A Link-Layer Tunneling Mechanism for Unidirectional Links
RFC 3077

Document Type RFC - Proposed Standard (March 2001; No errata)
Last updated 2013-03-02
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Network Working Group                                           E. Duros
Request for Comments: 3077                                        UDcast
Category: Standards Track                                     W. Dabbous
                                                  INRIA Sophia-Antipolis
                                                            H. Izumiyama
                                                                N. Fujii
                                                                    WIDE
                                                                Y. Zhang
                                                                     HRL
                                                              March 2001

       A Link-Layer Tunneling Mechanism for Unidirectional Links

Status of this Memo

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

Copyright Notice

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

Abstract

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

1. Introduction

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

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

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.
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