RFC2684 - Multiprotocol Encapsulation over ATM Adaptation Layer 5

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　　Network Working Group D. Grossman
Request for Comments: 2684 Motorola, Inc.
Obsoletes: 1483 J. Heinanen
Category: Standards Track Telia
September 1999

Multiprotocol Encapsulation over ATM Adaptation Layer 5

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

Abstract

This memo replaces RFC1483. It describes two encapsulations methods
for carrying network interconnect traffic over AAL type 5 over ATM.
The first method allows multiplexing of multiple protocols over a
single ATM virtual connection whereas the second method assumes that
each protocol is carried over a separate ATM virtual connection.

Applicability

This specification is intended to be used in implementations which
use ATM networks to carry multiprotocol traffic among hosts, routers
and bridges which are ATM end systems.

1. Introduction

Asynchronous Transfer Mode (ATM) wide area, campus and local area
networks are used to transport IP datagrams and other connectionless
traffic between hosts, routers, bridges and other networking devices.
This memo describes two methods for carrying connectionless routed
and bridged Protocol Data Units (PDUs) over an ATM network. The "LLC
Encapsulation" method allows multiplexing of multiple protocols over
a single ATM virtual connection (VC). The protocol type of each PDU
is identified by a prefixed IEEE 802.2 Logical Link Control (LLC)
header. In the "VC Multiplexing" method, each ATM VC carries PDUs of
exactly one protocol type. When multiple protocols need to be
transported, there is a separate VC for each.

The unit of transport in ATM is a 53 octet fixed length PDU called a
cell. A cell consists of a 5 octet header and a 48 byte payload.
Variable length PDUs, including those addressed in this memo, must be
segmented by the transmitter to fit into the 48 octet ATM cell
payload, and reassembled by the receiver. This memo specifies the
use of the ATM Adaptation Layer type 5 (AAL5), as defined in ITU-T
Recommendation I.363.5 [2] for this purpose. Variable length PDUs are
carried in the Payload field of the AAL5 Common Part Convergence
Sublayer (CPCS) PDU.

This memo only describes how routed and bridged PDUs are carried
directly over the AAL5 CPCS, i.e., when the Service Specific
Convergence Sublayer (SSCS) of AAL5 is absent. If Frame Relay
Service Specific Convergence Sublayer (FR-SSCS), as defined in ITU-T
Recommendation I.365.1 [3], is used over the CPCS, then routed and
bridged PDUs are carried using the NLPID multiplexing method
described in RFC2427 [4]. The RFC2427 encapsulation MUST be used in
the special case that Frame Relay Network Interworking or transparent
mode Service Interworking [9] are used, but is NOT RECOMMENDED for
other applications. 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 RFC2427.

This memo also includes an optional encapsulation for use with
Virtual Private Networks that operate over an ATM subnet.

If it is desired to use the facilities which are designed for the
Point-to-Point Protocol (PPP), and there exists a point-to-point
relationship between peer systems, then RFC2364, rather than this
memo, applies.

2. Conventions

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
they appear in this document, are to be interpreted as described in
RFC2119 [10].

3. Selection of the Multiplexing Method

The decision as to whether to use LLC encapsulation or VC-
multiplexing depends on implementation and system requirements. In
general, LLC encapsulation tends to require fewer VCs in a
multiprotocol environment. VC multiplexing tends to reduce
fragmentation overhead (e.g., an IPV4 datagram containing a TCP
control packet with neither IP nor TCP options exactly fits into a
single cell).

When two ATM end systems wish to exchange connectionless PDUs across
an ATM Permanent Virtual Connection (PVC), selection of the
multiplexing method is done by configuration. ATM connection control
signalling procedures are used to negotiate the encapsulation method
when ATM Switched Virtual Connections (SVCs) are to be used. [5] and
[8] specify how this negotiation is done.

4. AAL5 PDU Format

For both multiplexing methods, routed and bridged PDUs MUST be
encapsulated within the Payload field of an AAL5 CPCS-PDU.

ITU-T Recomendation I.363.5 [2] provides the complete definition of
the AAL5 PDU format and procedures at the sender and receiver. The
AAL5 message mode service, in the non-assured mode of operation MUST
be used. The corrupted delivery option MUST NOT be used. A
reassembly timer MAY be used. The following description is provided
for information.

The format of the AAL5 CPCS-PDU is shown 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 is not used by the
multiprotocol ATM encapsulation described in this memo and MAY be set
to any value.

The CPI (Common Part Indicator) field aligns the CPCS-PDU trailer to
64 bits. This field MUST 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. A
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 Encapsulation

LLC Encapsulation is needed when more than one protocol might be
carried over the same VC. In order to allow the receiver to properly
process the incoming AAL5 CPCS-PDU, the Payload Field contains
information necessary to identify the protocol of the routed or
bridged PDU. In LLC Encapsulation, this information MUST be 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 also applies to protocols operating over LLC
Type 2 (connection-mode) service. In the latter case the format and
contents of the LLC header would be as described in IEEE 802.1 and
IEEE 802.2.

5.1. LLC Encapsulation for Routed Protocols

In LLC Encapsulation, the protocol type of routed PDUs MUST be
identified by prefixing an IEEE 802.2 LLC header to each PDU. In
some cases, the LLC header MUST be followed by an IEEE 802.1a
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 be
set to 0x03, specifying a Unnumbered Information (UI) Command PDU.

The LLC header value 0xFE-FE-03 MUST be used to identify a routed PDU
in the ISO NLPID format (see [6] and Appendix B). For NLPID-formatted
routed PDUs, the content of the AAL5 CPCS-PDU Payload field MUST be
as follows:

Payload Format for Routed NLPID-formatted PDUs
+-------------------------------+
| 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 field
that is part of Protocol Data. NLPID values are administered by ISO
and ITU-T. 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 MUST
NOT be used.

Although there is a NLPID value (0xCC) that indicates IP, the NLPID
format MUST NOT be used for IP. Instead, IP datagrams MUST be
identified by a SNAP header, as defined below.

The presence of am IEEE 802.1a SNAP header is indicated by the LLC
header value 0xAA-AA-03. A SNAP header is of the form

+------+------+------+------+------+
| OUI | PID |
+------+------+------+------+------+

The SNAP header consists of a three octet Organizationally Unique
Identifier (OUI) and a two octet Protocol Identifier (PID). The OUI
is administered by IEEE and identifies an organization which
administers the values which might be assigned to the PID. The SNAP
header thus uniquely identifies a routed or bridged protocol. The
OUI value 0x00-00-00 indicates that the PID is an EtherType.

The format of the AAL5 CPCS-PDU Payload field for routed non-NLPID
Formatted PDUs MUST be as follows:

Payload Format for Routed non-NLPID formatted PDUs
+-------------------------------+
| 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 PDUs
+-------------------------------+
| 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 RFC1042 [7].

5.2. LLC Encapsulation for Bridged Protocols

In LLC Encapsulation, bridged PDUs are encapsulated by identifying
the type of the bridged media in the SNAP header. The presence of
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 code
0x00-80-C2. The type of the bridged media MUST be specified by the
two octet PID. The PID MUST also indicate whether the original Frame
Check Sequence (FCS) is preserved within the bridged PDU. Appendix B
provides a list of media type (PID) values that can be used in ATM
encapsulation.

The AAL5 CPCS-PDU Payload field carrying a bridged PDU MUST have one
of the following formats. The necessary number of padding octets
MUST be added after the PID field in order to align the
Ethernet/802.3 LLC Data field, 802.4 Data Unit field, 802.5 Info
field, FDDI Info field or 802.6 Info field (respectively) of the
bridged PDU to begin at a four octet boundary. The bit ordering of
the MAC address MUST be the same as it would be on the LAN or MAN
(e.g., in canoncial form for bridged Ethernet/IEEE 802.3 PDUs, but in
802.5/FDDI format for bridged 802.5 PDUs).

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) |
+-------------------------------+

The Ethernet/802.3 physical layer requires padding of frames to a
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 format
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, it
MUST be able to insert the necessary padding (if none is already
present) before forwarding to an Ethernet/802.3 subnetwork.

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) |
+-------------------------------+

Since the 802.5 Access Control (AC) field has no significance outside
the local 802.5 subnetwork, it is treated by this encapsulation as
the last octet of the three octet PAD field. It MAY be set to any
value by the sending bridge and MUST be ignored by the receiving
bridge.

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 |
| |
+-------------------------------+

In bridged 802.6 PDUs, the presence of a CRC-32 is indicated by the
CIB bit in the header of the MAC frame. Therefore, the same PID
value is used regardless of the presence or absence of the CRC-32 in
the PDU.

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.

Payload Format for BPDUs
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-0E |
+-------------------------------+
| |
| BPDU as defined by |
| 802.1(d) or 802.1(g) |
| |
+-------------------------------+

6. VC Multiplexing

VC Multiplexing creates a binding between an ATM VC and the type of
the network protocol carried on that VC. Thus, there is no need for
protocol identification information to be carried in the payload of
each AAL5 CPCS-PDU. This reduces payload overhead and can reduce
per-packet processing. VC multiplexing can improve efficiency by
reducing the number of cells needed to carry PDUs of certain lengths.

For ATM PVCs, the type of the protocol to be carried over each PVC
MUST be determined by configuration. For ATM SVCs, the negotiations
specified in RFC1755 [5] MUST be used.

6.1. VC Multiplexing of Routed Protocols

PDUs of routed protocols MUST be carried as the only content of 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) |
| . |
| . |
+-------------------------------+

6.2. VC Multiplexing of Bridged Protocols

PDUs of bridged protocols MUST be carried in the Payload of the AAL5
CPCS-PDU exactly as described in section 5.2, except that only the
fields after the PID field MUST be included. The AAL5 CPCS-PDU
Payload field carrying a bridged PDU MUST, 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.

7. Bridging in an ATM Network

A bridge with an ATM interface that serves as a link to one or more
other 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 point-to-multipoint 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 MUST conform to the
encapsulation described in section 5. 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.

8. Virtual Private Network (VPN) identification

The encapsulation defined in this section applies only to Virtual
Private Networks (VPNs) that operate over an ATM subnet.

A mechanism for globally unique identification of Virtual Private
multiprotocol networks is defined in [11]. The 7-octet VPN-Id
consists of a 3-octet VPN-related OUI (IEEE 802-1990 Organizationally
Unique Identifier), followed by a 4-octet VPN index which is
allocated by the owner of the VPN-related OUI. Typically, the VPN-
related OUI value is assigned to a VPN service provider, which then
allocates VPN index values for its customers.

8.1 VPN Encapsulation Header

The format of the VPN encapsulation header is as follows:

VPN Encapsulation Header
+-------------------------------+
| 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 MUST
be structured according to the appropiate format described in section
5 or 6 (i.e., the VPN encapsulation header is prepended to the PDU
within an AAL5 CPCS SDU).

8.2 LLC-encapsulated routed or bridged PDUs within a VPN

When a LLC-encapsulated routed or bridged PDU is sent within a VPN
using ATM over AAL5, a VPN encapsulation header MUST be prepended to
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 VPN

When a routed or bridged PDU is sent within a VPN using VC
multiplexing, the VPN identifier MAY either be specified a priori,
using ATM connection control signalling or adminstrative assignment
to an ATM interface, or it MAY be indicated using an encapsulation
header.

If the VPN is identified using ATM connection control signalling, all
PDUs carried by the ATM VC are associated with the same VPN. In this
case, the payload formats of routed and bridged PDUs MUST be as
defined in sections 6.1 and 6.2, respectively. If a PDU is received
containing a VPN encapsulation header when the VPN has been

identified using ATM signalling, the receiver MAY drop it and/or take
other actions which are implementation specific. Specification of
the mechanism in ATM connection control signalling for carrying VPN
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 are
associated with that VPN. In this case, the payload formats of
routed and bridged PDUs MUST be as defined in sections 6.1 and 6.2,
respectively. If a PDU is received containing a VPN encapsulation
header when the VPN identifier has been administratively assigned,
the receiver MAY drop it and/or take other actions which are
implementation specific. Specification of mechanisms (such as MIBs)
for assigning VPN identifiers to ATM interfaces is outside the scope
of this memo.

If the VPN identifier is to be indicated using an encapsulation
header, then a VPN encapsulation header MUST be prepended to the
appropriate routed or bridged PDU format defined in sections 6.1 and
6.2, respectively.

9. Security Considerations

This memo defines mechanisms for multiprotocol encapsulation over
ATM. There is an element of trust in any encapsulation protocol: a
receiver must trust that the sender has correctly identified the
protocol being encapsulated. There is no way to ascertain that the
sender did use the proper protocol identification (nor would this be
desirable functionality). The encapsulation mechanisms described in
this memo are believed not to have any other properties that might be
exploited by an attacker. However, architectures and protocols
operating above the encapsulation layer may be subject to a variety
of attacks. In particular, the bridging architecture discussed in
section 7 has the same vulnerabilities as other bridging
architectures.

System security may be affected by the properties of the underlying
ATM network. The ATM Forum has published a security framework [12]
and a security specification [13] which may be relevant.

Acknowledgements

This memo replaces RFC1483, which was developed by the IP over ATM
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 also
contributed to the work on RFC1483.

This material evolved from RFCs [1] and [4] from which much of the
material has been adopted. Thanks to their authors Terry Bradley,
Caralyn Brown, Andy Malis, Dave Piscitello, and C. Lawrence. Other
key contributors to the work included Brian Carpenter (CERN), Rao
Cherukuri (IBM), Joel Halpern (then at Network Systems), Bob Hinden
(Sun Microsystems, presently at Nokia), and Gary Kessler (MAN
Technology).

The material concerning VPNs was developed by Barbara Fox (Lucent)
and Bernhard Petri (Siemens).

References

[1] Piscitello, D. and C. Lawrence, "The Transmission of IP
Datagrams over the SMDS Service", RFC1209, 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 Specific
Convergence Sublayer (SSCS), November 1993.

[4] Brown, C. and A. Malis, "Multiprotocol Interconnect over Frame
Relay", RFC2427, September 1998.

[5] Perez M., Liaw, F., Mankin, E., Grossman, D. and A. Malis, "ATM
Signalling Support for IP over ATM", RFC1755, February 1995.

[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 J. Reynolds, "A Standard for the Transmission of
IP Datagrams over IEEE 802 Networks", STD 43, RFC1042, February
1988.

[8] Maher, M., "IP over ATM Signalling - SIG 4.0 Update", RFC2331,
April 1998.

[9] ITU-T Recommendation I.555, "Frame Relay Bearer Service
Interworking", September 1997.

[10] Bradner, S. "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.

[11] Fox, B. and B. Gleeson, "Virtual Private Networks Identifier",
RFC2685, 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.

Appendix A. Multiprotocol Encapsulation over FR-SSCS

ITU-T Recommendation I.365.1 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 RFC2427. 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/IEC TR 9577 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 RFC2427, the Q.922 Address field MUST be
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 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 MUST 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 RFC2427 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

Appendix D. Applications of multiprotocol encapsulation

Mutiprotocol encapsulation is necessary, but generally not
sufficient, for routing and bridging over the ATM networks. Since
the publication of RFC1483 (the predecessor of this memo), several
system specifications were developed by the IETF and the ATM Forum to
address various aspects of, or scenarios for, bridged or routed
protocols. This appendix summarizes these applications.

1) Point-to-point connection between routers and bridges --
multiprotocol encapsulation over ATM PVCs has been used to provide
a simple point-to-point link between bridges and routers across an
ATM network. Some amount of manual configuration (e.g., in lieu
of INARP) was necessary in these scenarios.

2) Classical IP over ATM -- RFC2225 (formerly RFC1577) provides an
environment where the ATM network serves as a logical IP subnet
(LIS). ATM PVCs are supported, with address resolution provided by
INARP. For ATM SVCs, a new form of ARP, ATMARP, operates over the
ATM network between a host (or router) and an ATMARP server.
Where servers are replicated to provide higher availability or
performance, a Server Synchronization Cache Protocol (SCSP)
defined in RFC2335 is used. Classical IP over ATM defaults to the
LLC/SNAP encapsulation.

3) LAN Emulation -- The ATM Forum LAN Emulation specification
provides an environment where the ATM network is enhanced by LAN
Emulation Server(s) to behave as a bridged LAN. Stations obtain
configuration information from, and register with, a LAN Emulation
Configuration Server; they resolve MAC addresses to ATM addresses
through the services of a LAN Emulation Server; they can send
broadcast and multicast frames, and also send unicast frames for
which they have no direct VC to a Broadcast and Unicast Server.
LANE uses the VC multiplexing encapsulation foramts for Bridged
Etherent/802.3 (without LAN FCS) or Bridged 802.5 (without LAN
FCS) for the Data Direct, LE Multicast Send and Multicast Forward
VCCS. However, the initial PAD field described in this memo is
used as an LE header, and might not be set to all '0'.

4) Next Hop Resolution Protocol (NHRP) -- In some cases, the
constraint that Classical IP over ATM serve a single LIS limits
performance. NHRP, as defined in RFC2332, extends Classical to
allow 'shortcuts' over a an ATM network that supports several
LISs.

5) Multiprotocol over ATM (MPOA) -- The ATM Forum Multiprotocol over
ATM Specification integrates LANE and NHRP to provide a generic
bridging/routing environment.

6) IP Multicast -- RFC2022 extends Classical IP to support IP
multicast. A multicast address resolution server (MARS) is used
possibly in conjunction with a multicast server to provide IP
multicast behavior over ATM point-to-multipoint and/or point to
point virtual connections.

7) PPP over ATM -- RFC2364 extends multiprotocol over ATM to the
case where the encapsulated protocol is the Point-to-Point
protocols. Both the VC based multiplexing and LLC/SNAP
encapsulations are used. This approach is used when the ATM
network is used as a point-to-point link and PPP functions are
required.

Appendix E Differences from RFC1483

This memo replaces RFC1483. It was intended to remove anachronisms,
provide clarifications of ambiguities discovered by implementors or
created by changes to the base standards, and advance this work
through the IETF standards track process. A number of editorial
improvements were made, the RFC2119 [10] conventions applied, and
the current RFCboilerplate added. The following substantive changes
were made. None of them is believed to obsolete implementations of
RFC1483:

-- usage of NLPID encapsulation is clarified in terms of the RFC2119
conventions

-- a pointer to RFC2364 is added to cover the case of PPP over ATM

-- RFC1755 and RFC2331 are referenced to describe how
encapsulations are negotiated, rather than a long-obsolete CCITT
(now ITU-T) working document and references to work then in
progress

-- usage of AAL5 is now a reference to ITU-T I.363.5. Options
created in AAL5 since the publication of RFC1483 are selected.

-- formatting of routed NLPID-formatted PDUs (which are called
"routed ISO PDUs"
in RFC1483) is clarified

-- clarification is provided concerning the use of padding between
the PID and MAC destination address in bridged PDUs and the bit
ordering of the MAC address.

-- clarification is provided concerning the use of padding of
Ethernet/802.3 frames

-- a new encapuslation for VPNs is added

-- substantive security considerations were added

-- a new appendix D provides a summary of applications of
multiprotocol over ATM

Authors' Addresses

Dan Grossman
Motorola, Inc.
20 Cabot Blvd.
Mansfield, MA 02048

EMail: dan@dma.isg.mot.com

Juha Heinanen
Telia Finland
Myyrmaentie 2
01600 Vantaa, Finland

EMail: jh@telia.fi

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