RFC 3077 (rfc3077) - Page 5 of 25

A Link-Layer Tunneling Mechanism for Unidirectional Links

RFC 3077            LL Tunneling Mechanism for UDLs           March 2001


   link-layer network allows the immediate use of any other network
   layer protocols, and most particularly allows the immediate use of
   ARP.

   A link-layer tunneling mechanism which emulates bidirectional
   connectivity in the presence of a unidirectional link will be
   described in the next Section.  We first consider the various
   communication scenarios which characterize a broadcast network in
   order to define what functionalities the link-layer tunneling
   mechanism has to perform in order to emulate a bidirectional
   broadcast link.

   Here we enumerate the scenarios which would be feasible on a
   broadcast network, i.e., if feeds and receivers were connected by a
   bidirectional broadcast link:

   Scenario 1: A receiver can send a packet to a feed (point-to-point
      communication between a receiver and a feed).

   Scenario 2: A receiver can send a broadcast/multicast packet on the
      link to all nodes (point-to-multipoint).

   Scenario 3: A receiver can send a packet to another receiver (point-
      to-point communication between two receivers).

   Scenario 4: A feed can send a packet to a send-only feed (point-to-
      point communication between two feeds).

   Scenario 5: A feed can send a broadcast/multicast packet on the link
      to all nodes (point-to-multipoint).

   Scenario 6: A feed can send a packet to a receiver or a receive
      capable feed (point-to-point).

   These scenarios are possible on a broadcast network.  Scenario 6 is
   already feasible on the unidirectional link.  The link-layer
   tunneling mechanism should therefore provide the functionality to
   support scenarios 1 to 5.

   Note that regular IP forwarding over such an emulated network (i.e.,
   using the emulated network as a transit network) works correctly; the
   next hop address at the receiver will be the unidirectional link
   address of another router (a feed or a receiver) which will then
   relay the packet.







Duros, et al.               Standards Track