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Network Working Group                                       M. CrawfordRequest for Comments: 2470                                     FermilabCategory: Standards Track                                     T. Narten                                                                    IBM                                                              S. Thomas                                                             TransNexus                                                          December 1998Transmission of IPv6 Packets over Token Ring 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.Copyright Notice   Copyright (C) The Internet Society (1998).  All Rights Reserved.1.  Introduction   This memo specifies the MTU and frame format for transmission of IPv6   packets on Token Ring networks. It also specifies the method of   forming IPv6 link-local addresses on Token Ring networks and the   content of the Source/Target Link-layer Address option used the   Router Solicitation, Router Advertisement, Redirect, Neighbor   Solicitation and Neighbor Advertisement messages when those messages   are transmitted on a Token Ring network.   Implementors should be careful to note that Token Ring adaptors   assume addresses are in non-canonical rather than canonical format,   requiring that special care be taken to insure that addresses are   processed correctly. See [CANON] for more details.   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].2.  Maximum Transmission Unit   IEEE 802.5 networks have a maximum frame size based on the maximum   time a node may hold the token. This time depends on many factors   including the data signaling rate and the number of nodes on the   ring. Because the maximum frame size varies, implementations mustCrawford, et. al.           Standards Track                     [Page 1]

RFC 2470                  IPv6 over Token Ring             December 1998   rely on manual configuration or router advertisements [DISC] to   determine actual MTU sizes. Common default values include   approximately 2000, 4000, and 8000 octets.   In the absence of any other information, an implementation should use   a default MTU of 1500 octets. This size offers compatibility with all   common 802.5 defaults, as well as with Ethernet LANs in an   environment using transparent bridging.   In an environment using source route bridging, the process of   discovering the MAC-level path to a neighbor can yield the MTU for   the path to that neighbor. The information is contained in the   largest frame (LF) subfield of the routing information field. This   field limits the size of the information field of frames to that   destination, and that information field includes both the LLC [LLC]   header and the IPv6 datagram. Since, for IPv6, the LLC header is   always 8 octets in length, the IPv6 MTU can be found by subtracting 8   from the maximum frame size defined by the LF subfield. If an   implementation uses this information to determine MTU sizes, it must   maintain separate MTU values for each neighbor.   A detailed list of the LF values and the resulting maximum frame size   can be found in [BRIDGE]. To illustrate the calculation of IPv6 MTU,   the following table lists several common values. Note that some of   the 802.1D LF values would result in an IP MTU less than 1280 bytes.   This size is less than the IPv6 minimum, and communication across   paths with those MTUs is generally not possible using IPv6.           LF (base)  LF (extension)  MAC MTU  IP MTU             001           000         1470     1462             010           000         2052     2044             011           000         4399     4391             100           000         8130     8122             101           000         11407    11399             110           000         17749    17741             111           000         41600    41592   When presented with conflicting MTU values from several sources, an   implementation should choose from those sources according to the   following priorities:      1.  Largest Frame values from source route bridging           (only for specific, unicast destinations), but only if not           greater than value from any router advertisements      2.  Router advertisements, but only if not greater than any manual           configuration (including DHCP)Crawford, et. al.           Standards Track                     [Page 2]

RFC 2470                  IPv6 over Token Ring             December 1998      3.  Manual configuration (including DHCP)      4.  Default of 15003.   Frame Format   IPv6 packets are transmitted in LLC/SNAP frames.  The data field   contains the IPv6 header and payload. The following figure shows a   complete 802.5 frame containing an IPv6 datagram.      +-------+-------+-------+-------+      |  SD   |  AC   |  FC   |       |      +-----------------------+       |      |      Destination Address      |      |       +-----------------------+      |       |     Source            |      +-------+    Address    +-------+      |                       | DSAP  |      +-------+-------+-------+-------+      | SSAP  |  CTL  |      OUI      |      +-------+-------+-------+-------+      |  OUI  |   EtherType   |       |      +-------+---------------+       |      |                               |      ~  IPv6 header and payload...   ~      |                               |      +-------------------------------+      |              FCS              |      +-------+-------+---------------+      |  ED   |  FS   |      +-------+-------+   Token Ring Header Fields      SD:  Starting Delimiter      AC:  Access Control      FC:  Frame Control      Destination Address: 48-bit IEEE address of destination           station      Source Address: 48-bit IEEE address of source station      DSAP: Destination Service Access Point (for LLC/SNAP           format, shall always contain the value 0xAA)Crawford, et. al.           Standards Track                     [Page 3]

RFC 2470                  IPv6 over Token Ring             December 1998      SSAP: Source Service Access Point (for LLC/SNAP format,           shall always contain the value 0xAA)      CTL: Control Field (for Unnumbered Information, shall           always contain the value 0x03)      OUI: Organizationally Unique Identifier (for EtherType           encoding, shall always contain the value 0x000000)      EtherType: Protocol type of encapsulated payload (for           IPv6, shall always contain the value 0x86DD)      FCS: Frame Check Sequence      ED:  Ending Delimiter      FS:  Frame Status   In the presence of source route bridges, a routing information field   (RIF) may appear immediately after the source address. A RIF is   present in frames when the most significant bit of the source address   is set to one. (This is the bit whose position corresponds to that of   the Individual/Group bit in the Destination Address.)   The RIF is a variable-length field that (when present) contains a   two-octet Routing Control (RC) header, followed by zero or more two-   octet Route Designator fields:                             0                   1                             0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5                            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       Routing Control:     |Bcast| Length  |D|  LF   |rsvd |                            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       Route Designator 1:  |    Segment 1          |Bridge1|                            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                            ~              ...              ~                            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       Route Designator N:  |    Segment N          |BridgeN|         (0 <= N <= 7)      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       Route Designator Fields:       Bcast:    Broadcast Indicator, Defined values:                 10x: All Routes Explorer                 11x: Spanning Tree Explorer                 0xx: Specifically Routed FrameCrawford, et. al.           Standards Track                     [Page 4]

RFC 2470                  IPv6 over Token Ring             December 1998       Length:  Total length of RIF field in octets       D:   Direction of source route. A value of 0 means that            the left-to-right sequence of Route Designators            provides the path from the sender to recipient. A            value of 0 indicates the sequence goes from            recipient to sender.       LF:  Largest Frame       rsvd: Reserved   On transmission, the Route Designator fields give the sequence of   (bridge, LAN segment) numbers the packet is to traverse. It is the   responsibility of the sender to provide this sequence for   Specifically Routed Frames, i.e., unicast IP datagrams.4.  Stateless Autoconfiguration   The Interface Identifier [AARCH] for a Token Ring interface is based   on the EUI-64 identifier [EUI64] derived from the interface's built-   in 48-bit IEEE 802 address. The OUI of the Token Ring 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 Token Ring   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.  Complementing   this bit will generally change a 0 value to a 1, since an interface's   built-in address is expected to be from a universally administered   address space and hence have a globally unique value.  A universally   administered IEEE 802 address or an EUI-64 is signified by a 0 in the   U/L bit position, while a globally unique IPv6 Interface Identifier   is signified by a 1 in the corresponding position.  For further   discussion on this point, see [AARCH].   For example, the Interface Identifier for a Token Ring interface   whose built-in address is, in hexadecimal and in canonical bit order,                             34-56-78-9A-BC-DE   would be                         36-56-78-FF-FE-9A-BC-DE.Crawford, et. al.           Standards Track                     [Page 5]

RFC 2470                  IPv6 over Token Ring             December 1998   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 of a   Token Ring interface must have a length of 64 bits.5.  Link-Local Address   The IPv6 link-local address [AARCH] for a Token Ring interface is   formed by appending the Interface Identifer, as defined above, to the   prefix FE80::/64.     10 bits            54 bits                  64 bits   +----------+-----------------------+----------------------------+   |1111111010|         (zeros)       |    Interface Identifier    |   +----------+-----------------------+----------------------------+6.  Address Mapping -- Unicast   The procedure for mapping unicast IPv6 addresses into Token Ring   link-layer addresses is described in [DISC]. The Source/Target Link-   layer Address option has the following form when the link layer is   Token Ring.               0                   1               0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+              |     Type      |    Length     |              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+              |                               |              +-         Token Ring          -+              |                               |              +-           Address           -+              |                               |              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      Option fields:         Type:     1 for Source Link-layer address.                   2 for Target Link-layer address.         Length:  1 (in units of 8 octets).Crawford, et. al.           Standards Track                     [Page 6]

RFC 2470                  IPv6 over Token Ring             December 1998         Token Ring Address: The 48 bit Token Ring 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.            When source routing bridges are used, the source route for            the path to a destination can be extracted from the RIF            field of received Neighbor Advertisement messages. Note that            the RIF field of received packets can be reversed into a            source route suitable for transmitting return traffic by            toggling the value of the 'D' bit and insuring that the            Bcast field is set to indicate a Specifically Routed Frame.7.  Address Mapping -- Multicast   All IPv6 packets with multicast destination addresses are transmitted   to Token Ring functional addresses. The following table shows the   specific mapping between the IPv6 addresses and Token Ring functional   addresses (in canonical form). Note that protocols other than IPv6   may use these same functional addresses, so all Token Ring frames   destined to these functional addresses are not guaranteed to be IPv6   datagrams.   MAC Addr (canonical)       IPv6 Multicast Addresses   03-00-80-00-00-00  All-Nodes (FF01::1 and FF02::1) and                      solicited node (FF02:0:0:0:0:1:FFXX:XXXX)                      addresses   03-00-40-00-00-00  All-Routers addresses (FF0X::2)   03-00-00-80-00-00  any other multicast address with three                      least significant bits = 000   03-00-00-40-00-00  any other multicast address with three                      least significant bits = 001   03-00-00-20-00-00  any other multicast address with three                      least significant bits = 010   03-00-00-10-00-00  any other multicast address with three                      least significant bits = 011   03-00-00-08-00-00  any other multicast address with three                       least significant bits = 100Crawford, et. al.           Standards Track                     [Page 7]

RFC 2470                  IPv6 over Token Ring             December 1998   03-00-00-04-00-00  any other multicast address with three                       least significant bits = 101   03-00-00-02-00-00  any other multicast address with three                       least significant bits = 110   03-00-00-01-00-00  any other multicast address with three                       least significant bits = 111   In a bridged token ring network, all multicast packets SHOULD be sent   with a RIF header specifying the use of the Spanning Tree Explorer.   Note: it is believed that some (very) old bridge implementations do   not properly support the Spanning Tree Explorer mechanism.  In such   environments, multicast traffic sent through bridges must use a RIF   with the All Routes Explorer. Consequently, an implementation MAY   wish to allow the sending of IP multicast traffic using an All Routes   Explorer. However, such an ability must be configurable by a system   administrator and the default setting of the switch MUST be to use   the Spanning Tree Explorer.8.  Security Considerations   Token Ring, like most broadcast LAN technologies, has inherent   security vulnerabilities. For example, any sender can claim the   identity of another and forge traffic. It is the responsibility of   higher layers to take appropriate steps in those environments where   such vulnerabilities are unacceptable.9.  Acknowledgments   Several members of the IEEE 802.5 Working Group contributed their   knowledge and experience to the drafting of this specification,   including Jim, Andrew Draper, George Lin, John Messenger, Kirk   Preiss, and Trevor Warwick. The author would also like to thank many   members of the IPng working group for their advice and suggestions,   including Ran Atkinson, Scott Bradner, Steve Deering, Francis Dupont,   Robert Elz, and Matt Thomas. A special thanks is due Steve Wise, who   gave the most relevant advice of all by actually trying to implement   this specification while it was in progress.Crawford, et. al.           Standards Track                     [Page 8]

RFC 2470                  IPv6 over Token Ring             December 199810.  References   [802.5]   8802-5 : 1995 (ISO/IEC) [ANSI/IEEE 802.5, 1995             Edition] Information technology--Telecommunications and             information exchange between systems--Local and             metropolitan area networks--Specific requirements-- Part 5:             Token ring access method and physical layer specification.   [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.   [BRIDGE]  10038: 1993 (ISO/IEC) [ANSI/IEEE Std 802.1D, 1993 Edition]             Information technology--Telecommunications and information             exchange between systems--Local area networks--Media access             control (MAC) bridges.   [CANON]   Narten, T. and C. Burton, "A Caution on Canonical Bit Order             Of Link-Layer Addresses",RFC 2469, December 1998.   [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 2461, December             1998.   [EUI64]  "64-Bit Global Identifier Format Tutorial", http:             //standards.ieee.org/db/oui/tutorials/EUI64.html.   [IPV6]    Deering, S. and R. Hinden, "Internet Protocol, Version 6             (IPv6) Specification",RFC 2460, December 1998.   [KWORD]   Bradner, S., "Key words for use in RFCs to Indicate             Requirement Levels,"BCP 14,RFC 2119, March 1997.   [LLC]     8802-2 : 1994 (ISO/IEC) [ANSI/IEEE 802.2, 1994 Edition]             Information technology--Telecommunications and information             exchange between systems--Local and Metropolitan area             networks--Specific requirements-- Part 2: Logical link             control.Crawford, et. al.           Standards Track                     [Page 9]

RFC 2470                  IPv6 over Token Ring             December 199811.  Authors' Addresses   Matt Crawford   Fermilab MS 368   PO Box 500   Batavia, IL 60510 USA   Phone: +1 630 840 3461   EMail: crawdad@fnal.gov   Thomas Narten   IBM Corporation   P.O. Box 12195   Research Triangle Park, NC 27709-2195 USA   Phone: +1 919 254 7798   EMail: narten@raleigh.ibm.com   Stephen Thomas   TransNexus   430 Tenth Street NW Suite N204   Atlanta, GA 30318 USA   Phone: +1 404 872 4745   EMail: stephen.thomas@transnexus.comCrawford, et. al.           Standards Track                    [Page 10]

RFC 2470                  IPv6 over Token Ring             December 1998Full 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   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Crawford, et. al.           Standards Track                    [Page 11]