IPng Working Group Matt Crawford
Internet Draft Fermilab
September 26, 1997
Transmission of IPv6 Packets over FDDI Networks
<draft-ietf-ipngwg-trans-fddi-net-03.txt>
Status of this Memo
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Distribution of this memo is unlimited.
1. Introduction
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 [KWORD].
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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.
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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 must not depend on support from MAC
bridges, unless those bridges are known to correctly implement IPv6
Path MTU Discovery [PMTU, 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.)
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,
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34-56-78-9A-BC-DE
would be
36-56-78-FF-FE-9A-BC-DE.
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
prefix FE80::/64.
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
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- FDDI -+
| |
+- Address -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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 to derive the Interface Identifier.
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
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.
10. References
[AARCH] R. Hinden, S. Deering "IP Version 6 Addressing
Architecture", Currently draft-ietf-ipngwg-addr-arch-v2-
02.txt.
[ACONF] S. Thomson, T. Narten, "IPv6 Stateless Address
Autoconfiguration", currently draft-ietf-ipngwg-addrconf-
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v2-00.txt.
[DISC] T. Narten, E. Nordmark, W. A. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", currently draft-ietf-ipngwg-
discovery-v2-00.txt.
[ETHER] M. Crawford, "Transmission of IPv6 Packets over Ethernet
Networks", currently draft-ietf-ipngwg-trans-ethernet-
02.txt.
[EUI64] "64-Bit Global Identifier Format Tutorial",
http://standards.ieee.org/db/oui/tutorials/EUI64.html.
[ICMPV6]A. Conta, S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)", RFC
1885
[IPV6] S. Deering, R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", currently draft-ietf-ipngwg-ipv6-spec-v2-
00.txt.
[KWORD] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels," RFC 2119.
[PMTU] J. Mogul, S. Deering "Path MTU Discovery", RFC 1191.
11. 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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