Network Working Group                                           E. Duros
Internet-Draft                                                W. Dabbous
June 1999
Feb 2000                                          INRIA Sophia-Antipolis
                                                            H. Izumiyama
                                                                N. Fujii
                                                                    WIDE
                                                                Y. Zhang
                                                                     HRL

       A Link Layer Tunneling Mechanism for Unidirectional Links
                   <draft-ietf-udlr-lltunnel-02.txt>
                   <draft-ietf-udlr-lltunnel-03.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   A version of this draft document is intended for submission to the
   RFC editor as a Proposed Standard for the Internet Community.

Note to the RFC editor

   Please replace all references to rfcXXXX with references to the new
   GRE specification when it is published as Proposed Standard. It
   currently exists as <draft-meyer-gre-update-03.txt>. The entry in the
   "References" section may also need updating at this time.

Abstract

   This document describes a mechanism to emulate bidirectional
   connectivity between 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 network. As it is implemented at the link layer, other
   protocols than in addition to IP may also use be supported by this tunneling 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
   nodes which are directly connected by a unidirectional link (feeds
   and receivers, see section Section 2 for terminology) to send datagrams as if
   they were connected to a bidirectional link. We present a generic
   topology with a tunneling mechanism that supports multiple feeds and
   receivers.

   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 on every feed and
   every receiver. link. The aim
   is to hide from higher layers, i.e. the network layer and above, the
   unidirectional feature nature of the link. The tunneling mechanism also
   includes an automatic tunnel configuration protocol that allows feeds and receivers nodes
   to come up/down at any time.

   The tunneling mechanism proposes to use

   Generic Routing Encapsulation
   [rfc1701] and [rfcXXXX] is suggested as the tunneling
   mechanism as it provides a means for carrying IP, ARP datagrams datagrams, and
   any other layer-3 protocol from receivers to feeds. between nodes.

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

2. Terminology

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

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

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

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

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

   Node: A receiver or a feed.

3. Topology

   In general, feeds

   Feeds and receivers are connected via a unidirectional link. Send-only 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 this document, we
   consider particular, the special
   case of several feeds (send-only and/or receive capable) a single send-only feed and many receivers. 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 receiver
   is required to
   MUST be a router.

   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.

   In

   Tunnels are constructed between the entire bidirectional interfaces of
   nodes, so these interfaces must be interconnected by an IP
   infrastructure. In this document we assume that feeds and receivers are
   connected to that infrastructure
   is the Internet via one of their bidirectional interfaces.
   A receiver and a feed can then communicate with each other using this
   specific bidirectional interface (Ethernet interface, PPP connection,
   etc.). 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.

   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

4. Problems related to recv1.

   Note that nodes have IP addresses on their unidirectional links

   Most protocols in and their
   bidirectional interfaces. The addresses on the Internet assume that links are bidirectional. unidirectional
   interfaces (f1u, f2u, r1u, r2u) will be drawn from the same IP
   network. In particular, routing protocols used by directly connected routers
   no longer behave properly in general the presence of a 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 link.
   Indeed, receivers cannot send requests/responses or routing messages links

   Receive-only interfaces are "dumb" and send-only interfaces are
   "deaf". Thus a datagram passed to feeds through their the link layer driver of a
   receive-only interface. 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 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. Recv 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.

   More generally, a datagram of any protocol that passes from

   Most protocols in the
   network layer to Internet assume that links are bidirectional.
   In particular, routing protocols used by directly connected routers
   no longer behave properly in the driver presence of a receive-only interface is simply
   discarded. unidirectional link.

5. Emulating a broadcast bidirectional network

   Some unidirectional links (e.g., satellite links) allow by nature
   communication from a feed

   The simplest solution is to emulate a set broadcast capable link layer
   network. This will allow the immediate deployment of receivers: a feed can send
   packets to existing higher
   level protocols without change. Though other network structures, such
   as NBMA, could also be emulated, a particular receiver and it can send broadcast packets.
   However, any other communication network is simply impossible: a receiver
   cannot send packets to a receiver or a feed, a feed cannot send more
   generally useful. Though a
   packet to layer 3 network could be emulated, a send-only feed. link
   layer network allows the immediate use of any other network layer
   protocols, and most particularly allows the immediate use of ARP.

   A solution to this problem based on a link layer (LL) tunneling mechanism which emulates a bidirectional
   connectivity in the presence of a unidirectional link will be
   described in the next section. 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 follows we enumerate the scenarios which would be feasible on a
   broadcast network, i.e 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 feed receiver and a receiver). feed).

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

   Scenario 6: A feed can send a packet to a receiver or a receive
     capable feed. This is the default communication over a unidirectional link.

   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 functionalities functionality to permit support
   scenarios 1 to 5 to happen. 5.

   Note that regular IP forwarding over such an emulated network (i.e.
   using the scenarios above allow emulated network as a
   node to send a packet to any destination IP address on transit network) works correctly; the Internet.
   The
   next hop address at the receiver will be in this case the unidirectional link
   address of another router (a feed or a receiver) which will then
   relay the packet.

6. Link layer tunneling mechanism

   This section describes a link layer tunneling mechanism allowing
   bidirectional communication between feeds and receivers in the
   presence of a unidirectional link. This mechanism has been designed
   to work with any topology regardless of the number of feeds and
   receivers. In particular, the special case of a single send-only feed
   and multiple receivers is among the topologies supported.

   This link layer tunneling mechanism operates underneath the network
   layer. Its aim is to emulate a bidirectional link layer connectivity. It
   This is transparent to the network layer: the link appears and
   behaves with 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 LL Tunneling Mechanism

6.1. Tunneling mechanism on the receiver

   A

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

   The packet is encapsulated behind an IP header whose destination is
   the IP address of a feed bidirectional interface (f1b or f2b), 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 6.3. 7. The type of encapsulation is
   described in Section 7.

   The new datagram is passed to the network layer that forwards it
   according to its destination address. The destination address of the
   encapsulated datagram is a feed bidirectional interface which is
   reachable via the Internet. The datagram is then forwarded via the
   receiver bidirectional interface (r1b).

   We have to distinguish several 8.

   In all cases as the datagram packet is to be
   tunneled according to 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 (Scenario 1). The datagram is encapsulated,
        the destination address of the new encapsulating datagram is the
        feed tunnel end-point (f1b referring to Figure 2).

     2) a MAC broadcast/multicast address (Scenario 2): the 2).  The datagram is
        encapsulated, the destination address of the new encapsulating
        datagram is the default feed tunnel end-point. See Section 6.3.4 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 3).  The datagram is encapsulated
        and sent to encapsulated,
        the tunnel end-point destination address of the default feed.

   At this point, encapsulating datagram is the network layer passes a
        default feed tunnel end-point.

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

6.2. Tunneling mechanism on the feed

   A feed processes packets in unidirectional link related packets in two different
   ways: packets must be
   forwarded over the unidirectional link (e.g.
     - packets generated by a local application or a packet packets routed as
        usual by the IP layer, see section layer may have to be forwarded over the
        unidirectional link (Section 6.2.1) and
     - encapsulated packets received from one of the receivers another receiver or
   the feeds that feed need particular
        tunnel processing (section (Section 6.2.2).

   A feed cannot send directly over the unidirectional link send a packet to a send-only feed. feed over the
   unidirectional link (Scenario 4).  In order to emulate this type of
   communication, a 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. In
   fact, administrator, as stated
   described in Section 3, the number of feeds is expected to be
   relatively small, therefore a manual configuration can be envisaged.
   How to use this list is detailed in the next section. 7.

6.2.1. Forwarding packets over the unidirectional link

   The way

   When a packet datagram is forwarded delivered to the link layer of the unidirectional
   interface of a feed for transmission, its treatment depends on its the
   packet's destination MAC address,
   if it 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 the 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 7. 8.

     3) a broadcast/multicast destination (Scenario 5). The packet is
        sent over the unidirectional link with a link layer header
        corresponding to the broadcast/multicast addressing scheme.
        Currently, 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 to for incoming encapsulated datagrams on their tunnel end-
   point. An encapsulated packet which is
   points. Encapsulated packets will have been received from the on a
   bidirectional interface, traverses and traversed their way up the IP stack and enters stack.
   They will then enter a decapsulation process (See Figure 2). Note that

   Decapsulation reveals the original datagram
   was encapsulated and therefore its payload (especially MAC and IP
   header) link layer packet. Note that this
   has not been modified in any way by intermediate routers. It is
   decapsulated and further routers; in
   particular, the original MAC header will be intact.

   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, this corresponds to Scenarios i.e. own MAC address (Scenarios 1 and 4. 4).
        The packet is passed to the link layer of the interface
        connected to the unidirectional link which can then delivers deliver it
        up to the network
        layer. higher layers. As a result, the datagram is processed as
        if it was coming from the unidirectional link. At this point, 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, this corresponds to Scenario 3. 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 with the proper link layer encapsulation link, to the receiver
        (note,
        indicated receiver.  Note, the packet must not be delivered to the network layer).
        locally.  Scenario 5 3 is now feasible, a receiver can send a
        packet to another receiver.

     3) a broadcast/multicast address, this corresponds to Scenario 2. 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):
        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 designed 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. An
           encapsulation process at the feed encapsulates Delivery
           to all nodes is accomplished by executing all 3 of the
           following actions:
           - The packet is encapsulated and
           sends a copy sent to the list of send-only send-
             only feed tunnel end-points.
           The
           - Also, the packet is also passed to the link layer of the
             interface which forwards it directly over the
             unidirectional link (all receivers and receive capable
             feeds receive it). The
           - Also, the link layer also delivers it locally to the network layer. 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.

        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 the network layer. 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.

6.3.

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, (e.g. Scenarios 1 and 4).

   The configuration number of the send-only feeds list is performed manually expected to be relatively small (Section 3),
   so at every feed the feed. 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 this 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) when New feed detection.
        When a new feed comes up, every receiver must create a tunnel to
        enable a bidirectional communication with it. Static tunneling
        configuration is not appropriate, as new feeds can be connected
        to the

     2) Loss of unidirectional link at any time.

     2) when 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. Indeed there are protocols Protocols that consider a link as operational if they
        receive datagrams from it (e.g., (e.g. the RIP protocol [rfc2453]). [rfc2453])
        require this behavior for correct operation.

     3) when Loss of feed detection.
        When a feed is down, receivers must disable their corresponding
        tunnel. This prevents unnecessary datagrams from being tunneled
        which would 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.

   Note that these tunnels have no existence at the IP layer and are not
   considered as tunnel interfaces.

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

6.3.1.

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 2. 3. Fields contain binary
   integers, in normal Internet order with the most significant byte 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 6.3.5: Section 7.5:

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

   Com (4 bits): 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 same layer-3 protocol. IP protocol in "IP Vers". Must be > 0. The
     recommended value is HELLO_INTERVAL. This field If this value is increased,
     the feed MUST not be changed while sending. continue to send HELLO messages at the old rate for
     at least the old HELLO_LEAVE period.

   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 bits): bit unsigned integer): IP protocol version of the feed
     bidirectional IP addresses (FBIP):
     4 = IP version 4
     6 = IP version 6
   Tunnel Type (8 bits): bit unsigned integer): tunneling protocol supported by feed,
     corresponds to
     the type feed; receivers MUST use this form of tunnel encapsulation used by receivers when
     tunneling to
     encapsulate packets which are tunneled: the feed.
     47 = GRE [rfc1701] [rfcXXXX] (recommended)
      x = any other tunneling supporting the UDL MAC packets.
     Other values may be used, but their interpretation is not specified
     here.

   Nb of FBIP (8 bits): 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 addr (32 or 128 bit field. bits). The feed bidirectional IP address feed
     is the IP address of a feed bidirectional interface (tunnel
     end-point) 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.

6.3.2.

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.

   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).

   The process in charge of sending HELLO packets fills every field of
   the datagram according to the description given in Section 6.3.1. 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 to: 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.

6.3.3.

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. The 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 entries
   (FUIP,FBIP1,...,FBIPn) and operation of
   this protocol. However, the method of discovery of this value is not
   specified here.

   Initially, the list of active feeds is initially 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.

   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, 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, the entry (FUIP,FBIP1,...,FBIPn) 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 is and Seq values are also read taken from the
      HELLO packet and recorded for this FUIP. 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 it is
      equal they
      are equal, the timer associated with this entry is reset to
      HELLO_LEAVE. Otherwise all the information corresponding to FUIP
      is reset. 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 which
      are (f1u,(f1b)) 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 (f2u,(f2b)). a list of tunnel end-points (f2b).

   - LEAVE:

      The process checks if there is an entry with the value for FUIP that
      belongs to in the list of
      active feeds. If it does, there is, the timer is disabled and the entry
      (FUIP,FBIP1,...,FBIPn) is
      deleted from the list and the
      corresponding timer is disabled. 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, Thus no more JOIN messages are
        received from any of the feeds. All the timeouts associated with
        entries (FUIP,FBIP1,...,FBIPn) expire at HELLO_LEAVE after the
        last JOIN message from the unidirectional link.

   In both cases, the associated (FUIP,FBIP1,...,FBIPn) entries are
   removed from the list feeds, and they will each timeout
        independently. The timeout of active feeds. As either each entry depends on its
        individual HELLO_LEAVE value, and when the feed does not
   route datagrams over last JOIN message was
        sent by that feed, before the unidirectional link or the link is down, went down.

   In either case, bidirectional connectivity can no longer be ensured
   between the receiver and the feed (FUIP). (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 the receivers with the a list of feeds, and a
   list of usable tunnel end-points (FBIP1,..., FBIPn) per for each feed. In
   the following
   section, Section, we describe how to integrate the HELLO
   protocol into the tunneling mechanism described in Sections 6.1 and
   6.2.

6.3.4.

7.4. Tunneling mechanism using the list of active feeds

   This section 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 done by the receiver administrator
   according to made independently at each
   receiver. The choice is a matter of local policy. This policy, and this policy is
   out of scope of in 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 feed, it chooses the must choose a tunnel end-
   point
   end-point from within the FBIP list. The 'preferred FBIP' is
   generally FBIP1
   (see 6.3.1). However, for some (Section 7.1). For various reasons, a receiver may have a better
   connectivity to another FBIPi of this feed. In that case, it is
   possible
   decide to use a different FBIP, say FBIPi instead of FBIP1 FBIP1, as the
   tunnel end-point. For example, the receiver may have better
   connectivity to FBIPi. This decision is taken by the receiver
   administrator.

   Several cases are enumerated in

   Here we show how the list of active feeds is involved when a receiver
   tunnels a link layer packet. Section 6.1 to determine where to
   forward a MAC packet according to its destination address. If 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 This is TRUE if the address matches with
        the MAC a FUMAC
        address of an FUIP in the list of active feeds. The
        datagram packet is encapsulated and sent tunneled to
        the preferred FBIP of the matching feed. The encapsulation type depends on the tunnel type required
        by the feed FUIP.

     2) the broadcast address of the unidirectional link or a multicast
        address: a copy of the datagram
        This is encapsulated determined by the MAC address format rules, and sent to the
        preferred FBIP list
        of active feeds is not involved. The packet is tunneled to the default feed. The encapsulation type
        depends on the tunnel type required by
        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 (i.e., it does not match a MAC address of any
        FUIP is neither broadcast nor multicast,
        nor found in the list of active feeds): the datagram feeds. The packet is encapsulated
        and sent tunneled to
        the preferred FBIP of the default feed. The

   In all cases, the encapsulation type depends on the tunnel type
   required by the
        default feed.

6.3.5. feed which is selected.

7.5. Constant definitions

   DTCP_VERSION is 1.

   HELLO_INTERVAL is 5 seconds.

   "DTCP announcement" multicast group is 224.0.1.124.

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

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

7.
      seconds if the default HELLO_INTERVAL was advertised.

8. Tunnel encapsulation format

   The tunneling mechanism operates at the link layer level and emulates
   bidirectional connectivity between 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 of
   about the encapsulated data type.

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

   The

   Generic Routing Encapsulation (GRE) [rfc1701] [rfcXXXX] suits our requirements
   because it specifies a protocol for encapsulating arbitrary packets within packets,
   and allows use of IP as the delivery protocol. Alternatively,
   we can also encapsulate

   Other encapsulations are possible, such as directly encapsulating a
   MAC level packet within an IP datagram.

   It is the

   The feed's local administrator who decides what encapsulation
   is used by a receiver to send a packet via a tunnel to this feed. The 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 either set to GRE or to any
   other encapsulation supporting UDL MAC level packets.

7.1. 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
   [rfc1701][rfc1702] [rfcXXXX] for
   details about the encapsulation. In our case, only the support of 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 level type of the UDL      |
           ----------------------------------------
           |            Payload packet            |
           |             MAC packet               |
           ----------------------------------------

                 Figure 4: Encapsulated packet

7.2.

8.2. Encapsulation of UDL MAC level packets

   An alternative is to encapsulate the MAC level packet within IP. The
   protocol field in the IP datagram is then set to the MAC level type of the
   unidirectional link. Figure 5 presents the entire encapsulated
   packet.

           ----------------------------------------
           |           IP delivery header         |
           |        destination addr = FBIP       |
           |    IP proto = MAC level type of the UDL    |
           ----------------------------------------
           |            Payload packet            |
           |             MAC packet               |
           ----------------------------------------

                 Figure 5: Encapsulated packet

8.

9. Issues

8.1.

9.1. Hardware address resolution

   Regardless of whether the link is unidirectional or bidirectional links, bidirectional, if
   a feed sends a packet over a broadcast non-point-to-point type network network, it
   requires the data link address of the destination. ARP [rfc826] is
   used on an Ethernet
   network networks for that 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 back-channel 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.

   Because of long round trip delays, current reactive address resolution
   methods such as ARP [rfc826] may not be efficient. E.g., work well. For example, a feed has
   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), 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.

   The inefficiency

   This problem of address resolution protocols is not addressed in this
   document. An ad-hoc solution is proposed possible when the MAC address is
   configurable (which
   configurable, which is the case possible in some satellite receiver cards). It
   consists in mapping cards. A
   simple transformation (maybe null) of the IP address on a can then be used
   as the MAC address. In this case, no senders do not need to "resolve" an
   IP address resolution protocol is required.

8.2. 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 layer and
   above higher
   layers the fact that feeds and receivers are connected by a
   unidirectional link. Communication is bidirectional bidirectional, but asymmetric
   in bandwidths and delays.

   In order to incorporate unidirectional links in the Internet, feeds
   and receivers have to must run routing protocols. They These protocols will work
   fine because feeds and receivers consider themselves as directly
   connected, the tunneling mechanism results in bidirectional
   connectivity between all feeds and can exchange receivers. Thus routing messages over the unidirectional
   link.
   can be exchanged as on any bidirectional network.

   The tunneling mechanism allows one to forward all the any IP traffic, and not only just routing
   protocol messages from receivers 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 can run multicast routing daemon daemons 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.

8.3.

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 a more general issue issues such as the
   maximum number of nodes which can be connected to a any link. This is
   out of scope of this document.

9.

10. Security Considerations

   Receivers send packets through tunnels to feeds. Because of
   scalability reasons, there

   Confidentiality and integrity concerns may arise from the lower layer
   technologies employed, e.g. if the unidirectional link is no specific mechanism in this document
   to ensure that a receiver satellite
   link and the backchannel is allowed the public internet.  Since this protocol
   aims to set support a link layer, link layer confidentiality and
   integrity mechanisms may be appropriate. In the case of the
   backchannel only, IPSEC [rfc2401] may provide appropriate services.

   Theft of service and denial of service attacks become possible to
   systems which can discover the feed tunnel end-point addresses and
   can direct packets to a feed. Any
   malicious individual that gains access them. It may be appropriate for feeds to
   authenticate tunnel sources, i.e. receivers. Feeds can validate the unidirectional link
   IP source addresses of tunneled packets, but this can
   get full connectivity be easily
   spoofed. MAC layer filtering may also be possible. Adequate
   protection can be ensured using IPSEC [rfc2401] AH [rfc2402] to feeds. Therefore it
   provide strong authentication of tunnel sources. For reasons of
   scalability, no particular mechanism is highly recommended on
   the feed site to have some firewall capabilities.

10. specified in this protocol.

11. 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 (INRIA) 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).

11.

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."

References

   [rfc826] 'An Ethernet Address Resolution Protocol', David C. Plummer,
             November 1982.

   [rfc1112] 'Host Extensions for IP Multicasting', S. Deering, Stanford
             University, August 1989

   [rfc1702] 'Generic Routing Encapsulation over IPv4 networks',

   [rfc2401] 'Security Architecture for the Internet Protocol', S.
             Hanks, NetSmiths, Ltd., T. Li, D. Farinacci, P. Traina,
             Cisco System, October 1994.

   [rfc1701] 'Generic Routing Encapsulation (GRE)', Kent,
             BBN Corp, R. Atkinson, @Home Network

   [rfc2402] 'IP Authentication Header', S. Hanks, NetSmiths,
             Ltd., T. Li, D. Farinacci, P. Traina, Cisco System, October
             1994. Kent, BBN Corp, R. Atkinson,
             @Home Network

   [rfc2453] 'RIP Version 2', G. Malkin, Bay Networks, November 1998

   [rfcXXXX] 'Generic Routing Encapsulation (GRE)', D. Farinacci, T. Li,
             S. Hanks, D. Meyer, P. Traina.

Author's address

   Emmanuel Duros
   INRIA Sophia Antipolis
   2004, Route des Lucioles BP 93
   06902 Sophia Antipolis
   France
   Phone : +33 4 92 38 79 42
   Fax   : +33 4 92 38 79 78
   Email : Emmanuel.Duros@inria.fr

   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
   Japan Satellite Systems Inc.
   Toranomon 17 Mori Bldg.5F
   1-26-5 Toranomon, Minato-ku
   Tokyo 105
   Japan
   Voice : +81-3-5511-7568
   Fax   : +81-3-5512-7181
   Email : izu@jcsat.co.jp

   Noboru Fujii
   Sony Corporation
   2-10-14 Osaki, Shinagawa-ku
   Tokyo 141
   Japan
   Voice : +81-3-3495-3092
   Fax   : +81-3-3495-3527
   Email : fujii@dct.sony.co.jp

   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