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, RFC 2460 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, RFC 2461 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.
Pv6 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.
The Frame Code must be in the range 50 to 57 hexadecimal, inclusive, with the three low order bits indicating the frame priority.
|DSAP, SSAP|| |
Both the DSAP and SSAP fields shall contain the value AA hexadecimal, indicating SNAP encapsulation.
The Control field shall be set to 03 hexadecimal, indicating Unnumbered Information.
The Organizationally Unique Identifier shall be set to 000000 hexadecimal.
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, RFC 2463].
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 [RFC 2373] for an FDDI interface is based on the EUI-64 identifier [“Guidelines For 64-bit Global Identifier (EUI-64)”, http://standards.ieee.org/db/oui/tutorials/EUI64.html.] 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 [RFC 2469] 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 [RFC 2464] and [RFC 2373].
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 [RFC 2462] of an FDDI interface must have a length of 64 bits.
6. Link-Local Addresses
7. Address Mapping — Unicast
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 to derive the Interface Identifier.
8. Address Mapping — Multicast