rfc3077.txt

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Network Working Group                                           E. Duros
Request for Comments: 3077                                        UDcast
Category: Standards Track                                     W. Dabbous
                                                  INRIA Sophia-Antipolis
                                                            H. Izumiyama
                                                                N. Fujii
                                                                    WIDE
                                                                Y. Zhang
                                                                     HRL
                                                              March 2001


       A Link-Layer Tunneling Mechanism for Unidirectional Links

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This document describes a mechanism to emulate full bidirectional
   connectivity between all nodes that are directly connected by a
   unidirectional link.  The "receiver" uses a link-layer tunneling
   mechanism to forward datagrams to "feeds" over a separate
   bidirectional IP (Internet Protocol) network.  As it is implemented
   at the link-layer, protocols in addition to IP may also be supported
   by this mechanism.

1. Introduction

   Internet routing and upper layer protocols assume that links are
   bidirectional, i.e., directly connected hosts can communicate with
   each other over the same link.

   This document describes a link-layer tunneling mechanism that allows
   a set of nodes (feeds and receivers, see Section 2 for terminology)
   which are directly connected by a unidirectional link to send
   datagrams as if they were all connected by a bidirectional link.  We
   present a generic topology in section 3 with a tunneling mechanism




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   that supports multiple feeds and receivers.  Note, this mechanism is
   not designed for topologies where a pair of nodes are connected by 2
   unidirectional links in opposite direction.

   The tunneling mechanism requires that all nodes have an additional
   interface to an IP interconnected infrastructure.

   The tunneling mechanism is implemented at the link-layer of the
   interface of every node connected to the unidirectional link.  The
   aim is to hide from higher layers, i.e., the network layer and above,
   the unidirectional nature of the link.  The tunneling mechanism also
   includes an automatic tunnel configuration protocol that allows nodes
   to come up/down at any time.

   Generic Routing Encapsulation [RFC2784] is suggested as the tunneling
   mechanism as it provides a means for carrying IP, ARP datagrams, and
   any other layer-3 protocol between nodes.

   The tunneling mechanism described in this document was discussed and
   agreed upon by the UDLR working group.

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

2. Terminology

   Unidirectional link (UDL): A one way transmission link, e.g., a
      broadcast satellite link.

   Receiver: A router or a host that has receive-only connectivity to a
      UDL.

   Send-only feed: A router that has send-only connectivity to a UDL.

   Receive capable feed: A router that has send-and-receive connectivity
      to a UDL.

   Feed: A send-only or a receive capable feed.

   Node: A receiver or a feed.

   Bidirectional interface: a typical communication interface that can
      send or receive packets, such as an Ethernet card, a modem, etc.







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3. Topology

   Feeds and receivers are connected via a unidirectional link.  Send-
   only feeds can only send data over this unidirectional link, and
   receivers can only receive data from it.  Receive capable feeds have
   both send and receive capabilities.

   This mechanism has been designed to work with any topology with any
   number of receivers and one or more feeds.  However, it is expected
   that the number of feeds will be small.  In particular, the special
   case of a single send-only feed and multiple receivers is among the
   topologies supported.

   A receiver has several interfaces, a receive-only interface and one
   or more additional bidirectional communication interfaces.

   A feed has several interfaces, a send-only or a send-and-receive
   capable interface connected to the unidirectional link and one or
   more additional bidirectional communication interfaces.  A feed MUST
   be a router.

   Tunnels are constructed between the bidirectional interfaces of
   nodes, so these interfaces must be interconnected by an IP
   infrastructure.  In this document we assume that that infrastructure
   is the Internet.

   Figure 1 depicts a generic topology with several feeds and several
   receivers.

                     Unidirectional Link

         ---->---------->------------------->------
          |          |               |           |
          |f1u       |f2u            |r2u        |r1u
      --------   --------        --------    --------   ----------
      |Feed 1|   |Feed 2|        |Recv 2|    |Recv 1|---|subnet A|
      --------   --------        --------    --------   ----------
          |f1b       |f2b            |r2b        |r1b      |
          |          |               |           |         |
         ----------------------------------------------------
         |                     Internet                     |
         ----------------------------------------------------
                     Figure 1: Generic topology

   f1u (resp. f2u) is the IP address of the 'Feed 1' (resp. Feed 2)
       send-only interface.





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   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



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   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.







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6. Link-layer tunneling mechanism

   This link-layer tunneling mechanism operates underneath the network
   layer.  Its aim is to emulate bidirectional link-layer connectivity.
   This is transparent to the network layer: the link appears and
   behaves to the network layer as if it was bidirectional.

   Figure 2 depicts a layered representation of the link-layer tunneling
   mechanism in the case of Scenario 1.

              Send-only Feed                       Receiver

               decapsulation                     encapsulation
        /-----***************----\       /-->---***************--\
        |                        |       |                       |
        |                        |       |                       |
      --|----------------------  |       |  ---------------------|---
      | |    f1b  |  f1u      |  |       |  |    x  r1u | r1b    |  |
      | |         |       ^   |  |   IP  |  |    |      |        v  |
      | ^         |       |   |  v       |  |    |      |        |  |
      | |         |       |   |  |       |  |    v      |        |  |
      |-|---------|-------|---|  |       |  |----|------|--------|--|
      | |         |       |   |  |       ^  |    |      |        |  |
      | |         |       |   |  |   LL  |  |    |      |        |  |
      | |         |       |   |  |       |  |    |      |        |  |
      | |         |       O------/       \<------O      |        |  |
      |-|---------|-----------|             |-----------|--------|--|
      | |         |           |             |           |        |  |
      | |         |           |     PHY     |           |        |  |
      | |         |           |             |           |        v  |
      | |         | |         |             |         | |        |  |
      --|-----------|----------             ----------|----------|---
        | Bidir     | Send-Only             Recv-Only |   Bidir  |
        ^ Interf    | Interf        UDL      Interf   |   Interf |
        |           \------------>------->------------/          |
        \----------------------<------------------------<--------/
                             Bidirectional network

     x : IP layer at the receiver generates a datagram to be forwarded
         on the receive-only interface.
     O : Entry point where the link-layer tunneling mechanism is
         triggered.

     Figure 2: Scenario 1 using the link-layer Tunneling Mechanism







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6.1. Tunneling mechanism on the receiver

   On the receiver, a datagram is delivered to the link-layer of the
   unidirectional interface for transmission (see Figure 2).  It is then
   encapsulated within a MAC header corresponding to the unidirectional
   link.  This packet cannot be sent directly over the link, so it is
   then processed by the tunneling mechanism.

   The packet is encapsulated within an IP header whose destination is