Transmission of IPv6 Packets Over FDDI
obsoleted by
- rfc-2467 — Transmission of IPv6 Packets over FDDI Networks
Extracted elements (19)
FDDI adjacency detection uses the LLC priority field of the FC octet to distinguish FDDI-native frames from frames that have traversed an Ethernet (which must have zero priority per IEEE 802.1d). This allows per-destination MTU determination without router support.
The default MTU of 4352 octets (rather than the theoretical maximum of 4470) was chosen to allow for variable sizes and possible future extensions to the MAC header and frame status fields, providing headroom without specifying exact future overhead.
An IPv6 frame originating on or traversing an Ethernet will have zero in the FDDI priority field when delivered via an 802.1d bridge, as required by IEEE 802.1d. This is the key property that makes FDDI adjacency detection work across bridged networks.
A node implementing FDDI adjacency detection must set a non-zero LLC priority in all Neighbor Advertisement, Neighbor Solicitation, and (if applicable) Router Advertisement frames on that FDDI interface. If detection is disabled, the priority field must be zero.
An IPv6 address prefix used for stateless autoconfiguration of an FDDI interface must be 80 bits in length.
For correct operation in bridged mixed-media environments, the smallest MTU of all bridged media must be advertised by routers in an MTU option. Multicast packets must not be larger than the smallest MTU of any bridged medium.
If a node N1 receives a valid Router Advertisement, Neighbor Advertisement, or Neighbor Solicitation from N2 in an FDDI frame with non-zero LLC priority, N1 may send unicast IPv6 packets to N2 up to the default FDDI MTU (4352 octets), overriding any smaller configured or advertised MTU.
If a Router Advertisement is received with an MTU option specifying a value larger than the default or manually configured MTU, that option may be logged to system management but must be otherwise ignored.
IPv6 packets shall be sent in asynchronous FDDI frames using unrestricted tokens. Nodes should also be able to receive synchronous frames and asynchronous frames sent using restricted tokens.
Nodes implementing FDDI adjacency detection must provide a configuration option to disable the mechanism; by default, FDDI adjacency detection should be enabled.
The address token for an FDDI interface for stateless autoconfiguration is the interface's built-in 48-bit IEEE 802 address in canonical bit order. A manually or software-set MAC address should not be used as the address token.
The default MTU size for IPv6 packets on an FDDI network is 4352 octets. This may be reduced by a Router Advertisement MTU option or by manual configuration; DHCP-supplied values are treated as manually configured.
An IPv6 multicast packet destined for address DST is transmitted to the FDDI multicast address 33-33-DST13-DST14-DST15-DST16, where the last four octets are the final four octets of the IPv6 destination address ordered from most to least significant.
IPv6 packets are transmitted in LLC/SNAP frames using long-format (48-bit) FDDI addresses. The data field carries the IPv6 header and payload, followed by FDDI FCS, Ending Delimiter, and Frame Status symbols.
The IPv6 link-local address for an FDDI interface is formed by appending the 48-bit IEEE 802 address to the 80-bit prefix FE80::, yielding a 128-bit address of the form FE80:0000:0000:0000:0000:XXXX:XXXX:XXXX.
Security considerations are not addressed in this memo.
DSAP and SSAP fields shall both contain AA hex indicating SNAP encapsulation; CTL shall be 03 hex (Unnumbered Information); OUI shall be 000000 hex; Ethertype shall be 86DD hex for IPv6.
The FC (Frame Code) field must be in the range 50–57 hex inclusive, with three low-order bits indicating frame priority. The recommended range is 51–57 hex to support FDDI adjacency detection.
The Source/Target Link-layer Address option for FDDI carries Type (1 or 2), Length (1, in units of 8 octets), and the 48-bit FDDI IEEE 802 address in canonical bit order. The address reflects the currently active address, which may differ from the built-in address token.