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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Internet Engineering Task Force Matt Crawford 2 INTERNET-DRAFT Fermilab 3 December 11, 1995 5 A Method for the Transmission of IPv6 Packets over FDDI Networks 7 Status of this Memo 9 This document is an Internet Draft. Internet Drafts are working 10 documents of the Internet Engineering Task Force (IETF), its Areas, 11 and its Working Groups. Note that other groups may also distribute 12 working documents as Internet Drafts. 14 Internet Drafts are draft documents valid for a maximum of six 15 months. Internet Drafts may be updated, replaced, or obsoleted by 16 other documents at any time. It is not appropriate to use Internet 17 Drafts as reference material or to cite them other than as a "working 18 draft" or "work in progress." 20 To learn the current status of any Internet-Draft, please check the 21 ``1id-abstracts.txt'' listing contained in the Internet Drafts Shadow 22 Directories on ds.internic.net (US East Coast), nic.nordu.net 23 (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific 24 Rim). 26 Distribution of this memo is unlimited. 28 Introduction 30 This memo specifies the MTU and frame format for transmission of IPv6 31 [IPV6] packets on FDDI networks, including a method for MTU 32 determination in the presence of 802.1d bridges to other media. It 33 also specifies the method of forming IPv6 link-local addresses on 34 FDDI networks and the content of the Source/Target Link-layer Address 35 option used the the Router Solicitation, Router Advertisement, 36 Neighbor Solicitation, and Neighbor Advertisement messages described 37 in [DISC], when those messages are transmitted on an FDDI network. 39 Maximum Transmission Unit 41 FDDI permits a frame length of 4500 octets (9000 symbols), including 42 at least 22 octets (44 symbols) of Data Link encapsulation when 43 long-format addresses are used. Subtracting 8 octets of LLC/SNAP 45 =0C 46 header, this would, in principle, allow the IPv6 packet in the 47 Information field to be up to 4470 octets. However, it is desirable 48 to allow for the variable sizes and possible future extensions to the 49 MAC header and frame status fields. The default MTU size for IPv6 50 packets on an FDDI network is therefore 4352 octets. This size may 51 be reduced by a Router Advertisement [DISC] containing an MTU option 52 which specifies a smaller MTU, or by manual configuration of a 53 smaller value on each node. If a Router Advertisement is received 54 with an MTU option specifying an MTU larger than the default or the 55 manually configured value, that MTU option may be logged to system 56 management but must be otherwise ignored. 58 For purposes of this document, information received from DHCP is 59 considered ``manually configured''. 61 Frame Format 63 FDDI provides both synchronous and asynchronous transmission, with 64 the latter class further subdivided by the use of restricted and 65 unrestricted tokens. Only asynchronous transmission with 66 unrestricted tokens is required for FDDI interoperability. 67 Accordingly, IPv6 packets shall be sent in asynchronous frames using 68 unrestricted tokens. The robustness principle dictates that nodes 69 should be able to receive synchronous frames and asynchronous frames 70 sent using restricted tokens. 72 IPv6 packets are transmitted in LLC/SNAP frames, using long-format 73 (48 bit) addresses. The data field contains the IPv6 header and 74 payload and is followed by the FDDI Frame Check Sequence, Ending 75 Delimiter, and Frame Status symbols. 77 +-------+ ^ 78 | FC | | 79 +-------+-------+-------+-------+-------+-------+ | 80 | Destination FDDI address | | 81 +-------+-------+-------+-------+-------+-------+ FDDI 82 | Source FDDI address | header 83 +-------+-------+-------+-------+-------+-------+ | 84 | DSAP | SSAP | CTL | OUI | | 85 +-------+-------+-------+-------+-------+-------+ | 86 | Ethertype | v 87 +-------+-------+-------+-------+-------+------+------+ 88 | IPv6 header and payload ... / 89 +-------+-------+-------+-------+-------+------+------+ 91 =0C 93 FDDI Header Fields: 95 FC The Frame Code must be in the range 50 to 57 hexadecimal, 96 inclusive, with the three low order bits indicating the 97 frame priority. The Frame Code should be in the range 51 to 98 57 hexadecimal, inclusive, for reasons given in the next 99 section. 101 DSAP, SSAP Both the DSAP and SSAP fields shall contain the value AA 102 hexadecimal, indictating SNAP encapsulation. 104 CTL The Control field shall be set to 03 hexadecimal, indicating 105 Unnumbered Information. 107 OUI The Organizationally Unique Identifier shall be set to 108 000000 hexadecimal. 110 Ethertype The ethernet protocol type ("ethertype") shall be set to the 111 value 86DD hexadecimal. 113 Interaction with Bridges 115 For correct operation when mixed media are bridged together, the 116 smallest MTU of all the media must be manually configured in each 117 node or advertised by routers in an MTU option. Nodes transmitting 118 IPv6 on FDDI should implement the following simple mechanism for 119 ``FDDI adjacency detection''. 121 If a node N1 receives, in an FDDI frame with a non-zero LLC priority, 122 a valid Router Advertisement or Neighbor Advertisement from a node 123 N2, then N1 may send unicast IPv6 packets to N2 with sizes up to the 124 default IPv6 FDDI MTU (4352 octets), regardless of any smaller MTU 125 configured manually or received in a Router Advertisement MTU option. 126 N2 may be the IPv6 destination or the next hop router to the 127 destination. 129 Nodes implementing FDDI adjacency detection must provide a 130 configuration option to disable the mechanism. This option may be 131 used when a smaller MTU is desired for reasons other than mixed-media 132 bridging. By default, FDDI adjacency detection should be enabled. 134 The only contemplated use of the LLC priority field of the FC octet 135 is to aid in per-destination MTU determination. It would be 136 sufficient for that purpose to require only that Router 137 Advertisements and Neighbor Advertisements sent on FDDI always have 138 non-zero priority. However, it may be simpler or more useful to 139 transmit all IPv6 packets on FDDI with non-zero priority. 141 =0C 143 Stateless Autoconfiguration and Link-Local Addresses 145 The address token [CONF] for an FDDI interface is the interface's 146 built-in 48-bit IEEE 802 address, in canonical bit order and with the 147 octet in the same order in which they would appear in the header of 148 an ethernet frame. (The individual/group bit is in the first octet 149 and the OUI is in the first three octets.) A different MAC address 150 set manually or by software should not be used as the address token. 152 An IPv6 address prefix used for stateless autoconfiguration of an 153 FDDI interface must be 80 bits in length. 155 The IPv6 Link-local address [AARCH] for an FDDI interface is formed 156 by appending the interface's IEEE 802 address to the 80-bit prefix 157 FE80::. 159 +-------+-------+-------+-------+-------+-------+------+------+ 160 | FE 80 00 00 00 00 00 00 | 161 +-------+-------+-------+-------+-------+-------+------+------+ 162 | 00 00 | FDDI Address | 163 +-------+-------+-------+-------+-------+-------+------+------+ 165 Address Mapping -- Unicast 167 The procedure for mapping IPv6 addresses into FDDI link-layer 168 addresses is described in [DISC]. The Source/Target Link-layer 169 Address option has the following form when the link layer is FDDI. 171 +-------+-------+-------+-------+-------+-------+------+------+ 172 | Type |Length | FDDI Address | 173 +-------+-------+-------+-------+-------+-------+------+------+ 175 Option fields: 177 Type 1 for Source Link-layer address. 178 2 for Target Link-layer address. 180 Length 1 (in units of 8 octets). 182 =0C 183 FDDI Address 184 The 48 bit FDDI IEEE 802 address, in canonical bit order. 186 Address Mapping -- Multicast 188 An IPv6 packet with a multicast destination address DST is 189 transmitted to the FDDI multicast address whose first two octets are 190 the value 3333 hexadecimal and whose last four octets are the last 191 four octets of DST, ordered from more to least significant. 193 +-------+-------+-------+-------+-------+-------+ 194 | 33 | 33 | DST13 | DST14 | DST15 | DST16 | 195 +-------+-------+-------+-------+-------+-------+ 197 Security Considerations 199 Security considerations are not addressed in this memo. 201 References 203 [AARCH] R. Hinden, S. Deering, IP Version 6 Addressing Architecture. 204 Currently draft-ietf-ipngwg-addr-arch-03.txt. 206 [CONF] S. Thomson, IPv6 Stateless Address Autoconfiguration. Currently 207 draft-ietf-addrconf-ipv6-auto-06.txt. 209 [DISC] T. Narten, E. Nordmark, W. A. Simpson, Neighbor Discovery for IP 210 Version 6 (IPv6). Currently draft-ietf-ipngwg-discovery-03.txt. 212 [IPV6] S. Deering, R. Hinden, Internet Protocol, Version 6 (IPv6) 213 Specification. Currently draft-ietf-ipngwg-ipv6-spec-02.txt. 215 =0C 217 Author's Address 219 Matt Crawford 220 Fermilab MS 368 221 PO Box 500 222 Batavia, IL 60510 223 USA 225 Phone: +1 708 840-3461 227 EMail: crawdad@fnal.gov