rfc1483
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　　Network Working GroupJuha Heinanen
　　Reguest for Comments: 1483Telecom 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 RFC 1483Multiprotocol over AAL5July 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 1483Multiprotocol over AAL5July 1993
　　AAL5 CPCS-PDU Format
　　　
　　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
　　
　　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.