Network Working Group Juha Heinanen Reguest for Comments: DRAFT Telecom Finland Expires: December 12, 1992 June 12, 1992 Multiprotocol Interconnect over ATM Status of this Memo The purpose of this memo is to define approaches for multi-protocol operation over ATM. After field experience is obtained, some of the approaches may or may not be progressed to Internet Standards. Distribution of this memo is unlimited. 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. 1. Introduction 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 LLC/SNAP header. It is in the following called "LLC/SNAP Encapsulation". The second method is functionally equivalent to the first, but approaches ATM from the WAN perspective by prefixing the carried PDU by an NLPID/SNAP header. It is in the following called "NLPID/SNAP Encapsulation". The third method does the higher-layer protocol multiplexing implicitly by ATM Virtual Circuits (VCs) and 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 Heinanen [Page 1] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 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 Convergence Sublayer (CS) 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/SNAP Encapsulation or NLPID/SNAP 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 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/SNAP Encapsulation has been earlier defined for SMDS [2], whereas NLPID/SNAP Encapsulation resembles the encapsulation defined for Frame Relay [3], X.25 and ISDN [4]. Selection between the two encapsulation based multiplexing methods 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 schemes. 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 CS-PDU. The format of the AAL5 CS-PDU is given below: Heinanen [Page 2] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 AAL5 CS-PDU Format +-------------------------------+ | . | | . | | Payload | | (up to 2^16 - 1 octets) | | . | | . | +-------------------------------+ | PAD ( 0 - 47 octets) | +-------------------------------+\ | Control (2 octets) | | +-------------------------------+ | | Length (2 octets) | > CS-PDU Trailer +-------------------------------| | | CRC 32 (4 octets) | | +-------------------------------+/ The Payload field contains user information up to 2^16 - 1 octets. The PAD field pads the CS-PDU to fit exactly into the ATM cells such that the last 48 octet cell payload created by the SAR sublayer will have the CS-PDU Trailer right justified in the cell. The Control field is coded 0x00-00 and it is reserved for new AAL5 functions (if required). The Length field indicates the length, in octets, of the Payload field. The maximum value for the Length field is 65535 octets. The CRC 32 protects the Payload field + the PAD field + the Control field + the Length field. 4. LLC/SNAP Encapsulation Encapsulation Based Multiplexing is needed when several protocols are carried over the same VC. In order to allow the receiver to properly process the incoming AAL5 CS-PDU, the Payload Field must contain information necessary to identify the protocol of the routed or bridged PDU. In LLC/SNAP Encapsulation this information is encoded in an LLC/SNAP header placed in front of the carried PDU. 4.1. LLC/SNAP Encapsulation for Routed Protocols In LLC/SNAP 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 SNAP header. The LLC header consists of three one octet fields: Heinanen [Page 3] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 +------+------+------+ | DSAP | SSAP | Ctrl | +------+------+------+ The LLC header value 0xFE-FE-03 identifies that a routed ISO PDU (see [6] and the Appendix) follows. For routed ISO PDUs the format of the AAL5 CS-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 Network Layer Protocol ID (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 the currently known ones are listed in the Appendix. An NLPID value of 0x00 is defined in ISO/IEC TR 9577 as the Null Network Layer or Inactive Set. Since it has no significance within the context of this encapsulation scheme, a NLPID value of 0x00 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 EtherType of 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 CS-PDU Payload field for routed non-ISO PDUs Heinanen [Page 4] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 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: 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/SNAP Encapsulation for Bridged Protocols In LLC/SNAP 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 FCS is preserved within the bridged PDU. The 802.1 organization has assigned the following media type values than can be used in ATM encapsulation: Heinanen [Page 5] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 PID Values for OUI 0x00-80-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 In addition, the PID value 0x00-0E, when used with OUI 0x00-80-C2, identifies Bridged Protocol Data Units (BPDUs). The AAL5 CS-PDU Payload field carrying a bridged PDU will, 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. 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) | +-------------------------------+ Heinanen [Page 6] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 Payload Format for Bridged 802.4 PDUs +-------------------------------+ | LLC 0xAA-AA-03 | +-------------------------------+ | OUI 0x00-80-C2 | +-------------------------------+ | PID 0x00-02 or 0x00-08 | +-------------------------------+ | PAD 0x00-00-00 | +-------------------------------+ | Frame Control (1 octet) | +-------------------------------+ | MAC destination address | +-------------------------------+ | | | (remainder of MAC frame) | | | +-------------------------------+ | LAN FCS (if PID is 0x00-02) | +-------------------------------+ Payload Format for Bridged 802.5 PDUs +-------------------------------+ | LLC 0xAA-AA-03 | +-------------------------------+ | OUI 0x00-80-C2 | +-------------------------------+ | PID 0x00-03 or 0x00-09 | +-------------------------------+ | PAD 0x00-00-00 | +-------------------------------+ | Frame Control (1 octet) | +-------------------------------+ | MAC destination address | +-------------------------------+ | | | (remainder of MAC frame) | | | +-------------------------------+ | LAN FCS (if PID is 0x00-02) | +-------------------------------+ Heinanen [Page 7] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 Payload Format for Bridged FDDI PDUs +-------------------------------+ | LLC 0xAA-AA-03 | +-------------------------------+ | OUI 0x00-80-C2 | +-------------------------------+ | PID 0x00-04 or 0x00-0A | +-------------------------------+ | PAD 0x00-00-00 | +-------------------------------+ | Frame Control (1 octet) | +-------------------------------+ | MAC destination address | +-------------------------------+ | | | (remainder of MAC frame) | | | +-------------------------------+ | LAN FCS (if PID is 0x00-04) | +-------------------------------+ Payload Format for Bridged 802.6 PDUs +-------------------------------+ | LLC 0xAA-AA-03 | +-------------------------------+ | OUI 0x00-80-C2 | +-------------------------------+ | PID 0x00-0B | +-------------------------------+ ------ | Reserved | BEtag | Common +---------------+---------------+ PDU | BAsize | Header +-------------------------------+ ------- | MAC destination address | +-------------------------------+ | | | (remainder of MAC frame) | | | +-------------------------------+ | | +- Common PDU Trailer -+ | | +-------------------------------+ 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. Heinanen [Page 8] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 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. 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 +-------------------------------+ | LLC 0xAA-AA-03 | +-------------------------------+ | OUI 0x00-80-C2 | +-------------------------------+ | PID 0x00-0E | +-------------------------------+ ---- | 802.1(d) Protocol Identifier | BPDU, as defined +-------------------------------+ by 802.1(d), | Version = 00 | BPDU Type | section 5.3 +-------------------------------+ | | | (remainder of BPDU) | | | +-------------------------------+ ---- | LAN FCS | +-------------------------------+ 5. NLPID/SNAP Encapsulation In NLPID/SNAP Encapsulation the carried protocol is identified by prefixing the carried PDU by a one octet Network Layer Protocol ID (NLPID), which is possibly followed by an SNAP header. 5.1. NLPID/SNAP Encapsulation for Routed Protocols In NLPID/SNAP encapsulation the protocol of a routed PDU is identified by Network Level Protocol ID (NLPID), which is a one octet field Heinanen [Page 9] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 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. The known 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 CS-PDU Payload field shall be as follows: Payload Format for Routed NLPID PDUs +-------------------------------+ | NLPID (1 octet) | +-------------------------------+ | PAD (3 octets) | +-------------------------------+ | . | | Carried PDU | | (up to 2^16 - 5 octets) | | . | +-------------------------------+ The three octet PAD field after the NLPID field is used to align the carried PDU at a four octet boundary. In the particular case of an Internet IP datagram, the NLPID is 0xCC: Payload Format for Routed IP PDUs +-------------------------------+ | 0xCC | +-------------------------------+ | PAD (3 octets) | +-------------------------------+ | . | | IP PDU | | (up to 2^16 - 5 octets) | | . | +-------------------------------+ 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 Heinanen [Page 10] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 +------+------+------+------+------+ | 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 CS-PDU Payload field for routed non-NLPID PDUs shall thus be as follows: Payload Format for Routed non-NLPID PDUs +-------------------------------+ | NLPID 0x80 | +-------------------------------+ | PAD (2 octets) | +-------------------------------+ | OUI 0x00-00-00 | +-------------------------------+ | EtherType (2 octets) | +-------------------------------+ | . | | Carried PDU | | (up to 2^16 - 5 octets) | | . | +-------------------------------+ The two octet PAD field after the NLPID 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/SNAP Encapsulation for Bridged Protocols In NLPID/SNAP 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 Heinanen [Page 11] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 PDU. The 802.1 organization has assigned the following media type values than can be used in ATM encapsulation: PID Values for OUI 0x00-80-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 In addition, the PID value 0x00-0E, when used with OUI 0x00-80-C2, identifies Bridged Protocol Data Units (BPDUs). The AAL5 CS-PDU Payload field carrying a bridged PDU will, 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. Payload Format for Bridged Ethernet/802.3 PDUs +-------------------------------+ | 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) | +-------------------------------+ Heinanen [Page 12] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 Payload Format for Bridged 802.4 PDUs +-------------------------------+ | 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) | +-------------------------------+ Payload Format for Bridged 802.5 PDUs +-------------------------------+ | NLPID 0x80 | +-------------------------------+ | OUI 0x00-80-C2 | +-------------------------------+ | PID 0x00-03 or 0x00-09 | +-------------------------------+ | PAD 0x00 | +-------------------------------+ | Frame Control (1 octet) | +-------------------------------+ | MAC destination address | +-------------------------------+ | | | (remainder of MAC frame) | | | +-------------------------------+ | LAN FCS (if PID is 0x00-02) | +-------------------------------+ Heinanen [Page 13] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 Payload Format for Bridged FDDI PDUs +-------------------------------+ | 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) | +-------------------------------+ Payload Format for Bridged 802.6 PDUs +-------------------------------+ | NLPID 0x80 | +-------------------------------+ | OUI 0x00-80-C2 | +-------------------------------+ | PID 0x00-0B | +-------------------------------+ | PAD 0x00-00 | +-------------------------------+ ------ | 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 Heinanen [Page 14] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 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. 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 +-------------------------------+ | NLPID 0x80 | +-------------------------------+ | OUI 0x00-80-C2 | +-------------------------------+ | PID 0x00-0E | +-------------------------------+ ---- | 802.1(d) Protocol Identifier | BPDU, as defined +-------------------------------+ by 802.1(d), | Version = 00 | BPDU Type | section 5.3 +-------------------------------+ | | | (remainder of BPDU) | | | +-------------------------------+ ---- | LAN FCS | +-------------------------------+ 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 CS-PDU. This results in minimal bandwidth and processing overhead. Heinanen [Page 15] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 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. 6.1. VC Based Multiplexing of Routed Protocols PDUs of routed protocols shall be carried as such in the Payload of the AAL5 CS-PDU. The format of the AAL5 CS-PDU Payload field thus becomes: +-------------------------------+ | . | | Routed 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 CS-PDU exactly as described in section 4.2 except that only the fields after the PID field are included. In other words: o Ethernet and 802.3 PDUs are prefixed with two octets of padding. o 802.4, 802.5, and FDDI PDUs are prefixed with three octets of padding. o 802.6 PDUs and BPDUs are carried without padding. In case of Ethernet, 802.3, 802.4, 802.5, and FDDI PDUs the presense or absence of the trailing LAN FCS shall be identified implicitly by the VC, since the PID field is not included. PDUs with the LAN FCS and PDUs without the LAN FCS are thus considered to belong to different protocols even if the bridged media type would be the same. 7. Address Resolution An ATM network provides virtual circuits (VC) 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 private ATM networks connected by 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. Heinanen [Page 16] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 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/SNAP encoded CS-PDU Payload field as described in section 3. 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 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. Further elaboration of address resolution mechanisms for large public ATM networks is outside the scope of this memo. In case of (semi)permanent VCs, it is possible to use VPI/VCIs as ATM hardware addresses in the same way as DLCIs can used as hardware addresses in Frame Relay networks [3]. In case of dynamic VCs, the address resolution process is likely to first yield an "ATM address" of the destination. This address can then be given to the signalling function that sets up a VC between the two stations. The ATM address format for public ATM networks has not yet been defined in CCITT. It is, however, likely that a public ATM addresses will consists of an E.164 address which can be followed by a subaddress similar to the ISDN addresses defined in Q.931. The E.164 component of the ATM address will identify the subscription point to the public ATM network whereas the subaddress can be used to identify a station in an attached private ATM network. A common addressing scheme for private (local) ATM networks doesn't currently exist, but is likely to be defined in near future in order to allow inter- operation between private ATM switches. It is also possible to apply a method similar to InARP [9] for resolving protocol addresses when the VPI/VCI of the peer station is already known. The details of InARP for ATM are left for further study. 8. Bridging in an ATM Network An ATM interface acting as a bridge must be able to flood, forward, and filter bridged PDUs. Heinanen [Page 17] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 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 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, Joel Halpern of Network Systems, and Bob Hinden of Sun Mircosystems and for their detailed contributions. Security Considerations Security issues are not addressed in this memo. References [1] ANSI, "A New High Speed Data Transfer AAL". ANSI T1S11.5/91- 449, November 1991. [2] Piscitello, D. and Lawrence, C., "The Transmission of IP Heinanen [Page 18] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 Datagrams over the SMDS Service". RFC-1209, Bell Communications 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] Malis, A., Robinson, D., Ullman, R., "Multiprotocol Interconnect on X.25 and ISDN in the Packet Mode". Internet Draft, BBN Communications, Computervision Systems Integration, Process Software Corporation, April 6, 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 the SMDS Service". Internet Draft, Bellcore, March 1992. [9] Bradley, T., and C. Brown, "Inverse Address Resolution Protocol". RFC-1293, Wellfleet Communications, Inc., January 1992. Appendix List of Known 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 Heinanen [Page 19] RFC DRAFT Multiprotocol Interconnect over ATM June 1992 0x00-05 0x00-0B 802.6 0x00-0D Fragments 0x00-0E BPDUs 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 Expiration date: December 12, 1992 Heinanen [Page 20]