rfc1042.txt

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Postel & Reynolds                                               [Page 5]

RFC 1042            IP and ARP on IEEE 802 Networks        February 1988


      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.







Postel & Reynolds                                               [Page 6]

RFC 1042            IP and ARP on IEEE 802 Networks        February 1988


   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).








Postel & Reynolds                                               [Page 7]

RFC 1042            IP and ARP on IEEE 802 Networks        February 1988


   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.





Postel & Reynolds                                               [Page 8]

RFC 1042            IP and ARP on IEEE 802 Networks        February 1988


            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



Postel & Reynolds                                               [Page 9]

RFC 1042            IP and ARP on IEEE 802 Networks        February 1988


               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



Postel & Reynolds                                              [Page 10]