Proposed Standard for the Transmission of IP Datagrams over FDDI Networks
obsoletes
- rfc-1103 — Proposed standard for the transmission of IP datagrams over FDDI Networks
Extracted elements (23)
ARP hardware addresses are carried in canonical (bit-reversed) order, with the Group bit as the low-order bit of the first octet, to maintain compatibility with Ethernet conventions in bridged environments. It is also recommended that all Network Layer protocols in FDDI frames use canonical MAC address order.
ARP uses hardware type code 1 (Ethernet) instead of 6 (IEEE 802) on FDDI because differing values between Ethernet and IEEE 802 networks cause interoperability problems in bridged environments, and the goal is not to preclude such interoperability.
The FDDI MAC maximum frame size is 4500 octets, but the MTU is set to 4352 rather than the theoretical maximum of ~4470 to reserve space for future extensions to the MAC header and frame status fields.
IEEE specifies numbers as bit-strings with the least significant bit first (bit-wise little-endian), while Internet protocols are documented in bit-wise big-endian order. This can cause confusion; the appendix provides conversion tables for key values such as UI (IEEE 11000000 = Internet 3), SAP for SNAP (IEEE 01010101 = Internet 170), and others.
The semantics of FDDI Frame Copied (C) and Address Recognized (A) indicators are not well defined as of the time of writing, especially in the presence of bridges. Implementors are urged to follow ANSI ASC X3T9.5 work on this issue to avoid interoperability problems.
This document covers single MAC stations only (single-attach or dual-attach). Operation on dual MAC stations is to be described in a forthcoming document.
All implementations must support IEEE 802.2 standard Class I service, including support for Unnumbered Information (UI) commands, XID commands and responses, and TEST commands and responses.
All IP and ARP frames shall be transmitted as Asynchronous LLC frames using Unrestricted tokens. The Priority value is a matter of local convention, but implementations should make it a tunable parameter.
ARP packets shall be transmitted with a hardware type code of 1 (Ethernet), not 6 (IEEE 802), to preserve interoperability in bridged environments. ARP packets must be accepted if received with hardware type codes of either 1 or 6.
FDDI implementations must be prepared to receive and ignore trailing pad octets, in order to not preclude interoperability with Ethernet in a bridged environment.
Gateway implementations must be prepared to accept packets as large as the MTU and fragment them when necessary. Host implementations must not send datagrams longer than 576 octets unless they have explicit knowledge that the destination will accept them.
Hardware addresses in ARP packets (ar$sha, ar$tha) must be carried in canonical bit order, with the Group bit as the low order bit of the first octet. Because FDDI normally places the Group bit in the high order position, addresses must be bit-reversed within each octet.
Hosts directly connected to FDDI networks shall not use trailer encapsulations (BSD Unix 4.x style). Trailers are prohibited.
IP datagrams and ARP requests/replies on FDDI shall be encapsulated within 802.2 LLC and SNAP data link layers. All frames must be transmitted in standard 802.2 LLC Type 1 Unnumbered Information format.
Only 48-bit FDDI addresses shall be used with IP and ARP, to avoid interoperability problems arising from incomplete interworking between 16-bit and 48-bit address stations.
The broadcast Internet address must be mapped to the FDDI broadcast address (all binary ones). This is an absolute requirement.
The MTU of FDDI networks shall be 4352 octets, providing 4096 octets of data and 256 octets of network-layer headers. Implementations must not send packets larger than the MTU.
When an XID or TEST command is received with a DSAP of SNAP SAP (170), Null SAP (0), or Global SAP (255), an appropriate response must be returned. Stations must be able to transmit XID and TEST responses though not necessarily commands.
ARP field values for use on FDDI: hardware type (hrd) = 1, protocol type (pro) = 2048 (IP), hardware address length (hln) = 6, protocol address length (pln) = 4, operation code (op) = 1 for request, 2 for reply.
IP multicast addresses are mapped to FDDI group addresses by placing the low order 23 bits of the IP address into the low order 23 bits of the FDDI group address 01-00-5E-00-00-00 (canonical order). When transmitted in native FDDI bit order, the multicast bit appears as the high order bit of the first octet.
Security issues are not discussed in this memo.
IEEE 802.2 LLC control field encoding (Internet/big-endian decimal): UI=3, XID=175 (Poll/Final off) or 191 (Poll/Final on), TEST=227 (Poll/Final off) or 243 (Poll/Final on). XID response frames must include an XID Information field of 129.1.0 indicating Class I service.
IP datagrams and ARP packets on FDDI are encapsulated using 802.2 LLC Type 1 UI frames with a SNAP header. DSAP and SSAP are both set to 170 (decimal), the Control field is 3 (UI), the 24-bit Organization Code in SNAP is 0, and the 16-bit EtherType identifies the network layer protocol (IP=2048, ARP=2054). The combined LLC and SNAP header is 8 octets.