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Terminology/intro draft
As promised, here's the draft. I'm interested in comments (things I left
out? Other UDLN examples other than satellites) and additional references.
James: VIPRE and Split-IP draft stauts?
And before you ask: No, this hasn't been submitted yet. Do I seem that
crazy?
-scooter
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INTERNET-DRAFT W. Dabbous
<draft-udlr-terminology-01.txt> INRIA, Sophia-Antipolis
S. Michel
The Aerospace Corporation
Normalized Terminology for Unidirectional Link Routing
1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working docu-
ments of the Internet Engineering Task Force (IETF), its areas, and its
working groups. Note that other groups may also distribute working
documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."
To view the entire list of current Internet-Drafts, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), ftp.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
2. Abstract
This document provides a normalized reference terminology for components
found in unidirectional link networks (UDLNs). Differing terminologies
often exist for these components, resulting in ambiguity and confusion.
This document also provides a framework with which to discuss the uni-
directional link routing (UDLR) problem.
3. Unidirectional Link Networks
Many existing computer network technologies support a bidirectional
traffic channel model: a traffic source channel and a traffic return
channel. A unidirectional link network (UDLN) is a network where a the
traffic source channel exists, but the traffic return channel has zero
capacity along the same data link path. The traffic return channel is
provided by an alternate interface on the host in order to complete the
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draft-udlr-terminology-01.txt 18 July 1997
communication loop.
4. Terminology
The following definitions compose the normalized terminology of UDLNs.
Examples of networks which utilize this terminology are explained in
subsequent sections.
UDD An unidirectional device (UDD) is a physical device which
implements one half of a bidirectional communications channel,
or implements a single communications channel.
UDLN An unidirectional link network (UDLN) is network where the
bidirectional traffic model is not supported by the data link
layer. In general, there exist a set of uplink nodes which
feeds traffic to a set of downlink nodes. Traffic from the
downlink nodes back to the uplink nodes is sent on a separate
interface or data link path.
Feed A feed is a traffic flow which transit through an uplink, and
is transmitted along a unidirectional link layer.
Reachback A reachback is a network which provides connectivity from
downlink nodes to uplink nodes.
Uplink An uplink is a node where feeds are concentrated for transmis-
sion to a set of downlink nodes. An uplink node must support a
least two interfaces: one interface for transmitting feeds,
one interface for receiving from the reachback. An uplink may
be a host or router.
Downlink A downlink is a feed destination. A downlink node must support
at least two interfaces: one interface to receive feeds, one
interface for transmitting to the reachback. A downlink may be
a host or a router.
5. Example: Unidirectional Satellite Networks
Unidirectional satellite networks (UDSN) are the principle example of a
UDLN. UDSNs offer high bandwidth to multiple receivers over a large geo-
graphical area (satellite footprint). It is inexpensive to construct a
commodity satellite receiver. It is more expensive to construct a satel-
lite transmitter, in addition to which there are governmental regula-
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draft-udlr-terminology-01.txt 18 July 1997
tions which are attached to the use of satellite transmitters. The
principle components of a UDSN are shown in the following figure, bor-
rowed from [1]:
___ ___
\__\OO\__\ Satellite
^^ vv Network
/ \
feed / \
/ \
/ \
uplink / \
---- / V ----
---======>| | | | downlink(s)
---- ----
/\ /\
|| ||
\/ ||
------------------------- ||
| Reachback |<==/
-------------------------
6. Problems Encountered in UDLNs
6.1. Downlink-to-Uplink Network Connectivity
Downlink nodes must divert packets destined for the uplink node or nodes
to the reachback. Several approaches exist for solving this problem and
include, but are not limited to:
Encapsulation
A packet trap is inserted between IP and the UDD. The packet
is encapsulated and re-injected into IP for delivery to the
uplink.
Interface Redirection
The interface route on a downlink node to the uplink is
removed or otherwise rendered inoperative. The interface route
is replaced with a static route to the uplink node via the
reachback.
6.2. Dynamic Routing
The frequently encountered problem is multiple paths to the downlink
nodes advertised by routers outside the UDLN. The uplink node is the
only point of reachability to the downlink nodes. Interface redirection
partially solves the problem, but introduces another problem when the
downlink nodes are routers: the downlink will advertise the reachback as
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draft-udlr-terminology-01.txt 18 July 1997
the route to the uplink when other paths to the uplink may be preferred.
6.3. Multicast
The example UDSN illustrates a mechanism for distributing multicast
traffic over a broadcast medium. The uplink node must be aware of all
multicast group joins in order that multicast routing to operate
correctly. However, downlink nodes may only participating in a subset of
all multicast groups being routed by the uplink. Multicast prune mes-
sages become problematic because the messages are sent to the uplink
when encapsulation and packet diversion are employed. Solutions to the
multicast problem include adjusting metrics in the DVMRP routing proto-
col [2].
An additional problem arises in the UDSN when the uplink and the down-
link are part of the same subnetwork and encapsulation to send multicast
packets through the reachback to the uplink. A downlink multicast source
transmits a packet, which is encapsulated and the UDSN uplink rebroad-
casts it. The downlink multicast receives it own multicast packet
regardless of whether it wants to receive it or not.
7. References
[1] Dabbous, Walid, et. al. "Supporting unidirectional links in the
Internet". http://zenon.inria.fr/rodeo/udlr/overview.html
[2] Partridge, C., Deering, S. E., "Distance Vector Multicast Routing
Protocol". RFC 1075, November 1, 1988.
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