RFC2492 - IPv6 over ATM Networks

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　　Network Working Group G. Armitage
Request for Comments: 2492 Lucent Technologies
Category: Standards Track P. Schulter
BrightTiger Technologies
M. Jork
Digital Equipment GmbH
January 1999

IPv6 over ATM Networks

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 document is a companion to the ION working group's architecture
document, "IPv6 over Non Broadcast Multiple Access (NBMA) networks".
It provides specific details on how to apply the IPv6 over NBMA
architecture to ATM networks. This architecture allows conventional
host-side operation of the IPv6 Neighbor Discovery protocol, while
also supporting the establishment of 'shortcut' ATM forwarding paths
(when using SVCs). Operation over administratively configured Point
to Point PVCs is also supported.

1. Introduction.

This document is an ATM-specific companion document to the ION
working group's, "IPv6 over Non Broadcast Multiple Access (NBMA)
networks" specification [1]. Terminology and architectural
descriptions will not be repeated here.

The use of ATM to provide point to point PVC service, or flexible
point to point and point to multipoint SVC service, is covered by
this document.

A minimally conforming IPv6/ATM driver SHALL support the PVC mode of
operation. An IPv6/ATM driver that supports the full SVC mode SHALL
also support PVC mode of operation.

2. Specification Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [16].

3. PVC Environments

When the ATM network is used in PVC mode, each PVC will connect
exactly two nodes and the use of Neighbor Discovery and other IPv6
features is limited. IPv6/ATM interfaces have only one neighbor on
each Link. The MARS and NHRP protocols are NOT necessary, since
multicast and broadcast operations collapse down to an ATM level
unicast operation. Dynamically discovered shortcuts are not
supported.

The actual details of encapsulations, MTU, and link token generation
are provided in the following sections.

This use of PVC links does not mandate, nor does it prohibit the use
of extensions to the Neighbor Discovery protocol which may be
developed for either general use of for use in PVC connections (for
example, Inverse Neighbor Discovery).

Since ATM PVC links do not use link-layer addresses, the link-layer
address options SHOULD not be included in any ND message [11]. If a
link-layer address option is present in an ND message, then the
option SHOULD be ignored.

A minimally conforming IPv6/ATM driver SHALL support the PVC mode of
operation. PVC only implementations are not required to support any
SVC mode of operation.

3.1 Default Packet Encapsulation

Following the model in RFC1483 [2], AAL5 SHALL be the default
Adaptation Layer service, and (LLC/SNAP) encapsulation SHALL be
default encapsulation used by unicast and multicast packets across
pt-pt PVC links. As defined in [1], the default IPv6 packet
encapsulation SHALL be:

[0xAA-AA-03][0x00-00-00][0x86-DD][IPv6 packet]
(LLC) (OUI) (PID)

3.2 Optional null encapsulation

IPv6/ATM drivers MAY also support null encapsulation as a
configurable option. When null encapsulation is enabled, the IPv6
packet is passed directly to the AAL5 layer. Both ends of the PVC
MUST be configured to use null encapsulation. The PVC will not be
available for use by protocols other than IPv6.

3.3 PPP encapsulation

The concatentation of IPv6 over PPP with PPP over AAL5 PVCs is not
covered by this specification.

3.4 MTU For PVC Environments

The default IP MTU size for PVC links is 9180 bytes as specified in
[7]. Other IP MTU values MAY be used.

3.5 Interface Token Formats in PVC Environments

When the ATM network is used in PVC mode interface tokens SHALL be
generated using one of the methods described in section 5. Interface
tokens need only be unique between the two nodes on the PVC link.

4 SVC environments

4.1 SVC Specific Code Points

4.1.1 ATM Adaptation Layer encapsulation for SVC environments

Following the model in RFC1483 [2], AAL5 SHALL be the default
Adaptation Layer service, and (LLC/SNAP) encapsulation SHALL be the
default encapsulation used by unicast and multicast packets across
SVC links.

4.1.2 Unicast Packet Encapsulation

As defined in [1], the default IPv6 unicast packet encapsulation
SHALL be:

[0xAA-AA-03][0x00-00-00][0x86-DD][IPv6 packet]
(LLC) (OUI) (PID)

4.1.3 Multicast packet encapsulation

As defined in [1], the default IPv6 multicast packet encapsulation
SHALL be:

[0xAA-AA-03][0x00-00-5E][0x00-01][pkt$cmi][0x86DD][IPv6
packet]
(LLC) (OUI) (PID) (mars encaps)

The IPv6/ATM driver's Cluster Member ID SHALL be copied into
the 2 octet pkt$cmi field prior to transmission.

4.1.4 Optional null encapsulation

IPv6/ATM drivers MAY also support null encapsulation as a
configurable option. Null encapsulation SHALL only be used for
passing IPv6 packets from one IPv6/ATM driver to another. Null
encapsulation SHALL NOT be used on the pt-pt SVC between the IPv6/ATM
driver and its local MARS.

If null encapsulation is enabled, the IPv6 packet is passed directly
to the AAL5 layer. Both ends of the SVC MUST agree to use null
encapsulation during the call SETUP phase. The SVC will not be
available for use by protocols other than IPv6.

If null encapsulation is enabled on data SVCs between routers,
inter-router NHRP traffic SHALL utilize a separate, parallel SVC.

Use of null encapsulation is not encouraged when IPv6/ATM is used
with MARS/NHRP/ND as described in [1].

4.1.5 MARS control messages

The encapsulation of MARS control messages (between MARS and MARS
Clients) remains the same as shown in RFC2022 [3]:

[0xAA-AA-03][0x00-00-5E][0x00-03][MARS control message]
(LLC) (OUI) (PID)

The key control field values are:

The mar$afn field remains 0x0F (ATM addresses)

The mar$pro field SHALL be 0x86DD (IPv6)

The mar$op.version field remains 0x00 (MARS)

The mar$spln and mar$tpln fields (where relevant) are either 0 (for
null or non-existent information) or 16 (for the full IPv6 protocol
address)

The way in which ATM addresses are stored remains the same as shown
in RFC2022 [3]

4.1.6 NHRP control messages

The encapsulation of NHRP control messages remains the same as shown
in RFC2332 [4]:

[0xAA-AA-03][0x00-00-5E][0x00-03][NHRP control message]
(LLC) (OUI) (PID)

The key control field values are:

The ar$afn field remains 0x0F (ATM addresses)

The ar$pro field SHALL be 0x86DD (IPv6)

The ar$op.version field remains 0x01 (NHRP)

The ar$spln and ar$tpln fields (where relevant) are either 0 (for
null or non-existent information) or 16 (for the full IPv6 protocol
address)

The way in which ATM addresses are stored remains the same as shown
in RFC2022 [3]

4.1.7 Neigbor Discovery control messages

Section 5.2 of [1] describes the ND Link-layer address option. For
IPv6/ATM drivers, the subfields SHALL be encoded in the following
manner:

[NTL] defines the type and length of the ATM number immediately
following the [STL] field. The format is as follows:

7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+
|0|x| length |
+-+-+-+-+-+-+-+-+

The most significant bit is reserved and MUST be set to zero. The
second most significant bit (x) is a flag indicating whether the
ATM number is in:

ATM Forum AESA format (x = 0).
Native E.164 format (x = 1).

The bottom 6 bits represent an unsigned integer value indicating
the length of the associated ATM address field in octets.

The [STL] format is the same as the [NTL] field. Defines the length
of the subaddress field, if it exists. If it does not exist this
entire octet field MUST be zero. If the subaddress exists it will be
in AESA format, so flag x SHALL be zero.

[NBMA Number] is a variable length field containing the ATM address
of the Link layer target. It is always present.

[NBMA Subaddress] is a variable length field containing the ATM
subaddress of the Link layer target. It may or may not be present.
When it is not, the option ends after the [NBMA Number] (or any
additional padding for 8 byte alignment).

The octet ordering of the [NBMA Number] and [NBMA Subaddress] fields
SHALL be the same as that used in MARS and NHRP control messages.

4.2 UNI 3.0/3.1 signaling issues (SVC mode).

When an IPv6 node places a call to another IPv6 node, it SHOULD
follow the procedures in [6] and [7] for signalling UNI 3.0/3.1 SVCs
[9] and negotiating MTU. The default IP MTU size on a LL is 9180
bytes as specified in [7].

Note that while the procedures in [7] still apply to IPv6 over ATM,
IPv6 Path MTU Discovery [8] is used by nodes and routers rather than
IPv4 MTU discovery. Additionally, while IPv6 nodes are not required
to implement Path MTU Discovery, IPv6/ATM nodes SHOULD implement it.
Also, since IPv6 nodes will negotiate an appropriate MTU for each VC,
Path MTU should never be triggered since neither node should ever
receive a Packet Too Big message to trigger Path MTU Discovery. When
nodes are communicating via one or more routers Path MTU Discovery
will be used just as it is for legacy networks.

5 Interface Tokens

For both PVC and SVC modes of operation, one of the following methods
SHALL be used to generate Interface Tokens as required by section 5.1
of [1].

5.1 Interface Tokens Based on ESI values

When the underlying ATM interface is identified by an ATM End System
Address (AESA, formerly known as an NSAPA), the interface token MAY
be formed from the ESI and SEL values in the AESA as follows:

[0x00][ESI][SEL]

[0x00] is a one octet field which is always set to 0.
Note that the bit corresponding to the EUI-64 Global/Local bit
[5] is always reset indicating that this address is not a
globally unique IPv6 interface token.

[ESI] is a six octet field.
This field always contains the six octet ESI value for the
AESA used to address the specific instance of the IPv6/ATM
interface.

[SEL] is a one octet field.
This field always contains the SEL value from the AESA used to
address the specific instance of the IPv6/ATM interface.

5.2 Interface Tokens Based on 48 Bit MAC Values

Where the underlying ATM NIC driver has access to a set of one or
more 48 bit MAC values unique to the ATM NIC (e.g. MAC addresses
configured into the NIC's ROM), the IPv6/ATM interface MAY use one of
these values to create a unique interface token as described in [10].

5.3 Interface Tokens Based on EUI-64 Values

Where the underlying ATM NIC driver has access to a set of one or
more 64 bit EUI-64 values unique to the ATM NIC (e.g. EUI-64
addresses configured into the NIC's ROM), the IPv6/ATM interface
SHOULD use one of these values to create a unique interface token.
after inverting the Global/Local identifier bit [10]. (Any
relationship between these values and the ESI(s) registered with the
local ATM switch by the ATM driver are outside the scope of this
document.)

When EUI-64 values are used for IPv6 interface tokens the only
modification allowed to the octet string read from the NIC is
inversion of the Global/Local identifier bit.

5.4 Interface Tokens Based on Native E.164 Addresses

When an interface uses Native E.164 addresses then the E.164 values
MAY be used to generate an interface token as follows:

[D14][D13D12][D11D10][D9D8][D9D6][D5D4][D3D2][D1D0]

[D14] A single octet containing the semi-octet representing the most
significant E.164 digit shifted left four bits to the most
significant four bits of the octet. The lower four bits MUST be set
to 0. Note that the EUI-64 Global/Local indicator is set to 0
indicating that this is not a globally unique IPv6 interface token.

[D13D12] A single octet containing the semi-octet representing the
second most significant E.164 digit [D13] shifted left four places to
the most significant bits of the octet, and the third most
significant semi-octet in the four least significant bits of the
octet.

[D11D10] - [D1D0] Octets each containing two E.164 digits, one in the
most significant four bits, and one in the least significant four
bits as indicated.

5.5 Nodes Without Unique Identifiers

If no MAC, EUI-64, AESA, or E.164 value is available for generating
an interface token, then the interface token SHALL be generated as
described in Appendix A of [10].

5.6 Multiple Logical Links on a Single Interface

A logical ATM interface might be associated with a different SEL
field of a common AESA prefix, or a set of entirely separate ESIs
might have been registered with the local ATM switch to create a
range of unique AESAs.

The minimum information required to uniquely identify each logical
ATM interface is (within the context of the local switch port) their
ESI+SEL combination.

For the vhost case described in section 5.1.2 of [1], vhost SHALL
select a different interface token from the range of 64 bit values
available to the ATM NIC (as described in 4.1). Each vhost SHALL
implement IPv6/ATM interfaces in such a way that no two or more
vhosts end up advertising the same interface token onto the same LL.
(Conformance with this requirement may be achieved by choosing
different SEL values, ESI values, or both.)

6. Conclusion and Open Issues.

This document is an ATM-specific companion document to the ION
working group's, "IPv6 over Non Broadcast Multiple Access (NBMA)
networks" specification [1]. It specifies codepoints for the
administratively configured PVC, and dynamically established SVC,
modes of operation.

There are no major open issues. Comments to the ION mailing list are
solicited (ion@nexen.com).

7. Security Considerations

While this proposal does not introduce any new security mechanisms
all current IPv6 security mechanisms will work without modification
for ATM. This includes both authentication and encryption for both
Neighbor Discovery protocols as well as the exchange of IPv6 data
packets.

Acknowledgments

The original IPv6/ATM work by G. Armitage occurred while employed at
Bellcore. Elements of section 4 were borrowed from Matt Crawford's
memo on IPv6 over Ethernet.

The authors would like to thank Kazuhiko Yamamoto, Kenjiro Cho,
Yoshinobu Inoue, Hiroshi Esaki, Yoshifumi Atarashi, and Atsushi
Hagiwara for their contributions based on actual PVC implementations.

Authors' Addresses

Grenville Armitage
Bell Laboratories, Lucent Technologies
101 Crawfords Corner Road
Holmdel, NJ 07733
USA

EMail: gja@lucent.com

Peter Schulter
BrightTiger Technologies
125 Nagog Park
Acton, MA 01720

EMail: paschulter@acm.org

Markus Jork
European Applied Research Center
Digital Equipment GmbH
CEC Karlsruhe
Vincenz-Priessnitz-Str. 1
D-76131 Karlsruhe
Germany

EMail: jork@kar.dec.com

References

[1] Armitage, G., Schulter, P., Jork, M. and G. Harter, "IPv6 over
Non-Broadcast Multiple Access (NBMA) networks", RFC2491, January
1999.

[2] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaption
Layer 5", RFC1483, July 1993.

[3] Armitage, G., "Support for Multicast over UNI 3.1 based ATM
Networks", RFC2022, November 1996.

[4] Luciani, J., Katz, D., Piscitello, D., Cole, B. and N. Doraswamy,
"NBMA Next Hop Resolution Protocol (NHRP)", RFC2332, April 1998.

[5] "64-Bit Global Identifier Format Tutorial",
http://standards.ieee.org/db/oui/tutorials/EUI64.html.

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

[7] Atkinson, R., "Default IP MTU for use over ATM AAL5", RFC1626,
May 1994.

[8] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for IP
version 6", RFC1981, August 1996.

[9] ATM Forum, "ATM User Network Interface (UNI) Specification
Version 3.1", ISBN 0-13-393828-X, Prentice Hall, Englewood
Cliffs, NJ, June 1995.

[10] Hinden, B. and S. Deering, "IP Version 6 Addressing
Architecture", RFC2373, July 1998.

[11] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC2461, December 1998.

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