Movatterモバイル変換

Network Working Group                                           E. DurosRequest for Comments: 3077                                        UDcastCategory: Standards Track                                     W. Dabbous                                                  INRIA Sophia-Antipolis                                                            H. Izumiyama                                                                N. Fujii                                                                    WIDE                                                                Y. Zhang                                                                     HRL                                                              March 2001A Link-Layer Tunneling Mechanism for Unidirectional LinksStatus 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, seeSection 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 insection 3 with a tunneling mechanismDuros, 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 20013. Topology   Feeds and receivers are connected via a unidirectional link.  Send-   only feeds can only send data over this unidirectional link, and   receivers can only receive data from it.  Receive capable feeds have   both send and receive capabilities.   This mechanism has been designed to work with any topology with any   number of receivers and one or more feeds.  However, it is expected   that the number of feeds will be small.  In particular, the special   case of a single send-only feed and multiple receivers is among the   topologies supported.   A receiver has several interfaces, a receive-only interface and one   or more additional bidirectional communication interfaces.   A feed has several interfaces, a send-only or a send-and-receive   capable interface connected to the unidirectional link and one or   more additional bidirectional communication interfaces.  A feed MUST   be a router.   Tunnels are constructed between the bidirectional interfaces of   nodes, so these interfaces must be interconnected by an IP   infrastructure.  In this document we assume that that infrastructure   is the Internet.   Figure 1 depicts a generic topology with several feeds and several   receivers.                     Unidirectional Link         ---->---------->------------------->------          |          |               |           |          |f1u       |f2u            |r2u        |r1u      --------   --------        --------    --------   ----------      |Feed 1|   |Feed 2|        |Recv 2|    |Recv 1|---|subnet A|      --------   --------        --------    --------   ----------          |f1b       |f2b            |r2b        |r1b      |          |          |               |           |         |         ----------------------------------------------------         |                     Internet                     |         ----------------------------------------------------                     Figure 1: Generic topology   f1u (resp. f2u) is the IP address of the 'Feed 1' (resp. Feed 2)       send-only interface.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, aDuros, 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 20016. 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 MechanismDuros, et al.               Standards Track                     [Page 6]

RFC 3077            LL Tunneling Mechanism for UDLs           March 20016.1. 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 inSection 7.  The type of encapsulation is   described inSection 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.  SeeSection 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   -  packets generated by a local application or packets routed as      usual by the IP layer may have to be forwarded over the      unidirectional link (Section 6.2.1)   -  encapsulated packets received from another receiver or feed need      tunnel processing (Section 6.2.2).   A feed cannot directly send a packet to a send-only feed over the   unidirectional link (Scenario 4).  In order to emulate this type of   communication, feeds have to tunnel packets to send-only feeds.  A   feed MUST maintain a list of all other feed tunnel end-points.  This   list MUST indicate which are send-only feed tunnel end-points.  This   is configured manually at the feed by the local administrator, as   described inSection 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 inSection 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:            -  The packet is encapsulated and sent to the list of send-               only feed tunnel end-points.            -  Also, the packet is passed to the link-layer of the               interface which forwards it directly over the               unidirectional link (all receivers and receive capable               feeds receive it).            -  Also, the link-layer delivers it locally to higher               layers.            Caution: a receiver which sends an encapsulated            broadcast/multicast packet to a default feed will receive            its own packet via the unidirectional link.  Correct            filtering as described in [RFC1112] must be applied.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 inSection 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 inAppendix 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 inSection 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 inSection 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:   -  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 inSection 3, both receivers (recv 1 and      recv 2) have a list of active feeds containing two entries: Feed 1      with a FUIP of f1u and a list of tunnel end-points (f1b); and Feed      2 with a FUIP of f2u and a list of tunnel end-points (f2b).   -  LEAVE:      The process checks if there is an entry for FUIP in the list of      active feeds.  If there is, the timer is disabled and the entry is      deleted from the list.  The LEAVE message provides a means of      quickly updating the list of active feeds.   A timeout occurs for either of two reasons:      1) a feed went down without sending a LEAVE message.  As JOIN         messages are no longer sent from this feed, a timeout occurs at         HELLO_LEAVE after the last JOIN message.      2) the unidirectional link is down.  Thus no more JOIN messages         are received from any of the feeds, and they will each timeout         independently.  The timeout of each entry depends on itsDuros, 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 Sections6.1 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   toSection 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.            ----------------------------------------            |           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 packet9. Issues9.1. 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 20019.3. 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 inSection 7.   IANA has reserved the udp port 652 for the HELLO_PORT as defined inSection 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 200112. 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 2001Appendix A: Conformance and interoperability   This document describes a mechanism to emulate bidirectional   connectivity between nodes that are directly connected by a   unidirectional link.  Applicability over a variety of equipment and   environments is ensured by allowing a choice of several key system   parameters.   Thus in order to ensure interoperability of equipment it is not   enough to simply claim conformance with the mechanism defined here.   A usage profile for a particular environment will require the   definition of several parameters:      - the MAC format used      - the tunneling mechanism to be used (GRE is recommended)      - the "tunnel type" indication if GRE is not used   For example, a system might claim to implement "the link-layer   tunneling mechanism for unidirectional links, using IEEE 802 LLC, and   GRE encapsulation for the tunnels."Appendix B: DTCP announcement address transition plan   Some older receivers listen for DTCP announcements on the 224.0.1.124   multicast address (the "old DTCP announcement" address).  In order to   support such legacy receivers, feeds SHOULD be configurable to send   all announcements simultaneously to both the "DTCP announcement"   address, and the "old DTCP announcement" address.  The default   setting is to send announcements to just the "DTCP announcement"   address.   In order to encourage the transition plan, the "old" feeds MUST be   updated to send DTCP announcements as defined in this section.  The   number of "old" feeds originally deployed is relatively small and   therefore the update should be fairly easy.  "New" receivers only   support "new" feeds, i.e., they listen to DTCP announcements on the   "DTCP announcement" address.Duros, et al.               Standards Track                    [Page 21]

RFC 3077            LL Tunneling Mechanism for UDLs           March 2001References   [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,RFC1700, 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",RFC2402, 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",BCP37,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 2001Authors' 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.jpDuros, 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.comDuros, et al.               Standards Track                    [Page 24]

RFC 3077            LL Tunneling Mechanism for UDLs           March 2001Full 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]