Internet Engineering Task Force Stephen Thomas INTERNET DRAFT AT&T Tridom January 22, 1996 A Method for the Transmission of IPv6 Packets over Token Ring 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 frame format for transmission of IPv6 [IPV6] packets and the method of forming IPv6 link-local addresses on Token Ring networks [802.5]. It also specifies the content of the Source/Target Link-layer Address option used by the Router Solicitation, Router Advertisement, Neighbor Solicitation, and Neighbor Advertisement messages described in [DISC], when those messages are transmitted on a Token Ring. The memo concludes with a description of the mapping of IPv6 multicast addresses to Token Ring functional addresses. 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, Thomas Expires July 1996 [Page 1] INTERNET-DRAFT IPv6 over Token Ring January 11, 1996 including Ran Atkinson, Scott Bradner, Matt Crawford, Steve Deering, Francis Dupont, Robert Elz, Thomnas Narten, and Matt Thomas. IPv6 Encapsulation 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 | +-------+-------+ 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.) Token Ring Header Fields SD - Starting Delimiter AC - Access Control FC - Frame Control Thomas Expires July 1996 [Page 2] INTERNET-DRAFT IPv6 over Token Ring January 11, 1996 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) 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 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 must rely on static 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 [802.2] header and the IPv6 datagram. Since, for IPv6, the LLC Thomas Expires July 1996 [Page 3] INTERNET-DRAFT IPv6 over Token Ring January 11, 1996 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 [802.1D]. 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 576 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) 2. Router advertisements 3. Static configuration 4. Default of 1500 Stateless Autoconfiguration and Link-local Addresses The address token [CONF] for a Token Ring interface is the built-in 48-bit IEEE 802 address associated with that interface, in canonical bit order. (Note: multiple interfaces on a system may share the same IEEE 802 address.) 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 a Token Ring interface must be 80 bits in length. Thomas Expires July 1996 [Page 4] INTERNET-DRAFT IPv6 over Token Ring January 11, 1996 The IPv6 Link-local address [AARCH] for a Token Ring 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 | | +-------+-------+ | | Token Ring Address | +-------+-------+-------+-------+ Address Mapping - Unicast The procedure for mapping 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. +-------+-------+-------+-------+ | 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) Token Ring Address The 48-bit IEEE 802 address, in canonical bit order. 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. Thomas Expires July 1996 [Page 5] INTERNET-DRAFT IPv6 over Token Ring January 11, 1996 MAC Func Addr (canonical) IPv6 Multicast Addresses 03 00 80 00 00 00 all nodes (FF0X::1) and solicited node (FF02::1:XXXX: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 = 100 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 Security Considerations Security considerations are not addressed in this memo. References [802.1D] IEEE Standards for Local and Metropolitan Area Networks: Media Access Control (MAC) Bridges. ANSI/IEEE Std 802.1D, 1993 Edition. [802.2] IEEE Standards for Local Area Networks: Logical Link Control. ANSI/IEEE Std 802.2-1985. Thomas Expires July 1996 [Page 6] INTERNET-DRAFT IPv6 over Token Ring January 11, 1996 [802.5] IEEE Standards for Local and Metropolitan Area Networks: Token Ring Access Method and Physical Layer Specifications. IEEE Std 802.5-1995. [AARCH] R. Hinden, S. Deering, IP Version 6 Addressing Architecture. RFC 1884. [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-03.txt. [IPV6] S. Deering, R. Hinden, Internet Protocol, Version 6 (IPv6) Specification. RFC 1883. Author's Address Stephen Thomas AT&T Tridom Phone: (770) 514-3522 840 Franklin Court Fax: (770) 514-3491 Marietta, GA 30067 USA Email: stephen.thomas@tridom.com Thomas Expires July 1996 [Page 7]