rfc1188.txt

RFC 的详细文档！

      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.



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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
      interoperability.

      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
      frames.

      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
   responses.

   Encodings

      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.




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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
                command
         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.









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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

References

   [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
       1982.

   [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
       X3.148-1988.

   [6] ISO, "Fiber Distributed Data Interface (FDDI) - Physical Layer
       Medium Dependent", ISO DIS 9314-3, 1989.  See also ANSI X3.166-
       199x.

   [7] ANSI, "FDDI Station Management", ANSI X3T9.5/84-49 Rev 6.0, 1990.




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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,
       1985.

   [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
       1983.

Security Considerations

   Security issues are not discussed in this memo.

Author's Address

   Dave Katz
   Merit/NSFNET
   1075 Beal Ave.
   Ann Arbor, MI  48109

   Phone: [phone]

   EMail: [e-mail]



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