As per Relevance of the word encapsulation, we have this rfc below:
Network Working Group Juha
Reguest for Comments: 1483 Telecom
July 1993
Multiprotocol Encapsulation over ATM Adaptation Layer 5
Status of this
This RFC specifies an IAB standards track protocol for the
community, and requests discussion and suggestions for improvements
Please refer to the current edition of the "IAB Official
Standards" for the standardization state and status of this protocol
Distribution of this memo is unlimited
This memo describes two encapsulations methods for carrying
interconnect traffic over ATM AAL5. The first method
multiplexing of multiple protocols over a single ATM virtual
whereas the second method assumes that each protocol is carried
a separate ATM virtual circuit
1.
Asynchronous Transfer Mode (ATM) based networks are of
interest for both local and wide area applications. This
describes two different methods for carrying connectionless
interconnect traffic, routed and bridged Protocol Data Units (PDUs),
over an ATM network. The first method allows multiplexing
multiple protocols over a single ATM virtual circuit. The
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
called "LLC Encapsulation" and a subset of it has been
defined for SMDS [1]. The second method does higher-layer
multiplexing implicitly by ATM Virtual Circuits (VCs). It is in
following called "VC Based Multiplexing".
ATM is a cell based transfer mode that requires variable length
information to be segmented and reassembled to/from short,
length cells. This memo doesn't specify a new Segmentation
Reassembly (SAR) method for bridged and routed PDUs. Instead,
PDUs are carried in the Payload field of Common Part
Sublayer (CPCS) PDU of ATM Adaptation Layer type 5 (AAL5) [2].
Note that this memo only describes how routed and bridged PDUs
carried directly over the CPCS of AAL5, i.e., when the
Specific Convergence Sublayer (SSCS) of AAL5 is empty. If
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RFC 1483 Multiprotocol over AAL5 July 1993
Relay Service Specific Convergence Sublayer (FR-SSCS), as defined
I.36x.1 [3], is used over the CPCS of AAL5, then routed and
PDUs are carried using the NLPID multiplexing method described in
1294 [4]. Appendix A (which is for information only) shows
format of the FR-SSCS-PDU as well as how IP and CLNP PDUs
encapsulated over FR-SSCS according to RFC 1294.
2. Selection of the Multiplexing
It is envisioned that VC Based Multiplexing will be dominant
environments where dynamic creation of large numbers of ATM VCs
fast and economical. These conditions are likely to first prevail
private ATM networks. LLC Encapsulation, on the other hand, may
desirable when it is not practical for one reason or another to
a separate VC for each carried protocol. This is the case,
example, if the ATM network only supports (semi) Permanent
Circuits (PVCs) or if charging depends heavily on the number
simultaneous VCs
When two ATM stations wish to exchange connectionless
interconnect traffic, selection of the multiplexing method is
either by manual configuration (in case of PVCs) or by B-
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
compatibility" information element, which will allow negotiation
AAL5 and the carried (encapsulation) protocol
3. AAL5 Frame
No matter which multiplexing method is selected, routed and
PDUs shall be encapsulated within the Payload field of AAL5 CPCS-PDU
The format of the AAL5 CPCS-PDU is given below
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AAL5 CPCS-PDU
+-------------------------------+
| . |
| . |
| CPCS-PDU Payload |
| up to 2^16 - 1 octets) |
| . |
| . |
+-------------------------------+
| PAD ( 0 - 47 octets) |
+-------------------------------+ -------
| CPCS-UU (1 octet ) |
+-------------------------------+
| CPI (1 octet ) |
+-------------------------------+CPCS-PDU
| 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
such that the last 48 octet cell payload created by the SAR
will have the CPCS-PDU Trailer right justified in the cell
The CPCS-UU (User-to-User indication) field is used to
transfer CPCS user to user information. The field has no
under the multiprotocol ATM encapsulation described in this memo
can be set to any value
The CPI (Common Part Indicator) field alings the CPCS-PDU trailer
64 bits. Possible additional functions are for further study
CCITT. When only the 64 bit alignment function is used, this
shall be codes as 0x00.
The Length field indicates the length, in octets, of the
field. The maximum value for the Length field is 65535 octets.
Length field coded as 0x00 is used for the abort function
The CRC field protects the entire CPCS-PDU except the CRC
itself
4. LLC
LLC Encapsulation is needed when several protocols are carried
the same VC. In order to allow the receiver to properly process
incoming AAL5 CPCS-PDU, the Payload Field must contain
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necessary to identify the protocol of the routed or bridged PDU.
LLC Encapsulation this information is encoded in an LLC header
in front of the carried PDU
Although this memo only deals with protocols that operate over
Type 1 (unacknowledged connectionless mode) service, the
encapsulation principle applies also to protocols operating over
Type 2 (connection-mode) service. In the latter case the
and/or contents of the LLC header would differ from what is
below
4.1. LLC Encapsulation for Routed
In LLC Encapsulation the protocol of the routed PDU is identified
prefixing the PDU by an IEEE 802.2 LLC header, which is
followed by an IEEE 802.1a SubNetwork Attachment Point (SNAP) header
In LLC Type 1 operation, the LLC header consists of three one
fields
+------+------+------+
| DSAP | SSAP | Ctrl |
+------+------+------+
In LLC Encapsulation for routed protocols, the Control field
always value 0x03 specifying Unnumbered Information Command PDU
The LLC header value 0xFE-FE-03 identifies that a routed ISO PDU (
[6] and Appendix B) follows. The Control field value 0x03
Unnumbered Information Command PDU. For routed ISO PDUs the
of the AAL5 CPCS-PDU Payload field shall thus be as follows
Payload Format for Routed ISO
+-------------------------------+
| LLC 0xFE-FE-03 |
+-------------------------------+
| . |
| ISO PDU |
| (up to 2^16 - 4 octets) |
| . |
+-------------------------------+
The routed ISO protocol is identified by a one octet NLPID field
is part of Protocol Data. NLPID values are administered by ISO
CCITT. They are defined in ISO/IEC TR 9577 [6] and some of
currently defined ones are listed in Appendix C
An NLPID value of 0x00 is defined in ISO/IEC TR 9577 as the
Network Layer or Inactive Set. Since it has no significance
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the context of this encapsulation scheme, a NLPID value of 0x00
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 0
defined for it. This format must not be used. Instead, IP
encapsulated like all other routed non-ISO protocols by
it in the SNAP header that immediately follows the LLC header
The presence of a SNAP header is indicated by the LLC header
0xAA-AA-03. A SNAP header is of the
+------+------+------+------+------+
| OUI | PID |
+------+------+------+------+------+
The three-octet Organizationally Unique Identifier (OUI)
an organization which administers the meaning of the following
octet Protocol Identifier (PID). Together they identify a
routed or bridged protocol. The OUI value 0x00-00-00 specifies
the following PID is an EtherType
The format of the AAL5 CPCS-PDU Payload field for routed non-ISO
shall thus be as follows
Payload Format for Routed non-ISO
+-------------------------------+
| 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
0x08-00:
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RFC 1483 Multiprotocol over AAL5 July 1993
Payload Format for Routed IP
+-------------------------------+
| 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
format specified in RFC 1042 should be followed by this memo
4.2. LLC Encapsulation for Bridged
In LLC Encapsulation bridged PDUs are encapsulated by identifying
type of the bridged media in the SNAP header. As with routed non-
protocols, the presence of the SNAP header is indicated by the
header value 0xAA-AA-03. With bridged protocols the OUI value in
SNAP header is the 802.1 organization code 0x00-80-C2 and the
type of the bridged media is specified by the two octet PID
Additionally, the PID indicates whether the original Frame
Sequence (FCS) is preserved within the bridged PDU. The media
(PID) values that can be used in ATM encapsulation are listed
Appendix B
The AAL5 CPCS-PDU Payload field carrying a bridged PDU shall
therefore, have one of the following formats. Padding is added
the PID field if necessary in order to align the user
field of the bridged PDU at a four octet boundary
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Payload Format for Bridged Ethernet/802.3
+-------------------------------+
| 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
+-------------------------------+
| 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) |
+-------------------------------+
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RFC 1483 Multiprotocol over AAL5 July 1993
Payload Format for Bridged 802.5
+-------------------------------+
| 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
outside the local 802.5 subnetwork. It can thus be regarded as
last octet of the three octet PAD field, which can be set to
value (XX).
Payload Format for Bridged FDDI
+-------------------------------+
| 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) |
+-------------------------------+
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Payload Format for Bridged 802.6
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-0B |
+---------------+---------------+ ------
| Reserved | BEtag |
+---------------+---------------+
| BAsize |
+-------------------------------+ -------
| MAC destination address |
+-------------------------------+
| |
| (remainder of MAC frame) |
| |
+-------------------------------+
| |
| Common PDU Trailer |
| |
+-------------------------------+
Note that in bridged 802.6 PDUs, there is only one choice for the
value, since the presence of a CRC-32 is indicated by the CIB bit
the header of the MAC frame
The Common Protocol Data Unit (PDU) Header and Trailer are
to allow pipelining at the egress bridge to an 802.6 subnetwork
Specifically, the Common PDU Header contains the BAsize field,
contains the length of the PDU. If this field is not available
the egress 802.6 bridge, then that bridge cannot begin to
the segmented PDU until it has received the entire PDU,
the length, and inserted the length into the BAsize field. If
field is available, the egress 802.6 bridge can extract the
from the BAsize field of the Common PDU Header, insert it into
corresponding field of the first segment, and immediately
the segment onto the 802.6 subnetwork. Thus, the bridge can
transmitting the 802.6 PDU before it has received the complete PDU
Note that the Common PDU Header and Trailer of the encapsulated
should not be simply copied to the outgoing 802.6 subnetwork
the encapsulated BEtag value may conflict with the previous
value transmitted by that bridge
An ingress 802.6 bridge can abort an AAL5 CPCS-PDU by setting
Length field to zero. If the egress bridge has already
transmitting segments of the PDU to an 802.6 subnetwork and
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notices that the AAL5 CPCS-PDU has been aborted, it may
generate an EOM cell that causes the 802.6 PDU to be rejected at
receiving bridge. Such an EOM cell could, for example, contain
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
In VC Based Multiplexing, the carried network interconnect
is identified implicitly by the VC connecting the two ATM stations
i.e. each protocol must be carried over a separate VC. There
therefore no need to include explicit multiplexing information in
Payload of the AAL5 CPCS-PDU. This results in minimal bandwidth
processing overhead
As indicated above, the carried protocol can be either
configured or negotiated dynamically during call establishment
signalling procedures. The signalling details will be defined
in other RFCs when the relevant standards have become available
5.1. VC Based Multiplexing of Routed
PDUs of routed protocols shall be carried as such in the Payload
the AAL5 CPCS-PDU. The format of the AAL5 CPCS-PDU Payload
thus becomes
Payload Format for Routed
+-------------------------------+
| . |
| Carried PDU |
| (up to 2^16 - 1 octets) |
| . |
| . |
+-------------------------------+
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RFC 1483 Multiprotocol over AAL5 July 1993
5.2. VC Based Multiplexing of Bridged
PDUs of bridged protocols shall be carried in the Payload of the AAL
CPCS-PDU exactly as described in section 4.2 except that only
fields after the PID field are included. The AAL5 CPCS-PDU
field carrying a bridged PDU shall, therefore, have one of
following formats
Payload Format for Bridged Ethernet/802.3
+-------------------------------+
| PAD 0x00-00 |
+-------------------------------+
| MAC destination address |
+-------------------------------+
| |
| (remainder of MAC frame) |
| |
+-------------------------------+
| LAN FCS (VC dependent option) |
+-------------------------------+
Payload Format for Bridged 802.4/802.5/FDDI
+-------------------------------+
| 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
outside the local 802.5 subnetwork. It can thus be regarded as
last octet of the three octet PAD field, which in case of 802.5
be set to any value (XX).
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RFC 1483 Multiprotocol over AAL5 July 1993
Payload Format for Bridged 802.6
+---------------+---------------+ -------
| Reserved | BEtag |
+---------------+---------------+
| BAsize |
+-------------------------------+ -------
| MAC destination address |
+-------------------------------+
| |
| (remainder of MAC frame) |
| |
+-------------------------------+
| |
| Common PDU Trailer |
| |
+-------------------------------+
Payload Format for
+-------------------------------+
| |
| 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
or absence of the trailing LAN FCS shall be identified implicitly
the VC, since the PID field is not included. PDUs with the LAN
and PDUs without the LAN FCS are thus considered to belong
different protocols even if the bridged media type would be the same
6. Bridging in an ATM
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
destinations. In the ATM environment this means sending the
through each relevant VC. This may be accomplished by
copying it to each VC or by using a multicast VC
To forward a PDU, a bridge must be able to associate a
MAC address with a VC. It is unreasonable and perhaps impossible
require bridges to statically configure an association of
possible destination MAC address with a VC. Therefore, ATM
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RFC 1483 Multiprotocol over AAL5 July 1993
must provide enough information to allow an ATM interface
dynamically learn about foreign destinations beyond the set of
stations
To accomplish dynamic learning, a bridged PDU shall conform to
encapsulation described within section 4. In this way, the
ATM interface will know to look into the bridged PDU and learn
association between foreign destination and an ATM station
7. For Further
Due to incomplete standardization of ATM multicasting, addressing
and signalling mechanisms, details related to the negotiation of
multiplexing method as well as address resolution had to be left
further RFCs
This document has evolved from RFCs [1] and [4] from which much
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
invaluable in completing the document. Special thanks
Carpenter of CERN, Rao Cherukuri of IBM, Dan Grossman of Motorola
Joel Halpern of Network Systems, Bob Hinden of Sun Mircosystems,
Gary Kessler of MAN Technology Corporation for their
contributions
Security
Security issues are not addressed in this memo
[1] Piscitello, D. and Lawrence, C., "The Transmission of
Datagrams over the SMDS Service". RFC 1209, Bell
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., "
Interconnect over Frame Relay". RFC 1294,
Communications, Inc. and BBN Communications, January 1992.
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RFC 1483 Multiprotocol over AAL5 July 1993
[5] CCITT, "Draft text for Q.93B". CCITT Study Group XI, 23
September - 2 October, 1992.
[6] Information technology - Telecommunications and
Exchange Between Systems, "Protocol Identification in
Network Layer". ISO/IEC TR 9577, October 1990.
[7] Postel, J. and Reynolds, J., "A Standard for the Transmission
IP Datagrams over IEEE 802 Networks". RFC 1042, ISI, February
1988.
Appendix A. Multiprotocol Encapsulation over FR-
I.36x.1 defines a Frame Relaying Specific Convergence Sublayer (FR
SSCS) to be used on the top of the Common Part Convergence
CPCS) of the AAL type 5 for Frame Relay/ATM interworking.
service offered by FR-SSCS corresponds to the Core service for
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,
the corresponding functions are provided by the AAL. The
below shows an FR-SSCS-PDU embedded in the Payload of an AAL5 CPCS
PDU
FR-SSCS-PDU in Payload of AAL5 CPCS-
+-------------------------------+ -------
| Q.922 Address Field | FR-SSCS-PDU
| (2-4 octets) |
+-------------------------------+ -------
| . |
| . |
| Q.922 Information field | FR-SSCS-PDU
| . |
| . |
+-------------------------------+ -------
| AAL5 CPCS-PDU Trailer |
+-------------------------------+
Routed and bridged PDUs are encapsulated inside the FR-SSCS-PDU
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
the remainder of the frame to a convenient boundary for the sender
The protocol of the carried PDU is then identified by prefixing
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
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RFC 1483 Multiprotocol over AAL5 July 1993
FR-SSCS-PDU Format for Routed IP
+-------------------------------+
| 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
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
+-------------------------------+
| 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
octet of the PDU itself and shall thus not be repeated
The above encapsulation applies only to those routed protocols
have a unique NLPID assigned. For other routed protocols (and
bridged protocols), it is necessary to provide another mechanism
easy protocol identification. This can be achieved by using an
value 0x80 to indicate that an IEEE 802.1a SubNetwork
Point (SNAP) header follows
See RFC 1294 for more details related to multiprotocol
over FRCS
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Appendix B. List of Locally Assigned values of OUI 00-80-C
with preserved FCS w/o preserved FCS
------------------ ----------------- --------------
0x00-01 0x00-07 802.3/
0x00-02 0x00-08 802.4
0x00-03 0x00-09 802.5
0x00-04 0x00-0A
0x00-05 0x00-0B 802.6
0x00-0D
0x00-0E
Appendix C. Partial List of
0x00 Null Network Layer or Inactive Set (not used with ATM
0x80
0x81 ISO
0x82 ISO
0x83 ISO
0xCC Internet
Author's
Juha
Telecom
PO Box 228
SF-33101
Phone: +358 49 500 958
Email: Juha.Heinanen@datanet.tele.
Heinanen [Page 16]
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