rfc1483.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

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



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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]

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                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



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   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



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