The Internet is mostly composed of networks interconnected via heterogeneous, but bidirectional and symmetric links. The links have different bandwidth and each of them generally have similar transmission characteristics in both directions. Internet routing and transmission control protocols have been designed to optimize the transmission of data over such an architecture by assuming that links offer bidirectional and symmetric communication.New transmission media have been proposed to provide higher bandwidth to the Internet, i.e. broadcast satellite links offering unidirectional access to the Internet. This link provides high speed bandwidth downstream with zero bandwidth upstream.
Most Internet protocols have been designed assuming that the underlying technology was bidirectional. As an example, a set of routers which are connected through a single physical network assume they can exchange routing information with each other. Exchanging routing information enables the discovery of the underlying network topology, and the routing of Internet traffic via discovered networks. However, if the link connecting these routers is unidirectional, we can say that all downstream routers have receive-only capabilities and therefore cannot send routing information to upstream routers. As a result, upstream routers cannot discover downstream network topologies dynamically and will therefore never forward Internet traffic towards them.
Generally, in the presence of a unidirectional link, many Internet protocols, such as IGMP and routing protocols, will fail to operate. The link cannot be used, therefore, to provide full Internet connectivity.
The UDLR working group was created in April 1997 at the IETF in order to provide a solution for the support of unidirectional links in the Internet. The working group issued the RFC 3077 which describes "A Link-Layer Tunneling Mechanism for Unidirectional Links" that allows Internet protocols to operate. The main objective of the working group, as described in the charter , has now been accomplished. Additional objectives, described below, have now been assigned to the working group.
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As of the publication of RFC 3077, the working group has decided to further their work by issuing the following objectives:
:q1- Produce informational RFCs that describe in detail the requirements of example case study scenarios as listed below. Note that these scenarios relate to field experiments that are being undertaken based on the LLTM mechanism:
a) configuration of multicast routing protocols over a unidirectional link2- Identify security holes
b) configuration of unicast routing protocols over a unidirectional link
c) securing operation of a UDLR network
d) other relevant experiments ?
The goals and milestones can be found at the UDLR page on the IETF web site .