Network Working Group M. Crawford
Request for Comments: 2019 Fermilab
Category: Standards Track October 1996

A Method for the 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.

IntrodUCtion

This memo specifies the MTU and frame format for transmission of IPv6
[IPV6] packets on FDDI networks, including a method for MTU
determination in the presence of 802.1d bridges to other media. It
also specifies the method of forming IPv6 link-local addresses on
FDDI networks and the content of the Source/Target Link-layer Address
option used the the Router Solicitation, Router Advertisement,
Neighbor Solicitation, and Neighbor Advertisement messages described
in [DISC], when those messages are transmitted on an FDDI network.

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 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 to 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 a
smaller value on each node. If a Router Advertisement is received
with an MTU option specifying an MTU larger than the default or the
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".

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.

+-------+ ^
FC
+-------+-------+-------+-------+-------+-------+
Destination FDDI address
+-------+-------+-------+-------+-------+-------+ FDDI
Source FDDI address header
+-------+-------+-------+-------+-------+-------+
DSAP SSAP CTL OUI
+-------+-------+-------+-------+-------+-------+
Ethertype v
+-------+-------+-------+-------+-------+------+------+
IPv6 header and payload ... /
+-------+-------+-------+-------+-------+------+------+

FDDI Header Fields:

FC The Frame Code must be in the range 50 to 57 hexadecimal,
inclusive, with the three low order bits indicating the
frame priority. The Frame Code should be in the range 51 to
57 hexadecimal, inclusive, for reasons given in the next
section.

DSAP, SSAP Both the DSAP and SSAP fields shall contain the value AA
hexadecimal, indictating SNAP encapsulation.

CTL The Control field shall be set to 03 hexadecimal, indicating
Unnumbered Information.

OUI The Organizationally Unique Identifier shall be set to
000000 hexadecimal.

Ethertype The ethernet protocol type ("ethertype") shall be set to the
value 86DD hexadecimal.

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 [PMTU],
many others do not, or do so incorrectly. Use of IPv6 in a bridged
mixed-media environment should not depend on support from MAC
bridges.

For correct operation when mixed media are bridged together, 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 larger
default MTU. Multicast packets on such a bridged network must not be
larger than the smallest MTU of any of the bridged media. Often, the
subnetwork topology will support larger unicast packets to be
exchanged between certain pairs of nodes. To take advantage of
high-MTU paths when possible, nodes transmitting IPv6 on FDDI should
implement the following simple mechanism for "FDDI adjacency
detection".

A node which implements FDDI adjacency detection and has it enabled
on an FDDI interface must set a non-zero LLC priority in all Neighbor
Advertisement, Neighbor Solicitation and, if applicable, Router
Advertisement frames transmitted on that interface. (In IEEE 802
language, the user_priority parameter of the M_UNITDATA.request
primitive must not be zero.) If FDDI adjacency detection has been
disabled on an FDDI interface, the priority field of those frames
must be zero.

Note that an IPv6 frame which originated on an Ethernet, or traversed
an Ethernet, before being translated by an 802.1d bridge and
delivered to a node's FDDI interface will have zero in the priority
field, as required by [BRIDGE]. (There's a fine point here: a
conforming bridge may provide a management-settable Outbound User
Priority parameter for each port. However, the author is unaware of
any product that provides this optional capability and, in any case,
the default value for the parameter is zero.)

If a node N1 receives, in an FDDI frame with a non-zero LLC priority,
a valid Router Advertisement, Neighbor Advertisement, or Neighbor
Solicitation from a node N2, then N1 may send unicast IPv6 packets to
N2 with sizes up to the default IPv6 FDDI MTU (4352 octets),
regardless of any smaller MTU configured manually or received in a
Router Advertisement MTU option. N2 may be the IPv6 destination or
the next hop router to the destination.

Nodes implementing FDDI adjacency detection must provide a
configuration option to disable the mechanism. This option may be
used when a smaller MTU is desired for reasons other than mixed-media
bridging. By default, FDDI adjacency detection should be enabled.

The only contemplated use of the LLC priority field of the FC octet
is to aid in per-destination MTU determination. It would be
sufficient for that purpose to require only that Router
Advertisements, Neighbor Advertisements, and Neighbor Solicitations
sent on FDDI always have non-zero priority. However, it may be
simpler or more useful to transmit all IPv6 packets on FDDI with
non-zero priority.

Stateless Autoconfiguration and Link-Local Addresses

The address token [CONF] for an FDDI interface is the interface's
built-in 48-bit IEEE 802 address, in canonical bit order and with the
octet in the same order in which they would appear in the header of
an ethernet frame. (The individual/group bit is in the first octet
and the OUI is in the first three octets.) A different MAC address
set manually or by software should not be used as the address token.

An IPv6 address prefix used for stateless autoconfiguration of an
FDDI interface must be 80 bits in length.

The IPv6 Link-local address [AARCH] for an FDDI interface is formed
by appending the interface's IEEE 802 address to the 80-bit prefix
FE80::.

+-------+-------+-------+-------+-------+-------+------+------+
FE 80 00 00 00 00 00 00
+-------+-------+-------+-------+-------+-------+------+------+
00 00 FDDI Address
+-------+-------+-------+-------+-------+-------+------+------+

Address Mapping -- Unicast

The procedure for mapping IPv6 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.

+-------+-------+-------+-------+-------+-------+------+------+
Type Length FDDI Address
+-------+-------+-------+-------+-------+-------+------+------+

Option fields:

Type 1 for Source Link-layer address.
2 for Target Link-layer address.

Length 1 (in units of 8 octets).

FDDI Address
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 as the
address token.

Address Mapping -- Multicast

An IPv6 packet with a multicast destination address 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 DST, ordered from more to least significant.

+-------+-------+-------+-------+-------+-------+
33 33 DST13 DST14 DST15 DST16
+-------+-------+-------+-------+-------+-------+

Security Considerations

Security considerations are not addressed in this memo.

Acknowledgments

Erik Nordmark contributed to the method for interaction with bridges.

References

[AARCH] Hinden, and S. Deering, "IP Version 6 Addressing
Architecture", RFC1884, December 1995.

[BRIDGE]ISO/IEC 10038 : 1993 [ANSI/IEEE Std 802.1D] Media Access
control (MAC) bridges.

[CONF] Thomson, S., and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC1971, August 1996.

[DISC] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6), RFC1970, August 1996.

[IPV6] Deering, S., and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC1883, August 1996.

[PMTU] Mogul, J., and S. Deering, "Path MTU Discovery", RFC1191,
November 1990.

Author's Address

Matt Crawford
Fermilab MS 368
PO Box 500
Batavia, IL 60510
USA

Phone: +1 708 840-3461

EMail: crawdad@fnal.gov