





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
   the IP address of a feed bidirectional interface (f1b or f2b).  This
   destination address is also called the tunnel end-point.  The
   mechanism for a receiver to learn these addresses and to choose the
   feed is explained in Section 7.  The type of encapsulation is
   described in Section 8.

   In all cases the packet is encapsulated, but the tunnel end-point (an
   IP address) depends on the encapsulated packet's destination MAC
   address.  If the destination MAC address is:

      1) the MAC address of a feed interface connected to the
         unidirectional link (Scenario 1).  The datagram is
         encapsulated, the destination address of the encapsulating
         datagram is the feed tunnel end-point (f1b referring to Figure
         2).

      2) a MAC broadcast/multicast address (Scenario 2).  The datagram
         is encapsulated, the destination address of the encapsulating
         datagram is the default feed tunnel end-point.  See Section 7.4
         for further details on the default feed.

      3) a MAC address that belongs to the unidirectional network but is
         not a feed address (Scenario 3).  The datagram is encapsulated,
         the destination address of the encapsulating datagram is the
         default feed tunnel end-point.

   The encapsulated datagram is passed to the network layer which
   forwards it according to its destination address.  The destination
   address is a feed bidirectional interface which is reachable via the
   Internet.  In this case, the encapsulated datagram is forwarded via
   the receiver bidirectional interface (r1b).

6.2. Tunneling mechanism on the feed

   A feed processes unidirectional link related packets in two different
   ways:






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   -  packets generated by a local application or packets routed as
      usual by the IP layer may have to be forwarded over the
      unidirectional link (Section 6.2.1)

   -  encapsulated packets received from another receiver or feed need
      tunnel processing (Section 6.2.2).

   A feed cannot directly send a packet to a send-only feed over the
   unidirectional link (Scenario 4).  In order to emulate this type of
   communication, feeds have to tunnel packets to send-only feeds.  A
   feed MUST maintain a list of all other feed tunnel end-points.  This
   list MUST indicate which are send-only feed tunnel end-points.  This
   is configured manually at the feed by the local administrator, as
   described in Section 7.

6.2.1. Forwarding packets over the unidirectional link

   When a datagram is delivered to the link-layer of the unidirectional
   interface of a feed for transmission, its treatment depends on the
   packet's destination MAC address.  If the destination MAC address is:

      1) the MAC address of a receiver or a receive capable feed
         (Scenario 6).  The packet is sent over the unidirectional link.
         This is classical "forwarding".

      2) the MAC address of a send-only feed (Scenario 4).  The packet
         is encapsulated and sent to the send-only feed tunnel end-
         point.  The type of encapsulation is described in Section 8.

      3) a broadcast/multicast destination (Scenario 5).  The packet is
         sent over the unidirectional link.  Concurrently, a copy of
         this packet is encapsulated and sent to every feed of the list
         of send-only feed tunnel end-points.  Thus the
         broadcast/multicast will reach all receivers and all send-only
         feeds.

6.2.2. Receiving encapsulated packets

   Feeds listen for incoming encapsulated datagrams on their tunnel
   end-points.  Encapsulated packets will have been received on a
   bidirectional interface, and traversed their way up the IP stack.
   They will then enter a decapsulation process (See Figure 2).

   Decapsulation reveals the original link-layer packet.  Note that this
   has not been modified in any way by intermediate routers; in
   particular, the original MAC header will be intact.





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   Further actions depend on the destination MAC address of the link-
   layer packet, which can be:

      1) the MAC address of the feed interface connected to the
         unidirectional link, i.e., own MAC address (Scenarios 1 and 4).
         The packet is passed to the link-layer of the interface
         connected to the unidirectional link which can then deliver it
         up to higher layers.  As a result, the datagram is processed as
         if it was coming from the unidirectional link, and being
         delivered locally.  Scenarios 1 and 4 are now feasible, a
         receiver or a feed can send a packet to a feed.

      2) a receiver address (Scenario 3).  The packet is passed to the
         link-layer of the interface connected to the unidirectional
         link.  It is directly sent over the unidirectional link, to the
         indicated receiver.  Note, the packet must not be delivered
         locally.  Scenario 3 is now feasible, a receiver can send a
         packet to another receiver.

      3) a broadcast/multicast address, this corresponds to Scenarios 2
         and 5.  We have to distinguish two cases, either (i) the
         encapsulated packet was sent from a receiver or (ii) from a
         feed (encapsulated broadcast/multicast packet sent to a send-
         only feed).  These cases are distinguished by examining the
         source address of the encapsulating packet and comparing it
         with the configured list of feed IP addresses.  The action then
         taken is:

         i) the feed was designated as a default feed by a receiver to
            forward the broadcast/multicast packet.  The feed is then in
            charge of sending the multicast packet to all nodes.
            Delivery to all nodes is accomplished by executing all 3 of
            the following actions:

            -  The packet is encapsulated and sent to the list of send-
               only feed tunnel end-points.
            -  Also, the packet is passed to the link-layer of the
               interface which forwards it directly over the
               unidirectional link (all receivers and receive capable
               feeds receive it).
            -  Also, the link-layer delivers it locally to higher
               layers.

            Caution: a receiver which sends an encapsulated
            broadcast/multicast packet to a default feed will receive
            its own packet via the unidirectional link.  Correct
            filtering as described in [RFC1112] must be applied.




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        ii) the feed receives the packet and keeps it for local
            delivery.  The packet is passed to the link-layer of the
            interface connected to the unidirectional link which
            delivers it to higher layers.

         Scenario 2 is now feasible, a receiver can send a
         broadcast/multicast packet over the unidirectional link and it
         will be heard by all nodes.

7. Dynamic Tunnel Configuration Protocol (DTCP)

   Receivers and feeds have to know the feed tunnel end-points in order
   to forward encapsulated datagrams (e.g., Scenarios 1 and 4).

   The number of feeds is expected to be relatively small (Section 3),
   so at every feed the list of all feeds is configured manually.  This
   list should note which are send-only feeds, and which are receive
   capable feeds.  The administrator sets up tunnels to all send-only
   feeds.  A tunnel end-point is an IP address of a bidirectional link
   on a send-only feed.

   For scalability reasons, manual configuration cannot be done at the
   receivers.  Tunnels must be configured and maintained dynamically by
   receivers, both for scalability, and in order to cope with the
   following events:

      1) New feed detection.
         When a new feed comes up, every receiver must create a tunnel
         to enable bidirectional communication with it.

      2) Loss of unidirectional link detection.
         When the unidirectional link is down, receivers must disable
         their tunnels.  The tunneling mechanism emulates bidirectional
         connectivity between nodes.  Therefore, if the unidirectional
         link is down, a feed should not receive datagrams from the
         receivers.  Protocols that consider a link as operational if
         they receive datagrams from it (e.g., the RIP protocol
         [RFC2453]) require this behavior for correct operation.

      3) Loss of feed detection.
         When a feed is down, receivers must disable their corresponding
         tunnel.  This prevents unnecessary datagrams from being
         tunneled which might overload the Internet.  For instance,
         there is no need for receivers to forward a broadcast message
         through a tunnel whose end-point is down.






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   The DTCP protocol provides a means for receivers to dynamically
   discover the presence of feeds and to maintain a list of operational
   tunnel end-points.  Feeds periodically announce their tunnel end-
   point addresses over the unidirectional link.  Receivers listen to
   these announcements and maintain a list of tunnel end-points.

7.1. The HELLO message

   The DTCP protocol is a 'unidirectional protocol', messages are only
   sent from feeds to receivers.

   The packet format is shown in Figure 3.  Fields contain binary
   integers, in normal Internet order with the most significant bit
   first.  Each tick mark represents one bit.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Vers  |  Com  |    Interval   |           Sequence            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | res |F|IP Vers|  Tunnel Type  |   Nb of FBIP  |    reserved   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Feed  BDL IP addr (FBIP1)    (32/128 bits)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             .....                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Feed  BDL IP addr (FBIPn)    (32/128 bits)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 3: Packet Format

   Every datagram contains the following fields, note that constants are
   written in uppercase and are defined in Section 7.5:

   Vers (4 bit unsigned integer): DTCP version number.  MUST be
      DTCP_VERSION.

   Com (4 bit unsigned integer): Command field, possible values are
      1 - JOIN   A message announcing that the feed sending this message
           is up and running.
      2 - LEAVE  A message announcing that the feed sending this message
           is being shut down.

   Interval (8 bit unsigned integer): Interval in seconds between HELLO
      messages for the IP protocol in "IP Vers".  Must be > 0.  The
      recommended value is HELLO_INTERVAL.  If this value is increased,
      the feed MUST continue to send HELLO messages at the old rate for
      at least the old HELLO_LEAVE period.



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   Sequence (16 bit unsigned integer): Random value initialized at boot
      time and incremented by 1 every time a value of the HELLO message
      is modified.

   res (3 bits): Reserved/unused field, MUST be zero.

   F (1 bit): bit indicating the type of feed:
      0 = Send-only feed
      1 = Receive-capable feed

   IP Vers (4 bit unsigned integer): IP protocol version of the feed
      bidirectional IP addresses (FBIP):
      4 = IP version 4
      6 = IP version 6

   Tunnel Type (8 bit unsigned integer): tunneling protocol supported by
      the feed.  This value is the IP protocol number defined in
      [RFC1700] [iana/protocol-numbers] and their legitimate
      descendents.  Receivers MUST use this form of tunnel encapsulation
      when tunneling to the feed.
      47 = GRE [RFC2784] (recommended)
      Other protocol types allowing link-layer encapsulation are
      permitted.  Obtaining new values is documented in [RFC2780].

   Nb of FBIP (8 bit unsigned integer): Number of bidirectional IP feed
      addresses which are enumerated in the HELLO message

   reserved (8 bits): Reserved/unused field, MUST be zero.

   Feed BDL IP addr (32 or 128 bits).  The bidirectional IP address feed
      is the IP address of a feed bidirectional interface (tunnel end-
      point) reachable via the Internet.  A feed has 'Nb of FBIP' IP
      addresses which are operational tunnel end-points.  They are
      enumerated in preferred order.  FBIP1 being the most suitable
      tunnel end-point.

7.2. DTCP on the feed: sending HELLO packets

   The DTCP protocol runs on top of UDP.  Packets are sent to the "DTCP
   announcement" multicast address over the unidirectional link on port
   HELLO_PORT with a TTL of 1.  Due to existing deployments a feed
   SHOULD also support the use of the old DTCP announcement address, as
   described in Appendix B.

   The source address of the HELLO packet is set to the IP address of
   the feed interface connected to the unidirectional link.  In the rest
   of the document, this value is called FUIP (Feed Unidirectional IP
   address).



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   The process in charge of sending HELLO packets fills every field of
   the datagram according to the description given in Section 7.1.

   As long as a feed is up and running, it periodically announces its
   presence to receivers.  It MUST send HELLO packets containing a JOIN
   command every HELLO_INTERVAL over the unidirectional link.

   Referring to Figure 1 in Section 3, Feed 1 (resp. Feed 2) sends HELLO
   messages with the FBIP1 field set to f1b (resp. f2b).

   When a feed is about to be shut down, or when routing over the
   unidirectional link is about to be intentionally interrupted, it is
   recommended that feeds:

      1) stop sending HELLO messages containing a JOIN command.

      2) send a HELLO message containing a LEAVE command to inform
         receivers that the feed is no longer performing routing over
         the unidirectional link.

7.3. DTCP on the receiver: receiving HELLO packets

   Based on the reception of HELLO messages, receivers discover the
   presence of feeds, maintain a list of active feeds, and keep track of
   the tunnel end-points for those feeds.

   For each active feed, and each IP protocol supported, at least the
   following information will be kept:

      FUIP              - feed unidirectional link IP address
      FUMAC             - MAC address corresponding to the above IP
                          address
      (FBIP1,...,FBIPn) - list of tunnel end-points
      tunnel type       - tunnel type supported by this feed
      Sequence          - "Sequence" value from the last HELLO received
                          from this feed
      timer             - used to timeout this entry

   The FUMAC value for an active feed is needed for the operation of
   this protocol.  However, the method of discovery of this value is not
   specified here.

   Initially, the list of active feeds is empty.

   When a receiver is started, it MUST run a process which joins the
   "DTCP announcement" multicast group and listens to incoming packets
   on the HELLO_PORT port from the unidirectional link.




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   Upon the reception of a HELLO message, the process checks the version
   number of the protocol.  If it is different from HELLO_VERSION, the
   packet is discarded and the process waits for the next incoming
   packet.

   After successfully checking the version number further action depends
   on the type of command:

   -  JOIN:

      The process verifies if the address FUIP already belongs to the
      list of active feeds.

      If it does not, a new entry, for feed FUIP, is created and added
      to the list of active feeds.  The number of feed bidirectional IP
      addresses to read is deduced from the 'Nb of FBID' field.  These
      tunnel end-points (FBIP1,...,FBIPn) can then be added to the new
      entry.  The tunnel Type and Sequence values are also taken from
      the HELLO packet and recorded in the new entry.  A timer set to
      HELLO_LEAVE is associated with this entry.

      If it does, the sequence number is compared to the sequence number
      contained in the previous HELLO packet sent by this feed.  If they
      are equal, the timer associated with this entry is reset to
      HELLO_LEAVE.  Otherwise all the information corresponding to FUIP
      is set to the values from the HELLO packet.

      Referring to Figure 1 in Section 3, both receivers (recv 1 and
      recv 2) have a list of active feeds containing two entries: Feed 1
      with a FUIP of f1u and a list of tunnel end-points (f1b); and Feed
      2 with a FUIP of f2u and a list of tunnel end-points (f2b).

   -  LEAVE:

      The process checks if there is an entry for FUIP in the list of
      active feeds.  If there is, the timer is disabled and the entry is
      deleted from the list.  The LEAVE message provides a means of
      quickly updating the list of active feeds.

   A timeout occurs for either of two reasons:

      1) a feed went down without sending a LEAVE message.  As JOIN
         messages are no longer sent from this feed, a timeout occurs at
         HELLO_LEAVE after the last JOIN message.

      2) the unidirectional link is down.  Thus no more JOIN messages
         are received from any of the feeds, and they will each timeout
         independently.  The timeout of each entry depends on its



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         individual HELLO_LEAVE value, and when the last JOIN message
         was sent by that feed, before the unidirectional link went
         down.

   In either case, bidirectional connectivity can no longer be ensured
   between the receiver and the feed (FUIP): either the feed is no
   longer routing datagrams over the unidirectional link, or the link is
   down.  Thus the associated entry is removed from the list of active
   feeds, whatever the cause.  As a result, the list only contains
   operational tunnel end-points.

   The HELLO protocol provides receivers with a list of feeds, and a
   list of usable tunnel end-points (FBIP1,..., FBIPn) for each feed.
   In the following Section, we describe how to integrate the HELLO
   protocol into the tunneling mechanism described in Sections 6.1 and
   6.2.

7.4. Tunneling mechanism using the list of active feeds

   This Section explains how the tunneling mechanism uses the list of
   active feeds to handle datagrams which are to be tunneled.  Referring
   to Section 6.1, it shows how feed tunnel end-points are selected.

   The choice of the default feed is made independently at each
   receiver.  The choice is a matter of local policy, and this policy is
   out of scope for this document.  However, as an example, the default
   feed may be the feed that has the lowest round trip time to the
   receiver.

   When a receiver sends a packet to a feed, it must choose a tunnel
   end-point from within the FBIP list.  The 'preferred FBIP' is
   generally FBIP1 (Section 7.1).  For various reasons, a receiver may
   decide to use a different FBIP, say FBIPi instead of FBIP1, as the
   tunnel end-point.  For example, the receiver may have better
   connectivity to FBIPi.  This decision is taken by the receiver
   administrator.

   Here we show how the list of active feeds is involved when a receiver
   tunnels a link-layer packet.  Section 6.1 listed the following cases,
   depending on whether the MAC destination address of the packet is:

      1) the MAC address of a feed interface connected to the
         unidirectional link: This is TRUE if the address matches a
         FUMAC address in the list of active feeds.  The packet is
         tunneled to the preferred FBIP of the matching feed.

      2) the broadcast address of the unidirectional link or a multicast
         address:



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         This is determined by the MAC address format rules, and the
         list of active feeds is not involved.  The packet is tunneled
         to the preferred FBIP of the default feed.

      3) an address that belongs to the unidirectional network but is
         not a feed address:
         This is TRUE if the address is neither broadcast nor multicast,
         nor found in the list of active feeds.  The packet is tunneled
         to the preferred FBIP of the default feed.

   In all cases, the encapsulation type depends on the tunnel type
   required by the feed which is selected.

7.5. Constant definitions

   DTCP_VERSION is 1.

   HELLO_INTERVAL is 5 seconds.

   "DTCP announcement" multicast group is 224.0.0.36, assigned by IANA.

   HELLO_PORT is 652.  It is a reserved system port assigned by IANA, no
      other traffic must be allowed.

   HELLO_LEAVE is 3*Interval, as advertised in a HELLO packet, i.e., 15
      seconds if the default HELLO_INTERVAL was advertised.

8. Tunnel encapsulation format

   The tunneling mechanism operates at the link-layer and emulates
   bidirectional connectivity amongst receivers and feeds.  We assume
   that hardware connected to the unidirectional link supports broadcast
   and unicast MAC addressing.  That is, a feed can send a packet to a
   particular receiver using a unicast MAC destination address or to a
   set of receivers using a broadcast/multicast destination address.
   The hardware (or the driver) of the receiver can then filter the
   incoming packets sent over the unidirectional links without any
   assumption about the encapsulated data type.

   In a similar way, a receiver should be capable of sending unicast and
   broadcast MAC packets via its tunnels.  Link-layer packets are
   encapsulated.  As a result, after decapsulating an incoming packet,
   the feed can perform link-layer filtering as if the data came
   directly from the unidirectional link (See Figure 2).

   Generic Routing Encapsulation (GRE) [RFC2784] suits our requirements
   because it specifies a protocol for encapsulating arbitrary packets,
   and allows use of IP as the delivery protocol.



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   The feed's local administrator decides what encapsulation it will
   demand that receivers use, and sets the tunnel type field in the
   HELLO message appropriately.  The value 47 (decimal) indicates GRE.
   Other values can be used, but their interpretation must be agreed
   upon between feeds and receivers.  Such usage is not defined here.

8.1. Generic Routing Encapsulation on the receiver

   A GRE packet is composed of a header in which a type field specifies
   the encapsulated protocol (ARP, IP, IPX, etc.).  See [RFC2784] for
   details about the encapsulation.  In our case, only support for the
   MAC addressing scheme of the unidirectional link MUST be implemented.

   A packet tunneled with a GRE encapsulation has the following format:
   the delivery header is an IP header whose destination is the tunnel
   end-point (FBIP), followed by a GRE header specifying the link-layer
   type of the unidirectional link.  Figure 4 presents the entire
   encapsulated packet.

            ----------------------------------------
            |           IP delivery header         |
            |        destination addr = FBIP       |
            |          IP proto = GRE (47)         |
            ----------------------------------------
            |             GRE Header               |
            |      type = MAC type of the UDL      |
            ----------------------------------------
            |            Payload packet            |
            |             MAC packet               |
            ----------------------------------------

                  Figure 4: Encapsulated packet

9. Issues

9.1. Hardware address resolution

   Regardless of whether the link is unidirectional or bidirectional, if
   a feed sends a packet over a non-point-to-point type network, it
   requires the data link address of the destination.  ARP [RFC826] is
   used on Ethernet networks for this purpose.

   The link-layer mechanism emulates a bidirectional network in the
   presence of an unidirectional link.  However, there are asymmetric
   delays between every (feed, receiver) pair.  The backchannel between
   a receiver and a feed has varying delays because packets go through
   the Internet.  Furthermore, a typical example of a unidirectional
   link is a GEO satellite link whose delay is about 250 milliseconds.



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   Because of long round trip delays, reactive address resolution
   methods such as ARP [RFC826] may not work well.  For example, a feed
   may have to forward packets at high data rates to a receiver whose
   hardware address is unknown.  The stream of packets is passed to the
   link-layer driver of the feed send-only interface.  When the first
   packet arrives, the link-layer realizes it does not have the
   corresponding hardware address of the next hop, and sends an ARP
   request.  While the link-layer is waiting for the response (at least
   250 ms for the GEO satellite case), IP packets are buffered by the
   feed.  If it runs out of space before the ARP response arrives, IP
   packets will be dropped.

   This problem of address resolution protocols is not addressed in this
   document.  An ad-hoc solution is possible when the MAC address is
   configurable, which is possible in some satellite receiver cards.  A
   simple transformation (maybe null) of the IP address can then be used
   as the MAC address.  In this case, senders do not need to "resolve"
   an IP address to a MAC address, they just need to perform the simple
   transformation.

9.2. Routing protocols

   The link-layer tunneling mechanism hides from the network and higher
   layers the fact that feeds and receivers are connected by a
   unidirectional link.  Communication is bidirectional, but asymmetric
   in bandwidths and delays.

   In order to incorporate unidirectional links in the Internet, feeds
   and receivers might have to run routing protocols in some topologies.
   These protocols will work fine because the tunneling mechanism
   results in bidirectional connectivity between all feeds and
   receivers.  Thus routing messages can be exchanged as on any
   bidirectional network.

   The tunneling mechanism allows any IP traffic, not just routing
   protocol messages, to be forwarded between receivers and feeds.
   Receivers can route datagrams on the Internet using the most suitable
   feed or receiver as a next hop.  Administrators may want to set the
   metrics used by their routing protocols in order to reflect in
   routing tables the asymmetric characteristics of the link, and thus
   direct traffic over appropriate paths.

   Feeds and receivers may implement multicast routing and therefore
   dynamic multicast routing can be performed over the unidirectional
   link.  However issues related to multicast routing (e.g., protocol
   configuration) are not addressed in this document.





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9.3. Scalability

   The DTCP protocol does not generate a lot of traffic whatever the
   number of nodes.  The problem with a large number of nodes is not
   related to this protocol but to more general issues such as the
   maximum number of nodes which can be connected to any link.  This is
   out of scope of this document.

10. IANA Considerations

   IANA has reserved the address 224.0.0.36 for the "DTCP announcement"
   multicast address as defined in Section 7.

   IANA has reserved the udp port 652 for the HELLO_PORT as defined in
   Section 7.

11. Security Considerations

   Many unidirectional link technologies are characterised by the ease
   with which the link contents can be received.  If sensitive or
   valuable information is being sent, then link-layer security
   mechanisms are an appropriate measure.  For the UDLR protocol itself,
   the feed tunnel end-point addresses, sent in HELLO messages, may be
   considered sensitive.  In such cases link-layer security mechanisms
   may be used.

   Security in a network using the link-layer tunneling mechanism should
   be relatively similar to security in a normal IPv4 network.  However,
   as the link-layer tunneling mechanism requires the use of tunnels, it
   introduces a potential for unauthorised access to the service.  In
   particular, ARP and IP spoofing are potential threats because nodes
   may not be authorised to tunnel packets.  This can be countered by
   authenticating all tunnels.  The authenticating mechanism is not
   specified in this document, it can take place either in the delivery
   IP protocol (e.g., AH[RFC2402]) or in an authentication protocol
   integrated with the tunneling mechanism.

   At a higher level, receivers may not be authorised to provide routing
   information even though they are connected to the unidirectional
   link.  In order to prevent unauthorised receivers from providing fake
   routing information, routing protocols running on top of the link-
   layer tunneling mechanism MUST use authentication mechanisms when
   available.








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12. Acknowledgments

   We would like to thank Tim Gleeson (Cisco Japan) for his valuable
   editing and technical input during the finalization phase of the
   document.

   We would like to thank Patrick Cipiere (UDcast) for his valuable
   input concerning the design of the encapsulation mechanism.

   We would like also to thank for their participation: Akihiro Tosaka
   (IMD), Akira Kato (Tokyo Univ.), Hitoshi Asaeda (IBM/ITS), Hiromi
   Komatsu (JSAT), Hiroyuki Kusumoto (Keio Univ.), Kazuhiro Hara (Sony),
   Kenji Fujisawa (Sony), Mikiyo Nishida (Keio Univ.), Noritoshi Demizu
   (Sony CSL), Jun Murai (Keio Univ.), Jun Takei (JSAT) and Harri
   Hakulinen (Nokia).




































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Appendix A: Conformance and interoperability

   This document describes a mechanism to emulate bidirectional
   connectivity between nodes that are directly connected by a
   unidirectional link.  Applicability over a variety of equipment and
   environments is ensured by allowing a choice of several key system
   parameters.

   Thus in order to ensure interoperability of equipment it is not
   enough to simply claim conformance with the mechanism defined here.
   A usage profile for a particular environment will require the
   definition of several parameters:

      - the MAC format used
      - the tunneling mechanism to be used (GRE is recommended)
      - the "tunnel type" indication if GRE is not used

   For example, a system might claim to implement "the link-layer
   tunneling mechanism for unidirectional links, using IEEE 802 LLC, and
   GRE encapsulation for the tunnels."

Appendix B: DTCP announcement address transition plan

   Some older receivers listen for DTCP announcements on the 224.0.1.124
   multicast address (the "old DTCP announcement" address).  In order to
   support such legacy receivers, feeds SHOULD be configurable to send
   all announcements simultaneously to both the "DTCP announcement"
   address, and the "old DTCP announcement" address.  The default
   setting is to send announcements to just the "DTCP announcement"
   address.

   In order to encourage the transition plan, the "old" feeds MUST be
   updated to send DTCP announcements as defined in this section.  The
   number of "old" feeds originally deployed is relatively small and
   therefore the update should be fairly easy.  "New" receivers only
   support "new" feeds, i.e., they listen to DTCP announcements on the
   "DTCP announcement" address.














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References

   [RFC826]  Plummer, D., "An Ethernet Address Resolution Protocol", STD
             37, RFC 826, November 1982.

   [RFC1112] Deering, S., "Host Extensions for IP Multicasting", STD 5,
             RFC 1112, August 1989

   [RFC1700] Reynolds, J. and J. Postel, "ASSIGNED NUMBERS", STD 2, RFC
             1700, October 1994.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2402] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
             2402, November 1998.

   [RFC2453] Malkin, G., "RIP Version 2", STD 56, RFC 2453, November
             1998.

   [RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
             Values In the Internet Protocol and Related Headers", BCP
             37, RFC 2780, March 2000.

   [RFC2784] Farinacci, D., Hanks, S., Meyer, D. and P. Traina, "Generic
             Routing Encapsulation (GRE)", RFC 2784, March 2000.

























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Authors' Addresses

   Emmanuel Duros
   UDcast
   1681, route des Dolines
   Les Taissounieres - BP 355
   06906 Sophia-Antipolis Cedex
   France

   Phone : +33 4 93 00 16 60
   Fax   : +33 4 93 00 16 61
   EMail : Emmanuel.Duros@UDcast.com


   Walid Dabbous
   INRIA Sophia Antipolis
   2004, Route des Lucioles BP 93
   06902 Sophia Antipolis
   France

   Phone : +33 4 92 38 77 18
   Fax   : +33 4 92 38 79 78
   EMail : Walid.Dabbous@inria.fr


   Hidetaka Izumiyama
   JSAT Corporation
   Toranomon 17 Mori Bldg.5F
   1-26-5 Toranomon, Minato-ku
   Tokyo 105
   Japan

   Phone : +81-3-5511-7568
   Fax   : +81-3-5512-7181
   EMail : izu@jsat.net


   Noboru Fujii
   Sony Corporation
   2-10-14 Osaki, Shinagawa-ku
   Tokyo 141
   Japan

   Phone : +81-3-3495-3092
   Fax   : +81-3-3495-3527
   EMail : fujii@dct.sony.co.jp





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   Yongguang Zhang
   HRL
   RL-96, 3011 Malibu Canyon Road
   Malibu, CA 90265,
   USA

   Phone : 310-317-5147
   Fax   : 310-317-5695
   EMail : ygz@hrl.com










































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Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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