IP over ATM AAL5: Multiprotocol Encapsulation over ATM Adaptation Layer 5
How routed and bridged packets are carried over an ATM network, either one protocol per circuit or many on one circuit.
Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation Layer 5", RFC 1483, DOI 10.17487/RFC1483, July 1993, <https://www.rfc-editor.org/info/rfc1483>.
@misc{rfc1483,
series = {Request for Comments},
number = 1483,
howpublished = {RFC 1483},
publisher = {RFC Editor},
doi = {10.17487/RFC1483},
url = {https://www.rfc-editor.org/info/rfc1483},
author = {J. Heinanen},
title = {{Multiprotocol Encapsulation over ATM Adaptation Layer 5}},
pagetotal = 16,
year = 1993,
month = jul,
} RFC text
Contents
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
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:
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
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
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:
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.
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) |
+-------------------------------+
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) |
+-------------------------------+
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
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) |
| . |
| . |
+-------------------------------+
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).
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
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.
[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:
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.
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
ISO Plain Language
On this page
§1Two Ways to Carry Traffic over ATM
Asynchronous Transfer Mode (ATM) based networks are of increasing interest for both local and wide area applications. The 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, LLC Encapsulation, allows multiplexing of multiple protocols over a single ATM virtual circuit. The protocol of a carried PDU is identified by prefixing the PDU with an IEEE 802.2 Logical Link Control (LLC) header. A subset of this method was earlier defined for SMDS.
The second method, VC Based Multiplexing, does higher-layer protocol multiplexing implicitly through ATM Virtual Circuits (VCs): each protocol is carried over a separate virtual circuit, so nothing has to be added to the payload.
ATM is a cell based transfer mode that requires variable length user information to be segmented and reassembled to and from short, fixed length cells. This memo does not specify a new Segmentation And Reassembly method for bridged and routed PDUs. Instead the PDUs are carried in the Payload field of the Common Part Convergence Sublayer (CPCS) PDU of ATM Adaptation Layer type 5 (AAL5). The memo only describes how routed and bridged PDUs are carried directly over the CPCS of AAL5, that is, when the Service Specific Convergence Sublayer of AAL5 is empty.
Two choices are on the table. Either every packet gets a small header saying what protocol it holds, so one ATM connection can carry several protocols, or each protocol gets its own ATM connection and the packet is left exactly as it is.
§2Choosing the Multiplexing Method
The RFC expects VC Based Multiplexing to be dominant in environments where dynamic creation of large numbers of ATM VCs is fast and economical, and says 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. That is the case, for example, if the ATM network only supports semi-permanent or 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, the selection of the multiplexing method is done either by manual configuration, in the case of PVCs, or by B-ISDN signalling procedures, in the case of switched VCs. The RFC notes that the details of B-ISDN signalling were still under study in CCITT, but that B-ISDN signalling messages can be assumed to include a low layer compatibility information element, which will allow negotiation of AAL5 and the carried encapsulation protocol.
Use one connection per protocol when setting up connections is cheap. Use a header on every packet when it is not, for example when the network only offers permanent connections or charges by connection.
| Aspect | LLC Encapsulation | VC Based Multiplexing |
|---|---|---|
| How the protocol is identified | By an LLC header, possibly followed by a SNAP header | Implicitly, by the virtual circuit |
| Virtual circuits needed | Several protocols can share one | One per protocol |
| Overhead | Extra header bytes in every payload | Minimal bandwidth and processing overhead |
| Suited to | Networks with only permanent circuits, or charging per circuit | Networks where creating many circuits is fast and economical |
§3The AAL5 Frame Format
No matter which multiplexing method is selected, routed and bridged PDUs are encapsulated within the Payload field of the AAL5 CPCS-PDU. 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, so that the last 48 octet cell payload created by the SAR sublayer has the CPCS-PDU Trailer right justified in the cell. It holds 0 to 47 octets.
The CPCS-UU, or 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 the memo and can be set to any value. The CPI, or Common Part Indicator, field aligns the CPCS-PDU trailer to 64 bits, and when only that alignment function is used the field shall be coded as 0x00.
The Length field indicates the length in octets of the Payload field. The maximum value for the Length field is 65535 octets, and 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.
Whatever is carried, it goes in the payload of an AAL5 frame. The frame ends with a small trailer that says how long the payload was and checks it for damage.
- PayloadThe user information: up to 2^16 - 1 octets.
- PAD0 to 47 octets, padding the CPCS-PDU to fit exactly into the ATM cells.
- CPCS-UU1 octet of user-to-user indication, with no function here.
- CPI1 octet aligning the CPCS-PDU trailer to 64 bits; coded as 0x00 when only alignment is used.
- Length2 octets giving the length of the Payload field; 0x00 is used for the abort function.
- CRC4 octets protecting the whole CPCS-PDU except the CRC field itself.
§4LLC Encapsulation
LLC Encapsulation is needed when several protocols are carried over the same VC. So that the receiver can process the incoming CPCS-PDU, the Payload field must identify the protocol of the routed or bridged PDU, and in LLC Encapsulation that information is encoded in an LLC header placed in front of the carried PDU.
For routed protocols the PDU is identified by prefixing it with an IEEE 802.2 LLC header, which may be 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 and Ctrl, and the Control field always has the value 0x03, specifying an Unnumbered Information Command PDU.
The LLC header value 0xFE-FE-03 identifies that a routed ISO PDU follows. The routed ISO protocol is then identified by a one octet NLPID field that is part of the Protocol Data, with NLPID values administered by ISO and CCITT and defined in ISO/IEC TR 9577. An NLPID value of 0x00 is defined as the Null Network Layer or Inactive Set, and since it has no significance in this encapsulation scheme it is invalid under the ATM encapsulation.
IP is not carried that way, even though it has an NLPID value of 0xCC: the RFC says that format must not be used, and 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 holds a three-octet Organizationally Unique Identifier (OUI) and a two octet Protocol Identifier (PID), which together identify a distinct routed or bridged protocol. The OUI value 0x00-00-00 specifies that the following PID is an EtherType, and for an Internet IP PDU the EtherType value is 0x08-00. The memo notes that this is compatible with RFC 1042.
For bridged protocols the type of the bridged media is identified in the SNAP header instead. The OUI value is the 802.1 organization code 0x00-80-C2 and the actual media type is specified by the two octet PID, which also indicates whether the original Frame Check Sequence is preserved within the bridged PDU. Padding is added after the PID field if necessary, to align the user information field of the bridged PDU at a four octet boundary.
Every packet on a shared connection starts with three bytes that name the protocol, and usually four more bytes that narrow it down. For IP those bytes are the LLC value 0xAA-AA-03 and the EtherType 0x08-00.
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-00-00 |
+-------------------------------+
| EtherType 0x08-00 |
+-------------------------------+
| . |
| IP PDU |
| (up to 2^16 - 9 octets) |
| . |
+-------------------------------+§5VC Based Multiplexing
In VC Based Multiplexing the carried network interconnect protocol is identified implicitly by the VC connecting the two ATM stations: 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, which results in minimal bandwidth and processing overhead. The carried protocol can be manually configured or negotiated dynamically during call establishment using signalling procedures.
PDUs of routed protocols are carried as they are in the Payload of the AAL5 CPCS-PDU. PDUs of bridged protocols are carried exactly as in the LLC Encapsulation case, except that only the fields after the PID field are included. Because the PID field is not included, the presence or absence of the trailing LAN FCS must be identified implicitly by the VC: PDUs with the LAN FCS and PDUs without it are considered to belong to different protocols even if the bridged media type would be the same.
The RFC also covers 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, which in the ATM environment means sending it through each relevant VC, either 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. Because it is unreasonable, and perhaps impossible, to require bridges to statically configure an association of every possible destination MAC address with a VC, ATM bridges must provide enough information for an ATM interface to learn dynamically about foreign destinations. To do that, a bridged PDU shall conform to the LLC encapsulation, so that the receiving ATM interface knows to look into the bridged PDU and learn the association between the foreign destination and an ATM station.
With one connection per protocol, the packet is copied into the frame with nothing added. The connection itself says what protocol it carries, so there is nothing to strip off at the other end.
§6Security Considerations
The RFC is brief on this point: security issues are not addressed in this memo. The document says only that the details related to the negotiation of the multiplexing method as well as address resolution had to be left for further RFCs, because of incomplete standardization of ATM multicasting, addressing and signalling mechanisms.
The RFC says nothing about security. It leaves the details of negotiating the method and of resolving addresses to later documents.