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6Bone is an IPv6 testbed to which
subnetworks located in most parts of the world are interconnected. It is a network
made up by interconnecting IPv6 islands, and is largely implemented as a network
layered on top of the current IPv4 Internet using the static tunneling technique.
The idea of setting up an experimental IPv6 backbone over the Internet originated as a
spontaneous initiative on the part of several research centers involved in testing the
first implementations of the IPv6 protocol. The network became a reality in March 1996,
when the first tunnels were established between the IPv6 laboratories run by G6 (France),
UNI-C (Denmark) and WIDE (Japan). Today, 6Bone is the network where the most interesting
geographical tests on the IPv6 protocol take place. These experimental activities are
coordinated by the IETF in order to assist technical specification work and help stabilize
protocol implementations through the experience gained in field trials.
The 6Bone network is organized in three hierarchical layers as illustrated in the figure
below: the core backbone layer, the transit node layer, and the peripheral or leaf node
layer. The 6Bone core backbone is made up of a mesh of IPv6 over IPv4 tunnels (with the
addition of several direct links) which connect only the backbone nodes. IPv6 routing in
the backbone is based on BGP4+ (Border Gateway Protocol version 4 extended to support IPv6)
(RFC2858).
The transit nodes are connected to one or more backbone nodes and, as their name implies,
provide transit service to the leaf nodes. Routing outside of the backbone is mainly
static, though the number of non-backbone nodes which employ protocols such as BGP4+ is
rapidly increasing.
IPv6 addressing on the 6Bone is based on the format established for IPv6 unicast addresses
(RFC2073).
The backbone nodes act as experimental TLAs (Top Level Aggregators) and for this reason are
also called pTLAs (pseudo TLAs). These nodes are responsible for assigning IPv6 addresses
to nodes belonging to lower hierarchical levels which are configured as their
"clients".
The entire 6Bone network is identified by a 16-bit prefix assigned directly by IANA, the
Internet Assigned Numbers Authority: each backbone node is assigned a 24- or 28-bit long
prefix ( the pTLA prefix) which identifies the address space which must be administered
according to the rules established by the IETF for TLAs. In this model, the pTLA performs
the function of an experimental top level ISP whose job is to manage a stable and reliable
network in a context where routing information aggregation must be guaranteed: ideally,
routing information which is more specific than that regarding pTLA prefixes should not
appear in the backbone.
Since 6Bone's creation in 1996, the number of nodes interconnected to the network has
grown steadily. In 1997, the network consisted of some 150 nodes; by February 2001, more
than 700 nodes in 50 countries around the world were officially registered in the 6Bone
database. In the same period, the number of backbone nodes rose from 36 to 82.
Participants in the 6Bone project include major manufacturers ( Cisco, 3COM, Compaq-Digital,
SUN, Microsoft, etc.), leading Service Providers (UUNET, MCI-Worldcom, AT&T, Verio, NTT,
Telecom Italia through CSELT, etc.), many academic or research networks and a large number
of university groups.
Telecom Italia Lab in 6Bone
Telecom Italia Lab (as CSELT) has been part of the 6Bone network since late 1996, when
it set up a network backbone node. Connectivity between the CSELT IPv6 laboratory and
other backbone nodes is guaranteed by a sufficiently large number of IPv6 over IPv4
tunnels established through the Internet and configured for the BGP4+ routing protocol.
In order to provide good connectivity performance, the main peering points were chosen
from the 6Bone backbone nodes which, through the IPv4-based Internet, guaranteed
the best reachability characteristics (i.e., the lowest packet losses and end-to-end delays).
From this position, CSELT offers experimental IPv6 access to a large number of
laboratories operated by universities, research centers, companies and Internet
Service Providers in Italy and abroad. In addition, CSELT has extended IPv6 "trial runs"
to a vast array of small users, who have been able to set up connections to the 6Bone
network through the IPv6 Tunnel Broker.
CSELT has also contributed to 6Bone testing by developing methods and tools for
monitoring BGP4+ routing performance on the 6Bone network. The most significant
achievement resulting from this effort to date is a software tool called ASpath-tree,
which provides useful information about network behavior and has also been made available
to the other 6Bone project participants.
Basically, ASpath-tree takes a snapshot of the BGP4+ routing table for an IPv6 router
and generates a set of Web pages that provide a graphic view of the AS (Autonomous System)
paths to other nodes. This snapshot also pinpoints any invalid or unaggregated
route entries. When ASpath-tree is executed periodically, moreover, it can use data
from snapshots taken over a period of time to analyze routing stability.
At CSELT, ASpath-tree is run at regular 5-minute intervals by one of the two border routers used for 6Bone access
(routing information).
For further off-line processing, a selected set of captured data is stored in a local archive, including:
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· The number of daily snapshots of pTLA prefixes for which the associated route entry is unavailable.
- The number of prefixes in the BGP4+ table (total and pTLA) at each snapshot.
- The number of daily route changes towards each pTLA prefix.
- The number of daily snapshots of pTLA prefixes for which the associated route entry is unavailable.
This procedure has been in place since September 1998, making it possible to collect a
huge amount of data on 6Bone routing quality and to generate reports that have been
extremely well-received by IETF and the Internet community
(6Bone routing history).
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