RFC 3077 (rfc3077) - Page 4 of 25

A Link-Layer Tunneling Mechanism for Unidirectional Links

RFC 3077            LL Tunneling Mechanism for UDLs           March 2001


   f1b (resp. f2b) is the IP address of the 'Feed 1' (resp. Feed 2)
       bidirectional interface connected to the Internet.

   r1u (resp. r2u) is the IP address of the 'Receiver 1' (resp. Receiver
       2) receive-only interface.

   r1b (resp. r2b) is the IP address of the 'Receiver 1' (resp. Receiver
       2) bidirectional interface connected to the Internet.

   Subnet A is a local area network connected to recv1.

   Note that nodes have IP addresses on their unidirectional and their
   bidirectional interfaces.  The addresses on the unidirectional
   interfaces (f1u, f2u, r1u, r2u) will be drawn from the same IP
   network.  In general the addresses on the bidirectional interfaces
   (f1b, f2b, r1b, r2b) will be drawn from different IP networks, and
   the Internet will route between them.

4. Problems related to unidirectional links

   Receive-only interfaces are "dumb" and send-only interfaces are
   "deaf".  Thus a datagram passed to the link-layer driver of a
   receive-only interface is simply discarded.  The link-layer of a
   send-only interface never receives anything.

   The network layer has no knowledge of the underlying transmission
   technology except that it considers its access as bidirectional.
   Basically, for outgoing datagrams, the network layer selects the
   correct first hop on the connected network according to a routing
   table and passes the packet(s) to the appropriate link-layer driver.

   Referring to Figure 1, Recv 1 and Feed 1 belong to the same network.
   However, if Recv 1 initiates a 'ping f1u', it cannot get a response
   from Feed 1.  The network layer of Recv 1 delivers the packet to the
   driver of the receive-only interface, which obviously cannot send it
   to the feed.

   Many protocols in the Internet assume that links are bidirectional.
   In particular, routing protocols used by directly connected routers
   no longer behave properly in the presence of a unidirectional link.

5. Emulating a broadcast bidirectional network

   The simplest solution is to emulate a broadcast capable link-layer
   network.  This will allow the immediate deployment of existing higher
   level protocols without change.  Though other network structures,
   such as NBMA, could also be emulated, a broadcast network is more
   generally useful.  Though a layer 3 network could be emulated, a



Duros, et al.               Standards Track