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Network Working Group                                      Juha Heinanen
Reguest for Comments: 1483                               Telecom Finland
                                                               July 1993

            Multiprotocol Encapsulation over ATM Adaptation Layer 5

Status of this Memo

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

Abstract

   This memo describes two encapsulations methods for carrying network
   interconnect traffic over ATM AAL5.  The first method allows
   multiplexing of multiple protocols over a single ATM virtual circuit
   whereas the second method assumes that each protocol is carried over
   a separate ATM virtual circuit.

1.  Introduction

   Asynchronous Transfer Mode (ATM) based networks are of increasing
   interest for both local and wide area applications.  This memo
   describes two different methods for carrying connectionless network
   interconnect traffic, routed and bridged Protocol Data Units (PDUs),
   over an ATM network.  The first method allows multiplexing of
   multiple protocols over a single ATM virtual circuit.  The protocol
   of a carried PDU is identified by prefixing the PDU by an IEEE 802.2
   Logical Link Control (LLC) header.  This method is in the following
   called "LLC Encapsulation" and a subset of it has been earlier
   defined for SMDS [1].  The second method does higher-layer protocol
   multiplexing implicitly by ATM Virtual Circuits (VCs).  It is in the
   following called "VC Based Multiplexing".


   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.  Instead, the
   PDUs are carried in the Payload field of Common Part Convergence
   Sublayer (CPCS) PDU of ATM Adaptation Layer type 5 (AAL5) [2].

   Note that this memo only describes how routed and bridged PDUs are
   carried directly over the CPCS of AAL5, i.e., when the Service
   Specific Convergence Sublayer (SSCS) of AAL5 is empty.  If Frame



Heinanen                                                        [Page 1]

RFC 1483                Multiprotocol over AAL5                July 1993


   Relay Service Specific Convergence Sublayer (FR-SSCS), as defined in
   I.36x.1 [3], is used over the CPCS of AAL5, then routed and bridged
   PDUs are carried using the NLPID multiplexing method described in RFC
   1294 [4].  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 1294.

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
   private ATM networks.  LLC 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.

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


















Heinanen                                                        [Page 2]

RFC 1483                Multiprotocol over AAL5                July 1993


                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.

   The CPCS-UU (User-to-User indication) field is used to transparently
   transfer CPCS user to user information.  The field has no function
   under the multiprotocol ATM encapsulation described in this memo and
   can be set to any value.

   The CPI (Common Part Indicator) field alings the CPCS-PDU trailer to
   64 bits.  Possible additional functions are for further study in
   CCITT.  When only the 64 bit alignment function is used, this field
   shall be codes 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 protects the entire CPCS-PDU except the CRC field
   itself.

4.  LLC Encapsulation

   LLC Encapsulation is needed when several protocols are carried over
   the same VC.  In order to allow the receiver to properly process the
   incoming AAL5 CPCS-PDU, the Payload Field must contain information



Heinanen                                                        [Page 3]

RFC 1483                Multiprotocol over AAL5                July 1993


   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 deals with 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 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 Appendix B) 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 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



Heinanen                                                        [Page 4]

RFC 1483                Multiprotocol over AAL5                July 1993


   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 must 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:










Heinanen                                                        [Page 5]

RFC 1483                Multiprotocol over AAL5                July 1993


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

   This is compatible with RFC 1042 [7].  Any changes in the header
   format specified in RFC 1042 should be followed by this memo.

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
   Appendix B.

   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
   field of the bridged PDU at a four octet boundary.

















Heinanen                                                        [Page 6]

RFC 1483                Multiprotocol over AAL5                July 1993


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











Heinanen                                                        [Page 7]

RFC 1483                Multiprotocol over AAL5                July 1993


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





Heinanen                                                        [Page 8]

RFC 1483                Multiprotocol over AAL5                July 1993


                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.

   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



Heinanen                                                        [Page 9]

RFC 1483                Multiprotocol over AAL5                July 1993


   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.

               +-------------------------------+
               |       LLC  0xAA-AA-03         |
               +-------------------------------+
               |        OUI 0x00-80-C2         |
               +-------------------------------+
               |         PID 0x00-0E           |
               +-------------------------------+
               |                               |
               |      BPDU as defined by       |
               |     802.1(d) or 802.1(g)      |
               |                               |
               +-------------------------------+

5.  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.


5.1.  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)    |
               |             .                 |
               |             .                 |
               +-------------------------------+




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RFC 1483                Multiprotocol over AAL5                July 1993


5.2.  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) |
               +-------------------------------+


                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).










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RFC 1483                Multiprotocol over AAL5                July 1993


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

   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.

6.  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
   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



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RFC 1483                Multiprotocol over AAL5                July 1993


   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 4.  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.

7. 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 RFCs.

Acknowledgements

   This document has evolved from RFCs [1] and [4] 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]  Piscitello, D. and Lawrence, C., "The Transmission of IP
        Datagrams over the SMDS Service".  RFC 1209, Bell Communications
        Research, March 1991.

   [2]  CCITT, "Draft Recommendation I.363".  CCITT Study Group XVIII,
        Geneva, 19 - 29 January, 1993.

   [3]  CCITT, "Draft Recommendation I.36x.1".  CCITT Study Group XVIII,
        Geneva, 19-29 January, 1993.

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




Heinanen                                                       [Page 13]

RFC 1483                Multiprotocol over AAL5                July 1993


   [5]  CCITT, "Draft text for Q.93B".  CCITT Study Group XI, 23
        September - 2 October, 1992.

   [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 Reynolds, J., "A Standard for the Transmission of
        IP Datagrams over IEEE 802 Networks".  RFC 1042, ISI, February,
        1988.

Appendix A.  Multiprotocol Encapsulation over FR-SSCS

   I.36x.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 1294.  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/CCITT Network Layer Protocol ID (NLPID).

   In the particular case of an IP PDU, the NLPID is 0xCC and the FR-
   SSCS-PDU has the following format:



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RFC 1483                Multiprotocol over AAL5                July 1993


                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 1294 the Q.922 Address field shall be
   either 2 or 4 octets, i.e., a 3 octet Address field is not supported.

   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 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 an IEEE 802.1a SubNetwork Attachment
   Point (SNAP) header follows.

   See RFC 1294 for more details related to multiprotocol encapsulation
   over FRCS.



Heinanen                                                       [Page 15]

RFC 1483                Multiprotocol over AAL5                July 1993


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

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



















Heinanen                                                       [Page 16]