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Versions: 03 04 05 RFC 1483

Network Working Group                                      Juha Heinanen
Reguest for Comments: DRAFT                              Telecom Finland
Expires March 16, 1993                                  October 16, 1992


         Multiprotocol Interconnect over ATM Adaptation Layer 5


Status of this Memo

   This document is an Internet Draft.  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.  Internet Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet
   Drafts as reference material or to cite them other than as a
   ``working draft'' or ``work in progress.''  Please check the 1id-
   abstracts.txt listing contained in the internet-drafts Shadow
   Directories on nic.ddn.mil, nnsc.nsf.net, nic.nordu.net,
   ftp.nisc.sri.com, or munnari.oz.au to learn the current status of any
   Internet Draft.

Abstract

   The purpose of this memo is to define approaches for multi-protocol
   operation over ATM AAL5.  After field experience is obtained, some of
   the approaches may or may not be progressed to Internet Standards.
   Distribution of this memo is unlimited.

1.  Introduction

   Asynchronous Transfer Mode (ATM) based networks are of increasing
   interest for both local and wide area applications.  This memo
   describes three different methods for carrying connectionless network
   interconnect traffic (routed and bridged PDUs) over an ATM network.
   The first method approaches ATM from the LAN perspective and does
   higher-layer protocol multiplexing by prefixing the carried PDU by an
   IEEE 802.2 Logical Link Control (LLC) header.  It is in the following
   called "LLC Encapsulation".  The second method is functionally
   equivalent to the first, but approaches ATM from the WAN perspective
   by prefixing the carried PDU by an ISO/CCITT Network Layer Protocol
   ID (NLPID).  It is in the following called "NLPID Encapsulation".
   The third method does higher-layer protocol multiplexing implicitly
   by ATM Virtual Circuits (VCs) and is in the following called "VC
   Based Multiplexing".



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   ATM is a cell based transfer mode that requires variable length user
   information to be segmented and reassembled to/from short, fixed
   length cells.  This memo doesn't specify a new Segmentation And
   Reassembly (SAR) method for bridged and routed PDUs, but proposes
   that they are carried in the Payload field of Common Part Convergence
   Sublayer (CPCS) PDU of AAL5 [1].  AAL5 is a new simple and efficient
   ATM Adaptation Layer currently being standardized both in ANSI and
   CCITT.

2.  Selection of the Multiplexing Method

   It is envisioned that VC Based Multiplexing will be dominant in
   environments where dynamic creation of large numbers of ATM VCs is
   fast and economical.  These conditions are likely to first prevail in
   ATM LANs.  Encapsulation based approach to protocol multiplexing (LLC
   Encapsulation or NLPID Encapsulation), on the other hand, may be
   desirable when it is not practical for one reason or another to have
   a separate VC for each carried protocol.  This is the case, for
   example, if the ATM network only supports (semi) Permanent Virtual
   Circuits (PVCs) or if charging depends heavily on the number of
   simultaneous VCs.

   As already mentioned in the introduction, the two proposed
   encapsulation based multiplexing methods are functionally equivalent.
   A subset of LLC Encapsulation has been earlier defined for SMDS [2],
   whereas NLPID Encapsulation is the same as that used for Frame Relay
   [3].  The later also allows interworking of ATM and Frame Relay
   stations according to I.555 [4].  Selection between the two
   encapsulation based multiplexing methods for ATM may thus depend on
   compatibility requirements with other network technologies.  This
   memo doesn't prefer one or the other, but leaves the choice to the
   market.  Later, when practical experience has been obtained and the
   market response is known, a new memo may be issued that states the
   Internet policy regarding the two encapsulation methods.

   When two ATM stations wish to exchange connectionless network
   interconnect traffic, selection of the multiplexing method is done
   either by manual configuration (in case of PVCs) or by B-ISDN
   signalling procedures (in case of Switched VCs).  The details of B-
   ISDN signalling are still under study in CCITT [5].  It can, however,
   be assumed that B-ISDN signalling messages include a "Low layer
   compatibility" information element, which will allow negotiation of
   AAL5 and the carried (encapsulation) protocol.

3.  AAL5 Frame Format

   No matter which multiplexing method is selected, routed and bridged
   PDUs shall be encapsulated within the Payload field of AAL5 CPCS-PDU.



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   The format of the AAL5 CPCS-PDU is given below:

             AAL5 CPCS-PDU Format
            +-------------------------------+
            |        CPCS-PDU Header        |
            +-------------------------------+
            |             .                 |
            |             .                 |
            |           Payload             |
            |    (up to 2^16 - 1 octets)    |
            |             .                 |
            |             .                 |
            +-------------------------------+
            |      PAD ( 0 - 47 octets)     |
            +-------------------------------+\
            |       Reserved (2 octets)     | |
            +-------------------------------+ |
            |        Length (2 octets)      |  > CPCS-PDU Trailer
            +-------------------------------| |
            |         CRC (4 octets)        | |
            +-------------------------------+/

   The need and the functions of the CPCS-PDU Header are currently for
   further study in CCITT.  It is assumed in this memo that if the
   Header is included, its length will be a multiple of four 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.

   The Reserved field is coded 0x00-00 and is used to achieve 32 bit
   alignment in the CPCS-PDU trailer.  Additional functions besides the
   32 bit alignment are for further study in CCITT.

   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 zero is used for the abort function.

   The CRC field protects the CPCS-PDU Header (if included) + the
   Payload field + the PAD field + the Reserved field + the Length
   field.

4.  LLC Encapsulation

   Encapsulation Based Multiplexing is needed when several protocols are
   carried over the same VC.  In order to allow the receiver to properly



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   process the incoming AAL5 CPCS-PDU, the Payload Field must contain
   information necessary to identify the protocol of the routed or
   bridged PDU.  In LLC Encapsulation this information is encoded in an
   LLC header placed in front of the carried PDU.

   Although this memo only covers protocols that operate over LLC Type 1
   (unacknowledged connectionless mode) service, the same encapsulation
   principle applies also to protocols operating over LLC Type 2
   (connection-mode) service.  In the latter case the format and/or
   contents of the LLC header would differ from what is shown below.

4.1.  LLC Encapsulation for Routed Protocols

   In LLC based encapsulation the protocol of the routed PDU is
   identified by prefixing the PDU by an IEEE 802.2 LLC header, which is
   possibly followed by an IEEE 802.1a SubNetwork Attachment Point
   (SNAP) header.  In LLC Type 1 operation, the LLC header consists of
   three one octet fields:

            +------+------+------+
            | DSAP | SSAP | Ctrl |
            +------+------+------+

   In LLC encapsulation for routed protocols, the Control field has
   always value 0x03 specifying Unnumbered Information Command PDU.

   The LLC header value 0xFE-FE-03 identifies that a routed ISO PDU (see
   [6] and the appendix) follows.  The Control field value 0x03
   specifies Unnumbered Information Command PDU.  For routed ISO PDUs
   the format of the AAL5 CPCS-PDU Payload field shall thus be as
   follows:

             Payload Format for Routed ISO PDUs
            +-------------------------------+
            |       LLC  0xFE-FE-03         |
            +-------------------------------+
            |             .                 |
            |           ISO PDU             |
            |     (up to 2^16 - 4 octets)   |
            |             .                 |
            +-------------------------------+

   The routed ISO protocol is identified by a one octet NLPID field that is
   part of Protocol Data.  NLPID values are administered by ISO and CCITT.
   They are defined in ISO/IEC TR 9577 [6] and some of the currently
   defined ones are listed in the appendix.

   An NLPID value of 0x00 is defined in ISO/IEC TR 9577 as the Null Network



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   Layer or Inactive Set.  Since it has no significance within the context
   of this encapsulation scheme, a NLPID value of 0x00 is invalid under the
   ATM encapsulation.

   It would also be possible to use the above encapsulation for IP, since,
   although not an ISO protocol, IP has an NLPID value 0xCC defined for it.
   This format shall, however, not be used.  Instead, IP is encapsulated
   like all other routed non-ISO protocols by identifying it in the SNAP
   header that immediately follows the LLC header.

   The presence of a SNAP header is indicated by the LLC header value
   0xAA-AA-03. A SNAP header is of the form

            +------+------+------+------+------+
            |         OUI        |     PID     |
            +------+------+------+------+------+

   The three-octet Organizationally Unique Identifier (OUI) identifies an
   organization which administers the meaning of the following two octet
   Protocol Identifier (PID).  Together they identify a distinct routed or
   bridged protocol.  The OUI value 0x00-00-00 specifies that the following
   PID is an EtherType.

   The format of the AAL5 CPCS-PDU Payload field for routed non-ISO PDUs
   shall thus be as follows:

             Payload Format for Routed non-ISO PDUs
            +-------------------------------+
            |       LLC  0xAA-AA-03         |
            +-------------------------------+
            |        OUI 0x00-00-00         |
            +-------------------------------+
            |     EtherType (2 octets)      |
            +-------------------------------+
            |             .                 |
            |         Non-ISO PDU           |
            |     (up to 2^16 - 9 octets)   |
            |             .                 |
            +-------------------------------+

   In the particular case of an Internet IP PDU, the Ethertype value is
   0x08-00:









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             Payload Format for Routed IP PDUs
            +-------------------------------+
            |       LLC  0xAA-AA-03         |
            +-------------------------------+
            |        OUI 0x00-00-00         |
            +-------------------------------+
            |       EtherType 0x08-00       |
            +-------------------------------+
            |             .                 |
            |           IP PDU              |
            |     (up to 2^16 - 9 octets)   |
            |             .                 |
            +-------------------------------+

4.2.  LLC Encapsulation for Bridged Protocols

   In LLC Encapsulation bridged PDUs are encapsulated by identifying the
   type of the bridged media in the SNAP header.  As with routed non-ISO
   protocols, the presence of the SNAP header is indicated by the LLC
   header value 0xAA-AA-03.  With bridged protocols the OUI value in the
   SNAP header is the 802.1 organization code 0x00-80-C2 and the actual
   type of the bridged media is specified by the two octet PID.
   Additionally, the PID indicates whether the original Frame Check
   Sequence (FCS) is preserved within the bridged PDU.  The media type
   (PID) values that can be used in ATM encapsulation are listed in the
   appendix.

   The AAL5 CPCS-PDU Payload field carrying a bridged PDU shall, therefore,
   have one of the following formats.  Padding is added after the PID field
   if necessary in order to align the user information of the bridged PDU
   at a four octet boundary.




















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             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)  |
            +-------------------------------+

             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)  |
            +-------------------------------+












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             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)  |
            +-------------------------------+

      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 can be set to
      any value (XX).

             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)  |
            +-------------------------------+





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             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       -+
            |                               |
            +-------------------------------+

      Note that in bridged 802.6 PDUs, there is only one choice for the
      PID value, since the presence of a CRC-32 is indicated by the CIB
      bit in the header of the MAC frame.

      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.






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             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)      |
            |                               |
            +-------------------------------+
            |           LAN FCS             |
            +-------------------------------+

5.  NLPID Encapsulation

   In NLPID Encapsulation, the protocol of the routed or bridged PDU is
   identified by prefixing the PDU by a one octet NLPID, which is possibly
   followed by a SNAP header.  For compatibility with RFC 1294 and I.555,
   the AAL5 CPCS-PDU Payload field always starts with three extra octets
   corresponding to the Address and Control fields of Q.922.  In some
   cases, additional padding is needed before the NLPID octet in order to
   align the routed PDU or the payload of the bridged PDU at a four octet
   boundary.

   Although this memo only describes how NLPID encapsulation is used to
   carry connectionless PDUs over an ATM network, the same encapsulation
   principle also applies to connection-oriented protocols, such as X.25
   and Q.931, for which appropriate NLPID values have been allocated.

5.1.  NLPID Encapsulation for Routed Protocols

   In NLPID encapsulation the protocol of a routed PDU is identified by
   Network Level Protocol ID, which is a one octet field administered by
   ISO and CCITT.  Values for this field are defined in ISO/IEC TR 9577
   [6].  A NLPID value of 0x00 is defined within 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 is
   invalid under the ATM encapsulation.  Some of the currently defined
   NLPID values are listed in the appendix.

   As it can be seen from the appendix, a unique NLPID value has been
   assigned for some routed protocols, such as IP and CLNP.  In such cases
   the format of the AAL5 CPCS-PDU Payload field shall be as follows:





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             Payload Format for Routed NLPID PDUs
            +-------------------------------+
            |   XXXXXXX0 (Q.922 Addr MSO)   |
            +-------------------------------+
            |   XXXXXXX1 (Q.922 Addr LSO)   |
            +-------------------------------+
            |      0x03 (Q.922 Control)     |
            +-------------------------------+
            |        NLPID (1 octet)        |
            +-------------------------------+
            |             .                 |
            |         Carried PDU           |
            |     (up to 2^16 - 5 octets)   |
            |             .                 |
            +-------------------------------+

   The value of the first two octets is constrained so that the they
   form a valid two octet Q.922 Address field.  This means that the last
   bit of the first octet must be set to 0 and the last bit of the
   second octet must be set to 1.  All other bits (marked with letter X)
   can be set to any value.  The third octet is the Q.922 Control field
   and shall according to RFC 1294 have the value 0x03.  Note that these
   three octets together with the NLPID octet align the carried PDU at a
   four octet boundary.

   In the particular case of an Internet IP PDU, the NLPID is 0xCC:

             Payload Format for Routed IP PDUs
            +-------------------------------+
            |   XXXXXXX0 (Q.922 Addr MSO)   |
            +-------------------------------+
            |   XXXXXXX1 (Q.922 Addr LSO)   |
            +-------------------------------+
            |      0x03 (Q.922 Control)     |
            +-------------------------------+
            |         NLPID  0xCC           |
            +-------------------------------+
            |             .                 |
            |           IP PDU              |
            |    (up to 2^16 - 5 octets)    |
            |             .                 |
            +-------------------------------+

   In the particular case of a CLNP PDU, the NLPID is 0x81:







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             Payload Format for Routed CLNP PDUs
            +-------------------------------+
            |   XXXXXXX0 (Q.922 Addr MSO)   |
            +-------------------------------+
            |   XXXXXXX1 (Q.922 Addr LSO)   |
            +-------------------------------+
            |      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 shall thus 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 a SNAP header follows.

   A SNAP header is of the form

            +------+------+------+------+------+
            |         OUI        |     PID     |
            +------+------+------+------+------+

   The three-octet Organizationally Unique Identifier (OUI) identifies
   an organization which administers the meaning of the two-octet
   Protocol Identifier (PID) which follows.  Together they identify a
   distinct protocol.  Note that OUI 0x00-00-00 specifies that the
   following PID is an EtherType.

   The format of the AAL5 CPCS-PDU Payload field for routed non-NLPID
   PDUs shall thus be as follows:












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             Payload Format for Routed non-NLPID PDUs
            +-------------------------------+
            |   XXXXXXX0 (Q.922 Addr MSO)   |
            +-------------------------------+
            |   XXXXXXX1 (Q.922 Addr LSO)   |
            +-------------------------------+
            |      0x03 (Q.922 Control)     |
            +-------------------------------+
            |       PAD 0x00-00-00          |
            +-------------------------------+
            |         NLPID  0x80           |
            +-------------------------------+
            |        OUI  0x00-00-00        |
            +-------------------------------+
            |     EtherType (2 octets)      |
            +-------------------------------+
            |              .                |
            |          Carried PDU          |
            |     (up to 2^16 - 9 octets)   |
            |              .                |
            +-------------------------------+

   The three octet PAD field is used to align the carried PDU at a four
   octet boundary.

   According to the above described encapsulation scheme, PDUs of those
   protocols, such as IP and CLNP, that have a unique NLPID assigned to
   them, could be encapsulated in two different ways.  In order to
   eliminate the conflict, the SNAP variation shall only be used if no
   NLPID value is defined for the given protocol.  This results in a
   smaller frame and less processing overhead at the receiver.

5.2.  NLPID Encapsulation for Bridged Protocols

   In NLPID Encapsulation bridged PDUs are encapsulated by identifying the
   type of the bridged media in the SNAP header.  As with routed non-NLPID
   protocols, the presence of the SNAP header is indicated by the NLPID
   value 0x80.  With bridged protocols the OUI value in the SNAP header is
   the 802.1 organization code 0x00-80-C2 and the actual type of the
   bridged media is specified by the two octet PID.  Additionally, the PID
   indicates whether the original FCS is preserved within the bridged PDU.
   The media type (PID) values that can be used in ATM encapsulation are
   listed in the Appendix.

   The AAL5 CPCS-PDU Payload field carrying a bridged PDU shall, therefore,
   have one of the following formats.  Padding is added before the NLPID
   field if necessary in order to align the user information of the bridged
   PDU at a four octet boundary.



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             Payload Format for Bridged Ethernet/802.3 PDUs
            +-------------------------------+
            |   XXXXXXX0 (Q.922 Addr MSO)   |
            +-------------------------------+
            |   XXXXXXX1 (Q.922 Addr LSO)   |
            +-------------------------------+
            |      0x03 (Q.922 Control)     |
            +-------------------------------+
            |           PAD 0x00            |
            +-------------------------------+
            |          NLPID  0x80          |
            +-------------------------------+
            |        OUI 0x00-80-C2         |
            +-------------------------------+
            |    PID 0x00-01 or 0x00-07     |
            +-------------------------------+
            |    MAC destination address    |
            +-------------------------------+
            |                               |
            |   (remainder of MAC frame)    |
            |                               |
            +-------------------------------+
            |  LAN FCS (if PID is 0x00-01)  |
            +-------------------------------+



























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RFC DRAFT       Multiprotocol Interconnect over ATM AAL5    October 1992


             Payload Format for Bridged 802.4 PDUs
            +-------------------------------+
            |   XXXXXXX0 (Q.922 Addr MSO)   |
            +-------------------------------+
            |   XXXXXXX1 (Q.922 Addr LSO)   |
            +-------------------------------+
            |      0x03 (Q.922 Control)     |
            +-------------------------------+
            |           PAD 0x00            |
            +-------------------------------+
            |          NLPID  0x80          |
            +-------------------------------+
            |        OUI 0x00-80-C2         |
            +-------------------------------+
            |    PID 0x00-02 or 0x00-08     |
            +-------------------------------+
            |           PAD 0x00            |
            +-------------------------------+
            |    Frame Control (1 octet)    |
            +-------------------------------+
            |    MAC destination address    |
            +-------------------------------+
            |                               |
            |   (remainder of MAC frame)    |
            |                               |
            +-------------------------------+
            |  LAN FCS (if PID is 0x00-02)  |
            +-------------------------------+























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             Payload Format for Bridged 802.5 PDUs
            +-------------------------------+
            |   XXXXXXX0 (Q.922 Addr MSO)   |
            +-------------------------------+
            |   XXXXXXX1 (Q.922 Addr LSO)   |
            +-------------------------------+
            |      0x03 (Q.922 Control)     |
            +-------------------------------+
            |           PAD 0x00            |
            +-------------------------------+
            |          NLPID  0x80          |
            +-------------------------------+
            |        OUI 0x00-80-C2         |
            +-------------------------------+
            |    PID 0x00-03 or 0x00-09     |
            +-------------------------------+
            |           PAD 0xXX            |
            +-------------------------------+
            |    Frame Control (1 octet)    |
            +-------------------------------+
            |    MAC destination address    |
            +-------------------------------+
            |                               |
            |   (remainder of MAC frame)    |
            |                               |
            +-------------------------------+
            |  LAN FCS (if PID is 0x00-03)  |
            +-------------------------------+

      Note that the 802.5 Access Control (AC) field has no significance
      outside the local 802.5 subnetwork.  It can thus be regarded as a
      PAD field, which can be set to any value (XX).



















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             Payload Format for Bridged FDDI PDUs
            +-------------------------------+
            |   XXXXXXX0 (Q.922 Addr MSO)   |
            +-------------------------------+
            |   XXXXXXX1 (Q.922 Addr LSO)   |
            +-------------------------------+
            |      0x03 (Q.922 Control)     |
            +-------------------------------+
            |           PAD 0x00            |
            +-------------------------------+
            |          NLPID  0x80          |
            +-------------------------------+
            |        OUI 0x00-80-C2         |
            +-------------------------------+
            |    PID 0x00-04 or 0x00-0A     |
            +-------------------------------+
            |           PAD 0x00            |
            +-------------------------------+
            |    Frame Control (1 octet)    |
            +-------------------------------+
            |    MAC destination address    |
            +-------------------------------+
            |                               |
            |   (remainder of MAC frame)    |
            |                               |
            +-------------------------------+
            |  LAN FCS (if PID is 0x00-04)  |
            +-------------------------------+























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             Payload Format for Bridged 802.6 PDUs
            +-------------------------------+
            |   XXXXXXX0 (Q.922 Addr MSO)   |
            +-------------------------------+
            |   XXXXXXX1 (Q.922 Addr LSO)   |
            +-------------------------------+
            |      0x03 (Q.922 Control)     |
            +-------------------------------+
            |        PAD 0x00-00-00         |
            +-------------------------------+
            |          NLPID  0x80          |
            +-------------------------------+
            |        OUI 0x00-80-C2         |
            +-------------------------------+
            |         PID 0x00-0B           |
            +-------------------------------+ ------
            |   Reserved    |     BEtag     |  Common
            +---------------+---------------+  PDU
            |            BAsize             |  Header
            +-------------------------------+ -------
            |    MAC destination address    |
            +-------------------------------+
            |                               |
            |   (remainder of MAC frame)    |
            |                               |
            +-------------------------------+
            |                               |
            +-    Common PDU Trailer       -+
            |                               |
            +-------------------------------+

      Note that in bridged 802.6 PDUs, there is only one choice for the
      PID value, since the presence of a CRC-32 is indicated by the CIB
      bit in the header of the MAC frame.

      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.



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      One should 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.

             Payload Format for BPDUs
            +-------------------------------+
            |   XXXXXXX0 (Q.922 Addr MSO)   |
            +-------------------------------+
            |   XXXXXXX1 (Q.922 Addr LSO)   |
            +-------------------------------+
            |      0x03 (Q.922 Control)     |
            +-------------------------------+
            |          NLPID  0x80          |
            +-------------------------------+
            |        OUI 0x00-80-C2         |
            +-------------------------------+
            |         PID 0x00-0E           |
            +-------------------------------+
            |                               |
            |      BPDU as defined by       |
            |     802.1(d) or 802.1(g)      |
            |                               |
            +-------------------------------+
            |           LAN FCS             |
            +-------------------------------+

5.3.  Fragmentation Issues

   In the event of I.555 interworking with Frame Relay, a fragmented frame
   may (rarely) be encountered.  Frame fragments have an OUI of 0x00-80-C2
   and a PID of 0x00-0D.  For optional fragmentation procedures, see
   section 9 of RFC 1294.  In general, fragmentation is not recommended.

6.  VC Based Multiplexing

   In VC Based Multiplexing, the carried network interconnect protocol is
   identified implicitly by the VC connecting the two ATM stations, i.e.
   each protocol must be carried over a separate VC.  There is therefore no
   need to include explicit multiplexing information in the Payload of the
   AAL5 CPCS-PDU.  This results in minimal bandwidth and processing overhead.

   As indicated above, the carried protocol can be either manually
   configured or negotiated dynamically during call establishment using
   signalling procedures.  The signalling details will be defined later in
   other RFCs when the relevant standards have become available.





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6.1.  VC Based Multiplexing of Routed Protocols

   PDUs of routed protocols shall be carried as such in 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 Based Multiplexing of Bridged Protocols

   PDUs of bridged protocols shall be carried in the Payload of the AAL5
   CPCS-PDU exactly as described in section 4.2 except that only the
   fields after the PID field are included.  The AAL5 CPCS-PDU Payload
   field carrying a bridged PDU shall, 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) |
            +-------------------------------+

















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             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)      |
            |                               |
            +-------------------------------+
            |           LAN FCS             |
            +-------------------------------+

   In case of Ethernet, 802.3, 802.4, 802.5, and FDDI PDUs the presense



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   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.  Address Resolution

   An ATM network provides VCs that form the basis for connections
   between stations attached to it.  A VC may also span over several ATM
   networks in an "ATM internet" consisting of an arbitrary
   concatenation of private ATM and public ATM networks.  ATM VCs can be
   establish either (semi)permanently by the operator of the ATM network
   or dynamically by an ATM signalling protocol being defined by CCITT.
   In either case, each VC is identified by a Virtual Path Identifier
   (VPI) and a Virtual Channel Identifier (VCI).  These identifiers have
   only local significance at each ATM interface.

   The support of multicasting in ATM networks is also presently under
   study in CCITT.  If an ATM network supports multicasting, a special
   VPI/VCI pair can be used to indicate the sending of ATM cells to all
   stations in a particular multicast group.  An ATM station may use the
   multicasting capability to dynamically resolve a protocol address to
   a hardware address using the standard Address Resolution Protocol
   (ARP) [7].  ARP packets are encapsulated within an LLC encoded CPCS-
   PDU Payload field as described in section 4.  The details of
   multicast based address resolution will be described in a future RFC
   when more information is available on the ATM multicast mechanism.

   Multicast based address resolution will not be practical over large
   public or private ATM networks.  In such cases it might be possible
   to apply a technique similar to "shortcut routing" [8] to augment the
   address resolution process.  Address resolution could also work using
   a "well known" VC that connects to one or more address resolution
   servers.  Another possibility might be to use DNS to store both the
   internet address and the physical ATM address of the destination.
   Finally, as proposed in [9], an ATM network could support signalling
   based on internet addresses in which case no address resolution would
   be needed.  Further elaboration of address resolution mechanisms is
   outside the scope of this memo.

8.  Bridging in an ATM Network

   An ATM interface acting as a 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



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   copying it to each VC or by using a multicast 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 shall conform to the
   encapsulation described within section 3.  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.

9. For Further Study

   Due to incomplete standardization of ATM multicasting, addressing,
   and signalling mechanisms, details related to the negotiation of the
   multiplexing method as well as address resolution had to be left for
   further study.  Also, this memo didn't try to specify which one (if
   any) of the two encapsulation based multiplexing methods should be
   preferred.  Practical experience with real ATM networks is needed
   before these issues can be fully resolved.

Acknowledgements

   This document has evolved from RFCs [2] and [3] from which much of
   the material has been adopted.  Thanks to their authors T.  Bradley,
   C.  Brown, A. Malis, D. Piscitello, and C. Lawrence.  In addition,
   the expertise of the ATM working group of the IETF has been
   invaluable in completing the document.  Special thanks Brian
   Carpenter of CERN, Rao Cherukuri of IBM, Dan Grossman of Motorola,
   Joel Halpern of Network Systems, Bob Hinden of Sun Mircosystems, and
   Gary Kessler of MAN Technology Corporation for their detailed
   contributions.

Security Considerations

   Security issues are not addressed in this memo.

References

   [1]  CCITT, "Report of WPXVIII".  CCITT Study Group XVIII, Working
        Party 8, TD 62, Annex 5, Section C, Geneva 8-19 June, 1992.

   [2]  Piscitello, D. and Lawrence, C., "The Transmission of IP
        Datagrams over the SMDS Service".  RFC 1209, Bell Communications



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        Research, March 1991.

   [3]  Bradley, T., Brown, C., and Malis, A., "Multiprotocol
        Interconnect over Frame Relay".  RFC 1294, Wellfleet
        Communications, Inc. and BBN Communications, January 1992.

   [4]  CCITT, "Draft Recommendation I.555".  CCITT Study Group XVIII,
        Working Party 2, TD 36, Annex 4, Geneva 8-19 June, 1992.

   [5]  CCITT, "Draft text for Q.93B".  CCITT Study Group XI, Working
        Party XI/6-37, March 9-20, 1992.

   [6]  Information technology - Telecommunications and Information
        Exchange Between Systems, "Protocol Identification in the
        Network Layer".  ISO/IEC TR 9577, October 1990.

   [7]  Plummer, David C., "An Ethernet Address Resolution Protocol".
        RFC 826, Symbolics, Inc., November 1982.

   [8]  Tsuchiya, Paul, "Discovery and Routing over Large Public Data
        Networks".  Internet Draft, Bellcore, July 1992.

   [9]  Lyon, T., Liaw, F., and Romanow, A., "Network Layer Architecture
        for ATM Networks".  Internet Draft, Sun Microsystems, July 1992.

Appendix

   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

   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




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Author's Address

   Juha Heinanen Telecom Finland, PO Box 228, SF-33101 Tampere, Finland

   Phone: +358 49 500 958

   Email: Juha.Heinanen@datanet.tele.fi












































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