IPv6 over FDDI: Transmission of IPv6 Packets over FDDI Networks
How IPv6 packets are framed, addressed and mapped to link-layer addresses on FDDI networks.
Crawford, M., "Transmission of IPv6 Packets over FDDI Networks", RFC 2467, DOI 10.17487/RFC2467, December 1998, <https://www.rfc-editor.org/info/rfc2467>.
@misc{rfc2467,
series = {Request for Comments},
number = 2467,
howpublished = {RFC 2467},
publisher = {RFC Editor},
doi = {10.17487/RFC2467},
url = {https://www.rfc-editor.org/info/rfc2467},
author = {M. Crawford},
title = {{Transmission of IPv6 Packets over FDDI Networks}},
pagetotal = 9,
year = 1998,
month = dec,
} RFC text
Contents
Network Working Group M. Crawford
Request for Comments: 2467 Fermilab
Obsoletes: 2019 December 1998
Category: Standards Track
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.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
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 [RFC 2119].
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.
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.
DSAP, SSAP Both the DSAP and SSAP fields shall contain the value AA
hexadecimal, indicating 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.
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, 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. See [CANON] 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 [ETHER] and [AARCH].
For example, the Interface Identifier for an FDDI interface whose
built-in address is, in hexadecimal,
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 -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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. Differences From RFC 2019
The following are the functional differences between this
specification and RFC 2019.
"FDDI adjacency detection" has been removed, due to recent work
in IEEE 802.1p.
The Address Token, which was a node's 48-bit MAC address, is
replaced with the Interface Identifier, which is 64 bits in
length and based on the EUI-64 format [EUI64]. An IEEE-defined
mapping exists from 48-bit MAC addresses to EUI-64 form.
A prefix used for stateless autoconfiguration must now be 64 bits
long rather than 80. The link-local prefix is also shortened to
64 bits.
10. 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.
11. References
[AARCH] Hinden, R. and S. Deering "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[ACONF] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[CANON] Narten, T. and C. Burton, "A Caution On The Canonical
Ordering Of Link-Layer Addresses", RFC 2469, December 1998.
[DISC] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[ETHER] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, December 1998.
[EUI64] "Guidelines For 64-bit Global Identifier (EUI-64)",
http://standards.ieee.org/db/oui/tutorials/EUI64.html.
[ICMPV6] Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", RFC 2463, December 1998.
[IPV6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC 1981] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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
13. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
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AI ELI5
On this page
§1What the Memo Covers
This memo specifies three things for IPv6 on FDDI: the frame format for IPv6 packets, how to form link-local and statelessly autoconfigured addresses, and the contents of the Source/Target Link-layer Address option used in Router Solicitation, Router Advertisement, Neighbor Solicitation, Neighbor Advertisement and Redirect messages.
It replaces RFC 2019, 'Transmission of IPv6 Packets Over FDDI', which the text says will become historic. The term FDDI here includes CDDI.
It is a short rulebook for sending IPv6 over one particular kind of network cable: what the envelope looks like, how a computer picks its name tag, and how to find a neighbour.
§2Maximum Transmission Unit
FDDI allows a frame length of 4500 octets, including at least 22 octets of data link encapsulation when long-format addresses are used. After the 8 octets of LLC/SNAP header, that would in principle allow an IPv6 packet of up to 4470 octets. To leave room for variable MAC header sizes and future extensions of the frame status fields, the default MTU is 4352 octets.
The MTU may be lowered by a Router Advertisement carrying an MTU option, or by manual configuration. If an advertisement specifies an MTU larger than 4352 or larger than the manually configured value, the option may be logged to system management but must otherwise be ignored. Information received from DHCP counts as manually configured.
The envelope has a size limit. The memo picks a safe size a little smaller than the largest possible, and it lets a router ask for smaller but never larger.
§3Frame Format
FDDI offers synchronous and asynchronous transmission. Only asynchronous transmission with unrestricted tokens is required for interoperability, so IPv6 packets are sent in asynchronous frames using unrestricted tokens. Nodes should still be able to receive synchronous frames and asynchronous frames sent with restricted tokens.
Packets travel in LLC/SNAP frames using long-format (48 bit) addresses. The data field holds the IPv6 header and payload and is followed by the FDDI Frame Check Sequence, Ending Delimiter and Frame Status symbols. The header fields have fixed values: the Frame Code FC must be in the range 50 to 57 hexadecimal (the three low-order bits give the priority), DSAP and SSAP are both AA, CTL is 03 (Unnumbered Information), OUI is 000000 and the Ethertype is 86DD.
Every packet goes into an envelope with a label saying 'this holds IPv6', so the receiving computer knows how to read what is inside.
- 50-57FC — Frame Code, hexadecimal; three low-order bits give the priority.
- AADSAP, SSAP — Both hold AA hexadecimal, meaning SNAP encapsulation.
- 03CTL — Control field: Unnumbered Information.
- 000000OUI — Organizationally Unique Identifier.
- 86DDEthertype — Marks the payload as IPv6.
§4Interaction with Bridges
MAC bridges that join different media, for example Ethernet and FDDI, are very widespread. Some fragment IPv4 packets or support IPv4 Path MTU discovery, and many do not or do it incorrectly. Use of IPv6 in a bridged mixed-media environment must not depend on bridge support, unless the bridges are known to implement IPv6 Path MTU Discovery correctly.
When bridges lack that support, routers must advertise the smallest MTU of all the media in an MTU option. If no routers are present, that MTU must be configured by hand in each node attached to a medium whose default MTU is larger than the smallest one.
If a mail chute between two buildings only fits small envelopes, everyone must use the small size, because the chute will not tell you when something is too big.
§5Interface Identifiers and Link-Local Addresses
The Interface Identifier for an FDDI interface is based on an EUI-64 identifier derived from the interface's built-in 48-bit IEEE 802 address. The first three octets of the MAC address (the OUI) become the company_id, the fourth and fifth octets of the EUI-64 are set to FFFE, and the last three MAC octets become the last three octets. The identifier is then obtained by complementing the Universal/Local bit, the next-to-lowest order bit of the first octet. The built-in address 34-56-78-9A-BC-DE gives 36-56-78-FF-FE-9A-BC-DE.
A MAC address set manually or by software should not be used to derive the identifier; if one must be used, its global uniqueness should be reflected in the U/L bit. A prefix used for stateless autoconfiguration on FDDI must be 64 bits long.
The link-local address is formed by appending the Interface Identifier to the prefix FE80::/64.
A computer builds its own name tag from the serial number printed on its network card, with a few fixed digits added in the middle and one bit flipped. Nobody has to hand out names.
built-in address: 34-56-78-9A-BC-DE
interface identifier: 36-56-78-FF-FE-9A-BC-DE§7Address Mapping
Unicast mapping from IPv6 addresses to FDDI link-layer addresses follows Neighbor Discovery. The Source/Target Link-layer Address option has Type 1 for a source address or 2 for a target address, Length 1 (in units of 8 octets), and then the 48-bit FDDI address in canonical bit order. That address is the one the interface currently responds to, which may differ from the built-in address used to derive the Interface Identifier.
For multicast, an IPv6 packet with destination address DST is sent to the FDDI multicast address whose first two octets are 3333 hexadecimal and whose last four octets are the last four octets of DST (DST[13] through DST[16]).
To find a neighbour's real card address you ask on the ring and they answer with it. For messages to a whole group, the group address is made by taking the last part of the IPv6 address and putting 3333 in front.
§9Differences from RFC 2019
The memo lists three functional changes from RFC 2019. 'FDDI adjacency detection' was removed because of recent work in IEEE 802.1p. The Address Token, which was a node's 48-bit MAC address, is replaced by the 64-bit Interface Identifier based on EUI-64. Prefixes used for stateless autoconfiguration must now be 64 bits long rather than 80, and the link-local prefix is likewise shortened to 64 bits.
This is the list of things fixed since the first edition: one old feature dropped, and the name tags made longer and more standard.
§10Security Considerations
Deriving Interface Identifiers from MAC addresses is intended to preserve global uniqueness where possible. The memo points out that there is no protection from duplication, whether by accident or by forgery.
Name tags are built from card serial numbers, but nothing stops two cards from having the same one by mistake, or someone from faking it on purpose.