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