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Network Working Group                                        M. CrawfordRequest for Comments: 2019                                      FermilabCategory: Standards Track                                   October 1996A Method for the Transmission of IPv6 Packets over FDDI NetworksStatus 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.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   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".Crawford                    Standards Track                     [Page 1]

RFC 2019         Transmission of IPv6 Packets Over FDDI     October 1996Frame 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 ...              /       +-------+-------+-------+-------+-------+------+------+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.Crawford                    Standards Track                     [Page 2]

RFC 2019         Transmission of IPv6 Packets Over FDDI     October 1996Ethertype   The ethernet protocol type ("ethertype") shall be set to the            value 86DD hexadecimal.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 should not depend on support from MAC   bridges.   For correct operation when mixed media are bridged together, 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 larger   default MTU.  Multicast packets on such a bridged network must not be   larger than the smallest MTU of any of the bridged media.  Often, the   subnetwork topology will support larger unicast packets to be   exchanged between certain pairs of nodes.  To take advantage of   high-MTU paths when possible, nodes transmitting IPv6 on FDDI should   implement the following simple mechanism for "FDDI adjacency   detection".   A node which implements FDDI adjacency detection and has it enabled   on an FDDI interface must set a non-zero LLC priority in all Neighbor   Advertisement, Neighbor Solicitation and, if applicable, Router   Advertisement frames transmitted on that interface.  (In IEEE 802   language, the user_priority parameter of the M_UNITDATA.request   primitive must not be zero.) If FDDI adjacency detection has been   disabled on an FDDI interface, the priority field of those frames   must be zero.   Note that an IPv6 frame which originated on an Ethernet, or traversed   an Ethernet, before being translated by an 802.1d bridge and   delivered to a node's FDDI interface will have zero in the priority   field, as required by [BRIDGE].  (There's a fine point here: a   conforming bridge may provide a management-settable Outbound User   Priority parameter for each port.  However, the author is unaware of   any product that provides this optional capability and, in any case,   the default value for the parameter is zero.)Crawford                    Standards Track                     [Page 3]

RFC 2019         Transmission of IPv6 Packets Over FDDI     October 1996   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   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               |      +-------+-------+-------+-------+-------+-------+------+------+Crawford                    Standards Track                     [Page 4]

RFC 2019         Transmission of IPv6 Packets Over FDDI     October 1996Address 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            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 |             +-------+-------+-------+-------+-------+-------+Crawford                    Standards Track                     [Page 5]

RFC 2019         Transmission of IPv6 Packets Over FDDI     October 1996Security Considerations   Security considerations are not addressed in this memo.Acknowledgments   Erik Nordmark contributed to the method for interaction with bridges.References   [AARCH] Hinden, and S. Deering, "IP Version 6 Addressing           Architecture",RFC 1884, December 1995.   [BRIDGE]ISO/IEC 10038 : 1993 [ANSI/IEEE Std 802.1D] Media access           control (MAC) bridges.   [CONF] Thomson, S., and T. Narten, "IPv6 Stateless Address          Autoconfiguration",RFC 1971, August 1996.   [DISC] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery          for IP Version 6 (IPv6),RFC 1970, August 1996.   [IPV6] Deering, S., and R. Hinden, "Internet Protocol, Version 6          (IPv6) Specification",RFC 1883, August 1996.   [PMTU] Mogul, J., and S. Deering, "Path MTU Discovery",RFC 1191,          November 1990.Author's Address   Matt Crawford   Fermilab MS 368   PO Box 500   Batavia, IL 60510   USA   Phone: +1 708 840-3461   EMail: crawdad@fnal.govCrawford                    Standards Track                     [Page 6]