IPng Working Group Matt Crawford
Internet Draft Fermilab
March 21, 1997
Transmission of IPv6 Packets over FDDI Networks
<draft-ietf-ipngwg-trans-fddi-net-00.txt>
Status of this Memo
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Distribution of this memo is unlimited.
1. Introduction
This memo specifies the MTU and frame format for transmission of
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 Router Solicitation, Router Advertisement,
Neighbor Solicitation and Neighbor Advertisement messages when those
messages are transmitted on an FDDI network.
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
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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 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".
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.
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+
| FC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination |
+- -+
| FDDI |
+- -+
| Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source |
+- -+
| FDDI |
+- -+
| Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DSAP | SSAP |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CTL | OUI ... |
+-+-+-+-+-+-+-+-+ +
| ... OUI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 |
+- -+
| header |
+- -+
| and |
+- -+
/ payload ... /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(Each tic mark represents one bit.)
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, indicating SNAP encapsulation.
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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.
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
[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.)
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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.
5. Stateless Autoconfiguration
The interface token [CONF] for an FDDI interface is the EUI-64
identifier [EUI64] derived from the interface's built-in 48-bit IEEE
802 address. The OUI of the Ethernet 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 Ethernet
address become the last three octets of the EUI-64.
For example, the interface token for an Ethernet interface whose
built-in address is, in hexadecimal and in canonical bit order,
34-56-78-9A-BC-DE
would be
34-56-78-FF-FE-9A-BC-DE.
A different MAC address set manually or by software should not be
used to derive the interface token.
An IPv6 address prefix used for stateless autoconfiguration of an
FDDI interface must have a length of 64 bits.
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6. Link-Local Addresses
The IPv6 link-local address [AARCH] for an FDDI interface is formed
by appending the interface token, as defined above, to the prefix
FE80::/64.
10 bits 54 bits 64 bits
+----------+-----------------------+----------------------------+
|1111111010| (zeros) | Interface Token |
+----------+-----------------------+----------------------------+
7. 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.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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.
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8. Address Mapping -- Multicast
An IPv6 packet with a multicast destination address DST, consisting
of the sixteen octets DST[1] through DST[16], 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.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 1 1 0 0 1 1|0 0 1 1 0 0 1 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DST[13] | DST[14] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DST[15] | DST[16] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9. Security Considerations
Security considerations are not addressed in this memo.
10. Acknowledgments
Erik Nordmark and Matt Thomas contributed to the method for
interaction with bridges.
11. References
[AARCH] R. Hinden, S. Deering "IP Version 6 Addressing
Architecture", RFC 1884.
[BRIDGE]ISO/IEC 10038 : 1993 [ANSI/IEEE Std 802.1D] Media access
control (MAC) bridges.
[CONF] S. Thomson, T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 1971.
[DISC] T. Narten, E. Nordmark, W. A. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 1970.
[EUI64] "64-Bit Global Identifier Format Tutorial",
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http://standards.ieee.org/db/oui/tutorials/EUI64.html.
[IPV6] S. Deering, R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 1883.
[PMTU] J. Mogul, S. Deering "Path MTU Discovery", RFC 1191.
12. Author's Address
Matt Crawford
Fermilab MS 368
PO Box 500
Batavia, IL 60510
USA
Phone: +1 630 840-3461
EMail: crawdad@fnal.gov
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