o    SHOULD be able to receive RFC-1042 packets, intermixed
              with RFC-894 packets; and

         o    MAY be able to send packets using RFC-1042 encapsulation.


         An Internet host that implements sending both the RFC-894 and
         the RFC-1042 encapsulations MUST provide a configuration switch
         to select which is sent, and this switch MUST default to RFC-
         894.




 



RFC1122                        LINK LAYER                   October 1989


         Note that the standard IP encapsulation in RFC-1042 does not
         use the protocol id value (K1=6) that IEEE reserved for IP;
         instead, it uses a value (K1=170) that implies an extension
         (the "SNAP") which can be used to hold the Ether-Type field.
         An Internet system MUST NOT send 802 packets using K1=6.

         Address translation from Internet addresses to link-layer
         addresses on Ethernet and IEEE 802 networks MUST be managed by
         the Address Resolution Protocol (ARP).

         The MTU for an Ethernet is 1500 and for 802.3 is 1492.

         DISCUSSION:
              The IEEE 802.3 specification provides for operation over a
              10Mbps Ethernet cable, in which case Ethernet and IEEE
              802.3 frames can be physically intermixed.  A receiver can
              distinguish Ethernet and 802.3 frames by the value of the
              802.3 Length field; this two-octet field coincides in the
              header with the Ether-Type field of an Ethernet frame.  In
              particular, the 802.3 Length field must be less than or
              equal to 1500, while all valid Ether-Type values are
              greater than 1500.

              Another compatibility problem arises with link-layer
              broadcasts.  A broadcast sent with one framing will not be
              seen by hosts that can receive only the other framing.

              The provisions of this section were designed to provide
              direct interoperation between 894-capable and 1042-capable
              systems on the same cable, to the maximum extent possible.
              It is intended to support the present situation where
              894-only systems predominate, while providing an easy
              transition to a possible future in which 1042-capable
              systems become common.

              Note that 894-only systems cannot interoperate directly
              with 1042-only systems.  If the two system types are set
              up as two different logical networks on the same cable,
              they can communicate only through an IP gateway.
              Furthermore, it is not useful or even possible for a
              dual-format host to discover automatically which format to
              send, because of the problem of link-layer broadcasts.

   2.4  LINK/INTERNET LAYER INTERFACE

      The packet receive interface between the IP layer and the link
      layer MUST include a flag to indicate whether the incoming packet
      was addressed to a link-layer broadcast address.




 



RFC1122                        LINK LAYER                   October 1989


      DISCUSSION
           Although the IP layer does not generally know link layer
           addresses (since every different network medium typically has
           a different address format), the broadcast address on a
           broadcast-capable medium is an important special case.  See
           Section 3.2.2, especially the DISCUSSION concerning broadcast
           storms.

      The packet send interface between the IP and link layers MUST
      include the 5-bit TOS field (see Section 3.2.1.6).

      The link layer MUST NOT report a Destination Unreachable error to
      IP solely because there is no ARP cache entry for a destination.

   2.5  LINK LAYER REQUIREMENTS SUMMARY

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