Network Working Group D. Katz
Request for Comments: 1390 cisco Systems, Inc.
STD: 36 January 1993
Transmission of IP and ARP over FDDI Networks
Status of this Memo
This RFC specifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
This memo defines a method of encapsulating the Internet Protocol
(IP) datagrams and Address Resolution Protocol (ARP) requests and
replies on Fiber Distributed Data Interface (FDDI) Networks.
This RFC is the product of the IP over FDDI Working Group of the
Internet Engineering Task Force (IETF).
This memo draws heavily in both concept and text from RFC 1042 ,
written by Jon Postel and Joyce K. Reynolds of USC/Information
Sciences Institute. The author would also like to acknowledge the
contributions of the IP Over FDDI Working Group of the IETF, members
of ANSI ASC X3T9.5, and others in the FDDI community.
The following language conventions are used in the items of
specification in this document:
"Must," "Shall," or "Mandatory"--the item is an absolute
requirement of the specification.
"Should" or "Recommended"--the item should generally be followed
for all but exceptional circumstances.
"May" or "Optional"--the item is truly optional and may be
followed or ignored according to the needs of the implementor.
The goal of this specification is to allow compatible and
interoperable implementations for transmitting IP datagrams  and
ARP requests and replies .
The Fiber Distributed Data Interface (FDDI) specifications define a
family of standards for Local Area Networks (LANs) that provides the
Physical Layer and Media Access Control Sublayer of the Data Link
Layer as defined by the ISO Open System Interconnection Reference
Model (ISO/OSI). Documents are in various stages of progression
toward International Standardization for Media Access Control (MAC)
, Physical Layer Protocol (PHY) , Physical Layer Medium
Dependent (PMD) , and Station Management (SMT) . The family of
FDDI standards corresponds to the IEEE 802 MAC layer standards [8, 9,
The remainder of the Data Link Service is provided by the IEEE 802.2
Logical Link Control (LLC) service . The resulting stack of
services appears as follows:
| IP/ARP |
| 802.2 LLC |
| FDDI MAC | F |
+-------------+ D S |
| FDDI PHY | D M |
+-------------+ I T |
| FDDI PMD | |
This memo describes the use of IP and ARP in this environment. At
this time, it is not necessary that the use of IP and ARP be
consistent between FDDI and IEEE 802 networks, but it is the intent
of this memo not to preclude Data Link Layer interoperability at such
time as the standards define it.
It is the explicit intent of this memo to allow the interoperability
of IP and ARP between stations on FDDI networks and stations on
Ethernet networks via translational bridges.
The FDDI standards define both single and dual MAC stations. This
document describes the use of IP and ARP on single MAC stations
(single-attach or dual-attach) only.
IP datagrams and ARP requests and replies sent on FDDI networks shall
be encapsulated within the 802.2 LLC and Sub-Network Access Protocol
(SNAP)  data link layers and the FDDI MAC and physical layers.
The SNAP must be used with an Organization Code indicating that the
SNAP header contains the EtherType code (as listed in Assigned
802.2 LLC Type 1 communication (which must be implemented by all
conforming 802.2 stations) is used exclusively. All frames must be
transmitted in standard 802.2 LLC Type 1 Unnumbered Information
format, with the DSAP and the SSAP fields of the 802.2 header set to
the assigned global SAP value for SNAP . The 24-bit Organization
Code in the SNAP must be zero, and the remaining 16 bits are the
EtherType from Assigned Numbers  (IP = 2048, ARP = 2054).
MAC Header | FDDI MAC
| DSAP=K1| SSAP=K1| Control| 802.2 LLC
|Protocol Id or Org Code =K2| EtherType | 802.2 SNAP
The total length of the LLC Header and the SNAP header is 8
The K1 value is 170 (decimal).
The K2 value is 0 (zero).
The control value is 3 (Unnumbered Information).
The mapping of 32-bit Internet addresses to 48-bit FDDI addresses
shall be done via the dynamic discovery procedure of the Address
Resolution Protocol (ARP) .
Internet addresses are assigned arbitrarily on Internet networks.
Each host's implementation must know its own Internet address and
respond to Address Resolution requests appropriately. It must also
use ARP to translate Internet addresses to FDDI addresses when
The ARP protocol has several fields that parameterize its use in any
specific context . These fields are:
hrd 16 - bits The Hardware Type Code
pro 16 - bits The Protocol Type Code
hln 8 - bits Octets in each hardware address
pln 8 - bits Octets in each protocol address
op 16 - bits Operation Code
The hardware type code assigned for IEEE 802 networks is 6 . The
hardware type code assigned for Ethernet networks is 1 .
Unfortunately, differing values between Ethernet and IEEE 802
networks cause interoperability problems in bridged environments. In
order to not preclude interoperability with Ethernets in a bridged
environment, ARP packets shall be transmitted with a hardware type
code of 1. ARP packets shall be accepted if received with a hardware
type code of 1.
The protocol type code for IP is 2048 .
The hardware address length is 6.
The protocol address length (for IP) is 4.
The operation code is 1 for request and 2 for reply.
In order to not preclude interoperability in a bridged environment,
the hardware addresses in ARP packets (ar$sha, ar$tha) must be
carried in "canonical" bit order, with the Group bit positioned as
the low order bit of the first octet. As FDDI addresses are normally
expressed with the Group bit in the high order bit position, the
addresses must be bit-reversed within each octet.
Although outside the scope of this document, it is recommended that
MAC addresses be represented in canonical order in all Network Layer
protocols carried within the information field of an FDDI frame.
The broadcast Internet address (the address on that network with a
host part of all binary ones) must be mapped to the broadcast FDDI
address (of all binary ones) (see ).
A method of supporting IP multicasting is specified in . This
method shall be used in FDDI networks if IP multicasting is to be
supported. The use of this method may require the ability to copy
frames addressed to any one of an arbitrary number of multicast
An IP multicast address is mapped to an FDDI group address 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 (in "canonical" order).
[See 13, page 29.]
For example, the IP multicast address:
maps to the FDDI group address:
in which the multicast (group) bit is the low order bit of the first
octet (canonical order). When bit-reversed for transmission in the
destination MAC address field of an FDDI frame (native order), it
that is, with the multicast (group) bit as the high order bit of the
first octet, that being the first bit transmitted on the medium.
Some versions of Unix 4.x bsd use a different encapsulation method in
order to get better network performance with the VAX virtual memory
architecture. Hosts directly connected to FDDI networks shall not
As described in Appendix B of the Internet Protocol specification
, the IP datagram is transmitted over FDDI networks as a series of
8-bit bytes. This byte transmission order has been called "big-
MAC Layer Details
The FDDI MAC specification  defines a maximum frame size of
9000 symbols (4500 octets) for all frame fields, including four
symbols (two octets) of preamble. This leaves roughly 4470 octets
for data after the LLC/SNAP header is taken into account.
However, in order to allow future extensions to the MAC header and
frame status fields, it is desirable to reserve additional space
for MAC overhead.
Therefore, the MTU of FDDI networks shall be 4352 octets. This
provides for 4096 octets of data and 256 octets of headers at the
network layer and above. Implementations must not send packets
larger than the MTU.
Gateway implementations must be prepared to accept packets as
large as the MTU and fragment them when necessary. Gateway
implementations should be able to accept packets as large as can
be carried within a maximum size FDDI frame and fragment them.
Host implementations should be prepared to accept packets as large
as the MTU; however, hosts must not send datagrams longer than 576
octets unless they have explicit knowledge that the destination is
prepared to accept them. Host implementations may accept packets
as large as can be carried within a maximum size FDDI frame. A
host may communicate its size preference in TCP-based applications
via the TCP Maximum Segment Size option .
Datagrams on FDDI networks may be longer than the general Internet
default maximum packet size of 576 octets. Hosts connected to an
FDDI network should keep this in mind when sending datagrams to
hosts that are not on the same local network. It may be
appropriate to send smaller datagrams to avoid unnecessary
fragmentation at intermediate gateways. Please see  for
There is no minimum packet size restriction on FDDI networks.
In order to not preclude interoperability with Ethernet in a
bridged environment, FDDI implementations must be prepared to
receive (and ignore) trailing pad octets.
Other MAC Layer Issues
The FDDI MAC specification does not require that 16-bit and 48-
bit address stations be able to interwork fully. It does,
however, require that 16-bit stations have full 48-bit
functionality, and that both types of stations be able to receive
frames sent to either size broadcast address. In order to avoid
interoperability problems, only 48-bit addresses shall be used
with IP and ARP.
The FDDI MAC specification defines two classes of LLC frames,
Asynchronous and Synchronous. Asynchronous frames are further
controlled by a priority mechanism and two classes of token,
Restricted and Unrestricted. Only the use of Unrestricted tokens
and Asynchronous frames are required by the standard for FDDI
All IP and ARP frames shall be transmitted as Asynchronous LLC
frames using Unrestricted tokens, and the Priority value is a
matter of local convention. Implementations should make the
priority a tunable parameter for future use. It is recommended
that implementations provide for the reception of IP and ARP
packets in Synchronous frames, as well as Restricted Asynchronous
After packet transmission, FDDI provides Frame Copied (C) and
Address Recognized (A) indicators. The use of these indicators is
a local implementation decision. Implementations may choose to
perform link-level retransmission, ARP cache entry invalidation,
etc., based on the values of these indicators and other
information. The semantics of these indicators, especially in the
presence of bridges, are not well defined as of this writing.
Implementors are urged to follow the work of ANSI ASC X3T9.5 in
regard to this issue in order to avoid interoperability problems.
IEEE 802.2 Details
While not necessary for supporting IP and ARP, all implementations
must support IEEE 802.2 standard Class I service in order to be
compliant with 802.2. Described below is the minimum functionality
necessary for a conformant station. Some of the functions are not
related directly to the support of the SNAP SAP (e.g., responding to
XID and TEST commands directed to the null or global SAP addresses),
but are part of a general LLC implementation. Implementors should
consult IEEE Std. 802.2  for details.
802.2 Class I LLC requires the support of Unnumbered Information (UI)
Commands, eXchange IDentification (XID) Commands and Responses, and
TEST link (TEST) Commands and Responses. Stations need not be able
to transmit XID and TEST commands, but must be able to transmit
Command frames are identified by having the low order bit of the
SSAP address reset to zero. Response frames have the low order
bit of the SSAP address set to one.
The UI command has an LLC control field value of 3.
The XID command/response has an LLC control field value of 175
(decimal) if the Poll/Final bit is off or 191 (decimal) if the
Poll/Final bit is on.
The TEST command/response has an LLC control field value of 227
(decimal) if the Poll/Final bit is off or 243 (decimal) if the
Poll/Final bit is on.
Elements of Procedure
UI responses and UI commands with the Poll bit set shall be
ignored. UI commands having other than the SNAP SAP in the DSAP
or SSAP fields shall not be processed as IP or ARP packets.
When an XID or TEST command is received, an appropriate response
must be returned. XID and TEST commands must be responded to only
if the DSAP is the SNAP SAP (170 decimal), the Null SAP (0
decimal), or the Global SAP (255 decimal). XID and TEST commands
received with other DSAP values must not be responded to unless
the station supports the addressed service. Responses to XID and
TEST frames shall be constructed as follows:
Destination MAC: Copied from Source MAC of the command
Source MAC: Set to the address of the MAC receiving the
DSAP: Copied from SSAP of the command
SSAP: Set to 171 decimal (SNAP SAP + Response bit) if the
DSAP in the command was the SNAP SAP or the Global SAP;
set to 1 decimal (Null SAP + Response bit) if the DSAP
in the command was the Null SAP
When responding to an XID or a TEST command, the value of the
Final bit in the response must be copied from the value of the
Poll bit in the command.
XID response frames must include an 802.2 XID Information field of
129.1.0 indicating Class I (connectionless) service.
TEST response frames must echo the information field received in
the corresponding TEST command frame.
Appendix on Numbers
The IEEE specifies numbers as bit strings with the least significant
bit first, or bit-wise little-endian order. The Internet protocols
are documented in bit-wise big-endian order. This may cause some
confusion about the proper values to use for numbers. Here are the
conversions for some numbers of interest.
Number IEEE Internet Internet
Binary Binary Decimal
UI 11000000 00000011 3
SAP for SNAP 01010101 10101010 170
Global SAP 11111111 11111111 255
Null SAP 00000000 00000000 0
XID 11110101 10101111 175
XID Poll/Final 11111101 10111111 191
XID Info 129.1.0
TEST 11000111 11100011 227
TEST Poll/Final 11001111 11110011 243
Differences between this document and RFC 1188
The following is a summary of the differences between RFC 1188 and
A reference to a future dual-MAC document has been removed.
A statement of explicit intent to support FDDI/Ethernet
interoperability has been added.
The acceptance of ARP frames bearing hardware type code 6 (IEEE
802) has been removed.
The references have been updated.
The author's address has been updated.
 Postel, J., "Internet Protocol", STD 5, RFC 791, USC/Information
Sciences Institute, September 1981.
 Plummer, D., "An Ethernet Address Resolution Protocol - or -
Converting Network Protocol Addresses to 48.bit Ethernet Address
for Transmission on Ethernet Hardware", RFC 826, MIT, November
 Postel, J., and J. Reynolds, "A Standard for the Transmission of
IP Datagrams over IEEE 802 Networks", RFC 1042, USC/Information
Sciences Institute, February 1988.
 ISO, "Fiber Distributed Data Interface (FDDI) - Media Access
Control", ISO 9314-2, 1989. See also ANSI X3.139-1987.
 ISO, "Fiber Distributed Data Interface (FDDI) - Token Ring
Physical Layer Protocol", ISO 9314-1, 1989. See also ANSI
 ISO, "Fiber Distributed Data Interface (FDDI) - Physical Layer
Medium Dependent", ISO DIS 9314-3, 1989. See also ANSI X3.166-
 ANSI, "FDDI Station Management", ANSI X3T9.5/84-49 Rev 7.1, 1992.
 IEEE, "IEEE Standards for Local Area Networks: Carrier Sense
Multiple Access with Collision Detection (CSMA/CD) Access Method
and Physical Layer Specifications", IEEE, New York, New York,
 IEEE, "IEEE Standards for Local Area Networks: Token-Passing Bus
Access Method and Physical Layer Specification", IEEE, New York,
New York, 1985.
 IEEE, "IEEE Standards for Local Area Networks: Token Ring Access
Method and Physical Layer Specifications", IEEE, New York, New
 IEEE, "IEEE Standards for Local Area Networks: Logical Link
Control", IEEE, New York, New York, 1985.
 IEEE, "Draft Standard P802.1A--Overview and Architecture", 1989.
 Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
USC/Information Sciences Institute, July 1992.
 Braden, R., and J. Postel, "Requirements for Internet Gateways",
STD 4, RFC 1009, USC/Information Sciences Institute, June 1987.
 Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
1112, Stanford University, August 1989.
 Cohen, D., "On Holy Wars and a Plea for Peace", Computer, IEEE,
 Postel, J., "The TCP Maximum Segment Size Option and Related
Topics", RFC 879, USC/Information Sciences Institute, November
Security issues are not discussed in this memo.
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