path: root/doc/rfc/rfc1042.txt

Network Working Group                                          J. Postel
Request for Comments:  1042                                  J. Reynolds
                                                                     ISI
Obsoletes: RFC-948                                         February 1988



 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, Jacob Rekhter of the
   T.J. Watson Research Center, IBM Corporation, and Joseph Cimmino of
   the University of Maryland.

Introduction

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

   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.

   This memo describes the use of IP and ARP on the three types of
   networks.  At this time, 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.  This may change in the future as new
   IEEE 802 standards are defined and the existing standards are revised



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   allowing for interoperability at the Data Link Layer.

   It is the goal of this memo to specify enough about the use of IP and
   ARP on each type of network to ensure 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].  The use of IEEE 802.1
   compatible Transparent Bridges to allow interoperability across
   different networks is not fully described pending completion of that
   standard.

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 IEEE 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]).

   Normally, all communication is performed using 802.2 type 1
   communication.  Consenting systems on the same IEEE 802 network may
   use 802.2 type 2 communication after verifying that it is supported
   by both nodes.  This is accomplished using the 802.2 XID mechanism.
   However, type 1 communication is the recommended method at this time
   and must be supported by all implementations.  The rest of this
   specification assumes the use of type 1 communication.

   The IEEE 802 networks may have 16-bit or 48-bit physical addresses.
   This specification allows the use of either size of address within a
   given IEEE 802 network.

   Note that the 802.3 standard specifies a transmission rate of from 1



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   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 for 802.3, 10 megabit/second for 802.4, and 4
   megabit/second for 802.5.  However, this specification for the
   transmission of IP Datagrams does not depend on the transmission
   rate.

Header Format
                                                                  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 an nice boundary.

   The K1 value is 170 (decimal).

   The K2 value is 0 (zero).

   The control value is 3 (Unnumbered Information).

Address Mappings

   The mapping of 32-bit Internet addresses to 16-bit or 48-bit IEEE 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 IEEE 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:



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         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 the IEEE 802 networks (of all
      kinds) is 6 (see [7] page 16).

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

      The hardware address length is 2 for 16-bit IEEE 802 addresses, or
      6 for 48-bit IEEE 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 IEEE
   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 IEEE 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 IEEE 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
   IEEE 802 networks.  In the following there are comments on the MTU
   for each type of IEEE 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.




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

      IEEE 802 packets may have a minimum size restriction.  When
      necessary, the data field should be padded (with octets of zero)
      to meet the IEEE 802 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 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.

      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].



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

      IEEE 802.2 Details

         While not necessary for supporting IP and ARP, all
         implementations are required to support IEEE 802.2 standard
         Class I service.  This requires supporting Unnumbered
         Information (UI) Commands, eXchange IDentification (XID)
         Commands and Responses, and TEST link (TEST) Commands and
         Responses.

         When either an XID or a TEST command is received a response
         must be returned; with the Destination and Source addresses,
         and the DSAP and SSAP swapped.

         When responding to an XID or a TEST command the sense of the
         poll/final bit must be preserved.  That is, a command 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
         175 (decimal) if poll is off or 191 (decimal) if poll is on.
         (See Appendix on Numbers.)

         The TEST command or response has an LLC control field value of
         227 (decimal) if poll is off or 243 (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.

         XID response frames should include an 802.2 XID Information
         field of 129.1.0 indicating Class I (connectionless) service.
         (type 1).

         TEST response frames should echo the information field received
         in the corresponding TEST command frame.







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   For IEEE 802.3

      A particular implementation of an IEEE 802.3 Physical Layer is
      denoted using a three field notation.  The three fields are data
      rate in megabit/second, medium type, and maximum segment length in
      hundreds of meters.  One combination of of 802.3 parameters is
      10BASE5 which specifies a 10 megabit/second transmission rate,
      baseband medium, and 500 meter segments.  This correspondes to the
      specifications of the familiar "Ethernet" network.

      The MAC header contains 6 (2) octets of source address, 6 (2)
      octets of destination address, and 2 octets of length.  The MAC
      trailer contains 4 octets of Frame Check Sequence (FCS), for a
      total of 18 (10) octets.

      IEEE 802.3 networks have a minimum packet size that depends on the
      transmission rate.  For type 10BASE5 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 type 10BASE5 802.3 networks the
      maximum packet size is 1518 octets including all octets between
      the destination address and the FCS inclusive.

      This allows 1518 - 18 (MAC header+trailer) - 8 (LLC+SNAP header) =
      1492 for the IP datagram (including the IP header).  Note that
      1492 is not equal to 1500 which is the MTU for Ethernet networks.

   For IEEE 802.4

      The MAC header contains 1 octet of frame control, 6 (2) octets of
      source address, and 6 (2) octets of destination address.  The MAC
      trailer contains 4 octets of Frame Check Sequence (FCS), for a
      total of 17 (9) octets.

      IEEE 802.4 networks have no minimum packet size.

      IEEE 802.4 networks have a maximum packet size of 8191 octets
      including all octets between the frame control and the FCS
      inclusive.

      This allows 8191 - 17 (MAC header+trailer) - 8 (LLC+SNAP header) =
      8166 for the IP datagram (including the IP header).








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   For IEEE 802.5

      The current standard for token ring's, IEEE 802.5-1985, specifies
      the operation of single ring networks.  However, most
      implementations of 802.5 have added extensions for multi-ring
      networks using source-routing of packets at the MAC layer.  There
      is now a Draft Addendum to IEEE 802.5, "Enhancement for Multi-Ring
      Networks" which attempts to standardize these extensions.
      Unfortunately, the most recent draft (November 10, 1987) is still
      rapidly evolving.  More importantly, it differs significantly from
      the existing implementations.  Therefore, the existing
      implementations of 802.5 [13] are described but no attempt is made
      to specify any future standard.

      The MAC header contains 1 octet of access control, 1 octet of
      frame control, 6 (2) octets of source address, 6 (2) octets of
      destination address, and (for multi-ring networks) 0 to 18 octets
      of Routing Information Field (RIF).  The MAC trailer contains 4
      octets of FCS, for a total of 18 (10) to 36 (28) octets.  There is
      one additional octet of frame status after the FCS.

      Multi-Ring Extension Details

         The presence of a Routing Information Field is indicated by the
         Most Significant Bit (MSB) of the source address, called the
         Routing Information Indicator (RII).  If the RII equals zero, a
         RIF is not present.  If the RII equals 1, the RIF is present.
         Although the RII is indicated in the source address, it is not
         part of a stations MAC layer address.  In particular, the MSB
         of a destination address is the individual/group address
         indicator, and if set will cause such frames to be interpreted
         as multicasts.  Implementations should be careful to reset the
         RII to zero before passing source addresses to other protocol
         layers which may be confused by their presence.

         The RIF consists of a two-octet Routing Control (RC) field
         followed by 0 to 8 two-octet Route-Designator (RD) fields.  The
         RC for all-routes broadcast frames is formatted as follows:

                         0                   1
                         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        |  B  |   LTH   |D|  LF |   r   |
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Note that each tick mark represents one bit position.





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            B - Broadcast Indicators: 3 bits

               The Broadcast Indicators are used to indicate the routing
               desired for a particular frame.  A frame may be routed
               through a single specified route, through every distinct
               non-repeating route in a multi-ring network, or through a
               single route determined by a spanning tree algorithm such
               that the frame appears on every ring exactly once.  The
               values which may be used at this time are (in binary):

                  000 - Non-broadcast (specific route)
                  100 - All-routes broadcast (global broadcast)
                  110 - Single-route broadcast (limited broadcast)

               All other values are reserved for future use.

            LTH - Length: 5 bits

               The Length bits are used to indicate the length or the RI
               field, including the RC and RD fields.  Only even values
               between 2 and 30 inclusive are allowed.

            D - Direction Bit: 1 bit

               The D bit specifies the order of the RD fields.  If D
               equals 1, the routing-designator fields are specified in
               reverse order.

            LF - Largest Frame: 3 bits

               The LF bits specify the maximum MTU supported by all
               bridges along a specific route.  All multi-ring broadcast
               frames should be transmitted with a value at least as
               large as the supported MTU.  The values used are:

                       LF (binary)   MAC MTU      IP MTU

                           000          552         508
                           001         1064        1020
                           010         2088        2044
                           011         4136        4092
                           100         8232        8188

               All other values are reserved for future use.

               The receiver should compare the LF received with the MTU.
               If the LF is greater than or equal to the MTU then no
               action is taken; however, if the LF is less than the MTU



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               the frame is rejected.

                  There are actually three possible actions if LF < MTU.
                  First is the one required for this specification
                  (reject the frame).  Second is to reduce the MTU for
                  all hosts to equal the LF.  And, third is to keep a
                  separate MTU per communicating host based on the
                  received LFs.

            r - reserved: 4 bits

               These bits are reserved for future use and must be set to
               0 by the transmitter and ignored by the receiver.

         It is not necessary for an implementation to interpret
         routing-designators.  Their format is left unspecified.
         Routing-designators should be transmitted exactly as received.

      IEEE 802.5 networks have no minimum packet size.

      IEEE 802.5 networks have a maximum packet size based on the
      maximum time a node may hold the token.  This time depends on many
      factors including the data signalling rate and the number of nodes
      on the ring.  The determination of maximum packet size becomes
      even more complex when multi-ring networks with bridges are
      considered.

      Given a token-holding time of 9 milliseconds and a 4
      megabit/second ring, the maximum packet size possible is 4508
      octets including all octets between the access control and the FCS
      inclusive.

      This allows 4508 - 36 (MAC header+trailer with 18 octet RIF) - 8
      (LLC+SNAP header) = 4464 for the IP datagram (including the IP
      header).

      However, some current implementations are known to limit packets
      to 2046 octets (allowing 2002 octets for IP).  It is recommended
      that all implementations support IP packets of at least 2002
      octets.

      By convention, source routing bridges used in multi-ring 802.5
      networks will not support packets larger than 8232 octets.  With a
      MAC header+trailer of 36 octets and the LLC+SNAP header of 8
      octets, the IP datagram (including IP header) may not exceed 8188
      octets.

      A source routing bridge linking two rings may be configured to



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      limit the size of packets forwarded to 552 octets, with a MAC
      header+trailer 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.  An implementation is not required to support an
      MTU of less than 576 octets, although it is suggest that the MTU
      be a user-configurable parameter to allow for it.

      IEEE 802.5 networks support three different types of broadcasts.
      All-Stations broadcasts are sent with no RIF or with the Broadcast
      Indicators set to 0 and no Routing Designators, and are copied
      once by all stations on the local ring.  All-Routes broadcasts are
      sent with the corresponding Broadcast Indicators and result in
      multiple copies equal to the number of distinct non-repeating
      routes a packet may follow to a particular ring.  Single-Route
      broadcasts result in exactly one copy of a frame being received by
      all stations on the multi-ring network.

      The dynamic address discovery procedure is to broadcast an ARP
      request.  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 an all-stations broadcast, without a Routing
      Information Field (RIF).  If the all-stations (local ring)
      broadcast is not supported or if the all-stations broadcast is
      unsuccessful after some reasonable time has elapsed, then send the
      ARP request as an all-routes or single-route broadcast with an
      empty RIF (no routing designators).  An all-routes broadcast is
      preferable since it yields an amount of fault tolerance.  In an
      environment with multiple redundant bridges, all-routes broadcast
      allows operation in spite of spanning-tree bridge failures.
      However, single-route broadcasts may be used if IP and ARP must
      use the same broadcast method.

      When an ARP request or reply is received, all implementations are
      required to understand frames with no RIF (local ring) and frames
      with an empty RIF (also from the local ring).  If the
      implementation supports multi-ring source routing, then a non-
      empty RIF is stored for future transmissions to the host
      originating the ARP request or reply.  If source routing is not
      supported them all packets with non-empty RIFs should be
      gracefully ignored.  This policy will allow all implementations in
      a single ring environment, to interoperate, whether or not they
      support the multi-ring extensions.

      It is possible that when sending an ARP request via an all-routes
      broadcast that multiple copies of the request will arrive at the
      destination as a result of the request being forwarded by several



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      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 and legal
      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.

      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 single-route broadcasts.  Unlike ARP, all-routes
      broadcasts are not desirable for IP.  Receiving multiple copies of
      IP broadcasts would have undesirable effects on many protocols
      using IP.  As with ARP, when an IP packet is received, all
      implementations are required to understand frames with no RIF and
      frames with an empty RIF.

      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.

      After packet transmission, 802.5 provides frame not copied and
      address not recognized indicators.  Implementations may use these



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      indicators to provide some amount of error detection and
      correction.  If the frame not copied bit is set but the address
      not recognized bit is reset, receiver congestion has occurred.  It
      is suggested, though not required, that hosts should retransmit
      the offending packet a small number of times (4) or until
      congestion no longer occurs.  If the address not recognized bit is
      set, an implementation has 3 options: (1) ignore the error and
      throw the packet away, (2) return an ICMP destination unreachable
      message to the source, or (3) delete the ARP entry which was used
      to send this packet and send a new ARP request to the destination
      address.  The latter option is the preferred approach since it
      will allow graceful recovery from first hop bridge and router
      failures and changed hardware addresses.

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.

   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.






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   Number          IEEE    IEEE            Internet        Internet
                   HEX     Binary          Binary          Decimal

   UI Op Code      C0      11000000        00000011          3
   SAP for SNAP    55      01010101        10101010        170
   XID             F5      11110101        10101111        175
   XID             FD      11111101        10111111        191
   TEST            C7      11000111        11100011        227
   TEST            CF      11001111        11110011        243
   Info            818000                                  129.1.0

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 M. Karels, "Trailer Encapsulations", RFC-893,
         University of California at Berkeley, April 1984.

   [10]  Postel, J., "The TCP Maximum Segment Size Option and Related



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         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.

   [13]  IBM, "Token-Ring Network Architecture Reference", Second
         Edition, SC30-3374-01, August 1987.







































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