Network Working Group M. Crawford
Request for Comments: 2467 Fermilab
Obsoletes: 2019 December 1998
Category: Standards Track
Transmission of IPv6 Packets over FDDI 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 (C) The Internet Society (1998). All Rights Reserved.
This document specifies the frame format for transmission of IPv6
packets and the method of forming IPv6 link-local addresses and
statelessly autoconfigured addresses on FDDI networks. It also
specifies the content of the Source/Target Link-layer Address option
used in Router Solicitation, Router Advertisement, Neighbor
Solicitation, Neighbor Advertisement and Redirect messages when those
messages are transmitted on an FDDI network.
This document replaces RFC 2019, "Transmission of IPv6 Packets Over
FDDI", which will become historic.
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 [RFC 2119].
2. Maximum Transmission Unit
FDDI permits a frame length of 4500 octets (9000 symbols), including
at least 22 octets (44 symbols) of Data Link encapsulation when
long-format addresses are used. Subtracting 8 octets of LLC/SNAP
header, this would, in principle, allow the IPv6 [IPV6] packet in the
Information field to be up to 4470 octets. However, it is desirable
to allow for the variable sizes and possible future extensions of the
MAC header and frame status fields. The default MTU size for IPv6
packets on an FDDI network is therefore 4352 octets. This size may
be reduced by a Router Advertisement [DISC] containing an MTU option
which specifies a smaller MTU, or by manual configuration of each
node. If a Router Advertisement received on an FDDI interface has an
MTU option specifying an MTU larger than 4352, or larger than a
manually configured value, that MTU option may be logged to system
management but must be otherwise ignored.
For purposes of this document, information received from DHCP is
considered "manually configured" and the term FDDI includes CDDI.
3. Frame Format
FDDI provides both synchronous and asynchronous transmission, with
the latter class further subdivided by the use of restricted and
unrestricted tokens. Only asynchronous transmission with
unrestricted tokens is required for FDDI interoperability.
Accordingly, IPv6 packets shall be sent in asynchronous frames using
unrestricted tokens. The robustness principle dictates that nodes
should be able to receive synchronous frames and asynchronous frames
sent using restricted tokens.
IPv6 packets are transmitted in LLC/SNAP frames, using long-format
(48 bit) addresses. The data field contains the IPv6 header and
payload and is followed by the FDDI Frame Check Sequence, Ending
Delimiter, and Frame Status symbols.
OUI The Organizationally Unique Identifier shall be set to
Ethertype The Ethernet protocol type ("ethertype") shall be set to
the value 86DD hexadecimal.
4. Interaction with Bridges
802.1d MAC bridges which connect different media, for example
Ethernet and FDDI, have become very widespread. Some of them do IPv4
packet fragmentation and/or support IPv4 Path MTU discovery [RFC
1981], many others do not, or do so incorrectly. Use of IPv6 in a
bridged mixed-media environment must not depend on support from MAC
bridges, unless those bridges are known to correctly implement IPv6
Path MTU Discovery [RFC 1981, ICMPV6].
For correct operation when mixed media are bridged together by
bridges which do not support IPv6 Path MTU Discovery, the smallest
MTU of all the media must be advertised by routers in an MTU option.
If there are no routers present, this MTU must be manually configured
in each node which is connected to a medium with a default MTU larger
than the smallest MTU.
5. Stateless Autoconfiguration
The Interface Identifier [AARCH] for an FDDI interface is based on
the EUI-64 identifier [EUI64] derived from the interface's built-in
48-bit IEEE 802 address. The EUI-64 is formed as follows.
(Canonical bit order is assumed throughout. See [CANON] for a
caution on bit-order effects in LAN interfaces.)
The OUI of the FDDI MAC address (the first three octets) becomes the
company_id of the EUI-64 (the first three octets). The fourth and
fifth octets of the EUI are set to the fixed value FFFE hexadecimal.
The last three octets of the FDDI MAC address become the last three
octets of the EUI-64.
The Interface Identifier is then formed from the EUI-64 by
complementing the "Universal/Local" (U/L) bit, which is the next-to-
lowest order bit of the first octet of the EUI-64. For further
discussion on this point, see [ETHER] and [AARCH].
For example, the Interface Identifier for an FDDI interface whose
built-in address is, in hexadecimal,
A different MAC address set manually or by software should not be
used to derive the Interface Identifier. If such a MAC address must
be used, its global uniqueness property should be reflected in the
value of the U/L bit.
An IPv6 address prefix used for stateless autoconfiguration [ACONF]
of an FDDI interface must have a length of 64 bits.
6. Link-Local Addresses
The IPv6 link-local address [AARCH] for an FDDI interface is formed
by appending the Interface Identifier, as defined above, to the
10 bits 54 bits 64 bits
|1111111010| (zeros) | Interface Identifier |
7. Address Mapping -- Unicast
The procedure for mapping IPv6 unicast addresses into FDDI link-layer
addresses is described in [DISC]. The Source/Target Link-layer
Address option has the following form when the link layer is FDDI.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
| Type | Length |
+- FDDI -+
+- Address -+
Type 1 for Source Link-layer address.
2 for Target Link-layer address.
Length 1 (in units of 8 octets).
The 48 bit FDDI IEEE 802 address, in canonical bit order.
This is the address the interface currently responds to,
and may be different from the built-in address used to
derive the Interface Identifier.
8. Address Mapping -- Multicast
An IPv6 packet with a multicast destination address DST, consisting
of the sixteen octets DST through DST, is transmitted to the
FDDI multicast address whose first two octets are the value 3333
hexadecimal and whose last four octets are the last four octets of
|0 0 1 1 0 0 1 1|0 0 1 1 0 0 1 1|
| DST | DST |
| DST | DST |
9. Differences From RFC 2019
The following are the functional differences between this
specification and RFC 2019.
"FDDI adjacency detection" has been removed, due to recent work
in IEEE 802.1p.
The Address Token, which was a node's 48-bit MAC address, is
replaced with the Interface Identifier, which is 64 bits in
length and based on the EUI-64 format [EUI64]. An IEEE-defined
mapping exists from 48-bit MAC addresses to EUI-64 form.
A prefix used for stateless autoconfiguration must now be 64 bits
long rather than 80. The link-local prefix is also shortened to
10. Security Considerations
The method of derivation of Interface Identifiers from MAC addresses
is intended to preserve global uniqueness when possible. However,
there is no protection from duplication through accident or forgery.
[AARCH] Hinden, R. and S. Deering "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[ACONF] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[CANON] Narten, T. and C. Burton, "A Caution On The Canonical
Ordering Of Link-Layer Addresses", RFC 2469, December 1998.
[DISC] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[ETHER] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, December 1998.
[EUI64] "Guidelines For 64-bit Global Identifier (EUI-64)",
[ICMPV6] Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", RFC 2463, December 1998.
[IPV6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC 1981] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
13. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
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