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Network Working Group D. Katz Request for Comments: 1188 Merit/NSFNET Obsoletes: RFC 1103 October 1990

            A Proposed Standard for the Transmission of
                  IP Datagrams over FDDI Networks

Status of this Memo

 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 specifies a protocol on the IAB Standards Track 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.
 This proposal is the product of the IP over FDDI Working Group of the
 Internet Engineering Task Force (IETF).  Comments on this memo should
 be submitted to the IETF IP over FDDI Working Group Chair.
 Distribution of this memo is unlimited.


 This document specifies a method for the use of IP and ARP on FDDI
 networks.  The encapsulation method used is described, as well as
 various media-specific issues.


 This memo draws heavily in both concept and text from RFC 1042 [3],
 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.

Katz [Page 1] RFC 1188 IP and ARP on FDDI Networks October 1990

    "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 [1] and
 ARP requests and replies [2].
 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)
 [4], Physical Layer Protocol (PHY) [5], Physical Layer Medium
 Dependent (PMD) [6], and Station Management (SMT) [7].  The family
 of FDDI standards corresponds to the IEEE 802 MAC layer standards
 [8, 9, 10].
 The remainder of the Data Link Service is provided by the IEEE 802.2
 Logical Link Control (LLC) service [11].  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.
 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.  Operation on dual MAC stations
 will be described in a forthcoming document.

Katz [Page 2] RFC 1188 IP and ARP on FDDI Networks October 1990

Packet Format

 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) [12] 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
 Numbers [13]).
 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 [11].  The 24-bit Organization
 Code in the SNAP must be zero, and the remaining 16 bits are the
 EtherType from Assigned Numbers [13] (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).

Address Resolution

 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) [2].
 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

Katz [Page 3] RFC 1188 IP and ARP on FDDI Networks October 1990

 The ARP protocol has several fields that parameterize its use in any
 specific context [2].  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 [13].  The
 hardware type code assigned for Ethernet networks is 1 [13].
 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.  Furthermore, ARP packets shall be accepted if received
 with hardware type codes of either 1 or 6.
 The protocol type code for IP is 2048 [13].
 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.

Broadcast Address

 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 [14]).

Multicast Support

 A method of supporting IP multicasting is specified in [15].  This

Katz [Page 4] RFC 1188 IP and ARP on FDDI Networks October 1990

 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
 (group) addresses.
 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 20.]
 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.

Trailer Formats

 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
 use trailers.

Byte Order

 As described in Appendix B of the Internet Protocol specification
 [1], the IP datagram is transmitted over FDDI networks as a series of
 8-bit bytes.  This byte transmission order has been called "big-
 endian" [16].

MAC Layer Details

 Packet Size
    The FDDI MAC specification [4] defines a maximum frame size of
    9000 symbols (4500 octets) for all frame fields, including four

Katz [Page 5] RFC 1188 IP and ARP on FDDI Networks October 1990

    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 [17].
    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 [17] for
    further information.
    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.

Katz [Page 6] RFC 1188 IP and ARP on FDDI Networks October 1990

    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 [11] 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.

Katz [Page 7] RFC 1188 IP and ARP on FDDI Networks October 1990

    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.

Katz [Page 8] RFC 1188 IP and ARP on FDDI Networks October 1990

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


 [1] Postel, J., "Internet Protocol", RFC 791, USC/Information
     Sciences Institute, September 1981.
 [2] 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
 [3] Postel, J., and Reynolds, J., "A Standard for the Transmission of
     IP Datagrams over IEEE 802 Networks", RFC 1042, USC/Information
     Sciences Institute, February 1988.
 [4] ISO, "Fiber Distributed Data Interface (FDDI) - Media Access
     Control", ISO 9314-2, 1989.  See also ANSI X3.139-1987.
 [5] ISO, "Fiber Distributed Data Interface (FDDI) - Token Ring
     Physical Layer Protocol", ISO 9314-1, 1989.  See also ANSI
 [6] ISO, "Fiber Distributed Data Interface (FDDI) - Physical Layer
     Medium Dependent", ISO DIS 9314-3, 1989.  See also ANSI X3.166-
 [7] ANSI, "FDDI Station Management", ANSI X3T9.5/84-49 Rev 6.0, 1990.

Katz [Page 9] RFC 1188 IP and ARP on FDDI Networks October 1990

 [8] 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,
 [9] IEEE, "IEEE Standards for Local Area Networks: Token-Passing Bus
     Access Method and Physical Layer Specification", IEEE, New York,
     New York, 1985.
[10] IEEE, "IEEE Standards for Local Area Networks: Token Ring Access
     Method and Physical Layer Specifications", IEEE, New York, New
     York, 1985.
[11] IEEE, "IEEE Standards for Local Area Networks: Logical Link
     Control", IEEE, New York, New York, 1985.
[12] IEEE, "Draft Standard P802.1A--Overview and Architecture", 1989.
[13] Reynolds, J.K., and J.  Postel, "Assigned Numbers", RFC 1060,
     USC/Information Sciences Institute, March 1990.
[14] Braden, R., and J.  Postel, "Requirements for Internet Gateways",
     RFC 1009, USC/Information Sciences Institute, June 1987.
[15] Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
     Stanford University, August 1989.
[16] Cohen, D., "On Holy Wars and a Plea for Peace", Computer, IEEE,
     October 1981.
[17] Postel, J., "The TCP Maximum Segment Size Option and Related
     Topics", RFC 879, USC/Information Sciences Institute, November

Security Considerations

 Security issues are not discussed in this memo.

Author's Address

 Dave Katz
 1075 Beal Ave.
 Ann Arbor, MI  48109
 Phone: (313) 763-4898

Katz [Page 10] RFC 1188 IP and ARP on FDDI Networks October 1990

Katz [Page 11]

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