R&D Information
OCN Introduces ENCORE Inter-AS Diagnostic
System Targeting the Construction of an Internet
Environment with Outstanding Stability

In April 2004, NTT and NTT Communications
introduced ENCORE*1, a new inter-AS (autonomoussystem) diagnostic system developed by NTT Laboratories. The system, which has been incorporated
into NTT Com’s Internet access network OCN*2, willautomatically monitor and diagnose anomalies inrouting information between multiple ISPs (Internetservice providers) on the Internet. It makes it possible
to quickly discover, analyze, and recover from anomalies in routing information between multiple ISPs—
which had been difficult in the past, when individual
ISPs performed monitoring and diagnosis manually—and to construct high-quality Internet environments that offer outstanding stability. The technicalfeatures of ENCORE are described below.
(1) ENCORE places agents with monitoring anddiagnostic capabilities at each ISP, integrates theinformation they gathered to infer routing information behavior, and analyzes the causes of anomalies.
When an abnormality is detected through regularmonitoring, the system diagnoses the anomaly andidentifies the factors inhibiting communications.
(2) If a route is omitted due to an erroneous filter
setting by the network operator or a difference inpolicies among ISPs, the agents at the two ISPs inquestion exchange and analyze observation information to automatically detect routes that are missing
from a given ISP’s routing table, enabling improved
stability of IP packet transmission.
(3) ENCORE detects cases in which a givenrouter’s routing table has been rewritten due to anerroneous routing information advertisement fromanother ISP, and identifies whether the source of theerror is being operated as a proper punching hole*3.
In this way, if a given ISP’s route has been takenover as a result of an illegal setting by a malicious
third party or an erroneous setting resulting from anoversight by a network operator, the situation can be

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quickly and automatically detected, and the ISP at theroot of the problem can be identified.
The Internet is a conglomeration of countless networks, and the stability of connections between networks is dramatically affected by the service qualityof individual ISPs. NTT Com has constructed a routing monitoring system using ENCORE as its engine,and has incorporated this system into OCN, a world’s
first for a commercial Internet service. This implementation is part of NTT Com’s efforts to improveconnectivity, which is a fundamental element of network quality.
NTT Laboratories will continue to promoteresearch and development in an autonomous network
management environment based on intelligent agentsas an extension of the ENCORE system. At the sametime, NTT Com will continue its efforts to provide
the world’s highest level of quality and service as aGlobal IP Solutions Company, aiming for even saferand more secure network applications.
For further information, please contact
NTT Information Sharing Laboratory Group
Musashino-shi, 180-8585 Japan
E-mail: koho@mail.rdc.ntt.co.jp*1 ENCORE: An inter-AS diagnostic ensemble system using cooperative reflector agents. NTT Network Service Systems Laboratories created an illegal route detection system based on an intelligent diagnostic system developed by NTT Network Innovation
Laboratories with additional new functions.
*2 OCN (open computer network): A large-scale domestic Internet
connection service provided by NTT Com. Service began inDecember 1996, and the number of subscribers surpassed four
million in November 2003.
*3 Punching hole: A multihomed site connected to multiple ISPs may
obtain a small prefix from an ISP and announce it from multiple
ISPs to achieve better reachability. This technique is called“punching hole”.

NTT Technical Review