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rfc:rfc3077

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

Duros, et al. Standards Track [Page 1] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

 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.

Duros, et al. Standards Track [Page 2] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

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
  1. —>———→——————→——

| | | |

        |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.

Duros, et al. Standards Track [Page 3] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

 f1b (resp. f2b) is the IP address of the 'Feed 1' (resp. Feed 2)
     bidirectional interface connected to the Internet.
 r1u (resp. r2u) is the IP address of the 'Receiver 1' (resp. Receiver
     2) receive-only interface.
 r1b (resp. r2b) is the IP address of the 'Receiver 1' (resp. Receiver
     2) bidirectional interface connected to the Internet.
 Subnet A is a local area network connected to recv1.
 Note that nodes have IP addresses on their unidirectional and their
 bidirectional interfaces.  The addresses on the unidirectional
 interfaces (f1u, f2u, r1u, r2u) will be drawn from the same IP
 network.  In general the addresses on the bidirectional interfaces
 (f1b, f2b, r1b, r2b) will be drawn from different IP networks, and
 the Internet will route between them.

4. Problems related to unidirectional links

 Receive-only interfaces are "dumb" and send-only interfaces are
 "deaf".  Thus a datagram passed to the link-layer driver of a
 receive-only interface is simply discarded.  The link-layer of a
 send-only interface never receives anything.
 The network layer has no knowledge of the underlying transmission
 technology except that it considers its access as bidirectional.
 Basically, for outgoing datagrams, the network layer selects the
 correct first hop on the connected network according to a routing
 table and passes the packet(s) to the appropriate link-layer driver.
 Referring to Figure 1, Recv 1 and Feed 1 belong to the same network.
 However, if Recv 1 initiates a 'ping f1u', it cannot get a response
 from Feed 1.  The network layer of Recv 1 delivers the packet to the
 driver of the receive-only interface, which obviously cannot send it
 to the feed.
 Many protocols in the Internet assume that links are bidirectional.
 In particular, routing protocols used by directly connected routers
 no longer behave properly in the presence of a unidirectional link.

5. Emulating a broadcast bidirectional network

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

Duros, et al. Standards Track [Page 4] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

 link-layer network allows the immediate use of any other network
 layer protocols, and most particularly allows the immediate use of
 ARP.
 A link-layer tunneling mechanism which emulates bidirectional
 connectivity in the presence of a unidirectional link will be
 described in the next Section.  We first consider the various
 communication scenarios which characterize a broadcast network in
 order to define what functionalities the link-layer tunneling
 mechanism has to perform in order to emulate a bidirectional
 broadcast link.
 Here we enumerate the scenarios which would be feasible on a
 broadcast network, i.e., if feeds and receivers were connected by a
 bidirectional broadcast link:
 Scenario 1: A receiver can send a packet to a feed (point-to-point
    communication between a receiver and a feed).
 Scenario 2: A receiver can send a broadcast/multicast packet on the
    link to all nodes (point-to-multipoint).
 Scenario 3: A receiver can send a packet to another receiver (point-
    to-point communication between two receivers).
 Scenario 4: A feed can send a packet to a send-only feed (point-to-
    point communication between two feeds).
 Scenario 5: A feed can send a broadcast/multicast packet on the link
    to all nodes (point-to-multipoint).
 Scenario 6: A feed can send a packet to a receiver or a receive
    capable feed (point-to-point).
 These scenarios are possible on a broadcast network.  Scenario 6 is
 already feasible on the unidirectional link.  The link-layer
 tunneling mechanism should therefore provide the functionality to
 support scenarios 1 to 5.
 Note that regular IP forwarding over such an emulated network (i.e.,
 using the emulated network as a transit network) works correctly; the
 next hop address at the receiver will be the unidirectional link
 address of another router (a feed or a receiver) which will then
 relay the packet.

Duros, et al. Standards Track [Page 5] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

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

Duros, et al. Standards Track [Page 6] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

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:

Duros, et al. Standards Track [Page 7] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

  1. 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)
  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.

Duros, et al. Standards Track [Page 8] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

 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:
  1. The packet is encapsulated and sent to the list of send-

only feed tunnel end-points.

  1. 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.

Duros, et al. Standards Track [Page 9] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

      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.

Duros, et al. Standards Track [Page 10] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

 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.

Duros, et al. Standards Track [Page 11] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

 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).

Duros, et al. Standards Track [Page 12] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

 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.

Duros, et al. Standards Track [Page 13] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

 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:
  1. 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).
  1. 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

Duros, et al. Standards Track [Page 14] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

       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:

Duros, et al. Standards Track [Page 15] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

       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.

Duros, et al. Standards Track [Page 16] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

 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.
  1. —————————————

| 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.

Duros, et al. Standards Track [Page 17] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

 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.

Duros, et al. Standards Track [Page 18] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

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.

Duros, et al. Standards Track [Page 19] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

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).

Duros, et al. Standards Track [Page 20] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

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:
  1. the MAC format used
  2. the tunneling mechanism to be used (GRE is recommended)
  3. 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.

Duros, et al. Standards Track [Page 21] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

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.

Duros, et al. Standards Track [Page 22] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

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

Duros, et al. Standards Track [Page 23] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

 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

Duros, et al. Standards Track [Page 24] RFC 3077 LL Tunneling Mechanism for UDLs March 2001

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.

Duros, et al. Standards Track [Page 25]

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