A Link-Layer Tunneling Mechanism for Unidirectional Links
RFC 3077
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(March 2001; No errata)
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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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