder of MAC frame)    |
                |                               |
                +-------------------------------+
                |  LAN FCS (if PID is 0x00-04)  |
                +-------------------------------+
              Payload Format for Bridged 802.6 PDUs
                +-------------------------------+
                |       LLC  0xAA-AA-03         |
                +-------------------------------+
                |        OUI 0x00-80-C2         |
                +-------------------------------+
                |         PID 0x00-0B           |
                +---------------+---------------+ ------
                |   Reserved    |     BEtag     |  Common
                +---------------+---------------+  PDU
                |            BAsize             |  Header
                +-------------------------------+ -------
                |    MAC destination address    |
                +-------------------------------+
                |                               |
                |   (remainder of MAC frame)    |
                |                               |
                +-------------------------------+
                |                               |
                |      Common PDU Trailer       |
                |                               |
                +-------------------------------+
 In bridged 802.6 PDUs, the presence of a CRC-32 is indicated by the
 CIB bit in the header of the MAC frame.  Therefore, the same PID
 value is used regardless of the presence or absence of the CRC-32 in
 the PDU.

Grossman & Heinanen Standards Track [Page 9] RFC 2684 Multiprotocol Over AALS September 1999

 The Common Protocol Data Unit (PDU) Header and Trailer are conveyed
 to allow pipelining at the egress bridge to an 802.6 subnetwork.
 Specifically, the Common PDU Header contains the BAsize field, which
 contains the length of the PDU.  If this field is not available to
 the egress 802.6 bridge, then that bridge cannot begin to transmit
 the segmented PDU until it has received the entire PDU, calculated
 the length, and inserted the length into the BAsize field.  If the
 field is available, the egress 802.6 bridge can extract the length
 from the BAsize field of the Common PDU Header, insert it into the
 corresponding field of the first segment, and immediately transmit
 the segment onto the 802.6 subnetwork.  Thus, the bridge can begin
 transmitting the 802.6 PDU before it has received the complete PDU.
 Note that the Common PDU Header and Trailer of the encapsulated frame
 should not be simply copied to the outgoing 802.6 subnetwork because
 the encapsulated BEtag value may conflict with the previous BEtag
 value transmitted by that bridge.
 An ingress 802.6 bridge can abort an AAL5 CPCS-PDU by setting its
 Length field to zero.  If the egress bridge has already begun
 transmitting segments of the PDU to an 802.6 subnetwork and then
 notices that the AAL5 CPCS-PDU has been aborted, it may immediately
 generate an EOM cell that causes the 802.6 PDU to be rejected at the
 receiving bridge.  Such an EOM cell could, for example, contain an
 invalid value in the Length field of the Common PDU Trailer.
                    Payload Format for BPDUs
                +-------------------------------+
                |       LLC  0xAA-AA-03         |
                +-------------------------------+
                |        OUI 0x00-80-C2         |
                +-------------------------------+
                |         PID 0x00-0E           |
                +-------------------------------+
                |                               |
                |      BPDU as defined by       |
                |     802.1(d) or 802.1(g)      |
                |                               |
                +-------------------------------+

6. VC Multiplexing

 VC Multiplexing creates a binding between an ATM VC and the type of
 the network protocol carried on that VC.  Thus, there is no need for
 protocol identification information to be carried in the payload of
 each AAL5 CPCS-PDU.  This reduces payload overhead and can reduce
 per-packet processing. VC multiplexing can improve efficiency by
 reducing the number of cells needed to carry PDUs of certain lengths.

Grossman & Heinanen Standards Track [Page 10] RFC 2684 Multiprotocol Over AALS September 1999

 For ATM PVCs, the type of the protocol to be carried over each PVC
 MUST be determined by configuration.  For ATM SVCs, the negotiations
 specified in RFC 1755 [5] MUST be used.

6.1. VC Multiplexing of Routed Protocols

 PDUs of routed protocols MUST be carried as the only content of the
 Payload of the AAL5 CPCS-PDU.  The format of the AAL5 CPCS-PDU
 Payload field thus becomes:
                  Payload Format for Routed PDUs
                +-------------------------------+
                |             .                 |
                |         Carried PDU           |
                |    (up to 2^16 - 1 octets)    |
                |             .                 |
                |             .                 |
                +-------------------------------+

6.2. VC Multiplexing of Bridged Protocols

 PDUs of bridged protocols MUST be carried in the Payload of the AAL5
 CPCS-PDU exactly as described in section 5.2, except that only the
 fields after the PID field MUST be included.  The AAL5 CPCS-PDU
 Payload field carrying a bridged PDU MUST, therefore, have one of the
 following formats.
           Payload Format for Bridged Ethernet/802.3 PDUs
                +-------------------------------+
                |         PAD 0x00-00           |
                +-------------------------------+
                |    MAC destination address    |
                +-------------------------------+
                |                               |
                |   (remainder of MAC frame)    |
                |                               |
                +-------------------------------+
                | LAN FCS (VC dependent option) |
                +-------------------------------+

Grossman & Heinanen Standards Track [Page 11] RFC 2684 Multiprotocol Over AALS September 1999

           Payload Format for Bridged 802.4/802.5/FDDI PDUs
                +-------------------------------+
                | PAD 0x00-00-00 or 0x00-00-XX  |
                +-------------------------------+
                |    Frame Control (1 octet)    |
                +-------------------------------+
                |    MAC destination address    |
                +-------------------------------+
                |                               |
                |   (remainder of MAC frame)    |
                |                               |
                +-------------------------------+
                | LAN FCS (VC dependent option) |
                +-------------------------------+
 Note that the 802.5 Access Control (AC) field has no significance
 outside the local 802.5 subnetwork.  It can thus be regarded as the
 last octet of the three octet PAD field, which in case of 802.5 can
 be set to any value (XX).
                Payload Format for Bridged 802.6 PDUs
               +---------------+---------------+ -------
               |   Reserved    |     BEtag     |  Common
               +---------------+---------------+  PDU
               |            BAsize             |  Header
               +-------------------------------+ -------
               |    MAC destination address    |
               +-------------------------------+
               |                               |
               |   (remainder of MAC frame)    |
               |                               |
               +-------------------------------+
               |                               |
               |     Common PDU Trailer        |
               |                               |
               +-------------------------------+
                   Payload Format for BPDUs
               +-------------------------------+
               |                               |
               |      BPDU as defined by       |
               |     802.1(d) or 802.1(g)      |
               |                               |
               +-------------------------------+

Grossman & Heinanen Standards Track [Page 12] RFC 2684 Multiprotocol Over AALS September 1999

 In case of Ethernet, 802.3, 802.4, 802.5, and FDDI PDUs the presense
 or absence of the trailing LAN FCS shall be identified implicitly by
 the VC, since the PID field is not included.  PDUs with the LAN FCS
 and PDUs without the LAN FCS are thus considered to belong to
 different protocols even if the bridged media type would be the same.

7. Bridging in an ATM Network

 A bridge with an ATM interface that serves as a link to one or more
 other bridge MUST be able to flood, forward, and filter bridged PDUs.
 Flooding is performed by sending the PDU to all possible appropriate
 destinations.  In the ATM environment this means sending the PDU
 through each relevant VC.  This may be accomplished by explicitly
 copying it to each VC or by using a point-to-multipoint VC.
 To forward a PDU, a bridge MUST be able to associate a destination
 MAC address with a VC.  It is unreasonable and perhaps impossible to
 require bridges to statically configure an association of every
 possible destination MAC address with a VC.  Therefore, ATM bridges
 must provide enough information to allow an ATM interface to
 dynamically learn about foreign destinations beyond the set of ATM
 stations.
 To accomplish dynamic learning, a bridged PDU MUST conform to the
 encapsulation described in section 5.  In this way, the receiving ATM
 interface will know to look into the bridged PDU and learn the
 association between foreign destination and an ATM station.

8. Virtual Private Network (VPN) identification

 The encapsulation defined in this section applies only to  Virtual
 Private Networks (VPNs) that operate over an ATM subnet.
 A mechanism for globally unique identification of Virtual Private
 multiprotocol networks is defined in [11].  The 7-octet VPN-Id
 consists of a 3-octet VPN-related OUI (IEEE 802-1990 Organizationally
 Unique Identifier), followed by a 4-octet VPN index which is
 allocated by the owner of the VPN-related OUI.  Typically, the VPN-
 related OUI value is assigned to a VPN service provider, which then
 allocates VPN index values for its customers.

Grossman & Heinanen Standards Track [Page 13] RFC 2684 Multiprotocol Over AALS September 1999

8.1 VPN Encapsulation Header

 The format of the VPN encapsulation header is as follows:
                     VPN Encapsulation Header
                +-------------------------------+
                |       LLC  0xAA-AA-03         |
                +-------------------------------+
                |        OUI 0x00-00-5E         |
                +-------------------------------+
                |        PID 0x00-08            |
                +-------------------------------+
                |          PAD 0x00             |
                +-------------------------------+
                |   VPN related OUI (3 octets)  |
                +-------------------------------+
                |    VPN Index (4 octets)       |
                +-------------------------------+
                |                               |
                |     (remainder of PDU)        |
                |                               |
                +-------------------------------+
 When the encapsulation header is used, the remainder of the PDU MUST
 be structured according to the appropiate format described in section
 5 or 6 (i.e., the VPN encapsulation header is prepended to the PDU
 within an AAL5 CPCS SDU).

8.2 LLC-encapsulated routed or bridged PDUs within a VPN

 When a LLC-encapsulated routed or bridged PDU is sent within a VPN
 using ATM over AAL5, a VPN encapsulation header MUST be prepended to
 the appropriate routed or bridged PDU format defined in sections 5.1
 and 5.2, respectively.

8.3 VC multiplexing of routed or bridged PDUs within a VPN

 When a routed or bridged PDU is sent within a VPN using VC
 multiplexing, the VPN identifier MAY either be specified a priori,
 using ATM connection control signalling or adminstrative assignment
 to an ATM interface, or it MAY be indicated using an encapsulation
 header.
 If the VPN is identified using ATM connection control signalling, all
 PDUs carried by the ATM VC are associated with the same VPN.  In this
 case, the payload formats of routed and bridged PDUs MUST be as
 defined in sections 6.1 and 6.2, respectively.  If a PDU is received
 containing a VPN encapsulation header when the VPN has been

Grossman & Heinanen Standards Track [Page 14] RFC 2684 Multiprotocol Over AALS September 1999

 identified using ATM signalling, the receiver MAY drop it and/or take
 other actions which are implementation specific.  Specification of
 the mechanism in ATM connection control signalling for carrying VPN
 identifiers is outside the scope of this Memo.
 If a VPN identifier is administratively assigned to an ATM interface,
 then all PDUs carried by any ATM VCs within that interface are
 associated with that VPN.  In this case, the payload formats of
 routed and bridged PDUs MUST be as defined in sections 6.1 and 6.2,
 respectively.  If a PDU is received containing a VPN encapsulation
 header when the VPN identifier has been administratively assigned,
 the receiver MAY drop it and/or take other actions which are
 implementation specific.  Specification of mechanisms (such as MIBs)
 for assigning VPN identifiers to ATM interfaces is outside the scope
 of this memo.
 If the VPN identifier is to be indicated using an encapsulation
 header, then a VPN encapsulation header MUST be prepended to the
 appropriate routed or bridged PDU format defined in sections 6.1 and
 6.2, respectively.

9. Security Considerations

 This memo defines mechanisms for multiprotocol encapsulation over
 ATM. There is an element of trust in any encapsulation protocol:  a
 receiver must trust that the sender has correctly identified the
 protocol being encapsulated.  There is no way to ascertain that the
 sender did use the proper protocol identification (nor would this be
 desirable functionality).  The encapsulation mechanisms described in
 this memo are believed not to have any other properties that might be
 exploited by an attacker. However, architectures and protocols
 operating above the encapsulation layer may be subject to a variety
 of attacks.  In particular, the bridging architecture discussed in
 section 7 has the same vulnerabilities as other bridging
 architectures.
 System security may be affected by the properties of the underlying
 ATM network.  The ATM Forum has published a security framework [12]
 and a security specification [13] which may be relevant.

Grossman & Heinanen Standards Track [Page 15] RFC 2684 Multiprotocol Over AALS September 1999

Acknowledgements

 This memo replaces RFC 1483, which was developed by the IP over ATM
 working group, and edited by Juha Heinanen (then at Telecom Finland,
 now at Telia).  The update was developed in the IP-over-NBMA (ION)
 working group, and Dan Grossman (Motorola) was editor and also
 contributed to the work on RFC 1483.
 This material evolved from RFCs [1] and [4] from which much of the
 material has been adopted.  Thanks to their authors Terry Bradley,
 Caralyn  Brown, Andy Malis, Dave Piscitello, and C. Lawrence.  Other
 key contributors to the work included Brian Carpenter (CERN), Rao
 Cherukuri (IBM), Joel Halpern (then at Network Systems), Bob Hinden
 (Sun Microsystems, presently at Nokia), and Gary Kessler (MAN
 Technology).
 The material concerning VPNs was developed by Barbara Fox (Lucent)
 and Bernhard Petri (Siemens).

References

 [1]  Piscitello, D. and C. Lawrence, "The Transmission of IP
      Datagrams over the SMDS Service", RFC 1209, March 1991.
 [2]  ITU-T Recommendation I.363.5, "B-ISDN ATM Adaptation Layer (AAL)
      Type 5 Specification", August 1996.
 [3]  ITU-T Recommendation I.365.1, "Frame Relaying Service Specific
      Convergence Sublayer (SSCS), November 1993.
 [4]  Brown, C. and A. Malis, "Multiprotocol Interconnect over Frame
      Relay", RFC 2427, September 1998.
 [5]  Perez M., Liaw, F., Mankin, E., Grossman, D. and A. Malis, "ATM
      Signalling Support for IP over ATM", RFC 1755, February 1995.
 [6]  Information technology - Telecommunications and Information
      Exchange Between Systems, "Protocol Identification in the
      Network Layer".  ISO/IEC TR 9577, October 1990.
 [7]  Postel, J. and J. Reynolds, "A Standard for the Transmission of
      IP Datagrams over IEEE 802 Networks", STD 43, RFC 1042, February
      1988.
 [8]  Maher, M., "IP over ATM Signalling - SIG 4.0 Update", RFC 2331,
      April 1998.

Grossman & Heinanen Standards Track [Page 16] RFC 2684 Multiprotocol Over AALS September 1999

 [9]  ITU-T Recommendation I.555, "Frame Relay Bearer Service
      Interworking", September 1997.
 [10] Bradner, S. "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [11] Fox, B. and B. Gleeson, "Virtual Private Networks Identifier",
      RFC 2685, September 1999.
 [12] The ATM Forum, "ATM Security Framework Version 1.0", af-sec-
      0096.000, February 1998.
 [13] The ATM Forum, "ATM Security Specification v1.0", af-sec-
      0100.001, February 1999.

Grossman & Heinanen Standards Track [Page 17] RFC 2684 Multiprotocol Over AALS September 1999

Appendix A. Multiprotocol Encapsulation over FR-SSCS

 ITU-T Recommendation I.365.1 defines a Frame Relaying Specific
 Convergence Sublayer (FR- SSCS) to be used on the top of the Common
 Part Convergence Sublayer CPCS) of the AAL type 5 for Frame Relay/ATM
 interworking.  The service offered by FR-SSCS corresponds to the Core
 service for Frame Relaying as described in I.233.
 An FR-SSCS-PDU consists of Q.922 Address field followed by Q.922
 Information field.  The Q.922 flags and the FCS are omitted, since
 the corresponding functions are provided by the AAL.  The figure
 below shows an FR-SSCS-PDU embedded in the Payload of an AAL5 CPCS-
 PDU.
              FR-SSCS-PDU in Payload of AAL5 CPCS-PDU
             +-------------------------------+ -------
             |      Q.922 Address Field      | FR-SSCS-PDU Header
             |         (2-4 octets)          |
             +-------------------------------+ -------
             |             .                 |
             |             .                 |
             |    Q.922 Information field    | FR-SSCS-PDU Payload
             |             .                 |
             |             .                 |
             +-------------------------------+ -------
             |      AAL5 CPCS-PDU Trailer    |
             +-------------------------------+
 Routed and bridged PDUs are encapsulated inside the FR-SSCS-PDU as
 defined in RFC 2427.  The Q.922 Information field starts with a Q.922
 Control field followed by an optional Pad octet that is used to align
 the remainder of the frame to a convenient boundary for the sender.
 The protocol of the carried PDU is then identified by prefixing the
 PDU by an ISO/IEC TR 9577 Network Layer Protocol ID (NLPID).

Grossman & Heinanen Standards Track [Page 18] RFC 2684 Multiprotocol Over AALS September 1999

 In the particular case of an IP PDU, the NLPID is 0xCC and the FR-
 SSCS-PDU has the following format:
              FR-SSCS-PDU Format for Routed IP PDUs
             +-------------------------------+
             |       Q.922 Addr Field        |
             |       (2 or 4 octets)         |
             +-------------------------------+
             |     0x03 (Q.922 Control)      |
             +-------------------------------+
             |          NLPID  0xCC          |
             +-------------------------------+
             |             .                 |
             |           IP PDU              |
             |    (up to 2^16 - 5 octets)    |
             |             .                 |
             +-------------------------------+
 Note that according to RFC 2427, the Q.922 Address field MUST be
 either 2 or 4 octets, i.e., a 3 octet Address field MUST NOT be used.
 In the particular case of a CLNP PDU, the NLPID is 0x81 and the FR-
 SSCS-PDU has the following format:
          FR-SSCS-PDU Format for Routed CLNP PDUs
             +-------------------------------+
             |       Q.922 Addr Field        |
             |       (2 or 4 octets)         |
             +-------------------------------+
             |     0x03 (Q.922 Control)      |
             +-------------------------------+
             |         NLPID  0x81           |
             +-------------------------------+
             |              .                |
             |       Rest of CLNP PDU        |
             |    (up to 2^16 - 5 octets)    |
             |              .                |
             +-------------------------------+
 Note that in case of ISO protocols the NLPID field forms the first
 octet of the PDU itself and MUST not be repeated.
 The above encapsulation applies only to those routed protocols that
 have a unique NLPID assigned.  For other routed protocols (and for
 bridged protocols), it is necessary to provide another mechanism for
 easy protocol identification.  This can be achieved by using an NLPID
 value 0x80 to indicate that an IEEE 802.1a SubNetwork Attachment
 Point (SNAP) header follows.

Grossman & Heinanen Standards Track [Page 19] RFC 2684 Multiprotocol Over AALS September 1999

 See RFC 2427 for more details related to multiprotocol encapsulation
 over FRCS.

Appendix B. List of Locally Assigned values of OUI 00-80-C2

     with preserved FCS   w/o preserved FCS    Media
    ------------------   -----------------    --------------
     0x00-01              0x00-07              802.3/Ethernet
     0x00-02              0x00-08              802.4
     0x00-03              0x00-09              802.5
     0x00-04              0x00-0A              FDDI
     0x00-05              0x00-0B              802.6
                          0x00-0D              Fragments
                          0x00-0E              BPDUs

Appendix C. Partial List of NLPIDs

     0x00    Null Network Layer or Inactive Set (not used with ATM)
     0x80    SNAP
     0x81    ISO CLNP
     0x82    ISO ESIS
     0x83    ISO ISIS
     0xCC    Internet IP

Appendix D. Applications of multiprotocol encapsulation

 Mutiprotocol encapsulation is necessary, but generally not
 sufficient, for routing and bridging over the ATM networks.   Since
 the publication of RFC 1483 (the predecessor of this memo), several
 system specifications were developed by the IETF and the ATM Forum to
 address various aspects of, or scenarios for, bridged or routed
 protocols.  This appendix summarizes these applications.
 1) Point-to-point connection between routers and bridges --
    multiprotocol encapsulation over ATM PVCs has been used to provide
    a simple point-to-point link between bridges and routers across an
    ATM network.  Some amount of manual configuration (e.g., in lieu
    of INARP) was necessary in these scenarios.
 2) Classical IP over ATM -- RFC 2225 (formerly RFC 1577) provides an
    environment where the ATM network serves as a logical IP subnet
    (LIS). ATM PVCs are supported, with address resolution provided by
    INARP.  For ATM SVCs, a new form of ARP, ATMARP, operates over the
    ATM network between a host (or router) and an ATMARP server.
    Where servers are replicated to provide higher availability or
    performance, a Server Synchronization Cache Protocol (SCSP)
    defined in RFC 2335 is used. Classical IP over ATM defaults to the
    LLC/SNAP encapsulation.

Grossman & Heinanen Standards Track [Page 20] RFC 2684 Multiprotocol Over AALS September 1999

 3) LAN Emulation -- The ATM Forum LAN Emulation specification
    provides an environment where the ATM network is enhanced by LAN
    Emulation Server(s) to behave as a bridged LAN.  Stations obtain
    configuration information from, and register with, a LAN Emulation
    Configuration Server;  they resolve MAC addresses to ATM addresses
    through the services of a LAN Emulation Server;  they can send
    broadcast and multicast frames, and also send unicast frames for
    which they have no direct VC to a Broadcast and Unicast Server.
    LANE uses the VC multiplexing encapsulation foramts for Bridged
    Etherent/802.3 (without LAN FCS) or Bridged 802.5 (without LAN
    FCS) for the Data Direct, LE Multicast Send and Multicast Forward
    VCCS.  However, the initial PAD field described in this memo is
    used as an LE header, and might not be set to all '0'.
 4) Next Hop Resolution Protocol (NHRP) -- In some cases, the
    constraint that Classical IP over ATM serve a single LIS limits
    performance.  NHRP, as defined in RFC 2332, extends Classical to
    allow 'shortcuts' over a an ATM network that supports several
    LISs.
 5) Multiprotocol over ATM (MPOA) -- The ATM Forum Multiprotocol over
    ATM Specification integrates LANE and NHRP to provide a generic
    bridging/routing environment.
 6) IP Multicast -- RFC 2022 extends Classical IP to support IP
    multicast.  A multicast address resolution server (MARS) is used
    possibly in conjunction with a multicast server to provide IP
    multicast behavior over ATM point-to-multipoint and/or point to
    point virtual connections.
 7) PPP over ATM -- RFC 2364 extends multiprotocol over ATM to the
    case where the encapsulated protocol is the Point-to-Point
    protocols.  Both the VC based multiplexing and LLC/SNAP
    encapsulations are used.  This approach is used when the ATM
    network is used as a point-to-point link and PPP functions are
    required.

Appendix E Differences from RFC 1483

 This memo replaces RFC 1483.  It was intended to remove anachronisms,
 provide clarifications of ambiguities discovered by implementors or
 created by changes to the base standards, and advance this work
 through the IETF standards track process.  A number of editorial
 improvements were made, the RFC 2119 [10] conventions applied, and
 the current RFC boilerplate added.  The following substantive changes
 were made.  None of them is believed to obsolete implementations of
 RFC 1483:

Grossman & Heinanen Standards Track [Page 21] RFC 2684 Multiprotocol Over AALS September 1999

  1. - usage of NLPID encapsulation is clarified in terms of the RFC 2119

conventions

  1. - a pointer to RFC 2364 is added to cover the case of PPP over ATM
  1. - RFC 1755 and RFC 2331 are referenced to describe how

encapsulations are negotiated, rather than a long-obsolete CCITT

    (now ITU-T) working document and references to work then in
    progress
  1. - usage of AAL5 is now a reference to ITU-T I.363.5. Options

created in AAL5 since the publication of RFC 1483 are selected.

  1. - formatting of routed NLPID-formatted PDUs (which are called

"routed ISO PDUs"

     in RFC 1483) is clarified
  1. - clarification is provided concerning the use of padding between

the PID and MAC destination address in bridged PDUs and the bit

    ordering of the MAC address.
  1. - clarification is provided concerning the use of padding of

Ethernet/802.3 frames

  1. - a new encapuslation for VPNs is added
  1. - substantive security considerations were added
  1. - a new appendix D provides a summary of applications of

multiprotocol over ATM

Authors' Addresses

 Dan Grossman
 Motorola, Inc.
 20 Cabot Blvd.
 Mansfield, MA 02048
 EMail: dan@dma.isg.mot.com
 Juha Heinanen
 Telia Finland
 Myyrmaentie 2
 01600 Vantaa, Finland
 EMail: jh@telia.fi

Grossman & Heinanen Standards Track [Page 22] RFC 2684 Multiprotocol Over AALS September 1999

Full Copyright Statement

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 This document and translations of it may be copied and furnished to
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 or assist in its implementation may be prepared, copied, published
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 Internet organizations, except as needed for the purpose of
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Grossman & Heinanen Standards Track [Page 23]

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