As per Relevance of the word encapsulation, we have this rfc below:
Network Working Group D.
Request for Comments: 2684 Motorola, Inc
Obsoletes: 1483 J.
Category: Standards Track
September 1999
Multiprotocol Encapsulation over ATM Adaptation Layer 5
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (1999). All Rights Reserved
This memo replaces RFC 1483. It describes two encapsulations
for carrying network interconnect traffic over AAL type 5 over ATM
The first method allows multiplexing of multiple protocols over
single ATM virtual connection whereas the second method assumes
each protocol is carried over a separate ATM virtual connection
This specification is intended to be used in implementations
use ATM networks to carry multiprotocol traffic among hosts,
and bridges which are ATM end systems
1.
Asynchronous Transfer Mode (ATM) wide area, campus and local
networks are used to transport IP datagrams and other
traffic between hosts, routers, bridges and other networking devices
This memo describes two methods for carrying connectionless
and bridged Protocol Data Units (PDUs) over an ATM network. The "
Encapsulation" method allows multiplexing of multiple protocols
a single ATM virtual connection (VC). The protocol type of each
is identified by a prefixed IEEE 802.2 Logical Link Control (LLC
header. In the "VC Multiplexing" method, each ATM VC carries PDUs
exactly one protocol type. When multiple protocols need to
transported, there is a separate VC for each
Grossman & Heinanen Standards Track [Page 1]
RFC 2684 Multiprotocol Over AALS September 1999
The unit of transport in ATM is a 53 octet fixed length PDU called
cell. A cell consists of a 5 octet header and a 48 byte payload
Variable length PDUs, including those addressed in this memo, must
segmented by the transmitter to fit into the 48 octet ATM
payload, and reassembled by the receiver. This memo specifies
use of the ATM Adaptation Layer type 5 (AAL5), as defined in ITU-
Recommendation I.363.5 [2] for this purpose. Variable length PDUs
carried in the Payload field of the AAL5 Common Part
Sublayer (CPCS) PDU
This memo only describes how routed and bridged PDUs are
directly over the AAL5 CPCS, i.e., when the Service
Convergence Sublayer (SSCS) of AAL5 is absent. If Frame
Service Specific Convergence Sublayer (FR-SSCS), as defined in ITU-
Recommendation I.365.1 [3], is used over the CPCS, then routed
bridged PDUs are carried using the NLPID multiplexing
described in RFC 2427 [4]. The RFC 2427 encapsulation MUST be used
the special case that Frame Relay Network Interworking or
mode Service Interworking [9] are used, but is NOT RECOMMENDED
other applications. Appendix A (which is for information only)
the format of the FR-SSCS-PDU as well as how IP and CLNP PDUs
encapsulated over FR-SSCS according to RFC 2427.
This memo also includes an optional encapsulation for use
Virtual Private Networks that operate over an ATM subnet
If it is desired to use the facilities which are designed for
Point-to-Point Protocol (PPP), and there exists a point-to-
relationship between peer systems, then RFC 2364, rather than
memo, applies
2.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD
SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL,
they appear in this document, are to be interpreted as described
RFC 2119 [10].
3. Selection of the Multiplexing
The decision as to whether to use LLC encapsulation or VC
multiplexing depends on implementation and system requirements.
general, LLC encapsulation tends to require fewer VCs in
multiprotocol environment. VC multiplexing tends to
fragmentation overhead (e.g., an IPV4 datagram containing a
control packet with neither IP nor TCP options exactly fits into
single cell).
Grossman & Heinanen Standards Track [Page 2]
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When two ATM end systems wish to exchange connectionless PDUs
an ATM Permanent Virtual Connection (PVC), selection of
multiplexing method is done by configuration. ATM connection
signalling procedures are used to negotiate the encapsulation
when ATM Switched Virtual Connections (SVCs) are to be used. [5]
[8] specify how this negotiation is done
4. AAL5 PDU
For both multiplexing methods, routed and bridged PDUs MUST
encapsulated within the Payload field of an AAL5 CPCS-PDU
ITU-T Recomendation I.363.5 [2] provides the complete definition
the AAL5 PDU format and procedures at the sender and receiver.
AAL5 message mode service, in the non-assured mode of operation
be used. The corrupted delivery option MUST NOT be used.
reassembly timer MAY be used. The following description is
for information
The format of the AAL5 CPCS-PDU is shown below
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
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The CPCS-UU (User-to-User indication) field is used to
transfer CPCS user to user information. The field is not used by
multiprotocol ATM encapsulation described in this memo and MAY be
to any value
The CPI (Common Part Indicator) field aligns the CPCS-PDU trailer
64 bits. This field MUST be coded 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 is used to detect bit errors in the CPCS-PDU. A CRC-32
is used
5. LLC
LLC Encapsulation is needed when more than one protocol might
carried over the same VC. In order to allow the receiver to
process the incoming AAL5 CPCS-PDU, the Payload Field
information necessary to identify the protocol of the routed
bridged PDU. In LLC Encapsulation, this information MUST be
in an LLC header placed 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 also applies to protocols operating over
Type 2 (connection-mode) service. In the latter case the format
contents of the LLC header would be as described in IEEE 802.1
IEEE 802.2.
5.1. LLC Encapsulation for Routed
In LLC Encapsulation, the protocol type of routed PDUs MUST
identified by prefixing an IEEE 802.2 LLC header to each PDU.
some cases, the LLC header MUST be followed by an IEEE 802.1
SubNetwork Attachment Point (SNAP) header. In LLC Type 1 operation
the LLC header MUST consist of three one octet fields
+------+------+------+
| DSAP | SSAP | Ctrl |
+------+------+------+
In LLC Encapsulation for routed protocols, the Control field MUST
set to 0x03, specifying a Unnumbered Information (UI) Command PDU
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The LLC header value 0xFE-FE-03 MUST be used to identify a routed
in the ISO NLPID format (see [6] and Appendix B). For NLPID-
routed PDUs, the content of the AAL5 CPCS-PDU Payload field MUST
as follows
Payload Format for Routed NLPID-formatted
+-------------------------------+
| LLC 0xFE-FE-03 |
+-------------------------------+
| NLPID (1 octet) |
+-------------------------------+
| . |
| PDU |
| (up to 2^16 - 4 octets) |
| . |
+-------------------------------+
The routed protocol MUST be identified by a one octet NLPID
that is part of Protocol Data. NLPID values are administered by
and ITU-T. 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
the context of this encapsulation scheme, a NLPID value of 0x00
NOT be used
Although there is a NLPID value (0xCC) that indicates IP, the
format MUST NOT be used for IP. Instead, IP datagrams MUST
identified by a SNAP header, as defined below
The presence of am IEEE 802.1a SNAP header is indicated by the
header value 0xAA-AA-03. A SNAP header is of the
+------+------+------+------+------+
| OUI | PID |
+------+------+------+------+------+
The SNAP header consists of a three octet Organizationally
Identifier (OUI) and a two octet Protocol Identifier (PID). The
is administered by IEEE and identifies an organization
administers the values which might be assigned to the PID. The
header thus uniquely identifies a routed or bridged protocol.
OUI value 0x00-00-00 indicates that the PID is an EtherType
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The format of the AAL5 CPCS-PDU Payload field for routed non-
Formatted PDUs MUST be as follows
Payload Format for Routed non-NLPID formatted
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-00-00 |
+-------------------------------+
| EtherType (2 octets) |
+-------------------------------+
| . |
| Non-NLPID formatted PDU |
| (up to 2^16 - 9 octets) |
| . |
+-------------------------------+
In the particular case of an IPv4 PDU, the Ethertype value is 0x08-
00, and the payload format MUST be
Payload Format for Routed IPv4
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-00-00 |
+-------------------------------+
| EtherType 0x08-00 |
+-------------------------------+
| . |
| IPv4 PDU |
| (up to 2^16 - 9 octets) |
| . |
+-------------------------------+
This format is consistent with that defined in RFC 1042 [7].
5.2. LLC Encapsulation for Bridged
In LLC Encapsulation, bridged PDUs are encapsulated by
the type of the bridged media in the SNAP header. The presence
the SNAP header MUST be indicated by the LLC header value 0xAA-AA-03.
The OUI value in the SNAP header MUST be the 802.1 organization
0x00-80-C2. The type of the bridged media MUST be specified by
two octet PID. The PID MUST also indicate whether the original
Check Sequence (FCS) is preserved within the bridged PDU. Appendix
provides a list of media type (PID) values that can be used in
encapsulation
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The AAL5 CPCS-PDU Payload field carrying a bridged PDU MUST have
of the following formats. The necessary number of padding
MUST be added after the PID field in order to align
Ethernet/802.3 LLC Data field, 802.4 Data Unit field, 802.5
field, FDDI Info field or 802.6 Info field (respectively) of
bridged PDU to begin at a four octet boundary. The bit ordering
the MAC address MUST be the same as it would be on the LAN or
(e.g., in canoncial form for bridged Ethernet/IEEE 802.3 PDUs, but
802.5/FDDI format for bridged 802.5 PDUs).
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) |
+-------------------------------+
The Ethernet/802.3 physical layer requires padding of frames to
minimum size. A bridge that uses uses the Bridged Ethernet/802.3
encapsulation format with the preserved LAN FCS MUST include padding
A bridge that uses the Bridged Ethernet/802.3 encapsulation
without the preserved LAN FCS MAY either include padding, or omit it
When a bridge receives a frame in this format without the LAN FCS,
MUST be able to insert the necessary padding (if none is
present) before forwarding to an Ethernet/802.3 subnetwork
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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) |
+-------------------------------+
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) |
+-------------------------------+
Since the 802.5 Access Control (AC) field has no significance
the local 802.5 subnetwork, it is treated by this encapsulation
the last octet of the three octet PAD field. It MAY be set to
value by the sending bridge and MUST be ignored by the
bridge
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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) |
+-------------------------------+
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 |
| |
+-------------------------------+
In bridged 802.6 PDUs, the presence of a CRC-32 is indicated by
CIB bit in the header of the MAC frame. Therefore, the same
value is used regardless of the presence or absence of the CRC-32
the PDU
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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
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
Payload Format for
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-0E |
+-------------------------------+
| |
| BPDU as defined by |
| 802.1(d) or 802.1(g) |
| |
+-------------------------------+
6. VC
VC Multiplexing creates a binding between an ATM VC and the type
the network protocol carried on that VC. Thus, there is no need
protocol identification information to be carried in the payload
each AAL5 CPCS-PDU. This reduces payload overhead and can
per-packet processing. VC multiplexing can improve efficiency
reducing the number of cells needed to carry PDUs of certain lengths
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For ATM PVCs, the type of the protocol to be carried over each
MUST be determined by configuration. For ATM SVCs, the
specified in RFC 1755 [5] MUST be used
6.1. VC Multiplexing of Routed
PDUs of routed protocols MUST be carried as the only content of
Payload of the AAL5 CPCS-PDU. The format of the AAL5 CPCS-
Payload field thus becomes
Payload Format for Routed
+-------------------------------+
| . |
| Carried PDU |
| (up to 2^16 - 1 octets) |
| . |
| . |
+-------------------------------+
6.2. VC Multiplexing of Bridged
PDUs of bridged protocols MUST be carried in the Payload of the AAL
CPCS-PDU exactly as described in section 5.2, except that only
fields after the PID field MUST be included. The AAL5 CPCS-
Payload field carrying a bridged PDU MUST, 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) |
+-------------------------------+
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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).
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) |
| |
+-------------------------------+
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RFC 2684 Multiprotocol Over AALS September 1999
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
7. Bridging in an ATM
A bridge with an ATM interface that serves as a link to one or
other 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 point-to-multipoint 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
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 MUST conform to
encapsulation described in section 5. In this way, the receiving
interface will know to look into the bridged PDU and learn
association between foreign destination and an ATM station
8. Virtual Private Network (VPN)
The encapsulation defined in this section applies only to
Private Networks (VPNs) that operate over an ATM subnet
A mechanism for globally unique identification of Virtual
multiprotocol networks is defined in [11]. The 7-octet VPN-
consists of a 3-octet VPN-related OUI (IEEE 802-1990
Unique Identifier), followed by a 4-octet VPN index which
allocated by the owner of the VPN-related OUI. Typically, the VPN
related OUI value is assigned to a VPN service provider, which
allocates VPN index values for its customers
Grossman & Heinanen Standards Track [Page 13]
RFC 2684 Multiprotocol Over AALS September 1999
8.1 VPN Encapsulation
The format of the VPN encapsulation header is as follows
VPN Encapsulation
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-00-5E |
+-------------------------------+
| PID 0x00-08 |
+-------------------------------+
| PAD 0x00 |
+-------------------------------+
| VPN related OUI (3 octets) |
+-------------------------------+
| VPN Index (4 octets) |
+-------------------------------+
| |
| (remainder of PDU) |
| |
+-------------------------------+
When the encapsulation header is used, the remainder of the PDU
be structured according to the appropiate format described in
5 or 6 (i.e., the VPN encapsulation header is prepended to the
within an AAL5 CPCS SDU).
8.2 LLC-encapsulated routed or bridged PDUs within a
When a LLC-encapsulated routed or bridged PDU is sent within a
using ATM over AAL5, a VPN encapsulation header MUST be prepended
the appropriate routed or bridged PDU format defined in sections 5.1
and 5.2, respectively
8.3 VC multiplexing of routed or bridged PDUs within a
When a routed or bridged PDU is sent within a VPN using
multiplexing, the VPN identifier MAY either be specified a priori
using ATM connection control signalling or adminstrative
to an ATM interface, or it MAY be indicated using an
header
If the VPN is identified using ATM connection control signalling,
PDUs carried by the ATM VC are associated with the same VPN. In
case, the payload formats of routed and bridged PDUs MUST be
defined in sections 6.1 and 6.2, respectively. If a PDU is
containing a VPN encapsulation header when the VPN has
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RFC 2684 Multiprotocol Over AALS September 1999
identified using ATM signalling, the receiver MAY drop it and/or
other actions which are implementation specific. Specification
the mechanism in ATM connection control signalling for carrying
identifiers is outside the scope of this Memo
If a VPN identifier is administratively assigned to an ATM interface
then all PDUs carried by any ATM VCs within that interface
associated with that VPN. In this case, the payload formats
routed and bridged PDUs MUST be as defined in sections 6.1 and 6.2,
respectively. If a PDU is received containing a VPN
header when the VPN identifier has been administratively assigned
the receiver MAY drop it and/or take other actions which
implementation specific. Specification of mechanisms (such as MIBs
for assigning VPN identifiers to ATM interfaces is outside the
of this memo
If the VPN identifier is to be indicated using an
header, then a VPN encapsulation header MUST be prepended to
appropriate routed or bridged PDU format defined in sections 6.1
6.2, respectively
9. Security
This memo defines mechanisms for multiprotocol encapsulation
ATM. There is an element of trust in any encapsulation protocol:
receiver must trust that the sender has correctly identified
protocol being encapsulated. There is no way to ascertain that
sender did use the proper protocol identification (nor would this
desirable functionality). The encapsulation mechanisms described
this memo are believed not to have any other properties that might
exploited by an attacker. However, architectures and
operating above the encapsulation layer may be subject to a
of attacks. In particular, the bridging architecture discussed
section 7 has the same vulnerabilities as other
architectures
System security may be affected by the properties of the
ATM network. The ATM Forum has published a security framework [12]
and a security specification [13] which may be relevant
Grossman & Heinanen Standards Track [Page 15]
RFC 2684 Multiprotocol Over AALS September 1999
This memo replaces RFC 1483, which was developed by the IP over
working group, and edited by Juha Heinanen (then at Telecom Finland
now at Telia). The update was developed in the IP-over-NBMA (ION
working group, and Dan Grossman (Motorola) was editor and
contributed to the work on RFC 1483.
This material evolved from RFCs [1] and [4] from which much of
material has been adopted. Thanks to their authors Terry Bradley
Caralyn Brown, Andy Malis, Dave Piscitello, and C. Lawrence.
key contributors to the work included Brian Carpenter (CERN),
Cherukuri (IBM), Joel Halpern (then at Network Systems), Bob
(Sun Microsystems, presently at Nokia), and Gary Kessler (
Technology).
The material concerning VPNs was developed by Barbara Fox (Lucent
and Bernhard Petri (Siemens).
[1] Piscitello, D. and C. Lawrence, "The Transmission of
Datagrams over the SMDS Service", RFC 1209, March 1991.
[2] ITU-T Recommendation I.363.5, "B-ISDN ATM Adaptation Layer (AAL
Type 5 Specification", August 1996.
[3] ITU-T Recommendation I.365.1, "Frame Relaying Service
Convergence Sublayer (SSCS), November 1993.
[4] Brown, C. and A. Malis, "Multiprotocol Interconnect over
Relay", RFC 2427, September 1998.
[5] Perez M., Liaw, F., Mankin, E., Grossman, D. and A. Malis, "
Signalling Support for IP over ATM", RFC 1755, February 1995.
[6] Information technology - Telecommunications and
Exchange Between Systems, "Protocol Identification in
Network Layer". ISO/IEC TR 9577, October 1990.
[7] Postel, J. and J. Reynolds, "A Standard for the Transmission
IP Datagrams over IEEE 802 Networks", STD 43, RFC 1042,
1988.
[8] Maher, M., "IP over ATM Signalling - SIG 4.0 Update", RFC 2331,
April 1998.
Grossman & Heinanen Standards Track [Page 16]
RFC 2684 Multiprotocol Over AALS September 1999
[9] ITU-T Recommendation I.555, "Frame Relay Bearer
Interworking", September 1997.
[10] Bradner, S. "Key words for use in RFCs to Indicate
Levels", BCP 14, RFC 2119, March 1997.
[11] Fox, B. and B. Gleeson, "Virtual Private Networks Identifier",
RFC 2685, September 1999.
[12] The ATM Forum, "ATM Security Framework Version 1.0", af-sec
0096.000, February 1998.
[13] The ATM Forum, "ATM Security Specification v1.0", af-sec
0100.001, February 1999.
Grossman & Heinanen Standards Track [Page 17]
RFC 2684 Multiprotocol Over AALS September 1999
Appendix A. Multiprotocol Encapsulation over FR-
ITU-T Recommendation I.365.1 defines a Frame Relaying
Convergence Sublayer (FR- SSCS) to be used on the top of the
Part Convergence Sublayer CPCS) of the AAL type 5 for Frame Relay/
interworking. The service offered by FR-SSCS corresponds to the
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,
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 2427. 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/IEC TR 9577 Network Layer Protocol ID (NLPID).
Grossman & Heinanen Standards Track [Page 18]
RFC 2684 Multiprotocol Over AALS September 1999
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
+-------------------------------+
| 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 2427, the Q.922 Address field MUST
either 2 or 4 octets, i.e., a 3 octet Address field MUST NOT be used
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 MUST 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
Grossman & Heinanen Standards Track [Page 19]
RFC 2684 Multiprotocol Over AALS September 1999
See RFC 2427 for more details related to multiprotocol
over FRCS
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
Appendix D. Applications of multiprotocol
Mutiprotocol encapsulation is necessary, but generally
sufficient, for routing and bridging over the ATM networks.
the publication of RFC 1483 (the predecessor of this memo),
system specifications were developed by the IETF and the ATM Forum
address various aspects of, or scenarios for, bridged or
protocols. This appendix summarizes these applications
1) Point-to-point connection between routers and bridges --
multiprotocol encapsulation over ATM PVCs has been used to
a simple point-to-point link between bridges and routers across
ATM network. Some amount of manual configuration (e.g., in
of INARP) was necessary in these scenarios
2) Classical IP over ATM -- RFC 2225 (formerly RFC 1577) provides
environment where the ATM network serves as a logical IP
(LIS). ATM PVCs are supported, with address resolution provided
INARP. For ATM SVCs, a new form of ARP, ATMARP, operates over
ATM network between a host (or router) and an ATMARP server
Where servers are replicated to provide higher availability
performance, a Server Synchronization Cache Protocol (SCSP
defined in RFC 2335 is used. Classical IP over ATM defaults to
LLC/SNAP encapsulation
Grossman & Heinanen Standards Track [Page 20]
RFC 2684 Multiprotocol Over AALS September 1999
3) LAN Emulation -- The ATM Forum LAN Emulation
provides an environment where the ATM network is enhanced by
Emulation Server(s) to behave as a bridged LAN. Stations
configuration information from, and register with, a LAN
Configuration Server; they resolve MAC addresses to ATM
through the services of a LAN Emulation Server; they can
broadcast and multicast frames, and also send unicast frames
which they have no direct VC to a Broadcast and Unicast Server
LANE uses the VC multiplexing encapsulation foramts for
Etherent/802.3 (without LAN FCS) or Bridged 802.5 (without
FCS) for the Data Direct, LE Multicast Send and Multicast
VCCS. However, the initial PAD field described in this memo
used as an LE header, and might not be set to all '0'.
4) Next Hop Resolution Protocol (NHRP) -- In some cases,
constraint that Classical IP over ATM serve a single LIS
performance. NHRP, as defined in RFC 2332, extends Classical
allow 'shortcuts' over a an ATM network that supports
LISs
5) Multiprotocol over ATM (MPOA) -- The ATM Forum Multiprotocol
ATM Specification integrates LANE and NHRP to provide a
bridging/routing environment
6) IP Multicast -- RFC 2022 extends Classical IP to support
multicast. A multicast address resolution server (MARS) is
possibly in conjunction with a multicast server to provide
multicast behavior over ATM point-to-multipoint and/or point
point virtual connections
7) PPP over ATM -- RFC 2364 extends multiprotocol over ATM to
case where the encapsulated protocol is the Point-to-
protocols. Both the VC based multiplexing and LLC/
encapsulations are used. This approach is used when the
network is used as a point-to-point link and PPP functions
required
Appendix E Differences from RFC 1483
This memo replaces RFC 1483. It was intended to remove anachronisms
provide clarifications of ambiguities discovered by implementors
created by changes to the base standards, and advance this
through the IETF standards track process. A number of
improvements were made, the RFC 2119 [10] conventions applied,
the current RFC boilerplate added. The following substantive
were made. None of them is believed to obsolete implementations
RFC 1483:
Grossman & Heinanen Standards Track [Page 21]
RFC 2684 Multiprotocol Over AALS September 1999
-- usage of NLPID encapsulation is clarified in terms of the RFC 2119
-- a pointer to RFC 2364 is added to cover the case of PPP over
-- RFC 1755 and RFC 2331 are referenced to describe
encapsulations are negotiated, rather than a long-obsolete
(now ITU-T) working document and references to work then
-- usage of AAL5 is now a reference to ITU-T I.363.5.
created in AAL5 since the publication of RFC 1483 are selected
-- formatting of routed NLPID-formatted PDUs (which are
"routed ISO PDUs
in RFC 1483) is
-- clarification is provided concerning the use of padding
the PID and MAC destination address in bridged PDUs and the
ordering of the MAC address
-- clarification is provided concerning the use of padding
Ethernet/802.3
-- a new encapuslation for VPNs is
-- substantive security considerations were
-- a new appendix D provides a summary of applications
multiprotocol over
Authors'
Dan
Motorola, Inc
20 Cabot Blvd
Mansfield, MA 02048
EMail: dan@dma.isg.mot.
Juha
Telia
Myyrmaentie 2
01600 Vantaa,
EMail: jh@telia.
Grossman & Heinanen Standards Track [Page 22]
RFC 2684 Multiprotocol Over AALS September 1999
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Grossman & Heinanen Standards Track [Page 23]
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