Internet Engineering Task Force Matt Crawford INTERNET-DRAFT Fermilab February 26, 1996 A Method for the Transmission of IPv6 Packets over FDDI Networks Status of this Memo This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress." To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). Distribution of this memo is unlimited. Introduction This memo specifies the MTU and frame format for transmission of IPv6 [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 the Router Solicitation, Router Advertisement, Neighbor Solicitation, and Neighbor Advertisement messages described in [DISC], when those messages are transmitted on an FDDI network. 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 Crawford Expires August 1996 [Page 1] INTERNET-DRAFT Transmission of IPv6 Packets Over FDDI 1996-02-26 header, this would, in principle, allow the 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 to 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''. 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. +-------+ ^ | FC | | +-------+-------+-------+-------+-------+-------+ | | Destination FDDI address | | +-------+-------+-------+-------+-------+-------+ FDDI | Source FDDI address | header +-------+-------+-------+-------+-------+-------+ | | DSAP | SSAP | CTL | OUI | | +-------+-------+-------+-------+-------+-------+ | | Ethertype | v +-------+-------+-------+-------+-------+------+------+ | IPv6 header and payload ... / +-------+-------+-------+-------+-------+------+------+ Crawford Expires August 1996 [Page 2] INTERNET-DRAFT Transmission of IPv6 Packets Over FDDI 1996-02-26 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, indictating 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. Interaction with Bridges For correct operation when mixed media are bridged together, the smallest MTU of all the media must be manually configured in each node or advertised by routers in an MTU option. Nodes transmitting IPv6 on FDDI should implement the following simple mechanism for ``FDDI adjacency detection''. 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 Crawford Expires August 1996 [Page 3] INTERNET-DRAFT Transmission of IPv6 Packets Over FDDI 1996-02-26 non-zero priority. Stateless Autoconfiguration and Link-Local Addresses The address token [CONF] for an FDDI interface is the interface's built-in 48-bit IEEE 802 address, in canonical bit order and with the octet in the same order in which they would appear in the header of an ethernet frame. (The individual/group bit is in the first octet and the OUI is in the first three octets.) A different MAC address set manually or by software should not be used as the address token. An IPv6 address prefix used for stateless autoconfiguration of an FDDI interface must be 80 bits in length. The IPv6 Link-local address [AARCH] for an FDDI interface is formed by appending the interface's IEEE 802 address to the 80-bit prefix FE80::. +-------+-------+-------+-------+-------+-------+------+------+ | FE 80 00 00 00 00 00 00 | +-------+-------+-------+-------+-------+-------+------+------+ | 00 00 | FDDI Address | +-------+-------+-------+-------+-------+-------+------+------+ 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. +-------+-------+-------+-------+-------+-------+------+------+ | 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 Crawford Expires August 1996 [Page 4] INTERNET-DRAFT Transmission of IPv6 Packets Over FDDI 1996-02-26 address token. Address Mapping -- Multicast An IPv6 packet with a multicast destination address DST 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, ordered from more to least significant. +-------+-------+-------+-------+-------+-------+ | 33 | 33 | DST13 | DST14 | DST15 | DST16 | +-------+-------+-------+-------+-------+-------+ Security Considerations Security considerations are not addressed in this memo. Acknowledgments Erik Nordmark contributed to the method for interaction with bridges. References [AARCH] R. Hinden, S. Deering, IP Version 6 Addressing Architecture. RFC1884. [CONF] S. Thomson, IPv6 Stateless Address Autoconfiguration. Currently draft-ietf-addrconf-ipv6-auto-07.txt. [DISC] T. Narten, E. Nordmark, W. A. Simpson, Neighbor Discovery for IP Version 6 (IPv6). Currently draft-ietf-ipngwg-discovery-05.txt. [IPV6] S. Deering, R. Hinden, Internet Protocol, Version 6 (IPv6) Specification. RFC1883. Crawford Expires August 1996 [Page 5] INTERNET-DRAFT Transmission of IPv6 Packets Over FDDI 1996-02-26 Author's Address Matt Crawford Fermilab MS 368 PO Box 500 Batavia, IL 60510 USA Phone: +1 708 840-3461 EMail: crawdad@fnal.gov Crawford Expires August 1996 [Page 6]