ftp.cse.unsw.edu.au/2014.06.ftp.cse.unsw.edu.au.tar/ftp.cse.unsw.edu.au/pub/doc/standards/rfc/rfc0948.Z/rfc0948

Text File | 1992-10-18 | 23.5 KB | 613 lines

Network Working Group                                          J. Postel
Request for Comments:  DRAFT                                 J. Reynolds
                                                                     ISI
Obsoletes: RFC-948                                             mmmm 1987



 A Standard for the Transmission of IP Datagrams over IEEE 802 Networks


Status of this Memo

   This RFC specifies a standard method of encapsulating the Internet
   Protocol (IP) [1] datagrams and Address Resolution Protocol (ARP) [2]
   requests and replies on IEEE 802 Networks.  This RFC specifies a
   protocol standard for the Internet community.  Distribution of this
   memo is unlimited.

Acknowledgment

   This memo would not exist with out the very significant contributions
   of Drew Perkins of Carnegie Mellon University and Jacob Rekhter of
   the T.J. Watson Research Center, IBM Corporation.

Introduction

   The goal of this specification is to have implementations for
   transmitting IP datagrams and ARP request and replies be compatible
   and interwork.  To achieve this it may be necessary in a few cases to
   limit the use that IP datagrams make of the capabilities of a
   particular IEEE 802 network.

   This memo describes the use of IP and ARP on three types of networks.
   It is not necessary that the use of IP and ARP be consistent across
   all three types of networks, only that it be consistent within each
   type.

   The IEEE 802 specifications define a family of standards for Local
   Area Networks (LANs) that deal with the Physical and Data Link Layers
   as defined by the ISO Open System Interconnection Reference Model
   (ISO/OSI).  Several Physical Layer standards (802.3, 802.4, and
   802.5) [3,4,5] and one Data Link Layer Standard (802.2) [6] have been
   defined.  The IEEE Physical Layer standards specify the ISO/OSI
   Physical Layer and the Media Access Control Sublayer of the ISO/OSI
   Data Link Layer.  The 802.2 Data Link Layer standard specifies the
   Logical Link Control Sublayer of the ISO/OSI Data Link Layer.

   All communication is performed using 802.2 type 1 communication.  The



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   802.2 type 2 communication is not used.

   The 802.x networks may have 16-bit or 48-bit physical addresses.

   It is the goal of this memo to specify enough about the use of IP and
   ARP on each type of network such that:

      (1) all equipment using IP or ARP on 802.3 networks will
      interoperate,

      (2) all equipment using IP or ARP on 802.4 networks will
      interoperate,

      (3) all equipment using IP or ARP on 802.5 networks will
      interoperate.

   Of course, the goal of IP is interoperability between computers
   attached to different networks, when those networks are
   interconnected via an IP gateway [8].

Description

   IEEE 802 networks may be used as IP networks of any class (A, B, or
   C).  These systems use two Link Service Access Point (LSAP) fields of
   the LLC header in much the same way the ARPANET uses the "link"
   field.  Further, there is an extension of the LLC header called the
   Sub-Network Access Protocol (SNAP).

   IP datagrams are sent on 802 networks encapsulated within the 802.2
   LLC and SNAP data link layers, and the 802.3, 802.4, or 802.5
   physical networks layers.  The SNAP is used with an Organization Code
   indicating that the following 16 bits specify the EtherType code (as
   listed in Assigned Numbers [7]).

   Note that the 802.3 standard specifies a transmission rate of from 1
   to 20 megabit/second, the 802.4 standard specifies 1, 5, and 10
   megabit/second, and the 802.5 standard specifies 1 and 4
   megabit/second.  The typical transmission rates used are 10
   megabit/second (802.3) or 4 megabit/second (802.5).  However, this
   specification for the transmission of IP Datagrams does not depend on
   the transmission rate.










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                                                                  Header

   ...--------+--------+--------+
              MAC Header        |                        802.{3/4/5} 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-octets,
   making the 802.2 protocol overhead come out on a nice boundary.

   The K1 value is 170 (decimal).

   The K2 value is 0 (zero).

   The control value is 3 (for Unnumbered Information).

Address Mappings

   The mapping of 32-bit Internet addresses to 16-bit or 48-bit 802
   addresses must 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 802 addresses when needed.

   The ARP Details

      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       Bytes in each hardware address
         pln      8 - bits       Bytes in each protocol address
         op      16 - bits       Operation Code

      The hardware type code assigned for the 802 networks (of all
      kinds) is 6 (see [7] page 16).




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      The protocol type code for IP is 2048 (see [7] page 14).

      The hardware address length is 2 (for 16-bit 802 addresses), or 6
      (for 48-bit 802 addresses).

      The protocol address length (for IP) is 4.

      The operation code is 1 for request and 2 for reply.

Broadcast Address

   The broadcast Internet address (the address on that network with a
   host part of all binary ones) should be mapped to the broadcast 802
   address (of all binary ones) (see [8] page 14).

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.  Consenting systems on the same 802 network may use
   this format between themselves.  Details of the trailer encapsulation
   method may be found in [9].  However, all hosts must be able to
   communicate using the standard (non-trailer) method.

Byte Order

   As described in Appendix B of the Internet Protocol specification
   [1], the IP datagram is transmitted over 802 networks as a series of
   8-bit bytes.  This byte transmission order has been called "big-
   endian" [11].

Maximum Transmission Unit

   The Maximum Transmission Unit (MTU) differs on the different types of
   802 networks.  In the following there are comments on the MTU for
   each type of 802 network.  However, on any particular network all
   hosts must use the same MTU.  In the following, the terms "maximum
   packet size" and "maximum transmission unit" are equivalent.

Frame Format and MAC Level Issues

   For all hardware types

      IP datagrams and ARP requests and replies are transmitted in
      standard 802.2 LLC Type 1 Unnumbered Information format, control
      code 3, with the DSAP and the SSAP fields of the 802.2 header set
      to 170, the assigned global SAP value for SNAP [6].  The 24-bit
      Organization Code in the SNAP is zero, and the remaining 16 bits



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      are the EtherType from Assigned Numbers [7] (IP = 2048, ARP =
      2054).

      IEEE 802.x packets may have a minimum size restriction.  When
      necessary, the data field should be padded (with octets of zero)
      to meet the 802.x minimum frame size requirements.  This padding
      is not part of the IP datagram and is not included in the total
      length field of the IP header.

      For compatibility (and common sense) the minimum packet size used
      with IP datagrams is 28 octets, which is 20 (minimum IP header) +
      8 (LLC+SNAP header) = 28 octets (not including the MAC header).

      The minimum packet size used with ARP is 24 octets, which is 20
      (ARP with 2 octet hardware addresses and 4 octet protocol
      addresses) + 8 (LLC+SNAP header) = 24 octets (not including the
      MAC header).

      In typical situations, the packet size used with ARP is 32 octets,
      which is 28 (ARP with 6 octet hardware addresses and 4 octet
      protocol addresses) + 8 (LLC+SNAP header) = 32 octets (not
      including the MAC header).

      IEEE 802.x packets may have a maximum size restriction.
      Implementations are encouraged to support full-length packets.

      For compatibility purposes, the maximum packet size used with IP
      datagrams or ARP requests and replies must be consistent on a
      particular network.  Each type of 802 network has a different
      specification for the maximum packet size.

      Gateway implementations must be prepared to accept full-length
      packets and fragment them when necessary.

      Host implementations should be prepared to accept full-length
      packets, however hosts must not send datagrams longer than 576
      octets unless they have explicit knowledge that the destination is
      prepared to accept them.  A host may communicate its size
      preference in TCP based applications via the TCP Maximum Segment
      Size option [10].

      Datagrams on 802.x networks may be longer than the general
      Internet default maximum packet size of 576 octets.  Hosts
      connected to an 802.x network should keep this in mind when
      sending datagrams to hosts not on the same 802.x network.  It may
      be appropriate to send smaller datagrams to avoid unnecessary
      fragmentation at intermediate gateways.  Please see [10] for
      further information.



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   For 802.3

      IEEE 802.3 networks have a minimum packet size that depends on the
      transmission rate.  For 10 megabit/second 802.3 networks the
      minimum packet size is 64 octets.

      IEEE 802.3 networks have a maximum packet size which depends on
      the transmission rate.  For 10 megabit/second 802.3 networks the
      maximum packet size is 1518 octets.

      The MAC header is 6 octets of source address, 6 octets of
      destination address, 2 octets of length, and (at the end of the
      packet) 4 octets of CRC, for a total of 18 octets.

      Note that 1518 - 18 (MAC header) - 8 (LLC+SNAP header) = 1492 for
      the IP datagram (including the IP header).

      One popular combination of 802.3 parameters is the "Ethernet"
      style in which networks use 48-bit physical addresses and 10
      megabit/second transmission rate.

   For 802.4

      IEEE 802.4 networks have no minimum packet size.

      IEEE 802.4 networks have no maximum packet size.

      The MAC header is 6 octets of source address, 6 octets of
      destination address, 2 octets of length, and (at the end of the
      packet) 4 octets of CRC, for a total of 18 octets.

      For compatibility, the maximum packet size used with IP datagrams
      or ARP requests and replies is 1492 octets for the IP datagram
      (including the IP header) plus 8 octets for the LLC+SNAP header,
      for a total of 1500 octets (not including the MAC header).

      In one combination of 802.4 parameters, 48-bit physical addresses
      and 10 megabit/second transmission rate are used.













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   For 802.5

      IEEE 802.5 networks have no minimum packet size.

      IEEE 802.5 networks have no maximum packet size.

      The MAC header is 6 octets of source address, 6 octets of
      destination address, 2 octets of length, plus another 18 octets of
      what ???, and (at the end of the packet) 4 octets of CRC, for a
      total of 36 octets.

      In one combination of 802.5 parameters, 48-bit physical addresses
      and 4 megabit/second transmission rate are used.

      There is a convention that IBM style 802.5 networks will not use
      packets larger than 8232 octets.  With a MAC header of 36 octets
      and the LLC+SNAP header of 8 octets, the IP datagram (including IP
      header) may not exceed 8188 octets.

      Note that a MAC level bridge linking two rings may limit the size
      of packets forwarded to 552 octets, with a MAC header of 36 octets
      and the LLC+SNAP of 8 octets, the IP datagram (including the IP
      header) may be limited to 508 octets.  This is less that the
      default IP MTU of 576 octets, and may cause significant
      performance problems due to excessive datagram fragmentation.

         One implementation will not support IP datagram communication
         across a MAC level bridge unless the bridge will allow an IP
         MTU of at least 1020 octets.

      The dynamic address discovery procedure is to do a ARP request.
      The IBM style 802.5 networks support two different types of
      broadcasts, local ring broadcasts and all rings broadcasts.  To
      limit the number of all rings broadcasts to a minimum, it is
      desirable (though not required) that an ARP request first be sent
      as a local ring broadcast, without a Routing Information Field
      (RIF).  If the local ring broadcast is not supported or if the
      local ring broadcast is unsuccessful after some reasonable time
      has elapsed, then send the ARP request as an all rings broadcast
      with an empty RIF.

      When an ARP request or reply is received, all implementations are
      required to understand at least local ring broadcasts (no RIF) and
      all ring broadcasts from the same ring (empty RIF).  If the
      implementation supports IBM style source routing, then a non-empty
      RIF is stored for future transmissions to the host originating the
      ARP request or reply.  If this source routing is not supported
      them all packets with non-empty RIFs should be gracefully ignored.



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      It is possible that when sending an ARP request via an all rings
      broadcast that multiple copies of the request will arrive at the
      destination as a result of the request being forwarded by several
      MAC layer bridges.  However, these "copies" will have taken
      different routes so the contents of the RIF will differ.  An
      implementation of ARP in this context must determine which of
      these "copies" to use and to ignore the others.  There are three
      obvious strategies: (1) take the first and ignore the rest (that
      is, once you have an entry in the ARP cache don't change it), (2)
      take the last, (that is, always up date the ARP cache with the
      latest ARP message), or (3) take the one with the shortest path,
      (that is, replace the ARP cache information with the latest ARP
      message data if it is a shorter route).  Since there is no problem
      of incompatibility for interworking of different implementations
      if different strategies are chosen, the choice is up to each
      implementor.  The recipient of the ARP request must send an ARP
      reply as a point to point message using the RIF information.

      Note that a MAC level bridge linking two rings may limit the size
      of packets forwarded to 552 octets, with a MAC header of 36 octets
      and the LLC+SNAP of 8 octets, the ARP request or reply may be
      limited to 508 octets.

      The RIF information should be kept distinct from the ARP table.
      That is, there is, in principle, the ARP table to map from IP
      addresses to 802 48-bit addresses, and the RIF table to map from
      those to 802.5 source routes, if necessary.  In practical
      implementations it may be convenient to store the ARP and RIF
      information together.

         Storing the information together may speed up access to the
         information when it is used.  On the other hand, in a
         generalized implementation for all types of 802 networks a
         significant amount of memory might be wasted in an ARP cache if
         space for the RIF information were always reserved.

      IP broadcasts (datagrams with a IP broadcast address) must be sent
      as 802.5 all ring broadcasts.

      Since current interface hardware allows only one group address,
      and since the functional addresses are not globally unique, IP and
      ARP do not use either of these features.  Further, in the IBM
      style 802.5 networks there are only 31 functional addresses
      available for user definition.

      IP precedence should not be mapped to 802.5 priority.  All IP and
      ARP packets should be sent at the default 802.5 priority.  The
      default priority is 3.



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      An 802.5 address not recognized report should be mapped an ICMP
      destination unreachable message.

      MAC Management Support

         While not necessary for supporting IP and ARP, IEEE 802.5
         devices should be able to respond to EXCHANGE ID (XID) and TEST
         LINK (TEST) frames.

         When either an XID or a TEST frame is received a response must
         be returned.

         When responding to an XID or a TEST frame the sense of the
         poll/final bit must be preserved.  That is, a frame received
         with the poll/final bit reset must have the response returned
         with the poll/final bit reset and vice versa.

         The XID command or response has an LLC control field value of
         245 (decimal) if poll is off or 253 (decimal) if poll is on.
         (See Appendix on Numbers.)

         The TEST command or response has an LLC control field value of
         199 (decimal) if poll is off or 207 (decimal) if poll is on.
         (See Appendix on Numbers.)

         A command frame is identified with high order bit of the SSAP
         address reset.  Response frames have high order bit of the SSAP
         address set to one.

         TEST command frames are merely echoed exactly as received,
         after swapping the Destination Address/Source Address and
         DSAP/SSAP and setting the response bit.

         XID commands frames received should return the 802.2 XID
         Information field in the response as 0.128.129 indicating
         connectionless service (type 1) and swap the addresses and set
         the response bit.

Interoperation with Ethernet

   It is possible to use the Ethernet link level protocol [12] on the
   same physical cable with the IEEE 802.3 link level protocol.  A
   computer interfaced to a physical cable used in this way could
   potentially read both Ethernet and 802.3 packets from the network.
   If a computer does read both types of packets, it must keep track of
   which link protocol was used with each other computer on the network
   and use the proper link protocol when sending packets.




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   One should note that in such an environment, link level broadcast
   packets will not reach all the computers attached to the network, but
   only those using the link level protocol used for the broadcast.

   Since it must be assumed that most computers will read and send using
   only one type of link protocol, it is recommended that if such an
   environment (a network with both link protocols) is necessary, an IP
   gateway be used as if there were two distinct networks.

   Note that the MTU for the Ethernet allows a 1500 octet IP datagram,
   with the MTU for the 802.3 network allows only a 1492 octet IP
   datagram.


Appendix on Numbers

   The IEEE likes to specify numbers in bit transmission order, or bit-
   wise little-endian order.  The Internet protocols are documented in
   byte-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    IEEE            Internet        Internet
                   HEX     Binary          Binary          Decimal

   UI Op Code      0xC0    11000000        00000011          3
   SAP for SNAP    0x55    01010101        10101010        170
   XID             0xAF    10101111        11110101        245
   XID             0xBF    10111111        11111101        253
   TEST            0xE3    11100011        11000111        199
   TEST            0xF3    11110011        11001111        207
   Info            0x810100                                0.128.129



















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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]   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.

   [4]   IEEE, "IEEE Standards for Local Area Networks: Token-Passing
         Bus Access Method and Physical Layer Specification", IEEE, New
         York, New York, 1985.

   [5]   IEEE, "IEEE Standards for Local Area Networks: Token Ring
         Access Method and Physical Layer Specifications", IEEE, New
         York, New York, 1985.

   [6]   IEEE, "IEEE Standards for Local Area Networks: Logical Link
         Control", IEEE, New York, New York, 1985.

   [7]   Reynolds, J.K., and J. Postel, "Assigned Numbers", RFC-1010,
         USC/Information Sciences Institute, May 1987.

   [8]   Braden, R., and J. Postel, "Requirements for Internet
         Gateways", RFC-1009, USC/Information Sciences Institute, June
         1987.

   [9]   Leffler, S., and Karels, M.,  "Trailer Encapsulations", RFC-
         893, University of California at Berkeley, April 1984.

   [10]  Postel, J., "The TCP Maximum Segment Size Option and Relate
         Topics", RFC-879, USC/Information Sciences Institute, November
         1983.

   [11]  Cohen, D., "On Holy Wars and a Plea for Peace", Computer, IEEE,
         October 1981.

   [12]  D-I-X, "The Ethernet - A Local Area Network: Data Link Layer
         and Physical Layer Specifications", Digital, Intel, and Xerox,
         November 1982.





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